Fabrication of the Photo-Resist Mask onto 3D Non-Planar Wafer for Micro Abrasive Jet Machining

Abstract

This paper presents a novel fabrication technique of a photo-resist mask onto 3D curved wafer for micro-abrasive jet machining (AJM) process. The photo-resist mask structure selectively blocks the abrasive jet at the portion of the surface that is not supposed to be machined.  Previous studies on the photo-resist mask fabrication process were mainly focused on planar wafers and it was difficult to adapt the fabrication technology directly to materials with 3D curved surfaces because besides UV power and scanning speed, the solidifying action is strongly affected by the beam size as well as hatching size. In this research work, a modeling algorithm that uses images obtained from 3D CAD or no CAD mask models of the wafer has been developed and validated through a number of experiments. SU-8 a negative, epoxy-type, near-UV photo-resist based on EPON SU-8 epoxy resin has been used for the photo-resist mask fabrication. Even though, the SU-8 photo-resist is highly sensitive epoxy resin to UV light, existing fabrication process onto the wafer surface is still an ill-defined problem relying on heuristic methods because of the lack of adequate knowledge on the sensitivity variation. The UV laser limits the SU-8 photo-resist mask to 300µm thickness where photo-resist is most sensitive; therefore a spin coating technique is desired to allow the resist to acquire uniform thickness. For 3D non-planar, the spinning technique is very much demanding and sometimes unfeasible depending on the wafer shape.  In this study, the spinning issue has been consciously analyzed and worked out through dilute, at defined ratio, of the concentrated SU-8 epoxy resin with Cyclopentanone (C₅H₈O). Through Taguchi method, the most effective controlling parameters for fabrication of the mask with appropriate hardness and surface quality as vital conditions for micro-AJM process have been ranked and reported in this paper.