Very thin ( < 100 nm) polycrystalline silicon (poly-Si) thin films were deposited on glass by plasma enhanced chemical vapor deposition (PECVD) using SiF4 and H-2 gas mixtures. Film structures, i.e. crystal fraction, nucleus density and/or grain size, were controlled by an alternating deposition and etching technique. By this technique, an improved crystal fraction of 52% was successfully obtained for a 50 nm-thick him with a grain diameter of 37 nm. By adopting a two-step-growth (TSG) technique in which these thin layers are used as seeds, larger size grains (220 nm in diameter) were obtained in 290 nm-thick films. Also, preferential orientation was observed in the RHEED pattern of films grown by the TSG. (C) 2001 Published by Elsevier Science B.V. All rights reserved.