POLISHING APPARATUS

Abstract: A compact polishing apparatus (30) can produce a high degree of flatness and cleanliness in the polished object. The polishing apparatus (30) comprises a polishing table having an abrading plate (59), a pressing device (32) for forcing a work surface of a workpiece (1) against an abrading surface (59) of the abrading plate. A driving device is provided for making the abrading surface (59) and the work surface undergo a relative circulative translation motion tracing a given pattern.

Description: DESCRIPTION POLISHING APPARATUS Technical Field The present invention relates in general to polishing apparatuses, and relates in particular to a polishing apparatus for processing workpieces, such as semiconductor wafers, glass plates and liquid crystal display panels which require a high surface flatness. Background Art In recent years, there has been a remarkable progress in the density of integrated circuit devices which leads to a narrower interline spacing of the wiring. Optical lithographic reproduction of circuits using stepper printing process for such highly integrated devices requires the depth of focus be very shallow so that the wafer surface must be extremely flat. This trend means also that if a particle of a size larger than the line spacing should remain on the fabricated device, it can cause short circuiting which may lead to device failure. Therefore, it is evident that workpiece processing must produce a flat and clean workpiece. These processing requirements apply equally to other workpiece materials in general, such as glass plates for photo-masking or liquid crystal display panels. Figure 4 shows a conventional polishing apparatus comprising: a polishing unit 10; a loading/unloading unit 21; a transfer robot 22; and two cleaning machines 23a, 23b. Figure 5 is a schematic illustration of the polishing unit 10 comprising a turntable 12 having a polishing cloth 11 attached thereto; and a top ring 13 for holding a workpiece (wafer) 1 and pressing the workpiece 1 onto the turntable 12. Polishing is carried out by holding a workpiece 1 at the bottom surface of the top ring 13, and pressing the workpiece 1 by means of a vertically movable cylinder onto the polishing cloth 11 mounted on the top surface of the rotating turntable 12. In the meantime, a polishing solution Q is supplied from a delivery nozzle 14 in such a way to retain the solution Q between the bottom surface of the workpiece 1 and the polishing surface of the polishing cloth 11. The turntable 12 and the top ring 13 are rotated independently at their individual controlled speed. As shown in Figure 5, the top ring 13 is positioned in relation to the turntable 12, so that the peripheral edge of the workpiece 1 is located at distances "a" and "b" , respectively, from the center and the peripheral edge of the turntable 12 so that the entire surface of the workpiece 1 can be polished uniformly at some high rotational speeds. It indicates that the diameter "D" of the turntable 12 is chosen according to the following relation to be more than twice the diameter d of the workpiece 1: D = 2 (d + a + b) The polished workpiece 1 is then processed in the cleaning machines 23a, 23b through several washing and drying steps, and is transferred onto the loading/unloading unit 21 to be stored in a portable workpiece cassette 24. A scrub washing is used, which involves the use of brushes made of nylon or mohair, or a sponge made from polyvinylalcohol (PVA) . The conventional polishing apparatus of the type described above has an advantage that, because of the elasticity of the polishing cloth 11 moderating the effects of waviness and bowing in the wafer, relatively uniform polishing is produced over the surface of the wafer, but it is susceptible to generating edge wear ( excessive polishing around the periphery edge of the wafer ) . Further, for polishing wafers with printed wiring patterns, it is necessary that polishing be carried out to attain less than 1,000 angstrom flatness by removing any micro-protrusions, but the polishing cloth 11 is unable to meet this requirement because the elastic nature of the cloth allows the cloth to deform, thus resulting in removing the material from recessed regions as well as from protruding regions. Disclosure of Invention It is an object of the present invention to provide a compact polishing apparatus to produce a high degree of flatness in the polished object. The object has been achieved in a polishing apparatus comprising: a polishing table having an abrading plate; a pressing device for forcing a work surface of a workpiece against an abrading surface of the abrading plate; and a driving device for making the abrading surface and the work surface undergo a relative circulative translation motion tracing a given pattern. Here, a "circulative translation motion" refers to a relative planar motion of two surfaces which involves no change in their relative orientation, in other words, with only a translation motion tracing a predetermined pattern and without any relative rotary motion, or a relative motion primarily consisting of such a circulative translation motion. The trace can be a linear reciprocating pattern, a polygonal pattern or an elliptical pattern, but from the practical standpoint of polishing efficiency and quality and ease of mechanical setup, a circular pattern, that is, an orbiting motion would be optimum. In a circulative translation motion, all the contacted regions of the polishing object is subjected to a same pattern. Because an abrading plate is used as a polishing tool, the present apparatus can satisfy a wide range of polishing needs, from rough grinding to finish polishing, by choosing an abrasive grain size, a method of supplying the polishing solution and a rotational speed to suit each work. That is, to perform rough polishing, abrading surface is made coarser and a relatively high speed and high pressing pressure are used, while to perform finish polishing, abrading surface is made finer and a relatively low speed and low pressing pressure are applied. Removal of micro-particles adhering to the workpiece surface may also be performed during the finish polishing by using a solution appropriate to the purpose . Specifically, for rough polishing, abrasive grains are used while for finish polishing, deionized water and solutions may be used. .Abrading grains are normally not used in finish polishing, but if they are needed, a small amount of ultra-fine micro-grains is used. In this type of arrangement of the apparatus, the size of the abrading plate needs to be only slightly larger than the workpiece size, and therefore, it is easier to produce high flatness over the entire surface of the polishing tool, compared with producing the same degree of flatness in a conventional large polishing table. The apparatus is compact and the drives can also be small and require only low power, and the installation space is minimized. The overall design of the polishing facility, including the cleaning and wafer inverting devices, is simplified and changes can be accommodated readily. These advantages become more important as the size of the wafer to be processed increases, as anticipated. Because the polishing tools are not rotated, relative rate of material removal is uniform throughout the wafer area so that it is easier to produce flatness even at low polishing speeds to provide a smooth surface of a superior quality. Because the abrading plate is hard, there is few distortion in the tool itself so that only the protruded regions of the patterned wafer can be selectively removed to produce an overall flat surface of the polished wafer. At least one of the workpiece and the polishing tool may be rotated with a period of revolution significantly in excess of a period of rotation of the circulative translation motion. Accordingly, the contact area between the work surface and the abrading surface gradually shift its location to produce uniform polishing over the entire surface of the wafer. The abrading surface may be provided with surface channels for supplying a polishing solution to an interface between the abrading surface and the work surface. Accordingly, the center region of the wafer, which is difficult for the polishing solution to reach from the outer periphery, can be supplied with sufficient amount of polishing solution from the inner locations of the polishing tool. The driving device may comprise: a base section; a surface plate having an upper surface for supporting the polishing table; a support section disposed on the base section for supporting the surface plate so as to produce a circulative translation motion of the surface plate; a driving section for producing the circulative translation motion of the surface plate. The support section may provide support to the surface plate at not less than three peripheral locations, such that, even if a local higher pressure is exerted, the surface plate is supported firmly to maintain precise flatness in the polished wafer, so that the quality of the polished wafer is improved. The support section may include a connecting member having a pair of shafts extending in opposite directions aligned on eccentric axes, the shafts being rotatably inserted into a depression section provided in each of the surface plate and the base section, to provide a simple and compact connection. The driving section may include a rotation shaft, a drive end formed eccentrically on the rotation shaft, and a depression section formed on the surface plate for accepting the drive end, thereby to transmit the torque effectively. The polishing apparatus may include a workpiece holding section provided with a flexible sheet member disposed between a workpiece and a workpiece holding surface of the workpiece holding section, so that surface condition of the wafer holding surface does not affect polishing of the wafer, thereby providing precise flatness in the polished wafer. The object is also achieved in a polishing facility comprising at least one main polishing unit and at least one finish polishing unit, wherein the main polishing unit comprises : a polishing table having an abrading plate; a pressing device for forcing a work surface of a workpiece against an abrading surface of the abrading plate; and a driving device for making the abrading surface and the work surface undergo a relative circulative translation motion tracing a given pattern, and the finish polishing unit comprises: a polishing table having a polishing cloth; a pressing device for forcing a work surface of a workpiece against a polishing surface of the polishing cloth; and a driving device for making the polishing cloth and the work surface undergo a relative circulative translation motion tracing a given pattern. Thus, the apparatus takes maximum advantage of using the performance features provided by a hard abrading plate and a soft polishing cloth. Brief Description of Drawings Figure 1 is an overall plan view of the arrangement of the polishing facility of the present invention; Figure 2 is a cross sectional view of a polishing unit of the present invention; Figure 3A is a plan view of the surface plate shown in Figure 2 looking towards the drive motor of the polishing apparatus; Figure 3B is a cross sectional view of the surface plate shown in Figure 2 ; Figure 4 is a perspective view of a conventional polishing facility; and Figure 5 is a cross sectional view of a conventional polishing unit. Best Mode for Carrying Out the Invention Figure 1 shows an embodiment of the arrangement of the component units in the polishing apparatus of the present invention. At one end of a rectangular shaped floor space, there is a loading/unloading unit 21 for delivery of workpieces which are to be polished or already polished to and from the polishing apparatus. At the opposite end of the floor space, there are two main polishing units 30a, 30b. The loading/unloading unit 21 and main polishing units 30a, 30b are connected with a workpiece transfer route for two robotic transfer devices 22a, 22b in this embodiment, and at one side of the transfer route, there is a workpiece inverter 25 for turning over the workpiece, and on the opposite side, there are disposed a finish polishing unit 30c and three cleaning machines 23a, 23b and 23c. The main polishing units 30a, 30b and the finish polishing unit 30c are basically of the same construction and are, as shown in Figures 2 and 3, respectively provided with a translation table section 31 which provides a circulative translation motion of the abrading surface of a polishing tool, and a top ring 32 for holding the workpiece 1 to direct its surface to be polished downwards and pressing it onto the polishing tool surface with a given pressure. The translation table section 31 comprises: a cylindrical casing 34 housing a motor 33 therein; an annular overhang plate section (base section) 35 protruding inwards at an upper portion of the cylindrical casing 34; three support sections 36 formed around the circumference of the overhang plate section 35; and a surface plate 37 supported on the support sections 36 and mounted with an abrading plate 59 or polishing cloth 59a attached thereon. The surface plate 37 and the abrading plate 59 or polishing cloth 59a constitute a polishing table. As shown in Figure 3B, the upper surface of the overhang plate section 35 and the bottom surface of the surface plate 37 respectively include a plurality of cavity sections 38, 39 which are equally spaced apart in the circumferential direction, together with corresponding bearings 40, 41 disposed therein. These bearings 40, 41 are respectively supporting each end portion of the upper and lower shafts 42, 43 of each of three connecting members 44. The axes of the upper shaft 42 of each connecting members 44 is displaced from the center of the lower shaft 43 by an eccentricity distance "e", as shown in Figure 3, thereby permitting the surface plate 37 to undergo a circulative translation motion over a distance of radius "e" . A cavity section 48 is provided in the central region of the bottom surface of the surface plate 37 for housing a drive bearing 47 for supporting the drive end 46 which is formed at a top surface of the main shaft 45 of the drive motor 33, whose axis Z 2 is displaced with respect to the axis Z α of the main shaft 45. The amount of offset is also "e". The drive motor 33 is housed in the motor chamber 49 provided in the casing 34, and its main shaft 45 is supported by the top and bottom bearings 50, 51. A pair of balancers 52a, 52b are provided for the purpose of dynamic compensation for the eccentric loading. The radius of the surface plate 37 is chosen to exceed the sum of the offset radius "e" plus the radius of the workpiece to be polished, and is constructed by overlaying two pieces of disc members 53, 54. A fluid passage 55 for carrying the polishing solution is formed between the overlaid two discs 53, 54, which communicates with a polishing solution inlet opening 56 provided on the lateral side of the surface plate 37 as well as with a plurality of polishing solution outlet openings 57 opening at the upper surface of the disc 53. An abrading plate 59 is attached to the top surface of the surface plate 37 of the main polishing units 30a, 30b, and polishing cloth 59a is attached to the surface plate 37 of the finish polishing unit 30c. These abrading plate 59 and polishing cloth 59a are also provided with a plurality of holes (solution outlet openings) 58 to correspond with the polishing solution outlet openings 57. The solution outlet openings 57, 58 are generally uniformly distributed across the entire surface of the surface plate 37, abrading plate 59 and polishing cloth 59a. The abrading plate 59 is bonded to the top surface of the surface plate 37 in the main polishing units 30a, 30b, and a polishing cloth 59a is bonded to the surface of the surface plate 37 in the finish polishing unit 30c. The abrading plate 59 is a circular disc made of a resin serving as a binder for abrasive grains of less than several micrometers (for example, Ce0 2 ). To assure that the abrading surface is flat, the material and manufacturing process are selected so that the abrading plate would not show bowing and deformation during manufacturing and storage. The surface of the abrading plate 59 is provided with some type of channels made of grooves, shaped in a lattice, spiral or radiating pattern (not shown) to distribute the polishing solution and to remove polishing debris, and the solution outlet openings 58 are aligned with the channels. The particle size of the abrasive grains included in the polishing solution is chosen so that the size is relatively large for the rough polishing units 30a, 30b, but is relatively small or not used in the finish polishing unit 30c. The top ring 32 serves as a pressing device for the workpiece 1 onto the translation table 31 and is attached to the bottom of a shaft 60 so as to permit a free tilting within a certain degree by way of a joint. The compression force exerted by an unshown air cylinder as well as the rotational force exerted by a motor are transmitted to the top ring 32 through the shaft 60. The top ring 32 comprises a flexible sheet member at its workpiece holding section 61 for preventing transcription of the micro-waviness of the workpiece holding section 61 onto the polished surface of the wafer 1. The top ring 32 is constructed similarly to those shown in Figures 4, 5, except that this top ring 32 rotates at a slower speed. On the outer top side of the casing 34, there is a solution collection tank 63 to collect the polishing solution supplied. The operation of the polishing facility of such a construction will be explained in the following. The wafer (workpiece) 1 in the wafer storage cassette 24 (refer to Figure 4) is transferred by the transfer robot 22a, 22b through the wafer inverter 25, as necessary, to be attached to the top ring 32 in the main polishing unit 30a or 30b where rough polishing is performed. Roughly polished wafer is transferred by the robot 22a, 22b to the cleaning section 23a to be washed and finish polished in the finish polishing unit 30c. Details of polishing action will be explained further. The surface plate 37 undergoes a circular translation motion by the action of the driving motor 33, and the wafer 1 attached to the top ring 32 is pressed against the surface of the abrading plate 59 bonded to the surface plate 37. Polishing solution is supplied through the solution inlet opening 56, fluid passage 55 and the solution outlet openings 57 to reach the work surface, and is ultimately supplied to the interface between the wafer 1 and the abrading plate 59 through the channels on the abrading plate 59. The action of the minute circular translation motion (of motion radius "e") between the wafer 1 and the rubbing surface of the abrading plate 59 produces a uniform polish on the entire work surface of the wafer 1. When the wafer 1 is processed by positioning the wafer 1 and the abrading plate 59 at a constant relationship, it causes some problems introduced by localized differences in the surface conditions of the abrading plate 59, and to avoid such problems, the top ring 32 is rotated slowly so as to change positioning of the wafer 1 relative to the abrading plate 59. Finish polishing is basically the same process as rough polishing. Here, in the main polishing process, polishing conditions are such that wafer 1 and the polishing tool (abrading plate) 59 are moved at a relatively fast speed, and that the pressing force is relatively high and polishing solution includes relatively coarse abrasive grains to produce a given amount of material removal. On the other hand, the purpose of the finish polishing process is, in addition to producing further leveling and smoothing of the work surface, to remove any adhered micro-particles from the wafer surface. Therefore, roughness of the work surface of the polishing tool (cloth) 59a is finer, and the relative motion speed and pressing force between the polishing tool and the wafer are made lower than those in the main polishing process. The polishing solution is usually deionized water, but occasionally a solution or slurry may be used when necessary. When using a slurry, use of polishing grains of the same material as the abrading plate in the slurry produces good results in some cases. Wafer 1 after the finish polishing process is subjected to several cleaning and drying steps in the cleaning machines 23a~23c, and is stored in the wafer cassette 24. In this polishing facility, two main polishing units 30a, 30b are provided to perform the main polishing process while one finish polishing unit 30c is provided. This arrangement is chosen because of the consideration that that the duration of the main polishing process is longer than that of the finish polishing process, so that, one of the two units can operate for the other ' s downtime to increase the operational efficiency. In this polishing facility, because the polishing process is carried out in two stages in parallel, particle size and solution outlet openings 57, 58 can be chosen to suit the nature of each polishing process, therefore, the duration of each polishing process is shortened. Accordingly, the overall throughput (work done) is significantly improved compared with the conventional polishing apparatus shown in Figures 4 and 5. Also, because the polishing units 30a-30c are moved in a circulative translation motion, the size of the surface plate 37 only needs to be larger than the wafer 1 by an amount of eccentricity "e" . Therefore, compared with the conventional polishing unit 10, the installation space is reduced significantly. Additionally, it is easier to design a combined layout of units such as cleaning machines and wafer inverters and well as to modify an existing layout. Furthermore, the surface plate 37 undergoes a circulative translation motion in the polishing units 30a-30c, the surface plate 37 is supported at several locations distributed along its peripheral edge as shown in Figure 2, and contributes to improved flatness of the polished wafer compared with the conventional polishing apparatus based on a highspeed turntable . In the following, some of the typical operating parameters in the first and second polishing steps are compared. First Polishing Step (rough polishing) Polishing solution Depends on material to be polished Abrasive grains material of abrading plate Ce0 2 Grain size 0.1-10 μm Pressing pressure 200-500 g/cm 2 Relative speed 0.07-0.6 m/sec Polishing duration Depends on removal rate Second Polishing Step (finish polishing) Polishing solution water, chemicals, slurry Polishing cloth soft cloth (non-woven cloth, nap lamination) Pressing pressure 0-200 g/cm 2 Relative speed 0.07-0.6 m/sec Polishing duration 10-120 sec In the above embodiment, the polishing tool is made to undergo a circulative translation motion but it is also permissible to have the workpiece undergo the same motion . Also, the circular translation motion was produced by an "eccentric" design provided at the end of the drive shaft of the motor, but other designs, for example, an X-Y stage and the resulting vectors in the X- and Y-directions may be utilized to produce a circulative translation motion of a similar trace for the surface plate. Also, a crank type of support was utilized to connect to the surface plate, but it is permissible to use other types of support such as magnetic bearings and dry bearings, provided that it gives the surface plate the translation motion while restricting the rotation . Summarizing the features of the present polishing apparatus, because the size of the abrading plate needs to be only slightly larger than the workpiece size, it is easier to produce precise flatness over the entire surface of the polishing tool, compared with producing the same degree of flatness in a conventional large polishing table. The apparatus is compact and the drives can also be small and require only low power, and the installation space is minimized. The overall design of the polishing facility, including the cleaning and wafer inverting devices, is simplified and changes can be accommodated readily. These advantages become more important as the size of the wafer to be processed increases, as anticipated. Because the polishing tools are not rotated, relative speed between the wafer and the abrading plate is uniform throughout the wafer area, thus it is easier to produce flatness even at low polishing speeds to provide a smooth surface of a superior quality. Industrial Applicability The present invention is useful for polishing workpieces, such as semiconductor wafers, glass plates and liquid crystal display panels which require a high surface flatness.

Claims: CLAIMS 1. A polishing apparatus comprising: a polishing table having an abrading plate; a pressing device for forcing a work surface of a workpiece against an abrading surface of said abrading plate; and a driving device for making said abrading surface and said work surface undergo a relative circulative translation motion tracing a given pattern. 2. An apparatus according to claim 1, wherein said relative circulative translation motion is an orbiting motion. 3. An apparatus according to claim 1, wherein at least one of said workpiece and said polishing tool is rotated with a period of rotation significantly in excess of a period of said circulative translation motion. 4. An apparatus according to claim 1, wherein said abrading surface is provided with surface channels for supplying a polishing solution to an interface between said abrading surface and said work surface. 5. An apparatus according to claim 1, wherein said driving device comprises: a base section; a surface plate having an upper surface for supporting said polishing table; a support section disposed on said base section for supporting said surface plate so as to produce a circulative translation motion of said surface plate; and a driving section for producing said circulative translation motion of said surface plate. 6. An apparatus according to claim 5, wherein said support section provides support to said surface plate at not less than three peripheral locations. 7. An apparatus according to claim 6, wherein said support section includes a connecting member having a pair of shafts extending in opposite directions aligned on eccentric axes, said shafts being rotatably inserted into a depression section provided in each of said surface plate and said base section. 8. An apparatus according to claim 5, wherein said driving section includes a rotation shaft, a drive end formed eccentrically on said rotation shaft, and a depression section formed on said surface plate for accepting said drive end. 9. An apparatus according to claim 1, wherein said apparatus includes a workpiece holding section provided with a flexible sheet member disposed between a workpiece and a workpiece holding surface of said workpiece holding section. 10. A polishing facility comprising at least one main polishing unit and at least one finish polishing unit, wherein said main polishing unit comprises: a polishing table having an abrading plate; a pressing device for forcing a work surface of a workpiece against an abrading surface of said abrading plate; and a driving device for making said abrading surface and said work surface undergo a relative circulative translation motion tracing a given pattern, and said finish polishing unit comprises: a polishing table having a polishing cloth; a pressing device for forcing a work surface of a workpiece against a polishing surface of said polishing cloth; and a driving device for making said polishing cloth and said work surface undergo a relative circulative translation motion tracing a given pattern.

Inventors: KIMURA NORIO, SHIMIZU NOBURU, KIMURA, NORIO, SHIMIZU, NOBURU
Applicants: EBARA CORP, KIMURA NORIO, SHIMIZU NOBURU, EBARA CORPORATION, KIMURA, NORIO, SHIMIZU, NOBURU
Application year: 1998
Main classification: B24B37
IPC codes: B08B1, B24B37, H01L21