1. Applications: CO Gas Sensor
CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Prof. Darrin Hanna

2. 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).

3. 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

4. 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

5. 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.

6. 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

7. CO Gas Sensors

8. 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

9. 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

10. 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 and protection
of the front side are necessary.

11. 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.