Study of the structure of titanium thin films deposited with a vacuum arc as a function of the thickness
Introduction
Polycrystalline titanium thin films have been widely employed as interlayer between the substrate and different coatings, on metallic, semiconductor or ceramic substrates and for metallic, metallic compounds or carbon coatings. The Ti interlayer has been included in order to improve adhesion strength, corrosion resistance and wear performance, as well as to promote the growth of crystalline phases of the coating [1], [2], [3]. In some works the influence of the Ti interlayer thickness on the properties of multilayer systems has been analyzed, the thickness of the Ti layer being relevant on the behavior of the coatings. However, very few studies about the dependence of the characteristics of Ti films on the thickness have been carried out.
At ambient condition, Ti is in the hexagonal-close-packed (hcp) crystal structure, which is referred to as α phase. This structure transforms to a body-centered-cubic (bcc) structure, called β phase, when the temperature is higher than 1155 K. At room temperature, the α phase transforms to the hexagonal ω phase (three atoms per unit cell) when the pressure is increased between 2 and 9 GPa. Other two high pressure phases, γ-Ti (distorted hcp) and δ-Ti (distorted bcc), have been found. At room temperature, the phase transition from ω-Ti to γ-Ti occurs at 116 GPa, and into δ phase at 140 GPa [4].
The face centered cubic (fcc) Ti phase has been only found in thin films and in high energy milled powder. Wawner Jr. and Lawless were the first to report the fcc Ti phase in Ti films epitaxially grown on NaCl single crystals [5]. Afterwards, some investigations about epitaxial growth of Ti thin films on metallic substrates and on semiconductors have been published (e.g. Al [6], [7] and SiC [8]). In all cases, at the onset of the growth the Ti atomic structure presented the fcc phase, which seemed to perfectly match the substrate lattice in the directions parallel to the surface plane. At a critical thickness the hcp phase appeared, this being the predominant phase for thicker films. The reported critical thickness values varied from 1 to 20 nm for epitaxial growth. Beyond this thickness, axial alignment with the substrate was only partially preserved, and off-normal alignment was lost. The disorder in the film at coverage larger than the critical thickness was associated with the formation of misfit dislocations or the relaxation to the hcp phase of Ti [6]. More recently, a study of the dependence of the crystalline structure on the thickness in polycrystalline Ti films deposited on Si substrates by DC magnetron sputtering has been presented [9]. The authors analyzed the structure, textures and stresses of Ti films with thickness in the range of 140–720 nm. In that work, from the deconvolution of Ti X ray diffraction peaks the fcc and hcp phases were detected in the thinnest films, while for films with a thickness larger than 500 nm only the hcp phase was identified. They suggested that fcc Ti is locally stable in highly stressed hcp Ti matrix at relatively low thin film thicknesses.
In view of the lack of information about the crystal structure of polycrystalline Ti films deposited with vacuum arcs on monocrystalline silicon wafers (100) as a function of the film thickness, a systematic study on this subject was carried out. The structure, the morphology and the residual stresses of the films were studied. Characteristics of films deposited in one and two steps were compared.
Section snippets
Experimental details
The system employed to produce the cathodic arc discharge had a high power DC source (model ARCC 18KW 150 A, ALTATEC) connected to a titanium cathode and to a grounded annular copper anode. In Fig. 1 a schematic view of the complete device is shown. The cathode was a cylinder of 55 mm in diameter and 40 mm thick, the anode had an internal diameter of 80 mm and was electrically insulated from the cathode and the vacuum chamber. The vacuum chamber consisted of a stainless steel cylinder (50 cm long,
Results
Fig. 2 corresponds to a typical image of the depth profile obtained by SEM for a film grown in a single discharge. The films displayed a compact columnar structure, with a column width of ~ 100 nm. The thickness of all the films was measured from images similar to that of Fig. 2, the growth rate resulting in (3.8 ± 0.3) nm/s. EDS compositional analysis indicated a minimum contamination of oxygen and carbon, and a titanium content higher than 80%.
The mass density of the films was estimated from the
Discussion
Based on XRD results and regarding the structure of the fcc and hcp lattices, the phase transformation could be analyzed. Fig. 6a shows the (111) surface of fcc-titanium with the corresponding interatomic distance obtained from the experimental interplanar spacing. The titanium atoms are represented with their corresponding metallic radius. The (111) fcc surface is equivalent to the (002) surface of α-titanium, depicted in Fig. 6b, except for the difference in stacking associated to the fcc and
Conclusions
The crystal structure of polycrystalline titanium films deposited with a vacuum arc discharge on monocrystalline silicon wafers (100) was found to depend on the film thickness. The presence of the fcc phase of titanium was observed for the thinnest films. The critical thickness was estimated in 300 nm, a much larger value than those reported for other deposition processes. For larger thicknesses, the films grew as α-titanium with a preferred orientation in the [100] direction. The obtained
Acknowledgments
This work was supported by grants from Universidad de Buenos Aires (PID 20020110100161) and CONICET (PIP 11220120100468CO). XRR measurements were performed at LNLS (under proposal XRD1-13700 - (XRD2)).
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