WO1998012020A1

WO1998012020A1 - Methods and apparatus for uniform polishing of a workpiece

Info

Publication number: WO1998012020A1
Application number: PCT/US1997/015560
Authority: WO
Inventors: Inki Kim
Original assignee: Speedfam Corporation
Priority date: 1996-09-19
Filing date: 1997-09-04
Publication date: 1998-03-26
Also published as: TW356436B

Abstract

An apparatus for evenly polishing or planarizing the surfaces of workpieces comprising a polishing pad with a plurality of grooves along the surface of the pad for uniformly guiding a slurry radially outward across the surface of the pad. Preferably, the grooves are arc-shaped and extend radially outward from the center of the polishing pad to the outer edge of the pad, the angle of the arc being in the same direction as the angular rotation of the polishing pad; the grooves along the pad are cut in a U-shape or V-shape; and the grooves are equally spaced apart from each other along the surface of the pad.

Description

METHODS AND APPARATUS FOR UNIFORM POLISHING OF A WORKPIECE

Technical Field

The present invention relates, generally, to apparatus for polishing or planarizing wor pieces such as semiconductor wafers, and more particularly, to methods and apparatus for uniformly distributing a slurry across the surface of the polishing pad.

Background Art and Technical Problems;

The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high- precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created at the wafer surface. As the size of the integrated circuits continues to decrease and the number of microstructures per integrated circuits increases, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.

In an exemplary prior art polishing method, one side of the wafer is attached to a flat surface of a wafer carrier or chuck and the other side of the wafer is pressed against a flat polishing surface. In general, the polishing surface comprises a polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives. Polishing pads can be formed of various commercially available materials such as a blown polyurethane.

During the polishing or planarization process, the workpiece (e.g., silicon wafer) is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated over the inner and outer radial surface of the polishing pad. Additionally, in a chemical mechanical planarization ("CMP") process, a slurry such as a water-based slurry comprising colloidal silica particles is deposited between the polishing pad and workpiece. For a more detailed description of CMP processes, see, for example, Aral, et al, U.S. Patent No. 4,805,348, issued February, 1989; Arai, et ai, U.S. Patent No. 5,099_:614, issued March, 1992; Karlsrud et al, U.S. Patent No. 5,329,732, issued July, 1994; Karlsrud, U.S. Patent No. 5,498,196, issued March, 1996; and Karlsrud et al., U.S. Patent No. 5,498,199, issued March, 1996. By this reference, the entire disclosures of the foregoing patents are hereby incorporated herein. When pressure is applied between the polishing pad and the workpiece being polished, mechanical stresses and the abrasive particles within the slurry creates mechanical strain on the chemical bonds comprising the surface being polished, rendering the chemical bonds more susceptible to chemical attack or corrosion (e.g., stress corrosion). Consequently, microscopic regions are removed from the surface being polished, enhancing planarity of the polished surface.

Presently known polishing techniques are unsatisfactory in several regards. For example, when the slurry is deposited at the polishing pad, often the slurry does not cover the entire surface of the polishing pad, leaving dry spots. Also, the slurry may no: be evenly distributed across the entire surface of the polishing pad. Consequently, the polishing effect of the pad can be non-uniform across the surface of the workpiece, resulting in a non-uniform workpiece surface.

Apparatus and methods are thus needed which will uniformly and even y distribute the slurry across the surface of the polishing pad and permit a higher degree of planarization and uniformity over the entire surface of the workpiece.

Summary of the Invention

The present invention provides methods and apparatus for controlling the distribution of a slurry over the surface of a polishing pad to thereby overcome many of the shortcomings of the prior art, such as uneven and non-uniform workpiece surfaces. In accordance with one aspect of the present invention, a polishing pad comprises a plurality of grooves running along the surface of the pad for uniformly guiding a slurry radially outward across the entire surface of the pad.

In accordance with a further aspect of the invention, the grooves extend radially outward from the center of the polishing pad to the pad's periphery; in a preferred embodiment the grooves are equally spaced apart from each other along the surface of the pad.

In accordance with yet a further aspect of the invention, the polishing pad rotates around its vertical axis during the polishing of a workpiece and the direction of the grooves at the surface of the pad correspond to the same angular direction as the pad rotation. In accordance with still a further aspect of the invention, the grooves are cut into the surface of the pad in a U-shape or V-shape with the grooves extending along the surface of the pad in an arc-shape configuration.

In accordance with still a further aspect of the invention, a wafer polishing apparatus comprises a polishing pad of the present invention, a lap wheel for rotating the polishing pad about its vertical axis, a slurry nozzle for depositing slurry at the surface of the polishing pad, and an individual carrier element that spins a workpiece about its respective vertical axis and moves the workpiece radially inward and outward across the polishing pad.

In accordance with a further aspect of the present invention, the distribution of slurry across the surface of a polishing and/or lapping pad may be optimally configured as a function of one or more of: the number, size, and spacing of slurry grooves on the pad; the texture of the pad, the cross-sectional contour of the grooves; the groove pattern; and the rotational speed of the pad

In accordance with another embodiment of the invention, the angular direction of the grooves along the surface of the pad are in a direction opposite to the angular direction of the rotating pad.

Brief Description of the Drawing Figures The present invention will hereinafter be described in conjunction with the appended drawings figures, wherein like numerals designate like elements, and

Figure 1 is a perspective view of a semiconductor wafer polishing and anarization machine in accordance with the present invention, Figure 2 is a top, cross-sectional view of the machine of Figure 1 showing a wafer carrying apparatus positioned over a polishing pad;

Figure 3 is another top view of the machine of Figure 1 showing the wafer carrying apparatus positioned over a wafer table;

Figure 4 is a side view of a polishing pad in accordance with the present invention, and

Figure 5 is a plan view of the polishing pad of Figure 4

Detailed Description of Preferred Exemplary Embodiment

The subject invention relates to an improved polishing pad for use h processing workpiece surfaces and an apparatus including same Although the workpiece n ay comprise virtually an device requiring a controlled finish, the present invention is conveniently described with reference to semiconductor wafers which require controlled and uniform sun ace finishes. It will be understood by those skilled in the art, however, that the invention is not limited to any particular type of workpiece, polishing pad or any particular type of workpiece surf tee finish

Referring now to Figures 1-3, a wafer polishing apparatus 100 is shown in accordance with the present invention Wafer polishing apparatus 100 suitably comprises a load station 102, a wafer transition station 104, a polishing station 106, and a wafer rinse and unloac station 108 In accordance with a preferred embodiment of the present invention, cassettes 1 10, each holding a plurality of wafers, are loaded into machine 100 at load station 102 Next, a robotic wafer carrier arm 1 12 removes the wafers from cassettes 1 10 and places them, one at a time, on a first wafer transfer arm 1 14 Wafer transfer arm 1 14 then lifts and moves the wafer into wafer transition section 104 That is, transfer arm 1 14 suitably places an individual wafer on one of a plurality of wafer pick-up stations or load cups 1 16 which reside on a rotatable index table 120 within wafer transition section 104 Rotatable table 120 also suitably includes a plurality of wafer drop-off stations or unload cups 1 18 which alternate with pick-up stations 1 16 After a wafer is deposited on one of the plurality of pick-up stations 1 16, table 120 will rotate so that a new station 1 16 aligns with transfer arm 1 14 Transfer arm 1 14 then places the next wafer on the new empty pick-up station 1 16 This process continues until all pick-up stations 1 16 are filled with wafers In a preferred embodiment of the invention, table 120 will include five pick-up stations 1 16 and five drop-off stations 1 18

With continued reference to Figures 1-3, a wafer carrier apparatus 122, comprising individual wafer earner elements 124, suitably aligns itself over table 120 so that respective carrier elements 124 are positioned directly above the wafers which reside in respective pick-up stations 1 16 The carrier apparatus 122 then drops down and picks up the wafers from their respective stations and moves the wafers laterally such that the wafers are positioned above polishing station 106 Once above polishing station 106, carrier apparatus 122 suitably lowers the wafers, which are held by individual elements 124, into operative engagement with a polishing pad 126 which sits atop a polishing table or lap wheel 128 During operation, lap wheel 128 causes polishing pad 126 to rotate about its vertical axes At the same time, individual carrier elements 124 spin the wafers about their respective vertical axis and oscillate the wafers radially inward and outward across polishing pad 126, the wafers being pressed against pad 126 for polishing and planaπzing the surface of the wafer With continued reference to Figures 1-3, the wafers are removed from polishing pad

126 after an appropriate period of time, and carrier apparatus 122 transports the wafers back to transition station 104 Carrier apparatus 122 then lowers individual carrier elements 124 and deposits the wafers onto drop-off stations 1 18 The wafers are then removed from drop-off stations 1 18 by a second transfer arm 130 Transfer arm 130 suitably lifts each wafer out of transition station 104 and transfers them into wafer rinse and unload station 108 In unload station 108, transfer arm 130 holds the wafers while they are rinsed by a scruber 131 After a thorough rinsing, the wafers are reloaded into cassettes 132, which then may be transported to subsequent stations for further processing or packaging Referring now to Figures 4 and 5, polishing pad 126 will be described in more detail in accordance with the present invention Figures 4 and 5 depict a side view and a plan view of pad 126, respectively Pad 126 preferably comprises a substantially flat surface characterized by relatively few surface irregularities Pad 126 may be comprised of a variety of materials such as polyurethane, felt, fabric, and the like. In a preferred embodiment, polishing pad 126 has a diameter D2 of 25 to 40 inches (most preferably, 32 inches or 812 8 mm) and a thickness Tl of 0 04 to 0 15 inches (most preferably, 0 050 inches or 1.27 mm)

With continued reference to Figures 4 and 5, pad 126 further comprises a cone 402 having a diameter Dl of, for example, 2 to 10 inches (most preferably, 6 inches) A slurry nozzle 404 is positioned over cone 402 so that when slurry is expelled from nozzle 404, it is dispersed onto pad 126.

With continued reference to Figures 4 and 5, pad 126 comprises a plurality of grooves 502 along the surface of pad 126 Grooves 502 are appropriately configured to uniformly and evenly distribute the slurry across pad 126 by helping guide or impel the slurry radially outward across the surface of pad 126 from cone 402 to an outer periphery 406 of the polishing pad Grooves 502, coupled with the angular rotation of pad 126 about its vertical axis, facilitate a more uniform and even distribution of the slurry across the entire surface of pad 126 than prior art techniques Consequently, under the present invention, the surface of the workpieces exhibit a higher degree of surface planarity and uniformity than achievable with prior art pads With continued reference to Figures 4 and 5, and in accordance with a particularly preferred embodiment, there are approximately 150 to 280 grooves (preferably about 230 grooves), suitably equally spaced apart on pad 126, each groove 502 having a depth of 0 45 to 0 75 mm (preferably, 0 63 mm), a width of 0 15 to 0 50 mm (preferably, 0 25 mm), and cut in such a manner as to facilitate slurry flow, for example in a U or V-shape In a preferred embadiment, the configuration of each groove comprises an arc taken from a point in the range of 3.5 to 7 5 inches (preferably 3 5 inches) from a center 506 of pad 126, the arc having a 0 to 15 inch (preferably, 10 inch or 254 mm) radius Grooves 502 preferably extend from outer periphery 406 of polishing pad 126 to at least as far as the outer perimeter of cone 402, and most preferably, from outer periphery

406 to a point one inch (25.4 mm) from center point 506 of pad 126.

With continued reference to Figures 4 and 5, in a preferred embodiment, pad 126 rotates in a direction corresponding to the direction of grooves 502; for example, pad 126 rotates in a counter-clockwise direction shown by arrow 504 with grooves 502 oriented in a similar direction. However, in another embodiment of the present invention (not shown), grooves 502 may be oriented in other directions, including a direction generally opposite to that of the angular rotation of pad 126.

With specific reference to Figure 4, the slurry distributed by nozzle 404 may comprise a chemically and mechanically active solution; for example, abrasive particles coupled with chemically reactive agents. Suitable reactive agents include hydroxides, but may also include highly basic or highly acidic ions. Suitable agents (e.g., hydroxides) are advantageously coupled to abrasive particles within the slurry solution. In the context of a particularly preferred embodiment, suitable abrasive particles within the slurry may be on the order of 10-200 nanometers in size in the source (dry) state, and most preferably about 30-80 nanometers. Suitable slurries in the context of the present invention may also include oxidizing agents (e.g. potassium iodide, or ferric nitride), for example in a concentration on the order of 5-20% by weight particle density, and most preferably about 1 1% by weight particle density.

Thus, in accordance with the present invention, a method and apparatus for uniformly and evenly distributing a slurry about a polishing pad and providing a higher degree of planarization of a workpiece surface is provided.

Although the present invention is set forth herein in the context of the appended drawing figures, it should be appreciated that the invention is not limited to the specific forms shown. Various other modifications, variations, and enhancements in the design and arrangement of the polishing apparatus as set forth herein may be made without departing from the spirit and scope of the present invention as set forth in the appended claims. For example, while the exemplary invention embodied a device for polishing semiconductor wafers, it should be understood that the invention is not limited to any particular type of workpiece and may be used for such things as polishing of device wafers, hard disks, and glass polishing, at both low and high material removal rates. Also, while the grooves in the polishing pad are shown in thi^* figures as arc-shaped, those skilled in the art will appreciate that other groove configurations are also possible for facilitating the even distribution of slurry across the polishing pad.

Claims

What is claimed is:

I . A polishing pad for use with a slurry in processing a workpiece surface, comprising: a plurality of grooves extending radially outward along the surface of the polishing pad for uniformly guiding the slurry across the surface of the polishing pad. 2. The polishing pad of claim 1 wherein the surface of the polishing pad is substantially planar.

3. The polishing pad of claim 1 wherein said plurality of grooves are equally spaced apart from each other.

4. The polishing pad of claim 1 wherein each of said plurality of grooves comprises a cut into the surface of the pad having a depth and a width, said depth being approximately 0.45 to 0.75 mm and said width being approximately 0.15 to 0.50 mm.

5. The polishing pad of claim 4 wherein said cut comprises a U-shape or V-shape cut in the surface of the pad.

6. The polishing pad of claim 1 wherein each of said plurality of grooves radially extends along the surface of the polishing pad in an arc-shaped configuration.

7. The polishing pad of claim 6 wherein said arc-shaped configuration comprises an arc taken from a point approximately 3.5 to 7.5 inches from a center of the polishing pad, said arc having an approximately 0 to 15 inch radius.

8. The polishing pad of claim 6 wherein the polishing pad rotates around a vertical axis in the same angular direction as the angle of said arc.

9. The polishing pad of claim 6 wherein the polishing pad rotates around a vertical axis in the opposite direction of the angle of said arc.

10. The polishing pad of claim 1 wherein the polishing pad has a diameter and a thickness, said diameter being approximately 25 to 40 inches and said thickness being approximately 0.04 to 0.15 inches.

I I . The polishing pad of claim 1 wherein the polishing pad comprises polyurethane, felt, or fabric. 12 The polishing pad of claim 1 further comprising a cone-shaped center portion for dispersing the slurry onto the polishing pad, said center portion having a diameter of 2 to 10 inches

13 The polishing pad of claim 1 wherein said plurality of grooves consists of approximately 150 to 280 grooves.

14 A method for planarizing a surface of a workpiece comprising the steps of providing a polishing pad having a plurality of grooves at said surface of said pad, said grooves extending radially outward from the center of said pad toward the periphery of said pad for evenly distributing a slurry across the surface of the Dad, rotating said polishing pad about its vertical axis, depositing said slurry on said polishing pad, and pressing the surface of the workpiece against said surface of sai i pad in the presence of said slurry

15 The method of claim 14 further compnsmg the step of rotating said workpiece about its vertical axis while pressing the surface of the workpiece against the pad

16 The method of claim 14 wherein said grooves gradually begin to curve radially inward as said grooves near the periphery of the pad

17 The method of claim 16 wherein said pad is rotated about its vertica axis in the same angular direction as the curvature of the grooves 18 The method of claim 16 wherein said pad is rotated about its vertica axis in the opposite angular direction as the curvature of the grooves

19 The method of claim 14 wherein the surface of said pad is substantially planar

20 The method of claim 14 wherein said grooves are spaced equidistan from each other 21 The method of claim 14 wherein each of said grooves comprises a cut in the surface of the pad having a depth and a width, said depth being approximately 0 45 to 0 75 nm and said width being approximately 0 15 to 0 50 mm

22. The method of claim 14 wherein said cut comprises a U-shape or V-shape cut in the surface of the pad.

23. The method of claim 14 wherein said pad has a diameter and a thickness, said diameter being approximately 25 to 40 inches and said thickness being approximately 0.040 to 0.15 inches.

24. The method of claim 14 wherein said grooves consist of approximately 150 to 280 grooves.

25. An apparatus for polishing a surface of a workpiece, comprising: a polishing pad having curved grooves extending radially outward across the surface of said polishing pad for evenly distributing a slurry across the surface of said polishing pad; a lap wheel for rotating said polishing pad about its vertical axis; a slurry nozzle for depositing said slurry at the surface of said polishing while said polishing pad is being rotated; and an individual carrier element for spinning a workpiece about its vertical axis and moving the workpiece radially inward and outward across said polishing pad.

26. The apparatus of claim 25 wherein the surface of said polishing pad is substantially planar.

27. The apparatus of claim 25 wherein said grooves are equally spaced apart from each other.

28. The apparatus of claim 25 wherein each of said grooves comprises a cut into the surface of said pad, said cut having a depth and a width and wherein said depth is approximately 0.45 to 0.75 mm and said width is approximately 0.15 to 0.50 mm.

29. The apparatus of claim 25 wherein said cut comprises a U-shape or V-shape cut in the surface of said pad.

30. The apparatus of claim 25 wherein said polishing pad rotates around its vertical axis in the same direction as the curvature of said curved grooves. 31 The apparatus of claim 25 wherein said polishing pad rotates around its vertical axis in the same direction as the curvature of said curved grooves

32 The apparatus of claim 25 wherein said polishing pad has a diameter and a thickness, said diameter being approximately 25 to 40 inches and said thickness being approximately 0 04 to 0 15 inches

33 The apparatus of claim 25 wherein said polishing pad comprises a mate lal from the group consisting of polyurethane, felt, and fabric

34 The apparatus of claim 25 wherein said polishing pad further comprises a cone- shaped center portion for dispersing said slurry onto said polishing pad, said center portion having a diameter of 2 to 10 inches

35 The apparatus of claim 25 wherein said grooves consists of approximately 150 to 280 grooves