MEMS真空封装内部放气失效机理与加速寿命模型研究

To achieve high Q-factor or rapid thermal response, many MEMS structures must be packaged and operated inside the cavity with controlled ambient of vacuum. Therefore, control and maintenance of the vacuum level inside the cavity are very important for long-term stability and performance robustness. Though die level MEMS packages such as ceramic and metal package are giving their way to wafer level encapsulation, they still dominate the vacuum package of a variety of MEMS devices such as infrared detectors, helium-neon lasers and digital mirror devices. Components such as transparent window, getter, TEC, shell and lid, which needed to be integrated into a die level MEMS package assembly, may outgas and progressively degrade the vacuum level over time. This kind of interior outgassing is one of the most dominant failure mechanisms of die level MEMS package. Though the outgassing failure is not only based on physical desorption and adsorption of gas molecules, but also based on chemical reaction between desorbed gases, the long-term stability and reliability caused by outgassing failure is still unclear, and there are a little reliable methods and models published to predict lifetime and to perform accelerated life testing. In this project, through the use of RGA and thermistor vacuum gauge, pressure of encapsulated package cavity will be continuously monitored to determine the outgassing characteristics of different components and their interactions. Accelerated life testing will be performed under different temperatures to get the empirical value of actuation energy, which is important in calculation of acceleration factor. And long-term life testing data will be achieved to fit a proper probability distribution model of failure.

为了获得稳定的Q值或热响应率，MEMS器件有比电子器件更高的气密性要求，且经常使用吸气剂来吸附泄漏气体以保证真空度不发生变化。对于高气密性要求下主要由内部放气导致吸气剂中毒或者吸气剂饱和的失效模式，由于其内部气源比较多样且内部放气还有可能互相反应，其物理原理甚至化学原理比较复杂，其失效机理还没有很好地确定，现存的标准很少，在很多模型和试验方法方面还没有达成共识。本研究拟利用在线残余气体分析系统，通过在线排气监测确定不同温度下的放气种类、对应气源和放气速度；确定内部放气是由气体渗出单一机制还是气体化学反应复合机制造成；确定高温加速下是否具有与使用条件完全相同的放气机理，并通过多个温度下的试验数据确定加速模型的激活能；根据计算得到的加速因子和加速条件下试验数据，选择合适的寿命模型推测使用条件下的寿命分布。从而为MEMS高气密性封装的可靠性设计、可靠性鉴定和寿命预测提供重要依据。

高气密性真空封装MEMS器件封装材料的内部放气使得其内部真空度发生变化，从而降低器件的性能和可靠性，故使用吸气剂来吸附泄漏气体以保证真空度不变，掌握放气失效机理并预测寿命对提高真空封装MEMS器件的可靠性有着重要的意义。课题利用在线残余气体分析系统，通过在线排气监测对封装内部材料的放气种类、对应气源和放气速度进行研究，从试验的角度解释了MEMS器件内部放气是由气体单一渗出机制和渗出气体发生化学反应复合机制所造成，并揭示了吸气剂中毒失效的机理；通过加速寿命试验确定了高温加速下与使用条件完全相同的放气机理，根据两组温度下的实验数据计算得出加速模型的激活能；利用计算得到的加速因子和加速条件下试验数据，建立合适的寿命模型预测使用条件下的寿命分布，同时建立可靠性鉴定模型，鉴定器件的可靠性。为MEMS真空封装的可靠性设计、可靠性鉴定和寿命预测提供重要依据。