Page 104 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH09.qxd 04/04/2005 19:42 Page 9.3

Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 9

MICROLITHOGRAPHY

Chris A. Mack
KLA-Tencor
Austin, Texas

The fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes per-
formed on a semiconductor (e.g., silicon) substrate. In general, the various processes used to make an
IC fall into three categories—film deposition, patterning, and semiconductor doping. Films of both
conductors (such as polysilicon, aluminum, and more recently copper) and insulators (various forms
of silicon dioxide, silicon nitride, and others) are used to connect and isolate transistors and their com-
ponents. Selective doping of various regions of silicon allows the conductivity of the silicon to be
changed with the application of voltage. By creating structures of these various components, millions
of transistors can be built and wired together to form the complex circuitry of a modern microelec-
tronic device. Fundamental to all of these processes is lithography, the formation of three-dimensional
relief images on the substrate for subsequent transfer of the pattern to the substrate.
The word “lithography” comes from the Greek lithos, meaning stones, and graphia, meaning to
write. It means quite literally writing on stones. In the case of semiconductor lithography our stones
are silicon wafers and our patterns are written with a light sensitive polymer called a photoresist. To
build the complex structures that make up a transistor and the many wires that connect the millions
of transistors of a circuit, lithography and etch pattern transfer steps are repeated at least 10 times,
but more typically are done 20 to 30 times to make one circuit. Each pattern being printed on the
wafer is aligned to the previously formed patterns and slowly the conductors, insulators, and selec-
tively doped regions are built up to form the final device.
The importance of lithography can be appreciated in two ways. First, due to the large number of
lithography steps needed in IC manufacturing, lithography typically accounts for about 30 percent of
the cost of manufacturing. Second, lithography tends to be the technical limiter for further advances in
feature size reduction and thus transistor speed and silicon area. Obviously, one must carefully under-
stand the trade-offs between cost and capability when developing a lithography process. Although lith-
ography is certainly not the only technically important and challenging process in the IC manufacturing
flow, historically, advances in lithography have gated advances in IC cost and performance.

9.1 THE LITHOGRAPHIC PROCESS

Optical microlithography is basically a photographic process by which a light sensitive polymer,
called a photoresist, is exposed and developed to form three-dimensional relief images on the sub-
strate. The general sequence of processing steps for a typical optical lithography process is as follows—
substrate preparation, photoresist spin coat, prebake, exposure, postexposure bake, development, and
postbake. A resist strip is the final operation in the lithographic process, after the resist pattern has

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) 9.3
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.