EP1542832A1

EP1542832A1 - Polishing pad with window for planarization

Info

Publication number: EP1542832A1
Application number: EP03759509A
Authority: EP
Inventors: Robert Swisher; William C. Allison; Alan E. Wang
Original assignee: PPG Industries Ohio Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 2002-09-25
Filing date: 2003-09-18
Publication date: 2005-06-22
Also published as: TW200522185A; TWI287836B; CN1684798A; WO2004028744A1; JP2005538571A; KR20050052513A; CN100417493C; JP2008229843A; AU2003275237A1; US20040102141A1

Abstract

A polishing pad is described having a window. The polishing pad comprises a first layer and a second layer. The first layer having an opening can function as the work surface or polishing layer of the pad. At least a portion of said second layer comprises an at least partially transparent window.

Description

POLISHING PAD WITH WINDOW FOR PLANARIZATION

The present invention relates to a polishing pad. In particular, the polishing pad of the present invention can include a window. The polishing pad of the present invention can be useful for polishing articles and can be especially useful for chemical mechanical polishing or planarization of a microelectronic device, such as a semiconductor wafer. The window of the polishing pad is at least partially transparent and thus, can be particularly useful with polishing or planarizing tools that are equipped with through-the-platen wafer metrology. In general, the polishing or planarization of a non-planar surface of a microelectronic device to an essentially planar surface involves rubbing the non-planar surface with the work surface of a polishing pad using a controlled and repetitive motion. A polishing slurry can be interposed between the rough surface of the article that is to be polished and the work surface of the polishing pad. The fabrication of a microelectronic device such as a semiconductor wafer generally involves the formation of a plurality of integrated circuits on the wafer comprising, for example, silicon or gallium arsenide. The integrated circuits can be formed by a series of process steps in which patterned layers of materials, such as conductive, insulating and semiconducting materials, are formed on the substrate. In order to maximize the density of integrated circuits per wafer, it is desirable to have an essentially planar polished substrate at various stages throughout the semiconductor wafer production process. Thus, semiconductor wafer production can include at least one, and more typically a plurality of polishing steps, which can use one or more polishing pads. A chemical mechanical polishing (CMP) process can include placing the microelectronic substrate in contact with a polishing pad; rotating the pad while a force is applied to the backside of the microelectronic device; and applying an abrasive- containing chemically-reactive solution commonly referred to as a "slurry" to the pad during polishing. A CMP polishing slurry can contain an abrasive material, such as silica, alumina, ceria or mixtures thereof. The rotational movement of the pad relative to the substrate as slurry is provided to the device/pad interface can facilitate the polishing process. In general, polishing can be continued in this manner until the desired film thickness is removed.

Depending on the choice of polishing pad and abrasive, and other additives, the CMP process can provide effective polishing at desired polishing rates while reducing or minimizing surface imperfections, defects, corrosion, and erosion.

Polishing or planarization characteristics can vary from pad-to-pad, and throughout the operating lifetime of a given pad. Variations in the polishing characteristics of the pads can result in inadequately polished or planarized substrates which are not useful. Thus, it is desirable in the art to develop a polishing pad that exhibits reduced pad-to-pad variation in polishing or planarization characteristics. It is further desirable to develop a polishing pad that exhibits reduced variations in polishing or planarization characteristics throughout the operating life of the pad.

Planarizing tools having the ability to measure the progress of the planarization process while the wafer is held in the tool and in contact with the pad are known in the art. Measuring the progress of planarizing a microelectronic device during the planarizing process can be referred to in the art as "in-situ metrology". United States Patents 5,964,643 and 6,159,073; and European Patent 1,108,501 describe polishing or planarizing tools and in-situ metrology systems. In general, in-situ metrology can include directing a beam of light through an at least partially transparent window located in the platen of the tool; the beam of light can be reflected off the surface of the wafer, back through the platen window, and into a detector. The polishing pad can include a window that is at least partially transparent to the wavelengths used in the metrology system, and essentially aligned with the planten window. Thus, it is desirable to develop a polishing pad that comprises a window area useful for in-situ metrology. It is further desirable that the window provides suitable transparency throughout the operating life of the pad.

One disadvantage with known pads having windows which are coplanar with the polishing surface, can include wearing of the window portion at a slower rate than the pad surface. A further disadvantage with known pads having a coplanar window can include scratching of the window as a result of its contact with abrasive particles in the slurry during the polishing or planarization process. A scratched window can generally reduce the transparency of the window and can cause an attenuation of the metrology signal.

The present invention includes a polishing pad having a window. In a non- limiting embodiment, the polishing pad can comprise a first layer and a second layer. The first layer can function as the work surface or polishing layer of the pad. At least a portion of the second layer can comprise a window which is at least partially transparent to wavelengths used by the metrology instrumentation of polishing tools. Furthermore, the first layer can absorb at least 2 percent by weight of polishing slurry based on the total weight of the first layer.

The polishing pad of the present invention can comprise a first layer and a second layer. The first layer can function as the polishing or working surface of the pad such that the first layer can at least partially interact with the substrate to be polished and the polishing slurry. In a non-limiting embodiment, the first layer can be-porous and permeable to polishing slurry. In a non-limiting embodiment, the second layer can be substantially nonporous and substantially impermeable to polishing slurry.

As used herein and the claims the term "substantially nonporous" means generally impervious to the passage of liquid, gas, and bacteria. On a macroscopic scale, a substantially nonporous material exhibits few if any pores. As used herein and the claims, the term "porous" means having pore(s) and the term "pore(s)" refers to minute opening(s) through which matter passes.

It is noted that, as used in this specification, the singular forms "a," "an," and "the" include plural referents unless expressly and unequivocally limited to one referent. For the purposes of this specification, unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

The first layer can include a variety of materials known in the art. Non-limiting examples of suitable materials comprising the first layer can include but are not limited to paniculate polymer and crosslinked polymer binder as described in United States Patent 6,477,926B1; particulate polymer and an organic polymer binder as described in United States Patent Application Serial Number 10/317,982; sintered particles of thermoplastic resin as described in United States Patents 6,062,968; 6,117,000; and 6,126,532; and pressure sintered powder compacts of thermoplastic polymer as described in United States Patents 6,231,434B1; 6,325,703B2; 6,106,754; and 6,017,265. Further non-limiting examples of suitable materials comprising the first layer can include polymeric matrices impregnated with a plurality of polymeric microelements, wherein each polymeric microelement can have a void space therein, as described in United States Patents 5,900,164 and 5,578,362. The thickness of the first layer can vary. In alternate non-limiting embodiments, the first layer can have a thickness of at least 0.020 inches, or at least 0.040 inches; or 0.150 inches or less, or 0.080 inches or less.

In another non-limiting embodiment, the first layer can include pores such that polishing slurry can be at least partially absorbed by the first layer. The number of pores can vary. In alternate non-limiting embodiments, the first layer can have a porosity, expressed as percent pore volume, of at least 2 percent by volume based on the total volume of the first layer, or 50 percent or less by volume based on the total volume of the first layer, or from 2 to 50 percent by volume based on the total volume of the first layer. The percent pore volume of the polishing pad layer can be determined using a variety of techniques known in the art. In a non-limiting embodiment, the following expression can be used to calculate percent pore volume:

100 x (density of the pad layer) x (pore volume of the pad layer).

The density can be expressed in units of grams per cubic centimeter, and can be determined by a variety of conventional methods known in the art. In a non-limiting embodiment, the density can be determined in accordance with ASTM D 1622-88. The pore volume can be expressed in units of cubic centimeters per gram, and can be determined using conventional methods and equipment known in the art. In a non- limiting embodiment, pore volume can be measured in accordance with the mercury porosimetry method in ASTM D 4284-88, using an Autopore III mercury porosimeter from Micromeritics can be used. In a further non-limiting embodiment, the pore volume measurements can be made under the following conditions: a contact angle of 140°; a mercury surface tension of 480 dynes/cm; and degassing of the polishing pad layer sample under a vacuum of 50 micrometers of mercury. In a non-limiting embodiment, the first layer can have an at least partially open structure such that it can absorb slurry. In alternate non-limiting embodiments, the first layer can absorb at least 2 percent by weight of polishing slurry based on the total weight of the first layer, or not more than 50 percent by weight, or from 2 percent by weight to 50 percent by weight. In another non-limiting embodiment of the present invention, the first layer of the polishing pad can have a compressibility greater than the second layer. As used herein, the term "compressibility" refers to the percent volume compressibility measurement. In a further non-limiting embodiment, the percent volume compressibility of the first layer can be at least 0.3 percent; or 3 percent or less; or from 0.3 to 3 percent; when a load of 20 psi is applied.

The percent volume compressibility of a pad layer can be determined using various methods known in the art. In a non-limiting embodiment, the percent volume compressibility of a pad layer can be determined using the following expression.

100 x (pad layer volume without load - pad layer volume under load)

(pad layer volume without load)

In a non-limiting embodiment, the area of the pad layer does not change when the load is placed on it; thus, the preceding equation for volume compressibility can be expressed in terms of pad layer thickness by the following expression.

100 x (pad layer thickness without load - pad layer thickness under load)

(pad layer thickness without load)

The pad layer thickness can be determined using a variety of known methods. In a non-limiting embodiment, the pad layer thickness can be determined by placing a load (such as, but not limited to, calibrated weights) on the pad sample and measuring the change in thickness of the pad layer as a result of the load. In a further non-limiting embodiment, a Mitutoyo Electronic Indicator, Model ID-C112EB can be used. The indicator has a spindle or threaded rod which can be fitted at one end with a flat contact under which the pad layer is placed. The spindle can be fitted at the other end with a device for applying specified loads to the contact area, such as but not limited to a balance pan which accepts calibrated weights. The Indicator displays the displacement of the pad layer resulting from applying the load. The Indicater display is typically representative of inches or millimeters. The Electronic Indicator can be mounted on a stand, such as a Mitutoyo Precision Granite Stand, to provide stability while taking the measurements. The lateral dimensions of the pad layer can be sufficient to permit measurements at least 0.5" from any edge. The surface of the pad layer can be flat and parallel over a sufficient area to permit uniform contact between the test pad layer and the flat contact. The pad layer to be tested can be placed under the flat contact. The thickness of the pad layer can be measured prior to applying the load. Calibrated balance weights can be added to the balance pan for a specific resultant load. The pad layer then can be compressed under the specified load. The Indicator can display the thickness/height of the pad layer under the specified load. The thickness of the pad layer prior to applying the load minus the thickness of the pad layer under the specified load can be used to determine the displacement of the pad layer. In a non-limiting embodiment, a load of 20 psi can be applied to the pad layer. Measurements can be made at a standardized temperature such as room temperature. In a non-limiting embodiment, measurements can be taken at a temperature of 22°C +/-2°C.

In alternate non-limiting embodiments, the above-described method of measuring pad layer thickness can be applicable to a stacked pad assembly or layer(s) comprising the stacked pad assembly.

In a non-limiting embodiment, a procedure for measuring percent volume compressibility can include placing the contact on the granite base and adjusting the indicator to read zero. The contact can then be raised and the specimen placed on the granite stand under the contact with the edge of the contact at least 0.5" from any edge of the specimen. The contact can be lowered onto the specimen and the specimen thickness measurement can be taken after 5 +/-1 seconds. Without moving the specimen or the contact, sufficient weight can be added to the pan to cause a force of 20 psi to be applied to the specimen by the contact. The reading for the specimen thickness under load measurement can be made after 15 +/-1 seconds. The measurement procedure can be repeated, making five measurements at different positions on the specimen at least 0.25" apart using 20 psi of compressive force.

In a non-limiting embodiment, the softness of the first layer can be determined. As used herein and the claims, the term "softness" refers to the Shore A Hardness of the material. In general, the softer the material, the lower the Shore A Hardness value. In alternate non-limiting embodiments, the first layer can have a Shore A Hardness of at least 85; or 99 or less, or from 85 to 99. The Shore A Hardness value can be determined using various methods and equipment known in the art. In a non-limiting embodiment, Shore A Hardness can be determined in accordance with the procedure recited in ASTM D 2240, using a Shore "Type A" Durometer having a maximum indicator (available from PCT Instruments, Los Angeles, CA). In a non-limiting embodiment, the test method for Shore A Hardness can include the penetration of a specific type of indentor being substantially forced into the test material under specified conditions. In this embodiment, the Shore A Hardness can be inversely related to the penetration depth and can be dependent on the elastic modulus and viscoelastic behavior of the test material. In a non-limiting embodiment, the first layer can comprise grooves or pattern on the work or polishing surface. The types of grooves and/or patterns can vary and can include the various types of grooves and/or patterns known in the art. The process for making the grooves and patterns can also vary and can include the various conventional methods known in the art. The polishing pad of the present invention further comprises a second layer. In a non-limiting embodiment, the second layer can be connected to at least a portion of the first layer. In a further non-limiting embodiment, the first layer can be connected to at least a portion of the second layer, and the second layer can be connected to at least a portion of an optional third layer. The second layer can include a variety of materials known in the art. The second layer can be selected from substantially non-volume compressible polymers and metallic films and foils. As used herein and the claims, "substantially non-volume compressible" means that the volume can be reduced by less than 1 % when a load of 20 psi is applied. In a non-limiting embodiment, the method for applying the load and measuring the reduction in volume is previously described herein can be employed. Non-limiting examples of substantially non-volume compressible polymers can include polyolefins, such as but not limited to low density polyethylene, high density polyethylene ultra-high molecular weight polyethylene and polypropylene; polyvinylchloride; cellulose-based polymers, such as but not limited to cellulose acetate and cellulose butyrate; acrylics; polyesters and co-polyesters, such as but not limited to PET and PETG; polycarbonate; polyamides, such as but not limited to nylon 6/6 and nylon 6/12; and high performance plastics, such as but not limited to polyetheretherketone, polyphenylene oxide, polysulfone, polyimide, and polyetherimide; and mixtures thereof.

Non-limiting examples of metallic films can include but are not limited to aluminum, copper, brass, nickel, stainless steel, and combinations thereof. The thickness of the second layer can vary. In alternate non-limiting embodiments, the second layer can have a thickness of at least 0.0005, or at least 0.0010; or 0.0650 inches or less, or 0.0030 inches or less.

In a non-limiting embodiment, the second layer can be flexible to enhance or increase the uniformity of contact between the polishing pad and the surface of the substrate being polished. A consideration in selecting the material for the second layer can be the capability of a material to provide compliant support to the work surface of the polishing pad such that the first layer substantially conforms to the macroscopic contour or long-term surface of the device being polished. A material having said capability can be desirable for use as the second layer in the present invention.

The flexibility of the second layer can vary. The flexibility can be determined using a variety of conventional techniques known in the art. As used herein and the claims the term "flexibility" (F) refers to the inverse relationship of second layer thickness cubed (t³) and the flexural modulus of the second layer material (E), i.e. F = l/t³E. In alternate non-limiting embodiments, the flexibility of the second layer can be at least 0.5 in 'lb ¹; or at least 100 in 'lb^'1; or from 1 ih'lb ¹ to 100 ih'lb ¹. In a non-limiting embodiment, the second layer can have a compressibility which allows the polishing pad to substantially conform to the surface of the article to be polished. The surface of a microelectronic substrate, such as a semiconductor wafer, can have a "wave" contour as a result of the manufacturing process. It is contemplated that if the polishing pad cannot adequately conform to the "wave" contour of the substrate surface, the uniformity of the polishing performance can be degraded. For example, if the pad substantially conforms the ends of the "wave", but cannot substantially conform and contact the middle portion of the "wave", only the ends of the "wave" can be polished or planarized and the middle portion can remain substantially unpolished or unplanarized. The compressibility of the second layer can vary. As previously described, the term "compressibility" refers to the percent volume compressibility measurement. In alternate non-limiting embodiments, the percent volume compressibility of the second layer can be at least one percent; or three percent or less; or from one to three percent. The percent volume compressibility can be determined using a variety of conventional methods known in the art. In a non-limiting embodiment, the percent volume compressibility is determined as previously described herein.

In another non-limiting embodiment, the second layer can distribute the compressive forces experienced by the first layer over a larger area of an optional third layer. In a non-limiting embodiment, the second layer is substantially non-volume compressible.

In another non-limiting embodiment, the second layer can function as a substantial barrier to fluid transport between the first layer and an optional third layer at least partially connected to the second layer. Thus, a consideration in selecting the material comprising the second layer can be the ability of the material to substantially reduce, minimize or essentially prevent the transport of polishing slurry from the first layer to an optional third layer. In a non-limiting embodiment, the second layer can be substantially impermeable to the polishing slurry such that the optional third layer does not become saturated with polishing slurry.

In an alternate non-limiting embodiment, the second layer can be perforated such that polishing slurry can penetrate the first and second layers to wet an optional third layer. In a further non-limiting embodiment, the optional third layer can be substantially saturated with polishing slurry. The perforations in the second layer can be formed by a variety of techniques known to the skilled artisan, such as but not limited to punching, die cutting, laser cutting or water jet cutting. The hole size, number and configuration of the perforations can vary. In a non-limiting embodiment, the perforation hole diameter can be at least 1/16 inch with at least 26 holes per square inch in a staggered hole pattern.

In a non-limiting embodiment, the first layer can be connected to at least a portion of the second layer to produce a stacked pad assembly. As used herein and the claims, the term "connected to" means to link together or place in relationship either directly, or indirectly by one or more intervening materials. In a non-limiting embodiment, the first and second layers are at least partially connected such that the opening of the first layer can be at least partially aligned with the at least partially transparent window of the second layer.

In a non-limiting embodiment, the first layer of the polishing pad can be connected to at least a portion of the second layer using an adhesive. A suitable adhesive for use in the present invention can provide sufficient peel resistance such that the pad layers essentially remain in place during use. Further, the adhesive can be selected to sufficiently withstand shear stresses which are present during the polishing or planarization process and moreover, can sufficiently resist chemical and moisture degradation during use. The adhesive can be applied using conventional techniques known to the skilled artisan. In a non-limiting embodiment, the adhesive can be applied to a lower surface of the first layer and/or an upper surface of the second layer which are parallel facing to one another.

The adhesive can be chosen from a wide variety of adhesive materials known in the art, such as but not limited to contact adhesives, pressure sensitive adhesives, structural adhesives, hot melt adhesives, thermoplastic adhesives, and curable adhesives, such as thermosetting adhesives. Non-limiting examples of structural adhesives can be chosen from polyurethane adhesives, and epoxy resin adhesives; such as those based on the diglycidyl ether of bisphenol A. Non-limiting examples of pressure sensitive adhesives can include an elastomeric polymer and a tackifying resin. The elastomeric polymer can be chosen from natural rubber, butyl rubber, chlorinated rubber, polyisobutylene, poly(vinyl alkyl ethers), alkyd adhesives, acrylics such as those based on copolymers of 2-ethylhexyl acrylate and acrylic acid, block copolymers such as styrene-butadiene-styrene, and mixtures thereof. In a non-limiting embodiment, a pressure sensitive adhesive can be applied to a substrate using an organic solvent such as toluene or hexane, or from a water-based emulsion or from a melt. As used herein, "hot melt adhesive" refers to an adhesive comprised of a nonvolatile thermoplastic material that can be heated to a melt, then applied to a substrate as a liquid. Non-limiting examples of hot melt adhesives can be chosen from ethylene-vinyl acetate copolymers, styrene-butadiene copolymers, ethylene-ethyl acrylate copolymers, polyesters, polyamides such as those formed from the reaction of a diamine and a dimer acid, and polyurethanes.

In a non-limiting embodiment of the present invention, the first layer can comprise an opening. In a further non-limiting embodiment, at least a portion of the second layer can comprise a window which is at least partially transparent to wavelengths used by the metrology instrumentation of the planarizing equipment. The size, shape, and positioning of the opening in the first layer and/or the window in the second layer can be dependent upon the metrology instrumentation and polishing apparatus being employed to polish and/or planarize the pad. The opening can be produced by a variety of conventional methods known in the art. In alternate non- limiting embodiments, the opening can be made by punching, die cutting, laser cutting or water jet cutting. In a further non-limiting embodiment, the opening can be formed by molding the first layer. In an alternate non-limiting embodiment, the openings can be die cut into the first layer using an NAEF Model B die press fitted with dies of suitable size and shape, which are commercially available from MS Instruments Company, Stony Brook, NY.

In a non-limiting embodiment, the opening in the first layer can be produced prior to stacking together and/or at least partially connecting the first layer with the second layer. At least a portion of the second layer can comprise an at least partially transparent window. In a non-limiting embodiment, the second layer can comprise an at least partially transparent material. In another non-limiting embodiment, the second layer can comprise a substantially non-transparent material; an opening can be cut into the second layer to remove a portion of the second layer; an at least partially transparent material can be inserted into the opening in the second layer. The opening can be made using a variety of methods previously described herein. In a non-limiting example, the second layer can include a metal foil; an opening can be cut into the metal foil to remove a portion of the metal foil; a piece of polyester can be cut into a size and shape that substantially corresponds to the opening; the polyester can be fitted into the opening in the metal foil to form an at least partially transparent window.

In a non-limiting embodiment, the second layer can comprise an adhesive assembly. The adhesive assembly can include interposing the second layer between an upper adhesive layer and a lower adhesive layer. In a non-limiting embodiment, the upper adhesive layer of the adhesive assembly can be at least partially connected to the lower surface of the first layer. The lower adhesive layer of the adhesive assembly can be at least partially connected to the upper surface of an optional third layer. The second layer of the adhesive assembly can be selected from the aforementioned suitable materials for the second layer of the polishing pad. The upper and lower adhesive layers of the adhesive assembly can be selected from the non-limiting examples of adhesives previously mentioned herein. In a non-limiting embodiment, the upper and lower adhesive layers each can be contact adhesives. The adhesive assembly can be referred to in the art as two-sided or double coated tape. Non-limiting examples of commercially available adhesive assemblies include those from 3M, Industrial Tape and Specialties Division.

In a further non-limiting embodiment, at least a portion of the adhesive layer can be removed from the second layer of the adhesive assembly exposing at least a portion of the at least partially transparent middle layer of the adhesive assembly, thereby forming an at least partially transparent window in the second layer. In alternate non- limiting embodiments, the removal of the adhesive can be performed prior to stacking the layers or after the layers are stacked. The removal process can include a variety of methods known to the skilled artisan, including but not limited to dissolution of the adhesive in solvent or aqueous detergent solution, or physically stripping the adhesive from the second layer. In a non-limiting embodiment, physically stripping the adhesive can be include contacting the adhesive with a material to which the adhesive substantially adheres, and then pulling the material from the second layer, whereby the adhesive is removed with the material.

In a further non-limiting embodiment, the window of the second layer can be recessed below the polishing surface of the pad by a distance equal to the thickness of the first layer of the pad.

In another non-limiting embodiment, the pad assembly can include a coating on at least a portion of the top and/or bottom sides of the window of the second layer. The coating can be at least partially applied with an adhesive in place or following removal of the adhesive. The coating can be at least partially applied prior to stacking the layers or after the layers have been stacked. The coating can provide any one of the following properties, for example: improved transparency of the window area, improved abrasion resistance, improved puncture resistance. In a non-limiting embodiment, the coating can include a resin film, or a cast-in-place resin coating.

Non-limiting examples of suitable resin films for use in the present invention can include the materials described above for the second layer. In alternative non-limiting embodiments, the resin film chosen for the coating can be the same material or different material as that comprising the second pad layer. The resin film can be at least partially adhered to the window area of the second layer by any means known to the skilled artisan, such as the adhesive methods and materials listed above for pad stack adhesives. In a non-limiting embodiment, the coating can be a layer of resin film that can be the same as the material used for the second layer. The coating can be at least partially applied after assembly of the pad stack. The coating can be at least partially applied to both the top and bottom surfaces of the window area of the second layer, and the adhesive can be at least partially adhered using a contact adhesive used as the stack adhesive. In a non-limiting embodiment, the coating can be a cast-in-place resin coating, which can be applied as a liquid, as a solvent solution, dispersion, or aqueous latex; as a melt, or as a blend of resin precursors that can react to form the coating. The application of the liquid can be accomplished by a variety of known methods, including spraying, padding, and pouring. Non-limiting examples of suitable materials for the coating include thermoplastic acrylic resins, thermoset acrylic resins, such as hydroxyl- functional acrylic latexes crosslinked with urea-formaldehyde or melamine-formaldehyde resins, hydroxyl-functional acrylic resins crosslinked with epoxy resins, or carboxyfunctional acrylic latexes crosslinked with carbodiimides or polyimines or epoxy resins; urethane systems, such as hydroxy functional acrylic resin crosslinked with polyisocyanate, moisture-cured isocyanate-terminated resins; carbamate-funtional acrylic resins crosslinked with melamine-formaldehyde resins; epoxy resins, such as polyamide resin crosslinked with bisphenol A epoxy resins, phenolic resins crosslinked with bisphenol A epoxy resins; polyester resins, such as hydroxyl-terminated polyesters crosslinked with melamine-formaldehyde resins or with polyisocyanates or with epoxy crosslinkers.

In a non-limiting embodiment, the coating can be an aqueous acrylic latex, which can be applied following stacking of the pad assembly. The coating can be at least partially applied to the top and bottom surfaces of the window area of the second layer. Application of the coating can be performed following removal of an adhesive from the window area.

The window pad of the present invention can be used with a variety of polishing equipment known in the art. In a non-limiting embodiment, a Mirra polisher, produced by Applied Materials Inc, Santa Clara CA, can be used wherein the shape of the opening is a rectangle, having a size 0.5" x 2", being positioned with the long axis radially oriented and centered 4" from the center of the pad. The platen for the Mirra polisher is 20" in diameter. A pad for use with this polisher can comprise a circle of a 20-inch diameter having a window located in the area as described.

In a further non-limiting embodiment, a Teres polisher commercially available from Lam Research Corporation, Fremont, CA, can be employed. This polisher uses a continuous belt instead of a circular platen. The pad for this polisher can be a continuous belt of 12" width and 93.25" circumference, which has a window area suitably sized and positioned to align with the metrology window of the Teres polisher.can be such that it can be at least partially aligned with the at least partially transparent window in the second layer. As identified previously herein, the polishing pad of the present invention can comprise additional optional layer(s). The additional layer(s) can contain opening(s) that are substantially aligned with the opening in the first layer and the at least partially transparent window in the second layer.

In a non-limiting embodiment, the polishing pad of the present invention can comprise a third layer. The third layer can function as the bottom layer of the pad which can be attached to the platen of the polishing apparatus.

In a non-limiting embodiment, the third layer can comprise a material that is softer than the first layer. As used herein, the term "softness" refers to the Shore A Hardness of the material. The softer the material, the lower the Shore A Hardness value. Thus, in the present invention, the Shore A Hardness value of the third layer can be lower than the Shore A Hardness value of the first layer. In a non-limiting embodiment, the third layer can have a Shore A Hardness of at least 15. In alternate non-limiting embodiments, the Shore A Hardness of the third layer can be at least 45, or 75 or less, or from 45 to 75. The Shore A Hardness can be determined using a variety of conventional methods known in the art. In a non-limiting embodiment, the Shore A Hardness can be determined as previously described herein.

In a non-limiting embodiment, the third layer can be used to increase the uniformity of contact between the polishing pad and the surface of the substrate undergoing polishing. In a non-limiting embodiment of the present invention, the material comprising the third layer of the polishing pad can have a compressibility greater than the material comprising the first layer. As previously described, the term "compressibility" refers to the percent volume compressibility measurement. Thus, the percent volume compressibility of the third layer is greater than the percent volume compressibility of the first layer. In a non-limiting embodiment, the percent volume compressibility of the third layer can be less than 20 percent when a load of 20 psi is applied. In alternate non- limiting embodiments, the percent volume compressibility of the third layer can be less than 10 percent when a load of 20 psi is applied, or less than 5 percent when a load of 20 psi is applied. As previously identified percent volume compressibility can be determined by a variety of conventional methods known in the art. In a non-limiting embodiment, percent volume compressibility can be determined as previously described herein.

The thickness of the third layer can vary. In general, the third layer thickness should be such that the pad is not too thick. A pad which is too thick can be difficult to place on and take off of the planarization equipment. Thus, in alternate non-limiting embodiments, the thickness of the first layer can be at least 0.040 inches, or at least 0.045 inches; or 0.100 inches or less, or 0.080 inches or less, or 0.065 inches or less. The sublayer can comprise a wide variety of materials known in the art. Suitable materials can include natural rubber, synthetic rubbers, thermoplastic elastomer, foam sheet and combinations thereof. The material of the sublayer can be foamed or blown to produce a porous structure. The porous structure can be open cell, closed cell, or combinations thereof. Non-limiting examples of synthetic rubbers can include neoprene rubber, silicone rubber, chloroprene rubber, ethylene-propylene rubber, butyl rubber, polybutadiene rubber, polyisoprene rubber, EPDM polymers, styrene-butadiene copolymers, copolymers of ethylene and ethyl vinyl acetate, neoprene/vinyl nitrile rubber, neoprene/EPDM/SBR rubber, and combinations thereof. Non-limiting examples of thermoplastic elastomers can include polyolefins, polyesters, polyamides, polyurethanes such as those based on polyethers and polyesters, and copolymers thereof. Non-limiting examples of foam sheet can include ethylene vinyl acetate sheets and polyethylene foam sheets, such as but not limited to those which are commercially available from Sentinel Products, Hyannis, NJ; polyurethane foam sheets, such as but not limited to those which are commercially available from Illbruck, Inc., Minneapolis, MN; and polyurethane foam sheets, and polyolefin foam sheets, such as but not limited to those which are available from Rogers Corporation, Woodstock, CT. In a further non-limiting embodiment, the sublayer can include non- woven or woven fiber mat, and combinations thereof; such as but not limited to polyolefin, polyester, polyamide, or acrylic fibers, which have been impregnated with a resin. The fibers can be staple or substantially continuous in the fiber mat. Non-limiting examples can include but are not limited to non-woven fabric impregnated with polyurethane as describe in United States Patent 4,728,552, such as polyurethane impregnated felt. A non- limiting example of a commercially available non- woven subpad can be Suba™ IN, from Rodel, Inc. Newark DE.

In the present invention, the optional third layer can comprise an opening. In alternate non-limiting embodiments, the opening can be produced by any suitable means known in the art, such as those previously identified relative to the opening in the first layer. Further, as previously identified, the size, shape and position of the opening can be dependent upon the metrology instrumentation and polishing apparatus employed.

In a non-limiting embodiment, the third layer can be at least partially connected to the second layer and can be in contact with the base of the planarizing machine. The third layer can contain an opening that is at least partially aligned with the opening of the first layer and the at least partially transparent or window area of the second layer.

In an alternate non-limiting embodiment, the first layer of the polishing pad can be connected to at least a portion of the second layer and the second layer can be connected to at least a portion of a third layer using an adhesive. Suitable adhesives can include those previously recited herein. In a further non-limiting embodiment, the polishing pad of the present invention can comprise a first layer, a second layer, and a third layer. The first and third layers each comprise an opening. The opening of the first and third layers can be at least partially aligned with one another. At least a portion of the second layer can include an at least partially transparent window. The window can be at least partially coated on both sides with contact adhesive, and the layers can be pressed together to form a stacked pad assembly. The adhesive can then be physically stripped from the top and bottom surface of the window area of the second layer using a material to which the adhesive substantially adheres. A non-limiting example of a material to which the adhesive substantially adheres is Teslin^® SP-100 0, a synthetic sheet material which is commercially available from PPG Industries, Inc, Pittsburgh, PA. The polishing pads of the present invention can be used in combination with polishing slurrys, such as polishing slurries, which are known in the art. Non-limiting examples of suitable slurries for use with the pad of the present invention, include but are not limited to the slurries disclosed in United States Patent Application having Serial Numbers 09/882,548 and 09/882, 549, which were both filed on June 14, 2001 and are pending. In a non-limiting embodiment, the polishing slurry can be interposed between the first layer of the pad and the substrate to be polished. The polishing or planarizing process can include moving the polishing pad relative to the substrate being polished. A variety of polishing slurrys or slurries are known in the art. Non-limiting examples of suitable slurries for use in the present invention include slurries comprising abrasive particles. Abrasives that can be used in the slurries include particulate cerium oxide, paniculate alumina, particulate silica and the like. Examples of commercial slurries for use in the polishing of semiconductor substrates include but are not limited to ILD1200 and ILD1300 available from Rodel, Inc. Newark DE and Semi-Sperse AM100 and Semi-Sperse 12 available from Cabot Microelectronics Materials Division, Aurora, IL. In a non-limiting embodiment, the polishing pad of the present invention can be utilized with an apparatus for planarizing an article having a non-planar surface. The planarizing apparatus can include a retaining means for holding the article; and a motive power means for moving the pad and the retaining means with respect to the other such that movement of the pad and the retaining means causes the slurry and the planarizing surface of the pad to contact and planarize the non-planar surface of the article. In a further non-limiting embodiment, the planarizing apparatus can include a means of renewing the polishing or planarizing surface of the pad. A non-limiting example of a suitable renewing means includes a mechanical arm equipped with an abrasive disk which abrades the work surface of the pad.

In an alternate non-limiting embodiment, the planarizing apparatus can include an apparatus for conducting in-situ metrology of the article being polished or planarized. Commercial polishing or planarizing apparatuses are available from equipment manufacturers such as Applied Materials, LAM Research, SpeedFam-IPEC, and Ebara Corp. In a non-limiting embodiment, the pad of the present invention can be placed on a cylindrical metal base; and can be connected to at least a portion of the base with a layer of adhesive. Suitable adhesives can include a wide variety of known adhesives. In a further non-limiting example, the pad can be placed on the cylindrical metal base or platen of a polishing or planarizing apparatus that includes a means of conducting in-situ metrology of the article being polished. The pad can be placed such that its window can be aligned with the metrology window of the platen.

EXAMPLE 1 A polishing pad with a window was prepared as follows:

1. A first layer was prepared according to Recipe A described below.

2. A 2" 2" rectangular hole was cut into the first layer using a straight-edge and a scalpel-style utility knife.

3. A second layer was formed using High Performance Double Coated Tape 9500PC, commercially obtained from 3M Industrial Tape and Specialties

Division. The adhesive layer of the tape was adhered to the bottom side of the first layer such that the rectangular opening in the first layer was substantially spanned by the tape.

4. A third layer was formed using a 0.060" thick sheet of polyurethane foam, having trade name PORON 4701-50, commercially obtained from Rogers

Corporation. A Vτ T rectangular hole was cut into the third layer using a straight-edge and a scalpel-style utility knife.

5. The third layer was adhered to the second layer by removing the release paper from the second layer and applying the third layer to the adhesive film thus exposed. The third layer was positioned such that the rectangular openings in the first layer and the third layer were substantially aligned.

6. The three-layer stack assembly was then pressed together and passed through a calendar roll set.

7. A window was formed by removing a portion of the adhesive on the upper and lower sides of the second layer. The adhesive was removed by contacting it with a V₂"x2" rectangular piece of Teslin SP-1000, commercially available from PPG Industries, Incorporated, pressing the piece by hand to ensure good contact between the adhesive and the Teslin SP-1000, then peeling away the Teslin SP-1000. The adhesive selectively adhered to the Teslin SP-1000, leaving the substantially clear film of the window free of adhesive.

The resulting pad stack had a rectangular window having a size of ¹/2"x2" .

Recipe A for Pad First Layer:

Step 1 Particulate crosslinked polyurethane was prepared using the ingredients listed in

Table A.

Table A

Ingredients Weight (grams)

Charge 1 diamine curative (a) 810 surfactant (b) 30.6 methyl isobutyl ketone solvent 822

Charge 2 isocyanate functional pre olymer (c :) 2112

(a) LONZACURE MCDEA diamine curative obtained from Air Products and Chemicals, Inc, which describes it as methylene bis(chlorodiethylanaline).

(b) PLURONIC F108 surfactant, obtained from BASF Corporation.

(c) ARITHANE PHP-75D prepolymer, obtained from Air Products and Chemicals, Inc, which describes it as the isocyanate functional reaction product of toluene diisocyanate and poly(tetramethylene glycol).

Charge 1 was added to an open container and warmed with stirring on a hot plate until the contents of the container reached a temperature of 35°C. Stirring was continued at this temperature until the ingredients formed a substantially homogeneous solution. The container was then removed from the hot plate. Charge 2 was warmed to a temperature of 55°C using a water bath then added to Charge 1. The contents were mixed for a period of three (3) minutes with a motor driven impeller until uniform. The contents of the container were then quickly poured into 10 kilograms of deionized water at a temperature of 40°C, with concurrent vigorous stirring of the deionized water. Upon completion of the addition of the contents of the container, vigorous mixing of the deionized water was continued for an additional 60 minutes. The wet particulate crosslinked polyurethane was classified using a stack of two sieves. The top sieve had a mesh size of 50 mesh (300 micron sieve openings), and the bottom sieve had a mesh size of 140 mesh (105 micron sieve openings). The isolated particulate crosslinked polyurethane from the 140 mesh was dried overnight in an oven at a temperature of 80°C.

Step 2:

A polishing pad comprising particulate crosslinked polyurethane and crosslinked polyurethane binder was prepared using the ingredients summarized in the following Table B.

Table B

Ingredients Weight (grams)

Charge 1 particulate crosslinked polyurethane of Step 1 918

Charge 2 isocyanate functional prepolymer (c) 265 aliphatic polyisocyanate (d) 8.5 additive (e) 8.5 acetone solvent 62

(d) DESMODUR N 3300 aliphatic polyisocyanate, obtained from Bayer Corporation, Coatings and Colorants Division, which describes it as a polyfunctional aliphatic isocyanate resin based on hexamethylene diisocyanate. (e) Lanco PP1362D micronized modified polypropylene wax, obtained from The Lubrizol Corporation. Charge 2 was mixed until substantially homogeneous, using a motor driven stainless steel impeller. The substantially homogenous mixture of Charge 2 was then combined with Charge 1 in a suitable container and mixed together by means of a motor driven mixer. A 1040 gram portion of the combination of Charges 1 and 2 was then introduced onto a 26" x 26" flat mold. The mold was fed through a pair of rollers at ambient temperature to form a sheet that was 0.100" thick. The sheet was cured at a temperature of 25° C and 80% relative humidity for 18 hours, followed by a temperature of 130°C for 1 hour. Circular pads with a 22.5" diameter were cut from the sheet then the upper and lower surfaces of the pad were made parallel using a milling machine.

The resulting pad was used as the first layer in Example 1.

Claims

In the claims:

1. A polishing pad comprising: a. a first layer having an opening; and b. a second layer wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein said first layer is at least partially connected to said second layer, and wherein said first layer absorbs at least two percent by weight of polishing slurry based on total weight of said first layer.

2. The polishing pad of claim 1 wherein said first layer absorbs 50% or less by weight of polishing slurry based on total weight of said first layer.

3. The polishing pad of claim 1 wherein said first layer is selected from particulate polymer and crosslinked polymer binder; particulate polymer and an organic polymer binder; sintered particles of thermoplastic resin; pressure sintered powder compacts of thermoplastic polymer; polymeric matrices impregnated with a plurality of polymeric microelements wherein each polymeric microelement can have a void space therein, or combinations thereof.

4. The polishing pad of claim 1 wherein said first layer has a thickness of at least 0.020 inches.

5. The polishing pad of claim 4 wherein said first layer has a thickness of 0.150 inches or less.

6. The polishing pad of claim 1 wherein said second layer is selected from substantially non- volume compressible polymers and metallic films and foils.

7. The polishing pad of claim 1 wherein said second layer is selected from polyolefins; cellulose-based polymers; acrylics; polyesters and co-polyesters; polycarbonate; polyamides; high performance plastics; or mixtures thereof.

8. The polishing pad of claim 1 wherein said second layer is selected from low density polyethylene, high density polyethylene ultra-high molecular weight polyethylene or polypropylene; cellulose acetate or cellulose butyrate; PET or PETG; nylon 6/6 or nylon 6/12; polyetheretherketone, polyphenylene oxide, polysulfone, polyimide, or polyetherimide; or mixtures thereof.

9. The polishing pad of claim 1 wherein said second layer has a thickness of at least 0.0005 inches.

10. The polishing pad of claim 9 wherein said second layer has a thickness of

0.0650 inches or less.

11. The polishing pad of claim 1 wherein said first and second layers are at least partially connected by an adhesive material.

12. The polishing pad of claim 11 wherein said adhesive material is selected from contact adhesives, pressure sensitive adhesives, structural adhesives, hot melt adhesives, thermoplastic adhesives, and curable adhesives, thermosetting adhesives; and combinations thereof.

13. The polishing pad of claim 1 wherein said opening in said first layer is at least partially aligned with said window in said second layer.

14. The polishing pad of claim 1 further comprising a third layer at least partially connected to said second layer, said third layer having an opening.

15. The polishing pad of claim 14 wherein said third layer is selected from natural rubber, synthetic rubbers, thermoplastic elastomer, foam sheet and combinations thereof.

16. The polishing pad of claim 14 wherein said third layer has a thickness of at least 0.04 inches.

17. The polishing pad of claim 16 wherein said third layer has a thickness of 0.100 inches or less.

18. The polishing pad of claim 14 wherein said first, second and third layers are at least partially connected by an adhesive material.

19. The polishing pad of claim 14 wherein said opening in said first layer, said window in said second layer and said opening in said third layer are at least partially aligned.

20. A polishing pad comprising: a. a first layer having an opening; and b. a second layer wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein said first layer is at least partially connected to said second layer, and wherein said first layer has a porosity of at least two percent by volume based on total volume of said first layer.

21. The polishing pad of claim 20 wherein said first layer has a porosity of 50% or less by volume based on total volume of said first layer.

22. The polishing pad of claim 20 wherein said first layer is selected from particulate polymer and crosslinked polymer binder; particulate polymer and an organic polymer binder; sintered particles of thermoplastic resin; pressure sintered powder compacts of thermoplastic polymer; polymeric matrices impregnated with a plurality of polymeric microelements wherein each polymeric microelement can have a void space therein, or combinations thereof.

23. The polishing pad of claim 20 wherein said first layer has a thickness of at least 0.020 inches.

24. The polishing pad of claim 23 wherein said first layer has a thickness of 0.150 inches or less.

25. The polishing pad of claim 20 wherein said second layer is selected from substantially non- volume compressible polymers and metallic films and foils.

26. The polishing pad of claim 20 wherein said second layer is selected from polyolefins; cellulose-based polymers; acrylics; polyesters and co-polyesters; polycarbonate; polyamides; high performance plastics; or mixtures thereof.

27. The polishing pad of claim 1 wherein said second layer is selected from low density polyethylene, high density polyethylene ultra-high molecular weight polyethylene or polypropylene; cellulose acetate or cellulose butyrate; PET or PETG; nylon 6/6 or nylon 6/12; polyetheretherketone, polyphenylene oxide, polysulfone, polyimide, or polyetherimide; or mixtures thereof.

28. The polishing pad of claim 20 wherein said second layer has a thickness of at least 0.0005 inches.

29. The polishing pad of claim 28 wherein said second layer has a thickness of 0.0650 inches or less.

30. The polishing pad of claim 20 wherein said first and second layers are at least partially connected by an adhesive material.

31. The polishing pad of claim 30 wherein said adhesive material is selected from contact adhesives, pressure sensitive adhesives, structural adhesives, hot melt adhesives, thermoplastic adhesives, curable adhesives, thermosetting adhesives and combinations thereof.

32. The polishing pad of claim 20 wherein said opening in said first layer is at least partially aligned with said window in said second layer.

33. The polishing pad of claim 20 further comprising a third layer at least partially connected to said second layer, said third layer having an opening.

34. The polishing pad of claim 33 wherein said third layer is selected from natural rubber, synthetic rubbers, thermoplastic elastomer, foam sheet and combinations thereof.

35. The polishing pad of claim 33 wherein said third layer has a thickness of at least 0.04 inches.

36. The polishing pad of claim 35 wherein said third layer has a thickness of 0.100 inches or less.

37. The polishing pad of claim 30 wherein said first, second and third layers are at least partially connected by an adhesive material.

38. The polishing pad of claim 30 wherein said opening in said first layer, said window in said second layer and said opening in said third layer are at least partially aligned.

39. A polishing pad comprising: a. a first layer having an opening; and b. a second layer wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein said first layer is at least partially connected to said second layer, and wherein said first layer has a percent volume compressibility greater than said second layer.

40. The polishing pad of claim 39 wherein said first layer has a percent volume compressibility of at least 0.3% when a load of 20 psi is applied.

41. The polishing pad of claim 40 wherein said first layer has a percent volume compressibility of 3% or less when a load of 20 psi is applied.

42. The polishing pad of claim 39 wherein said second layer is substantially non- volume compressible.

43. The polishing pad of claim 39 wherein said first layer is selected from particulate polymer and crosslinked polymer binder; particulate polymer and an organic polymer binder; sintered particles of thermoplastic resin; pressure sintered powder compacts of thermoplastic polymer; polymeric matrices impregnated with a plurality of polymeric microelements wherein each polymeric microelement can have a void space therein, or combinations thereof.

44. The polishing pad of claim 39 wherein said second layer is selected from substantially non- volume compressible polymers and metallic films and foils.

45. The polishing pad of claim 39 wherein said second layer is selected from polyolefins; cellulose-based polymers; acrylics; polyesters and co-polyesters; polycarbonate; polyamides; high performance plastics; or mixtures thereof.

46. The polishing pad of claim 39 wherein said second layer is selected from low density polyethylene, high density polyethylene ultra-high molecular weight polyethylene or polypropylene; cellulose acetate or cellulose butyrate; PET or PETG; nylon 6/6 or nylon 6/12; polyetheretherketone, polyphenylene oxide, polysulfone, polyimide, or polyetherimide; or mixtures thereof.

47. The polishing pad of claim 39 wherein said first and second layers are at least partially connected by an adhesive material.

48. The polishing pad of claim 47 wherein said adhesive material is selected from contact adhesives, pressure sensitive adhesives, structural adhesives, hot melt adhesives, thermoplastic adhesives, curable adhesives, thermosetting adhesives and combinations thereof.

49. The polishing pad of claim 39 wherein said opening in said first layer is at least partially aligned with said window in said second layer.

50. The polishing pad of claim 39 further comprising a third layer at least partially connected to said second layer, said third layer having an opening.

51. The polishing pad of claim 50 wherein said third layer is selected from natural rubber, synthetic rubbers, thermoplastic elastomer, foam sheet and combinations thereof.

52. The polishing pad of claim 50 wherein said first, second and third layers are at least partially connected by an adhesive material.

53. The polishing pad of claim 50 wherein said opening in said first layer, said window in said second layer and said opening in said third layer are at least partially aligned.

54. The polishing pad of claim 39 wherein at least a portion of said window comprises a coating.

55. The polishing pad of claim 54 wherein said coating comprises a resin coating.

56. The polishing pad of claim 55 wherein said resin coating is selected from thermoplastic acrylic resins, thermoset acrylic resins, urethane systems, epoxy resins, polyester resins, or mixtures thereof.

57. The polishing pad of claim 39 wherein said first layer comprises grooves on a polishing surface.

58. The polishing pad of claim 39 wherein said first layer comprises a pattern on a polishing surface.

59. A method of preparing a polishing pad comprising at least partially connecting a first layer having an opening to a second layer, wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein said first layer absorbs at least two percent by weight of polishing slurry based on total weight of said first layer.

60. The method of claim 59 further comprising at least partially connecting a third layer having an opening to said second layer.

61. The method of claim 59 wherein said first and second layers are at least partially connected by an adhesive material.

62. A method of preparing a polishing pad comprising at least partially connecting a first layer having an opening to a second layer, wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein said first layer has a porosity of at least two percent by volume based on total volume of said first layer.

63. The method of claim 62 further comprising at least partially connecting a third layer having an opening to said second layer.

64. The method of claim 62 wherein said first and second layers are at least partially connected by an adhesive material.

65. A method of preparing a polishing pad comprising at least partially connecting a first layer having an opening to a second layer, wherein at least a portion of said second layer comprises an at least partially transparent window, and wherein first layer has a percent volume compressibility greater than said second layer.

66. The method of claim 65 further comprising at least partially connecting a third layer having an opening to said second layer.

67. The method of claim 65 wherein said first and second layers are at least partially connected by an adhesive material.

68. A polishing pad comprising: a. a first layer having an opening; b. a second layer wherein at least a portion of said second layer comprises an at least partially transparent window; and c. a third layer having an opening, wherein said first layer is at least partially connected to said second layer and said second layer is at least partially connected to said third layer, and wherein said third layer is softer than said first layer.