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August 2005 EEEL Measurements Extend Gas Purity Instrumentation Range |
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The Optoelectronics Division in EEEL worked with a semiconductor gas manufacturer and an instrumentation manufacturer to extend the detection limit of water as an impurity in phosphine gas to a record low range. Contamination is a serious problem in phosphine, arsine, silane, ammonia, and similar gases used in the epitaxial growth of high-purity semiconductor layers. Semiconductor device manufacturers have expressed frustration with the irreproducibility of source material purity from vendor lot to vendor lot. The issue is described by industry as primarily a measurement problem in that manufacturers frequently report impurity levels as "not detected" for a series of lots that produce very different device results. Gas purity can also be compromised at the device manufacturing site through improper line purging and purifier saturation, and these failures can only be identified with on-site measurement equipment. The critical concentrations of the impurities are not well known; however, it is believed that >10 nmol/mol oxygen or water in phosphine is undesirable. A major commercial
manufacturer of semiconductor source gas wanted to respond to customer
requests for higher purity semiconductor gases, in this case phosphine,
and arranged to partner with a company that produces an instrument based
on optical cavity ring-down for measuring water contamination in gases
to high sensitivity. They both required NIST's specialized equipment for
handling phosphine, a toxic gas, and doping it with moisture, to test
the product and acquire the necessary gas coefficient data. EEEL researcher
Kris Bertness installed the commercial instrument in NIST labs and tested
its performance with both highly purified phosphine and phosphine with
added moisture contamination. Bertness worked with the gas supplier to
acquire the necessary gas absorption spectroscopy data, including pressure
broadening coefficients, to enable the use of the instrument with phosphine
for the first time. This gas coefficient data will be published in the
open literature and programmed into the commercial instruments. The data
demonstrated a sensitivity to changes in water concentration as low as
2 nmol/mol of water. To the best of our knowledge, this is the only demonstration
of measurement for water in phosphine with sensitivity below 10 parts
per billion. Detection
and control of water vapor contamination in specialty gases increases
manufacturing yields in the compound semiconductor components industry.
They reduce costs and increase performance for high power semiconductor
lasers, light emitting diodes, and high efficiency solar cells, which
are used in health care, industrial material processing, communications,
consumer electronics, and space exploration. Contact: Kris Bertness, (303) 497-5069 |