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.