Mixture of ZEP and PMMA with varying ratios for tunable sensitivity as a lift-off resist with controllable undercut

A lift-off process is a popular method to pattern metals, especially for the noble metals that are hard to dry-etch. For a "clean" lift-off process, an undercut profile is critical and is commonly achieved by using a bilayer resist stack. A resist with tunable sensitivity is apparently the most desirable, since it offers a controlled amount of undercut when used as the bottom layer, with the top layer being a less sensitive resist. In this study, the authors show that a simple mixture of poly (methyl methacrylate) (PMMA) and ZEP can offer tunable sensitivity by adjusting the ratio of the two resists dissolved in anisole. Higher sensitivity was attained by increasing the ZEP content in the mixture since ZEP is about 3 more sensitive than PMMA. However, the relationship is not a linear one, and the contrast curve for a mixture containing more PMMA (e.g., PMMA: ZEP ratio of 2: 1) is closer to that of pure ZEP than to PMMA. For dense line array patterns with a periodicity of 200 and 500 nm, a moderate undercut obtained by using a low ZEP concentration (PMMA: ZEP = 2: 1 as the bottom layer, PMMA as the top layer) gave the result for lift-off of 100 nm Cr. While using pure ZEP as the bottom layer, the undercut was often too large that the resist lines collapsed because of capillary force or even completely detached when the adjacent undercut merged together. (C) 2016 American Vacuum Society.