Chenghao Wang

Effect of Annealing Process on Properties of Pb(Zr0.52Ti0.48)O3 Thin Films Prepared by Sol-Gel Method

The effect of the annealing process on properties of Pb(Zr0.48Ti0.52)O3 (PZT) thin films prepared by sol-gel method was systematically studied. PZT film deposited by the single layer pre-crystallization processing followed by multilayer crystallization processing is easy to grow along (110) orientation. The PZT films were prepared by the single layer annealing process and show (100) preferred orientation at lower crystallization temperature, while (111) and (110) orientation growth is occurs easily in higher the crystallization temperature. The PZT films deposited at higher crystallization temperature have larger average grain, but the high temperature cause the volatilization of lead, film density to decrease.

Design and Fabrication of Piezoelectric Micromachined Ultrasound Transducer (pMUT) with Partially-Etched ZnO Film

A square piezoelectric composite diaphragm was analyzed by the finite element method to enhance the sensitivity of a piezoelectric micromachined ultrasound transducer (pMUT). The structures of electrode and piezoelectric film were optimized and a centric electrode was designed to avoid the counteraction of stress in the centre and edges. In order to further improve the sensitivity; a pMUT with partially-etched piezoelectric film was adopted. The receive and transmit sensitivities of the pMUT were analyzed in details. The receive sensitivity of pMUT with partially-etched ZnO film is 3.3 dB or 6.8 dB higher than those with a centric and whole electrode, respectively; and the amplitude of a partially-etched ZnO film pMUT under a certain voltage is 5.5 dB and 30 dB higher than those with centric and whole electrode separately. Two pMUT-based ZnO films were fabricated by micromachining technology and their receive and transmit sensitivities were tested. The ZnO films deposited by direct current (DC) magnetron sputtering exhibit a densely packed structure with columnar crystallites. The test results show that the structure of the square diaphragm with partially-etched piezoelectric layer can significantly improve the transducer sensitivity. The receive sensitivity and transmit sensitivity are −238.35 dB (ref. 1 V/μPa) and 150.42 dB (ref. 1 μPa/V); respectively.

High Q, high frequency, high overtone bulk acoustic resonator with ZnO films

Bulk acoustic wave resonators with piezoelectric films have been widely explored for the small size and high quality factor (Q) at GHz. This paper describes a high overtone bulk acoustic resonator (HBAR) based on Al/ZnO/Al sandwich layers and c-axis sapphire substrate. ZnO film with high quality c-axis orientation has been obtained using DC magnetron sputtering. The fabricated HBAR presents high Q at the multiple resonances from a 0.5–4.0 GHz wide band with a total size (including the contact pads) of 0.6 mm×0.3 mm×0.4 mm. The device exhibits the best acoustic coupling at around 2.4 GHz, which agrees with the simulation results based on the one-dimensional Mason equivalent circuit model. The HBAR also demonstrates Q values of 30 000, 25 000, and 6500 at 1.49, 2.43, and 3.40 GHz, respectively. It is indicated that the HBAR has potential applications for the low phase noise high frequency oscillator or microwave signal source.

Design and Analysis of MEMS Linear Phased Array

A structure of micro-electro-mechanical system (MEMS) linear phased array based on “multi-cell” element is designed to increase radiation sound pressure of transducer working in bending vibration mode at high frequency. In order to more accurately predict the resonant frequency of an element, the theoretical analysis of the dynamic equation of a fixed rectangular composite plate and finite element method simulation are adopted. The effects of the parameters both in the lateral and elevation direction on the three-dimensional beam directivity characteristics are comprehensively analyzed. The key parameters in the analysis include the “cell” number of element, “cell” size, “inter-cell” spacing and the number of elements, element width. The simulation results show that optimizing the linear array parameters both in the lateral and elevation direction can greatly improve the three-dimensional beam focusing for MEMS linear phased array, which is obviously different from the traditional linear array.

Influence of electrodes on the effective electromechanical coupling coefficient distributions of high-overtone bulk acoustic resonator.

In this paper, the influence of the electrodes on effective electromechanical coupling coefficient k(2)eff distribution of high-overtone bulk acoustic resonators (HBAR) is investigated using the four-layer thickness extension mode composite resonator model. The k(2)eff distributions and the spacing of the parallel resonance frequency (SPRF) distributions are calculated for HBARs with different electrode and substrate materials. The frequency of first peak fM in k(2)eff distribution curve is close to the frequency of first valley in SPRF distribution curve, which has the maximum deviation of about 20%. The fM ratio of Al0.1μm-ZnO0.6μm-Au0.1μm-Sapphire400μm to Au0.1μm-ZnO0.6μm-Al0.1μm-Sapphire400μm is 1.91 which indicated that the k(2)eff distribution of HBAR is affected significantly by different electrode materials. We discuss for the first time the influence of the acoustic impedance ratio of the electrodes to substrate to the k(2)eff distribution, which is independent on the thickness ratio of the electrodes to piezoelectric film. It is found that for the pure-hard and pure-soft substrate HBARs, the resonator frequency of piezoelectric sandwich structure can be used to estimate the resonator frequency fM of the first peak of k(2)eff curve, which has the maximum deviation of 7%. The fabricated HBAR(I) Al-ZnO-Al-Sapphire and HBAR(II) Al-ZnO-Au-Sapphire shows identical outlines of k(2)eff and SPRF curves with the simulations, which have fMs of about 2GHz and 3GHz. The k(2)t of ZnO films are extracted from the first peak of the k(2)eff curve. The ZnO film deposited on Al and Au electrodes have k(2)t of 0.0597 and 0.0615, respectively.

Design of MEMS Piezoelectric Vector Hydrophone

This paper presents a novel micro-electromechanical system (MEMS) piezoelectric vector hydrophone based on ZnO thin film. The approximate theory analysis is introduced in sensitivity and resonant frequency, and a simulation on the vector hydrophone is carried out by finite element method (FEM). Results of the theory analysis and FEM simulation agree well. The results show that the ratio of the piezoelectric layer thickness and Si layer thickness exists an optimum value for sensitivity, and MEMS piezoelectric vector hydrophones in smaller size have higher sensitivity than piezoresistive ones and are of passivity.

Design and Fabrication of a Novel PZT Films Based Piezoelectric Micromachined Ultrasonic Transducers

A novel piezoelectric micromachined ultrasonic transducer (pMUT), has been designed, and successfully fabricated .A bridge type structure in flexure mode was adopted for the design of the pMUT element. The transducers comprise a 4-6μm thick silicon layer, a 1μm thick thermal oxide layer, and 1.1μm thick piezoelectric PZT layer . The PZT films were deposited by a sol-gel technique. The influence of different annealing processes on the films properties had been investigated. The PZT films annealed at 650¿ for half an hour in air exhibit preferred orientation in the direction of the (100) plane. The piezoelectric coefficient d33 of PZT films was measured as 120 pC/N. The bridge diaphragms are much more compliant and free from the clamped stress in two side edges in vibrating diaphragms in comparison with diaphragms clamped on all four edges. The sensitivity of novel pMUT is expected to be higher.