Applications: CO Gas Sensor

1
ApplicationsCO 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.