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 grey 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 vapour-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.