Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
1998-09-16
2001-09-11
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S327000, C318S648000, C318S652000
Reexamination Certificate
active
06287735
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to semiconductor manufacturing, and more particularly to controlling the leveling (upper) table of a wafer stage in a wafer stepper.
2. Description of the Related Art
During the manufacture of integrated circuits, circuit patterns for multiple chips are made on a single semiconductor wafer using techniques such as e-beam or ultraviolet photolithography. The wafer rests on a wafer stage under the control of a feedback wafer controller. The wafer stage includes a lower XY stage and an upper leveling stage. To control the leveling stage, the feedback may be measured at the surface of the wafer, or alternatively at the actuators driving the leveling stage. The first configuration introduces inaccuracies into the system because of the delay between the measurement at the wafer surface and the actuation points below the leveling stage. By measuring position at the actuators themselves, the second technique eliminates this delay, but provides an inaccurate representation of the measurement at the wafer surface.
In particular, the leveling stage driving mechanism, including the actuators and the upper leveling stage itself, exhibits nonlinear dynamics. The nonlinear effects hamper the ability of the system to quickly and accurately position the wafer stage at a desired height and keep the wafer level as it moves. Improvements in positioning and leveling would result in a higher throughput and improved exposure image quality.
FIG. 1
is a simplified block diagram illustrating an example of a conventional wafer scanner-stepper, such as the Nikon Model NSR 201, used in the manufacture of semiconductor chips. A radiant energy source
100
, such as an ultraviolet light, is directed towards a reticle or mask
102
. The light passing through the mask falls on an exposure area of a wafer
104
. As a result, the area of the reticle illuminated by the light projects a corresponding pattern onto the exposure area of the wafer. The wafer
104
rests on a wafer stage
106
, which moves under the control of a feedback wafer controller
108
. The position of the wafer
104
is detected by a wafer position sensor
110
, which can be implemented with a laser interferometer for measuring position in the XY direction and an encoder for measuring position in the vertical direction, for example.
The reticle may be held by a two-part reticle stage structure which includes a fine motion stage
112
and a coarse motion stage
114
. The coarse stage motion is controlled by a coarse stage controller
116
, and the fine stage motion is controlled by a fine stage controller
118
. The XY position of the reticle is sensed by a reticle position sensor
120
, which can be implemented by a laser interferometer, for example. The present invention may be employed with this system or with many other scanner-steppers known in the art.
FIG. 2
illustrates the wafer stage
106
in more detail. The wafer stage
106
moves the wafer
104
in three dimensions. The wafer stage
106
includes a lower XY stage
200
and an upper leveling stage
202
. A wafer chuck
204
on the leveling stage
202
supports the wafer
104
. Interferometer mirrors
206
mounted on the leveling stage
202
reflect light back to the sensor circuitry
110
to determine the position of the leveling stage
202
in the XY direction. Interposed between the lower stage
200
and upper stage
202
are leveling drive mechanisms or actuators
208
.
As is well known in the art, the XY stage
200
carries the leveling stage
202
, and thus the wafer
104
, along a path in the XY plane. Typically, under control of the leveling stage
202
by three leveling mechanisms
208
, the wafer is positioned to a desired height and maintained in a level position as the wafer travels. As is known in the art, each leveling drive mechanism
208
may include a motor
210
that turns a lead screw
212
. The screw
212
is threaded into a wedge
214
, and also coupled to an encoder
216
of sensor
110
. Based upon rotation of the screw, the encoder
216
provides a measurement related to the height of a roller
218
supported by the wedge and thus related to the height of the leveling table
202
.
Rotation of the screw
212
translates rotational motion of the motor
210
into translational motion of the wedge
214
. The wedge
214
supports the roller
218
, which has a fixed axle. As the wedge
214
moves in the XY plane, that motion is translated into orthogonal vertical motion by the roller
218
moving up or down the wedge
214
. In this manner, three actuators
208
control the vertical position and leveling of the upper leveling stage
202
.
The scanner-stepper operates as follows. A control computer
122
generates commands specifying the position of the wafer. In response, the wafer controller
108
causes the wafer stage
106
to move toward the desired or target position. The actual position of the wafer
104
is detected by the wafer sensor
110
and is fed back to a first adder
124
. The difference between the commanded position and the sensed position is the following error of the wafer stage. The wafer controller
108
adjusts the position of the wafer stage
106
in response to this error.
Because of limitations on the resolving power of projection lenses used in the light source
100
, the wafer is typically exposed to only a small area of the reticle mask
102
to maintain a high resolution. The reticle motion is synchronized with the wafer motion to expose more of the reticle to the wafer. Typically, the coarse controller
116
first moves the coarse reticle stage
114
in a coarse adjustment. The reticle sensor
120
feeds the position of the reticle to a second adder
126
, which compares the sensed reticle position to the sensed wafer position. The difference is the synchronization error, which is used by the fine controller
118
to adjust the fine reticle stage
112
in order to minimize the synchronization error.
During exposure, the wafer
104
is scanned with the mask pattern at a constant velocity. Scanning is performed on a row of chip areas laid out in the Y direction. When the end of a row is reached, the control computer
122
inputs a command to step the wafer in the orthogonal X direction so that scanning may proceed on the next row. After stepping, motion in the X direction is halted and scanning continues in the reverse Y direction. As a result, the wafer is moved in a serpentine pattern. For more information on serpentine scanning, please refer to U.S. Pat. No. 4,818,885, issued to Davis, et al., which is incorporated by reference herein.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for controlling the leveling table of a wafer stage. More generally, the invention includes control circuitry for controlling motion of a stage, where the stage is adapted to support a workpiece. The control circuitry measures position in a vicinity of the workpiece. Based upon the measured position, the control circuitry drives the stage toward a target position while accounting for nonlinear dynamics of the stage. The nonlinear dynamics may include inertia, in which case the control circuitry adaptively estimates the inertia of the stage. The nonlinear dynamics may also include tilt due to acceleration or deceleration of the stage, in which case the circuitry adaptively estimates the tilt of the stage.
The stage generally travels in a plane, and the circuitry measures position in a direction orthogonal to the plane. The circuitry may measure the position of the workpiece itself, or the position of an upper surface of the stage. The workpiece may be a semiconductor wafer in an exposure system.
REFERENCES:
patent: 4221995 (1980-09-01), Barkman
patent: 4223257 (1980-09-01), Miller
patent: 4818885 (1989-04-01), Davis et al.
patent: 4977361 (1990-12-01), Phillips et al.
patent: 5379108 (1995-01-01), Nose et al.
patent: 5448332 (1995-09-01), Sakakibara et al.
patent: 5493402 (1996-02-01), Hirukawa
patent: 5543921 (1996-08-01), Uzawa et al.
patent: 5563798 (1996-
Duda Kathleen
Morrison & Foerster / LLP
Nikon Corporation
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