Integrated PMUT Fingerprint Sensor CMOS Wafer Bonding
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Solution Overview
Problem
Conventional methods for manufacturing microelectromechanical systems (MEMS) devices, particularly for fingerprint sensing, face challenges in achieving high resolution and reliability due to limitations in area array interconnection and transducer size, leading to inaccuracies in fingerprint detection caused by skin conditions and sensor contamination.
Innovation Solution
The development of integrated piezoelectric MEMS transducers (PMUTs) on integrated circuits (ICs) with a vertically stacked structure, including a CMOS wafer with cavities, piezoelectric material, and metal conductive layers, enables high fill factor arrays for improved fingerprint sensing by enhancing electrical coupling and mechanical anchoring, while minimizing parasitics and utilizing CMOS metal layers effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional manufacturing methods are used for MEMS devices, then fabrication costs and chip size are reduced through integration, but manufacturing precision and resolution are insufficient to achieve required 300 dpi or higher resolutions
Solution Approach 1:
The device is divided into separate CMOS wafer and MEMS wafer that are processed independently and then bonded together. This segmentation allows each wafer to be optimized separately - the CMOS wafer for circuit integration and the MEMS wafer for high-precision transducer arrays, thereby achieving both cost-effectiveness and high manufacturing precision
Solution Approach 2:
A bonding interface is introduced as an intermediary between the CMOS wafer and MEMS wafer. This bonding layer enables precise alignment and mechanical coupling while maintaining electrical connections, allowing the system to achieve high resolution without requiring the entire manufacturing process to occur on a single chip
2Manufacturing precision
If CMUT linear arrays are used for fingerprint detection, then required resolution of 300 dpi or higher can be achieved, but accuracy deteriorates due to skin condition and sensor contamination
Solution Approach 1:
The system uses multiple transducer arrays that can be selectively activated based on detection needs. The mechanical switching capability allows dynamic reconfiguration of the sensing array, enabling the system to adapt to different skin conditions and maintain high detection accuracy by selecting optimal transducer combinations
Solution Approach 2:
The patent implements variable impedance matching networks that dynamically adjust electrical parameters to optimize signal transmission. By changing impedance parameters in real-time, the system compensates for signal degradation caused by skin conditions and contamination, thereby maintaining detection accuracy
3Reliability
If vertically stacked structure with CMOS wafer and MEMS wafer is used, then electrical coupling and mechanical anchoring are enhanced, but device complexity increases
Solution Approach 1:
The patent combines electrical and mechanical functions at the bonding interface between CMOS and MEMS wafers. The same bonding structure that provides mechanical anchoring also serves as the electrical connection pathway, thereby enhancing reliability without proportionally increasing structural complexity
Solution Approach 2:
The bonding interface is designed to perform multiple functions simultaneously: mechanical support, electrical connection, and alignment reference. This multi-functionality reduces the need for separate structures for each function, thereby maintaining relatively simple device architecture while achieving enhanced electrical coupling and mechanical stability
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 approach allows for high-resolution fingerprint sensing with improved accuracy and reliability by reducing mechanical coupling suppression and anchor occupation ratio, increasing signal-to-noise ratio and array gain, thus overcoming limitations of conventional MEMS devices.
Implementation Method 1
a piezoelectric material disposed within the MEMS device comprising a piezoelectric MUT (PMUT) array of a fingerprint sensor
Implementation Method 2
adapted to sense a characteristic of a fingerprint placed adjacent to the MUT structure
Data Source
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.