Abstract:
A solid state imaging device includes a transfer transistor for transferring signal charges generated by photoelectric conversion to a floating diffusion layer, a reset transistor for resetting a potential of the floating diffusion layer, and an amplifying transistor for outputting a signal corresponding to the potential of the floating diffusion layer. A low concentration impurity region having an impurity concentration lower than that of the first conductivity type semiconductor region is formed in part of a surface portion of the first conductivity type semiconductor region which is located below a gate electrode of the amplifying transistor and serves as a well region of the amplifying transistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) of Japanese Patent Application No. 2008-133884 filed in Japan on May 22, 2008, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present disclosure relates generally to solid state imaging devices used in digital still cameras and the like and methods for manufacturing the same. 
         [0003]    MOS (Metal Oxide Semiconductor) solid state imaging devices, including a MOS transistor for amplifying a signal detected by a photodiode in each pixel, have higher sensitivity as compared with CCD (Charge Coupled Device)-type solid state imaging devices. 
         [0004]      FIG. 8  is a sectional view schematically illustrating the structure of a unit pixel and an amplifying transistor of a conventional MOS solid state imaging device. In  FIG. 8 , a photodiode and a transfer transistor are shown as the unit pixel. The structure of the amplifying transfer shown in  FIG. 8  is a typical structure of transistors (reset transistors, column selecting transistors, and other transistors: they are all N-type MOS transistors) formed on a semiconductor substrate other than the transfer transistors. 
         [0005]    As shown in  FIG. 8 , a unit pixel  150  and an amplifier transistor  170  are formed in regions defined by isolation regions  102  in a surface portion of a semiconductor substrate  100  where a P-type well region  101 ( a  P-well  101 ) is formed. 
         [0006]    A photodiode  110  constituting the unit pixel  150  includes a P-type impurity region  112  and an N-type impurity region  111  formed in this order from the surface of the substrate. A transfer transistor  120  constituting the unit pixel  150  includes the N-type impurity region  111  of the photodiode  110  as a source region, a floating diffusion layer  160  as a drain region, and a gate electrode  123  formed on the P-well  101  between the N-type impurity region  111  and the floating diffusion layer  160 . The floating diffusion layer  160  is formed of a low concentration impurity region  121  adjacent to the gate electrode  123  of the transfer transistor  120  and a high concentration impurity region  122  electrically connected to the low concentration impurity region  121 . 
         [0007]    The amplifying transistor  170  has a gate electrode  173  formed on the P-well  101 , and low concentration impurity regions  171  adjacent to the gate electrode  173  and high concentration impurity regions  172  electrically connected to the low concentration impurity regions  171  as source/drain regions formed in a surface portion of the P-well  101  on both sides of the gate electrode  173 . 
         [0008]    In order to reduce parasitic resistance of contacts with the source/drain regions, metal silicide layers  124  and  174  are formed on the high concentration impurity region  122  of the floating diffusion layer  160  and the high concentration impurity diffusion regions  172  of the amplifying transistor  170 , respectively. 
         [0009]    In the conventional unit pixel  150  shown in  FIG. 8 , signal charges (electrons) generated by photoelectric conversion by the photodiode  110  and accumulated at a PN junction are transferred to the floating diffusion layer  160  when the transfer transistor  120  is brought into conduction. As the floating diffusion layer  160  is connected to the gate electrode  173  of the amplifying transistor  170 , a potential of the floating diffusion layer  160  is read through the amplifying transistor  170  to output a pixel signal. 
         [0000]    [Patent Document 1] Published Japanese Patent Application No. H7-122733 
       SUMMARY OF THE INVENTION 
       [0010]    As the solid state imaging devices use more pixels, the size of pixel cells including the amplifying transistors is becoming smaller. However, in general, it has been known that RTS (Random Telegraph Signal) noise is proportional to 1/N 2  when N is the number of carriers in the amplifying transistor. This relationship is illustrated in  FIG. 9 . The number of carriers N is defined by the equation (1): 
         [0000]        N=W×Lg×Cox ×( Vg−Vth )/ q   (1) 
         [0011]    In the equation, W represents a gate width, Lg a gate length, Cox a capacity of a gate oxide film, Vg a gate voltage, Vth a threshold voltage, and q the amount of elementary charges. As expressed by the equation (1), when the size of the amplifying transistors is reduced due to the size reduction of the pixel cells, the number of carriers N decreases, and the RTS noise is likely to occur. As a result, an S/N characteristic of the circuit may deteriorate. 
         [0012]    A possible cause of the pronounced occurrence of the RTS noise resulting from the decrease in number of carriers N may be the influence of a carrier capturing/releasing phenomenon caused by a carrier trap that exists at an interface with the gate oxide film. Specifically, the influence of the carrier capturing/releasing phenomenon, which has been averaged and less significant when the number of carriers N is large, becomes prominent as the number of carriers N decreases. 
         [0013]    To cope with this, a method has been proposed as a measure against the RTS noise in the solid state imaging devices. According to this method, phosphorus is implanted by ion implantation to a channel region of the amplifying transistor at an implantation energy (an acceleration energy) of 300 keV to form an N-type layer in a region at a distance from the interface with the oxide film. Using a buried channel transistor structure obtained by the N-type layer, the influence of the carrier trap that exists at the interface with the oxide film is reduced (e.g., see Patent Document 1 and other like documents). 
         [0014]    However, when the aforementioned buried channel layer is formed in the case where the gate width of the amplifying transistor isolated by an STI (shallow trench isolation) region is reduced as the pixel cells are reduced in size, a breakdown voltage in the isolation region is reduced and leakage is likely to occur because the depth at which the buried channel layer is formed is almost the same as or greater than the depth at which the STI region is formed. 
         [0015]    Thus, as described above, a new technique that allows improving the RTS noise characteristic by increasing the number of carriers of the amplifying transistors has been demanded. 
         [0016]    In view of the above-described disadvantages of the conventional technologies, the present disclosure has been proposed. The present disclosure proposes a solid state imaging device that allows improving the RTS noise characteristic even when the amplifying transistors isolated by the STI regions are reduced in size, and a method for manufacturing the same. 
         [0017]    In order to achieve the aforementioned purpose, the present inventor has paid attention to a method of manufacturing a solid state imaging device including, on a semiconductor layer, a plurality of pixels for transferring signal charges generated by photoelectric conversion to a floating diffusion layer and outputting a signal corresponding to a potential of the floating diffusion layer through an amplifying transistor, and a transistor constituting a peripheral circuit. Based on this method, the present inventor has come up with the idea of forming an isolation region defining a photodiode formation region and a transistor formation region in a surface portion of the semiconductor layer, forming a first conductivity type well region in the photodiode formation region and the transistor formation region, and then implanting a second conductivity type impurity by ion implantation to reduce an impurity concentration in a top surface portion of the semiconductor layer serving as a channel region of the amplifying transistor. 
         [0018]    According to the disclosed configuration, an impurity region having an impurity concentration lower than that in the well region is formed in the channel region immediately below a gate insulating film of the amplifying transistor. Therefore, a threshold voltage of the amplifying transistor can effectively be reduced, and the number of carriers is increased. As a result, the influence of a carrier trap that exists at the interface with the gate insulating film can be reduced, and the RTS noise characteristic can be improved. Further, since the number of carriers can be increased without forming a buried channel layer, leakage resulting from the reduction in breakdown voltage in the isolation region is less likely to occur, and the RTS noise characteristic can be improved even when the amplifying transistor is isolated by an STI region for the size reduction. In the present disclosure, the ion implantation of the second conductivity type impurity for forming a low concentration impurity region serving as a channel region is performed under the conditions that do not bring about reduction in carrier mobility in the amplifying transistor. 
         [0019]    More specifically, the disclosed solid state imaging device includes a transfer transistor for transferring signal charges generated by photoelectric conversion to a floating diffusion layer, a reset transistor for resetting a potential of the floating diffusion layer, and an amplifying transistor for outputting a signal corresponding to the potential of the floating diffusion layer. The solid state imaging device further includes: a first conductivity type semiconductor region serving as a well region of the amplifying transistor; and a low concentration impurity region which is formed in part of a surface portion of the first conductivity type semiconductor region below a gate electrode of the amplifying transistor, and has an impurity concentration lower than that of the first conductivity type semiconductor region. 
         [0020]    Regarding the disclosed solid state imaging device, the low concentration impurity region may substantially be an intrinsic semiconductor region. 
         [0021]    In order to obtain the aforementioned advantages of the present disclosure with reliability in the disclosed solid state imaging device, the low concentration impurity region preferably has an impurity concentration of 3×10 16  cm 3  or lower, or an impurity concentration of ½ or lower of the impurity concentration of the first conductivity type semiconductor region. Further, the low concentration impurity region is preferably formed in a region ranging from an interface with a gate insulating film of the amplifying transistor to a depth of 60 nm. 
         [0022]    Regarding the disclosed solid state imaging device, an STI region may be formed in part of the first conductivity type semiconductor region between the transfer transistor and the amplifying transistor. 
         [0023]    The disclosed method is a method for manufacturing a solid state imaging device including a transfer transistor for transferring signal charges generated by photoelectric conversion to a floating diffusion layer, a reset transistor for resetting a potential of the floating diffusion layer, and an amplifying transistor for outputting a signal corresponding to the potential of the floating diffusion layer, the method including: (a) forming a first conductivity type semiconductor region serving as a well region of the amplifying transistor on a substrate; and (b) implanting a second conductivity type impurity to a surface portion of the first conductivity type semiconductor region to form a low concentration impurity region having an impurity concentration lower than that of the first conductivity type semiconductor region and serving as a channel region of the amplifying transistor. 
         [0024]    Regarding the disclosed method for manufacturing the solid state imaging device, the implantation (b) may be performed before the formation (a). Specifically, the low concentration impurity region serving as the channel region of the amplifying transistor may be formed first, and then the first conductivity type semiconductor region serving as the well region of the amplifying transistor may be formed. 
         [0025]    Regarding the disclosed method for manufacturing the solid state imaging device, the low concentration impurity region may substantially be an intrinsic semiconductor region. 
         [0026]    Regarding the disclosed method for manufacturing the solid state imaging device, the second conductivity type impurity is arsenic or phosphorus. 
         [0027]    According to the present disclosure described above, even when the gate width of the amplifying transistor isolated by the STI region is reduced as the size of the pixel cells is reduced, the number of carriers of the amplifying transistor can be increased, and the occurrence of the RTS noise can be suppressed. As a result, the S/N characteristic of the circuit can be improved. This allows manufacture of high sensitive solid state imaging devices with high yield. 
         [0028]    Thus, the present disclosure advantageously makes it possible to manufacture the high sensitive solid state imaging devices with stability, and is useful for solid state imaging devices and methods for manufacturing the same. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1A  is a circuit diagram schematically illustrating the structure of a first example solid state imaging device, and  FIG. 1B  is a circuit diagram illustrating an enlargement of one of unit pixels shown in  FIG. 1A . 
           [0030]      FIG. 2  is a sectional view schematically illustrating the structure of the unit pixel (a photodiode, a transfer transistor and an amplifying transistor) of the first example solid state imaging device. 
           [0031]      FIGS. 3A-3D  are sectional views illustrating the processes of a method for manufacturing the first example solid state imaging device. 
           [0032]      FIGS. 4A-4C  are sectional views illustrating the processes of the method for manufacturing the first example solid state imaging device. 
           [0033]      FIGS. 5A-5C  are sectional views illustrating the processes of the method for manufacturing the first example solid state imaging device. 
           [0034]      FIG. 6  is a graph illustrating the relationship between a threshold voltage of the amplifying transistor and the magnitude of RTS noise, which is a finding of the present inventor. 
           [0035]      FIG. 7  is a graph illustrating the relationship between substantial impurity concentration in a surface portion of a substrate serving as a channel region of the amplifying transistor and the magnitude of the RTS noise, which is a finding of the present inventor. 
           [0036]      FIG. 8  is a sectional view schematically illustrating the structure of a unit pixel and an amplifying transistor of a conventional MOS solid state imaging device. 
           [0037]      FIG. 9  is a graph illustrating the relationship between a reciprocal of a square of number of carriers N of the amplifying transistor and the magnitude of the RTS noise. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 
       [0038]    Hereinafter, an example solid state imaging device and an example method for manufacturing the same according to the present disclosure will be explained with reference to the drawings. 
         [0039]      FIG. 1A  is a circuit diagram schematically illustrating the structure of a first example solid state imaging device, more specifically, a MOS solid state imaging device.  FIG. 1B  is a circuit diagram illustrating an enlargement of one of unit pixels shown in  FIG. 1A . As shown in  FIG. 1A , a light sensitive region  50  of the example MOS solid state imaging device includes a plurality of unit pixels  51  arranged in a two-dimensional array. A peripheral circuit including, for example, a vertical shift register  52  for selecting the pixels in a column direction, a horizontal shift register  53  for selecting the pixels in a line direction, and a pulse generator circuit  54  for supplying a timing pulse to the vertical shift register  52  and the horizontal shift register  53 , is formed around the light sensitive region  50 . As shown in  FIG. 1B , the unit pixel  51  includes a photodiode  10 , a transfer transistor  20 , an amplifying transistor  30 , a reset transistor  70 , and a column selecting transistor  80 . More specifically, the transfer transistor  20  includes an impurity region of the photodiode  10  as a source region, a floating diffusion layer  60  as a drain region, and a gate electrode connected to a signal line  52   a  for selection in the column direction extending from the vertical shift register  52 . The floating diffusion layer  60  is connected to a gate electrode of the amplifying transistor  30  and a source region of the reset transistor  70 . A gate electrode of the reset transistor  70  is connected to a reset signal line  52   b  extending from the vertical shift register  52 . A drain region of the amplifying transistor  30  and a drain region of the reset transistor  70  are common and they are connected to a power supply line (not shown). A source region of the amplifying transistor  30  and a drain region of the column selecting transistor  80  are common. A source region of the column selecting transistor  80  is connected to a signal line  53   a  for selection in the line direction extending from the horizontal shift register  53 , and a gate electrode of the column selecting transistor  80  is connected to a control signal line  52   c  extending from the vertical shift register  52 . 
         [0040]    When the example MOS solid state imaging device is configured into a CMOS (Complementary-MOS) image sensor, the peripheral circuit is configured by a CMOS logic circuit obtained by combining an N-type MOS transistor and a P-type MOS transistor. The transfer transistor  20 , the amplifying transistor  30 , the reset transistor  70 , and the column selecting transistor  80  constituting the unit cell  51  are all N-type MOS transistors. 
         [0041]    When the example MOS solid state imaging device is configured into a MOS solid state imaging device including the peripheral circuit configured by N-type MOS transistors only, the MOS solid state imaging device can be configured by the N-type MOS transistors only. In this case, the production processes can be simplified. 
         [0042]      FIG. 2  is a sectional view schematically illustrating the structure of the unit pixel (the photodiode, the transfer transistor, and the amplifying transistor) of the example MOS solid state imaging device. In  FIG. 2 , the same components as those shown in  FIGS. 1A and 1B  are indicated by the same reference numerals. The structure of the amplifying transistor shown in  FIG. 2  is a typical structure of transistors formed on a semiconductor substrate other than the transfer transistors (reset transistors, column selecting transistors, and other transistors: they are all N-type MOS transistors). In the MOS solid state imaging device shown in  FIG. 2 , the photodiode  10 , the transfer transistor  20 , and the amplifying transistor  30  are arranged in this order from left to right. However, as described later, an N-type impurity region of the photodiode  10  is common with a source region of the transfer transistor  20 . Therefore, in  FIG. 2 , the gate electrode and the drain region (the floating diffusion layer  60 ) of the transfer transistor  20  are indicated as the transfer transistor  20 . 
         [0043]    As shown in  FIG. 2 , the photodiode  10 , the transfer transistor  20 , and the amplifying transistor  30  are formed in regions defined by isolation (STI) regions  3  in a surface portion of a semiconductor substrate  1  where a P-type well region  2  (a P-well  2 ) is formed. 
         [0044]    The photodiode  10  includes a P-type impurity region  12  and an N-type impurity region  11  formed in this order from the surface of the substrate. The transfer transistor  20  includes the N-type impurity region  11  of the photodiode  10  as a source region, the floating diffusion layer  60  as a drain region, and a gate electrode  23  formed on the P-well  2  between the N-type impurity region  11  and the floating diffusion layer  60 . A gate insulating film below the gate electrode  23  is omitted from the figure. The floating diffusion layer  60  includes a low concentration impurity region  21  adjacent to the gate electrode  23  of the transfer transistor  20 , and a high concentration impurity region  22  electrically connected to the low concentration impurity region  21 . Metal silicide layers  24  are formed in surface portions of the high concentration impurity region  22  of the floating diffusion layer  60  and the gate electrode  23 , respectively. An insulating sidewall spacer  25  is formed on one of the side surfaces of the gate electrode  23  closer to the floating diffusion layer  60 . The low concentration impurity region  21  is formed below the insulating sidewall spacer  25 . 
         [0045]    The amplifying transistor  30  has a gate electrode  33  formed on the P-well  2 , and low concentration impurity regions  31  adjacent to the gate electrode  33  and high concentration impurity regions  32  electrically connected to the low concentration impurity regions  31  as source/drain regions formed in the surface portion of the P-well  2  on both sides of the gate electrode  33 . A gate insulating film below the gate electrode  33  is omitted from the figure. Metal silicide layers  34  are formed in surface portions of the high concentration impurity region  32  and the gate electrode  33 , respectively. Insulating sidewall spacers  35  are formed on both side surfaces of the gate electrode  33 . The low concentration impurity regions  31  are formed below the insulating sidewall spacers  35 . 
         [0046]    As a feature of the present embodiment, a low concentration impurity region  36  is formed in part of the surface portion of the P-well  2  (a channel region) below the gate electrode  33  of the amplifying transistor  30 . The impurity concentration in the low concentration impurity region  36  (e.g., about 3×10 16  cm 3  or lower) is lower than that in the P-well  2  (e.g., about 2×10 17  cm 3 ). Therefore, a threshold voltage of the amplifying transistor  30  can effectively be reduced, and the number of carriers can be increased. As a result, the influence of a carrier trap that exists at the interface with the gate insulating film can be reduced, and the RTS noise characteristic can be improved. Since the number of carriers can be increased without forming a buried channel layer, leakage resulting from the reduction in breakdown voltage in the isolation region is less likely to occur, and the RTS noise characteristic can be improved even when the amplifying transistor  30  is isolated by an STI region  3  for the size reduction. 
         [0047]    According to the present embodiment described above, the number of carriers of the amplifying transistor  30  can be increased, and the occurrence of the RTS noise can be suppressed even when the gate width of the amplifying transistor  30  isolated by the STI region  3  is reduced due to the size reduction of the pixel cells. As a result, the S/N characteristic of the circuit can be improved. This allows manufacture of high sensitive solid state imaging devices with high yield. 
         [0048]    Hereinafter, an example method for manufacturing the example MOS solid state imaging device will be explained.  FIGS. 3A-3D ,  4 A- 4 C and  5 A- 5 C are sectional views illustrating the processes of the example method for manufacturing the example MOS solid state imaging device. In  FIGS. 3A-3D ,  4 A- 4 C and  5 A- 5 C, the same components as those shown in  FIGS. 1A ,  1 B and  2  are indicated by the same reference numerals. 
         [0049]    First, as shown in  FIG. 3A , isolation regions  3  are formed in a surface portion of a semiconductor substrate  1  made of N-type silicon, for example, by a known method. The isolation region  3  may have an STI structure formed to a depth of about 300 nm, for example (hereinafter, the isolation region  3  is referred to as an STI region  3 ). 
         [0050]    Then, P-type impurities, e.g., boron, are implanted by ion implantation into element formation regions of the semiconductor substrate  1  to form a P-well  2  as shown in  FIG. 3B . The P-well  2  has an impurity concentration of about 2×10 17  cm 3 , for example. 
         [0051]    Using a resist pattern  40  having an opening corresponding to a photodiode formation region R 1  only as a mask, N-type impurities are implanted into the surface portion of the semiconductor substrate  1  by ion implantation to form an N-type impurity region  11  constituting the photodiode  10  as shown in  FIG. 3C . 
         [0052]    Then, using a resist pattern  41  having an opening corresponding to an amplifying transistor formation region R 3  only as a mask, N-type impurities are implanted into the surface portion of the semiconductor substrate  1  (a portion serving as a channel region of the amplifying transistor  30 ) to form a low concentration impurity region  36  as shown in  FIG. 3D . In this process, arsenic (As) is implanted at an implantation energy of 60 keV and a dose amount of 4.1×10 12 /cm 2  by ion implantation. Under these conditions, the low concentration impurity region  36  does not show the N-type conductivity type. The low concentration impurity region  36  having an effective impurity concentration of 3×10 16  cm 3  or lower is formed in a region ranging from the substrate surface to a depth of about 60 nm. 
         [0053]    Then, a gate insulating film (not shown) made of a silicon oxide film of about 5-10 nm in thickness is formed on the surface of the semiconductor substrate  1 , for example, by thermal oxidation. A conductive polysilicon film of about 180-200 nm in thickness is formed on the gate insulating film, for example, by reduced-pressure CVD (Chemical Vapor Deposition) or other like technique. Then, the polysilicon film is etched by known photolithography and etching to form a gate electrode  23  of the transfer transistor  20  and a gate electrode  33  of the amplifying transistor  30  on the semiconductor substrate  1  as shown in  FIG. 4A . 
         [0054]    Then, as shown in  FIG. 4B , a resist pattern  42  having an opening ranging from a position at a certain distance from a source-side edge of the gate electrode  23  of the transfer transistor  20  to the STI region  3  defining an edge of the photodiode formation region R 1  (the source region of the transfer transistor  20 ) is formed by known photolithography. Using the resist pattern  42  as a mask, P-type impurities are implanted into an upper portion of the N-type impurity region  11  by ion implantation to form a P-type impurity region  12  constituting the photodiode  10 . 
         [0055]    As shown in  FIG. 4C , using a resist pattern  43  having an opening corresponding to the transfer transistor formation region R 2  and the amplifying transistor formation region R 3 , the gate electrode  23  of the transfer transistor  20 , and the gate electrode  33  of the amplifying transistor  30  as a mask, N-type impurities are implanted by ion implantation into the surface portion of the semiconductor substrate  1  to form an N-type low concentration impurity region  21  constituting part of the drain region of the transfer transistor  20 , and N-type low concentration impurity regions  31  constituting parts of the source and drain regions of the amplifying transistor  30 . In this process, phosphorus (P) is implanted by ion implantation at an implantation energy of 45 keV and a dose amount of 2×10 13 /cm 2  and arsenic (As) is implanted by ion implantation at an implantation energy of 45 keV and a dose amount of 1.2×10 13 /cm 2 . As a result, low concentration impurity regions  21  and  31  having a junction depth of about 60 nm and a maximum impurity concentration of about 1.0×10 18 /cm 3  are formed. 
         [0056]    Then, an insulating film  16  made of, for example, a silicon oxide film or a silicon nitride film, is deposited on the whole surface of the semiconductor substrate  1  by, for example, CVD, and a resist pattern  44  covering the photodiode formation region R 1  is formed by photolithography or other like technique. Subsequently, using the resist pattern  44  as a mask, the insulating film  16  is etched back by, for example, RIE (Reactive Ion Etching). As a result, an insulating sidewall spacer  25  is formed on a side surface of the gate electrode  23  on the drain side, and insulating sidewall spacers  35  are formed on both side surfaces of the gate electrode  33  as shown in  FIG. 5A . The insulating film  16  covering the photodiode formation region R 1  remains below the resist pattern  44 . 
         [0057]    Then, the resist pattern  44  is removed by ashing or other like technique. N-type impurities, e.g., phosphorus (P), are implanted into the surface portion of the semiconductor substrate  1  by ion implantation using the gate electrode  23  and the insulating sidewall spacer  25  of the transfer transistor  20 , the gate electrode  33  and the insulating sidewall spacers  35  of the amplifying transistor  30 , and the remaining insulating film  16  as a mask to form an N-type high concentration impurity region  22  constituting part of the drain region of the transfer transistor  20  (the floating diffusion layer  60 ), and N-type high concentration impurity regions  32  constituting parts of the source and drain regions of the amplifying transistor  30  as shown in  FIG. 5B . The phosphorus is implanted by ion implantation at an implantation energy of 10 keV and a dose amount of 1×10 15 /cm 2 . As a result, high concentration impurity regions  22  and  23  having a junction depth of about 200 nm and a maximum impurity concentration of about 1×10 20 /cm 3  are formed. After the phosphorus ion implantation, activation annealing is performed, for example, at 850° C. for about 10 minutes. 
         [0058]    Then, a refractory metal film made of cobalt, for example, is deposited on the whole surface of the semiconductor substrate  1 , and then lamp annealing is performed. As a result, as shown in  FIG. 5C , metal silicide layers are formed in the surface portions of the silicon substrate and the polysilicon film (the gate electrodes) which are in direct contact with the refractory metal film. In the present embodiment, metal silicide layers  24  made of cobalt silicide are formed in the surface portions of the high concentration impurity region  22  and the gate electrode  23  of the transfer transistor  20 . Likewise, metal silicide layers  34  made of cobalt silicide are formed in the surface portions of the high concentration impurity regions  32  and the gate electrode  33  of the amplifying transistor  30 . The metal silicide layers  24  and  34  are formed by two-step annealing. Specifically, first annealing is performed at 440° C. for 66 seconds, and then second annealing is performed at 750° C. for 36 seconds. Part of the refractory metal film unreacted with the silicon substrate and the polysilicon film is selectively removed by wet etching or other like technique after the first annealing. 
         [0059]    Subsequently, an interlayer insulating film is deposited on the whole surface of the semiconductor substrate  1 . Then, contact holes reaching the metal silicide layers  24  and  34  are formed in the interlayer insulating film. Further, upper wires are formed in the contact holes and on the interlayer insulating film. Thus, the MOS solid state imaging device is completed. 
         [0060]    In the present embodiment described above, ion implantation is performed to implant arsenic ions, for example, into the surface portion of the substrate serving as a channel region of the amplifying transistor  30 , so that the low concentration impurity region  36  having an impurity concentration lower than that of the P-well  2  is formed as the channel region of the amplifying transistor  30 . Therefore, the threshold voltage of the amplifying transistor  30  can effectively be reduced, and the number of carriers can be increased. This makes it possible to reduce the influence of the carrier trap that exists at the interface with the gate insulating film, and therefore the RTS noise characteristic can be improved. 
         [0061]    As described above, when the impurity concentration in the surface portion of the substrate serving as the channel region of the amplifying transistor  30  is substantially reduced, the threshold voltage is reduced. Therefore, the number of carriers is increased, and the RTS noise is reduced. In connection to this, as shown in  FIG. 6 , the present inventor has found that there is a local minimum value of the RTS noise in the plot of the relationship between the threshold voltage of the amplifying transistor  30  and the magnitude of the RTS noise. Referring to the graph of  FIG. 6 , a region where the RTS noise is reduced as the threshold voltage is reduced is indicated as R VA . However, when an arsenic concentration in the surface portion of the substrate serving as the channel region is increased and the conductivity type varies to N, the RTS noise gradually increases. This region is indicated as R VB  in  FIG. 6 . This is because the charge amount at the interface with the gate insulating film (oxide film) is increased when the conductivity type of the channel region varies from P to N. Then, the mobility of the charges at the interface is reduced, and the RTS noise is gradually increased. Therefore, the RTS noise is minimized at a threshold voltage V th  where the substantial impurity concentration in the surface portion of the substrate serving as the channel region of the amplifying transistor  30  is the lowest (in other words, where the low concentration impurity region  36  is a substantially intrinsic semiconductor region). Specifically, in the present embodiment, the RTS noise is minimized at a threshold voltage of −0.3 V where the impurity concentration in the low concentration impurity region  36  is set to about 3×10 16  cm 3  or lower. At this time, the RTS noise is effectively reduced by about 4 dB as compared with the conventional amplifying transistor whose channel region is formed of a high concentration P-well only. 
         [0062]    As described above, in order to effectively reduce the RTS noise, it is necessary to reduce the substantial impurity concentration in the surface portion of the substrate serving as the channel region of the amplifying transistor to be lower than the impurity concentration in the well region. Specifically, as shown in  FIG. 7 , the impurity concentration is preferably set to be not higher than Cs which is a concentration at which the RTS noise comes to the level L N , which is a level at which the S/N characteristic of the circuit does not deteriorate. According to the present embodiment, it has been found that the deterioration of the S/N characteristic of the circuit can be prevented by forming the low concentration impurity region  36  having an impurity concentration of about 1×10 17  cm 3  or lower, which is half the impurity concentration of the P-well  2  (about 2×10 17  cm 3 ), in a region ranging from the substrate surface (the interface with the gate insulating film) to a depth of 60 nm. 
         [0063]    As described above, in the present embodiment, N-type impurities are implanted into the surface portion of the substrate (the surface portion of the P-well  2 ) serving as the channel region of the amplifying transistor  30  at a relatively low concentration so as to form the low concentration impurity regions  36  having an impurity concentration substantially lower than that of the P-well  2 . As a result, the number of carriers of the amplifying transistor  30  can be increased, and the occurrence of the RTS noise can be suppressed. This allows manufacture of high sensitive solid state imaging device with high yield. 
         [0064]    It should be noted that the present disclosure is not limited to the above embodiment and various modifications and applications are possible within the spirit and essential features of the present disclosure. A feature of the present disclosure lies in reducing the impurity concentration in the surface portion of the substrate serving as the channel region of the amplifying transistor within such a range that does not reduce the charge mobility due to the increase in interface charge amount, thereby reducing the threshold voltage of the amplifying transistor. Therefore, within the extent of the technical concept, the aforementioned processes can be replaced with other equivalent processes. The process sequence may be changed, and a different material seed can be used. For example, in the present embodiment, the low concentration impurity regions  36  are formed after the P-well  2  is formed. Instead of this, the low concentration impurity regions  36  may be formed first, and then the P-well  2  may be formed. Further, in the present embodiment, the N-type impurity region  11  constituting the photodiode  10  is formed before the gate electrodes  23  and  33  are formed, more specifically, before the formation of the low concentration impurity regions  36  and after the formation of the P-well  2 . This sequence may be changed so that the N-type impurity region  11  is formed after the formation of the gate electrodes  23  and  33  using a resist pattern having an opening corresponding to the photodiode formation region R 1  only. Alternatively, the P-well  2  may be formed after the N-type impurity region  11  is formed. Further, the low concentration impurity regions  36  may be formed by ion implantation using phosphorus (P) instead of the ion implantation using arsenic (As) performed in the present embodiment. 
         [0065]    The present disclosure is particularly suitable for the manufacture of the MOS solid state imaging devices, but is also applicable to various kinds of solid state imaging devices including a floating diffusion layer and an amplifying transistor connected to the floating diffusion layer through a gate electrode thereof. More specifically, high sensitive solid state imaging devices can be realized by applying the channel structure of the amplifying transistor of the present embodiment to the channel structure of the amplifying transistor of the various kinds of solid state imaging devices.