Bimorph Capacitor for Temperature-Compensated MEMS Resonators

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Solution Overview

Problem

Micro-electromechanical (MEMs) resonators in lateral bulk extension mode face significant frequency variations due to process-induced and temperature-induced factors, such as variations in layer thicknesses and dimensions, and temperature coefficients of frequency, leading to unstable resonant frequencies.

Innovation Solution

Incorporating a temperature-compensation capacitor with a bimorph beam and a thin-film bulk acoustic resonator, where the bimorph beam increases spacing between electrodes with temperature changes, reducing capacitance to counteract negative temperature coefficients and stabilize resonant frequency.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If a thin-film bulk acoustic resonator is used, then high frequency operation is achieved, but temperature-induced frequency drift occurs due to negative temperature coefficient of frequency

Engineering Contradiction:

Improveresonant frequencyVSAvoidfrequency stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The patent changes the physical parameter of capacitance by using a bimorph beam structure that converts temperature changes into mechanical displacement, thereby changing the capacitor electrode spacing and capacitance value to compensate for frequency drift

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs composite material structure with a bimorph beam consisting of two different materials (e.g., silicon and silicon nitride) with different thermal expansion coefficients, creating a temperature-responsive mechanical structure that actively compensates for temperature effects

Inventive Principle:

Principle #40Composite materials

2Manufacturing precision

If layer thicknesses are controlled during deposition, then manufacturing precision is improved, but variations across the wafer still cause frequency deviations of several thousand ppm

Engineering Contradiction:

Improvelayer thickness controlVSAvoidfrequency consistency

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The patent implements a feedback mechanism where the bimorph capacitor senses temperature changes and automatically adjusts its capacitance to counteract the frequency drift, creating a self-correcting system that compensates for manufacturing variations

Inventive Principle:

Principle #23Feedback

3Manufacturing precision

If lateral dimensions are controlled through photolithography and etching, then dimensional precision is improved, but process-induced variations still cause frequency drift of several thousand ppm

Engineering Contradiction:

Improvelateral dimension controlVSAvoidfrequency stability

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The patent changes the operational parameter of capacitance dynamically through temperature-responsive mechanical displacement of the bimorph beam, allowing the system to adapt to and compensate for dimensional variations caused by manufacturing processes

Inventive Principle:

Principle #35Parameter changes

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 effectively reduces frequency drift caused by temperature fluctuations and process-induced variations, enhancing the stability of micro-electromechanical devices by compensating for temperature-induced frequency shifts and dimensional changes.

Implementation Method 1

The bimorph beam is configured to provide an increase in spacing between the first and second electrodes in response to an increase in temperature of the micro-electromechanical device

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

The thin-film bulk acoustic resonator and the bimorph beam may both contain respective first and second piezoelectric layers

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS7834524B2Micro-electromechanical devices having variable capacitors therein that compensate for temperature-induced frequency drift in acoustic resonators

Publication Date: 2010.11.16 SITIME CORP

AI summary

Micro-electromechanical devices include a temperature-compensation capacitor and a thin-film bulk acoustic resonator having a first terminal electrically coupled to an electrode of the temperature-compensation capacitor. The temperature-compensation capacitor includes a bimorph beam having a first electrode thereon and a second electrode extending opposite the first electrode. This bimorph beam is configured to yield an increase in spacing between the first and second electrodes in response to an increase in temperature of the micro-electromechanical device. This increase in spacing between the first and second electrodes leads to a decrease in capacitance of the temperature-compensation capacitor. Advantageously, this decrease in capacitance can be used to counteract a negative temperature coefficient of frequency associated with the thin-film bulk acoustic resonator, and thereby render the resonant frequency of the micro-electromechanical device more stable in response to temperature fluctuations.