Polycrystalline silicon films on SiO2 were deposited without external heating by plasma immersion ion implantation using bias voltages ranging from -5 to -15 kV and electron cyclotron resonance plasma of SiH4. Compared with films deposited without ion irradiation, ion implanted films have larger grains and lower grain density even when their deposition temperature was lower. Furthermore, ion irradiated films are fully dense and large-grained starting from the initial Si/SiO2 interface without the initial amorphous/microcrystalline seed layer often thought to be necessary to produce polycrystalline silicon films at low temperatures. Using plasma immersion ion implantation, deposition of fully polycrystalline Si films with thicknesses less than 30 nm was demonstrated at temperatures as low as 410 degrees C.