Patent Publication Number: US-2021187693-A1

Title: Polishing pads having selectively arranged porosity

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Application No. 62/951,938, filed on Dec. 20, 2019, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to polishing pads, and methods of manufacturing polishing pads, and more particularly, to polishing pads used for chemical mechanical polishing (CMP) of a substrate in an electronic device fabrication process. 
     Description of the Related Art 
     Chemical mechanical polishing (CMP) is commonly used in the manufacturing of high-density integrated circuits to planarize or polish a layer of material deposited on a substrate. A typical CMP process includes contacting the material layer to be planarized with a polishing pad and moving the polishing pad, the substrate, or both, and hence creating relative movement between the material layer surface and the polishing pad, in the presence of a polishing fluid comprising abrasive particles. One common application of CMP in semiconductor device manufacturing is planarization of a bulk film, for example pre-metal dielectric (PMD) or interlayer dielectric (ILD) polishing, where underlying two or three-dimensional features create recesses and protrusions in the surface of the layer to be planarized. Other common applications of CMP in semiconductor device manufacturing include shallow trench isolation (STI) and interlayer metal interconnect formation, where CMP is used to remove the via, contact or trench fill material from the exposed surface (field) of the layer having the STI or metal interconnect features disposed therein. 
     Often, polishing pads used in the above-described CMP processes are selected based on the material properties of the polishing pad material and the suitability of those material properties for the desired CMP application. One example of a material property that may be adjusted to tune the performance of a polishing pad for a desired CMP application is the porosity of a polymer material used to form the polishing pad and properties related thereto, such as pore size, pore structure, and material surface asperities. Conventional methods of introducing porosity into the polishing pad material typically comprise blending a pre-polymer composition with a porosity forming agent before molding and curing the pre-polymer composition into individual polishing pads or a polymer cake and machining, e.g., skiving, individual polishing pads therefrom. Unfortunately, while conventional methods may allow for the creation of uniform porosity and/or gradual porosity gradients, they are generally unable to provide precision placement of pores within the formed pad and the pad polishing performance-tuning opportunities that might result therefrom. 
     Accordingly, there is a need in the art for methods of forming discrete respective regions of higher and lower porosity within a polishing pad and polishing pads formed therefrom. 
     SUMMARY 
     Embodiments described herein generally relate to polishing pads, and methods for manufacturing polishing pads which may be used in a chemical mechanical polishing (CMP) process, and more particularly, to polishing pad having selectively arranged pores to define discrete regions that include porosity within a polishing element. 
     In one embodiment, a polishing pad features a plurality of polishing elements each comprising a polishing surface and sidewalls extending downwardly from the polishing surface to define a plurality of channels disposed between the polishing elements. Here, one or more of the polishing elements is formed of a continuous phase of polymer material having one or more first regions comprising a first porosity and a second region comprising a second porosity. Typically, the second porosity is less than the first porosity. In some embodiments, one or more regions of intermediate porosities which have corresponding porosities less than the relatively high porosity region A and more than the relatively low porosity region B may be interposed between the regions A and B. In some embodiments, one or more regions of either higher, lower, or a combination of higher and lower porosities may be interposed between the regions A and B. 
     In another embodiment, a method of forming a polishing pad includes dispensing droplets of a pre-polymer composition and droplets of a sacrificial material composition onto a surface of a previously formed print layer according to a predetermined droplet dispense pattern. The method further includes at least partially curing the dispensed droplets of the pre-polymer composition to form a print layer comprising at least portions of a polymer pad material having one or more first regions comprising first porosity and one or more second regions comprising a second porosity. At least one of the second regions is disposed adjacent to a first region and the second porosity is less than the first porosity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  is a schematic side view of an exemplary polishing system configured to use a polishing pad formed according to one of, or a combination of, the embodiments described herein. 
         FIG. 2A  is a schematic perspective sectional view of a polishing pad featuring selectively arranged pores, according to one embodiment. 
         FIGS. 2B-2I  are schematic sectional views of polishing elements that illustrate various selective pore arrangements. 
         FIGS. 3A-3F  are schematic plan view of various polishing pad designs which may be used in place of the pad design shown in  FIG. 2A , according to some embodiments. 
         FIG. 4A  is a schematic sectional view of an additive manufacturing system, which may be used to form the polishing pads described herein. 
         FIG. 4B  is a close-up cross-sectional view schematically illustrating a droplet disposed on a surface of a previously formed print layer, according to one or more, or a combination of, the embodiments described herein. 
         FIGS. 5A-5C  show portions of CAD compatible print instructions  500   a - c , which may be used to form the polishing pads, described herein. 
         FIG. 6  is a flow diagram setting forth a method of forming a polishing pad, according to one or more, or a combination of, the embodiments described herein. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one implementation may be beneficially incorporated in other implementations without further recitation. 
     DETAILED DESCRIPTION 
     Embodiments described herein generally relate to polishing pads, and methods for manufacturing polishing pads, which may be used in a chemical mechanical polishing (CMP) process, and more particularly, to polishing pads having selectively arranged pores to define discrete regions that include porosity within a polishing element. 
     Generally, the polishing pads described herein feature a foundation layer and a plurality of polishing elements disposed on, and integrally formed with, the foundation layer to form a unitary body comprising a continuous polymer phase. The polishing elements form a polishing surface of the polishing pad and the foundation layer provides support for the polishing elements as a to-be-polished substrate is urged against the polishing surface. 
     The polishing elements feature pores that are selectively arranged across the polishing surface and/or in a direction orthogonal thereto. As used herein, the term “pore” includes openings defined in the polishing surface, voids formed the polishing material below the polishing surface, pore-forming features disposed in the polishing surface, and pore-forming features disposed in polishing material below the polishing surface. Pore-forming features typically comprise a water-soluble-sacrificial material that dissolves upon exposure to a polishing fluid thus forming a corresponding opening in the polishing surface and/or void in the polishing material below the polishing surface. In some embodiments, the water-soluble-sacrificial material may swell upon exposure to a polishing fluid thus deforming the surrounding polishing material to provide asperities at the polishing pad material surface. The resulting pores and asperities desirably facilitate transporting liquid and abrasives to the interface between the polishing pad and a to-be-polished material surface of a substrate, and temporarily fixes those abrasives (abrasive capture) in relation to the substrate surface to enable chemical and mechanical material removal therefrom. 
     The term “selectively arranged pores” as used herein refers to the distribution of pores within the polishing elements. Herein, the pores are distributed in one or both directions of an X-Y plane parallel to the polishing surface of the polishing pad (i.e., laterally) and in a Z-direction which is orthogonal to the X-Y planes, (i.e., vertically). 
       FIG. 1  is a schematic side view of an example polishing system configured to use a polishing pad formed according to one or a combination of the embodiments described herein. Here, the polishing system  100  features a platen  104 , having a polishing pad  102  secured thereto using a pressure sensitive adhesive, and a substrate carrier  106 . The substrate carrier  106  faces the platen  104  and the polishing pad  102  mounted thereon. The substrate carrier  106  is used to urge a material surface of a substrate  108 , disposed therein, against the polishing surface of the polishing pad  102  while simultaneously rotating about a carrier axis  110 . Typically, the platen  104  rotates about a platen axis  112  while the rotating substrate carrier  106  sweeps back and forth from an inner diameter to an outer diameter of the platen  104  to, in part, reduce uneven wear of the polishing pad  102 . 
     The polishing system  100  further includes a fluid delivery arm  114  and a pad conditioner assembly  116 . The fluid delivery arm  114  is positioned over the polishing pad  102  and is used to deliver a polishing fluid, such as a polishing slurry having abrasives suspended therein, to a surface of the polishing pad  102 . Typically, the polishing fluid contains a pH adjuster and other chemically active components, such as an oxidizing agent, to enable chemical mechanical polishing of the material surface of the substrate  108 . The pad conditioner assembly  116  is used to condition the polishing pad  102  by urging a fixed abrasive conditioning disk  118  against the surface of the polishing pad  102  before, after, or during polishing of the substrate  108 . Urging the conditioning disk  118  against the polishing pad  102  includes rotating the conditioning disk  118  about an axis  120  and sweeping the conditioning disk  118  from an inner diameter the platen  104  to an outer diameter of the platen  104 . The conditioning disk  118  is used to abrade, rejuvenate, and remove polish byproducts or other debris from, the polishing surface of the polishing pad  102 . 
       FIG. 2A  is a schematic perspective sectional view of a polishing pad  200   a  featuring selectively arranged pores, according to one embodiment. The polishing pad  200   a  may be used as the polishing pad  102  of the exemplary polishing system  100  described in  FIG. 1 . Here, the polishing pad  200   a  comprises a plurality of polishing elements  204   a , which are disposed on and partially disposed within a foundation layer  206 . The polishing pad  200   a  has a first thickness T( 1 ) of between about 5 mm and about 30 mm. The polishing elements  204   a  are supported in the thickness direction of the pad  200   a  by a portion of the foundation layer  206  that has a second thickness of T( 2 ) of between about ⅓ to about ⅔ of the first thickness T( 1 ). The polishing elements  204   a  have a third thickness T( 3 ) that is between about ⅓ and about ⅔ the thickness T( 1 ). As shown, at least portions of the polishing elements are disposed beneath a surface of the foundation layer  206  and the remaining portions extend upwardly therefrom by a height H. In some embodiments, the height H is about ½ the first thickness T( 1 ) or less. 
     Here, the plurality of polishing elements  204   a  comprise a plurality of discontinuous (segmented) concentric rings  207  disposed about a post  205  and extending radially outward therefrom. Here, the post  205  is disposed in the center of the polishing pad  200   a . In other embodiments the center of the post  205 , and thus the center of the concentric rings  207 , may be offset from the center of the polishing pad  200   a  to provide a wiping type relative motion between a substrate and the polishing pad surface as the polishing pad  200   a  rotates on a polishing platen. Sidewalls of the plurality of polishing elements  204   a  and an upward facing surface of the foundation layer  206  define a plurality of channels  218  disposed in the polishing pad  200   a  between each of the polishing elements  204   a  and between a plane of the polishing surface of the polishing pad  200   a  and a surface of the foundation layer  206 . The plurality of channels  218  enable the distribution of polishing fluids across the polishing pad  200   a  and to an interface between the polishing pad  200   a  and the material surface of a substrate to be polished thereon. Here, the polishing elements  204   a  have an upper surface that is parallel to the X-Y plane and sidewalls that are substantially vertical, such as within about 20° of vertical (orthogonal to the X-Y plane), or within 10° of vertical. A width W( 1 ) of the polishing element(s)  204   a  is between about 250 microns and about 10 millimeters, such as between about 250 microns and about 5 millimeters, or between about 1 mm and about 5 mm. A pitch P between the polishing element(s)  204   a  is between about 0.5 millimeters and about 5 millimeters. In some embodiments, one or both of the width W( 1 ) and the pitch P vary across a radius of the polishing pad  200   a  to define zones of pad material properties. 
       FIGS. 2B-2I  are schematic sectional views of polishing elements  204   b - i  that illustrate various selective pore arrangements. Any one or combination of the selective pore arrangements shown and described in  FIGS. 2B-2I  may be used with, and/or in place of, the selective pore arrangements of the polishing elements  204   a  of  FIG. 2A . As shown in  FIGS. 2B-2I , each of the polishing elements  204   b - i  are formed of a continuous phase of polymer material  212  comprising relatively high porosity regions A and one or more relatively low porosity regions B disposed adjacent thereto. As used herein, “porosity” refers to the volume of void-space as a percentage of the total bulk volume in a given sample. In embodiments where a pore, as defined herein, comprises a pore-forming feature formed of a sacrificial material the porosity is measured after sacrificial material forming the feature is dissolved therefrom. Porosity and pore size may be measured using any suitable method, such as by methods using scanning election microscopy (SEM) or optical microscope. Techniques and systems for characterizing porosity (e.g., area density) and pore size are well known in the art. For example, a portion of the surface can be characterized by any suitable method (e.g., by electron microscope image analysis, by atomic force microscopy, by 3D microscopy, etc.). In one implementation, the porosity (e.g., percentage or ratio of the exposed pore area to exposed non-pore containing area of a sample&#39;s surface) and pore size analysis can be performed using a VK-X Series 3D UV Laser Scanning Confocal Microscope, produced by KEYENCE Corporation of America, located in Elmwood Park, N.J., U.S.A. 
     Typically, the porosity in a region of relatively high porosity A will be about 3% or more, such as about 4% or more, about 5% or more, about 10% or more, about 12.5% or more, about 15% or more, about 17.5% or more, about 20% or more, about 22.5% or more, or about 25% or more. The porosity in a relatively low porosity region B will generally be about 95% or less than the porosity of the region of relatively high porosity A adjacent thereto, such as about 90% or less, about 85% or less, about 80% or less, about 75% or less, about 70% or less, about 60% or less, about 50% or less, about 40% or less, about 30% or less, or about 25% or less. In some embodiments, the relatively low porosity region B will have substantially no porosity. Herein, substantially no porosity comprises regions having a porosity of about 0.5% or less. In some embodiments, the relatively low porosity region B will have a porosity of 0.1% or less. 
     In some embodiments, such as shown in  FIGS. 2B-2E , the relatively high porosity regions A comprise a plurality of pores  210  disposed proximate to one or more of the sides of the polishing elements  204   a - e  (when viewed from top down). The regions of relatively low (or substantially no) porosity B are disposed inwardly from the sidewalls of the polishing elements  204   a - e , i.e., inwardly from the relatively high porosity regions A (when viewed from top down). Here, the relatively high porosity regions A have a width W( 2 ) that is less than the width W( 3 ) of the relatively low porosity region B disposed adjacent thereto. In some embodiments, one or more of the relatively high porosity regions A have a width W( 2 ) in the range of about 50 μm to about 10 mm, such as about 50 μm to about 8 mm, about 50 μm to about 8 mm, about 50 μm to about 6 mm, about 50 μm to about 5.5 mm, about 50 μm to about 5 mm, about 50 μm to about 4 mm, about 50 μm to about 3 mm, about 50 μm to about 2 mm, such as about 50 μm to about 1.5 mm, about 50 μm to about 1 mm, about 100 μm to about 1 mm, or about 200 μm to about 1 mm. In some embodiments, the width W( 2 ) of the region of relatively high porosity A is about 90% or less of the width of the region of relatively low porosity B disposed adjacent thereto, such as 80% or less, 70% or less, 60% or less, or 50% or less. As shown, the relatively high porosity region A is adjacent to the relatively low porosity region B. In some embodiments, one or more regions of intermediate porosity (not shown) which has a porosity less than the relatively high porosity region A and more than the relatively low porosity region B may be interposed between the regions A and B. 
     Typically, the pores  210  used to form the relatively high porosity regions A will have one or more lateral (X-Y) dimensions which are about 500 μm or less, such as about 400 μm or less, 300 μm or less, 200 μm or less, or 150 μm or less. In some embodiments, the pores  210  will have at least one lateral dimension that is about 5 μm or more, about 10 μm or more, about 25 μm or more, or about 50 μm or more. In some embodiments, the pores will have at least one lateral dimension in the range of about 50 μm to about 250 μm, such as in the range of about 50 μm to about 200 μm, about 50 μm to about 150 μm. A pore height Z-dimension may be about 1 μm or more, about 2 μm or more, about 3 μm or more, about 5 μm or more, about 10 μm or more, such as about 25 μm or more, about 50 μm or more, about 75 μm, or about 100 μm. In some embodiments, the pore height Z-dimension is about 100 μm or less, such as between about 1 μm and about 50 μm, or between about 1 μm and about 25 μm, such as between about 1 μm and about 10 μm. 
     As shown in  FIGS. 2A-2I  the relatively high porosity regions A extend from the surface of the polishing elements  204   a  to a depth D which may be the same as the height H ( FIG. 2A ) or the thickness T( 3 ) of the polishing elements  204   a - i  or may be a fraction thereof. For example, in some embodiments, the relatively high porosity regions A may extend to a depth D that is 90% or less of the thickness T( 3 ), such as about 80% or less, 70% or less, 60% or less, or 50% or less. In some embodiments, the relatively high porosity regions A may extend to a depth D that is about 90% or less of the height H of the polishing element  204   a - i , such as 80% or less, 70% or less, 60% or less, or 50% or less. 
     The pores  210  used to form the relatively high porosity regions A may be disposed in any desired vertical arrangement when viewed in cross-section. For example, in some embodiments, the pores  210  may be vertically disposed in one or more columnar arrangements such as shown in  FIGS. 2B, 2D  where the pores  210  in each of the columns are in substantial vertical alignment. In other embodiments, the pores  210  may be vertically disposed in one or more staggered columnar arrangements where each pore  210  is offset in one or both of the X-Y directions with respect to a pore  210  that is disposed thereabove and/or therebelow. The orientation of the pores in a columnar arrangement can be used to adjust the compliance of the porosity region A, due to the relative alignment or non-alignment of the pores to a direction in which a load is provided during polishing by a substrate that is being polished. Thus, in one example, the columnar arrangement of pores can be used to adjust and/or control the polishing planarization results for a formed polishing pad. 
     Here, the pores  210  are spaced apart in the vertical direction by one or more printed layers of the polymer material  212  that has a total thickness T( 4 ) of the one or more printed layers of about 5 μm or more, such as about 10 μm or more, 20 μm or more, 30 μm or more, 40 μm or more, or 50 μm or more. In one example, spacing between pores  210  in a vertical direction in polishing feature is about 40 μm. In this example, the 40 μm spacing can be formed by disposing three or four layers of the polymer material  212  between printed layers that include the pores  210 . Thus, as shown, the pores  210  form a substantially closed-celled structure. In other embodiments one or more of the pores  210 , or portions thereof, are not spaced apart from one or more of the pores adjacent thereto and thus form a more open-celled structure. 
     In some embodiments, such as shown in  FIGS. 2F-2I , the polishing elements  200   f - i  comprise at least one relatively low porosity region B disposed proximate to the sidewall of the polishing element  204   f - i  and at least one adjacent relatively high porosity region A disposed inwardly therefrom. In some embodiments, such as shown in  FIGS. 2H-2I , the polishing elements  204   h - i  alternating relatively high porosity regions A and relatively low porosity regions B. In those embodiments, each of the high porosity regions A may have the same width W( 2 ), as shown, or have different widths (not shown). The alternating high porosity regions A are spaced apart by a low porosity region B and each of the low porosity regions B may have the same width (not shown) or different widths, such as W( 4 ) and W( 5 ) respectively where the widths W( 4 ) and W( 5 ) may be found the ranges set forth above for the width W( 3 ). 
       FIGS. 3A-3F  are schematic plan views of various polishing elements  304   a - f  shapes which may be used with or in place of the polishing elements  204   a  of the polishing pad  200   a  described in  FIG. 2A . Each of the  FIGS. 3A-3F  include pixel charts having white regions (regions in white pixels) that represent the polishing elements  304   a - f  and black regions (regions in black pixels) that represent the foundation layer  206 . Pores and related high porosity regions (not shown in  FIGS. 3A-3F ) comprise any one or combination of the selective pore arrangements set forth in  FIGS. 2B-2I  above. 
     In  FIG. 3A , the polishing elements  300   a  comprise a plurality of concentric annular rings. In  FIG. 3B , the polishing elements  300   b  comprise a plurality of segments of concentric annular rings. In  FIG. 3C , the polishing elements  304   c  form a plurality of spirals (four shown) extending from a center of the polishing pad  300   c  to an edge of the polishing pad  300   c  or proximate thereto. In  FIG. 3D , a plurality of discontinuous polishing elements  304   d  are arranged in a spiral pattern on the foundation layer  206 . 
     In  FIG. 3E , each of the plurality of polishing elements  304   e  comprise a cylindrical post extending upwardly from the foundation layer  206 . In other embodiments, the polishing elements  304   e  are of any suitable cross-sectional shape, for example columns with toroidal, partial toroidal (e.g., arc), oval, square, rectangular, triangular, polygonal, irregular shapes in a section cut generally parallel to the underside surface of the pad  300   e , or combinations thereof.  FIG. 3F  illustrates a polishing pad  300   f  having a plurality of discrete polishing elements  304   f  extending upwardly from the foundation layer  206 . The polishing pad  300   f  of  FIG. 3F  is similar to the polishing pad  300   e  except that some of the polishing elements  304   f  are connected to form one or more closed circles. The one or more closed circles create damns to retain polishing fluid during a CMP process. 
       FIG. 4A  is a schematic sectional view of an additive manufacturing system, which may be used to form the polishing pads described herein, according to some embodiments. Here, the additive manufacturing system  400  features a movable manufacturing support  402 , a plurality of dispense heads  404  and  406  disposed above the manufacturing support  402 , a curing source  408 , and a system controller  410 . In some embodiments, the dispense heads  404 ,  406  move independently of one another and independently of the manufacturing support  402  during the polishing pad manufacturing process. Here, the first and second dispense heads  404  and  406  are respectively fluidly coupled to a first pre-polymer composition source  412  and sacrificial material sources  414  which are used to from the polymer material  212  and the pores  210  described in  FIGS. 2A-2I  above. Typically, the additive manufacturing system  400  will feature at least one more dispense head (e.g., a third dispense head, not shown) which is fluidly coupled to a second pre-polymer composition source used to form the foundation layer  206  described above. In some embodiments, the additive manufacturing system  400  includes as many dispense heads as desired to each dispense a different pre-polymer composition or sacrificial material precursor compositions. In some embodiments, the additive manufacturing system  400  further comprises pluralities of dispense heads where two or more dispense heads are configured to dispense the same pre-polymer compositions or sacrificial material precursor compositions. 
     Here, each of dispense heads  404 ,  406  features an array of droplet ejecting nozzles  416  configured to eject droplets  430 ,  432  of the respective pre-polymer composition  412  and sacrificial material composition  414  delivered to the dispense head reservoirs. Here, the droplets  430 ,  432  are ejected towards the manufacturing support and thus onto the manufacturing support  402  or onto a previously formed print layer  418  disposed on the manufacturing support  402 . Typically, each of dispense heads  404 ,  406  is configured to fire (control the ejection of) droplets  430 ,  432  from each of the nozzles  416  in a respective geometric array or pattern independently of the firing other nozzles  416  thereof. Herein, the nozzles  416  are independently fired according to a droplet dispense pattern for a print layer to be formed, such as the print layer  424 , as the dispense heads  404 ,  406  move relative to the manufacturing support  402 . Once dispensed, the droplets  430  of the pre-polymer composition and/or the droplets of the sacrificial material composition  414  are at least partially cured by exposure to electromagnetic radiation, e.g., UV radiation  426 , provided by an electromagnetic radiation source, such as a UV radiation source  408  to form a print layer, such as the partially formed print layer  424 . 
     In some embodiments, dispensed droplets of the pre-polymer compositions, such as the dispensed droplets  430  of the first pre-polymer composition, are exposed to electromagnetic radiation to physically fix the droplet before it spreads to an equilibrium size such as set forth in the description of  FIG. 4B . Typically, the dispensed droplets are exposed to electromagnetic radiation to at least partially cure the pre-polymer compositions thereof within 1 second or less of the droplet contacting a surface, such as the surface of the manufacturing support  402  or of a previously formed print layer  418  disposed on the manufacturing support  402 . 
       FIG. 4B  is a close up cross-sectional view schematically illustrating a droplet  430  disposed on a surface  418   a  of a previously formed layer, such as the previously formed layer  418  described in  FIG. 4A , according to some embodiments. In a typically additive manufacturing process, a droplet of pre-polymer composition, such as the droplet  430   a  will spread and reach an equilibrium contact angle α with the surface  418   a  of a previously formed layer within about one second from the moment in time that the droplet  430   a  contacts the surface  418   a . The equilibrium contact angle α is a function of at least the material properties of the pre-polymer composition and the energy at the surface  418   a  (surface energy) of the previously formed layer, e.g., previously formed layer  418 . In some embodiments, it is desirable to at least the partially cure the dispensed droplet before it reaches an equilibrium size in order to fix the droplets contact angle with the surface  418   a  of the previously formed layer. In those embodiments, the fixed droplet&#39;s  430   b  contact angle θ is greater than the equilibrium contact angle α of the droplet  430   a  of the same pre-polymer composition which was allowed to spread to its equilibrium size. 
     Herein, at least partially curing a dispensed droplet causes the at least partial polymerization, e.g., the cross-linking, of the pre-polymer composition(s) within the droplets and with adjacently disposed droplets of the same or different pre-polymer composition to form a continuous polymer phase. In some embodiments, the pre-polymer compositions are dispensed and at least partially cured to form a well about a desired pore before a sacrificial material composition is dispensed thereinto. 
     The pre-polymer compositions used to form the foundation layer  206  and the polymer material  212  of the polishing elements described above each comprise a mixture of one or more of functional polymers, functional oligomers, functional monomers, reactive diluents, and photoinitiators. 
     Examples of suitable functional polymers which may be used to form one or both of the at least two pre-polymer compositions include multifunctional acrylates including di, tri, tetra, and higher functionality acrylates, such as 1,3,5-triacryloylhexahydro-1,3,5-triazine or trimethylolpropane triacrylate. 
     Examples of suitable functional oligomers which may be used to form one or both of the at least two pre-polymer compositions include monofunctional and multifunctional oligomers, acrylate oligomers, such as aliphatic urethane acrylate oligomers, aliphatic hexafunctional urethane acrylate oligomers, diacrylate, aliphatic hexafunctional acrylate oligomers, multifunctional urethane acrylate oligomers, aliphatic urethane diacrylate oligomers, aliphatic urethane acrylate oligomers, aliphatic polyester urethane diacrylate blends with aliphatic diacrylate oligomers, or combinations thereof, for example bisphenol-A ethoxylate diacrylate or polybutadiene diacrylate, tetrafunctional acrylated polyester oligomers, and aliphatic polyester based urethane diacrylate oligomers. 
     Examples of suitable monomers which may be used to from one or both of the at least two pre-polymer compositions include both mono-functional monomers and multifunctional monomers. Suitable mono-functional monomers include tetrahydrofurfuryl acrylate (e.g. SR285 from Sartomer®), tetrahydrofurfuryl methacrylate, vinyl caprolactam, isobornyl acrylate, isobornyl methacrylate, 2-phenoxyethyl acrylate, 2-phenoxyethyl methacrylate, 2-(2-ethoxyethoxy)ethyl acrylate, isooctyl acrylate, isodecyl acrylate, isodecyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, stearyl methacrylate, cyclic trimethylolpropane formal acrylate, 2-[[(Butylamino) carbonyl]oxy]ethyl acrylate (e.g. Genomer 1122 from RAHN USA Corporation), 3,3,5-trimethylcyclohexane acrylate, or mono-functional methoxylated PEG (350) acrylate. Suitable multifunctional monomers include diacrylates or dimethacrylates of diols and polyether diols, such as propoxylated neopentyl glycol diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, alkoxylated aliphatic diacrylate (e.g., SR9209A from Sartomer®), diethylene glycol diacrylate, diethylene glycol dimethacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, triethylene glycol dimethacrylate, alkoxylated hexanediol diacrylates, or combinations thereof, for example SR562, SR563, SR564 from Sartomer®. 
     Typically, the reactive diluents used to form one or more of the pre-polymer compositions are least monofunctional, and undergo polymerization when exposed to free radicals, Lewis acids, and/or electromagnetic radiation. Examples of suitable reactive diluents include monoacrylate, 2-ethylhexyl acrylate, octyldecyl acrylate, cyclic trimethylolpropane formal acrylate, caprolactone acrylate, isobornyl acrylate (IBOA), or alkoxylated lauryl methacrylate. 
     Examples of suitable photoinitiators used to form one or more of the at least two different pre-polymer compositions include polymeric photoinitiators and/or oligomer photoinitiators, such as benzoin ethers, benzyl ketals, acetyl phenones, alkyl phenones, phosphine oxides, benzophenone compounds and thioxanthone compounds that include an amine synergist, or combinations thereof. 
     Examples of polishing pad materials formed of the pre-polymer compositions described above typically include at least one of oligomeric and, or, polymeric segments, compounds, or materials selected from the group consisting of: polyamides, polycarbonates, polyesters, polyether ketones, polyethers, polyoxymethylenes, polyether sulfone, polyetherimides, polyimides, polyolefins, polysiloxanes, polysulfones, polyphenylenes, polyphenylene sulfides, polyurethanes, polystyrene, polyacrylonitriles, polyacrylates, polymethylmethacrylates, polyurethane acrylates, polyester acrylates, polyether acrylates, epoxy acrylates, polycarbonates, polyesters, melamines, polysulfones, polyvinyl materials, acrylonitrile butadiene styrene (ABS), halogenated polymers, block copolymers, and random copolymers thereof, and combinations thereof. 
     The sacrificial material composition(s), which may be used to form the pores  210  described above, include water-soluble material, such as, glycols (e.g., polyethylene glycols), glycol-ethers, and amines. Examples of suitable sacrificial material precursors which may be used to form the pore forming features described herein include ethylene glycol, butanediol, dimer diol, propylene glycol-(1,2) and propylene glycol-(1,3), octane-1,8-diol, neopentyl glycol, cyclohexane dimethanol (1,4-bis-hydroxymethylcyclohexane), 2-methyl-1,3-propane diol, glycerine, trimethylolpropane, hexanediol-(1,6), hexanetriol-(1,2,6) butane triol-(1,2,4), trimethylolethane, pentaerythritol, quinitol, mannitol and sorbitol, methylglycoside, also diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols, dibutylene glycol, polybutylene glycols, ethylene glycol, ethylene glycol monobutyl ether (EGMBE), diethylene glycol monoethyl ether, ethanolamine, diethanolamine (DEA), triethanolamine (TEA), and combinations thereof. 
     In some embodiments, the sacrificial material precursor comprises a water soluble polymer, such as 1-vinyl-2-pyrrolidone, vinylimidazole, polyethylene glycol diacrylate, acrylic acid, sodium styrenesulfonate, Hitenol BC10®, Maxemul 6106e, hydroxyethyl acrylate and [2-(methacryloyloxy)ethyltrimethylammonium chloride, 3-allyloxy-2-hydroxy-1-propanesulfonic acid sodium, sodium 4-vinylbenzenesulfonate, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, 2-acrylamido-2-methyl-1-propanesulfonic acid, vinylphosphonic acid, allyltriphenylphosphonium chloride, (vinylbenzyl)trimethylammonium chloride, allyltriphenylphosphonium chloride, (vinylbenzyl)trimethylammonium chloride, E-SPERSE RS-1618, E-SPERSE RS-1596, methoxy polyethylene glycol monoacrylate, methoxy polyethylene glycol diacrylate, methoxy polyethylene glycol triacrylate, or combinations thereof. 
     Here, the additive manufacturing system  400  shown in  FIG. 4A  further includes the system controller  410  to direct the operation thereof. The system controller  410  includes a programmable central processing unit (CPU)  434  which is operable with a memory  435  (e.g., non-volatile memory) and support circuits  436 . The support circuits  436  are conventionally coupled to the CPU  434  and comprise cache, clock circuits, input/output subsystems, power supplies, and the like, and combinations thereof coupled to the various components of the additive manufacturing system  400 , to facilitate control thereof. The CPU  434  is one of any form of general purpose computer processor used in an industrial setting, such as a programmable logic controller (PLC), for controlling various components and sub-processors of the additive manufacturing system  400 . The memory  435 , coupled to the CPU  434 , is non-transitory and is typically one or more of readily available memories such as random access memory (RAM), read only memory (ROM), floppy disk drive, hard disk, or any other form of digital storage, local or remote. 
     Typically, the memory  435  is in the form of a computer-readable storage media containing instructions (e.g., non-volatile memory), which when executed by the CPU  434 , facilitates the operation of the manufacturing system  400 . The instructions in the memory  435  are in the form of a program product such as a program that implements the methods of the present disclosure. 
     The program code may conform to any one of a number of different programming languages. In one example, the disclosure may be implemented as a program product stored on computer-readable storage media for use with a computer system. The program(s) of the program product define functions of the embodiments (including the methods described herein). 
     Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the methods described herein, are embodiments of the present disclosure. In some embodiments, the methods set forth herein, or portions thereof, are performed by one or more application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other types of hardware implementations. In some other embodiments, the polishing pad manufacturing methods set forth herein are performed by a combination of software routines, ASIC(s), FPGAs and, or, other types of hardware implementations. 
     Here, the system controller  410  directs the motion of the manufacturing support  402 , the motion of the dispense heads  404  and  406 , the firing of the nozzles  416  to eject droplets of pre-polymer compositions therefrom, and the degree and timing of the curing of the dispensed droplets provided by the UV radiation source  408 . In some embodiments, the instructions used by the system controller to direct the operation of the manufacturing system  400  include droplet dispense patterns for each of the print layers to be formed. In some embodiments, the droplet dispense patterns are collectively stored in the memory  425  as CAD-compatible digital printing instructions. Examples of print instructions which may be used by the additive manufacturing system  400  to manufacture the polishing pads described herein are shown in  FIGS. 5A-5C . 
       FIGS. 5A-5C  show portions of CAD compatible print instructions  500   a - c  which may be used by the additive manufacturing system  400  to form embodiments of the polishing pads described herein. Here, the print instructions  500   a - c  are for print layers used to form polishing elements  504   a - c  respectively. Each of the polishing elements  504   a - c  are formed of the polymer material  212  and comprise relatively high porosity regions A disposed proximate to the sidewalls of the polishing elements  504   a - c  and relatively low porosity regions B disposed inwardly of the relatively high porosity regions A. Droplets of the pre-polymer composition(s) used to form the polymer material  212  will be dispensed in the white regions and droplets of the sacrificial material composition(s) will be dispensed within the black pixels of the high porosity regions A. In this print layer, no droplets will be dispensed in the black regions between the polishing elements  504   a - c  (outside of the relatively high porosity regions A). The print instructions  500   a - c  may be used to form relatively high porosity regions A each having a porosity of 25%, 16%, and 11% respectively and relatively low porosity regions B having no intended porosity (e.g., less than about 0.1% porosity). Here, the width W( 1 ) of each polishing element  504   a - c  is about 2.71 mm, the widths W( 2 ) of the relatively high porosity regions A are each about 460 μm, and the width W( 3 ) of the relatively low porosity region B is about 1.79 mm. 
     Polishing pads formed according to embodiments described herein show unexpectedly superior performance in dielectric CMP processing when compared to similar polishing pads having uniformly distributed porosity. A comparison of CMP performance between continuous porosity and a selective porosity pad is set forth in Table 1 below. Sample polishing pad D in table 1 was formed using the print instructions  500   a  of  FIG. 5A . Sample polishing pads A-C were formed using the same material precursors and substantially the same print instructions as  500   a  except the pores of sample polishing pads A-C were informingly distributed across the polishing elements to achieve uniform porosities of 33%, 11%, and 5% respectively. Each of the sample polishing pads A-D were used to polish a blanket film of silicon oxide film layer disposed on a patterned substrate comprising a design architecture used in manufacture of logic and memory devices. The silicon oxide film was conventionally deposited using a tetraethylorthosilicate (TEOS) precursor. Surprisingly, the sample polishing pad D having selectively arranged regions of relatively high porosity disposed adjacent to regions of relatively low porosity provided desirably higher oxide removal rates when compared to polishing pads have uniformly distributed porosity values both higher and lower than that of the A regions of sample D. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Polish 
                   
                   
               
               
                 Sample 
                 Segment 
                 Feature 
                   
                   
                 Layer 
                   
                 Normalized 
               
               
                 Polishing 
                 Length 
                 Width 
                   
                 Porosity 
                 Hardness 
                 Foundation 
                 Maximum Oxide 
               
               
                 Pads 
                 (mm) 
                 (mm) 
                 Comments 
                 (%) 
                 (Shore D) 
                 Layer 
                 Removal Rate 
               
               
                   
               
             
            
               
                 A 
                 100 
                 2.71 
                 Continuous 
                 33% 
                 55D 
                 62D 
                 100.0% 
               
               
                 B 
                 100 
                 2.71 
                 Porosity 
                 11% 
                 63D 
                 62D 
                 161.5% 
               
               
                 C 
                 100 
                 2.71 
                   
                  5% 
                 71D 
                 62D 
                 138.5% 
               
               
                 D 
                 100 
                 2.71 
                 Porosity 
                 25% on 
                 55D 
                 62D 
                 200.0% 
               
               
                   
                   
                   
                 only on 
                 Edge 
                   
                   
                   
               
               
                   
                   
                   
                 edge of the 
                 Only 
                   
                   
                   
               
               
                   
                   
                   
                 pads 
               
               
                   
               
            
           
         
       
     
       FIG. 6  is a flow diagram setting forth a method of forming a print layer of a polishing pad according to one or more embodiments. Embodiments of the method  600  may be used in combination with one or more of the systems and system operations described herein, such as the additive manufacturing system  400  of  FIG. 4A , the fixed droplets of  FIG. 4B , and the print instructions of  FIGS. 5A-5C . Further, embodiments of the method  600  may be used to form any one or combination of embodiments of the polishing pads shown and described herein. 
     While  FIGS. 5A-5C  illustrate a configuration where a polishing feature includes a relatively high porosity regions A disposed proximate to the sidewalls of the polishing elements  504   a - c  and a relatively low porosity regions B disposed inwardly of the relatively high porosity regions A this configuration is not intended to be limiting as to the scope of the disclosure provided herein, since it may be desirable, depending on the polishing application, to alternately form the relatively high porosity regions A proximate to the inward region of the polishing elements  504   a - c  and form the relatively low porosity regions B proximate to the sidewalls of the polishing elements  504   a - c.    
     At activity  601  the method  600  includes dispensing droplets of a pre-polymer composition and droplets of a sacrificial material composition onto a surface of a previously formed print layer according to a predetermined droplet dispense pattern. 
     At activity  602  the method  600  includes at least partially curing the dispensed droplets of the pre-polymer composition to form a print layer comprising at least portions of a polymer pad material having one or more relatively high porosity regions and one or more relatively low porosity regions disposed adjacent to the one or more relatively high porosity regions. 
     In some embodiments, the method  600  further includes sequential repetitions of activities  601  and  602  to form a plurality of print layers stacked in a Z-direction, i.e., a direction orthogonal to the surface of the manufacturing support or a previously formed print layer disposed thereon. The predetermined droplet dispense pattern used to form each print layer may be the same or different as a predetermined droplet dispense pattern used to form a previous print layer disposed there below. 
     The polishing pads and polishing pad manufacturing methods described herein beneficially allow for selectively arranged pores and resulting discrete regions of porosity that enable fine tuning of CMP process performance. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.