Title: Reticle focus measurement system using multiple interferometric beams
Abstract: A first set of interferometric measuring beams is used to determine a location of a patterned surface of a reticle and a reticle focus plane for a reticle that is back clamped to a reticle stage. A second set of interferometric measuring beams is used to determine a map of locations of the reticle stage during scanning in a Y direction. The two sets of interferometric measuring beams are correlated to relate the reticle focal plane to the map of the reticle stage. The information is used to control the reticle stage during exposure of a pattern on the patterned surface of the reticle onto a wafer.
Patent Number: 6,850,330 Issued on 02/01/2005 to Roux, et al.
| Inventors:
|
Roux; Stephen (New Fairfield, CT);
Bednarek; Todd J. (Southbury, CT)
|
| Assignee:
|
ASML Holding N.V. (Veldhoven, NL)
|
| Appl. No.:
|
417257 |
| Filed:
|
April 17, 2003 |
| Current U.S. Class: |
356/510; 356/500 |
| Intern'l Class: |
G01B 009/02 |
| Field of Search: |
356/500,508,509,516,510
|
References Cited [Referenced By]
U.S. Patent Documents
| 5408320 | Apr., 1995 | Katagiri et al. | 356/490.
|
| 6359678 | Mar., 2002 | Ota.
| |
| 6406820 | Jun., 2002 | Ota.
| |
| Foreign Patent Documents |
| 1160629 | Dec., 2001 | EP.
| |
| WO 2004/012245 | Feb., 2004 | WO.
| |
Other References
Copy of Australian Search Report for European Appln. 200305649-6 mailed
Aug. 19, 2004.
English Language Abstract of JP 11-307436 dated Nov. 5, 1999.
English Language Abstract of JP 10-335234 dated Dec. 18, 1999.
|
Primary Examiner: Turner; Samuel A.
Assistant Examiner: Connolly; Patrick
Attorney, Agent or Firm: Sterne, Kessler, Goldstein & Fox P.L.L.C.
Parent Case Text
RELATED APPLICATIONS
This application is a divisional patent application of U.S. Ser. No.
10/235,499, filed Sep. 6, 2002, which is incorporated by reference herein
in its entirety.
Claims
What is claimed is:
1. A system comprising:
a moveable reticle stage holding a reticle, said reticle having a patterned
side;
a dual interferometer device that projects and detects a first set of
interferometer beams from said patterned side of said reticle and a second
set of interferometer beams from said reticle stage; and
a storage device that stores location data of said reticle measured by said
first set of interferometer beams and map data of said reticle stage
measured by said second set of interferometer beams.
2. The system of claim 1, further comprising a controller that controls
said reticle stage during exposure of a reticle pattern on a wafer based
on said stored map data and said stored location data.
3. The system of claim 1, wherein said reticle is back clamped to said
reticle stage.
4. The system of claim 1, wherein said reticle is side clamped to said
reticle stage.
5. The system of claim 1, wherein said reticle is front clamped to said
reticle stage.
6. The system of claim 1, wherein said dual interferometer device comprises
a single structure with two interferometer sections.
7. The system of claim 1, wherein said dual interferometer device comprises
two interferometers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to controlling a reticle stage during
exposure.
2. Background Art
Historically, in lithographic tools a mounting side and a patterned side of
a reticle are one and the same, establishing a reticle focal plane at a
plane of a reticle stage platen. Thus, knowledge of stage position in six
degrees-of-freedom (DOF) resulted in knowledge of the reticle patterned
surface position in six DOF. The six DOF are X, Y, Z, Rx, Ry, and Rz, as
shown in FIG. 1. However, mounting (or clamping) of an extreme ultra
violet (EUV) reticle will almost certainly be to a back surface of the
reticle (e.g., opposite from the patterned surface). Backside clamping
results in a reticle focal plane position relative to the reticle stage
that is a function of reticle flatness, reticle thickness, and reticle
thickness variation. Thus, in contrast to deep ultra violet (DUV) systems,
knowledge of the reticle stage position does not resolve where the pattern
of the reticle is located in all six DOF. The out-of-plane DOF (Z, Rx, and
Ry) cannot be easily determined due to the thickness variation of the
reticle. The position of the patterned side (opposite to the clamped side)
of the reticle needs to be known accurately in all six DOF.
In almost all steppers and scanners three in-plane DOF (X, Y, and Rz) are
determined from typical stage metrology schemes using interferometers.
However, three out-of-plane DOF (Z, Ry, and Rx) are more difficult to
measure. As discussed above, in an EUV tool, Z, Rx, and Ry have to be
known with much higher accuracy than in previous lithography tools. The
accuracy requirement stems from the need to position the pattern on the
reticle at a focal plane related to optics of the lithography tool. Also,
in some cases, optics are not telecentric at the reticle focal plane,
which increases the need for accuratley determining the reticle position
on the reticle stage to within six DOF. At the same time, it is critical
to accurately maintain focus on the pattern on the reticle even though the
reticle is not perfectly flat. Therefore, measuring the Z position and the
out of plane tilts (Rx and Ry) of the patterned side of the reticle in the
EUV tool requires tight accuracy.
Therefore, what is needed is a measuring system and method that can easily
calibrate or correlate a reticle focal plane (for a backside clamped
reticle) to a reticle stage to allow tracking of a patterned surface of a
reticle's position in six DOF using reasonably conventional stage
metrology methods. A measuring system and method is also needed that maps
a reticle surface to surfaces on a reticle stage, which allows feedback
for stage position to be based on surfaces on the stage instead of
surfaces on the reticle surface.
BRIEF SUMMARY OF THE INVENTION
Embodiments of the present invention provide a method including the steps
of measuring location data of a pattern side of a reticle based on a first
set of interferometer measuring beams, measuring map data of a reticle
stage during scanning of the reticle stage based on a second set of
interferometer measuring beams, and controlling the reticle stage during
exposure of a wafer with a pattern on the pattern side of the reticle
based on the location data and the map data.
Further embodiments of the present invention provide a method that includes
the steps of determining a reticle focal plane of a backside clamped
reticle on a reticle stage using a first interferometer, determining
positions of the reticle stage during scanning of the reticle stage using
a second interferometer, correlating the reticle focal plane to the
positions of the reticle stage, and controlling the reticle stage during
an exposure process based on the correlating step.
Still further embodiments of the present invention provide a system
including a moveable reticle stage holding a reticle, the reticle having a
patterned side, a dual interferometer device that projects and detects a
first set of interferometer beams from the patterned side of the reticle
and a second set of interferometer beams from the reticle stage, and a
storage device that stores location data of the reticle measured by the
first set of interferometer beams and map data of the reticle stage
measured by the second set of interferometer beams.
Further embodiments, features, and advantages of the present inventions, as
well as the structure and operation of the various embodiments of the
present invention, are described in detail below with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
The accompanying drawings, which are incorporated herein and form a part of
the specification, illustrate the present invention and, together with the
description, further serve to explain the principles of the invention and
to enable a person skilled in the pertinent art to make and use the
invention.
FIG. 1 shows an example orientation of a reticle according to embodiments
of the present invention.
FIG. 2A shows a portion of a lithographic system or tool using a dual
interferometer according to embodiments of the present invention.
FIG. 2B shows a portion of a lithographic system using two interferometers
according to embodiments of the present invention.
FIGS. 3A and 3B show various configurations of a reticle and a stage being
measured according to various embodiments of the present invention.
FIG. 4 shows a flowchart of an overall measuring and controlling method for
a lithography tool according to embodiments of the present invention.
FIG. 5 shows a flowchart of a measuring and controlling method for a
reticle according to embodiments of the present invention.
FIG. 6 shows a flowchart of a measuring and controlling method for a
reticle stage according to embodiments of the present invention.
FIG. 7 shows a portion of a lithographic system for measuring reticle and
stage positions according to embodiments of the present invention.
FIG. 8 shows a portion of a lithographic system for measuring reticle and
stage positions according to embodiments of the present invention.
FIG. 9A shows a portion of a lithographic system having a side held reticle
according to embodiments of the present invention.
FIG. 9B shows a portion of a lithographic system having a front held
reticle according to embodiments of the present invention.
The present invention will now be described with reference to the
accompanying drawings. In the drawings, like reference numbers indicate
identical or functionally similar elements. Additionally, the left-most
digit(s) of a reference number identifies the drawing in which the
reference number first appears.
DETAILED DESCRIPTION OF THE INVENTION
A first set of interferometric measuring beams is used to determine a
location of a patterned surface of a reticle and a reticle focus plane for
a reticle that is clamped (e.g., back, side, or front clamped) to a
reticle stage. A second set of interferometric measuring beams is used to
determine a map of locations of the reticle stage during scanning in a Y
direction. The two sets of interferometric measuring beams are correlated
to relate the reticle focal plane to the map of the reticle stage. The
information is used to control the reticle stage during exposure of a
pattern on the patterned surface of the reticle onto a wafer.
FIG. 1 shows six degrees of freedom (DOF) for a reticle 100 oriented in or
parallel to an X-Y plane according to embodiments of the present
invention. Again, the six DOF are X (along the X axis), Y (along the Y
axis), Z (along the Z axis), Rx (rotation around the X axis), Ry (rotation
around the Y axis), and Rz (rotation around the Z axis). The more easily
determinable DOF are the X, Y, and Rz based on a reticle stage's
movements. In the embodiments discussed below, the DOF that are the focus
of the discussion below are Z and Ry. It is to be appreciated that any DOF
can be determined by the appatarus and methods below if the orientation of
the reticle 100 is changed.
FIG. 2A shows a portion 200 of a lithography tool according to embodiments
of the present invention. Portion 200 includes a reticle stage 202 with a
backside clamped reticle 204 that has a pattern 206. Although not drawn to
scale, an interferometer system 208 includes two interferometers 208A and
208B. Each interferometer 208A and 208B projects illuminating (I) light
from illumination devices 210 towards portion 200. In various embodiments,
illumination devices 210 can be light sources, lasers, or the like with or
without focusing or expanding optical devices. A first set of
interferometric measuring beams RSZ1 and RSZ2 from first interferometer
208A are reflected from first 212 and second 214 positions, respectively,
on reticle 204. First position 212 is adjacent a first side of pattern 206
and second position 214 is adjacent a second side of pattern 206. The
reflected beams are received by detectors (D) 216. Signals corresponding
to the detected beams are stored in a storage device 218 either before or
after being processed by controller 220.
Again with reference to FIG. 2A, similarly, a second set of interferometric
measuring beams RSZ3 and RSZ4 from second interferometer 208B are
reflected from first 222 and second 224 points, respectively, on reticle
stage 202 and detected by detectors 216. Signals correlating to the
detected beams are then stored in storage 218. In the embodiments shown
and described above, all four measuring points, 212, 214, 222, and 224
substantially lie along a line having a same Y value. In other embodiments
this may be required.
FIG. 2B shows an interferometer 208' including a first interferometer 208A'
and a second interferometer 208B' according to embodiments of the present
invention.
FIGS. 3A and 3B show a first and second possible position of reticle 204
according to embodiments of the present invention. To calcuale the Z and
Ry values, interferometric techniques are performed by the interferometer
system 208 or 208' and values are determined by controller 220 (FIG. 2A).
Z can be determined by averging distances Z1 and Z2 and Ry can be
determined based on:
##EQU1##
In other embodiments, signals represent an interferometric measurement
based on either intensity, phase, distance, or the like of two related
beams (i.e., RSZ1 and RSZ2 or RSZ3 and RSZ4) being compared. A resulting
signal from the comparison corresponds to paramaters (e.g., position,
orientation, tilt, etc.) of either reticle stage 202 or reticle 204.
With reference to FIG. 3A, the calculation of Z and Ry is as follows for a
reticle 204 that lies on or parallel to the Y axis. In regards to Z, Z1 is
approximately equal to Z2 because reticle 204 lies in or parallel to the
Y-axis. Thus, Z.apprxeq.Z1.apprxeq.Z2. In regards to Ry, it is
substantially zero. This is because, if Z1.apprxeq.Z2, then
Z2-Z1.apprxeq.0.
With reference to FIG. 3B, the calculation of Z and RY is as follows for a
reticle that is rotated Ry around the Y axis. In regards to Z, it is equal
to (Z1+Z2)/2, or the average of the two values. In regards to Ry, it is
equal to (Z2-Z1)/L, as is shown in the equation above.
Therefore, in various embodiments, the four interferometer beams RSZ1-RSZ4
are used to determine two DOF (Z and Ry) of the patterned surface 206 of
reticle 204. In these embodiments, Z is a direction about normal to the
patterned surface 206 and parallel to the lithographic tool's optical
axis. Also, in these embodiments, Ry is a rotation about a scan axis of
the reticle stage 202. As described above, two interferometer beams (RSZ1
and RSZ2) reflect off of pattern surface 206 of reticle 204 on either side
of the pattern 206. These beams cannot be used during lithographic
printing because the reticle stage 202 has to travel (in the scan Y
direction shown as an arrow in FIGS. 2A and 2B) further than a physical
length of the reticle 204. This causes discontinuous signals from these
two interferometer beams (RSZ1 and RSZ2) as the beams run off of a reticle
surface. This discontinuity makes accurate stage control in Z and Ry
difficult to nearly impossible. Also, other masking functions at the
reticle focal plane (framing blades (not shown)) make the use of these two
beams (RSZ1 and RSZ2) impractical for control of reticle stage 202 under
lithography conditions because the blades will cut off the interferometer
beams (RSZ1 and RSZ2) every time a scan is made.
Also, in various embodiments, the other two interferometer beams (RSZ3 and
RSZ4) are positioned to reflect off of surfaces on the reticle stage 202.
There are numerous options for the configuration of these reflective
surfaces. In some embodiments, a first reflective surface (e.g., with
point 222) of reticle stage 202 can be oriented in or parallel to the X-Y
plane to give Z position feedback. Then, a second reflective surface
(e.g., with point 224) of reticle stage 202 can be oriented in or parallel
to the X-Y plane. Alternate configurations are possible where the second
reflective surface of reticle stage 202 can be oriented in or parallel to
a Y-Z plane. Then, the second surface yields Ry stage position
information. In further alternative embodiments, various other
orientations exist where calculations would yield Z and Ry values. The
lithographic tool would typically look at the difference between two
interferometers (e.g., dual interferometer 210 or interferometers 210A'
and 210B') with separation in either the X or Z directions, thus giving Ry
information.
FIGS. 4-6 show flowcharts of methods 400, 500, and 600 according to
embodiments of the present invention. A summary of those methods follows.
After loading reticle 204 (and occasionally during calibration or between
calibrations once or periodically) onto reticle stage 202 the data from
RSZ1 and RSZ2 can be used to locate the patterned surface 206 at a reticle
focal plane established by projection optics (not shown) of the
lithography tool or any other desired plane determined by machine setup.
Then, while reticle stage 202 is scanned in the Y direction so that
reticle 204 remains in the chosen plane, the values of RSZ3 and RSZ4 are
recorded and stored as a map. When the lithography tool is ready to do
exposures, the data from the map will be used to control the reticle stage
202, and thereby the reticle 204, in Z and Ry so that pattern 206 is
always in the chosen plane. Thus, even if beams RSZ1 and RSZ2 are
discontinuous due to running off of the reticle 204 at either end of the
scans, the stage control is not compromised because the control feedback
is coming from beams RSZ3 and RSZ4. In another embodiment, beams RSZ1 and
RSZ2 can be constantly monitored during lithography to verify the map and
to possibly do continuous updating of the map used for stage Z and Ry
control. It is to be appreciated that there are other ways of determining
stage position during scanning while maintaining pattern 206 of reticle
204 in a chosen plane, which are all contemplated by the invention.
FIG. 4 depicts a flowchart of method 400 according to embodiments of the
present invention (steps 402-410). At step 402, a reticle (e.g., reticle
204) is back clamped to a reticle stage (e.g., stage 202). At step 404, a
reticle focal plane is determined based on a first set of interferometric
measuring beams (e.g., RSZ1 and RSZ2). At step 406, a map of reticle stage
locations is determined during scanning of the reticle stage based on a
second set of interferometric measuring beams (e.g., RSZ3 and RSZ4). In
step 408, the measured reticle focal plane is correlated to the map of the
reticle stage. In step 410, the reticle stage is controlled based on the
correlation during exposure of a pattern on the reticle onto a wafer. The
exposure is accomplished through processes known in the art.
FIG. 5 depicts a flowchart of method 500 that can occur during step 406
according to embodiments of the present invention. At step 502, a first
beam (e.g., RSZ1) is reflected from a location (e.g., point 212) adjacent
a first side of a reticle pattern (e.g., pattern 206). At step 504, a
second beam (e.g., RSZ2) is reflected from a location (e.g., point 214)
adjacent a second side of the reticle pattern. At step 506, the two
reflected beams are detected in an interferometer (e.g., interferometer
208 or 208'). At step 508, an interferometric operation is performed
(e.g., in controller 220) on the received signals to determine a location
of the reticle pattern, and thus the reticle focus plane. At step 510,
location information is stored (e.g., in storage 218). At step 512, which
can be part of step 410, the location information is used (e.g., by stage
controller 228) to control a reticle stage (e.g., stage 202) during an
exposure process.
FIG. 6 depicts a flowchart of a method 600 that can occur during step 408
according to embodiments of the present invention. At step 602, a reticle
stage (e.g., stage 202) is scanned in a Y direction. At step 604, a first
measuring beam (e.g., RSZ3) is reflected off a point (e.g., point 222) on
the reticle stage that is parallel to or oriented in an X-Y plane. At step
606, a second measuring beam (e.g., RSZ4) is reflected off a point (e.g.,
point 224) on the reticle stage that is parallel to or oriented in the X-Y
or Y-Z plane. At step 608, the first and second measuring beams are
detected by an interferometer (e.g., interferometers 208 or 208'). At step
610, stage position information is determined (e.g., by processor 220)
based on interferometric values generated by the interferometer. At step
612, a map is generated (e.g., by controller 220) of the stage position
during the scan based on the interferometric values. At step 614, the map
is stored (e.g., in storage 218). At step 616, which can be part of step
410, data from the stored map is used (e.g., by stage controller 228) to
control the reticle stage during an exposure process.
FIG. 7 shows a portion 700 of a lithography tool used to measure stage 202
and reticle 204 positions according to embodiments of the present
invention. In this embodiment, although not shown, beams RSZ1-RSZ3 and
RSX1-RSX2 are produced by and detected by an interferometer similar to 208
or 208' discussed above, or any other interferometer. As discussed above,
RSZ1 and RSZ2 are used to determined characteristics about reticle 204 and
RSZ3 is used to determine Z of stage 202. RSX1 and RSX2 are used to
determined both an X position of stage 202 and Ry. Ry is determined by:
##EQU2##
FIG. 8 shows a portion 800 of a lithography tool used to measure stage 202
and reticle 204 positions according to embodiments of the present
invention. Again, in this embodiment, although not shown, beams RSZ1-RSZ5,
RSY1-RSY3, and RSX1 are produced by and detected by an interferometer
similar to 208 or 208' discussed above, or any other interferometer. This
embodiment shows beams that can enable determination of all six DOF for
stage 202 and/or reticle 204. Beams RSZ1 and RSZ2 allow for Z and Ry of
reticle 204 to be determined. Beams RSZ1 and RSZ5 allows for Rx of reticle
204 to be determined. Beams RSZ3 and RSZ4 allow for Z and Ry of stage 202
to be determined. Beam RSX1 allows for X of stage 202 to be determined.
Beam RSY1, RSY2, and/or RSY3 allow for Y of stage 202 to be determined.
Beams RSY2 and RSY3 allow for Rz of stage 202 to be determined. Beams RSY1
and RSY3 allow for Rx of stage 202 to be determined. These determination
are made based on the above formulas, similar formulas to the above, or
any other known interferometric formulas.
FIG. 9A shows a portion 900 of a lithography tool according to embodiments
of the present invention. Portion 900 includes reticle 204 that is clamped
at its sides to stage 902. In some embodiments, reticle 204 can be coupled
to a support device (e.g., a stiffener) 904 to counteract any warping
force on reticle 202. Beams RSZ1-RSZ4 can be used as described above to
determine Z and Ry of stage 902 and/or reticle 204.
FIG. 9B shows a portion 920 of a lithography tool according to embodiments
of the present invention. Portion 920 includes reticle 204 that is front
clamped to stage 922. In some embodiments, reticle 204 can be coupled to
support device 904 to counteract any warping force on reticle 202. Beams
RSZ1-RSZ4 can be used as described above to determined Z and Ry of stage
922 and/or reticle 204.
Conclusion
While various embodiments of the present invention have been described
above, it should be understood that they have been presented by way of
example only, and not limitation. It will be apparent to persons skilled
in the relevant art that various changes in form and detail can be made
therein without departing from the spirit and scope of the invention.
Thus, the breadth and scope of the present invention should not be limited
by any of the above-described exemplary embodiments, but should be defined
only in accordance with the following claims and their equivalents.
*
