MEMS Resonator Vertical Oscillation Gap Control
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Solution Overview
Problem
MEMS resonators for real-time-clock applications face challenges in reducing motional resistance, which affects energy consumption and frequency tunability due to technological limitations in controlling the gap between the mobile mass and fixed electrodes, leading to shifts in resonance frequency caused by process and temperature variations.
Innovation Solution
A MEMS resonant structure with a vertical oscillation configuration and a parallel-plate capacitive coupling design, where the mobile mass is suspended above the substrate and oscillates transversely, allowing for precise control and reduction of the gap between the mobile mass and fixed electrodes, achieved through the THELMA process, enabling efficient capacitive coupling and tunability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a parallel-plate configuration is used to enable frequency tunability, then the resonance frequency can be corrected for process and temperature shifts, but the gap control between mobile mass and fixed electrodes becomes difficult leading to high motional resistance
Solution Approach 1:
The patent transitions from lateral gap control (2D plane) to vertical gap control (3D dimension) by suspending the mobile mass above the substrate. This dimensional change enables precise gap control through vertical positioning while maintaining parallel-plate configuration for frequency tunability, resolving the contradiction between adaptability and manufacturing precision.
Solution Approach 2:
The patent implements a suspended mobile mass that can dynamically adjust its position vertically relative to the fixed electrodes. This dynamic positioning capability allows precise control of the gap distance, enabling both low motional resistance and frequency tunability through electrostatic actuation of the suspended structure.
2Use of energy by moving object
If the gap between mobile mass and fixed electrodes is reduced to lower motional resistance, then energy consumption decreases, but technological limitations make precise gap control difficult
Solution Approach 1:
By moving from lateral to vertical gap control, the patent achieves precise sub-micron gap control through the suspended structure's vertical positioning. This enables significantly reduced gaps that lower motional resistance and energy consumption while overcoming photolithographic resolution limitations.
Solution Approach 2:
The patent replaces mechanical contact or lateral positioning mechanisms with electrostatic actuation of a suspended mobile mass. This substitution enables precise, contactless gap control through electrical fields, achieving the required precision for low-energy operation without mechanical constraints.
3Ease of manufacture
If lateral oscillation configuration is used, then the structure is simpler to manufacture, but the resonance frequency cannot be tuned and shifts due to process and temperature variations cannot be corrected
Solution Approach 1:
The patent maintains manufacturing simplicity through suspended structure fabrication but adds dynamic frequency tunability by enabling electrostatic actuation of the mobile mass. The suspended configuration allows post-fabrication frequency adjustment, combining ease of manufacture with adaptability.
Solution Approach 2:
The suspended mobile mass structure serves multiple functions: it provides mechanical resonance, enables frequency tuning through electrostatic actuation, and allows correction of process and temperature shifts. This multi-functionality resolves the contradiction between manufacturing simplicity and frequency adaptability.
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 configuration reduces motional resistance, enhances the reproducibility of electromechanical characteristics, and allows for effective correction of frequency shifts with reduced energy consumption, improving the overall performance and tunability of the resonant structure.
Implementation Method 1
micromechanical structures obtained by means of micromachining techniques, which, due to external stresses (in the form of appropriate electrical biasing), are induced to vibrate at their natural resonance frequency
Implementation Method 2
The mobile mass forms, with a fixed-electrode structure coupled thereto, a capacitor, and the vibration in resonance conditions of the mobile mass causes a variation of the capacitance of this capacitor
Implementation Method 3
the variation of the biasing voltage enables modification of the electrostatic force acting on the resonant structure, modifying the state of stress and hence the effective elastic constant
Data Source
AI summary
The present disclosure is directed to a MEMS resonant structure, provided with a substrate of semiconductor material; a mobile mass suspended above the substrate and anchored to the substrate by constraint elements to be free to oscillate at a resonance frequency; and a fixed-electrode structure capacitively coupled to the mobile mass to form a capacitor with a capacitance that varies as a function of the oscillation of the mobile mass; the fixed-electrode structure arranged on a top surface of the substrate, and the constraint elements being configured in such a way that the mobile mass oscillates, in use, in a vertical direction, transverse to the top surface of the substrate, keeping substantially parallel to the top surface.