Apparatus and method for chemical mechanical polishing

A chemical mechanical polishing apparatus includes a rotatable carousel and multiple carrier head assemblies coupled to the carousel. Each carrier head assembly includes multiple carrier heads each of which can hold a single substrate. The apparatus includes multiple substrate processing stations separated from one another in substantially equal angular intervals. The carrier head assemblies can be positioned in angular alignment with the stations and can be rotated from one station to another station. At least one polishing station includes a fixed abrasive sheet and a fluid bearing surface that provides an upward pressure against the lower surface of the polishing sheet. The carrier head assembly is positioned so that the substrates are in angular alignment with the fluid bearing disposed below the polishing sheet, and the substrates are brought into contact with the polishing sheet. The carrier head assembly and the fluid bearing then are rotated at substantially the same speed and in angular alignment while the substrates are held in contact with the polishing sheet to polish the substrates.

BACKGROUND
 The present invention relates generally to chemical mechanical polishing of
 substrates.
 An integrated circuit is typically formed by the sequential deposition of
 conducting, semiconducting or insulating layers on a silicon wafer. One
 fabrication step involves depositing a filler layer over a patterned stop
 layer, and planarizing the filler layer until the stop layer is exposed.
 For example, trenches or holes in an insulating layer may be filled with a
 conductive layer. After planarization, the portions of the conductive
 layer remaining between the raised pattern of the insulating layer form
 vias, plugs and lines that provide conductive paths between thin film
 circuits on the substrate.
 Chemical mechanical polishing (CMP) is one accepted method of
 planarization. CMP typically requires that the substrate be mounted on a
 carrier or polishing head. The exposed surface of the substrate is placed
 against a rotating polishing pad. The polishing pad may be either a
 "standard" pad or a fixed-abrasive pad. A standard pad has a durable
 roughened surface, whereas a fixed-abrasive pad has abrasive particles
 held in a containment media. The carrier head provides a controllable
 load, in other words, pressure, on the substrate to push it against the
 polishing pad. A polishing slurry, including at least one
 chemically-reactive agent, and abrasive particles if a standard pad is
 used, is supplied to the surface of the polishing pad.
 An effective CMP process provides not only a high polishing rate, but also
 a substrate surface which is finished (lacks small-scale roughness) and
 flat (lacks large-scale topography). The polishing rate, finish and
 flatness are determined by the pad and slurry combination, the relative
 speed between the substrate and pad, and the force pressing the substrate
 against the pad. The polishing rate sets the time needed to polish a
 layer, which in turn sets the maximum throughput of the CMP apparatus.
 During CMP operations, the polishing pad needs to be replaced periodically.
 For a fixed-abrasive pad, the substrate wears away the containment media
 to expose the embedded abrasive particles. Thus, the fixed-abrasive pad is
 gradually consumed by the polishing process and, after a number of
 polishing runs (e.g., as few as about 40-50 and as many as several
 hundred), the fixed-abrasive pad needs to be replaced. For a standard pad,
 the substrate thermally and mechanically damages the polishing pad and
 causes the pad's surface to become smoother and less abrasive. Therefore,
 most standard pads must be periodically conditioned to restore a roughened
 texture to their surface. After a number of conditioning operations (e.g.,
 as few as several hundred and as many as several thousand), the
 conditioning process consumes the pad or the pad is unable to be properly
 conditioned. The pad must then be replaced. An advantage of fixed-abrasive
 polishing pads is that they may not need to be conditioned.
 One problem encountered in the CMP process is the difficulty in replacing
 the polishing pad. The polishing pad may be attached to the platen surface
 with an adhesive. Significant physical effort is often required to peel
 the polishing pad away from the platen surface. The adhesive then must be
 removed from the platen surface by scraping and washing with a solvent. A
 new polishing pad can then be adhesively attached to the clean surface of
 the platen. While this is happening, the platen is not available for the
 polishing of substrates, resulting in a decrease in polishing throughput.
 This problem is even more acute for fixed-abrasive pads, which need to be
 replaced more often than standard polishing pads. Thus, although the
 fixed-abrasive pads may not need to be conditioned, the use of
 fixed-abrasive pads in a CMP apparatus can result in a higher cost of
 operation.
 Another problem that can arise when using a CMP process is difficulty in
 achieving a high throughput for the polishing process. Typically,
 substrates must be loaded and unloaded into the CMP apparatus. In addition
 to polishing the substrates using a fixed-abrasive pad, it is sometimes
 desirable to buff the substrates as well using a standard pas. The
 polishing, buffing, and loading/unloading steps are often performed
 sequentially using an architecture that performs only a single one of the
 steps at a time. It would, therefore, be desirable to modify the
 architecture of existing CMP apparatus to improve the overall throughput.
 SUMMARY
 In general, according to one aspect, a chemical mechanical polishing
 apparatus includes a rotatable carousel and multiple carrier head
 assemblies coupled to the carousel. Each carrier head assembly includes
 multiple carrier heads each of which can hold a substrate. Each carrier
 head assembly also includes a motor-driven system for causing the carrier
 head assembly to rotate about its center axis.
 The CMP apparatus also can include multiple substrate processing stations.
 The processing stations can be separated from one another in substantially
 equal angular intervals, and the carrier head assemblies can be positioned
 in angular alignment with the stations. The carousel can be rotated to
 move each carrier head assembly from one station to another station.
 According to another aspect, a polishing station of a CMP apparatus
 includes a substantially fixed polishing sheet and a fluid bearing surface
 that provides an upward pressure against the lower surface of the
 polishing sheet. In one implementation, the station includes a fixed
 abrasive polishing sheet and a rotatable plate disposed below an exposed
 portion of the polishing sheet. An upper surface of the plate is
 positioned near a lower surface of the polishing sheet. The rotatable
 plate includes a pattern of holes in its upper surface through which a
 fluid can flow to provide an upward pressure against the lower surface of
 the polishing sheet.
 In some implementations, the polishing sheet includes a thin coated,
 micro-replicated, fixed-abrasive polishing pad having relatively high bulk
 compressibility and low bending stiffness.
 In some implementations, the CMP apparatus also includes a source for
 providing fluid to the holes in the rotatable plate and a
 computer-controlled valve for controlling the flow of fluid from the
 source to the pattern of holes in the rotatable plate. The pattern of
 holes in the upper surface of the rotatable plate can include an area
 whose size corresponds to a substrate to be polished by the polishing
 sheet. In some cases, the pattern of holes in the upper surface of the
 rotatable plate includes multiple areas of vertical holes. Each area of
 holes can have a size that corresponds to a substrate to be polished by
 the polishing sheet. Furthermore, the flow of fluid to each of the areas
 of holes can be controlled independently. A motor can be coupled to the
 rotatable plate for causing the plate to rotate during polishing.
 According to yet another aspect, a CMP apparatus includes a carrier head
 assembly with multiple carrier heads each of which can hold a substrate, a
 shaft about which the carrier head assembly can be rotated, and a
 motor-driven pulley system for causing the carrier head assembly to rotate
 about the shaft. The apparatus can further include a system of gears
 coupled to the carrier heads for causing each of the carrier heads to move
 in a circular path substantially without rotating with respect to a fixed
 point as the carrier head assembly is rotated about the shaft. In one
 implementation, the system of gears includes a central gear surrounding
 the shaft, idler gears coupled to the central gear, and outer gears. Each
 outer gear is coupled to one of the idler gears and a respective one of
 the carrier heads.
 A rotatable fluid coupling can be positioned about the shaft. The apparatus
 can include multiple channels having one end coupled to the fluid coupling
 and a second end rotarily coupled to a respective one of the carrier
 heads. The flow of fluid can be used to provide a downward pressure on
 substrates held by the carrier heads. In some implementations, the
 pressure on a substrate held by each carrier head can be controlled
 independently. Polishing of a substrate held by a first one of the carrier
 heads can be stopped while polishing of a substrate held by a second one
 of the carrier heads is continued.
 According to a further aspect, a CMP apparatus includes a polishing station
 which has a substantially fixed polishing sheet and a rotatable plate
 disposed below an exposed portion of the polishing sheet such that an
 upper surface of the plate is positioned near a lower surface of the
 polishing sheet. The plate includes multiple areas of holes in its upper
 surface through which a fluid can flow to provide an upward pressure
 against the lower surface of the polishing sheet. Each area of holes has a
 size that corresponds to a substrate to be polished by the polishing
 sheet. The apparatus also includes a rotatable carrier head assembly with
 multiple carrier heads each of which can hold a substrate. The carrier
 head assembly can be positioned to bring the substrates into contact with
 an upper surface of the polishing sheet. Also, the rotatable plate and the
 carrier head assembly can be rotated at substantially the same speed and
 in angular alignment when the substrates are held in contact with the
 polishing sheet so that the substrates remain positioned directly above
 the areas of holes in the plate during polishing.
 The invention also features methods of polishing one or more substrates
 held by a carrier head assembly. According to one aspect, the method
 includes positioning the carrier head assembly so that the substrates are
 in angular alignment with a fluid bearing disposed below a substantially
 fixed polishing sheet. The carrier head assembly is positioned to bring
 the substrates into contact with the polishing sheet. The carrier head
 assembly and the fluid bearing are rotated at substantially the same speed
 and in angular alignment while the substrates are held in contact with the
 polishing sheet to polish the substrates.
 In some implementations, the method further includes independently
 controlling, during polishing, a downward pressure with which each
 substrate is pressed against the polishing sheet. Also, a first one of the
 substrates can be lifted out of contact with the polishing sheet while a
 second one of the substrates continues to be polished by the polishing
 sheet. An upward pressure can be provided to counteract a downward
 pressure exerted on the substrates. The amount of upward pressure can be
 controlled independently for each of the substrates. Preferably, the
 substrates move in a circular path substantially without rotating with
 respect to the polishing sheet as they are polished.
 Various implementations include one or more of the following advantages.
 The overall throughput of the CMP apparatus can be increased because the
 architecture of the system allows multiple processes to be performed to a
 particular substrate without unloading the substrate from the CMP
 apparatus. Other than the time it takes to move the carrier head
 assemblies from between stations, most of the time can be used to polish,
 buff or load/unload the substrates. Therefore, the throughput can be
 increased further. Additionally, each carrier head assembly can
 simultaneously hold multiple substrates. For example, multiple substrates
 can be polished using a fixed abrasive sheet, while other substrates are
 buffed and yet other substrates are loaded or unloaded. The various
 processing steps can be accomplished in a single CMP apparatus, thereby
 increasing the overall throughput of the fabrication process.
 Additional advantages can be obtained as a result of using a substantially
 fixed web-type abrasive sheet in conjunction with a fluid bearing.
 Web-type abrasive polishing sheets are generally thin and incompressible
 relative to other polishing pad materials and have a low bending
 stiffness. Therefore, it is possible to use the web-type abrasive sheet
 either alone or in conjunction with a sub-pad to achieve a desired
 stiffness and compressibility. In particular, a web-type of abrasive sheet
 and a sub-pad can be selected to modify the overall bending modulus and,
 therefore, provide improved compliance with the surface of the substrate.
 Although an unreinforced web-type abrasive sheet does not generally provide
 a particularly good material against which to press solid, mechanical,
 moveable bearing surface, the fluid bearing can provide a comparatively
 gentle and substantially uniform bearing force against the downward
 pressure of the carrier heads. Therefore, the fluid bearing allows the CMP
 apparatus to take advantage of the beneficial features of a thin webtype
 of abrasive sheet. Rotating the fluid bearing at the same speed and in
 angular alignment with the carrier heads while the substrates are held in
 contact with the abrasive sheet helps assure that the substrates remain
 positioned directly above the fluid bearing surface of the plate during
 polishing. The quality of the polished substrates can, therefore, be
 improved.
 Use of the fluid bearing also makes possible a wide selection of materials
 as the web-type abrasive sheet and/or the sub-pad because the fluid
 bearing is not likely to damage the material it is supporting. Therefore,
 the optimum bending stiffness ad compressibility required to achieve a
 highly uniform polish for the finished substrates can be selected with
 less concern for the durability or other mechanical properties of the
 fixed abrasive sheet or the sub-pad. The sub-pad can be used to optimize
 local and global polishing uniformity across the substrate.
 Other features and advantages will be apparent from the detailed
 description, the drawings and the claims.

DETAILED DESCRIPTION
 As shown in FIG. 1, one or more substrates 10 can be polished by a chemical
 mechanical polishing apparatus 20. The polishing apparatus 20 includes a
 machine base 22 with a table top 24 that supports a series of polishing
 stations, including a first polishing station 25, a second polishing
 station 26, and a transfer station 27. The stations 25, 26, 27 are
 separated from one another by substantially equal angular intervals.
 The transfer station 27 serves multiple functions, including receiving one
 or two substrates 10 from a loading apparatus (not shown), washing the
 substrates, loading the substrates into carrier head assemblies 80,
 receiving the substrates from the carrier head assemblies, washing the
 substrates again, and finally, transferring the substrates back to the
 loading apparatus.
 The first polishing station 25 includes a polishing cartridge 38 detachably
 secured to the top of the table 24. The polishing cartridge 38 includes a
 feed roller 42, a take-up roller 44, and a generally linear sheet 46 of a
 polishing pad material. An unused or fresh portion 48 of the polishing
 sheet 46 is wrapped around the feed roller 42, and a used portion 50 of
 the polishing sheet is wrapped around the take-up roller 44. A rectangular
 exposed portion of the polishing sheet 46 is used to polish substrates and
 extends between the used and unused portions 48, 50.
 The polishing sheet 46 is preferably a web-type fixed-abrasive pad and is
 linearly advanceable between the feed roller 42 and the take-up roller 44.
 Thus, the sheet can be a thin coated, micro-replicated, fixed-abrasive
 polishing medium which has relatively high bulk compressibility and low
 bending stiffness. For example, the fixed-abrasive polishing sheet 46 can
 include an upper layer and a lower layer. The upper layer is an abrasive
 composite layer composed of abrasive grains held or embedded in a binder
 material. The abrasive grains may have a particle size between about 0.1
 and 1500 microns. Examples of such grains include silicon oxide, fused
 aluminum oxide, ceramic aluminum oxide, green silicon carbide, silicon
 carbide, chromium, alumina zirconium, diamond, iron oxide, cerium, cubic
 boron nitride, garnet and combinations thereof. The binder material may be
 derived from a precursor which includes an organic polymerizable resin
 which is cured to form the binder material. Examples of such resins
 include phenolic resins, urea-formaldehyde resins, melamine formaldehyde
 resins, acrylated urethanes, acrylated epoxies, ethylenically unsaturated
 compounds, aminoplast derivatives having at least one pendant acrylate
 group, isocyanurate derivatives having at least one pendant acrylate
 group, vinyl ethers, epoxy resins, and combinations thereof. The lower
 layer is a backing layer composed of a material such as a polymeric film,
 paper, cloth, a metallic film or the like.
 Additional details of the first polishing station 25 are described in
 greater detail below.
 The polishing station 26 includes a standard polishing pad 32 adhesively
 attached to a circular platen 30. The polishing station 26 also can
 include a combined slurry/rinse arm 52 that projects over the associated
 polishing surface. The slurry/rinse arm 52 can include slurry supply tubes
 to provide a polishing liquid, slurry, or cleaning liquid to the surface
 of the polishing pad. Typically, sufficient liquid is provided to cover
 and wet the entire polishing pad. Each slurry/rinse arm can include
 several spray nozzles (not shown) to provide a high-pressure rinse at the
 end of each polishing and conditioning cycle. The polishing station 26
 also can include an optional associated pad conditioner apparatus 40.
 Optional cleaning stations 54 may be positioned between polishing stations
 25, 26 and between the polishing stations 26 and the transfer station 27
 to clean the substrates as they move between the stations.
 A rotatable multi-head carousel 60 can be supported above the polishing
 stations by a center post 62 and is rotated about a carousel axis 64 by a
 motor 180 (FIG. 7). In the illustrated implementation, the carousel 60 can
 include three carrier head assemblies 80 mounted on a carousel support
 plate 66 at substantially equal angular intervals about the carousel axis
 64. During operation of the system, one carrier head assembly receives and
 holds substrates, and polishes the substrates by pressing them against the
 polishing sheet 46 of the first station 25. Another one of the carrier
 head assemblies polishes substrates by pressing them against the polishing
 pad 32 of the second station 26. The third carrier head assembly receives
 substrates from and delivers substrates to the transfer station 27. The
 carousel 60 can be rotated to move the carrier head assemblies 80 between
 the stations.
 In the illustrated embodiment, each carrier head assembly 80 includes two
 carrier heads 82. A respective rotation motor 154 (FIG. 5A) is provided
 for each of the carrier head assemblies 80 so that each carrier head
 assembly independently can rotate about its own axis. The carrier head
 assembly motors 154, as well as the motor 180 that controls rotation of
 the carousel 60, are controlled by a general purpose programmable digital
 computer 124 (FIG. 7).
 Providing each carrier head assembly 80 with multiple carrier heads 82
 allows a large number of substrates 10 to be processed within a relatively
 short period of time. Furthermore, in the illustrated implementation, the
 carousel 60 needs only four driving motors to rotate the three carrier
 head assemblies 80 and to rotate the carousel 60. The illustrated CMP
 apparatus can process up to six substrates at one time.
 Each carrier head 82 performs several mechanical functions. Generally, the
 carrier head holds a substrate 10 against the polishing surface, evenly
 distributes a downward pressure across the back surface of the substrate,
 transfers torque to the substrate, and ensures that the substrate does not
 slip out from beneath the carrier head during polishing operations. A
 retaining ring 72 (see FIG. 5A) is provided about the outer perimeter of
 each carrier head 82 to help hold the substrate 10 in place.
 Further details of the polishing station 25 are illustrated in FIGS. 2, 3,
 4A and 4B. A square or rectangular-shaped platen 90, supported by the
 table 24, has a circular hole surrounded by a seal, such as an O-ring 92
 (FIG. 4A). One or more grooves 98, shown in phantom in FIGS. 4A and 4B,
 are formed in a top surface 100 of the platen 90. The groove 98 can be a
 generally-rectangular or other pattern that extends along the edges of the
 top surface 100. A passage 102 through the platen 100 connects the groove
 98 to a vacuum source 104. When the passage 102 is evacuated, a portion of
 the polishing sheet 46 is vacuum-chucked to the top surface 100 of the
 platen 90. That 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.
 A pneumatic control line 106 connects the passage 102, and thus the groove
 98, to the vacuum source 104. Multiple pneumatic lines 106 can be used to
 vacuum-chuck the polishing sheet 46 and to activate a polishing sheet
 advancement mechanism. A pump or source of pressurized gas, for example,
 can be used as the vacuum source 104. The vacuum source 104 is connected
 by a fluid line 108 to a computer controlled valve 110. The computer
 controlled valve 110 is connected by a second fluid line 112 to a rotary
 coupling 114. The rotary coupling 114 connects the vacuum source 104 to an
 axial passage 116 in a rotating shaft 118, and a coupling 120 connects the
 axial passage 116 to one end of a flexible pneumatic line 122. The other
 end of the flexible pneumatic line 122 is connected to the pneumatic line
 106. The computer 124 is coupled to the valve 110 and controls whether the
 valve is open or closed.
 As further shown in FIG. 3, a rotatable shaft 96 supports a plate 94 and
 extends through the central hole in the platen 90. Bearings 128 permit the
 shaft 96 to be rotated about its central axis 130. A motor 146 (FIG. 7),
 controlled by the computer 124, causes the rotation of the shaft 96. As
 the shaft 96 is rotated about its axis 130, the plate 94 also rotates, as
 indicated by the arrow 144 (FIGS. 3 and 4A). As explained in greater
 detail below, the plate 94 rotates during polishing of the substrates 10.
 However, the platen 90, which provides a vacuum to hold down the abrasive
 sheet 46 during polishing, remains stationary.
 The rotatable plate 94 can be a flat, rigid metal plate which has a pattern
 of vertical holes 132 extending through it. The vertical holes 132 are
 located in two circular areas 134, 136 of the plate 94 and serve as a
 means for introducing a fluid underneath the abrasive sheet 46. The fluid
 serves as a fluid bearing. Each circular area 134, 136 is approximately
 the same size as a single substrate 10. If the retaining ring 72 in the
 carrier head 82 is designed to contact the polishing surface, then each
 circular area 134, 136 can be approximately the size of a single substrate
 10 plus its retaining ring 72. The top surface of the plate 94 can be at
 substantially the same height or slightly lower than the top surface 100
 of the platen 90 to allow a subpad 126 to be placed between the fluid
 bearing and the polishing sheet 46. The sub-pad 126, if used, can modify
 the effective compressibility of the abrasive sheet 46.
 The shaft 96 includes one or more channels, such as hollow tubes 138,
 through which a fluid, such as air or water, can be provided to the fluid
 bearing plate 94. A separate channel 138 can be used to provide air flow
 to each of the areas 134, 136 of holes 132 in the plate 94. The computer
 124 (FIG. 7) controls the positions of respective valves 142 to control
 the flow of a fluid, such as air or water, from a source 140 to the fluid
 bearing 94 via the channels 138. The amount of fluid flowing through each
 of the channels 138 can be controlled independently.
 When substrates 10 are placed against the upper surface of the polishing
 sheet 46, they are positioned opposite the circular areas 134, 136 of
 holes 132 on the plate 94. During polishing, the pressure created by the
 flow of air through the holes 132 counteracts the downward pressure
 exerted on the substrates. A pattern of holes 132 can be provided so that
 the amount of upward pressure provided by the air flowing through the
 vertical holes 132 varies across the substrate 10. If desired, the pattern
 of holes 132 can be designed so that the effective area of the holes
 locally produces a nonuniform pressure profile across the face of the
 substrate 10. Additionally, the amount of fluid exiting each hole can be
 varied to produce a desired pressure distribution on the face of the
 substrate.
 One or more in-situ monitors 200 are mounted below the plate 94 (see, e.g.
 FIG. 3). The monitors 200 use optical or other techniques to perform
 end-point or thickness monitoring and determine when polishing should be
 stopped with respect to the substrates 10. Preferably, a monitor 200 is
 mounted directly below the center of each area 134, 136 of holes 132 so
 that each substrate 10 held by one of the carrier heads 82 is positioned
 directly above a monitor during polishing (see, e.g., FIG. 4A).
 Alternatively, one or more monitors can be mounted in a fixed position so
 that the substrates pass directly above the monitor(s) during polishing.
 To allow optical signals, for example, to be reflected off the face of a
 substrate 10 and detected by one of the monitors 200, windows (not shown)
 can be provided in the plate 94. The abrasive sheet 46, as well as the
 sub-pad 126, can be provided with transparent regions to allow the
 end-point or thickness monitoring to be performed.
 Each of the three carrier head assemblies 80 is substantially the same. As
 shown in FIG. 5A, a stationary shaft 78, which can be mounted to a fixed
 cross-beam, has a central axis 220 that extends vertically through the
 carrier head assembly 80. Other mounting techniques also can be used. When
 a carrier head assembly 80 is positioned for polishing the substrates 10
 at the first station 25, the axes 220 and 130 are aligned (FIG. 5B). A
 first pulley 150 is provided about the central shaft 78, and a second
 pulley 152 is provided about a rotatable shaft 153. A belt 148 is provided
 around the pulleys 150, 152. The second pulley 152 can be rotated by one
 of the motors 154 which, as shown in FIG. 7, is controlled by the computer
 124. The carrier head assembly 80 includes bearings 156 which allow the
 carrier head assembly to rotate about the stationary shaft 78 when the
 motor 154 is operating. When the motor 154 causes the shaft 153 to rotate,
 the pair of pulleys 150, 152 and the belt 148 coordinate to rotate the
 entire carrier head assembly 80 about the stationary shaft 78.
 As further illustrated in FIGS. 5A and 5B, the carrier head assembly 80
 includes several gears, including a central stationary gear 158
 surrounding the stationary shaft 78, a pair of idler gears 160, and two
 outer gears 162. A respective vertical shaft 164 and bearings 166 are
 associated with each of the idler gears 160 to allow the idler gears to
 rotate. Similarly, a respective hollow shaft 168 and bearings 170 are
 associated with each of the outer gears 162 so that they can rotate as
 well. Each hollow shaft 168 is connected at its lower end to a respective
 one of the carrier heads 82 in the assembly 80.
 When the carrier head assembly 80 rotates about the axis 220, the outer
 gears 162 and associated shafts 168 are caused to rotate about their own
 axes 172 as well. The result is that as the assembly 80 rotates about its
 axis 130, the carrier heads 82, each of which can hold a substrate to be
 polished, rotate about the respective axes 172. Preferably, the outer
 gears 162 have approximately the same number of teeth as the central gear
 158. Therefore, each point on a substrate that is held by one of the
 carrier heads 82 can be caused to translate linearly in a circular path
 substantially without rotating with respect to the fixed polishing sheet
 46. FIGS. 6A, 6B and 6C illustrate several positions of the carrier heads
 82 to show how they move in a circular path as the assembly 80 is rotated
 about the axis 220. In FIGS. 6A, 6B and 6C, the carrier heads are labelled
 82A and 82B, respectively, and an "x" provides a reference point on each
 of the carrier heads 82A, 82B.
 A fluid coupling 174 in the form of a collar is positioned around the
 stationary shaft 78. One or more channels, such as tubes 176, are
 connected from the fluid coupling 174 to a rotary coupling inside each
 hollow shaft 168. During operation of the CMP apparatus 20, a fluid such
 as air is provided from a pneumatic or other source (not shown) and flows
 down the stationary shaft 78, through the channels 176 via the fluid
 coupling 174, and through the hollow shafts 168. The fluid is provided to
 the carrier heads 82 so that the substrates 10 are pressed into contact
 with the abrasive sheet 46. A vacuum can be provided through the channels
 176 to dechuck the substrates 10. When the carrier head assembly 80 is
 rotated about the axis 220, the fluid coupling 174 rotates about the shaft
 78.
 The computer 124 controls valves 178 (FIG. 7) to control the amount of air
 pressure to the carrier heads 82 during polishing. The air-flow through
 the channels 176 to each of the carrier heads 82 in the assembly 80 can be
 controlled independently. As a result, the amount of pressure applied to
 each substrate during polishing can be controlled independently.
 As previously mentioned, the station 25 can employ optical or other
 techniques for end-point or thickness monitoring to determine when
 polishing should be stopped with respect to a particular substrate 10. In
 some cases, the end-point detection may indicate that polishing of a
 substrate held by one of the carrier heads 82 should be stopped, even
 though polishing is not yet completed with respect to a substrate held by
 the second carrier head. In such a situation, the substrate for which
 polishing is completed can be lifted out of contact with the abrasive
 sheet 46 by providing a vacuum to dechuck the carrier head 82 holding the
 substrate. At substantially the same time as the substrate is lifted out
 of contact with the abrasive sheet 46, the computer 124 turns off the flow
 of air through the corresponding channel 138 in the shaft 96 to prevent
 the abrasive sheet 46 from being blown off the plate 94. Although one
 substrate has been lifted out of contact with the abrasive sheet 46,
 polishing of the substrate held by the second carrier head 82 can
 continue.
 In operation, the exposed portion of the polishing sheet 46 is
 vacuum-chucked to the platen 90 by applying a vacuum to the passage 102
 (FIG. 4B). Substrates 10 held by the carrier heads 82 are lowered into
 contact with the polishing sheet 46 by the carrier head assembly 80 (see
 FIG. 5B). The substrates 10 are positioned above the fluid bearing surface
 of the plate 94, in other words, above the circular areas 134, 136 with
 the holes 132. As previously discussed, the computer 124 controls the
 motors 146, 154 which respectively cause the fluid bearing plate 94 and
 the carrier head assembly 80 to rotate. The computer 124 controls the
 speed of the motors 146, 154 so that both the plate 94 and the carrier
 head assembly 80 rotate in the same direction and at substantially the
 same speed. In other words, the fluid bearing rotates at the same speed
 and in angular alignment with the carrier heads 82 while the substrates 10
 are held in contact with the abrasive sheet 46. Therefore, the substrates
 10 remain positioned directly above the fluid bearing surface of the plate
 94 during polishing. The web-type abrasive polishing sheet 46 remains in a
 fixed position during the polishing operation. A sub-pad 126 can be placed
 between the plate 94 and the underside of the polishing sheet 46 to modify
 the effective compressibility of the support for the polishing sheet. In
 that case, the sub-pad 126 will be stationary with respect to the
 polishing sheet 46.
 After polishing, the substrates 10 are lifted off the polishing sheet 46 by
 the carrier heads 82, and the vacuum on the passage 98 is removed. The
 polishing sheet 46 can be advanced to expose a fresh section of the sheet.
 The polishing sheet 46 is vacuum-chucked to the platen 90 and, after
 rotating the carousel 60, new substrates are lowered into contact with the
 polishing sheet. Thus, between each polishing operation, the polishing
 sheet 46 can be advanced incrementally. The polishing sheet may also be
 washed between polishing operations. An exemplary mechanism for advancing
 the polishing sheet 46 is described in co-pending U.S. application Ser.
 No. 09/244,456, filed Feb. 4, 1999 and assigned to the assignee of the
 present invention. That application is incorporated herein by reference.
 In some implementations, the platen 90 that holds down the abrasive sheet
 46 during polishing also can be oscillated back and forth slightly during
 polishing. The oscillation can help prevent the same area(s) of the sheet
 46 from being used during the polishing process and can provide a
 relatively smooth transition of abrasive material when the sheet 46 is
 moved forward incrementally between polishing steps. For example, as shown
 in FIG. 3, the platen 90 is supported by a set of linear bearings 210 and
 allows the platen to move back and forth as indicated by the arrow 212. In
 one implementation, the range of the oscillation is several inches. Except
 for the small amount of oscillation, the polishing sheet 46 remains
 substantially fixed.
 Several advantages can be obtained through the polishing station 25 using a
 web-type abrasive polishing sheet 46. Web-type abrasive sheets are thin
 and incompressible relative to other polishing pad materials. Such
 abrasive sheets have a low bending stiffness because they are thin.
 Therefore, it is possible to use the webtype abrasive sheet 46 either
 alone or in conjunction with a sub-pad 126 to achieve a desired stiffness
 and compressibility. In particular, a web-type of abrasive sheet and a
 sub-pad can be selected to modify the overall bending modulus and
 compressibility and, therefore, provide improved compliance with the
 surface of the substrate 10. The sub-pad 126 can, thus, be used to
 optimize local and global polishing uniformity across the substrate 10.
 Although the web-type abrasive sheet 46 can provide a desirable low bending
 stiffness, such a polishing sheet does not generally provide a
 particularly good material against which to press a solid, mechanical,
 moveable bearing surface. On the other hand, the rotatable plate 90, which
 serves as a fluid bearing, can provide a substantially uniform bearing
 force against the downward pressure of the carrier heads 82. Therefore,
 the fluid bearing is particularly suited for use with the thin web-type of
 abrasive sheet 46.
 Furthermore, use of the fluid bearing makes possible a wide selection of
 materials as the web-type abrasive sheet 46 and/or the sub-pad 126 because
 the fluid bearing is less likely to damage the material it is supporting.
 Therefore, the optimum bending stiffness and compressibility required to
 achieve a highly uniform polish for the finished substrates 10 can be
 selected with less concern for the durability or other mechanical
 properties of the fixed abrasive sheet 46 or the sub-pad 126.
 Although the CMP apparatus depicted in the FIGS. 1 and 2 show three
 stations 25, 26, 27, and three carrier head assemblies 80, additional
 stations and a corresponding number of carrier head assemblies can be
 added. Each of the stations can be separated from adjacent stations by
 substantially equal angular intervals. Similarly, the carrier head
 assemblies would be separated from one another by the same angular
 intervals. Furthermore, although the illustrated embodiment of FIG. 1
 shows a polishing station 25 and a buffing station 26, two polishing
 stations, 25A, 25B, each of which is substantially similar to the
 polishing station 25, can be provided instead (see FIG. 8). Additionally,
 although in many implementations it is desirable to use an abrasive sheet
 as the polishing sheet 46, in other implementations the sheet 46 can be a
 soft polishing sheet that is used to buff the substrate. Alternatively, a
 polishing or buffing material in roll form can be provided for one or both
 of the stations 25, 26.
 In some implementations, additional carrier heads 82 can be provided on
 each assembly 80 so that more than two substrates 10 can be processed by
 each assembly at the same time. Additional pairs of gears 160, 162 would
 be provided for each additional carrier head 82. Similarly, an additional
 circular pattern of holes 132 would be provided in the plate 94 for each
 additional substrate that can be held by the carrier head assembly 80.
 Other implementations are within the scope of the claims.