【作者】 甘志银；

【导师】 黄德修；

【作者基本信息】 华中科技大学 ， 物理电子学， 2008， 博士

【摘要】 MEMS器件不同于集成电路的典型特征是具有传感、致动、三维结构、功能多样性,使得MEMS封装技术更具挑战性,成为MEMS器件走向市场应用的一个技术瓶颈,其中真空封装技术是一个多年来未能攻克的技术难题,许多很有创意、具有潜在市场应用的MEMS器件由于没有可靠的真空封装技术,只能停留在实验室做功能演示。本文采用理论、模拟、实验相结合的方法系统地研究了MEMS真空封装中的关键技术问题,特别是一些共性的关键技术。在此基础上,研制了一台具有自主知识产权的电阻熔焊真空封装设备,开发了一套可靠的MEMS器件级真空封装工艺。研究的主要工作与创新点总结如下:(1)在国内首次系统地研究了MEMS真空封装工艺。从真空状态下的气体流动、气体吸附、渗透与扩散的理论入手定量分析了吸附气体的脱附、气体的渗透以及封装材料溶解气体的排放对真空封装效果的影响,研究结果表明MEMS器件由于体积小,封装壳体壁厚比较薄,这些因素的影响非常显著。从理论上分析了真空封装的要求,得到密封泄漏率水平、真空压力与寿命的相互关系,对真空封装工艺的实现具有重要的指导意义。从理论上揭示真空封装要求非常低的密封泄漏率水平,一方面超出了目前常用的测试泄漏率仪器的测试范围,另一方面在密封工艺上也很难达到。指出实现真空封装只有两个途径,方法一就是系统设计封装外壳使有效的泄漏率水平满足真空封装的要求;方法二就是采用吸气剂来吸收泄漏的气体,从而平衡泄漏,对于0.1Pa以下的真空封装只能采用吸气剂实现。(2)第一次应用音叉石英晶振实时监测封装壳体内的真空度变化,解决了MEMS器件由于体积小,无法采用商业真空规测试真空的技术难点。从压电晶体的动力学模型出发,得到了音叉晶振的谐振电阻与音叉晶振的关键结构尺寸、气体阻尼系数、气体压力的解析关系,对石英晶振作为空气压力传感器的优化设计、以及应用的拓展具有理论上的指导意义。对稀薄气体下各种阻尼进行了系统的研究,并通过石英晶振的谐振电阻的实测结果进行了对比验证。在此基础上,研制了晶振真空压力检测系统,对测试重复性、测试量程、温度的测试敏感性、以及老化特性进行了实验研究。为研究真空封装工艺提供了可靠的评价手段。(3)研制了国内第一台电阻熔焊真空封装设备,填补了国内真空封装设备的空白。该设备把电阻熔焊与真空封装工艺有机地结合在一起,在结构上将手套箱体、真空烘箱、抽真空系统、焊接机构等融为一体,为整个真空封装工艺提供了可重复的工作环境,保证了真空封装的质量。在该设备上研究了金属外壳的真空封装工艺如焊接工艺参数、除气工艺、夹具对真空分布的影响等,总结了一套可靠的基于电阻熔焊的MEMS器件级真空封装工艺。该设备已申请为专利产品。(4)详细研究了泄漏模式与泄漏率的定量测量。对He质谱检漏仪评价漏率的原理进行了详尽的分析,从理论和实验上证明He质谱检漏仪在真空封装研究中存在应用的局限,一方面He质谱检漏仪的检测灵敏度不满足真空封装对漏率的要求;另一方面在真空下检测漏率与实际漏率具有非线性关系,使得测试漏率只具有相对比较的意义,与实际漏率相差甚远。用晶振压力监测的方法检测的漏率与He质谱检漏仪的测试结果进行了比较。实验测试表明在空气中的实际漏率比He质谱检漏仪的测试漏率至少低3个数量级。对泄漏模式进行充分的理论探讨与实验分析,用基于漏孔的泄漏模型与基于材料吸附的放气模型,对真空封装壳体焊缝的泄漏实验结果进行了圆满的解释。指出仅仅通过提高焊接质量是很难达到真空封装5年以上长时间真空保持的要求,因为焊接质量的进一步提高存在现实的困难。(5)提出了带真空缓冲腔的真空封装外壳设计方案,从理论上系统地论证了该设计方案能有效延长真空保持寿命至少20倍以上。对真空封装的可靠性进行了研究,通过高温高湿(85℃/85%RH)的加速测试方法对真空保持寿命进行了评估,同时对带缓冲腔的真空保持效果进行了长达二年的真空度实时跟踪监测,验证了该方案的有效性。使得真空封装可以在不需要吸气剂的情况下得以实现。真空封装外壳的设计方案已申请为发明专利。更多还原

【Abstract】 Compared with ICs, MEMS devices have specific features such as sensing, actuating, 3D structure and function diversity, those make MEMS packaging technology facing more challenges and a bottleneck of mass production and wide applications for many MEMS devices. Of all the packaging technology, MEMS vacuum packaging is one of the most challenging technologies still under development, many MEMS devices which have potential market applications can only stay in laboratory for demonstration because of lack of adequate, reliable vacuum packaging technology. This dissertation conducted fundamental research into several key issues of MEMS vacuum packaging technology, especially focusing on a lot of common tough technologies by integrating theory, simulation, and experiments. Based on the fundamental work conducted, a vacuum pacakaging equipment by resistance fusion welding has been developed and patented, and MEMS device-level vacuum packaging processes have been developed and implemented in volume production. The major innovation points are summarized below.(1) Systematic research has been conducted on the MEMS vacuum packaging technology for the first time in China. Using permeance and diffuse theory, the disabsorption of surface absorbed gases, the gas penetration and degassing of dissolved gases in packaging materials have been studied and quantatively evaluated the effects on the vacuum packaging processes, especially the dramatic influence on MEMS device vacuum packaging because of MEMS device small volume package, thin package shell. The requirement of vacuum packaging has been theoretical analysed, a relationship between leak rate, vacuum pressure and vacuum maintenance lifetime was derived, which played an important, instructive role on implementing the vacuum packaging. The results demonstrated that the leak rate required for MEMS vacuum packaging is so much low that it is beyond the ability of an instrument to measure and difficult to find out an adequate welding way to achieve the hermeticity. MEMS vacuum packaging could only be implemented by two possible ways, one way is to decrease the effective leak rate by the package structure design, and another way is to use the getter to blance the leak by absorption of leaking gases, the vacuum packaging under 0.1Pa can only be realized by the aid of the getter.(2) The packaged cavity of MEMS devices is too small to find an ordinary vacuum gauge to monitor the vacuum degree, which has been an obstacle in MEMS vacuum research. Using tuning fork cystal in this dissertation gives a way to measure the pressure of the packaged cavity as a monitor chip. The mathematic physical model of tuning fork cystal was created by piezo-electric dynamics, the resonant resistance analytical solution has been derived and gives the relationship between crystal structure, gas damping coefficient, gas pressure and resonant resistance, which plays a great role in crystal optimum design as a sensing chip, and has an instructive significance in crystal new applications. The gas damping effects in rarefied gases have been theoretically studied and verified by the crystal vacuum measurement. Based on the fundamental research work, a crystal vacuum measurement system was designed and carried out, many experiments have been done on measurement repeatability, measuring range, temperature sensibility, and aging characteristic. This system acts as a key vacuum evaluation tool for MEMS vacuum packaging research.(3) Vacuum packaging equipment by resistance fusion welding has been developed for the first time in China. This equipment incorportated the resistance fusion welding process into vacuum packaging with a systematic design of vacuum baking, vacuum pumping, resistance welding,etc. All the subsystems were fitted in the working cabinet with pure gas circulation which supplied a repeatable working environment and guaranteed the vacuum packaging quality. The vacuum packaging process research such as welding parameter optimization, degassing, the fixure influence on the vacuum pressure distribution, etc. has been conducted, and finally established a reliable vacuum metal packaging processes for MEMS devices. The equipment has been patented.(4) The leak mode and leak rate measurement have been studied. The detailed analyse has been conducted on the leak evaluation by helium mass spectrometer leak detector, and theoretically and experimentally authenticated that the helium leak detector as a common leak rate measurement tool can not meet the requirements for leak detection during vacuum packaging research because of its limited sensitivity and measured nolinearity characteristic under vacuum, the measurement results have a big difference compared to the actual leak rate, and can be only taken as a rough reference for comparison with each other. The leak rate was also measured by crystal pressure monitoring. The results were contrasted with that by helium leak detector. The experiments showed that the measured leak rate by helium leak tester had more than three orders difference with the actual leak rate. The leak modes have been thoroughly analyzed based on the micro-tube leak model and degassing model of package materials, and the simulation results greatly satisfied with that from the experiment measurment. The leak rate measurement research showed that it was not a feasible direction to carry out reliable vacuum packaging with 5 more years vacuum maintenance only by increasing the welding hermeticity.(5) An innovated vacuum package design with a small vacuum buffer cavity was proposed, and has been proved in theory that this design can extend the vacuum maintenance at least 20 times more than the common package with the same volume and under the same welding means. The reliability research has been conducted on the vacuum packaging with the vacuum buffer cavity, using 85℃high temperature and 85% relative humidity soak, temperature and humidity cycling to accelerate fatigue. The lifetime of the package cavity vacuum has been evaluated at the same order as that in theory. Another experiment of one more year continued vacuum monitoring has also verified the validity of the vacuum package design. The reliable vacuum packaging has been realized with getter free. The design of the vacuum package has been patented.更多还原