Abstract:
A solid-state imaging device includes: a photoelectric conversion section (PCS) generating signal charge from light; a charge accumulating section (CAS) accumulating the signal charge; a first charge transfer section (CTS 1 ) between the PCS and the CAS transferring the signal charge from the PCS to the CAS responsive to a control signal; and a second charge transfer section (CTS 2 ) provided for the CAS to transfer the signal charge from the CAS in response to a control signal. The CAS includes: a charge accumulation gate electrode; and a gate insulating film between the charge accumulation gate electrode and a semiconductor substrate. The gate insulating film includes: a first region (R 1 ) provided on a side of CTS 1  in a region corresponding to the CAS; and a second region (R 2 ) provided on a side of CTS 2  in the region corresponding to the CAS. R 2&#39; s gate insulating film is thicker than R 1&#39; s.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This patent application claims a priority on convention based on Japanese Patent application No. 2009-136134 filed on Jun. 5, 2009. The disclosure thereof is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a solid-state imaging device. 
       BACKGROUND ART 
       [0003]    A solid-state imaging device is mounted on a linear sensor used for an image pick-up device such as an auto-focusing sensor and scanner. The solid-state imaging device is also called a CCD imaging sensor, and includes a photodiode for carrying out photoelectric conversion to received light and an accumulating section for carrying out accumulation of charges obtained through the photoelectric conversion. In the CCD imaging sensor, as the accumulation time becomes long, a dark current sometimes increases to considerably degrade an image. A solid-state imaging device which can reduce the dark current is known (patent literatures 1 to 3). 
         [0004]    The patent literature 1 discloses the solid-state imaging device which can reduce the dark current.  FIG. 1  is a plan view showing a configuration of the solid-state imaging device disclosed in the patent literature 1. The solid-state imaging device  101  includes a photoelectric conversion section  102 , an auxiliary charge accumulating section  103 , a first control section  104 , a charge accumulating section  105 , a second control section  106 , and a charge transfer section  108 .  FIG. 2  is a cross sectional view of the solid-state imaging device  101  along the X-X′ line in  FIG. 1 . 
         [0005]    In the solid-state imaging device  101 , a signal charge generated by incidence of light to the photoelectric conversion section  102  is accumulated in a potential well of the auxiliary charge accumulating section  103 . The accumulated signal charge moves into a potential well of the charge accumulating section  105  by opening a first control gate  104  in response to a clock signal φTG 1  of a “high” level. Then, the first control gate  104  is closed in response to the clock signal φTG 1  of a “low” level. Subsequently, the accumulated signal charge is transferred under a gate electrode  182  of the charge transfer section  108  by opening a second control gate  106  in response to a clock signal φTG 2  of the “high” level. 
         [0006]    As described above, since the potential of the second control section  106  on the side of the charge transfer section  108  is higher than that of the first control section  104 , and an electrode  143  of the first control section  104  is formed over a first region  111  and a second region  112 , the signal charge can be smoothly transferred from the photoelectric conversion section  102  to the auxiliary accumulating section  103 , the first control section  104 , the charge accumulating section  105 , the second control section  106 , and the charge transfer section  108 . Additionally, in the operation, a surface of a semiconductor substrate will be in a pinning state showing a P-type, which can suppress the generation of the dark current. This can be described as follows. 
         [0007]    A constant negative-voltage is applied to a first accumulate gate electrode  132  of the auxiliary accumulating section  103 . For this reason, an interface between the first region  111  and a first accumulation gate insulating film  131  is reversed into the P-type, that is, is set to the pinning state, and the surface potential becomes a GND potential. Accordingly, thermal electrons that are generated due to crystal defects present in interface of the gate insulating film  131  are recombined with holes present in the P-type semiconductor in the inverted state, to be hard to influence the accumulated signal charge. 
         [0008]    For this reason, degradation of the signal charge due to the dark current can be suppressed, and the accumulation of signal charge for a long time or under a high temperature can be made possible. The photoelectron conversion section  102  has a so-called PIN photodiode structure of a first region  111  of an N-type and a P′-type region  121  whose concentration is higher than that of the semiconductor (silicon) substrate  110 , and can have a characteristic substantially equal to that of the first region  111 . Meanwhile, since a constant negative voltage is also applied to the accumulation gate electrode  152  of the charge accumulating section  105 , the pinning state will appear in which an interface with a second accumulation gate insulating film  151  in the second region  112  will be inverted into the P-type. 
         [0009]      FIG. 3  is a plan view showing a configuration of a solid-state imaging device disclosed in the patent literature 3.  FIG. 4  is a cross sectional view of the solid-state imaging device along the line Y-Y′ in  FIG. 3 . According to the patent literature 3, in a Frame Interline Transfer (FIT) type solid-state imaging element, an electric field intensity in the accumulating section  202  is suppressed by reducing the thickness W 1  of a gate insulating film of a light receiving section  201  and increasing the thickness W 2  of a gate insulating film of the accumulating section  202 , and thus the dark current is prevented from generating in the accumulating section  202 . 
       CITATION LIST 
       [0010]    Patent literature 1: JP 2008-258571A
 
Patent literature 2: JP-B-Heisei 8-10760
 
Patent literature 3: JP-A-Heisei 4-137763
 
       SUMMARY OF THE INVENTION 
       [0011]    The solid-state imaging device having an electric accumulating section is applied to various application fields. However, in an auto-focusing image sensor, it is specially required that an accumulation time can be changed in a wide range because of necessity to focus the sensor under a condition that there is an extraordinarily wide difference of light intensity from imaging of a night view to imaging under sunlight. Moreover, as the periphery becomes dark, a signal charge amount from a subject is less. Accordingly, it is required to reduce a dark current generated in the photoelectric conversion section and the charge accumulating section, that is, noise generated from the sensor itself. 
         [0012]    However, since the signal charge obtained in the photoelectric conversion section is amplified in an output circuit, a leakage current generated in the charge accumulating section causes a problem as a noise, even if it is in an order of 1 to 10 fA. In a structure of the solid-state imaging device  101  disclosed in the patent literature 1, since the voltage of approximately −10V is applied to the accumulation gate electrode, the leakage current is generated as the noise due to electric field between the accumulation gate and the semiconductor substrate surface and between the accumulation gate and the control gate adjacent to the accumulation gate. When the voltage is changed from −10V toward 0V to ease the electric field, the pinning state in the interface of the gate insulating film cannot be maintained and accordingly a dark current characteristic is degraded. 
         [0013]    Additionally, in the case of the patent literature 3, a gate insulating film of a V register in the accumulating section  202  is made thick so that the charge transfer to an H register  203  becomes slow. 
         [0014]    In an aspect of the present invention, a solid-state imaging device includes: a photoelectric conversion section configured to generate signal charge from light inputted thereto; a charge accumulating section configured to accumulate the signal charge supplied; a first charge transfer section provided between the photoelectric conversion section and the charge accumulating section to transfer the signal charge from the photoelectric conversion section to the charge accumulating section in response to a control signal; and a second charge transfer section provided for the charge accumulating section to transfer the signal charge from the charge accumulating section in response to a control signal. The charge accumulating section includes: a charge accumulation gate electrode; and a gate insulating film provided between the charge accumulation gate electrode and a semiconductor substrate. The gate insulating film includes: a first region provided on a side of the first charge transfer section in a region corresponding to the charge accumulating section; and a second region provided on a side of the second charge transfer section in the region corresponding to the charge accumulating section. A thickness of the gate insulating film in the second region is thicker than that of the gate insulating film in the first region. 
         [0015]    In another aspect of the present invention, a method of manufacturing a solid-state imaging device, is achieved by forming a first insulating film on a semiconductor substrate in a region for a charge accumulating section; by selectively removing the first insulating film to expose the semiconductor substrate in a first region of the region for the charge accumulating section; by forming a second insulating film to cover the exposed semiconductor substrate and the remaining first insulating film; and by forming a charge accumulation gate electrode on the second insulating film. 
         [0016]    According to the present application, an accumulation time of a signal charge obtained through the photoelectric conversion can be controlled to enlarge a dynamic range in the solid-state imaging device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain embodiments taken in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a plan view showing a configuration of a solid-state imaging device disclosed in patent literature 1; 
           [0019]      FIG. 2  is a cross sectional view showing the solid-state imaging device shown in  FIG. 1 ; 
           [0020]      FIG. 3  is a diagram showing a configuration of a conventional solid-state imaging device; 
           [0021]      FIG. 4  is a cross sectional view showing the configuration of the conventional solid-state imaging device shown in  FIG. 3 ; 
           [0022]      FIG. 5  is a diagram showing a configuration of a solid-state imaging device according to an embodiment of the present invention; and 
           [0023]      FIGS. 6A to 6D  are cross-section views showing a manufacturing process of the solid-state imaging device according to the embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Hereinafter, a solid-state imaging device of the present invention will be described in detail with reference to the attached drawings. It should be noted that the same numerals are assigned to the same components, and the repetitive description thereof will be omitted. 
         [0025]    An upper portion of  FIG. 5  is a cross sectional view showing a configuration of a solid-state imaging device  1  according to the present embodiment. A lower portion of  FIG. 5  is a potential chart showing an operation of the solid-state imaging device  1 . 
         [0026]    Referring to  FIG. 5 , the solid-state imaging device  1  of the present embodiment includes a light-receiving section  2 , a charge accumulating section  3 , a CCD section  4 , a first charge transfer section  5 , and a second charge transfer section  6 . 
         [0027]    The first charge transfer section  5  includes a first gate electrode  5 - 1  formed a silicon substrate through a second gate insulating film  9  and a second gate electrode  5 - 2  formed on the silicon substrate through a first gate insulating film  7 . A part of the first gate electrode  5 - 1  extends above the second gate electrode  5 - 2 . Impurity is implanted into a region of the silicon substrate which is located below the second gate electrode  5 - 2  and its periphery. The second charge transfer section  6  includes a first gate electrode  6 - 1  formed on the silicon substrate through the first gate insulating film  8  and a second gate electrode  6 - 2  formed on the silicon substrate through the second gate insulating film  9 . A part of the second gate electrode  6 - 2  extends above the first gate electrode  6 - 1  through the second gate insulating film  9 . The charge accumulating section  3  includes a gate electrode  3 - 1  formed on the silicon substrate through the second gate insulating film  9 . A part of the gate electrode  3 - 1  extends above the second gate electrode  5 - 2  through the second gate insulating film  9  and another port of the gate electrode  3 - 1  extends above the first gate electrode  6 - 1  through the second gate insulating film  9 . The charge accumulating section  3  is composed of a first region  11  and a second region  12 . In the charge accumulating section  3 , the second gate insulating film  9  is formed to be thicker in the second region  12  than that in the first region  11 . In other words, the film thickness of the second gate insulating film  9  in the second region  12  is formed to be thicker than that of the second gate insulating film  9  in the first region  11 . A control voltage TG 1  is applied to the first and second gate electrode  5 - 1  and  5 - 2  of the first charge transfer section  5 . Also, a control voltage TG 2  is applied to the first and second gate electrode  6 - 1  and  6 - 2  of the second charge transfer section  5 . A control voltage ST 1  is applied to the gate electrode  3 - 1 . In this way, the potential well in the first transfer section  5  has a step. Also, in the charge accumulating section  3 , the potential well has a step. 
         [0028]    By the above-mentioned structure, in the solid-state imaging device having a 2-layer gate electrode configuration according to the present embodiment, the potential wells are formed as shown in  FIG. 5 . In the first and second charge transfer sections  5  and  6 , the depths of the potential wells are changed a level shown by a solid line and a level shown by a broken line depending on the control voltage TG 1  and TG 2 , respectively. 
         [0029]    Signal charges are generated based on light incident to the light-receiving section  2 . At this time, the first charge transfer section  5  is set to a state of a deep potential well based on the control voltage TG 1 , so that the signal charges move to the first charge transfer section  5 . Then, the first charge transfer section  5  is changed to a state of a shallow potential well based on the control voltage TG 1 , so that the signal charges move to the charge accumulating section  3 . At this time, since the first charge transfer section  5  has a step portion in the potential well, the signal charges rapidly move to the charge accumulating section  3 . Also, the second charge transfer section  6  is set to a state of a shallow potential well based on the control voltage TG 2 . Therefore, the signal charges are accumulated in the charge accumulating section  3 . In this case, since the step is formed in the potential well of the charge accumulating section  3  due to the difference in the thickness of the gate insulating film  9 , the signal charges are easy to be collected in the second region  12  of the charge accumulating section  3 . After that, the second charge transfer section  6  is set to a state of a deep potential well, so that the signal charges move from the charge accumulating section  3  to the CCD section  4  through the second charge transfer section  6 . 
         [0030]    In this way, the thickness of the gate insulating film  9  in the charge accumulating section  3  is increased from the first region  11  to the second region  12  in the charge transfer direction, to generate an electric field in that direction. Thus, the signal charges are smoothly transferred without generating noise. 
         [0031]    A leakage current functioning as noise is reduced by increasing the thickness of the gate including film  9  of the charge accumulating section  3  in the second region  12  to ease the electric field between the accumulation gate electrode  3 - 1 , the semiconductor substrate surface, and the adjacent transfer gates  5 - 2  and  6 - 1 . In addition, since the thickness of the gate insulating film  9  of the charge accumulating section  3  is increased in the charge transfer direction, the electric field is generated in a region of the gate electrode  3 - 1  so as to prevent decrease of the charge transfer speed. Control sections and the charge transfer sections have the same thickness of the gate insulating films as that of the conventional example, and accordingly other performances can be maintained. 
         [0032]    A manufacturing process of the solid-state imaging device  1  according to the present embodiment will be described below.  FIG. 6A  is a cross sectional view showing a first stage of the manufacturing process of the solid-state imaging device  1  according to the present embodiment. In the first stage, impurity implantation is performed in a region of the semiconductor substrate corresponding to the gate electrode  5 - 2 . Then, the first gate insulating film is formed on the semiconductor substrate to have the thickness of 800 Å and a polysilicon film is formed and patterned on the first gate insulating film. Thus, lower layer gates  22  and  23  are formed as the gate electrodes  5 - 2  and  6 - 1 . Then, the first gate insulating film is removed by using the lower layer gates  22  and  23  as a mask. Thus, the first gate insulating film  7  and the first gate insulating film  8  are formed on the semiconductor substrate  21 . Subsequently, a second oxide film  24  is formed to have the thickness of 200 Å on the whole semiconductor substrate. 
         [0033]      FIG. 6B  is a cross section view showing a second stage of the manufacturing process of the solid-state imaging device  1 . In the second stage, a portion of the second oxide film  24  other than the second region  12  of the charge accumulating section  3  is removed or etched in a lithography process. In this manner, a remaining oxide film  24   a  is formed in the second region  12  of the charge accumulating section  3 . 
         [0034]      FIG. 6C  is a cross section view showing a third stage of the manufacturing process of the solid-state imaging device  1 . In the third stage, a third oxide film  25  is formed on the whole semiconductor substrate  21  to have the thickness of 1000 Å. The third oxide film  25  is also formed on the remaining oxide film  24   a  that has been formed on the second region  12  of the charge accumulating section  3 . In this process, the thickness of the oxide film on the second region  12  of the charge accumulating section  3  will become 1100 Å. 
         [0035]      FIG. 6D  is a cross section view showing a fourth stage of the manufacturing process of the solid-state imaging device  1 . In the fourth stage, the first gate electrode  5 - 1  in the first charge transfer section  5 , the gate electrode  3 - 1  in the charge accumulating section  3 , and the second gate electrode  6 - 2  in the second charge transfer section  6  are formed. The oxide film  25  and the remaining oxide film  24   a  function as the second gate insulating film  9 . The second gate insulating film  9  has the thickness of 1100 Å only in the second region  12  of the charge accumulating section  3 , and has a thickness of 1000 Å in other regions. 
         [0036]    As described above, the solid-state imaging device  1  according to the present embodiment has the charge accumulating section  3  in a change transfer route from the photoelectric conversion section (a pixel section)  2  to the charge transfer section (a CCD section)  4 . In the solid-state imaging device  1 , the configuration where the thickness of the gate insulating film  9  in the second region  12  of the charge accumulating section  3  is increased to be thicker than the thickness of other region of the gate insulating film  9  toward the charge transfer direction  4 . In this manner, the accumulation time can be widely varied, and even when an output circuit having a high amplification factor is mounted, the solid-state imaging device without dark current can be provided. 
         [0037]    The embodiments of the present invention have been specifically described above. However, the present invention is not limited to the above-mentioned embodiments and can be variously modified within the scope of the present invention. For example, when a design rule for a gate electrode interval is reduced, a configuration of a single-layer gate electrode can be adopted. Even in such a case, by only increasing the thickness of the gate insulating film in a partial region of the accumulation gate, the same effect can be obtained.