CMOS-Integrated PMUT Array Layout for Reliable Fingerprint Sensing

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Solution Overview

Problem

Conventional MEMS devices for fingerprint sensing, particularly piezoelectric MUT devices, face challenges in achieving high resolution and accuracy due to manufacturing limitations and sensitivity to skin conditions and sensor contamination, which affects the reliability of fingerprint detection.

Innovation Solution

The development of integrated piezoelectric MEMS transducers (PMUTs) on integrated circuits (ICs) with a vertically stacked configuration, including a CMOS wafer and PMUT array, utilizing a piezoelectric material with metal conductive layers for improved electrical connections and high fill factor configurations such as rhombus and hexagonal arrangements to enhance mechanical coupling and resolution.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If conventional manufacturing processes (polishing and sawing from bulk piezoelectric materials) are used, then device fabrication is straightforward, but the required resolution of at least 300 dpi cannot be achieved

Engineering Contradiction:

Improvepixel size resolutionVSAvoidfabrication complexity

Core Design Contradiction:

Manufacturing precisionVSEase of manufacture

Solution Approach 1:

The device is divided into separate functional layers (CMOS circuit layer and PMUT array layer) that can be manufactured independently and then bonded together. This allows each layer to be optimized separately - the CMOS layer for integration and the PMUT layer for high-resolution acoustic transduction, thereby achieving 300 dpi resolution without compromising manufacturability

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Traditional mechanical polishing and sawing processes are replaced with a wafer bonding approach. The PMUT array is fabricated as a separate membrane structure on a support substrate, then bonded to the CMOS circuit wafer. This substitution enables precise control of pixel dimensions through photolithography and etching processes rather than mechanical removal, achieving the required 300 dpi resolution

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Manufacturing precision

If CMUT linear arrays are used for fingerprint detection, then required resolution of 300 dpi can be achieved, but accuracy deteriorates due to skin condition and sensor contamination

Engineering Contradiction:

Improvepixel size resolutionVSAvoiddetection accuracy

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The capacitive sensing mechanism (CMUT) is replaced with a piezoelectric ultrasonic transduction mechanism (PMUT). The piezoelectric material converts electrical signals to mechanical ultrasonic waves that penetrate skin barriers and contaminants, then detect acoustic impedance variations at fingerprint ridges and valleys. This mechanical acoustic approach is insensitive to electrical contamination and skin conditions, maintaining high detection accuracy while achieving 300 dpi resolution

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Manufacturing precision

If PMUT array is integrated on IC, then resolution and accuracy are improved, but device complexity increases

Engineering Contradiction:

Improvefingerprint sensing resolutionVSAvoidintegration structure

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The CMOS circuit wafer and PMUT array wafer are merged through wafer bonding to form a single integrated device. This merging consolidates the control electronics and sensing elements into one compact unit, achieving high-resolution fingerprint sensing while managing complexity through standardized bonding processes and unified device packaging

Inventive Principle:

Principle #5Merging (Combining)

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 solution improves the resolution and accuracy of fingerprint sensing by minimizing manufacturing limitations and contamination issues, enabling reliable detection with enhanced mechanical and electrical coupling in a compact, high-resolution PMUT array.

Implementation Method 1

piezoelectric material with metal conductive layers

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

piezoelectric material with metal conductive layers

Methodology Applied

Scientific EffectPiezoelectric effect: Converse Piezoelectric Effect

Data Source

PatentEP3292508B1Integrated piezoelectric microelectromechanical ultrasound transducer (PMUT) on integrated circuit (IC) for fingerprint sensing

Publication Date: 2020.12.23 INVENSENSE INC

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AI summary

Microelectromechanical (MEMS) devices and associated methods are disclosed. Piezoelectric MEMS transducers (PMUTs) suitable for integration with complementary metal oxide semiconductor (CMOS) integrated circuit (IC), as well as PMUT arrays having high fill factor for fingerprint sensing, are described.