Takaaki Yasumoto

Voltage shift phenomena in a heteroepitaxial BaTiO3 thin film capacitor

Voltage shift phenomena of the hysteresis loop were characterized for a c-axis oriented heteroepitaxial BaTiO3 film by means of switching current measurements using various types of pulse sequences. During application of voltage, the hysteresis loop gradually shifted along the voltage axis according to the polarity of the voltage. Even after the application of voltage, while the top and bottom electrodes were short-circuited, the hysteresis loop continued to move. Under certain conditions, a part of the hysteresis loop shifted back, whereas the rest shifted forward. These results were explained, assuming that there is a nonswitching layer between the ferroelectric layer and the bottom electrode, and that the discontinuity of polarization can be compensated by injection of negative charges from the electrode. It was suggested that the nonswitching layer is possibly formed by relaxation of lattice misfit strain in the heteroepitaxial ferroelectric thin film.

P2G-4 Suppression of Acoustic Energy Leakage in FBARs with Al Bottom Electrode: FEM Simulation and Experimental Results

One of the most challenging issues in designing film bulk acoustic wave resonators (FBARs) is how to realize high-Q resonators. According to our experimental results, an acoustic leakage is the dominant loss factor at antiresonance frequency for FBARs with an aluminum bottom electrode. In this paper, we report simulation results obtained using the 2-dimensional finite element method (2D FEM), which was employed in order to confirm the above-mentioned acoustic loss mechanisms and optimize the design parameters of the resonator. As a result, optimizing the aluminum bottom electrode thickness and properly designing an attenuation structure that reflects the laterally propagating Lamb waves inside the resonator areas suppress the acoustical leakage significantly. Comparisons between FEM simulation and measured results in terms of the relationship between the Q-factors at antiresonance frequency and the structural parameters of the resonators are shown.

Nonswitching Layer Model for Voltage Shift Phenomena in Heteroepitaxial Barium Titanate Thin Films

We have introduced a nonswitching layer model to explain voltage shift phenomena of hysteresis loops in a heteroepitaxial barium titanate thin film capacitor. A nonswitching layer, which has irreversible spontaneous polarization, was assumed to be present between the ferroelectric layer and the bottom electrode layer. Calculation based on the model demonstrated that the presence of the nonswitching layer would cause an inherent voltage shift of the hysteresis loop from the origin along the voltage axis. If free charges accumulate at the interface between the ferroelectric layer and the nonswitching layer to compensate the discontinuity of the polarization, the hysteresis loop would inversely move toward the opposite direction. A series of experimental results observed for a c-axis oriented barium titanate thin film seemed consistent with the model. It was suggested that the nonswitching layer may be formed in accordance with relaxation of lattice misfit stress in the early stage of the heteroepitaxial growth.

Epitaxial Growth of BaTiO3 Thin Films by High Gas Pressure Sputtering

Heteroepitaxial BaTiO3 thin films with lattice-misfit strain were prepared on SrRuO3/SrTiO3 substrates by rf magnetron sputtering under high gas pressure, in order to realize large area deposi-tion with uniform ferroelectric properties. Lattice constants, composition and ferroelectric properties of the epitaxial BaTiO3 films were characterized as a function of pressure as well as the incident angle of sputtered particles, under the assumption that the sputtered particles were eradiated from the eroded area of the target surface. Although ferroelectric properties of films sputtered under low gas pressure had remarkable place dependability, they have been drastically improved by high gas pressure sputtering. X-ray diffraction analyses revealed that the BaTiO3 film deposited with a pressure of 6.3 Pa had a large lattice strain of 5%, even though the substrate was placed at a position directly facing the erosion area of the target, where a number of high-energy ions such as oxygen ions are thought to collide against the film surface if the pressure is low. The BaTiO3 film exhibited a ferroelectric hysteresis loop with remnant polarization of 43 µC/cm2. These results indicate that high gas pressure sputtering is effective for achieving large area uniformity of ferroelectric properties in heteroepitaxial BaTiO3 thin films.

Ferroelectric properties of epitaxial barium titanate thin film capacitors on silicon substrate

Abstract A heteroepitaxial BaTiO3 film with a thickness of 65 nm was prepared on SrRuO3/Pt/TiAlN/Si. The Ti0.91Al0.09N film was grown on Si substrate by DC sputtering in Ar/N2 ambient at 700°C. The BaTiO3, SrRuO3 and Pt films were deposited by radio-frequency magnetron sputtering at substrate temperatures ranging from 450°C to 600°C. The BaTiO3 film has a c-axis 1.6% longer (0.410 nm) than that of bulk, due to lattice misfit between SrRuO3 and BiTiO3. When positive and negative voltage pulses with an amplitude of 5 V were applied, a switching charge density, Q sw, of 40 μC/cm2 was obtained. No fatigue was observed after 1011 cycles of polarization switching even when an amplitude of 7 V was applied. Remanence of polarization for 4 hours was confirmed in a retention test. These results indicate that the heteroepitaxial BaTiO3 film capacitors on Si substrates can be used for ferroelectric nonvolatile memory applications.

High-q thin film bulk acoustic wave resonator using highly [111]-oriented aluminum electrode

The selection of the bottom electrode materials for FBARs is a key issue in regard to the fabrication of highly c-axis oriented AlN thin films and high-Q resonators. In this paper, high-Q FBARs using aluminum electrodes are presented. An amorphous metal underlayer enables fabrication of highly -oriented Al thin films with XRD rocking curve FWHM of 0.6°. Using these high quality Al films, we have obtained highly c- axis oriented AlN films. These AlN films showed excellent crystallinity even at the initial stages of the AlN film deposition. The fabricated resonators showed good resonance characteristics, such as high-Q values (loaded-Q ~800) and high effective coupling constants (~6.7%). The impedance ratio (Zmax/Zmin) was more than 1500, corresponding to an FOM (Figure of merit) of 51.

Etching resistivity of AlN ceramics surface

The etching resistivity of aluminum nitride (AlN) surface was investigated on properties of pull-out for AlN ceramics grain and uneven dissolution in chemical agents. Maximum step irregularity (MSI) of AIN with Y/sub 2/O/sub 3/ finally reaches to 7/spl times/10/sup -7/m at 10 k s at 50/spl deg/C in the aqueous solution of 2.5 mol KCN. This value is below the grain size of AlN, 4/spl times/10/sup -6/m. AlN grains did not begin to pullout, when the MSI value was below the AlN grain size. MSI of AlN with CaO reached to 2/spl times/10/sup -2/m at 3 ks in the same solution. This value is equal to the limit value of 2/spl times/10/sup -4/m. No pit was observed on the surface of AIN with Y/sub 2/O/sub 3/ until 10 ks from SEM result. However, many pits were observed at the grain boundary even at 30 s on the surface of AlN with CaO. The etching resistivity for AlN with Y/sub 2/O/sub 3/ is about 300 times as long as that with CaO. The surface of AlN ceramics with Y/sub 2/O/sub 3/ was confirmed to be stable against the KCN aqueous solution. The surface of AlN with CaO received a great deal of damage in etching solution. The AlN with Y/sub 2/O/sub 3/ is suitable choice for a MCM substrate with wet etching process.

Densification of Tungsten Conductors in Cofired Aluminum Nitride Multilayer Substrates by the Addition of Manganese Oxide

The sintering behavior of tungsten (W) conductors embedded in aluminum nitride (A1N) multilayer substrates was improved by adding manganese dioxide (MnO2) to A1N green sheets. To obtain A1N suspensions, MnO2 powder (purity: 99.99%) was mixed with A1N powder, yttrium oxide (Y2O3) powder (as a sintering additive), casting organic additives, and organic solvents by ball milling. Each suspension was cast into A1N green sheets and tungsten conductors were then printed on them. To obtain A1N multilayer substrates, the green sheets were laminated and then sintered at 1780, 1820, and 1850°C for 4 hours in a nitrogen atmosphere. The microstructure of the fracture surface of the A1N multilayer substrate with added MnO2 indicated that the densified W conductor layers were without pores. Segregation of manganese (Mn) was observed among W grains by EPMA in the A1N multilayer substrate heated up to 1500°C. Moreover, rapid W grain growth was observed in the specimen, and Mn wetted these W grains well. These results indicated that molten Mn penetrated into W grain boundaries from the A1N matrix and accelerated W grain growth by liquid phase sintering during A1N multilayer substrate cofiring. Therefore, the A1N multilayer substrates, including the densified W conductors, were prepared by the addition of MnO2 into A1N green sheets.