Patent Publication Number: US-7214125-B2

Title: Method for controlling pH during planarization and cleaning of microelectronic substrates

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/806,765, filed Mar. 22, 2004 now U.S. Pat. No. 6,913,523. 

   TECHNICAL FIELD 
   The present invention relates to mechanical and chemical-mechanical planarization of microelectronic substrates. More particularly, the present invention relates to controlling the pH of a microelectronic substrate during planarization and post-planarization processing of the microelectronic substrate. 
   BACKGROUND OF THE INVENTION 
   Mechanical and chemical-mechanical planarization processes remove material from the surfaces of semiconductor wafers, field emission displays, and many other microelectronic substrates to form a flat surface at a desired elevation.  FIG. 1  schematically illustrates a planarizing machine  10  with a platen or base  20 , a carrier assembly  30 , a polishing pad  41  positioned on the platen  20 , and a planarizing liquid  44  on the polishing pad  41 . The planarizing machine  10  can also have an under-pad  25  attached to an upper surface  22  of the platen  20  for supporting the polishing pad  41 . In many planarizing machines, a drive assembly  26  rotates (arrow A) and/or reciprocates (arrow B) the platen  20  to move the polishing pad  41  during planarization. 
   The carrier assembly  30  controls and protects a substrate  12  during planarization. The carrier assembly  30  generally has a substrate holder  32  with a pad  34  that holds the substrate  12  via suction. A carrier drive assembly  36  typically rotates and/or translates the substrate holder  32  (arrows C and D, respectively). Alternatively, the substrate holder  32  can include a weighted, free-floating disk (not shown) that slides over the polishing pad  41 . 
   The combination of the polishing pad  41  and the planarizing liquid  44  generally defines a planarizing medium  40  that mechanically and/or chemically-mechanically removes material from the surface of the substrate  12 . The polishing pad  41  may be a conventional polishing pad composed of a polymeric material (e.g., polyurethane) without abrasive particles, or it may be an abrasive polishing pad with abrasive particles fixedly bonded to a suspension material. In a typical application, the planarizing liquid  44  may be a chemical-mechanical planarization slurry with abrasive particles and chemicals for use with a conventional non-abrasive polishing pad. In other applications, the planarizing liquid  44  may be a chemical solution without abrasive particles for use with an abrasive polishing pad. In any case, the planarizing liquid  44  can be pumped from a planarizing liquid supply  45  through a conduit  46 , and through orifices  43  to a planarizing surface  42  of the polishing pad  41 . 
   To planarize the substrate  12  with the planarizing machine  10 , the carrier assembly  30  presses the substrate  12  against the planarizing surface  42  of the polishing pad  41  in the presence of the planarizing liquid  44 . The platen  20  and/or the substrate holder  32  then move relative to one another to translate the substrate  12  across the planarizing surface  42 . As a result, the abrasive particles and/or the chemicals of the planarizing medium  40  remove material from the surface of the substrate  12 . 
   After the substrate  12  has been planarized, particulate matter, such as abrasive particles, particles removed from the polishing pad  41 , and/or particles removed from the substrate  12  may adhere to the substrate. Accordingly, the substrate  12  can be rinsed to remove the particulate matter before the substrate  12  undergoes additional processing. One conventional approach to rinsing the substrate  12  is to pump a rinsing solution  53  from a rinsing solution supply  54  through the orifices  43  to the planarizing surface  42  of the polishing pad  41 . The rinsing solution  53  rinses the substrate  12  while the substrate remains in situ on the polishing pad  41 . The rinsing solution  53  may be introduced to the polishing pad  41  as the relative velocity between the substrate  12  and the polishing pad  41  is reduced or ramped down. 
   Another rinsing approach, which can be used in addition to or in lieu of the in situ approach discussed above, can include removing the substrate  12  from the polishing pad  41  with a substrate transporter  60  and moving the substrate  12  to a rinse chamber  50 . The substrate transporter  60  can include a grasping device  62  that engages the substrate  12  after the substrate has been detached from the carrier assembly  30 . The substrate transporter  60  can further include one or more movable arms  61  that can robotically move the substrate  12  to the rinse chamber  50 . The rinse chamber  50  can include a plurality of opposing spray bars  51 , each having a plurality of nozzles  52  for directing a spray of the rinsing solution  53  onto the substrate  12 . The rinse chamber  50  shown in  FIG. 1  can simultaneously accommodate two substrates  12  positioned upright in adjacent bays  57 . 
   A third approach to removing particulate matter from the substrate  12  is to remove the substrate from the polishing pad  41  and place the substrate  12  on a separate buffing pad (not shown). The buffing pad then moves relative to the substrate and may also be supplied with a rinsing solution to convey the particulate matter away. 
   After the substrate  12  has been planarized and rinsed, the polishing pad  41  can be conditioned to restore its ability to planarize additional substrates. Accordingly, the planarizing machine  10  can include a conditioner  70  that removes polishing pad material from the planarizing surface  42  to expose new polishing pad material. The conditioner  70  can include an abrasive disk  71  for mechanically roughening the planarizing surface  42  of the polishing pad  41 . The conditioner  70  can also include a conditioning fluid source  72  that supplies conditioning fluid to the polishing pad  41  for chemically conditioning the planarizing surface  42  of the polishing pad  41 . 
   Planarizing processes must consistently and accurately produce a uniformly planar surface on the microelectronic substrate  12  to enable precise fabrication of circuits and photo-patterns. As the density of integrated circuits increases, the uniformity and planarity of the substrate surface is becoming increasingly important because it is difficult to form sub-micron features or photo-patterns to within a tolerance of approximately 0.1 microns on non-uniform substrate surfaces. Thus, planarizing processes must create a highly uniform, planar surface on the substrate. 
   One drawback with the conventional methods discussed above is that they may not create a sufficiently planer surface on the substrate because particulates may remain attached to the substrate as a result of contact between the substrate  12  and a variety of chemical solutions during and after planarization. For example, in one conventional method the planarizing solution is an ammonia-based solution, and the rinsing and conditioning fluids are deionized water. Each chemical solution may have different chemical characteristics and sequentially exposing the microelectronic substrate  12  to different chemical solutions may cause particulates to adhere to the surfaces of the substrate. These particulates may damage the wafer during subsequent polishing and handling steps, or may interfere with subsequent processing steps, such as masking and etching. Furthermore, the particulates may become incorporated into the devices formed on the substrate, potentially causing the devices to fail. 
   In the competitive semiconductor and microelectronic device manufacturing industries, it is desirable to maximize the throughput of finished substrates. Accordingly, a further drawback with the conventional processes described above is that they may require additional time to remove the particulates from the substrate. The additional time can be required because the substrate has additional particulate adhered to it as a result of exposure to various chemical solutions. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward methods and apparatuses for processing a microelectronic substrate. In one embodiment, the apparatus can include a polishing pad having a planarizing surface and a source of planarizing liquid in fluid communication with the planarizing surface of the polishing pad. The microelectronic substrate is planarized by engaging the substrate with the polishing pad while the planarizing liquid is disposed on the polishing pad, and moving one of the substrate and the polishing pad relative to the other of the substrate and the polishing pad. As the relative motion between the substrate and the polishing pad is decreased, rinsing fluid having a pH approximately the same as a pH of the planarizing liquid can be introduced to the planarizing surface to maintain the pH of the microelectronic substrate at an approximately constant level. 
   In another embodiment, the microelectronic substrate can be removed from the polishing pad and rinsed remotely with a rinsing liquid having a pH approximately the same as a pH of the planarizing liquid. The rinsing liquid in either of the foregoing embodiments can be selected to include tetramethyl ammonium hydroxide and deionized water, or other substances where a pH of the rinsing liquid is approximately the same as the pH of the planarizing liquid. 
   In still another embodiment, the polishing pad can include a non-abrasive polishing pad and the planarizing liquid can include an abrasive slurry. The pH of the microelectronic substrate can be maintained by maintaining the pH of the abrasive slurry at an approximately constant level as the relative velocity between the microelectronic substrate and the polishing pad is reduced to approximately zero. 
   In yet another embodiment of the invention, the polishing pad can be conditioned by supplying to the polishing pad a conditioning liquid having a pH approximately the same as the pH of the planarizing liquid. In still a further embodiment, the microelectronic substrate can be cleaned by engaging the microelectronic substrate with the polishing pad, after the polishing pad has been conditioned, and moving at least one of the polishing pad and the substrate relative to the other of the polishing pad and the substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic side elevation view of a planarizing machine in accordance with the prior art. 
       FIG. 2  is a schematic side elevation view of a planarizing machine having a source of rinsing liquid and a source of planarizing liquid, each liquid having an approximately equal pH in accordance with an embodiment of the present invention. 
       FIG. 3  is a schematic side elevation view of a planarizing machine having a source of conditioning liquid and a source of planarizing liquid, each liquid having an approximately equal pH in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is an apparatus and method for mechanical and/or chemical-mechanical planarization of substrates used in the manufacture of microelectronic devices. Many specific details of certain embodiments of the present invention are set forth in the following description and in  FIGS. 2–3  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments or that the invention may be practiced without several of the details described in the following description. 
     FIG. 2  is a schematic side elevation view of a CMP machine  110  having a platen  120  and a planarizing medium  140 . In one embodiment, the CMP machine can include a model number  676  manufactured by IPEC Corp. of Portland, Oreg., and in other embodiments, the CMP machine can include other devices, such as a web-format planarizing machine, manufactured by EDC Corporation. In the embodiment shown in  FIG. 2 , the planarizing medium  140  includes a polishing pad  141  and an under-pad  125  releasably attached to the platen  120 . The planarizing medium can further include a planarizing liquid  144  disposed on a planarizing surface  142  of the polishing pad  141 . The platen  120  can be movable by means of a platen drive assembly  126  that can impart a rotational motion (indicated by arrow A) and/or a translational motion (indicated by arrow B) to the platen  120 . As was discussed above, the CMP apparatus  110  can also include a carrier assembly  130  having a substrate holder  132  and a resilient pad  134  that together press a microelectronic substrate  112  against the planarizing surface  142  of the polishing pad  141 . A carrier drive assembly  136  can be coupled to the carrier assembly  130  to move the carrier assembly axially (indicated by arrow C) and/or rotationally (indicated by arrow D) relative to the platen  120 . 
   The planarizing liquid  144  can be supplied to the polishing pad  141  from a planarizing liquid supply  145  via a conduit  146 . In one embodiment, the conduit  146  can include a flexible coupling  147 , shown schematically in  FIG. 2 , to allow for translational or rotational motion of the platen  120  relative to the planarizing liquid supply  145 . The coupling  147  can be connected to a manifold  148  in the platen  120 . The manifold  148  can include a plurality of orifices  143  that extend upwardly through the under-pad  145  and the polishing pad  141  to the planarizing surface  142  of the polishing pad. As the carrier assembly  130  moves relatively to the platen  120 , the planarizing medium  140  (i.e., the polishing pad  141  and/or the planarizing liquid  144 ) removes material from the microelectronic substrate  112 . The process may also cause material to be removed from the polishing pad  141 . 
   Particulates removed from the microelectronic substrate  112  and the polishing pad  141 , as well as abrasive elements in the planarizing liquid  144  may tend to adhere to the microelectronic substrate  112 . Accordingly, the CMP machine  110  can include a rinsing liquid  153  that is pumped from a rinsing liquid supply  154  through the conduit  146  to the orifices  143  in the polishing pad  141 . The conduit  146  can include a valve  149  that can be adjusted to couple the rinsing liquid supply  154  and/or the planarizing liquid supply  145  with the orifices  143 , to selectively provide rinsing liquid  153  and/or planarizing fluid  144  to the polishing pad  141 . 
   It has been observed that particulates adjacent to the substrate  112  may have a greater tendency to adhere to the substrate  112  when the pH of the substrate changes suddenly. Accordingly, in one embodiment of the invention, the rinsing liquid  153  is selected to have a pH approximately the same as a pH of the planarizing liquid  144 , to maintain the pH of the substrate  112  at an approximately constant level as the substrate  112  is exposed to the rinsing liquid  153 . In one embodiment, the CMP machine  110  can include a pH meter  158  coupled to the rinsing liquid supply  154  and the planarizing liquid supply  145  to monitor the pH levels of both liquids. The pH meter can include a conductivity meter or other device that detects pH. 
   In one embodiment, the planarizing liquid  144  can include Klebosol, an ammonia-based solution available from Rodel Corp. of Newark, Del. having a pH in the range of approximately 10.6 to approximately 11.4, and more particularly, approximately 11.0. The pH of the rinsing liquid  153  can be selected to have a pH in approximately the same range. In one embodiment, the rinsing liquid  153  can include a mixture of deionized water provided by a deionized water supply  155  and tetramethyl ammonium hydroxide (TMAH) provided by a TMAH supply  156 . The relative amount of deionized water and TMAH included in the rinsing liquid  153  can be controlled by adjusting a rinsing liquid valve  157  coupled between the deionized water supply  155  and the TMAH supply  156 . In one embodiment, the rinsing liquid  153  can include 99.994% deionized water and 0.006% TMAH by volume, to have a pH of approximately 11.0. In other embodiments, the planarizing liquid  144  and the rinsing liquid  153  can include other compositions having other pHs, so long as the pH of the rinsing liquid  153  is selected to be approximately the same as the pH of the planarizing liquid  144 . 
   In another aspect of this embodiment, the rinsing liquid  153  can be selected to have an electrical charge that is approximately the same as an electric charge of the planarizing liquid  144 . For example, in one embodiment, the electrical charge of the rinsing liquid  153  and the planarizing liquid  144  can be selected to be approximately zero to reduce the likelihood of imparting unwanted electrical charges to the substrate  112 . In other embodiments, the substrate  112 , the rinsing liquid  153 , and/or the planarizing liquid  144  can have other non-zero electrical charges. 
   The CMP machine  110  can also include a substrate transporter  160  and a rinse chamber  150 . As was discussed above, the substrate transporter  160  can include a plurality of articulated movable arms  161  coupled to a grasping device  162 . The grasping device  162  can engage the substrate  112  after it has been released from the carrier assembly  130  and the arms  161  can be controlled to robotically transfer the substrate  112  to the rinse chamber  150 . The rinse chamber  150  can include spray bars  151  positioned on opposite sides of adjacent rinse bays  157 . The spray bars  151  can direct the rinsing liquid  153  through nozzles  152  toward the substrate  112  to clean opposing surfaces of the substrate  112  when the substrate is positioned in one of the rinse bays  157 . 
   The rinsing liquid  153  can have a pH that is approximately the same as the pH of the planarizing liquid  144 , to maintain the pH of the substrate  112  at an approximately constant level for an additional portion of the post-planarization processing operation. In one-aspect of this embodiment, the rinsing fluid  153  can be supplied from the same rinsing liquid supply  154  that supplies rinsing liquid the polishing pad  141 . Accordingly, the valve  149  can be adjustable to provide the rinsing solution  153  to the polishing pad  141  and/or the rinsing chamber  153 , as well as provide the planarizing liquid  144  to the polishing pad  141 . 
   In operation, the planarizing liquid  144  is pumped from the planarizing liquid supply  145  through the orifices  143 . The microelectronic substrate  112  engages the planarizing surface  142  of the polishing pad  141  while the platen  120  and/or the carrier assembly  130  are moved relative to each other to planarize the microelectronic substrate  112 . As the planarizing process nears completion, the relative velocity between the microelectronic substrate  112  and the polishing pad  141  is ramped down or reduced to zero by gradually halting the motion of the platen  120  and/or the carrier assembly  130 . 
   As the relative velocity between the substrate  112  and the platen  120  decreases, the flow of planarizing liquid  144  is halted and the rinsing liquid  153  is supplied to the polishing pad  141 . In one embodiment, the time required to halt the relative motion between the substrate  112  and the polishing pad  141  (and accordingly, the time during which the substrate  112  is rinsed on the polishing pad  141 ), is in the range of approximately twenty to approximately forty seconds, and preferably approximately forty seconds. In other embodiments, the substrate  112  can be rinsed on the polishing pad  141  for greater or lesser periods of time, depending upon, for example, the initial relative velocity between the substrate  112  and the polishing pad  141 , the normal force between the substrate  112  and the polishing pad  141 , and the fluid characteristics of the planarizing liquid  144  and the rinsing liquid  153 . 
   Once the relative motion between the microelectronic substrate  112  and the polishing pad  141  is halted, the carrier assembly  130  disengages from the substrate  112  and the substrate transporter  160  engages the substrate  112  and removes the substrate from the polishing pad  141 . The substrate transporter  160  moves the substrate  112  to the rinse chamber  150  where the substrate is sprayed with the rinsing solution  153 . In one embodiment, the substrate  112  can be rinsed for approximately five seconds in the rinse chamber, and in other embodiments, the substrate may be rinsed for greater or lesser periods of times. 
   An advantage of the CMP machine  110  and the process described above with reference to  FIG. 2  is that the substrate  112  can be maintained at an approximately constant pH level throughout the planarization, ramp down, and rinsing operations. This is advantageous because particulate matter, such as material removed from the substrate  112 , material removed from the polishing pad  141 , and/or abrasive particles in the planarizing liquid  144  may be less likely to adhere to the microelectronic substrate  112  when the pH of the substrate  112  remains approximately constant. Accordingly, the likelihood of contaminating the substrate  112  with particulate matter can be substantially reduced, increasing the number of defect-free substrates. The absence of particulate matter may also be advantageous because post-CMP processing steps, such as masking, may be more accurately performed without the interference created by the particulate matter. 
   Another advantage of the CMP machine  110  and process described above with reference to  FIG. 2  is that the machine can increase the throughput of substrates  112 . For example, conventional CMP methods that include changing the pH of the substrate  112  before all of the particulates have been removed may require that the substrate be rinsed in a rinse chamber for approximately thirty seconds. By contrast, the process described above can include rinse times in the rinse chamber  150  on the order of approximately five seconds. 
   Still referring to  FIG. 2 , the CMP machine  110  can be operated in accordance with another embodiment of the invention by supplying the planarizing fluid  144  to the polishing pad  141  during both the planarization and ramp-down steps. Accordingly, the pH of the microelectronic substrate  112  can remain approximately constant during both the planarization and ramp-down steps. The substrate  112  can then be moved directly to the rinse chamber  150  and rinsed with the rinsing liquid  153  without first rinsing the substrate  112  on the polishing pad  141 . An advantage of this process is that it does not require the rinsing solution supply  154  to be coupled to the polishing pad  141 , potentially simplifying the CMP machine  110 . Conversely, an advantage of rinsing the substrate  112  on the polishing pad  141  before moving the substrate to the rinse chamber  150  is that the additional rinse step may increase the likelihood that any particulate matter adhering to the substrate  112  is removed. 
     FIG. 3  is a schematic side elevation view of a CMP machine  210  having a conduit  246  that delivers fluid downwardly onto the planarizing surface  142  of the polishing pad  141 , in accordance with another embodiment of the invention. Accordingly, an advantage of the CMP machine  210  when compared with the CMP machine  110  shown in  FIG. 2  is that the need for orifices  143  ( FIG. 2 ) and a manifold  148  ( FIG. 2 ) is eliminated, potentially simplifying the construction and maintenance of the CMP machine  210 . Conversely, an advantage of the CMP machine  110  is that it may more uniformly distribute the planarizing fluid  144  over the planarizing surface  142 , and may distribute the planarizing fluid  144  independent of the location of the carrier assembly  130 . 
   As is also shown in  FIG. 3 , the CMP machine  210  can include a conditioner  270  to refurbish the polishing pad  141  after planarization. In one embodiment, the conditioner  270  can include an abrasive disk  271  that roughens the planarizing surface  142  of the polishing pad  141  and removes polishing pad material from the planarizing surface. The conditioner  270  can also include a conditioning fluid source  272  in addition to or in lieu of the abrasive disk  271 , for removing polishing pad material from the polishing pad  141 . In one embodiment, the conditioning fluid can be chemically active to chemically remove the polishing material. In another embodiment, the conditioning fluid can be chemically inactive, but can act to flush the removed polishing pad material away from the polishing pad  141 . In either case, a pH of the conditioning fluid can be selected to be approximately the same as the pH of the planarizing liquid  144 . For example, the conditioning fluid can have the same chemical composition as the rinsing liquid  153 . Accordingly, the conditioning fluid can be supplied by the rinsing solution supply  154  in one embodiment and the separate conditioning fluid source  172  can be eliminated. 
   In operation, the CMP machine  210  can be initially operated according to the steps discussed above with reference to  FIG. 2  to planarize the substrate  112 . In one embodiment, the substrate  112  can be moved directly to the rinse chamber after planarization (as was generally discussed above with reference to  FIG. 2 ). Alternatively, the polishing pad  141  can be conditioned after planarization, and the substrate  112  can be buffed or cleaned on the conditioned polishing pad  141 . For example, the polishing pad  141  can be conditioned by moving the abrasive disk  271  over the planarizing surface  142  and/or by flushing the planarizing surface  142  with the conditioning liquid. After the conditioning step has been completed, the substrate  112  can be buffed by moving the substrate  112  relative to the newly conditioned planarizing surface  142  in the presence of the rinsing solution  153 . The buffing step can remove substrate material from the substrate  112  and/or can remove particulates that adhere to the surface of the substrate  112 . In one embodiment, the ramp-down time can be reduced from a range of twenty to forty seconds to a range of approximately ten to approximately thirty seconds (preferably approximately fifteen seconds) when the ramp-down step is followed by the buffing step. Optionally, the substrate  112  can then be rinsed in the rinse chamber  150 . 
   One advantage of the CMP machine  210  and the process discussed above with reference to  FIG. 3  is that the conditioning fluid has a pH approximately the same as the pH of the planarizing liquid  144 . Accordingly, the pH of the polishing pad  141  can be maintained at an approximately constant level, increasing the likelihood that the substrates  112  contacting the polishing pad  141  also remain at an approximately constant pH. As discussed above, keeping the pH of the microelectronic substrate  112  at an approximately constant level can reduce the tendency for particulate matter to adhere to the substrate  112 . 
   Another advantage of the process described above with reference to  FIG. 3  is that the microelectronic substrate  112  can be buffed on the same polishing pad  141  as was used to planarize the substrate  112 , unlike some conventional methods which require a separate buffing pad. This is advantageous because it can reduce the number of pads necessary for CMP and post-CMP processing, and can also increase throughput by eliminating the step of moving the wafer from the polishing pad to a separate buffing pad. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.