Experimental Study and Empirical Modelling of Long Term Annealing of the ATLAS18 Sensors

In order to continue the program of the LHC, the accelerator will be upgraded to the HL-LHC, which will have a design luminosity 10e3 cm^2 s^-1, an order of magnitude greater than the present machine. In order to meet the occupancy and radiation hardness requirements resulting from this increase in luminosity, the present ATLAS tracking detector must be replaced. The ATLAS Collaboration is constructing a new central tracking system based completely on silicon sensors. The system comprises an inner pixel detector, a barrel strip detector and an endcap strip detector. In order to satisfy the radiation hardness requirements we have developed a new n-in-p sensor design. Extensive studies have shown that it results in detectors which comfortably reach the required end-of-life performance. The latest sensor layouts prepared for preproduction, known as ATLAS18, implement this design. However, as well as knowing the performance after a given irradiation fluence, operational considerations require an understanding of the time development of the annealing, and resulting variation of the collected charge, of irradiated detectors at different temperatures. This requirement results from the fact that there are various proposed temperature profiles for the operation of the detector over the expected lifetime of 14 years. Here we describe the measurement of charge collection performance as a function of irradiated fluence and long term annealing time. We also describe a semi-empirical model based on these measurements which allows us to predict the end-of-life charge collection as a function of the temperature profile during operation of the detector. The use of the model to study the effect of annealing on the strip detector at a radius of 40 cm and an integrated irradiation fluence of 16e14 MeV neutron equiv. is presented. This methodology can be applied to other regions of the detector, such as the pixels

Analysis of the Quality Assurance results from the initial part of production of the ATLAS18 ITk strip sensors

The production of strip sensors for the ATLAS Inner Tracker (ITk) started in 2021. Since then, a Quality Assurance (QA) program has been carried out continuously, by using specific test structures, in parallel to the Quality Control (QC) inspection of the sensors. The QA program consists of monitoring sensor-specific characteristics and the technological process variability, before and after the irradiation with gammas, neutrons, and protons. After two years, half of the full production volume has been reached and we present an analysis of the parameters measured as part of the QA process. The main devices used for QA purposes are miniature strip sensors, monitor diodes, and the ATLAS test chip, which contains several test structures. Such devices are tested by several sites across the collaboration depending on the type of samples (non-irradiated components or irradiated with protons, neutrons, or gammas). The parameters extracted from the tests are then uploaded to a database and analyzed by Python scripts. These parameters are mainly examined through histograms and time-evolution plots to obtain parameter distributions, production trends, and meaningful parameter-to-parameter correlations. The purpose of this analysis is to identify possible deviations in the fabrication or the sensor quality, changes in the behavior of the test equipment at different test sites, or possible variability in the irradiation processes. The conclusions extracted from the QA program have allowed test optimization, establishment of control limits for the parameters, and a better understanding of device properties and fabrication trends. In addition, any abnormal results prompt immediate feedback to the vendor

ATLAS ITk strip sensor quality assurance tests and results of ATLAS18 pre-production sensors

Towards the high luminosity (HL) operation of the Large Hadron Collider (LHC), the inner tracking system of the ATLAS detector is replaced by a fully silicon-based inner tracker (ITk). Its outer region consists of 17,888 $n^+$-in-$p$ silicon strip sensors. In order to confirm key properties of the production sensors as well as to establish a solid workflow of quality inspection and monitoring of the various sensor properties, about 5\% of the total strip sensors were produced in 2020 as a pre-production run. As a quality assurance (QA) program, irradiation of dedicated QA test pieces was periodically performed. The fluences of proton, neutron and $\gamma$-ray irradiations were up to $1.6 \times 10^{15}$ 1-MeV neutrons$/\mathrm{cm}^2$ and 0.66 MGy, which are equivalent to the maximum expected radiation fluences at the HL-LHC operation with a safety factor of 1.5. Results from 154 QA test pieces demonstrated high quality of the strip sensors through the pre-production. Detailed understanding of post-irradiated strip sensors was acquired, and the procedures of irradiation and post-irradiation QA testing were fully established. Consequently, the 3.8-year project of the strip sensor production for the ATLAS ITk detector was initiated in July 2021

Analysis of humidity sensitivity of silicon strip sensors for ATLAS upgrade tracker, pre- and post-irradiation

The ATLAS collaboration is working on a major upgrade of the Inner-Tracker, able to withstand the extreme operational conditions expected for the forthcoming High-Luminosity Large Hadron Collider (HL-LHC) upgrade. During the prototyping phase of the new large area silicon strip sensors, the community observed a degradation of the breakdown voltage (down to 200-500 V from >= 1 kV in bias voltage) when the devices with final technology options were exposed to high humidity, recovering the electrical performance prior to the exposure after a short period in dry conditions [J. Fernandez-Tejero, et al., NIM A 978 (2020) 164406]. These findings helped to understand the humidity sensitivity of the new sensors, defining the optimal working conditions and handling recommendations during production testing. In 2020, the ATLAS strip sensor community started the pre-production phase, receiving the first sensors fabricated by Hamamatsu Photonics K.K. using the final layout design. The work presented here is focused on the analysis of the humidity sensitivity of production-like sensors with different surface properties, providing new results on their influence on the humidity sensitivity observed during the prototyping phase. Additionally, the new production strip sensors were exposed to short (days) and long (months) term exposures to high humidity. This study allows to recreate and evaluate the influence of the detector integration environment expected during the Long Shutdown 3 (LS3) in 2025, where the sensors will be exposed to ambient humidity for prolonged times. A subset of the production-like sensors were irradiated up to fluences expected at the end of the HL-LHC lifetime, allowing the study of the evolution of the humidity sensitivity and influence of the passivation layers on sensors exposed to extreme radiation conditions

Analysis of humidity sensitivity of silicon strip sensors for ATLAS upgrade tracker, pre- and post-irradiation

During the prototyping phase of the new ATLAS ITk large area strip sensors, a degradation of the device breakdown voltage at high humidity was observed. Although the degradation was temporary, showing a fast recovery in dry conditions, the study of the influence of humidity on the sensor performance was critical to establish counter-measures and handling protocols during production testing in order to ensure the proper performance of the upgraded detector. The work presented here has the objective to study for the first time the breakdown voltage deterioration in presence of humidity of ATLAS ITk production layout sensors with different surface properties, before and after proton, neutron and gamma irradiations. The sensors were also exposed several days to high humidity with the aim to recreate and evaluate the influence of the detector integration environment expected during the Long Shutdown 3 (LS3) in 2025, where the sensors will be exposed to ambient humidity for prolonged times

Establishing the Quality Assurance Programme for the Strip Sensor Production of the ATLAS Tracker Upgrade Including Irradiation with Neutrons, Photons and Protons to HL-LHC Fluences

The successful pre-production delivery of strip sensors for the new Inner Tracker ITk for the upgraded ATLAS detector at the High Luminosity LHC CERN has completed and based on their performance full production has commenced. The overall delivery period is anticipated to last 4 years to complete the approximately 22000 sensors required for the ITk. For Quality Assurance QA, a number of test structures designed by the collaboration, along with a large area diode and miniature version of the main sensor, are produced in every wafer by the foundry Hamamatsu Photonics K.K HPK. As well as Quality Control QC checks on every main sensor, samples of the QA pieces from each delivery batch are tested both before and after irradiation with results after exposure to neutrons, gammas or protons to doses corresponding to those anticipated after operation at the HL-LHC to roughly 1.5 times the ultimate integrated luminosity of 4000~fb$^{-1}$. In this paper are presented the procedures and the studies carried out to establish that the seven ITk QA Strip Sensor irradiation and test sites meet all the requirements to support this very extensive programme throughout the strip sensor production phase for the ITk project