Concave Bottom Electrode CMUT for Increased Capacitance
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Solution Overview
Problem
Conventional Capacitive Micromachined Ultrasonic Transducers (CMUTs) with flat bottom electrodes suffer from limited capacitance change and sensitivity due to parasitic capacitance, leading to reduced effectiveness in ultrasound generation and reception, especially when the membrane collapses under high DC bias voltage.
Innovation Solution
A CMUT design featuring a concave-shaped bottom electrode, where the top electrode covers the entire membrane area, allowing for increased effective capacitance and reduced membrane collapse, enabling higher sensitivity and bandwidth by minimizing contact area and utilizing a smaller DC bias voltage for membrane deflection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a flat bottom electrode is used in conventional CMUTs, then the structure is simple and easy to manufacture, but the capacitance change is limited and sensitivity is reduced due to parasitic capacitance
Solution Approach 1:
The bottom electrode is designed with a concave spherical curvature that matches the membrane's deflection profile. This curvature allows the electrode surface to follow the membrane's movement more closely, increasing the capacitance change and reducing parasitic capacitance effects, thereby improving sensitivity while maintaining manufacturability through standard fabrication techniques.
Solution Approach 2:
The invention changes the geometric parameter of the bottom electrode from flat to concave spherical shape. This parameter change increases the effective capacitance area and reduces parasitic capacitance, leading to improved sensitivity and performance without significantly complicating the manufacturing process.
2Measurement precision
If high DC bias voltage is applied to increase sensitivity, then the membrane deflects closer to the bottom electrode and capacitance increases, but the membrane collapses onto the bottom electrode and effective capacitance contribution is reduced
Solution Approach 1:
The concave spherical curvature of the bottom electrode creates a larger gap distance in the central region compared to a flat electrode. This increased gap prevents membrane collapse at high DC bias voltages, maintaining membrane stability and reliability while still allowing sufficient capacitance change for high sensitivity operation.
Solution Approach 2:
The concave electrode geometry preemptively counteracts the collapse tendency by maintaining a larger initial gap distance in the critical central region. This preliminary geometric adjustment prevents the harmful collapse effect before it occurs, allowing the membrane to operate stably at high DC bias voltages.
3Measurement precision
If only 25% of the membrane area is patterned with top electrode to remove parasitic capacitance, then parasitic capacitance is reduced, but the effective capacitance area is limited and sensitivity is reduced
Solution Approach 1:
The concave spherical bottom electrode increases the capacitance change across the entire membrane area by better matching the electrode surfaces. This allows the top electrode to be extended to cover more or all of the membrane area while maintaining low parasitic capacitance effects, thereby increasing effective capacitance area and sensitivity.
Solution Approach 2:
The invention merges the functions of maximizing effective capacitance area and minimizing parasitic capacitance by combining the concave electrode geometry with extended top electrode coverage. This integration allows the entire membrane area to contribute effectively to capacitance change without significant parasitic losses.
4Quantity of substance
If the gap distance between top electrode and bottom electrode is reduced to increase capacitance, then effective capacitance increases, but the membrane collapses more easily onto the bottom electrode
Solution Approach 1:
The concave spherical bottom electrode creates a non-uniform gap distribution with larger spacing in the central region and smaller spacing at the periphery. This geometry maintains sufficient capacitance through the increased peripheral area contribution while preventing central membrane collapse, thereby balancing effective capacitance and membrane stability.
Solution Approach 2:
The invention changes the gap distance parameter from uniform (flat electrode) to non-uniform (concave electrode) distribution. This parameter change allows different regions of the membrane to have optimized gap distances, with peripheral regions contributing to capacitance and central regions maintaining stability, achieving both high effective capacitance and membrane reliability.
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 concave bottom electrode design enhances capacitance change by up to 79%, increases sensitivity, and improves bandwidth, allowing for more efficient ultrasound generation and reception, particularly in intravascular applications, with a maximum capacitance at collapsed mode reaching 0.7 pF, nearly ten times that of conventional CMUTs.
Implementation Method 1
the bottom electrode is configured to generate an electrostatic force to deflect the membrane towards the bottom electrode such that a peripheral edge region of the membrane is brought into close proximity with the bottom electrode and an electrostatic force proximal to the peripheral edge region of the membrane is increased
Data Source
AI summary
A Capacitive Micromachined Ultrasonic Transducer (CMUT) having a membrane operatively connected to a top electrode and having a bottom electrode having a concave void. When a DC bias voltage is applied, the membrane is deflected towards the bottom electrode such that a peripheral edge region of the membrane is brought into close proximity with the bottom electrode and an electrostatic force proximal to the peripheral edge region of the membrane is increased.