Methods for forming semiconductor fins include forming a sacrificial semiconductor structure around a hardmask fin on an underlying semiconductor layer. A first etch is performed that partially etches away a portion of the hardmask fin and the sacrificial semiconductor structure with a first etch chemistry. A second etch is performed that etches away remaining material of the portion of the hardmask fin and partially etches remaining material of the sacrificial semiconductor structure with a second etch chemistry. A semiconductor fin is etched from the semiconductor layer using the etched hardmask fin as a mask.