Avalanche Photodiodes in Submicron CMOS Technologies for High-Sensitivity Imaging

Single photon avalanche diodes (SPADs) are a key underpinning technology to many existing and emerging applications, including LIDAR for 3D imaging as well as quantum imaging, quantum encryption and quantum information applications. There is a growing demand for low-cost LIDAR systems for autonomous vehicles, particularly in the short-wave infrared (SWIR) spectral range, which enables long-range measurements whilst complying with eye-safety regulations and offers enhanced transmission through atmospheric obscurants like smoke and haze compared to systems operating in the near-infrared. Furthermore, for quantum-key distribution, single photons must be measured at telecoms wavelengths for compatibility with optical fibre networks. Ge-on-Si SPADs offer significant potential for low-cost SWIR single photon detection, with the ability to meet the price points required for these large emerging markets thanks to Si foundry compatibility. This contrasts with state-of-the-art SWIR SPADs based on InGaAs/InP, which are not only expensive but suffer from the effects of afterpulsing.Here, we present an overview of our work on the design, fabrication and characterisation of pseudo-planar Ge-on-Si detectors, with both operation in the Geiger mode (i.e. SPADs), as well as linear mode for avalanche photodiodes (APDs). The pseudo-planar SPAD design resulted in a step-change in performance compared to mesa-based SPADs, leading to high single-photon detection efficiencies (SPDEs) of 38 % at 1310 nm wavelength, and ultimately low noise equivalent powers of 7.7x10-17 W/Hz0.5 in 26 µm diameter pixels at 125 K, with single photon detection demonstrated up to 165 K. This was achieved by local ion-implantation of a charge-sheet layer, used to mediate the electric field between the Si avalanche layer and the Ge absorber, which in conjunction with a local p+Ge etched contact layer enabled a reduced E-field at etched sidewalls. These devices demonstrated over 100 X improvement in NEP compared to the most comparable Ge-on-Si mesa devices, and showed reduced afterpulsing compared to commercial InGaAs/InP devices when run in nominally identical operating conditions.In order to further optimise the technology, and gain insight into the device dynamics, we have simulated SPADs using TCAD process and device simulators, and developed custom-code to solve triggering probabilities using McIntyre’s model to understand the sensitivities of the detectors to the device design. Simulations are compared to experimental DCR measurements to probe sensitivities to surface passivation and geometry scaling and reveal that surfaces do not appear to be the limiting factor on performance, therefore validating the pseudo-planar architecture. Furthermore, with these simulation techniques, we have investigated potential enhancements achievable by the inclusion of an etched photonic crystal nano-hole array, which is known to enhance absorption and will therefore enhance SPDE. Here, we demonstrate the design trade-offs between enhanced SPDE performance, and the degradation of DCR that can be induced by etching surfaces into the device active area.Finally, we present recent results on surface normal Ge-on-Si APDs using the pseudo-planar architecture in devices with 2 µm thick Ge absorbers. Here, the benefits of the device architecture are demonstrated to be applicable to high performance in the linear mode. Devices are measured at room-temperature, and demonstrate ~0.40 A/W responsivity at unity gain at 1550 nm wavelength, with a maximum avalanche gain of ~100, and excess noise of 3.1 at a gain of 20; to our knowledge a record for any Ge-on-Si APD.

We design a single photon avalanche diode (SPAD) in a standard 180-nm CMOS technology. The SPAD consists of a P+/n-well junction with octagon shape, a p-well guard-ring and a ploy guard-ring. The two rings prevent SPAD from premature edge breakdown events and STI interface trap influence, respectively. The measurement results show that a typical dark count rate of SPAD is 8 kHz at room temperature and its PDP is 8% at wavelength of 640nm. The distance measurement is carried out by a SPAD direct Time of Flight method. A centimeter-level precision is achieved based on time-correlated single-photon counting technology.

A time-gated 4×128 Single Photon Avalanche Diode (SPAD) line array with a 512-channel Time-to-digital converter (TDC) has been designed and fabricated in a 0.35 μm high voltage CMOS technology for pulsed Raman spectroscopy. In Raman spectroscopy the SPAD array can be designed as a line array with a high fill factor because electronics can be designed on either side of the SPAD array. In this paper cross talk measurements of the array have been carried out to investigate the probability of the cross talk between adjacent SPADs in an array with high density. Measurements showed a cross talk probability of 0.11% with a pitch of adjacent SPADs of 32 μm. In addition, the largest probability of the cross talk was measured to occur with a short delay.

Scientific experiments often demand the detection of very weak light signals at high-speed or to precisely measure the time of arrival of single photons. Arrays of Single-Photon Avalanche Diodes (SPAD) are ideal candidates when high sensitivity is required together with high frame-rate or precise photon-timing resolution. We designed a linear 32x1 SPAD array using a high-voltage CMOS technology able to provide both good SPAD performance and fast electronics. During frame acquisition all pixels work in parallel, each of them being equipped with anything necessary for photon counting. The array architecture is capable of fully parallel operation of all pixels allowing free running acquisition at high frame-rate. With a low-speed 10 MHz clock frequency, one pixel is read out in 100 ns while the whole array is readout in 320 ns, corresponding to a frame-rate of 312.5 kframe/s. The frame-rate can top to 4 Mframe/s with a clock of 128 MHz. The photon timing modality employs the photon time-of-arrival information provided by each of the 32 outputs. All 32 "timing" outputs feed external Time-Correlated Photon Counting boards. The Full-Width at Half- Maximum using very short laser pulses is 55 ps with few kcps counting rate.

The integrated circuit design of Single Photon Avalanche Diode (SPAD) with quenching circuit in CMOS is highly desirable photon detection at high rates, but low counting rates constrain image acquisition rates and dynamic range. The characterization should primarily define the dead time of quenching SPAD circuit in order to estimate the SPAD performance prior fabrication. This paper reports the development and characterization of the mathematical model SPAD on passively quenched SPAD circuit with ballast resistor. An improved model in defining the dead time (tD) response for SPAD is used to characterize the performance of 8×1 passively quenched SPAD array. The time response for both quenching and recharging time are developed for 180 nm depletion layer which means low voltage technology. Hence that, the 8×1 passively SPAD array circuit is designed by using Silterra 180nm CMOS technology for uncorrelated time measurements with on-chip 4-bit counter to improve the counting rate. The passive quenching circuit design on-chip would enable the capability to perform at higher speed, which is more than 100 kHz. In addition, are presented in this paper.

In many fields and in particular in astrophysical observations, a chronic problem is the photon-starving condition, which becomes severe when images are to be obtained in short acquisition times (from micro to milliseconds), as happens in hot areas of astrophysics: optical counterparts of high-energy gamma-ray bursts, study and interpretation of Supernovae bursts. CCDs are inherently unable to provide accurate measurements of such fast low-intensity transients at high frame rates. To respond to single photons, suitable detectors must provide output signals that are sufficiently high to be individually processed by electronic circuits. Therefore, only detectors with an internal mechanism that provides a high multiplication of charge carriers are suitable, namely vacuum tube photomultipliers (PMTs), solid-state avalanche photodiodes (APDs) and electron-multiplying CCDs (EMCCDs). In PMTs, the photocathodes available for the visible spectral range provide fairly good quantum efficiency and low noise, whereas cathodes for the red and near-infrared range have lower quantum efficiency and must be cooled to reduce the dark-count rate. PMTs are bulky, and so not suitable for assembly in large arrays, fragile, sensitive to electromagnetic disturbances and mechanical vibrations, require high supply voltages (1–2 kV) and are costly devices, particularly the high-performance models. EM-CCDs exploit an internal multiplication process to achieve sub-electron readout noise, thus being able to detect single photons. Their quantum efficiency is very high, and they are inherently suited to imaging applications. However, due to their readout technique, they cannot provide frame rates higher than a few kilo-frames per second, and cannot be used in extreme timeresolved measurements. Single photons can be detected efficiently by avalanche diodes operating in Geiger mode, known as Single-Photon Avalanche Diodes (SPADs). Avalanche photodiodes have the typical advantages of solid state devices (small size, low bias voltage, low power consumption, ruggedness and reliability, suited to building integrated systems). In the last few years, a new kind of planar semiconductor device has slowly but steadily come out, namely the silicon photomultiplier (SiPM), with promising features that, in some respect, could even replace traditional photomultiplier tubes (Kovaltchouk et al, 2005). Based on a Geiger mode avalanche photodiode elementary cell, it consists of an array of n

We have designed and fabricated high-performance single-photon avalanche diodes (SPADs) by using 0.18-µm high-voltage CMOS technology. Without any technology customization, the SPADs have low dark-count rate, high photon-detection probability, low afterpulsing probability, and acceptable timing jitter and breakdown voltage. Our design provides a low-cost and high-performance SPAD for various applications.

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.

The operation of InP-based single photon avalanche diodes (SPADs) in Geiger mode provides great utility for the detection of single photons at near-infrared wavelengths between 1.0 and 1.6 μm. However, SPADs have performance limitations with respect to photon counting rate and the absence of photon number resolution that, at the most fundamental level, can be traced back to the positive feedback inherent in the impact ionization-driven avalanche process. In this paper, we describe the inclusion of negative feedback with best-in-class InP-based single photon avalanche diode (SPAD) structures to form negative feedback avalanche diodes (NFADs) in which many of the present limitations of SPAD operation can be overcome. The use of thin film resistors as monolithic passive negative feedback elements ensures rapid self-quenching with very low parasitic effects. We demonstrate a qualitative difference in the performance of NFADs in the two regimes of small and large negative feedback. With small feedback, we have studied the behavior of the persistent current prior to quenching, for which we have found oscillatory behavior as well as an exponentially distributed duration. For large feedback, we find rapid quenching, accompanied by evidence for a partial discharge of the detector capacitance, leading to charge flows as low as ~3 ×10⁵ carriers associated with each avalanche event.

Single Photon Avalanche Diodes (SPADs) are avalanche photodiodes specifically designed for reverse bias operation above the breakdown voltage (Geiger-mode operation), and used for detecting single optical photons. Studies were performed to relate the attainable timing resolution to the device geometry and operating conditions. A new silicon device structure was designed in order to obtain improved timing performance with respect to previous SPADs. Extensive tests were carried out in order to ascertain the timing resolution of the device in time-correlated photon counting. The SPAD timing resolution, in terms of its full-width at half­maximum (FWHM) contribution to the overall instrumental response width, is 20 ps with the detector cooled to -65 C, and 28 ps at room temperature. This is the highest resolution so far reported for solid-state single-photon detectors. Among vacuum tubes, comparable results are obtained only with special microchannel-plate photomultipliers (MCP-PMT). With the excellent timing resolution of the SPAD and the well-known advantages of Time Correlated Photon Counting systems (high sensitivity, linearity etc.), various applications are foreseen in areas so far dominated by streak cameras.

This paper investigates the Random Telegraph Signal (RTS) in Single Photon Avalanche Diodes (SPADs). The test-chip features SPADs with different architectures implemented in 150 nm CMOS technology. The test-chip has been irradiated with 21 MeV proton beam. RTS occurrence probability has been investigated in two different architectures. RTS measurements allowed to investigate the defect responsible for RTS phenomenon.

High-performance LiDAR is a key module to enable autonomous driving. In this talk, I shall address the cost issue hinder LiDAR from current vehicles and how to solve the problem by replacing current optical sensors in LiDAR with single-photon detectors, or called single-photon avalanche diodes (SPADs) in CMOS technology. SPADs attract increasing attentions due to the easy integration with CMOS circuits and the great potential for various applications, such as low-light/3-D imaging, fluorescence lifetime imaging microscopy, time-gated Raman spectroscopy, and LiDAR for vehicles, etc. In particular, in advanced driver assistance system (ADAS) or driverless cars, LiDAR using SPADs promises a highly reliable way for detecting distant pedestrians which may not be detectable by other means including camera and Radar. By making LiDAR modules affordable without sacrificing its performance, the traffic accidents could be significantly reduced and fully autonomous driving could be enabled in next years. I shall first discuss a few fundamental limits in LiDAR and then introduce the operation principle of SPADs and how this ultimate optical sensors could make cost of LiDAR module effective and available to each car in the near future.

Single photon avalanche diodes (SPAD) based on avalanche effect have been widely used in the detection of extremely weak light signals. Conventional SPAD devices manufactured with silicon-based CMOS technology have a high dark count rate (DCR), making it difficult to accurately detect single photon signals. This paper proposes a low dark count rate ring-gate SPAD (RG-SPAD) to solve above problems, and compares RG-SPAD with conventional SPAD (C-SPAD) and conventional dummy-gate SPAD (CDG-SPAD). Slivaco-Technology Computer Aided Design (TCAD) performs two-dimensional simulation of the device to verify the basic principles of SPAD. Three types of SPAD device are manufactured by standard BCD process. The passive quenching circuit is built to obtain the DCR of SPAD devices. The avalanche breakdown voltages of C-SPAD, CDG-SPAD and RG-SPAD are 11.55 V, 11.85 V and 11.4 V respectively. In order to test the dark count rate of SPAD devices, an over-bias voltage of 1 V was applied to three types of SPAD device under the same size condition (temperature: 22 °C). The DCR of RG-SPAD (186 Hz) with ring-gate structure is significantly lower than CDG-SPAD (378 Hz) with conventional dummy-gate structure and C-SPAD (498 Hz) with conventional structure. When the room temperature dropped to 18 °C, RG-SPAD still maintained the lowest DCR (148 Hz) among three types of device structure.

Silicon photomultipliers (SiPMs) are arrays of many single-photon avalanche diodes (SPADs), all connected in parallel. Each SPAD is sensitive to single photons and the SiPM gives an output proportional to the number of detected photons. These sensors are becoming more and more popular in different applications, from high-energy physics to spectroscopy, and they have been significantly improved over last years, decreasing the noise, increasing the cell fill-factor (FF) and thus achieving very high photon-detection efficiency (PDE). In FBK (Trento, Italy), we developed new SiPM technologies with high-density (HD) and, more recently, ultra-high-density (UHD) of cells (i.e. density of SPADs). These technologies employ deep-trenches between cells, for electrical and optical isolation. As an extreme case the smallest-cell, SiPM, i.e. with 5μm cell pitch, has about 40000 SPADs per squared millimeter. Such small SPAD dimensions gives a significantly high dynamic range to the SiPM. These small-cells SiPM have a lower correlated noise (including lower afterpulsing probability) and a faster recharge time (in the order of few nanoseconds), and they also preserve a very good detection efficiency (despite the small SPAD dimension).

We report on the design and characterization of a multipurpose 64 × 32 CMOS single-photon avalanche diode (SPAD) array. The chip is fabricated in a high-voltage 0.35-μm CMOS technology and consists of 2048 pixels, each combining a very low noise (100 cps at 5-V excess bias) 30-μm SPAD, a prompt avalanche sensing circuit, and digital processing electronics. The array not only delivers two-dimensional intensity information through photon counting in either free-running (down to 10-μs integration time) or time-gated mode, but can also perform smart light demodulation with in-pixel background suppression. The latter feature enables phase-resolved imaging for extracting either three-dimensional depth-resolved images or decay lifetime maps, by measuring the phase shift between a modulated excitation light and the reflected photons. Pixel-level memories enable fully parallel processing and global-shutter readout, preventing motion artifacts (e.g., skew, wobble, motion blur) and partial exposure effects. The array is able to acquire very fast optical events at high frame-rate (up to 100 000 fps) and at single-photon level. Low-noise SPADs ensure high dynamic range (up to 110 dB at 100 fps) with peak photon detection efficiency of almost 50% at 410 nm. The SPAD imager provides different operating modes, thus, enabling both time-domain applications, like fluorescence lifetime imaging (FLIM) and fluorescence correlation spectroscopy, as well as frequency-domain FLIM and lock-in 3-D ranging for automotive vision and lidar.