Quantitative analysis and depth profiling of rare gases in solids by secondary-ion mass spectrometry: Detection of (CsR)+ molecular ions (R=rare gas)

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Abstract
A new secondary-ion mass spectrometry technique is described for the quantitative analysis and depth profiling of rare gases incorporated in thin films and bulk solids. A Cs+ primary-ion beam incident on the sample surface was used to produce sputtered (CsR)+ (R=rare gas) molecular ions whose yield was proportional to the rare-gas concentration in the analyzed region of the sample. Depth profiles have been measured for Ar and Kr implants in single-crystal Si, Ge, and GaAs wafers and a Kr implant in polycrystalline Ni. Detection limits ranged from ∼4×1017 cm−3 for Ar in Ge to ∼1019 cm−3 for Kr in GaAs where the sensitivity was decreased due to interference with (Ga2As)+ ions at mass 217. The Cs+ primary-ion current density used in these experiments was 6 mA cm−2. It was not possible to obtain sufficiently high Cs+ current densities in our system to observe R+ secondary ions, although they were detectable using an O+2 primary ion beam incident at 35–40 mA cm−2. The high current densities required in the O+2 experiments, however, resulted in a correspondingly high sputter etching rate and hence a decrease in the depth resolution. Measurements of secondary-ion intensities as a function of primary-ion current density and secondary-ion energy distributions showed that (CsR)+ molecular ions were formed by surface-ionization processes, while R+ secondary ions were formed primarily by inelastic gas-phase collisions involving rare-gas atoms released from the samples with thermal velocities.

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