Probing the Chemical Nature of TiN

A full concentration profile for the constituent elements of the TiN/SiO₂ layers on a Si substrate is shown in Figure 2. It is immediately apparent from the concentration profile that the TiN shows a sharp boundary with the SiO₂ substrate at 40nm The second interface is observed between SiO₂ and Si at 150nm, with no degradation in the interface resolution.

Figure 2: Concentration depth profile for TiN/SiO2 thin films on a Si substrate.

The great advantage of XPS over other techniques is that chemical state information is also available from the spectra, allowing a chemical state concentration depth profile for the elements to be generated. Changes in the surface layer as a function of depth from the original surface are easily observed. Figure 3(a) shows the Ti 2p region after introduction of the sample into AMICUS, with Figure 3(b) showing the same region after a single sputter cycle. The Ti 2p peak is spin-orbit split into two peaks, separated by approximately 6eV, with each of these peaks showing further structure, reflecting different Ti chemical states. The Ti 2p peaks are deconvoluted into three component peaks corresponding to Ti in titanium monoxide (TiO), titanium nitride (TiN) and titanium dioxide (TiO₂), with a shift to higher binding energy relative to metallic Ti 2p quoted in the literature at 454eV binding energy.

Figure 3: Ti 2p spectra from TiN showing (a) sample surface as introduced into AMICUS, and (b) after first sputter cycle.

The X-ray photoelectron spectrum of the uppermost TiN layer shows that Ti is in fact predominantly present as TiO₂ and, as shown in the concentration depth profile, the titanium oxide species persist throughout the TiN film. The oxidation of titanium will decrease the hardness of the thin film and suggests that the deposition of the TiN film should be performed in a controlled, inert atmosphere.