Fabrication and Measurement of Test Structures to Monitor Stress in SU-8 Films

SU-8 is an epoxy-based, negative photoresist that is widely used in the manufacturing of micromechanical systems. The polymer cross-linking that occurs during the photolithographic processing of SU-8 can result in high levels of stress in the patterned film. This has significant implications for the yield and reliability of SU-8 structures and needs to be understood if the material is to be integrated with other technologies. This paper describes micromechanical test structures that provide the opportunity to wafer map the stress in SU-8 at different stages of the process. The structures are fabricated in a thick layer of SU-8 and are subsequently released from the underlying substrate using a dry chemical vapor etch process. An automated optical measurement system has been built to allow rapid optical inspection of many thousands of test structures fabricated on 200 mm wafers. Initial results indicate significant tensile stress in the SU-8, which demonstrates a radial variation along with a dependence on the process conditions.