Fabrication and Characterization of PZT Fibered-Epitaxial Thin Film on Si for Piezoelectric Micromachined Ultrasound Transducer.

Pham Ngoc Thao,Shuji Tanaka,Shinya Yoshida

doi:10.3390/mi9090455

Abstract

This paper presents a fibered-epitaxial lead zirconate titanate (PZT) thin film with intermediate features between the monocrystalline and polycrystalline thin films for piezoelectric micromachined ultrasound transducer (pMUT). The grain boundaries confirmed by scanning electron microscopy, but it still maintained the in-plane epitaxial relationship found by X-ray diffraction analyses. The dielectric constant (εr33 = 500) was relatively high compared to those of the monocrystalline thin films, but was lower than those of conventional polycrystalline thin films near the morphotropic phase boundary composition. The fundamental characterizations were evaluated through the operation tests of the prototyped pMUT with the fibered-epitaxial thin film. As a result, its piezoelectric coefficient without poling treatment was estimated to be e31,f = −10–−11 C/m2, and thus reasonably high compared to polycrystalline thin films. An appropriate poling treatment increased e31,f and decreased εr33. In addition, this unique film was demonstrated to be mechanically tougher than the monocrystalline thin film. It has the potential ability to become a well-balanced piezoelectric film with both high signal-to-noise ratio and mechanical toughness for pMUT.

Highlights

A piezoelectric micromachined ultrasound transducer based on the flexural vibration of a membrane has been developed for various applications such as ultrasonic diagnostic, nondestructive evaluation of structures, proximity sensing and fingerprint identification [1,2,3,4,5,6,7,8]
The membrane is vertically deflected by a stress generated in the piezoelectric thin film by applying a driving voltage, and transmits an ultrasonic wave by spring-back motion
A piezoelectric output constant, g constant (e31,f /εr33ε0), is important, where εr33 and ε0 correspond to the relative dielectric constant and dielectric constant of vacuum, respectively. Considering both modes, the figure of merit (FOM) for piezoelectric micromachined ultrasound transducer (pMUT) is expressed as (e31,f )2/εr33ε0 [2,9]. This expression shows that the piezoelectric thin film with a high piezoelectric constant and a low dielectric constant is desired to improve the performance of pMUTs

Summary

Introduction

A piezoelectric micromachined ultrasound transducer (pMUT) based on the flexural vibration of a membrane has been developed for various applications such as ultrasonic diagnostic, nondestructive evaluation of structures, proximity sensing and fingerprint identification [1,2,3,4,5,6,7,8]. The membrane is vertically deflected by a stress generated in the piezoelectric thin film by applying a driving voltage, and transmits an ultrasonic wave by spring-back motion. In order to break this relationship, we developed a highly c-axis-oriented monocrystalline thin film of PZT family near MPB composition [19,20] This monocrystalline thin film exhibits large piezoelectricity (e31,f = −10–−14 C/m2) and a small dielectric constant (εr33 = 200–300). The high c-axis orientation and elimination of the grain boundary suppress the extrinsic factors such as domain motion and charge traps, increasing the dielectric constant [21,22] This PZT monocrystalline thin film promises to create a high performance pMUT, considering the FOM [2,20]. Its applicability to pMUT was investigated through the operation experiment

Materials and Methods

PZT Buffer layers Si

PZT etched area

Mechanical Properties