Methods for forming a semiconductor device include forming a first spacer on a plurality of fins. A second spacer is formed on the first spacer, the second spacer being formed from a different material from the first spacer. Gaps between the fins are filled with a support material. The first spacer and second spacer are polished to expose a top surface of the plurality of fins. All of the support material is etched away after polishing the first spacer and second spacer. The plurality of fins is etched below a bottom level of the first spacer to form a fin cavity. Material from the first spacer is removed to expand the fin cavity. Fin material is grown directly on the etched plurality of fins to fill the fin cavity.