Fingerprint Sensor Protective Layer with Local Permittivity Control
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Solution Overview
Problem
Fingerprint sensors, particularly active self-capacitive sensors, require a larger window sensing distance due to the typical design, which can affect their sensitivity and accuracy in recognizing fingerprints.
Innovation Solution
A fingerprint sensor design with a sensor protective layer having regions of varying permittivity and hardness, incorporating inorganic fillers like barium titanate, alumina, or titanium dioxide, to enhance capacitance changes and improve sensing sensitivity by optimizing the distance between sensor electrodes and the user's hand.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional sensor protective layer is used, then the sensor substrate is protected, but the window sensing distance becomes too large reducing sensing sensitivity
Solution Approach 1:
The sensor protective layer is divided into two regions with different permittivities: a first region with higher permittivity positioned over the sensor electrode to enhance capacitance changes, and a second region with lower permittivity in peripheral areas to maintain protection while minimizing interference with the sensing field. This local differentiation allows the protective layer to simultaneously protect the sensor and enhance sensing sensitivity.
Solution Approach 2:
The sensor protective layer uses a composite structure combining regions of different permittivity values, effectively creating a multi-functional protective layer that optimizes both protection and sensing performance. The composite approach allows tailored electrical properties in different spatial zones.
2Measurement precision
If the sensor protective layer has high permittivity to enhance capacitance, then sensing sensitivity improves, but the capacitance ratio between sensor electrode and user fingerprint decreases
Solution Approach 1:
By positioning the high permittivity first region specifically over the sensor electrode area, the design enhances the capacitance change detection capability where it is most needed, while the lower permittivity second region in peripheral areas prevents excessive overall capacitance that would reduce the fingerprint detection ratio.
Solution Approach 2:
The protective layer is segmented into functionally distinct regions: the first region enhances sensing sensitivity locally over the electrode, while the second region manages overall capacitance characteristics. This segmentation allows independent optimization of competing requirements.
3Measurement precision
If the sensor protective layer is made softer to reduce sensing distance, then fingerprint recognition improves, but the protective capability decreases
Solution Approach 1:
The permittivity parameter is varied spatially within the protective layer, allowing the material to provide mechanical protection while creating electrical conditions that enhance capacitance changes for fingerprint recognition. The hardness is maintained for protection while permittivity is optimized for sensing.
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 design increases the ratio of capacitance between the sensor electrode and the user's fingerprint, enhancing the sensor's ability to accurately recognize fingerprints and palm prints with improved sensitivity.
Implementation Method 1
the active self-capacitive fingerprint sensor senses a fingerprint based on a change in capacitance of a capacitor formed between it and the hand of a user
Implementation Method 2
The first region has a first permittivity. The second region has a second permittivity lower than the first permittivity
Data Source
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AI summary
A fingerprint sensor includes a sensor substrate. A plurality of sensor pixels is configured to sense a change in capacitance corresponding to a touch of a user. Each of the plurality of sensor pixels includes a sensor electrode. A sensor protective layer is configured to protect the sensor substrate and the plurality of sensor pixels. The sensor protective layer includes a first region disposed over the sensor electrode, and a second region. The first region has a first permittivity. The second region has a second permittivity lower than the first permittivity.