16.1. Introduction

In Chapter 13, we reviewed a few important steps in the process of fabricating semiconductor devices, such as oxidation, diffusion and ion implantation of dopants. The resulting semiconductor wafer then undergoes a series of additional steps before the final device is obtained, which are shown in the flowchart in Fig. 16.1. The major ones include lithography, etching, metallization and packaging, each of which will be discussed in the present Chapter.

Figure 16.1: Basic fabrication process flowchart, including the important steps lithography, etching, metallization, passivation and packaging, which transform a semiconductor wafer into a device that can be used in electronic systems.

16.2. Photolithography

Lithography consists of preparing the surface of a semiconductor wafer in order to allow the subsequent transfer of a specific pattern. To do so, the surface of the semiconductor must be carefully prepared and a film called resist is conformally applied onto it. Parts of this resist film will be selectively "activated" through a number of processes, while others will be left untouched in order to transfer the desired pattern. This is generally achieved through what is called mask alignment and exposure. A mask is a template which contains the desired pattern to be transferred. Finally, the resist is developed to reveal the desired pattern before proceeding to the subsequent processing steps.

There are several types of lithography techniques depending on the method used to activate the resist film. The most common form of lithography uses ultraviolet light and is called photolithography. This is currently...