RETAINER FOR CHEMICAL MECHANICAL POLISHING CARRIER HEAD

Abstract

A retaining ring for a carrier head of a chemical mechanical polishing system includes an annular outer portion having an annular outer surface and a plurality of flanges projecting radially inward from the annular outer portion. Adjacent flanges are separated by a gap and inner ends of the plurality of flanges provide an inner surface to contact a substrate held in the carrier head. The plurality of flanges are canted relative to radial direction.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Application No. 63/403,228, filed on Sep. 1, 2022, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a retainer for use in chemical mechanical polishing of substrates.

BACKGROUND

An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. 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.

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 typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad.

The carrier head provides a controllable load on the substrate to push it against the polishing pad. A retaining ring is used to hold the substrate in place below the carrier head during polishing.

SUMMARY

In one aspect, a retaining ring for a carrier head of a chemical mechanical polishing system includes an annular outer portion having an annular outer surface and a plurality of flanges projecting radially inward from the annular outer portion. Adjacent flanges are separated by a gap and inner ends of the plurality of flanges provide an inner surface to contact a substrate held in the carrier head. The plurality of flanges are canted relative to radial direction.

Implementations may optionally include, but are not limited to, one or more of the following advantages. Polishing non-uniformity, e.g., particularly near the substrate edge, can be reduced. Defects can be reduced, e.g., without sacrificing ability to adjust the polishing rate at the substrate edge.

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional side view of a polishing system.

FIG. 2 is a bottom view of a retaining ring for a carrier head; an expanded portion is indicated by phantom lines.

FIG. 3 is a cross-sectional side view along line A-A in FIG. 2.

FIG. 4 is a cross-sectional side view along line B-B in FIG. 2.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

A reoccurring problem in CMP is achieving uniformity near the substrate edge, e.g., within 5-20 mm of the substrate edge for a 300 mm diameter wafer. First, the region immediately adjacent the substrate edge, e.g., within 1-5 mm of the edge, tends to be overpolished, although under-polishing can also occur. Second, many polishing processes result in an annular band that near but not immediately adjacent the substrate edge, e.g., a band from five to ten millimeters from the substrate, being polished at a different rate than the center of the substrate. Third, angular asymmetry is more likely to occur near the substrate edge, i.e., different angular regions of the substrate will be polished at different rates. For example, fast removal has been observed on the “trailing edge” of the carrier head.

During polishing, the friction between the polishing pad and the substrate applies a lateral force that drives the substrate edge into contact with the inner surface with the retaining. Without being limited to any particular theory, the contact between a circular substrate and the circular inner surface of the retaining ring concentrates the lateral side load at a single point, which can result in unexpected reactions, e.g., wafer stress, pressure waves in the polishing pad, or vertical deflection of the substrate. However, by dispersing lateral contact forces between the retainer and substrate which otherwise would be concentrated at a single point of contact, the maximum stress at any particular point on the edge of the substrate can be reduced, which may reduce non-uniformity near the substrate edge.

A variety of retaining rings have been proposed to attempt to disperse lateral side load forces. However, none of such retaining rings have achieved more than trivial commercial adoption by semiconductor fabricators. Having portions of the ring undergo flexion about a vertical axis rather than about a horizontal axis in order to disperse lateral contact forces may be more likely to achieve desired uniformity without sacrificing other desired characteristics.

FIG. 1 illustrates an example of a polishing station of a chemical mechanical polishing system 20. The polishing system 20 includes a rotatable disk-shaped platen 24 on which a polishing pad 30 is situated. The platen 24 is operable to rotate about an axis 25. For example, a motor 26 can turn a drive shaft 28 to rotate the platen 24. The polishing pad 30 can be a two-layer polishing pad with an outer polishing layer 32 and a softer backing layer 34.

The polishing system 20 can include a supply port or a combined supply-rinse arm 36 to dispense a polishing liquid 38, such as an abrasive slurry, onto the polishing pad 30. The polishing system 20 can include a pad conditioner apparatus 40 with a conditioning disk 42 to maintain the surface roughness of the polishing pad 30.

A carrier head 70 is operable to hold a substrate 10 against the polishing pad 30. The carrier head 70 is suspended from a support structure 50, e.g., a carousel or a track, and is connected by a drive shaft 54 to a carrier head rotation motor 56 so that the carrier head can rotate about an axis 58. Optionally, the carrier head 70 can oscillate laterally, e.g., on sliders on the carousel, by movement along the track, or by rotational oscillation of the carousel itself.

The carrier head 70 includes a housing 72, a flexible membrane 74 that defines a plurality of pressurizable chambers 76, and a retaining ring 100 that can be secured to the housing 72. The carrier head can include other components, e.g., the flexible membrane could be connected to another component which is itself movable relative to the housing, e.g., by an upper loading chamber and gimbal mechanism.

Referring to FIGS. 1 and 2, the retaining ring 100 is a generally annular body having a bottom surface 102 to contact the polishing pad 30, a top surface 104 to abut the carrier head, an outer surface 106, and inner surface 108 for contacting and retaining the substrate. For a 300 mm diameter substrate, the inner surface 108 can have a diameter of 302-305 mm. The retaining ring can have a width of 2 to 6 cm.

In some implementations, the retaining ring includes a lower portion 110 that provides the bottom surface 102 and an upper portion 112 formed of a more rigid material for structural reinforcement. For example, the lower portion 110 can be a plastic, e.g., polyphenylene sulfide (PPS), whereas the upper portion can be a metal, e.g., stainless steel. However, in some implementations the entire ring is a single seamless plastic part. The retaining ring 100 can be secured to the carrier head, e.g., to the housing, by mechanical fasteners. For example, bolts 114 can extend through apertures 78 in the housing 72 and into threaded receiving holes 116 in the top surface 104 of the retaining ring.

Referring to FIGS. 2 and 3, the retaining ring 110 includes an annular outer portion 120 and a plurality of flexible flanges 122 that project inwardly from the annular outer portion 120. Adjacent flanges 122 are separated by gaps 124; effectively the gaps 124 are recesses that extend inwardly from the inner surface 112 of the retaining ring 110 to form the flanges 122. In addition, the flanges 122 are vertically separated from a portion 128 above flanges 122 by a horizontally extending gap 126. Thus, each flange 122 can act as a flexure that is free to flex and deform independently of the other flanges when a side load is applied by the substrate 10.

Each flange 122 can form a sheet with thickness T, a height H, and a length L (the length is the longest dimension; the thickness is the smallest dimension). The dimensions can be the same for each flange 122. The flanges 122 are oriented substantially vertically, i.e., the height-length surfaces are substantially vertical, e.g., less than 10° off vertical. However, the flanges 122 are canted relative to the radial direction. In particular, for each flange 122 a midline M (a line segment extending along the length dimension) of the flange 122 forms an oblique angle α with a radial segment R that passes through the center C of the retaining ring 100 and through the midline M. The angle α can be 30°-60°, e.g., 40°-50°, e.g., 45°. The angle α can be the same for each flange 122. Unlike the bodies of the flanges 122, the inner end surfaces 129 of the flanges 122 can be angled normal to the radial direction to thus form the circular inner surface 112 which will contact the edge of the substrate 10.

The flanges 122 can be spaced uniformly about the center C of the retaining ring. There may be twenty to eighty flanges for a retaining ring having an inner diameter of 302 mm.

The flanges 122, and in some embodiments at least the entire lower portion 110 including the flanges 122, are formed of a plastic having sufficient flexibility to bend inwardly under the side load generated by typical polishing operations such that the substrate will contact two or more flanges, e.g., two to ten flanges. For example, the flanges 122 and lower portion 110 can be polyphenylene sulfide (PPS).

In operation, when the friction between the polishing pad and the substrate drives the substrate 10 against the inner surface 112, the substrate edge can press and cause one or more of the flanges 122 to flex. In particular, the inner end 129 of the flange is pressed closer to the annular outer portion 120 (and thus radially outward), effectively increasing the angle α. This permits the substrate edge to contact multiple flanges 112, and thus spreads the reaction force out over a circumferential zone on the substrate edge. This reduces the maximum stress at any particular point on the edge of the substrate 10, which can reduce non-uniformity near the substrate edge. As the carrier head 70 and retaining ring 100 rotate, new flanges 122 move into contact with the substrate edge; when the side load is removed the flanges 122 return to their regular angle.

Referring to FIGS. 2 and 4, in some implementations, the retaining ring 100 has one or more channels 140 formed in the lower surface 102. The channels 140 extend from the inner surface 112 to the outer surface 110 of the retaining ring 100 to allow the polishing liquid, e.g., slurry, to pass from the exterior to the interior of the retaining ring during polishing, and to allow for draining of excess slurry and polishing by-products. The channels 140 can be evenly spaced around the retainer 100.

Each channel 140 can be offset at an angle, 30°-60°, e.g., 40°-50°, relative to the radius passing through the channel 140. The offset angle for the channels 140 can be the same as the angle α for the flanges 122. The channels 140 can have a width of about 0.125 inches.

In some implementations, the channels 140 are aligned with some of the gaps 124 between the flanges 122. Thus, the channels 140 can have the same width as the gaps 124. However, not every gap 124 will have a channel 140; there may be one channel 140 for every four to forty gaps 124, e.g., one channel 140 for every ten gaps 124.

As used in the instant specification, the term substrate can include, for example, a product substrate (e.g., which includes multiple memory or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, e.g., the substrate can be a bare wafer, or it can include one or more deposited and/or patterned layers. The term substrate can include circular disks and rectangular sheets.

The above described retaining ring can be used in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the platen. The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. Terms of relative positioning are used; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientation.

Particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results.

Claims

1. A retaining ring for a carrier head of a chemical mechanical polishing system, the retaining ring comprising:

an annular outer portion, the annular outer portion having a cylindrical outer surface;

a plurality of flanges projecting radially inward from the annular outer portion with adjacent flanges separated by a gap and with inner ends of the plurality of flanges providing an inner surface to contact a substrate held in the carrier head, wherein each flange of the plurality of flanges is canted relative to a respective radial direction from the flange.

2. The retaining ring of claim 1, wherein the flanges are formed of a material sufficiently flexible that in operation one or more flanges contacted by the substrate will flex radially outward.

3. The retaining ring of claim 1, wherein for each flange of the plurality of flanges, a midline of the flange forms an oblique angle of 30°-60° with a radial segment that passes through the center of the retaining ring and through the midline.

4. The retaining ring of claim 3, wherein the oblique angle is between 40°-50°.

5. The retaining ring of claim 4, wherein the oblique angle is 45°.

6. The retaining ring of claim 1, wherein the flanges are spaced uniformly along the inner surface.

7. The retaining ring of claim 6, wherein the plurality of flanges consist of twenty to eighty flanges.

8. The retaining ring of claim 1, wherein the inner ends of the plurality of flanges define a circle having a diameter of 302 to 305 mm.

9. The retaining ring of claim 1, wherein an inner end surface of each flange of the plurality of flanges is angled normal to the respective radial direction of the flange.

10. The retaining ring of claim 1, wherein a plurality of channels are formed in a lower surface of the annular outer portion, each channel of the plurality of channels extending from a gap to the outer surface of the annular outer portion.

11. The retaining ring of claim 10, wherein there are fewer channels than gaps.

12. The retaining ring of claim 11, comprising one channel for every ten gaps.

13. The retaining ring of claim 10, wherein the plurality of channels have a height less than a height of the plurality of flanges.

14. The retaining ring of claim 13, wherein each channel has a width equal to a width of the gap.

15. The retaining ring of claim 9, wherein each channel is canted relative to a respective radial direction from the channel.

16. The retaining ring of claim 14, wherein each channel is canted at the same angle as the gap from which the channel extends.