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.