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Silicon Photomultipliers (SiPMs) are fabricated in two different configurations: p-on-n and n-on-p junctions. More particularly p-on-n SiPMs turn out to be more suitable in nuclear medical imaging applications like Positron Emission Tomography (PET), due to their higher sensitivity in blue wavelength range where common PET scintillators have their emission spectrum. Here we report on the preliminary results of the electro-optical characterization performed on the first STMicroelectronics p-on-n SiPMs with standard (45%) and enhanced (62%) fill factor. The performances of these devices are compared with standard n-on-p technology. The inversion of the junction results in a remarkable improvement of PDE in blue wavelength range. Moreover all the tested devices show very good single photoelectron resolution also for high overvoltage values confirming the excellent single photon detection capability of SiPM technology. In this work a comprehensive SiPM model integrating the electrical and statistical behavior of the device is also presented. We used a Monte Carlo model for statistical and a SPICE circuit model for the electrical behavior description of the SiPM.

In this contribution we present the results of the first morphological and electro-optical characterization of Silicon Photomultipliers (SiPM) for nuclear medical imaging applications fabricated in standard silicon planar technology at the STMicroelectronics Catania R&D clean room facility. We have improved our previous Geiger Mode Avalanche Photodiodes (GMAP) technology in order to realize a photodetector with relevant features in terms of single-photoelectron resolution, timing and photon detection efficiency. The performances of our devices, investigated in several experimental conditions and here reported make ST-SiPM suitable in many applications like for example PET (Positron Emission Tomography). Keywords: single photon counting, positron emission tomography, silicon photomultiplier, single photon avalanche photodiode, quenching resistor. 1. INTRODUCTION Photon counting techniques have been developed over many years by exploiting the remarkable performances of photomultiplier tubes (PMT) and avalanche photodiodes (APD) [1]. In r ecent years, a new kind of planar semiconductor photodetector, called Silicon PhotoMultiplier (SiPM), has been developed to detect single optical photons as a possible alternative to PMT and APD. SiPM have many advantages in terms of very fast timing response, high gain at low bias voltage, small recovery time, high detection efficiency in all the visible range and single photon response. Low bias voltage, low power consumption, good temperature and voltage stability, insensitivity to magnetic fields, reliability, ruggedness, compactness, no excess light damage and long lif espan are other favorable characteristics of these devices [2]. Moreover the solid-state detector approach has the typical advantages of a planar process integration, therefore arrays with a large number of pixels working in Geiger Mode can be manufactured at low cost

In this paper we report the results relative to the design and fabrication of Single Photon Avalanche Detectors (SPAD) operating at low voltage in planar technology. These silicon sensors consist of pn junctions that are able to remain quiescent above the breakdown voltage until a photon is absorbed in the depletion volume. This event is detected through an avalanche current pulse. Device design and critical issues in the technology are discussed. Experimental test procedures are then described for dark-counting rate, afterpulsing probability, photon timing resolution, quantum detection efficiency. Through these experimental setups we have measured the electrical and optical performances of different SPAD technology generations. The results from these measurements indicate that in order to obtain low-noise detectors it is necessary to introduce a local gettering process and to realize the diode cathode through in situ doped polysilicon deposition. With such technology low noise detectors with dark counting rates at room temperature down to 10c/s for devices with 10mm diameter, down to 1kc/s for 50mm diameter have been obtained. Noticeable results have been obtained also as far as time jitter and quantum detection efficiency are concerned. This technology is suitable for monolithic integration of SPAD detectors and associated circuits. Small arrays have already been designed and fabricated. Preliminary results indicate that good dark count rate uniformity over the different array pixels has already been obtained.