Overview of Phase-Change Chalcogenide Nonvolatile Memory Technology
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Overview of
Phase-Change Chalcogenide Nonvolatile Memory Technology S. Hudgens and B. Johnson
Abstract Phase-change nonvolatile semiconductor memory technology is based on an electrically initiated, reversible rapid amorphous-to-crystalline phase-change process in multicomponent chalcogenide alloy materials similar to those used in rewriteable optical disks. Long cycle life, low programming energy, and excellent scaling characteristics are advantages that make phase-change semiconductor memory a promising candidate to replace flash memory in future applications. Phase-change technology is being commercialized by a number of semiconductor manufacturers. Fundamental processes in phase-change semiconductor memory devices, device performance characteristics, and progress toward commercialization of the technology are reviewed. Keywords: chalcogenides, nonvolatile memory, phase change.
Introduction The use of phase-change chalcogenide alloy films to store data electrically and optically was first reported in 19681 and in 1972,2 respectively. Early phase-change memory devices used tellurium-rich, multicomponent chalcogenide alloys with a typical composition of Te81Ge15Sb2S2. Both the optical and electrical memory devices were programmed by application of an energy pulse of appropriate magnitude and duration. A short pulse of energy was used to melt the material, which was then allowed to cool quickly enough to “freeze in” the glassy, structurally disordered state. To reverse the process, a somewhat loweramplitude, longer-duration pulse was used to heat a previously vitrified region of the alloy to a temperature below the melting point, at which crystallization could occur rapidly, as shown in Figure 1. Differences in electrical resistivity and the optical constants between the amorphous and polycrystalline phases were used to store data.
MRS BULLETIN/NOVEMBER 2004
During the 1970s and 1980s, significant research efforts by many industrial and academic groups were focused on understanding the fundamental properties of chalcogenide alloy amorphous semiconductors.3,4 Prototype optical memory disks and electronic memory device arrays also were announced,5,6 beginning in the early 1970s. Rapidly crystallizing chalcogenide alloys were later reported7–10 by several optical memory research groups. These new material compositions, derived from the Ge-Te-Sb ternary system, did not phasesegregate upon crystallization like the earlier Te-rich alloys, but instead exhibited congruent crystallization11 with no largescale atomic motion. In the 1990s, researchers at Energy Conversion Devices Inc. and Ovonyx Inc. developed new, thermally optimized phasechange memory device structures that exploited rapidly crystallizing chalcogenide alloy materials to achieve increased pro-
gramming speed and reduced programming current.12,13 These devices14 could be programmed in 20 ns—about six orders of magnitude faster than the early phasechange memory cells, and their much lower programming current requirements permitted the design of memory ar
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