Lithography Basics
Photolithography is the foundation on which microelectronic devices and circuits are constructed. It is a process by which a pattern is transferred from a mask to a film on the silicon wafer. Although the term “photolithography” was originally related to optical imaging, the same term is now frequently used to describe x-ray and electron beam lithography as well. This section describes an optical lithography process used to produce pattern on the surface of a wafer.
The lithography is accomplished by coating the wafer with an organic compound called photoresist which changes in solubility when exposed to appropriate radiation. Photoresist has the important characteristic that its immunity to chemical etching can be modified by exposure to UV light.
There are basically two types of photo resist; positive photoresist and negative photoresist. Photoresist whose solubility increase when exposed to radiation are called positive photoresist, and those that become less soluble upon exposure are called negative photoresist.
The photolithographic process is especially sensitive to particles and it recommended that a face mask be worn during this procedure. It is very important to check the progress of the wafers at each step. Look at the wafers using a microscope, this will ensure quality control.
All lithographic processes follow the same general sequence as described in the next several sections.