Dr. Jörg Schwartz, firstname.lastname@example.org
BOULDER, Colo. – A highly sensitive spectral measurement technique has been extended
from biomedical breath analysis to industrial detection of impurities in semiconductor
gases, according to a group from JILA, a joint institute of the National Institute
of Standards and Technology (NIST) and the University of Colorado. Their method,
called cavity-enhanced direct-frequency comb spectroscopy, uses a wavelength comb
to increase speed and accuracy.
Gases such as arsine are used for epitaxial techniques including
molecular beam epitaxy or chemical vapor deposition to grow semiconductor devices
such as integrated circuits, laser diodes and solar cells. However, because high-quality
devices require extremely pure raw base materials, gases must have an extremely
high level of purity. Just 10 water molecules per billion molecules of arsine can
cause changes in the band structure and electrical properties of the semiconductor;
e.g., reducing the electro-optic conversion efficiency for LEDs or solar cells.
Beyond the challenge of detecting low concentration levels, the
processing gas itself has strong absorption bands, sometimes hiding the absorption
lines of the contaminant. In addition, there are multiple impurities to check, and
with most of today’s methods, this means separate instruments/setups for each.
The researchers say that Fourier transform infrared spectrometers
require long acquisition times, whereas negative-ion atmospheric pressure ionization
mass spectroscopy – another candidate technology – requires large, complex
setups and is not suitable for online testing. Current research focuses on laser-based
spectroscopy such as tunable diode laser spectroscopy or so-called cavity ring-down
spectroscopy, but those use only one or two specific absorption lines to get a concentration
figure. The downside is that other, unexpected contaminants with neighboring absorption
lines can mess up the measurement completely.
The approach demonstrated by the JILA group, which was led by
professor Jun Ye in collaboration with colleagues from NIST’s Boulder-based
campus and Matheson Tri-Gas Inc. of Longmont, overcame this issue by using a whole
comb of wavelengths to measure a large number of absorption lines simultaneously.
Direct frequency comb spectroscopy combines broad spectral coverage
and parallel detection with very high spectral resolution and detection sensitivity.
A broadband optical frequency is coupled into an optical enhancement cavity containing
the sample. The transmitted comb spectrum contains spectral signatures of various
molecules, revealing minute impurities such as water and methane molecules in the
arsine gas used in the semiconductor industry.
Mode-locked lasers produce a train of short pulses in the time
domain, equivalent to a closely spaced and regularly defined set of optical frequencies
– the “teeth” of the comb. In this case, the researchers used
an erbium-doped fiber ring laser, which, after being boosted by an optical amplifier,
produced 81-fs pulses at a repetition rate of approximately 250 MHz and average
power of 400 mW.
This power was required at the next step, where light was launched
into a 6-cm-long piece of highly nonlinear silica fiber. The nonlinearity of this
fiber broadens the light spectrum, resulting in a grid of emission lines extending
from 1.2 to 2.1 μm (8300 to 4700 cm—1). Noting that up to this point everything
could be realized in a rugged all-fiber configuration, they exposed the light to
the sample gas, which flowed through a cylinder surrounded by a “build-up
cavity” with high finesse.
They carefully adjusted the mirrors and cavity length to couple
with the laser frequency comb, matching the comb lines so that each one was in resonance
with its corresponding cavity resonance mode. This cavity enhancement, although
difficult for such a wide comb, increased the power and interaction length between
the light and the gas and intensified the sensitivity of the measurement. Finally,
they analyzed the transmitted light with a two-dimensional dispersive spectrometer
based on a virtually imaged phased array, a cross-dispersion grating and a 2-D camera,
enabling many wavelengths to be measured simultaneously.
By detecting which colors were absorbed by what amounts –
matched against a catalog of known absorption signatures for different atoms and
molecules – the researchers demonstrated water concentration measurements
to very low levels. To date, water at levels of seven molecules per billion in nitrogen
gas have been demonstrated and at 31 molecules per billion in arsine.
The group currently is working on parts-per-trillion sensitivity
– including extending the comb system even further into the infrared –
and toward applications such as atmospheric chemistry.