Abstract:
A system for chemical-mechanical polishing is described which includes a wafer backing film having concentric first and second portions. The portions of the wafer backing film are of different materials. The second portion of the wafer backing film has an annular shape and surrounds the first portion; a backing shim is used to adjust the first portion and second portion with respect to each other in a vertical direction. The first and second portions of the wafer backing film and the backing shim are mounted on an adhesive assembly film, thereby forming an assembly for mounting on a wafer carrier. The second portion of the wafer backing film is less compressible than the first portion, and is adjusted in the vertical direction so that the outer edge of the wafer is substantially sealed when backside air is applied to the wafer during a film removal process.

Description:
RELATED APPLICATION 
     This application is related to application Ser. No. 09/303,471, now U.S. Pat. No. 6,171,513, filed the same day and assigned to the same assignee as the present application. The disclosure of this related application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to semiconductor processing, and more particularly to improvement of uniformity in chemical-mechanical polishing (CMP) processes. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. Chemical-mechanical polishing (CMP) is widely used to reduce the thickness and planarize the topography of films on the substrate (generally a silicon wafer). 
     In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. FIG. 1 shows a conventional CMP arrangement wherein the wafer  1  is held against a polishing pad  11  using a wafer carrier  2 . Wafer carrier  2 , which often comprises a metal plate, is covered by a backing film  3  in contact with the backside of the wafer (that is, the side not being polished). The wafer, wafer carrier, and backing film are held in radial alignment by a retaining ring  4 . 
     Chemical-mechanical polishing using this standard arrangement does not result in a uniform polishing rate across the wafer, and thus does not produce a planar polished surface. Both radial and non-radial variations in uniformity have been observed. A number of techniques have been employed in attempts to equalize the polishing rate at different areas of the wafer, as detailed below. 
     CMP tools often use vacuum or backside air pressure at the surface of the wafer carrier to hold a wafer during loading on the tool and to eject a wafer after the process is finished. This may be done by providing a porous wafer carrier plate (as described in U.S. Pat. No. 5,645,474) and pre-punching holes in the backing film. Another typical arrangement (shown in FIG. 1B) uses a wafer carrier  2  with a plenum formed therein and holes  12  aligned with holes  13  in the backing film, to conduct air to the backside of the wafer. 
     Another known practice is to modulate the amount of backside air pressure during the polishing process to control and improve polishing uniformity (see, for example, Murakami et al., VMIC Conference, 1996). Air pressure applied to the backside of the wafer causes the wafer to flex outward, which in turn causes the wafer center to come into closer contact with the polishing pad. Generally, additional force on the wafer at the center reduces the polishing rate near the wafer perimeter relative to that at the center, thereby improving the overall polish uniformity. 
     Unfortunately, the use of backside air pressure has drawbacks. If the air is permitted to leak around the edge of the wafer, a substantial portion of the applied force is lost. 
     In addition, greater and greater amounts of backside air pressure are required as various tool elements (such as the polishing pad and backing film) degrade with repeated use. 
     Furthermore, since the use of backside air pressure reduces the relative polishing rate near the wafer edge, it aggravates a well-known radial non-uniformity called “edge bead.” FIG. 2 shows the radial variation in polishing rate on a 200 mm wafer in a typical CMP process. The polishing rate is generally higher near the periphery of the wafer than near the center, but drops sharply at a radius of 90-98 mm. This results in a sharp increase in film thickness (a bead) at the edge of the wafer after polishing. It is generally accepted that the edge bead is caused by deflection of the polishing pad as it meets the wafer edge; this is referred to as “pad dive.” As the pad moves under the wafer and wafer carrier, the wafer edge forces the pad to tilt locally. The pad pressure on the wafer is very high at the outer 2 mm of the wafer, but very low at a radial distance of 3 to 7 mm from the edge. This low pressure results in a low polish rate. This problem is aggravated by the use of a stacked pad arrangement (preferred for many processes for better overall planarization), wherein a hard polishing pad is in contact with the wafer and a soft pad is placed underneath. 
     Various tool modifications have been suggested to reduce the effect of pad dive. These include milling the carrier face to a predetermined concave profile (so that the perimeter of the carrier is in closer contact with the wafer) and placing shims behind the backing film in the 90-98 mm radius area. However, even if the effects of backside air pressure and pad dive are brought into balance, that balance cannot be maintained for repeated process cycles as various components of the polishing apparatus are subjected to wear. 
     An additional problem that appears at the wafer edge is called “slurry penetration.” If the wafer is not sealed to the wafer carrier at its edge, slurry may penetrate between the wafer edge and the retaining ring and deposit on the backside of the wafer near the edge. A cleaning process is then required after the CMP process to remove the deposited slurry. This problem is aggravated by backside air leaking radially outward, which dries the slurry and causes it to adhere to the wafer (as noted by Ikenouchi et al., CMP-MIC Conference, 1999). 
     There remains a need for a wafer carrier and wafer backing film arrangement which provides improved polishing uniformity and is simple and inexpensive to implement on a wide variety of tools. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-described need for improved CMP process uniformity by providing a bi-material wafer backing film assembly, with a wafer edge sealed against backside air leakage. 
     In accordance with the present invention, a film removal apparatus is provided which includes a wafer backing film having a first portion and a second portion composed of different materials. The wafer backing film is substantially circular in shape and the first portion and second portion are concentric; the first portion has a circular shape at the center of the backing film and the second portion has an annular shape and surrounds the first portion. One or more backing shims may be provided to adjust the first portion of the wafer backing film and the second portion of the wafer backing film with respect to each other in the vertical direction. The backing shim, the first portion of the wafer backing film and the second portion of the wafer backing film are mounted on an adhesive assembly film, thereby forming an assembly for mounting on a wafer carrier. The first and second portions of the wafer backing film are adjusted with respect each other in the vertical direction in accordance with the thickness of the backing shim or shims. 
     During a film removal process, the wafer is pressed by the wafer carrier and wafer backing film with greater pressure at the perimeter of the wafer than at its center. 
     According to a further aspect of the invention, the wafer backing film is assembled so that, during a film removal process, the wafer is in contact with the second portion of the wafer backing film. Furthermore, the second portion of the wafer backing film is substantially impermeable to air. When the wafer backing film is assembled as described just above, this permits a seal to be formed at the edge of the wafer during a film removal process. Accordingly, when backside air pressure is applied to the wafer, leakage of the air around the edge of the wafer is controlled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a polishing pad, wafer carrier and wafer backing film in a conventional chemical-mechanical polishing (CMP) arrangement. 
     FIG. 1B shows a typical chemical-mechanical polishing (CMP) arrangement in which backside air is used. 
     FIG. 2 is a plot of radial variation of film polishing rate, showing the edge bead effect in a typical CMP process. 
     FIG. 3A shows a bi-material wafer backing film in accordance with the present invention. 
     FIG. 3B is a detail view showing the bi-material wafer backing film assembled on an adhesive film. 
     FIG. 4 is a detail view showing the wafer edge sealing effect of the bi-material backing film of the present invention. 
     FIG. 5 is a detail view showing the effect of improper alignment of the surfaces of the two pieces of the backing film. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention includes a wafer backing film with an inner portion and an outer portion having different properties, where the outer portion of the backing film seals the edge of the wafer against backside air leakage. These two film portions are preassembled on a single adhesive film and then mounted on the wafer carrier. 
     Bi-Material Backing Film 
     The uniformity of a film polishing process may be improved by replacing the single-piece wafer backing film  3  with a bi-material backing film  30 , as shown schematically in FIG.  3 A. In order to show details of the backing film and wafer carrier more clearly, the wafer and polishing pad are not shown in the figure. 
     The backing film  30  includes two concentric pieces, a center piece  31  and an outer ring  32 . These pieces in general have different compressibilities, hardnesses, porosities and thicknesses. The outer diameter of ring  32  matches the diameter of the wafer being polished (e.g., 200 mm for a standard production Si wafer). In a CMP arrangement for 200 mm wafers, the outer ring  32  has a width of about 5 mm, so that the center piece  31  has a diameter of about 190 mm. The combination of two pieces  31  and  32  overcomes the problems of backside air leakage and pad dive, permitting substantial improvement in CMP process uniformity. 
     The center piece  31  is made from a relatively soft, compressible gas-permeable material, such as Rodel DF200. This material may have a fibrous or open-cell structure. Center piece  31  may have a film (such as mylar) on the backside thereof, with an adhesive coating to permit positive attachment to the surface of the wafer carrier  2 . Such a film, if not gas-permeable, must be perforated with holes to align with the holes  12  in the wafer carrier, so that backside air can penetrate the gas-permeable material. The thickness of center piece  31  is typically about 0.025 inch. 
     If the center piece  31  is substantially thinner than the outer ring  32 , a backing shim  33  may be placed behind the center piece  31 . In this embodiment, the thickness of the backing shim  33  is about 0.005 inch. If the backing shim  33  is made of a gas-impermeable material such as mylar, it must have holes  36  punched therein, aligned with holes  12 , to permit air to reach center piece  31 . As shown in FIG. 3A, the thickness of outer ring  32  exceeds the combined thickness of center piece  31  and backing shim  33 , so that there is a step  35  between the center and outer pieces of the backing film  30 . 
     The outer ring  32  is made from an elastic material which is harder and less compressible than that used for the center piece  31 . For example, the outer ring  32  may be made from Rogers Poron 4701-50, with a compressibility about half that of Rodel DF200. This material has a closed-cell structure that is relatively impermeable to air. The thickness of the outer ring is typically about 0.031 inch in this embodiment. 
     Alternatively, if the outer ring  32  is thinner than the center piece  31 , a backing shim is then provided for the outer ring  32  to maintain an appropriate step height  35 , as discussed in more detail below. 
     The center piece  31  and outer piece  32  must be centered and mounted concentrically on the wafer carrier  2 . In addition, the holes  36  in backing shim  33  must be aligned with the holes  12  in the wafer carrier. Furthermore, the polishing process requires that the backing film be mounted smoothly against the wafer carrier  2 , with no gaps or air bubbles. 
     To minimize the difficulties of assembling the two-piece backing film on the wafer carrier, the center piece  31 , backing shim  33  and outer piece  32  are pre-assembled on an adhesive assembly film  34 . The film  34  typically is composed of mylar with a suitable adhesive coating, and has holes  37  aligned with holes  36  and  12 . 
     In this embodiment, as shown in FIG.  3 A and in greater detail in FIG. 3B, the combined thickness of outer ring  32  and adhesive film  34  exceeds the combined thickness of center piece  31 , backing shim  33  and adhesive film  34 . Accordingly, there is a step  35  representing the vertical distance between surfaces  31   f  and  32   f  facing the wafer. 
     When a wafer is pressed against the polishing pad by the wafer carrier during a CMP process, both the center piece  31  and the outer ring  32  are compressed. The lower compressibility of outer ring  32  results in greater polishing pressure near the wafer edge, thereby increasing the polishing rate and counteracting the edge bead effect. 
     Wafer Carrier Film Change 
     Since the wafer backing film  30  is preassembled on the adhesive film  34 , changing the wafer backing film on the wafer carrier can be done quickly and accurately. Compared to the two pieces of the wafer backing film, the assembly is relatively stiff and easy to handle. In particular, the concentric alignment of wafer backing film pieces  31 ,  32  is maintained during the wafer carrier rebuild procedure. Accordingly, using a preassembled wafer backing film minimizes the time required for this procedure, as compared with aligning the two pieces of the wafer backing film directly on the wafer carrier surface. Any assemblies with misaligned pieces, air bubbles, gaps, or incorrect step heights are rejected before use, saving the tool downtime associated with reworking the wafer backing film. 
     The assembly has an overall diameter greater than or equal to that of the face  2   f  of the wafer carrier. When the diameter is greater than that of the wafer carrier, the entire wafer backing film is easily aligned to the wafer carrier by matching the edge of adhesive film  34  with the edge of wafer carrier  2 . When the diameter of the assembly is larger, alignment can easily be achieved by matching the punched holes in the film to the holes in the wafer carrier face and then trimming the excess portion with a razor. Alignment-aiding jigs may also be used wherein notches or holes in the excess portion of the film match pins on the jig. 
     Wafer Edge Seal 
     As shown in FIGS. 3A and 3B, backside air is introduced behind the wafer through holes  12  in wafer carrier  2 . Since the center piece  31  of the backing film  30  is gas-permeable (and films  31 ,  33  and  34  are perforated), the backside air penetrates center piece  31  so that backside air pressure is distributed over the area of center piece  31 . However, there are no holes  12  to apply backside air pressure behind outer ring  32 , and outer ring  32  is relatively gas-impermeable. Accordingly, when the wafer  1  is pressed against the backing film  30  during polishing, the edge of the wafer is effectively clamped against outer ring  32 , so that the escape of backside air between the wafer  1  and wafer carrier  2  is hindered (see FIG.  4 ). 
     It should be noted that in the present invention, the elastic outer ring  32  conforms to the backside of the wafer&#39;s outer edge when backside air pressure is applied. This is in contrast to the conventional arrangement, wherein radially leaking backside air tends to move the entire wafer in a vertical direction away from the face of the wafer carrier. When the outer edge of the wafer is sealed or partially sealed against the wafer carrier by the outer ring  32  during polishing, the flexure of the wafer tends to be a radially symmetric bowing of the wafer. Because the backside air is substantially trapped between the wafer and wafer carrier, the air pressure is uniformly distributed across the wafer backside, thereby improving polishing uniformity. Furthermore, the outer edge of the wafer is securely supported while backside air pressure is applied to the center region of the wafer. The combination of (1) control of backside air leakage and (2) mechanical support of the outer edge of the wafer permits improved control of polishing pressure on the wafer by modulating the backside air pressure. In particular, the backside air pressure at the center of the wafer may be adjusted to balance the mechanical pressure of the seal near the outer edge of the wafer, to obtain a uniform polishing rate across the wafer. 
     The tightness of the seal may be varied by varying the choice of material for the outer backing film piece  32  and/or the wafer carrier. 
     It is also noteworthy that the outer diameter of outer ring  32  is essentially the same as that of wafer  1 , so that the entire back side of the wafer is covered by the backing film  30 . In addition, since outer ring  32  is a relatively hard material, contaminants are prevented from penetrating the backing film and depositing on the back side of the wafer. In particular, the problem of slurry penetration is avoided. 
     Control of Step Height 
     It has been found that the height of the step  35  between the two pieces  31 ,  32  of the backing film  30  has a critical effect on the uniformity of the polishing process. In particular, as both pieces are compressed during the polishing process, the vertical alignment of the two pieces  31 ,  32  must be such that outer ring  32  remains in contact with the backside of the wafer. FIG. 5 shows a situation where the surface  31   a  of center piece  31 , when compressed during polishing and when backside air pressure is applied, is not in vertical alignment with the surface  32   a  of outer ring  32 . A gap  51  appears between the wafer  1  and the outer ring  32 . This results in inadequate polishing pressure on the wafer at the outer edge, and permits backside air to leak radially outward. The beneficial effects of the bi-material backing film are therefore lost. 
     In this embodiment of the present invention, the thicknesses of the wafer backing film pieces  31 ,  32  and the thickness of the backing shim  33  are selected so as to yield the correct step height  35  when assembled on the adhesive film  34 . 
     In this arrangement (see FIG.  3 B), the step height  35  may be characterized as the vertical distance between surfaces  31   f  and  32   f  of the center piece  31  and outer ring  32  of the wafer backing film  30  (that is, the surfaces facing the wafer during a polishing process). A positive value of step height  35  is defined as surface  32   f  being at a greater vertical distance, relative to surface  2   f  of the wafer carrier  2 , than surface  31   f.  When the center piece  31  and outer ring  32  are of the materials and dimensions given above, the optimum height of step  35  has been found to be about 0.001 inch. Furthermore, adjustment of the step height  35 , using various thicknesses of backing shim  33 , permits use of varying thicknesses of both backing film pieces  31 ,  32 . The thicknesses of backing film pieces  31 ,  32  may thus be varied to optimize the polishing process, without the need for modification of the wafer carrier. 
     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.