Abstract:
A method for selectively altering a polishing pad adhesive layer includes providing a mask having transparent regions and opaque regions and directing radiation toward the mask so that the radiation passes through the transparent regions and impinges onto the adhesive layer on the polishing pad. The area of the adhesive layer corresponding to the transparent regions of the mask is cured to be less adhesive. The area of the adhesive layer corresponding to the opaque regions remain adhesive.

Description:
This application is a divisional of U.S. application Ser. No. 09/207,793, filed Dec. 9, 1998 now U.S. Pat. No. 6,290,589. 
    
    
     BACKGROUND 
     The invention relates to chemical mechanical polishing of substrates, and more particularly to a polishing pad with a partial adhesive coating, and to methods and apparatus for producing such polishing pads. 
     Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative 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 successively more non-planar. If the outer surface is non-planar, then photolithographic techniques to pattern photoresist layers might not be suitable, as a non-planar surface can prevent proper focusing of the photolithography apparatus. Therefore, there is a need to periodically planarize this substrate surface to provide a planar layer surface. 
     Chemical mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head, with the surface of the substrate to be polished exposed. The substrate is then placed against a rotating polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface. Further, a polishing slurry, including an abrasive and at least one chemically-reactive agent, may be spread on the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate. 
     A typical polishing pad includes a hard top layer and a softer bottom layer. The top layer has a high friction polishing surface, capable of transporting slurry, and a second surface adhesively bonded to a first surface of the bottom layer. A second surface of the bottom layer is typically attached to a platen by a high strength pressure-sensitive adhesive layer. 
     One consideration in the production of integrated circuits is process and product stability. To achieve a low defect rate, each substrate should be polished under similar conditions, i.e., same polishing surface structure. A limitation on process stability, as well as polishing throughput, is “glazing” of the polishing pad. “Glazing” occurs when the polishing pad is frictionally heated and compressed in regions where the substrate is pressed against it, as well as worn as a result of the abrasive contact. The peaks of the polishing pad are pressed and worn down and the pits of the polishing pad are filled up, so the surface of the polishing pad becomes smoother and less abrasive. As a result, the polishing time required to polish a substrate increases. Therefore, the polishing pad surface must be periodically returned to a more uniform abrasive condition, with higher friction and ability to transport slurry. This process is defined as “conditioning” and serves the purpose of maintaining a high polishing rate. The conditioning process can be destructive for the polishing pad and results in reducing the lifetime of the polishing pad. Because of these reasons the polishing pad needs to be removed from the platen and replaced every 100 to 1000 substrates, depending upon the type of substrate and conditioning process. 
     In order to remove the pad, an operator reaches into the polishing area, grasps the polishing pad by hand or with mechanical aids and pulls it to peel it off the platen. Because of the high strength of the adhesive layer, the operator must apply a large force to pull the polishing pad off the platen. For example, the operator may need to apply pulling force of approximately 100 pounds. This large force can exceed the physical abilities of the operator and can pose a risk of injury. 
     SUMMARY 
     In general, in one aspect, the invention features a polishing pad including a first layer having a polishing surface, and a second layer having an adhesive region and a cured region disposed on a surface of the first layer opposite the polishing surface. 
     Implementations of the invention may include one or more of the following features. The polishing pad may have a plurality of adhesive and cured regions. The cured regions may be partially or entirely cured. The cured regions may be circles or arc segments arranged in a pattern. The cured region and the adhesive region may form concentric circles. A third layer may be disposed between the first and second layers opposite the polishing surface. 
     In general, in another aspect, the invention features an apparatus for selectively altering a polishing pad adhesive layer. The apparatus includes a radiation source generating a radiation beam, a support for the polishing pad, and a mask having a transparent region and an opaque region. The mask is disposed between the radiation source and the substrate holder, such that the radiation passes through the transparent region and is blocked by the opaque region. A shutter may be disposed between the radiation source and the mask. 
     In general, in another aspect, the present invention features a method for selectively altering a polishing pad adhesive layer. The method includes: providing a mask having a transparent region and an opaque region and directing radiation toward the mask so that the radiation is blocked by the opaque region and passes through the transparent region to impinge on the adhesive layer on the polishing pad, whereby the area of the adhesive layer corresponding to the transparent region of the mask is cured to be less adhesive. 
     Implementations of the invention may include one or more of the following features. The radiation may be ultraviolet light. The transparent region may be made of ultraviolet light transparent quartz or polymer material. The mask may be made of ultraviolet light blocking material such as paper metal, ceramic or polymer material. The transparent region may be an opening in the mask. The transparent region and the opaque region may form concentric circles. The mask may have a plurality of transparent and opaque regions. The transparent regions may be circles or arc segments arranged in a pattern. The ratio of the cured region surface to the adhesive region surface may be between about 10 to 30%. The polishing pad may be exposed to the radiation for a time varying between about 5 to 60 seconds. The radiation intensity may vary between about 100 to 1200 Watts/inch. 
     In general, in another aspect, the invention features a method for selectively altering a polishing pad adhesive layer. The method includes: providing a polishing pad having a layer of adhesive that covers substantially an entire surface of the pad, and curing selected portions of the adhesive layer to reduce adhesive strength of the layer. 
     Among the advantages of the invention may be one or more of the following. By selectively altering a polishing pad adhesive layer the strength of the adhesive layer is reduced. This reduces the force applied to remove the polishing pad off the platen and the risk of injury for an operator. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a chemical mechanical polishing apparatus. 
     FIG. 2 is a diagrammatic cross-sectional view of a polishing pad. 
     FIG. 3 is a flow diagram describing the method for fabricating a polishing pad with adhesive and non-adhesive regions. 
     FIG. 4 is a diagrammatic cross-sectional view of a polishing pad being exposed to ultraviolet(UV) radiation through a mask. 
     FIG. 4A is a diagrammatic cross-sectional view of a polishing pad having a paper mask disposed on the adhesive layer. 
     FIG. 5 is a top view of a UV mask having a “polka dot” pattern. 
     FIG. 6 is a top view of a UV mask having a ring pattern. 
     FIG. 7 is a top view of a UV mask having a ring and arc segment pattern. 
     FIG. 8 is a diagrammatic view of an apparatus for selectively removing an adhesive layer from a polishing pad. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a polishing apparatus  10  includes a housing  12  that contains three independently-operated polishing stations  14 , a substrate transfer station  16 , and a rotatable carousel  18  which choreographs the operation of four independently rotatable carrier heads  20 . A more complete description of the polishing apparatus  10  may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
     Each polishing station  14  includes a rotatable platen  110  which supports a polishing pad  100 . The platen  110  is mounted to a table top  57  inside the polishing apparatus  10 . In operation, a substrate  30  is loaded to the transfer station  16 , from which the substrate is transferred to a carrier head  20 . The carousel  18  then transfers the substrate through a series of one or more polishing stations  14  and finally returns the polished substrate to the transfer station  16 . Each carrier head  20  receives and holds a substrate, and polishes it by pressing it against the polishing pad  100  on platen  110 . 
     Referring to FIG. 2, one type of a polishing pad  100  includes a fifty mil thick hard layer  102 , a fifty mil thick softer layer  104 , and a pressure sensitive adhesive lower layer  106 . The hard layer  102  is typically a material composed of polyurethane mixed with other fillers. The softer layer  104  is typically a material composed of compressed felt fibers leached with urethane, and adhesive layer  106  is a rubber-based or acrylic based adhesive. The hard layer  102  is adhesively bonded to the softer layer  104  by a bonding layer  107 . The thickness of the polishing pad may vary between 30 to 250 mils. A polishing pad, with a hard layer composed of IC-1000, a softer layer composed of SUBA-4, and a layer composed of a rubber-based adhesive, is available from Rodel, Inc., located in Newark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.). The adhesive layer  106  covers the entire bottom area of the polishing pad and has one surface  111  attached to layer  104  and a second surface  109  which is to be attached to the platen. The polishing pad is supplied with this second surface  109  covered with a lining  108 , to prevent accidental adhesion of the pad. The lining  108  is removed before installing the polishing pad on the platen. 
     The force applied to remove the polishing pad from the platen depends upon the size of the area covered with the adhesive and the type of the adhesive. The entire bottom surface of the pad is usually covered with a continuous adhesive layer. The strength of the adhesive layer may be reduced by making selected portions of the adhesive layer less adherent. Specifically, selected areas of the adhesive layer may be partially or entirely cured to reduce their adhesive strength. For example, the selected areas of the adhesive layer may be exposed to ultra-violet (UV) light in the presence of air, as this cures the rubber-based adhesive to make it less adherent. 
     Referring to FIG. 3, the method  200  of producing a polishing pad with a partial adhesive layer includes providing a polishing pad with a layer of adhesive that covers the entire lower surface of the pad and removing the protective lining that covers the adhesive layer (step  202 ), positioning a mask with openings of a predetermined size over the adhesive layer (step  204 ), exposing the adhesive layer to UV light for a predetermined time (step  206 ), and then removing the mask and applying the pad onto the platen (step  208 ). After completing the polishing process the pad may be easily peeled away from the platen due to the reduced strength of the adhesive layer. 
     Referring to FIG. 4, the areas  105  of the adhesive layer  106  that are to be exposed to UV light are selected by using a mask  120 . The mask  120  includes windows  124  which are transparent to UV light, and opaque areas  122  which block UV light. The mask material may be paper, metal, polymer or ceramic material, or regular borosilicate glass that blocks UV light. The transparent windows may be openings formed in the mask, UV transparent suprasil type quartz, or certain types of polymer materials which are UV transparent. The areas  105  of the adhesive layer  106  that are exposed to the UV light are cured and the areas  103  that do not receive the UV light remain adhesive. In one example, the mask  120  may be a paper having openings  124  and being attached to the adhesive layer  106  (FIG.  4 A). The paper mask may be covered with a lining  108 . 
     Referring to FIG. 5, mask  120  is circular and has a diameter equal to or greater than the diameter of the polishing pad. Different patterns for the UV windows and complementing UV blocking areas are shown in FIGS. 5,  6  and  7 . They include transparent “polka dot” windows  124  (FIG. 5) having a diameter between ¼ to ½ inch, a UV blocking ring  122  (FIG. 6) or a narrow outer ring connected to an inner circle by arc segments  122  which alternate with UV windows  124  (FIG.  7 ). The pattern of transparent and opaque mask areas is replicated in the cured and adhesive areas of the polishing pad, respectively. 
     The ratio of the surface area of the cured areas  105  to the surface of the adhesive areas  103  determines the strength of the adhesive bond between the polishing pad and the platen. The ratio of the surface of the cured areas to the adhesive areas may vary between about 10% to 30%. Several cured areas, each cured area occupying a small surface, are used to prevent formation of wrinkles on the polishing pad. 
     The cured areas may be located near the center of the polishing pad to avoid large shear stresses and delamination of the polishing pad from the platen during polishing. 
     The degree of curing depends upon the exposure time and the intensity of the UV light source. Short exposure times and low UV light intensities cause partial curing, whereas long exposure times and high UV light intensities cause complete curing of the adhesive. Exposure times may vary between 5 to 60 seconds, and UV power levels may vary between 100 to 600 Watts/inch. In one example a UV light of 100 Watts/inch is used and the exposure time may vary between 5 to 30 seconds for a rubber based adhesive. 
     Referring to FIG. 8, an apparatus  150  for selectively curing an adhesive layer includes a UV source  140 , a mask  120  supported by a mask holder  128 , a shutter  130  for blocking the UV-light to the polishing pad  100  and a support or holder  132  supporting the polishing pad. The UV source has a fast linear ramp-up and ramp-down mechanism, and variable intensity high enough to fully cover the polishing pad surface and low enough to avoid build up of heat in the pad. In one example, the UV source  140  is a UV lamp from UV Systems, Gaithersburg, Md. having an intensity that can be linearly ramped-up to 600 Watts/inch. In other examples, a UV source having a 20 inch long bulb and being rated at 1200 Watts/inch is used to irradiate a circular polishing pad along its diameter. 
     One embodiment of the present invention has been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. 
     Accordingly, other embodiments are within the scope of the following claims.