Optimal conditions of SU-8 mask for micro-abrasive jet machining of 3-D freeform brittle materials

Abstract

Micro-abrasive jet machining (AJM), also called micro-blasting, is a mainstream machining process that uses abrasive particles for difficult-to-cut workpieces such as glass, carbides, and ceramics. During the micro-blasting process, non-machined areas are covered by a protective mask. Today, either mask fabrication practice or micro-blasting process is well suited and optimized for producing micro-features on planar workpieces. However, the demand for micro-features on three-dimensional (3-D) freeform substrates in micro electro-mechanical systems (MEMS) and lab-on-a-chip devices requires more refined non-planar micro-manufacturing techniques. We focused on devising an appropriate photoresist mask required by micro-AJM processes on the surface of a 3-D freeform workpiece. Fundamental erosion mechanisms based on SU-8 mask properties (hardness, surface roughness, and thickness) were investigated. The optimal conditions were found at an ultraviolet (UV) energy of 12.0752 μJ/μm, focus ratio of 4.8341, and hard baking time of 8.4974 min. Under these settings, the mask hardness and surface roughness were 25.04 HV and 1.14 μm, respectively. The reliability of the fabricated mask was verified through a micro-AJM process. With existing plant conditions, the engraved microfeature dimensions on the surface of a 3-D freeform workpiece were 535.3 μm (width) and 11.6 μm (depth).