Abstract:
A typical integrated circuit interconnects millions of microscopic transistors and resistors with aluminum wires buried in silicon-dioxide insulation. Yet, aluminum wires and silicon-dioxide insulation are less attractive than copper wires and polymer-based insulation, which promise both lower electrical resistance and capacitance and thus faster, more efficient circuits. Unfortunately, current techniques cannot realize the promise because copper reacts with the polymer-based insulation to form copper dioxide within the polymer, reducing effectiveness of the copper-polymer combination. Accordingly, the inventor devised a method which uses a non-acid-precursor to form a polymeric layer and then cures, or bakes, it in a non-oxidizing atmosphere, thereby making the layer resistant to copper-dioxidizing reactions. Afterward, the method applies a copper-adhesion material, such as zirconium, to the layer to promote adhesion with a subsequent copper layer. With reduced copper-dioxide, the resulting interconnective structure facilitates integrated circuits with better speed and efficiency.

Description:
This application is a Divisional of U.S. application Ser. No. 09/128,859, filed Aug. 4, 1998 now U.S. Pat. No. 6,284,656. 
    
    
     FIELD OF THE INVENTION 
     The present invention concerns methods of fabricating integrated circuits, particularly methods of forming interconnects from copper and other metals. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, known as a wafer. The components are then wired, or interconnected, together to define a specific electric circuit, such as a computer memory. 
     Interconnecting millions of microscopic components typically entails covering the components with an insulative layer of silicon dioxide, etching small holes in the insulative layer to expose portions of the components underneath, and digging trenches in the layer to define a wiring pattern. Then, through metallization, the holes and trenches are filled typically with aluminum, to form line-like aluminum wires between the components. The aluminum wires are typically about one micron thick, or about 100 times thinner than a human hair. 
     Silicon dioxide and aluminum are the most common insulative and conductive materials used to form interconnections today. However, at sub-micron dimensions, that is, dimensions appreciable less than one micron, aluminum and silicon-dioxide interconnection systems present higher electrical resistances and capacitances which waste power and slow down integrated circuits. Moreover, at these smaller dimensions, aluminum exhibits poor electromigration resistance, a phenomenon which promotes disintegration of the aluminum wires at certain current levels. This ultimately undermines reliability, not only because disintegrating wires eventually break electrical connections but also because aluminum diffuses through surrounding silcon-dioxide insulation to form short circuits with neighboring wires. Thus, at submicon dimensions, aluminum and silicon-dioxide interconnection systems waste power, slow down integrated circuits, and compromise reliability. 
     Copper appears, because of its lower electrical resistivity and higher electromigration resistance to be a promising substitute for aluminum. And, many polymers, for example, fluorinated polyimides, because of their lower dielectric constants, appear to be promising substitutes for silicon dioxide. Thus, a marriage of copper with these polymers promises to yield low-resistance, low-capacitance interconnective structures that will improve the efficiency and speed of integrated circuits. 
     Unfortunately, copper reacts with these polymers to form conductive copper dioxide within these polymers, reducing their effectiveness as low-capacitance insulators and ultimately leaving the copper-polymer promise of superior efficiency and speed unfulfilled. 
     SUMMARY OF THE INVENTION 
     To address these and other needs, the inventor has developed methods of making copper-polymer interconnection systems with reduced copper oxide. Specifically, one method uses a non-acid-based polymeric precursor, such as ester, instead of the typical acid precursor, to form a polymeric layer, and then cures the layer in a reducing or non-oxidizing atmosphere, thereby making the layer resistant to oxidation. Afterward, a zirconium, hafnium, or titanium layer is formed on the polymeric layer to promote adhesion with a subsequent copper layer. With the reduced formation of copper oxide, the method yields faster and more efficient copper-polymer interconnects. 
     Moreover, reducing copper-dioxidation facilitates micron and sub-micron spacing of polymer-insulated copper conductors, which would otherwise require spacings of 10 or more microns. Accordingly, another aspect of the invention is an integrated circuit including at least two conductors which are separated by no more than about one micron of a polymeric insulator. Thus, the inventor provides a method that not only yields copper-polymer interconnects of superior speed and efficiency, but also yields integrated circuits with unprecedented spacing of copper-polymer interconnects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described with reference to the following figures, wherein like numerals re -like features: 
     FIG. 1 is a cross-sectional view of an integrated-circuit assembly at an early fabrication stage, including transistors  14   a  and  14   b,  an insulative layer  16 , contacts  16   a  and  16   b  and a silicon nitride layer  17 ; 
     FIG. 2 is a cross-sectional view of the FIG. 1 integrated-circuit assembly after formation of a polymeric layer  18  over contacts  16   a  and  16   b  and layer  17  and after formation of a trench  20 ; 
     FIG. 3 is a top view of the FIG. 2 integrated-circuit assembly, showing position of the trench relative contacts  16   a  and  16   b;    
     FIG. 4 is another cross-sectional view of the FIG. 3 integrated-circuit assembly, taken along line  4 — 4  to show depth and width of the trench; 
     FIG. 5 is a cross-sectional view of the FIG. 4 assembly after formation of a copper-adhesion layer  22  and a copper layer  24 ; 
     FIG. 6 is a cross-sectional view of the FIG. 5 assembly after removal of excess portions of layers  22  and  24  leaves copper conductor  24 ′; 
     FIG. 7 is a cross-sectional view of the FIG. 6 assembly taken along line  7 — 7  after formation of another polymeric layer  26  having a via hole  28 ; 
     FIG. 8 is a top view of the FIG. 7 assembly, showing position of the via hole relative conductor  24 ′; 
     FIG. 9 is a cross-sectional view of the FIG. 8 assembly after formation of a copper-adhesion layer  30  and copper contact  32 ; 
     FIG. 10 is a cross-sectional view of an integrated-circuit assembly having two copper conductors  24   a ′ and  24   b ′ in a polymeric insulator  18 , with the conductors separated by no more than about one micron; and 
     FIG. 11 is a block diagram of an integrated memory circuit which incorporates the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description, which references and incorporates FIGS. 1-11, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach the invention, are shown and described in sufficient detail to enable those skilled in the art to practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. 
     FIGS. 1-9 show a number of exemplary integrated-circuit assemblies, which taken collectively and sequentially, illustrate an exemplary method of the present invention. The method, as shown in FIG. 1, a cross-sectional view, begins with a known integrated-circuit assembly or structure  11 , which can be within any integrated circuit, an integrated memory circuit, for example. Assembly  11  includes a substrate  12 . The term “substrate,” as used herein, encompasses a semiconductor wafer as well as structures having one or more insulative, semi-insulative, conductive, or semiconductive layers and materials. Thus, for example, the term embraces silicon-on-insulator, silicon-on-sapphire, and other advanced structures. 
     Substrate  12  supports a number of integrated elements  14 , such as transistors  14   a  and  14   b.  Transistors  14   a  and  14   b  are covered by a 100-nanometer-thick, insulative layer  16 , which, for example, comprises a silicon oxide. A silicon-nitride layer  17 , also 100-nanometers thick, covers layer  16 . Extending through layers  16  and  17  are two tungsten vias (or contact plugs)  16   a  and  16   b  electrically connected to respective transistors  14   a  and  14   b.  Although omitted from FIGS. 1-9 for clarity, assembly  11  preferably includes a titanium-nitride (TiN) diffusion barrier between vias  16   a  and  16   b  and transistors  14   a  and  14   b.    
     FIG. 2 shows that the first step of the exemplary method entails forming a polymeric layer  18  atop layer  17 . As used herein, the term “polymeric” encompasses organic polymers, organic oligomers, and organic monomers. Collectively, these may be described as materials containing one or more mer units having a carbon-chain backbone. In addition, “polymeric” encompasses materials having properties similar to those of organic polymers. For instance, organic polymers characteristically have high ductility, low elastic modulus, low compressive-yield strength, and/or low thermal-expansion coefficients. Moreover, as used herein, polymeric encompasses polymer precursors, or bases. 
     In the exemplary embodiment, polymeric layer  18  begins as a non-acid polymeric precursor, that is, a precursor with a pH greater than about 6. Examples of polymeric precursors include a polyimide ester, such as the type sold by E. I. du Pont de Nemours under the tradename PI-2801, or a polymeric-precursor derivative based on fluorine, bromine, or other elements from the fluorine periodic group. Other embodiments form polymeric layer  18  as a foamed polymer, which will generally have a lower dielectric constant than most non-foamed polymers and thus provides further capacitance reductions. An example of a foamed polymer is taught in co-pending and co-assigned patent application Ser. No. 08/892,114 filed Jul. 14, 1997. This application, entitled Method of Forming Insulating Material for an Integrated Circuit and Integrated Circuits Resulting from Same, is incorporated herein by reference. 
     Subsequent to its formation from a non-acid precursor, polymeric layer  18  is cured, with the resultant layer having a thickness of about 500 nanometers. In the exemplary embodiment, the curing procedure has three phases: the first at 125° C. for 15 minutes, the second at 250° C. for 30 minutes, and the third at 375° C. for 30 minutes, with the second and third phases occurring in a non-oxidizing, or reducing, atmosphere to inhibit oxidation. Some exemplary atmospheres are pure hydrogen or mixtures of about 3-to-10% hydrogen with nitrogen, argon, or helium. Starting with the non-acid precursor and curing according to this procedure ultimately reduces the amount of oxidization that occurs in subsequent processing by about four fold. Therefore, unlike conventional polymeric processing, this procedure prevents or reduces increases in the dielectric constant of polymeric layer  18 . 
     The next step, best illustrated in FIG. 3, a top view of the FIG. 2 assembly, is to define the stud and wiring patterns on layer  18 , using for example suitable masking and etching techniques. In the exemplary embodiment, this entails using reactive-ion etching or any selective-material-removal technique to form a trench  20 , which will ultimately define a conductor for connecting vias  16   a  and  16   b.  FIG. 3 shows that trench  20  has ends  20   a  and  20   b  which correspond with respective vias  16   a  and  16   b.  FIG. 4, a cross-section of assembly  11  taken along line  4 — 4  in FIG. 3, shows that trench  20  has a depth  20   d  of about 500 nanometers to expose vias  16   a  and  16   b,  and a width  20   w  of about    250   nanometers. Thus, in this exemplary embodiment, trench  20  has an approximate aspect ratio of 2-to-1. 
     The next step, shown in FIG. 5, entails applying a 10-nanometer-thick adhesion layer  22  on layer  18 , inside and outside trench  20 . The principle purpose of the adhesion layer, which may also be called a seed, contact, or cladding layer, is to promote adherence of a copper layer formed in the next step. Examples of suitable adhesion-layer materials include zirconium, hafnium, titanium. Zirconium, however, is preferable to titanium since it has at least a ten-fold lower solubility in copper. 
     Next, to form a copper conductor, the method fills trench  20 , as FIG. 5 shows, by depositing a 975-nanometer-thick copper layer  24  on layer  22  and inside trench  20 . In some embodiments another 10-nanometer-thick adhesion layer is formed atop copper layer  24  to inhibit copper oxidation during later processing, particularly during curing of subsequent polymeric layers. To reduce copper oxidation, one embodiment performs the adhesion-material and the copper deposition at temperatures less than 450° C., another embodiment at temperatures between about 250 and 350° C., and another at around 300° C. 
     In the 250-350° C. range, a thin layer of Cu 5 Zr (or Beta), tends to initially form at the interface of the copper and zirconium layers, inhibiting diffusion of zirconium into the copper and preventing it from significantly increasing resistance of the copper. A similar effect may be achieved by electroplating the copper and heat-treating the zirconium and copper layers at 250-350° C. for one to two hours, before curing the polymer. Inhibiting the diffusion of zirconium into the copper ultimately yields a copper conductor with a conductivity greater than 95 percent of IACS, or International Annealed Copper Standard. The International Annealed Copper Standard (IACS) is 1.7241 microhm-centimeters at 20 C, or 17.241 nanaohm-meters. Thus, the exemplary copper conductor has a conductivity greater than about 16.4 nanaohm-meters. However, in other embodiments the conductor is in the range of 14 nanohm-meters or greater. 
     Afterwards, excess copper and zirconium on the surface are then removed using a chemical-mechanical polishing technique. FIG. 6 shows the resulting metal conductor  24 ′, which electrically connects vias  16   a  and  16   b  and therefore connects transistors  14   a  and  14   b.  Formation of conductor  24 ′ completes the first level of metallization. 
     FIG. 7 shows that the second level metallization starts with formation and curing of a second polymeric layer  28  on layer  22 . In some embodiments, layer  28  has a composition similar to polymeric layer  18 . Subsequently, the method cures layer  28 , again following a three-phase curing procedure with temperatures similar to those used for layer  18 . The first phase preferably occurs in hydrogen, high-purity forming gas, or a non-oxidizing high-purity argon, and the second and third phases preferably occur in a non-oxidizing atmosphere of high-purity argon. In contrast to conventional curing procedures, this curing procedure, devised primarily for second and subsequent polymeric layers, takes particular care to avoid inciting reactions with and between existing polymeric and copper structures, for example, layer  18  and conductor  24 ′. Therefore, this procedure safeguards the dielectric strength of polymeric layer  18 . 
     After curing polymeric layer  28 , the method defines a stud and/or wiring pattern using any suitable technique. The exemplary embodiment defines a vertical stud, or via, hole  28   a  in layer  28 , using masking and etching techniques. (Hole  28   a  may also be viewed as the cross-section of a trench, defining a wire that intersects, or contacts, copper conductor  24 ′ which lies below.) Other embodiments form hole  28   a  along with other wiring trenches similar to trench  20 , according to conventional dual-damascene techniques, which fill via holes and trenches in one metallization. FIG. 8 shows the position of hole  28   a  relative transistors  14   a  and  14   b,  vias  16   a  and  16   b,  and conductor  24 ′ from the first metallization level. 
     The next steps form a 10-nanometer-thick adhesion layer  30 , similar to adhesion layer  22 , on polymeric layer  28 , as well as a copper layer  32  on layer  28 . (Other embodiments form an additional adhesion layer on copper layer  32 .) Layers  30  and  32 , in the exemplary embodiment, are deposited at approximately 300° C. As already noted, depositing zirconium and copper at this temperature tends to form a thin interfacial layer of Cu 5 Zr (not shown) between layers  30  and  32 , which ultimately enhances conductivity of the resulting conductor. Subsequent annealing also promotes formation of this interfacial layer. 
     After completion of layer  32 , excess copper and adhesion-layer material are removed, for example, by chemical-mechanical polishing. FIG. 9 shows that the resulting integrated-circuit assembly includes a copper via  34 ′ electrically connected to underlying conductor  24 ′ and thus also connected to transistors  14   a  and  14   b.  Subsequent metallizations would follow similarly. 
     In addition to preservation of the dielectric constant of the polymeric layers, the oxidation reductions stemming from the present invention also allow closer spacing of copper conductors in polymeric insulators, particularly micron and sub-micron spacing. FIG. 10, a juxtaposition of two of the assemblies shown in FIG. 6, shows two side-by-side copper structures (for example contact plugs or wires)  24   a ′ and  24   b ′ separated by a distance  25  which is less than about one micron in one embodiment. In various embodiments, distance  25  is less than about 0.75 microns, less than about 0.5 microns, or less than about 0.25 microns. In contrast, conventional techniques for forming copper-polymer interconnections require intrapolymer conductor spacings of 20 or more microns to maintain isolation of the conductors after uninhibited formation of conductive copper dioxide in the polymer between conductors. Thus, the micron and submicron spacings of the present invention provide a dramatic improvement. 
     Exemplary Embodiment of an Integrated Memory-Circuit Incorporating the Copper-Polymer Interconnection System 
     FIG. 11 shows one example of the unlimited number of applications for the copper-polymer interconnections of the present invention: a generic integrated memory circuit  40 . Circuit  40 , which operates according to well-known and understood principles, is generally coupled to a processor (not shown) to form a computer system. More precisely, circuit  40  includes a memory array  42  which comprises a number of memory cells  43 , a column address decoder  44 , and a row address decoder  45 , bit lines  46 , word lines  47 , and voltage-sense-amplifier circuit  48  coupled to bit lines  46 . 
     In the exemplary embodiment, each of the memory cells, the address decoders, and the amplifier circuit includes two or more zirconium-clad copper conductors embedded in polymeric insulation according to the present invention. In addition, connections between the address decoders, the memory array, the amplifier circuit are implemented using similar copper-polymer interconnects. The spacings of these conductors, in some embodiments, follow the micron and submicron spacings noted for FIG.  10 . 
     CONCLUSION 
     The present invention provides a method of forming copper-polymer interconnections systems, which reduces the tendency of polymers to react with copper and form undesirable copper oxides. Formation of these copper oxides would otherwise reduce the effectiveness of the polymers as low-capacitance insulators and thus offset their ability to improve speed and efficiency of integrated circuits. Thus, the present invention ultimately facilitates the fabrication of integrated circuits having superior speed and efficiency. Moreover, because the reduction in oxidation allows closer placement of polymer-insulated copper wires, the invention also facilitates denser integrated circuits, that is, circuits with greater numbers of components in the same space. 
     The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which encompasses all ways of practicing or implementing the invention, is defined only by the following claims and their equivalents.