An overview of RF MEMS switches

By Sarjak Shah and Sakina Fakhruddin(2010AAA4091H) (2010A3PS220H)

Presentation is About...

SEM Group of CSIR-CEERI

RF MEMS Switches

Work so far..

Future focus

SEM Group and its ActivitiesHead- Dr. Kamaljit RangraMembers:

1. Mr. Deepak Bansal2. Ms. K. Maninder3. Mr. Akshdeep Sharma4. Ms. Prachi Jhanwar5. Ms. Shilpi Pandey6. Mr. Sumit Khandelwal7. Ms. Priyanka Dwivedi8. Mr. Mahesh Angira9. Mr. Kushagra

Projects being pursued..

Design of Symmetric Toggle SwitchesMEMS Switches ConfigurationsMEMS FiltersPackaging of devices designedStudy of switch structure Digital Mirror Devices

Issues to be addressed

Size of capacitive switches

Designing of SPDT and Phase shifters

Liability issues in ohmic contact switches

Combination of both types on same board

What is MEMS?

MEMS is an abbreviation of microelectromechanical system.

Components vary from 1 micrometre to maximum of 1 mm.

Micro establishes the dimensional scale, Electro suggests either electricity or electronics or both and Mechanical implies ‘moving’ device components.

Originated from R P Feyman's hypothesis on miniaturization of devices.

RF Switches

Designed to operate at RF-to-millimeter-wave frequencies (0.1 to 100 GHz).

Key role in the development of the modern communications systems (wireless and satellite).

Considering RF system, simple wire replaced by coaxial and microstrip lines.

More complicated devises that introduce resistance, capacitance and inductance in signal-to-signal and signal-to-ground paths.

Different switch technologies for various frequency-bands of application.

In wireless communication, currently SPNT switches are being used.

Advantages of RF switches

Near-Zero Power Consumption: high actuation voltage(20 to 80V) but low current consumption.

Very High Isolation: low off-state capacitances (2-4 fF) resulting in excellent isolation at 0.1-40 GHz.

Very Low Insertion Loss: about -0.1dB upto 40 GHz.

Intermodulation Products: MEMS are linear devices.

Applications of RF switches

Transmit/Receive (T/R) RF switches: get fully integrated with other circuits and operate over very wide bandwidths, DPST used in radar and satellite communication, must be highly linear.

Programmable RF attenuators: reduce the level of the signal and provide impedance matching.

Phase shifters: produce a replica of the signal applied to its input, but with a modified phase done through routing the signal.

Capacitive switches

Thin layer of dielectric material separates two conducting electrodes. No direct metal-to-metal contact.

Bandwidth is limited by the capacitance ratio between the ON and OFF states and generally the RF signal range for capacitive RF MEMS switches is 4GHz-120 GHz.

Ohmic switchesConductive beam suspended over a break in the transmission line.

Non-contact part of the beam is used for switch actuation and can be configured as a single fixed end cantilever or a fixed-fixed cantilever.Τhe signal range for ohmic switches is DC to 40 GHz.

Parallel configuration

Series configuration

Switch configurations

Specifications of RF Switches

Insertion loss( S11 )Isolation( S21 )Switching speedPower handlingBandwidthActuation voltageResonant frequency

Design considerations of switch (cantilever type)

Mechanical modelling

Electrostatic modelling

Consider the two plates as a capacitor, we get the following equation for force.

Comparing with the mechanical model equation, we get

Fringing capacitance

Till now whatever equations were discussed had the assumption that the dimensions of plate are much larger than the gap, but in case of MEMS switches the gap is comparable to the dimension of the plate. So fringing capacitance does play a major role and has to be considered.

Perforation

The RF-MEMS switches, especially those with wide cantilever designs, use small diameter holes (2–10 μm) in the beam to speed up the release etch while allowing easier sacrificial layer removal. These holes also reduce the stiffness of the cantilever as well as the squeeze film damping increasing the switching speed of the MEMS switch.

RF switches reliability issuesContamination: Contact resistance increases due to organic

deposits, absorbed hydrocarbon layers and contamination around the contact area. Can be avoided by proper material selection.

Stiction: Arises when the surface interaction energy at a contact point is greater than the restoring force which tends to bring the switch into the equilibrium/open position. In such a case the RF MEMS switch will stick in the closed position. At high current conditions, stiction will generally be generated by hot-welding. Can be avoided by using stiff elements.

Reliability issues continued..

Wear: The contact may be damaged by a large impact force which can be much greater than the high static contact force needed for low contact resistance.

Cycling mode: In cold switching contact damage, pitting, and hardening of the metal occur since the same point is repeatedly hit during cycling. In the long term the contact area is reduced, increasing the contact resistance. In hot switching, metal arcs are produced at contact melting the asperities thereby increasing the area.

Dielectric charging : Trapping of charge due to asperities on the metal surface, non-stoichiometric defects on the dielectric surface and bulk polarization.

Our Work…

Analysis of a switch configuration, determining the switch parameters and suggest some improvements in the design.

Study of asperities and inclusion of roughness on a contact surface based on an AFM image in either ANSYS or COMSOL.

REFERENCES

K. J. Rangra, “Electrostatic Low Actuation Voltage RF MEMS Switches for Telecommunications”, PhD Thesis, University of Trento, Italy 2005.

G. M. Rabeiz, “RF MEMS switches and Switch Circuits”, IEEE Microwave magazine, December 2001.

G. M. Rebeiz, “RF MEMS Theory, Design and Technology”, Wiley Publications, Canada, 2003.

Michail N. Spasos, “RF MEMS switch for reconfigurable antennas”, PhD thesis, School of Engineering and Design, Brunel University, July 2011.

Minhang Bao, “Analysis and Design Principles of MEMS Devices”, Department of Electronic Engineering, Fudan University, China, Elsevier 2005.