Impact of SizE and StrAin on crystallization of chalcogenides for the ultimate scaling of phase change MEmories – SESAME

Phase Change Random Access Memories (PCRAM), which are based on the reversible amorphous-crystalline transition in phase change materials (PCMs), constitute a very promising alternative to Flash technology, which is reaching fundamental limits. One of their key advantages is their scalability but, for ultimate miniaturization, energy consumption is critical and a promising solution is the geometrical confinement of the memory points. Mastering this with PCMs at ultimate dimensions (typically 5 nm) is, however, a real challenge, which calls for a fundamental understanding of the interplay between strain (the amorphous-to-crystal transition is accompanied by density increase of several %) and interface energies at the nanoscale. The objective of the SESAME project is to study the influence of strain and size on the PCM phase transition at ultimate dimensions. To address these issues we will use advanced in situ characterization techniques applied to ultra-thin layers, confined nanostructures and nanoclusters in order to investigate the early stages of phase transition and also to measure local strains and microstructure changes at crystallization. Five partners with complementary know-how will participate in the project: IM2NP-Marseille, CEA-LETI-Grenoble, CEA-INAC-Grenoble, synchrotron SOLEIL – St Aubin, CINaM-Marseille. The SESAME project will be organized along 5 tasks: 1. Coordination, 2. Sample preparation and characterization, 3. High resolution synchrotron X-ray scattering, 4. Transmission Electron Microscopy (TEM), 5. Simulation. Thin/ultra-thin GeTe and Ge2Sb2Te5 (GST) PCM films and PCM materials in confined structures will be prepared at CEA-LETI. Various thickness (100 to 5 nm), size (down to 10 nm width) and capping materials (Ta, TaN, Ta2O5, SiN, SiO2, Ti, TiN …) will be studied. CEA-INAC has the unique capability of preparing sub-10 nm GeTe and GST clusters by gas phase condensation. This will allow us to address the ultimate sizes, far beyond existing capabilities of lithography. Clusters with different composition or doping will be embedded in matrices with various thermo mechanical properties in order to evaluate the impact of mechanical stress on PCM clusters properties. Preliminary in situ sample characterizations will be performed at CEA: in situ ellipsometry, reflectivity, Raman spectroscopy or four-point-probe resistivity measurements. On these well-characterized samples unique in situ High-resolution synchrotron x-ray scattering and state-of-the-art transmission Electron microscopy (TEM) investigations will be performed. An original combination of resistance, X-ray diffraction and X-ray reflectivity that allows correlating structural and electrical PCM properties upon crystallization has been developed jointly by IM2NP and ESRF and will be used at synchrotron SOLEIL to characterize in situ the phase transition of ultrathin PCMs. Also the in situ combination of X-ray diffraction and optical curvature measurements developed jointly by IM2NP and DiffAbs beamline at SOLEIL will allow for an in-depth understanding of the mechanics involved in the amorphous-to-crystal transition. State-of-the-art TEM performed at CEA-INAC and CEA-LETI will bring valuable knowledge on local distribution of elements, defects and strains. In situ TEM observations during crystallization will offer invaluable information on the nucleation sites for crystallization. It is worth noting that these highly original in situ techniques (based either on TEM or Synchrotron radiation) will be used also to investigate structural changes in the amorphous phase. The issue of resistance drift in the amorphous phase is a key point for the stability of stored information in the memory cell. Atomistic simulations (Density Functional Theory, Molecular Dynamics) will be performed at CINaM in order to simulate the atomistic structure and the properties (structural, electronic, spectroscopic) of phase change materials in amorphous and crystalline form.