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

TCAD simulation of the electrical performance of the ATLAS18 strip sensor for the HL-LHC

To cope with the increased occupancy and radiation dose expected at the High-Luminosity LHC, the ATLAS experiment will replace its current Inner Detector with the Inner Tracker (ITk), consisting of silicon-based pixel and strip sub-detectors. The strip detector will consist of $n^+$-in-$p$ sensors fabricated by Hamamatsu Photonics, with 300 $\mu$m signal-generation thickness and approximately 75 $\mu$m strip pitch. To guide the operation of these sensors in the ITk, it is desirable to understand the basic mechanisms underlying their performance, including the effects of the radiation fluence (up to $1.6 \times 10^{15}$ 1-MeV n$_\text{eq}$/cm$^2$) expected during operation. To this end, we have used Sentaurus TCAD to develop a 2D simulation of the ITk large-format strip sensor, based on detailed optical and electrical measurements of the sensors and of test devices fabricated on the same wafers. Current-voltage and capacitance-voltage behaviour is reproduced in the simulation by implementing charge trapping due to defects in the silicon, and the dependence of sensor behaviour on the location of these defects is investigated. Trapping parameters are informed by existing frameworks, such as the Perugia model of surface and bulk radiation damage, and by deep-level transient spectroscopy of test devices on the sensor wafers

Analysis of MOS capacitor with p layer with TCAD simulation

The ATLAS18 strip sensors of the ATLAS inner tracker upgrade (ITk) are in production since 2021. Along with the large-format n$^+$-in-p strip sensor in the center of 6-inch wafer, test structures are laid out in the open space for monitoring the performance of the strip sensor and its fabrication process. One of the structures is a 1.2$\times$1.0 cm$^2$ test chip that includes representative structures of the strips, and Metal-Oxide-Silicon (MOS) capacitors. In addition to the standard MOS capacitor, a MOS capacitor is designed with a p-implantation in the surface of silicon, representative of the p-stop doping for isolating the n$^+$ strips, the MOS-p capacitor. The capacitance measurement of the standard MOS capacitor as a function of bias voltage (C-V) shows characteristic behavior in the accumulation, depletion, and inversion regimes, from which one can deduce the amount of the interface charge. The MOS-p capacitor shows the C-V behavior modulated by the properties of the p-layer. With over 50% of the full production complement delivered, we have observed consistent characteristics in the MOS-p capacitors. Rarely and currently only in three batches, we have observed abnormalities which have implied lower density of p-implantation in the p-layer. To study the cause, we have simulated the MOS-p capacitor with a TCAD software, which successfully reproduces the normal behavior, with the p-density and the interface charge within the expected ranges, including a feature caused by a geometrical offset of the areas of the metal and the p-implantation. By contrast, overall shapes of the abnormal cases are only reproduced when introducing 1/10 of p-density, larger interface charge, charge traps in the p-layer, and/or n-type surface contamination. A smaller but distinctive feature in the C-V behavior might also be caused by non-uniform distribution of these or other components. These simulations help to take final acceptance decisions for the batches in production

Analysis of MOS capacitor with p-layer with TCAD simulation

The ATLAS18 strip sensors of the ATLAS inner tracker upgrade (ITk) are under production since 2021. Along with the large-format n^+-in-p strip sensor in the center of the wafer, test structures are laid out in the open space for monitoring the performance of the strip sensor and its fabrication process. One of the structures is a 1.2×1.0 mm^2 test chip that includes representative structures of the strips, and Metal-Oxide-Silicon (MOS) capacitors. In addition to the standard MOS capacitor, a MOS capacitor with a p-layer in the surface of silicon, the MOS-p capacitor, is designed with a p-density representative of the p-stop doping for isolating the n+ strips. The C-V curve of the MOS capacitor shows characteristic behavior in the accumulation, depletion, and inversion regions as a function of bias voltage, from which one can estimate the amount of the interface charge. The MOS-p capacitor shows the C-V curve modulated by the properties of the p-layer. With over 50% of the full production complement delivered, we have observed consistent characteristics in the MOS-p capacitors. Rarely and currently only in 3 batches, we have observed abnormalities. To further study them, we have simulated the MOS-p capacitor with TCAD software, which successfully reproduces the normal behavior, including a feature caused by a geometrical setback of the p-layer to the metal area, with the p-density and the interface charge within the expected range. By contrast, the overall shapes of the abnormal cases are only reproduced with 1/10 of the p-density to the specification and possible charge traps in the p-layer area. A smaller but distinctive feature in the behavior may require a non-uniform distribution of the p-density and the interface charge or something else. These simulations help to take final decisions for the batches in production

Evaluation of MOS and Gated Diode Devices of the ATLAS ITk Test Chip

The new ATLAS Inner Tracker (ITk) sub-detector is necessitated by the impending High Luminosity Large Hadron Collider (HL-LHC) upgrade. This replacement is part of the phase-II upgrade programme for the HL-LHC which will see a sevenfold increase in peak instantaneous luminosity with a total ionizing dose of 53 MRad. The fully solid state ITk will employ silicon n^{+}-in-p microstrip sensors in the outer layers of the tracking detector. The main sensors are manufactured on 6” diameter silicon wafers. Periphery wafer area (halfmoons) to which the main sensor does not extend serves as convenient venues for the implementation of test devices. The primary utility of the test devices is Quality Assurance (QA), that is, the monitoring of the consistency and reliability of the manufacturing process. A Metal-Oxide-Semiconductor (MOS) and Gate-Controlled Diode (GCD) are two such test devices aimed at characterizing the surface oxide and silicon-oxide interface. Measurement procedures and parameters for QA are established for these devices and the viability of these tests for gamma irradiated samples is evaluated. The suitability of these devices to further monitor the strip sensor fabrication process is also investigated

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

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

Identification and Recovery of ATLAS18 Strip Sensors with High Surface Static Charge

The new all-silicon Inner Tracker (ITk) is being constructed by the ATLAS collaboration to track charged particles produced at the High-Luminosity LHC. The outer portion of the ITk detector will include nearly 18,000 highly segmented and radiation hard silicon strip sensors (ATLAS18 design). Throughout the production of 22,000 sensors, the strip sensors are subjected to a comprehensive suite of mechanical and electrical tests as part of the Quality Control (QC) program. In a large fraction of the batches delivered to date, high surface electrostatic charge has been measured on both the sensors and the plastic sheets which sheathe the sensors for shipping and handling rigidity. Aggregate data from across QC sites indicate a correlation between observed electrical failures and the sensor/plastic sheet charge build up. To mitigate these issues, the QC testing sites introduced recovery techniques involving UV light or flows of ionizing gas. Significant modifications to sensor handling procedures were made to prevent subsequent build up of static charge. This publication details a precise description of the issue, a variety of sensor recovery techniques, and trend analyses of sensors initially failing electrical tests (IV, strip scan, etc.)