Patent Publication Number: US-8968531-B2

Title: Electro processor with shielded contact ring

Description:
BACKGROUND OF THE INVENTION 
     Microelectronic devices, such as semiconductor devices, and micro-scale mechanical, electro-mechanical, and optical devices, are generally fabricated on and/or in substrates using several different types of machines. In a typical fabrication process, an electroplating processor plates one or more layers of conductive materials, usually metals, onto a work piece, such as a semiconductor wafer or substrate. Electroplating processors generally use a contact ring having many contacts or fingers that make electrical connections to the surface of the substrate. Contact rings can be categorized into two groups: wet rings and dry rings. With a wet ring, the contact fingers are exposed to the plating bath, so that the contact fingers get “wet” during electro processing. A dry ring has a seal that seals the contact fingers, so that the contact fingers remain dry. 
     As semiconductor and similar micro-scale device feature sizes continue to decrease, the seed layers that can be used on wafers become thinner. This creates a high initial sheet resistance on the wafer which affects both reactor and contact fingers. In dry contact ring processors, thin seed layers are prone to inadvertent etching due to seal leaking and/or residual chemistry on the seal. Joule heating due to high currents passing through a thin seed layer can also be disruptive to uniform plating. In wet contact ring processors, a thief electrode at the edge of the wafer may be needed to control the “terminal effect” which results in a non-uniform electric field near the locations where the contact fingers touch the seed layer. However, using thief currents to control the terminal effect can deplate the seed layer around or between contact fingers, and make uniform plating problematic using wet ring processors. Thief currents also tend to cause more metal to plate onto the contact fingers. 
     Accordingly, engineering challenges remain in designing electroplating processors. 
     SUMMARY OF THE INVENTION 
     A new contact ring for an electro processor has now been invented which largely overcomes the challenges described above. The contact ring has a plurality of spaced apart contact fingers. In a first aspect, a shield at least partially overlies the contact fingers. The shield may be provided in the form of an annular ring substantially completely overlying and covering the contact fingers. In a second aspect, a shield may overlie or surround the outer edge of the workpiece. The shield changes the electric field around the outer edge of the workpiece and the contact fingers, which reduces or eliminates the negative aspects created by thin seed layers and high thief electrode currents used with a wet contact ring design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electro-processing chamber. 
         FIG. 2  is a perspective view of the contact ring shown in  FIG. 1 . 
         FIG. 3  is an enlarged section perspective view of the contact ring shown in  FIGS. 1 and 2 . 
         FIG. 4  is an inverted view of the contact ring shown in  FIG. 3 . 
         FIG. 5  is a schematic diagram of the shielded contact of  FIGS. 3 and 4  in a pre-processing position. 
         FIG. 6  is a schematic diagram of the shield contact ring of  FIG. 5  in a processing position. 
         FIG. 7  is a schematic diagram of a combination shield and contact ring. 
         FIG. 8  is a schematic diagram of an alternative shielded contact ring. 
         FIG. 9  is a plan view of an alternative shield design which may be used with contact rings having fewer and farther spaced apart contact fingers. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     As shown in  FIG. 1 , and electro processing chamber  20  has a head  22  including a rotor  24 . A motor  28  in the head  22  rotates the rotor  24 , as indicated by the arrow R. A contact ring assembly  30  on the rotor  24  makes electrical contact with a work piece or wafer  100  held into or onto the rotor  24 . The rotor  24  may include a backing plate  26 , and ring actuators  34  for moving the contact ring assembly  30  vertically (in the direction T in  FIG. 1  between a wafer load/unload position and a processing position. The head  22  may include bellows  32  to allow for vertical or axial movement of the contact ring while sealing internal head components from process liquids and vapors. 
     Referring still to  FIG. 1 , the head  22  is engaged onto a base  36 . A vessel or bowl  38  within the base  36  holds electrolyte. One or more electrodes are positioned in the vessel. The example shown in  FIG. 1  has a center electrode  40  and a single outer electrode  42  surrounding and concentric with the center electrode  40 . The electrodes  40  and  42  may be provided below a di-electric material field shaping unit  44  to set up a desired electric field and current flow paths within the processor  20 . Various numbers, types and configurations of electrodes may be used. The vessel may be divided by a membrane  170  into a lower anode compartment containing an anolyte liquid, and an upper compartment containing a catholyte. A thief electrode  190  is often provided in the processor  20  adjacent to the contact ring, to compensate for the terminal effect. 
       FIG. 2  shows the contact ring assembly  30  separated from rotor  24  and without any shield installed.  FIG. 2  shows the contact ring assembly  30  inverted. Accordingly, the contact fingers  82  on the contact ring assembly  30  which are shown at or near the top of the contact ring assembly  30  in  FIG. 2 , are at or near the bottom end of the contact ring assembly  30  when the contact ring assembly  30  is installed into the rotor  24 . A mounting flange  64  may be provided on the contact ring for attaching the contact ring assembly  30  to the rotor  24  with fasteners. 
       FIGS. 3 and 4  show a section view with the contact ring assembly  30  once again in the installed upright orientation shown in  FIG. 1 , and with a shield installed. In this example, the contact ring assembly  30  has a base ring  50  between an inner liner  56  and an outer shield ring  52 . Contact fingers  82  are attached to the ring base  50 . Referring now also to  FIG. 5 , the contact fingers  82  may be positioned onto a flat angled bottom surface  70  of the ring base  50 . Consequently, the contact fingers  82  extend inwardly (towards the center of the contact ring assembly  30 ) and also slightly upwardly in  FIGS. 1 and 3 . Alternatively, the bottom or mounting surface  70  may be horizontal, or even inclined downwardly. 
     The contact fingers  82  are electrically connected to the processor electrical system. This electrical connection may be achieved via an electrically conductive ring base  50 , e.g., with the ring base made partially or entirely of metal. Alternatively, the ring base  50  may also be an electrically non-conductive material or dielectric material, with one or more electrical leads extending through or alongside the ring base  50 , to electrically connect with the contact fingers  82 . The inner liner  56  may have an outwardly tapering surface  58 , to help to guide and center a wafer  100  into the contact ring assembly  30 . The inner liner  56 , which is generally plastic or another non-conductive material, may have an outwardly extending lip  60  that extends into a slot or recess in the ring base  50 . Alternatively the geometry of the inner liner  56  can also be incorporated into or made part of the base ring  50 . 
     A contact ring assembly  30  for use with a 12 inch diameter wafer may have 480 or even 720 contact fingers. Providing a large number of contact fingers may reduce adverse effects, such as current path variations and heating, when plating onto extremely thin seed layers. Typical contact finger dimensions are a length of about 0.25 inches, and thickness ranging from about 0.005 to 0.010 inch. A contact ring for a 450 mm diameter wafer may have 1080 or more contact fingers. 
     A shield  54 , covers part of, or the entire length of contact fingers  82 , as well as the entire edge of the wafer. In  FIGS. 3 and 4 , only the innermost tips  75  of the contact fingers  82  are not covered or shielded by the shield  54 . The inwardly extending length of the shield  54 , relative to the length of the contact fingers  82 , may be adjusted to vary the current thieving effect of the contact fingers. In some designs, the inner edge of the shield may nominally be substantially aligned with the innermost tips  75  of the contact fingers  82 . The shield may also optionally extend inwardly past the tips of the contact fingers  82  in some designs. Rinse holes  62  may be provided in the shield  54  to better allow for cleaning. The rinse holes  62  are small diameter to minimize their affect on the plating process, and may be provided to improve cleaning and rinsing. The shield  54  is made of a di-electric material and may be formed as part of the shield ring  52 . Alternatively, the shield  54  may be a separate ring attached to the contact ring assembly  30 . The ring base  50  may be made of metal, such as titanium. The shield ring  52  may include a ring section  66  and an attached or integral shield or shield section  54 . The shield  54  is omitted from  FIG. 2  for purpose of illustration. 
     The contact ring assembly  30  may be used in wet contact applications where the contact fingers are in contact with the electrolyte. In this type of application, the shield  54  reduces the build up of metal plated onto the contact fingers, and also help to prevent deplating on the wafer edge between contact fingers. This improves the performance of the plating chamber  20  and reduces the time required for contact finger de-plating. The shield  54  may be used various types of conventional fingers. The contact ring assembly  30  may also be used in sealed ring or dry contact applications. 
       FIGS. 5 and 6  schematically show a shielded wet contact ring assembly  200 . The contact ring assembly  200  is “wet” in that the individual contact fingers, and the edge of the wafer are exposed to the plating bath. A di-electric material shield  201  covers some or all of the contact ring  202 . The shield  201  is spaced apart from the contact fingers  206  by a gap  208 , when the contact ring assembly is in the load/unload position (before or after processing). During processing, the shield  201  may be in direct contact with the contact fingers, as shown in  FIG. 6 . The gap height SG may be about equal to the thickness CT of the contacts  206 . CT for example may range from about 0.05 to 1, 2 or 3 mm, and typically about 0.05 to 1 mm. The gap may be created by sandwiching the contacts  206  between the wafer  100  and the shield  201 . In the case of the contacts sandwiched between the wafer and the shield with a gap equal to CT, the fluid wets the contacts through the gap between the contacts. The gap may be readily controlled and be made uniform all around the circumference of the wafer. With the wafer sufficiently engaging the contacts against the shield, a uniform geometry is established. On the other hand, if the gap is too large, non-uniform current thieving may occur, because the gap will likely also be non-uniform. 
     The shield  201  may be made of a thin, resilient electrically non-conducting material, such as polyether ether ketone (PEEK).  FIG. 5  shows the positions of the components before electro processing begins. To initiate electro processing, the shield and contact fingers are moved into contact with the edge of the wafer  100 , as shown in  FIG. 6 . This movement is typically performed via an actuator in the head  22  pulling the support ring up into contact with the wafer held in a rotor in the head  22 . This movement causes the contacts deflect or bend down slightly. If the shield  201  is flexible, the force exerted on the wafer edge is limited, even if mechanical tolerances are not precisely maintained. In other designs, such as shown in  FIG. 3 , the shield may be less flexible. 
     The contacts touch the wafer about 0.75 to about 2 mm in from the wafer edge, or as close to the edge as possible, but generally not on the bevel itself. When a thief electrode is used, as is often necessary to control the terminal effect, the thief current can deplate the seed layer on the 0.75 to 2 mm wide area between the contact touch points and the wafer edge, leaving this area useless for manufacturing micro scale devices. The shield  54  or  201  acts to reduce the electrical field effects of the thief electrode  190 , shown in  FIG. 1 . This helps to prevent deplating at the outer 1 or 2 mm of the wafer. The contacts  206  are covered by the shield  201  which reduces plating onto the contacts  206 . The shield  201  may have a very low profile (i.e. with a thickness ET of about 0.020 inches) at the inner edge. This can minimize electric field disturbances and improve current density control at the wafer edge. A large number of contacts  206 , for example 360, 480, 720 or more, may be used to help control the terminal effect between the contacts on thin seed layers. The shield  201  may slightly overhang or extend inwardly past the tips of the contacts, to help control the thieving current to the contacts and edge of the wafer. 
     The design shown in  FIGS. 5 and 6  may allow for a reduced edge exclusion zone on the wafer (to for example 148 mm-148.5 mm radius on a 300 mm wafer) because it allows some thieving at the wafer edge. This avoids a spike in the plated film thickness at the edge of the wafer that often occurs with a sealed ring due to current crowding. The design shown in  FIGS. 5 and 6  also avoids the need for special treatment at the wafer notch that a seal would require. To seal around the notch, the seal must be brought radially inward from the notch, all the way around the wafer to keep a circular geometry, or the seal must have a local geometry for sealing the notch. If local geometry is used, wafer alignment is required, which is an added complication. Since no seal is used, the complications and additional space needs associated with seals are avoided. Contacts  206  may be positioned within 0.75-1 mm of the wafer edge, with the shield  201  having similar overhang onto the wafer. 
     With contacts placed very close to the wafer edge, and with no extra space taken up by a seal; a very low profile; control of the current density very close to contacts with the chamber thief; and a controlled amount of thieving onto the wafer edge and contact; the present shield-ring design offers advantages to yield good process results (i.e. good die) toward the very edge of the wafer (i.e. 1-2 mm from the wafer edge); closer to the edge than prior ring contact designs. 
     Since the contact ring assembly  200  is “wet”, the copper or other seed layer is “protected” as soon as the plating process starts because metal is being added at or around the contact. In contrast, seal rings are susceptible to any acidic moisture that can etch the seed layer behind the seal during the process. Contact forces are also not divided between the contacts and the seal when no seal is used. Instead, the whole engagement force is on the contact fingers. The contact fingers may deflect to down and touch the shield, as shown in  FIG. 6 , although this is not necessary. Electro plating may also be performed with the contact fingers and the shield spaced apart, as shown in  FIG. 5 . In contrast to a sealed ring, since the shield does not have to seal and engage with a sealing force it can be made with a thinner profile helping to reduce bubble trapping and improve edge uniformity by allowing the chamber thief to act more effectively at the wafer edge. 
     The contacts and the contact ring  202  may be coated with a non-conducting material, except at the tips, to prevent plating build up. With use of the shield  201 , in some cases coating may not be required, or the contact-to-contact tolerance of coated/uncoated areas can be enlarged. Using uncoated contacts allows for easier manufacture and eliminates some potential failure modes (i.e. pealing or pin holes in the coating). As the shield  201  reduces metal build up on the contact fingers  206 , the deplate time is reduced, in comparison to a non-shielded wet ring. 
     The shield  201  may slightly overhang or extend inwardly past the inner tips of the contact fingers to provide an adjustable parameter (i.e. the amount of overhang) that can be used to “dial-in” uniformity at the very edge of the wafer. In some cases the shield  201  may help to reduce metal build up on the edge exclusion zone of the wafer making it easier and quicker to bevel etch (compared to a non-shielded wet ring). Holes  228  can be added to outer region/diameter of the shield to help with sling off and rinsing. This can be an advantage over a sealed ring which cannot have holes, making the rinse/dry maintenance more difficult. 
     The size and shape of the shield depends upon the number of discrete contacts and the contact radial location. Contact rings with fewer contact points (e.g. less than roughly  220 ) may require circumferential variations in the shield geometry. For example, as shown in  FIG. 9 , a shield  230  may have individual projections  232  aligned between the contacts of the contact ring. Contact rings with larger number of contacts (greater than roughly  220 ) generally do not need individual projections and may be annular. 
       FIG. 7  shows an alternative design having electrically conducting contact fingers  272  embedded into, or formed integrally with, a shield  220 . The contact fingers may entirely contained within the shield  220 , except where the end tips  240  project outwardly at the inner edge of the shield. The shield  220  may optionally have an upright (vertical or near vertical) side wall  242  to help control deplating at the wafer edge. In this case, the inner edge of the shield may end at the side wall  242 , instead of extending all the way in to the end tips  240 . 
       FIG. 8  shows an alternative shield ring design having an annular shield  214  attached to the contact ring, with the annular shield vertically aligned with and surrounding the wafer  100 . The contacts  206  in this design may have an outer straight or horizontal segment and an inner angled segment contacting the wafer. The shield  214  may optionally have notches or slots to allow the contacts  206  to pass through it. In this case, straight contacts such as shown in  FIGS. 3-7  may be used. In the design of  FIG. 8 , the shield is generally vertically oriented, i.e., the shield  214  may be a cylindrical section having vertically oriented central axis. In contrast, the shields shown in  FIGS. 3-5  are largely flat horizontal rings. The height of the shield  214  may be equal to about 1, 2 or 3 times the thickness of the wafer  100 , typically about 1, 2 or 3 mm. The shield  214  may be formed as part of the inner liner  56 . The gap GG between the wafer edge and the shield  214  may vary from zero to 1 or 2 mm. The inside diameter of the shield  214  may be generally equal to the diameter of the wafer, plus tolerances. 
     Thus, novel methods and designs have been shown and described. Various changes, substitutions and use of equivalents may of course be made, without departing from the spirit and scope of the invention. The invention, therefore, should not be limited, except to the following claims and equivalents of them.