Abstract:
A method of producing a semiconductor device according to the present invention comprises steps of:
       (A) forming trenches ( 13 ) on the front surface (FS) of a semiconductor substrate ( 11 ) on the back surface (BS) of which a nitride film ( 12   b ) is formed;   (B) depositing an insulating film ( 15 ) to bury the trenches ( 13 );   (C) removing the nitride film ( 12   b ) on the back surface (BS) of the semiconductor substrate ( 11 ) after the step (B); and   (D) annealing before the insulating film ( 15 ) is etched after the step (C).

Description:
This application is based upon and claims the benefit of priority from Japanese patent application No. 2006-226250, filed on Aug. 23, 2006, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of producing a semiconductor device, and particularly, to a method of producing a semiconductor device comprising device isolation structure. 
     2. Related Art 
     There has been known a shallow trench isolation (STI) as device isolation structure in a semiconductor device. The STI structure is formed such that an insulating film is deposited on a semiconductor substrate in which trenches are formed to bury the trenches with the insulating film. A chemical vapor deposition (CVD) method is generally used to deposit the insulating film. 
     Hitherto, an insulating film has been deposited by a high-density plasma (HDP) CVD method, for example. However, further miniaturization of a device has increased the aspect ratio of a trench, which has made it difficult to bury trenches by the HDP-CVD method. For this reason, other vapor deposition methods having more excellent coverage has drawn attention instead of the HDP-CVD method in recent years. Such vapor deposition methods include a sub-atmospheric (SA) CVD method using gases containing O 3  (ozone)-TEOS (tetraethoxysilane) and an atomic layer deposition (ALD) method. These methods are excellent in coverage and characterized in that a conformal thin film can be formed. 
     A method of forming a semiconductor device according to a related art will be described below with reference to  FIGS. 1A to 1C  and  FIGS. 2A to 2E . 
     First, an oxide film (not shown) is formed on semiconductor substrate (semiconductor wafer)  1  by a thermal oxidation process. Then, silicon nitride film (SiN film)  2  is formed by a low-pressure (LP) CVD method. At this point, SiN film  2  is deposited on both sides of semiconductor substrate  1  as illustrated in  FIG. 1A . That is to say, on front surface FS of semiconductor substrate  1  is formed SiN film  2   a , and on back surface BF of semiconductor substrate  1  is formed SiN film  2   b . SiN film  2  exerts a strong tensile stress on semiconductor substrate  1 . However, the stress is in balance on the front and the back surface in a phase illustrated in  FIG. 1 , so that “warpage” does not appear on semiconductor substrate  1 . 
     A mask is formed from SiN film  2   a  on front surface FS of semiconductor substrate  1  through photolithography and etching. The mask is used to form an STI structure and has a pattern corresponding to the STI structure. SiN film  2   a , as illustrated in  FIG. 1B , is segmented by pattering to release the stress. As a result, stress is imbalanced between front surface FS and back surface BS to convexly warp front surface FS of semiconductor substrate  1 . In other words, a tensile stress of SiN film  2   b  on back surface BS convexly warps semiconductor substrate  1 . 
     Exposed portions on semiconductor substrate  1  are subjected to dry etching using the mask to form trenches  3  in front surface FS of semiconductor substrate  1  as illustrated in  FIG. 1B . Subsequently, oxidation treatment is performed to form 10 to 15 nm thick oxide film  4  on the inner walls of trenches  3 . After that, 6 to 8 nm thick SiN film is formed as liner film (not shown). The liner film plays a role to suppress the oxidation of semiconductor substrate  1  at the following steam anneal process and oxidation process. 
     As illustrated in  FIG. 1C , oxide film  5  is deposited to cover front surface FS of semiconductor substrate  1  by the SA-CVD method using gases containing O 3 -TEOS. As a result, trenches  3  are buried with oxide film  5 . Oxide film  5  deposited by this method is referred to as “O 3 -TEOS-USG (undoped silicate glass).” The above SA-CVD method has an excellent coverage, and O 3 -TEOS-USG to be formed is a conformal film. 
     Care must be devoted to portions poor in film quality referred to as “seam” when trenches  3  are buried with the conformal film. The term “seam” refers to a line where the edges of two films growing from both side walls in trench  3  touch each other. As illustrated in  FIG. 1C , oxide films  5  deposited in each trench  3  are joined with each other to form seam SE in each trench  3 . Large aspect ratio of trench  3  makes seams SE particularly conspicuous. The etching rate of wet etching to seams SE is higher than that to other portions of the film, which is unfavorable for processing. 
     Then, annealing is performed in an atmosphere of water vapor at temperatures of 700° C. to 950° C. (steam anneal process) to improve the film quality of the entire oxide film  5  including seams SE. Furthermore, annealing is performed in an atmosphere of inert gas (N 2 ) at a temperature of 1100° C. This intends oxide film  5  to be densified. However, seams SE cannot be completely removed by such an annealing process in the related art. Particularly, in recent years, the aspect ratio of trench  3  has tended to increase in accordance with miniaturization of a device, so that it is very difficult to eliminate seams SE. 
       FIG. 2A  illustrates an enlarged periphery of trench  3 . As illustrated in  FIG. 2A , trenches  3  are formed on front surface FS of semiconductor substrate  1 . Oxide film  4  is formed on the inner wall of trench  3 . Oxide film  5  is formed to bury trenches  3 . Seams SE are formed at positions corresponding to trenches  3  inside oxide film  5 . 
     As illustrated in  FIG. 2B , chemical mechanical polishing (CMP) is performed to such an extent that SiN film  2   a  is exposed. As a result, STI structure  6  is formed in which trench  3  is buried with oxide film  5 . Seams SE exist in STI structure  6 . 
     As illustrated in  FIG. 2C , oxide film  5  between adjacent masks (SiN film  2   a ) is removed by wet etching using fluorinated acid. The etching rate of wet etching to seams SE is higher than that to other portions, so that oxide film  5  around seam SE is etched faster. As a result, slit dents DE are formed at positions corresponding to seams SE on the surface of STI structure  6 . 
     As illustrated in  FIG. 2D , SiN film  2   a  used as a mask is removed by etching. Wet etching is performed by, for example, phosphoric acid heated to temperatures of 140° C. to 160° C. In this process, SiN film  2   b  formed on back surface BS of semiconductor substrate  1  is also removed at the same time. Incidentally, dents DE still remain on the surface of STI structure  6 . 
     As illustrated in  FIG. 2E , gate oxide film (not shown), gate polysilicon film  7 , tungsten film  8  and SiN film  9  are sequentially deposited in this order. After that, a gate electrode of an MOS transistor is formed by photolithography technique and dry etching technique. 
     However, in this case, a conductor (i.e., gate polysilicon film  7 ) used as a material of the gate electrode is deposited also in dents DE. Even after a gate electrode is formed by dry etching, the conductor in dents DE may not be completely removed. The conductor remaining inside dents DE causes a short circuit between adjacent gate electrodes to deteriorate yield. Better technique is demanded to solve problems resulting from seams SE in oxide film  5 . 
     There have been known the following techniques for solving problems resulting from a stress exerted on a semiconductor substrate. 
     Japanese Patent Laid-Open No. 2004-158711 describes that STI structure is formed with an SOI wafer concavely warped and thereafter the warp of the SOI wafer is corrected to generate a crack in the STI structure. The concave warp results from a compressive stress film (or a polycrystalline silicon film) formed on the back surface of the SOI wafer. According to a technique for solving the problem described in this document, the compressive stress film on the back surface of the SOI wafer is removed and then the STI structure is formed. 
     Japanese Patent Laid-Open No. 2005-340734 describes a technique for decreasing a stress resulting from a silicon nitride film for forming the side wall of a gate electrode. When a silicon nitride film is deposited, it is formed not only on the front surface of a semiconductor substrate, but also on the back surface thereof. The above document indicates that the stress resulting from the silicon nitride film on the back surface thereof deteriorates characteristics of an MOS transistor. According to the technique for solving the problem described in this document, the silicon nitride film is deposited and then an antifouling oxide silicon film is selectively formed only on the front surface of the semiconductor substrate. The silicon oxide film is used as a mask to selectively etch the silicon nitride film on the back surface thereof. After that, the silicon oxide film on the front surface is selectively etched. Then, the silicon nitride film on the front surface is subjected to anisotropic etching to form the side wall of a gate electrode. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a technique capable of solving problems resulting from seams in a deposited film. 
     Another object of the present invention is to provide a technique capable of improving the yield of a semiconductor device. 
     The present invention is described below using reference numbers and characters used in the drawings, but not limited to the description. 
     A method of producing a semiconductor device is provided in a first aspect of the present invention. The method of producing a semiconductor device comprises steps of: 
     (A) forming trenches ( 13 ) on the front surface (FS) of a semiconductor substrate ( 11 ) in which a nitride film ( 12   b ) is formed on the back surface (BS) thereof; 
     (B) depositing an insulating film ( 15 ) to bury the trenches ( 13 ); 
     (C) removing the nitride film ( 12   b ) on the back surface (BS) of the semiconductor substrate ( 11 ) after the step (B); and 
     (D) annealing the semiconductor substrate before the insulating film ( 15 ) is etched and after the step (C). 
     The insulating film ( 15 ) is formed by a vapor deposition method having an excellent coverage in the step (B). The insulating film ( 15 ) is formed by, for example, a sub-atmospheric chemical vapor deposition (SA-CVD) method or an atomic layer deposition (ALD) method. At this point, seams (SE) are probably generated in the insulating film ( 15 ) deposited inside the trenches ( 13 ). 
     The inventers&#39; attention is directed to a stress exerted on seams (SE) in the steam annealing step after the formation of the insulating film. The inventers have found that the stress is controlled to enable the degree of seams (SE) to be improved. The stress exerted on seams (SE) depends on the warpage of the semiconductor substrate ( 11 ). The degree of warpage is controlled to enable the stress exerted on seams (SE) to be indirectly controlled. 
     The nitride film ( 12   b ) is deposited on the back surface (BS) of the semiconductor substrate ( 11 ) in the above step (B). Tensile stress of the nitride film ( 12   b ) on the back surface (BS) convexly warps the front surface (FS) of the semiconductor substrate ( 11 ). A convex warpage acts in a direction to expand seams (SE) in the insulating film ( 15 ) inside the trenches ( 13 ). It is conceivable that the convex warpage is decreased to allow the bonding force of the film in the seam (SE) to be strengthened. The nitride film ( 12   b ) on the back surface (BS) exerting a strong tensile stress may be removed to decrease the convex warpage. 
     According to the present invention, the nitride film ( 12   b ) on the back surface (BS) of the semiconductor substrate ( 11 ) is removed in the step (C) after the step (B). This decreases a stress expanding the seams (SE). Since the film quality of the insulating film ( 15 ) inside the trench ( 13 ) is improved in a steam annealing (the step (D)), the step (C) is desirably implemented prior to the steam annealing. That is to say, annealing is performed in the step (D) after the step (C). The bonding force of the film in the seams (SE) is strengthened to enable the seams (SE) to be sufficiently destroyed by this annealing. 
     After the step (D), a part of the insulating film ( 15 ) is etched to form the device isolation structure ( 16 ). Since the seams (SE) in the insulating film ( 15 ) sufficiently disappear, conventional “dents (DE)” are not made on the surface of the device isolation structure ( 16 ) in the etching process. Short-circuit is therefore prevented resulting from conductors remaining in the “dents (DE)” when a semiconductor device is formed on the front surface (FS) of the semiconductor substrate ( 11 ). For example, short circuit between gate electrodes of adjacent MOS transistors is prevented. As a result, the yield of a semiconductor device is improved. Thus, according to the present invention, it is enabled to solve a problem resulting from a seam. 
     A method of producing a semiconductor device is provided in a second aspect of the present invention. The method of producing a semiconductor device comprises steps of: 
     (a) forming trenches ( 13 ) on the front surface (FS) of a semiconductor substrate ( 11 ); 
     (b) depositing an insulating film ( 15 ) to bury the trenches ( 13 ) while exerting a stress in such a direction that the front surface (FS) of the semiconductor substrate ( 11 ) is convexly warped; 
     (c) decreasing a stress after the step (b); and 
     (d) annealing after the step (c). 
     The step (d) is desirably implemented before the insulating film ( 15 ) is etched. 
     According to the present invention, a stress exerted on seams in the deposited film is controlled to promote the removal of the seams in the annealing step. For this reason, a problem resulting from seams in the deposited film is solved. For example, “dents” are prevented from being formed on the surface of the device isolation structure to prevent short circuit resulting from a conductor remaining in the “dents.” As a result, the yield of a semiconductor device is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are cross sections illustrating the production process of a semiconductor device according to a first related art; 
         FIGS. 2A to 2E  are cross sections illustrating the production process of a semiconductor device according to a second related art; 
         FIGS. 3A to 3D  are cross sections illustrating the production process of a semiconductor device according to an exemplary embodiment; 
         FIGS. 4A to 4E  are cross sections illustrating the production process of a semiconductor device according to another exemplary embodiment; and 
         FIG. 5  is a schematic flow chart illustrating a method of producing a semiconductor device according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method of producing a semiconductor device according to an exemplary embodiment is described below with reference to  FIGS. 3A to 3D  and  FIGS. 4A to 4E . 
     First, an oxide film (not shown) is formed on semiconductor substrate (semiconductor wafer)  11  by a thermal oxidation process. Then, silicon nitride film (SiN film)  12  is formed by an LP-CVD method. At this point, SiN film  12  is deposited on both sides of semiconductor substrate  11  as illustrated in  FIG. 3A . That is to say, on front surface FS of semiconductor substrate  11  is formed SiN film  12   a , and on back surface BF of semiconductor substrate  11  is formed SiN film  12   b . SiN film  12  exerts a strong tensile stress on semiconductor substrate  11 . However, the stress is in balance between front surface FS and back surface BS in the phase illustrated in  FIG. 3A , so that “warpage” is not generated on semiconductor substrate  11 . 
     A mask is formed from SiN film  12   a  on front surface FS of semiconductor substrate  11  through photolithography and etching. The mask is used to form the STI structure and has a pattern according to the STI structure. SiN film  12   a , as illustrated in  FIG. 3B , is segmented by pattering to release the stress. As a result, a stress is imbalanced between front surface FS and back surface BS to convexly warp front surface FS of semiconductor substrate  11 . In other words, a tensile stress of SiN film  12   b  on back surface BS convexly warps semiconductor substrate  11 . 
     Exposed portions on semiconductor substrate  11  are subjected to dry etching using the mask to form trenches  13  in front surface FS of semiconductor substrate  11  as illustrated in  FIG. 3B . Subsequently, oxidation treatment is performed to form 10 to 15 nm thick oxide film  14  on the inner walls of trenches  13 . After that, 6 to 8 nm thick SiN film is formed as a liner film (not shown). The liner film plays a role to suppress the oxidation of semiconductor substrate  11  at the following steam annealing process and oxidation process. 
     As illustrated in  FIG. 3C , insulating film (or oxide film)  15  is deposited to cover front surface FS of semiconductor substrate  11 . As a result, trenches  13  are buried with insulating film  15 . This insulating film (oxide film)  15  is formed by the SA-CVD method using gases containing O 3 -TEOS, for example. In this case, insulating film  15  is formed of O 3 -TEOS-USG. Insulating film  15  may be formed by the ALD or the LP-CVD method. The above vapor deposition methods have an excellent coverage, and insulating film  15  to be formed is a conformal film. Consequently, seams SE are formed in insulating film  15  inside trenches  13  as illustrated in  FIG. 3C . 
     Front surface FS of semiconductor substrate  11  is convexly warped in the deposition process of insulating film  15 . In other words, insulating film  15  is deposited with a stress exerted in such a direction that the front surface FS of semiconductor substrate  11  is convexly warped. Such a stress acts in such a direction as to “expand” seams SE. In order to strengthen the bonding force between the films in seam SE, such a stress should decrease. 
     Then, a step is implemented to decrease the stress exerted on semiconductor substrate  11  following the deposition step of insulating film  15  illustrated in  FIG. 3C . Specifically, SiN film  12   b  on back surface BS exerting a strong tensile stress is removed as illustrated in  FIG. 3D . Tool such as, for example, a back-surface spin etcher is used to remove only SiN film  12   b  on back surface BS. This removes a tensile stress caused by SiN film  12   b  to decrease the warpage of semiconductor substrate  11  and weaken a force to expand seams SE. Thus, controlling a degree of warpage of semiconductor substrate  11  indirectly controls a stress exerted on seams SE. 
     Then, annealing is performed in an atmosphere of water vapor at temperatures of 700° C. to 950° C. (steam anneal process) to improve the film quality of the entire insulating film  15  including seams SE. Furthermore, annealing is performed in an atmosphere of inert gas (such as N 2 ) at a temperature of 1100° C. Insulating film  15  is densified by this procedure. According to the present exemplary embodiment, the bonding force of the films in seams SE is strengthened to promote the removal of seams in the annealing step. That is, seams SE are enabled to be sufficiently removed in the annealing step. As a result, portions poor in film quality hardly exist in insulating film  15 . 
       FIG. 4A  illustrates an enlarged periphery of trenches  13  subjected to the annealing step. As illustrate in  FIG. 4A , trenches  13  are formed on front surface FS of semiconductor substrate  11 . Oxide film  14  is formed on the inner wall of trench  13 . Insulating film  15  is formed to bury trench  13 . It should be noted that seams SE do not exist in insulating film  15 . 
     As illustrated in  FIG. 4B , CMP is performed to such an extent that SiN film  12   a  is exposed. As a result, STI structure  16  is formed in which trenches  13  are buried with insulating film  15 . 
     As illustrated in  FIG. 4C , insulating film  15  between adjacent masks (SiN film  12   a ) is removed by wet etching using hydrofluoric acid. Since seams SE in insulating film  15  have disappeared, a conventional “dent DE” is not made on the surface of STI structure  16  in the etching step. 
     As illustrated in  FIG. 4D , SiN film  12   a  used as a mask is removed by etching. Wet etching is performed by, for example, phosphoric acid heated to temperatures of 140° C. to 160° C. 
     As illustrated in  FIG. 4E , gate oxide film (not shown), gate polysilicon film  17 , tungsten film  18  and SiN film  19  are sequentially deposited in this order. After that, a gate electrode of an MOS transistor is formed by photolithography technique and dry etching technique. According to the present exemplary embodiment, “dent DE” is not formed on the surface of STI structure  16 . For this reason, such a problem is prevented that a conductor remaining inside dent DE causes a short circuit between adjacent gate electrodes. As a result, the yield of a semiconductor device is improved. 
       FIG. 5  is a schematic flow chart illustrating a method of producing a semiconductor device according to an exemplary embodiment. First, nitride film  12  is formed, which is a material for a mask used in forming STI structure  16  (step S 1 ). Nitride film  12  is formed on both surfaces of semiconductor substrate  11 . Trenches  13  for STI structure  16  are then formed by etching using the mask (step S 2 ). Insulating film  15  is then deposited to bury trenches  13  (step S 3 ). Seams SE exist in deposited insulating film  15 . 
     Stress exerted on seams SE is controlled to promote the removal of seams SE in the steam annealing step. Specifically, nitride film  12   b  on back surface BS of semiconductor substrate  11  is removed (step S 4 ). After that, steam annealing step is implemented (step S 5 ). This effectively removes seams SE in insulating film  15 . Subsequently, a part of insulating film  15  is etched to form STI structure  16  (step S 6 ). Thereafter, a semiconductor device such as an MOS transistor is formed on front surface FS of semiconductor substrate  11  (step S 7 ). 
     According to the present exemplary embodiment, a stress exerted on seams SE in the deposited film is controlled to promote the removal of seams SE in the annealing step. For this reason, a problem resulting from seams in the deposited film is solved. For example, a “dent DE” is prevented from being formed on the surface of STI structure  16  to prevent short-circuit resulting from a conductor remaining in “dent DE.” As a result, the yield of a semiconductor device is improved. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.