US 20020151260 A1
A chemical mechanical polishing (CMP) device carrier head (54) utilizing a pressure pack (58) for transferring a polishing force (D) to a wafer (15) being polished. The pressure pack may include a silicon gel material (60) encapsulated within a sealed urethane casing (62). The pressure pack provides a desired fluid coupling in the polishing force load path, allowing the carrier head to function as a front side floating reference polishing apparatus. The pressure pack may include a plurality of casings (70, 72) defining a plurality of chambers (66, 68) in order to affect the distribution of pressure across an abutting pressure plate (56, 65).
1. A carrier head for a chemical mechanical polishing apparatus comprising:
a frame adapted for connection to a drive apparatus;
a pressure plate adapted for contact with a wafer; and
a pressure pack disposed between the frame and the pressure plate for transferring a force there between.
2. The carrier head of
3. The carrier head of
4. The carrier head of
5. The carrier head of
6. The carrier head of
7. The carrier head of
8. The carrier head of
9. The carrier head of
10. The carrier head of
11. The carrier head of
12. The carrier head of
13. A wafer polishing apparatus comprising:
a platen having a polishing pad affixed to a surface thereof, a wafer carrier head for holding a wafer to be polished and for abutting a surface of the wafer against the polishing pad;
a driving mechanism for polishing the surface of the wafer by moving the wafer carrier head with respect to the platen;
wherein the wafer carrier head further comprises:
a pressure plate adapted for urging the surface of the wafer against the polishing pad;
a frame for applying a force against the pressure plate; and
a pressure pack disposed between the frame and the pressure plate for transferring the force there between.
14. The wafer polishing apparatus of
15. The wafer polishing apparatus of
16. The wafer polishing apparatus of
17. The wafer polishing apparatus of
18. The wafer polishing apparatus of
19. The wafer polishing apparatus of
20. The wafer polishing apparatus of
21. The wafer polishing apparatus of
22. The wafer polishing apparatus of
23. The wafer polishing apparatus of
24. A method of modifying a back side reference polishing apparatus to function as a floating reference polishing device, the back side polishing apparatus having a polishing pad affixed to a platen, a wafer carrier for holding a wafer to be polished against the polishing pad, a driving mechanism for polishing the wafer by moving the wafer carrier with respect to the platen, the wafer carrier further including a carrier head frame disposed against a pressure plate for urging the wafer against the polishing pad, the method comprising:
removing the pressure plate; and
replacing the pressure plate with a replacement pressure plate and a pressure pack, the pressure pack being disposed between the replacement pressure plate and the carrier head frame for transferring a force there between for urging the wafer against the polishing pad.
25. The method of
26. The method of
27. The method of
28. A chemical mechanical polishing apparatus adapted for applying a polishing force to urge a substrate against a polishing pad, wherein the improvement comprises a pressure pack disposed in a load path of the polishing force.
29. The chemical mechanical polishing apparatus of
30. The chemical mechanical polishing apparatus of
31. The chemical mechanical polishing apparatus of
 This invention relates generally to the field of substrate polishing, and more specifically the field of polishing semiconductor wafers, and in particular to an improved carrier for a chemical mechanical polishing (CMP) apparatus.
 It is well known in the field of integrated circuit fabrication to polish a semiconductor wafer to achieve a planar surface on one or both sides of the wafer. One method used to polish such wafers is mechanical polishing, where a surface of the wafer is scoured with an abrasive polishing pad. It is also known to utilize a ID chemical slurry in conjunction with the abrasive polishing pad to increase the material removal rate, and such a process is commonly referred to as chemical mechanical polishing (CMP). Many styles of CMP machines are commercially available from manufactures including Applied Materials, Inc., Ebara, Strausbaugh, LAM Research, and SpeedFam-IPEC International.
FIG. 1 illustrates a prior art chemical mechanical polishing machine 10 including a polishing pad 12 disposed on a platen 14 being used to polish a semiconductor wafer 15 secured in a wafer carrier 16. The carrier 16 includes a driving mechanism 18 for rotating the wafer 15 about a central axis and for oscillating the wafer 15 in a horizontal direction and lifting it in a vertical direction. The CMP machine 10 also includes a driving mechanism 20 for rotating the platen about a central vertical axis. Various CMP machine designs may include one or both such driving mechanisms 18, 20 for moving the wafer carrier 16 and wafer 15 with respect to the platen 14 and polishing pad 12. A slurry delivery system 22 provides the chemical agent 24 used in the CMP process. As is known in the art, CMP machine 10 may include a plurality of carriers 16 and an automatic control system for moving a plurality of semiconductor wafers or other such substrates into and away from the carriers for batch processing.
FIG. 2 is a partial cross-sectional view of one type of wafer carrier head that may be used on a CMP machine. Wafer carrier head 26 includes a carrier head frame 28 adapted for connection to a drive apparatus 30. The drive apparatus 30 is operable to impart rotation and horizontal movement while applying downward force D during a wafer polishing operation. A wafer 15 is secured to the carrier head 26 by a retaining ring 32, as is known in the art. A pressure plate 34 is disposed between the frame 28 and the wafer 15 and is functional to transfer the downward force D to the wafer 15 to urge the surface to be polished against the polishing pad 12. It is known that pressure plate 34 may be formed of a metal or metal alloy and may be covered by a felt or polyurethane backing film (not illustrated) to protect the wafer 15.
 The prior art device of FIG. 2 firmly positions the wafer 15 against the pressure plate 34 during a polishing operation. This type of design is sometimes referred to as back-side reference polishing. The wafer surface that is being polished is urged toward the polishing pad 12 in direct response to the movement of the bottom side face 36 of the pressure plate 34. Such a system is known to produce uneven material removal across the polished face of wafer 15, since there is no mechanical flexibility provided to accommodate problems such as a lack of parallelism between the pressure plate bottom side face 36 and the top surface 38 of the polishing pad 12.
 In order to address the inflexibility of the carrier head 26 of FIG. 2, it is known to provide a floating or front-side reference carrier head 40, as is illustrated in FIG. 3. In this type of design, downward force D is transmitted to wafer 15 by a flexible membrane 42 that is forced against the wafer 15 by a pressurized fluid such as pressurized air 44 contained within a volume 46 within wafer carrier head frame 28. A felt or urethane film 43 may be disposed between the wafer 15 and the flexible membrane 42. The pressurized air is provided and vented via a delivery system 48 designed to be compatible with the driving mechanism 30. A plate 50 having one or more holes 52 formed there through is provided behind the flexible membrane 42. However, when pressurized by pressurized air 44, flexible membrane 42 “floats” away from the plate 50. This allows an even pressure to be applied across the wafer in spite of minor mechanical misalignments and results in a more consistent material removal rate across the polished face of the wafer 15.
 Although providing superior polishing performance, a front side reference CMP machine may be more costly to manufacture due to the necessity for incorporating a pressurized air delivery system 48 including associated electrical and pneumatic controls. Such systems may also be more expensive to operate, since even a small hole in the flexible membrane 42 will cause unacceptable polishing performance and will necessitate a maintenance outage to replace the membrane 42. Furthermore, while it would be desirable to retrofit older back side reference 15 machines with floating head capability, it is generally not practical to do so because of the difficulty of routing an air supply system 48 through an existing driving mechanism 30.
 Thus there is a particular need for a chemical mechanical polishing machine that offers the performance advantages of a front side reference machine without the expense and maintenance disadvantages of prior art machines. It is also desired to be able to upgrade an existing back side reference CMP machine to provide performance equivalent to a floating reference CMP machine.
 Accordingly, an improved carrier head for a chemical mechanical polishing apparatus is described herein as including: a frame adapted for connection to a drive apparatus; a pressure plate adapted for contact with a wafer; and a pressure pack disposed between the frame and the pressure plate for transferring a force there between. The pressure pack provides a desired degree of fluid coupling between the frame and the wafer to ensure that an even pressure is applied across the face of the wafer being polished. The pressure pack may be formed as a gelatinous material encapsulated within a sealed casing. The pressure pack may further be formed of a plurality of chambers to provide a varied force profile across the wafer surface, such as to provide a different down force at the edge of the wafer than at the center of the wafer in response to a single down force applied by the carrier driving mechanism. In order to reduce the risk of damage resulting from a leaking pressure pack, the pack may be formed to have a double casing surrounding an interior fluid.
 An improved wafer polishing apparatus is also described herein as incorporating such an improved carrier head.
 A method of modifying a back side reference polishing apparatus to function as a floating reference polishing device is also described herein. Such a back side polishing apparatus may have a polishing pad affixed to a platen, a wafer carrier for holding a wafer to be polished against the polishing pad, and a driving mechanism for polishing the wafer by moving the wafer carrier with respect to the platen, with the wafer carrier further including a carrier head frame disposed against a pressure plate for urging the wafer against the polishing pad. The method of modifying such a device is described as including the steps of removing the pressure plate; and replacing the pressure plate with a replacement pressure plate and a pressure pack, the pressure pack being disposed between the replacement pressure plate and the carrier head frame for transferring a force there between for urging the wafer against the polishing pad. The pressure pack provides a desired degree of fluid coupling to ensure that an even pressure is applied across the face of the wafer being polished without the necessity for adding a pressurized air supply system to the back side reference device.
 The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which:
FIG. 1 illustrates a prior art polishing device.
FIG. 2 is a partial cross-sectional view of a prior art back side reference wafer carrier head.
FIG. 3 is a partial cross-sectional view of a prior art front side reference wafer carrier head.
FIG. 4 is a partial cross-sectional view of an improved wafer carrier head incorporating a pressure pack for transmitting force to a wafer being polished against a polishing pad.
FIG. 5 is a cross-sectional view of a multi-chamber pressure pack.
FIG. 4 illustrates an improved wafer carrier head 54 that may be used in a chemical mechanical polishing machine such as the machine 10 of FIG. 1. The carrier head 54 includes a frame 28 adapted to be moved relative to a platen 14 and polishing pad 12 by a driving mechanism 30. The carrier head 54 includes a retaining ring 32 and is adapted to position a semiconductor wafer 15 for polishing against polishing pad 12. A force D is applied through frame 28 to urge the wafer 15 against the polishing pad 12 during the polishing operation. A pressure plate 56 is disposed between the frame 28 and the wafer 15 for transmitting polishing force D there between. As is known in the art, pressure plate 56 may be fabricated from a hard substance, such as metal or ceramic, but may include a covering of a softer substance, such as a felt pad or polyurethane in order to protect wafer 15 from scratching. In one embodiment, the hard material may be omitted and only a softer substance may be used to transfer the force to the wafer. The downward force D is transmitted to pressure plate 56 from the carrier frame 28 through a pressure pack 58. Pressure pack 58 provides a desired degree of fluid coupling between the frame 28 and the wafer 15 to ensure that an even force is applied across pressure plate 56 in spite of any slight misalignment between the Gus abutting surfaces of the frame 28, pressure plate 56 and wafer 15. Adequate space is made available between frame 28 and pressure plate 56 to permit the pressure plate 28 to more about freely to accommodate the interactions between wafer 15 and pressure pack 58. Pressure pack 58 includes a volume of fluid 60 encapsulated within a sealed outer casing 62. The outer casing 62 may be a high quality urethane and the fluid 60 may be water, silicone or other viscous material. The flexible, fluid properties of pressure pack 58 allow the pressure plate 56 to “float” relative to the frame 28, thus ensuring that carrier 54 functions as a front side reference polishing device.
 A back side reference polishing apparatus such as wafer carrier head 26 of FIG. 2 may be modified to function as a floating reference polishing device by incorporating a pressure pack 58 into an existing carrier frame 28. In order to provide space for pressure pack 58, the prior art pressure plate 34 of carrier 26 must be removed. Pressure pack 58 and a replacement pressure plate 56 are then installed into the carrier head frame 28 in order to provide the desired fluid coupling in the load path between driving mechanism 30 and wafer 15. Depending upon the particular carrier head being modified, other parts may have to be replaced or modified in order to accommodate the pressure pack 58. Importantly, however, there is no need to provide a supply of pressurized air behind a membrane, as is described in the prior art carrier head 40 of FIG. 3. Fluid 60 is sealed within casing 62 and therefore is available for repeated polishing operations without the need to be replenished.
 Contamination is a constant concern during semiconductor fabrication processes. It is important that pressure pack 58 have a low incidence of failure, and that in the event of a failure, the spread of contaminants is minimized. Toward that end, it is preferred that fluid 60 be a highly viscous material, such as a gelatinous material, so that in the event of a failure of casing 62, the spread of the fluid 60 will be minimized for easier clean up. It is also possible to manufacture pressure pack 58 from a highly elastic solid material that does not have liquid properties, such as a material having a consistency such as soft clay. Furthermore, materials of construction for pressure pack 58 should be selected to avoid any highly harmful or hard to clean materials.
FIG. 5 illustrates another embodiment of a pressure pack 64 for use in a wafer polishing apparatus. Pressure pack 64 contains two chambers, a first chamber 66 in a central portion of the pressure pack 64, and a second chamber 68 forming a donut-shaped volume around the perimeter of the pressure pack 64. One may appreciate that any number of individual chambers may be provided. The chambers 66, 68 of pressure pack 64 are defined by two separate casings 70, 72 respectively. Advantageously, the fluid contained in first chamber 68 is encapsulated by both casing 72 and casing 70, so that a leak in either one of the casings will not result in contamination of the associated machinery. Such a double casing design may also be incorporated on a single chamber pressure pack. An inner casing may be selected for its compatibility with an encapsulated fluid and an outer casing may be selected for its toughness or elasticity properties.
 Advantageously, the physical properties of the two chambers may be selected to be different so that the distribution of a force across an abutting pressure plate may be made non-equal while still maintaining a fluid coupling. Such a dual chamber pressure pack 64 may advantageously be used with a two-part pressure plate 65 having corresponding center 67 and edge 69 portions. In order to achieve a desired pressure loading on an abutting pressure plate, the fluid pressure in the respective chambers 66, 68 may be different in a relaxed state. For example, by filling fluid in chamber 66 to a higher pressure than fluid in chamber 68, the pressure transferred by the pressure pack 64 between abutting frame and pressure plate members will be higher at the center portion of the pressure pack 64 than at the periphery portion of the pressure pack 64. Similarly, the fluid material contained within the two chambers 66, 68 may be selected to be different to achieve a desired mechanical effect. For example, the two fluids may have a different viscosity. Furthermore, the material of casing 70 may be different than the material of casing 72 in order to provide a desired mechanical effect and/or to improve the compatibility with the encapsulated fluids.
 While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.