Eric Coffre

Investigations into the direct analysis of semiconductor grade gases by inductively coupled plasma mass spectrometry

Silane is the main gas used in the field of electronics to produce compounds of silicon. The direct analysis of silane by inductively coupled plasma mass spectrometry has been found to be a practical proposition for both the measurement and indentification of elemental impurities at the sub-parts per billion level. Several steps, however, need to be taken to optimise commercial instrumentation further for this task. Firstly, in order to minimise the amount of matrix material being deposited on the sampler orifice, an alloy sample cone was used which operated at a higher temperature than that of the commercially available nickel cones; additionally, the optimum carrier gas flow-rate with silane was found to be significantly lower than that required to achieve maximum sensitivity in argon alone. This too reduced sample deposition around the orifice. A further increase in sensitivity was achieved when the argon carrier gas was supplemented by the addition of hydrogen; the detection limits for 75As and 127l were 0.55 and 0.65 p.p.b., respectively, with a precision of 2–5%. In order to quantify impurities in the silane two techniques were employed. The first used the silicon matrix as an internal standard and the second involved direct comparison with a calibration graph obtained by the addition of impurities to the silane.