Tuesday, July 2, 2013

When are Standards Necessary?

The EDS spectrum shown was taken at 30 kV on a piece of GaAs wafer with a 2 µs shaping time-constant. The Ga and As Lα peaks are visible just above 1 keV while the Ga and As Kα and Kβ peaks are from 9.2 to 11.7 keV. These two sets of peaks have overlaps that can be resolved by deconvolution, and the peak sets differ by roughly 10 keV.

This is a very good test example to illustrate when standards are absolutely necessary for EDS quantitation. Ga and As were quantitated using standard ZAF corrections which account for such effects as the excitation volume, back-scattered current, absorption and fluorescence in the sample matrix. Obviously Ga:As should be 1:1 for a GaAs wafer-- and in this example we will look at Ga:As when quantitating with different combinations of these lines.

Common practice would be to quant using the Ga Kα and Kβ lines as there is less overlap with the K-lines than the L-lines. Ga Kα:As Kα yields 49.7:50.3-- which is quite close to the 1:1 expected from a GaAs wafer. If one is concerned about an accurate estimation of the As Kα intensity given the need to deconvolute the Ga Kβ peak, one can also quant Ga Kα:As Kβ-- which yields 49.3:50.7.  Both approaches are very close to the expected 1:1 value.  The error in any component is < 1%.

For the sake of exploration, consider quantitating with the low energy Lα lines. Ga Lα:As Lα is 48.6:51.4. The quant is now more than 1%, but just marginally so.

Let's consider quantitating with one high energy Kα line and one low energy Lα line. Ga Lα:As Kα is 44.7:55.3 and Ga Kα:As Lα is 53.0:47.0. In the first case the lighter element, Ga, is underestimated by 5.3% and in the later case the heavier element is underestimated by 3.0%-- but in each case the lower-energy component is the component underestimated.  We are now unable to argue convincingly that the GaAs wafer is stoichiometric 1:1.

What we find is that we can adequately quantitate without standards if the X-ray lines are close in energy.  When the lines differ by close to 10 keV-- then there are problems. Why?  Without resorting to standards we are not accounting for detector-specific losses such as absorption in the EDS detector window and the material that is cryo-pumped on its surface, as well as absorption in the body of the silicon EDS detector itself.  Such absorption is more significant at lower energies and thus we tend to underestimate those  components without resorting to standards. Another important effect is the energy dependent quantum efficiency of the EDS detector itself.

No comments:

Post a Comment