Abstract: Methods for forming a dual damascene dielectric structure in a porous ultra-low-k (ULK) dielectric material by using gas-cluster ion-beam processing are disclosed. These methods minimize hard-mask layers during dual damascene ULK processing and eliminate hard-masks in the final ULK dual damascene structure. Methods for gas-cluster ion-beam etching, densification, pore sealing and ashing are described that allow simultaneous removal of material and densification of the ULK interfaces. A novel ULK dual damascene structure is disclosed with densified interfaces and no hard-masks.