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无基底焦平面阵列的性能分析和优化
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摘要
红外成像技术在军事、工业、医疗、商业等方面有广泛应用。光学读出非制冷红外成像技术由于成本低、性能高而成为今年来研究的热点之一。本文在课题组工作的基础上,针对光学读出非制冷红外成像技术的核心元件无基底焦平面阵列(FPA)展开了一系列研究,建立了新的热机械分析理论模型,设计制造了新材料的FPA,为了改善FPA反光板的弯曲,提出了多种优化方法,并建立了嵌入式的软件平台,主要成果如下:
     (1)热机械性能分析方面,尽管有限元方法能够精确计算已知设计尺寸的无基底FPA的热机械性能,但是当系统有一个要达到的指标(如温度灰度响应),需要分析FPA的设计尺寸时,有限元方法就不再方便。本文利用整体分析方法和电学类比分析方法,建立了热流在FPA传递的电路模型,分析了FPA支撑框架的热阻,推导出无基底FPA的理论温度灰度响应和响应时间的计算公式。设计并指导制作了50μm×50μm像素尺寸的无基底FPA来验证理论模型,其热加载区域的平均温升的有限元和理论模型分析的结果误差为1.5%,两者非常吻合。其温度灰度响应理论分析结果与红外成像实验结果的误差为5.4%,此时系统的噪声等效灰度NETD为170mK。同时,理论模型对200μm×200μm和120μm×120μm像素尺寸无基底FPA的分析也证明这个解析模型适用于所有像素单元尺寸的无基底FPA。
     (2)基于无基底FPA的材料选择要求,在微电子机械系统(MEMS)制作工艺的约束下,设计并指导制作了像素尺寸为50μm×50μmAl和SiO2双材料无基底FPA。利用有限元方法定量分析了Al/SiO2双材料无基底FPA的热机械性能,并与相同结构及尺寸的Au/SiNx双材料无基底FPA的有限元分析结果进行了对比,发现Al/SiO2双材料FPA的热转化效率和热机械灵敏度都相较Au/SiNx双材料FPA有了大幅提升。在系统光学检测灵敏度一定时,新的材料能够将系统的灵敏度提高至原来的5.8倍。
     (3)为了改善FPA反光板的弯曲,本文提出了三种优化方案。A.减薄反光板上的金层厚度。在理论分析的基础上,设计并指导制作了单元尺寸为200μm的反光板金层减薄FPA,其反光板曲率半径提高至原来金层未减薄FPA的4.71倍,系统光学检测灵敏度提高至原来的5.2倍,实验验证了理论分析的结果;B.制作带加强筋的反光板。设计并指导制作了单元尺寸为60μm的反光板带加强筋FPA,其反光板曲率半径提高至原来没有加强筋FPA的4.29倍,系统光学检测灵敏度提高至原来的1.18倍,实验验证了理论分析的结果;C.利用冷热平台对像素尺寸为50μm×50μm的Au/SiNx双材料FPA进行退火处理,共试验了两种退火方式。第一种是直接退火法,对FPA进行退火都没有明显的纠正反光板的弯曲的效果,但FPA的均匀性都有所提升。第二种是循环退火方法,改善了反光板的弯曲,反光板的偏转更加均匀,FPA的温度灰度响应提高了约32%。
     (4)开发了嵌入式平台——红外热谱分析软件平台,整合了垂直聚焦平台的电机控制等多种硬件设备,实现了对红外成像设备的连接、调节、数据传输、数据分析处理、数据显示等功能。再加上联系客户端与服务器的注册服务模块,该嵌入式平台作为商业软件的必须功能已经完备。基于红外成像技术在监控方面的优越性,我们对红外热谱分析软件平台进行了二次开发,硬件上整合亚安云台,通过云台控制红外成像设备的视场;软件中增加云台控制模块,同时为云台携带双相机系统(一台可见光相机,一台红外成像设备)增加多通道图像处理模块。基于红外成像的广阔市场前景,其嵌入式平台的商业价值和发展空间非常大。
Infrared (IR) imaging technique plays a critical role in military, industrial, medical, commercial applications. Optical readout uncooled IR imaging technique becomes a hotspot of research because of its low cost and high performance. Substrate-free focal plane array (FPA) is the key component of this technique and a series studies about it are carried out. This dissertation has proposed a new theoretical analysis model for thermal-mechanical behaviour of substrate-free FPA. FPA was designed and fabricated with new materials, with various optimizations to improve the deformation of FPA reflectors. Furthermore, a thermal infrared spectral analysis embedded software platform has been developed. The main achievements of this dissertation are as follows:
     (1) Finite element analysis (FEA) is convenient and accurate in predicting a response when the parameters of the geometrical design are given. However, FEA hardly provides an effective methodology for the design of an FPA when a performance goal for a system, such as the temperature gray response, is desired. This paper presented an analytical model, based on an analogy with electrical circuits and a holistic approach, to analyze the performance of substrate-free FPA, using metrics such as the temperature gray response and the response time of the FPA. This analytical model was validated by FEA results. A substrate-free FPA with a unit size of50μm×50μum was fabricated. Its infrared imaging experiments validated the model and indicated a noise equivalent temperature difference (NETD) value of170mK has been achieved. At the same time, theoretical model analysis of FPA with unit sizes of120μm×120μm and200μm×200μm verified that this model can apply to substrate-free FPA with any unit size.
     (2) Based on the material selection requirements of substrate-free FPA, a substrate-free Al/SiO2bi-material FPA with a unit size of50μm×50μm was fabricated under the constraint of MEMS manufacturing process,. Its thermal-mechanical performance was analysed by FEA and compared with that of a substrate-free Au/SiNx bi-material FPA with the same unit size, which showed the heat conversion efficiency and the thermo-mechanical efficiency of the Al/SiO2bi-material FPA were improved significantly. This material optimization enhanced the sensitivity of the system by up to5.8times of the original.
     (3) Three optimization schemes for reducing the initial deformation of the FPA reflector were proposed in this dissertation:A. Reducing the thickness of the Au layer on the reflector. Based on the theoretical analysis, a FPA with a unit size of200μm was fabricated and the thickness of the Au layer of its reflector was reduced. The reflector curvature radius and optical detection sensitivity for the system were increased by4.71times and5.2times repectively compared to that of the FPA without thinning process. B. Fabricating reflector with stiffeners. A FPA with a unit size of60μm was fabricated, with stiffeners. The reflector curvature radius and optical detection sensitivity for the system were increased by4.29times and1.18times respectively comppared that of the FPA without stiffeners. The experiments agreed with the anticipation of the theory. C. Two annealing methods on substrate-free Au/SiNx bi-material FPA with a unit size of50μm×50μm using Hot-Cold Stage were carried out:direct annealing process and circular annealing process. The second treatment enhanced the temperature-gray response by32%。
     (4) A thermal infrared spectral analysis embedded software platform was developed. This software platform integrated the motor control of a vertical focused platform and a variety of hardware devices and realized functions on the infrared imaging device, such as camera connecting, adjusting, data transmission, data analysis and processing, data display, etc. With an extra registration service module to contact the client and server, this embedded platform was ready for commercial sale. Further development on this platform was based on the advantage of the IR imaging technique on monitoring. A YaAn PTZ device was integrated for controlling the field of view of the IR imaging device. The PTZ controlling module and multichannel image processing (visible light camera and IR imaging device) module were also embedded in the platform. Considering the broad market prospects of the IR imaging, the commercial value and development potential of this embedded platform would be great.
引文
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