A method and apparatus for treating a substrate is provided. A porous dielectric layer is formed on the substrate. In some embodiments, the dielectric may be capped by a dense dielectric layer. The dielectric layers are patterned, and a dense dielectric layer deposited conformally over the substrate. The dense conformal dielectric layer seals the pores of the porous dielectric layer against contact with species that may infiltrate the pores. The portion of the dense conformal pore-sealing dielectric layer covering the field region and bottom portions of the pattern openings is removed by directional selective etch.