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).
Figure 4592a. The ratio of O and Si X-rays signals, taken from quartz (SiO2), as a function of incident electron
beam energy (E0), detected with a Si(Li) ATW detector .
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 .
 Edward D. Boyes, Analytical potential of EDS at low voltages, Mikrochim. Acta 138, 225-234 (2002).