Two-Terminal CMUT Device with High-Impedance Resistor
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Solution Overview
Problem
The existing capacitive micromachined ultrasonic transducer (CMUT) devices face efficiency losses due to high parasitic capacitance in collapsed mode, leading to overheating and reduced sensitivity, particularly because of the large ratio between the collapsed area and active area capacitors, which complicates voltage supply and interconnects.
Innovation Solution
The introduction of a high impedance resistor electrically coupling the second and third electrodes, reducing AC current flow through the DC-impedance load, allowing for a two-terminal drive configuration and simplifying the electrical scheme, with the resistor's impedance value higher than the AC-impedance between the first and second electrodes, and its implementation using a thin film layer for integration into the CMUT cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the membrane is brought into a collapsed state by applying DC voltage, then the parasitic capacitance (Cc) increases, but this causes high AC current flow and overheating that reduces operating efficiency
Solution Approach 1:
The substrate electrode is segmented into two separate electrodes: a first electrode in the central region and a second electrode in the peripheral region. This segmentation allows independent electrical control of the collapsed area (first electrode) and the active area (second electrode), enabling the membrane to be held in collapsed state while minimizing parasitic capacitance effects on the active transmission/reception area.
Solution Approach 2:
Different electrical conditions are applied to different regions of the substrate electrode. The first electrode (central region) is biased to maintain membrane collapse, while the second electrode (peripheral region) is kept at a different potential to minimize parasitic capacitance. This local differentiation allows the collapsed state to be maintained only where needed without sacrificing overall operating efficiency.
2Reliability
If the membrane is collapsed to the substrate, then the collapsed area capacitor (Cc) becomes large, but this large capacitor acts as a low pass filter that reduces receive mode sensitivity
Solution Approach 1:
By segmenting the substrate electrode into first and second electrodes, the patent separates the collapsed area capacitor (associated with the first electrode) from the active area capacitor (associated with the second electrode). This separation ensures that the large Cc does not directly load the receive signal path, thereby maintaining sensitivity while still maintaining the collapsed state through the first electrode.
3Loss of energy
If a three-terminal CMUT device is used to isolate parasitic capacitance, then the operating efficiency improves, but the device complexity and interconnect requirements increase
Solution Approach 1:
The patent combines the benefits of the three-terminal device into a two-terminal implementation by integrating the first and second substrate electrodes into a single CMUT device structure that requires only two external interconnects. The segmented electrodes are fabricated as part of the device architecture, eliminating the need for additional external terminals while maintaining the electrical isolation benefits.
4Shape
If the ratio between collapsed area capacitor (Cc) and active area capacitor (Ca) is large, then the DC voltage can effectively collapse the membrane, but this large ratio causes high AC current flow back and forth
Solution Approach 1:
The segmentation of the substrate electrode allows the large Cc to be associated with the first electrode (central region) while the second electrode (peripheral region) maintains a different electrical potential. This segmentation effectively removes the large Cc from the AC signal path, preventing the high AC current flow that would otherwise occur due to the large capacitance ratio.
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 design enhances the operating efficiency of the ultrasound system by reducing AC current flow, simplifying the electrical scheme, and allowing for a single DC source per CMUT device, thereby improving power transmission and reception sensitivity while minimizing overheating.
Implementation Method 1
a high impedance resistor electrically coupling the second and third electrodes, reducing AC current flow through the DC-impedance load
Implementation Method 2
A commonly known capacitive micromachined ultrasonic transducer device is tens of micrometer size diaphragm-like cell comprising two electrodes opposing each other. For transmission the capacitive charge applied to the electrodes is modulated to vibrate/ move the diaphragm (membrane) of the device
Implementation Method 3
The reflected sound wave causes vibrations of the membrane, modulating the capacitance between two electrodes of the CMUT transducer, thereby generating an electric signal
Implementation Method 4
The DC actuation voltage applied across both electrodes is large enough deflect the membrane electrode towards the substrate electrode such that the membrane is brought into a physical contact with the substrate electrode
Data Source
Figure 1
Figure 2~3(b)
Figure 4(a)~5
AI summary
The invention related to an ultrasound system comprising: an ultrasound array including at least one capacitive micromachined ultrasonic transducer device comprising a membrane coupled to a first electrode, a substrate opposing the membrane with a gas or vacuum cavity there between and coupled to a second and a third electrodes, wherein the second electrode opposes the first electrode in a peripheral region and the third electrode opposes the first electrode in a central region; at least one drive circuit coupled to the array. The system further comprises a high impedance resistor, which electrically couples to the second electrode and the third electrode and has an impedance value higher than an AC impedance between the first and the second electrodes, when the membrane of the CMUT device is in the collapsed state and the CMUT devices is activated at operating frequency.