Fully Integrated Electronic-Photonic Ultrasound Receiver Array for Endoscopic Applications in a Zero-Change 45-nm CMOS-SOI Process

This article presents the first fully integrated 2-D array of electronic-photonic ultrasound sensors targeting low-power miniaturized ultrasound probes for endoscopic applications. Fabricated in a zero-change 45-nm CMOS-silicon-on-insulator (SOI) technology, this 5.53 $\times $ 3.03 mm electronic-photonic system on chip (EPSoC) utilizes micro-ring resonators (MRRs) as ultrasound sensors instead of the traditional piezoelectric or capacitive micromachined transducers (PMUTs or CMUTs). The photonic nature of the sensor enables remoting of the power-hungry receive electronics outside the probe tip, thus lowering the power dissipation inside the human body. Moreover, it eliminates electrical cabling, replacing the bulky micro-coax cables with thinner optic fibers. This EPSoC also reduces fiber count by 8 $\times $ with zero power and area overhead by performing wavelength division multiplexed interrogation of MRR sensors coupled onto the same waveguide in lieu of sub-array beamforming or in-pixel digitization. Leveraging the monolithic integration of photonic devices with CMOS circuitry, a complete receiver (RX) unit is built right next to the sensor MRRs, including a programmable gain transimpedance amplifier (TIA) and background current cancellation digital-to-analog converters ( $I_{\text {DAC}}\text{s}$ ), which allow for a sensor operating range of 74 dB. A 9-bit SAR ADC performs on-chip A/D conversion making for a self-contained endoscopic ultrasound receiver system. The photonic sensing element demonstrates 7.3 mV/kPa sensitivity while consuming 0.43 mW of power and occupying 0.01 mm of area. The functionality of the fabricated chip has been demonstrated in ultrasonic receiver beamforming experiments.