Giovanni Condorelli

Silicon Photomultiplier Technology at STMicroelectronics

In this paper we present the results of the first electrical and optical characterization performed on 1 mm2 total area Silicon Photomultipliers (SiPM) fabricated in standard silicon planar technology at the STMicroelectronics Catania R&D clean room facility. The device consists of 289 microcells and has a geometrical fill factor of 48%. Breakdown voltage, gain, dark noise rate, crosstalk, photon detection efficiency and linearity have been measured in our laboratories. The optical characterization has been performed by varying the temperature applied to the device. The results shown in the manuscript demonstrate that the device already exhibits relevant features in terms of low dark noise rate and inter-pixel crosstalk probability, high photon detection efficiency, good linearity and single photoelectron resolution. These characteristics can be considered really promising in view of the final application of the photodetector in the Positron Emission Tomography (PET).

Timing Performances of Large Area Silicon Photomultipliers Fabricated at STMicroelectronics

In this paper the results of charge and timing resolution characterization realized at Fermi National Accelerator Laboratory (Fermilab) on 3.5 × 3.5 mm2 Silicon PhotoMultipliers fabricated at STMicroelectronics Catania R&D clean room facilities are presented. The device consists of 4900 microcells and has a geometrical fill factor of 36%. Timing measurements were realized at different wavelengths by varying the overvoltage and the temperature applied to the photodetector. The results shown in this manuscript demonstrate that the device, in spite of its large area, exhibits relevant features in terms of low dark current density, fast timing and very good single photoelectron resolution. All these characteristics can be considered very appealing in view of the utilization of this technology in applications requiring detectors with high timing and energy resolution performances.

Usefulness of exercise tomographic myocardial perfusion imaging for detection of restenosis after coronary stent implantation.

P lacement of intracoronary stents during percutaneous transluminal coronary angioplasty (PTCA) decreases the morbidity of acute closure,1 the shortterm requirement for repeat revascularization by achieving a large lumen.2,3 However, stent implantation does not prevent neointimal hyperplasia, and instent restenosis still remains between 20% and 30%.4 Because of favorable initial results, stents are implanted in multivessel PTCA and as a partial revascularization in some patients, especially in elderly and high-risk patients, therefore leaving a potential source of ischemia irrespective of stent restenosis.5 Thus, in an era of cost savings, the ability to localize ischemia noninvasively and detect stent restenosis accurately is crucial in the selection of patients who require further angiographic evaluation. The effectiveness of perfusion scintigraphy in the follow-up of symptomatic and asymptomatic patients with PTCA has been defined by several studies.6–9 However, there are few prospective data regarding the usefulness of exercise perfusion imaging in patients with coronary stent implantation.10 Therefore, the aim of this study was to assess, prospectively, the accuracy of exercise technetium99m (Tc-99m)-tetrofosmin single-photon emission tomography (SPECT) for detecting restenosis after coronary stent implantation. • • • The study group consisted of 97 patients, 74 (76%) men (mean age 58 6 12 years) who, after successful stenting procedures in native coronary arteries, prospectively underwent Tc-99m-tetrofosmin tomographic myocardial scintigraphy and coronary angiography. The average interval between the stent implantation and the scintigraphic myocardial scintigraphy study was 128.4 6 41.0 days (range 88 to 191), whereas that from the scintigraphic study to coronary angiography was 57 6 25 days (range 30 to 64). Because coronary stent implantation in 10 patients was performed in 2 vascular territories, 107 stented vascular territories were included in the evaluation. Patients were excluded with stents in bypass grafts, intervening myocardial infarction, or for revascularization procedures between stent placement and scintigraphic imaging. Prior myocardial infarction (

Quantum Detection Efficiency in Geiger Mode Avalanche Photodiodes

1 month) corresponding to the stented artery was present in 51 cases. There were 31 patients (60%) with a Q-wave myocardial infarction, and 20 (40%) with a non–Q-wave myocardial infarction. Myocardial viability and residual ischemia assessed by dobutamine echocardiography was documented in most cases (89%) of infarcted territories. At angiography performed before stenting there were 38 patients (39%) with 1-vessel disease, 49 (51%) with 2-vessel disease, and 10 (10%) with 3-vessel disease. Two patients (2%) had previous coronary artery bypass graft surgery. One hundred thirty-five stents (mean length 19 6 7 mm) were implanted in 107 coronary arteries as previously described.11 The distribution of the stented arteries were: 59 (55%) left anterior descending, 20 (19%) left circumflex, and 28 (26%) right coronary artery. Stents were implanted electively in 49% of patients, for suboptimal PTCA in 30% and in bailout due to dissection or vessel closure in 21%. The mean stenosis before stenting was 85 6 12% and after stenting 8 6 7%. Stenting was performed as complete revascularization in 51 patients (53%) and as partial revascularization in the remaining 46 patients (47%). Treadmill testing was performed with the modified Bruce protocol after appropriate pharmacologic washout and according to general standards. Patients underwent 2-day SPECT scintigraphy using Tc-99mtetrofosmin. Representative sections composed of 3 short-axis sections (apical, midventricular, and basal), and of the midventricular vertical long axis combined to assess apical segments, were selected for analysis as described by Garcia et al.12 A 4-point scoring system for segmental uptake of Tc-99m-tetrofosmin was used in which 0 5 normal, 1 5 mildly reduced, 2 5 moderately reduced, and 3 5 severely decreased uptake. Patterns of perfusion defects were separately determined in each segment for rest and stress studies as previously described.13 Defect extension and severity were separately analyzed by 2 observers blinded to each other’s readings. Of the total 832 segments scored, agreement was reached in 790 (95%), whereas in the remaining segments a split decision was resolved by consensus. Myocardial segments were assigned to individual coronary arteries as previously described.13 The observers had knowledge of the vessel of stent implantation but were unaware of the results of the control coronary angiography or the results of exercise testing and clinical findings. Coronary artery stenosis was assessed specifically by 2 experienced cardiologists by means of manual calipers without knowledge of exercise scintigraphic study results. Significant disease From the Institute of Cardiology, Ferrarotto Hospital, University of Catania, Catania, Italy. Dr. Galassi’s address is: Via Antonello da Messina 75, Acicastello 95021, Catania, Italy. E-mail: argal @ ctonline.it. Manuscript received August 6, 1999; revised manuscript received and accepted December 14, 1999.

Crosstalk Characterization in Geiger-Mode Avalanche Photodiode Arrays

The fabrication of silicon shallow junction photodiodes is a relevant topic for the detection of blue and near ultraviolet weak photon fluxes. In this paper we present a simple model to calculate the quantum detection efficiency (QDE) of a Geiger mode avalanche photodiode (GMAP) as a function of the dead layer thickness above the junction depletion layer. A comparison between calculated and experimental data is also presented. Moreover, by using the same model, an analysis of the QDE at 420 nm wavelength of conventional GMAPs based on shallow N+-P and P+-N junctions is given.

Single Photon Avalanche Diodes: Towards the Large Bidimensional Arrays

Following our work on Geiger-mode avalanche photodiode arrays, we have recently been dealing with the crosstalk issue in newly developed dense arrays with a minimum distance between pixel centers of 84 mum. In this paper, we present our crosstalk measurement approach, including the experimental setup and the offline calculation methods. Different characterizations of the crosstalk probability PCT versus capacitance have been performed to extrapolate the PCT when no measurement setup loads the pixels. We also present results regarding the crosstalk probability versus pixel distance and bias. Moreover, by adopting a slightly different approach, the probability density decay time has been measured to investigate about crosstalk origin.

A New Generation of SPAD—Single-Photon Avalanche Diodes

Single photon detection is one of the most challenging goals of photonics. In recent years, the study of ultra-fast and/or low-intensity phenomena has received renewed attention from the academic and industrial communities. Intense research activity has been focused on bio-imaging applications, bio-luminescence, bio-scattering methods, and, more in general, on several applications requiring high speed operation and high timing resolution. In this paper we present design and characterization of bi-dimensional arrays of a next generation of single photon avalanche diodes (SPADs). Single photon sensitivity, dark noise, afterpulsing and timing resolution of the single SPAD have been examined in several experimental conditions. Moreover, the effects arising from their integration and the readout mode have also been deeply investigated.

Characterization of SPAD Arrays: First Results

Design and characterization of a new generation of single-photon avalanche diodes (SPAD) array, manufactured by ST-Microelectronics in Catania, Italy, are presented. Device performances, investigated in several experimental conditions and here reported, demonstrate their suitability in many applications. SPADs are thin p-n junctions operating above the breakdown condition in Geiger mode at low voltage. In this regime a single charged carrier injected into the depleted layer can trigger a self-sustaining avalanche, originating a detectable signal. Dark counting rate at room temperature is down to 10 s-1 for devices with an active area of 10 mum in diameter, and 103 s-1 for those 50 mum wide. SPAD quantum efficiency, measured in the range 350-1050 nm, can be comparable to that of a typical silicon based detector and reaches the values of about 50% at 550 nm. Finally, the low production costs and the integration possibility are other favorable features in sight of highly dense integrated 1-D or 2-D arrays.

Long-Term angiographic follow-up after successful repeat balloon angioplasty for in-stent restenosis

SPAD detectors are very promising for astrophysical applications. Large arrays are crucial to cover as much of the focal plane as possible. STMicroelectronics has developed various monolithic arrays of SPAD. At present, the manufacturing of 5×5 arrays (each with an active area of 40 μm and pitch between adjacent pixels of 240 μm) is completed and improvements in terms of pixels and technology are under study and development. An appropriate cryogenic system to host the packaged SPAD arrays and detection electronics to drive them have been designed and realized. Electro-optical characteristics of various arrays in different operating conditions have been measured and the obtained results are presented herein.