Interstrip Resistance Research Articles

Modeling of surface damage at the Si/SiO2-interface of irradiated MOS-capacitors

Surface damage caused by ionizing radiation in SiO2 passivated silicon particle detectors consists mainly of the accumulation of a positively charged layer along with trapped-oxide-charge and interface traps inside the oxide and close to the Si/SiO2-interface. High density positive interface net charge can be detrimental to the operation of a multi-channel n-on-p sensor since the inversion layer generated under the Si/SiO2-interface can cause loss of position resolution by creating a conduction channel between the electrodes. In the investigation of the radiation-induced accumulation of oxide charge and interface traps, a capacitance-voltage characterization study of n/γ- and γ-irradiated Metal-Oxide-Semiconductor (MOS) capacitors showed that close agreement between measurement and simulation were possible when oxide charge density was complemented by both acceptor- and donor-type deep interface traps with densities comparable to the oxide charges. Corresponding inter-strip resistance simulations of a n-on-p sensor with the tuned oxide charge density and interface traps show close agreement with experimental results. The beneficial impact of radiation-induced accumulation of deep interface traps on inter-electrode isolation may be considered in the optimization of the processing parameters of isolation implants on n-on-p sensors for the extreme radiation environments.

Effect of irradiation and annealing performed with bias voltage applied across the coupling capacitors on the interstrip resistance of ATLAS ITk silicon strip sensors

The powering configuration of the silicon strip modules developed for the new Inner Tracker of the ATLAS experiment includes a voltage of up to 0.5 V across the coupling capacitor of each individual strip. However, this voltage is usually not applied in the sensor irradiation studies due to the significant technical and logistical complications. To study the effect of an irradiation and a subsequent beneficial annealing on the strip sensors in real experimental conditions, four prototype ATLAS17LS miniature sensors were irradiated by 60Co source and annealed, both with and without the bias voltage of 0.5 V applied across the coupling capacitors. The values of interstrip resistance measured on irradiated samples before and after annealing indicate that increase of radiation damage caused by the applied voltage can be compensated by the presence of this voltage during annealing.

Influence of surface damage and bulk defects on the interstrip isolation of p-type silicon strip sensors

Silicon strip sensors of upcoming tracking detectors in high luminosity colliders usually consist of a p-doped bulk with n-type strip implants. The general consensus is that such a design requires an additional interstrip isolation structure such as a p-stop implant. If there is no additional implant between the strips, it is expected that the strip isolation will be insufficient. Before irradiation, impurities and defects in the material lead to positive charge in the oxide and Si/SiO_2 interface which attracts electrons from the bulk. Those electrons accumulate just beneath the surface and between the n+ strip or pixel implants, which decreases the interstrip resistance significantly. Ionising radiation introduces even more charge inside the silicon dioxide, which further decreases the interstrip resistance. Contrary to that expectation of a decreasing interstrip resistance due to irradiation, a high interstrip resistance was sometimes observed after proton irradiation. Hence, bulk defects induced by proton irradiation seem to have a non-negligible impact on the strip isolation. Therefore, an irradiation campaign with sensors without any interstrip isolation implant has been performed. The use of sensors without isolation structures provides deeper insight into the dominant isolation effects. The sensors' performance is first evaluated before irradiation. Afterwards, a set of sensors is irradiated with different particles in order to systematically introduce a mixture of surface and bulk defects. This enables the distinction and leads to a better understanding of the different effects. Finally, a combined TCAD model is derived which is able to describe the beneficial effect of bulk defects and the complicated interplay between surface and bulk damage.

Electrical characterization of surface properties of the ATLAS17LS sensors after neutron, proton and gamma irradiation

The high luminosity upgrade of the Large Hadron Collider, foreseen for 2027, requires the replacement of the ATLAS Inner Detector with a new all-silicon Inner Tracker (ITk). The expected total integrated luminosity of 4000 fb−1 means that the strip part of the ITk detector will be exposed to a large radiation fluence of up to Φeq = 1.6 × 1015 1 MeV neq/cm and an ionizing dose of 0.66 MGy, including a safety factor of 1.5. Radiation-hard n+-in-p micro-strip sensors for use in the ITk have been developed by the ATLAS ITk Strip Sensor collaboration and produced by Hamamatsu Photonics K.K. In this paper, the results obtained from the electrical characterization of the latest barrel ATLAS17LS sensor prototype, before and after irradiation, are shown.Surface properties of the long-strip barrel, full-sized and miniature sensors have been studied before and after proton, neutron and gamma irradiation up to the maximal fluences and radiation doses specified for the ITk Strip tracker. Sensors have been irradiated by protons at CYRIC, Tohoku University (Japan), the Proton Irradiation Facility at CERN, Karlsruhe Inst. Tech. (Germany) and at the University of Birmingham (UK), by neutrons from the Ljubljana TRIGA reactor (Slovenia) and by gamma rays from the 60Co source in UJP Praha (Czech Republic).It has been verified that the surface radiation damage does not influence the sensor functionality. The breakdown voltage is well above the maximum operational voltage. All the tested surface parameters, such as the inter-strip resistance and capacitance, coupling capacitance and bias resistance satisfy the ATLAS ITk specifications for strip sensors.

Strip sensor performance in prototype modules built for ATLAS ITk

ATLAS experiment is preparing an upgrade of its detector for High-Luminosity LHC (HL-LHC) operation. The upgrade involves installation of the new all-silicon Inner Tracker (ITk). In the context of the ITk preparations, more than 80 strip modules were built with prototype barrel sensors. They were tested with electrical readout on a per-channel basis. In general, an excellent performance was observed, consistent with previous ASIC-level and sensor-level tests. However, the lessons learned included two phenomena important for the future phases of the project. First was the need to store and test the modules in a dry environment due to humidity sensitivity of the sensors. The second was an observation of high noise regions for 2 modules.The high noise regions were tested further in several ways, including monitoring the performance as a function of time and bias voltage. Additionally, direct sensor-level tests were performed on the affected channels. The inter-strip resistance and bias resistance tests showed low values, indicating a temporary loss of the inter-strip isolation. A subsequent recovery of the noise performance was observed. We present the test details, an analysis of how the inter-strip isolation affects the module noise, and the relationship with sensor-level quality control tests.

Characterization of irradiated p-type silicon detectors for TCAD surface radiation damage model validation

In this work we address the effects of surface radiation damage on detectors fabricated on high-resistivity p-type FZ ⟨ 100⟩ substrates by Hamamatsu Photonics (HPK) and by Infineon Technologies (IFX). Test structures underwent a wide set of measurements before and after X-ray irradiation with doses from 0.05 to 100 Mrad (SiO2) in order to extract the integrated interface trap density (NIT) and the oxide charge (QOX) peculiar to different vendors, processes and technology options. These parameters can be then used as inputs to TCAD simulation tools. On the basis of the new experimental evidences at these high doses, the TCAD numerical model has been updated considering two main bands of defects, one acceptor and one donor, extended to the whole silicon bandgap and simulating the net effect of the radiation-induced acceptor-like and donor-like defects. The comparison between simulation findings and measured macroscopic electric behaviour, e.g. in terms of C–V curves of MOS capacitors has been used as cross-check for model validation purposes. By means of the same modelling scheme it is possible to reproduce the I–V curves of gated-diodes and the interstrip resistance as a function of the dose from 0.05 to 100 Mrad (SiO2) for the range of technology and design options investigated. The good agreement between simulations and measurements would support the use of this TCAD surface radiation damage model as a predictive tool to optimize the design and the operations of the new generation of silicon detectors for the future High Energy Physics experiments.

Field effect transistor test structures for inter-strip isolation studies in silicon strip detectors

Its radiation resilience has established p-type silicon as tracking detector baseline material in upcoming high-luminosity physics experiments. Electric isolation of n+ electrodes with p+ implants (p-stop or p-spray) is crucial for segmented p-type sensor quality. P+ doping concentration, implantation depth, and geometry determine the achievable resistance between segments. Typically, inter-strip resistance is measured directly on the strip sensors. Large resistances on the order of 100GΩ require precise, low-noise current measurements, which are strongly influenced by parasitic currents. To provide a comparably simple alternative to measurements on strip sensors, this contribution seeks to relate Metal-Oxide–Semiconductor Field-Effect Transistor (MOSFET) threshold voltage to sensor inter-strip resistance. We utilize circular MOSFET test structures fabricated on the same wafers as the strip sensors. This paper compares measurements of MOSFETs and strip sensors on wafers with different n- and p-spray implantations and presents comparative TCAD simulations. MOSFET test structures could present a fast option to judge silicon sensor inter-strip resistance and strip isolation properties. Process quality control during future series productions can benefit from this technique.

Advances on TCAD numerical modelling of radiation damage effects in silicon detectors for HL-LHC operations

Abstract In this work we present a surface radiation damage effects model based on the introduction of amphoteric, uniform energy band distributed deep-level defects. The main characteristic parameters of the surface damage, e.g. the equivalent oxide charge and interface trap densities, have been extracted from experimental measurements carried out on test structures manufactured by the three foundries: HPK, FBK and Infineon. The surface damage model has been coupled with a bulk damage model based on multiple, single-level defects with tunable capture-cross sections. The model is able to reproduce the measurements carried out on irradiated test structures in terms of C–V curves, interstrip resistance as well as charge collection of segmented detectors.

Development of AC-coupled, poly-silicon biased, p-on-n silicon strip detectors in India for HEP experiments

P-on-n silicon strip sensors having multiple guard-ring structures have been developed for High Energy Physics applications. The study constitutes the optimization of the sensor design, and fabrication of AC-coupled, poly-silicon biased sensors of strip width of 30μm and strip pitch of 55μm. The silicon wafers used for the fabrication are of 4 inch n-type, having an average resistivity of 2–5 kΩ cm, with a thickness of 300μm. The electrical characterization of these detectors comprises of: (a) global measurements of total leakage current, and backplane capacitance; (b) strip and voltage scans of strip leakage current, poly-silicon resistance, interstrip capacitance, interstrip resistance, coupling capacitance, and dielectric current; and (c) charge collection measurements using ALiBaVa setup. The results of the same are reported here.

Field effect transistor test structures for p-stop strip isolation studies

The strip isolation of AC-coupled n-on-p silicon strip sensors is compromised by positive charges located in the oxide above the silicon bulk. These positive oxide charges form an inversion layer electrically interconnecting the n+-implanted strips. To achieve a proper strip isolation though, a p+-implantation, the p-stop, is usually inserted in between the strips. The properties of this implantation like the concentration or implantation depth influence the final strip isolation. To investigate the isolation capability of the p-stop, field effect transistor (FET) test structures have been designed, measured and simulated using Synopsys TCAD. The main goal of these studies is to relate the threshold voltage of such transistors to the inter-strip resistance of actual sensors. Therewith, extensive measurements of usually large resistances could be transferred to comparable simple ones.

First production of new thin 3D sensors for HL-LHC at FBK

Owing to their intrinsic (geometry dependent) radiation hardness, 3D pixel sensors are promising candidates for the innermost tracking layers of the forthcoming experiment upgrades at the “Phase 2” High-Luminosity LHC (HL-LHC) . To this purpose, extreme radiation hardness up to the expected maximum fluence of 2 × 1016 neq.cm−2 must come along with several technological improvements in a new generation of 3D pixels, i.e., increased pixel granularity (50 × 50 or 25 × 100 μ m2 cell size), thinner active region (∼ 100 μm), narrower columnar electrodes (∼ 5 μm diameter) with reduced inter-electrode spacing (∼ 30 μm), and very slim edges (∼ 100 μm). The fabrication of the first batch of these new 3D sensors was recently completed at FBK on Si-Si direct wafer bonded 6” substrates. Initial electrical test results, performed at wafer level on sensors and test structures, highlighted very promising performance, in good agreement with TCAD simulations: low leakage current (< 1 pA/column), intrinsic breakdown voltage of more than 150 V, capacitance of about 50 fF/column, thus assessing the validity of the design approach. A large variety of pixel sensors compatible with both existing (e.g., ATLAS FEI4 and CMS PSI46) and future (e.g., RD53) read-out chips were fabricated, that were also electrically tested on wafer using a temporary metal layer patterned as strips shorting rows of pixels together. This allowed a statistically significant distribution of the relevant electrical quantities to be obtained, thus gaining insight into the impact of process-induced defects. A few 3D strip test structures were irradiated with X-rays, showing inter-strip resistance of at least several GΩ even after 50 Mrad(Si) dose, thus proving the p-spray robustness. We present the most important design and technological aspects, and results obtained from the initial investigations.

Study of surface properties of ATLAS12 strip sensors and their radiation resistance

A radiation hard n+-in-p micro-strip sensor for the use in the Upgrade of the strip tracker of the ATLAS experiment at the High Luminosity Large Hadron Collider (HL-LHC) has been developed by the “ATLAS ITk Strip Sensor collaboration” and produced by Hamamatsu Photonics.Surface properties of different types of end-cap and barrel miniature sensors of the latest sensor design ATLAS12 have been studied before and after irradiation. The tested barrel sensors vary in “punch-through protection” (PTP) structure, and the end-cap sensors, whose stereo-strips differ in fan geometry, in strip pitch and in edge strip ganging options. Sensors have been irradiated with proton fluences of up to 1×1016 neq/cm2, by reactor neutron fluence of 1×1015 neq/cm2 and by gamma rays from 60Co up to dose of 1MGy. The main goal of the present study is to characterize the leakage current for micro-discharge breakdown voltage estimation, the inter-strip resistance and capacitance, the bias resistance and the effectiveness of PTP structures as a function of bias voltage and fluence. It has been verified that the ATLAS12 sensors have high breakdown voltage well above the operational voltage which implies that different geometries of sensors do not influence their stability. The inter-strip isolation is a strong function of irradiation fluence, however the sensor performance is acceptable in the expected range for HL-LHC. New gated PTP structure exhibits low PTP onset voltage and sharp cut-off of effective resistance even at the highest tested radiation fluence. The inter-strip capacitance complies with the technical specification required before irradiation and no radiation-induced degradation was observed. A summary of ATLAS12 sensors tests is presented including a comparison of results from different irradiation sites. The measured characteristics are compared with the previous prototype of the sensor design, ATLAS07.

T-CAD analysis of electric fields in n-in-p silicon strip detectors in dependence on the p-stop pattern and doping concentration

Detectors based on silicon have been proven to be efficient for particle tracking in high energy physics collider experiments. The Compact Muon Solenoid (CMS) Tracker at CERN near Geneva has a silicon detector surface of about 200 m2. The increasing demand on more collected data for new physics studies requires an upgrade of the Large Hadron Collider to higher luminosity which is foreseen for 2023. The increase of particles per time and area also introduces a harsh environment for the silicon sensors which should withstand an integrated luminosity of about L=3000 fb−1. Several R&D studies have been undertaken to face the high luminosity challenge and n-in-p detectors have been found to be more radiation hard than p-in-n. However, n-in-p detectors necessarily need an isolation layer of the n+ strips due to an accumulation layer of electrons caused by positive charge in the SiO2 at the sensor surface. An additional implantation of acceptors like boron between the n+ strips cuts the conducting electron layer and ensures reliable strip isolation. However, the implantation dose as well as the implant energy have to be carefully calculated as they directly affect the breakdown behavior and inter-strip resistance of the sensors. Experimentally, the inter-strip resistance (Rint) and charge collection efficiency (CCE) as well as the cluster charge formation are a direct indicator whether the isolation layer is sufficient or not. Furthermore T-CAD simulation studies have been carried out in order to reproduce the measurements and to predict the performance of sensors before and after irradiation with protons, neutrons and a mixture of both. Simulations also allow detailed studies of the formation of electric fields in the sensor which influence the sensor performance. Experimentally obtained CCE and misidentified hits in n-in-p devices with p-stop isolation pattern can be explained after analysis of the electric fields in dependence on the p-stop geometry and doping concentrations.The comparison of data and simulation results allows to limit the parameter space of the p-stop isolation technique which significantly affects the performance of irradiated n-in-p type silicon sensors.

Evaluating the influence of MCM-D post-processing on silicon microstrip sensors

Silicon micro-strip modules are traditionally built as a composite object of sensors, front-end ASICs, electronics hybrid, pitch adapters and passive components. This paper reports on a novel approach to use Multi-chip Module-Deposited (MCM-D) technology to build the electronics hybrid and pitch adapter directly on the silicon sensor, by post-processing the sensor wafers. A first prototype was made to evaluate the influence of the post-processing on the sensors in terms of the electrical performance, mechanical stability and the radiation hardness. The processing was done on a wafer with miniature strip sensors and featured one dielectric layer, one metal layer and one passivation layer. This paper summarises the results from this prototype run. It reports on the sensor leakage current, strip capacitance, inter-strip resistance and charge collection efficiency. These measurements are done on non-irradiated devices and devices subjected to proton irradiation. Furthermore it reports on the mechanical integrity of the post-processed assembly.

Evaluation of test structures for the novel n+-in-p pixel and strip sensors for very high radiation environments

Radiation-tolerant n+-in-p silicon sensors were developed for use in very high radiation environments. Novel n+-in-p silicon strip and pixel sensors and test structures were fabricated, tested and evaluated, in order to understand the designs implemented. The resistance between the n+ implants (interstrip resistance), the electric potential of the p-stop, and the punch-through-protection (PTP) onset voltage were measured before and as a function of fluence after irradiation. The technology computer-aided design (TCAD) simulations were used to understand the radiation damage and fluence dependence of the structures. The decrease in the interstrip resistance is a consequence of increased leakage current. The decrease in the electric potential of the p-stop results from a build-up of positive charge in the silicon–silicon oxide interface. The decrease and subsequent increase in the PTP onset voltages results from the interface charge build-up and an increase in acceptor states.

Pixelized M-π-n CdTe detector coupled to Medipix2 readout chip

We have realized a simple method for patterning an M-π-n CdTe diode with a deeply diffused pn-junction, such as indium anode on CdTe. The method relies on removing the semiconductor material on the anode-side of the diode until the physical junction has been reached. The pixelization of the p-type CdTe diode with an indium anode has been demonstrated by patterning perpendicular trenches with a high precision diamond blade and pulsed laser. Pixelization or microstrip pattering can be done on both sides of the diode, also on the cathode-side to realize double sided detector configuration.The article compares the patterning quality of the diamond blade process, pulsed pico-second and femto-second lasers processes. Leakage currents and inter-strip resistance have been measured and are used as the basis of the comparison. Secondary ion mass spectrometry (SIMS) characterization has been done for a diode to define the pn-junction depth and to see the effect of the thermal loads of the flip-chip bonding process. The anode and cathode-sides of a 6 × 6 mm2 diodes were patterned with a diamond blade and flip-chip bonded to the Medipix2 readout chips. First imaging results with an X-ray source show reduced polarization effect and edgeless detector behavior for the anode-side patterned detector.

Testing of surface properties pre-rad and post-rad of n-in-p silicon sensors for very high radiation environment

We are developing n +-in-p, p-bulk and n-readout, microstrip sensors as a non-inverting radiation hard silicon detector for the ATLAS Tracker Upgrade at the super LHC experiment. The surface radiation damages of the sensors fabricated by Hamamatsu Photonics are characterized on the interstrip capacitance, interstrip resistance and punch-through protection evolution. The detector should provide acceptable strip isolation, exceeding the input impedance of the signal readout chip ∼1 kΩ, after the integrated luminosity of 6 ab −1, which is twice the luminosity goal.

Development of n-on-p silicon sensors for very high radiation environments

We have developed a novel and highly radiation-tolerant n-in-p silicon microstrip sensor for very high radiation environments such as in the Super Large Hadron Collider. The sensors are designed for a fluence of 1×10 15 neq/cm 2 and are fabricated from p-type, FZ, 6 in. (150 mm) wafers onto which we lay out a single 9.75 cm×9.75 cm large-area sensor and several 1 cm×1 cm miniature sensors with various n-strip isolation structures. By evaluating the sensors both pre- and post-irradiation by protons and neutrons, we find that the full depletion voltage evolves to approximately 800 V and that the n-strip isolation depends on the p + concentration. In addition, we characterize the interstrip resistance, interstrip capacitance and the punch-through-protection (PTP) voltage. The first fabrication batch allowed us to identify the weak spots in the PTP and the stereo strip layouts. By understanding the source of the weakness, the mask was modified accordingly. After modification, the follow-up fabrication batches and the latest fabrication of about 30 main sensors and associated miniature sensors have shown good performance, with no sign of microdischarge up to 1000 V.

Technology of p-type microstrip detectors with radiation hard p-spray, p-stop and moderated p-spray insulations

A technology for the fabrication of p-type microstrip silicon radiation detectors using moderated p-spray implant insulation has been developed at CNM-IMB. The p-spray insulation has been optimized in order to withstand the ionizing irradiation dose expected in the middle region of the SCT-ATLAS detector of the future Super Large Hadron Collider (s-LHC) during 10 years of operation. A dedicated mask was designed in order to fabricate pad diodes with different sizes and test structures to measure the surface resistivity. The best technological options for the moderated p-spray implants were found by using a simulation software package and a dedicated calibration run. Detectors have been fabricated with Float Zone (FZ) p-type high resistivity silicon substrates in the Clean Room facility of CNM-IMB, and characterized by reverse current and capacitance versus voltage measurements. The detectors fabricated with the moderated p-spray technology are compared to similar detectors fabricated with p-stop and p-spray insulation implants. A dedicated test structure was used to measure the interstrip resistance.

Study of the annealing effect on silicon microstrip detectors built on [formula omitted] and [formula omitted] substrates after [formula omitted] proton irradiation

The behaviour of the leakage current, interstrip resistance and capacitance have been studied on silicon microstrip detectors during an annealing period equivalent to ≃10 8 min at room temperature, after 34 MeV proton irradiation. A comparison between samples of the same geometry built on 〈1 0 0〉 and 〈1 1 1〉 substrates with different resistivity has been carried out. The samples were irradiated at 4 different fluences up to 1×10 14 p/cm 2 . After the irradiation the measurements were performed at room temperature and after heating the samples at 60°C, 80°C and 120°C to cover the complete annealing curve. The leakage current shows the same annealing behaviour typical of a simple diode. The interstrip resistance measured at full depletion voltage ( V dep) decreases in all structures, going down to few tens of MΩ at the highest fluence. It remains practically constant during the annealing period. The interstrip capacitance (at V dep) varies during the annealing period with the same behaviour in both substrates and for all the fluence values: it decreases during the annealing at room temperature, reaching a minimum value, and increases after each heat treatment. Bistable defects seem to contribute to the interstrip capacitance variation.