A SPAD Flash LiDAR Sensor With Push-Pull Analog Counter Achieving a 0.12% Depth Precision at a 70 m Depth Range

Single photon avalanche diodes (SPADs) manufactured through the standard CMOS process have a major advantage in reducing the cost and expendability in comparison with the SPAD made with the custom process. SPAD is a single photon sensitive diode using the avalanche phenomenon and operated in Geiger-mode which applies a voltage higher than the breakdown voltage of the device. Therefore, SPAD is exposed to a higher voltage than other devices and has a high electric field in the multiplication region. In SPAD operating with such a high electric field, the guard ring prevents edge breakdown and serves to focus the electric field on the intended multiplication region. Therefore, the optimized guard ring structure is required for the high efficiency SPAD designs. The deep virtual guard ring is designed to cope with deeper multiplication regions formed by deep n-well and p-well, thus providing higher photon detection probability (PDP) and enhanced response at a longer wavelength. In this paper, the four possible Silicon-based SPAD candidates of different deep virtual guard rings formed by the combination of shallow trench isolation (STI) and p-substrate layer fabricated through standard CMOS process are investigated and their electric characteristics of the SPAD, such as the current-voltage (I-V) characteristic, and optical characteristics, such as dark count rate (DCR), and PDP, are measured. In addition, TCAD simulations are used to verify the characteristic variation by analyzing the electric field profiles. Based on the results, the optimized structure of the SPAD with the deep virtual guard ring can be proposed.

We present the design and characterization of a fully-integrated array of 16 x 16 Single-Photon Avalanche Diodes (SPADs) with fast-gating capabilities and 16 on-chip 6 ps time-to-digital converters, which has been embedded in a compact imaging module. Such sensor has been developed for Non-Line-Of-Sight imaging applications, which require: i) a narrow instrument response function, for a centimeter-accurate single-shot precision; ii) fast-gated SPADs, for time-filtering of directly reflected photons; iii) high photon detection probability, for acquiring faint signals undergoing multiple scattering events. Thanks to a novel multiple differential SPAD-SPAD sensing approach, SPAD detectors can be swiftly activated in less than 500 ps and the full-width at half maximum of the instrument response function is always less than 75 ps (60 ps on average). Temporal responses are consistently uniform throughout the gate window, showing just few picoseconds of time dispersion when 30 ns gate pulses are applied, while the differential non-linearity is as low as 250 fs. With a photon detection probability peak of 70% at 490 nm, a fill-factor of 9.6% and up to 1.6 . 10(8) photon time-tagging measurements per second, such sensor fulfills the demand for fully-integrated imaging solutions optimized for non-line-of-sight imaging applications, enabling to cut exposure times while also optimizing size, weight, power and cost, thus paving the way for further scaled architectures.

With the growing importance of single-photon-counting (SPC) techniques, researchers are now designing high-performance systems based on single-photon avalanche diodes (SPADs). SPADs with high performances and low cost allow the popularity of SPC-based systems for medical and industrial applications. However, few efforts were put into the design optimization of SPADs due to limited calibrated models of the SPAD itself and its related circuits. This paper provides a perspective on improving SPAD-based system design by reviewing the development of SPAD models. First, important SPAD principles such as photon detection probability (PDP), dark count rate (DCR), afterpulsing probability (AP), and timing jitter (TJ) are discussed. Then a comprehensive discussion of various SPAD models focusing on each of the parameters is provided. Finally, important research challenges regarding the development of more advanced SPAD models are summarized, followed by the outlook for the future development of SPAD models and emerging SPAD modeling methods.

Single-photon avalanche diodes (SPADs) are very attractive devices in optical detector applications where both CMOS integration and low light detection are critical. An important SPAD performance parameter is the photon detection probability (PDP) which has to be carefully characterized for the sensor design. In this article, we present a comprehensive modeling and experimental characterization of the PDP to obtain its optical and electrical dependency on different parameters including the wavelength and light incidence angle as well as the biasing condition. By calculating the optical absorption and the avalanche triggering probabilities within the silicon, the PDP of an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p-well CMOS-implemented SPAD is accurately modeled. It is shown that due to the formation of a standing wave in different layers above the silicon, which significantly affects the optical transmission and the PDP spectrum, the application of the presented approach is necessary especially when an anti-reflection coating layer is not available. The obtained result shows an excellent agreement to our measurements and, therefore, the model can be reliably used for accurate design and optimization of SPAD-based detectors and to avoid very time-consuming experimental investigations, e. g. for obtaining the detector performance for deviations in the light incidence angle.

The near ultraviolet photon detection probability (PDP) of single photon avalanche diodes (SPADs) is very important for the fluorescence lifetime imaging. However, the PDP of traditional SPAD (T-SPAD) devices in the near-ultraviolet is not ideal, which is difficult to meet the requirements of fluorescence lifetime imaging. In response to the above problems, this paper realizes a near ultraviolet enhanced composite SPAD (NUEC-SPAD) based on photogate. The device is based on a photogate and a PN junction formed by P+/N-Well to detect photons. Therefore, the PDP of the device in the near ultraviolet is greatly improved. In addition, the shallow trench isolation (STI) and multiplication regions are isolated by photogate, and the dark count rate (DCR) of the device is greatly reduced. The principle of NUEC-SPAD device is simulated and verified based on the Technology-Computer-Aided-Design (TCAD). The NUEC-SPAD device and the T-SPAD device are fabricated based on the 0.18 μm BCD process. The experimental data show that the avalanche breakdown voltage of NUEC-SPAD device is 12 V. The device has good PDP in the range of 360 nm to 700 nm. Under the excess bias voltage of 0.5 V, the PDP of NUEC-SPAD device is 43.81% (@460 nm), which is 45.50% higher than that of T-SPAD device. Under the excess bias voltage of 1 V, the DCR of NUEC-SPAD device is only 0.24 Hz/μm2.

We present a comprehensive comparative study of passive quenching active recharge (PQAR) and active quenching active recharge (AQAR) analog front-ends (AFEs) designed for 64×64 pixel arrays of single-photon avalanche diode (SPAD) pixels. These AFEs were fabricated using a back-side illuminated (BSI) 3D-stacked 40 nm/40 nm CMOS image sensor process. While configured as an array, initial verification was conducted at the pixel level, comparing both AFE types. To enable a truly fair and direct performance evaluation, which was often difficult in prior independent studies due to the use of different devices, processes, architectures, or assumptions, both AFE types were fabricated under identical process, wafer, and layout conditions on the same die. The SPADs exhibit a breakdown voltage of 22.5 V, a low dark count rate (5.1 cps/μm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 2 V excess voltage), and a high photon detection probability (25.1% at 850 nm at 2 V excess voltage). The AQAR pixel occupies a larger area (88.56 μm²) compared to the PQAR pixel (55.75 μm²), reflecting an area-performance trade-off. We analyzed methodologies for defining SPAD dead time, adopting inter-arrival time histogram analysis under intensive light. Experimental results demonstrate precise dead time control for both AFEs, achieving a minimum dead time of 2 ns and confirming maximum count rates up to near 500 Mcps. Moreover, this study experimentally verifies the effectiveness of active quenching in afterpulse suppression, leading to reduced noise and consequently contributing to an improved signal-to-noise ratio. The results of this study provide useful information on the design trade-offs and performance of PQAR and AQAR AFEs for various high-performance single-photon detection applications.

Single-photon avalanche diodes (SPADs) must be integrated on modern highly-scaled process nodes to achieve high array fill factors and imager pixel counts. Integration also enables applications where rapid, complex data processing needs to be co-located with these detectors and other photonic components. In this work we implement 5μm, 10μm, and 15μm-diameter circular SPADs using a p+/n-well structure and an STI guard ring in a 90nm bulk CMOS process, and in contrast to previous work on silicon SPADs carry out detailed study of dark count rate (DCR) and afterpulsing at cryogenic temperatures. With passive quenching these SPADs are saturated by dark counts at room temperature and by afterpulsing at cryogenic temperatures, motivating the development of an active quenching circuit (AQC) to characterize and ultimately suppress the processes responsible for these non-idealities. A novel AQC topology based on a linear output stage, able to detect avalanches and drive SPADs connected by a coaxial cable faster than previously-demonstrated circuits, is proposed and demonstrated. This circuit enables device testing using a coaxial microprobe inside a Helium cryostat. To quantify the SPADs’ performance, we operate them in free-running mode and collect histograms of the pulse interarrival times from which we extract the DCR and afterpulsing probability. We use this technique to measure these parameters with varying temperature and overbias voltage, showing there is an optimal temperature for operating each SPAD. With active quenching we achieve 100Hz-level DCR, 10% 405nm quantum efficiency at 140K, and are presently characterizing the photon detection probability.

A monolithic resistorless circuit has been designed for active reset of the N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> P single-ended (common anode) single-photon avalanche diode (SPAD). This type of SPAD has enhanced the photon detection efficiency in the near-infrared (NIR) region relatively to the P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> N (two-ended) SPAD. Due to this fact, the N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> P SPAD is a better candidate for the light detection and range (LiDAR) working in the NIR region. The circuit is fabricated in a standard 0.18-μm CMOS image sensor process. The dead time after each photon detection is adjustable; the minimum measured value is 4 ns. The maximum photon count rate corresponding to this dead time is 2.5·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> photons/s. This circuit guarantees precise repeatable response, short and well-controlled dead time, and after-pulsing effect reduction. All these are essential for many SPAD applications and crucial for increasing the saturation level of the silicon photo multiplier (SiPM)based LiDAR systems working in strong background light conditions on sunny day.

We present a single-photon avalanche diode (SPAD) with a wide spectral range fabricated in an advanced 180 nm CMOS process. The realized SPAD achieves 20 % photon detection probability (PDP) for wavelengths ranging from 440 nm to 820 nm at an excess bias of 4 V, with 30 % PDP at wavelengths from 520 nm to 720 nm. Dark count rates (DCR) are at most 5 kHz, which is 30 Hz/μm2, at an excess bias of 4V when we measure 10 μm diameter active area structure. Afterpulsing probability, timing jitter, and temperature effects on DCR are also presented.

Single-photon avalanche diodes (SPADs), integrated in CMOS technology, offer high-gain detection of low-intensity optical signals. SPADs are widely adopted in imaging systems, including both 2D and 3D depth sensing. As SPAD arrays scale to higher resolutions, backside illumination (BSI) is one approach to improving fill factor (FF) and routing, though it increases fabrication complexity. Alternatively, in frontside illumination (FSI), the integration of circuitry within the pixel area results in a significant reduction of the FF. Thus, this brief proposes an external counter architecture with a photon event summing and coincidence detection scheme. Unlike conventional FSI designs, it preserves FF and improves routing efficiency, which facilitates scalability. This architecture is well suited for SPAD-based imaging and 3D sensing, which require high pixel resolution and compact signal integration.

We present a 16×16 single-photon avalanche diode (SPAD) image array prototype for super-resolution imaging under photon-starved environments. To take advantage of photon sparsity, we introduce a 16-to-1 multiplexing scheme at the column level which shares the same quenching circuitry among 16 SPAD pixels. This includes column-level address encoding to support super-resolution microscopy (SRM) despite multiplexing. The quenching circuit uses a 16 input pseudo-NMOS OR gate to decrease SPAD recovery time and circuitry area. The imager array was implemented in a 180nm high-voltage CMOS process and consists of SPADs with a pixel pitch of 21.3 μm and a fill factor of 17.4 % to provide high system detection efficiency. The total die area is 1.5×2.5 mm2 with a photosensitive SPAD array area of 340×340 μm2. At an excess bias of 3.6 V, we measured a photon detection probability up to 22 % at a wavelength of 520 nm. The mean dark count rate of the arrayed detectors is approximately 20 Hz/μm2. We measured a detector dead time of 4ns, which enables lifetime measurement at our target laser repetition rate of 80MHz. To demonstrate SRM with our multiplexing scheme, we localized gold nanoparticle displacement at 40nm, which is 3.7 times smaller than our pixel pitch.

This paper presents the effect of shallow trench isolation (STI) on the dark count rate (DCR) and after-pulsing probability (APP) of deep-junction-based single-photon avalanche diodes (SPADs). Two different SPADs were fabricated in 110 nm CMOS image sensor technology, one with STI and the other without STI between its anode and cathode. With TCAD simulations and measurements, we have clearly demonstrated that the SPAD without STI enables a dramatic decrease in DCR by more than three orders of magnitude without suffering from the lateral leakage current between the anode and cathode. By excluding the STI from the device, the proposed SPAD also achieves a negligible APP while the SPAD with STI shows a very high APP of 92%. Thanks to the low-noise performance, the proposed SPAD becomes operable with higher excess bias voltage so that it achieves good photon detection probability, 58.3% at 500 nm and 3% at 940 nm, and timing jitter, 71 ps full width at half maximum at 670 nm, when the reverse bias voltage is 17 V.

Time-coordinated SPAD-based receiver for high-speed optical wireless communication

This review concentrates on the state-of-the-art hardware-oriented receiver aspects for optical wireless communication (OWC), and points to the importance of BER performance analysis and modeling in presence of non-perpendicular light incidence. Receivers in OWC networks for 6G applications have to work for strongly different light incidence angles, to allow the formation of connections to locally separated transceivers without the need for rotation units and accurate adjustment. In turn, and in combination with fully integrated optical receivers, reduction of cost and increased comfort can be achieved. Fully integrated [bipolar] complementary metal-oxide-semiconductor ([Bi]CMOS) receivers with on-chip avalanche photodiodes (APDs) and single-photon avalanche diodes (SPADs) are presented and their performance in optical wireless communication is summarized. Impressive data rates up to 2 Gbit/s and free-space transmission distances up to 27 m at bit error ratios (BER) below 10−9 are reached with linear-mode APD receivers. The importance of optical interference in the isolation and passivation stack on top of the integrated photodiodes is illuminated. To be able to predict the dependence of the BER of single-photon avalanche diode (SPAD) receivers on the light incidence angle, a model, which includes a model for the photon detection probability and a standing-wave model for the isolation and passivation stack, is extended. The dependence of the BER on the light incidence angle onto the photodiodes is investigated by electromagnetic simulation for optical transmission of the layers on top of the photodiode, device simulation for the avalanche triggering probability and BER modeling with MATLAB. It is found that incidence angles up to 30° have moderate influence on the BER and that the BER degrades significantly for incidence angles larger than 50°.

We report on the world's first backside-illuminated silicon photomultiplier (SiPM) implemented in 3D integrated circuit technology. The SiPM was fabricated in a two-tier 130nm CMOS process; the top tier houses 1600 single-photon avalanche diodes (SPADs), organized in a dual 4×200 linear array; the bottom tier houses 2×100 time-to-digital converters (TDCs). Every 8 SPADs there is one shared TDC whose digital output is routed to a 1.04Gps readout interface that enables a total count rate of 80Mcps. Preliminary results show a photon detection probability of 11% at a wavelength of 725nm and a dark count rate of 1kHz at room temperature and nominal excess bias, which reduces to 80Hz if cooled to −40°C at 2V excess bias; the breakdown voltage, Vbd, was measured at 12.3V. The sensor is intended for time-of-flight vision, wide-field near-infrared imaging, and lifetime fluorescence spectroscopy used in medical, particle detection, and material analysis applications.