Dark Signal Non-uniformity Research Articles

Degradation analysis of the pinned photodiode CMOS image sensors induced by energetic proton radiation

The experiments of the 0.18-μm pinned photodiode (PPD) CMOS image sensors (CISs) irradiated by energetic proton radiation with energies of 20, 40, 60, and 100 MeV are presented. The degradations of the mean dark signal, dark signal non-uniformity (DSNU), and fixed pattern noise (FPN) induced by protons are presented. The experimental results show that the mean dark signal degradations of the CISs decrease with increasing proton energy at the same fluence. The degradations induced by proton radiation at different fluence are also presented. The mean dark signals increase with increasing integration time at the same proton fluence. The degradation mechanisms of the CISs induced by energetic protons are analyzed.

Analysis of displacement damage effects on the charge-coupled device induced by neutrons at Back-n in the China Spallation Neutron Source

Displacement damage effects on the charge-coupled device (CCD) induced by neutrons at the back-streaming white neutron source (Back-n) in the China Spallation Neutron Source (CSNS) are analyzed according to an online irradiation experiment. The hot pixels, random telegraph signal (RTS), mean dark signal, dark current and dark signal non-uniformity (DSNU) induced by Back-n are presented. The dark current is calculated according to the mean dark signal at various integration times. The single-particle displacement damage and transient response are also observed based on the online measurement data. The trends of hot pixels, mean dark signal, DSNU and RTS degradation are related to the integration time and irradiation fluence. The mean dark signal, dark current and DSNU2 are nearly linear with neutron irradiation fluence when nearly all the pixels do not reach saturation. In addition, the mechanisms of the displacement damage effects on the CCD are demonstrated by combining the experimental results and technology computer-aided design (TCAD) simulation. Radiation-induced traps in the space charge region of the CCD will act as generation/recombination centers of electron–hole pairs, leading to an increase in the dark signal.

Uniformity Correction of CMOS Image Sensor Modules for Machine Vision Cameras.

Flat-field correction (FFC) is commonly used in image signal processing (ISP) to improve the uniformity of image sensor pixels. Image sensor nonuniformity and lens system characteristics have been known to be temperature-dependent. Some machine vision applications, such as visual odometry and single-pixel airborne object tracking, are extremely sensitive to pixel-to-pixel sensitivity variations. Numerous cameras, especially in the fields of infrared imaging and staring cameras, use multiple calibration images to correct for nonuniformities. This paper characterizes the temperature and analog gain dependence of the dark signal nonuniformity (DSNU) and photoresponse nonuniformity (PRNU) of two contemporary global shutter CMOS image sensors for machine vision applications. An optimized hardware architecture is proposed to compensate for nonuniformities, with optional parametric lens shading correction (LSC). Three different performance configurations are outlined for different application areas, costs, and power requirements. For most commercial applications, the correction of LSC suffices. For both DSNU and PRNU, compensation with one or multiple calibration images, captured at different gain and temperature settings are considered. For more demanding applications, the effectiveness, external memory bandwidth, power consumption, implementation, and calibration complexity, as well as the camera manufacturability of different nonuniformity correction approaches were compared.

Smartphone Camera Identification from Low-Mid Frequency DCT Coefficients of Dark Images.

Camera sensor identification can have numerous forensics and authentication applications. In this work, we follow an identification methodology for smartphone camera sensors using properties of the Dark Signal Nonuniformity (DSNU) in the collected images. This requires taking dark pictures, which the users can easily do by keeping the phone against their palm, and has already been proposed by various works. From such pictures, we extract low and mid frequency AC coefficients from the DCT (Discrete Cosine Transform) and classify the data with the help of machine learning techniques. Traditional algorithms such as KNN (K-Nearest Neighbor) give reasonable results in the classification, but we obtain the best results with a wide neural network, which, despite its simplicity, surpassed even a more complex network architecture that we tried. Our analysis showed that the blue channel provided the best separation, which is in contrast to previous works that have recommended the green channel for its higher encoding power.

Total Ionizing Dose Effects of the Color Complementary Metal Oxide Semiconductor (CMOS) Image Sensor at Different Bias

The experiments of total ionizing dose radiation effects on color CMOS image sensors at different biases were presented. Two bias conditions operated during 60Co γ irradiation, biased and unbiased. In the data processing, the data of four channels were extracted according to the sequence of Bayer array respectively to study the difference between each channel. The full well capacity and dark signal non-uniformity versus the cumulative doses were investigated. After γ-ray irradiation, the full well capacity of the samples decreased along with dark signal non-uniformity increased. Meanwhile, the descending amplitude of full well capacity was different in each channel, but the variation of dark signal non-uniformity was almost consistent. Moreover, the degradations of full well capacity and dark signal non-uniformity were more seriously under-biased.

Mechanism of Ionization Damage in Large Eight-Transistor Complementary Metal–Oxide–Semiconductor Color Image Sensors

To study the degradation of remote-sensing cameras in terms of responsivity, dynamic range, and imaging performance due to exposure to space radiation, a γ-irradiation experiment was performed on a large eighttransistor (8T) complementary metal–oxide–semiconductor (CMOS) color image sensor. An in-depth investigation was thus conducted on the mechanisms by which ionization damage degrades radiation-sensitive parameters (i.e., dark current, dark signal non-uniformity, output signal, saturated output, and responsivity) of an image sensor, in its red, green, and blue channels. The findings of this study will serve as a theoretical and technical basis for the design of large radiation-hardened 8T-CMOS image sensors for space-based remote-sensing satellites.

A method for measuring digital camera noise by automatic segmentation of a striped target

Currently, cameras are widely used in scientific, industrial and amateur tasks. Thus, one needs to be able to quickly evaluate characteristics and capabilities of a particular camera. A method for measuring noise components of the camera photosensor is proposed. It allows one to estimate shot noise, dark temporal noise, photo response non-uniformity and dark signal non-uniformity. For noise measurement, just two images of the same scene need to be registered. The scene consists of several stripes (quasihomogeneous regions). Then the images are processed by automatic signal segmentation. The performance and accuracy of the proposed method are higher than or equal to other fast methods. The experimental results obtained are similar to those derived using a time-consuming standard method within a measurement error.

Effect of proton radiation on 8T CMOS image sensors for space applications

ABSTRACT As mainstream optical imaging electronic devices, image sensors are widely used in space missions under complex radiation environments. This study investigated the effects 60 and 10 MeV proton irradiations on dark current, dark signal non-uniformity (DSNU), and photon response non-uniformity (PRNU) for a commercial global shutter 8T complementary metal–oxide semiconductor (CMOS) image sensor. A fluence range from 9 × 109 p/cm2 to 7.26 × 1010 p/cm2 was considered. The total ionizing dose deposited reached 10 krad (Si), which significantly increased the dark current. DSNU and PRNU also followed a similar trend with an increase in the dose, which is based on the effects of the total dose and displacement damage. After irradiation, a TCAD simulation was conducted to identify the radiation-induced defects. The results showed that the trapped positive charges in the oxides, interface states in the Si/SiO2 interface, and bulk defects in a pinned photodiode depleted region significantly increased the dark current. The outcomes of this study form significant reference for advancements in radiation-hardened image sensor designs.

Analysis of Dark Signal Degradation Caused by 1 MeV Neutron Irradiation on Backside‐Illuminated CMOS Image Sensors

The purpose of this work is to investigate the influence of the epitaxial layer thickness of Backside-illuminated CMOS image sensors (BSI CISs) on dark signal behaviors. BSI CISs with the high quantum efficiency and sensitivity were irradiated by 1 MeV neutron up to the fluences of 10(9) cm(-2). The displacement damage induced variations of the mean dark signal, Dark signal nonuniformity (DSNU), dark signal spikes and Random telegraph signal (RTS) on the different epitaxial layer thicknesses are analyzed. The experimental results show that there is no obvious correlation between the degradations of dark signal parameters and the epitaxial layer thickness, suggesting that the electric-optical performance of BSI CISs can be improved by optimizing the epitaxial layer thickness.

Effect of Total Ionizing Dose Damage on 8-Transistor CMOS Star Sensor Performance

The effects of total ionizing dose (TID) radiation from 60Co gamma-rays on an 8-transistor global shutter exposure complementary metal-oxide semiconductor image sensor (CIS) within a star sensor is presented to analyze the sources of star sensor performance degradation and the decrease of attitude measurement accuracy. The dark current, dark signal non-uniformity, and photon response non-uniformity versus the TID are investigated. The signal-to-noise ratio, star diagonal distance accuracy, and star point centroid positioning accuracy of the star sensor versus the TID are also analyzed. By establishing the correlation between space radiation, CIS noise, and star sensor performance parameters, the transfer mechanism of CIS parameter degradation to star sensor parameter degradation is revealed.

Fixed Pattern Noise Reduction and Linearity Improvement in Time-Mode CMOS Image Sensors

In the paper, a digital clock stopping technique for gain and offset correction in time-mode analog-to-digital converters (ADCs) has been proposed. The technique is dedicated to imagers with massively parallel image acquisition working in the time mode where compensation of dark signal non-uniformity (DSNU) as well as photo-response non-uniformity (PRNU) is critical. Fixed pattern noise (FPN) reduction has been experimentally validated using 128-pixel CMOS imager. The reduction of the PRNU to about 0.5 LSB has been achieved. Linearity improvement technique has also been proposed, which allows for integral nonlinearity (INL) reduction to about 0.5 LSB. Measurements confirm the proposed approach.

Automotive 3.0 µm Pixel High Dynamic Range Sensor with LED Flicker Mitigation.

We present and discuss parameters of a high dynamic range (HDR) image sensor with LED flicker mitigation (LFM) operating in automotive temperature range. The total SNR (SNR including dark fixed pattern noise), of the sensor is degraded by floating diffusion (FD) dark current (DC) and dark signal non-uniformity (DSNU). We present results of FD DC and DSNU reduction, to provide required SNR versus signal level at temperatures up to 120 °C. Additionally we discuss temperature dependencies of quantum efficiency (QE), sensitivity, color effects, and other pixel parameters for backside illuminated image sensors. Comparing +120 °C junction vs. room temperature, in visual range we measured a few relative percent increase, while in 940 nm band range we measured 1.46x increase in sensitivity. Measured change of sensitivity for visual bands—such as blue, green, and red colors—reflected some impact to captured image color accuracy that created slight image color tint at high temperature. The tint is, however, hard to detect visually and may be removed by auto white balancing and temperature adjusted color correction matrixes.

Suppression of Spatial and Temporal Noise in a CMOS Image Sensor

This paper presents methodologies for suppressing the spatial and the temporal noise in a CMOS image sensor (CIS). First of all, it demonstrates by using a longer-length column bias transistor, both the fixed pattern noise (FPN) and temporal noise can be suppressed. Meantime, it employs column-level oversampling delta-sigma ADCs to suppress temporal noise as well as to facilitate the realization of the thermal compensation of dark signal non-uniformity (DSNU). In addition, the image pixels are re-configured as temperature sensors with inaccuracies within ±0.65 °C, between −20 and 80 °C. If the dark current and its non-uniformities are caused by thermal gradients, the obtained in-pixel thermal information can be employed to compensate for the measured dark current by 95 % and DSNU, up to 13 %. All the column-level 13 bit 2nd-order incremental delta-sigma ADCs are measured with SNR around 65 dB and INL around 1.5 LSB, when tested with a −8 dB input signal and sampling at 2 MHz with an oversampling ratio (OSR) of 128, when the full scale voltage is 2 Vp-p. The 4T Pinned Photodiode (PPD) CIS is measured to have a temporal noise of $34~\mu \text{V}$ rms (with an OSR of 128, or, an input referred temporal noise of 0.5 e− rms, with a conversion gain, CG, of $73~\mu \text{V}/ \text{e}^{-}$ ), a column gain FPN of 0.06 %, a dynamic range (DR) of 92 dB (with OSR = 512), as well as a linearity of 1 %. It has a measured DSNU of 3.2 %, after the thermal compensation using the in-pixel temperature sensors, a dark current of 290 pA/cm2 and 15 pA/cm2, measured at 60 °C, before and after the thermal compensation, respectively.

A Power-Efficient Digital Technique for Gain and Offset Correction in Slope ADCs

In this brief, a power-efficient digital technique for gain and offset correction in slope analog-to-digital converters (ADCs) has been proposed. The technique is especially useful for imaging arrays with massively parallel image acquisition where simultaneous compensation of dark signal non-uniformity (DSNU) as well as photo-response non-uniformity (PRNU) is critical. The presented approach is based on stopping the ADC clock by a specially prepared clock-enable pulse sequence. This brief describes the properties of ADCs utilizing this clock stopping technique, including power dissipation, integral, and differential nonlinearity. The experimental validation has been performed for the ASIC implementation of the 128-pixel imager containing photo-sensors integrated with ADCs. Finally, a modification is proposed that increases the accuracy of the gain correction. Measurements confirm functionality of the proposed approach. Reduction of the PRNU (to ~0.4 LSB) has been achieved as well.

A CMOS-Imager-Pixel-Based Temperature Sensor for Dark Current Compensation

This brief proposes employing each of the classical 4 transistor (4T) pinned photodiode (PPD) CMOS image sensor (CIS) pixels, for both imaging and temperature measurement, intended for compensating the CISs’ dark current, and dark signal non-uniformity (DSNU). The proposed temperature sensors rely on the thermal behavior of MOSFETs working in subthreshold region, when biased with ratiometric currents sequentially. Without incurring any additional hardware or penalty to the CIS, they are measured to have thermal curvature errors less than ±0.3 °C and $3~\sigma $ process variations within ±1.3 °C, from 108 sensors on 4 chips, over a temperature range from −20 °C to 80 °C. Each of them consumes 576 nJ/conversion at a conversion rate of 62 samples/s, when quantized by 1st-order 14 bit delta–sigma ADCs and fabricated using $0.18~\mu \text{m}$ CIS technology. Experimental results show that they facilitate digital compensation for average dark current and DSNU by 78% and 20%, respectively.

Compensation for Process and Temperature Dependency in a CMOS Image Sensor.

This paper analyzes and compensates for process and temperature dependency among a (Complementary Metal Oxide Semiconductor) CMOS image sensor (CIS) array. Both the analysis and compensation are supported with experimental results on the CIS’s dark current, dark signal non-uniformity (DSNU), and conversion gain (CG). To model and to compensate for process variations, process sensors based on pixel source follower (SF)’s transconductance gm,SF have been proposed to model and to be compared against the measurement results of SF gain ASF. In addition, ASF’s thermal dependency has been analyzed in detail. To provide thermal information required for temperature compensation, six scattered bipolar junction transistor (BJT)-based temperature sensors replace six image pixels inside the array. They are measured to have an untrimmed inaccuracy within ±0.5 °C. Dark signal and CG’s thermal dependencies are compensated using the on-chip temperature sensors by at least 79% and 87%, respectively.

Single particle transient response and displacement damage in CMOS image sensors induced by high energy neutrons at Back-n in CSNS facility

Neutron radiation experiments of the complementary metal-oxide semiconductor (CMOS) image sensors (CISs) at back-streaming white neutrons (Back-n) in China Spallation Neutron Source (CSNS) facility are presented. The displacement damages induced by neutron radiation are characterized by the analysis of the increase of dark signal and dark signal non-uniformity, and the occurrences of dark signal spike and random telegraph signal (RTS). The two-level and multi-level RTS behaviors induced by neutron displacement radiation are presented. The mechanisms of the dark signal spikes and RTS induced by neutron displacement radiation are analyzed respectively. The single particle transient response characterizations are described by analyzing the online dark images during neutron radiation. The mechanisms of the dark signal spikes and the white lines induced by the single transient neutron events are also demonstrated by analyzing the interactions between the higher energy neutrons and bulk silicon lattices of CISs.

Integration of 555 temperature sensors into a 64 × 192 CMOS image sensor

In this work, a novel approach for measuring relative temperature variations across the active area of a CMOS image sensor itself is presented. 555 Image pixels have been replaced by temperature sensors pixels (Tixels) in the same pixel array layer. Both sensors, pixels and Tixels, utilize the same readout structure to obtain the data. This approach of measuring temperature variations in the pixel array can be used to compensate for dark signal non-uniformity. Measurements of dark current and temperature have been performed in a temperature range of −40 and 90 °C. Results show that pixels and Tixels are working by using the same readout system based on source followers and column amplifiers. The average dark current of the image sensor increases with temperature in the temperature range of −40 and 60 °C, and at the same time, Tixels show high linear response, having a temperature error less than 0.6 °C after a 1st order best curve fitting.

Radiation damage evaluation of the CCD detector induced by high energy protons

In order to help the designers to improve the CCD detector design, reliability and applicability for applications in particle detection or space applications, the radiation damage evaluation of the CCD detector induced by high energy protons are presented. The radiation experiments were carried out at the cyclotron accelerator with energies of 100, 60, and 30 MeV. The degradations of the CCD parameters such as the overall system gain, linear saturation output, dark current, dark signal non-uniformity (DSNU), random noise, dynamic range (DR), and quantum efficiency (QE) induced by proton radiation are analyzed. The degradations versus the proton fluences are presented. The degradations induced by different proton energies are compared. The degradation mechanisms of the CCD parameters induced by proton radiation damage are demonstrated in detail.

Theoretical and experimental study of the dark signal in CMOS image sensors affected by neutron radiation from a nuclear reactor

The CMOS image sensors (CISs) are irradiated with neutron from a nuclear reactor. The dark signal in CISs affected by neutron radiation is studied theoretically and experimentally. The Primary knock-on atoms (PKA) energy spectra for 1 MeV incident neutrons are simulated by Geant4. And the theoretical models for the mean dark signal, dark signal non-uniformity (DSNU) and dark signal distribution versus neutron fluence are established. The results are found to be in good agreement with the experimental outputs. Finally, the dark signal in the CISs under the different neutron fluence conditions is estimated. This study provides the theoretical and experimental evidence for the displacement damage effects on the dark signal CISs.