Overview

Thin coatings are increasingly being used in many industries to improve surface properties of materials, such as increased resistance to friction and wear or increasing the hardness and strength. Hard coatings like TiN are particularly useful in the coating of machine tools in extending the working life of the tool, where the properties of the thin film depend on the thickness and the adhesion between the coating and the substrate. This information can be gained using X-ray photoelectron spectroscopy (XPS) which provides qualitative and quantitative information on the elements present in complex materials and whilst providing chemical state information from these elements.

The Use of AMICUS

AMICUS, with its high energy X-ray source, efficient electron transfer lens, electron energy analyser and high speed ion etching, is ideally suited to elemental depth profiling. The AMICUS instrument was used to determine the depth dependent concentration profile through a thin TiN/SiO₂ double layer film. To ensure good interface resolution between layers the X-ray photoelectron spectra were recorded from a small, 3mm² sample which was rotated during sputtering to ensure an even etch crater. Figure 1 shows the data acquisition manager of the fully automated VISION control software. Complex data collection procedures are performed by means of a simple flowchart which may be saved and recalled for future analysis of similar samples. The use of the integrated VISION control software means that the analysis/sputter routine may be left to generate a complete concentration depth profile dataset unsupervised.

Figure 1: VISION data collection window.

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.

Similar chemical state information is also contained in the Si 2p spectra shown in Figure 4, where the spectra are shown as a function of depth from the surface. At the surface, where the TiN overlayer is expected to completely attenuate the Si 2p signal it is noted that there is still some signal at 99eV binding energy. This is thought to be due to the Si substrate exposed during sample preparation, as it remains constant through the TiN and SiO₂ layers. After removal of the TiN overlayer, the Si 2p signal from the SiO2 layer is observed at 103.5eV binding energy. Further removal of surface material exposed the Si substrate, characterized by the elemental Si 2p photoemission peak at 99.3eV.

Figure 4: Si 2p spectra as a function of depth from the surface.

Summary

X-ray photoelectron spectroscopy using the AMICUS instrument is an invaluable tool for the characterization of thin film materials. The fully automated operation using VISION control software allows repetitive depth profiling to be completed with ease.
The key features of AMICUS demonstrated are:
•     Concentration depth profiling,
•     Unsupervised, fully computer controlled data collection using VISION software,
•     Chemical state determination of surface atoms.