Abstract:
Removing metal from a semiconductor substrate by dissolving ions of the metal into an electrolyte, comprising the steps of: applying a voltage across a polishing pad and the substrate, while an electropolishing electrolyte is dispensed at an interface of the substrate and the polishing pad, and while pooling the electrolyte about the substrate by the polishing pad.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of provisional application serial No. 60/249,995 filed Nov. 20, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to a method for planarization of a substrate by a combination of electropolishing and chemical mechanical planarization, CMP.  
         BACKGROUND OF THE INVENTION  
         [0003]    A semiconductor substrate comprises, a silicon wafer on which is deposited successive layers of materials. A dielectric layer is deposited, and is provided with multiple trenches that are recessed in the dielectric layer. A thin barrier film, for example, tantalum, tantalum nitride or a tantalum alloy, is deposited to cover the surface of the dielectric layer including the trenches, which provides a barrier to migration of metal ions into the dielectric layer. A metal layer is deposited to fill the trenches with metal to provide electrical circuit interconnects in the trenches.  
           [0004]    During a CMP operation, the substrate is polished by a rotating polishing pad and a polishing fluid at an interface of the substrate and the polishing pad. The polishing operation removes metal by a combination of, abrasion applied by the polishing pad, and chemical reaction of the metal with the polishing fluid.  
           [0005]    A single step polishing operation, or two step polishing operations, may be performed to provide complete removal of the metal layer, and complete removal of the barrier film, without spots of residual metal on the polished surface, while leaving all of the trenches filled with metal at a level that is planar with the smooth planar polished surface.  
           [0006]    U.S. Pat. No. 6,056,864 discloses removing metal from a metal layer on a semiconductor substrate by dissolving ions of the metal into an electopolishing electrolyte, followed by performing CMP to remove a remaining thickness of metal. A faster rate of removal of the metal is attained as compared to that attained solely by performing CMP.  
         SUMMARY OF THE INVENTION  
         [0007]    According to the invention, metal is removed from a semiconductor substrate by dissolving ions of the metal into an electrolyte, comprising the steps of: applying a voltage across a polishing pad and the substrate, while an electropolishing electrolyte is dispensed at an interface of the substrate and the polishing pad, and while pooling the electrolyte about the substrate by the polishing pad.  
           [0008]    According to an embodiment of the invention, an apparatus comprises both an electropolishing apparatus and a CMP apparatus. Further according to an embodiment of the invention, an electropolishing electrolyte comprises a CMP polishing composition.  
           [0009]    According to an embodiment of the invention, performance of both electropolishing and CMP on the same apparatus leads to a significant reduction in processing time per substrate. 
       
    
    
       [0010]    Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, according to which;  
       DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a cross section view of a portion of a semiconductor substrate having a dielectric layer provided with multiple trenches, one of which is shown;  
         [0012]    [0012]FIG. 2 is a view similar to FIG. 1 with the substrate having a deposited metal layer to fill each of the trenches with metal;  
         [0013]    [0013]FIG. 3 is a view similar to FIG. 2 with a reduced thickness of the metal layer obtained by electropolishing;  
         [0014]    [0014]FIG. 4 is a view similar to FIG. 3 with complete removal of the metal layer from a polished surface and a metal filled trench with a surface planar with the polished surface;  
         [0015]    [0015]FIG. 5 is a diagrammatic view of an electropolishing apparatus;  
         [0016]    [0016]FIG. 6 is a diagrammatic view of an apparatus that is both an electropolishing apparatus and a CMP apparatus;  
         [0017]    [0017]FIG. 7 is a graph of weight loss of copper metal versus time of electropolishing with an electropolishing electrolyte comprising a CMP slurry; and  
         [0018]    [0018]FIG. 8 is a graph of removed thickness of copper metal versus time of electropolishing with an electropolishing electrolyte comprising a CMP slurry. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 discloses a trench  10  that is etched or otherwise formed in a dielectric layer  11 , typically SiO 2 . The trench is then coated with an adhesion or barrier layer  12 , as shown in FIG. 2, followed by a metal layer  13  which fills the trench  10  with metal. FIG. 3 discloses a reduced thickness  14  of the metal layer  13  obtained by electropolishing FIG. 5 discloses an electropolishing apparatus  20  according to which multiple substrates  22  are placed in a tank  24  filled with electrolyte  26 . The substrates  22  are connected to the positive side of a DC voltage source  28  so that the wafers act as anodes. The negative side of the DC voltage source  28  is connected to a chemically inert electrode  30  that acts as a cathode. The voltage source  28  applies a DC voltage across the anodes and the cathode. A DC electrical current flows through the electrolyte, causing removal of ions of the metal from the substrates  22  and into solution with the electrolyte. The DC voltage source  28  produces the required current density for the metal layer to be reduced in thickness faster than by performing CMP. The electropolishing apparatus  20  is incapable of providing a planar polished surface on the substrates  22 . Accordingly, the substrates  22  are removed from the electropolishing apparatus  20 , and the polished surface is provided on a CMP apparatus, not shown. Because removal and transfer of the substrates  22  consumes time, the processing time for each substrate  22  is extended.  
         [0020]    [0020]FIG. 6 discloses an apparatus that comprises both an electropolishing apparatus and a CMP apparatus. Each semiconductor substrate  22  that is mounted to the apparatus for electropolishing and CMP, serves as an anode by having a conducting portion thereof being connected to a positive side of a DC voltage source  28 . A cathode is provided by a conducting portion of a polishing pad  30  connected to a negative side of the DC voltage source  28 . Electrolyte  26  is applied by a dispenser  27  at the interface of the substrate and the polishing pad  30 . The substrate  22  is held by an electrically insulating holder, and is positioned in close proximity to the conducting portion of the polishing pad  30 , while maintaining adequate separation between the same to allow for electrolyte flow. A voltage applied by the voltage source  28  generates a current flow through the known electropolishing electrolyte, which causes the metal of the metal layer on each respective substrate to dissolve in the electrolyte. An advantage of the invention is that, the polishing pad  30  itself will pool the electrolyte about the substrate  22  without requiring immersion in a tank, such as, the tank  24 . Electropolishing is performed until substantial reduction of the thickness  24  of the metal layer  15  is attained for each substrate  22  being polished. The removal rate of the metal layer  15  is faster than that resulting from CMP, even while the apparatus of FIG. 6 is not operating to perform CMP during electropolishing.  
         [0021]    The removal rate of the metal layer  15  is faster while the apparatus of FIG. 6, according to another embodiment thereof, performs CMP during electropolishing.  
         [0022]    According to another embodiment of the apparatus disclosed by FIG. 6, a nonconducting portion of the polishing pad  30  contacts the substrate, especially when the nonconducting portion is a polishing surface of the polishing pad  30 , which spaces the conducting portion of the polishing pad  30  away from the substrate  22 . According to an embodiment, the conducting portion of the polishing pad  30  is recessed away from the polishing surface, and is spaced away from the substrate  22  in contact with the polishing surface. A further embodiment of the electrolyte comprises a chemical composition for performance of CMP, as well as, for performance of electropolishing. The removal rate of the metal layer  15  is faster than that resulting from CMP solely. Further, by combining both electropolishing and CMP, the removal rate of the metal layer  15  is faster than that resulting from either electropolishing solely or CMP solely.  
         [0023]    When a substantial reduction in thickness  14  is attained, the apparatus of FIG. 6 operates solely to perform CMP. Metal is removed by abrasion applied by the polishing pad and by chemical reaction with the electrolyte that comprises a CMP polishing composition. Although the removal rate of the metal decreases, the CMP operation without electropolishing provides a smooth, planar polished surface on the substrate that is unattainable by performing electropolishing. Further, the removal rate of metal by electropolishing tends to cause dishing, which refers to unwanted recesses in the metal in the trenches, which are considered as damage to the circuit interconnects. Thus performing CMP without electropolishing obtain complete removal of the metal layer  15 , with minimized dishing.  
         [0024]    CMP without electropolishing is performed by having the DC voltage source turned off. Alternatively, the DC voltage source is reversible in polarity. By performing CMP with the DC voltage source providing a voltage of reversed polarity relative to electropolishing, the current direction is reversed relative to electropolishing, which clears away charged contaminants from the metal layer  15  during CMP. Any metal ions that tend to plate onto the metal layer  15  are removed by CMP.  
         [0025]    CMP is then performed in the presence of the applied electrical field at a downforce of up to about 10 psi to obtain a planar surface. The applied voltage difference will be of such low magnitude as to prevent local hot spots and corrosion on the substrate surface.  
         [0026]    According to an embodiment of the invention, CMP is performed simultaneously with electropolishing, when an electropolishing electrolyte further comprises a CMP polishing compositon. A CMP polishing composition comprises, a metal complexing agent, a metal oxidizing agent and/or a metal corrosion inhibitor. The use of conventional CMP polishing compositions reduces cross-contamination on the tool and the need for multiple rinsing steps. Further, the electrolyte comprising the polishing composition will perform for both electropolishing and CMP. The invention significantly reduces waste and cross contamination when the same apparatus performs electropolishing and CMP.  
         [0027]    A CMP polishing composition may or may not have abrasive particles, oxidizing agents, complexing agents, pH buffers, surfactants and dispersants. Examples of abrasive particles include but are not limited to ceria, alumina, silica, titania, germania, zirconia, diamond, silicon carbide and combinations thereof.  
         [0028]    Exemplary complexing agents comprise, mono- and dicarboxylic aliphatic or aromatic acids and their salts such as malic acid, malates, tartaric acid and tartarates, gluconic acid and gluoconates, citric acid and citrates, malonic acid and malonates, formic acid and formates, lactic acid and lactates, phthalic acid and phtalates. Polyhydroxybenzoic acid and its salts are also used.  
         [0029]    Examples of oxidizing agents comprise, hydrogen peroxide; and iodates, nitrates, carbonates, perchlorates, and/or persulfates of alkali, alkaline earth and rare earth metals.  
         [0030]    Examples of inhibitors include BTA (benzotriazole) and TTA (tolyltriazole) or mixtures thereof. Other inhibitors that can be used are 1-hydroxybenzotriazole, N-(1H-benzotriazole-1-ylmethyl)formamide, 3,5-dimethylpyrazole, indazole, 4-bromopyrazole, 3-amino-5-phenylpyrazole, 3-amino-4-pyrazolecarbonitrile, 1-methyimidazole, Indolin QTS and the like.  
         [0031]    The weight percentages of the complexing agents, oxidizing agents and corrosion inhibitors are adjusted to maximize the electropolishing rate desired.  
       EXAMPLE 1  
       [0032]    This example illustrates the removal rate due to electropolishing copper using a conventional, abrasive free CMP polishing fluid as the electrolyte. A current density of 0.1 Amp per sq. cm. was used during this experiment. Each data point was generated using a copper (Cu) disk, 1.2 cm, available from Johnson Matthey Company, immersed in the CMP polishing fluid for various contact times. The CMP polishing fluid contained about 5% of ammonium hydrogen phthalate, about 1% of iminodiacetic acid, about 0.08% of tolyltriazole and about 1.7% of hydrogen peroxide. The experimental results are tabulated below.  
                                                                     Hold Time   Initial                   (min)   Wt (gm)   Final Wt (gm)   Wt. Loss (gm)   Wt. Loss (A)                                1   1.26   1.2585   0.0015   59529.15       2   1.2386   1.237   0.0016   63452.92       3   1.2442   1.2417   0.0025   99215.25       4   1.2446   1.2413   0.0033   130941.7       5   1.2635   1.2595   0.004   158632.3                  
 
         [0033]    The results shown in FIGS. 7 and 8 indicate that electropolishing of copper utilizing conventional CMP polishing fluids achieves rapid removal of the metal layer from the substrate.  
         [0034]    CMP of copper results in a removal rate of about 2,000 to about 4,000 Angstroms per minute. Thus for copper layer about 15,000 Angstroms in thickness, the average duration is about 5 minutes. Traditionally, electropolishing requires the use of phosphoric acid or phosphate salt electrolytes or other electrolytes used in the plating industry. This choice of electrolyte makes integration of conventional electropolishing with CMP difficult due to cross-contamination and waste handling issues. Using a CMP slurry as the electrolyte enables integration of electropolishing with CMP resulting in a hybrid process that reduces processing time, significantly reduces wastes and yields substrates with highly planar surfaces.  
         [0035]    Embodiments and modifications of the invention in the disclosed embodiments are intended to be covered by the spirit and scope of the appended claims.