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 a less attractive combination than gold, silver, or copper wires combined with polymer-based insulation, which promise both lower electrical resistance and capacitance and thus faster, more efficient circuits. Unfortunately, conventional etch-based techniques are ineffective with gold, silver, or copper, and conventional polymer formation promote reactions with metals that undermine the insulative properties of polymer-based insulations. Accordingly, the inventor devised methods which use a liftoff procedure to avoid etching problems and a non-acid-polymeric precursor and non-oxidizing cure procedure to preserve the insulative properties of the polymeric insulator. The resulting interconnective structures facilitate integrated circuits with better speed and efficiency.

Description:
This application is a divisional of U.S. Ser. No. 09/145,012, filed Sep. 10, 1998. 
    
    
     FIELD OF INVENTION 
     The present invention concerns methods of fabricating integrated circuits, particularly methods of forming integrated-circuit wiring, or interconnects, from metals, such as gold, silver, and copper. 
     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 follows one of two different methods, both of which initially entail covering the components with an insulative layer. In the first method, fabricators dig small holes in the insulative layer to expose portions of the components underneath and then, through metallization, the process of depositing a metal, they cover the entire insulative layer with a thin layer, or sheet, of aluminum, filling the holes with aluminum. Fabricators then apply an etch-resistant mask, which defines a wiring pattern, to the aluminum layer and subsequently etch, or dissolve, away unwanted aluminum, leaving behind an aluminum wiring pattern. The second method, known as a damascene process, entails digging trenches between the small holes in the insulative layer and then covering the entire insulative layer with aluminum, filling the holes and trenches with aluminum. Fabricators then polish off the metal outside the holes and trenches, leaving aluminum in the holes and trenches to define the wiring pattern. Both methods typically yield aluminum wires that are 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 submicron dimensions, that is, dimensions appreciably 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 insufficient electromigration resistance, a characteristic 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 silicon-dioxide insulation, forming short circuits with neighboring wires. Thus, aluminum and silicon-dioxide interconnection systems waste power, slow down integrated circuits, and compromise reliability. 
     Several metals, such as gold, silver, and copper, appear, because of their lower electrical resistances and higher electromigration resistances, to be promising substitutes for aluminum. And, many polymeric insulators, for example, fluorinated polyimides, because of their lower dielectric constants—an indicator of how much capacitance they will introduce—appear to be promising substitutes for silicon dioxide. Lower capacitance translates into faster, more efficient integrated circuits. Thus, a marriage of these metals with polymers promises to yield low-resistance, low-capacitance interconnective structures that will improve the speed, efficiency, and reliability of integrated circuits. 
     Unfortunately, conventional etch-based interconnection techniques are impractical for malting gold, silver, and copper interconnects. Specifically, silver, gold, and copper, are very difficult to etch. In fact, conventional attempts to etch a layer of silver, gold, or copper covered with an etch-resistant mask usually dissolve the mask faster than the gold, silver, or copper. Additionally, conventional techniques of working with polymers promote chemical reactions between the polymers and metals, such as copper, which undermine the insulative and capacitance-reducing properties of the polymers. 
     Accordingly, to build smaller, faster, more-efficient, and more-reliable integrated circuits, there is not only a need for new fabrication methods that work with gold, silver, and copper but also a need for methods that effectively combine these metals with the advantages of polymeric insulators. 
     SUMMARY OF THE INVENTION 
     To address these and other needs, the inventor has developed methods of making integrated circuit wiring not only from superior metals such as gold, silver, and copper, but also from superior, capacitance-reducing polymeric insulators, thereby facilitating fabrication of integrated circuits with superior speed, efficiency, and reliability. In one method, the inventor incorporates a liftoff processing technique understood throughout the industry to be unsuitable for submicron applications, to successfully make submicron interconnections from gold, silver, and copper. 
     Conventional liftoff processing entails masking sections of an insulative layer to define a wiring pattern, and depositing a sheet of aluminum over both the masked and unmasked portions of the insulative layer. The mask and overlying deposits of aluminum are then removed or lifted off, leaving behind aluminum wires on the unasked surface of the insulative layer. Successful liftoff requires a distinct break or separation between the metal deposited on the mask and metal deposited on the insulative layer. Without this distinct break, lifting the mask pulls and breaks or otherwise damages the microscopic metal structures—the wires—deposited on the insulative layer. Ensuring this distinct break, which conventionally requires building a thick mask having openings with steep, high sidewalls, is thought impractical for micron and submicron metallization because of difficulties in forming thick masks with steep-walled micron and submicron openings. These difficulties stemmed principally from inadequate planarization techniques, photolithographic limitations, and etching-control problems. 
     To overcome these difficulties in forming narrow, steepwalled openings in masks, the inventor recognized not only that current planarization techniques, such as chemical-mechanical planarization, ameliorated the conventional liftoff requirement of using thick masks to ensure the distinct break, but also that the effective thickness of a thinner mask, which is easier to make, could be increased in two ways. First, the inventor uses the mask to form trenches in the underlying insulative layer, which corresponded to openings in the thin mask, before depositing metal, thereby increasing the effective mask thickness by the depth of the trenches. And second, the inventor deposits only a thin, seed, or starter, layer of metal over the mask and in the trenches, not only further ensuring the distinct break necessary for successful liftoff, but also reducing the amount of metal for liftoff. 
     More precisely, one embodiment of the method forms a mask on an insulative layer and forms a hole or trench in the insulative layer through an opening in the mask. The opening is less than one micron wide. Then, the method deposits metal, for example, silver, gold, or copper, through the mask opening into the hole or trench, only partially filling the trench. The mask is then removed or lifted off with little or no risk of destroying the metal wiring in the trenches. To finish filling the trenches, the method concludes with an electroless metal deposition or selective chemical-vapor deposition, with the earlier deposited metal serving as a seed layer for the post-liftoff deposition. 
     Moreover, in another embodiment, the invention applies this liftoff technique to form silver, gold, and copper conductors in a polymeric insulator formed and cured using a unique procedure that reduces reactions with metals, particularly copper. The new procedure preserves the insulative and capacitance-reducing properties of the polymeric insulator. In contrast, conventional polymer processing techniques promote the formation of conductive oxides which increase the conductivity of the polymer and thus undermines its insulative and capacitance-reducing properties. Thus, the present invention facilitates the fabrication of integrated circuits with superior speed, efficiency, and reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an integrated-circuit assembly  10  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 an insulative layer  18  over contacts  16   a  and  16   b  and layer  17 , a mask  20  having an opening  21  which exposes a portion of layer  18 ; 
     FIG. 3 is a top view of the FIG. 2 integrated-circuit assembly, showing position of opening  21  relative contacts  16   a  and  16   b;    
     FIG. 4 is a cross-sectional view of the FIG. 3 integrated-circuit assembly, taken along line  4 — 4  to show depth and width of opening  21 ; 
     FIG. 5 is a cross-sectional view of the FIG. 4 assembly after using mask  20  to form a trench  22  in layer  18 , extending widthwise underneath mask  20 ; 
     FIG. 6 is a cross-sectional view of the FIG. 5 assembly after using mask  20  to form a barrier layer  24   a  and a metal-seed layer  24   b  both of which overlie mask  20  and at least partially fill trench  22 ; 
     FIG. 7 is a cross-sectional view of the FIG. 6 assembly after removing, or lifting off, mask  20  and overlying portions of layers  24   a  and  24   b  and after forming conductive layer  24   c  atop layer  24   b  in trench  20 ; 
     FIG. 8 is a cross-sectional view of the FIG. 7 assembly showing that conductor  24 , which includes layers  24   a ,  24   b , and  24   c , connects not only contacts  16   a  and  16   b  but also transistors  14   a  and  14   b;    
     FIG. 9 is a cross-sectional view of the FIG. 8 assembly after formation of a second metallization level including insulative layer  26  and conductor  28 ; and 
     FIG. 10 is a block diagram of a generic integrated memory circuit incorporating one or more interconnect structures of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description, which references and incorporates FIGS. 1-10, 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  10 , which can be within any integrated circuit, an integrated memory circuit, for example. Assembly  10  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-to-500 nanometer-thick, insulative layer  16 , which, for example, comprises a silicon oxide. Layer  16  is planarized using chemical-mechanical polishing or other suitable technique. A silicon-nitride layer  17 , 50 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  10  preferably includes a titanium-silicide 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 an insulative layer  18  atop layer  17 . Examples of suitable insulative materials include silicon oxides, such as silicon dioxide, and polymeric materials, such as polyimides. 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. 
     The exemplary embodiment forms insulative layer  18  using a non-acid polymeric precursor, that is, a precursor with a pH greater than about 6, and follows a three-stage curing process which makes layer  18  resistant to oxidation reactions with metals such as copper. For more details on this polymer formation, see U.S. patent application entitled Copper Metallurgy in Integrated Circuits (attorney docket number 303.473US1, Micron reference 97-1060) which is incorporated herein by reference. Unlike conventional polymeric processing, this procedure inhibits undesirable increases in the dielectric constant of the resulting polymeric layer and thus yields faster, more efficient interconnect structures. However, the invention encompasses an unlimited number of insulative materials and procedures for making them. 
     The next step, shown also in FIG. 2 but best illustrated in the FIG. 3 top view, is to form a mask, or stencil,  20  on layer  18 , using any suitable masking material and technique, for example, photoresist and conventional photolithography. Although mask  20 , in the exemplary embodiment, is a single-layer structure, other embodiments provide a composite mask structure consisting of two or more photoresist layers with a non-resist material, such as silicon nitride, between each pair of photoresist layers. 
     Mask  20  includes at least one exemplary opening  21 , which nominally defines the perimeter of a desired conductor, or metal structure generally. In other embodiments, mask  20  includes hundreds, thousands, or even millions of openings of various shapes and sizes. FIG. 3 shows that opening  21  defines a channel having ends  21   a  and  21   b  which overlie respective vias  16   a  and  16   b.  FIG. 4, a cross-section of assembly  10  taken along line  4 — 4  in FIG. 3, shows that opening  21  has a depth  21   d  of about 500 nanometers and a width  20   w  of about 250 nanometers, providing an exemplary 2-to-1 aspect ratio. (In contrast conventional liftoff structures range from one to two microns in thickness and have openings with widths of about one micron, because of the conventional wisdom that liftoff is impractical and unreliable for submicron dimensions.) In various embodiments, width  20   w  is less than about 0.75 microns, less than about 0.5 microns, or less than about 0.25 microns. Opening  21 , in the exemplary embodiment, also has negative-sloping sidewalls, although other embodiments provide vertical or even positive sloping sidewalls. The negative-sloping sidewall are formed, for example, using an “image reversal” photographic process. The negative sloping sidewalls facilitate formation of undercut trenches in subsequent steps. 
     The next step of the exemplary method, shown in FIG. 5, entails using mask  19  to form a trench  22  in insulative layer  18 . Forming trench  22  entails undercutting mask  19  such that trench  22  has a width  22   w  which, in the exemplary embodiment, is about five percent greater than width  21   w  of opening  21 . As a consequence, mask  20  overhangs the sidewalls of trench  22  and facilitates subsequent liftoff of mask  20  and any overlying metal by ensuring a distinct break or separation between metal deposited on mask  20  and metal deposited in trench  22 . To create the overhang, one embodiment forms the trench using an etch process which is not completely anisotropic. Another embodiment uses a hard liftoff mask comprising a silicon-nitride layer between two resist layers and overdevelops the bottom resist layer to create the overhang. However, in other embodiments of the invention, there is no overhang because widths  21   w  and  22   w  are substantially equal. In these embodiments, the distinct break results primarily from keeping the thickness of trench deposits less than depth  22   d,  the depth of trench  22 . 
     The next step, also illustrated in FIG. 5, entails partially filling trench  22  with a conductive metal. More specifically, the exemplary method grows or deposits a 10-nanometer-thick barrier layer  24   a  and a 50-nanometer-thick metal-seed layer  24   b  on mask  20  and inside trench  22 , only partially filling trench  22 . Barrier layer  24   a  prevents diffusion of metal through insulative layer  18 . One suitable technique for forming layers  24   a  and  24   b  is physical vapor deposition. Examples of suitable barrier materials include zirconium, hafnium, titanium, tantalum, and tantalum nitride. The principle purpose of seed layer  24   b , which may also be called a starter layer, is to control placement of material deposited during subsequent selective deposition processes. In the exemplary embodiment, seed layer  24   b  comprises copper, however, in other embodiments, layer  24   b  comprises more-conductive metals, such as silver and gold. In general, the method is not limited to any particular metals, or type of metals. Indeed, the method can be practiced with non-metallic materials. 
     Next, as FIG. 7 shows, the method entails removing or lifting off mask  20  and overlying portions of layers  24   a  and  24   b , leaving behind the portions of layers  24   a  and  24   b  occupying trench  22 . One suitable method of lifting off mask  20  and the overlying portions of layers  24   a  and  24   b  is to use a two-stage tape-assisted liftoff, a technique which entails lifting off the overlying metal using a first adhesive strip or patch of material and then using a chemical-resist-removal process to remove any remaining mask material. Another suitable technique entails dissolving the mask, for example, in a liquid etchant. The invention encompasses all techniques for removing the mask and overlying metal without completely destroying the portions of layers  24   a  and  24   b  in trench  22 . 
     After removal of mask  20  and excess portions of layers  24   a  and  24   b , the method entails continued filling of trench  22  with a conductive metal, such as gold, silver, or copper, to form a conductive metal layer  24   c  atop layer  24   b.  The exemplary method fills the trench substantially flush with the top surface of insulative layer  18 , using a selective electroless plating or selective chemical-vapor deposition. (As used herein, electroless plating, or deposition, at least encompasses any autocatalytic deposition of a film through the interaction of a metal salt and a chemical reducing agent.) For an example of an electroless deposition of copper, see U.S. Pat. No. 4,574,095, which is entitled Electroless Copper Deposition on Metals and Silicides and incorporated herein by reference. 
     FIG. 8, a cross-sectional view taken along line  8 — 8  in FIG. 7, shows the resulting three-layer metal conductor  24 , which electrically connects vias  16   a  and  16   b  and therefore connects transistors  14   a  and  14   b.  In the exemplary embodiment, conductor  24  has a width  24   w  of about 0.15 microns, which is about 700 percent less than the width of conductors normally formed using conventional liftoff procedures. However, in other embodiments, width  24   w  is less than about 0.75 microns, less than about 0.5 microns, or less than about 0.25 microns. Formation of conductor  24  completes the first level of metallization. 
     FIG. 9 shows a completed second-level metallization, specifically a second insulative layer  28  and a second three-layer metal conductor  28 . In the exemplary embodiment, conductor  28  is a via, contact, or vertical stud electrically corrected to conductor  24  and thus also to transistors  14   a  and  14   b.  In other embodiments of the invention, conductor  28  is a conductive line which crosses, or intersects, conductor  24 . 
     Formation of both layer  28  and conductor  28  generally follows the exemplary procedure used for insulative layer  18  and conductor  24 . In brief, this entails forming layer  28  of an insulative material similar to layer  18 ; forming a mask having one or more openings that define, for example, studs, contacts, or vias. Afterward, the method etches a corresponding opening in layer  28 , which extends underneath the mask to create overhangs similar to those shown for mask  20  and trench  22  in FIG.  5 . With the mask in place, the next steps form a 10-nanometer-thick barrier layer  28   a  and a 50-nanometer-thick adhesion layer  28   b , both on the mask and on insulative layer  26 . Then after removing the mask, electroless deposition or chemical vapor deposition is used to form the third layer  28   c  of conductor  28 . Subsequent metallizations would follow similarly. 
     Although the embodiments described above follow a single damascene process, other embodiments follow a dual-damascene process. The dual-damascene process generally entails separate maskings to form the vias and trenches in an insulative layer. After formation of the via and trenches, the method proceeds with deposition of the barrier and seed materials (using the trench mask as a stencil) into the trenches, liftoff of the trench mask, and selective deposition of a metal onto the seed material. 
     Exemplary Embodiment of an Integrated Memory Circuit 
     FIG. 10 shows one example of the unlimited number of integrated circuits which would benefit from incorporation of the faster, more-efficient, and more-reliable interconnect structures of the present invention: a generic integrated memory circuit  40 . Memory 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 particularly, 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 in conventional fashion to bit lines  46 . 
     In the exemplary embodiment, the memory cells, the address decoders, and amplifier circuit are interconnected via at least one gold, silver, or copper conductor formed according to a method embodying the present invention. In other embodiments, only certain components, for example memory array  42  and voltage-sense-amplifier circuit  48 , are interconnected via such conductors. Moreover, some embodiments embed these conductors in non-acid-based polymeric insulators, which ultimately reduce capacitance and thus enhance efficiency and speed of the integrated circuit. 
     Conclusion 
     In furtherance of the art, the inventor has developed methods of making integrated-circuit wiring systems not only from superior metals such as gold, silver, and copper, but also from superior, capacitance-reducing polymeric insulators. To overcome the inability of conventional etch-based techniques to work with gold, silver and copper, the inventor devised a liftoff procedure, which in one embodiment entails forming a hole or trench through an opening in a mask structure, at least partially filling the hole or trench with gold, silver, or copper, lifting off the mask structure, and then finishing filling the trench with metal using an electroless deposition. And, to overcome problems with polymers reacting with certain metals, such as copper, the inventor devised a unique polymer-formation procedure involving a non-acid-polymeric precursor and non-oxidizing cure technique. In sum, various embodiments of the invention ultimately facilitate fabrication of integrated circuits, such as integrated memory circuits, with superior speed, efficiency, and reliability. 
     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 concepts of the invention, is defined only by the following claims and their equivalents.