APPARATUS AND METHODS FOR SLURRY REMOVAL IN CHEMICAL MECHANICAL POLISHING

Abstract: A substrate and carrier head cleaning station (55b) is part of a chemical mechanical polishing apparatus. The cleaning station (55b) is positioned between two polishing stations to prevent slurry contamination therebetween. The cleaner may include a wash cup (100), a rotating scrub brush (112), and a nozzle (122) to direct a cleaning fluid (124) onto the substrate. Alternately, in a CMP apparatus having three polishing stations, the second station may be equipped with a soft polishing pad and be used as an intermediate cleaning station.

Description: APPARATUS AND METHODS FOR SLURRY REMOVAL IN CHEMICAL MECHANICAL POLISHING Background The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to apparatus and methods for removing slurry from a substrate. An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer may be etched to create circuitry features. One fabrication step involves the formation of metal vias, plugs and lines to provide conductive paths between thin film circuits. Metal vias can be created by depositing a metal layer over a patterned insulative layer and then planarizing the metal layer until the insulative layer is exposed. The portions of the metal layer remaining between the raised pattern of the insulative layer form the metal vias, plugs and lines. Chemical mechanical polishing (CMP) is one accepted method of planariza ion. 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 rotating 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, or simply deionized water if a fixed abrasive 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) . In some fabrication processes that use CMP, particularly metal polishing, shallow trench isolation, and other damascene processes, two or more slurries are used to polish the substrate. The two slurries may have different compositions. One slurry may be more acidic than the other slurry, the slurries may contain colloidal particles with different compositions or sizes, or the slurries can have different concentrations of additives such as oxidants or corrosion inhibitors. If droplets of one slurry are mixed with the other slurry, the chemistry of the second slurry will change and the CMP process will become unreliable. For example, an initial polishing step may be performed with an acidic slurry on a first polishing pad, and a second polishing step may be performed with an alkaline slurry on a second polishing pad. When the substrate is transported from the first to the second polishing pad, slurry may cling to the substrate or carrier head. When the first, acidic slurry mixes with the second, alkaline slurry, the pH of the second slurry will change. The change in the pH of the second slurry chemistry decreases the polishing rate and throughput and lowers the process yield. One method of washing substrates as they are transported between the first and the second polishing pads is to spray water onto the underside of the substrate. Unfortunately, such a water spray may not remove all of the slurry from the substrate. 2- Summary In one aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a first rotatable platen to support a first polishing pad, a second rotatable platen to support a second polishing pad, a carrier head movable along a path between the first and second platens, and a cleaning station located on the path between the first and second platens. The cleaning station includes a wash cup, a brush disposed at least partially in the wash cup and positionable to contact a substrate held on the carrier head when the carrier head is located over the cleaning station, and a nozzle located in the wash cup and oriented to direct a washing fluid onto the substrate. Implementations of the invention may include the following. A source of cleaning fluid, e.g., deionized water, may be coupled to the nozzle. The wash cup may have a generally elongated shape, and a primary axis substantially perpendicular to the path. The brush may include a substantially cylindrical body, or it may be substantially disk-shaped. The axis of rotation of the brush may be substantially parallel to the primary axis of the wash cup, or it may be perpendicular to a surface of the substrate. The brush may have bristles formed of polyvinyl alcohol, or a surface formed of a napped poromeric material. A second rotatable brush can be positioned substantially parallel to the first brush. A dispenser may provide a chemical solution, e.g., a corrosion inhibitor or a pH neutralizer, to the brush. In another aspect, the invention is directed to a method of polishing a substrate. In the method, a substrate is chemical mechanical polished with a first polishing slurry at a first polishing station, transported in a carrier head to a cleaning station, and positioned in contact with a rotating brush. A washing fluid is directed onto the substrate. The substrate is transported in the carrier head to a second polishing station, and chemical mechanical polished with a second polishing slurry at the second polishing station. In another aspect, the invention is directed to another method of polishing a substrate. In the method, a substrate is loaded into a chemical mechanical polishing apparatus having a first, a second and a third station, each station including a rotatable platen. The substrate is chemical mechanical polished with a first polishing slurry and a first polishing pad at the first polishing station. The substrate is cleaned at the second station with a washing fluid and a second polishing pad which is softer than the first polishing pad. Then the substrate is chemical mechanical polished with a second polishing slurry and a third polishing pad at the third station. In one aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a first rotatable platen to support a first polishing pad, a second rotatable platen to support a second polishing pad, a carrier head movable along a path between the first and second platens, and a cleaning station located on the path between the first and second platens. The cleaning station includes a wash cup, a mechanical scrubber positionable to contact a substrate held on the carrier head when the carrier head is located over the cleaning station, and a nozzle located in the wash cup and oriented to direct a washing fluid onto the substrate. Advantages of the invention may include the following. Substrates are effectively cleaned between -4. polishing stations. Multiple slurries may be used in the CMP apparatus in a single fabrication process- with little danger of the slurries mixing. Thus, the CMP apparatus can be used in a variety of multi-slurry polishing operations at high throughput and with a high 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 top view a CMP apparatus according to the present application. FIG. 3A is a schematic cross-sectional view of a cleaning station of the CMP apparatus of FIG. 1. FIG. 3B is a schematic view in which the substrate and carrier head are positioned over the cleaning station of FIG. 3A. FIG. 4 is a schematic cross-sectional view of the cleaning station of FIG. 3A along the primary axis of the wash cup which also illustrates the washing fluid source. FIG. 5 is a schematic top view of another embodiment of a CMP apparatus according to the present invention. FIG. 6A is a schematic cross-sectional view of a polishing pad from a first polishing station of the CMP apparatus of FIG. 5. FIG. 6B is a schematic cross-sectional view of a polishing pad from a second polishing station of the CMP apparatus of FIG. 5. FIG. 7 is a schematic cross-sectional view of another embodiment of a cleaning station. -5- 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. Patent Application Serial No. 08/549,336, entitled RADIALLY OSCILLATING CAROUSEL PROCESSING SYSTEM FOR CHEMICAL MECHANICAL POLISHING, filed October 27, 1995 by Ilya Perlov, et al . , and assigned to the assignee of the present invention, 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 25a, a second polishing station 25b, and a final polishing station 25c, and a transfer station 27. Transfer station 27 forms a generally square arrangement with the three polishing stations 25a, 25b and 25c. 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 32. 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 -6 hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station 25a-25c 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. 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 70a, 70b, 70c, and 70d. Three of the carrier head systems receive and hold substrates, and polish them by pressing them against the polishing pads on the platens of polishing stations 25a-25c. One of the carrier head systems receives a substrate from and delivers a substrate to transfer station 27. The four carrier head systems 70a-70d 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 70a-70d and the substrates attached thereto about carousel axis 64. Each carrier head system 70a-70d 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 -7 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 Serial No. 08/745,679, entitled a CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR a CHEMICAL MECHANICAL POLISHING SYSTEM, filed November 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. Referring to FIG. 2, a first polishing slurry 50a is supplied to the polishing pad at polishing station 25a by a combined slurry/rinse arm 52a, a second polishing slurry 50b is supplied to the polishing pad at polishing station 25b by a slurry/rinse arm 52b, and a third polishing slurry 50c is supplied to the polishing pad at polishing station 25c by a slurry/rinse arm 52c. Each slurry/rinse arm may include two or more slurry supply tubes to provide slurry to the surface of the associate polishing pad. Sufficient slurry may be -8 provided to cover and wet the entirety of each polishing pad. Each slurry/rinse arm 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. The first polishing slurry 50a may contain deionized water, abrasive particles (e.g., silica or alumina for copper polishing) , an acidic component, and an oxidizer, whereas second polishing slurry 50b may contain deionized water, abrasive particles (e.g., silica or alumina), and an alkaline component. The third polishing slurry may be suitable for final polishing or buffing, e.g., it may be pure deionized water, with or without abrasive particles. Four intermediate cleaning stations 55a, 55b, 55c and 55d may be positioned between neighboring polishing stations 25a, 25b and 25c and transfer station 27. The cleaning stations rinse the substrates as they pass from one polishing station to another to remove slurry from the substrate and prevent contamination and mixing of the three slurries. Referring to FIGS. 2, 3A, 3B and 4, each intermediate cleaning station, e.g., cleaning station 55b, includes an elongated wash cup 100 with two elongated sides 104, two short sides 106, and an elastomeric lip seal 102 on the upper surface 108 of the wash cup. The elongated sides 104 define a primary axis (as shown by dashed line 118) of the wash cup which is aligned radially with the center axis of the carousel; i.e., the primary axis is generally perpendicular to the direction of motion of the carrier head as the carousel swings it over the wash cup. The carrier head 80 may be moved over the cleaning station 55b, and it may be lowered into contact with lip seal 102. The wash cup -9 may be vertically movable. In this case, when the substrate is located over the cleaning station, the wash cup may be raised to contact the substrate. Alternately, the substrate can be moved horizontally by the carrier head across the cleaning station. The two elongated sides 104 of wash cup 100 may be long enough to span the substrate when the center of the wash cup is aligned with the center of the substrate. When the substrate contacts lip seal 102, the sealed volume inside wash cup 100 forms a wash chamber 110 (see FIG 3B) . Two rotatable cylindrical scrub brushes 112a and 112b are suspended in wash cup 100. The primary axes of scrub brushes 112a and 112b are oriented generally parallel to the primary axis of the wash cup. The axis of rotation 116 of the scrub brushes may also be generally parallel to the primary axis of the wash cup. The scrub brushes are positioned with their outer surface 114 projecting slightly above lip seal 102. Thus, when substrate 10 is lowered into contact with wash cup 100, scrub brushes 112a and 112b firmly contact the underside of the substrate and retaining ring of the carrier head (see FIG. 3B) . A motor (not shown) may be used to rotate scrub brushes 112a and 112b in a direction shown by arrows A opposite to the direction of travel of the substrate between the first and second polishing stations. This ensures that overspray from the cleaning station is directed back toward the originating polishing station, i.e., the polishing station from which the substrate is being transferred. The scrub brushes may be rotated at the same speed and in the same direction. Each brush may have bristles formed of polyvinyl alcohol (PVA) . The surfaces 114 of scrub brushes 112a and 112b may also be formed of a napped poromeric synthetic material, such as Politex. The optimal characteristics for the -10 brushes, such as the length and stiffness of the bristles, may be determined experimentally. A spray pipe 120 having a number of vertically oriented nozzles 122 may be positioned between scrub brushes 112a and 112b. The nozzles may also be positioned on the sides of the brush assembly and be angled inwardly toward the originating platen. The spray pipe is coupled to a source of washing liquid 128. When substrate 10 is positioned over wash cup 100, a washing liquid 124, such as deionized water, is supplied under pressure through the nozzles, which are oriented to spray the underside of the substrate and carrier head. The lip seal provides a barrier to prevent the washing liquid from escaping the wash cup. Excess washing liquid and slurry rinsed from the substrate and brushed off of the substrate by scrub brushes 112a and 112b, fall to the bottom of wash cup 100 and drain through a drain opening 126 for disposal. Two drip dispensers 130a and 130b may be positioned in wash cup 100 and extend over scrub brushes 112a and 112b, respectively, to dispense chemicals onto the scrub brushes. The drip dispensers 130a and 130b may supply a fluid to neutralize the slurry of the previous polishing station. For example, if an acidic slurry is used at polishing station 25a, then drip dispensers 130a and 130b may dispense an alkaline solution onto the scrub brushes. This alkaline solution then neutralizes the acidic component in the slurry from polishing station 25a that adhered to the underside of the substrate. In addition, a corrosion inhibitor, such as benzotriazole, may be dispensed by drip dispensers 130a and 130b onto the scrub brushes. In operation, once the polishing step at polishing station 25a has been completed, the rotation of carrier head 11 80 is halted, the substrate is raised from the platen and polishing pad, and the carrier head is radially aligned with cleaning station 55b. Then carousel 60 is rotated to move the carrier head and the substrate over the center of cleaning station 55b, and the carrier head lowers substrate 10 to place it into low-pressure contact with lip seal 102. Then the carrier head rotates the substrate as scrub brushes 112a and 112b remove slurry from the underside of the substrate and from the underside of the carrier head retaining ring. The substrate rotates until the entire substrate surface has been cleaned by scrub brushes. Then the carrier head raises the substrate off the lip seal, and the carousel rotates to position the substrate over the next polishing station 25b. In an alternate embodiment, shown in FIG. 7, a plurality of pancake brushes 170 may be suspended in wash cup 100. The pancake brushes are generally disk-shaped, and when substrate 10 is lowered into contact with wash cup 100, the upper surfaces 174 of the pancake brushes 170 contact the substrate. The pancake brushes may be rotated by a motor (not shown) about an axis of rotation 172 that is perpendicular to the substrate surface to mechanically clean the substrate surface. The upper surface 174 of each pancake brush 170 may be formed of a napped poromeric synthetic material, such as Politex. In another embodiment, one of the polishing stations in CMP apparatus 20' may be used as a cleaning station to remove slurry from the substrate. Specifically, referring to FIG. 5, the first and third polishing stations 25a' and 25c' may include a relatively hard polishing pad 150, whereas the second polishing station 25b' may include a relatively soft polishing pad 160. Referring to FIG. 6A, at -12 first and third polishing stations 25a' and 25c', the platen may support a polishing pad 150 having a roughed surface 152, an upper layer 154 and a lower layer 156. Lower layer 156 may be attached to platen 30 by a pressure-sensitive adhesive layer 158. Upper layer 154 may be harder than lower layer 156. For example, upper layer 154 may be composed of microporous polyurethane or polyurethane mixed with a filler, whereas lower layer 156 may be composed of compressed felt fibers leached with urethane. A two-layer polishing pad, with the upper layer composed of IC-1000 or 1C-1400 and the lower layer composed of SUBA-4, is available from Rodel, Inc. of Newark, Delaware (IC-1000, IC-1400 and SUBA-4 are product names of Rodel, Inc.). At the second polishing station 25b' , the platen may support a polishing pad 160 having a generally smooth surface 162 and a single soft layer 164. Layer 164 may be attached to platen 30 by a pressure-sensitive adhesive layer 168. Layer 164 may be composed of a napped poromeric synthetic material. A suitable soft polishing pad is available from Rodel, Inc., under the trade name Politex. Polishing pad 160 may be embossed or stamped with a pattern to improve distribution of slurry across the face of the substrate. Polishing station 25b' may otherwise be identical to polishing stations 25a' and 25c' . Returning to FIG. 5, substrate 10 is initially polished at polishing station 25a' with a first metal polishing slurry, such as SSW-2000, available from Cabot Corp., Aurora, Illinois, and the hard, rough polishing pad 150. Next, the substrate is moved to second polishing station 25b' , where it is buffed with deionized water and soft polishing pad 160, e.g., for about 20 to 30 seconds. This buffing action removes any slurry that has accumulated 13 on the underside of the substrate and carrier head. Finally, the substrate is polished at final polishing station 25c' using a second metal or oxide polishing slurry, such as SS12, available from Cabot. The cleaning at the second polishing station may be performed instead of, or in addition to, cleaning by the intermediate cleaning stations. Thus, CMP apparatus 20' need not include intermediate cleaning stations 55a-55d. The scrub brushes and pancake brushes illustrated and described are exemplary, and other mechanical cleaning devices are possible. For example, a brush may vibrate or orbit rather than rotate; a brush could be a porous body rather than having bristles; or a brush could have a rectangular cross-section. The invention is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims. What is claimed is: 14.

Claims: 1. A chemical mechanical polishing apparatus, comprising: a first rotatable platen to support a first polishing pad; a second rotatable platen to support a second polishing pad; a carrier head movable along a path between the first and second platens; and a cleaning station located on the path between the first and second platens, the cleaning station including a wash cup, a brush disposed at least partially in the wash cup and positionable to contact a substrate held on the carrier head when the carrier head is located over the cleaning station, and a nozzle located in the wash cup and oriented to direct a washing fluid onto the substrate. 2. The apparatus of claim 1, further comprising a source of cleaning fluid coupled to the nozzle. 3. The apparatus of claim 2, wherein the cleaning fluid is deionized water. 4. The apparatus of claim 1, wherein the wash cup has a generally elongated shape. 5. The apparatus of claim 4, wherein a primary axis of the wash cup is substantially perpendicular to the path. 6. The apparatus of claim 4, wherein the brush includes 15 a substantially cylindrical body with a primary axis substantially parallel to a primary axis of the wash cup. 7. The apparatus of claim 4, wherein the brush has an axis of rotation substantially parallel to the primary axis of the wash cup. 8. The apparatus of claim 1, wherein the brush is substantially disk-shaped. 9. The apparatus of claim 8, wherein the brush has an axis of rotation substantially perpendicular a surface of the substrate. 10. The apparatus of claim 1, wherein the brush includes bristles formed of polyvinyl alcohol. 11. The apparatus of claim 1, wherein the brush includes a surface formed of a napped poromeric material . 12. The apparatus of claim 1, further including a second rotatable brush positioned substantially parallel to the first brush. 13. The apparatus of claim 1, further comprising a dispenser to provide a chemical solution to the brush. 14. The apparatus of claim 13, wherein the chemical solution includes a corrosion inhibitor. 15. The apparatus of claim 13, wherein the chemical solution includes a pH neutralizer. -16- 16. A method of polishing a substrate, comprising: chemical mechanical polishing a substrate with a first polishing slurry at a first polishing station; transporting the substrate in a carrier head to a cleaning station; positioning the substrate in contact with a rotating brush; directing a washing fluid onto the substrate; transporting the substrate in the carrier head to a second polishing station; and chemical mechanical polishing the substrate with a second polishing slurry at the second polishing station. 17. A method of polishing a substrate, comprising: loading a substrate into a chemical mechanical polishing apparatus having a first, a second and a third station, each station including a rotatable platen; chemical mechanical polishing the substrate with a first polishing slurry and a first polishing pad at the first polishing station; cleaning the substrate at the second station with a washing fluid and a second polishing pad which is softer than the first polishing pad; and chemical mechanical polishing the substrate with a second polishing slurry and a third polishing pad at the third station. 18. A chemical mechanical polishing apparatus, comprising: a first rotatable platen to support a first polishing pad; a second rotatable platen to support a second -17- polishing pad; a carrier head movable along a path between the first and second platens; and a cleaning station located on the path between the first and second platens, the cleaning station including a wash cup, a mechanically scrubber positionable to contact a substrate held on the carrier head when the carrier head is located over the cleaning station to remove slurry from the substrate, and a nozzle located in the wash cup and oriented to direct a washing fluid onto the substrate. 18

Inventors: ADDIEGO GINETTO, DAVENPORT ROBERT, ADDIEGO, GINETTO, DAVENPORT, ROBERT
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1999
Main classification: B24B37
IPC codes: B08B1, B24B37, B24B55, B24B57, H01L21