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Rf-PECVD a-C:H has been applied to a wide variety of applications which utilise the material's electronic, rather than mechanical, properties. Such applications include metalsemiconductor-metal (MSM) [2] and thin film transistor (TFT) [3] switches for large area electronics and 'low k' dielectric isolation in VLSI and ULSI circuitry [4][5][6]. The interest in a-C:H for large area electronic applications arises from the ability to deposit semiconducting and insulating layers at room temperature. Such materials will therefore be fully compatible with the next generation of flat panel displays on plastic. Diamond-like carbon (DLC) and fluorinated DLC (F-DLC) have also attracted much attention for use in interlayer isolation. The delay time associated with an integrated circuit is a key measure of performance. 'Low k' dielectrics (i.e. materials with a dielectric constant much lower than that of Si0 2 ) are needed to minimise parasitic capacitances in ULSI circuitry. The combination of low dielectric constant, smoothness, mechanical strength and chemical inertness make a-C:H an ideal candidate for this application. The critical growth parameter when depositing a layer of rf-PECVD a-C:H is the dc-bias which develops between the substrate and the plasma sheath [7]. The potential for the 'effective' dc-bias to be modified by the electrical properties of the substrate and the conductivity and

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Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society

topography of additional layers on the substrate surface is clear but, to date, no extensive investigation has been carried out. Such effects are likely to be particularly relevant when optimising layers of a-C:H for use in large area electronic and interlayer dielectric applications. The requirement to deposit layers of electronic grade a-C:H directly on top of narrow metallic lines (e.g. gate, source/drain or interconnect tracks) will make these applications very sensitive to such effects. The present paper investigates the adhesion, morphology and electrical and structural properties of a-C:H films grown on a range of substrates (c-Si and C7059 glass) with and without pre-defined metallic patterns. EXPERIMENTAL Thin films of a-C:H films were deposited at room temperature from CH4/Ar and CH4/He gas mixtures using a capacitively coupled rf-PECVD (13.56 M-z) system at a pressure of 100 mTorr.

A nominal a-C:H film thickness of 90 nm was maintained for all samples. The plasma power was varied to develop dc self-bias voltages ranging from 70 V to 240 V. Test structures were prepared by thermally evaporating 20 nm thick films of Cr on to c-Si and C7059 glass substrates. Thin Cr layers were used to minimise step coverage induced stress effects in the deposited layers. Substrates (2 cm x 2 cm) with a continuous metal coating were used to investigate the properties of a-C:H films deposited on to large area metallic substrates. In addition, thin metallic strip and pad structures, with feature sizes up to several umn,were patterned on to selected substrates using sha