Solid state image pickup device having photo-shield plate in contact with photo-electric converting region via Schottky barrier

Abstract: A solid state image pickup device is fabricated on a semiconductor substrate (1) and comprises a well (2) formed in a surface portion of the semiconductor substrate, photo-electric converting regions (3a to 3f) arranged in rows and columns and converting parts of an optical radia­ tion into electric charges, respectively, charge transfer­ ring regions (4a and 4b) associated with the columns of the photo-electric converting regions, respectively, and con­ veying the electric charges, charge transferring electrodes (9) allowing the charge transferring regions to convey the electric charges, and a metallic photo-shield plate (11) preventing the charge transferring regions from the optical radiation and exposing the photo-electric converting re­ gions to the parts of the optical radiation, in which the metallic photo-shield plate is in contact with the photo­ electric converting regions for forming Schottky barriers therebetween and excess electric charges flow into the metallic photo-shield plate over the Schottky barriers.

Description: FIELD OF THE INVENTION This invention relates to a solid state image pick up device and, more particularly, to a photo-shield plate not only preventing a charge transfer region from an optical radiation but also serving as a drain for excess electric charges. DESCRIPTION OF THE RELATED ART When an optical radiation with large luminous intensity is fallen onto a solid state image pickup device incorporated in an image displaying system, excess electric charges are produced in the array of photo-electric converting elements and are liable to overflow from the photo-electric converting elements into image signal propagating paths. These overflowing electric charges are causative of a false image carrying signal, and a blooming phenomenon takes place on a screen of an associated display unit. Various technologies have been proposed for restriction of the blooming phenomenon, and the prior art anti-blooming technologies were developed on the basis of the fact that the excess electric charges swept into a drain never produce the false image carrying signal. One of the prior art anti-blooming technology provides an overflow drain region formed in the vicinity of the array of the photo-electric converting elements, and a high frequency transferring pulse signal sweeps the excess electric charges into the overflow drain in a vertical blanking period. Another prior art anti-blooming technology forms a vertical overflow drain under the array of the photo-electric converting elements and removes a potential barrier between the photo-electric converting elements and a semiconductor substrate for sweeping the excess electric charges. Such a sweeping operation is known as "electronic shutter mode". An electronic shutter mode is described in EP-A-0 265 271. However, the overflow drain provided in the vicinity of the array occupies a substantial amount of real estate of the semiconductor substrate, and, for this reason, the first technology is less desirable for a large integration density of component elements. The second technology using the vertical overflow drain is free from the drawback inherent in the above explained lateral overflow drain technology, because the vertical overflow drain is overlapped with the photo-electric converting elements and, therefore, no additional real estate is consumed by the vertical overflow drain. The vertical overflow drain is advantageous from the aspect of occupation area and has a majority in the solid state image pickup devices. However, the vertical overflow drain structure needs a well provided between the photo-electric converting elements and the substrate, and the depth of the well directly concerns the amount of electric charges to be accumulated in the individual photo-electric converting elements. If irregularity takes place in the depth of the well, the photo-electric converting elements are dispersed in the accumulating capability, and this deteriorates the solid state image pickup device. For preventing the solid state image pickup device from deterioration, the formation process is strictly controlled so as to keep the depth constant. However, turbulences tend to take place in the formation process and deteriorate the production yield of the solid state image pickup device with the vertical overflow drain. JP-A-60-89967 discloses a solid state image pickup device according to the pre-characterizing part of claim 1, wherein the photo-shield plates are held in contact with the photo-electric converting regions. In this prior are device only excess electric charges flow from the photo-electric converting regions to the photo-shield plate. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a solid state image pickup device which is resistive against a blooming phenomenon. It is also an important object of the present invention to provide a solid state image pickup device the structure of which is relatively easy for fabrication and desirable for a large integration density. To accomplish these objects, the present invention proposes a solid state image pickup device according to claim 1. The dependent claims are related to further advantageous aspects of the present invention. The present invention proposes to sweep excess electric charges to a metallic photo-shield plate over Schottky barriers. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of a solid state image pickup 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 plan view showing a part of a solid state image pickup device according to the present invention; Fig. 2 is a cross sectional view taken along line II-II and showing the structure of the solid state image pickup device; Fig. 3 is a cross sectional view showing the biasing conditions of the solid state image pickup device shown in Fig. 2; Fig. 4 is a diagram showing the bottom edge of a conduction band taken along line A-A' of Fig. 3; Fig. 5 is a view showing excess electric charges overflowing the Schottky barrier produced between a metallic photo-shield plate and a photo-electric converting region; Fig. 6 is a diagram showing variations of the bottom edge of the conduction band in the photo-electric converting region taken along line A-A' under various biasing conditions; Fig. 7 is a diagram showing the bottom edge of the conduction band in the photo-electric converting region in an accumulating mode of operation and an electronic shutter mode of operation; Fig. 8A is a timing chart showing an ordinary operational sequence of the solid state image pickup device shown in Fig. 1; Fig. 8B is a timing chart showing another operational sequence where an electronic mode of operation is followed by an accumulating mode of operation; Fig. 9 is a plan view showing another solid state image pickup device according to the present invention; Fig. 10 is a cross sectional view taken along line X-X of Fig. 9 and showing the structure of the solid state image pickup device; Fig. 11 is a cross sectional view showing the structure of still another solid state image pickup device according to the present invention; Fig. 12 is a diagram showing a bottom edge of a conduction band taken along line B-B' of Fig. 11; Fig. 13 is a cross sectional view showing the structure of still another solid state image pickup device according to the present invention; and Fig. 14 is a diagram showing the bottom edge of a conduction band taken along line C-C' of Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Referring to Figs. 1 and 2 of the drawings, a solid state image pickup device embodying the present invention is fabricated on an n-type semiconductor substrate 1, and a p-type well region 2 is formed in the surface portion of the semiconductor substrate 1. In the surface portion of the p-type well region 2 are formed n-type photo-electric converting regions 3a 3b, 3c, 3d, 3e and 3f and charge transferring regions 4a and 4b at spacings. In this instance, the n-type impurity atoms of the photo-electric converting regions 3a to 3f range from about 10 15 cm -3 to about 10 16 cm -3 . Each of the photo-electric converting regions 3a to 3f converts a part of an optical radiation and produces electric charges. The amount of electric charges depends on the luminous intensity of the part of the optical radiation, and, therefore, images carried on the optical radiation are converted into a distribution pattern of electric charges by the photo-electric converting regions 3a to 3f. The n-type photo-electric converting regions 3a to 3f are arranged in rows and columns, and the columns of the photo-electric converting regions 3a to 3f are respectively associated with the charge transferring regions 4a and 4b. Namely, the column of the photo-electric converting regions 3a to 3c are associated with the charge transferring region 4b, and the column of the photo-electric converting regions 3d to 3f are associated with another charge converting region (not shown) located on the right side thereof. The charge transferring region 4a is provided for another column of photo-electric converting regions (not shown) on the left side thereof. Between the column of the photo-electric converting regions 3a to 3c and the charge transferring region 4b is formed a p-type transfer channel region 6 in which a conductive channel takes place for allowing the electric charges to flow from the photo-electric converting regions 3a to 3c to the charge transferring region 4b. The photo-electric converting regions 3a to 3c, the charge transferring region 4b and the transfer channel region 6 are surrounded by a heavily doped p-type channel stopper region 7, and the heavily-doped p-type channel stopper region 7 isolates the photo-electric converting regions 3a to 3c from one another as well as the column of the photo-electric converting regions 3a to 3c and the charge transferring region 4b from other component regions. The other columns of the photo-electric converting regions and the charge transferring regions are arranged in a similar manner to the photo-electric converting regions 3a to 3c associated with the charge transferring region 4b; however, no further description is incorporated for avoiding repetition. The surface of the p-type well region 2 is covered with a thin insulating film 8 which is transparent to the optical radiation. Gate electrodes including a gate electrode 9 are formed over the charge transferring regions 4a and 4b to form a charge transfer shift register and the transfer channel region 6 to form a transfer channel, and a multi-phase vertical driving signal is applied to the gate electrodes so that the charge transferring regions 4a and 4b convey the electric charges read out from the associated columns of the photo-electric converting regions, respectively. The gate electrodes including the electrode 9 serve as charge transferring means. The gate electrodes are covered with thin insulating films such as films 10a and 10b, and the thin insulating films 10a and 10b and the surface portion of the p-type well region 2 are further covered with an aluminum photo-shield plate 11 except for the photo-electric converting regions 3a to 3f. In the thin insulating film 8 are formed a plurality of frame-shaped contact windows which allows the aluminum photo-shield plate 11 to be in contact with the photo-electric converting regions 3a to 3f. For better understanding, the frame-shaped contact windows 12a to 12f are hatched at narrow spacings, and the aluminum photo-shield plate 11 is indicated by hatching lines at wide spacings in Fig. 1. As will be understood from Fig. 1, the aluminum photo-shield plate 11 exposes the portions of the thin insulating film 8 over the photo-electric converting regions 3a to 3f to the optical radiation, and the parts of the optical radiation penetrate the respective portions of the thin insulating film 8, thereby reaching the photo-electric converting regions 3a to 3f, respectively. However, the aluminum photo-shield plate 11 prevents the charge transferring regions 4a and 4b as well as the transfer channel regions such as the region 6 from the optical radiation. As described in conjunction with the photo-electric converting regions 3a to 3f, the n-type impurity atoms range between about 10 15 cm -3 and about 10 16 cm -3 . Schottky barriers take place at respective contacts STK between the photo-electric converting regions 3a to 3f and the aluminum photo-shield plate 11, and the Schottky barriers allow excess electric charges to flow thereover. In operation, the p-type well region 2 is grounded, and a predetermined voltage level V0 is applied to the aluminum photo-shield plate 11 as shown in Fig. 3. If the aluminum photo-shield plate 11 is negatively biased by dV with respect to the photo-electric converting region 3b, the bottom edge of the conduction band along line A-A' is expressed as shown in Fig. 4. Efm and Efs stand for the Fermi level of aluminum and the Fermi level of the semiconductor material, respectively, and the Schottky barrier is as high as dEstk. Since the p-n junction between the photo-electric converting region 3b and the p-type well region 2 is reversely biased, a build-in potential dEbi takes place at the p-n junction. The n-type photo-electric converting region 3b and the p-type well region 2 are doped in such a manner as to satisfy the following relationship. dEbi > dEstk + (Efm - Ec) where Ec is the bottom edge of the conduction band of the photo-electric converting region 3b. Under those biasing conditions, if a part of an optical radiation is fallen onto the photo-electric converting region 3b, the potential well between the Schottky barrier and the built-in potential is filled with electric charges produced through the photo-electric converting phenomenon. If the luminous intensity is within a regular range, no overflow takes place, and the electric charges are transferred to the charge transferring region 4b as an image carrying signal. However, if the luminous intensity exceeds a predetermined upper bound level, the electric charges overflow from the potential well as shown in Fig. 5, and excess electric charges CHGex flow beyond the Schottky barrier into the aluminum photo-shield plate 11. As will be understood from the above inequality, the built-in potential is higher than the Schottky barrier in terms of the electric charges accumulated in the photo-electric converting region 3b, and, for this reason, no excess electric charges overflows into the p-type well region 2. Fig. 6 shows the bottom edge of the conduction band on the assumption that the photo-shield plate 11 is negatively biased by dV with respect to the photo-electric converting region 3b. If the difference in voltage level between the photo-shield plate 11 and the photo-electric converting region 3b is decreased to dV1, the bottom edge of the conduction band is upwardly moved from Ec to Ec1, and the potential well shallows by q(dV - dV1). The amount of electric charges to be accumulated is, accordingly, decreased. On the other hand, if the difference in voltage level is increased from dV to dV2, the bottom edge of the conduction band is downwardly moved from the Ec to Ec2, and, accordingly, the amount of electric charges to be accumulated is increased as shown in Fig. 6 where V D is a diffusion potential in the photo-electric converting region 3b. This means that the amount of electric charges to be accumulated in the potential well is variable with the voltage level V0 applied to the aluminum photo-shield plate 11. Thus, the solid state image pickup device according to the present invention can regulate the amount of electric charges to be accumulated by changing the voltage level at the aluminum photo-shield plate 11, and no delicate process control is necessary for formation of the p-type well region 2. This results in that the solid state image pickup device according to the present invention is easy for fabrication without sacrifice of the production yield. Since the amount of the electric charges to be accumulated in the potential well is varied with the voltage level V0 at the aluminum photo-shield plate 11, an electronic shutter mode of operation is achieved by changing the voltage level V0. In detail, the bottom edge Ec of the conduction band is indicated by the real line in Fig. 7 as long as the aluminum photo-shield plate 11 is negatively biased by dV; however, when the aluminum photo-shield plate 11 is positively biased with respect to the photo-electric converting region 3b and the difference in voltage level therebetween is assumed to be about 15 volts by way of example, the bottom edge of the conduction band exceeds the Schottky barrier, and the bottom edge Ecx thus lifted sweeps electric charges CHG into the aluminum photo-shield plate 11. Description is made on two different operational sequences with reference to Figs. 8A and 8B. In the description, "signal charges" are used for electric charges produced in an accumulating mode of operation in the presence of an optical radiation within a regular range, and "ineffectual electric charges" means electric charges accumulated out of the accumulating mode of operation. If the solid state image pickup device according to the present invention is used in an ordinary operational sequence, an accumulation of electric charges is immediately followed by a read-out of the electric charges as shown in Fig. 8A. In the ordinary operational sequence, a vertical blanking signal periodically defines vertical blanking periods, and a read-out signal of an active level is supplied to the gate electrode 9 in every vertical blanking period. In this instance, the accumulation of electric charges is carried out over a sixtieth of a second between two read-out signals, and the electric charges to be accumulated are read out from the photo-electric converting region 3b through the transfer channel region 6 to the charge transferring region 4b in the presence of the readout signal. Then, all of the electric charges serve as the signal charges, and this operational sequence is less desirable for a high speed moving object, because the images to be reproduced on a screen are liable to be vague. If another operational sequence shown in Fig. 8B is established in the solid state image pickup device according to the present invention, the solid state image pickup device clearly catches images of a high speed moving object through an electronic shutter mode of operation. In the operational sequence shown in Fig. 8B, the vertical blanking signal periodically defines vertical blanking periods as similar to the ordinary operational sequence; however, a shutter signal is applied to the aluminum photo-shield plate 11 at a predetermined timing before every vertical blanking period, and a read-out signal is applied to the gate electrode 9 in the later stage of every vertical blanking period. In this instance, the shutter signal causes a difference in voltage level of about 15 volts to take place between the aluminum photo-shield plate 11 and the photo-electric converting regions including the region 3b. After the application of the read-out signal, electric charges are accumulated in the photo-electric converting region 3b but swept in the aluminum photo-shield plate 11 in the presence of the shutter signal. This means that the electric charges accumulated between the previous read-out signal and a shutter signal are rejected as the ineffectual electric charges. After the application of the shutter signal, the photo-electric converting region 3b starts accumulating electric charges, and the electric charges are read out to the charge transferring region 4b in the presence of the subsequent read-out signal. Thus, the signal charges are accumulated between the shutter signal and the subsequent read-out signal only, and, for this reason, the images to be reproduced on a screen are clear even if the images are taken from a high speed moving object. Since the shutter signal is applied at any timing between two read-out signals, a variable electronic shutter mode of operation is easily established. In the above mentioned example, the photo-shield plate 11 is supplied with about 5 volts; however, if the voltage level at the photo-shield plate 11 is varied in a predetermined range such as, for example, from 1 volt to 7 volts, the amount of charges to be accumulated is adjustable to a target level. Second Embodiment Turning to Figs. 9 and 10 of the drawings, another solid state image pickup device embodying the present invention is illustrated. The solid state image pickup device shown in Figs. 9 and 10 is similar in arrangement to that shown in Figs. 1 and 2 except for an aluminum photo-shield plate 101 and contact windows 102a to 102f, and, for this reason, other regions and films are denoted by reference numerals designating the corresponding parts of the first embodiment. The contact windows 102a to 102f are formed in the thin insulating film 8 and allow the aluminum photo-shield plate to be brought into contact with a part of a periphery of each of the photo-electric converting regions 3a to 3f. For better understanding, the aluminum photo-shield plate 101 is hatched at wide spacings, and the contact windows 102a to 102f are indicated by narrow hatching lines. In this instance, each of the contact windows 102a to 102f exposes most of the periphery, but the contact windows 102a to 102f may be decreased in area in another instance. The behavior of the solid state image pickup device shown in Figs. 9 and 10 is similar in both operational sequences to that of the first embodiment, and no description is incorporated for avoiding repetition. Third Embodiment Turning to Fig. 11 of the drawings, another solid state image pickup device embodying the present invention is fabricated on an n-type semiconductor substrate 1. The solid state image pickup device shown in Fig. 11 is similar to the first embodiment with the exception of a p-type spacer region 110 formed in a surface portion of each of the photo-electric converting regions such as the region 3b. Other component regions and films are designated by the same reference numerals used in Fig. 2 without any detailed description. The bottom edge of the conduction band taken along line B-B' is illustrated in Fig. 12. The p-type spacer region 110 is isolated from the p-type well region 2, and the p-type spacer region 110 physically separates the photo-electric converting region 3b from the boundary between the insulating film 8 and the semiconductor region. Dark current tends to takes place at the boundary, and the p-type spacer region 110 prevents the photo-electric converting region 3b from the dark current. This results in a solid state image pickup device free from the dark current. The behavior of the solid state image pickup device shown in Fig. 11 is similar in both operational sequences to the first embodiment, and description on the behavior is omitted from the specification. Fourth Embodiment Turning to Fig. 13 of the drawings, still another solid state image pickup device embodying the present invention is fabricated on an n-type semiconductor substrate 1. The solid state image pickup device shown in Fig. 13 is slightly different from the third embodiment. Namely, p-type spacer regions 120a and 120b are contiguous to the heavily doped p-type channel stopper region 7, and a photo-shield plate 121 is not brought into contact with the p-type contact regions 120a and 120b. Since the heavily doped channel stopper region 7 is in contact with the p-type well region 2, a predetermined voltage level is supplied from a source of voltage 122 through the p-type well region 2 and the channel stopper region 7 to the spacer regions 120a and 120b. The predetermined voltage level may be the ground voltage level. The photo-shield plate 121 is directly brought into contact with the photo-electric converting regions, and, therefore, each contact window does not expose all the periphery of each photo-electric converting region as similar to the second embodiment. The bottom edge of the conduction band taken along line C-C' is illustrated in Fig. 14. The solid state image pickup device shown in Fig. 13 behaves as similar to the first embodiment in both operational sequences, and no repetition is made hereinbelow for the sake of simplicity. As will be understood from the foregoing description, the solid state image pickup device according to the present invention effectively sweeps excess electric charges into the aluminum photo-shield plate via Schottky barrier and, for this reason, is free from the blooming phenomenon. Moreover, since the amount of electric charges to be accumulated in each photo-electric converting region is controllable by changing the potential level at the photo-shield plate, no delicate process control is necessary for formation of the p-type well region, and, therefore, the production yield is improved. If an electronic shutter mode of operation is requested, it is easily achieved by changing the voltage level at the photo-shield plate, and the solid state image pickup device is operable in a variable electronic mode of operation. 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. In the above described embodiments, the charge transferring regions are respectively associated with the columns of the photo-electric converting regions; however, the word "columns" are replaceable with word "rows" because the difference is merely an orientation. Moreover, the photo-shield plate may be formed of another metal or alloy for forming an appropriate Schottky barrier, and platinum is one of the candidates for the material used for the photo-shield plate. Furthermore, although area image pickup devices are employed for explanation of the present invention through the embodiments, the present invention may be similarly applicable to linear image pickup devices having a single line of the photo-electric converting regions.

Claims: A solid state image pickup device fabricated on a semiconductor substrate (1) of a first conductivity tpye, comprising: a) a well (2) having a second conductivity type opposite to said first conductivity type and formed in a surface portion of said semiconductor substrate; b) a plurality of photo-electric converting regions (3a to 3f) of said first conductivity type arranged in at least one line in a surface portion of said well and converting parts of an optical radiation into electric charges, respectively; c) a plurality of charge transferring regions (4a and 4b) of said first conductivity type associated with the line of said photo-electric converting regions, respectively, and conveying said electric charges, each of said charge transferring regions being spaced apart from the associated line of said photo-electric converting regions by a channel forming region (6); d) charge transferring means (9) provided over said charge transferring regions and allowing said charge transferring regions to convey said electric charges; and e) a metallic photo-shield plate (11) shielding said charge transferring regions each being contiguous to said channel forming region from said optical radiation and exposing said photo-electric converting regions to the parts of said optical radiation, characterized by a biasing means (dV) connected between said well and said metallic photo-shield plate for changing the potential of said metallic photo-shield plate at least between a charge accumulating mode of operation and an electronic shutter mode of operation, and in that, said metallic photo-shield plate is in contact with peripheral areas of said photo-electric converting regions for forming Schottky barriers therebetween, the difference between a first bottom edge of the conduction band in said well and a second bottom edge of the conduction band in said photo-electric converting regions being larger than the sum of the height of said Schottky barriers and the difference between the Fermi level and said second bottom edge in said accumulating mode, said biasing means biasing said photo-electric converting regions to a first selected value in said charge accumulating mode for causing excess electric charges to overflow beyond said Schottky barrier into said metallic photo-shield plate, said biasing means biasing said photo-electric converting regions to a second selected value in said electronic shutter mode for allowing all the electric charges accumulated in said photo-electric converting regions to flow into said metallic photo-shield plate. A solid state image pickup device as set forth in claim 1, in which said solid state image pickup device further comprises a spacer region (110) of said second conductivity type formed in each of said photo-electric converting regions (3a - 3f) and in which said metallic photo-shield plate (11) is in contact with said spacer region (110) for forming said Schottky barrier therebetween. A solid state image pickup device as set forth in claim 1, in which a predetermined number of said photo-electric converting regions (3a - 3f) are associated with each of said charge transferring regions (4a, 4b) and in which a channel stopper region (7) of said second conductivity type is formed on both sides of a combination of said photo-electric converting regions (3a - 3f) and said associated charge transferring region (4a, 4f), wherein said channel stopper (7) is in contact with a spacer region (120a) of said second conductivity type formed in a surface portion of each photo-electric converting region (3a - 3f), thereby propagating a voltage level of said well to said spacer region.

Inventors: NAGAYA KAZUHISA, NAGAYA, KAZUHISA, NAGAYA, KAZUHISA C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H01L27
IPC codes: H04N3, H01L27, H04N5