Polishing machine and method of dissipating heat therefrom

Abstract: A polishing machine for polishing a flat workpiece such as a semiconductor wafer has a rotatable reference table supporting an abrasive cloth disposed on a surface thereof, and a rotatable workpiece holder for holding a flat workpiece against the abrasive cloth. While the flat workpiece is being polished by the abrasive cloth, an abrasive compound is supplied between the abrasive cloth and the flat workpiece. The reference table has grooves defined therein for dissipating heat from the reference table and the abrasive cloth while the flat workpiece is being polished by the abrasive cloth. The grooves may be supplied with either the abrasive compound or a coolant.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a polishing machine for a surface of a flat workpiece such as a semiconductor wafer of silicon single crystal, and a method of dissipating heat from such a polishing machine. Description of the Prior Art: Recent years have seen semiconductor devices that are fabricated into high-density integrated circuits by the ever-advancing technology of defining intricate patterns in microscopic scale on semiconductor wafer surfaces. Designs on semiconductor devices that are available today have a line width ranging from 1µ m to 0.5µ m or even smaller. Unless semiconductor wafers which serve as substrates of such semiconductor devices have flat surfaces, they cannot be processed highly accurately by various semiconductor microcircuit fabrication processes including lithography, etching, and thin-film deposition. Naturally, as the interconnections to be formed on semiconductor wafers are required to be narrower, the semiconductor wafers should have flatter surfaces. Therefore, polishing processes and polishing machines for polishing semiconductor wafers to a flat finish are also required to be improved at all times. Fig 8 of the accompanying drawings schematically shows a conventional polishing machine for polishing a semiconductor wafer. As shown in Fig 8 the polishing machine has a disc-shaped reference table 55 with a flat upper surface which is supported on a reference table holder 56. The reference table holder 56 has an integral shaft 57 coupled to a rotary actuator (not shown) for rotating the reference table holder 56. The flat upper surface of the reference table 55 is substantially fully covered with an abrasive cloth 58. A wafer holder head 8 with a semiconductor wafer 7 held against its lower surface can be rotated about its own axis by another rotary actuator (not shown). The polishing machine also has an abrasive compound supply unit 59 for supplying an abrasive compound 9 to a position between the semiconductor wafer 7 and the abrasive cloth 58. The abrasive compound 9 may comprise, for example, a fluid dispersion that is composed of an abrasive grain such as colloidal silica distributed in an alkaline solution. The reference table holder 56 has an upwardly opening coolant reservoir 60 defined in its upper surface and closed by the lower surface of the reference table 55. The shaft 57 has a coolant supply passage 61 and a coolant discharge passage 62 which are defined therein in communication with the coolant reservoir 60. The coolant supply passage 61 and the coolant discharge passage 62 are connected to a cooler 63 and a coolant supply 64. The coolant supply 64 supplies a coolant to the cooler 63 which cools the coolant. The coolant cooled by the cooler 63 is supplied through the coolant supply passage 61 into the coolant reservoir 60. After having cooled the reference table 55, the coolant is discharged from the coolant reservoir 60 through the coolant discharge passage 62 back to the coolant supply 64 so that the coolant will be used in circulation. To polish the semiconductor wafer 7 highly flatwise, it is necessary for the reference table 55 including the abrasive cloth 58 to have a flat surface that is pressed against the semiconductor wafer 7 during the polishing process, and also to be free from abrasive wear and deformation due to mechanical stresses. To meet the above requirements, the reference table 55 is made of a material and has a structure such that the reference table 55 has a desired mechanical strength. If the semiconductor wafer 7 has a relatively large diameter, or the polishing machine is relatively large in size or operates at relatively high speed to increase its ability to polish the semiconductor wafer 7 for higher productivity, then the reference table 55 tends to be deformed by a localised temperature irregularity thereof due to a friction-induced heat generated in a local region where the semiconductor wafer 7 is in abrasive contact with the reference table 55. Such a deformation will prevent the semiconductor wafer 7 from being polished to a desired degree of flatness. To polish the semiconductor wafer 7 highly efficiently, it is necessary that the semiconductor wafer 7 be polished at high speed while being pressed against the reference table 55 under strong forces. However, such a high-speed, high-pressure polishing process results in an increase in the temperature of the semiconductor wafer 7 and the abrasive cloth 58, increasing the localised temperature irregularity of the reference table 55. To achieve a desired degree of flatness of the semiconductor wafer 7, the semiconductor wafer 7 and the abrasive cloth 58 be held in uniform contact with each other. More specifically, during the polishing process, the friction-induced heat is generated between the semiconductor wafer 7 and the abrasive cloth 58, heating them to a higher temperature. Unless the contacting surfaces of the semiconductor wafer 7 and the abrasive cloth 58 were kept at a uniform temperature, it would not be possible to polish the semiconductor wafer 7 to a uniform surface finish. The polishing capability of the abrasive compound 9 also depends on the temperature thereof. If the temperature of the abrasive compound 9 present between the semiconductor wafer 7 and the abrasive cloth 58 becomes irregular, the abrasive compound 9 can no longer polish the semiconductor wafer 7 to a uniform surface finish. The coolant reservoir 60 serves to cool the reference table 55 to prevent the semiconductor wafer 7 and the abrasive cloth 58 from being unduly heated. Fig 9 of the accompanying drawings shows a temperature distribution across the reference table 55 and the reference table holder 56. As shown in Fig 9 the region where the reference table 55 and the semiconductor wafer 7 are held in abrasive contact with each other has a relatively large flow of frictional-heat energy directed downwardly as indicated by the arrow A, and a relatively small flow of frictional-heat energy directed downwardly as indicated by the arrow B near the circumferential edge of the reference table 55. The same abrasive-contact region also has an upward flow of heat energy, as indicated by the arrow C, from the rotary actuator which rotates the shaft 57 of the reference table holder 56. As a result, as shown on the lefthand side of Fig 9, the abrasive-contact region on the reference table 55 contains an area that undergoes relatively high frictional heat as indicated by the solid line and an area which undergoes relatively low frictional heat as indicated by the dotted line. Since the reference table 55 usually has a thickness of several tens millimetres, only the coolant reservoir 60 cannot sufficiently cool the frictional face side of the reference table 55. As a consequence, the temperatures of the face and reverse sides of the reference table 55 differ widely from each other, causing the reference table 55 to be largely deformed as shown in Fig 10 of the accompanying drawings. The reference table 55 is normally made of SUS or a ceramic material, whereas the reference table holder 56 of cast iron. The reference table 55 is therefore also deformed due to different coefficients of thermal expansion of the reference table 55 and the reference table holder 56. For the above reasons, the reference table 55 cannot keep its face side as flat as desired for uniformly polishing the semiconductor wafer 7. Fig 12 of the accompanying drawings shows a process of polishing one semiconductor wafer 7 at a time with the abrasive cloth 58, and Fig 13 of the accompanying drawings shows a process of polishing a batch of four semiconductor wafers 7 supported on a single wafer plate 65 with the abrasive cloth 58. In either of the illustrated processes, the heat generated in the region where the reference table 55 is in abrasive contact with the semiconductor wafer or wafers 7 is responsible for a temperature irregularity on the surface of the reference table 55, and the abrasive cloth 58 imposes an abrasive load on the semiconductor wafer or wafers 7 in that region due to the abrasive action of the abrasive cloth 58 on the semiconductor wafer or wafers 7 during rotation of the abrasive cloth 58. In Figs 12 and 13, as the curve goes higher, the abrasive load is higher and so is the frictional head. Therefore, as shown in Fig 11 of the accompanying drawings, the centre of each semiconductor wafer 7 is higher in temperature than the circumferential area thereof, resulting in an irregular temperature distribution in each semiconductor wafer 7. Irrespective of whether one semiconductor wafer is polished at a time or a batch of semiconductor wafers 7 are polished simultaneously, it has been impossible to finish the semiconductor wafer or wafers 7 to a desired flat finish. SUMMARY OF THE INVENTION It is an object of the present invention to provide a polishing machine for polishing a flat workpiece to a desired degree of flatness while cooling an abrasive cloth and a reference table for minimising temperature irregularities and thermally induced deformations of the reference table, so that the flat workpiece can be polished under high pressure at high speed. Another object of the present invention is to provide a method of dissipating heat from such a polishing machine for polishing a flat workpiece. According to a first aspect of the present invention, there is provided a polishing machine for polishing a flat workpiece, comprising: a rotatable reference table supporting an abrasive cloth disposed on a surface thereof; a reference table holder upon which said reference table is mounted; a rotatable workpiece holder for holding a flat workpiece against said abrasive cloth; and means for supplying an abrasive compound between said abrasive cloth and the flat workpiece; said reference table having groove means defined therein for dissipating heat from said reference table and said abrasive cloth while the flat workpiece is being polished by said abrasive cloth, such a device being known from FR-A-2106809, but the device additionally having said groove means comprising a plurality of grooves defined substantially entirely in an opposite surface of the reference table adjacent said reference table holder; and said grooves extending from said opposite surface of said reference table toward, and terminating short of, said surface; (FR-A-2106809 discloses a polishing machine in which similar grooves are formed in the reference table holder, rather than in the reference table itself). In accordance with a second aspect of the invention, there is provided a method of dissipating heat from such a polishing machine for polishing a flat workpiece with an abrasive cloth disposed on a reference table having grooves while pressing the flat workpiece against the abrasive cloth and supplying an abrasive compound between the flat workpiece and the abrasive cloth, said method comprising the steps of: supplying a coolant to said grooves; and discharging the coolant from said grooves The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is an axial cross-sectional view, partly in a coolant circuit diagram, of a polishing machine according to an embodiment of the present invention; Fig 2 is an enlarged fragmentary cross-sectional view of a reference table of the polishing machine shown in Fig 1, the reference table having grooves; Fig 3 is a plan view of the reference table shown in Fig 1; Fig 4 is a plan view of a modified reference table that can be used in the polishing machine shown in Fig 1, the modified reference table having grooves; Fig 5 through 7 are schematic plan views of other reference tables with grooves defining different coolant path patterns; Fig 8 is an axial cross-sectional view, partly shown in a coolant circuit diagram, of a conventional polishing machine; Fig 9 is an enlarged cross-sectional view of a reference table of the conventional polishing machine shown in Fig 8 and a graph showing a temperature distribution across the reference table; Fig 10 is an axial cross-sectional view showing the manner in which the reference table shown in Fig 9 is deformed due to heat; Fig 11 is an enlarged cross-sectional view of the reference table shown in Fig 8 and a graph showing an abrasive load distribution in semiconductor wafers; Fig 12 is a plan view and a graph illustrative of an abrasive load distribution in a single semiconductor wafer that is polished by the conventional polishing machine; and Fig 13 is a plan view and a graph illustrative of an abrasive load distribution in a batch of semiconductor wafers that are simultaneously polished by the conventional polishing machine. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig 1 a polishing machine to which the principles of the present invention are applied includes a circular reference table 2 having a flat upper surface and is fixedly mounted on a reference table holder 29, having an integral central shaft 3 which is operatively coupled to a drive unit (not shown). The reference table 2 has a plurality of grooves 30 (described later on). The upper surface of the reference table 2 is covered with an abrasive cloth 6. A wafer holder head 8 with a semiconductor wafer 7 held against its lower surface is positioned such that the semiconductor wafer 7 faces toward the abrasive cloth 6. The wafer holder 8 can be rotated about its own axis by a rotary actuator (not shown). The polishing machine also has an abrasive compound supply nozzle 10 for supplying an abrasive compound 9 to a position between the semiconductor wafer 7 and the abrasive cloth 6. The abrasive compound 9 is used to both polish and cool the semiconductor wafer 7. An annular abrasive compound receiver 11 is attached to the reference table 2 around its outer circumferential edge for receiving the abrasive compound 9 that flows radially outwardly on the reference table 2 and falls off the outer circumferential edge thereof. An abrasive compound discharge pipe 12 has an upper end connected to the abrasive compound receiver 11 and a lower end opening into an abrasive compound tank 13. Therefore, the abrasive compound that flows off the reference table 2 returns through the abrasive compound receiver 11 and the abrasive compound discharge pipe 12 back to the abrasive compound tank 13. A coolant pipe 17, which has one end connected through a cooler 15 and a control valve 16 to a coolant supply 14, has a portion extending through the abrasive compound tank 13. The other end of the coolant pipe 17 is connected also to the coolant supply 14. A temperature control unit 18 is electrically connected to a temperature sensor 19 in the abrasive compound tank 13 and the control valve 16. In response to a detected temperature signal from the temperature sensor 19, the temperature control unit 18 opens or closes the control valve 16 to control the flow of a coolant in the coolant pipe 17 for keeping the abrasive compound 19 at a predetermined temperature in the abrasive compound tank 13. An abrasive compound supply pipe 20 with a pump 21 has one end connected to the abrasive compound tank 13 and the other end to the abrasive compound supply nozzle 10. The drive unit (not shown) coupled to the shaft 3 may comprise a drive motor mounted on a bottom panel of a container (not shown) which houses the reference table and the abrasive compound receiver 11, a first pulley mounted on the output shaft of the drive motor, a second pulley coupled through a transmission mechanism to the shaft 3, and an endless belt trained around the first and second pulleys. The rotational power from the drive motor can thus be transmitted through the first pulley, the endless belt, the second pulley, and the transmission mechanism to the shaft 3 for thereby rotating the reference table 2. The reference table 2 has its back or lower surface covered with a thermally insulating layer 26 which faces downwardly toward the drive unit. The abrasive compound receiver 11 has an inner edge joined to the thermally insulating layer 26. The polishing machine operates as follows: The abrasive compound 9 that has been cooled to a suitable temperature by the coolant supplied from the coolant supply 14 is delivered by the pump 21 from the abrasive compound tank 13 thorough the abrasive compound supply pipe 20 to the abrasive compound supply nozzle 10, from which the abrasive compound 9 is ejected onto the upper surface of the abrasive cloth 6. The ejected abrasive compound 9 flows between the semiconductor wafer 7 and the abrasive cloth 6, and then falls off the circumferential edge of the reference table 2 into the abrasive compound receiver 11, from which the abrasive compound flows through the abrasive compound discharge pipe 12 into the abrasive compound tank 13 for circulation. In the abrasive compound tank 13, the abrasive compound 9 is cooled by the coolant flowing through the coolant pipe 17, and is kept at a substantially constant temperature by the control valve 16 which is selectively opened and closed by the temperature control unit 18 in response to a detected temperature signal from the temperature sensor 19. During a polishing process, the reference table 2 is rotated by the drive unit, and at the same time the wafer holder head 8 is also rotated by its rotary actuator. Therefore, the semiconductor wafer 7 supported by the wafer holder head 8 is polished to a flat finish while in pressed sliding contact with the abrasive cloth 6. As shown in Fig 1, the lower surface of the reference table 2 is covered with the thermally insulating layer 26 substantially in its entirety. The heat generated by the drive motor and the transmission mechanism is radiated upwardly toward the reference table 2. However, the thermally insulating layer 26 is effective in preventing the heat from being transmitted to the reference table 2. Accordingly, the reference table 2 is prevented from suffering temperature irregularities which would otherwise be caused by the heat from the motor 22 and the transmission mechanism 25a. Since the reference table 2 and the abrasive cloth 6 are effectively cooled or prevented from being unduly heated, the polishing machine can polish the semiconductor wafer 7 under high pressure at high speed for greater productivity. The reference table 2 has a plurality of grooves 30 defined in its back held in contact with the reference table holder 29. The grooves 30, which are defined substantially entirely in the back of the reference table 2, extend upwardly but terminate short of the upper surface of the reference table 2. As shown in Fig 2, the upper ends or bottoms of the grooves 30 are spaced from the upper surface of the reference table 2 by a distance or thickness t which is selected to be as small as possible. Since the thickness t is small, the reference table 2 is of small rigidity and can absorb deformations when the reference table is thermally expanded. Therefore, the reference table 2 is prevented from being thermally deformed as a whole. The reference table holder 29 which is fixed to the reference table 2 is of such high rigidity that the assembly of the reference table 2 and the reference table holder 29 is rigid enough to withstand mechanical stresses. As shown in Fig 1 the reference table holder 29 and the shaft 3 jointly have a coolant supply passage 31 and a coolant discharge passage 32. The coolant supply passage 31 has one end connected to the grooves 30 and the other end to the coolant supply 14 thorough the cooler 15. The coolant discharge passage 32 has one end connected to the grooves 30 and the other end to the coolant supply 14. The coolant from the coolant supply 14 flows into the cooler 15 which cools the coolant to a suitable temperature. The cooled coolant is then supplied through the coolant supply passage 31 into the grooves 30 to cool the reference table 2 in its entirety for reducing thermally induced deformations thereof. The coolant in the grooves 30 that has absorbed the heat from the abrasive cloth 6 is discharged from the grooves 30 through the coolant discharge passage 32 to the coolant supply 14. The abrasive compound 9 from the abrasive compound tank 13 is supplied onto the abrasive cloth 6. The supplied abrasive compound 9 serves to polish the semiconductor wafer and also to cool the abrasive cloth 6 and the reference table 2. The abrasive cloth 6 is kept flatwise to polish the semiconductor wafer to a desired degree of flatness. Fig 3 shows in plan a grid pattern, ie, a pattern of parallel rows and columns, of grooves 30a defined in the reference table 2. Fig 4 shows in plan a pattern of radial grooves 30b and concentric grooves 30c which may be defined in the reference table 2. The reference table 2 may of course have any of various other different patterns or grooves. Figs 5 through 7 illustrate other reference tables with grooves defining different coolant path patterns. In Figs 5 through 7, the grooves 30a, 30b, 30c shown in Figs 3 and 4 are partly blocked to control flows of the coolant for uniformly and efficiently cooling the reference table 2. In Fig 5, the reference table 2 has radial straight coolant passages 33 directed radially inwardly from the outer circumferential edge of the reference table 2 toward the centre thereof for intensifying the coolant flows. In Fig 6, the reference table 2 is divided into quarter areas each having a curved coolant passage 34. In Fig 7, the reference table 2 is divided into half area each having a meandering coolant passage for smoothly and uniformly polishing the coolant substantially entirely through the reference table 2. Two of the reference tables with the abrasive cloth in each of the above embodiments may be employed to sandwich a semiconductor wafer for simultaneously polishing the opposite surfaces thereof to a flat finish. While only one semiconductor wafer is shown as being polished by the polishing machine according to the above embodiments, the principles of the present invention are also applicable to a polishing machine for simultaneously polishing a batch of semiconductor wafers. Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims: A polishing machine (1) for polishing a flat workpiece (7), comprising: a rotatable reference table (2) supporting an abrasive cloth (6) disposed on a surface thereof; a reference table holder (29) upon which said reference table (2) is mounted; a rotatable workpiece holder (8) for holding a flat workpiece (7) against said abrasive cloth (6); and means (10) for supplying an abrasive compound (9) between said abrasive cloth (6) and the flat workpiece (7); said reference table (2) having groove means (30) defined therein for dissipating heat from said reference table (2) and said abrasive cloth (6) while the flat workpiece (7) is being polished by said abrasive cloth (6); characterized by: said groove means (30) comprising a plurality of grooves (30) defined substantially entirely in an opposite surface of the reference table (2) adjacent said reference table holder (29); and said grooves (30) extending from said opposite surface of said reference table (2) toward, and terminating short of, said surface. A polishing machine according to Claim 1, wherein said grooves (30) include a first group of grooves and a second group of grooves crossing said first group of grooves. A polishing machine according to Claim 2, wherein said first and second groups of grooves (30a) are arranged in a grid pattern in criss-cross relationship. A polishing machine according to Claim 2, wherein said first group of grooves comprise radial grooves (30b) and said second group of grooves comprise concentric circular grooves (30c). A polishing machine according to claim 1, wherein said groove means comprises a plurality of radial straight grooves (33) defined in said opposite surface of the reference table (2). A polishing machine according to claim 1, wherein said groove means comprises a plurality of curved grooves (34) defined in said opposite surface of the reference table (2). A polishing machine according to claim 1, wherein said groove means comprises a plurality of meandering grooves (35) defined in said surface of the reference table (2). A polishing machine according to any preceding Claim, further including a thermally insulating layer (26) located on a surface of said reference table holder (29) remote from said reference table (2). A polishing machine according to any preceding Claim, further comprising means (14, 15, 17, 31, 32) for supplying a coolant to said groove means (30). A polishing machine as claimed in any preceding Claim, further including means'(11, 12, 13, 20, 21) for collecting waste abrasive compound (9) after use and for recirculating said waste compound to said supply means (10). A polishing machine as claimed in Claim 10, further including means (14 - 19) for regulating the temperature of said recirculated abrasive compound (9). A polishing machine as claimed in Claim 11, when dependent from Claim 9, wherein the coolant supplied to said grooves (30) is also used to regulate the temperature of said recirculated abrasive compound (9). A polishing machine as claimed in any preceding Claim, wherein said machine is adapted for polishing semiconductor wafers. A method of dissipating heat from a polishing machine (1), as claimed in any preceding Claim, for polishing a flat workpiece (7) with an abrasive cloth (6) disposed on a reference table (2) having grooves (30) while pressing the flat workpiece (7) against the abrasive cloth (6) and supplying an abrasive compound (9) between the flat workpiece (7) and the abrasive cloth (6), said method comprising the steps of: supplying a coolant to said grooves (30); and discharging the coolant from said grooves (30). A method as claimed in Claim 14, further including the steps of collecting and recirculating said abrasive compound (9) after use, and of regulating the temperature of said recirculated compound. A method as claimed in Claim 15, wherein the coolant supplied to said grooves (30) is also used to regulate the temperature of said abrasive compound (9).

Inventors: HASHIMOTO HIROMASA, SUZUKI FUMIO, TANAKA KOHICHI, HASHIMOTO, HIROMASA, SUZUKI, FUMIO, TANAKA, KOHICHI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1993
Main classification: B24B37
IPC codes: B24B37, B24B49, B24B55