Electrochemical Gas Sensors
Graduate Research Project
Mike Weimer
ECEN 5004 – Digital Packaging
Introduction
 Gas sensors used in several applications
 Detection of toxic vapors
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HCl
Cl2
H2S
O3
 Explosives/narcotics detection
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Airport sensors (GE EntryScan3)
Police/Government narcotics detection
Nuclear detection at U.S. ports
 Radon / Natural Gas detection (Methyl Mercaptan)
 O2 sensors on automobiles
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Introduction – Automotive O2 Sensors
 Most widely used application
 Detects O2 concentration in exhaust stream
 Promotes cleaner burning fuel/air mixture
 Reduces overall pollution
• Invented by Bosch (1976)
• First used by Volvo (1976)
• Introduced to U.S. (1980)
• Required in Europe (1993)
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Introduction – Airport/Toxin Detection
GE EntryScan3
Toxic Gas Sensors
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Introduction – Natural Gas Detection
MythBusters ‘Flatus Catcher’
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Introduction – Natural Gas Detection
 MythBusters captured and analyzed ‘flatus’
 Employed a bathtub-based flatus catcher
 Flatus contained in a Flatulence Containment Unit (FCU)
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Methyl Mercaptan (CH4S) – Highly Toxic, Highly Smelly
Methane (CH4) – Highly Flammable
Hydrogen Sulfide (H2S) – Flammable and Toxic
 Proved though ‘toxic,’ flatus inhalation won’t kill you
 Proved flatus is flammable
 Proved ‘pretty girls’ do produce flatus
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Introduction – Natural Gas Detection
 Useful for Natural Gas furnaces and fireplaces
 Leak detection
 Particularly useful during sleep (not able to smell)
 Radon detection (carcinogen)
 No odor
 Responsible for 21,000 lung cancer deaths/yr (U.S.)
 Usual prevention is plastic sheeting
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Operation
 Incoming vapor reacts with surface or electrolyte
 Causes changes in current or resistance
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Current: FET-type devices (‘micro fuel cells’)
Resistance: Film-based devices
 Anomalies in current/resistance  concentration
 Multi-layered design for high sensitivity
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1st Layer: Hydrophobic Membrane
2nd Layer: Electrodes
3rd Layer: Electrolyte
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Operation
Typical Electrochemical Gas Sensor Structure
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Operation – Anodic Reactions
CO + H2O  CO2 +
[CO]:
2H+ + 2e[H2S]:
H2S + 4H2O  H2SO4 + 8H+ +8e-
[NO]:
NO + 2H2O  HNO3 + 3H+ + 3e-
[H2]:
H2  2H+ + 2e-
[HCN]:
2HCN + Au  HAu(CN)2 + H+ + eECEN 5004 – Digital Packaging
Operation – Cathodic Reactions
[O2]:
[NO2]:
O2 + 4H+ + 4e-  2H2O
NO2 + 2H+ + 2e-  NO + H2O
[Cl2]:
Cl2 + 2H+ + 2e-  2HCl
[O3]:
O3 + 2H+ + 2e-  O2 + H2O
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Fabrication
 Thin films are becoming more prevalent
 Resistance measurement on film surface
 SnO2 films are widely used - high surface reactivity
 Chemical Vapor Deposition (CVD)
 Gas-phase technique
 Precursors introduced simultaneously
 Deposition is controlled by exposure time
 Films are granular and non-uniform
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Fabrication – CVD Films
CVD Deposited SnO2 Film
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Fabrication – PVD Films
 Physical Vapor Deposition (PVD)
 Solid/Gas-phase technique
 Block of SnO2 heated to vaporization (thermal evap.)
 Films are irregular and non-uniform
PVD Deposited SnO2 Film
(Actual Journal image)
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Fabrication – Wet Chemistry Films
 Wet Chemical Deposition (WCD)
 a.k.a. ‘Sol-gel’
 Substrate submersed in solution to form SnO2
WCD Deposited SnO2 Film
(speaks for itself)
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Fabrication – ALD Films
 Atomic Layer Deposition (ALD)
 Conformal, uniformly-deposited SnO2 thin films
 Deposition rate precisely controlled
ALD Deposited SnO2 Film
(on Al nanoparticles)
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Fabrication – ALD Films
ALD Deposited Al2O3 Film
(on Ni particle)
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Fabrication – ALD Films
Fluidized Bed ALD Reactor
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Fabrication – ALD Films
 Precursors introduced individually
 Prevent gas-phase reactions
 Usually deposited using SnCl4 + H2O2
SnOSnCl3* + HCl
SnOH* + SnCL4 
[A]
SnCl* + H2O2  SnOH* + HCl + ½ O2
[B]
 Resulting SnO2 film deposits at ~0.1 nm/AB cycle
 Operates from 250 – 400 °C
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Fabrication – ALD Films
 Electrochemical gas sensors fabricated via ALD have
superior electrical properties
 Uniform film deposition
 Uniform electrochemical properties
 Free of pinholes
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Packaging Considerations
 Sensor selectivity/sensitivity
 Environmental concerns
 Corrosive environment (metals)
 Oxidizing environment
 Humidity
 Temperature
 Electrolyte housing
 Chemical inertness of housing
 Sensor lifetime
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Packaging Considerations
 Surface Area
 Higher sensitivity = larger surface area
 Higher sensitivity = shorter lifetime
 Package Material
 Plastics (polyethylene, polypropylene)
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Chemically inert, inexpensive
 Metals (aluminum, tin)
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Lightweight, inexpensive, less porous
Apparently several metals grow whiskers (even Al)
Whisker growth inside package can alter sensitivity and
cause false concentration reports
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Sn-plated Cu surface in need of a shave
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Packaging Considerations
Typical gas sensor packages
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Typical Sensitivities
* More corrosive/reactive gases tend to have higher sensitivity sensors
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Summary
 Electrochemical gas sensors widely available
 Toxic gas sensing, automotive applications
 Explosives sensing
 Flatus testing
 Thin film sensors are the next generation
 Atomic Layer Deposition (ALD)
 High sensitivities achievable with correct packaging
 Chemical inertness of housing
 Temperature/humidity variations
 Sensor lifetime
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Alliance, Nebraska