Understanding the Humidity Sensitivity of Sensors with TCAD Simulations

The breakdown voltage of silicon sensors without special surface is known to be affected by the ambient humidity. To understand the sensor’s humidity sensitivity, Synopsys TCAD was used to simulate n-in-p test structures for different effective relative humidity. Photon emission of hot electrons was imaged with a microscope to locate breakdown in the edge-region of the sensor. The Top-Transient Current Technique was used to measure charge transport near the surface in the breakdown region of the sensor. Using the measurements and simulations, the evolution of the electric field, carrier densities and avalanche breakdown in the periphery of p-bulk silicon sensors is presented

Gamma irradiation of ATLAS18 ITk strip sensors affected by static charge

Construction of the new all-silicon Inner Tracker (ITk), developed by the ATLAS collaboration to be able to track charged particles produced at the High-Luminosity LHC, started in 2021 and is expected to continue until 2028. The ITk detector will include ~18,000 highly segmented and radiation hard n+-in-p silicon strip sensors, which are being manufactured by Hamamatsu Photonics. Upon their delivery, the ATLAS ITk strip sensor collaboration performs detailed measurements of sensors to monitor quality of all fabricated pieces. QC electrical tests include current-voltage (IV) and capacitance-voltage (CV) tests, full strip tests, and a measurement of the long-term stability of the sensor leakage current. While most sensors demonstrate excellent performance during QC testing, we have nevertheless observed that a number of sensors from several production batches failed the electrical tests. Accumulated data indicates a strong correlation between observed electrical test failures and high electrostatic charge measured on the sensor surface during initial reception tests. This electrostatic charge enhances the risk of "Local trapped charge" events during manufacturing, shipping, and handling procedures, resulting in failed electrical QC tests. To mitigate the above-described issues, the QC testing institutes modified the sensor handling procedures and introduced sensor recovery techniques. Despite the implementation of various recovery techniques, it is still possible that some affected sensors will not be identified by the sensor QC testing, or that "Local trapped charge" events could occur in later manipulation stages of the sensor. In the presented study, we have investigated whether the total ionizing dose (TID) expected in the real experiment can effectively resolve early breakdown or low interstrip isolation caused by the electrostatic charge. Selected charge-affected sensors were irradiated with gamma rays from the 60Co source for a number of TID values. The results of this study indicate that the negative effects of the electrostatic charge on the critical sensors characteristics disappear after a very small amount of an accumulated TID, which actually corresponds to one or two days in the experiment. This finding gives us confidence in mitigating the issue of electrostatic charge during the operation of the ITk strip sensors in the real experiment

Mapping the in-plane electric field inside irradiated diodes

A significant aspect of the Phase-II Upgrade of the ATLAS detector is the replacement of the current Inner Detector with the ATLAS Inner Tracker (ITk). The ATLAS ITk is an all-silicon detector consisting of a pixel tracker and a strip tracker. Sensors for the ITk strip tracker have been developed to withstand the high radiation environment in the ATLAS detector after the High Luminosity Upgrade of the Large Hadron Collider at CERN, which will significantly increase the rate of particle collisions and resulting particle tracks. During their operation in the ATLAS detector, sensors for the ITk strip tracker are expected to accumulate fluences up to $1.5\cdot10^{15}\,\text{n}_{\text{eq}}/\text{cm}^2$ (including a safety factor of 1.5), which will significantly affect their performance. One characteristic of interest for highly irradiated sensors is the shape and homogeneity of the electric field inside its active area. For the results presented here, diodes with edge structures similar to full size ATLAS sensors were irradiated up to fluences comparable to those in the ATLAS ITk strip tracker and their electric fields mapped using a micro-focused X-ray beam (beam diameter $2\times3$ $\mu$m). This contribution shows the extension and shape of the electric field inside highly irradiated diodes over a range of applied bias voltages. Additionally, measurements of the outline of the depleted sensor areas allow a comparison of the measured leakage current for different fluences with expectations for the corresponding active areas

Monitoring Quality of ATLAS ITk Strip Sensors/wafers through Database

High-Luminosity LHC upgrade necessitated a complete replacement of the ATLAS Inner Detector with a larger all-silicon tracker. The strip portion of it covers 165 m2 area, afforded by the strip sensors. Following several prototype iterations and a successful pre-production, a full-scale production started in 2021, to finish by the beginning of 2025. It will include over 20,000 wafers and a factor of 5 higher throughput than pre-production, with about 500 sensors produced and tested per month. The transition to production stressed the need to evaluate the results from the Quality Control (QC) and Quality Assurance (QA) tests quickly to meet the monthly delivery schedule. The test data come from 15 collaborating institutes, therefore a highly distributed system with standardized interfaces was required. Specialized software layers of QA and QC Python code were developed against the backend of ITkdatabase (DB) for this purpose. The developments included particularities and special needs of the Strip Sensors community, such as the large variety of different test devices and test types, the necessary test formats, and different workflows at the test sites. Special attention was paid to techniques facilitating the development and user operations, for example creation of “parallel” set of dummy DB objects for practice purpose, iterative verification of operability, and the automatic upload of test data. The scalability concerns, and automation of the data handling were included in the system architecture from the very inception. The full suite of functionalities include data integrity checks, data processing to extract and evaluate key parameters, cross-test comparisons, and summary reporting for continuous monitoring. We will also describe the lessons learned and the necessary evolution of the system

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

Characterization of the Polysilicon Resistor in Silicon Strip Sensors for ATLAS Inner Tracker as a Function of Temperature, Pre- And Post-Irradiation

The high luminosity upgrade of the Large Hadron Collider, foreseen for 2029, requires the replacement of the ATLAS Inner Detector with a new all-silicon Inner Tracker (ITk). The expected total integrated luminosity of $4000\, \mathrm{fb}^{−1}$ means that the strip part of the ITk detector will be exposed to the total particle fluences and ionizing doses reaching the values of $1.6 \cdot 10^{15}$ $1\, \mathrm{MeV}$ $\mathrm{n_{eq}/cm^2}$ and $0.66\, \mathrm{MGy}$, respectively, including a safety factor of $1.5$. Radiation hard n${}^+$-in-p micro-strip sensors were developed by the ATLAS ITk strip collaboration and are produced by Hamamatsu Photonics K.K. The active area of each ITk strip sensor is delimited by the n-implant bias ring, which is connected to each individual n${}^+$ implant strip by a polysilicon bias resistor. The total resistance of the polysilicon bias resistor should be within a specified range to keep all the strips at the same potential, prevent the signal discharge through the grounded bias ring and avoid the readout noise increase. While the polysilicon is a ubiquitous semiconductor material, the fluence and temperature dependence of its resistance is not easily predictable, especially for the tracking detector with the operational temperature significantly below the values typical for commercial microelectronics. Dependence of the resistance of polysilicon bias resistor on the temperature, as well as on the total delivered fluence and ionizing dose, was studied on the specially-designed test structures called ATLAS Testchips, both before and after their irradiation by protons, neutrons, and gammas to the maximal expected fluence and ionizing dose. The resistance has an atypical negative temperature dependence. It is different from silicon, which shows that the grain boundary has a significant contribution to the resistance. We will discuss the contributions by parameterizing the excitation energy of the polysilicon resistance as a function of the temperature for unirradiated and irradiated ATLAS Testchips

Characterization of the Polysilicon Resistor in Silicon Strip Sensors for ATLAS Inner Tracker as a Function of Temperature, Pre- And Post-Irradiation

The high luminosity upgrade of the Large Hadron Collider, foreseen for $2029$, requires the replacement of the ATLAS Inner Detector with a new all-silicon Inner Tracker (ITk). The expected ultimate total integrated luminosity of $4000\, \mathrm{fb}^{-1}$ means that the strip part of the ITk detector will be exposed to the total particle fluences and ionizing doses reaching the values of $1.6 \cdot 10^{15}$ $1\, \mathrm{MeV}$ $\mathrm{n_{eq}/cm^2}$ and $0.66\, \mathrm{MGy}$, respectively, including a safety factor of $1.5$. Radiation hard n${}^+$-in-p micro-strip sensors were developed by the ATLAS ITk strip collaboration and are produced by Hamamatsu Photonics K.K. The active area of each ITk strip sensor is delimited by the n-implant bias ring, which is connected to each individual n${}^+$ implant strip by a polysilicon bias resistor. The total resistance of the polysilicon bias resistor should be within a specified range to keep all the strips at the same potential, prevent the signal discharge through the grounded bias ring and avoid the readout noise increase. While the polysilicon is a ubiquitous semiconductor material, the fluence and temperature dependence of its resistance is not easily predictable, especially for the tracking detector with the operational temperature significantly below the values typical for commercial microelectronics. Dependence of the resistance of polysilicon bias resistor on the temperature, as well as on the total delivered fluence and ionizing dose, was studied on the specially-designed test structures called ATLAS Testchips, both before and after their irradiation by protons, neutrons, and gammas to the maximal expected fluence and ionizing dose. The resistance has an atypical negative temperature dependence. It is different from silicon, which shows that the grain boundary has a significant contribution to the resistance. We will discuss the contributions by parameterizing the activation energy of the polysilicon resistance as a function of the temperature for unirradiated and irradiated ATLAS Testchips

Monitoring Quality of ATLAS ITk Strip Sensors through Database

The high-Luminosity LHC upgrade necessitates a complete replacement of the ATLAS Inner Detector with a larger all-silicon tracker. The strip portion of it covers 165 m$^2$ area, afforded by the strip sensors. Following several prototype iterations and a successful pre-production, a full-scale production started in 2021, to finish in 2025. It will include about 21,000 wafers and a factor of 5 higher throughput than pre-production, with about 500 sensors produced and tested per month. The transition to production stressed the need to evaluate the results from the Quality Control (QC) and Quality Assurance (QA) tests quickly to meet the monthly delivery schedule. The test data come from 15 collaborating institutes, therefore a highly distributed system with standardized interfaces was required. Specialized software layers of QA and QC Python code were developed against the backend of the ITk database (DB) for this purpose. The developments included particularities and special needs of the Strip Sensors community, such as the large variety of different test devices and test types, the necessary test formats, and different workflows at the test sites. Special attention was paid to techniques facilitating the development and user operations, for example creation of “parallel” sets of dummy DB objects for practice purposes, iterative verification of operability, and the automatic upload of test data. The scalability concerns and automation of the data handling were included in the system architecture from the very inception. The full suite of functionalities include data integrity checks, data processing to extract and evaluate key parameters, cross-test comparisons, and summary reporting for continuous monitoring. We will also describe the lessons learned and the necessary evolution of the system

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