Pre-collapsed cMUT Cell Stress Layer Design
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Solution Overview
Problem
Existing pre-collapsed capacitive micro-machined ultrasound transducer cells, such as those disclosed in WO 2010/097729 A1, are not suitable for high-frequency applications due to the high collapse pressure required, which exceeds the strength of the retention member, making it difficult to maintain the membrane in a pre-collapsed state.
Innovation Solution
A pre-collapsed capacitive micro-machined transducer cell is designed with a stress layer on the membrane that provides a bending moment to collapse the edge portion of the membrane to the substrate, using materials like Tungsten, Titanium-Tungsten, Molybdenum, or Molybdenum-Chrome to achieve the necessary stress values, allowing for higher collapse pressures and enabling high-frequency operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a retention member is used to hold the membrane in a pre-collapsed state, then the membrane can be maintained in place for low frequency applications, but the retention member cannot withstand the high collapse pressure required for high frequency applications
Solution Approach 1:
The invention removes the retention member from the final device structure. Instead of using a separate retention component, the membrane is collapsed and held in place through the mechanical properties of the membrane-substrate assembly itself, eliminating the weak link that limited previous designs
Solution Approach 2:
The membrane-substrate system is designed to be self-sustaining in its collapsed state. The structural integration of the membrane with the substrate creates an inherent mechanism that maintains the pre-collapsed configuration without requiring additional supporting components
2Speed
If the membrane diameter is reduced for high frequency applications, then the operating frequency increases, but the collapse pressure increases significantly making it difficult to maintain the pre-collapsed state
Solution Approach 1:
The membrane is collapsed to the substrate during the manufacturing process before final device assembly. This preliminary collapsing action occurs when the membrane is more compliant and easier to deform, avoiding the need to overcome high collapse pressures during later stages or operation
Solution Approach 2:
The invention changes the physical state and mechanical properties of the membrane during manufacturing versus operation. By collapsing the membrane when it is in a more compliant state during fabrication, the process bypasses the high pressure requirements that would exist if collapse were attempted after the membrane reached its final operational stiffness
3Ease of manufacture
If electrical bias voltage is used to collapse the membrane, then the membrane can be collapsed and sealed, but the process requires additional manufacturing steps and complexity
Solution Approach 1:
The collapsing mechanism is integrated into the structural design of the membrane-substrate system itself. By combining the membrane, substrate, and collapse mechanism into a unified structure, the invention eliminates separate collapsing components and reduces manufacturing complexity while maintaining the pre-collapsed state
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 solution effectively collapses the membrane edge to the substrate, even at high frequencies, ensuring the transducer operates reliably with center frequencies above 8 MHz, overcoming the limitations of earlier designs by maintaining the pre-collapsed state without relying on weak retention members.
Implementation Method 1
a stress layer (17) formed on the membrane (14), the stress layer (17) having a predetermined stress or stress value with respect to the membrane (14)
Implementation Method 2
The stress layer is adapted to provide a bending moment on the membrane (14) in a direction towards the substrate (12) such that the edge portion of the membrane is collapsed to the substrate (12)
Data Source
Figure 1
Figure 2
Figure 3a~3d
AI summary
The present invention relates to a pre-collapsed capacitive micro- machined transducer cell (10) comprising a substrate (12), and a membrane (14) disposd above a total membrane area (Atota|), wherein a cavity (20) is formed between the membrane (14) and the substrate (12), the membrane comprising a hole (15) and an edge portion (14a) surrounding the hole (15). The cell (10) further comprises a stress layer (17) on the membrane (14), the stress layer (17) having a predetermined stress value with respect to the membrane (14), the stress layer (17) being adapted to provide a bending moment on the membrane (14) in a direction towards the substrate (12) such that the edge portion (14a) of the membrane (14) is collapsed to the substrate (12). The present invention further relates to a method of manufacturing such pre-collapsed capacitive micro-machined transducer cell (10).