Silicon is the raw material most often used in integrated circuit (IC)
fabrication. It is the second most abundant substance on the earth. It is
extracted from rocks and common beach sand and put through an exhaustive
purification process. In this form, silicon is the purist industrial substance
that man produces, with impurities comprising less than one part in a billion.
That is the equivalent of one tennis ball in a string of golf balls stretching
from the earth to the moon. Semiconductors are usually materials which have
energy-band gaps smaller than 2eV. An important property of semiconductors is
the ability to change their resistivity over several orders of magnitude by
doping. Semiconductors have electrical resistivities between 10-5 and 107 ohms.
Semiconductors can be crystalline or amorphous. Elemental semiconductors are
simple-element semiconductor materials such as silicon or germanium. Silicon is
the most common semiconductor material used today. It is used for diodes,
transistors, integrated circuits, memories, infrared detection and lenses,
light-emitting diodes (LED), photosensors, strain gages, solar cells, charge
transfer devices, radiation detectors and a variety of other devices. Silicon
belongs to the group IV in the periodic table. It is a gray brittle material
with a diamond cubic structure. Silicon is conventionally doped with Phosphorus,
Arsenic and Antimony and Boron, Aluminum, and Gallium acceptors. The energy gap
of silicon is 1.1 eV. This value permits the operation of silicon semiconductors
devices at higher temperatures than germanium. Now I will give you some brief
history of the evolution of electronics which will help you understand more
about semiconductors and the silicon chip. In the early 1900's before integrated
circuits and silicon chips were invented, computers and radios were made with
vacuum tubes. The vacuum tube was invented in 1906 by Dr.Lee DeForest.
Throughout the first half of the 20th century, vacuum tubes were used to
conduct, modulate and amplify electrical signals. They made possible a variety
of new products including the radio and the computer. However vacuum tubes had
some inherent problems. They were bulky, delicate and expensive, consumed a
great deal of power, took time to warm up, got very hot, and eventually burned
out. The first digital computer contained 18,000 vacuum tubes, weighed 50 tins,
and required 140 kilowatts of power. By the 1930's, researchers at the Bell
Telephone Laboratories were looking for a replacement for the vacuum tube. They
began studying the electrical properties of semiconductors which are
non-metallic substances, such as silicon, that are neither conductors of
electricity, like metal, nor insulators like wood, but whose electrical
properties lie between these extremes. By 1947 the transistor was invented. The
Bell Labs research team sought a way of directly altering the electrical
properties of semiconductor material. They learned they could change and control
these properties by "doping" the semiconductor, or infusing it with
selected elements, heated to a gaseous phase. When the semiconductor was also
heated, atoms from the gases would seep into it and modify its pure, crystal
structure by displacing some atoms. Because these dopant atoms had different
amount of electrons than the semiconductor atoms, they formed conductive paths.
If the dopant atoms had more electrons than the semiconductor atoms, the doped
regions were called n-type to signify and excess of negative charge. Less
electrons, or an excess of positive charge, created p-type regions. By allowing
this dopant to take place in carefully delineated areas on the surface of the
semiconductor, p-type regions could be created within n-type regions, and
vice-versa. The transistor was much smaller than the vacuum tube, did not get
very hot, and did not require a headed filament that would eventually burn out.
Finally in 1958, integrated circuits were invented. By the mid 1950's, the first
commercial transistors were being shipped. However research continued. The
scientist began to think that if one transistor could be built within one solid
piece of semiconductor material, why not multiple transistors or even an entire
circuit. With in a few years this speculation became one solid piece of
material. These integrated circuits(ICs) reduced the number of electrical
interconnections required in a piece of electronic equipment, thus increasing
reliability and speed. In contrast, the first digital electronic computer built
with 18,000 vacuum tubes and weighed 50 tons, cost about 1 million, required 140
kilowatts of power, and occupied an entire room. Today, a complete computer,
fabricated within a single piece of silicon the size of a child's fingernail,
cost only about $10.00. Now I will tell you the method of how the integrated
circuits and the silicon chip is formed. Before the IC is actually created a
large scale drawing, about 400 times larger than the actual size is created. It
takes approximately one year to create an integrated circuit. Then they have to
make a mask. Depending on the level of complexity, an IC will require from 5 to
18 different glass masks, or "work plates" to create the layers of
circuit patterns that must be transferred to the surface of a silicon wafer.
Mask-making begins with an electron-beam exposure system called MEBES. MEBES
translates the digitized data from the pattern generating tape into physical
form by shooting an intense beam of electrons at a chemically coated glass
plate. The result is a precise rendering, in its exact size, of a single circuit
layer, often less than one-quarter inch square. Working with incredible
precision , it can produce a line one- sixtieth the width of a human hair. After
purification, molten silicon is doped, to give it a specific electrical
characteristic. Then it is grown as a crystal into a cylindrical ingot. A
diamond saw is used to slice the ingot into thin, circular wafers which are then
polished to a perfect mirror finish mechanically and chemically. At this point
IC fabrication is ready to begin. To begin the fabrication process, a silicon
wafer (p-type, in this case) is loaded into a 1200 C furnace through which pure
oxygen flows. The end result is an added layer of silicon dioxide (SiO2),
"grown" on the surface of the wafer. The oxidized wafer is then coated
with photoresist, a light-sensitive, honey-like emulsion. In this case we use a
negative resist that hardens when exposed to ultra-violet light. To transfer the
first layer of circuit patterns, the appropriate glass mask is placed directly
over the wafer. In a machine much like a very precise photographic enlarger, an
ultraviolet light is projected through the mask. The dark pattern on the mask
conceals the wafer beneath it, allowing the photoresist to stay soft; but in all
other areas, where light passes through the clear glass, the photoresist
hardens. The wafer is then washed in a solvent that removes the soft photoresist,
but leaves the hardened photoresist on the wafer. Where the photoresist was
removed, the oxide layer is exposed. An etching bath removes this exposed oxide,
as well as the remaining photoresist. What remains is a stencil of the mask
pattern, in the form of minute channels of oxide and silicon. The wafer is
placed in a diffusion furnace which will be filled with gaseous compounds (all
n- type dopants), for a process known as impurity doping. In the hot furnace,
the dopant atoms enter the areas of exposed silicon, forming a pattern of n-type
material. An etching bath removes the remaining oxide, and a new layer of
silicon (n-) is deposited onto the wafer. The first layer of the chip is now
complete, and the masking process begins again: a new layer of oxide is grown,
the wafer is coated with photoresist, the second mask pattern is exposed to the
wafer, and the oxide is etched away to reveal new diffusion areas. The process
is repeated for every mask - as many as 18 - needed to create a particular IC.
Of critical importance here is the precise alignment of each mask over the wafer
surface. It is out of alignment more than a fraction of a micrometer
(one-millionth of a meter), the entire wafer is useless. During the last
diffusion a layer of oxide is again grown over the water. Most of this oxide
layer is left on the wafer to serve as an electrical insulator, and only small
openings are etched through the oxide to expose circuit contact areas. To
interconnect these areas, a thin layer of metal (usually aluminum) is deposited
over the entire surface. The metal dips down into the circuit contact areas,
touching the silicon. Most of the surface metal is then etched away, leaving an
interconnection pattern between the circuit elements. The final layer is "vapox",
or vapor-deposited-oxide, a glass-like material that protects the IC from
contamination and damage. It, too, is etched away, but only above the
"bonding pads", the square aluminum areas to which wires will later be
attached.