1. Field of the Invention
The present invention relates to an improvement of a solid state image pickup apparatus having a photoconductive film as a photoelectric converting section formed on a CCD scan substrate.
2. Description of the Related Art
The essential section of a monolithic type CCD image pickup apparatus generally has the structure as shown in FIG. 1. Numeral 3 is an n-type buried channel on which transfer gates 6 and 7 are formed respectively through gate insulation films 5.sub.1 and 5.sub.2, thus constituting a vertical CCD. A photodiode 23 serving as a photoelectric converting section for the individual pixels is disposed adjacent to the vertical CCD. Numeral 2 is a p-type channel stopper. The pattern for one pixel portion of this CCD image pickup apparatus is as illustrated in FIGS. 2A-2C. FIG. 2A illustrates a signal charge transfer section 1-a between photodiode 23 and buried channel 3 as a shaded region, FIG. 2B buried channel 3 as a shaded region, and FIG. 2C an opening 18 of the photoelectric converting section constituted by photodiode 23, as a shaded region. As should be understood from these diagrams, since the monolithic type CCD image pickup apparatus has both of buried channel 3 and photodiode 23 formed on the same semiconductor substrate, the area of opening 18 is small. To reduce the lateral size of a pixel cell, therefore, the width of buried channel 3 decreases and so does the area of photodiode 23, as shown in FIGS. 3A-3C. This results in reduction in sensitivity of the image pickup apparatus and reduction in signal charge transfer capacity.
In contrast, a solid state image pickup apparatus having a photoconductive film formed on a CCD scan substrate has a signal charge transfer section separated from a photoelectric converting section, so that it is effective in realizing an image pickup apparatus with a high pixel density.
FIG. 4 illustrates the structure of the essential section of such a laminated type CCD image pickup apparatus. N-type buried channel 3 is formed on a p-type Si substrate 1 and on the channel 3 are transfer gate electrodes 6 and 7 formed respectively through gate insulation films 5.sub.1 and 5.sub.2, thus constituting a vertical CCD. An n-type source layer 4 for reading out a signal charge of each pixel in buried channel 3 is disposed adjacent to the channel 3 of the vertical CCD. Since n-type source layer 4 is not used as photodiode 23 serving as the photoelectric converting section, it need not have a large area. Such a CCD scan substrate is covered with a first insulation film 8 in which a contact hole 9 is formed, and a relay electrode 11 that has a contact with each source layer 4 is formed in the hole 9. The resultant structure is covered with a second insulation film 12 in which another contact hole 13 is formed and a pixel electrode 14 is formed therein. Then, an undoped a-Si:H film 15 serving as a photoconductive film is disposed on the resultant structure, and a p-type a-SiC:H film 16 serving as a hole preventing layer is further disposed on the film 15, with a transparent electrode 17 being formed on the film 16.
FIGS. 5A-5C illustrate patterns of a unit pixel region of this laminated type CCD image pickup apparatus in correspondence with FIGS. 2A-2C. The area of an opening 19 (see FIGS. 5C, 6C and 7C) determined by pixel electrode 14 is significantly larger than that of a monolithic type structure. With the illustrated structure, since photoconductive film 15 performs photoelectric conversion, that part which is other than the vertical CCD section for transferring a signal charge on the CCD scan substrate can have a small area. This can therefore permit the CCD image pickup apparatus to have such a layout that the area of buried channel 3 is made as large as possible by reducing the area of channel stopper 2, as shown in FIGS. 6A-6C. Accordingly, it is easy to increase the amount of signal charges transferred to 1.5 to 2 times as compared with a monolithic type CCD image pickup apparatus.
In such a laminated type CCD image pickup apparatus, however, reducing the lateral pixel size as shown in FIGS. 7A-7C considerably changes the width of buried channel 3, which adversely influences to signal charge transfer for the following detailed reason.
FIG. 8 illustrates the results of measuring the channel potential (with zero gate voltage) with a change in channel width on a mask of a buried channel MOS transistor. FIG. 9 illustrates the structure of the MOS transistor, with the same numerals as used in FIGS. 1-4 denoting the corresponding sections. As should be obvious from FIG. 8, the channel potential of the buried channel becomes lower with a decrease in the channel width, and it can be hardly called a buried channel in a region with a channel width of less than 1 .mu.m. This is the so-called narrow channel effect. The measuring results show that, in the laminated CCD with a buried channel as shown in FIGS. 7A-7C, the channel potential at a region having a narrow channel width is low. With the transfer electrodes being arranged as shown in, for example, FIG. 10, the potential distribution at the C--C' cross section would be as illustrated in FIG. 11. More specifically, when the charges are transferred from a second layer transfer gate electrode 7-a to a region under a first layer transfer gate electrode 6, the distribution is as indicated by the one-dot chain line, and when charge transfer is from the electrode 6 to a region under a second transfer gate electrode 7-b, the distribution is as indicated by the broken line. In this state, a potential barrier leaves out some signal charge and the proper image reproduction cannot be provided. With the electrodes arranged as shown in FIG. 12, the potential distribution would be as illustrated in FIG. 13. In this case, incomplete charge transfer also occurs. To prevent such incomplete charge transfer, it is necessary to align the edge portions of first layer transfer gate electrode 6 with the boundaries at which the channel width varies, as shown in FIG. 14. This produces the potential distribution as shown in FIG. 15, which can prevent the aforementioned incomplete charge transfer. With such an electrode arrangement, however, the reverse charge transfer becomes impossible as per the case of two-phase driving structure in which a potential step is formed in one transfer gate electrode. For instance, an overflow drain is provided at the opposite edge of the vertical CCD to the horizontal CCD side, thus making the sweeping of the unnecessary charges impossible by the transfer function in the opposite direction to the transfer direction for reading signal charges.
As described above, according to the conventional laminated type CCD image pickup apparatuses, the narrow channel effect becomes prominent with a reduction in pixel cell size, and, particularly, a channel potential step is formed in one transfer gate electrode to thereby prevent bi-directional charge transfer.