Cheap as Chips
One reason for the excitement is that while microelectronics semiconductor chips are great at logic and memory, they are a brain without a body. “Computers think and think and think. But MEMS are becoming the eyes, ears, noses, mouths, hands and feet of computers,” says Markus. Adds Barbour: “All of the electronic components end up passive, but MEMS can respond to all kinds of inputs-chemical, light, heat, pressure, vibration, acceleration-all of the things that just about everybody needs to measure in just about every physical system that we have.”
These advantages of MEMS would be enough to entice researchers, but not enough to get MEMS devices onto the market. Yet another factor has also entered the picture: These systems are no longer exotic items that take microengineering specialists months to fabricate. TI’s micromirror arrays are, for example, made using lithographic techniques adapted from the microelectronics industry. Technicians start with silicon wafers, spin on thin coatings of polymer photoresists (a photosensitive material), expose plots of the photoresist to light through a stencil-like mask, and wash away the exposed photoresist to reveal a pattern of the underlying wafer that matches the pattern on the mask. After they expose the naked wafer surface, MEMS makers then etch into, around and underneath the surface, diffuse ions into the silicon, or deposit materials such as aluminum onto it. Clever sequences of masks, etching and deposition yield tiny 3-D structures that move on command.
Make no mistake: It’s still not easy. But the reliance on standard fabrication tools means manufacturing technology is already in place and, once you design a MEMS device, you can potentially turn them out as cheaply as semiconductor chips. At Analog Devices, headquartered in Norwood, Mass., some 1 million tiny accelerometers are fabricated every month, according to Jeffrey Swift, director of engineering for the company’s micromachined products. Buy a car today, and there’s almost a 50-50 chance that one of Analog Devices’ accelerometer-based sensors (each about the size of the period at the end of this sentence) will be inside the air bag systems.
Prior to micromachined accelerometers, motion sensors in air bags required up to five fist-sized components, each costing about $18. Analog Devices and several competitors sell MEMS accelerometers for less than $10 apiece, reflecting the companies’ ability to make them in huge batches. The accelerometers are relatively simple-suspended rectangular slabs of silicon with fingers extending out to form what looks like a double-sided comb. The fingers of these combs mesh with silicon fingers machined into the surrounding silicon framework. The normal motion of a car, as well as the violent and jerky motion during the split second of a crash, instantly sets the suspended accelerometer in motion. The overlapping areas of the meshed silicon fingers change, which causes instantaneous changes in the structure’s electrical capacitance. Those electrical changes then feed into circuitry programmed to discern potentially deadly crashes from potholes; when appropriate, the circuitry triggers the release of the air bag.
Analog Devices’ engineers are developing MEMS for other emerging automotive applications such as side-impact air bags. And Swift, a father of three boys, including a new driver, suggests another possibility: “What if you had a sensor in the car that your teenager was driving that would tell you how many g forces the car experienced? Did he peel out or do a harsh stop or go around corners at really high speeds?”