  The ATLAS experiment will undergo a major upgrade of the tracker system in
view of the high luminosity phase of the LHC (HL-LHC) foreseen to start around
2025. Thin planar pixel modules are promising candidates to instrument the new
pixel system, thanks to the reduced contribution to the material budget and
their high charge collection efficiency after irradiation. New designs of the
pixel cells, with an optimized biasing structure, have been implemented in
n-in-p planar pixel productions with sensor thicknesses of 270 um. Using beam
tests, the gain in hit efficiency is investigated as a function of the received
irradiation fluence. The outlook for future thin planar pixel sensor
productions will be discussed, with a focus on thin sensors with a thickness of
100 and 150 um and a novel design with the optimized biasing structure and
small pixel cells (50 um x 50 um and 25 um x 100 um). These dimensions are
foreseen for the new ATLAS read-out chip in 65 nm CMOS technology and the fine
segmentation will represent a challenge for the tracking in the forward region
of the pixel system at HL-LHC. To predict the performance of 50 um x 50 um
pixels at high eta, FE-I4 compatible planar pixel sensors have been studied
before and after irradiation in beam tests at high incidence angle with respect
to the short pixel direction. Results on cluster shapes, charge collection- and
hit efficiency will be shown.
