Piezoelectric films for high frequency ultrasonic transducers in biomedical applications

Abstract

Piezoelectric films have recently attracted considerable attention in the development of various sensor and actuator devices such as nonvolatile memories, tunable microwave circuits and ultrasound transducers. In this paper, an overview of the state of art in piezoelectric films for high frequency transducer applications is presented. Firstly, the basic principles of piezoelectric materials and design considerations for ultrasound transducers will be introduced. Following the review, the current status of the piezoelectric films and recent progress in the development of high frequency ultrasonic transducers will be discussed. Then details for preparation and structure of the materials derived from piezoelectric thick film technologies will be described. Both chemical and physical methods are included in the discussion, namely, the sol–gel approach, aerosol technology and hydrothermal method. The electric and piezoelectric properties of the piezoelectric films, which are very important for transducer applications, such as permittivity and electromechanical coupling factor, are also addressed. Finally, the recent developments in the high frequency transducers and arrays with piezoelectric ZnO and PZT thick film using MEMS technology are presented. In addition, current problems and further direction of the piezoelectric films for very high frequency ultrasound application (up to GHz) are also discussed.

Introduction

High frequency (HF) ultrasound is a promising imaging modality for providing a more detailed delineation of superficial anatomical structures, e.g., skin and eye or vascular structures via catheterization. Ultrasonic transducers with frequency higher than 30 MHz yield improved spatial resolution at the expense of a reduced depth of penetration. The resolution of ultrasound image is determined by the pulse bandwidth (axial resolution, R_axial) and the beam width (lateral resolution, R_lateral). Simple expressions for the ideal lateral and axial resolution of a focused transducer within the focal zone are given by:

$$\text{Axial resolution}(-6\text{dB}):R_{\text{axial}}=\frac{\lambda }{2\mathit{BW}}=\frac{c}{2f_c\mathit{BW}}$$

$$\text{Lateral resolution}:R_{\text{axial}}=\lambda F\#\frac{c}{f_c}F$$

where c is the speed of sound in the medium, f_c the center frequency of the transducer, BW the bandwidth of the transducer, F# the f-number (the ratio of focal distance to aperture dimension) and λ the wavelength. Thus, for a fixed number of cycles per pulse, an increase in frequency would result in a reduction in wavelength and pulse duration (increase in BW). If the transducer frequency is increased to 50 MHz, an axial resolution and lateral resolution of better than 25 μm and 60 μm for an f-number of 1.9 and bandwidth of 65% can be achieved respectively. However, at the same time, the penetration depth of ultrasound into the tissue must be sacrificed due to frequency-dependent attenuation. At 50 MHz, the depth of penetration for most tissues would be approximately to 8–9 mm.

HF single element transducers have been widely used in ultrasound imaging systems. However, mechanical scanning of a single element transducer shows poorer resolution out of focal zone and has limited frame rate. These problems can be overcome by adopting linear arrays, in which an image is formed by electronically sweeping a beam. Fabrication of high frequency ultrasonic linear arrays has remained a challenge following more than a decade of study. The task is even more difficult to build arrays at a frequency greater than 50 MHz which requires very small kerfs and thinner ceramic sheets [9]. Recent research has shown that micro-machining technology is a possible solution to such high-frequency applications although much improvement still needs to be made [10], [11], [12]. Piezoelectric films have already been widely used to fabricate micro-scale devices [1], [2], [3], [4], [5], [6], [7], [8], [13], [14], [15]. Among them, PZT thick film has been shown to exhibit good dielectric and piezoelectric properties, making it a possible ultrasonic transducer material candidate for high-frequency applications via micro-machined technology.

To date, there are a number of groups that have successfully fabricated PZT thick films for transducer applications. Tsuzuki et al. [16] prepared PLZT thick films by multiple electrophoretic deposition and sintering process. Barrow et al. [17] and Lukacs et al. [18] have reported thick PZT ceramic coatings using sol–gel derived porous 0–3 composites for high frequency transducer application. High frequency ultrasound array transducers using piezoelectric thin films on larger structures are being developed for high-resolution imaging systems by Trolier-McKinstry et al. [10]. In their work, metal-oxide semiconductor (CMOS) transceiver chip for a 16-element array was fabricated with ultrasound front-end chip containing beam-forming electronics and receiver circuitry. In addition, Dausch et al. reported the design and fabrication of 2D pMUT arrays [19]. They also described the vibrational modes for operation of flexure-mode pMUT elements, transmit output pressures, receive and pulse-echo characteristics, and B-mode imaging performance of the 81-element 2D arrays at a center frequency of approximately 7 MHz [20]. Despite above progress the improving quality of piezoelectric films and exploring optimum process for high frequency transducers are still challenge.

In this review, we focus on the design, fabrication and application of piezoelectric thick films for high frequency ultrasonic transducers in biomedical imaging application. Recent progress in the development and application of ultrasonic transducers is presented and the challenges in pushing the transducers/arrays to higher operating frequency (>100 MHz) are addressed. Section 2 gives the basic principles of piezoelectric effect. Section 3 reviews the important design issues for ultrasonic transducer. Section 4 addresses piezoelectric films for high frequency ultrasonic transducer application. Section 5 presents fabrication and characteristics of high frequency piezoelectric films transducers. The current problems and future direction for very high frequency transducer applications in the near future are also addressed.