Using mirror-confinement-type electron cyclotron resonance (ECR) plasma sputtering method, strontium titanate (SrTiO3) thin films have been prepared on Si and Pt/Ti/SiO2/Si substrates at a low substrate temperature (below 450 K) in a low pressure (2.7 x 10(-2) Pa) environment of pure Ar and Ar/O-2 mixture. Prepared film surfaces were very smooth regardless of high deposition rate (similar to 8.5 nm/min). The composition ratio Sr/Ti of Sr to Ti in the films Varied with the distance between the target and the substrate. AII as-deposited films on Si substrates were found to be amorphous and were crystallized by post-deposition annealing using an electric furnace at 650 K, i.e. approximately 250 K lower than annealing for films obtained by conventional RF magnetron sputtering. Post-deposition annealing of these films using millimeter-wave radiation decreased the crystallization temperature to a value of 550 K. Furthermore, all as-deposited films on Pt/Ti/SiO2/Si substrates by a plasma of Ar and O-2 gas mixture were found to be crystallized regardless of no substrate heating.