Application of Aluminum Nitride Thin Film for Micromachined Ultrasonic Transducers
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Application of Aluminum Nitride Thin Film for Micromachined Ultrasonic Transducers Qianghua Wang, Jianzeng Xu, Changhe Huang, and Gregory W Auner Department of Electrical and Computer Engineering, Wayne State University Detroit, MI 48202, U.S.A. ABSTRACT This paper reports the fabrication and characterization of micromachined ultrasonic transducers (MUT) based on piezoelectric aluminum nitride (AlN) thin films. The MUT device is composed of an Al/AlN/Al sandwiched structure overlaid on top of a silicon (Si) diaphragm. X-ray diffraction (XRD) scan shows that highly c-axis oriented AlN (002) thin films have been grown on Al/Si(100) substrates. Electrical impedance of the MUT devices is analyzed as a function of frequency. The fundamental resonant frequencies of the devices are found in the range of 65-70 kHz, which are in approximation to the theoretical calculation. The effective coupling factors of the devices are also derived as 0.18.
INTRODUCTION Ultrasonic transducers, operating from 20 kHz to a few 100 kHz, have been extensively used for level evaluation in containers, process feedback in industrial automation, and collision avoidance in traffic control [1]. Conventional transducers lack the flexibility to incorporate with IC or MEMS technology for large scale production. The piezoelectric thin film materials, on the other hand, can be easily combined with bulk and surface micromachining technology to produce miniaturized piezoelectric transducers [2, 3], which have the potential to further integrate with on-chip Si electronics. One of a promising choice of such thin film materials is hexagonal wurtize AlN, a material known for its good piezoelectricity, large acoustic velocity, and mechanical robustness [4, 5]. This paper presents the design and fabrication of MUT array based on AlN thin films. The transducer operates in a flexural mode actuated by the Al/AlN/Al sandwiched structure. The Si diaphragm underneath provides the structure support and defines the resonator cavity. Electrical impedance measurements have been carried out to determine the fundamental resonance frequency and effective coupling factor of the MUT devices. The work lays out the foundation for the development of ultrasonic transducer array for sensing applications. DESIGN AND FABRICATION The MUT device consists of a flat-diaphragm resonator and an Al/AlN/Al sandwich structure, as shown in figure 1. The device in flexural mode has a maximum response as an actuator or a sensor at its fundamental resonant frequency. The fundamental flexural resonant frequency of a fixed-edge square plate is given by [6] h E f = 1.654 2 , (1) a ρ (1 − ν 2 )
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where h is the plate thickness and a is a side of the square. If only silicon (100) diaphragm is considered as the plate material, we have density ρ = 230 kg/m3, Young’s modulus E = 130 GPa, and Poisson’s ratio ν = 0.278.
AlN film
a Al film
SiO2 film
h Figure 1. Schematic of a flat-diaphragm transducer with piezoelectric AlN thin film. A p-type silicon (100) wafer in 3”
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