Abstract:
A novel method for the activation of semiconductor substrates for highly selective electroless copper plating in multilayer interconnect metallization lines and vias/contact holes has been developed. A copper-seeded polysilicon layer is provided over the substrate to facilitate growth of copper into the vias. Subsequent rinsing and chemical-mechanical polishing processes allow removal of overgrowth of copper and the polysilicon layer to achieve overall smooth topography of the copper surface and the insulating layer surface of the substrate.

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/SG99/00076 which has an International filing date of Jul. 14, 1999, which designated the United States of America. 
    
    
     BACKGROUND OF THE INVENTION FIELD OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of fabrication used for semiconductor integrated circuit devices and more specifically to a method of activation prior to electroless copper plating for achieving highly selective complete interconnect metal line and via/contact hole fill. 
     2. Description of Related Art 
     As semiconductor integrated circuits are constantly being scaled down to the deep submicron regime, multiple layers of interconnections are thus required so as to meet the increase in the packing density. Vias/contact holes between successive layers of interconnections have had to have almost near vertical sidewalls with high aspect ratios and have had to be placed at smaller spacing intervals to fulfill such requirement. Subsequent metallization processes are thus necessary to fill the vias/contact holes completely without voids with high conductivity metal such as copper. 
     Various techniques of deposition of copper have been known which, in general, include electroplating, electroless plating, sputtered chemical and physical vapor deposition. Among these, the electroless method appears preferable based on considerations like the cost of the fabrication process, the ease of execution of the process and the complete filling capability of the process. However, in general, prior to such electroless plating, an adhesive/catalytic seeding layer like Pd or CVD (Chemical Vapor Deposition) Cu has had to be laid down to facilitate subsequent plating and high selectivity of the plating is very much desired such that a majority of the plated Cu is deposited in the interconnect metal lines and the vias/contact holes. 
     In U.S. Pat. No. 5,167,992 (issued to Charles W. C. Lin et al on Dec. 1, 1992), a selective electroless plating for metal conductors disposed on a dielectric surface is disclosed. The method includes removing a carbonized film from the dielectric surface by a plasma discharge, acid treating the metal conductor in a first acid solution, activating the metal conductor, and deactivating the dielectric surface in a second acid solution, followed by electroless plating to overcoat a metal on the metal conductor only. 
     Also, in U.S. Pat. No. 5,017,516 (granted to Andreas M. T. P. van der Putten), a method of performing electroless plating selectively in contact holes through selective nucleations using a low concentration of PdCl 2  is disclosed. 
     U.S. Pat. No. 5,443,865 entitled “Method for Conditioning a Substrate for Subsequent Electroless Metal Deposition” (issued to Tisdale, et al on Aug. 22, 1995), discloses a method whereby electrochemically generated reducing agents are adsorbed by a substrate which is then in contact with a seeding medium to deposit Palladium seed for subsequent electroless and electrolytic metallization. 
     U.S. Pat. No. 4,869,930 entitled “Method for Preparing Substrates for Deposition of Metal Seed from An Organometallic Vapor for Subsequent Blectroless Metallization” (granted to Clarke, et al on Sep. 26, 1989) describes a process whereby a volatile organometallic compound is allowed to react with active chemical sites initially adsorbed on a substrate to form a species of the metal constituent of the compound which may then be transformed into a free metal to serve as a seed for subsequent electroless deposition. When selective deposition is desired, a resist masking layer is used prior to forming the seed layer. 
     U.S. Pat. No. 5,674,787 entitled “Selective Electroless Copper Deposited Interconnect Plugs for USLI Applications” discloses a method utilising electroless copper deposition to selectively form encapsulated copper plugs in vias of a semiconductor, to connect conductive regions on the semiconductor. 
     With the above prior art techniques, growth of plugs has been achieved by activation of a surface of a barrier material in a base of the vias followed by electroless deposition of metal on the activated surface. In the case of U.S. Pat. No. 5,674,787 the barrier material is a diffusion barrier material such as titanium nitride (TiN) and the deposited metal is copper. In order to activate the TiN the semiconductor is treated in an activation solution which includes a surfactant to facilitate seeding of Cu on the TiN. Use of a surfactant such as polyethylene glycol or Triton X-100 can, however, introduce hydrocarbons into the vias which may effect reliable operation of the plug. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a new method of forming plugs in a via/contact hole in a semiconductor without requiring selective activation of a barrier surface in a base of the hole. 
     In accordance with the invention there is provided a method of fabrication for a semiconductor integrated circuit device, including: 
     providing a semiconductor substrate having an insulating layer with a contact hole formed therein and a diffusion barrier provided on sidewalls and a base of the hole; 
     forming a film of polysilicon on a surface of the substrate in which the hole is provided; 
     subjecting the said substrate to an activation aqueous solution; 
     forming an activation layer comprising copper seedings on the said polysilicon; 
     rinsing said semiconductor substrate to remove said aqueous solution; 
     depositing copper on the activation layer by electroless copper deposition such the copper forms into said hole to fill the contact hole and thereby provide a plug without voids; 
     rinsing said semiconductor substrate; and 
     applying chemical-mechanical polishing to remove said polysilicon and excess copper external of said hole to thereby provide the substrate with a smooth topography. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is more fully described, by way of non-limiting example only, with reference to the drawings, in which: 
     FIGS. 1A-1E are cross-sectional schematic representations illustrating the method of one embodiment of the present invention. 
     FIGS. 2A-2E are cross-sectional schematic representations illustrating the method of a second embodiment of the present invention. 
     FIGS. 3A-3E are cross-sectional schematic representations illustrating the method of a third embodiment of the present invention. 
     FIGS. 4A-4E are cross-sectional schematic representations illustrating the method of a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1A-1B, the semiconductor substrate  11  may be p- or n-type single crystal silicon, but is not limited thereto. An insulating layer  12  may be a high dielectric material like silicon dioxide (SiO 2 ), BPTEOS, spin-on-glass (SOC) and SABPSG having a thickness of between about 4000 and 12000 angstroms. A physical vapour deposition (PVD) or chemical vapour deposition (CVD) polysilicon  13  with thickness in the range of 500 and 1000 angstroms may be formed on the insulating layer. Contact holes/vias  15  having a width between about 3500 and 5000 angstroms may be formed in the dielectric insulating layers by conventional plasma etching processes and reactive ion etching (RIE) to reach the silicon substrate. A difflision barrier  14  may be Titanium nitride (TiN), Tungsten nitride (WN) and Tantalum nitride (TaN) deposited by sputtered, collimated, CVD, MOCVD, PVD and IMP techniques. The polysilicon layer is preferably doped with Boron, Arsenic or P +  and the dopant concentration is at least 1E15 cm −3  so that the layer has some degree of conductivity to facilitate plating of, for example, TiN. An activation approach using an aqueous solution containing HF of concentration between about 1% and 10% and Cu of concentration between about 100 ppm and 500 ppm at a temperature between about 25 and 80° C. may be used to outplate copper  16  on the polysilicon  13  by immersing the semiconductor substrate and surface layers ( 11 - 15 ) into the above-mentioned aqueous solution for a duration of between about 15 and 60 s and followed by subsequent thorough rinsing with ultrapure water. Next, copper  17  is deposited on previously deposited copper  16  and in the contact hole/vias  15  by electroless plating from a solution comprising of a cupric ion source (eg. CuSO 4 .5H 2 O), a reducing agent (eg. HCHO, DMAB), a complexing agent (eg. BDTA), a pH adjuster (eg. TMAH), a surfactant (eg. polyethylene glycol) and additives (eg. KCN, TEAC) at a temperature between about 25 and 80° C. Electroless Cu plating is continued for a duration sufficiently long such that an overgrowth of copper of between about 500 and 2000 angstroms above the surface plane of the contact hole/via is obtained. Subsequent rinsing with ultrapure water, which preferably has a resistivity of about 18.2MΩcm −1 , results in peeling off of Cu deposits from top of the insulating layer  12  only, thus allowing highly selective deposition in the via/contact hole. Finally, a CMP (Chemical-Mechanical-Polishing) process, which uses a polishing slurry comprising of alumina, ultrapure water and sulfuric acid in the pH range of between about 2 and 3, is performed to remove the excess copper  17  over the contact hole/ via and the polysilicon  13  to obtain coplanar topography of the copper surface  17 A and the insulating layer surface  12 A. Complete removal of the polysilicon layer ensures there is no electrical connection between the individual filled holes. 
     Referring to FIGS. 2A-2E, a second embodiment of the invention is illustrated, wherein a contact hole in contact with a multilayer metallization pattern is completely filled without voids with copper  26  using the activation method of the present invention. The semiconductor substrate  18  may be p- or n-type single crystal silicon. The insulating layer  19  may be a high dielectric material like silicon dioxide (SiO 2 ), BPTBOS, spin-on-glass (SOC) and SABPSG having a thickness of between about 4000 and 12000 angstroms. A metallization pattern  22 , with diffusion barrier layer  21 , is formed in the insulating layer  20  by conventional integrated circuit processes. A PVD or CVD polysilicon  24  with thickness in the range of between about 500 and 1000 angstroms is formed on the insulating layer  20 . The polysilicon layer may be doped, as described with reference to the first embodiment. Contact hole  23  is formed in the insulating layer  20  by conventional reactive ion etching (RIB) or plasma etching to reach the multilayer metallization pattern  22 . An activation approach using an aqueous solution containing HF of concentration between about 1% and 10% and Cu of concentration between about 100 ppm and 500 ppm at a temperature between about 25 and 80° C. may be used to outplate copper  25  on the polysilicon  24  by immersing the semiconductor substrate and surface layers ( 18  and  24 ) into the above-mentioned aqueous solution for a duration of between about 15 and 60 s and followed by subsequent thorough rinsing with ultrapure water. Next, copper  26  is deposited on  25  and in the contact hole/via  23  by electroless plating from a solution comprising of a cupric ion source (eg. CuSO 4 .5H 2 O), a reducing agent (eg. HCHO, DMAB), a complexing agent (eg. BDTA), a pH adjuster (eg. TMAH), a surfactant (eg. polyethylene glycol) and additives (eg. KCN, TBAC) at a temperature between about 25 and 80° C. Electroless Cu plating is continued for a duration sufficiently long such that an overgrowth of copper of between about 500 and 2000 angstroms above the contact hole/via is preferred. Subsequent rinsing with ultrapure water results in peeling off of Cu deposits from top of the polysilicon  24  only, thus allowing highly selective deposition in the vias/contact holes. Finally, a CMP (Chemical-Mechanical-Polishing) process, which uses a polishing slurry comprising of alumina, ultrapure water and sulfuric acid in the pH range of between about 2 and 3, is performed to remove the excess copper  26  over the contact hole/via and to completely remove the polysilicon  24  to obtain coplanar topography of the copper surface  26 A and the insulating layer surface  20 A. 
     Referring to FIGS. 3A-3E, a third embodiment of the present invention is demonstrated, wherein a dual damascene structure  30  is completely filled without voids with copper  33  using the activation method of the present invention. The semiconductor substrate  27  may be p- or n-type single crystal silicon. The insulating layer  28  may be a high dielectric material like silicon dioxide (SiO 2 ), BPTEOS, spin-on-glass (SOG) and SABPSG having a thickness of between about 4000 and 12000 angstroms deposited on the semiconductor substrate  27 . The diffusion barrier  31  may be Titanium nitride (TiN), Tungsten nitride (WN) and Tantalum nitride (TaN) deposited by sputtered, collimated, CVD, MOCVD, PVD and IMP techniques. The PVD or CVD polysilicon  29  with thickness in the range of 500 and 1000 angstroms is formed on the insulating layer  28 , preferably with dopants as referred in above. An activation approach using an aqueous solution containing HF of concentration between about 1% and 10% and Cu of concentration between about 100 ppm and 500 ppm at a temperature between about 25 and 80° C. may be used to outplate copper  32  on the polysilicon  29  by immersing the semiconductor substrate and surface layers ( 27 - 32 ) into the above-mentioned aqueous solution for a duration of between about 15 and 60 s and followed by subsequent thorough rinsing with ultrapure water. Next, copper  33  is deposited on  32  and in the dual damascene structure  30  by electroless plating from a solution comprising of a cupric ion source (eg. CuSO 4 .5H 2 O), a reducing agent (eg. HCHO, DMAB), a complexing agent (eg. BDTA), a pH adjuster (eg. TMAH), a surfactant (eg. polyethylene glycol) and additives (eg. KCN, TEAC) at a temperature between about 25 and 800° C. Electroless Cu plating is continued for a duration sufficiently long such that an overgrowth of copper of between about 500 and 2000 angstroms above the dual damascene structure is obtained. Subsequent rinsing with ultrapure water results in peeling off of Cu deposits from top of the polysilicon  29  only, thus allowing highly selective deposition to occur. Finally, a CMP (Chemical-Mechanical-Polishing) process, which uses a polishing slurry comprising of alumina, ultrapure water and sulfuric acid in the pH range of between about 2 and 3, is performed to remove the excess copper  33  over the dual damascene structure and the polysilicon  29  to obtain coplanar topography of the copper surface  33 A and the insulating layer surface  28 A. 
     Referring to FIGS. 4A-4E, a fourth embodiment of the present invention is illustrated, wherein a via in contact to the active regions of an integrated circuit transistor like a MOSFET is completely filled without voids with copper using the activation method of the present invention. A device with gate oxide  42 , polysilicon  41 , source and drain regions  40  is built on the semiconductor substrate  34  by conventional processes and is surrounded by a thick field oxide region  35 . Contact holes  38  are formed in the insulating layer  39  by conventional plasma etching and reactive ion etching. The insulating layer  39  may be a high dielectric material like silicon dioxide (SiO 2 ), BPTEOS, spin-on-glass (SOC) and SABPSG having a thickness of between about 4000 and 12000 angstroms deposited on the semiconductor substrate  34 . The diffusion barrier  37  may be Titanium nitride (TiN), Tungsten nitride (WN) and Tantalum nitride (TaN) deposited by sputtered, collimated, CVD, MOCVD, PVD and IMP techniques. The PVD or CVD polysilicon  36  with thickness in the range of between about 500 and 1000 angstroms, preferably with dopants as described above, is formed on the insulating layer  39 . An activation approach using an aqueous solution containing HF of concentration between about 1% and 10% and Cu of concentration between about 100 ppm and 500 ppm at a temperature between about 25 and 80° C. may be used to outplate copper  43  on the polysilicon  36  by immersing the semiconductor substrate and surface layers ( 34 - 38 ) into the above-mentioned aqueous solution for a duration of between about 15 and 60 s and followed by subsequent thorough rinsing with ultrapure water. Next, copper  44  is deposited on  43  and in the contact holes  38  by electroless plating from a solution comprising of a cupric ion source (eg. CuSO 4 .5H 2 O), a reducing agent (eg. HCHO, DMAB), a complexing agent (eg. BDTA), a pH adjuster (eg. TMAH), a surfactant (eg. polyethylene glycol) and additives (eg. KCN, TEAC) at a temperature between about 25 and 80° C. Electroless Cu plating is continued for a duration sufficiently long such that an overgrowth of copper of between about 500 and 2000 angstroms above the contact holes is preferred. Subsequent rinsing with ultrapure water results in peeling off of Cu deposits from top of the polysilicon  36  only, thus allowing highly selective deposition to occur. Finally, a CMP (Chemical-Mechanical-Polishing) process, which uses a polishing slurry comprising of alumina, ultrapure water and sulfuric acid in the pH range of between about 2 and 3, is performed to remove the excess copper  44  over the contact holes and the polysilicon  36  to obtain coplanar topography of the copper surface  44 A and the insulating layer surface  39 A. 
     As can be appreciated the invention provides for convenient and reliable formation of plugs in a semiconductor substrate, without the more complicated prior art techniques of selectively seeding only a base of the vias and the use of a surfactant in the activation solution, thereby simplifying fabrication and minimising the level of hydrocarbon contamination in the vias. 
     While the invention has been particularly shown and described with respect to the preferred embodiments set forth above, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and the scope of the invention.