Pressure and plasma effects on the properties of magnetron sputtered carbon films

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Abstract
Thin, amorphous films of carbon have been deposited by conventional and hollow cathode enhanced magnetron sputtering of a pyrolytic graphite target. Films in the range of 15 to 200 nm were deposited over a pressure range or 50 to 0.1 mTorr on fused silica, sapphire, and Si substrates at 20 °C. Langmuir probe measurements of the local electron temperature, density, and plasma potential were made at several locations in the plasma as well as the substrate location as a function of pressure in both the conventional mode (50–1 mTorr) and the hollow cathode enhanced magnetron mode (1–0.1 mTorr). Several film properties were found to vary markedly over the pressure range investigated. For example, electrical resistivity was reduced from about 1×106 Ω cm at 50-mTorr sputtering pressure to a value of 0.01 Ω cm at 0.1-mTorr pressure. The optical band gap was reduced over the same pressure range from 2.1 to 0.4 eV. The index of refraction increased from n=2 at 50 mTorr to n=2.7 at 0.1 mTorr. The electron temperature of the plasma at the substrate location changed from a value of less than 1 eV at the higher pressure to over 8 eV at the lowest pressure, and the electron density increased 1 to 2 orders of magnitude in the same range. The results suggest that the local energy imparted to the growing film from the plasma environment can dramatically change the properties of the carbon films.

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