U.S. patent application number 09/153935 was filed with the patent office on 2001-05-31 for method and apparatus for controlling the leveling table of a wafer stage.
Invention is credited to YUAN, BAUSAN.
Application Number | 20010002303 09/153935 |
Document ID | / |
Family ID | 22549319 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010002303 |
Kind Code |
A1 |
YUAN, BAUSAN |
May 31, 2001 |
METHOD AND APPARATUS FOR CONTROLLING THE LEVELING TABLE OF A WAFER
STAGE
Abstract
A method and apparatus for controlling the leveling table of a
wafer stage is described. 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 may
generally travel 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.
Inventors: |
YUAN, BAUSAN; (SAN JOSE,
CA) |
Correspondence
Address: |
ROBERT A SALTZBERG
MORRISON & FOERSTER
425 MARKET STREET
SAN FRANCISCO
CA
941052482
|
Family ID: |
22549319 |
Appl. No.: |
09/153935 |
Filed: |
September 16, 1998 |
Current U.S.
Class: |
430/30 ;
430/311 |
Current CPC
Class: |
G03F 7/70716 20130101;
G03F 7/70725 20130101; G03F 7/707 20130101; G03F 9/7034
20130101 |
Class at
Publication: |
430/30 ;
430/311 |
International
Class: |
G03F 007/20 |
Claims
What is claimed is:
1. A method for controlling motion of a stage, the method
comprising the steps of: driving the stage toward a target
position; measuring position of the stage; and based upon the
measured position, correcting for nonlinear dynamics of the
stage.
2. The method of claim 1, wherein the nonlinear dynamics include
inertia.
3. The method of claim 2, wherein the correcting step includes the
step of adaptively estimating the inertia of the stage.
4. The method of claim 2, wherein the nonlinear dynamics further
include tilt.
5. The method of claim 1, wherein the nonlinear dynamics include
tilt.
6. The method of claim 4, wherein the correcting step includes the
step of adaptively estimating the tilt of the stage.
7. The method of claim 5, wherein the correcting step includes the
step of adaptively estimating the tilt of the stage.
8. The method of claim 1, wherein the stage generally travels in a
plane, and the measuring step comprises the step of measuring
position in a direction orthogonal to the plane.
9. The method of claim 1, wherein the measuring step comprises the
step of measuring position at an actuator driving the stage, and
transforming the actuator position to the measured position of the
stage, wherein the measured position represents position at or near
an upper surface of the stage in the z direction.
10. The method of claim 1, wherein the measuring step comprises the
step of measuring the position of an upper surface of the
stage.
11. The method of claim 1, wherein the measuring step comprises the
step of measuring the position of a workpiece supported by the
stage.
12. The method of claim 11, wherein the workpiece is a
semiconductor wafer, further comprising the step of exposing a
pattern onto the wafer.
13. The method of claim 1, wherein the correcting step corrects for
nonlinear dynamics while the stage is driven toward a target
position in the Z direction.
14. The method of claim 1, wherein the correcting step corrects for
nonlinear dynamics while the stage is driven toward a target
position in the XY plane.
15. The method of claim 1, wherein the correcting step comprises
the step of applying a correction force in a feedforward path.
16. A method for controlling motion of a stage, the method
comprising the steps of: driving the stage toward a target
position; measuring position of the stage; and based upon the
measured position, applying a feedforward force to adaptively
control motion of the stage.
17. The method of claim 16, wherein the applying step comprises the
step of adaptively correcting for nonlinear dynamics of the
stage.
18. The method of claim 17, wherein the correcting step comprises
the step of correcting for nonlinear dynamics while the stage is
driven toward a target position in the Z direction.
19. The method of claim 17, wherein the correcting step comprises
the step of correcting for nonlinear dynamics while the stage is
driven toward a target position in the XY plane.
20. The method of claim 16, wherein the applying step comprises the
step of adaptively estimating inertia of the stage.
21. The method of claim 16, wherein the applying step comprises the
step of adaptively estimating tilt of the stage.
22. The method of claim 16, wherein the stage generally travels in
a plane, and the measuring step comprises the step of measuring
position in a direction orthogonal to the plane.
23. The method of claim 16, wherein the measuring step comprises
the step of measuring the position of an upper surface of the
stage.
24. The method of claim 16, wherein the measuring step comprises
the step of measuring the position of a workpiece supported by the
stage.
25. The method of claim 24, wherein the workpiece is a
semiconductor wafer, further comprising the step of exposing a
pattern onto the wafer.
26. A system for controlling motion of a stage, the system
comprising: circuitry for driving the stage toward a target
position; a sensor for measuring position of the stage; and control
circuitry for correcting for nonlinear dynamics of the stage based
upon the measured position.
27. The system of claim 26, wherein the nonlinear dynamics include
inertia.
28. The system of claim 27, the control circuitry for adaptively
estimating the inertia of the stage.
29. The system of claim 27, wherein the nonlinear dynamics further
include tilt.
30. The system of claim 26, wherein the nonlinear dynamics include
tilt.
31. The system of claim 29, the control circuitry for adaptively
estimating the tilt of the stage.
32. The system of claim 30, the control circuitry for adaptively
estimating the tilt of the stage.
33. The system of claim 26, wherein the stage generally travels in
a plane, the sensor for measuring position in a direction
orthogonal to the plane.
34. The system of claim 26, the sensor for measuring position at an
actuator driving the stage, the system further comprising
coordinate transformation circuitry for transforming the actuator
position to the measured position of the stage, wherein the
measured position represents position at or near an upper surface
of the stage in the z direction.
35. The system of claim 26, the sensor for measuring the position
of an upper surface of the stage.
36. The system of claim 26, the sensor for measuring the position
of a workpiece supported by the stage.
37. The system of claim 36, wherein the workpiece is a
semiconductor wafer, the system further comprising an energy source
for exposing a pattern onto the wafer.
38. The system of claim 26, the control circuitry for correcting
for nonlinear dynamics while the stage is driven toward a target
position in the Z direction.
39. The system of claim 26, the control circuitry for correcting
for nonlinear dynamics while the stage is driven toward a target
position in the XY plane.
40. The system of claim 26, the control circuitry for applying a
correction force in a feedforward path.
41. A system for controlling motion of a stage, the system
comprising: circuitry for driving the stage toward a target
position; a sensor for measuring position of the stage; and control
circuitry for applying a feedforward force to adaptively control
motion of the stage based upon the measured position.
42. The system of claim 41, the control circuitry for adaptively
correcting for nonlinear dynamics of the stage.
43. The system of claim 42, the control circuitry for correcting
for nonlinear dynamics while the stage is driven toward a target
position in the Z direction.
44. The system of claim 42, the control circuitry for correcting
for nonlinear dynamics while the stage is driven toward a target
position in the XY plane.
45. The system of claim 41, the control circuitry for adaptively
estimating inertia of the stage.
46. The system of claim 41, the control circuitry for adaptively
estimating tilt of the stage.
47. The system of claim 41, wherein the stage generally travels in
a plane, the sensor for measuring position in a direction
orthogonal to the plane.
48. The system of claim 41, the sensor for measuring the position
of an upper surface of the stage.
49. The system of claim 41, the sensor for measuring the position
of a workpiece supported by the stage.
50. The system of claim 49, wherein the workpiece is a
semiconductor wafer, the system further comprising an energy source
for exposing a pattern onto the wafer.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present invention relates to semiconductor
manufacturing, and more particularly to controlling the leveling
(upper) table of a wafer stage in a wafer stepper.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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
[0014] 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.
[0015] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a simplified block diagram illustrating a wafer
scanner-stepper.
[0017] FIG. 2 illustrates a wafer stage including a lower, XY stage
and an upper, leveling stage.
[0018] FIG. 3 is a block diagram of the adaptive control system of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention provides a method and apparatus for
controlling the leveling table of a wafer stage. In the following
description, numerous details are set forth in order to enable a
thorough understanding of the present invention. However, it will
be understood by those of ordinary skill in the art that these
specific details are not required in order to practice the
invention. Further, well-known elements, devices, process steps and
the like are not set forth in detail in order to avoid obscuring
the present invention.
[0020] The dynamics of the leveling mechanism of the wafer table of
FIG. 2 may be represented by the following simplified equation.
M(q){umlaut over (q)}+C(q,{dot over (q)}){dot over (q)}+Kq=T
(1)
[0021] where
[0022] q=[q.sub.1, q.sub.2, q.sub.3 ].sup.T is a generalized
coordinate measured by the encoders
[0023] T=[.tau..sub.1, .tau..sub.2, .tau..sub.3].sup.T is the
torque force applied by the three actuator motors
[0024] M is a 3.times.3 matrix representing the inertia of the
leveling assembly, including the leveling mechanism, the table
itself, attachments such as interferometer mirrors, etc.
[0025] C is a 3.times.3 matrix representing centripetal and
Coriolis forces of the leveling mechanism.
[0026] K represents the stiffness of the leveling mechanism,
including stiffness corresponding to springs (not shown) interposed
between the upper (leveling) table and the lower (XY) table.
[0027] Define the coordinate transformation matrix R as
Z=R(q) (2)
[0028] where Z=(z,.theta..sub.x,.theta..sub.y).sup.T represents the
position at the wafer surface. Alternatively, Z may represent the
upper surface of the leveling stage. Z may be measured by a
standard AL/AF (auto-level/auto-focus) technique. For further
reference, please see co-assigned U.S. Pat. No. 5,448,332, issued
to Sakakibara et al, incorporation by reference herein. The R
matrix transforms coordinates from q to Z, and may be calculated
using well-known mathematical techniques.
[0029] With respect to differential motion,
.DELTA.Z=J(q).DELTA.q (3)
[0030] where J is the Jacobian of R.
[0031] Now divide the control force T into what will be denoted a
"feedback" force and a "feedforward" force. FIG. 3 is a block
diagram of the adaptive control system 300 of the present invention
that illustrates feedback and feedforward forces applied to the
wafer stage. Note that the reticle mechanism and components
relating to XY stage control have been omitted so as to not obscure
the figure. The system comprises a feedback portion 302 and a
feedforward portion 304. The feedback portion 302 includes an
encoder 216, which is coupled to the actuator motor 210 and feeds
back a signal q.sub.enc representing the generalized coordinate at
the actuator motor 210. The generalized coordinate is subtracted
from the input position q, and transformed by coordinate transform
circuitry 305, e.g., using J(q), to represent differential motion
at or near the wafer.
[0032] Based on this transformed feedback measurement, a wafer
controller 306 outputs a torque force T.sub.fb, which is added to
feedforward forces (discussed below). The resulting sum is inversed
transformed by inverse transform circuitry 307 back to the
generalized coordinate domain, i.e., in coordinates corresponding
to the leveling mechanism. This force is applied to the leveling
assembly 308, which supports wafer 104. The leveling assembly 308
includes standard components such as the leveling mechanism 208
(e.g., motors, wedges, rollers, etc.) and the leveling stage 202 of
FIG. 2.
[0033] The feedforward portion 304 includes a sensor 310, such as
an AL/AF sensor, which provides a signal .DELTA.Z. The quantity
.DELTA.Z represents the change in height of the wafer 104 (or
alternatively the upper surface of the stage) over one servo cycle.
The servo cycle represents the time period between adjustments in
the position of the wafer stage. The use of two sensors (e.g.,
encoder and AL/AF) for measuring position in the z direction
distinguishes the invention from typical conventional systems.
[0034] The feedforward portion 304 also includes an adaptive
inertial controller 312 that provides a torque output T.sub.ff, and
an adaptive tilt controller 314 that provides a torque force
T.sub.ffxy. All of the torque forces T.sub.fb, T.sub.ff and
T.sub.ffxy are input to the wafer stage 308 (through inverse
transformer 307) to control the actuator motors 210. The feedback
force T.sub.fb is applied at all times. The feedforward force
T.sub.ff is applied when commands are input to move the leveling
stage in the Z direction. The feedforward force T.sub.ffxy is
applied when commands are input to move the XY stage in the XY
plane.
[0035] As is known in the art, feedback controllers such as the
feedback wafer controller 306 correct relatively small errors.
Conventional feedback controllers cannot completely correct large
errors, such as those caused by inertia, and act on such errors
only very slowly.
[0036] The feedforward control compensates for non-linear dynamics
of the leveling assembly (e.g., stage, motors, wedges, rollers,
etc.). Focusing first on the adaptive inertial controller 312, the
control force T can be rewritten as
T=T.sub.ff+T.sub.fb (4)
[0037] Referring back to Equation (1), the second and third terms
are small quantities compared to the first term, and for the most
part are corrected by the feedback force T.sub.fb. It is a good
assumption that the feedforward force T.sub.ff will compensate the
larger first term in Equation (1), as follows. Define
M(q){umlaut over (q)}.ident.T.sub.ff (5)
[0038] This equation illustrates that the feedforward force
compensates for the inertia of the leveling assembly. This inertia
includes all inertial errors between the encoders and the point
where Z is measured, e.g., the upper surface of the wafer or the
leveling stage. These inertial errors include, but are not limited
to, the heavy mass of the leveling stage and nonlinear forces such
as backlash, screw flexure, side force effects of the wedges, and
nonlinear actuator effects. Traditional feedback action cannot
effectively compensate for these errors.
[0039] In a real-time implementation, the acceleration {umlaut over
(q)} is computed with difficulty. It may contain high-magnitude
noise. The acceleration is calculated by taking the double
derivative of the input position q. The acceleration is provided by
the control computer of the system. The real inertia matrix M may
also be unknown. To resolve this problem, a self-tuning or adaptive
scheme is used. First, make the following approximation.
M(q){umlaut over (q)}.congruent.{circumflex over (M)}a (6)
[0040] where
[0041] a=[a.sub.z, a.sub..theta.x, a.sub..theta.y].sup.T
[0042] The acceleration a is defined as the acceleration in the Z
direction. Through this definition, the force T.sub.ff compensates
for nonlinear dynamics when attempting to move the leveling stage
in the Z direction.
[0043] Although the inertia is not time varying, the quantity
{circumflex over (M)} is assumed to be a time-varying system in
order to allow it to be adaptively updated. The matrix can be
thought of as a virtual inertial mass. The acceleration a is an
estimated desired acceleration input corresponding to {umlaut over
(q)}.
[0044] By applying the well-known LMS (least mean square) method,
{circumflex over (M)} can be updated by the following formula.
[0045] .DELTA.{circumflex over (M)}=.mu.(J.sup.-1.DELTA.Z)a (7)
[0046]
[0047] where .mu. is a symmetric positive definite matrix related
to the correlation function of the input acceleration. A small .mu.
requires a long convergence time, but typically indicates a stable
system. Conversely, a large .mu. indicates a fast convergence, but
is more likely to represent an unstable system. Calculation of .mu.
is well known in the art. For further information, please refer to
S. Haykin, Adaptive Filter Theory, Prentice Hall, 2d edition, 1991,
which is incorporated by reference herein.
[0048] To initialize the algorithm, {circumflex over (M)} can be
initialized with each diagonal element representing the mass of the
leveling stage.
[0049] During the servo cycle in which {circumflex over (M)} is
updated, the next value of {circumflex over (M)} is calculated as
follows.
{circumflex over (M)}.sub.i+1={circumflex over
(M)}.sub.i+.DELTA.{circumfl- ex over (M)} (8)
[0050] where i is the servo cycle time index. (Generally, the index
is included only where necessary for clarity, but otherwise is
omitted for the sake of convenience.)
[0051] Based upon the updated value of the inertia, the inertial
feedforward force may be calculated as follows.
T.sub.ff.ident.{circumflex over (M)}a (9)
[0052] The force is applied to the leveling mechanism to compensate
for nonlinear dynamics, such as the effect of the inertia on
control of the leveling stage. The known prior art ignores the
effect of inertia.
[0053] Another effect ignored by the known prior art is tilt. When
the lower (XY) stage accelerates or decelerates in the XY plane, a
nonlinear coupling force will disturb the leveling upper stage in
the z direction.
[0054] Using a technique similar to that employed to compensate for
inertia, the system of the invention first assumes that there
exists a virtual disturbance force D due to the effect of the lower
stage. D is unknown and is a function of the X and Y acceleration
on the lower stage: a.sub.x,a.sub.y. An additional feedforward
force T.sub.ffxy is added to compensate this disturbance. To
adaptively calculate D, the following approximation is made.
D(a.sub.x,a.sub.y).congruent.{circumflex over (D)}.alpha. (10)
[0055] The matrix {circumflex over (D)} is assumed to be a
time-varying system, and is initialized to zero. The matrix
{circumflex over (D)} can be thought of as a virtual disturbance
mass, and is associated with an acceleration:
.alpha.=[a.sub.x,a.sub.y].sup.T (11)
[0056] The XY table acceleration a is known from the control
computer command given to the lower stage to move the lower stage
along the scan and step path. Alternatively, .alpha. may be
measured using standard techniques, such as laser interferometry.
Through the definition of .alpha., the force T.sub.ffxy compensates
for nonlinear dynamics when the control computer commands the XY
stage to move in the XY plane.
[0057] {circumflex over (D)} may be updated as follows:
.DELTA.D=.GAMMA.(J.sup.-1.DELTA.Z.alpha. (12)
[0058] The matrix .GAMMA. is calculated using the same techniques
used to calculate the matrix .mu. in Equation (7).
[0059] During each servo cycle {circumflex over (D)} is updated as
follows:
{circumflex over (D)}.sub.i+1={circumflex over
(D)}.sub.i+.DELTA.{circumfl- ex over (D)} (13)
[0060] The feedforward force T.sub.ffxy, applied by the adaptive
tilt controller 314, is calculated as follows:
T.sub.ffxy.ident.{circumflex over (D)}.alpha. (14)
[0061] This tilt compensation force is added to the inertial
compensation force T.sub.ff, and the sum is applied to the leveling
stage actuators.
[0062] The present invention provides for feedforward compensation
of nonlinear dynamic characteristics of the leveling stage, such as
inertia and tilt. By doing so, the system of the present invention
provides for more accurate positioning and leveling in the z
direction, and a faster settling time than the prior art. In
particular, by transforming the position measured by the encoders
to position at the stage surface, the invention minimizes errors at
the surface while reducing measurement delay.
[0063] Although the invention has been described in conjunction
with particular embodiments, it will be appreciated that various
modifications and alterations may be made by those skilled in the
art without departing from the spirit and scope of the invention.
For example, the control techniques of the invention do not apply
only to a typical wafer stage. Therefore, the term "stage" as used
herein means not only a stage used to support a semiconductor
workpiece, but any object for which motion is controlled. Moreover,
the invention may be incorporated into (and thereby include) a
conventional semiconductor exposure system with appropriate
modifications. Further, please note that the term "circuitry" as
used herein includes any hardware, software or firmware that may be
used to achieve the desired functionality. The invention is not to
be limited by the foregoing illustrative details, but rather is to
be defined by the appended claims.
* * * * *