Analysis and Modeling of Uncooled Microbolometers with Tunable Thermal Conductance
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Uncooled microbolometers have attracted significant interest due to their small size, low cost and low power consumption. As the application range of microbolometers broadens, increasing the dynamic range becomes one of the main objectives of microbolometer research. Targeting this objective, tunable thermal conductance microbolometers have been proposed recently, in which the thermal conductance is tuned by electrostatic actuation. Being a new concept in the field, the current tunable thermal conductance microbolometers have significant potential for improvement in design and performance. In this thesis, an extensive analysis of tunable thermal conductance microbolometers is made, an analytical model is constructed for this purpose, and solutions are proposed to some potential problems such as in-use stiction and variation in spectral response. The current thermal conductance tuning mechanisms use the substrate for electrostatic actuation, which does not support pixel-by-pixel actuation. In this thesis, a new thermal conductance tuning mechanism is demonstrated, that enables pixel-by-pixel actuation by using the micromirror as an actuation terminal instead of the substrate. In addition, a stopper mechanism is used to decrease the risk of in-use stiction. With this new mechanism, the thermal conductance can be tuned by a factor of three at relatively low voltages, making it a promising thermal conductance tuning mechanism for adaptive infrared detectors. Effective estimation of the performance parameters of a tunable thermal conductance microbolometer in the design state requires an analytical model that combines the physics of infrared radiation detection and the thermal conductance tuning mechanisms. As a part of this research, an extensive analytical model is presented, which includes the electrostatic-structural modeling of the thermal conductance tuning mechanism, and electromagnetic and thermal modeling of the microbolometer. The accuracy of the thermal model is of significant importance as the operation of the tuning mechanism within the desired range should be verified in the design stage. A thermal model based on the solution of the microbolometer heat conduction equation is established, which is easily applicable to conventional and tunable thermal conductance microbolometers of various shapes. The constructed microbolometer model is validated by experiments and finite element model simulations. Furthermore, the effect of thermal conductance tuning on spectral response is analyzed. The present thermal conductance tuning mechanisms result in variations in spectral response, which is an undesired effect in many applications. As a solution, a new microbolometer architecture is proposed, in which the spectral response is not affected by thermal conductance. The microbolometer is designed using an analytical model and its performance is characterized by finite element model simulations. To realize the proposed design, a fabrication process flow is offered. It is shown that the proposed microbolometer exhibits high performance, tunable thermal conductance and constant spectral response.