Journal of Micromechanics and Microengineering vol:20 issue:9
The swelling behavior of SU-8 is studied. It is well known that negative resist swells under the influence of common processing liquids, developers, solvents, etc. However, when SU-8 is used as a construction material for plating molds, or for permanent structures, for instance in microfluidics or as active mechanical components in the MEM structure, a more quantitative investigation is needed. In this paper, the volume change is measured when the SU-8 epoxy is submersed in common processing liquids. An analytical model is derived to link the diffusion mechanism to the mechanical response of the SU-8 material. Using the obtained model, the diffusion constants are calculated from the mechanical displacement measurements. Also, the derived model can be used for the prediction of the mechanical behavior of SU-8 structures in the future. The swelling behavior is also correlated with the internal stresses that exist in the SU-8 film. This helps to understand crack formation and delamination of SU-8 patterns during processing. The results show that the built-in stress in the SU-8 epoxy is strongly dependent on the ambient or submersion liquid and that this effect in itself is strongly dependent on the softbake procedure of the polymer. The built-in stress in SU-8 was found to be maximum when submersed in propylene glycol methyl ether acetate and in isopropyl alcohol, both liquids that are used during the development, and found to be very low in water.