A flip–chip encapsulation method for packaging of MEMS actuators using surface micromachined polysilicon caps for BioMEMS applications

In this paper, we present a novel technique for encapsulation of MEMS devices. The technique is demonstrated to address two issues related to the use of in-plane thermal actuators for BioMEMS applications. First, an encapsulation process is described to provide protection to a MEMS actuator from debris and other particulate matter when deployed in a biological environment. The encapsulation structure consists of a multilayer wall around the actuator and a surface micromachined polysilicon cap. A small clearance is provided around a piston that transmits motion from the actuator to the external world. In air, the packaged actuator performance is comparable to that of an unpackaged actuator, thus indicating successful encapsulation without any damage to the actuator. Second, this packaging approach is used to address the issue of reduction in efficiency of the thermal actuator in liquids by coating the packaged actuator with a thin conformal hydrophobic layer. This prevents liquid from entering the encapsulation, thus isolating the hot actuator components from the liquid. Experimental results show that efficiency of the packaged actuator in water improved giving a performance similar to that observed in air, suggesting an isolation of the hot actuator components from the liquid. Although the technique is demonstrated for thermal actuators, it is also applicable to other MEMS devices and in-plane actuators such as electrostatic comb drives for engineering as well as biological applications.