Interesting educational read about the problem of depth of focus in EUV lithography with an interesting summary.
7. Summary and Outlook
In the process of research and development of the lithography machine, in order to ensure that the developed system meets the expected performance requirements, it is necessary to make an index budget and predict the working performance of the system. This process is very important, it is the key to ensuring that the system is developed without design (over- or under-design) problems. A lithography machine is a complete machine composed of thousands of parts. my country is currently developing an advanced lithography machine that requires a reduction in focusing accuracy. Therefore, it is necessary to clarify the important factors affecting the focus control accuracy in the key subsystems, and to propose the maximum error range caused by this factor that can be allowed in the focus control index, so as to meet the increasingly small focus budget space. This can reduce potential design risks in the research and development process, and is also a requirement of the current cutting-edge lithography technology background.In the most advanced mass production lithography machine - EUV lithography machine, there are many factors that affect the focusing accuracy of lithography. The light source, illumination light path, mask, projection light path and photoresist in the optical path all affect it. According to The optical path sequence involves the following influence items: the illumination mode of the light source, the laser bandwidth and polarization state, the thermal deformation of the mask, the three-dimensional effect, the absorption layer and the self-focusing offset, the flatness of the mask chuck, and the image of the projection optical path. Differences, thermal effects, stray light and coatings, and three-dimensional effects of photoresist, etc. Among them, the three-dimensional effect of the mask is an important factor. In view of the above, combined with the current development trend of the industry, in order to improve the focus control accuracy in advanced lithography, the optical path design can be improved in the following ways:
(1) In the projection objective system, additional aberrations caused by inherent aberrations and thermal effects can be compensated by wavefront correction and can effectively increase the depth of focus;
(2) In order to reduce the three-dimensional effect of the mask, auxiliary patterns can be added, or the optical parameters of the mask preparation material and the chemical parameters of the photoresist can be optimized. In addition, due to the correlation between the influencing factors, for example, the influence of the mask on the focusing is actually the influence of the mask on the optical phase, so the influence can be considered together with the image difference of the projection object to compensate;
(3) In actual development, considering that a certain influence may not act independently, the following two opinions can be put forward from the perspective of optical simulation:
First, when considering the three-dimensional effect of the mask in the EUV band, the light source-mask synergistic optimization (SMO) technology is used to continuously approach the best model. The advantage of this technology is that its pixel-based lighting distribution can use a reference lighting condition as a starting point, and iterate continuously to new lighting methods, providing the largest process window.
Second, guide the mask pattern design through Optical Proximity Correction (OPC), SMO is to find the best lighting method, OPC is to optimize the mask pattern design, and combined with the OPC (Process Window OPC, PWOPC) related to the process window, By analyzing the OPC-corrected graphics, you can quickly discover which graphics have less depth of focus.
Of course, there are still parts that need to be improved in this work, and this is also the future direction. For example, the specific action model of each factor affecting focus control is still unclear, and the combined effects of different factors still need to be further studied. These problems involve high technical difficulties, many bottlenecks, and few materials that can be used for reference, which are a major challenge to the development of lithography technology in my country. But at the same time, it should be noted that in the research and development of EUV lithography, the National Science and Technology Major Project "Research on Key Technologies of Extreme Ultraviolet Lithography" undertaken by the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences has successfully passed the acceptance [ 57 ] , and developed A high-performance projection exposure device can meet the requirements of line width of 32 nm. Recently, the Tsinghua University team reported the first verification experiment of a new type of particle accelerator light source "steady-state microbunch" [ 58 ] , which is expected to provide a new technical route for EUV lithography light sources, which also improves the research and development of domestic lithography machines. confidence. Although there is still a big gap between my country and foreign top lithography machine manufacturers in terms of equipment products. However, I believe that as long as we continue to learn from the excellent technologies of domestic and foreign counterparts and organize scientific research teams to conduct solid research, we will definitely be able to make innovations and breakthroughs in core technologies and provide assistance for solving the problem of "stuck neck".
edit: in other words they are not resting in their laurels
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