SET Characteristics of Phase Change Bridge Devices
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1072-G06-07
SET Characteristics of Phase Change Bridge Devices Daniel Krebs1,2, Simone Raoux2, Charles T. Rettner2, Robert M. Shelby2, Geoffrey W. Burr2, and Matthias Wuttig1 1 IBM - Macronix PCRAM Joint Project: I. Institute of Physics, RWTH Aachen University, Sommerfeldstraße 14, Aachen, 52074, Germany 2 IBM - Macronix PCRAM Joint Project: IBM Almaden Research Center, 650 Harry Road, San Jose, CA, 95120 ABSTRACT Scaling studies have demonstrated that Phase Change Random Access Memory (PCRAM) is one of the most promising candidates for future non-volatile memory applications. The search for suitable phase change materials with optimized properties is therefore actively pursuit. In this paper, SET (crystallization) characteristics of an ultra fast switching material Ge15Sb85 in phase change memory bridge cell devices are presented. It was found that reproducible switching between two stable states with one decade resistance contrast and current pulses as short as 10 ns for SET and RESET (reamorphization) operation is possible. Particular emphasis was placed on the difference in crystallization kinetics between the as-deposited and melt-quenched amorphous phase. Evidence is given for the existence of an electrical field as the critical parameter for threshold switching rather than a threshold voltage. For Ge15Sb85 a threshold switching field of 9MV/m was measured and it was shown that switching from the melt-quenched amorphous phase to the crystalline phase is about 600 times faster than crystallization from the as-deposited amorphous phase. INTRODUCTION Reversible switching between the crystalline and amorphous phase was first reported by Ovshinsky in 1968 [1]. It lead to the technological use of phase change materials in optical data storage after Yamada and coworkers [2] discovered fast switching materials that are based on the Sb2Te3-GeTe pseudo-binary system and exhibit significant optical contrast. The contrast in resistance of several orders of magnitude in phase change materials is used in electrical data storage. Favorable properties like good scalability, fast programming speed, low power consumption, threshold switching effect, and high endurance make PCRAM one of the most promising candidates to replace flash memory as the next-generation non-volatile memory. In this paper crucial SET characteristics are discussed on the basis of Ge15Sb85. The SET operation is the crystallization of the amorphous regions of a PCRAM cell thus switching it to the low resistivity state (SET state). EXPERIMENT Bridge cell devices with two TiN bottom electrodes and Ge15Sb85 as the active material were produced by electron beam lithography and magnetron sputtering from compound targets resulting in phase change memory cells with a thickness of 30 nm,
lengths ranging from 0 nm to 510 nm and different widths between 15 nm and 145 nm [3]. SET and RESET experiments were performed by applying voltage pulses to the bridge cell devices using a Tektronix AWG520 Arbitrary Waveform Generator (AWG), while a 500 Ω series resistor was us
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