![QCM
Frequency
Mixer
QCM1
Difference
Frequency
QCM2
Sensitization
Structure of a sensitized QCM Referenced QCM System
• Δ𝑓 ∝ Δ𝑚
• Frequency mixing gives high
f-resolution at lower
sampling frequency
• QCM used to monitor
depositions in clean rooms
Δ𝑚
[1]
9](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-9-320.jpg)
![FBAR
Si
SiO2
PZT
Sensing Area
Sensitization
Si
Sensing Area
PZT
Etched substrate FBAR Air gap FBAR
Sensitization
Working Principle
• Thin-film piezoelectric
resonator
• 1-2𝜇m
• 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 = −
𝜈0
2𝜌
1
𝑡
2
Δ𝑚
𝐴
Sauerbrey-Lotsis approximation 𝝂 𝟎=acoustic velocity
• 𝜈 𝐴𝑙𝑁=11345
𝑚
𝑠
𝝆=density
• 𝜌 𝐴𝑙𝑁=3260
𝑘𝑔
𝑚3 [1]
10](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-10-320.jpg)
![SAW
PZT Substrate
Sensitization
Layer
IDT Tx IDT Rx
𝝀
PZT Substrate
IDT TxIDT Rx1 IDT Rx2
Sensitization
Layer
• 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
𝑉 = 𝑉 𝑀 sin 𝜙 𝑟 − 𝜙𝑠
Frequency shift is related by
Δ𝑓
𝑓
≅ 𝜅
Δ𝜙
𝜙
𝜅=fractional sensitization/wave area
Delay Line SAW Device Referenced SAW Device
[1]
11](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-11-320.jpg)
![Micro-Cantilevers
Motion
Si Micro-Cantilever
Optical Laser
(a)
(b)
Si Micro-Cantilever
𝚫𝐳
𝝈
Operating Modes
(a) Static (bending)
• Deflection is a measure of adsorbed
molecules and related to strain
Δ𝑧 =
3𝑙2 1 − 𝑣
𝐸𝑡2
Δ𝜎
Stoney’s Equation
(b) Dynamic (resonant)
• Δ𝑓 is related to the adsorbed mass
Δ𝑚 =
𝑘Δ𝑓−2
4𝑛 𝑐 𝜋
𝒌 → spring constant
𝒏 𝒄 → 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 =
𝑡𝑙2
2π
𝐸
𝜌
−
1
2
12
[2]](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-12-320.jpg)
![CMUT
Si
Si
SiO2
SiN
Sensitization
CMUT with Si Electrodes
CMUT Array
• Resonant membrane structure
• Frequency is geometry dependent
𝑓 = 0.47
𝑡
𝑟2
𝐸
𝜌 1 − 𝑣2
• Capacitive readout
• 100s-1000s of CMUT in parallel
• Individual capacitance is very low
• Sealed cavity
• Increased Q
• CMOS compatible
Δ𝑚 = 2𝐴𝜌𝑡
Δ𝑓
𝑓
Adsorbed Mass Relationship
Frequency shift due to H2O
𝑓 = 0.47
𝑡
𝑟2
𝐸
𝜌 1 − 𝑣2
Resonant Frequency of CMUT
[3] 13](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-13-320.jpg)
![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.
16](https://image.slidesharecdn.com/f5c338d7-b143-49ef-915f-e786a7c62342-160705025555/85/MEMS-Approach-to-Low-Power-Wearable-Gas-Sensors-16-320.jpg)
MEMS Approach to Low Power Wearable Gas Sensors
- 1. MEMS Approach to Low Power Wearable Gas Sensors Michael Lim 04/21/2016 1
- 2. 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 2
- 3. ASSIST (NSF NERC) MISSION Use 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) 3
- 4. 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 1.4cm 2.3cm 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 4
- 5. Comparison of Commercial Sensors Electrochemical Semiconductor Optical Selective Sensitive Large High T RT Operation Large sensing area Small Size Lower Lifetime Diffusion-Limited Adsorption-Limited Low CostHigh Power 5
- 6. Adsorption Processes Absorption Molecules diffuse into the material Adsorption Molecules bind to surface Phsyisorption, Chemisorption 6
- 7. Adsorption Processes Cont BindingEnergy(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 7
- 8. MEMS Structures Si SiO2 PZT Sensing Area PZT Substrate Sensitizatio nLayer 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) 8
- 9. QCM Frequency Mixer QCM1 Difference Frequency QCM2 Sensitization Structure of a sensitized QCM Referenced QCM System • Δ𝑓 ∝ Δ𝑚 • Frequency mixing gives high f-resolution at lower sampling frequency • QCM used to monitor depositions in clean rooms Δ𝑚 [1] 9
- 10. FBAR Si SiO2 PZT Sensing Area Sensitization Si Sensing Area PZT Etched substrate FBAR Air gap FBAR Sensitization Working Principle • Thin-film piezoelectric resonator • 1-2𝜇m • 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 = − 𝜈0 2𝜌 1 𝑡 2 Δ𝑚 𝐴 Sauerbrey-Lotsis approximation 𝝂 𝟎=acoustic velocity • 𝜈 𝐴𝑙𝑁=11345 𝑚 𝑠 𝝆=density • 𝜌 𝐴𝑙𝑁=3260 𝑘𝑔 𝑚3 [1] 10
- 11. SAW PZT Substrate Sensitization Layer IDT Tx IDT Rx 𝝀 PZT Substrate IDT TxIDT Rx1 IDT Rx2 Sensitization Layer • 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 𝑉 = 𝑉 𝑀 sin 𝜙 𝑟 − 𝜙𝑠 Frequency shift is related by Δ𝑓 𝑓 ≅ 𝜅 Δ𝜙 𝜙 𝜅=fractional sensitization/wave area Delay Line SAW Device Referenced SAW Device [1] 11
- 12. Micro-Cantilevers Motion Si Micro-Cantilever Optical Laser (a) (b) Si Micro-Cantilever 𝚫𝐳 𝝈 Operating Modes (a) Static (bending) • Deflection is a measure of adsorbed molecules and related to strain Δ𝑧 = 3𝑙2 1 − 𝑣 𝐸𝑡2 Δ𝜎 Stoney’s Equation (b) Dynamic (resonant) • Δ𝑓 is related to the adsorbed mass Δ𝑚 = 𝑘Δ𝑓−2 4𝑛 𝑐 𝜋 𝒌 → spring constant 𝒏 𝒄 → 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 = 𝑡𝑙2 2π 𝐸 𝜌 − 1 2 12 [2]
- 13. CMUT Si Si SiO2 SiN Sensitization CMUT with Si Electrodes CMUT Array • Resonant membrane structure • Frequency is geometry dependent 𝑓 = 0.47 𝑡 𝑟2 𝐸 𝜌 1 − 𝑣2 • Capacitive readout • 100s-1000s of CMUT in parallel • Individual capacitance is very low • Sealed cavity • Increased Q • CMOS compatible Δ𝑚 = 2𝐴𝜌𝑡 Δ𝑓 𝑓 Adsorbed Mass Relationship Frequency shift due to H2O 𝑓 = 0.47 𝑡 𝑟2 𝐸 𝜌 1 − 𝑣2 Resonant Frequency of CMUT [3] 13
- 14. Wearable Application of MEMS Structures Requirements for wearable sensors • Small • Sensitive • Selective* • Robust • Long Lifetime/Reversible** • Low power operation Limit of detection ∝ resonant frequency Power consumption ∝ resonant frequency Sensitivity ∝ sensing area Structure Size Sensitivity Robust Power QCM FBAR SAW 𝝁Cantilever CMUT Fundamental Trade-offs Si SiO2 PZT Sensing Area PZT Substrate Sensitizatio nLayer IDT Tx IDT Rx 𝝀 Si Si SiO2 SiN 14
- 15. 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 15
- 16. 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. 16
Editor's Notes
- #12: Fig. 8.21 Response towards H2 and NO2 for a InOx/ZnO/XZLiNbO3 SAW sensor at an operating temperature of 2468C