Patent Publication Number: US-2006000708-A1

Title: Noble metal contacts for plating applications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation of co-pending U.S. patent application Ser. No. 10/349,761, filed on Jan. 22, 2002, entitled NOBLE METAL CONTACTS FOR PLATING APPLICATIONS. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to semiconductor substrate processing systems. More specifically, the present invention relates to an apparatus for performing an electrochemical plating process in a semiconductor substrate processing system.  
      2. Description of the Related Art  
      In ultra large scale integration integrated circuit (IC) devices (i.e., devices having more than one million logic gates), the multilevel interconnects are formed by filling the interconnect features (i.e., trenches, vias, and the like) with a metal, such as copper (Cu), aluminum (Al), and the like. Copper is the wiring material of choice in the interconnecting networks of advanced IC devices. In addition to superior electrical conductivity, copper is more resistant than aluminum (Al) to electromigration that, in operation, may destroy a thin film conductive line that carries an electrical current.  
      As dimensions of the interconnect features decrease and the aspect ratios increase, a void-free metal fill using conventional metallizing techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and the like, becomes increasingly difficult. As a result thereof, during manufacturing of advanced IC devices, electrochemical plating (ECP) has emerged as a production-worthy process for metallizing interconnect features.  
      The ECP process is, generally, a two-step process. During a first step, a seed layer (e.g., copper seed layer) is formed upon the interconnect features, as well as elsewhere on the substrate. The seed layer may extend from a device surface (i.e., plating surface) of the substrate around the beveled edges to a backside (i.e., non-plating) surface of the substrate. Generally, the seed layer is deposited using a CVD, PVD, evaporation, and the like process. Then, during a second step, the substrate is exposed to a plating solution, while an electrical bias is simultaneously applied between the substrate and an anode electrode positioned within the plating solution. The plating solution is rich in ions of the metal to be plated onto the substrate (i.e., copper) and, as such, the electrical bias causes ions of such metal to be urged out of the plating solution and deposited onto the seed layer.  
      The electrical bias is provided to the substrate using a plurality of electrical contacts. Typically, the same contacts are also used to provide a support to the substrate during the ECP process. Generally, such contacts are collectively bonded to a conductive support ring and engage the seed layer of the substrate. In operation, the electrical contacts apply a voltage to the seed layer, creating a current path through the plating solution. Such current path has an associated electrical resistance. The contacting surface of the tip of the electrical contact (i.e., portion of the contact having a contact with the seed layer) erodes as a result of exposure to the plating solution. Similarly, the resistance of the current path changes when a mechanical and electrical interface formed between the tip and a shank of the contact is degraded by the plating solution.  
      In the prior art, to extend longevity of the electrical contacts, the contacting tip may be coated with a protective layer of noble metals, such as platinum (Pt), indium (In), and the like, or with a layer of an alloy of such metals. Generally, the contacting tip is attached to a shank of the contact using fasteners, such as screws and the like. Still, the electrical contacts of the prior art have limited service life and variable contact resistance, e.g., due to the thinness of the protective coating, deterioration of the interface between the tip and shank of the contact, and the like. The changes in the contact resistance of the electrical contact result in the non-uniformity of the film plated upon the substrate and may cause the ECP process to be defective.  
      Therefore, there is a need in the art for an improved electrical contact that provides an electrical bias to a substrate during an electrochemical plating process.  
     SUMMARY OF THE INVENTION  
      The present invention is an apparatus for electrochemical plating, comprising a support ring having a plurality of inwardly directed shanks extended from an inner circumference of the ring, wherein each of the shanks comprises a contacting tip brazed to the distal end the shank.  
      In one embodiment, the contacting tip is formed from a platinum/iridium alloy and is brazed to the shank using a palladium/cobalt alloy. In another embodiment, the shanks are formed from a metal, such as niobium (Nb), tantalum (Ta), and the like, that oxidizes in a plating solution and produces a protective oxide layer upon the shank. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a schematic, partial perspective and sectional view of an exemplary plating apparatus according to one application of the present invention;  
       FIGS. 2A and 2B  are, respectively, schematic, cross-sectional and top plan views of an exemplary support ring according to one embodiment of the present invention;  
       FIGS. 3-5  are schematic, cross-sectional views of contacts of the support ring of  FIGS. 2A, 2B  according to embodiments of the present invention; and  
       FIGS. 6A-6F  illustrate an exemplary support ring at different steps of fabricating the contacts of  FIG. 3  according to one embodiment of the present invention. 
    
    
      To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
      It is to be noted, however, that the appended drawings illustrate only exemplary 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.  
     DETAILED DESCRIPTION  
      The present invention is an apparatus for providing an electrical bias to a substrate in a processing system performing an electrochemical plating process. The apparatus (e.g., support ring) comprises a conductive annular body supplied with a plurality of flexible current-carrying electrical contacts. The contacts are a part of or conductively bonded to the support ring. Each contact comprises a shank and a contacting tip that is brazed to the shank. In operation, the contacting tips engage a peripheral portion of the substrate. A plurality of such flexible electrical contacts may be formed using, e.g., a pre-formed ring of the platinum/iridium alloy that is brazed into the support ring and then machined to define the individual contacts.  
      In one embodiment, the contacting tips are formed from an alloy comprising at least two noble metals (e.g., a platinum/iridium (Pt/In) alloy and the like). In another embodiment, shanks are formed from a metal, such as niobium (Nb), tantalum (Ta), and the like, that oxidizes in a plating solution and produces a protective oxide layer upon the shank.  
       FIG. 1  is a schematic, partial perspective and sectional view of an exemplary electrochemical plating (ECP) apparatus  100  utilizing a support ring  150  with flexible electrical contacts  156  according to one embodiment of the present invention.  FIGS. 2A and 2B  are, respectively, schematic, cross-sectional and top plan views of the support ring  150 . For best understanding of the invention, the reader should refer simultaneously to  FIGS. 1, 2A  and  2 B. The images in  FIGS. 1, 2A  and  2 B are simplified for illustrative purposes and are not depicted to scale.  
      The ECP apparatus  100  generally includes a head assembly  102 , a substrate securing assembly  110 , and a plating bath assembly  161 . The head assembly  102  is attached to a base  104  using a support arm  106 . In operation, the head assembly  102  defines the position and movements of the substrate securing assembly  110  that places a substrate  120  in a plating solution  165  for plating.  
      The plating bath assembly  161  includes an inner basin  167  that is contained within a larger outer basin  163 , and an anode assembly  170 . A plating solution (electrolyte)  165  is supplied to the inner basin  167  through an inlet  166  at a bottom  169  of the basin. The inlet  166  is generally connected to a supply line to a reservoir (not shown) for the plating solution  165 . The outer basin  163  collects the plating solution from the inner basin  167  and drains the solution through a fluid drain  168  back to the reservoir.  
      The anode assembly  170  is positioned within a lower region of the inner basin  167  and provided with a diffusion plate  172  (e.g., a porous ceramic member or the like) positioned above the anode assembly  170 . An electrical connection to the anode assembly  170  is provided using an anode contact  174  formed from a conductive material that is insoluble in the plating solution (e.g., platinum, platinum-coated steel, and the like).  
      The anode contact  174  extends through the bottom  169  and is coupled to an anode terminal of an electrical power supply (not shown), while a cathode terminal of the power supply is coupled to a support ring  150  (see discussion in reference to the substrate securing assembly  110  below). As such, the power supply provides an electrical bias between the anode assembly  170  and the substrate  120 . In operation (i.e., when a substrate  120  is immersed into the plating solution), in response to the bias, an electrical ionic current (represented by current flux lines  180 ) flows from the anode assembly  170  to a plating surface  122  of the substrate  120 . The electrical ionic current deposits the plating material onto the surface  122  and, as such, metallized the interconnecting features on the substrate  120 .  
      The substrate securing assembly  110  comprises a mounting plate  146 , a thrust plate  144 , a seal plate  142 , a housing  116 , and a support ring  150 . The substrate securing assembly  110  may also comprise an optional inflatable bladder assembly or o-ring (not shown) that applies an evenly distributed downward force to a non-plating surface  124  of a substrate  120 . The mounting plate  146  and thrust plate  144  couple the substrate securing assembly  110  to the head assembly  102 .  
      The support ring  150  comprises a plurality of flexible electrical contacts  156  that support the substrate  120 . The contacts  156  are disposed around an inner circumference of the support ring  150  in a circular pattern and extend from the circumference substantially radially inward. Each contact  156  comprising a shank  301  and a contacting tip  316 . Generally, the contacting tips  316  engage the substrate  120  around the edge of the substrate. In the depicted embodiment, the shank  301  extends from a midpoint  320  of the support ring  150  that is thicker than the shank. In other embodiments, the shank  301  may be coplanar with either an upper surface  322  or a bottom surface  324  of the support ring  150 .  
      The support ring  150  is further supplied with an optional protective coating  130  to protect the contacts  156  from the plating solution  165 . The protective coating  130  may comprise at least one layer of material that is chemically resistant to the plating solution, e.g., polytetrafluoroethylene-based material, such as AFLON®, TEFZEL®, KALREZ®, VITON®, and the like. Such materials are available from AG Fluoropolymers USA, Inc., Pennsylvania and other suppliers. Alternatively, the housing  116  may also be provided with such protective coating (not shown).  
      Generally, the housing  116  is formed from an electrically conductive material (e.g., stainless steel) coated with an insulator. As such, the housing  116  may be used to couple the support ring  150  to the power supply that facilitates the plating process. Therefore, electrical power (e.g., in a form of a controlled DC current) may be supplied to the contacts  156  by coupling the power supply either to the housing  116  or directly to the support ring  150 . The contact  156  conducts an electrical current from the support ring  150  to a seed layer deposited on the substrate  120 . Generally, the electrical current is supplied to the contacts  156  cooperatively. Alternatively, the current may be supplied to groups of the contacts, or to individual contacts that are electrically isolated one from another.  
      The support ring  150  may further comprise scallops (not shown) that are disposed along the bottom surface  324  of the ring to increase uniformity of the flux towards the plating surface  122  of the substrate  120 . Such scallops are described in commonly assigned U.S. patent application Ser. No. 10/278,527, filed Oct. 22, 2002, which is incorporated herein by reference.  
       FIGS. 3-5  depict schematic, cross-sectional views of exemplary embodiments of contacts of the support ring  150 . For best understanding of these embodiments, the reader should simultaneously refer to  FIGS. 3-5 . The images in  FIGS. 3-5  are simplified for illustrative purposes and are not depicted to scale. Those skilled in the art will understand that the scope of the invention is not limited to such exemplary embodiments.  
       FIG. 3  depicts a flexible contact  350  where the contacting tip  316  is brazed into a recess  314  at the distal end of the shank  301 . Accordingly,  FIG. 4  depicts a flexible contact  450  where the contacting tip  316  is brazed to a sidewall  328  of the shank  301 , and  FIG. 5  depicts a flexible contact  550  where the contacting tip  316  is supplied with a shoulder  330  that is brazed to a sidewall  502  of the shank  301 .  
      In depicted embodiments, the support ring  150  and the shanks  301  are formed from a single piece of material, such as a stainless steel (e.g., steel “ 302 ”) and the like. In an alternative embodiment, the stainless steel shanks  301  may be bonded to the support ring  150  using, e.g., welding, brazing, and the like. Further, the protective coating  130  is applied to protect, in operation, the contacts and support ring from the plating solution. In a further alternative embodiment, the shanks  301  may be formed from such a metal (e.g., niobium (Nb), tantalum (Ta), and the like) that will oxidize in the plating solution  165  and produce a protective oxide layer (not shown) upon the shank. When the support ring  150  comprised such oxidized shanks, the protective coating  130  is considered optional.  
      The shank  301  has a length and cross-sectional form factor that are selected such that the contact  156  provides support and electrical contact to the substrate  120 , however, causes no damage to the surface of the substrate. In one exemplary embodiment, a length  306 , thickness  308 , and width  309  of the shank  301  are about 2 to 10 mm, 0.2 to 1 mm, and 0.5 to 10 mm, respectively. In this embodiment, the contacting tip  316  has a length  310  (measured on a contact surface  313 ), thickness  312  (measured from a surface  311  of the shank  301  to the contact surface  313 ), and width  315  of about 0.05 to 1 mm, 0.1 to 1 mm, and 0.5 to 10 mm, respectively. In the depicted embodiment, the width  309  of the shank  301  and the width  315  of the contacting tip  316  are the same, however, in other embodiments, the widths  309  and  315  may be different.  
      A number of the contacts  156  may vary, for example, according to a diameter and weight of the substrate  120 . In one particular embodiment, to support a 300 mm silicon (Si) wafer, the support ring  150  and the shanks  301  were formed from a single piece of stainless steel “ 302 ”, and the support ring comprised 500 contacts  350 . Each shank was 4 mm long, 0.8 mm thick, and 0.5 mm wide and comprised a contacting tip that was 0.2 mm long, 0.4 mm thick, and 0.5 mm wide.  
      The contacting tip  316  may be formed from an alloy that comprises at least two noble metals (e.g., platinum/indium (Pt/In) alloy having about 85% of platinum and about 15% of indium) and then bonded to the shank  301 . In one embodiment, the contacting tip  316  is brazed to the shank  301  using, e.g., a palladium/cobalt (Pd/Co) alloy comprising about 65% of palladium and about 35% of cobalt.  
      In operation, the plating solution may cause corrosion of a contact, as well as corrosion and electrical degradation of the contacting tip and interface between the tip and shank. However, in the contacts  156 , the entire contacting tip  316  is formed from a chemically resistant alloy and then brazed to the shank  301  using also a chemically resistant alloy. Brazing facilitates a high quality mechanical and electrical interface between the tip and shank. Additionally, in embodiment shown in  FIG. 5 , a position of a brazed interface is moved away from a contact surface  313  of the contacting tip  316  and, as such, from the plating solution. In each embodiment, the shank is covered with the protective coating  130 , as discussed above. In an alternative embodiment, the shank  301  may be formed from such a metal (e.g., tantalum or niobium) that, when exposed to the plating solution, develops a protective oxide layer on the shank in lieu of using a separate protective coating  130 . As such, in either embodiment, the contacts  156  provide greater longevity (service life), reliability, and performance (e.g., stability ands low value of electrical resistance) than other contacts used in the ECP apparatuses.  
       FIGS. 6A-6F  depict a support ring at different steps of fabricating the contacts of  FIG. 3  according to one embodiment of the present invention. In operation, uniform contact resistance promotes uniform plating thickness. As such, a process of fabricating the contacts intends to ensure that the contacts being formed have uniform contact resistance. The views in  FIGS. 6B-6E  are taken along a centerline  6 - 6  in  FIG. 6A . For best understanding of this embodiment of the invention, the reader should simultaneously refer to  FIGS. 6A-6F . The images in  FIGS. 6A-6F  are simplified for illustrative purposes and are not depicted to scale.  
       FIG. 6A  depicts a top plan view of a support ring  450  before the process of fabricating the contacts begins. The support ring  450  comprises an outer region  402  and inner region  404 . In the depicted embodiment, the regions  402  and  404  are formed from an annular piece of a conductive material, e.g., stainless steel “ 302 ”. The outer region  402  is thicker than the inner region  404 . In one embodiment, the inner region  404  has a thickness  412  that is equal to the thickness of a contact  428  being formed, while a width  414  of the inner region  404  is generally greater than a length  411  of the contact (discussed in reference to  FIG. 6E  below). Generally, the inner region  404  is disposed at a midpoint  406  of the outer region  402  (discussed in reference to  FIG. 6B  below). Alternatively, the inner region  404  may be coplanar (not shown) with either upper ( 408 ) or bottom ( 410 ) surface of the outer region  402 .  
       FIG. 6B  depicts a portion of the support ring  450  after a groove  416  is formed in the distal portion of the inner region  404 . In one embodiment, the groove  416  has a depth  418  of about 0.3 to 0.1 mm and a width  420  of about 0.5 to 10 mm. Generally, the groove  416  is adapted to receive a pre-formed ring  426  (discussed in reference to  FIG. 6D  below).  
       FIG. 6C  depicts a schematic, cross-sectional view of a portion of the support ring  450  having a brazing material  422  placed and melted in the groove  416 . A melting temperature of the brazing material is below melting temperatures of materials of the support ring  450  and contacting tip (discussed in reference to  FIG. 6D  below). When melted, the brazing material forms a layer  424  in the groove  416 . In one embodiment, the brazing material comprises a palladium/cobalt alloy having about 65% of palladium and about 35% of cobalt. Such brazing material wets stainless steel “302” (inner region  404 ) and platinum/indium alloy (ring  426 ) and possesses high corrosion resistance to the plating solution, high purity, and a low vapor pressure at the brazing temperature. The palladium/cobalt alloy has a melting temperature of approximately 1220 degrees Celsius that is substantially below the melting temperature of the stainless steel “ 302 ” (approximately 1620 degrees Celsius).  
       FIG. 6D  depicts a schematic, cross-sectional view of a portion of the support ring  450  after the pre-formed ring  426  is positioned in the groove  416  on a layer  424  of melted brazing material  472 . In one embodiment, the ring  426  fits into the groove  416 . The pre-formed ring  426  comprises, for example, a platinum/indium alloy having about 85% of platinum and about 15% of indium. Such alloy has a melting temperature of approximately 2230 degrees Celsius.  
      When heated above its melting temperature (i.e., above of approximately 1220 degrees Celsius), the platinum/indium alloy wets a bottom surface  415  of the groove  416  and a bottom surface  428  of the pre-formed ring  426 . In the depicted embodiment, a width  440  of the pre-formed ring  426  is selected to fit into the groove  416  and a height  444  of the ring is selected such that a height  442  of an exposed portion of the ring is equal to that of the contacting tip  425  (discussed in reference to  FIG. 6E  below).  
      After the platinum/indium alloy is then cooled below its melting temperature, the alloy bonds the pre-formed ring  426  to the groove  426 . The brazing process develops a high strength mechanical and high quality electrical interface between the pre-formed ring  426  and inner region  402 . After brazing the pre-formed ring  426  into the groove  416 , an extending inwardly portion of the inner region  404  (portion  430 ) is generally machined off to prevent, in operation, shielding of the edge of the substrate from the plating solution. Alternatively, the annular portion  430  may be removed using an EDM process (discussed in reference to  FIGS. 6E and 6F  below). In a further embodiment, the pre-formed ring  426  may also be machined, e.g., the edges and upper surface of the ring may be rounded or polished, and the like.  
       FIGS. 6E and 6F  depict, respectively, schematic, cross-sectional and top plan views of a portion of the support ring  450  after a contact  428  is formed in the inner region  404 . The contact  428  may be formed using, for example, an electric discharge machining (EDM) technique and the like. The EDM process removes portions  446 ,  448 , and the like of the inner region  404  between the adjacent contacts (e.g., contacts  428   a ,  428   b , and  428   c ). In a further embodiment, the EDM process provides surface finishing to the contacts and/or contacting tips.  
      After the EDM process, the remaining portions of the inner region  404  and ring  426  form shanks  432  (e.g., shanks  432   a ,  432   b , and  432   c ) and contacting tips  425  (e.g., contact tips  425   a ,  425   b ,  425   c ). The EDM process continues until all contacts  428  of the support ring  450  are fabricated as described above.  
      Those skilled in the art will appreciate that the contacts shown in  FIGS. 4 and 5  may be fabricated using techniques that are similar to the described above in reference to the contact of  FIG. 3 . Similarly, to fabricate the contacts  428  having the tantalum or niobium shanks  432 , the inner portion  404  of the support ring  450  may be formed from tantalum or niobium, respectively.  
      While foregoing is directed to the illustrative embodiment 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.