Stoichiometry behavior of TaN, TaCN and TaC thin films produced by magnetron sputtering
Abstract
Thin films were synthesized in a magnetron sputtering system using a target of Ta with 99.99% purity and silicon substrates (1 1 1). The gases used for the film growth were (Ar + N2), (Ar + CH4 + N2) and (Ar + CH4) mixtures for TaN, TaCN and TaC, respectively. The substrate temperature increased from room temperature to 500 °C. The chemical composition and bonding configuration were examined using X-ray photoelectron spectroscopy (XPS), revealing Ta
N, TaCN, TaC and CC bonds. Moreover, the crystallographic structure was analyzed using X-ray diffraction (XRD), indicating the presence of (1 1 1) and (2 0 0) planes belonging to a face-centered cubic structure. The stoichiometry variation dependence on the CH4 and N2 flow was analyzed, and the influence of the substrate temperature on the coatings was investigated. Finally, scanning electron microscopy (SEM) was used to determine the evolution on the grain formation in the coatings as the substrate temperature increased.
Introduction
The modification of surface properties using thin, hard coatings as a surface engineering process has long been a proven method for improving tribological properties. Furthermore, the refractory metal carbides and nitrides are promising ceramic materials because they exhibit the unusual combination of physical and chemical properties, such as high hardness and a high melting point (typical for semiconductors with covalent bonds) as well as good electrical and thermal conductivities (typical for compounds with metallic bonds) [1]. However, the use of transition metal carbides as a protective layer is limited because of the poor adherence and likelihood of fracture that occurs when these materials are deposited on certain substrates.
An example of the transition metal carbides is tantalum carbide, TaC, which presents a NaCl type structure. This material is characterized by high hardness, a high melting point (3880 °C), great electrical conductivity (42.1 μΩ cm at 25 °C), good resistance to chemical attack and thermal shock and excellent resistance to oxidation [2], [3]. These properties make TaC very attractive for high temperature and electronic applications. However, because TaN is a hard material with low ductility, it is expected that TaN coatings would have high toughness when combined with other materials [4]. Nevertheless, few studies have been conducted concerning the production and characterization of TaCN coatings.
The composition and structure of coatings depend on N and C atomic relative concentration [5]. Coating growth is a complex process controlled by the interplay of both thermodynamic and kinetic driving forces, and the fundamental factors of the deposition process governing growth and microstructure involve deposition process parameters. The deposition temperature, the sputtering gas pressure and the gas flux have an important effect on the coating structure and stoichiometry [6]. According to Chen et al. [7] properties of TaC coatings vary due to the change of deposition parameters, such as temperature, total pressure, deposition time, the gas system (including precursor gas, carrier gas and dilute gas) and substrate. By controlling the growth condition, it is possible to control the properties of the coatings. According to the authors, the most important factor controlling the coating structure is temperature. Gas flux is the second important factor to control the coating structure and it is mainly controlled by precursor partial pressure. The stoichiometry is an important characteristic of the coatings that influences their physical and chemical properties. For instance, mechanical and tribological and corrosion resistance properties of Cr1−xAlxN have been studied depending on the Al concentration [8].
In this paper, the study and characterization of TaN, TaCN and TaC thin films grown in graded form using magnetron sputtering is presented. The stoichiometry of the coatings was analyzed using X-ray photoelectron spectroscopy, the structure was examined using X-ray diffraction, and the morphology was determined using scanning probe microscopy.
Section snippets
Experimental procedure
TaN, TaCN and TaC films were grown in a reactive DC magnetron sputtering system, as illustrated in Fig. 1 [9]. A current-regulated DC power supply was used to provide a discharge with an input power of 70 W and a target-substrate separation of 3 cm. The target diameter and thickness were 5 cm and 0.6 cm, respectively. The power density was approximately 5 W/cm2 with an input current of 170 mA. The thin films were grown on silicon substrates (1 1 1) using Ta of 99.99% of purity. Before deposition, the
Results and discussion
The XRD analysis results using the Bragg Brentano geometry of representative TaN (at 4 sccm of N2), TaCN (at 4 sccm of N2 and 4 sccm of CH4) and TaC (at sccm of CH4) films grown at 500 °C are presented in Fig. 2. In this figure, a cubic NaCl structure consisting of (1 1 1), (2 0 0) and (2 2 0) planes is observed. The TaN compound exhibits orientations in the (1 1 1) and (2 0 0) planes centered at 35.7° and 41.8°, respectively, as was reported by Tseng et al. [10]. According to the results, when carbon is
Conclusions
TaN, TaCN and TaC coatings were grown using magnetron sputtering. Using XRD, the cubic structure NaCl consisting of (1 1 1), (2 0 0) and (2 2 0) was identified. The coatings were grown by varying the methane and nitrogen flow and observing the influence of this parameter on the coating stoichiometry. As the nitrogen flow increased, the nitrogen concentration in the TaN film also increased. A similar influence on the TaC coatings was observed when the methane flow was increased. However, as the
Acknowledgments
This work was partially supported by the projects CONACyT (Mexico) 50203-F and COLCIENCIAS (Colombia) RC566-2008. The authors are grateful to D. Domínguez, E. Aparicio, I. Gradilla and J. Díaz for their technical assistance.
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