Abstract:
A solid-state imaging device comprises: a semiconductor substrate; a plurality of photoelectric conversion elements formed in a surface portion of the semiconductor substrate in the form of a two-dimensional array so as to comprise a plurality of sets, each comprising a subset of the photoelectric conversion elements arranged in one direction; charge transfer paths each formed at a side portion of the subset of the photoelectric conversion elements to cause a signal charge of the photoelectric conversion elements be read out when a readout pulse is applied and cause the signal charge which has been read out to be transferred when a transfer pulse is applied; and an electrically conductive light shielding film which is laminated on a surface of the semiconductor substrate through an insulating layer and has openings immediately above each of the photoelectric conversion elements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a solid-state imaging device and a driving method thereof, and more particularly to a solid-state imaging device suitable for such as smear reduction and the lowering of a readout voltage as well as a driving method thereof and an imaging apparatus.  
         [0003]     2. Description of the Related Art  
         [0004]     On a semiconductor substrate of a solid-state imaging device, a multiplicity of n-type region portions are formed in a p-well layer of a surface portion by being arranged in the form of a two-dimensional array to form a multiplicity of photodiodes (pn junction portions: photoelectric conversion elements). A metallic light shielding film provided with an opening above each photodiode is laminated on the surface of the solid-state imaging device. In the related-art solid-state imaging device, a predetermined potential is adapted to be applied to this light shielding film.  
         [0005]     For example, in the related-art technique disclosed in JP-A-7-153932, a high-concentration impurity layer of an opposite conductivity type (p type) is formed on the surface of the above-described n-type region portion, and a contact hole is provided in an insulating layer laminated on the surface of the semiconductor substrate, so that the light shielding film and the high-concentration impurity layer are electrically connected through the contact hole. As a predetermined potential is applied to the light shielding film, the surface potential of the photodiode is set to the quasi-Fermi level or less of the high-concentration impurity layer.  
         [0006]     Alternatively, as a potential lower than the surface potential of the photodiode is applied to the light shielding film, minority carriers (holes) generated by photoelectric conversion are allowed to escape to the light shielding film so as to reduce the recombination of signal charges (electrons) and the minority carriers.  
         [0007]     With the related-art solid-state imaging devices, attempts have been made to reduce smear and the like by applying a predetermined potential to the light shielding film and by directly controlling the potential of the high-concentration impurity layer to a predetermined potential through the contact hole. However, in solid-state imaging devices in which millions of pixels or more are mounted as in the devices of recent years, the size of each single pixel (photodiode) is extremely small, so that it has become difficult to form a contact hole in each single pixel. In addition, the effect of metal contamination due to direct contact of the light shielding film with the semiconductor substrate through the contact hole has become such that it cannot be neglected. Accordingly, there has been a demand for improving the performance of the solid-state imaging device such as the reduction of smear through other methods.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the invention is to provide a solid-state imaging device which makes it possible to attain improvement of device performance such as the reduction of smear and the lowering of a readout voltage as well as a driving method thereof and an imaging apparatus.  
         [0009]     According to the invention, there is provided a solid-state imaging device comprising: a semiconductor substrate; a plurality of photoelectric conversion elements formed in a surface portion of the semiconductor substrate in the form of a two-dimensional array so as to comprise a plurality of sets, each comprising a subset of the photoelectric conversion elements arranged in one direction; charge transfer paths each formed at a side portion of the subset of the photoelectric conversion elements to cause a signal charge of the photoelectric conversion elements be read out when a readout pulse is applied and cause the signal charge which has been read out to be transferred when a transfer pulse is applied; and an electrically conductive light shielding film which is laminated on a surface of the semiconductor substrate through an insulating layer and has openings immediately above each of the photoelectric conversion elements, a potential of a same polarity as that of the readout pulse being applied to the light shielding film when the readout pulse is applied to a readout electrode of the charge transfer paths, and a potential of an opposite polarity to that of the readout pulse being applied to the light shielding film when the readout pulse is not applied to the readout electrode.  
         [0010]     In the solid-state imaging device according to the invention, the solid-state imaging device further comprises an impurity surface layer provided in a surface portion of each of the photoelectric conversion elements, and the impurity surface layer comprises a high-concentration region portion of a central portion and a low-concentration region portion of a peripheral portion, and an end of each of the openings of the light shielding film extends up to a position so that the light shielding film covers the low-concentration region portion.  
         [0011]     In the solid-state imaging device according to the invention, the potential of the same polarity is applied to the light shielding film by preceding the readout pulse for a predetermined time period, and the potential of the opposite polarity is applied to the light shielding film by lagging behind the termination of the readout pulse for a predetermined time period.  
         [0012]     According to the invention, there is provided a method of driving a solid-state imaging device, the device comprising: a semiconductor substrate; a plurality of photoelectric conversion elements formed in a surface portion of the semiconductor substrate in the form of a two-dimensional array so as to comprise a plurality of sets, each comprising a subset of the photoelectric conversion elements arranged in one direction; charge transfer paths each formed at a side portion of the subset of the photoelectric conversion elements to cause a signal charge of the photoelectric conversion elements be read out when a readout pulse is applied and cause the signal charge which has been read out to be transferred when a transfer pulse is applied; and an electrically conductive light shielding film which is laminated on a surface of the semiconductor substrate through an insulating layer and has openings immediately above each of the photoelectric conversion elements, the method comprising applying a potential of a same polarity as that of the readout pulse to the light shielding film when the readout pulse is applied to a readout electrode of the charge transfer paths; and applying a potential of an opposite polarity to that of the readout pulse to the light shielding film when the readout pulse is not applied to the readout electrode.  
         [0013]     In the method of driving a solid-state imaging device according to the invention, the potential of the same polarity is applied to the light shielding film by preceding the readout pulse for a predetermined time period, and the potential of the opposite polarity is applied to the light shielding film by lagging behind the termination of the readout pulse for a predetermined time period.  
         [0014]     In the method of driving a solid-state imaging device according to the invention, the potential of the same polarity is applied to the light shielding film as a square or trapezoidal potential waveform.  
         [0015]     According to the invention, there is provided an imaging apparatus comprising: a semiconductor substrate; a plurality of photoelectric conversion elements formed in a surface portion of the semiconductor substrate in the form of a two-dimensional array so as to comprise a plurality of sets, each comprising a subset of the photoelectric conversion elements arranged in one direction; charge transfer paths each formed at a side portion of the subset of the photoelectric conversion elements to cause a signal charge of the photoelectric conversion elements be read out when a readout pulse is applied and cause the signal charge which has been read out to be transferred when a transfer pulse is applied; an electrically conductive light shielding film which is laminated on a surface of the semiconductor substrate through an insulating layer and has openings immediately above each of the photoelectric conversion elements; and an imaging device driving section that applies a potential of a same polarity as that of the readout pulse to the light shielding film when the readout pulse is applied to a readout electrode of the charge transfer paths, and applies a potential of an opposite polarity to that of the readout pulse to the light shielding film when the readout pulse is not applied to the readout electrode.  
         [0016]     In the imaging apparatus according to the invention, the imaging apparatus further comprises an impurity surface layer provided in a surface portion of each of the photoelectric conversion elements, and the impurity surface layer comprises a high-concentration region portion of a central portion and a low-concentration region portion of a peripheral portion, and an end of each of the openings of the light shielding film extends up to a position so that the light shielding film covers the low-concentration region portion.  
         [0017]     The imaging device driving section of the imaging apparatus in accordance with the invention applies the potential of the same polarity to the light shielding film by preceding the readout pulse for a predetermined time period, and applies the potential of the opposite polarity to the light shielding film by lagging behind the termination of the readout pulse for a predetermined time period.  
         [0018]     The imaging device driving section of the imaging apparatus in accordance with the invention applies the potential of the same polarity to the light shielding film as a square or trapezoidal potential waveform. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a functional block diagram of a digital camera in accordance with an embodiment of the invention;  
         [0020]      FIG. 2  is a schematic plan view of a solid-state imaging device shown in  FIG. 1 ;  
         [0021]      FIG. 3  is a schematic cross-sectional view of a substantially one pixel portion of the solid-state imaging device shown in  FIG. 2 ;  
         [0022]      FIGS. 4A and 4B  are waveform diagrams explaining a method of driving a solid-state imaging device in accordance with the embodiment of the invention;  
         [0023]      FIG. 5  is a graph illustrating the relationship between the smear and incident light angular dependence when the voltage to be applied to a light shielding film is varied in the solid-state imaging device in accordance with the embodiment of the invention;  
         [0024]      FIG. 6  is a graph illustrating the relationship between a depletion voltage and the applied voltage of the light shielding film in the solid-state imaging device in accordance with the embodiment of the invention;  
         [0025]      FIG. 7  is a graph illustrating the relationship between the readout gate turn-off voltage and the applied voltage of the light shielding film in the solid-state imaging device in accordance with the embodiment of the invention; and  
         [0026]      FIG. 8  is a graph illustrating the relationship between the breakdown voltage and the applied voltage of the light shielding film in the solid-state imaging device in accordance with the embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Hereafter, a description will be given of an embodiment of the invention with reference to the accompanying drawings.  
         [0028]      FIG. 1  is a functional block diagram of a digital camera in which a solid-state imaging device in accordance with the embodiment of the invention is mounted. The illustrated digital camera is comprised of a taking lens  10 , a CCD type solid-state imaging device  100 , a diaphragm  12  provided between these members, an infrared cutoff filter  13 , and an optical low-pass filter  14 . A CPU  15  which performs supervisory control of the overall digital camera controls a flash-light emitting unit  16  and a light receiving unit  17 , adjusts the position of the taking lens  10  to a focus position by controlling a lens drive unit  18 , and effects adjustment of exposure amount by controlling the amount of opening of the diaphragm  12  through a diaphragm drive unit  19 .  
         [0029]     In addition, the CPU  15  drives the solid-state imaging device  100  through an imaging device drive unit  20  to cause the solid-state imaging device  100  to output a subject image captured through the taking lens  10  as chrominance signals. An instruction signal from a user is inputted to the CPU  15  through an operation unit  21 , and the CPU  15  performs various control in accordance with this instruction.  
         [0030]     An electric control system of the digital camera includes an analog signal processing unit  22  connected to an output of the solid-state imaging device  100 , as well as an A/D conversion circuit  23  for converting R, G, and B chrominance signals outputted from this analog signal processing unit  22 . The analog signal processing unit  22  and the A/D conversion circuit  23  are controlled by the CPU  15 .  
         [0031]     The electric control system of this digital camera further includes a memory control unit  25  connected to a main memory (frame memory)  24 ; a digital signal processing unit  26  for performing an interpolation operation, a gamma correction operation, RGB/YC conversion processing, and the like; a compression/expansion processing unit  27  for compressing the captured image into a JPEG image or expanding the compressed image; an integration unit  28  for integrating photometric data to determine a gain for white balance correction which is effected by the digital signal processing unit  26 ; an external memory control unit  30  to which a detachable recording medium  29  is connected; and a display control unit  32  to which a liquid crystal display unit  31  mounted on the rear surface or the like of the camera is connected. These units are connected to each other through a control bus  33  and a data bus  34 , and are controlled by commands from the CPU  15 .  
         [0032]      FIG. 2  is a schematic plan view of the solid-state imaging device  100  shown in  FIG. 1 . The illustrated solid-state imaging device  100  has a multiplicity of photodiodes (photoelectric conversion elements)  101  formed on a semiconductor substrate by being arranged in the form of a two-dimensional array, such that the photodiodes  101  in even-numbered rows are disposed in such a manner as to be respectively offset with respect to the photodiodes  101  in odd-numbered lines by a half pitch (in a so-called honeycomb pixel arrangement).  
         [0033]     The characters “R,” “G,” and “B” illustrated on the respective photodiodes  101  represent the colors (red being R, green being G, and blue being B) of color filters laminated on the respective photodiodes, and each photodiode  101  accumulates signal charges corresponding the quantity of the received light of one of the three primary colors. It should be noted that although an example in which primary color-based color filters are installed is illustrated, complementary color-based color filters may alternatively be installed.  
         [0034]     Vertical transfer electrodes V 1 , V 2 , . . . , and V 8  are laid in the horizontal direction on the surface of the semiconductor substrate in a meandering manner so as to avoid the respective photodiodes  101 . An unillustrated embedded channel is formed on the semiconductor substrate alongside each photodiode column arranged in the vertical direction in such a manner as to meander in the vertical direction so as to avoid the photodiodes  101 .  
         [0035]     A vertical transfer path (VCCD)  102  is formed by this embedded channel and the vertical transfer electrodes provided thereon and disposed in such a manner as to meander in the vertical direction. This vertical transfer path  102  is driven for transfer by vertical transfer pulses φV 1  to φV 8  (the illustrated example is that of 8-phase drive) which are outputted from the imaging device drive unit  20  shown in  FIG. 1 .  
         [0036]     A horizontal transfer path (HCCD)  103  is provided on a lower side portion of the semiconductor substrate. This horizontal transfer path  103  is also constituted by an embedded channel and horizontal transfer electrodes provided thereon. This horizontal transfer path  103  is two-phase driven by horizontal transfer pulses φH 1  and φH 2  which are outputted from the imaging device drive unit  20  shown in  FIG. 1 .  
         [0037]     An output amplifier  104  is provided at an output end portion of the horizontal transfer path  103 . The output amplifier  104  outputs as an image signal a voltage value signal corresponding to the quantity of signal charges transferred to the end portion of the horizontal transfer path  103 .  
         [0038]     It should be noted that although the terms “vertical” and “horizontal” have been used, the terms mean “one direction” along the surface of the semiconductor substrate and “a direction substantially perpendicular to this one direction.” 
         [0039]     On the substantially entire surface of the solid-state imaging device  10  shown in  FIG. 2 , a metallic light shielding film provided with an opening is laminated immediately above each photodiode  101 . Further, a pad  105  for applying a required control pulse voltage φMV to the light shielding film is provided at a predetermined portion of the surface of the semiconductor substrate. The control pulse voltage φMV is applied from the imaging device drive unit  20  shown in  FIG. 1  to the light shielding film through the pad  105 .  
         [0040]      FIG. 3  is a schematic cross-sectional view of a substantially one pixel portion of the solid-state imaging device shown in  FIG. 2 . A p-well layer  111  is formed in an n-type semiconductor substrate  110  of this solid-state imaging device  100 , and an n-type region portion  112  is provided on a surface portion of the p-well layer  111 , thereby forming the photodiode  101  for effecting photoelectric conversion with respect to the p-well layer  111  (hereafter the n-type region portion  112  will be also referred to as the photodiode).  
         [0041]     An element isolating region (p +  region)  113  is provided is provided on an adjacent pixel side of the n-type region portion (photodiode)  112 , and an n region  115  is provided on the opposite side to the photodiode  112  via a readout gate portion (p −  region)  114 . This n region  115  constitutes the embedded channel of the vertical transfer path  102  described with reference to  FIG. 2 .  
         [0042]     A high-concentration impurity surface layer  116  of an opposite conductivity type (p type) is provided on a surface portion of the n-type region portion  112 . As this high-concentration impurity surface layer  116  is provided, free electrons generated as a dark current are captured by holes in the high-concentration impurity surface layer  116 , thereby preventing dark current components from appearing as white streaks in an image.  
         [0043]     The high-concentration impurity surface layer  116  is provided by being divided into a central high-concentration portion (p +  region)  116   a  on the surface of the n-type region portion  112  and a low-concentration portion (p −  region)  116   b  in its surrounding portion. As the surrounding portion is formed as the low-concentration portion  116   b , the electric field in the surrounding portion is weakened, which makes it possible to lower the voltage at the time of reading out the accumulated charges in the photodiode (n-type region portion)  112  to the embedded channel  115  of the vertical transfer path.  
         [0044]     The outermost surface of the semiconductor substrate  110  in which the photodiodes  112 , the embedded channels  115 , and the like have been formed is covered with a transparent insulating layer  118  of an ONO (oxide film/nitride film/oxide film) structure or a single-layered structure of an oxide film. A vertical transfer electrode film (e.g., a polysilicon film)  119  is laminated on the insulating layer  118  immediately above the embedded channel  115 .  
         [0045]     A metallic light shielding film  121  is laminated over the vertical transfer electrode film  119  via an insulating layer  120 . An opening  121   a  is provided in the light shielding film  121  immediately above each photodiode  112 , and incident light passes through this opening  121   a  and is made incident into the n-type region portion  112 .  
         [0046]     In addition, in the solid-state imaging device  100  in accordance with this embodiment, an end of the light shielding film opening  121   a  extends up to a position covering the low-concentration portion  116   b  of the high-concentration impurity surface layer  116 . The pad (an input terminal with an external pulse φMV)  105  shown in  FIG. 2  is connected to this light shielding film  121 .  
         [0047]     An unillustrated transparent flattening layer is laminated on the light shielding film  121 , an unillustrated color filter layer is laminated on the surface of the flattening layer whose surface has been formed flat, and a microlens is laminated thereon.  
         [0048]     When an image is photographed with the digital camera having the above-described configuration, the incident light from a subject field which entered through the taking lens  10  is applied to a light receiving surface of the solid-state imaging device  100 . When the light is incident upon the photodiodes  112 , signal charges (electrons in this example) corresponding to the respective quantities of incident light are accumulated in the photodiodes  112 .  
         [0049]     When the CPU  15  outputs a command to the imaging device drive unit  20 , and the imaging device drive unit  20  outputs a readout pulse to the solid-state imaging device  100 , this readout pulse is applied to the vertical transfer electrode  119  which also serves as a readout electrode. As a result, the accumulated charges (signal charges) in each photodiode  112  are read out to the embedded channel  115  through the readout gate portion  114 .  
         [0050]     When the CPU  15  outputs the command to the imaging device drive unit  20 , and the imaging device drive unit  20  outputs a vertical transfer pulse φV and a horizontal transfer pulse φH to the solid-state imaging device  100 , the respective signal charges on the vertical transfer path  102  are transferred on the vertical transfer path for one transfer electrode at a time. When the signal charges corresponding to one line portion of the photodiodes have been transferred to the horizontal transfer path  103 , this one line portion of signal charges is transferred on the horizontal transfer path  103 , and a voltage value signal corresponding to the quantity of signal charges is read out by the amplifier  104 .  
         [0051]     In such a readout operation of the signal charges, in the digital camera in accordance with this embodiment, the CPU  15  outputs a command to the imaging device drive unit  20 , and performs control of a pulse voltage φMV to be applied to the light shielding film. Hereafter, a description will be given of applied voltage control of the light shielding film.  
         [0052]      FIG. 4A  is a diagram illustrating a pulse waveform which is applied to the vertical transfer electrode (also serving as a readout electrode), and  FIG. 4B  is a diagram illustrating a pulse waveform which is applied to the light shielding film.  
         [0053]     Before the signal charges are read out from the photodiode to the vertical transfer path, the vertical transfer path is driven by a high-speed sweep pulse (e.g., Vmid=0 V, Vlow=−8 V)  130 . Consequently, unwanted charges on the vertical transfer path are swept away from the vertical transfer path.  
         [0054]     Next, when a readout pulse (e.g., Vhigh=15 V)  131  is applied to the vertical transfer electrode also serving as the readout electrode, the accumulated charges in the photodiode are read out to the vertical transfer path. Then, as the vertical transfer path is driven by a transfer pulse  132 , the transfer of the signal charges in the direction of the horizontal transfer path is effected.  
         [0055]     At this time, as shown in  FIG. 4B , the CPU  15  applies a pulse voltage (φMV)  135  to the light shielding film  121  through the pad  105 . This pulse voltage  135  is such a pulse voltage that it is synchronized with the readout pulse  131 . A high-level potential is controlled to the potential of the same polarity as the readout pulse  131 , i.e., to a predetermined positive potential in this example, whereas a low-level potential is controlled to the potential of the opposite polarity to that of the readout pulse  131 , i.e., to a predetermined negative potential in this example.  
         [0056]     The light shielding film  121  is always controlled to a predetermined negative potential except when the signal charges are read out from the photodiode to the vertical transfer path. Further, in this embodiment, when the readout pulse is applied to the vertical transfer electrode also serving as the readout electrode, a predetermined positive potential is applied to the light shielding film  121  by preceding the readout pulse  131  for a predetermined time period t 1 . When the readout pulse  131  terminated, the light shielding film  121  is returned to the predetermined negative potential by lagging behind this point of time of termination for a predetermined time period t 2 . The setting provided may be such that t 1 =t 2 , or t 1 ≠t 2 .  
         [0057]     It should be noted that although in  FIG. 4B  the pulse waveform of the pulse voltage  135  applied to the light shielding film is set to be a square wave, but may be a trapezoidal wave.  
         [0058]      FIG. 5  is a graph on actually measured data illustrating the improvement of smear characteristics in the solid-state imaging device  100  in accordance with this embodiment. In a case where the applied voltage of the light shielding film is always fixed to “0 V,” the absolute amount of smear with respect to the incident angle of the incident light is shown by curves I and II in the graph. The characteristic curve I shows the smear characteristic included in the signal charge of red (R) or blue (B), and the characteristic curve II shows the smear characteristic included in the signal charge of green (G).  
         [0059]     In contrast, it can be appreciated that if the voltage to be applied to the light shielding film is controlled to a predetermined negative potential (e.g., −8 V) as in the solid-state imaging device  100  of this embodiment, in comparison with the characteristic curves I and II the smear characteristics can be improved by 20% or thereabouts as in the case of the characteristic curves III (smear characteristic of R or B) and IV (smear characteristic of G) shown in  FIG. 5 .  
         [0060]     This is conceivably attributable to the fact that electrons which enter the embedded channel  115  through the insulating layer  118  between the end of the opening  121   a  of the light shielding film and the semiconductor substrate  110  can be prevented by applying a negative potential to the light shielding film  121 . It can be expected that the smear improvement rate can be improved by further increasing the negative potential to be applied to the light shielding film  121 .  
         [0061]      FIG. 6  is a graph on actually measured data illustrating changes in a depletion voltage with respect to the applied voltage of the light shielding film. From the arrangement of the data on measurement points in  FIG. 6 , it can be seen that the higher the applied voltage of the light shielding film is set on the high voltage side, the more the depletion is improved. From the data, it can be seen that when the depletion voltage is to be set to 10 V or less, it suffices if the voltage to be applied to the light shielding film is set to +3 V or higher.  
         [0062]     In this embodiment, when the signal charges are read out from the photodiode to the vertical transfer path, a predetermined positive potential is applied to the light shielding film  121 . On the basis of the data in  FIG. 6 , the depletion voltage can be set to 10 V or less by setting the predetermined positive potential to “+3 V” or higher, and the movement of signal charges (electrons) from the photodiode to the vertical transfer path can be facilitated. Namely, the movement of electrons can be assisted.  
         [0063]     At this time, in this embodiment, since the light shielding film  121  is provided at the position covering the low-concentration portion  116   b  of the high-concentration impurity surface layer  116 , as shown in  FIG. 3 , the light shielding film  121  performs the function of a gate electrode, allowing the signal charges of the n-type region portion  112  to move to the embedded channel  115  more easily.  
         [0064]      FIG. 7  is a graph on actually measured data illustrating changes in a readout gate turn-off voltage with respect to the applied voltage of the light shielding film. In the solid-state imaging device  100  in accordance with this embodiment, the voltage to be applied to the light shielding film is controlled to a predetermined negative voltage at all timings except for the timing for reading out the signal charges from the photodiode to the vertical transfer path. In consequence, the potential of the readout gate portion  114  becomes low except at the time of reading out the signal charges, so that the off voltage can be set higher.  
         [0065]     According to  FIG. 7 , it can be understood that in a case where the off voltage is to be set to 0 V or higher, it suffices if the voltage to be applied to the light shielding film is set to −5 V or less. Namely, by applying a negative voltage to the light shielding film  121  except at the time of reading out the signal charges, it is possible to prevent a situation in which signal charges (electrons) undesirably move from the photodiode  112  to the vertical transfer path  115  at a timing which is irrelevant to the reading out of the signal charges. It can be expected that the off voltage characteristic improves further by further lowering the voltage to be applied to the light shielding film.  
         [0066]      FIG. 8  is a graph on actually measured data illustrating changes in a breakdown voltage with respect to the applied voltage of the light shielding film. As described above, the smear can be reduced if a negative voltage is always applied to the light shielding film  121 , as described above. However, if the potential at the light shielding film kept at the negative potential at the time of reading out the signal charges, a potential difference between the light shielding film  121  and the readout electrode (to which, for example, +15 V is applied) becomes undesirably large. Hence, there is a breakdown at the element isolating region (p +  region)  113  provided between the pixel and an adjacent pixel is feared.  
         [0067]     The data shown in  FIG. 8  indicates that the lower the applied voltage of the light shielding film, the lower the breakdown voltage which causes a breakdown, and the more the breakdown is likely to occur.  
         [0068]     Therefore, in the solid-state imaging device  100  in accordance with this embodiment, the voltage to be applied to the light shielding film is controlled to a predetermined positive voltage by preceding the turning on of the readout pulse  131  for the predetermined time period t 1  until lagging behind the turning off of the readout pulse  131  for the predetermined time period t 2 .  
         [0069]     As a result, it is possible to make small the potential difference between the light shielding film and the adjacent pixel electrode at the time of reading out of the signal charges, thereby making it possible to avoid the occurrence of a breakdown. According to the data in  FIG. 8 , the characteristic curve of the voltage at which the breakdown occurs changes greatly with a voltage of “+3 V” applied to the light shielding film as a boundary. Therefore, the occurrence of the breakdown can be effectively suppressed by setting the applied voltage of the light shielding film to at least “+3 V” or higher. In addition, it is also possible to increase the margin for breakdown by setting the positive voltage to be applied to the light shielding film  121  to an even higher level.  
         [0070]     As described above, according to this embodiment, the pulse voltage to be applied to the light shielding film is applied in tune with the readout pulse, and the high-level potential and the low-level potential of this pulse voltage are adjusted as described above. Therefore, advantages can be obtained in that it is possible to improve the smear characteristics of the imaging device, improve the breakdown voltage, improve the depletion voltage, and improve the readout gate turn-off voltage.  
         [0071]     According to the invention, the voltage applied to the light shielding film is set as a pulse voltage synchronized with the readout pulse, and the high-level potential is set to a potential of the same polarity as that of the readout pulse, while the low-level potential is set to a potential of an opposite polarity to that of the readout pulse. Therefore, advantages can be obtained in that the smear characteristics improve, that the breakdown voltage improves, that the depletion voltage improves, that low readout-voltage drive is made possible, and that the readout gate turn-off voltage improves.  
         [0072]     The solid-state imaging device in accordance with the invention offers an advantage of improved device performance, and is useful as a solid-state imaging device for mounted in a digital camera or the like and as a driving method thereof.  
         [0073]     The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.