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Essay/Term paper: Silicon

Essay, term paper, research paper:  Science Research Papers

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Silicon


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.


 

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