Deposition and Properties of Diamond-Like Carbons
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Diamond-like
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3 of sp and Fig. 1. Ternary phase diagram hydrogen contents of various forms of diamond-like carbon.
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Fig. 2 Schematic diagram of a filtered cathodic vacuum arc (FCVA)
Many different processes are used to grow DLC thin films such as magnetron sputtering, mass selected ion beam deposition (MSIB)[3-5], cathodic arc, or laser plasma deposition [6] for a-C and by sputtering and plasma enhanced chemical vapour deposition (PECVD) for a-C:H. The fraction of sp3 bonding is maximised for ion-dominated processes with ion energies around 100 eV. In MSIB, a beam of carbon ions is accelerated, mass- and energy-selected and then decelerated to condense on a substrate. The resulting ta-C film has up to 80% sp3 bonding [3-5]. The cathodic vacuum arc is an industrial or laboratory scale process to grow ta-C. This is a variant of the cathodic arc process used for many materials, such as TiN [7]. For ta-C, an arc is struck on a graphite cathode to create an intense plasma about 30% ionised. The plasma expands 291 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999 Materials Research Society
through the chamber to condense on the substrate. The arc also produces some particulates which can degrade the film, so these and any neutrals can be filtered by passing the plasma along a curved magnetic solenoid [7,8] as in Fig. 2, giving the filtered cathodic vacuum arc (FCVA). The magnetic field guides the electrons along the tube and this sets up an electrostatic field which causes the ions to follow. Better filtering can be achieved using a double S bend filter [9]. The basic growth rate of an FCVA is high, -3 nmis, but careful design is needed to maximise plasma transport into and along the filter. Some groups use unfiltered arcs [10] and others use lasers to initiate the arc [11]. In laser plasma deposition (LPD), a UV laser ablates a graphite target and creates a plasma plume similar to the cathodic arc which is condensed onto a substrate [5]. LPD is widely available because it is a versatile method used to deposit many materials such as oxide superconductors. Sputtering is the most common industrial method to produce DLCs. A graphite target is sputtered by Ar ions. Magnetron sputtering is used because the sputter yield of carbon is low. In unbalanced magnetron sputtering, the substrate is bombarded with Ar ions, and this increases the sp 3 content of the DLC [12,13]. Sputtering in Ar/hydrogen mixtures produces a-C:H films [12]. PECVD is another popular means to prepare a-C:H [14]. Various source gases can be used, such as methane or acetylene. Acetylene has the advantage of minimising the hydrogen content of the final film. The PECVD chamber is configured with cathode and anode plates and RF power is capacitively coupled to the smaller plate which becomes the cathode. The substrate is attached to the cathode to maximise ion bombardment. The need to maximise the ion compo
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