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Making
Faster Computer Chips
By Dorothy Wright
If then-Bryn Mawr Physics
Department Chair Walter (Mike) Michaels had told
Patricia M. Mooney Ph.D. ’72 that she would become
one of IBM's principal researchers, she would
not have believed him. Yet today Mooney is a research
staff member at IBM’s T.J. Watson Research
Center in Yorktown Heights,
N.Y., focusing on defects and impurities in semiconductors
and their effects on the electronic and optical
properties of these materials. Since she joined
the research center in 1980, Mooney has earned
two Outstanding Technical Achievement Awards from
IBM for her contributions to the development of
faster computer chips.
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Patricia
M. Mooney,
Ph.D. ’72
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"I think it is amazing that
I ended up here," Mooney observes. "Most people
working in research laboratories such as IBM's
went to MIT, Berkeley or other big research universities,
even as undergraduates. In the 1960s and '70s,
few women studied science at these kinds of universities,
and very few women earned Ph.D.s in physics. In
general, women were not encouraged to go into
the physical sciences. Even now, fewer than 10
percent of the people in my environment are women."
As an undergraduate at Wilson
College, a small women's college in Chambersburg,
Pa., Mooney excelled in math and science, graduating
with honors in physics. "I think what made me
decide to go on to graduate school was doing research
as an undergraduate for Paul Ganley, a physics
professor who came to Wilson from Bryn Mawr during
the summer before my junior year. I did a senior
project measuring the electroluminescence of oxide
films on aluminum electrodes. I realized that
I liked working in a lab, building and measuring
things."
Mooney's physics professors
encouraged her to apply to graduate school. "I
was encouraged to apply to Bryn Mawr by Paul,"
she recalls. "During my senior year, Paul had
also introduced me to Priscilla and Ken Laws,
Bryn Mawr Ph.D. alums who were teaching at nearby
Dickinson College."
After earning her Ph.D. in
solid-state physics in 1972, Mooney taught physics
for six years, first at Hiram College, then at
Vassar. "I had learned a lot about teaching from
Mike, and he encouraged me to go into college
teaching," she says. "He was well known for physics
teaching and was a very strong influence on the
whole Bryn Mawr Physics Department, which has
always taught physics in ways that are creative
and interesting."
The Rewards
of Research
While at Vassar, Mooney heard
about Jim Corbett, a physics professor who specialized
in semiconductors at the State University of New
York (SUNY) Albany. She joined Corbett's team
as a senior research associate in the summer of
1976 studying radiation-induced defects in silicon
(Si). This was of interest because satellites
were powered by Si solar cells, which degrade
over time as they are exposed to radiation in
the upper atmosphere. The team used a high-energy
electron beam to introduce and then identify defects
in Si. "There were visitors from all over the
world," she recalls. "I began to see that research
is an activity involving an international community
of scientists. That interaction is stimulating."
Mooney had her first taste
of life at an industrial lab in 1977 in a visiting
summer faculty position at IBM's T.J. Watson Research
Center, where teams of researchers were studying
the processes for fabricating the integrated circuits
used in computers. "The real heart of semiconductor
activity at the time was in places such as IBM,"
she says. "Semiconductors are their bread and
butter, so they had the best resources."
Three years later, on the
basis of the research she did at SUNY Albany and
during a year at the University of Paris working
on radiation-induced defects in Si, germanium
(Ge) and gallium arsenide (GaAs), Mooney was recruited
for a full-time position at IBM. There she continued
to develop her research specialties, initially
studying process-induced defects in Si and GaAs
and later investigating impurities in epitaxially
grown semiconductor films, most notably the "DX"
center in aluminum GaAs. "For most of the 1980s
I was part of a group studying these compound
materials to see whether they would be better
than silicon for use in digital electronic circuits,"
Mooney says.
Mysterious
Atom "X"
Aluminum gallium arsenide
(AlGaAS) was being considered for several applications.
"But the material is not a good conductor at low
temperature unless you shine a beam of light on
it," Mooney explains. "When the light is turned
off, the material remains conducting; we were
trying to understand this phenomenon, known as
persistent photoconductivity. People thought silicon,
the electron donor, was interacting with something
else in the material — substance 'X' —
to create the so-called DX center."
Mooney's team was competing
with physicists around the world to explain this
phenomenon. "We learned that as you add more and
more aluminum to the crystal, the silicon atoms
lowered their energy by changing their position
in the silicon crystal lattice," she says. "Mysterious
atom 'X' was not needed to explain this behavior."
As a result, IBM presented Mooney with her first
Outstanding Technical Achievement Award.
Mooney has discovered that
competition can be heady stuff. "You don't work
at IBM if you don't like competition," she laughs.
"But the competition is between teams, and I am
always working with collaborators on my own team.
As problems become more complex, the solutions
require people with different training and perspectives
to work together, and I like that."
Faster,
but Not Smaller
Recently Mooney has studied
the characteristics and effects of straining,
or stretching, semiconductor materials, which
permits electrons — and, thus, information
— to move faster. "Until now, we've made
silicon chips faster by shrinking the size of
the transistors," she explains. "At this point,
the dimensions are close to the atomic scale.
So we are studying ways to make transistors faster
without making them smaller. Using silicon germanium
as a template for growing strained silicon is
a possible alternative." Mooney earned her second
Outstanding Technical Achievement Award for demonstrating
this possibility.
Active in professional societies,
Mooney is a fellow of both the American Physical
Society (APS) and the American Association for
the Advancement of Science (AAAS) and a member
of the Materials Research Society (MRS). She is
past chair of the Division of Materials Physics
(DMP) of the APS. She currently serves on the
editorial boards of Physical Review B and
the Journal of Materials Science: Materials
in Electronics. Mooney has also authored more
than 100 research publications.
Mooney expects to retire within
10 years, but shows no sign of slowing down. "Every
year I tell myself that I am going to work less,
but it doesn't happen," she laughs. "One reason
is because I like what I'm doing; the other is
that it is a very competitive field, and we want
to be first."
Assessing her career, Mooney
concludes, "I was a slow starter. I got more involved
and more interested in what I was doing as time
went on. One thing always leads to another."
About the Author
Dorothy Wright contributes
news and feature articles on science, technology,
engineering and general interest topics to a variety
of publications, including Civil Engineering,
Engineering News Record and Bryn Mawr
Now.
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