Patent Application
20150079878
Embodiments of the present invention are in the field of chemical mechanical polishing (CMP) and, in particular, polishing pads with apertures.
Chemical-mechanical planarization or chemical-mechanical polishing, commonly abbreviated CMP, is a technique used in semiconductor fabrication for planarizing a semiconductor wafer or other substrate.
The process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The polishing pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated during polishing. This approach aids in removal of material and tends to even out any irregular topography, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom levels for the latest sub-50 nanometer technology nodes.
The process of material removal is not simply that of abrasive scraping, like sandpaper on wood. The chemicals in the slurry also react with and/or weaken the material to be removed. The abrasive accelerates this weakening process and the polishing pad helps to wipe the reacted materials from the surface. In addition to advances in slurry technology, the polishing pad plays a significant role in increasingly complex CMP operations.
However, additional improvements are needed in the evolution of CMP pad technology.
Embodiments of the present invention include polishing pads with apertures.
In an embodiment, a polishing apparatus for polishing a substrate includes a polishing pad having a polishing surface and a back surface. The polishing surface includes a pattern of grooves. An aperture is disposed in the polishing pad from the back surface through to the polishing surface. An adhesive sheet is disposed on the back surface of the polishing pad but not in the aperture. The adhesive sheet provides an impermeable seal for the aperture at the back surface of the polishing pad.
In another embodiment, a polishing pad for polishing a substrate includes a polishing body having a polishing surface and a back surface. The polishing surface includes a pattern of grooves. An aperture is disposed in the polishing body from the back surface through to the polishing surface. The aperture has a sidewall having a ramp feature with a slope to provide a narrowest region of the aperture at the back surface of the polishing body and a widest region of the aperture at the polishing surface of the polishing body.
In another embodiment, a polishing pad for polishing a substrate includes a polishing body having a polishing surface and a back surface. The polishing surface includes a pattern of grooves. An aperture is disposed in the polishing body from the back surface through to the polishing surface. A first groove of the pattern of grooves is a circumferential groove continuous with the aperture at a first sidewall of the aperture but discontinuous with a second sidewall of the aperture. A second groove of the pattern of grooves is continuous with the aperture at the second sidewall.
In another embodiment, a polishing pad for polishing a substrate includes a polishing body having a polishing surface and a back surface. The polishing surface includes a pattern of grooves. An aperture is disposed in the polishing body from the back surface through to the polishing surface. A first groove of the pattern of grooves is a first radial groove continuous with the aperture at a first sidewall of the aperture. A second groove of the plurality of grooves is a second radial groove continuous with the aperture at a second sidewall of the aperture. The first sidewall is opposite the second sidewall.
In another embodiment, a method of polishing a substrate includes disposing a polishing pad above a platen of a chemical mechanical polishing apparatus. The polishing pad has a polishing surface, a back surface, and an aperture disposed in the polishing pad from the back surface through to the polishing surface. The polishing surface includes a pattern of grooves. A chemical mechanical polishing slurry is dispensed on the polishing surface of the polishing pad. A substrate is polished with the chemical mechanical polishing slurry at the polishing surface of the polishing pad. Through the aperture, the polishing of the substrate is monitored with an optical monitoring device coupled with the platen.
In another embodiment, a method of fabricating a polishing pad for polishing a substrate includes mixing a set of polymerizable materials to form a mixture in a base of a formation mold. A lid of the formation mold and the mixture together are moved together. The lid has disposed thereon a pattern of protrusions and an aperture protrusion with a height greater than the pattern of protrusions. With the lid placed in the mixture, the mixture is at least partially cured to form a molded homogeneous polishing body having a back surface. The molded homogeneous polishing body also has a polishing surface having disposed therein a pattern of grooves and an opening defining an aperture region.
Polishing pads with apertures are described herein. In the following description, numerous specific details are set forth, such as specific polishing pad compositions and designs, in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known processing techniques, such as details concerning the delivery of a slurry to a polishing pad to perform CMP of a substrate, are not described in detail in order to not unnecessarily obscure embodiments of the present invention. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
Features may need to be introduced to polishing pads for advanced chemical mechanical polishing processing. For example, otherwise opaque polishing pads may have one or more “windows” included therein to allow a substantial transmission of visible light for various monitoring applications. One such monitoring application may involve use of an optical device mounted within or on a chemical mechanical polishing apparatus. The optical device is used to monitor a chemical mechanical polishing process by, e.g., reflectance changes in the substrate undergoing polishing. The process is monitored through the window of the polishing pad since the polishing occurs at a top polishing surface of the polishing pad. The window is typically formed by inserting a transparent plug into the pad or by molding a transparent region (e.g., a local area transparency region or LAT) into an otherwise opaque pad at the time of fabrication. In either case, the window is composed of a distinct material included in the pad.
In accordance with an embodiment of the present invention, a “windowless” polishing pad suitable for optical monitoring there through is provided. As an example, an aperture is provided in the polishing pad allowing for optical monitoring through the polishing pad. In one embodiment, the aperture is an opening or hole made in the pad that extends through the entire pad. Thus, in contrast to a pad including a window composed of a material, the windowless polishing pad is characterized by the absence of material.
Conventionally, a mere hole formed in a polishing pad would have been unsuitable for monitoring a chemical mechanical process. For example, slurry would have been able to escape through the pad, possibly eroding an underlying optical monitoring device. In another example, a hole that fills with an opaque slurry may be unsuitable for allowing sufficient light transmission for optical detection. However, advanced slurries now being tested or in use are relatively, if not entirely, transparent.
As such, in an embodiment of the present invention, filling of an aperture with a slurry does not detrimentally impact optical detection. Furthermore, in an embodiment, a clear sheet (e.g., a pressure sensitive adhesive or PSA) is included between a polishing pad with an aperture there through and a chemical mechanical polishing apparatus. In one such embodiment, the clear sheet provides a seal under the pad to protect the platen and, e.g., a quartz laser site. As described in more detail below, various aperture designs are provided. In some embodiments, the designs include provisions to keep a slurry flushing across the opening or aperture during a polishing process. In a specific such embodiment, an aperture designed for slurry flushing is used to prevent polishing debris from collecting, agglomerating, and potentially attenuating the laser or other optical signal.
Conventional “window” polishing pads typically have an insert or LAT region of a material suitably transparent included therein. For example,
Referring to
In an aspect of the present invention, a windowless polishing pad suitable for optical monitoring includes an aperture there through. For example,
Referring to
Referring to
In an embodiment, the adhesive sheet 210 includes an adhesive layer to bond a sheet portion to the polishing pad 203. For example, in one embodiment, a layer of acrylic glue (shown as interface 209) is disposed on the back surface 203 of the polishing pad 201 and a layer of polyethylene terephthalate (PET) (shown as 210 in this embodiment) disposed on the layer of acrylic glue 209. In a specific such embodiment, the adhesive sheet 210 further includes a layer of rubber glue (shown as interface 211) disposed on the layer of PET 210, opposite the first layer of acrylic glue 209. In an embodiment, a disposable layer 212, such as a 3 mils layer of PET, is used to protect the layer of rubber glue 211 until the polishing apparatus 200 is used, at which point the disposable layer 212 is removed.
In an embodiment, the layer of rubber glue 211 is for adhering the polishing pad 203 to a platen of a chemical mechanical polishing tool. In an embodiment, the adhesive sheet 210 is sufficiently transparent for performing optical monitoring through the adhesive sheet 210, which may include acrylic glue layer 209 and rubber glue layer 211, and the aperture 208. In one such embodiment, the adhesive sheet 210 is for protecting a quartz laser site of an optical monitoring device coupled with a platen of a chemical mechanical polishing tool. In an embodiment, the adhesive sheet 210 which may include one or more adhesive layers is used to form an impermeable seal (e.g., impermeable to slurry) between the polishing pad 203 and a platen, particularly at or near the location of aperture 208.
It is to be understood that an aperture may be included in a polishing pad having a polishing surface with any pattern of grooves suitable for a chemical mechanical polishing process. For example, referring to
Basic examples of possible embodiments contemplated for groove patterns having concentric polygons as circumferential grooves, include groove patterns based on a series of grooves that form similar polygons, all with the same center point, and all aligned with an angle theta of zero so that their straight line segments are parallel and their angles are aligned in a radial fashion. Nested triangles, squares, pentagons, hexagons, etc., are all considered within the spirit and scope of the present invention. There may be a maximum number of straight line segments above which the polygons will become approximately circular. Preferred embodiments may include limiting the groove pattern to polygons with a number of sides less than such a number of straight line segments. One reason for this approach may be to improve averaging of the polish benefit, which might otherwise be diminished as the number of sides of each polygon increases and approaches a circular shape. Another embodiment includes groove patterns with concentric polygons having a center that is not in the same location as the polishing pad center. Of course, in other embodiments, an aperture may be formed in a pad with circular circumferential grooves.
Referring again to
In a first such example,
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In a second such example,
Referring to
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In a third such example,
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In a fourth such example,
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In an embodiment, polishing pads described herein, such as polishing pad 203 of polishing apparatus 200, are suitable for polishing substrates. The substrate may be one used in the semiconductor manufacturing industry, such as a silicon substrate having device or other layers disposed thereon. However, the substrate may be one such as, but not limited to, a substrates for MEMS devices, reticles, or solar modules. Thus, reference to “a polishing pad for polishing a substrate,” as used herein, is intended to encompass these and related possibilities.
Also, polishing pads described herein, such as polishing pad 203 of polishing apparatus 200, may be composed of a homogeneous polishing body of a thermoset polyurethane material. In an embodiment, the homogeneous polishing body is composed of a thermoset, closed cell polyurethane material. In an embodiment, the term “homogeneous” is used to indicate that the composition of a thermoset, closed cell polyurethane material is consistent throughout the entire composition of the polishing body. For example, in an embodiment, the term “homogeneous” excludes polishing pads composed of, e.g., impregnated felt or a composition (composite) of multiple layers of differing material. In an embodiment, the term “thermoset” is used to indicate a polymer material that irreversibly cures, e.g., the precursor to the material changes irreversibly into an infusible, insoluble polymer network by curing. For example, in an embodiment, the term “thermoset” excludes polishing pads composed of, e.g., “thermoplast” materials or “thermoplastics”—those materials composed of a polymer that turns to a liquid when heated and returns to a very glassy state when cooled sufficiently. It is noted that polishing pads made from thermoset materials are typically fabricated from lower molecular weight precursors reacting to form a polymer in a chemical reaction, while pads made from thermoplastic materials are typically fabricated by heating a pre-existing polymer to cause a phase change so that a polishing pad is formed in a physical process. Polyurethane thermoset polymers may be selected for fabricating polishing pads described herein based on their stable thermal and mechanical properties, resistance to the chemical environment, and tendency for wear resistance.
In an embodiment, polishing pads described herein, such as polishing pad 203 of polishing apparatus 200, include a molded homogeneous polishing body. The term “molded” is used to indicate that a homogeneous polishing body is formed in a formation mold, as described in more detail below in association with
In an embodiment, polishing pads described herein, such as polishing pad 203 of polishing apparatus 200, include a polishing body having a plurality of closed cell pores therein. In one embodiment, the plurality of closed cell pores is a plurality of porogens. For example, the term “porogen” may be used to indicate micro- or nano-scale spherical or somewhat spherical particles with “hollow” centers. The hollow centers are not filled with solid material, but may rather include a gaseous or liquid core. In one embodiment, the plurality of closed cell pores is composed of pre-expanded and gas-filled EXPANCEL™ distributed throughout (e.g., as an additional component in) a homogeneous polishing body of the polishing pad. In a specific embodiment, the EXPANCEL™ is filled with pentane. In an embodiment, each of the plurality of closed cell pores has a diameter approximately in the range of 10-100 microns. In an embodiment, the plurality of closed cell pores includes pores that are discrete from one another. This is in contrast to open cell pores which may be connected to one another through tunnels, such as the case for the pores in a common sponge. In one embodiment, each of the closed cell pores includes a physical shell, such as a shell of a porogen, as described above. In another embodiment, however, each of the closed cell pores does not include a physical shell. In an embodiment, the plurality of closed cell pores is distributed essentially evenly throughout a thermoset polyurethane material of a homogeneous polishing body.
In an embodiment, the homogeneous polishing body is opaque. In one embodiment, the term “opaque” is used to indicate a material that allows approximately 10% or less visible light to pass. In one embodiment, the homogeneous polishing body is opaque in most part, or due entirely to, the inclusion of a particle filler such as an opacifying lubricant throughout (e.g., as an additional component in) the homogeneous thermoset, closed cell polyurethane material of the homogeneous polishing body. In a specific embodiment, the particle filler is a material such as, but not limited to: boron nitride, cerium fluoride, graphite, graphite fluoride, molybdenum sulfide, niobium sulfide, talc, tantalum sulfide, tungsten disulfide, or Teflon.
The sizing of the homogeneous polishing body may be varied according to application. Nonetheless, certain parameters may be used to make polishing pads including such a homogeneous polishing body compatible with conventional processing equipment or even with conventional chemical mechanical processing operations. For example, in accordance with an embodiment of the present invention, the homogeneous polishing body has a thickness approximately in the range of 0.075 inches to 0.130 inches, e.g., approximately in the range of 1.9-3.3 millimeters. In one embodiment, the homogeneous polishing body has a diameter approximately in the range of 20 inches to 30.3 inches, e.g., approximately in the range of 50-77 centimeters, and possibly approximately in the range of 10 inches to 42 inches, e.g., approximately in the range of 25-107 centimeters. In one embodiment, the homogeneous polishing body has a pore density approximately in the range of 6%-36% total void volume, and possibly approximately in the range of 15%-35% total void volume. In one embodiment, the homogeneous polishing has a porosity of the closed cell type, as described above, due to inclusion of a plurality of pores. In one embodiment, the homogeneous polishing body has a compressibility of approximately 2.5%. In one embodiment, the homogeneous polishing body has a density approximately in the range of 0.70-1.05 grams per cubic centimeter.
In another embodiment, a polishing pad having a polishing surface with an aperture further includes a secondary detection region for use with, e.g., an eddy current detection system. For example,
Referring to
Referring to
In an aspect of the present invention, polishing pads having apertures therein may be fabricated in a molding process. For example,
Referring to
In an embodiment, the polishing pad precursor mixture 906 is used to ultimately form a molded homogeneous polishing body composed of a thermoset, closed cell polyurethane material. In one embodiment, the polishing pad precursor mixture 906 is used to ultimately form a hard pad and only a single type of curative is used. In another embodiment, the polishing pad precursor mixture 906 is used to ultimately form a soft pad and a combination of a primary and a secondary curative is used. For example, in a specific embodiment, the pre-polymer includes a polyurethane precursor, the primary curative includes an aromatic diamine compound, and the secondary curative includes a compound having an ether linkage. In a particular embodiment, the polyurethane precursor is an isocyanate, the primary curative is an aromatic diamine, and the secondary curative is a curative such as, but not limited to, polytetramethylene glycol, amino-functionalized glycol, or amino-functionalized polyoxypropylene. In an embodiment, the pre-polymer, a primary curative, and a secondary curative have an approximate molar ratio of 100 parts pre-polymer, 85 parts primary curative, and 15 parts secondary curative. It is to be understood that variations of the ratio may be used to provide polishing pads with varying hardness values, or based on the specific nature of the pre-polymer and the first and second curatives.
Referring to
In an embodiment, the aperture forming feature 911 is also a protrusion. For example, in one embodiment, the aperture forming feature 911 is an aperture protrusion having a height greater than the height of the protrusions of the pattern of protrusions 910. In a specific embodiment, the aperture protrusion 911 has a height at least triple the height of the protrusions of the pattern of protrusions 910.
It is to be understood that embodiments described herein that describe lowering the lid 908 of a formation mold 900 need only achieve a bringing together of the lid 908 and a base of the formation mold 900. That is, in some embodiments, a base of a formation mold 900 is raised toward a lid 908 of a formation mold, while in other embodiments a lid 908 of a formation mold 900 is lowered toward a base of the formation mold 900 at the same time as the base is raised toward the lid 908.
Referring to
Referring to
The molded homogeneous polishing body 912 includes a polishing surface 914 having disposed therein a pattern of grooves 920 corresponding to the pattern of protrusions 910 of the lid 908. The pattern of grooves 920 may be a pattern of grooves as described above, e.g., with respect to
In an embodiment, the opening defining the aperture region 918 is made to ultimately extend through the entire polishing body 912. The opening defining the aperture region 918 may be formed to extend through the polishing body 912 during molding or during a subsequent removal of a portion of the material of polishing body 912. For example, in one embodiment, forming the molded homogeneous polishing body 912 includes forming an aperture disposed in molded homogeneous polishing body 912 from the back surface 916 through to the polishing surface 914 at the aperture region 918 at the time of molding. In another embodiment, however, a portion of the homogeneous polishing body 912 is removed from the back surface 916 to form a polishing pad having a second back surface and to form an aperture disposed in molded homogeneous polishing body 912 from the second back surface through to the polishing surface 914 at the aperture region 918. That is, the aperture is formed by removing a portion of the molded material from the backside. In a specific such embodiment, the portion of the molded material is removed from the backside by cutting or by grinding.
In an embodiment, forming the molded homogeneous polishing body 912 includes forming the aperture region 918 to include a sidewall having a ramp feature with a slope to provide a narrowest region of the aperture region 918 proximate to the back surface 916 of the molded homogeneous polishing body 912 and a widest region of the aperture region 918 at the polishing surface 914 of the molded homogeneous polishing body 912, as described above at least in association with
In an embodiment, referring again to
Thus, groove patterns contemplated in embodiments of the present invention may be formed in-situ. Furthermore, apertures may also be formed simultaneously in the molding fabrication process. For example, as described above, a compression-molding process may be used to form polishing pads with a grooved polishing surface having an aperture therein. By using a molding process, highly uniform groove dimensions within-pad may be achieved. Furthermore, extremely reproducible groove dimensions along with very smooth, clean groove surfaces may be produced. Other advantages may include reduced defects and micro-scratches and a greater usable groove depth.
Also, since the fabricated aperture is formed during the molding, the positioning of the resulting pad during formation of a pad in a mold can be determined after removal of the pad from the mold. That is, such an aperture can provide traceability back to the molding process. Thus, in one embodiment, the polishing body of a polishing pad is a molded polishing body, and an aperture included therein indicates a location of a region in a mold used for forming the molded polishing body.
Individual grooves of the groove patterns described herein, including grooves at or near a location of an aperture in a polishing pad, may be from about 4 to about 100 mils deep at any given point on each groove. In some embodiments, the grooves are about 10 to about 50 mils deep at any given point on each groove. The grooves may be of uniform depth, variable depth, or any combinations thereof. In some embodiments, the grooves are all of uniform depth. For example, the grooves of a groove pattern may all have the same depth. In some embodiments, some of the grooves of a groove pattern may have a certain uniform depth while other grooves of the same pattern may have a different uniform depth. For example, groove depth may increase with increasing distance from the center of the polishing pad. In some embodiments, however, groove depth decreases with increasing distance from the center of the polishing pad. In some embodiments, grooves of uniform depth alternate with grooves of variable depth.
Individual grooves of the groove patterns described herein, including grooves at or near a location of an aperture in a polishing pad, may be from about 2 to about 100 mils wide at any given point on each groove. In some embodiments, the grooves are about 15 to about 50 mils wide at any given point on each groove. The grooves may be of uniform width, variable width, or any combinations thereof. In some embodiments, the grooves of a concentric polygon pattern are all of uniform width. In some embodiments, however, some of the grooves of a concentric polygon pattern have a certain uniform width, while other grooves of the same pattern have a different uniform width. In some embodiments, groove width increases with increasing distance from the center of the polishing pad. In some embodiments, groove width decreases with increasing distance from the center of the polishing pad. In some embodiments, grooves of uniform width alternate with grooves of variable width.
In accordance with the previously described depth and width dimensions, individual grooves of the groove patterns described herein, including grooves at or near a location of an aperture in a polishing pad, may be of uniform volume, variable volume, or any combinations thereof. In some embodiments, the grooves are all of uniform volume. In some embodiments, however, groove volume increases with increasing distance from the center of the polishing pad. In some other embodiments, groove volume decreases with increasing distance from the center of the polishing pad. In some embodiments, grooves of uniform volume alternate with grooves of variable volume.
Grooves of the groove patterns described herein may have a pitch from about 30 to about 1000 mils. In some embodiments, the grooves have a pitch of about 125 mils. For a circular polishing pad, groove pitch is measured along the radius of the circular polishing pad. In CMP belts, groove pitch is measured from the center of the CMP belt to an edge of the CMP belt. The grooves may be of uniform pitch, variable pitch, or in any combinations thereof. In some embodiments, the grooves are all of uniform pitch. In some embodiments, however, groove pitch increases with increasing distance from the center of the polishing pad. In some other embodiments, groove pitch decreases with increasing distance from the center of the polishing pad. In some embodiments, the pitch of the grooves in one sector varies with increasing distance from the center of the polishing pad while the pitch of the grooves in an adjacent sector remains uniform. In some embodiments, the pitch of the grooves in one sector increases with increasing distance from the center of the polishing pad while the pitch of the grooves in an adjacent sector increases at a different rate. In some embodiments, the pitch of the grooves in one sector increases with increasing distance from the center of the polishing pad while the pitch of the grooves in an adjacent sector decreases with increasing distance from the center of the polishing pad. In some embodiments, grooves of uniform pitch alternate with grooves of variable pitch. In some embodiments, sectors of grooves of uniform pitch alternate with sectors of grooves of variable pitch.
Polishing pads described herein may be suitable for use with a variety of chemical mechanical polishing apparatuses. As an example,
Referring to
As described above, in an embodiment, modern slurries are essentially transparent and will not attenuate or scatter a detection beam as early-generation slurries may otherwise have. Constant flow of slurry across an aperture opening may keep the opening free of debris. In one embodiment, a molding process is suitable for creating the opening during molding, so no extra manufacturing operations are needed. For windowless design features, in an embodiment, the purpose of each feature is to enable constant flushing of the opening with slurry during use. Features may be used individually or in combination. As described above, and in accordance with one or more embodiments of the present invention, one such feature may be a wedge or ramp shape of one or more edges of the opening. Another such feature may include one or more grooves connected with the opening. Radial grooves, circumferential grooves, or a combination thereof may be connected or continuous with the opening. The groove depth may be equal to the opening depth where they connect, with the groove floor ramping up to normal groove depth. Blocked or diverted flow of some grooves may be used so that they do not drain into the opening. A rounded shape of some or all of the corners of the opening may also be used.
In reference to polishing apparatus 1000 and one or more polishing pads described in association with
In one embodiment, disposing the polishing pad above the platen includes adhering the polishing pad to the platen with an adhesive sheet. In a specific such embodiment, adhering the polishing pad to the platen with the adhesive sheet is for protecting a quartz laser site of the optical monitoring device. In another embodiment, polishing the substrate with the chemical mechanical polishing slurry includes flushing the chemical mechanical polishing slurry from the aperture. In another embodiment, polishing the substrate with the chemical mechanical polishing slurry includes dispensing a slurry of sufficient transparency for monitoring the polishing of the substrate with the optical monitoring device. In a specific such embodiment, dispensing the slurry of sufficient transparency includes dispensing a slurry having greater than approximately 80% transmission of a wavelength of light emitted from the optical monitoring device. In another specific such embodiment, dispensing the slurry of sufficient transparency includes dispensing a slurry having less than approximately 1% of opaque components.
Thus, polishing pads with apertures have been disclosed. In accordance with an embodiment of the present invention, a polishing apparatus for polishing a substrate includes a polishing pad having a polishing surface and a back surface. The polishing surface includes a pattern of grooves. An aperture is disposed in the polishing pad from the back surface through to the polishing surface. An adhesive sheet is disposed on the back surface of the polishing pad but not in the aperture. The adhesive sheet provides an impermeable seal for the aperture at the back surface of the polishing pad. In one embodiment, the aperture has a sidewall having a ramp feature with a slope to provide a narrowest region of the aperture at the back surface of the polishing pad and a widest region of the aperture at the polishing surface of the polishing pad. In one embodiment, a first groove of the pattern of grooves is a circumferential groove continuous with the aperture at a first sidewall of the aperture but discontinuous with a second sidewall of the aperture, and a second groove of the pattern of grooves is continuous with the aperture at the second sidewall. In one embodiment, a first groove of the pattern of grooves is a first radial groove continuous with the aperture at a first sidewall of the aperture, a second groove of the plurality of grooves is a second radial groove continuous with the aperture at a second sidewall of the aperture, and the first sidewall is opposite the second sidewall.
This application is a divisional of U.S. patent application Ser. No. 13/184,395, filed on Jul. 15, 2011, the entire contents of which are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13184395 | Jul 2011 | US |
| Child | 14550129 | US |
