Radiation damages in CMOS image sensors: testing and hardening challenges brought by deep sub-micrometer CIS processes

This paper presents a summary of the main results we observed after several years of study on irradiated custom imagers manufactured using 0,18 µm CMOS processes dedicated to imaging. These results are compared to irradiated commercial sensor test results provided by the Jet Propulsion Laboratory to enlighten the differences between standard and pinned photodiode behaviors. Several types of energetic particles have been used (gamma rays, X-rays, protons and neutrons) to irradiate the studied devices. Both total ionizing dose (TID) and displacement damage effects are reported. The most sensitive parameter is still the dark current but some quantum eficiency and MOSFET characteristics changes were also observed at higher dose than those of interest for space applications. In all these degradations, the trench isolations play an important role. The consequences on radiation testing for space applications and radiation-hardening-by-design techniques are also discussed

Similarities Between Proton and Neutron Induced Dark Current Distribution in CMOS Image Sensors

Several CMOS image sensors were exposed to neutron or proton beams (displacement damage dose range from 4 TeV/g to 1825 TeV/g) and their radiation-induced dark current distributions are compared. It appears that for a given displacement damage dose, the hot pixel tail distributions are very similar, if normalized properly. This behavior is observed on all the tested CIS designs (4 designs, 2 technologies) and all the tested particles (protons from 50 MeV to 500 MeV and neutrons from 14 MeV to 22 MeV). Thanks to this result, all the dark current distribution presented in this paper can be fitted by a simple model with a unique set of two factors (not varying from one experimental condition to another). The proposed normalization method of the dark current histogram can be used to compare any dark current distribution to the distributions observed in this work. This paper suggests that this model could be applied to other devices and/or irradiation conditions

Radiation Induced Variable Retention Time in Dynamic Random Access Memories

The effect of gamma-ray and neutron radiations on the Variable Retention Time (VRT) phenomenon occurring in Dynamic Random Access Memory (DRAM) is studied. It is shown that both ionizing radiation and non-ionizing radiation induce VRT behaviors in DRAM cells. It demonstrates that both Si/SiO2 interface states and silicon bulk defects can be a source of VRT. It is also highlighted that radiation induced VRT in DRAMs is very similar to radiation induced Dark Current Random Telegraph Signal (DC-RTS) in image sensors. Both phenomena probably share the same origin but high magnitude electric fields seem to play an important role in VRT only. Defect structural fluctuations (without change of charge state) seem to be the root cause of the observed VRT whereas processes involving trapping and emission of charge carriers are unlikely to be a source of VRT. VRT also appears to be the most probable cause of intermittent stuck bits in irradiated DRAMs

Characterisation of CMOS APS Technologies for Space Applications

In recent years, the performance of scientific CMOS active pixel sensors has been improved to the point that it is now approaching that of the current silicon sensor of choice, CCDs. For some applications, CMOS APSs is believed to present significant advantages over CCDs, such as improved radiation hardness. In this work, the effect of radiation damage on a ‘baseline’ commercial APS, e2v technologies’ Jade APS, is characterised in response to gamma, proton and heavy ion irradiation. Specific performance problems encountered during this radiation characterisation, such as dark current non-uniformity under gamma irradiation, random telegraph signals under proton irradiation, and single event effects under heavy ion irradiation are described and analyzed. The X-ray spectroscopic imaging performance of the device is measured and compared to the Ocean Colour Imager APS test array showing progress towards a high frame rate spectroscopic X-ray imager for space science. The implications of these results for using similar devices in space applications are considered. Furthermore, possible novel techniques for measuring inter-pixel responsivity non-uniformity, heavy ion detection and spectroscopy, and measuring the dynamics of radiation-induced trap formation are discussed

Characterising the CMOS Image Sensor for the JANUS Camera on ESA’s JUICE Mission to Jupiter

The subject of this thesis is the characterisation of a scientific Complementary Metal Oxide Semiconductor (CMOS) image sensor to be used on the JANUS camera on ESA’s JUICE mission to Jupiter. The first part of this thesis investigates the initial characteristics of the device to better understand how changes in these characteristics manifest themselves over a range of tests. Initially, following total ionising dose and displacement damage, an increase in the dark current is observed. At temperatures above room temperature, it is theorised that the dark current is proportional to the exponent of the band gap of silicon. Following thermal annealing of these irradiated devices a slight recovery in the average dark current is noticed, which can be credited to the annealing of some radiation induced defects. The second part of this work investigates how image lag manifests in the image sensor, where a transitionary point to high level image lag is observed, referred to as the image lag ‘knee-point’. The signal that this knee point occurs is studied with varying total ionising dose and transfer gate voltages, allowing the cause to be hypothesised and an optimum operating condition to be recommended. The image lag is also investigated on a pixel-by-pixel basis, which is a novel approach compared to the typical average level across the whole image sensor. Measurements with devices exposed to total non-ionising doses demonstrate the creation of a population of pixels that exhibit higher levels of image lag than average, an effect that has been attributed to displacement damage in the image sensor

Radiation Effects on CMOS Active Pixel Image Sensors

Today, Complementary-Metal-Oxide-Semiconductor (CMOS) Image Sensors (CIS), also called Active Pixel Sensors (APS), are the most popular imager technology with several billions manufactured every year. They represent about 90% of the imager market and should exceed 95% in a couple of years. Compared to the main alternative imager technology, the Charge Coupled Device (CCD), CISs have several major benefits such as low-power consumption, high-integration, high speed and the capacity to integrate advanced CMOS functions on-chip (and even inside the pixel). Thanks to the latest technology innovations, CISs are now matching the performances of CCDs in terms of image quality and sensitivity placing them at the forefront even in high-end applications such as digital single-lens reflex, scientific instruments, and machine vision. Thanks to these advantages, CISs are also used in harsh radiation environment for applications such as: space applications, X-ray medical imaging, electron microscopy, nuclear facility monitoring and remote handling (nuclear power plants, nuclear waste repositories, nuclear physics facilities…), particle detection and imaging, military applications etc.. Designing, hardening and testing a sensor for such applications require the understanding of the CIS behavior when exposed to radiation sources. Understanding and improving further the intrinsically good radiation hardness of APS has been a topic of interest since its invention. This interest has been recently growing with the coming of new behaviors brought by the profound evolution of CIS technologies (as discussed throughout this manuscript) compared to the older generation mainstream CMOS processes used in early work. The aim of this chapter is to give an overview of the parasitic effects that can undergo a modern CIS when it is exposed to a high energy particle radiation field

Phosphorus Versus Arsenic: Role of the Photodiode Doping Element in CMOS Image Sensor Radiation-Induced Dark Current and Random Telegraph Signal

This work the role of the phosphorus doping element in the radiation-induced dark current in a CMOS image sensor (CIS) photodiode. The neutron and proton irradiations on shallow arsenic-based photodiode CISs and deep phosphorus-based photodiodes CISs have been performed. The results highlight the applicability of the same dark current increase and random telegraph signal (RTS) models. Already verified on other photodiode structures, these results further extend the universality of these analytic tools. Moreover, it emphasizes that the phosphorus element does not play a significant role either in the radiation-induced dark current increase or in the dark current RTS. The results on RTS after annealing reveal the same recovery dynamic than those already observed in irradiated image sensors, suggesting that the phosphorus element does not play a significant role after annealing. Therefore, this work is a piece of experimental evidence supporting the idea that RTS induced by displacement damage is principally due to defect clusters mainly constituted of intrinsic silicon defects such as clusters of vacancies and interstitials

Analyse des effets des déplacements atomiques induits par l'environnement radiatif spatial sur la conception des imageurs CMOS

L' imagerie spatiale est aujourd'hui un outil indispensable au développement durable, à la recherche et aux innovations scientifiques ainsi qu à la sécurité et la défense. Fort de ses excellentes performances électro-optiques, de son fort taux d intégration et de la faible puissance nécessaire à son fonctionnement, le capteur d images CMOS apparait comme un candidat sérieux pour ce type d application. Cependant, cette technologie d imageur doit être capable de résister à l environnement radiatif spatial hostile pouvant dégrader les performances des composants électroniques. Un nombre important d études précédentes sont consacrées à l impact des effets ionisants sur les imageurs CMOS, montrant leur robustesse et des voies de durcissement face à de telles radiations. Les conclusions de ces travaux soulignent l importance d étudier les effets non-ionisants, devenant prépondérant dans les imageurs utilisant les dernières évolutions de la technologie CMOS. Par conséquent, l objectif de ces travaux de thèse est d étudier l impact des effets non-ionisants sur les imageurs CMOS. Ces effets, regroupés sous le nom de déplacements atomiques, sont étudiés sur un nombre important de capteurs d images CMOS et de structures de test. Ces dispositifs sont conçus avec des procédés de fabrication CMOS différents et en utilisant des variations de règle de dessin afin d investiguer des tendances de dégradation commune à la technologie d imager CMOS. Dans ces travaux, une équivalence entre les irradiations aux protons et aux neutrons est mise en évidence grâce à des caractéristiques courant-tension et des mesures de spectroscopie transitoire de niveau profond. Ces résultats soulignent la pertinence des irradiations aux neutrons pour étudier les effets non-ionisants. L augmentation et la déformation de l histogramme de courant d obscurité ainsi que le signal télégraphique aléatoire associé, qui devient le facteur limitant des futures applications d imagerie spatiale, sont évalué et modélisés. Des paramètres génériques d évaluation des effets des déplacements atomiques sont mis en évidence, permettant de prévoir le comportement des capteurs d images CMOS en environnement radiatif spatial. Enfin, des méthodes d atténuation et des voies de durcissement des imageurs CMOS limitant l impact des déplacements atomiques sont proposées.Today, space imaging is an essential tool for sustainable development, research and scientific innovation as well as security and defense. Thanks to their good electro-optic performances and low power consumption, CMOS image sensors are serious candidates to equip future space instruments. However, it is important to know and understand the behavior of this imager technology when it faces the space radiation environment which could damage devices performances. Many previous studies have been focused on ionizing effects in CMOS imagers, showing their hardness and several hardening-by-design techniques against such radiations. The conclusions of these works emphasized the need to study non-ionizing effects which have become a major issue in the last generation of CMOS image sensors. Therefore, this research work focuses on non-ionizing effects in CMOS image sensors. These effects, also called displacement damage, are investigated on a large number of CMOS imagers and test structures. These devices are designed using several CMOS processes and using design rule changes in order to observe possible common behaviors in CMOS technology. Similarities have been shown between proton and neutron irradiations using current-voltage characteristics and deep level transient spectroscopy. These results emphasize the relevance of neutron irradiations for an accurate study of the non-ionizing effects. Then, displacement damage induced dark current increase as well as the associated random telegraph signal are measured and modeled. Common evaluation parameters to investigate displacement damage are found, allowing imager behavior prediction in space radiation environment. Finally, specific methods and hardening-by-design techniques to mitigate displacement damage are proposed.TOULOUSE-ISAE (315552318) / SudocSudocFranceF

Radiation Damage in CMOS Image Sensors for Space Applications

The space radiation environment is damaging to silicon devices, such as Complementary Metal Oxide Semiconductor (CMOS) image sensors, affecting their performance over time or causing total failure. The first part of this work investigates a Charge Coupled Device (CCD) style CMOS image sensor designed for TDI (Time Delay and Integration) mode imaging, a mode commonly used for Earth observation. Damage from high energy protons in the space environment decreases the Charge Transfer Efficiency (CTE) and increases the dark current of such devices. Experimental work on proton damaged devices is presented, showing the effects on CTE and dark current. The results are compared to a standard CCD by a simulation to take into account the different dimensions and operating conditions of the two devices. The second part of this work describes an experimental campaign to determine the effects of process variations (namely the introduction of deep doping wells and the variation of epitaxial silicon thickness) on the rate of Single Event Latchup (SEL) in CMOS Active Pixel Sensor (APS) devices. SEL is a potentially destructive phenomenon which occurs in CMOS technology but not in CCDs. Test devices were subjected to heavy ion bombardement and SEL rates recorded for a range of heavy ions causing varying amounts of ionisation. A simulation using Technology Computer Aided Design (TCAD) was developed to predict the SEL rates due to heavy ions and to understand the characteristic shape of the SEL cross section vs. Linear Energy Transfer (LET) curves produced by SEL experiments. The simuation was carried out for structures representative of each of the design variants

Novel Developments in Scientific EMCCDs

This thesis presents a complete characterisation and an assessment of the technology readiness of a new Electron Multiplying Charge Coupled Devices (EMCCD) technology. Several factors of interest are studied here including, charge transfer efficiency, gain ageing and radiation effects from protons. Many light-starved and high-speed image applications (e.g. observation from space and automated visual inspection) can benefit from Time Delay Integration (\acrshort{tdi}) as it allows photoelectrons from multiple exposures to be summed in the charge domain with no added noise. Electron multiplication (EM) can be used to further increase the signal to noise ratio for extremely faint light signals. There is a growing demand for Complementary metal–oxide–semiconductor (\acrshort{cmos}) sensors with the same or greater functionality and even better performance. The research presented here analyses the functionality of a recently designed EMCCD in a CMOS process. This device (EMTC1) incorporates two novel EM pixel structures which enable high gain at relatively low voltages. The theory and architecture of Charge Coupled Devices (CCDs) and EMCCDs are discussed, providing a technical background for the results. Furthermore, the practical methods, including experimental techniques developed for this device's testing, are presented here. Ageing within the device is a primary focus within this thesis, as is the effect of proton irradiation. The effects of the radiation damage on parameters such as dark current and Charge Transfer Inefficiency (CTI) have been documented along with its effect on EM gain. These results have been corroborated with simulations of device operation in TCAD