Speaker
Description
Towards high-luminosity operation of the Large Hadron Collider (HL-LHC), starting in 2030, the inner detector of the ATLAS detector will be replaced by a fully-silicon-based inner tracker (ITk). The outer part of the ITk detector consists of $\sim$ 20,000 strip sensors with glued-on hybrids carrying the front-end electronics necessary for readout. A production version of the sensor (ATLAS18 design version) has a dimension of 9.8$\times$9.8 cm$^2$ with a strip pitch of 75.5 $\mu$m for barrel sensors, while endcap sensors have similar but slightly modified dimensions depending on the regions on the detector. During mass production, their electrical parameters are monitored after exposure of up to 1.6$\times$10$^{15}$ $n_\mathrm{eq}$/cm$^2$ or 660 kGy. This is done on a sampling basis for every batch (~ 40 wafers). Irradiation tests are performed as part of quality assurance (QA) using test structures, processed on the same wafer with the main sensor.
While the aforementioned QA programme with the test structures is being advanced to efficiently determine quality of the delivered main sensors and make a decision for their acceptance, detailed measurements of the production sensor properties provide important reference data for future operation of the ITk detector at HL-LHC, where only digitised data from the ITk strip detector are recorded in the data storage. We therefore emphasised studies of the following key parameters: interstrip resistance and charge responses to minimum-ionisation particles, using the mini-sensor with exactly the same strip structure as the main sensor but scaled down to 1$\times$1 cm$^2$. Irradiation tests were performed using 70 MeV proton beams at RARiS, Tohoku University, Japan. In addition to the standard laboratory measurements with radioactive sources, testbeam experiments were performed using 3 GeV electron beams at PF-AR Test Beam Line, KEK, Japan, to measure charge collection efficiency (CCE) using minimum-ionising particles (MIPs). From these measurements using several mini sensors, we found that the actual interstrip resistance is in the range of 200-300 MΩ, which is sufficiently higher than the bias resistance connecting between the strip and the bias ring, and CCE measured using MIPs well agrees with that from radioactive sources. In this presentation, these results are discussed in comparison with data from the QA tests.
