Thermal Oxidation Properties of TiNOx Materials for Solar Thermal Selective Surfaces

Air-stable high temperature solar selective surfaces have advantages on enhancing the performance of solar selective surfaces and simplifying solar thermal collector design. Many previous studies on the mechanical properties of TiN or TiAlN films indicated that the material might be a candidate for such solar energy application. This study does not support this suggestion. We found that the oxidation occurred at 400C and affected significantly the optical reflectance of the surface for solar thermal applications, resulting from replacing nitrogen with oxygen, predominately at the top layer of the surface of about 200-300A thick. Less dense TiN has lower oxidation resistance temperature. The hardness of the thermally treated films is not reduced compared with the as-deposited films. The results agree well with observations both on TiNOx solar thermal selective surfaces and on mechanical hard coatings. Introduction TiNOx has been used as solar absorbing materials for more than one decade. In the family of TiN and TiAlN associated solar selective surfaces, oxygen is easy to be incorporated into the materials during deposition unless costly equipment and procedures are applied, which is not desirable for solar energy applications. The commercially available TiNOx solar selective surfaces do not have high oxidation resistance temperature in air. At temperature of about 250 C it survives from a few hours to a few days. The degradation of solar thermal selective surfaces due to thermal oxidation effect is mainly characterized by its optical performance, and relatively much less attention was paid to other properties such as mechanical and microstructure properties. However, in literature on mechanical protection hard coatings using TiN and TiAlN materials, it has been reported that TiN or TiAlN coating could be a good candidate for oxidation resistance at temperature above 450 C [1]. It is important to understand the discrepancy in order to improve thermal oxidation resistance of solar selective surfaces. In mechanical protection applications, for example, in order to extend the working life of drilling tools, a thick coating is often required, which may be up to 10 μm. The hard coating is much thicker than that in solar selective surfaces, which may be less than 0.2 μm for the absorbing layer. The optical properties of mechanical hard coating may not be generally critical, and the working life of oxidation resistant hard coating at high temperatures such as 400 C or above may be on the order of hours or days. But the working life of oxidation resistance solar selective surfaces should be on the order of years. TiN is also widely used as a diffusion barrier thin film in semiconductor industry [2], in which the thickness required in applications is similar to the absorbing layer of solar selective surfaces. Thermal treatment after deposition of the barrier is also widely used in order to improve the barrier properties. This method involves thermal annealing in oxygen or water vapor. Oxygen is thus “stuffed” into the barrier and leads to an improvement of the barrier property. The optical and mechanical properties of the TiN or TiAlN related materials are not major concern in this application. Though above three applications of TiN or TiAlN materials are significantly different, the materials and their deposition methods are similar. In this work, various TiN and TiAlN thin films were deposited. Thermal oxidation experiment was performed in air at different temperatures. The materials, after thermal treatment in air, were measured for their oxygen content, optical and mechanical properties. Experimental TiN and TiAlN samples were deposited using the DC magnetron sputtering method. Four different samples were deposited: 1. TiN deposited at 3 mtorr nitrogen pressure at deposition rate about 100nm/min (denoted as D-TiN); 2. TiN deposited at 16mtorr nitrogen pressure with deposition rate about 100nm/min (denoted as P-TiN); 3. TiN deposited at 3mtorr nitrogen pressure with a collimated device to provide direct beam deposition at deposition rate about 50nm/min (denoted as C-TiN); 4. TiAlN sample deposited at 3mtorr nitrogen pressure and the Al to Ti atomic ratio was about 30%. The substrates used were (100) silicon wafer with films about 120nm thick. The TiN samples were deposited using a commercial system from Applied Materials-ENDURA 5500, with base pressure better than 2x10 Torr. Co-sputtering of Ti and Al targets was used to deposit TiAlN sample. Oxidation treatment was done at various temperatures and durations in air. EDX was used to analyse the oxygen contents for these films. RBS was used to measure the oxygen incorporation with 2MeV alpha ions. The hardness was assessed using an UMIS-2000 nanoindentor. A 1. 0μm diameter spherical indentor was used and the maximum penetration depth was about one third of the total TiN thickness. The contact pressure (or Meyers hardness, which is initially elastic) and modulus are calculated from the load-penetration data to using the analysis developed by Field and Swain [3]. Optical properties of the TiN and TiAlN materials before and after oxidation treatment were measured using optical spectrometry for wavelengths from 300nm to 25μm and spectroscopic ellipsometry for wavelength range between 300nm to 1000nm. Results The first set of thermal oxidation tests was to oxidise the TiN samples at 450C in air for about 5 minutes. RBS measurement showed that about a 200-250A thick layer at the top of the treated samples was converted into titanium oxide with oxygen atomic ratio to titanium about 1.3. Table 1 showed the oxygen content in the films before and after heat treatment in air obtained using RBS method for the top layer and bulk respectively. Sample Top Layer Bulk Layer D-TiN 10A thick, TiO1.3 TiN1.2O (<0.1) C-TiN 10A thick, TiO1.3 TiN1.2O (<0.1) P-TiN 10A thick, TiO1.3 TiN1.0O0.2 Heated D-TiN 200A thick, TiO1.3 TiN1.2O (<0.1) Heated C-TiN 250A thick, TiO1.3 TiN1.2O (<0.1) Heated P-TiN 200A thick, TiO1.3 TiN0.9O0. 3 Table 1: RBS oxygen content for different TiN samples before and after heating at 450C for 1 hour. In all cases, we found that the thermally treated TiN films all had significantly different colour from that of as-deposited samples. The untreated samples had oxide layer about 10A at the top of the surfaces. The sheet resistivity of the as-deposited TiN films increases in air at room temperature, and the rate of the increase depended on deposition conditions. Figure 1 shows the typical data of sheet resistivity as a function of time. The sheet resistivity of the D-TiN changed about 8% within about two weeks, while the PTiN changed over 40% during the same period. We believe this is due to the slow process of oxygen incorporation into the films through grain boundaries.