"Tiny Machines Replace Electronics in Cell Phones", Input/Output, ME Departments, March 2007

input/output

by Alan S. Brown, Associate Editor

Tiny Machines Replace Electronics in Cell Phones

The latest cell phones send pictures and text, play music and games, show movies, automate credit card purchases, and yes, even make phone calls. Yet next-generation phones will go even further. And they are likely to rely on chip-size mechanical devices for many future improvements.

Microelectromechanical systems date back about 50 years, when engineers first began attaching wires to thin silicon slivers. When the membranes flexed, their electrical resistance changed, giving engineers a way to measure pressure and strain.

Since then, MEMS have grown increasingly sophisticated. Fabricators now carve cavities and suspend moving cantilevers, bridges, combs, carved plates, and hinges on silicon wafers. They use many semiconductor manufacturing processes, as well as a grab-bag of specialized techniques. Today, MEMS accelerometers activate airbags and stabilize cameras. MEMS monitor blood pressure, measure weight on bathroom scales, squirt colors in inkjet printers, and reflect light onto high-definition televisions.

More than one million MEMS microphones per day go into cell phones, displacing electret condenser microphones. They consist of movable and static capacitor plates. Talking produces pressure waves that vibrate the top plate. This changes the distance (and capacitance) between the two plates to produce a signal. MEMS work as well as electrets, but are smaller and cheaper to assemble.

Digital capacitors turn on and off to regulate capacitance more precisely than analog versions.

Vibrating MEMS resonators help radio frequency filters remove unwanted noise from incoming signals. They work better than the electronic components they replace. They are also smaller, freeing designers to create thinner, more elegant cell phones.

Some high-end phones feature MEMS accelerometers similar to those used to activate air bags. They sense motion, letting a user scroll down a list of names or play a game by moving a wrist rather than pressing keys.

Even more striking capabilities are on the way, made possible by next-generation MEMS. They combine what had been separate chips—one for MEMS, one for MEMS electronics—into a single, powerful chip.

This achievement has taken decades. The processes used to make MEMS structures can contaminate or rip apart a chip's electronics. Pioneering engineers are overcoming these problems by first laying down integrated circuits, isolating them with an insulating layer, then building MEMS above the insulation. Some have even done away with specialized techniques and now use only the processes found in a semiconductor foundry.

An array of digital capacitors enables smart cell phone antennas to improve performance.

"The approach gives you more roughness and residual stress than you want, but using only the materials and manufacturing operations found at a high-volume semiconductor fabrication plant lets you push costs down far enough to get into consumer applications," according to Jeff Hilbert, president of WiSpry Inc. in Irvine, Calif.

WiSpry's initial product is an RF filter based on MEMS digital capacitors. Digital capacitors are arrays of switches, each with twice the capacitance of an adjacent switch. Hilbert claims that MEMS arrays are more precise, have greater range, produce less electrical noise, and are simpler to use than variable analog capacitors.

In addition to filtering incoming signals, the arrays can switch to different frequencies around the world and optimize antenna electronics to improve signals and save battery power.

Akustica Inc. in Pittsburgh has begun sampling single-chip microphones. "They are so small, your cell phone can use multiple microphones the way your brain uses your two ears to filter out background noise," said marketing vice president Davin Yuknis.

Fujitsu Computer Systems Corp. recently incorporated Akustica's microphones in a laptop. "Users don't have to wear a headset to talk. They could walk around the room and the mikes will track them while suppressing background noise," Yuknis said. He expects hands-free cell phones to follow soon.

Three California companies—Discera Inc., Silicon Clocks Inc., and SiTime Corp.—are building MEMS oscillators to control signal timing in electronic circuits. To compete with quartz clocks, SiTime CEO Kurt Petersen figures he will eventually have to sell his MEMS timers for 50 cents each.

Semiconductor-style manufacturing makes those prices possible, and could push next-generation MEMS sensors and actuators into many more consumer products.