Capacitive Sensor Movable Frame Spring Stiction Prevention
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Solution Overview
Problem
Current capacitive sensors, particularly MEMS microphone chips, face issues with stiction between electrodes during manufacturing, leading to reduced sensitivity and low yield due to residual stresses and small electrode clearance, affecting applications in acoustic transducers, pressure gauges, accelerometers, and RF switches.
Innovation Solution
A capacitive sensor design featuring a movable frame and small electrode clearance formed by bonding two substrates, with spring members connecting the movable frame, anchor base, and sensing device to prevent stiction and maintain sensitivity, utilizing a manufacturing method that includes etching and bonding processes to create a capacitive sensor with improved sensitivity and reduced stiction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the electrode clearance is reduced to improve sensitivity, then the sensitivity is improved, but stiction occurs between electrodes causing short circuits and reduced yield
Solution Approach 1:
The patent introduces a sacrificial layer (intermediary substance) between the conductive membrane and conducting electrode during the manufacturing process. This sacrificial layer maintains a controlled air gap that prevents stiction between electrodes while allowing the membrane to move freely. The sacrificial layer is subsequently removed through etching to create the final capacitive structure with the desired electrode clearance for high sensitivity without direct electrode contact causing stiction.
Solution Approach 2:
The patent carefully controls and optimizes the thickness of the air gap between electrodes as a critical parameter. By precisely adjusting the air gap distance to an optimal range, the patent achieves both high sensitivity (requiring small clearance) and prevents stiction (requiring sufficient separation). The air gap thickness is controlled through the deposition step and sacrificial layer thickness during manufacturing.
2Manufacturing precision
If the sacrificial layer is removed by etching to form air gap, then the electrode clearance is achieved, but residual stresses accumulate in the conductive membrane reducing sensitivity
Solution Approach 1:
The patent incorporates stress compensation features directly into the conductive membrane design during the manufacturing process. Special geometric patterns or structural modifications are built into the membrane to preemptively counteract the residual stresses that will arise from sacrificial layer removal. This preliminary stress balancing action prevents sensitivity degradation before it occurs.
Solution Approach 2:
The patent modifies the physical and geometric parameters of the conductive membrane to manage residual stresses. By changing membrane thickness, material composition, or geometric configuration, the patent optimizes stress distribution and minimizes the impact of residual stresses on sensitivity while maintaining the required air gap for proper capacitive operation.
3Reliability
If springs are added to counteract stiction force, then stiction is reduced, but device complexity increases
Solution Approach 1:
The patent extracts and eliminates the need for complex spring structures by using a simpler sacrificial layer approach. Instead of adding mechanical spring elements to counteract stiction, the patent removes the problematic component (sacrificial layer) after it has served its purpose of maintaining electrode separation during manufacturing. This extraction approach prevents stiction without adding device complexity.
Solution Approach 2:
The sacrificial layer functions as a temporary, disposable component used only during manufacturing. It provides the necessary electrode separation and stress management during the critical manufacturing phase, then is removed through etching. This disposable approach solves the stiction problem without requiring permanent, complex spring structures in the final device.
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 enhances the sensitivity of capacitive sensors while resolving stiction issues, allowing for higher manufacturing yield and reduced driving voltage requirements by maintaining a controlled distance between electrodes through elastic deformation of spring members.
Implementation Method 1
maintaining a controlled distance between electrodes through elastic deformation of spring members
Implementation Method 2
formed by bonding two substrates
Data Source
AI summary
A capacitive sensor includes a substrate, at least one first electrode, at least one second electrode, a sensing device, at least one anchor base, at least one movable frame, and a plurality of spring members. The first and second electrodes are disposed on the substrate, and the anchor base surrounds the first and second electrodes and is disposed on the substrate. The movable frame surrounds the sensing device. Some of the spring members connect the movable frame and the sensing device, and the other spring members connect the movable frame and the anchor base. The sensing device and the first electrode are both sensing electrodes. The movable frame is disposed above the second electrode, and cooperates with the second electrode to act as a capacitive driver.