CMUT Permanent Contact Mode for Stable Ultrasonic Operation
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Solution Overview
Problem
Conventional CMUTs face significant performance variations over a wide pressure range due to the plate's contact with or detachment from the substrate, leading to inefficient device operation, especially at low pressures, and require high DC bias for plate collapse or fabrication.
Innovation Solution
Implementing a CMUT in permanent contact mode where the plate remains in contact with the substrate due to pressure differences, eliminating the need for DC bias for collapse and using a partial electrode to reduce electric field and parasitic capacitance, thereby enhancing coupling efficiency and stability across varying pressures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the CMUT plate is designed to not touch the substrate at any pressure, then device reliability is improved, but coupling efficiency deteriorates at low pressures
Solution Approach 1:
The patent changes the operational parameter by applying a permanent DC bias voltage to the bottom electrode, which creates an electrostatic force that pulls the CMUT plate into permanent contact with the substrate. This parameter change (applying DC bias) resolves the contradiction by ensuring the plate remains in contact across all pressure conditions, thereby maintaining high coupling efficiency at low pressures while avoiding the reliability issues of plate detachment.
2Use of energy by moving object
If high DC bias is applied to collapse the CMUT plate, then coupling efficiency is improved, but device complexity increases
Solution Approach 1:
The patent applies preliminary action by permanently collapsing the CMUT plate onto the substrate during fabrication through a single high-voltage bias application. This preliminary collapse action eliminates the need for continuous high DC bias during operation, as the plate remains in permanent contact. This resolves the contradiction by achieving high coupling efficiency through a one-time fabrication step rather than requiring complex continuous high-voltage biasing circuitry.
3Adaptability or versatility
If the CMUT operates in conventional mode with pressure variations, then adaptability is improved, but performance stability deteriorates
Solution Approach 1:
The patent extracts the problematic variable contact behavior by permanently fixing the CMUT plate to the substrate through electrostatic attraction. By removing the plate's ability to detach and re-contact under varying pressures, the invention eliminates performance variations caused by contact state changes. This resolves the contradiction by maintaining performance stability across the pressure range while preserving adaptability through the permanent contact design.
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 provides stable frequency, improved coupling efficiency, reduced risk of electric breakdown, and design flexibility, enabling reliable operation under high and varying pressures without the need for high DC bias, suitable for medical, therapeutic, and ultrasonic flow metering applications.
Implementation Method 1
This contact is provided by the pressure difference between the environment (typically 1 atm or greater), and the pressure of the evacuated region between the CMUT plate and substrate (typically 1 mTorr or less)
Implementation Method 2
Due to this permanent contact, the electric field in the gap for a given DC bias voltage will be larger, which provides improved coupling efficiency at lower DC bias voltages
Data Source
AI summary
Capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode are provided. Such a CMUT always has its plate in contact with the substrate, even for zero applied electrical bias. This contact is provided by the pressure difference between the environment, and the pressure of the evacuated region between the CMUT plate and substrate. Due to this permanent contact, the electric field in the gap for a given DC bias voltage will be larger, which provides improved coupling efficiency at lower DC bias voltages. Furthermore, in an environment with high and varying pressure, the plate will not shift between the conventional mode and the collapsed mode, but will only be pushed down with varying contact radius. In some embodiments, an electrode shaped as an annulus is employed, so that only the active vibrating part of the CMUT plate sees the applied DC and AC voltages.