Abstract:
Electroplating apparatus agitates electrolyte to provide high velocity fluid flows at the surface of a wafer. The apparatus includes a paddle which provides uniform high mass transfer over the entire wafer, even with a relatively large gap between the paddle and the wafer. Consequently, the processor may have an electric field shield positioned between the paddle and the wafer for effective shielding at the edges of the wafer. The influence of the paddle on the electric field across the wafer is reduced as the paddle is spaced relatively farther from the wafer.

Description:
TECHNICAL FIELD 
       [0001]    The field of the invention is apparatus and methods for agitating liquid electrolyte in an electroplating apparatus. 
       BACKGROUND OF THE INVENTION 
       [0002]    In many plating processes, a diffusion layer forms in the liquid electrolyte at the surface of the wafer. The diffusion layer reduces the mass transfer rate of electrolyte components and reactants to the surface of the wafer, which degrades the quality and efficiency of the plating process. One technique for increasing the mass transfer rate is to increase the relative velocity between the liquid electrolyte and the surface of the workpiece. In the past, some processing apparatus have used a paddle which oscillates horizontally or vertically in the electrolyte. The paddle has spaced apart ribs or blades. As the paddle moves, a liquid vortex is formed in the spaces between adjacent ribs. The liquid vortex creates a high speed agitated flow at or against the lower (down-facing) surface of the workpiece, increasing the mass transfer rate. 
         [0003]    These types of paddle plating apparatus also often have an electric field shield provided to shield the edges of the wafer from the full electric field in the electrolyte, to achieve more uniform plating at the edges of the wafer. The shield is usually an annular ring of di-electric material. 
         [0004]    Both the paddle and the shield are most effective when positioned very close to the wafer, for example, within 5 mm. If the shield is positioned below the paddle, the shield is less effective. If the shield is positioned above the paddle, then the paddle is less effective, as the gap between the paddle and the wafer is larger. Accordingly, engineering challenges remain in designing electro-plating apparatus. 
       SUMMARY OF THE INVENTION 
       [0005]    Experimental and computation results disclose a relationship between the dimension of the gap between the paddle and the wafer, and the vortex size for achieving improved mass transfer. Specifically, the inventors have discovered that in processor designs having a larger gap, using a paddle which creates larger vortices provides improved results. Consequently, in designs having a shield is at a vertical position above the paddle, making the gap larger, a paddle having ribs spaced farther apart provides better mass transfer by creating larger vortices. The vortices may also be made more consistently across the wafer providing more uniform mass-transfer. 
         [0006]    In one aspect, an electroplating apparatus agitates electrolyte to provide high velocity fluid flow at the surface of a wafer that results in results in high, uniform mass transfer providing more uniform plating at high plating rates. The apparatus includes a paddle which can provide uniform high mass transfer over the entire wafer, even with a relatively large gap between the paddle and the wafer. Consequently, the processor may have an electric field shield positioned between the paddle and the wafer, where the shield is more effective. In this design, with the paddle below the shield, the paddle is also less likely to adversely influence the electric field across the wafer. This advantage is particularly significant in processing where the wafer does not rotate, where such disturbances cannot be averaged out with wafer rotation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    In the drawings, the same reference number indicates the same element in each of the views. 
           [0008]      FIG. 1  is a top perspective view of an electroplating apparatus. 
           [0009]      FIG. 2  is a top perspective view of the apparatus of  FIG. 1  with the head removed for purpose of illustration. 
           [0010]      FIG. 3  is a section view of the apparatus of  FIG. 1 . 
           [0011]      FIG. 4  is a top perspective view of the paddle shown in the apparatus of  FIGS. 1-3 . 
           [0012]      FIG. 5  is a schematic section view of the paddle shown in  FIGS. 1-3 . 
           [0013]      FIG. 6  is a schematic section view of a prior art paddle. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    As shown in  FIGS. 1-3 , a processor  10  for electroplating a wafer  30  includes a head  14  supported on a head lifter  16  and a vessel  24 . A membrane  40  may be included to divide the vessel  24  into a lower chamber  44  containing one or more anodes  28 , and a first liquid electrolyte, below the membrane  40 , and an upper chamber  42  containing a second liquid electrolyte. Alternatively the membrane  40  may be omitted with the vessel  24  having a single chamber holding a single electrolyte. Referring to  FIG. 3 , a field shaping element  46  made of a dielectric material may be provided in the vessel  24  primarily to support the membrane  40 , and distribute flow of catholyte. The electric field in the vessel  24  may be shaped via an anode shield  45 , a chamber shield  47 , and a weir shield  34 . The shields may be annular dielectric elements. The shields provide shielding of the electric field with the vessel. 
         [0015]    A contact ring  26  on the head  14  holds the wafer  30  and has a plurality of contact fingers for making electrical contact with a conductive layer, such as a metal seed layer, on the wafer  30 . The contact ring  26  may optionally have a seal  38  to seal the contact fingers from the electrolyte. The head  14  may include a rotor  36  for rotating the wafer  30  during processing, with the contact ring  26  on the rotor. Typically the contact ring has a seal and a backing plate, with the contact ring and the backing plate forming a wafer holder. The head  14  is movable to position the wafer holder into a processing position in the vessel, where the seed layer is in contact with electrolyte in the vessel. 
         [0016]    Referring now also to  FIG. 4 , a paddle  18  is at a fixed vertical position within the vessel  24  adjacent to the wafer  30 . The paddle  18  may be a generally circular plate of dielectric material having a plurality of parallel ribs or blades  60  spaced apart by slots  62 . A paddle actuator  32  moves the paddle  18  horizontally in a flat plane, parallel to the wafer, within the vessel  24  to agitate the electrolyte  50 . The paddle  18  and the paddle actuator  32  may be supported on a base plate  20  attached to the vessel  24 . 
         [0017]    As shown in  FIG. 5 , a weir shield  34  is provided in the vessel  24  between the paddle  18  and the seal  38  of the contact ring  26 . Positioning the weir shield  34  above the paddle requires the gap GG between the top surface of the ribs  60  of the paddle  18  and the wafer  30 , to be larger than if the weir shield  34  is positioned below the paddle  18 . Generally, as the gap GG increases, the agitation on the wafer due to the paddle is reduced, which reduces the mass transfer rate and uniformity and the quality of the plating process. 
         [0018]    With a seal  38  height of 2-3 mm (2.7 mm nominal), and allowing for a 1 mm gap SG between the seal  38  and the weir shield  34 , a weir shield  34  thickness of 1 mm, and a gap BG of 1 mm between the top of the ribs and the weir shield  34 , the minimum gap GG is about 5-6 mm (5.7 mm nominal). 
         [0019]    To achieve a smaller gap GG over most of the wafer  30 , a raised rib paddle  15  as shown in  FIG. 6  has been used, with the raised rib paddle  15  having taller ribs  60   a  over the interior portion of the paddle, where ribs are not at risk of hitting the weir shield  34 . Shorter ribs  60   b  are used at the front and back of the paddle  15  (in the direction MM of paddle movement). The shorter ribs  60 B on a first side of the paddle can move under the weir shield  34  at the limit of paddle travel in a first direction, to a position where the weir shield overlies one or more of the ribs, and the ribs do not collide with the weir shield  34 . As the paddle moves to the limit of paddle travel in the opposite or second direction, the shorter ribs  60 B on the first side of the paddle move out from under the weir shield, so that the weir shield then does not overlie the shorter ribs  60 B. With a raised rib paddle  15 , the gap GG over much of the wafer can be reduced to about 3-4 mm or less (3.7 mm nominal), rather than 5.7 mm. However, test results using the raised rib paddle  15  show a thinner plated film at the edges of the wafer, and that this results due to the shorter ribs  60   b , which provide reduced mass transfer relative to the taller ribs  60   c.    
         [0020]    Referring once again to  FIG. 5 , with the paddle  18 , plating is substantially uniform, including at the wafer edges. All of the ribs  60  on the paddle  18  may have the same height HH. Although the minimum gap GG is 5-6 mm, the paddle  18  achieves plating uniformity better than the raised rib paddle  15 . The paddle  18  creates larger vortices, which maintains a high level of mass transfer. The ribs  60  are spaced much further apart in comparison to existing designs. For example, in  FIG. 5  the ribs  60  may be equally spaced apart on at a pitch dimension PP (between centers of adjacent ribs) of 18-22 mm (20.6 mm nominal), with a rib height HH equal to 8-13 mm (10.5 mm nominal). As the paddle moves or oscillates in the vessel, the large space  68  between ribs  60  creates a large diameter vortex which reduces the diffusion layer at the wafer surface and improves mass transfer. 
         [0021]    All of the ribs  60  may have the same cross section shape, dimensions and spacing, with the length of the ribs varying with rib position, as shown in  FIG. 4 . Referring back to  FIG. 5 , each rib  60  has an upright section  64  joined perpendicularly to a base  66  via radii. The radii may be omitted with straight ribs joined perpendicularly to a flat base. The slots or openings  62  between adjacent bases  66  have a width SS of 4-6 mm (5 mm nominal). Each base  66  has a width BW of 14-17 mm (15.6 mm nominal), and a base height or floor thickness BB of 1-2 mm. The upright section  64  may also have a width or thickness of 1-2 mm and a plurality of equally spaced apart upright ribs. 
         [0022]    The inventors have discovered that there is a mathematical relationship between the gap GG and the pitch spacing PP (or alternatively the width of the space  68  formed between adjacent ribs). 
         [0023]    1. PP=2.72×GG+3.45 mm. 
         [0024]    2. Space aspect ratio=(HH−BB)/PP=0.3 to 0.5 (0.44 nominal). 
         [0025]    Consequently, in processor design, the gap GG may be first determined based on the shield requirements and other factors. Then the paddle  18  may be designed with the pitch and height of the ribs selected to have an aspect ratio of 0.3 or 0.35 to 0.5, and PP is greater than 16, 17 or 18 mm, and up to 22 or 24 mm. Using these equations, the thickness BB of the base  66  is added to obtain the total rib height HH. Although the gap GG varies depending on dimensions of other elements and the design of the electroplating processor, the ratio of PP/GG may typically range from about 2.5 to 3. 
         [0026]    Thus, a novel electroplating processor has been shown and described. Various changes and substitutions may of course be made without departing from the spirit and scope of the invention. The invention, therefore, should not be limited, except by the following claims, and their equivalents.