Abstract:
A wafer for backside illumination type solid imaging device having a plurality of pixels inclusive of a photoelectric conversion device and a charge transfer transistor at its front surface side and a light receiving surface at its back surface side is produced by a method comprising a step of forming a BOX oxide layer on at least one of a wafer for support substrate and a wafer for active layer, a step of bonding the wafer for support substrate and the wafer for active layer and a step of thinning the wafer for active layer, which further comprises a step of forming a plurality of concave portions on a bonding face of the BOX oxide layer to the other wafer and filling a polysilicon plug into each of the concave portions to form a composite layer before the step of bonding the wafer for support substrate and the wafer for active layer.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a method for producing a wafer for backside illumination type solid imaging device, and more particularly to a method for producing a wafer for backside illumination type solid imaging device used in mobile phones, digital video cameras and the like. 
         [0003]    2. Description of the Related Art 
         [0004]    Recently, a high-performance solid imaging device using a semiconductor is mounted onto a mobile phone, a digital video camera or the like. As the performance to be required in the solid imaging device are high-quality pixels and ability of taking moving images, and in order to take moving images, it is required to combine a high-speed computing device with a memory device, and hence a CMOS image sensor allowing System on Chip (SoC) easily is used and the downsizing of the CMOS image sensor is developed. 
         [0005]    With the downsizing of the CMOS image sensor, however, there is caused a problem that an aperture ratio of a photodiode as a photoelectric conversion device is necessarily reduced to lower a quantum efficiency of the photoelectric conversion device, which makes it difficult to improve S/N ratio of imaging data. Therefore, it is attempted to conduct a method for increasing incident light quantity by inserting an inner lens into a front side of the photoelectric conversion device, or the like. However, the remarkable improvement of S/N ratio can not be realized. 
         [0006]    In order to increase the incident light quantity to improve the S/N ratio of the image data, therefore, it is attempted to feed the incident light from a backside of the photoelectric conversion device. The greatest merit of the light incidence from the backside of the device lies in a point that there is no restriction in the reflection or diffraction on the front face of the device or the light receiving area of the device as compared with the light incidence from the front side. On the other hand, when the light is entered from the backside, the light absorption through a silicon wafer as a substrate for the photoelectric conversion device must be suppressed, and hence the thickness of the solid imaging device as a whole is required to be less than 50 μm. As a result, the working and handling of the solid imaging device become difficult, causing a problem of extremely low productivity. 
         [0007]    For the purpose of overcoming the above technical problems, there are mentioned backside illumination type solid imaging devices as disclosed, for example, in JP-A-2007-13089 and JP-A-2007-59755. 
         [0008]    In JP-A-2007-13089 is disclosed a method for producing a solid imaging device, which allows the production of a backside illumination type CMOS solid imaging device having a structure that electrodes are taken out from a surface opposite to an illuminated surface relatively simply and easily since a semiconductor substrate is thinned after bonding with a support substrate to ensure a strength and a through-hole interconnection is formed after the thinning of the support substrate. 
         [0009]    In JP-A-2007-59755 is disclosed a solid imaging apparatus wherein internal stress and strain of a semiconductor substrate can be made small but also the processing of a color filter, a micro-lens or the like onto a thinned surface of a semiconductor substrate can be conducted in a high accuracy as well as a production method thereof. 
         [0010]    In the solid imaging devices of these patent documents, however, the gettering ability of the substrate (wafer) is low, so that there are problems that white defects occur and that heavy metal contamination occurs in the production process. Therefore, it is required to solve these problems in order to put the backside illumination type solid imaging device into practical use. 
       SUMMARY 
       [0011]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0012]    It is, therefore, an object of the invention to provide a method for producing a wafer for backside illumination type solid imaging device, which is capable of effectively suppressing the occurrence of white defects and the heavy metal contamination. 
         [0013]    In order to achieve the above object, the summary and construction of the invention are as follows. 
         [0014]    (1) A method for producing a wafer for backside illumination type solid imaging device having a plurality of pixels inclusive of a photoelectric conversion device and a charge transfer transistor at its front surface side and a light receiving surface at its back surface side, comprising a step of forming a buried oxide (BOX) oxide layer on at least one of a wafer for support substrate and a wafer for active layer, a step of bonding the wafer for support substrate and the wafer for active layer, and a step of thinning the wafer for active layer, which further comprises a step of forming a plurality of concave portions on a bonding face of the BOX oxide layer to the other wafer and filling a polysilicon plug into each of the concave portions to form a composite layer before the step of bonding the wafer for support substrate and the wafer for active layer. 
         [0015]    (2) The method according to the item (1), wherein a single oxide layer is existent between the composite layer and the other wafer. 
         [0016]    (3) The method according to the item (1), wherein the wafer for support substrate is made of C-containing n-type semiconductor material. 
         [0017]    (4) The method according to the item (1), wherein the wafer for active layer is an epitaxial wafer obtained by forming an epitaxial film of Si on a substrate for active layer made of n-type semiconductor layer. 
         [0018]    (5) The method according to the item (3), wherein the wafer for support substrate has a C concentration of 1×10 16  to 1×10 17  atoms/cm 3 . 
         [0019]    (6) The method according to the item (1), which further comprises a step of forming a polysilicon film on a face opposite to the bonding face of at least one of the wafer for support substrate and the wafer for active layer before the step of forming the BOX oxide layer. 
         [0020]    (7) The method according to the item (1), which further comprises a step of subjecting each of the wafers to a heat treatment at 600 to 800° C. before the step of forming the BOX oxide layer. 
         [0021]    (8) The method according to the item (1), which further comprises a step of adsorbing a given organic substance onto the bonding face of the BOX oxide layer to the other wafer after the step of forming the BOX oxide layer and before the step of bonding the wafer for support substrate and the wafer for active layer. 
         [0022]    (9) The method according to the item (8), wherein the organic substance is an organic carbon compound. 
         [0023]    According to the production method of a wafer for backside illumination type solid imaging device of the invention, it is possible to provide a wafer for backside illumination type solid imaging device, which is capable of effectively suppressing the occurrence of white defects and the heavy metal contamination. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0024]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0025]      FIG. 1  is a flow chart illustrating a method for producing a wafer for backside illumination type solid imaging device according to the invention; 
           [0026]      FIG. 2A  is a schematic view showing a section of a wafer for support substrate; 
           [0027]      FIG. 2B  is a schematic view showing a section of a wafer for active layer; and 
           [0028]      FIGS. 3A-3C  are schematic views showing exemplary configurations of a polysilicon plug. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 
         [0030]    A method for producing a wafer for backside illumination type solid imaging device according to the invention will be described with reference to the drawings.  FIG. 1  is a flow chart illustrating a method for producing a wafer for backside illumination type solid imaging device according to the invention, and  FIGS. 2A and 2B  are schematic views showing sections of a wafer for support substrate and a wafer for active layer, respectively, which are used in the method for producing a wafer for backside illumination type solid imaging device according to the invention. Also,  FIGS. 3A-3C  are schematic plan views showing exemplary configurations of a polysilicon plug shown in  FIG. 1(   c ). 
         [0031]    As shown in  FIG. 1 , the method for producing a wafer for backside illumination type solid imaging device according to the invention comprises a step ( FIG. 1(   b )) of forming a BOX oxide layer  30  on at least one of a wafer  10  for support substrate ( FIG. 1(   a )) and a wafer  20  for active layer ( FIG. 1(   d )), a step ( FIG. 1(   e )) of bonding the wafer  10  for support substrate and the wafer  20  for active layer and a step ( FIG. 1(   f )) of thinning the wafer  20  for active layer, and is characterized by further comprising a step ( FIG. 1(   c )) of forming a plurality of predetermined concave portions  40  on a bonding face  30   a  of the BOX oxide layer  30  to the other wafer and filling a polysilicon plug  50  in each of the concave portions  40  to form a composite layer  60  before the step of bonding the wafer  10  for support substrate and the wafer  20  for active layer. When a wafer  100  for backside illumination type solid imaging device ( FIG. 1(   f )) is formed by this method, the polysilicon plugs  50  serve as gettering sinks of heavy metal. When the wafer  100  is used in a backside illumination type solid imaging device, the occurrence of white defects and the heavy metal contamination can be effectively suppressed by gettering the heavy metal as compared to the conventional imaging device. As an example in  FIG. 1 , the BOX oxide layer  30  is formed only on the wafer  10  for support substrate, and the predetermined concave portions  40  are formed on the bonding face  30   a  of the BOX oxide layer  30  to the wafer  20  for active layer, and the polysilicon plug  50  is filled in each of the concave portions  40  to form the composite layer  60 , which shows merely one embodiment of the invention. The BOX oxide layer  30  may be formed on the wafer  20  for active layer, or on both of the wafer  10  for support substrate and the wafer  20  for active layer. 
         [0032]    (Wafer for Support Substrate) 
         [0033]    The wafer  10  for support substrate shown in  FIG. 2A  is preferably made of a carbon (C)-containing n-type semiconductor material in view of strengthening the gettering ability and is preferable to have a specific resistance of 1 to 20 Ω·cm. 
         [0034]    Further, the C concentration of the wafer  10  for support substrate is preferable to be within a range of 1×10 16  to 1×10 17  atoms/cm 3 . When the C concentration is less than 1×10 16  atoms/cm 3 , the gettering ability can not be developed sufficiently and there is a fear that the white defects and heavy metal contamination can not be sufficiently suppressed, while when it exceeds 1×10 17  atoms/cm 3 , the size of the oxygen precipitates is less than 50 nm and strain energy capable of gettering heavy metal can not be retained. 
         [0035]    (Wafer for Active Layer) 
         [0036]    The wafer  20  for active layer  20  is preferable to be an epitaxial wafer  20  obtained by forming an epitaxial film  22  of Si on a substrate  21  for active layer made of n-type semiconductor layer as shown in  FIG. 2B . Also, the substrate  21  for active layer is preferable to contain C in view of strengthening the gettering ability and to have a specific resistance of 3 to 15 Ω·cm. Since the epitaxial film  22  being less in the defects and having a high quality can be formed on the substrate  21  for active layer made of C-containing n-type semiconductor material by the gettering effect of the substrate  21  for active layer, when such an epitaxial film  22  is formed on the composite layer  60  to produce a solid imaging device, the effect of suppressing the occurrence of white defects and heavy metal contamination can be further improved. 
         [0037]    Moreover, the C concentration of the substrate  21  for active layer is preferable to be within a range of 1×10 16  to 1×10 17  atoms/cm 3 . When the C concentration is less than 1×10 16  atoms/cm 3  likewise the case of the wafer  10  for support substrate, the gettering ability can not be developed sufficiently and there is a fear that the white defects and heavy metal contamination generated in the epitaxial film  22  can not be sufficiently suppressed, while when it exceeds 1×10 17  atoms/cm 3 , the size of the oxygen precipitates becomes minimal and it is difficult to retain strain energy required for the gettering and hence there is a fear that the gettering ability lowers. 
         [0038]    Furthermore, when the wafer  10  for support substrate contains C, it is preferable that the contained C atoms are existent as a high carbon concentration region just beneath an interface with the composite layer  60 . The high carbon concentration region means a region that the C concentration is locally large in the wafer  10  for support substrate. Since the high carbon concentration region effectively serves as a gettering sink, the effect of suppressing the occurrence of white defects and the heavy metal contamination can be further improved. 
         [0039]    As a method of including a given amount of C into the wafer  10  for support substrate and the wafer  20  for active layer, there are mentioned a method of doping a silicon substrate with C atoms, an ion implantation method and so on. Also, O atoms can be included into the wafer  10  for support substrate and the wafer  20  for active layer. The inclusion of O atoms can effectively suppress the diffusion of the C atoms included for the gettering effect into the epitaxial film  22 . 
         [0040]    Although it is not shown, the production method according to the invention is preferable to further comprise a step of forming a polysilicon film on a face opposite to the bonding face of at least one of the wafer  10  for support substrate and the wafer  20  for active layer before the step of forming the BOX oxide layer. The formation of the polysilicon film is expected to further enhance the gettering effect since it serves as a gettering sink. 
         [0041]    (Composite Layer) 
         [0042]    The BOX oxide layer  30  is preferably formed by wet oxidation. In this case, a dense film can be formed to form an interface suitable for bonding. 
         [0043]    The concave portions  40  provided on the bonding face  30   a  of the BOX oxide layer  30  to the other wafer  20  are preferably formed by dry etching without passing through the BOX oxide layer  30 . Thus, the shape accuracy of the concave portion is attained in a high precision. Also, the shape of the concave portion  40  can be an arbitrary shape, which may include, for example, an island shape ( FIG. 3A ), a line shape (FIG.  3 B)), or a texture shape ( FIG. 3C ). 
         [0044]    The polysilicon plug  50  filled in the concave portion  40  is formed by embedding polysilicon into the concave portion  40 . The thickness d of the polysilicon plug  50  is preferable to be 50 to 70% of a thickness D of the composite layer  60 . When the thickness d is less than 50%, there is a fear that the gettering ability of the polysilicon lowers, while when it exceeds 70%, the formation of polysilicon takes long time and the production efficiency lowers. Moreover, a total upper area s of the polysilicon plugs  50  is preferable to be 70 to 90% of an upper area S of the wafer  10  for support substrate. When the area s is less than 70%, there is a fear that the gettering ability lowers, while when it exceeds 90%, there is a fear that the bonding strength between the polysilicon plug and the substrate for active layer lowers. 
         [0045]    The production method of a wafer for backside illumination type solid imaging device according to the invention is preferable to further comprise a step of subjecting each of the wafers  10  and  20  to a heat treatment at 600 to 800° C. before the step of forming the BOX oxide film. Since the oxygen precipitation is promoted by this heat treatment, it is possible to form high-density oxygen precipitates. 
         [0046]    The production method of a wafer for backside illumination type solid imaging device according to the invention is preferable to further comprise a step of adsorbing a given organic substance on the bonding face of the BOX oxide layer  30  to the other wafer after the step of forming the BOX oxide layer  30  and before the step of bonding the wafer  10  for support substrate and the wafer  20  for active layer though it is not shown in  FIG. 1 . When bonding is conducted by adsorbing the organic substance on the bonding face, the organic substance forms a high carbon concentration region in the bonding interface by the heat treatment in the bonding, which is desired to further improve the gettering ability in the wafer  100  for backside illumination type solid imaging device according to the invention. 
         [0047]    The organic substance is preferable to be an organic carbon compound such as N-methyl pyrrolidone, polyvinyl pyrrolidone or the like. By using such an organic carbon compound can be simply conducted the formation of the high carbon concentration region. 
         [0048]    The production method of a wafer for backside illumination type solid imaging device according to the invention is preferable to have a single oxide layer between the composite layer  60  and the other wafer though it is not shown in  FIG. 1 . This is for facilitating the separation of the polysilicon plugs  50  filled in the composite layer  60  from the wafer  20  for active layer. 
         [0049]    Although the above is described with respect to only one embodiment of the invention, various modifications may be made without departing from the scope of the appended claims. 
         [0050]    Next, a wafer for backside illumination type solid imaging device according to the invention is prepared as a sample and its performances are evaluated as described below. 
       EXAMPLE 1 
       [0051]    As shown in  FIGS. 1 ,  2 A, and  2 B, a BOX oxide layer  30  ( FIG. 1(   b )) is formed on a wafer  10  for support substrate ( FIG. 1(   a ) and  FIG. 2A)  by wet oxidation, and a plurality of columnar concave portions  40  are formed in the BOX oxide layer  30  by dry etching, and a polysilicon plug is filled in each of these concave portions  40  by a CVD method to form a composite layer ( FIG. 1(   c )). On the other hand, there is provided an epitaxial wafer prepared by forming an epitaxial film  22  of Si on a substrate  21  for active layer by the CVD method as a wafer  20  for active layer ( FIG. 1(   d ) and  FIG. 2B) . 
         [0052]    Thereafter, the wafer  10  for support substrate and the wafer  20  for active layer are bonded ( FIG. 1(   e )), and then the wafer  20  for active layer is thinned by polishing and chemical etching to prepare a wafer  100  for backside illumination type solid imaging device as a sample ( FIG. 1(   f )). 
       EXAMPLE 2 
       [0053]    A sample of a wafer for backside illumination type solid imaging device is prepared in the same manner as in Example 1 except that the thickness of the polysilicon plug is changed. 
       EXAMPLE 3 
       [0054]    A sample of a wafer for backside illumination type solid imaging device is prepared in the same manner as in Example 1 except that a total upper area of the polysilicon plugs is changed. 
       EXAMPLE 4 
       [0055]    A sample of a wafer for backside illumination type solid imaging device is produced in the same steps as in Example 2 except that a total upper area of the polysilicon plug is changed. 
       COMPARATIVE EXAMPLE 1 
       [0056]    A sample of a wafer for backside illumination type solid imaging device is prepared in the same manner as in Example 1 except that the polysilicon plug  50  is not formed. 
         [0057]    (Evaluation Method) 
         [0058]    Each sample prepared in Examples 1 to 4 and Comparative Example 1 is evaluated by the following evaluation methods. 
         [0059]    (1) White Defects 
         [0060]    A backside illumination type solid imaging device is prepared by using each sample prepared in Examples 1-4 and Comparative Example 1, and thereafter the dark leakage current of a photodiode in the backside illumination type solid imaging device is measured and converted to pixel data (number data of white defects) with a semiconductor parameter analyzing apparatus, whereby the number of white defects per unit area (1 cm 2 ) is measured to evaluate the suppression on the occurrence of white defects. The evaluation standard is shown below, and the measured results and evaluation results are shown in Table 1. 
         [0061]    {circumflex over (∘)}: not more than 5 
         [0062]    ◯: more than 5 but not more than 50 
         [0063]    ×: more than 50 
         [0064]    (2) Heavy Metal Contamination 
         [0065]    A defect density (defects/cm 2 ) on the surface of the sample is measured by contaminating the sample surface with nickel (1.0×10 12  atoms/cm 2 ) by a spin coat contaminating method and thereafter subjecting to a heat treatment at 900° C. for 1 hour and then selectively etching the sample surface. The evaluation standard is shown below, and the measured results and evaluation results are shown in Table 1. 
         [0066]              : less than 5 
         [0067]    ∘: not less than 5 but less than 50 
         [0068]    ×: not less than 50 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Thickness 
                   
                 Total upper 
                 Upper area 
                   
               
               
                   
                 of 
                 Thickness 
                 area of 
                 of wafer for 
                 Evaluation results 
               
             
          
           
               
                   
                 polysilicon 
                 of composite 
                 polysilicon 
                 support 
                 White 
                 Heavy metal 
               
               
                   
                 plug 
                 layer 
                 plugs 
                 substrate 
                 defect 
                 contamination 
               
               
                   
                 d (Å) 
                 D (Å) 
                 s (cm 2 ) 
                 S (cm 2 ) 
                 Evaluation 
                 Evaluation 
               
               
                   
                   
               
             
          
           
               
                 Example 1 
                 150 
                 300 
                 1130 
                 1256 
                 ⊚ 
                 ⊚ 
               
               
                 Example 2 
                 250 
                 300 
                 1130 
                 1256 
                 ⊚ 
                 ⊚ 
               
               
                 Example 3 
                 150 
                 300 
                 754 
                 1256 
                 ◯ 
                 ◯ 
               
               
                 Example 4 
                 250 
                 300 
                 754 
                 1256 
                 ◯ 
                 ◯ 
               
               
                 Comparative 
                 0 
                 300 
                 0 
                 1256 
                 X 
                 X 
               
               
                 Example 1 
               
               
                   
               
             
          
         
       
     
         [0069]    As seen from the results of Table 1, Examples 1 to 4 can suppress the occurrence of white defects and heavy metal contamination as compared to Comparative Example 1. 
         [0070]    According to the production method of a wafer for backside illumination type solid imaging device of the invention, it is possible to provide a wafer for backside illumination type solid imaging device capable of effectively suppressing the occurrence of white defects and heavy metal contamination.