Electronic shutter controlling method for a two-dimensional charge-coupled device

Abstract: A controlling method for a charge-coupled device decreases the amount of superfluous electric charges by applying a photo shield film (46) over photo sensing elements (31) on a semiconductor substrate (30) with a certain voltage level (-5 volts or +25 volts), then accumulating effectual electric charges in the presence of an light radiation into the photo sensing elements (37) in applying a different voltage level (+5 volts or + 9 volts) to the photo shield film (46) for promoting the production of the effectual electric charges while the re­ duced superfluous electric charges are swept into the overflow drain (34), then transferring the effectual electric charges to the multiple-stage horizontal shift register (33).

Description: FIELD OF THE INVENTION This invention relates to a charge-coupled device (which is abbreviated as "CCD") and, more particularly, to a controlling method for a two dimensional charge coupled device of the interlined transfer type. DESCRIPTION OF THE RELATED ART A typical charge-coupled device of the interlined type largely comprises a plurality of photo-sensitive elements such as photo-diodes arranged in rows and columns, vertical shift registers interlined with adjacent two columns of the photo-sensitive elements, and a horizontal shift register coupled at one end to the vertical shift registers. When an optical image impinges on the photo-sensitive elements, the photo-sensitive elements respectively produces electric charges the amounts of which are depending on the intensities of the parts of the optical image. The electric charges are transferred from the photo-sensitive elements to the vertical shift registers which in turn transfer the electric charges to the horizontal shift register. The electric charges produced by the photo-sensitive elements are successively transferred from the horizontal shift register to the outside thereof during a time period between two vertical blanking time periods. According to the NTSC (National Television System Committee) color system, an effective accumulation time period is selected to be a sixtieth second for converting an optical image into the electric charges, so that the charge-coupled device is capable of reproduce the optical image without blurring in so far as the optical image is provided from the static or a low speed moving object. However, a distinctive image is hardly reproduced from an optical image from a high-speed moving object due to the relatively long effective accumulation time period. In order to provide a solution of the problem described above, an "electronic shutter" is proposed so as to shorten the effective accumulation time period. The electronic shutter is described by Hiromi Okumura in "TECHNICAL REPORT, NATIONAL CCD SOLID IMAGER", Video alpha published by Photo Industry Publishing Company, August 1987, pages 145 to 149. For better understanding of the present invention, the technical report is summarized hereinbelow with Figs. 1 and 2. Referring to Fig. 1, the charge-coupled device largely comprises photo diodes 1 arranged in rows and columns, vertical shift registers 2 interlined with the adjacent two columns of the photo diodes, a horizontal shift register 3 coupled to the vertical shift registers 2, and a drain provided on the opposite side to the horizontal shift register 3. Each of the photo diodes 1 is represented by a small rectangle. In an ordinary charge-coupled device, a series of vertical driving pulses 4 takes place at a certain interval, and all of the electric charges are transferred to the outside thereof in a driving period between two vertical blanking time periods 6 as shown in Fig. 2. However, the charge-coupled device reported by Okumura intermittently produces two read-out pulses 7 and 8 in a single vertical blanking time period 6, and reverse transferring pulses 9 are further produced between the two read-out pulses 7 and 8. When the first read-out pulse 7 takes place, ineffectual electric charges are discharged from the photo diodes 1 to the vertical shift registers 2, and the ineffectual electric charges are reversely swept into the drain 4 through the vertical shift registers 2 in response to the reverse transferring pulses 9. While sweeping the ineffectual electric charges into the drain 4, the photo diodes 1 accumulate fresh or effectual electric charges produced in the presence of an optical image. The electric charges thus accumulated in the photo diodes 1 are discharged to the vertical shift registers 2 in response to the second read-out pulse 8, and are successively transferred to the outside thereof in cooperation with the horizontal shift register 3. The vertical driving pulses 10 are produced in a driving time period between the two vertical blanking time periods 6 at a certain time interval. The photo diodes 1 are exposed to the optical image for an effective accumulation time period t1 between the two read-out pulses 7 and 8, and the shutter speed is then calculated from the effective accumulation time period t1. In this instance, the effective accumulation time period t1 is about a thousandth second. The effective accumulation time period t1 is thus shrunk to the small value, the charge-coupled device can reproduce an optical image from a high-speed moving object. All of the above behaviors should be completed in the vertical blanking time period 6, and, for this reason, the charge-coupled device illustrated in Fig. 1 hardly increases the effective accumulation time period to a value greater than a thousandth second. A charge-coupled device illustrated in Fig. 3 is proposed and aims at extension of the effective accumulation time period t1. The charge-coupled device shown in Fig. 3 largely comprises an interlined sensing-and-transferring stage 21, a horizontal shift register 22, an accumulating stage 23 provided between the interlined sensing-and-transferring stage 21 and the horizontal shift register 22, and a drain 24 provided on the opposite side of the accumulating stage 23. The charge-coupled device thus arranged completes the delivery of the ineffectual electric charges in the previous driving period, and only the reverse transferring pulses 9 and the second read-out pulse 8 take place in the vertical blanking time period 6. For this reason, the effective accumulation time period t1 is prolonged to a two hundreds and fiftieth second. In a charge-coupled device with a high speed shutter, the ineffective accumulation time period is four times to sixteen times longer than the effective accumulation time period t1, and, accordingly, the ineffectual electric charges are drastically increased to a saturation level which serves as a criterion of production of the blooming phenomenon. The saturation level is related to a blooming control voltage level. A problem is encountered in the prior art charge-coupled device with the electronic shutter in that the electronic charges are liable to remain in the photo-sensitive elements after the sweeping operation of the ineffectual electronic charges. In detail, since the sweeping operation as well as the reading-out operation to the vertical shift registers should be completed in the vertical blanking time period, the ineffectual electric charges are transferred at about 800 KHz higher than the ordinary vertical transferring operation of the effective electric charges. The electrodes of the vertical shift registers 2 are relatively large in resistance and in capacitance due to double or tipple structure of polysilicon, and, for this reason, the driving pulses are delayed and deformed at the high frequency transferring operation. Thus, the driving pulses are delayed and deformed, so that the vertical shift registers deteriorate in the charge transferring capacity. In other words, the amount of the ineffectual electric charges transferred to the drain is decreased to a half or a third, leaving residual charges in the photo-sensitive elements. These residual charges are causative of electric signals over the saturation level, and the electric signals produces a white small jamming image. If the relaying operation from stage to stage repeats certain times greater than the number of the stages of each vertical shift register, more ineffectual electric charges are swept into the drain, however, the length of the vertical blanking time period t1 sets a limitation on the number of the relaying operations for the sweeping operation. This results in insufficient sweeping operation. The wider screen the charge-coupled device is associated, the more residual charges the charge-coupled device holds. If the blooming control voltage is regulated to a high level, there is a trade-off between the dynamic range and the amount of the residual charges. A charge coupled device according to the pre-characterizing part of the claim and a corresponding control method is disclosed in IEEE Transaction on Electron Devices, vol. ED-31, No. 1, January 1984, pages 83-88, by Y. Ishihara et al. Here a constant, probably selected voltage is supplied to the substrate in order to reduce blooming effects. EP-A-0 265 271 discloses a similar solid state imaging device, wherein the voltage supplied to the substrate can be switched between two values for reducing the amount of "ineffectual" electric charges in the photo-sensing elements. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a charge-coupled device which produces an image signal less affected by the residual charges. It is another important object of the present invention to provide a charge-coupled device in which the residual charges are sufficiently swept into the drain. To accomplish these objects, the present invention proposes to make the photo sensing elements less sensitive or less retainable in the ineffective accumulation time period. In accordance with one aspect of the present invention, there is provided a controlling method for a charge-coupled device fabricated on a semiconductor substrate and comprising a plurality of photo sensing elements arranged in columns and operative to convert an optical radiation into electric charges, respectively, a multiple-stage horizontal shift register, an overflow drain, a plurality of multiple-stage vertical shift registers interlined with said columns of said photo sensing elements and operative to transfer said electric charges to one of said multiple-stage horizontal shift register and said overflow drain, and a photo shield film provided over said photo sensing elements in such a manner as to allow said optical radiation to impinge on the photo sensing elements, said controlling method being characterized by the steps of a) accumulating effectual electric charges in the presence of said optical radiation into said photo sensing elements by applying a first voltage level to said photo shield plate for promoting the production of said effectual electric charges while ineffectual electric charges are swept into said overflow drain through said multiple-stage vertical shift registers in synchronism with a first driving pulse train after said ineffectual electric charges are read out from said plurality of photo sensing elements to said plurality of multiple-stage vertical shift registers in the presence of a first read-out pulse, b) reading out said effectual electric charges in the presence of a second read-out pulse, and c) transferrring said effectual electric charges from said plurality of multiple-stage vertical shift registers in synchronism with a second driving pulse train to said multiple-stage horizontal shift register in applying a second voltage level to said photo shield plate for restricting the production of said ineffectual electric charges in said photo sensing elements. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of a controlling method for a two dimensional charge-coupled device according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a block diagram showing the arrangement of a prior art charge-coupled device; Fig. 2 is a diagram showing the waveforms of essential signals in a composite manner applied to the prior art charge-coupled device; Fig. 3 is a block diagram showing the arrangement of another prior art charge-coupled device proposed for a solution of the problem inherent in the charge-coupled device shown in Fig. 1; Fig. 4 is a block diagram showing the arrangement of a two-dimensional charge-coupled device to which the present invention appertains; Fig. 5 is a cross sectional view showing the structure of a photo-sensitive element incorporated in the charge-coupled device illustrated in Fig. 4; Fig. 6 is a graph showing the saturated output voltage level in terms of the voltage level at a photo shield plate incorporated in the charge-coupled device shown in Fig. 5; Fig. 7 is a graph showing the saturated output voltage level in terms of the voltage level at the substrate where the charge-coupled device shown in Fig. 5 is fabricated; Fig. 8 is a diagram showing the waveforms of a set of essential signals applied to the charge-coupled device shown in Fig. 4; and DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 4 of the drawings, a two-dimensional charge-coupled device is fabricated on an n-type single semiconductor substrate 30 and largely comprises a plurality of photo-sensing elements 31 each corresponding to a pixel and arranged in rows and columns, a plurality of multiple-stage vertical shift registers 32 each interlined with adjacent two columns of the photo-sensing elements 31, a multiple-stage horizontal shift register 33 coupled to first output nodes of the vertical shift registers 32, and an overflow drain 34 coupled to second output nodes of the vertical shift registers 32 and provided at the opposite ends to the horizontal shift register 33. The two-dimensional charge-coupled device thus arranged is classified into the interlined type with the vertical overflow drain structure. Turning to Fig. 5, each of the photo-sensing elements 31 is formed by a photo diode, and the vertical shift register 32 is of the vertical CCD register type. Although the plural photo diodes and the plural vertical shift registers are fabricated on the semiconductor substrate 30, description is focused upon a single photo diode and a single vertical shift register. In the n-type semiconductor substrate is formed a p-type well 35 where a plurality of n-type regions 36, 37, 38 and 39 are formed and isolated by p-type impurity regions 40, 41 and 42. The p-type well 35 is reversely biased with respect to the n-type substrate 30, and the reverse biasing voltage level is variable by means of a controller 60. The major surface of the substrate 31 is covered with an oxide film 43 which is transparent to an optical radiation. The optical radiation carries an optical image from an object. The oxide film 43 is partially covered with conductive strips 44 serving as electrodes of the vertical shift register 32, and the entire surface is overlain by an oxide film 45 which is also transparent to the optical radiation. On the oxide film 45 is provided a photo-shield film 46 which is supplied with an appropriate positive biasing voltage level. The n-type region 37 and the p-type well 35 beneath the region 37 form in combination the photo diode, and the p-type impurity region 41 and the gate electrode 45 provide an input gate of the vertical shift register 32 bridging between the n-type region 37 and one of the n-type regions 38. The n-type regions 38 (only one of which is shown in the cross sectional view) are spaced apart from one another, and are respectively associated with the conductive strips or the gate electrodes 44 to form a plurality of transferring stages. The p-type impurity region 42 serves as a channel stopper between the vertical shift register 32 and the adjacent photo diode. The p-type impurity region 41, the n-type regions 38, and the gate electrodes 44 form the vertical shift register 32. Each of the vertical shift registers 32 has 250 gate electrodes 44 provided in parallel to one another, and, accordingly, is provided with 250 transferring stages. Each transferring stage performs a single transferring operation with a vertical driving pulse SHFT , and the amount of the maximum electric charge transferred is tantamount to the output voltage level of about 0.9 volt at15.75KHz. The biasing voltage level directly affects the amount of the maximum electric charge produced by the photo diode forming each of the photo sensing elements 31, and is varied as the function indicated by plots A in Fig. 6. This is because of the fact that the lines of electric forces from the photo shield film 46 reach the surface of the photo diode and, accordingly, vary the voltage level of the surface. The amount of the maximum electric charge affected by the biasing voltage level is also capable of being represented by an output voltage level delivered from the horizontal shift register of the charge-coupled device to the outside thereof. The output voltage level thus representing the amount of the maximum electric charge produced by each photo diode is hereinbelow referred to as "saturated output voltage level". The saturated output voltage level of the photo diode is further varied as the function indicated by plots B of Fig. 7 with respect to the voltage level at the semiconductor substrate 30 when a certain voltage level is fixedly applied to the photo shield film 46. When using the characteristics indicated in either Fig. 6 or 7, an electronic shutter of about a thousandth second is implemented by the charge-coupled device controlled by a method according to the present invention without any sacrifice of quality of an image reproduced on a screen. Turning to Fig. 8 of the drawings, horizontal blanking periods and vertical blanking periods appropriately take place, and electric charges respectively produced by the photo sensing elements 31 are delivered in succession from the horizontal shift register 33 of the charge-coupled device to the outside thereof in a single read-out cycle. Two read-out pulses 61 and 62 are produced in each vertical blanking period, and a series of high speed vertical driving pulses SWP takes place between the two read-out pulses 61 and 62 for sweeping ineffectual electric charges into the overflow drain 34. Namely, when the first read-out pulse 61 takes place, the ineffectual electric charges accumulated in the photo sensing elements 31 are delivered to the vertical shift registers 32, and are transferred to the overflow drain 34 in the presence of the high speed vertical driving pulses SWP. While sweeping the ineffectual electric charges into the overflow drain 34, fresh effectual electric charges are respectively accumulated in the photo sensing elements 31 in the presence of the optical radiation fallen thereon, and the effectual electric charges are delivered to the vertical shift registers in response to the second read-out pulse 62. In the horizontal blanking periods, the effectual electric charges accumulated in the effective accumulation period are transferred from the vertical shift registers 32 through the horizontal shift register 33 to the outside of the charge-coupled device, and ineffectual electric charges are accumulated in the photo-sensing elements 31, respectively. As described hereinbefore, each of the vertical shift registers 32 has the two hundreds and fifty transferring stages, however, five hundreds of the transferring operations are achieved at the high speed driving frequency of about 800 KHz. At this high speed driving frequency, the amount of transferring charge in a single transferring operation is turned into an output voltage level of about 0.3 volt which is a third achieved by an usual vertical transferring operation for the effectual electric charge, and the amount of the transferring charges is measured for the photo diode around the center of the pixel array. The ineffectual electric charges are accumulated in each of the horizontal blanking periods, and the photo shield film 44 is negatively biased to -5 volts. As will be understood from Fig. 6, the saturated output voltage level is about 0.2 volt at the negative biassing voltage level of about -5 volts. The saturated output voltage level of about 0.2 volt is low enough to perfectly be swept with the high speed driving frequency of about 800 KHz, because the amount of the transferring charge is estimated as about 0.3 volt as described hereinbefore. This results in that no residual electric charge is left in the photo sensing elements. After the first read-out pulse 61 is recovered at time t0, fresh effectual electric charges are accumulated in the photo sensing elements 31. While accumulating the effectual electric charges, the positive biasing voltage level of about +5 volts is applied to the photo shield film 44, and the saturated output voltage level is increased to about 0.8 volt with the biasing level of about +5 volts as will be seen from plots A of Fig. 6. Since the amount of the maximum transferring charge in the usual transferring operation is turned into about 0.9 volt, the saturated output voltage level of about 0.8 volt is slightly lower than the maximum transferring capacity of the vertical shift register, and, accordingly, a sufficiently wide dynamic range is established in the photo sensing elements. No residual electric charge is left in the photo sensing elements, and, for this reason, the blooming phenomenon is less liable to take place even if the wide dynamic range is established. The effectual electric charges are read-out from the photo sensing elements 31 to the vertical shift registers 32 in the presence of the second read-out pulse 62. The vertical shift registers 32 and the horizontal shift register 33 are responsive to the driving pulses SHFT and blanking pulses (not shown), and the effectual electric charges are relayed from the vertical shift registers 32 through the horizontal shift register 33 to the outside of the charge-coupled device. In this period, the photo shield film 46 is lowered to the negative biasing voltage level of about -5 volts again, so that a relatively small amount of ineffectual electric charge is merely produced in each of the photo sensing elements. The biasing voltage level is recovered to the negative value at time t2, and time t2 should be later than time t1 defining the trailing edge of the second read-out pulse 62. The recovery to the negative biasing voltage level is preferably adjusted in the vertical blanking period or the horizontal blanking period so as to prevent an optical image on the screen from some noises produced upon shifting. The ascent of the biasing voltage at time t3 is allowed to be later than the trailing edge of the first read-out pulse at time t0. However, it is desirable that time t3 is earlier than time t0, because the effective accumulation time period or the electronic shutter speed is proportional to the sensitivity. However, if a substantial amount of time takes place between the ascent of the biasing voltage level and the trailing edge of the first read-out pulse 61, undesirable ineffectual electric charges are produced therebetween, and the amount of the undesirable ineffectual electric charges can not be ignored due to the high saturated output voltage level. It is desirable to regulate the ascent of the biasing voltage level and the trailing edge of the first read-out pulse in such a manner that the time therebetween is equal to or less than a tenth of the effective accumulation time period. Time t3 should be fallen within the horizontal blanking time period or the vertical blanking time period in view of reduction in noise. As will be understood from the foregoing description, the charge-coupled device controlled by the method according to the present invention is free from the blooming phenomenon by virtue of shifting the voltage level at either of the photo shield film or the semiconductor substrate for reduction in the amount of the ineffectual electric charges. Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. For example, the transferring operations are as twice as large in number as the transferring stages, and the vertical shift registers can sweep the ineffectual electric charges as twice large as the maximum transferring charges. For this reason, the biasing voltage level of the photo shield film 46 may be selected to be not greater than about 0.6 volt.

Claims: A controlling method for a charge-coupled device fabricated on a semiconductor substrate (30) and comprising a plurality of photo sensing elements (31) arranged in columns and operative to convert an optical radiation into electric charges, respectively, a multiple-stage horizontal shift register (33), an overflow drain (34), a plurality of multiple-stage vertical shift registers (32) interlined with said columns of said photo sensing elements and operative to transfer said electric charges to one of said multiple-stage horizontal shift register and said overflow drain, and a photo shield film (46) provided over said photo sensing elements in such a manner as to allow said optical radiation to impinge on the photo sensing elements, said controlling method being characterized by the steps of a) accumulating effectual electric charges in the presence of said optical radiation into said photo sensing elements by applying a first voltage level to said photo shield plate for promoting the production of said effectual electric charges while ineffectual electric charges are swept into said overflow drain through said multiple-stage vertical shift registers in synchronism with a first driving pulse train (SWP) after said ineffectual electric charges are read out from said plurality of photo sensing elements to said plurality of multiple-stage vertical shift registers in the presence of a first read-out pulse (61), b) reading out said effectual electric charges in the presence of a second read-out pulse (62), and c) transferring said effectual electric charges from said plurality of multiple-stage vertical shift registers in synchronism with a second driving pulse train (SHFT) to said multiple-stage horizontal shift register in applying a second voltage level to said photo shield plate for restricting the production of said ineffectual electric charges in said photo sensing elements.

Inventors: IIJIMA TAKASHI C O NEC CORPORA, KOHNO AKIYOSHI C O NEC CORPORA, IIJIMA, TAKASHI C/O NEC CORPORATION, KOHNO, AKIYOSHI C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1989
Main classification: H04N5
IPC codes: H04N5, H01L27, H04N3