Technology of Ferroelectric Thin-Film Formation with Large Coercive Field on Amorphous SiO2 by Ion-Bombardment-Assisted Sputtering and Oxygen Radical Treatment for Future Scaling Down of Ferroelectric Gate Field-Effect Transistor Memory Device

Abstract

Sr2(Ta1-x,Nbx)2O7 (perovskite STN; x=0.3) is one of the most practical candidates for one-transistor-type ferroelectric memory devices, because it has a low dielectric constant. However, in the application to metal–ferroelectric–insulator–Si field-effect transistor (MFIS-FET) memory devices, the fabrication of STN on an amorphous insulator, such as SiO2, is difficult. In particular, in the case of STN, because its crystallization annealing temperature is 950 °C, the metal elements of STN and Si react with each other during crystallization annealing. As a result, perovskite STN cannot be fabricated. To overcome this problem, we have developed an ion-bombardment-assisted sputtering method. Furthermore, to obtain universal conditions for forming perovskite STN based on plasma physics, plasma parameters such as ion bombardment energy and ion flux, were measured. Perovskite STN was obtained when the ion bombardment energy was 38 eV and the normalized Kr ion flux was 78 [ions/atom]. An IrO2/STN (140 nm)/STN seed layer (10 nm)/Si device whose STN was fabricated under these plasma conditions, shows square hysteresis curves and a memory window of 1.7 V under an 8 V writing operation. This value corresponds to a coercive field of 55 kV/cm.