Densifying a multi-layer substrate includes providing a substrate with a first dielectric layer on a surface of the substrate. The first dielectric layer includes a multiplicity of pores. Water is introduced into the pores of the first dielectric layer to form a water-containing dielectric layer. A second dielectric layer is provided on the surface of the water-containing first dielectric layer. The first and second dielectric layers are annealed at temperature of 600° C. or less. In an example, the multi-layer substrate is a nanoimprint lithography template. The second dielectric layer may have a density and therefore an etch rate similar to that of thermal oxide, yet may still be porous enough to allow more rapid diffusion of helium than a thermal oxide layer.