Abstract:
A method of fabricating a semiconductor device including an interconnection is provided. The method is composed of covering a substrate with a metal film stack including a lower refractory metal film over the substrate, a lower protective layer of a first compound including metal disposed on an upper surface of the lower refractory metal film, a core metal film of the metal on an upper surface of the lower protective layer, an upper protective layer of a second compound including the metal disposed on an upper surface of the core metal film, and an upper refractory metal film disposed on an upper surface of the upper protective layer, patterning the metal film stack; and forming a side protective layer of a third compound including the metal on a side of the patterned core metal film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a method of fabricating interconnections including a multi-layer metal film stack, particularly, to an improvement in corrosion and heat resistances of interconnections. 
     2. Description of the Related Art 
     TFT (thin film transistor) apparatuses, such as LCDs (liquid crystal displays), require low resistivity interconnections with high corrosion and heat resistances. A technique for fabricating such interconnections is disclosed in Japanese Patent Application No. Jp-A-Heisei 8-62628. The disclosed technique involves forming a refractory metal film, oxidizing the surface of the refractory metal film, forming an aluminum film on the oxidized surface, oxidizing the upper surface of the aluminum film, forming another refractory metal film on the oxidized surface of the aluminum film to complete a film stack, patterning the film stack, and oxidizing sides of the film stack. The oxides effectively avoids the aluminum film being corroded by stripping agent for stripping off resist patterns used as a mask. 
     Japanese Patent Application No. P2000-26335A discloses an interconnections structure composed of an aluminum film sandwiched by a pair of refractory metal films. Oxygen including aluminum films are disposed between the aluminum film and refractory metal films to prevent thermally induced counter diffusion between the aluminum film and refractory metal films. 
     Japanese Patent Application No. P2002-198360 discloses an etching technique for etching a structure including a silicon layer, and an aluminum layer disposed on the upper surface of the silicon layer. The disclosed etching technique involves etching the aluminum layer with Cl 2  gas and H 2  gas, and etching the silicon layer with SF 6  gas, and HCl gas and He gas. The document also discloses the use of Cl 2  gas in place of the HCl gas. 
     Japanese Patent Application No. P2002-90774A discloses a LCD fabrication process to reduce deterioration of liquid crystal within cells caused by pollution with material of gate electrodes. The disclosed process involves successively depositing an aluminum layer and a molybdenum layer, partially etching the molybdenum layer in an effective display region of the display panel, and oxidizing the aluminum layer in the effective display region through an anodization technique to complete the gate electrodes. 
     Japanese Patent Application No. 2000-252473 discloses a TFT structure for achieving low resistivity ohmic contact onto gate electrodes. The disclosed TFT structure is composed of gate electrodes including first through third metal layers, the first metal layer being formed of refractory metal such as Ta, Hf, Nb, and Zr, the second metal layer being formed of low resistivity metal such as Al, Ti, Cu, Cr, W, and Mo, and the third metal layer being formed of refractory metal such as Ta, Hf, Nb, and Zr. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention addresses an improvement in corrosion and heat resistances of interconnections, especially those integrated within TFT devices. 
     In an aspect of the present invention, a method of fabricating a semiconductor device including an interconnection is composed of:
         forming a metal film stack to cover a substrate; the film stack including:   a lower refractory metal film over the substrate,   a lower protective layer of a first compound including metal disposed on an upper surface of the lower refractory metal film,   a core metal film of the metal on an upper surface of the lower protective layer,   an upper protective layer of a second compound including the metal disposed on an upper surface of the core metal film, and   an upper refractory metal film disposed on an upper surface of the upper protective layer;   patterning the metal film stack; and   forming a side protective layer of a third compound including the metal on a side of the patterned core metal film.       

     At least one of the first, second, and third compounds may be oxide, nitride, or oxynitride of the metal. 
     In the event that the metal is selected from among the group consisting of aluminum and aluminum alloy, the first, second, and third compounds are preferably selected from the group consisting of oxide, nitride, and oxynitride of the metal. 
     For copper, silver, and an alloy thereof, by contrast, the first, second, and third compounds are preferably selected from the group consisting of nitride, and oxynitride of the metal. 
     The patterning may include:
         forming a resist pattern on the metal film stack, and   etching the metal film stack using the resist pattern as a mask, the formation of the side protective layer being implemented before the resist pattern is stripped off.       

     Alternatively, the patterning may include:
         forming a resist pattern on the metal film stack,   etching the metal film stack using the resist pattern as a mask, and   stripping off at least a portion of the resist pattern, the formation of the side protective layer being implemented after the stripping off.       

     The method may further includes:
         forming a semiconductor film stack to cover the substrate; the semiconductor film stack including a semiconductor layer and a heavily doped semiconductor layer disposed on an upper surface of the semiconductor layer, and the metal film stack being patterned so that the patterned metal film stack overlaps the semiconductor film stack;   patterning the semiconductor film stack using the patterned film stack as a mask.       

     When the method includes covering the substrate with a semiconductor film stack including a semiconductor layer and a heavily doped semiconductor layer disposed on an upper surface of the semiconductor layer, and patterning the semiconductor film stack, the patterning the metal film stack may include:
         forming a resist pattern on the metal film stack,   etching the metal film stack using the resist pattern as a mask so that the patterned metal film stack overlaps the semiconductor film stack, and   the patterning the semiconductor film stack may be achieved by using the resist pattern as a mask.       

     The above-mentioned method is especially effective in the case that the patterning the semiconductor film stack is achieved by using an etchant including fluorine and/or chlorine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1F  are cross sectional views illustrating a fabrication process of an inversely staggered TFT device in an embodiment of the present invention; and 
         FIGS. 2A through 2G  are cross sectional views illustrating a fabrication process in an alternative embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below in detail with reference to the attached drawings. 
     In one embodiment, as shown in  FIG. 1A , a process for fabricating a TFT device begins with forming a gate electrode  2  and a scan line (not shown) on a transparent insulating substrate  1 . In order to form the gate electrode  2 , a core metal film  21  of aluminum is firstly deposited on the substrate  1 , and then the surface of the metal film  21  is covered with a thin protective film  22 . The protective film  22  may be formed through oxidizing, nitriding, or oxinitriding the surface of the core metal film  21 . A refractory metal film  23  of chromium is then deposited on the protective film  22 . The core metal film  21 , the protective film  22 , and the refractory metal film  23  is then patterned. After the patterning, the sides of the patterned core metal film  21  are oxidized, nitrided, or oxinitrided to form thin protective films thereon, which typically have a thickness in the orders of tens or hundreds of nano meters. This completes the gate electrode  2 . 
     After the gate electrodes  2  are covered with a gate dielectric  5 , as shown in  FIG. 1B , a semiconductor film stack  6  of an amorphous silicon film  61  and a heavily doped amorphous silicon film  62  is then formed to cover the gate dielectric  5 . 
     As shown in  FIG. 1C , the semiconductor film stack  6  is then patterned to form a semiconductor film stack  6 . 
     After patterning the semiconductor film stack  6 , as shown in  FIG. 1D , a metal film stack of a lower refractory metal film  91 , a lower protective film  93   a , a core metal film  92 , an upper protective film  93   b , and an upper refractory metal film  94  is then formed to cover the patterned semiconductor film stack  6 . 
     The refractory metal films  91  and  94  are formed of a material selected from the group of chromium (Cr), titanium (Ti), tantalum (Ta), Niobium (Nb), hafnium (Hf), zirconium (Zr), molybdenum (Mo), tungsten (W), alloys thereof, and conductive nitrides thereof, such as titanium nitride. 
     The core metal film  92  is formed of a low resistivity metal, such as aluminum, copper, silver, and alloys mainly consisting of these metal, such as AlNd. 
     The protective films  93   a  and  93   b  are formed of oxides, nitrides, or oxynitrides of the metal or alloy used as the core metal films  92 . In the event that the core metal film  92  is formed of aluminum, or aluminum alloy, any of the oxides, nitrides, or oxynitrides thereof is suitable for the protective films  93   a  and  93   b . For copper, silver, and alloys thereof, by contrast, the use of the oxides as the protective films  93   a  and  93   b  is not preferable because of the poor corrosion resistivity thereof. 
     The lower protective film  93   a  may be formed through any of three processes described below. 
     A first process for forming the lower protective film  93   a  involves oxidizing the upper surface of the lower refractory metal film  91  through plasma modification or O 2  annealing after cleaning the upper surface, and depositing the core metal film  92  on the oxidized surface. The oxidized surface of the lower refractory metal film  91  provides oxygen for the bottom portion of the core metal film  92 , and thereby completes the lower protective film  93   a  of an oxide of the core metal film  92 . 
     A second process for forming the lower protective film  93   a  involves reactive sputtering with a sputtering gas including O 2 , N 2 , or N 2 O gas as well as Ar gas at the initial deposition stage of the core metal film  92 . This achieves deposition of the lower protective film  93   a  of an oxide, nitride, or oxynitride of the core metal film  92 . After the completion of the lower protective film  93   a , the sputtering gas is then switched to pure Ar gas to deposit the core metal film  92 . 
     A third process for forming the lower protective film  93   a  involves depositing the metal used as the core metal film  92 , and oxidizing or nitrizing the deposited metal through O 2  plasma treatment, N 2  plasma treatment, or annealing in an oxidizing atmosphere. The oxidizing or nitrizing is followed by deposition of the core metal film  92 . 
     The upper protective film  93   b  may be formed by oxidizing or nitrizing the upper surface portion of the core metal film  92  through O 2  plasma treatment, N 2  plasma treatment, or annealing in an oxidizing atmosphere. Alternatively, the upper protective film  93   b  may be formed through reactive sputtering with a sputtering gas including O 2 , N 2 , or N 2 O gas at the final deposition stage of the core metal film  92 . 
     As shown in  FIG. 1E , the metal film stack is then patterned through a photolithography technique using a resist pattern  10  as a mask to form source and drain electrodes  7 ,  8  and data lines (not shown) so that the source and drain electrodes  7 ,  8  overlap the heavily doped amorphous silicon film  62 . The source electrode  7  includes a lower refractory metal layer  71 , a lower protective layer  73   a , a core metal layer  72 , a upper protective layer  73   b , and an upper refractory metal layer  74 , which are respectively formed from the refractory metal film  91 , the lower protective film  93   a , the core metal film  92 , the upper protective film  93   b , and the upper refractory metal film  94 . Correspondingly, the drain electrode  8  includes a lower refractory metal layer  81 , a lower protective layer  83   a , a core metal layer  82 , a upper protective layer  83   b , and an upper refractory metal layer  84 . The patterning of the metal film stack exposes a portion of the heavily doped amorphous silicon film  62  of the semiconductor film stack  6 . 
     After patterning the metal film stack, the side surfaces of the core metal layer  72 , and  82  are then oxidized or nitrized through O 2  plasma treatment, N 2  plasma treatment, or annealing in an oxidizing atmosphere to form side protective layers  73   c , and  83   c . The lower, upper, and side protective layers  73   a ,  73   b , and  73   c  may be collectively referred to as a protective layer  73 . Correspondingly, the lower, upper, and side protective layers  83   a ,  83   b , and  83   c  may be collectively referred to as a protective layer  83 . 
     After the resist pattern  10  is stripped off, as shown in  FIG. 1F , the exposed portion of the heavily doped amorphous silicon film  62  is dry-etched using the source and drain electrodes  7 ,  8  as a mask. It should be noted that the surface portion of the amorphous silicon film  61  may be etched by the dry-etching. This dry-etching forms a channel region  9  to complete an inversely staggered TFT. An etchant used for this dry-etching includes fluorine based chemicals, such as fluorocarbon. The etchant may additionally include chlorine based chemicals. 
     Alternatively, the exposed portion of the heavily doped amorphous layer  62  may etched using the resist mask  10  as a mask. In this case, the resist mask  10  is stripped off after the etching. 
     The protective layers  73 , and  83  effectively avoids the corrosion of the core metal films  72  and  82  during and after the dry-etching using fluorine and/or chlorine based chemicals. The use of fluorine and/or chlorine based chemicals potentially causes corrosion of the sides of the core metal films  72  and  82  during dry-etching. Furthermore, subjecting the device structure to the atmosphere may cause undesirable production of hydrofluoric and/or hydrochloric acids through reaction of residual fluorine and/or chlorine based chemicals and moisture of the atmosphere, and the produced acids potentially corrodes the core metal films  72  and  82 . However, the protective layers  73 , and  83 , which is resistive against chemicals, effectively prevent the core metal films  72 , and  83  from being corroded. 
     In addition, the protective layers  73 , and  83 , which are disposed between the core metal films and the refractory metal films, effectively prevent the undesirable reaction therebetween, and thereby improve the heat resistance of the interconnections. Improvement of the heat resistance is of much importance for implementing the remaining fabrication processes, typically including heat treatment for stabilizing transistor characteristics, passivation using a plasma chemical vapor deposition, and so forth. 
     In another alternative embodiment, as shown in  FIG. 2A , the metal film stack of the refractory metal film  91 , the lower protective film  93   a , the core metal film  92 , the upper protective film  93   b , and the upper refractory metal film  94  are deposited before patterning the semiconductor film stack  6 . As described below, the metal film stack and the semiconductor film stack  6  are then patterned using a single photolithography process. The fabrication process in this embodiment preferably reduces the number of necessary photolithography steps. 
     In this embodiment, as shown in  FIG. 2B , after depositing the metal film stack, the resist pattern  10  is formed thereon through a photolithography technique using a gray tone mask so that the resist pattern  10  has a thinner portion  110 . 
     The metal stack is then patterned with the resist pattern  10  used as a mask to expose a portion of the semiconductor film stack  6 . After patterning the metal stack, the side surfaces of the core metal film  92  are then oxidized or nitrized through O 2  plasma treatment, N 2  plasma treatment, or annealing in an oxidizing atmosphere to form side protective films  93   c.    
     After forming the side protective layers  93   c , as shown in  FIG. 2C , the semiconductor film stack  6  is then etched with an etchant gas including fluorine based chemicals, such as fluorocarbon, using the resist pattern  10  used as a mask. The etchant may additionally include chlorine based chemicals. As is the case of the protective layers  73  and  83  described before, the protective films  93   a ,  93   b , and  93   c  effectively avoids corrosion of the core metal film  92  resulting from the fluorine and/or chlorine based chemicals. 
     After etching the semiconductor film stack  6 , as shown in  FIG. 2D , the resist pattern  10  is subjected to ashing to remove the top portion of the resist pattern  10 . This ashing exposes a portion of the upper refractory metal film  94  to form a pair of separated resist patterns  210 . 
     As shown in  FIG. 2E , the metal film stack is then patterned to form the source and drain electrodes  7 , and  8 . This patterning exposes a portion of the heavily doped amorphous silicon film  62  of the semiconductor film stack  6 . After patterning the metal film stack, the side surfaces of the core metal layers  72  and  82  are then oxidized or nitrized through O 2  plasma treatment, N 2  plasma treatment, or annealing in an oxidizing atmosphere to form the protective layers  73   c  and  83   c.    
     The resist patterns  210  is then stripped off as shown in FIG.  2 F. 
     As shown in  FIG. 2G , the exposed portion of the heavily doped amorphous silicon film  62  is dry-etched using the source and drain electrodes  7 ,  8  as a mask. The etchant may additionally include chlorine based chemicals. This dry-etching forms a channel region  9  to complete an inversely staggered TFT. It should be noted that the surface portion of the amorphous silicon film  61  may be etched by this etching. An etchant used for this dry-etching includes fluorine based chemicals, such as fluorocarbon. As mentioned above, the protective layers  73  and  83  are effective for avoiding corrosion potentially caused by fluorine and/or chlorine based chemicals. 
     In concludion, the aforementioned method for fabricating interconnections effectively improves corrosion resistance through forming the protective layers  73  and  83  around the core metal layers  72  and  82 . The protective layers  73  and  83 , which are disposed between the core metal layers  72 , and  82  and the refractory metal layers  71 ,  81 ,  74 , and  84 , are also effective for improving heat resistance of the interconnections. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the scope of the invention as hereinafter claimed.