Development trend of automatic testing technology

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；

introductionSince its birth, lithography technology has been widely used as a graphic 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, it is bound to undergo significant changes. The light source usually used in lithography process is ultraviolet light emitted by mercury vapor with wavelengths of 366, 405 and 436nm. 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. Mask aligner consists of exposure light source, optical system, electrical system, mechanical system and control system, among which the optical system is the core of mask aligner. Mask aligner's exposure methods are generally divided into three ways according to the distance between the mask and the wafer: contact, proximity and projection.A driving force for the development of lithography technology and equipmentEconomic benefit is the main factor for the transfer of Si wafer diameter from 200mm to 300mm. The yield of 300mm Si wafer is 2 5 times that of 200 mm. The investment of 300mm factory is $1.5 ~ 3 billion, of which about 75% is used for equipment investment, so users require that the equipment can be extended downward for 3~4 generations. The 300mm chip diameter is cut from the technical node of 180nm, which requires that the equipment can still be used at 150 nm, 130nm or even 100nm.In order to promote the mass production of 300mm Si wafers, equipment manufacturers began to solve this problem several years ago. Canon started the 300mm exposure machine in 1995, and introduced the EX3L and I5L stepper, which were used by Japan Semiconductor Advanced Edge Technology (SELETE) Group in 1997~1998. The 300mm step-by-step scanning exposure machine made by ASML Company, with a wavelength of 193nm, was FPA-500, and was also used by SELETE Group in 1999. Now the hybrid matching exposure capability of Canon's third generation 300mm exposure machine has reached (< 110nm). At present, IC equipment with 300mm chip diameter of 180, 150 and 130nm has entered the production line, and equipment with 100nm has also begun to be provided.Exposure is the most critical manufacturing process in chip manufacturing. Due to the improper innovation of optical exposure technology, it has repeatedly broken through the expected limit, making it the mainstream technology of exposure at present. In 1997, GCA Company of the United States launched the first distributed repetitive projection exposure machine, which was regarded as a major milestone in exposure technology. In 1991, SVG Company of the United States launched a step-by-step scanning exposure machine, which combines the high resolution of distributed projection exposure machine with the large field of view and efficiency of scanning projector, and is more suitable for large-scale production exposure of lines (< 0.25μm).In order to improve the resolution, the exposure wavelength of mask aligner is decreasing, from 436 and 365nm, near ultraviolet (NUV) to deep ultraviolet (DUV) at 246 and 193nm. The 246nmKrF excimer laser was first applied to the exposure of 0.25μm, and then the NSR-S204B and KrF were introduced by Nikon. The exposure of 0.15μm can be achieved by using MBI. ASML also introduced PAS.5500/750E, and the exposure of 0. 13μm can be solved by using the company's AERILALI lighting. However, in 1991, TRS suggested that the 0.13μm exposure scheme should use 193nm or 248nm plus resolution enhancement technology (RET); The exposure scheme of 0.10μm is to add RET at 157 and 193nm, proximity X-ray exposure (PXL) or ion beam projection exposure (IPL). The so-called RET technology refers to adopting measures such as phase shift mask (PSM) and optical proximity correction (OPC) to further improve the resolution.It is worth pointing out that modern exposure technology requires not only high resolution, but also process tolerance and economy. For example, when using Alternate Phase Shift Mask (alt PSM) in RET, we should consider its complexity, high price, inspection and correction.At present, people in the industry are worried that the post-optical technology may be difficult to meet the technical requirements of 70nm in 2008 and 50nm in 2011, and they are vigorously developing the next generation (NGL) non-optical exposure, and taking the 157nm F2 excimer laser exposure as a gap between the post-optical exposure and the next generation NGL.2 development trend of lithography technology and equipment2.1 Resolution and Depth of Focus (DOF)Due to the scattering, the pattern produced by lithography is not as sharp, clear and steep as the pattern on the mask. Using optical system to improve the focusing degree of light and reduce the scattering of light can improve the resolution of lithography process. According to the analysis of several factors affecting the resolution, it can be known that the resolution of lithography technology can be improved by increasing the size of optical lens, but the increase of the size of optical system means a great increase in cost. In addition, reducing the wavelength of exposure light source can also greatly improve the resolution of lithography technology, which is the main reason why the wavelength of light source used in lithography technology is getting smaller and smaller at present. However, there are some limitations to the reduction of wavelength. When the wavelength is reduced to a certain value, it will go beyond the scope of ultraviolet light and reach the wavelength range of X-ray. Compared with the traditional optical theory, X-ray still has a considerable area to be studied and developed.Another important optical system parameter is the depth of focus. In mask aligner alignment system, the greater the focal depth, the easier the alignment operation. However, the focal depth and resolution conflict with each other. In order to improve the resolution, short wavelength and large aperture are often needed, but at the same time, the focal depth is reduced, which brings great inconvenience to the operation. Immersion lithography is a key technology of extended 193nm lithography proposed in recent years.The mask aligner can be extended to smaller nodes by increasing the focal depth by immersion of liquid.In the advanced lithography process, the resolution requirement is very high, resulting in a very small depth of focus. It is required that the focus of the exposure light source falls right in the center of the photoresist layer thickness to get the best resolution. The thickness deviation of photoresist layer should be less than 0.25μm, and only CMP process can obtain wafer surface flatness meeting the requirements of 0.13μm lithography process.2.2 i-ray exposure and DUV (deep UV)Because short wavelength can obtain high resolution, stable and high-intensity short wavelength light source has been developed and has been applied to the exposure system of lithography technology. Mercury pressure lamps and excimer laser sources have been widely used in stepping mask aligner at present.Mercury lamps radiate with various wavelengths, among which I-line (365nm) is widely used in step exposure machines to realize the feature size of 0.35μm in IC processing and manufacturing. The characteristic wavelength of excimer laser source is 248nm, which can realize the processing of 0.25μm characteristic size like DUV source. Step exposure machine with 198nm wavelength using ArF excimer laser source has been applied to 0.18 and 0.10μm processes. Mask aligner, which uses 157nm DUV light source (F2) to realize feature size processing below 0.10μm, is currently being developed and hopefully will be widely used before the next generation lithography technology appears.Photoresist is divided into positive and negative, and it is processed and produced for different wavelength sensitive exposure light sources. The application of insulating anti-reflection coating is also related to the exposure light source. Different coating processes need to be developed for different exposure light sources and different photoresists. At present, the lithography technology using glass optical system has approached its limit. Because SiO absorbs ultraviolet light (UV) and shorter wavelength light very much. Therefore, it is impossible to use lenses and masks of glass optical system to manufacture feature sizes below 0.10μm or even smaller. It is very necessary to research and develop new optical materials and light sources to improve the resolution of current lithography technology. At present, the new technologies that are most likely to be applied are PSM technology and off-axis lighting technology. These two technologies can improve the current level of lithography technology, and the resolution can meet the technological requirements below 0.1μm or even 0.04 μ m.2.3 PSM technologyIt is not very difficult to realize the transfer of a single independent small-size figure, but it is difficult to transfer the figure when many large-size figures are gathered together, because in this case, the scattering or interference of light source will cause the distortion of the figure. The solution to this problem is to adopt PSM technology. The vast majority of PSM plates used in semiconductor technology are made of Shi Ying glass. Experiments show that the smallest feature size can reach 1/5 of the exposure wavelength by using PSM technology, which is also called sub-wavelength lithography technology.2.4 extreme ultraviolet lithography technology (EUV lithography)The next generation of sub-0.1μm pattern transfer lithography technology is extreme ultraviolet lithography technology, and the wavelength of this exposure light source is 11~14nm. Light waves with a wavelength of 1~50nm cover the ultraviolet and X-ray regions. Therefore, the exposure technology using this wavelength range is also called extreme ultraviolet exposure or soft X-ray exposure or vacuum ultraviolet exposure. The principle of extreme ultraviolet exposure is to reduce the numerical aperture of optical system by using the wavelength of exposure light source, and then improve the resolution of lithography technology. However, as far as the known materials are concerned, no suitable material can be used as the lens of the extreme ultraviolet exposure optical system, because the absorption effect of the current materials on short-wavelength light sources is very strong, and the extreme ultraviolet lithography technology can only be realized based on the optical system. In addition, the light source in extreme ultraviolet lithography is currently under development, and the most likely light source for this technology is laser-pumped xenon plasma light source. However, the mask required by extreme ultraviolet lithography technology needs to be coated with multiple layers of metal before it can be used.

2.5 x-ray exposure technologyWhen the exposure wavelength falls 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 people's research field since 1972. Because almost no material can reflect or refract X-rays, X-ray exposure technology is similar to direct writing printing technology. X-rays are directly transmitted through the transparent part of the mask, and the exposure is directly realized on the photoresist of the substrate. Because the wavelength is very short, the influence of reflection is almost negligible, so the resolution of this exposure technology can almost reach the resolution level of mask.From the traditional lithography technology to the X-ray lithography technology, the process flow must be redesigned, mainly because X-rays can not be focused by optical systems such as lenses and mirrors like ordinary light sources. In addition, the X-ray mask is very expensive and the process is very complicated, which is also an important reason that hinders the development of X-ray lithography technology. In addition, it is very difficult to realize a stable, parallel and strong single-frequency X-ray source. Using synchrotron radiation can achieve the light source required for X-ray exposure, but the cost is very expensive. A synchrotron can realize multiple wavelengths, but if the electron beam is turned off due to a fault during the process, all the exposure systems related to it must be turned off at the same time. What the semiconductor manufacturing industry needs is the stability of technology and equipment, so even if each Fab only has one synchrotron exposure system, it can only be used as a backup because of its high price.2.6 Electron beam exposure technologyThe wavelength of electron beam exposure depends on the electron energy. The higher the energy, the shorter the wavelength of exposure. The wavelength of electron beam with energy of 10~50 keV is much smaller than that of UV light source, so the electron beam exposure process has a wide range of application values, and has a resolution that traditional optical exposure technology cannot achieve. Electron beam exposure technology is widely used in mask manufacturing and cross wire manufacturing in some semiconductor factories.In manufacturing technology, like UV exposure system, electron beam can be reflected, refracted and focused by electromagnetic field effect of some electron optical systems. Therefore, the electron beam exposure system can be applied to the scanning exposure technology of the step exposure system. In the next generation of electron beam exposure technology, the most striking is Scalpel (Ang μ lar Limitation Projection Electron-Beam Lithography). SCALPEL integrates high-resolution and multi-level process application technology, which can meet the demand of high output of semiconductor manufacturing industry. The system is very similar to the existing scanning step exposure system. This lithography technology is the mainstream technology of pattern transfer technology in semiconductor manufacturing industry in the future.2.7 ion beam exposure technologyLike electron beam exposure technology, the resolution of ion beam exposure technology far exceeds that of traditional optical exposure technology. 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 operated at the same time of 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 it to large-scale industrial production. At present, the most possible application of this technology is mask manufacturing, and it can also be applied to the inspection and repair of device defects.3 analysisSince the late 1980s, the focus of optical lithography technology has shifted to I-ray and excimer, and by the early 1990s, I-ray exposure quickly reached its peak. Excimer laser lithography technology has entered a mature stage in the late 1980s, among which Japanese Nikon NSR-2005EX8A, Dutch ASML PAS-5500/70 and PAS-5500/90, Chinese GCA XLS-7500/29 and XLS7800/31 are the most typical. Both of them can achieve a resolution of 0.45μm or 0.35μm, which is suitable for 16M and 64MDRAM chips and lithography of 200 mm wafers.PSM exposure technology is a historic breakthrough in optical lithography. Japanese companies Nikon and Canon published the research results of phase-shifting photographic optical system at the SPIE international symposium on microlithography in March, 1992, and applied this technology to far-ultraviolet stepping exposure machines such as EPA-2500i3, NSR-2005i9T NSR-4425i, so as to adapt to 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 pattern making in the future. It can produce high-precision graphics of 0.2~0.1μm, which can be said to be another monument in the field of micro-machining technology in the near future. SORTECH of Japan and Panasonic have successfully developed the world's first high-performance SOR mask aligner, and NEC has also developed a SOR stepper with a processing capacity of 0.2 μ m..4 ConclusionElectron 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, extreme ultraviolet lithography technology will be the mainstream technology for mass production of integrated circuits with feature sizes of 70nm and smaller. At present, Intel has selected the next generation lithography technology as extreme ultraviolet lithography technology. At the upcoming 65nm node, the mainstream lithography equipment will be ArF dry mask aligner and ArF immersion mask aligner; By 2010, the mainstream lithography equipment for 45nm nodes will be ArF immersion mask aligner. ArF submerged mask aligner still has the potential to extend to smaller nodes. Extreme ultraviolet lithography, nano-imprint lithography, maskless lithography and other next-generation lithography technologies have also made great progress. After the 193nm immersion lithography technology reaches the limit, extreme ultraviolet lithography will most likely become the mainstream lithography technology, and nano-imprint lithography and maskless lithography will also be the next generation lithography technology with great competitiveness, which puts forward new topics and challenges for the research of lithography equipment and technology in China.