Abstract:
A method for fabricating a contact plug that has a superior step coverage and does not have internal micro-cracks, including a first step for forming an insulating film including a contact hole on a silicon substrate, a second step for forming a Ti film in the contact hole, a third step for forming a TiN film on the Ti film, and a fourth step for repeatedly performing the second and third steps.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority to Korean Patent Application No. 2000/5759, filed on Feb. 8, 2000, the disclosure of which is incorporated by reference herein in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for fabricating a semiconductor device, and in particular to a method for fabricating a contact plug that has a superior step coverage and does not have internal micro-cracks.  
           [0004]    2. Description of the Background Art  
           [0005]    Semiconductor devices can be formed on a silicon substrate having P+ type or N+ type doped regions. The doped regions must be electrically connected to form an electrical circuit. Accordingly, a conductive layer that includes a metal or a doped polycrystalline silicon is deposited and patterned. In a conventional method for fabricating a semiconductor device, an insulating layer formed on a wafer is patterned and etched to form a contact hole. A contact plug and an interconnecting lead are formed by a conductive material.  
           [0006]    It is known to use W to form a contact plug. However, there has been recent interest in using TiN instead of W as a material in the contact plug. TiN can better fill a narrow contact hole or via hole since TiN has a superior step coverage as compared to W. In addition, since TiN itself serves as a diffusion barrier layer, it is unnecessary to deposit an additional diffusion barrier layer consisting of Ti/TiN, and therefore the manufacturing process of the contact plug is simplified.  
           [0007]    FIGS.  1 ( a ) to  1 ( d ) illustrate the sequential steps of the conventional method for fabricating the Ti/TiN contact plug.  
           [0008]    As shown in FIG. 1( a ), an insulating film  2  including silicon oxide (SiO 2 ) is deposited on a silicon substrate  1  that includes a doped region (not shown). A photoresist film (not shown) is spread on the insulating film  2 , and patterned according to an exposure and developing process. A contact hole  10  is formed in the insulating film  2  by an etching process that uses the patterned photoresist film (not shown) as a mask. The silicon substrate  1  is oxidized forming a native oxide film  3  at the upper surface of the silicon substrate  1  exposed by the contact hole  10 .  
           [0009]    Thereafter, as depicted in FIG. 1( b ), the native oxide film  3  shown in FIG. 1( a ) is removed by a cleaning process.  
           [0010]    As illustrated in FIG. 1( c ), a Ti film  5  is deposited on the upper surface of the insulating film  2  and inside of the contact hole  10 . The deposition of the Ti film  5  is performed at a temperature of approximately 650° C. The silicon and titanium react with each other and form a titanium silicide (TiSi 2 ) film  4  at an interface between the silicon substrate  1  and the Ti film  5 .  
           [0011]    As shown in FIG. 1( d ), a TiN film  6  is formed thick on the Ti film  5  in order to fill the contact hole  10 , thereby completing the fabrication of the contact plug.  
           [0012]    However, when the contact plug is fabricated by forming the TiN film  6  thick, micro-cracks  20  form in the TiN film  6 . The micro-cracks  20  are formed at grain boundaries due to stress generated from a nucleation step that is an initial stage of the deposition of the TiN to a grain transition step that leads to a bulk layer growth step. A number of disadvantages arise due to the formation of the micro-cracks  20 .  
           [0013]    First, after the formation of the contact plug, when a tungsten line is formed by depositing the tungsten according to the chemical vapor deposition using WF 6 , the WF 6  gas can penetrate into the TiN film through the micro-cracks and react with the TiN film.  
           [0014]    Second, the WF 6  gas penetrating into the TiN film can be diffused into the device and cause the deterioration of the electric characteristics of the device.  
           [0015]    Third, when the micro-cracks are formed in the TiN film, the TiN film cannot be operated as the diffusion barrier film.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, it is an object of the present invention to provide a method for fabricating a contact plug which has a sufficient step coverage to fill a narrow contact hole or via hole.  
           [0017]    It is another object of the present invention to restrict the formation or generation of micro-cracks and thus prevent the deterioration of the characteristics of the device caused by the micro-cracks.  
           [0018]    It is still another object of the present invention to simplify the process of fabricating a contact plug by removing the necessity of an additional or specific diffusion barrier layer.  
           [0019]    In order to achieve these and other objects of the present invention, there is provided a method for fabricating a contact plug, including: a first step for forming an insulating film including a contact hole on a silicon substrate; a second step for forming a Ti film in the contact hole; a third step for forming a TiN film on the Ti film; and a fourth step for repeatedly performing the second and third steps a few times. In addition, the method for fabricating the contact plug may further include a step for nitriding a surface of the Ti film between the second step and the third step, in order to enhance adhesion between the Ti film and the TiN film.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate specific preferred embodiments of the present invention, and, together with the general descriptions given above and the detailed descriptions given below, serve to explain features of the present invention.  
         [0021]    FIGS.  1 ( a ) to  1 ( d ) illustrate the sequential steps of a conventional method for fabricating a Ti/TiN contact plug; and  
         [0022]    FIGS.  2 ( a ) to  2 ( f ) illustrate the sequential steps of a method for fabricating a Ti/TiN contact plug in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    An example of a structure of a semiconductor device and a fabrication method thereof in accordance with the present invention will now be described in detail with reference to the accompanying drawings.  
         [0024]    FIGS.  2 ( a ) to  2 ( f ) illustrate the sequential steps of a method for fabricating a contact plug in accordance with a preferred embodiment of the present invention.  
         [0025]    As shown in FIG. 2( a ), an insulating film  102  including silicon oxide (SiO 2 ) is deposited on a silicon substrate  100  that includes a doped region (not shown). A photoresist film (not shown) is spread on the insulating film  102 , and patterned by an exposure and developing process. The patterned photoresist film (not shown) is used as a mask in an etching process to form a contact hole  110  in the insulating film  102 . A native oxide film  103  is formed on an upper surface of the silicon substrate  100  that is exposed by the contact hole  110 .  
         [0026]    As illustrated in FIG. 2( b ), the native oxide film  103  is removed by a cleaning process. A solution made by adding a predetermined ratio of detergent into a buffered oxide etchant (BOE) where HF and NH 4 F are mixed can be used as a cleaning solution. The cleaning process can be carried out by using a spin etch.  
         [0027]    As shown in FIG. 2( c ), a Ti film  105  is deposited on an upper surface of the insulating film  102  and inside of the contact hole  110 . The deposition of the Ti film  105  can be performed by a variety of methods. The Ti film  105  can be deposited according to the chemical vapor deposition (CVD). The Ti film  105  can also be deposited by a plasma enhanced chemical vapor deposition (PECVD). A variety of gases that include Ti can be used as a source gas for growing the Ti film  105 . The source gas can include TiCl 4 . A thickness of the Ti film  105  can be less than about 100 Å. The deposition of the Ti film  105  can be performed at a temperature between about 500° C. and about 800° C., and can be preferably about 650° C. The silicon and titanium react to form a titanium silicide (TiSi 2 ) film  104  at an interface between the silicon substrate  100  and the Ti film  105 . The titanium silicide film  104  forms an ohmic contact between the silicon substrate  100  and the Ti film  105 .  
         [0028]    The titanium silicide film described above can be formed by simultaneously depositing and annealing the Ti film  105 , or can be formed by annealing the Ti film  105  after the deposition of the Ti film.  
         [0029]    Thereafter, as illustrated in FIG. 2( d ), a gas that includes nitrogen is provided. The surface of the Ti film  105  is nitrided, thereby forming a Ti film  105 ′ having a nitrided surface or a nitride film. The surface of the Ti film  105  can be nitrided by using the gas including NH 3 . The flow of NH 3  can be between about 800 and about 1200 sccm, and can be preferably about 1000 sccm. A temperature of the chamber where the nitriding reaction takes place can be controlled, and can be at least about 600° C., and can be preferably at least about 680° C. The nitride film formed on the surface of the Ti film enhances adhesion between a deposited TiN film  107  and the Ti film  105 ′. The nitride film also prevents the Ti film  105 ′ from being etched by a source gas used for depositing the TiN film  107 .  
         [0030]    As shown in FIG. 2( e ), the TiN film  107  is deposited on the Ti film  105 ′ having the nitrided surface. The deposition of the TiN film  107  can be performed by a variety of methods. The TiN film  107  can be deposited according to a process of chemical vapor deposition (CVD). The TiN film  107  can also be deposited by plasma enhanced chemical vapor deposition (PECVD). Various gases that include Ti and N can be used as a source gas for growing the TiN film  107 . The source gas can include TiCl 4  and NH 3 . A thickness of the TiN film  107  can be about less than a critical thickness at which micro-cracks start to develop due to an accumulation of stress in the TiN film  107  during deposition. The thickness of the TiN film  107  can also be less than about 100 Å. The deposition of the TiN film  107  can be carried out at a temperature between about 500° C. and about 800° C., and can be approximately 650° C.  
         [0031]    As illustrated in FIG. 2( f ), steps shown in FIGS.  2 ( c ) and  2 ( e ) are repeated, thereby alternately forming the Ti film  105 ′ having the nitrided surface and the TiN film  107 . The steps shown in FIGS.  2 ( c ) and  2 ( e ) can be repeated from 2 to 6 times. The Ti film  105 ′ having the nitrided surface and the TiN film  107  can be alternately deposited until the contact hole  105  is filled so that a film closer to the silicon substrate  100  can be the Ti film  105 ′ and an uppermost film can be the TiN film  107 .  
         [0032]    In accordance with the method for fabricating the contact plug of the present invention, the thickness of the TiN film is about less than the critical thickness at which the micro-cracks start to form, thereby preventing the micro-cracks from forming in the TiN film. As a result, the present invention prevents the undesired effects that are caused by the micro-cracks on the characteristics of the device and during the fabrication process.  
         [0033]    In addition, according to the present invention, the surface of the Ti film is nitrided before the formation of the TiN film, thereby enhancing adhesion between the Ti film and the TiN film and preventing the Ti film from being etched by the source gas that is used to grow the TiN film.  
         [0034]    Furthermore, in accordance with the present invention, the contact plug replaces the conventional TiN contact plug and overcomes the disadvantages of the conventional contact plug. A narrow contact hole or via hole can be filled since TiN has a superior step coverage. The fabrication process is simplified since the TiN film operates as a diffusion barrier film and therefore it is not necessary to deposit the diffusion barrier film in advance.  
         [0035]    As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiment is not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.