Fingerprint Sensor ESD Protection Layer Design
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Solution Overview
Problem
Fingerprint sensors employing the active thermal principle face challenges in optimizing electrostatic discharge (ESD) protection, mechanical stress tolerance, and thermal signal sensitivity due to the close proximity of the finger and sensor, which leads to irreversible damage and compromised image quality.
Innovation Solution
A fingerprint sensing device with a substrate and pixels arranged in a grid, featuring a conductive ESD protection layer directly over the active thermal sensing elements, connected to a bias potential, and an insulating layer between the ESD protection layers, which minimizes ESD damage and enhances mechanical robustness without compromising thermal signal sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ESD protection circuitry is added to the fingerprint sensor, then ESD protection effectiveness is improved, but device complexity increases
Solution Approach 1:
The ESD protection layer is merged with the addressing line metal layer, eliminating the need for separate ESD protection circuitry. The addressing lines themselves serve dual purposes: signal transmission and ESD protection, thereby reducing device complexity while maintaining ESD protection effectiveness
Solution Approach 2:
The addressing lines are designed to perform multiple functions: they serve as both signal transmission pathways and ESD protection elements. This multi-functionality reduces the overall device complexity by eliminating dedicated ESD protection circuitry while maintaining reliable ESD protection
2Reliability
If ESD protection layer is distributed in the form of individual conductive plates next to the electrode, then ESD protection is provided, but sensing resolution is reduced
Solution Approach 1:
The ESD protection function is merged with the addressing line structure. The addressing lines are configured to overlap with the active electrode area, providing ESD protection without requiring separate conductive plates that would interfere with sensing resolution
Solution Approach 2:
The ESD protection is achieved by utilizing the vertical stacking dimension rather than horizontal placement. The addressing lines are positioned in different metal layers that vertically overlap with the active electrode, providing ESD protection without occupying horizontal space that would reduce sensing resolution
3Reliability
If ESD protection layer vertically overlaps the active thermal sensing element, then ESD protection is improved, but thermal signal sensitivity may be compromised
Solution Approach 1:
An insulating layer is introduced as an intermediary between the ESD protection layer (addressing lines) and the active thermal sensing element. This insulating layer allows vertical overlap for ESD protection while preventing direct thermal interference that would compromise sensing sensitivity
Solution Approach 2:
The thermal interference issue is extracted and solved by removing the direct thermal coupling between the ESD protection layer and sensing element. The insulating layer extracts the harmful thermal conduction path while maintaining the beneficial ESD protection function through electrical field interaction
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 effectively reduces ESD damage, improves mechanical stress tolerance, and maintains thermal signal sensitivity, providing robust ESD protection and enhanced mechanical properties for the fingerprint sensor.
Implementation Method 1
an electrically conductive ESD protection layer... The ESD protection layer is electrically connected to a bias potential
Implementation Method 2
an insulating layer disposed between the ESD protection layer and the active thermal sensing elements
Implementation Method 3
the use of an array of PIN diodes as thermal sensors to differentiate the ridges and valleys of the human fingerprint since the heat transfer in these two areas are different
Data Source
Figure 1
Figure 2
Figure 2A
AI summary
A fingerprint sensing device includes a substrate; a plurality of pixels arranged in a grid of rows and columns, each pixel having an active thermal sensing element therein; a first metal layer forming first addressing lines for addressing the active thermal sensing elements; a second metal layer above the first metal layer and forming second addressing lines for addressing the active thermal sensing elements; an electrically conductive ESD protection layer; and an insulating layer disposed between the ESD protection layer and the active thermal sensing elements. The ESD protection layer is electrically connected to a bias potential. The ESD protection layer is disposed in a pattern such that it partially overlaps each pixel, the ESD protection layer at least partially overlapping the active thermal sensing element of each pixel.