Distance pm

Fig. 13 Oxide layer at the interface between an (Al,Ti)N coating and a high-speed steel substrate (Fe), disclosed by using crater edge profiling with scanning Auger microscopy. Source: Ref 54

The chemical shift in XPS can be used to disclose different chemical bonding states at interfaces. For example, the two phases ZrO2 and ZrN were found at the interface ZrN-Fe of a ZrN coating on a high-speed steel substrate (Ref 54). Another example is that different SiOx suboxides in the interface of ultrathin SiO2 layers on silicon were disclosed by high-resolution XPS (Ref 56).

Reactions at interfaces can be directly studied by AES and XPS depth profiling. An example is shown in Fig. 14 for a thin-film structure of Si-Ni-Si (each layer about 50 nm thick) sputter deposited on a Si(111) substrate (Ref 57). Figure 14(a) shows the Auger depth profile for the as-deposited sample. Although the apparent interface width increases with depth, the small peak of oxygen at a sputtering time of about 9 min indicates the native SiO2 layer of about 2 nm thickness on the silicon substrate. The shape of the profile after thermal treatment, with a linear temperature increase up to 205 °C at a heating rate of 40 °C/min, shows that mainly nickel was migrating into the adjacent silicon layers and that mainly a Si2Ni phase was formed, which was confirmed by selected area diffraction in TEM (Ref 57). Evaluating the increase of the measured interface width, Az = z (0.84 I0) - z (0.16 • I0), with increasing temperature, T, and time, t, Az(T) -Az(T0) » 2 • (D • t)12, allows determination of the interdiffusion constant, in this case D = D0 • exp(-Q/kT) with D0 = 2.5 • 10-2 m2/s and the activation energy Q = 0.5 eV (Ref 57). The error in this evaluation is mainly due to a change of the sputtering rate with composition (about +50% from pure nickel to pure silicon).

Si/Al/Si

Sputter Time (min.)

Sputter Time (min.)

Sputter Time (min.)

Fig. 14 Sandwich layer of 30 nm aluminum between two silicon layers, deposited on a silicon substrate, studied with Auger electron spectroscopy depth profiling. (a) As-deposited. (b) After heating to 200 °C. (c) After heating to 550 °C. Source: Ref 57

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