Monolithic Single PMUT-on-CMOS Ultrasound System With +17 dB SNR for Imaging Applications

This article presents a fully integrated ultrasound system based on a single piezoelectric micromachined ultrasonic transducer (PMUT) monolithically fabricated with a <inline-formula> <tex-math notation="LaTeX">$0.13~\mu \text{m}$ </tex-math></inline-formula> complementary metal oxide semiconductor (CMOS) process analog front-end circuitry. The PMUT consists of an aluminum nitride, AlN, squared device with <inline-formula> <tex-math notation="LaTeX">$80~\mu \text{m}$ </tex-math></inline-formula> side that resonates at 2.4 MHz in liquid environment. The monolithic integration of the PMUT with the CMOS circuitry allows a reduction of the parasitic capacitance, a reduction of the electronic noise contribution and a clear improvement in the Signal-to Noise ratio (SNR ~ 27 dB better) compared to a non-integrated equivalent system. A pulse-echo experiment with the single PMUT-on-CMOS for transmitting and sensing simultaneously is demonstrated, ensuring a 17.3 dB SNR, higher than the minimal necessary for accurate fingerprint images, paving the way towards a pixel sized imaging system with no need of multiple simultaneous PMUTs transmitters. Consuming only 0.3 mW and getting an input-referred noise of 3.26 mPa/<inline-formula> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula>Hz at 2.4 MHz, the proposed pulse-echo system achieves a competitive noise efficient factor in comparison with the state-of-the-art.