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The Background Technology and Working Principle of Lithography Machine

In the past two years, China's chip industry has suffered a severe blow, and the Chinese people are also aware of the importance of independent research and development of chips. Since 2008, chips have been China's largest import commodity in the past ten years, with imports far exceeding those of the second-ranked oil. In 2018, China imported 417.57 billion integrated circuits and 312.258 billion integrated circuits. This set of data clearly reflects the lack of technical capability of high-end chips in China and the severity of external dependence.

There is a big gap between the technology level of chip production in China and the advanced foreign enterprises, and the tools and processes of chip production are also monopolized by several foreign companies. Among them, lithography machine, known as one of the miracles of human invention in the 20th century, is the Pearl on the crown of the integrated circuit industry. The technical and financial threshold for research and development is very high. Today, only a few countries can manufacture lithographic machines, such as the Netherlands, the United States, Japan and so on. ASML in the Netherlands is the absolute leader in this field. Its lithographic machines occupy about 80% of the global market.

Lithography machine has a wide range of uses. In addition to front-end lithography machine, there are projection lithography machine used in the field of LED manufacturing and back-channel lithography machine used for chip packaging. This paper only introduces front-end lithography machine.

1. Background Technology and Working Principle

Lithography equipment is a projection exposure system, which is composed of ultraviolet light source, optical lens, alignment system and other components. In semiconductor fabrication, photolithography devices project light beams through patterned photomasks and optical lenses, exposing circuit diagrams to silicon wafers with photosensitive coatings. Grooves are formed by etching exposed or unexposed parts, and then deposited, etched and doped to construct circuits of different materials.

This process is repeated over and over again. Billions of MOSFETs or other transistors are constructed on silicon wafers to form what is commonly known as integrated circuits.

Lithography process is very important in the whole chip manufacturing process, which determines the nano-level processing degree of semiconductor circuits. It has very strict technical requirements for lithography machine, and high requirements for error and stability. The related components need the most advanced technology in the fields of integrated materials, optics, electromechanical and other fields. Therefore, the resolution and accuracy of lithographic machine become the evaluation index of its performance, which directly affects the process accuracy, power consumption and performance level of the chip.

Therefore, lithography machine is the largest, most sophisticated, most difficult and most expensive equipment in integrated circuit manufacturing.

The resolution of lithography machine determines the minimum linewidth of IC. In order to improve the imaging resolution of lithography machine, two methods are usually used: shortening the wavelength of exposure light source and increasing the numerical aperture of projection objective.

According to the improvement of the light source, the lithography machine has experienced 436 nm g-line in the first generation, 365 nm i-line in the second generation, 248 nm KrF in the third generation, 193 nm ArF in the fourth generation and 13.5 nm EUV in the latest generation.

Among them, 193 nm ArF is also called deep ultraviolet light source. The dry lithography machine with 193 nmArF light source can achieve 45/40 nm lithography process nodes. Because the light source wavelength was difficult to break through further, the industry adopted immersion technology to reduce the light source wavelength (193 nm changed to 134 nm) while improving the numerical aperture of lens in liquid (0.50-0.93 changed to 0.85-1.35), and applied optical proximity effect correction (OPC) technology. After that, the limit process node of 193 nm ARF dry lithography can reach 28 nm.

After the 28nm process node, the single exposure pattern spacing can not be further improved. The industry began to use Multiple Patterning (multiple exposure and etching) technology to improve the graphics density, but the mask introduced from it increased the production process, resulting in a significant increase in costs and yield problems.

It is reported that the 7Nm technology of Taiji and Intel is still using the immersed ArF lithography equipment, but the next generation technology node after 7Nm is difficult to develop again. EUV has become the key to solve this problem. At present, the main method of EUV lithography light source is to irradiate excimer laser on tin and other targets, and excite 13.5nm photons as a result. Photolithography light source.

All major Foundry factories will use EUV lithography in the highest technology below 7nm, among which Samsung has adopted EUV lithography on 7Nm nodes. At present, only ASML in the Netherlands can provide EUV lithography machine for mass production. Since the 1970s, the domestic lithography technology has been developed by 45 Institutes of precision instruments, Optoelectronic Technology Research Institute of Chinese Academy of Sciences and China Electrical Science. At present, only the electrical equipment under the banner of Shanghai Microelectronics (SMEE) and China National Science (CETC) is manufactured in China. The best performance is 90 nm (193 ArF) lithographic machine, which is quite different from the international level.

On the other hand, projection lens is one of the most expensive and complex parts of lithography machine. The key to improve the resolution of lithography machine is to increase the numerical aperture of projection lens. With the improvement of lithographic resolution and overlay accuracy, aberration and stray light of projection lens have more and more prominent effects on image quality. The axial aberrations, such as spherical aberration and field curvature, of immersion lens are n times larger than that of dry lens. After introducing polarization illumination, the polarization control performance of projection lens becomes more important. In the case of increasing numerical aperture, how to maintain the field of view and polarization control performance, and strictly control aberration and stray light is a difficult problem in the design of projection lens.

The projection objective of traditional lithography mostly adopts full refraction design scheme, that is, the objective is composed of all transmission optical elements which are rotated, aligned and calibrated. Its advantages are relatively simple structure, easy processing and calibration, and less local stray light. However, the design of large numerical aperture full refractive objective is very difficult.

In order to correct the field curvature, large-sized positive lens and small-sized negative lens must be used to satisfy the Petswar condition, that is, the Petswar number of each optical surface of the projection objective is zero. The increase of lens size will consume more lens materials and greatly increase the cost of objective lens, while the control of aberration is very difficult due to the small size of negative lens.

In order to achieve a larger numerical aperture, in recent years, designers generally adopt the reverse design scheme. Refractive projection objective consists of lens and reflector. The Petswar number of the mirror is negative. It is no longer necessary to increase the size of the positive lens to satisfy the Petswar condition, which makes it possible for the projection objective to obtain larger numerical aperture in a certain size range.

Digital aperture is an important index of optical lens. The working wavelength of industrialized lithographic objective has gone through 436 nmG, 365 nmline, 248 nmKRF, 193 nmArF and 13.5 nmUltraviolet. The corresponding objective design is constantly improving the numerical aperture.

In order to reach the level of 22 nanometers for the ultraviolet ray of deep ultraviolet immersion lithographic machine, which is the mainstream lithographic machine in the world, the numerical aperture of the objective lens should be over 1.35. It is difficult to reach this aperture because the maximum aperture of the lens used to process large sub-nanometer precision lens reaches 400 millimeters. At present, only German optical companies can achieve it, and Nikon in Japan can achieve it by purchasing German technology.

Although two kinds of ultra-precision polishing equipment, magnetorheological and ion beam, developed independently by the precision engineering team of National Defense University, have achieved Nano-precision in optical parts processing, the immersion lithography objective is extremely complex, covering the forefront of many disciplines such as optics, machinery, computer, electronics and so on. The initial structure design of more than 20 lenses is extremely difficult - not only need to be controlled. The polarization aberration of the objective system should be controlled in an all-round way. Therefore, at this stage, domestic objective lens can not completely replace imported products.

2. Patent Analysis

From the domestic and foreign market structure, ASML occupies the main market share in the world, while Nikon's advanced lithography machine is not favored by semiconductor manufacturers because of its performance problems, and currently mainly operates as panel lithography machine; Canon retains low-end semiconductor i-line and Kr-F lithography machine, and withdraws from the competition of high-end lithography machine, as can be seen from the 2009 ASML and Nikon's financial reports. 。

According to ASML's first quarter earnings in 2019, although it was down from the fourth quarter of 2018, it still had 1.689 billion euros in revenue, with ArF Dry accounting for 4%, KrF 9%, i-line accounting for 2%, Metrology & inspection accounting for 3%, EUV accounting for 22%, ArF Immersion accounting for 60%. In Nikon's 2019 financial report, the temporary profit of semiconductor lithography business is 1.5 billion yen, about 91.05 million yuan, which is far from ASML.

Although domestic lithography machines are far away from ASML, some achievements have been made in exposure system and dual-stage system. For example, in 2017, Changchun Institute of Optical Precision Machinery and Physics, Chinese Academy of Sciences, led the development and acceptance of the key technology of extreme ultraviolet lithography, and Beijing Huazhuo Jingyi Technology Co., Ltd. successfully broke the technical monopoly of ASML on the workbench.

Through the incopat tool to search and analyze the patents related to lithography, we get the trend map of application in this field from 2000 to now, ranking the number of key applicants, EUV lithography key applicants.

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Figure 1 Trends in global applications for lithography

Data source: incopat, 2000-2018

As can be seen from Figure 1, the first rapid growth of patent applications for lithographic machines was welcomed in 2000-2004. During this period, the performance of semiconductor chips designed by Intel, VIA and IBM companies improved rapidly, which put forward higher and higher requirements for the semiconductor manufacturing process. With the continuous improvement of lithographic technology, the application volume also increased.

However, when the lithography machine was developed to 193 nm, ASML cooperated with several chip giants to extend 193 infiltrating optical technology tree to 15 nm. During this period, the number of patent applications declined, but immersion lithography is difficult to develop after 7 nm. EUV lithography has become the key to solve this problem. Therefore, in recent years, the related technology patent applications of lithography have shown a growing trend.

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Figure 2 Geographical distribution of patent applications for lithography

Data source: incopat, 2000-2018

From the point of view of geographical distribution, Japan has the largest number of patent applications in the field of lithography. In addition to a large number of domestic patents, Japanese enterprises attach more importance to the patent layout in the United States, South Korea, Taiwan, China and the mainland of China. This shows that Japan, as the leader of traditional lithography, has invested a lot of energy in the research and development of low and medium-end lithography machines, and has laid out a large number of related patents. End lithography technology has strong strength. But in the field of high-end lithography, Japanese technology still needs to be improved. In contrast, the number of patent applications in China is relatively small, indicating that lithography technology threshold is high, and there is no excessive technology accumulation in China, and the development is slow.

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Figure 3 shows the application ranking of key lithography applicants on the left and EUV applicants on the right.

Data source: incopat, 2008-2018

Figure 3 compares the ranking of key applicants for EUV patents with that of key applicants for lithography in recent years. Among them, ASML ranks second in the number of key applicants for EUV lithography, Carl Zeiss, the top optical instrument company, and Hailishi and Samsung are ASML partners. Nikon and Canon rank fourth and fourth respectively. Sixth.

Comparing the number of patent applications of key lithographic applicants and the number of patent applications of key EUV lithographic applicants, it is not difficult to see that Canon and Nikon in Japan have opened a big gap with ASML in the research of EUV lithography, and gradually withdrawn from the competition of high-end lithographic applicants. The reasons are as follows:

(1) ASML has no upstream and downstream enterprises, focusing on R&D, and the core technology is absolutely confidential;

(2) Special provisions of ASML: Enterprises wishing to obtain the preferential use rights of ASML lithography machines need to invest in ASML, TSMC, Samsung, Intel and Hailishi in order to seek mutual benefit. For example, when the lithography machine enters 193 nm node, the wet lithography machine developed by ASML and TSMC is the key step to establish the absolute dominance of ASML.

(3) ASML devotes 15% of its annual turnover to R&D. The high cost of R&D has deterred Nikon and Canon from competing for high-end lithography.

3. Conclusion

Lithography machine plays an important role in the process of chip manufacturing. With the shrinking of device feature size, the requirement for the accuracy of lithography machine is getting higher and higher. As a giant chip manufacturer, Samsung, TSMC and Intel have invested in ASML in order to seek the joint development and priority of their high-end lithography equipment. Although some progress has been made in the field of lithography machine in China, they still have some progress. There is a huge gap with the international level. Only relying on imports, the domestic chip manufacturing industry is bound to be subjected to others. It is urgent to speed up the development of lithographic machines.