OPTICAL-SCATTERING OF DIAMOND FILMS

One of the important applications for diamond films is the protective coating for optical windows, which require good transparency. Light sc attering causes transmittance loss and cannot be ignored if the surfac e roughness or grain sizes are comparable with the wavelength. The mic rowave plasma technique was used to grow diamond films of well-defined {111} and {100} facets on silicon substrates by controlling the CH4/H -2 ratio at 0.7% and 2.0%, respectively. The flow rate was 100 seem an d substrate temperatures were at 600 degrees C. An etched, free-standi ng window was used to study optical scattering by FTIR spectra and an integrating sphere. The FTIR transmittance spectra show that the scatt ering effect starts at a wavelength of around 5 mu m, where well-grown crystallites appear. Such effect is even more serious for rougher {11 1} facets. This scattering can be eased by longer deposition and the s o-called two-step process to obtain higher nucleation density. Optical properties of the diamond windows studied by integrating sphere show that only about 5% of incident visible laser beam is kept on its direc tion after passing the films, this number being somewhat higher for 13 00 nm diode laser beam. Most of the loss can be attributed to body sca ttering inside the films. These results are in consistence with that o f FTIR studies. It is safe to say that more directional trasmittance c an be anticipated if a longer wavelength infrared beam is used as inci dent light source.