Effects of thermal annealing on thermal conductivity of LPCVD silicon carbide thin films

Abstract

The thermal conductivity (k) of polycrystalline silicon carbide thin films is relevant for thermal management in emerging silicon carbide applications like MEMS and optoelectronic devices. In such films k can be substantially reduced by microstructure features including grain boundaries, thin film surfaces, and porosity, while these microstructural effects can also be manipulated through thermal annealing. Here, we investigate these effects by using microfabricated suspended devices to measure the thermal conductivities of nine LPCVD silicon carbide films of varying thickness (from 120 - 300 nm) and annealing conditions (as-grown and annealed at 950 degrees Celsius and 1100 degrees Celsius for 2 hours, and in one case 17 hours). Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) spectra and density measurements are also used to characterize the effects of the annealing on the microstructure of selected samples. Compared to as-deposited films, annealing at 1100 degrees Celsius typically increases the estimated grain size from 5.5 nm to 6.6 nm while decreasing the porosity from around 6.5% to practically fully dense. This corresponds to a 34% increase in the measured thin film thermal conductivity near room temperature, from 5.8 W/m-K to 7.8 W/m-K. These thermal conductivity measurements show good agreement of better than 3% with fits using a simple theoretical model based on kinetic theory combined with a Maxwell-Garnett porosity correction. Grain boundary scattering plays the dominant role in reducing the thermal conductivity of these films compared to bulk single-crystal values, while both grain size increase and porosity decrease play important roles in the partial k recovery of the films upon annealing. This work demonstrates the effects of modifying the microstructure and thus the thermal conductivity of silicon carbide thin films by thermal annealing.