A current-leveling electrode for improving electroplating and electrochemical polishing uniformity in the electrochemical plating or electropolishing of metals on a substrate is disclosed. The current-leveling electrode includes a base electrode and at least one sub-electrode carried by the base electrode. The at least one sub-electrode has a width which is less than a width of the base electrode to impart a generally tapered, stepped or convex configuration to the current-leveling electrode.

FIELD OF THE INVENTION

The present invention relates to electrochemical plating (ECP)/electropolishing processes used to deposit/polish metal layers on semiconductor wafer substrates in the fabrication of semiconductor integrated circuits. More particularly, the present invention relates to a current-leveling electrode which includes a base electrode and one or multiple sub-electrodes which can be selectively added to the base electrode to improve uniformity in the electroplating or electropolishing of metals on a substrate.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor integrated circuits, metal conductor lines are used to interconnect the multiple components in device circuits on a semiconductor wafer. A general process used in the deposition of metal conductor line patterns on semiconductor wafers includes deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal conductor line pattern, using standard lithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby leaving the metal layer in the form of the masked conductor line pattern; and removing the mask layer typically using reactive plasma and chlorine gas, thereby exposing the top surface of the metal conductor lines. Typically, multiple alternating layers of electrically conductive and insulative materials are sequentially deposited on the wafer substrate, and conductive layers at different levels on the wafer may be electrically connected to each other by etching vias, or openings, in the insulative layers and filling the vias using aluminum, tungsten or other metal to establish electrical connection between the conductive layers.

Electrochemical deposition or electrochemical plating of metals on wafer substrates has recently been identified as a promising technique for depositing conductive layers on the substrates in the manufacture of integrated circuits and flat panel displays. Such electrodeposition processes have been used to achieve deposition of the copper or other metal layer with a smooth, level or uniform top surface. Consequently, much effort is currently focused on the design of electroplating hardware and chemistry to achieve high-quality films or layers which are uniform across the entire surface of the substrates and which are capable of filling or conforming to very small device features. Copper has been found to be particularly advantageous as an electroplating metal.

Copper provides several advantages over aluminum when used in integrated circuit (IC) applications. Copper is less electrically resistive than aluminum and is thus capable of higher frequencies of operation. Furthermore, copper is more resistant to electromigration (EM) than is aluminum. This provides an overall enhancement in the reliability of semiconductor devices because circuits which have higher current densities and/or lower resistance to EM have a tendency to develop voids or open circuits in their metallic interconnects. These voids or open circuits may cause device failure or burn-in.

A typical standard or conventional electroplating system for depositing a metal such as copper onto a semiconductor wafer includes a standard electroplating cell having an adjustable current source, a bath container which holds an electrolyte solution (typically acid copper sulfate solution), and a copper anode and a cathode immersed in the electrolyte solution. The cathode is the semiconductor wafer that is to be electroplated with metal. Both the anode and the semiconductor wafer/cathode are connected to the current source by means of suitable wiring. The electrolyte solution may include additives for filling of submicron features and leveling the surface of the copper electroplated on the wafer. An electrolyte holding tank may further be connected to the bath container for the addition of extra electrolyte solution to the bath container.

In operation of the electroplating system, the current source applies a selected voltage potential typically at room temperature between the anode and the cathode/wafer. This potential creates a magnetic field around the anode and the cathode/wafer, which magnetic field affects the distribution of the copper ions in the bath. In a typical copper electroplating application, a voltage potential of about 2 volts may be applied for about 2 minutes, and a current of about 4.5 amps flows between the anode and the cathode/wafer. Consequently, copper is oxidized at the anode as electrons from the copper anode and reduce the ionic copper in the copper sulfate solution bath to form a copper electroplate at the interface between the cathode/wafer and the copper sulfate bath.

The copper oxidation reaction which takes place at the anode is illustrated by the following reaction equation:
Cu→Cu+++2e−

The oxidized copper cation reaction product forms ionic copper sulfate in solution with the sulfate anion in the bath20:
Cu+++SO4−−→Cu++SO4−−

At the cathode/wafer, the electrons harvested from the anode flowed through the wiring reduce copper cations in solution in the copper sulfate bath to electroplate the reduced copper onto the cathode/wafer:
Cu+++2e−→Cu

In an electropolishing process, an electroplated metal is removed from a substrate. Therefore, the wafer becomes the anode and the electroplated metal on the wafer is oxidized to form metal cations. The metal cations enter the electrolyte solution and are reduced and electroplated onto the cathode.

A typical conventional electrochemical plating apparatus10is shown inFIG. 1. The apparatus10includes a tank12which contains an electroplating electrolyte solution (not shown). An anode14and a cathode16, which is the wafer to be electroplated with metal from the anode14, are immersed in the solution. The anode14typically has a planar surface14a.

During an electroplating process, metal from the anode14is electroplated onto the cathode/wafer16as electroplated metal18. However, the planar surface14aof the anode14causes non-uniform current distribution in the tank12, with current field lines concentrated at the edge regions relative to the center regions of the cathode/wafer16. This results in a higher deposition rate at the edge regions relative to the center regions of the cathode/wafer16, forming excess metal19at the edges of the cathode/wafer16. This, in turn, leads to overpolishing burden during subsequent CMP processing. Furthermore, non-uniform grain sizes between the wafer edges and center as a result of electroplating may degrade the EM or SM reliability.

One of the methods used to correct the non-uniform deposition of metal on a cathode/wafer has included the use of post-electroplating edge bead removal (EBR) processes. A common drawback of this method, however, is that EBR processes eliminate usable die areas on the wafer (about 2-5 mm for 8″ wafers). Other methods have included enhancing CMP uniformity and controlling electroplating current distribution by sheltering (applied ECP). However, these methods have shown limited effectiveness.

Copper electropolishing (EP) has been regarded as a possible candidate for replacement of copper CMP in future super low-k (k<2) integration because the stress-free characteristics of copper EP are compatible with the poor mechanical properties of low-k materials. However, like electroplating techniques, copper electropolishing techniques result in the formation of metal layers having a non-uniform thickness caused by non-uniform current distribution in the electrochemical plating tank. As shown inFIG. 1A, when an anode/wafer60having a metal layer60ais subjected to an electropolishing process in a conventional electroplating apparatus10, the non-uniform current distribution in the plating tank12results in a cathode65having a layer of electroplated metal66with excess metal67at the edge regions. The metal layer60aof the anode/wafer60has thin edge regions20relative to the remaining portion of the electroplated metal66.

In an electroplating or electropolishing process, the magnitude of the electric field is inversely proportional to the distance between the cathode and anode. Therefore, the deposition current density can be compensated for by using a stepped or non-planar electrode to enhance the plating or polishing uniformity. Accordingly, a current-leveling electrode having a stepped or non-planar profile is needed to facilitate a uniform current distribution during an electroplating or electropolishing process in order to provide an electroplating layer having a uniform thickness on a substrate.

SUMMARY OF THE INVENTION

The present invention is generally directed to a novel current-leveling electroplating/electropolishing electrode which facilitates uniform current distribution during an ECP or electrochemical polishing process to deposit or electropolish a metal layer having a substantially uniform thickness on a substrate. The current-leveling electrode includes a base electrode and one or multiple sub-electrodes which, depending on the desired electroplating or electropolishing profile to be formed on a substrate, can be attached to the base electrode to impart a stepped, convex or non-planar configuration to the electrode. Accordingly, during an electroplating or electropolishing process, the stepped profile of the electrode establishes higher current densities between the electrode and the center portion of a substrate as compared to the current density between the electrode and the edge of the substrate. This results in electroplating or electropolishing of a metal layer having a substantially uniform thickness on the substrate, thereby improving the yield and reliability of devices fabricated on a wafer. Furthermore, the flexible structure of the current-leveling electrode facilitates improved process throughput by widening the process window for copper electroplating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates a novel current-leveling electroplating/electropolishing electrode which has a flexible geometry to facilitate uniform current distribution between an anode and a cathode in an electroplating or electropolishing process. The current-leveling electrode includes a base electrode and multiple sub-electrodes a selected number of which can be attached to the base electrode to impart a stepped, convex or non-planar configuration to the electrode, depending on the desired electroplating or electropolishing profile to be formed on a substrate. Accordingly, during an electroplating or electropolishing process, the stepped profile of the electrode establishes higher current densities between the electrode and the center portion of a substrate as compared to the current density between the electrode and the edge portion of the substrate. Consequently, in an electroplating process, a metal layer having a substantially uniform thickness is deposited on the substrate. In an electropolishing process, metal is removed from a metal layer on a wafer at substantially equal rates. This improves the yield and reliability of devices fabricated on the wafer. Furthermore, the flexible stepped or tapered geometry of the current-leveling electrode facilitates improved process throughput by widening the process window for copper electroplating.

Referring toFIG. 2, an illustrative embodiment of the current-leveling electrode of the present invention is generally indicated by reference numeral24. The current-leveling electrode24includes a base electrode26and one or multiple sub-electrodes30,35and40, respectively, which can be individually or successively mounted on each other or the base electrode26to impart a non-planar, convex, stepped or tapered configuration to the current-leveling electrode24. While three sub-electrodes30,35and40, respectively, are shown inFIG. 2, it is understood that the current-leveling electrode24may have four or more sub-electrodes depending on the use requirements of the current-leveling electrode24. Furthermore, while they are described herein as having a generally disc-shaped configuration, the base electrode and sub-electrodes may have any desired shape or pattern which is consistent with the use requirements of the current-leveling electrode24.

The base electrode26includes an electrode body27which may be a soluble metal such as copper (Cu), an insoluble metal such as platinum (Pt), or may include both soluble and insoluble metals. The electrode body27typically has a disc-shaped configuration and includes a planar surface27a. Multiple peg openings28, the purpose of which will be hereinafter described, extend into the planar surface27a.

The sub-electrode30includes an electrode body31which may have a generally disc-shaped configuration and a width or diameter which is smaller than that of the electrode body27of the base electrode26. The electrode body31may be a soluble metal such as Cu, an insoluble metal such as Pt, or may include both soluble and insoluble metals. Multiple mount pegs33extend downwardly from the electrode body31for alignment with and insertion in the respective peg openings28provided in the upper surface27aof the electrode body27of the base electrode26. Multiple peg openings32extend into the typically planar surface31aof the electrode body31.

The sub-electrode35is typically similar in construction to the sub-electrode30. The sub-electrode35includes an electrode body36which is typically disc-shaped in configuration and has a width or diameter smaller than that of the electrode body31of the sub-electrode30. The electrode body36typically has a generally planar surface36aand may be a soluble metal such as Cu, an insoluble metal such as Pt, or both soluble and insoluble metals. Multiple mount pegs38extend downwardly from the electrode body36for alignment with and insertion in the respective peg openings32provided in the upper surface31aof the electrode body31of the sub-electrode30. Multiple peg openings37extend into the planar surface36aof the electrode body36.

The sub-electrode40is typically similar in construction to the sub-electrode30and the sub-electrode35and includes an electrode body41which is typically disc-shaped in configuration and has a width or diameter smaller than that of the electrode body36of the sub-electrode35. The electrode body41typically has a generally planar surface41aand may be a soluble metal such as Cu, an insoluble metal such as Pt, or both soluble and insoluble metals. Multiple mount pegs43extend downwardly from the electrode body41for alignment with and insertion in the respective peg openings37provided in the upper surface36aof the electrode body36of the sub-electrode35. Multiple peg openings (not shown) may extend into the planar surface41aof the electrode body41to facilitate attachment of an additional sub-electrode (not shown) to the sub-electrode40, depending on the use requirements of the current-leveling electrode24.

Referring next toFIG. 3, the current-leveling electrode24can be used in an electroplating or electropolishing process using an electrochemical plating apparatus50, which may be conventional. The apparatus50typically includes a tank52for containing an electrochemical plating electrolyte solution (not shown), such as copper sulfate, for example. Depending on the desired thickness profile of an electroplated metal56to be electroplated or electropolished on a wafer54, the current-leveling electrode24can be assembled to include the base electrode26and the sub-electrode30, either alone or in combination with the sub-electrode35or both the sub-electrode35and the sub-electrode40, with or without additional sub-electrodes stacked on the sub-electrode40. The greater the number of sub-electrodes which are mounted on the base electrode26, the greater the degree of uniformity achieved in the electroplated or electropolished metal56on the wafer54.

When the sub-electrodes30,35and40are mounted on the base electrode26and each other in the assembled current-leveling electrode24, the electrode24has a stepped, tapered or convex profile with exposed surface regions46on the base electrode26, sub-electrode30and sub-electrode35, respectively. The current-leveling electrode24and the wafer54are immersed in the electrochemical electrolyte solution (not shown) contained in the tank52. In an electroplating process, as a current is applied, the current-leveling electrode24functions as an anode and the wafer54functions as a cathode. Accordingly, metal oxidized at the current-leveling electrode24travels as metal cations through the electrolyte solution to the wafer54, where the metal cations are reduced and coat the wafer54as the electroplated metal56.

The exposed surface regions46of the base electrode26and sub-electrodes30and35, respectively, are increasingly proximate to the wafer54. Consequently, the current density of the electric current applied between the current-leveling electrode24and wafer54increases from the edge to the center of the wafer54. This counteracts the natural tendency for a larger quantity of metal to be electroplated onto the edge regions relative to the center region of the wafer54. As a result, the thickness of the electroplated metal56is substantially uniform across the entire surface of the wafer54.

In an electropolishing process, an electrical current of reverse polarity is applied such that the current-leveling electrode24is the cathode and the wafer54is the anode. Accordingly, the electroplated metal56is oxidized at the wafer54and metal cations are reduced at and electroplated onto the current-leveling electrode24. The stepped configuration of the current-leveling electrode24facilitates establishment of an electrical current having a higher current density at the center portion relative to the edge portion of the wafer54. Consequently, metal is removed from the electroplated metal56at substantially equal rates among all regions of the wafer54.