SOI-based CMUT with Buried Electrodes
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Solution Overview
Problem
There is a need for smaller and more reliable ultrasonic transducers, particularly in imaging and Non Destructive Testing, where existing piezo-electric transducers fall short in terms of size and reliability.
Innovation Solution
A capacitive micromachined ultrasonic transducer (CMUT) device is developed using a bonded SOI structure with hermetically sealed buried electrodes, comprising a wafer with semiconductor electrode layers and insulating dielectric layers, where the dielectric stand-offs provide electrical isolation and form sealed cavities, enabling the creation of a multi-layer stacked micro-electro-mechanical device with a simple fabrication method involving few mask steps and wafer bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If piezo-electric transducers are used, then ultrasonic transduction function is achieved, but device size and reliability are insufficient
Solution Approach 1:
The patent replaces traditional piezo-electric mechanical systems with a capacitive micromachined ultrasonic transducer (CMUT) system using suspended semiconductor membranes and buried electrodes. This substitution enables improved reliability through hermetic sealing while achieving smaller device dimensions suitable for modern imaging applications.
Solution Approach 2:
The patent employs thin semiconductor films (e.g., silicon membranes) as flexible diaphragms in the CMUT structure. These thin films enable miniaturization while maintaining structural integrity and acoustic performance, directly addressing the need for smaller transducer sizes without compromising reliability.
2Reliability
If hermetically sealed buried electrodes are implemented, then reliability and controlled internal pressure are achieved, but fabrication complexity increases
Solution Approach 1:
The patent implements hermetic sealing and buried electrode structures during the initial wafer fabrication stage using standard semiconductor processing techniques. By performing the sealing action preliminarily during manufacturing rather than as a post-processing step, the patent achieves high reliability without significantly increasing overall fabrication complexity.
Solution Approach 2:
The patent utilizes standard semiconductor fabrication parameters and materials (e.g., silicon wafers, thermal oxidation processes, photolithography) to create hermetically sealed structures. By working within established parameter ranges and using conventional semiconductor manufacturing techniques, the patent achieves hermetic sealing without requiring exotic or overly complex fabrication processes.
3Reliability
If dielectric stand-offs are used for electrical isolation, then electrical isolation is achieved, but processing steps increase
Solution Approach 1:
The patent combines multiple functions into the dielectric stand-off structure: electrical isolation between buried electrodes, mechanical support for the suspended membrane, and definition of the sealed cavity volume. By merging these functions into a single integrated structure formed during standard fabrication, the patent achieves reliable electrical isolation without proportionally increasing processing complexity.
Solution Approach 2:
The dielectric stand-off acts as an intermediary structure that simultaneously provides electrical isolation and mechanical support. This intermediary element enables the CMUT structure to achieve both electrical isolation and structural integrity without requiring separate complex isolation and support systems.
4Object-affected harmful factors
If surface is kept non-energized, then patient safety is improved, but transducer performance may be limited
Solution Approach 1:
The patent extracts the high-voltage electrode from the surface interface and places it as a buried electrode beneath the suspended membrane. This extraction allows the surface to remain non-energized for patient safety while the buried electrode maintains full transducer performance through capacitive coupling across the membrane gap.
Solution Approach 2:
The suspended semiconductor membrane serves as an intermediary between the non-energized surface and the buried electrode. This intermediary enables capacitive coupling that maintains transducer performance while keeping the patient-contacting surface at ground potential, thus ensuring safety without sacrificing performance.
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 CMUT device achieves improved performance and reliability with a hermetically sealed cavity, allowing for controlled internal pressure and enhanced sensitivity, suitable for medical and NDT applications, while requiring less complex processing and ensuring safety by avoiding energization of the surface interacting with patients or tissues.
Implementation Method 1
the dielectric stand-offs having a height sufficient to provide electrical isolation between the first semi-conductor electrode layer and the second semi-conductor electrode layer
Implementation Method 2
the dielectric stand-offs being continuous in at least selected portions of the CMUT pattern to define sealed CMUT cavities
Data Source
AI summary
A multi-layer stacked micro-electro-mechanical (MEMS) device that acts as a capacitive micromachined ultrasonic transducer (CMUT) with a hermetically sealed device cavity formed by a wafer bonding process with semiconductor and insulator layers. The CMUT design uses a doped Si SOI and wafer bonding fabrication method, and is composed of semiconductor layers, insulator layers, and metal layers. Conventional doped silicon may be used for electrode layers. Other suitable semi-conductor materials such as silicon carbide may be used for the electrode layers. The insulator may be silicon oxide, silicon nitride or other suitable dielectric.