Silicon Pad Research Articles

The influence of edge effects on the determination of the doping profile of silicon pad diodes

Edge effects for square p+n pad diodes with guard rings, fabricated on high-ohmic silicon, are investigated. Using capacitance-voltage measurements of two pad diodes with different areas, the planar and the edge contributions to the diode capacitance are determined separately. It is found that the edge contributions are significant and that they strongly influence the determination of the doping concentration using capacitance-voltage measurements. After edge correction, the bulk doping of the pad diodes is found to be uniform within ±1.5%, which agrees with expectations. The edge-correction method is verified using TCAD simulations of two circular pad diodes with different radii.

Construction and first beam-tests of silicon-tungsten prototype modules for the CMS High Granularity Calorimeter for HL-LHC

The High Granularity Calorimeter (HGCAL) is the technology choice of the CMS collaboration for the endcap calorimetry upgrade planned to cope with the harsh radiation and pileup environment at the High Luminosity-LHC . The HGCAL is realized as a sampling calorimeter, including an electromagnetic compartment comprising 28 layers of silicon pad detectors with pad areas of 0.5–01. cm2 interspersed with absorbers made from tungsten and copper to form a highly compact and granular device. Prototype modules, based on hexagonal silicon pad sensors, with 128 channels, have been constructed and tested in beams at FNAL and at CERN. The modules include many of the features required for this challenging detector, including a PCB glued directly to the sensor, using through-hole wire-bonding for signal readout and 5 mm spacing between layers—including the front-end electronics and all services. Tests in 2016 have used an existing front-end chip —Skiroc2 (designed for the CALICE experiment for ILC). We present results from first tests of these modules both in the laboratory and with beams of electrons, pions and protons, including noise performance, calibration with mips and electron signals.

Beam tests of full-size prototypes of silicon detectors for TOF heavy-ions diagnostics in Super-FRS

The full-size prototypes of large-area silicon detectors for the Time-Of-Flight (TOF) diagnostics of heavy ions were tested with 132Xe (600 MeV/u) beam. The obtained time resolution of the prototypes was about 13 ps, which satisfied the requirements of diagnostics for the Super Fragment Separator (Super-FRS) that is under development at GSI, Darmstadt, Germany. The irradiation effect on the timing properties of silicon detectors was studied with super-fast silicon pad detectors with a rise time of 190 ps. It was shown that the changes in the rise time of the leading edge of the detector current response to 40Ar ions (40.5 MeV/u) were negligible up to the fluence of 2 × 1011 ion/cm2 expected after one year of Super-FRS operation. This result confirms the model of the leading edge current pulse formation via a flow of the polarization current in dense tracks of heavy ions and shows the perspectives for application of silicon detectors for the TOF diagnostics of intensive heavy-ion beams.

Radiation hardness study of Silicon Detectors for the CMS High Granularity Calorimeter (HGCAL)

The high luminosity LHC (HL-LHC or Phase-II) is expected to increase the instantaneous luminosity of the LHC by a factor of about five, delivering 0∼25 fb −1 per year between 2025 and 2035. Under these conditions the performance degradation of detectors due to integrated radiation dose/fluence will need to be addressed. The CMS collaboration is planning to upgrade many detector components, including the forward calorimeters. The replacement for the existing endcap preshower, electromagnetic and hadronic calorimeters is called the High Granularity Calorimeter (HGCAL) and it will be realized as a sampling calorimeter, including 40 layers of silicon detectors totalling 600 m2. The sensors will be realized as pad detectors with cell size between 0.5 and 1.0 cm2 and an active thickness between 100 μm and 300 μm depending on their location in the endcaps. The thinner sensors will be used in the highest radiation environment. For an integrated luminosity of 3000 fb −1, the electromagnetic calorimeter will have to sustain a maximum integrated dose of 1.5 MGy and neutron fluences of 1.0×1016 neq/cm2. A tolerance study after neutron irradiation of 300 μm, 200 μm, 100 μm and 50 μm n-on-p and p-on-n silicon pads irradiated to fluences up to 1.6×1016 neq/cm2 is presented. The main properties of these diodes have been studied before and after irradiation: leakage current, capacitance, charge collection efficiency with laser and sensitivity to minimum ionizing particles with radioactive source (90Sr). The results show a good performance even after the most extreme irradiation.

Invar Electrodeposition for Controlled Expansion Interconnects

In this work, we developed an electroplating technique to produce uniform, low-internal-stress and controlled-expansion nickel iron alloy (36% of Nickel and 64% of Iron (INVAR)) films deposited on different substrates to fabricate redistribution layers (RDL), through silicon vias (TSV) interconnects, pillars, bumps, pads and free standing foils. The linear thermal expansion of the INVAR deposits was measured down to 0.4 ± 0.1 (x10−6, K−1) within a process window that corresponds to ultra-low internal stress (30 ± 2 MPa) and low surface roughness (Ra ∼2 nm). Thermal conductivity and specific heat capacity of the INVAR pinhole-free films were found to be in the range of 43 ± 5 W/mK and 0.45 ± 0.05 J/gK, respectively. Ultra-high ductility of about 40 ± 10 % has been obtained for free standing INVAR films, which possess high tensile strength of 500 ± 60 MPa and yield strength of 280 ± 50 MPa.

Experimental evaluation of the resolution improvement provided by a silicon PET probe

A high-resolution PET system, which incorporates a silicon detector probe into a conventional PET scanner, has been proposed to obtain increased image quality in a limited region of interest. Detailed simulation studies have previously shown that the additional probe information improves the spatial resolution of the reconstructed image and increases lesion detectability, with no cost to other image quality measures. The current study expands on the previous work by using a laboratory prototype of the silicon PET-probe system to examine the resolution improvement in an experimental setting. Two different versions of the probe prototype were assessed, both consisting of a back-to-back pair of 1-mm thick silicon pad detectors, one arranged in 32 × 16 arrays of 1.4 mm × 1.4 mm pixels and the other in 40 × 26 arrays of 1.0 mm × 1.0 mm pixels. Each detector was read out by a set of VATAGP7 ASICs and a custom-designed data acquisition board which allowed trigger and data interfacing with the PET scanner, itself consisting of BGO block detectors segmented into 8 × 6 arrays of 6 mm × 12 mm × 30 mm crystals. Limited-angle probe data was acquired from a group of Na-22 point-like sources in order to observe the maximum resolution achievable using the probe system. Data from a Derenzo-like resolution phantom was acquired, then scaled to obtain similar statistical quality as that of previous simulation studies. In this case, images were reconstructed using measurements of the PET ring alone and with the inclusion of the probe data. Images of the Na-22 source demonstrated a resolution of 1.5 mm FWHM in the probe data, the PET ring resolution being approximately 6 mm. Profiles taken through the image of the Derenzo-like phantom showed a clear increase in spatial resolution. Improvements in peak-to-valley ratios of 50% and 38%, in the 4.8 mm and 4.0 mm phantom features respectively, were observed, while previously unresolvable 3.2 mm features were brought to light by the addition of the probe. These results support the possibility of improving the image resolution of a clinical PET scanner using the silicon PET-probe.

Invar Electroplating for Controlled Expansion Interconnects

NANO3D SYSTEMS (“NANO3D”) has developed an electroplating technique to produce uniform, low-internal-stress and controlled-expansion nickel iron alloy (36 % of Nickel and 64% of Iron (INVAR)) films on various substrates to fabricate redistribution layers (RDL), through silicon vias (TSV) interconnects, pillars, bumps, pads and free standing foils. The linear thermal expansion of the INVAR deposits was measured down to 0.4 ± 0.1 (x10-6, (K-1)) within a process window that corresponds to the ultra-low internal stress (30±2 (MPa)) and uniform morphology (Ra ~ 2 (nm)). The thermal conductivity and specific heat capacity of the INVAR pinhole-free films were found to be in the range of 43 ± 5 (W/mK) and 0.45 ± 0.05 (J/gK), respectively. Ultra-high ductility of about 40 ± 10 % has been obtained for the free standing INVAR films, which possess the high tensile strength of 500 ± 60 (MPa) and yield strength of 280 ± 50 (MPa).

CsI–Silicon Particle detector for Heavy ions Orbiting in Storage rings (CsISiPHOS)

A heavy-ion detector was developed for decay studies in the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. This detector serves as a prototype for the in-pocket particle detectors for future experiments with the Collector Ring (CR) at FAIR (Facility for Antiproton and Ion Research). The detector includes a stack of six silicon pad sensors, a double-sided silicon strip detector (DSSD), and a CsI(Tl) scintillation detector. It was used successfully in a recent experiment for the detection of the β+-decay of highly charged 142Pm60+ ions. Based on the ΔE/E technique for particle identification and an energy resolution of 0.9% for ΔE and 0.5% for E (Full Width at Half Maximum (FWHM)), the detector is well-suited to distinguish neighbouring isobars in the region of interest.

Influence of colloidal silica dispersion on the decrease of roughness in silicon chemical mechanical polishing

Colloidal silica with different dispersion states during silicon chemical mechanical polishing (CMP) are investigated in detail. As the dispersion of colloidal silica is improved, the roughness (Ra ) decreases gradually during polishing. The silicon surface has a minimum Ra of 0.118 nm when the polydispersity index of particles is 0.078. Comparing with surface polished by colloidal silica in poor dispersion, the surface quality has great improvement. The important role of dispersant is presented by the results of investigations. The research results indicate that dispersant makes aggregate micelles dispersed into many smaller uniform colloidal silica particles. The uniformly dispersed colloidal silica forms a thin film between the surfaces of silicon and polishing pad during CMP, which is beneficial for the stability of friction, and therefore greatly decreases roughness of silicon surface after polishing.

Radiation hardness and precision timing study of silicon detectors for the CMS High Granularity Calorimeter (HGC)

The high luminosity upgraded LHC or Phase-II is expected to increase the instantaneous luminosity by a factor of 10 beyond the LHC's design value, expecting to deliver 250fb−1 per year for a further 10 years of operation. Under these conditions the performance degradation due to integrated radiation dose will need to be addressed.The CMS collaboration is planning to upgrade the forward calorimeters. The replacement is called the High Granularity Calorimeter (HGC) and it will be realized as a sampling calorimeter with layers of silicon detectors interleaved. The sensors will be realized as pad detectors with sizes of less that ∼1.0cm2 and an active thickness between 100 and 300μm depending on the position, respectively, the expected radiation levels.For an integrated luminosity of 3000fb−1, the electromagnetic calorimetry will sustain integrated doses of 1.5MGy (150Mrads) and neutron fluences up to 1016neq/cm2. A radiation tolerance study after neutron irradiation of 300, 200, and 100μm n-on-p and p-on-n silicon pads irradiated to fluences up to 1.6×1016neq/cm2 is presented. The properties of these diodes studied before and after irradiation were leakage current, capacitance, charge collection efficiency, annealing effects and timing capability. The results of these measurements validate these sensors as candidates for the HGC system.

Development of the FoCal-E PAD detector and its electronics for the ALICE experiment at the LHC

In order to study the initial state of high-energy heavy-ion collisions, a forward calorimeter as an upgrade to the ALICE experiment is in the planning stage. The first beam test of the forward electro-magnetic calorimeter (FoCal-E) prototype was carried out in 2014 at the CERN PS and SPS accelerators. With the aim of reading out a signal from the low-granularity silicon pad sensors of FoCal-E, electronic circuits such as a temperature monitor, a trigger-signal converting-circuit, a trigger-signal processor, an independent regulated power circuit and an isolated high-voltage generator were developed. The electric noise problem was solved by means of both the power circuit and the high-voltage generator, and the signal was successfully read out.

Characterization of silicon detectors through TCT at Delhi University

Transient Current Technique (TCT) is one of the important methods to characterize silicon detectors and is based on the time evolution of the charge carriers generated when a laser light is shone on it. For red laser, charge is injected only to a small distance from the surface of the detector. For such a system, one of the charge carriers is collected faster than the readout time of the electronics and therefore, the effective signal at the electrodes is decided by the charge carriers that traverse throughout the active volume of the detector, giving insight to the electric field profile, drift velocity, effective doping density, etc. of the detector. Delhi University is actively involved in the silicon detector R&D and has recently installed a TCT setup consisting of a red laser system, a Faraday cage, a SMU (Source Measuring Unit), a bias tee, and an amplifier. Measurements on a few silicon pad detectors have been performed using the developed system, and the results have been found in good agreement with the CERN setup.

Silicon detectors for monitoring neutron beams in n-TOF beamlines.

During 2014, the second experimental area (EAR2) was completed at the n-TOF neutron beam facility at CERN (n-TOF indicates neutron beam measurements by means of time of flight technique). The neutrons are produced via spallation, by means of a high-intensity 20 GeV pulsed proton beam impinging on a thick target. The resulting neutron beam covers the energy range from thermal to several GeV. In this paper, we describe two beam diagnostic devices, both exploiting silicon detectors coupled with neutron converter foils containing (6)Li. The first one is based on four silicon pads and allows monitoring of the neutron beam flux as a function of the neutron energy. The second one, in beam and based on position sensitive silicon detectors, is intended for the reconstruction of the beam profile, again as a function of the neutron energy. Several electronic setups have been explored in order to overcome the issues related to the gamma flash, namely, a huge pulse present at the start of each neutron bunch which may blind the detectors for some time. The two devices were characterized with radioactive sources and also tested at the n-TOF facility at CERN. The wide energy and intensity range they proved capable of sustaining made them attractive and suitable to be used in both EAR1 and EAR2 n-TOF experimental areas, where they became immediately operational.

Random Variation Analysis and Variation-Aware Design of Symmetric Tunnel Field-Effect Transistor

A variation-immune symmetric tunnel field-effect transistor (S-TFET) is proposed for the first time to implement bidirectional current flows ( $I_{\mathrm{{\scriptstyle ON}}}=3.6~\mu $ A/ $\mu $ m, $I_{\mathrm{{\scriptstyle OFF}}}=23$ pA/ $\mu $ m at $V_{\mathrm {DD}}= 0.5$ V) with the steep-switching feature of a subthreshold slope (SS) $ mV/dec ( ${\rm SS} = 47$ mV/dec) and to alleviate the impact of random variation. A random variation analysis with the three major random variation sources, i.e., line-edge roughness, random dopant fluctuation, and work-function variation, is performed to quantitatively evaluate the impact of each variation source on the performance of the device. To perform variation-aware design for the S-TFET, the key device parameter (i.e., the thickness of the intrinsically doped silicon pad layer) is optimized to minimize the impact of random variation on the threshold voltage ( $V_{T}$ ) and SS. For ultralow power applications with a sub-0.5 V power supply voltage ( $V_{{\rm {DD}}}$ ), the variation-robust S-TFET is one of several promising device structures.

Performance of fully instrumented detector planes of the forward calorimeter of a Linear Collider detector

Detector-plane prototypes of the very forward calorimetry of a future detector at an e+e− collider have been built and their performance was measured in an electron beam. The detector plane comprises silicon or GaAs pad sensors, dedicated front-end and ADC ASICs, and an FPGA for data concentration. Measurements of the signal-to-noise ratio and the response as a function of the position of the sensor are presented. A deconvolution method is successfully applied, and a comparison of the measured shower shape as a function of the absorber depth with a Monte-Carlo simulation is given.

Low-temperature technique of thin silicon ion implanted epitaxial detectors

A new technique of large-area thin ion implanted silicon detectors has been developed within the R&D performed by the FAZIA Collaboration. The essence of the technique is the application of a low-temperature baking process instead of high-temperature annealing. This thermal treatment is performed after B+ ion implantation and Al evaporation of detector contacts, made by using a single adjusted Al mask. Extremely thin silicon pads can be therefore obtained. The thickness distribution along the X and Y directions was measured for a prototype chip by the energy loss of α-particles from 241Am (〈E α 〉 = 5.5 MeV). Preliminary tests on the first thin detector (area ≈ 20 × 20 mm2) were performed at the INFN-LNS cyclotron in Catania (Italy) using products emitted in the heavy-ion reaction 84Kr (E = 35 A MeV) + 112Sn. The ΔE − E ion identification plot was obtained using a telescope consisting of our thin ΔE detector (21 μm thick) followed by a typical FAZIA 510 μm E detector of the same active area. The charge distribution of measured ions is presented together with a quantitative evaluation of the quality of the Z resolution. The threshold is lower than 2 A MeV depending on the ion charge.

Determination of the electronics transfer function for current transient measurements

We describe a straight-forward method for determining the transfer function of the readout of a sensor for the situation in which the current transient of the sensor can be precisely simulated. The method relies on the convolution theorem of Fourier transforms. The specific example is a planar silicon pad diode. The charge carriers in the sensor are produced by picosecond lasers with light of wavelengths of 675 and 1060nm. The transfer function is determined from the 1060nm data with the pad diode biased at 1000V. It is shown that the simulated sensor response convoluted with this transfer function provides an excellent description of the measured transients for laser light of both wavelengths. The method has been applied successfully for the simulation of current transients of several different silicon pad diodes. It can also be applied for the analysis of transient-current measurements of radiation-damaged solid state sensors, as long as sensors properties, like high-frequency capacitance, are not too different.

Optically Modulated Multiband Terahertz Perfect Absorber

Development of tunable, dynamic, and broad bandwidth metamaterial designs is a keystone objective for metamaterials research, necessary for the future viability of metamaterial optics and devices across the electromagnetic spectrum. Yet, overcoming the inherently localized, narrow bandwidth, and static response of resonant metamaterials continues to be a challenging endeavor. Resonant perfect absorbers have flourished as one of the most promising metamaterial devices with applications ranging from power harvesting to terahertz imaging. Here, an optically modulated resonant perfect absorber is presented. Utilizing photo‐excited free carriers in silicon pads placed in the capacitive gaps of split ring resonators, a dynamically modulated perfect absorber is designed and fabricated to operate in reflection. Large modulation depth (38% and 91%) in two absorption bands (with 97% and 92% peak absorption) is demonstrated, which correspond to the LC (0.7 THz) and dipole (1.1 THz) modes of the split ring resonators.

Test and characterization of a prototype silicon–tungsten electromagnetic calorimeter

New generation high-energy physics experiments demand high precision tracking and accurate measurements of a large number of particles produced in the collisions of elementary particles and heavy-ions. Silicon–tungsten (Si–W) calorimeters provide the most viable technological option to meet the requirements of particle detection in high multiplicity environments. We report a novel Si–W calorimeter design, which is optimized for γ/π0 discrimination up to high momenta. In order to test the feasibility of the calorimeter, a prototype mini-tower was constructed using silicon pad detector arrays and tungsten layers. The performance of the mini-tower was tested using pion and electron beams at the CERN Proton Synchrotron (PS). The experimental results are compared with the results from a detailed GEANT-4 simulation. A linear relationship between the observed energy deposition and simulated response of the mini-tower has been obtained, in line with our expectations.

Unconstrained Sleep Apnea Monitoring Using Polyvinylidene Fluoride Film-Based Sensor

We established and tested an unconstrained sleep apnea monitoring method using a polyvinylidene (PVDF) film-based sensor for continuous and accurate monitoring of apneic events occurred during sleep. Twenty-six sleep apnea patients and six normal subjects participated in this study. Subjects' respiratory signals were measured using the PVDF-based sensor during polysomnography. The PVDF sensor comprised a 4 × 1 array, and a thin silicon pad was placed over the sensor to prevent damage. Total thickness of the merged system was approximately 1.1mm which was thin enough to prevent the subject from being consciously aware of its presence. It was designed to be placed under subjects' backs and installed between a bed cover and mattress. The proposed method was based on the standard deviation of the PVDF signals, and it was applied to a test set for detecting apneic events. The method's performance was assessed by comparing the results with a sleep physician's manual scoring. The correlation coefficient for the apnea-hypopnea index (AHI) values between the methods was 0.94 (p < 0.001). The areas under the receiver operating curves at three AHI threshold levels (>5, >15, and >20) for sleep apnea diagnosis were 0.98, 0.99, and 0.98, respectively. For min-by-min apnea detection, the method classified sleep apnea with an average sensitivity of 72.9%, specificity of 90.6%, accuracy of 85.5%, and kappa statistic of 0.60. The developed system and method can be applied to sleep apnea detection in home or ambulatory monitoring.