A porous region structure and a method of fabrication thereof are disclosed. The porous region structure is characterized by having a hard mask interface region with non-uniform pores sealed and thereby excluded functionally from the structure. The sealing of the hard mask interface region is done using a hard mask deposited on top of an anodization hard mask used to define the porous region of the structure. By excluding the hard mask interface region, the porosity ratio and the equivalent specific surface of the porous region structure can be controlled or quantified with higher accuracy. Corrosion due to exposure of an underlying metal layer of the structure is also significantly reduced by sealing the hard mask interface region.