Abstract:
A new method of polishing very large diameter wafers. Multiple polishing pads are provided. Each polishing pad rotates around the Z-axis. Each pad can be individually controlled for Chemical Mechanical Planarization (CMP) process parameters such as pressure, rotation speed, slurry feed and slurry mixture. The planarization process can be controlled or optimized by individual rotating polishing pad or by a grouping of one or more rotating polishing pads. The wafer being processed can be rotated which further reduces the dependence on existing pad conditions which in turn translates into reduced use of slurry and prolonged life-time of the polishing pad.

Description:
FIELD OF THE INVENTION 
     The invention relates to the fabrication of Semiconductor Wafers, and more specifically to a method of Chemical Mechanical Polishing (CMP) of very large semiconductor wafers of the type used in the fabrication of Integrated Circuits. 
     DESCRIPTION OF THE PRIOR ART 
     Integrated Circuits are conventionally fabricated from semiconductor wafers, each wafer contains an array of individual integrated circuit dies. It is of key importance that the wafer be polished to a planar configuration at various stages of the wafer processing stages. This requirement becomes increasingly more difficult to adhere to as the size of the wafer increases. 
     One of the most serious problems inherent in the CMP process is non-uniformity of the polishing rate over the entire surface of an object to be polished, e.g. a semiconductor wafer. A non-uniform rate of polishing results in not all surface regions of the wafer being polished equally which has a serious detrimental effect on the yield and reliability of the produced semiconductor elements. It is therefore of paramount importance to develop technology which permits further improving the uniformity of the polishing rate over the entire surface of the wafer, a requirement which becomes even more important as the size of the wafer increases. 
     The present invention addresses the problems of wafer polishing using Chemical Mechanical Planarization for very large wafers. 
     U.S. Pat. No. 5,575,707 (Talieh et al.) teaches a polishing pad cluster for polishing semiconductor wafers, the pads do not rotate. 
     U.S. Pat. No. 5,230,184 (Bukhman) teaches a plurality of periodic polishing pads, the pads do not rotate. 
     When processing a wafer, a conventional wafer clamping arrangement secures a wafer to a wafer cooling pedestal with a circular wafer clamping ring. The clamping ring is used to press the edge of the wafer into the continuous (sealing abutment with the upper surface of the wafer pedestal. A port or opening can be provided to flow a supply of an inert coolant gas, such as argon, to the backside of the wafer, this to improve thermal transfer between the wafer and the heater chuck. This takes advantage of the large thermal mass of the heater chuck relative to the wafer for conducting temperature. In this way, a predictable and consistent temperature is maintained across the wafer surface during wafer processing, and the various process steps that are used to fabricate devices on the wafer surface may be carried out in a reliable manner. 
     During standard PVD processing, deposition of the metal film on the surface of the semiconductor wafer typically results in the deposition of a metal film on the surface of the clamping ring. This deposition alters the profile (height and inner diameter) of the clamping ring, which in turn results in the metal ring, that is its modified profile, being shadowed on the semiconductor wafer which is being processed. This shadowing has a negative effect on wafer yield and must therefore be restricted or eliminated. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a polishing pad cluster is provided for polishing very large semiconductor wafers comprising a plurality of integrated circuit dies. This cluster includes a pad support and a plurality of polishing pads, each of the polishing pads rotating in the plane of the wafer (around the vertical or Z axis) and each polishing pad individually controlled. 
     A principle object of the present invention is to provide a method of Chemical Mechanical Polishing (CMP) for very large wafers. 
     Another object of the present invention is to provide extended control over polishing rates of selected areas within the semiconductor wafer being polished. 
     Another object of the present invention is to maintain polishing uniformity across the wafer for very large wafers. 
     Another object of the present invention is to maintain process optimization by maintaining tight process parameter control for the processing of very large wafers. 
     Another object of the present invention is pad condition control and process parameter control across the area of the entire wafer for very large wafers. 
     In the first embodiment of the present invention the downward pressure of the rotating polishing pad is adjusted via a flexible membrane which is controlled by a pressure cavity. The interface between the the flexible membrane and the polishing pad is formed by ball-bearings. 
     In the second embodiment of the present invention the downward pressure of the rotating polishing pad is adjusted via a flexible membrane which is controlled by a pressure cavity. The interface between the the flexible membrane and the polishing pad is part of the membrane. 
     In the third embodiment of the present invention the downward pressure of the rotating polishing pad is controlled by magnets which form part of the rotating polishing pads. 
     In the fourth embodiment of the present invention the downward pressure of the rotating polishing pad is controlled by one large magnet which forms part of the wafer mount chuck assembly 
     In the fifth embodiment of the present invention the downward pressure of the rotating polishing pad is controlled by passive mechanical weights which are part of the polishing pads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, forming a material part of the description, there is shown: 
     FIGS. 1 through 7 schematically illustrate a preferred embodiment of the implementation of the present invention. 
     FIG. 1 is a plan view of the polishing pad assembly of the present invention. 
     FIG. 2 is a cross-sectional view taken along line  2 - 2 ′ of FIG. 1 . 
     FIG. 3 is a cross-sectional view of two polishing pads mounted in a flexible membrane via ball bearings. 
     FIG. 4 is a cross-sectional view of a polishing pad mounted in a flexible membrane where the mounting is part of the membrane. 
     FIG. 5 is a cross-sectional view of a polishing pad where the down-ward pressure on the polishing pad is exerted via magnets which form part of the polishing pad. 
     FIG. 6 is a cross-sectional view of a polishing pad where the down-ward pressure on the polishing pad is exerted via a large magnet which is mounted on the wafer mounting chuck. 
     FIG. 7 is a cross-sectional view of a polishing pad where the down-ward pressure on the polishing pad is exerted via mechanical weights. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIGS.  1 . through  7  relate to the preferred embodiment of the polishing pad assembly  10  of the present invention. The polishing pads  16  are designed for use in Chemical Mechanical Planarization of a wafer  14  that includes an array on integrated circuit dies. Typically, wafer  14  is mounted in a non-gimbaling wafer mount which provides a polishing force in the Z direction and rotates wafer  14  about the center of rotation C. 
     Referring now more particularly to FIG. 3 there is shown a cross-section view of an assemblage of polishing pads  16  in relation to the location of the wafer  14  which is being polished using the Chemical Mechanical Polishing (CMP) process. FIG. 3 presents, for reasons of drawing simplicity, only two of the multiplicity of possible rotating polishing pads. 
     Chuck  12  is made of a flat rigid material, such as stainless steel, so that it supports substrate  14 . Substrate  14  is typically held on chuck  12  by a vacuum force that is commonly used and well understood in the semiconductor art and is not important for understanding the present invention. Chuck  12  is attached to a shaft or a movement means C that allows movement of chuck  12  in a vertical direction, a horizontal direction, rotational, and vibrational. It should be understood that when substrate  14  is held by chuck  12  that movement of chuck  12  is transferred to substrate  14 . Additionally, any movement can be done simultaneously, such as vibrational movement while chuck  12  slowly rotates. 
     Turning now to the drawings, FIGS. 1 through 7 relate to the preferred embodiment of the polishing pad assembly  10  of the present invention. The polishing pads  16 , FIGS. 1 and 2, are designed for use in Chemical Mechanical Planarization of a wafer  14  that includes an array on integrated circuit dies (not shown). Typically, wafer  14  is mounted in a non-gimbaling wafer mount, which provides a polishing force in the Z direction and rotates wafer  12  around a center of rotation C. FIG. 2 is a cross section that is taken along line  2 - 2 ′ of FIG.  1  and further shows the rotation R of each of the polishing pads  16 . Each of the polishing pads  16  is mounted on an axis that is rotated in direction R. Pressure can be applied (not shown) between the surface of the wafer  14  that is being polished and the polishing pads  16 , this pressure can be controlled by individual polishing pads  16  or it can be controlled by combining one or more polishing pads in groups for the control and application of pressure. By applying this latter method of pressure control, polishing action can be controlled across the surface of the wafer, typically dependent on the radial distance of the polishing pads  16  and the center of rotation C of the wafer  14 . 
     Chuck  12  with substrate is moved so that contact is made between substrate  14  and polishing pads  16 . Pressure is allowed to enter cavity  22  through port  24 , thereby creating pressure  20  which pushes the flexible membrane  13  in a downward or outward direction. As a result of this motion, polishing pads  16  are pressed in a downward or outward direction and conform to the unevenness or irregularities of substrate  14 . If, in addition, each polishing pad  16  has approximately the same size as the die and if each polishing pad is positioned over a single die location on the substrate  14 , this allows for polishing or planarization of each individual die, regardless of how warped or uneven the substrate  14  may be. Since, in addition, the polishing membrane  13  pushes polishing pads  16  into the substrate  14  with equal force or pressure, polishing rates for each individual polishing pad are relatively equal even on an irregular surface. 
     The flexible member  13  is attached to the side of the walls of cavity  22  by means of an edge ring (not shown) which provides support for the flexible membrane. 
     The polishing pads for the present invention may be used in virtually any application of the chemical mechanical planarization of semiconductor substrates. Many of the operating parameters when using the polishing pads should be similar to the parameters using conventional polishing pads. The slurry composition, polishing pad rotational velocity and substrate rotational velocity are all expected to be within the normal operating parameters of polishers with conventional polishing pads. 
     The shaft  19  on which the polishing pads  16  are mounted protrude through the flexible membrane  13  and are supported at each protrusion by ball bearings  18  which enable the polishing pad to rotate R around its axis. The ball bearings  18  employed are not part of the present invention, they may consist of one unit per polishing pad shaft or of two separate units per polishing pad shaft. If two separate units are used for the ball bearings  18  each of these units is mounted on the flexible membrane on the opposite side of the companion unit with both units belonging to the same protrusion of the polishing shaft. 
     The assemblage shown in FIG. 3 contains a driver mechanism  26  which stimulates the rotation R for each of the polishing pads  16 . This driver mechanism  26  can drive all polishing pads  16  simultaneously or the driving of the polishing pads can be divided into one or more (multiple) zones. A zone in this context is understood to mean a functional grouping of one or more polishing pads such that these polishing pads exhibit the same characteristics of control, that is rotation R and downward pressure  20 , and operation. Multiple driver zones allow for selective polishing of the wafer substrate  14  and introduces one more level of control for the polishing process. This additional level of control allows for selective polishing of specific wafer areas or dies to include different rotating speeds R and different uses of slurries. The method of implementing driver mechanism  26  is not part of this invention although all normal design parameters for such a driver mechanism apply. Where this driver mechanism is unique is that it must rotate the polishing pads  16  while providing a loose mechanical coupling to the polishing pads so as not to inhibit the effectiveness of the downward pressure  20 . 
     Cavity  22  is further equipped with a perforated stabilizer plate  17  which restricts motion of the polishing pads in the X and Y direction. This plate may be required due to the relative length of the shaft or axis  19  of the polishing pads  16 . In combination with this, the polishing pad  16  to polishing pad axis  19  interface may be of a design which allows the polishing pad  16  to articulate or move in the X-Y field thus further allowing the pad to more closely adhere to the surface of the wafer that is being polished. This feature however is not part of the present invention and can follow standard semiconductor polishing practices and implementations. 
     The indicated stabilizer plate  17  is optional, this plate must be perforated so as not to inhibit the downward pressure  20 . The feed through of the polishing pad axis  19  through the stabilizer plate  17  must be rigid in the X-Y direction but must be loosely coupled in the Z direction, again so as not to inhibit or hinder pressure  20 . 
     FIG. 4 differs from FIG. 3 in the technique used for the protrusion of the shafts  19  of the polishing pads  16  through the flexible membrane  17 . In this embodiment of the present invention no ball bearings are used, the opening for protrusion is part of the flexible membrane. 
     FIG. 5 shows an apparatus in cross sectional view where magnets  30  are used. Each polishing pad  16  has one corresponding magnet  30 . The magnets  30  have an opening in the center which allows the shaft of the polishing pad  16  to move freely in the Z direction. The magnets  30  create a magnetic field which interacts with the polishing pad  16  so as to urge the polishing pad  16  toward wafer  14 . If desired, the magnets  30  can be designed to create magnetic fields which are not uniform for all the magnets  30  applied. For example, in the situation where polishing rates tend to be greater near the periphery of the wafer  14  than near the center, the magnets  30  can provide stronger magnetic forces near the center of the wafer  14  than near the periphery in order to make the polishing rate more nearly uniform across the surface of the wafer. The inverse is also possible. 
     To create the magnetic fields, both permanent magnets and electro magnets can be used. 
     FIG. 6 shows the cross-sectional view of a wafer polishing apparatus where a large magnet  36  is mounted on top of and as part of the wafer chuck assembly  12 . An insulating layer  38  insulates the magnetic field of magnet  36  from the chuck assembly  12  while also attaching the magnet  36  to the chuck assembly  12 . Magnet  36  creates a magnetic field which interacts with the polishing pad  16  so as to urge the polishing pad toward the wafer. If desired, the magnet  36  can be designed to create magnetic fields which are not uniform across the magnet. For example, in the situation where polishing rates tend to be greater near the periphery of the wafer  14  than near the center, the magnet can provide stronger magnetic forces near the center of the wafer  14  than near the periphery in order to make the polishing rate more nearly uniform across the surface of the wafer. The inverse is also possible. 
     To create the magnetic fields, both a permanent magnets and electro magnetics can be used. 
     FIG. 7 shows a cross-sectional view of the wafer polishing apparatus where mechanical weights have been used to enhance polishing pad to wafer contact. These weights can be varied in size or weight such that the downward pressure exerted on the polishing pad can be varied resulting in selectivity of polishing speed for selected bands or areas or dies within the semiconductor wafer which is being polished. 
     This invention is not limited to the preferred embodiments described above, and a wide variety of polishing pads and polishing pad to polishing-axis joints can be used. A wide variety of polishing pad material cans also be used combined with or separate from a large variety of methods to stimulate or move the polishing pads in either the motion of rotation or in motion in the Z direction, that is the direction perpendicular to the plane of the wafer being polished. 
     It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.