Abstract
Experimental measurement results of a 1.75 mm × 1.75 mm footprint area Capacitive Micromachined Ultrasonic Transducer (CMUT) planar array fabricated using a bisbenzocyclobutene (BCB)-based adhesive wafer bonding technique has been presented. The array consists of 40 × 40 square diaphragm CMUT cells with a cavity thickness of 900 nm and supported by 10 µm wide dielectric spacers patterned on a thin layer of BCB. A 150 µm wide one µm thick gold strip has been used as the contact pad for gold wire bonding. The measured resonant frequency of 19.3 MHz using a Polytec™ laser Doppler vibrometer (Polytec™ MSA-500) is in excellent agreement with the 3-D FEA simulation result using IntelliSuite™. An Agilent ENA5061B vector network analyzer (VNA) has been used for impedance measurement and the resonance and anti-resonance values from the imaginary impedance curve were used to determine the electromechanical coupling co-efficient. The measured coupling coefficient of 0.294 at 20 V DC bias exhibits 40% higher transduction efficiency as compared to a measured value published elsewhere for a silicon nitride based CMUT. A white light interferometry method was used to measure the diaphragm deflection profiles at different DC bias. The diaphragm center velocity was measured for different sub-resonant frequencies using a Polytec™ laser Doppler vibrometer that confirms vibration of the diaphragm at different excitation frequencies and bias voltages. Transmit and receive operations of CMUT cells were characterized using a pitch-catch method and a −6 dB fractional bandwidth of 23% was extracted from the received signal in frequency domain. From the measurement, it appears that BCB-based CMUTs offer superior transduction efficiency as compared to silicon nitride or silicon dioxide insulator-based CMUTs, and provide a very uniform deflection profile thus making them a suitable candidate to fabricate highly energy efficient CMUTs.
Highlights
The capacitive micromachined ultrasonic transducer (CMUT) is a type of microelectromechanical systems (MEMS) device that is constructed using processes similar to conventional VLSI technology to generate and receive ultrasound [1]
The measured coupling coefficient of 0.294 at 20 V DC bias exhibits 40% higher transduction efficiency as compared to a measured value published elsewhere for a silicon nitride based CMUT
A DC bias voltage superimposed with an AC signal of desired frequency is applied to the CMUT geometry to generate a time varying electrostatic force that causes the diaphragm to vibrate to create ultrasonic vibration in the surrounding medium
Summary
The capacitive micromachined ultrasonic transducer (CMUT) is a type of microelectromechanical systems (MEMS) device that is constructed using processes similar to conventional VLSI technology to generate and receive ultrasound [1]. Silicon nitride or silicon dioxide are commonly used to realize the dielectric spacer in a CMUT structure Both silicon nitride and silicon dioxide suffer from high ionic contamination, trapped charges, and wafer bow or cracking during the high temperature annealing process that eventually degrades the transduction efficiency and leads to device failure [10]. It is worth to mention here that BCB is an excellent adhesive wafer bonding agent with a fracture strength comparable to silicon [18,19,20] These observations lead to the conclusion that if BCB could be used as the dielectric spacer between the diaphragm and the backplate of a CMUT, the effects of trapped charges or fixed ionic charges at the interface of the dielectric and silicon can be highly minimized while achieving excellent structural and processing.
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