MEMS Digital Variable Capacitor Stress Control via Delayed Grounding
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Solution Overview
Problem
The connection of a moveable plate in MEMS digital variable capacitor devices to CMOS ground during fabrication causes stress issues, leading to shifts in actuation voltages and reduced wafer yield due to thermal and plasma effects, making it difficult to achieve controlled electrostatic actuation.
Innovation Solution
The moveable plate is decoupled from CMOS ground during fabrication, with electrical coupling established only after the plate is completely formed, using the same layer that forms the pull-up electrode, ensuring identical stress levels in the top and bottom plates and allowing for controlled actuation voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the moveable plate is connected to CMOS ground during fabrication, then electrical stability is improved, but stress control deteriorates leading to actuation voltage shifts
Solution Approach 1:
The patent applies preliminary action by forming the moveable plate structure completely before establishing its electrical connection to CMOS ground. The plate is fabricated with controlled stress levels while isolated, then connected to ground only after formation is complete. This sequencing ensures that ground connection does not introduce stress variations that would shift actuation voltages, resolving the contradiction between electrical stability and stress control precision.
2Ease of operation
If the moveable plate is connected to CMOS ground during fabrication, then electrical reference is established, but thermal and plasma effects cause stress variations
Solution Approach 1:
The patent establishes the electrical reference connection to CMOS ground as a preliminary action completed after the moveable plate structure is fully formed. By delaying the ground connection until after plate formation, the patent prevents thermal and plasma effects during fabrication from causing stress variations in the plate, while still ensuring the electrical reference is established for proper operation.
3Reliability
If the moveable plate is connected to CMOS ground during fabrication, then electrical connectivity is ensured, but wafer yield decreases due to actuation voltage shifts
Solution Approach 1:
The patent applies preliminary action by completing the moveable plate fabrication and stress stabilization before establishing electrical connectivity to CMOS ground. This sequencing ensures that actuation voltages are properly controlled without shifts, thereby maintaining high wafer yield while ensuring reliable electrical connectivity for device operation.
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 ensures tightly controlled actuation voltages and improved wafer yield by maintaining flatness of the moveable plate and matching stress levels, reducing the impact of thermal and plasma effects during fabrication.
Implementation Method 1
These voltages result in electrostatic forces, which pull the MEMS element either up or down in contact to provide a stable minimum or maximum capacitance to the RF-electrode.
Data Source
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AI summary
The present invention generally relates to a MEMS digital variable capacitor (DVC) (900) and a method for manufacture thereof. The movable plate (938) within a MEMS DVC should have the same stress level to ensure proper operation of the MEMS DVC. To obtain the same stress level, the movable plate is decoupled from CMOS ground during fabrication. The movable plate is only electrically coupled to CMOS ground after the plate has been completely formed. The coupling occurs by using the same layer (948) that forms the pull-up electrode as the layer that electrically couples the movable plate to CMOS ground. As the same layer couples the movable plate to CMOS ground and also provides the pull-up electrode for the MEMS DVC, the deposition occurs in the same processing step. By electrically coupling the movable plate to CMOS ground after formation, the stress in each of the layers of the movable plate can be substantially identical.