Sophie Petit

Displacement Damage Effects Due to Neutron and Proton Irradiations on CMOS Image Sensors Manufactured in Deep Submicron Technology

Displacement damage effects due to proton and neutron irradiations of CMOS image sensors dedicated to imaging are presented through the analysis of the dark current behavior in pixel arrays and isolated photodiodes. The mean dark current increase and the dark current nonuniformity are investigated. Dark current histogram observations are compared to damage energy distributions based on GEANT 4 calculations. We also discuss, through annealing analysis, which defects could be responsible for the dark current in CMOS image sensors.

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.

Theoretical Models of Modulation Transfer Function, Quantum Efficiency, and Crosstalk for CCD and CMOS Image Sensors

This paper proposes analytical models of modulation transfer function (MTF), quantum efficiency (QE), and crosstalk for charge-coupled device (CCD) and CMOS image sensors. A unified MTF model for a CCD sensor built on an epitaxial layer deposited on a highly doped substrate was developed by Stevens. The Stevens model uses sinusoidal illumination to calculate the sensor MTF degradation due to charge diffusion and sampling aperture as a function of spatial frequency. The drawback of this approach is the difficulty to evaluate analytically the electrical crosstalk distribution, which can be a good tool for predicting the detector performances, particularly for smaller pixels. In this paper, we use point-source illumination to evaluate the pixel response function (PRF). This approach is applied to the case of CMOS sensors and buried channel CCD sensors. The MTF model includes the impact of pixel size and charge diffusion. The QE model and crosstalk distribution are directly derived from the PRF expression. The models can take into account an electric field induced by a doping gradient.

Total Ionizing Dose Versus Displacement Damage Dose Induced Dark Current Random Telegraph Signals in CMOS Image Sensors

Dark current Random Telegraph Signals due to total ionizing dose (TID) and displacement damage dose (DDD) are investigated in CMOS image sensors. Discrepancies between both RTS are emphasised to better understand the microscopic origins of the phenomena.

Single-Event Effects in CMOS Image Sensors

In this paper, 3T active pixel sensors (APS) are exposed to heavy ions (N, Ar, Kr, Xe), and single-event effects (SEE) are studied. Devices were fully functional during exposure, no single-event latch-up (SEL) or single-event functional interrupt (SEFI) happened. However, single-event transient (SET) effects happened on frames: line disturbances, and half or full circular clusters of white pixels. The collection of charges in cluster was investigated with arrays of two pixel width (7 and 10 μm), with bulk and epitaxial substrates. This paper shows technological and design parameters involved in the transient events. It also shows that STARDUST simulation software can predict cluster obtained for bulk substrate devices. However, the discrepancies in epitaxial layer devices are large-which shows the need for an improved model.

Theoretical evaluation of MTF and charge collection efficiency in CCD and CMOS image sensors

Classical models used to calculate the Modulation Transfer function (MTF) of a solid-state image sensor generally use a sinusoidal type of illumination. The approach, described in this paper, consists in considering a point-source illumination to built a theoretical three-dimensional model of the diffusion and the collection of photo-carriers created within the image sensor array. Fourier transform formalism is used for this type of illumination. Solutions allow to evaluate the spatial repartition of the charge density collected in the space charge region, i.e. to get the Pixel Response Function (PRF) formulation. PRF enables to calculate analytically both MTF and crosstalk at every needed wavelengths. The model can take into account a uniformly doped substrate and an epitaxial layer grown on a highly doped substrate. The built-in electric field induced by the EPI/Substrate doping gradient is also taken into account. For these configurations, MTF, charge collection efficiency and crosstalk proportion are calculated. The study is established in the case of photodiode pixel but it can be easily extended to pinned photodiode pixels and photogate pixels.

A Statistical Method to Extract MBU Without Scrambling Information

This paper reports on a new method for extracting MBUs from standard SEE data without the need for the physical organisation of the memories. Efficiency of the method proposed here is quantified through a reduced set of parameters.

Analytical modeling of MTF and QE of CCD and CMOS image sensors

Today, CCD and CMOS image sensors have found many applications in general public domains. However their use for scientific and space applications requires high electro optical performances and strong abilities to predict them prior to the image sensors design and conception. Sensitivity and image quality are two important electro-optical characteristics for an image sensor. The Quantum Efficiency (QE) and the Modulation Transfer Function (MTF) are respectively the common metrics used to quantify them. Because of an important number of parameters influencing the MTF and the QE, their analytical calculation is not an easy task. This paper describes an analytical model of MTF and QE of CCD and CMOS image sensors. The model has been developed in order to take into account a maximum number of parameters: pixel size, photosensitive area size and shape, EPI-layer and substrate doping concentration, junction depth. The effect of top layer oxide stacks on the resulting optical transmission coefficient and so on QE can also be taken into account. The study is established in the case of CMOS photodiode pixels and buried channel CCD pixels. The MTF and QE modeling results are compared with experimental results. MTF and QE measurements are realized on different pixels types having different photosensitive area shapes and using different technologies. A part of these measurements are performed on a frontside-illuminated CMOS sensor and on a thinned backside-illuminated CMOS image sensor, both of them are manufactured using CMOS technology dedicated to image sensors. The other part of MTF and QE measurements are performed on thinned backside-illuminated N-buried channel CCD sensor. Finally the MTF and QE models are used to make performance predictions, and the effects of various parameters on the MTF and the QE are discussed.

Single Event Effects in 4T Pinned Photodiode Image Sensors

This paper describes how Single Event Effects (SEEs) produced by heavy ions disturb the operation of Pinned Photodiode (PPD) CMOS Image Sensors (CISs) in the frame of space and nuclear applications. Several CISs with 4T and 5T pinned photodiode pixels were exposed to ions with a broad Linear Energy Transfer range (3.3 to 67.7 MeV·cm2/mg). One sensor exhibited Single Event Latchups (SELs). Physical failure mechanism and latchup properties were investigated. SELs are caused by the level shifters of the addressing circuits, which create frame perturbations - following which, in some cases, parts of the addressing circuits need to be hardened. In the second part of the paper, the effects of anti-blooming capabilities on the Single Event Transient effects (SETs) are analyzed. SETs in pixels can be partially mitigated by anti-blooming through the transfer gate and/or a dedicated transistor. This work also shows that the number of pixels disturbed by SETs can be reduced by using appropriate anti-blooming techniques.

Influence of displacement damage dose on dark current distributions of irradiated CMOS image sensors

Dark current increase distributions due to displacement damages are modeled using displacement damage dose concept. Several CMOS image sensors have been exposed to neutrons or protons and we have characterized their degradation in terms of dark current increase. We have been able to extract a set of two factors from the experimental dark current increase distributions. These factors are used to predict and build dark current increase distribution and leads to a better understanding of displacement damage effects on CMOS image sensors.