Abstract:
A method for making a semiconductor device includes cleaning a semiconductor wafer after a chemical mechanical polishing (CMP) process to remove or reduce particles of copper, a corrosion inhibitor such as triazole, and a copper oxide layer on the copper layer. In order to prepare for plating the copper layer with a layer that functions as a barrier to copper migration or diffusion, the surface of the copper layer and the dielectric layer are treated with an oxidant, a surfactant, and copper-chelating agent. The copper-chelating is preferably a mild acid such as an organic acid. The oxidant is particularly useful in removing the corrosion inhibitor. The barrier layer, preferably conductive, is then plated on the surface of the copper layer. Subsequent interlayer dielectric layers and copper layers follow that can use the same process.

Description:
RELATED APPLICATIONS  
       [0001]     A related, copending application is entitled “Method of Using an Aqueous Solution and Composition Therefor”, by Cooper et al., application Ser. No. 10/430,987, assigned jointly to Freescale Semiconductor and Advanced Micro Devices, and was filed on May 7, 2003.  
         [0002]     A related, copending application is entitled “Method To Passivate Conductive Surfaces During Semiconductor Processing”, by Flake et al., application Ser. No. 10/431,053 assigned jointly to Freescale Semiconductor and Advanced Micro Devices, and was filed on May 7, 2003.  
         [0003]     A related, copending application is entitled “Method Of Forming A Semiconductor Device Having A Diffusion Barrier Stack And Structure Thereof”, by Michaelson et al., application Ser. No. 11/078,236, assigned to the assignee hereof, and was filed on Mar. 11, 2005.  
         [0004]     A related, copending application is entitled “Semiconductor Process and Composition for Forming a Barrier Material Overlying Copper”, by Mathew et al., application Ser. No. 10/650,002, assigned to the assignee hereof, and was filed on Aug. 27, 2003. == 
     
    
     FIELD OF THE INVENTION  
       [0005]     The present invention relates generally to semiconductors, and more particularly, to a method and composition for preparing a copper surface for deposition of a diffusion barrier by electroless plating.  
       BACKGROUND OF THE INVENTION  
       [0006]     In integrated circuits, a dielectric layer is used to provide insulation around the interconnect wiring of the chip. Just as faster interconnect materials such as copper allow a signal to move faster through the chip, decreasing the capacitance factor of the insulating material also allows signals to travel across the interconnect faster because they have less interference with each other. The most common dielectric material is silicon dioxide. However, the semiconductor industry is constantly searching for commercially useful, lower capacitance dielectric materials, commonly referred to as low dielectric constant or low k materials.  
         [0007]     Conventional cobalt (Co) films doped with elements like tungsten (W), molybdenum (Mo), rhenium (Re), etc. are reported to have barrier properties to prevent diffusion of copper into a surrounding dielectric material. This can enable integration of copper with low k materials. Also capping copper with these types of materials can enhance reliability by increasing electro-migration resistance. In order to be successful a very selective deposition of these films is required. Also, formation of the barrier is highly dependent on the condition of the copper surface.  
         [0008]     Chemical mechanical polishing (CMP) has been widely adopted in semiconductor manufacturing processes for planarization of a layer, especially a copper layer on a wafer surface. More specifically, a copper layer is deposited over a dielectric layer to fill openings within the dielectric layer. To remove portions of the copper layer that are not within the openings (i.e., to form interconnects that are electrically isolated from each other), a slurry and a pad are used. The wafer may be rinsed following planarization to remove any unwanted surface defects, or other residuals.  
         [0009]     There may be additives in the CMP solution for inhibiting corrosion such as an azole-based corrosion inhibitor. The azole-based corrosion inhibitor, such as benzotriazole, remains on the wafer after CMP and is included to prevent the copper from oxidizing. However, azole-based corrosion inhibitor may block nucleation sites for electroless deposition on the copper surface. Also, the azole-based corrosion inhibitor can be leached into a cobalt plating bath and can affect the plating process including stopping the plating process altogether. In addition, during the CMP process, copper particles can be smeared and trapped on the dielectric surfaces which could act as catalytic or nucleation centers for, for example, CoWB growth. This will lead to increased leakage after CoWB deposition. In addition, copper oxide on the surface may need to be removed or reduced before plating.  
         [0010]     Therefore, there is a need for a copper preparation process and solution that can reduce azoles and remove copper oxides without increased leakage and without significantly removing the copper itself.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates a cross-sectional view of a portion of a semiconductor wafer after formation of a metal layer.  
         [0012]      FIG. 2  illustrates a cross-sectional view of the portion of the semiconductor wafer of  FIG. 1  after a portion of a metal layer has been removed.  
         [0013]      FIG. 3  illustrates a cross-sectional view of the portion of the semiconductor wafer of  FIG. 2  after formation of a diffusion barrier.  
         [0014]      FIG. 4  illustrates a cross-sectional view of the portion of the semiconductor wafer of  FIG. 3  after formation of a dielectric layer.  
         [0015]      FIG. 5  illustrates a flow chart of method for forming the semiconductor device of  FIG. 4  in accordance with one embodiment of the present invention.  
         [0016]      FIG. 6  illustrates a flow chart of the step for applying the surface preparation solution of  FIG. 5  in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     Generally, the present invention provides, in one form, a method for preparing a semiconductor wafer for deposition of a diffusion barrier after the wafer has been planarized using a CMP process. In one embodiment, the method includes applying a surface preparation solution comprising an organic acid, a surfactant, and an oxidant to a semiconductor wafer after the CMP process. The surface preparation solution may be applied to the wafer as one solution, or may be applied to the wafer as two solutions that are applied separately in a two step process. In a first step of the two step process, a solution comprising an organic acid and a surfactant is applied to the wafer. In a second step, a solution comprising an organic acid and an oxidant is applied to the wafer.  
         [0018]     The surface preparation solution, when applied to a semiconductor wafer after CMP, will remove, or reduce, azole-based corrosion inhibitors such as triazole, surface oxide, and copper particles without removing an excessive amount of copper. Also, the copper that is removed is removed nearly uniformly independent of metal feature size and metal feature density.  
         [0019]      FIG. 1  illustrates a cross-sectional view of a portion of a semiconductor wafer  10 . The semiconductor wafer is processed to produce semiconductor devices having integrated circuits implemented thereon. Semiconductor wafer  10  includes a substrate  12  having an active region. An insulating layer  22  is formed on a surface of the substrate  12 . In the illustrated embodiment, the insulating layer  22  is a gate dielectric layer for implementing a plurality of transistors. A polysilicon layer  14  is formed on the insulating layer  22  and patterned to be, for example, a gate electrode of a complementary metal-oxide semiconductor (CMOS) transistor. An insulating layer  20  is formed over the polysilicon layer  14  and removed in areas where electrical contact to polysilicon layer  14  is needed. One or more interconnect layers comprising metal will be formed over the active circuitry to serve as wiring layers for the integrated circuits. A contact  16  is formed through insulating layer  20  and makes electrical contact with the polysilicon layer  14 . The contact  16  will connect the polysilicon layer to one or more metal layers that will be formed above the polysilicon layer  14 . The contact  16  may be formed from tungsten (W) or some other suitable conductive material. A barrier layer  24  is then formed over the insulating layer  20  and lines the sides and bottom of a trench  18  in the insulating layer  20 . The barrier layer functions as a barrier to electro-migration or diffusion. A metal layer  19  is formed over the barrier layer  24  and fills the trench  18 . In the illustrated embodiment, the metal layer  19  is formed from copper. However, in other embodiments, the metal layer  19  may be formed from another metal such as aluminum. Also in other embodiments, the metal layer  19  may be one of many metal interconnect layers.  
         [0020]      FIG. 2  illustrates a cross-sectional view of the semiconductor device  10  of  FIG. 1  after a portion of the metal layer  19  has been removed using a conventional chemical mechanical polishing (CMP) process. As illustrated in  FIG. 2 , all of metal layer  19  is removed except for the metal filling the trench  18 . There may be additives in the CMP solution for inhibiting corrosion such as an azole-based corrosion inhibitor. After the CMP process is complete, the surface of the semiconductor wafer is cleaned using a method illustrated in  FIG. 5 , which will be discussed later.  
         [0021]      FIG. 3  illustrates a cross-sectional view of the portion of the semiconductor wafer  10  of  FIG. 2  after formation of a diffusion barrier  26 . In the illustrated embodiment the diffusion barrier  26  includes a cobalt (Co) film doped with tungsten (W) and boron (B). Also, the diffusion barrier  26  may include nickel (Ni). Also, the diffusion barrier  26  may be doped with elements like molybdenum (Mo), rhenium (Re), and phosphorus (P). Thus, for example, the diffusion barrier  26  may comprise one or more of CoWP, CoWB, CoWPB, CoReP, CoReB, CoRePB, CoMoP, CoMoB, CoMoPB, NiWP, NiWB, NiWPB, NiReP, NiReB, NiRePB, NiMoP, NiMoB, NiMoPB, and the like The diffusion barrier functions to prevent copper from diffusing into the upper insulating layer  28  ( FIG. 4 ). Also, the diffusion barrier may function to reduce electro-migration.  
         [0022]      FIG. 4  illustrates a cross-sectional view of the portion of the semiconductor wafer  10  of  FIG. 3  after an insulating layer  28  is formed. Following the formation of the insulating layer  28 , one or more additional metal interconnect layers may be formed over the insulating layer  28 . The insulating layer  28  may be patterned to form one or more copper vias for subsequent metal layers if additional metal layers are needed. The metal layers, including metal layer  19  for providing conductors for electrically connecting the circuits in the active layers on the semiconductor device.  
         [0023]      FIG. 5  illustrates a flow chart  50  of a method for forming the semiconductor device  10  of  FIG. 4  in accordance with one embodiment of the present invention. At step  52 , a metal, such as for example copper is formed in a surface of the semiconductor wafer. Preferably, the metal layer is electroplated. After electroplating the copper on a semiconductor device such as a wafer including an integrated circuit, the copper is annealed, and then the surface of the metal is smoothed and polished during a planarizing step  54 . For example, the metal may be planarized using a chemical mechanical polishing (CMP) technique.  
         [0024]     After planarization, the surface may need to be “pre-cleaned” to remove impurities introduced during the CMP process. At step  56  of  FIG. 5 , a “one-step” surface preparation solution for pre-cleaning the surface of the wafer is applied to the semiconductor wafer. The solution may also be applied in two steps as illustrated in  FIG. 6  and discussed later. Generally, the solution comprises organic acids that function as copper-chelating agents, surfactants, and an oxidant. In one embodiment, the organic acids may be carboxylic and the oxidant is a persulfate such as for example ammonium persulfate. In another embodiment, the oxidant may be hydrogen peroxide.  
         [0025]     More specifically, the surface preparation solution includes 20-60 grams/liter malic acid, 20-60 grams/liter citric acid, 20-60 parts-per-million (ppm) of an anionic surfactant such as Zonyl® FSJ or Zonyl® FSP, 20-60 ppm of a nonionic surfactant such as Zonyl® FS300, and 20-60 grams/liter of ammonium persulfate. Zonyl® FSJ, Zonyl® FSP, and Zonyl® FS300 are available from the Dupont Corporation and Zonyl® is a registered trademark of Dupont Corporation. In the one-step method, surfactant Zonyl® FSP is preferred over Zonyl® FSJ because it has been shown to be more stable when mixed with the oxidant ammonium persulfate. The solution is applied by spraying the wafer for about 30 seconds to 300 seconds or more preferably around 120 seconds at temperatures ranging from 20 to 45 degrees Celsius or more preferably around 25 degrees Celsius. The solution may be sprayed on the wafer or the wafer may be immersed in the solution. Note that in other embodiments, the malic or citric acids may be substituted by other water soluble acids such as carboxylic acids, such as tartaric, oxalic acid, etc. Also, in other embodiments, only one surfactant may be used.  
         [0026]     At step  58 , after applying the solution at step  56 , a heated rinse operation may be optionally performed. The wafer is sprayed with de-ionized water that has been heated to about 30 to 70 degrees Celsius for about 30 to 120 seconds. At step  60 , a diffusion barrier is formed on the metal layer after step  58 . The diffusion barrier, such as the diffusion barrier  26  of  FIG. 4 , is formed on the metal layer using a conventional electroless plating technique. In another embodiment, a conductive material may be formed on the metal layer instead of the diffusion barrier. The conductive material is formed to improve electro-migration resistance. Alternately, the conductive material may be formed with a diffusion barrier. Then, at step  62 , an insulating layer, such as insulating layer  28  in  FIG. 4  is formed over the metal layer. At decision step  64 , it is determined if more metal layers are to be formed. If more layers are needed, the flow returns to step  52  and the method is repeated until all of the metal layers, including interconnects between the layers, are formed. When all of the metal layers have been formed on the semiconductor wafer, the flow ends.  
         [0027]      FIG. 6  illustrates a flow chart of a step  56 ′ for applying the surface preparation solution of  FIG. 5  in accordance with another embodiment of the present invention. In  FIG. 6 , the surface preparation solution is applied in two steps after step  54  in  FIG. 5  and instead of step  56 . In the first step, step  72 , a solution comprising organic acids and surfactants are sprayed on the wafer at temperatures ranging from 20 to 45 degrees Celsius or more preferably 25 degrees Celsius for 30 to 180 seconds or more preferably 90 seconds. The organic acids include malic acid and citric acid in the same quantities as described above for  FIG. 5 . Preferably, the surfactant comprises either Zonyl® FSJ or Zonyl® FSP. Note that in the embodiment of  FIG. 5 , Zonyl® FSP is preferred because it is more stable when combined with the ammonium persulfate. In the second step, step  74 , a solution comprising organic acids and an oxidant is sprayed on the wafer at temperatures ranging from 20 to 45 degrees Celsius or more preferably 25 degrees Celsius for 30 to 120 seconds or more preferably 60 seconds. The organic acids include malic acid and citric acid in the same quantities as described above for  FIG. 5 . The oxidant is preferably ammonium persulfate in the quantities described in  FIG. 5 . After step  74 , the flow continues with step  58  in  FIG. 5 .  
         [0028]     The surface preparation solution as described above has been determined to remove the azole-based corrosion inhibitors that are applied as part of a CMP process. Also, surface oxide and copper particles are removed while only removing a small amount of copper. Further, the amount of copper removed is removed nearly uniformly, independent of metal feature size and metal feature density.  
         [0029]     While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true scope of the invention.  
         [0030]     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.