Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating selected area
Reexamination Certificate
1999-07-15
2001-09-04
Mayekar, Kishor (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating selected area
C204S242000
Reexamination Certificate
active
06284121
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electroplating and more particularly, to electroplating additives. Still more particularly, the present invention relates to electroplating additives for use in electroplating integrated circuit wafers having sub-micron features.
BACKGROUND
Some conventional copper electroplating processes use additives in the electroplating bath to achieve electrodeposition of the copper with a smooth or level top surface. For example, these conventional processes may be used in printed circuit board fabrication to achieve copper deposits of uniform thickness across the surface of the circuit board, to level or increase the smoothness of the copper deposit, and to increase the rate at which copper deposits inside hole and vias in the circuit board (relative to the surface). Use of these additives allows consistent electrical and mechanical properties of the copper to be achieved across the circuit board's surface.
These conventional processes typically perform the copper electrodeposition from acid sulfate solutions with certain organic additives. For example, additives such as Selrex CuBath M® and Learonal Copper Gleam are commonly used. These organic additives help achieve the level top surface by increasing the deposition rate of the copper at the lower points of the deposition surface relative to the upper points on the deposition surface. It is believed that the mechanism for this leveling effect is that (a) the organic additives tend to absorb on to the plating surface, thus inhibiting the deposition of copper at the point of absorption, and (b) the mass transfer rate of the organic additives tend to be greater for higher points on the plating surface compared to the lower points on the plating surface. Consequently, the deposition rate at the lower points on the plating surface tends to be greater than the deposition rate at the higher points on the surface. This difference in deposition rate helps to achieve deposition with a level top surface.
However, the inventors have observed that these conventional organic additives are only marginally effective when the plating surface contains very small features (i.e., sub-micron) with high aspect ratios. In particular, the copper fill in a small feature tends to have voids, as shown in FIG.
1
. These voids may increase the resistance of (or even open circuit) the conductive path intended to be formed by the copper deposited in the feature. This problem becomes critical in applying copper electrodeposition processes in integrated circuit fabrication. For example, contact and via holes in an integrated circuit can be a quarter micron or less in width, with an aspect ratio of up to four-to-one or greater. In particular, voids in the contacts and vias may result in high resistance interconnects or even open-circuits.
Accordingly, there is a need for an electroplating system capable of filling, without voids, sub-micron high aspect ratio features in a plating surface.
SUMMARY
In accordance with the present invention, an electroplating system is provided that is capable of filling sub-micron features having a high aspect ratio. In one embodiment adapted for copper electrodeposition upon a plating surface, the electroplating system includes a standard electroplating apparatus using an acid copper bath with an additive providing for preferential filling of submicron features. In accordance with the present invention, the additive is chosen to have molecules of a size that is about the size (or larger) of the features to be filled by the electroplating process. The term feature is used herein to refer to holes, vias, openings and other indentations formed in the plating surface.
Because the relatively large size of these additive molecules tends to hinder the mass transfer of the additive molecules into the features, the additive molecules are preferentially absorbed by the surface of the plating surface relative to the inner surfaces of the features. As a result of this preferential absorption of the additive molecules, the rate of plating in the features tends to be greater than the plating rate at the other parts of the plating surface. Accordingly, the electroplating process tends to fill the features relatively quickly compared to the other parts of the target surface so that all of the surface area of the target is equivalent in height, thereby leveling the plating surface.
In addition, because little or no additive molecules are within the features, the features tend to be filled without the voids often produced using conventional systems. These conventional systems use additives with relatively small-sized molecules that can enter the features. It is believed that the relatively small molecules of these conventional systems slow down the plating rate in the features enough so that the features cannot be completely filled before the plating at the upper “corners” of the feature close off the feature, creating a void. In contrast, because substantially no additive molecules can enter the features in this embodiment of the present invention, the plating rate in the features is great enough so that the features can be completely filled before the plating of the corners can close off the features, thereby avoiding the formation of voids.
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Mayekar Kishor
Novellus Systems Inc.
Skjerven Morrill & MacPherson LLP
Steuber David E.
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