Fabrication and characterization of cMUTs for forward looking intravascular ultrasound imaging

We have fabricated ring annular capacitive micromachined ultrasonic transducer (cMUT) arrays for forward looking IVUS operation in the 10 to 20MHz range with a maximum process temperature of 250°C. Our process uses a low stress plasma enhanced chemical vapor deposition (PECVD) silicon nitride membrane and provides localized sealing. The process is demonstrated on both silicon and dielectric substrates showing the potential for post-CMOS electronics integration and optical detection techniques while providing test arrays with significantly reduced parasitic capacitance. Arrays of 64 and 32 elements with approximately 1mm diameter have been fabricated. Measured pulse echo response in oil show that the cMUT has more than 70% fractional bandwidth around 15MHz, which is suitable for IVUS. Initial imaging results of an ultrasound source using a 16 element receive-only array are presented.