ECEN 5004 – Digital Packaging
Download
Report
Transcript ECEN 5004 – Digital Packaging
Electrochemical Gas Sensors
Graduate Research Project
Mike Weimer
ECEN 5004 – Digital Packaging
Introduction
Gas 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 automobiles
ECEN 5004 – Digital Packaging
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)
ECEN 5004 – Digital Packaging
Introduction – Airport/Toxin Detection
GE EntryScan3
Toxic Gas Sensors
ECEN 5004 – Digital Packaging
Introduction – Natural Gas Detection
MythBusters ‘Flatus Catcher’
ECEN 5004 – Digital Packaging
Introduction – Natural Gas Detection
MythBusters captured and analyzed ‘flatus’
Employed a bathtub-based flatus catcher
Flatus 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 flammable
Proved ‘pretty girls’ do produce flatus
ECEN 5004 – Digital Packaging
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
ECEN 5004 – Digital Packaging
Operation
Incoming vapor reacts with surface or electrolyte
Causes 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
ECEN 5004 – Digital Packaging
Operation
Typical Electrochemical Gas Sensor Structure
ECEN 5004 – Digital Packaging
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
ECEN 5004 – Digital Packaging
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
ECEN 5004 – Digital Packaging
Fabrication – CVD Films
CVD Deposited SnO2 Film
ECEN 5004 – Digital Packaging
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)
ECEN 5004 – Digital Packaging
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)
ECEN 5004 – Digital Packaging
Fabrication – ALD Films
Atomic Layer Deposition (ALD)
Conformal, uniformly-deposited SnO2 thin films
Deposition rate precisely controlled
ALD Deposited SnO2 Film
(on Al nanoparticles)
ECEN 5004 – Digital Packaging
Fabrication – ALD Films
ALD Deposited Al2O3 Film
(on Ni particle)
ECEN 5004 – Digital Packaging
Fabrication – ALD Films
Fluidized Bed ALD Reactor
ECEN 5004 – Digital Packaging
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
ECEN 5004 – Digital Packaging
Fabrication – ALD Films
Electrochemical gas sensors fabricated via ALD have
superior electrical properties
Uniform film deposition
Uniform electrochemical properties
Free of pinholes
ECEN 5004 – Digital Packaging
Packaging Considerations
Sensor selectivity/sensitivity
Environmental concerns
Corrosive environment (metals)
Oxidizing environment
Humidity
Temperature
Electrolyte housing
Chemical inertness of housing
Sensor lifetime
ECEN 5004 – Digital Packaging
Packaging Considerations
Surface Area
Higher sensitivity = larger surface area
Higher sensitivity = shorter lifetime
Package Material
Plastics (polyethylene, polypropylene)
Chemically inert, inexpensive
Metals (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
ECEN 5004 – Digital Packaging
Sn-plated Cu surface in need of a shave
ECEN 5004 – Digital Packaging
Packaging Considerations
Typical gas sensor packages
ECEN 5004 – Digital Packaging
Typical Sensitivities
* More corrosive/reactive gases tend to have higher sensitivity sensors
ECEN 5004 – Digital Packaging
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
ECEN 5004 – Digital Packaging
Alliance, Nebraska