Patent Publication Number: US-6699104-B1

Title: Elimination of trapped air under polishing pads

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/154,377 filed Sep. 15, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a polishing pad which is useful for planarizing by polishing a substrate such as a semiconductor device or wafer, and in particular, to a polishing pad having an adhesive for adhering and securing the polishing pad to a platen or other mounting surface. 
     2. Background of the Invention 
     Semiconductor wafers having integrated circuits fabricated thereon must be polished to smooth and flat wafer surfaces that in some cases are permitted to vary from a given plane by as little as a fraction of a micron. Such polishing is usually accomplished in a chemical-mechanical polishing (CMP) operation, which utilizes a chemically active slurry that is buffed against the wafer surface by a polishing pad. 
     A polishing pad is a relatively thin, planar, disk-shaped article. As an example, a DPM (disk pad Mylar®) polishing pad that is commercially available from Rodel, Inc., of Newark, Del., has a thickness of about 0.0225 inch and a diameter of about 52 inches. Such a pad is not completely rigid across their diameter, that is, the pads are somewhat floppy to conform flatly against a mounting surface, such as that provided by a platen of a known polishing apparatus or machine. 
     The polishing pad must be placed on a platen of a polishing machine, or on another mounting surface, and secured to the platen or other mounting surface by a pressure sensitive adhesive (PSA) on the back side of the polishing pad. As the polishing pad is placed on the platen or other mounting surface, bubbles of air tend to get trapped between the adhesive and the platen or other mounting surface. Any trapped air will distend the relatively thin pad, thereby causing raised areas or bulges in the polishing surface of the polishing pad. The presence of trapped air prevents the air entrapping portion of the adhesive from contacting and adhering to the platen or other mounting surface. These bulges cannot be eliminated by forcing the air bubbles out from under the pad with a roller. Instead, the bulges must be manually pierced with a hand tool in order to let the trapped air escape, and then the pad can be pressed flat against the platen or other mounting surface to obtain the flattest possible polishing surface. This process is time-consuming, and some small bulges that are undetected cause bulges to remain in the polishing surface. Any bulges in the polishing surface will generate non-uniformities on the polished surface of the wafer workpiece during polishing, thereby causing defects in the polished surface of the wafer. There is a need for a polishing pad which overcomes these problems. There is a further need for a process of manufacturing a polishing pad that overcomes these problems. 
     SUMMARY OF THE INVENTION 
     According to the invention, a polishing pad comprises a polishing layer and a bottom adhesive layer below the polishing layer. The adhesive layer has an adhesive bottom surface and a hollow air transmitting pathway, for example, a channel, in the adhesive layer extending to an end of the adhesive layer. When the adhesive bottom surface is applied to a platen or other mounting surface, air which is trapped between the adhesive layer and the platen can escape through the channel. 
     According to one embodiment, the channel comprises an array of intersecting channel segments or grooves. The grooves may be embossed, screen-printed, or otherwise cut in the adhesive bottom surface. 
     According to another embodiment, a material strand is disposed in a hollow in the adhesive layer, and the channel comprises a portion of the hollow that is unoccupied by the material strand. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawings wherein: 
     FIG. 1 is a side elevation view of a polishing pad according to the invention; 
     FIG. 2 is an enlarged side view of a portion of the polishing pad mounted on a platen or other mounting surface; 
     FIG. 3 is an enlarged bottom view of a portion of the polishing pad; 
     FIG. 3A is an isometric view of a carrier film carrying double coat adhesive layers; 
     FIG. 3B is an isometric view with parts separated from one another of a transfer tape; 
     FIG. 4 is an enlarged side view of a portion of the polishing pad in an alternate embodiment; 
     FIG. 5 is an enlarged view of a portion of the polishing pad that is shown in FIG. 4; 
     FIG. 6 is a fragmentary side view of a tool applying compression on a perimeter edge margin of a polishing pad mounted on a platen or other mounting surface; 
     FIG. 7 is a fragmentary end view of the structure shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     There is shown in FIG. 1 a polishing pad  1  including a polishing layer  10  and an adhesive layer  20 . As shown, the adhesive layer  20  and the polishing layer  10  are contiguous along an interface  15 . However, it should be understood that one or more intermediate layers, for example, cushioning layers, may be disposed in the interface  15  between the adhesive layer  20  and the polishing layer  10  without departing from the scope of the invention. The adhesive layer  20  is laminated to the interface  15  between the polishing layer  10  and the adhesive layer  20  such that the adhesive layer  20  is the bottom layer of the polishing pad  1 . The polishing layer  10  is porous, which assists in preventing entrapment of air between the polishing layer  10  and the adhesive layer  20 . 
     The polishing layer  10  has an exposed polishing surface  12  which extends along a front face or a major face of the polishing pad. The polishing layer  10  also has an opposite or back face  14  which extends parallel to the polishing surface  12 . The polishing layer  10  may comprise any material which is suitable for polishing a semiconductor wafer. One example of a suitable polishing layer material is disk pad Mylar® (DPM) which is used to make polishing pads that are commercially available from Rodel, Inc. of Newark, Del. 
     The adhesive layer  20  has a bottom surface  22  which is adhesive. The adhesive bottom surface  22  faces in an opposite direction from the polishing surface  12 . The adhesive bottom surface  22  may comprise a suitable pressure sensitive adhesive (PSA) which can firmly grip a mounting surface  23 , FIG. 2, upon contact therewith. The mounting surface  23  is either a platen of a polishing machine, or a device other than the platen itself, such as, a thin plate of uniform thickness that, in turn, is mounted on a platen of a polishing machine. For ease in shipping and handling, the adhesive bottom surface  22  remains covered by a removable liner (not shown) until immediately prior to application of the polishing pad  1  on the platen or other mounting surface  23 , at which time the liner is removed to expose the adhesive bottom surface  22  for assembly to the platen or other mounting surface  23 . 
     With reference to FIGS. 2 and 3, the adhesive layer  20  has one or more than one hollow channel  24  that is in the adhesive layer  20 , and is recessed in the adhesive bottom surface  22 . Each channel  24  is a void in the adhesive bottom surface  22  that provides a hollow, air transmitting pathway  24  for the collection and escape of air that is likely to become trapped between an expected air entrapping portion of the adhesive bottom surface  22  and the platen or other mounting surface  23 , when the adhesive bottom surface  22  is applied to the platen or other mounting surface  23 . The presence of trapped air would prevent the air entrapping portion of the adhesive bottom surface  22  from contacting and adhering to the platen or other mounting surface  23 . The channel  24  avoids air entrapment, and permits the expected air entrapping portion of the adhesive bottom surface  22  to contact and adhere to the platen or other mounting surface  23 . However, in the event that air becomes entrapped between the adhesive bottom surface  22  and the mounting surface  23 , the pad is subjected to rolling by a puck or a pressure roller of one foot long or longer, which forces entrapped air to migrate along the expected air entrapping portion of the adhesive bottom surface  22  until collecting and escaping along a corresponding channel  24 . 
     Thus the planarity, i.e., capability to remain planar, of the polishing pad  1  is improved by collection and escape of air from under the expected air entrapping portion of the adhesive bottom surface  22 , and by enabling adherence of the expected air entrapping portion of the adhesive bottom surface  22  to the platen or other mounting surface  23 . In practice, each expected air entrapping portion of the adhesive bottom surface  22  is in communication with at least one channel  24 . However, more than one channel  24  would be desired, for example, should a channel  24  collapse under the application of a force that is exerted to press and adhere the adhesive bottom surface  22  against a platen or other mounting surface  23 . Each channel  24  extends along the adhesive layer  20  and is in communication with at least one opening  26  through an end  28  at the lateral perimeter of the adhesive layer  20 . 
     Each channel  24  may extend along the adhesive layer  20  in a linear or non-linear fashion, or some combination thereof. A single channel  24  may include a plurality of interconnected channel segments which permit communication and passage of air therethrough. Thus, a single channel  24  may comprise an array of intersecting channel segments in the form of intersecting grooves in the adhesive bottom surface  22  which open through at least one opening  26  in the end  28  of the adhesive layer  20 . Further, a single channel  24  may have multiple openings  26  through one or more ends  28  of the adhesive layer  20 . Each channel  24  provides a means for air beneath the bottom surface  22  of the adhesive layer  20  of the polishing pad  1  to escape. The air can be forced or expelled into and along the channel  24 , and through at least one opening  26  communicating with the end  28  of the polishing pad  1 , upon applying a compressive force on the polishing surface  12  of the polishing pad  1  to flatten and adhere the bottom surface  22  to a platen or other mounting surface. The compressive force is applied with sufficient force to adhere the adhesive bottom surface  22  to the mounting surface  23 , and until the compressive force is applied, the channel  24  provides sufficient interruptions of the adhesive bottom surface  22 , which prevent the adhesive bottom surface  22  from adhering to the mounting surface  23  in the absence of the applied pressure, and the pad  1  has the advantage of being repositioned on the mounting surface  23  until the application of such pressure. Further, the adhesive mounting surface  22  may be populated with small glass beads that provide interruptions of the adhesive mounting surface  22 , further assisting in allowing the pad  1  to be repositioned on the mounting surface  23 , until the application of pressure sufficient to adhere the adhesive bottom surface  22  to the mounting surface  23 . The beads protrude at the adhesive mounting surface  22 , and provide stand offs. Upon the application of pressure, the beads imbed in the adhesive mounting surface  22 , and enable the adhesive mounting surface to abut and adhere to the mounting surface  23 . The bottom surface  22  is relied upon to adhere to the platen or other mounting surface  23 . Although the channels  24  provide interruptions of the bottom surface  22 , the channels  24  are small, and are finely divided to minimize such interruptions. Accordingly, the bottom surface  22  adheres substantially evenly over its entire area, with insignificant interruptions. 
     An embodiment of the adhesive layer  20 , as disclosed by FIG. 3, is a thick film in which the channels are provided in the adhesive bottom surface  22  of the thick film. 
     As disclosed in FIG. 3A, another suitable adhesive layer  20  is fabricated as a carrier film  30  carrying double coat adhesive layers. The carrier film  30  is sandwiched between oppositely facing adhesive surfaces  31  and  22 . One of the adhesive surfaces  31  is smooth, and is bonded or adhered to the interface  15  at the polishing layer  10 , and the other of the adhesive surfaces  22  provides the adhesive bottom surface  22  for adhering to the platen or other mounting surface  23 . The channels  24  are provided in the adhesive bottom surface  22 . 
     The channels  24  may be formed in the adhesive layer  20  either before or after application of the adhesive layer  20  to the interface  15  at the polishing layer  10 . The channels  24  may be formed by a process, such as, cutting, embossing, or otherwise indenting the adhesive bottom surface  22 . Further, the channels  24  may be formed as the adhesive layer  20  itself is being formed. The channels  24  may be formed as the adhesive layer  20  is formed by screen printing, laminating over a textured roll, and Grauvier roll coating. 
     As disclosed by FIG. 3B, another suitable adhesive layer  20  is a removable transfer tape  32  that has the smooth adhesive surface  31  for bonding or adhering to the interface  15  at the polishing layer  10 . The transfer tape  32  has an interior surface  33  that covers the adhesive bottom surface  22  that can be adhered to the platen or other mounting surface  23 . Further, the transfer tape  32  covers the channels  24  in the adhesive bonding surface  22 . After bonding or adhering the adhesive surface  31  of the transfer tape  32  to the interface  15 , the transfer tape is removed to reveal the adhesive bottom surface  22  and the channels  24 . The channels  24  are formed in the adhesive bottom surface  22  prior to the bottom surface being attached to the interior. 
     A further embodiment of the invention can use a pressure sensitive adhesive coated on an embossed liner like that used in Controltac™ manufactured by 3M Company of St. Paul, Minn. One version of Controltac™ provides glass beads protruding at the adhesive bottom surface  22 . 
     According to another embodiment as shown in FIGS. 4 and 5, air transmitting pathways  34  in the form of channels  34  may be formed along sides of material strands  36  that are embedded in an adhesive bottom surface  42  of an adhesive layer  40  having the adhesive bottom surface  42 . The material strands  36  may be made of polypropylene. The material strands  36  may be arranged as individual strands  36 , arranged as a mesh, or arranged as a woven screen. The strands  36  are forced against the bottom surface  42  of the adhesive layer  40 , and make impressions or hollows  44  that are recessed in and below the adhesive bottom surface  42 , and in which impressions or hollows  44  the strands  36  reside and imbed in the adhesive layer  40 . The strands are recessed below the adhesive bottom surface  42 , rather than project above the adhesive bottom surface  42 . For example, the adhesive layer  40  may be highly viscous or in an amorphous state. The adhesive layer  40  may shear, as the strands  36  cut their way into the bottom surface  42  of the adhesive layer  40  to form the hollows  36 . Alternatively, the adhesive layer  40  may be displaced and indented by the strands  36 , as the strands make impressions or hollows  44  in the bottom surface  42  of the adhesive layer  40  to form the hollows  36 . 
     Each hollow  44  contains the strand  36  together with an air space along the sides of the strand  36 . Each strand  36  has a circular or other cross-sectional shape that does not entirely occupy an hollow  44 , i.e., each strand  36  occupies solely a portion of an hollow  44 . The unoccupied portion of each hollow  44  contains the air space, and serves as a channel  34  for the escape of air from under the adhesive bottom surface  22 . 
     FIG. 6 discloses a tool  34  applying compression on a perimeter edge margin  36  of a polishing pad  1  mounted on a platen or other mounting surface  23 . The tool  34  has a frame on which is rotatably mounted one or more than one pressure applying wheel  38 . The circular perimeter of each wheel  38  is forced to apply pressure against the perimeter edge margin  36  of the pad  1 , as the wheel  38  is forced to roll against the pad  1  and traverse along the edge margin  36 . The tool  34  has a handle  40  that is manually grasped to apply the pressure, and to urge the wheel  38  to traverse the edge margin  36 . The tool  34  has a depending guide  42 , for example, in the form of an idler roller mounted on a shaft  44  having threads that secure in the frame of the tool  34 . The guide  42  engages against the perimeters of the pad  1  and the mounting surface  23  to guide the wheel  38  along the perimeter edge margin  36 . The tool  34  applies relatively increased pressure along the edge margin  36  to flatten a substantial area of the adhesive bottom surface  22  into sealed abutment with the mounting surface  23 . It has been found that the channel  24  and  34  may leak polishing fluid and ionized water beneath the edge margin  36 , which would delaminate the pad  1  from the mounting surface  23  sufficiently to vary the planarity of the pad  1 . The adhesive bottom surface  22  would have portions of its area that would be insufficiently adhered to the mounting surface  23 , particularly where the adhesive bottom surface  22  is intercepted by each channel  24  and  34 . However, by applying increased pressure along the edge margin  36 , a substantial area of the adhesive bottom surface  22  is forced into sealed abutment with the mounting surface  23 , particularly along the edges of each channel  24  and  34 . The edges of each channel  24  and  34  become sealed, at the edge margin  36 , which reduces the open size of each channel  24  and  34  below the minimum size capillary for admitting the polishing fluid of a given or known surface tension. 
     The invention provides a polishing pad with a means for escape of trapped air. The polishing pad can be readily applied to a platen or other mounting surface without trapping air bubbles beneath the pad. Bulges in the polishing pad are eliminated, thereby resulting in a flat polishing surface which facilitates polishing and planarization of a wafer workpiece. 
     Although preferred embodiments of the invention are disclosed, other embodiments and modifications are intended to be covered by the spirit and scope of the appended claims.