Method of forming embedded copper interconnections

Abstract: An embedded interconnections of copper is formed by forming an insulating layer, forming an embedded interconnections of copper in the insulating layer, planarizing an exposed surface of the insulating layer including an exposed surface of the embedded interconnections of copper according to chemical mechanical polishing, and forming a protective film of silver on the exposed surface of the embedded interconnections of copper. These steps are repeated on the existing insulating layer thereby to produce multiple layers of embedded interconnections of copper. The exposed surface of the embedded interconnections of copper is plated with silver according to immersion plating.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a method of forming embedded interconnections of copper on a surface of a substrate such as a semiconductor wafer, and a structure of such embedded interconnections of copper. Description of the Related Art: Generally, aluminum alloys have heretofore been used as the materials of interconnections for use in semiconductor devices. For lower electric resistance and greater migration resistance, however, embedded interconnections of copper produced by a damascene process, and such embedded interconnections of copper arranged in multiple layers are employed. Conventional multilayer embedded interconnections of copper have suffered various problems. Such problems will be described below with reference to FIGS. 1A through 1C of the accompanying drawings which illustrate a process of successive steps of forming multilayer embedded interconnections of copper. As shown in FIG. 1A, an interconnections 111 of copper is embedded in the upper surface of an insulating layer 110 of silicon dioxide (SiO 2 ), and another insulating layer 120 of silicon dioxide (SiO 2 ) is disposed on the insulating layer 110 and the interconnections 111 for insulating the interconnections of copper (Cu) 111 in an upper layer. When the insulating layer 120 is deposited on the interconnections layer 111, an exposed upper surface 111a of the interconnections 111 is undesirably oxidized by oxygen. As shown in FIG. 1B, an etchant (etching gas) is applied to etch the insulating layer 120 through a hole 131 defined in a resist layer pattern 130 on the surface of the insulating layer 120 for thereby forming a hole 121 in the insulating layer 120, which will be filled with a plug for connection to the interconnections 111. When the insulating layer 120 is thus etched, the exposed upper surface 111a of the interconnections 111 is undesirably modified in the composition by the etchant. As shown in FIG. 1C, when the resist layer 130 (see FIG. 1B) is removed using oxygen, the exposed upper surface 111a of the interconnections 111 is undesirably oxidized by the applied oxygen. The Japanese Patent Application having a publication number of JP-4061125 A relates to a multi-layered interconnection by repeatedly laminating metal layers to a predetermined thickness to enhance the electrical characteristics of the laminated structure with respect to the use of alloys of the metals. Further, attention is drawn to US-A-5,545,927, which relates to a structure and a method for capping copper electrical interconnects such that the top and side walls of the copper electrical interconnects are capped. These capped interconnects can be a single-layer or multi-layer structure. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of forming embedded interconnections of copper while effectively preventing an exposed surface of an interconnections of copper from being modified or oxidized. According to the present invention, a method of forming embedded interconnections of copper, comprises the steps of forming an insulating layer, forming an embedded interconnections of copper in the insulating layer, planarizing an exposed surface of the insulating layer including an exposed surface of the embedded interconnections of copper, and forming a protective film of silver on the exposed surface of the embedded interconnections of copper. The protective film of silver on the embedded interconnections of copper prevents the embedded interconnections of copper from being oxidized and modified in the surface thereof. To form the protective film of silver on the exposed surface of the embedded interconnections of copper, the exposed surface of the embedded interconnections of copper may be plated with silver according to immersion plating. The immersion plating allows the protective film of silver to be selectively formed only on the exposed surface of the embedded interconnections of copper, and also allows the protective film of silver to be forming in a very small thickness. Therefore, the amount of silver in the protective silver may be relatively small. Since the protective film of silver and the embedded interconnections of copper does not form a solid solution, the resistance of the embedded interconnections remains relatively low even if silver and copper are diffused in each other. According to the present invention, an embedded copper interconnection structure comprises a substrate, and first insulating layer disposed on said substrate, an embedded interconnections of copper disposed in said first insulating layer, a protective film of silver disposed on said embedded interconnections of copper in said first insulating layers; and second insulating layer disposed on a surface of said first insulating layer including said embedded interconnections of copper having said protective film of silver thereon. The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A through 1C are fragmentary cross-sectional views showing a conventional process of successive steps of forming multilayer embedded interconnections of copper; FIG. 2 is a perspective view illustrative of a process of forming a protective film of silver (Ag) on an interconnections of copper disposed in the surface of an insulating layer of SiO 2 according to the present invention; FIG. 3 is a diagram showing the relationship between the composition of a eutectic alloy of copper and silver and the resistivity at a certain temperature; FIGS. 4A through 4C are fragmentary cross-sectional views showing the manner in which the protective film operates. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 2, an interconnections 15 of copper is formed by plating so as to be embedded in the surface of an insulating layer 10 of SiO 2 on the surface of a semiconductor wafer. After the entire surface of the insulating layer 10 including the exposed surface of the interconnections 15 is planarized by chemical mechanical polishing (CMP), the semiconductor wafer is dipped in an aqueous solution of silver cyanide to plate only the exposed surface of the interconnections 15 with a thin protective film 17 of silver (Ag) according to immersion plating. The protective film 17 is shown exaggerated as having an appreciably large thickness in FIG. 2, it has a very small actual thickness on the exposed surface of the interconnections 15. The immersion plating for forming the protective film 17 is carried out according to the following formula: Since the silver cyanide reacts only with the copper of the interconnections 15, but not with the silicon dioxide of the insulating layer 10, according to the immersion plating, the protective film 17 is selectively formed only on the exposed surface of the interconnections 15. The immersion plating allows the protective film 17 to be formed in a very small thickness, and may require a relatively small amount of silver for the protective film 17. FIG. 3 shows the relationship between the composition of a eutectic alloy of copper and silver, which do not form a solid solution, and the resistivity at a certain temperature T 0 . It can be seen from FIG. 3 that if the concentration of copper in the eutectic alloy is close to 100 % (point ρ B ), then the resistance of the eutectic alloy is of substantially the same small value as if the concentration of copper is 100%. Therefore, since the protective film 17 is very thin and contains a relatively small amount of silver, the overall resistance of the interconnections 15 remains essentially unchanged even if silver and copper diffuse in each other. As shown in FIG. 4A, another insulating layer 20 of SiO 2 is formed on the insulating layer 10, including the interconnections 15 and the protective film 17, for supporting an interconnections of copper in an upper layer. At this time, the protective film 17 prevents the interconnections 15 from being oxidized. As shown in FIG. 4B, a resist layer 30 is formed on the upper surface of the insulating layer 20, and then an etchant of fluorine gas is applied to the insulating layer 20 through a hole 31 defined in the resist layer 30 for forming a hole 21 in the insulating layer 20, which will be filled with a plug for connection to the interconnections 15. At this time, the etchant contacts the protective film 17, but not the interconnections 15. Therefore, the interconnections 15 is prevented from being modified by the etchant. The resist layer 30 shown in FIG. 4B is then oxidized and removed by ashing, as shown in FIG. 4C. At this time, the protective film 17 prevents the interconnections 15 from being oxidized. After a plug is formed in the hole 21 in the insulating layer 20 and a next interconnections (not shown) is embedded in the insulating layer 20, the overall surface of the insulating layer 20 is planarized by the chemical mechanical polishing. If another insulating layer of SiO 2 is to be formed on the planarized insulating layer 20, then a protective layer of Ag may be formed on the exposed surface of the next interconnections (not shown) by immersion plating. In the above embodiment, the overall surface of each of the insulating layers is planarized by the chemical mechanical polishing. However, it may be planarized by any of various planarizing processes other than the chemical mechanical polishing. While the aqueous solution of silver cyanide is employed in the immersion plating in the above embodiment, another solution such as an aqueous solution of silver nitrate may be employed in the immersion plating. The present invention offers the following advantages: The protective film of Ag on the exposed surface of the interconnections of Cu prevents the interconnections from being oxidized or modified. Therefore, the yield of the semiconductor devices will be improved, and the production throughput of semiconductor wafers will be increased. Because the protective film of Ag is formed on the exposed surface of the interconnections of Cu by the immersion plating, the protective film of Ag can be selectively formed only on the exposed surface of the interconnections of Cu without a mask. The protective film can be of a very small thickness, which will reduce the amount of Ag that diffuses in Cu of the interconnections, and hence will prevent the resistance of the interconnections from increasing. Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims: A method of forming embedded interconnections (15) of copper, comprising the steps of: (a) forming an insulating layer (10); (b) forming an embedded interconnections (15) of copper in said insulating layer (10); (c) planarizing an exposed surface of said insulating layer (10) including an exposed surface of said embedded interconnections (15) of copper; and (d) forming a protective film (17) of silver on the exposed surface of said embedded interconnections (15) of copper. A method according to claim 1, further comprising the step of: (e) repeating said steps (a) through (d) on the existing insulating layer (10) thereby to produce multiple layers of embedded interconnections (15) of copper. A method according to claim 1, wherein said step (d) comprises the step of plating the exposed surface of said embedded interconnections (15) of copper with silver according to immersion plating. A method according to claim 1, wherein said step (c) comprises the step of planarizing the exposed surface of said insulating layer (10) including the exposed surface of said embedded interconnections (15) of copper according to chemical mechanical polishing.

Inventors: INOUE HIROAKI, OGURE NAOAKI, INOUE, HIROAKI, OGURE, NAOAKI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1998
Main classification: H01L21
IPC codes: H01L21, H01L23