Novel 3D Pixel Sensors for the Upgrade of the ATLAS Inner Tracker

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.

Qualification of the first pre-production 3D FBK sensors with ITkPixV1 readout chip

“…The measured efficiency is >97.5% already with 0 V bias applied, as visible in Figure 5 left. This result is compatible with previous efficiency measurements [9] performed at DESY (∼5 GeV electron beam) on similar sensors (50×50 µm 2 pixel cell prototype by FBK) bump-bonded to a RD53A chip [24]. Figure 5 right shows the hit efficiency as a function of the particle impact point over the surface of a pixel cell (in-pixel efficiency).…”

“…Then they have been irradiated twice, in Bonn and at the CERN IRRAD facility, up to a total integrated not uniform fluence up to 1.9 • 10 16 n eq /cm 2 (1.5 • 10 16 n eq /cm 2 average) and tested in test beams (at CERN SPS) after each irradiation. The average hit efficiency measured at a fluence of 1 • 10 16 n eq /cm 2 is ∼98.5%, higher than the one observed in [9] (∼96%) at a similar irradiation value. The average hit efficiency measured on devices irradiated at ultimate fluence is as high as 97% at 80 V, 100 V, 110 V reverse bias applied for an average fluence of 1.59 • 10 16 n eq /cm 2 , 1.70 • 10 16 n eq /cm 2 , 1.78 • 10 16 n eq /cm 2 respectively.…”

“…The modules are expected to be exposed to a non-ionising fluence of up to 1.7 • 10 16 n eq /cm 2 (after 2000 fb −1 , including safety factors) and an ionising dose of up to 1 Grad. After several years of R&D [9], the 3D sensor technology was chosen due to its inherent radiation hardness. 3D sensors will be used in the L0 layers in the form of triplet modules (three front-ends bump-bonded onto three sensor dies).…”

A Modified 3D-Trench Pixel Detector: Proof of Concept by TCAD Simulations

“…As previously mentioned, a delayed signal would cause a degradation of the timing performance, exactly as it would happen in the existing device if a particle would hit inside a trench. This problem can be strongly mitigated by tilting the sensor plane with respect to the particle direction by a few degrees, that is a typical solution adopted with 3D sensors [23]: the tilt of the particles leads to average out the effects of the non-ideal distributions of the electric and weighting field, yielding very fast rise times. In addition, full 4D-tracking systems should preferably use multiple planes of 3D-trenched sensors with an offset between the electrodes, so that most particle tracks would traverse sensor volumes away from low field regions [7].…”

Cold temperature characterization of ring triplets based on RD53A readout chip

“…SINTEF [8]. To house these sensors, type R0.5 ring triplets will be assembled at INFN Genova and Milan, Italy.…”