Page 295 - A Practical Guide from Design Planning to Manufacturing

Semiconductor Manufacturing 265

layer of the desired material, to create a pattern in photoresist, and then
etch away unwanted material. Although semiconductor manufacturing
creates all the wires or devices of a particular layer simultaneously, the
individual layers are processed one at a time, gradually building the
required three-dimensional structure.
As feature sizes have been scaled down, semiconductor manufactur-
ing has grown steadily more sensitive to particles in the air far too
small to be seen with the naked eye. A single particle 0.5 microns (µm)
in diameter can easily cause shorts or breaks in a wire 1 µm wide. In a
typical city, a single cubic foot of outdoor air can easily contain millions
of particles of this size. To make feature sizes below 1 µm possible, all
manufacturing must be carried out in specially constructed clean-
rooms. These facilities constantly blow air down from the ceiling and
through special filters in the floor before returning the cleaned air to the
room. The level of cleanliness achieved is measured as the average
number of particles above 0.5 µm in diameter per cubic foot of air.
Modern fabrication can require a class 10 environment with only 10 par-
ticles per cubic foot or even a class 1 cleanroom with less than 1 particle
per cubic foot.
The most difficult part of achieving this incredibly low level of parti-
cles is allowing fab workers any access to the fab itself. Workers enter
through air locks to protect the cleanroom from outside air, but a person
in normal clothes standing motionless can still emit 100,000 particles
1
per minute. Bits of hair, skin, or clothing, even tiny water droplets in
a person’s breath, can all ruin modern chips. To keep the air clean,
workers must be covered from head to toe in specialized garments,
called bunny suits, designed to trap particles. To further limit contam-
ination, the latest fabrication equipment can load wafers from airtight
containers, process them, and then return them to their containers
without ever exposing them to the cleanroom air.
This chapter will discuss some of the details of the many different
types of deposition, lithography, and etching used in semiconductor fab-
rication. Any semiconductor process requires these basic capabilities.
The chapter ends with these steps being used in an example CMOS
process. The starting point for the manufacture of any integrated circuit
is the creation of semiconductor wafers.

Wafer Fabrication
Today, the manufacture of almost all semiconductor chips starts with sil-
icon. There are a small number of other semiconductor materials used,

1
Van Zant, Microchip Fabrication, 77.