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Extreme Energy Filtering

A Cameca IMS-3f secondary ion microanalyzer has been modified to allow analysis using secondary ions emitted with initial kinetic energies as high as 4500 eV. Cluster ion signals including hydrides are essentially absent when ions with initial energies <400eV are rejected, while atomic ion signals are sufficiently intense to allow trace element analysis. The useful ion yield for 31P- sputtered from a phosphorus-implanted silicon wafer is comparable under these conditions to the useful ion yield in a depth profile obtained at high mass resolution. Because the mass spectrometer is operated at low mass resolution, peak switching is possible to monitor several ion signals in the course of a single depth profile, and the energy acceptance of the mass spectrometer can also be adjusted for individual species so that interference-free ions can be sampled at low initial energy and consequently high sensitivity.

Examples of Extreme Energy Filtering

Two examples of energy spectra obtained using the EEF modification to our SIMS are shown below. The first spectrum shows the intensity of secondary Si+ ions sputtered from a silicon wafer by Ar+ from a duoplasmatron held at either 7.5, 12.5, or 17.5 kV as a function of sample voltage. Note that secondary ions with initial kinetic energies (4500V-sample voltage) up to 4500 eV can be detected if the impact energy (duoplasmtron voltage - sample voltage) is great enough. The second spectrum is of Ce ions and molecules sputtered from a silicate glass containing >3 wt.% Ce. Note that CeO and CeH ions are effectively eliminated from the mass spectrum at secondary ion energies approaching 400 eV, while the elemental ion signal is still present. The drop in sensitivity from conventional energy filtering (~75 eV ions) to extreme energy filtering (400 eV ions) is approximately a factor of 100.

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