Patent Abstract:
The force required to seal a surface of an object for electrodeposition may be controlled. For example, the object may rest on a support that carries the majority of the force required for surface sealing. Further, pads mounted on the ends of flexible beams may exert a variable force to establish electrical contact with the object that may be controlled. By controlling the forces exerted on an object damage to the object&#39;s surface may be minimized or eliminated.

Full Description:
BACKGROUND  
       [0001]     The present invention relates generally to electrodeposition and particularly to electrodeposition onto silicon wafers or other objects.  
         [0002]     With large scale circuit integration comes a need for smaller features and increased circuit density. As component density increases, the surface space available to connect the components decreases. One solution to this “wiring” problem is to layer insulating materials and conductive materials. Generally, the conductive layers are connected by conductive vias or plugs formed through the insulating material.  
         [0003]     The metal and insulating material used to interconnect device components may determine the overall device performance. For example, interconnect resistance (R) and capacitance (C), the RC constant, may be an indicator of circuit speed. For example, a high RC constant may indicate a slow circuit signal. Interconnecting components with metals having low resistivity may lower the RC value. Further, separating interconnects with a dielectric having a low dielectric constant may reduce capacitance, which would also lower the RC value. Thus, when resistivity and capacitance are both reduced, device performance may increase.  
         [0004]     Aluminum and aluminum alloys have enjoyed widespread use to interconnect components in integrated circuits. However, aluminum may limit the speed of some circuits. Further, aluminum may be difficult to deposit in vias having small depth to width or aspect ratios. In contrast, copper has a lower resistance than aluminum, hence it is a better conductor. Thus, copper layered with a low capacitance dielectric may be well suited for smaller, faster integrated circuits. Copper use in integrated circuits however, is not without its own unique challenges. For example, copper is not easily patterned or etched. Thus, copper deposition in vias and/or trenches that have been etched in a dielectric is one way to form copper interconnects and plugs.  
         [0005]     Copper may be deposited on a wafer via chemical vapor deposition (CVD), plasma enhanced CVD, sputtering, and electrodeposition such as electroplating. Electroplating generally takes place at lower temperatures and at a lower cost than other deposition techniques. Further, electroplating is a favored deposition technique when using dielectrics having low dielectric constant.  
         [0006]     To deposit a metal on a wafer via electrodeposition, the back surface of the wafer is sealed off and electrical contact is made with the front surface of the wafer. Sealing the back of the wafer off and establishing electrical contact with front of the wafer may require considerable force to be exerted on the wafer. As such, soft materials such as low dielectric insulators may be susceptible to damage.  
         [0007]     Accordingly, there is a need for a way to deposit a conductive material without causing significant damage to the object that the conductive material is to be deposited on. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]      FIG. 1  is a cross section of a simplified system for electrodeposition;  
         [0009]      FIG. 2  is a bottom-up view of a portion of the system of  FIG. 1  according to some embodiments of the present invention;  
         [0010]      FIG. 3  is a cross section of the portion of the system of  FIG. 2 ;  
         [0011]      FIG. 4  is a cross section of an alternate embodiment of a portion of the system of  FIG. 1 ;  
         [0012]      FIG. 5  is a partial cross section of the portion of  FIG. 3  before electrical contact is made with an object to be electroplated;  
         [0013]      FIG. 6  is a partial cross section of the portion of  FIG. 3  when initial contact is made with the object to be electroplated; and  
         [0014]      FIG. 7  is a partial cross section of the portion of  FIG. 3  when electrical contact for deposition is made with the object to be electroplated. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Referring to  FIG. 1 , an exemplary plating cell or system  10  for electrodeposition is shown in simplified form. Generally, a metal, metal alloy or other conductive material may be deposited on an object  12  while the object  12  is immersed in the plating cell  10 . For example, in some embodiments, the object  12  may be a wafer. Thus, copper, gold, lead, nickel or alloys thereof may be deposited on the wafer  12  using system  10 . The system  10  may include a container  14 , an anode  16 , a seal assembly  18 , a frame  20 , a base  22  and a power supply  24 .  
         [0016]     The container  14  may be any container for use in electrodeposition. As shown in  FIG. 1 , the container  14  is box-like having sides  26  and a bottom  28 , although embodiments of the present invention are not limited in this respect. The container  14  may also include a top (not shown). When in use, the container  14  may be filled with an electrolytic solution  32 . In some embodiments, ions in the electrolyte  32  facilitate electroplating. Further, in some embodiments the anode  16  may add ions to the electrolytic solution  32 . For example, the anode  16  may be disposed in the electrolyte  32  within container  14 . Thus, when a voltage potential is applied to the anode  16  and the object  12  to be plated, ions may be released into the electrolyte  32  via an oxidation reaction. Generally, the anode  16  is a metal or combination of metals and may be a single piece or segmented, although embodiments are not limited in this respect.  
         [0017]     The seal assembly  18  may also be disposed in the container  14 . In some embodiments, the seal assembly  18  may include a thrust plate and/or a seal plate (not individually shown) having a flexible seal material or mechanism such as a sealing ring  30 . In some embodiments, the sealing ring  30  may be positioned to contact the backside  36  of the wafer  12  to create a watertight seal to prevent deposition and/or contamination on the wafer backside  36 . For example, a force may be applied by the seal assembly  18  (force producing mechanism not shown) to the backside  36  of the wafer  12  while the base  22  supports the wafer  12  at the front side  36 , holding the wafer  12  stationary. Thus, when the sealing force is applied, a watertight seal is create by the sealing ring  30 . As such, conductive material is not deposited on the wafer backside  36 .  
         [0018]     During electrodeposition, the frame  20  and base  22  may oppose the seal assembly  18  to contact the front side  34  of the wafer  12 . Generally, current is supplied to the wafer  12  through the frame  20 . In contrast, there is no electrical connection between the base  22  and the wafer  12 . Thus, the base  22  may serve as a support for the wafer  12  during surface sealing and/or electrodeposition.  
         [0019]     The power supply  24  may connect the frame  20  and the anode  16 . Generally, the power supply  24  delivers a positive voltage to the anode  16  and a negative voltage to the frame  20 . In this way, when the wafer  12  and anode  16  are disposed in the electrolyte  32  an electric circuit may be completed from the wafer  12  to the anode  16 .  
         [0020]     When system  10  is in use, a conductive material such as copper may be deposited on the front side  34  of the wafer  12 , although embodiments of the invention are not limited with respect to the conductive material or a wafer. Generally, to deposit a metal on a wafer  12  via electroplating, the wafer  12  front side  34  is pre-coated with a seed layer (not shown). For example, if copper is to be deposited, a seed layer of copper may be deposited over a barrier layer by physical vapor deposition (PVD) or high density plasma PVD although embodiments are not so limited. Further, the backside  36  of the wafer  12  may be sealed off to prevent deposition on other than the front side  34 . The wafer  12 , backside seal assembly  18 , frame  20  and base  22 , or portions thereof may be immersed in the electrolytic solution  32  including ions of the metal (e.g., Cu 2+ ) to be deposited.  
         [0021]     While in solution  32 , the wafer  12  front side  34  may be electrically connected to the power supply  24  via the frame  20 . The anode  16  may also be electrically connected to the supply  24 . Thus, when an electric potential is applied, metal ions from the electrolytic solution  32  may be reduced at the wafer  12  front side  34  to deposit the conductive material, although embodiments are not limited in this respect. Further, oxidation at the anode  16  may replenish the supply of metal ions in the electrolyte  32 . Thus, in some embodiments, the system  10  may be utilized to deposit copper in vias and/or trenches on a wafer  12  front side  34  to form plugs and interconnects respectively. Overfill of the conductive material during electrodeposition may be removed by chemical mechanical polishing (CMP) or any other suitable removal technique.  
         [0022]     Referring to  FIGS. 2 and 3 , details of the frame  20  and base  22  are shown. With respect to  FIG. 2 , the frame  20  and base  22  are devoid of a coating  48  to better illustrate the frame  20  and base  22  in this view. Further, although not shown, the frame  20  and base  22  may be independently attached to a robot. In this way, in some embodiments the frame  20  and base  22  are not connected, which may enable independent movement.  
         [0023]     In some embodiments, the frame  20  may be circular and may include a circular inner portion  42  that defines an aperture. However, the frame  20  may be any shape such as a square, rectangle, pentagon, octagon and the like. One or more flexible or spring-like beams  38  may be connected to and extend from the frame inner portion  42 . For example, the individual spring-like beams  38  may have a first end  44  and a second end  46 . The beam  38  may be joined to the frame  20  at the first end  44  to project inwardly from the frame inner portion  42 . Further, the second end  44  may terminate with a contact point or pad  40 . The points  40  may be configured to minimize localized areas of high pressure on the front side  34  of the wafer  12 . As shown in  FIG. 2 , there are eight beams  38 , each associated with a contact pad  40 . However, embodiments are not limited with respect to the number of beams  38 .  
         [0024]     The contacts  40  and beams  38  may provide electrical contact to the wafer  12 . For example, the frame  20 , beams  38  and pads  40  may be a conductive metal such as stainless steel as one example, although embodiments are not so limited. However, the frame  20  and beams  38  may be coated with a soft, chemically resistant material  48  such as KALREZ of Dupont Dow Elastomers as one example. In some embodiments, the beams  38  may be independently coated to preserve resiliency. Generally, only a portion of each contact pad  40  is coated with the material  48 . For example, the surface  50  of the points  40  lack the coating  48 . In this way, the surface  50  may electrically contact the wafer  12 . The coating  48  on the pads  40  may be continuous with the coating  48  on the beams  38  in some embodiments of the present invention. Thus, there are many ways to coat the frame  20 , beams  38  and pads  40  and embodiments of the present invention are not limited in this respect.  
         [0025]     Still referring to  FIGS. 2 and 3 , the base  22  may also be annular having an inner portion  52  that defines an annular aperture. However, like the frame  20 , the base  22  may be any shape and the inner portion  52  may define an aperture of complementary shape. Further, in some embodiments, the base inner portion  52  may be serpentine, have “V&#39;s”, or the like. The shape of the base  22  and/or inner portion  52  may complement the shape of the wafer  12  and/or the frame  20  although embodiments of the invention are not so limited.  
         [0026]     The wafer  12  may be uniformly seated on the base inner portion  52 . For example, the base  22  may be a strong metal such as stainless steel or titanium, as a few examples. Further, the base  22  may be coated with the material  48 . Referring to  FIG. 3 , in some embodiments, the wafer  12  may be seated on the material  48  that covers the top surface  56  of the base inner portion  52 . However, as shown in  FIG. 4 , the base inner portion  52  may be bent toward the wafer  12 . As such, the wafer  12  may be seated on or be supported by the material  48  that covers the end portion  54  of the base inner portion  52 .  
         [0027]     The region of the base inner portion  52  (e.g., coated surface  56  or end portion  54 ) that supports the wafer  12  may substantially continuously contact the wafer front side  34  to uniformly seat the wafer  12  thereon. That is, in some embodiments the support region may make continuous contact with the periphery of the wafer  12 . Alternately, in other embodiments continuous contact with the wafer  12  may be interrupted. For example, the base  22  may have elevated surfaces that contact the wafer  12  at is periphery. Either way, the force required for sealing may be distributed about the periphery of the wafer  12  by the base inner portion  52 . In those embodiments including interrupted contact, it is preferable to have the maximal amount of base  22  surface area (coated or uncoated) contacting the wafer  12 . In this way, the force required for sealing may be distributed about the wafer  12  periphery without creating localized areas of high pressure that could damage the wafer  12 .  
         [0028]     Damage to the wafer  12  front side  34  may be reduced or eliminated by controlling the amount of force each contact pad  40  places on the wafer  12  front side  34 . Controlling the amount of force may be influenced by beam  38  design. For example, each beam  38  may be spring-like or flexible and may independently deflect relative to the wafer  12  front side  34 . That is, each beam  38  may approximate a cantilevered beam with an end load. Thus, the force supported on the beam  38  end or pad  40  may be calculated according to equation 1.  
               F   ″     =         -   3     ⁢   dEI       L   3               (     Equation   ⁢           ⁢   1     )             
 
 where F″ ( FIG. 7 ) is the supported force, d is the displacement of the contact  40 , E is the Modulus of Elasticity, I is the Moment of inertia of a cross-sectional area, and L is the length of the beam  38 . Thus, when working with materials with low mechanical strength the beams  38  and/or pads  40  may be designed to deliver a force that will enable electrical contact for deposition yet not exceed the mechanical strength of the front side  34 , including a front side  34  having one or more films disposed thereon. 
 
         [0029]     Referring to  FIGS. 3 and 4 , beams  38  may vary in design, as determined by the calculated force. For example, in some embodiments beams  38  may be a reduced thickness as compared to frame  20 . Further, beams  38  may be straight, bent, curved or any other configuration. As shown in  FIG. 3 , the beams  38  may be relatively long. As such, the pads  40  may contact the wafer  12  inward of the base  22 . For example, the wafer  12  may have a given diameter “D”. In some embodiments, the contacts  40  may touch the wafer  12  at a diameter that is about D-1.5 millimeters (mm) and the base  22  may touch the wafer  12  at a diameter of about D-0.9 mm. Thus, opposing pads  40  are closer to each other than opposing portion of the base  22 . Nevertheless, the front side  34  to be deposited upon is within the openings defined by the base  22  and the contacts  40 .  
         [0030]     As shown in  FIG. 4 , in some embodiments the length of the beams  38  may be relatively short. As such, the distance between opposing base inner portions  52  is less than the distance between opposing pads  40 . However, the distance between opposing base inner portions  52  should still define an area that will permit deposition on all desired surfaces of the wafer  12 .  
         [0031]     Referring to  FIG. 5 , when in use, the wafer  12  may initially rest only on the base  22 . For example, the wafer  12  may rest on the surface  56  or end  54  ( FIG. 4 ) of the base inner portion  52 . As such, front side  34  of the wafer  12  may be exposed. The frame  20  may be in a plane generally parallel to the plane of the base  22  without contacting the wafer  12 . As such, there may be a gap “g” between the wafer front side  34  and the contact pads  40 .  
         [0032]     In some embodiments, the surface seal may be established before the contact points  40  make electrical contact with the wafer  12 . For example, the seal material  30  may contact the backside  36  of the wafer  12  via the seal assembly  18 . A force “F” may be applied to the backside  36  of the wafer  12 , which is held stationary by the base  22 . A resultant counterforce “F′” may be applied to the front side  34  of the wafer  12  by the base  22 . The pressure on the front of the wafer  12  is distributed over the interface between the wafer  12  and the base  22 . Thus, damage to the front side  34  of the wafer  12  is minimized or eliminated during sealing.  
         [0033]     Referring to  FIG. 6 , reducing the gap “g” enables initial contact between wafer front side  34  and the contact pads  40 . There is little, if any force associated with initial contact between the wafer  12  and pad  40 . To enable initial wafer  12  and pad  40  contact, the frame  20  may be independently moved toward the base  22 . Alternately, the base  22 , wafer  12  and seal assembly  18  may be moved toward the contacts  40 , stopping with initial contact between the wafer  12  and contact pads  40 . In yet other embodiments, both the frame  20  and base  22  may be moved independently toward each other until initial contact between the points  40  and wafer front side  34  is made without substantial force.  
         [0034]     Referring to  FIG. 7 , electrical contact for electrodeposition may be made by continuing to move the frame  20  and/or base  22  as described above. However, as electrical contact is made, the wafer  12  and contact pads  40  may press against each other to deflect the beams  38  downward a distance “d” relative to the wafer front side  34 . Because the beams  38  and/or pads  40  have been designed to exert a calculated or predicted force on the wafer front side  34 , damage to the wafer  12  front side  34  is minimal if at all. In other words, the force F″ exerted by the contacts  40  for the displacement of a particular beam  38  may be predicted such that the mechanical strength of the wafer  12  and/or films disposed thereon is not exceeded.  
         [0035]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Technology Classification (CPC): 2