Development of n-in-p pixel modules for the ATLAS upgrade at HL-LHC

Abstract

Thin planar pixel modules are promising candidates to instrument the inner layers of the new ATLAS pixel detector for HL-LHC, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. 100-200 µm thick sensors, interconnected to FE-I4 read-out chips, have been characterized with radioactive sources and beam tests at the CERN-SPS and DESY. The results of these measurements are reported for devices before and after irradiation up to a fluence of 14 × 10 15 n eq /cm 2 . The charge collection and tracking efficiency of the different sensor thicknesses are compared. The outlook for future planar pixel sensor production is discussed, with a focus on sensor design with the pixel pitches (50x50 and 25x100 µm 2 ) foreseen for the RD53 Collaboration read-out chip in 65 nm CMOS technology. An optimization of the biasing structures in the pixel cells is required to avoid the hit efficiency loss presently observed in the punch-through region after irradiation. For this purpose the performance of different layouts have been compared in FE-I4 compatible sensors at various fluence levels by using beam test data. Highly segmented sensors will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. In order to reproduce the performance of 50x50 µm 2 pixels at high pseudo-rapidity values, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angle (80• ) with respect to the short pixel direction. Results on cluster shapes, charge collection and hit efficiency will be shown.

Investigation of thin n-in-p planar pixel modules for the ATLAS upgrade

“…The expected cluster width along η strongly depends on the sensor thickness as visualised in Fig. 3(a) and thinner sensors produce smaller clusters [13,15]. This is in good agreement with the observed cluster widths for the sensor thicknesses of 100, 150 and 200 µm displayed in Fig.…”

Thin n-in-p planar pixel modules for the ATLAS upgrade at HL-LHC

“…It is almost constant over the sensor depth, except for the entrance and exit pixels that are not entirely crossed by the track. The obtained hit efficiency of a single pixel for the 200 µm thick sensor is 94.9% and hence it is (4-5)% lower than the one of 100 µm thickness presented in [4]. The hit efficiency was calculated with varying assumptions on the allowed number of holes between two pixels inside the cluster, up to a maximum of 10.…”

“…In the present design of the ATLAS pixel sensors it is possible to bias the pixel via the punch-through mechanism with an n + implant dot implemented in the pixel cell, connected to the bias ring through an aluminium rail. It has been observed that these structures introduce a loss of efficiency after irradiation [3,4]. To reduce this effect, an optimization has been carried out, comparing the performance of the different designs shown in Fig.…”

“…Sensors with a thickness of 100 and 150 µm were found to reach the same hit efficiency as thicker sensors already at a bias voltage of 300 V shown in A highest hit efficiency of around 97% was obtained for perpendicular incident tracks at a fluence of 5 × 10 15 n eq /cm 2 , the Email address: Natascha.Savic@mpp.mpg.de (N. Savic) expected fluence for the second layer at HL-LHC [3]. The main inefficiencies are caused by the bias dot and the bias rail, as described in [4] for fluences up to 3 × 10 15 n eq /cm 2 . In this paper, different designs of n-in-p planar hybrid pixel modules are investigated at the expected fluence of the second layer.…”

A review of advances in pixel detectors for experiments with high rate and radiation

“…With increasing radiation fluence the bias voltage needed for sufficient depletion must be increased, eventually reaching values of more than 500 V at Note the small distance between readout chip and guard ring implants; (c) n + -in-p sensor hit efficiency as a function of bias voltage for different fluences and various thicknesses [52].…”