Estimation and Modeling of the Full Well Capacity in Pinned Photodiode CMOS Image Sensors

This letter presents a simple analytical model for the evaluation of the full well capacity (FWC) of pinned photodiode (PPD) CMOS image sensors depending on the operating conditions and on the pixel parameters. While in the literature and technical documentations FWC values are generally presented as fixed values independent of the operating conditions, this letter demonstrates that the PPD charge handling capability is strongly dependent on the photon flu

Estimation and Modeling of Key Design Parameters of Pinned Photodiode CMOS Image Sensors for High Temporal Resolution Applications

Ph.D Defence Alice Pelamatt

Charge Transfer Inefficiency in Pinned Photodiode CMOS image sensors: Simple Montecarlo modeling and experimental measurement based on a pulsed storage-gate method

The charge transfer time represents the bottleneck in terms of temporal resolution in Pinned Photodiode (PPD) CMOS image sensors. This work focuses on the modeling and estimation of this key parameter. A simple numerical model of charge transfer in PPDs is presented. The model is based on a Montecarlo simulation and takes into account both charge diffusion in the PPD and the effect of potential obstacles along the charge transfer path. This work also presents a new experimental approach for the estimation of the charge transfer time, called pulsed Storage Gate (SG) method. This method, which allows reproduction of a “worst-case” transfer condition, is based on dedicated SG pixel structures and is particularly suitable to compare transfer efficiency performances for different pixel geometries

On The Pixel Level Estimation of Pinning Voltage, Pinned Photodiode Capacitance and Transfer Gate Channel Potential

The pinning voltage extraction method proposed by Tan et al. is analyzed to clarify its benefits and limitations. It is demonstrated that this simple measurement can bring much more useful information than the pinning voltage, such as the pinned photodiode capacitance and the transfer gate channel potential. Objective criteria to compare the pinning voltage on different devices are also discussed

Temperature dependence and dynamic behaviour of full well capacity in pinned photodiode CMOS image sensors

This study presents an analytical model of the Full Well Capacity(FWC) in Pinned Photodiode (PPD) CMOS image sensors. By introducing the temperature dependence of the PPD pinning voltage, the existing model is extended (with respect to previous works) to take into account the effect of temperature on the FWC. It is shown, with the support of experimental data, that whereas in dark conditions the FWC increases with temperature, a decrease is observed if FWC measurements are performed under illumination. This study also shows that after a light pulse, the charge stored in the PPD drops as the PPD tends toward equilibrium. On the base of these observations, an analytical model of the dynamic behaviour of the FWC in non-continuous illumination conditions is proposed. The model is able to reproduce experimental data over six orders of magnitude of time. Both the static and dynamic models can be useful tools to correctly interpret FWC changes following design variations and to accurately define the operating conditions during device characterizations

Pixel Level Characterization of Pinned Photodiode and Transfer Gate Physical Parameters in CMOS Image Sensors

A method to extract the pinned photodiode (PPD) physical parameters inside a CMOS image sensor pixel array is presented. The proposed technique is based on the Tan et al. pinning voltage characteristic. This pixel device characterization can be performed directly at the solid-state circuit output without the need of any external test structure. The presented study analyzes the different injection mechanisms involved in the different regimes of the characteristic. It is demonstrated that in addition to the pinning voltage, this fast measurement can be used to retrieve the PPD capacitance, the pixel Equilibrium Full Well Capacity (EFWC) and both the Transfer Gate (TG) threshold voltage and its channel potential at a given gate voltage. An alternative approach is also proposed to extract an objective pinning voltage value from this measurement

Speed Analysis in Pinned Photodiode CMOS Image Sensors based on a Pulsed Storage-Gate Method

The charge transfer time represents the bottleneck in terms of temporal resolution in Pinned Photodiode (PPD) CMOS image sensors. This work focuses on the modeling and estimation of this key parameter. A simple numerical model of charge transfer in PPDs is presented. The model is based on a Montecarlo simulation and takes into account both charge diffusion in the PPD and the effect of potential obstacles along the charge transfer path. This work also presents a new experimental approach for the estimation of the charge transfer time, called pulsed Storage Gate (SG) method. This method, which allows reproduction of a “worst-case” transfer condition, is based on dedicated SG pixe

Comparison of Pinning Voltage Estimation Methods in Pinned Photodiode CMOS Image Sensors

The pinning voltage is a key design parameter in Pinned Photodiode CMOS Image Sensors which significantly affects the device performances and which is often used by manufacturers to monitor production lines and for the optimization of technological processes. This work presents a comparative study of pinning voltage estimation methods, which are based both on electrical measurements performed on isolated test structures (or on test structures arrays) and on in-pixel measurements. It is shown, with the support of simulations and experimental measurements, that not all the estimation methods provide an absolute value of the pinning voltage. Moreover, this work demonstrates that the commonly accepted theoretical definition of the pinning voltage does not correspond to the physical parameter which is measured with the existing methods

Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

This paper demonstrates the existence of dark current blooming in pinned photodiode CMOS image sensors 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 (i.e. when the photodiode becomes forward biased). In this work, it is shown that blooming can also appear in the dark by dark current leakage from hot pixels in reverse bias (i.e. below the full well capacity). 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 state-of-the-art pinned photodiode CMOS Image Sensors used in dark current limited applications such as low-light optical imaging and should be taken into account in the dark current subtraction process. This work 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

Don't be corrosive. A novel image analysis method for the validation of microplastic extraction procedures

Most applied procedures to extract microplastics from complex matrices – such as soils, sediments, or biological samples – imply a digestion step to eliminate the biogenic component. Since the digestion efficiency of different alkaline and oxidant agents can vary according to the composition of the examined matrix, possibly new digestion protocols will be developed in the future. The development of digestion protocols must consider that different chemicals at different incubation temperatures can attack different polymeric structures, potentially reducing the recovery of different microplastic types. In this view, corrosiveness tests represent a crucial key step in validating new microplastic extraction procedures. In this study we developed a methodological approach based on image analysis to detect the corrosive effect of digestive solutions. To reach this goal, we performed an experiment to verify the detection of shape variations in different microplastic types (tested polymers: nylon, polyethylene, polyethylene terephthalate, polypropylene, polystyrene, and polyvinylchloride) treated with 10% KOH at 60 °C or 30% H2O2 at 50 °C. A treatment with Milli-Q ultrapure water at room temperature was used as control treatment. Pictures of 540 microplastics (30 particles ∙ 6 polymers ∙ 3 treatments; size = 0.170 – 1.534 mm2) were taken before and after their treatment using a camera-equipped dissecting microscope (ZEISS SteREO Discovery.V20; AxioCam ERc5s camera). The 1080 images (2560 ∙ 1920 pixel, 1143 pixel ∙ mm-1) were processed using the open-source software ImageJ and the shapeR package for R to obtain data on pre-post shape variations. Results were in line with what was expected, showing that 10% KOH at 60 °C damaged polyethylene terephthalate, while nylon, polyvinylchloride, and polystyrene were susceptible to the 30% H2O2 treatment. According to our results, the proposed image analysis approach could represent a replicable method for quantifying the corrosiveness of digestive solutions on microplastics with different polymer compositions