Circuit Design for a Prototype Ultrasound Fingerprint Sensor
Authors:Sien Mao, Richard Przybyla
Mentor:Bernhard Boser, Professor, University of California Berkeley
Fingerprint sensors are widely used to secure electronic interfaces and secure areas. Capacitive fingerprint sensors use micro-fabrication technology to enable low-cost fingerprint sensors, but suffer from poor reliability due to electrostatic discharge sensitivity. Ultrasonic fingerprint sensors overcome these problems by measuring the differences between the acoustic impedance of the ridges and valleys on the user’s finger. However, state of the art ultrasonic fingerprint sensors are fabricated by milling and bonding, making their adoption cost-prohibitive in the consumer market. In this work, we are seeking to demonstrate the use of piezoelectric micromachined ultrasound transducers (pMUTs) in fingerprint recognition sensors. Since pMUTs, like capacitive sensors, are built using micro-fabrication technology, this results in simple, very-low cost systems that could increase privacy and security in consumer electronics. pMUT fingerprint sensors have the potential to deliver superior performance even in the presence of moisture or dirt on the fingers. Each pMUT sensor element operates by delivering an ultrasound wave to the finger’s surface. Ridges on the finger absorb the ultrasound wave, while valleys reflect the wave back to the sensor. By electrically detecting the reflected wave, each sensor element is able to determine whether it is beneath a ridge or a valley. An array of pMUT sensors is used to create an image of the fingerprint from the reflected signals. Calculation, simulation, and experimental verification will be carried out to design an electrical circuit capable of sensing and amplifying the signals bounced back from the finger surface.