Hanan Bar

Metal contact reliability of RF MEMS switches

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several important advantages for wireless communications. These include superior linearity, low insertion loss and high isolation. Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. However, none of these applications have been materialized in high volume products to a large extent because of reliability concerns, particularly those related to the metal contacts. The subject of the metal contact in a switch was studied extensively in the history of developing miniaturized switches, such as the reed switches for telecommunication applications. While such studies are highly relevant, they do not address the issues encountered in the sub 100μN, low contact force regime in which most MEMS switches operate. At such low forces, the contact resistance is extremely sensitive to even a trace amount of contamination on the contact surfaces. Significant work was done to develop wafer cleaning processes and storage techniques for maintaining the cleanliness. To preserve contact cleanliness over the switch service lifetime, several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from contamination was examined. The contact reliability is also very much influenced by the contact metal selection. When pure Au, a relatively soft metal, was used as the contact material, significant stiction problems occurred when clean switches were cycled in an N2 environment. In addition, various mechanical damages occurred after extended switching cycling tests. Harder metals, while more resistant to deformation and stiction, are more sensitive to chemical reactions, particularly oxidation. They also lead to higher contact resistance because of their lower electrical conductivity and smaller real contact areas at a given contact force. Contact reliability issues could also be tackled by improving mechanical designs. A novel collapsing switch capable of generating large contact forces (>300μN) was shown to be less vulnerable to contamination and stiction.

A Stamp-Sealed Microshell Package for RF MEMS Switches

This paper describes a unique method of encapsulating MEMS switches at the wafer level using a thin-film “microshell” lid and a novel micro-embossing, or “stamping” technique to seal the lid. After fabrication of the MEMS switch and subsequent formation of the microshell, the switches are released through gold tunnels that allow the penetration of a chemical etchant. In a controlled ambient, a “stamp” wafer is aligned to the device wafer, and the wafers are thermally compressed together. This process applies pressure across each tunnel to fuse the gold, thereby sealing the microshell packages. By sealing and passivating the switches at the wafer level, the wafers can be exposed to backend processing, packaging, and assembly steps such as dicing without damaging the sensitive MEMS devices. Furthermore, the size, cost, and complexity of the packaged system are significantly reduced compared to standard wafer bonding processes.Copyright