Seiichi Arai

Enhancement of remanent polarization in epitaxial BaTiO3/SrTiO3 superlattices with “asymmetric” structure

Enhancement of remanent polarization has been demonstrated in epitaxial “asymmetric” BaTiO3/SrTiO3 strained superlattices, in which the thickness ratio of BaTiO3 to SrTiO3 layers is changed at molecular layer order accuracy. The superlattices have been prepared on Nb-doped SrTiO3 (100) single-crystal substrates by a pulsed-laser deposition technique. The superlattice with a stacking periodicity of 15 unit cells BaTiO3/3 unit cells SrTiO3 shows the largest remanent polarization 2Pr of 46 μC/cm2, which is about three times that of the BaTiO3 single-phase film formed under the same condition. The increase in the remanent polarization is attributed both to the BaTiO3-rich structure and to the increase in lattice parameter c due to the mismatch of in-plane lattice parameters between BaTiO3 and SrTiO3.

Epitaxial growth and dielectric properties of (111) oriented BaTiO3/SrTiO3 superlattices by pulsed-laser deposition

Epitaxial BaTiO3(111)/SrTiO3(111) multilayered thin films have been investigated with various periods of stacking layer between 0.45/0.45 and 10/10 nm on Nb-doped SrTiO3(111) substrates by a pulsed-laser deposition technique. Upon decreasing the period of each layer, the spacing of (111) plane (d111) of the multilayered film increases, and the relative dielectric constant goes up to 594 which is twice as large as that of (111) oriented (Ba0.5, Sr0.5)TiO3 solid-solution film. The expansion of d111, which might be attributed to an in-plane pressure effect due to the large lattice strain in the heteroepitaxial interface, contributes to the enlargement of relative dielectric constant. Remanent polarization observed in polarization versus applied voltage hysteresis loop is no more than 2.7 μC/cm2 with 2.0/2.0 nm period of layer.

Effect of Buffer Electrodes in Crystallization of Zinc Oxide Thin Film for Thin Film Bulk Acoustic Wave Resonator

A thin film bulk acoustic wave resonator (TBAR) has been fabricated using a ZnO thin film on a SiO 2 diaphragm by MEMs techniques. The ZnO/SiO 2 structure TBAR can be designed to cancel a temperature coefficient of frequency (TCF) by the ZnO/SiO 2 thickness ratio, because the TCF of ZnO is negative, and that of SiO 2 is positive. The ZnO thin film on the SiO 2 shows a c-axis orientation almost equivalent to that of the ZnO thin film on a glass substrate by RF sputtering. However, the crystallinity of the ZnO thin film is influenced by the surface conditions of substrates. ZnO thin films have been deposited on Au/Cr, Au/NiCr and Au/Ti. The Au/Ti/ZnO/Au/Ti/SiO 2 structure TBAR shows the best resonant characteristics in this experiment. The resonant characteristics of the TBAR depend on the crystallinity of the ZnO thin film. The resonant resistance of the TBAR at 205MHz using a Au/Ti under electrode is about 10% less than that using an Au/Cr electrode. The x-ray diffraction result shows that the crystallinity of ZnO is greatly influenced by the crystallinity of the lower electrode. The buffer layer between an Au electrode and substrate has an influence on both the crystallinity of the ZnO thin film and the resonant characteristics of the TBAR through the Au electrode.

Fabrication of 5GHz band film bulk acoustic wave resonators using ZnO thin film

We successfully fabricated 5GHz band thin film bulk acoustic wave (BAW) resonators using ZnO thin film. Two types of BAW resonators utilizing fundamental resonance and secondary harmonics were examined. In the fundamental mode resonator, the quality factor (Q factor) and the effective electromechanical coupling coefficient (k/sub teff//sup 2/) were realized at 1000 and 6.7%, respectively. These values for the secondary mode resonator were 700 and 4.7%, respectively. Combining four fundamental mode resonators as a ladder type piezoelectric filter, the bandwidth at 3 dB was achieved to be 191 MHz at 4.9 GHz. Even with the filter using the secondary mode resonator, the bandwidth was 130 MHz at 5.2 GHz.

A 180nm lift-off process for 5GHz band surface acoustic wave filters using an i-line reduction stepper

Using an i-line reduction stepper, we successfully realized a lift-off resist pattern with a 180nm line and space (L/S) resolution. This pattern was achieved by combining the multilayer resist (MLR) process that we originally developed and optimum dipole off-axis illumination (OAI). The technology extends to resolution limit of i-line photolithography from 250nm to 180nm resolution. Using this technology, we fabricated 180nm L/S Al electrodes for a 5GHz band surface acoustic wave (SAW) filter. The characteristics of the filter were demonstrated.