Modeling, simulation and experimental verification of inclined UV lithography for SU-8 negative thick photoresists

Three-dimensional oblique microstructures of SU-8 negative thick photoresists have drawn more attention with the rapid development of inclined UV lithography technology. In this paper, a simple model based on the Fresnel diffraction theory is developed to simulate the inclined UV lithography and predict the profiles of developed oblique SU-8 structures by utilizing the paraxial approximation approach. The reflection, refraction, absorption of the SU-8 photoresists and the reflection at the wafer surface are integrally incorporated in the model to improve the accuracy. The parameters, which have significant influence on the profile quality, have been studied in simulation. Simulation results demonstrate good agreement with the experimental results, where the oblique SU-8 structures were fabricated on glass and silicon wafers. To eliminate reflected UV light at the wafer surface, Ti films were sputtered on the glass wafers, followed by wet oxidation, thus employed as an antireflection layer. By contrast, the silicon wafers were used to fabricate the oblique SU-8 structures with reflected induced patterns. This model is useful to optimize the inclined UV lithography process of SU-8 thick photoresists and improve the efficiency of the design of some micro-electro-mechanical system devices