Hard Carbon Coatings for IR-Optical Components,

Hydrogenated amorphous carbon films a-CH were deposited on glass, silicon and germanium substrates. The films are transparent in the IR and are extremely hard. The a-CH films were homogeneously deposited at a rate of about 500 Amin in an RF excited discharge from benzene vapour. The characteristic features of this deposition method are discussed. Thickness, short wavelength absorption, refractive index 2 - 10 micrometers and relative hydrogen content were determined. Variations in short wavelength absorption, IR-refractive index and relative hydrogen content could be correlated with deposition conditions. The refractive index of a-CH can be tuned to be exactly 2, thus a-CH is an ideal single layer AR-coating for germanium n 4. Laser calorimetric measurements of optical absorption at 10.6 micrometers give an absorption coefficient of 250cm. The application of a-CH as single layer AR coating on Ge and as outside layer of a multi-layer coating is demonstrated.