Physical models play a fundamental role in the evolution of SPAD technology. However, as the majority of the detector properties strongly depends on the electric field, a thorough knowledge of the doping profile is mandatory. Conventional doping extraction methods proved to be not accurate enough, hence the need to develop a new profiling technique which allows us to reach the desired level of accuracy. To this aim, we adopted an inverse modeling approach based on the combined use of capacitance-voltage measurements and electrical simulations. We applied the method to several SPADs, which differ both in terms of doping profile and internal structure, and we used the extracted profile to simulate their breakdown voltages. Calculated results very close to the experimental data were obtained, providing us a convincing validation of the proposed extraction technique.
Physical models are key tools for developing new SPADs structures. However, as most SPADs characteristics strongly depends on the electric field, a precise knowledge of the doping profile is required. Unfortunately, common profiling techniques are not accurate enough. To cope with this problem, we developed an inverse approach which resorts to the combined use of electrical simulations and capacitance-voltage measurements. We applied the technique to multiple SPADs and we used the extracted profile to calculate their breakdown voltages. Simulated results closely matching the experimental outcomes provide us a strong validation of the proposed extraction technique.
Recently, there has been a growing research interest towards multichannel Time Correlated Single Photon Counting systems. To break the tradeoff between number of channels and performance that currently affects available systems, we have proposed an approach based on resource sharing and on the exploitation of different technologies. Here, we present a new standalone high-performance Time-to-Amplitude Converter featuring a timing jitter of 4.3ps rms and a DNL lower than 1% rms. The area occupation is 0.2mm2 , the power dissipation 70mW and the conversion rate 13.7MHz, making this circuit suitable to be the building block of a densely integrated, high-performance system.
A widefield system for multidimensional fluorescence imaging capable of resolving space, time and wavelength is developed and validated on a synthetic fluorescence sample. The system enables structured illumination and compressing detection. A compression strategy based on an a-priori information obtained by a camera is validated and proved to be effective at compression ratio of about 90%.
The Time Correlated Single Photon Counting (TCSPC) technique is a powerful tool to analyze extremely fast and faint optical signals; however, its main drawback relies in its intrinsic slowness which is due to the necessity to acquire a large number of events to make an accurate reconstruction of the analog waveform in the time domain. In recent years, a significant research effort has been put in the design of multichannel systems: indeed, parallelization could in principle increase the overall speed of the acquisition system. In this scenario, we focused on the investigation of both singlechannel and multichannel systems potentiality to push the speed of TCSPC acquisition towards its ultimate limits. In this paper we report a solution to increase the speed of a single-channel system by almost an order of magnitude with respect to the state of the art and a smart routing architecture to provide a true increment of the acquisition speed based on the exploitation of a single photon detector array.
Single Photon Avalanche Diodes (SPADs) provide remarkable features including single-photon sensitivity and picosecond timing capability. For these reasons, they are the detectors of choice in a large variety of applicationsincluding Light Detection and Ranging, Fluorescence Lifetime Imaging, quantum cryptography and many others. In most cases, the electronics used to drive the SPAD plays a crucial role in determining the performance of the overall system. In particular, high speed is still an open challenge for SPAD-based systems. In this paper, we discuss fully integrated electronics and system architectures to maximize the speed of these systems while providing overall high performance both for counting and timing applications.
Single Photon Avalanche Diodes (SPADs) have been proven to be extremely powerful sensors for single photon detection. Thanks to the advantages of solid state devices (ruggedness, small size, low supply voltage, high reliability) combined to photon detection efficiency inherently higher than PMTs, especially in the red and near-infrared regions of the spectrum, SPADs have become the detectors of choice in a steadily increasing number of applications, such as Förster Resonance Energy Transfer (FRET), Laser Imaging Detection and Ranging (LIDAR) and Quantum Key Distribution (QKD). The development of specific fabrication processes, usually referred to as custom technologies, has given the designers the degrees of freedom necessary to pursue the best device performance. Nevertheless, custom processes do not easily allow the integration of complex front-end and processing electronics on the same chip of the detector. Therefore, external high-performance electronics is required to extract the best performance from these sensors. We report the latest results we achieved with a fully-planar custom technology process, that allows the fabrication of SPAD arrays, and specifically designed external front-end and timing electronics with particular focus on solutions to achieve high speed in counting and timing applications.
Single-photon detection capabilities of Single Photon Avalanche Diodes (SPADs), in laser ranging, enable the measurement of significantly long ranges, the minimization of the optical power of the laser source and the implementation of high frame rate 3D imaging systems, thanks to the possibility of using an array. However, some disadvantages intrinsically affect the Geiger mode operation: above all, each time a photon is detected, the SPAD cannot record another photon during the so-called dead time. The minimization of this dead time is of paramount importance in many cases. For example, in airborne LIDAR altimeters that scan the terrain topography through a semiporous obscuration (e.g. tree canopies, clouds, ground fog, etc.), the first photons that are reflected can mask the photons actually scattered by the terrain: in this scenario, a dead time in the nanosecond range allows the record of photons reflected by surfaces having a distance of few meters. Moreover, a fast recovery of the detector is crucial in presence of a strong background when the LIDAR receiver can fall into paralyzation due to the high rate of photon detections. Here, we present a new Active Quenching Circuit (AQC) able to operate external high-performance custom technology SPAD detectors at extremely high rates. In particular, the circuit can drive a thin custom-technology SPAD with a dead time as low as 6.2ns, corresponding to a maximum photon count rate of more than 160 Mcps, and a RED-Enhanced SPAD up to 100Mcps.
The Southern Connecticut Stellar Interferometer (SCSI) is a two-telescope astronomical intensity interferometer that was completed in June 2016 and has been taking photon correlation data since that time. It uses single-photon avalanche diode (SPAD) detectors at the telescope focal plane and a central timing module, which records the signals from both telescopes simultaneously. In the observations taken to date, single-pixel SPADs have been connected to signal cables that stretch from each telescope to the timing module. However, we are now in the process of making the instrument “wireless” by using a separate timing module at each telescope and synchronizing the signals recorded using GPS timing cards. We have also upgraded one of the two stations with an 8-pixel SPAD device, which allows us to achieve higher count rates in a variety of observing conditions. In this paper, we report on the current state of the instrument, including engineering tests made in preparation for wireless operation, and we discuss the expected capabilities in that mode.
Single-molecule fluorescence resonant energy transfer (smFRET) allows identifying sub-populations within doubly-labeled molecules, based on the distances between the donor (D) and acceptor (A) fluorescent labels. Solution-based smFRET allows measurement of binding-unbinding events or conformational changes of dye-labeled biomolecules without ensemble averaging and free from surface perturbations. When employing dual (or multi) laser excitation, smFRET allows resolving the number of fluorescent labels on each molecule, greatly enhancing the ability to study heterogeneous samples. A major drawback to solution-based smFRET techniques is their low throughput, which renders measurements time-consuming and prevents from studying kinetic phenomena in real-time.
Here we demonstrate a high-throughput smFRET setup which multiplexes acquisition by using 48 excitation spots and two 48-pixel single-photon avalanche diode (SPAD) arrays. Using two excitation lasers, one of which is alternated on the 10 us time scale, allows identifying and sorting species with one or two active fluorophores, extending the range of measurable FRET efficiencies and enabling proper fluorescence-aided molecular sorting. The performance of the system is demonstrated with a set of doubly-labeled double-stranded DNA oligonucleotides with different D-A distances. We show that the acquisition time for accurate subpopulation identification is reduced from several minutes to seconds, opening the way to high-throughput screening applications and real-time kinetics studies of enzymatic reactions.
In recent years the development of Single-Photon Avalanche Diodes (SPADs) had a big impact on single-photon counting applications requiring high-performance detectors in terms of Dark Count Rate (DCR), Photon Detection Efficiency (PDE), afterpulsing probability, etc. Among these, it is possible to find applications in singlemolecule fluorescence spectroscopy that suffer from long-time measurements. In these cases SPAD arrays can be a solution in order to shorten the measurement time, thanks to the high grade of parallelism they can provide. Moreover, applications in other fields (e.g. astronomy) demand for large-area single-photon detectors, able also to handle very high count rates. For these reasons we developed a new single-photon detection module, featuring an 8×8 SPAD array. Thanks to a dedicated silicon technology, the performance of the detector have been finely optimized, reaching a 49% detection efficiency at 550 nm, as well as low dark counts (2 kcount/s maximum all over the array). This module can be used in two different modes: the first is a multi-spot configuration, allowing the acquisition of 64 optical signals at the same time and considerably reducing the time needed for a measurement. The second operation mode instead exploits all the pixels in a combined mode, allowing the detection of a 64-times higher maximum photon rate (up to 2 Gcount/s). In addition, this configuration provides also an extended dynamic range and allows to attain photon number resolving capabilities. Dark counts, detection efficiency, linearity, afterpulsing and crosstalk probability have been characterized at different operating conditions.
Single-molecule spectroscopy on freely-diffusing molecules allows detecting conformational changes of biomolecules without perturbation from surface immobilization. Resolving fluorescence lifetimes increases the sensitivity in detecting conformational changes and overcomes artifacts common in intensity-based measurements. Common to all freely-diffusing techniques, however, are the long acquisition times. We report a time-resolved multispot system employing a 16-channel SPAD array and TCSPC electronics, which overcomes the throughput issue. Excitation is obtained by shaping a 532 nm pulsed laser into a line, matching the linear SPAD array geometry. We show that the line-excitation is a robust and cost-effective approach to implement multispot systems based on linear detector arrays.
The construction of a new prototype visible-light intensity interferometer for use in stellar astronomy is described. The instrument is located in New Haven, Connecticut, at Southern Connecticut State University, but key components of the system are also portable and have been taken to existing research-class telescopes to maximize sensitivity and baseline. The interferometer is currently a two-station instrument, but it is easily expandable to several stations for simultaneous measurement using multiple baselines. The design features single photon avalanche diode (SPAD) arrays, which increase the throughput and signal-to-noise ratio of the instrument. Predicted system performance and preliminary observations will be discussed.
In order to fulfill the requirements of many applications, we recently developed a new technology aimed at combining the advantages of traditional thin and thick silicon Single Photon Avalanche Diodes (SPAD). In particular we demonstrated single-pixel detectors with a remarkable improvement in the Photon Detection Efficiency in the red/nearinfrared spectrum (e.g. 40% at 800nm) while maintaining a timing jitter better than 100ps. In this paper we discuss the limitations of such Red-Enhanced (RE) technology from the point of view of the fabrication of small arrays of SPAD and we propose modifications to the structure aimed at overcoming these issues. We also report the first preliminary experimental results attained on devices fabricated adopting the improved structure.
In order to fulfill the requirements of many applications, we recently developed a new technology aimed at combining the advantages of traditional thin and thick silicon Single Photon Avalanche Diodes (SPAD). In particular we demonstrated single-pixel detectors with a remarkable improvement in the Photon Detection Efficiency at the longer wavelengths (e.g. 40% at 800nm) while maintaining a timing jitter better than 100ps. In this paper we will analyze the factors the currently prevent the fabrication of arrays of SPADs by adopting such a Red-Enhanced (RE) technology and we will propose further modifications to the device structure that will enable the fabrication of high performance RE-SPAD arrays for photon timing applications.
With the recent progresses in quantum technologies, single photon sources have gained a primary relevance. Here we present a heralded single photon source characterized by an extremely low level of noise photons, realized by exploiting low-jitter electronics and detectors and fast custom-made electronics used to control an optical shutter (a LiNbO3 waveguide optical switch) at the output of the source. This single photon source showed a second-order autocorrelation function g(2)(0) = 0:005(7), and an Output Noise Factor (defined as the ratio of noise photons to total photons at the source output) of 0:25(1)%, among the best ever achieved.
Nowadays, many research fields like biology, chemistry, medicine and space technology rely on high sensitivity imaging instruments that allow to exploit modern measurement techniques; among these, Time-Correlated Single-Photon Counting (TCSPC) provides extremely high time resolution. Single-photon detectors play a key role in these advanced imaging systems, and in recent years Single-Photon Avalanche Diodes (SPADs) have become a valid alternative to Photo Multiplier Tubes (PMTs). Moreover scientific research has recently focused on single photon detector arrays, pushed by a growing demand for multichannel systems. In this scenario, we developed a compact, stand-alone, 32-channel system for time-resolved single-photon counting applications. The system core is represented by a 32×1 SPAD array built in custom technology, featuring high time resolution, high photon detection efficiency (> 45%) and low dark count rate. The SPAD avalanche signal is exported through an integrated inverter which is placed close to the photo detector, this way the avalanche event is detected with high time resolution while achieving negligible crosstalk between adjacent pixels. SPAD proper operation is guaranteed by a 32×1 mixed passive-active quenching circuit (AQC) array built in 0.18 μm HV-CMOS technology; its digital outputs are fed to an FPGA that performs on-board processing of photon counting information. On the contrary, photon timing information is directly extracted from the pixel array and exported in Current Mode Logic (CML) standard. Preliminary experiments have been carried out on the developed system, resulting in a high time resolution (< 60 ps FWHM) and mean dark count rate lower than 8500 counts/s at 25°C.
Single-molecule fluorescence spectroscopy of freely diffusing molecules in solution is a powerful tool used to
investigate the properties of individual molecules. Single-Photon Avalanche Diodes (SPADs) are the detectors of choice
for these applications. Recently a new type of SPAD detector was introduced, dubbed red-enhanced SPAD (RE-SPAD),
with good sensitivity throughout the visible spectrum and with excellent timing performance. We report a
characterization of this new detector for single-molecule fluorescence resonant energy transfer (smFRET) studies on
freely diffusing molecules in a confocal geometry and alternating laser excitation (ALEX) scheme. We use a series of
doubly-labeled DNA molecules with donor-to-acceptor distances covering the whole range of useful FRET values. Both
intensity-based (μs-ALEX) and lifetime-based (ns-ALEX) measurements are presented and compared to identical
measurements performed with standard thick SPADs. Our results demonstrate the great potential of this new detector for
smFRET measurements and beyond.
Single-molecule Förster resonance energy transfer (smFRET) techniques are now widely used to address outstanding
problems in biology and biophysics. In order to study freely diffusing molecules, current approaches consist in exciting a
low concentration (<100 pM) sample with a single confocal spot using one or more lasers and detecting the induced
single-molecule fluorescence in one or more spectrally- and/or polarization-distinct channels using single-pixel Single-Photon Avalanche Diodes (SPADs). A large enough number of single-molecule bursts must be accumulated in order to
compute FRET efficiencies with sufficient statistics. As a result, the minimum timescale of observable phenomena is set
by the minimum acquisition time needed for accurate measurements, typically a few minutes or more, limiting this
approach mostly to equilibrium studies. Increasing smFRET analysis throughput would allow studying dynamics with
shorter timescales. We recently demonstrated a new multi-spot excitation approach, employing a novel multi-pixel
SPAD array, using a simplified dual-view setup in which a single 8-pixel SPAD array was used to collect FRET data
from 4 independent spots. In this work we extend our results to 8 spots and use two 8-SPAD arrays to collect donor and
acceptor photons and demonstrate the capabilities of this system by studying a series of doubly labeled dsDNA samples
with different donor-acceptor distances ranging from low to high FRET efficiencies. Our results show that it is possible
to enhance the throughput of smFRET measurements in solution by almost one order of magnitude, opening the way for
studies of single-molecule dynamics with fast timescale once larger SPAD arrays become available.
In this paper we present an array of 48 Single Photon Avalanche Diodes (SPADs) specifically designed for multispot
Single Molecule Analysis. The detectors have been arranged in a 12x4 square geometry with a pitch-to-diameter ratio of
ten in order to minimize the collection of the light from non-conjugated excitation spots. In order to explore the tradeoffs
between the detectors’ performance and the optical coupling with the experimental setup, SPADs with an active
diameter of 25 and of 50μm have been manufactured. The use of a custom technology, specifically designed for the
fabrication of the detectors, allowed us to combine a high photon detection efficiency (peak close to 50% at a wavelength
of 550nm) with a low dark count rate compatible with true single molecule detection.
In order to allow easy integration into the optical setup for parallel single-molecule analysis, the SPAD array has been
incorporated in a compact module containing all the electronics needed for a proper operation of the detectors.
We present the design and performances of a radiation detector based on plastic scintillating fibers with doubleside readout by means of large-area Single Photon Avalanche Diodes (SPAD). This can be the basic step toward the realization of a large-area, cost-effective position sensitive detector to be employed in future space gammaray observatories. SPADs are silicon devices operated above the junction breakdown voltage (with the typical overvoltage of 5V), for which a single photon interacting in the active region is sufficient to trigger a selfsustainable avalanche discharge. SPADs can thus be used for the detection of very low light levels with a fast time response around 50ps FWHM for single photon detection, without spectroscopic capabilities. Large-area SPAD (500 μm in diameter) have been designed and fabricated at the CNR-IMM facility, with an intrinsic noise lower than 10kHz at -15°C, and are optically coupled to both ends of 3-meter long scintillating fibers, with the same diameter. Double-side readout is required to operate the devices in coincidence (10ns coincidence window), in order to reduce the rate of false detections to the level of 1Hz. The detectors have been tested with minimum ionizing particles at CERN PS demonstrating a detection efficiency larger than 90% and a moderate position resolution along the fiber due to the difference in time of arrival between the two photodetectors. Radiation hardness tests on SPADs have also been carried out, showing that large-area SPADs can be safely employed in low-inclination low Earth orbits.
In the last years many progresses have been made in the field of silicon Single Photon Avalanche Diodes (SPAD) thanks to the improvements both in device design and in fabrication technology. Particularly, the Dipartimento di Elettronica e Informazione of Politecnico di Milano and the CNR-IMM of Bologna have been in the forefront of this research activity by designing and fabricating a new device structure enabling the fabrication of SPADs with red enhanced photon detection efficiency. In this paper we present a compact photon counting and timing module that fills the gap between the high temporal resolution and the high detection efficiency systems. The module exploits Red-Enhanced SPAD technology to attain a Photon Detection Efficiency (PDE) as high as 37% at 800 nm (peak of 58% at 600 nm) while maintaining a temporal resolution of about 100 ps FWHM, even with light diffused across the whole active area. A thermo-electric cooling system guarantees a noise as low as few counts per second for a 50 μm diameter SPAD while a low threshold avalanche pick-up circuit assures a limited shift in the temporal response.
Thanks to the steady improvement in the detectors' performance, single-photon techniques are nowadays employed in a
large number of applications ranging from single molecule dynamics to astronomy. In particular, silicon Single Photon
Avalanche Diodes (SPAD) play a crucial role in this field thanks to their remarkable performance in terms of Photon
Detection Efficiency (PDE), temporal response and Dark Count Rate (DCR). While CMOS technology allows the
fabrication of large arrays of SPAD with built-in electronics, it is only resorting to custom fabrication processes that is
possible to attain detectors with high-end performance required by most demanding applications. However, the
fabrication of arrays for timing applications, even with a small number of pixels, is quite challenging with custom
processes owing to electrical coupling between pixels. In this paper we will discuss technological solutions for the
fabrication of arrays of high-performance SPAD for parallel photon timing.
KEYWORDS: Single photon detectors, Power supplies, Field programmable gate arrays, Photodetectors, Signal detection, Polarization, Sensors, Picosecond phenomena, Data processing, Integrated circuits
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPAD)
for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in planar technologies offer the
typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they
have inherently higher photon detection efficiency than PMTs and are able to provide, beside sensitivities down to
single-photons, very high acquisition speeds. In order to make SPAD array more and more competitive in time-resolved
application it is necessary to face problems like electrical crosstalk between adjacent pixel, moreover all the singlephoton
timing electronics with picosecond resolution has to be developed.
In this paper we present a new instrument suitable for single-photon imaging applications and made up of 32 timeresolved
parallel channels. The 32x1 pixel array that includes SPAD detectors represents the system core, and an
embedded data elaboration unit performs on-board data processing for single-photon counting applications. Photontiming
information is exported through a custom parallel cable that can be connected to an external multichannel TCSPC
system.
KEYWORDS: Fluorescence resonance energy transfer, Molecules, Sensors, Liquid crystal on silicon, Field programmable gate arrays, Data transmission, Imaging systems, Resonance energy transfer, Luminescence, Objectives
Single-molecule Förster resonance energy transfer (smFRET) is a powerful tool for extracting distance information
between two fluorophores (a donor and acceptor dye) on a nanometer scale. This method is commonly used to monitor
binding interactions or intra- and intermolecular conformations in biomolecules freely diffusing through a focal volume
or immobilized on a surface. The diffusing geometry has the advantage to not interfere with the molecules and to give
access to fast time scales. However, separating photon bursts from individual molecules requires low sample
concentrations. This results in long acquisition time (several minutes to an hour) to obtain sufficient statistics. It also
prevents studying dynamic phenomena happening on time scales larger than the burst duration and smaller than the
acquisition time. Parallelization of acquisition overcomes this limit by increasing the acquisition rate using the same low
concentrations required for individual molecule burst identification. In this work we present a new two-color smFRET
approach using multispot excitation and detection. The donor excitation pattern is composed of 4 spots arranged in a
linear pattern. The fluorescent emission of donor and acceptor dyes is then collected and refocused on two separate areas
of a custom 8-pixel SPAD array. We report smFRET measurements performed on various DNA samples synthesized
with various distances between the donor and acceptor fluorophores. We demonstrate that our approach provides
identical FRET efficiency values to a conventional single-spot acquisition approach, but with a reduced acquisition time.
Our work thus opens the way to high-throughput smFRET analysis on freely diffusing molecules.
In the last years many progresses have been made in the field of Silicon Single Photon Avalanche Diodes (SPAD) thanks
to the improvements both in device design and in fabrication technology. For example, the use of custom fabrication
processes has allowed a steadily improvement of SPAD performance in terms of active area diameter, Dark Count Rate
(DCR), and Photon Detection Efficiency (PDE). Although a significant breakthrough has been achieved with the recent
introduction of a new device structure capable of combining a good timing resolution with a remarkable PDE in the near
infrared region, nevertheless there is still room for further improvements.
In this paper we will discuss further modifications to the device structure enabling the fabrication of arrays with red
enhanced photon detection efficiency.
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPAD)
for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in planar technologies offer the
typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they
have inherently higher photon detection efficiency than PMTs and are able to provide, beside sensitivities down to
single-photons, very high acquisition speeds. Although currently available silicon devices reached remarkable
performance, nevertheless further improvements are needed in order to meet the requirements of most demanding timeresolved
techniques, it is necessary to face problems like electrical crosstalk between adjacent pixel, high detection
efficiency in the red spectral range, large area, low dark counting rate. Moreover to develop array with high number of
pixel became more and more important to develop all the TCSPC electronics with picosecond resolution to create a new
family of detection system for TCSPC applications. Recent advances in our research on single photon time resolved
array is here presented.
KEYWORDS: Sensors, Molecules, Imaging spectroscopy, Signal detection, Fluorescence correlation spectroscopy, Photodetectors, Temporal resolution, Single molecule spectroscopy, Point spread functions, Signal to noise ratio
Solution-based single-molecule fluorescence spectroscopy is a powerful new experimental approach with applications in
all fields of natural sciences. Two typical geometries can be used for these experiments: point-like and widefield
excitation and detection. In point-like geometries, the basic concept is to excite and collect light from a very small
volume (typically femtoliter) and work in a concentration regime resulting in rare burst-like events corresponding to the
transit of a single-molecule. Those events are accumulated over time to achieve proper statistical accuracy. Therefore the
advantage of extreme sensitivity is somewhat counterbalanced by a very long acquisition time. One way to speed up data
acquisition is parallelization. Here we will discuss a general approach to address this issue, using a multispot excitation
and detection geometry that can accommodate different types of novel highly-parallel detector arrays. We will illustrate
the potential of this approach with fluorescence correlation spectroscopy (FCS) and single-molecule fluorescence
measurements. In widefield geometries, the same issues of background reduction and single-molecule concentration
apply, but the duration of the experiment is fixed by the time scale of the process studied and the survival time of the
fluorescent probe. Temporal resolution on the other hand, is limited by signal-to-noise and/or detector resolution, which
calls for new detector concepts. We will briefly present our recent results in this domain.
Multi-dimensional Time Correlated Single Photon Counting has nowadays reached a prominent position among
analytical techniques employed in the medical and biological fields. The development of instruments able to
perform simultaneously temporal and spectral fluorescence analysis (sFLIM) is limited by the performances of
single-photon detectors; in fact currently available arrays cannot satisfy simultaneously all the requirements. To
face this rising quest, a fully-parallel eight channel module, based on a monolithic Single Photon Avalanche Diode
(SPAD) array with great temporal resolution, high Photon Detection Efficiency (PDE) and low Dark Counting
Rate (DCR), has been designed and fabricated. The system relies on a novel architecture of the single pixel,
based on the integration of the timing pick-up circuit next to the photodetector, making the negative effects of
electrical and optical crosstalk on photon timing performance negligible. To this aim, the custom technological
process used to fabricate the SPAD has been modified, allowing the integration of MOS transistors without
impairing the structure and the performance of the detector. The single channel is complemented by an external
Active Quenching Circuit, fabricated in a standard CMOS technology, that ensures high maximum counting rate
(> 5MHz) and low afterpulsing (< 2%). Finally, the output timing signals are read and conditioned by a proper
CMOS electronics. The complete pixel shows a very good temporal resolution of about 45 ps (FWHM).
Many applications require high performance Single Photon Avalanche Diodes (SPAD) either as single pixels or as small
arrays of detectors. Although currently available silicon devices reached remarkable performance, nevertheless further
improvements are needed in order to meet the requirements of most demanding time-resolved techniques.
In this paper we present a new planar silicon technology for the fabrication of SPAD detectors, aimed at improving the
Photon Detection Efficiency (PDE) of classical thin SPAD in the near infrared range while maintaining a good Temporal
Resolution (TR). Experimental characterization showed a significant increase in the PDE with a remarkable value of
40% at 800nm; a photon timing jitter as low as 93ps FWHM as been also attained, while other device performances,
such as Dark Count Rate (DCR) and Afterpulsing Probability (AP) are essentially unchanged, compared to thin SPAD.
Being planar, the new technology is also intrinsically compatible with the fabrication of arrays of detectors.
We introduce a novel SPAD device with high photon detection efficiency and good performances in terms of temporal resolution and dark count rate. The designed detectors are able to attain a PDE as high as 40% at a wavelength of 800 nm while keeping photon detection jitter below 100 ps. The device was fabricated with a suitable planar silicon technology process that allows the development of detector arrays.
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes
(SPAD) for time resolved detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible planar
technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power,
etc.). Furthermore, they have inherently higher photon detection efficiency than PMTs and are able to provide, beside
sensitivities down to single-photons, very high acquisition speeds. They are in principle therefore ideal candidates for the
development of new parallel systems analysis. The birth of novel techniques and diagnostic instruments in fact has led
towards the parallelization of measurement systems and consequently to the development of monolithic arrays of
detectors. Unfortunately, the implementation of a multidimensional system is a challenging task, because optical and
electrical crosstalk between adjacent channels strongly affect the timing performances of the SPADs; for these reasons,
only a few number of commercial solutions are available and their performances are not comparable to the best single
channel ones. A new compact module based on a 8x1 high performance time resolved SPAD array with a new timing
approach is here presented.
Remarkable advances in semiconductor technology as long as improvements in device design resulted in today's Silicon
Single Photon Avalanche Diodes (SPADs) that are widely used in many demanding applications thanks to their excellent
performance. However a lot of work is still be done in order to simultaneously meet three requirements crucial in a large
number of applications, i.e. high Photon Detection Efficiency (PDE), good timing resolution and suitability for the
fabrication of arrays.
We will report on our advances on the development of a new planar silicon SPAD with high photon detection efficiency
(PDE) and good photon timing resolution. A thick epitaxial layer allows for the absorption of a significant fraction of
photons even at the longer wavelengths, while a suitable electric field profile limits the breakdown voltage value and the
timing jitter; biased guard rings are also included to prevent edge breakdown. Preliminary results show that the new
devices can attain a PDE as high as 30% at a wavelength of 800nm while keeping photon detection jitter below 100ps.
In recent years a growing number of applications demands always better timing resolution for Single Photon
Avalanche Diodes. The challenge is pursuing the improved timing resolution without impairing the other device
characteristics such as quantum efficiency and dark counts. This task requires a clear understanding of the
physical mechanisms necessary to drive the device engineering process.
Past studies state that in Si-SPADs the avalanche injection position statistics is the main contribution to the
photon-timing jitter. However, in recent re-engineered devices, this assumption is questioned. For the purpose
of assessing for good this contribution we developed an experimental setup in order to characterize the photontiming
jitter as a function of the injection position by means of TCSPC measurements with a laser focused
on the device active area. Results confirmed not only that the injection position is not the main contribution
to the photon-timing jitter but also evidenced a radial dependence never observed before. Furthermore we
found a relation between the photon-timing jitter and the specific resistance of the devices. To characterize
the resistances we studied the avalanche current density distribution in the device active area by imaging the
photo-luminescence due to hot-carrier emission.
KEYWORDS: Sensors, Photodetectors, Doping, Silicon, Absorption, Electric field sensors, Single photon detectors, Near infrared, Instrument modeling, Ionization
We will report on our advances on the development of a new planar silicon SPAD with high photon detection efficiency
(PDE) and good photon timing resolution. We will show that a 10μm thick epitaxial layer allows for the absorption of a
significant fraction of the incident photons even at the longer wavelengths, while a suitable electric field profile limits
the breakdown voltage value and the timing jitter. Simulations show that the new devices can attain a PDE higher than
30% at a wavelength of 800nm.
Solution-based single-molecule fluorescence spectroscopy is a powerful new experimental approach with applications in
all fields of natural sciences. The basic concept of this technique is to excite and collect light from a very small volume
(typically femtoliter) and work in a concentration regime resulting in rare burst-like events corresponding to the transit
of a single-molecule. Those events are accumulated over time to achieve proper statistical accuracy. Therefore the
advantage of extreme sensitivity is somewhat counterbalanced by a very long acquisition time. One way to speed up data
acquisition is parallelization. Here we will discuss a general approach to address this issue, using a multispot excitation
and detection geometry that can accommodate different types of novel highly-parallel detector arrays. We will illustrate
the potential of this approach with fluorescence correlation spectroscopy (FCS) and single-molecule fluorescence
measurements obtained with different novel multipixel single-photon counting detectors.
In this paper we present a physically-based model aimed at calculating the Photon Detection Efficiency (PDE) and the
temporal response of a Single-Photon Avalanche Diode (SPAD) with a given structure. In order to calculate these
quantities, it is necessary to evaluate both the probability and the delay with which a photon impinging on the detector
area triggers an avalanche. Three tasks are sequentially performed: as a first step, the electron-hole generation profile
along the device is calculated according to the silicon absorption coefficient at the considered wavelength; successively,
temporal evolution of the carriers distribution along the device is calculated by solving drift diffusion equations; finally,
the avalanche triggering probability is calculated as a function of the photon absorption point.
Validation of the model has been carried out by comparing simulation and experimental results of a few generations of
detectors previously realized in our laboratory. Photon detection efficiency has been measured and calculated for
wavelengths ranging from 400nm to 1000nm and for excess bias voltages ranging from 2 to 8V. Similarly, temporal
response has been investigated at two different wavelengths (520 and 820nm). A remarkable agreement between
experimental and simulation results has been obtained in the entire characterization domain simply starting from the
measured doping profile and without the need of any fitting parameter. Consequently, we think that this model will be a
valuable tool for the development of new detectors with improved performances.
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes
(SPAD) for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible
planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low
power, etc.). Furthermore, they have inherently higher photon detection efficiency than PMTs and are able to provide,
beside sensitivities down to single-photons, very high acquisition speeds (i.e. either high frame-rates or very short
integration time-slots). SPADs offer several advantages over other commercially available imagers. For example, CCDs
and similar imagers lack in speed because their readout process is based on a slow charge-transfer mechanisms. CMOS
APS, on the other hand, are unable to detect very faint optical signals, due to poor sensitivity and noisy electronics.
In order to make SPAD array more and more competitive it is necessary to face several issues: dark counts, quantum
efficiency, crosstalk, timing performance. These issues will be discussed in the context of two possible approaches to
such a challenge: employing a standard industrial CMOS technology or developing a dedicated technology. Advances
recently attained will be outlined with reference to both photon counting and Time correlated single photon counting
detector arrays.
The development of a very-compact DNA sequencer instrument based on Single Photon Avalanche Diode (SPAD) for
microchip electrophoresis is here reported. The planar epitaxial SPAD combines the typical advantages of microelectronic
devices with high sensitivity. We present a miniaturized system based on a custom array of SPAD, purposely designed
to be compatible with Amersham Biosciences commercial markers. This system is the first example of very compact,
ultra-sensitive, portable and low cost DNA sequencer. It may represent a breakthrough in DNA sequencing system and
open the way to the development of a new category of portable low-cost apparatus.
Single photon counting (SPC) and time correlated single photon counting (TCSPC) techniques have been developed in
the past four decades relying on photomultiplier tubes (PMT), but interesting alternatives are nowadays provided by
solid-state single photon detectors. In particular, silicon Single Photon Avalanche Diodes (SPAD) fabricated in planar
technology join the typical advantages of microelectronic devices (small size, ruggedness, low operating voltage and low
power dissipation, etc.) with remarkable basic performance, such as high photon detection efficiency over a broad
spectral range up to 1 μm wavelength, low dark count rate and photon timing jitter of a few tens of picoseconds. In
recent years detector modules employing planar SPAD devices with diameter up to 50 µm have become commercially
available. SPADs with larger active areas would greatly simplify the design of optical coupling systems, thus making
these devices more competitive in a broader range of applications. By exploiting an improved SPAD technology, we
have fabricated planar devices with diameter of 200 μm having low dark count rate (1500 c/s typical @ -25 °C). A
photon timing jitter of 35 ps FWHM is obtained at room temperature by using a special pulse pick-up network for
processing the avalanche current. The state-of-the-art of large-area SPADs will be reviewed and prospects of further
progress will be discussed pointing out the challenging issues that must be faced in the design and technology of SPAD
devices and associated quenching and timing circuits.
Photon counting was introduced and developed during four decades relying on Photomultiplier Tubes (PMT), but
interesting alternatives are nowadays provided by solid-state single-photon microdetectors. In particular, Silicon Single-Photon Avalanche-Diodes (SPAD) attain remarkable basic performance, such as high photon detection efficiency over a
broad spectral range up to 1 micron wavelength, low dark counting rate and photon timing jitter of a few tens of
picoseconds. In recent years SPADs have emerged from the laboratory research phase and they are now commercially
available from various manufactures. However, PMTs have much wider sensitive area, which greatly simplifies the
design of optical systems; they attain remarkable performance at high counting rate and can provide position-sensitive
photon detection and imaging capability. In order to make SPADs more competitive in a broader range of applications it
is necessary to face issues in semiconductor device technology. The present state of the art, the prospect and main issues
will be discussed.
Time-correlated single photon counting (TCSPC) is exploited in emerging scientific applications in life sciences, such as
single molecule spectroscopy, DNA sequencing, fluorescent lifetime imaging. Detectors with wide active area (diameter
> 100 μm) are desirable for attaining good photon collection efficiency without requiring complex and time-consuming
optical alignment and focusing procedures. Fiber pigtailing of the detector, often employed for having a more flexible
optical system, is also obtained more simply and with greater coupling efficiency for wide-area detectors. TCSPC,
however, demands to detectors also high photon-timing resolution besides low noise and high quantum efficiency.
Particularly tight requirements are set for single-molecule fluorescence analysis, where components with lifetimes of tens
of picoseconds are often met. Small photon timing jitter and wide area are considered conflicting requirements for the
detector.
We developed an improved planar silicon technology for overcoming the problem and providing a solid-state alternative
to MCP-PMTs in demanding TCSPC applications. We fabricated Single Photon Avalanche Diodes (SPADs) with 200
μm active area diameter and fairly low dark counting rate (DCR). At moderately low temperature (-25 °C with Peltier
cooler) the typical DCR is 1500 c/s and it is not difficult to select devices with less than 1000 c/s. The photon detection
efficiency peaks at 48% around 530 nm and stays above 30% over all the visible range. A photon timing resolution of 35
ps FWHM (full width at half maximum) is obtained by using our patented pulse pick-up for processing the avalanche
current.
Silicon Single-Photon Avalanche-Diodes (SPAD) are nowadays considered a solid-state alternative to Photomultiplier
Tubes (PMT) in single photon counting (SPC) and time-correlated single photon-counting (TCSPC) over the visible
spectral range up to 1 micron wavelength. SPADs implemented in planar epitaxial technology compatible with CMOS
circuits offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage and low power, etc.).
Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in
vacuum from a photocathode as PMT, but instead on the internal photoelectric effect. However, PMTs offer much wider
sensitive area, which greatly simplifies the design of optical systems; they provide position-sensitive photon detection
and imaging capability; they attain remarkable performance at high counting rate and offer picosecond timing resolution
with Micro-Channel Plate (MCP) models. In order to make SPADs more competitive in a broader range of SPC and
TCPC applications it is necessary to face both semiconductor technology issues and circuit design issues, which will be
here dealt with. Technology issues will be discussed in the context of two possible approaches: employing a standard
industrial high-voltage compatible CMOS technology or developing a dedicated CMOS-compatible technology. Circuit
design issues will be discussed taking into account problems arising from conflicting requirements set by various
required features, such as fast and efficient avalanche quenching and reset, high resolution photon timing, etc.
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