WO2003099518A1 - Microporous polishing pad - Google Patents

Abstract

The invention provides polishing pads for chemical-mechanical polishing comprising a porous foam and a method for their production. In one embodiment, the porous foam has an average pore size of 50 μm or less, wherein 75% or more of the pores have a pore size within 20 μm or less of the average pore size. In another embodiment, porous foam has an average pore size of 20 μm or less. In yet another embodiment, the porous foam has a mufti-modal pore size distribution. The method of production comprises (a) combining a polymer resin with a supercritical gas to produce a single-phase solution and (b) forming a polishing pad from the single-phase solution, wherein the supercritical gas is generated by subjecting a gas to an elevated temperature and pressure.

Description

MICROPOROUS POLISHING PADS

FIELD OF THE INVENTION

[0001] This invention pertains to polishing pads for chemical-mechanical polishing comprising a porous foam having a uniform pore size distribution.

BACKGROUND OF THE INVENTION

[0002] Chemical-mechanical polishing ("CMP") processes are used in the manufacturing of microelectronic devices to form flat surfaces on semiconductor wafers, field emission displays, and many other microelectronic substrates. For example, the manufacture of semiconductor devices generally involves the formation of various process layers, selective removal or patterning of portions of those layers, and deposition of yet additional process layers above the surface of a semiconducting substrate to form a semiconductor wafer. The process layers can include, by way of example, insulation layers, gate oxide layers, conductive layers, and layers of metal or glass, etc. It is generally desirable in certain steps of the wafer process that the uppermost surface of the process layers be planar, i.e., flat, for the deposition of subsequent layers. CMP is used to planarize process layers wherein a deposited material, such as a conductive or insulating material, is polished to planarize the wafer for subsequent process steps.

[0003] In a typical CMP process, a wafer is mounted upside down on a carrier in a CMP tool. A force pushes the carrier and the wafer downward toward a polishing pad. The carrier and the wafer are rotated above the rotating polishing pad on the CMP tool's polishing table. A polishing composition (also referred to as a polishing slurry) generally is introduced between the rotating wafer and the rotating polishing pad during the pohshing process. The polishing composition typically contains a chemical that interacts with or dissolves portions of the uppermost wafer layer(s) and an abrasive material that physically removes portions of the layer(s). The wafer and the polishing pad can be rotated in the same direction or in opposite directions, whichever is desirable for the particular polishing process being carried out. The carrier also can oscillate across the polishing pad on the polishing table.

[0004] Polishing pads used in chemical-mechanical polishing processes are manufactured using both soft and rigid pad materials, which include polymer-impregnated fabrics, microporous films, cellular polymer foams, non-porous polymer sheets, and sintered thermoplastic particles. A pad containing a polyurethane resin impregnated into a polyester non- woven fabric is illustrative of a polymer-impregnated fabric polishing pad. Microporous polishing pads include microporous urethane films coated onto a base material, which is often an impregnated fabric pad. These polishing pads are closed cell, porous films. Cellular polymer foam polishing pads contain a closed cell structure that is randomly and uniformly distributed in all three dimensions. Non-porous polymer sheet polishing pads include a polishing surface made -from solid polymer sheets, which have no intrinsic ability to transport slurry particles (see, for example, U.S. Patent 5,489,233). These solid pohshing pads are externally modified with large and/or small grooves that are cut into the surface of the pad purportedly to provide channels for the passage of slurry during chemical-mechanical polishing. Such a non-porous polymer polishing pad is disclosed in U.S. Patent 6,203,407, wherein the polishing surface of the polishing pad comprises grooves that are oriented in such a way that purportedly improves selectivity in the chemical-mechanical polishing. Also in a similar fashion, U.S. Patents 6,022,268, 6,217,434, and 6,287,185 disclose hydrophilic polishing pads with no intrinsic ability to absorb or transport slurry particles. The pohshing surface purportedly has a random surface topography including microaspersities that have a dimension of 10 μm or less and formed by solidifying the pohshing surface and macro defects (or macrotexture) that have a dimension of 25 μm or greater and formed by cutting. Sintered polishing pads comprising a porous open-celled structure can be prepared from thermoplastic polymer resins. For example, U.S. Patents 6,062,968 and 6,126,532 disclose polishing pads with open-celled, microporous substrates, produced by sintering thermoplastic resins. The resulting polishing pads preferably have a void volume between 25 and 50% and a density of 0.7 to 0.9 g/cm³. Similarly, U.S. Patents 6,017,265, 6,106,754, and 6,231,434 disclose pohshing pads with uniform, continuously interconnected pore structures, produced by sintering thermoplastic polymers at high pressures in excess of 689.5 kPa (100 psi) in a mold having the desired final pad dimensions.

[0005] In addition to groove patterns, pohshing pads can have other surface features to provide texture to the surface of the polishing pad. For example, U.S. Patent 5,609,517 discloses a composite pohshing pad comprising a support layer, nodes, and an upper layer, all with different hardness. U.S. Patent 5,944,583 discloses a composite polishing pad having circumferential rings of alternating compressibility. U.S. Patent 6,168,508 discloses a polishing pad having a first polishing area with a first value of a physical property (e.g., hardness, specific gravity, compressibility, abrasiveness, height, etc.) and a second polishing area with a second value of the physical property. U.S. Patent 6,287,185 discloses a polishing pad having a surface topography produced by a thermoforming process. The surface of the polishing pad is heated under pressure or stress resulting in the formation of surface features.

[0006] Polishing pads having a microporous foam structure are commonly known in the art. For example, U.S. Patent 4,138,228 discloses a pohshing article that is microporous and hydrophilic. U.S. Patent 4,239,567 discloses a flat microcellular polyurethane polishing pad for polishing silicon wafers. U.S. Patent 6,120,353 discloses a polishing method using a suede-like foam polyurethane polishing pad having a compressibility lower than 9% and a high pore density of 150 pores/cm² or higher. EP 1 108 500 Al discloses a polishing pad of micro-rubber A-type hardness of at least 80 having closed cells of average diameter less than 1000 μm and a density of 0.4 to 1.1 g/ml.

[0007] Although several of the above-described polishing pads are suitable for their intended purpose, a need remains for an improved pohshing pad that provides effective planarization, particularly in substrate polishing by chemical-mechanical pohshing. In addition, there is a need for pohshing pads having improved polishing efficiency, improved slurry flow across and within the polishing pad, improved resistance to corrosive etchants, and/or improved pohshing uniformity. Finally, there is a need for polishing pads that can be produced using relatively low cost methods and which require little or no conditioning prior to use.

[0008] The invention provides such a polishing pad. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

[0009] The invention provides polishing pads for chemical-mechanical polishing comprising a porous foam. In a first embodiment, the porous foam has an average pore size of 50 μm or less, wherein 75% or more of the pores have a pore size within 20 μm or less of the average pore size. In a second embodiment, the polishing pad comprises a porous foam with an average pore size of 1 μm to 20 μm. In a third embodiment, the pohshing pad comprises a thermoplastic polyurethane foam having an average pore size of 50 μm or less, wherein the thermoplastic polyurethane has a Melt Flow Index of 20 or less, a molecular weight of 50,000 g mol to 300,000 g/moL, and a polydispersity index of 1.1 to 6. In a fourth embodiment, the polishing pad is a polyurethane polishing pad comprising a polishing surface with no externally produced surface texture, which can polish a silicon dioxide wafer at a rate of at least 600 A/min with a carrier downforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100 mVmin, a platen rotation speed of 60 rpm, and a carrier rotation speed of 55 rpm to 60 rpm. In a fifth embodiment, the polishing pad comprises a porous foam having a multi-modal pore size distribution, wherein the multi-modal distribution has 20 or less pore size maxima.

[0010] The invention further provides a method for producing the polishing pads comprising (a) combining a polymer resin with a supercritical gas to produce a single-phase solution, wherein the supercritical gas is generated by subjecting a gas to an elevated temperature and pressure, and (b) forming a pohshing pad from the single-phase solution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a scanning electron microscopy (SEM) image (100X magnification) of a cross-section of an extruded porous foam rod produced with a C0₂ concentration of 1.26% and a melt temperature of 212°C (414°F).

[0012] FIG. 2 is a plot of carbon dioxide concentration versus density illustrating the relationship between the concentration of CO₂ in a single-phase solution of polymer resin and the density of the resulting porous foam prepared therefrom.

[0013] FIG. 3 is a scanning electron microscopy (SEM) image (80X magnification) of a cross-section of an extruded porous foam sheet having an average pore size of 8 μm, a density of 0.989 g/cm³, and a cell density of greater than 10⁶ cells per cm³.

[0014] FIG. 4 is a scanning electron microscopy (SEM) image (50X magnification) of the top surface of an extruded porous foam sheet having an average pore size of 15 μm, an density of 0.989 g/cm³, a cell density of greater than 10° cells per cm³, and no surface macrotexture.

[0015] FIG. 5 is a plot of silicon dioxide removal rate versus the number of silicon dioxide wafers polished using a microporous foam polishing pad of the invention.

[0016] FIG. 6 is a plot of silicon dioxide removal rate versus the number of silicon dioxide wafers polished comparing a microporous foam polishing pad of the invention and a solid, non-porous polishing pad, wherein the polishing pads are grooved and buffed.

[0017] FIG. 7a is a scanning electron microscopy (SEM) image (20X magnification) of the top surface of a solid, non-porous polymer sheet having a grooved macrotexture that is glazed and clogged with polishing debris after polishing 20 silicon dioxide wafers, wherein the polishing pads are buffed and conditioned.

[0018] FIG. 7b is a scanning electron microscopy (SEM) image (20X magnification) of the top surface of an extruded porous foam sheet having an average pore size of 15 μm, an density of 0.989 g/cm³, a cell density of greater than 10° cells per cm³, as well as a grooved macrotexture that is free of pohshing debris after pohshing 20 silicon dioxide wafers (buffed and conditioned).

[0019] FIG. 7c is a scanning electron microscopy (SEM) image (20X magnification) of the top surface of an extruded porous foam sheet having an average pore size of 15 μm, an density of 0.989 g/cm³, a cell density of greater than 10⁶ cells per cm³, as well as a grooved macrotexture that is free of pohshing debris after polishing 20 silicon dioxide wafers (buffed, no conditioning).

[0020] FIGS. 8a, 8b, and 8c are Energy Dispersive X-ray (EDX) silica mapping images of a solid pohshing pad (Fig. 8a), a microporous foam pohshing pad of the invention (Fig. 8b), and a conventional closed cell polishing pad (Fig. 8c) showing the extent of penetration of the silica abrasive through the thickness of the polishing pad after pohshing 20 silicon dioxide blanket wafers.

[0021] FIG. 9 is a plot of time (s) versus the remaining step height (in A) for a 40% dense feature of a patterned silicon dioxide wafer comparing the use of a solid, non-porous polishing pad, a microporous foam polishing pad of the invention, and a conventional microporous closed cell polishing pad.

[0022] FIG. 10 is a plot of time (s) versus the remaining step height (in A) for a 70% dense feature of a patterned silicon dioxide wafer comparing the use of a solid, non-porous polishing pad, a microporous foam polishing pad of the invention, and a conventional microporous closed cell polishing pad.

[0023] FIG. 11 is an SEM image of a solid thermoplastic polyurethane sheet at a magnification of 350X.

[0024] FIG. 12 is an SEM image of a solid thermoplastic polyurethane sheet at a magnification of 7500X that has been treated by pressurized gas injection to produce a foam having an average cell size of 0.1 μm.

[0025] FIG. 13 is an SEM image of a solid thermoplastic polyurethane sheet at a magnification of 20000X that has been treated by pressurized gas injection to produce a foam having an average cell size of 0.1 μm.

[0026] FIG. 14 is an SEM image of a solid thermoplastic polyurethane sheet at a magnification of 350X that has been treated by pressurized gas injection to produce a foam having an average cell size of 4 μm.

[0027] FIG. 15 is an SEM image of a solid thermoplastic polyurethane sheet at a magnification of 1000X that has been treated by pressurized gas injection to produce a foam having an average cell size of 4 μm.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In a first embodiment, the invention is directed to a pohshing pad for chemical- mechanical pohshing comprising a porous foam with an average pore size of 50 μm or less. Preferably, the porous foam has an average pore size of 40 μm or less, or even 30 μm or less (e.g. 20 μm or less). Typically, the porous foam has an average pore size of 1 μm or more (e.g., 3 μm or more, or 5 μm or more).

[0029] In a second, preferred embodiment, the porous foam has an average pore size of 1 μm to 20 μm. Preferably, the porous foam has an average pore size of 1 μm to 15 μm (e.g., 1 μmto 10 μm). [0030] The porous foam of the polishing pads described herein has a highly uniform distribution of pore sizes (i.e., cell sizes). Typically, 75% or more (e.g., 80% or more, or 85% or more) of the pores (e.g., cells) in the porous foam have a pore size distribution of ±20 μm or less (e.g., ±10 μm or less, ±5 μm or less, or ±2 μm or less). In other words, 75% or more (e.g., 80% or more, or 85% or more) of the pores in the porous foam have a pore size within 20 μm or less (e.g., ±10 μm or less, ±5 μm or less, or ±2 μm or less) of the average pore size. Preferably, 90% or more (e.g., 93% or more, 95% or more, or 97% or more) of the pores (e.g., cells) in the porous foam have a pore size distribution of ±20 μm or less (e.g., ±10 μm or less, ±5 μm or less, or ±2 μm or less).

[0031] Typically, the porous foam comprises predominantly closed cells (i.e., pores); however, the porous foam can also comprise open cells. Preferably, the porous foam comprises at least 5% or more (e.g., at least 10% or more) closed cells. More preferably, the porous foam comprises at least 20% or more (e.g., at least 40% or more, or at least 60% or more) closed cells.

[0032] The porous foam typically has a density of 0.5 g/cm³ or greater (e.g., 0.7 g/cm³ or greater, or even 0.9 g/cm³ or greater) and a void volume of 25% or less (e.g., 15% or less, or even 5% or less). Typically the porous foam has a cell density of 10⁵ cells/cm³ or greater (e.g., 10⁶ cells/cm³ or greater). The cell density can be determined by analyzing a cross- sectional image (e.g., an SEM image) of a porous foam material with an image analysis software program such as Optimas® imaging software and ImagePro® imaging software, both by Media Cybernetics, or Clemex Vision® imaging software by Clemex Technologies.

[0033] The porous foam can comprise any suitable material, typically a polymer resin. The porous foam preferably comprises a polymer resin selected from the group consisting of thermoplastic elastomers, thermoplastic polyurethanes, polyolefins, polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers, polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes, cross-linked polyolefins, polyethers, polyesters, polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes, polyethyleneteraphthalates, polyimides, polyara ides, polyarylenes, polystyrenes, polymethylmethacrylates, copolymers and block copolymers thereof, and mixtures and blends thereof. Preferably, the polymer resin is thermoplastic polyurethane. [0034] The polymer resin typically is a pre-formed polymer resin; however, the polymer resin also can be formed in situ according to any suitable method, many of which are known in the art (see, for example, Szycher's Handbook of Polyurethanes CRC Press: New York, 1999, Chapter 3). For example, thermoplastic polyurethane can be formed in situ by reaction of urethane prepolymers, such as isocyanate, di-isocyanate, and tri-isocyanate prepolymers, with a prepolymer containing an isocyanate reactive moiety. Suitable isocyanate reactive moieties include amines and polyols.

[0035] The selection of the polymer resin will depend, in part, on the rheology of the polymer resin. Rheology is the flow behavior of a polymer melt. For Newtonian fluids, the viscosity is a constant defined by the ratio between the shear stress (i.e., tangential stress, σ) and the shear rate (i.e., velocity gradient, dγ/dt). However, for non-Newtonian fluids, shear rate thickening (dilatent) or shear rate t-hinning (pseudo-plastic) may occur. In shear rate thinning cases, the viscosity decreases with increasing shear rate. It is this property that allows a polymer resin to be used in melt fabrication (e.g., extrusion, injection molding) processes. In order to identify the critical region of shear rate thinning, the rheology of the polymer resins must be dete-rtnined. The rheology can be dete-πrdned by a capillary technique in which the molten polymer resin is forced under a fixed pressure through a capillary of a particular length. By plotting the apparent shear rate versus viscosity at different temperatures, the relationship between the viscosity and temperature can be determined. The Rheology Processing Index (RPI) is a parameter that identifies the critical range of the polymer resin. The RPI is the ratio of the viscosity at a reference temperature to the viscosity after a change in temperature equal to 20°C for a fixed shear rate. When the polymer resin is thermoplastic polyurethane, the RPI preferably is 2 to 10 (e.g., 3 to 8) when measured at a shear rate of 1501/s and a temperature of 205°C.

[0036] Another polymer viscosity measurement is the Melt Flow Index (MFI) which records the amount of molten polymer (in grams) that is extruded from a capillary at a given temperature and pressure over a fixed amount of time. For example, when the polymer resin is thermoplastic polyurethane or polyurethane copolymer (e.g., a polycarbonate silicone-based copolymer, a polyurethane fluorine-based copolymers, or a polyurethane siloxane-segmented copolymer), the MFI preferably is 20 or less (e.g., 15 or less) over 10 minutes at atemperature of 210°C and a load of 2160 g. When the polymer resin is an elastomeric polyolefin or a polyolefin copolymer (e.g., a copolymer comprising an ethylene α-olefϊn such as elastomeric or normal ethylene-propylene, ethlene-hexene, ethylene- octene, and the like, an elastomeric ethylene copolymer made from metallocene based catalysts, or a polypropylene-styrene copolymer), the MFI preferably is 5 or less (e.g., 4 or less) over 10 minutes at a temperature of 210°C and a load of 2160 g. When the polymer resin is a nylon or polycarbonate, the MFI preferably is 8 or less (e.g., 5 or less) over 10 minutes at a temperature of 210°C and a load of 2160 g.

[0037] The rheology of the polymer resin can depend on the molecular weight, polydispersity index (PDI), the degree of long-chain branching or cross-linking, glass transition temperature (T_g), and melt temperature (T_m) of the polymer resin. When the polymer resin is thermoplastic polyurethane or polyurethane copolymer (such as the copolymers described above), the weight average molecular weight (M_w) is typically 50,000 g/mol to 300,000 g/mol, preferably 70,000 g/mol to 150,000 g/mol, with a PDI of 1.1 to 6, preferably 2 to 4. Typically, the thermoplastic polyurethane has a glass transition temperature of 20°C to 110°C and a melt transition temperature of 120°C to 250°C. When the polymer resin is an elastomeric polyolefin or a polyolefin copolymer (such as the copolymers described above), the weight average molecular weight (M_w) typically is 50,000 g/mol to 400,000 g mol, preferably 70,000 g/mol to 300,000 g/mol, with a PDI of 1.1 to 12, preferably 2 to 10. When the polymer resin is nylon or polycarbonate, the weight average molecular weight (M_w) typically is 50,000 g/mol to 150,000 g/mol, preferably 70,000 g/mol to 100,000 g mol, with a PDI of 1.1 to 5, preferably 2 to 4.

[0038] The polymer resin selected for the porous foam preferably has certain mechanical properties. For example, when the polymer resin is a thermoplastic polyurethane, the Flexural Modulus (ASTM D790) preferably is 350 MPa (-50,000 psi) to 1000 MPa (-150,000 psi), the average % compressibility is 7 or less, the average % rebound is 35 or greater, and the Shore D hardness (ASTM D2240-95) is 40 to 90 (e.g., 50 to 80).

[0039] In a third embodiment, the pohshing pad comprises a porous foam comprising a thermoplastic polyurethane, wherein the porous foam has an average pore size of 50 μm or less (e.g., 40 μm or less, or 25 μm or less) and wherein the thermoplastic polyurethane has a Melt Flow Index (MFI) of 20 or less, an RPI of 2 to 10 (e.g., 3 to 8), and a molecular weight (MW) of 50,000 g/mol to 300,000 g/mol, with a PDI of 1.1 to 6 (e.g., 2 to 4). Preferably, the thermoplastic polyurethane has a Flexural Modulus of 350 MPa (-50,000 psi) to 1000 MPa (-150,000 psi), an average % compressibility of 7 or less, an average % rebound of 35 or greater, and a Shore D hardness of 40 to 90 (e.g., 50 to 80). Such a polishing pad can have one or more physical characteristics (e.g., pore size and polymer properties) described herein for the other embodiments of the invention.

[0040] In a fourth embodiment, the polishing pad is a polyurethane polishing pad comprising a polishing surface, which in the absence of any externally produced surface texture, can polish a silicon dioxide wafer with a polishing rate of at least 600 A min with a carrier downforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100 ml/min, a platen rotation speed of 60 rpm, and a carrier rotation speed of 55 rpm to 60 rpm. The pohshing pad of the fourth embodiment does not contain abrasive particles suspended in the foam and is used in conjunction with a polishing composition (i.e., slurry) containing metal oxide particles, in particular, Semi-Sperse® D7300 pohshing composition sold by Cabot Microelectronics Corporation. Typically, the polishing pad can polish a silicon dioxide wafer with a pohshing rate of at least 800 A/min or even at least 1000 A/min using the pohshing parameters recited above. The pohshing pad has a void volume of 25% or less and comprises pores having an average pore size of 50 μm or less (e.g., 40 μm or less). The polishing pad also can polish silicon dioxide blanket wafers such that the silicon dioxide blanket wafers have low non-uniformity values of only 2% to 4%. Such a polishing pad can have one or more physical characteristics (e.g., pore size and polymer properties) described herein for the other embodiments of the invention.

[0041] In a fifth embodiment, the pohshing pad comprises a porous foam having a multi-modal distribution of pore sizes. The term "multi-modal" means that the porous foam has a pore size distribution comprising at least 2 or more (e.g., 3 or more, 5 or more, or even 10 or more) pore size maxima. Typically the number of pore size maxima is 20 or less (e.g., 15 or less). A pore size maxima is defined as a peak in the pore size distribution whose area comprises 5% or more by number of the total number of pores. Preferably, the pore size distribution is bimodal (i.e., has two pore size maxima).

[0042] The multi-modal pore size distribution can have pore size maxima at any suitable pore size values. For example, the multi-modal pore size distribution can have a first pore size maximum of 50 μm or less (e.g., 40 μm or less, 30 μm or less, or 20 μm or less) and a second pore size maximum of 50 μm or more (e.g., 70 μm or more, 90 μm or more, or even 120 μm or more). The multi- modal pore size distribution alternatively can have a first pore size maximum of 20 μm or less (e.g., 10 μm or less, or 5 μm or less) and a second pore size maximum of 20 μm or more (e.g., 35 μm or more, 50 μm or more, or even 75 μm or more).

[0043] The porous foam of the fifth embodiment can comprise any suitable polymer resin, for example, the porous foam can comprise any of the polymer resins described herein. Preferably, the porous foam comprises a thermoplastic polyurethane. The polymer resin can have any of the physical, mechanical, or chemical properties described herein with respect to the other embodiments.

[0044] The porous foam of the pohshing pads described herein optionally further comprises a water absorbent polymer. The water absorbent polymer desirably is selected from the group consisting of amorphous, crystalline, or cross-linked polyacrylamide, polyacrylic acid, polyvinylalcohol, salts thereof, and combinations thereof. Preferably, the water absorbent polymers are selected from the group consisting of cross-linked polyacrylamide, cross-linked polyacrylic acid, cross-linked polyvinylalcohol, and mixtures thereof. Such cross-linked polymers desirably are water-absorbent but will not melt or dissolve in common organic solvents. Rather, the water-absorbent polymers swell upon contact with water (e.g., the liquid carrier of a pohshing composition).

[0045] The porous foam of the polishing pads described herein with respect to the first, second, third, and fifth embodiments optionally contains particles that are incorporated into the body of the pad. Preferably, the particles are dispersed throughout the porous foam. The particles can be abrasive particles, polymer particles, composite particles (e.g., encapsulated particles), organic particles, inorganic particles, clarifying particles, and mixtures thereof.

[0046] The abrasive particles can be of any suitable material, for example, the abrasive particles can comprise a metal oxide, such as a metal oxide selected from the group consisting of silica, alumina, ceria, zirconia, chromia, iron oxide, and combinations thereof, or a silicon carbide, boron nitride, diamond, garnet, or ceramic abrasive material. The abrasive particles can be hybrids of metal oxides and ceramics or hybrids of inorganic and organic materials. The particles also can be polymer particles many of which are described in U.S. Patent 5,314,512, such as polystyrene particles, polymethylmethacrylate particles, liquid crystalline polymers (LCP, e.g., Vectra® polymers from Ciba Geigy), polyetheretherketones (PEEK's), particulate thermoplastic polymers (e.g., particulate thermoplastic polyurethane), particulate cross-linked polymers (e.g., particulate cross-linked polyurethane or polyepoxide), or a combination thereof. If the porous foam comprises a polymer resin, then the polymer particle desirably has a melting point that is higher than the melting point of the polymer resin of the porous foam. The composite particles can be any suitable particle containing a core and an outer coating. For example, the composite particles can contain a solid core (e.g., a metal oxide, metal, ceramic, or polymer) and a polymeric shell (e.g., polyurethane, nylon, or polyethylene). The clarifying particles can be phyllosilicates, (e.g., micas such as fluorinated micas, and clays such as talc, kaolinite, montmorillonite, hectorite), glass fibers, glass beads, diamond particles, carbon fibers, and the like.

[0047] The porous foam of the pohshing pads described herein optionally contains soluble particles incorporated into the body of the pad. Preferably, the soluble particles are dispersed throughout the porous foam. Such soluble particles partially or completely dissolve in the liquid carrier of the polishing composition during chemical-mechanical pohshing. Typically, the soluble particles are water-soluble particles. For example, the soluble particles can be any suitable water-soluble particles such as particles of materials selected from the group consisting of dextrins, cyclodextrins, mannitol, lactose, hydroxypropylcelluloses, methylcelluloses, starches, proteins, amorphous non-cross-linked polyvinyl alcohol, amorphous non-cross-linked polyvinyl pyrrohdone, polyacrylic acid, polyethylene oxide, water-soluble photosensitive resins, sulfonated polyisoprene, and sulfonated polyisoprene copolymer. The soluble particles also can be an inorganic water- soluble particles of materials selected from the group consisting of potassium acetate, potassium nitrate, potassium carbonate, potassium bicarbonate, potassium chloride, potassium bromide, potassium phosphate, magnesium nitrate, calcium carbonate, and sodium benzoate. When the soluble particles dissolve, the polishing pad can be left with open pores corresponding to the size of the soluble particle. [0048] The particles preferably are blended with the polymer resin before being formed into a foamed polishing substrate. The particles that are incorporated into the polishing pad can be of any suitable dimension (e.g., diameter, length, or width) or shape (e.g., spherical, oblong) and can be incorporated into the polishing pad in any suitable amount. For example, the particles can have a particle dimension (e.g., diameter, length, or width) of 1 nm or more and/or 2 mm or less (e.g., 0.5 μm to 2 mm diameter). Preferably, the particles have a dimension of 10 nm or more and/or 500 μm or less (e.g., 100 nm to 10 μm diameter). The particles also can be covalently bound to the polymer resin of the porous foam.

[0049] The porous foam of the polishing pads described herein optionally contains solid catalysts that are incorporated into the body of the pad. Preferably, the solid catalysts are dispersed throughout the porous foam. The catalyst can be metallic, non-metallic, or a combination thereof. Preferably, the catalyst is chosen from metal compounds that have multiple oxidation states, such as but not limited to metal compounds comprising Ag, Co, Ce, Cr, Cu, Fe, Mo, Mn, Nb, Ni, Os, Pd, Ru, Sn, Ti, and V.

[0050] The porous foam of the polishing pads described herein optionally contains chelating agents or oxidizing agents. Preferably, the chelating agents and oxidizing agents are dispersed throughout the porous foam. The chelating agents can be any suitable chelating agents. For example, the chelating agents can be carboxylic acids, dicarboxylic acids, phosphonic acids, polymeric chelating agents, salts thereof, and the like. The oxidizing agents can be oxidizing salts or oxidizing metal complexes including iron salts, aluminum salts, peroxides, chlorates, perchlorates, permanganates, persulfates, and the like.

[0051] The polishing pads described herein have a polishing surface which optionally further comprises grooves, channels, and/or perforations which facilitate the lateral transport of pohshing compositions across the surface of the polishing pad. Such grooves, channels, or perforations can be in any suitable pattern and can have any suitable depth and width. The pohshing pad can have two or more different groove patterns, for example a combination of large grooves and small grooves as described in U.S. Patent 5,489,233. The grooves can be in the form of slanted grooves, concentric grooves, spiral or circular grooves, XY cross-hatch pattern, and can be continuous or non-continuous in connectivity. Preferably, the polishing pad comprises at least small grooves produced by standard pad conditioning methods. [0052] The polishing pads described herein have a polishing surface which optionally further comprises regions of different density, porosity, hardness, modulus, and/or compressibility. The different regions can have any suitable shape or dimension. Typically, the regions of contrasting density, porosity, hardness, and/or compressibility are formed on the polishing pad by an ex situ process (i.e., after the polishing pad is formed).

[0053] The polishing pads described herein optionally further comprise one or more apertures, transparent regions, or translucent regions (e.g., windows as described in U.S. Patent 5,893,796). The inclusion of such apertures or translucent regions is desirable when the polishing pad is to be used in conjunction with an in situ CMP process monitoring technique. The aperture can have any suitable shape and may be used in combination with drainage channels for minimizing or eliminating excess polishing composition on the polishing surface. The translucent region or window can be any suitable window, many of which are known in the art. For example, the translucent region can comprise a glass or polymer-based plug that is inserted in an aperture of the polishing pad or may comprise the same polymeric material used in the remainder of the polishing pad.

[0054] The polishing pads of the invention can be produced using any suitable technique, many of which are known in the art. For example, the polishing pads can be produced by (a) a mucell process, (b) a phase inversion process, (c) a spinodal or bimodal decomposition process, or (d) a pressurized gas injection process. Preferably, the polishing pads are produced using the mucell process or the pressurized gas injection process.

[0055] The mucell process involves (a) combining a polymer resin with a supercritical gas to produce a single-phase solution and (b) forming a pohshing pad substrate of the invention from the single-phase solution. The polymer resin can be any of the polymer resins described above. The supercritical gas is generated by subjecting a gas to an elevated temperature and pressure sufficient to create a supercritical state in which the gas behaves like a fluid (i.e., a supercritical fluid, SCF). The gas can be a hydrocarbon, chlorofluorocarbon, hydrochlorofluorocarbon (e.g., freon), nitrogen, carbon dioxide, carbon monoxide, or a combination thereof. Preferably, the gas is a non-flammable gas, for example a gas that does not contain C-H bonds. More preferably, the gas is nitrogen, carbon dioxide, or a combination thereof. Most preferably, the gas comprises, or is, carbon dioxide. The gas can be converted to the supercritical gas before or after combination with the polymer resin. Preferably, the gas is converted to the supercritical gas before combination with the polymer resin. Typically, the gas is subjected to a temperature of 100°C to 300°C and a pressure of 5 MPa (-800 psi) to 40 MPa (-6000 psi). When the gas is carbon dioxide, the temperature is 150°C to 250°C, and the pressure is 7 MPa (-1000 psi) to 35 MPa (-5000 psi) (e.g., 19 MPa (-2800 psi) to 26 MPa (-3800 psi)).

[0056] The single-phase solution of the polymer resin and the supercritical gas can be prepared in any suitable manner. For example, the supercritical gas can be blended with molten polymer resin in a machine barrel to form the single-phase solution. The single- phase solution then can be injected into a mold, where the gas expands to form a pore structure with high uniformity of pore size within the molten polymer resin. The concentration of the supercritical gas in the single-phase solution typically is 0.01% to 5% (e.g., 0.1% to 3%) of the total volume of the single-phase solution. The concentration of the supercritical fluid will determine the density of the porous foam and the pore size. As the concentration of the supercritical gas is increased, the density of the resulting porous foam increases and the average pore size decreases. The concentration of the supercritical gas also can affect the ratio of open cells to closed cells in the resulting porous foam. These and additional process features are described in further detail in U.S. Patent 6,284,810.

[0057] The pohshing pad is formed by creating a thermodynamic instability in the single-phase solution sufficient to produce greater than 10^s nucleation sites per cm³, of the solution. The thermodynamic instability can result from, for example, a rapid change in temperature, a rapid drop in pressure, or a combination thereof. Typically, the thermodynamic instability is induced at the exit of the mold or die which contains the single-phase solution. Nucleation sites are the sites at which the dissolved molecules of the supercritical gas form clusters from which the cells in the porous foam grow. The number of nucleation sites is determined by assuming that the number of nucleation sites is approximately equal to the number of cells formed in the polymer foam. The polishing pad can be formed from the single-phase solution by any suitable technique. For example, the pohshing pad can be formed using a technique selected from the group consisting of extrusion into a polymer sheet, co-extrusion of multilayer sheets, injection molding, compression molding, blow molding, blown film, multilayer blown film, cast film, thermoforming, and lamination. Preferably, the polishing pad is formed by extrusion into a polymer sheet or by injection molding.

[0058] The phase inversion process involves the dispersion of extremely fine particles of a polymer resin that have been heated above the T_m or T_g of the polymer in a highly agitated non-solvent (e.g., give examples here). The polymer resin can be any of the polymer resins described above. As the number of fine polymer resin particles added to the non-solvent increases, the fine polymer resin particles connect to form initially as tendrils and ultimately as a three-dimensional polymer network. The non-solvent mixture is then cooled causing the non-solvent to form into discrete droplets within the three-dimensional polymer network. The resulting material is a polymer foam having sub-micron pore sizes.

[0059] The spinodal or binodal decomposition process involves controlling the temperature and/or volume fraction of a polymer-polymer mixture, or a polymer-solvent mixture, so as to move the mixture from a single-phase region into a two-phase region. Within the two-phase region, either spinodal decomposition or binodal decomposition of the polymer mixture can occur. Decomposition refers to the process by which a polymer mixture changes from a nonequilibrium phase to an equilibrium phase. In the spinodal region, the free energy of mixing curve is negative such that phase separation of the polymers (i.e., formation of a two-phase material), or phase separation of the polymer and the solvent, is spontaneous in response to small fluctuations in the volume fraction. In the binodal region, the polymer mixture is stable with respect to small fluctuations in volume fraction and thus requires nucleation and growth to achieve a phase-separated material. Precipitation of the polymer mixture at a temperature and volume fraction within the two- phase region (i.e., the binodal or spinodal region) results in the formation of a polymer material having two phases. If the polymer mixture is laden with a solvent or a gas, the biphasic polymer material will contain sub-micron pores at the interface of the phase- separation. The polymers preferably comprise the polymer resins described above.

[0060] The pressurized gas injection process involves the use of high temperatures and pressures to force a supercritical fluid gas into a solid polymer sheet comprising an amorphous polymer resin. The polymer resin can be any of the polymer resins described above. Solid extruded sheets are placed at room temperature into a pressure vessel. A supercritical gas (e.g., N₂ or CO₂) is added to the vessel, and the vessel is pressurized to a level sufficient to force an appropriate amount of the gas into the free volume of the polymer sheet. The amount of gas dissolved in the polymer is directly proportional to the applied pressure according to Henry's law. Increasing the temperature of the polymer sheet increases the rate of diffusion of the gas into the polymer, but also decreases the amount of gas that can dissolve in the polymer sheet. Once the gas has thoroughly saturated the polymer, the sheet is removed from the pressurized vessel. If desired, the polymer sheet can be quickly heated to a softened or molten state if necessary to promote cell nucleation and growth. U.S. Patents 5,182,307 and 5,684,055 describe these and additional features of the pressurized gas injection process.

[0061] The polishing pads of the invention are particularly suited for use in conjunction with a chemical-mechanical polishing (CMP) apparatus. Typically, the apparatus comprises a platen, which, when in use, is in motion and has a velocity that results from orbital, linear, or circular motion, a pohshing pad of the invention in contact with the platen and moving with the platen when in motion, and a carrier that holds a substrate to be polished by contacting and moving relative to he surface of the polishing pad intended to contact a substrate to be polished. The polishing of the substrate takes place by the substrate being placed in contact with the polishing pad and then the pohshing pad moving relative to the substrate, typically with a pohshing composition therebetween, so as to abrade at least a portion of the substrate to polish the substrate. The CMP apparatus can be any suitable CMP apparatus, many of which are known in the art. The pohshing pad of the invention also can be used with linear pohshing tools.

[0062] The pohshing pads described herein can be used alone or optionally can be used as one layer of a multi-layer stacked pohshing pad. For example, the polishing pads can be used in combination with a subpad. The subpad can be any suitable subpad. Suitable subpads include polyurethane foam subpads (e.g., Poron® foam subpads from Rogers Corporation), impregnated felt subpads, microporous polyurethane subpads, or sintered urethane subpads. The subpad typically is softer than the pohshing pad of the invention and therefore is more compressible and has a lower Shore hardness value than the pohshing pad of the invention. For example, the subpad can have a Shore A hardness of 35 to 50. In some embodiments, the subpad is harder, is less compressible, and has a higher Shore hardness than the polishing pad. The subpad optionally comprises grooves, channels, hollow sections, windows, apertures, and the like. When the polishing pads of the invention are used in combination with a subpad, typically there is an intermediate backing layer such as a polyemyleneterephthalate film, coextensive with and inbetween the polishing pad and the subpad. Alternatively, the porous foam of the invention also can be used as a subpad in conjunction with a conventional polishing pad.

[0063] The polishing pads described herein are suitable for use in polishing many types of substrates and substrate materials. For example, the polishing pads can be used to polish a variety of substrates including memory storage devices, semiconductor substrates, and glass substrates. Suitable substrates for polishing with the polishing pads include memory disks, rigid disks, magnetic heads, MEMS devices, semiconductor wafers, field emission displays, and other microelectronic substrates, especially substrates comprising insulating layers (e.g., silicon dioxide, silicon nitride, or low dielectric materials) and or metal- containing layers (e.g., copper, tantalum, tungsten, aluminum, nickel, titanium, platinum, ruthenium, rhodium, iridium or other noble metals).

[0064] The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.

EXAMPLE 1 [0065] This example illustrates a method for producing microporous foam rods having a uniform pore size.

[0066] Thermoplastic polyurethane (TPU) foam rods (1A and IB) were produced by an extrusion method. Each TPU foam rod was prepared using TPU (TT 1072 Tecothane® polyurethane, Thermedics Polymer Products) having a weight average molecular weight of 90,000 g/mol to 110,000 g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placed in an extruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L D single screw extruder) at elevated temperature and pressure to form a polymer melt. Carbon dioxide gas was injected into the polymer melt (using a Trexel TR30-5000G delivery system equipped with P7 trim and 4 standard injectors) under the elevated temperature and pressure resulting in formation of a supercritical fluid CO₂ that blended with the polymer melt to form a single-phase solution. The CO₂/polymer solution was extruded through a converging die (0.15 cm (0.060 inch) diameter, 12.1° angle) to form a porous foam rod. The concentration of CO₂ was 1.51% and 1.26% for rods 1A and IB, respectively.

[0067] The temperatures for each zone of the extruder, the gate, die and melt temperatures, die pressure, screw speed, and concentration of CO₂ are summarized in Table 1. A scanning electron microscopy (SEM) image for Rod Sample IB is shown in FIG. 1.

Table 1:

[0068] This example illustrates that icroporous foam materials having uniform cell sizes can be produced using supercritical fluid microceU technology.

EXAMPLE 2 [0069] This example illustrates a method for preparing pohshing pads of the invention.

[0070] A series of thermoplastic polyurethane (TPU) foam sheets (2A, 2B, 2C, and 2D) were produced by an extrusion method. Each TPU sheet was prepared using TPU (TT 1072 Tecothane® polyurethane, Thermedics Polymer Products) having a weight average molecular weight of 90,000 g/mol to 110,000 g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placed in an extruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L/D single screw) at elevated temperature and pressure to form a polymer melt. Carbon dioxide gas was injected into the polymer melt under the elevated temperature and pressure resulting in formation of a supercritical fluid CO₂ that blended with the polymer melt to form a single-phase solution. The CO₂/polymer solution was extruded through a flat die (30.5 cm (12 inch) wide, 0.005 - 0.0036 cm (0.002 - 0.0014 inch) flex gap, 6° converging) to form a porous foam sheet. The concentration of CO₂ was 0.50%, 0.80%, 1.70%, and 1.95% for sheets 2A, 2B, 2C, and 2D, respectively.

[0071] The temperatures for each zone of the extruder, the gate, die and melt temperatures, die pressure, screw speed, concentration of CO₂, and sheet dimensions are summarized in Table 2.

Table 2:

[0072] Porous TPU foam sheets having good uniformity of cell size (± 25 μm) were produced using each series of the extrusion parameters shown in Table 2. Samples 2A and 2B had large average cell sizes (> 100 μm). Sheets 2C and 2D had small average cell sizes (< 100 μm).

[0073] This example demonstrates that porous foam sheets having small cell sizes can be produced by the supercritical fluid method. EXAMPLE 3 [0074] This example illustrates a method for preparing polishing pads of the invention.

[0075] A series of thermoplastic polyurethane (TPU) foam sheets (3A, 3B, 3C, and 3D) were produced by an extrusion method. Each TPU sheet was prepared using TPU (TT 1072 Tecothane® polyurethane, Thermedics Polymer Products) having a weight average molecular weight of 90,000 g/mol to 110,000 g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placed in an extruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L/D single screw) at elevated temperature and pressure to form a polymer melt. Carbon dioxide gas was injected into the polymer melt under the elevated temperature and pressure resulting in formation of a supercritical fluid CO₂ that blended with the polymer melt to form a single-phase solution. The CO₂/polymer solution was extruded through a flat die (30.5 cm (12 inch) wide, 0.005 - 0.0036 cm (0.002 - 0.0014 inch) flex gap, 6° converging) to form a porous foam sheet. The concentration of CO₂ was 1.38%, 1.50%, 1.66%, and 2.05% for sheets 3 A, 3B, 3C, and 3D, respectively.

[0076] The temperatures for each zone of the extruder, the gate, die, and melt temperatures; die pressures, screw speed, and concentration of CO₂ are summarized in Table 3. The average cell size produced in the porous TPU foam sheets depends on the concentration of the CO₂ gas. A plot of the CO₂ concentration in the single-phase solution versus the density of the resulting sheets is shown in FIG. 2.

Table 3:

[0077] Porous TPU foam sheets having good uniformity of cell size were produced using each series of the extrusion parameters shown in Table 3. Scanning electron microscopy (SEM) images of Sample 3D are shown in FIGS. 3 (cross-section) and 4 (top surface). The physical properties of Sample 3D were determined, and the data are summarized in Table 4.

[0078] The pohshing pad density was determined in accordance with the ASTM D795 test method. The Shore A hardness of the pohshing pad was determined in accordance with the ASTM 2240 test method. The peak stress of the polishing pad was deteπnined in accordance with the ASTM D638 test method. The % compressibility was determined at 0.031 MPa (4.5 psi) pressure using an Ames meter. The probe of the Ames tester was first zeroed (without the sample), and then the sample thickness was measured (Dl). A 5-pound weight (0.031 MPa) was placed on the probe and the sample thickness was measured after 1 minute (D2). The compressibility is the ratio of the difference in thickness (D1-D2) to the initial sample thickness (Dl). The % compressibility was also measured using an Instron technique at a pressure of 0.5 MPa (72 psi). The % rebound was determined using a Shore Resiliometer (Shore Instrument & MFG). The % rebound was measured in the height of the travel of a metal slug as it rebounds off the specimen preformed at 0.031 MPa (4.5 psi). The % rebound is reported as an average over 5 measurements. The flexural modulus was determined in accordance with the ASTM D790 test method. The air permeability was determined using a Genuine Gurley 4340 Automatic Densometer.

[0079] The T_g was determined either by Dynamic Mechanical Analyzer (DMA) or by Thermomechanical Analysis (TMA). For DMA, a TA 2980 model instrument was used at an operating temperature of-25°C to 130°C, a frequency of 3 Hz, and a heating rate of 2.5°C/min. The T_g was calculated from the midpoint of the storage modulus versus temperature plot. For TMA, the test was performed in accordance with the ASTM E831 test method. The T_m was determined by Differential Scanning Colorimetry (DSC). A TA 2920 model instrument was used at an operating temperature of-50°C to 230°C and a heating rate of 10°C/min. The T_m value was calculated form the peak melting point of the exothermic wave. The Storage Modulus was determined by DMA at 25°C. The Taber Wear is the amount of the porous foam sheet that is removed in 1000 cycles of pohshing. The average pore size and pore density of the porous foam sheets were determined using SEM micrographs at 50X and 100X magnification.

[0080] The average pore size and pore size distribution were measured by counting closed cell pores in a given unit area and then averaging the pore diameters using the imaging software, Clemex Vision software available from Clemex Technologies. The size and percentages for the pores are reported with respect to both width and length reflecting the non-spherical nature of the pores in the sample. The pore density was determined by the following formula:

Number of cells/cm³ = ( p_pad li_ai " ^l ) * ( ^p where p_SOij_d is the density of the solid thermoplastic polyurethane pads (without SCF gas) equal to 1.2 g/cm³, p^_{d mater}i_al is the density of the microcellular thermoplastic polyurethane pads (with SCF gas), and d is the diameter of the cell (in cm, assumed to be spherical).

Table 4:

[0081] The average pore size and pore size distribution of the porous foam of Sample 3D were also determined after the sample was conditioned with a silicon oxide block for 5 hours. The values for the average pore size and the percentage of pore having a dimension within ±20 mm of the average (7.7±9.3 x 13.2±15.5 (w x 1) and 98%/91% (w 1), respectively) were substantially the same as the values obtained prior to conditioning and abrasion. These results indicate that the pore size and pore size distribution was consistent through the cross-sectional area of the porous foam sheet.

[0082] This example demonstrates that microporous pohshing pads having a uniform pore size can be prepared using the method of the invention.

EXAMPLE 4 [0083] This example illustrates that microporous foam pohshing pads of the invention have good polishing properties.

[0084] A microporous foam polyurethane polishing pad produced according to the method recited in Example 3 for Sample 3D, having a density of 0.989 g/ml and a thickness of 0.107 cm (0.0423 in), was used to chemically-mechanically polish blanket silicon dioxide wafers. The polishing pad was used without any conditioning (i.e., formation of microgrooves or microstructure), buffing, or external macrogrooves (i.e., macrotexture). The removal rate and within wafer non-uniformity was determined for the polishing pad as a function of the number of silicon dioxide wafers that were pohshed. The removal rates were measured for four wafers in a row followed by polishing of four "dummy" silicon dioxide wafers, for which removal rates were not recorded. A plot of removal rate versus the number of silicon dioxide wafers polished is shown in FIG. 5. The polishing parameters were carrier downforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100 ml min, a platen speed of 60 rpm, a carrier speed of 55-60 rpm.

[0085] The data depicted in FIG. 5 shows that polishing pads comprising a microporous foam having a uniform cell size distribution produce substantial pohshing removal rates of silicon dioxide blanket wafers, even in the absence of any conditioning, buffing, or groove macrotexture. Moreover, the polishing pads produce very low within wafer non-uniformity.

EXAMPLE 5 [0086] This example illustrates that microporous foam pohshing pads of the invention have good pohshing properties.

[0087] Different pohshing pads were used to polish silicon dioxide blanket wafers in the presence of the same pohshing composition (i.e., Semi-Sperse® D7300 pohshing composition sold by Cabot Microelectronics). Polishing Pad 5 A (control) was a solid, non- porous polyurethane pohshing pad having microgrooves and macrogrooves. Polishing Pad 5B (invention) was a microporous foam polyurethane polishing pad having a uniform pore size of 20 ± 10 μm or less, which was produced according to the method recited in Example 3 for Sample 3D, and having a density of 0.989 g/ml and a thickness of 0.107 cm (0.0423 in) that was buffed, conditioned (to form microgrooves), and grooved (macrogrooves). The removal rates and non-uniformity were determined for each of the polishing pads as a function of the number of silicon dioxide wafers that were polished. A plot of removal rate versus the number of silicon dioxide wafers polished for each of the Polishing Pads 5 A and 5B is shown in FIG. 6. The pohshing parameters were carrier downforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100 -ml min, a platen speed of 60 rpm, a carrier speed of 55-60 rpm. Scanning Electron Microscopy (SEM) images of the top grooved surfaces of the solid polishing pad and microporous foam polishing pad of the invention are shown in FIG. 7a and FIGS. 7b-7c, respectively.

[0088] The plot of FIG. 6 shows that microporous foam polishing pads having a uniform cell size distribution have superior removal rates for silicon dioxide blanket wafers compared to solid, non-porous polishing pads. Moreover, the microporous polishing pad of the invention had a very consistent removal rate and low non-uniformity over the course of polishing 20 wafers or more, indicating that the polishing pad did not become glazed over time. The SEM images in FIGS. 7a-c illustrate that the microporous foam polishing pads of the invention (FIGS. 7b and 7c) are less prone to glazing during pohshing as is observed with conventional polishing pads (FIG. 7a).

EXAMPLE 6 [0089] This example illustrates that the microporous foam polishing pads of the invention are permeable to and can transport the polishing composition during polishing.

[0090] A solid polyurethane pohshing pad (Pad 6A, comparative), a microporous foam polyurethane polishing pad (Pad 6B, invention), and a conventional closed cell polyurethane polishing pad (Pad 6C, comparative) were used in a chemical-mechanical pohshing experiment using aqueous fumed silica abrasive at a pH of 11. After polishing 20 silicon dioxide wafers, each of the pohshing pads were studied by a SEM X-ray mapping technique, Energy Dispersive X-ray (EDX) Spectroscopy, to determine the extent of penetration of the silica-based pohshing composition. The EDX images are shown in FIGS. 8a, 8b, and 8c for Pads 6A, 6B, and 6C, respectively.

[0091] The extent of penetration of the silica abrasive was only 10 or 15% of the pad thickness for the solid polishing pad (Pad 6A) as shown in FIG. 8a. For the microporous foam polishing pad (Pad 6B), the silica abrasive penetrated through at least 40% of the pad thickness. For the conventional closed-cell polishing pad (Pad 6C), the silica abrasive penetrated through only 20% to 25% of the pad thickness.

[0092] This example demonstrates that the microporous foam polishing pads of the invention are capable of transporting pohshing composition abrasive particles well into the body of the polishing pad, while conventional solid and closed-cell polishing pads do not transport the polishing composition into the body of the pohshing pad. EXAMPLE 7 [0093] This example shows that the microporous foam polishing pads of the invention have superior pohshing rates compared to conventional closed cell microporous polishing pads.

[0094] Similar patterned silicon dioxide wafers were polished with an aqueous fumed silica abrasive at a pH of 11 using different pohshing pads (Pohshing Pads 7 A, 7B, and 7C). Polishing Pad 7A (comparative) was a solid non-porous polyurethane pohshing pad. Polishing Pad 7B (invention) was a microporous foam polyurethane polishing pad of the invention. Polishing Pad 7C (comparative) was a conventional microporous closed cell polyurethane pohshing pad. Each of the polishing pads were buffed, conditioned, and grooved. The planarization rates for a 40% density region having a step height of 8000 A and a 70% density region having a step height of 8000 A were pohshed by each of the polishing pads, and the remaining step height of the feature was determined after 30, 60, 90, 120, and 150 seconds. The results for the 40% dense feature and the 70% dense feature are plotted in FIGS. 9 and 10, respectively.

[0095] The results depicted in FIGS. 9 and 10 show that for a region of 40% density, all of the pohshing pads (Polishing Pads 7A-7C) have less than 1000 A remaining step height after 60 seconds. However, for a region of 70% density, only Polishing Pads 7 A and 7B have less than 1000 A remaining step height after 90 seconds. Thus, the microporous foam pohshing pad of the invention has a superior pohshing rate compared to the conventional microporous foam closed cell polishing pad.

EXAMPLE 8 [0096] This example illustrates a method for preparing polishing pads of the invention using a pressurized gas injection process.

[0097] Two samples of solid extruded TPU sheets were placed in a pressurized vessel with 5 MPa CO₂ gas at room temperature for 30 hours. The solid TPU sheets each absorbed 5 wt.% CO₂. The TPU samples (Samples 8A and 8B) were then heated to 50°C and 97.6°C, respectively, at a saturation pressure of 5 MPa to produce a sheet with an average cell size of 0.1 μm and 4 μm (99 cells counted, min 2 μm, max 8 μm, standard deviation 1.5), respectively. The average cell sizes were determined using image analysis software. An SEM image of an untreated solid TPU sheet is shown in FIG. 11. SEM images of the foamed TPU sheets (Samples 8 A and 8B, invention) are shown in FIGS. 12-15. FIGS. 12 and 13 are at a magnification of 7500X and 20000X, respectively. FIGS. 14 and 15 are at a magnification of 350X and 1000X, respectively.

[0098] This example demonstrates that the pressurized gas injection process can be used to produce porous foam polishing pad materials having an average pore size less than 20 μm and a highly uniform pore size distribution.

Claims

1. A polishing pad for chemical-mechanical polishing comprising a porous foam with an average pore size of 50 μm or less, wherein 75% or more of the pores have a pore size within 20 μm or less of the average pore size.

2. The polishing pad of claim 1, where the average pore size is 40 μm or less.

3. The polishing pad of claim 1 , wherein the porous foam has a density of 0.5 g/cm³ or greater.

4. The pohshing pad of claim 1 , wherein the porous foam has a void volume of 25% or less.

5. The polishing pad of claim 1, wherein the porous foam comprises closed cells.

6. The pohshing pad of claim 1 , wherein the porous foam has a cell density of 10⁵ cells/cm³ or greater.

7. The pohshing pad of claim 1, wherein the porous foam comprises a polymer resin selected from the group consisting of thermoplastic elastomers, thermoplastic polyurethanes, polyolefins, polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers, polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes, cross-linked polyolefins, polyethers, polyesters, polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes, polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes, polystyrenes, polymethylmethacrylates, copolymers and block copolymers thereof, and mixtures and blends thereof.

8. The pohshing pad of claim 7, wherein the polymer resin is a thermoplastic polyurethane.

9. The polishing pad of claim 8, wherein the thermoplastic polyurethane has a Melt Index of 20 or less, a weight average molecular weight (M_w) of 50,000 g/mol to 300,000 g/mol, and a polydispersity index (PDI) of 1.1 to 6.

10. The polishing pad of claim 8, wherein the thermoplastic polyurethane has a Rheology Processing Index (RPI) of 2 to 8 at a shear rate (γ) of 1501/s and a temperature of 205°C.

11. The polishing pad of claim 8, wherein the thermoplastic polyurethane has a Flexural Modulus of 350 MPa to 1000 MPa.

12. The polishing pad of claim 8, wherein the thermoplastic polyurethane has a glass transition temperature of 20°C to 110°C and a melt transition temperature of 120°C to 250°C.

13. The polishing pad of claim 7, wherein the porous foam further comprises a water absorbent polymer.

14. The polishing pad of claim 13, wherein the water absorbent polymer is selected from the group consisting of cross- linked polyacrylamide, cross-linked polyacrylic acid, cross-linked polyvinylalcohol, and combinations thereof.

15. The pohshing pad of claim 7, wherein the porous foam further comprises particles selected from the group consisting of abrasive particles, polymer particles, composite particles, liquid carrier-soluble particles, and combinations thereof.

16. The pohshing pad of claim 15, wherein the porous foam further comprises abrasive particles selected from the group consisting of silica, alumina, ceria, and combinations thereof.

17. A pohshing pad for chemical-mechanical pohshing comprising a porous foam with an average pore size of 1 μm to 20 μm.

18. The pohshing pad of claim 17, wherein 90% or more of the pores have a pore size within 20 μm or less of the average pore size.

19. The pohshing pad of claim 17, wherein the porous foam has a density of 0.5 g/cm³ or greater.

20. The polishing pad of claim 17, wherein the porous foam has a void volume of 25% or less.

21. The pohshing pad of claim 17, wherein the porous foam comprises closed cells.

22. The polishing pad of claim 17, wherein the porous foam has a cell density of 10⁵ cells/cm³ or greater.

23. The polishing pad of claim 17, wherein the porous foam comprises a polymer resin selected from the group consisting of thermoplastic elastomers, thermoplastic polyurethanes, polyolefins, polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers, polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes, cross-linked polyolefins, polyethers, polyesters, polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes, polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes, polystyrenes, polymethylmethacrylates, copolymers and block copolymers thereof, and mixtures and blends thereof.

24. The polishing pad of claim 23, wherein the polymer resin is a thermoplastic polyurethane.

25. The polishing pad of claim 24, wherein the thermoplastic polyurethane has a Melt Index of 20 or less, a weight average molecular weight (M_w) of 50,000 g/mol to 300,000 g/mol, and a polydispersity index (PDI) of 1.1 to 6.

26. The polishing pad of claim 24, wherein the thermoplastic polyurethane has a Rheology Processing Index (RPI) of 2 to 8 at a shear rale (γ) of 150 1/s and a temperature of 205°C.

27. The pohshing pad of claim 24, wherein the thermoplastic polyurethane has a Flexural Modulus of 350 MPa to 1000 MPa.

28. The pohshing pad of claim 24, wherein the thermoplastic polyurethane has a glass transition temperature of 20°C to 110°C and a melt transition temperature of 120°C to 250°C.

29. The pohshing pad of claim 23, wherein the porous foam further comprises a water absorbent polymer.

30. The polishing pad of claim 29, wherein the water absorbent polymer is selected from the group consisting of cross-linked polyacrylamide, cross-linked polyacrylic acid, cross-linked polyvinylalcohol, and combinations thereof.

31. The polishing pad of claim 23, wherein the porous foam further comprises particles selected from the group consisting of abrasive particles, polymer particles, composite particles, liquid carrier-soluble particles, and combinations thereof.

32. The polishing pad of claim 31, wherein the porous foam further comprises abrasive particles selected from the group consisting of silica, alumina, ceria, and combinations thereof.

33. A polishing pad for chemical-mechanical polishing comprising a porous foam having a multi-modal pore size distribution, wherein the multi-modal distribution has 20 or fewer pore size maxima.

34. The polishing pad of claim 33, wherein the multi-modal pore size distribution is a bimodal pore size distribution.

35. The polishing pad of claim 34, wherein the bimodal distribution has a first pore size maximum of 50 μm or less and a second pore size maximum at 50 μm or more.

36. The polishing pad of claim 34, wherein the bimodal distribution has a first pore size maximum at 20 μm or less and a second pore size maximum at 20 μm or more.

37. The polishing pad of claim 33, wherein the porous foam comprises a polymer resin selected from the group consisting of thermoplastic elastomers, thermoplastic polyurethanes, polyolefins, polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers, polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes, cross-linked polyolefins, polyethers, polyesters, polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes, polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes, polystyrenes, polymethylmethacrylates, copolymers and block copolymers thereof.; and mixtures and blends thereof.

38. The polishing pad of claim 37, wherein the polymer resin is a thermoplastic polyurethane.

39. The pohshing pad of claim 33, wherein the porous foam comprises a water- absorbent polymer.

40. The pohshing pad of claim 33, wherein the porous foam comprises particles selected from the group consisting of abrasive particles, polymer particles, composite particles, liquid carrier-soluble particles, and combinations thereof.

41. A polishing pad for chemical-mechanical polishing comprising a thermoplastic polyurethane foam with an average pore size of 50 μm or less, wherein the thermoplastic polyurethane has a Melt Flow Index (MFI) of 20 or less, a molecular weight of 50,000 g mol to 300,000 g/mol, and a polydispersity index of 1.1 to 6.

42. The polishing pad of claim41, wherein the polyurethane foam has a Flexural Modulus of 350 MPa to 1000 MPa.

43. The polishing pad of claim 41, wherein the thermoplastic polyurethane has a Rheology Processing Index of 2 to 10 at a shear rate of 150 1 s and a temperature of 205°C.

44. The polishing pad of claim 41 , wherein the thermoplastic polyurethane foam has an average % compressibility of 7 or less, an average % rebound of 35 or greater, and a Shore D hardness of 40 to 90.

45. A polyurethane polishing pad for chemical-mechanical polishing which can polish a silicon dioxide wafer at a rate of at least 600 A min with a carrier downforce pressure of 0.028 MPa, a slurry flow rate of 100 ml min, a platen rotation speed of 60 rpm, and a carrier rotation speed of 55 rpm to 60 rpm, wherein the polishing pad does not contain abrasive particles and comprises no externally produced surface texture.

46. The polishing pad of claim 45, wherein the polishing pad has a void volume of 5% or less.

47. The polishing pad of claim 45, wherein the pohshing pad comprises pores having an average pore size of 50 μm or less.

48. A method for producing a pohshing pad substrate comprising:

(a) combining a polymer resin with a supercritical gas to produce a single-phase solution, wherein the supercritical gas is generated by subjecting a gas to an elevated temperature and pressure, and

(b) forming a pohshing pad from the single-phase solution.

49. The method of claim 48, wherein the gas does not contain C-H bonds.

50. The method of claim 49, wherein the gas comprises -nitrogen, carbon dioxide, or combinations thereof.

51. The method of claim 48, wherein the polymer resin is selected from the group consisting of thermoplastic elastomers, thermoplastic polyurethanes, polyolefins, polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers, polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes, cross-linked polyolefins, polyethers, polyesters, polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes, polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes, polystyrenes, polymethylmethacrylates, copolymers and block copolymers thereof, and mixtures and blends thereof.

52. The method of claim 48, wherein the polishing pad is formed by creating a thermodynamic instability in the single-phase solution sufficient to produce greater than 10⁵ nucleation sites per cm³ of the solution.

53. The method of claim 48, wherein the amount of supercritical gas is 0.01% to 5% of the total volume of the single-phase solution.

54. The method of claim 48, wherein the gas is converted to the supercritical gas before combination with the polymer resin.

55. The method of claim 48, wherein the gas is converted to the supercritical gas after combination with the polymer resin.

56. The method of claim 50, wherein the gas is carbon dioxide, the temperature is 150°C to 250°C, and the pressure is 7 MPa to 35 MPa.

57. The method of claim 48, wherein the pohshing pad is formed from the single-phase solution using a technique selected from the group consisting of extrusion into a polymer sheet, co-extrusion of multilayer sheets, injection molding, compression molding, blow molding, blown film, multilayer blown film, cast film, thermoforming, and lamination.