A method of forming high density contact array is disclosed. The method includes providing a first dielectric layer and forming a hard mask stack over the first dielectric layer. The hard mask stack includes first, second and third hard mask layers. The first and second hard mask layers are processed to form high density array of hard mask stack structures using a double patterning process. The hard mask stack structures include patterned first and second hard mask layers having a first width F1. The width of the patterned second hard mask layers is reduced to a second width F2 to form high density array of hard mask posts. A fourth hard mask layer is formed over the third hard mask layer and surrounding the hard mask posts. The hard mask posts and portions of the third hard mask layer and first dielectric layer underlying the hard mask posts are removed to form high density contact hole array.