Abstract:
Systems and methods for polishing a substrate with reduced contamination are described. Moist air is directed to one or more surfaces in proximity to the polishing surface and exposed to airborne slurry particles generated during polishing. By maintaining the atmosphere in the vicinity of the exposed surfaces at an elevated relative humidity level, airborne slurry particles adhering to the exposed surfaces remain in suspension and, therefore, may be easily cleaned, e.g., during a high pressure rinse cycle. This feature reduces the likelihood that slurry particles will accumulate on exposed surfaces of the polishing apparatus and flake off while a substrate is being polished, reducing the likelihood of substrate defects caused by such slurry contamination.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to substrate polishing techniques, including chemical mechanical polishing (CMP). 
     Chemical mechanical polishing is a process by which a substrate surface is smoothed (planarized) to a uniform level by a polishing pad and an abrasive slurry. A substrate to be polished is usually mounted on a rotatable carrier head and pressed against a moving polishing pad. The polishing pad typically consists of an abrasive sheet. An abrasive chemical solution (slurry) may be introduced onto the polishing pad to assist in the polishing process. Typically, a rinse arm supplies de-ionized water or other rinse fluid to the polishing pad to remove coagulated slurry and other material from the polishing pad surface. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features systems and methods for polishing a substrate with reduced contamination. A substrate polishing system comprises: a substrate polishing apparatus including a substrate carrier, a polishing surface, and a slurry dispenser; and a humidifier configured to direct moist air to one or more surfaces in proximity to the polishing surface and exposed to airborne slurry particles generated during polishing. A method of polishing a substrate with a substrate polishing system comprises: supporting a substrate above a polishing surface; dispensing slurry onto the polishing surface; polishing the substrate against the polishing surface; and directing moist air to one or more surfaces of the substrate polishing system in proximity to the polishing surface and exposed to airborne slurry particles generated during polishing. 
     Embodiments may include one or more of the following features. 
     The humidifier may be configured to direct moist air in the vicinity of the substrate carrier. In one embodiment, the humidifier comprises a liquid atomizer. In another embodiment, the humidifier comprises a nozzle. In accordance with this embodiment, the humidifier is configured to receive a liquid and a gas, to mix received liquid and gas, and direct the mixed liquid and gas through the nozzle to generate a fine mist in the vicinity of the one or more exposed surfaces. 
     In operation, the humidifier may be configured to maintain the atmosphere in the vicinity of the one or more exposed surfaces at a relative humidity level of about 80% or greater. In one mode of operation, the humidifier may be configured to maintain a layer of liquid on the one or more exposed surfaces. 
     A humidistat may be coupled to the humidifier to control the humidity in the vicinity of the exposed surfaces. 
     Among the advantages of the invention are the following. By maintaining the atmosphere in the vicinity of the exposed surfaces at an elevated relative humidity level, airborne slurry particles adhering to the exposed surfaces remain in suspension and, therefore, may be easily cleaned, e.g., during a high pressure rinse cycle. This feature reduces the likelihood that slurry particles will accumulate on exposed surfaces of the polishing apparatus and flake off while a substrate is being polished, reducing the likelihood of substrate defects caused by such slurry contamination. 
    
    
     Other features and advantages will become apparent from the following description, including the drawings and the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is perspective view of a polishing apparatus. 
     FIG. 1B is an exploded view of the polishing apparatus of FIG.  1 A. 
     FIG. 2A is a diagrammatic side view of a substrate being polished and airborne slurry particles depositing onto exposed surfaces of the polishing apparatus. 
     FIG. 2B is a diagrammatic side view of a substrate being polished while moist air is being directed to surfaces of the polishing apparatus in proximity to the polishing surface and exposed to airborne slurry particles generated during polishing. 
     FIG. 3 is a diagrammatic view of the bottom portion of a substrate polishing apparatus that includes four misting nozzles. 
     FIG. 4 is a cross-sectional side view of a misting nozzle. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1A and 1B, a polishing apparatus  10  includes a housing  12  that defines a substantially enclosed polishing environment and contains 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 . Attached to one side of housing  12  is a substrate loading apparatus  22  that includes a tub  24  that contains a liquid bath  26  in which cassettes  28  of substrates  30  are immersed before polishing. 
     Carousel  18  has a support plate  42  with slots  44  through which shafts  46  of carrier heads  20  extend. Carrier heads  20  can independently rotate and oscillate back-and-forth in slots  44  to achieve a uniformly polished substrate surface. Carrier heads  20  are rotated by respective motors  48 , which are normally hidden behind removable sidewalls  50  of carousel  18 . In operation, a substrate is loaded from tub  24  to transfer station  16 , from which the substrate is transferred to a carrier head  20 . Carousel  18  then transfers the substrate through a series of one or more polishing stations  14  and finally returns the polished substrate to transfer station  16 . 
     Each polishing station  14  includes a rotatable platen  52 , which supports a polishing pad  54 , and a fluid delivery system  80 . Platen  52  and fluid delivery system  78  are both mounted to a tabletop  57  of polishing apparatus  10 . A pad conditioner (not shown) also may be provided to condition the surface of polishing pad  54 . 
     For further details regarding the general features and operation of polishing apparatus  10 , please refer to co-pending application Ser. No. 08/549,336, filed, Oct. 27, 1995, by Perlov et al., entitled “Continuous Processing System for Chemical Mechanical Polishing,” and assigned to the assignee of the present invention, which is herein incorporated by reference. 
     Referring to FIG. 2A, substrate  30  is polished as it is pressed against and moved relative to polishing pad  54 . Slurry particles in a colloidal suspension  80  assist in the polishing process. As substrate  30  is polished, portions of colloidal suspension  80  may evaporate or otherwise become airborne. This airborne substance may adhere to exposed surfaces in proximity to polishing pad  54 . Once adhered to an exposed surface, the colloidal suspension tends to dry, leaving deposits  82 ,  84 ,  86 ,  88 ,  90  and  92  which may accumulate over time and occasionally flake off onto polishing pad  54 . If such contamination becomes trapped between substrate  30  and polishing pad  54  while substrate  30  is being polished, it would likely cause a defect in the substrate surface. 
     As shown in FIG. 2B, this problem is substantially reduced by supplying moist air  94  to surfaces in proximity to polishing pad  54  and exposed to airborne slurry particles. Moist air  94  maintains the atmosphere in the vicinity of the exposed surfaces at a relative humidity level of about 80% or greater, and preferably at a relative humidity level of about 90% or greater, up to a relative humidity level of about 99%. At these humidity levels, colloidal suspensions adhering to the exposed surfaces do not dry and, instead, the slurry particles remain in suspension during polishing. In some modes of operation, moist air stream  94  maintains a layer of liquid (e.g., deionized water) on the exposed surfaces to prevent adhered colloidal suspensions from drying. Once a substrate has been polished the exposed surfaces may be rinsed with a flow of deionized water or other liquid that entrains the suspended slurry particles and washes them from the exposed surfaces. In this way, deposits of slurry particles do not accumulate on the exposed surfaces, reducing the amount of contamination that might cause substrate defects. The exposed surfaces may be rinsed after one or more substrates have been polished, or less often, e.g., while polishing pad  54  is being replaced or during some other routine maintenance procedure. One or more humidistats may be coupled to a controller in respective feedback loops to enable humidifiers 96-100 to maintain the atmosphere in the vicinity of the exposed surfaces at the desired humidity level. 
     Moist air stream  94  may be supplied to the exposed surfaces in proximity to polishing pad  54  in a variety of ways, including ultrasonic humidification and fine mist generation. 
     Referring back to FIGS. 1A and 1B, in one embodiment, three ultrasonic humidifiers  96 ,  98  and  100  are incorporated into three of the side walls of housing  12 . Humidifiers  96 - 100  include air blowers that direct streams of air through respective atmospheres of atomized deionized water to generate moist air streams  94 . Each humidifier  96 - 100  is configured to direct its moist air stream to a respective polishing station  14 . The moist air streams preferably are directed in the vicinity of substrate carriers  20  and associated surfaces, including the surfaces of support plate  42  and slots  44 , to prevent slurry deposits from accumulating on these surfaces. Humidifiers  96 - 100  may supply moist air streams to the exposed surfaces before, during and after substrate polishing. Humidistats  97 ,  99  and  101  (hidden) measure the relative humidity adjacent to exposed surfaces of support plate  42 . Signals from humidistats  97 ,  99  and  101  are sent to a controller which controls the operation of humidifiers  96 ,  98  and  100  to achieve a desired relative humidity level in the vicinity of the exposed surfaces. 
     Referring to FIG. 3, in another embodiment, polishing apparatus  10  includes four misting nozzles  102 ,  104  and  106 , each positioned adjacent to a respective polishing station  14 . Misting nozzles are configured to direct a fine water mist in the vicinity of surfaces in proximity to polishing pad  54  and exposed to airborne slurry particles generated during polishing. Each misting nozzle  102 - 106  directs its respective moist air stream in the vicinity of a respective substrate carrier  20  and its associated surfaces, including the associated surfaces of support plate  42  and slot  44 , to prevent slurry deposits from accumulating on these surfaces. Misting nozzles  102 - 106  may be moved to other locations within polishing apparatus  10 , or additional misting nozzles may be installed, to provide moisture sufficient to prevent slurries from drying on the exposed surfaces within the polishing apparatus. 
     Misting nozzles  102 - 106  are coupled to a controller (not shown) that is configured to choreograph the operation of nozzles  102 - 106 . In one mode of operation, nozzles  102 - 106  supply moist air streams to the exposed surfaces only after a substrate has been polished, while the exposed surfaces are being cleaned (e.g., with a high pressure deionized water rinse), or before polishing while polishing apparatus  10  is idle. In another mode of operation, a humidistat is coupled to the controller in a feedback loop to enable misting nozzles  102 - 106  to maintain the atmosphere in the vicinity of the exposed surfaces at the desired relative humidity level. The controller is preferably programmable to enable operators to select the times during which nozzles  102 - 106  are supplying moist air streams  94  to the exposed surfaces. 
     As shown in FIG. 4, each misting nozzle  102 - 106  includes an inlet  110  for receiving a gas (e.g., nitrogen or “clean dry air”), an inlet  112  for receiving a liquid (e.g., deionized water), a mixing chamber  114 , and an outlet  116  for supplying a fine mist of moist air to exposed surfaces of polishing apparatus  10 . The pressures at which gas and liquid are supplied to nozzles  102 - 108  are selected to achieve a desired humidity level at the exposed surfaces. An exemplary gas pressure range is about 30-80 psi, and an exemplary liquid pressure range is about 25-55 psi. 
     Although a rotating polishing system has been described above, the invention also may be implemented in connection with a linear polishing system, such as the linear polishing system described in U.S. application Ser. No. 08/568,188, filed Dec. 5, 1995, and entitled “Substrate Belt Polisher,” which is incorporated herein by reference. 
     Other embodiments are within the scope of the claims.