Introduction

Abstract

In the early days of silicon Integrated Circuit technology circuit dimensions were large (10 pm and up) and wafers were small (2 inch and down) Mask patterns were transferred mostly by means of hard contact exposure of a photoresist. After developing the resist pattern, the wafers were immersed in some aggressive liquid to remove the material not covered by resist. Since wet chemical etching is in principle isotropic, the pattern etched is wider than the original hole. This so-called undercutting is shown in figure 1.1.