Capacitive Image Sensor Dual-Plate Array for Fingerprint Accuracy
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Solution Overview
Problem
Capacitive-type fingerprint sensors face challenges in achieving high accuracy without the use of a separate conductive drive structure, which is desirable for certain applications, and existing designs often suffer from issues related to sensor complexity and form factor, particularly in portable devices.
Innovation Solution
A capacitive image sensor design featuring an array of sensing units with a protective layer, multiple insulating layers, and active semiconductor circuitry, including switches and a voltage follower, that transforms the distance between sensing units and a finger into an output electric potential, allowing for accurate fingerprint imaging without a separate drive structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a separate conductive drive structure is used, then sensor protection and image quality are improved, but device complexity increases
Solution Approach 1:
The patent combines the drive structure function with the sensing electrode structure itself. The sensing electrode is designed to serve dual purposes: as the sensing element for fingerprint detection and as the drive structure for capacitive coupling. This integration eliminates the need for separate drive electrodes while maintaining sensor protection and image quality through the patented switching sequence and voltage control methods.
2Measurement precision
If sensing electrode area is increased to improve signal strength, then measurement precision improves, but sensor protection decreases due to direct finger contact
Solution Approach 1:
The patent segments the sensing electrode into multiple independently controllable sensing units arranged in an array. Each sensing unit can be individually controlled through specific switching sequences, allowing the system to achieve high measurement precision through signal integration while maintaining protection by controlling which units are active at any given time and using insulating layers to isolate direct contact paths.
Solution Approach 2:
The patent introduces insulating layers as intermediary elements between the sensing electrode and direct finger contact. These insulating layers mediate the capacitive coupling, allowing the sensing electrode to maintain a larger effective area for improved signal strength while preventing direct harmful contact, thus preserving sensor protection.
3Measurement precision
If multiple sensing plates are used to improve accuracy, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent merges multiple sensing plates (first sensing plate and second sensing plate) into a unified capacitive sensing structure where the plates work together as a single integrated unit. The specific switching sequence controls the plates to function collectively, achieving improved measurement precision through differential capacitance measurement while avoiding the complexity of fully independent plate control systems.
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 solution enhances the signal-to-noise ratio and improves the accuracy of fingerprint imaging, enabling high-quality image acquisition in a compact form factor suitable for portable devices without the need for a separate conductive drive structure.
Implementation Method 1
A capacitive image sensor consists of an array of sensing units. Each sensing unit contains a sensing electrode. By using the sensing electrode as one plate of the two-plated capacitor and a dermal tissue as another plate, ridges and valleys of a fingerprint can be located by measuring the different capacitances.
Data Source
AI summary
A capacitive image sensor and a method for operating the capacitive image sensor are provided. The capacitive image sensor has an array of capacitive sensing units for transforming a distance between each of the capacitive sensing units and a surface of an adjacent finger into an output electric potential. The capacitive sensing unit comprises: a protective layer; a first sensing plate, formed under the protective layer; a second sensing plate, formed under the first sensing plate; an active semiconductor circuitry, formed under the second sensing plate and connected to the first and second sensing plates; at least one first insulating layer, formed between the first sensing plate and the second sensing plate; and at least one second insulating layer, formed between the second sensing plate and the active semiconductor circuitry.