Integrated Gas Sensor - Studies On Sensing Film Deposition, Microheater Design And Fabrication, Interface Electronics Design And Testing
Recently, there has been an increasing interest in the electronics world for those aspects related to semiconducting gas sensor (SGS) materials. In view of the increasingly strict legal limits for pollutant gas emissions, there is a great interest in developing high performance gas sensors for applications such as controlling air pollution and exhaust gases in automotive industry. In this way, semiconductor gas sensors offer good advantages with respect to other gas sensor devices, due to their simple implementation, low cost and good stability and sensitivity. The first part of the thesis is dedicated to the synthesis, film structural and sensitivity study of the Tin Oxide film deposited by RF sputtering, doped with noble metal Palladium (Pd). Effects on the Gas Sensitivity due to the deposition parameters like thickness of the film, Substrate temperature, Ar /O2 ratio of the sputtering environment, annealing temperature and duration and doping metal weight % into the Tin Oxide films are studied and the results are shown in detail. The sensitivity and selectivity of the gas sensing film is decided by the operating temperature i.e. the temperature of the gas sensing film while it is in the target gas ambience, Microheaters happen to be the very important component in the gas sensor especially with wide band gap semiconducting metal oxides films such as tin oxide, gallium oxide or indium oxides. Other than gas sensing microheater also finds applications in many areas like thermal dip pen nanolithography, polymerase chain reaction (PCR), fluid pumping with bubbles, in vitro fertilization etc. So in this report due importance was given for the design and fabrication of the microheater. Microheaters are the most power consuming element of the integrated Gas sensors. This is also an important reason for the extensive microheater work in this research. Six different heater patterns were simulated by considering low power and temperature uniformity as an important goals. Among them the best three patterns named Double spiral, “Fan” Shape and “S” shape were chosen for fabrication and both thermal and electrical characterization results of them were presented in detail in the Microheater section of the thesis. It is believed that the intelligent design and integration of the electronic circuitry (for drive, signal conditioning/compensation, and read-out) with the gas sensing element can mitigate some of the significant issues inherent in solid-state gas sensors, such as strong temperature and humidity dependence, signal drift, aging, poisoning, and weak selectivity. The sensitivity of the gas sensors which has been indicated as the dynamic change of resistance in wide range should be read out properly. Towards this aim a low cast high efficient readout circuit is designed and implemented. Temperature monitoring and controlling is a key phenomenon in the metal Oxide based gas sensors since the selectivity mainly depends on the operating temperature of the sensing film. So focus was also shown on the design and implementation of the temperature monitoring and control unit, which been presented in the last part of this thesis.
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