PPM analysis by using quadrupole mass spectrometers with closed ion source

Abstract

Quadrupole mass spectrometers, used for residual gas analysis are commonly provided with an ion source, where the gas pressure in the source is the same like in the quadrupole system. In this traditional role as a residual gas analyser (RGA) the mass spectrometer is used to test the quality of the residual gas in vacuum process chambers. Recent trends show mass spectrometer being used for process monitoring, for instance in sputtering processes, or for thermodesorption-measurements in the pressure range of about 1 × 10 −2 mbar. Although traditional mass spectrometers designed for residual gas analysis have adequate sensitivity to detect ppm-level contaminants in principle, interferences from process gases and background interferences from the sensor itself can make detection of the ppm-level difficult in practice. By using a closed ion source where the pressure is about two orders of magnitude higher than in the RGA this problem can be solved and ppm-levels can be detected.