Review of Design, Modeling, Testing and Fabrication of RF MEMS

Structure of RF MEMS

Generally, MEMS switch consist metal bridge of length l, width w, and thickness the fixed at the both ends. The bridge is suspended at a distance g + td from the central line CPW which has the width W, where td is the thickness of the dielectric layer deposited on the CPW central line and g is the air gap between the bridge and the dielectric layer. The central line of the CPW is centered under the suspended bridge [1].

rfsw

MEMS capacitive switches are fabricated with a metal dielectric-air gap-metal cross section. The upper metal plate (also known as bridge) can be actuated from an up state to a down state. In the up-state, the plate is relaxed and the air gap is present between the dielectric and the upper plate. In the down state, an electrostatic force is applied by an external control voltage to the upper plate causing it to collapse and eliminate the air gap between the dielectric and upper plate [2].

Modeling of RF MEMS for Eliminating Problem of Reliability Properties

Even though RF MEMS have certain advantages over solid-state switches, it still have any problem of reliability properties. The problem of bouncing upon closer and failure of electrostatically actuated caused decreasing signal quality and increasing switch times, and decreasing the lifetime of capacitive RF MEMS. For solving this problem, reliability modeling and simulation can be used [1,4].

For eliminating the bouncing of switch, it was proposed two numerical models: SDOF model and 3D Finite Element Model [1]. By using SDOF model the minimum required hold voltage for deflection x0 has the form where x0 is the initial position of plat

{V}_{h}=\sqrt{\frac{2{k}_{eff}{x}_{0}}{\varepsilon A}}\left(g-{x}_{0} \right)

The discussion hitherto has assumed that the plate stays parallel to the substrate at all times that only the springs undergo flexible deflection and that oscillatory deflection and vibrations are negligible. Using the soft-landing waveform generated by the SDOF model, a 3D Finite Element Model is used the remove this assumption.

The other way for increasing reliability is using RF MEMS reliability test set [4]. By using this reliability test set, it was founded a failure phenomenon related to the chargers trapping within the dielectric. The model describes the kinetic of the dielectric charging effect and it can be used as a methodology to provide the suitable parameters which drive the reliability of RF MEMS in order to compare and optimize their lifetime.

Design and Fabrication of RF MEMS

Some steps are required as a design procedure to provide the circuit designer with a simple set of parameters, while avoiding the level detail that is generally the domain of the device engineer. The procedure are: 1. Identify type of implementation, 2. Determine size ration, 3. Size capacitor and allocate chip area and 4. Adjust switch for linearity. This procedure had been used for circuit of dual-band LNA and tunable power amplifier [2].

Fabrication will be doing while design of RF MEMS and substrate choice has been done. The fabrication procedure are using three steps: 1. Bottom wafer (Signal Control lines), 2. Top wafer (Switching Structure) and 3. Assembly of the prototypes [5].

Reference:

[1] Simion, S., Modeling and Design Aspect of The MEMS Switch, IEEE, 2003.

[2] Danson, John, Plett, Calvin, Tait, Niall, Design and Characterization of a MEMS Capacitive Switch for Improved RF Amplifier Circuit, IEEE 2005 Custom Integrated Circuits Conference, Page 251-254, 2005.

[3] Massad, Jordan E., Sumali, Hartono, EPP, David S., DYCK, Christopher W., Modeling, Simulation, and Testing of the Mechanical Dynamics of an RF MEMS Switch, Proceeding of the 2005 International Conference on MEMS, NANO, and Smart System (ICMENS’05), 2005.

[4] Melle, Samuel, De Conto, David, Dubuc, David, Grenier, Katia, Vendier, Olivier, Muraro, Jean-Luc, Cazaux, Jean-Louis, Plana, Robert, Reliability Modeling of Capacitive RF MEMS,IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 11, November 2005.

[5] Oberhammer, Joachim, Stemme, Goran, Design and Fabrication Aspects of an S-Shaped Film Actuator Based DC to RF MEMS Switch, Journal of Microelectromechanical System, Vol. 13, No. 3, June 2004.

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