Patent Publication Number: US-2020295078-A1

Title: Solid-state imaging device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-044028, filed on Mar. 11, 2019; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments relate to a solid-state imaging device. 
     BACKGROUND 
     A solid-state imaging device has a structure in which multiple pixels are provided in a semiconductor body such as silicon or the like. The solid-state imaging device may achieve higher sensitivity by increasing the light amount entering each pixel. Thus, it is important to provide, for example, an anti-reflection film to reduce the light loss due to reflections at the silicon surface. However, there may be a case where the dark output increases in the pixel to which the anti-reflection film is applied, and reduces the sensitivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic views showing a solid-state imaging device according to a first embodiment; 
         FIGS. 2A and 2B  are schematic views showing a solid-state imaging device according to a comparative example; 
         FIG. 3  is a schematic view showing a solid-state imaging device according to a modification of the first embodiment; and 
         FIGS. 4A and 4B  are schematic views showing a solid-state imaging device according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a solid-state imaging device includes a light detector provided inside a semiconductor body; a first insulating film provided on a front surface of the semiconductor body; a plurality of second insulating films provided between the light detector and the first insulating film, the plurality of second insulating films arranged in a first direction along the front surface of the semiconductor body; and a third insulating film provided between the semiconductor body and the second insulating films, the third insulating film having a refractive index lower than a refractive index of the second insulating films. 
     Embodiments will now be described with reference to the drawings. The same portions inside the drawings are marked with the same numerals; a detailed description is omitted as appropriate; and the different portions are described. The drawings are schematic or conceptual; and the relationships between the thicknesses and widths of portions, the proportions of sizes between portions, etc., are not necessarily the same as the actual values thereof. The dimensions and/or the proportions may be illustrated differently between the drawings, even in the case where the same portion is illustrated. 
     There are cases where the dispositions of the components are described using the directions of XYZ axes shown in the drawings. The X-axis, the Y-axis, and the Z-axis are orthogonal to each other. Hereinbelow, the directions of the X-axis, the Y-axis, and the Z-axis are described as an X-direction, a Y-direction, and a Z-direction. Also, there are cases where the Z-direction is described as upward and the direction opposite to the Z-direction is described as downward. 
     First Embodiment 
       FIGS. 1A and 1B  are schematic views showing a solid-state imaging device  1  according to a first embodiment. The solid-state imaging device  1  is, for example, a Charge Coupled Device (CCD).  FIG. 1A  is a plan view showing the arrangement of pixels P X  of the solid-state imaging device  1 .  FIG. 1B  is a cross-sectional view showing the structure of the pixel P X  of the solid-state imaging device  1 . 
     As shown in  FIG. 1A , the solid-state imaging device  1  includes the pixels P X , a light-shielding film SF 1 , and a light-shielding film SF 2 . For example, the pixels P X  are arranged in a transparent region TPR between the light-shielding film SF 1  and the light-shielding film SF 2 . The light-shielding films SF 1  and SF 2  are, for example, aluminum films, and extend in the Y-direction. The pixels P X  are arranged in the Y-direction in the transparent region TPR. 
     As shown in  FIG. 1B , in the solid-state imaging device  1 , a pixel P X  includes a light detector PD, a first insulating film  13 , a second insulating film  15 , and a third insulating film  17 . 
     The light detector PD is provided inside a semiconductor body, e.g., a semiconductor substrate  10 . The semiconductor substrate  10  is, for example, a silicon substrate. The light detector PD is, for example, a photodiode, which includes a p-type semiconductor and an n-type semiconductor. The light detector PD has, for example, the long side along the X-direction and the short side along the Y-direction (see  FIG. 1A ). 
     The first insulating film  13  is provided on the front surface of the semiconductor substrate  10 . The first insulating film  13  is, for example, a silicon oxide film. The light-shielding films SF 1  and SF 2  are provided on the first insulating film  13 . The first insulating film  13  is provided between the light-shielding film SF 1  and the semiconductor substrate  10  and between the light-shielding film SF 2  and the semiconductor substrate  10 . The first insulating film  13  also includes a portion positioned in the transparent region TPR between the light-shielding film SF 1  and the light-shielding film SF 2  when viewed along the Z-direction. 
     The second insulating film  15  is provided between the light detector PD and the first insulating film  13 . The second film  15  is provided in a plurality. The plurality of second insulating films  15  are arranged in a first direction (e.g., the X-direction) with slits SL interposed. The first direction is, for example, a direction along the front surface of the semiconductor substrate  10 . The second insulating film  15  is, for example, silicon nitride film, and has a refractive index higher than the refractive index of the first insulating film  13 . For example, the second insulating film  15  has a thickness of 500 angstroms (Å) in the Z-direction. For example, the slits SL have widths in the range of 0.2 to 0.4 micrometers (μm) in a direction along the front surface of the semiconductor substrate  10  (i.e., the X-direction or the Y-direction). The plurality of second insulating films are arranged in two rows along the X-direction (see  FIG. 1A ). 
     The third insulating film  17  is provided between the semiconductor substrate  10  and the second insulating films  15 . The third insulating film  17  is, for example, a silicon oxide film, and has a refractive index lower than the refractive index of the second insulating films  15 . 
     The solid-state imaging device  1  further includes a control electrode  20  and an insulating film  19 . For example, the control electrode  20  is provided on the outer edge of the light detector PD. For example, the control electrode  20  is placed between the semiconductor substrate  10  and the light-shielding film SF 2 . The control electrode  20  electrically connects the light detector PD and a charge transfer portion (not illustrated). The insulating film  19  is provided to cover the light-shielding film SF 1 , the light-shielding film SF 2 , and the first insulating film  13 . The insulating film  19  is, for example, a silicon oxide film. 
     The third insulating film  17  includes a first portion  17   a  and a second portion  17   b . The first portion  17   a  is positioned between the semiconductor substrate  10  and the second insulating films  15 . The second portion  17   b  is positioned between the semiconductor substrate  10  and the control electrode  20 . The second portion  17   b  is provided to be thicker than the first portion  17   a  in a direction perpendicular to the front surface of the semiconductor substrate  10  (i.e., the Z-direction). For example, the second portion  17   b  is provided to have a thickness of 500 Å in the Z-direction. For example, the first portion  17   a  is provided to have a thickness of 100 Å in the Z-direction. 
     For example, the solid-state imaging device  1  operates by sending the electron to the charge transfer portion (not illustrated) through a channel induced in the front surface of the semiconductor substrate  10 , which are excited by the light entering the light detector PD. The control electrode  20  functions as a gate electrode which induces the channel in the front surface of the semiconductor substrate  10 ; and the second portion  17   b  of the third insulating film  17 , for example, serves as the gate insulating film. 
     To increase the sensitivity of the solid-state imaging device  1 , it is desirable to increase the light amount that passes through the transparent region TPR and enters each pixel P X . In the embodiment, the second insulating films  15  that have a larger refractive index than the third insulating film  17  are provided to reduce the light not entering the light detector PD due to reflections at the interface between the light detector PD and the third insulating film  17 . That is, the light reflected at the interface between the light detector PD and the third insulating film  17  is reflected again at the interface between the third insulating film  17  and the second insulating films  15 , and returns toward the light detector PD. Thereby, it is possible to make the sensitivity of the pixel P X  higher by increasing the light amount entering the light detector PD. 
       FIGS. 2A and 2B  are schematic views showing a solid-state imaging device  2  according to a comparative example.  FIG. 2A  is a plan view showing the arrangement of pixels P X  in the solid-state imaging device  2 .  FIG. 2B  is a cross-sectional view showing a structure of the pixel P X  in the solid-state imaging device  2 . Other than the pixel P X , the solid-state imaging device  2  has the same structures as those of the solid-state imaging device  1 . 
     In the solid-state imaging device  2 , the pixel P X  includes the third insulating film  17 , the second insulating film  15 , the first insulating film  13 , and the light detector PD that is provided in the semiconductor substrate  10 . The second insulating film  15  is positioned between the light detector PD and the first insulating film  13 ; and the third insulating film  17  is positioned between the light detector PD and the second insulating film  15 . 
     The third insulating film  17  is, for example, a silicon oxide film formed by thermal oxidation of the silicon substrate. For example, the second insulating film  15  is a silicon nitride film deposited on the third insulating film  17  by Chemical Vapor Deposition (CVD). 
     In the second insulating film  15  of the solid-state imaging device  2 , stress is generated due to the differences of the linear thermal expansion coefficient between the semiconductor substrate  10  and the second insulating film  15 . For example, in the case where the second insulating film  15  is a silicon nitride film, the second insulating film  15  has a tensile stress of 200 to 300 megapascals (MPa). For example, the stress concentrates at the end portions of the second insulating film  15 ; and stress concentration portions SCP are generated in the semiconductor substrate  10 . 
     For example, the lattice strain of silicon crystal is generated at the stress concentration portion SCP. When the stress concentration portion SCP is positioned inside the light detector PD, the electrons generated by thermal excitation increases; and the dark output of the pixel P X  increases. For example, the dark output of the pixel P X , increases to 1.5 times the dark output in the case where the stress concentration portion SCP is not positioned inside the light detector PD; and the sensitivity of the pixel P X  degrades. 
     As shown in  FIG. 1A , the second insulating films  15  are subdivided into a plurality by slits SL 1  and SL 2  in the solid-state imaging device  1  according to the embodiment. Therefore, the stress is dispersed in the second insulating films  15 ; and the lattice strain can be suppressed in the semiconductor substrate  10 . The sensitivity of the pixel P X  can be increased thereby. 
     The slits SL 1  and SL 2  may be provided to have a depth not enough to completely subdivide the second insulating film  15 , i.e., a depth not enough to reach the third insulating film  17  from the front surface of the second insulating film  15  on the first insulating film  13  side. 
     In the example shown in  FIG. 1A , the second insulating film  15  is subdivided into a plurality by the slit SL 1 ; and the plurality of second insulating films  15  are arranged in the X-direction. The second insulating films  15  are further subdivided by the slit SL 2 , and are arranged also in the Y-direction. 
     The embodiment is not limited to the example. For example, in the example shown in  FIG. 1A , the end portions in the Y-direction of the second insulating film  15  are positioned outside the light detector PD. That is, in the case where the slit SL 2  is not provided, a width W AR  in the Y-direction of the second insulating film  15  is wider than a width W PD  in the Y-direction of the light detector PD. There is lower risk of making the dark output of the pixel P X  increase, when the stress concentration portion SCP is positioned outside the light detector PD. Accordingly, the slit SL 2  can be omitted, which subdivides the second insulating film  15  in the Y-direction, 
       FIG. 3  is a schematic view showing a solid-state imaging device  3  according to a modification of the first embodiment.  FIG. 3  is a plan view showing the arrangement of the pixels P X . Other than the second insulating film  15 , the solid-state imaging device  3  has the same structures as those of the solid-state imaging device  1 . 
     The second insulating film  15  is selectively provided between the first insulating film  13  and the third insulating film  17  (referring to  FIG. 1B ) and has a refractive index higher than the refractive index of the third insulating film  17 . For example, the second insulating film  15  has a thickness of 500 Å in the Z-direction. For example, the second insulating film  15  has the length along the X-direction, which is shorter than the length of the light detector PD along the X-direction. 
     As shown in  FIG. 3 , the second insulating film  15  includes a slit PSL having a hole configuration. For example, the slit PSL is provided to extend through the second insulating film  15  in the direction from the first insulating film  13  toward the third insulating film  17  (the −Z direction). For example, the slit PSL has a width in the range of 0.2 to 0.4 μm in a direction along the front surface of the semiconductor substrate  10  (the X-direction or the Y-direction). The slit PSL may be provided to have a depth not enough to reach the third insulating film  17 . 
     For example, the slit PSL is arranged in the X-direction and the Y-direction along the front surface of the semiconductor substrate  10 . Multiple columns of the slits PSL are arranged in the X-direction, in which the slits PSL are arranged in the Y-direction. 
     In the example shown in  FIG. 3 , there is one column of the slits PSL arranged in the X-direction; but multiple such columns may be arranged in the Y-direction. Also, the column of the slits PSL arranged in the X-direction may be omitted. 
     In the example, the stress can be reduced by arranging the multiple slits PSL in the second insulating film  15 ; and the lattice strain can be suppressed in the semiconductor substrate  10 . Thereby, the dark output of the pixel P X  can be reduced; and the sensitivity of the pixel P X  can be increased. 
     Second Embodiment 
       FIGS. 4A and 4B  are schematic views showing a solid-state imaging device  4  according to a second embodiment.  FIG. 4A  is a plan view showing the arrangement of the pixels P X  in the solid-state imaging device  4 .  FIG. 4B  is a cross-sectional view showing the structure of the pixel P X  in the solid-state imaging device  4 . 
     In the solid-state imaging device  4 , as shown in  FIG. 4A , the arrangement of the pixels P X  in the transparent region TPR between the light-shielding film SF 1  and the light-shielding film SF 2  is the same as that of the solid-state imaging device  1 . The second insulating film  15  of the solid-state imaging device  4  does not include the slits SL or PSL. 
     As shown in  FIG. 4B , the solid-state imaging device  4  includes the light detector PD, the first insulating film  13 , the second insulating film  15 , the third insulating film  17 , and a fourth insulating film  21 . 
     The first insulating film  13  is provided above the semiconductor substrate  10 . The first insulating film  13  is, for example, a silicon oxide film. The first insulating film  13  is provided between the light-shielding film SF 1  and the semiconductor substrate  10  and between the light-shielding film SF 2  and the semiconductor substrate  10 . The first insulating film  13  also includes a portion positioned in the transparent region TPR between the light-shielding film SF 1  and the light-shielding film SF 2  when viewed in the Z-direction. 
     The second insulating film  15  is provided between the light detector PD and the first insulating film  13 . For example, the second insulating film  15  has a refractive index higher than the refractive index of the first insulating film  13 . The second insulating film  15  is, for example, a silicon nitride film. For example, the second insulating film  15  has a thickness of 500 Å in the Z-direction. 
     The third insulating film  17  is provided between the semiconductor substrate  10  and the second insulating film  15 . The third insulating film  17  is, for example, a silicon oxide film and has a refractive index lower than the refractive index of the second insulating film  15 . The third insulating film  17  includes a portion positioned between the semiconductor substrate  10  and the control electrode  20 . For example, the third insulating film  17  has a thickness in the Z-direction of 100 Å at the portion positioned between the light detector PD and the second insulating film  15 . 
     The fourth insulating film  21  is provided between the second insulating film  15  and the third insulating film  17 . The fourth insulating film  21  has a refractive index having a value between the refractive index of the second insulating film  15  and the refractive index of the third insulating film  17 . The fourth insulating film  21  is, for example, a silicon oxynitride film. The fourth insulating film  21  includes a portion positioned between the first insulating film  13  and the control electrode  20 . For example, the fourth insulating film  21  has a thickness in the Z-direction in the range of 100 to 200 Å at the portion positioned between the second insulating film  15  and the third insulating film  17 . 
     The third insulating film  17  is, for example, a silicon oxide film formed by thermal oxidation of a silicon substrate. For example, the fourth insulating film  21  is formed using CVD and has a film density lower than the film density of the third insulating film  17 . For example, the fourth insulating film  21  includes a dangling bond of a silicon atom terminated with hydrogen. For example, when the observation using TEM (transmission electron microscopy) is performed, the fourth insulating film  21  exhibits higher brightness than the brightness of the third insulating film  17 . 
     In the embodiment, the influence of the stress in the second insulating film  15  can be suppressed on the semiconductor substrate  10  by providing the fourth insulating film  21  between the second insulating film  15  and the third insulating film  17 . In other words, the lattice strain in the semiconductor substrate  10  can be suppressed; and the dark output of the pixel P X  can be reduced. As a result, the sensitivity of the pixel P X  can be increased in the solid-state imaging device  4 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.