Trends of Lithography Technology & Equipments for Semiconductor Fabrication

Abstract：Lithography technology and equipments are in a significant improvement with high chip integration and the device size scaling

down. The development trends of lithography and equipments for semiconductor fabrication are discussed through the current requirements for next generation lithography technology of lithography equipment manufacturers domestic and abroad, and by comparing the lithography technology and equipments applied to advanced production line, and reasonable proposal development trend is given.

Keyword：lithography；mask aligner；resolution；mask；depth of focus (DOF) ；exposure；

0 Introduction

Since its birth, lithography has been widely used as a pattern transfer technology in the semiconductor processing and manufacturing industry. With the continuous improvement of chip integration, the continuous reduction of device size and the continuous improvement of device functions, as the most critical lithography technology and lithography process equipment in semiconductor processing technology, significant changes will inevitably occur. The light source commonly used in the photolithography process is ultraviolet light emitted by mercury vapor, with wavelengths of 366, 405, and 436 nm. At present, in order to improve the exposure resolution, reducing the exposure light source used is also a trend in the development of lithography technology and equipment. The main components of the lithography machine include exposure light source, optical system, electrical system, mechanical system and control system, among which the optical system is the core of the lithography machine. The exposure method of the lithography machine is generally divided into three methods according to the distance between the reticle and the wafer: contact type, proximity type and projection type.

1 The driving force for the development of lithography technology and equipment

Economic benefit is the main factor behind the shift of Si wafer diameter from 200mm to 300mm. The output rate of 300mm Si wafers is 25 times that of 200mm. The investment of the 300mm factory is 1.5-3 billion US dollars, of which about 75% is used for equipment investment, so users require that the equipment can be extended down to 3-4 generations. The 300mm chip diameter is cut in from the 180nm technology node, which requires that the equipment can still be used at 150, 130nm, and even 100nm.

In order to promote the mass production of 300mm Si wafers, equipment manufacturers began to solve this problem a few years ago. Canon started 300mm exposure machine in 1995 and launched EX3L and I5L stepper, which was used by Japan Semiconductor Advanced Edge Technology (SELETE) group from 1997 to 1998. ASML's 300mm stepper scan exposure machine used 193nm wavelength, the model is FPA-500, also supplied to SELETE Group in 1999. Now Canon's third-generation 300mm exposure machine has a mix-and-match exposure capability (<110nm). At present, IC equipment with 300mm chip diameter to produce 180, 150, and 130 nm has entered the production line, and 100 nm equipment has also begun to be provided.

Exposure is the most critical manufacturing process in chip manufacturing. Due to the misguided innovation of optical exposure technology, it has repeatedly broken the limit expected by people, making it the mainstream technology of current exposure. In 1997, GCA Corporation of the United States launched the first distributed repetitive projection exposure machine, which was regarded as a major milestone in exposure technology. The large field of view and efficiency of the scanning projector are combined, and it is more suitable for mass production exposure of lines (<0.25μm).

In order to improve the resolution, the exposure wavelength of the lithography machine is continuously reduced, from 436, 365nm, near ultraviolet (NUV) to 246, 193nm deep ultraviolet (DUV). The 246nm KrF excimer laser was first used for 0.25μm exposure, and later Nikon introduced NSR-S204B, KrF, which can achieve 0.15μm exposure using anamorphic illumination (MBI). ASML also introduced the PAS.5500/750E, which uses the company's AERILALI illumination to solve 0.13μm exposures. However, the 19991TRS suggested that the 0.13μm exposure scheme is to use 193nm or 248nm plus resolution enhancement technology (RET); the 0.10μm exposure scheme is to use 157, 193nm plus RET, proximity X-ray exposure (PXL) or ion beam projection exposure (IPL) . The so-called RET technology refers to the use of phase shift mask (PSM), optical proximity correction (OPC) and other measures to further improve the resolution.

It is worth pointing out that modern exposure technology not only requires high resolution, but also requires process tolerance and economy. For example, when using alternating phase shift mask (alt PSM) in RET, its complexity and price must be considered. Disadvantages such as expensive, inspection, correction, etc.

At present, people in the industry are worried that post-optical technology may be difficult to meet the technical requirements of 70nm in 2008 and 50nm in 2011. They are vigorously developing next-generation (NGL) non-optical exposure, and using 157nm F2 excimer laser exposure as a fill-in post-optical exposure and down Gap between generations of NGLs.

2 The development trend of lithography technology and equipment

2.1 Resolution and Depth of Focus (DOF)

Due to scattering, the pattern produced by photolithography is not as sharp and sharp as the pattern on the reticle. Using an optical system to improve the focusing of light and reduce light scattering can improve the resolution of the lithography process. According to the analysis of several factors that affect the resolution, it can be seen that the resolution of lithography can be improved by increasing the size of the optical lens, but the increase in the size of the optical system also means a great increase in cost. In addition, reducing the wavelength of the exposure light source can also greatly improve the resolution of the lithography technology, which is also the main reason why the wavelength of the light source used in the current lithography technology is getting smaller and smaller. However, the reduction of the wavelength also has certain limitations . When the wavelength is reduced to a certain value, it will exceed the range of ultraviolet light and reach the wavelength range of X-rays. Compared to traditional optical theory, X-rays still have a considerable area to be researched and developed.

Another important optical system parameter is the depth of focus. In the lithography machine alignment system, the larger the focal depth, the easier the alignment operation. However, the depth of focus and the resolution are in conflict with each other. In order to improve the resolution, a short wavelength and a large numerical aperture are often required, but at the same time, the depth of focus is reduced, which brings great inconvenience to the operation. Immersion lithography is the key technology of extended 193nm lithography proposed in recent years.

Using immersion liquid to increase the depth of focus allows the lithography machine to extend to smaller nodes.

In the advanced photolithography process, the resolution requirements are very high, resulting in a very small depth of focus, and the focus of the exposure light source is required to be exactly in the center of the photoresist layer thickness to obtain the best resolution. The thickness deviation of the photoresist layer should be less than 0.25μm, and only the CMP process can obtain the wafer surface flatness that meets the requirements of the 0.13μm photolithography process.

2.2 i-line exposure and DUV (deep UV)

Because short wavelengths can achieve higher resolution, stable, high-intensity short-wavelength light sources have been developed and applied to exposure systems in lithography. Mercury intrusion lamps and excimer laser light sources are currently widely used in stepper lithography machines.

Mercury lamps have various wavelengths of radiation, of which i-line (365nm) is commonly used in steppers to achieve feature sizes of 0.35μm in IC manufacturing. The characteristic wavelength of the excimer laser light source is 248nm, which can realize the processing of 0.25μm feature size like the DUV light source. The stepper exposure machine using the ArF excimer laser light source with a wavelength of 198 nm has been applied to the process of 0.18 and 0.10 μm. A lithography machine that uses a 157nm DUV light source (F2) to achieve sub-0.10μm feature size processing is currently under development and is expected to be widely used before the emergence of next-generation lithography technology.

Photoresists are divided into positive and negative types, and are processed and produced for exposure light sources that are sensitive to different wavelengths. The application of the insulating anti-reflection coating is also related to the exposure light source. For different exposure light sources and different photoresists , different coating processes need to be developed. Currently, lithography using glass optical systems is approaching its limits. Because SiO absorbs ultraviolet light (UV) and shorter wavelength light very greatly. Therefore, lenses and masks using glass optical systems cannot be used to manufacture feature sizes below 0.10 μm or even smaller. It is very necessary to research and develop new optical materials and light sources to improve the resolution of current lithography techniques. The new technologies that are most likely to be applied at present are PSM technology and off-axis lighting technology. These two technologies can improve the level of current lithography technology, and the resolution can meet the process requirements of less than 0.1μm or even 0.04μm.

2.3 It is not very difficult for PSM technology to realize the transfer of a single independent small-sized pattern. What is difficult is the pattern transfer when many size patterns are gathered together, because in this case, the scattering or interference of the light source will cause the distortion of the pattern. . The solution to this problem is to use PSM technology. The vast majority of PSM plates used in semiconductor processes are fabricated from quartz glass. Experiments have shown that by using PSM technology, the smallest feature size can reach 1/5 of the exposure wavelength, which is also known as subwavelength lithography.

2.4 EUV lithography

The next generation of sub- 0.1μm pattern transfer lithography technology that may be realized is extreme ultraviolet lithography technology. The wavelength of this exposure light source is 11~14nm. Light waves with wavelengths from 1 to 50 nm cover the ultraviolet and X-ray regions. Therefore, exposure techniques using this wavelength range are also called extreme ultraviolet exposure or soft X-ray exposure or vacuum ultraviolet exposure. The principle of extreme ultraviolet exposure is mainly to use the wavelength of the exposure light source to reduce the numerical aperture of the optical system, thereby improving the resolution of lithography technology. However, as far as the currently known materials are concerned, there is no suitable material that can be used as the lens of the EUV exposure optical system, because the current materials have a very strong absorption effect on short-wavelength light sources, and EUV lithography technology must also be based on optical systems. accomplish. In addition, a light source for EUV lithography is currently under development, and the most likely source for this technique is a laser-pumped xenon plasma light source. The mask required by EUV lithography also needs to be coated with multiple layers of metal before it can be used.

2.5 X-ray exposure technology

When the exposure wavelength is reduced to below 5nm, it belongs to the X-ray range, and the wavelength of the X-ray range is shorter than that of UV, so higher lithography resolution can be obtained in the lithography process. X-ray exposure technology has been in the field of research since 1972. Because almost no material can reflect or refract X-rays, X-ray exposure techniques approximate direct-write printing techniques. The X-rays are transmitted directly through the transparent part of the reticle, and the exposure is directly achieved on the photoresist of the substrate. Since the wavelength is so short, the effect of reflections is almost negligible, so the resolution of this exposure technique is almost reticle-level.

From traditional lithography to X-ray lithography, the process flow has to be redesigned, mainly because X-rays cannot be focused through optical systems such as lenses and mirrors like ordinary light sources. In addition, the cost of X-ray reticle is very expensive, and the process is also very complicated, which is also an important reason for hindering the development of X-ray lithography technology. In addition, a stable, parallel, and sufficiently intense single-frequency X-ray light source is very difficult to achieve. The light source required for X-ray exposure can be achieved using synchrotron radiation, but the cost is very expensive. A synchrotron can achieve multiple wavelengths, but if the electron beam is turned off due to a malfunction during the process, all exposure systems associated with it must be turned off at the same time. What the semiconductor manufacturing industry needs is the stability of the process and equipment, so even if each Fab only has one synchrotron exposure system, it can only be used as a backup due to its high price.

2.6 Electron beam exposure technology

The wavelength of electron beam exposure depends on the electron energy. The higher the energy , the shorter the wavelength of exposure. The wavelength of the electron beam with energy between 10 and 50 keV is much smaller than that of the UV light source. Therefore, the electron beam exposure process has a wide range of application value and has a resolution that cannot be achieved by traditional optical exposure technology. Electron beam exposure technology is widely used in reticle manufacturing and cross-hair manufacturing in some semiconductor manufacturing plants.

In the manufacturing technology, like the UV exposure system, the electron beam can be reflected, refracted and focused by the electromagnetic field effect of some electron optical systems. Therefore, the electron beam exposure system can be applied to the scanning exposure technology of the stepper exposure system. Among the next generation electron beam exposure technologies, the most striking is SCALPEL (angμlar limitation projection electron-beam lithography). SCALPEL combines high-resolution and multi-level process application technology to meet the high-volume needs of semiconductor manufacturing, and the system is very similar to existing scanning stepper exposure systems. This lithography technology is the mainstream technology of pattern transfer technology in the semiconductor manufacturing industry in the future.

2.7 Ion beam exposure technology

Like electron beam exposure, the resolution of ion beam exposure far exceeds that of traditional optical exposure. Ion beam exposure technology can also be applied to direct writing exposure and projection exposure. The advantage of ion beam exposure is that the etching process can be performed at the same time as the exposure. This will greatly save the operation steps of the process and simplify the process flow. However, the efficiency of ion beam exposure is particularly low, and it is impossible to apply to large-scale industrial production. The most likely application of this technology today is reticle manufacturing, but it can also be applied to the inspection and repair of device defects.

3 Analysis

Since the late 1980s, the focus of optical lithography has shifted to i-line and excimer, and by the early 1990s, i-line exposure has rapidly reached its heyday. Excimer laser lithography technology has entered a mature stage in the late 1980s, among which Japan's Nikon NSR-2005EX8A, Netherlands ASML PAS-5500/70 and PAS-5500/90, China GCA XLS-7500/29 and XLS7800 /31 type is the most typical. All of them can achieve 0.45μm or 0.35μm resolution, which can meet the requirements of 16M, 64M DRAM chips, and 200 mm lithography chips.

PSM exposure technology is a historic breakthrough in optical lithography. Two Japanese companies, Nikon and Canon, published the research results of the phase-shift photographic optical system at the SPIE International Symposium on Microlithography in March 1992, and applied the technology to EPA-2500i3, NSR-2005i9T NSR-4425i, etc. On the far ultraviolet stepper exposure machine, it is suitable for the mass production of 64M DRAM and the development of 256M DRAM.

Synchrotron radiation (SOR) X-ray lithography is the most promising main processing method for sub-micron patterning in the future. It can produce high-precision patterns of 0.2~0.1μm, which can be said to be another milestone in the technology in the field of microfabrication in the near future. Japan's SORTECH and Panasonic have jointly developed the world's first high-performance SOR lithography machine, and NEC has also developed a SOR stepper with 0.2μm processing capability.

4 Conclusion

Electron beam exposure technology is developing towards high precision, high brightness, high-speed blanking and high-speed scanning. In recent years, foreign electron beam exposure technology has developed from submicron to nanometer (10~100nm) processing.

According to the current development of lithography technology, EUV lithography technology will be the mainstream technology for mass production of integrated circuits with feature sizes of 70nm and finer line widths. At present, Intel Corporation has selected the next-generation lithography technology as extreme ultraviolet lithography technology. In the upcoming 65nm node, the mainstream lithography equipment will be ArF dry lithography machine and ArF immersion lithography machine; by 2010, the mainstream lithography equipment for 45nm node will be ArF immersion lithography machine. ArF immersion lithography machines still have the potential to extend to smaller nodes. Great progress has also been made in the research of next-generation lithography technologies such as extreme ultraviolet lithography, nanoimprint lithography, and maskless lithography. After the 193nm immersion lithography technology reaches its limit, EUV lithography will most likely become the mainstream lithography technology, and nanoimprint lithography and maskless lithography will also be extremely competitive next-generation lithography technologies , which presents new topics and challenges for my country's lithography equipment and process research.