We present a Montecarlo simulator developed in Matlab® for the analysis of a Single Photon Avalanche Diode (SPAD)-based Complementary Metal-Oxide Semiconductor (CMOS) flash Light Detection and Ranging (LIDAR) system. The simulation environment has been developed to accurately model the components of a flash LIDAR system, such as illumination source, optics, and the architecture of the designated SPAD-based CMOS image sensor. Together with the modeling of the background noise and target topology, all of the fundamental factors that are involved in a typical LIDAR acquisition system have been included in order to predict the achievable system performance and verified with an existing sensor.

https://www.mdpi.com/1424-8220/20/18/5203/pdf

Numerical Model of SPAD-Based Direct Time-of-Flight Flash LIDAR CMOS Image Sensors.

This article presents a 36-channel scanning light detection and ranging (LiDAR) sensor with an on-chip single-photon avalanche diode array. The sensor has an area-efficient 11-bit in situ histogramming time-to-digital converter with a $3000 \times 78\,\,\mu \text {m}^{2}$ per channel area based on a mixed-signal accumulator, though it is incorporated with histogramming and filtering capabilities. Furthermore, owing to its embedded interference (IF) filter, the sensor can perform reliable direct time-of-flight measurements even with IF from 32 different LiDAR sensors. The LiDAR system also has a beam scanner that comprises dual laser diodes for IF elimination and a hybrid mirror such that high-resolution images with a resolution of $2200 \times 36$ can be acquired with a wide field-of-view of $120^{\circ } \times 8^{\circ }$ .

Direct TOF Scanning LiDAR Sensor With Two-Step Multievent Histogramming TDC and Embedded Interference Filter

A video graphics array (VGA) (640 $\times $ 480) indirect time-of-flight (ToF) CMOS image sensor has been designed with 4-tap 7- $\mu \text{m}$ global-shutter pixel in 65-nm back-side illumination (BSI) process. With a 4-tap pixel structure, we achieved motion artifact-free depth map. Peak current during exposure time has been reduced by current spreading with constant delay chain in the photo-gate driver. Column fixed-pattern phase noise (FPPN) from the constant delay chain is self-compensated by the proposed time-interleaving technique with the two inversely directional clock chains in the photo-gate driver. Quantum efficiency (QE) and demodulation contrast (DC) have been optimized by using appropriate optical engineering techniques with an optimal silicon thickness. As a result, QE of 34% at 940-nm near-infrared and high DC of 86% at 100-MHz modulation frequency have been achieved. In addition, motion artifact and column FPPN are successfully removed in the depth map. The proposed ToF sensor shows depth noise less than 0.57% with 940-nm illuminator over the working distance up to 4 m, and consumes only 160 mW for VGA output at 60 frames/s.

A VGA Indirect Time-of-Flight CMOS Image Sensor With 4-Tap 7- $\mu$ m Global-Shutter Pixel and Fixed-Pattern Phase Noise Self-Compensation

This article presents a 320 $\times $ 240 indirect time-of-flight (iToF) CMOS image sensor (CIS) with on-chip motion artifact suppression and background light cancelling (BGLC). The proposed iToF CIS uses a backside-illuminated trident pinned photodiode (PPD) that assists charge transfer with a built-in lateral electric field for enhanced depth accuracy. To overcome the limitation of the conventional iToF CIS that exhibits motion artifact, we propose a pseudo-four-tap (P4-tap) demodulation method with alternate phase driving using a conventional two-tap pixel structure with a high fill factor of over 43%. In addition, by combining the advantages of both the P4-tap and conventional two-tap demodulation schemes, we propose hybrid depth imaging with reduced motion artifact for moving objects, while providing high-depth precision for the static background. For outdoor mobile applications of the iToF CIS, we integrated on-chip BGLC circuits to eliminate the BGL-induced depth error. A prototype chip is fabricated using a 90-nm backside illumination (BSI) CIS process. The BSI trident PPD enabled a low depth error of under 0.54% over the range of 0.75–4 m, with a modulation frequency of 100 MHz. Motion artifact was suppressed at 60 frames/s of hybrid depth imaging owing to the proposed P4-tap scheme. With the on-chip BGLC circuit, the experimental results demonstrate a 0.55% depth error at a 1-m distance, even under 120-klx BGL illumination.

Indirect Time-of-Flight CMOS Image Sensor With On-Chip Background Light Cancelling and Pseudo-Four-Tap/Two-Tap Hybrid Imaging for Motion Artifact Suppression

An 8-tap CMOS lock-in pixel image sensor that has seven carrier-capturing and a draining time window was developed for short-pulse time-of-flight (TOF) measurements. The proposed pixel for the short-pulse TOF measurements has seven consecutive time-gating windows, each of which has the width of 6 ns, which is advantageous for high-resolution range imaging, particularly for relatively longer distances (>5 m) and under high ambient light operations. In order to enhance the depth resolution, a technique for the depth-adaptive time-gating-number assignment (DATA) for the short-pulse TOF measurement is proposed. A prototype of the 8-tap CMOS lock-in pixel image sensor is implemented with a 1POLY 4METAL 0.11-μm CIS process. The maximum non-linearity error of 1.56%FS for the range of 1-6.4 m and the depth resolution of 6.4 mm was obtained at 6.2 m using the DATA technique.

An 8-Tap CMOS Lock-In Pixel Image Sensor for Short-Pulse Time-of-Flight Measurements.

The evolution of 3D depth-sensing technology enables various applications in mobile devices, from conventional photography enhancement (e.g., autofocus and bokeh effect) to new applications such as augmented reality and 3D scanning. Usually, 3D depth sensors operate with RGB color sensors for image fusion; the spatial resolution of depth sensors should be compatible to that of RGB sensors for better image fusion quality, which requires mega-pixel depth sensors. Among different depth-sensing techniques, only indirect time-of-flight (ToF) sensors can generate higher resolution depth maps with smaller system cost and power [1,2]. Recent indirect ToF sensors have achieved continuous improvements in depth performance [1–4]. However, one of the issues with indirect ToF sensors is peak current during exposure time [1,2]. To solve the issue, current spreading can be adopted by driving each column group in a different time point; the resulting column fixed-pattern phase noise (FPPN) is compensated by time-interleaving two opposite-directional delay chains [2]. However, this approach suffers from demodulation contrast (DC) degradation around the edge of the pixel array, by combining two different phase signals at the pixel level [2]. Another issue for indirect ToF sensors is multi-user interference [5]. When multiple ToF cameras with the same modulation frequency shine light onto the same object, the reflected light signals interfere each other, which may distort depth information. In this work, a mega-pixel indirect ToF sensor is reported with the solutions for the two issues: 1) peak current mitigation with minimal side-effect, and 2) multi-user interference cancellation.

A 4-tap 3.5 μm 1.2 Mpixel Indirect Time-of-Flight CMOS Image Sensor with Peak Current Mitigation and Multi-User Interference Cancellation

${A}640 \times 480$ indirect Time-of-Flight (ToF) CMOS image sensor has been designed with 4-tap $7-\mu \mathrm{m}$ global-shutter pixel in 65-nm back-side illumination (BSI) process. With novel 4tap pixel structure, we achieved motion artifact-free depth map. Column fixed-pattern phase noise (FPPN) is reduced by introducing alternative control of the clock delay propagation path in the photo-gate driver. As a result, motion artifact and column FPPN are not noticeable in the depth map. The proposed ToF sensor shows depth noise less than 0.62% with 940-nm illuminator over the working distance up to 400 cm, and consumes 197 mW for VGA, which is 0.64 pW/pixel.

A 640×480 Indirect Time-of-Flight CMOS Image Sensor with 4-tap 7-μm Global-Shutter Pixel and Fixed-Pattern Phase Noise Self-Compensation Scheme

A 1.2-Mpixel indirect time-of-flight (ToF) CMOS image sensor is presented to lower peak current and to cancel out multi-user interference. The proposed 4-tap 3.5- $\mu \text{m}$ demodulation pixel is optimally designed to improve quantum efficiency (QE) and demodulation contrast (DC). A new “multiple-interleaving” scheme is proposed to reduce the peak current and self-compensate fixed-pattern phase noise (FPPN) with the minimal side effect of DC imbalance. To cancel out interference from the other ToF sensors, a multi-user interference cancellation (MUIC) scheme based on pseudorandom modulation is adopted. From the measurement results, we achieve a high QE of 38% at 940 nm and a high DC of 96% and 80% at 100- and 200-MHz modulation frequencies, respectively. With the superior pixel performance, the depth noise of $\times $ 2 binning modes, respectively. The multiple-interleaving scheme suppresses the peak current to less than 0.9 A and removes the column FPPN with constant DC for the whole pixel array. The proposed MUIC proves that at least two interferers can be successfully rejected. The sensor chip is fabricated in a two-stack process with 65 nm for the top die and 65 nm for the bottom die.

Yeomyung Kim

Papers

A VGA Indirect Time-of-Flight CMOS Image Sensor With 4-Tap 7- $\mu$ m Global-Shutter Pixel and Fixed-Pattern Phase Noise Self-Compensation

A 4-tap 3.5 μm 1.2 Mpixel Indirect Time-of-Flight CMOS Image Sensor with Peak Current Mitigation and Multi-User Interference Cancellation

A 640×480 Indirect Time-of-Flight CMOS Image Sensor with 4-tap 7-μm Global-Shutter Pixel and Fixed-Pattern Phase Noise Self-Compensation Scheme

A 1.2-Mpixel Indirect Time-of-Flight Image Sensor With 4-Tap 3.5- μ m Pixels for Peak Current Mitigation and Multi-User Interference Cancellation