Purity of ingredients is a constant concern for the semiconductor
industry, because a mere trace of contaminants can damage or ruin tiny
devices. In a step toward solving a long-standing problem in
semiconductor manufacturing, scientists at JILA and collaborators have
used their unique version of a “fine-toothed comb” to detect minute
traces of contaminant molecules in the arsine gas used to make a variety
of photonics devices.
A NIST invention may help purify a process for making semiconductors used in devices such as light-emitting diodes (LEDs).
©Igor Stepovik/courtesy Shutterstock
JILA is a joint institute of the National Institute of Standards and
Technology (NIST) and the University of Colorado at Boulder (CU). The
research was conducted with collaborators from NIST’s Boulder campus and
Matheson Tri-Gas (Longmont, Colo.).
The research, described in a new paper,* used a NIST/CU invention
called cavity-enhanced direct frequency comb spectroscopy (CE-DFCS).**
It consists of an optical frequency comb—a tool for accurately
generating different colors, or frequencies, of light—adapted to analyze
the quantity, structure and dynamics of various atoms and molecules
simultaneously. The technique offers a unique combination of speed,
sensitivity, specificity and broad frequency coverage.
The semiconductor industry has long struggled to find traces of water
and other impurities in arsine gas used in manufacturing of III-V
semiconductors for light-emitting diodes (LEDs), solar-energy cells and
laser diodes for DVD players. The contaminants can alter a
semiconductor’s electrical and optical properties. For instance, water
vapor can add oxygen to the material, reducing device brightness and
reliability. Traces of water are hard to identify in arsine, which
absorbs light in a complex, congested pattern across a broad frequency
range. Most analytical techniques have significant drawbacks, such as
large and complex equipment or a narrow frequency range.
The JILA comb system, previously demonstrated as a “breathalyzer” for
detecting disease***, was upgraded recently to access longer
wavelengths of light, where water strongly absorbs and arsine does not,
to better identify the water. The new paper describes the first
demonstration of the comb system in an industrial application.
In the JILA experiments, arsine gas was placed in an optical cavity
where it was “combed” by light pulses. The atoms and molecules inside
the cavity absorbed some light energy at frequencies where they switch
energy levels, vibrate or rotate. The comb’s “teeth” were used to
precisely measure the intensity of different shades of infrared light
before and after the interactions. By detecting which colors were
absorbed and in what amounts—matched against a catalog of known
absorption signatures for different atoms and molecules—the researchers
could measure water concentration to very low levels.
Just 10 water molecules per billion molecules of arsine can cause
semiconductor defects. The researchers detected water at levels of 7
molecules per billion in nitrogen gas, and at 31 molecules per billion
in arsine. The researchers are now working on extending the comb system
even further into the infrared and aiming for parts-per-trillion
The research was funded by the Air Force Office of Scientific
Research, Defense Advanced Research Projects Agency, Defense Threat
Reduction Agency, Agilent Technologies, and NIST.
* K.C. Cossel, F. Adler, K.A. Bertness, M.J. Thorpe, J. Feng, M.W.
Raynor, J. Ye. 2010. Analysis of Trace Impurities in Semiconductor Gas
via Cavity-Enhanced Direct Frequency Comb Spectroscopy. Applied Physics B. Published online July 20.
** U.S. Patent number 7,538,881: Sensitive, Massively Parallel,
Broad-Bandwidth, Real-Time Spectroscopy, issued in May 2009, NIST docket
number 06-004, CU Technology Transfer case number CU1541B. Licensing
rights have been consolidated in CU.
*** See “Optical ‘Frequency Comb’ Can Detect the Breath of Disease”, in NIST Tech Beat Feb 19, 2008, at www.nist.gov/public_affairs/techbeat/tb2008_0219.htm#comb.
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