Abstract: A series of polycrystalline diamond films with different surface roughness were successfully deposited on silicon substrates using the HFCVD method with varying carbon content in the range of 2%～3%. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman scattering spectroscopy, and optical transmission spectroscopy and spectroscopic ellipsometry were employed to characterize the asgrown films. With increase of carbon content, average roughness of the diamond films was reduced from 45 nm to 21 nm while their grain size was lowered to tens of nanometers. The refractive index (n) came to offset the idealized value as well. The effects of film's surface roughness and content of the nondiamond phase on the optical property were studied. It was observed that the films' quality determines their optical performance when they have similar surface roughness.

Key words: carbon content, optical performance, surface roughness, diamond film