Negative cesium sputter ion source for generating cluster primary ion beams for secondary ion mass spectrometry analysis

Abstract

A cesium sputter ion source has been used to generate novel cluster and monoatomic primary ion beams for secondary ion mass spectrometry (SIMS). The source produces a variety of primary ion beam species with sufficient flux to be usable for both organic surface analysis and semiconductor depth profiling. The primary focus of this work is on the generation and use of carbon and carbon-containing cluster primary ion beams for SIMS. Stability of the sputter ion source is a few percent over 20 min, has useful lifetimes of weeks to months, and produces total primary ion beam currents for C2− ions, measured at the sample, of >1 μA at an extraction voltage of 10 kV. Larger cluster ions (Cx−x=4–10 and CsCx−x=2–8) are produced with tens of nA of beam current. Due to the divergence of the source, focused beam operation gives current densities under optimal conditions of 0.4–0.5 mA/cm2. Cluster bombardment studies of organic films using carbon clusters Cx−x=1–10 indicate that large enhancements (up to a factor of 800) in the secondary ion yield for characteristic molecular ions from organic samples can be obtained with the larger cluster ions. The signal enhancement can also be utilized in microfocus operation of the source for organic secondary ion imaging studies. For favorable organic samples, cluster bombardment with Cx−, x>6 shows little evidence of degradation of the sample from the accumulation of primary beam-induced damage. This effect can be potentially utilized for depth profiling of organic thin films and for further enhancements in sensitivity for organic SIMS analysis. Depth profiling of low energy As implants in silicon with the CsC6− primary ion demonstrates that as much as a factor of 6 improvement in apparent depth resolution can be obtained compared to profiles obtained under standard conditions using Cs+ bombardment. The flexibility of the source to produce monoatomic primary ion beams from virtually any target material is also being exploited to prepare low energy in situ ion implant standards for quantitative SIMS analysis.