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

Abstract

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 3Â 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 ¼ 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. V

Single Layer Lift-Off of CSAR62 for Dense Nanostructured Patterns

“…There have been comparatively few studies investigating which resist type would be optimal for a lift-off process, and especially what constitutes the optimal resist-to-metal aspect ratio for a successful lift-off. Thicker resist layers seem to offer a possibility for thicker metal layers [5]. For thinner resist layers options such as nanoparticle lift-off methods have been suggested for the fabrication of gold nanostructures [4].…”

“…A common method of pattern definition is electron beam lithography (EBL), which is suitable for defining very fine structured patterns or patterns with a high demand on precision and small feature size. For EBL systems, single lift-off processes have previously been made with the resists Poly(methyl methacrylate) (PMMA) [4,5] or Hydrogen silsesquioxane (HSQ) [6]. Recent developments in resist technology has lead to chemically amplified (CAR) and chemically semi-amplified (CSAR) resists being used for EBL applications and in pattern transfer processing [7].…”

Adjustable sidewall slopes by electron-beam exposure layout

“…The undercut prevents resist sidewalls from being coated during the directed deposition of, e.g., a metal layer, which enables a subsequent removal of the resist mask and, thus, a selective lifting of the metal. 2,3 In addition, controlling the sidewall slope enhances fabrication possibilities for 3D resist patterns.…”

“…Another possibility is the exposure of a single positive tone resist layer using a low energy electron beam (about 3 keV) so that the electron forward scattering leads to a significantly widened energy deposition at the bottom of the resist layer. 3 In optical lithography, the depth dependent energy deposition due to absorption and bleaching of the resist is well known and even deployed to achieve undercuts. However, in EBL, the depth dependent intensity is usually not considered even though it has a large impact on resist profiles due to lateral resist erosion.…”