Charge diffusion in the field-free region of charge-coupled devices

The potential well in back-illuminated charge-coupled devices (CCDs) does not reach all the way to the back surface. Hence, light that is absorbed in the field-free region generates electrons that can diffuse into neighboring pixels and thus decreases the spatial resolution of the sensor. We present data for the charge diffusion from a near point source by measuring the response of a back-illuminated CCD to light emitted from a submicron diameter glass fiber tip. The diffusion of electrons into neighboring pixels is analyzed for different wavelengths of light ranging from 430 to 780 nm. To find out how the charge spreading into other pixels depends on the location of the light spot; the fiber tip could be moved with a piezoelectric translation stage. The experimental data are compared to Monte Carlo simulations and an analytical model of electron diffusion in the field-free region. The presented analysis can be used to predict the charge diffusion in other back-illuminated sensors, and the experiment is universally applicable to measure any type of sensors

Dynamic CCD pixel depletion edge model and the effects on dark current production

The depletion edge in Charge-Coupled Devices (CCD) pixels is dependent upon the amount of signal charge located within the depletion region. A model is presented that describes the movement of the depletion edge with increasing signal charge. This dynamic depletion edge is shown to have an effect on the amount of dark current produced by some pixels. Modeling the dark current behavior of pixels both with and without impurities over an entire imager demonstrates that this moving depletion edge has a significant effect on a subset of the pixels. Dark current collected by these pixels is shown to behave nonlinearly with respect to exposure time and additionally the dark current is affected by the presence of illumination. The model successfully predicts unexplained aspects of dark current behavior previously observed in some CCD sensors

Dark current modeling with a moving depletion edge

Within a pixel in a digital imager, generally either a chargecoupled device or complementary metal oxide semiconductor device, doping of the semiconductor substrate and application of gate voltages create a region free of mobile carriers called the depletion region. This region fills with charge after incoming photons or thermal energy raise the charges from the valence to the conduction energy band. As the signal charge fills the depletion region, the electric field generating the region is altered, and the size of the region is reduced. We present a model that describes how this dynamic depletion region, along with the location of impurities, will result in pixels that produce less dark current after being exposed to light and additionally show nonlinear production rates with respect to exposure time. These types of effects have been observed in digital imagers, allowing us to compare empirical data with the modeled data

Temperature dependence of dark current in a CCD

We present data for dark current of a back-illuminated CCD over the temperature range of 222 to 291 K. Using an Arrhenius law, we found that the analysis of the data leads to the relation between the prefactor and the apparent activation energy as described by the Meyer-Neldel rule. However, a more detailed analysis shows that the activation energy for the dark current changes in the temperature range investigated. This transition can be explained by the larger relative importance at high temperatures of the diffusion dark current and at low temperatures by the depletion dark current. The diffusion dark current, characterized by the band gap of silicon, is uniform for all pixels. At low temperatures, the depletion dark current, characterized by half the band gap, prevails, but it varies for different pixels. Dark current spikes are pronounced at low temperatures and can be explained by large concentrations of deep level impurities in those particular pixels. We show that fitting the data with the impurity concentration as the only variable can explain the dark current characteristics of all the pixels on the chip

PSF Measurements on Back-Illuminated CCDs

The spatial resolution of an optical device is generally characterized by either the Point Spread Function (PSF) or the Modulation Transfer Function (MTF). To directly obtain the PSF one needs to measure the response of an optical system to a point light source. We present data that show the response of a back-illuminated CCD to light emitted from a sub-micron diameter glass fiber tip. The potential well in back-illuminated CCD s does not reach all the way to the back surface. Hence, light that is absorbed in the field-free region generates electrons that can diffuse into other pixels. We analyzed the diffusion of electrons into neighboring pixels for different wavelengths of light ranging from blue to near infrared. To find out how the charge spreading into other pixels depends on the location of the light spot, the fiber tip could be moved with a piezo-electric translation stage. The experimental data are compared to Monte Carlo simulations and an analytical model of electron diffusion in the field-free region

Interpreting Activation Energies in Digital Image Sensors

We present a study demonstrating the dependence of apparent activation energies on signal charge in digital image sensors. The data presented in this paper are for a charge-coupled device imager, but the analysis can be applied to CMOS sensors or generally to any system that shows nonlinearity with respect to time. Activation energies for some pixels are observed to vary between values at about half the bandgap of silicon, when calculated at low signal-charge levels, to values approaching the bandgap when calculated at high signal-charge levels. As such, the traditional method of calculating activation energies using a single exposure time at varying temperatures will result in different activation energies dependent on the exposure time chosen, making it difficult to draw physically meaningful conclusions from its value. Therefore, a method of calculating activation energies using a single signal-charge level is proposed, making it easier to correlate activation energies with impurities. Further, we demonstrate how a model of a pixel with a fixed location impurity in conjunction with a moving depletion edge, due to a changing depletion region size, can lead to this behavior

[pt] INFLUÊNCIA DO CUPOM CAMBIAL SOBRE A CONTA CAPITAL BRASILEIRA

Dark current is caused by electrons that are thermally exited into the conduction band. These electrons are collected by the well of the CCD and add a false signal to the chip. We will present an algorithm that automatically corrects for dark current. It uses a calibration protocol to characterize the image sensor for different temperatures. For a given exposure time, the dark current of every pixel is characteristic of a specific temperature. The dark current of every pixel can therefore be used as an indicator of the temperature. Hot pixels have the highest signal-to-noise ratio and are the best temperature sensors. We use the dark current of a several hundred hot pixels to sense the chip temperature and predict the dark current of all pixels on the chip. Dark current computation is not a new concept, but our approach is unique. Some advantages of our method include applicability for poorly temperature-controlled camera systems and the possibility of ex post facto dark current correction