Structure and Mechanical Properties of Nitrogen Incorporated Diamond-Like Carbon Films
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INTRODUCTION Diamond-like carbon (DLC) films have a unique combination of physical and chemical properties such as high hardness, optical transparency, low coefficient of friction and chemical inertness [1]. Combined with ease of large area deposition and low deposition temperature, DLC films have been considered as a candidate for various applications. Much work has thus been done on the relations between preparation conditions and corresponding structure and properties of DLC films. It is now well known that the kinetic energy of activated species in the deposition reactor is the most important parameter which determines the structure and properties of DLC 2 films [1]. Many theoretical studies on the structure showed that the film consists of sp clusters 3 of about 15 A connected by sp inter-links [2, 3, 4]. Optical and mechanical properties are well understood by this structure model. Recently, it was reported that nitrogen doping or implantation modifies the chemical and electronic structure of the films [5-15]. One of the motivations of the investigation is that hypothetical carbon nitride (C 3 N4 ) of O-Si 3 N4 structure can have properties superior to those of diamond [16]. Changes in electrical and optical properties by nitrogen incorporation has been extensively investigated. However, only a few results were published about the change in mechanical properties such as hardness and residual compressive stress [12, 13, 14, 15]. Franceschini et al and Metin et al have reported the reduction of residual stress by nitrogen incorporation while the hardness remains unchanged or rather increases [12, 13]. On the other hand, Schwan et al and Lee et al showed that both the hardness and the residual stress decreases with the nitrogen incorporation [14, 15]. More investigations are thus required to clarify the effect of nitrogen on the mechanical properties of the films. Various combinations of hydrocarbon and nitrogen source 233 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
gas have been used for the nitrogen incorporated DLC film deposition. It is thus necessary to investigate the effect of nitrogen source gases on the structure and properties of deposited films. In the present work, nitrogen and ammonia gases were used as the nitrogen sources under the same deposition condition. Distinct differences in deposition rate, composition, mechanical properties and atomic bond structures were observed according to the nitrogen source.
EXPERIMENTAL METHOD Nitrogen incorporated DLC films were prepared by r.f.-PACVD method using mixtures of benzene (C 6 H6 ) and ammonia (NH 3 ) or nitrogen (N 2) as the reaction gases. Details of the experimental set-up were previously described [17]. Substrates were mounted on the water cooled cathode where 13.56 MHz r.f. power was delivered through the impedance matching network. Prior to the deposition process, the substrates were sputter cleaned by an Ar discharge at the
negative bias voltage -400 V and the pressure 3.7 mTorr for 15 min. The films were then deposi
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