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September, 2002 EEEL Researchers Measure Low-level Water Impurities in Phosphine |
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Kris Bertness and Susan Lehman of EEEL's Optoelectronics Division, in collaboration with Joseph Hodges of CSTL, have recently demonstrated the ability to measure extremely small concentrations of water as an impurity in phosphine gas, using cavity ring-down spectroscopy (CRDS). This is the first time that CRDS has been applied to impurity measurements in the highly toxic gases used in the growth of compound semiconductors. The resolution of the NIST technique is currently 20 nmol/mol of water in nitrogen, comparable to the best results from other techniques but with much simpler calibration and instrument preparation.
Water contamination in phosphine, arsine, silane, ammonia, and other gases, has been widely identified as a major problem facing the compound semiconductor industry. Levels of water contamination that have been below the threshold of detectability seriously degrade the quality and yield of materials. Manufacturers have thus resorted to “test growths” to evaluate water impurity levels.
The CRDS technique is based on the measurement of the decay time of laser light tuned to resonate with an impurity absorption line and a high-finesse cavity. It offers high sensitivity and high accuracy without the necessity to calibrate with gas mixtures or to control water vapor concentrations outside the active gas cell. The NIST system is placed in the phosphine line of a gas-source molecular beam epitaxy growth system, permitting the gas to be tested as it enters the growth chamber. Cylinders of phosphine from commercial vendors were found to contain water impurity levels of several mmol/mol.
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