Special Plenary Session

After briefly reviewing phonon scattering at dielectric crystal boundaries, a method is described by which phonon scattering in thin films on substrates can be measured below 1 K. The Monte Carlo technique used to evaluate the measurements is tested on polished and sandblasted silicon wafers. It is then applied to thermally oxidized a-SiO 2 films, in which the same phonon scattering is determined as is known for bulk a-SiO 2 . Crystalline dielectric or metal films produced by e-beam evaporation, however, have up to over ten times shorter phonon mean free paths, indicative of far greater disorder than existing even in amorphous solids. The origin of this disorder, and in particular, ways to control it, are as yet unknown.