Patent Publication Number: US-2013241017-A1

Title: Solid-state image pickup device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 11/774,726 filed on Jul. 9, 2007, which claims foreign priority to Japanese patent application No. 2006-188904 filed on Jul. 10, 2006. The entire contents of each of these applications are hereby expressly incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solid-state image pickup device. 
     2. Description of the Related Art 
     Japanese Patent Laid-Open No. 2002-33469 describes a back surface incident type of solid-state image pickup device. In the solid-state image pickup device, light incident on the back surface of a semiconductor substrate from an object is photoelectrically converted inside the semiconductor substrate. Electric charges produced by the conversion are received by a light receiving unit to image the object. 
     Japanese Patent Laid-Open No. 2000-217803 in addition to Japanese Patent Laid-Open No. 2002-33469 is known as a prior art document related to the present invention. 
     The present inventor has recognized as follows. The solid-state image pickup device described in Japanese Patent Laid-Open No. 2002-33469 leaves room for improvement in terms of easy release of excess charges. In this respect, the resistivity of the whole substrate needs to be reduced to easily release excess charges. This, however, makes it difficult to directly use a platform in an existing device process without modification when a light receiving unit is formed in the substrate. 
     SUMMARY 
     According to one aspect of the present invention, there is provided a solid-state image pickup device photoelectrically converting light incident on a back surface of a semiconductor substrate into signal electric charges to image an object, the solid-state image pickup device comprising: a first semiconductor layer forming a part or whole of the semiconductor substrate and having a first resistivity; a second semiconductor layer provided on a front surface of the semiconductor substrate and having a second resistivity higher than the first resistivity; and a light receiving unit provided in the second semiconductor layer and receiving the signal electric charges produced by the photoelectric conversion. 
     The solid-state image pickup device is provided with a first semiconductor layer with a relatively lower resistivity (a first resistivity). This enables excess charges to be easily released from the surface of the semiconductor substrate. On the other hand, a second semiconductor layer on which the light receiving unit is provided has a higher resistivity (a second resistivity) than the first semiconductor layer. Unlike the case where both the first and the second semiconductor layer have the first resistivity, a platform in an existing device process can be directly used when a light receiving unit is formed on the second semiconductor substrate. 
     According to another aspect of the present invention, there is provided a solid-state image pickup device comprising: a first semiconductor layer of first conductivity type having first and second main surface portions; a plurality of diffusion regions of second conductivity type formed in the first main surface portion of the first semiconductor layer; and a second semiconductor layer of said first conductivity type formed over the second main surface portion of the first semiconductor layer and receiving light incident thereon, the second semiconductor layer being lower in resistivity than the first semiconductor layer. 
     The present invention realizes a solid-state image pickup device which can be easily produced and has a structure capable of easily releasing excess charges. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  are cross sections illustrating a first embodiment of a solid-state image pickup device according to the present invention; 
         FIGS. 2A and 2B  are cross sections describing one example of operation of the solid-state image pickup device in  FIG. 1 ; 
         FIGS. 3A and 3B  are cross sections illustrating a second embodiment of a solid-state image pickup device according to the present invention; 
         FIGS. 4A and 4B  are cross sections illustrating a solid-state image pickup device according to one modification in an embodiment; and 
         FIGS. 5A and 5B  are cross sections illustrating a solid-state image pickup device according to another modification in the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a solid-state image pickup device according to the present invention are described in detail below with reference to the drawings. In the description of the drawings the same constituent elements are given the same reference numerals to omit duplicated description. 
     First Embodiment 
       FIG. 1A  is a cross section illustrating a first embodiment of a solid-state image pickup device according to the present invention. A solid-state image pickup device  1  is back surface incident type including a semiconductor substrate  10  (a first semiconductor layer), semiconductor layer  20  (a second semiconductor layer) and light receiving unit  30 . The solid-state image pickup device  1  images an object in such a manner that light incident on the back surface S 2  of the semiconductor substrate  10  is photoelectrically converted into signal electric charges inside the semiconductor substrate  10  or the semiconductor layer  20 . 
     The semiconductor substrate  10  in the present invention is a p + -type silicon substrate. The semiconductor substrate  10  has a resistivity ρ 1  (a first resistivity). The resistivity ρ 1  is, for example, 0.01 Ωcm and is preferably ρ 1 ≦0.1 Ωcm. The thickness of the semiconductor substrate  10  is preferably not greater than the absorption length of the above light, that is, of light used in image pickup. If the wavelength of the above light is 1 μm, for example, the absorption length is approximately 100 μm. 
     The semiconductor layer  20  is provided on a front surface S 1  of the semiconductor substrate  10 . The semiconductor layer  20  in the present invention is a p-type silicon layer. The semiconductor layer  20  has a resistivity ρ 2  (a second resistivity). Where, ρ 2 &gt;ρ 1 . The resistivity ρ 2  is, for example, 10 Ωcm. It is preferably that 5 Ωcm≦ρ 2 ≦100 Ωcm. The semiconductor layer  20  is formed by an epitaxial growth method, for example. 
     The light receiving unit  30  is formed in the semiconductor layer  20 . Specifically, the light receiving unit  30  is provided in the surface layer on the front surface (or, the surface on the opposite side of the semiconductor substrate  10 ) side of the semiconductor layer  20 . The light receiving unit  30  receives signal electric charges produced by the above photoelectric conversion. The light receiving unit  30  in the present invention is an n-type impurity diffusion layer. The light receiving unit  30  and the semiconductor layer  20  adjacent thereto form a photodiode. 
     A MOSFET  50  is also formed in the semiconductor layer  20 . That is to say, the solid-state image pickup device  1  includes a MOS image sensor formed of the light receiving unit  30  and a logic circuit unit formed of the MOSFET  50 . The MOSFET  50  includes an n-type impurity diffusion layer  52  functioning as source and drain area and a gate electrode  54 . 
     An interconnect layer  60  is provided on the surface of the semiconductor layer  20 . An interconnect (not shown) is formed in the interconnect layer  60 . 
     One example of operation of the solid-state image pickup device  1  is described with reference to  FIG. 2A . In the figure, a finger  90  being an object to be imaged is brought into contact with the back surface S 2  of the semiconductor substrate  10 . When light L 1  from a light source such as a fluorescent lamp or LED is caused to be incident on a finger  90 , a transmitted light L 2  is incident on the back surface S 2 . At that point, the transmitted light L 2  includes information on a fingerprint  92  of the finger  90 . The transmitted light L 2  is photoelectrically converted into signal electric charges inside the semiconductor substrate  10  or the semiconductor layer  20 . The light receiving unit  30  receives the signal charges produced by the photoelectric conversion to pick up the image of the fingerprint  92 . Incidentally, any of visible light, near infrared light or infrared light may be used as the light L 1 . A fixed electric potential (for example, ground potential) is preferably applied to the semiconductor substrate  10  while the solid-state image pickup device  1  is operating.  FIGS. 1B and 2B  illustrate examples where the semiconductor substrate  10  is connected to the ground in  FIGS. 1A and 2A . 
     The effects of the present invention are described below. The semiconductor substrate  10  with a small resistivity ρ 1  is provided on the solid-state image pickup device  1 . This causes excess charges introduced by electro-static discharge (ESD) when the finger is brought into contact with the back surface S 2  to be readily released from the semiconductor substrate  10  connected to the fixed electric potential (for example, ground potential), precluding circuit elements (MOSFET  50  or the like) in the solid-state image pickup device  1  from being electrostatically broken down. If ρ 1 ≦0.1 Ωcm, a marked effect can be obtained. 
     The semiconductor layer  20  on which the light receiving unit  30  is provided has a higher resistivity ρ 2  than that of the semiconductor substrate  10 . This enables the platform in an existing device process to be used without modification when the light receiving unit  30  and MOSFET  50  are formed on the semiconductor layer  20 , unlike the case where both the semiconductor substrate  10  and the semiconductor layer  20  have the resistivity ρ 1 . This is because the resistivity ρ 2  can be set to a value within an available range of the existing device process because the value of the resistivity ρ 2  does not need following the resistivity ρ 1  to be reduced. If 5 Ωcm≦p 2 ≦100 Ωcm, it is particularly preferable to use the existing device process without modification. If the existing device process cannot be used, a problem is caused in that a standard logic process is prevented from being used and circuit components such as a standard macro from being used. However, according to the present embodiment, this problem can be avoided. 
     Part of signal electric charges produced by photoelectric conversion disappears due to recombination before reaching the light receiving unit  30 . In this respect, if the thickness of the semiconductor substrate  10  is not greater than the absorption length of the light used for image pickup, the ratio of recombining signal charges can be sufficiently lowered, providing the solid-state image pickup device  1  having extra-high sensitivity. 
     A fixed electric potential is applied to the semiconductor substrate  10  to allow excess charges to be more easily released. This further suppresses the occurrence of electrostatic breakdown. 
     When the semiconductor layer  20  is formed by the epitaxial growth method, or when the semiconductor layer  20  is formed of an epitaxial layer, the semiconductor layer  20  having a higher resistivity than the semiconductor substrate  10  is easily formed. The resistivity ρ 1  of the semiconductor substrate  10  can be sharply changed to the resistivity ρ 2  of the semiconductor layer  20 . 
     Second Embodiment 
       FIG. 3A  is a cross section illustrating a second embodiment of a solid-state image pickup device according to the present invention. A solid-state image pickup device  2  is also back surface incident type including a semiconductor substrate  10 , semiconductor layer  20  and light receiving unit  30 . The solid-state image pickup device  2  is different from the solid-state image pickup device  1  in the configuration of the semiconductor substrate  10 . The semiconductor substrate  10  in the solid-state image pickup device  2  is a p-type or p − -type silicon substrate. A p + -type impurity diffusion layer  12  (a first semiconductor layer) is formed on the surface layer on the side of the back surface S 2  of the semiconductor substrate  10 . The p + -type impurity diffusion layer  12  has the above described resistivity ρ 1 . That is, the whole of the semiconductor substrate  10  is formed by the first semiconductor layer in the solid-state image pickup device  1 , whereas, only part of the semiconductor substrate  10  is formed by the first semiconductor layer in the solid-state image pickup device  2 . The resistivity ρ 1  of the p + -type impurity diffusion layer  12  is lower than a resistivity ρ 3  of semiconductor substrate  10 . The resistivity ρ 3  of semiconductor substrate  10  may be lower than the resistivity ρ 2  of the semiconductor layer  20 , or the resistivity ρ 3  of semiconductor substrate  10  may be higher than the resistivity ρ 2  of the semiconductor layer  20 . Other configuration and operation of the solid-state image pickup device  2  are the same as those of the solid-state image pickup device  1 . Also in  FIG. 3A , a fixed electric potential is preferably applied to the semiconductor substrate  10 , it is particularly preferable to apply a fixed electric potential to the p + -type impurity diffusion layer  12 .  FIG. 3B  illustrates an example where the p + -type impurity diffusion layer  12  is connected to the ground. 
     When the whole semiconductor substrate  10  corresponds to the first semiconductor layer as in the solid-state image pickup device  1 , the thickness of the whole semiconductor substrate  10  needs to be reduced from the viewpoint that the ratio of recombining signal charges is lowered as described above. On the other hand, the thickness of the semiconductor substrate  10  needs to be increased from the viewpoint that the strength of the semiconductor substrate  10  or the strength of the solid-state image pickup device is sufficiently secured. 
     The present embodiment can satisfy these opposing requirements. That is to say, a configuration in which the first semiconductor layer is provided only at part of the semiconductor substrate  10  enables the ratio of recombining signal electric charges to be lowered even if the thickness of the whole semiconductor substrate  10  is increased, as long as the first semiconductor layer is positioned in the vicinity of the back surface S 2  of the semiconductor substrate  10 . Specifically, it is preferable that a distance d 1  between the face of the first semiconductor layer on the side of the semiconductor layer  20  (refer to  FIGS. 3A and 3B ) and the back surface S 2  of the semiconductor substrate  10  is not greater than the absorption length of the light used for image pickup. In the present embodiment, the distance d 1  is equal to the thickness of the p + -type impurity diffusion layer  12 . Other effects of the solid-state image pickup device  2  are the same as those of the solid-state image pickup device  1 . 
     The solid-state image pickup device according to the present invention is not limited to the above embodiment, but may be modified in various forms. Various configurations in addition to those illustrated in the above embodiments are possible as the configuration of the semiconductor substrate  10  as long as at least part of the semiconductor substrate  10  is formed by the first semiconductor layer with the resistivity ρ 1 . For example, as illustrated in  FIG. 4A , the semiconductor substrate  10  may include a p + -type silicon substrate  14  corresponding to the first semiconductor layer and an epitaxial layer  15  provided thereon. The epitaxial layer  15  is a p-type or p − -type silicon layer. 
     Alternatively, as illustrated in  FIG. 5A , the semiconductor substrate  10  may include a silicon substrate  16  and epitaxial layers  17  and  18  deposited thereon in this order. The silicon substrate  16  is of p-type or p − -type. The epitaxial layer  17  is a p + -type silicon layer and corresponds to the first semiconductor layer. The epitaxial layer  18  is a p-type or p − -type silicon layer. That is, the resistivity of the silicon substrate  10  is higher than the resistivity of the epitaxial layer  17 , and the resistivity of the epitaxial layer  18  is higher than the resistivity of the epitaxial layer  17 . In this case, a distance d 2  in  FIG. 5A  denotes “a distance between the face of the first semiconductor layer on the side of the semiconductor layer  20  and the back surface S 2  of the semiconductor substrate  10 ”. In  FIG. 5A , the epitaxial layer  18  need not be provided. In that case, the epitaxial layer  17  being the first semiconductor layer is positioned on the surface layer of the semiconductor substrate  10  on the side of a surface S 1 . Also in  FIGS. 4A and 5A , a fixed electric potential is preferably applied to the semiconductor substrate  10 , it is particularly preferable to apply a fixed electric potential to the first semiconductor layer (to the p + -type silicon substrate  14  in  FIG. 4A  and to the epitaxial layer  17  in  FIG. 5A ).  FIG. 4B  illustrates an example where the p + -type silicon substrate  14  is connected to the ground in  FIG. 4A .  FIG. 5B  illustrates an example where the epitaxial layer  17  is connected to the ground in  FIG. 5A . 
     Although the above embodiments show examples where the N-channel MOSFET  50  is provided, a P-channel MOSFET may be provided instead of the MOSFET  50  or in addition to the MOSFET  50 . 
     Although the p-type semiconductor substrate, p-type semiconductor layer and n-type light receiving unit are exemplified in the above embodiments, the n-type semiconductor substrate, n-type semiconductor layer and p-type light receiving unit may be used. 
     A portion excluding the first semiconductor layer in the semiconductor substrate may be a high-resistance semiconductor layer with a resistivity of 1000 Ωcm or more. The portion corresponds to the semiconductor substrate  10  (except the portion where the p + -type impurity diffusion layer  12  is formed) in  FIGS. 3A and 3B , the epitaxial layer  15  in  FIGS. 4A and 4B  and the silicon substrate  16  and the epitaxial layer  18  in  FIGS. 5A and 5B . 
     The present invention can be suitably applied to a charge coupled device (CCD) type of solid-state image pickup device. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.