Interface Characterization of Metals and Metal-nitrides to Phase Change Materials
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Interface Characterization of Metals and Metal-nitrides to Phase Change Materials Deepu Roy1, Dirk J. Gravesteijn1 and Rob A. M. Wolters1,2 1 NXP Semiconductors, High Tech Campus 4, 5656AE, Eindhoven, The Netherlands E-mail: [email protected], phone: +31(0)402726872. 2 MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands. ABSTRACT We have investigated the interfacial contact properties of the CMOS compatible electrode materials W, TiW, Ta, TaN and TiN to doped-Sb2Te phase change material (PCM). This interface is characterized both in the amorphous and in the crystalline state of the doped-Sb2Te. The electrical nature of the interface is characterized by contact resistance measurements and is expressed in terms of specific interfacial contact resistance (ρC). These measurements are performed on four-terminal Kelvin Resistor test structures. Knowledge of the ρC is useful for selection of the electrode in the integration and optimization of the phase change memory cells. INTRODUCTION Embedded Phase Change Random Access Memory (ePCRAM) technology is a one transistor one resistor memory concept [1]. The resistive element is a thin PCM layer which can be switched between amorphous and crystalline states. The large resistance contrast between these two states is utilized to achieve the memory functionality. The total resistance of a ePCRAM line cell in the current path includes the resistance of the switching part of the line (RL) and the metal-to-crystalline PCM contact resistance (RC). For efficient cell switching at maximum power (reset pulse) the total resistance of the memory cell should match the impedance of the accompanying transistor, where RC acts as a parasitic resistance. The larger RC, the more the power will be dissipated at the contacts and the cell will be less efficient during switching. As the memory cell size should scale with the technology node and the relative contribution of the contacts to the cell resistance will increase. In addition, for the same technology node, the metal electrode at the contact also influences the contact resistance. Although important in switching and scaling of phase change memories, very little attempts have been performed to describe the metal-to-PCM contact properties [2]. The phase change material of choice suitable for a line cell is a fast-growth material like doped-Sb2Te. In this work the metal to doped-Sb2Te contact resistance is studied for different CMOS compatible electrodes materials W, TiW, Ta, TaN and TiN. This is done by contact resistance measurements on Kelvin resistor structures fabricated with these different electrodes. From these measurements ρC is extracted both in the amorphous and crystalline states of the PCM. The metal to PCM interface properties are described with a standard metal semiconductor model.
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EXPERIMENT The Kelvin resistor structure is a planar test structure suitable for measurement of the interfacial contact resistance with minimum parasitic resistance interference [3]. The schematic of a Kelvin resi
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