Patent Grant
9868185
Embodiments of the present invention are in the field of chemical mechanical polishing (CMP) and, in particular, polishing pads having a foundation layer and a window attached to the foundation layer, and methods of fabricating such polishing pads.
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 having a foundation layer and a window attached to the foundation layer, and methods of fabricating such polishing pads.
In an embodiment, a polishing pad for polishing a substrate includes a foundation layer having a first modulus. A polishing layer is attached to the foundation layer and has a second modulus less than the first modulus. A first opening is through the polishing layer and a second opening is through the foundation layer. The first opening exposes at least a portion of the second opening and exposes a portion of the foundation layer. A window is disposed in the first opening and is attached to the exposed portion of the foundation layer.
In another embodiment, a method of fabricating a polishing pad for polishing a substrate includes forming a first opening through a polishing layer. The polishing layer has a polishing side and a back side and having a modulus. The method further includes attaching a foundation layer to the back side of the polishing layer. The foundation layer has a modulus greater than the modulus of the polishing layer. The method further includes, subsequent to attaching the foundation layer to the back side of the polishing layer, forming a second opening through the foundation layer. The first opening exposes at least a portion of the second opening and exposes a portion of the foundation layer. The method further includes inserting a window in the first opening and attaching the window to the exposed portion of the foundation layer.
In another embodiment, a method of fabricating a polishing pad for polishing a substrate includes forming a first opening through a polishing layer. The polishing layer has a polishing side and a back side and has a modulus. The method also includes forming a second opening through a foundation layer. The foundation layer has a polishing side and a back side and has a modulus greater than the modulus of the polishing layer. The method also includes attaching a window to the polishing side of the foundation layer. The window covers at least a portion of the second opening. The method also includes, subsequent to attaching the window to the polishing side of the foundation layer, attaching the polishing layer to the foundation layer. The first opening formed through the polishing layer surrounds the window.
Polishing pads having a foundation layer and a window attached to the foundation layer, and methods of fabricating such polishing pads, are described herein. In the following description, numerous specific details are set forth, such as specific polishing pad architectures, designs and compositions, 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 combination of a slurry with a polishing pad to perform CMP of a semiconductor 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.
One or more embodiments described herein are directed to polishing pads having a window attached to an underlying to the foundation layer of the polishing pad. The window provides an avenue for optical end-point detection of a CMP process as monitored through the polishing pad, even in the case that the polishing pad includes an opaque polishing layer.
To provide context, traditional approaches to incorporating a window into a polishing pad have included attaching the window to an underlying sub pad. However, the sub pad is typically a low density foam (or low density impregnated felt) and does not necessarily offer good mechanical strength. For example, attaching the window to a sub pad has been known to lead to window issues during CMP processing, such as slurry leakage and window pop-out. Another approach has been to attach the window to the pad itself. However, such an approach requires that the pad is thicker than the window, which can lead to either an overly thin window or an overly thick pad. An overly thin window can lead to CMP issues at the end of the pad life. An overly thick pad, on the other hand, provides limited design options for tuning CMP performance. One or more embodiments, described herein address such issues by including a window attached to a foundation layer of a polishing pad.
To provide further context, polishing pads for CMP operations may have trade-offs in performance such as a trade-off between across-wafer polishing uniformity versus within die polishing uniformity. For example, hard polishing pads may exhibit good die-level planarization, but poor across-wafer uniformity. They may also scratch a substrate being polished. On the other hand, soft polishing pads may exhibit poor die-level planarization (e.g., they may cause dishing within die), but good wafer-level uniformity. An approach to mitigating the above performance trade-off may be to decouple within-wafer and within-die polishing effects. One or more embodiments, described herein address such issues by including a foundation layer together with a polishing layer.
A first exemplary polishing pad includes a foundation layer having a window attached to the foundation layer by an adhesive layer. For example,
Referring to
With reference to
Referring again to
Referring again to
Referring to
Referring again to
In an embodiment, the window 112 is composed of a material transparent to a broad spectrum irradiation approximately in the range of 300-800 nanometers. In an embodiment, the window 112 is composed of a material such as, but not limited to, a polyethylene terephthalate material, a polyurethane material, a cyclic olefin copolymer material, a polycarbonate material, a polyester material, a polypropylene material, or a polyethylene material.
In an embodiment, the foundation layer 102 is composed of a material such as, but not limited to, a polycarbonate material, an epoxy board material, a polyurethane material, a composite fiber board, a polymethylmethacrylate (PMMA) material, or a cyclic olefin copolymer material. In an embodiment, the foundation layer 102 has an energy loss factor of less than approximately 100 KEL at 1/Pa at 40° C. KEL is parameter for predicting polishing performance. ASTM D4092-90 (“Standard Terminology Relating to Dynamic Mechanical Measurements of Plastics”) defines this parameter as the energy per unit volume lost in each deformation cycle. In other words, it is a measure of the area within the stress-strain hysteresis loop. The Energy Loss Factor (KEL) is a function of both tan δ and the elastic storage modulus (E′) and may be defined by the following equation: KEL=tan δ*1012/[E′*(1+tan δ2)] where E′ is in Pascals. The ratio of elastic stress to strain is the storage (or elastic) modulus and the ratio of the viscous stress to strain is the loss (or viscous) modulus. When testing is performed in tension, flex, or compression, E′ and E″ designate the storage and loss modulus, respectively. The ratio of the loss modulus to the storage modulus is the tangent of the phase angle shift (δ) between the stress and the strain. Thus, E″/E′=tan δ and is a measure of the damping ability of the material.
In an embodiment, the polishing layer 104 is composed of a thermoset polyurethane material. In one such embodiment, the foundation layer 102 is composed of a polycarbonate layer, and the window 112 is composed of a polyethylene terephthalate material. In an embodiment, the polishing layer 104 has an energy loss factor of greater than approximately 1000 KEL at 1/Pa at 40° C. Referring to
In an embodiment, the polishing layer 104 is a homogeneous polishing layer. In one such embodiment, the homogeneous polishing layer is composed of a thermoset polyurethane material. For example, in a specific embodiment, the homogeneous polishing layer 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 layer 104. For example, in an embodiment, the term “homogeneous” excludes polishing layers 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, although the polishing layer 104 is composed of a thermoset material, the corresponding foundation layer 102 is composed of a thermoplastic material, such as a polycarbonate.
The material of polishing layer 104 may be molded. The term “molded” may be used to indicate that the polishing layer 104 is formed in a formation mold. In an embodiment, the molded polishing layer 104, upon conditioning and/or polishing, has a polishing surface roughness approximately in the range of 1-5 microns root mean square. In one embodiment, the molded polishing layer 104, upon conditioning and/or polishing, has a polishing surface roughness of approximately 2.35 microns root mean square.
The material of polishing layer 104 may include pore-forming features. In an embodiment, the polishing layer 104 has a pore density of closed cell pores approximately in the range of 6%-50% total void volume. 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) the polishing layer 104 of the polishing pad 100. 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 the polishing layer 104. In an embodiment, although the polishing layer 104 includes pore-forming features, the corresponding foundation layer 102 does not and is non-porous.
In an embodiment, the polishing layer 104 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 polishing layer 104 is opaque in most part, or due entirely to, the inclusion of an opacifying particle filler, such as a lubricant, throughout (e.g., as an additional component in) the polishing layer 104. In a specific embodiment, the opacifying 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 grooves 105 of the polishing layer 104 may have a pattern suitable for polishing during a CMP operation. Individual grooves 105 may be from about 2 to about 100 mils wide at any given point on each groove. In some embodiments, the grooves 105 are about 15 to about 50 mils wide at any given point on each groove. The grooves 105 may be of uniform width, variable width, or any combinations thereof. In some embodiments, the grooves 105 of a groove pattern are all of uniform width. In some embodiments, however, some of the grooves 105 of a groove 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 layer 104. In some embodiments, groove width decreases with increasing distance from the center of the polishing layer 104. In some embodiments, grooves 105 of uniform width alternate with grooves 105 of variable width.
In accordance with the previously described depth and width dimensions, individual grooves 105 may be of uniform volume, variable volume, or any combinations thereof. In some embodiments, the grooves 105 are all of uniform volume. In some embodiments, however, groove volume increases with increasing distance from the center of the polishing layer 104. In some other embodiments, groove volume decreases with increasing distance from the center of the polishing layer 104. In some embodiments, grooves of uniform volume alternate with grooves of variable volume.
Grooves 105 of the groove patterns described herein may have a pitch from about 30 to about 1000 mils. In some embodiments, the grooves 105 have a pitch of about 125 mils. For a circular polishing layer 104, groove pitch is measured along the radius of the circular polishing layer 104. In CMP belts, groove pitch is measured from the center of the CMP belt to an edge of the CMP belt. The grooves 105 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 layer 104. In some embodiments, the pitch of the grooves 105 in one sector varies with increasing distance from the center of the polishing layer 104 while the pitch of the grooves 105 in an adjacent sector remains uniform. In some embodiments, the pitch of the grooves 105 in one sector increases with increasing distance from the center of the polishing layer 104 while the pitch of the grooves in an adjacent sector increases at a different rate. In some embodiments, the pitch of the grooves 105 in one sector increases with increasing distance from the center of the polishing layer 104 while the pitch of the grooves 105 in an adjacent sector decreases with increasing distance from the center of the polishing layer 104. 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.
In an embodiment, when the grooves 105 of the polishing layer 104 are formed during a molding process, the positioning of the resulting polishing layer 104 during formation of the polishing layer 104 in a mold can be determined after removal of the polishing layer 104 from the mold. That is, such a polishing layer 104 may be designed (e.g., with clocking marks) to provide traceability back to the molding process. Thus, in one embodiment, the polishing layer 104 is a molded polishing layer, and a feature included therein indicates a location of a region in a mold used for forming a resulting polishing layer 104.
In an embodiment, as a pairing, a combination of the foundation layer 102 and the polishing layer 104 has an energy loss factor of less than approximately 1000 KEL at 1/Pa at 40° C. In an embodiment, the polishing layer 104 has an elastic storage modulus (E′) at 40 degrees Celsius approximately in the range of 50 MPa-100 MPa, and the foundation layer 102 has an elastic storage modulus (E′) at 40 degrees Celsius approximately in the range of 1500 MPa-3000 MPa. In an embodiment, the foundation layer 102 has a hardness approximately in the range of 70-90 Shore D, and the polishing layer 104 has a hardness approximately in the range of 20-65 Shore D.
As mentioned in an embodiment above, the polishing layer 104 may be attached to the foundation layer 102 by an adhesive layer 116 such as a PSA layer. In another embodiment, however, the polishing layer 104 is bonded directly to the foundation layer 102. That is, the polishing layer 104 is in direct contact with the foundation layer 102. In one embodiment, then, “bonded directly to” describes direct contact with no intervening layers (such as pressure sensitive adhesive layers) or otherwise glue-like or adhesive films. In a specific such embodiment, the polishing layer 104 is covalently bonded to the foundation layer 102. In an embodiment, the term “covalently bonded” refers to arrangements where atoms from a first material (e.g., the material of a polishing layer) are cross-linked or share electrons with atoms from a second material (e.g., the material of a foundation layer) to effect actual chemical bonding. Covalent bonding is distinguished from mechanical bonding, such as bonding through screws, nails, glues, or other adhesives. In another specific embodiment, the polishing layer 104 is not covalently bonded, but is rather only electrostatically bonded, to the foundation layer 102. Such electrostatic bonding may involve van der Waals type interactions between the foundation layer 102 and the polishing layer 104.
In either case, whether the polishing layer 104 is attached to the foundation layer 102 by an adhesive layer 116 or is bonded directly to the foundation layer 102, peel resistance may provide an indication of the strength and extent to which a polishing layer 104 is coupled with a foundation layer 102. In an embodiment, the foundation layer 102 and the polishing layer 104 have a peel resistance sufficient to withstand a shear force applied during the useful lifetime of the polishing pad 100.
In an embodiment, a surface roughness is used at the interface of the polishing layer 104 and the foundation layer 102 to enhance bond strength of these two portions of the polishing pad 100. In one such embodiment, the foundation layer 102 has a surface roughness greater than approximately 1 micrometer Ra (root mean square). In a specific such embodiment, the surface roughness is approximately in the range of 5-10 micrometers Ra (root mean square). However, in another embodiment, substantial surface roughness is not included and the interface of a polishing layer 104, and the foundation layer 102 is particularly smooth. The strength of such a smooth interface may be independent of surface roughness or may not need further strengthening by the inclusion of such surface roughness. In one such embodiment, the foundation layer 102 has a smooth surface with a surface roughness less than approximately 1 micrometer Ra (root mean square).
In an embodiment, materials of polishing layer 104 and corresponding foundation layer 102 may each have defined dimensions suitable to provide desired polishing characteristics, either as individual components or collectively for the polishing pad 100 as an entirety. Referring to
A first exemplary method of fabricating a polishing pad includes attaching a window to a foundation layer subsequent to attaching a polishing layer to the foundation layer. As an example,
Referring to
Referring to
Referring to
In another embodiment, the window 112 is attached to the exposed portion 110 of the foundation layer 102 by welding a portion of the window 112 to the exposed portion 110 of the foundation layer 102, as is described below in association with
A second exemplary polishing pad includes a foundation layer having a window attached to the foundation layer by a welded region. For example,
Referring to
Referring again to
A third exemplary polishing pad includes a foundation layer having a window attached to the foundation layer by a snap-fit feature. For example,
Referring to
Referring again to
It is to be appreciated that other approaches may be used to fabricate polishing pads, such as polishing pads 100, 300 and 400, in addition or as an alternate to the method described above in association with
Referring to
Referring to
In an embodiment, the window 112 is attached to the polishing side 504 of the foundation layer 102 with an adhesive layer, as was described in association with
In another embodiment, the window 112 is attached to the polishing side 504 of the foundation layer 102 by welding a portion of the window 112 to the foundation layer 102, as was described in association with
Referring to
In an embodiment, polishing pads described herein, such as polishing pads 100, 300 or 400, 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. In an embodiment, a polishing pad 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.
Providing context for one or more embodiments described herein, it is to be appreciated that conventional approaches to fabricating and using soft polishing pads may have limitations. For example, casted soft pads may offer low defect characteristics but compromised planarization performance. There may be a need for polishing pads that offer both low defect characteristics yet high planarization performance during polishing operations. Similarly, conventional approaches to fabricating and using hard polishing pads may have limitations. For example, faster gelling speeds possibly inherent in harder urethane formulations may force process compromises that impact pad uniformity and limit formulation options. There may be a need for an approach suitable to produce and implement hard pads that avoid such compromises. Additionally, as noted above, it may be desirable to decouple the properties of the polishing surface of a pad from its bulk properties, such that the properties of each may be separately optimized.
In accordance with an embodiment of the present invention, polishing pads with foundation layers of a material different from the material of the polishing surface are described above. Such polishing pads may be fabricated or implemented in polishing approaches suitable to address the above described compromises made for conventional pads. In one embodiment, a composite polishing pad includes a foundation fabricated from a stable, essentially non-compressible, inert material to which a polishing layer is attached. A foundation layer having a relatively higher modulus may provide support and strength for pad integrity while a polishing layer having relatively lower modulus may reduce scratching, enabling decoupling of the material properties of the polishing layer and the remainder of the polishing pad.
Polishing pads described herein may be suitable for use with a variety of chemical mechanical polishing apparatuses. As an example,
Referring to
Thus, polishing pads having a foundation layer and a window attached to the foundation layer, and methods of fabricating such polishing pads, have been disclosed. In accordance with an embodiment of the present invention, a polishing pad for polishing a substrate includes a foundation layer having a first modulus. A polishing layer is attached to the foundation layer and has a second modulus less than the first modulus. A first opening is through the polishing layer and a second opening is through the foundation layer. The first opening exposes at least a portion of the second opening and exposes a portion of the foundation layer. A window is disposed in the first opening and is attached to the exposed portion of the foundation layer.
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