RETAINING RING FOR CMP

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims priority to U.S. Provisional Application No. 63/688,711, filed on Aug. 29, 2024, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

[0002]The present disclosure relates to a retaining ring for a carrier head for chemical mechanical polishing.

BACKGROUND

[0003]Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non-planar surface. In addition, planarization of the substrate surface is usually required for photolithography.

[0004]Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, such as a slurry with abrasive particles, is typically supplied to the surface of the polishing pad.

[0005]The substrate is typically retained below the carrier head by a retaining ring. However, because the retaining ring contacts the polishing pad, the retaining ring tends to wear away, and is occasionally replaced. Some retaining rings have an upper portion formed of metal and a lower portion formed of a wearable plastic, whereas some other retaining rings are a single plastic part.

SUMMARY

[0006]In a general aspect, a retaining ring includes: a generally annular body having an inner surface to constrain a substrate and a bottom surface, the bottom surface having multiple channels extending from an outer surface to the inner surface. Each channel of the multiple channels can include an inner channel and outer channel connected by a reservoir channel, the reservoir channel having an annular shape within the bottom surface. The outer channel can have two opposing sidewalls and both opposing sidewalls of the outer channel have an inflection point.

[0007]In another general aspect, a retaining ring includes: a generally annular body having an inner surface to constrain a substrate and a bottom surface, the bottom surface having multiple channels extending from an outer surface to the inner surface. Each channel of the multiple channels can include an S-shaped channel extending from an outer edge of the bottom surface to a reservoir channel, the reservoir channel extending in an annular shape around the bottom surface and connected to an inner edge of the bottom surface.

[0008]Implementations may include one or more of the following features.

[0009]In some implementations, the inner channel has a first orientation, the outer channel has a second orientation, and the first and second orientations are different.

[0010]In some implementations, the outer channel has a first width where the outer channel meets the reservoir channel, a second width where the outer channel meets an outer diameter surface of the retaining ring, and the second width is greater than the first width.

[0011]In some implementations, the first width is in a range of 10-15 mm, and the second width is in a range of 20-30 mm.

[0012]In some implementations, the outer channel has a third width in an intermediate portion of the outer channel that is less than both the first and second widths.

[0013]In some implementations, the third width is in a range of 5-10 mm.

[0014]In some implementations, sidewalls of the outer channel are S-shaped.

[0015]In some implementations, intermediate portions of sidewalls of the inner channel are substantially parallel.

[0016]In some implementations, a width between the intermediate portions of the sidewalls of the inner channel is in a range of 2-5 mm.

[0017]In some implementations, ends of the sidewalls of the inner channels flare where the inner channel meets an inner diameter surface of the retaining ring and the reservoir channel.

[0018]In some implementations, a width between the ends of the sidewalls of the inner channels is in a range of 2-5 mm.

[0019]In some implementations, a length of the inner channel is in a range of 10-20 millimeters.

[0020]In some implementations, inner and outer islands are separated by the inner and outer channels, respectively, a contact area is a sum of areas of inner and outer islands divided by a surface area of the generally annular body, and the contact area is about 50% or less.

[0021]In some implementations, the multiple channels are spaced uniformly around the annular body.

[0022]In some implementations, there are eight to thirty channels.

[0023]In some implementations, each channel of the multiple channels includes a leading edge and a trailing edge. The leading and trailing edges can be not symmetric across a centerline of the channel.

[0024]In some implementations, sidewalls of the inner channel are oriented at an angle of about 30-60° relative to a radial direction.

[0025]In some implementations, a width of the annular shape of the reservoir channel is less than 2 mm.

[0026]In some implementations, a carrier head includes: a base; a flexible membrane coupled to the base, the flexible membrane having a mounting surface for a substrate; and any of the retaining rings described above, where the retaining ring is coupled to the base.

[0027]In some implementations, using the disclosed retaining ring can reduce edge nonuniformity. For example, the geometry of the retaining ring, e.g., a reduced number of grooves and/or reduced groove sizes compared to conventional retaining rings, can lead to more uniform contact between the polishing pad and the edge of the wafer being polished. The geometry can also reduce edge pressure variation at the wafers edge interface.

[0028]In some implementations, the channel configuration of the retaining ring can create a more uniform slurry distribution at the wafer's edge. As a result, slurry transport can have controlled and laminar flow, e.g., flow through a passive slurry flow channel. Further, a transient slurry reservoir within the retaining ring can provide adequate slurry supply during polishing.

[0029]The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0030]FIG. 1 is a schematic cross-sectional view of a carrier head.

[0031]FIG. 2A is a bottom view of a conventional retaining ring.

[0032]FIG. 2B is a bottom view of a retaining ring with a reservoir and inner and outer channels.

[0033]FIG. 2C is a close-up view of the inner and outer channels of the retaining ring of FIG. 2B.

[0034]FIGS. 3A, 3B, 4A, and 4B are graphs illustrating experimental results for the removal rate as a function of distance for various retaining rings.

[0035]Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0036]A retaining ring can be used to press on the surface of the polishing pad in the region around a substrate. In particular, the retaining ring presses with sufficient force so that the substrate cannot accidentally slide beneath the ring and become effectively trapped. However, if the retaining ring presses with too much force, the polishing pad can be over-compressed, resulting in non-uniform polishing. Non-uniform contact between the polishing pad and an edge of the wafer being polished can lead to wafer edge nonuniformity, e.g., at a radius of and beyond 146 to 149 mm for a 300 mm diameter wafer.

[0037]A retaining ring can have channels on the bottom surface to permit flow of slurry from outside the retaining ring to the substrate. A variety of channel configurations have been attempted, but room remains for improvement.

[0038]The present disclosure provides a retaining ring to reduce edge nonuniformity. For example, the geometry of the retaining ring can lead to more uniform contact between the polishing pad and the edge of the wafer being polished and reduce edge pressure variation at the wafers edge interface. Further, the channel configuration of the retaining ring can create a more uniform slurry distribution between the inner surface of the retaining ring and the wafer's edge. As a result, slurry transport can have controlled and laminar flow, e.g., flow through a passive slurry flow channel. Further, a transient slurry reservoir within the retaining ring can provide adequate slurry supply during polishing.

[0039]During a polishing operation, one or more substrates can be polished by a chemical mechanical polishing (CMP) apparatus that includes a carrier head 100.

[0040]Referring to FIG. 1, in an exemplary simplified view, a carrier head 100 includes a base 102, a flexible membrane 104 that provides a mounting surface for the substrate, a pressurizable chamber 106 between the membrane 104 and the base 102, and a retaining ring 110 secured near the edge of the base 102 to hold the substrate below membrane 104. Although FIG. 1 illustrates the membrane 104 as clamped between the retaining ring 110 and the base 102, one or more other parts, e.g., clamp rings, could be used to hold the membrane 104. A drive shaft 120 can rotate and/or translate the carrier head 100 across a polishing pad. A pump may be fluidly connected to the chamber 106 though a passage 108 in the base to control the pressure in the chamber 106 and thus the downward pressure of the flexible membrane 104 on the substrate.

[0041]The retaining ring 110 may be a generally annular ring secured at the outer edge of the base 102, e.g., by screws or bolts 136 that extend through passages 138 in the base 102 into aligned threaded receiving recesses in the upper surface 112 of the retaining ring 110. In some implementations, the drive shaft 120 can be raised and lowered to control the pressure of a bottom surface 114 of the retaining ring 110 on a polishing pad. Alternatively, the retaining ring 110 can be movable relative to the base 102, and the carrier head 100 can include an internal chamber which can be pressurized to control a downward pressure on the retaining ring 110. The retaining ring 110 is removable from the base 102 (and the rest of the carrier head 100) as a unit. In the case of the retaining ring 110 of FIG. 1, this means that an upper portion 142 of the retaining ring 110 remains secured to a lower portion 140 of the retaining ring while the retaining ring 110 is removed, without requiring disassembly of the base 102 or removal of the base 102 from the carrier head 100.

[0042]An inner surface 116 of retaining ring 110 defines, in conjunction with the lower surface of the flexible membrane 104, a substrate receiving recess 101. The retaining ring 110 prevents the substrate from escaping the substrate receiving recess 101.

[0043]The retaining ring 110 can include multiple vertically stacked sections, including the annular lower portion 140 having the bottom surface 114 that may contact the polishing pad, and the annular upper portion 142 connected to base 102. The lower portion 140 can be bonded to the upper portion 142 with an adhesive layer 144.

[0044]The lower portion 140 is a plastic, e.g., polyphenol sulfide (PPS). The plastic of the lower portion 140 is chemically inert in a CMP process. In addition, the lower portion 140 should be sufficiently elastic such that contact of the substrate edge against the retaining ring does not cause the substrate to chip or crack. On the other hand, the lower portion 140 should be sufficiently rigid to have a sufficiently long lifetime under wear from the polishing pad (on the bottom surface 114) and substrate (on the inner surface 116). The plastic of the lower portion 140 can have a durometer measurement of about 80-95 on the Shore D scale. In general, the elastic modulus of the material of lower portion 140 can be in the range of about 0.3-1.0×10⁶ psi. Although the lower portion can have a low wear rate, it is acceptable for the lower portion 140 to be gradually worn away, as this appears to prevent the substrate edge from cutting a deep grove into the inner surface 116.

[0045]The plastic of the lower portion 140 may be (e.g., consist of) polyphenylene sulfide (PPS), polyaryletherketone (PAEK), polyetheretherketone (PEEK), or polyetherketoneketone (PEKK).

[0046]The upper portion 142 of retaining ring 110 can be a harder material, e.g., a metal, ceramic or harder plastic, than the lower portion 140. An adhesive layer 144 can be used to secure the upper portion 142 to the lower portion 140, or the upper portion 142 and the lower portion 140 can be connected with screws (e.g., that extend through an aperture in the upper portion 142 into a receiving threaded recess or screw sheath in the lower portion 140), press-fit together, or joined by sonic molding. Alternatively, rather than having a lower portion and an upper portion, the entire retaining ring 110 can be formed of the same material, e.g., a plastic of the lower portion as described above.

[0047]The inner surface 116 of the lower portion 140 of the retaining ring can have an inner diameter D just larger than the substrate diameter, e.g., about 1-3 mm larger than the substrate diameter, so as to accommodate positioning tolerances of the substrate loading system. The retaining ring 110 can have a radial width of about half an inch.

[0048]FIG. 2A illustrates a conventional retaining ring 110a in which the bottom surface includes a plurality of channels 150a that extend from the inner diameter surface 116a to an outer diameter surface 118a. The bottom surface also includes islands 152, each pair of adjacent islands 152 separated by one of the channels 150a. Each channel 150a can have the same shape in the plan bottom view as the other channels; similarly, each island 152 can have the same shape as the other islands.

[0049]The islands 152 provide the contact area of the bottom surface against the polishing pad. The islands 152 can provide a contact area that is about 80-95%, e.g., 90%, of the plan area of the bottom surface. The plan area of the bottom surface can be calculated as π(R_O²-R_I²), where R_O is the radius of the outer diameter, e.g., at outer diameter surface 118a, and R_I is the radius of the inner diameter, e.g., at inner diameter surface 116a, of the retaining ring 110a.

[0050]Each channel 150a includes a leading edge 160a and a trailing edge 170a. Assuming the carrier head, and thus the retaining ring 110a, is rotating in a clockwise direction (as viewed from the bottom side of the ring, shown by arrow A), for the labeled channel 150a at the bottom of FIG. 2A, the leading edge 160a is the right edge and the trailing edge 170 is the left edge (again, as viewed from the bottom side of the ring).

[0051]FIG. 2B illustrates a retaining ring 110b with a reservoir channel 151 between an inner diameter surface 116b and an outer diameter surface 118b to form a reservoir for the polishing liquid. The reservoir channel 151 can be an annular channel that extends entirely around the retaining ring 110b. A plurality of liquid transfer channels 150b extend from the inner diameter surface 116b to the outer diameter surface 118b to permit transport of polishing liquid from outside the retaining ring 110b to the substrate edge. These liquid transfer channels 150b pass through the reservoir channel 151. Each liquid transfer channel 150b includes an inner channel 154 extending between the inner diameter surface 116b and the reservoir channel 151 and an outer channel 156 extending between the reservoir channel 151 and the outer diameter surface 118b. The inner and outer channels 154, 156 are aligned at the reservoir channel 151. This permits some polishing liquid to pass directly between the channels 154 and 156 without having to flow angularly through the reservoir channel 151, e.g., in a clockwise or counterclockwise direction.

[0052]The channels 150b and 151 divide the bottom surface 114b of the retaining ring 110b into multiple islands, including inner islands 153 adjacent the inner diameter surface 116b and outer islands 155 adjacent the outer surface 118b. The reservoir channel 151 separates the inner islands 153 from the outer islands 155. Each inner channel 154 separates a pair of angularly adjacent inner islands 153 from each other, and similarly each outer channel 156 separates a pair of angularly adjacent outer islands 155 from each other.

[0053]The outer channel 156 is wider than the inner channel 154 and the reservoir channel 151, which can help “scoop” slurry from the polishing pad and direct the slurry inwardly toward the substrate. As slurry flows inward toward the substrate, the slurry will encounter the reservoir channel 151, which redirects the slurry in a tangential direction, Thus, some of the slurry enters the reservoir channel 151 rather than traveling directly toward a respective inner channel 154. Redirecting slurry flow into the reservoir channel 151 can increase the uniformity of slurry flow toward the substrate.

[0054]Each inner channel 154 can have the same shape in the plan bottom view as the other inner channels; each outer channel 156 can have the same shape in the plan bottom view as the other outer channels; each inner island 153 can have the same shape in the plan bottom view as the other inner islands; and each outer island 155 can have the same shape in the plan bottom view as the other outer islands. However, other geometries are possible.

[0055]Along a depth direction, e.g., into the page of FIGS. 2A and 2B, the side walls of the channels 150b can be substantially perpendicular to the surfaces of the inner and outer islands 153 and 155 so that the contact area as a percentage of the plan area does not change as the retaining ring 110b wears. The channels 150b (and thus the inner and outer islands 153 and 155) can be positioned with uniform spacing circumferentially around the retaining ring 110b. There may be eight to thirty channels 150b, e.g., eighteen channels.

[0056]The leading and trailing edges 160b and 170b of each channel 150b are not symmetric across a centerline of the channel 150b, e.g., along any of lines 115a, 115b, or 115c.

[0057]FIG. 2C illustrates a close-up of the channel 150b from FIG. 2B. The reservoir channel 151 can have an annular shape, e.g., have constant inner and outer diameters. The difference between the inner and outer diameters is the width W_r of the reservoir channel 151. In some implementations, the width of the reservoir channel W_r is less than the width of either opening to the inner and outer channels 154 and 156, e.g., W_r<W_o1 and W_r<W_i3. The width W_r can be less than 3 mm, e.g., 2 mm or 1 mm.

[0058]Most of the inner channel 154 extends linearly from the inner diameter surface 116b to the reservoir channel 151. On each end of the inner channel 154, the width of the inner channel 154 can be greater than the width of the intermediate portion, e.g., between the ends, of the inner channel 154. In particular, one or both ends of the inner channel 154 can be flared. For example, where the inner channel 154 begins at the inner diameter surface 116b, the inner channel 154 can have a first width Wil greater than a second width Wiz of the intermediate portion. Where the inner channel 154 ends at the reservoir channel 151, the inner channel 154 can have a third width Wis greater than the second width Wiz of the intermediate portion. In some implementations, the first width W_i1 is in a range of 2-3 mm, the second width Wie is in a range of 2-3 mm, in the third W_i3 width is in a range of 2-3 mm.

[0059]The sides of the inner and outer channels 154 and 156 that intersect the reservoir channel 151 can be disposed relative to each other in various ways. For example, a pair of inner and outer channels 154 and 156 can overlap along the radial direction, e.g., a line originating from the center of the retaining ring 110b can intersect a single inner channel 154 and a single outer channel 156, which form a pair. In this example, for each pair, the outer channel 156 extends through a greater range of angular values, e.g., from sidewall 162a to sidewall 162b along the reservoir channel 151, compared to the inner channel 154, e.g., from sidewall 158a to sidewall 158b along the reservoir channel 151. For example, in FIG. 2B, lines 115a and 115b mark a first edge of the outer and inner channels, respectively, and there is an angle θ between lines 115a and 115b. The trailing edges of the inner and outer channels 154 and 156 have roughly the same angular location in this example.

[0060]The inner channel 154 can be angled relative to the radial direction of the retaining ring. As an example, sidewalls 158a and 158b can be oriented at an angle of about 30-60° relative to a radial direction. The direction of canting, e.g., the tilt of the inner channel 154 relative to a radial direction, can be in the opposite direction of rotation during use. For example, with reference to FIG. 2B, line 115c extends along the radial direction from the center of the retaining ring 110b to the beginning of the inner channel 154. For example, assuming the retaining ring 110b is to rotate in the clockwise direction during polishing, then from the inner surface 116 to the reservoir channel 151, the inner channel 154 extends in a counterclockwise direction, e.g., there is an acute angle between sidewall 158b and an inner wall 159 of the reservoir channel 151. During this clockwise rotation, from the outer surface 118 to the reservoir channel 151, the outer channel 156 extends in a clockwise direction, such that the inner and outer channels 154 and 156 have opposite orientations.

[0061]In this example, the sidewalls 158a and 158b of the inner channel 154 are substantially parallel and straight in the intermediate portion of the inner channel 154. Near the ends of the inner channel 154, the sidewalls 158a and 158b curve outward, increasing in width. However, other geometries are possible. The inner channel 154 can have a length Li, e.g., parallel to the sidewalls 158a and 158b. In some implementations, the length of the inner channel is in a range of 10-30 mm.

[0062]In this example, the inner channel 154 shares structural similarities to the channel 150a of the conventional retaining ring 110a, although the direction of canting is reversed, e.g., there would be an obtuse angle between sidewall 158b and an inner wall 159 of the reservoir channel 151.

[0063]In some implementations, the retaining ring has no inner channel. For example, a retaining ring can have a reservoir connected to a channel having a shape similar to outer channel 156b. In some implementations, the annular reservoir channel 151 is present when the inner channel is absent.

[0064]The outer channel 156 extends from the reservoir channel 151 to the outer diameter surface 118b. In this example, the outer channel 156 has an offset hourglass shape. For example, where the outer channel 156 begins at the reservoir channel 151, the outer channel 156 can have a first width W_o1 greater than a second width W_o2 of intermediate portion of the outer channel 156. Where the outer channel 156 ends at the outer diameter surface 118b, the outer channel 156 can have a third width W_o3 greater than the second width W_o2 of intermediate portion of the outer channel 156.

[0065]In other words, the outer channel 156 has a constriction between the reservoir channel 151 and the outer diameter surface 118. In some implementations, the first width W_o1 is in a range of 10-15 mm, the second width W_o2 is in a range of 5-10 mm, in the third width W_o3 is in a range of 20-30 mm.

[0066]In a plan view, the sidewalls 162a and 162b are curved and can have an inflection point, e.g., a location where the curvature of the sidewall changes from concave to convex. In this example, the sidewalls 162a and 162b are S-shaped, e.g., somewhat sinusoidal. However, other geometries are possible. For example, each of sidewalls 162a and 162b can include two linear segments that meet at an angle to create an angular hourglass shape.

[0067]The inner and outer channels 154 and 156 can be angled in opposite directions relative to the annular reservoir channel 151 or relative to a radial line extending from the retaining ring center through the channel 150b. For example, the inner channel 154 can begin at the inner diameter surface 116b at a position further counterclockwise (in this bottom view) than where the inner channel 154 ends at the reservoir channel 151. In contrast, the outer channel 156 begins at the reservoir channel 151 at a position further clockwise (again in this bottom view) than where the outer channel 156 ends at the outer diameter surface 118b. Consequently, a first line connecting the beginning and end of the inner channel 154 can have a different slope than a second line connecting the beginning and end of the outer channel 156.

[0068]The inner and outer channels 154 and 156 being angled in opposite directions can slow down the fluid flow rate of the slurry as fluid travels from the outside to the inside of the retaining ring 110b. For example, the slurry flow changes directions, which involves colliding with the sidewalls of the inner and outer channels 154 and 156, therefore losing energy in the collisions. However, this slurry need not be lost, as the slurry can flow into the reservoir channel 151.

[0069]The size of the contact area, e.g., the relative surface area of the inner and outer islands 153 and 155 compared to the overall surface area of the retaining ring, e.g., of an annulus determined by radii of inner and outer surfaces 116 and 118, can impact the performance of the retaining ring. For example, the larger the contact area more stable the retaining ring is during polishing. Additionally, a larger contact area slows down how quickly the retaining ring wears out.

[0070]A small contact area can encourage laminar flow if the channels and reservoirs of the retaining ring are designed as described. In some implementations, the contact area can be less than that of a conventional retaining ring, e.g., 90%. For example, the contact area, e.g., the sum of the areas of the inner and outer islands 153 and 155, can be about 50% or less of the overall surface area of the retaining ring.

[0071]The third width W_o3 being greater the first width W_o1 and the second width W_o2 means that the retaining ring 110b can draw in a sufficient amount of slurry without overly reducing the contact area. For example, if the entire outer channel 156 had a width equal to the third width W_o3, the contact area might be too small to maintain stability while polishing. The third width W_o3 being greater the second width W_o2 means that the retaining ring 110b will efficiently “scoop” slurry as it rotates in a clockwise direction. The first width W_o1 being greater than the second width W_o2 means that the slurry will slow down and spread out as the slurry approaches the reservoir channel 151. In some implementations, the first width W_o1 being greater than the third width W_o3 reduces how much flow of slurry is restricted.

[0072]The material choice of the lower portion 140 of the retaining ring, e.g., the inner and outer islands 153 and 155, can be selected given rigidity and friction constraints. For example, the material of the retaining ring can be rigid enough to not deform while polishing. As another example, the material of the retaining ring can have a low frictional coefficient to avoid undesired heating of the substrate and/or slurry during polishing. In some implementations, the retaining ring is composed of polyphenol sulfide or polyetheretherketone (PEEK, “P”) material.

[0073]FIGS. 3A, 3B, 4A, and 4B are experimental results for the removal rate as a function of distance. FIGS. 3A and 3B depict the removal rates for three different slurry recipes for a first, conventional retaining ring and a second retaining ring with a reservoir and inner and outer channels, respectively. Each of the first and second retaining rings are composed of polyphenol sulfide. As can be seen in FIG. 3A, as the distance from the pad center increases, the removal rate rapidly increases for each of the slurry recipes. In contrast, while the removal rate does increase as the distance from the pad center increases in FIG. 3B, the removal rate does not increase as rapidly compared to FIG. 3A. For example, the removal rate at 148 mm and 149 mm decreases by about 700 Å/min, e.g., an 85% reduction, for each of the three slurry recipes.

[0074]FIGS. 4A and 4B depict the removal rates for different slurry recipes for a first, conventional retaining ring and a second retaining ring with a reservoir and inner and outer channels, respectively. The removal rates in each of FIGS. 4A and 4B increase as the distance from the center of the wafer being polished increases. However, the rate of increase near the wafer edge, e.g., 146-149 mm, is not as rapid for the second retaining ring compared to the first retaining ring. For example, the removal rate at 148 mm and 149 mm decreases by about 700 Å/min, e.g., an 90% reduction, for each of the four slurry recipes.

[0075]A number of embodiments have been described. However, many variations are possible. For example, rather than the complicated channel and island shapes described above, the islands can be simple linear stripes with a width selected to provide the desired percentage of contact area. For example, the trailing and leading edge of each island can be provided by two parallel linear segments. The edges can be rounded at the inner and outer diameter surfaces. Thus, the scope of the invention is defined by the appended claims.