EP1694466A1

EP1694466A1 - A polishing pad for chemical mechanical polishing

Info

Publication number: EP1694466A1
Application number: EP04784014A
Authority: EP
Inventors: Yaw S. Obeng
Original assignee: psiloQuest Inc
Current assignee: psiloQuest Inc
Priority date: 2003-09-15
Filing date: 2004-09-14
Publication date: 2006-08-30
Also published as: WO2005028157A1; US20050055885A1; TW200525017A; CN1863644A; JP2007505749A

Abstract

The present invention provides in one embodiment, a polishing pad (100) for chemical mechanical polishing. The polishing pad comprises a polishing body (110). The polishing body comprises a thermoplastic foam substrate (115) having a surface (120) comprising concave cells (125). A polishing agent (130) coats an interior surface (135) of the concave cells. The polishing agent comprises an inorganic metal oxide that includes carbides or nitrides. Yet another embodiment of the present invention is a method for preparing a polishing pad (200).

Description

APOLISHING PAD FOR CHEMICAL MECHANICAL POLISHING

TECHNICAL FIELD OF THE INVENTION The present invention is directed to chemical mechanical polishing for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectrics, metals and composites thereof, magnetic mass storage media and integrated circuits .

BACKGROUND OF THE INVENTION Chemical mechanical polishing (CMP) has been successfully used for planarizing both metal and dielectric films. In one plausible mechanism of planarizing, the polishing process is thought to involve intimate contact between high points on the wafer surface and the pad material, in the presence of slurry. In this scenario, corroded materials, produced from reactions between the slurry and wafer surface being polished, are removed by shearing at the pad-wafer interface. The elastic properties of pad material significantly influence the final planarity and polishing rate. In turn, the elastic properties are a function of both the intrinsic polymer and its foamed structure.

Historically, polyurethane-based pads have been used for CMP because of their high strength, hardness, modulus and high elongation at break. While such pads can achieve both good uniformity and efficient topography reduction, their ability to rapidly and uniformly remove surface materials drops off rapidly as a function of use. The drop off in material removal rates as a function of time observed for polyurethane-based pads has been attributed to changes in the mechanical response of such polishing pads under conditions of critical shear. It is generally believed that the loss in functionality of polyurethane-based CMP pads is due to pad decomposition from the interaction between the pad and the slurries used in the polishing. Moreover, decomposition produces a surface modification in and of itself in the case of the polyurethane pads which can be detrimental to uniform polishing. Alternatively, in some instances, the surface modification of materials used for CMP polishing pads may improve the application performance. Such modifications, however may be temporary, thus requiring frequency replacement or retreatment of the CMP pad. Polyurethane pads also generally require a break-in period before polishing, in addition to the reconditioning and retreatment after a period of use. It is often also necessary to keep traditional pads wet in while polishing equipment is in idle mode. These characteristics undesirably reduce the overall efficiency of CMP when using polyurethane or similar conventional pads . Accordingly, what is needed is an improved CMP pad capable of providing a highly planar surface during CMP and having improved longevity, while not experiencing the above-mentioned problems .

SUMMARY OF THE INVENTION To address the above-discussed deficiencies of the prior art, the present invention provides in one embodiment, a polishing pad for chemical mechanical polishing. The polishing pad comprises a polishing body comprising a thermoplastic foam substrate. The thermoplastic foam substrate has a surface comprising concave cells. A polishing agent coating an interior surface of the concave cells comprises an inorganic metal oxide that includes carbides or nitrides. Another embodiment of the present invention is directed to a method for preparing a polishing pad. The method comprises exposing closed cells within a thermoplastic foam substrate to provide a substrate surface comprising concave cells. The method further includes coating an interior surface of the concave cells with a polishing agent comprising an inorganic metal oxide, wherein carbides and nitrides are incorporated into the inorganic metal oxide during the coating.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: FIGURE 1 illustrates a cross sectional view of a polishing pad of the present invention; FIGURES 2-4 illustrate cross sectional views of selected step in a method of the present invention for preparing a polishing pad; FIGURE 5 presents representative near infrared spectra of samples of thermoplastic foam polishing pads after variable periods of coating with a polishing agent precursor comprising tetraethoxy silane (TEOS) ; FIGURE 6 illustrates changes in the near infrared signal for representative thermoplastic foam polishing pads exposed to different coating periods with TEOS; FIGURE 7 illustrates exemplary indentation curves for a thermoplastic foam polishing pad after being coated with TEOS; FIGURE 8 illustrates the representative change in (Xa) , ^Λpop-in' for thermoplastic foam polishing pads as a function of coating time with TEOS; FIGURE 9 illustrates the representative change in hardness for thermoplastic foam polishing pads as a function of coating time with TEOS; FIGURE 10 illustrates the representative change in elastic modulus for thermoplastic foam polishing pads as a function of coating time with TEOS; FIGURE 11 illustrates the representative change in storage and loss modulus for thermoplastic foam polishing pads as a function of coating time with TEOS; FIGURE 12 presents representative XPS spectra of polishing pads after various periods of coating time with TEOS; FIGURE 13 presents the change in the Oxygen to Si intensity ratio values calculated from the XPS spectra of polishing pads after various periods of coating times with TEOS; and FIGURE 14 presents the change in relative Blanket Tungsten Removal Rate (WRR) and the Static Coefficient of Friction (COF) for thermoplastic foam polishing pads as a function of coating time with TEOS.

DETAILED DESCRIPTION The present invention benefits from the previously unrecognized advantages of using a thermoplastic polymer as the substrate for depositing a uniform coating of a polishing agent on concave cells . The interior surface of the concave cells was discovered to form excellent receptacles for receiving a uniform coating of the polishing agent. It is hypothesized that the center of the concave cell serves as an excellent nucleating point for coating because the surface energy of the cell at the center is lowest. It is believed that the initiation of coating at this location facilitates the uniform coverage of the interior surface of the concave cell with the polishing agent, thereby facilitating the polishing performance of a pad having such a surface. The polishing agent of the present invention comprises an inorganic metal oxide that includes nitrides or carbides . Using such metal-oxides to coat the pad surface also advantageously makes the polishing pad surface permanently hydrophilic. Preferably, the inorganic metal oxide has a lattice of atoms that incorporate the nitrides or carbides into the lattice. The use of such polishing agent surface coatings enhances polishing by modifying the surface mechanical properties of polishing pads . For instance, altering the nitride or carbide content of particular inorganic metal oxides allows the mechanical properties of the polishing pad surface to be fine tuned so as to match the mechanical properties the surface being polished. In turn, matching the mechanical properties of the polishing pad surface to the surface being polished improves polishing rate selectivity and reduces process-induced defects, such as scratching. Tuning the surface mechanical properties is accomplished by taking advantage of thermoplastic polymer substrate alterations produced by the plasma enhanced chemical vapor deposition (PECVD) surface coating and the secondary thermal induced reactions in the bulk of the thermoplastic substrate. One embodiment of the present invention is a polishing pad for chemical mechanical polishing semiconductor devices. FIGURE 1 presents an exemplary polishing pad 100 of the present invention. The polishing pad 100 comprises a polishing body 110. The polishing body 110 comprises a thermoplastic foam substrate 115 having a surface 120 comprising concave cells 125. A polishing agent 130 coating an interior surface 135 of the concave cells 125 comprises an inorganic metal oxide that includes carbides or nitrides. The polishing agent 130 comprises ceramic compounds composed of one or more inorganic metal oxides formed by the grafting of secondary reactants on the thermoplastic foam substrate 115 surface 120 in a plasma-enhanced chemical vapor deposition (PECVD) process. As further explained below, in the present invention, the PECVD process can be altered to promote the inclusion of carbides or nitride in the inorganic metal oxide. It is preferable for one or both of the carbides or nitrides to be incorporated into a lattice of the inorganic metal oxides. For instance, when the inorganic metal oxide comprises a silicon oxide, then the lattice can comprise silicates in with polymeric Si-O-Si structures having tetrahedral and distorted tetrahedral configurations . The nitrides can comprise silicon nitrides that are incorp'orated into these lattices . Alternatively, when the inorganic metal oxide comprises a titanium oxide, then the nitrides can comprise titanium nitrides incorporated into titanium oxide lattices . Similarly, silicon carbides and titanium carbides can be incorporated into a polishing agent 130 whose inorganic metal oxides comprise silicon oxide and titanium oxide, respectively. In some preferred embodiments, nitrides, such as silicon nitride, comprise about 10 mol percent of the polishing agent 130, while in other embodiments, carbides, such as silicon carbide, comprise about 10 mol percent of the polishing agent. - In some preferred embodiments the polishing agent 130 can comprise both nitride and carbides at these concentrations. As the PECVD process is extended to longer periods, the silanol concentration in the polishing agent 130 decreases, resulting in a decrease in the ratio of oxygen to silicon. In some advantageous embodiments of the polishing pad 100 where the polishing agent 130 comprises silicon oxide, the 0:Si ratio is at least about 8:1, and in some cases, at least about 9.9:1. Including nitrides and carbides in the polishing agent 130 provides an additional, heretofore unrecognized, means to alter the mechanical properties of the polishing pad 100, and thereby alter the pad's polishing properties. In some preferred embodiments, the polishing pad 100 has a hardness of greater than 60 KPa, and more preferably greater than 70 KPa. In other preferred embodiments, the polishing pad 100 has a hardness between about 62 KPa and about 70 KPa. In certain preferred embodiments, the polishing pad 100 has an elastic modulus of about greater than about 3 MPa, and more preferably, 4 MPa or greater. In other preferred embodiments, the polishing pad has an elastic modulus of between about 4 MPa and 8.2 MPa, respectively. In yet other embodiments, the polishing pad 100 has a loss modulus of about 0.4 MPa or greater, and more preferably at least about 0.6 MPa. As disclosed in U.S. Patent 6,579,604 and U.S. Application No. 10/241,074, incorporated herein by reference, the inorganic metal oxide of the polishing agent 130 can be produced from a variety of oxygen- containing organometallic compound used as the secondary reactant in a PECVD process. For example, the secondary plasma mixture may include a transition metal such as titanium, manganese, or tantalum. However, any metal element capable of forming a volatile organometallic compound, such as metal ester contain one or more oxygen atoms, and capable of being grafted to the polymer surface is suitable. Silicon may also be employed as the metal portion of the organometallic secondary plasma mixture. In these embodiments, the organic portion of the organometallic reagent may be an ester, acetate, or alkoxy fragment. In preferred embodiments, the inorganic metal oxide of the polishing agent 130 comprises silicon oxides or titanium oxides, such as silicon dioxide or titanium dioxide, respectively; tetraethoxy silane polymer; or titanium alkoxide polymer. Other secondary plasma reactants include ozone, alkoxy silanes, water, ammonia, alcohols, mineral sprits or hydrogen peroxide. In some preferred embodiments, the secondary plasma reactant comprises titanium esters; tantalum alkoxides, including tantalum alkoxides wherein the alkoxide portion has 1-5 carbon atoms; manganese acetate solution in water; manganese alkoxide dissolved in mineral spirits; manganese acetate; manganese acetylacetonate; aluminum alkoxides; alkoxy aluminates; aluminum oxides; zirconium alkoxides, wherein the alkoxide has 1-5 carbon atoms; alkoxy zirconates; magnesium acetate; and magnesium acetylacetonate. Other embodiments are also contemplated for the secondary plasma reactant, for example, alkoxy silanes and ozone, alkoxy silanes and ammonia, titanium esters and water, titanium esters and alcohols, or titanium esters and ozone . Some preferred embodiments of the thermoplastic foam substrate 115 comprise cross-linked polyolefins, such as polyethylene, polypropylene, and combinations thereof. In certain preferred embodiments, the thermoplastic foam substrate 115 comprises a closed-cell foam of crosslinked homopolymer or copolymers. Examples of closed-cell foam crosslinked homopolymers comprising polyethylene (PE) include: Volara™ and Volextra™ from Voltek (Lawrence, MA); Aliplast™, from JMS Plastics Supply, Inc. (Neptune, NJ) ; or Senflex T-Cell™ (Rogers Corp., Rogers, CT) . Examples of closed-cell foams of crosslinked copolymers comprising polyethylene and ethylene vinyl acetate (EVA) include: Volara™ and Volextra™ (from Voltek Corp.); Senflex EVA™ (from Rogers Corp.); and J-foam™ (from JMS Plastics JMS Plastics Supply, Inc.) In other preferred embodiments, the closed-cell thermoplastic foam substrate 115 comprises a blend of crosslinked ethylene vinyl acetate copolymer and a low density polyethylene copolymer (i.e., preferably between about 0.1 and about 0.3 gm/cc) . In yet other advantageous embodiments, the blend has a ethylene vinyl acetate :polyethylene weight ratio between about 1:9 and about 9:1. In certain preferred embodiments, the blend comprises EVA ranging from about 5 to about 45 wt%, preferably about 6 to about 25 wt% and more preferably about 12 to about 24 wt% . Such blends are thought to be conducive to the desirable production of closed cells 140 of the thermoplastic foam substrate 115, having a small size (e.g., diameters between about 10 and about 500 microns, and more preferably between about 50 to 150 microns). In still more preferred embodiments, the blend has an ethylene vinyl acetate : polyethylene weight ratio between about 0.6:9.4 and about 1.8:8.2. In even more preferred embodiments, the blend has an ethylene vinyl acetate :polyethylene weight ratio between about 0.6:9.4 and about 1.2:8.8. As further illustrated in FIGURE 1, the thermoplastic foam substrate 115 comprises closed cells 140. The term closed cell 140 as used herein, refers to any volume defined by a membrane within the substrate 115 occupied by air, or other gases used as blowing agents, such as nitrogen or helium. The closed cells 140 form a substantially concave cell 135 formed upon skiving of the substrate 115. The concave cells 135 need not have smooth or curved walls, however. Rather, the concave cells 135 may have irregular shapes and sizes. Several factors, such as the composition of the thermoplastic foam substrate 115 and the procedure used to prepare the thermoplastic foam substrate 115, may affect the shape and size of the closed cells 140 and the concave cells 135. As further illustrated in FIGURE 1, the thermoplastic foam substrate 115 can be coupled to an optional backing material 145. In some preferred embodiments the backing material 145 is stiff. A stiff backing advantageously limits the compressibility and elongation of the foam during polishing, which in turn, reduce erosion and dishing effects during metal polishing via CMP. In some cases, the backing material 145 comprise a high density polyethylene (i.e., greater than about 0.98 gm/cc), and more preferably a condensed high density polyethylene. In certain cases, coupling to the thermoplastic foam substrate 115 is achieved via chemical bonding using a conventional adhesive 150, such as epoxy or other materials well known to those skilled in the art. In some preferred embodiments, coupling is achieved via extrusion coating of the molten backing material 145 onto the thermoplastic foam substrate 115. In still other preferred embodiments, the backing material 145 is thermally welded to the thermoplastic foam substrate 115. Another aspect of the present invention is a method for preparing a polishing pad for chemical mechanical polishing. FIGURES 2 to 4 present selected steps in an exemplary method of preparing a polishing pad 200. Any of the embodiments of the polishing pad and its component parts, including the above described primary and secondary plasma reactants, can be incorporated into the method of preparing the polishing pad 200. Turning now to FIGURE 2, shown is the partially constructed polishing pad 200 after exposing closed cells 210 within a thermoplastic foam substrate 220 of the polishing pad 200 to provide a substrate surface 230 comprising concave cells 240. The concave cells 240 are formed on the substrate's surface 230 by skiving. The term skiving as used herein means any process to cut away a thin layer of the surface of the substrate 220 so as to expose concave cells 240 within the thermoplastic foam substrate 220. Skiving may be achieved using any conventional technique well-know to one of ordinary skill in the art . FIGURES 3 and 4 illustrate selected stages in a surface coating process. Turning first to FIGURE 3, illustrated is the partially completed polishing pad 200 after exposing the substrate surface 230 to an initial plasma reactant, followed by exposure to a secondary plasma reactant in a PECVD process. Exposure to the initial plasma reactant forms a modified surface 310 of the thermoplastic foam substrate 220. It is important to carefully control the conditions and duration of the plasma treatment to avoid excessive damage to the thermoplastic foam substrate 220. For example, an excessively high or uncontrolled radio flow discharge electrode temperature can cause the thermoplastic foam substrate 220 to melt, warp or crack. In some preferred embodiments of the method, the radio flow discharge electrode temperature is maintained between about 20 °C and 100 °C and more preferably between about 30°C and 50°C. In some cases an RF operating power between about 250 and about 1000 Watts, and more preferably about between about 300 Watts and 400 Watts is used. In certain preferred embodiments, the initial plasma reactant comprises an inert gas such as neon, and more preferably, argon^' or helium. In some cases, exposure to the initial plasma proceeds for between about 1 second and 60 seconds, and more preferably about 30 seconds. In some embodiments, the PECVD reaction chamber is maintained at between about 300 mTorr and about 400 mTorr, and more preferably about 350 mTorr. FIGURE 3 also shows the partially completed polishing pad 200 after exposing the modified surface 310 to a secondary plasma reactant. In some preferred embodiments the secondary plasma reactant comprises tetraethoxy silane (TEOS) or titanium alkoxide (TYZOR) . In some cases, the secondary plasma reactant also includes the first plasma reactant, for example, TEOS or TYZOR vapor mixed with helium or argon gas . Exposure to the secondary plasma reactant results in the grafting of the secondary plasma reactant to the modified surface 310 to form a polishing agent 320 comprising inorganic metal oxides. The polishing agent 320 coats an interior surface 330 of the concave cells 240. Again, the conditions and duration of exposure to the secondary plasma reactant is carefully controlled to avoid damaging the thermoplastic foam substrate 220, or the polishing agent 320, and to achieve long-lasting coatings of polishing agent 320. In some embodiments, the PECVD reaction chamber is maintained at between about 300 mTorr and about 400 mTorr, and more preferably about 350 mTorr. In some instances, the radio flow discharge electrode temperature is maintained at between about 20°C and 100 °C, and more preferably between about 30 °C and 50 °C. In some cases, an RF operating power between about 50 and about 500 Watts, and more preferably, about 250 to about 350 Watts, is used. Turning now to FIGURE 4, illustrated is the partially completed polishing pad 200 after a period of exposure to the secondary plasma reactant of at least about 30 minutes. In some preferred embodiments, exposure to the secondary plasma reactant is for between about 30 minutes and about 60 minutes. In other preferred embodiments, exposure to the secondary plasma reactant is for between about 30 minutes and about 45 minutes. Such periods of exposure advantageously enhance the incorporation of nitrides or carbides, or both, into the inorganic metal oxide of the polishing agent 320. In some embodiments of the method, an interior of closed cells 410 of the thermoplastic foam substrate 220 comprise nitrogen gas. The nitrogen gas can react with the secondary plasma reactant to form nitrides . In other embodiments of the method, at least a portion of the thermoplastic foam substrate 220 reacts with the secondary plasma reactant to form carbides. For example, in some embodiments, carbon radical species within in an about 1 micron depth of the thermoplastic foam substrate 220 from the modified surface 310 can react with the secondary plasma reactant. As discussed above and further illustrated in the example section to follow, the deposition of the polishing agent 320 via PECVD modifies the surface properties of certain thermoplastic foams 230, such as polyolefin foams . Surface coating of the thermoplastic foam surface 310 with a polishing agent 320 for up to about 30 minutes occurs by one mechanism. After this period, however, surface coating occurs by a different mechanism. This, in turn, results in the production of a polishing pad surface 410 having distinct differences in surface micromechanics and chemistry, depending on the coating time . In some cases, as the coating time increases, the temperature thermoplastic foam substrate 220 temperature increases. This, in turn, causes out-gassing of the nitrogen gas used in foaming the substrate 220 and located in the closed cells 410 of the substrate 240. In some embodiments, for instance where the polishing agent 320 comprises silicon oxides, the out-gassed nitrogen reacts with the silicon species on the pad surface, which causes formation of Si₃N₄ species in stoichiometric conversion from Si0₄ to Si₃N₄. Of course, analogous reactions can occur in embodiments where the polishing agent comprises other inorganic metal oxides such as titanium oxides . Similarly, at long coating times, the ion bombardment of the thermoplastic foam substrate 240 surface 310 generates appreciable amounts of carbon radicals on the surface 310 of the pad 200. In some embodiments, for instance where the polishing agent 320 comprises silicon oxides, these radicals react with the silicon species to form silicon carbide (SiC) , which are subsequently incorporated into the polishing agent 320 coating the pad 200. The incorporation of species such as Si₃N₄ and Sic into the polishing agent 320 modifies the polishing pad's 200 properties, such as enhancing its stiffness, hardness, and altering its modulus of elasticity, as compared to the starting thermoplastic foam substrate or substrates subject to brief coating periods. Having described the present invention, it is believed that the same will become even more apparent by reference to the following experiments. It will be appreciated that the experiments are presented solely for the purpose of illustration and should not be construed as limiting the invention. For example, although the experiments described below may be carried out in a laboratory setting, one skilled in the art could adjust specific numbers, dimensions and quantities up to appropriate values for a full-scale plant setting.

Experiments Experiments were conducted to: 1) characterize the chemical composition of thermoplastic foam substrates coated with polishing agents as a function of coating time; 2) characterize the mechanical properties of the foam substrate coated with polishing agents; and 3) measure the polishing properties of the polishing pads coated with polishing agents as a function of coating time . A thermoplastic foam substrate was formed into circular polishing pads of approximately 120 cm diameter of about 0.3 cm thickness. The commercially obtained thermoplastic foam substrate (J-foam from JMS Plastics, Neptune NJ) , designated as "J-60SE," comprised a blend of about 18% EVA, about 16 to about 20% talc, and balance polyethylene and other additives, such as silicates, present in the commercially provided substrate. The J-60 sheets were skived with a commercial cutting blade (Model number D5100 Kl, from Fecken-Kirfel, Aachen, Germany) . The sheets were then manually cleaned with an aqueous/isopropyl alcohol solution. The J-60SE substrate was then coated with a polishing agent comprising Tetraethoxy Silane (TEOS) , by placing the skived substrate into a reaction chamber of a conventional commercial Radio Frequency Glow Discharge (RFGD) plasma reactor having a temperature controlled electrode configuration (Model PE-2; Advanced Energy Systems, Medford, NY) . The plasma treatment of the substrate was commenced by introducing the primary plasma reactant, Argon, for 30 seconds within the reaction chamber maintained at 350 mTorr. The electrode temperature was maintained at 30 °C, and a RF operating power of 300 Watts was used. Subsequently, the secondary reactant was introduced, for periods ranging from about 0 to about 45 minutes at 0.10 SLM, and comprising TEOS mixed with He or Ar gas. The amount of secondary reactant in the gas stream was governed by the vapor back pressure (BP) of the secondary reactant monomer at the monomer reservoir temperature (MRT; 50 ± 10°C). The polishing properties of the J60SE polishing pads were examined by polishing wafers having an about 4000 Angstrom thick tungsten surface and an underlying about 250 Angstrom thick tantalum barrier layer. Tungsten polishing properties were assessed using a commercial polisher (Product No. EP0222 from Ebara Technologies, Sacramento, CA) . Unless otherwise noted, the removal rate of tungsten polishing was assessed using a down force of about 25 kPa of substrate, table speed of about 100 to about 250 rpm (Product Number MSW2000, from Rodel, Newark DE) . A conventional slurry (Product Number MSW2000, from Rodel, Newark DE) adjusted to a pH of about 2 was used. FIGURE 5 illustrates FTIR spectra of the substrate' s surface after different periods of coating with TEOS. Spectra were obtained on a FTIR spectrometer (FTIR 1727, Perkin-Elmer System detector, equipped with a Series-I FTIR Microscope (MCT detector) and having a spectral range from 10,000 to 370 cm-1. Signals at about 1010 and about 950 cm-1 were assigned to the asymmetric Si-O-Si stretch of silica and the Si-O-X (where X refers to polymeric - (Si-O-Si) n- structures not in the tetrahedral configuration), stretch of silicates, respectively. A signal at 850 cm-1 is due to free and associated silanols (Si-O-H) . The silanols associate through hydrogen bonding with the extent of association increases with increasing surface concentration of silanols. As illustrated in FIGURE 6, as coating time increases up to about 30 minutes, both of these signals montonically decreased, due to a net decrease in the surface concentration of Si-0 Thereafter, there was a change in deposition kinetics and mechanism, indicating an increased surface Si-0 concentration. The latter observation is inconsistent with generally accepted notions of TEOS deposition mechanisms and kinetics, and promoted further investigation of the coating process, especially in the post-30 minutes coating period. Nanoindentation testing was used to assess the mechanical properties of the surface coating coatings, and more specifically to measure the elastic modulus and the hardness. Indentations were carried out on the thermoplastic foam substrates coated with polishing agent for different periods. A NANOTEST 600®, Nanoindenter located at Advanced Material and Characterization Facility (AMPAC, Orlando, FL) was used for all measurements. The machine rests upon a vibration isolation table and is enclosed in a temperature- controlled cabinet. Two separate heaters placed on either side in front of the cabinet provide a thermal barrier. The temperature controller was set to a value about 2 or 3 °C above the room temperature, with expected stability at ± 0.1 °C. The indenter was allowed to settle for at least half an hour to attain thermal stability before starting the experiment. Indentation parameters such as the type of indenter, the maximum depth and loading /unloading rate, were established by performing preliminary tests of the coated polishing pads . The polishing pad surfaces were found to contain asperities and pores of varying sizes of the order of few microns. Based on these observations, a spherical indenter with tip diameter of ~1 mm was chosen, so that the indenter would sample enough pad material. For the same reason, a spatial resolution of greater than 500 microns was chosen. Indentations were performed under ultra low load range, with an initial load of 0.1 mN, which is a machine parameter. The control parameter was set to depth controlled and each pad was indented for varying depths with a maximum depth of 10,000 nanometers. The results were typically examined as an average of 10 indentations. Both Oliver-Pharr method and Hertz method were used to evaluate and validate the results. Load- depth (P-h) curves obtained from the nanoindenter were analyzed using the Oliver-Pharr method. The polishing agent-coated polishing pads exhibited non-uniform penetration during the indentation experiments. Visual examination of the indentation (P-h) curves shows some unique characteristics . As shown in FIGURE 7, distinct events, labeled as ''pop-in' (Xb) , ^Λpop-out' or ""kink-back' (Xc) , are discernable from the curves. For example, ^λpop-in' occurs during the compression cycle when there is a sudden penetration of the indenter tip into the sample. These events correlate with several experimental parameters, such as coating time, loading /unloading rate and depth of indentation. The mixed response exhibited by the coated polishing pads revealed that ^Λpop-in' events occurred more often for indenter penetration depth around 1000 nanometers, ^λpop- out' events seemed not to be affected by indentation depths, and rates of loading /unloading were affected both of these events. Further analysis of the P-h curve for various depths and different coating times, revealed that the loading curve increase steeply and decreases, this X coordinates, or the depth of this transition point, is designated as Xa. Similarly, Xb and Xc are the corresponding X coordinates or depth in nanometers. Such non-uniform penetration of the indenter tip into the coatings probably results from the onset of plastic deformation. It is thought that plastic deformation is a critical attribute of CMP pads, affecting the efficiency of the CMP process. Thus, the above-described events in the load-depth curves was hypothesized to be predictors of pad performance. The initial surface penetration events (Xa) were found to be a function of PECVD coating time, maximum penetration depth and load rate. For instance, FIGURE 8 shows a representative correlation of Xa with the TEOS coating time. The data suggests that Xa is related to the thickness of surface foam that has been modified by the dielectric coating. FIGURES 9 and 10, respectively illustrate the hardness and elastic modulus of polishing pads for different TEOS coating times, calculated using the Oliver-Pharr method. The effective surface modulus and hardness were found to increase with increasing coating time. For coating times of 30 minutes, 40 minutes and 45 minutes, the polishing pads had hardness values of about 65 KPa, 62 KPa and 70 KPa, respectively. For shorter coating times, the hardness was 60 KPa or less. For coating times of 30 minutes, 40 minutes and 45 minutes, the polishing pads had elastic modulus values of about 4 MPa, 5.5 MPa and 8.2 MPa, respectively. For shorter coating times, the elastic modulus values was 3 MPa or less . The changes in the mechanical properties of the pad surface are attributed to the effect of the coatings deposited on the foam substrate. These data indicate that the previously noted discontinuities in the FTIR data are indicative of changes in the pad surface chemistry, rather than net removal of TEOS-derived coatings . Dynamic mechanical analysis (DMA) was carried out on samples of coated polishing pads using commercial equipment operating in the tension mode from -125 to 200°C at a frequency of 1 Hz, at 10 micron amplitude with a programmed heating rate of 5°C/min. Liquid nitrogen was used to achieve the sub-ambient temperature. Samples were equilibrated at a predefined initial temperature for 10 minutes before measurements were made. All of the polishing pad samples were prepared to have the same dimensions of 15 cm x 5 cm, and were vacuum dried (30 °C at ~1 x 10-2 Torr) for 24 hours prior to DMA measurements, so as to avoid having to consider moisture effects . As illustrated in FIGURE 11, the DMA studies indicate an abrupt change in the loss modulus at long PECVD coating times. At a coating time of 45 minutes the loss modulus increase to about 0.6 MPa, as compares to values ranging from 0.37 to 0.23 for coating times of 10 minutes to 40 minutes. This is contrary to the generally accepted view that PECVD coatings only modify the surface of substrates. This surprising result suggests that other processes occur that alter the bulk mechanical properties of the foam substrate during the surface coating. It was hypothesized that residual reactants in the thermoplastic foam substrate were reacting during PECVD coatings in a time dependent fashion. The surface modification of polishing -pads, subjected to different coating times, was further characterized using X-ray photoelectron spectroscopy (XPS) . A commercial X-ray photoelectron spectrometer was operated at a base pressure of 10^~10 Torr and the spectrometer was calibrated using a metallic gold standard (Au (4_f7/2) : 84.0 ± 0.1 eV) . A non-monochromatic Mg K oc X-ray source with an energy of 1253 eV at a power of 250 W, was used for the arialysis. Charging shift produced by the polishing pad samples were removed by using binding energy scale referenced with respect to the binding energy of the hydrogen part of adventitious carbon line at 285.0 eV. Peak deconvolution was carried out using commercial software. The XPS analysis provides several insights about the chemical nature of the topography resulting from TEOS adsorption and dissociation. The Carbon (Is) signal was resolved into three major peaks: two peak at -285.0 eV, corresponding to C-C and C-H bonds and peak observed at ~286.5 eV corresponding to C-0 bonds. A peak centered at ~289 to ~289.3 eV was attributed to carbamide [-0- C(NH₂)=0] functional group from residual blowing agents used in the thermoplastic foam substrate manufacturing process. For the specimens coated for 40 and 45 minutes respectively, another peak near ~283.6 eV was observed, and was tentatively assigned to C-Si bonds. FIGURE 12 presents exemplary peak fitted XPS signals of the Si (2p) envelop obtained from pads after TEOS coating times of: (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, and (e) 45min. Peaks were identified as: (1) Si- O, (2) Silicate, (3) Si-N, and (4) Si-C bonds. Each spectrum was deconvoluted into two major peaks at -102.3 and -103.4 eV, corresponding to bonds in silicate and Si- 0 species, respectively. These data indicate that for short coating times (e.g., less than ~30 minutes) the pad surface is rich in silanol, consistent with TEOS films deposited at low process temperatures. As further illustrated in FIGURE 13 the Oxygen to Si intensity ratio, calculated from the XPS data, is high early during coating indicative of a high the concentration of the silanol in the deposited coatings. The silanol concentration decreases with coating times up 30 minutes, then starts increasing. For example, as shown in FIGURE 9, the 0:Si ratio equals about 7.4, 4.8, 3.6, 7.1 and 9.9, after coating times of about 10 min, 20 min, 30 min, 40 min and 45 min, respectively. Turning again to FIGURE 12, for coating times of 30, 40 and 45 min, a small peak is observed at -102.1 eV, corresponding to Si-N bonds. Over this same period, there is an abrupt reduction in Si to N ratio, which indicates an increase in nitrogen species on the surface. These observations suggest that PECVD-based coating involving competition between several processes. The PECVD-based coating produces both silica and silicates on the (SiO_x and Si0₂) on the foam surface. Moreover, surface chemistry of the substrates changes as a function the coating time. For coating times below 30 minutes, it there is net etching of the deposits from the Ar-ion bombardment of the surface. The sample also heats up from the plasma, so thermal processes also occur. As the coating time increases, the silicate content on the substrate surface starts to decrease and the pad becomes denser, so as to increase the hardness of the pad. For coating times of 30 minutes of longer, the substrate temperature is high enough to cause out-gassing of the nitrogen gas used in foaming the substrate or decomposition of any residual blowing agent left in the foam to produce nitrogen gas . The nitrogen reacts with Si-containing intermediates in the gas phase or Si- species on the pad surface, to form nitrides such as Si₃N₄ at the expense of Si0₂. Such nitrides are incorporated into the polishing agent in concentrations up to 10 mol% . Furthermore, for such coating times, the ion bombardment of the foam surface generates appreciable amounts of ^' Carbon radicals on the pad surface. These radicals react with the silicon species to form carbides, such as silicon carbide (SiC) , which is incorporated into the polishing agent in concentrations up to 10 mol% . FIGURE 14 compares the Relative Blanket Tungsten Removal Rate (W-RR) and the Static Coefficient of Friction (COF) for thermoplastic foam substrate subjected to different periods of coating times with TEOS . Both the W-RR and COF both increase with increased coating times up to 30 minutes, signifying an increase in the thickness of the polishing agent. For coating times between 30 and 60 minutes, the W-RR and COF both decrease, and then increase. These results suggest that the pad appears polishes by one mechanism for surfaces coated for up to 30 minutes, and by a different mechanism for surfaces coated for more than 30 minutes, due to differences in surface micromechanics and chemistry.

Claims

WHAT IS CLAIMED IS: 1. A polishing pad for chemical mechanical polishing comprising: a polishing body comprising a thermoplastic foam substrate having a surface comprising concave cells; and a polishing agent coating an interior surface of said concave cells, wherein said polishing agent comprises an inorganic metal oxide that includes carbides or nitrides . 2. The polishing pad as recited in Claim 1, wherein said carbides or nitrides are incorporated into a lattice of said inorganic metal oxides. 3. The polishing pad as recited in Claim 2, wherein said nitrides comprise silicon nitrides or titanium nitrides and said carbides include silicon carbides or titanium carbides . 4. The polishing pad as recited in Claim 1, wherein said polishing pad has a hardness of 70 KPa or greater. 5. The polishing pad as recited in Claim 1, wherein said polishing pad has an elastic modulus of 5 MPa or greater. 6. A method for preparing a polishing pad for chemical mechanical polishing, comprising: exposing closed cells within a thermoplastic foam substrate to provide a substrate surface comprising concave cells; and coating an interior surface of said concave cells with a polishing agent comprising an inorganic metal oxide, wherein carbides and nitrides are incorporated into said inorganic metal oxide during said coating. 7. The method as recited in Claim 6, wherein coating comprises: exposing said substrate surface to an initial plasma reactant in a plasma enhanced chemical vapor deposition (PECVD) process to produce a modified surface thereon; and exposing said modified surface to a secondary plasma reactant in said PECVD process to form said polishing agent . 8. The method as recited in Claim 7, wherein exposure to a secondary plasma reactant proceeds for between 30 minutes and 60 minutes. 9. The method as recited in Claim 7, wherein an interior of said closed cells comprise nitrogen gas and said nitrogen gas reacts with said secondary plasma reactant to form said nitride. 10. The method as recited in Claim 7, wherein said thermoplastic foam substrate reacts with said secondary plasma reactant to form said carbide.