Capacitive Membrane Ultrasonic Transducer Bulk Wave Suppression
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Solution Overview
Problem
Capacitive membrane ultrasonic transducers (cMUTs) face issues with bulk wave generation due to substrate ringing modes, which cause undesirable frequency response notches and ringing in the time-domain response, especially when the operating frequency range includes substrate resonant cavity frequencies, and existing solutions like bonding with a thick impedance-matched backing are impractical for space-constrained applications.
Innovation Solution
The substrate is modified by reducing its thickness, creating a diffraction grating pattern of grooves on the backside to suppress specific substrate bulk wave frequencies, or by roughening the substrate surfaces to scatter energy incoherently, and bonding to a high-impedance backing with a low-impedance filling material to ensure out-of-phase reflections, allowing for flexible suppression of substrate ringing modes without requiring a thick backing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick impedance-matched backing is bonded to the substrate, then substrate ringing modes are suppressed, but the transducer becomes too thick for space-constrained applications
Solution Approach 1:
A thin layer of epoxy is used as an intermediary bonding material between the cMUT transducer and the acoustic matching layer. This thin intermediary layer is sufficient to suppress substrate ringing modes when combined with the acoustic matching layer, avoiding the need for a thick backing that would increase transducer thickness
Solution Approach 2:
The solution combines multiple materials with different acoustic impedances (epoxy, silicon substrate, acoustic matching layer) to achieve effective suppression of substrate ringing modes. The composite structure of these materials with specific impedance relationships provides the necessary suppression while maintaining thin overall dimensions
2Reliability
If the substrate thickness is reduced to eliminate ringing modes, then bulk wave generation is suppressed, but the substrate may become too thin to support the membrane structure
Solution Approach 1:
The substrate thickness is optimized to a specific range that balances two competing requirements: thin enough to push substrate ringing modes above the operating frequency range, but thick enough to provide sufficient structural support for the membrane. This parameter optimization eliminates the need for thick backings while maintaining structural integrity
Solution Approach 2:
The substrate thickness is predetermined and controlled during fabrication to be within a specific range that inherently suppresses bulk wave generation at operating frequencies. This preliminary design choice prevents the need for additional thick backing layers, as the substrate itself is designed to avoid resonant cavity effects
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
These modifications effectively suppress bulk wave generation at the operating frequency, preventing frequency response notches and ringing, while maintaining practical transducer dimensions and impedance matching, thus enhancing the operational efficiency of cMUTs.
Implementation Method 1
creating a diffraction grating pattern of grooves on the backside to suppress specific substrate bulk wave frequencies
Implementation Method 2
roughening the substrate surfaces to scatter energy incoherently
Implementation Method 3
bonding to a high-impedance backing with a low-impedance filling material to ensure out-of-phase reflections
Implementation Method 4
A DC bias voltage 14 is applied between the membrane and the bottom electrodes to create electrostatic attraction, pulling the membrane toward the substrate
Implementation Method 5
An AC voltage 16 is applied to the biased membrane to generate harmonic membrane motion and ultrasonic waves
Data Source
AI summary
A capacitive membrane ultrasonic transducer which includes a membrane supported by a substrate in which ultrasonic bulk waves at the frequency of operation of the transducers are suppressed by configuring the substrate and a method of suppressing the ultrasonic bulk waves.