Piezoelectric Nanoscale Ultrasound Transducers (pnuts) for Miniaturized Internet of Things Receivers

The landscape of connected devices has both grown in size and changed in nature in the last few decades. When the internet came along, it represented a first wave of connectivity that enabled communication between desktop computers. As computers moved to smaller form factors such as laptops and more recently smartphones, the need for connectivity was extended to this class of devices. This second wave was wireless in nature, and came with the

additional constraint of lower power consumption as these devices are powered by batteries. We are now going through a third wave of connectivity, where everyday appliances and sensor nodes distributed in the environment are equipped with the ability to remotely receive and

report back information. This third wave is commonly referred to as the internet of things (IoT), and as it rolls out, the constraints on form-factor and power consumption become increasingly tight and challenging to engineer.

Because of the high density of wireless nodes, radio frequency bands are becoming a scarce resource and they constitute yet another constraint to account for during the system design. The number of wireless nodes is increasing exponentially and we expect to have a trillion sensors deployed by the year 2030. To make the deployment of a trillion sensors feasible, these nodes must be small and function on limited power budgets. For a subset of IoT applications, a network of sensors would need to be installed in a localized area, where the required communication range is less than 10 m and where a central hub can orchestrate the connection between different nodes.

For such applications, ultrasound between 40 kHz and 100 kHz becomes an attractive candidate to overcome several of the constraints outlined above. This frequency range offers

two important advantages: 1) The acoustic waves attenuation is low and permits communication over several meters (loss < 3 dB/m). 2) The node electronics can operate at extremely low power, potentially extending its lifetime to several years while drawing power from a miniaturized battery. If ultrasound is used in place of RF waves, we must replace the node antenna with an ultrasound transducer. In this work we introduce a novel ultrasound sensor that we dub the piezoelectric nano-scale ultrasound transducer (pNUT). The pNUTs are characterized by piezoelectric films that are only 100 nm thick and by a footprint that is 1 to 2 orders of magnitude smaller than the state of the art, while showing similar performance. A model for the scaling of ultrasound transducers is presented to offer guidelines for the design of miniaturized ultrasound sensors. Finally, we design a circuit with accessible off-the-shelf components to demonstrate a low-power pNUT-based ultrasound wake-up receiver (WuRx). The WuRx presents a communication range of several meters, confirming the feasibility of

miniaturized ultrasound tags for distributed sensors networks.