Abstract:
A wafer polishing tool is disclosed which includes a polishing platen which is rotatable about a central platen axis, and a wafer carrier which supports a wafer for rotational movement to cause a portion of a surface of the wafer to only intermittently contact a polishing surface of the platen while the wafer rotates. The polishing tool may include a plurality of vertically stacked polishing platens which are rotatable about a central platen axis, and a plurality of stacked wafer carriers, wherein each carrier supports a wafer for rotational movement and vertical movement into contact with one of the polishing platens. During polishing, the carrier pack maintains the wafers in uninterrupted contact with the platen over less than entire surfaces of the wafers.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to equipment used in fabricating semiconductor devices, and more specifically to equipment for performing chemical mechanical polishing (CMP) of semiconductor wafers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Chemical mechanical polishing (CMP) has become an indispensable step in the fabrication of integrated circuit (IC) devices. In some steps of the fabrication process of ICs, later layers cannot be applied to a semiconductor substrate unless an earlier applied layer presents a planar surface. A CMP process is used to planarize such layers. In a step of the fabrication process, it may be desired to obtain a planar surface on an oxide layer. Alternatively, in a different step in fabricating a semiconductor device, a conformal layer of metal is deposited in blanket manner over a dielectric layer to fill vias therein. Then, by a CMP process, the blanket metal layer is polished down to the surface of the dielectric layer. In such a CMP process, it is important for the removal rates of the metal and dielectric materials to be dissimilar in order that polishing stops at the dielectric so that the metal inside the vias does not become overly “dished”, i.e. overly removed below the upper surface of the dielectric layer, nor does the dielectric layer become overly thinned, either of which may lead to failure at a later time.  
           [0003]    Conventionally, the chemical composition of the slurry is selected in order to adjust a removal rate according to the composition of a specific layer and features therein to be planarized. Apart from the chemical composition of the slurry provided to the CMP tool, two mechanical parameters play a critical role in determining the removal rate. These are the rotational velocity between the wafer and the polishing pad, and the downforce applied to press the wafer against the polishing pad. An increase in either the rotational velocity or the downforce results in a higher removal rate. Conversely, a decrease in the rotational velocity or the downforce results in a lower removal rate.  
           [0004]    Currently available CMP polishers process only one or at most a few wafers at one time. The number of wafers which can be polished at one time is limited because conventional CMP polishers require the entire surface of each wafer to be placed in contact with the polishing pad. At current 200 mm wafer diameters, some existing CMP tools polish at most two wafers concurrently. A very large CMP polisher can polish as many as five 200 mm wafers on a single large disc-shaped polishing pad at one time.  
           [0005]    With reference to FIG. 14, CMP is conventionally performed on equipment having a large rotating disc-shaped platen  118  of approximately 60 cm in diameter. A wafer  114  is held face down by a carrier  116  on a pad  119  covering the rotating platen  118 . The wafer  114  is positioned between the outer disc perimeter  121  and an inner circle  123  at a set radius R from the center  125 . Because the rotational velocity of the platen  118  is higher near the outer perimeter  121  of the platen than at the inner circle  123 , the wafer is rotated during polishing in order to reduce position-dependent velocity variations which could result in polishing rate differentials at different locations on the wafer surface. However, despite this practice, a difference in rotational velocity still remains between the outer perimeter of the wafer and points near the wafer center. Consequently, a consistent polishing rate is not achieved between the outer perimeter and the interior surface of the wafer.  
           [0006]    Because of this difference in rotational velocity at different wafer locations, it is considered undesirable to perform CMP at rotational velocities greater than 140 rpm. The rotational velocity of the disc platen in conventional CMP polishers is generally kept within a range between 10 and 140 rpm.  
           [0007]    At conventional platen rotational velocities of 10 to 140 rpm, a force of at least 5 and up to 9 psi must be applied by a wafer carrier  16  to press the wafer towards the platen  118  (“downforce”) in order to perform CMP to attain even a marginal wafer processing rate. The application of a downforce of 5 to 9 psi is not uncommon to achieve desirable process throughput. A known consequence of high downforce at the wafer/platen interface is a tendency for differentials in the removal rates of different composition features to increase. Higher downforce results in increased dishing of metal features within an oxide layer, and ultimately reduced planarity when polishing layers which contain features of different composition or pattern density.  
           [0008]    Wafer throughput is one measure of the desirability of a CMP tool. There are other measures too. Optimally, CMP tools should be inexpensive to own and operate, occupy little space in a semiconductor foundry, polish to adequate and consistent local planarity, as well as global uniformity, and provide high and consistent throughput.  
           [0009]    Existing CMP polishers are larger and more expensive than necessary and provide much lower throughput than that which is made possible by the multi-level polishing tool of the present invention disclosed in the following.  
           [0010]    It is therefore an object of the present invention to provide a CMP polisher which provides greater throughput than existing CMP polishers.  
           [0011]    A further object of the present invention is to provide a CMP polisher which is smaller than existing CMP polishers.  
           [0012]    Another object of the present invention is to provide a CMP polisher which is less expensive to own and operate than existing CMP polishers.  
           [0013]    Still another object of the present invention is to provide a CMP polisher which processes wafers in a consistent, quality manner.  
           [0014]    Another object of the present invention is to provide a CMP polisher which polishes to superior planarity than that provided by existing CMP polishers.  
           [0015]    An additional object of the present invention is to provide a fully integrated CMP polisher which performs in-situ post measurements and endpoint detection as well as wafer clean and dry operations.  
         SUMMARY OF THE INVENTION  
         [0016]    These and other objects of the invention are provided by the wafer polishing tool of the present invention. In a first aspect of the invention, the wafer polishing tool includes a polishing platen which is rotatable about a central platen axis, and a wafer carrier which supports a wafer for rotational movement to cause a portion of a surface of the wafer to only intermittently contact a polishing surface of the platen while the wafer rotates.  
           [0017]    According to a second aspect of the invention, a wafer polishing tool is provided which includes a polishing platen which is rotatable about a central platen axis, and a wafer carrier which supports a wafer for rotational movement and in uninterrupted contact with the platen over less than the entire surface of the wafer.  
           [0018]    According to another aspect of the invention, a wafer polishing tool is provided which includes a plurality of vertically stacked polishing platens which are rotatable about a central platen axis, and a plurality of stacked wafer carriers, wherein each carrier supports a wafer for rotational movement and vertical movement into contact with one of the polishing platens.  
           [0019]    According to another aspect of the invention, a wafer polishing tool includes a plurality of vertically stacked polishing platens which are rotatable about a central platen axis, and a wafer carrier pack which imparts rotational motion to a plurality of wafers, wherein the carrier pack maintains the wafers in uninterrupted contact with the platen over less than entire surfaces of the wafers.  
           [0020]    Further preferred embodiments of the invention are disclosed herein.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a top view of the wafer polishing tool of the present invention showing a platen assembly  10  which is flanked by left and right wafer carrier packs  12 .  
         [0022]    [0022]FIG. 2 is a side view of the wafer polishing tool of the present invention.  
         [0023]    [0023]FIG. 3 is a detailed view of a mechanism constructed according to a first embodiment of the invention for applying upward and rotational forces to wafers to bring them into polishing relation with polishing pads  16  of platen assembly  10 .  
         [0024]    [0024]FIGS. 4A and 4B show respective top and side views of a housing  80  containing motors which drive the central platen assembly  10  and left and right carrier packs  12 .  
         [0025]    [0025]FIG. 5 is a top view of a wafer carrier pack and polishing platen assembly showing a first, pivotable alternative engaging mechanism.  
         [0026]    [0026]FIG. 6 is a top view of a wafer carrier pack and polishing platen assembly showing a second, slidable, alternative engaging mechanism.  
         [0027]    [0027]FIG. 7 is a side view of a three-level wafer carrier pack in an embodiment in which a drive shaft and drive pulleys are used to impart wafer rotation.  
         [0028]    [0028]FIG. 8 is a top view of a wafer carrier  22  used in the embodiment shown in FIG. 7.  
         [0029]    [0029]FIG. 9 is a cross-section of the view in FIG. 8 through lines  9 ′- 9 ′.  
         [0030]    [0030]FIG. 10 is a close-up of the view shown in FIG. 9.  
         [0031]    [0031]FIG. 11 is a top view of a wafer carrier  22 , including top and bottom base members  24   a,    24   b,  which is used in an embodiment in which a drive gear is used to impart wafer rotation.  
         [0032]    [0032]FIG. 12 is a cross-section of the view in FIG. 11 through lines  12 ′- 12 ′.  
         [0033]    [0033]FIG. 13 is a close-up of the view in FIG. 12.  
         [0034]    [0034]FIG. 14 is a perspective drawing of a prior art CMP polisher.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    [0035]FIG. 1 is a top view of the wafer polishing tool of the present invention showing a platen assembly  10  which is flanked by left and right wafer carrier packs  12 . With reference to a side view thereof in FIG. 2, the platen assembly  10  includes a plurality of polishing platens  14  to the underside of each a polishing pad  16  is attached. Each polishing platen is a hard flat disc having a central opening which engages a central driveshaft  18  of platen assembly  10 . A platen  14  is held at a fixable vertical spacing relative to other platens  14  by one or more spacers  20  which fit over the central shaft  18 . Platens are preferably substantially rigid and constructed with sufficient mass relative to shaft  18  and spacer ( 20 ) to provide rotational inertia for stabilizing rotational motion. A stable system which is capable of rotating the platens at speeds at several hundred to a few thousand revolutions per minute with good inertial characteristics is all that is required. One such rotational system that has been studied in the development of this invention is the Model No. 3380 multi-disk direct access storage device (DASD) drive manufactured by IBM.  
         [0036]    With reference to FIG. 2, wafer carrier packs  12  each include a plurality of wafer carriers  22 . Each wafer carrier  22  includes a base  24  which supports the wafer and has internal components to be described below which cause or permit the wafer to rotate. Each wafer carrier  22  includes a ring  26  which encloses the outer perimeter of a wafer over a majority of the wafer circumference, in order to hold the wafer in position notwithstanding the rotation of the wafer and the platen  14 . Ends  28  of rings  26 , as shown in FIG. 1, are preferably located at positions slightly to the same side of the center of the wafer bed  38  which is enclosed by the ring  26 . Referring to FIG. 1, carrier packs  12  are movable along fixed rails  68  towards and away from platen assembly  10 . During polishing, carrier packs  12  oscillate along rails  68  such that the surface of each wafer is polished for substantially the same amount of time regardless of the particular location on the wafer surface.  
         [0037]    As further shown in FIG. 1, there are optical endpoint detection mechanisms  21 , strobe lights  23 , and cleaning brushes  25  located above each wafer of the carrier pack  12 . The purpose of the optical measurement and endpoint detection mechanism  21  is to permit in situ endpoint detection while the wafer is engaged in a wafer carrier  22  or even during polishing. Strobe lights  23  fix an image of a moving wafer in position for capture by an imaging lens within optical measurement and endpoint detection mechanism  21 . Measurement, detection mechanism  21  can then accurately gauge the stage of the polishing process and provide data for feedback to the operator and/or automated control over the polishing process. Brushes  25  are preferably driven opposite a direction of wafer rotation in order to maximize cleaning effect.  
         [0038]    [0038]FIG. 3 is a detailed view of a mechanism constructed according to a first embodiment of the invention for applying upward and rotational forces to wafers to bring them into polishing relation with polishing pads  16  of platen assembly  10 . Upward movement of wafer carriers  22  is imparted by a vertical lifting force applied at lifting sleeves  29 . Lifting sleeves  29  are linked to each other at wafer carrier  22  nearest base  32  and to a lift shaft  33  which, in turn, is vertically moved, preferably by a voice coil motor  88  (FIG. 4B) which allows for precise control over the amount and timing of vertical force applied. Lifting sleeves  29  enclose support shafts  31  and vertically carry the lifting force to higher placed wafer carriers  22  within carrier pack  12 .  
         [0039]    As further shown in FIG. 3, a carrier assembly  12  is provided with a driveshaft  30  which extends from a base  32  of the carrier assembly  12  through a plurality of wafer carriers  22  to a top  34  of the carrier assembly  12 . Driveshaft  30  is provided with a plurality of drive gears  36 , each of which is positioned to engage a secondary drive gear  42  coupled to a wafer carrier  22 .  
         [0040]    [0040]FIG. 11 is a top view of a wafer carrier  22 , including top and bottom base members  24   a,    24   b,  wafer bed  38 , secondary drive gear  42  and guide gears  46 . Wafer carrier  22  is engageable to receive a rotational force through secondary drive gear  42  from drive gear  36  secured to drive shaft  30  of wafer carrier pack  12 . Rotation of secondary drive gear  42 , in turn, causes gear  40  secured to wafer bed  38  to rotate. Guide gears  46  are provided along a periphery of gear  40  to guide the motion of wafer bed  38  in response to secondary drive gear  42 .  
         [0041]    Referring to FIG. 11, wafer bed  38  and gear  40  engaged thereto are held in place laterally by guide gears  46 . FIG. 12 is a cross-section of the view in FIG. 11 through lines  12 ′ - 12 ′. FIG. 13 is a close-up of the view in FIG. 12. As shown in FIGS.  12 - 13 , for guiding wafer beds  38 , ball bearings  44  are provided, preferably, within fixed concavities  48  within top and bottom base members  24   a,    24   b  of base  24 . Ball bearings  44  ride within a groove (not shown) located within wafer bed  38 . Alternatively, a race (not shown) housing a set of ball bearings  44  can be secured within corresponding grooves in top member  24   a  and wafer bed  38 , with a second race of ball bearings  44  secured within corresponding grooves in bottom member  24   b  and wafer bed  38 .  
         [0042]    [0042]FIGS. 4A and 4B show respective top and side views of a housing  80  containing motors which drive the central platen assembly  10  and left and right carrier packs  12 . As shown in FIG. 4A, housing  80  contains a primary motor  82  which, by a belt, drives a pulley  84  which is fastened to platen drive shaft  18 . Wafer carrier drive motors  86  which impart rotational force are also shown in approximate positions, as well as voice coil motors  88 , which impart a lifting force to wafer carriers  22 , as described in the foregoing.  
         [0043]    An alternative to the rotational drive mechanism shown and described in the foregoing with respect to FIGS. 11 through 13 will now be described, with respect to FIGS. 7 through 10. In this embodiment, the vertical lift mechanism is substantially the same as that shown with respect to FIGS. 3 and 11 through  13  and need not be described further. FIG. 7 is a side view of a three-level wafer carrier pack having a drive shaft  31  and drive pulleys  52  secured thereto under the base  24  for each of three wafer carriers  22 . Drive pulleys  52  are each linked by a drive belt  56  to a wafer bed pulley  54  secured to a wafer bed  38  of a wafer carrier  22 .  
         [0044]    [0044]FIG. 8 is a top view of a wafer carrier  22  for this embodiment of the drive mechanism, including top and bottom base members  24   a,    24   b,  wafer bed  38  and guide rollers  58 . Wafer bed  38  is caused to rotate by a wafer bed pulley  54  secured thereto. Guide rollers  58  provided along a periphery of wafer bed  38  guide the motion of wafer bed  38  in response to rotation of wafer bed pulley  54 .  
         [0045]    [0045]FIG. 9 is a cross-section of the view in FIG. 8 through lines  9 ′- 9 ′. FIG. 10 is a close-up of the view shown in FIG. 9. As in the embodiment described with respect to FIGS.  11 - 13 , ball bearings  44  are provided to guide the rotation of the wafer beds  38 . However, ball bearings  44  are preferably provided in concavities  60  located at asymmetric positions within top and bottom base members  24   a,    24   b  of base  24 . In this manner, forces are more evenly distributed over the circumference of wafer bed  38 , which may make fabrication of the required hardware simpler and/or if fewer bearings are used, can reduce mass along the periphery of the wafer bed  38  and thereby increase rotational stability.  
         [0046]    [0046]FIGS. 5 and 6 show respective embodiments of engaging mechanisms which bring wafer carrier packs  12  into position with platen assembly  10  so that wafers can be polished. FIG. 5 shows the relationship of carrier pack  12  to platen assembly  10  in an embodiment in which carrier pack  12  pivots with respect to a fixed pin  62  generally along an arc  64  towards and away from platen assembly  10 . In this manner, once wafers are loaded into carrier pack  12 , the entire carrier pack  12  is pivoted into position for polishing of individual wafers by respective platens  14 . During polishing, carrier pack  12  oscillates slightly around its pivot point to provide even polishing of the entire wafer surface, as in the embodiment described in the foregoing with reference to FIG. 1.  
         [0047]    [0047]FIG. 6 shows the relationship of carrier pack  12  to platen assembly  10  in which carrier pack  12  is movable along fixed rails  68  towards and away from platen assembly  10 . In this embodiment, carrier pack  12  includes a plurality of rail guides  70  which maintain carrier pack  12  in a fixed relation along axis  72 . Once wafers are loaded into the carrier pack  12  as shown in FIG. 6, the entire carrier pack  12  is moved along rails  68  into position for polishing of individual wafers by respective platens  14 .  
         [0048]    In operation, the wafer carrier pack  12  is disengaged from platen assembly  10  by movement along rails  68  (FIG. 6) or about pivot shaft  62 . Wafers are then loaded onto carriers  22  of the carrier pack  12  by hand or by automated means. A preferred automated loader includes a robot which has multiple pairs of wafer “pencils” (the digits of the robotic hand), each pair of which clutches a wafer so that several wafers are loaded onto the polisher with one sweeping movement of the robotic arm from the wafer cassette to the carriers  22 . Alternatively, wafers may be picked up and held by vacuum by vacuum fingers and then deposited by the robot into wafer carriers  22 .  
         [0049]    After the wafers have been loaded, wafer carrier packs  12  are then slid (FIG. 6) or pivoted (FIG. 5) into and engaged position (FIGS. 1, 2) with respect to platen assembly. Rotational motion is imparted to platens  14  and to wafer beds  38  through respective drive motors  82 ,  86  and wafer carriers are then lifted into polishing position by vertical drive motors  88  linked to lifting sleeves  29  coupled to wafer carriers  22 . By appropriate signals provided to vertical drive motors, which are preferably voice coil motors, the wafer to platen polishing pressure is finely controlled and can be increased, reduced or cycled during polishing through different levels to meet the particular polishing objective. In addition, a feedback signal from a force transducer secured to a wafer carrier can be provided to the voice coil motor to more finely control the vertical force applied thereto.  
         [0050]    Because the rotational drive mechanism of the present invention permits wafer to platen rotational speeds which are in the hundreds to thousands of revolutions per minute (rpm) and are much greater than heretofore, the wafer to polishing pressure can be vastly reduced while still preserving desirable removal rates. In this manner, greater planarity and much less dishing are achieved during polishing.  
         [0051]    During polishing, a polishing slurry is applied to the wafer or, alternatively, to the underside of polishing pad  16  through a porous (e.g sponge-like) applicator which engages platen assembly  10 . Brushes  25  remove abrasive materials from wafers during polishing to reduce scratching and provide better control over the polishing. To provide polishing uniformity across the wafer surface, oscillating motion towards and away from platen assembly  10  is provided in the direction of rails  68  (FIG. 6) or about the pivot shaft  62  (FIG. 5).  
         [0052]    While carrier pack  12  is engaged to platen assembly  10  or during polishing, measurement and detection systems  21 , with aid of strobe lights  23  provided above the wafer surfaces, provide real-time measurements for monitoring or endpoint detection purposes. Rather than relying on guesswork or samples, an endpoint detection signal is provided directly from the wafer being polished at the time that the wafer polishing is being performed.  
         [0053]    While the invention has been described herein in accordance with certain preferred embodiments thereof, those skilled in the art will recognize the many modifications and enhancements which can be made without departing from the true scope and spirit of the invention set forth in the appended claims.