The widespread use of Liquid Petroleum Gas (LPG) for cooking and as fuel for automobile vehicles requires fast and selective detection of LPG to precisely measure the leakage of gas for preventing the occurrence of accidental explosions. The adoption of Micro-Electro-Mechanical-System (MEMS) technology for fabricating the gas sensor provides other potential advantages for sensing applications, which includes low power consumption, low fabrication cost, high quality, small size and reliability. MEMS based gas sensor requires a sensitive layer of oxide material like ZnO, SnO2, TiO2, Fe2O3, etc. The tin oxide material used in the present work changes its electrical properties, as it interacts with the reducing gas like LPG. The sensor material becomes active only at high temperature such as 400ºC, thereby realizing the need of a micro heater to reach the desired temperature. To control the temperature of micro heater and to determine the change in electrical properties of the sensor due to its interaction with LPG an Application Specific Integrated Circuit (ASIC) forms an essential constituent of the MEMS based gas sensor. In the present work, an attempt has been made to improve the sensitivity of LPG gas sensor and it is correlated with other properties by different characterization techniques. The work also includes the design as well as testing of ASIC for gas sensor system. Process parameters particularly deposition time and substrate temperature have a profound influence on the microstructure of the tin oxide film, which in turn affects the gas sensing properties. To study the effects of these parameters, RF magnetron sputtering system is used for depositing tin oxide films onto the silicon substrate, which is compatible with CMOS technology. The effects of structural properties, optical properties and the porosity of the films are also studied and correlated with the gas sensing properties. In this direction the deposited films are characterized using X-Ray Diffraction (XRD) to determine the structure orientation. The morphology of the sensor films are analyzed by Scanning Electron Microscope (SEM) while the refractive index, thickness and porosity of the films are determined using ellipsometry studies. The thickness of the deposited films is also confirmed by the surface profilometer. The change in composition of the deposited film along its depth is determined using Secondary Ion Mass Spectrometer (SIMS). Maximum sensitivity 5.5 is obtained for 470 nm thick films, which corresponds to a grain size of 38nm at the operating temperature of 4000C. Following these studies, an ASIC has been designed using Tanner EDA Tools on AMIS 0.7 µm CMOS process, fabricated through Euro practice’s ASIC prototyping service, Belgium and tested successfully after fabrication. The temperature control module of ASIC has been designed using relaxation oscillator technique to control the temperature of the in house developed heater. The resistance to period conversion technique is explored for the design of the sensor read out module of ASIC. The heater is integrated successfully with the sensor film, ASIC and microcontroller based LCD module. The test results show good agreement with the simulation results.

With the proliferation of industries world-wide, there is a growing need and interest in sensing and monitoring environmental pollutants and monitoring the concentration of chemicals/gases in industrial process control. There is also an increasing demand for chemical sensors in other applications such as home security, breath analysis and food processing. Design and development of metal-oxide based gas sensor system is reported in this thesis. The system consists of three components viz. micro heater(which aids inheating the sensor film to required temperatures), CMOS ASIC (the sensor interface circuit) and the thin film transducer(a semiconducting metal oxide thin film whose resistance changes with the concentration of the target gas). Microheaters were realized through PolyMUMPs process. Thermal characterization of surface-micromachined microheaters is carried out from their dynamic response to electrothermal excitations. An electrical equivalent circuit model is developed for the thermo-mechanical system. The mechanical parameters are extracted from the frequency response obtained using a Laser Doppler Vibrometer. The resonant frequencies of the microheaters are measured and compared with FEM simulations. The thermal time constants are obtained from the electrical equivalent model by fitting the model response to the measured frequency response. Microheaters with an active area of140m × 140m have been realized on two different layers(poly-1 andpoly-2) with two different air-gaps (2m and 2.75m). The effective time constants, combining thermal and mechanical responses, are intherangeof0.13msto0.22msforheatersonpoly-1,and1.9s to0.15ms for microheaters on poly-2 layer. The thermal time constants of the best microheaters are in the range of a few s, thus making them suitable for sensor applications that need faster thermal response. The mechanical deformation of the microheaters subjected to an electrothermal excitation, due to thermal stress, is also analyzed using lensless in-line digital holographic microscopy (DHM). The numerically reconstructed holographic images of the micro-heaters clearly indicate the regions under high stress. Double exposure method has been used to obtain the quantitative measurements of the deformations, from the phase analysis of the hologram fringes. The measured deformations correlate well with the theoretical values predicted by a thermo-mechanical analytical model. The results show that lensless in-line DHM with Fourier analysis is an effective method for evaluating the thermo-mechanical characteristics of MEMS components. A sensor interface circuit comprising a resistance-to-time period converter as the front-end circuit and a proportional temperature controller to control the microheater temperature is designed and realized in 130nm UMC CMOS technology. The impact of biasing the transistors in subthreshold versus saturation conditions on analog circuit performance is systematically analyzed. A cascode current mirror, designed in 130nm CMOS technology, is biased in subthreshold and saturation regions and its performance has been analyzed through rigorous analytical modeling. The analytical results have been validated with SPICE simulations. It is demonstrated that the subthreshold operation provides better performance in terms of linearity, power, area, output impedance and tolerance to temperature variation, making it a preferable option for applications such as signal conditioning circuitry for environmental sensors. On the other hand, biasing the circuit in saturation is preferable for applications like transceivers and data converters where high bandwidth, SNR and low sensitivity to process variations are the key requirements. Based on this analysis, a sensor interface circuit has been prototyped for resistance measurement on 130nm CMOS technology, using subthreshold cascode current mirrors as the key building blocks. This current mirror results in 14X lower power compared to above-threshold operation. The interface circuit spans 5 orders of magnitude of resistance, and consumes an ultra low power of 326W. A proportional temperature controller with an integrated on-chip power MOSFET is also realized on the same chip for heating and temperature control of microheaters. The microheater is reused as temperature sensor. The entire circuit works with 1.2V supply, except the power MOSFET and the heater driver circuit, which operate with 3.3V supply. ZnO, a semiconducting metal-oxide, is used as the sensing material. Thin films of ZnO are spin-coated over insulating substrates using sol-gel processing technique. Gold pads deposited over the sensing film act as electrodes. The sensor film is characterized at different temperatures for its sensitivity to ethanol. A peak response of 14% change in resistance is observed for 5ppm ethanol, at a working temperature of 275◦C.