Flexible Ultrasonic Fingerprint Stack for Foldable Displays
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Solution Overview
Problem
Existing flexible fingerprint sensors face challenges in achieving acceptable performance levels due to low-power transmitted ultrasonic waves and low resolution of fingerprint image data, particularly in the absence of a rigid thin-film transistor substrate.
Innovation Solution
A flexible fingerprint sensor stack is designed with acoustic resonators configured to produce a local maximum of ultrasonic wave transmission at frequencies between 1 MHz to 20 MHz, incorporating layers with specific acoustic impedances and thicknesses to enhance power and resolution, suitable for attachment to non-planar surfaces and foldable display devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a rigid thin-film transistor substrate is used, then manufacturing precision and structural stability are improved, but flexibility and adaptability to non-planar surfaces deteriorate
Solution Approach 1:
The patent replaces the rigid thin-film transistor substrate with a flexible polymer substrate that has a modulus of elasticity between 2-5 GPa. This flexible substrate supports the piezoelectric layer and electrode layers while enabling the sensor to conform to non-planar surfaces and foldable displays, resolving the contradiction between structural stability and flexibility.
2Length of moving object
If the sensor stack is made thinner, then flexibility and ease of lamination are improved, but ultrasonic wave transmission power deteriorates
Solution Approach 1:
The patent employs a piezoelectric layer that converts electrical signals to mechanical vibrations at ultrasonic frequencies (1-20 MHz). This mechanical vibration mechanism generates high-power ultrasonic waves despite the thin sensor stack thickness, resolving the contradiction between thinness and transmission power.
Solution Approach 2:
The patent optimizes the thickness of the piezoelectric layer and the acoustic impedance matching of various layers to enhance ultrasonic wave transmission. By carefully controlling these parameters, the sensor achieves high transmission power in a thin configuration suitable for foldable devices.
3Power
If acoustic impedance matching layers are added, then ultrasonic wave transmission is improved, but device complexity increases
Solution Approach 1:
The patent designs the polymer substrate to serve multiple functions: it provides mechanical flexibility, structural support, and acoustic impedance matching for ultrasonic wave transmission. This multi-functionality reduces the need for separate acoustic matching layers, resolving the contradiction between transmission power and device complexity.
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 flexible fingerprint sensor stack achieves higher-power transmitted ultrasonic waves and higher resolution of fingerprint image data, is thinner than previous designs, more durable, and easier to laminate, with reduced bubble trapping, making it suitable for foldable devices.
Implementation Method 1
a first piezoelectric layer residing between the first fingerprint sensor electrode layer and the polymer layer
Implementation Method 2
one or more acoustic resonators configured to produce a local maximum of ultrasonic wave transmission at a frequency in a range from 1 MHz to 20 MHz
Data Source
AI summary
An apparatus may include a flexible fingerprint sensor stack. The flexible fingerprint sensor stack may include a first fingerprint sensor electrode layer, a polymer layer, a second fingerprint sensor electrode layer residing on the polymer layer and a first piezoelectric layer residing between the first fingerprint sensor electrode layer and the polymer layer. The flexible fingerprint sensor stack may have a modulus of elasticity in a range from 2-5 gigapascals (GPa). The apparatus may include an adhesive layer residing adjacent the flexible fingerprint sensor stack. The first fingerprint sensor electrode layer, the polymer layer and the first piezoelectric layer may be layers of one or more acoustic resonators configured to produce a local maximum of ultrasonic wave transmission at a frequency in a range from 1 MHz to 20 MHz. The adhesive layer may reside between the flexible fingerprint sensor stack and a display stack.