Abstract:
A typical integrated-circuit fabrication requires interconnecting millions of microscopic transistors and resistors with metal wires. Making the metal wires flush, or coplanar, with underlying insulation requires digging trenches in the insulation, and then filling the trenches with metal to form the wires. The invention provides a new “trench-less” or “self-planarizing” method of making coplanar metal 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 aluminum or an aluminum alloy with the non-oxidized region. The reaction substitutes, or replaces, the non-oxidized region with aluminum to form a metallic wire 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.

Description:
[0001]     This application is a Continuation of U.S. application Ser. No. 10/338,178, filed Jan. 7, 2003, which is a Continuation of U.S. application Ser. No. 09/651,471, filed Aug. 30, 2000, now U.S. Pat. No. 6,504,224, which is a Divisional of U.S. application Ser. No. 09/030,430, filed Feb. 25, 1998, now U.S. Pat. No. 6,121,126, all of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention concerns methods of making, or fabricating, integrated circuits, particularly methods of forming interconnects.  
         [0003]     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.  
         [0004]     Interconnecting millions of microscopic components typically entails covering the components with an insulative layer, etching small holes in the insulative layer to expose portions of the components underneath, and then actually connecting the components through metallization. Metallization is the process of depositing a metal, usually an aluminum alloy, into the small holes and onto the insulative layer to form line-like, metallic paths, or wires, between the components. Photolithography, an optical-patterning technique, defines the particular wiring or interconnection pattern.  
         [0005]     Frequently, several levels of metallization, sometimes as many as six or seven, are necessary to make a particular circuit. Such cases require covering each metallization level with insulation, etching holes to expose portions of an underlying metallization, and then depositing more metal to form additional wires, connecting the exposed portions of the underlying metallization.  
         [0006]     Current interconnection techniques suffer from two significant shortcomings. First, because of limitations in the photolithography used to define metallization patterns, current techniques require digging trenches to ensure that deposited metallic lines 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.  
         [0007]     Secondly, current techniques yield interconnective structures of insulation and metal that are highly capacitive. High capacitance wastes power and slows the response of integrated circuits to electrical signals. Thus, current interconnection techniques stand in the way of faster and more-efficient integrated circuits.  
         [0008]     Accordingly, there is not only a need for new interconnection methods that eliminate the trench-digging step, but also a need for new methods and interconnective structures that yield faster and more-efficient integrated circuits.  
       SUMMARY OF THE INVENTION  
       [0009]     To address these and other needs, the present invention provides a new method of making coplanar metallic and insulative members 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 aluminum or an aluminum alloy with the non-oxidized region. The reaction substitutes, or replaces, the non-oxidized region with aluminum alloy to form a metallic member flush or coplanar with the first layer. Another step removes germanium oxide from the oxidized region to form a porous insulative member having a low dielectric constant.  
         [0010]     Thus, the method of the present invention yields a self-planarizing metallic structure that not only eliminates the time-consuming, trench-digging step of conventional methods, but also places the metallic structure within a low-dielectric insulation that allows faster, more-efficient integrated circuits. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The following figures are used to describe many aspects of the invention:  
         [0012]      FIG. 1  is a cross-sectional view of an integrated-circuit assembly;  
         [0013]      FIG. 2  is a cross-sectional view of the  FIG. 1  integrated-circuit assembly after formation of silicon-germanium layer;  
         [0014]      FIG. 3A  is a cross-sectional view of the  FIG. 2  integrated-circuit assembly after formation of an oxidation mask;  
         [0015]      FIG. 3B  is a top view of the  FIG. 3A  integrated-circuit assembly, showing the oxidation mask;  
         [0016]      FIG. 4  is a cross-sectional view of the  FIG. 3  integrated-circuit assembly after removal of the oxidation mask and formation of an aluminum alloy layer and a titanium layer;  
         [0017]      FIG. 5  is a cross-sectional view of the  FIG. 4  assembly after reaction of the aluminum alloy layer and the silicon-germanium layer.  
         [0018]      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;  
         [0019]      FIG. 6B  is a top view of the  FIG. 6A  integrated-circuit assembly, showing the oxidation mask;  
         [0020]      FIG. 7  is a cross-sectional view of the  FIG. 6  integrated-circuit assembly after oxidation, removal of the oxidation mask, and formation of an aluminum alloy layer and a titanium layer;  
         [0021]      FIG. 8  is a cross-sectional view of the  FIG. 7  assembly after reaction of the aluminum alloy layer and the silicon-germanium layer;  
         [0022]      FIG. 9  is a cross-sectional view of an integrated-circuit assembly embodying a coplanar hybrid interconnect system; and  
         [0023]      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  
       [0024]     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 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.  
         [0025]     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.  
         [0000]     Preferred Method of Making Single- and Multi-Level Interconnects  
         [0026]      FIGS. 1-8  show a number of preferred integrated-circuit assemblies, which taken collectively and sequentially, illustrate the preferred method of the present invention. The method, as shown in  FIG. 1 , begins with a known integrated-circuit assembly or structure  10 . Assembly  10  includes a substrate  12  which supports a number of integrated elements  14 , preferably transistors  14   a  and  14   b . (The term “substrate” encompasses semiconductor wafers as well as structures having one or more insulative, conductive, or semiconductive layers and materials, for example, silicon-on-insulator or silicon-on-sapphire structures.) 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 aluminum vias  16   a  and  16   b  electrically connected to respective transistors  14   a  and  14   b.    
         [0027]     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  is approximately one micron thick, and comprises between 10 and 40 percent germanium. The layer is 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.  
         [0028]     Deposition temperatures lower than 400° C. are possible through plasma-enhanced chemical-vapor deposition. The silicon-germanium film can also be grown using other gases, such as germanium fluoride (GeF 4 ). See, for example, the J. I. Hanna article entitled “Early Stage of Polycrystalline Growth of Ge and SiGe by Reactive Thermal CVD from GeF 4  and Si 2 H 6  (Microcrystalline and Nanocrystalline Semiconductors Symposium, Boston 1994, pp. 877-881, 1995), which is incorporated herein by reference.  
         [0029]     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. 3B .  
         [0030]     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.  
         [0031]     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 plasma enhanced to occur at temperatures below 200° C. The results of oxidation are shown as regions  18 ′ in  FIG. 3A .  
         [0032]     In  FIG. 4 , the method entails removing mask  20 , preferably through chemical etching or chemical-mechanical polishing, to leave silicon-germanium layer  18  with a planar surface. Afterward, physical or chemical vapor deposition forms a 1.5-micron-thick, metal layer  22  on the oxidized and non-oxidized regions of film  16 . In the preferred embodiment the metal is an aluminum alloy that includes 0.3 to 4.0 percent copper (Al—Cu), but in another embodiment, the aluminum alloy also includes 0.3 to 1.6 percent silicon. The copper and silicon additions reduce electro-migration of the aluminum at high current levels.  
         [0033]     Next, the method forms a 200-nanometer-thick, titanium layer  24  on layer  18  by physical vapor deposition. In other embodiments, layer  24  is between 500 and 2500 Angstroms thick and comprises zirconium or hafnium, instead of titanium. Layer  24 , which is optional, reduces the temperature and time necessary to complete the next step.  
         [0034]     The next step forces a metal-substitution reaction between metal layer  22  and the non-oxidized, or desired metallic, regions of silicon-germanium layer  18 . To force this reaction, the method heats or anneals the integrated-circuit assembly to 400° C. in a nitrogen, forming gas, or other non-oxidizing atmosphere for approximately 60 minutes. The substitution temperature, that is, the annealing temperature, should be lower than the eutectic (lowest melting) temperature of the ternary aluminum-silicon-germanium system for the reaction or substitution to occur. Heating urges or causes diffusion or dissolution of the non-oxidized regions of metal layer  20  into silicon-germanium layer  18  and silicon-germanium into the metal layer, ultimately substituting the non-oxidized regions of silicon-germanium layer  20  with metal from metal layer  22 , an aluminum-copper alloy in the preferred embodiment. Consequently, a coplanar metallic structure  20 ′, consisting essentially of metal from layer  22  with small amounts of silicon and germanium, forms in silicon-germanium layer  18 . Thus, unlike conventional techniques that require trench-digging (or other pre-metallization steps) to promote co-planarity of deposited metallic structures and underlying insulative structures, the present method is “self-planarizing” or “self-trenching.” 
         [0035]     After the reaction, a small amount of germanium may remain on metallic structure  20 ′. Additionally, any metal from metal layer  22  that remains on the surface of layer  20 , such as an aluminum alloy of the preferred embodiment, may contain titanium silicide and silicon-germanium agglomerates. These are removed by chemical-mechanical polishing or other suitable techniques.  
         [0036]     Finally, to reduce the dielectric constant of oxidized regions  18 ′, germanium oxide in the oxidized regions of silicon-germanium layer  20  is removed using a wet etchant, such as distilled or deionized water. This leaves a porous oxide, insulative structure with a low dielectric constant which ultimately reduces capacitance and allows a faster, more efficient integrated circuit.  
         [0037]     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, as illustrated in  FIG. 6A , forms a second silicon-germanium layer and then an oxidation mask  28  defining the size, shape, and position of the via relative conductor  22 ′.  FIG. 6B  shows a top view of the mask and its position relative conductor  22 ′.  
         [0038]     After oxidizing the silicon-germanium film and then removing oxidation mask  28 , the method forms a second metal (aluminum-alloy) layer  30  and a second titanium layer  32  as shown in  FIG. 7 . The integrated-circuit assembly is then heated to force the metal-substitution reaction, thereby substituting a portion of metal layer  30  for the non-oxidized of layer  26 , forming a substantially coplanar via  30 ′. Notably, this technique, 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, proceeds without etching and filling holes. Thus, the present invention provides an “etchless” or “hole-less” method of forming vias.  
         [0039]     After, forming via  30 ′, wet etching removes germanium oxide from regions  26 ′ to form porous low-dielectric insulation  26 ″ around via  30 ′. The resulting integrated-circuit assembly is depicted in  FIG. 8 . Subsequent metallizations would follow similarly.  
         [0040]     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.  
         [0000]     Preferred Hybrid Interconnects Incorporating Low-Capacitance Air Bridges  
         [0041]      FIGS. 9 and 10  show two integrated-circuit assemblies which respectively combine the low-capacitance 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. 3 , connects transistors  14   a  and  14   b  on substrate  12  via conductive vias (or contacts)  16   a  and  16   b  and aluminum 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. 1-3 .  
         [0042]     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.  
         [0043]     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 Region, both issued to Fitch et al. and both incorporated herein by reference.  
         [0044]     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 aluminum may occur.  
         [0045]      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 .  
         [0046]     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  
       [0047]     The present invention overcomes at least two significant shortcomings of previous interconnection techniques. First, unlike previous techniques that require digging trenches in an insulative layer before metallization to ensure a coplanar metallic structure, the present invention provides a self-planarizing metallization process, which 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.  
         [0048]     Secondly, unlike previous techniques which yielded highly-capacitive interconnective structures, the present invention puts metallic structures in a low-dielectric insulator to form low-capacitance structures which permit integrated circuits to operate with greater speed and economy.  
         [0049]     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 scope of the invention, intended to encompass all ways of practicing or implementing the invention, is defined only by the following claims and their equivalents.