Nanometer Resolution XANES Imaging of Individual PC-RAM Devices
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Nanometer Resolution XANES Imaging of Individual PC-RAM Devices Jan H. Richter1,2, Milos Krbal1,2, Alexander V. Kolobov1,2,3, Paul Fons1,2,3, Xiaomin Wang1,2, Kirill V. Mitrofanov1, Robert E. Simpson1,2, Junji Tominaga1,2, Hitoshi Osawa3 and Motohiro Suzuki3 1Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, 305-8562, Ibaraki, Japan 2Collaborative Research Team Green Nanoelectronics Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, 305-8562, Ibaraki, Japan 3SPring-8, Japan Synchrotron Radiation Institute (JASRI), Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ABSTRACT We introduce a technique to permit x-ray absorption spectroscopy studies focusing on individual phase-change (Ge2Sb2Te5) memory cells in fully integrated PC-RAM structures. Devices were investigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell and enabled us to investigate individual devices without interference from non-switching material surrounding the area of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon energy corresponding to the Ge K-edge absorption edge white line position, we were successful in producing 2D area maps of the active cell region, which clearly show the imbedded tungsten heater element and the switched region of the phase change material. Additionally, position dependent changes in the phase change material could be traced by taking an array of XANES spectra at the Ge K-edge on and in the vicinity of individual devices. INTRODUCTION Due to the continuous development in consumer electronics towards increased performance and smaller device sizes, Si based flash will soon reach its physical limit as a memory material. The unsatiated demand for higher density memory material will ultimately lead to the emergence of new types of memory storage systems based upon novel materials. Phase-change materials are a promising candidate for fulfilling this role [1] and have already successfully been used for years in optical storage technology (DVD and Blu-ray). Phase-change random-access memory (PCRAM) utilizes a different data storage mechanism from Si technology with inherent scaling ability to a much lower ultimate scaling limit [2, 3]. In PCRAM materials, information is stored in the local structure of the material as opposed Si based technology, where the method of information storage is electronic charge trapping [4]. By injecting energy into the system, be it by laser or electrical pulses, a reversible and stable transition between amorphous and crystalline states can be induced [5, 6]. Originating from the atomic scale structural differences the two phases display large optical and electrical differences [7], with the crystalline phase usually possessing higher refractive index, optical absorption and electrical conductivity. For phase change memory s
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