We have been developing the hard X-ray telescope using depth graded multilayer. In our balloon experiment launched in 2001 July, we obtained hard X-ray image of Cygnus X-1 in 20-40 keV energy band. We are trying to expand the energy band of X-ray telescope up to 80 keV by optimizing supermirror design. Design of supermirror should achieve high reflectivity and flat response. Moreover, total number of bilayers have to be smaller than 200 for quality of multilayer and fabrication process. In such high energy region (~80 keV), interfacial roughness is serious problem. In practical use, interfacial roughness has to be smaller than 0.3 nm. In our balloon borne experiment, average interfacial roughness of supermirror is about 0.38 nm. We try to improve interface roughness of platinum-carbon multilayer with ion beam sputtering.