Abstract:
A platen for chemical mechanical polishing of a substrate includes a surface upon which a polishing pad can be placed, a support structure, and a controller. The surface has a first region and a second region and is operable to exert force against the polishing pad during polishing. The support structure is located beneath the second region and is operable to cause the second region to exert more force than the first region. The controller is operable to adjust the amount of force that is exerted by the second region.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to chemical mechanical polishing of substrates.  
           [0002]    Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semi-conductive or insulating layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photo-resist layer placed thereon is also non-planar. A photo-resist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photo-resist. If the outer surface of the substrate is sufficiently non-planar, the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. Then it will be impossible to properly focus the light image onto the entire outer surface. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface for photolithography.  
           [0003]    Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing.  
           [0004]    An effective CMP process generates a substrate surface that is finished (lacks small-scale roughness) and flat (lacks large-scale profile). The polishing finish and flatness are determined in part by the force pressing the substrate against the pad and in part by the relative velocities of the substrate and the pad. However, a variety of factors, including non-uniform velocities, non-uniform slurry distribution and distortions in the polishing pad can cause the rate of polishing to vary spatially, resulting in non-uniform polishing of a semiconductor substrate surface.  
         SUMMARY OF THE INVENTION  
         [0005]    In one aspect, the invention is directed to a chemical mechanical polishing apparatus comprising a platen that includes a surface upon which a polishing pad can be placed, a support structure, and a controller. The surface has a first region and a second region and is operable to exert force against the polishing pad during polishing. The support structure is located beneath the second region and is operable to cause the second region to exert more force than the first region. The controller is operable to adjust the amount of force that is exerted by the second region.  
           [0006]    Particular implementations can include one or more of the following features. The support structure is a mechanical structure that is operable to position the second region such that the second region is elevated with respect to the first region. The controller is operable to adjust the height at which the mechanical structure positions the second region.  
           [0007]    The support structure is a pressurized chamber and the controller is a valve operable to adjust the amount of pressure in the chamber. The pressure within the chamber is created by adding fluid into the chamber. The fluid is gaseous. The fluid is air. The valve is operable to control the amount of fluid that is added or released from the chamber. The second portion is formed of a flexible membrane and the valve is operable to allow enough fluid to enter the chamber such that the pressure within the chamber causes the flexible membrane to become distended.  
           [0008]    The second region is a groove and the support structure is a pressurized chamber formed within the groove when the polishing pad is placed over the groove. The controller is a valve operable to adjust the amount of pressure in the chamber. The platen is configured to rotate during polishing. The surface of the platen is circular in shape. The polishing pad has edges that are attached to the platen.  
           [0009]    In another aspect, the invention is directed to a method for chemical mechanical polishing that calls for placing a polishing pad on a platen, the platen having a first surface region and a second surface region; using the platen to exert force against the polishing pad during polishing; and adjusting the force that is exerted by the second surface region such that the second region exerts more force than the first region.  
           [0010]    Particular implementations can include one or more of the following features. Adjusting the amount of force includes adjusting the amount of pressure within a pressure chamber located beneath the second surface region. The method further calls for placing a substrate on the polishing pad; and adjusting the placement of the substrate relative to the second surface region.  
           [0011]    Particular implementions of the invention can realize one or more of the following advantages. The invention can provide improved control of polishing rates across the substrate surface (ie., polishing profile control). The backside of the polishing pad can be pressurized and the pressure can be applied at selected regions of the polishing pad. The location of the selected pressurized regions relative to the substrate can be varied by varying the location of the substrate relative to the polishing pad. More pressure can be applied in regions of the substrate where the polishing rate is lower. The polishing pad can transfer pressure to the front surface of the substrate substantially without distortion, e.g., spreading, of the shape or size of the selected region.  
           [0012]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic top view of a platen.  
         [0014]    [0014]FIG. 2 is a schematic cross-sectional side view of a platen with mechanically adjustable surface.  
         [0015]    [0015]FIGS. 3A and 3B are schematic cross-sectional side views of a platen with pneumatically adjustable surface.  
         [0016]    [0016]FIG. 4A is a cross-sectional side view of a platen having a grooved surface.  
         [0017]    [0017]FIG. 4B is a cross-sectional side view of a platen having a grooved surface and covered by a polishing pad.  
         [0018]    [0018]FIG. 5 is a schematic top view illustrating a possible location for the groove relative to the substrate.  
     
    
       [0019]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0020]    Referring to FIG. 1, a chemical mechanical polishing apparatus includes a rotatable platen  10  for supporting a polishing pad (not shown). In other respects, the CMP apparatus can be configured as described in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated by this reference.  
         [0021]    The surface of the platen  10  can include an outer region  12 , a center region  14 , and a middle region  16  that lies in between the outer region  12  and center region  14 . In one implementation, the surface of the platen  10  can be shaped as a circle. The center  14 , middle region  16 , and outer  12  regions can represent radial regions of the circular surface, with the circular center region  14  being closest to the center, the annular outer region  12  being furthest from the center and the annular middle region  16  being between the annular outer  12  and the circular center  14 . The amount of pressure that the platen exerts against the polishing pad can vary among regions of the platen. A given region, for example, the middle region  16 , can exert more pressure than another region. The difference in pressure between regions can be implemented, for example, by varying the height of a given region relative to other regions of the platen.  
         [0022]    In one implementation, shown in FIG. 2, the platen  10  includes a base  22  that supports the center region  14 , middle region  16  and outer region  12 . In addition, the platen  10  includes an actuator  20 , such as an electric or pneumatic actuator, that can extend or contract to adjust the height of a portion of the platen. For example, the middle region  16  of the platen can be physically separable from the surrounding regions and can rest upon or be attached to the actuator  20  so that as the actuator  20  extends or contracts, the middle region  16  is raised or lowered accordingly. The actuator  20  can be connected by a control line  22  to unillustrated control elements, such as pneumatic or electronic controllers. The actuator  20  can be configured to position the middle region  16  such that the middle region  16  is elevated with respect to the adjacent regions  12 ,  14 . The difference in height between the middle region  16  and the adjacent regions  12 ,  14  can be increased or decreased by extending or contracting the actuator  20 .  
         [0023]    In another implementation, shown in FIG. 3A, the platen  10  includes a pneumatic support structure such as a pressurized chamber  30 . The chamber  30  can be located underneath the middle region  16 , which can be formed by a flexible membrane  34  that seals the pressurized chamber  30 . The edges of the flexible membrane  34  can be attached to the adjacent regions  12 ,  14  of the platen  10 . The pressure within the chamber  30  can be created by forcing a fluid, typically a gas such as air, into or out of the chamber. As the pressure within the chamber  30  increases, the membrane  34  can become distended, rising the most near the center of the membrane  34  and rising less near the ends of the membrane  34 . The chamber  30  can be connected by tubing or passages  32  to unillustrated pressure control elements, such as valves, pumps, pressure lines, vacuum lines, and pressure regulators. Alternatively, the pressure control elements can be located inside the platen. The chamber  30  is pressurized so that the pressure at the center of the membrane  34  is greater than the pressure at the adjacent regions  12 ,  14 . The difference in pressure between the middle region  16  and the adjacent regions  12 ,  14  can be adjusted by adjusting the amount of fluid in the chamber.  
         [0024]    In FIG. 3A, the membrane  34  is shown having a uniform thickness. Alternatively, the membrane  34  can have portions of varying thickness. For example, one portion of the membrane can be thicker than another portion of the membrane, as depicted in FIG. 3B. In such cases, the thicker portion of the membrane rises less than the thinner portions of the membrane. The thicker portion of the membrane can be located radially outside or radially inside the thinner portion of the membrane.  
         [0025]    In yet another implementation, shown in FIGS. 4A and 4B, the middle region  16  of the platen&#39;s  10  surface contains a recess or groove  40 . The groove  40  is open at the top, but the placement of a polishing pad  42  over the platen  10  completely covers and seals the opening of the groove  40  to form a pressurizable chamber  44 . The polishing pad  42  needs to be sufficiently flexible that it will undergo some distortion in response to the pressure in the chamber  44 . The thickness and composition of the polishing pad  42  can affect the pad&#39;s flexibility. Also, the width of the groove  40  can affect the amount of pressure that is produced within the pressurized chamber  44 . In one implementation, the polishing pad is formed of a porous polyurethane material measuring 0.05 inches thick and the pad completely covers a groove that measures 0.1 inches wide. The level of pressure in the chamber  44  can be controlled, e.g., by unillustrated pressure control elements, such as valves, pumps, pressure lines, vacuum lines, and pressure regulators that are connected to the chamber  44  by a passage or tubing  46 . Changing pressure in the chamber  44  proportionally changes the force by which the portion of the polishing pad covering the groove is pressed against the substrate.  
         [0026]    The number of grooves, the location of each groove, and the location of the substrate relative to the grooves can be varied to produce different polishing profiles. For example, FIG. 5 shows a single circular groove  50  located near the center of the platen  10 . A polishing pad (not shown) covers the groove  50  and a substrate  52  is positioned on top of the polishing pad such that only the edge portion of the substrate  52  overlaps with the groove  50 . With the groove  50  and the substrate  52  so positioned, more force can be exerted during polishing against the edge portion of the substrate  52  than against portions of the substrate which do not overlap with the groove  50 .  
         [0027]    Although specific implementations have been described herein, those skilled in the art will recognize that the implementations disclosed herein may be changed without deviating from the scope of the invention. For example, instead of having a grooved surface, the platen can have a ridged surface formed by attaching a rigid ring to the surface of the platen.