Thermal conductivity-structure-processing relationships for amorphous nano-porous organo-silicate thin films

While numerous thermal conductivity investigations of amorphous dielectrics have been reported, relatively few have attempted to correlate to the influence of processing conditions and the resulting atomic structure. In this regard, we have investigated the influence of growth conditions, post deposition curing, elemental composition, atomic structure, and nano-porosity on the thermal conductivity for a series of organo-silicate (SiOCH) thin films. Time-domain thermoreflectance (TDTR) was specifically utilized to measure thermal conductivity while the influence of growth conditions and post deposition curing on composition, mass density, atomic structure, and porosity were examined using nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS), Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), ellipsometric porosimetry (EP), and positronium annihilation lifetime spectroscopy (PALS). Analytical models describing the thermal conductivity dependence on mass density and vol% porosity were found to generally over-predict the measured thermal conductivity, but improved agreement was obtained when considering only the heat carrying network density determined by FTIR. Ashby’s semi-empirical relation, which assumes only 1/3 of the heat carrying bonds are aligned to the heat transport direction, was also found to reasonably describe the observed trends. However, the thermal conductivity results were best described via a model proposed by Sumirat (J Porous Mater 9:439 (2006)) which considers the effect of both vol% porosity and phonon scattering by nanometer sized pores. Post-deposition curing was additionally observed to increase thermal conductivity despite an increase in nano-porosity. This effect was attributed to an increase in the Si–O–Si network bonding produced by the cure.