Thick Metal Film Patterning by Bi-Layer Lift-Off Process for MEMS Application

An integrated spray-coated bi-layer lift-off process using the same positive photoresist is investigated for thick Ti patterning on structured MEMS wafers. Compared with conventional spin coating, spray coating provides more conformal photoresist coverage on wafers with 30–50 µm surface topography and reduces the local top–bottom thickness variation from approximately 30% to below 5%. By combining spray coating, heated-chuck drying, and flood exposure of the first resist layer, the integrated process improves thickness retention of the bi-layer resist and enables controlled lateral dissolution during development. After optimizing the development time, a lateral undercut of approximately 3.5 µm was obtained, which is consistent with the expected “T-shaped” resist profile. Representative 600 nm thick Ti patterns were fabricated on structured silicon wafers using this process. The results indicate that the proposed process is a feasible route for local thick-metal patterning on structured MEMS substrates under the present experimental conditions.