EDS Measurement of Copper (Cu) and its Artifacts
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The values of characteristic X-ray energies of copper (Cu) can be obtained, e.g. for Cu K, by subtracting the energy of the L shell (binding) energy from that of the K shell (binding) energy as shown in Figure 4479a.

Extraction of values of characteristic X-ray energies of Cu

Figure 4479a. Extraction of values of characteristic X-ray energies of Cu.

As examples, according to Figure 4479a, we can calculate,
                ΔEKα1 = 8979 eV - 933 eV = 8.046 keV
                ΔEKα2 = 8979 eV - 952 eV = 8.027 keV

Figure 4479b shows a typical EDS profile of pure copper. The natural peak widths are much narrower than the measured ones in the figure. A noise peak is also shown at very low energies.

Typical EDS profile of pure copper

Figure 4479b. Typical EDS profile of pure copper. Here, Lα = 0.93 keV, Kα = 8.04 keV, and Kβ = 8.91 keV.

Cu grids are widely used as supporting grids in FIB (focused ion beam) prepared TEM specimens. However, artificial Cu signals can be induced due to the Cu grid itself, especially when Cu interconnect structures are analyzed in semiconductor industry. The origin of the artificial Cu signal is mainly electron scattering induced by the analyzing materials (see Figure 4479c). Therefore, the artificial Cu signal is higher when heavier elements, e.g. W and Pt, are measured because of their stronger scattering. On the other hand, the stray radiations originated from the analyzing heavy elements can also enhance artificial Cu signal. Other grids, such as nickel (Ni) and molybdenum (Mo) grids, can be used to avoid the artificial Cu signal but those grids induce their own artificial EDS signals. For this reason, depending on the interest, proper grids are usually selected for FIB-TEM specimen preparation. On the other hand, in practice, Cu X-rays are also generated from SEM/TEM specimen chamber, column and apertures by Rutherford backscattered electrons because these parts of the SEM/TEM systems contain such element. These spurious X-rays can often be detected by the EDS detector. EDS-TEM specimen holders and stray aperture are normally used to reduce such spurious X-rays.

Cu Grid for FIB-TEM
Electron beam at heavier elements
Electron beam at light elements
(a)
(b)
(c)
Figure 4479c. (a) Copper (Cu) grid for FIB-TEM specimen preparation, (b) Electron beam at heavier elements, and (c) Electron beam at light elements. The scattered electrons generate X-rays from the Cu grid. For heavier analyzing elements, more electrons are scattered, and thus more artificial Cu X-rays are generated from the Cu grid.

The EDS spectrum in Figure 4479d was taken from a nanoparticle (with Pt and Pd compositions of 50 at.% : 50 at.%) on holey carbon (C) film supported by a copper TEM grid. The copper signal in the spectrum was from the Cu grid (see page4638).

EDS spectrum taken from a nanoparticle with Pt and Pd compositions

Figure 4479d. EDS spectrum taken from a nanoparticle with Pt and Pd compositions.

As discussed on page4650, X-ray absorption is a function of the energy of X-rays. Low energy peaks will be more strongly absorbed than high energies ones. For thick TEM samples, k-factor correction due to X-ray absorption is needed in order to accurately quantify EDS measurements. Table 4479 lists Cu-examples of thicknesses at which the thin-film approximation is no longer valid due to X-ray absorption effects in specific materials.

Table 4479. Cu-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)
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)

 

 

 

 

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