Investigation of Image Sensors for Ultra High Resolution Imaging Systems

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(1) We proposed a new photodiode to increase the charge handling capability of solid state image sensors. This photo diode includes an N^+ sidewall that surrounds the charge strage region of a conventional photo diode with a vertical overflow drain. The operation and performance of the diode were analyzed using the three dimensional numerical analysis. We found that an abrupt potential increase, produced around the boundary of the channel stop and the N^+ sidewall creates a flat potential profile that spread all over the storage region, at the beginning of the charge storage. This profile suppresses electron over-flow into the substrate that occurs due to the small amount of signal electron concentrated around the center of the storage region. Therefore, the N^+ sidewall increases the storable charge. For example, the charge handling capability of a conventional 3.0-mum diameter photodiode was improved by more than a factor of four by adding N^+ sidewall. The validity of the analysis w … More as confirmed from the results of one dimensional analys. This technology is useful for small diameter photodiode.(2) It is well known that the saturation signal level of a CCD image sensor including photodiode with vertical overflow rain varies strongly with the substrate impurity concentration. However, the analyti-I results between the saturation signal level and substrate impurity concentration has not been ob-Tained yet. The relation between them was calculated using the drift-diffusion model as a function of photodiode structure and operation condition. By comparing analytical and experimental results obtained from CCD image sensor chips, we found that the impuriy fluctuation at the peripherals of CZ, MCZ, and epitaxial wafers *11%, *16% abd *6%, respectivery. Those at the center were *7%, *9% and *6%, respectively. The distnctive features of the impurity fluctuation pattern for these wafers were therefore clarified. The minimum imuruty fluctuation level that can can be detected is almost 0.1%. Moreover, a novel photodide strucre that suppresses the fluctuation was proposed.(3) CCD shift register suitable for highly photo sensitive frame transfer CCD image sensors was proposed and the performance of it was analized using three-dimensional numerical analysis.(4) A digital noise reduction method applicable to video signal was proposed and tested. Less