Lithography technology is crucial for manufacturing all kinds of semiconductor integrated circuits. Overlay, a major performance indicator, is critical to monitor the lithography quality. Together with the increasing density of integrated circuit (IC) chips and continuously shrinking critical dimension, alignment accuracy for lithographic overlay is required to be extreme. Overlay usually refers to the process where each layer of the pattern needs to be accurately transferred to the correct position on the silicon wafer so that its position error relative to the previous layer of the pattern is within the tolerance range. The position error among different layers mainly depends on the alignment system situated inside the lithographic equipment. Thus, the measurement capability of an alignment system is very important, since the budget of the overlay budget is set to be just one-third to one-fifth of the resolution of a node, and the budget of alignment is only allowed to be within one-third of the overlay.

For each lithography step, the alignment system measures special marks at certain targeted locations. By calculating the mark positions, microscopic aligning errors can be captured dynamically and compensated when necessary. Moreover, considering the wafer deformation during the process, such as the warpage caused by thin film deposition, the partition is needed with 20-40 marks placed in each region of the wafer. By these means, every exposure field is measured and controlled precisely.

With the continuous development of lithography, alignment systems have achieved measurement accuracy from a sub-micrometer level in the 1980s to a nanometer level in 2002 and then reached a sub-nanometer level in 2016. Advanced lithography companies, such as ASML, Nikon, and Canon, evolve distinctly with their alignment technologies. At the same time, the designs of the alignment marks vary significantly based on the characteristics of specific alignment systems. Consequently, it is crucial to categorize and analyze the measurement principles and technology paths of the alignment systems. It is also important to provide references and insights for successive development.

The high-end litho-equipment global market has been dominated by ASML, Nikon, and Canon. Since the 1970s, lithography machines have briefly been through five generations of products, featured by advanced light-source technologies and process innovations. These improvements successively reduced critical dimensions and refined overlay. To address the technical problems, the three companies have continuously developed their alignment technologies. We summarize the characteristics of alignment hardware systems (Table 1), the corresponding alignment mark designs (Table 2), and the evolutionary roadmap of each company's alignment technology (Fig. 1).

ASML built its alignment system based on the phase grating principle. In the beginning, its single stage system adopted the coaxial through-the-lens (TTL) aligning method, for which only the first-order diffraction signals were considered. The advanced technology using high-order enhanced alignment (ATHENA) system was invented to reduce the influence of the production process on diffraction signals. Later, smart alignment sensor hybrid (SMASH) was introduced to ensure compatibility with the alignment marks of Nikon and Canon. Furthermore, ORION was developed to reduce the effect of mark asymmetry on alignment accuracy and was released together with ASML's commercial extreme ultraviolet (EUV) lithography machines.

ASML conducted research to improve alignment accuracy, such as special mark-design software, color weighted or polarization algorithms, high-order deformation models, and layout optimized via error separation or grid mapping.

Nikon applied various aligning methods based on specific scenarios, including phase grating intensity, image processing, and heterodyne interference. Canon then adopted either phase grating or image processing for its alignment system.

Besides the above international giants, we also investigate the domestic teams who are actively exploring alignment improvements. Shanghai Micro Electronics Equipment (Group) Co., Ltd. (SMEE) proposed multi-grating marks with large and small periods for coarse and fine alignment. Institute of Optoelectronics Technology, Chinese Academy of Sciences (IOE) conducted an overlapped grating equivalent comparing with the transmission type.Harbin Institute of Technology (HIT) put forward a multi-channel and multi-order grating interferometry for stable position measurement and alignment. Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (SIOM) proposed Moiré fringes to enhance the detection sensitivity. Image processing methods were employed to avoid motion errors. Institute of Microelectronics, Chinese Academy of Sciences (IME) proposed a mark design method that makes zero and even order diffraction automatically miss while the diffraction efficiency of higher odd orders was enhanced. The team also provided a depolarizer-compensation method based on an optimized reflective film layer. Additionally, they investigated the effect of mark asymmetry and proposed a weighted optimization for different diffraction orders.

The rapid development of the IC industry has triggered increasingly higher demands for lithographic alignment accuracy and overlay. The development of alignment technology poses challenges to the diffraction field, such as extraction and analysis of higher diffraction orders, recognition and compensation of asymmetric signals, and interactions with mixed optical structures. To realize higher alignment accuracy, technology therefore could evolve through improving optical components, analyzing polarization states and wavelength influences, optimizing the interaction structures and layouts, and even considering suitable positioning mechanisms. We comprehensively investigate and summarize the development of alignment technology from perspectives of demands and problems, solutions, and improvements. The future improvement directions are pointed out to provide a meaningful reference for relevant studies.

Coherent combining of fiber lasers by active phase control is an effective way to break through the power limit of a single fiber laser and achieve higher output power while maintaining good beam quality. Based on the research progress in China and abroad, this paper introduces the representative achievements in the past 20 years made by the coherent beam combination research group in National University of Defense Technology and presents the prospect of coherent beam combining (CBC) of fiber lasers.

We present our representative achievements in CBC of fiber lasers in this paper, which are organized as follows.

First, the high power key components for CBC were designed and manufactured. Various types of fiber amplifiers have achieved power breakthroughs. For example, a 500 W level single-frequency fiber amplifier, 7 kW level narrow line-width fiber amplifier, and 500 W femtosecond fiber amplifier were obtained. High-power phase modulators based on piezoelectric ceramics were developed. We also designed two kinds of high power adaptive fiber-optics collimators (AFOC), which were based on flexible hinges and piezoelectric bimorph actuators respectively.

Second, the active phase control of fiber lasers was studied. Various phase control methods were deeply researched, including the stochastic parallel gradient descent (SPGD) algorithm, dithering technique, heterodyne interference measurement technique, and deep learning algorithm. Some innovative phase control techniques were proposed to increase the control bandwidth, such as the single dithering technique, orthogonal dithering technique, and cascaded phase control technique.

Third, we also studied the high precision control of other optical parameters for CBC, including optical path difference control, tilt-tip control, and defocus aberration control. For example, we proposed an all-fiber optical path difference adaptive control method and simultaneously controlled phase and optical path in coherent combing of broadband light sources based on spectral filtering. In addition, a collimator was designed for defocus aberration compensation.

Fourth, beam combination techniques were demonstrated. Beam combination can be classified into tiled aperture and filled aperture. In the aspect of tiled aperture, a series of beam combination methods with high fill factor were designed and developed. For example, we proposed a coherent fiber-optics-array collimator that was mainly composed of a single unitary collimating lens and a prism. We also proposed a novel scheme of fiber collimator based on rod lens, which had good application prospects in the CBC of a large number of fiber lasers. In the aspect of filled aperture, we experimentally testified coherent polarization beam combining (CPBC) of eight low power fiber lasers, and 5.02 kW output power was obtained by CPBC of four fiber lasers with combining efficiency of 93.8% and beam quality of M²<1.3.

Fifth, based on the enabling technology mentioned above, a number of experimental systems were built. For high power fiber laser CBC systems, 1.08 kW output power was obtained by coherent combing of nine fiber lasers in 2011; CBC of a seven-channel fiber laser array with 7.1 kW overall output power was reported in 2020, and 21.6 kW was generated by CBC of 19 fiber lasers in 2021. For a large number of fiber laser CBC systems, phase locking of 32, 60, and 107 fiber lasers was realized by using the SPGD algorithm in 2014, 2019, and 2020 respectively. Based on the heterodyne interference measurement technique, efficient phase compensation of 397 and 1027 laser channels were realized in 2022 and 2023 respectively. For the pulsed fiber laser CBC system, 1.2 kW average power was generated by the coherent combining of seven nanosecond fiber amplifiers array in 2013; CPBC of two-femtosecond fiber lasers was realized with 313 W average power in 2018, and CPBC of two ultrafast laser channels was realized based on fiber stretcher and SPGD algorithm in 2022. For target-in-the-loop CBC systems, CBC of a fiber laser array with nine channels and 100 W level was reported in 2013, and atmospheric turbulence compensation was realized over a 1 km level propagation path for a six-channel fiber laser array based on target-in-the-loop CBC in 2018. In addition, CBC of fiber lasers with special wavelengths such as 1018 nm and 2 μm has also been achieved.

Sixth, the novel compact internal sensing phase locking techniques were presented. By using those techniques, the phase noises in the laser channels can be detected and compensated for before the lasers form the laser array. Based on spatial structure, internal phase locking of 12 fiber lasers was realized, and 1.5 kW output power was generated by CBC of three fiber lasers. Based on an all-fiber network, methods to compensate for π‑ambiguity between channels were proposed, and CBC of three fiber lasers was experimentally verified.

Seventh, CBC technique was employed for light field control, and special light fields such as vortex beams and vectorial beams were generated. For example, by CBC of six fiber lasers, a vortex beam with an output power of more than 1.5 kW has been generated.

Our group has researched CBC for nearly 20 years. Some representative results have been achieved. Artificial intelligence and light field control have been integrated with CBC. Some innovative breakthroughs have also been made in interdisciplinarity. The scientific research results have been continuously added to undergraduate and graduate courses such as Physical Optics and Advanced High Energy Laser Technology. A large number of graduate students have become the backbone force of scientific research. In the future, we will focus on the development of science and technology, student education, and talent cultivation integrally and make unremitting efforts to produce innovative results in this field.

The photonic lantern is a new type of photonic device that combines the advantages and characteristics of single-mode fiber and multi-mode fiber. It has important applications in the fields such as astronomical photonics, optical fiber communication mode division multiplexing, and optical fiber laser mode control.

This review introduces the structure and mode evolution theory of photonic lanterns, fabrication technology, the mode adaptive control based on photonic lanterns, their application in high-power fiber laser amplifiers to suppress the transverse mode instability, and utilization in large mode area fiber to excite special structural beams (such as orbital angular momentum modes). Through theoretical simulations and experimental exploration, the original design and fabrication criteria of photonic lanterns are improved. Meanwhile, two key design criteria for mode adaptive control are added: 1) optimizing the input fiber arrangement to improve the control bandwidth; 2) selecting the appropriate input core cladding ratio to expand the optional range of the output fiber. According to the above design requirements, N×1 photonic lanterns with excellent performance are prepared (N=3,5,6,7,…), as shown in Fig. 12. The phase of the input beams is actively modulated by the stochastic parallel gradient descent (SPGD) algorithm. The output beam of the optimized 3×1 photonic lantern with 30/125 μm output fiber is stable, and the M² factor is lower than 1.18 (Fig. 15). Orbital angular momentum modes (OAM₀¹ or OAM₀² modes) and higher-order linear polarization modes (LP₁₁or LP₂₁ modes) are obtained, and the corresponding modes purities are more than 0.85, as shown in Figs. 16 and 26(a). The mode adaptive control system based on photonic lanterns achieves stable fundamental mode output with M² factor ~1.4 in large mode area fiber with a core diameter of 50 μm. By adopting photonic lanterns, the transverse mode instability is suppressed in a fiber amplifier with a core diameter of 42 μm (Figs. 22 and 23). Finally, a possible technical solution is provided for further increasing the power of near-diffraction-limit fiber lasers with large mode areas and high brightness.

The mode adaptive control system based on photonic lanterns can effectively suppress TMI in the 42 μm core fiber amplifier. The results of selective amplification of high-order mode and OAM beams achieved by this technique has a wide application prospect in the fields requiring high power special beams. Further research will be focused on the design and fabrication of the photonic lanterns with more channels and better performance, as well as increasing the modulated parameters of the adaptive control system such as polarization and intensity.