Patent Publication Number: US-11664402-B2

Title: Semiconductor device having a trench and camera with semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/267,626, filed Sep. 16, 2016, which is a continuation of U.S. patent application Ser. No. 14/224,288, filed Mar. 25, 2014, now U.S. Pat. No. 9,508,768, which claims the benefit of Japanese Patent Application No. 2013-080836, filed Apr. 8, 2013. The entire contents of these prior applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor device and a method of manufacturing the same. 
     Description of the Related Art 
     Japanese Patent Laid-Open No. 2012-15316 describes a method of manufacturing a semiconductor device such as a solid-state image sensor. In this method, a structure in which a plurality of single-crystal silicon layers each including a semiconductor element such as a photodiode are separated by gaps is prepared, and a light-shielding film such as a tungsten film, a planarizing layer, color filters, and microlenses are formed on the plurality of single-crystal silicon layers. 
     In the method described in Japanese Patent Laid-Open No. 2012-15316, the light-shielding film such as a tungsten film is formed on the plurality of single-crystal silicon layers without filling the gaps formed between the plurality of single-crystal silicon layers. Accordingly, a conductive film as the light-shielding film may be formed in the gaps. Also, the degree of the formation of the light-shielding film in the gaps may change in accordance with a position in a substrate, or may change from one substrate to another. This may produce variations in characteristics of semiconductor devices such as solid-state image sensors. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique advantageous in reducing variations in a plurality of spaces formed for element isolation in a semiconductor device. 
     One of the aspects of the present invention provides a method of manufacturing a semiconductor device, comprising: forming a trench in a semiconductor substrate having a first face and a second face by processing the first face of the semiconductor substrate, the trench including a first portion and a second portion located between the first portion and a plane including a first face; filling an insulator in the second portion such that a space remains in the first portion and the trench is closed; and forming a plurality of elements between the first face and the second face, wherein the space and the insulator form element isolation. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a sectional view showing a portion of a pixel array of a solid-state image sensor of the first embodiment; 
         FIG.  2    is a plan view showing the portion of the pixel array of the solid-state image sensor of the first embodiment; 
         FIG.  3    is a sectional view for explaining a method of manufacturing the solid-state image sensor of the first embodiment; 
         FIG.  4    is a sectional view for explaining the method of manufacturing the solid-state image sensor of the first embodiment; 
         FIG.  5    is a sectional view for explaining the method of manufacturing the solid-state image sensor of the first embodiment; 
         FIG.  6    is a sectional view for explaining the method of manufacturing the solid-state image sensor of the first embodiment; 
         FIG.  7    is a sectional view for explaining the method of manufacturing the solid-state image sensor of the first embodiment; 
         FIG.  8    is a sectional view showing a portion of a pixel array of a solid-state image sensor of the second embodiment; and 
         FIG.  9    is a sectional view for explaining a method of manufacturing the solid-state image sensor of the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be explained below by way of its exemplary embodiments with reference to the accompanying drawings. Note that an example in which the present invention is applied to a solid-state image sensor will be explained below, but one aspect of the present invention has features in the structure of element isolation and a method of manufacturing the same. Accordingly, this aspect of the present invention is also applicable to a semiconductor device other than the solid-state image sensor. 
       FIG.  1    is a sectional view showing a portion of a pixel array of a solid-state image sensor  100  of the first embodiment.  FIG.  2    is a plan view showing the portion of the pixel array of the solid-state image sensor  100 . Note that  FIG.  1    is a schematic sectional view taken along a line A-B in  FIG.  2   , and shows three photoelectric converters PEC. 
     The solid-state image sensor  100  can include a semiconductor substrate  1  having a first face S 1  and second face S 2 ′, an insulator  22  arranged on the side of the first face S 1  of the semiconductor substrate  1 , and a space  7  which is also called an air gap or just a gap, extending from the insulator  22  to the side of the second face S 2 ′ in the semiconductor substrate  1 . The solid-state image sensor  100  can also include a plurality of elements formed between the first face S 1  and second face S 2 ′. The plurality of elements can include, for example, the photoelectric converters PEC, floating diffusions  28 , and transfer gates  27 . The plurality of elements can also include a reset transistor for resetting the potential of the floating diffusion  28 , and an amplification transistor for outputting a signal corresponding to the potential of the floating diffusion  28  to a column signal line. The reset transistor and amplification transistor may be formed for each photoelectric converter PEC, and may also be shared by a plurality of photoelectric converters PEC or a plurality of pixels. Each pixel of the solid-state image sensor  100  includes at least one photoelectric converter PEC. The solid-state image sensor  100  can include channel stops  21  formed in positions adjacent to the insulators  22  and spaces  7 . 
     The insulator  22  forms, for example, STI (Shallow Trench Isolation). The space  7  may be filled with a gas, and may also be maintained in a low-pressure state or vacuum state. The insulator  22  and space  7  form element isolation for isolating elements. Isolation between elements can be, for example, isolation between adjacent photoelectric converters PEC. Alternatively, isolation between elements can be isolation between pixels. Furthermore, isolation between elements can be isolation between the photoelectric converter PEC and a transistor (for example, the reset transistor or amplification transistor) adjacent to each other. The width (the width in the horizontal direction in  FIG.  1   ) of the space  7  in a direction in which adjacent elements of a plurality of elements (for example, the photoelectric converters PEC) are isolated is smaller than the width of the insulator  22  in the same direction. The width of the space  7  may be constant along the depth direction (a direction from the side of the first face S 1  to the side of the second face S 2 ′) of the semiconductor substrate  1 , and may also increase or decrease in the direction away from the first face S 1 . In the following description, the width means a dimension in the direction in which adjacent elements of a plurality of elements (for example, the photoelectric converters PEC) are isolated. 
     As exemplified in  FIG.  2   , the insulators  22  forming element isolation are so arranged as to surround the elements such as the photoelectric converters PEC. The spaces  7  forming this element isolation are also so arranged as to surround the elements such as the photoelectric converters PEC. 
     The photoelectric converter PEC can include, for example, a charge accumulation region  25  made of a first-conductivity-type semiconductor region, a first-conductivity-type semiconductor region  23  arranged in contact with the charge accumulation region  25 , and a second-conductivity-type semiconductor region  24  arranged in contact with the semiconductor region  23 . The photoelectric converter PEC may also include a second-conductivity-type semiconductor region  26  between the charge accumulation region  25  and first face S 1 . The second conductivity type is a p type when the first conductivity type is an n type, and an n type when the first conductivity type is a p type. The transfer gate  27  is a gate electrode. The transfer gate  27 , charge accumulation region  25 , and floating diffusion  28  form a MOS structure. 
     On the first-face side of the semiconductor substrate  1 , a wiring structure including a plurality of interlayer insulation films  10  and a plurality of wiring layers  9  and a passivation film  11  formed on the wiring structure can be arranged. On the second-face side of the semiconductor substrate  1 , a planarizing film  12 , color filter layer  13 , capping layer  14 , and microlens layer  15  can be stacked. Note that a material forming the planarizing film  12  hardly enters the spaces  7 . 
     The solid-state image sensor  100  exemplified in  FIGS.  1  and  2    is a backside irradiation type solid-state image sensor designed such that light enters the side opposite to the first-face side on which the wiring structure is arranged, that is, enters the second-face side. Light having passed through the microlens of the microlens layer  15  enters the second face S 2 ′ of the semiconductor substrate  1  through the color filter layer  13 , and generates an electric charge in the photoelectric converter PEC. The generated electric charge is accumulated in the charge accumulation region  25 , and transferred to the floating diffusion  28  through a channel formed by the transfer gate  27 . The potential of the floating diffusion  28  changes in accordance with the amount of transferred electric charge. The amplification transistor outputs a signal corresponding to the potential of the floating diffusion  28  to the column signal line. 
     When the semiconductor substrate  1  is made of silicon and the insulator  22  is made of silicon oxide, the refractive indices of the silicon oxide and space  7  are smaller than that of silicon. Accordingly, light having entered the photoelectric converter PEC is reflected by the interface between the space  7  and photoelectric converter PEC, and the interface between the insulator  22  and photoelectric converter PEC. Consequently, it is possible to reduce the penetration of light having entered the photoelectric converter PEC into the photoelectric converter PEC and/or the floating diffusion  28  of an adjacent pixel. This makes it possible to reduce a mixture of colors, and increase the resolution of an image. 
     A method of manufacturing the solid-state image sensor  100  will exemplarily be explained below with reference to  FIGS.  3  to  7   .  FIGS.  3  to  7    illustrate the photoelectric converter PEC and transistors AT (for example, the reset transistor and amplification transistor) of one pixel, and portions corresponding to an NMOS transistor PTN and PMOS transistor PTP of a peripheral circuit. 
     Steps S 101  to S 105  are steps of forming a trench T including a space (first portion)  7  and second portion  6  by processing the first face S 1  of the semiconductor substrate  1 . The semiconductor substrate  1  is typically a silicon substrate and has the first face S 1  and a second face S 2 . 
     In step S 101 , a laminated film  18  is formed on the first face of the semiconductor substrate  1 . The laminated film  18  can include, for example, a silicon oxide film formed on the first face S 1  of the semiconductor substrate  1 , a polysilicon film formed on the silicon oxide film, and a silicon nitride film formed on the polysilicon film. The silicon oxide film can have a thickness of, for example, 5 to 30 nm, and can be formed by, for example, normal-pressure thermal oxidation. This thermal oxidation can be performed, for example, in oxygen or in a hydrogen/oxygen gas mixture at a temperature of 700° C. to 1,150° C. The polysilicon film can have a thickness of, for example, 10 to 100 nm, and can be deposited by, for example, using monosilane (SiH 4 ) at a temperature of 600° C. to 700° C. and a pressure of 5 to 100 Pa. The silicon nitride film can have a thickness of, for example, 30 to 200 nm, and can be deposited by using a gas mixture containing ammonia and monosilane or dichlorosilane (Si 2 H 2 Cl 2 ) at a temperature of 700° C. to 900° C. and a pressure of 5 to 100 Pa. 
     Also, in step S 101 , a first photoresist PR 1  having first openings OP 1  is further formed on the laminated film  18 . The first photoresist PR 1  having the first openings OP 1  can be formed by a photolithography step. The first opening OP 1  defines the region of the space (first portion)  7  (that is, a width and a dimension in a direction perpendicular to the width). 
     In step S 102 , a second photoresist PR 2  having second openings OP 2  is formed on the first photoresist PR 1 . The second photoresist PR 2  having the second openings OP 2  can be formed by a photolithography step. The second opening OP 2  defines the region of the second portion  6  (that is, a width and a dimension in a direction perpendicular to the width). 
     In step S 103 , the laminated film  18  on the semiconductor substrate  1  is etched by using the first photoresist PR 1  and second photoresist PR 2  as etching masks. In step S 104 , the semiconductor substrate  1  is etched by using the first photoresist PR 1  and second photoresist PR 2  as etching masks. 
     As described previously, the laminated film  18  can include the silicon oxide film, polysilicon film, and silicon nitride film in this order from the side of the semiconductor substrate  1 . The silicon nitride film can be, for example, plasma-etched by using a Freon-based gas such as methane tetrafluoride (CF 4 ), methane difluoride (CH 2 F 2 ), methane trifluoride (CHF 3 ), or sulfur hexafluoride (SF 6 ). It is possible to use the Freon-based gas singly, by combining it with another Freon-based gas, and/or by mixing it with an inert gas such as argon or helium or with hydrogen. 
     The polysilicon film can be plasma-etched by using a gas mixture of hydrogen bromide (HBr)/chlorine (Cl 2 )/oxygen. This gas mixture may also be used as it is mixed with an inert gas such as argon or helium. The silicon oxide film can be plasma-etched by mixing a Freon-based gas similar to that of the silicon nitride film with oxygen. It is possible to use this gas mixture singly, by combining it with another Freon-based gas, and/or by mixing it with an inert gas such as argon or helium. 
     In step S 104 , the semiconductor substrate  1  is etched through the first openings OP 1 , thereby forming trenches  20  corresponding to the width of the first openings OP 1  in the semiconductor substrate  1 . The tranches  20  are formed between a plane including the first face S 1  and a plane including the second face S 2 . Also, in step S 104 , in addition to the formation of the trenches  20 , those portions of the first photoresist PR 1 , which are exposed in the second openings OP 2 , are removed, thereby forming openings OP corresponding to the dimension of the second openings OP 2  in the first photoresist PR 1 . In step S 104 , a Bosch process that performs etching by alternately supplying sulfur hexafluoride and Freon 318C (C 4 F 8 ) by using an inductively coupled plasma (ICP) apparatus can be adopted. As an example, it is possible to set the width of the trench  20  at 0.1 μm, and the depth of the trench  20  at 3 to 5 μm. In step S 104 , it is also possible to perform plasma etching using a dipole ring magnetron (DRM) method by using a sulfur hexafluoride/oxygen gas mixture. In this case, only the resist pattern in an element formation region can be left behind by the resist etch back technique. 
     In step S 105 , the laminated film  18  and semiconductor substrate  1  are etched through the openings OP formed in the first photoresist PR 1 , thereby forming first portions  7 ′ and second portions  6  in the semiconductor substrate  1 . The first portions  7 ′ are defined by the openings OP, and the second portions  6  are defined by the trenches  20 . Therefore, the region of the first portion  7 ′ (that is, a width and a dimension in a direction perpendicular to the width) is defined by the first opening OP 1  formed in the first photoresist PR 1 . Also, the region of the second portion  6  (that is, a width and a dimension in a direction perpendicular to the width) is defined by the second opening OP 2  formed in the second photoresist PR 2 . The etching conditions of the laminated film  18  can follow those of the laminated film  18  in step S 103 . The semiconductor substrate  1  can be, for example, plasma-etched by using a gas mixture of hydrogen bromide (HBr)/chlorine (Cl 2 )/oxygen. This gas mixture may also be used as it is mixed with an inert gas such as argon or helium. The depth from the first face S 1  to the bottom of the second portion  6  of the semiconductor substrate  1  can be set at, for example, 200 to 300 nm. 
     On at least a portion of the side surface of the trench T formed by the first portion  7 ′ and second portion  6 , it is possible to form a semiconductor region serving as a barrier against an electric charge to be accumulated in the charge accumulation region  25  of the photoelectric converter PEC. 
     In step S 106 , channel stops  21  are formed below the first portions  7 ′. The channel stops  21  can be formed by, for example, implanting ions for forming a second-conductivity-type semiconductor region by using the first photoresist PR 1  as a mask. When the second conductivity type is a p type, the channel stops  21  can be formed by, for example, obliquely implanting boron ions into the semiconductor substrate  1  at an acceleration voltage of 25 keV, and a dose of 3.0×10 12  to 2.5×10 13  ions/cm 2 . The channel stops  21  can also be formed below the second portions  6 . 
     In step S 107 , an insulator  22  is filled in the second portions  6  so that the spaces  7  remain in the first portions  7 ′ and the trenches T are closed. In other words, in step S 107 , the insulator  22  that covers the first portions  7 ′ is formed in the second portions  6 . That is, the insulator  22  formed in the second portions  6  is the cover of the first portion  7 ′. Before the insulator  22  is filled in the second portions  6 , an oxidation process may also be performed in order to smooth the edges in the trenches T. As this oxidation process, it is possible to apply, for example, radical oxidation performed in a hydrogen/oxygen gas mixture at a temperature of 900° C. to 1,100° C. and a pressure of 0.8 to 2.0 kPa, or thermal oxidation performed in normal-pressure oxygen. The insulator  22  is, for example, silicon oxide, and can be filled by, for example, a high-density plasma CVD technique. More specifically, the insulator  22  can be filled in a monosilane/oxygen/helium gas mixture at a temperature of 550° C. to 700° C. and a pressure of 0.4 to 1.3 Pa by using an ICP apparatus. Under the conditions, almost no silicon oxide film is formed inside the trenches T, and silicon oxide as the insulator  22  is deposited on the trenches T while the interiors of the trenches T are kept in a low-pressure state. To leave the space  7  behind without filling the insulator  22  inside the first portion  7 ′, it is favorable to, for example, set the aspect ratio (height/width) of the first portion  7 ′ at 30 to 50, and the width of the first portion  7 ′ on a submicron order. The thickness of the insulator  22  can be set at, for example, 500 to 800 nm on the laminated film  18 . 
     In step S 108 , the insulator (silicon oxide)  22  is planarized as it is removed from the surface of the laminated film  18 . In this step, after the insulator  22  is plasma-etched, annealing can be performed in a nitrogen ambient at a temperature of 900° C. for 20 to 90 min. Consequently, it is possible to stabilize the characteristics by thermally tightening the insulator (silicon oxide)  22 , and activate the channel stops  21 . After that, the insulator (silicon oxide)  22  can be planarized by two-step CMP (Chemical Mechanical Polish) using silica and ceria as polishing agents. 
     In step S 109 , the surface (first face S 1 ) of the semiconductor substrate  1  in the element formation region is exposed by removing the laminated film  18 . The same conditions as in step S 103  can be adopted to remove the laminated film  18 . 
     In step S 110 , an element is formed between the first face S 1  and second face S 2  of the semiconductor substrate  1 . A portion of the element may be formed on the first face S 1 . Also, another element may be formed on the first face S 1 . As elements forming a pixel,  FIG.  6    shows the photoelectric converter PEC, transfer gate  27 , floating diffusion  28 , and transistor AT (for example, the reset transistor and amplification transistor).  FIG.  6    also shows the NMOS transistor PTN and PMOS transistor PTP in the peripheral circuit. That is,  FIG.  6    shows the gates  8  of the transistor AT, NMOS transistor PNT, and PMOS transistor PTP. The elements can be formed by, for example, an FEOL (Front-End Of Line) process. In step S 110 , an interlayer insulation film  10  of the first layer is further formed. 
     In step S 111 , a wiring structure including a plurality of interlayer insulation films  10  and a plurality of wiring layers  9  is completed, and a passivation film  11  is formed on the wiring structure. The wiring layers  9  can be formed by, for example, Cu. The interlayer insulation films  10  can include a Cu diffusion preventing film or a silicon nitride film serving as an etching stopper, in addition to a silicon oxide film. The wiring layers  9  may also be formed by a material other than Cu, for example, Al. 
     In step S 112 , the semiconductor substrate  1  is thinned from the side of the second face S 2  so as to expose the spaces  7 . The semiconductor substrate  1  can be thinned by, for example, polishing by a grinder, or CMP. It is also possible to apply a well-known substrate thinning technique adopted in three-dimensional packaging or TSV (Through-Silicon Via) formation process. The second face after the thinning is indicated by a second face S 2 ′. 
     The thinning can be performed in a state in which a support substrate containing a resin such as plastic, glass, or silicon as a main material is bonded to the side of the first face S 1  of the semiconductor substrate  1 . The support substrate can be bonded to the semiconductor substrate  1  by using an adhesive such as an epoxy resin. When using an adhesive for bonding the support substrate to the semiconductor substrate  1 , the semiconductor substrate  1  is preferably thinned after the wiring structure is formed. 
     In step S 113 , a light-transmitting planarizing film  12 , color filter layer  13 , capping layer  14 , and microlens layer  15  are formed on the side of the second face S 2 ′ of the semiconductor substrate  1 . 
     The solid-state image sensor  100  as an example of a semiconductor device is obtained through the manufacturing method described above. In this manufacturing method, the alignment of element isolation formed by the spaces  7  and insulators  22  and the alignment of the photoelectric converters PEC and the elements such as the transistors can be performed by using alignment marks formed on the same face (first face S 1 ) of the semiconductor substrate  1 . This is advantageous for high integration because the margin required for the layout can be decreased. 
     The second embodiment of the present invention will be explained below. Note that matters not mentioned in the second embodiment can follow the first embodiment.  FIG.  8    is a sectional view showing a portion of a pixel array of a solid-state image sensor  100 ′ of the second embodiment. In the solid-state image sensor  100 ′ of the second embodiment, spaces  7  (first portions) do not reach a second face S 2  of the semiconductor substrate  1 , and a planarizing film  12  is omitted. In the second embodiment, no element isolation by the space  7  is performed between the second face S 2  and the end portion of the space  7  on the side of the second face S 2 . However, a photoelectric converter PEC and floating diffusion  28  in a pixel are isolated from another pixel by the space  7  and insulator  22 . 
     A method of manufacturing the solid-state image sensor  100  will exemplarily be explained below with reference to  FIG.  9   .  FIG.  9    shows the photoelectric converter PEC and transistors AT (for example a reset transistor and amplification transistor) of one pixel, and portions corresponding to an NMOS transistor PTN and PMOS transistor PTP in a peripheral circuit. 
     Processing until the end of step S 111 ′ is the same as that until the end of step S 111  in the first embodiment. In step S 112 ′, the semiconductor substrate  1  is thinned from the side of the second face S 2 , but the spaces  7  are not exposed. This thinning can be stopped when, for example, channel stops  21 , which exist on the side of the second face S 2 , of the spaces  7  are exposed. The final thickness (the thickness after the thinning) of the semiconductor substrate  1  can suitably be set in accordance with a purpose, for example, securing the flatness when performing a process of forming the color filter layer  13 . The thinning can be performed in a state in which a support substrate is bonded to the semiconductor substrate  1 . 
     In step S 113 , a color filter layer  13 , capping layer  14 , and microlens layer  15  are formed on the side of the second face S 2 ′ of the semiconductor substrate  1 . In the second embodiment, no planarizing film  12  needs to be formed because the spaces  7  are not exposed in the thinning step. 
     As an application example of the solid-state image sensor according to each of the above-mentioned embodiments, a camera incorporating the solid-state image sensor will exemplarily be explained below. The concept of the camera includes not only an apparatus whose main purpose is imaging, but also an apparatus (for example, a personal computer or portable terminal) having an imaging function as an auxiliary function. The camera includes the solid-state image sensor according to the present invention exemplified as the above-mentioned embodiments, and a processing unit that processes an output signal from the solid-state image sensor. This processing unit can include an A/D converter, and a processor that processes digital data output from the A/D converter. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.