3-Port PMUT Sensor Background Signal Reduction
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Solution Overview
Problem
Ultrasonic imaging sensors face challenges in accurately determining fingerprint ridges and valleys due to interference from background signals, which reduces the accuracy of fingerprint recognition and introduces noise.
Innovation Solution
The use of piezoelectric micromechanical ultrasonic transducers (PMUTs) with a 3-port configuration, where a set of electrodes above the piezoelectric layer exhibits opposite voltage signs during deflection, effectively reduces background signals and sustains the intensity of the delta signal by employing a sensing scheme that involves distinct electrode configurations during transmit and receive modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional ultrasonic imaging sensors are used to capture fingerprint signals, then the sensor can detect ridges and valleys, but background signals interfere with the delta signal causing less accurate measurements
Solution Approach 1:
The sensor divides the detection function into separate transmit and receive modes using distinct electrode configurations. During transmit mode, first electrodes generate ultrasonic waves; during receive mode, second electrodes detect reflected signals. This segmentation allows independent optimization of each function and enables background signal rejection through differential measurement.
Solution Approach 2:
Different regions of the sensor have specialized functions: transmit electrodes are optimized for generating ultrasonic energy, while receive electrodes are optimized for detecting reflected signals. This local specialization allows each region to perform its specific task with high efficiency while minimizing interference from other regions.
2Reliability
If a 3-port PMUT with opposite poling is used, then background signals are reduced and delta signal intensity is sustained, but the device complexity increases
Solution Approach 1:
The patent combines transmit and receive functionality into a single integrated sensor array, where the same physical structure serves both functions at different times. The multi-port PMUT structure merges the advantages of separate transmit and receive sensors while maintaining a compact form factor suitable for mobile devices.
Solution Approach 2:
The sensor alternates between transmit mode and receive mode in periodic cycles. During each transmit phase, ultrasonic waves are generated; during the subsequent receive phase, reflected signals are detected. This periodic operation allows the system to function as both transmitter and receiver using the same hardware, reducing overall system complexity while maintaining reliable signal measurement.
3Measurement precision
If opposite poling is applied to reduce background signals, then the intensity of delta signal is sustained, but manufacturing precision requirements increase
Solution Approach 1:
The sensor structure is designed to be self-aligning, where the physical constraints of the substrate and layer deposition processes automatically ensure proper alignment of transmit and receive electrodes. The opposite poling configuration is achieved through standard fabrication techniques that self-correct minor variations, reducing the need for post-manufacturing alignment adjustments.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the accuracy of fingerprint recognition by minimizing background noise and maintaining the intensity of the delta signal, leading to improved image quality and reduced interference.
Implementation Method 1
piezoelectric micromechanical ultrasonic transducers (PMUTs)
Implementation Method 2
an ultrasonic wave may propagate through a surface of the smartphone on which a person's finger may be placed to obtain a fingerprint image. After passing through the surface, some portions of the wave may encounter skin that is in contact with the surface (e.g., fingerprint ridges), while other portions of the ultrasonic wave encounter air (e.g., valleys between adjacent ridges of a fingerprint) and may be reflected with different intensities back towards the ultrasonic fingerprint imager
Data Source
AI summary
The descried techniques may support a sensing scheme for reducing background signals in imaging sensors. A device may include a sensor configured to determine ridges and valleys of a fingerprint. The sensor may include a pixel array with each pixel of the pixel array having a set of electrodes. To reduce the background signals, the device may sense, e.g., during a transmit mode, a first set of signals associated with the pixel array using at least one electrode of the set of electrodes, and sense, e.g., during a receive mode, a second set of signals associated with the pixel array using the at least one electrode. The device may reduce a background signal associated with the sensor according to the sensing of the first set of signals and the second set of signals.