Absorption of Outgoing X-Ray in EDS Analysis
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In EDS measurements, X-rays can be generated when an incident electron beam irradiates at the surface of materials. X-ray, however, can also be absorbed by the materials themselves (especially for thick TEM samples and bulk SEM samples) during the EDS data acquisition process. This process is called self-absorption of outgoing X-ray. This process can be minimized if a thin film is used, e.g. thin samples in TEM measurement. However, for thick samples in TEM/STEM measurements and bulk samples in SEM measurement, due to this absorption effect, the measured X-ray intensities are a function of the escape depth and of the absorption characteristics of the sample. In fact, for SEM-EDS measurements on bulk materials, the absorption of X-rays is also increased with higher accelerating voltage. Especially, for the samples with higher Z and the X-rays at lower energies, this correction becomes even more important. Note that the abrupt appearance of an absorption edge in the continuum of the spectrum represents a severe deviation from linearity even though these abrupt steps are smooth in appearance due to the low energy resolution of the X-ray detector.

When the X-ray absorption is significant, especially for SEM-EDS measurements on bulk samples, the shape of the depth distribution of X-ray production is crucial, and thus ZAF and Phi–Rho–Z models should be used. However, the mass absorption coefficients are still problematic because significant uncertainties exist even in absorption-corrected ZAF and Phi–Rho–Z models.

X-ray absorption is a function of the energy of X-rays. Low energy peaks will be more strongly absorbed than high energies ones. Figure 4650a shows EDS analysis of materials consisting of Al, Pt, and O elements. At high accelerating voltage (30 keV), the spectrum shows a dominant Pt M (~2.1 keV) peak. The O K (~0.5 keV) peak is very weak due to the matrix absorption. That is, low energy O K (~0.5 keV) X-rays are absorbed more significantly than high energy Pt M X-rays, and thus the X-ray escape range is reduced sufficiently for the O K X-ray. As the accelerating voltage is reduced to middle voltage, and then to low voltage (4 keV), the absorption of O K X-rays becomes less significantly because of its shallow interaction volume.

EDS analysis of materials consisting of Al, Pt, and O elements.

Figure 4650a. EDS analysis of materials consisting of Al, Pt, and O elements, performed at accelerating voltages of: (A) 30 keV and (B) 4 keV [1].

Figure 4650b shows 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 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).

ratio of O and Si X-rays signals, taken from quartz (SiO2)

Figure 4650b. 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 [1].

In accurate EDS quantifications, the appropriate corrections such as stopping power, back-scattering, X-ray absorption and secondary X-ray fluorescence within the sample, should be evaluated and applied to the raw EDS data.

Note that the absorptions of x-rays can also affect the calibration of x-ray signals and the effects of absorption are nearly always much more serious than characteristic fluorescence for samples of similar thicknesses. For instance, Si and Fe are often used as standards, however, the absorptions of their x-rays are not equal. The error of the detected Si x-ray intensity, due to Si Kα absorption in the sample, is larger than that of Fe Kα. Furthermore, determination of the correction for X-ray absorption within the sample requires the knowledge of the sample thickness. There are some techniques which can be used to measure the sample thickness.

For thick TEM samples, k-factor correction due to X-ray absorption is needed in order to accurately quantify EDS measurements. Table 4650 lists examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.

Table 4650. Examples of limits to the thin-film approximation caused by X-ray absorption: Maximum thicknesses of thin specimens for which the absorption correction (or error) is less than ±10% and ±3%.

Material

10% error in kAB
3% error in kAB
Absorbed X-ray lines
Primary X-ray lines
Thickness (nm)
Al-7% Zn
336 94 Al Kα Al Kα (1.486 keV) and Zn Kα(8.63 keV)
CuAl2
40 12 Al Kα Al Kα (1.486 keV) and Cu Kα (8.04 keV)
CuAu
  11 Cu Kα and Au Mα Cu Kα (8.027) and Au Lα (9.628 keV)
NiAl
32 9 Al Kα Al Kα (1.486 keV) and Ni Kα (7.471 keV)
Ag2Al
33 10 Al Kα and Ag Lα Al Kα (1.486 keV) and Ag Lα (2.984 keV)
Ag3Al
31 9 Al Kα and Ag Lα Al Kα (1.486 keV) and Ag Lα (2.984 keV)
Fe–5%Ni
  89 Ni Kα Fe Kα (6.391 keV) and Ni Kα (7.461 keV)
FeP
  34 P Kα Fe Kα (6.391 keV) and P Kα (2.014 keV)
Fe3P
  22 P Kα Fe Kα (6.391 keV) and P Kα (2.014 keV)
FeS
180 50 S Kα Fe Kα (6.398 keV) and S Kα (2.307 keV)
Al2O3
113 14 Al Kα and O Kα Al Kα (1.486 keV) and O Kα (0.525 keV)
MgO
  25 Mg Kα, O Kα Mg Kα (1.253 keV) and O Kα (0.525 keV)
SiO2
167 14 Si Kα and O Kα Si Kα (1.739 keV) and O Kα (0.525 keV)
Si3N4
413 6 Si Kα and N Kα Si Kα (1.739 keV) and N Kα (0.392 keV)
SiC
13 3 Si Kα and C Kα Si Kα (1.739 keV) and C Kα (0.277 keV)

 

 

 

 

[1] Edward D. Boyes, Analytical potential of EDS at low voltages, Mikrochim. Acta 138, 225-234 (2002).

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