MEMS Accelerometer Decoupling Structure for Stress Isolation
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Solution Overview
Problem
MEMS accelerometers are sensitive to mechanical constraints, leading to measurement errors due to stress-induced deformations, and existing decoupling solutions either constrain design, increase device size, or are application-specific, with challenges in maintaining low manufacturing costs and controlling parallelism and temperature-dependent stresses.
Innovation Solution
An accelerometer with a decoupling structure featuring a top portion with a smaller width than the bottom portion, both made from the same semiconductor material, forming a hermetically closed cavity with a seismic mass, and using direct bonding to minimize stress and plastic deformations, allowing for better parallelism and reduced material usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the sensor is made more rigid to resist stress-induced deformations, then measurement precision is improved, but device complexity and design constraints increase
Solution Approach 1:
The device is segmented into a sensor chip and a separate mounting structure with a decoupling layer. This segmentation allows the sensor chip to maintain its rigidity for accurate measurements while the mounting structure absorbs external stresses, resolving the contradiction between measurement precision and design constraints.
Solution Approach 2:
A decoupling layer is introduced as an intermediary between the sensor chip and the mounting structure. This intermediary layer mechanically decouples the sensor from external stresses while maintaining thermal and electrical connections, enabling the sensor to remain rigid without adding complex design constraints to the overall system.
2Reliability
If a frame is built around the sensor chip to mechanically decouple it, then stress decoupling is improved, but device size increases
Solution Approach 1:
The decoupling function is extracted from the sensor chip itself and placed in a separate decoupling layer within the mounting structure. This extraction allows the sensor chip to remain compact while the decoupling layer, positioned at the interface, provides the necessary stress isolation without increasing the sensor chip's footprint.
Solution Approach 2:
Instead of adding a frame around the sensor chip in the planar dimension, the decoupling is achieved by adding a layer in the vertical dimension at the interface between the sensor and mounting structure. This dimensional shift provides stress decoupling without increasing the device's planar footprint.
3Reliability
If different materials are used for the sensor chip and mounting structure to reduce stress, then stress decoupling is improved, but temperature-dependent stresses increase
Solution Approach 1:
The mounting structure has non-uniform mechanical properties: the decoupling layer is designed to be mechanically compliant (softer) to decouple stresses, while the upper portion providing thermal management and electrical connections is made rigid. This local differentiation allows stress decoupling without significant temperature-dependent stresses.
Solution Approach 2:
The mechanical properties of the mounting structure are carefully controlled by adjusting the thickness and material composition of the decoupling layer. By changing these parameters, the layer provides sufficient stress decoupling while maintaining thermal stability and minimizing temperature-dependent stresses.
Data Source
Figure 1a~1b
Figure 2a~3b
Figure 4~5
AI summary
Accelerometer comprising a decoupling structure (3) for fixing the accelerometer on a package and a MEMS sensor chip (10) for measuring an acceleration, wherein the MEMS sensor chip (10) is supported by the decoupling structure (3) and comprises a first sensor wafer layer (11) of a semiconductor material. The decoupling structure (3) forms a bottom portion (30) for fixing the decoupling structure (3) on the package and a top portion (31) fixed to the first sensor wafer layer (11) so that the MEMS sensor chip (10) is arranged above the decoupling structure (3). A width of the top portion (31) in a first planar direction (41) is smaller than a width of the bottom portion (30) and/or of the first sensor wafer layer in the first planar direction (41). The decoupling structure (3) is made out of the same semiconductor material as the first sensor wafer layer (11). The top portion (31) and the bottom portion (30) are manufactured from the same wafer layer. The centre point of the top portion (31) in the first planar direction (41) is arranged in a central region of the bottom portion (30) in the first planar direction (41). The decoupling structure (3) is made out of the same semiconductor material as the first sensor wafer layer (11). The MEMS sensor chip (10) comprises a hermetically closed cavity (16) which includes a seismic mass (14) of the MEMS sensor chip (10)