Abstract:
An improvement in a polishing apparatus for planarizing substrates comprises a tenacious coating of a low-adhesion material to the platen surface. An expendable polishing pad is adhesively attached to the low-adhesion material, and may be removed for periodic replacement at much reduced expenditure of force. Polishing pads joined to low-adhesion materials such as polytetrafluoroethylene (PTFE) by conventional adhesives resist distortion during polishing but are readily removed for replacement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/478,692, filed Jan. 6, 2000, pending, which is a continuation of application Ser. No. 09/124,329, filed Jul. 29, 1998, now U.S. Pat. No. 6,036,586, issued Mar. 14, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to polishing methods and apparatus. More particularly, the invention pertains to apparatus and methods for polishing and planarizing semiconductor wafers, optical lenses and the like. 
     2. State of the Art 
     In the manufacture of semiconductor devices, it is important that the surface of a semiconductor wafer be planar. 
     For high density semiconductor devices having features with extremely small sizes, i.e. less than 1 μm, planarity of the semiconductor wafer is particularly critical to the photolithographic forming of the extremely small conductive traces and the like. 
     Methods currently used for planarization include (a) reflow planarization, (b) application of a sacrificial dielectric followed by etch back planarization, (c) mechanical polishing, and (d) chemical-mechanical polishing (CMP). Methods (a) through (c) have some applications but have disadvantages for global wafer planarization, particularly when fabricating dense, high speed devices. 
     In U.S. Pat. No. 5,434,107 of Paranjpe, a planarization method consists of applying an interlevel film of dielectric material to a wafer—and subjecting the wafer to heat and pressure so that the film flows and fills depressions in the wafer, producing a planar wafer surface. An ultraflat member overlying the dielectric material ensures that the latter forms a flat surface as it hardens. The ultraflat member has a non-stick surface such as polytetrafluoroethylene so that the interlevel film does not adhere thereto. 
     In a similar method shown in European Patent Publication No. 0 683 511 A2 of Prybyla et al. (AT&amp;T Corp.), a wafer is covered with a hardenable low-viscosity polymer and an object with a highly planar surface is placed in contact with the polymer until the polymer is cured. The object is separated from the polymer, which has cured into a highly planar surface. 
     The planarization method of choice for fabrication of dense integrated circuits is typically chemical-mechanical polishing (CMP). This process comprises the abrasive polishing of the semiconductor wafer surface in the presence of a liquid or slurry. 
     In one form of CMP, a slurry of an abrasive material, usually combined with a chemical etchant at an acidic or alkaline pH, polishes the wafer surface in moving compressed planar contact with a relatively soft polishing pad or fabric. The combination of chemical and mechanical removal of material during polishing results in superior planarization of the polished surface. In this process it is important to remove sufficient material to provide a smooth surface, without removing an excessive quantity of underlying materials such as metal leads. It is also important to avoid the uneven removal of materials having different resistances to chemical etching and abrasion. 
     In an alternative CMP method, the polishing pad itself includes an abrasive material, and the added “slurry” may contain little or no abrasive material, but is chemically composed to provide the desired etching of the surface. This method is disclosed in U.S. Pat. No. 5,624,303 of Robinson, for example. 
     Various methods for improving wafer planarity are directed toward the application of interlayer materials of various hardness on the wafer surface prior to polishing. Such methods are illustrated in U.S. Pat. No. 5,618,381 of Doan et al., U.S. Pat. No. 5,639,697 of Weling et al., U.S. Pat. No. 5,302,233 of Kim et al., U.S. Pat. No. 5,643,837 of Hayashi, and U.S. Pat. No. 5,314,843 of Yu et al. 
     The typical apparatus for CMP polishing of a wafer comprises a frame or base on which a rotatable polishing pad holder or platen is mounted. The platen, for example, may be about 20-48 inches (about 50-122 cm) or more in diameter. A polishing pad is typically joined to the platen surface with a pressure-sensitive adhesive (PSA). 
     One or more rotatable substrate carriers are configured to compress e.g. semiconductor wafers against the polishing pad. The substrate carrier may include non-stick portions to ensure that the substrate, e.g. wafer, is released after the polishing step. Such is shown in U.S. Pat. No. 5,434,107 of Paranjpe and U.S. Pat. No. 5,533,924 of Stroupe et al. 
     The relative motion, whether circular, orbital or vibratory, of the polishing pad and substrate in an abrasive/etching slurry may provide a high degree of planarity without scratching or gouging of the substrate surface, depending upon wafer surface conditions. Variations in CMP apparatus are shown in U.S. Pat. No. 5,232,875 of Tuttle, U.S. Pat. No. 5,575,707 of Talieh, U.S. Pat. No. 5,624,299 of Shendon, U.S. Pat. No. 5,624,300 of Kishii et al., U.S. Pat. No. 5,643,046 of Katakabe et al., U.S. Pat. No. 5,643,050 of Chen, and U.S. Pat. No. 5,643,406 of Shimomura et al. 
     In U.S. Pat. No. 5,575,707 of Talieh et al., a wafer polishing system has a plurality of small polishing pads which together are used to polish a semiconductor wafer. 
     As shown in U.S. Pat. No. 5,624,304 of Pasch et al., the polishing pad may be formed in several layers, and a circumferential lip may be used to retain a desired depth of slurry on the polishing surface. 
     A CMP polishing pad has one or more layers and may comprise, for example, felt fiber fabric impregnated with blown polyurethane. Other materials may be used to form suitable polishing pads. In general, the polishing pad is configured as a compromise polishing pad—that is a pad having sufficient rigidity to provide the desired planarity, and sufficient resilience to obtain the desired continuous tactile pressure between the pad and the substrate as the substrate thickness decreases during the polishing process. 
     Polishing pads are subjected to stress forces in directions both parallel to and normal to the pad-substrate interfacial surface. In addition, pad deterioration may occur because of the harsh chemical environment. Thus, the adhesion strength of the polishing pad to the platen must be adequate to resist the applied multidirectional forces during polishing, and chemical deterioration should not be so great that the pad-to-platen adhesion fails before the pad itself is in need of replacement. 
     Pores or depressions in pads typically become filled with abrasive materials during the polishing process. The resulting “glaze” may cause gouging of the surface being polished. Attempts to devise apparatus and “pad conditioning” methods for removing such “glaze” materials are illustrated in U.S. Pat. No. 5,569,062 of Karlsrud and U.S. Pat. No. 5,554,065 of Clover. 
     In any case, polishing pads are expendable, having a limited life and requiring replacement on a regular basis, even in a system with pad conditioning apparatus. For example, the working life of a typical widely used CMP polishing pad is about 20-30 hours. 
     Replacement of polishing pads is a difficult procedure. The pad must be manually pulled from the platen, overcoming the tenacity of the adhesive which is used. The force required to manually remove a 30-inch diameter pad from a bare aluminum or ceramic platen may exceed 100 lbf (444.8 Newtons) and may be as high as 150 lbf (667.2 Newtons) or higher. Manually applying such high forces may result in personal injury as well as damage to the platen and attached machinery. 
     SUMMARY OF THE INVENTION 
     The invention comprises the application of a permanent, low adhesion, i.e. “non-stick,” coating of uniform thickness to the platen surface. Exemplary of such coating materials are fluorinated compounds, in particular fluoropolymers including polytetrafluoroethylene (PTFE) sold under the trademark TEFLON by DuPont, as well as polymonochlorotrifluoroethylene (CTFE) and polyvinylidene fluoride (PVF 2 ). The coating retains its tenacity to the underlying platen material, and its relatively low adhesion to other materials, at the temperatures, mechanical forces, and chemical action encountered in CMP processes. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale: 
     FIG.  1 . is a perspective partial view of a polishing apparatus of the prior art; 
     FIG. 2 is a cross-sectional view of a portion of a polishing apparatus of the prior art, as taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a cross-sectional view of a portion of a polishing apparatus of the invention; 
     FIG. 4 is a cross-sectional view of a portion of a platen and polishing pad of the invention, as taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is a top view of a polishing platen and pad of another embodiment of the invention; and 
     FIG. 6 is a cross-sectional view of a portion of a platen and polishing pad of the invention, as taken along line  6 — 6  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Portions of a typical prior art chemical-mechanical polishing (CMP) machine  10  are illustrated in drawing FIGS. 1 and 2. A platen  20  has attached to its upper surface  12  a polishing pad  14  by a layer of adhesive  16 . If it is desired to rotate platen  20 , its shaft  18 , attached to the platen  20  by flange  48 , may be turned by a drive mechanism, such as a motor and gear arrangement, not shown. 
     A substrate  30  such as a semiconductor wafer or optical lens is mounted on a substrate carrier  22  which may be configured to be moved in a rotational, orbital and/or vibratory motion by motive means, not shown, through shaft  24 . In a simple system, shafts  18  and  24  may be rotated in directions  26  and  28  as shown. The substrate  30  is held in the carrier  22  by friction, vacuum or other means resulting in quick release following the polishing step. A layer  38  of resilient material may lie between the substrate  30  and carrier  22 . The surface  32  of the substrate  30  which is to be planarized faces the polishing surface  34  of the pad  14  and is compressed thereagainst under generally light pressure during relative movement of the platen  20  (and pad  14 ). 
     In chemical-mechanical polishing (CMP), a polishing slurry  40  is introduced to the substrate-pad interface  36  to assist in the polishing, cool the interfacial area, and help maintain a uniform rate of material removal from the substrate  30 . The slurry may be introduced e.g. via tubes  42  from above, or may be upwardly introduced through apertures, not shown, in the polishing pad  14 . Typically, the slurry  40  flows as a layer  46  on the pad polishing surface  34  and overflows to be discarded. 
     Upward removal of a polishing pad  14  from the platen surface  12  is generally a difficult operation requiring high removal forces. Pad replacement is necessary on a regular basis, and the invention described herein and illustrated in drawing FIGS. 3 through 6 makes pad replacement easier, safer and faster. 
     Turning now to drawing FIGS. 3 and 4, the prior art polishing apparatus of drawing FIG. 2 is shown with a platen  20  modified in accordance with the invention. Parts are numbered as in drawing FIG. 2, with the modification comprising a permanent coating  50  of a “non-stick” or adhesion material applied to the upper surface  12  of the platen  20 , along coating/adhesive interface  54 . The polishing pad  14  is then attached to the coating  50  using a pressure-sensitive adhesive (PSA)  16 . It is common practice for manufacturers of polishing pads to supply pads with a high-adhesion PSA already fixed to the attachment surface  44  of the pads. It has been found that the adhesion of polishing pads  14  to certain low-adhesion coatings  50  with conventional high adhesion adhesives results in a lower release force, yet the bond strength is sufficient maintain the integrity of the polishing pads  14  during the polishing operations. Typically, variables affecting the release force include the type and surface smoothness of the coating  50 , the type and specific adhesion characteristics of the adhesive material  16 , and pad size. 
     Referring to drawing FIGS. 5 and 6, depicted is another version of the platen  20  which is coated with a low-adhesion coating  50  in accordance with the invention. In this embodiment, the platen  20  includes a network of channels  58 , and slurry  40  is fed thereto through conduits  60 . The low-adhesion coating  50  covers the platen  20  and, as shown, may extend into at least the upper portions of channels  58 . Apertures  64  through the coating  50  match the channels  58  in the platen  20 . The polishing pad  14  and attached pressure-sensitive adhesive (PSA)  16  have through-apertures  62  through which the slurry  40  may flow upward from channels  58  onto the polishing surface  34  of the pad  14 . 
     The surface area of coating  50  to which the adhesive  16  may adhere is reduced by the apertures  64 . This loss of contact area between adhesive  16  and platen coating  50  may be compensated by changing the surface smoothness of the coating or using an adhesive material with a higher release force. 
     Materials which have been found useful for coating the platen  20  include coatings based on fluoropolymers, including polytetrafluoroethylene (PTFE or “Teflon”), polymonochloro-trifluoroethylene (CTFE) and polyvinylidene fluoride (PVF 2 ). Other materials may be used to coat the upper surface  12  of platen  20 , provided that the material has the desired adherence, i.e. release properties, with available adhesives, may be readily cleaned, and has a long life in the mechanical and chemical environment of polishing. 
     Various coating methods may be used. The platen  20  may be coated, for example, using any of the various viable commercial processes, including conventional and electrostatic spraying, hot melt spraying, and cementation. 
     In the application of one coating process to a modification of the platen  20 , the upper surface  12  of the platen is first roughened to enhance adhesion. The coating material  50  is then applied to the upper surface  12  by a wet spraying or dry powder technique, as known in the art. In one variation of the coating process, white-hot metal particles, not shown, are first sprayed onto the uncoated base surface and permitted to cool, and the coating  50  is then applied. The metal particles reinforce the coating  50  of low-adhesion material which is applied to the platen  20 . 
     The result of this invention is a substantial reduction in release force between polishing pad  14  and platen  20  to a level at which the pad may be removed from the platen with minimal effort, yet the planar attachment of the pad to the platen during polishing operations will not be compromised. The particular combination(s) of coating  50  and adhesive material  16  which provide the desired release force may be determined by testing various adhesive formulations with different coatings. 
     Another method for controlling the release force is the introduction of a controlled degree of “roughness” in the coating surfaces  52  (including surfaces of fluorocarbon materials) for changing the coefficient of friction. The adhesion of an adhesive material  16  to a coating  50  may be thus controlled, irrespective of the pad construction, size or composition. 
     The use of a coating  50  of the invention provides useful advantages in any process where a polishing pad  14  must be periodically removed from a platen  20 . Thus, use of the coating  50  is commercially applicable to any polishing method, whether chemical-mechanical polishing (CMP), chemical polishing (CP) or mechanical polishing (MP), where a polishing pad  14  of any kind is attached to a platen  20 . 
     EXAMPLE 
     A piece of flat aluminum coated with polytetrafluoroethylene (PTFE) was procured. The particular formulation of PTFE was Malynco 35011 Black Teflon™, applied to the aluminum. 
     Conventional CMP polishing pad samples were obtained in a size of 3.7×4.2 inches (9.4×10.67 cm.). The area of each pad was 15.54 square inches (100.3 square cm.). These pads were identified as SUBA IV psa 2 adhesive pads and were obtained from Rodel Products Corporation of Scottsdale, Ariz. 
     The polishing pads included a polyurethane-based pressure-sensitive adhesive (PSA2) on one surface. The pads were placed on the coated aluminum, baked at 53° C. for two hours under slight compression, and cooled for a minimum of 45 minutes, thereby bonding the pads to the PTFE surface. 
     Samples of the same pad material were similarly adhered to an uncoated aluminum surface of a polishing platen for comparison as test controls. 
     Tests were conducted to determine the force required to remove each pad from the surface coating and the uncoated surfaces. The average measured removal forces were as follows: 
     Removal force from Malynco 35011 Black Teflon™ coated aluminum: 1.08 1bf. 
     Removal force from uncoated aluminum: 11.5 1bf. 
     Extrapolation to actual production size platens of 30 inch diameter indicates that pad removal forces may be reduced from about 100-150 1bf. (about 444.8-667.2 Newtons) to about 15 1bf. to about 25 1bf. (about 66 to 112 Newtons). This force is sufficient to maintain pad-to-platen integrity during long-term polishing but is a significant reduction in the force required for pad removal and replacement. 
     It is apparent to those skilled in the art that various changes and modifications, including variations in pad type and size, platen type and size, pad removal procedure, etc. may be made to the polishing apparatus and method of the invention as described herein without departing from the spirit and scope of the invention as defined in the following claims.