Phase Change Memories challenges: a material and process perspective S. Maitrejean 1 , G. Ghezzi 1,2 , E. Gourvest 1,3,4 , G. Betti Beneventi 1 , A. Fantini 1 , N. Pashkov 1 , G. Navarro 1 , A. Roule 1 , F. Fillot 1 , P. Noé 1 , S. Lhostis 4 , O. Cueto 1 , C. Jahan 1 , JF Nodin 1 , A. Persico 1 , M. Armand 1 , L. Dussault 3 , C. Vallé 3 , Ph. Michallon 1 , R. Morel 5 , A. Brenac 5 , M. Audier 2 , JY Raty 6 , F. Hippert 2 , L. Perniola 1 , V. Sousa 1 , B. de Salvo 1 1 CEA, LETI, MINATEC Campus, 17 rue des Martyrs, 38054 Grenoble, France 2 Laboratoire des Matériaux et du Génie Physique (Grenoble-INP, CNRS), Minatec, 3 Parvis L. Neel, 38016 Grenoble, France 3 LTM-Leti, Minatec Campus, F38054 Grenoble, France 4 STMicroelectronics, 38926 Crolles, France 5 INAC/SP2M and Université Joseph Fourier, CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble, France 6 Physics Department B5, University of Liège, B-4000 Sart-Tilman, Belgium ABSTRACT Among all the new memories concepts, Phase Change Memories (PCM) is one of the most promising. However, various challenges remain. This paper reviews the materials and processes required to face these challenges. As an example, attention will be made on the effect of Phase change material composition on stability of the amorphous phase i.e. on the retention of the information. Additionally, it is showed how specific processes such as CVD or ALD can be developed in order to minimize the current required to amorphize the phase change material i.e. to reset the device. Finally, with the perspectives of the advanced integration nodes, experimental results on the effect of scaling on phase transformation are presented and discussed. INTRODUCTION As stated in various technological review papers [1-3], new concepts are required to face the scaling challenges of advanced memories. Phase Change Memories (PCM) rely on the large resistivity variation that occurs during the reversible amorphous-to-crystalline phase transition of chalcogenide material such as GeTe or GeSbTe (GST). The memory cell is a simple resistor as illustrated in the insert of Fig. 1. Current pulses with various intensities are used to switch the material from one phase to the other. Information is read thanks to the resistance variation. During the past 10 years, PCM attracts large attention thanks to (i) their maturity, i.e. industrial prototypes have been released, and (ii) their amazing performances. They combine scalability, high speed, high cyclability and non-volatility [4]. Nevertheless, numerous challenges remain. Indeed, a switching PCRAM module requires: - A high retention time at elevated temperature. This implies a high thermal stability of the amorphous phase. - A high transformation speed with a low pulse current in order to minimize driving transistor foot print i.e. memory cell footprint. - A high cyclability of the cell. Thereby, reversible phase transformation between amorphous and crystalline phase are mandatory. - A high scalability. The switching properties have to be maintained at dimensions well below tenth of nm. In the following parts, we will review the materials and processes required facing these challenges. Fig. 1: Main, Simulation of the effect of confinement on PCM cell resistance switching (from crystal state to amorphous). Insert, schematic cross section of plug and confined cell. [14] PHASE CHANGE MATERIAL ENGINEERING Two main axes have been explored in order to increase the stability of the amorphous phase and consequently the ability of the cell to retain information. One can change or adjust the film composition: compared to classical Ge 2 Sb 2 Te 5 , the binary compound GeTe demonstrates higher retention time [5]. If you explore non- stoichiometric composition, you can further increase crystallization temperature and make functional devices [6]. However, during crystallization, you may have precipitation that may compromise the device reliability during cycling [7, 8]. It is worth noting that usually, a tradeoff between speed and data retention needs to be made. Recently, the demonstration of a “golden composition” of GST film was announced. [9] 300 250 200 150 100 20 22 24 26 28 30 32 34 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 101 °C E a = 3.0 eV 87 °C E a = 2.9 eV 124 °C E a = 2.26 eV 154 °C E a = 3.1 eV Fail time [s] 1/kT [eV -1 ] GST225 GeTe GeTeN 2% GeTeN 4% (b) Temperature [°C] Fig. 2: Effect of doping element on retention time. Arrehenius extrapolation at 10 years for GST, GeTe and GeTeN devices: GeTeN2% extrapolated fail temperature after 10 years is 154 °C. [10] 978-1-4673-1137-3/12/$31.00 ©2012 IEEE