Write your abstract here.Nanoscale blasting adjusts resistance in
magnetic sensors.
August 18th, 2007.
A
new process for adjusting the resistance of semiconductor devices by
carpeting a small area of the device with tiny pits, like a yard dug up
by demented terriers, may be the key to a new class of
magnetic sensors, enabling new, ultra-dense data storage devices. The technique
demonstrated by researchers at the National Institute of Standards and
Technology (NIST)* allows engineers to tailor the electrical resistance
of individual layers in a device without changing any other part of the
processing or design.
Caption: Cartoon
illustrates new NIST technique for selectively modifying resistance of
a semiconductor device layer. (Top) First layer — in this case a
composite of copper and cobalt — and an insulating buffer layer of
aluminum oxide is deposited. Buffer is about one nanometer thick.
(Middle) Highly charged xenon +44 ions strike the buffer layer, digging
nanoscale pits. (Bottom) Top conducting layer of cobalt and copper is
deposited. Pits reduce the electrical resistance of the layers and may
function as nanoscale GMR sensors embedded in a MTJ sensor.
Credit: NIST
The
tiny magnetic sensors in modern disk drives are a sandwich of two
magnetic layers separated by a thin buffer layer. The layer closest to
the disk surface is designed to switch its magnetic polarity quickly in
response to the direction of the magnetic “bit” recorded on the disk
under it. The sensor works by measuring the electrical resistance
across the magnetic layers, which changes depending on whether the two
layers have matching polarities.
As
manufacturers strive to make disk storage devices smaller and more
densely packed with data, the sensors need to shrink as well, but
current designs are starting to hit the wall. To meet the size
constraints, prototype sensors measure sensor resistance perpendicular
to the thin layers, but depending on the buffer material in the sensor,
two different types of sensors can be made. Giant magneto-resistance
(GMR) sensors use a low-resistance metal buffer layer and are fast, but
plagued by very low, difficult to detect, signals. On the other hand,
magnetic tunnel junction (MTJ) sensors use a relatively high-resistance
insulating buffer that delivers a strong signal, but has a slower
response time, too slow to keep up with a very high-speed,
high-capacity drive.
What’s
needed, says NIST physicist Josh Pomeroy, is a compromise. “Our
approach is to combine these at the nanometer scale. We start out with
a magnetic tunnel junction—an insulating buffer—and then, by using
highly charged ions, sort of blow out little craters in the buffer
layer so that when we grow the rest of the sensor on top, these craters
will act like little GMR sensors, while the rest will act like an MTJ
sensor.” The combined signal of the two effects, the researchers argue,
should be superior to either alone.
The
NIST team has demonstrated the first step—the controlled pockmarking of
an insulating layer in a multi-layer structure to adjust its total
resistance. The team uses small numbers of highly charged xenon ions
that each have enormous potential energies—and can blast out surface
pits without damaging the substrate. With each ion carrying more than
50 thousand electron volts of potential energy, only one impact is
needed to create a pit—multiple hits in the same location are not
necessary. Controlling the number of ions provides fine control over
the number of pits etched, and hence the resistance of the
layer—currently demonstrated over a range of three orders of magnitude.
NIST researchers now are working to incorporate these modified layers
into working magnetic sensors.
The
new technique alters only a single step in the fabrication process—an
important consideration for future scale-up—and can be applied to any
device where it’s desirable to fine-tune the resistance of individual
layers. NIST has a provisional patent on the work, number 60,905,125.
Source: National Institute of Standards and Technology (NIST)