Study of crack formation in high-aspect ratio SU-8 structures on silicon

Abstract

The high-aspect ratio capability of SU-8 photoresist led to the successful use of this epoxy based material in a diversity of microfabricated devices as a construction material as well as for micromolding purposes. Throughout the literature it was noticed that the thermal mismatch of SU-8 and the substrate material silicon generates high film stress in the spin-coated SU-8 causing crack formation in the microstructures. Using baking parameters this crack formation can be minimized but will remain a critical aspect of design. In this study the process was first optimised on non-patterned wafers. Secondly, we transferred this optimised process to a pre-patterned wafer containing deep silicon etch pits to account for a specific application in micromolding. We discuss the behaviour of film stress, number of cracks and crack length. The number of cracks as well as the length of cracks in concave corner designs can be significantly decreased, while round holes resulted even in crack-free microstructures. In the case of pre-patterned wafers no cracks appear around the features, however we observed unsatisfied development within the resist features caused by insufficient solidification in the deep etch pits during Soft Bake. Increased Soft Bake time can overcome these problems but will require more systematic investigations.