5.2 Composition Determination
5.2.5 X-Ray Analysis
the same numerical value but with opposite signs. It shows that when an electron in the M shell with a potential energy of E2 jumps down to the L shell with a potential energy of E1, the released potential energy, E2 – E1, excites one of the outer electrons in the N shell with a potential energy of E3 to eject it from the atom as an Auger electron. Auger electrons are only emitted from the specimen surface, and have very low kinetic energies, E = (E2 – E1) – E3 = (E2 – E1) + E3 . Therefore, the resulting Auger electron energy spectra can be used to identify the element and the surface information about the specimen. Today Auger electron spectroscopy is one of the most effective surface analytical techniques for determining the composition of the surface layers of a specimen in reverse engineering.
in shells, K, L, M, and N, respectively. When the vacancy is in the K shell, as shown in Figure 5.6a, the emitted X-rays are referred to as K-line X-rays in the X-ray spectra. The emitted X-ray is further distinguished as Kα X-ray if the electron is dropped from the L shell to the K shell, and Kβ X-ray when the electron falls from the M shell to the K shell. The emitted X-rays are referred to as L-line X-rays if the vacancy is located in the L shell, as illustrated in Figure 5.6b. Similarly, the L-series X-rays are further distinguished as Lα or Lβ X-rays depending on whether the electron falls from the M or N shell.
The energy dispersive X-ray analysis (EDXA), also referred to as energy dis- persive X-ray spectroscopy (EDS), is usually used in conjunction with SEM. An SEM sample has to be conductive. A nonconductive sample will accumulate the incident electrons on its surface and repel other electrons that follow. This effect is referred to as discharging and results in poor imaging. For a noncon- ductive sample, a thin layer of carbon or gold is applied. A word of caution:
Only carbon coating is suitable for EDXA. The gold coating enhances the SEM imaging, but will absorb the incident X-ray and weaken the analytical capabil- ity because gold is a heavy metal with an atomic number of 79, while carbon only has an atomic number of 6. EDXA offers a convenient nondestructive method for preliminary chemical composition determination in reverse engi- neering. The electron beam, typically with an energy of 10 to 20 keV, strikes the sample surface and stimulates X-ray emission. The energy of the X-rays emit- ted depends on the individual atomic structure of each element, and forms a characteristic X-ray histogram or profile. Figure 5.7a is the EDXA spectrum of a paint chip. It shows multiple characteristic X-ray lines of the same elements, such as Fe, Pb, Ca, and Ti, because X-rays originated from different shells have been emitted. This is a real-life example of paint analysis. The EDAX provides a quick analysis of how many layers of paint there are and the composition of each layer in reverse engineering a piece of artwork. This technology is applied for environment protection as well. If an old house was painted over with a layer of new paint, an EDAX examination over the cross section can verify if the old lead-containing paint was removed. The quantity and energy of the X-rays can be measured by an energy dispersive spectrometer, and the con- stituent elements can be semiquantitatively measured. A quantitative analysis is achievable but requires delicate calculations and comparative corrections with standards. These standards are the materials containing a known con- centration of an analyte. The primary standards are usually extremely pure and stable. They provide a reference to determine unknown concentrations or to calibrate analytical instruments. The National Institute of Standards and Technology provides a wide variety of standard reference materials for vali- dating and calibrating analytical instruments. When the atomic number of an element decreases, this element’s detectability gets progressively worse. Any element below sodium (Na) that has an atomic number of 11 in the periodic table of elements cannot be detected by standardless analysis.
Spectral resolution and detection limit are two important parameters in composition analysis. Spectral resolution is the capability of an analytical
Element OC AlSi Ca ?K ? FeTi TotalPb
? These elements are statistically insignificant.
Quantitative Results for Paint Chip Analysis: Bulk Method: Standardless Acquired 03-Jun-2009, 20.0 keV@10 eV/channel
0.00 1.28
Fe C
Counts O
Paint Chip Realtime: 206.2
Livetime: 180.0
Si Pb Al Pb
K Ca Ca Ti Ti
Fe Fe
2.56 3.84 5.12
X-Ray Energy (keV)
6.40 7.68 8.96 10.24
525 1051 1576 2102
Weight % 12.64 31.53 12.898.05 0.781.19 13.053.11 16.76 100.00
Std. Dev.
0.921.29 0.990.74 0.290.41 0.880.81 0.92
MDL3.72 0.370.76 0.481.63 1.210.99 0.610.65
Atomatic % 24.99 46.82 10.907.09 0.470.70 1.545.55 1.92
k-Ratio 0.0885 0.1137 0.0482 0.0853 0.0064 0.0103 0.0264 0.1178 0.1268
Intensities 1534.2 6518.7 5107.8 9313.7 524.5 802.7 1579.4 4345.9 7659.4
Probability 0.880.93 1.000.99 0.000.98 0.990.98 0.00 (a)
(b)
FIgurE 5.7
(a) EDXA spectrum of a paint chip. (Reprinted from SEMTech Solutions, Inc. With permission.) (b) EDXA analysis on a Mn-rich particle.
instrument to separate two test data or areas. The detection limit is the smallest quantity an analytical instrument can detect. For EDXA, the elec- tron beam is used and the spectral resolution can be as fine as just 1 micron.
For a laser beam that is commonly used in mass spectroscopy, the spectral resolution might be as large as 20 microns. However, mass spectroscopy has a detection limit usually in the range of 1 to 2 ppm, which is much better than the typical range of 50 to 100 ppm for EDXA.
An EDXA detector is used to convert X-ray energy into voltage signals, and separate the characteristic X-rays of different elements. This information is then sent to an analyzer for further analysis and display. Figure 5.7b is an EDXA on a manganese-rich particle in an aluminum alloy. The data are reported as a plot of X-ray intensity or counts on the vertical axis vs. energy, usually in keV, on the horizontal axis. Each peak corresponds to an individual characteristic X-ray from different elements, which reflect their respective identities. The height of the peak, the area under the peak, and the full width at half maximum all have their respective roles in quantitative analysis. The quantity of each element can be semiquantitatively estimated by comparing the relative peak-height ratio or the area under the peak to a standard. The peaks in wavelength dispersive X-ray spectroscopy (WDS) usually are the narrow Lorentzian distributions, and their heights are often measured to reflect the intensity for quantitative analysis. However, the peaks in EDXA are closer to Gaussian distributions, and the areas under the peak are often measured to reflect the intensity for quantitative analysis. The full width at half maximum intensity of the peak accounts for the spectral resolution.
EDXAs are subject to some limitations. For example, multiple peaks, such as Mn-Ka and Cr-Kb, might closely overlap and make it difficult to resolve them. It is also worth noting that the traditional SiLi detector used for EDXA is often protected by a beryllium (Be) window, and the absorption of the soft X-rays by beryllium could preclude the detection of elements below sodium.
The EDXA analytical capability increases in a windowless system. However, it generally cannot detect the presence of elements with an atomic number of less than 5. In other words, EDXA has difficulty detecting hydrogen, helium, lithium, and beryllium.
WDS is another technique utilized for elemental chemical analysis in reverse engineering. EDXA and WDS are usually used in conjunction with SEM, or an electron probe microanalyzer (EPMA). EPMA is a nondestructive elemental analysis technique, similar to SEM but with a more focused analy- sis area. It works by rastering a micro volume of the sample with an electron beam typical of an energy level of 5 to 30 keV. It then collects the induced X-ray photons emitted by the various elemental species and quantitatively analyzes the spectrum with precise accuracy, up to ppm. In contrast to EDXA, WDS analyzes the electron diffraction patterns based on Bragg’s law and has a much finer spectral resolution and better accuracy. WDS also avoids the problems associated with artifacts in EDXA, such as the false peaks and the background noise from the amplifiers. The noise intensity that appears in
most EDXAs partially results from the interaction between the incident elec- trons and the outermost-shell valence electrons of the sample atoms, which slows down the speed of the incident electrons and releases their kinetic energy to form the background noise. WDS is a high-quality technique com- monly used for quantitative spot analysis. EDXA shows a spectrum of ele- ments of a sample simultaneously, as illustrated in Figure 5.7a and b. WDS, however, can only read a single wavelength and analyze one element at a time. The X-ray intensity in any quantitative analysis should be corrected for the matrix effects associated with atomic number (Z), absorption (A), and flu- orescence (F), the so-called ZAF factor. In reverse engineering applications, EDXA and WDS are best used as complementary analytical tools. EDXA can be used first to scan the general chemical makeup of an unknown sample, and then WDS is applied to more accurately conduct a quantitative analysis of specific constituent elements of the sample.