A hard mask is formed on an interconnect structure comprising a low-k material layer and a metal feature embedded therein. A block polymer is applied to the hard mask layer, self-assembled, and patterned to form a polymeric matrix of a polymeric block component and containing cylindrical holes. The hard mask and the low-k material layer therebelow are etched to form cavities. A conductive material is plated on exposed metallic surfaces including portions of top surfaces of the metal feature to form metal pads. Metal silicide pads are formed by exposure of the metal pads to a silicon containing gas. An etch is performed to enlarge and merge the cavities in the low-k material layer. The metal feature is protected from the etch by the metal silicide pads. An interconnect structure having an air gap and free of defects to surfaces of the metal feature is formed.