This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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ELNES mapping is enabled by the fact that chemical bonding and short-range order in the environment of a specific chemical element induce a chemical shift of the energy-loss edge in the range between 0.1 and 8 eV and modifies the near-edge fine structure (ELNES) of the electron energy-loss spectrum. Therefore this method can differentiate between compounds of the same binary or multi-component system but with different bonding and valency.

Figure 3391a shows the Si L_2,3 EEL spectra of Si, SiO₂ and Si₃N₄. The positions of the edge onsets clearly indicate chemical shifts and differences in ELNES present. In Figure 3391a, for pure Si, the ELNES shows a first peak at 101.5 eV followed by a small plateau between 105 and 114 eV, and a very broad peak with the maximum at 135 eV. The SiO₂ ELNES has a strong peak at 108.5 eV, which is also followed by a small plateau between 112 and 116 eV. However, the second peak located at 128 eV is far less intensive than the one for Si. An additional small peak presents at 161 eV. The ELNES of Si₃N₄ is very similar to the SiO₂ ELNES. The two main peaks are located at 105 and 124.5 eV.

Figure 3391a. Si L_2,3 EEL spectra of Si, SiO₂ and Si₃N₄. [1]

In order to distinguish different compounds containing a specific element, e.g. silicon in pure Si, SiO₂ and Si₃N₄, the positions of the energy windows (in three-window method) and respective slit widths have to be chosen according to the ELNES as mentioned above. Table 3391a shows the optimized imaging filter settings in three-window EFTEM mode for Si, SiO₂ and Si₃N₄. For Si mapping in SiO₂ the post-edge window was placed above the second peak at 143 eV (See Table 3391a). The two pre-edge windows were placed in the small plateau region between the first (110 eV) and the second (125 eV) peaks. The optimal slit width was found to be 5 eV. The filter settings for Si₃N₄ were determined in a similar way by considering the shift of the Si L_2,3 spectrum towards lower energy loss energies. Therefore the pre-edge windows were placed at 107 and 110 eV. The post-edge window was placed at 114 eV in order to avoid interference with Si and SiO₂ signals. A slit width of 3 eV was used. Those settings were used to take Si images in a MOSFET structure as shown in Figure 3391b. Note that in those images, SiO₂ and Si₃N₄ can be distinguished without mapping O and N elements. It is clearly shown that the polycrystalline Si gate (a) is separated by a thin dielectric layer (b) from the single crystalline Si (c). The spacers (d) consisting of two different types of material can be seen on both sides of the gate. Between the polycrystalline gate and the single crystalline Si, the gate oxide layer is also seen clearly. The spacers consist of two layers, namely a thin SiO₂ layer and a thicker Si₃N₄ layer. Note that the bright contrast at the top of image (c) can be an artifact because of the signal of some elements overlapping the Si energy windows of SiO₂.

Table 3391a. The imaging filter settings of EFTEM for Si, SiO₂ and Si₃N₄. [1]

The valence state of Co can be indirectly determined by mapping the Si ELNES characteristics for the cases of CoSi_x (x = 1 or 2). Figure 3391c shows the as-measured and background-subtracted Si L_2,3 EELS spectra of CoSi and CoSi₂. The spectra for both silicides have Si L_2,3 edge onsets located at 100.6 eV, while the Si ELNESs are slightly different. The CoSi₂ ELNES has one broad peak with a characteristic pre-edge peak, while the CoSi ELNES does not show this pre-edge peak. Based on those characteristics, the parameters for three-window EFTEM method in Table 3391b are used. Here, in order to distinguish between CoSi and CoSi₂, the pre-edge peak of CoSi₂ was used, while the third peak of CoSi at 145 eV was used for the post-edge window. Using those settings, the Si ELNES maps of CoSi and CoSi₂ were obtained together with TEM image as shown in Figure 3391d. Note that the Ti, Co, and Si elements had overlapped layers as indicated in Figure 3391d (a). Although the Ti film also gave some artefacts in the maps, it is very important that the grains consisting of CoSi and CoSi₂ were still distinguished and the images revealed that a CoSi₂ layer was formed between CoSi and Si substrate.

Figure 3391c. The as-measured (a), and background-subtracted (b) Si L_2,3 EELS spectra of CoSi and CoSi₂. Adapted from [1]

Table 3391b. Three-windows imaging filter settings for Si L_2,3 ELNES for CoSi and CoSi₂. [1]

Figure 3391d. The TEM image (a), CoSi map (b), and CoSi₂ map (c) of Ti/CoSi/CoSi₂ layer stack. Adapted from [1].

[1] M. Worch, H.J. Engelmann, W. Blum, E. Zschech, Cross-sectional thin film characterization of Si compounds in semiconductor device structures using both elemental and ELNES mapping by EFTEM, Thin Solid Films 405 (2002) 198–204.