Abstract:
A typical integrated-circuit fabrication requires interconnecting millions of microscopic transistors and resistors with aluminum wires. Making the aluminum wires flush, or coplanar, with underlying insulation requires digging trenches in the insulation, and then filling the trenches with aluminum to form the aluminum wires. Trench digging is time consuming and costly. Moreover, aluminum has higher electrical resistance than other metals, such as silver. Accordingly, the invention provides a new “self-trenching” or “self-planarizing” method of making coplanar silver wires. Specifically, one embodiment forms a first layer that includes silicon and germanium; oxidizes a region of the first layer to define an oxidized region and a non-oxidized region; and reacts silver with the non-oxidized region. The reaction substitutes, or replaces, the non-oxidized region with silver to form silver wires coplanar with the first layer. Another step removes germanium oxide from the oxidized region to form a porous insulation having a very low dielectric constant, thereby reducing capacitance. Thus, the present invention not only eliminates the timing-consuming, trench-digging step of conventional methods, but also reduces resistance and capacitance which, in turn, enable faster, more-efficient integrated circuits.

Description:
This application is a Divisional of U.S. Ser. No. 09/030,113, filed Feb. 25, 1998 now U.S. Pat. No. 6,143,655. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention concerns methods of making, or fabricating, integrated circuits, particularly methods of forming silver interconnects. 
     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 or microprocessor. 
     Interconnecting millions of microscopic components typically entails covering the components with an insulative layer, digging small holes in the insulative layer to expose portions of the components underneath, and digging trenches from each hole to one or more other holes in the layer. Then, through metallization, the holes and trenches are filled with aluminum (or an aluminum alloy) to form aluminum interconnects, or wires, between the components. 
     To fill the trenches and holes, fabricators cover the entire insulative layer with a thin layer, or film, of aluminum, and then selectively dissolve, or etch, away the aluminum that lies outside the holes and trenches. The selective etching requires the use of photolithography, a photographic-patterning technique, to form an etch-resistant mask, which protects the aluminum-filled holes and trenches from the etchant. The resulting aluminum wires, intended to be flush, or coplanar, with the surface of the underlying insulative layer, are typically about one micron thick, or about 100 times thinner than a human hair. 
     These conventional interconnection techniques suffer from at least three significant shortcomings. First, because of the difficulty of using photolithography to form high-precision masks on bumpy, uneven surfaces, conventional techniques require digging trenches to ensure that the deposited aluminum wires are flush, or coplanar, with the surface of the underlying insulation. However, digging these trenches is a time-consuming step which ultimately increases the cost of manufacturing integrated circuits. 
     Secondly, conventional techniques produce wires of aluminum, which not only has a higher electrical resistance, but also a lower electromigration resistance than other metals, such as silver. High electrical resistance wastes power, and low electromigration resistance means that, at certain electric current levels, the aluminum readily diffuses, or migrates, into neighboring structures, eventually thinning or breaking the wires and thus undermining reliability of integrated circuits. 
     Moreover, although silver has a 40-percent lower electrical resistivity and at least 100-percent higher electromigration resistance than aluminum, conventional interconnection techniques are impractical for making silver interconnects. In a particular, silver, a noble metal, is immune to most etchants. In fact, attempts to selectively etch a layer of silver covered with an etch-resistant mask usually dissolve the mask, not the silver. Thus, conventional etch-based techniques are wholly inadequate to form silver interconnects. 
     Thirdly, in addition to being time-consuming because of the trench-digging step and ineffective with more desirable metals such as silver, conventional techniques place aluminum wires in relatively high-capacitance insulators, typically solid silicon oxide. High capacitance slows the response of integrated circuits to electrical signals, a great disadvantage in computers and other systems including the integrated circuits. 
     Accordingly, there is not only a need for new interconnection methods that eliminate the trench-digging step, but also methods that yield less-resistive, less-capacitive, and more-reliable silver-based interconnects for faster and more-efficient integrated circuits. 
     SUMMARY OF THE INVENTION 
     To address these and other needs, the present invention provides a new method of making coplanar silver and insulative structures for an integrated circuit. Specifically, one embodiment of the method entails forming a first layer that includes silicon and germanium, and then oxidizing a region of the first layer to define an oxidized region and a non-oxidized region. After oxidation, the method reacts silver with the non-oxidized region. The reaction substitutes, or replaces, the non-oxidized region with silver to form a silver structure flush or coplanar with the first layer. Another step removes germanium oxide from the oxidized region to surround the silver structure in a porous insulative member which reduces capacitance. 
     Thus, the method of the present invention yields a self-planarizing silver structure that not only eliminates the time-consuming, trench-digging step of conventional methods, but also places the low-resistance, highly-reliable silver structure within a capacitance-reducing insulation that allows faster, more-efficient integrated circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following figures are used to describe many aspects of the invention: 
     FIG. 1 is a cross-sectional view of an integrated-circuit assembly; 
     FIG. 2 is a cross-sectional view of the FIG. 1 integrated-circuit assembly after formation of a silicon-germanium layer; 
     FIG. 3A is a cross-sectional view of the FIG. 2 integrated-circuit assembly after formation of an oxidation mask; 
     FIG. 3B is a top view of the FIG. 3A integrated-circuit assembly, showing the oxidation mask; 
     FIG. 4 is a cross-sectional view of the FIG. 3 integrated-circuit assembly after oxidation, removal of the oxidation mask, and formation of a silver layer and a zirconium layer; 
     FIG. 5 is a cross-sectional view of the FIG. 4 assembly after reaction of the silver layer and the silicon-germanium layer. 
     FIG. 6A is a cross-sectional view of the FIG. 5 integrated-circuit assembly after formation of a silicon-germanium layer and an oxidation mask; 
     FIG. 6B is a top view of the FIG. 6A integrated-circuit assembly, showing the oxidation mask; 
     FIG. 7 is a cross-sectional view of the FIG. 6 integrated-circuit assembly after oxidation, removal of the oxidation mask, and formation of a silver layer and a titanium layer; 
     FIG. 8 is a cross-sectional view of the FIG. 7 assembly after reaction of the silver layer and the silicon-germanium layer; 
     FIG. 9 is a cross-sectional view of an integrated-circuit assembly embodying a coplanar hybrid interconnect system; and 
     FIG. 10 is a cross-sectional view of an integrated-circuit assembly embodying a two-level (non-coplanar) hybrid interconnect system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description, which references and incorporates FIGS. 1-10, describes and illustrates 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 implement or 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. 
     The detailed description includes two sets of separate but overlapping embodiments of the invention. The first set of embodiments, illustrated principally with FIGS. 1-8, pertains to a preferred method of making single- and multi-level interconnective structures. The second set of embodiments, illustrated principally with FIGS. 9 and 10, concern hybrid interconnect structures which combine air bridges with structures exemplified in the first set of embodiments. 
     Preferred Method of Making Single- and Multi-Level Interconnects 
     FIGS. 1-8 show a number of preferred integrated-circuit assemblies, which taken collectively and sequentially, illustrate the preferred method of making single- and multi-level interconnects. The method, as shown in FIG. 1, begins with a known integrated-circuit assembly or structure  10 . Assembly  10  includes a substrate  12 . The term “substrate” encompasses a semiconductor wafer as well as structures having one or more 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, preferably transistors  14   a  and  14   b . Transistors  14   a  and  14   b  are covered by an insulative layer  16 , which preferably comprises silicon oxide, nitride, or oxynitride. Layer  16  includes two tungsten vias  16   a  and  16   b  electrically connected to respective transistors  14   a  and  14   b . Although omitted from FIGS. 1-8 for clarity, assembly  10  preferably includes a titanium-nitride diffusion barrier between vias  16   a  and  16   b  and transistors  14   a  and  14   b.    
     Next, as FIG. 2 shows, the preferred method forms a polycrystalline, silicon-germanium (SiGe) layer  18  on insulative layer  16  and over vias  16   a  and  16   b . In the preferred embodiment, silicon-germanium layer  18  consists of 10-60 percent germanium (by weight). Because of the low solubility of silicon in silver, compared to silicon in germanium, as well as to produce a low-dielectric insulator, a high germanium content, such as 50 percent, is preferred. The thickness of the silicon-germanium layer, approximately one micron, matches the thickness of the desired silver structure. 
     Silicon-germanium layer  18  is preferably formed through conventional low-pressure chemical-vapor deposition (LP-CVD), specifically the pyrolysis of silicon hydride (SiH 4 ) and germanium hydride (GeH 4 ) gases at a temperature as low as 400° C. For more details on this technique, which controls the silicon-germanium composition by varying the gas flow rates, refer to the T. J. King article, entitled “Deposition and Properties of Low-Pressure Chemical-Vapor Deposited Polycrystalline Silicon-Germanium Films,” in the Journal of the Electro-Chemical Society (pp. 2235-41, August 1994). This article is incorporated by reference. 
     In FIG. 3A, the method forms an oxidation mask  20  on silicon-germanium layer  18 . This mask defines a narrow, rectangular channel which ultimately defines the periphery of a conductor connecting vias  16   a  and  16   b  and thus transistors  14   a  and  14   b . A top view of mask  20  and its relation to transistors  14   a  and  14   b  and vias  16   a  and  16   b  is shown in FIG.  3 B. 
     To form mask  20 , the method deposits a layer of silicon nitride (Si 3 N 4 ), using low-pressure or plasma-enhanced chemical-vapor deposition. Conventional lithographic techniques define the mask, finally formed through reactive-ion or plasma etching of the silicon-nitride layer. The actual mask thickness, which preferably ranges between 30 and 100 nanometers, depends on the etchant used to remove the unwanted silicon nitride. 
     After completion of mask  20 , the method oxidizes silicon-germanium layer  18 . This oxidization defines regions under mask  20  as non-oxidized regions and unmasked regions as oxidized regions. The oxidized regions contain an extractable or removable germanium oxide, which can be removed to leave a porous insulator having a low dielectric constant. The non-oxidized regions eventually become the metallic or conductive member connecting transistors  14   a  and  14   b . In the preferred embodiment, the oxidation is performed in microwave-enhanced plasma to occur at temperatures as low as 200° C. The results of oxidation are shown as regions  18 ′ in FIG.  3 A. 
     In FIG. 4, the method entails removing mask  20 , preferably through reactive-ion etching, chemical etching, or chemical-mechanical polishing, to leave silicon-germanium layer  18  with a planar surface for depositing silver. Afterward, physical or chemical-vapor deposition forms a 1.5-micron-thick, silver layer  22  on the oxidized and non-oxidized regions of layer  16 . 
     Next, the method forms a 200-nanometer-thick, zirconium layer  24  on layer  22  by physical vapor deposition. In other embodiments, layer  24  is between 20 and 250 nanometers thick and comprises titanium or hafnium, instead of zirconium. Zirconium is preferred because of its lower solubility in silver. Layer  24  not only reduces the temperature and time necessary to complete the next step, but also improves the contact resistance between the silver layer and any subsequent via level. 
     The next step forces a metal-substitution reaction between silver layer  22  and the non-oxidized regions of silicon-germanium layer  18 , thereby substituting, or replacing, the non-oxidized regions of silicon-germanium layer  18  with silver from silver layer  22 . Consequently, a coplanar (or at least partially submerged) metallic structure  22 , consisting essentially of silver with,small amounts of silicon and germanium, forms in silicon-germanium layer  18 . Thus, unlike conventional techniques that require trench-digging before metal deposition to ensure co-planarity of the metallic structures and underlying insulative structures, the present method is “self-planarizing” or “self-trenching.” 
     More specifically, to force the metal-substitution reaction, the method heats the integrated-circuit assembly to 500-600° C. in a vacuum, nitrogen, argon, or other non-oxidizing atmosphere for approximately 60 minutes. Heating urges diffusion of portions of metal layer  22  into adjacent non-oxidized portions of silicon-germanium layer  18  and vice versa. The substitution temperature, that is, the annealing temperature, should be lower than 651° C., which is the eutectic (lowest melting) temperature of the ternary silver-silicon-germanium system for the reaction or substitution to occur. 
     Forming-gas or other hydrogen-containing atmospheres are not used at the 500-600° C. temperatures because of the tendency to saturate zirconium layer  24  with hydrogen and form zirconium hydride. However, if the process is performed below 400° C., a forming-gas atmosphere is feasible. 
     After the metal-substitution reaction, a small amount of germanium may remain on silver structure  22 . Additionally, any silver that remains on the surface of layer  18  may contain zirconium silicide and silicon-germanium agglomerates. These are removed by chemical mechanical polishing or other suitable techniques. 
     Then, to reduce the dielectric constant of oxidized regions  18 ′, germanium oxide in the oxidized regions of silicon-germanium layer  18  is removed using a wet etchant, such as distilled or deionized water. This leaves a porous oxide, insulative structure having a dielectric constant about one half that of conventional solid oxide insulators. (However, this reduction in dielectric constant depends on the original composition of silicon-germanium layer  18 , which in the preferred embodiment is 50 percent germanium.) The lowered dielectric constant ultimately reduces capacitance which, in turn, yields a faster, more efficient integrated circuit. 
     Further fabrication of more interconnection or metallization levels would entail repeating the steps already described and illustrated. For example, to form a second level that includes a via, the preferred embodiment, illustrated in FIG. 6A, forms a second silicon-germanium layer  26  and then an oxidation mask  28  defining a position of the via relative conductor  22 ′. FIG. 6B shows a top view of mask  28  and its position relative conductor  22 ′. After oxidizing silicon-germanium layer  26  and then removing oxidation mask  28 , the method forms a second silver layer  30  and a second zirconium layer  32  as FIG. 7 shows. The integrated-circuit assembly is then heated to force the metal-substitution reaction, thereby substituting a portion of silver layer  30  for non-oxidized region of layer  26 , and forming a substantially coplanar via  30 ′. 
     Notably, unlike conventional techniques that require forming an insulative layer, etching holes to define the vias, and then finally filling these holes with metal to form the vias, this technique proceeds without etching holes and without filling holes. Thus, the present invention provides an “etchless” or “hole-less” method of forming vias. 
     After forming via  30 ′, wet etching removes germanium oxide from regions  26 ′ to form porous low-dielectric insulation around via  30 ′. The resulting integrated-circuit assembly is depicted in FIG.  8 . Subsequent metallizations would follow similarly. (However, one should take care to ensure that both the deposition and oxidation temperatures for subsequent silicon-germanium layers are lower than the substitution temperature, 651° C. in the preferred embodiment, to avoid causing substitution from underlying silver structures.) 
     With completion of the desired number of metallization levels, the preferred method ultimately concludes by heat-treating the integrated circuit for one to six hours at a temperature between 100 and 200° C. This heat treatment, which preferably occurs after packaging the integrated circuit in a protective housing, ensures that the metallic structures have minimum resistivity. 
     Preferred Hybrid Interconnects Incorporating Low-Capacitance Air Bridge 
     FIGS. 9 and 10 show two integrated-circuit assemblies which respectively combine the low-capacitance interconnect structures of FIGS. 3 and 8 with low-capacitance air bridges to yield new hybrid structures. In particular, FIG. 9 shows an integrated-circuit assembly  40  embodying a coplanar hybrid interconnect system which combines two distinct types of low-capacitance interconnect structures: a first interconnect structure  42  (on the left), and a second interconnect structure  43  (on the right). Structure  42 , identical to the previously described structure of FIG. 5, connects transistors  14   a  and  14   b  on substrate  12  via conductive vias (or contacts)  16   a  and  16   b  and silver conductor  22 ′. For reduced capacitance, conductor  22 ′ is embedded in porous insulation  18 ″ which has a low dielectric constant. Structure  42  is preferably formed as detailed through the above-description of FIGS. 1-5. 
     Interconnect structure  43  comprises an air bridge  44 , which in turn comprises an aluminum conductor  44   a  and air cavity  44   b . Conductor  44   a , which is substantially coplanar to conductor  22 ′, electrically connects vias  16   c  and  16   d  and thus electrically connects corresponding transistors  14   c  and  14   d . The presence of air cavity  44   b , which has a unity or near-unity dielectric constant lower than even that of porous insulation  18 ″, minimizes line capacitance between these transistors. 
     Air bridge  44  is built conventionally by forming conductor  44   a  on an underlying, sacrificial support structure (not shown) and then removing the support structure to leave air cavity  44   b . In conventional processing, the sacrificial support structure consists of photoresist or other material which can be easily dissolved or etched away. For further details on this known process, see U.S. Pat. No. 5,510,645 entitled Semiconductor Structure Having an Air Bridge and Method of Forming the Semiconductor Structure and U.S. Pat. No. 5,324,684 entitled Method of Forming Semiconductor Structure Having an Air Bridge, both issued to Fitch et al. and both incorporated herein by reference. 
     Fabrication of air bridge  44  may occur before, after, or concurrent with the fabrication of structure  42 . For example, vias  16   c  and  16   d  may be formed simultaneous with vias  16   a  and  16   b  or started during the metal substitution reaction that forms conductor  22 ′. However, one should take care to avoid exceeding the substitution temperature; otherwise undesired substitution of silver may occur. 
     FIG. 10 shows an integrated-circuit assembly  50  embodying a two-level (non-coplanar) hybrid interconnect system which, like assembly  40 , combines two types of low-capacitance interconnect structures. Assembly  50  includes an air bridge  54  which connects integrated-circuit assemblies  52  and  53 . Assemblies  52  and  53 , both of which are structurally identical to the previously described assembly of FIG. 8, embed aluminum conductors  22 ′ in porous insulation  26 ″. Conductors  22 ″ are connected to vias  30 ′, which are electrically connected via air bridge  54 . 
     Air bridge  54  comprises aluminum conductor  54   a  and air cavity  54   b . Conductor  54   a , which occupies a plane above that of conductors  22 ′, electrically connects vias  30 ′ and thus electrically connects transistors  14   a  and  14   b  to transistors  14   c  and  14   d . Air bridge  54  as well as assemblies  52  and  53  are fabricated according the methods described above. 
     CONCLUSION 
     The present invention overcomes at least three significant shortcomings of conventional interconnection techniques. First, unlike previous techniques that require digging trenches in an insulative layer before metallization to ensure a coplanar metallic structure, the invention provides a self-planarizing or self-trenching metallization process, which directly substitutes metal for select portions of an insulative layer, thereby skipping the time-consuming trench-digging step. Relatedly, the invention forms vias without the conventional steps of etching and filling holes with metal. 
     Second, unlike conventional techniques which are limited to forming interconnects from aluminum, the invention forms interconnects from silver, which has 40-percent lower electrical resistance and at least 100-percent higher electromigration resistance. Thus, the invention yields integrated circuits with superior efficiency and reliability. 
     Third, unlike conventional techniques that bury aluminum wiring in solid high-capacitive insulation, the invention places silver wiring in porous low-capacitive insulation which ultimately yields faster integrated circuits. In short, the invention not only eliminates the time-consuming, trench-digging step, but also yields integrated circuits that operate with superior speed, reliability, and economy. 
     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 embraces all ways of practicing or implementing the invention, is defined only by the following claims and their equivalents.