Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-07-02
2003-09-30
Bell, Bruce F. (Department: 1746)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S280000, C204S292000, C164S459000, C164S476000, C164S122000, C148S529000, C148S553000, C148S554000, C148S679000, C148S680000, C148S681000, C148S684000, C029S526300, C029S526400, C029S527500, C029S527700
Reexamination Certificate
active
06627055
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to making fine-grained electroplating anodes especially useful for producing copper interconnects in silicon semiconductor chips.
BACKGROUND
Copper interconnects in multi-layer silicon wafers and semiconductor chips are often made by the damascene process. This process is described in U.S. Pat. No. 4,789,648 and U.S. Pat. No. 5,539,255, the disclosures of which are incorporated herein by reference.
In this process, copper is selectively electrodeposited onto a silicon wafer from an electroplating anode made from copper or copper alloy. Before electrodeposition, an intricate circuit pattern of trenches is etched into the wafer to define the interconnects to be formed. The anode is then mounted in close proximity to, but not touching, the wafer. Both are immersed in an electrolytic bath, where copper from the anode is electrodeposited onto the wafer.
Typical electroplating anodes for use in the damascene process take the form of squat, cylindrical copper discs 200 to 300 mm in diameter and 2 to 6 cm thick. In some instances, the anode is formed with a hollow interior so that it is annular in configuration rather than cylindrical. In either case, the surface of the anode is machined very flat to provide uniform deposition over the entire silicon wafer. Uniform deposition is critical because the wafer will be sectioned to make several chips and each chip is intended to be identical to the next.
Electroplating anodes for the damascene process are produced commercially by sectioning copper rods and tubes and then machining the sections to the desired flatness on one face and to a mounting configuration on the other opposite face. The mounting configuration is dependent on the particular electrodeposition system in which the anode is used. These copper rods and tubes, in turn, are typically made by a multi-step step process including casting, hot working, cold working and annealing.
In order to achieve optimal performance in the damascene process, the average size of the copper grains in these anodes, and hence the rods and tubes used to make these anodes, should be no more than about 150 &mgr;m. In addition, the grain size distribution should be fairly uniform throughout the cross section of the rod or tube and anode. A fine, uniform grain structure is important in maintaining smoothness (or, more accurately, “local flatness”) of the anode face. Moreover, a finer grain structure may be machined and polished to a smoother initial surface finish and, during deposition, the anodes will erode more uniformly and stay smooth for a longer time. A rough anode face is deleterious to uniform copper deposition.
Unfortunately, conventional manufacturing processes can only produce average grain sizes as small as 200 &mgr;m in rods and tubes with diameters of 200 mm or more. Average grain size is often much larger. Moreover, grain size distributions in such rods and tubes are not particularly uniform. Furthermore, conventional billet manufacturing processes are inherently expensive, since they require multiple working steps including at least one cold working step.
In this connection, there are basically two different ways as a practical matter for reducing grain size of copper rods and tubes produced by conventional continuous casting procedures. The first is to hot work several times including reheating the billet between the hot working steps. The second, which is the technique normally used commercially, is to hot work and then cold work the billet followed by annealing. Both require a substantial amount of mechanical working—on the order of 10 to 1 or more in terms of reduction in cross sectional area. Accordingly, these techniques can be very expensive. Furthermore, above a section thickness of about three inches or more, conventional cold working equipment cannot accomplish grain size reduction uniformly. In addition, cracks and other imperfections often occur during cold working leading to the production of large amounts of scrap and/or unacceptable product. As a practical matter, therefore, conventional manufacturing processes cannot consistently and reliably achieve average grain sizes as small as 200 &mgr;m in copper rods and tubes with diameters of 200 mm or more.
Accordingly, there is a need for a new manufacturing process which can consistently and reliably produce copper rods and tubes having average grains sizes significantly less than 200 &mgr;m, typically about 150 &mgr;m or less, in rods and tubes with diameters of 200 to 300 mm or even more. In addition, it would also be desirable if such a process could provide rods and tubes with fairly uniform grain size distributions. And, it would be especially desirable if such a process could be done using less working steps than required in conventional processes, so that manufacturing costs could be reduced.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that copper ingots can be directly formed into rods and tubes having diameters of 200 mm or more and average grain sizes of 150 &mgr;m and less by simple hot working, provided that the ingots are made by a continuous casting procedure in which turbulence is imparted to the metal/solid interface during the casting operation.
Accordingly, the present invention provides a new process for producing a copper or copper alloy billet comprising forming an ingot by a continuous casting procedure in which turbulence is imparted to the metal/solid interface in the casting die and thereafter hot working the ingot so formed to produce the billet.
In addition, the present invention also provides a new copper or copper alloy billet which is made by this process and which has a diameter of at least about 200 mm and an average grain size of about 180 &mgr;m or less, preferably 150 &mgr;m or less.
Similarly, the present invention also provides new electroplating anodes made from such rods and tubes.
DETAILED DESCRIPTION
In accordance with the present invention, copper and copper alloy rods and tubes having diameters of at least 200 mm and average grain sizes of 150 &mgr;m or less are made by hot working ingots formed by a continuous casting procedure in which turbulence is imparted to the metal/solid interface during the casting operation.
Composition
The same coppers and copper alloys used to make conventional electroplating anodes for the damascene process and other plating processes can be used to make the rods and tubes and anodes of the present invention. Examples of such coppers and copper alloys are the deoxidized high phosphorous alloys (C12200, C12210 and C12220), the phosphorous deoxidized tellurium-bearing alloys (C14500, C14510 and C14520) and the phosphorous deoxidized sulfur-bearing alloys (C14700, C14710 and C14729).
In general, any copper or copper alloy can be used which does not contain ingredients, or amounts of ingredients, imparting an adverse impact on the silicon wafers and chips produced by the anodes of the present invention. The copper or copper alloy should also be compatible with the equipment used in the continuous casting process in the sense that no adverse interaction occurs between the two. For example, if a graphite mold is used, coppers or copper alloys which stick to graphite should be avoided.
Turbocasting
Conventional continuous casting is a well known technology in which molten metal flows through a vertically-arranged mold whose inlet is continuously fed with molten metal while frozen, solid metal is being withdrawn from the mold bottom. A cooler is provided to cool the mold and, thus, the metal passing through the mold. Pinch rollers or other withdrawal mechanisms are provided for controlling the rate at which the solidified billet passes out of the die while maintaining the liquid/solid interface of the metal being cast within the confines of the mold.
When copper and copper alloys are continuously cast following this general procedure, gross directional solidification occurs during transition of the metal from liquid to solid. This results in large, coarse, elongated cr
Bishop William J.
Krus, Jr. David
Bell Bruce F.
Brush Wellman Inc.
Calfee Halter & Griswold LLP
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