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RF MEMS Switches

RF MEMS Switches - The Holy Grail of MEMS devices is the micromechanical switch. For more than a decade, researchers have endeavored to perfect the development of microminiature relays using micromachining techniques.  With the recent boom in wireless communications, research has intensified in the quest to develop low cost, ultra-low loss switches.  The goal is to have these switches replace traditional FETS for reduced loss and improved linearity in key components.  There are fundamentally two types of switch contact mechanisms – ohmic contact and capacitive contact.  With ohmic switches, two metal electrodes are brought into contact to create a low-resistance connection.  In capacitive switches, a metal membrane is pulled down onto a dielectric layer, usually by electrostatic means, to form a capacitive sandwich.  At high frequencies, the capacitive suseptance of this sandwich acts like a short circuit.  In either case, the mechanical action of the switch causes the switch to efficiently change from high impedance to short circuit.

Micromechanical switches can utilize one of many actuation mechanisms, including magnetic, piezoelectric, thermal, and most commonly electrostatic forces.  Switches that operate electrostatically require very little energy, usually on the order of tens of nanojoules per switch cycle.  The Achilles Heel of all MEMS switches is their switching speed, which is determined by their mechanical resonant frequency.  Actuation is typically accomplished in microseconds to 10s of microseconds for electrostatically operated devices, and 100s of microseconds to milliseconds for thermal actuators.  Despite the fact that MEMS switches operate slower than their electronic counterparts, they are still useful in many applications.  One important advantage of MEMS switches is their linearity.  Unlike electronic switches made with metal-semiconductor or p-n junctions, the contact area for MEMS switches is perfectly linear.  This means that well-designed MEMS switches do not create nonlinearities or distortion such as harmonics or intermodulation products.  In many cases, these nonlinearities are unmeasurable.

MEMS Capacitive SwitchMEMS Capacitive Switch

RF MEMS capacitive switches were first invented and reduced to practice in 1993 as a means of achieving the low RF loss afforded by MEMS and micromachining technology. The use of a capacitive coupling mechanism entirely eliminates issues associated with dry contact, metal-metal ohmic switching. Over the years, investments by government and corporate IR&D have evolved these switches considerably. Presently, capacitive MEMS switches, and MEMS switches in general, provide the lowest loss means for switching and routing RF, microwave, and millimeter-wave signals. Over the past decade processing improvements, material refinements, and RF and mechanical design changes have allowed MEMS capacitive switches to demonstrate ultra-low loss (<0.1 dB through 40 GHz), miniscule power consumption (tens of nanojoules per switch cycle), and very high linearity (>+66 dBm).

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