Patent Publication Number: US-11638979-B2

Title: Additive manufacturing of polishing pads

Description:
TECHNICAL FIELD 
     This disclosure relates to polishing pads utilized in chemical mechanical polishing. 
     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 techniques employ planarization of a layer on the substrate. For example, 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 technique of planarization. In application, this planarization technique may mount the substrate 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 the substrate against the polishing pad. A polishing liquid, such as slurry with abrasive particles, may be supplied to the surface of the polishing pad. One objective of a chemical mechanical polishing may be 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 for CMP may include “standard” pads and fixed-abrasive pads. A standard pad may have as a polyurethane polishing layer with a durable roughened surface, and can also include a compressible backing layer. In contrast, a fixed-abrasive pad has abrasive particles held in a containment media, and can be supported on a generally incompressible backing layer. 
     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. More recently, 3D printing techniques have been proposed for manufacturing of polishing pads. The pads may be formed from acrylate polymers, as described in U.S. Pat. Nos. 10,399,201 and 10,384,330, and US Patent Application Publication Nos. 2020/0001433 and 2019/0047112. 
     SUMMARY 
     In implementations described herein, an interpenetrating network is used to form a polishing pad with a window region. The properties of the polishing pad and the window region may be accurately controlled by adjusting a ratio of cationic materials to free-radical materials in a blend that is photopolymerized to form the polishing pad. Further, different blends may be used in different nozzles of a three-dimensional printer to adjust the properties in different regions of the polishing pad, such as the window region, the polishing surface, the body, or the backing, among others. 
     An implementation described herein provides a method of generating a formulation for forming a polishing pad including an interpenetrating polymer network. The method includes blending a cationically polymerized material with a free-radically polymerized material to form a precursor blend, wherein a ratio of the cationically polymerized material to the free-radically polymerized material in the oligomer blend is selected to control properties of the polishing pad. A first amount of a mixture of a free radical photoinitiator and a cationic photoinitiator is blended with the precursor blend to form a polishing pad blend. A second amount of the mixture of a free radical photoinitiator and a cationic photoinitiator is blended with the precursor blend to form a window blend. The polishing pad blend and the window blend are provided to a manufacture to generate a polishing pad having a window using a three-dimensional printer. 
     Another implementation described herein provides a method of manufacturing a polishing pad including an interpenetrating polymer network. The method includes obtaining a polishing pad blend including a free radical photoinitiator, a free-radically polymerized material, a cationic photoinitiator, and a cationically polymerized material, wherein a ratio of the free-radically polymerized material to the cationically polymerized material is selected to control properties of the polishing pad. A window blend is obtained, wherein the window blend includes the free radical photoinitiator, wherein a concentration of the free radical photoinitiator is lower than the free radical photoinitiator in the polishing pad blend. The window blend includes the free-radically polymerized material. The window blend includes the cationic photoinitiator, wherein the concentration of the cationic photoinitiator is lower than the concentration of the cationic photoinitiator in the polishing pad blend. The window blend also includes the cationically polymerized material, wherein a second ratio of the free-radically polymerized material to the cationically polymerized material in the window blend is selected to control the properties of a window in the polishing pad. A raw polishing pad is formed in a polishing pad region from the polishing pad blend by printing droplets of the polishing pad blend in the polishing pad region. A window is formed in a window region from the window blend by printing droplets of the window blend in the window region. The raw polishing pad is irradiated to initiate free radical photopolymerization of the free radically polymerized material and cationic photopolymerization of the cationically polymerized material. 
     Another implementation described herein provides a polishing pad for a semiconductor fabrication operation. The polishing pad includes a polishing region and a window region, wherein both regions include an interpenetrating polymer network formed from a free-radically polymerized material and a cationically polymerized material. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a schematic cross-sectional side view of an example polishing pad. 
         FIG.  1 B  is a schematic cross-sectional side view of another example polishing pad. 
         FIG.  1 C  is a schematic cross-sectional side view of yet another example polishing pad. 
         FIG.  2    is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station. 
         FIG.  3    is a schematic side view illustrating a substrate in contact with the polishing pad of  FIG.  1 A . 
         FIG.  4    is a drawing of an interpenetrating polymer network. 
         FIG.  5    is a block flow diagram of a method of forming a polishing pad by the 3D printing of an interpenetrating polymer network. 
         FIG.  6    is a block flow diagram of a method for forming a polishing pad with an inherent window region by the 3D printing of an interpenetrating polymer network. 
         FIG.  7 A  is a plot of Tan δ measured by dynamic mechanical analysis (DMA) of pure cationic polymers and interpenetrating polymer networks as a function of increasing cationic polymer composition. 
         FIG.  7 B  is a plot of modulus values for E30 and E90 as a function of Tg and cationic polymer composition. 
         FIG.  8 A  is a plot of percent cure at the bottom of a droplet after UV curing as a function of polymer composition, 
         FIG.  8 B  is a plot of percent cure at the bottom of a droplet after UV curing and annealing as a function of polymer composition. 
         FIG.  9 A  is a plot of percent cure at the top of a droplet after UV curing as a function of polymer composition. 
         FIG.  9 B  is a plot of percent cure at the top of the droplet after UV curing and annealing as a function of polymer composition. 
         FIG.  10    is a plot of light transmittance as a function of wavelength (nm). 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     To polish a wafer, a polishing pad and polishing head apply mechanical energy to the substrate. The pad also helps to control the transport of a slurry which interacts with the substrate during the polishing process. Because polishing pads are typically made from viscoelastic polymeric materials, the mechanical properties of a polishing pad (e.g., elasticity, rebound, hardness, and stiffness), and the chip manufacturing process (CMP) processing conditions have a significant impact on the CMP polishing performance on both an integrated circuit (IC) die level (microscopic or nanoscopic) and wafer or global level (macroscopic), For example, CMP process forces and conditions, such as pad compression, pad rebound, friction, and changes in temperature during processing, and abrasive aqueous slurry chemistries will impact polishing pad properties and thus CMP performance. 
     Current pad materials and methods to produce them limit the manipulation and fine control bulk pad properties such as storage modulus (E′) and loss modulus (E″), which play critical roles in pad performance. Therefore, uniform CMP requires a pad material and polishing surface with a predictable and finely controlled balance of storage modulus E′ and loss modulus E″, that are further maintained over a CMP processing temperature range, from, for example, about 30° C. to about 90° C. The polishing surface also requires features, such as grooves and channels, to carry polishing liquid. 
     However, conventional pad production via traditional bulk polymerization and casting and molding techniques only provide a degree of pad property (e.g., modulus) control, because the pad is a random mixture of phase separated macromolecular domains that are subject to intramolecular repulsive and attractive forces and variable polymer chain entanglement. For example, the presence of phase separated macroscopic structural domains in the bulk pad may yield an additive combination of non-linear material responses, such as a hysteresis in the storage modulus (E′) over multiple heating and cooling cycles that typically occur during the CMP processing of batches of substrates, which may result polishing non-uniformities and unpredictable performance across the batch of substrates. 
     Because of the drawbacks associated with conventional polishing pads and their methods of manufacture, there is a need for new polishing pad materials and new methods of manufacturing polishing pads that provide control of pad feature geometry, and fine control of the pad&#39;s material, chemical and physical properties. Such improvements are expected to yield improved polishing uniformity at both a microscopic level and macroscopic level, such as over the entire substrate. 
     Some aspects of the present disclosure are directed to a formulation comprising monomers that form an interpenetrating polymer network (IPN) for the formation of a polishing layer of a polishing pad. In one implementation, the formulation is applied to form the polishing pad by an additive manufacturing process, such as three dimensional (3D) printing. In 3D printing, a printhead ejects droplets of the formulation onto a surface from a nozzle, then cures the droplets with a light, e.g., ultraviolet light, from a light source, such as an LED or focused lamp in the printer. The 3D printing allows for different materials to be deposited in different portions of the polishing layer, which provides significant advantages over casting. For example, as described herein, a window blend is deposited in a window region used for end point determination during the polishing operation. A polishing pad blend is used for regions outside of the window region. The window blend will generally have a lower concentration of the photoinitiators, allowing more transmission of light. 
     The technique may include preparing a blend that includes free radically polymerized materials, such as urethane acrylate oligomers, ester acrylates, or ether acrylates, among others. The blend also includes cationically polymerized materials, such as epoxy oligomers. Other components, including monomers, stabilizers, and the like, are then added to adjust properties of the blend, such as the viscosity. Free radical and cationic photoinitiators are added to form an active blend. With the oligomers of the present techniques, the active blend may have a relatively low viscosity to facilitate 3D printing of the formulation for manufacturing CMP polishing pads. The formulation may then be 3D printed to make the CMP Pads. The CMP pads may have a high modulus and good elasticity for semiconductor applications. 
     Referring to  FIG.  1 A- 1 C , a polishing pad  102  includes a polishing layer  104 . As shown in  FIG.  1 A  the polishing pad can be a single-layer pad that consists of the polishing layer  104 , or as shown in  FIG.  1 C  the polishing pad can be a multi-layer pad that includes the polishing layer  104  and at least one backing layer  106 . 
     The polishing layer  104  can be a material that is inert in the polishing. The material of the polishing layer  104  can be a plastic, e.g., an interpenetrating network formed from a blend of a urethane acrylate oligomer and in epoxy oligomer. In some implementations the polishing layer  104  is a relative durable and hard material. For example, the polishing layer  104  can have a hardness of about 40 to 80, e.g., 50 to 65, on the Shore D scale. 
     As shown in  FIG.  1 A , the polishing layer  104  can be a layer of homogeneous composition, e.g., formed from the IPN with no additional components. In some implementations, the polishing layer includes pores, e.g., small voids. The pores can be 50-100 microns wide. Such pores need not detract from the generally homogeneous composition of the polishing layer  140 . 
     As shown in  FIG.  1 B  the polishing layer  104  can include abrasive particles  108  held in a matrix  110  of plastic material, e.g., an interpenetrating network of polyurethane and epoxide. The abrasive particles  108  are harder than the material of the matrix  110 . The abrasive particles  108  can be from 0.05 weight percent (wt. %) to 75 wt. % of the polishing layer. For example, the abrasive particles  108  can be less than 1 wt. % of the polishing layer  104 , e.g., less than 0.1 wt. %. Alternatively, the abrasive particles  108  can be greater than 10 wt. % of the polishing layer  104 , e.g., greater than 50 wt. %. The material of the abrasive particles can be a metal oxide, such as cerin, alumina, or silica, or any combinations thereof. 
     The polishing layer  104  can have a thickness D 1  of 80 mils or less, 50 mils or less, or 25 mils or less. Because the conditioning process tends to wear away the cover layer, the thickness of the polishing layer  104  can be selected to provide the polishing pad  102  with a useful lifetime, e.g., 3000 polishing and conditioning cycles. 
     On a microscopic scale, the polishing surface  112  of the polishing layer  104  can have rough surface texture, e.g., a root mean squared (rms) surface-roughness of 2-4 microns. For instance, the polishing layer  104  can be subject to a grinding or conditioning process to generate the rough surface texture. In addition, 3D printing can provide small uniform features, e.g., down to 200 microns. 
     Although the polishing surface  112  can be rough on a microscopic scale, the polishing layer  104  can have good thickness uniformity on the macroscopic scale of the polishing pad itself. This uniformity may refer to the global variation in height of the polishing surface  112  relative to the bottom surface of the polishing layer, and does not count any macroscopic grooves or perforations deliberately formed in the polishing layer. The thickness non-uniformity can be less than 1 mil. 
     In some implementations, at least a portion of the polishing surface  112  can include a plurality of grooves  114  formed therein for carrying slurry. The grooves  114  may be of nearly any pattern, such as concentric circles, straight lines, a cross-hatched, spirals, and the like. In implementations with grooves present, then on the polishing surface  112 , the plateaus between the grooves  114  can be, for example, 25-90% of the total horizontal surface area of the polishing pad  102 . Thus, the grooves  114  can occupy 10%-75% of the total horizontal surface area of the polishing pad  102 . The plateaus between the grooves  114  can have a lateral width of about 0.1 to 2.5 mm. 
     In some implementations, e.g., if there is a backing layer  106 , the grooves  114  can extend entirely through the polishing layer  104 . In some implementations, the grooves  114  can extend through about 20-80%, e.g., at 40-60%, of the thickness of the polishing layer  104 . The depth of the grooves  114  can be 0.25 to 1 mm. For example, in a polishing pad  102  having a polishing layer  104  that is 40-60 mils thick, e.g., 50 mils thick, the grooves  114  can have a depth D 2  of about 15-25 mils, e.g., 20 mils. 
     The backing layer  106  can be softer and more compressible than the polishing layer  104 . The backing layer  106  can have a hardness of 80 or less on the Shore A scale, e.g., a hardness of about 60 Shore A or less. The backing layer  106  can be thicker or thinner than (or the same thickness as) the polishing layer  104 . 
     In certain implementations, the backing layer  106  can be an open-cell or a closed-cell foam, such as polyurethane or polysilicone with voids, so that under pressure the cells collapse and the backing layer compresses. Examples of material for the backing layer are PORON 4701-30 from Rogers Corporation, in Rogers, Conn., or SUBA-IV from Rohm &amp; Haas. The hardness of the backing layer  106  can generally be adjusted by selection of the layer material and porosity. Alternatively, the backing layer  106  can be formed from the same precursor and have the same porosity as the polishing layer, but have a different degree of curing, or a different IPN composition, so as to have a different hardness. In these implementations, the ratio between the free radically polymerized oligomer and the cationically polymerized oligomer can be adjusted to change the modulus of the backing material. 
     Turning now to  FIG.  2   , one or more substrates  202  can be polished at a polishing station  204  of a CMP apparatus. A description of an applicable polishing apparatus can be found in U.S. Pat. No. 5,738,574. 
     The polishing station  204  can include a rotatable platen  206  on which is placed the polishing pad  102 . During polishing, a polishing liquid  208 , e.g., abrasive slurry, can be supplied to the surface of polishing pad  102  by a slurry supply port or combined slurry/rinse arm  210 . The polishing liquid  208  can contain abrasive particles, a pH adjuster, or chemically active components. 
     The substrate  202  is held against the polishing pad  102  by a carrier head  212 . The carrier head  212  is suspended from a support structure, such as a carousel, and is connected by a carrier drive shaft  214  to a carrier head rotation motor so that the carrier head can rotate about an axis  216 . As the carrier head  212  is rotated about the axis  216 , the rotatable platen  206  is rotated by a platen axis  218  around a vertical axis  220 . The relative motion of the polishing pad  102  and the substrate  202  in the presence of the polishing liquid  208  results in polishing of the substrate  202 . 
     An in-situ monitoring system, e.g., an optical endpoint monitoring system, eddy current monitoring system, motor current monitoring system, etc., can be used in a CMP process to determine when a substrate  202  or a bulk film has been polished to a desired thickness when the contact material has been removed from the field (upper surface) of a layer. This can be performed using optical techniques, for example, in which a light beam  222  is directed towards the substrate  202  through a window  224  in the rotatable platen  206  and through a polishing pad window  226 . The light beam  222  may be intermittently reflected from the surface of the substrate  202  as the rotatable platen  206  rotates. An optical system  228  may use interferometry or changes in the reflectance of the substrate  202  to determine the removal of material from the surface of the substrate  202 . As described herein, the polishing pad window  226  is generally made from the same material as the polishing layer  104 , for example, formed by 3D printing of the polishing pad window  226  during the 3D printing of the polishing pad  102 . For optical clarity, the polishing pad window  226  may include a lower amount of photoinitiators, which absorb light. Further, the polishing pad window  226  may not include the abrasive particles  108 , described with respect to  FIG.  1   . 
     Referring to  FIG.  3   , at least the polishing layer  104  ( FIG.  1   ) of the polishing pad  102  is manufactured utilizing 3D printing. In the manufacturing, thin layers of material are progressively deposited and fused. For example, droplets  302  of a formulation of pad precursor material can be ejected from a nozzle  304  of a droplet ejecting printer  306  to form a layer  308 . The droplet ejecting printer  306  is similar to an inkjet printer, but employs the pad precursor material rather than ink. The nozzle  304  translates (shown by arrow A) across a support  310 . 
     For a first layer  312  deposited, the nozzle  304  can eject onto the support  310 . For subsequently deposited layers  314 , the nozzle  304  can eject onto the already solidified material  316 . After each layer  308  is solidified, a new layer is then deposited over the previously deposited layer until the full 3-dimensional polishing layer  104  is fabricated. Each layer is applied by the nozzle  304  in a pattern stored in a 3D drawing computer program that runs on a computer  318 . Each layer  308  is less than 50% of the total thickness of the polishing layer  104 , e.g., less than 10%, e.g., less than 5%, e.g., less than 1%. 
     During the deposition of the layers  312  and  314 , a window region  320  may be deposited. The window region  320  may be formed from an IPN blend with a lower concentration of photoinitiators to improve the transparency for the measurement wavelengths, e.g., with a cutoff below 350 nm, below 375 nm, or below 400 nm. As used herein, a cutoff indicates a point wherein the transmittance drops below 50% of the transmittance at 800 nm as the wavelength is decreased. Further, the material used in the window region may not include abrasive particles, further increasing the light transmittance. The window region  320  can be located in the groove(s) between raised plateaus as illustrated, or the raised plateaus can provide part of the window region  320 . 
     To change the material deposited on the pad, the droplet ejecting printer  306  may include additional nozzles that are coupled to reservoirs holding different materials. For example, one reservoir may hold a normal IPN blend material used to form most of the polishing layer  104 , while a second reservoir feeding another nozzle may include an IPN blend material with a lower concentration of the photoinitiators, and no abrasives. For further customization of the properties of the pads, additional nozzles may print only the epoxy oligomer blend with a cationic photoinitiator, while other nozzles may print only the urethane acrylic oligomer with the free radical photoinitiator, allowing customization of the blend used for a region. Other combinations, such as changes in a blend ratio that includes both epoxy oligomers and urethane acrylic oligomers, may be used. 
     The support  310  can be a rigid base, or be a flexible film, e.g., a layer of polytetrafluoroethylene (PTFE). If the support  310  is a film, the support  310  can form a portion of the polishing pad  102 . For example, the support  310  can be the backing layer  106  or a layer between the backing layer  106  and the polishing layer  104 . Alternatively, the polishing layer  104  can be removed from the support  310 , and applied to a backing layer  106 . 
     Solidification can be accomplished by polymerization. For example, the layer  308  of pad precursor material can be a blend that includes an epoxy oligomer and a urethane oligomer, along with the relevant photoinitiators. The blend can be polymerized in-situ by light, e.g., ultraviolet (UV) curing using a UV source  322 . The UV source  322  may emit a broad UV spectrum, including, for example, UV-A and UV-B, or UV-A, UV-B, and UV-C. In some implementations, a narrow bandwidth source, such as a UV LED laser may be used. 
     The pad precursor material can be cured effectively immediately upon deposition, or an entire layer  308  of pad precursor material can be deposited and then the entire layer  308  be cured simultaneously. Further, a partial cure may be effected during printing, with a full cure being carried out by annealing under a large UV source, or in an oven. As the epoxy oligomers cure more slowly that the urethane acrylate oligomers, a hold time after printing, even without additional light or heat annealing, may be used to achieve full properties. 
     In addition to 3D printing, other technologies may be used to form the polishing layer  104  from the IPN blend, as described herein. For example, the polishing layer  104  may be cast by placing the IPN blend in a mold, and irradiating to form the final shape. A window may be formed using leaving a circular mold to form a hole in the polishing layer  104  during the casting procedure. After the polishing layer  104  is cured, the hole can be filled with an IPN blend that includes a lower amount of photoinitiator. Curing the window material may couple it to the polymer structure of the surrounding polishing layer  104 . 
     The 3D printing generally avoids the need for making molds, which can be relatively expensive and add time in the manufacturing. The 3D printing may eliminate several conventional-pad manufacturing steps, such as molding, casting, and machining. Further, in conventional methods, windows may pop out during CMP or may not be coplanar with respect to the surface of the pads resulting in uneven surfaces. Additionally, tight tolerances can generally be achieved in 3D printing due to the layer-by-layer printing. Also, one printing system (with printer  306  and computer  318 ) can be employed to manufacture a variety of different polishing pads, simply by changing the pattern stored in the 3D drawing computer program in certain implementations. Further, the use of additional nozzles can allow for customization of properties during the printing process. 
     In some implementations, the backing layer  106  can also be fabricated by 3D printing. For example, the backing layer  106  and polishing layer  104  could be fabricated in an uninterrupted operation by the printer  306 . The backing layer  106  can be provided with a different hardness than the polishing layer  104 , for example, by applying a different amount of curing, e.g., a different intensity of UV radiation. In some implementations, the backing layer is formed using a different IPN blend. The backing layer  106  may also include a window region, aligned with the window region  320  in the polishing layer  104 . 
     In other implementations, the backing layer  106  is fabricated by a conventional process and then secured to the polishing layer  104 . For instance, the polishing layer  104  can be secured to the backing layer  106  by a thin adhesive layer, e.g., as a pressure-sensitive adhesive. 
       FIG.  4    is a schematic drawing of an interpenetrating polymer network (IPN)  400  of two cross-linked polymers. As used herein, an IPN  400  may be defined as an interpenetrating network of two or more polymers, such as polymers  402  and  404 , in which each of the polymers  402  and  404  is crosslinked with other polymer chains of substantially the same type, i.e., polymer chains of polymer  402  are cross-linked with other polymer chains of polymer  402 , and polymer chains of polymer  404  are cross-linked with other polymer chains of polymer  404 . Some small amount of cross-linkage between polymers  402  and  404  is permissible, e.g., less than 5%, e.g., less than 1% of the linkages. An IPN  400  may be formed from a blend of monomers or oligomers, e.g., the IPN blend, as used herein, with at least one of the polymers synthesized in the presence of another. 
     The IPN  400  may be formed using cross-linking polymerization of two multifunctional oligomers that polymerize through different mechanisms, such as free radical polymerization and cationic polymerization. In the example of  FIG.  4   , a polyurethane acrylate oligomer is cross-linked though free-radical links  406  to form polymer  402 , while an epoxy oligomer is cross-linked through cationic links  408  to form an interpenetrating polymer  404 . This produces a “physically cross-linked” network wherein cross-linked polymer chains of one polymer, e.g., polymer  402 , are entangled with and/or penetrate the network formed by cross-linked chains of the other polymer, e.g.,  404 . Each individual network retains its individual properties, so that synergistic improvements in properties including E′30, E′90, E′30/E′90, strength, toughness, compression and elongation may be made through changes to the concentrations of the components. 
     An IPN  400  can be distinguished from the other multiple systems or networks through their multi-continuous structure ideally formed by the physical entanglement or interlacement of at least two polymers that are in intimate physical contact but may or may not be chemically bounded to one another. Photoinitiated polymerization has been a very efficient way to obtain IPNs and is initiated by a monomer mixture such as acrylates, which polymerize by a radical mechanism, and epoxides, which polymerize by a cationic mechanism. UV irradiation has been explored heavily to produce IPNs. 
       FIG.  5    is a method  500  of forming a polishing pad from an IPN. The polishing pad may be a CMP pad. The method  500  begins at block  502 , with the selection of a urethane acrylate oligomer and an epoxy oligomer. The oligomers are chosen to control the properties of a polishing pad. 
     Typical material composition properties that can be selected using the methods and material compositions described herein include storage modulus E′, loss modulus E″, hardness, tan δ, yield strength, ultimate tensile strength, elongation, glass transition temperature (Tg) and other related properties. It is desirable for the window material to have a similar storage modulus as the surrounding polishing elements so that the window material wears at a similar rate and does not extend above or below the surface or the polishing pad over the lifetime. The properties that can be controlled by include the modulus, or stiffness, of the polishing pad. Different moduli can be selected for different applications and lifespan. Generally, three modulus ranges may be used, as shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Moduli used for polishing pads 
               
            
           
           
               
               
               
               
            
               
                   
                 Low Modulus 
                 Medium Modulus 
                 High Modulus 
               
               
                   
                 Compositions 
                 Compositions 
                 Compositions 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 E′30 
                 5 MPa-100 MPa 
                 100 MPa-500 MPa 
                 500 MPa-3000 MPa 
               
               
                   
               
            
           
         
       
     
     The moduli may be controlled by the selection of the urethane acrylate oligomer and the epoxy oligomer as well as by the ratio of the urethane acrylate oligomer to the epoxy oligomer. Other properties may be controlled by the selections of the oligomers as well as the ratios of the oligomers used to form the IPNs. Accordingly, the control of ratios and oligomers allow the properties of polishing pads to be tuned to create a desired composite of properties within a layer and/or layer by layer, such as those properties including E′30, E′90, E′30/E′90, strength, toughness, compression, and elongation. 
     In various implementations, the IPN blend, or active blend, includes a material that is polymerized by a free radical mechanism, such as a urethane acrylate oligomer, an ester acrylate, an ether acrylate, or monomer. The IPN blend also includes a material that is polymerized by a cationic mechanism, such as an epoxy oligomer. The IPN blend also includes a free radical photoinitiator and a cationic photoinitiator. Each of the oligomers, such as the urethane acrylate oligomer and the epoxy oligomer, can be part of a blend of components. In some implementations, the IPN blend also includes a monomer and an additive. The additive may be, for example, silicones, polyether silicones, or surfactants. In implementations, the formulation need not include a solvent as solvent can act as plasticizer to reduce or adversely affect mechanical properties. 
     In various implementations described herein, the free radically polymerized formulation includes a polyester urethane acrylate, such as BR744SD, BR744BT, BRC 841, BRC 843, BR 741, BR742S, BR 771F, or BR 7432G130, among others available in the Bomar product line from Dymax Oligomers and Coatings of Torrington, Conn., USA. Other materials that may be used in the free radically polymerized formulation include trimethylolpropane triacrylate (TMPTA) (available as SR351H from Sartomer Arkema of Exton, Pa., USA), and CN 8881, 9004, 9009, 99030, 9031, 964, 981, 991, or 996 (available in the Sartomer N3XT dimension series from Sartomer). Any number of other monomers and oligomers, such as acrylic acid, may be included in the formulation. Further, crosslinking agents used in the formulations could include EB40 from Allnex, TPGDA, or multifunctional acrylates, among others. 
     In various implementations described herein, the free-radical, or acrylate, photoinitiators used include an acyl phosphine oxide (such as Omnirad 819 available from IGM Resins of Waalwijk, The Netherlands), an alpha hydroxy ketone (such as Omnirad 2959, available from IGM Resins), or 2-Methyl-4′-(methylthio)-2-morpholinopropiophenone (available as Omnirad 907 from IGM Resins). Similar free radical photoinitiators may be used in addition to, or in combination with, these initiators. The choice of the photoinitiator may be made based on the viscosity of the resulting solution and the absorbance of the photoinitiator, such as in a window application as described herein. 
     In various implementations described herein, the cationic photoinitiator is Omnicat 250, available from IGM Resins. Omnicat 250 is a 75% blend of (4-methylphenyl)[4-(2-methylpropyl) phenyl]-, hexafluorophosphate(1-) in 25% propylene carbonate. Other cationic photoinitiators may be used instead of, or in addition to, Omnicat 250, such as other cationic photoinitiators in the Omnicat product line from IGM Resins, including, for example, sulfonium hexafluro phosphate (Omnicat 270), or Omnicat 250, 440, 432, BL 550, and 320, among others. The choice of the photoinitiator may be made based on the viscosity of the resulting solution, and the absorbance of the photoinitiator, such as in a window application as described herein. 
     At block  504 , the IPN blend is made from the urethane acrylate oligomer and the epoxy oligomer at a ratio to control the properties of the polishing pad. For example, increasing the amount of the epoxy oligomer in the IPN blend may increase the stiffness of the resulting polishing pad. Further, the IPN blend may be a viscous liquid, in which the viscosity can be adjusted by modifying the ratio of the urethane acrylate oligomer to the epoxy oligomer. The choice of the urethane acrylate oligomer and the epoxy oligomer may also be used to adjust the viscosity of the IPN blend. For example, the IPN blend used for 3D printing may be a free-flowing viscous liquid at a viscosity in a range of about 10 centipoise (cP) to a maximum value of about 20 cP at a temperature of 70° C. The polishing layer fabricated by the 3D printing utilizing the formulation may have an elongation at break of at least 8% and ultimate tensile strength (UTS) of at least about 30 megapascals (MPa). The correlation of properties to the ratio of the oligomers in the IPN plant is discussed further with respect to the examples. 
     At block  506 , the IPN blend is mixed with the photoinitiators to form an active blend. As described herein the photoinitiators include free radical initiators, for example, used to initiate the polymerization of the urethane acrylate oligomer. The photoinitiators also include cationic photoinitiators, for example, used to initiate the polymerization of the epoxy oligomers. Further, as described with respect to  FIG.  6   , a different IPN blend may be used for different portions of the polishing pad, such as a window used for endpoint detection in the polishing process. 
     At block  508 , the polishing pad is made, for example, by a manufacturer at a semiconductor fabrication plant. This may be a single step, for example, during a 3D printing process using the printing of droplets of the active blend to form the polishing pad. In some implementations, a polishing pad may be cast from an IPN blend. This polishing pad is formed as described at block  510 . At block  512  the polishing pad is irradiated to initiate photopolymerization, e.g., curing. At block  510 , droplets may be ejected from the nozzles at target locations as determined by a file in the computer that includes 3D plans for the polishing pad. 
     At block  512 , the formulation having the urethane acrylate oligomer and epoxy oligomer is subjected to photopolymerization to form the layer of the polishing pad. The irradiation may be performed at different levels during the process. For example, an initial radiation may be performed to fix droplets in place, while a more intense radiation may be used to cure the entire layer, either after the layer is deposited, or after the entire structure is created. 
     Once the 3D printing of the polishing pad is completed, the polishing pad may be annealed under a UV lamp or in an oven for further curing. It can be noted that the curing will continue to take place after the formation of the polishing pad, even without annealing. Accordingly, the polishing pad may be allowed to rest before use to allow for further cure time. As described herein, the additives and oligomer ratio may be used to adjust the viscosity and curability of the formulation and the mechanical properties of the polishing pad. 
       FIG.  6    is a block flow diagram of a method  600  for forming a polishing pad with an inherent window region by the 3D printing of an interpenetrating polymer network. The method begins at block  602  with the formation of the polishing layer IPN blend. This may be done as described herein, for example, using the ingredients discussed with respect to  FIG.  5   . The proportions of the ingredients used may be as described with respect to the examples. 
     At block  604 , the window IPN blend is formed. Generally, the proportions of the oligomers and other ingredients in the window IPN blend will match the polishing layer IPN blend, so that the properties will also match. However, the amounts of photoinitiators in the window IPN blend may be lower than in the polishing layer IPN blend, as the photoinitiators may absorb light in a region used for the endpoint detection for the polishing operation. For example, in various implementations, the photoinitiators may be decreased by about 5% with respect to the polishing layer IPN, or by about 10%, or by about 20%, or more. This will slow down the curing of the window region with respect to the rest of the polishing layer. The blends are then loaded into the 3D printer, for example, in reservoirs for at least two different droplet ejectors in the nozzle. Once the blends are loaded, the polishing pad with the inherent window region is printed. 
     At block  606 , the location of the nozzle is determined and compared to a stored 3D plan of the polishing pad. At block  608 , a determination is made as to whether the nozzle is over a window region of the polishing pad. If so, at block  610 , a droplet of the window IPN blend is deposited at the target location. If not, at block  612 , a droplet of the polishing layer IPN blend is deposited with target location. 
     Once the droplet is deposited, at block  614 , the target location is irradiated to initiate photopolymerization and fix the droplet in place. This may be done using a number of different techniques. In an implementation, a lower power irradiation may start a top cure of the photopolymerization, which may be used just to fix the droplet in place while other droplets are deposited. This may be then followed by a more intense irradiation to initiate a full cure, or bottom cure, once the entire polishing pad is formed. In another implementation, a curing lamp may be passed over the polishing pad once a full layer has been printed to start a top cure of that layer. It can be noted that upper layers printed on lower layers that have already started to cure may bond to the lower layers shortly after being printed, through the reactions that have already initiated in the lower layers. 
     Examples 
     The examples are given only as examples and not meant to limit the present techniques. A number of different mixtures were made to determine the properties of the IPNs as the ratios of the components were varied. The components used were as discussed with respect to  FIG.  5   . 
     The experiments were carried out by mixing compositions containing the radical and cationic components in the proportions shown in Table 1. The formulations were cured bulk in a silicone mold of Type V dogbones (with thickness of 2 mm) by exposing to UV light of about 1150 mJ/cm 2  in a Heraeus UV curing station with  3  passes at 18 ft/min (each pass exposed at 380 mJ/cm2). In practice, these formulations may be 3D printed and then cured to make CMP Pads, including CMP Pads having a high modulus and good elasticity for semiconductor applications. Accordingly the UV dosage can be reduced depending up on the thickness of samples. 
     Dynamic mechanical analysis (DMA) was performed on the UV-cured films. The main parameters that can be obtained are E and tan δ. E′ is the storage modulus, the elastic component of the complex modulus, E. Tan δ is the damping factor, calculated as the ratio E″/E′, in which E″ is the loss modulus, the dissipative component of the complex modulus E′ of the viscoelastic material. Importantly, the molecular weight, chain length and branching of the polymer may play a role in the weight percent of polymer due to such factors that include polymer miscibility and mixture viscosity. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Compositions used to test properties of IPNs for polishing pads. 
               
            
           
           
               
               
               
            
               
                   
                 Radical composition 
                 Cationic composition 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Radical 
                 Cationic 
                 BR 
                 SR 
                 Celloxide 
                   
                   
               
               
                 composition 
                 composition 
                 744SD 
                 351H 
                 2021 P 
                 OXT 101 
                 PCL-U 
               
               
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 100 
                 0 
                 20 
                 80 
                 0 
                 0 
                 0 
               
               
                 90 
                 10 
                 18 
                 72 
                 5.7 
                 1.3 
                 3 
               
               
                 75 
                 25 
                 15 
                 60 
                 14.25 
                 3.25 
                 7.5 
               
               
                 55 
                 45 
                 11 
                 44 
                 25.65 
                 5.85 
                 13.5 
               
               
                 35 
                 65 
                 7 
                 28 
                 37.05 
                 8.45 
                 19.5 
               
               
                 0 
                 100 
                 0 
                 0 
                 57 
                 13 
                 30 
               
               
                   
               
               
                 Note that 0.5 wt. % of each of the cationic and radical photoinitiators were used. 
               
            
           
         
       
     
     As described herein, IPNs are used to tune and adjust the properties of polishing pads to create a desired composite of properties within a layer and/or layer by layer, such as those properties including E′30, E′90, E′30/E′90, strength, toughness, compression, and elongation. 
       FIG.  7 A  is a plot of Tan δ as measured by dynamic mechanical analysis (DMA) of pure cationic polymers and interpenetrating polymer networks as a function of increasing cationic polymer composition. As shown in  FIG.  7 A , the 100% cationic, or pristine cationic polymer  702 , has the highest Tg at 115° C. However, a 10% cationic polymer  704  is nearly as high, at a Tg of 100° C. Increasing to a 25% cationic polymer  706  drops the Tg further, to a value of 78° C. A 45% cationic polymer  708  has a Tg of about 62° C., and a 65% cationic polymer has a Tg of about −65° C. Thus, as the cationic polymer content increases (arrow  712 ) from 10% to 65%, the Tg falls. It can be noted that the kinetics of polymerization are faster for the free-radical materials, inhibiting the formal of the epoxy network. Accordingly, the properties are not linearly related to the concentration ratios of the free-radically polymerized materials to the cationically polymerized materials. 
     In the glass-transition-temperature (Tg) region a strong decrease of E′ can be observed as the cationic polymer content increases (arrow  712 ), while tan δ shows a maximum. The temperature corresponding to such a maximum was assumed to be the Tg of the cured film. In  FIG.  7 A , the tan δ curves for the pure-epoxy and the IPN system are shown. In the case of the hybrid system, a broad Tg peak can be observed, with intervals in between the Tg interval of the pure acrylic and the pure epoxy UV-cured resins. This indicates a complicated polymer-network structure. As described herein, the properties of the IPN networks are not an average of the properties of the two component networks. The IPN networks have a complicated polymer network structure owing to the differences in kinetics of polymerization as a function of individual compositions. Faster polymerization of one component limits mass transfer and movement of other fragments through the vitrified matrix. 
       FIG.  7 B  is a plot of modulus values for E30 and E90 as a function of Tg and cationic polymer composition. The selection, formulation and/or formation of materials that have a desirable storage modulus E and E30:E90 ratio in desirable regions of an advanced polishing pad by use of an additive manufacturing process is an important factor in assuring that the polishing results achieved by the advanced polishing pad are uniform across a substrate. It is noted that storage modulus E′ is an intrinsic material property of a formed material, which results from the chemical bonding within a cured polymeric material, Storage modulus may be measured at a desired temperature, such as 30° C. and 90° C. using a dynamic mechanical analysis (DMA) technique. Examples of formulations that contain different storage moduli are illustrated in  FIG.  1 B ). 
     As shown in the plots of  FIGS.  7 A and  7 B , increasing the cationic ratio affords IPNs with lower Tg. This is dictated by the kinetics of acrylate vs epoxy polymerization during the UV curing process. This is further explored in the measurements shown in  FIGS.  8 A,  8 B,  9 A, and  9 B . 
       FIG.  8 A  is a plot of percent cure at the bottom of a droplet after UV curing as a function of polymer composition.  FIG.  8 B  is a plot of percent cure at the bottom of a droplet after UV curing and annealing as a function of polymer composition.  FIG.  9 A  is a plot of percent cure at the top of a droplet after UV curing as a function of polymer composition.  FIG.  9 B  is a plot of percent cure at the top of the droplet after UV curing and annealing as a function of polymer composition. 
     As shown in these figures, the top cures for cationic polymers are better while through cures are better for radical curing. Acrylates cure faster than epoxy and inhibit the epoxy network formation by limiting diffusion (glassy), resulting in materials with the lowest Tg. The UV cured samples were annealed for two hours and this was found to increase top surface cures due to reduced moisture and oxygen inhibition. Further, the formulations were tuned to have viscosities between 13-15 cP which makes them amenable to inkjet printing. 
     Window Formulation Tests 
     In a similar fashion to the more general polishing pad IPN formulations, the window region formulations were tested by mixing compositions containing the free radical and cationic components, and curing films of about 2 mm in thickness by exposing them to UV light of 1150 mJ/cm 2 . The major difference was the amount of photoinitiator used in each test. For example, results for a window formulation using 0.1 weight percent Omnirad 819 are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 mechanical properties of IPN formulations for pad 
               
               
                 window cured using 0.1 weight percent OmniRad 819. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Viscosity 
                   
                   
                   
                   
                   
                   
               
               
                   
                 after 
               
               
                   
                 stability 
               
               
                 Visc@70 
                 test@70 
                 UTS 
                   
                 E30 
                 E90 
                 % epoxy 
                 % acrylate 
               
               
                 (cP) 
                 (cP) 
                 (MPa) 
                 % EL 
                 (MPa) 
                 (MPa) 
                 conversion 
                 conversion 
               
               
                   
               
               
                 12.9 
                 13.2 
                 29.6 
                 2.8 
                 1147 
                 700 
                 T: 85 
                 T: 89 
               
               
                   
                   
                   
                   
                   
                   
                 B: 75 
                 B: 93 
               
               
                   
               
            
           
         
       
     
     It can be further added that the tuning of the Tg can be used to obtain pad window compositions with varying mechanical properties. The viscosities of the above formulations are in the optimum range for inkjet printing, for example, between about 12 cP and about 16 cP, depending on the printhead. The thermal stability of these formulations can be improved by adding inhibitors, for example, Proton Sponge®, 1,8-bis(dimethylamino)naphthalene from Sigma-Aldrich, or a hindered amine light stabilizer (HALS), such as Omnistab L5292 from IGM Resins. 
     The jetting performance of these formulations were tested, and they were found to stably jet at 70 degrees with a drop velocity of 5.87 m/s. The print performance and the mechanical properties were validated using a Connex 500 printer from Stratasys at a concentration of 0.2 wt. % of the Omnirad 819 photoinitiator, providing the mechanical properties tabulated in Table 3. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 mechanical properties of IPN formulations for pad window printed 
               
               
                 on Connex 500 using 0.2 weight percent Omnicure 819. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Viscosity 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 after 
                 UTS 
                 % EL 
                 UTS 
                 % EL 
               
               
                   
                 stability 
                 (Type 
                 (Type V, 
                 (Type 
                 (Type 
               
               
                 Visc@70 
                 test@70 
                 V) 
                 5 
                 IV) 
                 IV, 5 
                 E30 
                 E90 
                 % epoxy 
                 % acrylate 
               
               
                 (cP) 
                 (cP) 
                 (MPa) 
                 mm/min) 
                 (MPa) 
                 mm/min) 
                 (MPa) 
                 (MPa) 
                 conversion 
                 conversion 
               
               
                   
               
               
                 13.4 
                 13.6 
                 48.9 
                 6.4 
                 44 
                 6.2 
                 1380 
                 530 
                 T: 70 
                 T: 65 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 B: 77 
                 B: 69 
               
               
                   
               
               
                 Note that the dogbone test samples are prepared according to ASTM D638 (Type V and Type IV). 
               
            
           
         
       
     
       FIG.  10    is a plot of polymer transmittance (%) as a function of wavelength (nm). Finally, the cutoff wavelength and transmittance data for the pad window are shown in  FIG.  10   . The cutoff wavelength was 375 nm with an optical transmittance of 60% at wavelength &gt;400 nm while those for commercial windows is 395 nm with a transmittance of 50% at wavelengths &gt;400 nm. Further, the cutoff does not change after 6 million flashes of UV which is after 45 second exposure accounting for one pad life or even after 90 second exposure indicating the stability of the pad window. 
     An implementation described herein provides a method of generating a formulation for forming a polishing pad including an interpenetrating polymer network. The method includes blending a cationically polymerized material with a free-radically polymerized material to form a precursor blend, wherein a ratio of the cationically polymerized material to the free-radically polymerized material in the oligomer blend is selected to control properties of the polishing pad. A first amount of a mixture of a free radical photoinitiator and a cationic photoinitiator is blended with the precursor blend to form a polishing pad blend. A second amount of the mixture of a free radical photoinitiator and a cationic photoinitiator is blended with the precursor blend to form a window blend. The polishing pad blend and the window blend are provided to a manufacture to generate a polishing pad having a window using a three-dimensional printer. 
     In an aspect, the properties include elongation at break, ultimate tensile strength (UTS), storage modulus, or glass transition temperature, or any combinations thereof. In an aspect, a maximum value of a viscosity of the polishing pad blend, the window blend, or both is 20 centipoise (cP). 
     Another implementation described herein provides a method of manufacturing a polishing pad including an interpenetrating polymer network. The method includes obtaining a polishing pad blend including a free radical photoinitiator, a free-radically polymerized material, a cationic photoinitiator, and a cationically polymerized material, wherein a ratio of the free-radically polymerized material to the cationically polymerized material is selected to control properties of the polishing pad. A window blend is obtained, wherein the window blend includes the free radical photoinitiator, wherein a concentration of the free radical photoinitiator is lower than the free radical photoinitiator in the polishing pad blend. The window blend includes the free-radically polymerized material. The window blend includes the cationic photoinitiator, wherein the concentration of the cationic photoinitiator is lower than the concentration of the cationic photoinitiator in the polishing pad blend. The window blend also includes the cationically polymerized material, wherein a second ratio of the free-radically polymerized material to the cationically polymerized material in the window blend is selected to control the properties of a window in the polishing pad. A raw polishing pad is formed in a polishing pad region from the polishing pad blend by printing droplets of the polishing pad blend in the polishing pad region. A window is formed in a window region from the window blend by printing droplets of the window blend in the window region. The raw polishing pad is irradiated to initiate free radical photopolymerization of the free radically polymerized material and cationic photopolymerization of the cationically polymerized material. 
     In an aspect, the free-radically polymerized material is a urethane acrylate oligomer. In an aspect, the cationically polymerized material is an epoxy oligomer. In an aspect, the polishing pad is annealed by irradiating the polishing pad, heating the polishing pad in an oven, or both. In an aspect, a maximum viscosity of the polishing pad blend, or the window blend, or both is 20 centipoise (cP). 
     Another implementation described herein provides a polishing pad for a semiconductor fabrication operation. The polishing pad includes a polishing region and a window region, wherein both regions include an interpenetrating polymer network formed from a free-radically polymerized material and a cationically polymerized material. 
     In an aspect, the free-radically polymerized material is a urethane acrylate oligomer. In an aspect; the cationically polymerized material is an epoxy oligomer. In an aspect, the polishing pad includes a free radical photoinitiator and a cationic photoinitiator. In an aspect, the window region includes a lower amount of the free radical photoinitiator, the cationic photoinitiator, or both than the polishing region. 
     In an aspect, a ratio of the cationically polymerized material to the free-radically polymerized material is controlled to achieve a property target. In an aspect, the ratio of the cationically polymerized material to the free-radically polymerized material is between 10 wt. % and 65 wt. % for a polishing region. In an aspect, the ratio of the cationically polymerized material to the free-radically polymerized material is about 25 wt. %. 
     In an aspect, an E′30 modulus of the polishing pad is between 5 megapascals (MPa) and 100 MPa. In an aspect, an E′30 modulus of the polishing pad is between 101 megapascals (MPa) and 500 MPa. In an aspect, an E′30 modulus of the polishing pad is between 501 megapascals (MPa) and 3000 MPa. In an aspect, the ratio of the cationically polymerized material to the free-radically polymerized material in the window region is controlled to match a property target with the polishing region. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular or some other shape. An adhesive layer can be applied to the bottom surface of the polishing pad to secure the pad to the platen, and the adhesive layer can be covered by a removable liner before the polishing pad is placed on the platen. In addition, although terms of vertical positioning are used, it should be understood that the polishing surface and substrate could be held upside down, in a vertical orientation, or in some other orientation. Accordingly, other implementations are within the scope of the following claims.