BERKELEY, CA -- With help from Ernest Orlando Lawrence BerkeleyNational Laboratory researchers, the storage capacity of your computer'shard drive is about to advance dramatically.
Simone Anders of the Accelerator and Fusion Research Division atBerkeley Lab and her colleagues from IBM and UC Berkeley have found away to shield disks and sliders, or reader heads, with ultra-thin"overcoats" of diamond-like carbon that can survive repeated crashlandings at 3600 rpm. IBM has already brought to market disks that store 2.64 gigabytes ofdata per square inch; densities almost twice that have beendemonstrated, and researchers are aiming for 10 gigabytes per squareinch and more. To read a disk where magnetic domains are packed only 25nanometers apart, disk surface and slider will have to move so close toeach other that it's almost a matter of semantics whether they willactually be touching. Typical high-quality commercial overcoats now in use are made ofsputtered-on, hydrogenated carbon 12 to 15 nanometers thick. Higher datadensities require reduced magnetic spacing between heads and disks --thus disk coatings must be thinner and made of even harder material.Sputtering can't do the job.
"For decades, tool manufacturers have put titanium nitride and otherhard coatings on the cutting edges of their tools using a techniquecalled cathodic arc deposition," says Anders. Unlike sputtering -- inwhich the coating material is knocked off the cathode of the plasmasource by ions, forming a plasma mixed with un-ionized (electricallyneutral) atoms -- cathodic arcs produce a fully ionized plasma ofwhatever material, including carbon, is used for the cathode. "Since the1970s it has been known that carbon deposited this way is almost as hardas diamond," Anders notes. A fully ionized carbon plasma allows electrons and carbon nuclei toreassemble themselves as diamond, in a three-dimensional lattice inwhich each atom is bound to four others by electron pairs -- atetrahedral bond. By contrast, atoms in graphite are bound to only threeother atoms, forming a much less stable configuration. By tuning theenergy of the incoming carbon ions, the tetrahedral-bond content of thedeposited film can be optimized; thus films have been made that, whiletechnically amorphous, are 85 percent diamond.
"Still, the method hasn't been practical for coating disks," saysAnders, "because micron-sized chunks of the cathode boil off andcontaminate the films." A micron-sized macroparticle in a nanometer-scale overcoat is like a mountain in a mud puddle, a thousand timesbigger. For cathodic arc deposition to be useful in coating disks andsliders, a way must be found to completely filter out themacroparticles.
"What our team has done is to devise a filter so good that all ourgoals" -- of thin, flat, hard, macroparticle-free carbon -- "werefulfilled," Anders says. The secret is a magnetic coil that looks muchlike a Slinky toy, placed between the plasma source and the substrate tobe coated. The fully ionized plasma is easily bent through this S-shapedmagnetic field -- effectively two fields at right angles -- but themassive macroparticles of carbon can't turn easily; they fly rightthrough the sides of the coil or pile up on its walls. A coil that hasbeen used for some time is thickly coated with a dust of macroparticlesnear the plasma source, yet dust-free at the substrate end.
After perfecting the filtering method, Anders and her colleaguesperformed a series of brutal endurance tests, pitting disks withcathodic arc-deposited carbon coatings against samples with sputtered,hydrogenated carbon coats.
They found that disks coated with cathodic arc carbon had acoefficient of friction half that of those coated with hydrogenatedcarbon and caused 20 times less wear on the slider. In additionalstudies, when a silicon wafer coated with cathodic arc carbon wasexamined at nanometer scale after repeated loading, it showed virtuallyno scratches. Slider tests were equally dramatic. Weighted sliders were repeatedlyset down on spinning disks coated with hydrogenated carbon. As could beexpected, uncoated sliders failed after only 7500 cycles -- they blew upand dug trenches in the disks -- but sliders coated with cathodic arccarbon were still going strong after 100,000 cycles, with no visiblewear on the disk. The team announced these spectacular results in July, "and we'vereceived lots of requests for information," says Anders. "We are quitehopeful that cathodic arc-deposited carbon will find wide use inindustry."
Details and results of the cathodic-arc carbon deposition andfiltering system will be published in the October edition of DataStorage magazine. Berkeley Lab is a U.S. Department of Energy national laboratorylocated in Berkeley, California. It conducts unclassified scientificresearch and is managed by the University of California.
Materials provided by Lawrence Berkeley National Laboratory. Note: Content may be edited for style and length.
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