Abstract:
A method of fabricating a polishing layer of a polishing pad includes successively depositing a plurality of layers with a 3D printer, each layer of the plurality of polishing layers deposited by ejecting a pad material precursor from a nozzle and solidifying the pad material precursor to form a solidified pad material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/591,051, filed Aug. 21, 2012, which claims priority to U.S. Provisional Application No. 61/638,461, filed on Apr. 25, 2012, the entire disclosures of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This present invention relates to polishing pads used in during 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 processes require 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 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”). 
     Conventional polishing pads include “standard” pads and fixed-abrasive pads. A standard pad has 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 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. 
     In addition to planarization, polishing pads can be used for finishing operations such as buffing. 
     SUMMARY 
     In order to provide polishing uniformity, the polishing pad needs to form a uniform contact with the substrate being polished, such that uniform pressure can be applied across the substrate surface. A variation in thickness of the pads can create non-uniform pressure across the substrate surface. Even small variations in thickness lead to variations in the applied pressure, and hence non-uniform removal as well as higher defects such as micro-scratches on the substrate surface. This effect is more acute for hard polishing pads, and is also more acute at low pressure polish processes. Although a soft polishing pad can accommodate larger thickness variations, the process of forming grooving in the pad is more likely to generate non-uniformity for the soft polishing pad. 
     A technique for manufacturing polishing pads that can provide improved thickness uniformity is to use a 3D printing process. In a 3D printing process a thin layer of pad precursor, e.g., a powder, is progressively deposited and fused to form a full 3-dimensional polishing pad. 
     In one aspect a method of fabricating a polishing layer of a polishing pad includes successively depositing a plurality of layers with a 3D printer, each layer of the plurality of polishing layers deposited by ejecting a pad material precursor from a nozzle and solidifying the pad material precursor to form a solidified pad material. 
     Implementations of the invention may include one or more of the following features. A thickness of each layer of the plurality of layers may be less than 50% of a total thickness of the polishing layer. A thickness of each layer of the plurality of layers may be less than 1% of a total thickness of the polishing layer. Recesses may be formed in the polishing layer by controlling ejection of the pad material precursor with 3D drawing program run on a computer to form a pattern in at least some of the plurality of layers. The recesses may be 10%-75% of the total horizontal surface area of the polishing pad. Plateaus between the recesses may have lateral dimension of 0.1 to 2.5 mm. The recesses may have a depth of 0.25 to 1.5 mm. The recesses may have a widest lateral dimension of 0.1 mm to 2 mm. The recesses may be shaped as one or more of a cylindrical, a truncated pyramid or a prism. The recesses may be grooves. Solidifying the pad material precursor may include curing the pad material precursor. Curing the pad material precursor may include ultraviolet (UV) curing. The pad material precursor may include a urethane monomer. The solidified pad material may include polyurethane. Abrasive particles may be supplied in the solidified pad material. The abrasive particles may be metal oxide particles. A backing layer of the polishing pad may be formed by successively depositing a plurality of layers with the 3D printer. Forming the backing layer may include curing the plurality of layers of the backing layer by a different amount than the plurality of layers of the polishing layer. Forming the backing layer may include ejecting a different material than the pad precursor material. The solidified polishing material may have a hardness between about 40 to 80 Shore D. The pad material precursor may be a melted pad material and solidifying the pad material precursor may include cooling the melted pad material. 
     Potential advantages of the invention may include one or more of the following. A polishing pad can be manufactured with very tight tolerance, i.e., good thickness uniformity. Grooves can be formed in the polishing pad without distorting the thickness uniformity. Polishing uniformity across the substrate may be improved, particularly at low pressures such as below 0.8 psi, or even below 0.5 psi or 0.3 psi. The pad manufacturing process is adaptable to different polishing pad configuration and groove patterns. A polishing pad can be manufactured faster and cheaper. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic cross-sectional side view of an example polishing pad. 
         FIG. 1B  is a schematic cross-sectional side view of another example polishing pad. 
         FIG. 1C  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 cross-sectional side view illustrating a substrate in contact with the polishing pad of  FIG. 1A . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A-1C , a polishing pad  18  includes a polishing layer  22 . As shown in  FIG. 1A  the polishing pad can be a single-layer pad that consists of the polishing layer  22 , or as shown in  FIG. 1C  the polishing pad can be a multi-layer pad that includes the polishing layer  22  and at least one backing layer  20 . 
     The polishing layer  22  can be a material that is inert in the polishing process. The material of the polishing layer  22  can be a plastic, e.g., a polyurethane. In some implementations the polishing layer  22  is a relative durable and hard material. For example, the polishing layer  22  can have a hardness of about 40 to 80, e.g., 50 to 65, on the Shore D scale. 
     As shown in  FIG. 1A , the polishing layer  22  can be a layer of homogeneous composition, or as shown in  FIG. 1B  the polishing layer  22  can include abrasive particles  28  held in a matrix  29  of plastic material, e.g., polyurethane. The abrasive particles  28  are harder than the material of the matrix  29 . The abrasive particles  28  can be from 0.05 wt % to 75 wt % of the polishing layer. For example, the abrasive particles  28  can be less than 1 wt % of the polishing layer  22 , e.g., less than 0.1 wt %. Alternatively, the abrasive particles  28  can be greater than 10 wt % of the polishing layer  22 , e.g., greater than 50 wt %. The material of the abrasive particles can be a metal oxide, such as ceria, alumina, silica or a combination thereof. 
     In some implementations, the polishing layer includes pores, e.g., small voids. The pores can be 50-100 microns wide. 
     The polishing layer  22  can have a thickness D 1  of 80 mils or less, e.g., 50 mils or less, e.g., 25 mils or less. Because the conditioning process tends to wear away the cover layer, the thickness of the polishing layer  22  can be selected to provide the polishing pad  18  with a useful lifetime, e.g., 3000 polishing and conditioning cycles. 
     On a microscopic scale, the polishing surface  24  of the polishing layer  22  can have rough surface texture, e.g., 2-4 microns rms. For example, the polishing layer  22  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  24  can be rough on a microscopic scale, the polishing layer  22  can have good thickness uniformity on the macroscopic scale of the polishing pad itself (this uniformity refer to the global variation in height of the polishing surface  24  relative to the bottom surface of the polishing layer, and does not count any macroscopic grooves or perforations deliberately formed in the polishing layer). For example, the thickness non-uniformity can be less than 1 mil. 
     Optionally, at least a portion of the polishing surface  24  can include a plurality of grooves  26  formed therein for carrying slurry. The grooves  26  may be of nearly any pattern, such as concentric circles, straight lines, a cross-hatched, spirals, and the like. Assuming grooves are present, then the polishing surface  24 , i.e., the plateaus between the grooves  26 , can be 25-90% of the total horizontal surface area of the polishing pad  18 . Thus, the grooves  26  can occupy 10%-75% of the total horizontal surface area of the polishing pad  18 . The plateaus between the grooves  26  can have a lateral width of about 0.1 to 2.5 mm. 
     In some implementations, e.g., if there is a backing layer  20 , the grooves  26  can extend entirely through the polishing layer  22 . In some implementations, the grooves  26  can extend through about 20-80%, e.g., 40%, of the thickness of the polishing layer  22 . The depth of the grooves  26  can be 0.25 to 1 mm. For example, in a polishing pad  18  having a polishing layer  22  that is 50 mils thick, the grooves  26  can have a depth D 2  of about 20 mils. 
     The backing layer  20  can be softer and more compressible than the polishing layer  22 . The backing layer  20  can have a hardness of 80 or less on the Shore A scale, e.g., a hardness of about 60 Shore A. The backing layer  20  can be thicker or thinner or the same thickness as the polishing layer  22 . 
     For example, the backing layer 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. A suitable material for the backing layer is PORON 4701-30 from Rogers Corporation, in Rogers, Conn., or SUBA-IV from Rohm &amp; Haas. The hardness of the backing layer can be adjusted by selection of the layer material and porosity. Alternatively, the backing layer  20  can be formed from the same precursor and have the same porosity as the polishing layer, but have a different degree of curing so as to have a different hardness. 
     Turning now to  FIG. 2 , one or more substrates  14  can be polished at a polishing station  10  of a CMP apparatus. A description of a suitable polishing apparatus can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
     The polishing station  10  can include a rotatable platen  16  on which is placed the polishing pad  18 . During a polishing step, a polishing liquid  30 , e.g., abrasive slurry, can be supplied to the surface of polishing pad  18  by a slurry supply port or combined slurry/rinse arm  32 . The polishing liquid  30  can contain abrasive particles, a pH adjuster, or chemically active components. 
     The substrate  14  is held against the polishing pad  18  by a carrier head  34 . The carrier head  34  is suspended from a support structure, such as a carousel, and is connected by a carrier drive shaft  36  to a carrier head rotation motor so that the carrier head can rotate about an axis  38 . The relative motion of the polishing pad  18  and the substrate  14  in the presence of the polishing liquid  30  results in polishing of the substrate  14 . 
     Referring to  FIG. 3 , at least the polishing layer  22  of the polishing pad  18  is manufactured using a 3D printing process. In the manufacturing process, thin layers of material are progressively deposited and fused. For example, droplets  52  of pad precursor material can be ejected from a nozzle  54  of a droplet ejecting printer  55  to form a layer  50 . The droplet ejecting printer is similar to an inkjet printer, but uses the pad precursor material rather than ink. The nozzle  54  translates (shown by arrow A) across a support  51 . 
     For a first layer  50   a  deposited, the nozzle  54  can eject onto the support  51 . For subsequently deposited layers  50   b , the nozzle  54  can eject onto the already solidified material  56 . After each layer  50  is solidified, a new layer is then deposited over the previously deposited layer until the full 3-dimensional polishing layer  22  is fabricated. Each layer is applied by the nozzle  54  in a pattern stored in a 3D drawing computer program that runs on a computer  60 . Each layer  50  is less than 50% of the total thickness of the polishing layer  22 , e.g., less than 10%, e.g., less than 5%, e.g., less than 1%. 
     The support  51  can be a rigid base, or be a flexible film, e.g., a layer of polytetrafluoroethylene (PTFE). If the support  51  is a film, then the support  51  can form a portion of the polishing pad  18 . For example, the support  51  can be the backing layer  20  or a layer between the backing layer  20  and the polishing layer  22 . Alternatively, the polishing layer  22  can be removed from the support  51 . 
     Solidification can be accomplished by polymerization. For example, the layer  50  of pad precursor material can be a monomer, and the monomer can be polymerized in-situ by ultraviolet (UV) curing. The pad precursor material can be cured effectively immediately upon deposition, or an entire layer  50  of pad precursor material can be deposited and then the entire layer  50  be cured simultaneously. 
     However, there are alternative technologies to accomplish 3D printing. For example, the droplets  52  can be a polymer melt that solidifies upon cooling. Alternatively, the printer creates the polishing layer  22  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., the abrasive particles  28 . 
     The 3D printing approach eliminates the need for making expensive and time consuming molds. The 3D printing approach also eliminates several conventional pad manufacturing steps such as molding, casting and machining Additionally, tight tolerances can be achieved due to the layer-by-layer printing approach. Also, one printing system (with printer  55  and computer  60 ) can be used to manufacture a variety of different polishing pads, simply by changing the pattern stored in the 3D drawing computer program. 
     In some implementations, the backing layer  20  can also be fabricated by a 3D printing process. For example, the backing layer  20  and polishing layer  22  could be fabricated in an uninterrupted operation by the printer  55 . The backing layer  20  can be provided with a different hardness than the polishing layer  22  by using a different amount of curing, e.g., a different intensity of UV radiation. 
     In other implementations, the backing layer  20  is fabricated by a conventional process and then secured to the polishing layer  22 . For example, the polishing layer  22  can be secured to the backing layer  20  by a thin adhesive layer, e.g., as a pressure-sensitive adhesive. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. 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.