Elsevier

Applied Surface Science

Volume 420, 31 October 2017, Pages 681-690
Applied Surface Science

Full Length Article
Influence of the sputtering pressure on the morphological features and electrical resistivity anisotropy of nanostructured titanium films

https://doi.org/10.1016/j.apsusc.2017.05.175Get rights and content

Highlights

  • Ti films were produced by GLAD with α = 80° and increasing Pwork (0.2-1.5 Pa).

  • Obtained column angles β decrease from 40° to 0°.

  • The thermalization degree Ξ increases from 0.17 to 1.29.

  • Oxidation of the porous films occurs after a 25–300–25 °C annealing.

  • Higher ρ anisotropy is obtained for the porous films (1.6 vs. 1.25).

Abstract

Titanium films were DC sputtered with a particle flux incidence angle of 80°, using the Glancing Angle Deposition (GLAD) technique with increasing sputtering pressures from 0.2 to 1.5 Pa. This range of pressures is typically implemented for the deposition of thin films by the magnetron sputtering process. The main objective of this work was to study the anisotropic electrical resistivity behaviour of the different thin film nanostructures that were obtained. It is shown that low sputtering pressures (0.2–0.5 Pa) promote higher column angles β with respect to the substrate normal (15°  β  40°), as well as better defined porous structures. On the other hand, intermediate and high pressures (0.6-0.8 Pa) originate secondary growth effects on the columnar structures perpendicular to the substrate normal (β = 0°). No defined columns can be seen when the films are sputtered using the highest pressure (1.5 Pa). The electrical resistivity is significantly affected by the differences in the columnar microstructure. Porous films exhibit higher room temperature (RT) resistivity values (0.95–1.5 × 10−5 Ω m), when compared to the more compact ones (0.6–0.9 × 10−5 Ω m). When a temperature cycle of RT(25)-300-RT(25) °C was applied, a more significant oxidation is evidenced in the more porous structures, as well as a higher resistivity anisotropy (maximum of 1.6) than in the more compact ones (minimum of 1.25).

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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