Surface Analysis Studies of Yield Enhancements in Secondary Ion Mass Spectrometry by Polyatomic Projectiles

In this paper we examine the mechanism of secondary ion yield enhancements previously observed for polyatomic projectiles by measuring the weight loss, volume loss, and surface composition of poly(methyl methacrylate) (PMMA) films sputtered by keV SF5 + and Ar+ projectile ions. The sputter yieldthe amount of material removed from the surface by 3.0 keV SF5 + projectileswas found to be 2.2 ± 0.8 higher than for Ar+ projectiles, measured by weight loss in the PMMA film with a quartz crystal microbalance. This result is consistent with sputter yield measurements reported here using 5.5 keV ions and stylus profilometry. Thus, the >10× enhancement in secondary ion yield in secondary ion mass spectrometry observed for polyatomic ion projectiles is not attributable to the modest ∼2× enhancements observed in the sputter yields for this molecular solid. Surface chemical measurements by X-ray photoelectron spectroscopy also indicated fundamental differences in atomic versus polyatomic sputtering mechanisms at 3.0 keV, but not at 0.7 keV. These results provide a reasonable explanation for the depth profiling capability demonstrated here on PMMA films for 5.5 keV SF5 + ions that is not possible with isoenergetic Ar+ ions.