dToF Sensor with In-pixel Processing

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In a recent preprint (https://arxiv.org/pdf/2209.11772.pdf) Gyongy et al. describe a new 64x32 SPAD-based direct time-of-flight sensor with in-pixel histogramming and processing capability.

3D flash LIDAR is an alternative to the traditional scanning LIDAR systems, promising precise depth imaging in a compact form factor, and free of moving parts, for applications such as self-driving cars, robotics and augmented reality (AR). Typically implemented using single-photon, direct time-of-flight (dToF) receivers in image sensor format, the operation of the devices can be hindered by the large number of photon events needing to be processed and compressed in outdoor scenarios, limiting frame rates and scalability to larger arrays. We here present a 64 × 32 pixel (256 × 128 SPAD) dToF imager that overcomes these limitations by using pixels with embedded histogramming, which lock onto and track the return signal. This reduces the size of output data frames considerably, enabling maximum frame rates in the 10 kFPS range or 100 kFPS for direct depth readings. The sensor offers selective readout of pixels detecting surfaces, or those sensing motion, leading to reduced power consumption and off-chip processing requirements. We demonstrate the application of the sensor in mid-range LIDAR.

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Direct ToF Single-Photon Imaging (IEEE TED June 2022)

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The June 2022 issue of IEEE Trans. Electron. Devices has an invited paper titled Direct Time-of-Flight Single-Photon Imaging by Istvan Gyongy et al. from University of Edinburgh and STMicroelectronics. 

This is a comprehensive tutorial-style article on single-photon 3D imaging which includes a description of the image formation model starting from first principles and practical system design considerations such as photon budget and power requirements.

Abstract: This article provides a tutorial introduction to the direct Time-of-Flight (dToF) signal chain and typical artifacts introduced due to detector and processing electronic limitations. We outline the memory requirements of embedded histograms related to desired precision and detectability, which are often the limiting factor in the array resolution. A survey of integrated CMOS dToF arrays is provided highlighting future prospects to further scaling through process optimization or smart embedded processing.

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Photonics magazine article on Pi Imaging SPAD array

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Photonics magazine has a new article about Pi Imaging Technology's high resolution SPAD sensor array; some excerpts below.

As the performance capabilities and sophistication of these detectors have expanded, so too have their value and impact in applications ranging from astronomy to the life sciences.

As their name implies, single-photon avalanche diodes (SPADs) detect single particles of light, and they do so with picosecond precision. Single-pixel SPADs have found wide use in astronomy, flow cytometry, fluorescence lifetime imaging microscopy (FLIM), particle sizing, quantum computing, quantum key distribution, and single- molecule detection. Over the last 10 years, however, SPAD technology has evolved through the use of standard complementary metal-oxide-semiconductor (CMOS) technology. This paved the way for arrays and image sensor architectures that could increase the number of SPAD pixels in a compact and scalable way. 

Compared to single-pixel SPADs, arrays offer improved spatial resolution and signal-to-noise ratio (SNR). In confocal microscopy applications, for example, each pixel in an array acts as a virtual small pinhole with good lateral and axial resolution, while multiple pixels collect the signal of a virtual large pinhole.


Early SPADs produced as single-point detectors in custom processes offered poor scalability. In 2003, researchers started using standard CMOS technology to build SPAD arrays. This change in design and production platform opened up the possibility to reliably produce high-pixel-count SPAD detectors, as well as invent and integrate new pixel circuity for quenching and recharging, time tagging, and photon-counting functions. Data handling in these devices ranged from simple SPAD pulse outputting to full digital signal processing.
Close collaboration between SPAD developers and CMOS fabs, however, has helped SPAD technology overcome many of its sensitivity and noise challenges by adding SPAD-specific layers into the semiconductor process flow, design innovations in SPAD guard rings, and enhanced fill factors made possible by microlenses. 


Research on SPADs also focused on the technology’s potential in biomedical applications, such as Raman spectroscopy, FLIM, and positron emission tomography (PET).

FLIM [fluorescence lifetime imaging microscopy] benefits from the use of SPAD arrays, which allow faster imaging speeds by increasing the sustainable count rate via pixel parallelization. SPAD image sensors enhanced with time-gating functions can further expand the implementation of FLIM to nonconfocal microscopic modalities and thus establish FLIM in a broader range of potential applications, such as spatial multiplexed applications in a variety of biological disciplines including genomics, proteomics, and other “-omics” fields.

One additional application where SPAD technology is forging performance enhancements is high-speed imaging, in which image sensors typically suffer from low SNR. The shorter integration times in these operations lead to lower photon collection and pixel blur, while the faster readout speeds increase noise in the collected image. SPAD image sensors fully eliminate this noise to offer Poisson-maximized SNR. 

A signal-to-noise ratio (SNR) comparison between a SPAD with 50% sensitivity and a typical photodiode with 80% sensitivity, both with equivalent readout noise of 10 e− (representative only for high-speed readout mode). Courtesy of Pi Imaging Technology.

A demonstration of SNR differences between a typical photodiode with 80% sensitivity and 10 e− signal (representative only for high-speed readout mode) equivalent readout noise (top) and a SPAD with 50% sensitivity (bottom), both at 10 impinging photons average. Courtesy of Pi Imaging Technology.

A SPAD array system implementation for image-scanning microscopy applications.

A fluorescence lifetime imaging microscopy (FLIM) image of mouse embryo tissue recorded with a single-photon-counting confocal microscope. Courtesy of PicoQuant.

About Pi Imaging:

Pi Imaging Technology is fundamentally changing the way we detect light. We do that by creating photon-counting arrays with the highest sensitivity and lowest noise.

We enable our partners to introduce innovative products. The end-users of these products perform cutting-edge science, develop better products and services in life science and quantum information.

Pi Imaging Technology bases its technology on 7 years of dedicated work at TU Delft and EPFL and 6 patent applications. The core of it is a single-photon avalanche diode (SPAD) designed in standard semiconductor technology. This enables our photon-counting arrays to have an unlimited number of pixels and adaptable architectures.

Full article here: https://www.photonics.com/Articles/Single-Photon_Avalanche_Diodes_Sharpen_Spatial/p5/vo211/i1358/a67902

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"Photon counting cameras for quantum imaging applications" Prof. Edoardo Charbon

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"Photon counting cameras for quantum imaging applications" 

Prof. Edoardo Charbon, Full Professor, Advanced Quantum Architecture Lab


Photon counting has entered the realm of image sensing with the creation of deep-submicron CMOS SPAD technology. The format of SPAD image sensors has expanded from 8×4 pixels in 2004 to the recent megapixel camera in 2019, and the applications have literally exploded in the last few years, with the introduction of proximity sensing and portable telemeters. SPAD image sensors are today in almost every smartphone and will soon be in every car. The introduction of Quanta Burst Photography has created a great opportunity for photon counting cameras, which are ideally suited for it, given its digital nature and speed; it is however computationally intensive. A solution to this problem is the use of 3D stacking, introduced for SPADs in 2015, where large silicon real estate is now available to host deep-learning processors, neural networks directly on chip, thus enabling complex processing in situ and reducing the overall power consumption. But the real opportunity is the emergence of a variety of quantum imaging modalities, including ghost imaging, quantum plenoptic vision, quantum LiDAR, to name a few. The talk will conclude with a technical and economic perspective on SPAD imagers and the vision for massively parallel solid-state photon counting in scientific and consumer applications.

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90-min Tutorial on Single Photon Detectors

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Krister Shalm of National Institute of Standards and Technologies presented a tutorial: Single-photon detectors at the 2013 QCrypt Conference in August. http://2013.qcrypt.net

This is from a while back but an excellent educational resource nevertheless!

The video is roughly 90-minutes long but has several gaps that can be skipped ahead. Or play it at >1x speed!

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Two new papers on 55 nm Bipolar-CMOS-DMOS SPADs

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The AQUA research group at EPFL together with Global Foundries have published two new articles on 55 nm Bipolar-CMOS-DMOS (BCD) SPAD technology in the upcoming issues of IEEE Journal of Selected Topics in Quantum Electronics.

Engineering Breakdown Probability Profile for PDP and DCR Optimization in a SPAD Fabricated in a Standard 55 nm BCD Process

CMOS single-photon avalanche diodes (SPADs) have broken into the mainstream by enabling the adoption of imaging, timing, and security technologies in a variety of applications within the consumer, medical and industrial domains. The continued scaling of technology nodes creates many benefits but also obstacles for SPAD-based systems. Maintaining and/or improving upon the high-sensitivity, low-noise, and timing performance of demonstrated SPADs in custom technologies or well-established CMOS image sensor processes remains a challenge. In this paper, we present SPADs based on DPW/BNW junctions in a standard Bipolar-CMOS-DMOS (BCD) technology with results comparable to the state-of-the-art in terms of sensitivity and noise in a deep sub-micron process. Technology CAD (TCAD) simulations demonstrate the improved PDP with the simple addition of a single existing implant, which allows for an engineered performance without modifications to the process. The result is an 8.8 μ\mu m diameter SPAD exhibiting ∼\sim 2.6 cps/ μ\mu m 2^2 DCR at 20 ∘^{\circ} C with 7 V excess bias. The improved structure obtains a PDP of 62% and ∼\sim 4.2% at 530 nm and 940 nm, respectively. Afterpulsing probability is ∼\sim 0.97% and the timing response is 52 ps FWHM when measured with integrated passive quench/active recharge circuitry at 3 V excess bias.



On Analog Silicon Photomultipliers in Standard 55-nm BCD Technology for LiDAR Applications

We present an analog silicon photomultiplier (SiPM) based on a standard 55 nm Bipolar-CMOS-DMOS (BCD) technology. The SiPM is composed of 16 x 16 single-photon avalanche diodes (SPADs) and measures 0.29 x 0.32 mm2. Each SPAD cell is passively quenched by a monolithically integrated 3.3 V thick oxide transistor. The measured gain is 3.4 x 105 at 5 V excess bias voltage. The single-photon timing resolution (SPTR) is 185 ps and the multiple-photon timing resolution (MPTR) is 120 ps at 3.3 V excess bias voltage. We integrate the SiPM into a co-axial light detection and ranging (LiDAR) system with a time-correlated single-photon counting (TCSPC) module in FPGA. The depth measurement up to 25 m achieves an accuracy of 2 cm and precision of 2 mm under the room ambient light condition. With co-axial scanning, the intensity and depth images of complex scenes with resolutions of 128 x 256 and 256 x 512 are demonstrated. The presented SiPM enables the development of cost-effective LiDAR system-on-chip (SoC) in the advanced technology. 

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[Updated] 2022 International SPAD Sensor Workshop Final Program Available

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About ISSW 2022

Devices | Architectures | Applications

The International SPAD Sensor Workshop focuses on the study, modeling, design, fabrication, and characterization of SPAD sensors. The workshop welcomes all researchers, practitioners, and educators interested in SPADs, SPAD imagers, and associated applications, not only in imaging but also in other fields.

The third edition of the workshop will gather experts in all areas of SPADs and SPAD related applications using Internet virtual conference technology.  The program is under development, expect three full days of with over 40 speakers from all over the world. This edition is sponsored by ams OSRAM.

Workshop website: https://issw2022.at/

Final program: https://issw2022.at/wp-content/uploads/2022/03/amsOSRAM_ISSW22_Program_3003.pdf

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Photonics Spectra article about Gigajot’s QIS Tech

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The March 2022 edition of Photonics Spectra magazine has an interesting article titled "Photon-Counting CMOS Sensors: Extend Frontiers in Scientific Imaging" by Dakota Robledo, Ph.D., senior image sensor scientist at Gigajot Technology.

While CMOS imagers have evolved significantly since the 1960s, photon-counting sensitivity has still required the use of specialized sensors that often come with detrimental drawbacks. This changed recently with the emergence of new quanta image sensor (QIS) technology, which pushes CMOS imaging capabilities to their fundamental limit while also delivering high-resolution, high-speed, and low-power linear photon counting at room temperature. First proposed in 2005 by Eric Fossum, who pioneered the CMOS imaging sensor, the QIS paradigm envisioned a large array of specialized pixels, called jots, that are able to accurately detect single photons at a very fast frame rate . The technology’s unique combination of high resolution, high sensitivity, and high frame rate enables imaging capabilities that were previously impossible to achieve. The concept was also expanded further to include multibit QIS, wherein the jots can reliably enumerate more than a single photon. As a result, quanta image sensors can be used in higher light scenarios, versus other single-photon detectors, without saturating the pixels. The multibit QIS concept has already resulted in new sensor architectures using photon number resolution, with sufficient photon capacity for high-dynamic-range imaging, and the ability to achieve competitive frame rates.

The article uses "bit-error-rate" metric for assessing image sensor quality.

The photon-counting error rate of a detector is often quantified by the bit error rate. The broadening of signals associated with various photo charge numbers causes the peaks and valleys in the overall distribution to become less distinct, and eventually to be indistinguishable. The bit error rate measures the fraction of false positive and false negative photon counts compared to the total photon count in each signal bin. Figure 4 shows the predicted bit error rate of a detector as a function of the read noise, which demonstrates the rapid rate reduction that occurs for very low-noise sensors. 


The article ends with a qualitative comparison between three popular single-photon image sensor technologies.

Interestingly, SPADs are listed as "No Photon Number Resolution" and "Low Manufacturability". It may be worth referring to previous blog posts for different perspectives on this issue. [1] [2] [3]

Full article available here: https://www.photonicsspectra-digital.com/photonicsspectra/march_2022/MobilePagedReplica.action?pm=1&folio=50#pg50

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CMOS SPAD SoC for Fluorescence Imaging

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Hot off the press! An article titled "A High Dynamic Range 128 x 120 3-D Stacked CMOS SPAD Image Sensor SoC for Fluorescence Microendoscopy" from the research group at The University of Edinburgh and STMicroelectronics is now available for early access in the IEEE Journal of Solid-State Circuits.

A miniaturized 1.4 mm x 1.4 mm, 128 x 120 single-photon avalanche diode (SPAD) image sensor with a five-wire interface is designed for time-resolved fluorescence microendoscopy. This is the first endoscopic chip-on-tip sensor capable of fluorescence lifetime imaging microscopy (FLIM). The sensor provides a novel, compact means to extend the photon counting dynamic range (DR) by partitioning the required bit depth between in-pixel counters and off-pixel noiseless frame summation. The sensor is implemented in STMicroelectronics 40-/90-nm 3-D-stacked backside-illuminated (BSI) CMOS process with 8-μm pixels and 45% fill factor. The sensor capabilities are demonstrated through FLIM examples, including ex vivo human lung tissue, obtained at video rate.

Full article is available here: https://ieeexplore.ieee.org/document/9723499 

Open access version: https://www.pure.ed.ac.uk/ws/portalfiles/portal/252858429/JSSC_acceptedFeb2022.pdf

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