Essay/Term paper: Semiconductors

Essay, term paper, research paper:  Science

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.