PZT Thick Films for 100 MHz Ultrasonic Transducers Fabricated Using Chemical Solution Deposition Process
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PZT Thick Films for 100 MHz Ultrasonic Transducers Fabricated Using Chemical Solution Deposition Process Naoto Kochi1,2, Takashi Iijima2, Takashi Nakajima1 and Soichiro Okamura1 1 Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan 2 National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ABSTRACT To achieve ultrasonic transducers operating above 100 MHz, square pillar shaped Pb1.1(Zr0.53Ti0.47)O3 thick film structures were fabricated using a chemical solution deposition (CSD) process. The fabricated sample showed well-saturated P-E hysteresis curve and butterflyshaped longitudinal displacement curve. The fabricated samples generated more than 100 MHz ultrasonic waves with a pulser/receiver. Electrical impedance properties of the samples were measured with an impedance analyzer. A number of spurious resonant modes were observed in the frequency range from 40 to 300 MHz. The characteristics of the sample were investigated by finite element method (FEM). The FEM simulations were in good agreement with the experimental results. For free-standing (substrate free) 10-μm-thick PZT film models, the resonant frequency of the thickness vibration mode was estimated to be 160 MHz with the FEM simulations. These results indicate that the substrate affects the behavior of the spurious resonant modes. Therefore, a sample structure was designed using the FEM simulation. The FEM result suggests that the backside of the substrate should be removed to reduce the substrate effects. Consequently, the thickness vibration mode was observed clearly at 160 MHz. This structure is applicable to the micromachined ultrasonic transducers (MUT) operating in the thickness vibration mode above 100 MHz. INTRODUCTION Micromachined ultrasonic transducers (MUT) are attractive devices for several applications such as sonars for underwater exploration [1], nondestructive evaluation, and medical imaging systems. There are two types of MUT: capacitive MUT (cMUT) [2, 3] and piezoelectric MUT (pMUT). By comparison with cMUT, pMUT have advantages such as simplicity of fabrication, higher capacitance, and lower impedance [4]. The MUT operating at high frequency have possibilities to enhance the spatial resolution of the medical imaging devices. For example, transducers operating in the range of 20 to 30 MHz have been studied for skin, intravascular, and ophthalmic imaging [5]. In order to observe biological tissue images clearly, ultrasonic waves above 100 MHz are required. However, it is difficult to realize high frequency transducers using the technologies based on bulk ceramic materials because the operating frequency of the transducers generally depend on the geometry of piezoelectric materials such as thickness. Therefore, it is important to develop microfabricated piezoelectric films in the range of several microns. Recently, preparation techniques of the piezoelectric thin films have been well studied for microelectromechanical systems (MEMS) [6, 7].
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