Abstract
The ATLAS experiment will undergo a full replacement of its inner detector to face the challenges posed by the High Luminosity upgrade of the Large Hadron Collider (HL-LHC). The new Inner Tracker (ITk) will have to deal with extreme particle fluences. Due to its superior radiation hardness the 3D silicon sensor technology has been chosen to instrument the innermost pixel layer of ITk, which is the most exposed to radiation damage. Three foundries (CNM, FBK, and SINTEF), have developed and fabricated novel 3D pixel sensors to meet the specifications of the new ITk pixel detector. These are produced in a single-side technology on either Silicon On Insulator (SOI) or Silicon on Silicon (Si-on-Si) bonded wafers by etching both n- and p-type columns from the same side. With respect to previous generations of 3D sensors they feature thinner active substrates and smaller pixel cells of 50 × 50 and 25 × 100 µm2. This paper reviews the main design and technological issues of these novel 3D sensors, and presents their characterization before and after exposure to large radiation doses close to the one expected for the innermost layer of ITk. The performance of pixel modules, where the sensors are interconnected to the recently developed RD53A chip prototype for HL-LHC, has been investigated in the laboratory and at beam tests. The results of these measurements demonstrate the excellent radiation hardness of this new generation of 3D pixel sensors that enabled the project to proceed with the pre-production for the ITk tracker.
Highlights
The Large Hadron Collider (LHC), located at CERN, is the world’s largest and most powerful particle accelerator
The HL-LHC presents two key challenges to the ATLAS experiment: while the detector has to cope with unprecedented radiation levels, it has to be able to disentangle the huge amount of simultaneous events that will be generated in the proton-proton beam collisions
Even if in some productions the thickness is still less with respect to the design specifications of the final sensor (130 μm in FBK, and 50 μm or 100 μm in SINTEF sensors), most of the results can be considered as worst-case scenarios due to the reduced number of initial charge carriers generated by Minimum Ionising Particles (MIPs)
Summary
The Large Hadron Collider (LHC), located at CERN, is the world’s largest and most powerful particle accelerator. 3D sensors are an established technology that has been already employed in experiments at the LHC such as in the ATLAS Insertable B-Layer (IBL) [1] and for the tracker of the AFP experiment [2] With respect to these designs the new ITk 3D sensors feature a reduced pixel cell size of 25 × 100 and 50 × 50 μm with one collecting electrode. A 25 × 100 μm pixel cell design with two electrodes (25 × 100−2E) was investigated, but eventually dropped because of technical difficulties in achieving a satisfactory production yield All these sensor designs are compatible with the future ATLAS readout chip, the ITkPix. in order to lower the occupancy, the thickness of the active substrate of these new sensors is reduced to 150 μm in comparison to the previous generation of 230 μm thick 3D sensors. This paper will describe the design and the technological challanges of the recent 3D sensor productions for ITk as well as the results of the characterization performed on assembled module prototypes to validate the performance of these novel 3D sensor designs
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