1996 Electronics Industry Environmental Roadmap - Civil and ...

Emerging Technologies
wafer. 18 Generally, the process involves the creation of 10 to 20 patterned layers on and into the
substrate, ultimately forming the complete IC. This layering process creates electrically active
regions in and on the semiconductor wafer surface.
Wafer Production: The process starts with a thin silicon wafer 19 —currently in the range of 150
mm to 200 mm in diameter. The larger chips demanded by future performance requirements will
be cut from larger wafers. Wafers are forecast to grow from today’s sizes to wafer sizes of 400
mm by 2010.
To start, purified polycrystalline silicon, created from sand, is heated to a molten liquid. A small
piece of solid silicon (seed) is placed on the molten liquid, and as the seed is slowly pulled from
the melt the liquid cools to form a single crystal ingot. The surface tension between the seed and
molten silicon causes a small amount of the liquid to rise with the seed and cool. The crystal
ingot is then ground to a uniform diameter and a diamond saw blade cuts the ingot into thin
wafers. The wafer is processed through a series of machines, where it is ground smooth and
chemically polished to a mirror-like luster. The wafers are then ready to be sent to the wafer
fabrication area where they are used as the starting material for manufacturing integrated circuits.
Wafer Fabrication: Silicon wafer processing is predominantly based on the Complementary-
Metal-Oxide-Semiconductor (CMOS) process. CMOS technology is advantageous in that it can
be easily powered by batteries for portable electronics, yet is extendible to high-performance
applications as well. As the demand for electronics is increasing, it is expected that CMOS will
remain the dominant technology.
The central activity of semiconductor manufacturing is the wafer fabrication area where the IC is
formed in and on the wafer. The fabrication process, which takes place in a clean room area,
involves a series of operations called oxidation, masking, developing, doping, dielectric deposition,
metallization, etching, and passivation. Because of the number and complexity of steps in
this process, more time and labor is invested here than in any other area of semiconductor manufacturing.
Typically it takes from 10 to 30 days to complete the fabrication process. Following
are the principle steps involved in semiconductor wafer fabrication.
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Thermal Oxidation or Deposition: Wafers are pre-cleaned using high purity chemicals
required for high-yield products. The silicon wafers are heated and exposed to ultra-pure
oxygen in diffusion furnaces under carefully controlled conditions forming a silicon dioxide
film of uniform thickness on the surface of the wafer. The thermal oxide will form
the important first layer on which circuitry in the IC will be based.
Masking: Masking is used to protect one area of the wafer while working on another.
This process is referred to as photolithography or photo-masking. A photoresist or lightsensitive
layer is applied to the wafer, giving it characteristics similar to a piece of
photographic paper. A photo aligner aligns the wafer to a mask and then projects an
18 Much of the material that follows is taken from “How Semiconductors are Made,” prepared by Harris Corporation
and included in the Lexicon of Semiconductor Terms (available from Harris) as well as on the Harris
Corporation Web site, found at http://rel.semi.harris.com/docs/lexicon/manufacture.html.
19 For high-speed and high-performance devices (particularly in photonic or optical applications), gallium arsenide
will be increasingly used as the starting material.