aspects of the invention are directed to a method for forming a semiconductor device . a dielectric layer is formed on a semiconductor substrate . subsequently , a metallic contact is formed in the dielectric layer such that it lands on the semiconductor substrate . a masking layer comprising a block copolymer is then formed on the dielectric layer . this block copolymer is caused to separate into two phases . one of the two phases is selectively removed to leave a patterned masking layer . the patterned masking layer is used to etch the dielectric layer . the patterned air gaps reduce the interconnect capacitance of the semiconductor device while leaving the dielectric layer with enough mechanical strength to serve as a middle - of - line dielectric .