Design of Capacitive Micromachined Ultrasonic Transducers for Application in Electronic Travel Aid for the Blind

A low cost, compact Capacitive Micromachined Ultrasonic Transducer (CMUT) based Electronic Travel Aid (ETA) to identify obstacle has been proposed. The aim is to enable free movement of the blind in an environment ridden with stationary obstacles with a target distance of 1-2m. In the present work, SU-8 has been chosen as the membrane material of the CMUT. This has enabled drastic reduction in the DC operating voltage to just 22V- an essential limit for handheld devices to be carried by the user. The CMUT is designed to operate at around 70kHz which minimises frequency dependent loss in air. The simulations are carried out in CoventorWare and COMSOL. The results show that on superimposing an AC of 0.5V on the DC, displacements as large as the gap distance of 5um are obtained resulting in output pressures of 140dB on surface of CMUT. On travelling a to and fro distance of 2m, this pressure drops to about 1uPa. To be able to sense this small pressure, a receiver with thin membrane is designed. A circuit has been implemented off chip to process the received signal and generate a PWM signal to drive the vibrator

Characterization of a CMUT Array

Ultrasound transducers are used in a broad range of applications covering from underwater communications to medical imaging and treatment. The ultrasonic transducer determines the key specifications such as resolution, sensitivity and signal to noise ratio. The capacitive micromachined ultrasonic transducer (CMUT) has emerged as an alternative to standard piezoelectric transducers due to advanced microelectronics fabrication technology and methods. Comparing to piezoelectric transducers, the CMUT is superior to it\u27s competitor with higher acoustic bandwidth, higher sensitivity and greater coupling with the acoustic medium. Design, fabrication, and characterization of a capacitive micromachined ultrasonic transducer (CMUT) array have been presented along this thesis. The array is designed to operate in the frequency range of 113-167 kHz. The CMUT array is fabricated using an SOI based fabrication technology and includes 6x6 CMUTs. Necessary test setups and readout circuitry is designed in order to carry out the characterization process. Static analysis results are verified with Wyko optical profilometer, Agilent LCR meter and SEM analysis. Dynamic characterizations are done with Polytec MSA-4 laser Doppler vibrometer. An efficient and low noise capacitive readout circuit is designed using transimpedance amplifier scheme with 75 kilo ohm gain and fabricated on a PCB. The developed analytical models, FEA and experimental results are in very good agreement to exhibit accuracy of the design methodology