AN ALUMINUM-METAL SILICIDE INTERCONNECT STRUCTURE FOR INTEGRATED CIRCUITS AND METHOD OF MANUFACTURE THEREOF

Abstract: An interconnect structure for integrated circuits having a layer of aluminum (16) and a layer of refractory metal silicide (17) on the aluminum layer (16) to form an aluminum-silicide composite layer. Ranges of suitable thicknesses for the aluminum and metal silicide layers are disclosed. Molybdenum (Mo) and tantalum (Ta) may be used as the refractory metal in the metal silicide layer. Also disclosed is a method of manufacturing an interconnect structure in an integrated circuit providing a first insulating layer (12) over a semiconductor substrate having a plurality of active regions (11) at the surface of the semiconductor substrate (10), forming a first aluminum layer (16) over the first insulating layer (12), forming a metal silicide layer (17) over the aluminum layer (16), and selectively removing parts of the first aluminum layer (16) and the metal silicide layer (17) in a predetermined pattern whereby an aluminum-metal silicide interconnect structure is formed coupling at least some of the active regions (11).

Description: AN ALUMINUM-METAL SILICIDE INTERCONNECT STRUCTURE FOR INTEGRATED CIRCUITS AND METHOD OF MANUFACTURE THEREOF BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an interconnect structure for integrated circuit devices and, more particularly, to a low-resistance aluminum-metal silicide interconnect structure for connecting elements within an integrated circuit and to a method of manufacturing the interconnect structure. Description of the Prior Art Most integrated circuits are formed by active regions at the surface of a semiconductor substrate, typically, on monocrystalline silicon. These active regions are interconnected in part by conducting regions formed at the surface of the substrate. Other interconnecting lines are formed by the deposition of a conducting layer over an insulating layer covering the surface of the substrate. The conducting layer is delineated into interconnecting lines which contact the active regions of the substrate at preselected locations through holes in the insulatin . layer. Some integrated circuits have another insulating layer covering the interconnecting lines upon which a second set of interconnecting lines is formed. This second interconnecting layer contacts the interconnecting layer below through holes in the second insulating layer or may contact the substrate itself through holes in the first and second insulating layers. Still more complex integrated circuits have more alternating layers of insulation and conducting material to form more sets of interconnecting lines. Generally, though, most complex integrated circuits have two sets of interconnecting lines, a top layer and an intermediate layer. These interconnecting lines form the interconnect structure by which the elements, active and passive, of the integrated circuit are electrically coupled for the functioning of the circuit. In present integrated circuits, the top conducting layer is usually formed from aluminum. The intermediate conducting layer or layers below the top conducting layer are formed from aluminum or polysilicon. Another material which has been examined as a possible candidate for intermediate conducting layers is some refractory metal silicide. All of these materials have various shortcomings. Refractory metal silicides and polysilicon have the same general shortcomings as intermediate conducting layers. These materials, for example, have high sheet resistances compared with aluminum. Polysilicon sheet resistance is usually equal to or greater than fifteen ohms per square and silicides have sheet reisistances usually equal to or greater than three ohms per square. Such high sheet resistances slow down the speed of integrated circuits. Secondly, these materials typically form high resistance or nonohmic contacts with either or both N+ or P+ regions in, the substrate of the semiconductor substrate. In CMOS t-echnology both N+ and P+ regions must contact the conducting lines. Polysilicon and refractory metal silicide interconnecting lines are particularly troublesome because the P or N doping of the interconnecting lines will create a P-N junction with an oppositely doped contact substrate region normally found in a CMOS layout. A metal silicide or polysilicon interconnecting line must completely cover the contact region to the substrate, sometimes called "dog boning". The complete overlapping of the contact area by the silicide or polysilicon interconnecting line prevents the silicon substrate surface from gouging because etchants for polysilicon and silicides typically will also etch silicon. Gouging of the substrate silicon is avoided by "dog boning" the interconnecting layer to completely cover the contact region, even with the occurrence of masking misalignments. However, this reduces the density of the integrated circuit. See FIG. 2 which illustrates an interconnection line 20 having "dog bone" contacts and a line 21 which does not. It is easy to see that a set of interconnecting lines having "dog O PI bones" contacts could be less tightly integrated compared to interconnecting lines of the same width W. Another disadvantageous requirement for refractory metal silicide and polysilicon interconnecting lines is that of high temperature annealing. Any high temperature annealing ' step, normally at or above 900°C, will cause the junction depths of the doped regions of the silicon substrate to diffuse deeper. This increases the parasitic capacitances in the integrated circuit which ultimately reduces its speed. Another material which has been used in the prior art for an intermediate interconnect layer to avoid the problems above is aluminum. Aluminum has the disadvantage of having bumps on its surface which cause the insulating layer placed over the aluminum to have hillocks. These hillocks, or irregularities, may cause interdielectric short circuits between the intermediate conducting layer and the conducting layer above. Aluminum also does not provide adequate step coverage in many applications. For example, when aluminum is deposited on the insulating layer covering the substrate, it will not deposit evenly on the sides of a contact opening to the substrate. Rather, the aluminum tends to accumulate in certain places and avoid other places, thereby increasing the likelihood of open circuits in the interconnecting lines formed. Furthermore, since present day production technology has not yet produced a totally satisfactory plasma etch for aluminum, wet etching is still widely used. Wet etching results in a large masking pitch between . neighboring aluminum interconnect lines. This reduces the overall density of the integrated circuit. Another disadvantage of aluminum is that the material when in contact with silicon, readily absorbs silicon to form an aluminum-silicon compound. This increases the possibility of aluminum spiking into the substrate through shallow P-N junctions used in present day OS VLSI integrated circuits to cause short circuits. Still another disadvantage of an intermediate aluminum interconnect layer is that it does not permit the reworking of the top aluminum layer when mistakes in photolithography occur. Since both materials are aluminum, the task of removing the top layer without affecting the intermediate interconnect layer becomes very difficult, if not impossible. An etch of the top layer will invariably remove part of the intermediate layer where contact is made between the two layers. Finally, since both interconnection layers are aluminum, the top layer is required to have a "dog bone" contact to the lower aluminum layer. Otherwise, the exposed open contact area of the lower aluminum layer will be etched when the top layer is defined into interconnecting lines. As mentioned previously, this increases the masking pitch of the top aluminum layer and reduces the overall density of the integrated circuit. The present invention overcomes or substantially mitigates these semiconductor process, structural and electrical shortcomings. SUMMARY OF THE INVENTION The present invention provides an interconnect structure for integrated circuit having a layer of aluminum and a layer or refractory metal silicide on the aluminum layer to form an aluminum-silicide composite layer. Ranges of suitable thicknesses for the aluminum and metal silicide layers are also provided. Molybdenum and tantalum may be used as the refractory metal in the metal silicide layer. The present invention also provides for a method of manufacturing an interconnect structure in an integrated circuit providing a first insulating layer over a semiconductor substrate having a plurality of active regions at the surface of the semiconductor substrate, forming a first aluminum layer over the first insulating layer, forming a metal silicide layer over the aluminum layer, and selectively removing parts of the first aluminum layer and the metal silicide layer in a predetermined pattern whereby an aluminum-metal silicide interconnect structure is formed coupling at least some of the active regions. BRIEF DESCRIPTION OF THE DRAWINGS A more detailed understanding of the present invention may be achieved by the perusal of the following detailed specifications and reference to the following drawings: FIGS. 1A to IE detail the steps of a process used in forming a novel integrated circuit device structure having a triple layer of interconnecting lines of polysilicon, aluminum -refractory metal silicide, and aluminum. FIG. 2 is a top view of an interconnecting line having two "dog bone" contacts and an interconnecting line without such contacts. DETAILED DESCRIPTION An embodiment for manufacturing the novel interconnect structure is described in FIGS. 1A to IE. FIG. 1A illustrates the state of a semiconductor wafer 10 just before an intermediate aluminum-metal silicide interconnection layer is deposited. The substrate 10 has active or diffused regions 11, which are regions heavily doped with N or P type impurities, formed within the surface of substrate 10. Separating these regions 11 are insulating field oxide regions 12. To further isolate the active regions 11, field implant regions, or channel stops, may be placed under the field oxide layers 12. These are not shown in FIG. 1A. An optional conducting layer 13 of polysilicon is deposited upon the field oxide layer 12 to add anoth-er set of interconnecting lines to the completed integrated circuit. Over the entire surface of a processed substrate is deposited a layer of silicon dioxide 15. A typical way of deposition is by chemical vapor deposition. Holes 14 are then formed in the oxide layer 15 to expose the active regions 11 and the polysilicon layer 13. FIG. IB shows a deposited aluminum-metal silicide composite layer. The composite layer is formed from a thin aluminum layer 16 and an overlying layer of a refractory metal silicide 17. The layer of aluminum 16 is deposited by sputteringj other deposition techniques may be used. Pure aluminum or aluminum with small amounts of silicon or copper have been found to be satisfactory for the layer 16. The aluminum deposition is followed by the sputter deposition of a refractory metal silicide layer 17. Molybdenum silicide and tantalum silicide have been used successfully as the refractory metal silicide material. Other refractory metal silicides may also be suitable. To avoid the formation of an oxide layer on the aluminum the refractory metal silicide layer 17 is deposited by sputtering without breaking the system vacuum used in depositing the aluminum layer 16. This ensures good electrical contact between the aluminum 16 and silicide OMPI • Λ VΫIPO 17. Other deposition methods may also be used but steps must be taken to ensure that an oxide layer is not formed on the aluminum layer 16 when the metal silicide layer 17 is deposited. The constraints on the thicknesses for the aluminum layer 16 and silicide layer 17 are flexible. In the present exemplary process, a thickness range of 500 to 3000 Angstroms for the aluminum 16 and a range of 1000 to 2500 Angstroms for the silicide 17 works quite well. The aluminum-silicide composite layer is then defined and etched to yield the desired interconnect structure as shown in FIG. 1C. The desired aluminum-silicide pattern is defined using conventional photoresist and exposure techniques. The metal silicide layer 17 is etched in a suitable wet or plasma etch. A plasma of CCI2F2+SF,- has been used successfully. The layer 17 is then used as a mask to define the aluminum layer 16. Since the aluminum layer 16 is thin, it is possible to get a fine resolution pattern for the aluminum. A phosphoric aluminum etch has proved to be a suitable wet etch in the process. Furthermore, it has been found that the wet etch of the aluminum layer 16' can be done either before or after the removal of the photoresist used in defining the silicide layer 17 because the defined silicide layer 17 can be used as a mask for the aluminum layer 16. The aluminum layer 16 in turn, provides a nearly perfect etch stop for the plasma etch of the metal silicide so that the contact holes 14, 16 (FIG.1A) do not have to be completely covered by the aluminum-silicide pattern to prevent etching of the exposed silicon substrate 10 or polysilicon layer 13. The aluminum etchant for the layer 16 will not attack silicon or polysilicon material. This permits greater integration of the semiconductor device. After the aluminum-metal silicide layer (FIG. IB) is deposited and delineated, an interdielectric insulating layer 18 is deposited (FIG. ID). A silicon dioxide layer from 7,000 to 8,000 Angstroms thick deposited by low pressure chemical vapor deposition techniques has been used. Other thicknesses may also be used. The layer 18 may be formed in an undoped state and in doped states; the layer 18 with % and 8% phosphorous concentrations have been used successfully. Other materials may also be used for the layer 18 but the material should be conform al and compatible with integrated circuit operation and technology. One constraint of the deposition is that the temperature should be below *\-50°C so as to avoid excessive aluminum- silicon eutectic formation so that the shallow junctions of the active regions 11 are not shorted out by aluminum spikes into the substrate 10 from the aluminum layer 16. Using well-known photolithographic t-εchniques the layer 18 is masked and etched so as to open contacts 19 through the layer 18 to the aluminum -metal silicide composite layer. FIG ID illustrates the contact openings 19 only in areas over the aluminum-silicide pattern. Contacts may also be opened to diffused or active regions 11, the substrate itself and the polysilicon layer 13. FIG. IE shows the state of the integrated circuit after a top layer 19 of aluminum has been deposited and defined using standard semiconductor photolithographic technology. It is understood that more alternating layers of aluminum-metal silicide and insulating layers may be placed on the interdielectric layer 18 to provide additional interconnecting layers before the top aluminum layer 20 is deposited. The integrated circuit structure having an intermediate interconnection layer of aluminum-metal silicide has many advantages over the prior art discussed previously. While the description of the resulting integrated circuit structure is described with reference to FIG. IE, the structure should not be considered to be as so limited. Unlike polysilicon layers, the composite layer formed by the aluminum layer 16 and metal silicide layer 17 has low sheet resistances in the range of O.I to 0.5 ohms per square, or even less. The composite layer forms the desired low resistance ohmic contacts to the active regions 11 of the substrate 10 through the lower aluminum layer 16. The composite layer can form desirable contacts with N-doped and P-doped silicon, polysilicon, silicide and even other aluminum materials. The present invention is well suited for CMOS technology. Since the bottom layer 16 is aluminum, "dog bone" contacts are not required. Even if the layer 16 does not completely cover the contact OMPI area of the substrate 10, aluminum etchant typically used in defining the layer 16 into an interconnection line will not gouge the substrate 10. Thus the present structure permits greater integration. No high temperature annealing is required for the composite layer. This is useful in maintaining the speed of the integrated circuit by keeping the junction depths of the active regions shallow and avoiding increased parasitic capacitance. The composite layer has advantages over an aluminum intermediate interconnection layer. The top layer 17 is metal silicide, to permit a smooth coverage by the insulating layer 18. This decreases the likelihood of open circuits in the top conducting layers over the insulating layer 18. Step coverage problems are also reduced with the present invention. Any inconsistencies of coverage by the aluminum layer 16 is compensated by the metal silicide layer 17. Furthermore, the layer 17 is a source of silicon for the aluminum layer 16 below. This reduces the possibility of aluminum spiking through shallow P-N junctions in the substrate. The thinness of the aluminum layer 16 also reduces the amount of silicon absorbed from the substrate contact areas. With the composite layer the top aluminum layer 20 can be reworked, since the metal silicide layer 17 is exposed to the etchants for removing the layer 20. The layer 17 is an effective barrier against the aluminum etchants reaching the underlying layer 16. Furthermore, since the layer 17 acts as a barrier against the etchants for the top layer 20, the top layer contact areas are not required to be a "dog bone", which permits the top aluminum layer to be defined more densely by photolithographic techniques. Finally, since the interconnect structure has the silicide layer 17 electrically in parallel with the aluminum layer 16, it is believed that the likelihood of current density induced aluminum electromigration, which may cause open circuits to form, is reduced. Although the invention has been described relative to a specific embodiment, it is not so limited and, in the light of the above teaching, numerous variations and modifications thereof will be readily apparent to those skilled in the art. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. OMPI

Claims: CLAIMS What is claimed is: 1. An interconnect structure in an integrated circuit device comprising a layer of aluminum, and a layer of refractory metal silicide on said aluminum layer. 2. The device as in claim 1 wherein said metal silicide layer comprises a refractory metal from the group consisting of molybdenum and tantalum. 3. The device as in claim 1 wherein said aluminum layer has a thickness in the range of 500 to. 3000 Angstroms. -r. The device as in claim 2 wherein said metal silicide layer has a thickness in the range of 1000 to 2500 Angstroms. 5. An integrated circuit device having a substrate, a plurality of active regions within the surface of said substrate, a first insulating layer over said substrate surface, a first interconnect structure on said first insulating layer coupling preselected active regions, said interconnect layers comprising a first layer of aluminum on said first insulating layer, and a layer of refractory metal silicide on said first aluminum layer. 6. The device as in claim 5 further comprising a second insulating layer over said first insulating layer and said interconnect structure, and a second interconnect structure of aluminum on said second insulating layer further coupling said active regions and said first interconnect structure. 7. The device as in claim 6 wherein said metal silicide layer comprises a refractory metal from the group consisting of molybdenum and tantalum. 8. The device as in claim 7 wherein said first aluminum layer has a thickness in the range of 500 to 3000 Angstroms and said metal silicide layer has a thickness in the range of 1000 to 2500 Angstroms. 9. A integrated circuit having a silicon substrate, a plurality of active regions within the surface of said substrate, a first silicon dioxide layer over said substrate surface and having at least one aperture exposing a preselected active region, a first conducting layer on said first silicon dioxide layer in a first predetermined pattern so that said first conducting layer contacts said preselected active region, said first conducting layer comprising a first layer of aluminum on said first silicon dioxide layer, and a layer of refractory metal silicide on said first aluminum layer. 10- The device as in claim 9 further comprising a second silicon dioxide layer over said first silicon dioxide layer and said first conducting layer, said second silicon dioxide layer having at least one opening exposing a preselected portion of said first conducting layer, and a second layer of aluminum on said second silicon dioxide layer in a second predetermined pattern so that second aluminum layer contacts said first conducting layer, whereby a multilayer interconnected conducting structure is found. 11. The device as in claim 10 further comprising a polysilicon layer in a third predetermined pattern between said first silicon dioxide layer and said substrate, and a third silicon dioxide layer between said polysilicon layer and said substrate whereby a third conducting layer is added to said multilayer conducting structure. 12. A method of manufacturing an interconnect structure in an integral circuit device comprising providing a first insulating layer over a semiconductor substrate having a plurality of active regions at the surface of said semiconductor substrate, forming a first aluminum layer over said first insulating layer, forming a metal silicide layer over said aluminum layer, and selectively removing parts of said first aluminum layer and said metal silicide layer in a predetermined pattern, whereby an aluminum-silicide interconnect structure is formed coupling together at least some of said active regions. 13. The method as in claim 12 wherein said metal silicide layer is formed over said aluminum layer in such a manner that said metal silicide directly contacts said aluminum layer. 1 . The method as in claim 13 wherein said aluminum layer and metal silicide layer removing step comprises selectively etching said metal silicide layer into said predetermined pattern, selectively etching said aluminum layer into said predetermined pattern by using said etched metal silicide layer as a mask. O FI 15. The method as in claim 14 wherein said metal silicide layer etching step comprises using an etchant which does not etch aluminum and said aluminum layer etching step comprises using an etchant which does not etch said first insulating layer nor said metal silicide layer. 16. The method as in claim 15 wherein said metal silicide layer comprises a refractory metal selected from the group consisting of molybdenum and tantalum. 17. The method as in claim 16 wherein said aluminum layer is formed to a thickness in the range of 500 to 3000 Angstroms. 18. The method as in claim 17 wherein said metal silicide layer is formed to a thickness in the range of 1000 to 2500 Angstroms. 19. A method of manufacturing a semiconductor device comprising forming a plurality of active regions at the surface of a silicon substrate, forming a first layer of silicon dioxide over said substrate, said insulating layer having at least one aperture exposing one preselected active region, depositing a first aluminum layer on said first silicon dioxide layer, depositing a refractory metal silicide layer on said first aluminum layer in a manner so that said silicide layer and said first aluminum layer are in direct contact, selectively etching said silicide layer into a predetermined pattern over said aperture, selectively etching said first aluminum layer into said predetermined pattern so that first aluminum layer contacting said preselected active region through said aperture, depositing a second layer of silicon dioxide over said selectively etched first aluminum and silicide layers, said second silicon dioxide layer having at least one aperture exposing said silicide layer at a predetermined location, depositing a second layer of aluminum over said second silicon dioxide layer, selectively etching said second aluminum layer into a second predetermined pattern so that said second aluminum layer contacts said silicide layer through said second silicon dioxide aperture, whereby a semiconducting having an intermediate conducting layer of aluminum and metal silicide is formed. 20. The method as in claim 19 wherein said aluminum sputtering step is performed in a vacuum, and said metal silicide deposition step is performed by sputtering refractory metal silicide after said aluminum sputtering step without breaking said vacuum so as to prevent the formation of an insulating oxide layer on said aluminum before the sputtering of said metal silicide. 21. The method as in claim 20 wherein said selectively etched metal silicide layer is used as a mask for said first aluminum layer etching steps. 22. The method as in claim 21 wherein first aluminum layer deposition step is by sputtering aluminum to a thickness in range of 500 to 3000 Angstroms. 23. The method as in claim 22 wherein said metal silicide deposition step is by sputtering on a refractory metal-silicide layer in range of 1000 to 2500 Angstroms. 2*%. The method as in claim 19 wherein said silicide layer etching step comprises placing a photoresist layer over said silicide layer, defining said photoresist layer into said predetermined pattern, etching said silicide layer using said photoresist layer as a mask; and wherein said aluminum layer etching step comprises etching said aluminum layer using said photoresist layer as a mask. 25. The method as in claim 24 wherein said etched silicide layer is also used as a mask in said aluminum layer etching step.

Inventors: HASKELL JACOB D, SANDER CRAIG S, WOLLESEN DONALD L, HASKELL, JACOB, D., SANDER, CRAIG, S., WOLLESEN, DONALD, L.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1983
Main classification: H01L23
IPC codes: H01L21, H01L23, H01L27, H01L29