Localization of Dark Current Random Telegraph Signal sources in pinned photodiode CMOS Image Sensors

This work presents an analysis of Dark Current Random Telegraph Signal (DC-RTS) in CMOS Image Sensors (CIS). The objective is to provide new insight on RTS in modern CIS by determining the localization of DC-RTS centers and the oxide interfaces involved. It is shown that DC-RTS centers are located near the transfer gate. In particular, it is demonstrated that both gate oxide and Shallow Trench Isolation (STI) contribute to this parasitic dark current variation

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

The total ionizing dose 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 pre-metal dielectric and in the transfer gate spacer vicinity which modifies the potential diagram at the photodiode/transfer gate 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 pre-metal dielectric 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

Influence of Pixel Design on Charge Transfer Performances in CMOS Image Sensors

The influence of pixel design on image lag is investigated by focusing on two different aspects which impact the charge transfer. First it is confirmed that the transfer gate channel doping profile strongly affects image lag. Introducing a step under the TG in the potential diagram, due to the doping implant differences in the channel, enables very good transfer performances by limiting spill-back of the charge to the photodiode. On the other hand, it is demonstrated that the overlap between the two implants used to create the step can produce a potential barrier under the transfer gate which extension increases the image lag. Then, the influence of pixel layout geometrical parameters (e.g. of the photodiode size, the transfer gate length and floating diffusion area) on the charge transfer efficiency is clarified. The whole study conclusions allow identifying the design parameter limiting the transfer efficiency in a given design and the possible design based solutions to improve it

Dark Current Sharing and Cancellation Mechanisms in CMOS Image Sensors Analyzed by TCAD Simulations

TCAD simulations are conducted on a 4T PPD pixel, on a conventional gated photodiode, and finally on a radiation hardened pixel. Simulations consist in demonstrating that it is possible to reduce the dark current due to interface states brought by the adjacent gate, by means of a sharing mechanism between the photodiode and the drain. The sharing mechanism is activated and controlled by polarizing the adjacent gate at a positive \itshape off \upshape voltage, and consequently the dark current is reduced and not compensated. The drawback of the dark current reduction is a reduction of the full well capacity of the photodiode, which is not a problem when the pixel saturation is limited by the readout chain. Some measurement performed on pixel arrays confirm the TCAD results

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

In this work, 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 is analysed 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 same considering a low or high X-rays irradiation. Moreover, a statistical analysis is realized in order to better caracterize the defects responsible for RTS. It turns out that electric field enhancement signature can appear in some specific cases

High Displacement Damage Dose Effects in Radiation Hardened CMOS Image Sensors

CMOS image sensors (CISs) hardened by design against total ionizing dose (TID) are exposed to neutron fluences beyond 1014 n(1 MeV)/cm2. Neutron-irradiated devices show a huge increase in the dark current affecting uniformly the pixel array which leads to Gaussian shape dark current distributions. Moreover, random telegraph signal behavior is hardly detectable at these very high neutron fluences since the fluctuation amplitudes are hidden by the dark current shot noise. It is observed that neutrons induce a change in the depleted volume in impacted photodiodes because of doping profile modifications which are responsible for the decrease in the charge-to-voltage conversion factor and quantum efficiency. Even if neutron-induced degradations affect all the image sensor performances, results show that this technology is still functional after having absorbed 8.1 × 1014 n(1 MeV)/cm2. Image sensors are still able to capture an image without significant degradation compared to nonirradiated devices. Such TID radiation-hardened CISs are thus highly promising for applications where both high TID and high neutron fluence radiation tolerance are required

Real time monitoring of water level and temperature in storage fuel pools through optical fibre sensors

We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0–to–5 m) and temperature (20–to–100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants

Radiation resistant single-mode fiber with different coatings for sensing in high dose environments

A radiation resistant single-mode optical fiber has been specifically developed for distributed sensing in harsh environments associated with MGy(SiO2) dose radiation. Different types of coating have been used: acrylate, polyimide, aluminum that allow extending the range of accessible temperatures up to 400°C. Various characterizations were performed: radiation inducted attenuation (offline and online), fiber mechanical strength and coating thermal degradation post irradiation. Safe operation is demonstrated for almost all coating types up to the MGy(SiO2) range of cumulated dose

Vulnerability of OFDR-based distributed sensors to high γ-ray doses

Vulnerability of Optical Frequency Domain Reflectometry (OFDR) based sensors to high γ-ray doses (up to 10 MGy) is evaluated with a specific issue of a radiation-hardened temperature and strain monitoring system for nuclear industry. For this, we characterize the main radiation effects that are expected to degrade the sensor performances in such applicative domain: the radiation-induced attenuation (RIA), the possible evolution with the dose of the Rayleigh scattering phenomenon as well as its dependence on temperature and strain. This preliminary investigation is done after the irradiation and for five different optical fiber types covering the range from radiation-hardened fibers to highly radiation sensitive ones. Our results show that at these high dose levels the scattering mechanism at the basis of the used technique for the monitoring is unaffected (changes below 5%), authorizing acceptable precision on th

Radiation-Induced Leakage Current and Electric Field Enhancement in CMOS Image Sensor Floating Diffusions

Radiation-Induced Leakage Current and Electric Field Enhancement in CMOS Image Sensor Floating Diffusions