Preliminary evaluation of an SF5+ polyatomic primary ion beam for analysis of organic thin films by secondary ion mass spectrometry.

Abstract

Organic vapor deposited thin films of pure biomolecules, polymer films and biomolecules dispersed in gelatin and biological tissue have been analyzed in a magnetic sector secondary ion mass spectrometer using an SF5+ primary ion beam at keV impact energies. In comparison to Ar+ bombardment under identical conditions, bombardment with SF5+ gives a 10 to 50 fold enhancement in the secondary ion yields for characteristic molecular ions. The SF5+ primary ion beam can be focussed to a small spot allowing molecular ion images to be obtained at micrometer spatial resolution with enhanced sensitivity. More importantly, the decay in molecular ion signal as a function of primary ion dose commonly observed in SIMS using monoatomic primary ions is either eliminated or greatly reduced, allowing molecular depth profiles to be obtained of organic thin films. By continuing to sample intact molecules as sputtering proceeds into the sample, the total number of detected characteristic secondary ions is increased by as much as a factor of approximately 700 for SF5+ bombardment as compared to Ar+ bombardment under identical analytical conditions. This effect is thought to be a result of the high erosion rate and the low penetration depth inherent in the use of a polyatomic primary projectile.