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Depth Profiling
Monitoring the secondary ion count rate of selected elements as a function of time leads to depth profiles. The following figure shows the raw data for a measurement of phosphorous in a silicon matrix. The sample was prepared by ion implantation of phosphorous into a silicon wafer. The analysis uses Cs+ primary ions and negative secondary ions.
To convert the time axis into depth, the SIMS analyst uses a profilometer to measure the sputter crater depth. A profilometer is a separate instrument that determines depth by dragging a stylus across the crater and noting vertical deflections. At the end of the above phosphorous depth profile, profilometry gives 0.74 um for the crater depth. Total crater depth divided by total sputter time provides the average sputter rate.
Relative sensitivity factors (RSFs) convert the vertical axis from ion counts into concentration. The appropriate RSF value for the above phosphorous implant is 1.07E23 atoms per cubic centimeter and the matrix current (IM) is 2.2E8 silicon ion counts per second. The following figure shows the above phosphorous depth profile plotted on depth and concentration axes.
