Abstract:
Grooves are formed in a COD pad by positioning the pad on a supporting surface with a working surface of the pad in spaced relation opposite to a router bit and at least one projecting stop member adjacent to the router bit, an outer end portion of the bit projecting beyond the stop. When the bit is rotated, relative axial movement between the bit and the pad causes the outer end portion of the bit to cut an initial recess in the pad. Relative lateral movement between the rotating bit and the pad then forms a groove which extends laterally away from the recess and has a depth substantially the same as that of the recess. The depths of the initial recess and the groove are limited by applying a vacuum to the working surface of the pad to keep it in contact with the stop member(s). Different lateral movements between the bit and the pad are used to form a variety of groove patterns, the depths of which are precisely controlled by the stop member(s).

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of making polishing pads, and more specifically to providing macrotextured surfaces on polishing pads used in the chemical-mechanical planarization (CMP) of semiconductor substrates. 
     BACKGROUND OF THE INVENTION 
     Chemical-mechanical polishing has been used for many years as a technique for polishing optical lenses and semiconductor wafers. More recently, chemical-mechanical polishing has been developed as a means for planarizing intermetal dielectric layers of silicon dioxide and for removing portions of conductive layers within integrated circuit devices as they are fabricated on various substrates. For example, a silicon dioxide layer may cover a metal interconnect conformably such that the upper surface of the silicon dioxide layer is characterized by a series of non-planar steps corresponding in height and width to the underlying metal interconnects. 
     The step height variations in the upper surface of the intermetal dielectric layer have several undesirable characteristics. Such non-planar dielectric surfaces may interfere with the optical resolution of subsequent photolithographic processing steps, making it extremely difficult to print high resolution lines. Another problem involves the step created in the coverage of a second metal layer over the intermetal dielectric layer. If the step height is relatively large, the metal coverage may be incomplete such that open circuits may be formed in the second metal layer. 
     To combat these problems, various techniques have been developed to planarize the upper surface of the intermetal dielectric layer. One such approach is to employ abrasive polishing to remove the protruding steps along the upper surface of the dielectric layer. According to this method, a silicon substrate wafer is mounted face down beneath a carrier and pressed between the carrier and a table or platen covered with a polishing pad that is continuously coated with a slurried abrasive material. 
     Means are also provided for depositing the abrasive slurry on the upper surface of the pad and for forcibly pressing the substrate wafer against the polishing pad, such that movement of the platen and the substrate wafer relative to each other in the presence of the slurry results in planarization of the contacted face of the wafer. Both the wafer and the table may be rotated relative to each other to rub away the protruding steps. This abrasive polishing process is continued until the upper surface of the dielectric layer is substantially flat. 
     Polishing pads may be made of a uniform material such as polyurethane or nonwoven fibers impregnated with a synthetic resin binder, or may be formed from multilayer laminations having non-uniform physical properties throughout the thickness of the pad. Polyurethane polishing pads are typically formed by placing a reactive composition in a mold, curing the composition to form the pad material, and then die cutting the pad material into the desired size and shape. The reagents that form the polyurethane or the resin binder also may be reacted within a cylindrical container. After forming, a cylindrically shaped piece of pad material is cut into slices that are subsequently used as the polishing pad. A typical laminated pad may have a plurality of layers, such as a spongy and resilient microporous polyurethane layer laminated onto a firm but resilient supporting layer comprising a porous polyester felt with a polyurethane binder. Polishing pads typically may have a thickness in the range of 50-80 mils, preferably about 55 mils, and a diameter in the range of 10 to 36 inches, such as about 22.5 inches. 
     Polishing pads also may have macrotextured work surfaces made by surface machining using various techniques, many of which are expensive and produce undesirable surface features of widely varying depths. Surface features include waves, holes, creases, ridges, slits, depressions, protrusions, gaps, and recesses. Some other factors which influence the macroscopic surface texture of a polishing pad are the size, shape, and distribution frequency or spacing of the surface features. Polishing pads typically may also have microtextured surfaces cause by a microscopic bulk texture of the pad resulting from factors intrinsic to the manufacturing process. Since polishing does not normally occur across the entire pad surface, any microtexture of the pad and the macrotextures made by surface machining, may only be formed into the portion of the pad over which polishing is to take place. 
     During the polishing process, the material removed from the wafer surface and the abrasive, such as silica, in the slurry tend to become compacted and embedded in the recesses, pores, and other free spaces within the microscopic and macroscopic bulk texture of the polishing pad at and near its surface. One factor in achieving and maintaining a high and stable polishing rate is providing and maintaining the pad surface in a clean condition. Another factor is reducing or preventing a hydroplaning effect caused by the buildup of a layer of water between the abutting surfaces of the pad and the wafer. It has also been determined that increasing the flexibility of the pad in a controlled manner will increase polishing uniformity, i.e., the uniformity of the polished wafer surface. 
     Thus, consistently achieving uniform and high quality polishing of wafer surfaces by conventional pads has presented three problems. The first of these is the buildup of abrasive particles and debris between the pad and the wafer causing uneven polishing and damage to both the pad and the wafer. Secondly, uneven polishing due to hydroplaning between the wafer and the pad during conventional processes has resulted in the relatively high loss of product yield due to the resulting wafer damage. Thirdly, uneven polishing and wafer damage has also resulted from overly rigid pads produced by prior art manufacturing techniques. Therefore, there is a need for a method and apparatus for providing polishing pads capable of consistently producing high quality wafers with uniformly polished surfaces. 
     SUMMARY OF THE INVENTION 
     The present invention, therefore, provides a pad grooving method and apparatus for producing a polishing pad that is capable of consistently forming uniformly polished surfaces on high quality wafers. The apparatus comprises a platen with positioning post for holding a polishing pad in position for engagement by a router to machine grooves in the working surface of the pad. In order to precisely control the depth of the grooves as they are routed in the pad, a spacing mechanism provides a constant and precise separation between the working surface of the pad and the chuck for holding and rotating the router. 
     The pad is placed on the supporting surface of the platen with its working surface in spaced relation opposite to the router bit. The router chuck and drive motor are supported opposite to the pad by a frame. The spacing mechanism comprises at least one, preferably two or more, stop members mounted on the frame adjacent to an aperture through which passes the router bit. An outer end portion of the bit projects beyond the stop member(s), which preferably are pins threaded within the frame so as to be axially adjustable. A vacuum system is provided for applying a vacuum to the working surface of the pad to pull the pad first against the outer end of the router bit and then against the stop member(s). 
     Rotation of the router bit by the motor while the vacuum is applied to the pad causes the outer end portion of the bit to cut an initial recess (hole) into the pad to a depth below its working surface. The recess depth is precisely limited by the stop member(s), which comes into contact with the working surface of the pad as the rotating bit cuts into the pad to form the initial recess. After formation of the initial recess, a lateral motion mechanism causes relative lateral movement between the rotating router bit and the pad while the vacuum maintains the pad in contact with the stop member(s). 
     This lateral movement causes the rotating bit to cut a groove in the pad extending away from the initial recess and having a depth substantially the same as the initial recess depth. The lateral motion mechanism may comprise upper and lower plates suspended from an overhead beam and arranged for relative movement in the x-y plane. For example, the upper plate may be mounted on the overhead beam and driven in the X-direction (along the X-axis) by one or more motorized screws; and the router frame suspended from the lower plate which, in turn, is mounted on the upper plate and driven in the Y-direction by one or more motorized screws. As an alternative, the platen may be similarly mounted for such x-y movement instead of the router frame, or both the platen and router frame may be mounted for such movement In addition, the platen may be rotated by a drive motor to provide an additional means for causing lateral movement between the router bit and the pad. 
     It follows from the foregoing that relative movement between the stop member(s) and the pad in the Z-direction (along Z-axis) may be provided by the vacuum as it pulls the pad toward the router bit and the stop member(s). Where the polishing pad is flexible due to its large diameter and small thickness, there may be no need to guide this pad movement. Furthermore, significant pad movement along the Z-axis may be avoided by instead moving the router bit along the Z-axis, and then using the vacuum to maintain the bit depth during lateral movement between the bit and pad. 
     However motion of the pad along the Z-axis may be guided by a plurality, preferably two or more, posts projecting outward from the platen along axes parallel to the rotational axis of the router bit. These guideposts also may secure the pad for rotation when the platen is rotated by a platen drive motor, and are particularly useful for grooving disks other than polishing pads, such as rigid disks of greater thickness and smaller diameter. As already indicated, the upper and lower lateral motion plates provide for lateral movement of the router bit relative to the pad along the X-axis and along the Y-axis. Therefore, the router bit may be moved relative to the pad in accordance with the Cartesian coordinates x, y and z, or in accordance with the cylindrical coordinates R, θ and Z. 
     The foregoing relative lateral movements permit the grooves cut in the working surface of the pad to have either left or right spiral patterns, zigzag patterns with different groove densities, each following a constant radius around the pad at different radii, inner and outer circle grooves with spiral grooves or zigzag in areas therebetween, inner and outer sectors at different radii and having different spiral or zigzag patterns, or any combinations of these and other patterns. In addition, the patterned portions of the working surface of the pad may be confined only to those areas over which polishing of a wafer is to take place. 
     The depth of the grooves may also be varied for different patterns by axially adjusting the projecting length of the stop members, which are preferably symmetrical pins, or by axially adjusting the projecting length of the router bit relative to axially fixed stop members. To provide pads of increased flexibility, the grooves may penetrate into the pad for a depth up to 80% of the pad thickness. Pad flexibility may also be adjusted by the overall number of grooves provided, such as, for example, a pattern of 8, 32, or 64 spirals. 
     Grooves in the working surface of a CMP pad made according to the invention significantly reduce the hydroplaning effect during wafer polishing and, as a result, a much higher polishing rate can be achieved. A pattern with a higher number of spiral grooves can reduce the hydroplaning effect more efficiently than a pattern with a lower number of spiral grooves because more grooves will pass across the wafer surface being polished in the same period of time. An increase in pad flexibility due to the groove pattern selected may also help improve the polishing uniformity of the wafer surface. The groove density of zigzag groove patterns also may be varied to control the polishing rate distribution within different segments of the polishing pad surface and this may also improve polishing uniformity within the wafer surface. 
     The polishing pad provided by the present invention is ideal for polishing wafers of dielectric materials such as silicon dioxide, diamond-like carbon (DLC), spin-on-glass (SOG), polysilicon, and silicon nitride. The polishing pads also may be used to polish other wafers or disks such as those made of copper, aluminum, tungsten, and alloys of these and other metals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, operation, and advantages of the invention may be better understood from the following detailed description of the preferred embodiments taken in conjunction with the attached drawings, in which: 
     FIG. 1 is an elevational view of the invention in partial section and in which its major components are illustrated diagrammatically; 
     FIG. 2 is a planar cross-sectional view as taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged partial sectional view of a portion of FIG. 1; 
     FIG. 4 shows a polishing pad made according to the present invention wherein the groove pattern comprises 8 left-hand spiral grooves beginning near the center of the pad and ending near the outer edge of the working surface of the pad; 
     FIG. 5 shows a polishing pad made according to the present invention wherein the groove pattern comprises 32 left-hand spiral grooves beginning near the center and ending near the outer edge of the working surface of the pad; 
     FIG. 6 shows a polishing pad made according to the invention wherein the groove pattern comprises 64 right-hand spiral grooves beginning near the center and ending near the outer edge of the working surface of the pad; and, 
     FIG. 7 shows a polishing pad made according to the invention wherein the groove pattern comprises a plurality of radially spaced zigzag grooves each formed symmetrically along a substantially constant radius around the pad surface, and wherein the groove density of the innermost and outermost grooves are varied from each other and from intermediate grooves. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The polishing pad grooving method and apparatus of the present invention are illustrated best in FIGS. 1-3. The polishing apparatus has a platen  10  on which a polishing pad  12  is supported and held in a fixed radial position by a plurality of holding posts  14 . Each of the holding posts  14  fits within a channel or recess  16  (FIG. 4) formed within the pad body or in the pad periphery and extending parallel to the central axis C of the pad so that the pad may be guided for axial movement away from the surface of the platen, as illustrated by the arrows Z and the air gap  17  shown in FIG.  3 . However, for axially adjustable routers and/or flexible pads of sufficiently large diameter and small thickness to movement of the portion thereof being grooved, the holding posts  14  may be replaced by non-guiding clamps. 
     Positioned opposite to the working surface  22  of pad  12  is a router bit  24  replaceably held in a chuck  26  and driven in rotation by a router motor  28 . Router motor  28  is carried by a frame  30  surrounded by a casing  32 , such that an annular space  34  is provided between the concentric walls of the frame and the casing, both of which are preferably cylindrical. A vacuum, represented by arrows V, V is provided in the annular space  34  by a blower  36  attached to the casing  32  by a flexible hose  38 . The platen  10  is carried for rotation in either direction by a drive shaft  18  driven by a platen motor  20 . Motors  20  and  28  may both be of the reversible type, such that the router bit  24  may be rotated in either direction, as indicated by the arrow R 1 , and the platen  10  also may be rotated in either direction, as indicated by the arrow R 2 . 
     Mounted on the bottom wall  31  of the frame  30  adjacent to a passage  35  for the router bit  24  is a plurality of stop pins  33 , which project parallel to the router bit for a distance that is less than the projecting distance of the router bit itself. The difference between the projecting distance of the pins  33  and the projecting distance of the router bit define the length of an end portion  37  of the bit equal to the desired depth of the groove to be cut by this end portion, as described more fully below in connection with operation of the invention. The projecting length of bit end portion  37  may be changed by rotating a pair of pinions  27 ,  27  that engage a corresponding pair of racks  29 ,  29  mounted on router motor  28  as shown in FIG.  1 . The pins  33  are preferably threaded into the bottom wall  31  for axial adjustment, as an alternative means for changing the projecting length of bit end portion  37 . Pins  33  may have a hex head portion  39  permitting engagement for rotation by a corresponding tool. 
     The router is mounted to an overhead support or carrying member  40  by a lateral motion mechanism, generally designated  42 , to provide for lateral movement of the router bit in an x-y plane perpendicular to the axis of router bit rotation and the corresponding central axis C of the polishing pad. The lateral motion mechanism  42  may be any structure providing precise lateral movement of the router  24  in the x-y plane, and may not be needed in instances where the router support member  40  is itself movable in the x-y plane, such as where the member  40  is attached to or part of a precisely controllable robotic arm. 
     By way of example, the motion device illustrated in FIGS. 1 and 2 comprises a lower plate  44  suspended from an upper plate  46  by two pairs of threaded eyelets  48 ,  48  and  50 ,  50 . In turn, the upper plate  46  is suspended from two pairs of brackets  52 ,  52  and  53 ,  53  by another two pair of threaded eyelets  54 ,  54  and  56 ,  56 . Each eyelet pair  48 ,  48  and  50 ,  50  is threadedly engaged by a corresponding drive screw  58  driven in rotation by a reversible y-axis motor  59  to provide reciprocal motion of lower plate  44  along the y-axis, as illustrated by the double-ended arrow Y. Similarly, the eyelet pairs  54 ,  54 , and  56 ,  56  are each threadedly engaged by a corresponding drive screw  60  rotated by a reversible x-axis electric motor  62  to provide reciprocal motion of upper plate  46  along the x-axis, as illustrated by the double-ended arrow X in FIG.  2 . 
     Operation of the pad grooving apparatus will now be described with reference to FIGS. 1-3. The blower  36  is turned on to generate a vacuum V in the annular passage  34 . This vacuum generates an upward force in the direction of arrows Z, Z to uplift and/or hold the pad  12  against the axially adjustable stop pins  33 , which are thereby used to control the groove depth. The router bit  24  extends beyond the ends of stop pins  33  by the length of bit end portion  37 , and will cut into the pad  12  when the bit is rotated by turning on the router motor  28 . The router is preferably turned on and vertically adjusted after the vacuum is applied. Any upward movement of the pad, in response to the vacuum V, is guided by the engagement between the holding posts  14  and corresponding recesses or channels  16 , which may be in the body or the periphery of the pad  12 . The end portion  37  of the bit  24  may project beyond the tips of pins  33  by a length of up to 80% of the pad thickness, such that the end portion of the bit may penetrate to a depth up to 80% of the thickness of the pad. The projecting length of bit end portion  37  may be changed to thereby change the groove depth by turning the pinions  27 ,  27  or by turning the pins  33 ,  33 , or by a combination of these adjustments 
     After the router bit  24  has penetrated fully into the pad, as determined by abutment between the tips of stop pins  33  and the working surface  22  of pad  12 , the bit is then moved radially relative to the pad in an x-y plane, as illustrated by the double-end arrows X and Y in FIG.  2 . This x-y movement may be achieved solely by moving the lower plate  44  and the upper plate  46  relative to each other by operation of the motors  59  and  62 , or these lateral movements may be combined with rotation of the platen  10  about the center axis C, while the router bit  24  is moved in a radial direction to form spiral grooves. 
     Lateral movement of the lower plate  44  along the y-axis is produced by the rotation of screws  58 ,  58  in threaded engagement with the respective eyes  48 ,  48  and  50 ,  50 . Lateral movement of the upper plate  46  along the x-axis is produced by rotation of screws  60 ,  60  in threaded engagement with the eyes  54 ,  54  and  56 ,  56 . Rotation of the platen  10  is provided by rotation of the shaft  18  by platen motor  20 . Accordingly, the router bit  24  may be moved laterally in the x, y plane in the Cartesian coordinates x, y, or in the cylindrical coordinates R, θ with respect to the polishing pad  12 . In addition, the router bit may be moved up and down along the Z-axis in both Cartesian and cylindrical coordinates by either hand or motorized rotation of the pinions  27  by conventional mechanisms that are not seen. 
     Upward movement along the z-axis in both Cartesian and cylindrical coordinates is also provided by movement of the pad  12  away from the surface  22  of platen  10  and against the tips of pins  33  in response to the creation of vacuum within annular passage  34 . The pad moves downward along the z-axis when the vacuum ceases upon stopping blower  36 . Such movement of the pad  12  along the z-axis is therefore produced by the pressure differential across the pad thickness as generated by the vacuum V. As an alternative, a pressure differential for causing such pad movement could be generated by ejecting pressurized air under the pad through a series of air holes or nozzles (not shown). 
     Thus, the spiral grooves formed by the present invention preferably (but not necessarily) start from the center of the pad and end near the outer edge thereof. The direction of the spiral pattern can either be to the left, as shown by the eight spiral grooves in FIG.  4  and the 32 spiral grooves in FIG. 5, or to the right, as illustrated by the 64 spiral grooves in FIG.  6 . In FIGS. 4-7, the grooves are represented by heavy solid black lines for clarity because the opposing edges of the actual grooves are too close to be shown as double lines. As careful examination will reveal, a single continuous groove forms the patten  70  of FIG. 4, the pattern  72  of FIG. 5, and the pattern  74  of FIG. 6, such that, once inserted, the router bit does not have to be withdrawn until the pattern is completed. 
     The spiral grooves in the surface of the pad will reduce the hydroplaning effect during polishing and, as a result, a much higher polishing rate can be achieved. A higher number of spiral grooves within the same surface area can reduce the hydroplaning effect more efficiently than a lower number of spiral grooves because in the same period of time more grooves will pass across the surface of a wafer pressed against the pad surface during polishing of the former. It follows from this that the rate of removal of the slurried abrasive, which is used in combination with the pad for wafer polishing, will be greater the higher number of the spiral grooves per unit area of the pad working surface. A high number of grooves can also make the pad more flexible, which can help improve the uniformity of wafer polishing. 
     FIG. 7 illustrates a zigzag groove pattern consisting of an outer groove  76 , an inner groove  78 , and three intermediate grooves  80 ,  81 , and  82 . These grooves are made separately by stopping the blower to withdraw the bit from the pad, repositioning the bit laterally relative to the pad, and then restarting the blower to insert the bit into the pad. However, the grooves  76 ,  78 ,  80 ,  81 , and  82  could be interconnected, in which case the pattern could instead be made by a single continuous groove to eliminate intermediate withdrawals of the bit from the pad. The groove pattern of FIG. 7 illustrates that the groove density may be varied over different portions of the pad surface. Such variations in groove density can be used to control the polishing rate distribution in accordance with where a wafer is pressed against the polishing pad surface, and this, too, can help improve the uniformity of wafer polishing. For generating the patterns shown in FIGS. 4-7 and other complex groove patterns, the positioning motors  20 ,  59 , and  62  are preferably controlled by a microprocessor (not shown). 
     Person skilled in the art, upon learning of the present disclosure, will recognize that various changes and modifications to the elements and steps of the invention are possible without significantly affecting their functions. For example, the support structures for the pad and for the router, the nature and shape of the stop members for controlling the depth of the grooves, the arrangement for applying a pressure differential for holding the pad against the stop members, and the structures for providing relative lateral movement between the router bit and the pad, all as described above by way of example, may be varied widely in accordance with current and future technology for providing the functions of these systems and components. For example, the platen may include an array of air passages and outlets for providing a cushion of pressurized air under the pad to provide all or part of the pressure differential for holding the pad against the stop members. Also, in addition to being rotated, both the platen and the pad may be moved in an x-y plane by mounting the platen drive motor on a lateral movement mechanism similar to mechanism  42  for mounting the router motor as described above. Accordingly, while the preferred embodiments have been shown and described above in detail by way of example, further modifications and embodiments are possible without departing from the scope of the invention as defined by the claims set forth below.