Electron-Beam Lithographic Studies of the Scaling of Phase-Change Memory
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Lithographic Studies of the Scaling of Phase-Change Memory Simone Raoux, Charles T. Rettner, Yi-Chou Chen, and Geoffrey W. Burr
Abstract Phase-change random-access memory (PCRAM) is a promising technology for future nonvolatile storage with the added potential for possible impact on dynamic random-access memory technologies. To be successful, however, PCRAM must be able to scale to dimensions on the order of a few tens of nanometers, considering the increasingly tiny memory cells that are projected for future technology nodes. The experiments discussed in this article directly address these scaling properties, examining both the materials themselves and the operation of nanoscale devices. One series of experiments is time-resolved x-ray diffraction studies of ultrathin films and nanostructures. Electron-beam lithography was applied to pattern thin films into nanostructures with dimensions down to 20 nm. The article also includes descriptions of prototype PCRAM devices, successfully fabricated and tested down to phase-change material cross sections of 3 nm × 20 nm. The measurements provide a clear demonstration of the excellent scaling potential offered by this technology.
Introduction Phase-change materials generally have two phases with remarkably different properties. The 30% or so change in reflectivity between phases has been exploited for many years in optical recording. However, the change in electrical resistivity is far more remarkable, reaching as much as five orders of magnitude in some materials. The basic principles of using this resistance change in a memory device have been known for over 40 years.1 Only recently has the community recognized the potential of using these materials in crosspoint memory devices to compete with and replace existing solid-state nonvolatile devices. The renewed interest in phase-change random-access memory (PCRAM) technology was triggered by the discovery of fast-crystallizing materials such as Ge2Sb2Te5 (GST) and Ag- and
In-doped Sb2Te.2,3 These materials can crystallize in less than 100 ns, as opposed to the materials used for early PCRAM demonstrations such as Te48As30Si12Ge10, which could require 10 µs or more to crystallize.1 These newer alloys switch more rapidly because the crystallization process generates very little atomic motion,4 as compared to the phase segregation that occurred in the earlier, Te-rich alloys.5 When data are written to a PCRAM element, the phase-change material is cycled between a highly conductive crystalline state and a resistive amorphous state. The amorphous state can be crystallized by heating it to a temperature above its crystallization temperature (a so-called SET operation) and returned to the amorphous state by melt-quenching (in a RESET operation). Both SET and RESET operations
MRS BULLETIN • VOLUME 33 • SEPTEMBER 2008 • www.mrs.org/bulletin
are controlled by electrical current: highpower pulses place the memory cell into the high-resistance RESET state, and moderate power but longer duration pulses return the cell to the low-re
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