Influence of photo-initiator concentration on residual mechanical stress in SU-8 thin films

In this paper, experimental results on the residual mechanical stress generated during the different steps of the photolithography process of SU-8 thin films using different photo-initiator concentrations are presented. The main aim of the reported research work has been firstly to investigate and identify the causes for the generation of residual stresses and secondly to optimise the fabrication process in order to minimise the residual stresses. It was found that the major parameters that influence the generation of internal stresses were the exposure time, the post-baking temperature and time and the concentration of the photoinitiator which is used (Cyracure UVI). At lower concentrations of the photo-initiator, the measured stress levels after the post-baking step were consistently found to be significantly lower than the ones corresponding to higher photo-initiator concentrations. In addition, there was an additional improvement by using a ramping hot-plate. Finally, preliminary experiments were carried out in order to encapsulate nanoparticles in these low stress SU-8 thin films.