摘要
各种射频、惯性、机械谐振器等MEMS器件通常在高真空环境下才能保证优良的性能和较高的品质因数，因而需要进行真空封装。本文提出采用真空电阻熔焊来实现MEMS器件真空封装的新方法。通过理论、数值模拟、实验等方法系统地研究了MEMS器件真空电阻封装过程中的关键技术问题，制定MEMS器件的真空电阻熔焊封装的工艺标准。其主要研究内容和创新如下：
通过大量封装试验和对真空封装技术分析的基础上，摸索出真空电阻熔焊影响真空的关键因素。根据真空环境进行焊接的新特点，研制了具有自主知识产权的真空电阻熔焊设备；该设备将手套箱体、真空烘箱、抽真空系统、焊接机构融为一体，实现了电阻熔焊技术与真空封装工艺的有机结合，保证了真空封装的质量。
真空封装过程中需监测MEMS器件的真空度。本文采用石英晶振的谐振电阻随环境真空度变化而变化的原理实现了MEMS器件小体积内的真空度测量，并深入研究了各种影响真空度测量的因素。
对真空电阻凸焊过程进行热-电-结构三场耦合有限元模拟。对电阻凸焊预压阶段的接触过程进行数值模拟，研究了其中凸焊筋位移场、应力场、温度场、电流场的分布规律。
为实现真空度在10Pa以下的MEMS器件真空封装，创造性地提出了带缓冲腔的真空壳体来保证真空腔体泄漏率，设计出了MEMS器件真空封装专用外壳。为提高真空封装的成品率，分析了金属镀层对封装质量的影响，进行了真空封装外壳焊接后的焊接强度、真空度保持实验，保证了真空封装工艺的可靠性。提出了一套基于真空电阻熔焊的MEMS器件真空封装工艺。
应用真空物理的相关理论，建立了MEMS器件真空电阻熔焊封装模型，分析引起封装腔体的真空度降低的主要原因。通过真空封装专用外壳封装泄漏率的理论计算，对真空封装MEMS器件进行真空寿命计算。试验结果显示自行设计的真空封装壳体可以满足长时间真空度保持的要求，证明了真空电阻熔封装工艺的可靠性。
The MEMS devices are widely used in various radio frequency, inertia and mechanicalresonators which are often operated under the high vacuum environment to ensure goodvibration performance, high quality and long-term stablity and reliability. Therefore,vacuum packaging is necessary for these MEMS devices. In this article, a new method toachieve the vacuum packaging of MEMS devices using vacuum resistance welding wasdeveloped. To establish a standard crafts of the MEMS devices in a vacuum resistancepackaging process, the key technologies were investigated intensively by theory,numerical simulation and experimental methods. The main research and innovation are asfollows:
Through a large number of encapsulation testing and analysis about characteristics ofhermetic package, the key factors that influence the vacuum of MEMS encapsulation werefound. According to the new features of welding in the vacuum environment, the vacuumresistance welding equipment, combined the resistance fusion welding device togetherwith the vacuum encapsulation, with independent intellectual property rights has beendeveloped. The glove box, vacuum drying oven, vacuum-pumping system and weldingmechanism of the equipment form as an organic whole which provides repeatable workingenvironment for the whole vacuum encapsulation process and guarantees the quality ofvacuum encapsulation.
The vacuum of small volume in MEMS devices should be measured before vacuumpackaging. This paper makes use of the existing theory that the resonant resistance ofquartz crystal changes with the vacuum of the environment to measure the vacuum ofthese MEMS devices and studies various factors that influence the vacuum measurementaccuracy.
The thermo-electricity-structure finite element simulation on vacuum resistancewelding process under projection welding was performed. The displacement field, stressfield, temperature field and the current distribution by contact analysis at the prepressingstage of the projection welding, the process of the convex welding steel conquassation and the forming process of the nugget was further discussed. Results indicated that the meltnuclear form is influenced by the prepressing and the size of the convex welding steel.
In order to achieve the vacuum packaging of the MEMS devices below10Pa, avacuum shell with cushion chamber to ensure the leakage rate and a special shell of thevacuum encapsulation of MEMS devices were developed. To improve the rate of finishedproducts of the vacuum packaging, the effects of the metal coating on the vacuumpackaging was further investigated. And then experiments on the weld strength, thevacuum maintenance of the vacuum packaging hull were performed to ensure thetechnical reliability of the vacuum packaging.
The vacuum resistance welding of the MEMS device package models based on thetheory of vacuum physics and the primary factors leading to the loss of vacuum wereanalyzed. According to the theoretical calculation of the leak rate for the special vacuumpackaging shell, the lifetime estimation approach of the vacuum packaging MEMSdevices was established. At last, a packaging technology for resistance welding MEMSdevices was found. Experimental results showed that the self-designed vacuum packagingshell meets the requirements of long time of vacuum maintenance.
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