With this work we have assessed the minimum thickness and grain size realizable for a polysilicon (poly-Si) layer deposited with a low-pressure chemical vapour deposition technique. Three different approaches using pure silane in a standard horizontal reactor have been evaluated: (i) a direct poly-Si deposition, (ii) a hemispherical silicon grain deposition and (iii) an amorphous silicon (a-Si) deposition followed by crystallization thermal annealing. It has been demonstrated that the a-Si/crystallization process seems to be the best candidate for the deposition of ultra-thin poly-Si films. However, it involves process innovation in both the nucleation and annealing phases. With this method we have succeeded in the deposition of uniform 6 nm poly-Si layers with grain sizes around 10-20 nm.