  A new approach to increase the downsize scalability of perpendicular STT-MRAM
is presented. It consists in significantly increasing the thickness of the
storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to
conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in
this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer
on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the
thickness of the storage layer becomes of the order or larger than the diameter
of the MTJ pillar. In contrast to conventional spin transfer torque magnetic
random access memory (STT-MRAM) wherein the demagnetizing energy opposes the
interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory
cells, both PSA and iPMA contributions favor out-of-plane orientation of the
storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAM
has several advantages. Thanks to the PSA, very high and easily tunable thermal
stability factors can be achieved, even down to sub-10 nm diameters. Moreover,
low damping material can be used for the thick FM material thus leading to a
reduction of the write current. The paper describes this new PSA-STT-MRAM
concept, practical realization of such memory arrays, magnetic characterization
demonstrating thermal stability factor above 200 for MTJs as small as 8nm in
diameter and possibility to maintain thermal stability factor above 60 down to
4nm diameter.
