Mechanical Behavior and Anisotropy of Spin-Coated SU-8 Thin Films for MEMS

Spin-coated SU-8 thin films used in Microelectromechanical Systems (MEMS) devices are investigated using tensile experiments on freestanding microscale specimens to understand their mechanical response. Modified in situ optical microscope experiments are performed on 2 mu m thick transparent SU-8 films in the strain rate range of 2 x 10(-5) /s to 2 x 10(-2) /s and accurate strain measurements are extracted using Digital Image Correlation. The effects of strain rate and anisotropy, on mechanical properties, in hard baked and non-hard baked films were studied in conjunction with Fourier Transform Infrared Spectroscopy. The Young's modulus and tensile strength were found to increase with strain rate significantly, with the stress vs. strain behavior changing from viscoelastic to linear elastic. Hard baking of the films resulted in better mechanical properties, viz., elastic modulus and tensile strength, due to increased cross-linking density. The average moduli for hard baked and non-hard baked films were 3.48 +/- 0.57 GPa and 2.92 +/- 0.43 GPa, respectively, obtained from 30 experiments each, over the entire strain rate range. Furthermore, the spin-coat process used to make the films developed anisotropy in the plane of the films due to molecular orientation, which was independent of hard baking temperature and duration.