Properties of Tetrahedral Amorphous Carbon Deposited by a Filtered Cathodic Vacuum Arc
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ABSTRACT The properties of a highly sp3 bonded form of amorphous carbon denoted ta-C deposited from a filtered cathodic vacuum arc (FCVA) are described as a function of ion energy and deposition temperature. The sp 3 fraction depends strongly on ion energy and reaches 85% at an ion energy of 100 eV. Other properties such as density and band gap vary in a similar fashion, with the optical gap reaching a
maximum of 2.3 eV. These films are very smooth with area roughness of order 1 nm. The sp 3 fraction falls suddenly to almost zero for deposition above about 2000C. INTRODUCTION
Diamond-like carbon (DLC) is of interest for the formation of transparent, hard films. DLC is a metastable, higher density form of amorphous C (a-C) or hydrogenated3 amorphous C (a-C:H) containing a significant fraction of tetrahedrally coordinated sp
C-C bonding [ 11. It is known that a highly sp 3 bonded form of a-C denoted tetrahedral amorphous carbon, or ta-C is obtained by deposition from a highly ionized beam of medium energy, from for example a filtered ion beam [2], the laser ablation of graphite [31 or from a filtered cathodic vacuum arc (FCVA) [4]. The most complete form of ion filtering is found in mass selected ion beam deposition (MSIB). On the other hand the FCVA is a lower cost source of ta-C suitable for laboratory or industrial use. This paper describes the dependence of the properties of ta-C on deposition conditions, in particular the ion energy and deposition temperature. EXPERIMENTAL DETAILS In the FCVA, an arc is struck on a graphite target, forming a highly ionised plasma. The plasma beam is led around a 900 solenoid magnetic filter which removes neutral species and particulates. The filter does not provide mass or charge selection. The plasm beam is then condensed on a substrate of Si or quartz held on a temperature
controlled copper block. The effective ion energy is varied by applying a DC bias to the substrate, or an RF bias for the quartz substrate.
The sp 2 fraction is found by electron energy loss spectroscopy (EELS), from the C
K edge spectrum [5). The microscopic density is derived from the valence plasmon
energy, also measured by EELS. The films can possess a high intrinsic compressive stress, which is derived from the substrate curvature by Stoney's equation. More details of the methods are given elsewhere [6]. RESULTS AND DISCUSSION The properties of ta-C deposited at room temperature depend strongly on ion 299 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
energy. Fig. 1 shows that the sp 3 fraction, density and compressive stress each pass through a maximum as a function of ion energy. The maximum occurs at 60 eV. The maximum sp3 fraction is 85%, which compares well with that found by others [4,51. The maximum occurs at an ion energy of 100 eV, slightly lower than found by Fallon [51 by FCVA. A broader maximum is found for ta-C prepared by MSIB by Lifshitz et al [7]. The reason for their broader maximum is not known, but may be related to their slower growth rates. The films a
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