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A carrier head for chemical mechanical polishing of a substrate has a base and a retaining ring positioned beneath the base. The retaining ring includes a main portion with a first surface to apply a load to a perimeter portion of the back surface of the substrate and an annular projection with a se

A carrier head for chemical mechanical polishing of a substrate has a base and a retaining ring positioned beneath the base. The retaining ring includes a main portion with a first surface to apply a load to a perimeter portion of the back surface of the substrate and an annular projection with a second surface to retain the substrate. A bottom surface of the projection is separated from a top surface of a polishing pad by a gap.

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1. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; an annular retaining ring positioned beneath the base and having a main portion with a first surface to apply a load to a perimeter portion of the back surface of

1. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; an annular retaining ring positioned beneath the base and having a main portion with a first surface to apply a load to a perimeter portion of the back surface of the substrate and having an annular projection with a second surface to retain the substrate, wherein the retaining ring is configured such that a bottom surface of the projection is separated from a top surface of a polishing pad by a gap during polishing. 2. The carrier head of claim 1 wherein the projection extends downwardly from the main portion.3. The carrier head of claim 2 wherein the second surface circumferentially surrounds the edge of the substrate.4. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; a first flexible membrane extending beneath the base to define at least a portion of a first pressurizable chamber, a lower surface of the first flexible membrane providing a first surface to apply a first load to a center portion of the back surface of the substrate; and a retaining ring positioned beneath the base and having a main portion with a second surface to apply a second load to a perimeter portion of the back surface of the substrate and having an annular projection protruding downwardly from the main portion with a third surface to circumferentially surround the edge of the substrate to retain the substrate wherein the retaining ring is configured such that a bottom surface of the projection is separated from a top surface of a polishing pad by a gap during polishing. 5. The carrier head of claim 4, further comprising a housing portion to be secured to a drive shaft, wherein the base is joined to the housing.6. The carrier head of claim 5, wherein the retaining ring is vertically movable relative to the base.7. The carrier head of claim 6, wherein the base includes a flange which circumferentially surrounds the retaining ring.8. The carrier head of claim 4, further comprising a second pressurizable chamber between a top surface at the retaining ring and the base.9. The carrier head of claim 8, wherein pressurization of the second pressurizable chamber applies a downward second load to the retaining ring.10. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; a first flexible membrane extending beneath the base to define at least a portion of a first pressurizable chamber, a lower surface of the first flexible membrane providing a first surface to apply a first load to a center portion of the back surface of the substrate; a retaining ring positioned beneath the base and having a main portion with a second surface to apply a second load to a perimeter portion of the back surface of the substrate and having an annular projection protruding downwardly from the main portion with a third surface to circumferentially surround the edge of the substrate to retain the substrate; and a housing and a second flexible membrane, wherein the base is movably connected to the housing by the second flexible membrane. 11. The carrier head of claim 10, wherein the retaining ring is fixed to the base.12. The carrier head of claim 11, wherein a volume between the base and the housing defined by the second flexible membrane forms a second pressurizable chamber.13. The carrier head of claim 12, wherein pressurization of the second pressurizable chamber applies a download pressure to the retaining ring.14. The carrier head of claim 13, wherein the first flexible membrane further comprises a perimeter portion and a rim portion.15. The carrier head of claim 14, wherein the rim portion of the first flexible membrane has a thickness greater than the perimeter portion.16. The carrier head of claim 14, wherein a rim portion of the first flexible membrane is clamped between the housing and the base.17. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; a retaining ring positioned beneath the base and having a main portion with a first surface to apply a first load to a perimeter portion of the back surface of the substrate and an annular lower projection protruding downwardly from the main portion with a second surface to circumferentially surround the edge of the substrate to retain the substrate; and a flexible membrane defining a pressurizable chamber between the base and the retaining ring, the chamber configured to apply a downward force on the retaining ring and the edge of the substrate when pressurized, wherein the retaining ring is configured such that a bottom surface of the retaining ring is separated from a top surface of a polishing pad by a gap during polishing. 18. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; a housing; a first flexible membrane extending beneath the base to define at least a portion of a first pressurizable chamber, a lower surface of the first flexible membrane providing a first surface to apply a first load to a center portion of the back surface of the substrate; a retaining ring positioned beneath the base, the retaining ring including a main portion with a second surface to apply a second load to a perimeter portion of the back surface of the substrate and an annular lower projection with a third surface to retain the substrate, wherein the carrier head is configured such that during polishing a bottom surface of the retaining ring is separated from a top surface of a polishing pad by a gap; and a second flexible membrane movably connecting the base and the housing and defining a second pressurizable chamber to apply a second load to a retaining ring. 19. A carrier head for chemical mechanical polishing of a substrate having a front surface, a back surface and an edge, comprising:a base; a first load applying member extending beneath the base, a lower surface of the first load applying member providing a first surface to apply a first load to a center portion of the back surface of the substrate; and a second load applying member positioned beneath the base and having a main portion with a second surface to apply a second load to a perimeter portion of the back surface of the substrate and having an annular projection protruding downwardly from the main portion with a third surface to circumferentially surround the edge of the substrate to retain the substrate wherein the carrier head is configured such that a bottom surface of the projection is separated from a top surface of a polishing pad by a gap during polishing. 20. A method of polishing of a substrate having a front surface, a back surface and an edge, comprising:holding a substrate with an inner diameter surface of an annular projection from a retaining ring without the retaining ring contacting the polishing pad, wherein the retaining ring includes a main portion with a first surface to contact a perimeter portion of a back surface of the substrate; pressurizing a first chamber in a carrier head to press a center portion of the substrate against the polishing pad; and creating a relative motion between the substrate and the polishing pad to polish the substrate. 21. The method of claim 20, wherein:pressurizing a first chamber includes pressing a membrane against a back surface of the substrate. 22. The method of claim 20, wherein:holding a substrate including applying a first load to a perimeter portion of the back surface of the substrate; pressurizing a first chamber includes applying a second load to the center portion of the back of the substrate; and the first load is not equal to the second load. 23. The method of claim 20, wherein:pressurizing a first chamber includes pressurizing a chamber formed between a substrate support structure and a base, and the substrate support structure has a projection that applies pressure to the center portion of the back surface of the substrate. 24. The method of claim 20, further comprising:pressurizing a second chamber located between a base and the retaining ring to move the retaining ring relative to the base. 25. The method of claim 20, wherein:holding a substrate includes transferring pressure from the retaining ring through a high friction compressible material to the perimeter portion of the back surface of the substrate. 26. The method of claim 20, wherein:holding a substrate includes applying a first load to an perimeter portion of the back surface of the substrate; of the carrier head pressurizing a first chamber includes pressurizing a chamber formed between a substrate support and a base, wherein the substrate support structure has a projection that applies a second load to the center portion of the back surface of the substrate; and pressurizing a first chamber includes pressurizing a chamber formed between a membrane and the base, such that a third load is applied to an inner annular portion of the back surface of the substrate.