Diamond growth on copper rods from polymer composite nanofibres
Introduction
The exceptional properties of diamond film make it an admirable material for various engineering and scientific areas and the potential uses can be found in a diverse range of industrial fields. Diamond films are usually employed in cutting tool industry or as protective layers of various implements and/or customisable surfaces (adhesiveness, controllable electrical conductivity, patterning, etc.) of commonly used materials [1], [2], [3], [4]. However, the deposition of diamond films onto some materials is still limited due to difficulties with its adhesion to foreign substrates (alumina, sapphire, etc.), by the damaging of the materials itself (polymers, ultra thin metals, etc.) or by substantial differences in thermal expansion coefficients (copper, gold, etc.). Usually, the diamond deposition is performed at high temperatures (800 °C) and during the cooling step down to room temperature, high thermal stress is induced what causes diamond film cracking or even its delamination from the substrate. Up to now, the variety of Chemical Vapour Deposition (CVD) techniques have been widely used for growing diamond films onto different substrate materials. Notwithstanding the great industrial potential and importance, a practical application of these CVD techniques is in some cases limited due to difficulties in achieving well-adhered diamond films on metals, such as copper, which is one of the most common and widely used metal material in today's electronic industry. So copper and diamond materials and their combination are in the centre of interest of many research groups. However, the combination of these materials is usually performed as some copper/diamond-particles composite or by deposition of the thin copper film on diamond [5]. Besides, copper, as a substrate exhibits a good lattice match with diamond. The face-centred-cubic lattice constants of Cu and diamond are 0.361 nm and 0.357 nm, respectively [6].
However, as drawbacks can be mentioned the large mismatch of thermal expansion coefficients that exist between diamond and copper may induce high stress within the deposited diamond film with a consequence of severe cracking and film delamination. In addition, the established fact that Cu does not form carbide usually results in a weak adhesion of the diamond film [7]. Moreover, high diffusivity of carbon in metals often leads to a long incubation time for diamond nucleation and growth [2], [8]. In order to solve such interface problems and/or increase of the diamond film adhesion, the great efforts have been made to achieve adherent diamond coating by proper modification of the substrate surface or by employing interlayers [9], [10], [11], [12], [13], [14]. However, implementing additional technological steps makes the deposition process time-consuming and costly.
In general, the diamond growth process can be separated into two steps: (i) the substrate pre-treatment (i.e. nucleation or seeding), and (ii) the growth process itself. Nucleation or seeding is a very important step prior to the diamond thin film deposition. It is a necessary step because of low intrinsic nucleation density of diamond on foreign substrates. For example, the nucleation density on silicon substrate is approximately 105 cm−2 [15]. The nucleation process influences not only the interlayer between the diamond film and substrate but it also affects the size of formed grains, their crystallographic orientation, chemical purity (i.e. sp3 to sp2 ratio), adhesion to the substrate, etc. [16]. The deposition of thin continuous diamond film (<200 nm) requires a nucleation/seeding density at least 1010 cm−2 or higher. The nucleation density can be improved by various pre-treatment processes which enhance the formation of diamond nuclei on the substrates and improve the growth process. These techniques include polishing of the substrate with ultra-dispersed diamond powder (UDD) produced by a detonation method, ultrasonic treatment of the substrate in the slurry of micrometer or smaller sized diamond particles, coating of the substrate with diamond-containing materials, or a pre-coating of the substrate with different carbon materials, etc. [17]. However, employing these techniques as a pre-treatment step is not possible in particular cases. For example mechanically soft and/or unstable substrates (polymers, ultra thin metals, etc.) can be damaged during the ultrasonic seeding. As mentioned above, a high thermal expansion mismatch between substrate (Al, Cu) and diamond makes the situation more unfavourable.
The objective of this study is to investigate the fabrication of adherent diamond films on copper rods by using polyvinyl alcohol (PVA) composite nanofibre textile employed as the nucleation/seeding layer. Moreover, the comparison between the samples nucleated by polymer nanofibres with the ultrasonically seeded ones is discussed too. Diamond CVD depositions were carried out by linear antenna MWCVD system, which is characterized by cold plasma due to larger distance of hot plasma region from the substrate [18].
Section snippets
Pre-treatment of substrates
The virgin PVA and PVA + UDD solutions were electrospun using a roller electrospinner on polypropylene spunbond supporting web with an areal weight of 18 g/m2. The average nanofibre diameter was about 100 nm. In the case of PVA + UDD solution the weight concentration of diamond powder in PVA solution was 0.023 wt.%. Concentration of PVA was 14 wt.% [16]. Such prepared PVA textile was further used for pre-treatment of copper substrates. Four differently pre-treated copper rods (treatment S1–S4) were
Growth rate and diamond film vertical homogeneities
Homogeneity of the diamond film grown over large areas is a common issue in all plasma CVD systems (the deposition area depends on the applied plasma system). Of course, the growth homogeneity is not only related to the deposition area size but is also affected by the temperature gradients and distribution of active species in plasma/in deposition chamber. These effects are more critical in the case of vertically positioned substrates. Fig. 1a shows photos of vertically positioned Si substrates
Conclusions
In this article the homogeneity of diamond film growth on vertically positioned substrates and the diamond growth on copper rods with the assistance of differently prepared polyvinyl alcohol composite nanofibre textile were studied. The diamond growth was performed by the pulsed linear antenna MWCVD system using various gas mixtures, specifically with various C:O:H species ratio. It was found that the gas composition significantly influence the growth rate as well as homogeneity of deposited
Acknowledgements
The authors would like to gratefully appreciate M. Kotlar for SEM measurements. This work was financially supported by the research GAČR project P108/12/G108 (M.V., A.K.).
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