E*PCOS2007 X-ray diffraction studies of the crystallization of phase change nanoparticles produced by self-assembly-based techniques Simone Raoux 1 , Yuan Zhang 2 , Delia Milliron 1 , Jennifer Cha 1 , Marissa Caldwell 2 , Charles T. Rettner 1 , Jean L. Jordan-Sweet 3 , and H.-S. Philip Wong 2 1 IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120 2 Department of Electrical Engineering and Center for Integrated Systems, Stanford University, 420 Via Palou, Stanford, CA 94305 3 IBM T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY 10598 e-mail: simone_raoux@almaden.ibm.com ABSTRACT The crystallization behavior of phase change nanoparticles can provide useful insight into the scaling properties of phase change materials and of related memory devices. While such nanoparticles can be fabricated using electron-beam lithography, this method is expensive and time-consuming. We have therefore developed fabrication techniques based on self-assembly-based lithographic processes. We have successfully made three types of phase change nanoparticle samples, and have applied time-resolved X-ray diffraction to study their crystallization behavior. Two kinds of self-assembling diblock-copolymers were applied to pattern the phase change materials. In one case self-assembled PS-b-P4VP (polystyrene-b-polyvinylpyridine) was formed on top of an amorphous GeSb phase change film and was used to locally grow SiO 2 dots on top of the P4VP domains. The SiO 2 dots then served as a hard mask to transfer the pattern into the GeSb film using reactive ion etching and ion milling, and isolated 15 nm diameter dots were formed. It was found that these GeSb nanoparticles have a crystallization temperature that is 15ºC lower than comparable blanket film. In the second case cylindrical-phase PS-b- PMMA (polystyrene-b-poly(methylmethacrylate)) films were used. After removal of the PMMA domains, AgInSbTe phase change material was deposited by sputtering using a substrate rf bias and a collimator for better conformality. After lift-off of the PS, isolated AgInSbTe nanoparticles of about 20nm diameter were obtained that showed a crystallization temperature of 175ºC, slightly higher than blanket film (165ºC). The same PS-b-PMMA template was used to deposit newly-developed spin-on phase change material. A GeSeSb precursor was synthesized by dissolving GeSe and Sb 2 Se 3 in hydrazine in the presence of additional elemental selenium. The template was filled with precursor by spin casting, then annealed to form an array of ~20 nm diameter GeSbSe nanoparticles. Finally, the PS template was dissolved to leave the nanoparticles. These arrays crystallized at about 215°C which is 35 °C lower than a blanket film. All of these experiments confirm that phase change devices should scale to well below 20nm. Key words: phase change nanoparticles, scaling 1. INTRODUCTION Phase change random access memory (PCRAM) is a promising technology that applies phase change materials, which are successfully used in optical storage in re-writable CDs and DVDs, to solid state memory technology. While in optical storage the large change in reflectivity between the amorphous and crystalline phase of these materials is used to store the information, in PCRAM the large (several orders of magnitude) change in resistivity is applied. The state of the PCRAM cell can be switched using current pulses. If the cell is in the high-resistive OFF state a current pulse is applied that heats the phase change material above its crystallization temperature for a long enough duration to transform it into the crystalline state (SET operation). If the cell is in