Silicon Photomultipliers (SiPMs) are emerging single photon detectors used in many applications requiring large active
area, photon-number resolving capability and immunity to magnetic fields. We present three families of analog SiPM
fabricated in a reliable and cost-effective fully standard planar CMOS technology with a total photosensitive area of 1×1
mm<sup>2</sup>. These three families have different active areas with fill-factors (21%, 58.3%, 73.7%) comparable to those of
commercial SiPM, which are developed in vertical (current flow) custom technologies. The peak photon detection
efficiency in the near-UV tops at 38% (fill-factor included) comparable to commercial custom-process ones and dark
count rate density is just a little higher than the best-in-class commercial analog SiPMs. Thanks to the CMOS processing,
these new SiPMs can be integrated together with active components and electronics both within the microcell and on-chip,
in order to act at the microcell level or to perform global pre-processing. We also report CMOS digital SiPMs in
the same standard CMOS technology, based on microcells with digitalized processing, all integrated on-chip. This
CMOS digital SiPMs has four 32×1 cells (128 microcells), each consisting of SPAD, active quenching circuit with
adjustable dead time, digital control (to switch off noisy SPADs and readout position of detected photons), and fast
trigger output signal. The achieved 20% fill-factor is still very good.
Advanced Driver Assistance Systems (ADAS) are the most advanced technologies to fight road accidents. Within
ADAS, an important role is played by radar- and lidar-based sensors, which are mostly employed for collision avoidance
and adaptive cruise control. Nonetheless, they have a narrow field-of-view and a limited ability to detect and
differentiate objects. Standard camera-based technologies (e.g. stereovision) could balance these weaknesses, but they
are currently not able to fulfill all automotive requirements (distance range, accuracy, acquisition speed, and frame-rate).
To this purpose, we developed an automotive-oriented CMOS single-photon camera for optical 3D ranging based on
indirect time-of-flight (iTOF) measurements.
Imagers based on Single-photon avalanche diode (SPAD) arrays offer higher sensitivity with respect to CCD/CMOS
rangefinders, have inherent better time resolution, higher accuracy and better linearity. Moreover, iTOF requires neither
high bandwidth electronics nor short-pulsed lasers, hence allowing the development of cost-effective systems. The
CMOS SPAD sensor is based on 64 × 32 pixels, each able to process both 2D intensity-data and 3D depth-ranging
information, with background suppression. Pixel-level memories allow fully parallel imaging and prevents motion
artefacts (skew, wobble, motion blur) and partial exposure effects, which otherwise would hinder the detection of fast
moving objects. The camera is housed in an aluminum case supporting a 12 mm F/1.4 C-mount imaging lens, with a
40°×20° field-of-view. The whole system is very rugged and compact and a perfect solution for vehicle’s cockpit, with
dimensions of 80 mm × 45 mm × 70 mm, and less that 1 W consumption. To provide the required optical power (1.5 W,
eye safe) and to allow fast (up to 25 MHz) modulation of the active illumination, we developed a modular laser source,
based on five laser driver cards, with three 808 nm lasers each.
We present the full characterization of the 3D automotive system, operated both at night and during daytime, in both
indoor and outdoor, in real traffic, scenario. The achieved long-range (up to 45m), high dynamic-range (118 dB), highspeed
(over 200 fps) 3D depth measurement, and high precision (better than 90 cm at 45 m), highlight the excellent
performance of this CMOS SPAD camera for automotive applications.
An array of 32x32 Single-Photon Avalanche-Diodes (SPADs) and Time-to-Digital Converters (TDCs) has been
fabricated in a 0.35 μm automotive-certified CMOS technology. The overall dimension of the chip is 9x9 mm2. Each
pixel is able to detect photons in the 300 nm – 900 nm wavelength range with a fill-factor of 3.14% and either to count
them or to time stamp their arrival time. In photon-counting mode an in-pixel 6-bit counter provides photon-numberresolved
intensity movies at 100 kfps, whereas in photon-timing mode the 10-bit in-pixel TDC provides time-resolved
maps (Time-Correlated Single-Photon Counting measurements) or 3D depth-resolved (through direct time-of-flight
technique) images and movies, with 312 ps resolution. The photodetector is a 30 μm diameter SPAD with low Dark
Count Rate (120 cps at room temperature, 3% hot-pixels) and 55% peak Photon Detection Efficiency (PDE) at 450 nm.
The TDC has a 6-bit counter and a 4-bit fine interpolator, based on a Delay Locked Loop (DLL) line, which makes the
TDC insensitive to process, voltage, and temperature drifts. The implemented sliding-scale technique improves linearity,
giving 2% LSB DNL and 10% LSB INL. The single-shot precision is 260 ps rms, comprising SPAD, TDC and driving
board jitter. Both optical and electrical crosstalk among SPADs and TDCs are negligible. 2D fast movies and 3D
reconstructions with centimeter resolution are reported.
We present our latest results concerning CMOS Single-Photon Avalanche Diode (SPAD) arrays for high-throughput parallel single-photon counting. We exploited a high-voltage 0.35 μm CMOS technology in order to develop low-noise CMOS SPADs. The Dark Count Rate is 30 cps at room temperature for 30 μm devices, increases to 2 kcps for 100 μm SPADs and just to 100 kcps for 500 μm ones. Afterpulsing is less than 1% for hold-off time longer than 50 ns, thus allowing to reach high count rates. Photon Detection Efficiency is > 50% at 420 nm, > 40% below 500 nm and is still 5% at 850 nm. Timing jitter is less than 100 ps (FWHM) in SPADs with active area diameter up to 50 μm.
We developed CMOS SPAD imagers with 150 μm pixel pitch and 30 μm SPADs. A 64×32 SPAD array is based on pixels including three 9-bit counters for smart phase-resolved photon counting up to 100 kfps. A 32x32 SPAD array includes 1024 10-bit Time-to-Digital Converters (TDC) with 300 ps resolution and 450 ps single-shot precision, for 3D ranging and FLIM. We developed also linear arrays with up to 60 pixels (with 100 μm SPAD, 150 μm pitch and in-pixel 250 ps TDC) for time-resolved parallel spectroscopy with high fill factor.
SPADs (Single Photon Avalanche Diodes) are emerging as most suitable photodetectors for both single-photon counting (Fluorescence Correlation Spectroscopy, Lock-in 3D Ranging) and single-photon timing (Lidar, Fluorescence Lifetime Imaging, Diffuse Optical Imaging) applications. Different complementary metal-oxide semiconductor (CMOS) implementations have been reported in literature. We present some figure of merit able to summarize the typical SPAD performances (i.e. Dark Counting Rate, Photo Detection Efficiency, afterpulsing probability, hold-off time, timing jitter) and to identify a proper metric for SPAD comparison, both as single detectors and also as imaging arrays. The goal is to define a practical framework within which it is possible to rank detectors based on their performances in specific experimental conditions, for either photon-counting or photon-timing applications. Furthermore we review the performances of some CMOS and custom-made SPADs. Results show that CMOS SPADs performances improve as the technology scales down; moreover, miniaturization of SPADs and new solutions adopted to counteract issues related with the SPAD design (electric field uniformity, premature edge breakdown, tunneling effects, defect-rich STI interface) along with advances in standard CMOS processes led to a general improvement in all fabricated photodetectors; therefore, CMOS SPADs can be suitable for very dense and cost-effective many-pixels imagers with high performances.
We designed and characterized Silicon Single-Photon Avalanche Diodes (SPADs) fabricated in a high-voltage 0.35 μm
CMOS technology, achieving state-of-the-art low Dark Counting Rate (DCR), very large diameter, and extended Photon
Detection Efficiency (PDE) in the Near Ultraviolet. So far, different groups fabricated CMOS SPADs in scaled
technologies, but with many drawbacks in active area dimensions (just a few micrometers), excess bias (just few Volts),
DCR (many hundreds of counts per second, cps, for small 10 μm devices) and PDE (just few tens % in the visible
range). The novel CMOS SPAD structures with 50 μm, 100 μm, 200 μm and 500 μm diameters can be operated at room temperature and show DCR of 100 cps, 2 kcps, 20 kcps and 100 kcps, respectively, even when operated at 6 V excess
bias. Thanks to the excellent performances, these large CMOS SPADs are exploitable in monolithic SPAD-based arrays
with on-chip CMOS electronics, e.g. for time-resolved spectrometers with no need of microlenses (thanks to high fillfactor).
Instead the smaller CMOS SPADs, e.g. the 10 μm devices with just 3 cps at room temperature and 6 V excess
bias, are the viable candidates for dense 2D CMOS SPAD imagers and 3D Time-of-Flight ranging chips.
Combined 2D imaging and 3D ranging sensors provide useful information for both long (some kms) and short (few tens of m) distance, in security applications. To this aim, we designed two different monolithic imagers in a 0.35 μm costeffective CMOS technology, based on Single Photon Avalanche Diodes (SPADs), for long-range time-of-flight (TOF)
and short-range phase-resolved depth ranging. The single pixel consists of a SPAD (30 μm diameter), a quenching
circuit, and a Time-to-Digital Converter (TDC) for TOF measurements or three up/down synched counters for phaseresolved depth assessments. Such smart pixels operate in two different modalities: single photon-counting for 2D “intensity” images; while either photon-timing or phase-resolved photon-counting for 3D “depth” images. In 2D
imaging, each pixel has a counter that accumulates the number of photons detected by the SPAD in the pixel, thus
providing single-photon level sensitivity and high (100 kframe/s) frame-rate. In the TOF 3D imager, each pixel measures
the photon arrival time with a 312 ps resolution, thanks to a two-stage TDC (with 6 bit coarse counter plus a 4 bit fine
interpolator), with a 320 ns full-scale range. The resulting spatial resolution is 9 cm within a 50 m range, centered at any user-selectable distance (e.g. 100 m – 5 km), with linearity of DNLrms=4.9% LSB and INLrms=11.7% LSB, and 175 ps
precision. In the phase-resolved 3D imager, the in-pixel electronics measures the phase difference between the
modulated light emitted by a laser and the back-reflected light, with both continuous-wave and pulsed-light modulation techniques.
The growing interest for fast, compact and cost-effective 3D ranging imagers for automotive applications has prompted to explore many different techniques for 3D imaging and to develop new system for this propose. CMOS imagers that exploit phase-resolved techniques provide accurate 3D ranging with no complex optics and are rugged and costeffective. Phase-resolved techniques indirectly measure the round-trip return of the light emitted by a laser and backscattered from a distant target, computing the phase delay between the modulated light and the detected signal. Singlephoton detectors, with their high sensitivity, allow to actively illuminate the scene with a low power excitation (less than 10W with diffused daylight illumination). We report on a 4x4 array of CMOS SPAD (Single Photon Avalanche Diodes) designed in a high-voltage 0.35 μm CMOS technology, for pulsed modulation, in which each pixel computes the phase difference between the laser and the reflected pulse. Each pixel comprises a high-performance 30 μm diameter SPAD, an analog quenching circuit, two 9 bit up-down counters and memories to store data during the readout. The first counter counts the photons detected by the SPAD in a time window synchronous with the laser pulse and integrates the whole echoed signal. The second counter accumulates the number of photon detected in a window shifted with respect to the laser pulse, and acquires only a portion of the reflected signal. The array is readout with a global shutter architecture, using a 100 MHz clock; the maximal frame rate is 3 Mframe/s.
Systems for 3D image acquisition are the enabling technology for a number of applications such as architectural studies,
safety and security, automotive. Single-sensor active-illumination cameras are the most promising system, ensuring a
good depth measurement accuracy combined with a simple structure (no double sensor required), simplest measurement
algorithm and night and daytime operation. These systems are based on the measurement of the time delay between the
emission of light signal and the detection of the back-reflected signal (Time of Flight - TOF). The direct measurement of
the time delay between two adjacent pulses is called direct TOF (dTOF), while if the time delay is obtained starting from
the phase delay of a periodic waveform we speak of indirect TOF (iTOF). We present two different 0.35μm CMOS
Silicon mini-arrays for iTOF 3D ranging based on square and sinusoidal waveforms, in which the sensitive element is a
Single-Photon Avalanche Diode (SPAD).