Significance With the continuous exploration of novel materials, especially nanomaterials, in developing advanced flexible and high-performance micro/nano optical and electrical devices, high-quality nanojoint formation within nanomaterials has become a key issue for device nanofabrication. However, with the restriction of the size and microstructures of nanomaterials, conventional macro-and microjoining technologies cannot achieve highly controlled spatial energy input within the structures, and therefore fail to produce low-damage joints. Optical nanojoining (i.e. plasmonic nanojoining) technology, arising from the surface plasmon resonance generated at the metal-dielectric interface, is advantageous for joining nanomaterials. Specifically, the spatial energy input is confined at the locations with geometric discontinuities, owing to the strong localized plasmonic effect. Therefore, material damage is minimized, even when the entire nanostructure is covered by a large laser beam. This noncontact laser nanojoining technique permits precise and low-damage material interconnection at the nanoscale. Compared with other joining methods, such as nanobrazing and focused ion/electron-beam nanojoining, laser nanojoining can greatly simplify the joining process and reduce the demand for high-precision operation of the energy input. In addition, it is known that the introduction of an ultrafast laser with a pulse duration of femtoseconds or picoseconds can further enhance the electromagnetic field intensity generated by the surface plasmonic effect, which can extend the processed materials selection from metal to oxide/semiconductor. Therefore, ultrafast laser nanojoining can enable heterogeneous material integration, which is of vital importance to the implementation of advanced nanomaterials in micro/nanoelectronic applications.

Progress Since the plasmonic nanowelding of silver nanowires was demonstrated in detail by E. C. Garnett in 2012, the nanojoining of metal nanomaterials, including nanoparticles, nanorods, and nanowires, has been widely studied. Self-limited energy inputs within the nanostructures during the nanojoining process have been observed for low-damage nanojoint formation. Initially, most research work only focused on improving the electrical conduction of the joined metal nanowire networks by using lamps in the visible spectrum. However, because of its low efficiency of energy conversion and high dissipation of incident energy, the lamp was replaced with a laser beam. Based on the processed nanomaterials, laser nanojoining has shown high efficiency and low material damage due to thermal accumulation, even for temperature-and environment-sensitive materials. Therefore, the production rate has been improved by several orders of magnitude, as has the electrical performance.

Although laser nanojoining has been used widely in the fabrication of nanowire-based transparent electrodes, the material selection is limited to metals, owing to the low photon-absorption efficiency of dielectric materials (e.g., oxides and semiconductors) under conventional laser-beam irradiation. A. Hu and Y. Zhou proposed applying an ultrafast laser for nanojoining a broad range of materials. Because of the effects of nonlinear photon absorption and intense electromagnetic fields, oxides and semiconductors can be processed accordingly under ultrafast laser irradiation. On this basis, L. C. Lin systematically studied the ultrafast laser nanojoining of materials in different combinations. Specifically, heterogeneous metal and oxide nanowire joining was demonstrated using femtosecond laser irradiation. The as-received heterogeneous nanowire joint shows robust joint strength and improves electrical conduction, further demonstrating the effectiveness of using an ultrafast laser to join metal and oxide materials. Notably, this ultrafast laser nanojoining process is a generic joining technology, in which the joined materials are not limited to metals and oxides, but can also be applied to other metal and dielectric combinations.

With the formation of low-damage homogeneous and heterogeneous nanowire joints, the development of nanowires in nanodevices has become possible. Notably, ultrafast laser nanojoining has shown great advantages over the nanosecond laser in the fabrication of transparent electrodes, as the substrate can be protected well during ultrafast laser irradiation (Fig. 10). Further, the robust and stable nanowire joints show various applications in nanoelectronics, including single-nanowire electrical units or even nanowire sensors. However, as summarized, the material combination by ultrafast laser nanojoining is limited presently to metal-metal and metal-oxide/semiconductor applications, which are based on the surface plasmonic effects during laser-matter interaction. Broader combinations (e.g., oxide-oxide) by ultrafast laser nanojoining with low damage have not been studied, as the plasmonic effect no longer exists in such dielectric environments under optical excitation. Therefore, other mechanisms (e.g., nonlinear photon absorption) during laser-matter interaction may be an alternative to extend ultrafast laser nanojoining to dielectric-dielectric material joining.

Conclusion and Prospect Ultrafast laser nanojoining has been used successfully in low-damage nanowire joining with broad material combinations, including metal-metal and metal-oxide/semiconductor. The spatial energy input within the nanowire structures, arising from the localized plasmonic effects, can be confined precisely at the junction area, which greatly simplifies the operation of the laser beam and thus allows mass production of high-quality nanowire joints. By constructing nanoscale homogeneous and heterogeneous joints, ultrafast laser nanojoining can be used not only in fabricating individual functional nanowire devices, but also in scalable material integration, which shows great potential in applications including small-scale additive manufacturing and integrated nanoelectronics manufacturing.

Significance Nanomaterials have been researched and developed in the fields of solar cells, biological detection, sensors, and information storage. However, the interconnection between nanomaterials and external units is limited to simple mechanical contact, and many nanoscale features, such as excellent electrical, optical, and magnetic properties, are not exhibited. The rapid development of nanotechnology has high demands on the joining technology of nanomaterial units to realize complex functional systems. The interconnection of nanomaterials is the basis of nanoscale product integration and will immensely enrich its functionality.

Progress According to the size of the joining materials, if the size is at least in the range 1--100 nm, it is called nanojoining. The essence of nanojoining technology is material interconnection, and conventional joining methods via the force/heat strategy are still applicable in nanojoining. Compared with traditional macro-joining, nanomaterials are melted or interdiffused to obtain effective joints. By using the nanosize effect, the sintering temperature of metallic nanoparticles (NPs) will be much lower than the melting point of the bulk metal, they will be interconnected by sintering at a low temperature, and the metallurgical interface will be formed by diffusion. Surface diffusion is the main sintering mechanism of NPs, while the grain boundary diffusion is the sintering mechanism of large particles.

The metallurgical connection between the metal materials is realized via cold welding without external direct energy input. In situ transmission electron microscopy shows that the joining is almost perfect (Fig. 7). Compared with the traditional joints, the cold-welding joint has the same crystal orientation, strength, and conductivity. For nanowires, the size that can be cold-welded is about 10 nm, whereas that of nanofilms is limited to 2--3 nm.

Laser irradiation is one of the most common joining methods in nanomaterials. This method can avoid the high requirement for mechanical manipulation in cold welding. Surface plasmon heated local nanomaterials, which could achieve cross-scale, cross-material low-damage joining. Owing to surface excitation, the electromagnetic field occurring in the metal nanostructures and the enhanced plasmon contributes to heat and join nanomaterials. In addition to the strong thermal effect of surface plasmon, the electromagnetic field will promote interconnection. If a femtosecond laser with low power density is irradiated, particles will achieve an orderly arrangement. If the laser power density is high, the ends of the nanorod will be arranged under the action of local heat, and the crystal faces will match to realize interconnection.

Numerous studies have been conducted on the interconnection of various metals and nonmetals with the formation of electrical signal connections in the printed electronic products as the main driving force. The interconnection of heterogeneous and homogeneous nanomaterials has the same diffusion mechanism, but the challenge of heterogeneous material interconnection is the lattice matching at the interface. When an ultrafast laser irradiates Ag and Pt NPs, Ag NPs are first melted and interconnected with the surrounding Pt NPs. Ag NPs act as metal solder, and the interface shows good Ag-Pt lattice matching (Fig. 12).

Conclusions and Prospects Nanoscience provides many strategies for building high-performance materials and devices. The bottom-up manufacturing process is conducive to large-scale synthesis, the joining and interconnections, especially heterogeneous nanomaterials, still need further development. The joining between materials should be extended to different systems to ensure the versatility of interconnected nanomaterials and devices and meet the design function requirements. An essential factor in the interconnection of nanomaterials is to precisely control the melting depth to prevent NPs from merging to form a single particle. To avoid excessive damage, space-limited energy input will become necessary. Ultrafast laser-precise irradiation may be an ideal method for joining and interconnection of nanomaterials.

Significance Due to the unique mechanical, electrical, thermal and optical properties, one-dimensional nanomaterials have a wide range of applications in micro- and nano- electro-mechanical systems, flexible transparent conductive devices, and sensors. Different from zero-dimensional and two-dimensional nanomaterials, one-dimensional nanomaterials have larger ratios of length to diameter. To build nano-structures or nano-devices with one-dimensional nanomaterials, both the position and posture (or rotation angle) need to be considered. Therefore, how to precisely control the pose of one-dimensional nanomaterials and then connect them with nanoscale, microscale, or bulk materials is crucial to realize the functionalization and deviceization of nanostructures.

Although one-dimensional nanomaterials have a variety of applications, precise manipulation of their pose and forming a reliable nanoscale interconnection with other materials are still great challenges. It is well known that nanomaterials have a small size, which makes them difficult to manipulate their poses. Manipulation methods, such as mechanical clamping, can easily cause damage to nanomaterials. In addition, the small size of nanomaterials results in a large specific surface area and high surface energy, which significantly reduces the melting point of nanomaterials and makes the nanomaterials susceptible to oxidation. How to connect nanomaterials and other materials without affecting the non-connection parts of nanomaterials is crucial for preparing high-performance nano-joints. To connect nanomaterials, many methods have been proposed, including mechanical pressing, thermal annealing, chemical treatment, cold welding, and light-induced plasmonic nanojoining. One of the potential nanojoining technologies is light-induced plasmonic nanojoining, which utilizes white light or laser to excite local surface plasmon resonances, thereby resulting in local heating. By precisely adjusting the intensity of the incident laser, the connection position of nanomaterials can be slightly melted, resulting in nanojoining of nanomaterial with other materials. Light-induced plasmonic nanojoining is a high-efficiency and low-damage nanojoining method.

In the past few years, various methods have been developed to manipulate the pose of one-dimensional nanomaterials, and then to join them by laser. According to the principle of nano-manipulation, these nano-manipulation methods are summarized into three types: probe method, self-assembly, and optical tweezers. Combining the pose manipulation with laser-induced plasmonic nanojoining of one-dimensional nanomaterials, we introduce the principles and characteristics of pose regulation of one-dimensional nanomaterials in detail, as well as the new development in laser-induced plasmonic nanojoining.

Progress The probe method utilizes probes to move and rotate one-dimensional nanomaterials to adjust their poses precisely. Due to the small size of one-dimensional nanomaterials, the probe tip is generally on the nanoscale. In addition, to manipulate the poses of one-dimensional nanomaterials, the probe method usually requires high-resolution microscope to locate nanomaterials. The microscopes used in the probe method include optical microscope (Fig.1), atomic force microscope (Fig.3), and scanning electron microscope (Fig.4), and these probes include nano-fiber probes, atomic force microscope probes, and nano-tungsten needles.

Joining larger-scale one-dimensional nanomaterials is one of the keys for the realization of deviceization. A potential manipulation method is self-assembly. According to the principle of self-assembly of nanomaterials, there are three types of self-assembly methods. First, nanomaterials are subject to gravitational or repulsive force in solutions, and then self-assembly is realized under dynamic equilibrium. Second, nanomaterials are self-assembled through the Langmuir-Blodgett technology by the surface tension of liquid-gas or solid-liquid interfaces. Third, under the action of external fields, such as electric field, magnetic field, and light field (Fig.6), the polarized nanomaterials are moved along the gradient direction, and the moving direction of polarized nanomaterials is determined by the relative permittivity of the solution and the nanomaterials (Fig.5).

To further improve the precision and efficiency of nano-operation, optical tweezers are used to manipulate the poses of one-dimensional nanomaterials. Single-beam optical tweezer has been used to manipulate the poses of semiconductor nanowires (Fig.7). To improve the stability of nano-manipulation, holographic optical tweezers or optical tweezer arrays are used for nano-manipulation (Fig.8). Photons have momentum. The exchange of momentum between photons and nanomaterials produces scattering forces. Unless the gradient force generated by the optical tweezers can overcome the scattering force acting on nanomaterials, or the nanomaterials are pushed away by the scattering force. For wide-bandgap semiconductor materials, infrared lasers can pass through one-dimensional nanomaterials. Therefore, the scattering force acting on one-dimensional nanomaterials is very weak. But for metal nanomaterials, the scattering force acting on them increases significantly due to their good conductivity. As a result, it is difficult to use conventional optical tweezers to manipulate the poses of one-dimensional metal nanomaterials. To manipulate the poses of metal nanomaterials, plasma tweezers are developed (Fig.9).

Conclusions and Prospects According to the characteristics of the aforementioned nano-manipulation technology, it is observed that the probe technology can be used to manipulate the pose of one-dimensional nanomaterials, to test the conductivity and reliability of the laser-prepared nanostructures. However, the probe technology has a low efficiency of nano-manipulation efficiency. Self-assembly technology has unique advantages in the larger-scale operation of one-dimensional nanomaterials, but the commonly used dielectrophoresis methods need to be performed under a specific electrode structure, and the precision of nano-manipulation is relatively low. Optical tweezer technology has high nano-operation accuracy. The laser source can be used for nano-operation and nanojoining, but it needs to be carried out in solutions. These problems indicate that improving the compatibility of self-assembly and optical tweezer technology with the existing microelectronic processes is the focus of future research. Based on the above research results, we believe that multi-technology integration is the future development trend. It is found that nano-manipulation technology is merging with laser, microscopy, and nano-testing technology to realize nano-observation, nano-operation, nano-joining, and nano-testing. This kind of multi-functional technology and equipment will further promote the development and application of nano-technology, promote multi-disciplinary cross, and produce more innovative applications.

Objective With manufacturing being downsized to the nanoscale, the welding and joining of nanoscale materials (“nanowelding” for short) has become key in the integration of advanced nanodevice fabrication and packaging. As a bottom-up process, nanowelding can flexibly combine nanostructures with different chemical compositions and selectively modify the interfaces of semiconductor heterojunctions in order to obtain complex electrical and optical properties. In recent years, although the research on nanowelding has made significant progress, the basic theory of low-damage joining of heterojunctions at the nanoscale has not been well-established. Femtosecond (fs) laser is now widely used in the precision machining of materials with high melting point or high damage threshold due to its unique “cold” processing characteristics. In particular, optical irradiation-induced surface plasmonic effect on the metal-dielectric interface can affect the energy redistribution in the nanostructures, which is beneficial for heterogeneous interface modification of semiconductors at the nanoscale. In this study, the nanowelding process of Pt electrodes and TiO₂ nanowires under localized fs laser irradiation has been reported. The creation of this welded structure shows good performance of the synaptic response, which indicates a new method to realize the modification of a metal-oxide heterointerface.

Methods Pt/Ti (the thick is 200 nm/5 nm) electrodes with special finger spacing were fabricated by optical lithography and lift-off processes on oxidized Si substrate. TiO₂ nanowires with pure rutile phase were synthesized by a hydrothermal process, dispersed and diluted in a high-purity acetone solution, which was then drop-cast on the chip with Pt electrodes. The fs laser beam, with 50 fs pulse duration, 800 nm wavelength, and 1 kHz frequency, was generated from a Ti:sapphire laser system and focused by an objective lens with a numerical aperture of 0.5 at a nanoscale spot overlapping a small portion of the nanowire at the junction. COMSOL Multiphysics v5.2 with a RF module was used to simulate the electric field distribution around the Pt-TiO₂ nanoscale structure under polarized Gaussian beam excitation. The morphology of the welded structure was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrical characteristic was measured by a precision source and s measuring unit (Angilent B2901A) in a voltage sweeping mode at room temperature.

Results and Discussions Under focused fs laser irradiation, the TiO₂ nanowire deposited on a Pt substrate exhibits a limited damaging effect. By adjusting the incident laser fluence to 5.02 mJ/cm ², the nanowelding of Pt-TiO₂ was obtained (Fig. 3). The mechanical strength of the welded Pt-TiO₂ bonding can be evaluated by “contact AFM”. The coincidence of the loading/unloading force curves, corresponding to the extend/retract process of the probe, indicates a reliable welded bonding at the irradiated location. In contrast, the separation of the loading/unloading force curves indicates that the nanowire does not remain bound to the Pt electrode without laser irradiation and moves away during the extend/retract process (Fig. 4). The bonding between the TiO₂ nanowire and the Pt electrode is mainly facilitated by the formation of “hotspots”, which is known to result from localized plasmonic field enhancement (Fig. 6). During the welding process, the Magnéli phase (oxygen deficient TiO_2-x) is formed at the Pt-TiO₂ contact by redox reactions under high intensity excitation. The layer contains a high defect concentration, which is beneficial for the wettability of the Pt-TiO₂ interface as well as the reduction of the heterointerface barrier height. The electrical characteristics show that after fs laser welding on one side of the TiO₂ nanowire contacted with the two Pt electrodes, there is an obvious self-rectifying current response at a bias of -10/+10 V (Fig. 7). Besides, the introduced TiO_2-x layer during nanowelding is helpful for the synaptic plasticity of the TiO₂nanowire as an artificial synapse. For the initial TiO₂ synapses, multilevel excitatory postsynaptic current amplification is observed under voltage cycling. Unlike the unwelded units, the maximum amplified current in the welded structure is stabilized during the first ten cycles (Fig. 10). The current accumulation and decay properties of the artificial synapse to simulate the learning/forgetting response of human memory are also investigated. The repeated application of input pulses induces an enhancement in the current response stability, which suggests the transition from short-term potentiation to long-term potentiation in the TiO₂ synapse by repeated stimulation.

Conclusions We have demonstrated a method for welding TiO₂ nanowires on Pt electrodes that utilizes plasmonic effect induced by fs laser irradiation. The welded bonding of Pt-TiO₂ can be obtained at a fluence of 5.02 mJ/cm ², which has been confirmed by the “contact AFM”. Strong plasmonic-enhanced electric fields induced by tightly-focused fs laser irradiation exist at the Pt-TiO₂ interface and contribute to the formation of localized oxygen deficiencies (Magnéli phases). This introduced component improves the wettability of the TiO₂ nanowire on the Pt electrodes and reduces the interfacial barrier height, which results in the stability of the TiO₂ synapse. Based on the welded Pt-TiO₂ structure, the synaptic plasticity of a single TiO₂ nanowire is presented, which shows potential in replicating the complicated learning/forgetting process.

Objective With the rapid development of nanotechnology, new devices are gradually developing toward miniaturization, complexity, multimaterial, and multifunction. Selective nanojoining of nanowires is essential for the fabrication and assembly of high-performance functional nanounits. The development of good quality nanojoined structures based on new material systems has attracted considerable attention. Owing to its high peak power and small heat-affected zone, the femtosecond laser has unique advantages in accurate selective nanojoining. It is difficult to choose the parameters of laser processing joint fabrication, and material selection of nanowires, thus far, laser irradiation has only realized the nanojoining of metal-metal nanowires, metal-semiconductor nanowires, and n-n type semiconductor nanowires. The nanojoining system of nanowires under laser irradiation is still imperfect and needs further improvement. Therefore, we propose a method to successfully nanojoin two p-type copper oxide (CuO) nanowires using the local energy field of femtosecond laser with high spatial and temporal accuracy. Simultaneously, we investigate the influence of different femtosecond laser energy inputs on the interconnection joint and fabricate optoelectronic devices based on the nanojoined structure. The results show that the electrical response and photoelectric properties of the nanowire structure fabricated under femtosecond laser irradiation are significantly improved compared with those before nanojoining and can reach the level of the base material on electric properties.

Methods The CuO joints are prepared using the dry transfer method. The nanowires are ultrasonically dispersed into an ethanol solution and spread on the surface of a polydimethylsiloxane(PDMS) film. The suitable target nanowires are obtained using an optical imaging system and aligned with the test electrode of the substrate using light transmittance of the film. The substrate is heated to 120 ℃ and held for 10--20 min to ensure that homogeneous joint of CuO nanowires is formed at the designated position on the substrate. (Fig.1(a)--(f)). The femtosecond laser is focused on the surface of the sample using a focusing microscope, and a CCD camera is used for real-time observation to ensure that the laser spot is focused accurately at the joint (Fig.1(g)). The continuous adjustment of laser power is achieved using a polarizer. The main characterization methods include a scanning electron microscope (SEM, Zeiss Supra 55), transmission electron microscope (TEM,JEM-2100F), and energy dispersive spectrometer (EDS). COMSOL Multiphysics 5.4 is used for simulation software, and Keithley 2636B is used for electrical tests.

Results and Discussions The prepared CuO nanowires are cylindrical with diameters ranging from 100--250 nm (Fig. 2). SEM is used to observe the morphology of CuO nanowire joints under different femtosecond laser parameters. When the single pulse energy density of the laser reaches 22.3 mJ/cm ², the melting and wetting of the nanowires can be observed at the joint while the two base CuO nanowires remain intact, and almost no damage occurs, indicating that femtosecond laser can nanoweld two CuO nanowires with a minimal heat-affected zone (Fig.3(a)(d)). When the laser energy density is increased to 27 or 30 mJ/cm ², a partial ablation or fracture of nanowires occurs, respectively, resulting in joint failure (Fig. 3 (e) and Fig. 3 (f)). The light field enhancement caused by geometric factors occurs at the contact area of nanowires by simulating the electric field distribution under laser irradiation using COMSOL, which is conducive to forming joints with a minimal heat-affected zone while nanojoining (Fig.4). The current response of the CuO homojunction device fabricated using this method is more than three orders of magnitude higher than that of the sample without nanojoining at 10 V bias, indicating that the properties of the nanowelded device are restored to the base material level (Fig. 5 and 6). CuO is a common optical sensing material. After femtosecond laser nanojoining, the fabricated CuO homojunction photoelectric sensor reaches the photoelectric performance of the base material, and the current growth ratio under 25.3 mW halogen lamp irradiation is the same as that of the base CuO nanowire (Fig.7).

Conclusions In this paper, we have successfully achieved the nanojoining between two p-type semiconductor CuO nanowires by combining the method of dry transfer and femtosecond laser irradiation. Under the influence of the laser energy field, the cylindrical CuO nanowires generate local energy field enhancement at the contact area due to the geometric factors, promoting the nanowelded joint formation with the minimal heat-affected zone. Under laser irradiation with a single pulse energy density of 22.3 mJ/cm ², atomic-scale diffusion occurs at the joint of CuO nanowires to form a wetting structure, which transits the contact condition of nanowires from point contact to surface contact, greatly reduces the interface barrier, and widens the carrier transmission channel. This process increases the current level by more than three orders of magnitude compared with samples without nanojoining at 10 V bias, which almost reaches the current level of the base material. The photodetector based on the nanowelded structure obtains the same current growth ratio as that of the base material under a power of 25.3 mW of a halogen lamp. This study broadens the material system of semiconductor nanowires, which can be nanowelded, and provides a basis for the fabrication of miniaturized, high-performance, and multifunctional nanowire networks nanojoining.

Objective As the most common structural unit in electronic devices, the interface contact state of metal-semiconductor heterostructures plays an important role in the performance of electronic devices. Some traditional nanojoining technologies, such as ultrasonic nanojoining, joule heating, and ion and electron beam irradiation, have been used to improve the interface contact state of metal-semiconductor heterostructures. However, these methods easily cause additional damage to nanomaterials and require a complex processing environment, which poses a great challenge to the control of energy inputs. Femtosecond lasers with short pulse time and high peak intensity, as a method of nanojoining, are suitable to process nanomaterials with high melting points and damage thresholds. In recent years, most research studies have focused on the fabrication of metal-semiconductor heterostructure devices using femtosecond laser technology, but the material systems need to be broadened. In this study, the dry transfer method is used to fabricate Au electrode-CuO/ZnO nanowires (NWs) heterostructures and a semiconductor inverter based on p-type CuO NWs and n-type ZnO NWs. The nanojoining of the heterostructures based on the localized energy input of femtosecond lasers caused by the surface plasmon effect is successfully realized. The laser-treated semiconductor inverter had obtained stable voltage regulation capability based on the p-type and n-type field-effect transistor characteristics of CuO and ZnO NWs. This study provides a new idea for the top-down assembly of micro/nanoelectronic devices. We hope that our design provides a new idea for the top-down assembly of micro/nanoelectronic devices.

Methods CuO nanowires (NWs) were synthesized by thermal oxidation at 400 ℃ for 4 h; however, ZnO NWs and lithography electrodes were purchased directly from XFNANO Materials. Au electrode-CuO/ZnO NWs heterostructures and a semiconductor inverter based on p-type CuO NWs and n-type ZnO NWs were fabricated using a continuous dry transfer method. The nanojoining of the heterostructures was achieved using femtosecond laser irradiation (50 fs pulse duration, 800 nm wavelength, and 1 kHz frequency). The surface morphology and crystalline structure of nanowires were analyzed using scanning electron microscopy (SEM, Zeiss Supra 55) and X-ray diffraction (XRD, Bruker D8). The electrical characterization of the heterostructures and the semiconductor inverter in a three-terminal configuration were examined by a probe station (Keithley 2636B). In addition, the commercial finite element analysis (COMSOL Multiphysics 5.4) was used to simulate the electric ?eld distribution of the heterostructures under linearly polarized Gauss light to further elucidate the interfacial modification mechanism of Au electrode-CuO/ZnO under femtosecond lasers.

Results and Discussions The stable welded joints of the Au electrode-CuO/ZnO NWs heterostructures are obtained by femtosecond laser irradiation with a power intensity of 12.9 mJ/cm ² and 20.2 mJ/cm ², respectively. The nanowires form a degree of wetting angle on the surface of the Au electrode (the red dotted line area), and the surface organic matter is effectively removed, greatly improving the interface contact state (Fig. 4). The simulation results show that the enhancement region with a high electric field intensity mainly occurred in the interface between the Au electrode and nanowires on both sides, while the suspended nanowires in the middle have lower electric field intensity (Fig. 5). The strong surface plasmon effect due to femtosecond laser irradiation confines the energy to the interface of the heterostructures, promoting the formation of low damage welded joints. Compared with the electrical measurement results of pristine and laser-treated heterostructures, the voltage dependence of the p-type and n-type NW field-effect transistors (FETs) irradiated by femtosecond lasers is significantly improved. When the gate voltage (VG) reaches +20 V and -20 V, there is an obvious trend of current suppression for CuO and ZnO NWs, respectively (Fig. 6). In addition, the semiconductor inverter is fabricated based on the p-type and n-type FET characteristics of the CuO and ZnO NWs (Fig. 7). The laser-treated inverter had obtained stable voltage regulation capability, with a full voltage output swing of ~79.5% (Fig. 8).

Conclusions In this study, the Au electrode-CuO/ZnO NWs heterostructures are successfully fabricated through the dry transfer method. The strong plasmon interaction of metal-semiconductor heterostructural interface induced by femtosecond laser irradiation is demonstrated using COMSOL multi-physical field simulation to elucidate nanojoining mechanism. The laser energy is effectively limited in the interface area between the Au electrode and nanowires, achieving a low damage nanojoining of the heterogeneous materials. The device units of the two kinds of nanowire heterostructures have obvious electrical characteristics of p-type and n-type FETs with a great back gate control performance. The laser-treated semiconductor inverter had obtained stable voltage regulation capability based on p-type CuO NWs and n-type ZnO NWs, with a full voltage output swing of ~79.5%. The fabrication of Au electrode-CuO/ZnO nanowires heterostructures through dry transfer and femtosecond laser irradiation extends the material systems of metal-semiconductor heterostructures. In the end, we hope that the design of the inverter provides a new idea for the top-down assembly of micro/nanoelectronic devices.

Objective With the rapid development of information technology, aerospace technology, biotechnology and other fields, there are more demands for the miniaturization, integration, low power consumption and application of its equipment. Due to the unique small size effect, surface effect and quantum size effect, nanomaterials have a very broad application prospect in the new generation of semiconductor devices. As a bottom-up forming method of nanomaterials, welding and joining of nanoscale materials provide a technical mean for high-performance micro/nano devices and various cross-scale applications, such as memristors, field effect transistors, sensors and so on. Although the concept of nanowire bonding has been put forward very early, the theory of nanowire bonding has not been fully established, and the existing nanojoining methods made high demands on spatial accuracy of energy input and external environment. In view of this, we considered graphene oxide (GO) as the intermediate layer to join nanowires. In this paper, we introduce graphene oxide as an auxiliary conductive path at a fixed point in space, and use the reduction effect of femtosecond laser and the characteristics of local energy input to reduce graphene oxide locally. As a way of direct energy input, ultrafast laser can quickly adjust the energy input in space domain and time domain through optical lens and photoelectric shutter. The theoretical point contact joint was optimized as line contact and area contact, so as to improve the current level and performance. In addition, the protective effect of reduced graphene oxide (rGO) nanofilms on the structure was further studied, and a series of nanowire network device such as ultraviolet (UV) detectors and flexible transparent conductive films were prepared.

Methods SiC nanowire-GO film-SiC nanowire structure was prepared by dry transfer method (Fig. 1). The transfer process of a single nanomaterial is as follows: 10 μL of SiC nanowires suspension was placed on polydimethylsiloxane (PDMS) with a pipette gun. After evaporation of alcohol, SiC nanowires were moved to the designated position by using the nano transfer platform under the light microscope. The structure of SiC nanowire-GO film-SiC nanowire can be obtained by repeating the above steps. The reduction of GO films are achieved by femtosecond laser irradiation (50 fs pulse duration, 800 nm wavelength and 1 kHz frequency). The surface morphology and crystalline structure of SiC nanowire-GO film-SiC nanowire structures were obtained by scanning electron microscope (SEM, Zeiss Supra 55) and X-ray diffraction (XRD, Bruker D8). The electrical characterization of SiC nanowire-GO film-SiC nanowire structures was examined by a probe station (Keithley 2636B). The simulation of light field irradiation was carried out by multi-physical field finite element simulation software (COMSOL multiphysics 5.4). In the frequency domain module of wave optics, the wavelength of incident light was set to 800 nm, the intensity of incident electric field was set to 1, and the material parameters were inquired through related literature.

Results and Discussions After femtosecond laser irradiation, the current levels at both ends of the SiC nanowire-GO film-SiC nanowire structures are significantly improved (Fig. 3). The improvement of electrical properties of SiC nanowire-SiC nanowire-GO film structures is mainly due to the formation of intralayer conductive paths of rGO (Fig. 2), while the improvement of SiC nanowire-GO film-SiC nanowire structure is due to the formation of interlayer and intralayer conductive paths of rGO. After the rGO formed by laser reduction is in contact with SiC semiconductor, the Fermi energy levels of SiC and rGO will move simultaneously due to a small amount of carrier transport at the interface, and reach equilibrium. The contact barrier between rGO and SiC is significantly lowered (Fig. 5). The spatial electric field distribution shows that the electric field at the interface of graphene and nanowires will be enhanced, which further promotes the two-photon absorption of GO for femtosecond laser, and improves the reduction efficiency (Fig. 6). For the ultraviolet sensor device constructed by SiC nanowire network, the photoelectric response characteristic before femtosecond laser irradiation is less than 10 ^-5 A/W when it is irradiated by ultraviolet light with wavelength of 375 nm. After femtosecond laser irradiation, although the dark current of SiC nanowire network increased significantly, it showed a good response intensity and faster response speed to ultraviolet light. The responsivity of the optical sensor was about 0.11 A/W, which was improved by more than four orders of magnitude. Moreover, we used SiC nanowires and GO film to construct a transparent flexible conductive film on PDMS. After femtosecond laser scanning irradiation, the regional current of the conductive film was increased by more than five orders of magnitude (Fig. 7). This kind of flexible and transparent conductive film can be used in the fabrication of extensible flexible electrode or touch screen panel in the future.

Conclusions In this paper, the SiC nanowire-GO nanofilm-SiC nanowire structures were prepared by dry transfer method, then graphene oxide was reduced by femtosecond laser irradiation, which reduced the barrier between SiC and GO, and a wider carrier channel was formed through the way of intra layer and inter layer conduction, which significantly increased the current level of this structure. In addition, the obtained rGO nanofilms can wrap and protect the joints of SiC nanowires, which makes the joint have better radiation resistance and heat conduction, so as to improve the stability and service life of the device. Finally, the field effect transistor with low loss and high stability, UV sensor with good response and fast response, and transparent flexible conductive film were fabricated from SiC nanowire network with GO film by femtosecond irradiation.

Objective Copper (Cu) nanoparticle exhibits high potential as an interconnecting material in electronic devices due to its relatively lower cost and similar conductivity compared with other noble metals. The interconnection between Cu nanoparticles can optimize the electrical conductivity and optical and mechanical properties of fabricated Cu microstructures. Compared with other traditional joining technologies, laser-induced nanojoining has the advantages of high precision, low damage, and high efficiency. In particular, femtosecond lasers with high peak power and ultrashort pulse duration would limit the heat-affected zone and result in less damage of joint than other lasers with longer pulse duration or continuous wave. When femtosecond laser pulse interacts with metallic nanomaterials, electrons absorb photons and quickly reach a higher temperature, while the lattice remains unchanged, resulting in less thermal effect and local melting during processing. It is expected to have potential in joining materials at nanoscale. At present, some reports focus on the reduction of Cu nanoparticles by femtosecond laser irradiation, whereas the effect of femtosecond laser on the joining process of Cu nanoparticles is yet to be understood. The joining mechanism and laser thermal effect on the joining of Cu nanoparticles need to be given more effort to optimize the femtosecond laser processing. In this work, femtosecond laser direct-writing is used to in-situ reduce Cu nanoparticles and join them to form a conductive copper microstructure. The effect of laser power on the composition, microstructure, and conductivity of Cu microstructures are investigated. Furthermore, the effect of single-shot laser pulses on the electron and lattice temperature in the “hotspot” between a Cu nanoparticle dimer is calculated. Simulation experimental results are compared to understand the joining process and mechanism of Cu nanoparticles under femtosecond laser irradiation.

Methods In a typical experimental procedure, the aqueous solution of polymethacrylic acid sodium salt (PMAA-Na, 30%, 1 μL), and polyvinyl pyrrolidone (PVP, 0.25 g/mL, 1200 μL) are added to the aqueous solution of copper nitrate hydrate (Cu(NO₃)₂·3H₂O, 1.208 g/mL, 1000 μL) to form a Cu ion precursor. The as-prepared Cu ion precursor (200 μL) is coated on a polycarbonate flexible substrate (PC, 2.5 cm × 5 cm) and then dried at 50 ℃ in an oven. The femtosecond laser is used to scan the dry precursor film to reduce Cu ion to Cu nanoparticles and join the nanoparticles to form a conductive Cu microstructure. After laser writing, deionized water is used to clean the Cu microstructure to leave the as-written structures on a substrate. The electrical properties of the Cu microstructure are measured with a source meter using the four-point probe method. Then, the morphology of the Cu microstructure is characterized by field emission scanning electron microscopy and high-resolution transmission electron microscopy. X-ray diffraction is used to verify the chemical composition of the Cu microstructure. The effect of laser power on the chemical composition, microstructure, and conductivity of Cu are studied. Finally, the electric field and temperature field distribution characteristics of the Cu nanoparticle dimer under femtosecond laser irradiation are simulated using COMSOL Multiphysics, and the effect of single-shot laser pulses on the electron and lattice temperature of Cu nanoparticles is calculated.

Results and Discussions The sheet resistance of as-fabricated Cu microstructure presents a tendency to decrease first and then slowly increase with the increase of laser power (Fig.1). The Cu microstructure obtained at 960 mW laser power exhibits the lower sheet resistance of 11.2 Ω·sq ^-1. When the laser power is 322 mW, insufficient laser energy input results in the reduction of only a few dispersed Cu nanoparticles from the precursor, leading to high sheet resistance. As the laser power increases to 960 mW, more Cu nanoparticles are reduced and joined to form a dense network structure because of more hot-spots induced by the plasmonic effect, which greatly enhances its conductivity (Fig.3). Further increasing the laser power to 1690 mW or above, the high local temperature can melt Cu nanoparticles to form large micron-sized Cu, resulting in the increased sheet resistance. The simulation results show that the lattice temperature at the contact area of the Cu nanoparticle dimer increases as the incident laser power increases (Fig.5). When the laser power is 960 mW, the lattice temperature of the “hotspot” between the Cu nanoparticle dimer is up to 698 K. It induces surface melting of Cu nanoparticles and facilitates their interconnection (Fig.6). As the laser power increases to 1690 mW, the lattice temperature increases to 1175 K, resulting in intensive melting and interconnection of nanoparticles. These also have been observed in the experiment.

Conclusions In this work, femtosecond laser direct-writing was used to reduce Cu nanoparticles and in-situ joins them to fabricate the Cu microstructure with high conductivity. The Cu microstructure obtained at 960 mW laser power and 3 mm/s scan rate exhibited the lowest sheet resistance of 11.2 Ω sq ^-1. A two-temperature model during single-pulse femtosecond laser irradiation was employed to calculate the electron and lattice temperature of Cu nanoparticles using COMSOL Multiphysics. Laser-induced localized surface plasmon effect on the Cu nanoparticle dimer enhanced the local temperature greatly at the contact area of Cu nanoparticles, contributing to the interconnection of nanoparticles. As the incident laser power increased, the lattice temperature at the contact area of the Cu nanoparticle dimer increased, leading to intensive joining. When the laser power was 960 mW, the lattice temperature of the “hotspot” between the Cu nanoparticle dimer was up to 698 K, which can cause surface melting to facilitate joining. The consistent experimental and simulation results provide a further understanding of the joining process and mechanism of Cu nanoparticles under femtosecond laser irradiation.

Objective Owing to high porosity and large specific surface areas, nanoporous metal materials have attracted considerable attention for applications in sensors, energy storage, etc. With the rapid development of flexible sensors and energy storage devices, the fabrication of flexible nanoporous metal materials is becoming more and more important in recent years. However, nanoporous metal materials prepared by dealloying, spark plasma sintering, and heat pressing exhibit an extremely high elastic modulus and yield strength that do not meet the requirements of flexible devices. To overcome these problems, herein, flexible nanoporous metal materials were prepared by irradiating Ag nanowires (NWs) using a femtosecond (fs) laser.

Ag NWs are ideal nanomaterials for fabricating flexible nanoporous metal materials because of their good flexibility, conductivity, and oxidation resistance. The fs laser offers several advantages such as ultrashort pulse width and ultrahigh peak power. Flexible nanoporous metal materials prepared by irradiating Ag NWs using the fs laser show ultralow yield strength, which can meet the requirements of flexible devices. In this study, a novel method for preparing flexible nanoporous silver materials is reported and the influence of fs laser power on the yield strength of the resulting flexible nanoporous Ag materials is revealed.

Methods The following steps are involved in the fabrication process of flexible nanoporous Ag materials. The as-synthesized Ag NW solution and deionized (DI) water were pipetted into a centrifuge tube using a pipettor. Then, the centrifuge tube containing the Ag NWs solution and DI water was placed in an ultrasonic cleaner to strip the polyvinylpyrrolidone (PVP) coating the surface of Ag NWs. Subsequently, the centrifuge tube was centrifuged to obtain highly concentrated Ag NW pastes. After adding DI water, ultrasound and centrifugation were performed several times to remove excess PVP. The Ag NW paste was dropped onto a silicon substrate and completely dried at room temperature. Then, the dried Ag NWs were irradiated using the fs laser to fabricate the flexible nanoporous Ag materials. Nanoindentation experiments were performed to analyze the mechanical properties of the as-prepared flexible nanoporous Ag material. X-ray diffractometer (XRD) and high-resolution transmission electron microscopy were used to analyze the crystalline structure of the flexible nanoporous Ag materials. Additionally, the yield strength of nanoporous metal materials fabricated using different methods was compared.

Results and Discussions The fs laser irradiation of nanowires can induce localized surface plasmon resonance on the Ag NW surface. When the intensity of the fs laser is low, nanojoining occurs owing to the melting of the Ag NWs at the contact region. By increasing the intensity of the fs laser, the ends of the Ag NWs begin to melt and the linear ends of Ag NWs gradually transform into a spherical structure (Fig. 2). When the fs laser power is 60 mW, nanojoining occurs at the gaps between the Ag NWs because of local fusion. The volume and number of nanojoining increase with an increase in the fs laser power, and spherical structures are observed at the end of Ag NWs (Fig. 3). The yield strength of flexible nanoporous Ag materials is determined using depth-sensing nanoindentation equipped with a Berkovich indenter. Different peak loads (150, 300, and 450 μN) are investigated. Plastic deformation is found to only occur at the residual indentation impression, and no nanoporous material deformation adjacent to the contact impression is observed. This result indicates the occurrence of significant densification of the flexible nanoporous Ag materials under the Berkovich indenter (Fig. 4). With increasing fs laser power, the elastic modulus and yield strength of flexible nanoporous Ag materials increase, while their grain size decreases (Fig. 6). The yield strength of flexible nanoporous metal materials prepared using different methods is compared in this study. The experimental data clearly show that nanoporous Ag materials prepared using the fs laser exhibit the smallest yield strength at the same grain size (Fig. 7).

Conclusions The flexible nanoporous Ag materials are prepared by irradiating Ag NWs using the fs laser. The mechanical properties and grain size of the synthesized flexible nanoporous Ag materials are evaluated using a nanoindenter and XRD, respectively. Based on the experimental results, several important conclusions can be drawn.

1) The elastic modulus and yield strength of nanoporous Ag materials increase with an increase in the fs laser power. At a peak load of 450 μN and when the fs laser power is increased from 60 to 100 mW, the elastic modulus and yield strength of the flexible nanoporous Ag materials increase from 88.4 to 235.2 MPa and from 1102.0 to 2737.0 MPa, respectively.

2) The grain size of the flexible nanoporous Ag materials decreases with an increase in the fs laser power. When the fs laser power is increased from 60 to 100 mW, the grain size decreases from 44.6 to 41.5 nm.

3) Compared with Ag materials prepared by hot pressing, plasma sintering, and dealloying, flexible nanoporous Ag materials prepared by irradiating Ag NWs using the fs laser exhibit the lowest yield strength at the same grain size.

Objective With the rapid development and low cost of pulsed lasers, the preparation of high-performance films by pulsed laser deposition (PLD) has become a research hotspot recently. Compared with other film preparation technologies, PLD has many advantages. First, PLD can fabricate most film materials, such as metal films, alloy films, carbon films, compound films, and composite films, due to the high energy density of laser. The crystal structure, micromorphology, and particle dimension of films are controllable and designable by regulating the processing parameters such as laser energy density, background gas pressure, background gas type, substrate material, substrate temperature, and deposition tilt angle. The multicomponent films with desired stoichiometric ratio can be easily obtained by PLD, which contributes to the preparation of multicomponent compound and alloy films. Owing to the high velocity and energy of plasma plume from the laser ablation, the substrate temperature required for film growth is relatively low, even at room temperature. In addition, PLD possesses a high deposition rate, which can attain more than 10 μm/min. Therefore, PLD has become one of the best film deposition technologies. In the past decade, the mechanism of PLD was revealed, and the most cutting-edge research in this field was mainly focused on the preparation and application of film materials. The applications cover many relevant fields such as optoelectronics, sensing, biology, superconductivity, new energy, tribology, catalysis, and electronic packaging. The material forms include zero-dimensional quantum dot doping, one-dimensional nanowires (rods), two-dimensional thin films, and three-dimensional thick films. Current studies show that the film materials prepared by PLD technology possess a very large material system. Therefore, the research status of high-performance films by PLD recently is reviewed systematically from the perspective of material systems, and the application fields are summarized.

Progress Five types of film materials by PLD—metal films, alloy films, carbon films, compound films, and composite films—are summarized. Notably, metal films are one of the simplest film materials. Researchers commonly employ metals to study the effect of deposition parameters on film structure and morphology. Metal films are easily oxidized in the deposition process with high energy input. Therefore, most metal film materials were prepared under a high vacuum or in an inert atmosphere. At present, metal materials by PLD mainly include inert metals such as gold, silver, and copper along with active metals such as niobium, aluminum, and iron ( Fig. 2). Compared with metal films, alloy films can exploit the advantages and characteristics of various metals to obtain better performance or new properties. Therefore, alloy films have significant research and application value. PLD can prepare not only simple binary system alloy films such as AgCu ( Fig. 3), AuAg, and PtAg but also complex multisystem alloy films such as Heusler alloy and high-entropy alloys. Recently, PLD has become a significant method for preparing carbon films, including graphene, diamond-like carbon films, and nanostructured porous carbon films ( Fig. 4). Compound film is currently one of the most common and widely used material types. Because PLD has the characteristic of keeping the composition of the target and film consistent, the film composition can be controlled by the target to fabricate oxide, nitride, sulfide, and compound films with more complex compositions. Sometimes composition control is also performed by reaction with background gas during the PLD process. Compared with elemental metal films, the mechanism of preparing compound films is more complicated, and the requirements for composition and crystal structure control are more stringent. Therefore, the richness of the material system is far greater than that of elemental metal films. At present, compound films mainly include metallic and nonmetallic compound films ( Fig. 5). In addition, there are a few reports on the preparation of phthalocyanine organic compound films by PLD recently. Composite films prepared by PLD possess good design flexibility, which can combine the advantages of multiple materials by structural and material designs ( Fig. 6). Composite films have become a hotspot.

The application of the films by PLD in optoelectronics, new energy, biology, superconductivity, and electronic packaging fields has attracted much attention. In the optoelectronics field, two-dimensional compound thin films are mainly used to acquire excellent photoelectric detection performance. In the new energy field, high-performance functional films are mainly applied as photoluminescent materials and electrodes in photovoltaic cells, fuel cells, and lithium batteries. In the biology field, the film prepared by PLD is mainly used as a bacteriostatic coating. In the superconductivity field, high-temperature superconducting films such as Y-Ba-Cu-O(YBCO) films and low-temperature superconducting films such as niobium films are fabricated and their superconducting properties are explored and controlled. In the electronic packaging field, organic-free nanostructure films are prepared for interconnecting SiC power dies and substrates.

Conclusion and Prospect High-performance films by PLD are becoming a hot research direction due to the advantages of PLD, and they have been applied in many relevant fields. However, there are some technical and engineering problems, such as large particle splash and the bottleneck of large-area uniform deposition. From the perspective of process, the development of PLD particle control technology can immensely improve the surface problems due to the large particle splash. In addition, large-area deposition technology is continuously developing and an 8-inch large-area uniform deposition in the literature has been achieved. From the perspective of technology, with the continuous progress of laser technology, the type of laser used in PLD will develop from long pulse nanosecond laser to picosecond and femtosecond laser. Therefore, PLD preparation of high-performance film materials has potentials for industrial application.

Significance With the rapid development of electronic industry, new problems and challenges in electronic packaging appeared. Sustained improvement of power density and expansion of application fields express a requirement for electronic devices that should have higher operating temperatures. The development of the third-generation semiconductor technology represented by SiC and GaN provides a feasible solution for high-temperature applications. Correspondingly, the traditional electronic packaging technology exposes many problems under high-temperature conditions, such as material fusing and fatigue cracking. It would seriously affect the high-temperature reliability of electronic devices. Thus, it is paramount to investigate the packaging technology to realize better reliability for high-temperature applications.

The die-attachment between the chip and substrate is an essential process for electronic packaging technology. The reliability of the joint not only determines the topology structure of the internal circuit of the packaging, but also directly relates to the electrical and thermal characteristics of the module. Traditional connection materials and technologies, such as solder and transient liquid phase bonding, cannot meet the packaging requirements of electronic devices under high-temperature conditions, so it is urgent to explore and develop new interconnection technologies for high-temperature operation.

Metal nanoparticles (NPs) paste sintering with excellent thermal/electric properties and “low-temperature sintering and high-temperature operation” characteristics has become a relevant development direction of interconnection technologies. The sintering mechanism of metal NPs is the size effect of metal NPs, which could reduce the temperature required for interconnection. The operation temperature of the interconnection layer after sintering can approach the melting point of the bulk material. Since the 1980s, metal NP pastes have been used in electronic packaging as interconnection materials. To investigate the problems of NP paste sintering technology in practical applications, the joint reliability of nanometal particles after sintering has become a research hotspot.

Progress This article presented the sintering mechanism, composition, and process of NP paste-sintering technology of NP pastes recently. The advantages and disadvantages of the existing research were discussed. The sintering process of metal NP pastes is generally in the early stage, and the main process to accomplish interconnection is the formation and growth of the sintering neck (Fig. 2). After many years of research and accumulation, the formula selections of Ag nanosolder pastes, including dispersants, passivation layer, coating agent, binder, and solvents, have been investigated. Moreover, the different particle sizes (nano, micro, or micro-nano-hybrid), the process method (with pressure or pressureless assistant), and the respect of joint organization have been analyzed and improved (Table 1). Based on these, many researchers have attempted to modify the composition of metal NP pastes and improve the performance of a particular aspect. The Cu NP and composite NP pastes, such as Ag-Cu and Ag-Pd NP pastes, have been developed. However, owing to the lack of mechanism research, although these joints have excellent performance in a certain aspect, they still have shortcomings in comprehensive performance, such as high requirements of the sintering process and lack of reliability at high temperature (over 200 ℃), which need further optimization and theoretical analysis.

In addition, with the reliability test and results of the joint after sintering, this paper indicated the failure mechanism of existing research in high temperature and high power applications, and the development direction of metal NP paste-sintering technology in the future. Regarding joint reliability, according to internationally accepted standard of reliability tests, researchers have investigated and analyzed the sintering joint reliability with different processes and materials of NP pastes. Compared with the traditional assembly interconnection materials, the reliability of the metal NP paste after low-temperature sintering has significantly been improved (Fig.7). Moreover, some researchers proposed solutions to improve joint reliability according to reliability test results and achieved remarkable success. However, there are still few studies on the reliability under high-temperature conditions (over 200 ℃). To promote the industrial application of metal NP pastes, researchers should further investigate the reliability test under practical application conditions. With the increase in temperature requirement for electronic device packaging, further research on the joint reliability of metal NP pastes needs to be conducted.

Conclusions and Prospect Metal NP pastes provide an opportunity to develop electronic devices with higher power density. It has great prospects in the rapid development of the third-generation semiconductor industry at home. Recently, domestic researchs groups have made some achievements in the field of metal NP pastes, but there is still a certain gap, compared with overseas, especially the leading company in the industry. Therefore, based on further analysis of the studies, we should focus on promoting producer-university-researcher combination and seek suitable materials and processes for metal NP pastes in industrial applications and development, integrally considering the connection performance, process and material cost, high-temperature reliability, and other factors.

Significance Owing to excellent adaptability to different working conditions, flexible electronics have attracted significant attention in many fields, such as wireless communication, human-machine interaction, and personal healthcare. Functional parts and conductive circuits are the basic components of electronics that respond to external stimulus and conduct signals, respectively.

Nanomaterials with unique physical and chemical properties are widely used for developing flexible electronics. Noble metals, such as silver, gold, and platinum are good candidates for manufacturing conductive parts because of their high conductivity and chemical stability. However, the high price of these metals limits their large scale production. Recently, copper has been considered a good alternative to noble metals for developing conductive component owing to its low-cost and excellent electrical properties. Furthermore, copper oxides (cuprous oxide and cupric oxide) are important transition metal oxides because of their semiconductive properties. They have been widely used as functional parts owing to their high sensitivity for external stimulation, such as humidity, temperature. Efficient manufacturing methods for materials play a major role in developing high-performance devices.

The typical “bottom-up” process, such as hydrothermal and chemical precipitation, provides a low-cost, precise control, and large-scale synthesis route to manufacture the micro/nanostructured copper. However, post-treatment processes, such as printing and sintering, are required to obtain the desired properties in a device. Such step-by-step manufacturing requires the cooperation of various techniques, which increases the process cost and complexity. Thus, developing a low-cost process for manufacturing the micro/nanostructures has attracted significant attention.

Direct laser writing, as an advanced processing technology developed recently, provides a novel approach for micro/nanostructure manufacturing. This technology has been used to process the structure, including noble metals, metal oxides, and carbon-based materials. In this study, the technical characteristics of manufacturing copper-based micro/nanostructures with direct laser writing have been summarized.

Progress The typical laser processing of micro/nanostructures, such as laser assembly, sintering, and synthesis, has been elaborated (Fig. 1). The advantages of laser processing compared with other processing technologies are presented. Then, the studies of laser sintering, reduction, and synthesis, for manufacturing copper structures are reviewed. The challenges of laser processing for copper-based materials, especially for the conductive copper structure, are highlighted. Subsequently, one-step direct laser writing technology based on the ionic precursor has been discussed. The typical manufacturing process and mechanism of the one-step direct laser writing of copper structures are revealed (Fig. 2). The effects of process parameters, such as precursor compositions, reducing agent type, laser wavelength, and laser parameters on the structure and electrical properties of patterns are discussed (Fig. 3). The tuning methods of the copper micro/nanostructures, such as topography, composition, and joining behavior during the writing process are demonstrated. The conductivity of the written structure and its influencing factors, such as porosity and composition, are comprehensively summarized according to the previously reported studies (Fig. 4). Besides, the manufacturing method of antioxidation copper structures with direct laser writing is described (Fig. 5). Typical applications of the copper-based structures in conductive (Fig. 6) and functional parts (Fig. 7) in microelectronic devices are listed. The working mechanisms of these typical devices, such as an electrode, antenna, heater, capacitor, and sensors, and their influencing factors in performance have been clarified. Finally, the development trend of laser direct writing of micro/nano copper structures has prospected.

Conclusions and Prospect In summary, direct laser writing has been an efficient manufacturing process for copper micro/nanostructures owing to its noncontact, maskless, and rapid processing characteristics. Direct laser writing based on ionic precursors integrates the synthesis, positioning, assembly, and joining of copper nanomaterials into a one-step, which shows unique advantages in structure and composition control. This process still faces challenges in processing copper structures, such as the accurate control of products, diversification of composite structures, and further expansion of application. Further, in-depth study is needed to explore the writing mechanism and fully understand the processing characteristics for copper-based micro/nanostructures.

Significance Micro-holes with a diameter of tens to few hundreds of microns are widely used in different industrial fields, such as injection nozzles in the automotive industry, cooling holes in jet engine components, and interconnecting micro-via in electronic packages. Methods, such as electro discharge, mechanical, electrochemical, and continuous or pulsed laser drilling, are used for micro-hole machining. However, these methods cannot be used in the micro-hole drilling with a diameter smaller than 100 μm. For micro-holes larger than this size, these methods have their limitations, such as poor accuracy, low efficiency, and incapable of drilling in non-conductive materials (e.g., glass). For the past decades, ultrafast laser has been a reliable tool for such processing due to its unique characteristics. Various hard-to-machine and newer materials, such as glass, diamond, biological materials, and superalloy, can be ablated by ultrafast laser through nonlinear absorption of energy. The pulse is too short that only a small amount of heat transfers to the surrounding of the irradiated zone, leading to minimized heat-affected zone and high processing precision.

Although the ultrafast laser has advantages in micro-drilling, some points need to be carefully investigated for practical applications. The quality and processing efficiency of ultrafast laser drilling are affected by processing methods, materials, auxiliary methods and beam characteristics such as pulse energy, frequency, pulse width, polarization, etc. Different process parameters will result in different roundness, taper, defects,and surface quality. The study on process technology parameters is one of the core issues of ultrafast laser micro-hole machining.

Recently, many advances have been achieved in the processing technology of ultrafast laser drilling in various materials. However, due to the differences in materials, processing, and auxiliary methods, different studies have different conclusions about the same process parameters. Thus, it is essential to summarize and compare the results of the same process parameter.

Progress The study of ultrafast laser micro-hole machining began in the 1990s. Early studies focused on the interaction mechanism between the laser and material. A series of models of interaction between ultrafast laser and material were proposed. Many studies have focused on the investigation of aspect ratio and surface morphology of the micro-holes. With the progress of laser technology, the laser power has been increased, and the pulse width is further compressed. Besides, high-speed and high-precision laser processing technologies, such as helical drilling, have appeared. The study focuses on micro-hole processing gradually turn to the acquisition of higher precision, larger aspect ratio, and drilling on various hard-to-machine materials. Recently, due to its super-short time resolution, several research groups have employed the high-speed observation technique, such as pump-probe, to study the mechanism of laser-matter interaction. There has been an increasing interest in parallel processing, high-speed scanning, and other technologies with high efficiency suitable for micro-hole array drilling.

Typical materials and applications of ultrafast laser micro-hole drilling are summarized (Table 1). The influence of main process parameters in ultrafast laser micro-hole drilling is summarized (Table 2). The pulse energy and frequency are the dominant factors (Figs.1 and 3). Increasing the pulse energy and frequency can increase the drilling speed, but the defects become more prominent at same time. Thus, the determination of process parameters in processing is the process of finding the balance point of pulse energy and frequency. The effect of polarization on high aspect ratio micro-holes is more obvious (Fig. 4) since the high-speed and stable rotation of linear polarization is not easy to achieve. Thus, it is preferable to use circularly polarized light for processing. There are many auxiliary ways (Fig. 8); the most economical and convenient way is the coaxial air-assist, which can increase heat conduction and reduce debris adhesion.

For stable energy transmission in the micro-hole, some researchers have proposed water-guided laser processing technology. The laser beam is coupled into the water jet, and the laser travels forward due to total reflection in the water jet. The water-guided laser has great advantages in high aspect ratio micro-holes processing. Recently, multi-beam parallel processing and high-speed scanning technology have appeared in the field of efficient micro-hole drilling. For example, the diffractive optical element (DOE), spatial light modulator (SLM), and acousto-optic modulator (AOM) are used for beam splitting and energy modulation. By using these devices, parallel processing of 16 laser beams is realized. Polygon laser scanning technology is another approach to enhance the processing speed by increasing the laser scanning speed from several meters per second to a hundred meters per second.

Conclusions and Prospects With the development of laser technology, the pulse width is getting shorter, while the frequency and power are getting higher. The ultrafast laser has gradually become a reliable tool for high aspect ratio and precision micro-hole machining; however, several problems remained. Thus, in-depth and detailed explorations are essential to enhance the development of this micro-hole drilling technology.

Objective There is an increasing demand for die attach materials with the rapid development of SiC devices, which can be bonded at low-temperature and function at high temperature. Nano-Ag sintering has been extensively investigated for application in high-temperature power electronics. However, the electrochemical-migration of Ag ions is the main drawback. Pd is famous for its chemical stability, and various studies have focused on the influence of Pd content on the effectiveness and its mechanism. Recently, researchers have been trying to mix Pd and Ag nanoparticles (NPs) to improve the resistance to electrochemical-migration of the sintered layer. However, Pd has a melting point higher than that of Ag, whereas the alloying process needs high temperature (~850 ℃) to form Ag-Pd alloy. Pulsed laser deposition (PLD) is a physical method feasible for fabricationg Ag-Pd nanoalloy without using organic additives such as polyvinylpyrrolidone, which is required in the chemical method. In this work, Ag-10%Pd nanoalloy was fabricated by the PLD method, which can be used to connect SiC and Ag-coated direct bonding copper (DBC) substrates. The sintered layer enhances resistance to electrochemical-migration with low-temperature bonding characteristics. The microstructure of the bonding, shear properties, and its electrochemical-migration resistance are studied.

Methods Ag-10%Pd NPs were fabricated using PLD with a pressure of 750 Pa of Ar atmosphere. The Ag-Pd target was fabricated by powder sintering with weight ratio of 90∶10. A picosecond laser with a pulse width of 10 ps was employed to ablate the target. Ag-Pd NPs were deposited on the back side of SiC chip (G.P.Tech, Ti/Ni/Ag metallization), then the SiC chip was removed from the substrate and placed on the Ag-coated DBC (HuaSemi Electronics, Ni/Au metallization). The interconnecting process is performed at a temperature range of 200 ℃-350 ℃ assisted with a pressure of 5 MPa for 30 min in air. The shear test is conducted using Dage 4000. The electrochemical-migration test is conducted using a water drop test.

Results and Discussions The microstructure of as-deposited Ag-Pd film comprises various NPs with diameters less than 1 μm (Fig. 3). Element results indicate that these deposited NPs are in alloy state with a uniform composition distribution. The sintered joint comprises SiC chip, bondline and Ag-coated substrates (Fig. 4). The bondline thickness is about 27 μm, which is only 31.6% of the as-deposited state. Thus, the Ag-Pd film had excellent deformability. The bondline exhibited Ag-9.57%Pd alloy microstructure without obvious element segregation. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which is higher than the US military standard MIL-STD-883K(7.8 MPa). Therefore, Ag-Pd nanoalloy film can be used as die attach material for low-temperature bonding. The sintering temperature provides the driving force for sintering process, as a denser bondline is achieved when the temperature is increased to 300 ℃ (Fig. 6). Fracture surface reveals that the failure mainly occurred at the bondline, indicating that high bonding quality interface is realized (Fig. 7). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple resistance to electrochemical-migration during the water drop test (Fig. 8). For pure Ag electrode, the current reached 1 mA with only 81.4 s, while the Ag-Pd electrode required 349.7 s for the short-circuit process. The dissolution of Ag ion was blocked by PdO formation on the anode, which played a paramount role in extending the short-circuit time, whereas the migration product was cloud-like instead of dendritic growth. This work proposed a method for fabricating Ag-Pd nanoalloy films as die attach material without the high alloying temperature. It should be noted that, Pd has a higher melting point (1554 ℃) than Ag (961.7 ℃), and Ag-Pd nanoalloy sintering requires higher sintering temperature than pure Ag NPs. Moreover, adding Pd is costly. Consequently, the sintering temperature, demand of electrochemical-migration resistance and its cost should be balanced when applying Ag-Pd nanoalloy in electronic packaging.

Conclusions Ag-10%Pd nanoalloy was successfully fabricated as die attach material using PLD. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which was higher than the US military standard MIL-STD-883K (7.8 MPa). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple electrochemical-migration resistance. The dissolution of Ag ion was blocked by PdO formation on the anode with obviously extended short-circuit time, whereas the migration product was cloud-like. Compared with conventional direct sintering of Ag and Pb nanoparticles, pulsed laser deposited Ag-Pd nanoalloy sintering avoids high-temperature alloying process (850 ℃), which is promising for Ag-Pd low-temperature bonding and is expected to provide a solution for the high-reliability power electronic packaging.

Objective With the application of new generation power electronic devices, their advantages in severe working conditions are gradually emerging. For instance, SiC power devices can serve in high-temperature conditions over 300 ℃, and their packaging materials should be bonded at low temperature (≤250 ℃) and work reliably in high-temperature conditions. Sintering silver nanoparticles (NPs) technology is an effective method that can form joints with excellent electrical and thermal properties between SiC chips and metallized substrates. The traditional silver NP-sintering technology is synthesizing NPs by a chemical method and mixing them with organic components to prepare pastes. However, the adverse effects of organic components in the pastes can reduce the performance of sintered joints; thus, organic-free solutions have emerged recently. Pulsed laser deposition (PLD) with ultrafast laser as the light source is an efficient method to prepare organic-free silver-nanostructured films with large areas. Ultrafast laser with a very high peak power can ablate a target and produce ions with high kinetic energy, which can be controlled by the atmosphere to form films with unique nanostructures. In this study, organic-free silver micro-particle and NP composite film (SMNCF) was prepared by a PLD method on SiC chips. The films were used as intermediate layers for the low-temperature sintering process to bond SiC chips and metallized ceramic substrates. The size distribution of the as-prepared nanosilver films is diverse and controllable, and the sintering temperature can be as low as 180 ℃, which meets the shear strength requirements of SiC power devices.

Methods In this study, SiC chips with deposited SMNCF were mounted on silver metallized substrates to form modules. These modules were sintered at 250 ℃ with 10 MPa applied pressure for 30 min. In addition, samples in control groups were prepared with hybrid silver NP pastes. The modules were dried at 150 ℃ for 5 min and then maintained at 250 ℃ for 30 min under 10 MPa applied pressure to complete the sintering process. High-temperature storage (HTS) is a general method to verify the reliability of sintered layers using SMNCF, and it mainly focuses on the change of shear strength after tests and the influence of microstructure evolution in sintered layer. The HTS tests were performed at 300 ℃, and cross-sectional samples of sintered joints after tests were prepared to observe the microstructure of the sintered layer. Shear strength, porosity, and average pore area of sintered layer were measured. In addition, the HTS tests were divided into two categories: long-term tests in the atmospheric and vacuum environments for 2000 h (holding time of the control group using hybrid paste is 1500 h) and short-term tests at varying oxygen concentrations for 400 h. In the HTS test with varying atmosphere, three groups of the atmosphere were set for comparison: inert atmosphere using argon, which was used to confirm whether the pore evolution in the sintered layer was consistent with the samples in the vacuum environment; atmosphere with 20% oxygen concentration, which was used for comparison with the atmospheric environment; and a pure oxygen environment, which was used to observe the effect of pure oxygen on the microstructure evolution of sintered layer.

Results and Discussions Test results (Fig.3--5) show that the shear strength of sintered joints was always higher than 20 MPa in the HTS test at 300 ℃ for 2000 h, which was significantly higher than the standard MIL-STD-883K. Besides, although the shear strength of sintered joints using hybrid NP paste is also higher than the standard MIL-STD-883K, it is significantly lower than the control groups using SMNCF. This is attributable to the residual organic components in sintered layer with decomposition temperature higher than 300 ℃, which would affect the mechanical properties of sintered joints. During 0--400 h in the atmospheric environment, the pores in sintered layer gradually accumulated and led to densification of sintered layer, which increased the shear strength of the joints; during 400--2000 h, the accumulation of pores led to a continuous expansion of the pores, which increased the porosity and decreased the shear strength of joints. The vacuum environment hindered the evolution of pores in sintered layer (Fig.6). The increase in oxygen concentration in the atmosphere can accelerate the evolution process of the sintered layer during HTS tests. The influence of oxygen concentration on the mechanism of the microstructure evolution was discussed (Fig.7).

Conclusions In this study, organic-free SMNCF was fabricated by the PLD method and a low-temperature sintering process was performed to obtain porous joints between SiC chips and silver metallized substrates. HTS tests were performed in different atmospheres. The results show that the shear strength of the sintered joints is always higher than 20 MPa at 300 ℃ for 2000 h. The pore aggregation and connectivity during the tests can be observed in the atmosphere, and this phenomenon is mostly influenced by the oxygen concentration in the atmosphere, while the ambient pressure and other gas components in the atmosphere have an insignificant influence on the pore evolution. In vacuum and inert atmosphere, the microstructure of the sintered layer is relatively stable in the HTS tests, and there was no significant change in shear strength of the joints. This indicates that both vacuum and inert atmosphere are beneficial to improve the high-temperature reliability of the porous sintered joints using SMNCF.

Objective Owing to the increasing demand for microelectronic devices, micro-nanostructures obtained using nanomaterials have great advantages in reducing size and achieving characteristic performance. Because a structure characteristic size is reduced to the nanoscale, its light absorption, melting point, and several other physical and chemical properties are different from macroscopic bulk materials, demonstrating the unique size effect of nanomaterials. Therefore, nanojoining often requires low-energy conditions. Chemical, light, electrical, and thermal energies can be used as energy sources to achieve the low-temperature or room-temperature joining of nanomaterials. To date, the methods to achieve nanojoining mainly include self-joining, thermal sintering, and laser sintering. In this study, the performance of the abovementioned three methods in joining silver nanomaterials was evaluated. The electrical properties and microstructure of silver nanomaterials under different joining methods were compared, and the potential joining mechanism was analyzed. Laser sintering, with the advantages of high precision, high efficiency, and low damage to a substrate, is applied in flexible device preparation, dissimilar material combination device preparation, and electronic packaging.

Methods Silver nanomaterials (Ag NP) were obtained by the hydrothermal method. A mixture of 30-mL AgNO₃ aqueous solution (5.1-g AgNO₃) with 200-mL glucose (14 g) and PVP (8 g) aqueous solution was heated to 90 ℃ for 20 min under vigorous stirring and then naturally cooled. After the ultrasonication and centrifugation of the reaction solution, the solid matter was extracted and dried at 50 ℃ to obtain silver nanomaterials.

Silvernanobelts were synthesized by a one-step solution method at ambient temperature (~25 ℃). The aqueous solution of AgNO₃ (4 mol/L,5 mL) was successively added to the aqueous solution of VC (0.25 mol/L,20 mL) and PMAA-Na (mass fraction of 30%,5 μL). The mixture was then washed using water, and the solid material was extracted after centrifugation to obtain the silver nanobelts.

Moreover, the conductive inks (solid content with mass fraction of 30%) with different quantities of silver nanobelts were prepared. Two materials, glass and PI films, were used as substrates. Electrodes were printed using a direct writing platform. Laser sintering was performed using an 808-nm diode laser with a spot diameter of 600 μm and a constant power density of 15.3 W/mm². Their morphologies were characterized using a scanning electron microscope (SEM, Merlin Compact, Germany) and a transmission electron microscope (TEM, JEOL 2100F, Japan). Resistance of the silver structure was measured using a source meter (Keithley, 2400), and the electrode resistivity σ was calculated using σ=RS/L, where R is measured resistance, S is cross-sectional area, and L is the length of the electrodes. The 20-mm electrodes were used for bending tests using a homemade bending device. The bending frequency was 30 cycles/min, bending degree was 50%, and bending speed was 10 mm/s.

Results and Discussions Chemical energy of a reduction reaction can drive silver nanomaterials to self-join. The self-joined structure was composed of a large number of silver nanobelts with a smooth surface (Fig. 2). The TEM image revealed that the configured nanoparticles were joined using perfectly aligned (111) lattices. Resistivity of the self-joined silver foam was 5.56×10^-5 Ω·m. Thermal sintering can significantly reduce the resistivity of silver electrodes. When the sintering temperature increased to 300 ℃, resistivity was stabilized to 5.4×10^-7 Ω·m. However, high temperature resulted in a spheroidization effect, leading to resistivity increase to 6.98×10^-6 Ω·m (Fig. 3). Laser sintering exhibited unique advantages in reducing resistivity and maintaining nanostructures compared with self-joining and thermal sintering. The silver nanobelts could be joined at a low temperature, forming a cross-linked network structure to reduce the resistivity of the electrodes, and improving its flexibility significantly. The resistivity of the laser-sintered electrodes was 1.88×10^-7 Ω·m (Fig. 4), and the resistance change after 3000 bending cycles was 21.26% (Fig. 6).

Conclusions In this study, the performance of the three methods, including self-joining, thermal sintering, and laser sintering, in the joining of silver nanomaterials was evaluated. The electrical properties and microstructure of the silver nanomaterials under different joining methods were compared. Although self-joining can promote the joining of nanoparticles at room temperature(~25 ℃), it results in a large number of dielectric substances in a system. The resistivity of the self-joined structure was 5.56×10^-5 Ω·m and that of the nanobelt electrodes after thermal sintering was 5.4×10^-7 Ω·m. However, the sintered structure was uncontrollable at high temperatures and not suitable for flexible substrates. In contrast, laser sintering can induce the joining of silver nanomaterials at low temperatures without destroying a substrate. Under the laser irradiation, the silver nanobelts were interconnected to form a network structure, electrode resistivity was 1.88×10^-7 Ω·m, and electrode resistance change rate was 21.26% after 3000 bending cycles.

Objective Taper-free or negative conical micro-holes with a diameter of tens to few hundreds of microns are of great interest in a variety of industries. Conventional methods, such as electro-discharge drilling, mechanical drilling, electrochemical drilling, and continuous or pulsed laser drilling, have their limitations, including poor accuracy, low efficiency, as well as they are incapable of drilling in non-conductive materials, such as glass. Although ultrafast laser is believed to be a reliable tool for drilling processes due to its unique characteristics, the control of the diameter and taper of micro-holes is one of the greatest challenges encountered in the process. At present, the diameter and taper of micro-holes can be controlled using a 5-axis Galvano scanner, but the equipment is expensive, and a complex drilling strategy is required. Ultrafast laser helical drilling technology is an effective and simple way to adjust the taper and diameter of micro-holes since the laser spot is rotated at a high speed, and the roundness of the holes can be improved in the meantime. However, the taper and diameter are affected by several factors in helical drilling, and these factors vary with material, aspect ratio, laser characteristics, and drilling strategy. Hence, it is important and necessary to investigate the relationship between the diameter, taper, and beam rotation of micro-holes and to show the control mechanism of the diameter and taper in ultrafast laser helical drilling.

Methods With the ultrafast laser focused in the middle of a 0.5 mm thick copper plate, various micro-holes with different diameters and taper are drilled by adjusting the angle of the focused beam and rotation diameter of the laser spot. Then the rotation diameter of the laser spot at a focal plane, ±250 μm above and below the focal plane, is in situ monitored by a CCD. The diameters of the micro-holes in the entrance and exit are compared with the CCD measurement results at ±250 μm above and below the focal plane, respectively. The dependence of the spot rotation diameter on the angle of the wedge prism is analyzed based on geometric optics.

Results and Discussions Micro-holes with an aspect ratio of 6∶1 and taper of -2.6°-2.3° are obtained, and the relationship between the micro-hole diameter, taper, and beam rotation characteristics is shown (Table 1). The rotation diameter of the spot measured by the CCD and the diameter of the micro-hole drilled under the same parameters are investigated (Fig. 6(a)). When the position of the movable mirror changes from -4 mm to 3 mm, the diameter of the rotation beam changes from a small value to a large value at 250 μm above the focal plane and from a large value to a small value at 250 μm below the focal plane, while the diameter remains constant at the focal plane. This means that the focused beam rotates around the focus point (Fig. 6(b)). However, the angle of the focused beam does not equal the actual taper. Under a fixed wedge prism angle, the change in the hole diameter on the exit side is similar to that of the spot rotation diameter on the focal plane, the changes vary slightly, and the difference between them is almost constant. In contrast, the diameter of the hole entrance increases with the beam rotation diameter at 250 μm above the focal plane. That is, the hole diameter of the exit side remains unchanged, and the diameter change of the entrance side will lead to the change of the taper (Fig. 6(c)). It is based on this principle that micro-holes with a taper varying from -2.6° to 2.3° are machined in which a negative taper represents that the entrance of the micro-hole is smaller than the exit. Besides, when the rotation diameter ratio of the spot at ± 250 μm from the focal plane is 0.66, the hole taper is close to 0°, and a negative taper can be obtained when the ratio is smaller. When the ratio is 1, although the spot rotation diameter is constant, there is still a positive taper in the micro-hole. The diameter of the exit changes marginally no matter how much the rotation diameter changes because the laser energy is attenuated during the transmission in the micro-hole. The rotation diameter of the spot at the entrance must be smaller than that at the exit to get a straight hole with zero tapers. The pulse energy, frequency, and focus position will affect the processing efficiency and the shape of the hole. If the laser is focused on the surface of the workpiece, the micro-holes cannot penetrate completely under the same drilling time.

Conclusions In this study, we showed the relationship between the micro-hole diameter, taper, and beam rotation in a Dove-prism-based helical drilling system. The results show that the micro-hole size is dominated by the angle of the wedge prism. As for the taper of the micro-hole, adjusting the position of the movable mirror can change the exit angle of the focused beam, and then the taper can be adjusted. Since the laser energy will affect the ablation rate of material, the focus position, laser power, and spot overlap rate will slightly contribute to the micro-hole size and taper.

Significance Continuous miniaturization of traditional silicon electronic devices and photoelectric components increases the integration and performance of devices and introduces some undesirable problems caused by the size and quantum effects, and increased power consumption. Thus, the development of multifunctional next-generation nano-devices with more excellent performance than traditional devices is inevitable and significant in the post-Moore era. Owing to the excellent mechanical, thermal, electrical, and optical properties of nanomaterials, such as nanoparticles, quantum dots, nanowires, nanotubes, and two-dimensional (2D) materials, many studies have suggested that these materials are suitable for channel or electrode of multifunctional and high-performance nano-devices. Thus, the study and development of nano-devices based on nanomaterials are crucial for solving the bottleneck problems of electronics in the future.

Recently, abundant theoretical and experimental results have demonstrated that bending, folding, twisting a single nanomaterial, and arranging, assembling, connecting several nanomaterials can improve properties further or bring extraordinary characteristics of nano-devices. For instance, compared with chemical doping and contact engineering, deformation of 2D materials can solve the Fermi-level pinning and carrier concentration decreasing in nano-devices and may introduce new phenomena, such as piezotronics and piezo-phototronics. Thus, methods and accompanying systems for moving, arranging, deforming nanomaterials, and fabricating nano-structures and nano-devices will be crucial and indispensable in the electronics field in the future. The most “top-down” approaches for fabricating electronic and optoelectronic devices, such as ultraviolet lithography, electron beam lithography, and laser writing, are unfit for the mentioned purpose. Instead, nano-manipulation technology, as a “bottom-up” method, is proposed to move or spin atoms, nanomaterials, and cells in the nanoscale resolution. Based on this, it is promising in moving, deforming, and assembling nanomaterials in high-precision than other methods. For example, some indirect methods for bending 2D materials (e.g., thermal expansion mismatch, deformation of flexible substrates, and substrate surface topography modification) exist some problems, such as slipping of materials, small deformation, and uncontrollability. Nano-manipulation can use probes to push or fold materials in nano-/micro-scale directly and achieve large, complex, and controllable deformation. With electron beam-induced deposition, laser processing, and nano-welding, this technology can also develop nano-structures with excellent properties, weld a single material onto an electrode to fabricate devices, and test properties of a single material and device. Thus, it provides a new idea for the development of new-generation nano-devices with excellent performance.

Among many nano-manipulation techniques, methods and systems based on the microscopes with nano-level imaging accuracy, e.g., scanning probe microscope (SPM) and electron microscope (EM), are widely used. With the microscope monitor, the system controls the motion module to move the probes, tweezers and other manipulation tools in high-precision, and then moves, picks up, and bends nanoparticles (NPs), nanowires (NWs), and 2D materials. Besides, optical tweezers, magnetic tweezers, and acoustic tweezers can apply force to materials and trap or move these further by controlling the optical, magnetic, and sound fields. To develop nano-devices, manipulation methods based on SPM, EM, and optical tweezers are promising and anticipated. Thus, it is necessary and significant to introduce and summarize the recent studies in these nano-manipulation methods and understand their application in nano-devices.

Progress This study introduces the recent research in nano-manipulation based on scanning probe microscope(SPM), electron microscope(EM), and optical tweezers. For SPM manipulation, the principles and typical process demonstrate the capacity for accurately moving particles of tens to a few nanometers in diameter and weakness in real-time imaging, efficient and complex manipulation. For real-time imaging during manipulation, representative improvements include strategy optimization and development of parallel imaging/manipulation system (Fig. 2). The strategy optimization can also improve the success rate and efficiency, like sequential parallel pushing and virtual nano-hand strategies. To achieve three-dimensional (3D) and complex manipulation, researchers proposed new approaches to pile up nanoparticles and nanowires using a special substrate or two probes. With the development of strategies and systems, SPM manipulation applications were proposed in the field of nano-devices, such as testing mechanical and electronic properties, building plasma nano-structures, and fabricating electrodes of nano-devise. To achieve real-time imaging, Fukuda' group concentrated on EM manipulation. They invented nanorobotic manipulators inside the scanning electron microscope (SEM) and used it to stretch nanotubes, cut cells, and test mechanical properties (Fig. 5). EM manipulation is more suitable than SPM manipulation for complicated works due to real-time imaging and flexible manipulators. Complex manipulators also cause noise and drift in EM imaging. Sun' group invented various manipulators for compatibility with EM. Another major drawback of EM manipulation is the lack of depth information, which is critical to judge the contact between probes and materials. Thus, touchdown sensor, piezo fine positioner, laser displacement sensor, and other improvements are used to acquire precise 3D information. EM manipulation is wildly used in bending and twisting materials, nano-welding, fabricating transistors because of its flexibility. Optical tweezers are used in trapping nanoparticles and nanowires. Especially, near-field optical tweezers can trap particles up to several nanometers in diameter. To achieve a successful and superior manipulation, researchers concentrated on the enhancement of the optical field. Besides, fiber probes were suggested for trapping then moving nanomaterials. These researches showed optical tweezers are potential for manipulating smaller objectives to develop nano-devices.

Conclusions and Prospects SPM manipulation, EM manipulation, and optical tweezers have their advantages, limitations, and applications (Table 1). These provide new ideas and technical means for moving and assembling nanomaterials and manufacturing nano-structures and nano-devices. However, these methods and systems also face technical and scientific challenges. Thus, the development of integrated manipulation methods and systems will facilitate the applications in the field of nanoscience and electronics.

Objective Carbon nanotubes (CNTs) have become an essential electronic material to replace silicon materials in the post-Moore era due to their unique electrical properties and one-dimensional nanostructures. The micro/nano electronic devices fabricated by CNTs have the advantages of small size, high speed, and low power consumption. The type of effective method to be used to achieve a reliable and effective connection between CNTs and metal electrodes has always been a difficulty and key point in the construction of CNTs electronic devices. To achieve an effective and reliable connection between CNTs and metals, a series of connection technologies have been proposed, such as metal welding, local annealing, ultrasonic welding, electron beam, and ion irradiation. However, some additional elements, such as graphitized carbon, hydrogen, and other solders, are introduced into the interface between CNTs and metal, which affects its connection performances. The process of some methods is complicated, and they required accurate positioning. Thus, an effective and reliable connection technology between CNTs and metal electrodes on a large-scale without damaging the metal electrodes or other structures is highly needed. The laser-processing technology is widely used to manufacture electronic devices with the advantages of high peak power, noncontact processing, and good controllability. Femtosecond pulse laser is considered a cold processing technology since its pulse width is less than the cooling time of the electron. It can avoid damage to the metal electrode structure caused by heat accumulation. Besides, it is an ideal high-energy beam interconnection technology. We use femtosecond pulse laser irradiation technology to realize an effective and repeatable connection between Multi-walled CNTs (MWCNTs) and different metal electrodes (Au and Ni), which provides a certain experimental basis for subsequent large-scale preparation of high-performance CNTs field-effect transistors.

Methods In this experiment, mass concentration of 0.1 mg/mL sodium dodecyl sulfate is applied as a surfactant, and the MWCNTs powder is dispersed uniformly and stably in an aqueous solution using ultrasonic vibration. Spin coating is adopted to deposit the MWCNTs after laying electrodes. In this process, the metal electrode is the carrier of MWCNTs and test probe in the subsequent electrical performance test. Thus, the metal electrode should have a larger area for electrical performance test and smaller channel width for MWCNTs and metal contact. The morphology and size of the electrode are designed. The electrode is a rectangle of 200 μm×200 μm, and the channel's width is 8 μm. The required electrode group is obtained by photolithography, as shown in Fig. 2(a)--Fig. 2(b). Further, MWCNTs are deposited on the metal electrode by the spin coating process with a mass concentration of 0.005 mg/mL MWCNTs dispersion. In this experiment, scanning electron microscope is used to characterize the intrinsic structure, metal electrode structure, and metal morphology of MWCNTs. The electrical properties are tested using a semiconductor device analyzer.

Results and Discussions Femtosecond laser irradiation technology can achieve an effective and repeatable connection between MWCNTs and metal electrodes. The results show that there is no linear relationship between the laser power and irradiation time. Considering MWCNTs and Au as examples, it can be divided into three stages. The first is the nonaction stage. In this stage, even if the laser irradiation time is continually increased, the effect on the Au surface is ignored. The second is the selective modification stage. When the laser power is increased to 220 mW and the irradiation time is set at 60 s, the local plasma enhancement effect between MWCNTs and Au surface modifies the metal surface. When laser irradiates at the Au surface, the free-electron in Au collides with the photon of laser inelastic. Then, the free-electron absorbing photon energy migrates to a high-energy level, which increases the lattice thermal shock. Further, the local area is heated and softened, and micro molten pool formed MWCNTs are “embedded” in the metal electrode, forming a good “embedded” connection (Fig. 4(b)). The Au electrode will be ablated at 65 s in 220 mW. The last stage is material removal. When the laser power increases to 235 mW, the electrode surface will be damaged and ablated in different degrees within 50 ms (Fig. 4(d)). The same state also occurred in the Ni-MWCNTs structure. The results of the electrical test show that the contact resistance between MWCNTs and Au or Ni has been greatly decreased (Fig. 9), indicating that the connection is effective and repeatable.

Conclusions The effects of processing parameters of femtosecond pulse laser, such as irradiation, time, and laser power, on the morphology of different metal electrodes and MWCNTs, are investigated experimentally. When the laser power is 220 mW, and irradiation time is 30 s, an embedded connection formed between Au and MWCNTs. When the laser power is 28 mW and the irradiation time is 30 s, the cladding connection formed between Ni and MWCNTs. Under the same laser power, as the irradiation time continues to increase, the metal electrode surface would be ablated, and the structure of the electrode would be destroyed. The MWCNTs deposited on the electrode surface would be peeled off by the laser shock wave. The contact resistance of Au-MWCNTs-Au structure is reduced from 454--658 kΩ to 78.9--397 kΩ and that of Ni-MWCNTs-Ni structure is reduced from 505--612 kΩ to 21.1--64.6 kΩ. The latter structure is reduced by an order of magnitude, verifying the effectiveness of the connection. It also show that the bonding force and wettability of metal to MWCNTs influence electronic transport before and after interconnection.

Significance With the development of aeronautics, astronautics, communication, and instrument fields, conventional optical and electrical systems can hardly meet the demand for information transmission with high capacity and rate. Integrated optics system gradually develops under this historical circumstance. Its superiority is that optical devices with different functions can be integrated into a limited area, and the optical signal can be transmitted and processed. Comparing with a conventional optical system, an integrated optics system has the superiorities of small size, compact structure, high stability, and strong anti-interference capability. The optical waveguide device is the most basic unit within the integrated optics system. Its principle is that total reflection takes place at the media interface when light transmits in media with different refractive indexes, and light can be trapped in the microstructures. A channel forms correspondingly, in which light is capable of transmitting along a specific direction. Optical waveguide devices can trap and guide light and provide extra functions such as nonlinearity and active gain. Besides, optical waveguide devices can cooperate with other components in the integrated optics system, forming a photonic integrated circuit with various functions. Therefore, the performance of the optical waveguide device has a considerable influence on the performance of an integrated optics system. The development of manufacturing technology for optical waveguide devices with high quality and precision of great importance for the innovation of photocommunication, optical information processing, optical calculation, and optical sensing.

Optical waveguide devices can be divided into passive and active devices. The former devices are the basic units of integrated optoelectronics, and they are more widely used. Passive waveguides are mainly fabricated using semiconductors and organic polymers. There are three types of semiconductor waveguides, listed as silicon-based waveguides, III-IV group compounds waveguides, and ferroelectric oxides waveguides. Specifically, silicon-based waveguides have the advantages of good heat conductivity, chemical stability, mechanical strength, and low absorption loss. However, there is a large difference between the refractive indexes of cladding and core. The advantage of III-IV group compounds waveguides is that they can be integrated on the same chip with active devices. However, the large transmission loss and cost limit their further application. The advantages of ferroelectric oxides waveguides are large electrooptic coefficient, high response speed, and excellent heat and chemical stability. However, they are still troubled by high cost and large size. In terms of organic polymer waveguides, they are characterized by low optical loss and birefringence, high thermooptical and electrooptical coefficients as well as simple fabrication techniques. Unfortunately, they age easily, which is unfavorable for improving the device stability.

In 1996, Davis K M manufactured an optical waveguide with optical glass for the first time. In the following decades, with the development of ultrafast laser systems and optimization of manufacturing technology, researchers from home and abroad have successfully manufactured various optical waveguides in different materials. Although several remarkable advances have been made in improving device performance, the industrial fields are still troubled with several problems, such as non-negligible machining defects and transmission loss. Hence, it is of great significance to summarize the existing research to guide the future development of manufacturing technology for passive waveguide devices.

Progress The interaction mechanism between laser and waveguide materials is first explained. On these bases, the application of ultrafast laser in the manufacture of optical path converter is firstly interpreted. As a common optical path converter, the morphologies of curved waveguides before or during the performance test are shown (Fig. 1). Then, the morphologies and performances of three kinds of power splitters, listed as a branched waveguide (Fig. 2), directional coupler (Fig. 3), and multimode interference waveguide (Fig. 4), fabricated using ultrafast laser and other auxiliary means are illustrated. The waveguide lens can be divided into Fresnel lens and microlens array, the corresponding morphologies fabricated by ultrafast laser are demonstrated as well (Figs. 5--6). At last, the representative applications of ultrafast laser in the manufacture of passive optical waveguide devices are summarized (Table 1), in which the leading research teams and their achievements are highlighted.

Conclusions and Prospect Since the 1990s, a large number of theoretical and experimental studies have pointed out that ultrafast lasers are effective tools to fabricate various passive optical waveguides in dielectrics and polymers. Especially in recent years, based on the investigation of the interaction mechanism between ultrafast lasers and different waveguide materials, researchers have achieved the fabrication of passive waveguides with multiple structures and excellent performance by adjusting the temporal and spatial distribution of laser pulses. The fabrication techniques are optimized, and the process is simplified. To further improve the fabrication quality and precision, there are still several existing problems that need to be solved from the aspects of mechanism and techniques. For instance, the quantitative analysis and precise modulation of the spatial distribution of thermal stress induced by ultrafast lasers within the same material as well as the interface between different materials and the controllable fabrication of different materials with a precision that exceeds the diffraction limit.

Nowadays, integrated optoelectronic devices are developing toward multifunctions and high integrated levels, which raises great challenges for passive optical waveguides. On the one side, their volumes need to be minimized, and production efficiency needs to be improved further. On the other side, their applications are not only limited to treating light signals, but also integrated with active optical waveguides, such as lasers, modulators, and detectors to construct new-generation optoelectronic devices. Besides, novel materials such as graphene have been used to fabricate passive waveguides. To deal with these challenges, the transient evolution mechanisms of light, heat, electrical, and mechanical properties of different waveguide materials irradiated by ultrafast lasers should be investigated deeply. Under the guidance of theoretical studies, nanofabrication and parallel fabrication technologies can be developed that are beneficial for extending the application of ultrafast lasers in modern industrial fields, such as integrated optics, quantum information, and nonlinear optics.

Significance With the development of new electronic devices for miniaturization, flexibility and intelligence, the diversity of nanomaterial properties and limitations of traditional electrical connection methods bring new challenges in new electronic device preparation. Researchers are encouraged to continue to explore ways to break the limit of the device size. The manufacturing technology has gradually developed to the nanoscale level. Nanowelding is one of the key technologies for integrating nanomaterials with micro and macro systems.

Metal nanomaterials (e.g., Ag, Au, and Cu) and some carbon-based nanomaterials (e.g., carbon nanotubes, and graphene) exhibit excellent electrical and thermal properties. Besides, some wide bandgap semiconductor nanomaterials (e.g., ZnO) have shown great potential in future electronic devices. Not only for homogenous connections but also for the study of electrical and mechanical properties of heterogeneous connections, evaluating their mechanical and electrical properties is crucial for predicting the failure modes of electronic devices.

Stable device performance depends on reliable nanointerconnected structures. The size effect and high specific surface area of nanomaterials make them exhibit different welding characteristics from bulk materials during the welding process. The study on the electrical performance of nanowelding consists of single joints and interconnection networks. The study of a single nanojoint is essential to deepen the understanding of the welding mechanism. For interconnection networks, especially with the rapid development of industries, such as smart touch interactive terminals and wearable electronic equipment flexible solar devices, their performance has attracted significant attention.

Progress Currently, the electrical and mechanical characterization of nanoconnection quality consists of two methods. The first method is aimed at electrical testing and characterization of nanoconnected single-welded joints, such as direct in-suit measurement of one-dimensional (1D) nanowire and nanotube-welding points. The second method indirectly characterizes macroscopic devices based on nanointerconnections, especially for some flexible film structures. For the study on the performance of 1D nanowires and single-nanometer connection joints of tubes, some researchers have used molecular dynamics-related simulation software to simulate the mechanical and electrical properties of their interface and perform atomic simulation of the entire welding process. The morphology and influencing factors are analyzed to obtain theoretical electrical performance before and after welding. For experimental measurement, if the electrical and mechanical properties are to be directly characterized at such a small scale, with the development of characterization technology, direct mechanical measurement of solder joints can be achieved. However, there are still many challenges in the actual measurement process.

The current nanowelding methods are low-temperature cold welding, pressure welding, ultrasonic welding, electric field and chemical-assisted welding, high temperature and Joule welding, high-energy beam welding (e.g., electron and ion beam), and laser-induced plasma welding at local low temperature. During the preparation of nanointerconnection devices, especially for the new generation of flexible nanoelectronics, it is necessary to prepare interconnect joints with high electrical performance and a low-temperature and low-stress welding environment, which does not cause damage to other surrounding nanodevices and substrates. The nanojoints obtained using high-temperature melting are often accompanied by a relatively large heat-affected area, which will also have a thermal impact or even damage to the structure of the nonconnected parts, and then reduce the electrical performance of the overall interconnection structure.

Conclusion and Prospect This study summarizes and prospects the electrical and mechanical properties of different materials from the atomic scale to single welded joints, and then to macroscopic multinanoscale welded joints by combining the characteristics of current different nanowelding technologies and their welding interfaces. The discussion of welding structure and deformation mechanism, welding strength, fatigue characteristics, and electrical performance showed that laser-induced plasma welding with characteristics of self-limiting and low-temperature has great potential in fabricating nanodevices and flexible electronic devices.

Although the current study on laser-induced plasma self-limiting low-temperature welding technology has achieved a certain progress, it still faces huge challenges for achieving high-efficiency, high-precision, and high-resolution laser-induced nanocontrollable interconnection manufacturing. The realization of the energy precise control of the nanoscale joints and interconnection mechanism of materials at the nanoscale still needs further study. Besides, for interconnection functional structures with nanoscale line widths, effective manipulation techniques are often required to arrange and assemble them before the connection. It is necessary to achieve subsequent high-precision positioning. This process relies on the integration of high-precision laser nanowelding equipment; however, related technologies still need further study and development. It is believed that the continuous development of laser nanowelding technology will play a significant role in the next generation of electronic device interconnection packaging.

Objective Due to the advantage of high electrical conductivity, thermal conductivity, and superior surface area ratio, graphene has become the current research focus in the application of flexible energy storage and sensor. Comparing the chemical vapor deposition, mechanical exfoliation, and epitaxial growth, the direct reduction of graphene oxide (GO) can satisfy the demand for graphene production in the industrial field. Currently, the methods of GO reduction are chemical, thermal, and photon reductions. Based on reduction efficiency and cost-benefit, laser irradiation is an efficient way to remove the surface oxygen group for GO reduction without special physical and chemical conditions. Thus, laser reduction can be considered a highly effective method for graphene production. Some study has focused on different methods of GO through laser reduction, such as KrF excimer, ultraviolet, and femtosecond laser. Despite these investigations on GO reduction, simultaneous reduction and nanopattern of GO through laser irradiation are still challenging. To further investigate the morphology and structural properties of reduced GO, this study compares the morphology of the reduced GO with different nanostructures through femtosecond laser irradiation with 1030 nm and 257 nm. Besides, the influence of different laser-induced nanostructures on the electrochemical impendence will be discussed.

Methods GO can be obtained using Hummers methods. Different from graphene, surface oxygen-containing groups located at the surface and margin of GO nanosheets improve the hydrophilia property. By the preparation of GO dispersion, spray coating was used to form a uniform GO film on the polyethylene terephthalate (PET) substrate. After that, a femtosecond laser with 1030 nm and 257 nm was irradiated on the GO surface to construct the nanostructure. The morphology and characteristics of nanostructure were compared to show the difference of GO through femtosecond laser irradiation. The all-solid-state interdigital micro-supercapacitors were constructed with the assistance of PVA/H₂SO₄ to obtain the electrochemical performance of GO by femtosecond laser.

Results and Discussions The surface ablated morphology of GO using femtosecond laser irradiation was observed. The comparison results showed that the morphology evolution in GO has not followed the linear change with an increase in the incident laser energy and pulses number (Figs. 3 and 5). Under the 1030 nm laser irradiation, the ablated region of GO occurred in the layered annular structure, resulting from energy deposition and thermal diffusion. However, a large number of nanosheets located at the ablated margin of GO were obtained by 257 nm laser irradiation. The photochemical effect plays a significant role in laser irradiation. Two surface laser-induced nanostructures were further investigated to obtain the mechanism in the morphology evolution of GO (Fig. 7). Femtosecond laser-induced periodic surface structures (LIPSSs) with a high and low spatial frequency contributes to the surface plasmon polaritons (SPPs) on the GO surface. The coupling effect of SPPs and LIPSSs can result in the formation of nanostructure by 1030 nm femtosecond laser irradiation. The photomechanical effect induced by photochemical action is the main reason for the groove nanostructure’s formation by 257 nm laser irradiation. Combined with the results of the Raman spectrum of GO (Fig. 8), the ratio of the intensity of D and 2D peaks relative to that of G peak was calculated. Thus, the 1030 nm laser irradiation is essential for improving the transformation of graphite structure from sp ³ to sp ² and removing surface oxygen-containing groups. Through the electrochemical impendence spectra (Fig. 9), the impendence spectra of different nanostructures induced by laser irradiation with 1030 nm and 257 nm display apparent distinct. The ohmic resistance value nanostructure with LIPSSs or stripe is 40 Ω, which is lower than that of the nanostructure with groove morphology. According to the test data fitting, the nanostructure with LIPSSs or stripe morphology demonstrates the process of charge transformation at the high frequency and ion diffusion at the low frequency. The results suggested that the nanostructure by femtosecond laser irradiation with 1030 nm can improve the electrochemical action of micro-supercapacitors.

Conclusions In this study, the morphology and characteristics of GO nanostructures were investigated using femtosecond laser irradiation. Under the 1030 nm laser irradiation, the interference effect of SPPs and incident laser results in the formation of stripe nanostructure with the period of subwavelength. The groove nanostructure by 257 nm laser irradiation contributes to the photochemical effect. Based on the analysis of the Raman spectra of GO by femtosecond laser irradiation, the GO reduction level by 1030 nm femtosecond laser irradiation is higher than that of GO by 257 nm laser irradiation. Compared with the results of electrochemical impendence of different nanostructures by femtosecond laser irradiation, the GO nanostructure by 1030 nm laser irradiation improves the rate of ion diffusion of electrodes and decreases the ohmic resistance. This study will strengthen the practical application of simultaneous reduction and nanopatterning of GO by femtosecond laser in microelectronic devices.

Significance Since the end of the 18th century, mankind has experienced three industrial revolutions, represented by the applications of the steam engine, electric power, and electronic information technology. Each revolution has brought about a huge increase in productivity. Now, the fourth industrial revolution has quietly occurred and industrial production has changed from mechanization and automation to informatization and intelligence. The semiconductor integration industry has become an important carrier of the fourth industrial revolution. Currently, semiconductor integrated circuits (ICs) are developing toward high integration, high density, high performance, and low power consumption, and their manufacturing process has entered the era of the 5-nm node. However, the reduction in feature size has led the bottom-up development model based on photolithography to face huge challenges, such as the restriction of manufacturing processes and applications in ICs. The latest international semiconductor technology roadmap shows that the feature size of ICs will approach its physical limit and the size effect will greatly affect the performance of the device. This will cause electronic devices to fail according to traditional semiconductor physics principles. Solving the size effect caused by the ever-decreasing IC feature size has become the frontier and hot spot of domestic and foreign scholars.

Carbon nanotubes (CNTs) have become ideal next-generation electrical wire materials, owing to their unique electrical, mechanical, and thermal properties, and have attracted the attention of scholars worldwide. The CNT is a typical one-dimensional nanostructured material that only propagates along the axial direction, which greatly reduces the probability of scattering during electron transport. CNTs can withstand 70% higher carrier mobility than silicon materials and their current density is more than 1000 times that of copper wires after interconnection. Therefore, CNTs can not only replace both copper wires and the doped silicon to become the next generation of semiconductor device materials, but also unify semiconductor device materials, which will greatly simplify the manufacturing processes and reduce the cost of ICs. Electrical contact is an indispensable part of ICs. Because of the small contact area between CNTs and metal electrodes, electrical coupling between CNTs and metal electrodes is difficult. Although CNTs have high conductivity, their large interface-contact resistance hinders their practical electronic applications. Therefore, to realize the various applications of CNTs in the field of micro-nanoelectronics in the future, a key prerequisite is to establish reliable and stable mechanical and electrical connections between CNTs and micro-nanoelectrodes. Hence, it is important and necessary to summarize the existing research on CNT-metal connections to guide the future development of this field rationally.

Progress As can be seen from the interface behavior of CNTs and metal, there are two contact modes between them: weak contact in physics and strong contact in chemistry. Experimental and simulation results show that strong chemical contact can not only ensure high mechanical connection strength, but also ensure stable and efficient energy transfer. However, the precise application of an energy source to form a stable chemical connection between CNTs and metal remains an urgent technical problem. Because CNTs are one-dimensional nanomaterials, physical or chemical methods are currently used to achieve closer contact or connections between CNTs and metals in the microscopic fields, such as annealing, deposition, ultrasonic welding, and high-energy beam irradiation. At present, the lowest interface contact resistance and resistivity between the CNT bundle and metal electrode based on interconnection technology are approximately 0.6 Ω and 10^-3-10^-2 Ω·cm, respectively. In contrast, the resistivity of a copper wire interconnection in 22-nm technology is 5.8×10^-6 Ω·cm. Interconnection technologies, such as high-temperature annealing, electron- or ion-beam deposition, and ultrasonic welding, are not suitable for the above applications. For CNT-based micro-nanodevices, achieving high-quality connections between a single CNT or multiple CNTs and metal electrodes is still an urgent problem. So far, the minimum contact resistance between a single CNT and the metal electrode is 116 Ω, and the interface contact resistivity varies from 10^-5 Ω·cm to 7.5×10^-4 Ω·cm. Therefore, CNT interconnection technology based on IC applications still requires much work.

Conclusions and Prospects Compared with other interconnection methods, such as electron- and ion-beam interconnection technology, laser-beam irradiation technology has the advantages of shape control and versatility. Laser near-net shaping technology, selective laser sintering technology, and other laser irradiation technologies can effectively form strong and effective connections between CNTs and nanoscale metal powder, nanoscale metal particles, micro-nanoscale metal bulk materials, etc. When the laser beam is coupled with the nanoscale operating system, heterogeneous joints of different geometric shapes can also be created across scales, and the performance of each joint can be controlled according to the application requirements. When the laser processing system and high-speed automation system are combined, the focused laser beam can irradiate or process various nanomaterials with high efficiency and high precision in a large work area. This is a good method to prepare high-quality heterogeneous connections between CNTs and metal in large quantities and areas. The interconnection technology for CNTs and metal electrodes is developing toward stability, convenience, large area, and environmental friendliness to effectively reduce the contact resistance of the CNT/metal interface and promote its industrial applications.

Objective In recent years, nanowires have achieved great progress in the fields of nanoelectronics, nanophotonics, nanomedicine, and nanoelectromechanical systems. Nanowires play an important role in the miniaturization of micro/nanoelectronic devices. However, the effective interconnection of nanowires is an extremely critical concern in nanomanufacturing. Thus, a good welding method is required to attain effective interconnection of the nanowires. Recently, among the nanowire interconnection methods, the braze welding method presents less damage to the base material than other methods and has the advantages of homogenous and heterogeneous connections of the nanowires. Because of that, it has the greatest potential among all methods. Nevertheless, it is very difficult to locate and transport the nanosolders, which limit the development of nano brazing technology. Therefore, it is important to study the positioning and assembling of flexible nanosolders and nanowires to solve this problem. As a “bottom-up” manufacturing, assembling, and testing method, nanomanipulation technology is an important method to obtain a new generation of electronic components, optical device manufacturing, and nanomaterial manipulation and testing. Among the nanomanipulation technologies, the nanomanipulation technology based on scanning electron microscope (SEM) is suitable for the assembly of nanosolders and nanowires because it has several benefits such as real-time imaging and large operating space. Simultaneously, it has a focused electron beam with high energy, appropriating for performing the work of nanosolders melting.

Methods Based on the above-mentioned limitations of the nanowire braze welding, a new method that can facilitate in situ assemble and braze the interconnection structure of nanowires and nanosolders under SEM is proposed. This method is basically divided into three steps. At first, a self-built SEM nanomanipulation platform was used to finish all steps (Fig.1). This platform consists of an SEM observation system, a double probe nanomanipulation system, the electrical test module, the nanomanipulation sample stage, and the tungsten probe with a tip diameter of 300 nm. With the ZnO nanowires as the target nanowires and Ag nanoclusters as the solder, the nanomanipulation uses the nanoprecision manipulator that clamps the tungsten probe to assemble the interconnection structure of ZnO nanowires and push Ag nanoclusters to the interconnection joint of the interconnection, which complete the assembly of brazing interconnection structure for nanowires and nanosolder on the same base. Secondly, this study uses the SEM in situ focused electron beam to accurately irradiate the nanosolders, making them absorb energy and produce melting, thus achieving the brazing of nanowire interconnections. Finally, double probes of the nanomanipulation system were used to finish the I-V tests on the soldered nanowires and to verify whether the soldering was successfully achieved.

Results and Discussions The melting experiment of a single Ag nanocluster irradiated by the electron beam was completed. Thus, through focused electron beam irradiation for approximately 5 min, the nanocluster achieved local melting, which confirmed the feasibility of melting the Ag nanosolders with the electron beam (Fig.5). Using the nanomanipulation platform, the assembly experiment that includes the interconnection structure of Ag nanoclusters with diameters of ~188, 200, 243, and 290 nm and nanowires with a diameter of ~100 nm was completed using tungsten probes (Fig.7). The electron beam focusing was then used to obtain joints with different sizes. The nanoclusters composed of multiple nanoparticles achieved edge melting and bonding after approximately 5 min of irradiation (Fig.8). As the size of the solder increases, the irradiation time is longer, and the nanowires are also slightly damaged by the irradiation. But if the size is too small, the difficulty of nanomanipulation increases. Therefore, for 100-nm nanowires, the transportation of nanosolders of 200--250 nm can ensure high efficiency of pushing and can also that of the joint connection is not too large at the same time. Finally, the I-V electrical test was accomplished by double-probe contacting the two ends of the interconnected nanowires. The curve is basically linear and symmetrical, indicating that the ZnO nanowires and Ag solder have formed a satisfactory ohmic contact (Fig.9). The resistance of the loop calculated using the I-V curve is ~0.3 MΩ and the resistivity ρ=1.69 Ω·cm; this value is very similar to that previously reported, confirming the reliability of this method.

Conclusions In this study, based on the SEM nanomanipulation platform, the assembly and interconnection of nanowires and nanosolders were achieved successfully. The assembly and interconnection processes were integrated flexibly in situ. Moreover, the electrical testing of the structure after welding was completed in situ. The nanomanipulator manipulated the nanoscale probe to complete the two-dimensional operation of the ZnO nanowires with a diameter of ~100 nm and Ag nanosolders with a diameter of 180--300 nm, and the ZnO-Ag-ZnO nanowire interconnection structure was assembled successfully. The experiment of the electron beam focusing melting was processed and a high beam brazed joint of the nanowire interconnection structure achieved a good interconnection of the ZnO nanowires. The in situ electrical performance test was performed using the double-probe as the electrode. The I-V curve of the ZnO nanowires after brazing was obtained and the calculated resistivity is ρ=1.69 Ω·cm, confirming the reliability of the electron beam irradiation brazing method. The proposed research method is flexible and has a high success rate, providing an important reference for future homogenous and heterogeneous nanowire soldering interconnections and their applications.

Objective As a technology of welding nanomaterials, nano-welding is not only an important “bottom-up” means for manufacturing nanostructures, but also a key technology for the development of high-performance integrated circuits with reliable interconnection points. Among all nano-welding methods, the nanometer brazing technology of melting nanomaterials under laser irradiation, as one of most reliable methods, is utilized to realize nano-device-level interconnection. This technology reduces the damage to the welding base materials, achieves the interconnection points with high mechanical strength, and even maintains the excellent electrical performance of the devices. However, the previous theoretical models of nano-welding have only considered the atomic configuration evolution process of nanoparticles at different temperatures, ignoring the effect of substrate materials on the energy exchange process for achieving the best welding quality. Moreover, the simulation of nanoparticle melting under laser irradiation without substrates cannot completely represent the evolution of actual atomic configuration of nanoparticles as a reliable interconnection node during the brazing process. Therefore, in view of the actual brazing process of nanometer brazing, the change of atomic configuration of Ag nanoparticles on a SiO₂ substrate under laser irradiation is simulated and analyzed. More importantly, the adsorption energy between the substrate and nanoparticles during the melting process is discussed in detail. These results provide a theoretical basis for the realization of actual nanometer brazing.

Methods To obtain the melting evolution process of nanoparticles at the atomic level under laser irradiation, molecular dynamics (MD) simulation based on classical mechanics is used for establishing the simulation model. In the simulation model, single and multiple Ag nanoparticles are considered. Also, amorphous silica is obtained by the energy minimization process for supporting an energy-exchanging substrate in the melting evolution process of nanoparticles. This paper simulates the melting process of silver nanoparticles induced by a laser. In the melting simulation, geometric structure optimization is first executed as an initial system state. The laser irradiation energy is applied by controlling the corresponding evolution temperature of an atomic structure. The melting process utilizes a canonical ensemble NVT to carry out the relaxation of an atomic configuration. The Nose-Hoover thermostat method is used to set the temperature and bath time for matching the requirement of energy exchange. The boundary condition is an aperiodic boundary. Three bottom atoms of the amorphous SiO₂ substrate are selected to exert fixed constraints in three directions in the simulation. After simulation, the atomic configuration and energy change are extracted and analyzed for the subsequent discussion of contact length, contact angle, and adhesion energy.

Results and Discussions When the applied temperature is low, the shape of silver nanoparticles is spherical. With the increase of applied temperature, the shape of silver nanoparticles gradually changes to a hemispherical shape (Fig. 2). The hemispherical shape is attributed to the restriction of the substrate at the interface between nanoparticles and SiO₂ substrate during the evolution of atomic configuration. The changes of contact length and contact angle between silver nanoparticles and substrate at different temperatures are analyzed [Fig. 3(a)]. The contact length and contact angle increase first and then reach a flat state with the increase of temperature. The adsorption energy between a single silver nanoparticle and an amorphous SiO₂ substrate versus temperature is discussed [Fig. 3(b)]. When the applied temperature is 400--1000 K, the adsorption energy increases linearly with temperature. When the temperature is higher than 1000 K, the adsorption decreases rapidly. The changes of the atomic configuration of two silver nanoparticles on the amorphous SiO₂ substrate at different time are conducted (Fig. 4). The original two nanoparticles fuse into one after high-temperature relaxation. The adsorption energy of two silver nanoparticles melted on the substrate is significantly higher than that of a single nanoparticle, which was attributed to the increase of contact area (Fig. 7).

Conclusions In summary, based on the molecular dynamics method, the evolution process of the atomic configuration of 4 nm-diameter nanoparticles on SiO₂ substrate at different temperatures is discussed, and the melting process of nanoparticles caused by laser irradiation in the actual brazing process is simulated. When the temperature reaches 800 K, the atomic configuration of a single Ag nanoparticle forms a hemispherical shape, and the adsorption capacity of a single Ag nanoparticle reaches the maximum at 1000 K. At a temperature of 1200 K, the atomic lattice change, sintering neck formation, and melting of two nanoparticles into a single particle occur. The atomic configuration can completely form a single nano-interconnection node. The adsorption capacity with a SiO₂ substrate can reach the maximum, higher than the adsorption capacity of a single particle as an interconnection node. In addition, the adsorption energy increases first and then decreases with temperature based on different numbers of Ag nanoparticles and the SiO₂ substrate. Therefore, there is an optimal critical temperature to maximize the adsorption energy and to ensure a stable nano-interconnect structure after welding. The above simulation results lay a theoretical foundation for the subsequent realization of laser melting of nanoparticles and nanomaterial brazing.