A HIGH-PACKING DENSITY PIXEL WITH PUNCHTHROUGH READ-OUT METHOD FOR AN HDTV INTERLINE CCD

A new pixel structure for a high-packing-density interline CCD is proposed, in which signal charges are read out from the photodiodes to the vertical CCD by a punchthrough mechanism. This read-out method makes it possible to reduce the depth of the VCCD channel and the second p-well by implanting these two layers after diffusion of the photodiode n layer. Spreading resistance measurements on dummy wafers show that the depths of these layers are 0.28 mum and 0.6 mum, respectively. Moreover, the photodiode n-layer is covered with a surface p+-layer, even at the transfer region. We describe the results of simulations and experiments on a test image sensor with pixel dimensions of 7.3 mum (H) x 7.6 mum (V). From the experimental data, we estimate the characteristics of an image sensor with pixel dimension 5.0 mum (H) x 5.2 mum (V). Such a device should have a maximum charge handling capability of 1.4 x 10(5) electrons, a smear level of -88 dB, a sensitivity of 1.5 x 10(3) electrons/lx with a 30% fill factor, no image lag, and a low photodiode dark signal of less than 14 electrons at 60-degrees-C. These results indicate that an IL-CCD with a punchthrough readout structure is suitable for image sensors with a high pixel density such as 2/3 inch 2 million pixel image sensors for high-definition TV applications.