Diamond-like carbon films prepared by rf substrate biasing in an ECR discharge
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Diamond-like carbon (DLC) films have attracted considerable attention due to the fact that some of their mechanical, chemical, optical, and electrical properties are similar to those of single crystal diamond. They have been the subject of several recent review articles.1"5 Today the majority of research in this area has been redirected to studying diamond film growth. Recent results have demonstrated low temperature polycrystalline diamond growth.6"8 Ultimately, heteroepitaxial diamond growth appears feasible. In the near term, however, for particular applications such as the encapsulation of integrated circuits, the properties of DLC films may be adequate or desired over that of polycrystalline material. Therefore continued research on DLC films is merited. Holland and Ojha were the first to deposit DLC films on the powered electrode in an rf sustained discharge.9 Bubenzer et al.10 showed the dramatic effect that energetic ion bombardment had on the properties of the deposited film by correlating these findings with the negative dc bias developed on the powered electrode. This study found that for biases in the range of 100 to 600 V, DLC films were obtained. Below this region the films were polymer-like and above this range they were graphite-like. For all the films studied, the optical band gap decreased monotonically with increasing bias voltage. Tamor et al.n performed a similar study and found comparable results with the exception that for bias voltages above 1000 V, the optical band gap made a reversal and began to increase with bias voltage. Other methods of depositing DLC films include monoenergetic ion beams and dual ion beam sputtering. These materials are often referred to as a-C or i-C because they contain no hydrogen. Kasi et al.,12 using a monoenergetic ion beam, found that the optimum energy for the deposition of DLC films is in the energy range of 30 to 175 eV. In this case the beam is assumed to be completely ionized, containing only positive C ions. Commercially developed ion beam enhanced deJ. Mater. Res., Vol. 5, No. 11, Nov 1990
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position processes such as "DiondSM" use ion bombardment energies on the order of 100 eV to obtain good quality DLC films.13 These films are typically harder, denser, and have a lower optical band gap than DLC films produced by glow discharge of hydrocarbon source gases. In this investigation a highly ionized discharge of methane is obtained with a microwave electron cyclotron resonance source. The higher coupling efficiencies obtained with this method over that of conventional rf techniques allow for plasma densities in the range of 10n-1012 cm"3. The ions in the discharge are then accelerated by the diverging magnetic field toward the substrate and are expected to strike the substrate with ion energies in the range of 20 eV.1415 Additional ion acceleration can be obtained by rf biasing the substrate. This develops a negative dc self bias on the substrate relative to the plasma potential. It is not possible to s
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