Light scattering from slightly rough dielectric films

It is shown that the long-scale (smooth) component of the roughness spectru m of a slightly rough dielectric layer critically affects the angular distr ibution of radiation scattered fi om 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 lambda/10) Variation of the thickness of the dielectric layer. I t is shown that when interference phenomena are significant and the dielect ric film has long-scale roughness, conventional perturbation theory is inva lid, even if the rms of roughness is much smaller than the wavelength. A mo del is presented that correctly predicts the measured angular intensity dis tributions in the scattered-light field for samples that possess arbitrary scales of roughness. (C) 1999 Optical Society of America [S0740-3232(99)020 02-5].