Philips Research claims record-breaking MEMS capacitors
By Peter Clarke - Silicon Strategies - February 3, 2004 (1:47 p.m. ET)
EINDHOVEN, Netherlands — Philips Research scientists have made MEMS (microelectromechanical systems) capacitors that could be used to enhance the performance and reduce the size of RF circuits in mobile phones.
The devices are the microscopic equivalent of air-spaced variable capacitors, but can be integrated into silicon ICs using conventional wafer fabrication processes. Philips Research claims to have achieved a ratio between maximum and minimum capacitance as high as 17, and Q-factors as high as 500, thereby outperforming virtually all other types reported so far.
The MEMS are fabricated by under-etching an area of the silicon chip's top metal layer to create a metal beam that moves up and down through electrostatic attraction in response to an applied voltage. In the case of the switched MEMS capacitor, it moves until it contacts a dielectric layer situated underneath the beam.
Philips can integrate MEMS capacitors alongside high Q-factor inductors and fixed value MIM (metal-insulator-metal) capacitors using a manufacturing process called PASSI.
The PASSI/MEMS should reduce the size of the impedance-matching network that sits between a mobile phone's power amplifier and its antenna to about half the pc-board area needed to accommodate today's discrete component networks, Philips said.
To save power, the dynamic matching can be used to accurately match the amplifier to the antenna at all RF power levels. In GSM systems, for example, where the RF power transmitted by a handset varies according to local signal conditions and the distance to the nearest basestation, this will significantly increase the power amplifier's efficiency and, hence, conserve battery power.
Philips Research is also employing MEMS capacitor technology to produce RF MEMS switches. Philips said it intends to replace PIN-diode switches with the MEMS devices.
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