A fabrication method for an air-gap, in which method hard mask is used, is described. A patterned hard mask layer is formed on a semiconductor substrate. Taking advantage of the etching selectivity of the hard mask layer to the dielectric layer, an opening with a high aspect ratio is formed in the dielectric layer. A conductive plug is then formed in the opening, followed by forming a conductive layer on the hard mask layer to cover the conductive plug. The hard mask layer is further removed. A silicon oxide layer with poor step coverage is formed to cover the substrate. Using the space remaining after the removal of the hard mask layer, an air-gap is formed between the conductive layer and the dielectric layer to enhance the insulation effect.