Testing the viscoelastic properties of SU8 photo resist thin films at different stages of processing by nanoindentation creep and stress relaxation

The viscoelastic properties of SU8-25 photo resist coatings have been investigated by means of nanoindentation creep and stress relaxation tests. Different stages of processing of SU8 have been tested: (a) Spin coating and pre-bake only, (b) exposure to ultraviolet light for different durations, (c) different cross-linking times of the resist and finally (d) the completely processed sample (post-bake, develop and hard-bake). At each stage creep and stress relaxation tests have been performed with loads of 2 mN or indentation depth of 500 nm covering creep/relaxation times from 0.1 s to 1000 s. From the raw data the following viscoelastic functions have been computed: the creep compliance J(t) and relaxation modulus G(t), the relaxation and retardation time spectra H() and L(), and the dynamic viscosity (t). It has been found that different pre-bake times influence the viscosity of the film. The cross-linking of the SU8 starts immediately after exposure even without applying a post-bake. A strong increase in viscosity can be observed within the first 4 h after exposure due to the progress of cross-linking at room temperature. After the finish of cross-linking the dynamic viscosity has increased by 50 times. Observation of impression recovery after unloading over 3 days indicated up to 75 % immediate and retarded elastic recovery for the fully processed film, while the simply pre-baked sample only exhibied 25 % recovery and 75 % irreversible viscous flow.