Nanoblasting fine-tunes sensors

Nov. 8, 2007
Carpeting small areas of a semiconductor device with tiny pits adjusts its electrical resistance and could usher in a new class of magnetic sensors and ultradense data storage, say researchers with the National Institute of Standards and Technology (NIST).

The technique lets engineers tailor the electrical resistance of individual layers in a device without changing any other part of its processing or design.

For example, the magnetic sensors in disk drives contain a sandwich of two magnetic layers separated by a thin buffer layer. The layer closest to the disk switches magnetic polarity in response to the direction of the magnetic bit recorded on the disk beneath it. The sensor measures electrical resistance across the magnetic layers, which changes depending on whether layers have matching polarities.

Of course, ever-smaller storage devices need ever-smaller sensors, a trend at odds with sensor performance. To get around the size issue, NIST prototypes combine two different sensors based on the buffer material.

Magnetoresistance sensors use a low-resistance metal buffer layer. Though fast, they are plagued by weak and difficult-todetect signals. Magnetic tunnel junction sensors use a relatively high-resistance insulating buffer that delivers strong signals. But they respond more slowly, too slow to keep up with high-speed, high-capacity drives. The combined signal, researchers argue, should work better than either alone.

“Our approach combines these sensors at the nanometer scale,” explains NIST physicist Josh Pomeroy. “We start with a magnetic tunnel junction — an insulating buffer — and use highly charged ions to ‘blow out’ little craters in the buffer layer.” Xenon ions, each carrying more than 50,000 eV of potential energy, make the surface pits without damaging the substrate. Manipulating ion flux controls pit count and layer resistance. “Growing the rest of the sensor atop the pitted surface makes the pits act as GMR sensors, while the remaining area behaves as an MTJ sensor,” says Pomeroy. The group is now working to incorporate these modified layers into magnetic sensors.

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