Characteristics of OUM Phase Change Materials and Devices for High Density Nonvolatile Commodity and Embedded Memory App
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Characteristics of OUM Phase Change Materials and Devices for High Density Nonvolatile Commodity and Embedded Memory Applications Tyler A. Lowrey, Stephen J. Hudgens, Wally Czubatyj, Charles H. Dennison, Sergey A. Kostylev, and Guy C. Wicker Ovonyx, Inc., 1030 E. El Camino Real #276, Santa Clara, CA 94087 Tel: 408-653-9742 Fax: 408-653-5244 Email: [email protected] ABSTRACT Phase change memory devices were originally reported by S. R. Ovshinsky [1] in 1968. A 256-bit phase-change memory array based on chalcogenide materials was reported in 1970 [2]. Recent advances in phase change materials, memory device designs, and process technology have resulted in significant advances in phase change device performance, and a new memory device, called Ovonic Unified Memory (OUM), has been developed. This paper will discuss various device and materials characteristics of OUM phase change memory materials of interest in applications for nonvolatile high-density memories. These materials are generally Te chalcogenide based, exploiting the congruent crystallization of the FCC phase and the associated reduction in resistivity that results from crystallization from the quenched amorphous state. Data storage is a thermally initiated, rapid, reversible structural phase change in the film. While rewriteable DVD disks employ laser heat to induce the phase change and modulate reflectivity, OUM technology uses a short electrical current pulse to modulate resistivity. The device geometry and thermal environment dictate the power and energy required for memory state programming. We will review the device structure, characterization data, device performance, and reliability as well as device modeling results. Characteristics of key material and device performance metrics such as crystallization and amorphization programming speed will be presented. Programming algorithms will be discussed for optimizing operating margin for high-density applications. Data retention characteristics will be reviewed along with means for accelerated statistical evaluation. Potential high-density memory array data disturb issues will be identified, and their impact on scalability will be assessed. Scaling attributes and issues will be reviewed. INTRODUCTION OUM is a random access memory with read/write for a user addressed bit, byte, or word without the requirement for Flash-like block erase. Low voltage and energy read/write requirements are compatible with CMOS feature- and power supply scaling and attractive for mobile and wireless applications. The ultra-small volume of phase change material programmed minimizes the power and energy needed for heating to produce the phase transition. The chalcogenide phase change alloys in OUM contain at least one element from Group VI in the periodic table: Te, Se, or S. Early phase change devices were based on materials containing substantial amounts of Te, which phase segregated into a “filament like” electrically conductive region between two contacts. These devices were inherently slow and power hungry, taking
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