Pinned Photodiode CMOS Image Sensors 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.

Dark current in pinned photodiode CMOS image sensors: a pre-fabrication physics-based model

The present technique to find a dark current source in an image sensor, i.e., Dark Current Spectroscopy, is not sufficient to conclusively differentiate between the contribution of interface and bulk defect generation in a pixel. Also, the dark current data is available only post-fabrication. In this work, the Dark current versus Transfer Gate-off voltage curve is used to extract the defect density-cross-section product, activation energy and maximum carrier lifetime for two important device generation sites, i.e., interface under the Transfer Gate and Pinned Photodiode depletion region. A comprehensive analytical dark current model is formulated and validated using TCAD simulation and experimental data reported in the literature. The temperature variation is incorporated using the Arrhenius relation corresponding to the specific activation energy level. The proposed model along with the Arrhenius relation provides a sufficient pre-fabrication dark current estimate.

A simulation result of trapped charge in PPD CIS induced by total ionizing dose effect

This paper describes a method for simulating the total ionizing dose (TID) radiation effect of a PPD CMOS image sensor (CIS) using TCAD. This paper takes the CIS pixel unit as the research object, modeling the device structure and combining it with a gamma radiation model to simulate the distribution of radiation-induced charge generation rate, electron concentration, and potential at each interface in the entire region before and after irradiation with TID equal to 5, 20, 50, 80, 100, 200, and 300 krad(Si). At the same time, the curves of the above parameters changed with the increase of TID radiation have been simulated. Furthermore, the influence of different TIDs on the parameters mentioned above at each interface has been investigated based on the two main damage mechanisms of the TID radiation effect: radiation-induced oxide trapped charge and radiation-induced interface trapped charge. It was also discovered that as the TID increases, the depletion region of the pinned photodiode (PPD) becomes broader, causing the dark current to increase.

Influence of Pixel Design on Charge Transfer in Pinned Photodiode CMOS Image Sensors

The influence of pixel design on charge transfer, mainly on potential barrier and potential pocket, is investigated in this paper. By simplifying the interface region between the pinned photodiode (PPD) and the transfer gate (TG), an intuitive and simple method for analyzing the potential barrier and pocket is presented. Through TCAD simulations, the validity of the analysis method is verified, and the influence of some key dimension and process parameters in PPD pixel design on the potential barrier and pocket is clarified. The whole study help improve the understanding of the charge transfer limitation mechanism, and can achieve high charge transfer efficiency and low image lag noise in the PPD pixel design of CMOS image sensors (CISs).

Characterization and analysis of electrical crosstalk in a linear array of CMOS image sensors.

In this paper, the influences of the depth and width of the oxide trench isolation between pixels, pixel epitaxial layer thickness for different impurity doping concentrations, and light exposure time on electrical crosstalk are characterized in an array of pinned photodiode CMOS image sensor pixels. The simulation results show that with a proper and simultaneous selection of epitaxial layer doping concentration and epitaxial layer thickness, the electrical crosstalk at long wavelengths can be reduced above 66%. The use of oxide trench isolation depth less than pixel p-well depth leads to an increase in electrical crosstalk of more than 12%. The effect of increasing light exposure time on increasing electrical crosstalk can be minimized by selecting proper epitaxial layer thicknesses.

Analytical Modeling of Potential Barrier for Charge Transfer in Pinned Photodiode CMOS Image Sensors

A potential barrier is one of the most common lag sources for charge transfer in pinned photodiode (PPD) CMOS image sensors (CISs). In this article, an analytical model of the potential barrier is proposed for the PPD combined with the transfer gate (TG). Through detailed electrostatic analysis of the PPD and TG, the potential barrier is analytically expressed as a function of the doping concentrations, TG voltage, spatial dimensions, and other physical parameters. The proposed model is validated by technology computer-aided design (TCAD) simulations, and the results show that the model data are in good agreement with the TCAD simulations in terms of the magnitude and location of the potential barrier. The model can be used for the design, simulation, and optimization of PPD-based pixels in CISs to improve the charge transfer efficiency (CTE) and reduce the image lag noise.

Probing Dark Current Random Telegraph Signal in a Small Pitch Vertically Pinned Photodiode CMOS Image Sensor After Proton Irradiation

The dark current degradation and dark current random telegraph signal (DC-RTS) after proton irradiation are studied in new scale silicon microvolumes by using a commercial CMOS image sensor. Results show that previously reported empirical models describing the displacement damage-induced degradations are still valid despite the 10–100 times smaller depletion volume used. In addition, no evidence of significant total ionizing dose effects is observed. Finally, the reduction of the fraction of RTS pixels detected and the fraction of multilevel RTS pixels is directly linked to the reduction in pixel volume.

Single-Event Effects in Pinned Photodiode CMOS Image Sensors: SET and SEL

Single-event transient (SET) and single-event latchup (SEL) of a 4T pinned photodiode (PPD) complementary metal&#x2013;oxide&#x2013;semiconductor (CMOS) image sensor (CIS) fabricated with a 0.18-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS process were investigated using heavy ions and a picosecond pulsed laser. The SET bright spot characteristics were further studied, and the transient bright spots formed by laser and heavy ions were analyzed. The sensor exhibited SELs and micro-SEL. To determine the precise sensitive location where SEL occurred in the CIS, a pulsed laser with a 2-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> beam spot was used to scan the entire surface of the device. The results revealed that SEL occurred in the peripheral circuits of the CIS, including the analog signal processing circuit, row-addressing circuit, and analog-to-digital conversion (ADC). The cross section of the SELs for the sample were measured using a linear energy transfer (LET) ranging from 8.62 to 81.35 MeV<inline-formula> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>cm² mg^&#x2212;1. The SEL rates in space for the used CIS were predicted using the CREME96 tool.

Proton and Gamma Radiation Effects on a Fully Depleted Pinned Photodiode CMOS Image Sensor

The radiation hardness of a fully depleted pinned photodiode (PPD) CMOS image sensor (CIS) has been evaluated with gamma and proton irradiations. The sensors employ an additional n-type implant in pixel which allows full depletion to be achieved by reverse biasing the substrate and have been irradiated alongside reference devices in which the implant has been omitted. The results show no visible degradation of the sensor’s radiation hardness by the additional implant. The main degraded parameters caused by ionizing radiation, from both gamma and proton irradiation, are the image lag and the surface dark current. High dark current in the sense node (SN) is identified and is found to be caused by the shallow trench isolation (STI) around the SN. The dark current increase after proton irradiation has been shown to be dominated by displacement damage-induced bulk defects and the value is consistent with those from other PPD CISs in the literature. The charge collection efficiency is not significantly affected by the bulk traps even after 10-MeV-equivalent proton fluence of up to 1012 p/cm2. However, the ionizing damage from the highest proton fluence reached causes a considerable drop in the charge-to-voltage conversion factor.

Leakage Current Non-Uniformity and Random Telegraph Signals in CMOS Image Sensor Floating Diffusions Used for In-Pixel Charge Storage.

The leakage current non-uniformity, as well as the leakage current random and discrete fluctuations sources, are investigated in pinned photodiode CMOS image sensor floating diffusions. Different bias configurations are studied to evaluate the electric field impacts on the FD leakage current. This study points out that high magnitude electric field regions could explain the high floating diffusion leakage current non-uniformity and its fluctuation with time called random telegraph signal. Experimental results are completed with TCAD simulations allowing us to further understand the role of the electric field in the FD leakage current and to locate a high magnitude electric field region in the overlap region between the floating diffusion implantation and the transfer gate spacer.

Comparison of X-Ray and Electron Radiation Effects on Dark Current Non-Uniformity and Fluctuations in CMOS Image Sensors

This article investigates the dark current as well as the dark current random telegraph signal (RTS) after 1-MeV electron, 3-MeV electron, and 10-keV X-ray irradiations in a pinned photodiode CMOS image sensor (CIS). A large range of deposited ionizing dose from 10 to 525 krad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is considered. The displacement damage dose deposited through electron irradiation ranges from 60 to 1200 TeV · g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Results on dark current distributions highlight the predominance of the ionizing damage in opposition to the displacement damage induced by the electron irradiations. Moreover, the dark current distributions also suggest that if the ionizing dose is high enough [i.e., beyond 50 krad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )], the trapped positive charges in the silicon oxides create high magnitude electric field regions leading to an electric field enhancement (EFE) of the dark current which is neither present at lower doses nor in pristine image sensors. This EFE mechanism also seems to have a strong influence on the RTS leading to a clear discrepancy from the existing dark current nonuniformity model developed for amplitude distributions in CISs as well as from what is reported in the literature in the more studied ionizing dose range. Annealing treatments after electron irradiations have highlighted the existence of specific population of pixels sharing the same well-defined maximum transition amplitudes (i.e., maximum amplitude between two dark current levels). The results suggest the use of maximum transition amplitude spectroscopy applied to dark current RTS to push forward the investigation on radiation-induced defects creation and identification.

Exploration of Pinned Photodiode Radiation Hardening Solutions Through TCAD Simulations

The use of pinned photodiode (PPD)-based CMOS image sensors (CIS) in harsh radiation environment (such as space) is limited by their tolerance to ionizing radiation. Technology computer-aided design (TCAD) simulations are performed to reproduce the radiation-induced defect and therefore the dark current increase in PPD pixels up to 1 kGy (i.e., 100 krad) of total ionizing dose (TID). To do so, the TCAD models are calibrated with measurements performed on irradiated pixels. Then, the influence on the PPD radiation hardness of various manufacturing process and pixel design modifications are explored. This work shows that the proposed modification can improve the radiation hardness of PPD CIS against ionizing.

Transient Response in PPD CMOS Image Sensors Irradiated by Gamma Rays: Variation of Dose Rates and Integration Times

The transient responses in CMOS image sensors (CISs) during radiation are one of the most important key issues to the detector design, reliability, and applicability. The radiation experiments of transient responses in CISs are presented. The CISs have a four Megapixels and pinned photodiode (PPD) pixel architecture with a standard 0.18- $\mu \text{m}$ CMOS technology. The transient responses induced by the gamma radiation at the dose rates of 0.01, 0.02, 0.05, 0.1, 0.2, and 0.3 rad(Si)/s are compared to analyze the influence of dose rates. The transient responses of the PPD CISs at different integration times ranged from 1.02 to 20.51 ms during radiation are also analyzed. The transient response characteristics of the PPD CISs induced by the gamma radiation present a transient disturbance and a random signal distribution. The experimental results show that the mean signal levels increase linearly with increasing dose rates or integration times during radiation, which indicates a method to improve the CIS detector precision. The mechanisms of the transient responses in the PPD CISs are also demonstrated.

Comparison of the dark signal degradation induced by Gamma ray, proton, and neutron radiation in pinned photodiode CMOS image sensors

Comparison of the dark signal degradation induced by Gamma ray, proton, and neutron radiation in pinned photodiode CMOS image sensors

Measurement and analysis of the conversion gain degradation of the CIS detectors in harsh radiation environments

The conversion gain of the CMOS image sensor (CIS) is one of the most important key parameters to the CIS detector. The conversion gain degradation induced by radiation damage will seriously affect the performances of the CIS detector. The experiments of the CISs irradiated by protons, neutrons, and gamma rays are presented. The CISs have 4 Megapixels and pinned photodiode (PPD) pixel architecture with a standard 0.18 μm CMOS technology. The conversion gains versus the proton fluence (including the proton ionizing dose), neutron fluence and gamma total ionizing dose are presented, respectively. The mechanisms of the conversion gain degradation induced by radiation damage are analyzed in details. The investigations will help to improve the PPD CIS detector design, reliability and applicability for applications in the harsh radiation environments such as space and nuclear environments.

Total-Ionizing Dose Effects on Charge Transfer Efficiency and Image Lag in Pinned Photodiode CMOS Image Sensors

The total-ionizing dose (TID) effects on image lag in pinned photodiode CMOS image sensors are investigated thanks to various device variants in order to isolate the major radiation-induced effects on the charge transfer. It is shown that the main cause of the charge transfer degradation is the radiation-induced defects generation in the premetal dielectric (PMD) and in the transfer gate (TG) spacer vicinity which modifies the potential diagram at the photodiode/TG interface by the creation of a potential pocket retaining the electrons that are not transferred. For 0.1 kGy(SiO2) 5 kGy(SiO2), the defects generated in the PMD influence the whole photodiode potential inducing a pinning voltage increase and degrading the charge transfer by enlarging the potential pocket effect which becomes the main image lag source. The reported results clarify the impact of ionizing radiation on the charge transfer, suggesting radiation hardened by design solutions for future space or nuclear applications.

Total Ionizing Dose Radiation-Induced Dark Current Random Telegraph Signal in Pinned Photodiode CMOS Image Sensors

In this paper, several studies on total ionizing dose effects on pinned photodiode CMOS images sensors are presented. More precisely, the evolution of a parasitic signal called random telegraph signal (RTS) is analyzed through several photodiode designs. It is shown that the population of pixels exhibiting this fluctuation depends on the design variants. This population also increases in a different way with the dose: the effects are not the same considering a low or high X-rays irradiation. Moreover, a statistical analysis is realized in order to better characterize the defects responsible for RTS. It turns out that an electric-field enhancement signature can appear in some specific cases.

Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

This paper demonstrates the existence of dark current blooming in pinned photodiode (PPD) CMOS image sensors (CISs) with the support of both experimental measurements and TCAD simulations. It is usually assumed that blooming can appear only under illumination, when the charge collected by a pixel exceeds the full well capacity (FWC) (i.e., when the photodiode becomes forward biased). In this paper, it is shownthat blooming can also appear in the dark by dark current leakage from hot pixels in reverse bias (i.e., belowthe FWC). The dark current blooming is observed to propagate up to nine pixels away in the experimental images and can impact hundreds of pixels around each hot pixel. Hence, it can be a major image quality issue for the state-of-the-art PPD CISs used in dark current limited applications, such as low-light optical imaging and should be considered in the dark current subtraction process. This paper also demonstrates that one of the key parameter for dark current optimization, the transfer gate bias during integration, has to be carefully chosen depending on the application because the optimum bias for dark current reduction leads to the largest dark current blooming effects.

Simulation and measurement of total ionizing dose radiation induced image lag increase in pinned photodiode CMOS image sensors

This paper presents an investigation of total ionizing dose (TID) induced image lag sources in pinned photodiodes (PPD) CMOS image sensors based on radiation experiments and TCAD simulation. The radiation experiments have been carried out at the Cobalt −60gamma-ray source. The experimental results show the image lag degradation is more and more serious with increasing TID. Combining with the TCAD simulation results, we can confirm that the junction of PPD and transfer gate (TG) is an important region forming image lag during irradiation. These simulations demonstrate that TID can generate a potential pocket leading to incomplete transfer.

Fully Depleted Pinned Photodiode CMOS Image Sensor With Reverse Substrate Bias

A new pixel design using fully depleted pinned photodiode (PPD) in a 180-nm CMOS image sensor (CIS) process has been developed and the first experimental results from a test chip are presented. The sensor can be fully depleted by means of reverse bias applied to the substrate, and the principle of operation is applicable to very thick sensitive volumes. Additional n-type implants under the in-pixel p-wells have been added to the manufacturing process in order to eliminate the large parasitic substrate current that would otherwise be present in a normal device. The new design shows the same electro-optical performance as the PPD pixel it is based on, and can be fully depleted without significant leakage currents. This development has the potential to greatly increase the quantum efficiency of scientific PPD CIS at near-infrared and soft X-ray wavelengths.