**2. Single photon avalanche diodes**

In the last decades, the possibility to build a silicon photo-sensor suitable for single photon counting applications was investigated by several research teams (Cova et al. 1983 and references therein). The original idea, firstly proposed by R.J. Mc Intyre since the 1960s, was to implement a semiconductor photodiode with characteristics suitable for the triggered avalanche operation mode, known as Geiger mode of operation, and therefore able to detect single photons.

Carriers generated by the absorption of a photon in the p–n junction, are multiplied by impact ionization thus producing an avalanche. APDs can reach timing uncertainties as low as a few tens of picoseconds thanks to the speed at which an avalanche evolves from the initial carrier pair forming in the multiplication region. An APD is implemented as photodiode reverse biased near or above breakdown, where it exhibits optical gains greater than one. When an APD is biased below breakdown it is known as proportional or linear APD. It can be used to detect clusters of photons and to determine their energy. When biased above breakdown, the optical gain becomes virtually infinite (see fig. 1). Thus, with relatively simple ancillary electronics, the APD becomes capable of detecting single photons. The APD operating in this regime is called single-photon avalanche diode - SPAD. If the primary carrier is photo-generated, the fast leading edge of the avalanche pulse marks the arrival time of the detected photon. After the avalanche is triggered, the current keeps flowing until the avalanche is quenched by lowering the bias voltage down to breakdown voltage or below. The bias voltage is then restored in order to detect another photon. This operation requires a suitable circuit that is usually referred to as a quenching circuit.

Individual detectors and detector arrays based on the SPAD technology have received renewed interest in recent years due to the versatility of their applications.

Time Resolved Camera: The New Frontier of Imaging Devices 307

more disadvantageous when the detector is cooled to reduce the dark-counting rate, since

In conclusion, a really suitable technology for producing SPADs must not only reduce the generation and regeneration centers to very low concentration level, but also eliminate trapping level or at least minimize their concentration. The technological challenge is to design a process with such characteristics and still compatible with standard microelectronic

There exist several implementation styles for APDs, of which two are the most used. In the first style, known as reach-through APD, one builds a *p*+–π–*p*–*n* structure where denotes very lightly *p*-doped (McIntyre, 1985). When reverse biased, the depletion region extends from the cathode to the anode. Thus, the multiplication region is deep in the *p*/*n*+ junction. Due to the depth of the multiplication region, this device is indicated for absorption of red and NIR photons up 1.1 m (for silicon). Since the photoelectrons drift until the

The second implementation style is compatible with planar CMOS processes and it involves a shallow or medium depth *p* or *n* layer to form high-voltage *pn* junctions. Cova and others have investigated devices designed in this style since the 1970s, yielding a number of structures (Cova et al. 1981). All these structures have in common a *pn* junction and a zone designed to prevent premature edge breakdown. An example of the early structures is reported in the work of Zappa et al. (1997) *n*+/*p*+ enrichment in p-substrate was used, while premature edge breakdown was prevented by confining *p*+ enrichment in the centre of the

More recently, many authors have developed APDs, both in linear and Geiger mode, using dedicated planar and non planar processes, achieving superior performance in terms of

The main disadvantage of using dedicated processes is generally the lack of libraries that can support complex functionalities and deep-submicrometre feature sizes, thus limiting array sizes. An interesting alternative is the use of a hybrid approach whereby the APD array and ancillary electronics are implemented in two different processes, each optimized for APD performance and speed, respectively. If the ancillary electronics is implemented in CMOS, high degrees of miniaturization are possible. The price to pay is increased

multiplication region, a larger timing uncertainty is generally observed.

the release from trapping states becomes slower.

Fig. 2. Junction scheme of SPAD device

**2.1 Fabrication and structure design** 

sensitivity and noise (Kindt 1999).

industry processes.

device.

There are two main lines of research in silicon SPADs: one that advocates the use of highly optimized processes to boost performance and one that proposes to adapt SPAD design to existing processes to reduce cost and to maximize miniaturization. Both approaches have advantages and drawbacks.

Fig. 1. Typical multiplication regions of reverse polarization junction.

The basic goals of the SPAD fabrication technology are to: *(i)* keep low the dark counting rate; *(ii)* keep low the afterpulsing probability; *(iii)* make uniform the electric field over the whole active area in order to have a photo-detection efficiency (PDE) independent from the absorption position; *(iv)* keep low the photo-generation of carriers outside the multiplication region in order to minimize the time uncertainty.

As it happens in PMTs, thermal generation effects produce current pulses even in the absence of illumination, and the Poissonian fluctuation of these dark counts represents the internal noise source of the detector. The dark pulses are due to carriers thermally generated in the SPAD junction, so that the count rate increases with the temperature, as does the dark current in ordinary photodiodes. The rate also increases with the excess bias voltage (EBV, which is the voltage over the breakdown) because of two effects: *i*) field-assisted enhancement of the emission rate from generation centers (Hurkx et al. 1996) and *ii*) increase of the avalanche triggering probability (Oldham et al. 1972).

The SPAD count rate includes also secondary pulses due to afterpulsing effects that in the case of dark count may strongly enhance the total measured rate.

During the avalanche some carriers are captured by deep levels in the junction depletion layer and subsequently released with a statistically fluctuating delay, whose mean value depends on the deep levels actually involved (Cova et al. 1991). Released carriers can retrigger the avalanche, generating afterpulses correlated with a previous avalanche pulse. The number of carriers captured during an avalanche pulse increases with the total number of carriers crossing the junction, that is, with the total charge of the avalanche pulse. Therefore afterpulsing increases with the delay of avalanche quenching and with the current intensity, which is proportional to the EBV (usually dictated by photon detection efficiency or time resolution requirements, or both (Cova et al. 1996)) so that the trapped charge per pulse first has to be minimized by minimizing the quenching delay. The situation is even

There are two main lines of research in silicon SPADs: one that advocates the use of highly optimized processes to boost performance and one that proposes to adapt SPAD design to existing processes to reduce cost and to maximize miniaturization. Both approaches have

The basic goals of the SPAD fabrication technology are to: *(i)* keep low the dark counting rate; *(ii)* keep low the afterpulsing probability; *(iii)* make uniform the electric field over the whole active area in order to have a photo-detection efficiency (PDE) independent from the absorption position; *(iv)* keep low the photo-generation of carriers outside the multiplication

As it happens in PMTs, thermal generation effects produce current pulses even in the absence of illumination, and the Poissonian fluctuation of these dark counts represents the internal noise source of the detector. The dark pulses are due to carriers thermally generated in the SPAD junction, so that the count rate increases with the temperature, as does the dark current in ordinary photodiodes. The rate also increases with the excess bias voltage (EBV, which is the voltage over the breakdown) because of two effects: *i*) field-assisted enhancement of the emission rate from generation centers (Hurkx et al. 1996) and *ii*) increase

The SPAD count rate includes also secondary pulses due to afterpulsing effects that in the

During the avalanche some carriers are captured by deep levels in the junction depletion layer and subsequently released with a statistically fluctuating delay, whose mean value depends on the deep levels actually involved (Cova et al. 1991). Released carriers can retrigger the avalanche, generating afterpulses correlated with a previous avalanche pulse. The number of carriers captured during an avalanche pulse increases with the total number of carriers crossing the junction, that is, with the total charge of the avalanche pulse. Therefore afterpulsing increases with the delay of avalanche quenching and with the current intensity, which is proportional to the EBV (usually dictated by photon detection efficiency or time resolution requirements, or both (Cova et al. 1996)) so that the trapped charge per pulse first has to be minimized by minimizing the quenching delay. The situation is even

Fig. 1. Typical multiplication regions of reverse polarization junction.

region in order to minimize the time uncertainty.

of the avalanche triggering probability (Oldham et al. 1972).

case of dark count may strongly enhance the total measured rate.

advantages and drawbacks.

more disadvantageous when the detector is cooled to reduce the dark-counting rate, since the release from trapping states becomes slower.

In conclusion, a really suitable technology for producing SPADs must not only reduce the generation and regeneration centers to very low concentration level, but also eliminate trapping level or at least minimize their concentration. The technological challenge is to design a process with such characteristics and still compatible with standard microelectronic industry processes.
