Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1998-07-08
2001-09-04
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559400
Reexamination Certificate
active
06285035
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method for detecting an endpoint polishing layer of a semiconductor wafer, a more particularly to an apparatus for detecting an endpoint layer of a semiconductor wafer having a wafer carrier with independent concentric sub-carriers and associated method.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated on or in a surface of a wafer, or alternatively, on a surface of a previous layer. This fabrication process typically requires subsequent layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be uneven due to an uneven topography associated with an underlying layer. As a result, a layer may need to be polished in order to present a smooth, planar surface for a subsequent processing step. For example, a layer may need to be polished prior to formation of a conductor layer or pattern on an outer surface of the layer.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as crystal lattice damage, scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes top and bottom platens (e.g. a polishing table and a wafer carrier or holder), between which the semiconductor wafer is positioned. The platens are moved relative to each other thereby causing material to be removed from the surface of the wafer. This polishing process is often referred to as mechanical planarization (MP) and is utilized to improve the quality and reliability of semiconductor devices. The polishing process may also involve the introduction of a chemical slurry to facilitate higher removal rates, along with the selective removal of materials fabricated on the semiconductor wafer. This polishing process is often referred to as chemical mechanical planarization or chemical mechanical polishing (CMP).
In these polishing processes, it is often important to determine an endpoint of the polishing process. Overpolishing (removing too much) of a conductive layer results in increased circuit resistance and potential scrapping of the semiconductor wafer. Since many processing steps have occurred prior to the polishing process, scrapping a semiconductor wafer during fabrication may undesirably result in significant financial loss. Underpolishing (removing too little) of a conductive layer on the other hand leads to failure in isolating circuits and results in electrical shorts. Presence of such electrical shorts leads to rework (redoing the CMP process) thereby disadvantageously increasing costs (e.g. production costs) associated with the semiconductor wafer. Thus, a precise endpoint detection technique is needed.
A typical method employed for determining the endpoint in polishing systems is to measure the amount of time needed to planarize a first wafer, and thereafter polishing the remaining wafers for a similar amount of time. In practice this method is extremely time consuming since machine operators must inspect each wafer after polishing. In particular, it is extremely difficult to precisely control the removal rate of material since the removal rate may vary during the polishing of an individual wafer. Moreover, the removal rate may be diminished in the process of polishing a number of wafers in sequence.
Another method employed for determining endpoint in polishing systems is to form a polishing endpoint layer in the semiconductor device, and thereafter polish the semiconductor device down to the polishing endpoint layer. To this end, polishing systems detect when the polishing process reaches the polishing endpoint layer and terminate the polishing process in response to reaching the polishing endpoint layer. Various techniques have been used to detect when the polishing process reaches the polishing endpoint layer. For example, U.S. Pat. No. 5,668,063 issued to Fry et al polishes a semiconductor device down to a tracer layer of detectable material. The polishing system of Fry determines that the tracer layer has been reached when a chemical element detector detects if materials, such as boron or phosphorous, associated with the tracer layer have been removed by the polishing process. However, such techniques undesirably require relatively complex chemical analysis equipment thereby undesirably increasing costs associated with the semiconductor wafers.
Thus, a continuing need exists for a method and an apparatus which accurately and efficiently detects when a polishing system polishes a semiconductor device down to a desired polishing endpoint layer.
SUMMARY OF THE INVENTION
In accordance with a first embodiment of the present invention, there is provided an apparatus for polishing a first side of a semiconductor wafer down to a desired level. The apparatus includes a polishing platen having a polishing surface. The apparatus also includes a wafer carrier having a first sub-carrier and a second sub-carrier which is concentric to the first sub-carrier. The first sub-carrier is configured to engage a first radial portion of the wafer by a second side of the wafer and apply first pressure to the first radial portion in order to press the first radial portion against the polishing surface of the polishing platen. The second sub-carrier is configured to engage a second radial portion of the wafer by the second side of the wafer and apply second pressure to the second radial portion in order to press the second radial portion against the polishing surface of the polishing platen. The apparatus also includes a first endpoint detector operable to determine when the first radial portion of the wafer has reached the desired level and cause the first sub-carrier to adjust the first pressure applied to the first radial portion of the wafer in response to determining that the first radial portion has reach the desired level. Moreover, the apparatus includes a second endpoint detector operable to determine when the second radial portion of the wafer has reached the desired level and cause the second sub-carrier to adjust the second pressure applied to the second radial portion of the wafer in response to determining that the second radial portion has reach the desired level.
Pursuant to a second embodiment of the present invention, there is provided a method of operating a polishing system to polish a first side of a semiconductor wafer down to a desired level. The polishing system has a wafer carrier which includes a first sub-carrier and a second sub-carrier which is concentric to the first sub-carrier. The method includes the step of rotating the wafer while the first side of the wafer contacts a polishing surface of a polishing platen in order to remove material from the first side of the wafer. The method also includes the step of applying first pressure to a first radial portion of the wafer with the first sub-carrier in order to press the first radial portion against the polishing surface of the polishing platen. The method further includes the step of applying second pressure to a second radial portion of the wafer with the second sub-carrier in order to press the second radial portion against the polishing surface of the polishing platen. Moreover, the method includes the step of determining when the first radial portion of the wafer has been polished down to the desired level and adjusting the first pressure applied to the first radial portion by the first sub-carrier in response thereto. Yet further, the method includes the step of determining when the second radial portion of the wafer has been polished down to the desired level and adjusting the second pressure applied to the second radial portion by the second sub-carrier in response thereto.
Pursuant to a third embodiment of the present invention, there is provided an apparatus for polishing a first side of a semiconductor wafer down to a desired level. The apparatus
Le Que T.
LSI Logic Corporation
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