A low background secondary ion mass spectrometer with quadrupole analyser.

A simple secondary ion mass spectrometer has been developed to allow trace analysis at impurity concentrations down to the parts per billion range. The spectrometer consists of an aperture bounded plate capacitor followed by a quadrupole mass flter with an off-axis detector. This arrangement allows suppression of sputtered neutrals and high energy sputtered or backscattered ions which otherwise produce a high background intensity of more than 1% of the target peak intensity in our set-up. After optimization of the geometry of the plate capacitor an intensity ratio of peak to background of 2 × 108 could be obtained for an aluminium specimen. The capability of the arrangement is demonstrated by trace analysis of boron in silicon. The process of background formation by neutrals and high energy ions is studied and discussed