In-situ pad conditioning apparatus for CMP polisher

Information

  • Patent Grant
  • 6733370
  • Patent Number
    6,733,370
  • Date Filed
    Wednesday, June 27, 2001
    23 years ago
  • Date Issued
    Tuesday, May 11, 2004
    20 years ago
Abstract
An apparatus and method for conditioning a pad used for chemical-mechanical planarization (CMP) are provided, that allow the conditioning to be performed in situ without stopping the polishing. A retractable pad-conditioning structure, e.g., conditioning tips, is positioned along the bottom perimeter of a wafer carrier. While polishing a surface of a wafer held in the middle of the wafer carrier, whenever the removal rate drops below a permissible value, the pad-conditioning structure, which rotates in unison with the wafer carrier, is lowered to contact the pad to condition the pad's surface. Since an area of the pad used for polishing the wafer is always surrounded by already conditioned pad areas and the area for polishing moves as the wafer carrier moves around on the pad surface, a substantially uniform removal rate is maintained. When the pad is sufficiently conditioned, the conditioning structure is retracted until the pad needs to be conditioned again.
Description




FIELD OF THE INVENTION




This invention relates generally to chemical-mechanical planarization (CMP) and, in particular, to the pad conditioning aspect of CMP.




BACKGROUND OF THE INVENTION




Chemical-mechanical planarization (CMP) is a well known process used in semiconductor fabrication to polish a surface, for instance a dielectric film surface formed on a silicon wafer or other workpiece. The CMP process removes small elevated features on the surface without significantly thinning the film on, for instance, the flat areas lower than the elevated features.




As shown in a cross-sectional front view (FIG.


1


), a typical CMP system uses a flat, rotating disk (“platen”


112


) with a pliable polishing pad


110


mounted on its upper surface


112


S. As platen


112


is rotated, a slurry (not shown) is deposited near the center of the pad's surface


110


S and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer


106


(or substrate), held by a carrier


102


positioned above pad


110


, is then pressed downward against pad's surface


110


S such that the rotating polishing pad


110


moves the slurry over the wafer's surface


106


S. In this manner, elevated spots of the wafer surface


106


S are removed and a substantially planar surface is achieved. It is to be noted that although pad surface


110


S is illustrated as a smooth surface for simplicity of illustration, in fact pad


110


has a rather roughly textured surface


110


S to be rubbed against wafer surface


106


S along with the acidic (or basic) slurry containing abrasives.




Pad


110


is made of, e.g., polyurethane impregnated felt, cast and sliced polyurethane with filler, cast and sliced polyurethane without filler, or composite of two or more types of pad material. For effective polishing, the pad's surface


110


S needs to have a flat or bell-shaped profile. After a certain period of CMP polishing, the profile of the pad's surface


110


S is altered to be no longer useful for polishing, resulting in inefficient and sometimes even ineffective polishing. Deterioration of the profile happens as the pad surface loses its rough texture through mesa formation. Therefore, to maintain the quality of planarization, pad's surface


110


S needs to be rejuvenated (i.e., a desirable profile of surface


110


S restored) once in a while by, for example, raising the naps of surface


110


S by scraping surface


110


S.




One problem with prior art pad rejuvenation is that it has to be done “off-line” and ex situ, i.e., either before or after the polishing event with the wafer removed from the polishing station, reducing the throughput of the polishing process. Another problem with prior art pad rejuvenation (which is performed ex situ) is that a uniform removal rate is not maintained, since areas of pad surface


110


S are conditioned only in the beginning of the polishing process and, as areas of pad surface


110


S are used one or more times for polishing, the removal rates of those areas gradually drop.




SUMMARY




In accordance with the present invention, an apparatus and method for rejuvenating a polishing pad in situ without interrupting the CMP (or other types of) polishing are disclosed. Embodiments of the present invention achieve uniform removal rate and a higher throughput relative to prior art pad conditioning which cannot be performed during the CMP polishing.




In one embodiment, the inventive apparatus includes a wafer carrier carrying a wafer facing down a platen on which a polishing pad is placed. Along the bottom perimeter of the carrier are a number of conditioning tips, which are retractable upward in the direction normal to the upper surface of the platen by, say for example, spring loaded mechanism known to a person skilled in the art. During the polishing of the wafer surface (which faces the pad on the platen), the wafer carrier rotates in one direction and the pad on the platen rotates in the same or other direction. When the removal rate of the polishing process drops below a permissible value (i.e., the surface of the pad needs conditioning), the retractable tips attached to the rotating wafer carrier are lowered onto the pad to start conditioning the pad. The conditioning tips are pushed against the pad by air pressure so that the tips, while in constant contact with the pad, flexibly conform to the varying topography of the pad surface.




When the surface of the pad is sufficiently conditioned, the retractable tips are raised from the pad to stop conditioning. Later, whenever the state of the pad surface deteriorates after a period of polishing, the process of lowering the tips to condition the pad and raising the tips when the pad surface is conditioned is repeated.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

illustrates a cross-sectional front view of a prior art wafer carrier and platen with a CMP pad.





FIG. 2

illustrates, in accordance with the present invention, a cross-sectional front view of a wafer carrier and platen with a CMP pad in one embodiment, where the wafer carrier is lifted from the platen.





FIG. 3A

illustrates a front view of a conditioning structure (attached to a wafer carrier) in contact with the pad in the embodiment of

FIG. 2

, where the conditioning structure has multiple tips.





FIG. 3B

illustrates a front view of a conditioning structure (attached to a wafer carrier) in contact with the pad in the embodiment of

FIG. 2

, where the conditioning structure has a serrated blade.





FIG. 3C

illustrates a front view of a conditioning structure (attached to a wafer carrier) in contact with the pad in the embodiment of

FIG. 2

, where the conditioning structure has a nickel film with embedded diamond grit.





FIG. 4

illustrates a cross-sectional front view of the embodiment of

FIG. 2

, where the conditioning tips are retracted while the wafer surface is in contact with the pad surface for polishing.





FIG. 5

illustrates a cross-sectional front view of the embodiment of

FIG. 2

, where the pad is being conditioned while the wafer surface is being polished.











DETAILED DESCRIPTION




In accordance with the present invention,

FIG. 2

shows a cross-sectional front view of one embodiment of a wafer carrier


202


, with a conditioning structure


208


, holding a wafer (workpiece)


206


under a carrier insert


204


, where carrier


202


is positioned above a pad


210


laid on top of a platen


212


.

FIG. 2

shows a state wherein wafer carrier


202


is sufficiently lifted from platen


212


so that neither of a surface


206


S of wafer


206


or conditioning structure


208


are in contact with pad surface


210


S. It is to be noted that although pad surface


210


S in

FIGS. 2

,


3


A-


3


C,


4


and


5


is shown as a smooth surface for simplicity of illustration, in fact pad surface


210


S is conventionally a rather roughly textured surface for polishing.




The Y-R coordinate in the upper left-hand side corner of

FIGS. 2

,


4


and


5


denotes that upward direction is the positive Y direction, and counterclockwise direction (when looking down at the apparatus) is the positive R (rotational) direction. Conditioning structure


208


, defining a ring structure, is retractable in the positive Y direction and placed along the bottom perimeter of wafer carrier


202


. Mechanisms to retract and lower conditioning structure


208


are known to a person skilled in the art. For example, conditioning structure


208


is in the retracted position using springs when no power is supplied, and pushed down to the lowered position using air pressure or by one or more motor drives. Although a cross-section of conditioning structure


208


is shown to be wedge-shaped in this embodiment, the cross-section can be shaped differently as long as scraping pad surface


210


S with the bottom tips of conditioning structure


208


performs effective conditioning.





FIGS. 3A

,


3


B and


3


C respectively illustrate three different embodiments


208


A,


208


B and


208


C of conditioning structure


208


(in an enlarged front view) attached to wafer carrier


202


and in contact with pad surface


210


S.

FIGS. 3A

,


3


B and


3


C respectively illustrate conditioning structure


208


having conditioning tips


208


A, a serrated blade


208


B, and a nickel film


208


CN with protruding embedded diamond grit particles


208


CD. In the embodiment of

FIG. 3A

with conditioning tips


208


A, each of the tips is movable independently in the Y direction so that the ends of the tips conform to the changing topography and texture of pad surface


210


S. Besides using one of the three embodiments of conditioning structure


208


, more than one embodiment can be used simultaneously for pad conditioning. For example, the embodiment of serrated blade


208


B can be used along with the embodiment


208


C including nickel film and diamond grit.




As shown in

FIG. 4

(another cross-sectional front view of the embodiment of FIG.


2


), the CMP polishing is initiated by rotating wafer carrier


202


, for example, in the positive R direction and lowering wafer carrier


202


(in the negative Y direction) to the point where wafer surface


206


S is pressed against the top surface


210


S of pad


210


. Air pressure (denoted by a number of downward arrows above carrier insert


204


) is supplied within wafer carrier


202


and via the carrier insert


204


the downward force due to air pressure is transmitted to wafer


206


so that wafer surface


206


S and pad surface


210


S are pressed against each other with a force sufficient to enable the CMP polishing but not excessive, e.g., to damage wafer


206


. Note that, in

FIG. 4

, although CMP polishing is proceeding (i.e., wafer surface


206


S is in contact with pad surface


210


S), retractable conditioning structure


208


is in its retracted position, not in contact with pad surface


210


S.




After a certain period of CMP polishing (e.g., after polishing a number of wafers), the profile of pad surface


210


S deteriorates, resulting in drop of removal rate (usually measured in Å/minute), i.e., inefficient CMP polishing. The deterioration of surface


210


S happens in several forms including the naps of surface


210


S being polished away thus forming mesas. Drop of removal rate can be detected by methods known to a person skilled in the art. Several points in time can be selected to initiate conditioning of surface


210


S, including: (1) immediately after a preset length of polishing time, (2) immediately after a preset number of wafers being polished, (3) as soon as the removal rate drops under a preset value, or (4) when the polishing starts (i.e., simply from the beginning of polishing).




When it is the time to condition pad surface


210


S (choosing whichever point in time to start conditioning), conditioning structure


208


, attached to now rotating wafer carrier


202


, is lowered to contact and condition pad surface


210


S as seen in FIG.


5


. Besides being rotated, wafer carrier


202


is moved in a plane of motion parallel to platen surface


212


S (e.g., back and forth along a diameter of pad surface


210


S as denoted by two arrows directed to the left and the right, respectively). The rotational movement and the translational movement of wafer carrier


202


make the entire area of pad surface


210


S evenly used for polishing wafer (or workpiece)


206


.




In this embodiment, platen


212


also rotates in the direction (denoted “−R”) opposite to the direction (denoted “R”) in which carrier


202


rotates, to expedite the polishing as well as the pad conditioning. But, in another embodiment, platen rotates in the R direction.




Note that the air pressure is also exerted on conditioning structure


208


. In case conditioning structure


208


has conditioning tips


208


A, each of conditioning tips


208


A is allowed a certain extent of individual movement in the Y direction, and contacts pad surface


210


S with variable pressure to flexibly conform to the dynamically changing topography and texture of pad surface


210


S while not losing contact with the surface.




Depending on the implementation, two different levels of air pressure (possibly from two pressure mechanisms) may be (directly or indirectly) exerted on wafer


206


and conditioning structure


208


, or the same air pressure may be exerted on both. In case two different pressures on conditioning tips


208


A are used, in one embodiment, one of the pressures is exerted on a ring structure (not shown) that holds conditioning tips


208


A to indirectly exert that pressure on conditioning tips


208


A.




After a period of conditioning pad


210


, the conditioning process is terminated by retracting the conditioning structure


208


(e.g., conditioning tips


208


A or ring structure with a serrated blade


208


B) while the polishing process can continue with wafer surface


206


S remaining in contact with pad surface


210


S. Depending on the implementation, the time to terminate the conditioning process varies, e.g., (1) as soon as the removal rate is raised back to or above a preset adequate value, (2) when the profile of the pad surface


210


S is restored to a desired form, or (3) after a preset period of conditioning.




With prior art pad conditioning, the entire surface of the polishing pad is in a conditioned state only at the beginning of a polishing session. As areas of the pad surface are used multiple times for polishing, the areas gradually lose their texture and accordingly the effectiveness for polishing. In contrast, in accordance with the present invention, since the area of pad surface


210


S being conditioned always surrounds the area of pad surface


210


S used for polishing, and conditioning structure


208


(along with wafer carrier


202


) is moved across pad surface


210


S, wafer surface


206


S is always polished by the areas of pad surface


210


S that are in the conditioned state. This aspect brings an advantage that the removal rate is substantially uniform when compared to that of prior art pad conditioning. Another relative advantage is that higher polishing throughput is obtained, since the polishing process need not be stopped whenever pad surface


210


S needs to be conditioned.




This disclosure is illustrative and not limiting; further modifications will be apparent to one skilled in the art, in the light of this disclosure, and are intended to fall within the scope of the appended claims.



Claims
  • 1. A method for rejuvenating a pad used for polishing, the method comprising:placing a pad on an upper surface of a platen; placing a workpiece in a carrier so that the workpiece faces the upper surface of the platen; positioning a conditioning structure along a bottom perimeter of the carrier, the conditioning structure opposing the upper surface of the platen; and rejuvenating an upper surface of the pad using the conditioning structure, the rejuvenating including: rotating the conditioning structure attached to the carrier by rotating the carrier around an axis normal to the upper surface of the platen; lowering the carrier so that a surface of the workpiece contacts the upper surface of the pad to polish the surface of the workpiece; lowering the conditioning structure so that the conditioning structure contacts the upper surface of the pad; and conditioning the upper surface of the pad using the conditioning structure.
  • 2. The method of claim 1, wherein the conditioning structure includes a plurality of conditioning tips.
  • 3. The method of claim 2 further comprising:applying pressure to the tips so that the tips conform to the upper surface of the pad while maintaining contact with the upper surface of the pad during the conditioning.
  • 4. The method of claim 3, wherein the pressure is gas pressure.
  • 5. The method of claim 1, wherein the conditioning structure includes a plurality of serrated blades.
  • 6. The method of claim 1, wherein the conditioning structure includes a film with embedded particles protruding from the surface of the film.
  • 7. The method of claim 6, wherein the conditioning structure further includes a plurality of serrated blades.
  • 8. The method of claim 1, wherein lowering the conditioning structure occurs in response to detecting a predetermined planarization removal rate.
  • 9. The method of claim 1, wherein lowering the conditioning structure coincides with lowering the carrier for polishing.
  • 10. The method of claim 1, wherein lowering the conditioning structure occurs after polishing a predetermined number of workpieces.
  • 11. The method of claim 1, wherein lowering the conditioning structure occurs after polishing for a predetermined time.
  • 12. The method of claim 1, wherein conditioning the upper surface of the pad comprises:scraping the upper surface of the pad with the conditioning structure to raise naps on the upper surface of the pad.
  • 13. The method of claim 1, wherein rejuvenating further includes:translating the conditioning structure in a plane parallel to that of the upper surface of the platen.
  • 14. The method of claim 1 further comprising stopping polishing by:raising the carrier; and stopping the rotation of the carrier.
  • 15. The method of claim 1 further comprising:rotating the pad by rotating the platen.
  • 16. The apparatus of claim 1, wherein the conditioning is monitored by monitoring a removal rate of polishing the workpiece.
  • 17. An apparatus for conditioning a pad used for polishing, the apparatus comprising:a platen having an upper surface, the pad being on the upper surface of the platen; and a carrier for holding a workpiece which opposes the upper surface of the platen, the carrier including: a conditioning structure positioned along a perimeter of the carrier, the conditioning structure opposing an upper surface of the pad, wherein the carrier is rotated and lowered so that the conditioning structure contacts and conditions the upper surface of the pad while a surface of a workpiece held by the carrier is polished by the pad, and wherein the duration of conditioning is adjusted automatically depending on conditioning the upper surface of the pad.
  • 18. The apparatus of claim 17, wherein the conditioning structure includes a plurality of conditioning tips.
  • 19. The apparatus of claim 18, wherein the plurality of conditioning tips are positioned near the perimeter of the carrier in a line.
  • 20. The apparatus of claim 18, further including a source of pressure applied to the tips, whereby the tips, while in contact with the pad, conform to the upper surface of the pad while the upper surface of the pad is being conditioned.
  • 21. The apparatus of claim 20, wherein the pressure is gas pressure.
  • 22. The apparatus of claim 17, wherein the conditioning structure includes a serrated blade.
  • 23. The apparatus of claim 17, wherein the conditioning structure includes a film with embedded particles protruding from the surface of the film.
  • 24. The apparatus of claim 23, wherein the conditioning structure further includes a plurality of serrated blades.
  • 25. The apparatus of claim 17, wherein the pad includes polyurethane.
  • 26. The apparatus of claim 17, further including a mechanism which retracts the conditioning structure, thereby to lose contact with the upper surface of the pad, while the surface of the workpiece is still in contact with the upper surface of the pad and being polished by the upper surface of the pad.
  • 27. The apparatus of claim 17, wherein the platen and the pad are rotated in unison while the carrier is separately rotated during polishing of the surface of the workpiece.
  • 28. The apparatus of claim 17, further including a mechanism which translates the carrier in a plane of motion parallel to the upper surface of the platen during polishing of the surface of the workpiece.
  • 29. A method for rejuvenating a pad used for polishing, the method comprising:placing a pad on an upper surface of a platen; positioning a conditioning structure along a bottom perimeter of the carrier, the conditioning structure opposing the upper surface of the platen; and rejuvenating an upper surface of the pad using the conditioning structure, the rejuvenating including: rotating the conditioning structure by rotating the carrier around an axis normal to the upper surface of the platen; lowering the conditioning structure so that the conditioning structure contacts the upper surface of the pad; and conditioning the upper surface of the pad using the conditioning structure.
Parent Case Info

This application is a Divisional of Ser. No. 09/123,946 filed on Jul. 28, 1998.

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