Renata Melamud

                               

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Silicon Microelectromechanical Resonators

 

 

Double-Ended Tuning Fork Resonator (DETF): Vibration of beams produces a timing signal

 

Silicon micromechanical resonators provide a periodic signal that can be used as a frequency reference in timing applications.

 

Silicon resonators have the potential to displace the $12 billion timing market currently dominated by quartz crystals.

 

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Temperature Dependence Problem                                      

 

 

Silicon Resonator Behavior:  The frequency of a silicon resonator decreases with increasing temperature.

 

 

Commercialization of silicon frequency references has been hindered by the strong temperature dependence of silicon resulting in frequency instability.

 

Unlike quartz crystals, whose temperature dependence can be reduced by an appropriate cut of the crystal, the silicon structure does not possess a temperature dependent cut.

 

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Compensation Methods

 

Active Temperature Compensation: The temperature of the resonator is maintained at a constant temperature using joule heating of a serpentine resistive structure

 

 

Compensation methods to reduce the temperature dependence of frequency can be broadly characterized as passive or active.

 

Active compensation schemes rely on characterization of the temperature dependent behavior and circuitry to correct for temperature changes.

 

Passive compensation schemes seek to reduce the inherent temperature dependence of the resonator.

 

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Material Compensation

 

Composite Resonator: Cross-section of composite beam with sense and actuation electrodes on either side.

 

 

Material compensation is a passive compensation method and is the primary focus of my research.

 

Temperature compensation is achieved by creating composite structures containing both silicon and silicon dioxide.  Composite resonators show a 20X reduction in the temperature sensitivity of frequency that is comparable to quartz crystal resonator behavior.

 

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Single Wafer Encapsulation Process           

 

Encapsulated Resonator: SEM of a DETF showing resonator under the surface of a wafer with top metallization.

 

 

Composite resonators were fabricated in a CMOS compatible, single-wafer encapsulation process. This high-yield process encapsulates resonators in a hermetic environment whose long-term stability has been established.

 

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Enabling Technologies

 

 

High Accuracy Temperature Sensor:  Multiple composite resonators with different temperature dependencies are combined to achieve a extremely temperature sensitive beat frequency.

 

Material compensated composite resonators can be combined with active temperature compensation schemes to create high stability frequency references.

 

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