Using Plasma Immersion Ion Implantation (PIII) during Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD), fully polycrystalline silicon thin films were deposited on $SiO_2$ at temperature much lower than is possible without implantation. At same deposition temperature, PIII-deposited films have larger grain size than films deposited without PIII. PIII-deposited film was fully polycrystalline right down to Si/$SiO_2$ interface, while film deposited without PIII had a mixed amorphous/polycrystalline layer at interface. The very thin fully polycrystalline thin film(~30 nm) were fabricated in this method and the deposition temperatures of fully polycrystalline silicon thin film were lowered by nearly 200 ℃. The PIII-enhanced grain growth directly observed and identified as being as a major reason for the observed effects