Abstract:
An in-situ method for synthesis of a vapor type of copper or other metal precursor from a solid source of metal in an oxidation state of 1 or greater The solid source is localized above the wafer and its temperature is controlled independently from the wafer temperature. The solid source may be located, for example, in a showerhead. A metal precursor vapor is produced, and this vapor is drawn onto the wafer, allowing deposition to occur on the wafer and a solid thin metal film to form on the wafer. The invention overcomes the problem of low partial pressure of copper precursors in copper CVD.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to methods for chemical vapor deposition of a metal, particularly copper, onto a substrate. 
     BACKGROUND 
     In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are deposited upon the surface of a semiconductor substrate or wafer. The films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via in a semiconductor substrate. The film, passing through the insulative layers of the substrate, provides plugs of conductive material for the purpose of making interconnections across the insulating layers. 
     One well known process for depositing a thin metal film is by chemical vapor deposition (CVD). In CVD, reactant or deposition gas precursors are pumped into proximity to a substrate inside a reaction chamber. The precursors are activated using either thermal energy or electrical energy, and subsequently undergo chemical reactions at the surface of the substrate. This results in one or more reaction by-products, which are deposited on the exposed substrate or wafer surface to form a film. 
     Copper CVD reactions are limited mainly by the low partial pressure of potential copper precursors. In fact, most of the potential precursors have such a low partial pressure that use of a gas mass flow controller (MFC) and vapor draw from a liquid precursor in a vessel is difficult. Instead, direct liquid injection (DLI) of the liquid copper precursor must be used. 
     DLI of a copper precursor, however, is undesirable for several reasons. One reason is that it requires use of two devices: a first device to form a mist or aerosol of the precursor liquid, and a second device to transform the mist into a precursor vapor. Additionally, during this process the stability of the precursor source is generally a problem; the precursor tends to decompose or change composition along the path of its liquid injection and vaporization. Another reason is that control of the deposition rate is difficult in DLI of the precursor liquid. Also, the DLI device may clog if precursor deposition occurs in the device itself. Moreover, a device such as a showerhead, which is supposed to aid in uniform distribution of precursor at the surface of the wafer or substrate, might become hot enough to participate in decomposition of the copper precursor, and the precursor may deposit on the showerhead. Such an occurrence would make control of precursor delivery to the wafer even more difficult. 
     What is needed is a way to generate a copper or other metal precursor vapor to avoid problems that arise with injecting a precursor in a liquid state. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of generating a metal precursor, such as copper precursor, used for CVD within the deposition reactor. This method generates the vapor precursor from simple elements in-situ, and thus reduces or eliminates the need to obtain a sufficient partial pressure to deliver the copper or other metal precursor. The present invention is also directed to a method of delivering the vapor precursor to the wafer to deposit a thin film. To this end, and in accordance with the principles of the present invention, a solid source of a metal in an oxidation state of 1 or greater is heated in the chamber above the substrate to a temperature at which the source will react with a gas injected into the chamber to form a vapor metal precursor. This precursor is directed toward the substrate, which is heated so as to decompose the precursor onto the substrate surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a reaction chamber in the practice of the invention; 
     FIG. 2 is an enlarged view of the encircled area  2  of FIG. 1; 
     FIG. 3 is an alternative embodiment of the showerhead of FIG. 2; 
     FIG. 4 is another alternative embodiment of the showerhead of FIG. 2; and 
     FIG. 5 is yet another alternative embodiment of the showerhead of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     A copper or other metal vapor precursor is generated in-situ in a CVD chamber without the aforementioned problem of low partial pressure of the precursor. The present invention forms and uses the vapor precursor directly above the substrate surface thereby minimizing travel of the vapor and the resulting pressure drop. With flow control, the vapor need not travel through the chamber, so vapor conductive loss is less. By the method of the present invention, the partial pressure of the formed vapor becomes less of a limiting factor in the CVD process. The principles of the present invention may be best understood with reference to the drawings and illustrative examples. 
     With reference to FIG. 1, a reactor  45  used for copper deposition on the surface  28  of a semiconductor substrate  22  by CVD is illustrated. The reactor  45  includes a reaction chamber  46  which encloses a processing space  48 . In the reaction chamber  46 , which is shown as containing a substrate  22  on a susceptor  20 , reactant gases for CVD are delivered to a processing space  48 . A gas delivery system, such as the system described in U.S. Pat. No. 5,628,829 METHOD AND APPARATUS FOR LOW TEMPERATURE DEPOSITION OF CVD AND PECVD REACTIONS, which is assigned to the Assignee of the present invention and is hereby specifically incorporated in its entirety by reference, provides the proper flow and distribution of the gases for the CVD process. Generally, gas delivery systems contain gas-dispersing elements, such as a flat showerhead  50 , in the reaction chamber  46 . The showerhead  50  spreads the entering reactant gases around the processing space  48  of the reaction chamber  46  to ensure a uniform distribution and flow of the gases proximate the susceptor  20  and substrate  22 . Uniform gas distribution and flow is desirable for a uniform and efficient deposition process, a dense plasma, and a uniformly deposited film. 
     According to one embodiment of the present invention, the reactor  45  is equipped with a plasma producing apparatus  51  for exposing the susceptor  20  to a hydrogen/argon plasma either prior or subsequent to placing of the substrate  22  thereon for processing. The apparatus  51  to expose the susceptor  20  to the hydrogen/argon plasma may be the type described in U.S. Pat. No. 5,906,866 entitled PROCESS FOR CHEMICAL VAPOR DEPOSITION FOR TUNGSTEN ONTO A TITANIUM NITRIDE SUBSTRATE SURFACE, which is assigned to the Assignee of the present invention and is expressly incorporated herein in its entirety by reference. The apparatus  51  preferably includes a radiofrequency (RF) generator  52 , capable of generating 450 KHz, which is attached to the showerhead  50 . 
     In the present invention and with reference to FIG. 2, a showerhead  50  apparatus, as is known to one skilled in the art and as previously described, may be used. The showerhead  50 , located above the wafer  22 , has two plates. A top plate  54   a  is made of copper and a bottom plate  54   b  is made of an inert material with respect to the chemical introduced into the showerhead. For example,  54   b  may be made of stainless steel or aluminum. The copper of the top plate  54   a  of the showerhead  50  serves as the main source for the copper that is deposited on the surface  28  of the wafer  22 . If a metal other than copper is to be deposited, such as palladium, chromium, nickel or iron, then top plate  54   a  is made of that metal to provide the source metal for deposition. The showerhead  50  is maintained at a temperature T SH  of about 50° C. to about 500° C., and preferably at about 200° C. to 300° C. 
     A gas mixture that has the property of reacting with copper and forming a volatile copper compound at T SH  is injected at a site  56 , which is the plenum of showerhead  50 . 
     A gas mixture that has the property of reacting with copper and forming a volatile copper compound at T SH  is injected at a site  56 , which is a site  56 , which is a plenum of showerhead  50 . A mixture of an oxidizer gas, a bidendate ligand and a Lewis-based ligand is the gas injected in the plenum of showerhead  50 . The oxidizer gas may be any of Cl 2 , O 2 , HCl, SiCl 4  or H 2 O 2 , and is preferably Cl 2 . The bidendate ligand may be a diketonate gas (i.e., hexafluoroacetylacetone) or acetylacetone (acac), and is preferably Hhfac. The Lewis-based ligand may be any of trimethylvinylsilane (tmvs), cyclooctadiene (cod) or triethyl phosphine (TEP 3 ), and is preferably tmvs. This mixture of gases react with the copper of the top plate  54   a  of the showerhead  50  and forms a volatile copper compound, for example, Cu(hfac) 2  or hfac—Cu—tmvs. The volatile compound passes through the holes  58  of the showerhead  50  and is directed above the surface  28  of the wafer  22 . 
     The wafer  22  is positioned on the heated susceptor  20 , also referred to as a hot plate, which is positioned under the bottom plate  54   b  of the showerhead  50 . The temperature of the wafer  22  is maintained at T W , which is in the range of about 100°-400° C., and preferably about 200° C. T W  is selected to allow the copper precursor to decompose on the wafer surface  28  and form a solid copper film  60 . In this process, the copper on the top plate  54   a  of the showerhead  50  is consumed. Because the method involves metal purification principles, it should be possible to have a very pure vapor of the copper precursor with a minimum of the impurities that could be contained in the solid copper of the top plate  54   a  of the showerhead  50 . 
     As an example of one chemical path that may be used in the present invention to form the vapor copper precursor, solid copper of the top plate  54   a  is brought from a zero oxidation state (Cu 0   solid ) to a higher oxidation state (Cu 1+  or Cu 2+ ) by an oxidation reaction with a gaseous compound (“A”). “A” is the same as the oxidizer gas described above, namely Cl 2 , O 2 , HCl, SiCl 4  or H 2 O 2 , and preferably Cl 2 . If “A” is a chloride molecule, copper chloride in a solid form CuCl (solid)  is formed at T SH  at the surface of the top copper plate  54   a  of the showerhead  50 . The oxidation reaction is as follows:                           
     At the same time or thereafter, a second compound (“B”) that can react with the oxidized copper is introduced in the showerhead plenum  56 . “B” can bind to the oxidized copper and form a volatile compound (“C”). Compound “B” may be any bidendate ligand, such as Hhfac. Compound “B” may also be a mixture of Hhfac or other bidendate ligand and another Lewis-based ligand (“L”) such as tmvs, cod, or TEP 3 . Where “B” includes Hhfac, compound “C” may be volatile Cu 1+ (hfac) or a Cu 2+ (hfac)-based molecule. The reaction is as follows:                           
     The newly generated volatile compound “C” is then drawn through the holes  58  of the showerhead  50  above the wafer  22 . At T W , two molecules of Cu 1+ (hfac) (tmvs) react together to generate copper metal (Cu 0 ) and by-products according to the following disproportionation reaction:                           
     For Cu 2+ , that is, when copper is oxidized to a higher oxidation state, a reduction reaction, for instance with hydrogen, is necessary to generate copper metal (Cu 0 ). The reduction reaction is as follows:                           
     Volatile compound “B” may be either drawn away, such as by pumping, or recycled in the showerhead  50  together with compound “A”. 
     EXAMPLE 1 
     The solid source for copper can be either pure copper (Cu 0 ) or copper in a higher oxidation state, either Cu 1+  or Cu 2+ , such as CuO or Cu 2 O. The shape and state of the solid copper source in showerhead  50  may be a top plate  54 , as shown in FIG. 2; a cylinder  62 , as shown in FIG. 3; a wire  64 , as shown in FIG. 4, or a compressed powder of any shape or a powder  66  in a receptacle  68 , as shown in FIG.  5 . The cylinder  62  and receptacle  68  may be secured within the showerhead  50  by an electrically conductive wire or rod  70 . 
     Heating of the copper source could be thermally assisted, for example, by thermal convection, or radiation from a heating or radiative source. Heating of the copper source may also be electrically assisted. Since copper is a good electrical conductor, the copper source may be connected to an electrical power generator and heated by the Joule effect. The temperature could be controlled through a current flowing in the copper source, as shown in FIGS. 3-5, by connecting wire  64  or wire or rod  70  to the electrical power generator. 
     Copper in a solid form in a zero oxidation state (Cu 0 ) may react with Cl 2 . Copper is thereby oxidized to, for example, a Cu 1+ Cl compound, which is a solid. The reaction is as follows: 
     
       
         Cu 0   (solid) +Cl 2 →Cu 1+ Cl (solid)   (1) 
       
     
     The resulting Cu 1+ Cl (solid)  then reacts with Hhfac and a Lewis-based ligand, tmvs. The products are a hfac-Cu 1+ -tmvs gas and an acid HCl. The reaction is as follows: 
     
       
         Cu 1+ -Cl (solid) +Hhfac+tmvs (gas) →hfac—Cu 1+ —tmvs (gas) +HCl  (2) 
       
     
     The resulting copper-ligand compound decomposes to form solid copper in a zero oxidation state (Cu 0 ) and a copper compound with two hfac and two molecules of tmvs. This disproportionation reaction is as follows: 
     
       
         2hfac-Cu 1+ —tmvs (gas) →Cu 0   (solid) +hfac—Cu—hfac+2tmvs  (3) 
       
     
     EXAMPLE 2 
     Copper may be in a higher oxidation state than in Example 1, that is, Cu 2− . Then, two Hhfac molecules react with Cu 2+ Cl 2  to form hfac—Cu 2+ —hfac+HCl. A protic solvent is then necessary to reduce hfac—Cu 2+ +hfac. Two molecules of the acid Hhfac will be formed as follows: 
     
       
         hfac—Cu 2+ —hfac+H 2 →Cu 0   (solid) +2Hhfac 
       
     
     It should be understood that the embodiments of the present invention shown and described in the specification are only preferred embodiments of the inventors who are skilled in the art and are not limiting in any way. Therefore, various changes, modifications or alterations to these embodiments may be made or resorted to without departing from the spirit of the invention and the scope of the following claims.