Transcript Document
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.