Abstract:
An apparatus for supplying a slurry to a polishing surface has a slurry source, a slurry supply line, and a slurry return line. The slurry supply line and slurry return line are configured so that slurry may be directed from the outlet of the slurry supply line onto the polishing surface during a chemical mechanical polishing operation, or into an inlet of the slurry return line after the polishing operation is stopped. This permits continuous circulation of slurry through the slurry supply line to prevent coagulation.

Description:
BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to an apparatus and method for supplying slurry to a polishing pad. 
     Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e, the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents a photolithography problem for the integrated circuit manufacturer. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface. 
     Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a moving polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad. 
     An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between t.-ie substrate and pad, and the force pressing the substrate against the pad. 
     One problem in CMP is coagulation of the polishing slurry. Specifically, small abrasive particles in the slurry tend to conglomerate to form larger particulates. These large particulates create scratches, e.g., shallow grooves on the order of 300 angstroms (A) deep, in the substrate surface. These scratches render the substrate finish unsuitable for integrated circuit fabrication, decreasing process yield. 
     SUMMARY 
     In one aspect, the invention is directed to an apparatus for supplying a slurry to a polishing surface. The apparatus has a slurry source, a slurry supply line, and a slurry return line. The slurry supply line extends from the slurry source and has an outlet that may be fluidly coupled to a dispensing port positionable over the polishing surface to deliver slurry thereto during a chemical mechanical polishing operation. The slurry return line extends between the dispensing port and the slurry source, and has an inlet that may be fluidly coupled to the outlet of the slurry supply line to direct slurry away from the dispensing port and to the slurry supply. 
     In another aspect, the slurry supply line extends from the slurry source and has an outlet located at or proximate to a slurry dispensing point. The slurry return line extends from the slurry source and has an inlet. The slurry supply line and slurry return line are configured so that slurry may be directed from the outlet of the slurry supply line onto the polishing surface during a chemical mechanical polishing operation, and from the outlet of the slurry supply line into the inlet of the slurry return line after the polishing operation is stopped to return slurry to the slurry supply. This substantially eliminates deadleg from the slurry supply line. 
     Implementations of the invention may include the following. A pump may provide a flow of slurry through the slurry supply line, e.g., during the polishing operation. The pump may also direct slurry through the slurry supply line and the slurry return line, e.g., after the polishing operation is stopped. Thus, the pump may operate to provide a substantially continuous flow of slurry through the slurry supply line. A filter may be located between the slurry source and the pump. 
     A valve, e.g., a ball valve or a plunger valve, at the outlet of the slurry supply line may be operable between a first position in which the outlet of the slurry supply line is fluidly coupled to the port to dispense slurry onto the polishing pad and a second position in which the outlet of the slurry supply line is fluidly coupled to the inlet of the slurry return line. A portion of the slurry supply line may be flexible and moveable between a first position in which the outlet of the slurry supply line dispenses slurry to the polishing surface and a second position in which the slurry supply line is fluidly coupled to the supply return line. 
     The inlet of the slurry return line may be located adjacent to the polishing surface to receive slurry from the slurry supply line. The outlet of the slurry supply line may be movable between a first position in which it is positioned over the polishing surface and a second position in which it positioned over the inlet of the slurry return line. 
     An arm may extend over the polishing surface and support at least a portion of the slurry supply line. The outlet of the slurry supply line may be located at the end of the arm. The slurry supply line can be a passage in the arm or tubing supported by the arm. A machine base may support the polishing surface, and the arm may be pivotally connected to the base. 
     A second slurry supply line may extend from the slurry source and have a second outlet proximate to a second slurry dispensing point. A second slurry return line may extend from the slurry source and have an inlet. The second slurry supply line and second slurry return line may be configured so that slurry may be directed from the outlet of the slurry supply line to a second polishing surface during a chemical mechanical polishing operation, and into the inlet of the slurry return line after the polishing operation is stopped to return slurry to the slurry supply. This substantially eliminates deadleg from the second slurry supply line. 
     In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, slurry is pumped from a slurry source to an outlet of a slurry supply line that is positionable over a polishing surface. The slurry is directed from the outlet to the polishing surface. The outlet of the slurry supply line is fluidly coupled to an inlet of a slurry return line after the polishing operation has stopped to return the slurry to the slurry source. 
     Implementations of the invention may include the following. The pumping may create a flow of slurry through the slurry supply line and the slurry return line after polishing operation has stopped. The pumping may create a substantially continuous flow of slurry through the slurry supply line. 
     Advantages of the invention may include the following. Coagulation of slurry is reduced or eliminated, thereby reducing scratch defects and increasing process yield. 
    
    
     Other features and advantages will be apparent from the following description, including the drawings and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic exploded perspective view of a chemical mechanical polishing apparatus. 
     FIG. 2 is a schematic diagram of a prior art slurry delivery system. 
     FIG. 3A is a schematic diagram of a slurry delivery system according to the present invention. 
     FIGS. 3B and 3C are enlarged views of a valve from the slurry delivery system of FIG.  3 A. 
     FIG. 4 is a schematic diagram of a slurry delivery system having a flexible slurry supply line. 
     FIG. 5 is a schematic diagram of a slurry delivery system having a plunger valve. 
     FIG. 6 is a schematic diagram of a slurry delivery system having a slurry catch inlet. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, one or more substrates  10  will be polished by a chemical mechanical polishing apparatus  20 . A description of polishing apparatus  20  may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. Polishing apparatus  20  includes a lower machine base  22  with a table top  23  mounted thereon and a removable outer cover (not shown). Table top  23  supports a series of polishing stations, including a first polishing station  25   a , a second polishing station  25   b , a final polishing station  25   c , and a transfer station  27 . Transfer station  27  forms a generally square arrangement with the three polishing stations  25   a ,  25   b  and  25   c . Transfer station  27  serves multiple functions, including receiving individual substrates  10  from a loading apparatus (not shown), washing the substrates, loading the substrates into carrier heads, receiving the substrates from the carrier heads, washing the substrates again, and finally, transferring the substrates back to the loading apparatus. 
     Each polishing station includes a rotatable platen  30  on which is placed a polishing pad. The first and second stations  25   a  and  25   b  may include a relatively hard polishing pad  32 , whereas the final polishing station may include a relative soft polishing pad  34 . If substrate  10  is an “eight-inch” (200 millimeter) or “twelve-inch” (300 millimeter) diameter disk, then the platens and polishing pads will be about twenty inches or thirty inches in diameter, respectively. Each platen  30  may be a rotatable aluminum or stainless steel plate connected to a platen drive motor (not shown). For most polishing processes, the platen drive motor rotates platen  30  at thirty to two hundred revolutions per minute, although lower or higher rotational speeds may be used. 
     Each polishing station  25   a - 25   c  may further include an associated pad conditioner apparatus  40 . Each pad conditioner apparatus  40  has a rotatable arm  42  holding an independently-rotating conditioner head  44  and an associated washing basin  46 . The pad conditioner apparatus  40  maintains the condition of the polishing pad so that it will effectively polish substrates. 
     At each polishing station, a polishing slurry  50  containing deionized water, abrasive particles (e.g., silica particles for oxide polishing) and a chemically reactive component (e.g., potassium hydroxide for oxide polishing) is supplied to the polishing pad surface by a slurry delivery system  200 . As described in greater detail below, the slurry delivery system is designed to prevent coagulation of the slurry. 
     Two or more intermediate washing stations  55   a  and  55   b  may be positioned between neighboring polishing stations. The washing stations rinse the substrates as they pass from one polishing station to another. 
     A rotatable multi-head carousel  60  is positioned above lower machine base  22 . Carousel  60  is supported by a center post  62  and is rotated thereon about a carousel axis  64  by a carousel motor assembly located within machine base  22 . Center post  62  supports a carousel support plate  66  and a cover  68 . Carousel  60  includes four carrier head systems  70   a ,  70   b ,  70   c , and  70   d . Three of the carrier head systems receive and hold substrates, and polish them by pressing them against the polishing pads on the platens of the polishing stations. One of the carrier head systems receives a substrate from and delivers a substrate to transfer station  27 . 
     The four carrier head systems  70   a - 70   d  are mounted on carousel support plate  66  at equal angular intervals about carousel axis  64 . Center post  62  allows the carousel motor to rotate carousel support plate  66  and to orbit carrier head systems  70   a - 70   d  and the attached substrates thereto about carousel axis  64 . 
     Each carrier head system  70   a - 70   d  includes a carrier or carrier head  80 . A carrier drive shaft  74  connects a carrier head rotation motor  76  (shown by the removal of one quarter of cover  68 ) to carrier head  80  so that each carrier head  80  can independently rotate about its own axis. There is one carrier drive shaft and motor for each head. In addition, each carrier head  80  independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A slider (not shown) supports each drive shaft in its associated radial slot. A radial drive motor (not shown) may move the slider to laterally oscillate the carrier head. 
     The carrier head  80  performs several mechanical functions. Generally, the carrier head holds the substrate against the polishing pad, evenly distributes a downward pressure across the back surface of the substrate, transfers torque from the drive shaft to the substrate, and ensures that the substrate does not slip out from beneath the carrier head during polishing operations. 
     The carrier head  80  may include a flexible membrane (not shown) which provides a substrate receiving surface. A description of a suitable carrier head  80  may be found in U.S. patent application Ser. No. 08/745,679, entitled a CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR a CHEMICAL MECHANICAL POLISHING SYSTEM, filed Nov. 8, 1996, by Steven M. Zuniga et al., assigned to the assignee of the present invention, the entire disclosure of which is incorporated herein by reference. 
     In order to more clearly explain the invention, a conventional slurry delivery system will first be described. Referring to FIG. 2, a conventional slurry delivery system  100  includes a slurry reservoir  102 , a pump  104 , a coarse filter  106  located upstream of pump  104 , and a point-of-use (POU) filter  108  located downstream of pump  104 . Slurry is pumped through filters  106  and  108  by pump  104 , and returned to reservoir  102  through a slurry manifold  110 . Pump  104  may be operated so that slurry from reservoir  102  is continuously circulated through the slurry line and the filters. The continuous motion of the slurry helps prevent coagulation, and filters  106  and  108  remove slurry particle conglomerates from slurry manifold  110 . 
     A plurality of peristaltic pumps  112   a ,  112   b  and  112   c , associated with polishing stations  25   a ,  25   b  and  25   c , respectively, are fluidly coupled to slurry manifold  110  by intake lines  114   a ,  114   b  and  114   c , respectively. Three supply lines  116   a ,  116   b  and  116   c  deliver slurry from peristaltic pumps  112   a ,  112   b  and  112   c , respectively, to the polishing pads at the polishing stations. Each supply line extends through a combined slurry/rinse arm  118  that extends over platen  30 . Although arm  118  is illustrated with only one supply line, the arm may include two or more supply lines to distribute multiple slurries to the surface of the polishing pad. The arm  118  also includes several spray nozzles (not shown) which provide a high pressure rinse of the polishing pad at the end of each polishing and conditioning cycle. 
     Unfortunately, the portion of the slurry delivery system extending between slurry manifold  110  and each polishing pad, e.g., intake line  114   a , peristaltic pump  112   a  and supply line  116   a , constitutes a so-called “deadleg”. When slurry is not required at one of the polishing stations, e.g., polishing station  25   a , the peristaltic pump associated with that polishing station is stopped, and the slurry in the deadleg sits stagnant and coagulates. When the peristaltic pump is restarted, coagulated slurry will be delivered to the polishing pad, where it can scratch the substrate and cause defects. 
     Referring to FIGS. 3A-3C, a slurry delivery system  200  is constructed without a deadleg. Slurry delivery system  200  includes a slurry reservoir  202 , a primary pump  204 , and a coarse filter  206  located between primary pump  204  and reservoir  202 . Reservoir  202 , primary pump  204  and coarse filter  206  may be located in machine base  22  or in a separate slurry supply module  220 . Three peristaltic pumps  208  are connected to primary pump  204  by a slurry supply manifold  210 . A slurry/rinse arm  218  extends over each polishing pad, and a three-way valve  214  is located at the end of the each arm. Each peristaltic pump  208  is fluidly coupled to a first port  228   a  of the three-way valve by a slurry supply line  212 . A point-of-use filter  216  may be located in each slurry supply line  212  between the peristaltic pump and the three-way valve. A slurry return line  222  extends back through the arm to fluidly couple a second port  288   b  of each valve  214  to a slurry return manifold  224 , which returns the slurry to reservoir  202 . A third port  228   c  of valve  214  is connected to an exit port  226  (see FIGS. 3A and 3B) in the arm to dispense slurry onto the polishing pad. 
     In the configuration illustrated in FIGS. 3A-3C, valve  214  is a ball valve rotatable between a first position (shown in FIG. 3A) in which slurry supply line  212  is fluidly coupled to exit port  226 , and a second position (shown in FIG. 3B) in which slurry supply line  212  is fluidly coupled to exit port  226 . Thus, when the valve is in the first position, slurry is directed through slurry supply line  212  and exit port  226  and onto the polishing pad. In contrast, when the valve is in the second position, slurry is pumped out to the end of arm  218  via slurry supply line  212  and returned to reservoir  202  via slurry return line  222 . Pumps  204  and  208  are operated to provide a substantially continuous, i.e., both during and between polishing operations (but not when slurry delivery system  200  is shut down for maintenance and the like), flow of slurry through the slurry supply line, thereby reducing coagulation and substrate defects. 
     The slurry supply line  212  may be a passageway formed integrally through arm  218 , or it may be a flexible or rigid tube supported by the arm (either inside or outside the arm housing). Alternately, the slurry supply line may be sufficiently rigid that an arm is not required. Similarly, slurry return line  222  may be a passage formed through the arm, a flexible tube supported by the arm, or a rigid self-supporting tube. 
     FIG. 4 illustrates a slurry delivery system  200 ′ in which the ball valve is replaced with a moveable tubing. For clarity, only the portion of the slurry delivery system associated with a single polishing station is illustrated. Additionally, the slurry reservoir, the coarse filter, the primary pump, the peristaltic pump and the point-of-use filter are not shown. A slurry/rinse arm  218 ′ supports a slurry supply line  230  having an outlet  234  near the end of the arm. The slurry supply line  230  includes a flexible portion  232  located adjacent an aperture  238  in the arm  218 ′. The flexible portion of slurry supply line  230  is moveable between a first position in which the outlet of the slurry supply line dispenses slurry onto the polishing pad via outlet  234 , and a second position (shown in phantom) in which the outlet of the slurry supply line is connected to an inlet  239  of a slurry return line  236 . Inlet  239  may be provided with a seal (not shown) to prevent leakage of the slurry when the slurry supply line is connected to the slurry return line. Alternately, inlet  239  may be slightly wider than outlet  234 . The flexible portion  232  of slurry supply line  230  may be actuated between the first and second positions by a pneumatic actuator  237 . Between polishing operations at this particular polishing station, slurry supply line  230  is fluidly coupled to slurry return line  236  so that the pumps continuously recirculate slurry through the slurry delivery system. On the other hand, during polishing operations, flexible portion  232  is shifted so that slurry flows through outlet  234  and aperture  238  onto the polishing pad. 
     Referring to FIG. 5, in another configuration, a slurry delivery system  200 ″ includes a slurry supply line  240  to transport slurry to a plunger valve  242  located adjacent an aperture or port  244  at the end of a slurry/rinse arm  218 ″. The plunger valve may be operated between a first position in which a first valve passage  250  directs slurry from slurry supply line  240  onto the polishing pad, and a second position (shown in phantom) in which a second valve passage  252  fluidly couples slurry supply line  240  to a slurry return line  248 . Thus, during polishing at this particular polishing station, the plunger valve is in the first position to dispense slurry onto the pad. On the other hand, between polishing operations, the plunger valve is in the second position so that slurry is continuously circulated through the slurry delivery system. The plunger valve may be equipped with a lip-seal (not shown) to prevent leakage of the slurry from slurry supply line  240  and slurry return line  248 . Plunger valve  242  may be actuated by a pneumatic actuator  246  in such a fashion as to minimize particle generation. 
     Referring to FIG. 6, in another embodiment (illustrated for a single polishing station  25 ), a slurry delivery system  300  includes a slurry reservoir  302 , a primary pump  304 , a coarse filter  306 , a peristaltic pump  308  and a POU filter  310 . Slurry delivery system  300  also includes a generally funnel-shaped slurry catch cup  322  located adjacent platen  30 . The slurry catch cup  322  is fluidly coupled to reservoir  302  by a slurry return line  324 . A slurry supply line  314  extends through a moveable slurry/rinse arm  318  to direct slurry onto polishing pad  32 . The arm  318  is pivotally connected to table top  23  and may be moved between a first position in which an outlet  320  at the end of slurry supply line  314  is located over polishing pad  32 , and a second position (illustrated in phantom) in which outlet  320  is positioned over slurry catch cup  322 . A motor or pneumatic actuator  316  may be connected at the base of arm  318  to pivot the arm. Thus, during polishing, slurry delivery system  300  may position arm  318  over polishing pad  32 , whereas between polishing operations, pneumatic actuator  316  may rotate or pivot arm  318  over slurry catch cup  322  so that slurry is continuously recirculated though slurry supply line  314  and slurry return line  324 . 
     The invention is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims.