MEMS Sensor Piezoresistance Orientation and Direct Silicon Junction
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Solution Overview
Problem
Existing MEMS sensors face limitations in enhancing piezoresistance characteristics due to restrictions in plane orientation and accuracy in cavity formation, leading to instability and sensitivity issues.
Innovation Solution
A MEMS sensor design where the plane orientation of the silicon substrate and diaphragm differ, allowing optimal piezoresistance orientation without restriction, and employing a Si—Si direct junction to stabilize characteristics and prevent gas entry, enabling accurate cavity formation and improved sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the plane orientation of the silicon substrate is fixed, then the manufacturing process is simplified, but the piezoresistance characteristics are limited
Solution Approach 1:
The invention divides the silicon structure into two separate components with different plane orientations: the silicon substrate maintains a conventional orientation for easy manufacturing, while the silicon diaphragm is formed with a different plane orientation (e.g., (110) surface) to optimize piezoresistance characteristics. This segmentation allows each component to have its orientation optimized independently.
Solution Approach 2:
Different regions of the silicon structure are assigned different plane orientations based on their functional requirements. The silicon substrate uses an orientation suitable for manufacturing, while the silicon diaphragm uses an orientation that provides optimal piezoresistance response. This local differentiation of material properties resolves the contradiction between manufacturing simplicity and sensor performance.
2Reliability
If an oxide layer is used to join the silicon substrate and diaphragm, then the sealing is easier, but gas can penetrate through the oxide layer
Solution Approach 1:
The invention removes the oxide layer from the joining interface between the silicon substrate and diaphragm, extracting this intermediate layer that caused gas penetration. The silicon diaphragm is joined directly to the silicon substrate without the oxide layer, eliminating the pathway for gas to penetrate into the sealed cavity.
Solution Approach 2:
The invention uses a composite structure where the silicon diaphragm and silicon substrate are joined through a Si-Si direct junction, creating a sealing interface that combines the benefits of silicon compatibility with gas-tight sealing. This direct silicon-to-silicon junction eliminates the permeability issue of oxide layers while maintaining manufacturing feasibility.
3Manufacturing precision
If the cavity is formed in the silicon diaphragm, then the structure is compact, but the cavity formation accuracy is reduced
Solution Approach 1:
Instead of forming the cavity in the thin silicon diaphragm, the invention inverts the approach by forming the cavity in the thicker silicon substrate. This inversion allows for more accurate and easier cavity formation due to the substrate's greater thickness, while the overall structure remains compact through proper spatial arrangement of the cavity and diaphragm.
4Reliability
If the silicon substrate and diaphragm have the same plane orientation, then the junction is simpler, but stress occurs due to temperature changes
Solution Approach 1:
The invention changes the plane orientation parameter of the silicon diaphragm relative to the silicon substrate. By forming the diaphragm with a different plane orientation (e.g., (110) surface instead of (100) surface), the thermal expansion characteristics are optimized to reduce stress at the junction interface during temperature variations, improving long-term reliability.
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 design enhances piezoresistance sensitivity and stability by optimizing plane orientations and using a direct silicon junction, preventing gas entry and maintaining vacuum integrity even in gaseous environments with small atomic radius atoms.
Implementation Method 1
a piezoresistance formed at a first surface of the silicon diaphragm
Implementation Method 2
the substrate and the diaphragm are equal to each other in linear expansion coefficient, and therefore it is possible to restrain the occurrence of stress in the junction interface that results from a temperature change
Data Source
AI summary
A MEMS sensor includes a silicon substrate that has a first surface and a second surface on a side opposite to the first surface and that has a cavity in the first surface, a silicon diaphragm that has a first surface and a second surface on aside opposite to the first surface and in which the second surface is joined directly to the first surface of the silicon substrate, and a piezoresistance formed at the first surface of the silicon diaphragm, and, in the MEMS sensor, a plane orientation of the first surface of the silicon substrate and a plane orientation of the first surface of the silicon diaphragm differ from each other.