Patent Publication Number: US-6217429-B1

Title: Polishing pad conditioner

Description:
BACKGROUND 
     This invention relates generally to the planarization of semiconductor substrates and, more particularly to the conditioning of polishing pads. 
     Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes successively less planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer as a non-planar surface can prevent proper focusing of the photolithography apparatus. Therefore, there is a need to periodically planarize the substrate surface to provide a planar surface. Planarization, in effect, polishes away a non-planar, outer surface, whether a conductive, semiconductive, or insulative layer, to form a relatively flat, smooth surface. 
     Chemical mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head, with the surface of the substrate to be polished exposed. The substrate is then placed against a rotating polishing pad. The carrier head may also rotate and/or oscillate to provide additional motion between the substrate and polishing surface. Further, a polishing slurry, including an abrasive and at least one chemically-reactive agent, may be spread on the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate. 
     Important factors in the chemical mechanical polishing process are: substrate surface planarity and uniformity, and the polishing rate. Inadequate planarity and uniformity can produce substrate defects. The polishing rate sets the time needed to polish a layer. Thus, it sets the maximum throughput of the polishing apparatus. 
     It is important to take appropriate steps to counteract any deteriorative factors which either present the possibility of damaging the substrate (such as by scratches resulting from accumulated debris in the pad) or reduce polishing speed and efficiency (such as results from glazing of the pad surface after extensive use). The problems associated with scratching the substrate surface are self-evident. The more general pad deterioration problems both decrease polishing efficiency, which increases cost, and create difficulties in maintaining consistent operation from substrate to substrate as the pad decays. 
     The glazing phenomenon is a complex combination of contamination, thermal, chemical and mechanical damage to the pad material. When the polisher is in operation, the pad is subject to compression, shear and friction producing heat and wear. Slurry and abraded material from the wafer and pad are pressed into the pores of the pad material and the material itself becomes matted and even partially fused. These effects reduce the pad&#39;s roughness and its ability to apply fresh slurry to the substrate. 
     It is, therefore, desirable to continually condition the pad by removing trapped slurry, and unmatting or re-expanding the pad material. 
     A conventional conditioning apparatus places an abrasive material in contact with the moving polishing pad. For example, a diamond coated screen or disk may be used to scrape and abrade the pad surface, and to expand and re-roughen the pad. The diamond coated disk can be attached to a rotatable backing element. 
     Two manufacturers of diamond coated disks are Abrasive Technology, Inc., of Westerville, Ohio, and TBW Industries, Inc. of Furlong, Penn. The disks from Abrasive Technology are thicker than those of TBW. Also, mechanical means are used to secure the Abrasive Technology disks to the backing element, whereas magnetic means are used to secure the TBW disks to the backing element. Different disk holders are needed to hold these different types of disks. 
     SUMMARY 
     In one aspect, the invention is directed to a conditioner head to condition the polishing surface of a polishing pad. The conditioner head has a disk having an abrasive surface to contact a polishing pad, a disk holder to carry the disk and to hold it in contact with the polishing pad, and a drive element to rotate the disk about an axis. The disk holder has a generally flat mounting surface. 
     Implementations of the invention may include one or more of the following features. A mechanical fastener may secure the disk to the disk holder. The mechanical fastener may include a plurality of holes provided around a periphery of the disk holder. The holes may extend through the disk holder. A plurality of first cavities may be provided around a periphery of the disk corresponding to the plurality of holes of the disk holder. A plurality of screws may be inserted into the plurality of holes and the plurality of first cavities to secure the disk to the disk holder. The disk holder may include an upwardly protruding rim. A ring may be positioned on the rim of the disk holder, and the ring may have a plurality of holes at its edges. A membrane cover may be secured between the ring and the rim to prevent contaminants from falling into an interior of the disk holder. Each of the plurality of holes on the ring may include an upper cavity and a lower cavity, with the lower cavity extending radially further into the ring than the upper cavity. The disk holder may include a generally convex spherical portion protruding upward on an opposing side of the mounting surface. The disk holder may be secured to the disk by a plurality screws. A plurality of drive pins on the disk receiving surface of the disk holder may transfer torque to the disk, and a plurality of drive bores on the disk may receive the drive pins. A generally annular, flat adapter may be positioned between the disk and the disk holder. A plurality of second cavities may be provided around a periphery of the adapter to correspond to the plurality of holes of the disk holder, and a plurality of screws may be inserted into the plurality of holes and the plurality of second cavities to secure the adapter to the disk holder. The adapter may include a plurality of drive bores to receive the drive pins of the disk holder. The disk holder may include a generally convex spherical portion protruding upward on an opposing side of the mounting surface. A generally flat lower surface of the disk may be secured to the adapter and the disk holder defines a disk plane, and the disk, the adapter and the disk holder may be configured such that the center of the spherical portion is located substantially at the disk plane. A magnetic plate may be positioned between the adapter and the disk to magnetically couple the disk to the adapter. 
     In another aspect, the invention is directed to a disk holder of a conditioner head to carry and hold an abrasive disk against a polishing surface of a polishing pad. The disk holder has a generally convex spherical portion protruding upward from an upper surface of the disk holder, a generally flat mounting surface provided on a lower surface of the disk holder, and a plurality of holes around a periphery of the disk holder. The holes are configured to receive a plurality of screws which are used to secure the disk or an adapter to the mounting surface of the disk holder. 
     In another aspect, the invention is directed to a conditioner head to condition the polishing surface of a polishing pad. The conditioner head has a disk having an abrasive surface to contact a polishing pad, a disk holder to carry the disk and to hold it in contact with the polishing pad, and a drive element to rotate the disk about a longitudinal axis. The disk holder has a generally flat mounting surface on one side and a generally convex spherical portion protruding upward on an opposing side of the mounting surface. A generally flat adapter is secured to the disk on one side and mounted to the mounting surface of the disk holder on an opposing side thereof. A generally flat lower surface of the disk secured to the adapter and the disk holder defines a disk plane, and the disk, the adapter and the disk holder are configured such that the center of the spherical portion is located substantially at the disk plane. 
     In another aspect, the invention is directed to a conditioner head to condition the polishing surface of a polishing pad. The conditioner head has a disk having an abrasive surface to contact a polishing pad, a disk holder to carry the disk and to hold it in contact with the polishing pad, and a drive element to rotate the disk about a longitudinal axis. The disk has a plurality of cavities at its periphery, and the disk holder has a generally flat mounting surface to mount the disk thereon and a plurality of holes at its periphery. A plurality of screws are inserted into the plurality of holes and the plurality of cavities to secure the disk and the disk holder together. 
     Advantages of the invention may include the following. The disk holder of the present invention may carry the disks of different thicknesses, including disks manufactured by both Abrasive Technology and TBW. The disk holder allows either magnetic or mechanical means to secure the disks to the backing element. 
     Other advantages and features of the invention will be apparent from the following description, including the drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially exploded view of the polishing apparatus of FIG.  1 . 
     FIGS. 2A and 2B are diagrammatic top views of a substrate being polished and a polishing pad being conditioned by the polishing apparatus of FIG.  1 . 
     FIG. 3A is a diagrammatic cross-sectional view of a conditioner head with an end effector in a retracted position. 
     FIG. 3B is a diagrammatic cross-sectional view of a conditioner head with an end effector in an extended position. 
     FIG. 4 is a diagrammatic cross-sectional view of the end effector of the conditioner head of FIGS. 3A and 3B. 
     FIG. 5 is a top view of a clamp ring of the end effector of FIG.  4 . 
     FIG. 6 is a cross-sectional view of the clamp ring of FIG. 5 taken along the line  6 — 6 . 
     FIG. 7 is a cross-sectional view of the clamp ring of FIG. 5 taken along the line  7 — 7 . 
     FIG. 8 is a top view of a plurality of spokes of the end effector of FIG.  4 . 
     FIG. 9 is a top view of a disk holder of the end effector of FIG.  4 . 
     FIG. 10 is a bottom view of a disk holder of the end effector of FIG.  4 . 
     FIG. 11 is a top view of a disk of the end effector of FIG.  4 . 
     FIG. 12 is a diagrammatic cross-sectional view of an end effector having an adapter and a magnetic plate. 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. A primed reference number indicates that an element has a modified function, operation or structure. 
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a polishing apparatus  10  includes three independently-operated polishing stations  14 , a substrate transfer station  16 , and a rotatable carousel  18  which choreographs the operation of four independently rotatable carrier heads  20 . 
     The carousel  18  has a support plate  42  with slots  44  through which shafts  46  of the carrier heads  20  extend. The carrier heads  20  can independently rotate and oscillate back-and-forth in the slots  44  to achieve a uniformly polished substrate surface. The carrier heads  20  are rotated by respective motors  48 , which are normally hidden behind removable sidewalls  50  of the carousel  18 . In operation, a substrate is loaded by the transfer station  16  into a carrier head  20 . The carousel  18  then transfers the substrate through a series of one or more polishing stations  14  and finally returns the polished substrate to the transfer station  16 . 
     Each polishing station  14  includes a rotatable platen  52 , which supports a polishing pad  54 , and a pad conditioner  56 . The platen  52  and conditioner  56  are both mounted to a table top  57  inside the polishing apparatus  10 . Each pad conditioner  56  includes a conditioner head  60 , an arm  62 , and a base  64 . The arm  62  has a distal end coupled to the conditioner head  60  and a proximal end coupled to the base  64 . The base  64  rotates to sweep the conditioner head  60  across the polishing pad surface  76  to condition the surface  76 . Each polishing station  14  also includes a cup  66 , which contains a cleaning liquid for rinsing or cleaning the conditioner head  60 . 
     Referring to FIGS. 2A and 2B, in one implementation, a polishing pad  54  is conditioned by a pad conditioner  56 . The conditioner head  60  sweeps across the polishing pad  54  with a reciprocal motion that is synchronized with the motion of the carrier head  20  across the polishing pad  54 . For example, before polishing, the carrier head  20  may be positioned in the center of the polishing pad  54  and the conditioner head  60  may be immersed in the cleaning liquid contained within the cup  66 . During polishing, the cup  66  may pivot out of the way as shown by arrow  69 , and the conditioner head  60  and the carrier head  20  may be sweep back-and-forth across the polishing pad  54  as shown by arrows  70  and  72 , respectively. Three water jets  74  may direct streams of water toward the polishing pad  54  to rinse slurry from the polishing or upper pad surface  76 . 
     Further details regarding the general features and operation of polishing apparatus  10  may be found in U.S. Pat. No. 5,738,574, the entirety of which is incorporated herein by reference. 
     Referring to FIGS. 3A and 3B, a conditioner head  60  includes an actuation and drive mechanism  78  which rotates an end effector  80  carrying a diamond impregnated conditioning disk  82  (manufactured, for example, by Abrasive Technology) about a central vertically-oriented longitudinal axis  300  of the head. The actuation and drive mechanism further provides for the movement of the end effector  80  and disk  82  between an elevated retracted position (FIG. 3A) and a lowered extended position (FIG.  3 B). In the substantially extended position, the lower surface  84  of the disk  82  may be brought into engagement with the polishing surface  76  of the pad  54 . 
     The actuation and drive mechanism  78  includes a vertically-extending drive shaft  86  which, at its upper end, includes a unitarily-formed, radially-extending web  88 . In the exemplary embodiment, the drive shaft may be formed of heat treated  440 C stainless steel. A pulley  90  is secured to the web and carries a belt  92  which extends along the length of the arm  62  and is coupled to a remote motor (not shown) for rotating the shaft  86  about the longitudinal axis  300 . A rotary union  94  is secured to the upper end of the shaft for introducing and withdrawing air from an actuation chamber via a longitudinal channel  96  in the shaft. A collar, having upper and lower pieces  98  and  100 , respectively, coaxially encompasses the shaft, defining a generally annular space  102  therebetween. The upper collar piece  98  depends from the web  88 . The shaft, pulley, and collar form a generally rigid structure which rotates as a unit about the longitudinal axis  300 . To permit rotation, the shaft/pulley/collar unit is carried within the head by a bearing system  104  comprising upper and lower ball bearing units  104 A and  104 B. The bearing system  104  couples the lower collar  100  of the collar piece to an inner head housing  106  which is fixed to the structure of the arm. An annular clamp  114  is secured to the base of lower collar piece  100  so as to vertically clamp an inner portion of the bearing system  104  between the clamp  114  and upper collar piece  98 . The inner head housing  106  is held within a centrally-apertured cup-shaped outer head housing  108  and secured thereto to vertically clamp an outer portion of the bearing system  104  between the inner and outer head housings. The outer head housing  108  is secured to a lower arm housing  110  so that the arm  62  supports the head  60 . An upper arm housing  112  provides additional structural support. 
     A generally-annular drive sleeve  120  couples the end effector  80  to the drive shaft  86 . The drive sleeve  120  is accommodated within the annular space  102  between the collar and drive shaft. The drive sleeve  120  is keyed to the drive shaft  86  so as to permit relative longitudinal translation therebetween while preventing relative rotation. In the illustrated embodiment, this is achieved by a keying member  122  having an outwardly projected keying tab  124 . The keying member  122  is secured within a vertical slot  126  in the periphery of shaft  86 . The tab  124  rides within a vertical slot  128  in the interior of sleeve  120  and interacts with the sides of the slot  128  to prevent relative rotation of the shaft and sleeve. Thus the shaft transmits torque and rotation from the pulley to the sleeve  120 . To provide a smooth sliding vertical engagement between the drive shaft  86  and drive sleeve  120 , a bearing having a cage  130  and a plurality of balls  132  is interposed between the inner cylindrical surface of the sleeve  120  and the outer cylindrical surface of the shaft  86 . 
     A generally-annular elastomeric diaphragm  134  having an outer periphery  136  and an inner periphery  138  off an upper portion of the annular space  102  to form a pressure chamber  102 A. The diaphragm has an upper surface  140 A generally interior to the pressure chamber  102 A and a lower surface  140 B generally exterior to the pressure chamber. Along its inner periphery  138 , the diaphragm is sealingly secured between an upward facing shoulder of the drive sleeve  120  and a lower face an annular internally threaded clamp  142 . The clamp  142  (which may be formed as a nut) is engaged to an externally threaded reduced diameter portion  144  at the upper end of the drive sleeve  120 . The diaphragm extends radially outward from between the clamp and shoulder and then curves downward along a round  146  formed between the shoulder and a cylindrical outer surface portion  148  of the drive sleeve. The diaphragm disengages the circular cylindrical outer surface portion and continues radially outward, traversing a gap (the annular space  102 ) between the drive sleeve and the collar. Continuing and curving upwardly, the lower surface  140 B of the diaphragm engages a circular cylindrical inner surface  150  of the lower collar piece  100  and extends upward therealong. The diaphragm wraps over a round  152  formed between the cylindrical inner surface  150  and an upward facing shoulder of the lower collar piece and is clamped between the upward facing shoulder and a downward facing shoulder of the upper collar piece  98 . Inboard of the inner cylindrical surface  150 , an annular lip  154  projects downward from the upper collar piece, sandwiching a portion of the diaphragm between an outer cylindrical surface of the lip  154  and the inner cylindrical surface  150  of the lower collar piece. 
     In operation, the chamber  102 A may be inflated to move the drive sleeve  120  and end effector  86  from the retracted position (FIG. 3A) to the extended position (FIG.  3 B). The chamber may be deflated, such as by applying a vacuum through the rotary union  94 , move the drive sleeve and end effector from the extended position to the retracted position. Because gravity naturally biases the end effector and drive sleeve toward the extended position, vacuum is provided for retraction. During transition between the retracted and extended positions, the lower surface  140 B of the diaphragm rolls off the cylindrical outer surface  148  of the drive sleeve, traverses the gap formed by annular space  102 , and rolls onto the cylindrical inner surface  150  of the lower collar piece. The amount of downforce applied to the end effector will be proportional to the pressure applied to the chamber. Optionally, a spring (not shown) may be provided to bias the drive sleeve toward the retracted position and, thereby, eliminate or reduce the need for applying a vacuum to retract the end effector. 
     The drive sleeve couples the end effector to the drive shaft to transmit torque and rotation from the drive shaft and downforce from the pressure chamber to the end effector shown in FIG.  4 . The end effector  80  transmits the torque, rotation, and downward force to the conditioning disk  82 . A central cylindrical projection  160  depends from the base of the drive sleeve  120  and is received by a cylindrical well  162  in a hub  164  of the end effector  80  and is secured thereto by means such as screws (not shown). A centrally-apertured annular elastomeric membrane cover  166  prevents contaminants from falling into the interior of the end effector. The cover  166  is clamped at its aperture between a horizontal shoulder  168  of the drive sleeve base and an annular surface of the top of the hub  164 , outboard of the projection  160  and well  162 . The cover  166  is also clamped at its edges between a clamp ring  198  and a disk holder  182 . In the exemplary embodiment, the cover may be formed of ethylene propylene diene terpolymer (EPDM) rubber. 
     Referring to FIGS. 5,  6  and  7 , the ring  198  includes a plurality of inner holes  200  arranged around the ring  198  and extend from an upper surface to a lower surface of the ring  198 . The holes  200  are configured to receive a plurality of screws to clamp the edges of the cover  166  between the ring  198  and the disk holder  182 . The ring  198  also includes a plurality of outer holes  202 , outboard of the inner hole  200 . The outer holes  202  extend from the upper surface to the lower surface of the ring  198 . The holes  202  are configured to receive a plurality of mechanical fasteners, e.g., securing screws  207  (see FIG.  4 ), to secure the disk holder  182  to the disk  82 . Providing the holes  202  on the ring allows for direct access to the screws  207 , so that an operator can attach or detach the disk from the disk holder without first removing the cover  166  or other components positioned above the inner part of the disk holder. 
     Each of the holes  202  includes a generally annular upper cavity  204  and a generally annular lower cavity  206  extending further inward into the ring than the upper cavity  204  (FIG.  6 ). The lower cavity has slightly greater dimensions than the head of screw  207  to contain it therein. The upper cavity  204 , however, has a smaller diameter than the head of the screw  207  to keep the screw  207  secured within the lower cavity  206 . The ring  198  further includes an orientation depression  210  which is used to properly align the ring  198  to the disk holder  182  (FIG.  7 ). In the exemplary embodiment, the ring  198  has an outer diameter of about 4¼ inches, an inner diameter of about 3½ inches, and thickness of about ⅛ of an inch. 
     Referring back to FIG. 4, a central downward facing socket  170  having a concave spherical surface portion is formed in the bottom of the hub  164 . In the illustrated embodiment, the socket is a sector comprising approximately 63.5° degrees of arc. Extending radially outward from the hub  164  are four generally flat sheet-like spokes  172  (see also FIG.  8 ), each oriented so as to have generally upper and lower surfaces. At the proximal end of each spoke, the spoke&#39;s upper surface is in contact with an annular downward facing shoulder  176  of the hub  164  radially outboard of the socket  170 . Each spoke&#39;s proximal end is secured to the hub  164  such as by rivets, screws, or other fastening means (not shown). The distal ends of the spokes are secured to a flat horizontal annular band  178 . 
     With their low profile, the spokes  172  are resiliently flexible upward and downward so as to permit tilting of the rim, relative to the axis  300  from the otherwise neutral horizontal orientation. However, the configuration of the spokes makes them substantially inflexible transverse to the axis  300 , so that they effectively transmit torque and rotation about the axis  300  from the hub  164  to the inner rim  178 . Optionally, to increase vertical flexibility without compromising lateral strength and ability to transmit torque, the spokes may each be provided with a transversely extending wave or ruffle  180 . Three to five spokes are preferred to balance torque transmission and flexibility. 
     Referring to FIGS. 4,  9  and  10 , the disk holder  182  which is rigid and generally disk-shaped is provided immediately below the spokes. In an exemplary embodiment, the disk holder  182  is formed of polyethylene terepthalate (PET). The disk holder has a central upward facing projection  184  having a convex spherical surface portion  186  of equal radius to and in sliding engagement with the concave spherical surface portion of the socket  170 . Interaction of the projection  184  and socket  170  can transmit compressive force between the drive sleeve  120  and the disk holder  182  while permitting the disk holder to rotate about axes orthogonal to the axis  300 . The disk holder  182  has a generally flat lower surface or mounting surface  188  in contact with an upper surface  83  of the disk  82 . 
     The disk holder  182  extends radially outward to a generally annular rim  194 . The rim  194  is secured to the outer periphery of the cover  166  such as by screws extending through the holes  200  of the clamp ring  198 . The screws are received by a plurality of cavities  212  provided around the rim  194  of the disk holder  182 . An upwardly protruding orientation pin  218  is provided on the rim  194 . The disk holder  182  and the ring  198  may be quickly aligned by inserting the pin  218  into the depression  210 . A plurality of holes  216  which extend to the lower surface  222  of the disk holder  182  are also provided on the rim  194 , outboard the cavities  212 . The holes  214  correspond to the holes  206  of the ring  198  and are configured to receive the screws  207  to secure the disk  82  to the disk holder  182 . The disk holder  182  further includes two indentations  220  at the edges to provide a grip for manually detaching the disk holder from the disk. 
     On the lower surface or the mounting surface, the disk holder  182  includes a plurality of drive pins  224  protruding downward at the periphery of the disk holder. In the exemplary embodiment, two drive pins  224  are provided, each on the opposing edges of the disk holder. When the disk holder  182  is mated to the disk  82 , the drive pins  224  are received by associated bores  226  on an upper surface  228  of the disk (FIG. 11) and serve to prevent rotation of the disk  82  relative to the disk holder. 
     Referring to FIG. 11, the disk  82  includes a plurality of cavities  230  corresponding to the holes  214  of the disk holder  182 . The cavities  230  receives the screws  207  inserted into the hole  214  to secure the disk  82  to the disk holder  182 . The disk  82  and the disk holder  182  are configured to receive the screws  207  at the periphery, so that the disk may be detached or attached to the disk holder removing the cover  166 . A flat lower surface  84  of the disk  82  is embedded with diamond particles to abrade and condition the polishing pad. 
     In operation, with the conditioner head located above the polishing pad as described above, the drive shaft  86  is caused to rotate, transmitting torque to the disk  82 . The end effector  80  is then shifted from the retracted position to an extended position to bring the lower surface  84  of the disk into engagement with the polishing surface  76  of the pad. The downward force compressing the disk against the pad is controlled by modulating the pressure in the pressure chamber  102 A. The downward force is transmitted through the drive sleeve, the hub, between the concave and convex spherical surface portions to the disk holder and then to the disk. Torque to rotate the disk relative to the pad is supplied from the drive shaft to the drive sleeve, the hub, the spokes, the rim of the disk holder, and then to the disk via the drive pins. The rotating disk  82  is reciprocated in a path along the rotating polishing pad. 
     The end effector  80  is configured to maintain its lower surface flat against the polishing surface of the pad even if a precise perpendicular alignment between the axis  300  and the polishing surface  76  of the pad is not provided. For this purpose, the concave and convex spherical surface portions of socket  170  and projection  184 , respectively, have a common center of curvature  304  at the intersection of a disk plane  302  (the flat lower surface  84  of the disk) with the longitudinal axis  300 . In a neutral orientation, the disk plane is perpendicular to the longitudinal axis  300  which extends through the center of the disk. 
     If the polishing surface of the pad is not perpendicular to the axis  300 , the disk, and disk holder may tilt relative to the axis via sliding of the convex spherical surface of the projection  184  relative to the concave spherical surface of the socket  170 . The hub  164  remains fixed relative to the axis  300 . To accommodate the tilt, the spokes  172  flex either upward or downward depending on their location at any given point in time. The location of the common center  304  in the disk plane  302  minimizes fluctuations in the compression force between the disk and the pad when the end effector  80  tilts to maintain engagement between the end effector and pad. The shear force applied to the disk by friction with the polishing pad is directed in the disk plane  302  and, thereby, does not exert a moment about the center  304  which would otherwise tend to pivot the disk and produce an uneven pressure distribution between the disk and pad. The cover  166  is free to flex and stretch to accommodate the tilting. 
     Referring to FIG. 12, the disk holder  182  may be used to hold a disk  82 ′ manufactured by TBW while maintaining the common center of curvature  304  at the intersection of the disk plane  302  with the axis  300 . The disk  82 ′ is generally thinner than the disk  82 , so that a generally annular, flat adapter  230  is secured between the disk holder  182  to compensate the thickness variation of the disk  82 ′. The upper surface of the adapter  230  has substantially identical configuration as with the upper surface  228  of the disk  82 , as shown in FIG.  11 . The adapter includes a plurality of holes (not shown) on its upper surface to receive the screws  207 , so that the adapter may be secured to the disk holder. The adapter also has two bores (not shown), similar to the bores  226  of the disk  82 , to receive the drive pins  224  of the disk holder. A generally annular, flat magnetic plate  240  is inserted between the adapter  230  and the disk  82 ′ to magnetically secure the disk  82 ′ to the adapter  230 , which in turn is mechanically secured to the disk holder  182 . The adapter  230  and the magnetic plate  240  are configured so that the total thickness of the adapter  230 , the magnetic plate  240  and the disk  82 ′ is substantially identical to the thickness of the disk  82 . Thus, the common center of curvature  304  is maintained at the intersection of the disk plane  302  with the axis  300 . The disk holder  182 , therefore, may carry disks of varying thickness by using complementary adapters accordingly. The operation of the conditioner head  60  using the disk  82 ′ is substantially identical to that of using the disk  82  described above. 
     Other embodiments are within the scope of the present invention. The scope of the invention is defined by the appended claims.