Study On The Internal Stress And Thermal Swelling Of SU-8 Photoresist In UV-LIGA Process

With the rapid development of MEMS, UV-LIGA technology based on SU-8 photoresist has been applied extensively. Thick SU-8 photoresist has been widely used to fabricate high aspect ratio structure. However, the aspect ratio and dimension error of electroforming are largely limited by the well-know internal stress and thermal swelling of SU-8 photoresist induced by processing. The primary work of this study is mainly concentrated on the two aspects. The research result is beneficial for improving the stability of SU-8 processing and dimensional precision of electroforming.Based on the theory of curvature method, a theoretical formula for SU-8 internal stress is established. And a finite element analysis (ANSYS) is performed to investigate the influence of curvature which involves substrate diameter, film thickness and post-exposure bake (PEB) temperature. Internal stresses of SU-8 under different temperatures are measured through experiments. The results show that the PEB temperature is the main factor in developing the resulted internal stress, and the effects of film and substrate diameter thickness are subordinate. Moreover, experimental results show that lower PEB temperature can effectively reduce the internal stress.The processing parameters of negative SU-8 photoresist are optimized in terms of internal stress. A 3~3 factorial orthogonal array technique is designed to measure the internal stress of SU-8 photoresist under nine different process parameters. According to the results of orthogonal experiment, a neural network is employed to investigate the influence of process parameters on the internal stress. And the prediction model is built between the internal stress and three main process parameters: soft bake temperature, exposure energy and PEB temperature. The prediction results are in good agreement with the experimental results. And the lithographic process of SU-8 is optimized with the application of neural network.SU-8 photoresist is the electroforming mould for metal structure, and it keeps the dimensional precision of micro structure. However, the feature width of microchannel will be seriously reduced after electroforming due to the thermal swelling of patterned SU-8 in electrolyte. Based on the UV-LIGA process of micro-mold insert, the deformation of thermal swelling of SU-8 photoresist is investigated by ANSYS simulation. And the swelling trend of micro channel's sidewall is represented. Combined with the deforming rate of feature width and the equation of first-order dynamics, a velocity model of thermal swelling is established. Top width of micro-mold under different electroforming time is able to calculate through simulative results and the velocity model.The influence of different PEB temperature on thermal swelling is measured through swelling experiments. Experimental results show that, the swelling deformation of SU-8 is larger when at a lower PEB temperature. However, lower PEB temperature can effectively reduce the internal stress. So, the influence of the two effects above should be considered interactively, and proper step should be done under the insurance of stability of micro structure to improve the dimensional error of electroforming.