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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 )
2
( 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]
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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.