A vertical ultrathin-channel (UTC) formation process using a low-energy neutral beam etching (NBE) for a double-gate (DG) metal-oxide-semiconductor field-effect transistor (MOSFET) is proposed for the first time. The NBE can perfectly eliminate the charge build-up and photon radiation damages from the plasma. By utilizing the NBE, fin-type vertical MOSFETs with damage-less smooth sidewalls were successfully fabricated. The fabricated FinFFTs realized higher electron mobility than that using a conventional reactive ion etching. The improved mobility is well explained by the atomically-flat surface utilizing by the NBE.