Electronic Design

MEMS Switch Puts SoC Radios On The Cusp

This MEMS switch technology for multiband/multimode RF systems heralds the anticipated system-on-a-chip radio.

Slowly, microelectromechanical systems (MEMS) devices have found homes, mostly in optical switching and processing. STMicroelectronics has now stretched their reach by demonstrating an RF MEMS switch for use as a band and circuit reconfiguration switch in multiband/multimode RF systems.

The device also has potential as a transmit/receive (Tx/Rx) switch in cell phones and other radios. Today, most Tx/Rx switches are separate devices made with gallium-arsenide MESFETs or PIN diodes. STM's MEMS version can be fully integrated into the silicon RF chip, drawing closer to a system-on-a-chip (SoC) radio while lowering costs.

Benefits derived include superior insertion loss and isolation figures. STM's switch measured an insertion loss of only 0.18 dB with an isolation factor of -57 dB at 2 GHz on a 15-Ù-cm substrate. Other vital benefits include 3 W of continuous RF power handling and superior linearity. A two-tone test showed the intermodulation products to be 82 dB below the tone levels at 2.4 GHz.

The STM switch uses an unusual combination of thermal and electrostatic actuation. It consists of a floating 400- by 50- by 1-µm nitride beam with two contacts designed to provide a metal-to-metal bridge between two conductors of a 50-Ù line. At each end of the beam is a small resistive heating element. When 20 mA is applied to the elements, they produce a temperature that causes the beam to bend and ultimately make the interconnection. The bimorph effect produces bending because of different thermal expansion coefficients of two materials in the structure. Actuation time is about 200 µs.

To hold the switch contacts in place, the heating current is removed and a static field is set up by dc voltage applied between a capacitor element within the beam and the circuit ground plane. Removing the voltage causes the contacts to separate within about 30 µs.

As developments unfold in this technology, particularly in switching time, the switch will suit an even wider range of RF applications.

See associated figure


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