RF-MEMS aims to tune mobile wireless

More challenging problems
WiSpry was spun-off from Coventor Inc.--the MEMS design-software maker--in 2002. The company began by pursuing a MEMS version of the venerable reed relay, which is the standard way of switching RF (for instance, to switch an antenna between two different radio-frequency front ends). After solving the MEMS-design problems for RF switches, the company turned to more challenging problems, such as tunable RF filters, and eventually designed the array of digital capacitors on a MEMS chip that the company is currently sampling.

Along the road to its first chip, WiSpry has accumulated a portfolio of MEMS technology, including integration with CMOS, SiGe and GaAs technologies used in various types of RF front ends. The company has also perfected a wafer-scale encapsulation technique that isolates its MEMS structures from the environment, and permits standard CMOS-fabrication equipment to handle MEMS wafers as cheaply as are ASICs. The company has filed multiple patent claims for its MEMS techniques--including contact welding for long-term reliability and a triple-layer beam structure that avoids static charging effects. The company is also retaining its most difficult-to-duplicate technologies as trade secrets. So far, WiSpry has had two rounds of financing, netting more than $8 million in venture capital, which it has invested in the RF-MEMS infrastructure for its chips.

WiSpry's first RF-MEMS chip, the digital capacitor array, is being fabricated at Jazz Semiconductor in 0.18 micron CMOS on 200 millimeter wafers. The tiny picofarad capacitors change their value 10-fold--either between .1 and 1 picofarads or between .2 and 2 picofarads--in response to a voltage that changes the distance between capacitor plates to adjust their value. The digital capacitors can change their values in increments as small as five femtofarads. Besides tuning the radio better, the variable capacitor array can add 10 to 20 percent to a cell phone battery's lifetime, since a matched antenna does not require as much broadcasting power to go the same distance.

WiSpry's first customer, the unnamed cell phone handset maker, plans to have digital capacitors doing dynamic antenna impedance matching in its handsets by 2008. In addition to tuning antennas, WiSpry's digital capacitor arrays can be used for tunable RF filters, power amplifier tuning and other programmable impedance-matching applications, according to the company.