Capacitive Image Sensor With Isolated Well For Noise Reduction
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Solution Overview
Problem
Capacitive fingerprint sensors face challenges in achieving high accuracy and compact form factor, particularly in portable devices, due to the need for a separate conductive drive structure and high-density semiconductor manufacturing processes.
Innovation Solution
A capacitive image sensor design featuring a semiconductor substrate with an array of capacitive sensing units, including a protective layer, sensing electrode, voltage follower, comparative capacitor, and bias voltage supply circuit, all formed in an isolated well to prevent current flow and enhance noise reduction, along with a method for operating the sensor using positive and negative waveforms to improve measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a separate conductive drive structure is used to improve fingerprint sensing accuracy, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent merges the drive structure function with the sensing electrode structure itself. The sensing electrode serves dual purposes: it acts as both the sensing element and the drive structure by applying drive signals directly to the finger through the capacitive coupling, eliminating the need for a separate conductive drive structure while maintaining fingerprint sensing accuracy
Solution Approach 2:
The sensing electrode is designed to perform multiple functions simultaneously: it serves as the capacitive sensing element for detecting fingerprint ridges and valleys, and also functions as the drive structure by applying drive signals to the finger. This multi-functionality reduces overall device complexity while preserving measurement precision
2Measurement precision
If high-density semiconductor processes are used to improve sensing element accuracy, then measurement precision is improved, but manufacturing complexity increases
Solution Approach 1:
The patent changes the design parameters of the sensing electrode structure to achieve high-density sensing capability without requiring complex manufacturing processes. By optimizing the electrode geometry, spacing, and material properties, the patent achieves improved sensing accuracy using standard semiconductor fabrication techniques
3Volume of moving object
If a compact form factor is achieved for portable devices, then device size is reduced, but sensing accuracy may be compromised
Solution Approach 1:
The patent implements a nested structure where the drive signal application and sensing functions are integrated within the same electrode structure. The sensing electrode is nested within the overall sensor architecture, allowing compact form factor while maintaining the functional separation needed for accurate fingerprint sensing through capacitive coupling
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 image acquisition without a separate conductive drive structure, enhancing accuracy and suitability for portable devices by reducing noise and improving the compactness of the sensor design.
Implementation Method 1
The isolated well is configured in such a way that current is able to be prevented from flowing across an interface the isolated well and surrounding structures
Implementation Method 2
A capacitive fingerprint sensor consists of an array of capacitive sensing units. Each capacitive sensing unit contains a sensing plate. By using the sensing plate 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
Implementation Method 3
a voltage follower, wherein an input node of the voltage follower is connected to the sensing electrode
Implementation Method 4
A change of output electric potential of the driving source is caused by a positive and/or negative waveform
Data Source
AI summary
A noised-reduced capacitive image sensor and a method operating the capacitive image sensor are provided. In order to generate a noise-reduced image of a fingerprint, the capacitive image sensor has an array of capacitive sensing units which each has a protective layer; a sensing electrode; a voltage follower; a comparative capacitor; and a bias voltage supply circuit. The comparative capacitor, a portion of the bias voltage supply circuit, and the voltage follower are formed in an isolated well which is configured in such a way that current is able to be prevented from flowing across an interface in the isolated well and surrounding structures. The driving source is connected to a bulk node of the isolated well such that well potential of the isolated well equals to the output electric potential of a driving source.