THPM 13.6: Interline CCD Image Sensor with an Anti Blooming Structure

THE APPLICATION OF AN overflow drain and barrier positioned beside the photosensitive area’ has made it possible to suppress blooming of CCD image sensors. However, this method was found to sacrifice photosensitivity and dynamic range. This paper will describe an interline CCD sensor where the vertical overflow drain is positioned under, rather than beside the photodiode. Thus the cell area can now be used effectively for photoelectron generation and storage, and increased photosensitivity and dynamic range can be maintained. Furthermore, this technique eliminates the blooming phenomenon. A unit cell cross-sectional view of the vertical overflow drain (VOD) CCD image sensor is shown in Figure 1. The cell consists of a photodiode (PD) and a half stage of a 4-phase driven buried channel vertical CCD register (V-CCD) including a threshold controlled transfer gate (TG) region. The PD is made in a lightly doped P-well (lPW), while the rest of the cell is made in a more highly doped thick P-well(2PW). N-type substrate is reverse biased at VSUB from these grounded P-wells. The potential profiles under the PD and the TG region are shown in Figure 2(a) for two operating periods of V-CCD clock pulse (@v) which is shown in Figure 2(b). T1 and T2 are charge transfer and charge integration periods, respectively. During TI, as the photogenerated signal charges are transferred to the corresponding V-CCD, the N-region of the PD is reset at $TGH (TG channel potential at @V = VH) as shown by the curve indicated as Empty. During the following charge integration period T2, PD potential decreases with increasing signal charges. In time, the PD potential reaches ($PB + $bi) where $p~ is the minimum potential in a completely depleted 1PW corresponding to the supplied VSUB and $bi is the built-in potential of the PD junction. When VSUB is adjusted in such a way that ( $PB + $bi) is always higher than $TGM (TG channel potential at @V = V,) as shown by the curve indicated as Full, all of the photogenerated excess charges are drained into the N-type substrate, without flowing into the V-CCD. As a result, the overflow blooming is suppressed under strong illumination.