Full Length ArticleInfluence of the sputtering pressure on the morphological features and electrical resistivity anisotropy of nanostructured titanium films
Graphical abstract
Introduction
Titanium-based thin films are still among the most actively studied materials due to their excellent biocompatibility, their thermal, electrical, chemical and mechanical properties, together with good wear and corrosion resistance [1], [2], [3]. These attractive multifunctional properties can be further improved and tailored by nanostructuring the thin film’s columnar features [4], [5]. Nanostructured materials are emerging as potential candidates to be used as multifunctional materials in applications such as nanoelectronics, biomaterials and biosensors [6]. In recent years, considerable research has been focused on the design of nanoscale materials based on metals and semiconductors with controlled morphology in order to change their physical properties [4], [5], [6].
Traditionally, thin films fabricated using the Physical Vapour Deposition (PVD) technique are deposited with a perpendicular incidence of the particle flux to promote a typical columnar growth perpendicular to the substrate. The microstructure of the films is strongly dependent on the experimental parameters selected for their fabrication [7], which affects significantly their physical characteristics and in-service performance [4], [8], [9], [10], [11]. By finely controlling the processing parameters, different nanostructures can be created. For that purpose, the glancing angle deposition (GLAD) technique [12] can be used to change the typical columnar growth in order to obtain different 2D or 3D nanostructures, such as inclined columns, zigzags or spirals, among others [5], [13], [14], [15]. The preparation of thin films under oblique, fixed or mobile substrate has been successfully applied to numerous materials including metals, alloys, oxides and fluorides, among others [14], [16]. The application of the obtained novel microstructures in the field of photonics, mechanics or sensors, easily explains the growing interest of the GLAD approach.
The elaboration of these novel thin film architectures mainly depend on the materials used in the deposition process, the particle incident angle and the substrate rotation. However, few studies report on the relation between the sputtering pressure conditions used during the GLAD process and the obtained morphologies [7], [17], [18] as well as on the anisotropic behaviour of the resulting properties [19], [20], [21], namely the electrical resistance [22]. The understanding of these anisotropic electrical properties is in turn critical for the functional use of these materials in pressure and temperature sensors based on thin films [4], [5].
Therefore, the present work studies the influence of the sputtering pressure conditions on the obtained microstructures and, consequently, on the anisotropy of the electrical properties of pure titanium (Ti) films sputter-deposited by GLAD. For that purpose, a set of Ti films was sputtered with a fixed particle incidence angle α = 80° and increasing sputtering pressures. The coatings were then characterized in terms of their morphological and structural properties. Afterwards, the method proposed by Bierwagen et al. [23], which makes use of the van der Pauw configuration [24], was used to obtain the anisotropic electronic transport properties of the Ti thin films during an annealing treatment in air.
Section snippets
Experimental details
Titanium thin films were deposited by DC magnetron sputtering from a titanium target (51 mm diameter and 99.9 at.% purity), using a custom-made vacuum chamber. The 40 L sputtering chamber was equipped with a turbo-molecular pump backed by a primary pump, allowing a constant residual vacuum of approximately 10−6 Pa in all depositions. The Ti target, located at 65 mm from the substrate holder (Fig. 1 a), was sputtered with a constant argon flow rate of 5 sccm. The working pressure was changed from a
Morphological and structural features
The Ti films were deposited with increasing working (argon) pressures and using a fixed particle incidence angle α = 80°. The cross-section and top (plane) view of the coatings obtained in the low-pressure (0.2–0.5 Pa) and high-pressure (0.6–1.5 Pa) ranges are presented in Fig. 2, Fig. 3, respectively.
In an overall analysis, it is observed that the pressure variation gives rise to significant changes on the morphology, growth rate, and column angle of the prepared Ti films, Fig. 4. The films
Conclusion
The effect of increasing sputtering pressures on the morphology, microstructure and electrical evolution of Ti thin films deposited with α = 80° was studied. Significant thermalization effects occur with increasing Pwork, leading to the formation a less energetic plasma environment. Consequently, the particle incidence gradually loses its directionality and thus the resulting films exhibit less inclined (45° ≥ β ≥ 0° for 0.2 ≤ Pwork ≤ 0.8 Pa) and less porous columnar features. For the highest pressure and
Acknowledgements
This work was supported by the Region of Franche-Comté, the French RENATECH network and performed in cooperation with the Labex ACTION program (contract ANR-11-LABX-01-01). Funding was also provided by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013 and project PTDC/EEI-SII/5582/2014. Armando Ferreira acknowledges the FCT for the SFRH/BPD/102402/2014 grant. The authors thank financial support from the Basque Government
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