Plasma deposition of amorphous carbon films from methane atmospheres highly diluted in argon
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Plasma deposition of amorphous carbon films from methane atmospheres highly diluted in argon
L. G. Jacobsohn and F. L. Freire Jr. Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro Caixa Postal 38071, 22452-970 Rio de Janeiro, RJ, Brazil.
ABSTRACT We investigated the deposition, structure and mechanical properties of a-C:H films grown in Ar-CH4 mixtures with the Ar partial pressure ranging from 0 to 99 %. The deposition rate strongly decreased with progressive Ar dilution of the CH4 atmosphere. Films deposited in pure CH4 atmospheres have a hydrogen content of 20 at.% that showed a trend to decrease for lower CH4 partial pressures, while the density remained nearly constant at around 1.4x1023 at./cm3. Raman spectroscopy and x-ray diffraction revealed the amorphous character of the films. The compressive internal stress remained constant around 2.5 GPa and the hardness decreases for Ar rich precursor atmospheres.
INTRODUCTION In hydrogenated amorphous carbon (a-C:H) films deposited by CH4 plasma decomposition, the structure is composed of sp2 hybridized nanoclusters interconnected by sp3 hybridized carbons, and it is usually accepted that surface chemiosorption of CH3 radicals is the main channel for film growth [1]. Only recently a more systematic comparison between the effects of different noble gases dilution of CH4 atmospheres was carried out [2,3]. Sun et. al. deposited a-C:H films by magnetically enhanced PECVD with a gas pressure of ~7 Pa [2], while Tomasella et. al. employed a PECVD system with total pressure of 1 Pa [3]. Both works investigated the structure, mechanical and optical properties of the films, though the investigation by Sun et. al. was restricted to CH4 rich atmospheres only, i.e., noble gases dilution down to 50 %. Moreover, recent gas kinetics and transport phenomena in low pressure CH4 rfplasmas were numerically calculated to simulate gas phase chemistry and provided strong indications for C2 species to be the building block of a-C:H films when CH4 is highly diluted in argon [4]. In fact, C2 is the building block of diamond films deposited by microwave decomposition of gas mixtures containing CH4. Considering that the growth of diamond films requires high substrate temperatures (~700 oC) and that this fact limits the choice of substrate materials, the possibility of obtaining carbon coatings deposited at low substrate temperatures with new structural arrangement deserves investigation. Among the works dealing with the deposition of a-C:H films by rf capacetively coupled plasma decomposition of CH4-Ar mixtures, the Cambridge group reported the observation of a nanocrystalline diamond phase in films deposited at low substrate temperatures [5]. In this work, a-C:H films were deposited in conditions similar to those used in the computer calculations mentioned above and the films were investigated on their structure and mechanical properties. P9.2.1
EXPERIMENTAL PROCEDURES A-C:H films were deposited by PECVD in CH4-Ar mixtures with a total pressure of 13 Pa, total flu
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