Nicola Serra

Characterization of the First FBK High-Density Cell Silicon Photomultiplier Technology

In this paper, we present the results of the characterization of the first high-density (HD) cell silicon photomultipliers produced at FBK. The most advanced prototype manufactured with this technology has a cell size of 15 × 15 μm2 featuring a nominal fill factor of 48%. To reach this high area coverage, we developed a new border structure to confine the high electric-field region of each single-photon avalanche diode. The measured detection efficiency approaches 30% in the green part of the light spectrum and it is above 20% from 400 to 650 nm. At these efficiency values, the correlated noise is very low, giving an excess charge factor below 1.1. We coupled a 2 × 2 × 10- mm3 LYSO scintillator crystal to a 2.2 × 2.2- mm2 silicon photomultiplier, obtaining very promising results for PET application: energy resolution of less than 11% full-width at half maximum (FWHM) with negligible loss of linearity and coincidence resolving time of 200-ps FWHM at 20°C.

Performance of FBK high-density SiPM technology coupled to Ce:LYSO and Ce:GAGG for TOF-PET

This paper presents the performance, in terms of energy and timing resolution, of high-density silicon photomultipliers (SiPMs) produced at Fondazione Bruno Kessler for time-of-flight positron emission tomography application. The new SiPM technology allows us to produce devices with a small cell size maintaining a high fill factor (FF). The sensors considered in this paper are composed by 30 × 30 μm(2) cells with a FF exceeding 70% to cover a total area of 4 × 4 mm(2). The SiPM performance was evaluated using two types of scintillators (Ce:LYSO and Ce:GaGG) both with a short height (5 mm) in order to minimize the time jitter caused by light propagation in the crystal. With Ce:LYSO, an energy resolution of 9.0% FWHM at 511 keV and a coincidence resolving time (CRT) of 125 ps FWHM were obtained at -20 °C. With Ce:GaGG, an energy resolution of 6.4% FWHM and a CRT of 260 ps FWHM were achieved at the same temperature. The novel SiPM technology, combining a high PDE with a low correlated noise (i.e., crosstalk and afterpulse), allows us to improve the state-of-the-art of energy and timing resolution with both the tested crystals.

Development of an automatic procedure for the characterization of silicon photomultipliers

In this paper, we show the analysis software we created for a fast and reliable automatic characterization of a silicon photomultiplier. The program can be used both in dark and under continuous low-level light illumination. It grabs 1ms-long waveforms, each containing many single cell pulses, from an oscilloscope connected to the PC via Ethernet. On-line data analysis is done both on the raw waveform as well as on a filtered (with DLED technique) version in order to facilitate the pulse identification and the extraction of some parameters also in cases of high count rates or cross-talk. The main outcomes of the program are: single-cell signal shape, gain, primary dark count rate, after-pulse probability, direct and delayed cross-talk probability and excess charge factor. It can be used also to determine the actual photo-detection efficiency with the proper hardware set-up.

Experimental and TCAD Study of Breakdown Voltage Temperature Behavior in

In this paper, the breakdown voltage in avalanche photodiodes operating in Geiger mode is investigated. In particular, the breakdown voltage dependence on the structural characteristics of n+/p avalanche diodes, such as implant dose and epitaxial layer thickness and doping concentration, as well as the temperature behavior are analyzed. The study includes both experimental data and numerical simulations. The first are acquired in a controlled temperature environment on samples featuring different electrical structures. Simulations are performed both at the technological process as well as at the electrical level using a commercial TCAD software. Agreement between experiments and numerical analysis is found to be quite good and first hints on how to improve the breakdown voltage uniformity and temperature dependence are given. Moreover, with the aid of TCAD simulations, the potential sources of device breakdown voltage fluctuations related to both device process variability and the properties of the starting silicon wafers are investigated.

n^{+}/p

In this paper we show new coincidence resolving time measurements with large area SiPMs coupled to LYSO scintillators in which we compensate the baseline fluctuation generated by dark events. In particular, we present experimental data obtained with the Differential Leading Edge Discriminator (DLED) method in which the arrival time of the event is not extracted directly from the signal but rather from the difference between the signal and its delayed replica. Dark events are effectively compensated allowing very low thresholds to be set. Timing performances have been measured by the DLED method on different SiPM-scintillator configurations at various temperatures. As an example, coupling a 4×4mm2 SiPM to a 3×3×5mm3 LYSO crystal, a coincidence resolving time of 190ps FWHM was obtained at room temperature.

SiPMs

In this paper, the breakdown voltage (VBD) temperature behavior in Geiger-Mode avalanche photodiodes (GM-APDs) is investigated by means of both experimental characterization of silicon photomultiplier (SiPMs) fabricated at FBK-IRST and one-dimensional TCAD simulations of GM-APDs. The analysis aims at relating both the VBD and its temperature coefficient to relevant technological device parameters, such as epitaxial layer thickness and doping concentration, in the context of device performance optimization for PET (positron emission tomography) applications. We show that the properties of the epitaxial layer regulate the extension of the depleted region and play therefore a crucial role in determining the device breakdown voltage and its temperature coefficient. We also show that only by recurring to an energy-balance transport model, the TCAD simulations are able to capture the dependences of VBD on the device structural characteristics and on temperature that are experimentally observed. We finally report on TCAD simulations of VBD variability in correlation to possible fluctuations in the epitaxial layer properties.

Timing performance of large area SiPMs coupled to LYSO using dark noise compensation methods

In this paper, we present a novel SiPM technology, developed at FBK, characterized by small cells with very high fill factor, called RGB-HD (red-green-blue high-density SiPM). The first prototypes feature a cell size of 15×15 μm² and 30×30 μm² with nominal fill factors of 48 % and 75 %, respectively. Here we focus on the 30 μm cell size, which was used to produce a 4×4mm² SiPM. We coupled different samples to both LYSO and GAGG scintillators, with sizes of 3×3×5 mm³ and 4×4×22 mm³, in order to characterize the RGB-HD SiPM performance for TOF-PET application. The results are extremely promising. We measured energy resolution of 9 % and coincidence resolving time of 170 ps FWHM with thick LYSO while 7 % and 375 ps FWHM with thick GAGG.