An implantable ultrasonic doppler blood flowmeter

A miniature ultrasonic transducer array for minimally invasive blood flow measurement has been developed. Compare with laser Doppler flowmeter, this tool is much smaller in size and is capable of reaching deep inside the tissue to monitor blood flow not accessible by the laser beam. Compared with traditional ultrasound Doppler flowmeter, this miniature tool is able to achieve more accurate measurement by using high frequency ultrasounds. The silicon substrate of the transducers array was micromachined into a probe shape using double-sided dry etching. The typical cross-sectional dimension of the silicon probe is 40μm×80μm. Arrays comprising 24-32 transducers were prototyped and tested. The 46μm- diameter drum shaped ultrasound transducers operate at a voltage lower than 40 volts, generate and receive ultrasound at a central frequency of 5.8MHz, and has a bandwidth of about 80%. A new polysilicon surface micromachining process was developed for fabrication of the transducer. The new process separates all the conductors from the silicon nitride film and reduces the charging problem associated with nitride-conductor interface.