Large-Mass MEMS Resonant Beam Accelerometer with Vacuum Sealing
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Solution Overview
Problem
Existing resonant beam accelerometers face challenges in achieving large proof masses for greater sensitivity while navigating limitations in fabrication processes and material mismatches, leading to issues like thermal mechanical noise, cross-axis sensitivity, and reliability.
Innovation Solution
A resonant beam accelerometer design utilizing a pair of double-ended tuning forks and a proof mass formed in the handle layer of a silicon-on-insulator (SOI) wafer, with a meso-scale mass, and sealed in a vacuum by a lid and stator cap, allowing for increased sensitivity and reduced thermal mechanical noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large proof mass is used to increase sensitivity, then sensitivity is improved, but device aspect ratio increases which decreases reliability and increases cross-axis sensitivity
Solution Approach 1:
The patent transitions from planar device architecture to three-dimensional bulk silicon structures. By utilizing vertical depth in thick silicon wafers (500-1000 micrometers), the design achieves large proof mass without increasing lateral footprint, thereby maintaining acceptable aspect ratios and improving reliability while enhancing sensitivity through increased mass.
Solution Approach 2:
The resonant beam structures are nested within the bulk silicon substrate, with tuning forks positioned within trenches etched into the thick wafer. This nesting approach allows the sensitive elements to be embedded within the robust bulk material, providing mechanical support and stress relief while maintaining large proof mass for high sensitivity.
2Reliability
If the device footprint is reduced to improve reliability and cross-axis rejection, then reliability is improved, but critical features such as flexures must become thinner which increases thermal sensitivities and fragility
Solution Approach 1:
The patent exploits the third dimension (vertical depth) by using thick silicon wafers to accommodate all critical features within the bulk substrate. Flexures, tuning forks, and proof mass are all defined within the vertical thickness of the wafer, eliminating the need to reduce lateral dimensions and avoiding the associated thermal sensitivity issues.
Solution Approach 2:
The invention changes the fundamental parameter of substrate thickness from thin (conventional SOI) to thick (500-1000 micrometers). This parameter change allows all device features to be defined in the bulk silicon with adequate dimensions, reducing thermal sensitivities while maintaining small lateral footprint for improved reliability.
3Volume of moving object
If a thin device layer is used to reduce mechanical envelope, then device size is reduced, but large proof mass requires high aspect ratio which decreases reliability
Solution Approach 1:
The patent inverts the conventional approach by using thick substrates rather than thin layers. All device features are defined within the vertical thickness of thick silicon wafers, achieving large proof mass without high aspect ratios while maintaining compact lateral dimensions for small mechanical envelope.
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 design achieves enhanced sensitivity and reliability by leveraging the meso-scale mass and vacuum sealing, reducing thermal mechanical noise and improving cross-axis rejection, while maintaining robustness and stability.
Implementation Method 1
The resonant frequencies of the first and second resonant beam structures are a function of an externally applied acceleration in a direction parallel to the acceleration measurement axis
Implementation Method 2
the resonant frequency of a member, such as a string or a beam, is a function of its tensile stress... the resonant frequency would become a function of the inertial loading of the mass by an externally applied acceleration
Implementation Method 3
sealed in a vacuum by a lid and stator cap, allowing for increased sensitivity and reduced thermal mechanical noise
Data Source
AI summary
A resonant beam accelerometer employing a pair of double ended tuning forks and a proof mass having meso-scale mass as it is formed in the handle layer of a SOI wafer and a method of fabricating same is disclosed. The resonant frequencies of the pair of double ended tuning forks change in opposite directions when an externally applied acceleration is in a direction parallel to the pair of double ended tuning forks. Due to the proof mass having meso-scale mass, the resonant beam accelerometer is very sensitive. By employing a lid and a stator cap, the active portions of the resonant beam accelerometer may be sealed in a vacuum to further increase sensitivity.