Publications – 2021

Tiago Costa, Chen Shi, Kevin Tien, Jeffrey Elloian, Filipe A. Cardoso and Kenneth L. Shepard, “An Integrated 2D Ultrasound Phased Array Transmitter in CMOS with Pixel Pitch-Matched Beamforming,” IEEE Transactions on Biomedical Circuits and Systems, 2021 (Early Access)

Emerging non-imaging ultrasound applications, such as ultrasonic wireless power delivery to implantable devices and ultrasound neuromodulation, require wearable form factors, millisecond-range pulse durations and focal spot diameters approaching 100 m with electronic control of its three-dimensional location. None of these are compatible with typical handheld linear array ultrasound imaging probes. In this work, we present a 4 mm x 5 mm 2D ultrasound phased array transmitter with integrated piezoelectric ultrasound transducers on complementary metal-oxide-semiconductor (CMOS) integrated circuits, featuring pixel-level pitch-matched transmit beamforming circuits which support arbitrary pulse duration. Our direct integration method enabled up to 10 MHz ultrasound arrays in a patch form-factor, leading to focal spot diameter of ~200 m, while pixel pitch-matched beamforming allowed for precise three-dimensional positioning of the ultrasound focal spot. Our device has the potential to provide a high-spatial resolution and wearable interface to both powering of highly-miniaturized implantable devices and ultrasound neuromodulation.

Chen Shi, Victoria Andino-Pavlovsky, Stephen A. Lee, Tiago Costa, Jeffrey Elloian, Elisa E. Konofagou, and Kenneth L. Shepard, “Application of a sub-0.1-mm3 implantable mote for in vivo real-time wireless temperature sensing,” Science Advances 7, No. 19 (2021)

There has been increasing interest in wireless, miniaturized implantable medical devices for in vivo and in situ physiological monitoring. Here, we present such an implant that uses a conventional ultrasound imager for wireless powering and data communication and acts as a probe for real-time temperature sensing, including the monitoring of body temperature and temperature changes resulting from therapeutic application of ultrasound. The sub–0.1-mm3, sub–1-nW device, referred to as a mote, achieves aggressive miniaturization through the monolithic integration of a custom low-power temperature sensor chip with a microscale piezoelectric transducer fabricated on top of the chip. The small displaced volume of these motes allows them to be implanted or injected using minimally invasive techniques with improved biocompatibility. We demonstrate their sensing functionality in vivo for an ultrasound neurostimulation procedure in mice. Our motes have the potential to be adapted to the distributed and localized sensing of other clinically relevant physiological parameters.

“Electron beam lithography (EBL) is the state-of-the-art technique for rapid prototyping of nanometer-scale devices. Even so, processing speeds remain limited for the highest resolution patterning. Here, we establish Mr-EBL as the highest throughput negative tone electron-beamsensitive resist. The 10 μC cm−2 dose requirement enables fabricating a 100 mm2 photonic diffraction grating in a ten minute EBL process. Optimized processing conditions achieve a critical resolution of 75 nm with 3× faster write speeds than SU-8 and 1–2 orders of magnitude faster write speeds than maN-2400 and hydrogen silsesquioxane. Notably, these conditions significantly differ from the manufacturers’ recommendations for the recently commercialized Mr-EBL resist. We demonstrate Mr-EBL to be a robust negative etch mask by etching silicon trenches with aspect ratios of 10 and near-vertical sidewalls. Furthermore, our optimized processing conditions are suitable to direct patterning on integrated circuits or delicate nanofabrication stacks, in contrast to other negative tone EBL resists. In conclusion, Mr-EBL is a highly attractive EBL resist for rapid prototyping in nanophotonics, MEMS, and fluidics.”

Sajjad Moazeni, Eric H. Pollmann, Vivek Boominathan, Filipe A.Cardoso,Jacob T. Robinson, Ashok Veeraraghavan, Kenneth L. Shepard, A Mechanically Flexible Implantable Neural Interface for Computational Imaging and Optogenetic Stimulation over 5.4×5.4mm2 FoV, International Solid-State Circuits Conference, 2021

Sajjad Moazeni, Kevin Renehan, Eric Pollmann, Kenneth Shepard, Integrated-Circuit Node for Time-Domain Near-infrared Diffuse Optical Tomography Imaging Arrays with On-chip Histogramming and Integrated VCSELs, IEEE Custom Integrated Circuits Conference (CICC), 2021