XPS chemical state analysis of sputter depth profiling measurements for annealed TiAl‐SiO2 and TiAl‐W layer stacks

For the application of surface acoustic wave sensors at high temperatures, both a high‐temperature stable piezoelectric substrate and a suitable metallization for the electrodes are needed. Our current attempt is to use TiAl thin films as metallization because this material is also known to be high temperature stable. In this study, Ti/Al multilayers and Ti‐Al alloy layers were prepared in combination with an SiO2 cover layer or a W barrier layer at the interface to the substrate (thermally oxidized Si or Ca3TaGa3Si2O14) as an oxidation protection. To form the high‐temperature stable γ‐TiAl phase and to test the thermal stability of the layer systems, thermal treatments were done in vacuum at several temperatures. We used X‐ray photoelectron spectroscopy (XPS) sputter depth‐profiling to investigate the film composition and oxidation behavior. In this paper, we demonstrate how the semiautomatic peak fitting can help to extract beside the elemental information also the chemical information from the measured depth profiles.

For the application of surface acoustic wave sensors at high temperatures, both a high-temperature stable piezoelectric substrate and a suitable metallization for the electrodes are needed. Our current attempt is to use TiAl thin films as metallization because this material is also known to be high temperature stable. In this study, Ti/Al multilayers and Ti-Al alloy layers were prepared in combination with an SiO 2 cover layer or a W barrier layer at the interface to the substrate (thermally oxidized Si or Ca 3 TaGa 3 Si 2 O 14 ) as an oxidation protection. To form the high-temperature stable γ-TiAl phase and to test the thermal stability of the layer systems, thermal treatments were done in vacuum at several temperatures. We used X-ray photoelectron spectroscopy (XPS) sputter depth-profiling to investigate the film composition and oxidation behavior. In this paper, we demonstrate how the semiautomatic peak fitting can help to extract beside the elemental information also the chemical information from the measured depth profiles.

K E Y W O R D S
CTGS, depth-profiling, peak fit, SAW, surface acoustic wave, TiAl, XPS

| INTRODUCTION
High-temperature sensors based on the surface acoustic wave (SAW) principle are of high interest for automatization and process control in many fields. One general demand for such devices is to find material systems for the metallization which are compatible with the complex piezoelectric substrates and withstand the high application temperatures. One approach we discussed earlier is the use of Ru-Al layers and appropriate barrier systems. [1][2][3][4] Currently, we concentrate on the Ti-Al 5-7 system known from aerospace applications. The next challenge is to find barrier layers for the protection of the metallization from both oxidation with the surrounding atmosphere and chemical reaction with the substrate at the high temperatures. Sputter depth profiling investigations with electron spectroscopic methods are an effective tool to investigate material changes in such layer structures. In a previous study, 7 we dealt with Auger electron and X-ray photoelectron spectroscopy measurements (AES, XPS) to analyze the Ti-Al system with AlN barriers, also already using peak fit during XPS depth profiling to separate the Ti and N signal. The main conclusion was that the AlN barriers were stable up to 600 C (high vacuum); however, a partial preferential oxidation of Al was already observed.
In this paper, we study two film systems: A Ti/Al multilayer system with SiO 2 cover layer on thermally oxidized Si substrates and a Ti-Al alloy film with a W barrier layer between the film and the Keeping the possible influence of an ion beam damage in mind, from analyzing the characteristic peak shape changes the phase stability or degradation of the film was followed. A detailed discussion of the high temperature behavior of the Ti-Al thin film system is presented in a separate paper, 6 which includes also a large series of Auger electron depth profiles for different annealing procedures.
In contrast to this, the main focus of the present paper is to demonstrate the possibilities of the peak shape analysis for the Ti-Al material system under investigation by peak fitting of spectra series from XPS depth profiling, because this methodology is not used regularly. Such studies mostly are focused on the investigation of damages from the ion sputtering effects. 8,9 In this context, also the reduced damaging of material by use of alternative ion sources (C 60 , Ar-cluster) is in focus. 9,10 Another attempt for chemical shift identification in spectral series from XPS depth profiling would be the use of multivariate methods like factor analysis applied by us earlier. 11 The samples were annealed under high vacuum for 10 h at up to 800 C. The heating rate was 6.5 K min −1 .
The XPS investigations were performed with a PHI 5600 CI (Physical Electronics) system using nonmonochromatic Mg-Kα X-rays (400 W) for excitation. The spectra were recorded with a hemispherical electron analyzer working at a pass energy of 29 eV. For depth profiling Ar + ions with an energy of 3.5 keV scanned over an area of 2 mm × 2 mm were used. The effective sputtering rates were  For the semiautomatic peak fit of the spectra from the depth profile measurements, also the PHI-Multipak software 14 was used.
The peak fits were done after the background subtraction (Shirley type) by using the Gauss-Lorentz-peak shape. As discussed later using the example of the results shown in Figure 1 individual constraints for the peak shape/position were defined so that the fitting of the whole depth profile data was then possible without further efforts.  15 The reduction of SiO 2 in contact with Al was observed also by other authors (e.g., Dadabhai et al. 16 and references therein). Using the chemical information for the peak fit of the spectra during the depth profile, one can observe this formation of elemental Si on both top and bottom interfaces. For the semiautomatic peak fit, the peak positions and the full-width-half-maximum of the Al and Si species were determined for selected spectra from the depth profile, where the signal-to-noise ratio of the measured data allows a reliable calculation-as in Figure 1C for Al2p. These values then were fixed for the fit procedure, which leads to low a scattering in the data for the regions in the depth profile with no well-defined spectra.

| Ti/Al-multilayers with SiO 2 barrier
At the bottom interface between the bottommost Ti layer and the SiO 2 of the substrate, no Ti oxidation was observed despite the presence of elemental Si. It is known that Ti is able to reduce SiO 2 , the residual O is then solved within the Ti layer.
In Figure 2

| SUMMARY
It is demonstrated that XPS sputter depth profiling measurements in combination with semiautomatic peak fits are a powerful tool to study chemical reactions in layer structures and to help to overcome distortions by peak overlap in multielement samples. In this work, the semiautomatic peak fitting was applied in the first part to separate the oxidized and elemental state of Al and Si and with this to determine the respective content in the sample. In the second example it was used to distinguish between W and Ti. The analyses reveal that in the Ti-Al system studied for an application as a high-temperature metallization in SAW devices the preferential oxidation of Al is the main damaging effect. Ti is initially not taking part in the oxidation. Furthermore, it was not possible to investigate the interface between the W layer and the insulating CTGS substrate due to the extremely strong and instable charging which makes measurements in general very challenging.

ACKNOWLEDGEMENT
The work was supported by German Federal Ministry of Education and Research under grant InnoProfile-Transfer 03IPT610Y.