Currently, commercial EUV lithography utilizes a laser plasma-type-extreme ultraviolet (LPP-EUV) light source system, which is mainly composed of a drive laser, a droplet tin target, and a collector mirror. After two precise bombardments of the droplet tin target by the drive laser, the tin will be completely ionized and generate high-energy EUV radiation, which will be reflected and focused to a focal point (IF point) by the collecting mirror and then entered into the subsequent transmission of the light path.
The process of excitation and focusing of EUV is often accompanied by the generation and convergence of other bands of light (Out-of-band, OoB). Some of these lights can be removed using background hydrogen or are insensitive to the photoresist, so their impact is minimal. However, there are other bands of light that can cause serious damage to the entire lithography system and affect the final imaging performance, such as deep ultraviolet (DUV) and infrared (IR) light below 300 nm. The former arises from laser bombardment of the tin target, which causes a reduction in the contrast of the lithographic pattern because the photoresist is very sensitive to this band of light; while the latter arises from the driving laser, whose high energy will cause different degrees of heating of the optical elements, masks, and wafers, which reduces the precision of the pattern and damages the optical elements. In addition, the reflectivity of the collection mirror surface on the former is almost the same as that of EUV, while the reflectivity of the latter is close to 100%, as shown in Figure 1. Take IR as an example, as the driving light source laser power requirements for 20 kW, after the collection mirror reflection and convergence, its power to reach the IF point is still nearly 10%, that is, about 2 kW; however, in order to make the IR on the entire system has almost no effect, it is necessary to further reduce the power at the IF point of at least 1%, that is, only 20 W below. With such a high demand, it is necessary to filter out OoB radiation, which would greatly degrade the performance of the light source system if it were not filtered out so that it would be reflected by the collector mirrors and enter the subsequent light path.
Fig. 1 Calculated reflectance of different wavelength bands of light from a 50-layer molybdenum/silicon multilayer with a period of 6.9 nm and a molybdenum/silicon ratio of 0.4 on the surface of the collector mirror.
Filter structure in EUV lithography light source system
The team of Nan Lin and Yuxin Leng from the State Key Laboratory of Intense Field Laser Physics, Shanghai Institute of Optical Machinery, Chinese Academy of Sciences (SIOM), has systematically elaborated the key technologies, main challenges and future trends of EUVL filtering systems with respect to the out-of-band wavelengths in EUV lithography light source systems.
The results are published in the article of High Power Laser Science and Engineering 2023, No. 5 (Nan Lin, Yunyi Chen, Xin Wei, Wenhe Yang, Yuxin Leng. Spectral purity systems applied for laser-produced plasma extreme ultraviolet lithography sources: a review[J]. High Power Laser Science and Engineering, 2023, 11(5): 05000e64).
In EUVL light source systems, the plasma-generated DUV and the IR originating from the driving light source usually have a large impact on the lithography performance and the lifetime of the optical system, and the molybdenum/silicon multilayer film structure on the surface of the collector mirrors has a high reflectance to them, so the EUVL light source filtering system is mainly designed for them. DUV low energy intensity, the use of transmissive or reflective independent film structure can achieve good filtering effect, but due to the low mechanical strength of the film structure is easy to lead to film rupture and other problems, the service life is shorter. In contrast, IR with high energy cannot be filtered simply by using thin-film filters. Instead, multi-layer grating structures need to be processed and coated on the collector mirror substrate (shown in Fig. 2), in order to filter IR of specific wavelengths by diffraction and retain as much EUV radiation as possible (shown in Fig. 3). This method places very high demands on the design, processing and measurement of the grating structure, especially in the control of the grating surface roughness and the uniformity of the multilayer film, as well as the influence of the height-based parameters of the grating structure on the reflectivity, which we need to measure to only a few nanometers or even sub-nanometers. In terms of the entire EUVL light source system, the filtering object determines that the final filtering system is difficult to exist in a single structure, which needs to consider both the freestanding thin-film structure and the built-in grating structure of the collecting mirror, in order to realize the impact on the lithographic performance of the OoB for the overall filtration, so as to ensure the purity of the EUV light source.
Fig. 2 Schematic diagram of the grating structure built into the collector mirror.
Fig. 3 Schematic diagram of the principle of IR filtering by the built-in grating structure of collecting mirror.
The article summarizes the mainstream technical solutions of EUVL light source filtering system, analyzes the key technology of filtering OoB radiation, and discusses the main challenges and future development trends in the light of practical applications.The performance of EUV light source determines the performance of lithographic patterns, and in order to ultimately obtain a high-purity EUV light source, it is necessary to improve the design of filtering system, the advanced manufacturing process, and the advanced measurement method. In order to obtain high purity EUV light source, it is indispensable to improve the design of filtering system, process manufacturing, and measurement method.
