MEMS Variable Gap Width for Parasitic Capacitance Reduction
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Solution Overview
Problem
Conventional MEMS capacitive sensors face challenges in high acceleration environments due to parasitic capacitance and damping, which affect accuracy and sensitivity.
Innovation Solution
A method is developed to produce MEMS devices with variable gap widths, where the gap between the proof mass and substrate is larger at non-sensing regions and smaller at sense regions, reducing parasitic capacitance and damping, achieved through a specific fabrication process involving dielectric and structural layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a uniform gap width is used between proof mass and substrate, then manufacturing is simplified, but parasitic capacitance and damping increase reducing sensor accuracy
Solution Approach 1:
The patent applies local quality by creating different gap widths in different regions of the MEMS device. Specifically, the gap between the proof mass and substrate is made non-uniform, with smaller gaps at support regions and larger gaps at sensing regions. This local variation reduces parasitic capacitance and damping at critical sensing locations while maintaining structural integrity at support locations, thereby improving sensor accuracy without requiring complete redesign of the entire gap structure.
2Reliability
If larger gap width is used between proof mass and substrate, then damping is reduced improving dynamic behavior, but capacitance sensing sensitivity decreases
Solution Approach 1:
The patent resolves this contradiction by implementing local quality through spatially varying gap widths. At sensing regions where capacitance measurement occurs, larger gap widths reduce damping and improve dynamic behavior. At support regions near the substrate, smaller gap widths maintain structural stability. This localized differentiation allows the device to simultaneously achieve improved dynamic behavior and maintained sensing sensitivity.
3Measurement precision
If smaller gap width is used at sense regions, then parasitic capacitance is reduced improving accuracy, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies preliminary action by pre-forming the non-uniform gap structure during the fabrication process using sacrificial layers and selective etching. The variable gap geometry is established in advance through carefully designed deposition and etching steps, rather than requiring post-fabrication adjustment. This preliminary formation of the desired gap profile reduces the need for high-precision control during final assembly and operation.
Solution Approach 2:
The patent uses local quality by creating smaller gaps specifically at sense regions where parasitic capacitance must be minimized. This localized gap reduction is achieved through selective removal of sacrificial material or targeted etching processes that create the desired non-uniform profile. By concentrating precision requirements only at critical sensing locations rather than uniformly across the entire device, the overall manufacturing complexity is reduced.
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 enhances noise reduction and sensitivity by minimizing parasitic capacitance and damping, leading to improved accuracy and dynamic behavior in MEMS devices.
Implementation Method 1
a conductive proof mass suspended above said base structure to yield a first gap between said proof mass and said top surface of said substrate and a second gap between said proof mass and said sense plate, wherein the proof mass is movable relative to said base structure for capacitive sensing of movement of the proof mass by said sense plate
Data Source
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AI summary
A MEMS device (40) includes a base structure (42) and a microstructure (44) suspended above the structure (42). The base structure (42) includes an oxide layer (50) formed on a substrate (48), a structural layer (54) formed on the oxide layer (50), and an insulating layer (56) formed over the structural layer (54). A sacrificial layer (112) is formed overlying the base structure (42), and the microstructure (44) is formed in another structural layer (116) over the sacrificial layer (112). Methodology (90) entails removing the sacrificial layer (112) and a portion of the oxide layer (50) to release the microstructure (44) and to expose a top surface (52) of the substrate (48). Following removal, a width (86) of a gap (80) produced between the microstructure (44) and the top surface (52) is greater than a width (88) of a gap (84) produced between the microstructure (44) and the structural layer (54).