Fingerprint Sensor Electrode Structure for Fast Stray-Capacitive Scanning
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Solution Overview
Problem
Existing capacitive sensing methods for fingerprint recognition face challenges such as slow scan speeds, noise immunity issues, and limitations in high dots per inch (dpi) due to capacitance to voltage conversion and capacitive loading of sensing lines, particularly in self-capacitive and mutual-capacitive sensing.
Innovation Solution
A capacitive sensor design featuring a first electrode with an opening covered by a second electrode and a conductive layer underneath, utilizing stray-capacitive sensing to achieve large capacitance changes with a simple read-out circuit, enabling fast scanning and reduced interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If self-capacitive sensing is used, then large capacitance change is achieved, but circuit complexity increases due to capacitance to voltage converter
Solution Approach 1:
The patent extracts the capacitance-to-voltage conversion function from the pixel circuit and relocates it to an external converter circuit. The pixel only contains the sensing capacitor and readout transistor, while the complex capacitance-to-voltage conversion is performed externally, reducing in-pixel complexity while maintaining large capacitance change detection capability.
Solution Approach 2:
The patent introduces an intermediary charge transfer mechanism where charge is transferred from the sensing capacitor to the fingerprint (ridge or valley) and then to an external converter. This intermediary approach allows the pixel to maintain simplicity while still achieving accurate capacitance measurement through the external conversion process.
2Device complexity
If charge transfer through sensing lines is used, then capacitance to voltage converter outside active area is achieved, but scan speed decreases due to capacitive loading
Solution Approach 1:
The patent segments the sensing function by separating the sensing electrode from the conversion function. The sensing electrode captures charge directly, and the conversion is performed externally, eliminating the need for long sensing lines that cause capacitive loading. This segmentation allows faster scanning by removing the bottleneck in the charge transfer path.
3Device complexity
If mutual-capacitive sensing is used, then voltage output is achieved, but large capacitance change is not expected due to lateral electrical field
Solution Approach 1:
The patent inverts the conventional mutual-capacitive approach by using a single sensing electrode that directly interacts with the fingerprint through vertical field lines. Instead of using transmitter and receiver electrode pairs with lateral fields, the invention uses a direct capacitance change approach where the sensing electrode's capacitance changes vertically due to fingerprint contact, achieving large signal changes with simplified circuitry.
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 enables high-speed scanning with improved noise immunity and supports high dpi capabilities without additional components like light sources, utilizing in-plane-switching liquid crystal arrays.
Implementation Method 1
The two major capacitive sensing methods are self-capacitive sensing and mutual-capacitive sensing. Self-capacitive sensing may be a good solution for fingerprint sensing because a large capacitance (signal) changed will be expected due to vertical electrical field
Implementation Method 2
Self-capacitive sensing may be a good solution for fingerprint sensing because a large capacitance (signal) changed will be expected due to vertical electrical field
Data Source
AI summary
An electronic structure includes a first electrode, a first insulating layer, a second electrode, a third electrode, and a conductive layer. The first electrode has an opening. The first insulating layer is disposed on the first electrode. The second electrode is disposed on the first insulating layer. The third electrode is disposed on the first insulating layer and surrounding the second electrode. The conductive layer disposed under the first electrode. The second electrode and the third electrode are in the same layer. An area of the conductive layer is greater than an area of the second electrode.