Abstract:
A system and method for chemical mechanical polishing of a substrate is disclosed in which a polishing pad is conditioned by directing a fluid jet to the surface of the polishing pad. Thus, the use of expensive consumables, like conditioning pads comprising diamonds, can be avoided. Furthermore, the risk of substrates being scratched by diamonds lost from the conditioning pad is avoided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of fabrication of microstructures, and, more particularly, to a tool for conditioning the surface of a polishing pad in a system for chemical mechanical polishing of substrates.  
           [0003]    2. Description of the Related Art  
           [0004]    In microstructures such as integrated circuits, a large number of elements, e.g., transistors, capacitors and resistors, are fabricated on a single substrate by depositing semi-conductive, conductive and insulating material layers and patterning those layers by photolithography and etch techniques. The individual circuit elements are electrically connected by means of metal lines. In the formation of these metal lines, a so-called inter-layer dielectric is deposited and vias and trenches are thereafter formed in this dielectric layer. The vias and trenches are then filled with a metal, e.g., copper, to provide electrical contact between the circuit elements. In modem integrated circuits, a plurality of such metallization layers comprising metal lines must be stacked on top of each other to maintain the required functionality. The repeated patterning of material layers, however, creates a non-planar surface topography, which may deteriorate subsequent patterning processes, especially for such microstructures including features with minimum dimensions in the submicron range, as is the case for sophisticated integrated circuits.  
           [0005]    It has turned out to be necessary to planarize the surface of the substrate between the formation of subsequent layers. A planar surface of the substrate is desirable for various reasons, one of them being the limited optical depth of the focus in photolithography which is used to pattern the material layers of microstructures. Chemical mechanical polishing is an appropriate and widely used process to achieve global planarization of a substrate.  
           [0006]    [0006]FIG. 1 schematically shows a schematic sketch of a conventional system  100  for chemical mechanical polishing. The system  100  comprises a platen  101  on which a polishing pad  102  is mounted. Frequently, polishing pads are formed of a cellular microstructure polymer material having numerous voids, such as polyurethane. A polishing head  130  comprises a body  104  and a substrate holder  105  for receiving and holding a substrate  103 . The polishing head  130  is coupled to a drive assembly  106 . The device  100  further comprises a slurry supply  112  and a pad conditioner  131 . The pad conditioner  131  comprises a conditioning head  107  and a conditioning pad  108  attached to the conditioning head  107 . The conditioning head  107  is coupled to a drive assembly  109 .  
           [0007]    In operation, the platen  101  rotates. The slurry supply  112  supplies slurry to a surface of the polishing pad  102  where it is dispensed by centrifugal forces. The slurry comprises a chemical compound reacting with the material or materials on the surface of the substrate  103 . The reaction product is removed by abrasives contained in the slurry and/or the polishing pad  102 . The polishing head  130 , and thus the substrate  103 , is rotated by the drive assembly  106  in order to substantially compensate for the effects of different angular velocities of parts of the polishing pad  102  at different radii. In advanced systems  100 , the rotating polishing head  130  is additionally moved across the polishing pad  102  to further optimize the relative motion between the substrate  103  and the polishing pad  102  and to maximize pad utilization. The drive assembly  109  rotates the conditioning head  107  and thus the conditioning pad  108  attached to it. The conditioning pad  108  may comprise an abrasive component like, e.g., diamonds embedded in a matrix. Thus, the surface of the polishing pad  102  is abraded and densified slurry, as well as particles that have been polished away from the surface of the substrate, are removed from voids in the porous polishing pad  102 . This process is denoted as conditioning.  
           [0008]    Without conditioning, densified slurry and particles abraded from the substrate  103  would clog pores in the polishing pad  102 . Thus, the polishing pad  102  would lose its absorbency such that most of the slurry would flow off the polishing pad  102  too quickly. Due to this degradation of the polishing pad  102 , the removal rate in the polishing process would steadily decrease.  
           [0009]    Conditioning may be performed after polishing one or more substrates  103 . This, however, leads to significant variations of the removal rate due to the difference between the reworked surface of a freshly conditioned polishing pad  102  compared to the exhausted surface present immediately before the conditioning. Alternatively, the pad conditioner  131  is continuously in contact with the polishing pad  102  while the substrate  103  is polished. Thus, a more uniform rate of removal of substrate material is achieved.  
           [0010]    Various designs of chemical mechanical polishing devices are known in the art. For example, the rotating platen  101  may be replaced with a continuous belt kept in tension by rollers moving at high speed, or slurry may be injected through the polishing pad  102  in order to deliver slurry directly to the interface between the polishing pad  102  and the substrate  103 .  
           [0011]    One problem with conventional systems for chemical mechanical polishing is that conditioning pads are consumables, which typically have lifetimes of less than 2,000 substrates. Thus, conditioning pads are expensive consumables, the price of which significantly contributes to the cost of operating a chemical mechanical polishing device.  
           [0012]    Another problem with conventional systems for chemical mechanical polishing is that conditioning pads comprising diamonds tend to lose single diamonds, which then may cause serious scratches on the surface of the polished substrate. Depending on the type of polishing system and the control strategy thereof, a large number of substrates can be affected until the problem is either detected and removed by pad changes, or the diamond is removed by pad conditioning. This can result in high costs for scratched substrates.  
           [0013]    In view of the above-mentioned problems, a need exists for a system for chemical mechanical polishing which comprises an improved pad conditioner.  
         SUMMARY OF THE INVENTION  
         [0014]    According to one embodiment of the present invention, a system for chemical mechanical polishing comprises a polishing pad and a pad conditioner being adapted to direct a fluid jet towards the polishing pad.  
           [0015]    According to another embodiment of the present invention, a method comprises chemical mechanical polishing using a polishing pad and directing a high pressure fluid jet towards the polishing pad to condition a surface portion of the polishing pad.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:  
         [0017]    [0017]FIG. 1 shows a sketch of a conventional system for chemical mechanical polishing;  
         [0018]    [0018]FIG. 2 a  shows a sketch of a system for chemical mechanical polishing according to an illustrative embodiment of the present invention;  
         [0019]    [0019]FIG. 2 b  shows a sketch of a pad conditioner in a system for chemical mechanical polishing according to another illustrative embodiment of the present invention;  
         [0020]    [0020]FIG. 3 shows a sketch of a system for chemical mechanical polishing according to yet another illustrative embodiment of the present invention;  
         [0021]    [0021]FIG. 4 shows a sketch of a pad conditioner in a system for chemical mechanical polishing according to yet another illustrative embodiment of the present invention;  
         [0022]    [0022]FIG. 5 shows a sketch of a pad conditioner in a system for chemical mechanical polishing according to yet another illustrative embodiment of the present invention; and  
         [0023]    [0023]FIG. 6 shows a sketch of a system for chemical mechanical polishing according to yet another illustrative embodiment of the present invention.  
         [0024]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0026]    The present invention will now be described with reference to the attached figures. Although the various regions and structures of a semiconductor device are depicted in the drawings as having very precise, sharp configurations and profiles, those skilled in the art recognize that, in reality, these regions and structures are not as precise as indicated in the drawings. Additionally, the relative sizes of the various features and doped regions depicted in the drawings may be exaggerated or reduced as compared to the size of those features or regions on fabricated devices. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.  
         [0027]    A system for chemical mechanical polishing according to the present invention comprises a pad conditioner which is adapted to direct one or more fluid jets towards the polishing pad. Thus, a mechanical force entry into the polishing pad is achieved which leads to the desired removal of densified slurry and particles abraded from the substrate from the polishing pad and to a recreation of absorbency of the polishing pad.  
         [0028]    [0028]FIG. 2 a  shows a schematic side view of a system  200  for chemical mechanical polishing according to an illustrative embodiment of the present invention. The system  200  comprises a platen  201 , a polishing pad  202 , a polishing head  230 , a drive assembly  206 , a slurry supply  212  and a pad conditioner  231 . The polishing head  230  comprises a substrate  203 , a substrate holder  205  and a body  204 .  
         [0029]    The pad conditioner  231  comprises a high pressure fluid supply  213 , a movable mount  214  and a nozzle  215 . The high pressure fluid supply  213  can comprise well-known means for generating fluids having high pressure, e.g., a pump or a bottle of compressed gas, and well-known means for supplying the fluid to the nozzle  215  and controlling the flow of the fluid, e.g., tubes and valves. The movable mount  214  is connected to a drive device  217 , which is adapted to move the movable mount  214 .  
         [0030]    In operation, the platen  201  and the polishing head  230  rotate, and the slurry supply  212  supplies slurry to the polishing pad  202 , where it is distributed by centrifugal forces. Prior to and/or during and/or after polishing a substrate, the high pressure fluid supply  213  supplies a fluid having high pressure to the nozzle  215 . As the fluid passes through the nozzle  215 , the pressure of the fluid decreases. Thereby, elastic energy is released and the fluid is accelerated to high velocity, and a fluid jet  216  is formed which impinges on the polishing pad  202 .  
         [0031]    [0031]FIG. 2 b  shows a schematic perspective view of the pad conditioner  231 . For the sake of convenience, like reference numerals have been used in FIGS. 2 a  and  2   b.  The fluid jet  216  may impinge at an approximately perpendicular angle to the polishing pad  202 . In other embodiments of the present invention, the fluid jet  216  impinges at an incline to the polishing pad  202 . As the fluid jet  216  impinges on the polishing pad  202 , the fluid is decelerated and exhibits force to an area on the polishing pad  202  such that densified slurry and particles abraded from the substrate  203  are removed from voids in the porous pad material. A fluid jet  216  having a high velocity may also abrade the pad material itself.  
         [0032]    In one illustrative embodiment, the fluid jet  216  can have a substantially cylindrical shape. It can have a diameter that is small compared to the radius of the polishing pad  202 . Of course, other shapes or configurations are possible for the fluid jet  216 .  
         [0033]    The pad conditioner  231  comprises a drive device  217  being connected to the mobile mount  214 , which can rotate the mobile mount  214  around an axis substantially perpendicular to the surface of the polishing pad  202 . Thus, the nozzle  215  and the fluid jet  216  move within a plane that is substantially parallel to the polishing pad surface, ensuring a constant distance between the nozzle  215  and the polishing pad  202 .  
         [0034]    In one illustrative embodiment of the present invention, the drive device  217  comprises a servo motor that is controlled by a microprocessor in coordination with the rotation of the platen  201 .  
         [0035]    The drive device  217  is adapted to change the direction of rotation of the mobile mount  214  as the fluid jet  216  approaches the edge of the polishing pad  202  in order to ensure that the fluid jet  216  impinges on the polishing pad  202 . Thus, the fluid jet  216  oscillates in a bi-directional circular motion over the polishing pad  202 .  
         [0036]    Moving the fluid jet  216  over the rotating polishing pad  202  allows a substantially uniform conditioning of the surface of the polishing pad  202  with a fluid jet  216  having a diameter which is small compared to the radius of the polishing pad  202  if the rotational frequency of the platen  201  and the frequency of the oscillating motion of the fluid jet  216  are coordinated. The motion of the fluid jet  216  relative to the polishing pad  202  may be advantageously controlled so as to avoid parts of the polishing pad  202  from being frequently exposed to the fluid jet  216  while other parts of the polishing pad  202  are rarely or never exposed to the fluid jet  216 . In one embodiment, this can be achieved if the motion of the fluid jet  216  is controlled to be slow enough such that the fluid jet  216  moves over a distance equal to or less than the diameter of the fluid jet  216  during one revolution of the platen  201 .  
         [0037]    In other embodiments of the present invention, the ratio between the frequency of the oscillating motion of the fluid jet  216  and the rotational frequency of the platen  201  is a fraction a/b of integers a, b, where a is not an integer multiple of b. Then, the motion of the fluid jet  216  relative to the polishing pad  202  repeats after b revolutions of the platen  201 . In one particular embodiment, b is equal to or greater than the ratio between the radius of the polishing pad  202  and the diameter of the fluid jet  216 .  
         [0038]    The angular velocity of the circular motion of the fluid jet  216  need not be constant. It may be desirable to move the fluid jet  216  faster if it impinges on a point close to the center of the polishing pad  202  and slower if it impinges on a point closer to the perimeter of the polishing pad  202 . Thus, a more uniform exposure of the surface of the polishing pad  202  to the fluid jet  216  is obtained.  
         [0039]    In other embodiments of the present invention, the mobile mount  214  performs a unidirectional circular motion over the polishing pad  202 . The drive device may be provided over the surface of the polishing pad  202 , similar to the drive assembly  109  shown in FIG. 1, and the dimensions of the mobile mount  214  are such that the fluid jet  216  always impinges on the polishing pad  202  as the mobile mount  214  performs a complete revolution.  
         [0040]    If desired, conditioning of the polishing pad  202  can be performed continuously or intermittently while a substrate  203  is polished. To this end, in one embodiment, the high pressure fluid supply  213  is configured to supply one or more high pressure gas streams as the fluid jet  216 . With this configuration, dilation and/or a chemical change of the slurry may be substantially avoided. Appropriate gases may include, without limiting the present invention air, nitrogen, carbon dioxide or a noble gas. Alternatively, polishing and conditioning can be performed successively. For example, conditioning can be performed after one or more substrates have been polished.  
         [0041]    The fluid jet  216  can comprise water, for example, provided as ultra pure water. In other embodiments of the present invention, the fluid jet  216  may comprise another liquid, e.g., an organic solvent. The fluid jet  216  may also comprise a mixture of a liquid and a gas. The fluid jet  216  may also comprise abrasive particles which abrade the surface of the polishing pad  202 . Conditioning with a fluid jet  216  comprising abrasive particles and polishing of the substrate  203  may be performed successively to prevent the substrate  203  from being scratched by abrasive particles remaining on the polishing pad.  
         [0042]    The high pressure fluid supply can be adapted to supply different fluids to the nozzle  215 . In one embodiment of the present invention, the polishing pad  202  is conditioned by a fluid jet which consists of pure water while a substrate  203  is polished. After the polishing of one or more substrates, conditioning with a fluid jet  216  comprising abrasive particles is performed.  
         [0043]    [0043]FIG. 3 shows a schematic side view of a system  300  for chemical mechanical polishing according to another embodiment of the present invention. The system  300  comprises a platen  301 , a polishing pad  302 , a polishing head  330 , a drive assembly  306 , a slurry supply  312  and a pad conditioner  331 . The polishing head  330  comprises a substrate  303 , a substrate holder  305  and a body  304 .  
         [0044]    The pad conditioner  331  contains a high pressure fluid supply  313 , a mobile mount  314 , a nozzle  315  and a drive device  317 . The high pressure fluid supply  313  is configured to supply a fluid having high pressure to the nozzle  315  to form a fluid jet  316 . The drive device  317  is adapted to move the mobile mount  314  back and forth in a radial direction of the platen  301 .  
         [0045]    In operation, the fluid jet  316  oscillates in a bi-directional linear motion over the polishing pad  302 . Similar to the embodiment of the present invention described with reference to FIGS. 2 a  and  2   b,  the frequency of the oscillation of the fluid jet  316  and the rotational frequency of the platen  301  are coordinated such that a substantially uniform conditioning of the surface of the polishing pad  302  is achieved.  
         [0046]    Advantageously, the pad conditioner  331 , with the fluid jet  316  performing a linear motion, requires a smaller amount of free space above the polishing pad  302  than, for example, the pad conditioner  231  where the fluid jet  216  performs a circular motion.  
         [0047]    [0047]FIG. 4 shows a schematic perspective view of a pad conditioner  431  in a system for chemical mechanical polishing according to yet another illustrative embodiment of the present invention. The pad conditioner  431  comprises a high pressure fluid supply  413 , a mobile mount  414  and a drive device  417 . A plurality of nozzles  415 ,  418 ,  420  is attached to the mobile mount  414 . In operation, a fluid flows through the nozzles  415 ,  418 ,  420  such that a plurality of fluid jets  416 ,  419 ,  421  is formed. These fluid jets  416 ,  419 ,  421  are directed to a polishing pad (not shown). The drive device  417  is adapted to rotate the mobile mount  414  around an axis substantially perpendicular to a surface of the polishing pad such that the fluid jets  416 ,  419 ,  421  and the nozzles  415 ,  418 ,  420  perform a bi-directional circular motion within a plane essentially parallel to the polishing pad surface. The direction of the fluid jets  416 ,  419 ,  421  can be perpendicular to this plane.  
         [0048]    An advantage of a pad conditioner  431  that is adapted to direct a plurality of fluid jets  416 ,  419 ,  421  to the polishing pad is that it is sufficient to pivot the mobile mount  414  by a smaller angle to condition the whole surface of the polishing pad compared to a pad conditioner with only one fluid jet. Thus, the pad conditioner  431  requires a smaller amount of free space above the polishing pad. A further advantage of the pad conditioner  431  described with reference to FIG. 4 is that the force entry into the polishing pad is more evenly distributed over the area of the polishing pad.  
         [0049]    In a further embodiment, the drive device  417  is adapted to move the mobile mount  414  in a bi-directional linear motion similar to the embodiment described with reference to FIG. 3.  
         [0050]    In other embodiments of the present invention, the system for chemical mechanical polishing  400  may comprise a plurality of nozzles that are attached to a plurality of mobile mounts that can be moved independently by a plurality of drive devices (not shown) to produce the plurality of fluid jets.  
         [0051]    [0051]FIG. 5 shows a pad conditioner  531  in a system for chemical mechanical polishing according to further embodiments of the present invention. The pad conditioner  531  comprises a nozzle  515  being attached to a mount  514 . In operation, a high pressure fluid supply  513  supplies fluid at high pressure to the nozzle  515 . An opening of the nozzle  515  has an elongated shape, such that it emits a line-shaped fluid jet  516 . The shape of the fluid jet  516  can be characterized by a first diameter d 1  in a cross-direction and a second diameter d 2  in a lengthwise direction, wherein d 2 &gt;d 1 . In one embodiment, d 2  is equal to the radius of the polishing pad used. Then, the whole surface of the rotating polishing pad can be conditioned without moving the fluid jet  516 . Thus, the number of moving parts may be reduced.  
         [0052]    A further embodiment of the present invention is described with reference to FIG. 6. A system  600  for chemical mechanical polishing comprises a polishing pad  602  being attached to a platen  601  which rotates during operation. The system  600  further comprises a slurry supply  612  and a polishing head  630  comprising a substrate  603 , a substrate holder  605  and a body  604 . A drive assembly  606  rotates the polishing head  630  during operation of the system  600 . A plurality of nozzles  615 ,  618  are attached to the polishing head  630 . A high pressure fluid supply  613  supplies fluid at high pressure to the nozzles  615 ,  618  such that fluid jets  616 ,  619  are created that are directed to the surface of the polishing pad  602 . The high pressure fluid supply  613  and the nozzles  615 ,  618  together form a pad conditioner, which is attached to the polishing head  630 .  
         [0053]    The fluid jets  616 ,  619  are moved over the surface of the polishing pads  602 , as the polishing head  630  and the platen  601  rotate. In this embodiment, the rotation of the polishing head  630  is advantageously employed for the motion of the fluid jets  616 ,  619 , such that no additional drive device is required for the pad conditioner. A further advantage of this embodiment is that the surface of the polishing pad is conditioned directly before it encounters the substrate  603 , such that it is ensured that a freshly conditioned polishing pad surface is used for polishing the substrate  603 . In one embodiment, the rotational frequency of the platen  601  and the polishing head  630  are coordinated to ensure a substantially uniform conditioning of the polishing pad  602 .  
         [0054]    In other embodiments of the present invention, the nozzles  615 ,  618  are arranged around the polishing head  630  so as to form a substantially ring-shaped nozzle assembly. In a further embodiment, one or more of the nozzles  615 ,  618  may have an arcuate shape to provide an arcuate line-shaped fluid jet, or, in still a further embodiment, the plurality of arcuated nozzles may be replaced by a single substantially ring-shaped nozzle. In operation, fluid at high pressure is supplied to the nozzles  615 ,  618  such that a fluid jet around the polishing head is created.  
         [0055]    In the embodiments described above, it may be advantageous to use a fluid that substantially maintains the chemistry of the slurry, i.e., the fluid may be a gas, or a chemical reagent may be supplied along with the fluid jet.  
         [0056]    In a system for chemical mechanical polishing according to the present invention, the pressure of the fluid being supplied to a nozzle, the size of an opening of the nozzle and the angle at which a fluid jet impinges on the polishing pad can be adapted to the individual application and the used pad material. In a pad conditioner comprising a plurality of nozzles, the individual nozzles may have different diameters, and the individual fluid jets may impinge on the surface of the polishing pad at different angles. The individual fluid jets may comprise different fluids.  
         [0057]    A jet moving unit for moving one or more fluid jets over the surface of a polishing pad need not comprise a mobile mount as in the embodiments described above. In other embodiments of the present invention, the position at which a fluid jet impinges on the polishing pad may be controlled by changing a direction of the fluid jet by pivoting a fixed nozzle.  
         [0058]    In further embodiments of the present invention, one or more pivoting nozzles are attached to a mobile mount which may be coupled to a drive device. Thus, both the angle at which the one or more fluid jets emitted by the nozzle or nozzles impinges on the polishing pad and the position where it impinges can be varied.  
         [0059]    The present invention is not limited to systems for chemical mechanical polishing comprising a rotating platen and a slurry supply as shown in FIGS. 1, 2 a,    3  and  6 . Pad conditioners that are adapted to direct a fluid jet to the surface of a polishing pad may also be used in a sequential linear polisher, which comprises a polishing pad being attached to a continuous belt kept in tension by rollers, wherein this belt moves at high speed. Slurry may also be supplied directly to the interface between a polishing pad and a polished substrate by injecting it through the polishing pad instead of using a slurry supply above the polishing pad as shown in FIGS. 1, 2 a,    3  and  6 .  
         [0060]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.