Capacitive Fingerprint Sensor Air Gap Parasitic Capacitance

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Capacitive fingerprint sensors face limited sensitivity due to parasitic capacitance from the covering layer and interconnect layers, which introduces noise into the sensing signal, limiting the choice of covering material thickness.

Innovation Solution

Incorporating an air gap under the sensing electrode reduces parasitic capacitance by creating a dielectric environment with a lower dielectric constant than the surrounding interconnect structure, enhancing the signal-to-noise ratio and allowing for thicker covering materials without compromising sensitivity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a covering layer is added to protect the sensing electrode, then the sensing electrode is protected, but parasitic capacitance increases and sensitivity decreases

Engineering Contradiction:

Improveprotection of sensing electrodeVSAvoidsensing sensitivity

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

An air gap is introduced as an intermediary layer between the sensing electrode and the covering layer. The air gap has a low dielectric constant that reduces parasitic capacitance while still allowing the covering layer to provide protection. This mediator resolves the contradiction by enabling both protection and sensitivity to coexist.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The dielectric constant parameter is changed by introducing an air gap (dielectric constant ≈ 1.0) between the sensing electrode and covering layer. This parameter change reduces the overall parasitic capacitance in the system, allowing the covering layer to be thicker while maintaining sensing sensitivity.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If the covering layer thickness is increased for better protection, then protection is improved, but parasitic capacitance increases and sensitivity decreases

Engineering Contradiction:

Improveprotection strengthVSAvoidsensing sensitivity

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The air gap serves as a mediator that allows the covering layer to be thicker without directly increasing parasitic capacitance. The low dielectric constant of air reduces the capacitive coupling between the sensing electrode and the thicker covering layer, enabling enhanced protection while maintaining sensitivity.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

By changing the dielectric constant parameter through the introduction of an air gap, the system allows for increased covering layer thickness without a proportional increase in parasitic capacitance. This parameter change enables the covering layer to be optimized for protection while the air gap maintains sensing performance.

Inventive Principle:

Principle #35Parameter changes

3Ease of operation

If interconnect layers are added for electrical connection, then electrical connectivity is improved, but parasitic capacitance increases and signal-to-noise ratio decreases

Engineering Contradiction:

Improveelectrical connectivityVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The air gap acts as an intermediary that reduces parasitic capacitance between the sensing electrode and the interconnect layers. This allows the interconnect layers to be positioned closer to the sensing electrode without increasing noise, thereby improving electrical connectivity while maintaining signal-to-noise ratio.

Inventive Principle:

Principle #24Intermediary (Mediator)

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 air gap configuration effectively reduces parasitic capacitance, improving the sensitivity and flexibility of the capacitive fingerprint sensor by minimizing noise and allowing for increased covering layer thickness while maintaining or improving signal quality.

Implementation Method 1

parasitic capacitance from the covering layer and interconnect layers, which introduces noise into the sensing signal

Methodology Applied

Scientific EffectParasitic capacitance: Parasitic Capacitance

Implementation Method 2

creating a dielectric environment with a lower dielectric constant than the surrounding interconnect structure

Methodology Applied

Scientific EffectDielectric: Dielectric

Implementation Method 3

A capacitive fingerprint sensor recognizes a fingerprint by detecting capacitance variations between an embedded electrode and ridges and valleys that make up the fingerprint

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS10025442B2Capacitive fingerprint sensor

Publication Date: 2018.07.17 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

The present disclosure relates to a capacitive fingerprint sensor having an air gap, which is disposed under a sensing electrode and that is configured to improve sensing sensitivity by reducing a parasitic capacitance of the capacitive fingerprint sensor, and an associated method. In some embodiments, the capacitive fingerprint sensor includes a plurality of semiconductor devices disposed over a substrate. An interconnect structure is disposed over the semiconductor devices. A sensing electrode is disposed over the interconnect structure to form a capacitance between the sensing electrode and finger skins. An air gap is disposed under the sensing electrode and enclosed by the sensing electrode and the interconnect structure.