Method of driving an interline transfer type CCD imager

Abstract: A method for driving an interline transfer type CCD imager by reading out a signal charge accumulated in one of first and second light receiving pixels to corresponding one of first and second accumulation regions. Part of the signal charge is transferred to the other one of the first and second accumulation regions which is disposed adjacently to the one of the accumulated regions. Another signal charge accumulated in the other one of the first and second light receiving pixels is read out so as to add the other signal charge to the part of the signal charge. Thereafter, the remaining part of the signal charge is transferred to the adjacent other one of the first and second accumulation regions.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates generally to a method of driving an interline transfer type charge-coupled device (CCD) imager. More particularly, the present invention relates to a method of driving an interline transfer type CCD imager which operates in a field accumulation mode. 2. Description of the Prior Art: In an interline transfer type CCD imager, signals are generally read out by a 2/1-interlaced reading system in which two fields constitute one frame. The accumulation of signals in each of plural light receiving pixels is conducted in either a frame accumulation mode in which one light accumulation period for each pixel coincides with one frame; or a field accumulation mode in which one light accumulation period coincides with one field. The field accumulation mode has a high dynamic resolution, as is described in "Proc. of Conf. on Charge Coupled Device Tech. and Appl., p. 152 (1976)", and is commonly used in movie cameras. Methods of driving a CCD imager in the field accumulation mode are generally classified as being a single transfer (ST) method or a double transfer (DT) method (see, for example, Technical Report of Television Engineering Society of Japan, TEBS87-3 (1983)). In the ST method, the process of reading signals from the light receiving pixels into a vertical transfer unit is conducted in one step after the light accumulation period of one field. In contrast, in the DT method, the process of reading signals from the light receiving pixels is conducted in two steps. In the ST method, as is graphically represented in Figures 5A and 5B , the respective signal charges S 1 and S 2 from all of the light receiving pixels PD are transferred to a vertical transfer unit V (Figure 5A ). Thereafter, the signal charges S 1 and S 2 of two vertically adjacent pixels are added in the vertical transfer unit V (Figure 5B ; see also, for example, Japanese patent publication JP-B-6240910). The addition in odd fields is conducted in a manner different from that in even fields, as is known, thereby complying with the interlaced reading system. In the DT method, as is graphically represented in Figures 6A and 6B , the signal charges S 1 of half of the light receiving pixels are transferred initially to the vertical transfer unit V (Figure 6A ). Typically, this half of the light receiving pixels consists of every other pixel along the vertical direction as shown in Figure 6A . Thereafter, the signal charges S 1 are transferred forward or backward in the vertical transfer unit V by a distance equal to one pixel pitch. The signal charges S 2 from the remaining light receiving pixels are subsequently read out and are added to the previously transferred signal charges S 1 (Figure 6B ). The transfer direction in odd fields along which signal charges S 1 are transferred by one pixel pitch is made different from that in even fields, thereby complying with the interlaced reading system. One particular drawback associated with the above-described ST method is that signal charges of all of the light receiving pixels must first be retained in the vertical transfer unit V without mixing the signal charges. This often causes a decrease in the maximum transferable amount of signal charges. Problems can arise, for example, when a vertical transfer unit V is operated by a four-phase driving system. The signal charges transferred from one pixel must be held under one electrode in the vertical transfer unit V and, therefore, the signal charges are strongly affected by a fringe electric field as compared to another method in which the signal charges transferred from one pixel are held under two or more electrodes. Hence, the amount of signal charges transferred from each pixel in the ST method typically will be less than half of that in another method. As a result, even after signal charges transferred from two pixels are added according to the ST method, the amount of the resulting signal charges will not reach the amount of signal charges as compared to another method. As the difference in the available amounts of signal charges to be added in the ST method becomes greater, the degree of the reduction in signal charges as compared to other methods becomes even larger as will be appreciated. Using the DT method, typically the maximum transferable amount of signal charges is not a problem, but signal charges in the vertical transfer unit V must be transferred in reverse directions depending on whether it is in relation to an odd or even field. Such reverse charge transfer of signal charges often causes a problem of erroneously transferring signal charges. Such problem does not exist in a forward charge transfer. For example, in a situation where there is a small potential dip in one portion of the vertical transfer unit V , forward charge transfers cause this potential dip to be filled always with a signal of the former stage, so as to prevent the potential dip from appearing externally. If, however, a reverse charge transfer is first conducted, the signal charge in the potential dip is swept away. This swept amount of signal charge from the potential dip is recovered by a signal of the next stage, resulting in an erroneous charge transfer which adversely affects the quality of output signals. The applicant in this application has developed an imager which can eliminate the above-mentioned problems of the ST and DT methods (Japanese patent application JP-A-1298973). In such an improved imager (represented in part by Figures 7A and 7B ), the potential distribution in a vertical transfer unit V is controlled such that the signal charges of two vertically adjacent pixels will be added. Then, signal charges of all light receiving pixels are read out sequentially or simultaneously to be added. As a result, the above-described problems are eliminated, i.e., the problems of a reduced maximum transferable amount of signal charges, and of erroneous charge transfer caused by a reverse charge transfer. However, when signal charges of the light receiving pixels PD are read out, most of the electrodes of the vertical transfer unit V (in the four-phase driving system, for example, three electrodes) are set to a high potential. Hence, the potentials of light receiving pixels become unstable. Therefore, the improved imager has a drawback that, when reading out signal charges of light receiving pixels, the charge transfer may erroneously function. An object of the present invention is to provide a method of driving an interline transfer type CCD imager by which the maximum transferable amount of signal charges in the field accumulation mode can be increased, and the above-described problems of an erroneous charge transfer due to a reverse charge transfer or the reading of signal charges from light receiving pixels can be eliminated. SUMMARY OF THE INVENTION In one aspect, the present invention provides a method, defined by claim 1, for driving an interline transfer type CCD imager, and an imaging apparatus, defined by claim 8, including an interline transfer type CCD imager operated according to that method. In another aspect, the present invention provides a method, defined by claim 6, for transferring signal charges from a plurality of charged elements in an interline transfer type CCD imager. According to the present invention, the maximum transferable amount of signal charges in the field accumulation mode can be increased. Furthermore, it is possible to stabilize the potential of a light receiving pixel in the reading operation. Therefore, an erroneous charge transfer in a reverse charge transfer or when reading signal charges from the light receiving pixels can be eliminated. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of an interline transfer type CCD imager to be driven in accordance with the present invention; Figures 2(a) - 2(h), 2(b′), 2(f′) and 2(g′) diagrammatically illustrate the time-dependent variations of the potential distributions for an odd field along a forward direction of a vertical transfer unit in accordance with the present invention; Figures 3(a) - 3(h) diagrammatically illustrate the time-dependent variations of the potential distributions for an even field along a forward direction of a vertical transfer unit in accordance with the present invention; Figures 4(a) and 4(b), respectively, are timing charts representing the driving pulses  V1 to  V4 in odd and even fields in accordance with the present invention; Figures 5A and 5B illustrate a conventional single transfer method for driving a CCD imager in the field accumulation mode; Figures 6A and 6B illustrate a conventional double transfer method for driving a CCD imager in the field accumulation mode; and Figures 7A and 7B illustrate schematically an improved imager. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described with reference to the drawings wherein like reference labels are used to identify like elements throughout. Figure 1 shows an exemplary interline transfer type CCD imager 10 which is driven by the method of the present invention. The interline transfer type CCD imager 10 includes photodiodes PD ij used as light receiving pixels which are arranged in a matrix form consisting of plural rows and columns. Between columns of the photodiodes PD ij , corresponding columns of electrode groups are formed. More specifically, each of the columns of electrode groups constitutes a vertical transfer unit VT , and each of the electrode groups consists of four electrodes VT 1 , VT 2 , VT 3 and VT 4 as is illustrated. The lower end of each of the electrode group columns shown in Figure 1 is connected to a horizontal transfer unit (not shown). Four driving pulses  V1 ,  V2 ,  V3 and  V4 are respectively applied to the four electrodes VT 1 , VT 2 , VT 3 and VT 4 of each electrode group, so that the imager 10 is driven in a four-phase driving system. The driving pulses  V1 -  V4 are generated by a vertical transfer driving pulse generation circuit VG. A signal charge accumulated in each of the photodiodes PD ij is read out to a transfer region TG of the vertical transfer unit VT , such transfer region TG being common to either the electrodes VT 1 and VT 3 . Namely, signal charges of odd-numbered photodiodes PD ij (e.g., i is odd) of each column are read out through a transfer region TG which extends from the corresponding electrodes VT 1 . Signal charges of even-numbered photodiodes PD ij (e.g., i is even) in each column are read out through another transfer region TG which extends from the corresponding electrodes VT 3 . Therefore, the regions of the electrodes VT 1 and VT 2 as referred to herein correspond to a first accumulation region, and the regions of the electrodes VT 3 and VT 4 correspond to a second accumulation region. In Figure 1 , the transfer regions TG are drawn hatched. The manner of driving the imager 10 will be described with reference to Figures 2(a) to 4(b) . Figures 2(a) - 2(h) and 3(a) - 3(h) , respectively, show the time-dependent variations of the potential distributions along the forward direction of the vertical transfer unit VT (i.e., from the electrode VT 1 to the electrode VT 4 ) in odd and even fields. Figures 2(b′) , 2(f′) ) and 2(g′) show the related time-dependent potential distributions along horizontal lines A-A and B-B (Figure 1 ) which extend along the photodiode PD ij , the transfer region TG , and the vertical transfer unit VT . Figures 4(a) and 4(b) are timing charts representing the driving pulses  V1 to  V4 in odd and even fields, respectively. In the exemplary embodiment described with respect to the Figures, the signal charges are electrons. In an odd field, at time t 1 as is shown in Figure 4(a) , the second and fourth driving pulses  V2 and  V4 , which are respectively supplied to the electrodes VT 2 and VT 4 in each vertical transfer unit VT , are set to a low level V L . The first and third driving pulses  V1 and  V3 , which are respectively supplied to the electrodes VT 1 and VT 3 in each vertical transfer unit VT , are set to a first intermediate level V I13 , resulting in the potential distribution shown in Figure 2(a) . At time t 2 , the first driving pulse  V1 is set to a high level V H (Figure 4(a) ). As a result, the potential of the region under the electrode VT 1 is made deeper so that a signal charge Q 1 accumulated in the photodiode PD ij of an odd-numbered line (i.e., where i is an odd number) is transferred through the transfer region TG to the region under the electrode VT 1 , as is illustrated in Figures 2(b) and 2(b′) . Hereinafter, the regions under the electrodes VT 1 to VT 4 are referred to simply as "the regions VT 1 to VT 4 ", respectively. With such charge transfer, since the potentials of the regions VT 4 and VT 2 adjacent to the region VT 1 into which the signal charge Q 1 is transferred are maintained at the low level V L , the potential of the photodiode PD ij will be stable. Then, while maintaining the first driving pulse  V1 at the high level V H , the second driving pulse  V2 is raised at time t 3 from the low level V L to a second intermediate level V I24 (where V I24 ≧ V I13 ) as is shown in Figure 2(c) . In addition, the first driving pulse  V1 is lowered at time t 4 from the high level V H to the first intermediate level V I13 as is represented in Figure 2(d) . Preferably, the second intermediate level V I24 is higher by several volts than the first intermediate level V I13 . At time t 5 , the second driving pulse  V2 is lowered from the second intermediate level V I24 to the low level V L as is shown in Figure 2(e) . As a result, a divisional signal charge Δ Q 1a of the signal charge Q 1 is transferred to the region VT 1 , and the remaining divisional signal charge Δ Q 1b is transferred to the region VT 3 . The result is that the potential distribution becomes as shown in Figure 2(e) where Δ Q 1a + Δ Q 1b = Q 1 . According to the above-described series of operations, when the signal charge Q 1 , which has been read out into a deep potential region, is returned to a potential state for a normal transfer, the signal charge Q 1 is surely held by a plurality of electrode regions (i.e., regions VT 1 and VT 3 as shown in Figure 2(e) ). Even if an excess signal charge enters one region of the vertical transfer unit, therefore, the signal charge is prevented from overflowing into adjacent regions. Thereafter, the third driving pulse  V3 is raised from the first intermediate level V I13 to the high level V H at time t 6 . This causes the potential of the region VT 3 to become deeper as illustrated in Figure 2(f) so that a signal charge Q 2 accumulated in the photodiode PD ij of an even-numbered line (i.e., in this case, where i is an even number) is transferred through the transfer region TG to the region VT 3 , and is added to the divisional signal charge Δ Q 1b which exists in the region VT 3 . Since the potentials of the regions VT 2 and VT 4 adjacent to the region VT 3 are maintained at the low level V L , the potential of the photodiode PD ij is stable. Then, while maintaining the third driving pulse  V3 at the high level V H , the second driving pulse  V2 is raised at time t 7 from the low level V L to the second intermediate level V I24 as is shown in Figure 2(g) . As a result, the divisional signal charge Δ Q 1a in the region VT 1 is transferred to the region VT 3 and is added to the total signal charge Δ Q 1b + Q 2 which exists in the region VT 3 such that Δ Q 1b + Q 2 + Δ Q 1a = Q 1 + Q 2 . Therefore, the signal charges in the region VT 3 amount to the total of the signal charges Q 1 and Q 2 (i.e., Q 1 + Q 2 ). When this total signal charge Q 1 + Q 2 is large, a portion thereof is held in the region VT 2 . The amount of the total signal charge Q 1 + Q 2 held in the region VT 3 (and the region VT 2 ) is restricted to the upper limit at which the signal charge read out from the photodiode PD ij is not caused to flow back to that photodiode. As shown in Figure 2(g) , the second intermediate level V I24 is set to a slightly higher level, so that the potential of the region VT 2 is made deeper than that of the transfer region TG of the region VT 3 by a margin Δ  m . Since a large amount of signal charges is transferred to the region VT 2 before a portion of the total signal charge flows back to the photodiode, the upper limit of the total signal charge Q 1 + Q 2 is sufficiently large. Finally, the third driving pulse  V3 is lowered at time t 8 from the high level V H to the first intermediate level V I13 as illustrated in Figure 2(h) , whereby the total signal charge Q 1 + Q 2 is distributed to the three regions VT 1 , VT 2 and VT 3 and is held therein. Thereafter, the signal transfer is continued to be conducted in a usual manner. In an even field, the imager 10 is driven in the same manner as in an odd field except that the function of the region VT 2 is replaced with that of the region VT 4 , as is illustrated in Figures 3(a) - 3(h) and 4(b) . The vertical transfer driving pulse generation circuit VG generates the driving pulses  V1 -  V4 on the basis of a clock signal, a horizontal synchronizing signal and a vertical synchronizing signal. The vertical transfer driving pulse generation circuit VG can be easily configured by using usual logic circuits or the like, and therefore its detailed description is omitted. The above embodiment in which the second intermediate level V I24 is set to be higher than the first intermediate level V I13 (V I24 ≧ V I13 ) can achieve another effect that the maximum transferable amount of signal charges in a normal transfer which is conducted after time t 8 can be increased. Furthermore, if the imager 10 is constructed so that the electrodes VT 2 and VT 4 are formed as lower electrodes and the electrodes VT 1 and VT 3 as upper electrodes, the effect that, when a signal charge of a photodiode PD ij is read out to a vertical transfer unit VT , the potential variation of this photodiode P ij is suppressed is more pronounced, with the result that the present invention becomes further effective. The configuration of an imager 10 to which the present invention can be applicable is not restricted to that which is described above, but the present invention can also be applicable to an interline transfer type CCD imager having another configuration. For example, the transfer region TG may be constructed in the form of an independent gate. The driving system of vertical transfer unit VT is not restricted to the four-phase driving system, and three or more electrodes may be formed per one pixel as will be appreciated. The invention may be embodied in other specific forms and manners without departure from its scope as defined by the claims.

Claims: A method of driving an interline transfer type CCD imager (10) which comprises a plurality of light receiving pixels (PD ij ) arranged in a matrix form and a plurality of vertical transfer units (VT1-VT4) respectively disposed between columns of the light receiving pixels, each of the columns of light receiving pixels having a plurality of first light receiving pixels (PD ij , i = odd) and a plurality of second light receiving pixels (PD ij , i = even), the first and second light receiving pixels being alternately arranged in each column of light receiving pixels, each of the columns of the vertical transfer units having a plurality of first accumulation regions (VT1,VT2) respectively corresponding to the first light receiving pixels and a plurality of second accumulation regions (VT3,VT4) respectively corresponding to the second light receiving pixels, the first and second accumulation regions being alternately arranged in each column of vertical transfer units, the method comprising the steps of: reading out a signal charge (Q 1 ) accumulated in one (PD 11 ) of the first and second light receiving pixels to the corresponding one of the first and second accumulation regions (VT1,VT2), transferring part (ΔQ 1b ) of the signal charge (Q 1 ) accumulated in the one of the first and second accumulation regions (VT1,VT2) to the other one of the first and second accumulation regions (VT3,VT4) which is disposed adjacently on a prescribed side of the one of the accumulation regions, reading out another signal charge (Q 2 ) accumulated in the other one PD 21 ) of the first and second light receiving pixels to the corresponding other one of the first and second accumulation regions (VT3, VT4), so as to add the other signal charge (Q 2 ) to the part (ΔQ 1b ) of the signal charge which is accumulated in the other accumulation region (VT3, VT4), and transferring the remaining part (ΔQ 1a ) of the signal charge which is accumulated in the one accumulation region (VT1, VT2) to the adjacent other one of the first and second accumulation regions (VT3,VT4), so as to add the remaining part (ΔQ 1a ) of the signal charge to the added signal charge (ΔQ 1b +Q 2 ) which is accumulated in the other accumulation region (VT3, VT4). The method of claim 1, wherein said signal charges (Q 1 , Q 2 ) consist of electrons. The method of claim 1, further comprising the step of generating a plurality of driving pulses ( v1 - v4 ) for effecting said reading steps and said transferring steps. The method of claim 3, wherein said imager (10) is driven according to a multi-phase driving system. The method of claim 4, wherein said multi-phase driving system is a four-phase driving system. A method of transferring signal charges (Q 1 ,Q 2 ) from a plurality of charged elements (PD ij ) in an interline transfer type CCD imager (10), the method comprising the steps of: reading out one of said signal charges (Q 1 ) from a first (PD 11 ) of said charged elements to a first accumulation region (VT1, VT2); transferring part (ΔQ 1b ) of said one of said signal charges (Q 1 ) from said first accumulation region (VT1, VT2) to a second accumulation region (VT3, VT4); reading out another of said signal charges (Q 2 ) from a second (PD 21 ) of said charged elements to said second accumulation region (VT3, VT4) so as to add said other of said signal charges (Q 2 ) to said part (ΔQ 1b ) of said one of said signal charges (Q 1 ); and transferring the remaining part (ΔQ 1a ) of said one of said signal charges (Q 1 ) which is accumulated in said first accumulation region (VT1, VT2) to said second accumulation region (VT3, VT4) so as to add said remaining part (ΔQ 1a ) of said one of said signal charges to said added signal charge (ΔQ 1b + Q 2 ) in said second accumulation region (VT3, VT4). The method of claim 6, further comprising the steps of generating a plurality of driving pulses ( v1 -  v4 ) for effecting said reading steps and said transferring steps. An imaging apparatus comprising an interline transfer type CCD imager (10) which includes: a plurality of light receiving pixels (PD ij ) arranged in a matrix form and a plurality of vertical transfer units (VT1 - VT4) respectively disposed between columns of the light receiving pixels, each of the columns of light receiving pixels having a plurality of first light receiving pixels (PD ij , i = odd) and a plurality of second light receiving pixels (PD ij , i = even), the first and second light receiving pixels being alternately arranged in each column of light receiving pixels, each of the columns of the vertical transfer units having a plurality of first accumulation regions (VT1, VT2) respectively corresponding to the first light receiving pixels and a plurality of second accumulation regions (VT3, VT4) respectively corresponding to the second light receiving pixels, the first and second accumulation regions being alternately arranged in each column of vertical transfer units, and a control device (VG) arranged to control the operation of said interline transfer type CCD imager (10), the control device comprising: means for reading out a signal charge (Q 1 ) accumulated in one (PD 11 ) of the first and second light receiving pixels to the corresponding one of the first and second accumulation regions (VT1, VT2), means for transferring part (ΔQ 1b ) of the signal charge (Q 1 ) accumulated in the one of the first and second accumulation regions (VT1, VT2) to the other one of the first and second accumulation regions (VT3, VT4) which is disposed adjacently on a prescribed side of the one of the accumulation regions, means for reading out another signal charge (Q 2 ) accumulated in the other one (PD 21 ) of the first and second light receiving pixels to the corresponding other one of the first and second accumulation regions (VT3, VT4), so as to add the other signal charge (Q 2 ) to the part (ΔQ 1b ) of the signal charge which is accumulated in the other accumulation region (VT3,VT4), and means for transferring the remaining part (ΔQ 1a ) of the signal charge which is accumulated in the one accumulation region (VT1, VT2) to the adjacent other one of the first and second accumulation regions (VT3,VT4), so as to add said remaining part (ΔQ 1a ) of the signal charge to the added signal charge (ΔQ 1b + Q 2 ) which is accumulated in the other accumulation region (VT3, VT4).

Inventors: WATANABE TAKASHI, WATANABE, TAKASHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N3
IPC codes: H04N5, H01L27, H04N3