Polishing method

A semiconductor wafer (W) and a polishing table (100) are rotated. The polishing table (100) has a polishing surface thereon. The semiconductor wafer (W) is pressed against the polishing surface on the polishing table (100) rotated at a first rotational speed to polish the semiconductor wafer (W). The semiconductor wafer (W) is separated from the polishing surface after the semiconductor wafer (W) is pressed against the polishing surface. Before the semiconductor wafer (W) is separated from the polishing surface, a rotational speed of the polishing table (100) is reduced to a second rotational speed lower than the first rotational speed to provide a difference between a rotational speed of the semiconductor wafer (W) and the rotational speed of the polishing table (100).

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Description
TECHNICAL FIELD

The present invention relates to a polishing method, and more particularly to a method of polishing a workpiece such as a semiconductor wafer to a flat mirror finish.

BACKGROUND ART

As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. In the case of photolithography, which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because the depth of focus of an optical system is relatively small. However, conventional apparatuses for planarizing semiconductor wafers, such as self-planarizing CVD apparatuses and etching apparatuses, cannot produce semiconductor wafers having completely planarized surfaces. Recently, it has been attempted to planarize semiconductor wafers with a polishing apparatus, which is expected to achieve complete planarization of semiconductor wafers more easily as compared to the above conventional apparatuses.

FIG. 1 shows a basic arrangement of this type of polishing apparatus. As shown in FIG. 1, the polishing apparatus has a polishing table 151 having a polishing pad 161 thereon and a top ring 152 for holding a semiconductor wafer W as a workpiece to be polished in a manner such that a surface to be polished faces the polishing pad 161. The polishing pad 161 has an upper surface which serves as a polishing surface for polishing a workpiece. The top ring 152 is connected to a lower end of a top ring shaft 153 via a ball joint 154 in a manner such that the top ring 152 is tiltable with respect to the top ring shaft 153. The top ring 152 presses the semiconductor wafer W against the polishing table 151 under a desired pressure while the polishing table 151 and the top ring 152 are independently rotated. A polishing liquid Q such as abrasive liquid or pure water is supplied onto the polishing pad 161 from a polishing liquid supply nozzle 155. Thus, a surface of the semiconductor wafer W is polished by the polishing liquid Q. At that time, the surface of the semiconductor wafer W is brought into sliding contact with a surface of the polishing pad 161 so as to follow the surface of the polishing pad 161 via the ball joint 154.

There has heretofore employed a polishing pad made of non-woven fabric as a polishing pad attached to a polishing table. Higher levels of integration achieved in recent years for ICs and LSI circuits demand smaller steps or surface irregularities on a surface of a semiconductor wafer. In order to meet such a demand, there has been used a polishing pad made of a hard material such as polyurethane foam.

After the semiconductor wafer W is thus polished by the polishing apparatus, it is necessary to remove (or separate) the semiconductor wafer W from the polishing surface (polishing pad 161) on the polishing table 151. However, a large surface tension acts between the polishing pad 161 and the semiconductor wafer W due to the polishing liquid Q interposed therebetween. Accordingly, if the top ring 152 holding the semiconductor wafer W is lifted at a polishing position in order to remove the semiconductor wafer W from the polishing pad 161, there are some cases in which only the top ring 152 is lifted and the semiconductor wafer W adheres to the polishing pad 161 so as to be left on the polishing pad 161.

Such a problem can be solved by an overhanging action of the top ring. In the overhanging action, after a polishing process is completed, the top ring 152 is not lifted at the polishing position, but is moved to an outer circumferential edge of the polishing pad 161 to partly expose a polished surface of a semiconductor wafer W beyond the outer circumferential edge of the polishing pad 161, and is then lifted to remove the semiconductor wafer W from the polishing pad 161. This overhanging action allows surface tension between the polishing pad 161 and the semiconductor wafer W to be reduced, and the semiconductor wafer W can reliably be removed or separated from the polishing pad 161.

As described above, the overhanging action can reduce the surface tension between the polishing pad 161 and the semiconductor wafer W. However, the top ring 152 may tilt when the polished semiconductor wafer W projects from the outer circumferential edge of the polishing pad 161. In such a case, the semiconductor wafer W is intensively pressed at the outer circumferential edge of the polishing pad 161, so that the semiconductor wafer W is cracked or scratched.

Polishing capability of a polishing pad is gradually deteriorated due to deposits of abrasive particles and polishing wastes of semiconductor material, and due to changes in characteristics of the polishing pad. Therefore, if the same polishing pad is used to repeatedly polish semiconductor wafers, a polishing rate of the polishing apparatus is lowered, and polished semiconductor wafers tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing pad according to a dressing process of recovering a surface of the polishing pad with a diamond dresser or the like before, after, or during polishing.

When a dresser dresses a polishing surface of a polishing pad, it scrapes a thin layer off the polishing pad. Therefore, after the polishing surface of the polishing pad has been dressed many times, it becomes irregular, i.e. loses its planarity, thereby causing formation of steps. As a result, during movement of a polished semiconductor wafer to the outer circumferential edge of the polishing pad in the aforementioned overhanging action, the semiconductor wafer may be cracked or scratched because of the irregularities of the polishing pad.

DISCLOSURE OF INVENTION

The present invention has been made in view of the above drawbacks. It is, therefore, an object of the present invention to provide a polishing method which can easily and safely separate a polished workpiece form a polishing surface without an overhanging action and can increase throughput.

In order to attain the above object, according to a first aspect of the present invention, there is provided a polishing method. A workpiece and a polishing table are rotated. The polishing table has a polishing surface thereon. The workpiece is pressed against the polishing surface on the polishing table rotated at a first rotational speed to polish the workpiece. The workpiece is separated from the polishing surface after the workpiece is pressed against the polishing surface. Before the workpiece is separated from the polishing surface, a rotational speed of the polishing table is reduced to a second rotational speed lower than the first rotational speed to provide a difference between a rotational speed of the workpiece and the rotational speed of the polishing table.

According to the first aspect of the present invention, the rotational speed of the polishing table is reduced from a rotational speed of the polishing table rotated at the time of polishing to provide a difference between the rotational speeds of the polishing table and the workpiece. At that time, the workpiece floats on a liquid layer of polishing liquid which is formed between the lower surface of the workpiece and the polishing surface. Specifically, a hydroplaning phenomenon occurs. The hydroplaning phenomenon reduces surface tension between the polishing surface and the workpiece due to the polishing liquid. Therefore, even if the workpiece is lifted directly at a position where the polishing process is completed, the workpiece can readily be removed or separated from the polishing surface without an overhangining action. Thus, the workpiece is not left on the polishing surface and is prevented from being cracked or scratched, which would be caused by an overhanging action. Further, since it is not necessary to perform an overhanging action, polishing tact time can be shortened, and throughput can be improved.

According to a preferred aspect of the present invention, the reduction of the rotational speed of the polishing table is performed during the polishing process. When the rotational speed of the polishing table is reduced, a cleaning process of the polishing surface is performed. In this case, separation of the workpiece from the polishing surface is facilitated, and conditions for the next workpiece can timely be adjusted. As a result, throughput is improved.

According to a preferred aspect of the present invention, a polishing liquid to the polishing surface is supplied at a first flow rate during the polishing process. Before or during the separation of the workpiece from the polishing surface, a flow rate of the polishing liquid is increased to a second flow rate higher than the first flow rate. In this case, it is possible to enhance effects of a hydroplaning phenomenon and to easily separate the workpiece from the polishing surface.

According to a preferred aspect of the present invention, a mixture of gas and pure water or chemical liquid is ejected to the polishing surface to clean the polishing surface before the separation of the workpiece from the polishing surface. In this case, it is possible to enhance effects of a hydroplaning phenomenon and to start a cleaning process of the polishing surface, which would be performed after the polishing process, during the polishing process.

According to a preferred aspect of the present invention, the workpiece is lifted from the polishing surface to separate the workpiece form the polishing surface. A lifting speed of the workpiece is reduced until the workpiece has substantially been separated from the polishing surface. Thus, stresses caused to the workpiece when the workpiece is separated from the polishing surface can be reduced.

According to a second aspect of the present invention, there is provided a polishing method. A workpiece and a polishing surface are moved relative to each other. The workpiece is pressed against the polishing surface moved at a first frequency to polish the workpiece. The workpiece is separated from the polishing surface after the workpiece is pressed against the polishing surface. Before the workpiece is separated from the polishing surface, a frequency of movement of the polishing surface is reduced to a second frequency lower than the first frequency. The workpiece and the polishing surface may make an orbital movement.

According to the second aspect of the present invention, the workpiece floats on a liquid layer of polishing liquid which is formed between the lower surface of the workpiece and the polishing surface. Specifically, a hydroplaning phenomenon occurs. The hydroplaning phenomenon reduces surface tension between the polishing surface and the workpiece due to the polishing liquid. Therefore, even if the workpiece is lifted-directly at a position where the polishing process is completed, the workpiece can readily be removed or separated from the polishing surface without an overhangining action.

According to a third aspect of the present invention, there is provided a polishing method. A polishing surface is moved relative to a workpiece. The workpiece is pressed against the polishing surface moved at a first speed to polish the workpiece. The workpiece is separated from the polishing surface after the workpiece is pressed against the polishing surface. Before the workpiece is separated from the polishing surface, a speed of movement of the polishing surface is reduced to a second speed smaller than the first speed. The polishing surface may be moved linearly.

According to the third aspect of the present invention, when the speed of movement of the polishing surface is reduced, a hydroplaning phenomenon occurs to reduce surface tension between the polishing surface and the workpiece due to the polishing liquid. Therefore, even if the workpiece is lifted directly at a position where the polishing process is completed, the workpiece can readily be removed from the polishing surface without an overhangining action.

According to a fourth aspect of the present invention, there is provided a polishing method. A workpiece and a polishing table are rotated. The polishing table has a polishing surface thereon. The workpiece is pressed against the polishing surface on the polishing table. The workpiece is separated from the polishing surface after the workpiece is pressed against the polishing surface. A ratio of a rotational speed of the top ring to a rotational speed of the polishing table is reduced before the workpiece is separated from the polishing surface.

The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic view showing a basic arrangement of a polishing apparatus;

FIG. 2 is a front view showing a polishing apparatus employing a polishing method according to an embodiment of the present invention;

FIG. 3 is a plan view of the polishing apparatus shown in FIG. 2;

FIG. 4 is a cross-sectional view showing a top ring in the polishing apparatus shown in FIG. 2;

FIG. 5 is a bottom view of the top ring shown in FIG. 4;

FIG. 6 is a timing chart showing an example of a polishing process to polish a semiconductor wafer and a separating process to separate the semiconductor wafer from a polishing surface according to an embodiment of the present invention; and

FIG. 7 is a perspective view showing a polishing apparatus having a web-type (belt-type) polishing table which can employ the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A polishing method according to embodiments of the present invention will be described below with reference to FIGS. 2 through 7.

FIG. 2 is a front view showing an arrangement of a polishing apparatus employing a polishing method according to an embodiment of the present invention, and FIG. 3 is a plan view of the polishing apparatus shown in FIG. 2. As shown in FIGS. 2 and 3, the polishing apparatus has a top ring 1 for holding a semiconductor wafer W and a polishing table 100 disposed below the top ring 1. The polishing table 100 has a polishing pad 101 attached to an upper surface of the polishing table 100. The polishing pad 101 has an upper surface which is brought into sliding contact with the semiconductor wafer W. Thus, the upper surface of the polishing pad 101 serves as a polishing surface for polishing a semiconductor wafer W. The polishing apparatus also has a polishing liquid supply nozzle 102 disposed above the polishing table 100 for supplying a polishing liquid Q onto the polishing pad 101, and an atomizer 103 having a plurality of ejection nozzles 103a connected to a nitrogen gas supply source and a liquid supply source. In FIG. 3, the atomizer 103 may be disposed at a position indicated by solid lines or at a position indicated by chain double-dashed lines.

The polishing liquid supply nozzle 102 supplies a polishing liquid Q, such as abrasive liquid or pure water, which is used for polishing the semiconductor wafer W, onto the polishing surface on the polishing table 100. The atomizer 103 ejects a mixture of nitrogen gas and pure water or chemical liquid to the polishing surface on the polishing table 100. The nitrogen gas and the pure water or chemical liquid are regulated in pressures to predetermined values through regulators or air operator valves (not shown) connected to the nitrogen gas supply source and the liquid supply source and are then supplied to the ejection nozzles 103a of the atomizer 103 in a mixed state. In this case, the ejection nozzles 103a of the atomizer 103 should preferably eject the mixture toward a peripheral edge of the polishing table 100. The atomizer 103 may employ any inert gas other than nitrogen gas. The atomizer 103 may eject only liquid such as pure water or chemical liquid.

The mixture of the nitrogen gas and the pure water or chemical liquid may be supplied in a state of (1) liquid fine particles, (2) solid fine particles as a result of solidification of the liquid, or (3) gas as a result of vaporization of the liquid. These states (1), (2), and (3) are referred to as atomization. In these states, the mixture may be ejected from the ejection nozzles 103a of atomizer 103 toward the polishing surface on the polishing table 100. For example, pressure or temperature of the nitrogen gas and/or the pure water or chemical liquid, or the shape of the nozzles determines which state of the mixture is to be ejected, i.e., liquid fine particles, solid fine particles, or gas. Therefore, the state of the mixture to be ejected can be varied, for example, by properly adjusting pressure or temperature of the nitrogen gas and/or the pure water or chemical liquid with use of a regulator or the like, or by properly adjusting the shape of the nozzles.

Various kinds of polishing pads are available on the market. For example, some of these are SUBA800, IC-1000, and IC-1000/SUBA400 (two-layer cloth) manufactured by Rodel Inc., and Surfin xxx-5 and Surfin 000 manufactured by Fujimi Inc. SUBA800, Surfin xxx-5, and Surfin 000 are non-woven fabrics bonded by urethane resin, and IC-1000 is made of rigid polyurethane foam (single-layer). Polyurethane foam is porous and has a large number of fine recesses or holes formed in its surface.

As shown in FIG. 2, the top ring 1 is connected to a top ring drive shaft 11 via a universal joint 10. The top ring drive shaft 11 is coupled to a top ring air cylinder 111 fixed to a top ring head 110. The top ring air cylinder 111 operates to move the top ring drive shaft 11 vertically to thereby lift and lower the top ring 1 as a whole and to press a retainer ring 3 fixed to a lower end of a top ring body 2 against the polishing table 100. The top ring air cylinder 111 is connected to a compressed air source 120 via a regulator R1, which can regulate pressure of compressed air or the like which is supplied to the top ring air cylinder 111. Thus, it is possible to adjust a pressing force to press the polishing pad 101 with the retainer ring 3.

The top ring drive shaft 11 is connected to a rotary sleeve 112 by a key (not shown). The rotary sleeve 112 has a timing pulley 113 fixedly disposed at a peripheral portion thereof. A top ring motor 114 is fixed to the top ring head 110. The timing pulley 113 is coupled to a timing pulley 116 mounted on the top ring motor 114 via a timing belt 115. Therefore, when the top ring motor 114 is energized for rotation, the rotary sleeve 112 and the top ring drive shaft 11 are rotated in unison with each other via the timing pulley 116, the timing belt 115, and the timing pulley 113 to thereby rotate the top ring 1. The top ring head 110 is supported on a top ring head shaft 117 fixedly supported on a frame (not shown).

The top ring 1 of the polishing apparatus will be described below. FIG. 4 is a cross-sectional view showing the top ring 1 of the polishing apparatus, and FIG. 5 is a bottom view of the top ring 1 shown in FIG. 4. As shown in FIG. 4, the top ring 1 has a top ring body 2 in the form of a cylindrical housing with a receiving space defined therein, and a retainer ring 3 fixed to the lower end of the top ring body 2. The top ring body 2 is made of a material having high strength and rigidity, such as metal or ceramics. The retainer ring 3 is made of highly rigid synthetic resin, ceramics, or the like.

The top ring body 2 has a cylindrical housing 2a, an annular pressurizing sheet support 2b fitted into the cylindrical portion of the housing 2a, and an annular seal 2c fitted over an outer circumferential edge of an upper surface of the housing 2a. The retainer ring 3 is fixed to the lower end of the housing 2a of the top ring body 2. The retainer ring 3 has a lower portion projecting radially inwardly. The retainer ring 3 may be formed integrally with the top ring body 2.

The top ring drive shaft 11 is disposed above the central portion of the housing 2a of the top ring body 2. The top ring body 2 is coupled to the top ring drive shaft 11 by the universal joint 10. The universal joint 10 has a spherical bearing mechanism by which the top ring body 2 and the top ring drive shaft 11 are tiltable with respect to each other, and a rotation transmitting mechanism for transmitting rotation of the top ring drive shaft 11 to the top ring body 2. The spherical bearing mechanism and the rotation transmitting mechanism transmit a pressing force and a rotating force from the top ring drive shaft 11 to the top ring body 2 while allowing the top ring body 2 and the top ring drive shaft 11 to be tilted with respect to each other.

The spherical bearing mechanism has a hemispherical concave recess 11a defined centrally in the lower surface of the top ring drive shaft 11, a hemispherical concave recess 2d defined centrally in the upper surface of the housing 2a, and a bearing ball 12 made of a highly hard material such as ceramics and interposed between the concave recesses 11a and 2d. The rotation transmitting mechanism includes drive pins (not shown) fixed to the top ring drive shaft 11 and driven pins (not shown) fixed to the housing 2a. Even if the top ring body 2 is tilted with respect to the top ring drive shaft 11, the drive pins and the driven pins remain in engagement with each other while contact points are displaced because the drive pins and the driven pins are vertically movable relative to each other. Thus, the rotation transmitting mechanism reliably transmits rotational torque of the top ring drive shaft 11 to the top ring body 2.

The top ring body 2 and the retainer ring 3 secured to the top ring body 2 have a space defined therein, which accommodates therein a seal ring 4 having a lower surface brought into contact with a peripheral portion of the semiconductor wafer W held by the top ring 1, an annular holder ring 5, and a disk-shaped chucking plate 6 which is vertically movable within the receiving space in the top ring body 2. The seal ring 4 has a radially outer edge clamped between the holder ring 5 and the chucking plate 6 secured to the lower end of the holder ring 5 and extends radially inwardly so as to cover the lower surface of the chucking plate 6 near its outer circumferential edge. The lower end surface of the seal ring 4 is brought into contact with the upper surface of the semiconductor wafer W to be polished. The semiconductor wafer W has a recess defined in an outer edge thereof, which is referred to as a notch or orientation flat, for recognizing (identifying) orientation of the semiconductor wafer. The seal ring 4 should preferably extend radially inwardly of the chucking plate 6 from the innermost position of such as a notch or orientation flat.

The chucking plate 6 may be made of metal. However, when the thickness of a thin film formed on a surface of a semiconductor wafer is measured by a method using eddy current in a state such that the semiconductor wafer to be polished is held by the top ring, the chucking plate 6 should preferably be made of a non-magnetic material, e.g. an insulating material such as fluororesin, epoxy resin, or ceramics.

A pressurizing sheet 7 comprising an elastic membrane extends between the holder ring 5 and the top ring body 2. The pressurizing sheet 7 has a radially outer edge clamped between the housing 2a and the pressurizing sheet support 2b of the top ring body 2, and a radially inner edge clamped between an upper end portion 5a and a stopper 5b of the holder ring 5. The top ring body 2, the chucking plate 6, the holder ring 5, and the pressurizing sheet 7 jointly define a pressure chamber 21 in the top ring body 2. As shown in FIG. 4, a fluid passage 31 comprising tubes and connectors communicates with the pressure chamber 21, which is connected to the compressed air source 120 via a regulator R2 provided on the fluid passage 31. The pressurizing sheet 7 is made of a highly strong and durable rubber material such as ethylene propylene rubber (EPDM), polyurethane rubber, or silicone rubber.

In a case where the pressurizing sheet 7 is made of an elastic material such as rubber, if the pressurizing sheet 7 is fixedly clamped between the retainer ring 3 and the top ring body 2, then a desired horizontal surface cannot be maintained on the lower surface of the retainer ring 3 because of elastic deformation of the pressurizing sheet 7 as an elastic material. In order to prevent such a drawback, the pressurizing sheet 7 is clamped between the housing 2a of the top ring body 2 and the pressurizing sheet support 2b provided as a separate member in the present embodiment. The retainer ring 3 may be movable vertically with respect to the top ring body 2, or the retainer ring 3 may have a structure capable of pressing the polishing surface independently of the top ring body 2. In such cases, the pressurizing sheet 7 is not necessarily fixed in the aforementioned manner.

A cleaning liquid passage 51 in the form of an annular groove is defined in the upper surface of the housing 2a near its outer circumferential edge over which the seal 2c of the top ring body 2 is fitted. The cleaning liquid passage 51 communicates with a fluid passage 32 through a through-hole 52 formed in the seal 2c. A cleaning liquid (pure water) is supplied through the fluid passage 32 to the cleaning liquid passage 51. A plurality of communication holes 53 are defined in the housing 2a and the pressurizing sheet support 2b in communication with the cleaning liquid passage 51. The communication holes 53 communicate with a small gap G defined between the outer circumferential surface of the seal ring 4 and the inner circumferential surface of the retainer ring 3.

A central bag 8 and a ring tube 9 which serve as abutment members brought into contact with the semiconductor wafer W are mounted in a space defined between the chucking plate 6 and the semiconductor wafer W. In the present embodiment, as shown in FIGS. 4 and 5, the central bag 8 is disposed centrally on the lower surface of the chucking plate 6, and the ring tube 9 is disposed radially outwardly of the central bag 8 in surrounding relation thereto. Each of the seal ring 4, the central bag 8, and the ring tube 9 is made of a highly strong and durable rubber material such as ethylene propylene rubber (EPDM), polyurethane rubber, or silicone rubber.

The space defined between the chucking plate 6 and the semiconductor wafer W is divided into a plurality of spaces by the central bag 8 and the ring tube 9. Accordingly, a pressure chamber 22 is defined between the central bag 8 and the ring tube 9, and a pressure chamber 23 is defined radially outwardly of the ring tube 9.

The central bag 8 has an elastic membrane 81 brought into contact with the upper surface of the semiconductor wafer W, and a central bag holder 82 for detachably holding the elastic membrane 81 in position. The central bag holder 82 has threaded holes 82a defined therein, and the central bag 8 is detachably fastened to the center of the lower surface of the chucking plate 6 by screws 55 threaded into the threaded holes 82a. The central bag 8 has a central pressure chamber 24 defined therein by the elastic membrane 81 and the central bag holder 82.

Similarly, the ring tube 9 has an elastic membrane 91 brought into contact with the upper surface of the semiconductor wafer W, and a ring tube holder 92 for detachably holding the elastic membrane 91 in position. The ring tube holder 92 has threaded holes 92a defined therein, and the ring tube 9 is detachably fastened to the lower surface of the chucking plate 6 by screws 56 threaded into the threaded holes 92a. The ring tube 9 has an intermediate pressure chamber 25 defined therein by the elastic membrane 91 and the ring tube holder 92.

In the present embodiment, the pressure chamber 24 is formed by the elastic membrane 81 of the central bag 8 and the central bag holder 82, and the pressure chamber 25 is formed by the elastic membrane 91 of the ring tube 9 and the ring tube holder 92. The pressure chambers 22 and 23 may also be formed by an elastic membrane and a holder for fixing the elastic membrane, respectively. Further, elastic membranes and holders may appropriately be added to increase the number of the pressure chambers.

Fluid passages 33, 34, 35 and 36 comprising tubes and connectors communicate with the pressure chambers 22 and 23, the central pressure chamber 24, and the intermediate pressure chamber 25, respectively. The pressure chambers 22 to 25 are connected to the compressed air source 120 as a supply source via respective regulators R3, R4, R5 and R6 connected respectively to the fluid passages 33 to 36. The fluid passages 31 to 36 are connected to the respective regulators R1 to R6 through a rotary joint (not shown) mounted on the upper end of the top ring shaft 11.

The pressure chamber 21 above the chucking plate 6 and the pressure chambers 22 to 25 are supplied with pressurized fluids such as pressurized air or atmospheric air or evacuated, via the fluid passages 31, 33, 34, 35 and 36 connected to the respective pressure chambers. As shown in FIG. 2, the regulators R2 to R6 connected to the fluid passages 31, 33, 34, 35 and 36 of the pressure chambers 21 to 25 can respectively regulate the pressures of the pressurized fluids supplied to the respective pressure chambers. Thus, it is possible to independently control the pressures in the pressure chambers 21 to 25 or independently introduce atmospheric air or vacuum into the pressure chambers 21 to 25. In this manner, the pressures in the pressure chambers 21 to 25 are independently varied with the regulators R2 to R6, so that the pressing forces to press the semiconductor wafer W against the polishing pad 101 can be adjusted in local areas of the semiconductor wafer W. In some applications, the pressure chambers 21 to 25 may be connected to a vacuum source 121.

In this case, the pressurized fluid or the atmospheric air supplied to the pressure chambers 22 to 25 may independently be controlled in temperature. With this configuration, it is possible to directly control the temperature of a workpiece such as a semiconductor wafer from the backside of the surface to be polished. Particularly, when each of the pressure chambers is independently controlled in temperature, the rate of chemical reaction can be controlled in the chemical polishing process of CMP.

The chucking plate 6 has radially inner suction portions 61 extended downwardly therefrom between the central bag 8 and the ring tube 9. The chucking plate 6 has radially outer suction portions 62 extended downwardly therefrom outside of the ring tube 9. In the present embodiment, eight suction portions 61, 62 are provided.

The inner suction portions 61 and the outer suction portions 62 have communication holes 61a, 62a communicating with fluid passages 37, 38, respectively. The inner suction portions 61 and the outer suction portions 62 are connected to the vacuum source 121 such as a vacuum pump via the fluid passages 37, 38 and valves V1, V2. When the communication holes 61a, 62a of the suction portions 61, 62 are connected to the vacuum source 121, a negative pressure is developed at the lower opening ends of the communication holes 61a, 62a thereof to attract a semiconductor wafer W to the lower ends of the inner suction portions 61 and the outer suction portions 62. The inner suction portions 61 and the outer suction portions 62 have elastic sheets 61b, 62b, such as thin rubber sheets, attached to their lower ends, for thereby elastically contacting and holding the semiconductor wafer W on the lower surfaces thereof.

Since there is a small gap G between the outer circumferential surface of the seal ring 4 and the inner circumferential surface of the retainer ring 3, the holder ring 5, the chucking plate 6, and the seal ring 4 attached to the chucking plate 6 can vertically be moved with respect to the top ring body 2 and the retainer ring 3, and hence are of a floating structure with respect to the top ring body 2 and the retainer ring 3. The stopper 5b of the holder ring 5 has a plurality of teeth 5c projecting radially outwardly from the outer circumferential edge thereof. Downward movement of members including the holder ring 5 is limited to a predetermined range by engaging the teeth 5c with the upper surface of the radially inwardly projecting portion of the retainer ring 3.

Operation of the top ring 1 thus constructed will be described in detail below.

In the polishing apparatus thus constructed, when a semiconductor wafer W is to be delivered to the polishing apparatus, the top ring 1 as a whole is moved to a position to which the semiconductor wafer W is transferred, and the communication holes 61a and 62a of the suction portions 61 and 62 are connected through the fluid passages 37 and 38 to the vacuum source 121. The semiconductor wafer W is attracted under vacuum to the lower ends of the suction portions 61 and 62 by suction effect of the communication holes 61a and 62a. With the semiconductor wafer W attracted to the top ring 1, the top ring 1 as a whole is moved to a position above the polishing table 100 having the polishing surface (polishing pad 101) thereon. The outer circumferential edge of the semiconductor wafer W is held by the retainer ring 3 so that the semiconductor wafer W is not dislodged from the top ring 1.

For polishing the semiconductor wafer W, the attraction of semiconductor wafer W by the suction portions 61 and 62 is released, and the semiconductor wafer W is held on the lower surface of the top ring 1. Simultaneously, the top ring air cylinder 111 connected to the top ring drive shaft 11 is actuated to press the retainer ring 3 fixed to the lower end of the top ring 1 against the polishing surface on the polishing table 100 under a predetermined pressure. In such a state, pressurized fluids are respectively supplied to the pressure chambers 22, 23, the central pressure chamber 24, and the intermediate pressure chamber 25 under respective pressures, thereby pressing the semiconductor wafer W against the polishing surface on the polishing table 100. The polishing liquid supply nozzle 102 supplies a polishing liquid Q onto the polishing pad 101 in advance, so that the polishing liquid Q is held on the polishing pad 101. Thus, the semiconductor wafer W is polished by the polishing pad 101 with the polishing liquid Q being present between the (lower) surface of the semiconductor wafer W to be polished and the polishing pad 101. At the time of polishing, the rotational speed of the polishing table 100 is maintained to be substantially the same as the rotational speed of the top ring 1.

The local areas of the semiconductor wafer W that are positioned beneath the pressure chambers 22 and 23 are pressed against the polishing surface under the pressures of the pressurized fluids supplied to the pressure chambers 22 and 23. The local area of the semiconductor wafer W that is positioned beneath the central pressure chamber 24 is pressed via the elastic membrane 81 of the central bag 8 against the polishing surface under the pressure of the pressurized fluid supplied to the central pressure chamber 24. The local area of the semiconductor wafer W that is positioned beneath the intermediate pressure chamber 25 is pressed via the elastic membrane 91 of the ring tube 9 against the polishing surface under the pressure of the pressurized fluid supplied to the intermediate pressure chamber 25.

Therefore, the polishing pressures acting on the respective local areas of the semiconductor wafer W can be adjusted independently by controlling the pressures of the pressurized fluids supplied to the respective pressure chambers 22 to 25. Specifically, the respective regulators R3 to R6 independently regulate the pressures of the pressurized fluids supplied to the pressure chambers 22 to 25 to thereby adjust the pressing forces applied to press the local areas of the semiconductor wafer W against the polishing pad 101 on the polishing table 100. With the polishing pressures on the respective local areas of the semiconductor wafer W being adjusted independently to desired values, the semiconductor wafer W is pressed against the polishing pad 101 on the polishing table 100 that is being rotated. Similarly, the pressure of the pressurized fluid supplied to the top ring air cylinder 111 can be regulated by the regulator R1 to adjust the force with which the retainer ring 3 presses the polishing pad 101. While the semiconductor wafer W is being polished, the force with which the retainer ring 3 presses the polishing pad 101 and the pressing force with which the semiconductor wafer W is pressed against the polishing pad 101 can appropriately be adjusted to thereby apply polishing pressures in a desired pressure distribution to a central area (C1 in FIG. 5), an inner area (C2) between the central area and an intermediate area, the intermediate area (C3), a peripheral area (C4) of the semiconductor wafer W, and a peripheral portion of the retainer ring 3 which is positioned outside of the semiconductor wafer W.

In this manner, the semiconductor wafer W is divided into the four concentric circular and annular areas (C1 to C4), which can respectively be pressed under independent pressing forces. A polishing rate depends on a pressing force applied to a semiconductor wafer W against a polishing surface. As described above, since the pressing forces applied to those areas can independently be controlled, the polishing rates of the four circular and annular areas (C1 to C4) of the semiconductor wafer W can independently be controlled. Consequently, even if the thickness of a thin film to be polished on the surface of the semiconductor wafer W suffers radial variations, the thin film on the surface of the semiconductor wafer W can be polished uniformly without being insufficiently or excessively polished over the entire surface of the semiconductor wafer. More specifically, even if the thickness of the thin film to be polished on the surface of the semiconductor wafer W differs depending on the radial position on the semiconductor wafer W, the pressure in a pressure chamber positioned over a thicker area of the thin film is made higher than the pressure in other pressure chambers, or the pressure in a pressure chamber positioned over a thinner area of the thin film is made lower than the pressure in other pressure chambers. In this manner, the pressing force applied to the thicker area of the thin film against the polishing surface is made higher than the pressing force applied to the thinner area of the thin film against the polishing surface, thereby selectively increasing the polishing rate of the thicker area of the thin film. Consequently, the entire surface of the semiconductor wafer W can be polished exactly to a desired level over the entire surface of the semiconductor wafer W irrespective of the film thickness distribution produced at the time when the thin film is formed.

Any unwanted edge rounding on the circumferential edge of the semiconductor wafer W can be prevented by controlling the pressing force applied to the retainer ring 3. If the thin film to be polished on the circumferential edge of the semiconductor wafer W has large thickness variations, then the pressing force applied to the retainer ring 3 is intentionally increased or reduced to thus control the polishing rate of the circumferential edge of the semiconductor wafer W. When the pressurized fluids are supplied to the pressure chambers 22 to 25, the chucking plate 6 is subjected to upward forces. In the present embodiment, the pressurized fluid is supplied to the pressure chamber 21 through the fluid passage 31 to prevent the chucking plate 6 from being lifted under the forces due to the pressure chambers 22 to 25.

As described above, the pressing force applied by the top ring air cylinder 111 to press the retainer ring 3 against the polishing pad 101 and the pressing forces applied by the pressurized air supplied to the pressure chambers 22 to 25 to press the local areas of the semiconductor wafer W against the polishing pad 101 are appropriately adjusted to polish the semiconductor wafer W. When the polishing of the semiconductor wafer W is finished, the semiconductor wafer W is attracted to the lower ends of the inner suction portions 61 and the outer suction portions 62 under vacuum in the same manner as described above. At that time, the supply of the pressurized fluids into the pressure chambers 22 to 25 to press the semiconductor wafer W against the polishing surface is stopped, and the pressure chambers 22 to 25 are vented to the atmosphere. Accordingly, the lower ends of the suction portions 61 and 62 are brought into contact with the semiconductor wafer W. The pressure chamber 21 is vented to the atmosphere or evacuated to develop a negative pressure therein. If the pressure chamber 21 is maintained at a high pressure, then the semiconductor wafer W is strongly pressed against the polishing surface only in areas brought into contact with the suction portions 61 and 62. Therefore, it is necessary to decrease the pressure in the pressure chamber 21 immediately. Accordingly, a relief port 39 penetrating from the pressure chamber 21 through the top ring body 2 may be provided for decreasing the pressure in the pressure chamber 21 immediately, as shown in FIG. 4. In this case, when the pressure chamber 21 is pressurized, it is necessary to continuously supply the pressurized fluid into the pressure chamber 21 via the fluid passage 31. The relief port 39 comprises a check valve for preventing an outside air from flowing into the pressure chamber 21 at the time when a negative pressure is developed in the pressure chamber 21.

After the semiconductor wafer W is thus attracted under suction to the suction portions 61 and 62 as described above, it is necessary to remove (or separate) the semiconductor wafer W from the polishing pad 101. In this case, a large surface tension adversely acts between the polishing pad 101 and the semiconductor wafer W as described above. According to the present invention, in order to reduce surface tension between the polishing pad 101 and the semiconductor wafer W, the rotational speed of the polishing table 100 is reduced from a rotational speed: of the polishing table 100 rotated at the time of polishing. In this case, the rotational speed of the top ring 1 is maintained at the same rotational speed as a rotational speed of the top ring 1 rotated at the time of polishing.

Specifically, the rotational speed of the polishing table 100 is reduced from a rotational speed of the polishing table 100 rotated at the time of polishing to provide a difference between the rotational speeds of the polishing table 100 and the top ring 1. At that time, the semiconductor wafer W held by the top ring 1 floats on a liquid layer of the polishing liquid Q which is formed between the lower surface of the semiconductor wafer W and the upper surface of the polishing pad 101. This phenomenon is referred to as a hydroplaning phenomenon. The hydroplaning phenomenon reduces surface tension between the polishing surface and the semiconductor wafer W due to the polishing liquid. Therefore, even if the top ring 1 is lifted directly at a position where the polishing process is completed, the semiconductor wafer W can readily be removed or separated from the polishing pad 101 without an overhangining action. Thus, the semiconductor wafer W is not left on the polishing pad 101 and is prevented from being cracked or scratched, which would be caused by an overhanging action.

When the top ring 1 is lifted to separate the semiconductor wafer W from the polishing surface, the speed of lifting the top ring 1 is made relatively low until the semiconductor wafer W has substantially been separated from the polishing surface. After the semiconductor wafer W has been separated from the polishing surface, the speed of lifting the top ring 1 is increased. Specifically, the top ring 1 is lifted at a low speed when the semiconductor wafer W is separated from the polishing surface and at a high speed after the semiconductor wafer W has been separated from the polishing surface. Thus, pressure applied to the semiconductor wafer W when the semiconductor wafer W is separated from the polishing surface is made as small as possible to minimize stresses applied to circuit elements formed on the semiconductor wafer W. The lifting speed of the top ring 1 can be controlled by adjusting pressure of compressed air to be supplied to the top ring air cylinder 111. For example, a sensor (not shown) may be provided near a rod of the top ring air cylinder 111 for detecting a position of the semiconductor wafer W when the semiconductor wafer W has substantially been separated from the polishing surface to control the lifting speed of the top ring 1.

According to the present invention, since it is not necessary to perform an overhanging action, polishing tact time can be shortened, and throughput can be improved.

It is desirable that the rotational speed of the polishing table 100 is reduced to provide a difference between the rotational speeds of the top ring 1 and the polishing table 100 during the polishing process and before the semiconductor wafer W is separated from the polishing surface, and that the top ring 1 is lifted as it is when the polishing process has been completed. Specifically, the rotational speed of the polishing table 100 is reduced immediately before the polishing process has been completed to provide a difference between the rotational speeds of the top ring 1 and the polishing table 100. The polishing process is continued in this state. When the polishing process is completed, the top ring 1 is lifted as it is to separate the semiconductor wafer W from the polishing surface. According to this method, preparation for separating the semiconductor wafer W from the polishing surface can be started during the polishing process. Therefore, it is possible to shorten polishing tact time and improve throughput.

It is desirable that the amount of polishing liquid Q supplied from the polishing liquid supply nozzle 102 is increased before or during the separation of the semiconductor wafer W from the polishing surface. In such a case, it is possible to enhance effects of a hydroplaning phenomenon and to easily separate the semiconductor W from the polishing surface. The atomizer 103 should preferably start to eject a mixture of gas (e.g. inert gas such as N2 gas) and pure water or a chemical liquid for cleaning the polishing surface toward the polishing surface immediately before the semiconductor wafer W is separated from the polishing surface. Thus, it is possible to enhance effects of a hydroplaning phenomenon and to start a cleaning process of the polishing surface, which would be performed after the polishing process, during the polishing process.

After the top ring 1 has been lifted as described above, the top ring 1 is moved to a position at which the semiconductor wafer is transferred. Then, a fluid (e.g. compressed air or a mixture of nitrogen and pure water) is ejected through the communication holes 61a and 62b of the suction portions 61 and 62 to the semiconductor wafer W to release the semiconductor wafer W.

The polishing liquid Q used to polish the semiconductor wafer W tends to flow through the small gap G between the outer circumferential surface of the seal ring 4 and the retainer ring 3. If the polishing liquid Q is firmly deposited in the gap G, then the holder ring 5, the chucking plate 6, and the seal ring 4 are prevented from smoothly moving vertically with respect to the top ring body 2 and the retainer ring 3. To avoid such a drawback, a cleaning liquid (pure water) is supplied through the fluid passage 32 to the cleaning liquid passage 51. Accordingly, the pure water is supplied through a plurality of communication holes 53 to a region above the gap G, thus cleaning the gap G to prevent the polishing liquid Q from being firmly deposited in the gap G. The pure water should preferably be supplied after the polished semiconductor wafer W is released and until the next semiconductor wafer to be polished is attracted to the top ring 1. It is desirable to discharge all the supplied pure water out of the top ring 1 before the next semiconductor wafer is polished, and hence to provide the retainer ring 3 with a plurality of through-holes 3a shown in FIG. 4. Furthermore, if a pressure buildup is developed in a space 26 defined between the retainer ring 3, the holder ring 5, and the pressurizing sheet 7, then it acts to prevent the chucking plate 6 from being elevated in the top ring body 2. Therefore, in order to allow the chucking plate 6 to be elevated smoothly in the top ring body 2, the through-holes 3a should preferably be provided for equalizing the pressure in the space 26 with the atmospheric pressure.

In the present embodiment, as shown in FIG. 2, the fluid passages 37 and 38 as a vacuum line communicating with the vacuum evacuation source 121 is connected to the top ring 2. Thus, the semiconductor wafer W is held on the lower surface of the top ring 2 by vacuum suction. When the semiconductor wafer W is to be separated from the polishing surface, the top ring 1 is mechanically lifted. If the semiconductor wafer W is dislodged from the top ring 1 and is left on the polishing pad 101, a pressure of the vacuum line (fluid passages 37 and 38) changes to a vacuum nearer to atmospheric pressure. Therefore, pressure of the vacuum line is monitored to judge whether or not the semiconductor wafer W has normally been separated from the polishing pad 101.

In the present embodiment, as shown in FIG. 2, a vacuum pressure sensor 40 is provided in the vacuum line to measure and monitor pressure of the vacuum line, to thereby judge whether or not the semiconductor wafer W has normally been separated from the polishing pad 101. Specifically, after a lifting operation of the top ring 1 is started, pressure of the vacuum line is measured with the vacuum pressure sensor 40. If the measured value is less than a predetermined pressure value, it is judged that the semiconductor wafer W has normally been separated from the polishing pad 101 in such a state that the semiconductor wafer W is attracted to the top ring 1. More specifically, if pressures that are at least 10 kPa larger than the predetermined pressure value, at which the semiconductor wafer W is judged to be normally attracted to the top ring 1, are measured for a predetermined period or longer, then it is judged that the semiconductor wafer W has been dislodged from the top ring 1.

FIG. 6 is a timing chart showing an example of a polishing process to polish a semiconductor wafer and a separating process to separate the semiconductor wafer from the polishing surface in the present embodiment. With reference to FIG. 6, there will be described processes including rotation of the top ring, stop of the rotation of the top ring, rotation of the polishing table, reduction of the rotational speed of the polishing table, stop of the rotation of the polishing table, supply of the polishing liquid from the polishing liquid supply nozzle, stop of the supply of the polishing liquid, supply of a mixture of nitrogen gas and pure water or chemical liquid from the atomizer, and stop of the supply from the atomizer.

A polishing process is continued until T2. For example, the rotational speed of the top ring 1 is 71 rpm (71 min−1), and the rotational speed of polishing table 100 is 70 rpm (70 min−1). A polishing liquid is supplied from the polishing liquid supply nozzle 102 at a flow rate of 150 ml/min. The atomizer 103 does not supply a mixture of gas and pure water or chemical liquid at that time.

Conditions are changed between T1 and T2 before the top ring 1 is lifted (i.e. the semiconductor wafer is separated from the polishing surface). At that time, the rotational speed of the top ring is 71 rpm (71 min−1), which is the same as the rotational speed of the top ring rotated at the time of the polishing process. The rotational speed of the polishing table 100 begins to be reduced from 70 rpm (70 min−1) at T1 and reaches 25 rpm (25 min−1) in a short period of time after T1. Then, the polishing table 100 maintains a rotational speed of 25 rpm (25 min−1). The flow rate of polishing liquid supplied from the polishing liquid supply nozzle 102 begins to be increased at T1 and reaches 300 ml/min in a short period of time after T1. Then, the polishing liquid supply nozzle 102 maintains a flow rate of 300 ml/min. The atomizer 103 begins to eject a mixture of nitrogen gas and pure water or chemical liquid onto the polishing surface at T1.

The top ring 1 begins to be lifted at T2 to separate the semiconductor wafer W from the polishing surface. The lifting operation of the top ring 1 is completed at T3. At that time, the semiconductor wafer W has been separated from the polishing surface by a predetermined distance.

The top ring 1 begins to stop its rotation at T4, and the polishing table 100 also begins to stop its rotation at T4. The rotations of the top ring 1 and the polishing table 100 are completely stopped at T′. At that time, the supply of the polishing liquid is also completely stopped. The supply of the mixture of nitrogen gas and pure water or chemical liquid from the atomizer 103 is continued until T5 and stopped at T5. The atomizer usually begins to supply the mixture at T′ when the rotations of the top ring and the polishing table are completely stopped and continues to supply the mixture for a while after T5 as indicated by chain double-dashed lines.

As described above, according to the present embodiment, the atomizer 103 begins to eject the mixture before the top ring is lifted. Therefore, the atomizer 103 can complete a cleaning process of the polishing surface earlier as compared to a conventional method. Thus, conditions for the next workpiece can timely be adjusted, and hence throughput is improved.

In the above example, the amount of polishing liquid supplied from the polishing liquid supply nozzle 102 is increased at T1. In this case, a cleaning liquid (pure water) for cleaning the retainer ring 3 may be supplied in synchronism with the increase of the amount of polishing liquid supplied from the polishing liquid supply nozzle 102.

The present invention is applicable not only to a polishing apparatus having a rotatable polishing table as shown in FIGS. 2 and 3, but also to a polishing apparatus having a scroll-type polishing table or a web-type (belt-type) polishing table.

A scroll-type polishing table has a diameter considerably smaller than the diameter of the polishing table 100 shown in FIGS. 2 and 3. Specifically, a scroll-type polishing table has a diameter equal to or larger than the sum of the diameter of a semiconductor wafer and a twofold eccentric distance. Thus, the semiconductor is moved within the polishing table even if the polishing table makes a translational movement (scrolling movement or orbital movement) with a radius of the eccentric distance. The scroll-type polishing table makes a translational orbital movement (scrolling movement) at a predetermined frequency by actuation of a motor. In a polishing apparatus having a scroll-type polishing table, a semiconductor wafer is held and pressed against a polishing surface on the scroll-type polishing table by the same top ring as that shown in FIGS. 2 and 4. A polishing liquid is supplied through holes defined in the polishing table to the polishing surface. The polishing apparatus provides a relative movement between the polishing surface and the semiconductor wafer to make a small translational orbital movement with a radius of an eccentric distance. The semiconductor wafer is uniformly polished over the entire surface thereof. If the polishing apparatus continuously has the same positional relationship between the surface of the semiconductor wafer and the polishing surface, a polished surface of the semiconductor wafer may be influenced by local differences of the polishing surface. Therefore, the top ring is gradually rotated to prevent the surface of the semiconductor wafer from being polished only by the same portion of the polishing surface.

When the scroll-type polishing table is used in a polishing apparatus, a semiconductor wafer is separated from the polishing surface as follows. Before the semiconductor wafer is separated from the polishing surface, the frequency of the orbital movement of the polishing table is reduced. When the polishing process is completed, the top ring is lifted as it is. Thus, the semiconductor wafer held by the top ring floats on a liquid layer of the polishing liquid which is formed between the lower surface of the semiconductor wafer and the polishing surface. Specifically, a hydroplaning phenomenon occurs. Accordingly, surface tension due to the polishing liquid is reduced between the polishing surface and the semiconductor wafer. Therefore, even if the top ring is lifted directly at a position where the polishing process is completed, the semiconductor wafer can readily be removed or separated from the polishing surface without an overhangining action.

FIG. 7 is a perspective view showing a polishing apparatus having a web-type (belt-type) polishing table. As shown in FIG. 7, the web-type polishing table has a belt 215 having abrasive particles attached on a surface of the belt 215, a pair of rotatable drums 216 and 217 spaced from each other, and a supporting table 218 disposed between an upper belt surface 215a and a lower belt surface 215b. The belt 215 is wound on the rotatable drums 216 and 217. When the rotatable drums 216 and 217 are rotated, the belt 215 is linearly moved in a direction indicated by an arrow A. The polishing apparatus has the same top ring 1 as that shown in FIGS. 2 and 4. A semiconductor wafer held by the top ring 1 is pressed against an upper surface of the belt 215 (i.e., polishing surface). A polishing liquid Q is supplied from a polishing liquid supply nozzle 102 onto the upper surface of the belt 215 to polish the semiconductor wafer.

With the polishing apparatus shown in FIG. 7, the semiconductor wafer is separated from the polishing surface as follows. Before the semiconductor wafer is separated from the polishing surface, the speed of movement of the belt 215 is reduced. When the polishing process is completed, the top ring is lifted as it is. Thus, the semiconductor wafer held by the top ring 1 floats on a liquid layer of the polishing liquid which is formed between the lower surface of the semiconductor wafer and the upper surface of the belt 215. Specifically, a hydroplaning phenomenon occurs. Accordingly, surface tension due to the polishing liquid is reduced between the polishing surface and the semiconductor wafer. Therefore, even if the top ring is lifted directly at a position where the polishing process is completed, the semiconductor wafer can readily be removed or separated from the polishing surface without an overhangining action. The amount of polishing liquid supplied from the polishing liquid supply nozzle 102 may be increased before or during the separation of the semiconductor wafer from the polishing surface.

In the embodiment described above, the polishing surface is formed by the polishing pad. However, the polishing surface is not limited to the polishing pad. For example, the polishing surface may be formed by a fixed abrasive. The fixed abrasive is formed into a flat plate comprising abrasive particles fixed by a binder. With the fixed abrasive, a polishing process is performed by abrasive particles that are self-generated from the fixed abrasive. The fixed abrasive comprises abrasive particles, a binder, and pores. For example, cerium dioxide (CeO2) having an average particle diameter of 0.5 μm or less is used as an abrasive particle, and epoxy resin is used as a binder. Such a fixed abrasive forms a harder polishing surface. The fixed abrasive includes a fixed abrasive pad having a two-layer structure formed by a thin layer of a fixed abrasive and an elastic polishing pad attached to a lower surface of the thin layer of the fixed abrasive.

With the polishing apparatus shown in FIGS. 2 through 5, stresses caused to a semiconductor wafer were measured. Strain gauges were attached at a central portion and a peripheral portion of a semiconductor wafer held by the top ring. Rotational speeds of the polishing table and the top ring were varied when the semiconductor wafer was separated from the polishing surface. At that time, stresses caused to the semiconductor wafer were measured for several examples under the following experimental conditions. The measured results are shown in Table 1 below.

1. A semiconductor wafer having a diameter of 300 mm was used.

2. The semiconductor wafer was separated from the polishing surface under the following conditions.

EXAMPLE 1

The rotational speed of the polishing table was maintained at 60 rpm (60 min−1), and the rotational speed of the top ring was maintained at 60 rpm (60 min−1). These conditions are the same as those in a conventional method. The top ring was lifted for 0.5 second.

EXAMPLE 2

The rotational speed of the polishing table was increased from 60 rpm (60 min−1) to 70 rpm (70 min−1), and the rotational speed of the top ring was increased from 60 rpm (60 min−1) to 71 rpm (71 min−1). The top ring was lifted for 0.5 second.

EXAMPLE 3

The rotational speed of the polishing table was increased from 70 rpm (70 min−1) to 120 rpm (120 min−1), and the rotational speed of the top ring was increased to 120 rpm (120 min−1). The top ring was lifted for 0.5 second.

EXAMPLE 4

The rotational speed of the polishing table was reduced from 70 rpm (70 min−1) to 25 rpm (25 min−1), and the rotational speed of the top ring was maintained at 71 rpm (71 min−1). The top ring was lifted for 1.5 seconds.

EXAMPLE 5

The rotational speed of the polishing table was reduced from 70 rpm (70 min−1) to 25 rpm (25 min−1), and the rotational speed of the top ring was maintained at 71 rpm (71 min−1). The top ring was lifted for 0.5 second.

EXAMPLE 6

The rotational speed of the polishing table was reduced from 70 rpm (70 min−1) to 25 rpm (25 min−1), and the rotational speed of the top ring was reduced from 70 rpm (70 min−1) to 50 rpm (50 min−1). The top ring was lifted for 0.5 second.

EXAMPLE 7

The rotational speed of the polishing table was reduced from 70 rpm (70 min−1) to 25 rpm (25 min−1), and the rotational speed of the top ring was reduced from 70 rpm (70 min−1) to 40 rpm (40 min−1). The top ring was lifted for 0.5 second.

EXAMPLE 8

The rotational speed of the polishing table was reduced from 70 rpm (70 min−1) to 50 rpm (50 min−1), and the rotational speed of the top ring was maintained at 71 rpm (71 min−1). The top ring was lifted for 0.5 second.

TABLE 1 Time Major Principal Stress [MPa] Level of Rotational to Peripheral Stress Speed of Rotational Lift Central Portion Portion in Polishing Speed of Top Strain [μ] Mea- Mea- Wafer Example Table Top Ring Ring Central Portion Peripheral Portion sured Average sured Average (1 to No. [min−1] [min−1] [s] εa εb εc εa εb εc Value Value Value Value 11) Note 1 60 60 0.5 −65.8 −18.7 −36.4 23.4 2.0 40.6 −13.0 −12.4 9.1 8.3 5 Conventional −69.1 −39.4 −34.3 20.1 29.6 46.5 −11.7 7.6 Method 2 70 71 0.5 −28.8 −32.1 −36.1 195.2 181.5 153.9 −6.4 −7.9 34.6 38.0 8 −40.7 −54.3 −48.3 292.9 48.9 −47.8 −9.3 41.3 3 120 120 0.5 — — — 264.9 −12.0 −117.9 — — 34.6 40.7 10 High Rotational — — — 289.3 19.1 −51.4 — 41.8 Speed — — — 315.6 24.8 −51.7 — 45.8 4 25 71 1.5 20.0 10.0 4.1 23.0 36.0 51.9 3.1 2.0 8.4 7.6 1 Change in Time 3.4 −1.8 2.4 11.4 28.3 43.9 1.0 6.8 to Lift Top Ring 5 25 71 0.5 — — — 32.0 35.9 53.0 — — 9.1 9.5 2 Ratio of — — — 21.7 26.9 45.7 — 7.6 Rotational Speeds — — — 17.5 37.4 55.0 — 8.6 of Top Ring to — — — 25.0 57.3 81.4 — 12.7 Polishing Table: 2.8 6 25 50 0.5 — — — 36.9 69.2 93.9 — — 15.0 10.8 3 Ratio of — — — 6.4 26.3 44.4 — 6.6 Rotational Speeds of Top Ring to Polishing Table: 2.0 7 25 40 0.5 — — — 22.2 45.9 66.1 — — 10.4 11.7 4 Ratio of — — — 27.2 60.4 82.2 — 13.0 Rotational Speeds of Top Ring to Polishing Table: 1.6 8 50 71 0.5 — — — 284.8 14.0 −91.1 — — 38.5 35.6 7 Ratio of — — — 250.1 −12.4 −110.9 — 32.8 Rotational Speeds of Top Ring to Polishing Table: 1.4

In Table 1, levels of stresses in a semiconductor wafer are represented by 11 steps. The minimum level of stress is represented by 1, and the maximum level of stress is represented by 11.

It can be seen from Table 1 that Example 4 had the lowest level of stress caused to the semiconductor wafer and thus required the smallest forces to lift the top ring. In this case, the semiconductor wafer can easily be separated from the polishing pad. Example 5 had the second lowest level of stress caused to the semiconductor wafer and thus required the second smallest forces to lift the top ring. Thus, when a ratio of rotational speeds of the top ring to the polishing table is large, the semiconductor wafer can easily be separated from the polishing pad.

From the above experiments, it is desirable that the rotational speed of the polishing table is reduced to 50 rpm (50 min−1) or less when the semiconductor wafer is separated from the polishing surface. If the polishing table is rotated at a rotational speed higher than 50 rpm (50 min−1), pure water or polishing liquid on the polishing table scatters under centrifugal forces, so that effects of a hydroplaning phenomenon are weakened.

As described above, according to the present invention, even if a top ring is lifted directly at a position where a polishing process is completed, a workpiece can readily be removed or separated from a polishing surface without an overhangining action. Thus, the workpiece is not left on the polishing surface and prevented from being cracked or scratched, which would be caused by an overhanging action.

Further, since it is not necessary to perform an overhanging action, polishing tact time can be shortened, and throughput can be improved.

When the rotational speed of the polishing table is reduced, a cleaning process of the polishing surface may be performed. In this case, separation of the workpiece from the polishing surface is facilitated, and conditions for the next workpiece can timely be adjusted. As a result, throughput is improved.

In a case of a scroll-type polishing table, when the workpiece is separated from the polishing surface, the frequency of movement of the scroll-type polishing table is reduced. In a case of a web-type (belt-type) polishing table, the speed of movement of a belt is reduced. In either case, it is possible to easily separate a workpiece from the polishing surface.

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.

INDUSTRIAL APPLICABILITY

The present invention is suitable for use in a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.

Claims

1. A polishing method comprising:

rotating a workpiece;
rotating a polishing table having a polishing surface thereon;
pressing the workpiece against the polishing surface on the polishing table rotated at a first rotational speed to polish the workpiece;
separating the workpiece from the polishing surface after said pressing; and
before said separating, reducing a rotational speed of the polishing table to a second rotational speed lower than the first rotational speed to provide a difference between a rotational speed of the workpiece and the rotational speed of the polishing table.

2. The polishing method as recited in claim 1, wherein said reducing is performed during said pressing.

3. The polishing method as recited in claim 1, further comprising:

supplying a polishing liquid to the polishing surface at a first flow rate during said pressing; and
before or during said separating, increasing a flow rate of the polishing liquid to a second flow rate higher than the first flow rate.

4. The polishing method as recited in claim 1, further comprising ejecting a mixture of gas and pure water or chemical liquid to the polishing surface to clean the polishing surface before said separating.

5. The polishing method as recited in claim 1, wherein said separating comprises lifting the workpiece from the polishing surface.

6. The polishing method as recited in claim 5, wherein said lifting comprises reducing a lifting speed of the workpiece until the workpiece has substantially been separated from the polishing surface.

7. A polishing method comprising:

moving a workpiece and a polishing surface relative to each other;
pressing the workpiece against the polishing surface moved at a first frequency to polish the workpiece;
separating the workpiece from the polishing surface after said pressing; and
before said separating, reducing a frequency of movement of the polishing surface to a second frequency lower than the first frequency.

8. The polishing method as recited in claim 7, wherein said moving comprising moving the workpiece and the polishing surface with an orbital movement.

9. A polishing method comprising:

moving a polishing surface relative to a workpiece;
pressing the workpiece against the polishing surface moved at a first speed to polish the workpiece;
separating the workpiece from the polishing surface after said pressing; and
before said separating, reducing a speed of movement of the polishing surface to a second speed smaller than the first speed.

10. The polishing method as recited in claim 9, wherein said moving comprising linearly moving the polishing surface.

11. A polishing method comprising:

rotating a workpiece;
rotating a polishing table having a polishing surface thereon;
pressing the workpiece against the polishing surface on the polishing table;
separating the workpiece from the polishing surface after said pressing; and
before said separating, reducing a ratio of a rotational speed of the top ring to a rotational speed of the poslishing table.

12. The polishing method as recited in claim 11, wherein said reducing is performed during said pressing.

13. The polishing method as recited in claim 11, further comprising:

supplying a polishing liquid to the polishing surface at a first flow rate during said pressing; and
before or during said separating, increasing a flow rate of the polishing liquid to a second flow rate higher than the first flow rate.

14. The polishing method as recited in claim 11, further comprising ejecting a mixture of gas and pure water or chemical liquid to the polishing surface to clean the polishing surface before said separating.

15. The polishing method as recited in claim 11, wherein said separating comprises lifting the workpiece from the polishing surface.

16. The polishing method as recited in claim 15, wherein said lifting comprises reducing a lifting speed of the workpiece until the workpiece has substantially been separated from the polishing surface.

Patent History
Publication number: 20050054272
Type: Application
Filed: Dec 3, 2003
Publication Date: Mar 10, 2005
Inventors: Nobuyuki Takahashi (Tokyo), Hiroomi Torii (Tokyo), Mikihiko Masaki (Tokyo), Hiroshi Biwata (Tokyo)
Application Number: 10/495,562
Classifications
Current U.S. Class: 451/41.000