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					CEA Grenoble (France) - postdoc position available at LETI-MINATEC (nanoscience &
nanotechnology)

Position :"Electrical Characterization and Physical Modeling of Chalcogenide Materials for
Application to Sub-45nm Embedded Phase-Change Non-Volatile Memories"

Starting date: position open

   Applicants should have a PhD degree in Physics, Electronic Engineering, Chemistry or equivalent
knowledge. Candidates with a strong background in physics, microelectronics, and chemistry are
encouraged to apply.
   A previous experience on Phase Change Memories would be a plus.


Postdoc Topic:

   The general context

    In an effort to overcome the scaling limit of the floating gate non-volatile memory (NVM)
technology below the 30nm design rule, the semiconductor industry has been forced to find alternative
NVM. Phase change random access memory (PCRAM) attracts great interest not only because it
satisfies the various demands for NVM devices but also because its fabrication process is relatively
simple.

   For the digital data storage PCRAM uses the reversible phase change between the crystalline and
amorphous state of chalcogenide materials, such as Ge2Sb2Te5 (GST), by joule heating. The crystalline
GST has a low resistivity and the amorphous GST has a high resistivity, which corresponds to the data
“0” (crystalline) and data “1” (amorphous). In PCRAM cells the reversible switching between these
two states can be achieved by applying a short (~ 50ns) and high current pulse for the transition from
the crystalline to the amorphous state (reset process) and a relatively long (Iset ~ 100ns) and low
current pulse for switching from the amorphous to the crystalline state (set process). Recently, there
have been great advances in PCRAM development, but several reliability issues still remain to be
solved before PCRAM will become a reliable commercial product. Main issues are data retention at
high temperature (150°C), degradation related to back-end process, cycling endurance and disturbance
immunity. Moreover, the high level of Ires has been the major obstacle for further scaling of PCRAM
because of the limited on-current drive capability of the cell transistor (< 1mA/µm).

   To address these issues, a big effort in different research topics is still needed, as the evaluation of
new phase change materials (GST, SbTe, GeSb, Nitrogen doped...), process technology innovations
(PVD, CVD, ...), engineering of various device structures (planar Phase Change Bridge cell and
vertical pillar or µ-trench cells), and device designs. This topic is targeted at LETI/MINATEC with
our nanotechnology facilities.




    Basic PCRAM cell structure and images of amorphous/crystalline states of chalcogenide material
   The postdoc objectives and the local context
  Main objectives of this postdoc position will be the electrical characterization and physical
modeling of chalcogenide materials for application to sub-45nm embedded Phase-Change Non-
Volatile Memories.
   Electrical characterization (write/erase, endurance, data-retention at different temperature,
disturb during cell reading and programming of nearby cells...) on test structures will be performed in
order to put in evidence the main operating/degradation modes and underline the related physical
mechanisms. The performance of the single cell will be thoroughly assessed in terms of minimum
current pulse coupled to the quenching speed to obtain an amorphous cell, data retention at high
temperature and endurance. Several splits concerning the chalgogenide as well as the heater material
on single cells will be analyzed.
   The electrical characterization will be performed also on arrays of cells to address the disturb
immunity of these cells fabricated with extremely aggressive pitch.

    The postdoc will be involved in a detailed experimental work, but he will have also to face the
theoretical principles governing the functionality of a Phase-Change Memory. In particular, the
obtained experimental data will be coupled to detailed physical modeling of the chalcogenide
materials integrated in the test structure, considering the electrical & thermal dynamics governing
the phase change process of PCRAM devices. This task will be performed by means of commercial
software as well as developing in-house semi-analytical physical modeling. The factors affecting
temperature distribution of a PCRAM cell structure such as geometry, device structure and the
properties of the individual materials will be investigated. Final result will be the physical
understanding of the developed materials through suitable parameters and the fundamental design of
robust cell structures with improved performance and reliability. Finally, a SPICE modeling approach
will be also considered, so that the electrical device properties and operation will be quickly and
reliably evaluated by circuit designers.

    The postdoc will benefit of in-depth morphological analyses of chalcogenide materials performed
in the CEA lab (ie X-ray fluorescence will be used to characterize the chemical composition of the
materials. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and advanced Transmission
Electron Microscopy (TEM), x-ray photoelectron spectroscopy (XPS) will be used to characterize the
structural and bonding properties of chalcogenide materials) obtained with different process
technologies in order to put in evidence the main properties of the materials.
    The postdoc will interact in strict correlation with the material engineering group to obtain properly
tailored chalcogenide materials integrated in test device structures (from simple chalcogenide
storage elements, to individual cells including the storage element and driving device to arrays of
cells).

The work will be performed in LETI-MINATEC (micro and nanoelectronics laboratories
LSCDP and LNDE)

                    For further information about the position, please contact:

L. Perniola                                          V. Sousa
17, rue des Martyrs, 38054 Grenoble                  17, rue des Martyrs, 38054 Grenoble
Tel. : 04.38.78.10.72, Fax. : 04.38.78.51.40         Tel. : 04.38.78.43.07
e-mail : luca.perniola@cea.fr                        e-mail : veronique.sousa@cea.fr

                                                     B. De Salvo
                                                     17, rue des Martyrs, 38054 Grenoble
                                                     Tel. : 04.38.78.64.97, Fax. : 04.38.78.94.56
                                                     e-mail : barbara.desalvo@cea.fr