Extreme ultraviolet (EUV) light sources are considered to be an important milestone in the production of wafers with tens of watts of power. It is expected that large quantities will be available later this year. However, for large-scale production of chips, its functions still have serious challenges.
Looking back at the history of semiconductor lithography, the 248nm KrF laser source replaced the mercury lamp, and later developed a smaller geometry 193nm ArF laser. By 2003, EUV light source began to occupy the semiconductor development roadmap. Now more than a decade has passed, EUV The actual production requirements have not yet been reached. However, the extreme ultraviolet light source will control the future of semiconductor lithography for a long time to come. The EUV light source is far more technically challenging than we think, and the semiconductor lithography roadmap has been modified and delayed, but it is worthy of recognition that since 2011, the complete EUV lithography system has been established with ASML, which has accelerated the development of EUV.
ASML will invest more in EUV light sources, including the acquisition of Cymer, the leading supplier of light sources. At the SPIE Advanced Lithography Conference 2013, Cymer reported that the EUV source of 13.5nm band has a uniform output power of 40~55W and is based on this technology. A new generation of light sources has emerged. However, the output power has not yet reached the level of the chip produced by the trade.
Lithography technology and its requirements
Lithography has been crucial in the advancement of electronics for more than a decade. The light source exposes the photoresist applied to the surface of the chip, the exposure department is removed, the surface is etched according to defined characteristics, and then the other layers repeat the process to create an integrated circuit. . The feature size depends on the wavelength of the source, the focusing optics and the photoresist.
The lithographic light source is integrated into a lithography machine or system, including an optical system that illuminates the pattern mask and optical elements that will illuminate the surface of the photoresist. As shown in Figure 5. The length of the system is often large, complex, and expensive, and economics requires that the light source emit enough light exposure to reach 100 wafers per hour.
EUV light source
The tin plasma emitted by the EUV is generated by the carbon dioxide laser output pulse bombarding the molten tin droplets. The laser technology used by Cymer and Gigaphoton of Japan, Xtreme Technologies uses laser beam direct discharge excitation.
Cymer has produced 10 prototypes of 3100EUV light source, Q-switch CO2 main oscillator/power amplifier (MOPA), three-stage amplification, working repetition frequency 40~50kHz, each pulse bombards a single tin drop, the laser's uniform power is 15kW, last year The EUV is evenly outputted with 11W power to the photoresist, and 7 wafers are exposed per hour.
At this year's SPIE enhanced predecessor lithography conference, Cymer reported that by adding a four-stage amplification of the CO2 laser, pre-pulse irradiation of tin droplets was added before the main pulse, so that the output power of the EUV lithography machine reached 40~55W. This power is maintained for a period of time during long-term exposure. This pre-pulse technique allows the size of the tin drop to be amplified from 30 μm to 100 μm, as shown in Figure 6.
The new light source NXE3300 will be added to increase the size of the collection mirror, and the EUV output power can be improved to 70~75W. In the technical roadmap developed by Cymer, two versions of high-power light source are included, and a 31kW CO2 laser pump is used. The output is 125W, and the other is a 43kW laser pumping output of 250W.
At the 2013 SPIE conference, Gigaphoton reported that its EUV source developed a uniform output power of 10W and a maximum peak power of 20W.
Optical components and other major topics
Working in the EUV band can cause a lot of problems in optical components and power transmission. As shown in Figure 6, the tin plasma is scattered in all directions, and the collecting mirror can only illuminate the light scattered toward the lithography system, so the transmissive optics Components cannot be used in EUV lithography systems, so lithographic light sources must use multiple layers of reflective film mirrors. At present, EUV optical components have become relatively mature, and the specular reflectance is close to 70% of the theoretical maximum incident light. However, the loss of the oval collecting mirror is the largest, and it is also necessary to maintain the cleanliness of sputtered tin.
The loss includes the EUV source illumination mask and the optical components required to project the mask pattern onto the photoresist. The system uses a 10-sided mirror as shown in Figure 6. If each mirror reflects 70% of the incident light, only 2.8%. The original power remains, and this does not calculate the reflection loss of the collection mirror, the mask, and the loss of light absorbed by the photoresist. These losses result in an EUV uniform power delivered to the wafer surface that does not meet the production requirements for more than 100 wafers per hour.
EUV has also caused some other technical problems, such as its photon energy is more than ten times that of 193nm. It is necessary to use a new type of photoresist, and electrons released by energy photons can be dispersed in the material. As the size of the chip shrinks, defects become an increasingly serious problem.
Other lithography
The main competitor of EUV lithography is the 193nm intrusive lithography system. For this, large companies are maintaining a two-way choice, continuing 193nm technology and exploring new EUV technologies.
Another alternative lithography technique is directed self-assembly, which uses a 193 nm source to create a template to guide small building modules to be automatically assembled into a semiconductor structure. In the past few years, this technology has been greatly improved, which is to propagate 193 nm invasive. One way of lithography is of course that it takes a long time to implement this technology, but it is a great candidate as a candidate for lithography.
However, this directional self-assembly technique does not compete with EUV sources. It is a replacement for traditional photoresists and cannot replace the entire device. It can use EUV sources or 193 nm sources to provide self-assembled structures from the bottom up. Top to bottom control.
Looking to the future
The current problems make the future of lithography technology complicated, and different parts of the semiconductor industry have different needs to make the problem more complicated. The most embarrassing EUV technology is the foundry, they can not control the design of their manufacturing chips, so the single exposure EUV manufacturing technology is more attractive than the multiple exposure 193nm lithography.
The real major breakthrough now is that technology has moved from the laboratory stage to the production floor. Engineers will use the latest tools to develop new processes and test how to produce next-generation chips.
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