Abstract:
A method and an apparatus for automatically replacing a used polishing pad in a chemical mechanical polishing system are described. A controller places a mechanical device against the used polishing pad while the pad is on the polishing platen and activates a pad chucking mechanism that affixes the used pad to the mechanical device. The controller then moves the mechanical device and the pad toward a used pad receptacle, where the pad chucking mechanism is deactivated to release the used pad into the receptacle. The controller then places the mechanical device against a clean polishing pad in a clean pad dispenser and reactivates the pad chucking mechanism to affix the clean pad to the mechanical device. The mechanical device and the clean pad are moved toward the platen, where the pad chucking mechanism is deactivated to release the clean polishing pad onto the platen.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a method and apparatus for automatically changing a polishing pad in a chemical mechanical polishing system. 
     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 increasingly more non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. It will then be impossible to properly focus the light image onto the entire outer surface. 
     It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a planar surface. 
     Chemical mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier head or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing surface. A polishing slurry, including an abrasive and at least one chemically-reactive agent, is supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate. 
     Chemical mechanical polishing is a fairly complex process, and it differs from simple wet sanding. In a chemical mechanical polishing process, a reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles at the reactive sites on the substrate results in polishing. 
     Polishing pads used in a chemical mechanical polishing process must be replaced periodically to insure efficient polishing of substrates. In general, pad replacement requires a person to remove a used pad from a platen, to remove excess adhesive remaining on the platen, to place fresh adhesive over the surface of the platen, and to affix a clean polishing pad to the platen. 
     An additional consideration in the production of integrated circuits is process and product stability. To achieve a high yield, i.e., a low defect rate, each successive substrate should be polished under substantially similar conditions. Each substrate, in other words, should be polished approximately the same amount so that each integrated circuit is substantially identical. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features a method and an apparatus for automatically removing a used polishing pad from a chemical mechanical polishing (CMP) system. A mechanical device is placed against the used polishing pad on a platen in the CMP system, and the pad is chucked to the mechanical device. The mechanical device and the pad are moved toward a used pad receptacle, into which the pad is released from the mechanical device. 
     Embodiments of the invention may include the following features. A lifting mechanism, such as a pneumatic actuator, may be used to lift the used polishing pad from the platen. A vacuum pump may be used to chuck the pad to the platen. 
     In another aspect the invention features a method and an apparatus for automatically placing a polishing pad on a polishing platen in a CMP system. A mechanical device is placed against the polishing pad in a pad dispenser, and the pad is chucked to the mechanical device. The mechanical device and the pad then are moved toward the polishing platen, and the pad is released from the mechanical device onto the platen. 
     Embodiments of the invention may include the following features. The polishing pad may be chucked to the platen. A vacuum pump may be used to chuck the pad to the mechanical device or to the platen. The platen may be aligned at a predetermined orientation as the polishing pad is placed onto the platen. 
     In another aspect, the invention features a CMP apparatus having a platen adapted to hold the polishing pad, a mechanical device operable to remove the polishing pad from the platen automatically, and a pad receptacle positioned to receive the polishing pad from the mechanical device after the pad is removed from the platen. 
     In yet another aspect, the invention features a CMP apparatus having a platen adapted to hold the polishing pad, a pad dispenser adapted to house the polishing pad temporarily, and a mechanical device operable to retrieve the polishing pad from the pad dispenser and place the polishing pad onto the platen automatically. 
     Embodiments of the invention may include the following features. The platen may include a pad chucking mechanism that affixes the polishing pad to the platen. The mechanical device may include a pad chucking mechanism that affixes the polishing pad to the mechanical device. Each of the pad chucking mechanisms may include a vacuum pump. The CMP apparatus also may include a platen alignment mechanism that holds the platen at a predetermined orientation, and a pad alignment mechanism that positions the polishing pad at a predetermined orientation before it is placed onto the platen. A controller may be used to govern the operation of the mechanical device. 
     In another aspect, the invention features a method and an apparatus for replacing a used polishing pad in a chemical mechanical polishing system. A mechanical device is placed against the used polishing pad while the pad is on a polishing platen, and the pad is chucked to the mechanical device. The mechanical device and the pad then are moved toward a used pad receptacle, into which the pad is released from the mechanical device. The mechanical device then is placed against a clean polishing pad in a clean pad dispenser, and the clean pad is chucked to the mechanical device. The mechanical device and a clean pad then are moved toward the platen, onto which the clean pad is released from the mechanical device. 
     Advantages of the invention may include one or more of the following. Manual labor may be eliminated from routine replacement of polishing pads. Throughput and efficiency of a chemical mechanical polishing system may be improved, and accidental damage to clean polishing pads during pad replacement may be minimized or even eliminated. Critical alignment between the polishing pad and platen may be controlled automatically. 
     Other features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized by means of the instrumentalities and combinations particularly pointed out in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic exploded perspective view of a chemical mechanical polishing apparatus. 
     FIGS. 2 and 3 are partial perspective views of a chemical mechanical polishing apparatus with an automatic pad changing mechanism. 
     FIG. 4 is a partial cross-sectional view of a platen having an endpoint detection system also used as a platen homing device. 
     FIG. 5A is a cross-sectional view of a platen with pad lifting and homing detection capabilities. 
     FIG. 5B is a partial cross-sectional view of a platen homing sensor and a platen having a homing flag. 
     FIGS. 6A through 6G are a flow diagram of a control system for a chemical mechanical polishing apparatus with an automatic pad changing mechanism. 
     FIGS. 7 and 8 are partial perspective views of an alternative embodiment of an automatic pad changing mechanism. 
     FIGS. 9,  10 , and  11  are perspective views of an alternative embodiment of a chemical mechanical polishing apparatus with automatic pad changing capabilities. 
     FIGS. 12A,  12 B, and  12 C are cross-sectional views of an alternative embodiment of an automatic pad changing mechanism. 
     FIGS. 13A,  13 B, and  13 C are top views of a chemical polishing apparatus with automatic pad exchanging capabilities. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a chemical mechanical polishing (CMP) apparatus  30  in which the present invention may be implemented is shown. The CMP apparatus  30  includes a lower machine base  32  with a table top  33  mounted thereon and a removable upper outer cover (not shown). Table top  33  supports a series of polishing stations  35   a ,  35   b  and  35   c , and a transfer station  37 . Transfer station  37  forms a generally square arrangement with polishing stations  35   a ,  35   b  and  35   c . Transfer station  37  serves multiple functions, including receiving individual substrates  10  from a loading apparatus (not shown), washing the substrates, loading the substrates into carrier or polishing heads  80  (described below), receiving the substrates from the carriers, washing the substrates again, and finally transferring the substrates back to the loading apparatus. Additional details of the CMP apparatus  30  may be found in U.S. application Ser. No. 08/549,336, filed Oct. 27, 1995, entitled “CAROUSEL PROCESSING SYSTEM FOR CHEMICAL MECHANICAL POLISHING” and assigned to Applied Materials, Inc., which is incorporated by reference. 
     Each polishing station  35   a - 35   c  includes a rotatable platen  40  having a polishing pad  42 . A slurry  50  containing a reactive agent (e.g., deionized water for oxide polishing), abrasive particles (e.g., silicon dioxide for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) is supplied to the surface of polishing pad  42  by a slurry supply tube  52 . Sufficient slurry is provided to cover and wet the entire polishing pad  42 . Two or more intermediate washing stations  55   a  and  55   b  may be positioned between neighboring polishing stations  35   a ,  35   b  and  35   c . The washing stations rinse the substrates as they pass from one polishing station to another. 
     A rotatable multi-head carousel  60  is positioned above lower machine base  32 . Carousel  60  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly (not shown) located within base  32 . Center post  62  supports a carousel support plate  66  and a cover  68 . Multi-head carousel  60  includes four carrier head systems  70   a ,  70   b ,  70   c , and  70   d . Three of the carrier head systems receive and hold substrates and polish them by pressing them against polishing pads  42  on platens  40  of polishing stations  35   a - 35   c . One of the carrier head systems receives a substrate from and delivers the substrate to transfer station  37 . 
     The four carrier head systems  70   a - 70   d  are mounted on carousel support plate  66  at equal angular intervals about carousel axis  64 . Center post  62  allows the carousel motor to rotate the carousel support plate  66  and to orbit the carrier head systems  70   a - 70   d , and the attached substrates, about carousel axis  64 . 
     Each carrier head system  70   a - 70   d  includes a carrier or carrier head  80 . Each carrier head  80  independently rotates about its own axis and independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A carrier drive shaft  74  connects a carrier head rotation motor  76  to carrier head  80  (shown by the removal of one-quarter of cover  68 ). There is one carrier drive shaft and motor for each head. 
     Referring to FIG. 2, an automatic pad exchanging mechanism  100  is mounted to the table top  33  of the chemical mechanical polishing apparatus. The pad exchanging mechanism  100  is a robot that replaces polishing pads  42  on the platens  40  at polishing stations  35   a  and  35   b . The pad exchanging mechanism  100  places used pads in a receptacle  116  mounted to the polishing apparatus and retrieves clean pads from a dispenser  118  also mounted to the polishing apparatus. The used pad receptacle  116  and the clean pad dispenser  118  preferably are indexing cassettes, as described below. Alternatively, the “dispenser” and “receptacle” may simply be stacks of new pads and used pads, respectively. An additional pad exchanging mechanism, used pad receptacle, and clean pad dispenser are similarly mounted to the CMP apparatus between polishing stations  35   b  and  35   c  (FIG.  1 ). 
     The pad exchanging mechanism  100  consists of a movable planar manipulator  102  mounted onto a fixed base  104 . The planar manipulator  102  includes a movable arm  106  and an end effector  108  that acts as a pad chucking mechanism. The end effector  108  is a standard Venturi blade that uses suction generated by a vacuum pump to chuck the polishing pad  42 , as described below. The movable arm  106  pivots about three axes  110 ,  112 , and  114  to remove and replace polishing pads at polishing stations  35   a  and  35   b . Referring also to FIG. 3, the planar manipulator  102  as a whole rotates about the fixed base  104  along horizontal axis  120  to access the used pad receptacle  116  and the new pad dispenser  118 . Planar manipulator robots are made by several companies, including Rorze Corporation. One such planar manipulator robot is shown in Rorze documents 1VRR8140-008-101 and 1VRR8151. 
     Within the used pad receptacle  116  and the clean pad dispenser  118 , the new and used pads are tilted to an angle of 5° away from the polishing apparatus to allow gravity to hold the pads in place. As a result, the planar manipulator  102  must rotate a total of 95° around the horizontal axis  120  to retrieve clean pads from the new pad dispenser  118  and to place used pads into the used pad receptacle  116 . The clean pad dispenser  118  also includes an indexing mechanism  119  that moves the stack of pads in the dispenser forward by the thickness of one pad (generally ¼ inch) when a pad is removed from the dispenser  118 . The indexing mechanism  119  preferably includes a leadscrew and linear drive assembly  121  driven by an encoder equipped motor  123 . A similar indexing mechanism may be included in the used pad receptacle  116  to allow the used pads to move away from the CMP apparatus each time a pad is placed in the receptacle. The position of the indexing mechanism  119  is governed by a controller  175 , as described below. 
     To prevent slurry on the used pads from contaminating the end effector  108 , one or more pad lifting mechanisms  122  are built into the outer surface  126  of each platen  40 . The lifting mechanisms  122  slightly elevate the used polishing pad  42  and allow the end effector  108  to contact the bottom surface of the pad  42 . The lifting mechanisms  122  should lift the pad  42  enough to allow the end effector  108  to move freely under the pad  42 , but should not lift the pad  42  so high that it contacts the carrier head  80 . In a standard CMP apparatus, the lifting mechanisms  122  may lift a 20-inch pad  42  to a 2° tilt without causing the pad  42  to contact the carrier head  80  and still allow the end effector  108  to move freely under the pad  42 . As shown in FIG. 3, the lifting mechanisms  122  may be built into the body of each platen  40  instead of or in addition to the outer surface  126 . The construction and operation of the lifting mechanisms are described below. 
     The lifting mechanisms  122  may be eliminated altogether if the end effector  108  is a double-sided blade. In this situation, the planar manipulator  102  removes a used pad by applying suction to the top surface of the pad and lifting the pad away from the platen  40 . The planar manipulator  102  places a new pad on the platen by applying suction to the pad&#39;s lower surface and laying the pad on the platen. Because the pads are flexible, the new pad can be held to the platen  40  by activating the platen&#39;s pad chucking mechanism before or as soon as the pad makes contact with the platen  40 . 
     Some chemical mechanical polishing systems include an endpoint detector alignment feature, such as described in U.S. patent application Ser. No. 08/605,769, filed Feb. 22, 1996, entitled “APPARATUS AND METHOD FOR INSITU ENDPOINT DETECTION FOR CHEMICAL MECHANICAL POLISHING” and assigned to Applied Materials, Inc., which is incorporated by reference. In these systems, the pad exchanging mechanism  100  must align an opening  136  (FIG. 3) in each platen  40  with a transparent “window”  132  (FIG. 2) in each polishing pad  42  to allow operation of the endpoint detection system in the platen. To insure that the platens  40  and the pads  42  are aligned, each platen  40  includes a homing flag  130  that is detected by a homing sensor  128  mounted to the table top  33 . When the homing sensor  128  detects the homing flag  130 , the homing sensor  128  sends a signal to the controller  175  that instructs the controller  175  to stop the rotation of the platen  40 , as described below. Each polishing pad  42  includes a notch or groove  134  on its outer edge  140  that fits around a corresponding ridge  142  in the new pad dispenser  118  to properly align the pads in the dispenser. 
     Referring to FIG. 4, the endpoint detection system  330  in the platen  40  may be used instead of the homing sensor and flag to align the platen  40  and the pad  42 . The endpoint detection system  330  includes a laser source  332  that projects a laser beam  334  directly upward through the opening  136  in the platen  40 . The laser beam passes through a 45° beam splitter  336  and encounters the pad  42  when the pad  42  is on the platen  40 . Whether light is transmitted through the pad  42  or reflected back into the platen  40  depends upon the orientation of the pad  42 . 
     When the pad  42  is properly aligned, most of the light in the laser beam  334  passes through the transparent window  132 , but some of the light reflects from the window  132  back into the platen  40 . When the pad  42  is above the platen  40  but is not yet properly aligned, most of the light in the laser beam reflects back into the platen  40 . 
     The beam splitter  336  redirects the portion  338  of the laser beam  334  that is reflected back into the platen  40  toward a receiver  340 . The receiver  340  is an intensity threshold detector that determines whether the reflected light  338  has relatively high, relatively low, or no intensity. When the reflected light  338  has relatively low intensity, the pad  42  is properly aligned and the receiver  340  generates a signal that instructs the controller to halt the rotation of the platen  40 . Otherwise, the receiver  340  sends no signal. 
     All motors and pumps used in the automatic pad exchange system are controlled by the control system  175 . The controller system  175  may be a single control unit, or it may comprise multiple control units. Preferably, the control system  175  includes a programmable controller, such as a microprocessor running a program code. The operation of the control system  175  during the pad changing process is described below. 
     Referring to FIGS. 5A and 5B, each platen  40  includes a vacuum driven pad chucking mechanism  164 . The pad chucking mechanism  164  includes a hollow chamber  165  in the platen that opens into passageways  166  and  167 , each of which leads to multiple openings  168  and  169  in the top surface  170  of the platen  40 . The pad chucking mechanism  164  is coupled to a vacuum device  171 , such as an air pump, through a stationary conduit  172 . The connection between the stationary conduit  172  and the rotating platen  40  is a standard rotary union coupling sealed by bearings  173 . The vacuum device  171  is controlled by controller  175 . Vacuum driven pad chucking mechanisms are described in a U.S. patent application entitled “HOLDING A POLISHING PAD ON A PLATEN IN A CHEMICAL MECHANICAL POLISHING SYSTEM,” filed on Jul. 12, 1996, with Express Mail Label No. TB888889881US and assigned to Applied Materials, Inc., which is incorporated by reference. 
     The platen  40  also includes a system  174  that drives the pad lifting mechanisms  122 . This system  174  includes a hollow chamber  176  surrounding the hollow chamber  165  of the pad chucking mechanism  164 . Passageways  177  and  178  connect the hollow chamber  176  to the lifting mechanisms  122 . The lifting mechanisms  122  are activated by a pressure source  179 , such as a pneumatic pump, that is coupled to the hollow chamber  176  through a stationary conduit  180 . The connection between the hollow chamber  176  and the stationary conduit  180  is a standard rotary union coupling sealed by bearings  181 . Like the vacuum source  171 , the pressure source  179  is controlled by the controller  175 . 
     Alternatively, both the vacuum source  171  and the pneumatic pressure source  179  could be coupled to one hollow chamber  165  and could apply suction pressure and pneumatic pressure to the openings  168  and  169  and to the pad lifting mechanisms  122 , respectively, through the same passageways  166  and  167 . In this embodiment, the passageways  166  and  167  would deliver suction pressure to the pad when the vacuum source  171  is activated and would deliver pneumatic pressure to the lifting mechanisms  122  when the pneumatic pressure source  179  is activated. 
     As noted above, the platen  40  may include a homing signal flag  130 , which is a tab that protrudes from the outer surface  126  of the platen  40 . The homing sensor  128  mounted to the baseplate  125  of the CMP system&#39;s table top  33  detects the homing flag  130  and instructs the controller  175  to stop the rotating platen  40 . The homing sensor  128 , when activated by the controller, is an optical coupler  320 , or “optoisolator,” that provides a continuous signal to the controller that is broken only when the homing flag  130  passes through the optoisolator  320 . The controller  175  halts the platen&#39;s rotation when the signal is broken. Alternatively, the homing sensor  128  may be a capacitive or an inductive device that generates a pulse when the homing flag  120  passes by. 
     Referring to FIGS. 6A through 6F and again to FIG. 2, the control system  175  monitors the number of polishing cycles that the pads have undergone since the last pad exchange. In a CMP system using fixed abrasive pads and running at an optimal polishing rate of approximately sixty wafers per hour per pad, the pads should be changed once every thirty minutes (after approximately thirty polishing cycles). Conventional non-fixed-abrasive pads that are conditioned periodically will be changed less often that fixed abrasive pads. 
     The control system  175  continuously watches (step  400 ) for a predetermined number (e.g., thirty) of polishing cycles to occur. When the predetermined number of polishing cycles have occurred, the control system  175  slows (step  402 ) the platens and waits (step  404 ) for a signal from each homing sensor  128 . When the control system  175  receives the signal from a homing sensor  128 , the control system  175  immediately stops (step  406 ) the encoder-equipped motor driving the corresponding platen. The control system  175  then deactivates (step  407 ) the platen&#39;s pad chucking mechanism and activates (step  408 ) the pad lifting mechanisms  122  for the center platen, if pad lifting mechanisms  122  are used in the system. The controller begins moving (step  410 ) the planar manipulator  102  toward the pad on the center platen and then watches (step  412 ) for the planar manipulator to reach the pad. When the planar manipulator reaches the pad, the control system  175  activates (step  414 ) the chucking mechanism on the end effector  108  to secure the pad to the end effector  108 . If the system includes pad lifting mechanisms, the controller places the planar manipulator under the pad and activates a chucking mechanism on the upper surface of the end effector. If the system does not include pad lifting mechanisms, the controller places the planar manipulator on the upper surface of the pad and activates a chucking mechanism on the lower surface of the end effector. The control system  175  then moves (step  416 ) the planar manipulator and the pad toward the used pad receptacle  116 . The controller  175  monitors the planar manipulator to determine (step  418 ) when it reaches the used pad receptacle  116  and, when it does, the controller deactivates (step  420 ) the chucking mechanism and releases (step  422 ) the pad into the used pad receptacle  116 . At the same time, the control system  175  activates (step  424 ) the motor that drives the indexing mechanism in the used pad receptacle  116 . 
     Once the used pad has been placed in the receptacle, the control system  175  begins to move (step  426 ) the planar manipulator toward the platen at polishing station  35   b  and activates (step  428 ) the lifting mechanisms  122 , if any, to lift the corresponding pad. The control system  175  waits (step  430 ) for the planar manipulator  102  to reach the pad and then activates (step  432 ) the chucking mechanism on the end effector  108  to secure the pad. The controller then moves (step  434 ) the pad toward the receptacle  116  and waits (step  436 ) for the pad to reach the receptacle  116 . When the planar manipulator  102  reaches the receptacle  116 , the control system  175  deactivates (step  438 ) the chucking mechanism, places (step  440 ) the pad in the receptacle  116 , and activates (step  442 ) the indexing mechanism in the receptacle  116 . 
     The control system  175  then moves (step  444 ) the planar manipulator  102  to the new pad dispenser  118  and activates (step  446 ) the chucking mechanism to secure a new pad to the end effector  108 . In both systems with and without pad lifting mechanisms, the upper surface of the end effector is placed against the lower surface of the pad. The control system  175  activates (step  448 ) the indexing mechanism in the new pad dispenser  118  to reposition the pads in the dispenser  118  and moves (step  450 ) the planar manipulator and the new pad toward the center platen. The controller then waits (step  452 ) for the pad to reach the platen, and when it does, the control system  175  deactivates (step  454 ) the chucking mechanism on the end effector and activates (step  456 ) the chucking mechanism on the platen. The planar manipulator then moves (step  458 ) again to the new pad dispenser  118 , activates (step  460 ) the chucking mechanism to secure another new pad, and activates (step  462 ) the indexing mechanism in the new pad dispenser  118 . The control system  175  then moves (step  464 ) the new pad toward the platen at polishing station  35   b  and waits (step  466 ) for the pad to reach the platen. When the pad reaches the platen, the controller deactivates (step  468 ) the chucking mechanism on the end effector and activates (step  470 ) the chucking mechanism on the platen. The planar manipulator  102  then returns (step  472 ) to its normal position, and the control system  175  instructs (step  474 ) the platen motors to begin the polishing process again. 
     While the pad changing mechanism  100  changes the pads at polishing stations  35   a  and  35   b , the other pad changing mechanism (not shown) changes the pad at polishing station  35   c  in similar fashion, also under control of control system  175 . The control system instead may be configured to replace each pad immediately after it is removed from the CMP apparatus and before the next pad is removed. The control system also may cause the planar manipulator to tilt the used pad immediately after it secures the pad to insure that slurry drips onto the baseplate of the CMP system and not onto other portions of the CMP apparatus. 
     Referring to FIGS. 7 and 8, an alternative embodiment of the pad exchanging mechanism  100  is a rack-and-pinion mechanism  200  mounted to the table top  33  of the CMP apparatus. The rack-and-pinion mechanism  200  rotates about a horizontal axis  202  to move pads  206  between the polishing platen  40  and two indexing cassettes  208  and  210  mounted to the apparatus. A planar manipulator  204  connected to the rack-and-pinion mechanism  200  rotates about axis  207  to grip the pads  206  on the platen  40  and in the indexing cassettes  208  and  210 . Three lifting mechanisms  212  in each platen  40  lift a used pad  218  from the platen  40  and then lower the pad  218  onto the planar manipulator  204  after the planar manipulator  204  has moved into place. The lifting mechanisms  212  preferably are pneumatically actuated, as described above. Also as discussed above, the planar manipulator  204  includes a standard Venturi blade end effector  220  that uses suction to secure the pad  218 . 
     Referring to FIGS. 9 and 10, in an alternative embodiment, the CMP apparatus  30  includes two movable indexing cassettes  250  and  252  mounted to the CMP apparatus  30  by leadscrew and linear guide assemblies  254 . The leadscrew  256  in each assembly is driven by a motor  258  mounted to the corresponding indexing cassette  250 . The leadscrews  256  are fully extended (FIG. 9) to expose polishing pads in the cassettes when the polishing pads are being replaced, and are fully retracted (FIG. 10) during the polishing process. 
     Referring also to FIG. 11, polishing pads  260  in the indexing cassettes  250  and  252  are placed onto a center platen  266  of the CMP apparatus  30  by “robots”  262  and  264  in the indexing cassettes  250  and  252 , respectively. Each robot  262  and  264  includes an end effector  272  and  274 , respectively, that uses suction to secure the pads  260  and carry them from the indexing cassettes  250  and  252  to the center platen  266 . Each time a pad is removed from one of the cassettes, an indexing mechanism  265  in the cassette moves the next pad in the cassette forward to be placed on the center platen  266 . Each robot  262  and  264  moves vertically in the corresponding indexing cassette  250  along a pair of linear tracks  276  and  278  (only one linear track is shown in each indexing cassette). 
     Referring also to FIGS. 12A,  12 B, and  12 C, the robot  262  in cassette  250  is positioned so that its end effector  272  contacts the rear surface  280  of the pad  260 . Suction then is applied through the end effector  272 , and the robot  262  moves down the linear track  276  toward the platen  266 . At the same time, the end effector  272  begins to pivot upward by 90° so that the pad  260  is held adjacent and parallel to the center platen  266 . The robot then lays the pad  260  onto several lift mechanisms  282  protruding from the platen  266 , as discussed above, and retracts back into the indexing cassette  250 . The pad  260  is affixed to the surface of the platen  266  by suction as the lifting mechanisms  282  retract into the platen  266 . 
     Two additional robots  284  and  286  are mounted to the CMP apparatus by linear tracking mechanisms  288  and  290 . Each of these robots  284  and  286  carries new polishing pads placed on the center platen  266  by robots  262  and  264  from the center platen  266  to the outer platens  268  and  270 , respectively. These robots  284  and  286  also remove used pads from the platens  266 ,  268 , and  270  and place them in used pad receptacles (not shown) adjacent the CMP apparatus  30 . The linear tracking mechanisms  288  and  290  are driven by motors  292  and  294  controlled by an electronic controller  296 . The robots  262  and  264  in the indexing cassettes also are driven by motors (not shown) controlled by the controller and are operated as discussed above. The robots  284  and  286  preferably include Venturi blade end effectors  285  and  287 , respectively, that apply suction to the lower surface of the pads after the pads are lifted by pad lifting mechanisms  282 . 
     Referring to FIGS. 13A,  13 B, and  13 C, after robot  284  has secured a new or used polishing pad  298  to its end effector  300 , the robot  284  must rotate clockwise and move linearly away from the destination platen  302  (i.e., in the direction of arrow  304 ) to insure that the pad  298  does not contact the center post  62  of the CMP apparatus  30 . The robot  284  then moves linearly toward the destination platen  302  (i.e., in the direction of arrow  306 ) after the pad  298  has cleared the center post  62 . As the robot  284  approaches the destination platen  302 , the robot rotates counter-clockwise to position the pad  298  correctly over the platen  302 . Robot  284  moves in this manner both to move new pads from center platen  302  to outer platen  298  and to move used pads from outer platen  298  to center platen  302 . The other robot  286  of FIG. 11 must move in the same manner to avoid collisions between the pads and the center post  62 . 
     The present invention has been described in terms of one or more preferred embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the following claims.