High-lying (Rydberg) electronic levels of sputtered Na atoms are spectrally shifted by the strong ion extracting electric field (≥12 kV/cm) present in a resonance ionization mass spectrometer (Stark effect). The Stark-shifted lines are a beneficial diagnostic to probe the ion source's local electric field, which is related to sample alignment and hence ion transmission efficiency. Changes in electric field of 1% can be detected spectrally, so the sample position is controlled to ≤10 μm by this technique. Results show that by the use of the Na spectrum as an alignment aid prior to sputter depth profiling, the precision of the RIMS instrument, as measured by the reproducibility of either the peak of the integrated signals from Be-implanted GaAs, is improved to about ±3%—at least a fourfold improvement over the best previous results. Spatial variations in the Be-ion implant dose over a GaAs wafer may be detected because of improved precision.
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