Hiroshi Koike

Application of the ferroelectric materials to ULSI memories

Abstract Memory is essential to electronic data processing and continuous efforts are being made to develop improved memory devices. In the era of VLSI, difficulties have arisen with respect to storage capacitance, which must be kept to a certain value while the device dimensions are reduced. This has prompted the adoption of complicated structures like the trench or stack, causing the number of process steps to be increased. The use of high dielectric constant materials has been researched for the extension of DRAM development. Recently, the development of the memories which use the polarization reversal current of the ferroelectric material is rapidly progressing because it enables high speed nonvolatile memory action which generally needed in recent electronic systems. These memories will replace a large portion of the existing memory systems in the near future. However, this is not a perfect solution to the problem, because they are not in accordance with the scaling rule. In this paper, it is shown that ferroelectric memories using the field effect current of a semiconductor by the remanent polarization of the ferroelectric material are in accordance with the scaling rule. The first experimental verification of the non-volatile memory action was reported by Moll and Tarui in 1963 [1]. This basic memory action has been successively used in MFS (metal-ferroelectric-semiconductor) transistors. The ferroelectric memories are nonvolatile and are expected to be high-speed devices, making them suitable for universal applications. However, it is necessary to optimize the interface between the semiconductor and ferroelectric material. Experiments for the prospective devices using CeO2 or CexZr(1−x)O2 as the buffer insulator layers of the MFIS (metal-ferroelectric-insulator-semiconductor) transistors are described.

Initial Stage and Growth Process of Ceria, Yttria-Stabilized-Zirconia and Ceria-Zirconia Mixture Thin Films on Si(100) Surfaces

Initial stages and growth processes of ceria ( CeO2), yttria-stabilized-zirconia (YSZ) and ceria-zirconia mixture ( CeZrO2) thin films on Si(100) surfaces were studied in conjunction with O2 pressure. We showed that (110)-oriented films of CeO2, YSZ and CeZrO2 grew obeying the arrangements of Si atoms in (2×1) or (1×2) structures on a Si(100) surface under about 5×10-7 Torr O2 gas at about 900° C. On the other hand, under 1×10-5 Torr O2 gas, (111)-oriented CeO2 polycrystal films grew because of formation of Ce2O3 and Si O2 layers at the interface between CeO2 and Si(100). Furthermore, the (100)-epitaxial YSZ and CeZrO2 films grew, reducing the SiO2 layer on Si(100) under 1×10-5 Torr O2 gas. We attempted to explain these processes in terms of standard formation enthalpies of CeO2, Ce2O3, ZrO2 and SiO2.

Crystal and electrical characterizations of oriented yttria-stabilized zirconia buffer layer for the metal/ferroelectric/insulator/semiconductor field-effect transistor

Using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and X-ray pole figure measurements, we evaluated the crystallinities of yttria-stabilized zirconia (YSZ) thin films as an intermediate layer for metal/ferroelectric/insulator/semiconductor-structure field-effect transistors (MFIS-FETs). A highly oriented YSZ film was grown on a Si(100) substrate by the vacuum evaporation method. The [100] axes of the YSZ crystals were aligned parallel to [100] axes of Si crystals in the plane. In addition, electrical characterizations of the highly oriented YSZ thin films on Si(100) were evaluated from current–voltage ( I–V ) and capacitance–voltage ( C–V ) measurements. The I–V measurement indicated a breakdown field of about 3 MV/cm (at I=1 nA/cm2). The C–V measurement results suggest that mobile ions were present in the YSZ films. Oriented perovskite PbTiO3 films were deposited on YSZ crystal and YSZ/Si(100) substrates by the digital chemical vapor deposition (CVD) method. These PbTiO3 films included many PbTiO3 grains with their [100] axes parallel to the [100] or [110] axis of YSZ crystals in the plane of the PbTiO3/YSZ interface.

Crystal and Electrical Characterizations of Epitaxial CeXZr1-XO2 Buffer Layer for the Metal/Ferroelectric/Insulator/Semiconductor Field Effect Transistor

We evaluated the crystallinities of Ce X Zr 1-X O 2 (X = 0.10-0.20) thin films used as intermediate layer for metal/ferroelectric/insulator/semiconductor field effect transistors (MFIS-FETs), using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and X-ray pole figure measurement. The crystal alignments of Ce X Zr 1-X O 2 films on Si(100) were Ce X Zr 1-X O 2 [100]//Si[100] or Ce X Zr 1-X O 2 [100]//Si[001], in the plane. The breakdown field of Ce X Zr 1-X O 2 was about 3 MV/cm (at I = 1 nA/cm 2 ). From C-V measurements, it was found that the electrical properties of the intermediate layers of the MFIS-FETs were good. Oriented perovskite PbTiO 3 films were deposited on Ce X Zr 1-X O 2 /Si(100) substrates by digital chemical vapor deposition (CVD). These PbTiO 3 films included many PbTiO 3 grains aligned with the [100] or [001] axis parallel to the [101] axis of the Ce X Zr 1-X O 2 crystals at the plane in the PbTiO 3 /Ce X Zr 1-X O 2 interface. From C-V measurements of an Al/PbTiO 3 /Ce X Zr 1-X O 2 /Si(100) sample, we obtained a threshold hysteresis (memory window) of about 1.4 V.

Characteristics of Metal/Ferroelectric/Insulator/Semiconductor Structure Using SrBi 2Ta 2O 9 as the Ferroelectric Material

We fabricated a metal/ferroelectric/insulator/semiconductor (MFIS) structure using SrBi2Ta2O9 (SBT) as the ferroelectric material for the first time. The SBT thin film on CeO2/Si (100) had (105), (110) and (200) orientation. Threshold hysteresis (called ?memory window?) was observed in the capacitance-voltage ( C?V ) characteristics of this sample. The memory window was about 0.5 V. From the calculation of the C?V characteristics, it is estimated that the density of surface states at the CeO2/Si interface increases upon deposition of SBT. By X-ray diffraction (XRD) analysis, diffusion of Si to the surface of SBT was detected. In order to prevent such diffusion it may be necessary to reduce the firing temperature.

Influence of Ce Content on Crystal and Electrical Properties of CexZr1-xO2 Thin Films on Si(100) Substrates

We report on the crystalline and electrical properties of Cex Zr1-x O2 thin films having various Ce content. The Cex Zr1-x O2 thin films were deposited on Si(100) substrates by electron-beam evaporation. X-ray diffraction (XRD) and inductively coupled plasma emission spectroscopy (ICPES) analysis revealed that the crystal structure and lattice constants of Cex Zr1-x O2 thin films depend on the Ce content. An increased Ce content brings about the (111)-preferred orientation structure. Capacitance-voltage (C-V) measurements results indicate that the interface state density decreased with increasing Ce content to bring about good electrical properties of Cex Zr1-x O2/Si interface as a gate oxide.