LASER SCAN MASS-SPECTROMETRY - A NOVEL METHOD FOR IMPURITY SURVEY ANALYSIS

Thin layer and bulk semiconductor materials are analysed, by raster sc an erosion of the sample surface under a focused Q-switched Nd-YAG las er beam, in the source chamber of a high resolution MS702 mass spectro meter. Interpretation of the spectra produced by the laser plasma give s a complete impurity survey of the material down to detection limits of approximately 1 part in 10(9) (mid 10(13) cm-3). Results have shown that surface impurities are effectively removed in the first scan and subsequent scans over the same area have given true measurements of i mpurities in typical materials. The method gives automatic successive erosion of sample surface areas from 0.1-130 mm2 with ionisation and m ass analysis of the sample material removed. The depth of penetration per scan is dependent on the material being analysed and the laser bea m power at the sample surface. In general it is variable between 0.3 m um and 4 mum for each scan. Most materials, including insulators, can be analysed providing they are not completely transparent to the laser light. Quantitative measurements of important dopants such as iodine and phosphorus in cadmium mercury telluride, difficult to make by othe r assessment methods, can be simply performed by laser scan mass spect rometry.