Oct. 10, 1997 BERKELEY, CA -- With help from Ernest Orlando Lawrence Berkeley National Laboratory researchers, the storage capacity of your computer's hard drive is about to advance dramatically.
Simone Anders of the Accelerator and Fusion Research Division at Berkeley Lab and her colleagues from IBM and UC Berkeley have found a way to shield disks and sliders, or reader heads, with ultra-thin "overcoats" of diamond-like carbon that can survive repeated crash landings at 3600 rpm. IBM has already brought to market disks that store 2.64 gigabytes of data per square inch; densities almost twice that have been demonstrated, and researchers are aiming for 10 gigabytes per square inch and more. To read a disk where magnetic domains are packed only 25 nanometers apart, disk surface and slider will have to move so close to each other that it's almost a matter of semantics whether they will actually be touching. Typical high-quality commercial overcoats now in use are made of sputtered-on, hydrogenated carbon 12 to 15 nanometers thick. Higher data densities 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 other hard coatings on the cutting edges of their tools using a technique called cathodic arc deposition," says Anders. Unlike sputtering -- in which the coating material is knocked off the cathode of the plasma source by ions, forming a plasma mixed with un-ionized (electrically neutral) atoms -- cathodic arcs produce a fully ionized plasma of whatever material, including carbon, is used for the cathode. "Since the 1970s it has been known that carbon deposited this way is almost as hard as diamond," Anders notes. A fully ionized carbon plasma allows electrons and carbon nuclei to reassemble themselves as diamond, in a three-dimensional lattice in which each atom is bound to four others by electron pairs -- a tetrahedral bond. By contrast, atoms in graphite are bound to only three other atoms, forming a much less stable configuration. By tuning the energy of the incoming carbon ions, the tetrahedral-bond content of the deposited film can be optimized; thus films have been made that, while technically amorphous, are 85 percent diamond.
"Still, the method hasn't been practical for coating disks," says Anders, "because micron-sized chunks of the cathode boil off and contaminate the films." A micron-sized macroparticle in a nanometer- scale overcoat is like a mountain in a mud puddle, a thousand times bigger. For cathodic arc deposition to be useful in coating disks and sliders, a way must be found to completely filter out the macroparticles.
"What our team has done is to devise a filter so good that all our goals" -- of thin, flat, hard, macroparticle-free carbon -- "were fulfilled," Anders says. The secret is a magnetic coil that looks much like a Slinky toy, placed between the plasma source and the substrate to be coated. The fully ionized plasma is easily bent through this S-shaped magnetic field -- effectively two fields at right angles -- but the massive macroparticles of carbon can't turn easily; they fly right through the sides of the coil or pile up on its walls. A coil that has been used for some time is thickly coated with a dust of macroparticles near the plasma source, yet dust-free at the substrate end.
After perfecting the filtering method, Anders and her colleagues performed a series of brutal endurance tests, pitting disks with cathodic arc-deposited carbon coatings against samples with sputtered, hydrogenated carbon coats.
They found that disks coated with cathodic arc carbon had a coefficient of friction half that of those coated with hydrogenated carbon and caused 20 times less wear on the slider. In additional studies, when a silicon wafer coated with cathodic arc carbon was examined at nanometer scale after repeated loading, it showed virtually no scratches. Slider tests were equally dramatic. Weighted sliders were repeatedly set down on spinning disks coated with hydrogenated carbon. As could be expected, uncoated sliders failed after only 7500 cycles -- they blew up and dug trenches in the disks -- but sliders coated with cathodic arc carbon were still going strong after 100,000 cycles, with no visible wear on the disk. The team announced these spectacular results in July, "and we've received lots of requests for information," says Anders. "We are quite hopeful that cathodic arc-deposited carbon will find wide use in industry."
Details and results of the cathodic-arc carbon deposition and filtering system will be published in the October edition of Data Storage magazine. Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
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