Optical and electrical properties of polymerizing plasmas and their correlation with DLC film properties
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Diamond-like carbon (DLC) films were grown from radiofrequency plasmas of acetylene-argon mixtures, at different excitation powers, P. The effects of this parameter on the plasma potential, electron density, electron temperature, and plasma activity were investigated using a Langmuir probe. The mean electron temperature increased from about 0.5 to about 7.0 eV while the mean electron density decreased from about 1.2x10(9) to about 0.2x10(9) cm(-3) as P was increased from 25 to 150 W. Both the plasma potential and the plasma activity were found to increase with increasing P. Through actinometric optical emission spectrometry, the relative concentrations of CH, [CH], and H, [H], in the discharge were mapped as a function of the applied power. A rise in [H] and a fall in [CH] with increasing P were observed and are discussed in relation to the plasma characteristics and the subimplantation model. The optical properties of the films were calculated from ultraviolet-visible spectroscopic data; the surface resistivity was measured by the two-point probe method. The optical gap, E(G), and the surface resistivity, rho(s), fall with increasing P. E(G) and rho(s) are in the ranges of about 2.0-1.3 eV and 10(14)-10(16) Omega/square, respectively. The plasma power also influences the film self-bias, V(b), via a linear dependence, and the effect of V(b) on ion bombardment during growth is addressed together with variation in the relative densities of sp(2) and sp(3) bonds in the films as determined by Raman spectroscopy.