Carrier head with a multilayer retaining ring for chemical mechanical polishing

Information

  • Patent Grant
  • 6251215
  • Patent Number
    6,251,215
  • Date Filed
    Wednesday, June 3, 1998
    26 years ago
  • Date Issued
    Tuesday, June 26, 2001
    23 years ago
Abstract
A carrier head for a chemical mechanical polishing apparatus includes a retaining ring having a flexible lower portion and a rigid upper portion.
Description




BACKGROUND




The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing apparatus.




Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.




Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” or a fixed-abrasive pad. A standard polishing pad has durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.




The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.




A reoccurring problem in CMP is the so-called “edge-effect”, i.e., the tendency of the edge of the substrate to be polished at a different rate than the center of the substrate. The edge effect typically results in over-polishing (the removal of too much material from the substrate) at the substrate perimeter, e.g., the outermost five to ten millimeters of a 200 mm wafer. Over-polishing reduces the overall flatness of the substrate, causing the edge of the substrate to be unsuitable for integrated circuit fabrication and decreasing the process yield.




SUMMARY




In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a substrate mounting surface and a retaining ring to maintain a substrate beneath the mounting surface during polishing. The retaining ring includes a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material.




Implementations of the invention may include the following. The first material may be a plastic, e.g., polyphenylene sulfide, polyethylene terephthalate, polyetheretherketone, or polybutylene terephthalate, which is substantially inert to a chemical mechanical polishing process. The second material may be a metal, e.g., steel, aluminum, or molybdenum, or a ceramic. The lower portion may be thicker than a substrate to be polished, e.g., between about 100 and 400 mils thick. The first material may provide a durometer measurement between about 80 and 95 on the Shore D scale. The second material may have an elastic modulus about ten to one-hundred, e.g., fifty times the elastic modulus of the first material. The lower portion may be adhesively attached, e.g., with a slow curing epoxy, or press fit to the upper portion.




In another aspect of the carrier head, the lower portion is made of a first material having a first elastic modulus and the upper portion is made of a second material having a second elastic modulus, and the second elastic modulus is selected to be sufficiently larger than the first elastic modulus to substantially prevent deflection of the lower surface of the retaining ring during polishing.




In another aspect of the carrier head, the lower portion is made of a first material having a first elastic modulus and the upper portion is made of a second material having a second elastic modulus, and the second elastic modulus is selected to be sufficiently larger than the first elastic modulus to substantially prevent deformation of the lower surface of the retaining ring where the retaining ring is joined to the carrier head.




In another aspect, the invention is directed to a retaining ring for a carrier head having a mounting surface for a substrate. The retaining ring has a generally annular lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material which is inert in a chemical mechanical polishing process, and a generally annular upper portion joined to the lower portion and made of a second material which is more rigid than the first material.




In another aspect, the invention is directed to a chemical mechanical polishing system with a rotatable polishing pad, a slurry supply to dispense a slurry onto the polishing pad, and a carrier head having a substrate mounting surface and a retaining ring to maintain a substrate beneath the mounting surface during polishing. The retaining ring includes a lower portion for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material.




Advantages of the invention may include the following. The edge effect is reduced, and the resulting flatness and finish of the substrate are improved.




Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is an exploded perspective view of a chemical mechanical polishing apparatus.





FIG. 2

is a schematic cross-sectional view of a carrier head according to the present invention.





FIG. 3

is an enlarged view of the carrier head of

FIG. 2

showing a retaining ring.











DETAILED DESCRIPTION




Referring to

FIG. 1

, one or more substrates


10


will be polished by a chemical mechanical polishing (CMP) apparatus


20


. A description of a similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is hereby incorporated by reference.




The CMP apparatus


20


includes a lower machine base


22


with a table top


23


mounted thereon and a removable upper outer cover (not shown). Table top


23


supports a series of polishing stations


25




a


,


25




b


and


25




c


, and a transfer station


27


for loading and unloading the substrates. Transfer station


27


may form a generally square arrangement with the three polishing stations


25




a


,


25




b


and


25




c.






Each polishing station


25




a


-


25




c


includes a rotatable platen


30


on which is placed a polishing pad


32


. If substrate


10


is an eight-inch (200 millimeter)or twelve-inch (300 millimeter) diameter disk, then platen


30


and polishing pad


32


will be about twenty or thirty inches in diameter, respectively. Platen


30


may be connected to a platen drive motor (not shown) located inside machine base


22


. For most polishing processes, the platen drive motor rotates platen


30


at thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station


25




a


-


25




c


may further include an associated pad conditioner apparatus


40


to maintain the abrasive condition of the polishing pad.




A slurry


50


containing a reactive agent (e.g., deionized water for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) may be supplied to the surface of polishing pad


32


by a combined slurry/rinse arm


52


. If polishing pad


32


is a standard pad, slurry


50


may also include abrasive particles (e.g., silicon dioxide for oxide polishing). Typically, sufficient slurry is provided to cover and wet the entire polishing pad


32


. Slurry/rinse arm


52


includes several spray nozzles (not shown) which provide a high pressure rinse of polishing pad


32


at the end of each polishing and conditioning cycle.




A rotatable multi-head carousel


60


, including a carousel support plate


66


and a cover


68


, is positioned above lower machine base


22


. Carousel support plate


66


is supported by a center post


62


and rotated thereon about a carousel axis


64


by a carousel motor assembly located within machine base


22


. Multi-head carousel


60


includes four carrier head systems


70




a


,


70




b


,


70




c


, and


70




d


mounted on carousel support plate


66


at equal angular intervals about carousel axis


64


. Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pads of polishing stations


25




a


-


25




c


. One of the carrier head systems receives a substrate from and delivers the substrate to transfer station


27


. The carousel motor may orbit carrier head systems


70




a


-


70




d


, and the substrates attached thereto, about carousel axis


64


between the polishing stations and the transfer station.




Each carrier head system


70




a


-


70




d


includes a polishing or carrier head


100


. Each carrier head


100


independently rotates about its own axis, and independently laterally oscillates in a radial slot


72


formed in carousel support plate


66


. A carrier drive shaft


74


extends through slot


72


to connect a carrier head rotation motor


76


(shown by the removal of one-quarter of cover


68


) to carrier head


100


. There is one carrier drive shaft and motor for each head. Each motor and drive shaft may be supported on a slider (not shown) which can be linearly driven along the slot by a radial drive motor to laterally oscillate the carrier head.




During actual polishing, three of the carrier heads, e.g., those of carrier head systems


70




a


-


70




c


, are positioned at and above respective polishing stations


25




a


-


25




c


. Each carrier head


100


lowers a substrate into contact with a polishing pad


32


. Generally, carrier head


100


holds the substrate in position against the polishing pad and distributes a force across the back surface of the substrate. The carrier head also transfers torque from the drive shaft to the substrate.




Referring to

FIG. 2

, carrier head


100


includes a housing


102


, a base


104


, a gimbal mechanism


106


, a loading chamber


108


, a retaining ring


110


, and a substrate backing assembly


112


. A description of a similar carrier head may be found in U.S. application Ser. No. 08/745,670 by Zuniga, et al., filed Nov. 8, 1996, entitled A CARRIER HEAD WITH A FLEXIBLE MEMBRANE FOR A CHEMICAL MECHANICAL POLISHING SYSTEM, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference.




The housing


102


can be connected to drive shaft


74


to rotate therewith during polishing about an axis of rotation


107


which is substantially perpendicular to the surface of the polishing pad during polishing. The loading chamber


108


is located between housing


102


and base


104


to apply a load, i.e., a downward pressure, to base


104


. The vertical position of base


104


relative to polishing pad


32


is also controlled by loading chamber


108


.




The substrate backing assembly


112


includes a support structure


114


, a flexure diaphragm


116


connecting support structure


114


to base


104


, and a flexible member or membrane


118


connected to support structure


114


. The flexible membrane


118


extends below support structure


114


to provide a mounting surface


120


for the substrate. Pressurization of a chamber


190


positioned between base


104


and substrate backing assembly


112


forces flexible membrane


118


downwardly to press the substrate against the polishing pad.




The housing


102


is generally circular in shape to correspond to the circular configuration of the substrate to be polished. A cylindrical bushing


122


may fit into a vertical bore


124


extending through the housing, and two passages


126


and


128


may extend through the housing for pneumatic control of the carrier head.




The base


104


is a generally ring-shaped body located beneath housing


102


. The base


104


may be formed of a rigid material such as aluminum, stainless steel or fiber-reinforced plastic. A passage


130


may extend through the base, and two fixtures


132


and


134


may provide attachment points to connect a flexible tube between housing


102


and base


104


to fluidly couple passage


128


to passage


130


.




An elastic and flexible membrane


140


may be attached to the lower surface of base


104


by a clamp ring


142


to define a bladder


144


. Clamp ring


142


may be secured to base


104


by screws or bolts (not shown). A first pump (not shown) may be connected to bladder


144


to direct a fluid, e.g., a gas, such as air, into or out of the bladder and thereby control a downward pressure on support structure


114


and flexible membrane


118


.




Gimbal mechanism


106


permits base


104


to pivot with respect to housing


102


so that the base may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism


106


includes a gimbal rod


150


which fits into a passage


154


through cylindrical bushing


122


and a flexure ring


152


which is secured to base


104


. Gimbal rod


150


may slide vertically along passage


154


to provide vertical motion of base


104


, but it prevents any lateral motion of base


104


with respect to housing


102


.




An inner edge of a rolling diaphragm


160


may be clamped to housing


102


by an inner clamp ring


162


, and an outer clamp ring


164


may clamp an outer edge of rolling diaphragm


160


to base


104


. Thus, rolling diaphragm


160


seals the space between housing


102


and base


104


to define loading chamber


108


. Rolling diaphragm


160


may be a generally ring-shaped sixty mil thick silicone sheet. A second pump (not shown) may be fluidly connected to loading chamber


108


to control the pressure in the loading chamber and the load applied to base


104


.




The support structure


114


of substrate backing assembly


112


is located below base


104


. Support structure


114


includes a support plate


170


, an annular lower clamp


172


, and an annular upper clamp


174


. Support plate


170


may be a generally disk-shaped rigid member with a plurality of apertures


176


therethrough. In addition, support plate


170


may have a downwardly-projecting lip


178


at its outer edge.




Flexure diaphragm


116


of substrate backing assembly


112


is a generally planar annular ring. An inner edge of flexure diaphragm


116


is clamped between base


104


and retaining ring


110


, and an outer edge of flexure diaphragm


116


is clamped between lower clamp


172


and upper clamp


174


. The flexure diaphragm


116


is flexible and elastic, although it could be rigid in the radial and tangential directions. Flexure diaphragm


116


may formed of rubber, such as neoprene, an elastomeric-coated fabric, such as NYLON™ or NOMEX™, plastic, or a composite material, such as fiberglass.




Flexible membrane


118


is a generally circular sheet formed of a flexible and elastic material, such as chloroprene or ethylene propylene rubber. A portion of flexible membrane


118


extends around the edges of support plate


170


to be clamped between the support plate and lower clamp


172


.




The sealed volume between flexible membrane


118


, support structure


114


, flexure diaphragm


116


, base


104


, and gimbal mechanism


106


defines pressurizable chamber


190


. A third pump (not shown) may be fluidly connected to chamber


190


to control the pressure in the chamber and thus the downward forces of the flexible membrane on the substrate.




Retaining ring


110


may be a generally annular ring secured at the outer edge of base


104


, e.g., by bolts


194


(only one is shown in the cross-sectional view of FIG.


2


). When fluid is pumped into loading chamber


108


and base


104


is pushed downwardly, retaining ring


110


is also pushed downwardly to apply a load to polishing pad


32


. An inner surface


188


of retaining ring


110


defines, in conjunction with mounting surface


120


of flexible membrane


118


, a substrate receiving recess


192


. The retaining ring


110


prevents the substrate from escaping the substrate receiving recess.




Referring to

FIG. 3

, retaining ring


110


includes multiple sections, including an annular lower portion


180


having a bottom surface


182


that may contact the polishing pad, and an annular upper portion


184


connected to base


104


. Lower portion


180


may be bonded to upper portion


184


with an adhesive layer


186


.




The lower portion is formed of a material which is chemically inert in a CMP process. In addition, lower portion


180


should be sufficiently elastic that contact of the substrate edge against the retaining ring does not cause the substrate to chip or crack. On the other hand, lower portion


180


should not be so elastic that downward pressure on the retaining ring causes lower portion


180


to extrude into substrate receiving recess


192


. Specifically, the material of the lower portion


180


may have a durometer measurement of about 80-95 on the Shore D scale. In general, the elastic modulus of the material of lower portion


180


may be in the range of about 0.3-1.0×10


6


psi. The lower portion should also be durable and have a low wear rate. However, it is acceptable for lower portion


180


to be gradually worn away, as this appears to prevent the substrate edge from cutting a deep grove into inner surface


188


. For example, lower portion


180


may be made of a plastic, such as polyphenylene sulfide (PPS), available from DSM Engineering Plastics of Evansville, Indiana, under the trade name Techtron™. Other plastics, such as DELRIN™, available from Dupont of Wilmington, Del., polyethylene terephthalate (PET), polyetheretherketone (PEEK), or polybutylene terephthalate (PBT), or a composite material such as ZYMAXX™, also available from Dupont, may be suitable.




The thickness T


1


of lower portion


180


should be larger than the thickness T


S


of substrate


10


. Specifically, the lower portion should be thick enough that the substrate does not brush against the adhesive layer when the substrate is chucked by the carrier head. On the other hand, if the lower portion is too thick, the bottom surface of the retaining ring will be subject to deformation due to the flexible nature of the lower portion. The initial thickness of lower portion 180 may be about 200 to 400 mils (with grooves having a depth of 100 to 300 mils). The lower portion may be replaced when the grooves have been worn away. Thus, the thickness T


1


of lower portion


180


may vary between about 400 mils (assuming an initial thickness of 400 mils) and about 100 mils (assuming that grooves 300 mils deep were worn away). If the retaining ring does not include grooves, the lower portion may be replaced when it's thickness is about equal to the substrate thickness.




The bottom surface of the lower portion


180


may be substantially flat, or it may have a plurality of channels or grooves


196


(shown in phantom in

FIG. 3

) to facilitate the transport of slurry from outside the retaining ring to the substrate.




The upper portion


184


of retaining ring


110


is formed of a rigid material, such as a metal, e.g., stainless steel, molybdenum, or aluminum, or a ceramic, e.g., alumina, or other exemplary materials. The material of the upper portion may have an elastic modulus of about 10-50×10


6


psi, i.e., about ten to one hundred times the elastic modulus of the material of the lower portion. For example, the elastic modulus of the lower portion may be about 0.6×10


6


psi, the elastic modulus of the upper portion may be about 30×10


6


psi, so that the ratio is about 50:1. The thickness T


2


of upper portion


184


should be greater than the thickness T


1


of lower portion


182


. Specifically, the upper portion may have a thickness T


2


of about 300-500 mils.




The adhesive layer


186


may be a two-part slow-curing epoxy. Slow curing generally indicates that the epoxy takes on the order of several hours to several days to set. The epoxy may be Magnobond-6375™, available from Magnolia Plastics of Chamblee, Ga. Alternately, instead of being adhesively attached the lower layer may be connected with screws or press-fit to the upper portion.




It appears that the flatness of the bottom surface of the retaining ring has a bearing on the edge effect. Specifically, if the bottom surface is very flat, the edge effect is reduced. If the retaining ring is relatively flexible, it can be deformed where it is joined to the base, e.g., by bolts


194


. This deformation creates a non-planar bottom surface, thereby increasing the edge effect. Although the retaining ring can be lapped or machined after installation on the carrier head, lapping tends to embed debris in the bottom surface which can damage the substrate or contaminate the CMP process, and machining is time-consuming and inconvenient. On the other hand, an entirely rigid retaining ring, such as a stainless steel ring, can cause the substrate to crack or contaminate the CMP process.




With the retaining ring of the present invention, the rigidity of upper portion


184


of retaining ring


110


increases the overall flexural rigidity of the retaining ring, e.g., by a factor of 30-40 times, as compared to a retaining ring formed entirely of a flexible material such as PPS. The increased rigidity provided by the rigid upper portion reduces or eliminates this deformation caused by the attachment of the retaining ring to the base, thereby reducing the edge effect. Furthermore, the retaining ring need not be lapped after it is secured to the carrier head. In addition, the PPS lower portion is inert in the CMP process, and is sufficiently elastic to prevent chipping or cracking of the substrate edge.




Another benefit of the increased rigidity of the retaining ring of the present invention is that it reduces the sensitivity of the polishing process to pad compressibility. Without being limited to any particular theory, one possible contribution to the edge effect, particularly for flexible retaining rings, is what may be termed “deflection” of the retaining ring. Specifically, the force of the substrate edge on the inner surface of the retaining ring at the trailing edge of the carrier head may cause the retaining ring to deflect, i.e., locally twist slightly about an axis parallel to the surface of the polishing pad. This forces the inner diameter of the retaining ring more deeply into the polishing pad, generates increased pressure on the polishing pad and causes the polishing pad material to “flow” and be displaced toward the edge of the substrate. The displacement of the polishing pad material depends upon the elastic properties of the polishing pad. Thus, a relatively flexible retaining ring which can deflect into the pad, makes the polishing process extremely sensitive to the elastic properties of the pad material. However, the increased rigidity provided by the rigid upper portion decreases the deflection of the retaining ring, thereby reducing pad deformation, sensitivity to pad compressibility, and the edge effect.




The present invention has been described in terms of a number of embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims.



Claims
  • 1. A carrier head for a chemical mechanical polishing apparatus, comprising:a substrate mounting surface; and a retaining ring to maintain a substrate beneath the mounting surface during polishing, the retaining ring including a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material and an upper portion made of a second material which is more rigid than the first material; wherein the first material is polyphenylene sulfide with a durometer measurement between about 80 and 95 on the Shore D scale, the second material is metal, and the lower portion is affixed to the upper portion by an epoxy.
  • 2. The carrier head of claim 1, wherein the first material is substantially inert to a chemical mechanical polishing process.
  • 3. The carrier head of claim 1, wherein the lower portion is thicker than a substrate to be polished.
  • 4. The carrier head of claim 3, wherein the lower portion is between about 100 and 400 mils thick.
  • 5. The carrier head of claim 1, wherein the upper and lower portions are substantially annular in shape.
  • 6. The carrier head of claim 1, wherein the second material is selected from the group consisting of steel, aluminum, and molybdenum.
  • 7. The carrier head of claim 1, wherein the epoxy is a slow curing epoxy.
  • 8. A retaining ring for a carrier head having a mounting surface for a substrate, comprising:a generally annular lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material which is inert in a chemical mechanical polishing process; and a generally annular upper portion joined to the lower portion and made of a second material which is more rigid than the first material; wherein the first material is polyphenylene sulfide with a durometer measurement between about 80 and 95 on the Shore D scale, the second material is metal, and the lower portion is affixed to the upper portion by an epoxy.
  • 9. A chemical mechanical polishing system, comprising:a rotatable polishing pad; a slurry supply to dispense a slurry onto the polishing pad; and a carrier head having a substrate mounting surface and a retaining ring to maintain a substrate beneath the mounting surface during polishing, the retaining ring including a lower portion for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material; wherein the first material is polyphenylene sulfide with a durometer measurement between about 80 and 95 on the Shore D scale, the second material is metal, and the lower portion is affixed to the upper portion by an epoxy.
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5205082 Shendon et al. Apr 1993
5423558 Koeth et al. Jun 1995
5584751 Kobayashi et al. Dec 1996
5605488 Ohashi et al. Feb 1997
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5695392 Kim Dec 1997
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