The present invention relates generally to a substrate polishing apparatus for chemical mechanical polishing of a semiconductor substrate. More specifically, the present invention is directed to a substrate supporting pad, to the various constructions and compositions of such a pad, and to polishing apparatus using such a pad to support a substrate during polishing.
Chemical mechanical polishing (CMP) is a process performed by a substrate polishing apparatus for planarizing or polishing semiconductor substrates, also known as substrates, to provide substantially planar front and/or backsides thereon. The CMP process is used to remove high elevation features, or other discontinuities, which are created during the fabrication of semiconductors on the substrate.
The CMP process is performed by a substrate polishing apparatus having a carrier head assembly that removably retains a substrate to be polished, the carrier head urging the substrate against a rotating polishing pad. Further, the carrier head assembly may or may not rotate and translate the substrate relative to the rotating polishing pad. As the polishing pad rotates, it tends to pull the substrate from beneath the carrier head assembly. To eliminate this problem, the carrier head assembly includes a substrate confining ring that extends circumferentially around the lateral edge of the substrate, and retains the substrate and limits movement of the substrate relative to the carrier head assembly. In addition, a controllable down force is applied on the substrate to urge it against the polishing pad.
A down force is applied according to U.S. Pat. No. 5,795,215 that discloses a carrier head assembly with a vertically expandable bellows chamber wherein fluid, preferably air, is pumped into and out of a primary pressure chamber to control the load to be applied to a substrate. When air is pumped into the primary pressure chamber, the pressure in the chamber increases and a base assembly is pushed downwardly. A substrate backing member is attached by a vertically expandable bellows. The bellows and the substrate backing member may be formed of stainless steel.
A down force is applied according to U.S. Pat. No. 5,449,316 that discloses a carrier head assembly for polishing a substrate by providing a downwardly opening plenum covered by a flexible membrane. When pressurized fluid is introduced to the plenum, the membrane applies a uniform downward pressure across a backside of a substrate to be polished. The membrane is several hundred microns in thickness and is composed of a synthetic rubber.
A down force is applied to a substrate according to U.S. Pat. No. 5,931,719 that discloses an inflatable bladder located beneath the polishing pad. The inflated bladder is used to vary the pressure exerted against the bottom of the polishing pad, which results in varying the down force of a substrate that is held against the top surface of the polishing pad. Varying the pressure is said to compensate for center slow polishing arising when a polishing pad has been used repeatedly for polishing multiple substrates.
Undesired overpolishing of the substrate can be attributed to a warped or otherwise unevenly planar substrate, and by the manner in which the substrate is retained to the carrier head assembly. The carrier head assembly includes a generally planar lower carrier face or platen on which is mounted a conformable and resilient carrier pad film in the form of a thick, resilient and solid film that contacts the backside of the substrate. The conformable and resilient carrier pad film may be a poromeric carrier pad film, commercially available from Rodel, Inc., Newark, Del. USA, and known as xe2x80x9cDF-200xe2x80x9d and xe2x80x9cR200T3xe2x80x9d or xe2x80x9cR200T4-470-510xe2x80x9d. Such a carrier pad film is a dense, porous, closed-cell polymer foam having a thickness generally less than 0.030 in. thick. When wetted, a fluid surface tension is provided for adhesion of a substrate in contact with the carrier pad film. Alternatively, a conformable carrier pad film may be provided by a wax mound against which the substrate is pressed to form a conformable film surface to receive a substrate for polishing. The wax material has limited resiliency.
The lower carrier face may lack a desired planar orientation, and may include protrusions or be misaligned from planar, all of which are irregularities that contribute to uneven distribution of a down force against the carrier pad film, and, in turn, against the backside of the substrate during polishing. Such uneven distribution will exert localized higher down force distribution on a portion of the substrate, which contributes to uneven rates of polishing and deviation from a planar polished surface on the substrate. Further, the substrate may become over-polished at a locations of localized higher down force distribution, which reduces the yield of useable planar semiconductor areas on the substrate. A conformable and resilient carrier pad film provides only limited improvement in attaining an even distribution of the applied down force. For example, the load distributing properties of the carrier pad are limited, and are often inadequate to compensate or overcome misalignment of the lower carrier face itself, and misalignments of the carrier pad film and the substrate on the lower carrier face. The carrier pad film itself may have variations in its thickness and constituent construction that might contribute to nonuniform distribution of down force. Further, the compressibility of a carrier pad film is limited by the amount of compression provided by its limited resiliency. A need exists for a conformable carrier pad that is more compressible than the resiliency provided by a carrier pad film, and that can be sufficiently compressible and/or resilient to compensate for uneven distribution of applied down force, as provided by a misaligned carrier face and by protrusions on the carrier face.
A recurring problem in chemical mechanical polishing is the so-called xe2x80x9cedge effectxe2x80x9d, i.e., the tendency of the substrate edge to be polished at a different rate than the center of the substrate. The edge effect typically results in overpolishing or under-polishing in spite of the use of carrier films. Further, variations in the composition and construction of the carrier film itself can contribute to nonuniformly distributed, localized pressure zones at certain areas across the contacting surface on the carrier film and, hence, across the substrate to be supported by the contacting surface.
The compressibility of a resilient film carrier pad is limited, and the resultant pressure on the substrate is directly proportional to the degree of resilient compression by the carrier pad. Since in practice, the film carrier film must be changed regularly, it would be desirable to provide carrier pads of uniform construction and of uniform composition from one carrier pad to another, which enables the same level of internal pressurization.
The present invention provides a carrier head assembly for a substrate polishing apparatus applying uniform down force distribution against a substrate by using an internally pressurizable carrier pad. Advantageously, the internally pressurized carrier pad conforms to, and thereby compensates for, variations in substrate thickness, carrier assembly irregularities, and avoids the limits to compressibility and the nonuniform distribution of down force due to variations in composition and thickness, as provided by conformable and resilient carrier pad films.
Embodiments and advantages of the invention will become apparent by way of example from the following detailed description taken in conjunction with the accompanying drawings.