In-Plane MEMS Accelerometer Vertical Gap Design
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Solution Overview
Problem
Current MEMS fabrication techniques struggle to create gun-hard, high-performance, three-axis accelerometers that include both in-plane and out-of-plane accelerometers on a single chip, due to limitations in gap size and fragility, as well as inherent nonlinearity issues in conventional comb-finger configurations.
Innovation Solution
A gun-hard, in-plane MEMS accelerometer design using a thick proof mass and vertical gaps as small as 1.0 μm, fabricated using techniques similar to out-of-plane accelerometers, which measures differential capacitance through area change instead of gap change, allowing for linear output and robustness suitable for high shock applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional MEMS comb-finger accelerometer fabrication techniques are used, then in-plane accelerometers can be manufactured, but the lateral gap cannot be reduced below 10 μm due to DRIE technology limitations
Solution Approach 1:
The patent transitions from measuring lateral gap changes (in-plane) to measuring vertical gap changes (out-of-plane). By orienting the proof mass movement perpendicular to the substrate rather than parallel, the invention achieves sub-micron precision using vertical DRIE etching capabilities, which can produce gaps as small as 1.0 μm, bypassing the 10 μm lateral gap limitation of conventional techniques
Solution Approach 2:
Instead of forming the sensing gap laterally between interdigitated fingers as in conventional comb-finger accelerometers, the patent inverts the approach by forming the gap vertically between the proof mass and substrate. This inversion allows utilization of superior vertical etching precision and enables much smaller effective gap dimensions
2Manufacturing precision
If polysilicon deposition techniques are used to reduce lateral gap to 1.1 μm, then manufacturing precision improves, but the process flow becomes very complicated resulting in low yield and fragile structures
Solution Approach 1:
The patent eliminates the need for complex polysilicon deposition by changing the measurement dimension from lateral to vertical. The vertical gap configuration uses standard silicon DRIE etching processes that produce robust, shock-resistant structures while achieving 1.0 μm precision, avoiding the fragile polysilicon layers required for lateral gap reduction
Solution Approach 2:
The invention changes the critical parameter from lateral gap dimension to vertical gap dimension. This parameter change allows use of proven, robust silicon etching processes rather than fragile polysilicon deposition, maintaining manufacturing precision while significantly improving structure reliability for high-shock applications
3Measurement precision
If conventional comb-finger accelerometer configuration is used, then in-plane acceleration can be measured, but inherent nonlinearity issues arise due to limited lateral gap change range
Solution Approach 1:
The patent resolves nonlinearity by measuring vertical displacement rather than lateral displacement. The vertical gap change between proof mass and substrate provides a larger, more linear measurement range, eliminating the inherent nonlinearity of lateral comb-finger configurations where the sensing gap is constrained by finger geometry
Solution Approach 2:
The invention changes the measurement parameter from lateral gap variation to vertical gap variation. This parameter change enables larger displacement ranges with improved linearity, as the vertical dimension allows greater proof mass travel without the geometric constraints that limit lateral comb-finger accelerometer performance
4Adaptability or versatility
If both out-of-plane and in-plane accelerometers are built on a single chip using the same fabrication techniques, then integration is achieved, but current techniques cannot fabricate in-plane accelerometers with sufficient precision and robustness
Solution Approach 1:
The patent makes the out-of-plane accelerometer fabrication technique universally applicable to both out-of-plane and in-plane accelerometer types. By using vertical gap measurement with DRIE etching for both configurations, the same fabrication process achieves high precision and robustness in both sensor types, enabling true three-axis integration on a single chip
Solution Approach 2:
The invention applies the vertical gap measurement approach to both in-plane and out-of-plane accelerometer configurations. This dimensional change allows the same fabrication techniques to produce both sensor types with equivalent precision and shock resistance, solving the integration problem that previously required different fabrication methods
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 enables the fabrication of a high-performance, low-noise, gun-hard, three-axis accelerometer on a single chip, with improved sensitivity and robustness, capable of withstanding extreme vibrations and shock loads, while minimizing damping and maintaining micro-gravity resolution.
Implementation Method 1
changing the capacitance between the proof mass and the substrate. This change in capacitance can be used to measure the displacement and to determine the acceleration
Implementation Method 2
A proof mass movably positioned a predetermined distance above the substrate by a pair of opposing springs
Data Source
AI summary
A system for determining in-plane acceleration of an object. The system includes an in-plane accelerometer with a substrate rigidly attached to an object, and a proof mass—formed from a single piece of material—movably positioned a predetermined distance above the substrate. The proof mass includes a plurality of electrode protrusions extending downward from the proof mass to form a gap of varying height between the proof mass and the substrate. The proof mass is configured to move in a direction parallel to the upper surfaces of each of the plurality of substrate electrodes when the object is accelerating, which results in a change in the area of the gap, and a change in capacitance between the substrate and the proof mass. The in-plane accelerometer can be fabricated using the same techniques used to fabricate an out-of-plane accelerometer and is suitable for high-shock applications.