First production of new thin 3D sensors for HL-LHC at FBK

Abstract

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 2 × 1016 neq.cm−2 must come along with several technological improvements in a new generation of 3D pixels, i.e., increased pixel granularity (50 × 50 or 25 × 100 μ m2 cell size), thinner active region (∼ 100 μm), narrower columnar electrodes (∼ 5 μm diameter) with reduced inter-electrode spacing (∼ 30 μm), and very slim edges (∼ 100 μm). The fabrication of the first batch of these new 3D sensors was recently completed at FBK on Si-Si direct wafer bonded 6” 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 GΩ 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.