Worlds of David Darling > Children's
Encyclopedia of Science > Nanotechnology > 2. Computers, Crystals, and
MICROMACHINES AND NANOTECHNOLOGY:
The Amazing New World of the Ultrasmall
a book in the Beyond 2000 series by David Darling
2. Computers, Crystals, and Chips
If you are sitting at home or at school reading this, you are probably surrounded
by dozens, or perhaps even hundreds, of MICROCHIPS. They are hidden away
inside TVs, hi-fis, wrist-watches, computers, calculators, kitchen appliances,
and many other modern gadgets. They help to control the latest car engines
and passenger aircraft.
|A complete, special-purpose computer can
be formed on a silicon chip smaller than an ordinary spider
A microchip is also known as an INTEGRATED CIRCUIT. It is made from a single
crystal of a substance called SILICON. The crystal is no bigger than your
little fingernail and thin enough to pass through the eye of a needle. On
the surface of this sliver of silicon, a complicated pattern of tiny electronic
components and connecting pathways has been laid down.
Microchips are the most advanced products of miniaturization yet made. Their
story began roughly a century ago.
The Dawn of Electronics
When a piece of metal is heated in empty space – a vacuum –
the metal gives off a stream of particles called ELECTRONS. Think of electrons
as being the tiniest possible packets of electricity. A flow of electrons,
therefore, creates an electric current. If you imagine moving electricity
to be like water flowing over a waterfall, electric current is equivalent
to the amount of water that goes over the waterfall each second.
In 1904 an English scientist, John Fleming, found a simple way to control
an electron flow. He invented the DIODE. Built from two pieces of metal,
known as electrodes, inside a vacuum-filled glass tube, the diode let current
pass through only in one direction. The flow of electrons from the first
electrode, which was hot, could be stopped or started depending on the voltage
on the second electrode. If you think again of a flow of electricity as
being like a waterfall, then voltage is equivalent to the height of the
In 1906 an American scientist, Lee de Forest, took Fleming's idea a step
further with his invention of the TRIODE. By placing a third, wire-mesh,
electrode between the other two, de Forest was able to control the amount
of current passing through the device. Very small changes in the voltage
on the third electrode would cause big changes in current across the tube.
This fact made triodes very useful as electronic components – devices
that regulate a flow of electricity. Later they found their way into radio
receivers, phonographs (record players), and television sets.
Scientists also realized that triodes could work as switches. In other words,
they could be made to flip from "on" (when a current passes through them)
to "off" (when no current passes through). This was very important because
switches are the most vital parts needed inside computers. Sets of switches
can be used to represent instructions and information.
|Glass triodes and other large components,
such as these from a computer of 1960, have now been replaced by small
silicon chips containing many millions of components
In the late 1940s the first electronic computers were built using triodes.
One of the earliest, called ENIAC, contained 18,000 triodes and weighed
30 tones! These computers were marvels of their age, but they suffered from
some major problems.
Triodes are bulky, use a lot of power, and become hot when working. Computers
made from them filled whole rooms, used a great deal of electricity, and
had to be cooled by big fans. They were always breaking down, too, since
each triode had a fairly short life.
In 1947 three American scientists, William Shockley, John Bardeen, and Walter
Brittain, invented an important new electronic device. It worked just like
a triode but was solid, small, and needed very little power. The invention
was a TRANSISTOR.
To work as an electronic switch, a transistor did not have to be heated.
Instead, it relied on the special electrical properties of a SEMICONDUCTOR.
This is a substance whose ability to pass a current improves as tiny amounts
of impurities, known as DOPANTS, are added to it. The first transistors
were built from a semiconductor called germanium. Eventually, however, silicon
became the main semiconductor from which transistors and other electronic
components were made.
Different parts of a crystal of silicon can be set up to act in the same
way as the electrodes in a triode or a diode. This change is made by adding
various tiny amounts of dopants to the silicon, such as boron in one region
and phosphorus in another region. The result is a device that is small,
stays cool, and last for a very long time.
In the 1950s transistors began replacing triodes as the basic building blocks
of all computers. As a result, computers quickly shrank in size and became
more dependable. Because they were smaller, information could travel around
them more quickly, so computers also increased in speed.
At the same time, people began to realize all the different ways in which
small, reliable computers might be applied. The space age had just begun,
and computers were needed aboard spacecraft. But before they could be used
for such tasks, computers would have to be reduced still further in size.
The Quarter-Inch-Square Marvel
Scientists began to ask: Instead of making components like transistors one
at a time, why not make many of them together on the same crystal of silicon?
The result would then be an integrated circuit – or a silicon chip.
In 1960 the first chip was made in the United States. Within a few years,
chips containing several dozen transistors and other components were being
manufactured. By 1975 the record stood at about thirty thousand components
per chip. Because each component was now microscopic in size, the chips
on which they were made became known as microchips. By 1994 it was possible
to place over three million components on a single piece of silicon about
a quarter-of-an-inch square.
In order to make a modern microchip, one draws a plan of all the components
and connections. This plan is reduced in size hundreds of times, trough
a series of photographs, until it is only about five or ten times bigger
than the actual chip. From the miniaturized plan of the chip, a template,
or mask, is created.
Fire and Light
Wafers of pure silicon are sliced from a rod-shaped crystal and put into
a red-hot furnace. The heat causes the wafers to produce a thin outer layer
of silicon dioxide – a substance through which electricity cannot
flow. Then the wafers are taken out of the furnace and coated with a type
of chemical known as a polymer. The particular kind of polymer used in microchip
manufacture is soft and sensitive to light.
|A worker checks a mask to be used in the
manufacture of silicon chips
A coated wafer is carefully positioned under the mask containing the details
of the chip to be manufactured. Ultraviolet light is shone through the mask
and focused by an arrangement of lenses so that it forms a tiny, sharp image
of the circuit pattern on the light-sensitive polymer. Ultraviolet light
is used because the waves that make up ordinary light are too far apart
to pick out the very fine details on a modern chip. In ultraviolet light
the waves are closer together and so can produce a sharper outline through
After the first exposure, the wafer is moved to a slightly different position
so that another tiny region can be exposed to the ultraviolet light shining
through the mask. In this way many copies of the chip are formed from one
The Acid Test
The exposure of the polymer to light changes the polymer's solubility –
the ease with which it can be dissolved by a special chemical. This chemical
is known as a developer and is used to remove the polymer in the regions
where it has been exposed.
The remaining polymer is baked to make it chemically resistant, and then
the whole wafer is bathed in acid. Where the polymer has been dissolved,
the acid is able to attack the underlying silicon dioxide and so expose
the silicon below. As a result, the pattern of exposed silicon exactly matches
the details contained on the mask.
Tiny amounts of different dopants are put onto the exposed silicon, at specific
points. In this way, regions of the chip are formed that will work as diodes,
transistors, and other components. Finally, very thin aluminum pathways
are laid down to provide electrical connections.
Modern microchips may have ten or more different layers of components and
connections. All the manufacturing stages just described must be repeated
to produce each of the circuit layers. This mean the chips must go back
into the furnace to begin the whole process again and again.
Challenges of Tomorrow Chip's
For a number of years scientists have been asking: How much more can chips
be miniaturized? Today's most advanced chips contain several billion transistors.
At some point, researchers are likely to run up against problems that prevent
a further reduction in size.
|Visible and invisible light. The light we
can see is just a small part of a complete spectrum of waves that
also includes infrared rays, ultraviolet rays, and X-rays.
Already, microchips produce more heat for their area than the hotplate of
an electric over. Since the chips would fail if they became too hot, cooling
systems are needed that can remove heat as fast as it is produced. In the
future, a limit may be reached on how quickly excess heat from chips can
be carried away.
Another problem arises from excessive electrical effects. The voltage across
a single transistor is quite small, but it occurs across such a tiny distance
that it amounts to an extremely steep voltage drop. Such a voltage drop
produces a powerful electrical effect in its immediate neighborhood. As
the components on a chip are crowded closer and closer together, the electrical
effects they give rise to may become so large that they prevent the transistors
from working properly.
Many other difficulties confront the chip manufacturers of tomorrow. Perhaps
the biggest challenge of all will be to find ways to create the ever-tiner
components and pathways on the chip's surface. Its fineness of detail is
limited by the size of the waves that are used to shine on the mask. Ordinary
light has already been replaced by ultraviolet light. Now even the limits
of ultraviolet rays are being reached, and some scientists believe that
the next generation of integrated circuits will have to make use of X-rays,
which consist of even shorter waves.
Limits to the miniaturization of chips may eventually be reached, but not,
scientists believe, before you will be able to carry a computerized copy
of all the books in the Library of Congress around in your pocket.
How can computers and other electronic devices be made even faster
and more powerful than they are today? One way is by miniaturizing
the components on a silicon chip still further. This would reduce
the distances that signals have to travel and so increase the speed
with which information can be handled. An alternative approach is
to look for new materials through which electrical signals can move
A basic limit to the speed of a computer is the rate at which its
transistors can flip from "off" to "on" and back again. Some scientists
are investigating materials in which this so-called switching speed
is much higher than it is in pure silicon. One of the new possibilities
that are looking at is an alloy of silicon and germanium. New transistors
made from a silicon-germanium alloy have been shown to switch on and
off about three times faster than the fastest silicon transistor ever
made. Researchers are now experimenting with ways to incorporate these
superspeed switches into practical circuits.