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3.2.5 Consistency of Secondary Ion Mass Spectrometry and Neutron Depth Profiling Measurements

Kevin J. Coakley

Statistical Engineering Division, ITL

George Lamaze

Analytical Chemistry Division, CSTL

David S. Simons

Surface and Microanalysis Science Division, CSTL

Neutron Depth Profiling (NDP) is a nondestructive method for analysis of the concentration profile of an element in material. Inferences about the concentration depth profile are based on the observed energy spectrum of charged particles emitted due to specific nuclear reactions. The detector response function (DRF) is a probability transition matrix which relates the depth of emission to the expected energy spectrum of the detected particles. The DRF depends on the geometries of the emitter and detector, and assumed models for the stopping power of the material, energy straggling, multiple scattering and random detector measurement error and detector calibration. In previous work, we developed a computer code to predict the DRF.

We check the consistency and validity of the NDP method as follows. The depth profile of boron in a silicon sample was measured by Secondary Ion Mass Spectrometry (SIMS). In a separate experiment, the NDP energy spectrum was measured for the same sample. Based on the measured SIMS profile and the modeled DRF, we predict the NDP energy spectrum.

We attribute the observed differences in the predicted and observed NDP spectra to imperfect knowledge in one or more of the following: stopping power of silicon, density of silicon, calibration of NDP detector, energy resolution of the detector, straggling in silicon, and calibration the SIMS instrument. Based on the current data, we can not resolve which of the factors is responsible for the discrepancy. To better explain the discrepancy, a new experimental study is underway.


\begin{figure}
\epsfig{file=/proj/sedshare/panelbk/97/data/projects/dex/sims_ndp.ps,width=6.0in}\end{figure}

Figure 15: Top: The concentration profile of boron in silicon measured by SIMS. This profile is fit with a smoothing spline. We convolve the fitted concentration profile with the modeled detector response function to get a predicted energy spectrum. Bottom: Predicted and observed NDP energy spectra.



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Date created: 7/20/2001
Last updated: 7/20/2001
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