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MEMS Approach to Low Power Wearable Gas Sensors

Michael Lim04/21/2016

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Outline• Advanced Self-Powered Systems of Integrated Sensors and

Technologies (ASSIST)• Commercial Gas Sensors• Adsorption Processes• MEMS structures

– QCM– FBAR– SAW– Micro-Cantilever– CMUT

• Application to wearables• Conclusion

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ASSIST (NSF NERC)MISSIONUse nanotechnology to improve global health by enabling correlation between personal health and personal environment and by empowering patients and doctors to manage wellness and improve quality of life. (www.assist.ncsu.edu)

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SGX Sensortech MOX CO2/VOC Sensor• 400-20ppm, 0.1ppm resolution• 0-1000ppb isobutylene VOC• 5-20s response time• Resistive sensing• Heated MOX• Read-out circuit integrated

1.4cm2.3cm

Commercial Technology

SGX Sensortech Electrochemical NO2 Sensor• 0-20ppm, 0.1ppm resolution• 35s response time• Amperiometric sensing

2cm

2cm

SGX Sensortech IR CO2

• 0-3000ppm, 100ppm resolution

• 20s response time• Fractional IR

absorbance

2cm

2.4cm

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Comparison of Commercial Sensors

Electrochemical Semiconductor

Optical

Selective

Sensitive

Large

High T

RT Operation

Large sensing area

Small SizeLower Lifetime

Diffusion-LimitedAdsorption-Limited

Low CostHigh Power

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Adsorption ProcessesAbsorption Molecules diffuse into the material

Adsorption Molecules bind to surfacePhsyisorption, Chemisorption

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Adsorption Processes ContBi

ndin

g En

ergy

(eV)

Distance (nm)

Surface

(a)

(b)

(a) Physisorption is due to van der Waals forces• Non-selective• Typical EB = 10-100 meV

(b) Chemisorption is due to electron exchange between substrate and adsorbed molecule• Binding site must be favorable• Typical EB = 1-10 eV

Lennard-Jones Potential

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MEMS Structures

Si

SiO2

PZT

Sensing Area

PZT Substrate

Sens

itiza

tion

Laye

r

IDT Tx IDT Rx

𝝀

Motion

Si Micro-Cantilever

Optical Laser

(a)

(b) Si

Si

SiO2

SiN

Quartz Crystal Microbalance (QCM)

Film Bulk Acoustic Resonator (FBAR)

Surface Acoustic Wave (SAW)

Micro-Cantilever Capacitive Micromachined Ultrasonic Transducer (CMUT)

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QCM

Freq

uenc

y M

ixer

QCM1

Difference Frequency

QCM2

Sensitization

Structure of a sensitized QCM Referenced QCM System

• Frequency mixing gives high f-resolution at lower sampling frequency

• QCM used to monitor depositions in clean rooms

Δ𝑚

[1]

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FBAR

Si

SiO2

PZT

Sensing Area

Sensitization

Si

Sensing Area

PZT

Etched substrate FBAR Air gap FBAR

Sensitization

Working Principle

• Thin-film piezoelectric resonator• 1-2m

• GHz range resonant frequency

• 3X mass sensitivity compared to QCM

• Trade off of Q and f• Higher f better mass resolution• Higher Q better SNR

• CMOS compatible • AlN or ZnO as piezoelectric film

Δf =−𝜈02𝜌 ( 1𝑡 )

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( Δ𝑚𝐴 ) Sauerbrey-Lotsis approximation =acoustic velocity

• =11345 =density• =3260

[1]

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SAW

PZT Substrate

Sens

itiza

tion

Laye

r

IDT Tx IDT Rx

𝝀PZT Substrate

IDT TxIDT Rx1 IDT Rx2

Sens

itiza

tion

Laye

r

• Acoustic waves travel across the surface from Tx to Rx (70-800MHz)• Multi-layer SAW devices can be used for acoustic wave properties

Surface layers create a delay line

Frequency shift is related by

=fractional sensitization/wave area

Delay Line SAW Device Referenced SAW Device

[1]

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Micro-Cantilevers

Motion

Si Micro-Cantilever

Optical Laser

(a)

(b)

Si Micro-Cantilever

Sensitization

𝚫𝐳𝝈

Operating Modes(a) Static (bending)

• Deflection is a measure of adsorbed molecules and related to strain

Δ 𝑧=3 𝑙2 (1−𝑣 )𝐸𝑡2

Δ𝜎

Stoney’s Equation

(b) Dynamic (resonant)• is related to the adsorbed mass

Δ𝑚=𝑘 Δ 𝑓 − 24𝑛𝑐 𝜋

spring constant geometric correction factor (0.24 for rectangular beams)

Transduction by optical laser or piezoresistive implantation• Laser is most common/sensitive• Piezoresistors are CMOS compatible

𝑓 =𝑡 𝑙22 π ( 𝐸𝜌 )−

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[2]

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CMUT

Si

Si

SiO2

SiN

Sensitization

CMUT with Si Electrodes

CMUT Array• Resonant membrane structure

• Frequency is geometry dependent

• Capacitive readout• 100s-1000s of CMUT in parallel

• Individual capacitance is very low• Sealed cavity

• Increased Q• CMOS compatible

Δ𝑚=2𝐴 𝜌𝑡 Δ 𝑓𝑓

Adsorbed Mass Relationship

Frequency shift due to H2O

𝑓 =0.47 𝑡𝑟2 √ 𝐸

𝜌 (1−𝑣2 )

Resonant Frequency of CMUT

[3]

Wearable Application of MEMS StructuresRequirements for wearable sensors• Small• Sensitive• Selective*• Robust• Long Lifetime/Reversible**• Low power operation

Limit of detection resonant frequencyPower consumption resonant frequency

Sensitivity sensing area

Structure Size Sensitivity Robust Power

QCM

FBAR

SAW

Cantilever

CMUT

Fundamental Trade-offs

Si

SiO2

PZT

Sensing Area

PZT Substrate

Sens

itiza

tion

Laye

r

IDT Tx IDT Rx

𝝀

Si

Si

SiO2

SiN

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Conclusion

• sorption process concepts• MEMS structures for gas sensing– Transduction methods– Mass relationship

• Evaluated candidate structures for wearables– FBAR, SAW, CMUT show promise for long term low

power operation

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Graph References• [1]E. Comini, G. Faglia and G. Sberveglieri, Solid state gas sensing. New York, NY: Springer,

2009, pp. 261-304.• [2]S. Singamaneni, M. LeMieux, H. Lang, C. Gerber, Y. Lam, S. Zauscher, P. Datskos, N. Lavrik,

H. Jiang, R. Naik, T. Bunning and V. Tsukruk, "Bimaterial Microcantilevers as a Hybrid Sensing Platform", Adv. Mater., vol. 20, no. 4, pp. 653-680, 2008.

• [3]K. Park, H. Lee, M. Kupnik, Ö. Oralkan, J. Ramseyer, H. Lang, M. Hegner, C. Gerber and B. Khuri-Yakub, "Capacitive micromachined ultrasonic transducer (CMUT) as a chemical sensor for DMMP detection", Sensors and Actuators B: Chemical, vol. 160, no. 1, pp. 1120-1127, 2011.