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Semiconductor Silicon: The Extraordinary made Ordinary
We humans have employed and improved materials for millennia, but it required the
Industrial Revolution of the last century to birth the systematic, science-based
development of materials. During this time, effort expended in understanding the
process-microstructure-properties relationships of materials conferred great economic
and military advantage upon the successful. The introduction of machine power in
this era created great leverage for improvements in this strength, ductility, corrosion
resistance, formability, and similar properties of materials. Response to this
opportunity led to the emergence of the materials profession. Stimulated by
opportunity, materials scientists and engineers of the day met many of the challenges
by first understanding and then controlling the composition and microstructure of
materials. In the process, they defined the materials-engineering profession In fact the
understanding and control of microstructure is the hallmark of materials science and
engineering. Of course the ancient art of finding, mining, concentrating, and refining
materials from the earth’s crust does not apply to this definition since we wish to
focus on the engineering of materials.
Five decades ago, a new chapter in the evolution of this profession began by the
invention of the transistor. This invention and the development of integrated circuitry
that followed from it spawned a new era of materials achievement, again stimulated
by the enormous economic and performance grains available. In this arena however,
the object of the game was to completely eliminate microstructure while doing with
impurities, save for a desired few, to levels previously unimagined. Today a materials
thus prepared is a blank slate upon which we can write the microstructure of an
integrated circuit. The story of integrated-circuit fabrication is by now a very familiar
one. Its telling employs vocabulary such as diffusion, ion implanation, lithography,
oxidation, metallization, and passivation.
Single-crystalline silicon is the enabling material of the information Age (see Figure
1), just as steel was the enabling material of the Industrial Revolution. While other
materials are also essential to the technology, silicon is the cornerstone material, and
the manufacture of silicon wafers is a significant economic activity. As Fig. 2 shows,
in 1995 semiconductor products exceeded the dollar value of steel-mill products. For
a product category essentially unknown just 40 years earlier, this trend is amazing,
both for the speed with which it has risen and the height to which it has climbed.
Implicit in the growth illustrated in Fig 2 is a host of activities: technology
development, production-facility development and construction, supplier
development, and ongoing continuous improvement. Because the product of this
continuing effort is so ubiquitous, like steel, contemptuous familiarity besets us all,
and we take as given the availability of this remarkable product of materials research
and engineering.
To begin, notice that the specifications given in Table I, when compared to materials
employed in other technologies, appear to be absurd. For example, iron is specified to
be present in the material at a concentration of less than 1011 atoms/cm3. Given that
silicon contains roughly 5 x 1022 atom/cm3, this concentration corresponds to about 2
parts per million atoms. In short the silicon crystals in use today are impuritycontrolled to amazing levels; are flat, clean, smooth, and free of crystallograpghic
defects to incomprehensible standards; and will continue to improve over the next
decade. Of course these specifications are neither amazing nor incomprehensible.
They are every-day and ordinary.
The intellectual and physical investment in semiconductor silicon and its processing
technology is enormous. Naturally this investment was not made without
consideration of alternatives. That single-crystal silicon remains the material of choice
for the fabrication of most integrated circuits speaks to the wisdom and good luck of
those who made the first few steps leading to the result shown in Table I. The
remainder of this article highlights these people, their environment, and their
motivation.
The first solid-state electronic device, the galena point-contact rectifier, familiar as the
key component in the “crystal” radio, was described by Ferdinand Braun of Marburg
university in 1874. This device was used for a time as the detector of spark-gap radio
transmitters but was eclipsed by the vacuum-tube rectifier in the early years of the
20th centyry. Following World War I and the development of quantum mechanics,
research interest once again turned to solid-state devices. The cuprous oxide rectifier,
for example, was first described by L.O. Grondahl and P.H. Geiger in 1927. These
rectifiers, produced simply by heating a copper disk in air at 1000ŗ C, found use as
modulators in telephone carrier systems. In 1931 Walter H. Brattain, who would later
play a leading role in the invention of the transistor, began studying the cuprous pxide
rectifier at Bell Labs with J. A. Becker. The two determined that current in Cu2O is
transported by positive holes excited from acceptor centers, which are probably
vacant copper sites. Futher they determined that the rectification occurred at the
junction between the metal and the oxide, and that current flow through the bulk
materials was ohmic by photoexcitation of the junction, which produced a current.
Meanwhile theoretical progress on semiconductors was in a state of dramatic
acceleration. Mott in England, Schottky in Germany, and Frenkel in the Soviet Union
all agreed that a semiconductor rectifier loses carriers at the junction, which creates an
effective barrier to equilibrium electron flow across the junction. Armed with this
understanding, inventors around the globe struggled with the notion of a solid-state
amplifier.
World War II was the first large-scale conflict in which electronics technologies
figured heavily, and none was more prominent than the development and perfection
of radar. On a scale comparable to the atomic-bomb projects the development of radar
spawned the development of subsidiary technologies, among them semiconductor
rectifiers. Rectifiers were vital in this effort as mixers, direct-current restorers, and
video detectors. This application was so important that, by 1943, 30 university and
industrial laboratories were engaged in research and development in the area. Among
these was the Bell Labs metallurgical group, led by E.E. (Earl) Schumacher, and later,
J.H. (Jack) Scaff. This group included W.G. (Bill) Pfann, H.C. (Henry) Theurer, and
R.S. (Russell) Ohl. Together these materials researchers would prepare and fertilize
the field of semiconductor materials for subsequent bountiful harvests. Their
motivation in the beginning was the improvement of varistors and thermistors, both of
which were extensively employed in the telephone system. Later of coursed the
requirements of war supplicated their motivation.
In their first experiment, Scaff and Ohl melted commercially available silicon of
99.80/0 purity under helium and allowed solidification to occur slowly and radially
inward. With a point contact of the galena and cat’s whisker sort they examined a
vertical section of the ingot thus prepared and discovered that the polarity of
rectification reversed during a traverse toward the ingot center. The knowledge that
“metalloids” sometimes rectified when contacted with a metal probe was already
available. The notion that one lump of such stuff rectifies with different polarity in
different areas was unheard of. Further experiments revealed that a photovoltage was
produced if the region of reversal was illuminated, just as in the case of Brattain’s
cuprous oxide rectifier.5 Scaff and Ohl named the outer region “p-type" and the inner
region n-type." The then young William G. (Bill) Pfann joined in an effort with Jack
Scaff to apply classical metallographic techniques to the solidified silicon, which they
were able to reveal by etching.6 By this point, the existence of a junction between nand p-type material was established, but no reason for its existence had surfaced.
Henry Theurer (Figure 3) showed via additional experiments that the formation of the
junction could be the result of normal segregation of impurities during solidification.
Theurer did not know, of course, which impurities might apply. Walter Brattain
discovered these developments through Bell Lab’s research director, M.J. (Mervin)
Kelly, who arranged a meeting of the groups in early 1940. Brattain was greatly
impressed with the photovoltage generated at the p-n junctions which was 10 times
larger than had been seen in other materials. For his own edification he subsequently
obtained a sample of silicon containing a junction.
Theurer continued to develop methods for the manufacture of silicon with uniform
conductivity for use in point- contact rectifiers, several types of which were being
manufactured by Western Electric-the manufacturing arm of the Bell System at the
time-for use in radar applications. He discovered that the addition of boron increased
the p-type conductivity of silicon and that the p-n junction moved toward the lastfrozen end of the ingot if more boron was present. He also suspected that phosphorus
was involved in p-n junction formation because he had occasionally smelled traces of
phosphine during ingot preparation. With these puzzle pieces in front of him, Theurer
postulated that phosphorus opposed boron and produced n- type conductivity. He
further asserted that phosphorus segregated preferentially in the liquid phase during
solidifi- cation. Therefore it would be incorporated only in the last part of the ingot to
freeze in amounts sufficient to offset the effect of boron and produce n-type material.
This stroke of scientific deduction could have been ascribed to genius, had Theurer
desired to be so labeled. Fortunately he preferred to remain unpretentious and
approachable, and the wisdom of his deduction diffused into the general font of
knowledge. The basic generalization of course is that elements of Group III and
Group V produce p-type and n-type conduction in silicon, respectively, and thatatom-for-atom-these impurities compensate for each other so that the conduction is
proportional to the difference in atomic concentrations. This profoundly valuable
insight was developed without benefit of chemical analysis, which at the time was too
insensitive for the concentrations involved. The classic paper describing this work
was completed in 1945 but was not published until 1949 because of military-security
restrictions.7 Contrast this level of understanding with that available for a material
like Cu20, for which nearly no predictive postulates existed.
Well before the end of World War II and well before the invention of the transistor,
the metallurgical group had amassed considerable understanding of semiconductor
silicon. This included knowledge of germanium, whose properties had been
thoroughly explored and described by Karl Lark-Horovitz and colleagues at Purdue
University during the war. It has often been speculated that, had the Lark- Horovitz
group continued to exist following the war, Purdue would have been the natural
birthplace of the transistor. Germanium proved to be amenable to the production of
higher quality rectifiers, primarily because it was available in more pure form. Again
the metallurgical group developed a germanium point- contact rectifier, and limited
numbers were manufactured by Western Electric. Theurer was responsible for
preparing the germanium material. This he accomplished by hydrogen reduction of
GeO; in a graphite crucible. The ingots were directionally solidified as in the case of
silicon and were polycrystalline but with a rather large grain size. Theurer*s stock of
germanium ingots was used subsequently in the discovery of the transistor effect.
While the war effort had diverted attention from the quest for the solid-state
amplifier, Mervin Kelly had not forgotten.
On April 9, 1945, a meeting on germanium crystals took place in Murray Hill. Three
months later, an “Authorization for Work” or “Case” had been developed to justify
the creation of a solid-state-physics group under the direction of Bill Shockley and
Stanley O. Morgan. The purpose of this group was to discover “new knowledge that
can be used in the development in completely new and improved components... of
communications systems”. Kelly maintained that advances in the understanding of
materials, as well as their role in components. The newly formed semiconductor
group decided at the outset to focus its attention on the elemental semiconductors
silicon and germanium, and to ignore selenium and copper oxide, which had received
considerable attention previously. Shockley and Morgan believed that the simpler
nature of these materials would make them easier to understand. The fact that bell
labs materials engineers had experience with the materials made them a logical choice
to renew the quest for solid-state amplification. These decisions – the choice of
semiconductors as a primary field of research and the choice of silicon and
germanium as primary materials within the field – were almost necessary
preconditions to the discovery of solid-state amplification. Today, the iniquitousness
of silicon makes this choice seem obvious. It was not John Bardeen, a solid-state
physicist who had spent the war years at the Naval Ordnance laboratory, joined the
group and would later share the first transistor patent with Brattain. In his Nobel Prize
lecture of 1956, Bardeen said, “We were able to take advantage of the important
advances made in that (war) period in connection with the development of silicon and
germanium detectors...”
It is well-known that the point-contact transistor was invented at Bell Labs on
December 23, 1947, by Bardeen and Brattain,8 and the events leading up to the
discovery have been chronicled often. What is not as well-appreciated is the essential
role played by the metallurgical group at Bell labs, whose understanding, both
practical and theoretical, was crucial to the event. had the wartime effort of Scaff,
Ohl, Pfann, Theurer, and Shumacher not been made and the experience thus gained
not been made available, the device-interested community at Bell labs, as elsewhere,
would have been stymied.
Bill Shockley, while still a firm believer in the inherent advantage of a field-effect
device, turned his attention to an improvement of the point-contact transistor through
removal of the emitter and collector from the influence of the crystal surface and
surface states. He worked out the theory of p-n junctions and the junction transistor9
in short order, and had applied for a patent before the point-contact transistor was
announced to the public on June 30, 1948. Of course there was no technology
available to fabricate the planar n-p-n or p-n-p devices described so elegantly in his
patent application.
Once the transistor effect had been demonstrated and Shockley’s simple junctiontransistor geometry had been understood, demand for crystals of higher purity and
greater uniformity increased. By this time, it was understood that conductivity-type
control was essential and that material purity was consequently an important issue.
Because germanium had been used in the inventive work of Bardeen and Brattain, it
was the object of early effort. Gordon Teal and J.B. Little10 adapted and improved a
crystal-growth technique first used by J. Czochralski in 191711. (See Figures 4 and 5).
The technique, which survives as today’s method for the high-volume production of
semiconductor silicon, consists of slowly pulling a seed crystal out of a melt while
atoms from the melt attach themselves to the crystal. Because crystal growth is
seeded, boules of predeterminated orientation can be produced, and with careful
control, large crystals are possible. At that time, one-inch-diameter crystals were
considered large. Teal and Little’s innovations included the use of a radio-frequency
heating source and a carbon susceptor fitted to a fused silica crucible containing the
melt. These elements remain at the center of today’s technology.
Teal focused on crystal-growth techniques because of his belief that single crystals
would make better transistors, a view that was not prevalent at the time. Nevertheless
his belief was vindicated when the minority carrier lifetime of his single crystals
proved to be much greater than was the case for polycrystalline material. Once single
crystals had been obtained, Teal set out to fabricate the n-p-n transistor Shockley had
envisioned. By 1950 he had succeeded.12.
In order to fabricate Shockley’s junction transistor, Teal developed a method for
adding pellets of gallium-germanium and antimony/germanium to the melt while a
crystal was being pulled.13. This resulted in the growth of a single n-p-n structure and
the experimental verification of Shockley’s earlier theory. Later Bob Hall of General
Electric invented a method of growing a series of n-p-n junctions into germanium by
varying the rate of crystal growth.14
Figure 6. Bill Pfann invented zone refining in 1951. He realized that impurities
could be eliminated from a material by passing a molten zone through it and
that at each pass, the difference between the liquidus and solidus concentration
would be instrumental in further removing the impurity. This method remains
important for controlling and purifying a wide variety of materials, and was
crucial to the development of the transistor era. Here Bill Pfann and Jack Scaff
(foreground) observe the zone melting of a germanium ingot.
These techniques were however a far cry from a manufacturing technology. The
transistors had to be sawed out of the crystal, and control of the base layer thickness
was poor. Nevertheless the devices were much less noisy than their point-contact
counterparts and could handle more current. J.E. Saby of General Electric improved
the art substantially by inventing the alloy junction transistor,** which was made by
alloying indium pellets on opposite sides of a thin piece of germanium. By March of
1952, licensees of Western Electric’s transistor patents numbered 26 domestic and
nine foreign. In addition to the expected licensees such as General Electric, the
technology attracted a number of very adventurous small firms, among them Texas
Instruments of Dallas, Texas, and Sony of Tokyo, Japan.
This era of commercially available germanium transistors came about because of
another materials-engineering innovation that stands today as a nearly universal
method for purification of a wide variety of materials. The innovation, zone
refining,16 was invented by Bill Pfann (Figure 6), who recognized that the typically
larger solubility of an impurity in the melt compared to that in the solid could be put
to use if the melt were confined to a small volume. Rather than attempt to control
impurity' segregation by mere directional solidification, he reasoned, why not purify a
boule by repeated passes of a small molten zone? In his process, each pass of the zone
results in a decrease in the amount of impurity, which segregates preferentially in the
liquid (phase where its solubility is larger. Pfann showed that extraordinary
purification could be obtained by repeated passes of a confined molten zone (Figure
7). Furthermore, he taught that material of uniform doping level could be produced by
the passage of a single zone in which sufficient dopant was present. This technique,
known as zone leveling, was key to the manufacture of semiconductor materials of
known conductivity and played a key role in the manufacture of reproducible devices.
In addition to this contribution to solid-state electronics, Pfann`s work on the growth
of pure germanium led to the discovery that dislocations emerging from a crystal of
germanium at a low angle boundary could be revealed by etching.17 This was the first
direct evidence of dislocations in crystals and provided enormous impetus for the
continued study of these heretofore only postulated crystal defects. J.H. (Jack)
Wernick18 applied the method to the preparation of pure metals, which were
indispensable for basic studies of electrical and magnetic properties.
The manufacture of germanium. transistors in commercial volumes touched off a
frenzy of interest in circuit applications for the new devices, as well as new variants
of the transistor itself. These included the unijunction transistor, the semiconductor-
controlled rectifier, the junction field-effect transistor, and the tunnel diode.
Meanwhile back at Bell Labs, Gordon Teal and Ernie Buchler had successfully
grown single crystals of silicon by the Czochralski technique.19 The new material
showed low minority carrier lifetime, evidence of impurity contamination from the
quartz crucible, and resistivity far from intrinsic. Nevertheless the material was
fashioned into an alloy junction by Pearson and Sawyer.20 This junction proved to be
superior to those made in germanium (Figure 8). It exhibited 1,000- times less reverse
currents a high and stable reverse voltage, and could be operated at 300°C.
Immediately silicon became a rival to germanium, and methods for making good
materials were sought
Meanwhile in Dallas, Texas Instruments (TI) had set up a semiconductor laboratory
and had recruited Gordon Teal from Bell Labs to head it. In short order, TI produced
germanium transistors land by 1954 had announced the development of the first
silicon transistor. This transistor was made using the grown unction technique that
Teal had developed at Bell Labs, and its production gave TI a three-year jump on the
competition in silicon-transistor manufacture. The clear superiority of silicon devices
in power handling and temperature range of operation made them a favorite of
military customers' which comprised the bulk of the market at that time.
Back in Murray Hill, the search for pure silicon continued. Zone refining proved to
be problematic because the molten material reacted with all of the (potential crucible
materials. Henry Theurer solved the problem by inventing the float-zone crystal
growth process.21 In this process, the crucible is eliminated. An uncontained molten
zone is (formed in a vertical rod of material and is stabilized by the surface tension of
the melt while the zone is moved along the rod (Figure 9). The method permits the
use of a seed crystal so that the 'crystal orientation can be controlled. Theurer`s
process permitted both refinement and crystal growth in one operation, and proved
capable of producing material of quality comparable to that obtained in germanium.
Float zoning (FZ) emerged as the production method of choice. During the last 20
years, the FZ process has been eclipsed by the Czochralski method, which is capable
of growing larger diameter material. It remains however the method of choice for
material of high purity. Given the availability of good silicon turned to the
development of diffusion-doping technology. and Theurer22 had proposed solid-state
diffusion from a vapor source as an attractive method for doping in 1947, but
methods for eliminating unwanted impurities were not developed until almost a
decade later. Finally diffusion was used to fabricate large-area silicon diodes in
1954.23 These diodes, whose p-n functions were near the crystal surface, commanded
considerable media attention for solar conversion efficiency of 6%. At this point,
diffusion doping and silicon made an unbeatable combination. While germanium
continued to be used in previously designed devices, virtually all subsequent
development effort was spent on silicon, at least within Bell Labs.
In a very short period following World War II, the basic understanding of the nature
of elemental semiconductors and their dopants permitted the invention of the
transistor, which then stimulated the development of methods for its manufacture.
Within six years of the invention, a relative handful of materials people had
developed the basic crystal-growth techniques that survive to this day. At the end of
this period, the centrifugal force of opportunity, created by the Bell system's policy of
low-cost nonexclusive licensing, had served to scatter semiconductor expertise across
the United States. Shockley had formed his own company, Shockley Semiconductor,
in California and had recruited a collection of very bright people. Eight of these,
called by Shockley the “traitorous eight" subsequently left to form their own
company, Fan-child Semiconductor, with funding from Fairchild Camera and
Instrument Company. These eight went on to commercialize the diffused base bipolar
transistor and to develop the planar process as well. This latter innovation made
transistors manufacturable via batch processing, which soon saturated the market for
discrete devices. The stage thus set, Kilby25 at TI reasoned that the technology then
developed would support more than a single transistor. While still a new employee
and therefore not entitled to vacation during the traditional July recess, Kilby in 1958
conceived of integrated circuits and had demonstrated the principle by October of that
year. From that point, one advance built upon another, and the geographical
dispersion of the innovators became marked. Fairchild`s Jr.A. Hoerni26 introduced
planar technology, which permitted contact to be made on the top planar surface of
the crystal. Also at Fairchild, Noyce27 utilized an aluminum-over oxide scheme to
contact circuit elements. Soon advances in integrated circuit design and fabrication
were being made by Sylvania, RCA, Westinghouse, Motorola, Faichild, and others.
The race was on. The race, of course, simply stimulated demand for the highest
possible quality starting crystals. Today the understanding and the tools invented
nearly 50 years ago continue to support the industry, though they are now greatly
refined, exquisitely engineered, and beautifully supported by a foundation of
scientific understanding and practical experience.
The emergence of transistor and integrated-circuit technology lent impetus to
efforts to extend the early understanding of crystal growth achieved by Gibbs.* In
addition to these fundamental advances, the growth of semiconductor silicon has
become an art of extreme sophistication. Molecular-beam epitaxial growth, ab initio
computations of surface structures, and direct observation of silicon surfaces via
scanning tunneling microscopy are all supported ultimately by the value of this
material in its integrated-circuit embodiment. While we know where this revolution
began, we do not yet know where it will end. As Figure 2 suggests, integrated-circuit
technology has yet to reach an inflection point in its growth rate. Furthermore
projections of future demand for the product do not indicate that an inflection is in
sight. Through comparison with the development of steel, neglecting the stasis that
occurred during the Great Depression, it might seem that the age of silicon has
several decades still ahead, with many innovations yet to be made. Of course the advances in lithography that have been made in order to fulfill the expectations of
Moore*s Law have been spectacular in their own right. The question is: Where does
this game end? After 50 years, we don't really know.
Instead, futuristic thrills are giving way to more pragmatic preoccupations. Industry
has focused on a few, basic micromachines that are starting to have a profound impact
on the lives of ordinary people. They are appearing in cars, in homes, in operating
theatres and in entirely new and unexpected areas. "The firm is also using
micromachining techniques to make the nozzles and inner workings of the next
generation of inkjet printers.
And microParts is not the only company exploiting micromachining techniques. Last
year. Analog Devices of Massachusetts, which makes accelerometers that trigger car
alrbags to inflate, launched a new micromodel. This fits onto a chip measuring just 3
millimetres square and includes not only the silicon sensor, but also the electronics
for calibrating and controlling the device It is already being touted as a diagnostic
tool for electric motors, which will quickly detect damaging vibrations, and as an
automatic trigger for switching off gas supplies during earthquakes.
In May, Lucas NovaSensor, a Californian subsidiary of Lucas Industries, unveiled
an air-pressure sensor that is small and light enough to fit inside a car tyre. The device
goes hand-in-glove with a new type of tyre that allows a car to
Then there is Texas Jnstruments which over I the past six months has introduced a
series of micromachines based on chips packed with 16 micrometre-square mirrors
that can be moved by electronic signals. These systems are being built into overhead
projectors, home movie equipment and *vide-screen TVs.
One of the critical elements of today's generation of microelectromechanical
systems (MEMS) is that, they are simple enough to mass-produce. This makes them
cheap. A single silicon wafer, for example, can hold tens of thousands of simple
pressure sensors and a smaller but still impressive number of accelerometers. The
cost of making one micrometre-scale device is nearly the same as making an entire
wafer-full. A mass-produced, micromachined accelerometer sells for about $10,
compared with up to $1000 for its larger, I conventional cousins.
And once one big market exists for a device, others are free to exploit it. Lucas
NovaSensor, for example, is seeing its* pressure sensors included in diving * gauges
for measuring depth and air-tank pressure, altimeters for hang-gliders, and even in
computers attached to mountain bikes for those who want to know how high they've
climbed.
MEMS may have only started to appear in products over the past few years, but the
idea of using silicon for building machines has been around for much longer. In
1982, Kurt Peterson of IBM published a paper called "Silicon as a mechanical
material", in which he noted that as well as being a good semiconductor, silicon has
some remarkable mechanical properties. Pure silicon, for example, has a hardness,
Young’s modulus-a measure of elasticity- and tensile yield strength similar to
stainless steel
In the years after Peterson’s paper, MEMS research took off, helped in
large measure by the army of experts who had learnt to manipulate silicon for the
electronics industry. Most MEMS are built the same way as microprocessors-using
photolithography. In this process, ultra- thin layers of silicon, silicon oxide or metal
are painted with a substance called photoresist, the desired pattern is masked off and
the surface exposed to light, which chemically alters the photoresist. The unchanged
areas of photoresist are then etched away to leave, say, an electronic circuit. By
etching away "sacrificial" layers of silicon oxide, engineers can create structures that
have gaps beneath them, such as a cantilevered "beam" that overhangs a pit in the
silicon wafer, or a cog that turns.
Up to now, the biggest impact of MEMS has been in devices that sense forces and
pressures around them, particularly where size and weight need to be kept to a
minimum. Lucas NovaSensor's blood pressure sensor, for example, is only 700
micrometres across.
It attaches to a saline-filled catheter that is inserted into a blood vessel. The sensor
itselt is a diaphragm of silicon, typically less than 10 micrometres thick, which covers
a sealed pit etched out of the wafer. Embedded in the diaphragm are piezoresistors
which change their resistance when subjected to stress. When the external pressure
alters, the diaphragm deforms and the variation in resistance gives a measure of the
pressure change.
The other main commercial sensor, the accelerometer, is a more complex machine.
Analog Devices’ design consists of a silicon beam mounted a micrometre or so above
a wafer. it is connected to the wafer by “tethers”, each about 2 micrometres thick and
200 micrometres long, which act as springs (see Diagram). The beam has dozens of
“fingers” sprouting from its sides, each of which acts as a capacitor plate, and sits
between two other plates that are fixed to the wafer. Normally, each pair of capacitors
is balanced, but when the beam moves-in response to a sudden acceleration-the
capacitance of one of one changes relative to the other, giving a measure of the
change in motion.
One of those specialist niches is (manipulating light, a task that needs only tiny
forces. Take Texas Instruments' digital light processor *DLP), a component of projectors and wide-screen TVs. A DLP chip, which measures lx5 by 1centimetres, is
covered by some 500 000 aluminium mirrors each of which can be moved
individually. Each mirror is attached to a (thin metal strip which acts as a hinge, with
its position controlled by a switch and two electrodes etched into the silicon below it
*see Diagram over page). The whole structure is built using standard
photolithographic techniques.
The mirror's position is controlled by electrostatic attraction to the electrodes. To
project a large TV picture, a bright light is shone onto the chip and a TV signal fed to
the switches. The signal controls the pattern of reflected light that passes through a
projection lens and onto a screen.
Another intriguing application for actuators is in controlling large- scale effects
that begin as tiny perturbations. An aircraft's wing, for example, creates and sheds
vortices as it travels through the air such devices could change the air
With MEMS to control tilt, roll and pitch, an aircraft could consist of a single wing
with only tiny flaps. But such an aircraft, although possible, is not going to roll onto
the apron in the near future. "We're talking ten years in terms* of real
implementation," says Gabriel.
There are, however, some problems that must be overcome before actuators and
more complex sensors become commonplace. One big advantage of MEMS is that
they are made with the same terials and methods as
devices, and so can be integrated with the control circuits that are needed to make
them work. For some devices, such as micro accelerometers, this closeness is vital.
And integrating electronic and mechanical components helps to keep down costs.
By exploiting advances in I the electronics industry, MEMS can reap the benefits of
those advances without footing the whole research bill. That's important, at least in
the US where last year the Department of Defense found that 60 of the 80 companies
engaged in MEMS activities were small concerns, with typical revenues of less than
$5 million a year.
\ But in some ways, the marriage of microelectronics and micromachines is not a
happy one. For example, certain components of MEMS, such as tiny cogs, need to be
treated at high temperatures.
But exposing nearly completed microelectronic circuits to high temperatures ruins
them because it melts the aluminium wiring and drives off the chemical dopants that
are added to semiconductors to create transistors and other components.
Pioneers in a microworld
At first sight, the solution appears to be to build the MEMS first, putting all the
destructive heat at the start of the manufacturing cycle. But this works only in some
cases. The problem is that micro- . machines are often not level with the \ plane of the
wafer, which disrupts the I focusing of the patterns of light and I \ shade that are cast
onto the wafer as part * lot the photolithographic process.
To avoid this, engineers have tried a number of approaches, such as lowering the
temperature of the heat treatment. But this limits the designs that can be
used. Now, however, a new technique developed at Sandia may provide a better
solution.
McWhorter and his colleagues etch a (trench in a silicon wafer, build their
(micromachine inside it, and then heat (treat it. The trench is then filled in with
(silicon dioxide to leave a flat surface. (The microelectronics are then built (nearby
and, as a last step, the silicon oxide is etched away to free the machinery. This
innovation promises to improve the sensitivity of MEMS by allowing the machinery
and the electronics to be optimised independently.
This phase will bring researchers together to work on two projects. One is to make a
catheter intended to carry a camera and other sensors into, the brain. The second is a
microrobot intended to travels through the pipes of power stations, diagnosing and
fixing problems.
Another difference Sood. noted is that "the Japanese believe the.future is not in
silicon. Silicon is just one of the materials that you can work with."Indeed Sood
believes that MITI cllose to fund theRMIT project precisely because theAustralian
team didn't want to work-in silicon gloves in virtual reality systems or input devices
for computers," says McWhorter.
Another problem facing engineers using photolithography is that they are largely
restricted to two dimensions. The height of microelectronic circuits and MEMS is
typically between 2 and 10 micrometres. Also, silicon is not the best substance for
every task. It would be useful to be able to use a polymer or a ceramic for certain
tasks. There are two promising solutions (here. The first is a technique called
(reactive ion etching. This process, which (uses chlorine or fluorine-based plasmas
(rather than a liquid to etch, is driven *y radio frequency power and can cut (out deep
structures with nearly vertical (sidewalls .*" We're going about 200 micro- metres
deep now," said Dale Gee, director of new products at Lucas Nova- Sensor, which is
working on the technique with Greg Kovacs at Stanford University. *The third
dimension opens up a range of new potential applications, such as building relatively
large capacitor plates, which boosts capacitance. This in turn could help to make
acce- lerometers more sensitive.
The second solution does away with silicon altogether. Developed by researchers at
the Karlsruhe Research Centre in Germany, the technique is known as LIGA. The
process uses X-rays to carve out deep patterns in a polymer. These patterns can then
be electroplated to produce tiny, 3D metallic structures, or metallic moulds. The
moulds can be injected with a variety of materials- including a number of polymers to produce micromechanical devices. Among the devices made with LIGA are gear
trains and magnetic motors.
Although the technique is still experimental, microParts has licensed the process
and is using it to make inkjet nozzles for the next generation of printers, which will
produce 2400 dots per inch. For such high resolution, these printers will need four
times the nozzle density of today’s printers, moving them into the realm where high
precision, low-cost micromachining comes into its own. Whether LIGA will be
widely adoted isn’t yet clear. Silicon micromachining is more mature and offers
easier integration with microelectronics.
A micromachine may be only a small fraction of the cost of a new device-such as an
altimeter for mountainbikers-but its action, size and weight will be the key to its
competitiveness, he says.
Increasingly sophisticated sensors will also appear, says McWhorter. Much
attention is now being given, for example, to developing inertial guidance
systems for cars. Along with accelerometers, these systems will need
gyroscopes, which require constantly rotating or vibrating parts. Such active
devices will help to bridge the gap between sensors and actuators.
More complex actuators, born of the tiny steam engine and motors designed at
Sandia, will also arrive eventually, reckons McWhorter. Their first use is likely to be
in the defence industry, for locking and arming weapons, because they would be
difficult to defeat and should improve reliability, he says.
Meanwhile one of the more popular images of MEMS is still the tiny robot free to
roam human blood vessels. How close is such an device? McWhorter is in no doubt:
"I think that would be further down the road than ten years."
\ source and a carbon susceptor fitted to a *fused silica crucible containing the melt.
*These elements remain at the center of (today's technology. J Teal focused on
crystal-growth techniques because of his belief that single *crystals would make
better transistors' a j view that was not prevalent at the time. [ Nevertheless his belief
was vindicated I when the minority carrier lifetime of his I single crystals proved to
be much greater I than was the case for polycrystalline ma- ( ferial. Once single
crystals had been abstained, Teal set out to fabricate the n-p-n *transistor Shockley
had envisioned. By 1950 he had succeeded.** , In order to fabricate Shockley*s junc'' tion transistor. Teal developed a method for adding pellets of gallium-germanium
and antimony/germanium to the melt while a crystal was being pulled." This resulted
in the growth of a single 'f tl-jy-n structure and the experimental verification of
Shockley*s earlier theory. Later Bob Hall of General Electric invented a method of
growing a series of n-p-rl junctions into germanium by varying the rate of crystal
growth.** These techniques were however a far cry