A shallow flat p-well structure for interline-transfer CCD image sensors

This paper describes a newly-developed image-sensing element having a shallow flat p-well (SFPW) structure, and further mentions its application to a 490 × 510- and a 580 × 500-element device for use in the ⅔-in optical format. The structure consists of a shallow flat p-well in the imaging area that incorporates not only photodiodes for blooming suppression but also CCD shift registers for reduction of smearing. Desirable features such as 1) high resolution, 2) high aperture ratio, and 3) low smearing level were facilitated simultaneously by a combination of the SFPW structure with a transfer gate-less (TGL) and clock-line-isolated photodiode (CLIP) structure. Here, the conventional n⁺-p photodiode is replaced by an n⁺-n^--p structure, in which the n^-region serves as an overflow duct to the substrate for excess charge generated by optical overloads. It also contributes to maintaining the sensitivity at medium wavelengths. Either of these two image sensors have horizontal resolution in excess of 370 TV lines, an aperture ratio of over 32 percent, and a smearing level of less than -70 dB.