This paper investigates the limits of a low pressure chemical vapour deposition (LPCVD) technique for the deposition of a nanometre scale ultra-thin polysilicon (poly-Si) film with sub-10nm grain sizes. Three different processes using pure silane (SiH4) in a standard horizontal hot-wall reactor are presented: (i) a direct poly-Si deposition, (ii) a Hemispherical Silicon Grain (HSG) deposition and (iii) an amorphous silicon (a-Si) deposition followed by a thermal crystallization anneal. The direct poly-Si deposition gives a minimum film thickness achievable around 20 nm with grain sizes ranging from 20 to 30 nm. The HSG deposition process leads to the formation of grains with diameters varying from 5 to 50 nm and heights ranging from 5 to 20 nm. The best results are obtained with the third process (a-Si / crystallization), which allows the formation of 6 nm poly-Si thick films with grain sizes ranging from 10 to 20 nm.