Composite MEMS Resonator Structure for Temperature-Stable Frequency
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Solution Overview
Problem
Conventional MEMS resonators made of homogeneous materials suffer from significant temperature-induced frequency drift, making them less suitable for timing devices due to their high temperature sensitivity, which is several orders of magnitude higher than that of quartz resonators.
Innovation Solution
The use of a resonator structure comprising a first material with a negative Young's modulus temperature coefficient and a second material with a positive Young's modulus temperature coefficient, where the second material is selectively located and dimensioned to specific regions of the resonator to tailor the temperature coefficient of frequency (TCF) without affecting the resonator's quality factor (Q).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a homogeneous material is used for the MEMS resonator, then the device complexity is low and manufacturing is simple, but the temperature sensitivity is high resulting in significant frequency drift
Solution Approach 1:
The patent employs a composite material structure consisting of a first material (e.g., SiGe) and a second material (e.g., SiO2 or Si3N4) with opposite Young's modulus temperature coefficients. The second material is selectively positioned in isolated regions within the resonator structure to provide temperature compensation. This composite approach reduces the overall temperature coefficient of frequency (TCF) to be an order of magnitude lower than homogeneous resonators, achieving frequency stability comparable to quartz resonators while maintaining MEMS fabrication compatibility.
Solution Approach 2:
The patent applies local quality by selectively placing the second compensation material in specific isolated regions within the resonator structure rather than using it uniformly throughout. The second material is positioned in regions where it can effectively counteract the temperature-dependent stiffness changes of the first material, such as in trenches or isolated zones. This localized approach allows temperature compensation without requiring the entire resonator to be reconstructed, thus maintaining manufacturing simplicity while reducing temperature sensitivity.
2Reliability
If the second material is added to the resonator for temperature compensation, then the temperature coefficient of frequency is reduced, but the quality factor (Q) may be affected
Solution Approach 1:
The second compensation material is confined to isolated regions rather than being distributed throughout the entire resonator. This localized placement minimizes the volume of material that could potentially dampen mechanical vibrations, thereby preserving the quality factor while still achieving effective temperature compensation in the critical regions where the compensation is most needed.
Solution Approach 2:
The patent uses a partial amount of the second compensation material, precisely where needed in isolated regions, rather than fully replacing or uniformly distributing the material throughout the resonator. This partial action approach provides sufficient temperature compensation to achieve the desired frequency stability without excessive material presence that would unnecessarily increase energy loss and reduce the quality factor.
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 approach significantly reduces the temperature sensitivity of MEMS resonators, achieving a TCF magnitude comparable to or better than quartz resonators, thereby enhancing their suitability for IC timing devices.
Implementation Method 1
a first material with a negative Young's modulus temperature coefficient and a second material with a positive Young's modulus temperature coefficient
Data Source
AI summary
MEMS resonators containing a first material and a second material to tailor the resonator's temperature coefficient of frequency (TCF). The first material has a different Young's modulus temperature coefficient than the second material. In one embodiment, the first material has a negative Young's modulus temperature coefficient and the second material has a positive Young's modulus temperature coefficient. In one such embodiment, the first material is a semiconductor and the second material is a dielectric. In a further embodiment, the quantity and location of the second material in the resonator is tailored to meet the resonator TCF specifications for a particular application. In an embodiment, the second material is isolated to a region of the resonator proximate to a point of maximum stress within the resonator. In a particular embodiment, the resonator includes a first material with a trench containing the second material. In a specific embodiment, the shape, dimensions, location and arrangement of a second material comprising silicon dioxide is tailored so that the resonator comprising a first material of SiGe will have a TCF of a much lower magnitude than that of either a homogeneous SiGe or homogeneous silicon dioxide resonator.