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06/27/2002
Solubility
After last week''s column on colliding branes and the cosmic
consequences, I find that only a couple of weeks ago there was
almost a collision that would have really rocked our little speck
of the cosmos. An asteroid "the size of a soccer field" sailed by
our planet only 75,000 miles away from us. That''s over three
times closer to us than the moon! To me, the disturbing thing
was that we didn''t even know the asteroid was in our area until
after it had passed us by! Brian Trumbore''s Week in Review for
last week also expressed concern about this near catastrophe.
Let''s turn away from such monumental topics to the other
extreme, the miniaturization of the transistors on the silicon chip.
A few weeks ago, I wrote about the centennial meeting of The
Electrochemical Society (ECS) in Philadelphia. The next ECS
meeting is in Salt Lake City, a venue I first visited over 50 years
ago. Four of us graduate students at Pitt drove to California and
back and we stopped to swim (actually, float) in the Great Salt
Lake. I came out covered with salt and thought that the lake
must be a saturated solution of sodium chloride in water. A
saturated solution is one in which a solvent (water in this case)
has dissolved as much as it can of a solute (salt here).
A year before the Salt Lake float, I was engaged in an ill-fated
Master''s degree project to measure the solubility of barium
sulfate in water at different temperatures. Barium sulfate is the
compound used in so-called "barium" enemas. Aptly, my
aspirations for a Master''s degree were flushed away by the
failure of that project. Later, I received my Ph.D. for another
totally different kind of project. Ironically, at Bell Labs I would
be best known for my work on solubility.
At Bell Labs, the solvents I studied were solids such as silicon.
How can silicon be a solvent? Well, just as we have liquid
solutions involving solvents like water, we have solid solutions.
In 1960, I published a paper titled "Solid Solubilities of Impurity
Elements in Germanium and Silicon". Recently, a friend asked
if I had seen my picture in the May 2002 issue of the Journal of
The Electrochemical Society (ECS). The article, by Howard
Huff, is a retrospective of activities in electronic materials and
devices over the past 50 years and cited my 1960 paper.
The photo of yours truly followed pictures of such eminent
figures as Bardeen, Brattain and Shockley gathered around the
first transistor, of Jack Kilby, recent Nobel Prize winner for his
invention of the integrated circuit, and of Gordon Moore of
Moore''s Law fame. My likeness in such stellar company
certainly puffed up my ego. However, I was quickly depuffed
upon reading an article in the April 2002 issue of Nature by P.
M. Voyles and coworkers at Bell Labs and Agere Systems.
This article did not include my photo or even mention my name.
What was worse, they were talking about silicon that contained
ten times more antimony than I said was soluble in my 1960
paper. How could they get more antimony into silicon than that?
Could it be that my data were wrong?
Let''s not panic, I tell myself. Let''s go back to Salt Lake. The
solubility of common table salt, sodium chloride, is 36 grams in a
hundred grams of water at room temperature. For most salts, as
with antimony in silicon, the solubility goes up with temperature.
For sodium chloride, the solubility at the boiling point of water is
almost 40 grams in 100 grams of water. Let''s take a solution of
40 grams of salt in 100 grams of boiling water and cool it down
to room temperature. The excess 4 grams of salt will precipitate
out of solution until we have only 36 grams in solution.
But, if we use a scratch-free beaker and keep all dust and other
dirt out of the solution, we might just be able to cool that solution
down to room temperature and keep the excess 4 grams of salt in
solution. Without anything to "seed" precipitation, we might
maintain this "supersaturated" solution for some time. However,
it won''t take much to get precipitation started and supersaturated
solutions aren''t typically very stable.
Let''s turn to silicon and make a silicon layer saturated with
antimony at a thousand degrees. When we cool the silicon down
to room temperature, where our silicon chip operates, the solid
solubility of antimony is much less. We expect the antimony,
like the salt in water, to precipitate out, forming clumps of
antimony. But silicon is a solid and water is a liquid. Big
difference! When you cool down your salt solution, it''s still a
liquid and the sodium and chloride ions can move around and get
together with others to form precipitates pretty readily. With
silicon, the antimony atoms sit down in their places (sites) in the
silicon crystal lattice and are "frozen" in those sites as the silicon
cools down. Their ability to move is not very great and it might
take many years, perhaps centuries, before enough antimony
atoms can get together to form a precipitate. We have a
supersaturated solid solution that is stable enough in our lifetimes
for the silicon chips in our computers to perform quite nicely.
I know many, perhaps most of you are saying, "Bortrum, who
cares and what about miniaturization of devices that you
mentioned?" Well, solubility and supersaturated solutions may
play an important role in the fate of Moore''s Law, which has
"driven" the semiconductor industry for some 37 years. There is
concern that the number of transistors on a silicon chip soon may
not double every couple of years. This in an era when security
concerns require increased computer capabilities in fields such as
facial or speech recognition, faster analysis of e-mail or other
communications, etc. Obstacles to a continuation of Moore''s
Law include such things as just the manufacturing problems in
making these devices any smaller. The "wires" connecting these
devices on the chip also must be made smaller. And when the
transistors get too tiny they may not act normally but weird
quantum effects will take over. Just a few of the problems.
Where does solubility come in? There is also a need to make
the silicon more electrically conducting as the devices get
smaller. One means of doing this is to put more electrons into
the silicon. The more electrons, the more current the silicon can
carry. Antimony is what is known as a "donor" impurity - it has
an extra electron that it can donate to the silicon. Stick in one
antimony atom and you get one more electron. Stick in 10
antimony atoms and you get 10 electrons. To really jazz up the
conductivity, add lots of antimony!
But Voyles and coworkers are talking about amounts of
antimony that are over ten times greater than I said was soluble
at any temperature! They''ve had to use trickery that fools the
silicon into accepting lots more antimony atoms than should
dissolve in silicon. One method they''ve used is "molecular beam
epitaxy". In this method a layer of silicon doped with antimony
is laid down atom by atom on a silicon wafer. Apparently, if the
temperature is low enough, the antimony atoms don''t know that
there are more of them being incorporated into the silicon layer
than there should be. As with our supersaturated solutions, they
get frozen in, so to speak.
But trickery only goes so far. It turns out that, at these very high
concentrations, they don''t get one electron for every antimony
atom. Electrical measurements indicate that there are only about
70 percent of the electrons there should be. In other words,
about 30 percent of the antimony atoms are slackers and not
doing their share. Upon reading this, I felt better about my 1960
data and that these antimony atoms were trying to precipitate out
from solution.
Well, Voyles and colleagues were quite clever and managed to
use a sophisticated form of electron microscopy to actually
image individual antimony atoms within the devices. Sure
enough, it seems that the antimony atoms were trying to get
together to form a precipitate. What the electron microscope
studies showed was that there were clusters of antimony atoms
forming. The studies indicated that about 30 percent of the
antimony atoms were in clusters of just two antimony atoms,
possibly combined with what is known as a vacancy, a missing
silicon atom. Now, 30 percent is the just the number of missing
electrons.
While I really don''t hope that solubility is the problem that brings
down Moore''s Law, I admit that it''s satisfying to see that people
haven''t been able to flout the concept of solubility with total
abandon. Nature can only be tested so far! Now I can get back
to worrying about those wandering asteroids.
Allen F. Bortrum
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