Abrading – Work holder
Reexamination Certificate
1999-07-26
2001-04-10
Scherbel, David A. (Department: 3723)
Abrading
Work holder
C451S288000
Reexamination Certificate
active
06213855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an apparatus for polishing or planarizing semiconductor workpieces such as silicon wafers. More particularly, the present invention relates to a wafer carrier for planarizing or polishing wafers on a polishing pad.
2. Description of the Related Art
Silicon workpieces or wafers, which are typically flat and circular in shape, are used in manufacturing semiconductor devices. Wafers are initially sliced from a silicon ingot and, thereafter, undergo multiple masking, etching, and dielectric and conductor deposition processes to create microelectronic structures and circuitry. The surface of a wafer undergoing these processes typically are polished or planarized between processing steps to ensure proper flatness to facilitate the use of photo lithographic processes for building additional dielectric and metallization layers on the wafer surface.
Chemical Mechanical Planarization (“CMP”) machines have been developed to polish or planarize silicon wafer surfaces to the flat condition desired for manufacture of integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued in February 1989 to Arai, et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,414, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March 1996 to Karlsrud et al.; and U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al.
Typically, a CMP machine includes a wafer carrier configured to hold and to rotate a wafer during the polishing or the planarizing of the wafer. For example, with reference to
FIG. 1
, a conventional wafer carrier
100
includes an upper housing
101
and a pressure plate
104
mounted underneath a lower or secondary housing
106
. A plurality of fasteners
108
fix pressure plate
104
to lower housing
106
. A plurality of vacuum holes
110
hold the wafer to be planarized to the planar lower surface of pressure plate
104
. Wafer carrier
100
then presses the wafer against a polishing pad (not shown) to polish or to planarize the wafer. More particularly, pressure plate
104
applies pressure to the wafer such that the wafer engages the polishing pad with a desired amount of pressure. The pressure plate and the polishing pad are also rotated, typically with differential velocities, to cause relative lateral motion between the polishing pad and the wafer to produce a more uniform thickness. Additionally, an abrasive slurry, such as a colloidal silica slurry, is often provided to enhance the polishing or planarizing process.
Conventional wafer carriers are typically rotated by a drive motor through a central drive shaft and a mechanical bearing assembly. For example, conventional wafer carrier
100
includes a bearing assembly
112
disposed between lower housing
106
and upper housing
101
and a drive shaft
114
connected to a drive motor (not shown). Bearing assembly
112
permits the movement of lower housing
106
and pressure plate
104
relative to upper housing
101
in order to maintain the surface of the wafer in parallel contact with the polishing pad even when the pad deviates from planarity. This motion is often referred to as “gimballing”, and the “gimbal point” is defined as the intersection of the plane in which the pressure plate
104
gimbals and the vertical central axis of the carrier. The gimbal point of wafer carrier
100
, for example, is at point
116
. The location of the gimbal point above the lower or backing surface of the pressure plate, however, can result in excessive tipping of the wafer with respect to the polishing pad, thus causing uneven edge polishing and detracting from uniform pressure distributed across the wafer.
Another shortcoming of conventional wafer carriers which arc rotated by a central drive shaft is the lag in response time due to the inertia of the wafer carrier. For example, when a torque is initially applied to drive shaft
114
to begin to rotate wafer carrier
100
, the mass of wafer carrier
100
results in a lag in response time of the wafer carrier
100
. Accordingly, the outer diameter portions of the wafer carrier
100
may initially rotate slower than the inner diameter portions of the wafer carrier
100
, thus contributing to uneven polishing or planarizing of the wafer. Additionally, the mass of wafer carrier
100
may result in undesired vibrations when the rotational speed of drive shaft
114
is increased or decreased, thus further contributing to uneven polishing or planarizing of the wafer.
An additional shortcoming of conventional wafer carriers is that the downward pressure applied to the drive shaft is not ideally distributed across the wafer. For example, in carrier
100
, upper housing
101
is connected to outer ring
118
of bearing assembly
112
by fasteners
120
, while inner ring
122
of bearing assembly
112
is connected to lower housing
106
by fasteners
124
. Hence, the pressure distribution path is as follows: downward pressure applied from the drive shaft is transmitted into upper housing
101
, transmitted through fasteners
120
and into outer bearing ring
118
, transmitted through bearing assembly
112
to inner bearing ring
122
, and transmitted through fasteners
124
to the narrow central body portion
126
of lower housing
106
and pressure plate
104
. Consequently, the downward pressure is concentrated at the central portion of the wafer and effects excessive material removal in the inner diameter portions of the wafer, while bowing and inadequate removal occurs at the outside diameter portions of the wafer.
SUMMARY OF THE INVENTION
In accordance with an exemplary embodiment of the present invention, a wafer carrier for polishing or planarizing semiconductor workpieces or wafers includes a pressure plate, an upper housing, and a lower housing. In accordance with one aspect of the present invention, the pressure plate is configured to hold a wafer to be polished or to be planarized against a polishing pad, and further configured to rotate about the lower housing to rotate the wafer during the polishing or the planarizing process. In accordance with another aspect of the present invention, the wafer carrier includes an electric direct drive motor, with the stators of the motor disposed in the lower housing and the rotors of the motor disposed in the pressure plate, to rotate the pressure plate about the lower housing. Accordingly, when electric power is supplied to the stators of the electric direct drive motor, the rotors of the motor rotate the pressure plate in response to the electromagnetic flux generated by the stators. The torque generated by the motor is developed in close proximity to the wafer, thus lowering the gimballing point of the carrier and thereby reducing the amount of gimballing or tilting force imparted to the wafer. The wafer thus tends to remain essentially parallel with the polishing pad surface.
In accordance with still another aspect of the present invention, a compliant material is disposed between the upper housing and the lower housing of the wafer carrier to form a flexible joint, or bellows, which maintains the wafer in substantially parallel and in substantially full contact with the polishing pad. In accordance with yet another aspect of the present invention, the lower housing of the wafer carrier is pressurized to apply pressure across substantially all of the surface area of the pressure plate and substantially uniformly across the surface area of the wafer.
REFERENCES:
patent: 4805348 (1989-02-01), Arai et al.
patent: 4811522 (1989-03-01), Gill, Jr.
patent: 5099614 (1992-03-01), Arai et al.
patent: 5329732 (1994-07-01), Karlsrud et al.
patent: 5476414 (1995-12-01), Hirose et al.
patent: 5498196 (1996-03-01), Karlsrud et al.
patent: 5498199 (1996-03-01), Karlsrud et al.
pate
Nguyen Dung Van
Scherbel David A.
Snell & Wilmer L.L.P.
SpeedFam-IPEC Corporation
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