CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Surface & Coatings Technology 205 (2011) 3698–3702 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Titanium diboride thin films produced by dc-magnetron sputtering: Structural and mechanical properties C.M.T. Sanchez, B. Rebollo Plata, M.E.H. Maia da Costa, F.L. Freire Jr. ⁎ Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, 22453-900, Rio de Janeiro, RJ, Brazil article info abstract Article history: The use of titanium diboride films as protective coatings was proposed for several applications because of its Received 6 September 2010 mechanical and tribological properties, as well as chemical and thermal stabilities. The aim of this work is to Accepted in revised form 11 January 2011 evaluate the effects of the deposition parameters on the microstructure and mechanical properties of titanium Available online 15 January 2011 diboride films. All films were deposited on silicon substrates by dc-magnetron sputtering from a stoichiometric TiB2 target in argon atmospheres. The chemical composition was determined by Rutherford Keywords: backscattering spectroscopy (RBS), while structural information was obtained by X-ray photoelectron Titanium diboride fi X-ray diffraction spectroscopy (XPS) and X-ray diffraction (XRD). The intrinsic stress of the lms was determined by measuring Hardness the change of the substrate curvature due to film deposition. Surface roughness was studied by Atomic Force Internal stress Microscopy (AFM). The film hardness and elastic modulus were determined by nanoindentation AFM measurements. The correlation between the mechanical properties with the film density is presented. The XPS internal stress reduction occurs with substantial reduction of the film hardness, and it occurs for films with low mass density. © 2011 Elsevier B.V. Open access under the Elsevier OA license. 1. Introduction cracking depending on the state of stress. Low-stress super-hard TiB2 films were obtained by controlling both the substrate temperature Titanium diboride, TiB2, is a well-known ceramic compound with and ion bombardment [17]. Films with hardness of 77 GPa and with hexagonal structure. The high hardness and Young's modulus of TiB2 low macro stress, as determined by X-ray diffraction (XRD), were as well as its chemical resistance are attributed to crystal structure obtained by argon sputtering of a TiB2 target with the substrate kept at o and atomic bonding of the compound [1]. Due to these properties, TiB2 550 C and biased with −50 V [17]. The results were explained as due films have been considered as protective coatings for several to an increase in the adatom mobility due to the energy supplied by applications, including wear and corrosion protection of magnetic both ion bombardment and heating. Enhancement of the adatom recording media and cutting tools [2,3]. Titanium diboride thin films mobility in the growing film surface was also obtained by electron were obtained by various techniques, such as reactive sputtering [4], irradiation during film growth when a positive bias was applied to the plasma enhanced CVD [5], ion beam sputtering [6], pulsed laser substrate, resulting in TiB2 films with lower stress levels [16]. Another deposition [7,8] and rf- and dc-magnetron sputtering [9–14]. Among route to limit the stress that is built-up in coatings and the crack these techniques, dc-magnetron sputtering appears to be the most initiation and propagation in ceramic films, like TiB2, is the use of a appropriate method for protective coatings applications due to the DLC (diamond-like carbon)-ceramic multilayer [18]. In this case, the low deposition temperature, the possibility of using substrates with substrate must be kept at low temperature since heating causes complicated geometries and the relatively high deposition rate deleterious effects on DLC film properties [19]. The incorporation of without the use of poison gases. The TiB2 films deposited using this nitrogen during film growth, resulting in nanocomposite TiBN technique have attracted increasing attention due to their mechanical coatings with low stress was also proposed [20]. and tribological properties [14–17]. The main purpose of this work was to analyze how some Despite intensive investigation, only a few commercial applica- deposition parameters, more specifically, the deposition pressure tions of TiB2 films exist. The main reason is the observed high and the bias applied to the substrate, modify the structural and compressive stress of the films. It has been recognized that the stress mechanical properties of TiB2 films deposited by dc-magnetron in coatings, built-up either during or after their deposition, may sputtering with the substrate kept at low temperatures. significantly affect their performance. Films can fail by buckling or 2. Experimental procedures fi ⁎ Corresponding author. The lms were deposited on Si (100) substrates by dc-magnetron E-mail address: lazaro@vdg.fis.puc-rio.br (F.L. Freire). sputtering from a 3-inch diameter TiB2 target (99.5% pure) in argon 0257-8972 © 2011 Elsevier B.V. Open access under the Elsevier OA license. doi:10.1016/j.surfcoat.2011.01.014 C.M.T. Sanchez et al. / Surface & Coatings Technology 205 (2011) 3698–3702 3699 atmosphere. Prior to the deposition, the substrates were ultrasonically probe tip with a curvature radius of 50 nm. In each test, an indentation cleaned in acetone, isopropyl alcohol and distilled water. The curve is obtained by plotting the applied force P, against the tip substrates were placed on a water-cooled copper cathode. The penetration depth h. The applied force is controllably changed with substrate temperature was kept lower than 100 °C during deposition. time following a three-segment load function while the value of h is The target-substrate distance was 18 cm. In all cases, the voltage being recorded. The applied force is first linearly increased from zero applied to the target was −350 V. Prior to deposition, the background to a maximum value in 5 s, it is kept at the maximum value for 2 s − 4 pressure was pumped down to approximately 10 Pa. Then, the TiB2 (hold time), and then it is linearly decreased down to zero in 5 s. The target was cleaned with argon rf-etching (150 W) for 30 min at a segment times were kept constant in all measurements, regardless of pressure of 1 Pa with a shield between the substrates and the TiB2 the value of the maximum applied force. To minimize the experi- target to avoid contamination of the substrates during the cleaning mental error, each test was repeated 10 times. The hardness and procedure. A residual gas analyzer model RGA100 from Stanford elastic modulus characteristics of the material were derived from the Research Systems can be connected to the chamber in order to control nanoindentation curves [24]. The drift during the nanoindentation the presence of contaminants in the deposition atmosphere. tests was measured and compensated for each test. Before an actual Two series of samples were prepared. In the first series, TiB2 films indentation, the system software holds the tip on the surface and were deposited in argon atmosphere while varying the dc voltage measures the tip displacement during 40 s. The last 20 s are used to applied to the substrate in the range between −100 V and +100 V calculate the drift rate, which is used to correct for drift in subsequent and keeping the deposition pressure fixed at 0.8 Pa. For the second displacement measurements. A fused silica sample was used to check series, TiB2 films were deposited in argon atmosphere at different the experimental procedure and the calibration of the apparatus. deposition pressures in the range between 0.15 and 2 Pa, keeping the substrate holder grounded. Within 5%, the films used in the present 3. Results and discussion study are 240-nm thick. Rutherford Backscattering Spectroscopy (RBS) was used to The chemical composition of the films was determined by RBS. The determine the composition of the films. The film density was obtained RUMP code [25] was used to simulate the spectrum and obtain the by combining the areal atomic density (atoms/cm2), provided by RBS, film composition. The simulation was done assuming a constant depth and the film thickness, determined by profilometry. The boron, profile for the elements, including oxygen and takes into account the titanium and oxygen contents were determined by RBS using a natural boron isotopic ratio. The results of the RBS analyses, together 1.8 MeV He+ beam that impinged the sample normal to the surface. with the density, are presented in Table 1. The typical absolute errors Up to this energy, the alpha particle elastic scattering by 11Bis in the atomic concentrations are 2 at.% for titanium, 7 at.% for boron essentially Rutherford. He+ beams were provided by a 4 MV Van de and 2 at.% for the oxygen concentration. As a general rule, the Graaff accelerator KN-4000 from High Voltage Engineering Corpora- compositions of the films are over-stoichiometric, within the tion. Details of RBS characterization are published elsewhere [21]. experimental sensitivity of the technique, with all samples showing The chemical environment of the elements was determined by X-ray oxygen contamination. Despite large scattering in the data, the over- photoelectron spectroscopy (XPS). The photoelectron spectra of B1s and stoichiometric character of the TiB2 films deposited by sputtering was Ti2p core levels were monitored using a VG CLAM4 hemispherical reported by other groups [15,26]. Reduction of the oxygen contam- analyzer with a Mg Kα X-ray source (hν=1253.6eV). The surface ination in the film was correlated with the etching time of TiB2 target sample was positioned at 90° with respect to the electron analyzer. A before film deposition. However, oxygen concentration lower than 1.0 keV-Ar+ beam was used for surface cleaning.