Patent Publication Number: US-2011058076-A1

Title: Solid state imaging device and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2009-205038 filed on Sep. 4, 2009, the disclosure of which including the specification, the drawings, and the claims is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a solid state imaging device, and a method for manufacturing the same, particularly to a solid state imaging device having a light shielding film and an anti-reflection film, and a method for manufacturing the same. 
     In recent years, solid state imaging elements having higher resolution and a higher number of pixels have been demanded, and manufacturers have been pursuing size reduction of cells. Simultaneously, high sensitivity and low smear comparable to those of conventional solid state imaging devices have been demanded. For these purposes, a method for alleviating reduction of an amount of incident light, and increase of smear due to the size reduction of the cells have been considered. For example, a method for forming a light shielding film has been known, in which an anti-reflection film and a planarization film are formed on a light receiving portion, a light shielding material is laminated thereon, and the laminated light shielding material is polished to expose the planarization film, thereby forming a light shielding film (see, e.g., Japanese Patent Publication No. 2004-140309). The anti-reflection film and the light shielding film formed by this method allow provision of a low-reflection, anti-reflection film on the entire surface of the light receiving portion, thereby increasing the amount of incident light. Further, a semiconductor substrate would not be damaged by etching because the light shielding film is not etched, and therefore, an insulating film formed below the light shielding film can be thinned. This allows reduction of a distance between a lower surface of the light shielding film and the semiconductor substrate, thereby reducing light which enters a transfer channel obliquely, and causes the smear. 
     SUMMARY 
     However, the conventional solid state imaging device has the following disadvantages. In the conventional solid state imaging device, a side surface of the light shielding film is in contact with a side surface of the anti-reflection film, and a distance between a lower surface of the light shielding film and the substrate is small. Therefore, light entering the transfer channel can be reduced. However, since the side surface of the light shielding film is almost perpendicular, light obliquely entering the anti-reflection film is blocked by the light shielding film, thereby causing so-called vignetting. 
     The present disclosure is intended to overcome the disadvantages described above to reduce smear, and to provide a solid state imaging device in which vignetting of incident light by a light shielding film is reduced. 
     For the above-described purposes, the present disclosure is directed to a solid state imaging device, wherein a light shielding film is in contact with a side surface of an anti-reflection film, and height of the light shielding film is equal to, or smaller than height of the anti-reflection film at a contact between the light shielding film and the side surface of the anti-reflection film. 
     Specifically, the disclosed solid state imaging device includes: a light receiving portion and a transfer channel formed in a semiconductor substrate; a transfer electrode formed on the transfer channel; an anti-reflection film formed on the light receiving portion; and a light shielding film which covers the transfer electrode, and is in contact with a side surface of the anti-reflection film, wherein an upper surface of the light shielding film at a contact between the light shielding film and the side surface of the anti-reflection film is located below an upper surface of the light shielding film on the transfer electrode. 
     In the disclosed solid state imaging device, open space where the light shielding film is not formed is provided obliquely above the anti-reflection film. Therefore, light obliquely entering the anti-reflection film is not blocked by an upper end of the light shielding film, and a range of the light entering the anti-reflection film can be increased, i.e., so-called vignetting can be reduced. Further, since the light shielding film is in contact with the side surface of the anti-reflection film, light traveling in the oblique direction is less likely to enter the transfer channel. In addition, an opening formed in the light shielding film is wholly constituted as a low-reflection region, thereby increasing the amount of incident light. 
     A method for manufacturing the disclosed solid state imaging device includes: forming a light receiving portion and a transfer channel in a semiconductor substrate; forming a first insulating film on the entire surface of the semiconductor substrate; forming a transfer electrode on the transfer channel after the formation of the first insulating film; forming a second insulating film on the entire surface of the semiconductor substrate to cover the transfer electrode; forming an anti-reflection film on the light receiving portion after the formation of the second insulating film; forming a light shielding film material on the entire surface of the semiconductor substrate after the formation of the anti-reflection film; and forming a light shielding film which covers the transfer electrode, and is in contact with a side surface of the anti-reflection film by selectively removing a portion of the light shielding film material formed on the anti-reflection film, wherein in the formation of the light shielding film, an upper surface of the light shielding film at a contact between the light shielding film and the side surface of the anti-reflection film is located below an upper surface of the light shielding film on the transfer electrode. 
     The disclosed method for manufacturing the solid state imaging device allows forming the light shielding film to be in contact with the side surface of the anti-reflection film. This makes it possible to reduce an amount of light entering the transfer channel, and to reduce the smear. Further, since the upper surface of the light shielding film at the contact between the light shielding film and the side surface of the anti-reflection film is located below the upper surface of the light shielding film on the transfer electrode, so-called vignetting can be reduced, thereby increasing the amount of incident light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) is a plan view illustrating a solid state imaging device of an embodiment, and  FIG. 1(   b ) is a cross-sectional view taken along the line Ib-Ib in  FIG. 1(   a ). 
         FIG. 2  is a cross-sectional view illustrating an alternative example of the solid state imaging device of the embodiment. 
         FIGS. 3(   a ) to  3 ( d ) are cross-sectional views sequentially illustrating steps for manufacturing the solid state imaging device of the embodiment. 
         FIG. 4  is an enlarged cross-sectional view illustrating one of the steps for manufacturing the solid state imaging device of the embodiment. 
         FIG. 5  is a cross-sectional view illustrating an alternative example of the solid state imaging device of the embodiment. 
         FIGS. 6(   a ) to  6 ( c ) are cross-sectional views sequentially illustrating steps for manufacturing the alternative example of the solid state imaging device of the embodiment. 
         FIG. 7  is a cross-sectional view illustrating an alternative example of the solid state imaging device of the embodiment. 
         FIG. 8  is a cross-sectional view illustrating an alternative example of the solid state imaging device of the embodiment. 
         FIGS. 9(   a ) to  9 ( c ) are cross-sectional views illustrating steps for manufacturing the alternative example of the solid state imaging device of the embodiment. 
         FIG. 10  is a plan view illustrating an alternative example of the solid state imaging device of the embodiment. 
         FIG. 11  is a plan view illustrating an alternative example of the solid state imaging device of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1(   a ) is a plan view illustrating a solid state imaging device of an embodiment, and  FIG. 1(   b ) is a cross-sectional view taken along the line Ib-Ib in  FIG. 1(   a ). In  FIG. 1(   a ), layers above an upper interlayer insulating film  113  are not shown. As shown in  FIGS. 1(   a ) and  1 ( b ), light receiving portions  103 , which are photodiodes, are formed in a matrix pattern in a semiconductor substrate  101  made of silicon (Si) etc. Transfer channels  105  extending in a column direction are formed between the light receiving portions  103 . Transfer electrodes  121  are formed on the transfer channels  105  with a first insulating film  111 , which is part of a lower interlayer insulating film  110 , interposed therebetween. The transfer electrodes  121  extend in a line direction not to overlap with the light receiving portions  103 . An upper surface and a side surface of each of the transfer electrodes  121  are covered with a second insulating film  112 , which is part of the lower interlayer insulating film  110 . The first insulating film  111  and the second insulating film  112  are in contact with each other on the light receiving portions  103 , and anti-reflection films  123  are formed in a matrix pattern on the light receiving portions  103  with the first insulating film  111  and the second insulating film  112  interposed therebetween. Upper surfaces of the anti-reflection films  123  are located below upper surfaces of the transfer electrodes  121 . 
     A light shielding film  125  is formed on the second insulating film  112 . The light shielding film  125  is formed to cover the side surface and the upper surface of each of the transfer electrodes  121 , and includes a protruding portion  125   a  on each of the transfer electrodes  121 , and a recessed portion  125   b  on the periphery of the transfer electrodes  121 . The recessed portion  125   b  includes an opening  125   c  in which the anti-reflection film  123  is exposed. The opening  125   c  is filled with the anti-reflection film  123 , and the light shielding film  125  and a side surface of the anti-reflection film  123  are in contact with each other. 
     An upper interlayer insulating film  113  is formed on the light shielding film  125  and the anti-reflection film  123 . The upper interlayer insulating film  113  includes a protruding portion formed on each of the transfer electrodes  121 , and a recessed portion formed on each of the anti-reflection films  123 . Intralayer lenses  131  are formed on the upper interlayer insulating film  113 , and a planarization layer  133  is formed on the intralayer lenses  131 . A color filter layer  135 , and microlenses  137  are formed on the planarization layer  133 . 
     Incident light collected by the microlens  137  and the intralayer lens  131  which are convex lenses passes through the opening  125   c  formed in the light shielding film  125  to enter the light receiving portion  103 , and is converted to a signal charge. In a general solid state imaging device, the light shielding film and the anti-reflection film are arranged to have a distance of 100 nm or larger therebetween. Therefore, the anti-reflection film is formed to cover only about 60% of an area of the opening. In contrast, in the solid state imaging device of the present embodiment, the light shielding film  125  is in contact with the side surface of the anti-reflection film  123 . Thus, the area of the opening  125   c  is equal to the area of the anti-reflection film  123 , i.e., the anti-reflection film  123  is formed to cover 100% of the area of the opening  125   c.  Therefore, light entering the opening  125   c  can completely be admitted into the anti-reflection film  123  having an anti-reflection effect, thereby reducing loss of light by the reflection. 
     In the solid state imaging device of the present embodiment, height h 1  of the light shielding film  125  is not larger than height h 2  of the anti-reflection film  123  at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . Specifically, an upper surface of the light shielding film  125  is located below an upper surface of the anti-reflection film  123  at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . Thus, sidewalls of the recessed portion  125   b  are separated from the side surfaces of the anti-reflection film  123 , i.e., open space where the light shielding film  125  is not formed is provided obliquely above the anti-reflection film  123 . In other words, planar dimension LI of an upper end of the recessed portion  125   b  of the light shielding film  125  is larger than planar dimension L 2  of the anti-reflection film  123 , i.e., of the opening  125   c.  Thus, a tangent passing an upper end of the anti-reflection film  123  and the sidewall of the recessed portion  125   b  forms an angle smaller than 90° with a principle surface of the semiconductor substrate  101 . This can reduce vignetting, which is a phenomenon in which light obliquely entering the light receiving portion is blocked by an upper end of the light shielding film  125 . 
     In the solid state imaging device of the present embodiment, the light shielding film  125  and the side surface of the anti-reflection film  123  are in contact with each other. Thus, at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 , a distance between the light shielding film  125  and the semiconductor substrate  101  can advantageously be reduced. For providing a distance between the light shielding film and the anti-reflection film, the light shielding film on the periphery of the anti-reflection film has to be removed. In this case, an insulating film formed under the light shielding film has to be thickened for the purpose of protecting the surface of the semiconductor substrate from damage caused by etching the light shielding film. In the solid state imaging device of the present embodiment, however, the light shielding film  125  and the side surface of the anti-reflection film  123  are in contact with each other. Thus, the light shielding film  125  is not etched, and the semiconductor substrate  101  is not damaged by etching. Therefore, the first insulating film  111  and the second insulating film  112  near the contact between the light shielding film  125  and the anti-reflection film  123  can be thinned down. This can reduce a distance t 1  between the light shielding film  125  and the semiconductor substrate  101 , and can reduce light entering the transfer channel  105  by passing below the light shielding film  125 . This can further reduce the smear. 
       FIGS. 1(   a ) and  1 ( b ) show an example in which the upper surface of the light shielding film  125  is located below the upper surface of the anti-reflection film  123  at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . However, as long as the upper surface of the light shielding film  125  at the contact between the light shielding film  125  and the anti-reflection film  123  is located below the upper surface of the light shielding film on the transfer electrode  121 , the sidewalls of the recessed portion  125   b  can be separated from the side surfaces of the anti-reflection film  123 . The light shielding film  125  is preferably not formed on the upper surface of the anti-reflection film  123 . However, as shown in  FIG. 2 , the light shielding film  125  may cover an upper surface of a peripheral portion of the anti-reflection film  123 . 
     A method for manufacturing the solid state imaging device of the present embodiment will be described below. 
     First, as shown in  FIG. 3(   a ), a plurality of light receiving portions  103  arranged in a matrix pattern, and a plurality of transfer channels  105  extending in a column direction are formed in a semiconductor substrate  101 , such as a Si substrate etc. Then, a first insulating film  111  made of a SiO 2  film etc., is formed on the semiconductor substrate  101  by CVD (chemical vapor deposition) etc. Then, transfer electrodes  121  extending in a line direction are formed not to overlap with the light receiving portions  103 . 
     As shown in  FIG. 3(   b ), the first insulating film  111  is selectively etched using the transfer electrodes  121  as a mask. Thus, a portion of the first insulating film  111  on which the transfer electrode  121  is not formed is made thinner than a portion of the first insulating film  111  on which the transfer electrode  121  is formed. If wet etching is employed to etch the first insulating film  111 , the semiconductor substrate  101  would hardly be damaged. Then, a second insulating film  112  is formed on the semiconductor substrate  101  by CVD etc. The thickness of the second insulating film  112  is determined to keep a dielectric breakdown voltage required between the transfer electrode  121  and the light shielding film  125 . For example, when a dielectric breakdown voltage of 30 V is required between the transfer electrode  121  and the light shielding film  125 , a 30 nm thick SiO 2  film having a dielectric breakdown voltage of 10 MV/cm may be formed by CVD as the second insulating film  112 . In this case, the sum of the thicknesses of the first insulating film  111  and the second insulating film  112  on the light receiving portion  103  may be about 40 nm. Then, as shown in  FIG. 3(   c ), an anti-reflection film  123  and an etch stop layer  141  are selectively formed on the light receiving portions  103 . The anti-reflection film  123  may be a silicon nitride film etc., formed by CVD. The etch stop layer  141  may be a silicon oxide film etc. Then, a light shielding film material  142  is provided on the semiconductor substrate  101 . The light shielding film material  142  may be aluminum, refractory metal, etc. The light shielding film material  142  is provided to completely fill recesses between the transfer electrodes  121  and the anti-reflection films  123 . Then, a resist mask  143  having openings on the anti-reflection films  123  is formed. 
     Then, as shown in  FIG. 3(   d ), exposed portions of the light shielding film material  142  are removed by dry etching using the resist mask  143 , and the etch stop layer  141  and the resist mask  143  are removed. The etch stop layer  141  may not be removed. In this case, the remaining etch stop layer  141  becomes part of the upper interlayer insulating film  113 . 
     In the method described above, the light shielding film material  142  formed on the anti-reflection film  123  can reliably be removed. The light shielding film material  142  formed on the anti-reflection film  123  is preferably removed completely. However, the light shielding film  142  may be left on a peripheral portion of the anti-reflection film  123 . 
     Even if the resist mask  143  is misaligned, a portion of the light shielding film material  142  formed between the transfer electrodes  121  and the anti-reflection films  123  is not completely etched, but remains there because the portion is thicker than the other portion of the light shielding film material  142 . Thus, the light shielding film  125  and the anti-reflection film  123  would not form a gap therebetween which exposes the lower interlayer insulating film  110 , and the light shielding film  125  is in contact with the side surface of the anti-reflection film  123 . Accordingly, the light would never enter through a gap between the light shielding film  125  and the anti-reflection film  123 , thereby reducing the smear. At the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 , the height of the light shielding film  125  is not larger than the height of the anti-reflection film  123 . Thus, the side surface of the light shielding film  125  is separated from the side surface of the anti-reflection film, thereby providing open space where the light shielding film  125  is not formed obliquely above the anti-reflection film  123 . Therefore, light traveling in the oblique direction can enter the anti-reflection film  123  without being blocked by an upper end of the light shielding film  125 . This can reduce vignetting, and can alleviate reduction in amount of the incident light. 
     In this case, the resist mask  143  may be formed to overlap with the anti-reflection film  123  by 20 nm to 30 nm as shown in  FIG. 4 . In etching the light shielding film material  142 , etching proceeds also in the lateral direction. Therefore, the light shielding film material  142  on the anti-reflection film  123  can completely be removed, and the side surface of the anti-reflection film  123  and the light shielding film  125  can easily be brought into contact. However, the resist mask  143  may not always overlap with the anti-reflection film  123 . Further, in this example, height hl of the light shielding film  125  is smaller than height h 2  of the anti-reflection film  123  at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . However, the height h 1  of the light shielding film  125  may be equal to the height h 2  of the anti-reflection film  123 . Alternatively, the height h 1  may be larger than the height h 2 . However, in general, the light shielding film material  142  is etched until the entire upper surface of the anti-reflection film  123  is fully exposed. Therefore, the height of the light shielding film  125  is generally smaller than the height of the anti-reflection film  123  at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . Thus, an upper end of the side surface of the anti-reflection film  123  is uncovered with the light shielding film  125 . This would not cause any disadvantages. 
     After the etch stop layer  141  and the resist mask  143  are removed, an upper interlayer insulating film  113 , intralayer lenses  131 , a planarization layer  133 , a color filter layer  135 , microlenses  137 , etc., are formed, although not shown. 
     In view of reducing the smear, a distance t 1  between the light shielding film  125  and the semiconductor substrate  101  is preferably small at the contact between the light shielding film  125  and the side surface of the anti-reflection film  123 . For this reason, the first insulating film  111  is thinned except for a portion thereof on which the transfer electrode  121  is formed. However, the second insulating film  112  functions to insulate the transfer electrodes  121  and the light shielding film  125 , and has to have a certain thickness. To reduce the distance between the light shielding film  125  and the semiconductor substrate  101  to a further extent, a portion of the second insulating film  112  covering the side surfaces and the upper surface of the transfer electrodes  121  may be thickened, and a portion of the second insulating film  112  under the anti-reflection film  123  may be thinned. In this manner, thickness t 1  of the first insulating film  111  and the second insulating film  112  under the anti-reflection film  123  can be reduced to a further extent, while ensuring a required dielectric breakdown voltage. 
     For example, as shown in  FIG. 5 , the second insulating film  112  may be constituted of a laminate of a first silicon oxide film  112   a,  a silicon nitride film  112   b,  and a second silicon oxide film  112   c,  and the second silicon oxide film  112   c  and the silicon nitride film  112   b  may be removed from the periphery of the anti-reflection film  123 . Due to the difference in etch rate of the silicon oxide film and the silicon nitride film, the second silicon oxide film  112   c  and the silicon nitride film  112   b  can easily be removed, with the first silicon oxide film  112   a  kept remained. Specifically, as shown in  FIG. 6(   a ), the first silicon oxide film  112   a,  the silicon nitride film  112   b,  and the second silicon oxide film  112   c  are formed sequentially on the semiconductor substrate  101 , and then a resist mask  151  which does not cover a region for forming the anti-reflection film  123  is formed. Then, an exposed portion of the second silicon oxide film  112   c  is removed as shown in  FIG. 6(   b ). Further, an exposed portion of the silicon nitride film  112   b  is removed as shown in  FIG. 6(   c ). If the silicon nitride film  112   b  is removed using hot concentrated phosphoric acid etc., only the silicon nitride film  112   b  can be removed without etching the first silicon oxide film  112   a.    
     As shown in  FIG. 7 , the light shielding film  125  and the transfer electrode  121  are connected through a contact  127 , and the light shielding film  125  may be used as a shunt wire. In this case, the lower interlayer insulating film  110  between the light shielding film  125  and the semiconductor substrate  101  has to be thickened to ensure a dielectric breakdown voltage between the light shielding film  125  and the semiconductor substrate  101 . For this reason, the first insulating film  111  under the light shielding film  125  is not thinned, but is kept thick. However, the first insulating film  111  may be thinned to such a degree that the dielectric breakdown voltage between the light shielding film  125  and the semiconductor substrate  101  can be ensured. 
     With the lower interlayer insulating film  110  under the anti-reflection film  123  made thin, the effect of the anti-reflection film  123  is enhanced. Therefore, the first insulating film  111  under the anti-reflection film  123  is preferably thinned. If the sum of the thicknesses of the first and second insulating films  111  and  112  under the anti-reflection film  123  is in the range of 10 nm to 20 nm, the effect of the anti-reflection film  123  can further be enhanced. In the case where the light shielding film  125  is used as a shunt wire, the second insulating film  112  may be constituted of a laminate of layers. 
     Even when the light shielding film  125  is used as the shunt wire, the first insulating film  111  under the light shielding film  125  can be thinned, and the smear can be reduced to a further extent by employing the configuration shown in  FIG. 8 . Specifically, a first light shielding film  125 A connected to the transfer electrode  121  through the contact  127  is formed on the transfer electrode  121 . A second shielding film  125 B is formed to fill a recess between the transfer electrode  121  and the anti-reflection film  123 . A third light shielding film  125 C is formed to overlap with both the first light shielding film  125 A and a second light shielding film  125 B. The first light shielding film  125 A, the second light shielding film  125 B, and the third light shielding film  125 C are insulated from each other by a third insulating film  114 . With this configuration, a voltage is not applied to the second light shielding film  125 B. Therefore, an insulating film between the second light shielding film  125 B and the semiconductor substrate  101  can be thinned. Further, since the third light shielding film  125 C is formed to overlap with both the first light shielding film  125 A and the second light shielding film  125 B, light would not pass between the first light shielding film  125 A and the second light shielding film  125 B to enter the transfer channel  105 . 
     The first light shielding film  125 A, the second light shielding film  125 B, and the third light shielding film  125 C may be formed in the following manner. As shown in  FIG. 9(   a ), after the light shielding film material  142  is formed, a resist mask  153  covering the transfer electrodes  121  is formed. Planar dimension of the resist mask  153  is preferably smaller than planar dimension of the transfer electrode  121 . Then, as shown in  FIG. 9(   b ), the light shielding film material  142  is etched until the etch stop layer  141  and the second insulating film  112  are partially exposed, thereby forming a first light shielding film  125 A and a second light shielding film  125 B. Then, as shown in  FIG. 9(   c ), a third insulating film  114  is formed on the semiconductor substrate  101 . Thereafter, a third light shielding film  125 C is formed on the third insulating film  114 , and a portion of the third light shielding film  125 C on the anti-reflection film  123  is selectively removed. In the case where the light shielding film is constituted of the first, second, and third light shielding films, the second insulating film  112  may be constituted of a laminate of layers. 
     When the light shielding film  125  is used as the shunt wire, the light shielding film  125  is not formed between the light receiving portions  103  adjacent to each other in the column direction. Therefore, as shown in  FIG. 10 , the anti-reflection film  123  may be increased in length in the column direction to overlap with the transfer electrodes  121 . Further, as shown in  FIG. 11 , the anti-reflection films  123  adjacent to each other in the column direction may be integrated. 
     In  FIGS. 5 ,  7 ,  8 ,  10 , and  11 , the light receiving portions  103 , the transfer channels  105 , and layers above the upper interlayer insulating film  113  are not shown. 
     According to the disclosed solid state imaging device and the method for manufacturing the same, the smear can be reduced, and vignetting of incident light caused by the light shielding film can be reduced. The present disclosure is particularly useful for solid state imaging devices including multiple pixels, and for a method for manufacturing the same. 
     The term “on” used in the specification and claims does not indicate that a first layer “on” a second layer is directly on, and in immediate contact with the second layer unless otherwise stated. A third layer or other structure may be present between the first layer and the second layer on the first layer. 
     Although the invention has been described with reference to specific embodiments, the description is intended to be illustrative of the invention, and is not intended to be limiting. 
     Various modifications and applications may occur to those skilled in the art without departing from the true spirit of the invention as defined in the appended claims.