Chapter/Index: Introduction | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Appendix
| Silicon dioxide (SiO2, also called quartz) is a very important material which is used, e.g. in semiconductors and MEMS. Figure 4592a shows the ratio of O and Si X-rays signals, taken from SiO2, as a function of incident electron beam energy (E0), detected with a Si(Li) ATW detector. The plot extends from a very low O/Si ratio (0.067) at 30 kV through an analysis at 5 kV which closely reflects the correct chemistry of the sample to analyses at low voltages of < 5 kV with O/Si >> 1. The final data point is 2.6 at 2.7 kV. Above 5 kV the accuracy of the analyses are limited primarily by differential absorption of the O(K) X-ray signal. At low voltage the under representation of Si reflects the reduction in the fluorescent yield with reducing overvoltage (U).
The effect on the analysis of SiO2 in terms of the measured O/Si ratio at different takeoff angles (20 to 65°) is plotted as a function of E0 in Figure 4592b. In the E0 range of 10 - 30 kV the effect is very large with the change of the takeoff angle because of the large change of X-ray absorption path, inducing significant change of absorption of low energy O X-rays. At low E0 of 5 kV, the interaction volume is small, so the change of X-ray absorption with the takeoff angle is limited; as a result, the takeoff angle has only a minor effect on the accuracy of analysis by measuring the O/Si ratio.
Figure 4592b. The O/Si ratio, obtained from SiO2, as a function of E0 [1].
[1] Edward D. Boyes, Analytical potential of EDS at low voltages, Mikrochim. Acta 138, 225-234 (2002).
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