Origin of the Chemical Enhancement of Positive Secondary Ion Yield in SIMS

Abstract

It is well known that the presence of electronegative atoms like oxygen or nitrogen on a sample surface can enhance the probability p⁺ of positive secondary ion emission in the sputtering process under noble gas ion bombardment by several orders of magnitude. There is, however, still some uncertainty about the physical origin of this large chemical enhancement. The strong increase of p⁺ could be either due to local bonding configurations upon chemisorption of electronegative gases (“bond breaking model” [1]), or, on the other hand, caused by changes in the (global) electronic band structure, as it has been shown for the enhancement of the negative ionization probability p⁻ on alkali adsorption (“resonant tunneling mechanism” [2]). In this paper, we present energy and angle-resolved static mode SIMS data from oxygenated and nitrogenated Si(100) surfaces obtained under well defined experimental conditions, which strongly favor the local bond breaking model.