Adhesion and mechanical properties of a-CSi:H and a-CSiO:H thin films prepared by PECVD

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The adhesion of the thin film (prepared by e.g. PVD or CVD) to the substrate is one of the most important properties and crucial attribute in determining of the thin film applications. Thin films with controlled adhesion are essential as anti-scratch, barrier, transparent, wear-resistant and antireflective coatings for surface modified materials. The greatest potential of this research for industrial applications is in glass-fiber-reinforced polymer composites without sharp interfaces and protective coatings. In this study, hydrogenated amorphous silicon-carbon (a-CSi:H) and hydrogenated amorphous carbon-silicon oxide (a-CSiO:H) thin films were characterized in terms of adhesion to the silicon substrate. Silicon wafers were pretreated with argon plasma (5.7 Pa, 10 sccm, 5 W, 10 min) using continuous wave to reach reproducible thin film adhesion. Thin films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using pure tetravinylsilane (TVS) monomer or TVS in a mixture with argon or oxygen gasses (2.7 Pa, total mass flow rate 3.8 sccm), employing a radiofrequency (RF) helical coupling system, using a pulsed regime with effective power ranging from 2 W to 150 W for each above mentioned mixtures. Prepared films were 0.1 μm thick for nanoscratch tests and 1.0 μm thick for nanoindentation measurement. The nanoscratch test employs a conical diamond indenter (90°, 1 μm tip radius) submersed into the film under linearly increasing normal loading, ranging from 1 μN up to 10 mN in 30 s. The scratch length was set to be constant 10 μm. The value of the load at which adhesion failure is detected is known as the critical normal load (Lc) and is used as a semiquantitative measure of adhesion. Scratch tracks were also observed by atomic force microscopy (AFM) and results were correlated with loading curve. Young's modulus and hardness were obtained by instrumented nanoindentation technique using Berkovich indenter (50 nm tip radius). Analysis of resulting load-displacement curves was based on Oliver-Pharr method. It was observed that the critical load varies from 1.6 mN (2 W, TVS) up to 6.5 mN (10 W, TVS/O2). The nanoindentation measurements showed changes of Young's modulus ranging from 10 GPa up to 141 GPa for samples prepared from pure TVS and powers 2 W and 150 W respectively. Obtained values of critical load (as a measure of adhesion) of three tested power series showed the given trends. However, it was noted, that calculated work of adhesion gives different trends. This is due to the considerable changes of Young's modulus, which then effects the critical load figures. Also friction coefficient and thickness has not negligible influence. Based on model Burnett and Rickerby [1], modified by Bull et al. [2] and later by Palesch and Cech [3], the work of adhesion, eliminating the influence of variable thickness, Young's modulus and friction coefficient, was used in this study. [1] P. J. Burnett, D. S. Rickerby, Thin Solid Films 157 (1988). [2] S. J. Bull et al., Surf. Coat. Technol. 36 (1988). [3] E. Palesch, V. Cech, Thin Solid Films 636 (2017).

adhesion; scratch test; PECVD; a-CSi:H; a-CSiO:H; thin films

PLICHTA, T.; BRÁNECKÝ, M.; ČECH, V.

18. 10. 2018