An intermediate semiconductor structure in fabrication includes a silicon semiconductor substrate, a hard mask of silicon nitride (SiN) over the substrate and a sacrificial layer of polysilicon or amorphous silicon over the hard mask. The sacrificial layer is patterned into sidewall spacers for mandrels of a filler material substantially different in composition from the sidewall spacers, such as a flowable oxide. The mandrels are removed such that the sidewall spacers have vertically tapered inner and outer sidewalls providing a rough triangular shape. The rough triangular sidewall spacers are used as a hard mask to pattern the SiN hard mask below.