Abstract:
A cleaning and slurry distribution assembly for use in a chemical mechanical polishing apparatus. The cleaning assembly includes a plurality of nozzles for directing a cleaning fluid against a polishing pad. The cleaning assembly further includes a housing for containing residual droplets, slurry and contaminants. The slurry distribution assembly includes a ring for optimally distributing slurry on the polishing pad.

Description:
BACKGROUND 
     The invention relates chemical mechanical polishing of substrates, and more particularly to dispensing slurry onto a polishing pad and cleaning the polishing pad. 
     Chemical mechanical polishing (CMP) is a process by which a substrate surface is planarized to a uniform level. In a conventional CMP apparatus, substrate is mounted on a rotatable carrier head and pressed against a rotating polishing pad. An abrasive chemical solution (slurry) is applied onto the polishing pad to aid in the polishing of the substrate to achieve a desired surface finish. Over time, the polishing process glazes the polishing pad and creates irregularities in the polishing pad surface that can adversely affect the substrate surface finish. The polishing pad surface is typically “conditioned” by scouring the polishing pad surface with an abrasive device known as a conditioning disk to deglaze and roughen the polishing pad surface. Periodically conditioning the pad maintains the pad surface at a consistent state of roughness to achieve consistent polishing uniformity. 
     One problem encountered in CMP is the generation of contaminants on the polishing pad surface during the polishing and conditioning procedures. These contaminants have a material adverse affect on the polishing process. For example, contaminants include (but are not limited to) abraded polishing pad material, dried slurry particles, conditioning disk material and airborne contaminants. Adverse material effects include (but are not limited to) scratching of the substrate and embedding of the particles in the polishing pad or substrate. It would be advantageous if the polishing apparatus cleaned the polishing pad to provide a substantially contaminant-free polishing pad. 
     Another problem in CMP is that slurry is an expensive consumable. A CMP system may use more than two hundred milliliters of slurry per minute. In general, the substrate takes two to three minutes to polish. Thus, a CMP system can use up to a sixth of a gallon of slurry per substrate. The per substrate cost of CMP could be reduced considerably by reducing the amount of slurry used. In addition, where excessive slurry is applied, the substrate can hydroplane over the surface of the polishing pad, thereby reducing the polishing rate. It would be advantageous if the CMP apparatus that reduced slurry consumption in the polishing process. 
     SUMMARY 
     In one aspect, the invention is directed to an apparatus for use in a chemical mechanical polishing system. The apparatus has a housing positionable over a polishing pad and at least one nozzle covered by the housing to spray a cleaning fluid against the polishing pad. 
     Implementations of the invention may include the following. The cleaning fluid may be deionized water, and may be sprayed by the nozzle under hydraulic pressure. The housing may extends toward the center of the polishing pad, and may be configured to be raised and lowered over a region of the polishing pad. A retainer may be joined to a lower surface of the housing, and may contact a surface of the polishing pad, e.g., at a pressure less than about 5 psi. A first feed line may supply the cleaning fluid to the assembly, a second feed line may supply a solution of deionized water and an agent selected from the group consisting of a corrosion inhibitor, a cleaner, an oxidizer, a pH adjustor, a dilution fluid, and a surface wetting agent, and a third feed line may supply an abrasive solution. 
     In another aspect, the invention is directed to a method of cleaning the surface of a polishing pad in a chemical mechanical polishing system. A cleaning fluid is directed from a cleaning assembly against a polishing pad that has residual contaminants, and the cleaning fluid is substantially contained within a housing of the cleaning assembly. 
     Implementations of the invention may include the following features. The cleaning fluid may be deionized water, and droplets of the cleaning fluid may be produced by subjecting the deionized water to a hydraulic pressure, e.g., of less than about 60 psi, such as less than about 10 psi. 
     In another aspect, the invention is directed to an apparatus for distributing slurry onto a polishing surface. The apparatus has a retainer having a lower surface in close proximity to the polishing surface and enclosing a region, and an outlet to distribute slurry to the enclosed region to form a reservoir of slurry in the enclosed region. The slurry is distributed to a region not enclosed by the retainer by traveling between the polishing surface and the lower surface of the retainer. 
     In another aspect, the invention is directed to a method of preparing the surface of a polishing pad in a chemical mechanical polishing system for polishing a substrate. In the method, a cleaning fluid impinges against the polishing pad having at least one of residual polishing slurry, contaminants and fluid. the cleaning fluid, residual polishing slurry, contaminants and fluid are substantially contained by means of a housing. The housing is lifted to expel at least a portion of the residual polishing slurry, contaminants and fluid from the polishing pad. A polishing slurry is applied to the polishing pad, and the polishing slurry is spread over the polishing pad with a lower surface of the housing. 
     The present invention advantageously cleans the polishing pad to provide a substantially contaminant-free polishing pad. The invention also can apply a uniform layer of polishing slurry to the polishing pad to provide improved polishing and planarization of the substrate while minimizing/optimizing the amount of slurry used. 
     Other features and advantages will become apparent from the following description, including the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic exploded view of a chemical mechanical polishing system. 
     FIG. 2 is a schematic top view of the CMP system of FIG. 1 showing a carrier head, a conditioning apparatus, and a cleaning and slurry distribution arm assembly. 
     FIG. 3 is a cross-sectional view of the cleaning and slurry distribution assembly of FIG. 2 taken along line  3 — 3 . 
     FIG. 4 is a cross-sectional view of the cleaning and slurry distribution assembly of FIG. 2 taken along line  4 — 4 . 
     FIG. 5 is a cross-sectional view of the cleaning and slurry distribution assembly being used to distribute slurry on the polishing pad. 
     FIG. 6 is a flow chart showing the process performed with the cleaning and slurry distribution assembly. 
     FIG. 7 is a cross-sectional view of a cleaning and slurry distribution assembly that includes multiple slurry delivery lines. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a chemical mechanical polishing apparatus  10  includes three independently-operated polishing stations  14 , a substrate transfer station  16 , and a rotatable carousel  18  which choreographs the operation of four independently rotatable carrier heads  20 . A similar polishing apparatus is discussed in U.S. Pat. No. 5,738,574, the entirety of which is incorporated herein by reference. 
     The carousel  18  has a support plate  42  with slots  44  through which drive shafts  46  for the carrier heads  20  extend. The carrier heads  20  independently rotate and oscillate back-and-forth in the slots  44 . The carrier heads  20  are rotated by the respective motors  48 , which are normally hidden behind removable sidewalls  50  of the carousel  18 . In operation, a substrate is transferred from the transfer station  16  to a carrier head  20 . The carousel  18  then transfers the carrier head and substrate through a series of one or more polishing stations  14 , and finally returns the substrate to the transfer station  16 . 
     Each polishing station  14  includes a rotatable platen  52  having secured thereto a polishing pad  54 . The polishing station  14  optionally includes a pad conditioner  56  mounted to a tabletop  57  of the polishing apparatus  10 . Each pad conditioner  56  includes a conditioner head  30 , an arm  32 , and a base  34  for positioning the conditioner head  30  over the surface of the polishing pad to be conditioned. Each polishing station  14  also includes a cup  36  containing a fluid for rinsing the conditioner head  30 . 
     Referring to FIG. 2, the polishing pad  54  is conditioned by the pad conditioner  56  while the polishing pad  54  polishes a substrate  5  (shown in phantom) mounted on the carrier head  20 . The conditioner head  30  sweeps across the polishing pad  54  with a motion that is synchronized with the motion of the carrier head  20  to avoid collision. Such synchronization may be controlled, for example, by a general purpose computer. For example, the carrier head  20  may be positioned in the center of the polishing pad  54  and the conditioner head  30  may be immersed in a rinsing fluid contained within the cup  36 . During polishing, the cup  36  may pivot out of the way, and the carrier head  20  and the conditioner head  30  may be swept back-and-forth across the polishing pad  54  (e.g., between the positions shown in solid and phantom) as shown by arrows  28  and  38 , respectively. 
     Each polishing station  14  also includes a corresponding slurry delivery and cleaning arm assembly  60  mounted to the table top  57  by a support post  62 . The arm assembly  60  serves two main purposes: to spread slurry over the surface of the pad in a thin layer, and to remove residues and contaminants, such as residual slurry, dirt, dust, abraded substrate material, abraded polishing pad material and other contaminants that would have a material adverse affect on the polishing process, from the polishing pad surface. The arm assembly  60  extends over the polishing pad from the pad edge to the pad center. The arm assembly  60  may be designed and configured to pivot about the support post  62  so as to sweep across over the surface of the polishing pad  54 . Specifically, the motion of the arm assembly  60  may be synchronized with the motion of the carrier head  20  and the conditioner head  30  to avoid collisions therebetween. Alternately, if the carrier head does not move over the pad center, the arm assembly  60  can remain stationary during polishing. 
     As shown in FIGS. 2,  3  and  4 , the slurry dispensing/cleaning arm assembly  60  includes an elongated housing  64  that extends from the platen edge to near the platen center. The housing  64  is supported by the support post  62 , and has a recess with an opening on the side of the housing that faces the polishing pad  54 . The volume between the polishing pad  54  and the housing  64  defines a chamber  66 . The chamber  66  contains the streams of cleaning fluid, and serves as a container for the slurry. 
     To clean the polishing pad, a spray of cleaning fluid is directed from the arm assembly  60  onto the polishing pad surface. Specifically, a set of fluid dispensing nozzles  72  are located inside the chamber  66  to spray streams  76  of a cleaning fluid, such as deionized water, against the top surface of the polishing pad  54 . Although four nozzles are illustrated, the assembly  60  could include more or fewer nozzles. The assembly may include 4-6 nozzles. The stream  76  from each nozzle  72  cleans and loosens residues and contaminants (such as residual liquid slurry, dust, dried slurry, abraded polishing pad material, abraded substrate, etc.) from the polishing pad  54 , particularly from any grooves or holes in the polishing pad  54 . Such cleaning advantageously prepares the polishing pad  54  for polishing. The cleaning fluid is supplied to the nozzles by a feed line  80 . Although illustrated as a passage through the housing  64 , the feed line  80  could be implemented as tubing inside or outside the chamber  66 . 
     The nozzles  72  may be any conventional nozzle capable of atomizing the cleaning fluid. For example, each nozzle may be an airless nozzle in which the cleaning fluid is forced through a small orifice under hydraulic pressure, such as less than about 60 psi, e.g., about 10-60 psi. The nozzles may also be air-assisted nozzles in which the cleaning fluid is forced through a small orifice under pressure (such as 60 psi) and the resultant fluid stream is further atomized and propelled by a compressed gas, such as compressed air. The compressed air may be pressurized, e.g., up to 10 psi, or about 5 psi. As such, the cleaning liquid may be sprayed at a rate in the range of about 0.2 to 1.0 gal/min. The nozzles  72  may be constructed from a chemical and corrosion resistant material, such as a polyvinyldene fluoride (PVDF) thermoplastic. For example, each nozzle may be a KYNAR4® Series Spray Nozzle, Model HVV-KY. 
     The assembly also includes a lower retainer  78  that projects downwardly from the housing  64 , and can be lowered to contact the polishing pad  54 . The housing  64  and the retainer  78  may be a unitary body, or the retainer  78  may be secured (e.g., by an adhesive or by screws or bolts) to the housing  64 . When the lower retainer  78  contacts the polishing pad  54 , it forms a dam to retain slurry and rinse water within a reservoir formed by the retainer and pad. The lower retainer  78  may contact the pad  54  at pressure of about 1 psi. The retainer  78  and the housing  64  may be constructed from a chemically resistant and wear resistant material, such as a polyphenylsulfide (PPS), a polytetrafluoroethylene (PTFE) or DELRIN™. 
     The arm assembly  60  is adapted to move up and down (i.e., to be raised and lowered with respect to the polishing pad  54 ) by a pneumatic or mechanical actuator  70 . The arm assembly  60  is lowered in contact with the polishing pad  54  to enclose the streams  76  of deionized water and prevent the resulting waste materials (e.g., polishing slurry, residues, contaminants, waste water, etc.) from splashing and collecting on the landing on exterior surfaces of the polishing apparatus  10 . These materials might otherwise form dried deposits which can flake off and land on the polishing pad  54  causing a defect in the substrate. The splashed liquids may also penetrate the interior workings of the polishing apparatus  10 , causing corrosion and other damage. When cleaning is completed, the arm assembly  60  may be raised to allow the contained liquid and residual materials to be centrifugally expelled from the polishing pad  54  as the pad rotates. Expelling the water, diluted slurry, residues and contaminants from the arm assembly  60  prevents the substrate from being polished with diluted slurry. 
     The arm assembly  60  is also be used to distribute a polishing slurry to the polishing pad  54 . A slurry delivery line  82  may connect one or more slurry outlets  84  to a slurry source for the polishing slurry. As shown in FIG. 5, after the pad has been cleaned, assembly  60  is lowered so that the retainer  78  contacts the polishing pad  54 . Then the polishing slurry is fed from the slurry delivery line  82  through the slurry outlets  84  so that it accumulates in a reservoir  86  contained by the retainer  78  and the housing  64 . The polishing slurry in the reservoir then either seeps out between a thin gap between the retainer  78  and the polishing pad  54 , or is carried beneath the lower retainer  78  by grooves or perforations in the polishing pad  54 . In either case, this arm assembly  60  leaves a thin layer of slurry  88  on the polishing pad  54 . The assembly housing  64  also prevents the polishing slurry from splattering and coating the exterior surfaces or penetrating the interior surfaces of the polishing apparatus  10 . 
     Referring to FIG. 6, a method  100  performed with the arm assembly  60  begins with a polishing operation when the assembly  60  is lowered into contact with the polishing pad  54  (step  102 ). The polishing slurry is directed through the slurry delivery line  82  to create the reservoir  86  of slurry on the polishing pad inside the housing  64  (step  104 ). The polishing proceeds for a period of time, such as about 15 seconds to 2 minutes, during which the reservoir  86  can be periodically or intermittently refilled. Specifically, slurry can be supplied at a flow rate equal to or slightly greater than the consumption rate of the slurry for a given set of polishing parameters. For example, slurry may be dispensed through the slurry outlets  84  at a flow rate in the range of about 50 to 200 ml/min. A well-distributed and uniform thin layer of slurry is deposited the pad  54  by the wiping action of the retainer  78 . By depositing a thin layer of slurry, excessive slurry usage can be greatly reduced. 
     After polishing has been completed, the arm assembly  60  is lifted and the remaining slurry is centrifugally expelled (step  106 ). During the cleaning operation, the arm assembly  60  is lowered back into contact with the polishing pad (step  108 ). Then the cleaning fluid (e.g., deionized water) is forced through the nozzles to direct a spray of cleaning fluid onto the polishing pad  54  inside the housing  64  (step  110 ). The cleaning fluid may be sprayed at a rate of about 0.5 gal./min. The arm assembly  60  may be held in a horizontal position, or it may be swept horizontally across a portion of the polishing pad  54  adjacent the region conditioned by the conditioner head  32 . In the later application, the assembly  60  may pivot over a fixed area above the polishing pad  54 . If the fixed area does not overlap the area swept by the conditioner arm  32  and head  30 , there is no need for a process controller to control the movements of the assembly  60 , the carrier head  20 , and the pad conditioner  56 . The cleaning mode is run for a period of time sufficient to suitably clean the pad in preparation for polishing a substrate, e.g., ten seconds. Once the cleaning operation is completed, the arm assembly  60  is lifted away from the polishing pad so that the waste water inside the housing  64  can be centrifugally expelled from the rotating polishing pad  54  (step  112 ). It is important for such fluids and materials to be removed from the pad to ensure that the pad is free of contaminants prior to polishing a substrate. 
     FIG. 7 shows another embodiment of the slurry delivery/rinse arm assembly that includes dual slurry delivery lines. The first slurry delivery line  82 ′ delivers a first slurry component to the polishing pad  54  via one or more of the slurry outlets  84 ′. A second slurry delivery line  90  delivers a second slurry component to the polishing pad via one or more outlets  92 . The first and second slurry components are mixed together in the reservoir formed by the retainer  78 . Both slurry delivery lines could deliver abrasive solutions. Alternately, the second slurry delivery line could be used to supply a chemical to control the polishing process, such as a corrosion inhibitor, an oxidizer, a dilution fluid, a pH adjustor, or a surface wetting agent. 
     For example, in CMP applications to polish a tungsten film layer, the first slurry component may include a solution of ferric nitrate and additives, such as buffers. The second slurry component may include an abrasive solution, such as fumed or colloidal silica, or alumina. Chemical reactions take place between constituents of the first and second slurry components that may age the resultant mixture. Thus, the first and second slurry components are mixed just prior to being utilized as a polishing medium to polish the tungsten. 
     The lower surface of retainer  78  can be roughened, or an abrasive material can be coated on the lower surface of the retainer  78 . When the arm assembly  60  is lowered into contact with the polishing pad  54 , the abrasive lower surface of the retainer  78  roughens and deglazes the polishing pad. Thus, the arm assembly  60  can be used to condition the polishing pad. In this implementation, the polishing apparatus  10  need not include a separate pad conditioner  56 . 
     The invention has been described with reference to various drawings, aspects and preferred embodiments. It is to be understood that the above descriptions are made by way of illustration, and that the invention may take other forms within the spirit of the structures and methods described herein. The invention includes variations and modifications thereof as defined in the claims attached hereto.