Open Volume Defects (Measured by Positron Annihilation Spectroscopy) in Thin Film Hydrogen-Silesquioxane Spin-on-Glass: Correlation with Dielectric Constant

We used Doppler broadening positron annihilation spectroscopy as a nanovoid characterization tool in the study of low dielectric constant (low-k) hydrogensilsequioxane (HSSQ) thin films. The high void sensitivity of this method, combined with depth-resolving capability, enables one to profile the local electronic environment in a thin film. We established a correlation between the annihilation parameters and the dielectric properties for a series of samples subjected to various annealing conditions in nitrogen. Open volume densities ranging from 10^-6 to 10^-4 were calculated from the measured positron diffusion length. We found qualitative differences between the pore structures of isochronally and isothermally annealed films, suggesting a complex relationship between film dielectric constant and pore size and pore density. Results showing changes in the chemical environment caused by exposure to various processing environments are also presented. First, changes in HSSQ films were determined after a year exposure to ambient air in which film properties changed with time due to water absorption. Second, intentional oxidation of HSSQ films by exposure to a oxygen plasma were found to be resulted in a 130 nm-thick layer of oxidized HSSQ, with a corresponding high k (= 3.6). A decrease in k (to 3.3), with a subsequent 400°C/30 minute forming gas anneal was attributed to void formation at the HSSQ/Si interface.