Pt/SrBi2Nb2O9 (SBN)/Al2O3/Si (MFIS) ferroelectric gate oxide structures were prepared with the rf (radio fre-quency) magnetron sputtering method for the application of non-destructive read-out ferroelectric RAM (NDRO-FRAM) devices. An Al2O3 intermediate layer between the perovskite SrBi2Nb2O9 film and Si substrate prevents the serious inter-diffusion of the SrBi2Nb2O9 (SBN) into the Si substrate. The coercive field that decisively affects the memory window was increased by inserting the Al2O3 insulator between the SBN and Si, and thus the memory window also increased with the increase in the electric field to the SBN. The memory windows of the metal-ferroelectric-insulator-semiconductors (MFIS) structures were in the range of 0.7-3.4 V when the gate voltage varied from 3 to 9 V. The memory windows of the MFIS structures were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in an MFIS with an insulator of 11.4 nm in thickness deposited in the deposition condition of a 15:5 flow ratio (Ar:O2) during sputtering.