Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
1998-03-05
2001-11-20
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S592000
Reexamination Certificate
active
06320345
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to controlling a stage in a high precision positioning system and in particular to a command trajectory that minimizes structural vibrations and discontinuities during stage travel.
BACKGROUND
High precision positioning systems, such as a microlithographic systems, require a smooth stage motion that produces a minimal amount of structural vibration or oscillation in the system's structure. Although most conventional positioning systems are supported by anti-vibration devices in an attempt to minimize disturbances, the unavoidable acceleration and deceleration of the stage produces forces on the positioning system which often result in small oscillations of the positioning system's structure.
FIGS. 1A-1D
show respectively the position, velocity, acceleration, and “jerk” of a positioning system's stage (not shown) moving during a conventional scan. The X axes of
FIGS. 1A-1D
represent the position, velocity, acceleration, and jerk of the stage with respect to time, which is represented along the Y axes.
As shown in
FIG. 1A
, the stage moves from position 0 to X between times to and t
0
and t
5
. The stage is moving at a constant velocity V
C
between times t
2
and t
3
and is stationary before time t
0
and after time t
5
as illustrated in FIG.
1
B. However, a settling period exists between time t
2
and t
set
, during which any oscillations generated during the acceleration of the stage are allowed to dissipate.
FIG. 1C
illustrates the acceleration and deceleration of the stage during a conventional scan. As shown in
FIG. 1C
, the acceleration increases in a linear manner until at time t
1
the acceleration is at +A, at which time the acceleration of the stage decreases until it is back at 0 at time t
2
. Thus, at time t
2
the stage will have a constant velocity V
C
shown in FIG.
1
B. The stage then linearly decelerates between times t
3
to t
5
with a maximum deceleration at time t
4
.
FIG. 1D
illustrates the jerk on the stage. The jerk is equal to the derivative of the acceleration of the stage with respect to time. As shown in
FIG. 1D
, during acceleration the jerk on the stage is at +J from time t
0
to t
0
, at −J from time t
1
to t
2
, and 0 at time t
2
. Thus, as illustrated in
FIG. 1D
, during acceleration there are discontinuities in the jerk at times t
0
, t
1
and t
2
. Further, when the acceleration of the stage begins and ends at times t
0
and t
2
, the jerk is equal to +J and −J, respectively. A similar, but opposite jerk, occurs during deceleration of the stage, as illustrated in FIG.
1
D.
A discontinuous jerk on a stage will create discontinuities in the motion of the stage during the scan, thus vibrating the stage. Moreover, a large jerk at the beginning and end of the acceleration and deceleration of the stage will produce a large reactive force, which consequently excites the positioning system's structure and creates large oscillations. Small vibrations or oscillations in a positioning system, such as a microlithography machine, will have a large deleterious effect where the system is expected to position stages with sub-micron accuracy.
Consequently, in a conventional positioning system in which oscillations occur, a settling period, between times t
2
and t
set
, is often required during which the oscillations are allowed to dissipate. The amount of time the stage needs to reach scanning stability at a constant velocity increases due to the increase or need for settling. Thus, the settling period reduces the throughput of a system.
SUMMARY
A position control apparatus that drives a stage with a trajectory command in accordance with the present invention provides a motion to the stage that minimizes any discontinuities and disturbances in the motion that create vibration or oscillations of the structure. A trajectory command in accordance with the present invention provides a continuous and piece-wise linear or sine wave function jerk during acceleration and deceleration of the stage, which generates a smooth continuous motion of the stage. Moreover, the values of the acceleration (and deceleration) and the jerk are equal to zero at the start and finish of the acceleration (and deceleration), which reduces the influence of reactive forces and thus minimizes oscillation of the system's structure.
The jerk on the stage during acceleration and deceleration has an adjustable time period (duration) to reduce the structural disturbances. The ratio between the positive peak value and negative peak value of the jerk during acceleration is adjustable, but is equal to the ratio between the negative jerk period and the positive jerk period during acceleration. The same is true during deceleration of the stage. These ratios can be adjusted to minimize structural vibration and decrease settling time of individual systems. Moreover, the ratios used during acceleration may differ from the ratios used during deceleration of the stage and these ratios may be individually adjusted.
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Chau Henry Kwok Pang
Saiki Kazuaki
Yuan Bausan
Duda Rina I.
Halbert Michael J.
Klivans Norman R.
Nappi Robert E.
Nikon Corporation
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