The ATLAS experiment will replace the entire inner tracking detector with a completely new silicon-only system. 3D silicon pixel sensors are promising candidates for the innermost layers of the Pixel detector due to their excellent radiation hardness and low power dissipation. 3D pixel sensors with 50x50 and 25x100 µm² pixel pitches have been produced at CNM Barcelona and studied by IFAE. The smaller pixel size will allow to cope with the increased particle rate at HL-LHC and in addition the reduced electrode distance is expected to increase the radiation hardness. A chip with pixel size of 50x50 µm² (RD53 - compatible with both small pixels) is still under development and the initial studies of the small-pitch 3D pixel sensor prototypes were made using the FEI4 readout chip (pixel size 50x250 µm²), presently used on the current innermost layer of ATLAS (IBL).
The hit efficiency of 50x50 µm2 sensors was measured in beam tests at CERN SPS after uniform (at KIT to 5e15 neq/cm2) and non uniform irradiations (at CERN PS up to 1.4e16 neq/cm2). The 5e15 neq/cm2 irradiation point is ideal to compare to IBL qualification devices due to the same fluence and irradiation facility. The non uniform irradiation allows to study a range of different fluences within the same device. The operation voltage of the new irradiated small-pixel sensors will be compared with the IBL-type 3D sensors.