Patent Publication Number: US-2022226956-A1

Title: Method of manufacture of polishing pads having two or more endpoint detection windows

Description:
FIELD OF THE INVENTION 
     The field of this invention is a method of making polishing pads having more than one endpoint detection windows. 
     BACKGROUND 
     Chemical Mechanical Planarization (CMP) is a variation of a polishing process that is widely used to flatten, or planarize, the layers of construction of an integrated circuit or similar structure. The process is often used as part of the manufacture when using lithography with multiple deposition steps, in order to precisely build multilayer three-dimensional structure or circuitry. The layer to be polished is typically a thin film (e.g. less than 10,000 Angstroms) that has been deposited on an underlying substrate. The objectives of CMP are to remove excess material on the substrate (e.g. wafer) surface to produce an extremely flat layer of a uniform thickness, the uniformity extending across the entire substrate (e.g. wafer) area. 
     CMP utilizes a liquid, often called slurry, which can contain nano-sized particles. The slurry is fed onto the surface of a rotating multilayer polymer pad (sometimes referred to as polishing sheet), the pad being mounted on a rotating platen. Substrates (e.g. wafers) are mounted into a separate fixture, or carrier, which has a separate means of rotation, and pressed against the surface of the pad under a controlled load. This can lead to a high rate of relative motion between the substrate (e.g. wafer) and the polishing pad and a resulting high rate of shear or abrasion at both the substrate and the pad surface. The shear in combination with the slurry particles trapped at the pad/substrate junction abrade the substrate (e.g. wafer) surface, leading to removal of material from the substrate surface. Control of removal rate and the uniformity of removal are important. 
     Commercial CMP pads are multilayer composites. In addition to an upper polishing layer, which contacts the substrate (e.g. wafer) to be polished, one or more subpad layers are employed to adjust the compliance of the pad under pressure, particularly over the full area of the pad, to better achieve the required film uniformity across the full substrate (e.g. wafer) surface. Controlled increase in compliance can be achieve by using a subpad material of lower modulus than the upper layer and adjusting the relative thicknesses of both layers to achieve the desired result. 
     In order to precisely control the final thickness of the polished film (the endpoint), film measurement during polishing is widely employed. There are two main metrology approaches for endpointing. One approach for endpoint detection uses transmittance of desired wavelengths of light through the polishing pad, the light reflects from the substrate being polished, and the reflected light signal then passes back to the interferometer, which processes the reflectance signal to determine if the polishing has reached its desired goal (e.g., film thickness, intended reveal of an underlying structure). The metrology equipment can be located within the body of the platen that holds the pad. In some instances, the optical equipment may extend above the platen into the plane of the subpad layers of the CMP pad. Thus, a recess may be desired in the subpad where the window is placed. 
     This method requires the polishing pad to have at least a portion to be transparent to the wavelengths of light being used in the interferometer. In some pad designs this is achieved by providing a window material of a different composition than the upper pad layer is disposed in an aperture within the pad since many polishing layer materials are opaque. Use of such windows requires alignment of apertures in both the polishing layer and the subpad layer(s). 
     As integrated circuit dimensions scale down, increasingly accurate real-time film thickness measurements are critical for process control. This has led to the adoption of multiple endpoint units per platen and the use of multispectral optics to allow accurate real-time measurements of film thickness profiles via triangulation and signal processing. Further, to avoid reducing the area for polishing, it can be desirable to reduce the aperture size (window size). As a result, the manufacturing tolerances of current CMP pad windows are becoming more stringent. In addition, maintaining alignment of windows with respect to polishing layer and subpad layer apertures can be challenging. For instance, when forming the multi-layer pad (e.g. by lamination or the like) the differing materials of the polishing and subpad layers may shrink or stretch in different proportions causing misalignment such that the aperture in the subpad does not align with the window or does not align with the aperture in the polishing layer. Also, in forming separate apertures in each layer and then laminating, the pre-formed apertures may not have been in completely consistent locations to enable full alignment of more than two apertures or the aperture and windows in the layers. For example, if there are two apertures in each of the polishing layer and the subpad layer(s) if those are formed at a slightly distinct distance in each layer, then it will be impossible to have full alignment. Total misalignment—e.g., where there is no overlap of the apertures of the subpad and polishing layer or no overlap of the window with one of the apertures—would prevent any use of the window. Partial misalignment—e.g., where the subpad aperture overlaps partially with the window—can narrow the transparent region enough to make it difficult to use for end detection (for example by producing attenuation and noise during measurements that can lead to endpoint detection errors). The misalignment issues can be particularly problematic in pads with multiple windows with smaller aperture sizes. Accordingly, a pad manufacturing process capable of producing a multilayer window pad in a simplified manufacturing process with increased window location precision in all dimensions that is useful for a wide variety of top pad materials would represent a significant improvement in the art. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a method of manufacturing a chemical mechanical polishing pad having two or more end point detection windows comprising providing a structure that includes a polishing layer having a top surface defining a horizontal direction, a bottom surface, and two or more apertures extending from the top surface to the bottom surface wherein each of the two or more apertures has a transparent window located in the aperture, wherein each of the windows has a portion extending outward from the bottom surface of the polishing layer, wherein the two or more apertures are located at select distance from each other, applying a subpad material on the bottom surface of the polishing layer and wherein the two or more apertures remain at the select distance from each other and the portion of the windows extending outward from the bottom surface, and revealing the windows by removing a portion of the subpad material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike. 
         FIG. 1 a    is a top view of a pad having three windows. 
         FIG. 1 b    is a cross-section view of a portion of the pad of  FIG. 1 . a . showing the layers of the pad and one window. 
         FIG. 2  is a schematic view of a window. 
         FIG. 3  is a side view of a window. 
         FIG. 4 a -4 i    represent a sequential series of cross section views that illustrate forming a pad and subpad with multiple windows in accordance with the method of the invention. 
         FIG. 5  is a cross section of a portion of a pad having a window as in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present application is a method of forming a pad that allows for precise locating of more than one window in the pad. The method avoids the problems of misalignment of apertures in the subpad with the apertures in the polishing layer and misalignment of the windows in the apertures. Such misalignment may be caused for example by one or more of the following: by different coefficients of thermal expansion in the layers, different stretching of deformation of the layers during manufacture, imprecise location of apertures in two different layers, imprecise alignment of apertures during lamination, or the like, or a combination of two or more of those causes. 
     Thus, the method comprises providing a polishing layer having a top surface defining a horizontal direction, a bottom surface (also referred to herein as an interface surface), and two or more apertures extending from the top surface to the bottom surface, wherein each of the two or more apertures is located at a select distance from the other aperture(s). Located in the aperture, is a transparent window. The windows each include a portion extending outward from the bottom surface of the polishing layer. The method includes applying a subpad material on the bottom surface and the portion of the windows extending outward from the bottom surface, and then removing a portion of the subpad material to expose the windows. This method ensures alignment of the windows with the apertures in the polishing layer as each window is located in one of such apertures. This method further ensures alignment of the polishing layer aperture windows with the aperture in the subpad. 
       FIG. 1 a    is a top view of one example of a pad made by the method disclosed herein. The pad  01  has a top surface  03 . Although not shown, the top surface  03  can have a surface roughness or a surface pattern such as grooves or perforations. This pad has three windows  04  located at select distances, d s1 , d s2 , and d s3 , from each other. The select distances can be the same or different. As examples of window placement and orientation: each window could be placed at the same radial distance from the center  07  of the pad or each window could be placed at a different distance from the center  07  of the pad; the windows could be equidistant from each other or the windows can be non-equidistant from each other. Most advantageously all windows are at the same radial distance from the center  07 . The select distance(s) can be selected to correspond to the placement of endpoint detection sensors in a tool or platen for chemical mechanical polishing. Each window  04  can be the same size. Each window  04  can be of the same shape. Each window  04  can be of the same material. Alternatively, a first window can be of a different size, a different shape, or a different material from a second window or any third or more windows. At least two of the windows are transparent to the wavelength of radiation (e.g. light) used in the sensor. The windows can be made from a polymer or polymer blends desired so long as it has sufficient transmission at the wavelengths used by the optical metrology. It can be helpful if that window material has a hardness and thermal expansion coefficient similar to that of the material used in the polishing layer. Examples of window materials include polyurethanes, acrylic polymers, cyclic olefin co-polymers (e.g. TOPAS 8007, etc.). Use of polyurethane materials can be helpful in pads where the polishing layer and subpad layer(s) are also polyurethanes. Advantageously, the window has transmission to as low of wavelength as possible, such as that achieve with windows manufactured with aliphatic polyurethanes or other materials having optical transparency. 
       FIG. 1 b    shows a cross-section of a portion of a pad. The pad includes a polishing layer  05  and a subpad  06 . The window  04  shown in  FIG. 1 b    has a plug shape. The polishing layer  05  can comprise a polymeric material. It can optionally include pores and or particulates. Pores can be provided, for example, by addition of hollow flexible polymer elements (e.g. hollow microspheres), blowing agents, frothing or supercritical carbon dioxide. Examples of polymeric materials for the polishing layer include polyurethanes, polycarbonates, polysulfones, nylons, polyethers, polyesters, polystyrenes, acrylic polymers, polymethyl methacrylates, polyvinylchlorides, polyvinyl fluorides, polyethylenes, polypropylenes, polybutadienes, polyethylene imines, polyether sulfones, polyamides, polyether imides, polyketones, epoxy resins, silicones, copolymers thereof (such as, polyether-polyester copolymers), and combinations or blends thereof. The polishing layer  05  can comprise a polymer that is a polyurethane formed by reaction of one or more polyfunctional isocyanates and one or more polyols. 
     The subpad layer  06  can comprise one or more layers. The subpad layer(s) can comprise a material that is more compliant than the material of the polishing layer  05 . The subpad layer(s)  06  can comprise a polymeric material. The subpad  06  can comprise a porous layer. Examples of polymeric materials for the subpad layer(s) include polyurethanes, polycarbonates, polysulfones, nylons, epoxy resins, polyethers, polyesters, polystyrenes, acrylic polymers, polymethyl methacrylates, polyvinylchlorides, polyvinyl fluorides, polyethylenes, polypropylenes, polybutadienes, polyethylene imines, polyether sulfones, polyamides, polyether imides, polyketones, silicones, copolymers thereof (such as, polyether-polyester copolymers), and combinations or blends thereof. 
       FIGS. 2 and 3  show other window shapes. The window  204  in  FIG. 2  has an upper portion  204   u  and a lower portion  204   l , each portion being cylindrical or oval. The upper portion  204   u  has a dimension (e.g. diameter) d u  that is smaller than the dimension (e.g. diameter), d l , of the lower portion  204   l  such that a rim  204   r  is formed. The lower portion can form a flange portion. In  FIG. 3  a cross section of a similar window  304  is shown that include an upper portion  304   u , a middle portion  304   m , and a lower portion  304   l . The middle portion  304   m  has a dimension (e.g. a diameter) in the horizontal direction that is larger than the dimension of either the upper portion  304   u  or the lower portion  304   l , thus forming an upper rim  304   ur  and a lower rim  3041   r . The middle portion can form a flange portion. While  FIG. 3  shows the lower portion  304   l  having a dimension in the horizontal direction that is smaller than the dimension in the horizontal direction of the upper portion  304   u , those dimensions could in the alternative be approximately the same size. Generally the window dimensions in the horizontal direction are on the order of 8 to 18 mm. Generally the overall height of the window is less than the overall thickness of the pad. For example, the overall height (e.g. h total ) of the window can be 1 to 5 mm, or 1 to 4 mm, where the height of the upper and lower portions can be 1 to 2 mm and the height of a middle portion can be 0.3 to 2, 0.4 to 1, or 0.5 to 0.8 mm. 
       FIGS. 4 a - i    represent a sequential series of schematic showing an example of herein using a mold and a window as disclosed in  FIG. 2 . A mold  208  is provided having at least two recess  211  (one is shown). The two recesses are located at a select distance, ds, (not shown) from each other. The location of the recesses can correspond to the location of endpoint detection sensors elements in a chemical mechanical polishing tool. The recess dimension  210  and the lower portion  204   l  of the window  204  are selected such that the lower portion  204   l  of the window  204  fits, preferably fits precisely and snuggly without any or any significant gaps between sides of the window  204  and the sides of the recess  211 . As shown, the depth  209  of the recess  211  can be the same or substantially the same as the height, hi, of the lower portion  204   l  of the window  204 . If a window  304  as shown in  FIG. 3  is used, the recess can mate with either the lower portion  304   l  or with both the lower portion  304   l , and the middle portion  304   m . If a plug type window  04  as is shown in  FIG. 1 b    is used, the depth  209  is less than the full height of the window  04 . In each instance, an upper portion of the window (e.g.  204   u  or  304   u , latter not shown) protrudes above a surface  212  of the mold  208 . The mold can be made from any material, for example, a metal such as steel. The mold can also be fitted with a removable collar around its outer perimeter that can shape the outer perimeter of the layer  205 . The mold includes a top surface  212 . As shown in  FIG. 4 c   , a polishing material  205  is applied over the top surface  212  and, optionally, over the upper portion (e.g.,  204   u  as shown) of the window (e.g.  204  as shown). The polishing material can be applied by coating (e.g., spray coating), casting, molding (e.g. injection molding), additive manufacture (e.g. 3D printing), or laminating. The polishing layer material  205  can be applied in polymerized form or can be applied as a pre-polymer composition and polymerized or can be applied in a curable form and cured while in the mold  208 . The polishing layer  205  as initially formed can have thickness  215  equal to, substantially equal to, or greater than the height of the upper portion of the window  204  that is extending above the surface  212 . In  FIG. 4 c   , the polishing layer  205  as initially formed has a thickness greater than the height of the upper portion of the window  204 . In this instance, the polishing layer material  205  can cover the top surface of the window  204  by an excess amount  213  and the top surface of the window  204  can be exposed by a reveal step to arrive at the structure in  FIG. 4 d   . In other instances—e.g., where injection molding is used where a top portion of a mold contacts a top surface of the window, coating with a mask over the window  204 , or additive printing are used—the polishing material  205  may not be on the top surface of the window  204  such that the applying step directly forms the structure of  FIG. 4 d   . When a reveal step is used this step can also provide desired texture or controlled surface roughness to the top surface of the polishing layer  205 . The structure can be such that the windows do not protrude above the top surface of the polishing layer  205 . The top surface of the windows can be coplanar with the top surface  205   t  of the polishing layer  205 . 
     After forming the polishing layer  205  (before or after any needed reveal step), the polishing layer with at least two windows in two apertures formed in the preceding steps is removed from the mold  208  exposing an interface surface  205   i  and the lower portion of the windows  204 . (See  FIG. 4 e    showing the polishing layer and window removed from mold  208  and inverted). Note, that while  FIG. 4  shows the method using a window with a structure as in  FIG. 2  with a smaller upper portion and a larger lower portion, the method is equally applicable to a window that has a simple plug shape or to a window that has three portions as shown in  FIG. 3 . Note further that while  FIG. 4  shows that the entire larger lower portion fits in the recess, it is also contemplated that a portion of the larger lower portion could extend above the top surface  212  of the mold. Similarly, if a window with three portions is used, the mold could have a recess for only the lowest portion  304   l  (or a portion there) or a stepped recess to accept both the lowest portion  304   l  and at least a portion of the middle portion  304   m.    
     Alternatively, the structure as shown in  FIG. 4 e   , can be made by forming two or more apertures in a pre-formed polishing layer the apertures being formed at a select distance, ds, from each other. The location of the apertures can correspond to the positioning of endpoint detection sensors in a chemical mechanical polishing tool. A window can then be inserted into each aperture such that a portion of the window extends above the interface surface  205   i  of the polishing layer. In this instance, an adhesive can be applied to at least a portion of the regions of the polishing layer contacting the window and at least a portion of the regions of the window contacting the polishing layer. Particularly, the adhesive can be applied at rim portions  204   r  and  304   ur  (as shown in  FIG. 2  and  FIG. 3 , respectively) of the window and the adjacent polishing layer. 
     As shown in  FIG. 4 f   , the polishing layer with windows can then have subpad  206  formed on the interface surface  205   i  and the lower portion  204   u . During this process the polishing layer  205  with windows can be supported on a support  218  (not shown). The support could be a mold. This support mold could also be fitted with a removable collar around its outer perimeter, which support can be used to define the outer perimeter and the thickness of the subpad material  206 . The subpad material can be applied in already polymerized form or can be polymerized or cured after application to the interface  205   i  and the windows. While the subpad material  206  is shown with a planar top surface and extending above the window at an excess thickness amount  213 , the top surface can alternatively generally follow the contour of the interface surface  205   i  and the window portion extending from that surface such that there is a raised portion of subpad material where the window is found. This raised portion can be used to identify where to apply the reveal step to form a structure as shown in  FIG. 4 h    with a recess area  214 . 
     As shown in  FIG. 4 g   , the excess subpad material  206  can be removed to reveal the window  204 , by machining, grinding or etching. As shown in  FIGS. 4 h  and 4 i   , ( FIG. 4 i    is  FIG. 4 h    inverted) the window material can be further etched back to provide a recess area  214 . Alternatively, if a raised portion of subpad  206  is used to identify the location of the window, a selective etch or well machining can be done in that location to reveal the window and form the recess. 
       FIG. 5  shows use of a window  504  similar to that in  FIG. 3  with adhesive  520  applied in certain portions (for example, the adhesive can be applied on at least one rim or flange) to hold the window in the polishing layer  505 . The extended middle portion of the window  504  serves as a flange that further can hold the window in its desired location. 
     This disclosure further encompasses the following aspects. 
     Aspect 1: A method of manufacturing a chemical mechanical polishing pad having two or more end point detection windows comprising providing a structure that includes a polishing layer having a top surface defining a horizontal direction, a bottom surface, and two or more apertures extending from the top surface to the bottom surface wherein each of the two or more apertures has a transparent window located in the aperture, where each of the windows has a portion extending outward from the bottom surface of the polishing layer, wherein the two or more apertures are located at select distance from each other, applying a subpad material on the bottom surface and the portion of the windows extending outward from the bottom surface, and revealing the windows by removing a portion of the subpad material. 
     Aspect 2. The method of Aspect 1 wherein the providing of the structure comprises providing a mold with a surface and two or more recesses in the surface each recess formed to receive a portion of one of the windows, the recesses being located at the select distance from each other, inserting the windows into the recesses such that a top portion of each the windows extends above the mold surface, forming the polishing layer on the mold surface and around a periphery of the top portion of the windows, and removing the polishing layer with the windows from the mold. 
     Aspect 3. The method of Aspect 2 wherein in forming the polishing layer a top surface of the windows is covered by a portion of the polishing layer and further comprising revealing the top surface of the windows by removal of a portion of the polishing layer. 
     Aspect 4. The method of Aspect 1 wherein the providing of the structure comprises forming the two or more apertures in the polishing layer at the select distance from each other, providing the windows for each aperture, each window having a top portion and a flange portion, wherein the top portion has an area in the horizontal direction that is smaller than an area of the flange portion in the horizontal direction such that the second portion forms a rim, applying adhesive to the bottom surface of the polishing layer adjacent the aperture, to the rim of the windows, or both, and inserting into each of the two or more the apertures one of the windows such that the polishing layer surrounds a periphery of the top portion of each of the windows in the horizontal direction. 
     Aspect 5. The method of Aspect 4 wherein at least a segment the flange portion of the windows forms the portion of the window that extends from the bottom surface of the polishing layer. 
     Aspect 6. The method of Aspect 4 wherein the flange portion of the windows is between the top portion of the windows and a bottom portion of the windows, wherein the bottom portion has an area in the horizontal direction that is smaller than the area of the flange portion in the horizontal direction, wherein the bottom portion, or the bottom portion and at least a segment of the flange portion form the portion of the windows extending outward from the bottom surface of the polishing layer and the area of the flange portion in the horizontal direction is larger than the area of the portion of the window extending outward from the bottom surface of the polishing layer. 
     Aspect 7. The method of any one of Aspects 1-6 wherein the subpad material is applied by coating, injection molding, printing. 
     Aspect 8. The method of any one of Aspects 1-7 wherein an amount of the portion of the window extending outward from the bottom surface of the polishing layer is removed during or after the removing of a portion of the subpad material to form a recessed window. 
     Aspect 9. The method of any one of Aspects 1-8 wherein there are three or more apertures and windows and the select distance between adjacent apertures is the same or different. 
     Aspect 10. The method of any one of Aspects 1-9 wherein the select distance corresponds to a distance of placement of endpoint detectors in a chemical mechanical polishing apparatus having two or more endpoint detectors. 
     Aspect 11: The method of any one of Aspects 1-10 wherein each of the apertures in the polishing layer and each of the windows in the pad are at a distance from a center of the polishing layer, preferably a radial distance, which is the same for each aperture and equidistant from the center. 
     Aspect 12: The method of any one of Aspects 1-11 wherein there are three windows that are equidistant from each other. 
     Aspect 13: The method of any one of Aspects 1-12 wherein the windows are of the same size. 
     All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other (e.g., ranges of “up to 25 wt. %, or, more specifically, 5 wt. % to 20 wt. %”, is inclusive of the endpoints and all intermediate values of the ranges of “5 wt. % to 25 wt. %,” etc.). Moreover, stated upper and lower limits can be combined to form ranges (e.g. “at least 1 or at least 2 weight percent” and “up to 10 or 5 weight percent” can be combined as the ranges “1 to 10 weight percent”, or “1 to 5 weight percent” or “2 to 10 weight percent” or “2 to 5 weight percent”). 
     The disclosure may alternately comprise, consist of, or consist essentially of, any appropriate components herein disclosed. The disclosure may additionally, or alternatively, be formulated so as to be devoid, or substantially free, of any components, materials, ingredients, adjuvants or species used in the prior art compositions or that are otherwise not necessary to the achievement of the function or objectives of the present disclosure. 
     All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference. 
     Unless specified to the contrary herein, all test standards are the most recent standard in effect as of the filing date of this application, or, if priority is claimed, the filing date of the earliest priority application in which the test standard appears. 
     References numerals stand for the element/component as listed below:
       01 ,  201  polishing pad     03 ,  205   t  top surface of polishing pad     04 ,  204 ,  304 ,  504  windows     05 ,  205 ,  505  polishing layer     06 ,  206 ,  506  subpad     204   u ,  304   u  upper portion of window, generally surrounded in its periphery by polishing   player     204   l ,  304   l  lower portion of window, generally at least part of its height is surrounded in its   periphery by subpad material.  204   l  can be flange     304   m  middle portion of window or flange     208  mold     209  depth of recess in mold     210  horizontal dimension of recess in mold     211  recess in mold     212  top surface of mold     205   i  interface surface of polishing layer upon which subpad is formed     213  excess height of subpad material applied over window     214  recess formed on bottom side (subpad side) of window     215  excess height of polishing layer material applied over window     520  adhesive