A mask used for exposing a porous substrate to form a first region and a second
region, the first region being filled with a conductive material piercing through
the entire thickness of the porous substrate to constitute an interfacial conductive
portion, the second region being filled with a conductive material not piercing
the entire thickness of the porous substrate to constitute a non-interfacial conductive
portion. The mask includes a first light-transmitting region for exposing the first
region, and a second light-transmitting region for exposing the second region,
said second light-transmitting region including an aggregation of fine patterns
of which an average aperture ratio is not more than 50% of an average aperture
ratio of the first light-transmitting region and a size of said fine patterns of
the second light-transmitting region being in the range of 0.1 m to 10 m.
