Patent Publication Number: US-2023154848-A1

Title: Semiconductor structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 17/034,129, filed on Sep. 28, 2020, which claims the priority of Chinese Patent Application No. 201911012609.X, filed on Oct. 23, 2019, the content of all of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to the field of semiconductor manufacturing and, more particularly, relates to a semiconductor structure and its fabrication method. 
     BACKGROUND 
     In integrated circuit (IC) manufacturing, standard cells, used as basic elements for fabricating IC devices, are arranged and wired to form functional circuits. Each standard cell needs a power (Vdd) input and a ground (Vss) connection when in use. In order to supply power to a variety of components, each standard cell is coupled to a power rail, which is electrically connected to an active layer of the standard cell, to supply the power (Vdd). In certain cases, a plurality of power rails may be provided to each standard cell to respectively provide the power (Vdd) input and the ground (Vss) connection. 
     The standard cell may be designed to be a stacked structure having various metal layers, and each metal layer may include wiring traces. Various interconnection elements may be formed in the wiring traces to allow connections between various components of the standard cell and to allow connections of various components of the standard cell with other standard cells. 
     In the existing technology, the power rail may be arranged in one metal layer of various metal layers and then be connected to the active layer of the standard cell through a conduction pass, which may occupy much space in the metal layer for forming wiring traces and various interconnection elements. As such, in order to improve the response speed of the integrated circuit, the dimension of the standard cell needs to be enlarged to provide a larger wiring trace space, which may not be beneficial for the advancement to the next process node of the semiconductor technology. 
     SUMMARY 
     One aspect of the present disclosure provides a semiconductor structure. The semiconductor structure includes a substrate; a plurality of fins, formed on the substrate; a plurality of isolation structures, each formed on a top surface of the substrate between adjacent fins; and a power rail, formed in at least one isolation structure of the plurality of isolation structures and further in the substrate, where a top surface of the power rail is lower than a top surface of the plurality of fins. 
     Optionally, the semiconductor structure further includes a metal layer, formed on the power rail. 
     Optionally, the metal layer is made of a material including cobalt, tungsten, copper, ruthenium or platinum, or a combination thereof. 
     Optionally, a bottom surface of the power rail is lower than the top surface of the substrate. 
     Optionally, the power rail is made of a material including ruthenium, copper, graphene, or a combination thereof. 
     Optionally, the power rail is formed through the at least one isolation structure having portions of the at least one isolation structure on sidewalls of the power rail; and a mask layer is formed on the portions of the at least one isolation structure and on the plurality of fins. 
     Optionally, the mask layer is made of a material including silicon nitride, aluminum nitride, silicon carbide, or a combination thereof. 
     Compared with the existing technology, the technical solutions provided by the present disclosure may achieve at least the following beneficial effects. 
     The power rail may be formed in the isolation structure and the substrate between any adjacent fins, and the top surface of the power rail may be lower than the top surface of the fins. The power rail and the active layer of the standard cell may be in a same layer, which may release the space occupied by the wiring traces in the metal layer. When a same response speed of the semiconductor structure is satisfied, the dimension of the standard cell may be reduced, which is beneficial for the advancement to the next process node of the semiconductor technology; or under a same dimension of the standard cell, more space may be available for the wiring traces, thereby improving the performance of the semiconductor structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure. 
         FIGS.  1 - 10    illustrate structural schematics corresponding to certain stages of a method for forming an exemplary semiconductor structure according to various disclosed embodiments of the present disclosure; and 
         FIG.  11    illustrates a flowchart of an exemplary fabrication method for forming a semiconductor structure according to various disclosed embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments of the disclosure, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     A semiconductor structure and its fabrication method are provided in the present disclosure. The method includes providing a substrate, forming a plurality of fins on the substrate, and forming an isolation structure layer including a plurality of isolation structures on the substrate, each isolation structure being formed between adjacent fins. The method further includes forming a first opening by etching at least one isolation structure of the plurality of isolation structures and a portion of the substrate, and forming a power rail by filling the first opening with a conductive material, where a top surface of the power rail is lower than a top surface of the plurality of fins. 
     When a power rail is used to supply power to the standard cell in the integrated circuit, the power rail is arranged in one or more of the metal layers of the standard cell. A conduction path is then used to connect the metal layer and the power rail that is connected between metal layers or used to connect the power rail to the active layer of the standard cell. The metal layer includes wiring traces; and the more wiring traces are, the faster the response speed of the semiconductor structure is. The arrangement of the power rail in the metal layer may cause the reduction of the space available for wiring traces, resulting in a decrease in the semiconductor speed and affecting the device performance of the semiconductor structure. In order to improve the semiconductor device performance, the dimension of the standard cell is increased to provide more space for wiring traces, which however may not be beneficial for further advancement of the technology node of the semiconductor process. 
     Various embodiments of present disclosure provide semiconductor structures and formation methods thereof. For example, a first opening may be formed in an isolation structure layer, including a plurality of isolation structures, and a substrate between any adjacent fins. A power rail may be formed in the first opening. A portion of the power rail may be buried in the substrate, and the power rail may be at a same layer as the active layer of the semiconductor structure, without being connected through a conduction path. The space available for wiring traces in the metal layer of the standard cell may be released. Under a same response speed, the dimension of the standard cell may be reduced; or under a same dimension of the standard cell, more space may be available for the wiring traces, thereby improving the response speed of the semiconductor structure. 
     In order to clearly illustrate the above-mentioned described objectives, features, and advantages of the present disclosure, various embodiments of the present disclosure are described in detail with reference to the accompanying drawings hereinafter. 
       FIGS.  1 - 10    illustrate structural schematics corresponding to certain stages of a method for forming an exemplary semiconductor structure according to various disclosed embodiments of the present disclosure. 
     Referring to  FIG.  1   , a substrate  100  may be provided (e.g., in S 801  of  FIG.  11   ), and fins  110  may be formed on the substrate (e.g., in S 802  of  FIG.  11   ). 
     In one embodiment, the substrate  100  may be a silicon substrate. In other embodiments, the substrate  100  may be any one of a germanium substrate, a silicon germanium substrate, a silicon carbide substrate, a silicon-on-insulator, a germanium-on-insulator substrate, and/or any other suitable substrate(s). 
     In one embodiment, the fins  110  may be made of silicon. In other embodiments, the fins  110  may be made of a material including germanium, silicon germanium, and/or any other suitable substrate(s). 
     In one embodiment, forming the fins  110  on the substrate  100  may include forming a patterned mask layer (not shown), corresponding to the positions of the fins  110  needed to be formed, on the substrate  100 ; and etching a portion of the substrate  100  using the patterned mask layer as a mask to form a plurality of discrete fins  110  on the substrate  100 . 
     It should be noted that the spacings between adjacent fins  110  shown in  FIG.  1    may be equal to each other. However, in actual semiconductor devices, the spacings between the fins may not be necessarily equal to each other. 
     In one embodiment, after forming the fins  110 , the pattern mask layer may be directly removed. In other embodiments, the patterned mask layer may be removed after a power rail is subsequently formed. 
     It should be noted that the standard cells may be any types of cells and have different device architectures. In one embodiment, the standard cell may be a fin FET, which may not be limited according to various embodiments of the present disclosure. 
     Referring to  FIG.  1   , an isolation structure layer  200 , including a plurality of isolation structures  200   a,  may be formed on the substrate  100  between adjacent fins  110 ; and each isolation structure  200   a  may be formed between any adjacent fins (e.g., in S 803  of FIG.  11 ). The top surface of the isolation structure layer  200  may be coplanar with the top surface of the fins  110 . 
     In one embodiment, the isolation structure layer  200  may be made of silicon oxide. In another embodiment, the isolation structure layer  200  may be made of silicon oxynitride. 
     In one embodiment, forming the isolation structure layer  200  may include depositing an isolation material layer on the substrate  100  between adjacent fins  110 , where the isolation material layer may cover the surface of the substrate  100 , and the sidewalls and top surfaces of the fins  110 ; and performing a chemical mechanical planarization process on the isolation material layer to make the top surface of the isolation material layer coplanar with the top surface of the fins  110  to form the isolation structure layer  200 . 
     After the isolation structure layer  200  is formed, the isolation structure  200   a  and the substrate  100  between any adjacent fins  110  may be etched to form a first opening. 
     It should be noted that, in one embodiment, etching the isolation structure  200   a  and the substrate  100  may be a stepwise etching process. The isolation structure  200   a  may be first etched till exposing the surface of the substrate  100 , and then a portion of the substrate  100  may be etched. 
     For example, etching the isolation structure  200   a  between any adjacent fins  110  may be described hereinafter. 
     Referring to  FIG.  2   , a patterned photoresist layer  300  may be formed on the surface of the isolation structure layer  200 . 
     In one embodiment, forming the patterned photoresist layer  300  may include depositing the photoresist on the surface of the isolation structure layer  200 , and developing and exposing the photoresist to form the patterned photoresist layer. The patterned photoresist layer  300  may expose at least one isolation structure  200   a  between adjacent fins  110 . 
     The exposed position of the patterned photoresist layer  300  may be the position for forming the power rail subsequently. 
     Referring to  FIG.  3   , using the patterned photoresist layer  300  as a mask, a portion of the isolation structure  200   a  between any adjacent fins  110  may be etched to form a second trench  210 . 
     In one embodiment, the thickness of the etched isolation structure  200   a  may be about 1 nm to about 10 nm. 
     In one embodiment, the second trench  210  may provide positioning to the subsequent formation of a first mask layer for etching the isolation structure  200   a.    
     After the second trench  210  is formed, the patterned photoresist layer  300  may be removed by a wet etching process or an ashing process. 
     Referring to  FIG.  4   , a first mask layer  400  may be formed on the surface of the isolation structure layer  200 , and on the sidewalls  211  and the bottom  212  of the second trench  210 . 
     In one embodiment, the first mask layer  400  may be formed by an atomic layer vapor deposition method. In other embodiments, the first mask layer  400  may be formed by a chemical vapor deposition method, a physical vapor deposition method, or a combination thereof. 
     In one embodiment, the first mask layer  400  may be made of silicon nitride. In other embodiments, the first mask layer  400  may be made of a material including aluminum nitride, silicon carbide, and/or other suitable material(s). 
     After the first mask layer  400  is formed, the first mask layer  400  may be etched to form a second opening. 
     For example, etching the first mask layer  400  may be described hereinafter. 
     Referring to  FIG.  5   , a second mask layer  500  may be formed on the first mask layer  400 . The second mask layer  500  may expose the first mask layer  400  at the bottom  212  of the second trench  210 . 
     In one embodiment, the second mask layer  500  may be formed by an atomic layer vapor deposition. In other embodiments, the second mask layer  500  may be formed by a chemical vapor deposition, a physical vapor deposition, or a combination thereof. 
     In one embodiment, the second mask layer  500  may be made of a hydrocarbon polymer. 
     Referring to  FIG.  6   , using the second mask layer  500  as a mask, the first mask layer  400  at the bottom  212  of the second trench  210  may be etched to form a second opening  410 . 
     In one embodiment, the first mask layer  400  may be etched by a dry etching process. 
     After removing the first mask layer  400  at the bottom  212  of the second trench  210 , the second mask layer  500  may be removed by a wet etching process or an ashing process. 
     Referring to  FIG.  7   , the isolation structure  200   a  may be etched along the second opening  410  till exposing the surface of the substrate  100  to form a first trench  220  (e.g., in S 804  of  FIG.  11   ). 
     In one embodiment, the isolation structure  200   a  may be etched by a plasma dry etching process. The process parameters of the plasma dry etching process may include the etching gas including trifluoromethane (CHF 3 ), tetrafluoromethane Carbon (CF 4 ), fluoromethane (CH 3 F), oxygen, or a combination thereof, the etching pressure of about 5 mTorr to about 100 mTorr, and the etching temperature of about 10° C. to about 120° C. 
     Referring to  FIG.  8   , a portion of the substrate  100  may be etched along the first trench  220  to form a first opening  120  (e.g., in S 804  of  FIG.  11   ). 
     In one embodiment, the etched thickness of the substrate  100  may be about 300 Å to about 1000 Å. The thickness to be etched of the substrate  100  may be selected according to actual process requirements. 
     In one embodiment, the substrate  100  may be etched by a plasma dry etching process. The process parameters of the plasma dry etching process may include the etching gas including hydrogen bromide (HBr), trifluoride Nitrogen (NF 3 ), chlorine, oxygen, or a combination thereof, the etching pressure of about 5 mTorr to about 100 mTorr, and the etching temperature about 10° C. to about 120° C. 
     In one embodiment, since the portion of the first mask layer  400  on the sidewalls  211  of the second trench  210  is remained, when the isolation structure  200   a  is subsequently etched using the first mask layer  400  as a mask, a portion of the isolation structure  200   a  may be remained on two sides of the first trench  220 . On the one hand, the damage to the fins  110  may be avoided when the isolation structure  200   a  is etched. On the other hand, the width of the first trench  220  may also be controlled by controlling the thickness of the first mask layer  400  deposited on the sidewalls  211  of the second trench  210 . The width of the first trench  220  corresponds to the dimension of the formed power rail, such that the dimension of the formed power rail may be controlled. 
     After the first opening  120  is formed, a conductive material may be filled in the first opening  120  to form a power rail. 
     For example, forming the power rail may be described hereinafter. 
     Referring to  FIG.  9   , the first opening  120  may be filled with the conductive material to form a conductive material layer  121 (e.g., in S 805  of  FIG.  11   ); and performing a chemical mechanical polishing process on the conductive material layer, such that the top surface of the conductive material layer  121  may be coplanar with the surface of the first mask layer  400 . 
     In one embodiment, the first mask layer  400  may serve as a polishing stop layer of the conductive material layer  121  and may also protect the fins  110  from being damaged. 
     In one embodiment, the conductive material may be ruthenium (Ru). In other embodiments, the conductive material may be copper (Cu), graphene, and/or any other suitable material(s). 
     In one embodiment, ruthenium may be used as the material for forming the power rail. Ruthenium has desirable electrical conductivity, high temperature and corrosion resistance, and excellent electrical performance. It may be an excellent choice for replacing original conductive materials such as copper in the next semiconductor technological process. 
     In one embodiment, filling the conductive material may be performed by an electrochemical deposition process. 
     Referring to  FIG.  10   , a portion of the conductive material layer  121  may be etched to form a power rail  600 , where the top surface of the power rail  600  may be lower than the top surface of the fin  110  (e.g., in S 805  of  FIG.  11   ). 
     In one embodiment, the conductive material layer  121  may be etched by a dry etching process. The etching process parameters may include the etching gas including carbon tetrafluoride (CF 4 ), boron trichloride (BCl 3 ), oxygen, chlorine, helium, or a combination thereof, where the flow rate of carbon tetrafluoride is about 0 sccm to 100 sccm, the flow rate of boron trichloride is about 30 sccm to 500 sccm, the flow rate of oxygen is about 0 sccm to about 100 sccm, the flow rate of chlorine is about 0 sccm to about 100 sccm, and the flow rate of helium is about 50 sccm to about 500 sccm, and may also include the etching pressure of about 2 mTorr to about 100 mTorr, and the etching temperature of about 0° C. to about 150° C. 
     After the power rail  600  is formed, the first mask layer  400  may be removed by a wet etching process or an ashing process. 
     In one embodiment, after the power rail  600  is formed, a metal layer (not shown) may be formed on the power rail  600 . 
     The metal layer may be made of a metal material including cobalt, tungsten, copper, ruthenium or platinum, and/or any other suitable material(s). 
     According to the method for forming the semiconductor structure provided by the embodiments of the present disclosure, the power rail may be formed in the isolation structure and the substrate between any adjacent fins, and the top surface of the power rail may be lower than the top surface of the fin. The power rail and the active layer of the standard cell may be located in a same layer, which may release the space occupied by wiring traces in the metal layer. When a same response speed is satisfied, the dimension of the standard cell may be reduced, which is beneficial for the advancement of the semiconductor technology to a next process node. 
     Referring to  FIG.  10   , correspondingly, the embodiments of the present disclosure also provide a semiconductor structure formed by the above-mentioned fabrication methods. The semiconductor structure may include the substrate  100 , fins  110  formed on the substrate  100 ; the plurality of isolation structures  200   a  on the substrate  100  between adjacent fins  110 ; the power rail  600  in at least one isolation structure  200   a  and in the substrate  100  between any adjacent fins  110 , where the top surface of the power rail  600  may be lower than the top surface of the fins  110 . 
     In one embodiment, a portion of the isolation structure  200  may still be remained between the power rail  600  and the fins  110 . 
     In one embodiment, the power rail  600  may be made of ruthenium. In other embodiments, the power rail  600  may be made of copper, graphene, and/or any other suitable material(s). 
     In one embodiment, the semiconductor structure may further include the metal layer (not shown), and the metal layer may be on the power rail  600 . 
     Although the present disclosure has been disclosed above, the present disclosure is not limited thereto. Any changes and modifications may be made by those skilled in the art without departing from the spirit and scope of the disclosure, and the scope of the disclosure should be determined by the scope defined by the appended claims.