Electrochemical Gas Sensors ECEN 5004 – Digital Packaging Mike Weimer Graduate Research Project.

Electrochemical Gas Sensors

ECEN 5004 – Digital Packaging

Mike Weimer

Graduate Research Project

IntroductionGas sensors used in several applications

Detection of toxic vapors HCl Cl2

H2S

O3

Explosives/narcotics detection Airport sensors (GE EntryScan3) Police/Government narcotics detection Nuclear detection at U.S. ports

Radon / Natural Gas detection (Methyl Mercaptan)

O2 sensors on automobilesECEN 5004 – Digital Packaging

Introduction – Automotive O2 SensorsMost widely used application

Detects O2 concentration in exhaust streamPromotes cleaner burning fuel/air mixtureReduces overall pollution


• Invented by Bosch (1976)

• First used by Volvo (1976)

• Introduced to U.S. (1980)

• Required in Europe (1993)

Introduction – Airport/Toxin Detection


GE EntryScan3 Toxic Gas Sensors

Introduction – Natural Gas Detection


MythBusters ‘Flatus Catcher’



MythBusters captured and analyzed ‘flatus’Employed a bathtub-based flatus catcherFlatus contained in a Flatulence Containment Unit

(FCU) 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 flammableProved ‘pretty girls’ do produce flatus



Useful for Natural Gas furnaces and fireplacesLeak detectionParticularly useful during sleep (not able to

smell)

Radon detection (carcinogen)No odorResponsible for 21,000 lung cancer deaths/yr

(U.S.)Usual prevention is plastic sheeting

OperationIncoming vapor reacts with surface or

electrolyteCauses changes in current or resistance

Current: FET-type devices (‘micro fuel cells’) Resistance: Film-based devices

Anomalies in current/resistance concentration

Multi-layered design for high sensitivity 1st Layer: Hydrophobic Membrane 2nd Layer: Electrodes 3rd Layer: Electrolyte


Operation


Typical Electrochemical Gas Sensor Structure

Operation – Anodic Reactions


[CO]: CO + H2O CO2 + 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+ + e-

Operation – Cathodic Reactions


[O2]: O2 + 4H+ + 4e- 2H2O

[NO2]: NO2 + 2H+ + 2e- NO + H2O

[Cl2]: Cl2 + 2H+ + 2e- 2HCl

[O3]: O3 + 2H+ + 2e- O2 + H2O

FabricationThin films are becoming more prevalent

Resistance measurement on film surfaceSnO2 films are widely used - high surface

reactivity

Chemical Vapor Deposition (CVD)Gas-phase techniquePrecursors introduced simultaneouslyDeposition is controlled by exposure timeFilms are granular and non-uniform


Fabrication – CVD Films


CVD Deposited SnO2 Film

Fabrication – PVD FilmsPhysical Vapor Deposition (PVD)

Solid/Gas-phase techniqueBlock of SnO2 heated to vaporization (thermal

evap.)Films are irregular and non-uniform


PVD Deposited SnO2 Film

(Actual Journal image)

Fabrication – Wet Chemistry FilmsWet Chemical Deposition (WCD)

a.k.a. ‘Sol-gel’Substrate submersed in solution to form SnO2


WCD Deposited SnO2 Film

(speaks for itself)

Fabrication – ALD FilmsAtomic Layer Deposition (ALD)

Conformal, uniformly-deposited SnO2 thin films

Deposition rate precisely controlled


ALD Deposited SnO2 Film

(on Al nanoparticles)

Fabrication – ALD Films


ALD Deposited Al2O3 Film

(on Ni particle)



Fluidized Bed ALD Reactor



Precursors introduced individuallyPrevent gas-phase reactionsUsually deposited using SnCl4 + H2O2

SnOH* + SnCL4 SnOSnCl3* + HCl [A]

SnCl* + H2O2 SnOH* + HCl + ½ O2

[B]

Resulting SnO2 film deposits at ~0.1 nm/AB cycleOperates from 250 – 400 °C



Electrochemical gas sensors fabricated via ALD have superior electrical propertiesUniform film depositionUniform electrochemical propertiesFree of pinholes

Packaging Considerations


Sensor selectivity/sensitivityEnvironmental concerns

Corrosive environment (metals)Oxidizing environmentHumidityTemperature

Electrolyte housingChemical inertness of housing

Sensor lifetime



Surface AreaHigher sensitivity = larger surface areaHigher sensitivity = shorter lifetime

Package MaterialPlastics (polyethylene, polypropylene)

Chemically inert, inexpensiveMetals (aluminum, tin)

Lightweight, inexpensive, less porous Apparently several metals grow whiskers (even Al) Whisker growth inside package can alter

sensitivity and cause false concentration reports


Sn-plated Cu surface in need of a shave



Typical gas sensor packages

Typical Sensitivities


* More corrosive/reactive gases tend to have higher sensitivity sensors

Summary


Electrochemical gas sensors widely availableToxic gas sensing, automotive applicationsExplosives sensingFlatus testing

Thin film sensors are the next generationAtomic Layer Deposition (ALD)

High sensitivities achievable with correct packagingChemical inertness of housingTemperature/humidity variations

Sensor lifetime

Alliance, Nebraska

Electrochemical Gas Sensors ECEN 5004 – Digital Packaging Mike Weimer Graduate Research Project.

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