High sensitivity single photon avalanche diodes for timing applications

Nowadays many scientific applications, which require the detection of very faint and fast decaying optical signals, rely on measurement systems with single photon sensitivity. Fluorescence Lifetime Imaging Microscopy (FLIM), diffuse optical tomography and Forster Resonance Energy Transfer (FRET) are just few examples. Today Single Photon Avalanche Diodes (SPAD) are the best choice when it comes to single photon detection. In the last decade they were able to overcome Photomultiplier Tubes (PMT) thanks to the advantages of solid state devices (compact size, lower voltage operation, insensitivity to magnetic fields) and in terms of photon detection efficiency. Single channel single photon modules featuring SPAD devices are already a well proven commercial product. This generation of SPADs shows great performances in terms of timing resolution (35ps), dark counting rate (10cps for a cooled 50um diameter detector) and afterpulsing (1%). But science still wants more and a strong need for detectors with both high sensitivity and high temporal resolution and for multidimensional system showed up. On one hand, up to now, only detectors with high temporal resolution but limited sensitivity in a one part of the spectral range or detectors with high sensitivity over the whole visible spectrum but poor temporal resolution can be found: a device capable of combining both high photon detection efficiency and high temporal resolution does not exist. On the other hand, large SPAD arrays were demonstrated only using a standard CMOS technology, which led to a severe worsening of the overall device performance. As a matter of fact, two different approaches for SPAD fabrication stand out: the first one is based on custom technology and the second one is based on standard high voltage CMOS technology. CMOS SPADs are fabricated by reusing the technological steps of a standard CMOS technology. The big advantage of this approach is the possibility to exploit state-of-the-art CMOS electronics to design large arrays of smart pixels. However a major drawback concerning the SPAD performances turns out to be hard to overcome: the use of a standard CMOS process makes it impossible for the designer to develop devices with both a high photon detection efficiency and a low dark counting rate. On the other hand, a custom technology process, i.e. a technology process suitably tunable by the designer, allows the development of high performance SPADs but makes electronic circuit integration, and thus array fabrication, more difficult. However, since the detector performances, especially in terms of sensitivity, matter the most in almost every application, the choice of a custom technology for SPAD fabrication is mandatory. Aim of this work is the design of a new SPAD device able to combine both high photon detection efficiency and high timing resolution, while preserving other features, such as a low dark count rate, a low afterpulsing probability and a large active area. The target performances are a 40% photon detection efficiency at a 800nm wavelength and a resolution under the 100ps barrier. The second goal is the development of solutions aimed at the fabrication of large custom SPAD arrays, including the new designed detector, able to preserve the high performances of a single discrete detector.

Al giorno d'oggi molte applicazioni scientifiche, che richiedono il rilevamento di segnali molto deboli e veloce ottici, si basano su sistemi di misura con la sensibilità del singolo fotone. Fluorescence Lifetime Imaging Microscopy (FLIM), diffuse optical tomography and Forster Resonance Energy Transfer (FRET) sono solo alcuni esempi. Oggi i singoli Photon Avalanche Diodes (SPAD) sono la scelta migliore quando per la rilevazione di singoli fotoni. Nell'ultimo decennio sono stati in grado di superare i tubi fotomoltiplicatori (PMT) grazie ai tipici vantaggi dei dispositivi a stato solido (dimensioni compatte, funzionamento a tensione più bassa, insensibilità ai campi magnetici) e in termini di efficienza quantica. Moduli a singolo canale sono stati sviluppati con successo. Questa generazione di SPAD mostra grandi prestazioni in termini di risoluzione temporale (35ps), conteggi di buio (10cps per un rivelatore di diametro 50 um) e afterpulsing (1%). Ora si presenta tuttavia una forte necessità per rivelatori con alta sensibilità e alta risoluzione temporale e per sistemi multicanale. Scopo di questo lavoro è la progettazione di un nuovo dispositivo SPADin grado di coniugare sia una alta efficienza quantica che unai risoluzione temporale elevata, pur mantenendo le altre caratteristiche, come ad esempio un basso tasso di conteggio di buio, una bassa probabilità di afterpulsing e una area attiva estesa. Le prestazioni di riferimento sono una efficienza quantica del 40% ad una lunghezza d'onda di 800nm e una risoluzione inferiore ai 100ps. Il secondo obiettivo è lo sviluppo di soluzioni per la realizzazione di grandi array di SPAD in tecnologia custom, incluso il nuovo rivelatore progettato, in grado di preservare le elevate prestazioni di un singolo rivelatore.