Significance electronic and information devices are becoming increasingly miniaturized and portable with technological advancements. These advancements require high-density distribution of device function units. This introduces new challenges to the electrical and optical interconnection technology among function units. Some techniques such as photolithography and electron beam have been developed for fabricating microelectrical and micro-optical devices. Although these methods have high resolution, they are inflexible for three-dimensional (3D) fabrication. Ultrafast pulse lasers are a versatile tool for fabricating microelectrical/optical devices owing to their high resolution, minimal thermal effect, and flexibility. In this study, we briefly introduce the basic mechanism of ultrashort pulse lasers for microelectrical/optical interconnection, including multiphoton-induced reduction, surface plasmon resonant, and two-photon photopolymerization. Furthermore, this study focuses on the application of ultrafast laser manufacturing in microelectrical/optical interconnection.
According to different applications, femtosecond laser interconnect technology can be categorized into electrical and optical interconnections. Between them, electrical interconnection technology can be used to connect zero-, one-, and two-dimensional nanomaterials.
For zero-dimensional nanomaterials, ultrafast laser-induced interconnection mechanisms include multiphoton reduction, photodynamic assembly, and selective laser sintering. Multiphoton reduction is a high-resolution approach for 3D electrical interconnection owing to the multiple absorptions induced in the metal-ion precursor (Fig. 1). To improve the quality of electrical structures, surfactant (Fig. 2) or polymeric matrix (Fig. 3) is added to the precursor to avoid the diffusion of ions. In addition, photodynamic assembly for electrical interconnection is developed to address the diffusion of metal ions in the precursor. This method uses laser-driven force to capture and connect nanoparticles (Fig. 4). Furthermore, selective laser sintering can be used to fabricate patterned electrodes in the atmosphere using surface plasma resonance (Fig. 5).
In nanowire electrical interconnection, femtosecond laser-induced local plasma resonance can be used to weld homogeneous nanowires or nanowires and substrate. Studies have shown that local-field enhancement appears at the ends of nanowires or coupled gap regions during femtosecond laser irradiation, inducing localized plasmon resonance to generate localized high temperatures, which can be used for nanowire joining, cutting, or reshaping. For example, silver nanowire networks will have local plasma resonance at junctions during femtosecond laser irradiation, resulting in a localized high temperature, to realize nanowire welding and reduce the sheet resistance of silver nanowire transparent conductive films (Figs. 7 and 8). The welding between heterogeneous material interfaces can also be realized to form electrical interconnection using local plasmon resonance induced via femtosecond lasers, such as Ag-TiO2 nanowire welding and TiO2 nanowire-Au electrode welding (Fig. 12). In two-dimensional material electrical interconnection, femtosecond laser direct writing induced reduction of graphene oxide can be used for electrode repairing or adjustment. To realize one- and two-dimensional material electrical interconnection, femtosecond laser has the advantages of small thermal impact, almost no thermal damage occurs to substrates, and high processing resolution. Therefore, the method of welding nanomaterials using femtosecond laser irradiation has important application prospects in developing flexible electronic devices and functional micro-nano devices.
In optical interconnection, femtosecond laser modification processing can often induce refractive index changes in glass and crystalline materials. Two-photon polymerization can be used for additive manufacturing outside the base material, which can process complex 3D structures compared with femtosecond laser modification(Fig. 13). The annealing treatment after modification processing can effectively reduce the transmission loss of a waveguide; beam shaping technology can improve the processing efficiency of the waveguide. However, efforts are still required to improve the compatibility of waveguide manufacturing. Among discrete components, relatively simple couplers, beam splitters, and microlenses have been extensively studied. However, further research is required to fabricate complex devices such as on-chip light source, modulator, and detector component.
Electrical/optical interconnection can be realized via femtosecond laser irradiation primarily through the principles of photon reduction, photodynamic assembly, laser-induced surface plasmon resonance, two-photon polymerization, or material phase transition. The interconnection process is complex, involving photon absorption, energy transfer or transformation, material phase transformation, etc. Laser processing involves the interaction between light, heat and materials. The welding of materials is usually the result of a combination of various mechanisms; therefore, further research is required. In addition, the smallest structure size can reach the submicron level. However, further reducing the characteristic size, reducing resistivity or transmission loss, and improving oxidation resistance and processing efficiency are still the challenges faced by the electrical/optical interconnection. With more understanding of ultrashort pulse laser processing, related technologies will play a more important role in the field of microelectrical/optical interconnection.
