referring to fig1 and 2 , one or more substrates 10 will be polished by a chemical mechanical polishing apparatus 20 . a description of a similar polishing apparatus may be found in u . s . pat . no . 6 , 244 , 935 , the description of which is incorporated by reference . polishing apparatus 20 includes a machine base 22 with a table top 23 that supports a series of polishing stations , including a first polishing station 25 a , a second polishing station 25 b , and a final polishing station 25 c , and a transfer station 27 . each polishing station includes a rotatable platen . at least one of the polishing stations , such as first station 25 a , includes a polishing cartridge 102 mounted to a rotatable , rectangular platen 100 . the polishing cartridge 102 includes a linearly advanceable sheet or belt of polishing material . the remaining polishing stations , e . g ., second polishing station 25 b and final polishing station 25 c , may include “ standard ” circular polishing pads 32 and 34 , respectively , each adhesively attached to a circular platen 30 . each platen may be connected to a platen drive motor that rotates the platen . each polishing station 25 a , 25 b and 25 c also includes a slurry delivery port , a pad rinse system ( which can be a combined slurry / rinse arm 52 that projects over the associated polishing surface ) and a pad conditioner apparatus 40 . a rotatable multi - head carousel 60 is supported above the polishing stations by a center post 62 and is rotated about a carousel axis 64 by a carousel motor assembly ( not shown ). carousel 60 can include four carrier head systems mounted on a carousel support plate 66 at equal angular intervals about carousel axis 64 . three of the carrier head systems receive and hold substrates , and polish them by pressing them against the polishing sheet of station 25 a and the polishing pads of stations 25 b and 25 c . one of the carrier head systems receives a substrate from and delivers a substrate to transfer station 27 . each carrier head system includes a carrier or carrier head 80 . a carrier drive shaft 78 connects a carrier head rotation motor 76 ( shown by the removal of one quarter of the carousel cover ) to carrier head 80 so that each carrier head can independently rotate about its own axis . in addition , each carrier head 80 independently laterally oscillates in a radial slot 72 formed in carousel support plate 66 . referring to fig3 a , 3 b , and 3 c , polishing cartridge 102 is detachably secured to rectangular platen 100 at polishing station 25 a . polishing cartridge 102 includes a feed roller 130 , a take - up roller 132 , and a generally linear sheet 110 ( which can be considered to form a polishing belt or web ) of a polishing pad material . the polishing sheet ( if completely unwound from the rollers ) has a length significantly greater than its width . an unused or “ fresh ” portion 120 of the polishing sheet is wrapped around feed roller 130 , and a used portion 122 of the polishing sheet is wrapped around take - up roller 132 . a rectangular exposed portion 124 of the polishing sheet that is used to polish substrates extends between the used and unused portions 120 , 122 over a top surface 140 of rectangular platen 100 . the rectangular platen 100 can be rotated ( as shown by phantom arrow “ a ” in fig3 a ) to rotate the exposed portion of the polishing sheet and thereby provide relative motion between the substrate and the polishing sheet during polishing . between polishing operations , the polishing sheet can be advanced ( as shown by phantom arrow “ b ” in fig3 a ) to expose an unused portion of the polishing sheet . when the polishing material advances , polishing sheet 110 unwraps from feed roller 130 , moves across the top surface of the rectangular platen , and is taken up by take - up roller 132 . referring to fig4 , polishing sheet 110 includes a polishing layer with a polishing surface 112 , and a leader 160 and a trailer 162 that extend past the polishing layer ( thus , polishing surface 112 does not extend to the ends of the polishing sheet ). the leader and trailer can be formed of a material that is more flexible , and optionally less compressible , than the polishing layer . the leader 160 and trailer 162 can be attached to the feed and take - up rollers 130 and 132 by tape or a pressure sensitive adhesive on the back of the leader . a peelable liner may be placed over the tape or adhesive and removed prior to attaching the polishing sheet 110 to the rollers . the polishing layer can be a porous polyurethane , and the leader and trailer can be a thin tear - resistant material , e . g ., polyethylene terephthalate . the polishing layer can be opaque , whereas the leader and trailer can be transparent . the leader 160 and trailer 162 can be printed or embossed with information , such as a part number , material type , lot number , or polishing layer length . grooves can be formed in the polishing surface 112 running across the width and transverse to the direction of travel of the sheet the grooves can be about half the depth of the polishing layer , e . g ., 18 - 20 mils deep . the grooves can have a uniform width and pitch , e . g ., about one millimeter wide and spaced apart by about two to three millimeters . referring to fig9 , the grooves 128 can stop short of the edge of the polishing layer . such a groove configuration can improve resistance of the polishing pad to tearing . the backing layer 116 can be about five mils thick . the polishing layer can be between about twenty to thirty feet long , e . g ., between twenty and twenty five feet long , and can be between twenty and thirty inches wide , e . g ., about twenty - eight inches wide . the leader and trailer 160 and 162 have same width as the polishing layer , and can be about six feet long . referring to fig5 a , in one implementation , the polishing sheet includes a backing layer 116 and a polishing layer 114 formed on the backing layer 116 . in this implementation , the backing layer 116 can be a continuous sheet that spans the polishing layer 114 . in the transverse direction ( perpendicular to the length and direction of travel of the sheet ), the wide edges 164 ( along the length of the sheet ) of the polishing layer 114 and backing layer 116 can be aligned so that the polishing layer and backing layer 116 have the same width . in contrast , in the longitudinal direction ( parallel to the length and direction of travel of the sheet ), the backing layer 116 extends past the narrow edges 166 ( edges across the width of the sheet ) of the polishing layer 114 to provide the leader 160 and trailer 162 . in the region of the leader 160 and trailer 162 , the top surface of the backing layer 116 can be the outermost surface , i . e ., there are no other layers on top of the backing layer 116 . as noted above , the leader 160 and trailer 162 can be printed or embossed with information , such as a part number , material type , lot number , or polishing layer length . the polishing layer 114 can be a porous polyurethane , and the backing layer 116 can be a thin , flexible , generally incompressible , fluid - impermeable sheet , e . g ., polyethylene terephthalate . the backing layer 116 can be transparent . the polyurethane layer an be 35 - 40 mils thick . the polishing layer 114 can be laid down as a single continuous unbroken layer . alternatively , the polishing layer 114 can be tiled in adjacent strips , e . g ., two to four feet long . the polishing layer 114 can be secured to the top surface of the backing layer 116 by an adhesive 170 , e . g ., a pressure - sensitive adhesive layer . for example , the polishing layer 114 and backing layer 116 can be fabricated separately and then adhered together . alternatively , the polishing layer 114 can adhere to the backing layer 116 backing layer without an adhesive . for example , a liquid polishing layer precursor can be dispensed onto the backing layer 116 and cured so as to form a polishing layer 114 adhering to the backing layer 116 . referring to fig5 b , in another implementation , the polishing sheet includes a backing layer 116 and a polishing layer 114 formed on the backing layer 116 , but the leader 160 and trailer 162 are separate pieces 172 bonded to the backing layer 116 . in this implementation , the backing layer 116 can be a continuous sheet that spans and has coextensive edges with the polishing layer 114 . the polishing layer 114 can be a porous polyurethane . the backing layer 116 can be either softer or less compressible than the polishing layer and can be transparent or opaque . for example , the backing layer can be a soft subpad such as a urethane - impregnated fibrous mat , or a generally incompressible layer such as polyethylene terephthalate . the separate pieces 172 can be thin , flexible , tear - resistant , generally incompressible , fluid - impermeable sheets , e . g ., polyethylene terephthalate . as discussed above , the polishing layer 114 can be secured to the backing layer with or without an adhesive . the leader and trailer pieces 172 can be secured to the backing layer 116 with a butt joint and a thin tape 174 on the underside of the polishing sheet . referring to fig5 c , in another implementation , the polishing sheet includes a polishing layer 114 without any backing layer , and the leader 160 and trailer 162 are separate pieces bonded to the polishing layer 114 . the polishing layer 114 can be a porous polyurethane . the tensile strength of the polishing layer 114 can be about 22 mpa (& gt ; 3000 psi ), which should exceed the average stresses applied while vacuum chucking the web onto the platen (˜ 14 psi ) or while advancing the web (˜ 100 psi ). the separate pieces 172 can be thin , flexible , tear - resistant , generally incompressible , fluid - impermeable sheets , e . g ., polyethylene terephthalate . the leader and trailer pieces 172 can be bonded to the polishing layer 114 by a thin tape 174 on the underside of the polishing sheet . in each of the above implementations , since the polishing layer does not extend to the edges of the polishing sheet , the polishing sheet can be fabricated using less polishing layer material , and thus at lower cost in comparison to a polishing sheet having the polishing layer across the entire length . referring to fig3 b and 6 , feed and take - up rollers 130 and 132 should be slightly longer than the width of polishing sheet 110 . the rollers 130 , 132 may be plastic or metal cylinders slightly longer than the width of the polishing sheet and about 2 ″ in diameter . the opposing end faces 134 of feed roller 130 ( only the feed roller is shown in fig6 , but the take - up roller would be constructed similarly ) can each include a recess 136 which will engage support and drive mechanisms on the platen . for example , one end face can include keyed slot into which an alignment pin will fit , and the other end face can include a keyed star pattern into which a drive gear will fit . in addition , both ends of each roller can include a circular flange 138 that projects above the surface of the roller to hold the polishing sheet in place and prevents it from sliding off either side of the roller . because the leader and trailer can be more flexible than the polishing layer , they can be easier to manipulate and thus easier to install on the rollers . in addition , because the leader and trailer can be more flexible than the polishing layer , the leader and trailer can be wound more tightly on the roller . this permits the diameter of the roller to be decreased , thus either reducing the volume required by the cartridge or permitting more polishing material to be stored in the cartridge . referring to fig7 , in another implementation , the polishing sheet is a single layer , specifically a polishing layer 114 with a polishing surface 112 but without any backing layer or carrier film , and without a leader or trailer . the polishing layer 114 is a contiguous and monolithic layer ; the entire length of the polishing layer between the feed and take - up roller is an uninterrupted seamless part , e . g ., there is no adhesive connection between separate pieces , and the entire polishing layer 114 is generally of uniform composition . in some implementations , the polishing layer is formed without heat - based molding of separate pieces ( such molding should generate regions of slightly different consistency where the molding occurs , and the polishing layer 114 can be sufficiently homogeneous to lack such regions ). the polishing layer 114 has sufficient cohesive strength and mechanical integrity to avoid tearing under the mechanical forces applied in typical cmp process conditions . the polishing layer 114 can be a porous polyurethane . the length of the polishing layer 114 can be at least six times greater than the width . for example , the polishing layer 114 can be about two to three feet wide , e . g ., 2 . 5 feet wide , and thirty to forty feet long . although illustrated in fig7 in a flat state , in use the end portions of the polishing sheet 114 would be wound in a roll around take - up and feed rollers , e . g ., as shown in fig3 b . a ration of length to width of the polishing sheet can be at least 4 : 1 , e . g ., 12 : 1 . the polishing layer 114 can be about 30 to 50 mils thick . because the backing layer and adhesive that would hold the backing layer to the polishing layer are eliminated , the polishing layer can be made thicker and yet the same length of polishing sheet can be wound around a spindle without increasing the diameter of the polishing magazine . since the polishing layer 114 is thicker , the number of substrates polished per unit length of the polishing sheet can increase , and thus the pad life can increase . for example , in comparison to a polishing sheet with a 40 mil thick polishing layer , a 5 mil thick backing layer and a 5 mil thick adhesive , a polishing sheet 50 mil thick polishing layer ( but no backing layer or adhesive ) should have about a 20 - 33 % increase in capacity . grooves extending partially but not entirely through the polishing layer pad , e . g ., having a depth between about 15 and 30 mils , e . g ., about half the thickness of the polishing layer , can be formed in the polishing surface 112 of the polishing layer 114 . where a window stripe is formed in the polishing sheet , the two portions on either side of the stripe can each be a single contiguous layer . the sides of the window can be molded to the sides of the polishing layer portions . referring to fig8 , a method of producing such a monolithic polishing sheet is to deposit a liquid precursor 200 onto a moving belt 210 . the layer of liquid precursor on the belt 210 can be subjected to a first curing process , e . g ., the belt and layer can pass through an oven 212 or other heat source , to at least partially cure the precursor into a semi - solid sheet 202 . the semi - solid sheet 202 can then be detached from belt and pass between pinch - rollers 214 , 216 that compress the sheet to a desired thickness . in addition , protrusions on one of the pinch rollers 216 can imprint grooves into the surface of the semi - solid sheet 202 . then the sheet 202 can be subjected to a second curing process , e . g ., additional heat , such as from the second over 220 to be cured to a final desired rigidity . edges of the sheet 202 can be trimmed to provide a polishing sheet of uniform width , and if necessary the polishing sheet can be smoothed and / or thinned , e . g ., with a grinder or by passing between blades that will skive off top and bottom portions of the sheet , to remove burrs or other defects . a similar process can be used to produce the polishing layer 114 for the implementation of fig5 c . in some implementations , a polishing sheet can be formed by casting the polyurethane precursor material on a carrier sheet ( e . g ., with the carrier sheet on the moving belt or supported on spaced - apart rollers so that the carrier sheet provides a moving belt - like surface ), and subjecting the precursor to the first curing step . the carrier sheet can peeled off either before or after the final cure step . both sides of the polish sheet can then be skived , ground and / or sanded to adjust pad thickness and remove any cured crust layer . grooves can be formed on the cured polishing sheet using a cutting tool such as a knife blade . if the grooves are formed by imprinting with patterned rollers and a subsequent cure step , by selecting an appropriate degree of compression , the grooves could be formed with a smooth or non - porous surface layer in the grooves ( in contrast , the polishing surface itself can be porous , and grooves formed in a porous pad with a knife have porous walls and bottoms ). if so , waste products or polishing debris can be less likely to remain trapped in the grooves , thus reducing the potential for defects , e . g ., scratches . returning to fig3 a , 3 b and 3 c , rectangular platen 100 includes a generally planar rectangular top surface 140 bounded by a feed edge 142 , a take - up edge 144 , and two parallel lateral edges 146 . a groove 150 ( shown in phantom in fig3 a and 3c ) is formed in top surface 140 . the groove 150 can be a generally - rectangular pattern that extends along edges 142 , 144 , 146 of top surface 140 . alternatively , vacuum groove 150 can be circular and about 29 ″ in diameter , and the polish area described by the motion of the head and platen contained within area described by the dashed lines 150 in fig3 a . a passage through platen 100 connects groove 150 to a vacuum source . when passage is evacuated , exposed portion 124 of polishing sheet 110 is vacuum - chucked to top surface 140 of platen 100 . this vacuum - chucking helps ensure that lateral forces caused by friction between the substrate and the polishing sheet during polishing do not force the polishing sheet off the platen . optionally , a central region 148 of top surface 140 can be free from grooves to prevent potential deflection of the polishing sheet into the grooves from interfering with the polishing uniformity . an unillustrated compressible backing pad can be placed on the top surface of the platen to cushion the impact of the substrate against the polishing sheet . in addition , platen 100 may include an unillustrated shim plate . shim plates of differing thickness may be attached to the platen to adjust the vertical position of the top surface of the platen . the compressible backing pad can be attached to the shim plate . in some implementations , the rectangular platen 100 also includes four retaining flanges 156 that hold feed and take - up rollers 130 and 132 at feed and take - up edges 142 and 144 , respectively . each retaining flange 156 includes a projection or detent that can engage the corresponding feature on the end of the rollers 130 or 132 . the rollers 130 and 132 can be positioned sufficiently below top surface 140 so that the polishing sheet stays in contact with the feed and take - up edges 142 and 144 of the platen when the entire polishing sheet is wound around either roller . this assists in the creation of a seal between the polishing sheet and the rectangular platen when vacuum is applied to the passage to vacuum - chuck the polishing sheet to the platen . furthermore , feed edge 142 and take - up edge 144 of the platen are rounded to prevent abrasion of the underside of the polishing sheet as it moves across the platen . a transparent strip 118 can be formed along the length of polishing sheet 110 . the transparent strip may be positioned at the center of the sheet , and may be about 0 . 6 inches wide . with respect to fig5 a and 5b , transparent strip 118 may be formed by removing the upper layer 114 from this region of the transparent backing layer 116 . with respect to fig5 c , a transparent strip 118 may be formed by molding a window into the polishing layer 114 . the transparent strip 118 can be aligned with an aperture or transparent window 154 in rectangular platen 100 to provide optical monitoring of the substrate surface for end point detection . referring again to fig3 a , 3 b and 3 c , in operation , exposed portion 124 of polishing sheet 110 is vacuum - chucked to rectangular platen 100 by applying a vacuum to the passage . a substrate is lowered into contact with polishing sheet 110 by carrier head 80 , and both platen 100 and carrier head 80 rotate to polish the exposed surface of the substrate . after polishing , the substrate is lifted off the polishing pad by the carrier head . the vacuum on passage 152 is removed . the polishing sheet is advanced to expose a fresh segment of the polishing sheet . the polishing sheet is then vacuum - chucked to the rectangular platen , and a new substrate is lowered into contact with the polishing sheet . thus , between each polishing operation , the polishing sheet may be advanced incrementally . if the polishing station includes a cleaning apparatus the polishing sheet may be washed between each polishing operation . the amount that the sheet may be advanced will depend on the desired polishing uniformity and the properties of the polishing sheet , but should be on the order of 0 . 05 to 1 . 0 inches , e . g ., 0 . 4 inch , per polishing operation . assuming that the exposed portion 124 of polishing sheet is 20 inches long and the polishing sheet advances 0 . 4 inches after each polishing operation , the entire exposed portion of the polishing sheet will be replaced after about fifty polishing operations . in another embodiment , a polishing sheet can be fabricated with a polishing layer disposed on a carrier layer , e . g ., a polyethylene terephthalate polyester film , e . g . a mylar sheet . in some implementations , the polishing sheet can be formed by casting the polyurethane precursor material on a carrier layer as discussed above ( e . g ., with the carrier layer on the moving belt ). however , rather than removing the carrier layer , the polishing layer remains on the carrier layer to form the final polishing sheet . in this case , grinding and surface finishing operations to adjust the polishing layer thickness or roughness would take place only on the top surface . in some implementations , the initial carrier layer is removed , grinding and surface finishing operations are performed , and the polishing layer can then be attached to a new carrier layer for the final polishing sheet the invention is not limited to the embodiment depicted and described . rather , the scope of the invention is defined by the appended claims .