电容式RF MEMS开关自热效应的多物理场协同仿真

Cooperative multi-physics simulation on self-heating effect of capacitive RF MEMS switch

随着信号输入功率的升高,电容式RF MEMS开关会发生自热效应使膜片变形,引起开关气隙高度的改变,导致开关驱动电压漂移,严重影响其可靠性。由于自热效应的失效机理涉及到复杂的多物理场耦合,因此提出了“电磁-热-应力”的多物理场协同仿真方法描述其失效模式,并分析其失效机理。首先利用HF-SS软件建立开关的电磁仿真模型,得到不同输入功率下膜片的耗散功率;再以此作为热源,利用ePhysics软件建立开关的热仿真模型,得到膜片上的温度分布;然后将温度梯度作为载荷,利用ePhysics软件建立开关的应力仿真模型,得到开关的形变行为;最后,根据膜片形变所致的气隙高度变化,得到驱动电压漂移的失效预测模型。以一种具有矩形膜片结构的典型电容式RFMEMS开关为例,利用该方法得到：矩形膜片表面电流密度主要分布在膜片的长边的边缘;温度沿膜片长边逐渐降低,且膜片中心处温度最高、锚点处温度最低;膜片的热应力变形呈马鞍面形,且最大形变点发生在膜片长边的边缘处,仿真还得到0~5 W输入功率下膜片的最大形变量;并拟合出了0~5W输入功率下的开关驱动电压-输入功率漂移曲线,该曲线具有线性特征并与文献实测数据极为吻合,由此证明了该方法的有效性。

The self-heating effect of the capacitive RF MEMS switch is caused by increasing incident power of the RF signal,which leads to deformation of the membrane. Thus, the air gap of the switch is changed. Eventually the drift of the actuationvoltage of the switch is prompted, which seriously affect the reliability of the switch. Because the failure mechanisms of self-heat-ing effect involves complex multi-physics coupling, the failure mechanisms are analysed and the failure modes are described byproposing the electromagnetic-thermo-mechanic multi-physics cooperative simulation method. Firstly, the dissipation power ofthe membrane under different incident power is got by constructing the electromagnetic simulation model of the switch in HFSS,which is taken as a heat source. Then the distribution of the surface temperature of the membrane is got by constructing the ther-mal simulation model of the switch in ePhysics, which is taken as a load. Next, the deformation behavior model of the switch isobtained by constructing the stress simulation model of the switch in ePhysics. At last, according to the change of the air gapcaused by deformation, the failure prediction model of the drift of the actuation voltage is obtained. Taking a typical capacitive RFMEMS switch with rectangular membrane geometry for an instance, the distribution along the edge of the length of the surfacecurrent density of the membrane is got with this method. And the temperature gradually reduces along the edge of the length,with the highest temperature in the center and the lowest at the anchor. It is found that the maximum deformation point of themembrane appears on the edges of the long side. And the deformation presents a saddle surface. The linear relationship of thedrift between the actuation voltage of the switch and the incident power （0-5 W） of the RF signal is fitted by getting maximum de-formation value of the membrane under different temperature incident power （0-5 W）. The effectiveness of the proposed methodis proved by comparing with the measured data of the references.