Capacitive Fingerprint Sensor Ground Coupling for Thicker Dielectric Layers
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Solution Overview
Problem
Capacitive fingerprint sensors face challenges in enhancing imaging capability due to increased dielectric layer thickness and varying electrical characteristics of individuals, leading to discomfort and field inconsistencies when directly coupling the finger with a driving signal.
Innovation Solution
A capacitive fingerprint sensor design that couples the ground end of the sensor with a driving signal, using a sensor ground generator, charge accumulation unit, and comparison circuit to enhance imaging without directly acting on the finger, thus avoiding discomfort and field inconsistencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the dielectric layer thickness is increased for industrial design integrity, then the mechanical strength and screen protection are improved, but the capacitance coupling between the sensing electrode and finger deteriorates, resulting in blurred fingerprint images
Solution Approach 1:
A driving electrode is introduced as an intermediary component between the sensing electrode and the finger. This driving electrode couples with the finger through capacitance and receives a driving signal, which indirectly enhances the coupling between the sensing electrode and finger without requiring direct contact or thinning the dielectric layer. The driving electrode acts as a mediator that transfers electrical energy through the thick dielectric layer to improve fingerprint imaging while maintaining the original dielectric thickness for mechanical strength.
2Measurement precision
If the amplitude of the driving signal is increased to enhance the measured signal, then the fingerprint imaging capability is improved, but discomfort or harm to the human body occurs when the peak-peak value exceeds 4 V
Solution Approach 1:
The driving electrode serves as an intermediary that isolates the high-voltage driving signal from direct contact with the finger. By coupling the driving signal to the driving electrode rather than directly to the finger, the system can use higher amplitudes to enhance imaging capability while the driving electrode and dielectric layer act as protective barriers that prevent direct harmful effects on the user's skin.
Solution Approach 2:
The system changes the electrical parameters by applying AC driving signals with controlled amplitudes and frequencies to the driving electrode. By optimizing the amplitude and frequency parameters of the driving signal, the system enhances the coupling effect and imaging capability while keeping the peak-peak voltage within safe limits (below 4 V) to avoid human discomfort or harm.
3Measurement precision
If the frequency of the driving signal is increased to improve signal coupling, then the imaging sensitivity is enhanced, but an amplitude gradient forms on the finger surface causing field inconsistency in fingerprint imaging
Solution Approach 1:
The driving electrode acts as an intermediary that distributes the driving signal uniformly across the sensing area before coupling to the finger. This intermediate structure helps to equalize the signal distribution and prevents the formation of amplitude gradients on the finger surface, thereby maintaining field consistency while still achieving enhanced imaging sensitivity through proper frequency selection.
4Measurement precision
If direct coupling of the finger with the driving signal is implemented to enhance imaging, then the fingerprint imaging capability is improved, but the design becomes inconvenient for different individuals with varying electrical characteristics
Solution Approach 1:
The driving electrode serves as a universal intermediary component that mediates between the driving signal and the finger for all users. This intermediate structure provides a consistent coupling interface that works across different individuals with varying electrical characteristics, eliminating the need for individualized direct coupling designs. The driving electrode standardizes the interaction while the system can still adapt to different users through software-based signal processing and parameter adjustment.
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
This approach improves fingerprint imaging by allowing the ground end to change with the driving signal, ensuring consistent field potentials and avoiding direct finger coupling, which maintains the integrity of the mobile phone design and enhances imaging without discomfort.
Implementation Method 1
a sensing unit, a charge accumulation unit and a comparison circuit... couples the ground end of the sensor with a driving signal
Implementation Method 2
The capacitive fingerprint sensor images fingerprints by measuring differences between the coupling capacitance formed between ridge lines of fingerprints and planar sensing electrode array units and that formed between valley lines of the fingerprints and the planar sensing electrode array units
Implementation Method 3
When a dielectric layer between a sensing electrode array and a finger becomes thicker, the capacitance may decay
Data Source
AI summary
The present invention provides a capacitive fingerprint sensor which images fingerprints by measuring differences between coupling capacitance formed between ridge lines of the fingerprints and corresponding units on planar sensing electrode arrays and that formed between valley lines of the fingerprints and corresponding units on the planar sensing electrode arrays. A conventional “C-Q-T” type capacitive fingerprint sensor employs twice conversion from capacitance to charge quantity and then to integrating time to indirectly measure the capacitance; and the capacitance-charge quantity conversion efficiency can be improved by coupling a human body with a driving signal so as to improve the sensitiveness of the sensor. According to the improved “C-Q-T” type capacitive fingerprint sensor provided coupling a ground potential of the fingerprint sensor with a reversed-phase driving signal equivalently substitutes for coupling the driving signal to the human body, so that the sensitiveness of the sensor is further enhanced.