Patent Description:
In some electroplating processors, a current thief electrode, also referred to as an auxiliary cathode, is used to better control the plating thickness at the edge of the wafer and for control of the terminal effect on thin seed layers. The terminal effect for a given seed layer increases as the electrical conductivity of the electrolyte bath increases. Hence, a current thief electrode can be effectively used with thinner seed layers combined with high conductivity electrolyte baths. The use of thin seed layers is increasing common with redistribution layer (RDL) and wafer level packaging (WLP) plated wafers. For example, it is expected that RDL wafers may soon have copper seed layers as thin as 500A-1000A and copper bath conductivities of <NUM>/cm or higher.

In WLP processing, a relatively large amount of metal is plated onto each wafer. Consequently, in a WLP electrochemical processor having a current thief electrode, a large amount of metal will also be plated on the current thief electrode. This metal must be deplated or otherwise removed from the current thief electrode at frequent intervals, with the processor removed from use during the deplating operation. Deplating the current thief electrode can also result in contamination particles in the electrolyte bath.

Damascene electroplating processors have used a current thief electrode, in the form of a platinum wire, inside of a membrane tube. The membrane tube holds a separate electrolyte (referred to as thiefolyte) having no metal (e.g., a <NUM>% sulfuric acid and deionized water solution). The thief cathode reaction mostly evolves hydrogen rather than plating copper onto the wire. The hydrogen is swept out of the tube by the flowing thiefolyte. However, some metal does cross the membrane into the thiefolyte and plates onto the platinum wire (especially when using a lower conductivity bath). Consequently, the thiefolyte is only used once and flows to drain after passing through the membrane tube. The platinum wire is deplated after processing each wafer. However, under certain conditions using high thief current, it may be difficult to fully deplate the platinum wire. For example, document <CIT> describes a method and apparatus for dynamic current distribution controlled during electroplating. A thieving secondary cathode <NUM> resides in its own chamber filled with electrolyte and is in ionic communication with electrolyte in the cathode chamber. An auxiliary electrode chamber is defined by the sidewall of the apparatus on one side, by an ionic current collimator on the bottom and on the other side and by a cationic membrane on top. The membrane resides directly above an auxiliary electrode, so that a chamber is formed. As another example, document <CIT> describes an electrochemical processor having a head with a rotor configured to hold a workpiece, with the head moveable to position the rotor in a vessel. Inner and outer anodes are in inner and outer anolyte chambers within the vessel. An upper cup in the vessel, has a curved upper surface and inner and outer catholyte chambers. A current thief is located adjacent to the curved upper surface.

The amp-minutes involved in processing RDL and WLP wafers can be <NUM> to <NUM> times higher than for damascene. As a result, the wire in a membrane tube thief electrode used in damascene electroplating may not suitable for electroplating RDL and WLP wafers, due to excessive metal plating onto the thief electrode wire, and excessive consumption of thiefolyte. Accordingly, engineering challenges remain in designing apparatus and methods for electroplating RDL and WLP wafers, and other applications, using a thief electrode.

An aspect of the present disclosure is provided by an electroplating processor according to independent claim <NUM>. The electroplating processor has a vessel holding a first electrolyte or catholyte containing metal ions. A head has a wafer holder, with the head movable to position the wafer holder in the vessel. One or more anodes are in the vessel. A second electrolyte or isolyte in a second compartment is separated from the catholyte by a first membrane. A third electrolyte or thiefolyte in a third compartment is separated from the isolyte by a second membrane. A current thief electrode is in the thiefolyte. The current thief electrode is connected to an auxiliary cathode and provides a current thieving function during electroplating. Build-up of metal on the current thief electrode is reduced or avoided via the membranes preventing metal ions from passing from the catholyte into the thiefolyte.

In the drawings, the same element number indicates the same element in each of the views.

Turning now in detail to the drawings, as shown in <FIG>, an electrochemical processor <NUM> has a head <NUM> positioned above a vessel assembly <NUM>. A single processor <NUM> may be used as a stand alone unit. Alternatively, multiple processors <NUM> may be provided in arrays, with workpieces loaded and unloaded in and out of the processors by one or more robots. The head <NUM> may be supported on a lift or a lift/rotate unit <NUM>, for lifting and/or inverting the head to load and unload a wafer into the head, and for lowering the head <NUM> into engagement with the vessel assembly <NUM> for processing. Electrical control and power cables <NUM> linked to the lift/rotate unit <NUM> and to internal head components lead up from the processor <NUM> to facility connections, or to connections within multi-processor automated system. A rinse assembly <NUM> having tiered drain rings may be provided above the vessel assembly <NUM>.

Referring to <FIG>, a current thief electrode assembly <NUM> is provided at a central position towards the bottom of the vessel assembly <NUM>. The current thief electrode assembly <NUM> allows thief current to be distributed uniformly around the edge of the wafer <NUM> while having a relatively small electrode area. Any membranes used may be small, making sealing around the membranes easier. The current thief electrode has a relatively small diameter (e.g. an effective diameter less than about <NUM>, <NUM>, or <NUM>). However, the current thief electrode assembly functions as a virtual annular thief with a much larger diameter (e.g. larger than wafer diameter). For a processor designed for <NUM> diameter wafers, the virtual annular thief has a diameter greater than <NUM>, for example, <NUM>, <NUM>, <NUM> or <NUM>. The virtual thief electrode is created by placing the thief source near or at the chamber centerline, so that thief current flows radially outward and up to the level of the wafer.

The current thief electrode assembly <NUM> may be used in a processor <NUM> having anodes <NUM> and <NUM> in the form of a wire-in-a-tube. A thief electrode wire <NUM> is provided in the thiefolyte channel <NUM> in the current thief electrode assembly <NUM>. Virtual thief current channels <NUM> extend up through the vessel from the current thief electrode assembly <NUM> to a virtual thief position <NUM> near the top of the vessel, beyond the edge of the wafer <NUM>.

<FIG> shows an example of a processor designed using the concepts of <FIG>. In <FIG>, the processor <NUM> includes an outer ring <NUM> around an inner ring or cup <NUM> within a vessel assembly <NUM>. The inner ring <NUM> may have a top surface <NUM> which curves downward from an outer perimeter of the inner ring <NUM> towards a central opening <NUM> of the inner ring <NUM>. Holes or passageways <NUM> extend vertically through the inner ring <NUM>, from anode compartments in an anode plate <NUM> below the inner ring <NUM> to a catholyte chamber or space above the inner ring <NUM>. A first anode <NUM> in an inner anode compartment is provided in the form of a wire in a membrane tube.

Similarly, one or more second anodes <NUM> in an outer anode compartment are also provided in the form of an inert anode wire in a membrane tube. The anodes Flow diffusers <NUM> and <NUM> may be used, with the anode tubes on the outlet side of the diffusers. The diffusers may have tabs for holding the membrane tubes down against the floor of the anode compartment. During use, the catholyte chamber holds a liquid electrolyte, referred to as catholyte. Typically, a solution of sulfuric acid and deionized water, referred to as anolyte, circulates through the membrane tubes of the anodes <NUM> and <NUM>. The circulating anolyte sweeps oxygen evolved off the inert anode wires within the tubes. The anolyte also provides a conductive path for the electric field from the inert anode wire to the catholyte.

Referring still to <FIG>, the current thief electrode assembly <NUM> is supported on a thief plate <NUM> attached to the anode plate <NUM> and/or the outer ring <NUM>. The current thief electrode assembly <NUM> includes a thief electrode wire <NUM> in a thiefolyte channel <NUM>. The thief electrode wire <NUM> is connected to an auxiliary cathode. The auxiliary cathode is a second cathode channel or connection to the processor which is independent of the first cathode channel connected to the wafer. The thiefolyte channel <NUM> is separated from the catholyte <NUM> in the vessel by a membrane. The channels <NUM> are filled with catholyte and function as virtual thief channels. The thiefolyte channel is separated from an isolyte, i.e., another electrolyte providing an isolation function, by a membrane. The isolyte is then separated from the catholyte by another membrane.

The catholyte <NUM> in the channels <NUM> conducts the electric field created by the current thief electrode assembly <NUM> to the virtual thief position <NUM>. In this way, the current thief electrode assembly <NUM> simulates having an annular thief electrode near the top of the vessel assembly <NUM>.

<FIG> show embodiments of thief electrodes. The electric current flowing through the thief electrode wire <NUM> is relatively small compared to the wafer current (<NUM>-<NUM>%), i.e., the current flowing from the anodes <NUM> and <NUM> through the catholyte <NUM> to the wafer <NUM>. Hence, the current thief electrode assembly <NUM> may use a small electrode and membrane area. Also because the current thief electrode assembly <NUM> is remote from the wafer <NUM>, the current thief electrode assembly <NUM> may be provided in varying shapes, other than annular. For example, the current thief electrode assembly <NUM> may be provided as a platinum wire that is <NUM> to <NUM> long. In comparison, a circumferential wire-in-a-tube thief electrode as used in existing electroplating processors is approximately <NUM> long.

In <FIG>, the thief electrode wire <NUM> extends through a flat membrane 95A. In <FIG>, the thief electrode wire <NUM> is within a membrane tube 95B. In <FIG> the thief electrode wire <NUM> is replaced by a metal plate or disk <NUM> is within a membrane cover 95C. In each case the thief electrode wire <NUM> or thief disk <NUM> is electrically connected to an auxiliary cathode. Metal mesh may be used in place of the thief electrode wire <NUM> or the thief disk <NUM>.

Turning to <FIG> and <FIG>, another membrane and isolation solution may be added to the current thief electrode assembly <NUM>. In this design, an isolation solution or isolyte <NUM> is separated from the catholyte by the first membrane 100B, and the isolyte <NUM> is separated from the thiefolyte <NUM> by a second membrane 100A. The isolyte <NUM> may also be a sulfuric acid and deionized water solution. If the isolyte is used in the processor of <FIG> having anodes in the form of a wire-in-a-tube, then the isolyte <NUM> may be the same liquid as the anolyte flowing through the membrane tubes of the anodes <NUM> and <NUM>. Therefore, besides the plumbing to the small fluid volume in the current thief electrode assembly <NUM>, using the isolyte <NUM> does not add significant cost or complexity to the processor.

The isolyte <NUM> greatly reduces the amount of metal ions that are carried into the thiefolyte <NUM>. In the case of a processor plating copper, because the isolyte <NUM> has a low pH and a very low copper concentration (as copper is only carried across the second membrane 100B) a low number of copper ions will be transported across the first membrane 100A and into the thiefolyte <NUM> touching the thief electrode wire <NUM>. Thus, any plating onto the thief electrode wire will be very small. The catholyte solution for WLP has a low pH (high conductivity) and so the copper flow across the membrane separating the catholyte and the isolyte is low. In turn, the isolyte has both a low pH and a low copper concentration. These factors combine to yield an even lower flow of copper across the membrane separating the isolyte and the thiefolyte.

If the isolyte <NUM> is also the anolyte solution flowing through the membrane tubes of the anodes <NUM> and <NUM>, some of the copper ions that get into the anolyte/isolation solution will pass through the anode membrane tubes and back into the catholyte <NUM>. Furthermore, by greatly reducing the amount of copper transported into the thiefolyte <NUM>, the thiefolyte <NUM> may be recirculated rather than used only once. Recirculating the thiefolyte <NUM> greatly reduces processing costs compared to using the thiefolyte only once as is done with damascene wafer processors. The small amount of copper that does make it to the thiefolyte <NUM> may plate onto the thief electrode wire <NUM>, but only in small amounts that can be quickly deplated between wafers.

The fluid compartments illustrated in <FIG> can be small so that the fluid turnover is high. In the thiefolyte <NUM>, this turnover sweeps hydrogen bubbles out of the fluid volume. The isolyte <NUM> (which may also be the anolyte) and the thiefolyte <NUM> may be replaced on a bleed and feed schedule. Large quantities may be economically replaced because of the low cost of sulfuric acid and deionized water solutions. As the volumes of the isolyte <NUM> and thiefolyte <NUM> are low, less solution is sent to drain compared to single use thiefolyte.

<FIG> shows a design similar to <FIG>, with an inner membrane tube 106A within an outer membrane tube 106B, to form an isolation flow path <NUM>.

As shown in <FIG>, a single membrane <NUM> may be used, with the thiefolyte <NUM> flowing through an electrowinning cell or channel <NUM> to remove any metal getting into the thiefolyte across the membrane <NUM>. This reduces thief maintenance and also avoids single use thiefolyte. The electrowinning electrode involves maintenance to remove plated on metal build up, but this electrode may be centralized for all the chambers on the thiefolyte fluid loop. This configuration may be used without the electrowinning cell or channel <NUM>, but with the membrane <NUM> being a monovalent type or anionic type membrane.

<FIG> shows a processor <NUM> as described above with the thiefolyte channel <NUM> connected to a first chamber <NUM> of a replenishment cell <NUM> via a replenishment catholyte tank <NUM>. The catholyte <NUM> in the catholyte chamber of the processor <NUM> flows through a third chamber <NUM> having a consumable anode <NUM>, such as bulk copper pellets, and optionally through a catholyte tank <NUM>. Anolyte from the anodes <NUM> and <NUM> flows through a second central chamber <NUM> of the replenishment cell <NUM>, and optionally through an anolyte tank <NUM>. The second central chamber <NUM> is separated from the first and third chambers via first and second membranes <NUM> and <NUM>.

<FIG> shows a design similar to <FIG> but using an annular thief electrode wire within a membrane tube, closer to the top of the vessel. This design allows a paddle or agitator to be used in the vessel.

The apparatus and methods described provide a current thieving technique for plating WLP wafers, while overcoming the maintenance issue of copper plate-up on the thief electrode. This may be achieved by a two-membrane stack using cationic membranes and high conductivity (low pH) electrolytes. The copper containing catholyte is separated from a low-copper isolyte by a cationic membrane, which in turn is separated from the lower-copper thiefolyte by another cationic membrane. The thief electrode resides within the thiefolyte. The combination of chemistries and membranes resists migration of copper ions to the thief electrode.

This two-membrane design, with the thief electrode separated from the catholyte in the vessel by two membranes and two electrolytes, is suitable for preventing copper build on the thief electrode during long amp-minute wafer level packaging electroplating. The two separating electrolytes can be the same conductive fluid (i.e. acid and water). The two separating membranes can be cation or monovalent membranes. The separating isolyte and thiefolyte chambers can be formed as a stack with planar membranes, or the two membranes can be formed using co-axial tubular membranes with the inner tube membrane containing the thiefolyte and a wire thief electrode. The thief assembly mid -compartment can be the same electrolyte as the anolyte flowing over inert anodes within the process chamber.

Alternatively, a single membrane may be used to separate the catholyte from the thiefolyte. The catholyte contains copper but has a low pH. The thiefolyte is intended to have no copper. The membrane can be an anionic membrane that prevents copper ions from passing or a monovalent membrane that offers more resistance to Cu++ ions. In the single membrane design, the thief electrode is separated from the catholyte <NUM> by a single membrane, such as a flat or planar anionic membrane, and the thief electrode assembly has a single compartment. As used here, separated from means that the electrolytes on either side of a membrane are both touching the membrane, to allow the membrane to pass selected species as intended.

In <FIG> and <FIG>, with the thief electrode assembly located below the center of vessel, the designs described above are achieved with smaller membranes that are easier to seal.

Conceptually, a centrally located thief acts circumferentially, beyond the edge of the wafer though a virtual anode channel. Since the thief current is relatively small compared to the anode currents, it is adequate to have a small, centrally located thief electrode (and its associated structure) rather than a thief electrode or assembly equal to or greater the circumference of the wafer as in currently used processor designs.

In a processor <NUM> without a paddle agitator, the virtual thief position or opening <NUM> may be below the wafer plane as shown in <FIG>. In a processor with a paddle agitator, the virtual thief position <NUM> may be at or above the wafer plane. The virtual thief position or opening <NUM> may be provided as a continuous annular opening, a segmented opening, or as one or more arcs. For example, a virtual thief position or opening <NUM> may subtend an arc of <NUM> degrees, so that the current thief acts over only a relatively small sector of the wafer. This design may be useful of non-symmetry edge control in a location like a notch, or for processors not having sufficient room for a circumferential current thief opening. In these designs, if the wafer rotates during processing, the current thieving at the edge of the wafer averages out over the entire circumference of the wafer.

Referring back to <FIG>, when coupled to a three-compartment replenishment cell, the three electrolytes within the chamber assembly can be matched to the three compartments in the replenishment cell. Catholyte <NUM> flows to replenishment anolyte (with consumable anodes). Thief assembly isolyte flows to replenishment cell mid-compartment isolyte (as does the chamber anolyte). Thief assembly thiefolyte flows to replenishment cell catholyte. The thief electrode can be run in reverse current for periodic maintenance.

Claim 1:
An electroplating processor (<NUM>), comprising:
a vessel holding a first electrolyte being a catholyte (<NUM>) containing metal ions;
a head (<NUM>) having a wafer holder, with the head movable to position the wafer holder in the vessel;
at least one anode (<NUM>, <NUM>) in the vessel;
an isolyte compartment containing a second electrolyte being an isolyte (<NUM>) providing an isolation solution, with the isolyte separated from the catholyte by a first membrane (100B);
a thiefolyte compartment containing a third electrolyte for a current thief electrode assembly and being a thiefolyte, with the thiefolyte separated from the isolyte by a second membrane (100A), the isolyte and the thiefolyte consisting of acid and water, wherein the first and second membranes are configured to prevent metal ions from passing from the catholyte into the thiefolyte; and
a current thief electrode (<NUM>) in the thiefolyte compartment, wherein the current thief electrode (<NUM>) is connected to an auxiliary cathode and is configured to provide a current thieving function during electroplating.