Fingerprint Sensor Cell ESD Clamping With Low Parasitic Capacitance
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Solution Overview
Problem
Fingerprint sensors in resource-constrained devices face challenges with electrostatic discharge (ESD) protection, which often requires external components and can result in higher parasitic capacitance, reducing sensor sensitivity and image contrast.
Innovation Solution
A fingerprint sensing device with integrated sensor cells featuring a capacitive sense plate, discharge electrodes, and a charge reservoir capacitor formed by polysilicon and n-well layers, implemented in a voltage clamping circuit to provide effective ESD protection without external components, reducing parasitic capacitance and maintaining sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external ESD protection components are used, then ESD protection capability is improved, but device complexity and parasitic capacitance increase
Solution Approach 1:
The ESD protection function is merged with the existing sensor structure by integrating discharge electrodes and charge reservoir capacitors directly into the sensor cell. This eliminates the need for separate external ESD protection components, thereby reducing device complexity while maintaining ESD protection capability.
Solution Approach 2:
The sensor device provides its own ESD protection through integrated discharge paths and charge reservoirs that are part of the sensor structure itself. The sensor cell discharges static electricity charges through its own embedded discharge electrode and capacitor, eliminating dependence on external protection components.
2Reliability
If external ESD protection components are used, then ESD protection capability is improved, but parasitic capacitance increases reducing sensor sensitivity
Solution Approach 1:
The ESD protection components (discharge electrode and capacitor) are merged into the sensor cell structure itself. The discharge electrode is formed in the same semiconductor substrate as the sense plate, and the charge reservoir capacitor is integrated within the sensor cell, minimizing additional parasitic capacitance compared to external components.
Solution Approach 2:
The ESD protection structure is localized within each sensor cell or small groups of sensor cells. The discharge electrode and charge reservoir are positioned locally near the sense plate, allowing ESD protection to be provided at the source without adding significant parasitic capacitance across the entire sensor array.
3Reliability
If external ESD protection devices are used, then ESD protection capability is improved, but manufacturing cost increases
Solution Approach 1:
The ESD protection function is merged into the standard sensor manufacturing process. The discharge electrode and charge reservoir capacitor are formed using the same semiconductor fabrication steps as the sensor cell structures, eliminating the need for separate external components and reducing assembly costs.
Solution Approach 2:
The integrated discharge electrode and charge reservoir serve multiple functions: they provide ESD protection while also being part of the sensor cell structure. This multi-functionality reduces the total component count and simplifies manufacturing compared to adding separate external ESD protection devices.
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 low-cost, effective ESD protection within the fingerprint sensing device, enhancing sensitivity and image contrast while eliminating the need for external protection devices.
Implementation Method 1
a charge reservoir coupled between the first electric potential terminal and the second electric potential terminal, and configured to take up electrostatic charge originating from the capacitive sense plate
Implementation Method 2
a first discharge path for discharging a first static electricity charge from the capacitive sense plate to a first electric potential terminal; a second discharge path for discharging a second static electricity charge from the capacitive sense plate to a second electric potential terminal
Data Source
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AI summary
In accordance with a first aspect of the present disclosure, a fingerprint sensing device is provided, comprising a plurality of sensor cells, wherein each sensor cell comprises: at least one sense plate and a discharge electrode insulated from the sense plate; a first discharge path for discharging a first static electricity charge to a first electric potential terminal; a second discharge path for discharging a second static electricity charge to a second electric potential terminal; a charge reservoir coupled between the first electric potential terminal and the second electric potential terminal. In accordance with a second aspect of the present disclosure, a corresponding method of producing a fingerprint sensing device is conceived.