Friction, nanostructure, and residual stress of single-layer and multi-layer amorphous carbon films deposited by radio-f
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Single- and multi-layer amorphous carbon (a-C) films of varying thickness were deposited on Si(100) substrates by radio-frequency sputtering in a pure Ar atmosphere. The thickness, roughness, coefficient of friction, and residual stress of the a-C films were measured by profilometry, atomic force microscopy, surface force microscopy, and curvature method, respectively. The through-thickness nanostructure and elemental composition of the films were examined by cross-sectional transmission electron microscopy and electron energy loss spectroscopy. The multi-layer a-C films, consisting of alternating ;10-nm-thick hard and soft a-C layers deposited under 0 and 200 V substrate bias, respectively, were found to exhibit lower roughness, coefficient of friction, and residual stress and slightly higher tetrahedral carbon atom hybridization than single-layer a-C films of similar thickness. The results of this study reveal a strong correlation of the friction characteristics with the surface roughness and nanostructure of single- and multi-layer a-C films.
I. INTRODUCTION
Amorphous carbon (a-C) films are extensively used as protective overcoats in numerous applications, mainly due to their excellent physical properties and chemical inertness. Among various film deposition techniques,1–4 radio-frequency (rf) sputtering is a widely used film deposition method, in which the film precursors (carbon atoms and/or clusters of atoms) require low input energy compared to deposition methods using energetic ions as film-forming precursors. Moreover, inert ion bombardment of the film surface during sputtering allows the film properties to be tailored without changing the film chemical environment. Energetic Ar1 ion bombardment of the growing film is commonly used in sputter deposition to enhance the film density and hardness.4–6 However, ion bombardment usually produces films with high residual stress and varying microstructures, depending on the dominance of competing kinetic processes occurring under nonequilibrium film-growth conditions.4,7–9 Previous studies have shown that ion bombardment produces compressive residual stresses up to about 16 GPa,7,9,10 which may lead to film delamination, buckling, agglomeration (clustering), and other adhesion failures when a critical film thickness is reached.8,10,11 Therefore, preventing the development of
Contributing Editor: Mohd Fadzli Bin Abdollah a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.404
high residual (intrinsic) stresses in films deposited under sputtering conditions of energetic ion bombardment is critical to the maximum film thickness. Because the film roughness and friction characteristics affect the tribological performance of contacting surfaces, methods of depositing relatively smooth and hard films are of particular interest. Several methods have been used to prevent the development of undesirable high residual stresses in carbon films including postprocess treatments (e.g., thermal annealing12–15), the addition of various elements (e.g., Si, Al
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