Patent Publication Number: US-2005121329-A1

Title: Thrust pad assembly for ECP system

Description:
FIELD OF THE INVENTION  
      The present invention relates to electrochemical plating 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 electrochemical plating system having a thrust pad assembly which reduces the quantity of metal electroplated on the edge regions of a cathode/wafer by the application of variable pressure to the center and edge regions of the wafer.  
     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.  
      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.  
      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.  
      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.  
       FIG. 1  schematically illustrates a typical standard or conventional electroplating system  10  for depositing a metal such as 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 the 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 .  
      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 .  
      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 ++ +2 e   −   (1) 
 
      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) 
 
      At the cathode/wafer  18 , the electrons harvested from the anode  16  flow through the wiring  38  and reduce copper cations in solution in the copper sulfate bath  20  to electroplate the reduced copper onto the patterned surface  19   a  of the cathode/wafer  18 : 
 
Cu ++ +2 e   − →Cu   (3) 
 
      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 on the oxidizing surface  22 .  
      As shown in  FIG. 2 , during the electroplating process air pressure  26  is applied against a thrust pad  24 , which in turn applies pressure through a contact ring (not shown) that is disposed in electrical contact with the current source  12  and presses against the backside  19  of the cathode/wafer  18 . The pressure exerted by the contact ring against the backside  19  of the cathode/wafer  18  increases the ohmic contact between the contact ring and the cathode/wafer  18 , thus enhancing electroplating of the copper or other metal on the patterned surface  19   a  of the cathode/wafer  18 . The air pressure  26  is typically the same throughout all regions on the entire surface of the thrust pad  24 . Accordingly, substantially equal quantities of the electroplated copper are applied to both the center region  18 a and the edge regions  18   b  of the cathode/wafer  18 .  
      After the electroplating process, some excess electroplated metal must typically be removed from the edge regions  18   b  of the cathode/wafer  18  since excess metal in the edge regions  18   b  is potentially a significant source of contaminant particles during subsequent processing of the wafer  18 . This excess metal removal process is typically carried out using an edge bevel clean process that is integrated into the electrochemical plating apparatus. However, such edge bevel cleaning of wafers required after electroplating is a common source of process flow bottlenecking and hinders orderly and efficient flow of the electroplating process sequence.  
      It has been found that the quantity of metal electroplated onto the edge region of a substrate can be reduced by the application of reduced-magnitude pressure to the edge region of the substrate during electrochemical plating, thus reducing the ohmic contact between the contact ring and the edge region of the substrate. This eliminates the need for edge bevel cleaning of substrates after electrochemical plating and facilitates efficient and orderly flow of substrates and increases throughput of electrochemically-plated substrates throughout a process flow sequence during the fabrication of semiconductor integrated circuits.  
      Accordingly, an object of the present invention is to provide a new and improved thrust pad assembly which can be adapted to an electrochemical plating system.  
      Another object of the present invention is to provide a new and improved thrust pad which is capable of applying pressure of reduced magnitude against the edge region of a substrate to reduce or eliminate the deposition of excess quantities of a metal on the edge region during electrochemical plating of the substrate.  
      Still another object of the present invention is to provide a new and improved thrust pad assembly which generates zones of variable pressure against a substrate in the electrochemical plating of the substrate.  
      Yet another object of the present invention is to provide a new and improved thrust pad assembly which is capable of reducing or eliminating the need for edge bevel cleaning of substrates after electrochemical plating.  
      A still further object of the present invention is to provide a new and improved thrust pad assembly which significantly increases throughput of substrates during electrochemical plating.  
      Another object of the present invention is to provide a novel method for electroplating a metal onto a substrate.  
     SUMMARY OF THE INVENTION  
      In accordance with these and other objects and advantages, the present invention is generally directed to a new and improved thrust pad assembly which is capable of reducing the quantity of metal electroplated onto the edge region of a substrate to eliminate or reduce the need for edge bevel cleaning or removal of excess metal from the substrate after the electroplating process. The thrust pad assembly typically includes an air platen through which air is applied at variable pressures to the central and edge regions, respectively, of a thrust pad. The thrust pad applies pressure to a contact ring connected to an electroplating voltage source. The contact ring applies relatively less pressure to the edge region than to the central region of the substrate, thereby reducing the ohmic contact between the contact ring and the edge region of the substrate. Therefore, excess electroplating of the metal onto the edge regions of the substrate is eliminated or substantially reduced.  
      The present invention further includes a method of electroplating a metal on a substrate. The method includes providing a substrate, providing a contact ring in contact with the substrate, providing the contact ring and the substrate in an electrolyte bath, providing an anode in the electrolyte bath, applying a voltage to the contact ring and the anode, applying a central pressure to a central region on the substrate, and applying a peripheral pressure which is less than the central pressure to an edge region on the substrate.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic view of a typical conventional electrochemical plating system for the electrochemical plating of a metal layer onto a substrate;  
       FIG. 2  is a schematic view of a typical conventional thrust pad assembly for an electrochemical plating system;  
       FIG. 3  is a cross-sectional, partially schematic, view of a thrust pad assembly in accordance with the present invention;  
       FIG. 4  is a schematic view of an electrochemical plating system in implementation of the thrust pad assembly of the present invention;  
       FIG. 5  is a top view of an air platen element in an illustrative embodiment of the thrust pad assembly of the present invention; and  
       FIG. 6  is a graph illustrating the relationship between plating thickness (on the Y-axis) and pressure (on the X-axis) applied by the thrust pad assembly of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention has particularly beneficial utility in the electroplating of copper or other metals onto a semiconductor wafer substrate in the fabrication of integrated circuits on the substrate. However, the invention is not so limited in application, and while references may be made to such semiconductor wafer substrate and integrated circuits, the invention may be more generally applicable to electroplating metals on substrates in a variety of industrial applications.  
      The present invention is generally directed to a new and improved thrust pad assembly which is suitable for preventing deposition of excess quantities of metal onto the edge or peripheral region of a substrate as copper or other metal is electroplated onto the substrate in the fabrication of semiconductor integrated circuits on the substrate. The thrust pad assembly eliminates the need for edge bevel cleaning or removal of excess metal from the edge region of the substrate after the electroplating process. The thrust pad assembly typically includes an air platen through which air is applied at variable pressures to the central and edge regions, respectively, of a thrust pad. The thrust pad, in turn, transmits this variable pressure to a contact ring which is electrically connected to the electroplating current source. The thrust pad applies relatively less pressure to the edge region than to the central region of the substrate, thus reducing ohmic contact between the contact ring and the edge region of the substrate. Electrical resistance between the anode and the edge region of the substrate is reduced with respect to the electrical resistance between the anode and the central region of the substrate. This variable pressure application to the substrate, and resulting disparity in electrical resistance, is used to control the substrate plating shape such that excess electroplating of the metal onto the edge region of the substrate is reduced or eliminated while electroplating of the metal onto the central region of the substrate remains optimal.  
      The present invention further contemplates a method of electroplating a metal on a substrate. The method includes providing a substrate, providing a contact ring in contact with the substrate, immersing the contact ring and the substrate in an electrolyte bath, immersing an anode in the electrolyte bath, applying a voltage to the contact ring and the anode, applying a central pressure to a central region on the substrate, and applying a peripheral pressure which is less than the central pressure to an edge region on the substrate.  
      Referring to  FIGS. 3-5 , an illustrative embodiment of the thrust pad assembly of the present invention is generally indicated by reference numeral  40 . The thrust pad assembly  40  includes a generally disk-shaped air platen  42  having a circular central region  42   a  and an annular edge region  42   b  surrounding the central region  42   a , as shown in  FIG. 5 . In a typical embodiment, the central region  42   a  represents typically from about 50% to about 80% of the total surface area of the air platen  42 , whereas the encircling edge region  42   b  represents typically from about 20% to about 50% of the total surface area of the air platen  42 . Multiple central air openings  44  extend through the central region  42   a  of the air platen  42 , and multiple peripheral air openings  46  extend through the edge region  42   b  of the air platen  42 . As shown in  FIG. 3 , the central air openings  44  are provided in pneumatic communication with a central air source  76  of central air pressure  45 , and the peripheral air openings  46  are provided in pneumatic communication with a peripheral air source  77  of peripheral air pressure  47 .  
      As further shown in  FIG. 3 , an electrically-conductive contact ring  50  extends downwardly from the bottom surface of the air platen  42 , and a thrust pad  48  is provided in the contact ring  50 . The thrust pad  48  is typically a resilient material such as rubber. As shown in  FIG. 3 , the central air openings  44  and the peripheral air openings  46  of the air platen  42  communicate with the upper surface  49  of the thrust pad  48 . A wafer clamp  52  (shown in phantom) removably secures a cathode/wafer  54  to the bottom surface of the contact ring  50 , typically in conventional fashion.  
      Referring to  FIG. 4 , a schematic of an electroplating system  60  which is suitable for implementation of the present invention is shown. The electroplating system  60  typically includes a bath container  64  in which a typically copper anode  66  and the thrust pad assembly  40  to which is mounted the cathode/wafer  54  are placed, the cathode/wafer  54  being the semiconductor wafer that is to be electroplated with the copper or other metal. A negative terminal  62   a  of an adjustable current source  62  is connected to the contact ring  50  of the thrust pad assembly  40  through wiring  67 . A positive terminal  62   b  of the adjustable current source  62  is connected to the anode  66  through wiring  68 . The bath container  64  holds an electroplating bath  70  typically of acid copper sulfate (CuSO 4 ) solution, for example, which may include an additive for filling of submicron features and leveling the surface of the copper electroplated on the wafer  54 , as is known by those skilled in the art. The electroplating system  60  may include additional features such as a bypass pump/filter (not shown) connected to the bath container  64  and an electrolyte holding tank (not shown) connected to the the bypass pump/filter and to the bath container  64  to facilitate the addition of electrolytes to the bath  70  and circulation of the bath  70 , as needed.  
      Referring next to  FIGS. 3 and 4 , in application of the thrust pad assembly  40 , the thrust pad assembly  40  is initially assembled in the bath container  64 , with the clamp  52  ( FIG. 3 ) attaching the wafer  54  to the contact ring  50 , and the anode  66  and the thrust pad assembly  40  with the cathode/wafer  66  are immersed in the electrolyte bath  70 . The electroplating system  60  is operated typically in conventional fashion to electroplate the metal from the metal electrolyte solution in the bath  70 , onto the patterned surface  57   a  of the wafer  54 . Accordingly, the current source  62  applies a selected voltage potential, typically at room temperature, between the anode  66  and the cathode/wafer  54 . This voltage potential creates a magnetic field around the anode  66  and the cathode/wafer  54 , which magnetic field affects the distribution of the metal ions in the electrolyte bath  70 . 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  66  and the cathode/wafer  54 . Consequently, the metal is oxidized typically at the upper oxidizing surface of the anode  66  as electrons from the metal anode  66  reduce the ionic metal in the electrolyte solution bath  70  to form a substantially corrosion-resistant electroplated metal layer  74  on the patterned surface  57   a  of the wafer  54 , as shown in  FIG. 4 , at the interface between the cathode/wafer  54  and the electrolyte bath  70 .  
      As the metal layer  74  is electroplated onto the wafer  54 , the contact ring  50  of the thrust pad assembly  40  applies pressure of variable magnitude against the backside  57  of the wafer  54 , as follows. As shown in  FIG. 3 , central air pressure  45  is directed from the central air source  76  through the respective central air openings  44  of the air platen  42  and against the upper surface  49  of the thrust pad  48  at a pressure of typically greater than about 14 psi. Similarly, peripheral air pressure  47  is directed from the peripheral air source  77  through the respective peripheral air openings  46  of the air platen  42  and against the upper surface  49  of the thrust pad  48  at a pressure of typically less than about 14 psi. Accordingly, the central portion of the contact ring  50  applies a pressure of typically greater than about 14 psi to the backside  57  of the wafer  54 , whereas the peripheral portion of the contact ring  50  applies a pressure of typically less than about 14 psi to the backside  57  of the wafer  54 . Because the ohmic contact between the contact ring  50  and the wafer  54  is directly proportional to the pressure applied by the contact ring  50  against the wafer backside  57 , the electrical resistance between the anode  66  and the cathode/wafer  54  at the edge region  54   b  of the wafer  54  is correspondingly less than the electrical resistance between the anode  66  and the cathode/wafer  54  at the center region  54   a  of the wafer  54 . Consequently, the electroplated metal  57   a  is correspondingly thicker at the center region  54   a  than at the edge region  54   b  of the wafer  54  for a given period of electroplating time. Typically, the electroplating process is carried out for a period of typically about 2 minutes to deposit an electroplated metal  74  having a thickness of typically at least about 7,000 angstroms at the center region  54   a  and a thickness of typically about 500-1000 angstroms at the edge region  54   b  of the wafer  54 .  
      It will be appreciated by those skilled in the art that because the thickness of the electroplated metal  74  at the edge region  54   b  of the wafer  54  is attenuated with respect to the thickness of the electroplated metal  74  at the center region  54   a  throughout the electroplating process, electroplating of excessive quantities of the metal layer  74  at the edge region  54   b  of the wafer  54  is prevented. Accordingly, there is no need to subject the wafer  54  to edge bevel clean methods which would otherwise be needed to remove excess electroplated metal from the edge region  54   b . This eliminates process bottlenecking at the electroplating station and promotes an orderly and efficient flow of wafers through the electroplating process.  
      Referring next to the graph of  FIG. 6 , wherein the relationship of pressure applied against the backside of a wafer is shown in relation to the thickness of metal electroplated onto the wafer. As indicated by reference numeral  50 , when a pressure of from about 0 psi to about 13 psi is applied to the backside of the wafer, the thickness of metal electropated onto the wafer is from typically about 100 to about 500 angstroms. When a pressure of greater than about 14 psi is applied to the backside of the wafer, the thickness of metal electroplated onto the wafer is about 7,000 angstroms. As indicated by reference numeral  60 , this thickness gradually increases at pressures above about 23 psi.  
      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.