Crystallization Behavior of Ge-Rich Ge-Sb-Te Thin Films for Embedded Memory Applications—Insights from Synchrotron X-Ray Scattering

Phase-Change Memory is a very promising non-volatile memory that is being considered by several companies for a wide range of applications (storage-class memory, in-memory computing, neuromorphic computing, eNVM for microcontrollers …). At STMicroelectronics a new Ge-rich Ge-Sb-Te alloy (GGST) has been developed with a crystallization temperature above 350°C [1] for addressing the specific needs of the automotive market where high operating temperatures are needed. Although this new alloy meets the requirements of the industry (robustness, stability at elevated temperatures, endurance…) an in-depth understanding of the materials physics of the memory cell at the nanoscale is needed for technology evolution as well as future downscaling. Among important issues it is important to underline that GGST composition does not correspond to a single crystal phase, which implies that crystallization is accompanied by phase separation (Ge followed by Ge₂Sb₂Te₅) as observed experimentally [2]. This has important consequences regarding elemental segregation during the operation of memory cells.<br/>The high X-ray photon flux provided by synchrotron facilities allow investigating <i>in situ </i>ultra thin (50 nm and below) GGST thin films during crystallization either using X-ray diffraction (XRD) or X-ray fluorescence (XRF). <i>In situ</i> XRD experiments [2-4] performed at DiffAbs beamline (SOLEIL) provide detailed information about phase change kinetics, stress and texture evolution as a function of film thickness or nature of underlayer. XRF microscopy [5] allows monitoring elemental segregation during the phase transformation (ID16B beamline at ESRF) and its dependance on doping. Finally, laser induced crystallization as well as time-resolved pump probe dynamics of Ge and GST have been investigated at ID09 beamline (ESRF). These latter experiments help bridging the huge time gap between furnace annealing experiment and memory cells operation (more than 10 orders of magnitude). These X-ray scattering results bear important consequences for the understanding of the crystallization process in memory cells and demonstrate the usefulness of synchrotron radiation for investigating these complex materials.<br/><br/><b>Acknowledgments:</b><br/>The authors gratefully acknowledge the ESRF and SOLEIL synchrotrons for allocating beam time. This research is supported by IPCEI/Nano 2022 and Nano 2025 programs.<br/><br/><b>References:</b><br/>[1] P. Zuliani <i>et al.</i>, Solid State Electronics 111, 27 (2015).<br/>[2] O. Thomas <i>et al</i>., Microelectronic Engineering 244, 11573 (2021).<br/>[3] P. Hans <i>et al</i>., Phys. Satus Solidi RRL 2100658 (2022).<br/>[4] P. Hans <i>et al</i>., J. Appl. Phys. 134, 105102 (2023).<br/>[5] T. Fernandes<i> et al</i>., Phys. Satus Solidi RRL (2024). doi: 10.1002/pssr.202300408.