Application of epoxy-based photosensitive polymers for optical MEMS and subsequent reliability evaluation

Epoxy-based negative photosensitive polymers, commercially available EPON SU-8 and SU-8-2000, are used for fabrication of the fiber-optical micro-electromechanical system (MEMS), consisting of a thin membrane silicon wafer bonded to a SU-8 pre-patterned silicon substrate. In such a structure, the uniformity and adhesion of the polymeric spacer at the interface between the membrane and the substrate is crucial. To evaluate such physical properties at the interface, the devices are constructed, diced, and then tested under the combination of the damp-heat aging and the drop-off test. From the processing point of view, SU-8-2000 is much beneficial for rapid processability and better uniformity. However, the adhesion strength of SU-8-2000 is much lower than that of SU-8. It is interesting to note that the longer soft baking for SU-8-2000 enhances the adhesion strength. In addition, delamination of the photosensitive film induced by compressive stress resulting from the damp-heat is analytically modeled using buckling analysis to evaluate the adhesion strength. Prior to manufacturing stage, a set of product qualification methodology is developed and reliability evaluation that provides a fast turn-around result is conducted. Optical transmission and spectrum of the device are measured and used as key parameters to monitor degradation process at the interface induced by the given test conditions. Specific experimental results and the influence of the given test conditions on reliability of the MEMS are discussed and presented in detail.