Patent Publication Number: US-2015075976-A1

Title: Electroplating processor with geometric electrolyte flow path

Description:
PRIORITY CLAIM 
     This Application is a Divisional of U.S. patent application Ser. No. 13/468,273, filed May 10, 2012, now pending and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is chambers, systems, and methods for electrochemically processing semiconductor material wafers and similar substrates having micro-scale devices integrated in and/or on the work piece. 
     BACKGROUND OF THE INVENTION 
     Microelectronic devices are generally fabricated on and/or in wafers or similar substrates. In a typical fabrication process, an electroplating processor applies one or more layers of conductive materials, typically a metals, onto the substrate. The substrate is then typically subject to etching and/or polishing procedures (e.g., planarization) to remove a portion of the deposited conductive layers, to form contacts and/or conductive lines. Plating in packaging applications may be performed through a photoresist or similar type of mask. After plating, the mask may be removed, with the metal then reflowed to produce bumps, redistribution layers, studs, or other interconnect features. 
     Many electroplating processors have a membrane separating anolyte plating liquid from a catholyte plating liquid within a bowl or vessel. In these processors, bubbles in the plating liquid may collect and stick to the bottom surface membrane. The bubbles act as an insulator, disrupting the electric field in the processor, and leading to inconsistent plating results on the work piece. Accordingly, engineering challenges remain in designing electroplating processors providing consistent plating results. 
     SUMMARY OF THE INVENTION 
     A new electroplating processor has now been invented that largely overcomes bubble-related variations in electroplating. This new electroplating processor includes an electrode tray or plate having a continuous flow path formed in a channel. The flow path may optionally be coiled. One or more electrodes are positioned in the channel, or multiple separate flow channels may be provided with a separate electrode in each channel. A membrane plate is attached to the electrode plate with a membrane in between them. Electrolyte moves through the flow path at a high velocity, preventing bubbles from sticking to the bottom surface of membrane. Any bubbles in the flow path are entrained in the fast moving electrolyte and carried away from the membrane. In an alternative design, a metal electrode, such as a platinum wire, may be positioned inside of a tubular membrane, with electrolyte flowing through the tubular membrane. The flow channels may be curved, or provided with straight segments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, the same element number indicates the same element in each of the views. 
         FIG. 1  is a perspective view of a new electroplating processor. 
         FIG. 2  is a perspective view of the processor of  FIG. 1  with the head removed, for purpose of illustration. 
         FIG. 3  is a section view through the vessel of the processor shown in  FIGS. 1 and 2 . 
         FIG. 4  is another section view through the vessel of the processor shown in  FIGS. 1 and 2 . 
         FIG. 5  is a top perspective view of the channel plate shown in  FIGS. 3 and 4 . 
         FIG. 6  is a top perspective view of the membrane plate shown in  FIGS. 3 and 4 . 
         FIG. 7  is a top perspective view of an alternative design using a membrane tube. 
         FIG. 8  is a top perspective view of an alternative design having an electrolyte flow channel formed as a linear array. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings, as shown in  FIGS. 1 and 2 , an electroplating processor includes a head  14  and a base  12 . A head lifter  16  lifts and lowers the head to move a work piece held in the head into a vessel or bowl  18  in the base. The vessel holds electroplating liquid. An agitator plate  24  may optionally be provided near the top of the vessel  18  to agitate the electroplating liquid adjacent to the work piece. 
     Referring now also to  FIGS. 3 and 4 , the vessel  18  may be divided via a membrane  32  into upper and lower chambers. A channel plate  30  is provided at the bottom of the vessel  18 . The channel plate is typically an insulator, such as plastic. A channel  42  may be provided in the channel plate  30 , with an anode material  52  in the channel  42 . Alternatively, the channel plate  30  may be metal, such as platinum plated titanium, with a flow channel machined into the metal plate. The membrane  32  is clamped between the channel plate  30  on the bottom and a membrane plate  60  on top. As shown in  FIGS. 4 and 5 , a circular or coiled flow path  40  is formed in the top surface of the channel plate  30 . Specifically, the coiled flow path  40  is formed via a coiled channel, groove or slot  42  in the channel plate, and by a corresponding coiled wall  44  which separates adjacent rings of the flow path  40 . 
     The flow path  40  may be continuous and extend uninterrupted from an inlet  36  adjacent to an outer edge of the channel plate  30 , to a drain  35  at or near the center of the channel plate, as shown in  FIG. 5 . Generally, the clamping force on the membrane  32  is highest adjacent to outside of the channel plate  30 , closer to the fasteners or bolts clamping the channel plate and the membrane plate  60  against the membrane  32 . Since the fluid pressure in the flow path  40  is highest at the inlet, in some designs locating the inlet towards the outside of the channel plate  30 , closer to the fasteners, may provide a better seal against the membrane. In other designs, the inlet and outlet positions may optionally be switched, with the inlet adjacent to an outer edge of the channel plate  30 . An alternative to the face-to-face seal shown in  FIG. 4  is to install a long circular elastomer that seals the membrane to the anode surface. 
     The membrane plate  60  is designed as a relatively stiff structure so that it is not deflected or deformed by the fluid pressure under the membrane that is required to pump the anolyte through the spiral flow path. Upward deflection of the membrane plate  60  would create leak paths over the spiral walls and underneath the membrane that would short circuit the spiral flow path. While some fluid leakage over the wall is tolerable (i.e. a perfect seal is not required), excessive flow over the walls decreases the flow velocity in the spiral path and reduces the ability to entrain and carry away bubbles. 
     In the design shown in  FIG. 5 , the channel  42  has a rectangular cross section, with the height of the channel greater than the width of the channel. For example, the height of the channel may be twice the width of the channel  42 . Other channel shapes, such as square and curved cross section channels may also be used. The cross section of the channel  42  may also vary between the inlet and outlet. The wall thickness of the channel wall  44  may also vary between the rings. 
     Referring still to  FIG. 5 , the coiled flow path  40  may be a true spiral in a mathematical sense, or other variations of a spiral. In  FIG. 5 , the rings of the flow path are circular, with a straight segment  46  providing the offset to have each ring of the flow path transition into adjacent rings. Similarly, the flow path may also have other shapes such as oval, elliptical, etc. The flow path  40  may also simply formed via concentric circles, or more properly circular or curved annular channels, connected by segments of any shape. Accordingly, the terms coil or coiled are used here to collectively include spirals and any other pathways having progressively expanding rings, regardless of their shape. 
     In  FIG. 5 , the rings are labeled  1 - 9 . For a processor designed to electroplate a 300 mm diameter work piece, the flow path may have 5-15 or 7-12 rings. Processors designed to electroplate a 450 mm diameter work piece may have proportionally more rings, i.e., 7-22 rings or 10-18 rings. The flow path  40  shown in  FIG. 5  having 9 rings may have a total length of about 3-6 or 4-5 meters. In selecting the number or rings and the total length of the flow path  40 , as well as the cross section(s) of the channel  42 , the pressure required to move anolyte through the flow path may be a limiting factor. 
     The channel wall  44  in the example shown has a generally flat top. A corresponding coiled plate support  62  on the bottom surface of the membrane plate  60 , shown in  FIG. 6 , may match the shape and position of the channel wall  44 . When the membrane plate  60  is clamped to the channel plate  30 , with the membrane  32  between them, the top surface of the channel wall  44  aligns with bottom surface of the coiled plate support, with the membrane clamped between them. The coiled plate support  62  may be a mirror image of the channel wall  44 , although they do not necessarily have the same height. 
     As shown in  FIGS. 3 and 4 , an inner or first anode  50  is positioned on the floor of the channel  42  in the inner rings of the flow path  40 . A second or outer anode  52  is positioned on the floor of the channel  42  in the outer rings of the flow path  42 . As shown in  FIG. 5 , a first electrical contact  54  connects to the first anode  50  and a second electrical contact  56  separately connects to the second anode  52 . The first and second anodes do not connect to each other. However, they are electrically connected through the electrolyte, so that they are not fully electrically isolated from each other. A small gap may be provided between them. On the other hand, both the first and second anodes are in the single continuous flow path  40 . While two anodes are shown, in some designs a single anode may be used, or three or more anodes may be used. 
     The electrical contact for each anode may be approximately centered on its length to help insure uniform electric current along the anode. For a long, thin anode spiral connected at one end, the current density along the anode may drop moving away from the contact because of the electrical resistance of the anode, itself. For very thin and/or very long electrodes, multiple connections can be made to each anode to help distribute the current uniformly. 
     The anodes  50  and  52  may be provided as flat strips of metal. In an inert anode design, where the anodes are not consumed during electroplating, the anodes may be platinum plated titanium. Alternatively, in an active anode design, where the anode is consumed, the anodes may be copper, or other metals. 
     Referring to  FIG. 6 , the membrane plate  60  may have an outer ring of ribs  64 , and inner ring of ribs  66 , and a center ring  68 . The coiled membrane support  62  on the bottom surface of the membrane plate  60  may be attached to ribs. Alternatively, the coiled membrane support  62  may be integrally formed as part of the membrane plate, along with ribs and other features of the membrane plate  60 . The rings of ribs provide a membrane plate  60  having a largely open cross section, to minimize affecting the electric field in the vessel, while also providing a rigid structure to clamp and seal against the membrane. The membrane plate and the channel plate are generally a di-electric material, such as polypropylene or other plastic. The membrane plate  60  may have catholyte inlets  70  and  72  in inner and outer annular sidewalls, to introduce catholyte into the vessel at a position immediately above the membrane  32 . 
     The rings of ribs  66  can have special provision for helping to minimize disturbances to the electric field that may be detrimental to plating uniformity. For example the vertical height of the center post and inner-most ribs maybe reduced to create a larger gap between the structure and the workpiece. The center region can be particularly influenced by the structure because wafer spinning does not help average out disturbances in this region. In another example, the circular ribs may be made as thin as possible, or made thinner at the top of the structure to help minimize their disturbance of the electric field, since their influence on the wafer also cannot be averaged out by wafer rotation. 
     In conventional electroplating membrane processors, the anolyte, or other electrolyte, moves slowly along the membrane. This allows gas bubbles to stick to the membrane and degrading plating performance, especially with substantially horizontally oriented membranes. Using an inert anode tends to generate substantial amounts of gas bubbles, as a electrolysis reaction occurs at the surface of the inert anode releasing oxygen gas. 
     Gas evolution from the anode can be especially problematic for processes that have a high plating rate (and therefore a high anode current and large gas creation) necessary so that the process can finish quickly and throughout can be maximized.” 
     In the processor  10  having a circular flow path  40  anolyte is pumped to the inlet at sufficient pressure so that it moves through the flow path at a high velocity. The velocity of the anolyte flowing through the channel is sufficient to prevent bubbles from sticking to the bottom surface of membrane  32 . Rather, the bubbles are entrained in the fast moving liquid and cannot stick or collect on the membrane. Therefore, bubbles created by the process are quickly carried out of the chamber preventing them from partially or completely blocking the electrical flow path between the anode and the cathode, helping to provide a reliable process. 
     As shown in  FIG. 7 , alternative design is to use a membrane tube  80  with a wire  82  inside the tube as the anode material. Optionally multiple membrane tubes  80  may be used. The membrane tube  80  may be in a coil or other shape. This approach avoids the need for the membrane plate  60  because there is no need to clamp a planar membrane. The chamber can then be more open to for electrical current flow. This approach also avoids the risk of flow leaking between adjacent channels. Rather, the flow is confined to within the membrane tube and is forced to follow the path of the tube. The design of  FIG. 7  may also enable more efficient draining of the catholyte chamber because there is flat divider between the anolyte and catholyte. The tubes can reside within the catholyte and so catholyte can be drained from a low spot below the elevation of the membrane tubes. 
     For the case of a constant area channel, the spiral flow path created by the clamping the membrane to the divider walls  44  can be thought of as similar to the flow within a spiraled tube. For a constant area channel, the flow velocity in the channel and over the anode and the membrane is constant and high throughout its entire length. In contrast, with existing conventional processors, the anolyte flow might be high near the flow inlet, but the velocity dissipates as the flow is distributed over the volume of the anode compartments making it difficult to use the flow to help sweep away bubbles. 
     The coiled electrolyte path of  FIGS. 1-6  may be used in various types of electroplating processors, other than the processor shown in  FIGS. 1 and 2 . Specifically, it may be used in any electroplating processor having a vessel and a membrane. Where the membrane tube of  FIG. 7  is used, no other separate membrane is needed. 
     The electrolyte flow channel need not be a spiral, have concentric rings, or even include largely curved shapes. Rather, as shown in  FIG. 8 , the channel  42  may have an array or other arrangement of straight segments  84 . As one example, the channel may be formed as an array of progressively larger quadrilateral or other geometric shapes, generally matching the shape of the substrate. If desired, curved transition sections may be used at the ends of the straight segments  84 , to reduce pressure loss through the channel. Similar designs using straight segments may also be used with the membrane tube as described above. 
     A method for electroplating a workpiece may include pumping an electrolyte through a continuous flow path formed in a channel extending between an inlet and an outlet. The channel may be formed in an electrode plate, with a membrane on the electrode plate. If the membrane is used, then a membrane plate may be attached to the electrode plate, with the membrane in between the electrode plate and the membrane plate. 
     Thus, novel electroplating apparatus and methods have been shown and described. Various changes and substitutions may of course be made without departing from the spirit and scope of the invention. The invention, therefore, should not be limited except by the following claims and their equivalents.