Research Of MEMS Based Thermal Imaging Technique With Optical Readout

Much attention has been paid to thermal irradiation detectors for their wide applications. Compared with cooled detectors, uncooled thermal imaging devices have the advantages such as small dimension, low cost, and superior reliability. With the development of Micro Electro-Mechanical System (MEMS) technology, there has recently been an increasing interest in uncooled thermal imaging with an optical readout for optical readout systems don't require highly sensitive readout integrated circuits, thus producing no additional heat energy, having satisfactory thermal isolation, and reducing fabrication complexity of a focal plane array (FPA). Based on the special application status of thermal imaging devices in many fields, it is of great significance to study MEMS based thermal imaging technique with an optical readout.Based on the optical readout technique with a knife-edge filtering operation in the spectrum plane, an improved optically readable uncooled thermal imaging device is constructed in the present work. Besides having high spatial resolution, favorable optically detecting sensitivity, and superior vibration-resistance, the readout becomes more compact for reducing an optical component. And an intensity-stable LED is used as the light source, which reduces obviously the system noise and improves the detective performance of the imaging device. On this platform, the thermal imaging experiments of fabricated substrate-free SiNx/Au bimaterial microcantilever FPAs, which have deformation amplifying structures, have been performed.Following the analysis of the basic FPA design models, a multi-fold interval metallized leg structure is presented in FPA design. In the structure, metallized bimaterial legs alternately connect with SiNx legs, which can produce an amplified deformation for a temperature rise of a microcantilever element and then improve the thermo-mechanical sensitivity of the element. In the design of the 120μm×120μm element, the fold number of deformation amplifying structure has been optimized considering the heat conversion efficiency of the element, thus the detective performance of the microcantilever element is optimized. Also the influence of dimension parameters of the element on its detective performance is discussed.In FPA microfabrication, several key issues are researched by experiment, such as selection of the bulk silicon etching technology and the technology of avoiding blind elements in the bulk silicon etching process. Based on the above key processes and other researches, the integrated stantard fabrication process of the MEMS based free-standing FPA is developed, which is proved having satisfactory stability and repeatability. And the rate of blind elements is below 10%.Thermal imaging experiments of the fabricated FPAs, whose element sizes are 200μm×200μm, 120μm×l20μm, 60μm×60μm respectively, are performed using the constructed optical readout. Thermal images of room temperature objectors in the range from 2m to 2km are obtained successfully. Noise Equivalent Temperature Differences (NETD) of the corresponding FPA are measured to be 670mK, 270mK, 116mK, respectively, which means that NETD of the present imaging device has approached to 100mK, satisfying approximately the requirements of application. For the 200μm×200μm FPA, a good agreement between the theoretic grey response and the measured value has been obtained. Further, the influences of background fluctuation noise, temperature fluctuation noise, thermo-mechanical vibration noise and optical readout noise on the system noise is discussed.