Kiloelectron volt cluster impacts: prospects for cluster-SIMS

Abstract

This paper describes part of our continuing efforts in examining the ability of polyatomic or ‘cluster’ projectiles to improve secondary ion yields in SIMS. Negative secondary ion yields resulting from keV bombardment of lanthanum and bismuth oxide projectiles (composition (M 2O 3) mMO +) were measured and compared for both organic and inorganic targets. Each metal oxide projectile produces an increase in secondary ion yield when compared to the same number of Cs + projectiles at similar energies. In addition, the nonlinear increase in secondary ion yield per number of constituent atoms in the primary ion is compared for polyatomic projectiles within a given metal oxide series. The high secondary ion yields combined with efficient time-of-flight mass analysis translate into prospects for SIMS under ‘superstatic’ conditions, which should be particularly attractive for the detection of organics. In considering analytical applications it must be recognized that when a projectile becomes more efficient at ejecting molecular or quasimolecular ions (intact emission), it may also become more efficient at ‘manufacturing’ ions (recombination processes). Moreover, the number of molecules destroyed per analyte-specific secondary ions detected varies with projectile characteristics. The challenge for the practice of ‘cluster-SIMS’ is to optimize the projectiles for maximizing sensitivity and eventually limit-of-detection.