Some practical remarks on the i n s i t u calibration of the sensitivity factor S of a quadrupole mass spectrometer for hydrogen and argon during rf sputtering for the growth of thin a-Si:H layers

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Abstract
Calibration curves for the sensitivity factor S in a quadrupole mass spectrometer for Ar and H2 are presented. The data are gathered in a sputtering system for which the partial pressures of Ar and H2 (10−2–10−3 Torr and 10−3–10−4 Torr, respectively) are appropriate for sputtering of hydrogenated amorphous silicon (a-Si:H). A continuous sampling system added to the sputtering system enables in situ mass spectrometry before and during sputtering. The method can be used for different vacuum systems, mass spectrometers, and residual gases. It enables identification of contaminants which may have a considerable effect on the semiconductor properties. This can serve to obtain both a better quality material and to bring about more reproducible results. The relationship between the mass spectrometer ion current and the partial pressure of Ar is found to be nonlinear. However, it is possible to approximate the curve by two straight line segments, with a break point at 1.6×10−5 Torr. The pressure reduction ratios of the sampling aperture is not constant for Ar, but is a slightly decreasing function of pressure at a rate of 8.6×105 Torr−1 for an aperture with a diameter of 1.5 mm. These effects were not observed for hydrogen over the range of pressures investigated. With more than one gas present, it was observed that the introduction of argon into the vacuum system influences the ion current reading of the mass spectrometer for the other gases. This phenomenon is thought to occur as a result of a charge exchange interaction.

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