Abstract:
Methods are disclosed to improve the planarization of copper damascene by the steps of patterning on the copper damascene a photoresist using a reverse tone photo mask or a reverse tone photo mask of the metal lines, removing excess copper by reverse current plating or by dry or wet chemical etching, stripping the photo resist, and a subsequent chemical mechanical planarization of the copper damascene. Lastly a cap layer is applied to the planarized surface. In a variant of the disclosed method a more relaxed reverse tone photo mask of the metal lines is used, which may be more desirable for practical use. These steps provide benefits such as improved uniformity of the wafer surface, reduce the dishing of metal lines (trenches) and pads, and reduce oxide erosion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to methods of fabricating copper damascene wiring on a semiconductor wafer, and more particularly to methods of planarizing, without dishing, damascene copper and similar metals. 
     2. Description of the Related Art 
     Copper damascene wiring is one of the most promising technologies to reduce the RC delay as well as to perform the shrinkage of interconnect structures. Metal filling not only submicron trenches but submicron holes is the key for damascene wiring. To realize the Cu-damascene wiring, new technologies with excellent step coverage such as metal-organic chemical vapor deposition (MOCVD) and electroplating deposition have been studied. After Cu deposition, the task of the polishing process is to remove only the top of the surfaces and not to encroach on the trench of a patterned sample. 
     There are several problems which have to be solved before the method of damascening can be widely used. FIGS. 1 a-c  illustrate typical problems for planarizing damascene using chemical mechanical planarization (CMP) techniques: there is 1) metal remaining on the larger open area(s) and 2) dishing in the wide field regions of the patterns. 
     Prior art FIG. 1 a  shows a cross-section of dual damascene trenches  12 ,  13 ,  14 , and  15  formed in a silicon oxide (SiO 2 )  10  on a silicon semiconductor substrate. A conformal barrier metal layer  16  is deposited on-top of the thus patterned silicon oxide. Layer  16  stops contamination of the SiO 2  by copper  18 , or other metal, which is next deposited over the barrier metal layer, filling the trenches. It can be seen that the surface of the copper damascene is quite uneven as the copper fills the trenches. FIG. 1 b  shows the silhouette of the copper during CMP. In area  11  (plateau) without trenches the copper is higher than in areas with trenches. The result of further planarization is shown in FIG. 1 c . On the plateau there is still copper residue, while the planarization has resulted in dishing of the copper in the trenches. In fact the surface has not been “planarized” as intended. 
     U.S. Pat. No. 5,602,423 (Jain) shows a damascene process using CMP and electroplating which uses an embedded pillar to prevent damage (e.g. dishing, smearing, overetching). However, this reference differs from the proposed invention. 
     U.S. Pat. No. 5,567,300 (Datta et al.) describes a high speed electrochemical metal removal technique for planarization of multilayer copper interconnections in thin film modules. 
     U.S. Pat. No. 5,494,857 (Cooperman et al.) presents a shallow trench isolation planarization method using an etch back process with a reverse tone filler mask and an oxide block in the depressions above the trenches and CMP process. 
     U.S. Pat. No. 5,346,584 (Nasr et al.) discloses a shallow trench isolation planarization method using an etch back process with a reverse tone filler mask and an oxide filler in the depressions above the trenches and CMP process. 
     Other patents bear on the forming of copper damascene but do not appear to bear directly on the proposed invention. U.S. Pat. No. 5,693,563 (Teong) shows a method of forming an etch stop for a copper damascene process with CMP. 
     U.S. Pat. No. 4,954,459 (Avanzino et al.) shows a planarization technique using a reverse mask and etch back only technique. 
     U.S. Pat. No. 4,789,648 (Chow et al.) shows a method for forming metal interconnects that uses CMP of metal (W or Al alloys). 
     U.S. Pat. No. 4,702,792 (Chow et al.) discloses a method for producing coplanar multi-level metal/insulator films on a substrate and for forming patterned conductive lines simultaneously with stud vias. 
     C. Y. Chang, S. M. Sze, in  ULSI Technology , by The McGraw-Hill Company, Inc. copyright 1997, pp. 444-445 discusses damascene and dual damascene techniques. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method to reduce the CMP generated non-uniformity of the wafer surface, the dishing of the copper damascene lines and pads, and the erosion of the silicon oxide layer. 
     Another object of the present invention is to reduce the time for copper polishing in CMP. 
     A further object of the present invention is to reduce metal line erosion and to improve metal continuity. 
     It is yet another object of the present invention to improve the subsequent photo processing by providing a global planar surface. 
     These objects have been achieved by patterning on the copper damascene a photoresist using a reverse tone photo mask or a reverse tone photo mask of the metal lines, removing excess copper by reverse current plating or dry or wet chemical etching, stripping the photo resist, and planarized by chemical mechanical planarization of the copper damascene. In one embodiment a more relaxed reverse tone photo mask of the metal lines is used, which may be more desirable for practical use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a-c , labeled “prior art” are a series of cross-sectional views showing a method of planarizing copper damascene. 
     FIGS. 2 a-h  are a series of cross-sectional views showing one method of the invention of planarizing copper damascene. 
     FIGS. 3 a-g  are a series of cross-sectional views showing another method of the invention of planarizing copper damascene. 
     FIGS. 4 a-d  are a series of cross-sectional views showing a variation of the method depicted in FIGS. 3 a-g.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Damascene is becoming important for future interconnection of devices on silicon semiconductor substrates. One major problem associated with the formation of damascenes is chemical mechanical planarization (CMP). During CMP the complete removal of metal film on silicon oxide (SiO 2 ) is difficult. Although copper (Cu) or aluminum (Al) on silicon oxide can be cleared out by overpolishing, dishing appears. Moreover, erosion of dense metal lines may occur as well. Thus, in this invention, a reverse tone photo mask is proposed which allows the removal of the redundant Cu or Al from the silicon oxide. When the Cu or Al on the silicon oxide is etched off, the margin of the CMP process becomes improved because overpolish is not critical. 
     Referring now to FIG. 2, a preferred method of forming and planarizing copper damascene is described. The cross-section of FIG. 2 a  demonstrates, by way of example, the forming of dual damascene trenches  12 ,  13 ,  14 , and  15  in a silicon oxide (SiO 2 ) layer  10  on a silicon semiconductor substrate (not shown). This is followed by depositing a conformal barrier metal layer  16  on top of the thus patterned silicon oxide to avoid contamination of the SiO 2  layer by the migration of copper into SiO 2 . In FIG. 2 b , a seed layer  17  is deposited on top of the barrier metal layer  16 . The seed layer is a nucleation layer, comprised of metals such as chromium, titanium, copper, aluminum, or gold which causes metals such as copper to adhere more readily to the barrier layer. In FIG. 2 c  copper  18  or other such suitable metal is electroplated on top of the seed layer by means of forward current electroplating sufficient to more than fill the trenches. The seed layer is not shown in this and subsequent FIGS. because it is now considered part of the copper layer. The next step is FIG. 2 d , where photoresist is applied and a reverse tone photoresist mask  19  is formed over the copper layer. 
     In FIG. 2 e  subsequent electroplating by reverse current etches away the copper layer  18  and barrier metal layer  16  from areas like area  20  not covered by photoresist. The areas not covered by photoresist are larger areas of the silicon wafer devoid of metal lines (trenches). Since the etching is isotropic it must be assured that there is a sufficient margin of copper  18  between area  20  and the adjacent trench, here trench  12 . 
     Electroplating of copper or other metals occurs by connecting a first and second metal electrode to the minus and plus side of a voltage source such a battery, respectively. The electrode connected to the minus side is called the cathode and the electrode connected to the plus side is called the anode. Placing both electrodes into a cell with an electrolyte causes electric current to flow from the anode to the cathode. The amount of current flowing is given by Ohm&#39;s law: 
     
       
         I=E/R 
       
     
     where I is the current in amperes, E is the electric potential between the two electrodes in volts, and R is the resistance of the electrolyte in ohms. 
     When the anode is copper, copper ions flow from the anode to the cathode and are deposited there according to the equation: 
     
       
         Cu 2+ +2 e →Cu 
       
     
     This is the plating process. 
     If a wafer is attached to the cathode, copper will be deposited on the wafer as a function of the current flowing. When the polarity of the voltage source is reversed then copper will be removed from the wafer and will be redeposited on the copper electrode according to the equation: 
     
       
         Cu→Cu 2+ +2 e   
       
     
     This is the plating process by reverse current or etching. 
     The following is one example of power supply setpoints for electroplating (deposition) of copper which were used: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 forward current 
                 9.0 
                 Amp for 200 mm wafers 
               
               
                   
                   
                 5.1 
                 Amp for 150 mm wafers 
               
               
                   
                 forward on 
                 95 
                 ms 
               
               
                   
                 forward off 
                 5 
                 ms 
               
               
                   
                   
               
             
          
         
       
     
     The following is one example of power supply setpoints for electroplating (removal) of copper which were used: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 reverse current 
                 4.8 
                 Amp for 200 mm wafers 
               
               
                   
                   
                 2.7 
                 Amp for 150 mm wafers 
               
               
                   
                 reverse on 
                 25 
                 ms 
               
               
                   
                 reverse off 
                 5 
                 ms 
               
               
                   
                   
               
             
          
         
       
     
     peak cathodic current density is: 29 mA/cm 2    
     In the next step, FIG. 2 f , the photoresist is stripped away, exposing the electroplated copper  18 . The copper dual damascene  18  and barrier metal layer  16  are next planarized by CMP. FIG. 2 g  shows the copper partially removed by CMP and FIG. 2 h  shows the copper dual damascene  18  and metal barrier layer  16  after completion of CMP where the copper dual damascene  18  is completely coplanar with the upper surface and without any dishing in the trenches  12 ,  13 ,  14 , and  15 . In a last step the copper dual damascene is sealed with a cap layer  25 , such as nitride or oxynitride. The copper layer  18  can be replaced by a layer of metal from the group comprising gold, aluminum, tungsten, titanium, or silver. 
     Next is described another preferred method of forming and planarizing copper damascene. The cross-section of FIG. 3 a  demonstrates, by way of example, the forming of damascene trenches  32 ,  33 ,  34 , and  35  in a silicon oxide (SiO 2 ) layer  10  on a silicon semiconductor substrate (not shown). This is followed by depositing a conformal barrier metal layer  16  on top of the thus patterned silicon oxide layer  10  to avoid contamination of the SiO 2  layer by the migration of copper into SiO 2 . In FIG. 3 b  a copper layer  18  or other such suitable metal is deposited on top of the barrier metal layer  16  by means such as plating or chemical vapor deposition (CVD), filling the damascene trenches. 
     Now referring to FIG. 3 c , the photoresist is applied and a reverse tone photoresist mask  39  is formed. Note that the same numbers in FIG.  2  and FIG. 3 identify the same component. As shown in FIG. 3 c , there is no photoresist between trenches  32 ,  33 ,  34 , and  35 , because the reverse tone photo mask of the metal lines is used which by definition covers only those parts of the copper layer that are in the trenches. Next, see FIG. 3 d , follows etching the copper layer  18  and barrier metal layer  16  from areas not covered by the reverse tone photoresist mask. Etching can be achieved using a dry anisotropic etch typically with chlorine (CI2) as the etchant or using a wet isotropic etch typically with nitric acid (HNO3) as an etchant. Note that copper has been etched away from areas  20 ,  21 ,  22 , and  23 . In FIG. 3 e  the photoresist has been stripped exposing the deposited copper  18  underneath. The copper damascene  18  and barrier metal layer  16  are next planarized by CMP. FIG. 3 f  shows the copper partially removed during CMP and FIG. 3 g  shows the copper damascene  18  and metal barrier layer  16  after completion of CMP where the copper damascene  18  is completely coplanar with the upper surface and without any dishing in trenches  32 ,  33 ,  34 , and  35 . A last step is the sealing of the copper damascene with a cap layer  25 , such as nitride or oxynitride. The copper layer  18  can be replaced by a layer of metal from the group comprising gold, aluminum, tungsten, titanium, or silver. 
     Another preferred method, and a modification to the method illustrated in FIGS. 3 a-g , is shown in FIGS. 4 a-d . The steps represented by FIGS. 3 a  and  3   b  are the same for this method and precede the steps represented by FIGS. 4 a-d . It may be more practical, simpler and cheaper to provide a relaxed tone photoresist mask of metal lines. “Relaxed” is here defined as a photoresist mask where spaces between the trenches  21 ,  22 , and  23  of FIG. 3 d  are covered if the trenches are separated by less than a critical distance. The critical distance ranges from 0.05 μm to 0.2 μm. In those areas containing trenches and pads the copper layer  18 , therefore, would not be etched away. FIG. 4 a  shows a relaxed tone photoresist mask  49  as compared to the “not relaxed” tone photoresist mask  39  of FIG. 3 c . Next, see FIG. 4 b , follows etching of the copper layer  18  and barrier metal layer  16  from area  20  not covered by photoresist  49 . In FIG. 4 c  photoresist has been stripped away by chemical means, exposing copper layer  18 . Note that the copper is one continuous layer from trench  32  to  35 . FIG. 4 d  shows the copper partially removed during CMP. Since there is little copper on area  20 , there is no problem with copper residue being left on area  20  nor silicon oxide  10  erosion after this step. After completion of CMP the top surface is planar as in FIG. 3 g.    
     Advantages of these disclosed methods are: 
     Reduced time for copper polishing operation (CMP), 
     Reduction of non-uniformity of the wafer surface, 
     Reduced dishing for wide metal lines and pads, 
     Reduced oxide erosion after damascene CMP process, 
     Reduced metal line erosion and improved metal continuity, 
     Relieve CMP pattern density sensitivity, 
     Improved reliability and subsequent photo processing. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.