Abstract:
A method and apparatus for adjusting an electric field of an electrochemical processing cell are provided. In one embodiment, a capacitive element is disposed in the processing solution. The strength, shape, or direction of the electric field in the processing solution may be modulated by charging and discharging the capacitive element in a controlled manner. Because the electric field is modulated with out passing a current from the capacitive element to the processing solution, electrochemical reactions do not occur on the interface of the capacitive element and the processing solution, thus, reduces complications caused by unwanted electrochemical reactions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to methods and apparatus for modulating of electric field in an electrochemical process. One embodiment of the invention relates to an electrolytic capacitor disposed in an electrochemical processing cell, wherein the electrolytic capacitor is configured to modulate the electric field without inducing deleterious electrochemical reactions. 
     2. Description of the Related Art 
     Metallization of high aspect ratio 90 nm and smaller sized features, such as 45 nm, is a foundational technology for future generations of integrated circuit manufacturing processes. Metallization of these features is generally accomplished via an electrochemical plating process. However, electrochemical plating of these features presents several challenges to conventional gap fill methods and apparatuses. One such problem, for example, is that electrochemical plating processes generally require a conductive seed layer to be deposited onto the features to support the subsequent plating process. Conventionally, these seed layers have had a thickness of between about 1000 Åand about 2500 Å; however, as a result of the high aspect ratios of 90 nm features, seed layer thicknesses must be reduced to less than about 300 Å. This reduction in the seed layer thickness has been shown to cause a “terminal effect,” which is generally understood to be decrease in the deposition rate of an electrochemical plating (ECP) process as a function of the distance from the electrical contacts at the edge of a substrate being plated. The impact of the terminal effect is that the deposition thickness near the edge of the substrate is substantially greater than the deposition thickness near the center of the substrate. The increase in deposition thickness near the edge of the substrate as a result of the terminal effect presents difficulties to subsequent processes, e.g., polishing, bevel cleaning, etc., and as such, minimization of the terminal effect is desired. 
     Attempts have been made to use conventional plating apparatus and processes to overcome the terminal effect through various apparatus and methods. Conventional configurations have been modified to include passive shield or flange members, or segmented anodes configured to control the terminal effect. These configurations were generally unsuccessful in controlling the terminal effect, which resulted in poor control over the deposition thickness near the perimeter. 
     Active thief electrodes have been used to adjust the current density near the perimeter of a substrate during a plating process to overcome the terminal effect generated by thin seed layers in electrochemical plating processes. An active thief electrode in conventional plating cells is generally configured to pass a current into the solution using an independent power supply. The current passed from the active thief modulates the strength, shape, or direction of the electric field in the solution to achieve desired results. Because a current passes from the thief/auxiliary electrode to the solution, an electrochemical reaction occurs at the interface between the electrode and the solution. This electrochemical reaction may cause several undesired complications. For example, the electrode may need to be cleaned and/or replaced frequently, defects may generate loose metal particles and other products from the electrochemical reaction, and bath additives may be electrochemically broken down. 
     Therefore, there exists a need for an apparatus and a method for overcoming he terminal effect without unwanted complications during an electrochemical processing. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an electrochemical plating cell with a capacitive element that satisfies these needs. One embodiment of the invention provides an apparatus for electrochemically processing a substrate with an electrolyte. The apparatus comprises a capacitive element in contact with the electrolyte, wherein the capacitive element is independently biased from the substrate. The apparatus further comprises a substrate support member configured to support the substrate, and a counter electrode in contact with the electrolyte, wherein the counter electrode is coupled to a power supply configured to provide an electric bias between the substrate and the counter electrode. 
     Embodiments of the invention further provide an apparatus for electroplating a substrate. The apparatus comprises a fluid basin configured to contain a plating solution therein, an anode in fluid communication with the plating solution, wherein the anode is adapted to a power supply configured to apply a plating bias between the anode and the substrate, and a capacitive element in fluid communication with the plating solution. 
     Another embodiment of the invention further provides a method for processing a substrate electrochemically with an electrolyte. The method comprises providing a counter electrode in contact with the electrolyte, providing a capacitive element in contact with the electrolyte, contacting the substrate with the electrolyte, processing the substrate by applying an electric bias between the substrate and the counter electrode, and passing a current to the capacitive element during processing the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  illustrates a schematic view of one embodiment of an electrochemical processing cell of the present invention. 
         FIG. 2A  illustrates enlarged view of an interface of an electrolytic capacitor and an electrolyte of the electrochemical processing cell of  FIG. 1 . 
         FIG. 2B  illustrates enlarged view of an interface of an electrolytic capacitor and an electrolyte of the electrochemical processing cell of  FIG. 1 . 
         FIG. 3  illustrate a schematic circuit of one embodiment of an electrochemical processing cell of the present invention. 
         FIG. 4  illustrates a sectional view of one embodiment of an electroplating cell of the present invention. 
         FIGS. 5A-D  illustrates exemplary charging/discharging sequences for an electrolytic capacitor used in an electroplating cell of the present invention. 
         FIG. 6  illustrates exemplary profiles of plating rate may be obtained by the electroplating cell of the present invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally provides an electrochemical plating cell, with an encased counter electrode assembly in fluid communication with the cathode compartment, configured to uniformly plate metal onto a substrate. 
       FIG. 1  illustrates a schematic view of an electrochemical processing cell  100 . An electric field in the electrochemical processing cell  100  may be adjusted without having to pass a current into the electrolyte. The electrochemical processing cell  100  generally comprises a fluid volume  102  configured to contain an electrolyte  110 . In one embodiment, the fluid volume  102  is defined by a fluid basin  101 . In other embodiments, the fluid volume  102  may be defined by a permeable and porous structure, for example, a polishing pad in an electrochemical polishing system. Two electrodes are configured to be in contact with the electrolyte  110  contained in the fluid volume  102  during process. In one embodiment, a counter electrode  103  is disposed in the fluid basin  101  and a substrate support member  105  is configured to form a working electrode along with a substrate  104  supported therein. The substrate support member  105  and the substrate  104  are in electrical contact on via one or more contact pins  106 . The substrate support member  105  is configured to transport the substrate  104  in and out the fluid volume  102 . 
     A processing power supply  108  is coupled between the substrate support member  105  and the counter electrode  103 . In one embodiment, the electrochemical processing cell  100  is configured to electroplate a metal layer on the substrate  104 , thus the substrate support member  105  is cathodically biased and the counter electrode  103  serves as an anode. In another embodiment, the electrochemical processing cell  100  is configured to electropolishing a metal layer from the substrate  104 , thus the substrate support member  105  is positively biased, and the counter electrode  103  is negatively biased. It should be noted that electroplating and electropolishing processes can be performed alternatively in the electrochemical processing cell  100  by simply alternating directions of the processing power supply  108 . 
     During processing, an electric field may be generated between the counter electrode  103  and the assembly of the substrate  104  and the substrate support member  105 . A capacitive element  107  is disposed in the fluid volume  102  and configured to have an interface in contact with the processing electrolyte during processing. The capacitive element  107  may be charged and discharged by a capacitor power supply  109 . In one embodiment, the power supplies  108  and  109  may be independent controllable outputs of a multiple power supply. 
     The capacitive element  107  is configured to have a large surface area and high electrolytic capacitance. When the capacitive element  107  is charged, a large amount of charge can be stored within the interface of the capacitive element  107  and the electrolyte. Therefore, the strength, shape, or direction of the electric field in the fluid volume  102  may be modulated by charging and discharging the capacitive element  107  disposed therein. 
       FIGS. 2A and 2B  illustrate enlarged views of an interface of the capacitive element  107  and the electrolyte  110  of the electrochemical processing cell  100  shown in  FIG. 1 . The capacitive element  107  has a surface  111  which is in contact with the electrolyte  110 . The electrolyte  110  contains positive ions  113  and negative ions  114 . 
     In  FIG. 2A , the capacitive element  107  is being charged negatively. A current of electrons is flowing into the capacitive element  107  from the capacitor power supply  109 . Electrons  112  accumulate inside the capacitive element  107  near the surface  111 . The electrons  112  attract the positive ions  113  in the electrolyte  110  producing positive-negative poles disturbed relative to each other across the surface  111  over an extremely short distance. This phenomenon is known as an “electrical double-layer”. While the positive ions  113  are flowing to the surface  111 , a current is generated in the electrolyte  110  near the surface  111 . The current can be supplied to the capacitive element  107  in such a way that voltage difference between the capacitive element  107  and the electrolyte  110  do not exceed an overvoltage for the onset of faradic reactions, such as metal depositions and breakdown of electrolytic compound, in the electrolyte  110 . Hence, faradic reactions do not occur near the surface  111 . In one embodiment, the voltage of the capacitive element  107  may be controlled by flowing a predetermined current for a predetermined period of time using the following relation: 
                   i   =     C   ⁢       ⅆ   V       ⅆ   t                 (   1   )               
wherein i denotes current, C denotes capacitance, V denotes electric potential, and t denotes time. Therefore, the electric field in the electrolyte  110  can be modified by charging the capacitive element  107  disposed therein without inducing electrochemical reactions.
 
     Similarly, the electric field of the electrolyte  110  may be adjusted while the charged capacitive element  107  is being discharged. As shown in  FIG. 2B , the electrons  112  are flowing out of the capacitive element  107  while a current is applied. The “electrical double-layer” neutralizes or switches signs releasing the positive ions  113  back to the electrolyte  110 , thus, creates another current in the electrolyte  110 . 
     In one embodiment, the capacitive element  107  may consist of a highly porous material, such as carbon aerogels, embedded in an inert but conductive matrix such as carbon paper. A carbon aerogel is a monolithic three-dimensional mesoporous network of carbon nanoparticles obtained by pyrolysis of organic aerogels based on resorcinol-formaldedhyde. Carbon aerogels have high surface area (on the order of several m 2 /g), low density, good electrical conductivity, high electrolytic capacitance (several F/g). It should be noted that other materials can also be used to make a capacitive element for an electrochemical system. In one embodiment, the capacitive element  107  may be encased in a polymeric sheath. 
     Through proper optimization of geometry, conductivity and capacitance, a capacitive structure, such as the capacitive element  107  in  FIG. 1 , may be used in an electrochemical processing system to modulate the strength, shape or direction of the processing electric field to achieve desired results, such as improving deposit uniformity, protecting substrates from corrosion, or enabling nucleation for an electrodeposition process. The capacitive element s of the present invention may be used to achieve different purposes by using different designs, applying different charging/discharging sequences, or positioning in different locations. 
       FIG. 3  illustrates one embodiment of an electrochemical processing cell of the present invention in form of an electronic circuit  300 . A substrate  304  having a layer of conductive material on a surface is generally connected to a processing power supply  308 . The power supply  308  is further connected to a counter electrode  303  disposed in an electrolyte  310 . The electrolyte  310  may be considered as a network of resistors  310 R. When the substrate  304  is immerged into the electrolyte  310 , the substrate  304 , the processing power supply  308 , the counter electrode  303  and the network of resisters  310 R form a closed circuit, and a processing current i p  flows in the closed circuit for processing, i.e., plating and/or deplating, the conductive layers on the substrate  304 . 
     A capacitive element disposed in the electrolyte  310  is equivalent of a capacitor  307  having a first electrode  307   1  and a second electrode  307   2 . Generally, the first electrode  307 , is a chargeable area inside the surface of the capacitive element and the second electrode  307   2  is a chargeable area outside the capacitor element in the electrolyte  310 . The capacitor  307  forms another circuit with the network of resisters  310 R, the counter electrode  303  and a capacitor power supply  309 . When the capacitor  307  is charged or discharged, a capacitor current i c  flows between the networks of the resisters  310 R and the capacitor  307 . The capacitor current i c  alters the electric fields in the electrolyte  310 , therefore, changing the processing current i p  at least in the region near the capacitor element. 
     As shown in  FIG. 3 , the first electrode  307   1 , is connected to the negative terminal of the capacitor power supply  309 , thus the first electrode  307   1  is configured to be charged negatively. During a charging process, the current i c  flows from the network of resisters  310  to the second electrode  307   2 . During a discharge processing, the current i c  flows from the second electrode  307   2  to the network of resisters  310 . It should be noted that the capacitor power supply  309  may be connected in a reversed manner so that the capacitor  307  can be charged either positively or negatively. 
     A capacitor element may be used to achieve different effects to an electrochemical processing cell depending charging and discharging sequences applied to the capacitor. More detailed description may be found in  FIGS. 5A-D . 
       FIG. 4  illustrates a sectional view of one embodiment of an electrochemical processing cell  400 . The electrochemical processing cell  400  is illustratively described below in reference to modification of a SlimCell™ system, available from Applied Materials, Inc., Santa Clara, Calif. Detailed description of an electroplating cell used in a SlimCell™ may be found in co-pending U.S. patent application Ser. No. 10/268,284, filed on Oct. 9, 2002, entitled “Electrochemcial Processing Cell”, which is herein incorporated by reference. 
     The electrochemical processing cell  400  generally includes a basin  401  defining a processing volume  402  configured to contain a plating solution. An anode  403  is generally disposed near the bottom of the processing volume  402 . In one embodiment, a membrane assembly  406  containing an ionic membrane is generally disposed on top of the anode  403  forming an anodic chamber near the anode  403 . A diffuser plate  405  configured to direct the fluid flow in the processing volume  402  may be positioned above the membrane assembly  406 . The electrochemical processing cell  400  further comprises a substrate support member  410  configured to transfer a substrate  404  and contact the substrate  404  electrically via one or more contact pins  411  near the edge of the substrate  404 . A processing power supply  408  is coupled between the contact pins  411  and the anode  403 . 
     During processing, the substrate support member  410  transders the substrate  404  into the processing volume  402  so that the substrate  404  is in contact with or immerged in a plating solution contained therein. The processing power supply  408  provides the substrate  404 , via the contact pins  411 , a plating bias relative to the anode  403 . An electric field is generated between the substrate  404  and the anode  403  and one or more conductive materials may be plated on the substrate  404 . 
     In one embodiment, a capacitive element  407  is disposed in the processing volume  402 . The capacitive element  407  is configured to adjust the electric field between the substrate  404  and the anode  403 . In one embodiment, the capacitive element  407  is shaped like a ring and positioned in a way that when the substrate  404  is in processing position, the capacitive element  407  is near the edge of the substrate  404 . In one embodiment, the capacitive element  407  is connected to a capacitor power supply  409  which is also connected to the anode  403 . The capacitor power supply  409  is configured to charge and discharge the capacitive element  407 . In another embodiment, the capacitor power supply  409  is in electrical communication with the contact pins  411  and the capacitive element  407 . In one embodiment, the capacitive element  407  is configured to adjust the electric field between the substrate  404  and the anode  403  during electroplating to improve plating uniformity. 
     It should be noted that the capacitor element  407  may have a variety of shapes and locations in an electrochemical processing cell. For example, the capacitor element  407  may include a plurality of capacitors in strips, or a continuous ring, or other shapes. The capacitor element  407  may be disposed on the diffuser plate  405 , attached to the substrate support member  410  near the contact pins  411 , or near the substrate. 
     An electroplating process performed in an electroplating cell, such as the electrochemical processing cell  400 , may be generally divided into four stages. In stage I, a substrate support member, such as the substrate support member  410 , is in a non-process position, and a substrate may be loaded into the substrate support member. In stage II, the substrate support member transfer and immerge the substrate into a plating solution in a processing volume, such as the processing volume  402  of  FIG. 4 . In stage III, a plating process is performed by applying a plating bias to the substrate an anode by a processing power supply, such as the processing power supply  408  of  FIG. 4 . In stage IV, the plating process is completed and the substrate support member transferred the substrate out of the plating solution. 
     Different effects on plating results may be achieved by charging/discharging a capacitor element at different stages of the plating process.  FIGS. 5A-D  illustrates exemplary charging/discharging sequences for a capacitor element used in an electrochemical processing cell of the present invention. 
       FIG. 5A  illustrates an exemplary charging/discharging sequence for a capacitor element, such as the capacitor element  407  of  FIG. 4 , during an electroplating process. The horizontal axis indicates time and the vertical axis indicates voltage. The stages I-IV indicate the plating stages described above. Curve  501  represents changes of supply voltage supplied to the capacitor element  407  by the capacitor power supply  409  during the plating process. In stage I, from time zero to t1, the curve  501  increases from V 1A  to V 2A , indicating the capacitive element  407  is being charged positively. In one embodiment, the charging may be performed by supplying to the capacitive element  407  a predetermined current for a predetermined time period. In stage I, the substrate  404  is not in contact with the electrolyte. In stage II, when the substrate  404  is being immersed into the electrolyte, the capacitive element  407  is kept in the positively voltage V A . In stage III, the plating processing starts in the electrochemical processing cell  400  and the capacitive element  407  is discharged as a function of time in a controlled manner to adjust the electric field in the vicinity of the capacitive element  407 , i.e. near the edge of the substrate. In one embodiment, the voltage is lowered from V 3A  to V 4A  in a linear manner as discharge continues. In one embodiment, the discharge continuous until the capacitive element  407  reaches a neutral condition or a predetermined voltage. In one aspect, the discharge of the capacitive element  407  may cover the whole process of plating. In another aspect, the discharge may only occur at the beginning of the plating process when the seed layer is thin and the terminal effect is most obvious. In stage IV, the capacitive element  407  is kept static, for example in the neutral condition, while the plating process is completing and the substrate  404  is removed from the electrolyte. The charge and discharge process may start again for a new substrate to be plated. 
     In the sequence shown in  FIG. 5A , during electroplating, a positively charged capacitive element is discharged negatively, which generates a current towards the capacitive element in the electrolyte, therefore reducing a plating rate near the capacitive element. 
       FIG. 5B  illustrates another exemplary charging/discharging sequence for a capacitor element, such as the capacitor element  407  of  FIG. 4 , during an electroplating process. Curve  502  represents changes of supply voltage supplied to the capacitor element by the capacitor power supply  409  during the plating process. In stage I, while the substrate is not in the electrolyte, the curve  502  decreases from V 1B  to V 2B , indicating the capacitive element  407  is being charged negatively. In stage II, when the substrate  404  is being immersed into the electrolyte, the capacitive element  407  is kept in the negatively charged voltage VB. In stage II, the plating processing starts in the electrochemical processing cell  400  and the capacitive element  407  is discharged as a function of time in a controlled manner. In stage IV, the capacitive element  407  is kept static, for example in the neutral condition, while the plating process is completing and the substrate  404  is removed from the electrolyte. The charge and discharge process may start again for a new substrate to be plated. 
     In the sequence shown in  FIG. 5B , during electroplating, a negatively charged capacitive element is discharged positively, which generates a current outward from the capacitive element in the electrolyte, therefore increasing a plating rate near the capacitive element. 
     Similarly, in the sequence shown in  FIG. 5C , the capacitive element is discharged in stage I and charged positively in stage III, i.e. the plating stage. Therefore, during electroplating, a capacitive element is positively charged, which generates a current outward from the capacitive element in the electrolyte, therefore increasing a plating rate near the capacitive element. 
     In the sequence shown in  FIG. 5D , the capacitive element is discharged in stage I and charged negatively in stage III, i.e. the plating stage. Therefore, during electroplating, a capacitive element is negatively charged, which generates a current towards the capacitive element in the electrolyte, therefore decreasing a plating rate near the capacitive element. 
     As described in  FIGS. 5A-D , a capacitive element in an electroplating cell may be used to adjust the electric field of the electroplating cell, hence adjusting a plating rate near the capacitive element.  FIG. 6  illustrates exemplary profiles of plating rates that may be obtained by an electroplating cell having a capacitive element near the edge of the substrate being processed. The horizontal axis indicates the distance from the center of the substrate and the vertical axis indicates a plating rate. Curves  620 - 625  illustrate a plurality of plating rate profiles along a radius of the substrate being processed. The curves  620 - 625  illustrate plating effects ranged from edge thick to edge thin which may be applied to different substrates or during a different time period of the plating process. The curves  620 - 625  may be obtained by charging/discharging a capacitive element near the edge of the substrate at different current settings or directions. 
     It should be noted that the present invention may be used to achieve good quality metal deposition, for example deposition with a uniform profile. The present invention may also be used to achieve specific deposition profiles, such as an intentionally non-uniform profile. The present invention may also be used for corrosion protection, for example by applying a protective bias to the substrate through the capacitive element. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.