INTRODUCTION
RF-MEMS devices like switches and tenable capacitors contain movable fragile parts that must be mechanically protected and packaged in a clean and stable environment [1]. To ease wafer dicing, (chip) handling and plastic moulding, the packaging is preferably carried out during wafer processing, prior to die singulation. This packaging step is referred to as wafer-level or 0-level packaging [2], [3], The 0-level package defines the first protective interface for the MEMS device, achieved through on-wafer encapsulation of the movable parts in a sealed cavity as illustrated in Fig. 1. A key consideration in the design of the 0-level package for RF-MEMS is the impact the package has on the high frequency characteristics. Ideally, the high frequency characteristics before and after packaging are not degraded. There are four main issues that determine the high frequency characteristics of the 0-level package; (1) the (resistive) losses of the RF feedthroughs, (2) the reflection at the package boundary and, (3) the loss and (4) the detuning of the transmission lines due to proximity coupling to the capping chip. Considering the relevance of the package for the RF-MEMS device it is surprising to see that only few research groups have addressed this important issue [2] [5]. Margomenos et al. [4] successfully demonstrate a 0-level packaged capacitive RF-MEMS switch implementing substrate RF vias through thick high resistivity silicon wafers. The designed RF transition displays a low insertion loss of better than 0.1 dB and a return loss of 32dB at 20GHz, but the fabrication process for the through-wafer via's seems rather complex and no indication is given concerning the influence of the capping chip on the switch characteristics. Another example of a 0-level packaged RF-MEMS relay also implementing substrate RF vias is due to Fujii et al. [5]. The reported measured insertion loss of the packaged relay is quite poor (1.3dB at 20GHz) but moreover, it is not at all clear what the influence of the 0-level package becomes on the device characteristics. Jourdain et al. [2] describe a 0-level package for RF-MEMS devices implementing buried RF feedthroughs. The impact of the 0-level package, including the feedthroughs, on the device characteristics are modeled and experimentally verified. Actual implementation of the package for a RF-MEMS device however was not demonstrated. Furthermore, as the buried RF feedthroughs are implemented in the RF-MEMS substrate, the fabrication process for the RF-MEMS is exceedingly complicated. An example of a 0-level packaged RF-MEMS switch implementing very straightforward coplanar RF feedthroughs is described in [3]. Here however the 0-level package is merely developed as a switch protection without special attention for optimizing the microwave characteristics. This is noticeably displayed by the degrading influence of the 0-level package (the capping chip) on the microwave characteristics of the switch.