On evaporation via an inclined rotating circular lift-off shadow or stencil mask

Abstract

A mathematical model is developed to calculate the topography of a mesa obtained by evaporation of matter via inclined rotating lift-off shadow masking. Two types of masking are considered: a circular mask and a cylindrical mask—the latter involves sidewall deposition. The model is able to predict various topographic profiles obtained via the evaporative deposition of matter, e.g., metals, when using a physical mask, e.g., a photoresist or an electron beam-sensitive resist patterned onto a flat wafer. The model predicts a range of profiles, e.g., sharp cones, round-tipped cones, spikes, irregular bumps, flat-topped features, “bagel-shaped” features, flat rings, and cylinders (fixed to the wafer and releasable)—depending on the aspect ratio of the circular opening, the deposition thickness, and the evaporation tilt angle. The ideas are extended to model an idealized resist-based lift-off mask involving overhang and undercut features. The model is simple to implement and should be of use for predicting the shape of deposited matter when using lift-off and stencil procedures—even at sub-micrometer dimensions. Despite its simplicity, the model goes some way in helping to understand the sensitivity of the various parameters on the final topography of the deposited matter. For example, the tilt angle—even when small—has an influence on the curvature radius of cone tips. In this way, the prediction—and even optimization—of the shape of the deposited material is possible prior to embarking on time-consuming, and perhaps costly, experimentation.