|
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.
|
