Light scattering from slightly rough dielectric films

It is shown that the long-scale (smooth) component of the roughness spectrum of a slightly rough dielectric layer critically affects the angular distribution of radiation scattered from the surface. The interference pattern obtained from a sample with only small-scale roughness differs drastically from a sample with the same small-scale roughness but possessing slight (of the order λ/10) variation of the thickness of the dielectric layer. It is shown that when interference phenomena are significant and the dielectric film has long-scale roughness, conventional perturbation theory is invalid, even if the rms of roughness is much smaller than the wavelength. A model is presented that correctly predicts the measured angular intensity distributions in the scattered-light field for samples that possess arbitrary scales of roughness.