MEMS and Nanotechnology for Gas Sensors

4

Photolithography: Pattern Transfer

4.1  Introduction

Photolithography is the photographic technique to transfer replica of a master pattern into a substrate of a different material (usually a silicon wafer). In the case of silicon, a SiO₂ insulating layer is used to cover the substrate. A thin layer of an organic polymer, which is sensitive to ultraviolet radiation, is then deposited on the oxide layer; this is called a photoresist (PR). A photomask, which consists of a transparent plastic coated with a chromium pattern (opaque), is placed in contact with the PR-coated surface. The masks are of two types: dark film mask and bright film mask. The wafer is exposed to the ultraviolet radiation transferring the pattern on the PR, which is then ...