Ultrasonic sensing using thermal-mechanical noise recorded on monolithic CMUT-on-CMOS arrays

Monolithic integration of miniature CMUT arrays and CMOS electronics minimizes interconnect parasitics and enables detection of the thermal-mechanical component of the ultrasonic noise field. Consequently, an estimate of the pulse-echo response (or Green's function) between two CMUT array elements can be obtained from the cross-correlation of ambient noise recorded by these two sensors. This provides a foundation for passive ultrasound sensing and imaging using only the thermal-mechanical noise field, without the use of active transmitter elements. We designed and fabricated monolithic a 32 element CMUT-on-CMOS array for intravascular imaging with low noise transimpedance amplifiers. Demonstration experiments were conducted by immersing the CMUT array in a water bath to sense/image the water-air interface from noise signals in the frequency band 12MHz-22MHz. The normalized cross-correlations function computed between selected pairs of receivers show a clear echo associated with the water interface consistent with the sensor and target location. These target echoes were found to shift in a consistent symmetric fashion when we reduced the thickness of the water layer from 2.0mm to 1.5mm. Using low noise amplifier designs in a CMUT-on-CMOS implementation allows for extracting pulse-echo-like arrivals from thermal-mechanical noise cross-correlations between CMUT array elements. In particular, this totally passive technique could improve ultrasound imaging of near-field targets in the deadzone created by active transmitters biasing the receivers and may lead to imaging using evanescent waves.