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Radiation effects on complementary metal–oxide–semiconductor (CMOS) active pixel sensors (APS) induced by proton and γ-ray are presented. The samples are manufactured with the standards of
Recent developments in complementary metal–oxide–semiconductor (CMOS) active pixel sensors (APS) indicate that CMOS imagers have reached or even exceeded the performance of charge-coupled devices (CCDs) in domains of low power consumption, high levels of integration, and low cost. Since CMOS APS can be used in applications with low light level (with few electrons) and extreme environment (especially for space, scientific, nuclear, medical, and military instruments applications), radiation damage of such applications becomes a primary concern.[1]
Important efforts have been made to reduce the radiation effect on CMOS APS during the last decade. It is known that that γ-ray irradiations can only induce ionization damage effects but proton irradiations induce both displacement damage and ionization damage effects.[2] Displacement damage effects are a key issue for solid state image sensors exposed to space radiation environments or used in nuclear physics experiments. Therefore, displacement damage effects on solid state image sensors are a subject of ongoing research,[3–5] particularly the degradation under illumination. Most of the previous work has been performed on devices in the dark during measurement,[6–11] which may hide some effects related to the photo-response of the devices.
In this paper, we compared the sensor performances under the radiation effects of proton and γ-ray. We studied the dark current, fixed-pattern noise under illumination (caused by the dark signal non-uniformity and photo-response non-uniformity), and conversion gain (CVG) of the sensors. The degradation mechanisms of CMOS APS image sensors were analyzed.
The device used for our test is AptinaTMMT9M001 with an active imaging pixel array of 1280H×1024V. The image-sensing element has a pixel size of
The devices have been exposed to 23 MeV protons in un-biased condition with fluences of
The annealing tests were carried out under the same biased conditions as the radiation tests at room temperature. The annealing temperatures are room temperature at 23 °C and high temperature at 150 °C.
An integrating sphere uniform light source was used during the measurement. The parameters of CMOS APS were measured before and after the irradiation.
Conversion gain (CVG) can be obtained by the slope of the photo transfer curve, which is a quantity to describe the process that the charge units accumulated by photo irradiance are converted into a voltage, amplified, and finally converted into a digital signal by an analog-to-digital converter (ADC). The unit of CVG is DN/e.[12] The values of CVG before and after irradiation are presented in Table
From Table
The dark signal is defined as the carriers generated in the potential wells under the pixels when no light is incident on the device. The dark signal is not constant. The main source of the dark signal is the thermally induced electrons. Therefore, the dark signal should be linearly dependent on the exposure time. Dark signal
Figure
Figure
Figure
Besides this, the ionization damage effects can also cause a build-up of trapped charge in the CMOS APS inter-electrode and gate oxide. The creation of trapped charges in the gate oxide or field oxide, which are close to the interface, broadens the width of the depletion region by changing the potential of the interface; this results in an increase of the generation process, which induces the dark current increase.[18] However, the oxide-trap charge is easy to be annealed at room temperature, while the interface traps and bulk traps are not readily annealed at room temperature.[19] Therefore, the dark current in this experiment associated with long time room temperature annealing after irradiation is mainly induced by the interface traps and bulk traps.
Since proton irradiations induce both displacement damage and ionization damage effects, the dark current induced by proton is much larger than that induced by γ-ray. The generation of bulk traps increases the dark current drastically.
The fixed-pattern noise under illumination is due to the photo-response gain mismatch of different pixels (photo-response non-uniformity, PRNU) and the dark signal non-uniformity (DSNU).[16] DSNU represents the distribution of the dark signal output of each individual pixel in the whole array when no light is incident on the device.
As illustrated in Figs.
Besides hot pixels, slight degradation of output under illumination has been observed for some pixels after irradiation, especially for those irradiated by protons. The main reason is the traps outside the depletion region in the p–n junction induced by displacement damage. The p–n junction is used to collect the photo-generated carriers, as shown in Fig.
As aforementioned, the displacement damage brings stable bulk traps with energy levels within the band-gap. The radiation-induced energy levels in the Si band-gap give rise to the following processes: (i) enhanced thermal generation, (ii) enhanced recombination, (iii) enhanced temporary trapping, (iv) reduced carrier concentration.[19–23] The primary effect produced by process (i) is an increase in dark current, which is induced hot pixels. Process (ii) causes effects such as reduced output. Process (iii) affects the charge collection efficiency. The photo-generated carriers may be recombined or trapped by these bulk traps before diffusing to the depletion region. As a result, bulk traps outside the depletion region induced by proton irradiation increase PRNU.
Quantum efficiency (QE) is an important parameter of the image sensor, which indicates the ability of the sensorr to convert an incident photon of a certain wavelength into an effective electrical signal.[24]
Figures
Radiation effects on CMOS APS caused by proton and γ-ray are presented. We study the dark current, FPN under illumination (caused by the dark signal non-uniformity and photo-response non-uniformity), quantum efficiency, and conversion gain. After exposure to irradiations, all devices have a performance penalty due to the radiation damage. It is found out that the dark current induced by proton irradiation is considerable. Both γ-ray and proton irradiations increase the signal non-uniformity, but the non-uniformity induced by proton is even worse. The bulk traps induced by the displacement damage increase DSNU and PRNU and both of them make FPN under illumination worse. The degradation of QE is induced by the displacement damage effect but not ionization damage. In order to investigate the degradation in different types of CMOS APS image sensors induced by γ-ray and proton, more radiation experiments will be carried out in our future works.
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