Capacitive Fingerprint Sensor Circuit for Parasitic Capacitance Control
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Solution Overview
Problem
Capacitive fingerprint sensors are sensitive to scratching and ESD due to thin protective layers, and existing active sensors with excitation pulses applied to the finger struggle with dry fingers and require a conductive drive structure, which is not robust enough for many applications.
Innovation Solution
A capacitive fingerprint sensing device with a semiconductor substrate and sensing elements featuring a protective dielectric top layer, an electrically conductive sensing structure, and a charge amplifier with excitation signal providing circuitry that reduces parasitic capacitance by controlling the potential of the well adjacent to the sensing structure, allowing for a thicker protective coating and improved robustness without a conductive drive electrode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a thin protective layer is used to enable passive capacitive sensing, then capacitance reading is possible, but the sensor becomes sensitive to scratching and ESD
Solution Approach 1:
The patent changes the sensing approach from passive to active by applying excitation pulses to the sensing electrode, enabling the use of a thicker protective coating (10-100 times thicker than passive sensors) while maintaining sensing capability. This parameter change in the sensing method allows the protective layer thickness to be increased from micrometer to millimeter scale.
2Measurement precision
If excitation pulse is applied to the finger, then fingerprint image quality is improved, but a separate conductive drive structure is required
Solution Approach 1:
The patent merges the sensing electrode with the excitation signal application function. The same sensing electrode that detects capacitance changes also receives the excitation pulse, eliminating the need for a separate conductive drive structure. This integration is achieved through the charge amplifier configuration where the sensing electrode is connected to the inverting input and the excitation signal is applied through the non-inverting input.
3Reliability
If excitation pulse is applied to the sensing electrode, then robust protective coating is possible, but parasitic capacitance between sensing structure and well increases
Solution Approach 1:
The patent applies equipotentiality by connecting the well to the excitation signal source, ensuring that the well potential changes in sync with the sensing electrode potential. This creates a virtual ground effect where the potential difference between the sensing structure and well remains constant during excitation, thereby minimizing parasitic capacitance effects. The charge amplifier configuration inherently maintains this equipotential relationship through its feedback mechanism.
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 enables high-quality fingerprint representation from 'difficult' fingers, such as dry ones, while providing enhanced protection against wear and tear and ESD, and simplifies integration into devices like mobile phones by eliminating the need for a conductive drive structure.
Implementation Method 1
all capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor
Implementation Method 2
changing a potential at the positive input from a first potential to a second potential, to thereby change a potential of the sensing structure, thereby providing the change in potential difference between the finger and the sensing structure
Data Source
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Figure 4a~4b
AI summary
The present invention relates to a capacitive fingerprint sensing device comprising a semiconductor substrate; and an array of sensing elements formed on the semiconductor substrate. Each of the sensing elements comprises a protective dielectric top layer;a sensing structure arranged underneath the top layer; and a charge amplifier connected to the sensing structure. The charge amplifier comprises a negative input connected to the sensing structure;a positive input;an output providing a sensing signal;a feedback capacitor; and a sense transistor having a gate constituting the negative input. The sense transistor is formed in an insulated well in the semiconductor substrate. The fingerprint sensing device further comprises excitation signal providing circuitry connected to the positive input of the charge amplifier and the well for changing electric potentials of the sensing structure and the well, to thereby reduce the influence of parasitic capacitances in the sensing element.