Daisuke Akai

Ferroelectric properties of sol-gel delivered epitaxial Pb(Zrx,Ti1−x)O3 thin films on Si using epitaxial γ-Al2O3 Layers

This letter reports on the fabrication and investigation of ferroelectric epitaxial Pb(Zr,Ti)O3(PZT)∕Pt films on Si substrates using epitaxial γ-Al2O3 buffer layer for Si integrated ferroelectric devices. (001) and (111) epitaxial γ-Al2O3 films were grown on Si(001) and Si(111) substrates, respectively, using chemical vapor deposition. PZT films with various compositions were epitaxially grown on epitaxial Pt coated substrates using a sol-gel method. Epitaxial PZT films exhibited better ferroelectric and pyroelectric properties than polycrystalline PZT films. In particular, maximum pyroelectric coefficients of the epitaxial films were obtained, with values of 1.8×10−8C∕cm2K for the PZT(001) film with a Zr∕Ti ratio of 40∕60 and 1.4×10−8C∕cm2K for the PZT(111) film with a Zr∕Ti ratio of 52∕48.

Ferroelectric and pyroelectric properties of highly (110)-oriented Pb(Zr0.40Ti0.60)O3 thin films grown on Pt∕LaNiO3∕SiO2∕Si substrates

Pb(Zr0.4Ti0.6)O3 thin films with a thickness of 500nm were spin coated on (110) preferred Pt bottom electrodes using a sol-gel method, in which the (110) preferred Pt bottom electrode was developed by using a (100)-oriented conductive oxide electrode LaNiO3 film as an adhesion layer on a SiO2∕Si substrate. X-ray diffraction analysis and field emission scanning electron microscopy show that the as-grown Pb(Zr0.4Ti0.6)O3 films are highly (110)-oriented with a columnar structure. It indicates that the (110) preferred Pt bottom electrode is effective for growing highly (110)-oriented Pb(Zr0.4Ti0.6)O3 films. The as-grown Pb(Zr0.4Ti0.6)O3 films show excellent dielectric and ferroelectric properties with dielectric constant e33T∕e0=1620, loss tangent tanδ=2.1%, spontaneous polarization 2Ps=158μC∕cm2, and remnant polarization 2Pr=92μC∕cm2. Excellent pyroelectric properties are also detected in the (110)-oriented films. At room temperature, the pyroelectric coefficient and the figure of merit for detectivity can r...

Miniature ultrasound acoustic imaging devices using 2-D pMUTs array on epitaxial PZT/SrRuO 3 /Pt/³-Al 2 O 3 /Si structure

In this paper, we demonstrate ultrasound acoustic imaging using a miniature 2-D piezoelectric micromachined ultrasonic transducers (pMUTs) array on an epitaxial PZT/SrRuO3(SRO)/Pt/³-Al2O3/Si structure for the first time. 2-D pMUTs arrays containing 64 ch (8 × 8 arrays) with 100 µm-diameter circle shape element were fabricated. Detection properties of ultrasonic were measured in water with a ceramics PZT transducer as a point ultrasonic source. The epitaxial structure indicates higher sensitivity than the conventional SiO2/Si structure. 2-D acoustic images for acrylic targets were successfully observed using pMUTs array, which shows potentials for high resolution 3-D acoustic imaging.

Fabrication of the Si/Al2O3/SiO2/Si Structure Using O2 Annealed Al2O3/Si Structure

The formation of a SiO2 layer in the interface between epitaxial Al2O3(100) and a Si(100) substrate, in order to improve the insulation, was studied using O2 annealing at 1000°C, and the Al2O3(100)/SiO2/Si(100) structure was proposed as a Si-on-insulator (SOI) substrate. The breakdown voltage property of Al2O3/SiO2/Si, rather than that of the Al2O3/Si structure, was improved from 8 V to 50 V. Moreover, Si(100) growth on the Al2O3/SiO2/Si structure was carried out, and the morphology of the Si top layer was very smooth. Metal-oxide-semiconductor field effect transistors (MOSFETs) on the substrates show excellent electrical properties, which indicates the high crystallinity of SOI.

Vibration Analysis and Transmission Characteristics of Piezoelectric Micromachined Ultrasonic Transducers Using Epitaxial Pb(Zr,Ti)O3 Thin Films on γ-Al2O3/Si Substrate

Low-voltage (<12 V)-operation piezoelectric micromachined ultrasonic transducers (pMUTs) are hoped to be integrated with Si-LSI. Realizing this requires not only improving the piezoelectric properties, but also clarifying the vibration modes of transducers. In this study, we experimentally investigated the vibration mode and transmission ultrasonic waves by pMUTs using epitaxial Pb(Zr,Ti)O3 thin films on the epitaxial γ-Al2O3/Si substrates to radiate ultrasonic waves effectively. Five resonance vibration modes appeared in 100-µm-diameter circular pMUTs at frequencies in the range from 1 to 10 MHz in air. The frequency of mode (0,1) was 2.2 MHz, and this mode radiates ultrasonic waves very effectively. Ultrasonic transmission was also achieved in water using the above vibration mode, applying voltage of less than 10 Vp–p. These results show the potential for high-frequency pMUT integration with Si devices.

Ultrasonic beam formation by pMUTs array using epitaxial PZT thin films on γ-Al 2 O 3 /Si substrates

We have proposed γ-Al₂O₃ thin films as an epitaxial buffer layer on Si substrates for obtaining epitaxial Pb(Zr,Ti)O₃ (PZT) thin films and reported a 2-D ultrasonic image sensor and measurement and analysis of ultrasonic transmission using piezoelectric micromachined ultrasonic transducers (pMUTs) on γ-Al₂O₃/Si substrates. -6 dB beamwidth and sound pressure at 5 mm from surface of the 4 channel pMUT array were exbited 10 times larger than signle pMUT. These experimental results good agree with analysis results. It will be applied to design miniature 2-D pMUTs array ultrasonic probe.

Fabrication of MEMS diaphragm transducer array based on epitaxial PZT thin film for 2-D hydrophone application

A piezoelectric ultrasonic hydrophone array was fabricated using diaphragm transducer on Si substrates. A two dimensional (2-D) array of 8 times 8 elements with diaphragm transducer has been developed by XeF2 etching from the topside of Si. In this paper, we describe the fabrication process of the diaphragm transducer array. A finite-element analysis (FEA) was calculated to evaluate the resonant frequency of the diaphragm transducer. The potential for this hydrophone array is discussed.

P1J-3 Evaluation of 2D Hydrophone Array System Using Epitaxial PZT Thin Film Grown on Epitaxial GAMMA-Al2O3/Si Substrate

A piezoelectric ultrasonic hydrophone array was fabricated using epitaxial lead zirconate titanate, Pb(Zx, Ti1-x)O3 (PZT) thin film grown on epitaxial SrRuO3/Pt/gamma-Al2O3/Si substrates. PZT film with 3 mum in thickness was prepared by sol- gel method. An experimental ultrasonic hydrophone array measurement system, a one dimensional (1-D) array of 10 elements and a two dimensional (2D) array of 4 times 4 elements have been developed to evaluate the hydrophone array system performance. In this paper, we describe the fabrication process of the hydrophone arrays and the data acquisition system. The beam direction was estimated and the B-mode image was reconstructed to evaluate the array sensitivity. The potential applications for this hydrophone array are discussed.

Sensitivity and resonance frequency with changing the diaphragm diameter of piezoelectric micromachined ultrasonic transducers

In this work, we investigate the sensitivity and resonance frequency of pMUTs by changing the diameter of the diaphragm in order to improve sensitivity. Five types of pMUTs which have different diaphragm diameters and three types of ultrasonic source which have differing transmitting frequencies were used in the evaluation. The pMUT with an 80-μm-diameter diaphragm showed the largest sensitivities with the 3.5-MHz ultrasonic source. The 60-μm-diameter and 90-μm-diameter pMUTs exhibited the highest sensitivities with the 1.75-MHz ultrasonic source. This is in good agreement with the analytical results and it could be expected that the sensitivities were improved by changing the diaphragm diameter of the pMUTs.

Quantifying modal shapes in smart piezoelectric ultrasonic transducer array: Modeling and experiment

In this paper, a novel technique has been developed to quantify modal shapes in piezoelecric functional thin films-based micromachined ultrasonic transducers (pMUTs) for ultrasonic diagnostics. To obtain high sensitivity, an array of smart pMUTs is designed and successfully fabricated with epitaxially grown functional eutectic Pb(Zr0.52Ti0.48)O3 (PZT) thin film on Si(111)/γ - Al2O3(111)/SrRuO3(111) substrates. Incorporating crystalline γ- Al2O3 insulator on Si substrates permitted to control smartly the orientation of piezoelectric film. The characterization is performed employing advanced 3D finite element modeling taking crystallographic anisotropy into account. Modal shapes were captured experimentally using Laser Doppler Vibratometry (LDV). In this pioneering work, an excellent correlation is realized between different resonance frequencies and mode shapes corresponding to different energy states obtained in computations and experiments. Quantification of eigenfrequencies with mode shapes has shown significant influence on transmit-receive characteristics of pMUTs. Utilizing the correlation results, ultrasonic wave transmitting experiment was successfully carried out using fabricated pMUTs as a transmitter and a commercial PZT hydrophone as a receiver.