Nicola Zorzi

Double-Sided, Double-Type-Column 3-D Detectors: Design, Fabrication, and Technology Evaluation

We report on the latest results from the development of 3-D silicon radiation detectors at Fondazione Bruno Kessler of Trento (FBK), Italy (formerly ITC-IRST). Building on the results obtained from previous devices (3-D Single-Type-Column), a new detector concept has been defined, namely 3-D-DDTC (Double-sided Double-Type Column), which involves columnar electrodes of both doping types, etched from alternate wafer sides, stopping a short distance (d) from the opposite surface. Simulations prove that, if d is kept small with respect to the wafer thickness, this approach can yield charge collection properties comparable to those of standard 3-D detectors, with the advantage of a simpler fabrication process. Two wafer layouts have been designed with reference to this technology, and two fabrication runs have been performed. Technological and design aspects are reported in this paper, along with simulation results and initial results from the characterization of detectors and test structures belonging to the first 3-D-DDTC batch.

Slim edges in double-sided silicon 3D detectors

Minimization of the insensitive edge area is one of the key requirements for silicon radiation detectors to be used in future silicon trackers. In 3D detectors this goal can be achieved with the active edge, at the expense of a high fabrication process complexity. In the framework of the ATLAS 3D sensor collaboration, we produced modified 3D silicon sensors with a double-sided technology. While this approach is not suitable to obtain active edges, because it does not use a support wafer, it allows for a new type of edge termination, the slim edge. In this paper we report on the development of the slim edge, from numerical simulations to design and testing, proving that it works effectively without increasing the fabrication complexity of silicon 3D detectors, and that it could be further optimized to reduce the insensitive edge region to less than 100 μm.

Characterization of new FBK SiPM technology for visible light detection

This paper presents the characterization of the new n-on-p SiPM technology developed at Fondazione Bruno Kessler (FBK, Trento-Italy). Several device aspects such as dark count rate, photo detection efficiency, breakdown voltage uniformity, and temperature stability have been significantly improved with respect to the original FBK SiPM technology. The modifications introduced involve the internal device structure and are based on an electric-field engineering approach. We report on the dark characterization, the visible light detection efficiency and 511 keV gamma ray energy resolution, when reading out small LYSO or Ce:GAGG crystals, of the new devices. In parallel, a comparison to the original SiPMs is done in order to underline the main advancements that have been obtained. We refer this new technology to as RGB-SiPMs because of the high detection efficiency for the whole red, green, and blue part of the spectrum.

Development of Double-Sided Full-Passing-Column 3D Sensors at FBK

We report on the main design and technological characteristics related to the latest 3D sensor process developments at Fondazione Bruno Kessler (FBK, Trento, Italy). With respect to the previous version of this technology, which involved columnar electrodes of both doping types etched from both wafer sides and stopping at a short distance from the opposite surface, passing-through columns are now available. This feature ensures better performance, but also a higher reproducibility, which is of concern in medium volume productions. In particular, this R&D project was aimed at establishing a suitable technology for the production of 3D pixel sensors to be installed into the ATLAS Insertable B-Layer. An additional benefit is the feasibility of slim edges, which consist of a multiple ohmic column termination with an overall size as low as 100 μm. Eight batches with two different wafer layouts have been fabricated using this approach, and including several design options, among them the ATLAS 3D sensor prototypes compatible with the new read-out chip FE-I4.

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.

Characterization of Single-Photon Time Resolution: From Single SPAD to Silicon Photomultiplier

In this paper, we report on the characterization of the single-photon time resolution (SPTR) of the RGB (Red-Green-Blue) type silicon photomultipliers (SiPM) produced at FBK. We measured and compared single-photon timing jitter of 1 ×1 mm2 and 3 ×3 mm2 SiPMs, and also of square SPADs with integrated passive quenching, identical to the cells composing the SiPMs. We reached a single-photon time resolution of about 180 ps full-width at half-maximum for 3 ×3 mm2 SiPM, 80 ps for 1 ×1 mm2 SiPM and less than 50 ps for single cells. From measurements with pinholes placed in front of 1 ×1 mm2 detector we see a very good cell-to-cell uniformity: it is not a limiting factor for time resolution. We also characterized the timing jitter of SiPMs as a function of the number of photons per laser pulse (N) finding that it does not decrease exactly with the square root of N because of the optical crosstalk between cells.

NUV Silicon Photomultipliers With High Detection Efficiency and Reduced Delayed Correlated-Noise

In this paper, we present the characteristics and performances of new silicon photomultipliers (SiPMs), produced at FBK, for the near-ultraviolet (NUV) light detection, with reduced afterpulsing and delayed optical crosstalk. To study these components of the correlated noise, we manufactured SiPMs on silicon wafers featuring different substrate minority-carrier lifetime. This parameter proved to be crucial in determining the amount of delayed optical crosstalk and afterpulsing caused by photo-generated carriers diffusing from the substrate to the cell active region. With a very low substrate lifetime, we were able to minimize this correlated noise component to few percent at room temperature. Besides reducing the excess noise factor, the lower delayed correlated noise allows biasing the SiPM at higher voltages, reaching higher values of photon detection efficiency.

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.

Performance of NUV-HD Silicon Photomultiplier Technology

In this paper, we present the full characterization of a new high-density (HD) cell silicon photomultiplier (SiPM) technology for ultraviolet (UV) and blue light detection, named near UV HD SiPM. Thanks to an optimized border region around each cell, we were able to develop devices having a very high detection efficiency and, at the same time, a high dynamic range. We produced SiPMs with a square cell pitch of 15, 20, 25, and 30 μm featuring a peak efficiency in the violet region ranging from 40% to 55%, according to the cell size. We tested this technology for time-of-flight positron emission tomography. Using two 4 × 4 mm2 SiPMs with a 25 × 25 μm2 cell pitch coupled to 3 × 3 × 5 mm3 LYSO scintillators, we reached for the first time 100-ps full-width at half-maximum coincidence time resolution. This result was independent of the temperature in a range from 20 °C to -20 °C. At the same time, thanks to the high dynamic range and low correlated noise, we obtained an energy resolution lower than 9% for 511-keV γ-rays.

Radiation hardness of silicon detectors for high-energy physics applications

Oxygenated and standard (not oxygenated) silicon diodes processed by CNM and IRST have been irradiated by 27 MeV protons and compared with standard devices from ST Microelectronics. As expected, the leakage current density increase rate (/spl alpha/) and its annealing do not show any significant dependence on starting material, oxygenation and/or device processing. On the contrary, oxygenation improves the radiation hardness by decreasing the acceptor introduction rate (/spl beta/) and mitigating the depletion voltage (V/sub dep/) increase, with the /spl beta/ parameter depending also on starting material and/or effects related to device processing for standard diodes. Finally, these results are included in a general review on the state of the art for silicon detector radiation hardening, confirming the good performance of the considered technologies.