Abstract:
A chemical mechanical polishing machine is provided with a platen that has an integral sub-pad. A fixed abrasive polishing layer is mounted, without adhesive between the polishing layer and the sub-pad, to the top surface of the platen and the sub-pad. The polishing layer is vacuum mounted, for example, to the integral sub-pad of the platen. A cooling arrangement is provided in the platen that cools the polishing layer and improves product quality.

Description:
FIELD OF THE INVENTION 
     The present invention relates to polishing and planarizing of surfaces, such as semiconductor wafer surfaces, and more particularly, to fixed abrasive pad and platen arrangements for chemical-mechanical polishing/planarizing of semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     In the process of fabricating modem semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously formed layers and structures. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems when forming the next layer. For example, when printing a photographic pattern having small geometries over previously formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface, otherwise, some parts of the pattern will be in focus and other parts will not. In fact, surface variations on the order of less than 1000 angstroms over a 25×25 mm die would be preferable. In addition, if the irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, it is desirable to effect some type of planarization, or leveling, of the IC structures. In fact, most high-density IC fabrication techniques make use of some method to form a planarized wafer surface at critical points in the manufacturing process. 
     One method for achieving semiconductor wafer planarization or topography removal is the chemical-mechanical planarization/polishing (CMP) process. In general, the CMP process involves holding the wafer against and moving the wafer relative to a polishing platen under a controlled pressure or friction force. A typical CMP apparatus includes a polishing head for holding the semiconductor wafer against the polishing platen. The polishing platen is covered with a pad. The pad is usually either an open cell foamed polyurethane (e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface (e.g. Rodel EX2000). The pad surface is wetted with a polishing slurry containing chemicals and, optionally, abrasive particles. This pad typically has a backing layer called a sub-pad that interfaces with the surface of the platen. 
     Another type of polishing pad is the “fixed-abrasive pad” made from abrasive particles fixedly dispersed in a suspension medium. Unlike conventional CMP slurries, the planarizing liquid employed with a fixed abrasive pad may be free of abrasive particles, i.e., a polishing chemical solution. Chemical-mechanical polishing/planarization with a chemical solution on a fixed abrasive pad offers several desirable aspects: low chemical cost, low dishing, and a good top surface finishing. 
     One concern with metal CMP is the heat generated during the polishing process, caused by chemical reaction of the metal with the polishing chemical and the friction of the wafer against the fixed abrasive pad surface. Heat build-up may degrade the quality of the wafer being polished. Fixed abrasive pads with thick stacks exhibit poor thermal conductivity and, therefore, wafers are not readily cooled during CMP on such pads. 
     Another concern is the dishing observed generally in metal CMP. Dishing can be high if the pad modulus is low. A conventional fixed abrasive pad may include polishing elements of 40 μm in height that are mounted in a thin Mylar™ sheet. The other layers below the Mylar™ sheet form a sub-pad stack, and may have five layers, for example. A top adhesive layer joins the Mylar™ sheet to the sub-stack. A polycarbonate layer, which is supposed to provide the necessary pad modulus, is underneath the top adhesive layer, and a foam layer, which is supposed to provide long range flexibility, is joined to the polycarbonate layer by a middle adhesive layer. A bottom adhesive layer on the foam layer attaches the polishing pad formed by the Mylar™ sheet and sub-pad stack to the top of the platen. However, due to the existence of the layer of very soft adhesive material between the very thin Mylar™ sheet and the polycarbonate layer, the modulus of the integrated pad of six layers cannot be high enough for strict dishing requirements. Elimination of this adhesive layer would greatly increase the pad modulus and help the pad dishing performance. 
     A further concern is the consumable cost of fixed abrasive polishing pads. Although only the top Mylar™ layer is subjected to wear during polishing, the entire pad is normally replaced when the Mylar™ pad is excessively worn. This increases the cost of ownership. 
     Another concern is the system vibration that may occur when polishing with conventional fixed abrasive pads. Such vibrations can occur due to the typical construction of fixed abrasive pads, and their attachment on top of a platen. The foam pad that forms part of the polishing pad is not readily changeable to damp the system vibrations of an individual polisher. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes problems with existing fixed abrasive polishing pads by providing a platen for a fixed abrasive pad arrangement, comprising a platen housing, a sub-structure integrally mounted on the platen housing, and a mounting arrangement configured to removably mount a fixed abrasive layer to the integral sub-structure. 
     One of the aspects of certain embodiments of the fixed abrasive pad arrangement of the present invention is that the fixed abrasive layer may be readily replaced since it is removably mounted to the sub-structure, or sub-pad. As the platen has an integral sub-pad, the sub-pad does not have to be replaced whenever the top polishing layer requires replacing. This reduces consumable expenses, as only the top polishing layer is replaced, rather than an entire polishing pad with sub-layers together. 
     As the sub-structure is integrally mounted on the platen housing, a cooling arrangement may be provided that improves the cooling of the overall sub-structure and fixed abrasive layer, in comparison to conventional fixed abrasive polishing pads that include a sub-structure stack and a fixed abrasive layer. Since the sub-structure is integral to the platen housing in certain embodiments of the invention, a cooling arrangement in accordance with embodiments of the invention may be provided that better cools the sub-structure and the fixed abrasive layer. 
     An aspect of the present invention provides a chemical-mechanical polishing machine comprising a platen housing, a sub-pad integral with the platen housing, and a mounting arrangement configured to removably mount a fixed abrasive layer to the sub-structure. In certain embodiments of the invention, the mounting arrangement includes a vacuum mounting arrangement that removably and quickly mounts the fixed abrasive layer to the sub-structure, without the use of adhesive. Eliminating the adhesive between the fixed abrasive layer and the sub-structure reduces the undesirable phenomenon of dishing. 
     The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of a fixed abrasive pad arrangement constructed in accordance with an embodiment of the present invention. 
     FIG. 2 depicts the arrangement of FIG. 1, with a fixed abrasive layer mounted to the platen and sub-pad of FIG.  1 . 
     FIG. 3 depicts a cross-section of a fixed abrasive pad arrangement in accordance with another embodiment of the present invention, with a fixed abrasive layer mounted to the platen and sub-pad. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses and solves problems related to fixed abrasive polishing pads, including difficulties in pad cooling, vibration damping, and dishing and further reduces the cost of the pad. The present invention achieves this, in part, by providing a fixed abrasive pad arrangement in which the sub-pad is integral to the platen. The fixed abrasive polishing layer is removably mountable on the sub-pad. The integral nature of the sub-pad allows pad cooling to be established since the sub-pad is not replaced when the fixed abrasive polishing layer is replaced. Improved vibration damping may be achieved since the foam pad layer of the sub-pad may be selected specifically to damp system vibrations. Dishing is reduced when the fixed abrasive polishing layer is mounted by a vacuum mount, or by adhesive at the edges. This avoids the use of an adhesive layer between the fixed abrasive polishing layer and the sub-pad, which is advantageous as such an adhesive layer can contribute to dishing. 
     FIG. 1 is a cross-section of a fixed abrasive pad arrangement in accordance with an embodiment of the present invention. The arrangement includes a platen  30  on which a sub-pad  32  is integrally provided. The sub-pad  32  is fixedly mounted, by adhesive  33 , for example, within a recess  35  in the platen  30 . The sub-pad  32  may be integrated into the platen  30  by other mechanisms, such as mechanical fasteners, etc. 
     The sub-pad  32  comprises a foam layer  34  that is attached to the bottom of the recess  35 . An adhesive layer  36  connects a structural layer  38  to the top of the foam layer  34 . The foam layer  34  provides long-range polishing uniformity adjustment, while the structural layer  38  provides hardness to increase characteristic length and reduce dishing. In certain embodiments, the structural layer  38  can be a polycarbonate layer. 
     The platen  30 , with the integral sub-pad  32 , is configured to receive a fixed abrasive polishing layer (not shown in FIG. 1) on the top surface  40  of the structural layer  38 . However, since the sub-pad  32  is integral to the platen  30 , the fixed abrasive polishing layer is the only layer that needs frequent replacement due to wear. Since frequent replacement of the fixed abrasive polishing layer is contemplated, it is advantageous to avoid the use of adhesive to mount the fixed abrasive polishing layer to the top surface  40  of the structural layer  38 . Embodiments of the present invention, such as that depicted in FIG. 1, include a mounting arrangement that removably mounts the fixed abrasive polishing layer. 
     In certain embodiments of the present invention, the mounting arrangement is a vacuum mounting arrangement that includes a vacuum source  46  that communicates through the platen  30  to vacuum conduits  44 . The vacuum conduits  44  open at vacuum openings  45  at the top surface  40  of the structural layer  38 , to thereby form a vacuum-mounting surface. The very edge of the fixed abrasive polishing layer  50  can be adhered to the edge of the platen  30 , to make sure the polishing layer  50  will not peel off during CMP operation. 
     FIG. 2 depicts the platen  30  of FIG. 1, with a fixed abrasive polishing layer  50  vacuum-mounted thereon. The fixed abrasive polishing layer  50 , in certain embodiments, comprises a Mylar™ sheet on which abrasive elements  52  are fixedly dispersed in a suspension medium. The fixed abrasive polishing layer  50  is easily mounted by applying vacuum, which creates negative pressure at the vacuum opening  45 , thereby covering the top surface  40  with the fixed abrasive polishing layer  50 . 
     Removal of the polishing layer  50  is accomplished by cutting off the vacuum and lifting off the polishing layer  50 . Providing slight positive pressure through the vacuum conduits  44  is employed in certain embodiments to aid in removal of the polishing layer  50 . The avoidance of adhesive directly between the fixed abrasive polishing layer  50  and the structural layer  38  reduces dishing, in addition to making the replacement of the fixed abrasive polishing layer  50  much easier. 
     Referring back to FIG. 1, the platen  30  also includes a cooling arrangement that serves to cool the fixed abrasive polishing layer  50  when it is mounted and the polishing process occurs. Since the sub-pad  32  is integral to the platen  30 , and is not meant to be frequently replaced, a cooling arrangement that includes cooling fluid conduits  42  through the sub-pad  32  can be employed. The cooling fluid conduits  42  are coupled to a cooling fluid source  48 . The cooling fluid conduits  42  form a closed-loop system and extend through the foam layer  34  to bear against the structural layer  38 . Since the foam layer  34  is not normally highly thermally conductive, directly cooling the structural layer  38  through the conduits  42  provides a more efficient and increased cooling of the structural layer  38 , and therefore, a mounted fixed abrasive polishing layer  50 . If the platen is rotatable, rotary couplings (not shown) are used to couple the vacuum conduits  44  to the vacuum source  46  and the cooling fluid conduits  42  to the cooling fluid source  48 . 
     FIG. 3 illustrates another embodiment of the present invention, which employs an alternative mounting arrangement. In the embodiment of FIG. 3, a seal is provided on the platen edge  54 . The seal may be an o-ring seal  60 , as shown in FIG.  3 . Alternatively, adhesive may be provided on the platen edge  54 , and the edges of the fixed abrasive polishing layer  50  pressed down on the adhesive at the platen edge  50  to hold the fixed abrasive polishing layer  50  on the sub-pad  32 . There is no adhesive between the polishing layer  50  and the structural layer  38 , so that increased dishing does not become a problem. 
     The present invention, described above, increases cooling of the polishing layer, allows for improvements in damping of system vibration, reduces dishing, and lowers the costs of operation, as only a single polishing layer needs to be replaced frequently and not the sub-pad, which is integral to the platen. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.