Patent Application
20100120335
Wafer polishing is often performed during semiconductor fabrication operations to planarize the surface of the wafer between process steps. Common components of wafer polishing systems include a polishing surface, a clamping system for holding the wafer against the polishing surface, and a wafer retaining ring to restrict lateral motion of the wafer while it is being polished.
When polishing a wafer, it is often desired that the material removal rate be uniform across the diameter of the wafer, and that the profile of material removal be consistent between wafers. However, in light of the extremely small scale of modern integrated circuits, and the corresponding accuracy and precision required in fabrication steps such as polishing, desired levels of wafer-to-wafer reproducibility in polishing profiles may be difficult to achieve.
Accordingly, various embodiments herein related to wafer polishing are disclosed. For example, one disclosed embodiment provides a wafer retaining ring comprising a ring for retaining the wafer, the ring having an inner diameter surface configured to restrict lateral wafer motion, and at least one interface surface configured to interface with a polishing surface. The interface surface comprises a recessed section adjacent to the ring inner diameter, configured to preclude contact between the recessed section and the polishing surface.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.
Prior to discussing the partial contact wafer retaining ring apparatus, an example use environment is described.
Retaining ring 102 comprises any ring shaped device suitable for retaining a wafer during polishing. In one embodiment, retaining ring 102 comprises a partial contact wafer retaining ring. The term “partial contact” as used herein refers to a configuration of a retaining ring 102 in which the interface surface that contacts the polishing surface 104 comprises a recessed section located adjacent an inner diameter of the retaining ring 102, configured to preclude contact between the recessed section and the polishing surface. The partial contact retaining ring configuration is described in more detail below.
Polishing surface 104 comprises any flat surface suitable for polishing a wafer. In one embodiment, polishing surface 104 comprises a polishing pad which is affixed to a wafer polishing head.
Wafer 106 comprises any flat disc shaped object suitable for polishing. In one embodiment, wafer 106 comprises a semiconductor wafer with one or more layers of thin film deposition product on its surface. In other embodiments, wafer 106 may comprise a similarly shaped piece of an alternate material such as glass, plastic, or metal, with or without a coating of thin film deposition product.
Clamping device 108 comprises any device suitable for urging a wafer and a wafer retaining ring against a polishing surface. In one embodiment, clamping device 108 comprises a wafer carrier head with a pressurized bladder. In other embodiments, clamping device 108 may comprise a mechanically driven rigid wafer carrier head.
During polishing, a relative lateral motion is established between polishing surface 104, and clamping device 108. Retaining ring 102, which is configured to move laterally with clamping device 108, restricts the lateral motion of wafer 106. This results in a relative lateral motion also being established between wafer 106 and polishing surface 104. In one embodiment, the relative lateral motion is an orbital motion. In some embodiments, the relative lateral motion may also include additional in-plane rotations or translations which are superimposed over the orbital motion.
When polishing a wafer, it is often desired that the material removal rate be uniform across the diameter of the wafer, and that the profile of material removal be consistent between wafers. Material removal rates near the edge of the wafer may be influenced when the wafer comes into contact with areas of the polishing surface that have also contacted the wafer retaining ring, or localized surface deformations on the polishing surface which are caused by interaction between the wafer retaining ring and the polishing surface. When either or both of these conditions occur, the uniformity of material removal across the diameter of the wafer, and the consistency of the material removal profile between wafers may be adversely affected. Additionally, areas of polishing surface 104 that contact both wafer 106 and retaining ring 102 may experience different wear characteristics than areas of polishing surface 104 that contact only wafer 106. This may lead to further difficulties in achieving uniform removal profiles.
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The use of a partial contact wafer retaining ring may reduce the presence of such localized surface deformations near the edge of the wafer.
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Moving now to the discussion of the partial contact wafer retaining ring apparatus,
Wafer retaining ring 102 may be constructed of any material or combinations of materials suitable for retaining a wafer. In one example, wafer retaining ring 102 is constructed entirely of a polymer such as polyethylene terapthalate, polyetheretherketone, polyacetyl, or polyamideimide. In other embodiments, wafer retaining ring 102 may be formed from a ceramic material such as silicon carbide, or aluminum oxide. In still other embodiments, wafer retaining ring 102 may be constructed of a combination of ceramic and polymer materials.
Inner diameter surface 504 serves to restrict lateral motion of a wafer, and may be configured in any manner suitable for this purpose. For example, in one embodiment, inner diameter surface 504 comprises a continuous cylindrical surface which is perpendicular to interface surface 506. In this embodiment, the vertical distance from surface 506 to the top edge of inner diameter surface 504 is equal to or larger than the thickness of a wafer.
In another embodiment, inner diameter surface 504 is configured to restrict the lateral motion of a wafer with one or more discrete surfaces of equal or non-equal size.
In yet another embodiment, inner diameter surface 504 comprises a removable surface 510, which is configured to restrict lateral wafer motion.
The use of the removable surface 510 may allow the surface 510 to be removed and replaced separately from the body 102 of the retaining ring. Further, the use of a softer material for the removable surface 510 to contact the wafer and a harder material for the body 102 of the retaining ring may allow the removable surface 510 and the body 102 of the retaining ring to be serviced or replaced at different intervals, and therefore may help to save costs compared to the use of a soft material for the entire retaining ring.
Interface surface 506 provides an interface between wafer retaining ring 102 and a wafer polishing surface, and may be configured in any manner suitable for this purpose. For example, in one embodiment, interface surface 506 comprises an outermost planar surface on one side of wafer retaining ring 102. In this embodiment, interface surface 506 forms a continuous planar surface, which is perpendicular to the cylindrical axis of wafer retaining ring 102, and which spans the radial distance between recessed surface 302 and the outer diameter of wafer retaining ring 102. It will be understood that this embodiment is described for the purpose of example, and that the interface surface may have any other suitable configuration.
Recessed section 302 provides a geometric buffer between interface surface 506, and inner diameter surface 504. During wafer polishing, this buffer creates a separation between the outer edge of the wafer being polished, and the inner diameter of interface surface 506. By maintaining this separation, it may be possible to control the interaction between the edge of the wafer, and interface surface 506 during wafer polishing. For example, the separation distance may be configured to preclude contact between the wafer and any portion of the polishing surface that has come into contact with interface surface 506. In another example the separation distance may be configured to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface.
Recessed section 302 may be configured in any manner suitable for the above described purpose. For example, in one embodiment, recessed section 302 is defined by a cylindrical recess which is concentric to the inner diameter of wafer retaining ring 102. In this embodiment the recessed surface is approximately planar, and is parallel to interface surface 506. The walls of the cylinder defining the outer diameter of recessed section 302 are concentric with the axis of wafer retaining ring 102. The depth of recessed section 302 is fixed with respect to interface surface 506, and may have any suitable dimension. For example, in embodiments configured for use with semiconductor wafers, recessed section 302 may have a maximum depth of one half the thickness of a wafer due to the location of the bevel on such wafers. In other embodiments, the recessed section 302 may have depths outside of this range, depending upon the specific configuration of the wafers for which retaining ring 102 will be used. The radial width of recessed section 302 is configured to preclude contact between the wafer and any portion of the polishing surface that has come into contact with the retaining ring, and to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface.
In another embodiment, recessed section 302 comprises one or more discrete surfaces of equal or non equal size.
In yet another embodiment, recessed section 302 comprises a varied diametric width.
As described above, the embodiments of partial contact wafer retaining rings detailed herein may be used during wafer polishing to preclude contact between the wafer and any portion of the polishing surface that has come into contact with the wafer retaining ring, and to preclude contact between the wafer and localized surface deformations on the polishing surface caused by the interaction between the wafer retaining ring and the polishing surface.
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Polishing the wafer at 1008 may additionally be performed by polishing such the wafer such that the wafer does not come into contact with any localized surface deformations on the polishing surface that are caused by the interaction between the wafer retaining device and the polishing surface. Any suitable method for polishing the wafer in this manner may be used. For example in one embodiment, the wafer is restrained during polishing with a partial contact wafer retaining ring that has been configured to preclude contact between the wafer and any area of the polishing surface that has also contacted the retaining device.
It will be understood that the configurations and/or approaches described herein are exemplary in nature and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein as well as an and all equivalents thereof.
