Abstract:
A polishing pad include a polishing layer having a polishing surface and a backing layer on a side of the polishing layer opposite the polishing surface. An outer edge of the polishing layer overhangs an outer edge of the backing layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/043,361, filed on Jan. 26, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 10/956,617, filed on Oct. 1, 2004, which claims priority to U.S. Provisional Application Ser. No. 60/508,321, filed on Oct. 3, 2003. The entire disclosure of each of the above-referenced applications is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    This present invention relates to polishing pads used in during chemical mechanical polishing. 
         [0003]    An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization is needed to planarize the substrate surface for photolithography. 
         [0004]    Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against the polishing surface of a polishing pad, such as a rotating polishing disk or linearly advancing belt. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, which can include abrasive particles, is supplied to the surface of the polishing pad, and the relative motion between the substrate and polishing pad results in planarization and polishing. 
         [0005]    Conventional polishing pads include “standard” pads and fixed-abrasive pads. A typical standard pad has a polyurethane polishing layer with a durable roughened surface, and can also include a compressible backing layer. In contrast, a fixed-abrasive pad has abrasive particles held in a containment media, and can be supported on a generally incompressible backing layer. 
         [0006]    One objective of a chemical mechanical polishing process is to achieve topology uniformity across the substrate. Another object is to achieve polishing uniformity. If different areas on the substrate are polished at different rates, then it is possible for some areas of the substrate to have too much material removed (“overpolishing”) or too little material removed (“underpolishing”), which can result in non-uniform topography across the substrate. 
       SUMMARY 
       [0007]    In one aspect, the invention is directed to a polishing pad with a polishing layer and a backing layer secured to the polishing layer. The polishing layer has a polishing surface, a first thickness, a first compressibility, a hardness between about 40 to 80 Shore D, and a thickness non-uniformity. The backing layer has a second thickness and a second compressibility greater than the first compressibility. The first thickness, first compressibility, second thickness and second compressibility are such that the polishing surface deflects, under an applied pressure of 1.5 psi or less, more than the thickness non-uniformity of the polishing layer. 
         [0008]    Implementations of the invention can include one or more of the following features. The second thickness may be greater than the first thickness, or about the same as the first thickness. The backing layer may have a hardness between about 1 and 10 Shore A. The backing layer may have second thickness between about 30 and 200 mils, e.g., between about 30 and 90 mils. A plurality of grooves may be formed in the polishing surface. A recess may be formed in a bottom surface of the polishing layer, and an aperture may be formed in the backing layer aligned with the recess. A conductive sheet may be secured to the backing layer on a side opposite the polishing layer. A plurality of holes may be formed through the polishing layer and the backing layer to expose the conductive sheet. A solid light-transmissive portion may be positioned in the polishing layer. An aperture may be formed in the backing layer aligned with the light-transmissive portion. A light-transmissive adhesion layer may be on the side of the backing layer opposite the polishing layer, and the adhesion layer may span the aperture on the backing layer. A fluid-impermeable transparent sheet may be between the backing layer and the polishing layer. An outer edge of the polishing layer may overhang an outer edge of the backing layer. The backing layer may have a product of the second thickness and second compressibility of 2 mils or more, at an applied pressure of 1.5 psi or less. The backing layer may include a polyurethane, polyether or polysilicone foam. 
         [0009]    In another aspect, the invention is directed to a polishing pad with a polishing pad having a polishing surface, a solid light-transmissive portion positioned in the polishing layer, a backing layer on a side of the polishing layer opposite the polishing surface, and a light-transmissive adhesion layer on the side of the backing layer opposite the polishing layer. The backing layer has an aperture aligned with the light-transmissive portion, and the light-transmissive adhesion layer spans the aperture on the backing layer. 
         [0010]    Implementations of the invention can include one or more of the following features. The adhesion layer may abut the backing layer. The backing layer may be connected directly to the polishing layer by an adhesive. A conductive layer may be on a side of the adhesion layer opposite the backing layer, e.g., the conductive layer may abut the adhesion layer. The backing layer may be more compressible than the polishing layer. The adhesion layer may include a double-sided adhesive tape. The adhesion layer may include a polyethylene terephthalate film. The window may be integrally molded in the polishing layer, or may be secured in an aperture in the polishing layer by an adhesive. A fluid-impermeable transparent sheet may be between the backing layer and the polishing layer. 
         [0011]    In another aspect, the invention is directed to a polishing pad with a polishing layer having a polishing surface and a backing layer on a side of the polishing layer opposite the polishing surface. An outer edge of the polishing layer overhangs an outer edge of the backing layer. 
         [0012]    Implementations of the invention can include one or more of the following features. The polishing layer and backing layer may be substantially circular, and a diameter of the backing layer may be less than a diameter of the polishing layer. The backing layer may be more compressible than the polishing layer. The outer edge of the polishing layer may overhang the outer edge of the backing layer by about one-quarter inch. The polishing layer and backing layer may be secured by an adhesive. 
         [0013]    In another aspect, the invention is directed to a polishing pad that has a polishing layer having a polishing surface, a solid light-transmissive portion positioned in the polishing layer, a backing layer on a side of the polishing layer opposite the polishing surface, a fluid-impermeable transparent sheet between the backing layer and the polishing layer, a light-transmissive adhesion layer on the side of the backing layer opposite the polishing layer, and a conductive layer on a side of the adhesion layer opposite the backing layer. The backing layer has an aperture aligned with the light-transmissive portion, and the transparent sheet spans the solid light-transmissive portion. 
         [0014]    In another aspect, the invention is directed to a substrate processing apparatus. The apparatus can include a pad support, a polishing pad according to one of the aspects discussed above, a carrier head to hold a substrate in contact with the polishing pad, a supply of processing fluid, and a motor connected to at least one of the pad support and the carrier head to cause relative motion between the processing pad and the substrate. 
         [0015]    Implementations of the invention can include one or more of the following features. The apparatus may include an electrode positioned to contact the substrate, a cathode contacting the processing fluid, and a power supply coupled between the electrode and the cathode to create a bias. 
         [0016]    In another aspect, the invention is directed to a method of chemical mechanical processing. The method includes bringing a substrate into contact with a polishing surface of a polishing layer of a polishing pad according to one of the aspects discussed above, supplying a polishing liquid to the polishing surface, creating relative motion between the substrate and the polishing surface, and applying a pressure to the substrate to press the substrate against the polishing pad. 
         [0017]    Implementations of the invention can include one or more of the following features. The applied pressure may be 1.5 psi or less, and the polishing surface may deflects more than a thickness non-uniformity of the polishing layer under the applied pressure. Supplying the polishing liquid may include supplying an electrolyte, and the method may further include applying a bias between a cathode exposed to the electrolyte and the substrate. 
         [0018]    Any of the various implementations discussed above may also be applicable to any of the various aspects of the invention. 
         [0019]    Potential advantages of the invention may include one or more of the following. Polishing uniformity across the substrate may be improved, particularly at low pressures, e.g. below 1.5 or 1.0 psi or 0.8 psi, or even below 0.5 psi or 0.3 psi. Consequently, materials, such as low-k dielectric materials, that require low-pressure polishing to avoid irreversible damage, such as delamination, can be polished with an acceptable degree of uniformity. In addition, the polishing pad can provide good mechanical contact to the substrate surface when the substrate is polished at low down force and/or the substrate is not flat due to internal stress induced by multiple levels of conductive and dielectric layers. The likelihood of premature pad failure, such as premature detachment of the pad from the platen in the region around the window, can be reduced, thereby increasing polishing pad lifetime. The likelihood of polishing liquid seeping into the backing layer can be reduced. 
         [0020]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1A  is a schematic cross-sectional side view illustrating a conventional polishing pad. 
           [0022]      FIG. 1B  is a schematic cross-sectional side view illustrating a substrate in contact with the polishing pad of  FIG. 1A . 
           [0023]      FIG. 2  is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station. 
           [0024]      FIG. 3A  is a schematic cross-sectional side view illustrating the polishing pad of  FIG. 2 . 
           [0025]      FIG. 3B  is a schematic cross-sectional side view illustrating a substrate in contact with the polishing pad of  FIG. 3A . 
           [0026]      FIG. 3C  is a schematic cross-sectional side view illustrating another implementation of the polishing pad in which the covering layer and backing layer have about the same thickness. 
           [0027]      FIG. 3D  is a schematic cross-sectional side view illustrating another implementation of the polishing pad in which an adhesive layer and a liner are attached to the backing layer. 
           [0028]      FIG. 3E  is a schematic cross-sectional side view illustrating another implementation of the polishing pad in which the covering layer overhangs the backing layer. 
           [0029]      FIG. 4  is a schematic cross-sectional side view illustrating another implementation of the polishing pad in which a recess is formed in the bottom surface of the covering layer. 
           [0030]      FIG. 5  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a transparent sheet. 
           [0031]      FIG. 6A  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a window and an adhesive layer that spans the window. 
           [0032]      FIG. 6B  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a window, an adhesive layer that spans the window, and a transparent sheet. 
           [0033]      FIG. 7  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a conductive layer. 
           [0034]      FIG. 8  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a window and a conductive layer. 
           [0035]      FIG. 9  is a schematic cross-sectional side view illustrating another implementation of the polishing pad including a window, a transparent sheet, and a conductive layer. 
       
    
    
       [0036]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0037]    As mentioned above, and referring to  FIG. 1A , a conventional polishing pad  60  can have a polyurethane covering layer  64  with a durable rough polishing surface  66  and a compressible backing layer  62  with about the same thickness as the covering layer. In addition, there may be small variations in the thickness of the covering layer  64 , e.g., on the order of a few mils, e.g., about 1-2 mil, across the polishing pad (for clarity, the variations are significantly exaggerated in  FIG. 1A ). 
         [0038]    For example, one polishing pad commercially available from Rodel, Inc., has a covering layer formed of polyurethane with embedded hollow microspheres (IC1000) and a backing layer formed of polyurethane impregnated polyester felt (Suba IV). The covering layer has a thickness of 50 or 80 mils and a hardness of 52-62 on the Shore D scale, whereas the backing layer has a thickness of 50 mils and a hardness of about 61 on the Shore A scale. 
         [0039]    Unfortunately, the conventional polishing pad can result in unacceptable polishing uniformity at low pressures, e.g., below 1.5 psi or below 1.0 psi, and particularly at very low pressures, e.g., below 0.5 psi. Without being limited to any particular theory, it may be that the dimensions and physical properties of the standard polishing pad are such that, at low polishing pressures, the backing layer remains rigid enough that the downward pressure of the substrate  14  is not sufficient to completely “flatten out” the covering layer. Consequently, as shown in  FIG. 1B , any thickness variation in the covering layer  64  results in pressure being transmitted to the substrate in only the thick portions  66  of the covering layer  64 , thus causing the non-uniformity in the polishing rate. In addition, a typical substrate is not perfectly flat due to internal stresses, and the applied load may not be great enough to conform the substrate against the polishing pad so as to create uniform contact between the substrate and the polishing surface. 
         [0040]    In contrast to these conventional polishing pads, one implementation of the polishing pad of the present invention has a thinner covering layer and a thicker and more compressible backing layer. Again without being limited to any particular theory, the reduced thickness of the covering layer may make it easier to deflect. In addition, the increased thickness and compressibility of the backing layer may make the covering layer easier to deflect. As a result, even at very low polishing pressures, the covering layer can be conformed to the substrate (for example, if the substrate is flat as shown then the covering layer will be flattened out, whereas if the substrate is warped then the covering layer will conform to have the same shape) so that thickness variations in the covering layer need not adversely impact the polishing uniformity, and so that good mechanical contact is created between the substrate and the polishing surface so as to provide a high polishing rate and a shorter polishing time. 
         [0041]    Turning now to  FIG. 2 , one or more substrates  14  can be polished at a polishing station  10  of a CMP apparatus. A description of a suitable polishing apparatus can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
         [0042]    The polishing station  10  includes a rotatable platen  16  on which is placed a polishing pad  18 . As described below, the polishing pad  18  is a two-layer polishing pad with a soft backing layer  20  and a hard durable outer layer  22  with a substantially uniform composition. The durable outer covering layer  22  provides a polishing surface  24 . The polishing station can also include a pad conditioner apparatus to maintain the condition of the surface of the polishing pad so that it will effectively polish substrates. 
         [0043]    During a polishing step, a polishing fluid  30 , e.g., a slurry, can be supplied to the surface of polishing pad  18  by a slurry supply port or combined slurry/rinse arm  32 . Slurry  30  can contain abrasive particles, a pH adjuster, or chemically active components. 
         [0044]    The substrate  14  is held against the polishing pad  18  by a carrier head  34 . The carrier head  34  is suspended from a support structure, such as a carousel, and is connected by a carrier drive shaft  36  to a carrier head rotation motor so that the carrier head can rotate about an axis  38 . 
         [0045]    Referring to  FIG. 3A , the covering layer  22  of the polishing pad  18  is a relative durable and hard polishing material that is inert in the polishing process, e.g., a cast polyurethane. For example, the covering layer  22  can have a hardness of about 30 to 80, e.g., 40 to 80, e.g., 50 to 65, on the Shore D scale. The polishing surface  24  of the covering layer  22  can have rough surface texture, e.g., hollow microspheres can be embedded in the polyurethane so that when the covering layer is skived from a cast polyurethane block, the microspheres at the exposed surface are ruptured to provide a pitted and rough surface texture. 
         [0046]    The covering layer  22  is thin, e.g., less than 50 mils, such as 40 mils or less, or 25 mils or less, or 20 mils or less, or 15 mils or less. In general, the covering layer  22  is as thin as possible, subject to manufacturability. However, the conditioning process tends to wear away the cover layer. Therefore, a thickness of the covering layer can be selected to provide the polishing pad with a useful lifetime, e.g., 3000 polishing and conditioning cycles. For example, the covering layer can have a thickness of 5 to 10 mils. A thickness between about 5 and 20 mils should be appropriate. There may be thickness non-uniformities across the pad of about 1-3 mils, although larger non-uniformities are possible (these non-uniformities refer to the global variations in thickness across the polishing pad caused by the pad fabrication process, rather than discrete thickness variations at a small scale (e.g., less than 100 mils), such as grooves, perforations, or surface roughness). 
         [0047]    Optionally, at least a portion of the polishing surface  24  can include a plurality of grooves  26  formed therein for carrying slurry. The grooves may be of nearly any pattern, such as concentric circles, straight lines, a cross-hatched, spirals, and the like. The grooves  26  can extend through about 20-80%, e.g., 25%, of the thickness of the covering layer  22 . For example, in a polishing pad having an covering layer  22  that is 20 mils thick, the grooves  26  can have a depth D 1  of about 5 mils. 
         [0048]    The backing layer  20  is a compressible material that is softer and more compressible than the covering layer  22 . For example, the backing layer can be an open-cell or a closed-cell foam, such as polyurethane, polyether or polysilicone with voids, so that under pressure the cells collapse and the backing layer compresses. It is permissible for the material of the backing layer  20  to be laterally displaced under pressure from the substrate. The backing layer  20  can have a hardness of 20 or less on the Shore A scale, e.g., 12 or less, e.g., between 1 and 10 Shore A, e.g., 5 or less. 
         [0049]    As mentioned above, the backing layer  20  should be more compressible than the covering layer  22 . Compressibility may be measured as a percentage thickness change at a given pressure. For example, under a pressure of about 0.5 psi, the backing layer  20  can undergo about 3% compression. A suitable material for the backing layer is PORON 4701-30 from Rogers Corporation, in Rogers, Conn. (PORON is a trademark of Rogers Corporation). 
         [0050]    In addition, the backing layer  20  is thick, e.g., 90 mils or more. For example, the backing layer may be about 95 to 500 mils thick, such as 95 to 200 mils, or 95 to 150 mils, or 95 to 125 mils. In particular, the backing layer  20  may be about 2 to 15 times as thick as the covering layer  22 , e.g., 4.5 to 8 times as thick (particularly for a 20 mil thick covering layer). 
         [0051]    In general, the thickness of the backing layer  20  is selected to ensure that, given the compressibility of the backing layer  20  and the rigidity of the covering layer  22 , the covering layer will deflect at very low pressures, e.g., pressures of 0.5 psi or less, an amount at least equal to any non-uniformity in the thickness of the covering layer, e.g., a few mils, e.g., about 2 mil (the non-uniformities are not shown in  FIG. 3A ). For example, a 100 mil thick backing layer should have a compression of at least 2% at 0.5 psi, whereas a 200 mil thick backing layer should have a compression of at least 1% at 0.5 psi. 
         [0052]    Moreover, the backing layer should be sufficiently compressible that at the operating pressures of interest, e.g., at 1.5 psi to 0.1 psi, the polishing pad is below the maximum compressibility of the polishing pad. The backing layer can have a maximum compressibility greater than 10%, or greater than 20%. In one implementation, the backing layer can have a compressibility of 25% at pressures of 3 to 8 psi, with a maximum compressibility that is even higher. 
         [0053]    The backing layer can have a compression force deflection range of 1 to 10 psi (0.2 inches strain rate force at 25% deflection). 
         [0054]    In brief, at pressures of 1.5 psi or below (and possibly at 1.0 psi or below, 0.8 psi or below, or 0.5 psi or below, or 0.3 psi or below), the backing layer can have a product of the compressibility and thickness (C·D) that is greater than the non-uniformities in thickness of the cover layer. For example, at pressures of 0.8 psi or below (and possibly at 0.5 psi or below), the backing layer can have a product of the compressibility and thickness (C·D) of a few mils, e.g., 2 mils, or more (and possibly 3 mils or more). 
         [0055]    Hydrostatic modulus K may be measured as applied pressure (P) divided volumetric strain (ΔV/V), i.e., K=PV/ΔV. Assuming that the backing layer undergoes pure compression (i.e., material is not displaced laterally under the applied pressure), then the hydrostatic modulus K equals the applied pressure divided by the compression (ΔD/D). Thus, assuming that the backing layer undergoes at least 2% pure compression at 0.5 psi, the backing layer would have a compressibility modulus K of 25 or less. On the other hand, if even lower pressures are to be use, e.g., pressures of 0.1 psi, then the backing layer  20  should have a compressibility modulus of 5 or less. The backing layer may have a compressibility modulus K of 50 psi or less per psi of applied pressure in the range of 0.1 to 1.0 psi. Of course, if the material of the backing layer does undergo lateral displacement under compression, then the volumetric strain will be somewhat less than the compression, so the hydrostatic modulus may be somewhat higher. 
         [0056]    Referring to  FIG. 3B , and without being limited to any particular theory, this configuration permits the downward force from the substrate to “flatten out” the covering layer at low pressures, even at pressures of 0.5 psi or less, such as 0.3 psi or less, such as 0.1 psi, and thus substantially compensate for the thickness non-uniformity of the polishing layer and for warp in the substrate. For example, as illustrated, the variations in thickness of the covering layer  22  are absorbed by the compression of the backing layer  20  (for clarity, the variations are significantly exaggerated in  FIG. 3A ), so that the polishing surface remains in substantially uniform contact with the substantially planar substrate across the substrate surface. As a result, a uniform pressure can be applied to the substrate, thereby improving polishing uniformity during low pressure polishing. Consequently, materials, such as low-k dielectric materials, that require low-pressure polishing to avoid damage, such as delamination, can be polished with an acceptable degree of uniformity. 
         [0057]    In one implementation, the covering layer  22  can be manufactured, e.g., by a molding process, with grooves preformed in the upper surface of the covering layer. In a molding process, e.g., injection molding or compression molding, the pad material cures or sets in a mold that has indentations that form the grooves recess. Alternatively, the covering layer  22  can be manufactured by a more conventional technique, e.g., by skiving a thin sheet of pad material from a cast block. The grooves can then be formed by machining or milling the top surface of the covering layer, respectively. 
         [0058]    Once the backing layer  20  and covering layer  22  have been manufactured, they can be secured, e.g., with a thin adhesive layer  28 , such as a pressure-sensitive adhesive. 
         [0059]    Referring to  FIG. 3C , in another implementation, the backing layer is the same thickness or is thinner than the covering layer, but is softer and more compressible than the covering layer. In particular, the backing layer can be sufficiently compressible to provide the same functionality as the polishing pad discussed with reference to  FIG. 3A . For example, the covering layer will deflect at very low pressures an amount at least equal to any non-uniformity in the thickness of the covering layer (the non-uniformities are not shown in  FIG. 3C ). In brief, at pressures of 1.5 psi or below (and possibly at 1.0 psi or below, or 0.8 psi or below, or 0.5 psi or below, or 0.3 psi or below), the backing layer can have a product of the compressibility and thickness (C·D) that is greater than the non-uniformities in thickness of the cover layer, for example a few mils, e.g., about 2 mil. For example, under a pressure of about 0.5 psi, the backing layer  20  can undergo about 1% to 30% compression, e.g., 3% compression. 
         [0060]    For example, the covering layer  22  can have a hardness of about 30 to 80, e.g., 50 to 65, on the Shore D scale, and can have a thickness between about 30 and 90 mils, e.g., 50 or 80 mils. The backing layer can be an open-cell foam or a closed-cell foam, such as polyurethane, polyether or polysilicone with voids. The backing layer  20  can have a hardness of 20 or less on the Shore A scale, e.g., 12 or less, e.g., between 1 and 10 Shore A, e.g., 5 or less, and can have a thickness that is about the same or less than that of the cover layer, e.g., 30 to 90 mils, e.g., 50 mils. 
         [0061]    In use, the polishing pad  18  can be secured to the platen with an adhesive layer. Referring to  FIG. 3D , a polishing pad otherwise constructed as described with respect to  FIG. 3A  or  3 C can be manufactured with an adhesive layer  50 , e.g., a double-sided adhesive tape, e.g., a Mylar sheet coated on both sides with adhesive, covering the bottom of backing layer  20 . In addition, a non-adhesive liner  52  may be placed under the adhesive layer  50 . The liner  52  is removed prior to attaching the polishing pad  18  to the platen. The adhesive layer  50  can provide additional structural integrity to the polishing pad so that the pad can be removed from the platen as a single unit without tearing the backing layer. 
         [0062]    Referring to  FIG. 3E , in another implementation, which can otherwise be constructed as described with respect to  FIG. 3A ,  3 C or  3 D, the backing layer  20  can have a diameter that is smaller than the diameter of the covering layer  22 . For example, the backing layer  20  can have a diameter of 30.0 inches, and the covering layer  22  can have a diameter of 30.5 inches. The outer edge of the backing layer can be evenly recessed by a distance D 2  of about 0.25 inches from the outer edge of the cover layer. Having the outer edge of the cover layer  22  overhang the outer edge of the backing layer  20  can help prevent polishing fluid, e.g., deionized water, from entering the backing layer  20  due to capillary action or the like, which could change the compressibility of the backing layer  20  and impact the uniformity of the polishing process. 
         [0063]    Referring to  FIG. 4 , in another implementation, which can otherwise be constructed as described with respect to  FIG. 3A ,  3 C,  3 D or  3 E, one or more recesses  70  can be formed in the bottom surface  72  of the covering layer  22  to provide a thin section  74 . These recesses  70  can extend through 20 to 80%, e.g., 50%, of the thickness of the covering layer  22 . For example, in a polishing pad having a covering layer  22  that is 20 mils thick, the recess  52  can have a depth of about 10 mils, leaving the thin section  74  with a thickness of about 10 mils. In addition, one or more apertures  76  can be formed in the backing layer  20  to permit sensor elements to extend through the backing layer  20  and partially into the covering layer  22 . 
         [0064]    In this implementation, the grooves  26  do not extend over the thin section  74  in the covering layer  22 . Thus, the polishing surface  24  of the polishing pad includes portions with and without grooves, and the indentation is located in one of the portions without grooves. The grooves  26  can be sufficiently deep that they extend to or past the plane defined by the inner surface of the recess  70 . 
         [0065]    Referring to  FIG. 5 , in another implementation which can otherwise be constructed as described with respect to  FIG. 3A ,  3 C- 3 E or  4 , a thin sheet  80  of fluid-impermeable, tear-resistant material, e.g., polyethylene terephthalate, for example, Mylar, is positioned between the backing layer  20  and the covering layer  22 . The sheet  80  may be secured to the cover layer  22  by an adhesive layer  28 , or the covering layer  22  can be deposited directly on the sheet  80 . The sheet  80  may be secured to the backing layer  20  by a thin adhesive layer  88 . The sheet  80  can be a transparent material, and aligned portions  82  and  84  of the covering layer  22  and backing layer  20 , respectively, can be removed to provide an optical port through the polishing pad. 
         [0066]    Alternatively, a window could be formed in the polishing pad without use of the transparent sheet. For example, a solid transparent portion can be formed in the covering layer  22 , and an aperture can be formed in the backing layer  20  that is aligned with the solid transparent portion. The transparent portion can be formed by cutting an aperture in the covering layer  22  and securing a transparent plug with an adhesive. Alternatively, the transparent portion can be formed by placing an insert of transparent material in a liquid pad material, curing the liquid pad material so that the insert is integrally molded into the block of solidified pad, and then skiving off the covering layer from the block. 
         [0067]    In both these implementations, the adhesive layer  50  can be removed from the region of the optical port or window. 
         [0068]    In addition, instead of being removed from the region of the optical port or window, the adhesive layer  50  can be substantially transparent and can span the optical port. For example, referring to  FIG. 6A , the polishing pad  18  can include a solid transparent portion  56  that is integrally molded into the covering layer or is held by adhesive in an aperture in the covering layer. An aperture  58  is formed in the backing layer  20  aligned with the solid transparent portion  56 . Assuming that the transparent portion is secured by adhesive, the edges of the transparent portion  56  can project over and rest on a rim of the backing layer  20  around the aperture  58  and be secured to the backing layer  20  by an adhesive  59 , which can be part of the adhesive  28 . On the other hand, if the transparent  56  portion is integrally molded into the covering layer  22 , then adhesive  59  is not needed and aperture  58  can be the same size or a different size than the transparent portion. This implementation can otherwise include the features described with respect to any of  FIGS. 3A ,  3 C- 3 E and  4 . 
         [0069]    The adhesive layer  50  spans the bottom surface of the backing layer  20 , including the aperture  58 . The adhesive layer can be a double-sided adhesive tape, such a thin, e.g., 2 mil thick, polyethylene terephthalate sheet coated on both sides with adhesive. To manufacture the polishing pad shown in  FIG. 6A  the aperture can be formed in the backing layer  20  before the adhesive layer  50  is applied to the bottom surface of the polishing pad. The aperture can be formed in the backing layer  20  before or after the backing layer  20  is secured to the covering layer  22 . 
         [0070]    A potential advantage of having the adhesive layer  50  span the aperture  58  is reduced likelihood of failure of the window and consequently increased lifetime of the polishing pad. Without being limited to any particular theory, if the adhesive layer  50  does not span the aperture  58 , then adhesion of the polishing pad to the platen around the window is reduced, such that cycling of pressure during substrate loading and unloading from the pad can cause failure of the attachment of the polishing pad to the platen around the window, thus causing the portion of the pad around the window to distort and create polishing nonuniformities. In contrast, the adhesive layer  50  spanning the window reinforces adhesion to the platen surface, thereby reducing the likelihood of pad failure. 
         [0071]    Optionally, as shown in  FIG. 6B , a thin sheet  80  of fluid-impermeable, tear-resistant material, such as polyethylene terephthalate, can be positioned between the backing layer  20  and the covering layer  22  with the solid transparent portion  56 . The transparent portion  56  can be integrally molded in the cover layer  22 , or can be a separate transparent element adhesively secured to the fluid-impermeable sheet  80 . The transparent portion  56  can be adhesively secured to the fluid-impermeable sheet  80  by adhesive  59 , which can be the same material as the adhesive layer  28  or a different material. If the transparent portion  56  is integrally molded in the cover layer, the adhesive  59  can optionally be removed. In addition, the portion of the adhesive layer  88  above the aperture  58  can be removed or can remain in place. 
         [0072]    Referring to  FIG. 7 , in another implementation which can otherwise be constructed as described with respect to any of  FIGS. 3A-5 , a conductive layer  90 , e.g., a thin metal layer metal, such as stainless steel, e.g., SST 410, is secured to the bottom surface of the backing layer  22 , e.g., with an adhesive layer  98 . The metal layer  90  may also be magnetic. A plurality of perforations  94  extend through both the cover layer  22  and the backing layer  20  to expose the top surface  92  of the metal layer. In addition, one or more holes  96  extend through the cover layer  22 , backing layer  20  and metal layer  90 , to permit electrodes secured to the platen to project through the polishing pad and contact the substrate. 
         [0073]    Referring to  FIG. 8 , if the polishing pad includes a conductive layer  90  as described with reference to  FIG. 7  and an adhesive layer  50  spanning a window as described with reference to  FIG. 6A , then the conductive layer  90  can be positioned below the adhesive layer  50 . In addition, apertures can be formed in the adhesive layer  50  in the perforations  94  to expose the top surface  92  of the metal layer.  FIG. 8  illustrates an implementation in which the transparent  56  portion is integrally molded into the covering layer  22  and aperture  58  is the same dimensions as the transparent portion. 
         [0074]    Referring to  FIG. 9 , if the polishing pad includes a conductive layer  90  as described with reference to  FIG. 7 , and an adhesive layer  50  spanning a window and a transparent sheet as described with reference to  FIG. 6B , then the conductive layer  90  can be positioned below the adhesive layer  50 . In addition, apertures can be formed in the adhesive layers  50 ,  28  and  88  and the transparent sheet  80  in the perforations  94  to expose the top surface  92  of the metal layer. 
         [0075]    The polishing pads of  FIGS. 7-9  (which can also use the various features described with respect to  FIGS. 3A-6B  in addition or in alternative to the features illustrated) may be used for electrochemical processing, such as electrochemical mechanical polishing (ECMP) or simultaneous electrochemical deposition and polishing, in addition to chemical mechanical polishing. 
         [0076]    In electrochemical mechanical polishing, conductive material, such as copper, is removed from the substrate surface by electrochemical dissolution while the substrate surface is concurrently polished. The substrate surface is placed in an electrolyte (which also serves as the polishing fluid), and a bias is applied between the substrate and a cathode that is in contact with the electrolyte. The ECMP can be performed at low or very low pressures, such as less than 1 psi, such 0.8 psi or less, or 0.5 psi or less, or 0.3 psi or less. 
         [0077]    For example, referring to  FIGS. 7-9 , the metal sheet  90  can be connected to a first electrode to serve as the cathode (the holes  94  provide access for the electrolyte to the metal sheet  90 ), and a second electrode can extend through the aperture  96  to contact the substrate so that the substrate serve as an anode. 
         [0078]    In electrochemical deposition, the bias voltage is reversed, so that the substrate surface becomes the cathode, the electrode in contact with the electrolyte becomes the anode, and conductive material is electrodeposited onto the substrate. If this is performed while the substrate is contacting a moving processing pad at low pressure, then material will be preferrentially deposited into any trenches in the dielectric layer. 
         [0079]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. 
         [0080]    For example, an adhesive layer can be applied to the bottom surface of the polishing pad to secure the pad to the platen, and the adhesive layer can be covered by a removable liner. In implementations using the transparent sheet, the transparent sheet need not span the entire polishing pad; the transparent sheet could be just large enough to span each aperture to seal the window. 
         [0081]    Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the platen, a tape extending between supply and take-up rollers, or a continuous belt. The polishing pad can be affixed on a platen, incrementally advanced over a platen between polishing operations, or driven continuously over the platen during polishing. The pad can be secured to the platen during polishing, or there could be a fluid bearing between the platen and polishing pad during polishing. In addition, although terms of vertical positioning are used, it should be understood that the polishing surface and substrate could be held upside down, in a vertical orientation, or in some other orientation. 
         [0082]    Accordingly, other embodiments are within the scope of the following claims.