A NEW ATMOSPHERIC-PRESSURE IONIZATION MASS-SPECTROMETER FOR THE ANALYSIS OF TRACE GAS IMPURITIES IN SILICON SOURCE GASES USED FOR SEMICONDUCTOR FABRICATION

A new atmospheric pressure ionization mass spectrometer (APIMS) was developed for the measurement of trace gas impurities in silicon source gases, such as monosilane and disilane. To prevent ionization instability caused by Si compound formation, the ion source is divided into a discharge chamber and a reaction chamber (bi-compartment). Impurity species are ionized in the reaction chamber by reactant ions which are generated by corona discharge using an inert gas in the discharge chamber. As the deposition was greatly reduced by this method, stable measurement was possible for over 60 h. Ions of SiH3OH2+ and Si2H5OH2+ were found to be produced from trace water in monosilane. The detection limit (S/N) for water in nitrogen was found to be 5 ppt, which is comparable to that obtained with our previous single-compartment ion source.