Patent Publication Number: US-11642757-B2

Title: Using sacrificial material in additive manufacturing of polishing pads

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/873,851, filed Jan. 17, 2018, which claims priority to U.S. Application Ser. No. 62/511,276, filed on May 25, 2017, the entire disclosures of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This specification relates to additive manufacturing, particularly additive manufacturing of chemical mechanical polishing pads. 
     BACKGROUND 
     An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. A variety of fabrication processes require planarization of a layer on the substrate. For certain applications, e.g., polishing of a metal layer to form vias, plugs, and lines in the trenches of a patterned layer, an overlying layer is planarized until the top surface of a patterned layer is exposed. In other applications, e.g., planarization of a dielectric layer for photolithography, an overlying layer is polished until a desired thickness remains over the underlying layer. 
     Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, such as slurry with abrasive particles, is typically supplied to the surface of the polishing pad. 
     One objective of a chemical mechanical polishing process is polishing uniformity. If different areas on the substrate are polished at different rates, then it is possible for some areas of the substrate to have too much material removed (“overpolishing”) or too little material removed (“underpolishing”). In addition to planarization, polishing pads can be used for finishing operations such as buffing. 
     Polishing pads are typically made by molding, casting or sintering polyurethane materials. In the case of molding, the polishing pads can be made one at a time, e.g., by injection molding. In the case of casting, the liquid precursor is cast and cured into a cake, which is subsequently sliced into individual pad pieces. These pad pieces can then be machined to a final thickness. Grooves can be machined into the polishing surface, or be formed as part of the injection molding process. 
     SUMMARY 
     The present disclosure describes manufacturing polishing pads with an additive manufacturing system. 
     In one aspect, a method of fabricating a polishing pad using an additive manufacturing system includes receiving data indicative of a desired shape of the polishing pad to be fabricated by droplet ejection by the additive manufacturing system. The data includes a desired shape defining a desired profile including a polishing surface having one or more partitions separated by one or more grooves on the polishing pad. Data indicative of distortions from the desired profile caused by dispensing of layers by droplet ejection by the additive manufacturing system is generated. Data indicative of an initial layer to dispense by droplet ejection is generated to at least partially compensate for the distortions from the desired profile. The initial layer is dispensed on a support by droplet ejection. Overlying layers are dispensed on the initial layer by droplet ejection by the additive manufacturing system to form the polishing pad. 
     Implementations may include one or more of the following features. 
     The polishing pad may include the initial layer. The polishing pad may include the support. The polishing pad may be removed from the support. The distortions may include regions expected to be thin relative to the desired profile. The initial layer may include voxels corresponding to the regions. The regions may correspond to edges of the one or more partitions. 
     In another aspect, a method of fabricating a polishing pad using an additive manufacturing system includes depositing successive layers by droplet ejection to form the polishing pad. The polishing pad includes a polishing surface having one or more partitions separated by one or more grooves. Depositing a layer of the successive layers includes dispensing first regions corresponding to edges of the one or more partitions by a first droplet ejection process. After curing the first regions, a second region corresponding to interior of the one or more partitions is dispensed between the edges by a different second droplet ejection process. 
     Implementations may include one or more of the following features. 
     The first droplet ejection process may include a first polymer and the second droplet ejection process may include a second polymer of different composition. The first droplet ejection process may include a first curing radiation and the second droplet ejection process may include a second curing radiation that cures the layer slower than the first curing radiation. The first curing radiation and the second curing radiation may be at different wavelengths. The first curing radiation may have a higher intensity than the second curing radiation. Droplets need not be ejected into regions corresponding to the grooves. 
     In another aspect, a method of fabricating a polishing pad using an additive manufacturing system includes depositing a first set of successive layers onto a support by droplet ejection. Depositing the first set of successive layers includes dispensing a polishing pad precursor to first regions corresponding to partitions of the polishing pad and dispensing a sacrificial material to second regions corresponding to grooves of the polishing pad. A second set of successive layers is deposited by droplet ejection over the first set of successive layers. The second set of successive layers corresponds to a lower portion of the polishing pad. The first set of successive layer and the second set of successive layers provide a body. The body is removed from the support. Removing the sacrificial material from the body provides the polishing pad with a polishing surface that has the partitions separated by the grooves. 
     Implementations may include one or more of the following features. 
     Depositing the second set of successive layers may include dispensing the polishing pad precursor. The second set of successive layers may correspond to a lower portion of the polishing layer. A third set of successive layers may be deposited by droplet ejection over the second set of successive layers. The third set of successive layers may have a different composition than the second set of successive layers. The second set of successive layers may correspond to a sub-pad of the polishing layer. The second set of successive layers may span both the first regions and the second regions. 
     The polishing pad precursor may be cured. The sacrificial material may be cured. The polishing pad precursor may be a first polymer precursor and the sacrificial material may be a second polymer precursor of different composition. Depositing the third plurality of successive layers may include droplet ejection of the polishing pad precursor and curing the third plurality of successive layers with a different wavelength or intensity of light than the second plurality of successive layers. Depositing the third plurality of successive layers may include droplet ejection of a polymer precursor of different composition than the polishing pad precursor. Removing the sacrificial material may include etching or lifting the polishing pad off the sacrificial material. 
     In another aspect an additive manufacturing system includes a support, a first dispenser configured to dispense a polishing pad precursor by droplet ejection, a second dispenser configured to dispense a sacrificial material, at least one energy source, and a controller. The controller is configured to receive data indicative of a desired shape of a polishing pad to be fabricated by droplet ejection by an additive manufacturing system. The polishing pad has a polishing surface having one or more partitions separated by one or more grooves. The controller is configured to cause the first dispenser to deliver the polishing pad precursor to first regions corresponding to partitions of the polishing pad, and cause the second dispenser to deliver the sacrificial material to second regions corresponding to grooves of the polishing pad. 
     Implementations may include one or more of the following features. 
     The controller may be configured to cause the first dispenser to deposit a second plurality of successive layers by droplet ejection over the first plurality of successive layers. The second plurality of successive layers may correspond to a lower portion of the polishing pad. A third dispenser may be configured to dispense a second polishing pad precursor by droplet ejection. The controller may be configured to cause the third dispenser to deposit a second plurality of successive layers by droplet ejection over the first plurality of successive layers. The second plurality of successive layers may correspond to a lower portion of the polishing pad. 
     The controller may be configured is configured to cause the first dispenser to deposit a second plurality of successive layers by droplet ejection over the first plurality of successive layers, and to cause the third dispenser to deposit a third plurality of successive layers by droplet ejection over the second plurality of successive layers, the first and second pluralities of successive layers corresponding to a lower portion of the polishing pad. 
     The second dispenser may be configured to dispense the sacrificial material by droplet ejection. 
     In another aspect, a method of fabricating an object using an additive manufacturing system includes depositing onto a support a first plurality of successive layers by droplet ejection, depositing a second plurality of successive layers by droplet ejection over the first plurality of successive layers with the second plurality of successive layers corresponding to a lower portion of the object, the first plurality of successive layer and the second plurality of successive layers providing a body; removing the body from the support, and removing the sacrificial material from the body to provide the object comprising the surface having the projections separated by the recesses. Depositing the first plurality of successive layers includes dispensing a polymer precursor to first regions corresponding to protrusions on a surface of the object, dispensing a sacrificial material to second regions corresponding to recesses on the surface of the object, and curing the polymer precursor. 
     Advantages of the foregoing may include, but are not limited to, the following. The geometry of a polishing pad can be more precisely controlled, thereby improving polishing efficacy of the polishing pad. Furthermore, a correction profile can compensate for potential distortions by adjusting data that the additive manufacturing apparatus uses to form an article, e.g., a polishing pad, rather than removing material after the article has been initially formed. The amount of post-processing of the article after it is formed by the additive manufacturing apparatus can be decreased. As a result, an amount of feed material waste can be reduced, and yield and throughput can be increased. The need for a secondary machining step may be eliminated. The change in slurry capture volume of the polishing pad as the surface is removed and the groove depth decreases may be reduced, thus improving wafer-to-wafer uniformity. The capture material may also be removed through a selective etching process to leave only the desired material, which can be beneficial where an optically transparent material used as in the formation of a CMP window and in fixed abrasive, roll format pad designs. 
     The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other potential features, aspects, and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic side view of a polishing system. 
         FIG.  2    is a schematic side view of an additive manufacturing apparatus. 
         FIG.  3 A  is a top view of an example of a polishing pad. 
         FIG.  3 B  is a side view of the polishing pad of  FIG.  3 A . 
         FIG.  4    is a flowchart of a process to form an article. 
         FIG.  5    illustrates an example of an actual shape formed based on a desired shape. 
         FIG.  6    illustrates an actual shape formed based on modification of the desired shape of  FIG.  5   . 
         FIG.  7    illustrates another example of an actual shape formed based on a desired shape. 
         FIGS.  8 A- 8 D  are side view representations of an example pattern and method for depositing. 
         FIGS.  9 A- 9 E  are side view representations of an example pattern and method for depositing. 
         FIG.  10    is a schematic side view of another implementation of an additive manufacturing apparatus. 
         FIG.  11 A  is a schematic side view of another implementation of an additive manufacturing apparatus. 
         FIG.  11 B  is a schematic side view of another implementation of an additive manufacturing apparatus. 
         FIG.  12    is a schematic side view of another implementation of an additive manufacturing apparatus. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     An additive manufacturing apparatus can be used to form a polishing pad. The additive manufacturing apparatus can be provided with an initial pattern to dispense feed material. The initial pattern corresponds to a desired shape of the polishing pad to be formed. Unfortunately, when the polishing pad is formed by the additive manufacturing apparatus using the initial pattern, an actual shape of the polishing pad may include distortions relative to the desired shape of the polishing pad. However, several techniques can be used to compensate for such distortions. 
     The initial pattern provided to the additive manufacturing apparatus can be modified by a correction profile to generate a modified pattern to at least partially compensate for these distortions. The resulting shape formed using the modified pattern can thus more closely match the desired shape of the polishing pad. The modified pattern can include an initial layer onto which additional layers are deposited. 
     Portions of the polishing pad corresponding to the edges of the partitions can be a deposited by a different technique than a center of the partitions, e.g., to provide improved verticality of side walls. 
     A sacrificial material can be deposited, and layers of the pad can be deposited between and over the sacrificial. This sacrificial material can then be removed to provide the grooves, surface texture to reduce break-in times, and to manufacture a polishing pad in a roll-to-roll format. 
     Turning now to  FIG.  1   , a polishing system  100  includes a polishing pad  102  that can be used to polish one or more substrates  104 . The polishing system  100  can include a rotatable platen  106  on which the polishing pad  102  is placed. During a polishing step, a polishing liquid  108 , e.g., abrasive slurry, can be supplied to a polishing surface  103  of polishing pad  102  by a slurry supply port or combined slurry/rinse arm  110 . The polishing liquid  108  can contain abrasive particles, a pH adjuster, or chemically active components. 
     The substrate  104  is held against the polishing pad  102  by a carrier head  112 . The carrier head  112  is suspended from a support structure, such as a carousel, and is connected by a carrier drive shaft  114  to a carrier head rotation motor so that the carrier head can rotate about an axis  116 . The relative motion of the polishing pad  102  and the substrate  104  in the presence of the polishing liquid  108  results in polishing of the substrate  104 . 
     Referring to  FIG.  2   , in some examples, an additive manufacturing apparatus  120  that dispenses successive layers of feed material can be used to form the polishing pad  102 . Referring to  FIGS.  1  and  2   , the additive manufacturing apparatus  120  is operated to form at least a polishing layer  122  of the polishing pad  102 . In the manufacturing process, thin layers of feed material are progressively dispensed and cured. For example, droplets  124  of feed material, e.g., polishing pad precursor material, can be ejected from a nozzle  126  of a dispenser  128 , e.g., a droplet ejector printer, to form a layer  130  of the feed material. The dispenser  128  is similar to an inkjet printer, but uses the feed material for forming the polishing pad  102  rather than ink. 
     A controller  129  is operable to control dispensing operations of the dispenser  128  and, if applicable, control curing operations using an energy source  131  such as a lamp or a laser. The nozzle  126  is translated (shown by arrow A) across a support  134  to dispense feed material at any portion of a build area on the support  134 . 
     In some implementations, the energy source  131  trails the nozzle  126  as the nozzle  126  is translated across the support  134 , such that feed material dispensed through the nozzle  126  can be immediately cured. In some implementations, the energy source  131  leads the nozzle  126  as the nozzle  126  is translated across the support  134  in a first scanning direction while dispensing feed material. The energy source  131  can cure this dispensed feed material as the energy source  131  is scanned across the support  134 , e.g., in a second scanning direction opposite the first scanning direction, thereby providing the feed material additional time to reach a stable state before being exposed to radiation of the energy source  131 . In some implementations, the energy source  131  leads the nozzle  126  as the nozzle  126  is translated across the support  134  in a first scanning direction, and the energy source  131  is used to cure the dispensed feed material as the energy source is scanned in the first scanning direction. Thus, the previously dispensed layer of feed material can be cured almost immediately before another layer is dispensed through the nozzle  126 . In some implementations, there are multiple energy sources, with an energy source  131  that trails the nozzle  126  and an energy source  131  that leads the nozzle  126 . 
     For a first layer  130   a  deposited, the nozzle  126  can eject the feed material onto the support  134 . For subsequently deposited layers  130   b , the nozzle  126  can eject onto already solidified feed material  132 . After each layer  130  is solidified, a new layer is then deposited over the previously deposited layer until the full 3-dimensional polishing layer  122  is fabricated. Each layer is applied by the nozzle  126  in a pattern stored in a 3D drawing computer program that runs on a computer. Each layer  130  is less than 50% of the total thickness of the polishing layer  122 , e.g., less than 10%, e.g., less than 5%, e.g., less than 1%. 
     The polishing layer  122  can be formed on a support  134 . In some examples, the support  134  includes a rigid base, or includes a flexible film, e.g., a layer of polytetrafluoroethylene (PTFE). If the support  134  includes a flexible film, then the support  134  forms a portion of the polishing pad  102 . For example, the support  134  can include a backing layer  136  (shown in  FIG.  1   ) of the polishing pad  102  or a layer between the backing layer and the polishing layer  122 . If the support  134  includes the backing layer  136  of the polishing pad  102 , the support  134  is not removed from the polishing pad  102  after manufacturing of the polishing pad  102  is complete. Referring to  FIG.  1   , the polishing pad  102  is mounted to the polishing system  100  with the backing layer  136  (e.g., the support  134 ) facing the rotatable platen  106 . 
     If the support  134  does not include the backing layer  136  of the polishing pad  102 , the polishing layer  122  can be removed from the support  134  after manufacturing of the polishing pad  102  is complete. In some implementations, the support  134  can include a rigid base that is covered by a protective film. The polishing pad  102  can be fabricated on the protective film. Thereafter, the protective film can be replaced on the rigid base, and a new polishing pad fabricated on the new protective film. The protective film can be removed from the polishing pad, e.g., the protective film can remain on the rigid base as the polishing pad is removed, or the protective film can detach from the rigid base and then be peeled off the polishing pad. 
     Solidification of the layers  130  of feed material can be accomplished by polymerization. For example, the layer  130  of feed material can be a monomer, and the monomer can be polymerized in-situ by ultraviolet (UV) curing. The feed material can be cured effectively immediately upon depositing, or an entire layer  130  of pad precursor material can be deposited and then the entire layer  130  be cured simultaneously. Alternatively, the droplets  124  can be a polymer melt that solidifies upon cooling. In further implementations, the apparatus  120  creates the polishing layer  122  by spreading a layer of powder and ejecting droplets of a binder material onto the layer of powder. In this case, the powder could include additives, e.g., abrasive particles. 
     In some implementations, the backing layer  136  can also be fabricated by a 3D printing process. For example, the backing layer  136  and polishing layer  122  could be fabricated in an uninterrupted operation by the apparatus  120 . The backing layer  136  can be provided with a different hardness than the polishing layer  122  by using a different amount of curing, e.g., a different intensity of UV radiation, or by using a different material. In other implementations, the backing layer  136  is fabricated by a conventional process and then secured to the polishing layer  122 . For example, the polishing layer  122  can be secured to the backing layer  136  by a thin adhesive layer, e.g., as a pressure-sensitive adhesive. 
     In some implementations, referring to  FIGS.  2 ,  3 A, and  3 B , when the polishing layer  122  is formed, the apparatus  120  can selectively dispense and/or selectively cure portions of the feed material to form grooves  138  in the polishing layer  122 . The grooves  138  can carry the polishing liquid  108  (shown in  FIG.  1   ). The grooves  138  may be of nearly any pattern, such as concentric circles, straight lines, cross-hatching, spirals, and the like. Assuming grooves are present, partitions  140  between the grooves  138  define the polishing surface  103 . The polishing surface  103 , e.g., including the partitions  140  between the grooves  138 , can be about 25-90%, e.g., 70-90%, of the total horizontal surface area of the polishing pad  102 . Thus, the grooves  138  can occupy 10%-75%, e.g., 10-30%, of the total horizontal surface area of the polishing pad  102 . The partitions between the grooves  138  can have a lateral width of about 0.1 to 2.5 mm. 
     Referring to examples illustrated in  FIGS.  3 A and  3 B , in some implementations, the grooves  138  include concentric circular grooves. These grooves  138  can be uniformly spaced with a pitch P. The pitch P is the radial distance between adjacent grooves  138 . The partitions  140  between the grooves  138  have a width W p . Each groove  138  are defined by side walls  142  extending from a bottom surface  144  of the groove  138  and terminate in at the polishing surface  103 , e.g., at the partition  140 . Each groove  138  may have a depth D g  and a width W g . 
     The side walls  142  can extend downwardly from and be generally perpendicular to the polishing surface  103 . In this regard, the side walls are substantially perpendicular to the layers  130  of feed material dispensed on the support  134 . In addition, the partitions  140  extend substantially parallel to the layers  130  of feed material dispensed on the support  134 . 
     Each polishing cycle results in wear of polishing pad  102 , generally in the form of thinning of the polishing pad  102  as the polishing surface  103  is worn down. The width W g  of a groove with substantially perpendicular side walls  142  does not change as the polishing pad is worn. Thus, the generally perpendicular side walls  142  ensure that the polishing pad  102  has a substantially uniform surface area over its operating lifetime. As described herein, the manufacturing process to form the polishing pad  102  can include compensatory operations to prevent the polishing surface  103  from being nonplanar, e.g., to ensure planarity or flatness of the polishing surface  103 , and to fabricate the side walls  142  as perpendicular to the polishing surface  103 . 
     The grooves  138  can have a minimum width W g  of about 0.34 mm. Each groove  138  can have a width W g  between 0.34 mm and 2.71 mm, e.g., between about 0.38 mm and 1.02 mm. Specifically, the grooves  138  may have a width W g  of approximately 0.51 mm or 0.68 mm. The pitch P between the grooves  138  may be between about 0.68 and 6.10 mm, e.g., between about 2.29 mm and 5.40 mm. Specifically, the pitch may be approximately 2.03 or 3.05 mm. Each partition  140  between the grooves  138  may have a width W p  of at least 0.34 mm. The ratio of groove width W g  to partition width W p  may be selected to be between about 0.10 and 0.4. The ratio may be approximately 0.2 or 0.3. 
     In some implementations, if the polishing pad  102  includes the backing layer  136 , the grooves  138  can extend entirely through the polishing layer  122 . In some implementations, the grooves  138  can extend through about 20-80%, e.g., 40%, of the thickness of the polishing layer  122 . The depth D g  of the grooves  138  can be 0.25 to 1 mm. The polishing layer  122  can have a thickness T between about 1 mm and 3 mm. The thickness T should be selected so that the distance D p  between the bottom surface  144  of the groove  138  and the backing layer  136  is between about 0.5 mm and 4 mm. Specifically, the distance D p  may be about 1 or 2 mm. 
     Referring to  FIG.  4   , a manufacturing process  200  to form the polishing pad  102  is illustrated. For example, the additive manufacturing apparatus  120 , including the controller  129 , can perform operations the manufacturing process  200 . 
     Data indicative of a desired shape of the polishing pad  102  to be fabricated is received ( 202 ). Data indicative of shapes, including the data indicative of the desired shape, can be defined by a two-dimensional or three-dimensional bitmap. For example, each bit can indicate whether material should be present in a corresponding voxel in the object. In some implementations, the shape data includes data representing a computer-aided design (CAD) model. For example, if the shape data corresponds to the data indicative of the desired shape, the CAD model can be representative of the polishing pad  102  to be fabricated. 
     In some examples, referring to  FIG.  5   , the desired shape includes a desired feature  300 . Absent further manipulation of the data indicative of the desired shape, when the additive manufacturing apparatus  120  forms the desired shape, e.g., dispenses the feed material and cures or allows the feed material to cure to form the desired shape, an actual feature  310  may be formed based on the data indicative of the desired shape including the desired feature  300 . For example, to form the rectangular desired feature  300 , the dispenser  128  is controlled to dispense parallel layers  130  of feed material. For each layer, a selected portion of feed material having a uniform width corresponding to a width of the rectangular desired feature  300  is cured. The recesses, e.g., grooves, can be provided by simply not dispensing any feed material into the corresponding regions on the object, e.g., the polishing pad. 
     During this dispensing and curing process, material properties of the feed material and the depositing technique of the additive manufacturing apparatus  120  can cause edges of the actual feature  310  to become undesirably rounded or beveled. In particular, if the layers  130  of feed material are dispensed in accordance to an original pattern determined based on the data indicative of the desired shape, the resulting shape includes rounding or beveling as depicted with respect to the actual feature  310 . 
     For example, as shown in  FIG.  5   , while a top surface  302  of the desired feature  300  is planar, a corresponding top surface  312  of the actual feature  310  is nonplanar. Due to a beveling effect on the top surface  312 , lateral edges  304   a ,  304   b  of the desired feature  300  have a greater length than actual lateral edges  314   a ,  314   b  of the actual feature formed by the additive manufacturing apparatus  120 . The desired feature  300  can correspond to the partitions  140  between the grooves  138  (shown in  FIGS.  3 A and  3 B ). In this regard, the rounding or beveling effect on the top surface  312  can cause the polishing surface  103  defined by the partitions  140  to become nonplanar. Without being limited to any particular theory, the liquid droplets of feed material, e.g., the liquid pad precursor material, ejected onto the previously deposited layer can spread and run down the sides of the feature  300 , e.g., due to wetting, resulting in the rounding. 
     Referring back to  FIG.  4   , data indicative of the distortions from the desired profile (caused by a dispensing of multiple layers by droplet ejection by the additive manufacturing system) is generated ( 204 ). That is, an expected distortion profile is produced. To reduce the rounding or beveling effect (that which is predicted with the expected distortion profile), the data indicative of the desired shape can be modified. In this regard, data indicative of a modified pattern of dispensing feed material to compensate for polishing pad distortions is generated or received ( 206 ). The distortions include distortions of the polishing surface  103  of the polishing pad  102 . These distortions, in some cases, are caused by the additive manufacturing apparatus  120 , as described herein. The modified pattern differs from the original pattern of dispensing the feed material in that the modified pattern accounts for the distortions in the actual feature  310  relative to the desired feature. In this regard, in some implementations, the data indicative of the modified pattern is determined based on relative differences between the actual feature  310  and the desired feature  300 . 
     For example, as shown in  FIG.  6   , the data indicative of the modified shape includes data indicative of a modified feature  320 . Even though the top surface  302  of the desired feature  300  is planar, a top surface  322  of the modified feature  320  is nonplanar to compensate for the distortions of the top surface  312  of the actual feature  310  formed from the original pattern. The modified feature  320  is determined based on relative differences between the desired feature  300  and the actual feature  310 . The top surface  322  of the modified feature  320  is concave to compensate for the convexity of the top surface  312  of the actual feature  310 . In this regard, the data indicative of the modified shape is determined based on a combination of the data indicative of the desired shape and data indicative of the actual shape formed using the original pattern. 
     Referring back to  FIG.  4   , an initial layer of the feed material is dispensed by droplet ejection in accordance to a modified pattern ( 208 ). A resulting actual feature  330  is formed based on data indicative of the modified pattern to dispense the feed material, the modified pattern being determined based on the data indicative of the modified shape. 
     When the dispenser  128  is controlled to dispense the layers  130  of feed material in accordance to the data indicative of the modified pattern ( 210 ), a size and shape of a selected portion of the layers  130  of feed material that is cured can vary through a height of the feature. This is in contrast to the process to form the actual feature  310  in which the selected portion of cured feed material is consistent from layer to layer because the width of the desired feature  300  is consistent from layer to layer. 
     The modified feature  320  includes a concave portion  326  having a width that varies from layer to layer. A modified pattern to dispense the feed material to form the concave portion  326  differs from the corresponding portion of the original pattern to form the top portion of the desired feature  300  in that the selected cured portions of the layers  130  of feed material for the modified pattern have varying widths and shapes. These varying widths and shapes compensate for the distortions present in the actual feature  310  such that the resulting actual feature  330  formed using the modified pattern has reduced convexity compared to the actual feature  310  formed using the original pattern. For example, a top surface  332  of the actual feature  330  has increased planarity and flatness compared to the top surface  312  of the actual feature  310 . By intentionally controlling where feed material is being dispensed and cured, this correction defined by the modified pattern can better match the shape of the resulting polishing pad  102  to the original desired shape for the polishing pad  102 . 
     For example, the controller  129  can receive an initial data object, e.g., a computer aided design (CAD)-compatible file, e.g., a bitmap, that specifies the initial or intended shape of the object to fabricated. The data object can be stored on a non-transitory computer readable medium. The controller  129  can be programmed to generate a modified data object, e.g., a modified bitmap, that includes a feature to reduce rounding or beveling. The modified data object can be based on the intended shape as indicated by the initial data object as well as data indicating variations from the intended shape that are introduced by the additive manufacturing procedure. Thus, when the polishing pad  102  is fabricated using the modified data object, e.g., the modified bitmap, it more closely matches the desired design. 
       FIG.  7    illustrates another example of a desired feature  400  and an actual feature  410  formed based on dispensing and curing patterns determined in accordance to the data indicative of the desired feature  400 . In this particular example, the desired feature  400  had a width of 680 μm and a height of 500 μm, although other dimensions are appropriate in an implemented process. As illustrated, the actual feature  410  had a non-planar top surface and slanted side walls. 
     The desired feature  400  is a constant width feature, e.g., the partition  140  separating the grooves  138  of the polishing pad  102 . A constant width of the partitions  140  can improve wafer-to-wafer polishing uniformity. Furthermore, the polishing efficacy of the polishing pad  102  can be dependent on planarity of the polishing surface  103 . Using the processes described herein, data indicative of a modified pattern can be generated so that the resulting actual feature formed using the modified pattern more closely matches the desired feature  400 . In particular, the modified pattern corresponds to the original pattern with an additional correction profile determined using processes described herein. The additional correction profile compensates for the distortions of the actual feature  410  formed using the original pattern. 
     The examples of  FIGS.  8 A- 8 C  are cross-sections of the layers dispensed and cured by the manufacturing apparatus  120 . In some implementations, the data indicative of the shapes described herein include bitmap representations of the shapes to be formed or the shapes formed. Each bit of the bitmap can correspond to a voxel of a feature of the polishing pad  102  to be formed. 
     For example,  FIG.  8 A  illustrates first layer  804  to be deposited to form the desired feature  400 . To compensate for distortion, two outer regions of a partition  806  along two opposite edges of the feature are dispensed and deposited to form a perimeter of the partition  806 . The two outer regions can be defined by a first set of voxels  802   a  that provide a first region adjacent the first edge, and second set of voxels  802   b  that provide a second region adjacent the second edge. 
     As illustrated by  FIG.  8 B , after curing the first edge  802   a  and the second edge  802   b , a successive layer  808  is deposited atop the initial layer  804 . The successive layer  808  has sufficient material to fill in the remaining portion of the partition  806  between the first set of voxels  802   a  and the second set of voxels  802   b  that provide the edges, and deposit an additional layer atop the first set of voxels  802   a  and the second set of voxels  802   b . The concave shape that is formed at least partially compensates for distortions to the desired feature caused by the depositing the plurality of successive layers. 
     As illustrated by  FIG.  8 C , after depositing a plurality of successive layers  810 , the resulting feature, e.g., the partition  806 , can have a top surface  812  with edges  814  that are less subject to rounding and/or other forms of distortion. 
     The initial data object need not include the initial layer, whereas the modified data object includes the initial layer. In particular, the controller can determine the distortions that would occur to the object being fabricated, and then generate the initial layer to compensate for these distortions. For example, the controller can identify regions expected to be thin relative to the desired profile. These regions can be made thicker by assigning voxels corresponding to the regions to the initial layer. 
     Alternatively, the initial data object might include an initial layer, whereas the modified data object includes a modified initial layer. For example, regions that are expected to be thin relative to the desired profile can be made thicker by modifying voxels corresponding to the regions in the initial layer to deposit more material so as to make the initial layer thicker in those regions. 
     In some implementations, the second layer  808  can be formed by a different, second droplet ejection process. For example, the first layer  804  can be formed by ejection of droplets of a first composition and the second layer can be formed by ejection of droplets of a different second composition. For example, the first material can be a first polymer, whereas the second layer  808  can be a second polymer. The first material can be a composition that cures faster than the second material in otherwise similar environmental conditions. 
     As another example, depositing the first layer  804  can include applying a first curing radiation to the first layer  804 , whereas depositing the second layer  808  can include applying second curing radiation that cures the second layer  808  slower than the first curing radiation cures the first layer  804 . In some implementations, the first curing radiation and the second curing radiation have different wavelengths, different intensities, or different delays between ejection of the droplet and application of the respective curing radiation. 
     Although  FIGS.  8 A- 8 C  illustrate the first layer  804  as deposited directly on the support  134 , as shown in  FIG.  8 D , the first layer  804  could be formed over a plurality of layers  820  that form the body of the object being fabricated, e.g., the main body of the polishing pad. In this case, the first layer  820  is the first layer of the feature, e.g., the partition, that projects above the main body. In this case, the layer  804  still is located at the perimeter of the partition  806 . 
     The examples of  FIGS.  9 A- 9 C  are cross-sections of the layers dispensed and cured by the additive manufacturing apparatus  120 . In some implementations, the data indicative of the shapes described herein include bitmap representations of the shapes to be formed or the shapes formed. Each bit of the bitmap can correspond to a voxel of a feature of the polishing pad  102  to be formed. 
     For example,  FIG.  9 A  illustrates first layer  904  to be deposited to form the desired feature  400 . To compensate for distortion, two outer regions, including a first region adjacent the first edge and a second region adjacent the second edge, are dispensed and deposited to form a perimeter of a partition  906 . A first set of voxels  902   a  can provide the first region and a second set of voxels  902   b  can provide the second region. 
     As shown in  FIG.  9 B , after curing the first set of voxels  902   a  and the second set of voxels  902   b , a third set of voxels  908  is deposited between the boundary formed by the first layer  904 . That is, the successive layer  908  has sufficient material to fill in the remaining portion of the partition  906  and deposit region between the first set of voxels  902   a  and the second set of voxels  902   b . In this example, little to no concave shape is formed. Instead, the edge portions ( 902   a  and  902   b ) are formed by a process that better maintains verticality of the side wall. This at least partially compensates for distortions of the actual feature  410 . In effect, the edge portions ( 902   a  and  902   b ) serve as a wall to retain the remainder of the polishing pad precursor that will form a center portion of the partition  908 . 
     In some implementations, the third set of voxels  908  may be deposited by a different, second droplet ejection process. For example, the first and second set of voxels  902   a  and  902   b  be may be formed using droplets of a material, such as a first polymer, that cures faster than a material, e.g., a second polymer, used for the droplet that form the third set of voxels  908 . 
     In some implementations, depositing the first and second sets of voxels  902   a  and  902   b  can include a first curing radiation, while depositing the third set of voxels  908  can include a second curing radiation that cures the third set of voxels  908  slower than the first curing radiation cures first and second sets of voxels  902   a  and  902   b . In such an implementation, the first curing radiation and the second curing radiation can be at different wavelengths or different intensities. The apparatus can include different energy sources, e.g., different UV lights, to provide the different wavelengths or intensities. Alternatively, the same energy source can driven at different power levels to provide the different intensities. 
     As shown in  FIG.  9 C , this process can be repeated until a plurality of successive layers  910  are deposited. The edge portions  902   a ,  902   b  serve to form a wall that provides the vertical outer surface of partition  906 , with the interior portion of the partition provided by the third set of voxels  908 . Assuming that the capture material is not removed and provides a part of the polishing pad, then the edge portions  902   a ,  902   b  would be regions inside and abutting a perimeter of the partitions. The controller can be programmed to determine these regions from a data file. 
     As shown in  FIG.  9 D , in some implementations, the capture material, i.e., the material of the first and second set of voxels  902   a  and  902   b , can be removed, e.g., through a selective etching process. This leaves only the material of the third set of voxels  908  remaining. In this case, the edge portions  902   a ,  902   b  would be regions outside and abutting a perimeter of the partitions. Again, the controller can be programmed to determine these regions from a data file. 
     This technique can be advantageous if the third set of voxels  908  are formed of an optically transparent material, e.g., for the formation of a CMP window. This technique can also be advantageous for fixed abrasive, roll format pad designs. This technique can also be used for secondary polymer curing of the third set of voxels  908  where the materials of voxels  902   a  and  902   b  are used as masks. 
     In some implementations, at least the center portion of the partition, i.e., the third set of voxels, is subject to a secondary polymer curing process. The capture material, i.e., the material of the first and second set of voxels  902   a  and  902   b , can be removed after the secondary curing. 
     Although  FIGS.  9 A- 9 C  illustrate the layers  910  as deposited directly on the support  134 , as shown in  FIG.  9 E , the plurality of layers  910  that provide the partition can be formed over a plurality of layers  920  that form the body of the object being fabricated, e.g., the main body of the polishing pad. In this case, the first layer  904  is the first layer of the feature, e.g., the partition, that projects above the main body. 
       FIG.  10    illustrates an example process depositing a first set of successive layers  1010  by droplet ejection onto the support  134 . Depositing the first set of successive layers  1010  involves dispensing a polishing pad precursor  1008   a  from a first ejector  1006   a  to first set of regions, such as region  1004   b , corresponding to partitions of the polishing pad. In addition, depositing the first set of successive layers  1010  involves dispensing a sacrificial material  1008   b  from a second injector  1006   b  to a set of second regions, such as region  1004   a , corresponding to grooves of the polishing pad. The first ejector  1006   a  and the second ejector  1006   b  can draw from different feed material sources. In particular, the sacrificial material  1008   b  can be deposited simultaneously with the pad pre-cursor  1008   a . The simultaneous depositing allows an entire layer to be deposited with one pass of the first ejector  1006   a  and the second ejector  1006   b . The sacrificial material  1008   b  at least partially reduced the distortion that can occur if the pad-precursor  1008   a  is deposited on its own by keeping the deposited pad pre-cursor  1008   a  in place before and during curing. 
     After the first set of successive layers  1010  is deposited, a second plurality of successive layers  1012  is deposited by droplet ejection over the first set of successive layers  1010 . The second set of successive layers  1012  spans both the first regions  1004   b  and the second regions  1004   a . In some implementations, some or all of the second set of successive layers  1012  correspond to a lower portion of the polishing layer of the polishing pad and are formed from the polishing pad pre-cursor  1008   a . In some implementations, some or all of the second set of successive layers  1012  are formed to have a different material composition than the first set of successive layers  1010 , and can correspond to a backing layer, e.g., a sub-pad, of the polishing pad. Such layers can be formed from a different material, e.g., a different precursor, or the same precursor can be ejected but treated differently, e.g., subjected to more or less curing to provide a different degree of polymerization and thus a different hardness. 
     In this implementation, the polishing pad is manufactured upside down. That is the uppermost layer of deposited material corresponds with a base, or lower portion, of the polishing pad. The first set of successive layers  1010  and the second set of successive layers  1012  provide a body of the polishing pad. 
     Once depositing of the polishing pad material is complete, the body of the polishing pad is removed from the support  134 . The sacrificial material  1008   b  is removed from the body, e.g., by selectively etching the sacrificial material, or by lifting away the body of the polishing pad while the sacrificial material remains on the support, to provide the polishing pad having a polishing surface with partitions separated by grooves. 
     In some implementations, a third set of successive layers is deposited by droplet ejection over the second set of successive layers  1012 . The third set of successive layers can have a different composition than the second set of successive layers  1012 . The second set of successive layers  1012  can correspond to a lower portion of the polishing pad (also known as a sub-pad of the polishing layer). 
     Referring to  FIG.  11 A , in some implementations the top surface of the support  134  includes a texture, e.g., projections  1100 , and the polishing pad precursor  1008  is ejected to fill the space between the projections so as to generate a complementary texture, e.g., the grooves, on the polishing pad. In particular, depositing a first set of successive layers  1110  involves dispensing a polishing pad precursor  1108  from an ejector  1106  to form the partitions first set of regions, such as region  1104 , corresponding to partitions of the polishing pad. After the first set of successive layers  1110  is deposited, a second plurality of successive layers  1112  is deposited over the first set of successive layers  1010 . The second set of successive layers  1112  spans both the first regions  1004  and the projections  1110 . 
     In some implementations, some or all of the second set of successive layers  1112  correspond to a lower portion of the polishing layer of the polishing pad and are formed from the polishing pad pre-cursor  1008 . In some implementations, some or all of the second set of successive layers  1112  are formed to have a different material composition than the first set of successive layers  1110 , and can correspond to a backing layer, e.g., a sub-pad, of the polishing pad. Such layers from can be formed from a different material, e.g., a different precursor, or the same precursor can be ejected but treated differently, e.g., subjected to more or less curing to provide a different degree of polymerization and thus a different hardness. 
     Referring to  FIG.  11 B , rather than the texture being formed in the support  134 , the texture can be provided by a film  1120  that is placed on the support  134 . In this case, the polishing pad precursor is ejected onto the film  1120 , and the film  1120  is removed from polishing pad after manufacture. 
     For any of the various implementations discussed above, instead of the dispenser  128  scanning over the support  134 , the support can be moveable. For example, referring to  FIG.  12   , support  134  could be a continuous belt. The belt  134  can be driven by drive wheels  160  that are powered by one or more actuators to move the belt (as shown by arrow B) to carry the dispensed polishing pad precursor under the energy source  131  to cure the precursor to form the polishing pad as a sheet. Although only one dispenser  128  and energy source  131  are shown, there can be multiple dispensers and energy sources arranged in series along the belt  134  such that multiple layers can be formed in succession on the belt to form the full thickness of the polishing pad. The cured polishing pad sheet  162  can then be lifted from the belt  134  and wound around a receiving roller  164 . 
     The controller, e.g., the controller  129 , can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations of them. The controller can include one or more computer program products, i.e., one or more computer programs tangibly embodied in an information carrier, e.g., in a non-transitory machine readable storage medium or in a propagated signal, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
     The controller  129  and other computing devices part of systems described can include non-transitory computer readable medium to store a data object, e.g., a computer aided design (CAD)-compatible file that identifies the pattern in which the feed material should be formed for each layer. For example, the data object could be a STL-formatted file, a 3D Manufacturing Format (3MF) file, or an Additive Manufacturing File Format (AMF) file. For example, the controller could receive the data object from a remote computer. A processor in the controller  129 , e.g., as controlled by firmware or software, can interpret the data object received from the computer to generate the set of signals necessary to control the components of the additive manufacturing apparatus  120  to deposit and/or cure each layer in the desired pattern. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. 
     The approach shown in  FIG.  6    is to deposit one or more additional layers of feed material to compensate for the height of the feature being less than desired. For example, one or more additional layers of feed material can be deposited in regions where rounding or beveling occurs. However, alternatively or in addition, the amount of feed material deposited in a layer can compensate for the height feature being less than desired. For example, the size of droplets or the number of droplets ejected can be increased for voxels located in regions where the height feature is less than desired, e.g., where rounding or beveling occurs. 
     In some implementations, a distribution of volume of the feed material is modified depending on a location at which the droplets  124  are to be dispensed. A volume of the droplets  124  of feed material is varied during the dispensing operation. For example, referring back to  FIG.  6   , the volume of the droplets  124  for forming edges  322   a ,  322   b  of the feature can be less than the volume of droplets  124  for forming the interior portion  322   c  of the feature. The controller  129  determines the appropriate weight for forming the edges  322   a ,  322   b  and the weight for forming the interior portion  322   c  based on the material properties of the feed material. The dispenser  128  can dispense less feed material to minimize roll off of the feed material. As the dispenser  128  moves to form the interior portion  322   c  of the feature, the volume of droplets is increased. In some implementations, the volume of the droplets  124  define a gradient from the edges  322   a ,  322   b  to the center of the feature. Depending on the wetting effect of the feed material, this type of volume control can be used to modulate an amount of feed material cured when the energy source  131 , if present, is operated. For example, if the energy source  131  is scanned across the support  134  to cure different portions of the dispensed feed material, drop volume control can allow for less feed material to roll off for each pass of the energy source  131  while injecting more feed material at the edges  322   a ,  322   b  of the feature to reduce the beveling effect described herein. 
     In some implementations, multiple types of feed material are dispensed. The additive manufacturing apparatus  120  includes, for example, two or more dispensers, each dispenser dispensing a different type of feed material. In some cases, a single dispenser, e.g., the dispenser  128 , receives multiple types of feed material and dispenses a mixture of the multiple types of feed material. Because properties of a first type of feed material may vary from properties of a second type of feed material, the modification to the original pattern to dispense the first type of feed material may include a greater or smaller amount of scaling than the modification to the original pattern to dispense the second type of feed material. Alternatively, if droplet weight is controlled, the weights of the droplets of the first type of feed material can be controlled to be higher or lower than the weights of the droplets of the second type of feed material. In some cases, the size of the droplets of the first type of feed material can be controlled to be larger or smaller than the sizes of the droplets of the second type of feed material. 
     In some implementations, multiple types of feed material form different portions of the polishing pad  102 , for example, to form the polishing layer  122  and the backing layer  136 , or to form different portions of the polishing layer  122 , e.g., to provide a polishing layer with polishing properties that vary laterally across the polishing surface. The second type of feed material can include the first type of feed material with an additive that alters the properties of the second type of feed material relative to the first type of feed material. The additive includes, for example, a surfactant that can adjust properties of the uncured feed material, for example, zeta potential, hydrophilicity, etc. 
     Thickness of each layer of the layers of feed material and size of each of the voxels may vary from implementation to implementation. In some implementations, when dispensed on the support  134 , each voxel can have a width of, for example, 10 μm to 50 μm (e.g., 10 μm to 30 μm, 20 μm to 40 μm, 30 μm to 50 μm, approximately 20 μm, approximately 30 μm, or approximately 50 μm). Each layer can have a predetermined thickness. The thickness can be, for example, 1 to 80 um, e.g., 2 to 40 μm (e.g., 2 μm to 4 μm, 5 μm to 7 μm, 10 μm to 20 μm, 25 μm to 40 μm). 
     Although the method and apparatus have been described in the context of fabrication of a polishing pad, the method and apparatus can be adapted for fabrication of other articles by additive manufacturing. In this case, rather than a polishing surface, there would simply be a top surface of the object being fabricated, and there would be recesses in the top surface. The modified pattern can at least partially compensate for distortions caused by the additive manufacturing system. 
     In addition, although the method and apparatus haves been described in the context of fabrication by droplet ejection, the method apparatus can be adapted for fabrication by other additive manufacturing techniques, e.g., selective powder dispensing followed by sintering. 
     Accordingly, other implementations are within the scope of the claims.