Invited Paper AS-MoA6
Cluster Primary Ions: Spikes, Sputtering Yields, Secondary Ion Yields and Inter-Relationships for Secondary Molecular Ions for Static SIMS

Monday, October 15, 2007, 3:40 pm, Room 610

A framework is provided to describe the enhanced sputtering yields and secondary ion yields of molecular fragments, from molecules on substrates, achieved when using cluster primary ions. Analysis of published sputtering yield data shows that one particular model of sputtering, that includes spikes, is an excellent quantitative description of the yields for a wide range of monatomic and polyatomic primary ions. Evidence will be shown of the model validity for clusters of up to more than 10 atoms over 3 orders of magnitude in sputtering yield. Using data from one primary ion, within this model, extremely good descriptions of measurements reported with other primary ions is achieved. This theory is then used to evaluate the important molecular ion yield behaviour for static SIMS. This leads to universal dependencies for the de-protonated molecular ion yields, valid for all primary ions, both single atom and cluster, over 5 decades of emission intensity. This formulation permits the prediction of the (M-H)^- secondary ion yield for different, or new, primary ion sources, e.g. Bi_n⁺ and C₆₀⁺, for the analysis of organic materials. Optimal primary ion sources are predicted and discussed. For analysing materials, raising the molecular secondary ion yield is extremely helpful but it is the ratio of this yield to the disappearance cross section that is critical. Data are evaluated and a description is given to show how this varies with the cluster type to provide a further universal dependence. Optimal analytical conditions are discussed.