Mechanical properties evaluation of fluor-doped diamond-like carbon coatings by nanoindentation
Introduction
Diamond-like carbon (DLC) is the given name of many materials that have amorphous carbon in its structure. This coating material contains carbon bonds, mainly sp3 and sp2 hybridizations, and can incorporate hydrogen in its structure. These recombine in order to minimize the total energy, making the DLC properties close to those presented by diamond [1], [2]. The DLC presents many technological applications due to a vast range of unique characteristics. Among these, the transparency at visible and infrared region makes optical antireflective applications possible [2]. It is also used as a protective layer on hard disks and magnetic tapes once it is very hard [1], [2]. By including fluor in DLC films, changes occur on the material's characteristics. By varying the fluor concentration in the film, the electrical, optical and mechanical characteristics can be tailored. In fact, the amorphous carbon fluorinated materials have as main characteristic a low value of dielectric constant (approx. 2.0) [3], [4], [5], giving place to applications like passivation layers, dielectric gates and as an isolation material for integration into ULSI (Ultra Large Scale Integrated) devices, allowing circuits’ better efficiency [3], [6], [7], [8]. The thermal stability up to 400 °C of F-doped DLC films is improved when compared to undoped ones [9]. In what regards to gains in the optical characteristics, F-doping increases the refractive index and the optical band gap [9]. This, together with the high abrasion resistance, makes its application as anti-reflection coating for infrared optics possible, as a result of the wider absorption band of CFx up to 9 μm compared to ones of CHn in the range of 3–3.5 μm [10]. Fluor incorporation in DLC is also advantageous as a fluorinated lubricant layer to reduce film wear of computer hard disks [3], [10].
The mechanical properties of DLC films are determinant for the above-referred applications. The superior hardness of DLC coatings results not only from the presence of sp3 bonds but also depends on the film's structure, namely the number and size of graphite clusters and the imperfections and absorbed gas occurrence at the cluster grain boundaries [11]. Such variety of the DLC structure characteristics, added to the wide range of hydrogen content in the films, leads to a very broad range of hardness and Young's modulus values. Moreover, the dependence of these properties on the applied load used for their determination also contributes to this scattering. Indentation is the most used technique for such evaluation in thin films, but it forces to take much care due to the very small film thickness and the consequent influence of the substrate. This imposes the use of nanoindentation. For DLC hard films an additional care in the interpretation of the results must be take into account due to cracking events and delamination phenomena [12]. In the recent review of J. Robertson [1], one can find update values for DLC mechanical properties depending on the film processing technique and composition. The maximum nanohardness and Young's modulus of the a-C:H films are 17 GPa and 300 GPa, respectively, while for ta-C (85–90% sp3) these properties can attain 88 GPa and 822.9 GPa, respectively.
As far as the fluorinated DLC coatings are concerned, their mechanical characterization by nanoindentation has never been evaluated. F.L. Freire Jr. et al. [3] studied the dependence of hardness on the PECVD self-bias and CF4 partial pressure, but they used microhardness (5 gf of applied load) resulting in underestimated hardness values, in the range of 5.5–15 GPa, due to the influence of the substrate. Thus, in the present work, nanoindentation technique was used to estimate the mechanical properties, namely hardness and Young's modulus, of thin DLC fluorinated films grown by CF4/CH4 assisted reactive sputtering method. The CF4 concentration on the gas mixture varied from 0 to 80% at three distinct process pressures of 0.5 kPa, 1.0 kPa and 1.5 kPa. The F-doped DLC mechanical properties were correlated with the structural characteristics of the films that were assessed by FTIR and Raman spectroscopies.
Section snippets
Experimental
The equipment used for the growth of the F-doped DLC films is a RF magnetron sputtering system home made at LSI, University of São Paulo, Brazil, as described elsewhere [13]. A rotary/turbo-molecular vacuum pump system allowed a working pressure of 9.3×10−5 kPa. The RF (13.56 MHz) power was fixed at 150 W. The sputtering process was done with a 99.9999% pure graphite target, located at 10 mm of the Si p-type {100} substrate wafers (7.62 cm diameter and 381±50 μm thickness), with an electric
Results and discussion
Table 1 presents the processing conditions and the thickness of the fluorinated DLC films on silicon. Fig. 1 is a typical plot of a loading/unloading nanoindentation curve of F-doped DLC coatings. Under a load up to the very small value of 5 mN, the displacement behaviour does not give a completely regular and smooth line, namely at the final section of the unloading curve. This is due to the combination of two effects: (i) the brittle nature of the coating/substrate system, that leads to
Conclusions
The mechanical evaluation of thin and hard DLC films requires the use of the nanoindentation technique with very small indentation loads. Despite the brittle nature of the films that often conducted to irregular loading/unloading displacement curves, it was possible to measure the hardness and the Young's modulus of fluorinated DLC films as a function of the CF4 percentage in the reactant gas mixture and total processing pressure.
SEM observations of the indentation impression patterns allowed
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