Angular dependence of ion yields and cesium surface coverage in Cs+ attachment secondary ion mass spectrometry (CsAMS)

Abstract

The yields of Cs+, Cs2+, Cs3+ and MCs+ secondary ions emitted from eight different elemental and compound targets (M) under 6 keV Cs+ bombardment have been measured as a function of the impact angle ϑ, from 0° (normal incidence) to 75°. For targets featuring a high sputtering yield (like Pt) the stationary Cs+ and MCs+ yields are independent of ϑ (except for a small ϑ-dependence of instrument transmission). Moreover, the Cs+ yield is maximized. The Cs2+ and Cs3+ yields slightly decrease as ϑ is varied from normal to glancing incidence. In the absence of cesium agglomeration the intensity ratio R21 = I(Cs2+)I(Cs+) appears to be proportional to the Cs coverage NCs. The Cs+ and MCs+ yields for Al, Si and Ti increase strongly with increasing ϑ (up to a factor of 2000). Concurrently the R21 ratio for Si (as well as for Ge and GaAs) decreases monotonically by up a factor of 50. Comparison of the results with literature data for vapor deposited Cs suggests that at normal beam incidence the stationary coverage NCs(Si, 6 keV) may be as high as 4 × 1014atomscm2 but decreases with increasing ϑ to 8 × 1012atomscm2 at 75°. The R21 ratio for the metals Al, Ti, Cu and Zn exhibits a maximum at angles between 25° and 45°. This anomalous behaviour is attributed to the polycrystalline structure of the samples. The absence of a ϑ-dependence of the PtCs+Cs+ intensity ratio appears to indicate that for total sputtering yields y in excess of about 10 atomsion the MCs+ yield might become independent of Y. This finding needs to be incorporated in models for quantitative analysis based on the detection of MCs+ secondary ions.