Polymer penetration and pore sealing in nanoporous silica by CHF3 plasma exposure

The polymerization and pore sealing that occurs during fluorocarbon plasma treatment of nanoporous silica xerogel was investigated experimentally by Rutherford backscattering spectroscopy and successfully modeled using a diffusion-reaction analysis. CHF3 was used as a reactant gas to expedite the rate of polymerization due to the presence of hydrogen in its structure and its low C/F ratio. Knudsen diffusion was assumed to be the dominant mechanism for the motion of polymer precursor species through the nanoporous material over the range of pressures used in the plasma experiments. The amount of fluorine atoms deposited on the sidewalls of the pores was measured as a function of depth in the dielectric film and that amount was assumed to correspond with the mass of the polymer layer formed inside the pores. By applying a Thiele-type analysis to this system, we successfully matched model calculations with measured fluorine amounts, predicted the time required to reach a steady-state concentration of the polymer precursor in a pore (similar to 10(-7) s) and predicted the time required to seal off pore necks at the surface of the dielectric (similar to 70 s). Both the model and experimental results show a greater depth of penetration and an enhanced deposition of polymer at higher porosities, confirming the need for pore sealing during back-end-of-the-line processing of nanoporous materials. (c) 2005 The Electrochemical Society. All rights reserved.