  Owing to their intrinsic (geometry dependent) radiation hardness, 3D pixel
sensors are promising candidates for the innermost tracking layers of the
forthcoming experiment upgrades at the Phase 2 High-Luminosity LHC (HL-LHC). To
this purpose, extreme radiation hardness up to the expected maximum fluence of
2e16 neq.cm-2 must come along with several technological improvements in a new
generation of 3D pixels, i.e., increased pixel granularity (50x50 or 25x100 um2
cell size), thinner active region (~100 um), narrower columnar electrodes (~5
um diameter) with reduced inter-electrode spacing (~30 um), and very slim edges
(~100 um). The fabrication of the first batch of these new 3D sensors was
recently completed at FBK on Si-Si direct wafer bonded 6-inch substrates.
Initial electrical test results, performed at wafer level on sensors and test
structures, highlighted very promising performance, in good agreement with TCAD
simulations: low leakage current (<1 pA/column), intrinsic breakdown voltage of
more than 150 V, capacitance of about 50 fF/column, thus assessing the validity
of the design approach. A large variety of pixel sensors compatible with both
existing (e.g., ATLAS FEI4 and CMS PSI46) and future (e.g., RD53) read-out
chips were fabricated, that were also electrically tested on wafer using a
temporary metal layer patterned as strips shorting rows of pixels together.
This allowed a statistically significant distribution of the relevant
electrical quantities to be obtained, thus gaining insight into the impact of
process-induced defects. A few 3D strip test structures were irradiated with
X-rays, showing inter-strip resistance of at least several GOhm even after 50
Mrad(Si) dose, thus proving the p-spray robustness. We present the most
important design and technological aspects, and results obtained from the
initial investigations.
