3D Resonant MEMS Motion Sensor With Through-Cap Signal Routing
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Solution Overview
Problem
Existing MEMS gyroscopes face limitations due to small sensing masses and springs, leading to low sensitivity, thermal noise, and the need for separate angular rate transduction methods for different axes, with packaging requiring wire bonding and limited to two-dimensional architectures.
Innovation Solution
A MEMS motion sensor with a proof mass enclosed by top and bottom cap wafers, allowing electrodes on both sides for 3D motion detection, using flexible springs and insulated conducting pathways to route electrical signals, enabling 3-axis measurement and angular rate sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polysilicon is used as mechanical material with thin film deposition, then manufacturing is simplified, but mass becomes too small leading to low sensitivity and high thermal noise
Solution Approach 1:
The patent changes the material parameter from polysilicon to single crystal silicon, and increases the thickness parameter from a few micrometers to 5-10 micrometers or more. This allows the proof mass to achieve sufficient mass for high sensitivity while maintaining manufacturability through established single crystal silicon processing techniques.
Solution Approach 2:
The patent employs a composite structure combining single crystal silicon for the proof mass with separate materials for springs and comb electrodes. This allows optimization of each component independently - using single crystal silicon for mass and other suitable materials for mechanical elements - achieving both high sensitivity and manufacturability.
2Measurement precision
If mass thickness is increased to increase sensitivity, then sensitivity improves, but spring stiffness increases counteracting the mass increase
Solution Approach 1:
The patent separates the proof mass from the spring structure, allowing independent optimization. The proof mass is formed as a distinct block of single crystal silicon, while springs are created as separate elements. This segmentation enables the mass to be thick and heavy without directly increasing spring stiffness, as the springs are independent components with their own geometric design freedom.
Solution Approach 2:
The patent changes the design approach by decoupling mass thickness from spring stiffness. Instead of increasing both together as in conventional designs, the proof mass thickness is increased independently for sensitivity, while spring stiffness is controlled through separate geometric parameters of the spring elements, allowing the two to be optimized independently.
3Ease of manufacture
If 2D planar architecture is used, then manufacturing is simplified, but 3D motion detection is limited requiring separate transduction methods
Solution Approach 1:
The patent extends the sensor architecture from 2D planar to 3D vertical structure. The proof mass is suspended above the substrate by flexural beams, creating a third dimension for motion detection. Capacitive sensors are positioned both above and below the proof mass, enabling detection of motion in multiple directions (x, y, and z axes) while maintaining compatibility with planar fabrication processes.
4Reliability
If cap is placed over MEMS for protection, then protection is achieved, but electrical contact requires wire bonding increasing complexity
Solution Approach 1:
The patent extracts the electrical contact function from the traditional wire bonding approach by integrating contact openings directly through the cap structure. These openings allow electrical connections to be made at the top surface of the cap without requiring wire bonding, simplifying the packaging process while maintaining the protective function of the cap.
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 provides enhanced sensitivity, reduced thermal noise, and efficient signal transmission without wire bonding, allowing for 3-axis acceleration and angular rate measurements with improved sensitivity and reduced vulnerability to temperature variations.
Implementation Method 1
Top and bottom cap electrodes are respectively provided in the top and bottom cap wafers and form capacitors with the proof mass, the top and bottom cap electrodes being configurable to detect a motion of the proof mass
Implementation Method 2
flexible springs suspending the proof mass relative to the outer frame and enabling the proof mass to move relative to the outer frame along mutually orthogonal x, y and z axes
Implementation Method 3
The proof mass and flexible springs form a resonant structure having resonant frequencies fx, fy and fz for motion along the x, y and z axes, respectively
Implementation Method 4
The electrical contact of the second set are connected to the respective bottom cap electrodes by way of respective insulated conducting pathways, each extending along the z axis from one of the respective bottom cap electrodes and upward successively through the bottom cap wafer, the outer frame of the MEMS wafer and the top cap wafer
Data Source
AI summary
A MEMS motion sensor and its manufacturing method are provided. The sensor includes a MEMS wafer including a proof mass and flexible springs suspending the proof mass and enabling the proof mass to move relative to an outer frame along mutually orthogonal x, y and z axes. The sensor includes top and bottom cap wafers including top and bottom cap electrodes forming capacitors with the proof mass, the electrodes being configured to detect a motion of the proof mass. Electrical contacts are provided on the top cap wafer, some of which are connected to the respective top cap electrodes, while others are connected to the respective bottom cap electrodes by way of insulated conducting pathways, extending along the z axis from one of the respective bottom cap electrodes and upward successively through the bottom cap wafer, the outer frame of the MEMS wafer and the top cap wafer.