Nanocomposite tantalum–carbon-based films deposited by femtosecond pulsed laser ablation
Introduction
The most recent research activities on diamond-like carbon (DLC) films are devoted to improve some of their most critical properties, including both mechanical properties (adhesion, stress, hardness, Young modulus, friction and wear) and other functional properties towards applications in microelectronics or optics. For this purpose, various elements, including Si, N, F, B, various metals and more recently Ge, P and I, are now introduced in the carbon network, thus leading to doped and alloyed DLC films and labeled a-C:X (see [1] and [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] related to published works from 2002). An increasing use of these films is observed in various applications, e.g. as microelectrodes in micro-electrochemical analysis, emitters in advanced field emission devices, solid lubricant films or magnetic films. The introduction of a doping/alloying element in the carbonaceous network of the DLC film leads to a wider range of nanostructures and compositions available, in comparison to “pure” DLC films (a-C and a-C:H well known films). The chemistry (in particular the chemical affinity with carbon) and the concentration of the doped/alloyed element strongly affect the nature and the properties of the film, and this is strongly dependent on the deposition process, due to the high versatility from one deposition technique to the other one.
Pulsed laser deposition (PLD) has proved to be an effective technique for low temperature deposition, at low temperature, of a wide variety of thin film materials, extending from DLC to oxides or nanostructured materials [15], [16], [17]. In particular, PLD allows for precise control of the concentration of elements impinging upon the growing films, and this is of particular interest to deposit doped and alloyed DLC films in a controlled and reproducible way, compared to other deposition techniques. Recent papers have highlighted the interest of nanosecond PLD to deposit doped and alloyed DLC and other carbon-based films. Wei et al. [18] have compared the structure and the mechanical properties of a-C:Cu, a-C:Ti and a-C:Si films deposited by nanosecond PLD. Contrary to adhesion, the stress, hardness, friction and wear depend on the alloying element. Mominuzzaman et al. [19] have shown the influence of P doping (1–7 wt.%) on the optical gap and electrical resistivity of the films. Zhu et al. [20] have shown how Ni and Co catalysts in ablated graphite targets may control the formation of both carbon nanotubes and metal nanocrystals. Trusso et al. [21] have performed reactive (N2) PLD on a SiC target to study the optical gap and the index of refraction of the deposited SiCN-based film. Suda et al. [22] have investigated a-C:B films deposited by nanosecond PLD, with a required boron concentration making the film suitable for electron emission studies. Morstein et al. [23] and Willmott and Spillmann [24] use the versatility of PLD to deposit MeCxN1−x (Me = Zr, Zr–Al, Ti, V) films, with highlights on their composition, microstructure, optical and mechanical properties. In particular, the friction coefficient of some of these films was found to be significantly lower than the friction coefficient of most other coatings with similar high hardness values. Orlianges et al. [25] have shown the influence of Ni and Ta doping on the electrical properties of metal-doped carbon films.
Femtosecond lasers challenge excimer lasers for high-quality thin film preparation and high-precision micromachining. The pulse duration makes possible to achieve higher spatial resolution but also higher laser intensity than nanosecond lasers. In this case, the kinetic energy of the ejected species can be increased up to a few keV [26] leading to lower stress values in the film [27]. Femtosecond laser ablation is used from 1999 to deposit various kinds of films, in particular pure DLC films [26], [28], [29], [30], [31], [32], [33]. These films exhibit sp3 content in the range 40–75%, with interesting mechanical properties, low friction and high wear resistance.
From our knowledge, the present work belongs to the first attempts to deposit an a-C:X film by femtosecond PLD. Some of us have already deposited a-C:Ni in similar deposition conditions, thus leading to a distribution of metallic Ni clusters embedded in the carbon network [34]. In the present work, tantalum has been selected due to its high chemical affinity with carbon, contrary to nickel. The films have been investigated by coupling X-ray photoelectron spectroscopy (XPS), grazing incident angle X-ray diffraction (GIXRD), high resolution scanning and transmission electron microscopies (SEM, HRTEM) and energy filtered transmission electron microscopy (EFTEM).
Section snippets
Experimental details
We have synthetized films containing 85 at.% carbon and 15 at.% tantalum by ablating respectively a target of pure graphite (purity 99.997%) and a target of pure tantalum (purity 99.9%). The principle of this co-deposition consists to focus alternatively, by using a shutter, the femtosecond laser (Concerto, BMI/TCL, λ = 800 nm, pulse duration 150 fs, repetition rate 1 kHz, energy per pulse 1.5 mJ) during 9s onto the graphite's target, and thus during 1s onto the tantalum's target. Those
Results and discussion
Some characteristics and basic properties of the investigated a-C:Ta films containing 15 at.% of tantalum deposited on the silicon or copper grid substrates are summarized in Table 1. The compressive stress, quantified by measuring the surface curvature of the substrate before and after deposition of 1 μm film of a-C:Ta, are in the GPa range, as for undoped DLC obtained in the same deposition conditions [31]. The surface energy of the a-C:Ta film is 34.2 mN/m, with polar and dispersive
Conclusion
The deposition of nanostructured coatings of tantalum-doped diamond-like carbon (a-C:Ta) by femtosecond PLD has been investigated. Tantalum has been selected due to the well-known chemical affinity between tantalum and carbon through the formation of carbides. The main conclusions of this study are the following
- (a)
Tantalum is synthetized as clusters with a size distribution in the range of 15 to 175 nm.
- (b)
These tantalum clusters appear under three distinct phases: the first crystalline phase (α-Ta)
Acknowledgments
The authors acknowledge Béatrice VACHER (Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes UMR 5513) for EFTEM investigations. The authors also acknowledge Mr Paul JOUFFREY for the use of the SEM–FEG apparatus (EMSE: Ecole des Mines de Saint-Etienne), Mr Gilles BLANC (EMSE) for his help for SEM–FEG imaging and Mr Pierre PASSET (EMSE) for XPS analysis.
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