wafer-level vacuum-encapsulated ultra-low voltage tuning fork mems

Junjun Huan , George Xereas, Vamsy Chodavarapu

Department of Electrical EngineeringUniversity of Dayton, OH

Email: huanj1@udayton.edu

WAFER-LEVEL VACUUM-ENCAPSULATED ULTRA-LOW

VOLTAGE TUNING FORK MEMS RESONATOR

Integrated Microsystems Laboratoryiml

RESONATORS: TIMING & FREQUENCY REFERENCES

Today, we have stringent requirements of low cost, low complexity, compact system integration, low power consumption, and shock resistance in mobile, IoT and wearable applications that cannot be satisfied with Quartz devices.• High spectral purity (High Q > 10,000)• Low temperature sensitivity (<5 ppm/oC)• High Stability over lifetime (material, aging

issues)• MEMS Piezoelectric Vs MEMS Electrostatic (SiLabs/IDT/Sand9 Vs SiTime)• Small size (1mm3)

3 oscillators in Apple Watchiml

5 oscillators in Apple iPhone

SINGLE CHIP MULTI-FUNCTION INTEGRATION

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Single-band Multi-chip

Multi-band Single-chip

CDMA

GSM

PCS

MEMS ELECTROSTATIC RESONATORS

524 kHz MEMS tuning fork resonator with Q of 52,000 by SITIME [2]

32 kHz MEMS tunable capacitive-comb driven folded-beam resonator with Q of 57,000 by Dr. Clark T.-C. Nguyen’s Group [3]

10 MHz MEMS ring resonator with Q of 473000 by SITIME [1]

6.35 MHz MEMS LAME-MODE resonator with Q of 3240000[2]

[1] S. Wang, T.W. Kenny, "Nonlinearity of hermetically encapsulated high-Q double balanced breathe-mode ring resonator," 23rd IEEE International Conference on Micro Electro Mechanical Systems, Hong Kong, p. 715-718, 2010.

[2] G. Xereas and V. P. Chodavarapu, "Wafer-Level Vacuum-EncapsulatedLame Mode Resonator With f-Q Product of 2.23 x 10(13) Hz," Ieee Electron Device Letters, vol. 36, pp. 1079-1081, Oct 2015.

[3] S. Zaliasl, J. C. Salvia, G. C. Hill, L. Chen, K. Joo, R. Palwai, et al., "A 3 ppm 1.5 x 0.8 mm(2) 1.0 mu A 32.768 kHz MEMS-Based Oscillator," Ieee Journal of Solid-State Circuits, vol. 50, pp. 291-302, Jan 2015.

[4] H. G. Barrow, T. L. Naing, R. A. Schneider, T. O. Rocheleau, V. Yeh, Z. Y. Ren, et al., "A Real-Time 32.768-kHz Clock Oscillator Using a 0.0154-mm(2) Micromechanical Resonator Frequency-Setting Element," 2012 Ieee International Frequency Control Symposium (Fcs), 2012

MEMS INTEGRATED DESIGN FOR INERTIAL SENSORS (MIDIS)

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MIDIS process from Teledyne DALSA Semiconductor Inc.

CMOS Compatible with flip chip bonding

Vacuum Encapsulation at 10mTorr

Reproducible Transduction Gap: 1.5um Device Layer Thickness: 30um

World’s most ultra-clean MEMS vacuum cavity demonstrated to date (Leak rate of 4 to 45 molecules/second)

MIDIS FABRICATION PROCESS

3 wafer process. Top: Interconnect WaferMiddle: Membrane WaferBottom: Handle Wafer

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MIDIS FABRICATION PROCESS

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Material: silicon <100>Resonance Frequency:

32.77kHzOverall Dimension:700 µm × 550 µm × 30 µmShuttle Finger Number:

89Transduction Gap Size (1) Fabrication: 1.5 µm(2) Post-Fabrication

silicon fusing: 50 nm DC Polarization Voltage:

1V

LOW POWER WEARABLE APPLICATIONS: TUNING FORK

RESONATOR

imlStop Anchors

Anchors

Driving Electrode

Sensing Electrode

Serpentine Spring

Folded Beams

TUNING FORK RESONATOR

3D Resonator Schematic

1. A pull-in voltage of over 50V applied to stop anchors

2. A contact formed between the welding pads and isolated stop anchors

3. Using a strong current pulse generated through electrical discharge of a capacitor at 100V to melt the connection joints

4. The final locking (permanent fusion bond connection) realized with a deflection of Movable Electrodes of 1.45 µm (50 nm gap between fingers)

SILICON FUSING: TRANSDUCTION GAP REDUCTION TECHNIQUE

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TRANSDUCTION GAP REDUCTION TECHNOLOGY

@5V and 1.5µm gap on one side

iml@1V and 50nm gap on one

side

3D Animation

• k: spring constant of spring system• m: dynamic mass• Q: quality factor• : coupling factor• : DC polarization voltage• N: number of f inger gaps• : dielectric constant of free space• t: thickness of fingers• : transduction gap• : overlapping finger length

RESONATOR ELECTRICAL EQUIVALENT MODEL

iml[5]

[5] C. T.. Nguyen and R. T. Howe, "An integrated CMOS micromechanical resonator high-q oscillator," IEEE Journal of Solid-State Circuits, vol. 34, no. 4, pp. 440–455, Apr. 1999.  

TRANSIMPEDANCE AMPLIFIER: BIASING CIRCUIT (GFUS 180NM

PROCESS)High Resistance

High DC Gain of CMOS OPAMP

Low Power Consumption of Oscillator Circuit

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SUSTAINING TRANSIMPEDANCE AMPLIFIER

Telescopic Differential Amplifier(high DC gain and low power

consumption) +

Push-pull Output Stage (low power consumption and

rail to rail output swing)

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OPERATIONAL AMPLIFIER SIMULATION RESULT

High DC Gain

Enough Bandwidth(minus 3dB frequency >>32kHz)

Good Phase Margin(60)

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CMOS OSCILLATOR CIRCUIT (180NM PROCESS)

A Colpitts oscillator configuration utilizinga capacitive voltagedivider as a feedbacksource with an overall loop phase shift of 360 between outputand input port of OPAMP (180 of OPAMP plus 180 of two capacitors )

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OSCILLATOR SIMULATION RESULTS

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OSCILLATOR SIMULATION RESULTS

:=((M1a) +) =((4.63+6+352.7)5)W=1.86mW

Transient Analysis

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ULTIMATE GOAL: SINGLE CHIP IMPLEMENTATION

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MEMS Resonator

CMOS Amplifie

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THANK YOU!