Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Reexamination Certificate
1997-11-13
2001-01-30
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
C205S118000, C205S157000, C204S227000, C204S228100, C204S230200, C204S230300, C204S242000, C204SDIG007
Reexamination Certificate
active
06179983
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to an apparatus for treating the surface of a substrate and more particularly to an apparatus for electroplating a layer on a semiconductor wafer.
BACKGROUND OF THE INVENTION
The manufacture of semiconductor devices often requires the formation of electrical conductors on semiconductor wafers. For example, electrically conductive leads on the wafer are often formed by electroplating (depositing) an electrically conductive layer such as copper on the wafer and into patterned trenches.
Electroplating involves making electrical contact with the wafer surface upon which the electrically conductive layer is to be deposited (hereinafter the “wafer plating surface”). Current is then passed through a plating solution (i.e. a solution containing ions of the element being deposited, for example a solution containing Cu
++
) between an anode and the wafer plating surface (the wafer plating surface being the cathode). This causes an electrochemical reaction on the wafer plating surface which results in the deposition of the electrically conductive layer.
To minimize variations in characteristics of the devices formed on the wafer, it is important that the electrically conductive layer be deposited uniformly (have a uniform thickness) over the wafer plating surface. However, conventional electroplating processes produce nonuniformity in the deposited electrically conductive layer due to the “edge effect” described in Schuster et al., U.S. Pat. No. 5,000,827, herein incorporated by reference in its entirety. The edge effect is the tendency of the deposited electrically conductive layer to be thicker near the wafer edge than at the wafer center.
To offset the edge effect, Schuster et al. teaches non-laminar flow of the plating solution in the region near the edge of the wafer, i.e., teaches adjusting the flow characteristics of the plating solution to reduce the thickness of the deposited electrically conductive layer near the wafer edge. However, the range over which the flow characteristics can be thus adjusted is limited and difficult to control. Therefore, it is desirable to have a method of offsetting the edge effect which does not rely on adjustment of the flow characteristics of the plating solution.
Another conventional method of offsetting the edge effect is to make use of “thieves” adjacent the wafer. By passing electrical current between the thieves and the anode during the electroplating process, electrically conductive material is deposited on the thieves which otherwise would have been deposited on the wafer plating surface near the wafer edge where the thieves are located. This improves the uniformity of the deposited electrically conductive layer on the wafer plating surface. However, since electrically conductive material is deposited on the thieves, the thieves must be removed periodically and cleaned, thus adding to the maintenance cost and downtime of the apparatus. Further, additional power supplies must be provided to power the thieves, adding to the capital cost of the apparatus. Accordingly, it is desirable to avoid the use of thieves.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a “virtual” anode between the actual anode (hereinafter “the anode”) and the wafer plating surface. This virtual anode, made of an electrically insulating material, acts to modify the electric current flux and the plating solution flow between the anode and the wafer plating surface in a manner which can be controlled by the shape and location of this virtual anode. Since the thickness of the deposited electrically conductive layer at any particular region of the wafer plating surface is determined by the electric current flux to the particular region, this virtual anode permits any desired thickness profile of the deposited electrically conductive layer.
In one embodiment, the virtual anode takes the form of a member positioned between the anode and the wafer plating surface, this member having at least one opening therein through which plating solution flows. This virtual anode has the effect of regulating both the electric current flux and the plating solution flow between the anode and the wafer plating surface, depending upon the shape and location of the virtual anode. The virtual anode also has the effect of “decoupling” the electric current flux from the plating solution flow so that the two variables may be controlled independent of each other.
In one embodiment of the invention, the virtual anode has a plurality of openings therein, at least one of which is of a different cross-sectional area than at least one of the others, or is of a different length, or both. In general, a change in the cross-sectional area of an opening produces a greater change in the plating solution flow than in the electric current flux through the opening. Thus, by using openings of different cross-sectional area, the plating solution flow can be decoupled (independently varied) from the electric current flux through the openings. In contrast, a change in the length of an opening produces a linear change in both the plating solution flow and the electric current flux through the opening.
In one particular embodiment the openings are cylindrical. In this embodiment, the electric current through any particular opening is inversely proportional to the length of the opening and is directly proportional to the square of the radius of the opening. The plating solution flow through any particular opening is also inversely proportional to the length of the opening. However, in contrast to the electric current flux which is directly proportional to the square of the radius of the opening, the plating solution flow through any particular opening is directly proportional to the cube of the radius of the opening. Similar relations exist for openings of other shapes. Thus, by combining various openings of variable length and variable cross-sectional area, electric current flux and plating solution flow to the wafer can be controlled and, if desired, decoupled from one another. This allows any desired thickness profile of the deposited electrically conductive layer on the wafer plating surface to be obtained.
In a first alternate embodiment, the virtual anode is in the form of an annulus attached to an anode cup of the anode. This virtual anode acts as a shield to limit the amount of electric current flux at the edge region of the wafer by forcing the electric current flux to pass around the virtual anode, thereby reducing the thickness of the deposited electrically conductive layer on the wafer edge region.
In the second alternative embodiment, intended for use when it is desired to have a relatively thick deposit on the edge region of the wafer and a relatively thin deposit on the center region, the virtual anode comprises a disk overlying the center of the anode. This virtual anode effectively shields the center region of the wafer from the electric current flux thereby reducing the thickness of the deposited electrically conductive layer on the center region.
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Reid Jonathan David
Taatjes Steve
Novellus Systems Inc.
Phasge Arun S,.
Skjerven Morrill & MacPherson LLP
Steuber David E.
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