Pre-collapsed cMUTs via Controlled Membrane Collapse
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Solution Overview
Problem
Current methods for producing capacitive micro-machined ultrasonic transducers (cMUTs) face challenges such as non-linear behavior, narrow operating range, low capacitance, high sensitivity to manufacturing variability, and the need for high bias voltages, which can lead to membrane collapse and reliability issues.
Innovation Solution
The method involves producing pre-collapsed cMUTs by defining holes through the membrane layer, applying a controlled bias voltage to collapse the membrane, and sealing it relative to the substrate using a cost-effective process that replaces traditional sacrificial etch techniques, allowing for larger hole sizes and reduced voltage requirements, thereby improving uniformity and reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If traditional sacrificial etch processes are used to create vacuum cavities beneath the membrane, then the cMUT structure is formed, but the process requires sub-100 nm precision holes and channels that need expensive and complicated equipment like high-resolution e-beam lithography
Solution Approach 1:
The patent changes the critical dimension parameter from sub-100 nm to larger dimensions (1-10 micrometers), which allows using standard lithography equipment instead of expensive e-beam lithography. This parameter change fundamentally simplifies the manufacturing process while maintaining functional performance.
Solution Approach 2:
The patent uses a sacrificial layer (such as silicon dioxide or silicon nitride) that is intentionally deposited and then removed through etch holes. This sacrificial material serves a temporary purpose during fabrication and is then discarded, enabling the formation of the vacuum cavity without requiring precision holes through the final membrane structure.
2Shape
If high bias voltage is applied to collapse the membrane in traditional cMUTs, then the desired collapsed structure is achieved, but the process increases sensitivity to charging and creates reliability issues
Solution Approach 1:
The patent performs the membrane collapse action during the fabrication process itself, before the device is finalized and packaged. By applying bias voltage to collapse the membrane while the structure is still accessible and controllable, the desired shape is achieved as part of manufacturing rather than as a post-processing step, ensuring consistency and reducing later reliability issues.
Solution Approach 2:
The patent replaces the mechanical approach of trying to maintain large vacuum cavities with the electrical approach of actively collapsing the membrane using bias voltage. This substitution allows precise control over the membrane position and eliminates the need for complex mechanical support structures, improving reliability.
3Volume of stationary object
If uncollapsed cMUT structures are used, then the vacuum cavity is maintained, but the devices exhibit non-linear behavior, narrow operating range, low capacitance, and high sensitivity to manufacturing variability
Solution Approach 1:
Instead of trying to maintain the uncollapsed state with large vacuum cavities, the patent inverts the approach by intentionally collapsing the membrane during fabrication. This inverted approach—collapsing rather than maintaining—produces devices with superior electrical characteristics including higher capacitance, improved linearity, and broader operating ranges.
Solution Approach 2:
The patent changes the structural parameter from uncollapsed to collapsed membrane state, which fundamentally alters the electrical characteristics. The collapsed configuration provides higher capacitance values and improved linearity, directly addressing the performance limitations of uncollapsed structures.
4Volume of stationary object
If smaller hole sizes are used in the membrane for sacrificial etch, then the vacuum cavity can be formed, but the manufacturing process becomes more complex and costly
Solution Approach 1:
The patent segments the fabrication process into distinct steps: first forming the sacrificial layer, then creating access holes through the membrane to reach the sacrificial layer, etching the sacrificial material, and finally removing the holes. This segmentation allows each step to be optimized independently, with the access holes being larger and easier to manufacture than the final functional dimensions would require.
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 results in cMUTs with improved linearity, higher array density, reduced operating voltage, and lower charging sensitivity, enabling consistent and cost-effective production of collapsed cMUTs with enhanced performance.
Implementation Method 1
Application of a bias voltage effects a charge which pulls the membrane and substrate closer together
Data Source
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AI summary
Methods are provided for production of pre-collapsed capacitive micro-machined ultrasonic transducers (cMUTs). Methods disclosed generally include the steps of obtaining a nearly completed traditional cMUT structure prior to etching and sealing the membrane, defining holes through the membrane of the cMUT structure for each electrode ring fixed relative to the top face of the membrane, applying a bias voltage across the membrane and substrate of the cMUT structure so as to collapse the areas of the membrane proximate to the holes to or toward the substrate, fixing and sealing the collapsed areas of the membrane to the substrate by applying an encasing layer, and discontinuing or reducing the bias voltage. CMUT assemblies are provided, including packaged assemblies, integrated assemblies with an integrated circuit/chip (e.g., a beam-steering chip) and a cMUT/lens assembly. Advantageous cMUT-based applications utilizing the disclosed pre-collapsed cMUTs are also provided, e.g., ultrasound transducer-based applications, catheter-based applications, needle-based applications and flowmeter applications.