A quantitative model for the interpretation of secondary ion mass spectra of dilute alloys

Abstract

A model is developed for the emission of singly-charged positive secondary ions of trace elements from metallic matrices under perpendicular ion bombardment. The model combines the sputtering theory of Sigmund with the adiabatic surface ionization model of Schroeer , and provides analytical expressions for the energy distribution and the yield of sputtered atomic particles (eqns. (7) and (8)) of a trace element, and of the fraction thereof with single positive charge (eqns. (16) and (17)). While the expressions for the ions still contain one fitting parameter (a modified form of Schroeer's “surface thickness”), this parameter falls away in expressions for the relative secondary ion yield of any two elements sputtered from the same matrix. Of the different solid phases that can be present at a metallic surface two important case are considered, viz. homogeneous solutions and pure precipitates of trace elements. The influence of the presence of the different phases on the relative ion yields and on an “average” analysis is discussed. It is also shown that in order to obtain meaningful analyses the band of emission energies which is accepted by the SIMS instrument has to be known exactly. Analytical expressions are developed which take into account this effect.