PORTLAND, Ore. Magnetic materials for hard-disk heads use the same magnetic tunnel junctions (MTJs) as do the magnetic random access memories (MRAMs) IBM announced it was developing with TDK Monday (Aug. 20). Now, the National Institute of Standards and Technology (NIST) claims to have invented a process for fine-tuning MTJs for the next generation of HD heads, and perhaps even to enable denser MRAMs.
"Our process was developed to show hard-disk head manufacturers how to fine tune the resistance of the buffer layer in their heads, but it may also help to improve the MTJs for denser MRAMs," said NIST physicist Josh Pomeroy.
With every semiconductor generation, engineers shrink devices more and more, which, unfortunately, changes the resistance of insulators--so-called buffer layers--especially when these films go below one nanometer thick, at which point they comprise of only a few atomic layers.
Particularly affected by this phenomenon, according to NIST, are hard-disk recording heads, which are currently caught between having too much resistance and too little--with an unreachable gap in-between. However, NIST claims to have a solution that enables engineers to regain fine-control over the resistance of films no matter how thin, and retarget their resistance anywhere in the gap, thereby improving MTJs for hard-disk heads and MRAMs.
"As device engineers make semiconductors smaller and smaller, there are fewer and fewer tunable properties," said Pomeroy. "We are showing engineers how they can vary resistance anywhere between that of a pure conductor and that of pure insulator by using a composite material that can span the gap that currently exists in between."
The new process directs a controlled beam of xenon +44 ions, each with 50,000 electron volts of potential energy, onto the insulating film, thereby precisely perforating it and lowering its resistance. The process--which typically creates from 1000 to 100,000 tiny holes in the aluminum oxide insulating film in a single hard-disk recording head--yields access to nearly any resistance value between zero and infinity. This gives engineers control over the resistance of films even if they are only a few atomic layers thick.
"We are offering engineers a way of scaling films down to smaller sizes without affecting their resistance by virtue of tuning it with a different process--our ion-pitting process," said Pomeroy. "This means that engineers won't have to redesign all their circuitry every time they shrink down a hard-disk head's size, because we give them a new knob to turn that will compensate for the smaller size."
Today's giant magnetoresistance (GMR) disk drive heads work use a low-resistance metal buffer layer, like copper, between their two magnetic layers--one pinned and the other free, to sense the magnetic pole of a bit. Unfortunately, today's current-in-the-plane heads cannot economically be produced small enough for the latest media. Going to current-perpendicular-to-the-plane in GMR heads is the obvious solution, but that makes the resistance plummet to unusable ranges.
To the rescue are MJT heads, which, instead of a metal buffer layer, use an insulator, such as aluminum oxide, between the two magnetic layers of the head. Unfortunately, here again the resistance of the insulator cannot be finely tuned.
However, using an insulator between the two layers--MTJ--and adjusting the resistance with xenon ion perforations, enables the best of both worlds, according to NIST. By precisely controlling the number of perforations, hard-disk heads with almost any desirable property are said to be produced.
Other groups are attempting to solve this problem by first depositing aluminum and then only partially oxidizing the layer, thus adjusting the resistance in the gap between an insulator and a conductor. However, NIST claims its ion perforation technique should be able to adjust the resistance of insulating films in MTJ heads anywhere in the gap between conductor and insulator.
NIST has a provisional patent on the process (number 60,905,125).