Simulation Research Of Exposure And Post Exposure Bake Processes Based On UV-LIGA Photolithography Technology

With the fast development of research and application of MEMS in recent years, micromachining technology has made great progress as an important part of MEMS as well. UV-LIGA technique is a hot technology of modern micromachining, which adopt tradition ultraviolet light system as illumination source. Comparing to the LIGA technique using X ray, it has virtue of briefness of technology, low system cost, and possibility of large-scale mass production, so it has been paid wide attention to.Photolithography technology is a key step of UV-LIGA, and it always is a hotspot of theory simulation research of it. Along with the more and more requirements of MEMS devices with complex high aspect ratio structures, simulations of lithography technique with cyber-simulant methods can reduce cost of design and shorten research cycle. Besides, it is of great significance for lithography simulation to increase micromachining product quality, guide the actual manufacturing process and so on.UV-LIGA technology mainly uses proximity lithography. It has proximity exposure and post exposure bake(PEB) as the two important components, which should be considered first. The study of exposure process mainly focus on the exposure figure on the surface of photoresist and the deep exposure figure inside the photo resist. PEB process is a kind of continued process of exposure, and it is a very important process in which photoresist changes essentially. The SU-8 photoresist is a kind of negative chemically amplified resists, which has been paid attention to in micromachining field because of its excellent character, and it is a kind of photoresist mainly used in UV-LIGA technology.This paper deals with reaction mechanisms of photoresist in exposure and PEB processes in lithography technology, and its purpose is to get appropriate models which can inextenso reflect the exposure and PEB process of SU-8 in UV-LIGA technology. Not only exposure process and also PEB process, can not be explained by the single discipline theory. It is necessary to be supported by interdisciplinary knowledge, which right is the characteristic of research in MEMS field. This paper based on the Chinese National Natural Science Foundation(No.60473133)—Simulation research of MEMS manufacturing error correction method based on UV-LIGA technology, make in-depth research on the theories of exposure and PEB processes in proximity UV-lithography technology. We overcomed the difficulties brought by interdisciplinary application in lithography, and built theory models which can fully simulate exposure and PEB processes.As for exposure on the surface of photoresist, this paper based on the optical scalar diffraction theory, rectified the binary gray-tone coding mask within the limit of most effective on the illumination by simulated annealing algorithm and division of wave front method, combined with the research of our group. Through the local searching and partially optimization on the binning grid of binary gray-tone coding mask, the optical proximity corrected mask code was obtained finally.Furthermore, for the simulation of deep exposure process, the photochemistry exposure model represented by the Dill's exposure model is more reasonable and applicable. This paper based on the classical Dill's exposure model, generalized its time and depth exposure model, and the more reasonable and integrated emulational theory methods were proposed. To generalize the time axis exposure model, the components of SU-8 photoresist and its photopolymerisable reaction in exposure process were analyzed, and a reaction kinetics model befitting SU-8 was built. In the depth axis, based on the Kirchhoff's diffraction formula generalized by the complex refractive index, the illumination distributing inside the photoresist at some moment was calculated by stages, using the beam propagation method. In this paper, the optical exposure model built on the depth axis can solve the problem of the jumping refractive index in the propagating path in proximity lithography, and the scalar diffraction theory was generalized to apply in micro-heterogeneous medium. Finally, the two models were coupled by complex refractive index, forming a new photochemistry exposure model which can reflect the essential process of SU-8 exposure. As for simulation of PEB, this paper started with analysis of SU-8 photopolymerisable reaction, discussed the main mechanism of the reactions in PEB process. Based on the Ferguson PEB reaction kinetics model, the diffusion model of photoacid was considered, and a new PEB reaction-diffusion model was built. In this paper, the Vrentas-Duda free volume diffusion theory of solvent-polymer system was introduced. According to the simplification to the Vrentas-Duda free volume diffusion coefficient calculating formula and combining with the Ferguson PEB reaction kinetics model, the PEB reaction-second-style-diffusion model of photoacid-epoxy crosslinked chain system was built to describe SU-8 PEB process finally.The experimental results indicate that the improved exposure and PEB model built in this paper can reflect the actual situation of experiment figure in a certain degree, and it is an integrated and effective simulation theory of micromachining process.