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Applications:
CO Gas Sensor
CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Prof. Darrin Hanna
CO Gas Sensors
• many gas sensors operate by adsorption of gas molecules to
surface
• resistance of a metaloxide changes
•. adsorbed gas molecules interact with the surface
• one or more conduction electrons get trapped
• reduces surface conductivity
• resistance is inversely proportional to a fractional power
of the gas concentration
• class of sensor materials include the oxides of tin (SnO2),
titanium (TiO2), indium (In2O3), zinc (ZnO), tungsten (WO3),
and iron (Fe2O3).
CO Gas Sensors
• Each metal oxide is sensitive to different gases
• tin oxide -- detecting alcohol, hydrogen, oxygen,
hydrogen sulfide, and carbon monoxide
• indium oxide -- ozone (O3)
• zinc oxide -- detecting halogenated hydrocarbons
• humidity
• variations in material properties require individual
calibration
CO Gas Sensors
• The MiCS series of carbon monoxide sensors from
MicroChemical Systems
• a tin-oxide, thin-film sense resistor over a polysilicon
resistive heater
CO Gas Sensors
• heater maintains the sensor at an operating temperature
between 100° and 450ºC,
• reduces effects of humidity
• The sense resistor and heater reside over a 2-µm-thick silicon
membrane to minimize heat loss through the substrate.
CO Gas Sensors
• only 47 mW is sufficient to maintain the membrane
at 400ºC!
• four electrical contacts
• two connect to the tin-oxide resistor
• two connect to the polysilicon heater
• flow a constant current through the sense element and
record the output voltage
CO Gas Sensors
CO Gas Sensors
Sample Fabrication
• form heavily doped, p-type, 2-µm-thick layer
• epitaxial growth or ion implantation
• deposit silicon nitride
• CVD to deposit a polysilicon film
• shape into heater
• polysilicon film is doped during the CVD process
CO Gas Sensors
Sample Fabrication
• deposit oxide layer and etch contact holes
• electrically isolate the polysilicon heater from the tin-oxide
• tin-oxide layer is deposited by sputtering tin and oxidize
• pattern tin-oxide layer and etched in the shape of the sense element
• sputter and pattern aluminum for contacts
CO Gas Sensors
Sample Fabrication
• etch from the back side in potassium hydroxide
• thin membrane by stopping on the p-type layer
• a masking layer (e.g., silicon nitride) on the back side of the substrate a
of the front side are necessary.
CO Gas Sensors
Specification
• apply to the heater a 5-V pulse for 5s, followed by a 1-V pulse
lasting 10s
• 400ºC during the first interval
• decreasing to 80ºC during the second pulse
• resistance measurement at 9.5s into the second 10-s long pulse
• demonstrates a response from 10 to 1,000 parts per million (ppm)
of carbon monoxide (CO)
• humidity range of 5 to 95%.
• output signal shows a square-root dependence on CO
concentration, with little dependence on
humidity for CO concentrations above 60 ppm.