Abstract:
A solid-state image sensor in which an interface area between a vertical charge coupled device (VCCD) and a horizontal charge coupled device (HCCD) is formed under the HCCD, thereby maximizing charge-transferring efficiency is disclosed, including a substrate; a well formed in the substrate; a first impurity region formed in the well under the VCCD and the HCCD; and second impurity regions selectively formed in the first impurity region to have a border from the first impurity region under the HCCD, wherein the second impurity regions have a different ion concentration from the first impurity region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solid-state image sensor and, more particularly, to a solid-state image sensor in which an interface area between a vertical charge coupled device (VCCD) and a horizontal charge coupled device (HCCD) is formed under the HCCD, thereby maximizing charge-transferring efficiency. 
     2. Discussion of the Related Art 
     FIG. 1 is a layout of a general solid-state image sensor, which includes photodiode regions for converting light signals into electric signals, a VCCD formed at right angle to the photodiode regions for transferring the converted signal charges, an HCCD formed at right angle to the VCCD for transferring the signal charges transferred from the VCCD, and a sensing amplifier SA for sensing and amplifying the signal charges transferred from the HCCD. 
     A VCCD-HCCD (V-H) interface area is placed between a VCCD and an HCCD. FIG. 2 is a layout of conventional V-H interface areas and FIG. 3 shows potential levels of V-H interface areas. 
     As shown in FIG. 2, signal charges, generated by photoelectric conversion in a photodiode region (PD), are transferred toward an HCCD by means of a VCCD. The transferred signal charges are then transferred to a sensing amplifier SA by clock signals Hφ 1 , Hφ 2  applied to polygates of the HCCD. At this time, the HCCD and the VCCD have different clock levels, as shown in FIG.  3 . 
     The VCCD generally carries out negative clocking. Since the VCCD is heightened to be in an accumulation mode during blanking of the HCCD, noise is made. Thus, potential barrier of a channel stop layer is not lowered. 
     As described above, signal charges are transferred from a VCCD to an HCCD because of difference of their clock levels. 
     In such a conventional solid-state image sensor, it was fine that VCCDs and HCCD are formed using one mask. However, since it is tendency to reduce a size of a device, any improvement of performance of a solid-state image sensor can hardly be expected. 
     FIG. 4 is a solid-state image sensor in which a VCCD and an HCCD are formed using one mask. Only one mask on a substrate  11  is used to form a p-well  13  over which a VCCD and an HCCD will be formed. A buried charge coupled device (BCCD)  15  and a channel stop (CST) layer  17  are formed in the p-well  13 . In case a VCCD and an HCCD are formed at a time using one mask, a size of the semiconductor device can be a problem. That is, when the size of the semiconductor device is diminished, and its packing density is heightened, a size of a pixel defining photodiodes and VCCDs is diminished as well. Accordingly, a VCCD becomes much smaller than an HCCD. For this problem, each of a VCCD and an HCCD is formed using its own mask to improve the performance of a semiconductor device. 
     FIG. 5 is a cross-sectional view taken along line I-I′ of FIG.  2 . As shown in FIG. 5, each of p-wells for a VCCD and an HCCD is formed using a mask. To obtain such a solid-state image sensor, a first p-well  13  for a VCCD is formed in a semiconductor substrate  11  using a mask. Subsequently, a second p-well  13   a  for an HCCD is formed using another mask. Next, ion-implanting process is performed over the first and second p-wells  13  and  13   a,  thus forming a BCCD  15  and a channel stop layer  17  therein. 
     However, since p-wells for VCCD and HCCD are formed using different masks, Hence, impurity ions having different concentrations for a VCCD and an HCCD are implanted in a diminished the size of a semiconductor device, thereby enhancing its packing density. 
     However, a conventional solid-state image sensor has the following problems. Since each of a VCCD and an HCCD is formed using a mask, V-H interface area is a problem. In other words, a p-well for a VCCD partially overlaps another p-well for an HCCD and thus a relatively big potential pocket is formed in the overlapped area than the other areas. Hence, on transferring signal charges, discontinuity is generated as though a well is formed in a transfer channel. Such discontinuity generates fixed pattern noise such as vertical black lines, thereby causing degradation of picture quality. 
     SUMMERY OF THE INVENTION 
     Therefore, the present invention is directed to a solid-state image sensor that substantially obviates one or more of problems due to limitations and disadvantages of the related art. 
     An object of the invention is to provide a solid-state image sensor in which a V-H interface area is formed in a VCCD to remove fixed pattern noise such as vertical black lines, thus enhancing its performance. 
     Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a solid-state image sensor having a VCCD and an HCCD includes a substrate, a well formed in the substrate, a first impurity region formed in the well under the VCCD and HCCD, and a second impurity region selectively formed in the first impurity to have a border from the first impurity region under the HCCD and having a different ion concentration from the first impurity region. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and various other objects, features, and advantages of the present invention will be readily understood with reference to the following detailed description read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a layout of a general solid-state image sensor; 
     FIG. 2 is a layout of V-H interface area in a conventional solid-state image sensor; 
     FIG. 3 shows potential levels of V-H interface area in a conventional solid-state image sensor; 
     FIG. 4 is a cross-sectional view of the conventional solid-state image sensor, taken along line I-I′ of FIG. 2 in accordance with a first embodiment; 
     FIG. 5 is a cross-sectional view of the solid-state image sensor, taken along line I-I′ of FIG. 2, in accordance with a second embodiment; 
     FIG. 6A is a layout of a solid-state image sensor in accordance with a first embodiment of the present invention; 
     FIG. 6B is a cross-sectional view, taken along line I-I′ of FIG. 6A, showing a structure of the solid-state image sensor in accordance with the first embodiment of the present invention; 
     FIG. 7A is a layout of a solid-state image sensor in accordance with a second embodiment of the present invention; 
     FIG. 7B is a cross-sectional view, taken along line I-I′ of FIG. 7A, showing a structure of the solid-state image sensor in accordance with the second embodiment of the present invention; 
     FIG. 8 is a cross-sectional view of a solid-state image sensor in accordance with a third embodiment of the present invention; 
     FIG. 9 is a cross-sectional view of a solid-state image sensor in accordance with a fourth embodiment of the present invention; and 
     FIG. 10 shows a potential profile of a solid-state image sensor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 6A is a layout of a solid-state image sensor in accordance with the first embodiment of the present invention, and FIG. 6B is a cross-sectional view taken along line I-I′ of FIG.  6 A. As shown in FIGS. 6A and 6B, only one mask is used to form a p-well  23  for an HCCD and a VCCD and then a first impurity region  25  is formed in the p-well  23 . At this time, the first impurity region  25  is a first BCCD. Next, in the first impurity region  25 , there is formed a second impurity region  27 , which is a second BCCD. At this time, the second impurity region extends to a predetermined area of the first impurity region under the HCCD. 
     Referring to FIG. 6B, the aforementioned solid-state image sensor having a VCCD and an HCCD includes an n conductive type semiconductor substrate  21 , a p-well  23  formed in the semiconductor substrate  21 , a first impurity region  25  formed in the p-well  23  under the HCCD and the VCCD, and a second impurity region  27  formed in the first impurity region under the VCCD and extending to a predetermined area of the first impurity region under the HCCD. A channel stop impurity region  29  is formed in the p-well  23  under the HCCD. The first and second impurity regions  25  and  27  are first and second BCCDs, respectively. 
     In the solid-state image sensor of the first embodiment, the overlapping area of the first and second impurity regions  25  and  27  has a different concentration of the impurity ions from that of the first impurity region  25 . Thus, owing to the concentration difference, discontinuity is generated at the boundary between the overlapping area and the first impurity region  25 . Such a discontinuity is a factor hindering the transfer of signal charges. However, since the discontinuity is generated in the HCCD, it doesn&#39;t cause any problems in the transfer of signal charges. That is to say, since the potential well is formed in the same direction as charges are transferred, there is caused no problem in transferring signal charges along the potential well to a sensing amplifier SA. 
     Besides, since impurity regions have different concentrations of impurity ions under the VCCD and the HCCD, there is no problem in miniaturizing devices. 
     FIG. 7A is a layout of a solid-state image sensor in accordance with the second embodiment of the present invention and FIG. 7B is a cross-sectional view taken along line I-I′ of FIG.  7 A. 
     As shown in FIGS. 7A and 7B, discontinuity generated by concentration difference of impurity ions is caused in an HCCD in the solid-state image sensor of the second embodiment, like that of the first embodiment. That is to say, as shown in FIG. 7B, the solid-state image sensor having a VCCD and an HCCD includes an n conductive type semiconductor substrate  21 , a p-well  23  in the semiconductor substrate  21 , a first impurity region  25  formed in the p-well  23  under the VCCD and HCCD, and a second impurity region  27  formed in the first impurity region  25  under the HCCD, wherein the second impurity region  27  under the HCCD is spaced apart from the VCCD by a predetermined distance. 
     According to the second embodiment of the present invention, potential well becomes big where the second impurity region  27  under the HCCD is formed. At this time, the first and second impurity regions  25  and  27  are first and second BCCDs, respectively. 
     However, since the potential well is formed in the same direction as signal charges are transferred, there is no problem in transferring signal charges to a sensing amplifier SA. 
     Besides, since impurity regions have different concentrations of impurity ions under the VCCD and the HCCD, the solid-state image sensor of the present invention advantageously applies to miniaturized semiconductor devices and enough margins can be provided in design. 
     FIG. 8 is a cross-sectional view showing a structure of a solid-state image sensor in accordance with the third embodiment of the present invention. As shown in FIG. 8, a solid-state image sensor having a VCCD and an HCCD includes an n conductive type semiconductor substrate  21 , a first p-well  23  formed in the semiconductor substrate  21 , an impurity region formed in the first p-well  23  under the HCCD and the VCCD, and a second p-well  23   a  formed in the first p-well  25  under the HCCD, wherein the second p-well  23   a  is spaced apart from the VCCD by a predetermined distance. At this time, the impurity region  25  is a BCCD. In addition, a channel stop impurity region  29  is formed in the second p-well  23   a  at one side of the HCCD. 
     In accordance with the third embodiment of the present invention, like the first and second embodiments, the second p-well  23   a  is formed inside the first p-well  23  under the HCCD so that discontinuity area is formed under the HCCD. 
     As shown in FIG. 8, a potential barrier is formed under the second p-well  23   a.  Since the potential barrier is formed under the HCCD, there is caused no problem in transferring signal charges. 
     FIG. 9 is a cross-sectional view showing a structure of a solid-state image sensor in accordance with the fourth embodiment of the present invention, where two discontinuities are generated under an HCCD. As shown in FIG. 9, a solid-state image sensor having a VCCD and an HCCD includes an n conductive type semiconductor substrate  21 , a first p-well  23  formed in the semiconductor substrate  21 , a second p-well  23   a  formed in the first p-well  23  under the HCCD, a first impurity region  25  formed in the first p-well  23  inclusive of the second p-well  23   a,  and a second impurity region  27  formed in the first impurity region  25  under the VCCD and extending to a predetermined area of the first impurity region under the HCCD. A channel stop impurity region  29  is formed in the second p-well  23   a  at one side of the HCCD. 
     In the same way as the first, second, and third embodiments, since discontinuity is generated under the HCCD because of difference concentrations of impurity ions, it doesn&#39;t affect the transferring of signal charges. 
     FIG. 10 shows a potential profile in a three-dimension. As shown in FIG. 10, as described in the first, second, third, and fourth embodiments, impurity regions have different concentrations of impurity ions under a VCCD and an HCCD, and discontinuity generated due to concentration difference of impurity ions is formed under the HCCD, there is caused no problem in transferring signal charges into a sensing amplifier. 
     Such a solid-state image sensor of the present invention has the following advantages. First, since impurity regions have different concentrations of impurity ions under a VCCD and an HCCD, suitable tunings can be provided. Second, there is removed a fixed pattern noise which may be generated in a V-H interface area between a VCCD and an HCCD, thereby improving picture quality. Finally, charge-transferring efficiency is maximized. 
     It will be apparent to those skilled in the art that various modification and variations can be made in the solid-state image sensor of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.