Nanoindentation Analysis of SU-8 Coated Wafers at Different Baking Phases

SU-8 photoresist is extensively used as a structural and passivation layer in microelectromechanical systems, microfluidic devices, and related microscale technologies. The long-term reliability of these devices critically depends on the mechanical integrity and viscoelastic behaviour of the SU-8 coating. In this study, the mechanical and viscoelastic behaviour of SU-8 polymer thin films was systematically investigated using nanoindentation at different baking stages representative of standard photolithographic processing. SU-8 layers were spin-coated on silicon wafers and subjected to pre-bake, post-bake, and hard-bake treatments to evaluate the effects of progressive cross-linking. Static nanoindentation revealed that the elastic modulus did not change significantly during the baking phases and remained near 6.2 GPa; however, a significant change in hardness was observed from 0.173 ± 0.012 GPa after pre-bake to 0.365 ± 0.011 GPa and 0.364 ± 0.016 GPa after post- and hard bake, respectively. Creep tests analysed by the Burgers viscoelastic model showed a significant increase in both the retarded modulus and viscosity parameters with thermal curing, indicating the suppression of long-term viscoelastic deformation. The combined results demonstrate that nanoindentation provides a sensitive, nondestructive tool for monitoring the evolution of cross-linking and viscoelastic stability in SU-8 films, offering valuable insight for process optimization and mechanical reliability in MEMS and microfluidic applications.