Effects of primary ion polyatomicity and kinetic energy on secondary ion yield and internal energy in sims

Abstract

The translational energy, mass, and polyatomicity of the bombarding ion affects the yield and extent of fragmentation of secondary ions in molecular secondary ion mass spectrometry (SIMS). These effects are examined using Ar + , N + 2, CO + 2, CF + 3, and SF + 5 primary ions with analytes which include ammonium, arsonium, and phosphonium salts. Increased yields, but decreased fragmentation, of secondary ions are observed as the primary ion energy is increased from 0.5 to 5.0 keV. The observed secondary ion yield also increases with the mass of the primary ion in the order N +. 2 < Ar +. < CO +. 2 < CF + 3 < SF + 5. The extent of fragmentation of the secondary ions, however, is dependent upon the polyatomicity of the bombarding ion and follows the order Ar +. < N +. 2 < CO +. 2 < CF + 3 <SF + 5 at a given energy. This latter effect may be due to the independent action of the constituent atoms of the projectile. For example, a 5-keV N +. 2 ion appears to act like two 2.5-keV N + ions during internal energy deposition, thus promoting increased fragmentation of the desorbed ions. It is shown that the degree of fragmentation is roughly proportional to the factor n/v, where n is the number of constituent atoms in the primary ion and v is the velocity of the primary ion.