Method of chemical mechanical polishing with edge control

Details Method of chemical mechanical polishing with edge control Method of chemical mechanical polishing with edge control

2000-02-08

2002-03-26

Rose, Robert A. (Department: 3723)

Abrading

Machine

Rotary tool

C451S398000

Reexamination Certificate

active

06361420

ABSTRACT:

BACKGROUND
The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
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 nonplanar. This nonplanar 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 a 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. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. 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 substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in overpolishing (the removal of too much material from the substrate) at the substrate perimeter, e.g., the outermost five to ten millimeters of a 200 millimeter (mm) wafer.
Another related problem, specifically in the polishing of so-called “flatted” substrates, i.e., substrates with a flat perimeter portion, is overpolishing of a region located adjacent the flat. In addition, the corners of the flat are often overpolished. Overpolishing reduces the overall flatness of the substrate, causing the edge, corners and flat of the substrate to be unsuitable for integrated circuit fabrication and decreasing process yield.
Another problem, particularly in polishing of flatted wafers using a carrier with a flexible membrane, is that the wafer flat contacts and abrades the bottom surface of the membrane, thereby reducing the membrane lifetime.
SUMMARY
In general, in one aspect, the invention is directed to a carrier head for chemical mechanical polishing. The carrier head has a base, a flexible membrane extending beneath the base to define a pressurizable chamber, an edge load ring, and a retaining ring. A lower surface of the flexible membrane provides a first surface for applying a first load to a center portion of a substrate. A lower surface of the edge load ring provides a second surface for applying a second load to perimeter portion of the substrate. The retaining ring surrounds the edge load ring to maintain the substrate beneath the first and second surfaces.
Implementations of the invention may include one or more of the following. The flexible membrane may be joined to a support structure, and the support structure may be movably connected to the base by a flexure. The flexible membrane may extend between an outer surface of the support structure and an inner surface of the edge load ring. A rim portion of the edge load ring may abut the support structure to maintain a gap between the inner surface of the edge load ring and the flexible membrane, and may extend over a portion of the support structure. A top surface of the edge load ring may abut a lower surface of the flexure, and pressurization of the chamber may apply a downward force on the edge load ring through the flexure. The surface area of the top surface of the edge load ring may be greater or less than the surface area of the lower surface of the edge load ring. An outer edge of the flexure may be clamped between the retaining ring and the base. An annular flexure support may be removably connected to the retaining ring and may support a perimeter portion of the flexure. The flexure support may be formed as an integral part of the retaining ring. The edge load ring may be joined to the support structure.
The support structure may include a support plate, a lower clamp, and an upper clamp, and the flexible membrane may be clamped between the support plate and the lower clamp. The flexure may be clamped between the lower clamp and the upper clamp, and the edge load ring may be joined to the lower clamp. The carrier head may have a layer of compressible material disposed on the lower surface of the edge load ring. The lower surface of the edge load ring may include an annular projection with an inner diameter which is larger than an outer diameter of the first surface. The edge load ring may include an annular flange located inwardly of the annular projection and may protrude downwardly to prevent the flexible membrane from extending beneath the edge load ring. The edge load ring may be configured to extend over a flat of the substrate. The lower surface of the edge load ring may include an annular projection which may extend over at least a portion of the flat. The carrier head may be constructed so that( RI+RO )/2>RF, where RI represents an inner radius of the annular projection, RO represents an outer radius of the annular projection, and RF represents the distance between the substrate center and the substrate flat.
A second edge load ring may surround the second surface, and a lower surface of the second edge load ring may provide a third surface for applying a third load to a second perimeter portion of the substrate. A third edge load ring may surround the third surface, and a lower surface of the third edge load ring may provide a fourth surface for applying a fourth load to a third perimeter portion of the substrate. A portion of the flexible membrane may extend beneath the lower surface of the edge load ring, may include a plurality of grooves, and may be secured to the edge load ring. An outer surface of the edge load ring may be separated from an inner surface of the retaining ring by a gap positioned such that frictional forces between the substrate and a polishing pad may urge a trailing edge of the substrate into the gap.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing. The carrier head has a base, a flexible membrane, and a rigid member. The flexible membrane extends beneath the base to define a pressurizable chamber, and a lower surface of the flexible membrane provides a first surface for applying a first load to a first portion of the substrate. The rigid member is movable relative to the base, and a lower surface of the rigid member provides a second surface for applying a second load to a second portion of the substrate.
In another aspect, the invention is directed to a method of polishing a substrate. In the method, the substrate is brought into contact with a polishing surface, a first load is applied to a center portion of the substrate with a flexible membrane, and a second load is applied to a perimeter portion of the substrate with an edge

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