Encapsulation of Mems Devices

This paper presents a innovative technique for encapsulation of MEMS devices. The technique is established 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. The encapsulation structure consists of a multilayer wall and a surface micromachined polysilicon cap. Second, this packaging approach is used to address the issue of decrease 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. Although the technique is established 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.

Actuator and wall design

A commercially available multi-user polysilicon surface micromachining MEMS process [26] was used for fabrication of the actuator and the encapsulation structures. Three polysilicon layers (Poly0, Poly1, and Poly2), two oxide layers (Oxide1 and Oxide2), a metal layer (Au with Cr adhesion layer) and a nitride isolation layer are available for the process. The nominal thicknesses of these layers are listed in Fig. 1. It has been shown that structures of various heights can be made-up by using different combinations of these layers [27]. These stacked layers were used to provide a uniform gap between the MEMS structure and an encapsulating cap. In Fig. 1(a), the MEMS device to be encapsulated is represented by stacked Poly1 and Poly2 layers. A multilayer wall structure was built around the MEMS device. The wall was designed to be taller than the actuator structure by stacking all the polysilicon layers and trapping both the oxide layers. Gaps were provided in the wall structure for