Measurement and optimization of clock-induced charge in electron multiplying charge-coupled devices

Electron multiplying charge-coupled devices (EMCCDs) are a variant of standard CCD technology capable of single-optical photon counting at MHz pixel readout rates. For photon counting, thermal dark signal and clock-induced charge (CIC) are the dominant source of noise and must be minimized to reduce the likelihood of coincident events. Thermal dark signal is reduced to low levels through cooling or operation in inverted mode (pinning). However, mitigation of CIC requires precise tuning of both parallel and serial clock waveforms. Here, we present a detailed study of CIC within Teledyne-e2v EMCCDs with a goal of better understanding the physical mechanisms that dominate CIC production in both noninverted and inverted mode operations (IMO). Measurements are presented as a function of parallel and serial clock timings, clock amplitudes, and device temperature. The effects of radiation damage and annealing are also discussed. A widely accepted view is that CIC is signal generated through impact ionization of energetic holes as the clock phase is driven high. While this explanation holds for IMO, we propose that the majority of CIC generated in noninverted mode is in fact due to a secondary effect of light emission from hot carriers. The information from this study is then used to optimize CIC on Teledyne e2v CCD201s operating at 1-MHz pixel rate in NIMO. For the CCD201, we obtained total CIC levels as low as 6.9  ×  10  −  4  e  −    /  pix  /  frame with ≥90  %   detective quantum efficiency. We conclude with proposals to further reduce CIC based upon modifications to clocking schemes and device architecture

Comparison of Back-Thinned Detector Ultraviolet Quantum Efficiency for Two Commercially Available Passivation Treatments

Back-thinned silicon detectors offer a high response over a very broad spectrum for direct detection by providing an efficient optical path into the sensing silicon avoiding front face structures manufactured from metal, polysilicon, nitrides, and oxides that may absorb the incident light before reaching the sensing silicon. We have tested two CCDs with different back-surface shallow p+ implant thicknesses (basic and enhanced) at the M4 line (wavelength between 40 and 400 nm) at Physikalisch-Technische Bundesanstalt (PTB)’s Metrology Light Source in Berlin. This characterization in the ultraviolet spectral range extends the soft X-ray quantum efficiency (QE) data set previously acquired with the exact same devices. Due to the short absorption depth and the scope for many types of interactions of the device materials with ultraviolet photons, QE measurement and stability of the device against extended exposure in the UV are of ongoing interest. Therefore, QE measurements have been carried out before and after exposures to quantify any change in behavior. To allow characterization of the passivation processes only, the devices have no antireflection coating. The measured QE of the standard back-thinned CCD is below 10% between 70 and 370 nm. An average additional 5% efficiency is achieved in the enhanced device within the same range. At the limits of the measured spectrum, toward soft X-rays or toward the visible range, the QE increases and the difference between the standard and the enhanced process is reduced as the photon absorption length increases beyond the immediate back-surface. The measured QE after long high-flux exposures at 200 nm shows remarkable improvement

Development of a photon-counting near-fano-limited x-ray CMOS image sensor for THESEUS' SXI

THESEUS (Transient High Energy Sky & Early Universe Surveyor) is one of the three candidates for the M5 mission of the European Space Agency. The favoured mission will be announced in 2021 for an expected launch in 2032. THESEUS will be equipped with a Soft X-ray Imager (SXI) composed of a set of two telescopes using micro-pore optics offering an overall field of view of 0.5 sr (<2' accuracy) for X-ray energies between 300 eV and 5 keV. The focal plane of each SXI telescope has a 16 x 16 cm² cooled detector area. However, the limited radiator accommodation on the spacecraft prohibits the use of CCDs since cooling the focal planes to an optimal temperature for radiation hardness (<-100 °C) is not feasible. Therefore, the development of a suitable CMOS Image Sensor (CIS), capable of handling the expected levels of radiation at higher operating temperatures (approximately -30 °C) has been proposed. To demonstrate the performance required for the THESEUS SXI detector, a 2 x 2 cm² prototype is under development using Open University pixel designs in a Teledyne-e2v digital CMOS platform. The pixel design will allow full depletion over silicon thickness of 35 µm for optimal soft X-ray quantum efficiency and instrument background suppression, and will be capable of near-Fano-limited spectral resolution that will also be of prime interest for synchrotron and Free Electron Lasers (FEL) applications. In this paper, we will present the design considerations and simulations leading to the implemented structures complying with THESEUS' SXI requirements

Development of LGAD sensors with a thin entrance window for soft X-ray detection

We show the developments carried out to improve the silicon sensor technology for the detection of soft X-rays with hybrid X-ray detectors. An optimization of the entrance window technology is required to improve the quantum efficiency. The LGAD technology can be used to amplify the signal generated by the X-rays and to increase the signal-to-noise ratio, making single photon resolution in the soft X-ray energy range possible. In this paper, we report first results obtained from an LGAD sensor production with an optimized thin entrance window. Single photon detection of soft X-rays down to 452~eV has been demonstrated from measurements, with a signal-to-noise ratio better than 20.Comment: 10 pages, 6 figure

Characterization of iLGADs using soft X-rays

Experiments at synchrotron radiation sources and X-ray Free-Electron Lasers in the soft X-ray energy range ($250$eV--$2$keV) stand to benefit from the adaptation of the hybrid silicon detector technology for low energy photons. Inverse Low Gain Avalanche Diode (iLGAD) sensors provide an internal gain, enhancing the signal-to-noise ratio and allowing single photon detection below $1$keV using hybrid detectors. In addition, an optimization of the entrance window of these sensors enhances their quantum efficiency (QE). In this work, the QE and the gain of a batch of different iLGAD diodes with optimized entrance windows were characterized using soft X-rays at the Surface/Interface:Microscopy beamline of the Swiss Light Source synchrotron. Above $250$eV, the QE is larger than $55\%$ for all sensor variations, while the charge collection efficiency is close to $100\%$. The average gain depends on the gain layer design of the iLGADs and increases with photon energy. A fitting procedure is introduced to extract the multiplication factor as a function of the absorption depth of X-ray photons inside the sensors. In particular, the multiplication factors for electron- and hole-triggered avalanches are estimated, corresponding to photon absorption beyond or before the gain layer, respectively.Comment: 16 pages, 8 figure

Désertion de capteurs à pixels CMOS : étude, caractérisations et applications

An architecture of CMOS pixel sensor allowing the depletion of the sensitive volume through frontside biasing is studied through the characterization in laboratory of a prototype. The charge collection performances confirm the depletion of a large part of the sensitive thickness. In addition, with a modest noise level, the sensor features an excellent energy resolution for photons below 20 keV at positive temperatures. These results demonstrate that such sensors are suited for soft X-ray spectroscopy and for charged particle tracking in highly radiative environment. A simplified analytical model and finite elements calculus are used to predict the depletion depth reached. An indirect measurement method to evaluate this depth is proposed. Measurements confirm predictions for a thin highly resistive epitaxial layer, which is fully depleted, and a 40micrometers thick bulk less resistive substrate, for which depletion reached 18 micrometers but which still offers correct detection over its full depth. Two sensor designs dedicated to X-ray imaging and in-brain neuroimaging on awake and freely moving rats are presented.Une architecture de capteurs à pixels CMOS permettant la désertion du volume sensible par polarisation via la face avant du circuit est étudiée à travers la caractérisation en laboratoire d’un capteur prototype. Les performances de collection de charge confirment la désertion d‘une grande partie de l’épaisseur sensible. De plus, le bruit de lecture restant modeste, le capteur présente une excellente résolution en énergie pour les photons en dessous de 20 keV à des températures positives. Ces résultats soulignent l’intérêt de cette architecture pour la spectroscopie des rayons X mous et pour la trajectométrie des particules chargées en milieu très radiatif. La profondeur sur laquelle le capteur est déserté est prédite par un modèle analytique simplifié et par des calculs par éléments finis. Une méthode d’évaluation de cette profondeur par mesure indirecte est proposée. Les mesures corroborent les prédictions concernant un substrat fin, très résistif, qui est intégralement déserté et un substrat moins résistif et mesurant 40 micromètres, qui est partiellement déserté sur 18 micromètres mais détecte correctement sur la totalité de l’épaisseur. Deux développements de capteurs destinés à l’imagerie X et à la neuro-imagerie intracérébrale sur des rats éveillés et libres de leurs mouvements sont présentés

Calibrating Teledyne-e2v’s ultraviolet image sensor quantum efficiency processes

Teledyne-e2v's sensors and wafer-scale processing are widely used for high performance imaging across soft X-ray and optical bands. In the ultraviolet spectral range, the combination of short absorption lengths (below 10 nm) and high reflectance (up to 75 %) can strongly limit the quantum efficiency. Direct detection capability relies on back-illumination and back-thinning processes to be applied to a sensor to remove dead layers from the optical path. As the thinning process leaves an unacceptably thick backside potential well as well as a highly reflective surface, in-house ultraviolet-specific (e.g. for WUVS) or third-party processes (e.g. delta-doping for FIREBall) are required. We have calibrated Teledyne-e2v's latest in-house wafer-scale proprietary processes with monochromatic synchrotron radiation over a wide spectral range in the ultraviolet domain (λ=40 nm – 400 nm) at the Metrology Light Source of the Physikalisch-Technische Bundesanstalt. The first process is a shallow p+ implantation that permits the thinning of the backside potential well. It is available in two different levels: basic and enhanced. The second type of enhancement is a specific anti-reflective coating to increase the back-surface transmittance for distinct spectral ranges. In this paper, we will present comparative quantum efficiency calibration of both passivation stages and of two different ultraviolet specific anti-reflective coatings (applied on enhanced passivation devices). Also, their stability after intense ultraviolet illumination will be shown. These measurements will permit Teledyne-e2v to extend the quantum efficiency data of their most recent processes across the soft X-ray to near-infrared spectrum

A CMOS image sensor for soft x-ray astronomy

A monolithic CMOS image sensor based on the pinned photodiode (PPD) and optimized for X-ray imaging in the 300 eV to 5 keV energy range is described. Featuring 40 µm square pixels and 40 µm thick, high resistivity epitaxial silicon, the sensor is fully depleted by reverse substrate bias. Backside illumination (BSI) processing has been used to achieve high X-ray QE, and a dedicated pixel design has been developed for low image lag and high conversion gain. The sensor, called CIS221-X, is manufactured in a 180 nm CMOS process and has three different 512×128-pixel arrays on 40 µm pitch, as well as a 2048×512 array of 10 µm pixels. CIS221-X also features per-column 12-bit ADCs, digital readout via four high-speed LVDS outputs, and can be read out at 45 frames per second. CIS221-X achieves readout noise of 2.6 e- RMS and full width at half maximum (FWHM) at the Mn-Kα 5.9 keV characteristic X-ray line of 153 eV at -40 °C. This paper presents the characterization results of the first backside illuminated CIS221-X, including X-ray response and readout noise. The newly developed sensor and the technology underpinning it is intended for diverse applications, including X-ray astronomy, synchrotron, and X-ray free electron laser light sources

Resolving soft X-ray photons with a high-rate hybrid pixel detector

Due to their high frame rates and dynamic range, large area coverage, and high signal-to-noise ratio, hybrid silicon pixel detectors are an established standard for photon science applications at X-ray energies between 2 keV and 20 keV. These properties also make hybrid detectors interesting for experiments with soft X-rays between 200 eV and 2 keV. In this energy range, however, standard hybrid detectors are limited by the quantum efficiency of the sensor and the noise of the readout electronics. These limitations can be overcome by utilizing inverse Low-Gain Avalanche Diode (iLGAD) sensors with an optimized X-ray entrance window. We have developed and characterized a prototype soft X-ray iLGAD sensor bonded to the charge integrating 75 µm pixel JUNGFRAU chip. Cooled to −22°C, the system multiplication factor of the signal generated by an impinging photon is ≥ 11. With this gain, the effective equivalent noise charge of the system is ≤5.5 electrons root-mean-square at a 5 µs integration time. We show that by cooling the system below −50°C, single photon resolution at 200 eV becomes feasible with a signal-to-noise ratio better than 5

Characterization of iLGADs using soft X-rays

Experiments at synchrotron radiation sources and X-ray Free-Electron Lasers in the soft X-ray energy range (250 eV–2 keV) stand to benefit from the adaptation of the hybrid silicon detector technology for low energy photons. Inverse Low Gain Avalanche Diode (iLGAD) sensors provide an internal gain, enhancing the signal-to-noise ratio and allowing single photon detection below 1 keV using hybrid detectors. In addition, an optimization of the entrance window of these sensors enhances their quantum efficiency (QE). In this work, the QE and the gain of a batch of different iLGAD diodes with optimized entrance windows were characterized using soft X-rays at the Surface/Interface:Microscopy beamline of the Swiss Light Source synchrotron. Above 250 eV, the QE is larger than 55% for all sensor variations, while the charge collection efficiency is close to 100%. The average gain depends on the gain layer design of the iLGADs and increases with photon energy. A fitting procedure is introduced to extract the multiplication factor as a function of the absorption depth of X-ray photons inside the sensors. In particular, the multiplication factors for electron- and hole-triggered avalanches are estimated, corresponding to photon absorption beyond or before the gain layer, respectively