PHONON-SCATTERING IN THIN SILICA FILMS BELOW 1 K

A method is described for studying thermal phonon scattering in thin f ilms on dielectric substrates below a few Kelvin. It is used to study silica films on silicon substrates. Using Monte Carlo simulations with no free parameters, we find that thermally grown silica films 0.1 to 1.0 mu m thick scatter the phonons as strongly as bulk silica, and hen ce, have the same thermal conductivity as bulk silica,,while a 0.1 mu m thick e-beam evaporated silica film has a thermal conductivity five times smaller than bulk silica, indicative of additional defects.