Arrays of single-photon avalanche diode (SPAD) detectors were fabricated, using a 0.35 μm CMOS technology process, for use in applications such as time-of-flight 3D ranging and microscopy. Each 150 x 150 μm pixel comprises a 30 μm active area diameter SPAD and its associated circuitry for counting, timing and quenching, resulting in a fill-factor of 3.14%. This paper reports how a higher effective fill-factor was achieved as a result of integrating microlens arrays on top of the 32 x 32 SPAD arrays. Diffractive and refractive microlens arrays were designed to concentrate the incoming light onto the active area of each pixel. A telecentric imaging system was used to measure the improvement factor (IF) resulting from microlens integration, whilst varying the f-number of incident light from f/2 to f/22 in one-stop increments across a spectral range of 500-900 nm. These measurements have demonstrated an increasing IF with fnumber, and a maximum of ~16 at the peak wavelength, showing a good agreement with theoretical values. An IF of 16 represents the highest value reported in the literature for microlenses integrated onto a SPAD detector array. The results from statistical analysis indicated the variation of detector efficiency was between 3-10% across the whole f-number range, demonstrating excellent uniformity across the detector plane with and without microlenses.
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
mm2. 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.
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.