Investigation of adhesion properties between SU-8 photoresist and stainless steel substrate

During a SU-8 lithographic process, poor adhesion performance between the SU-8 photoresist and metal substrate usually contributes to the failure of the binding. More seriously, the lithography can even be completely compromised. This significantly restricts the improvement of image resolution and enhancement of depth-to-width ratio. Concerning this problem, an indentation method was adopted to test the adhesion strength between the SU-8 and stainless steel substrate in this study. By a dimensional analysis method, empirical formula accounting for indentation stress was deduced from Evans model. Interface fracture energy release rate was also obtained to characterise interface adhesion strength. Simulation was performed based on the commercial finite element package ANSYS, and its results were compared with corresponding indentation experiment results. The good agreement of these two results demonstrated the applicability of the modified empirical formula. Besides, the influence factors of adhesion performance were discussed. The result indicated that the existence of internal stress could prompt interfacial fracture, whereas increasing substrate roughness could improve interface adhesion strength.