Ultrasonic Fingerprint Resonator Stack for Foldable Display Attenuation
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Solution Overview
Problem
Designing an under-display ultrasonic sensor system, particularly for foldable devices, is challenging due to the attenuation of ultrasonic waves by display stiffeners and other layers, which affects performance.
Innovation Solution
Incorporating an acoustic resonator with alternating high- and low-impedance layers, where the low-impedance layers have thicknesses corresponding to multiples of half or quarter wavelengths, to enhance ultrasonic wave transmission in the 1 MHz to 20 MHz frequency range suitable for ultrasonic fingerprint sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If display stiffeners and other layers are added to protect the display structure, then structural strength and protection are improved, but ultrasonic wave transmission is attenuated
Solution Approach 1:
An acoustic resonator structure is introduced as an intermediary component between the display stack and the ultrasonic sensor. This resonator includes alternating high-impedance and low-impedance layers that act as an acoustic lens to focus and transmit ultrasonic waves through the display stiffeners and other protective layers, thereby maintaining structural strength while improving ultrasonic wave transmission.
Solution Approach 2:
The acoustic resonator is designed with specific layer thicknesses corresponding to multiples of half or quarter wavelengths of the ultrasonic frequency. By adjusting these dimensional parameters, the resonator creates constructive interference patterns that amplify ultrasonic wave transmission at the target frequency range (1 MHz to 20 MHz), effectively counteracting the attenuation caused by the display structure.
2Loss of energy
If an acoustic resonator with multiple layers is introduced to enhance ultrasonic transmission, then ultrasonic wave transmission is improved, but device complexity increases
Solution Approach 1:
The acoustic resonator serves multiple functions simultaneously: it acts as an acoustic lens to focus ultrasonic waves, provides mechanical support as part of the display structure, and can be integrated with existing display components. This multi-functionality allows the resonator to improve ultrasonic transmission without requiring separate dedicated components, thereby limiting the increase in device complexity.
Solution Approach 2:
The acoustic resonator is integrated within the existing display stack structure, with its layers nested between the display components and the ultrasonic sensor. This nesting approach allows the resonator to be incorporated into the existing device architecture without adding significant external complexity, as the resonator layers are embedded within the existing structural framework.
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
The acoustic resonator configuration amplifies and filters ultrasonic waves, enhancing transmission and maintaining performance without modifying the existing device structure, thus improving ultrasonic fingerprint sensing capabilities.
Implementation Method 1
the acoustic resonator may include one or more higher-impedance layers having a thickness corresponding to a multiple of a half wavelength at a peak frequency of the acoustic resonator
Implementation Method 2
the acoustic resonator may include one or more first low-impedance layers residing between a first side of one or more higher-impedance layers and the ultrasonic sensor stack
Data Source
AI summary
Some disclosed implementations include an ultrasonic sensor stack and an acoustic resonator. The acoustic resonator may be configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack in an ultrasonic frequency range that is suitable for ultrasonic fingerprint sensors. In some examples, the acoustic resonator may include one or more low-impedance layers residing between a first higher-impedance layer and a second higher-impedance layer. Each of the one or more low-impedance layers may have a lower acoustic impedance than an acoustic impedance of the first higher-impedance layer or an acoustic impedance of the second higher-impedance layer. At least one low-impedance layer may have a thickness corresponding to a multiple of a half wavelength at a peak frequency of the acoustic resonator. The peak frequency may be within a frequency range from 1 MHz. to 20 MHz.