Semiconductor devices and sidewall image transfer methods with a spin on hardmask. Methods for forming fins include forming a trench through a stack of layers that includes a top and bottom insulator layer, and a layer to be patterned on a substrate; isotropically etching the top and bottom insulator layers; forming a hardmask material in the trench to the level of the bottom insulator layer; isotropically etching the top insulator layer; and etching the bottom insulator layer and the layer to be patterned down to the substrate to form fins from the layer to be patterned.