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We explore compatibility of Ru with Al-Ge eutectic wafer bonding. We first present experiments to check for the presence of Ru ternary alloy poisoning inhibiting Al-Ge melting as well as evaluations of Al-Ge melt wettability on Ru and diffusion outcomes following bond-simulating anneals. Results show that Ru is stable with no observed microstructural changes or dissolution in the melt, indicating no ternary poisoning for the applied thermal budget. Ru was found to act as an effective barrier offering good melt wettability in all considered configurations with Al and Ge. From inspection of the binary constituents of Al-Ge-Ru we propose that Al-Ge eutectic melting temperature will decrease marginally for Ru contamination in a 1-2% range before a drastic increase in melting temperature (>10°C/% Ru) at higher Ru compositions. We then demonstrate wafer-level packaged 200 mm devices and MEMS with strong bond outcomes of devices bearing Ru contacts. We conclude that Ru has high compatibility with Al-Ge eutectic bonding.

A Vernier ring-oscillator-based time-to-digital converter (TDC) with a new prelogic is presented. Experimental results show that the proposed architecture achieve a 5.5 ps RMS timing jitter with a 5.1 ps LSB within an area of 0.00151 mm 2 . Thanks to the new prelogic circuit, the power consumption of the circuit was optimised to 22 μ W at a rate of 1 Mevents/s for a dynamic range of 4 ns. The area, timing jitter and power consumption make the TDC suitable for an array of electronic readout in a position emission tomography single photon avalanche diode based detectors.

Author(s): Chambers, C; Walton, T; Fairbank, D; Craycraft, A; Yahne, DR; Todd, J; Iverson, A; Fairbank, W; Alamare, A; Albert, JB; Anton, G; Arnquist, IJ; Badhrees, I; Barbeau, PS; Beck, D; Belov, V; Bhatta, T; Bourque, F; Brodsky, JP; Brown, E; Brunner, T; Burenkov, A; Cao, GF; Cao, L; Cen, WR; Charlebois, SA; Chiu, M; Cleveland, B; Coon, M; Cree, W; Cote, M; Dalmasson, J; Daniels, T; Darroch, L; Daugherty, SJ; Daughhetee, J; Delaquis, S; Mesrobian-Kabakian, A Der; DeVoe, R; Dilling, J; Ding, YY; Dolinski, MJ; Dragone, A; Echevers, J; Fabris, L; Farine, J; Feyzbakhsh, S; Fontaine, R; Fudenberg, D; Giacomini, G; Gornea, R; Gratta, G; Hansen, EV; Heffner, M; Hoppe, EW; Hosl, J; House, A; Hufschmidt, P; Hughes, M; Ito, Y; Jamil, A; Jessiman, C; Jewell, MJ; Jiang, XS; Karelin, A; Kaufman, LJ; Kodroff, D; Koffas, T; Kravitz, S; Krucken, R; Kuchenkov, A; Kumar, KS; Lan, Y; Larson, A; Leonard, DS; Li, G; Li, S; Li, Z; Licciardi, C; Lin, YH; Lv, P; MacLellan, R; Michel, T; Mong, B; Moore, DC | Abstract: The search for neutrinoless double beta decay probes the fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct. Double beta detectors are large and expensive, so background reduction is essential for extracting the highest sensitivity. The identification, or 'tagging', of the $^{136}$Ba daughter atom from double beta decay of $^{136}$Xe provides a technique for eliminating backgrounds in the nEXO neutrinoless double beta decay experiment. The tagging scheme studied in this work utilizes a cryogenic probe to trap the barium atom in solid xenon, where the barium atom is tagged via fluorescence imaging in the solid xenon matrix. Here we demonstrate imaging and counting of individual atoms of barium in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser sits on an individual atom, the fluorescence persists for $\sim$30~s before dropping abruptly to the background level, a clear confirmation of one-atom imaging. No barium fluorescence persists following evaporation of a barium deposit to a limit of $\leq$0.16\%. This is the first time that single atoms have been imaged in solid noble element. It establishes the basic principle of a barium tagging technique for nEXO.

Single photon avalanche diodes (SPAD) are detectors used for applications requiring high timing resolution and single photon sensitivity. Medical imaging modalities such as positron emission tomography benefit from higher timing resolution enabling time-of-flight (ToF) measurements. One of the current challenge in the development of SiPM detectors is combining both high photodetection efficiency (PDE) and high single photon timing resolution (SPTR). Although SiPM have a high fill factor, they are limited for sub-10 ps FWHM SPTR for two reasons: the high output capacitance directly affects the rise time and a timing skew is associated with the SPAD position in the array. The proposed solution to obtain ToF capabilities with both high SPTR and PDE is to make a 3D digital SiPM where every SPAD is individually connected to a quenching circuit (QC) and a time-to-digital converter (TDC). While the 3D integration process is underway, we designed and tested a 2D version of a single channel of the detector composed of a SPAD, a QC and a TDC in TSMC CMOS 65 nm. This paper presents a SPAD and QC with 9 ps FWHM SPTR and a chain composed of a SPAD, a QC and a TDC with 17.5 ps FWHM SPTR.

Silicon photomultipliers are regarded as a very promising technology for next-generation, cutting-edge detectors for low-background experiments in particle physics. This work presents systematic reflectivity studies of Silicon Photomultipliers (SiPM) and other samples in liquid xenon at vacuum ultraviolet (VUV) wavelengths. A dedicated setup at the University of M\"unster has been used that allows to acquire angle-resolved reflection measurements of various samples immersed in liquid xenon with 0.45{\deg} angular resolution. Four samples are investigated in this work: one Hamamatsu VUV4 SiPM, one FBK VUV-HD SiPM, one FBK wafer sample and one Large-Area Avalanche Photodiode (LA-APD) from EXO-200. The reflectivity is determined to be 25-36% at an angle of incidence of 20{\deg} for the four samples and increases to up to 65% at 70{\deg} for the LA-APD and the FBK samples. The Hamamatsu VUV4 SiPM shows a decline with increasing angle of incidence. The reflectivity results will be incorporated in upcoming light response simulations of the nEXO detector., Comment: 18 pages, 11 figures

Analog and digital SiPMs have revolutionized the field of radiation instrumentation by replacing both avalanche photodiodes and photomultiplier tubes in many applications. However, multiple applications require greater performance than the current SiPMs are capable of, for example timing resolution for time-of-flight positron emission tomography and time-of-flight computed tomography, and mitigation of the large output capacitance of SiPM array for large-scale time projection chambers for liquid argon and liquid xenon experiments. In this contribution, the case will be made that 3D photon-to-digital converters, also known as 3D digital SiPMs, have a potentially superior performance over analog and 2D digital SiPMs. A review of 3D photon-to-digital converters is presented along with various applications where they can make a difference, such as time-of-flight medical imaging systems and low-background experiments in noble liquids. Finally, a review of the key design choices that must be made to obtain an optimized 3D photon-to-digital converter for radiation instrumentation, more specifically the single-photon avalanche diode array, the CMOS technology, the quenching circuit, the time-to-digital converter, the digital signal processing and the system level integration, are discussed in detail.

nEXO is a proposed tonne-scale neutrinoless double beta decay ($0\nu\beta\beta$) experiment using liquid ${}^{136}Xe$ (LXe) in a Time Projection Chamber (TPC) to read out ionization and scintillation signals. Between the field cage and the LXe vessel, a layer of LXe ("skin" LXe) is present, where no ionization signal is collected. Only scintillation photons are detected, owing to the lack of optical barrier around the field cage. In this work, we show that the light originating in the skin LXe region can be used to improve background discrimination by 5% over previous published estimates. This improvement comes from two elements. First, a fraction of the $\gamma$-ray background is removed by identifying light from interactions with an energy deposition in the skin LXe. Second, background from ${}^{222}Rn$ dissolved in the skin LXe can be efficiently rejected by tagging the $\alpha$ decay in the ${}^{214}Bi-{}^{214}Po$ chain in the skin LXe., Comment: 11 pages, 12 figures

In the fabrication of microelectronic devices, sputtering of thin films leads to isotropic rough surfaces described by (i) a Gaussian height distribution and (ii) a constant power spectral density (PSD) up to a roll-off frequency of q0 from which it decays as a power law. This paper introduces a robust approach to estimate the topography parameters such as the density, the mean radius of curvature and the height distribution of summits for isotropic Gaussian rough micro-surfaces. The proposed approach consists of computing the PSD function from the atomic force microscopy measurements, approximating the value of q0 and the magnitude P0 of the plateau by curve fitting algorithms, and analytically deriving the topography parameters using the random process theory. The robustness of the proposed methodology is assessed on a sputtered gold micro-surface. The investigations show that the proposed PSD-based approach reduces the dependence of the topography parameters on the sampling length compared to the deterministic method. Moreover, the PSD-based estimated density of summits is in a good agreement with scanning electron microscopy analysis, in contrast to both deterministic and autocorrelation-based methods. The proposed approach is therefore more suitable to extract topography parameters from measurements of micro-surfaces for the development of contact asperity-based models.

International audience; Through-Silicon-Vias (TSV) are the key to 3D integrated microsystems. Their fabrication leads to reliability issues linked to the thermo-mechanical stress induced in the silicon around the vias. In this work, we propose to reduce the silicon residual stress in high aspect ratio copper TSVs (HAR TSV) using an electrografted polymer insulator, poly-4-vinylpyridine (P4VP), in replacement of the traditional silicon oxide layer. We use Raman spectroscopy to make the first investigation of the residual stress in the Si around P4VP insulated TSVs and compare it to SiO2 insulated TSVs. The results show that P4VP acts as a stress buffer layer because of its particular mechanical properties as the measured residual stress in Si is significantly reduced at room temperature around the polymer insulated HAR TSVs. The potential benefits of such a technology are not only a better thermo-mechanical reliability of 3D integrated microsystem, but also a greater integration density.

In this paper, a design and optimization method for submillimeter-wave Schottky diode is proposed. Parasitic capacitance is significantly reduced to under 20% of the total capacitance of the diode. The parasitic capacitance value is measured to be 0.6 fF for 1 μm anode radius which increased the cut-off frequency to 1.5 THz. A corresponding microfabrication process that provides higher degrees of freedom for the anode diameter, air-bridge dimensions and distance to the substrate is introduced and implemented. The DC and RF measurements are provided and compared with the simulations. In order to provide a better understanding of the diode behavior, the limiting factors of the cut-off frequency for different applications are studied and compared. For the mixer/multiplier mode, an improved and expanded formulation for calculation of the cut-off frequency is introduced. It is shown that the usable voltage bias range (with acceptable cut-off frequency) is limited by the exponential reduction of junction resistance, R j , in mixer/multiplier mode.

The 3M-ESR multilayer polymer film is a widely used reflector in scintillation detector arrays. As specified in the datasheet and confirmed experimentally by measurements in air, it is highly reflective ( > 98 % ) over the entire visible spectrum (400–1000 nm) for all angles of incidence. Despite these outstanding characteristics, it was previously found that light crosstalk between pixels in a bonded LYSO scintillator array with ESR reflector can be as high as ∼30–35%. This unexplained light crosstalk motivated further investigation of ESR optical performance. Analytical simulation of a multilayer structure emulating the ESR reflector showed that the film becomes highly transparent to incident light at large angles when surrounded on both sides by materials of refractive index higher than air. Monte Carlo simulations indicate that a considerable fraction (∼25–35%) of scintillation photons are incident at these leaking angles in high aspect ratio LYSO scintillation crystals. The film transparency was investigated experimentally by measuring the scintillation light transmission through the ESR film sandwiched between a scintillation crystal and a photodetector with or without layers of silicone grease. Strong light leakage, up to nearly 30%, was measured through the reflector when coated on both sides with silicone, thus elucidating the major cause of light crosstalk in bonded arrays. The reflector transparency was confirmed experimentally for angles of incidence larger than ~ 60 ° using a custom designed setup allowing illumination of the bonded ESR film at selected grazing angles. The unsuspected ESR film transparency can be beneficial for detector arrays exploiting light sharing schemes, but it is highly detrimental for scintillator arrays designed for individual pixel readout.

Measurements of electron drift properties in liquid and gaseous xenon are reported. The electrons are generated by the photoelectric effect in a semi-transparent gold photocathode driven in transmission mode with a pulsed ultraviolet laser. The charges drift and diffuse in a small chamber at various electric fields and a fixed drift distance of 2.0 cm. At an electric field of 0.5 kV/cm, the measured drift velocities and corresponding temperature coefficients respectively are 1 . 97 ± 0 . 04 m m ∕ μ s and ( − 0 . 69 ± 0 . 05 ) %/K for liquid xenon, and 1 . 42 ± 0 . 03 m m ∕ μ s and ( + 0 . 11 ± 0 . 01 ) %/K for gaseous xenon at 1.5 bar. In addition, we measure longitudinal diffusion coefficients of 25 . 7 ± 4 . 6 cm 2 /s and 149 ± 23 cm 2 /s, for liquid and gas, respectively. The quantum efficiency of the gold photocathode is studied at the photon energy of 4.73 eV in liquid and gaseous xenon, and vacuum. These charge transport properties and the behavior of photocathodes in a xenon environment are important in designing and calibrating future large scale noble liquid detectors.

In this paper we report on the characterization of the Hamamatsu VUV4 (S/N: S13370-6152) Vacuum Ultra-Violet (VUV) sensitive Multi-Pixel Photon Counters (MPPC)s as part of the development of a solution for the detection of liquid xenon scintillation light for the nEXO experiment. Various MPPC features, such as: dark noise, gain, correlated avalanches, direct crosstalk and Photon Detection Efficiency (PDE) were measured in a dedicated setup at TRIUMF. MPPCs were characterized in the range 163 K ≤ T ≤ 233 K . At an over voltage of 3 . 1 ± 0 . 2 V and at T = 163 K we report a number of Correlated Avalanches (CAs) per pulse in the 1 μ s interval following the trigger pulse of 0 . 161 ± 0 . 005 . At the same settings the Dark-Noise (DN) rate is 0 . 137 ± 0 . 002 Hz/mm 2 . Both the number of CAs and the DN rate are within nEXO specifications. The PDE of the Hamamatsu VUV4 was measured for two different devices at T = 233 K for a mean wavelength of 189 ± 7 nm . At 3 . 6 ± 0 . 2 V and 3 . 5 ± 0 . 2 V of over voltage we report a PDE of 13 . 4 ± 2 . 6 % and 11 ± 2 % , corresponding to a saturation PDE of 14 . 8 ± 2 . 8 % and 12 . 2 ± 2 . 3 % , respectively. Both values are well below the 24 % saturation PDE advertised by Hamamatsu. More generally, the second device tested at 3 . 5 ± 0 . 2 V of over voltage is below the nEXO PDE requirement. The first one instead yields a PDE that is marginally close to meeting the nEXO specifications. This suggests that with modest improvements the Hamamatsu VUV4 MPPCs could be considered as an alternative to the FBK-LF Silicon Photo-Multipliers for the final design of the nEXO detector.

Characterization of the vacuum ultraviolet (VUV) reflectance of silicon photomultipliers (SiPMs) is important for large-scale SiPM-based photodetector systems. We report the angular dependence of the specular reflectance in a vacuum of SiPMs manufactured by Fondazionc Bruno Kessler (FBK) and Hamamatsu Photonics K.K. (HPK) over wavelengths ranging from 120 nm to 280 nm. Refractive index and extinction coefficient of the thin silicon-dioxide film deposited on the surface of the FBK SiPMs are derived from reflectance data of a FBK silicon wafer with the same deposited oxide film as SiPMs. The diffuse reflectance of SiPMs is also measured at 193 nm. We use the VUV spectral dependence of the optical constants to predict the reflectance of the FBK silicon wafer and FBK SiPMs in liquid xenon.

nEXO is a proposed experiment to search for the neutrino-less double beta decay ($0\nu\beta\beta$) of $^{136}$Xe in a tonne-scale liquid xenon time projection chamber (TPC). The nEXO TPC will be equipped with charge collection tiles to form the anode. In this work, the charge reconstruction performance of this anode design is studied with a dedicated simulation package. A multi-variate method and a deep neural network are developed to distinguish simulated $0\nu\beta\beta$ signals from backgrounds arising from trace levels of natural radioactivity in the detector materials. These simulations indicate that the nEXO TPC with charge-collection tiles shows promising capability to discriminate the $0\nu\beta\beta$ signal from backgrounds. The estimated half-life sensitivity for $0\nu\beta\beta$ decay is improved by $\sim$20$~(32)\%$ with the multi-variate~(deep neural network) methods considered here, relative to the sensitivity estimated in the nEXO pre-conceptual design report.

Understanding reflective properties of materials and photodetection efficiency (PDE) of photodetectors is important for optimizing energy resolution and sensitivity of the next generation neutrinoless double beta decay, direct detection dark matter, and neutrino oscillation experiments that will use noble liquid gases, such as nEXO, DARWIN, DarkSide-20k, and DUNE. Little information is currently available about reflectivity and PDE in liquid noble gases, because such measurements are difficult to conduct in a cryogenic environment and at short enough wavelengths. Here we report a measurement of specular reflectivity and relative PDE of Hamamatsu VUV4 silicon photomultipliers (SiPMs) with 50 micrometer micro-cells conducted with xenon scintillation light (~175 nm) in liquid xenon. The specular reflectivity at 15 deg. incidence of three samples of VUV4 SiPMs is found to be 30.4+/-1.4%, 28.6+/-1.3%, and 28.0+/-1.3%, respectively. The PDE at normal incidence differs by +/-8% (standard deviation) among the three devices. The angular dependence of the reflectivity and PDE was also measured for one of the SiPMs. Both the reflectivity and PDE decrease as the angle of incidence increases. This is the first measurement of an angular dependence of PDE and reflectivity of a SiPM in liquid xenon., Comment: 21 pages, 15 figures, 6 tables. As accepted by JINST

The challenge to reach 10 ps coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) is triggering major efforts worldwide, but timing improvements of scintillation detectors will remain elusive without depth-of-interaction (DOI) correction in long crystals. Nonetheless, this momentum opportunely brings up the prospect of a fully time-based DOI estimation since fast timing signals intrinsically carry DOI information, even with a traditional single-ended readout. Consequently, extracting features of the detected signal time distribution could uncover the spatial origin of the interaction and in return, provide enhancement on the timing precision of detectors. We demonstrate the validity of a time-based DOI estimation concept in two steps. First, experimental measurements were carried out with current LSO:Ce:Ca crystals coupled to FBK NUV-HD SiPMs read out by fast high-frequency electronics to provide new evidence of a distinct DOI effect on CTR not observable before with slower electronics. Using this detector, a DOI discrimination using a double-threshold scheme on the analog timing signal together with the signal intensity information was also developed without any complex readout or detector modification. As a second step, we explored by simulation the anticipated performance requirements of future detectors to efficiently estimate the DOI and we proposed four estimators that exploit either more generic or more precise features of the DOI-dependent timestamp distribution. A simple estimator using the time difference between two timestamps provided enhanced CTR. Additional improvements were achieved with estimators using multiple timestamps (e.g. kernel density estimation and neural network) converging to the Cramér–Rao lower bound developed in this work for a time-based DOI estimation. This two-step study provides insights on current and future possibilities in exploiting the timing signal features for DOI estimation aiming at ultra-fast CTR while maintaining detection efficiency for TOF PET.

The 3D vertical integration of SPAD and readout electronics is a promising avenue to high performance photodetectors. This approach will minimize most limitations of current SiPM and lead to better performances in terms of effective PDE, timing and added functionalities. In this paper, we present a new integrated digital SiPM electronic architecture. This specific architecture aims to replace conventional analog SiPM with the added benefits of a significantly lower and constant output capacitance as well as immunity to SPAD to SPAD gain variation on the single photon resolution while providing an analog-like output signal waveform representing the sum of all triggered SPADs. This electronic architecture also offers options to turn off noisy SPADs, to reduce afterpulsing through a programmable hold-off time and to adjust the dynamic range of the output depending on the photon flux. While the 3D integration process development is underway, we realized a 2D version as a proof of principle of the electronic architecture. The 2D detector presented in this article is composed of an array of 242 pixels for a size of $1.1\times 1.1$ mm2. The output capacitance of the device is 5.6 pF/mm2, the timing jitter associated to the quenching circuit is about 40 ps FWHM and the maximum timing skew within the array is about 70 ps, which could still be optimized.

Via-Last metallization of High Aspect Ratio Through Silicon Via (HAR TSV) for 3D integration is challenging. Indeed, the formation of a uniform and conformal dielectric to insulate HAR TSVs in Via-Last process flow is difficult to achieve for any physical deposition process. In this study, we present the first reported HAR copper TSVs, insulated by highly conformal electrografted poly-4-vinylpyridine (P4VP) in Via-Last process-flow. As a demonstration, this allowed the metallization of HAR copper TSVs for die-to-die 3D integration of a 22 × 22 photodiode array tier onto a CMOS control electronics ASIC. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0321606jss] All rights reserved.

The characterization of nuisance parameters in digital silicon photomultipliers (SiPMs) is important to their understanding and future development. Methods able to distinguish the types of events are necessary to obtain fair and legitimate measurements. In this work, the zero photon probability (ZPP) method and the time delay (TD) method are used to measure the dark noise of digital SiPMs free from the contribution of correlated noise such as afterpulsing and crosstalk. It highlights the unique features of digital SiPMs such as the holdoff delay, the digital output signal, and the embedded processing (e.g. the selection of the interval sampling width). The two methods correctly separate the correlated and uncorrelated events in digital SiPMs and therefore the determination of a true photon detection efficiency (PDE) is possible. The ZPP method is also implemented inside a digital SiPM using embedded digital signal processing.

Coincidence timing resolution (CTR) of 10 ps FWHM would drastically increase the contrast of images in pre-clinical and clinical positron emission tomography (PET) due to the capability to localize the annihilation site along the line of response with unprecedented accuracy. Currently, the scintillators and the photodetectors with their electronic readout are the two limiting factors to reach such a timing resolution. On the photodetector side, the single photon timing resolution (SPTR) must be improved below 4 ps RMS to reach a CTR of 10 ps FWHM. To this end, we propose to use a 3D digital silicon photomultiplier (3DdSiPM) where each single photon avalanche diode (SPAD) has its own quenching circuit (QC) and time-to-digital converter (TDC). To reach an SPTR of 4 ps RMS in an array of SPAD requires to correct each readout pixel individually to minimize the impact of TDC least significant bit (LSB) non-uniformities and SPAD-to-SPAD skew. For this purpose, we developed an array of 256 pixels of SPAD readout circuits optimized for high timing precision with embedded digital signal processing. The latter is a code-to-time conversion where each pixel is individually corrected for TDC LSB variation and skew. Measurement results on single pixels (QC and TDC) show timing jitter down to 8 ps RMS. The uncorrected array timing jitter is about 87 ps RMS and is reduced to 18 ps RMS after skew correction and TDC LSB correction.

Single Photon Avalanche Diode (SPAD) and Silicon PhotoMultiplier (SiPM) are detectors of high interest in radiation instrumentation. They provide high sensitivity in the UV-visible range and sub-nanosecond timing resolution. Systems such as large area detectors in particle physics also require the detectors to be low noise, low cost and suitable for large scale integration. We report on a 3D vertically integrated digital SiPM process made with industrial semiconductor technologies. The 3D digital SiPM architecture integrates a SPAD array with CMOS electronics using exclusively industrial technologies to ensure large scale fabrication capability. Three main development runs are ongoing: the custom SPAD fabrication (Teledyne DALSA CCD foundry), the electronics readout integrated circuit design (TSMC CMOS 180 nm), and the wafer scale 3D integration (Teledyne DALSA MEMS foundry). This paper focuses on the SPAD design and the 3D process made inside Teledyne DALSA’s facilities. First wafers from both the SPAD and 3D runs have been completed recently. We report on the characterization of those wafers. For the SPAD run, IV curves, dark noise, detection efficiency and timing resolution will be presented. For the 3D wafers, the studied aspects are the bonding strength, the uniformity and the interconnects electrical characteristics. The results will be discussed in the context of low background neutrino and dark matter physics experiments.

Single Photon Avalanche Diodes (SPAD) are known for their excellent timing performance which enables Time of Flight capabilities in positron emission tomography (PET). However, current array architectures juxtapose the SPAD with its ancillary electronics at the expense of a poor fill factor of the SPAD array. The 3D vertical integration of SPADs and readout electronics represents a solution to the aforementioned problem. Compared to systems with external electronics readout, 3D vertical integration reduces the SPAD interconnect parasitic capacitance while greatly increasing the photosensitive area and improving overall performances. This paper presents the implementation of two SPAD structures designed for PET. The SPAD structures are designed using Teledyne DALSA high voltage (HV) CMOS technology targeted for a 3-dimensional single photon counting module (3DSPCM). SPAD with two types of guard ring (diffusion-based and virtual guard ring) are designed, fabricated and characterized. All structures are based on a $p + $ anode in an $n$ -well cathode and are implemented along with active quenching circuits for proper characterization. The results show that the contact distribution and the anode-cathode spacing impact the dark count rate (DCR). The design of SPADs with a diffusion guard ring have a DCR down to $3~\hbox{s}^{- 1}\mu \hbox{m}^{-2}$ at room temperature, afterpulsing probability of $ , timing resolution of 27 ps FWHM and PDE of 49% at 480 nm.

The next-generation Enriched Xenon Observatory (nEXO) is a proposed experiment to search for neutrinoless double beta ($0\nu\beta\beta$) decay in $^{136}$Xe with a target half-life sensitivity of approximately $10^{28}$ years using $5\times10^3$ kg of isotopically enriched liquid-xenon in a time projection chamber. This improvement of two orders of magnitude in sensitivity over current limits is obtained by a significant increase of the $^{136}$Xe mass, the monolithic and homogeneous configuration of the active medium, and the multi-parameter measurements of the interactions enabled by the time projection chamber. The detector concept and anticipated performance are presented based upon demonstrated realizable background rates., Comment: v2 as published

Individually coupled scintillation detectors used in positron emission tomography (PET) imaging suffer from important signal losses due to the suboptimal light collection from crystals. As only a fraction of the light is generally extracted from long and thin scintillators, it is important to identify and understand the predominant causes of signal loss in order to eventually recover it. This simulation study investigates the multiple factors affecting the light transport in high-aspect ratio LYSO scintillators wrapped in specular reflectors through a full factorial design. By exploring various combinations of crystal geometry, readout conditions and wrapping conditions, it was found that an optimum light output can only be achieved through a careful selection of highly reflective material along with high-transmittance optical adhesive used to bond the reflector. Decreasing the adhesive thickness was also found to have a positive outcome in most explored configurations, however to a much lesser extent. Suboptimal reflectivity and adhesive transmittance also lead to an asymmetric light output distribution dependent on the depth of interaction of the radiation, potentially degrading energy resolution. By identifying the factors causing the most significant scintillation light losses through a factorial design, the most promising detector configurations have been identified in the quest for optimal light collection from scintillators.

A new design for the anode of a time projection chamber, consisting of a charge-detecting "tile", is investigated for use in large scale liquid xenon detectors. The tile is produced by depositing 60 orthogonal metal charge-collecting strips, 3~mm wide, on a 10~\si{\cm} $\times$ 10~\si{\cm} fused-silica wafer. These charge tiles may be employed by large detectors, such as the proposed tonne-scale nEXO experiment to search for neutrinoless double-beta decay. Modular by design, an array of tiles can cover a sizable area. The width of each strip is small compared to the size of the tile, so a Frisch grid is not required. A grid-less, tiled anode design is beneficial for an experiment such as nEXO, where a wire tensioning support structure and Frisch grid might contribute radioactive backgrounds and would have to be designed to accommodate cycling to cryogenic temperatures. The segmented anode also reduces some degeneracies in signal reconstruction that arise in large-area crossed-wire time projection chambers. A prototype tile was tested in a cell containing liquid xenon. Very good agreement is achieved between the measured ionization spectrum of a $^{207}$Bi source and simulations that include the microphysics of recombination in xenon and a detailed modeling of the electrostatic field of the detector. An energy resolution $\sigma/E$=5.5\% is observed at 570~\si{keV}, comparable to the best intrinsic ionization-only resolution reported in literature for liquid xenon at 936~V/\si{cm}., Comment: 18 pages, 13 figures, as published

In this paper, dielectric ribbon waveguides (DRWs) are designed and characterized to work in the $F$ (90–140 GHz) and $G$ (140–220 GHz) frequency bands. High resistivity silicon (HR-silicon) due to its high transparency in these bands, and its low-cost fabrication process, were selected for the fabrication of these waveguides. The guided modes of DRW are studied using Marcatili and multi-line methods. A design procedure for optimal single-mode propagation was introduced. The wave attenuation inside the waveguide is discussed. Our experiments show that HR-silicon DRW has less than 0.087 dB/cm (0.01 ${\hbox{cm}}^{-1}$ ) of loss over the guiding bandwidth. The designed DRWs were realized using microfabrication techniques. The measurements are in very good agreement with theoretical predictions.

We developed a configurable model of single photon avalanche diodes (SPAD) array photodetectors with intelligent control and active quenching. In this model individual components can be simulated independently and subsequently linked to provide the overall detector response. The model enables the simulation of the entire detector and analysis of performance, including photon detection efficiency, timing and energy resolution. It can be used to optimize detector performance for specific applications, such as positron emission tomography (PET). The simulator consists of multiple configurable and interchangeable modules to model the array geometry as well as physical and optical characteristics based on physical models and statistical equations. Readout electronics are also simulated in an algorithmic form. Monte Carlo simulations are used to model the 511 keV annihilation photon interactions and the optical photon transport in the scintillator, as well as carrier random walk in the silicon. Different methods to extract information from the digital output signal can be investigated. The simulator paves the way to the developement of new algorithms to extract relevant information in PET, but also for other applications such as Cerenkov radiation and fluorescence microscopy. Simulation results for photon detection efficiency, energy resolution and timing resolution are reported, showing the functionality of the simulator.

Electromagnetic simulators are essential tools for the design of complex microwave and RF systems. They are also extremely useful for educational purposes. One of their main advantages is the possibility to visualize the electromagnetic fields in any structure. Through this visualization, we can help the students comprehend the mathematical and physical concept involved and provide them with an easily understandable image of the field distribution inside any structure. Commercially available electromagnetic simulators [High Frequency Simulation Software (HFSS), Computer Simulation Technology Microwave Studio (CST), Sonnet electromagnetic field solver, etc.] allow us to pass from the somewhat obscure mathematical representation of the electromagnetic field theory, i.e., Maxwell's equations, to accurate images and animations. By understanding the modeling properties of the simulators, students can explore more details of complex structures [1].

We report on the fabrication of Josephson junction and superconducting bridges using a damascene CMP process applied for a first time to superconductors. The demonstrated industrial reliability of damascene CMP processes on large scale semiconductor circuits is a major incentive for our research that should allow large numbers of nanometric Josephson junctions to be fabricated in both Nb and Al, the two main material employed in superconducting quantum computing (qubit) and RSFQ electronics fabrication.We carried out a Chemical-Mechanical Polishing (CMP) process on Nb and Al films deposited on a SiO2 layer patterned with trenches of 100 to 300nm of nominal depth. The process formed long bridges, 1 to 4μm wide. The susceptibility and resistive transitions showed that CMP has no observable influence on superconductivity.We have also developed a hybrid technique that uses Al/Al2O3/Al shadow evaporation in the trenches before the damascene CMP process. This allows for high quality “in-situ” junction oxidation with the size reduction benefit provided by the damascene CMP process. We easily reach junctions sizes near 0.5μm2 which are difficult to fabricate by other methods.We describe these techniques and report on measurements on large bridges and junctions and on the fabrication and measurements of Al and Nb nanobridges.

This paper presents a new quenching circuit (QC) and single photon avalanche diode (SPAD) implemented in TSMC CMOS 65 nm technology. The QC was optimized for single photon timing resolution (SPTR) with a view to an implementation in a 3D digital SiPM. The presented QC has a timing jitter of 4 ps full width at half maximum (FWHM) and the SPAD and QC has a 7.8 ps FWHM SPTR. The QC adjustable threshold allows timing resolution optimization as well as SPAD excess voltage and rise time characterization. The adjustable threshold, hold-off and recharge are essential to optimize the performances of each SPAD. This paper also provides a better understanding of the different contributions to the SPTR. A study of the contribution of the SPAD excess voltage variation combined to the QC time propagation delay variation is presented. The proposed SPAD and QC eliminates the SPAD excess voltage contribution to the SPTR for excess voltage higher than 1 V due to its fixed time propagation delay.

We report on the performance of silicon photomultiplier (SiPM) light sensors operating in electric field strength up to 30 kV/cm and at a temperature of 149K, relative to their performance in the absence of an external electric field. The SiPM devices used in this study show stable gain, photon detection efficiency, and rates of correlated pulses, when exposed to external fields, within the estimated uncertainties. No observable physical damage to the bulk or surface of the devices was caused by the exposure., Comment: 16 pages, 12 figures, 2 tables and two conferences (INPC2016 and TIPP2017)

Self-switching devices (SSDs) are new nano-scale field effect active components. In the present work, these devices are made in siliconon-insulator (SOI) technology and operate at room temperature. We investigate their current-voltage (I-V) characteristics which show a diode-like behaviour due to electrostatic effects. Thermal activation measurements are presented and discussed. We also present simulations to gain better understanding of device physics and also to optimize the critical parameters of the fabrication process. (c) 2007 Elsevier Ltd. All rights reserved.

The performance of scintillation detectors used in Positron Emission Tomography imaging strongly depends on the scintillation light transport from the crystal to the photodetector. In highly pixelated scintillator arrays with individual pixels approaching millimetric cross section, the loss of signal is compounded with crosstalk effects, squandering valuable signal to adjacent pixels, and with light absorption in lateral faces adhesive materials and imperfect reflectors. The purpose of this simulation study is to uncover processes responsible for light losses in scintillator arrays. Four sources of losses through crosstalk between pixels were identified, namely 1) escaping photoelectrons to other pixels after photoelectric interactions, 2) X-ray fluorescence and Auger emission, 3) reflector transparency to scintillation light, and 4) light leakage to other crystals due to adhesive material between reflectors and scintillators in which optical photons can propagate to other crystals. An important source of signal loss and energy resolution degradation was found to be related to the transmittance of the adhesive material used to bond reflectors to scintillators. Moreover, the angular distribution of scintillation photons impinging on the detection face was assessed in order to weigh the proportion of trapped photons through total internal reflection due to the refractive index difference between scintillators and optical coupling medium.

Single Photon Avalanche Diodes (SPAD) are promising candidates for fast timing applications in high energy physics (HEP) and positron emission tomography (PET) for their excellent timing resolution and detection efficiency [1], [2]. However, as with many technologies, designing the best SPAD array involves many trade-offs. To facilitate the selection of optimal parameters for a specific application, as well as study the behavior of SPAD arrays, we developed a custom simulator. The size and number of individual SPAD cells affect the fill factor, which in turn changes the photon detection probability, the photon statistics and the dynamic range. However, the search for optimal pixel and array size has only been investigated with preexisting geometries; here we present a detailed parametric study. Fill factor is, as expected, a driving force for high coincidence time resolution (CTR), as is the choice of applied bias. The best combination of parameters with LYSO crystal leads to a CTR of 120 ps FWHM. The addition of prompt photons to the scintillation process could significantly improve CTR, reaching 34 ps FWHM with 500 prompt photons.

Most proposed realizations of a high temperature superconductor (HTS) qubit require the use of very small Josephson junctions. The properties of bicrystal junctions are especially interesting since they make it possible to implement several types of flux qubits in a relatively simple way. We have developed a technique that allows us to produce high quality sub-micrometer junctions in a reproducible way using bicrystal technology. We have successfully fabricated and characterized a large number of YBCO junctions and SQUIDs with bridge width as small as 0.2 micrometer on 0/spl deg/-3/spl deg/, 0/spl deg/-40/spl deg/ and 0/spl deg/-45/spl deg/ bicrystal STO substrates. The properties of these junctions have been extensively examined at temperatures down to 20 mK. The effects of external magnetic fields on these structures have been investigated. Figures of merit for the proposed qubits were also extracted from these measurements.

In this paper, a submillimeter-wave Schottky diode is designed and simulated. Effect of Schottky layer thickness on cut-off frequency is studied. A novel microfabrication process is proposed and implemented. The presented microfabrication process avoids electron-beam (e-beam) lithography which reduces the cost. Also, this process provides more flexibility in selection of design parameters and allows significant reduction in the device parasitic capacitance. A key feature of the process is that the Schottky contact, the air-bridges, and the transmission lines, are fabricated in a single lift-off step. This process relies on a planarization method that is suitable for trenches of 1–10 μm deep and is tolerant to end-point variations. The fabricated diode is measured and results are compared with simulations. A very good agreement between simulation and measurement results are observed.

In this paper, the performance of dielectric slab waveguides in the Terahertz (THz) frequency band is studied. High-Resistivity silicon (HR-silicon) is selected as the host material due to its high transparency in THz region. The low-cost fabrication process of the waveguide is described. Finite Element Method (FEM) based simulator (HFSS) and THz Time Domain Spectroscopy (THz-TDS) system were employed to characterize the waveguide both theoretically and experimentally. It is shown that the proposed transmission line is low-loss and low-dispersive for a wide bandwidth.

In 3D integration of microsystems, electrical insulation of High Aspect Ratio Through Silicon Via (HAR TSV) is a major challenge. This insulation is traditionally realized through the deposition of a thin SiO2 layer. Electrografted poly-4-vinylpyridine (P4VP) has been demonstrated to be considered as an alternative to SiO2 dielectric, traditionally used, for HAR TSVs insulation1. Electrografted P4VP has comparable electrical performances than SiO2 based dielectric and is capable to smoothly cover the scalloped side walls of the TSV with better coverage uniformity than most dielectrics considered for Via-Middle or Via-Last processes2. Electrografting methods are electro-initiated processes. The commercial electrografting solution contains organic reactants in an aqueous acidic media: 4-nitrobezen diazonium (NBD) and monomers of 4 vinylpyridine (4VP). The electrografting mechanism of P4VP onto Si in aqueous media through reduction of diazonium salts is complex. However, a mechanism based on electrochemical initiation followed by a purely chemical polymerization has been proposed. In a first electro-initiated step, aryl radicals must be generated by the electrochemical reduction of NBD on the Si electrode. This can be achieved using different technics such as electrochemical reduction. The generated aryl radicals can covalently bond to the silicon surface through an electron transfer or can initiate radical polymerization of 4VP monomers into P4VP. Most of the published work about electrografting on silicon substrate is realized on oxide free Si-H functionalized surface. Such experimental condition do not usually permit an electrografted polymer to grow up to hundreds of nanometres, which is not suitable to meet electrical requirement for TSV insulation. Experimentally, reverse potential pulse conditions applied on p-type Si-OH functionalized sample allow the electrografting process of P4VP to grow faster and the grafted layer to be thicker. This phenomenon is enhanced if the process is realized under illumination (with the proper wavelength source with the respects of the silicon band gap). One of the hypotheses to explain this increase of kinetic and thickness could be the rapid inversion of the surface charge during the reverse pulse: the polymer structure would stay open by the creation of channels in the film, through desorption of physisorbed species, allowing the reactive species to diffuse more easily from the electrografting solution to the silicon surface. This phenomenon could facilitate the generation of aryl radicals for the 4VP polymerization process to be maintained. And thus, for the P4VP electrografting technic to be a versatile method for HAR TSV electrical insulation. We will present and discuss the electrografting experimental conditions and the impact of the surface preparation on the P4VP to understand their role on the kinetic and the thickness of the grafted P4VP. Acknowledgement Université de Sherbrooke, Natural Sciences and Engeeniring Research Council of Canada (NSERC), Institut National de la Recherche Scientifique (INRS), Teledyne Dalsa and aveni are gratefully acknowledged for their help and their financial support. References 1. T. Dequivre, E. Al Alam, J. Plathier, A. Ruediger, G. Brisard, and S. Charlebois, ECS Trans., 69, 91–97 (2015). 2. C. Truzzi, F. Raynal, and V. Mevellec, 2009 IEEE Int. Conf. 3D Syst. Integr., 1–6 (2009).

We present the realization of Single Photon Avalanche Diode (SPA D) arrays for Positron Emission Tomography (PET). These SPAD arrays are designed in Teledyne BALSA High Voltage (HV) CMOS technology targeted for a 3 dimensional (3D) heterogeneous integration with deep-submicron CMOS readout electronics to realize a 3D Single Photon Counting Module (3DSPCM). We are developing a post-process Through Silicon Vias (TSV) technology at Universite de Sherbrooke to implement the 3D heterogeneous bonding. The 3D integration of SPAD and electronics reduces the SPAD interconnect parasitic capacitance, greatly increases the photosensitive area and improves overall performances. Also, SPAD are known for their excellent timing performance which enables Time of Flight capabilities in PET, significantly improving image contrast and spatial resolution. For this purpose, we designed, fabricated and characterized SPAD arrays with active quenching circuit using the HV CMOS 0.8 μm technology. The chosen structure is a p+ anode in an n-well using a p-well isolation layer to achieve 54 % of fill factor and takes full advantage of the 3D integration scheme. SPAD evaluation results show a Photodetection Efficiency (PDE) up to 49 % at 480 nm, Dark Count Rate (DCR) of 50 kcps, afterpulsing probability

In this paper, a tunable parallel plate waveguide with a metal plate and a sheet of graphene is presented. The anisotropic conductivity of the graphene sheet submitted to a magnetic field is modeled by a conductivity tensor in which the diagonal and Hall conductivities are derived from the Kubo formula. The Maxwell equations are solved assuming the graphene sheet been biased both with an electric and a magnetic fields. It is shown that in this case a hybrid mode propagates inside the waveguide. The amount of power which propagates in each TE and TM modes can be adjusted by the magnitude of applied fields. Considering the attenuation, the proposed waveguide demonstrate good characteristics in comparison to metal waveguides in the 0.4 to 1 THz frequency band.

Hole electrical transport in a p-doped nanochannel defined between two L-shape etched trenches made on a silicon-on-insulator substrate is investigated using a TCAD-Medici simulator. We study the impact of the etched trenches' geometry and dielectric filling materials on the current-voltage characteristics of the device. Carrier accumulation on frontiers defined by the trenches causes a modulation of the hole density inside the conduction channel as the bias voltage varies and this gives rise to a diode-like characteristic. For a 1.2 µm-long channel, plots of the electric field distribution show that a nonlinear transport regime is reached at a moderate reverse and forward bias of ± 2 V. Plots of the carrier velocity along the conduction channel show that holes remain hot for a few hundreds of nm outside the nanometre-wide channel, at a bias of ± 10 V. Filling the etched trenches with a high-κ dielectric material gives rise to a lower threshold voltage, V(th). A similar decrease of V(th) is also achieved by reducing the longitudinal and/or the transverse trench width. Our simulation results provide useful design guidelines for future integrated self-switching-diode-based circuits.