A procedure for the vapor deposition of a thick layer of silicon oxide was developed to confine thin polymer films between two rigid flat walls. For silicon oxide overlayers thicker than approximately 1.5 mum the deposited silicon oxide layer is mechanically stable against heating above the glass transition temperature of the polymer. Neutron reflectivity measurements show that the interface between the polymer and the deposited silicon oxide is sharp, having a characteristic width of 1.5 nm.