  In view of the high luminosity phase of the LHC (HL-LHC) to start operation
around 2026, a major upgrade of the tracker system for the ATLAS experiment is
in preparation. The expected neutron equivalent fluence of up to 2.4 * 1e16 1
MeV neq./cm2 at the innermost layer of the pixel detector poses the most severe
challenge. Thanks to their low material budget and high charge collection
efficiency after irradiation, modules made of thin planar pixel sensors are
promising candidates to instrument these layers. To optimise the sensor layout
for the decreased pixel cell size of 50 * 50 {\mu}m2, TCAD device simulations
are being performed to investigate the charge collection efficiency before and
after irradiation. In addition, sensors of 100-150 {\mu}m thickness,
interconnected to FE-I4 read-out chips featuring the previous generation pixel
cell size of 50 * 250 {\mu}m2, are characterised with testbeams at the CERN-SPS
and DESY facilities. The performance of sensors with various designs,
irradiated up to a fluence of 1 * 1e16 neq./cm2, is compared in terms of charge
collection and hit efficiency. A replacement of the two innermost pixel layers
is foreseen during the lifetime of HL-LHC. The replacement will require several
months of intervention, during which the remaining detector modules cannot be
cooled. They are kept at room temperature, thus inducing an annealing. The
performance of irradiated modules will be investigated with testbeam campaigns
and the method of accelerated annealing at higher temperatures.
