Fabrication of test structures to monitor stress in SU-8 films used for MEMS applications

SU-8, an epoxy based negative photoresist, is widely used in the manufacture of micromechanical systems but can exhibit significant levels of stress build-up during processing. This paper describes micromechanical test structures that provide the opportunity to spatially characterise stress the 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 vapour etch process. The initial results indicate that there is significant tensile stress in the SU-8, and that this demonstrates a radial variation along with a dependence on the process conditions.