Oh, the wonders of the Internet! If you read my last column you
may recall that I was wondering why the X-ray discoverer,
Roentgen, had his name listed among the famous composers in
the Concertgebouw in Amsterdam. Well, I was so troubled by
this that I logged onto the Concertgebouw website and via e-mail
asked if anyone could enlighten me. On the 4th of July, I
received e-mail from a Frits de Haen at the Concertgebouw
informing me that not only is there a well-known Mozart scholar
named Einstein, but also the Roentgens are a reputed family of
musicians in the Netherlands. The mystery is solved and it has
nothing to do with X-rays!
More troubling to many is the possible breakdown of Moore\’\’s
Law roughly a decade from now. This has been a subject of
discussion for many years in the technical media but now is
increasingly being mentioned in the financial press such as the
Wall Street Journal. For those unfamiliar with Moore\’\’s Law,
back in 1965 Gordon Moore, a co-founder of Intel, proposed that
the computing power of the veritable silicon chip would roughly
double every year or so (later modified to 18 months). What he
was saying was that the transistors on the chip would decrease in
size over the years so that the number of them on the chip would
keep increasing over time. The fact that you can now buy high
power computers for such low prices attests to the fact that,
amazingly, Moore\’\’s Law has held for more than three decades
and should continue to be true for at least a fourth. What makes
it so amazing is that the prediction was made when there were
only about 50 transistors on a chip and now the Pentium II chip,
for example, has over 7 million!
When I first joined Bell Labs in 1952, I sat in on meetings
attended by William Shockley and Walter Brattain, two of the
co-inventors of the transistor, Ian Ross (later president of Bell
Labs), George Dacey (some years ago, as a stockholder I cast my
ballot for him as a director of W. R. Grace), John Hornbeck (later
president of Bellcomm and then Sandia Laboratories), and others
who would contribute heavily to the silicon revolution spurred by
the transistor. As a young chemist who wasn\’\’t expert in
anything, I sat in awe of these truly brilliant scientists discussing
an idea for what to me was an incomprehensible device called a
junction field effect transistor (FET). In 1953, Dacey and Ross
made such an FET using germanium. However, the germanium
device performed poorly due to the presence of so-called
"surface states" which loused up the FET\’\’s performance.
To understand one contributor to these surface states, let\’\’s talk
about the crystal structure of germanium, which is the same as
that of diamond. In the diamond structure carbon, as with silicon
and germanium, has 4 electrons that are relatively "loose" or as
we chemists call them, "valence" electrons. Each of these 4
electrons wants to get together with another electron to form a
chemical bond. It\’\’s rather like each carbon atom has 4 arms
stretched out trying to shake hands with the electrons from 4
other carbon atoms. These "handshakes" form very strong bonds
as we might judge from the well-known hardness of diamond.
So, we have each carbon shaking hands with 4 other carbons
around it, EXCEPT AT THE SURFACE. Here, some of the
carbon atoms\’\’ arms are reaching out, dangling in space with
nothing to shake hands with. These are known in the trade,
appropriately, as "dangling bonds" and tend to interfere with the
desired flow of electron in the FET. The bothersome "surface
states" are partly due to these dangling bonds.
Serendipity often plays a role in science and technology. Two of
my good friends and colleagues at Bell Labs were Carl Frosch
and "Linc" Derick. Carl was older and somewhat of a father
figure to me and was a chain smoker who always had two or
three cigarettes during lunch. Linc was a tough-as-nails former
fighter pilot who flew cover for Patton in World War II and was
a meticulous experimentalist. Carl and Linc had been trying to
diffuse impurities into silicon by placing silicon wafers in a
furnace with the vapor of the particular impurity, say
phosphorus, flowing over the hot silicon. But, they kept having
problems with pitting of the silicon and nonuniform penetration
of the impurities. (Good device performance depends on control
and uniformity of impurities in the silicon.) They tried using all
kinds of gases to improve the situation. One day in 1955, Derick
was running an experiment using hydrogen. The regulator on the
hydrogen tank wasn\’\’t working properly and the pressure kept
falling. At one point, Derick turned the valve to increase the
pressure. However, when he returned later, the pressure had
dropped to zero. Expecting the worst, he took out the silicon
wafers and, instead of being pitted, he found them beautifully
smooth and colored, "looking like jewels", he told me. Frosch
immediately recognized that what had happened was that the
hydrogen had "flashed back", reacting with the oxygen in the air
to form water, actually steam. The hot steam reacted with the
silicon to form very thin, smooth layers of silicon dioxide (the
sand on a beach is silicon dioxide). The pretty colors were
interference colors just like the rainbow effect you get with very
thin oil films on water.
Frosch and Derick quickly tried adding water vapor deliberately
to the hydrogen and got the same "jewels". They also showed
that the oxide film could mask, or block the diffusion of certain
impurities into the silicon. Not only that, but within a few weeks
Jules Andrus and Walter Bond showed that, using something
called photolithography, windows could be etched in the oxide
layers. By using these windows, impurities could be diffused
into the silicon only in the selected areas. Thus, within the space
of a few weeks, the whole process was created for oxide masking
and photolithography to control the precise geometry of
impurities in silicon. This fundamental, but not too widely
appreciated work laid the foundation for the manufacture of the
integrated circuits on the silicon chips as we know them today.
"What does this have to do with the FET?" you ask. Well, in
1959 John Atalla at Bell Labs showed that this oxide layer of
Frosch and Derick could be incorporated into a very well
behaved FET that became known as the metal-oxide-
semiconductor FET or MOSFET. OK, you say this is just
another complicated device, why should we care? It happens
that the year before, a chap at Texas Instruments, Jack Kilby had
invented the integrated circuit, which incorporated several
devices on a single chunk of silicon. (Robert Noyce, another co-
founder of Intel, then at Fairchild, had a similar idea around the
same time and is acknowledged as making an important
contribution.) Eventually, the MOSFET was married to the
integrated circuit and the MOSFET became the key computer
memory device. Jean Hoerni, also at Fairchild, showed that
leaving the masking oxide in place after fabrication of the device
led to more stable performance, serving to "passivate" the
surface.
What about Moore\’\’s Law? The reason Moore\’\’s Law has held all
these years is that refining of the masking, etching and diffusion
techniques has permitted a continuous reduction in the size of the
MOSFETs so that today\’\’s chips contain those millions of FETs.
But what are the limits? Today, the thickness of the oxide layer
is only about 15 silicon atoms thick. By 2010, this thickness is
expected to be closer to 5 silicon atoms thick. Without going
into details here, this insulating silicon oxide layer has served to
"contain" the electrons in their proper place when they flow
through the MOSFET. Unfortunately, there is a phenomenon in
quantum mechanics known as "tunneling". This tunneling is sort
of like the following situation. You come to the end of a road at
the bottom of a hill and you want to get to the other side.
Normally, you would have to push or drive your car to the top of
the hill and hopefully coast down. If you were an electron,
however, there is this weird thing in quantum mechanics that
says there is a definite chance that you can tunnel through the hill
and be on the other side. This is the problem for Moore\’\’s Law.
The electrons can tunnel through the silicon oxide layer when it
gets too thin and when that happens it\’\’s a whole new ballgame.
Needless to say, many clever people are already working
feverishly on ideas to keep Moore\’\’s Law intact well into the 21st
century!
Are there any loose ends we\’\’ve left hanging? You\’\’re right, what
about those dangling bonds? Well, it turns out that the answer
lies again with Frosch and Derick\’\’s oxide. Those outstretched
arms of the surface silicon atoms find in the oxygen plenty of
electrons to shake hands with and, with the oxide formation, the
dangling bonds disappear. Problem solved!
Neither Frosch nor Derick are with us anymore and just recently
I had to retire one of the first red LED digital clocks, which Linc
made for me on one of my service anniversaries at Bell. If the
editor could tell me how to incorporate pictures in this column, I
would finish with a picture that Carl Frosch used when he gave
the first paper on the oxide masking process. It shows a masked
and etched silicon chip that, in contrast to today\’\’s crisply
executed integrated circuits, spells out in very ragged fashion
"THE END".
Allen F. Bortrum
