Patent Publication Number: US-9893109-B2

Title: Method for manufacturing a solid state image sensor with pixels having photodiodes patterned through overlapping divided exposure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/077,011, filed Mar. 22, 2016, which claims priority from Japanese Patent Application No. 2015-067714 filed on Mar. 27, 2015 including the specification, drawings, and abstract is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a semiconductor device, in particular, to a technology effective when applied to a semiconductor device including a solid state image sensor. 
     When an image sensor (picture element) to be used in digital cameras or the like has a large chip size in order to have an improved image quality, divided exposure treatment is performed a plurality of times during a manufacturing step of it because single exposure treatment is insufficient for the exposure of the entire chip. 
     It is known that in a solid state image sensor used in digital cameras having an autofocus system function to which an image-plane phase-detection technology has been applied, a plurality of pixels configures the image sensor and they are each equipped with two or more photodiodes. In this case, at the time of focusing, two photodiodes in a pixel having one microlens have an equal imaging output in principle. 
     Patent Document 1 (Japanese Unexamined Patent Application Publication No. 1994-324474) describes discrete and irregular arrangement of pixels at the connection making inconspicuous image abnormalities at the connection due to divided exposure. 
     Patent Document 2 (Japanese Unexamined Patent Application Publication No. 1997-190962) describes a boundary of divided exposure in nonlinear form. 
     Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2003-005346) describes a method of dividing a pixel pattern with a zigzag dividing line into a plurality of divided regions and forming a double exposure pattern, which is a pattern exposed doubly, between the divided regions adjacent to each other. 
     Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2014-102292) describes a photomask having an overlapped region between two divided regions and equipped with a plurality of light shielding patterns, a light transmission portion, and a light reduction portion. The light reduction portion has a light transmittance greater than that of the light shielding pattern and smaller than that of the light transmission portion. 
     Patent Document 5 (Japanese Unexamined Patent Application Publication No. 2008-008729) describes a connector exposure region placed so as to locate the width-direction center of the connector exposure region at the center on a line connecting the respective centers of oscillators above and below the connector exposure region. 
     PATENT DOCUMENTS 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 1994-324474 
     [Patent Document 2] Japanese Unexamined Patent Application Publication No. 1997-190962 
     [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-005346 
     [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2014-102292 
     [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2008-008729 
     SUMMARY 
     When a chip having a large area is formed by divided exposure, exposure treatment is performed using different masks according to exposure steps to be performed twice or more. Size variation or misregistration may then occur due to the masks or an exposure apparatus used. In this case, due to misregistration in a distance among respective patterns formed using a plurality of masks, there may occur problems such as image abnormalities resulting from a difference in output value in an image sensor or prevention of normal autofocus detection. In particular, in an image or picture obtained by imaging, linear image abnormalities may appear at a position of a solid state image sensor corresponding to a boundary between regions exposed through masks. 
     Another object and novel features will be apparent from the description herein and accompanying drawings. 
     Of the embodiments disclosed herein, a typical one will next be outlined simply. 
     In a semiconductor device according to one embodiment, a first exposure region having a first region and a second exposure region having a second region overlap with each other in a third region between the first region and the second region; and in a pixel formed in the third region, a photodiode formed through a mask for the first exposure region is placed at a position closer to the side of the second region than a photodiode formed through a mask for the second exposure region is. 
     According to the one embodiment disclosed by the present application, a semiconductor device having improved performance can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing the configuration of a semiconductor device of First Embodiment of the invention; 
         FIG. 2  is a planar layout showing a partially enlarged view of  FIG. 1 ; 
         FIG. 3  is a planar layout showing the semiconductor device of First Embodiment of the invention; 
         FIG. 4  is a cross-sectional view taken along the line A-A of  FIG. 3 ; 
         FIG. 5  is an equivalent circuit diagram showing the semiconductor device of First Embodiment of the invention; 
         FIG. 6  is a plan view for describing a manufacturing step of the semiconductor device of First Embodiment of the invention; 
         FIG. 7  is a plan view for describing a manufacturing step of the semiconductor device following that of  FIG. 6 ; 
         FIG. 8  is a plan view for describing a manufacturing step of the semiconductor device following that of  FIG. 7 ; 
         FIG. 9  is a plan view for describing a manufacturing step of the semiconductor device following that of  FIG. 8 ; 
         FIG. 10  is a planar layout showing a semiconductor device of Modification Example 1 of First Embodiment of the invention; 
         FIG. 11  is a planar layout showing another semiconductor device of Modification Example 1 of First Embodiment of the invention; 
         FIG. 12  is a planar layout showing a further semiconductor device of Modification Example 1 of First Embodiment of the invention; 
         FIG. 13  is a planar layout showing a semiconductor device of Modification Example 2 of First Embodiment of the invention; 
         FIG. 14  is a planar layout showing another semiconductor device of Modification Example 2 of First Embodiment of the invention; 
         FIG. 15  is a planar layout showing a further semiconductor device of Modification Example 2 of First Embodiment of the invention; 
         FIG. 16  is a planar layout showing a semiconductor device of Modification Example 3 of First Embodiment of the invention; 
         FIG. 17  is a planar layout showing another semiconductor device of Modification Example 3 of First Embodiment of the invention; 
         FIG. 18  is a planar layout showing a further semiconductor device of Modification Example 3 of First Embodiment of the invention; 
         FIG. 19  is a planar layout showing a semiconductor device of Modification Example 4 of First Embodiment of the invention; 
         FIG. 20  is a planar layout showing another semiconductor device of Modification Example 4 of First Embodiment of the invention; 
         FIG. 21  is a planar layout showing a further semiconductor device of Modification Example 4 of First Embodiment of the invention; 
         FIG. 22  is a planar layout showing a still further semiconductor device of Modification Example 4 of First Embodiment of the invention; 
         FIG. 23  is a planar layout showing a semiconductor device of Second Embodiment of the invention; 
         FIG. 24  is a planar layout showing a semiconductor device of a modification example of Second Embodiment of the invention; 
         FIG. 25  is a planar layout showing a semiconductor device of another modification example of Second Embodiment; and 
         FIG. 26  is a planar layout showing a semiconductor device of Comparative Example. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention will hereinafter be described in detail based on drawings. In all the drawings for describing the embodiments, members having the same function will be identified by the same reference numerals and overlapping descriptions will be omitted. In the present invention, a photodiode inside each of pixels identified by the same reference numeral has the same configuration. 
     In the following embodiments, a description on the same or similar portions will not be repeated in principle unless particularly necessary. The term “mask” as used herein is a photomask (reticle) to be used during exposure in a photolithography step except for a hard mask or photoresist film to be used as a protecting film for etching or ion implantation. 
     First Embodiment 
     A semiconductor device of the present embodiment will hereinafter be described referring to  FIGS. 1 to 5 . The semiconductor device of the present embodiment relates to a solid state image sensor, in particular, a solid state image sensor having a plurality of photodiodes in one pixel. 
       FIG. 1  is a schematic view showing the configuration of the solid state image sensor according to the present embodiment. The solid state image sensor included in the semiconductor device of the present embodiment is a CMOS (complementary metal oxide semiconductor) image sensor. As is shown in  FIG. 1 , it is equipped with a pixel array portion PEA, readout circuits CC 1  and CC 2 , an output circuit OC, a row selection circuit RC, and a control circuit COC. 
     The pixel array portion PEA has therein a plurality of pixels PE in matrix form. An X-axis direction shown in  FIG. 1  is a direction along the main surface of a semiconductor substrate configuring the solid state image sensor and is also a direction along a row direction along which pixels PE have been arranged. A Y-axis direction along the main surface of the semiconductor substrate and orthogonal to the X-axis direction is a direction along a column direction along which the pixels PE have been arranged. In short, the pixels PE are juxtaposed in matrix form. 
     The pixels PE each generate a signal depending on the intensity of irradiated light. The row selection circuit RC selects the pixels PE on a row unit basis. The pixels PE selected by the row selectin circuit RC output the signal thus generated to an output line (refer to  FIG. 5 ) which will be described later. The readout circuits CC 1  and CC 2  are opposite to each other in the Y-axis direction so as to sandwich therebetween the pixel array portion PEA. The readout circuits CC 1  and CC 2  each read out a signal output from the pixel PE to the output line OL and output it to the output circuit OC. 
     The readout circuit CC 1  reads out the signal of half of the pixels PE on the side of the readout circuit CC 1  and the readout circuit CC 2  reads out the signal of the remaining half pixels on the side of the readout circuit CC 2 . The output circuit OC outputs the signal of the pixels PE readout by the readout circuits CC 1  and CC 2  to outside the solid state image sensor. The control circuit COC integrally manages the operation of the entire solid state image sensor and controls the operation of the other constituent elements of the solid state image sensor. 
     Next,  FIGS. 2 and 3  each show a planar layout of pixels PE.  FIG. 4  shows a cross-sectional view taken along the line A-A of  FIG. 3 .  FIG. 2  is a planar layout showing a partially enlarged view of the pixel array portion PEA shown in  FIG. 1 .  FIG. 3  is an enlarged planar layout showing three pixels PE 1  to PE 3  shown in  FIG. 2 .  FIGS. 2 and 3  omit an interlayer insulating film, a wiring, and the like provided on photodiodes and transistors at the periphery thereof.  FIG. 2  shows only a microlens possessed by each pixel and two photodiodes formed in each pixel. 
     As shown in  FIG. 2 , a semiconductor substrate configuring the solid state image sensor has, on the upper surface thereof, a plurality of pixels PE 1 , PE 2 , and PE 3  arranged in matrix (array) form in the X-axis and Y-axis directions. The pixels PE 1 , PE 2 , and PE 3  correspond to the plurality of pixels PE shown in  FIG. 1 .  FIG. 2  shows a first exposure region IG 1  and a second exposure region IG 2  configuring the pixel array portion PEA (refer to  FIG. 1 ) and also shows a first region  1 A, a second region  2 A, and a third region  3 A that divide the first exposure region IG 1  and the second exposure region IG 2  into three regions. 
     In the plan view or planar layout of the present application, a photodiode formed through a mask for first exposure region IG 1  is hatched to facilitate understanding of them. A photodiode formed through a mask for second exposure region IG 2  is, on the other hand, not hatched. 
     The first exposure region IG 1  and the second exposure region IG 2  overlap, at each end portion thereof, with each other at the center portion of the pixel array portion PEA in the X-axis direction. The first region  1 A is a region which is in the first exposure region IG 1  but does not overlap with the second exposure region IG 2  in plan view; the second region  2 A is a region which is in the second exposure region IG 2  but does not overlap with the first exposure region IG 1  in plan view; and the third region  3 A is a region where the first exposure region IG 1  and the second exposure region IG 2  overlap with each other in plan view. 
     In other words, the first exposure region IG 1  has the first region  1 A and the third region  3 A and the second exposure region IG 2  has the second region  2 A and the third region  3 A. For example, the third region  3 A is a region having an X-axis direction width smaller than that of the first region  1 A or the second region  2 A. The first region  1 A and the second region  2 A are almost equal in area. This means that the first exposure region IG 1  and the second exposure region IG 2  are almost equal in area. 
       FIG. 2  shows the contour of each of the first exposure region IG 1  and the second exposure region IG 2  by a dotted line.  FIG. 2  shows the structure in which five pixels are arranged in both the X-axis direction and the Y-axis direction, but the number of pixels in an actual structure is larger in both the X-axis direction and in the Y-axis direction. 
     In the first region  1 A, a plurality of pixels PE 1  is placed in matrix form in both the X-axis direction and Y-axis direction. In the second region  2 A, a plurality of pixels PE 2  is arranged in matrix form in both the X-axis direction and Y-axis direction. In the third region  3 A between the first region  1 A and the second region  2 A, a plurality of pixels PE 3  is arranged in the Y-axis direction. The pixels PE 1 , PE 2 , and PE 3  are arranged in matrix form. This means that a plurality of pixels PE 1  and a plurality of pixels PE 2 , and a pixel PE 3  are arranged in the X-axis direction (first direction). The pixels PE 1  to PE 3  arranged in the X-axis direction configure a single row and a plurality of this rows arranged in the Y-axis direction (second direction) configure the pixel array portion PEA (refer to  FIG. 1 ). 
     The pixels PE 1  to PE 3  each have a microlens ML. The pixels PE 1  to PE 3  each have two photodiodes that overlap with the microlens ML in plan view. More specifically, the pixels PE 1  each have photodiodes PD 1  and PD 2  formed on the main surface of a semiconductor substrate; the pixels PE 2  each have photodiodes PD 3  and PD 4  formed on the main surface of the semiconductor substrate; and the pixels PE 3  each have photodiodes PD 3  and PD 2  formed on the main surface of the semiconductor substrate. The photodiodes PD 1  to PD 4  each have a substantially rectangular shape in plan view. 
     When the first direction is a direction extending from the side of the first region  1 A to the side of the second region  2 A, the photodiodes PD 1  and PD 2  in the pixel PE 1  are juxtaposed in this order in the first direction and the photodiodes PD 3  and PD 4  in the pixel PE 2  are juxtaposed in this order in the first direction. In other words, in the pixel PE 1 , the photodiode PD 2  is placed in a region closer to the second region  2 A than the photodiode PD 1  is and in the pixel PE 2 , the photodiode PD 3  is placed in a region closer to the first region  1 A than the photodiode PD 4  is. 
     In the pixel PE 3 , the photodiode PD 2  is placed in a region closer to the second region  2 A than the photodiode PD 3  is. This means that in the pixel PE 3 , the photodiode PD 3  is placed in a region closer to the first region  1 A than the photodiode PD 2  is. The photodiodes PD 1  and PD 2  are arranged in the first direction in the pixel PE 1  and the photodiodes PD 3  and PD 4  are arranged in the first direction in the pixel PE 2 , while the photodiodes PD 3  and PD 2  are, in a strict sense, not arranged in the first direction in the pixel PE 3  and one of the photodiodes PD 3  and PD 2  is arranged at a position deviated from the other one in one direction. 
     Strictly speaking, the photodiodes PD 1  and PD 2  in the pixel PE 1  and the photodiodes PD 3  and PD 4  in the pixel PE 2  are not juxtaposed in the first direction and the photodiodes PD 1  and PD 2  are arranged at a position deviated from the photodiodes PD 3  and PD 4  in one direction. This means that the photodiodes PD 1  and PD 2  in the pixels PE 1  and PE 3  are arranged at a position deviated from the photodiodes PD 3  and PD 4  in the pixels PE 2  and PE 3  in the same direction. 
     A distance between the photodiodes PD 1  and PD 2  in the pixel PE 1  is equal to that between the photodiodes PD 3  and PD 4  in the pixel PE 2 . In the pixel PE 3 , on the other hand, since there is a deviation between the respective formation positions of the photodiode PD 2  and the photodiode PD 3 , a distance between these two photodiodes in the pixel PE 3  is different from that between two photodiodes in the pixel PE 1  or the pixel PE 2 . 
     As described above, among the photodiodes PD 1  to PD 4  formed on the main surface of the semiconductor substrate, the photodiodes PD 1  and PD 2  and the photodiodes PD 3  and PD 4  have therebetween a deviation in the formation position. The deviation occurs because the formation position of the photodiodes PD 1  and PD 2  and the formation position of the photodiodes PD 3  and PD 4  are defined by exposure using respective masks to be used in a step of forming a solid state image sensor. More specifically, the positions of the photodiodes PD 1  and PD 2  are defined by a pattern of a mask used for exposure of the first exposure region IG 1 , while the positions of the photodiodes PD 3  and PD 4  are defined by a pattern of another mask used for exposure of the second exposure region IG 2 . 
     In the solid state image sensor configuring the semiconductor device of the present embodiment, a semiconductor chip (mage sensor) has a considerably large area and this area is larger than that exposable through a single mask. The first exposure region IG 1  and the second exposure region IG 2  on the main surface of the semiconductor chip are therefore formed by divided exposure with two masks, respectively. In this case, when these two masks are used for respective exposure steps, these masks cannot easily be aligned correctly and there occurs a deviation in the formation position between the photodiodes PD 1  and PD 2  formed in the first exposure region IG 1  and the photodiodes PD 3  and PD 4  formed in the second exposure region IG 2 . 
     A specific layout of a plurality of photodiodes deviated from each other in a formation position will hereinafter be described referring to  FIG. 3 , that is, an enlarged plan view. 
     As shown in  FIG. 3 , the pixels PE 1  to PE 3  each have one microlens ML and two photodiodes in a light receiving portion. In the pixel PE 1 , the microlens ML and the photodiodes PD 1  and PD 2  are arranged so as to overlap with each other in plan view. Similarly in the pixel PE 2 , the microlens ML and the photodiodes PD 3  and PD 4  overlap with each other in plan view. Similarly, in the pixel PE 3 , the microlens ML and the photodiodes PD 2  and PD 3  overlap with each other in plan view. In this drawing, the contour of the microlens ML is shown by a dotted line. 
     The pixel PE 1  has, around the light receiving portion thereof, a plurality of peripheral transistors and a substrate contact portion (not shown) and the light receiving portion, peripheral transistors, and substrate contact portion are each, at the peripheral portion of the active region thereof, surrounded by an element isolation region EI. The term “peripheral transistors” as used herein means a reset transistor RST, an amplifier transistor AMI, and a selection transistor SEL. 
     The active region AR including the light receiving portion has almost a rectangular shape in plan view. In one of the pixels PE 1 , peripheral transistors are formed in one active region and this active region extends in the X-axis direction along one side of the active region AR of the light receiving portion. Although not shown here, an active region configuring the substrate contact portion, for example, extends in the Y-axis direction along the other side of the active region AR of the light receiving portion or, for example, is formed in land form in the vicinity of the active region AR. 
     The active region AR has, along the other side thereof opposite to the side where the peripheral transistors are, a transfer transistor TX 1  having, as a source region, the photodiode PD 1  of the active region AR and a transfer transistor TX 2  having, as a source region, the photodiode PD 2  of the active region AR. In other words, in the active region AR, the photodiodes PD 1  and PD 2  are juxtaposed in the X-axis direction and the transfer transistors TX 1  and TX 2  are juxtaposed in the X-axis direction while corresponding to the photodiodes PD 1  and PD 2 , respectively. 
     The peripheral transistors each have a gate electrode GE extending in the Y-axis direction and the transfer transistors TX 1  and TX 2  each have a gate electrode GE extending in the X-axis direction. The gate electrodes GE are each made of, for example, polysilicon and are formed via a gate insulating film (not shown) on the semiconductor substrate. 
     In the active region having therein the peripheral transistors, the reset transistor RST, the amplifier transistor AMI, and the selection transistor SEL are juxtaposed successively in the X-axis direction. The reset transistor RST and the amplifier transistor AMI have a drain region in common. The source region of the reset transistor RST is coupled to the drain of the transfer transistors TX 1  and TX 2 , that is, a floating diffusion (floating diffusion portion) FD. The source region of the amplifier transistor AMI functions as a drain region of the selection transistor SEL. The selection transistor SEL has a source region coupled to an output line OL as will be described later in  FIG. 5 . 
     As shown in  FIG. 3 , the respective drain regions of the transfer transistors TX 1  and TX 2 , the source region of the selection transistor SEL, the source region of the reset transistor RST, and the drain region of the amplifier transistor AMI are N +  type semiconductor regions formed in the main surface of the semiconductor substrate and the substrate contact portion (not shown) is a P +  type semiconductor region formed in the main surface of the semiconductor substrate. These semiconductor regions have, on the upper surface thereof, contact plugs CP, respectively. Although not illustrated here, contact plugs are also on the upper surface of each of the gate electrodes GE. 
     The substrate contact portion is a region to which a ground potential GND (refer to  FIG. 5 ) is to be applied. This portion plays a role of preventing variation in the threshold voltage of the peripheral transistors by fixing the potential of a well in the upper surface of the semiconductor substrate to 0 V. 
     The photodiodes PD 1  and PD 2  arranged in the X-axis direction in the active region AR which is a light receiving portion are each a semiconductor element extending in the Y-axis direction. This means that the longer direction of each of the photodiodes PD 1  and PD 2  extends along the Y-axis direction. 
     As will be described later referring to  FIG. 4 , the photodiode PD 1  is comprised of an N −  type semiconductor region N 1  formed in the main surface of the semiconductor substrate and a well region WL which is a P type semiconductor region. Similarly, the photodiode PD 2  is comprised of an N −  type semiconductor region N 2  formed in the main surface of the semiconductor substrate and the well region WL. The photodiodes PD 1  and PD 2  which are light receiving elements shown in  FIG. 3  can be regarded as those formed in the formation regions of the N −  type semiconductor regions N 1  and N 2 . A region, in the active region AR, other than the region having therein the N −  type semiconductor regions N 1  and N 2  is a P −  type well region WL. 
     The active region AR has almost a rectangular shape in plan view and one of four sides configuring the rectangle has two protrusions. One of the protrusions has a drain region (floating diffusion FD) of the transfer transistor TX 1  and the other protrusion has drain region (floating diffusion FD) of the transfer transistor TX 2 . Gate electrodes GE straddle over these two protrusions, respectively. 
     These two protrusions are coupled to each other. This means that the active region AR has a closed layout including the rectangular pattern and two protrusion patterns protruding from one of the side of the rectangular pattern and coupled to each other. A region surrounded by the closed active region AR has therein an element isolation region EI similar to that outside the active region AR. The two protrusions are not necessarily coupled to each other on the main surface of the semiconductor substrate SB. In other words, the active region AR does not necessarily have a closed structure. In this case, the floating diffusions FD of the transfer transistors TX 1  and TX 2  are electrically coupled to each other via a contact plug or a wiring on the semiconductor substrate. 
     The pixel PE 2  has a structure similar to that of the pixel PE 1  which has been described above. Described specifically, the pixel PE 2  has the photodiodes PD 3  and PD 4  juxtaposed in the X-axis direction in the active region AR that overlaps with the microlens ML in plan view. The active region AR has, in the vicinity thereof, peripheral transistors. The pixels PE 1  and PE 2  each have no level difference at the center of two sides of the active region AR parallel to the X-axis direction except for the above-described protrusions. This means that no deviation of the layout occurs in these pixels. 
     The pixel PE 3  has a structure substantially similar to that of the pixel PE 1  or PE 2 , but the rectangular active region AR of the pixel PE 3  has, on one of four sides thereof having two protrusions, a level difference DP at the center between these two protrusions. Similarly, another level difference DP is on a side of the active region AR parallel to the above-described one side. These level differences DP on these two sides of the active region AR of the pixel PE 3  is at a position overlapping with a predetermined line in plan view. This straight line is indicated by a two-dot chain line in  FIG. 3 . This also applies to other pixels PD 3  arranged in the Y-axis direction (refer to  FIG. 2 ). 
     This straight line is a boundary line (which may hereinafter be called “boundary line DL” simply) between regions exposed through two different masks during exposure of a photoresist film in a lithography step for forming the element isolation region EI and defining the active region AR. Although not indicated by a two-dot chain line, a boundary line between regions exposed through two respectively different masks is present also between the pixels PE 1  and PE 3  or the pixels PE 3  and PE 2 . This means that the end portion of the second exposure region IG 2  that overlaps with the first exposure region IG 1  is a boundary between exposure regions. Similarly, the end portion of the first exposure region IG 1  that overlaps with the second exposure region IG 2  is a boundary between exposure regions. 
     A region of the third region  3 A including the photodiode PD 3  of the pixel PE 3  is a region where the layout of each element is defined by a mask for exposing the second exposure region IG 2 , while a region of the third region  3 A including the photodiode PD 2  of the pixel PE 3  is a region where the layout of each element is defined by a mask for exposing the first exposure region IG 1 . 
     This means that the photodiodes PD 1  and PD 2  of the pixel PE 1  and the photodiode PD 2  of the pixel PE 3  are light receiving elements formed through a mask for exposing the first exposure region IG 1  and the pixel PE 1  and the photodiode PD 2  of the pixel PE 3  have therebetween a photodiode PD 3  formed through a mask for exposing the second exposure region IG 2 . Similarly, the photodiodes PD 3  and PD 4  of the pixel PE 2  and the photodiode PD 3  of the pixel PE 3  are light receiving elements formed through a mask for exposing the second exposure region IG 2  and the pixel PE 2  and the photodiode PD 3  of the pixel PE 3  have therebetween a photodiode PD 2  formed through a mask for exposing the first exposure region IG 1 . 
     In the present application, such a state is called mixed arrangement of photodiodes formed by two masks (right and left masks) to be used for exposing the first exposure region IG 1  and the second exposure region IG 2 , respectively. 
     In other words, in the third region  3 A where the first exposure region IG 1  overlaps with the second exposure region IG 2 , an element in a region on the side closer to the second region  2 A with respect to the boundary line DL is formed through the mask for exposing the first exposure region IG 1  and an element in a region on the side closer to the first region  1 A with respect to the boundary line DL is formed through the mask for exposing the second exposure region IG 2 . The photodiodes PD 1  and PD 2  in the first region  1 A and the third region  3 A are therefore arranged in matrix form along the X-axis direction and the Y-axis direction, while the photodiodes PD 3  and PD 4  in the second region  2 A and the third region  3 A are arranged in matrix form along the X-axis direction and the Y-axis direction. 
     The photodiodes PD 3  and PD 4  are, on the other hand, at a position deviated from the photodiodes PD 1  and PD 2  in a specific one direction. In the present embodiment, since a plurality of photodiodes is formed by divided exposure in the pixel array portion of the solid state image sensor, formation position of some photodiodes is deviated from that of the other photodiodes. The distance between two photodiodes in each of the pixels PE 1  and PE 2  formed in the first region  1 A and the second region  2 A is constant, but it is different from the distance between the photodiodes PD 2  and PD 3  in the pixel PE 3  in the third region  3 A. 
     The boundary line DL overlaps with all the pixels PE 3  of a specific column, but does not overlap with the pixels PE 1  and PE 2  of another column. The boundary line DL overlaps with the active region AR of each of the pixels PE 3 , but does not overlap with the photodiodes PD 2  and PD 3 . This means that the deviation caused by divided exposure occurs at a position along the Y-axis direction between the photodiodes PD 2  and PD 3  of the pixel PE 3 . 
     The boundary line DL extends in the Y-axis direction, that is, in a longer direction of each of the photodiodes PD 1  to PD 4 . In the vicinity of the active region AR of the pixel PE 3 , a level difference is present at a position which is in the active region having therein the peripheral transistors and lying between the amplifier transistor AMI and the selection transistor SEL and overlaps with the boundary line DL. No contact plug is coupled to the main surface of the semiconductor substrate configuring a drain region between the amplifier transistor AMI and the selection transistor SEL so that a coupling failure of the contact plug CP due to a level difference, if any, can be prevented. 
     The pixels PE 1  and PE 2  each have a configuration similar to that of the pixel PE 3  except that the active region AR has no level difference DP, the peripheral transistors have, in the active region thereof, no level difference, and they do not overlap with the boundary line DL. 
       FIG. 4  is a cross-sectional view along the arrangement direction of the photodiodes PD 3  and PD 2  in one of the pixels PE 3  (refer to  FIG. 3 ). The cross-sectional view of  FIG. 4  omits a boundary between interlayer insulating films stacked one after another over the semiconductor substrate SB. As shown in  FIG. 4 , the semiconductor substrate SB made of N type single crystal silicon or the like has, in the upper surface thereof, a P −  type well region WL. The well region WL has thereover an active region AR and element isolation regions EI for separating it from another active region. The element isolation region EI is made of, for example, a silicon oxide film and it is buried in a trench formed in the upper surface of the semiconductor substrate SB. 
     The well region WL has, in the upper surface thereof, N type semiconductor regions N 1  and N 2  sandwiched between the element isolation regions EI. The well region WL forming a PN junction with the N −  type semiconductor region N 1  functions as an anode of the photodiode PD 3 . The well region WL forming a PN junction with the N −  type semiconductor region N 2  functions as an anode of the photodiode PD 2 . The N −  type semiconductor region N 1  and the N −  type semiconductor region N 2  are provided in one active region AR sandwiched between the element isolation regions EI. 
     Thus, the active region AR formed in the pixel has the photodiode PD 3  comprised of the N −  type semiconductor region N 1  and the well region WL and the photodiode PD 2  comprised of the N −  type semiconductor region N 2  and the well region WL. In the active region AR, the photodiodes PD 3  and PD 2  are juxtaposed via a portion of the well region WL exposed from the upper surface of the semiconductor substrate SB. The well region WL on the upper surface of the semiconductor substrate SB between the photodiode PD 3  and the photodiode PD 2  overlaps with the boundary line DL shown in  FIG. 3  in plan view. The formation positions of the N type semiconductor regions N 1  and N 2  correspond to the formation positions of the photodiodes PD 3  and PD 2  shown in  FIG. 3 , respectively. This means that portions having therein the N −  type semiconductor regions N 1  and N 2  function as a photoelectric conversion portion. 
     The formation depth of the N −  type semiconductor region N 1  or N 2  is shallower than that of the well region WL. The depth of the trench in the upper surface of the semiconductor substrate SB having the element isolation region EI buried therein is shallower than the formation depth of the N −  type semiconductor region N 1  or N 2 . 
     The semiconductor substrate SB has thereon an interlayer insulating film IF that covers the element isolation region EI and the photodiodes PD 3  and PD 2  therewith. The interlayer insulating film IF is obtained by stacking a plurality of insulating films. The interlayer insulating film IF has therein a plurality of wiring layers stacked one after another and the lowermost wiring layer has therein a wiring M 1  covered with the insulating film IF. The wiring M 1  has thereon a wiring M 2  via the interlayer insulating film IF and the wiring M 2  has thereon a wiring M 3  via the interlayer insulating film IF. The interlayer insulating film IF has, in the upper portion thereof, a color filter CF and the color filter CF has thereon the microlens ML. During operation of the solid state image sensor, light is irradiated to the photodiodes PD 3  and PD 2  via the microlens ML and the color filter CF. 
     The active region AR including the photodiodes PD 3  and PD 2  have, right thereabove, no wiring in order to prevent light incident from the microlens ML from being blocked by the wiring and not irradiated to the photodiodes PD 3  and PD 2  which are light receiving portions of a pixel. On the contrary, wirings M 1  to M 3  are placed in a region other than the active region AR to prevent photoelectric conversion from occurring in the active region having peripheral transistors and the like therein. 
     Not only formation of the active region AR and the element isolation region EI but also formation of the N −  type semiconductor regions N 1  and N 2 , the gate electrode GE (refer to  FIG. 3 ), the interlayer insulating film IF, and the wirings M 1  to M 3  is achieved by a plurality of times of exposure treatment using divided exposure. This exposure treatment is performed for respective exposure regions isolated by the boundary line DL. This means that in any step such as ion implantation step for forming the N −  type semiconductor regions N 1  and N 2  or a step of forming a contact hole to be filled with a contact plug, a dividing position in exposure treatment is set at a position overlapping with a region between the photodiode PD 3  and the photodiode PD 2  possessed by each of the pixels PE 3  (refer to  FIG. 3 ) arranged in a column in the Y-axis direction. 
     The planar layout of each of the N −  type semiconductor regions N 1  and N 2 , the gate electrode GE, the contact hole, and the wirings M 1  to M 3  is therefore deviated in each of regions sandwiching therebetween the boundary line DL. 
     With regard to management of mask misalignment in respective steps of forming the N −  type semiconductor regions N 1  and N 2 , the gate electrode GE, the contact hole, and the wirings M 1  to M 3 , variations in the performance of the solid state image sensor can be reduced by managing only the misalignment due to an overlaying (overlapping) error at a dividing position of the active region in each step. 
       FIG. 3  shows the structure in which the photodiode PD 2  and the gate electrode GE and the contact plug CP at the periphery thereof are formed at a position deviated from the photodiode PD 3  in a direction similar to that of the layout of the active region AR by divided exposure. On the other hand, pattern formation of the active region AR and that of the photodiode PD 2 , the gate electrode GE, and the contact plug CP are performed by respective exposure steps using different masks so that these patterns are not always deviated at an equal deviation amount in the same direction. This means that patterns of the active region, semiconductor region, gate electrode, and wiring formed by different steps are not deviated in one direction due to mask misalignment but deviated in various directions with the vicinity of the boundary line DL as a boundary. 
     The reason why the solid state image sensor included in the semiconductor device of the present embodiment has two photoelectric conversion portions (for example, photodiodes) in one pixel is that when a digital camera having an image-plane phase-detection type autofocusing system uses the solid state image sensor of the present embodiment, the resulting digital camera can have improved focusing precision and focusing speed. Such a digital camera can achieve focusing in a short time based on a drive amount of the lens necessary for focusing determined from a signal deviation amount, that is, phase difference detected by one of the photodiodes in the pixel and the other photodiode respectively. By providing the pixel with a plurality of photodiodes, therefore, a larger number of minute photodiodes can be formed in the solid state image sensor. This leads to improvement in autofocusing precision. 
     When a photographed image is output, signals (charges) of two photodiodes in a pixel are output collectively as a single signal. This makes it possible to obtain an image comparable to that of a solid state image sensor equipped with a plurality of pixels having only one photodiode. 
     The present embodiment describes the structure using a P type well region as an anode and a diffusion layer which is an N −  type semiconductor region as a cathode. Not only it but also a solid state image sensor having a photodiode comprised of an N type well and a P −  type diffusion layer in the N type well or a photodiode having, on the surface thereof, a diffusion layer with a conductivity type equal to that of the pixel well can produce a similar advantage. In addition, the kind of the solid state image sensor is not limited to a CMOS image sensor and the above-described advantage can also be achieved by actualizing a structure similar to that of CCD (charge coupled device). 
     Next, an equivalent circuit diagram of a pixel is shown in  FIG. 5 . A plurality of the pixels PE shown in  FIG. 1  each has the circuit shown in  FIG. 5 . Here, the circuit and operation of the pixel PE 1  (refer to  FIG. 2 ) will be described as one example, but they equally apply to the circuit and operation of the pixel PE 2  or PE 3  (refer to  FIG. 2 ). 
     As shown in  FIG. 5 , the pixel has photodiodes PD 1  and PD 2  at which photoelectric conversion is performed, a transfer transistor TX 1  for transferring charges generated at the photodiode PD 1 , and a transfer transistor TX 2  for transferring charges generated at the photodiode PD 2 . The pixel also has a floating diffusion (floating diffusion portion) FD for accumulating charges transferred from the transfer transistors TX 1  and TX 2  and an amplifier transistor AMI for amplifying the potential of the floating diffusion FD. 
     The pixel further has a selection transistor SEL for selecting whether or not the potential amplified by the amplifier transistor AMI is output to the output line OL coupled to one of the readout circuits CC 1  and CC 2  (refer to  FIG. 1 ) and a reset transistor RST for initializing the potentials of the respective cathodes of the photodiodes PD 1  and PD 2  and the floating diffusion FD to predetermined ones. The transfer transistors TX 1  and TX 2 , the reset transistor RST, the amplifier transistor AMI, and the selection transistor SEL are each, for example, an N type MOS transistor. 
     A ground potential GND which is a minus-side power supply potential is applied to the respective anodes of the photodiodes PD 1  and PD 2  and the cathodes of the photodiodes PD 1  and PD 2  are coupled to the sources of the transfer transistors TX 1  and TX 2 , respectively. The floating diffusion FD is coupled to the respective drains of the transfer transistors TX 1  and TX 2 , the source of the reset transistor RST, and the gate of the amplifier transistor AMI. To the drain of the resent transistor RST and the drain of the amplifier transistor AMI are applied a plus-side power supply potential VCC. The source of the amplifier transistor AMI is coupled to the drain of the selection transistor SEL. The source of the selection transistor SEL is coupled to the output line OL coupled to either one of the readout circuits CC 1  and CC 2 . 
     Next, operation of the pixel will be described. When a predetermined potential is applied to the respective gate electrodes of the transfer transistors TX 1  and TX 2  and the reset transistor RST, the transfer transistors TX 1  and TX 2  and the reset transistor RST are all turned ON. Then, charges remaining in the photodiodes PD 1  and PD 2  and charges accumulated in the floating diffusion FD flow toward the plus-side power supply potential VCC to initialize the charges of the photodiodes PD 1  and PD 2  and the floating diffusion FD. The reset transistor RST is then turned OFF. 
     Next, an incident light is irradiated to the PN junction of the photodiodes PD 1  and PD 2  to cause electric conversion at the photodiodes PD 1  and PD 2 . As a result, charges are generated at each of the photodiodes PD 1  and PD 2 . These charges are all transferred to the floating diffusion FD by the transfer transistors TX 1  and TX 2 . The floating diffusion FD accumulates the charges thus transferred. This changes the potential of the floating diffusion FD. 
     Next, when the selection transistor SEL is turned ON, the potential of the floating diffusion FD after change is amplified by the amplifier transistor AMI and then, output to the outline OL. One of the readout circuits CC 1  and CC 2  reads out the potential of the output line OL. When image plane phase detection type autofocusing is performed, charges of each of the photodiodes PD 1  and PD 2  are not simultaneously transferred to the floating diffusion FD by means of the transfer transistors TX 1  and TX 2  but they are successively transferred and readout. In such a manner, charges of each of the photodiodes PD 1  and PD 2  are read out. During image pickup, the charges of the photodiodes PD 1  and PD 2  are transferred simultaneously to the floating diffusion FD. This means that the output for a static image is determined by the sum of outputs of the active regions of two photodiodes in each pixel. 
     The advantage of the semiconductor device of the present embodiment will next be described using Comparative Example shown in  FIG. 26 .  FIG. 26  is a planar layout showing a pixel array portion of a solid state image sensor included in a semiconductor device of Comparative Example. 
     In order to form a solid state image sensor having a chip size exceeding the maximum exposure region of an exposure apparatus, it is necessary to carry out divided exposure for forming, in a region of a semiconductor wafer in which a single chip is formed, patterns by performing exposure a plurality of times while changing an exposure site. In this case, exposure treatment is performed using respectively different masks for a plurality of times of exposure so that there may occur size variations or overlaying errors due to the mask or exposure apparatus among the resist patterns formed through the plurality of masks even in lithography in the same step. A difference in an area or distance among the photodiodes formed through the plurality of masks may therefore occur and then, the resulting solid state image sensor may cause image abnormalities due to a difference in output value. 
     Described specifically, when divided exposure is performed through two masks, deviation in pattern formation position occurs between an exposure region exposed through one of the masks and an exposure region exposed through the other mask and this leads to a difference in light receiving characteristics of a pixel at the boundary between these exposure regions. This difference in characteristics of a pixel in the vicinity of the boundary is a visible abnormality of an image or picture obtained by imaging using a solid state image sensor and becomes a cause for a linear imaging abnormality at a position corresponding to the boundary. If such an abnormality occurs, an image obtained by imaging has a deteriorated quality. 
     At the time of performing image-plane phase-detection type autofocusing with two photodiodes provided in one pixel, an output difference occurs between these two photodiodes and as a result, an autofocusing detection error increases. This leads to an increase in time required for focusing. In addition to this problem, by providing an extra circuit for image correction, the resulting semiconductor device may have other problems such as increase in electric power consumption and delay in operation. 
     As shown in  FIG. 26 , in the semiconductor device of Comparative Example, by partially overlapping the exposure regions to be subjected to divided exposure, image abnormalities that occur at the boundary among the exposure regions are made inconspicuous. In  FIG. 26 , hatched pixels PEB are pixels exposed through a first mask and hatch-free pixels PEW are pixels exposed through a second mask different from the first mask. 
     In Comparative Example, similar to the layout shown in  FIG. 2 , a first exposure region IG 1  has a first region  1 A and a third region  3 A and a second exposure region IG 2  has a second region  2 A and a third region  3 A. In the third region  3 A, the first exposure region IG 1  and the second exposure region IG 2  overlap with each other. In this example, in the third region  3 A, the pixels PEB are arranged so that the number of them decreases gradually toward the side of the second region  2 A and the pixels PEW are arranged so that the number of them decreases gradually toward the side of the first region  1 A. 
     Thus, since at the boundary region of divided exposure, the respective pixels PEB and PEW of the first exposure region IG 1  and the second exposure region IG 2  to be exposed through two masks are mixedly placed, it becomes difficult to visibly recognize an output level difference in the vicinity of the boundary of divided exposure on an image and as a result, the image can have an improved quality in the boundary region. 
     In a solid state image sensor having, in a pixel thereof, two photodiodes below a microlens, a difference in output between these two photodiodes is presumed to lead to out of focus. When out of focus, image-plane phase-detection system autofocusing is performed by selecting photodiodes one by one from different pixels and searching the positions of two adjacent pixels equal in output of a plurality of the selected photodiodes. Based on an operation amount of the lens necessary for focusing thus determined, autofocusing can be achieved in a short time. 
     The semiconductor device of Comparative Example however has a problem. Described specifically, an output difference between the pixels PEB formed through a mask for the first exposure region IG 1  and the pixels PEW formed through a mask for the second exposure region IG 2  is increased due to deviation in finished size and overlapping position between right and left exposure regions and it may take longer time to find pixels having an equal output. 
     In addition, since among the pixels PEB and PEW arranged in matrix form, the number of pixels mixedly placed in one row differs perpendicularly (in the Y-axis direction), time for searching an output difference between pixels differs according to rows in the above-described image-plane phase-detection system autofocusing. In the vicinity of the boundary between exposure regions, therefore, the optimum focus correction amount becomes different according to rows. This considerably increases time necessary for focusing. 
     The semiconductor device of the present embodiment is similar to that of Comparative Example in that a semiconductor chip is formed by divided exposure and in that two divided exposure regions partially overlap with each other. The semiconductor device of the present embodiment is however different from that of Comparative Example in that as shown in  FIG. 3 , the first exposure region IG 1  and the second exposure region IG 2  overlap with each other only in a column of the pixels PE 3  extending in the Y-axis direction. The semiconductor device of the present embodiment is a solid state image sensor having, in each pixel thereof, two photodiodes for image-plane phase-detection system autofocusing. 
     In the present embodiment different from Comparative Example, in the pixels PE 3  in an overlapping region of exposure regions, the photodiodes PD 2  formed through the mask for first exposure region IG 1  are placed in a region separated from the first region  1 A where the pixels PE 1  formed through the mask for first exposure region IG 1  are arranged but close to the side of the second region  2 A where the pixels PE 2  formed through the mask for second exposure region IG 2  are arranged. Similarly, the photodiodes PD 3  formed through the mask for second exposure region IG 2  are placed in a region separated from the second region  2 A where the pixels PE 2  formed through the mask for second exposure region IG 2  are arranged but close to the side of the first region  1 A where the pixels PE 1  formed through the mask for first exposure region IG 1  are arranged. 
     This means that when divided exposure of a solid state image sensor is performed by dividing it into the first exposure region IG 1  and the second exposure region IG 2  that partially overlap with each other, inserted between photodiodes of the endmost column in matrix-form pixels formed through a mask for one of the exposure regions and photodiodes of a column adjacent thereto are photodiodes of a column formed through a mask for the other exposure region. Two photodiodes PD 2  and PD 3  in the pixels PE 3  are therefore formed through respectively different masks. This equally applies not only to photodiodes in the pixel PE 3  but also an active region, peripheral transistors, and wirings near these photodiodes (refer to  FIGS. 3 and 4 ). 
     With regard to a column of the pixels PE 3  at the boundary portion, two photodiodes PD 2  and PD 3  are mixedly placed in the pixels PE 3  in the third region  3 A so that relative positions with the microlens ML become same. The term “mixedly placed” as used herein means that patterns of an element and the like formed through a mask for one of the exposure regions are placed in a region where an element and the like to be formed through a mask for the other exposure region have been formed. 
     One of the advantages of the present embodiment is to make it difficult to recognize, on an image, a level difference of a static image output at the boundary portion between the first exposure region IG 1  and the second exposure region IG 2 . The image available from the resulting solid state image sensor can have an improved quality and therefore, the semiconductor device can have improved performance. 
     An output for a static image is determined from an output sum of the active regions of two photodiodes in each pixel, but in the present embodiment, photodiodes formed through two masks, respectively, are mixedly placed in the pixel PE 3 . 
     It is presumed that in a solid state image sensor where divided exposure is performed, there appears a difference in formation position and output characteristics between the photodiodes PD 1  and PD 2  in the first exposure region IG 1  and between the photodiodes PD 3  and PD 4  in the second exposure region IG 2 , in the pixels formed in the pixel array portion. In the present embodiment, however, the pixel PE 3  has therein the photodiodes PD 2  and PD 3  formed through respectively different masks so that the output sum of these two photodiodes PD 2  and PD 3  of the pixel PE 3  approximates the output sum of the photodiodes PD 1  and PD 2  of the pixel PE 1  and at the same time approximates the output sum of the photodiodes PD 3  and PD 4  of the pixel PE 2 . 
     This makes it possible to prevent appearance of a marked difference in output characteristics of pixels between the first exposure region IG 1  and the second exposure region IG 2  at the boundary portion; and therefore makes it difficult to recognize, on an image, a level difference in static image output at the boundary portion between the first exposure region IG 1  and the second exposure region IG 2 . 
     When the photodiodes PD 2  and PD 3  are formed through respectively different masks in the pixel PE 3 , one of the photodiodes in the pixel PE 3  may functionally stop due to misalignment of the masks or the like. When the other photodiode functions in the pixel PE 3 , however, the output of it approximates an average output of the photodiodes PD 1  and PD 2  in the pixel PE 1  and also approximates an average output of the photodiodes PD 3  and PD 4  in the pixel PE 2 . As a result, on an image available by imaging, an output difference due to divided exposure cannot be recognized easily at a corresponding position between the first region  1 A and the third region  3 A and between the third region  3 A and the second region  2 A. 
     Another advantage of the present embodiment is to reduce, in an image-plane phase-detection system autofocusing operation, a calculation time by simplifying the determination treatment in detecting a correction amount in the vicinity of the boundary portion. This makes it possible to increase the autofocusing speed so that the semiconductor device thus obtained can have improved performance. 
     Described specifically, in the calculation of a focusing correction amount in an image-plane phase-detection system autofocusing, the first exposure region IG 1  and the second exposure region IG 2  are formed under predetermined exposure conditions, respectively in the first region  1 A and the second region  2 A other than the boundary portion of divided exposure so that the image-plane phase-detection system autofocusing can be achieved using a pixel having two photodiodes and focusing position information can be calculated in a short time. 
     On the other hand, two photodiodes PD 2  and PD 3  formed through respectively different masks are placed in the pixel PE 3  at the boundary portion so that there may occur a difference in the finished size of the active region in the pixel PE 3  due to process variations under exposure conditions through these masks. In this case, even after focusing during imaging, it may be judged that unfocusing, that is, out-of-focus, occurs in the pixel PE 3 . The pixel judged as described above is only a column in the third region  3 A in the pixel array portion in the present embodiment so that it has less influence on the image-plane phase-detection system autofocusing treatment in moving image and the focusing information can be calculated in a short time by using pixel columns adjacent thereto. 
     This means that at a position adjacent, in the X-axis direction, to the pixel PE 3  of the third region  3 A, a pixel having two photodiodes formed through the same mask under the same exposure conditions is present so that even in an image phase difference-system autofocusing position detection algorism for searching until the outputs of the photodiodes on one side agree, searching can be completed in a short time. In short, information on a focusing position can be calculated in a short time. 
     In the present embodiment, the pixels PE 3  are juxtaposed in the Y-axis direction. This means that the number of the pixels PE 3  arranged in each row does not vary. It is therefore possible to prevent variations, for each row, in searching time of an output difference between pixels in the image-plane phase-detection system autofocusing which will otherwise occur due to difference in the number of pixels mixedly placed in every perpendicularly placed row (Y-axis direction) as in Comparative Example. Time necessary for focusing can therefore be shortened. 
     Here, a column of pixels including two photodiodes formed through respectively different masks is arranged in the Y-axis direction. Even without arranging the pixels in such a vertical straight line, almost similar function can be obtained by stepwise arrangement or zigzag arrangement in plan view. In principle, however, pixels arranged in a perpendicularly straight line are more effective for shortening the focusing position calculation time. 
     An advantage similar to that described above can be produced not only by applying the present embodiment to a solid state image sensor for detecting light irradiated from the main surface side of a semiconductor substrate but also by applying it to a back illuminated type solid state image sensor for detecting light irradiated from the back side of a semiconductor substrate. The above description essentially relates only to the arrangement of pixel layout. It is needless to say that with regard to the layer information of the pixel layout that determines the arrangement position, the pixels can be placed as the present embodiment by selecting a specific layer or some layers for all the layers and all the steps, for example, an element isolation step, a gate electrode formation step, an implantation step for source-drain regions, an implantation step of photodiode, and a wiring step. 
     Next, a method of manufacturing a solid state image sensor included a semiconductor device of the present embodiment will be described referring to  FIGS. 6 to 9 .  FIGS. 6 to 9  are plan views of the semiconductor device of the present embodiment during manufacturing steps thereof. A method of manufacturing a pixel will hereinafter be described mainly. 
     First, as shown in  FIG. 6 , a semiconductor substrate SB including a plurality of regions which will be a semiconductor chip is provided. Next, a P type impurity (for example, B (boron)) is implanted into the main surface of the semiconductor substrate SB by ion implantation or the like to form a well region WL in the main surface of the semiconductor substrate SB. 
     Next, an element isolation region EI is formed on the well region WL by photolithography to separate the upper surface of the semiconductor substrate into active regions AR configuring pixels, respectively, in a pixel array portion. At this time, also active regions in a region in which readout circuits CC 1  and CC 2 , an output circuit OC (refer to  FIG. 1 ) and the like outside the pixel array portion are defined in the main surface of the semiconductor substrate. Here, the element isolation region EI made of a silicon oxide film is formed by STI (shallow trench isolation) system. Alternatively, the element isolation region EI may be formed by LOCOS (local oxidation of silicon) system. 
     For the formation of the element isolation region EI, first a protecting film (not shown) having a stacked structure of a silicon oxide film and a silicon nitride film is formed on the semiconductor substrate SB. Then, a photoresist film (not shown) is formed on the protecting film. The photoresist film is then exposed through two photomasks having a predetermined mask pattern. At this time, the photoresist film is exposed by divided exposure. 
     The term “divided exposure” as used herein means not exposure of the first exposure region IG 1  and the second exposure region IG 2  juxtaposed on the surface of the semiconductor substrate SB by single exposure treatment but exposure of the entire region which will be a semiconductor chip by exposing each of these two regions once, that is, twice exposure in total. In the description of the present embodiment, the entire region which will be a single semiconductor chip, in a semiconductor wafer, is divided into two exposure regions and exposure is performed twice. The exposure frequency and the number of exposure regions thus divided for exposing the entire region which will be a single semiconductor chip may be three or more. 
     When divided exposure of a photoresist film is performed, the first exposure region IG 1  is exposed first through a first mask to transfer a mask pattern and then, the second exposure region IG 2  is exposed through a second mask to transfer a mask pattern. At this time, the first exposure region IG 1  and the second exposure region IG 2  overlap with each other in the third region  3 A. Then, the photoresist film after exposure is developed to pattern the photoresist film. 
     With the photoresist film as a mask, the protecting film exposed from the photoresist film is removed by etching. Then, the photoresist film used as an etching mask is removed. By dry etching with the protecting film as a mask, a trench for element isolation is formed in the main surface of the semiconductor substrate SB exposed from the protecting film. The trench is then filled with a silicon oxide film, followed by removal of the silicon oxide film and protecting film on the semiconductor substrate SB by polishing or the like to form an element isolation region EI partitioning a plurality of active regions including an active region AR. This means that from a region not covered with the element isolation region EI but covered with the protecting film, the main surface of the semiconductor substrate SB which is an active region is exposed. 
       FIG. 6  shows three regions arranged in the X-axis direction and in these regions, pixels are to be formed. A region to be a pixel has an active region AR which will be a light receiving portion and the other active region for peripheral transistors formed around the active region AR. The active region AR is a region in which two photodiodes are to be formed in a later step. 
     The pattern of an active region formed through a mask for exposing the first exposure region IG 1  is deviated in one direction from the pattern of an active region formed through a mask for exposing the second exposure region IG 2 . This occurs due to misalignment at the time of placing masks to be used for divided exposure. 
     The misalignment between these active regions occurs between a region, in an active region AR of the third region  3 A, in which one photodiode is to be formed and a region, in the active region AR, in which the other photodiode is to be formed. Due to this misalignment, a level difference DP is formed at the X-axis direction center of each of two sides of the active region AR of the third region  3 A. 
     In addition, misalignment between the active regions occurs between a portion of the first exposure region IG 1  that does not overlap with the second exposure region IG 2 , that is, the first region  1 A and a portion, of the active region AR of the third region  3 A in which two photodiodes are to be formed, closer to the side of the first region  1 A. Similarly, misalignment between the active regions occurs between a portion of the second exposure region IG 2  that does not overlap with the first exposure region IG 1 , that is, the second region  2 A, and a portion, of the active region AR of the third region  3 A in which two photodiodes are to be formed, closer to the side of the second region  2 A. 
     No misalignment occurs between the first region  1 A and a portion, of the active region AR of the third region  3 A in which two photodiodes are to be formed, closer to the side of the second region  2 A, because the patterns of these regions are formed through a first mask used for exposure of the first exposure region IG 1 . Similarly, no misalignment occurs between the second region  2 A and a portion, of the active region AR of the third region  3 A in which two photodiodes are to be formed, closer to the side of the first region  1 A, because the patterns of these regions are formed through a second mask used for exposure of the second exposure region IG 2 . 
     Next, as shown in  FIG. 7 , a gate electrode GE is formed via a gate insulating film (not shown) on respective active regions where various MOS transistors such as transfer transistor, reset transistor, amplifier transistor, and selection transistor are formed. More specifically, after stacking an insulating film and a polysilicon film on the semiconductor substrate SB by CVD (chemical vapor deposition) or the like, the polysilicon film and the insulating film are patterned by etching using photolithography to form the gate insulating film made of the insulating film and the gate electrode GE made of the polysilicon film. 
     A plurality of the gate electrodes and the gate insulating films therebelow has a rectangular pattern extending in the Y axis direction in plan view and is formed on a predetermined active region. The gate electrode GE of the transfer transistor adjacent to the active region AR is formed right above a semiconductor region protruding from the active region AR in the Y-axis direction. In the present embodiment, two photodiodes are formed for each pixel and two transfer transistors corresponding to these photodiodes are formed so that there are two protrusions and two gate electrodes GE for the transfer transistor. The two protrusions configuring a portion of the active region AR are coupled to each other at a place to which they extend. Two transfer transistors in one pixel may have one gate electrode GE in common. 
     The reset transistor, the amplifier transistor, and the selection transistor which are peripheral transistors are juxtaposed on the other active region adjacent to the active region AR serving as a light receiving portion in a region of one pixel. Three gate electrodes GE of these peripheral transistors are therefore formed so as to straddle over the other active region. These three gate electrodes GE are juxtaposed in the X-axis direction right above the other active region extending in the X-axis direction. 
     When the polysilicon film and the insulating film are patterned in the step of forming the gate electrode GE, divided exposure treatment is performed as in the above step of forming the element isolation region EI to define the active region AR. Misalignment in formation position therefore occurs between the gate electrode GE formed through the mask for first exposure region IG 1  and the gate electrode GE formed through the mask for second exposure region IG 2 . 
     Next, as shown in  FIG. 8 , various ion implantation steps are performed. By these steps, N −  type semiconductor regions N 1  and N 2  and a drain region of the transfer transistors are formed in the upper surface of the well region WL in each active region AR; and source-drain regions of each peripheral transistor are formed in the other active region. The N −  type semiconductor regions N 1  and N 2  are formed by implanting and introducing an N type impurity (for example, P (phosphorus) or As (arsenic)) into the main surface of the semiconductor substrate SB. 
     By the above ion implantation, in the active region AR of the first region  1 A, a photodiode PD 1  comprised of the N −  type semiconductor region N 1  and the well region WL and a photodiode PD 2  comprised of the N −  type semiconductor region N 2  and the well region WL are formed. In addition, in the active region AR of the second region  2 A, a photodiode PD 3  comprised of the N −  type semiconductor region N 1  and the well region WL and a photodiode PD 4  comprised of the N −  type semiconductor region N 2  and the well region WL are formed. Further, in the active region AR of the third region  3 A, a photodiode PD 3  comprised of the N −  type semiconductor region N 1  and the well region WL and a photodiode PD 2  comprised of the N −  type semiconductor region N 2  and the well region WL are formed. 
     In each active region AR, transfer transistors TX 1  and TX 2  each comprised of the gate electrode GE and source-drain regions on both sides of the gate electrode GE are formed by the above ion implantation. In the other active region, a reset transistor RST, an amplifier transistor AMI, and a selection transistor SEL each comprised of the gate electrode GE and the source-drain regions on both sides of the gate electrode GE are formed. 
     As a result, a pixel PE 1  including the photodiodes PD 1  and PD 2  and the peripheral transistors is formed in the first region  1 A. A pixel PE 2  including the photodiodes PD 3  and PD 4  and the peripheral transistors is formed in the second region  2 A. A pixel PE 3  including the photodiodes PD 3  and PD 2  and the peripheral transistors is formed in the third region  3 A. 
     In the pixel PE 1 , the transfer transistor TX 1  adjacent to the photodiode PD 1  is formed in the active region AR of the first region  1 A and the transfer transistor TX 2  is formed adjacent to the photodiode PD 2  in the active region AR of the first region  1 A. In the pixel PE 2 , the transfer transistor TX 1  adjacent to the photodiode PD 3  is formed in the active region AR of the second region  1 A and the transfer transistor TX 2  adjacent to the photodiode PD 4  is formed in the active region AR of the second region  2 A. In the pixel PE 3 , the transfer transistor TX 1  adjacent to the photodiode PD 3  is formed in the active region AR of the third region  3 A and the transfer transistor TX 2  adjacent to the photodiode PD 2  is formed in the active region AR of the third region  3 A. 
     In the step of forming the above-described various semiconductor regions, ion implantation is performed with a photoresist film (not shown) as a mask. During formation of a pattern of this photoresist film, divided exposure treatment is performed as in the above-described step of forming the element isolation region EI. The boundary for divided exposure treatment is defined at the same position as that in the step of forming the active region AR. Misalignment in the formation position therefore occurs between the N −  type semiconductor region N 1  formed in the first region  1 A and the N −  type semiconductor region N 1  formed in the second region  2 A, with the boundary line DL as a boundary. Misalignment in the formation position also occurs between the N −  type semiconductor region N 1  formed in the third region  3 A and the N −  type semiconductor region N 2  formed in the third region  3 A, with the boundary line DL as a boundary. 
     Next, as shown in  FIG. 9 , after formation of an interlayer insulating film (not shown) on the semiconductor substrate SB, a contact plug CP penetrating the interlayer insulating film is formed. 
     Wirings M 1  to M 3  (refer to  FIG. 4 ) are then formed. More specifically, after formation of a first-layer interlayer insulating film on the semiconductor substrate SB, a plurality of contact plugs CP penetrating the interlayer insulating film is formed. A lower wiring M 1  coupled to the contact plug CP is then formed on the first-layer interlayer insulating film. After formation of a second-layer interlayer insulating film on the first-layer interlayer insulating film, a via plug penetrating the second-layer interlayer insulating film and a wiring M 2  on the via plug are formed. By a similar step, a third-layer interlayer insulating film, a via plug, a wiring M 3 , and a fourth-layer interlayer insulating film are formed on the wiring M 2  to complete formation of upper wirings. The stacked films comprised of the first to fourth interlayer insulating films configure an interlayer insulating film IF. 
     As a result, a solid state image sensor included in the semiconductor device of the present embodiment is completed. As shown in  FIG. 4 , a color filter CF and a microlens ML may be formed successively on the interlayer insulating film IF. 
     In the step of forming the interlayer insulating film IF, the contact plug CP, the via plug, and the wirings M 1  to M 3 , patterning is performed by etching using a photoresist film (not shown) as a mask. When the pattern of this photoresist film is formed, divided exposure treatment is performed as in the step of forming the element isolation region EI. The boundary for divided exposure treatment is defined at the same position as that used in the step of forming the active region AR shown in  FIG. 6 . 
     The method of manufacturing a semiconductor device according to the present embodiment can produce an advantage similar to that produced by the semiconductor device of the embodiment described using Comparative Example of  FIG. 26 . When a solid state image sensor is formed by subjecting the first exposure regions IG 1  and the second exposure region IG 2  that partially overlap with each other in the third region  3 A to divided exposure, two photodiodes PD 2  and PD 3  in the pixel PE 3  of the third region  3 A are formed through respectively different masks. 
     At this time, the photodiode PD 2  formed through a first mask is placed, in the pixel PE 3 , not on the side of the first region  1 A including the pixel PE 1  formed through the first mask but on the side of the second region  2 A including the pixel PE 2  formed through a second mask. Similarly, the photodiode PD 3  formed through a second mask is, in the pixel PE 3 , not on the side of the second region  2 A including the pixel PE 2  formed through the second mask but on the side of the first region  1 A including the pixel PE 1  formed through the first mask. This means that in the pixel PE 3 , the photodiodes PD 2  and PD 3  formed through the respective masks are mixedly placed. 
     This makes it difficult to recognize, on an image, a level difference in static image output at the boundary between the first exposure region IG 1  and the second exposure region IG 2 . An image available by the solid state image sensor can have an improved quality and as a result, the semiconductor device can have improved performance. 
     Even when one of the photodiodes in the pixel PE 3  does not function due to misalignment of a mask or the like, if the other photodiode in the pixel PE 3  functions, the output approximates the average output of the photodiodes PD 1  and PD 2  in the pixel PE 1  and approximates the average output of the photodiodes PD 3  and PD 4  in the pixel PE 2 . As a result, an output difference due to divided exposure at a corresponding position between the first region  1 A and the third region  3 A or between the third region  3 A and the second region  2 A cannot easily be recognized from an image obtained by imaging. 
     The manufacturing method of the present embodiment can reduce a calculation time in an image-plane phase-detection system autofocusing operation of the solid state image sensor thus manufactured by simplifying the determination treatment in detecting a correction amount in the vicinity of the boundary portion. This leads to an increase in autofocusing speed and as a result, the semiconductor device thus obtained can have improved performance. 
     Modification Example 1 
     Modification Example of the present embodiment will next be described referring to  FIG. 10 .  FIG. 10  is a planar layout showing a semiconductor device of Modification Example 1 of the present embodiment. 
     The present modification example is different from the layout described referring to  FIG. 2  in that the third region  3 A has therein a pixel PE 4  and in the pixel PE 4 , photodiodes PD 2  and PD 3  are arranged without being mixedly placed. This means that in the third region  3 A, the pixel PE 4  and the pixel PE 3  having a similar structure to that of  FIG. 2  are arranged alternately in the Y-axis direction. In the pixel PE 4 , the photodiode PD 2  formed through the mask for first exposure region IG 1  is placed on the side of the first region  1 A having the pixel PE 1  formed through the mask for first exposure region IG 1 . In addition, in the pixel PE 4 , the photodiode PD 3  formed through the mask for second exposure region IG 2  is placed on the side of the second region  2 A having the pixel PE 2  formed through the mask for second exposure region IG 2 . 
     In the third region  3 A, therefore, two columns in which photodiodes PD 2  and PD 3  formed through respectively different masks are alternately arranged a plurality of times in the Y-axis direction are juxtaposed in the X-axis direction. 
     Such arrangement and configuration enable an output difference between two photodiodes due to horizontal asymmetry of the microlens ML to average in the Y-axis direction (column direction). The present modification example therefore can produce, in addition to the advantage described referring to  FIGS. 1 to 9 , an advantage of making inconspicuous an output level difference between the first exposure region IG 1  and the second exposure region IG 2  in the X-axis direction (row direction). 
     More specifically, in image-plane phase-detection system focusing detection, when a microlens on the solid state image sensor is not symmetric, there occurs an output difference between two photodiodes in a pixel. It is however very difficult to obtain a completely symmetric microlens due to a problem in its manufacture and an output difference, though very small, appears between photodiodes. 
     In the present modification example, the positions of the photodiodes PD 2  and PD 3  are changed between the pixels PE 3  and PE 4  adjacent to each other in the Y-axis direction to average the output information between the pixels PE 3  and PE 4 . This makes it possible to prevent generation of the output difference and thereby reduce an output level difference on an image available by imaging. 
     An advantage similar to that obtained by the solid state image sensor shown in  FIG. 10  can be obtained even by alternately placing pixels PE 3  and PE 5  in the Y-axis direction in the third region  3 A and placing photodiodes PD 1  and PD 4  in the pixel PE 5  as shown in  FIG. 11 . In other words, in the pixel PE 5 , the photodiode PD 1  is placed on a side closer to the first region  1 A and the photodiode PD 4  is placed on a side closer to the second region  2 A.  FIG. 11  is a planar layout showing another semiconductor device of Modification Example 1 of the present embodiment. 
     The photodiode PD 1  in the pixel PE 5  is, similar to the photodiodes PD 1  and PD 2  in the pixel PE 1 , a light receiving element formed through the mask for first exposure region IG 1  and the photodiode PD 4  is, similar to the photodiodes PD 3  and PD 4  in the pixel PE 2 , a light receiving element formed through the mask for second exposure region IG 2 . 
     An advantage similar to that obtained by the solid state image sensor shown in  FIG. 10 or 11  can be obtained even by, as shown in  FIG. 12 , alternately placing pixels PE 4  and PE 8  in the Y-axis direction in the third region  3 A and placing the photodiodes PD 4  and PD 1  in the pixel PE 8 . The pixel PE 4  has a configuration similar to that of  FIG. 10 . In the pixel PE 8 , the photodiode PD 4  is placed on a side closer to the first region  1 A and the photodiode PD 1  is placed on a side closer to the second region  2 A.  FIG. 12  is a planar layout showing a further semiconductor device of Modification Example 1 of the present embodiment. 
     The photodiode PD 1  in the pixel PE 8  is, similar to the photodiodes PD 1  and PD 2  in the pixel PE 1 , a light receiving element formed through the mask for first exposure region IG 1  and the photodiode PD 4  is, similar to the photodiodes PD 3  and PD 4  in the pixel PE 2 , a light receiving element formed through the mask for second exposure region IG 2 . 
     Modification Example 2 
     Modification Example 2 of the present embodiment will hereinafter be described referring to  FIG. 13 .  FIG. 13  is a planar layout showing a semiconductor device of Modification Example 2 of the present embodiment. 
     The layout of the present modification example is different from that described referring to  FIG. 2  in that in the third region  3 A, a column comprised of a plurality of pixels PE 4  arranged in the Y-axis direction is added to a column comprised of a plurality of pixels PE 3  arranged in the Y-axis direction. The pixel PE 4  has a structure similar to that of  FIG. 10 . A column comprised of a plurality of pixels PE 3  and a column comprised of a plurality of pixels PE 4  are juxtaposed in the X-axis direction. The column of pixels PE 3  is placed on a side closer to the second region  2 A and the column of pixels PE 4  is placed on a side of the first region  1 A. 
     In short, the layout shown in  FIG. 13  has a configuration in which two columns each comprised of pixels arranged in the Y-axis direction in the third region  3 A as shown in  FIG. 2  are axially symmetrically arranged in the third region  3 A. 
     When in the present modification example, the third region  3 A has only one column of pixels having a structure in which formation positions of the right and left photodiodes have been replaced with each other, that is, in which the photodiodes have been mixedly placed, an output level difference on an image between the first exposure region IG 1  and the second exposure region IG 2  can be made inconspicuous by gradually changing the output level difference. The output level difference on an image is however likely to be recognized because a width of a region for gradually changing the output level difference is small. 
     In the present modification example, therefore, the output difference between right and left exposure regions is averaged and reduced by forming two columns of pixels in the third region  3 A. In addition to the advantage described referring to  FIGS. 1 to 9 , it is possible to make an output level difference more inconspicuous at a portion, on an image, corresponding to the boundary portion of the divided exposure. In other words, a region capable of averaging an output level difference at the boundary portion can be widened and therefore, the output difference between right and left exposure regions can be made more inconspicuous. 
     In addition, such arrangement of the pixel PE 3  and the pixel PE 4  along the X-axis direction means formation of columns in which arrangement of the two photodiodes PD 2  and PD 3  is exchanged for the microlens ML. This makes it possible to average an output difference due to the asymmetric shape of the microlens and thereby makes it difficult to recognize the output difference on an image. 
     As shown in  FIG. 14 , in each of two columns of the third region  3 A, a pixel PE 3  and a pixel PE 4  may be arranged alternately a plurality of times along the Y-axis direction. In this case, the pixel PE 3  and the pixel PE 4  are juxtaposed in the X-axis direction.  FIG. 14  is a planar layout showing another semiconductor device of Modification Example 2 according to the present embodiment. 
     In short, the layout shown in  FIG. 14  has a configuration in which two columns, each column comprised of pixels arranged along the Y-axis direction in the third region  3 A as shown in FIG.  10  are axially symmetrically arranged in the third region  3 A. 
     This makes it possible to average an output difference between columns arranged in the X-axis direction in the third region  3 A and between rows arranged in the Y-axis direction in the third region  3 A. Compared with the solid state image sensor shown in  FIG. 13 , that of the present example can make an output level difference on an image more inconspicuous. 
     Even when the pixels PE 3 , PE 4 , PE 5 , and PE 8  are arranged in the third region  3 A as shown in  FIG. 15 , an advantage similar to that of the solid state image sensor shown in  FIG. 14  can be obtained.  FIG. 15  is a planar layout showing the semiconductor device of Modification Example 2 of the present embodiment. 
     In the third region  3 A, the pixels PE 5  and PE 8  are juxtaposed in a certain row and the pixels PE 3  and PE 4  are juxtaposed in a row adjacent to the above row in the Y-axis direction. In other words, the pixel PE 8  is placed between the pixels PE 3  adjacent to each other in the Y-axis direction and the pixel PE 5  is placed between the pixels PE 4  adjacent to each other in the Y-axis direction. In the third region  3 A, a column including the pixels PE 4  and PE 5  is placed on the side of the first region  1 A and a column including the pixels PE 3  and PE 8  is placed on the side of the second region  2 A. 
     Modification Example 3 
     Modification Example 3 of the present embodiment will hereinafter be described referring to  FIG. 16 .  FIG. 16  is a planar layout showing a semiconductor device of Modification Example 3 of the present embodiment. 
     The layout of the present modification example is different from that described referring to  FIG. 2  in that in the third region  3 A, three columns each comprised of a plurality of pixels PE 3  arranged in the Y-axis direction are juxtaposed in the X-axis direction. The output difference between the first exposure region IG 1  and the second exposure region IG 2  is averaged and reduced over a wide range by increasing the number of columns of the third region  3 A to three. By such a configuration, an output level difference on an image between the divided regions can be made more inconspicuous. 
     Here, the pixels PE 3  are arranged in matrix form in the third region  3 A, but instead, the pixels PE 4  shown in  FIG. 10  may be arranged in matrix form. More specifically, in the pixels arranged in the third region  3 A, the photodiodes PD 2  formed through the mask for first exposure region IG 1  may be placed on the side of the first region  1 A and the photodiodes PD 3  formed through the mask for second exposure region IG 2  may be placed on the side of the second region  2 A. In this case, since there are some pixels in which respective positions of two photodiodes are replaced with each other for the microlens ML, an output difference due to asymmetric shape of the microlens can be averaged. 
     Here, described is the third region  3 A in which three columns are arranged in the X-axis direction, but the number of the columns may be four or more. 
     As shown in  FIG. 17 , in each of a plurality of columns in the third region  3 A, a pixel PE 3  and a pixel PE 4  may be alternately arranged a plurality of times in the Y-axis direction. In this case, only a plurality of pixels PE 3  is juxtaposed in the X-axis direction in a certain row in the third region  3 A and in a row adjacent thereto in the Y-axis direction, only a plurality of pixels PE 4  is juxtaposed in the X-axis direction.  FIG. 17  is a planar layout showing another semiconductor device of Modification Example 3 of the present embodiment. 
     In this case, in addition to an output difference between the first exposure region IG 1  and the second exposure region IG 2  in the X-axis direction, an output difference in the Y-axis direction can be averaged. 
     As shown in  FIG. 18 , in each of a plurality of columns of the third region  3 A, a pixel PE 3  and a pixel PE 8  may be juxtaposed alternately a plurality of times in the Y-axis direction.  FIG. 18  is a planar layout of a further semiconductor device of Modification Example of the present embodiment. 
     In this case, in a certain row in the third region  3 A, only a plurality of pixels PE 3  is juxtaposed in the X-axis direction and in the row adjacent thereto in the Y-axis direction, only a plurality of pixels PE 8  is juxtaposed in the X-axis direction. The pixels PE 8  have a structure similar to that of the pixel PE 8  described referring to  FIG. 12 . 
     In this case, in addition to an output difference between the first exposure region IG 1  and the second exposure region IG 2  in the X-axis direction, an output difference in the Y-axis direction can be averaged. Further, since photodiodes differ in each row, an output difference between the first exposure region IG 1  and the second exposure region IG 2  in the X-axis direction and also that in the Y-axis direction can be averaged. 
     Modification Example 4 
     Modification Example 4 of the present embodiment will hereinafter be described referring to  FIG. 19 .  FIG. 19  is a planar layout showing a semiconductor device of Modification Example 4 according to the present embodiment. 
     The layout of the present modification example is different from that described referring to  FIG. 10  in that an area, in plan view, of each of photodiodes PD 5  and PD 6  formed in pixels PE 6  and PE 7  in the third region  3 A is larger than that of the photodiodes PD 1  to  4  in the first region  1 A and the second region  2 A. 
     The pixels PE 6  and PE 7  each have a photodiode PD 5  and a photodiode PD 6 . In the pixel PE 6 , the photodiode PD 5  is placed on the side of the second region  2 A and the photodiode PD 6  is placed on the side of the first region  1 A. On the contrary, in the pixel PE 7 , the photodiode PD 5  is placed on the side of the first region  1 A and the photodiode PD 6  is placed on the side of the second region  2 A. The photodiode PD 5  is, similar to the photodiodes PD 1  and PD 2 , a light receiving element formed through the mask for first exposure region, and the photodiode PD 6  is, similar to the photodiodes PD 3  and PD 4 , a light receiving element formed through the mask for second exposure region IG 2 . 
     This means that the layout of the present modification example is similar to that described referring to  FIG. 10  in that inside each of the pixels PE 6  and PE 7 , the photodiodes PD 5  and PD 6  are formed through respectively different masks and in each of the pixels PE 6  and PE 7  arranged along the Y-axis direction, the photodiodes are replaced with each other. 
     Here, an area of the photodiodes PD 5  and PD 6  of only the pixels PE 6  and PE 7  in the boundary region (third region  3 A) is made greater. When divided exposure causes misalignment, only the area of one of the photodiodes in one pixel may substantially decrease. In this case, the output of some photodiodes in the third region  3 A decreases, leading to deterioration in image quality and delay of autofocusing. In the present modification example, on the other hand, since the photodiodes PD 5  and PD 6  configuring the pixels PE 6  and PE 7  in the third region  3 A have a larger area according to their layout design, an influence of the output reduction can be reduced. As a result, an output level difference on an image in the divided region can be made inconspicuous. 
     The photodiode can be enlarged by increasing the formation area of the N −  type semiconductor regions N 1  and N 2  in the active region AR shown in  FIG. 3 . The area of the active region AR shown in  FIG. 3 , as well as the area of the photodiode, may be enlarged. 
     As shown in  FIG. 20 , the area of each of the photodiodes PD 5  and PD 6  in plan view may be smaller than that of the photodiodes PD 1  to PD 4  in the first region  1 A and the second region  2 A.  FIG. 20  is a planar layout showing another semiconductor device of Modification Example 4 of the present embodiment. This makes it possible to prevent the distance between the active regions or between the photodiodes from narrowing due to misalignment of masks used respectively for the first exposure region IG 1  and the second exposure region IG 2  and thereby preventing generation of leakage between the active regions or between photodiodes. Accordingly, generation of an output level difference in the third region  3 A and delay in autofocusing resulting from the leakage can be prevented. 
     In this example, the distance between the photodiodes PD 5  and PD 6  can be kept large in each of the pixel PE 6  and the pixel PE 7 . In addition, the distance between each of the photodiodes PD 5  and PD 6  and the end portion of the active region including the photodiodes PD 5  and PD 6  can be kept large. It is therefore possible to prevent narrowing of an area of the photodiode when due to misalignment of masks used for the first exposure region IG 1  and the second exposure region IG 2 , respectively, the formation region of the active region or the photodiode is misaligned. An output level difference between the divided regions on an image can be prevented. 
     Photodiodes having a smaller area may be formed by reducing the areas for the formation of the N −  type semiconductor region N 1  and N 2  in the active region AR shown in  FIG. 3 . 
     As shown in  FIG. 21 , the area of the photodiode PD 5 , of the photodiodes PD 5  and PD 6  in each pixel in the third region  3 A, may be made larger than that of each of the photodiodes PD 1  to PD 4  and the area of the photodiode PD 6  may be made smaller than that of the photodiodes PD 1  to PD 4 .  FIG. 21  is a planar layout showing a further semiconductor device of Modification Example 4 of the present embodiment. 
     Defining in advance the photodiode PD 5  having a large layout and the photodiode PD 6  having a small layout inside each of the pixels PE 6  and PD 7  in the third region  3 A is effective for specifying a measurement position easily when the pattern size of the photodiode at shot end of the first exposure region IG 1  or the second exposure region IG 2  is monitored for size measurement at the time of manufacturing. 
     In each of the pixel PE 6  and the pixel PE 7 , the distance between the photodiodes PD 5  and PD 6  can be kept large. In addition, the distance between the photodiode PD 6  and an end portion of the active region including the photodiode PD 6  can be kept large. 
     Even when formation positions of the active region, the photodiode PD 5 , or the photodiode PD 6  is misaligned due to misregistration of respective masks used for the first exposure region IG 1  and the second exposure region IG 2 , the area of the photodiode PD 6  can be prevented from decreasing. In addition, the leakage between the photodiodes PD 5  and PD 6  can be prevented by decreasing the size of the photodiode PD 6 . As a result, generation, on an image, of an output level difference in the divided regions can be prevented. 
     It is also possible to juxtapose a plurality of pixels PE 3  in one column in the Y-axis direction in the third region  3 A and decrease the area of one of two photodiodes which each pixel in the first region  1 A or the second region  2 A has, as shown in  FIG. 22 .  FIG. 22  is a planar layout showing a still further semiconductor device of Modification Example 4 of the present embodiment. 
     The configuration shown in  FIG. 22  is different from the configuration shown in  FIG. 2  in that in the first region  1 A and the second region  2 A, some photodiodes PD 1  to PD 3  have an area smaller than that of the other photodiodes. 
     This means that in the pixels PE 1  of a certain row arranged in the X-axis direction, the area of the photodiode PD 1  is smaller than that of the photodiode PD 2 . In the pixels PE 1  of a row adjacent to the above certain row in the Y-axis direction, the area of the photodiode PD 2  is smaller than that of the photodiode PD 1 . 
     Thus, the pixel PE 1  in the first region  1 A has a photodiode having a relatively small area and the area of this photodiode is smaller than the area of each of the photodiodes PD 2  and PD 3  in the pixel PE 3 . In the pixel PE 1 , a photodiode juxtaposed with the above-described photodiode having a relatively small area has an area equivalent to that of the photodiode PD 2  or PD 3  in the pixel PE 3  (which may hereinafter be called “standard area”). 
     In a certain column in the first region  1 A, a photodiode PD 1  having a standard area and a photodiode PD 1  having an area smaller than that of the above photodiode PD 1  are juxtaposed alternately in the Y-axis direction. In a column adjacent to the certain column in the first region  1 A, a photodiode PD 2  having a standard area and a photodiode PD 2  having an area smaller than that of the above photodiode PD 2  are alternately juxtaposed in the Y-axis direction. 
     In a certain row in the first region  1 A, a photodiode PD 1  having a standard area and a photodiode PD 2  having an area smaller than that of the photodiode PD 1  are alternately juxtaposed in the X-axis direction. In a column adjacent to the certain column in the first region  1 A, a photodiode PD 2  having a standard area and a photodiode PD 1  having an area smaller than that of the photodiode PD 2  are alternately juxtaposed in the X-axis direction. 
     Similarly, the pixel PE 2  in the second region  2 A has a photodiode having a relatively small area and the area of this photodiode is smaller than the respective areas of the photodiodes PD 2  and PD 3  in the pixel PE 3 . In the pixel PE 2 , a photodiode juxtaposed with the above photodiode having a relatively small area has an area equal to the respective areas of the photodiodes PD 2  and PD 3  in the pixel PE 3 . 
     In a certain column in the second region  2 A, a photodiode PD 3  having a standard area and a photodiode PD 3  having an area smaller than that of the above photodiode PD 3  are alternately juxtaposed in the Y-axis direction. In a column adjacent to the certain column in the second region  2 A, a photodiode PD 4  having a standard area and a photodiode having an area smaller than that of the above photodiode PD 4  are alternately juxtaposed in the Y-axis direction. 
     In a certain row in the second region  2 A, a photodiode PD 3  having a standard area and a photodiode PD 4  having an area smaller than that of the above photodiode PD 3  are alternately juxtaposed in the X-axis direction. In a row adjacent to the certain row in the second region  2 A, a photodiode PD 4  having a standard area and a photodiode PD 3  having an area smaller than that of the photodiode PD 4  are alternately juxtaposed in the X-axis direction. 
     In a certain row in the pixel array portion, the pixel PE 1  has a photodiode PD 1  having a small are and the pixel PE 2  has a photodiode PD 3  having a small area and in a row adjacent to the certain row, the pixel PE 1  has a photodiode PD 2  having a small area and the pixel PE 2  has a photodiode PD 4  having a small area. 
     Thus, in the layout shown in  FIG. 22 , a difference is provided in the area of the active region or photodiode formed in each of the pixel PE 1  and PE 2  in regions other than the boundary region (the third region  3 A). In the third region  3 A, there may occur an output difference between the photodiodes PD 2  and PD 3  in one pixel PE 3  due to an overlapping error of two masks used for divided exposure. In such a case, by providing a size difference in advance between two photodiodes of the pixel PE 1  or PE 2  in regions other than the third region  3 A as shown in  FIG. 22 , the outputs of all the pixels PE 1  to PE 3  arranged in the pixel array portion can be averaged. 
     This means that even when there occurs an output difference between two photodiodes in the pixel PE 3 , the output difference can be made inconspicuous in the resulting solid image pickup device as a whole. In this example, the photodiodes having a larger area and a smaller area are alternately arranged in each row so that an output difference in the boundary region cannot be recognized easily when an image obtained using the solid state image sensor is viewed as a whole. This makes it possible to prevent generation of image abnormalities attributable to divided exposure. 
     When two photodiodes with the same area are formed for each pixel, one of the photodiodes manufactured by some semiconductor device manufacturing apparatuses unintentionally becomes larger. When a manufacturing apparatus having such a characteristic is used, it is possible to prevent variations in the area between these photodiodes and thereby prevent generation of image abnormalities attributable to divided exposure by designing the photodiodes PD 1  and PD 3  inside each of the pixels PE 1  and PE 2  to have a small area in advance as shown in  FIG. 22 . Such a characteristic may occur in a manufacturing step of a semiconductor device, for example, a step of forming photodiodes on a semiconductor substrate having a main surface with a level difference. 
     The two photodiodes in each of the pixels PE 1  and PE 2  have areas different from each other so that in image-plane phase-detection system autofocusing of the solid state image sensor shown in  FIG. 22 , whether it is focused or not is determined by comparing an average output of photodiodes on the left side in four pixels adjacent to each other in the X-axis direction and Y-axis direction with an average output of photodiodes on the right side in these four pixels. 
     Second Embodiment 
     A semiconductor device of Second Embodiment will hereinafter be described referring to  FIG. 23 .  FIG. 23  is a planar layout showing the semiconductor device of the present embodiment. 
     The configuration of the present embodiment differs from that of the above embodiment described referring to  FIG. 2  in that the pixels in the third region  3 A each have only one photodiode having an area greater than that of the photodiodes PD 1  to PD 4  and do not have another photodiode. 
     As shown in  FIG. 23 , in the third region  3 A, a pixel PE 9  and a pixel PE 10  are juxtaposed alternately a plurality of times along the Y-axis direction. The pixel PE 9  and the pixel PE 10  each have only one photodiode. In other words, in the third region  3 A, only one photodiode overlaps with one microlens ML in plan view. The pixel PE 9  has a photodiode PD 7  having an area greater than that of each of the photodiodes PD 1  to PD 4  in plan view. The pixel PE 10  has a photodiode PD 8  having an area greater than that of each of the photodiodes PD 1  to PD 4  in plan view. 
     The photodiodes PD 7  and PD 8  have areas equal to each other. The area of each of the photodiodes PD 7  and PD 8  approximates a sum of the area of the photodiode PD 1  and the area of the photodiode PD 2 . This means that the area of each of the photodiodes PD 7  and PD 8  approximates the sum of the area of the photodiode PD 3  and the area of the photodiode PD 4 . 
     The photodiode PD 7  is, similar to the photodiodes PD 1  and PD 2 , a light receiving element formed through the mask for first exposure region IG 1 . The photodiode PD 8  is, similar to the photodiodes PD 3  and PD 4 , a light receiving element formed through the mask for second exposure region IG 2 . In the third region  3 A, the photodiode PD 7  and the photodiode PD 8  formed through respectively different masks are arranged alternately in the Y-axis direction. 
     In order to perform image-plane phase-detection system autofocusing of the solid state image sensor, almost all the pixels in the pixel array portion have two photodiodes, but in the third region  3 A, one pixel has only one photodiode. The image-plane phase-detection system autofocusing is therefore not performed in the pixels PE 9  and PE 10  in the third region  3 A. 
     When two photodiodes are formed in the pixel in the third region  3 A, there may occur an output difference between the two photodiodes due to an overlapping error of two masks used for divided exposure. This may cause abnormalities on an image available by imaging. In the present embodiment, on the other hand, since the number of a photodiode formed in each of the pixels PE 9  and PE 10  in the third region  3 A is limited to one, occurrence of an output difference between pixels in the third region  3 A due to divided exposure can be prevented. 
     In the present embodiment, the active region and the photodiode of the pixel PE 9  are formed using only the mask for first exposure region IG 1  so that there occurs no variation in the area of the photodiode due to partial misalignment of the active region in the pixel PE 9 . Similarly, the active region and the photodiode of the pixel PE 10  are formed using only the mask for second exposure region IG 2  so that no variation in the area of the photodiode occurs in the pixel PE 10 . It is therefore possible to effectively prevent occurrence of a level difference at a position, on a static image available by the solid state image sensor, corresponding to the divided region. 
     Modification Example 
     As shown in  FIGS. 24 and 25 , a pixel having two photodiodes may be provided between the pixel PE 9  and the pixel PE 10  in the third region  3 A.  FIGS. 24 and 25  are planar layouts of semiconductor devices, each a modification example of the present embodiment. 
       FIG. 24  shows a structure in which a pixel PE 3  having photodiodes PD 3  and PD 2  is placed between the pixel PE 9  and the pixel PE 10  in the third region  3 A. This means that in the third region  3 A, pixels PE 3 , PE 9 , PE 3 , PE 10 , and PE 3  are arranged in the order of mention in the Y-axis direction. The configuration of the pixel PE 3  is similar to the pixel PE 3  described referring to  FIG. 2 . 
       FIG. 25  shows a structure in which a pixel PE 5  having photodiodes PD 1  and PD 4  is placed between the pixels PE 9  and PE 10  in the third region  3 A. This means that in the third region  3 A, the pixels PE 9 , PE 5 , PE 10 , PE 5 , and PE 9  are arranged in the order of mention in the Y-axis direction. The configuration of the pixel PE 5  is similar to that of the pixel PE 5  described referring to  FIG. 11 . 
     In the present modification example shown in  FIGS. 24 and 25 , an output level difference of pixels can be reduced by placing a pixel having only one photodiode in the third region  3 A where exposure regions overlap with each other. By placing a pixel having two photodiodes in addition, focusing detection can be achieved in a portion of the third region  3 A. 
     The invention made by the present inventors has been described based on some embodiments. It is needless to say that the invention is not limited to or by these embodiments but can be changed without departing from the gist of the invention.