Method of driving a solid state imager device

Abstract: The solid state imager device has a substrate (1) of a first conductivity type, a charge storage area (2) of a second conductivity type formed on the substrate (1) of the first conductivity type, and a forward electrode (4) formed on the charge storage area (2) of the second conductivity type through an insulating layer (3), and the forward electrode (4) is alternately supplied with a first voltage (V₁) for accumulating charges of the first conductivity type on a boundary between the charge storage area (2) of the second conductivity type and the insulating layer (3) and a second voltage (V₂) so as to extend the depletion layer formed beneath the charge storage area (2) of the second conductivity type.

Description: This invention relates generally to a method of driving a solid state imager device and is directed more particularly to a method of driving an interline transfer type solid state imager device which has a light receiving section formed of a buried channel MOS structure. A solid state imager device generally comprises a semiconductor device and a voltage supplying means. In a solid state imager device having a buried channel MOS structure, it is desirable to reduce, as much as possible the dark current which may be a source of noise. A conventional solid state imager device which can reduce the dark current, for example, is disclosed in a collection of papers presented at ′84 National Conference of the Institute of Television Engineers of Japan, on pages 41-42. Such known solid state imager device has a light receiving section which is formed of a buried channel MOS structure. Specifically, the device is constructed such that an n-type charge storage area is formed over the surface of a p-type silicon substrate and a forward electrode is formed on the charge storage area through an insulating layer, so that it can be operated under conditions where a hole storage layer is produced on a boundary between the n-type charge storage area and the insulating layer. In this solid state imager device, the forward electrode is applied with a predetermined direct current voltage so as to form a channel within the n-type charge storage area from holes previously stored on the surface of the n-type charge storage area. Thus, the holes are kept in a so-called pinning state, so as to suppress the generation of hole-electron pairs which are the main component of the dark current from the surface level. A solid state imager device as described above can advantageously reduce the dark current. As described above, this prior art solid state imager device comprises: a semiconductor substrate of a first conductivity type; a charge storage area of the second conductivity type, formed below a surface of said substrate; an insulating, light transmittable layer formed on said surface of said substrate; a forward electrode formed on said insulating layer and extending above said charge storage area and being made of a conductive, light transmitting material; and a voltage supplying means for supplying said forward electrode with a first voltage for accumulating charges corresponding to the first conductivity type in the boundary between said charge storage area and said insulating layer. However, such conventional solid state imager device has disadvantages in that, since the forward electrode is always supplied with a direct current voltage so as to store holes on the boundary between the n-type charge storage area and the insulating layer, the depletion layer under the charge storage area is not extended, making it impossible to suppress disturbances in images which are picked-up, which disturbances are caused by signal charges flowing from the inside of the substrate into the charge transfer section, and which are called "smear". Accordingly, is the object of the present invention to provide a method for driving a solid state imager device which can effectively suppress dark current als well as smear. To achieve the above object, the method of the present invention makes use of a solid state imager device comprising the above listed features of the prior art device, and it is characterized by supplying said forward electrode alternately with said first voltage during a first time period T1, and a second voltage during a second time period T2 for extending the depletion layer formed beneath said charge storage area, thereby using a time duty ratio T2/T1 of 0 < T2/T1 ≦ 5. In the prior art method the voltage supplying means never supplies the second voltage, and therefore, there the time duty ratio T2/T1 equals zero. As it is discussed below, a smear level value decreases very rapidly, as soon as said i second voltage is supplied to the forward electrode. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, throughout which like reference numerals designate like elements and parts. FIG. 1 is a diagram showing a cross-sectional view of a main portion of a solid state imager device driven by a method according to the present invention; FIGS. 2A and 2B are timing charts showing waveforms of a pulse signal v and a dark signal, respectively; FIG. 3 is a graph showing the relationship between the duty ratio T₂/T₁ of the pulse signal v and a smear level and a relationship between the duty ratio T₂/T₁ and a dark signal output; and FIG. 4 is a chart showing a waveform of a pulse signal v′. FIG. 1 shows a p-type silicon substrate 1 into which a p-type impurity is diffused with a relatively low concentration. It is assumed in the present embodiment that a light receiving section 5, a vertical register section 6, a horizontal register section (not shown) and an output circuit section (not shown) are all formed on the p-type silicon substrate 1. In the structure, a plurality of the light receiving sections 5 are provided which are arranged in the horizontal and vertical directions. The vertical register section 6 is arranged to be parallel to each of the vertically aligned light receiving sections 5, and the horizontal register section is arranged to be parallel to the horizontal direction, so as to form an interline transfer solid state imager device. The light receiving section 5 comprises an n-type charge storage area 2 on the surface of the p-type silicon substrate 1. A forward electrode 4 is formed of a light transmissible polysilicon which is deposited over the charge storage area 2 through an insulating layer 3 made of SiO₂, so that the light receiving section 5 is formed by the buried channel MOS structure. When the solid state imager device of the present embodiment is operated so as to pick up an image, the forward electrode 4 is supplied through a terminal B with a pulse signal v, as shown in FIG. 2A. When the forward electrode 4 is supplied with a low level voltage V₁, e.g. -12V, a storage layer of holes is formed on the boundary between the charge storage area 2 and the insulating layer 3, and when a high level voltage V₂, e.g. 2V is supplied, the depletion layer beneath the charge storage area 2 is extended in the thickness direction of the substrate 1. It should be noted that the pulse signal v is selected such that its rising edge and its falling edge are within a horizontal blanking period so that the image signal is not influenced by the pulse signal v. Also, a relatively high concentration n-type overflow drain region 7 is formed adjacent to the charge storage area 2 through a p-type overflow control region 8 so as to prevent so-called blooming where the image picked up is made unclear by excesive signal charges which are produced when an especially strong light is incident on the imager device, and which flow into other charge storage areas. A p-type channel stopper region 9 is arranged adjacent to the overflow drain region 7 as well as in regions between respective charge storage areas which are vertically adjacent to each other. The vertical register section 6 is formed of an n-type charge transfer region 11 formed on a p-type well region 10 which is selectively formed on the p-type silicon substrate 1 and a transfer electrode 12 made of polysilicon is formed on the charge transfer region 11 through the insulating layer 3. Thus, the vertical register section 6 is formed as a so-called bulk charge coupled device (BCCD) structure. FIG. 1 shows only one transfer electrode 12. However, as is well known in the art, the requried number of transfer electrodes 12 are provided so that the vertical register section 6 of the present embodiment is driven in a 4-phase driving manner. On the insulating layer 3 between the light receiving section 5 and the vertical register section 6, there is formed a gate electrode 13 for controlling the transfer of signal charges which accumulate in the charge storage area 2 to the charge transfer region 11 in a manner such that it is integral with a portion of the transfer electrode 12, to thus form a gate section 14. The signal charges are transferred from the charge storage area 2 to the charge transfer region 11 during a vertical flyback period using a low level voltage V₁ which is supplied to the forward electrode 4. On the upper surface of the forward electrode 4 except over the light receiving section 5 there is deposited a light shielding layer 15 made e.g. of Aℓ. Also, over all of the forward electrode 4 there is deposited a protective layer 16 made of SiO₂. Other sections required for the interline transfer solid state imager device, such as the horizontal register section, the output circuit section are formed in the same manner as the conventionally known device, and these are not shown in FIG. 1. In the solid state imager device of the present invention constructed as described above, the forward electrode 4 is supplied with the pulse signal v as shown in FIG. 2A. When the forward electrode 4 is supplied with the low level voltage V₁, holes will accumulate on the surface of the charge storage area 2 and the the holes are kept in the pinning state, to suppress hole-electron pair generation from the surface level. If the forward electrode 4 is supplied with the high level voltage V₂, the holes which have accumulated on the surface of the charge storage area 2 will be released. However, once the holes accumulate on the surface of the charge storage area 2, the hole-electron pair generation is suppressed for a predetermined time during which the holes are released. Therefore, if the pulse signal v shown in FIG. 2A is applied to the forward electrode 4, a dark signal such as shown in FIG. 2B will be generated in the charge storage area 2. If the duty ratio T₂/T₁ of the pulse voltage v is varied from T₂/T₁ = 0 to T₂/ T₁ 10, the dark current output will increase as indicated by the solid line curve X in FIG. 3. Specifically, the dark current output will be low when the duty ratio T₂/T₁ is smaller, and the dark current output goes higher when the duty ratio T₂/T₁ becomes larger. When the low level voltage V₁ is supplied to the forward electrode 4, the depletion layer formed beneath the charge storage area 2 becomes shallow, which allows smear to be easily produced. On the other hand, when the high level voltage V₂ is supplied to the forward electrode 4, the depletion layer formed beneath the charge storage area 2 becomes thicker, which suppresses the generation of smear. The solid line curve Y in FIG. 3 shows how the smear level is varied when the duty ratio T₂/T₁ of the pulse voltage v is varied from T₂/T₁ = 0 to T₂/T₁ = 10. In FIG. 3, a smear level value when the duty ratio T₂/T₁ is 0 is defined as 1, and the curve illustrates variations in the smear level value as this ratio is varied. As will be understood from a consideration of FIG. 3, it is possible to reduce the dark current as well as the smear level by selecting the duty ratio T₂/T₁ of the pulse signal v to fall in a range of T₂/T₁ = 2 to T₂/T₁ = 5. As described above, the present embodiment has advantages over the prior art in that the dark current and smear level can be effectively reduced by using a simple construction such that the forward electrode 4 is alternately supplied with the pulse signal v consisting of a low level voltage V₁, e.g. -12V, for accumulating holes on the surface of the charge storage area 2 and a high level voltage V₂, e.g. 2V, for extending the depletion layer beneath the charge storage area 2 and by using an appropriate duty ratio in the range of 2 - 5. A further advantage of the present embodiment is that the dark current from the surface level 8 is not completely suppressed but is produced in a degree such that the quality of the image is not influenced by the dark current which makes it difficult to distinguish minute local dark signals (white flaw), produced by crystal defects and the like and other factors than the surface level, which appear on an image if the dark current is completely suppressed. The above described embodiment is for a case where the low level voltage period T₁ is determined as one horizontal period and the pulse signal v raises and falls within the horizontal blanking period. Alternatively, a pulse signal v′ which has its low level voltage arranged within the horizontal blanking period, as shown in FIG. 4, may be applied to the forward electrode. In this case, the duty ratio T₂/T₁ can be set to have a sufficiently large value so that smear can be satisfactorily suppressed, and so that the surface is not depleted continuously for more than one horizontal period so that the dark current can also be sufficiently suppressed. The above described present embodiment is for the case where the signal charges are electrons. However, the present invention may be applied to a case where the signal charges are holes. For that case, the conductivity types of the respective sections are selected so as to opposite to those shown in FIG. 1. The same effects as described above can be obtained in this case.

Claims: A method of driving a solid state imager device, said device comprising: a semiconductor substrate (1) of a first conductivity type; a charge storage area (2) of the second conductivity type, formed below a surface of said substrate (1); an insulating, light transmitting layer (3) formed on said surface of said substrate (1); a forward electrode (4) formed on said insulating layer (3) and extending above said charge storage area (2) and being made of a conductive, light transmitting material; and a voltage supplying means for supplying said forward electrode (4) with a voltage ; characterized by supplying said forward electrode (4) alternately with a first voltage (V₁) during a first time period T1 for accumulating charges, corresponding to the first conductivity type in the boundary between said charge storage area (2) and said insulating layer (3), and a second voltage (V₂) during a second time period T2 for extending the depletion layer formed beneath said charge storage area, thereby using a time duty ratio T2/T1 of 0 < T2/T1 ≦ 5. A method as claimed in claim 1, characterized by supplying said forward electrode (4) with said first voltage (V₁) within one horizontal blanking period.

Inventors: FUKUSYO TAKASHI, KUMESAWA TETSURO, TERAKAWA HIROSHI, YAMAMURA MICHIO, FUKUSYO, TAKASHI, KUMESAWA, TETSURO, TERAKAWA, HIROSHI, YAMAMURA, MICHIO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1987
Main classification: H01L27
IPC codes: H04N5, H01L27