The sputtering rate method described below can be used for bulk analysis and quantitative depth profiling or better Content Depth profiling (CDP).
For bulk analysis it has the advantage that it will work for a wide range of materials but the disadvantage that the sputtering rates of the calibration materials have to be measured and this is time consuming and introduces significant measurement uncertainties.
For composition depth profiling (CDP) of layered materials, we have no choice, we must use the sputtering rate method.
We do this by measuring the sputtering rates in the calibration samples and plotting ci.qM versus Ii.
We must also deal with the fact that Ri can vary. There are three ways to do this
- choose a mode of operation where the change in Ri is negligible (no such mode exits, but some come close)
- choose calibration samples similar to the unknown samples so that they have the same Ri. This is called matrix matching
- choose a mode of operation where the change in Ri is known and correct for Ri. For this approach to work long term, the correction must be done as part of the calibration.
Two examples of the sputtering rate method are shown below, for zinc coatings on steel: materials used for calibration include stainless steel (SS...), brass (CTIF...), steel (NBS...), Zn-Al (43Z...), etc.
Nickel 341.477 nm, in first order. Vertical axis: cNi.qM;
horizontal axis: INi (V)
Aluminium 396.152 nm, in second order, because of self-absorption. Vertical axis: cAl.qM; horizontal axis: IAl (V)
Some problems inherent in calibration methods can be found by going to the examples section.
First published on the web: 1 March 2000.
Authors: Richard Payling & Thomas Nelis