IMPROVED PRECISION IN A RESONANCE IONIZATION MASS-SPECTROMETER BY THEUSE OF STARK-SHIFTED SPECTRAL-LINES AS A PROBE FOR EXTRACTION FIELD

High-lying (Rydberg) electronic levels of sputtered Na atoms are spect rally shifted by the strong ion extracting electric field (greater-tha n-or-equal-to 12 kV/cm) present in a resonance ionization mass spectro meter (Stark effect). The Stark-shifted lines are a beneficial diagnos tic to probe the ion source's local electric field, which is related t o sample alignment and hence ion transmission efficiency. Changes in e lectric field of 1% can be detected spectrally, so the sample position is controlled to less-than-or-equal-to 10 mum by this technique. Resu lts 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 m easured by the reproducibility of either the peak of the integrated si gnals from Be-implanted GaAs, is improved to about +/-3%-at least a fo urfold improvement over the best previous results. Spatial variations in the Be-ion implant dose over a GaAs wafer may be detected because o f improved precision.