Quantitative surface analysis using electron beam SNMS: Calibrations and applications

A system of optics for electron impact post‐ionization sputtered neutral mass spectrometry (SNMS) has been designed and constructed. The design is based on an existing set of ‘Wittmaack‐box’‐type secondary ion mass spectrometry (SIMS) optics, commercially operational at UMIST. The system operates with ‘state of the art’ efficiency in the SNMS mode (a post‐ionization efficiency of 10⁻³–10⁻⁴), without significant degradation of the SIMS operation. The development stages described demonstrate many of the practical problems in the design of an electron beam post‐ionizer, involving characterization of the energy analyser used and the conditions from the optics for best secondary ion and residual gas suppression in the SNMS mode. Calibration of the SNMS mode of the system has been undertaken using standardized samples, cross‐calibrated by optical emission spectroscopy. (Typical spectra of standard stainless steel and some CuZn and CuAl alloys are presented.) These studies revealed a reproducibility of better than 10% in the relative sensitivity factors, which have been assigned to several elemental species. From these studies, a detection limit of better than 0.01 at.% was obtained for most elemental species under 10 μA Ar⁺ primary beam bombardment. The effects of surface roughness on the efficiency of sputtered neutral collection are reported. Studies of CuZn alloys have revealed effects on the absolute calibration and on data analysis of multicomponent materials where the individual components have differing sputter yields. This study also revealed SNMS to be a useful tool in the measurement of the sputter yields of composite materials. Studies of InAs_xP_1−x have been used to demonstrate the linearity of SNMS for quantitative analysis, as well as revealing the homogeneity of the layers analysed.