Determination of oxygen in semiconductor materials with a cryogenically pumped spark-source mass spectrometer

A liquid helium cryogenic pump, which is mounted directly on to the ion-source chamber of the mass spectrometer, is described. Base pressures down to about 8 × 10–10 torr are rapidly obtained within 1 hour of the commencement of cryogenic pumping and pressures of about 10–8 torr are maintained during the analysis. The analytical blank for oxygen is shown to be directly dependent on the residual gas pressure in the ion-source chamber and with liquid helium cryogenic pumping the blank for the determination of oxygen in germanium is not more than 0·02 p.p.m.a. (parts per million atomic).The quantitative performance of the method has been evaluated by using germanium and silicon samples that had previously been analysed by infrared spectrophotometry. A mean standard deviation of 19 per cent. was obtained from measurements of oxygen present in silicon covering the range 11 to 35 p.p.m.a. The method has also been applied to the determination of oxygen and carbon in indium phosphide, and satisfactory agreement at the 0·1 p.p.m.a. level has been found with results independently obtained by gamma-photon activation analysis.