Open volume defects (measured by positron annihilation spectroscopy) in thin film hydrogen-silsesquioxane spin-on-glass; correlation with dielectric constant

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Abstract
We used Doppler broadening positron annihilation spectroscopy as a nanovoid characterization tool in the study of low dielectric constant (low-k) hydrogen-silsesquioxane (HSSQ) thin films. The high void sensitivity of this method, combined with depth-resolving capability, enables one to observe changes in 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. Qualitative differences are found between the pore structures of isochronally and isothermally annealed films, suggesting a complex relationship between the 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, the changes in the HSSQ films were determined after a year of exposure to ambient air in which the film properties changed with time due to water absorption. Second, the intentional oxidation of the HSSQ films by exposure to an oxygen plasma were found to propagate from the film surface towards the substrate. A 10 min oxygen-plasma exposure 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 min forming gas anneal was attributed to the void formation at the HSSQ/Si interface.