  Oxide-based Random Access Memory (OxRAM), is part of the larger family of
Resistive RAM (RRAM) memories. Generally OxRAM cells consist of a transition
metal oxide (typically HfO2, Ta2O5, TiO2) sandwiched between two metal
electrodes (typically TiN, TaN, W but also Pt, Ir). This thesis describes the
experimental investigation performed on OxRAM memories during a 6-months
internship project at imec research institute (Leuven, Belgium). From previous
studies, HfO-based OxRAM memories showed good endurance properties (with more
than 10e9 write cycles), low operating current (as low as 10 uA), and very fast
switching (programming pulses of 100 ns). However, it has been observed that,
under low programming current condition, data stability becomes challenging.
Therefore, this programming state stability needed further investigation. This
thesis addresses the problem of state stability, first, by presenting the
equipment and algorithms used to study OxRAM state loss over short time
intervals. Secondly, various experiments and tests are performed searching for
a key parameter in order to control memory retention. Finally, this work
demonstrates that resistance evolution over time is affected by both a
determinist drift (logarithmically dependent on time) and a random stochastic
fluctuation component. It also demonstrates that this behavior is intrinsic in
the nature of the device itself and does not depend on processing issues or
variation in the programming conditions. This experimental study proved the
intrinsic nature of the program instability phenomena in OxRAM devices, which
in turns leads to the ineffectiveness of verify algorithm for low current
operation.
