Vacuum 81 (2007) 1412–1415 DC substrate bias effects on the physical properties of hydrogenated amorphous carbon films grown by plasma-assisted chemical vapour deposition J.G. Buijnsters à , M. Camero, L. Va´zquez, F. Agullo´-Rueda, C. Go´mez-Aleixandre, J.M. Albella Instituto de Ciencia de Materiales de Madrid (CSIC), C/ Sor Juana Ine´s de la Cruz 3, 28049 Madrid, Spain Abstract Hydrogenated amorphous carbon (a-C:H) films have been grown from argon/methane gas mixtures by electron cyclotron resonance chemical vapour deposition (ECR-CVD) on silicon substrates. The effects of the application of a DC substrate bias on the structural, morphological and mechanical properties of the films have been explored by multiple analysis techniques such as infrared and micro- Raman spectroscopy, atomic force microscopy, nanoindentation and pin-on-disk wear testing. In general, within the range of applied substrate bias (i.e. from 300 up to +100 V) we have observed a strong correlation between all measured properties of the a-C:H films and the ion energy. This work shows that the properties can differ greatly and indicates a threshold energy in the order of 90 eV. For the production of hard, low-friction coatings energies above this value are required. r 2007 Published by Elsevier Ltd. Keywords: Amorphous carbon; Bias; PACVD; Smoothness; Hydrogen; Tribology 1. Introduction Hard amorphous carbon (a-C) films exhibit a variety of useful properties such as a high wear resistance, low- friction coefficient, chemical inertness, surface smoothness and biocompatibility. In particular, the surface smoothness becomes a crucial property for developing protective coatings for magnetic storage devices and low-friction coatings. Therefore, over the last years special attention has been paid to the production of ultrasmooth a-C films. In addition, in magnetic storage technology, a slightly hydrogen-rich surface is desired because of the lubricant work [1]. Since the optimum hydrogen content of the carbon layers does not necessarily coincide with their maximum hardness [2], the development of ultrasmooth and hard hydrogenated amorphous carbon (a-C:H) films becomes essential. In earlier works on ion-assisted deposi- tion of a-C:H films, it has been demonstrated that the ion energy plays a crucial role in the formation of a-C:H films with desirable properties [3–5]. The addition of argon to a methane ECR plasma results in increased hydrocarbon ion concentrations and dangling bond densities at the deposit- ing surface [6]. Therefore, in this work a-C:H films have been deposited with a fixed methane-to-argon gas flux at varying external bias and their corresponding film proper- ties have been studied. 2. Experimental a-C:H films have been grown by an ECR-CVD reactor operating with a 2.45 GHz microwave plasma source at 205–210 W input power. Gas mixtures of methane/argon (15/35 sccm) are applied keeping the operating pressure at 1.1  10 2 Torr. A DC bias varying from 300 to +100 V is applied to the p-type (1 0 0) silicon substrates (1  1 cm 2 ) while no intentional heating is employed. The substrate temperatures were measured by a thermocouple attached to the substrate holder. A maximum substrate temperature as low as 120 1C was found for 1-h deposition at the ARTICLE IN PRESS www.elsevier.com/locate/vacuum 0042-207X/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.vacuum.2007.04.021 à Corresponding author. Present address: Institute for Molecules and Materials (IMM), Radboud University Nijmegen, The Netherlands. E-mail address: j.buijnsters@science.ru.nl (J.G. Buijnsters).