Abstract:
An electroplating anode including a substantially convex oxidizing surface for oxidation of metal atoms in a semiconductor wafer electroplating process. The electroplating anode of the present invention substantially prolongs the lifetime of the anode and contributes to the prevention of wafer contamination due to generation of potential wafer-contaminating precipitate particles during a wafer electroplating process.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to electroplating systems used in the deposition of metal layers on semiconductor wafer substrates in the fabrication of semiconductor integrated circuits. More particularly, the present invention relates to an anode having a convex profile which prevents buildup of potential wafer-contaminating precipitate or sludge on the anode and significantly prolongs the lifetime of the anode in an electroplating system.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    Deposition of conductive layers on the wafer substrate can be carried out using any of a variety of techniques. These include oxidation, LPCVD (low-pressure chemical vapor deposition), APCVD (atmospheric-pressure chemical vapor deposition), and PECVD (plasma-enhanced chemical vapor deposition). In general, chemical vapor deposition involves reacting vapor-phase chemicals that contain the required deposition constituents with each other to form a nonvolatile film on the wafer substrate. Chemical vapor deposition is the most widely-used method of depositing films on wafer substrates in the fabrication of integrated circuits on the substrates.  
           [0004]    Due to the ever-decreasing size of semiconductor components and the ever-increasing density of integrated circuits on a wafer, the complexity of interconnecting the components in the circuits requires that the fabrication processes used to define the metal conductor line interconnect patterns be subjected to precise dimensional control. Advances in lithography and masking techniques and dry etching processes, such as RIE (Reactive Ion Etching) and other plasma etching processes, allow production of conducting patterns with widths and spacings in the submicron range. Electrodeposition or electroplating 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.  
           [0005]    Electroplated copper provides several advantages over electroplated 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.  
           [0006]    [0006]FIG. 1 schematically illustrates a typical standard or conventional electroplating system  10  for depositing copper onto a semiconductor wafer  18 . The electroplating system  10  includes a standard electroplating cell having an adjustable current source  12 , a bath container  14 , a copper anode  16  and a cathode  18 , which cathode  18  is the semiconductor wafer that is to be electroplated with copper. The anode  16  and semiconductor wafer/cathode  18  are connected to the current source  12  by means of suitable wiring  38 . The bath container  14  holds a bath  20  typically of acid copper sulfate solution which may include an additive for filling of submicron features and leveling the surface of the copper electroplated on the wafer  18 .  
           [0007]    As illustrated in FIGS. 1 and 2, the electroplating system  10  typically further includes a pair of bypass filter conduits  24  which extend through the anode  16  and open to the upper, oxidizing surface  22  of the anode  16  through respective sludge openings  26  at opposite ends of the anode  16 . The bypass filter conduits  24  connect to a bypass pump/filter  30  located outside the bath container  14 , and the bypass pump/filter  30  is further connected to an electrolyte holding tank  34  through a tank inlet line  32 . The electrolyte holding tank  34  is, in turn, connected to the bath container  14  through a tank outlet line  36 .  
           [0008]    In operation of the electroplating system  10 , the current source  12  applies a selected voltage potential typically at room temperature between the anode  16  and the cathode/wafer  18 . This potential creates a magnetic field around the anode  16  and the cathode/wafer  18 , which magnetic field affects the distribution of the copper ions in the bath  20 . 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  16  and the cathode/wafer  18 . Consequently, copper is oxidized typically at the oxidizing surface  22  of the anode  16  as electrons from the copper anode  16  and reduce the ionic copper in the copper sulfate solution bath  20  to form a copper electroplate (not illustrated) at the interface between the cathode/wafer  18  and the copper sulfate bath  20 .  
           [0009]    The copper oxidation reaction which takes place at the oxidizing surface  22  of the anode  16  is illustrated by the following reaction formula (1): 
           Cu→Cu  ++ +2e −   (1) 
           [0010]    The oxidized copper cation reaction product forms ionic copper sulfate in solution with the sulfate anion in the bath  20 : 
           Cu ++ +SO 4   −− →Cu ++ SO 4   −−   (2) 
           [0011]    At the cathode/wafer  18 , the electrons harvested from the anode  16  flowed through the wiring  38  reduce copper cations in solution in the copper sulfate bath  20  to electroplate the reduced copper onto the cathode/wafer  18 : 
           Cu ++ +2e − →Cu  (3) 
           [0012]    As the anode  16  is consumed during the electroplating process, small quantities of solid copper sulfate or “anode fines” tend to precipitate at the interface between the copper sulfate bath  20  and the oxidizing surface  22  of the anode  16  to form a copper precipitate or sludge  28  on the oxidizing surface  22 , as illustrated in FIG. 2.  
           [0013]    Various problems can be caused by the sludge  28  on the anode  16 . For example, the sludge  28  may cause a voltage drop in the electroplating cell because oxidixed copper ions must migrate through the sludge in order to reach the bath solution  20 . The sludge  28  may also affect deposit uniformity of the copper on the wafer  18 . Additionally, the anode sludge  28  can be the source of potential wafer-contaminating particles which may contaminate the copper plated onto the wafer  18 .  
           [0014]    Copper sludge  28  can normally be effectively removed from the oxidizing surface  22  by operation of the bypass pump/filter  30 , wherein the bath solution  20  is continually drawn through the sludge openings  26  of the anode  16  and to the electrolyte holding tank  34  through the bypass filter conduits  24 , bypass pump/filter  30  and tank inlet line  32 , respectively. The bypass pump/filter  30  removes the particulate precipitate/sludge  28  from the bath solution  20  before entry of the bath solution  20  into the electrolyte holding tank  34 . The filtered bath solution  20  is typically distributed from the electrolyte holding tank  34  back into the bath container  14  through a tank outlet line  36  to replenish the supply of the bath solution  20  in the bath container  14 .  
           [0015]    As further illustrated in FIG. 2, in its original condition the anode  16  is typically rectangular in cross-section and has a uniformly flat oxidizing surface  22 . During prolonged use of the anode  16  in the electroplating system  10 , however, copper from the oxidizing surface  22  of the anode  16  is oxidized and enters the copper sulfate solution in the bath  20 , as indicated by reactions (1) and (2), respectively, above. Consequently, as the copper is gradually removed from the oxidizing surface  22  of the anode  16 , the oxidizing surface  22  gradually assumes a concave profile, as illustrated in FIG. 3. The sludge  28  tends to accumulate on the concave oxidizing surface  22 , as illustrated in FIG. 3, and is more difficult to remove from the concave oxidizing surface  22  than from the relatively flat oxidizing surface  22 . Accordingly, small particles from the sludge  28  may break off and enter the bath  20  and potentially contaminate the wafer  18  during the electroplating process. Consequently, the concave anodes  16  must be frequently replaced during periods of frequent usage of the electroplating system  10 .  
           [0016]    Accordingly, an electroplating anode is needed which is more resistant to concave profiling during prolonged wafer electroplating and which extends the lifetime of the anode in the electroplating system.  
         SUMMARY OF THE INVENTION  
         [0017]    An object of the present invention is to provide an anode for use in an electroplating system and which is characterized by extended lifetime.  
           [0018]    Another object of the present invention is to provide an anode which is capable of substantially preventing contamination of work-in-progress (WIP) semiconductor wafers by precipitate particles generated during an electroplating process.  
           [0019]    Still another object of the present invention is to provide an electroplating anode which is more resistant to concave profiling over prolonged periods of electroplating in the processing of semiconductor wafers.  
           [0020]    Yet another object of the present invention is to provide an electroplating anode which at least doubles the anode lifetime during electroplating of metals on a wafer substrate in the fabrication of semiconductor integrated circuits on the substrate.  
           [0021]    A still further object of the present invention is to provide an electroplating anode which is constructed with a substantially convex configuration on the oxidizing surface thereof to at least double the useful lifetime of the anode.  
           [0022]    Yet another object of the present invention is to provide a method for preventing contamination of WIP integrated circuits on semiconductor wafer substrates by precipitate particles during a wafer electroplating process.  
           [0023]    Still another object of the present invention is to provide a method for significantly prolonging the useful lifetime of an electroplating anode in an electroplating system for semiconductors.  
           [0024]    In accordance with these and other objects and advantages, the present invention comprises an electroplating anode including a substantially convex oxidizing surface for oxidation of metal atoms in a semiconductor wafer electroplating process. The electroplating anode of the present invention substantially prolongs the lifetime of the anode and contributes to the prevention of wafer contamination due to generation of potential wafer-contaminating precipitate particles during a wafer electroplating process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
         [0026]    [0026]FIG. 1 is a schematic view of a typical standard or conventional electroplating system for semiconductors suitable for implementation of the present invention;  
         [0027]    [0027]FIG. 2 is a side view of a typical standard or conventional anode used in an electroplating system for semiconductors, with the anode in a relatively new or unused condition;  
         [0028]    [0028]FIG. 3 is a side view of a standard or conventional anode, after prolonged use, more particularly illustrating a concave profile of the anode;  
         [0029]    [0029]FIG. 4 is a side view of a convex profile electroplating anode of the present invention; and  
         [0030]    [0030]FIG. 5 is a flow diagram illustrating a typical progression in anode profiles during prolonged use of the convex electroplating anode of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    When used herein, the term, “metal anode body” means an anode body constructed of a magnetic or non-magnetic metal suitable for electroplating purposes and including but not limited to gold, silver, aluminum, zinc, cadmium, iron, nickel or chromium. When used herein, the term “convex” means any arched, bulging, protruding, raised or rounded surface or any non-concave and non-planar surface.  
         [0032]    Referring to FIG. 4 of the drawings, an illustrative embodiment of the convex profile electroplating anode of the present invention is generally indicated by reference numeral  1 . The convex anode  1  includes an anode body  2  typically constructed of soluble CuP for copper electroplating applications, although the anode body  2  may alternatively be constructed of other magnetic or non-magnetic metals including gold, silver, aluminum, zinc, cadmium, iron, nickel or chromium, in non-exclusive particular, depending upon the desired electroplating application. The anode body  2  typically includes a flat bottom surface  7  and a continuous, annular side surface  5 , although the anode body  2  may have alternative configurations. An upper, oxidizing surface  3  of the anode body  2  has a convex, arched, bulging, protruded, rounded or raised profile or configuration when the anode body  2  is viewed from the side or in cross-section, and the anode body  2  is typically thickest at a center apex  4  of the oxidizing surface  3 , which tapers downwardly from the center apex  4  to the circumscribing side surface  5  the center apex  4  may be or curved, as illustrated, or truncated, and the oxidizing surface  3  may angle or curve gradually or sharply from the center apex  4 . Accordingly, that portion of the anode body  2  between the bottom surface  7  and the center apex  4  of the oxidizing surface  3  is typically at least as thick as that portion of the anode body  2  at the side surface  5 . The convex profile of the oxidizing surface  3  may be casted into the anode body  2  or shaped in the anode body  2  according to methods which are known by those skilled in the art. As illustrated in FIG. 4, a pair of bypass filter conduits  24  typically extends through the anode body  2  adjacent to respective edges thereof, and each bypass filter conduit  24  includes a sludge opening  26  which opens onto the oxidizing surface  3  of the anode body  2 .  
         [0033]    Referring next to FIGS. 1, 4 and  5  of the drawings, in typical application the convex anode  1  of the present invention is positioned in a bath solution  20  containing the metal cation in electrolyte solution with a cation such as sulfate or phosphate. For a copper electroplating process, the bath solution  20  may be acidic copper sulfate. The current source  12  is connected to the anode  1  and to the cathode/wafer  18 , and as the voltage potential is applied by the current source  12  between the anode  16  and the cathode/wafer  18 , copper on the anode  16  is oxidized at the convex upper oxidizing surface  3  of the anode body  2  as the copper cations dissociate from the oxidizing surface  3  and enter the bath solution  20 . The electrons harvested from the anode body  2  reduce the copper cations in the copper sulfate solution to electroplate copper atoms onto the cathode/wafer  18  at the interface of the cathode/wafer  18  and the bath  20 . Any sludge  28  forming on the upper, oxidizing surface  3  of the anode body  2  slides down the sloped oxidizing surface  3 , through the respective sludge openings  26  in the oxidizing surface  3  and into the bypass filter conduits  24 , which conduct the copper precipitate/sludge  28  through the bypass pump/filter  30  and to the electrolyte holding tank  34  for re-entry into the bath container  14 .  
         [0034]    After a prolonged period of electroplating, the oxidizing surface  3  of the anode body  2  assumes a substantially straight profile, as illustrated in the middle diagram of FIG. 5, due to sustained oxidation and removal of copper from the oxidizing surface  3 . At this point, the anode  1  is still useful for continued electroplating, since sludge  28  can still be effectively removed from the oxidizing surface  3  via constant suction applied through the sludge openings  26  by operation of the bypass pump/filter  30 . Continued electroplating, however, eventually generates a concave profile on the oxidizing surface  3  due to the sustained copper oxidation and removal, and the sludge  28  has a tendency to accumulate in the concave ozidizing surface  3  at a faster rate than the sludge  28  can be removed from the oxidizing surface  3  by operation of the bypass pump/filter  30 . At that point, the anode  1  is removed from the electroplating system  10  and replaced by a new, concave anode  1  for continued electroplating.  
         [0035]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.  
         [0036]    Having described our invention with the particularity set forth above, we claim: