U.S. patent application number 10/946334 was filed with the patent office on 2006-03-23 for lithographic apparatus, alignment system, and device manufacturing method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Arie Jeffrey Den Boef, Andre Bernardus Jeunink, Stefan Carolus Jacobus Antonius Keij, Henricus Petrus Maria Pellemans, Irwan Dani Setija, Cas Johannes Petrus Maria Van Nuenen.
Application Number | 20060061743 10/946334 |
Document ID | / |
Family ID | 36073568 |
Filed Date | 2006-03-23 |
United States Patent
Application |
20060061743 |
Kind Code |
A1 |
Den Boef; Arie Jeffrey ; et
al. |
March 23, 2006 |
Lithographic apparatus, alignment system, and device manufacturing
method
Abstract
A lithographic apparatus according to one embodiment includes an
alignment system for aligning a substrate. The alignment system
comprises an illuminator system configured to illuminate an
alignment mark on the substrate with an illumination spot, the
alignment mark comprising a plurality of lines and spaces. The
system also includes a combiner system configured to transfer
two-images of the illuminated alignment mark without spatial
filtering of the images, rotate the images 180.degree. relatively
to each other, and combine the two images; and a detection system
configured to detect an alignment signal from the combined images
and to determine a unique alignment position by selecting a
specific one of extreme values in the detected alignment
signal.
Inventors: |
Den Boef; Arie Jeffrey;
(Waalre, NL) ; Jeunink; Andre Bernardus; (Bergeyk,
NL) ; Maria Pellemans; Henricus Petrus; (Veldhoven,
NL) ; Setija; Irwan Dani; (Utrecht, NL) ; Van
Nuenen; Cas Johannes Petrus Maria; (Eindhoven, NL) ;
Keij; Stefan Carolus Jacobus Antonius; (Breda, NL) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
36073568 |
Appl. No.: |
10/946334 |
Filed: |
September 22, 2004 |
Current U.S.
Class: |
355/53 ;
355/55 |
Current CPC
Class: |
G03F 9/7092 20130101;
G03F 9/7088 20130101 |
Class at
Publication: |
355/053 ;
355/055 |
International
Class: |
G03B 27/42 20060101
G03B027/42 |
Claims
1. A lithographic apparatus comprising: an illumination system
configured to condition a radiation beam; a support constructed to
support a patterning device, the patterning device being capable of
imparting the radiation beam with a pattern in its cross-section to
form a patterned radiation beam; a substrate table constructed to
hold a substrate; a projection system configured to project the
patterned radiation beam onto a target portion of the substrate;
and an alignment system configured to illuminate an alignment mark
on the substrate with an illumination spot, the alignment mark
comprising a plurality of lines and spaces, the alignment system
comprising: a combiner system configured to transfer two images of
the illuminated alignment mark without spatial filtering of the
images, rotate the images 180.degree. relatively to each other, and
combine the two images, and a detection system configured to detect
an alignment signal from the combined images and to determine a
unique alignment position by selecting one of a plurality of
extreme values in the detected alignment signal.
2. The lithographic apparatus according to claim 1, wherein the
length of a line segment passing through the center of the
illumination spot and terminating at the periphery of the
illumination spot is less than the length of the alignment mark in
a direction of a scribe lane of the substrate.
3. The lithographic apparatus according to claim 1, wherein the
alignment mark comprises N pairs of lines and spaces with a
periodicity p, and wherein a width of the illuminating spot, as
measured through the center of the illumination spot and in the
direction of a scribe lane of the substrate, is substantially
equivalent to M pairs of lines and spaces, and wherein the detected
alignment signal includes a plurality of global extreme values
having substantially equal amplitudes, and wherein the selected
extreme value is one of the plurality of global extreme values.
4. The lithographic apparatus according to claim 1, wherein the
alignment mark comprises a combination of a first set of N.sub.1
pairs of lines and spaces with a periodicity p, and a second set of
N.sub.2 pairs of lines and spaces with the periodicity p, and
wherein the first and second sets are separated by a space having a
width greater than one of said spaces of the first set.
5. The lithographic apparatus according to claim 4, wherein N.sub.2
is less than N.sub.1, and wherein a width of the illuminating spot,
as measured through the center of the illumination spot and in the
direction of a scribe lane of the substrate, is substantially
equivalent to M pairs of lines and spaces, and wherein the detected
alignment signal includes a plurality of global extreme values
having substantially equal amplitudes, and wherein the detection
system is configured to select the largest extreme value in a
segment of the detected alignment signal corresponding to the
second set of N.sub.2 pairs, and wherein the detection system is
configured to select the one of a plurality of extreme values using
the selected largest extreme value as a reference.
6. The lithographic apparatus according to claim 4, wherein N.sub.2
is equal to N.sub.1, and wherein a width of the illuminating spot,
as measured through the center of the illumination spot and in the
direction of a scribe lane of the substrate, is substantially
equivalent to M pairs of lines and spaces, M being less than
(N.sub.1+N.sub.2), and wherein the detected alignment signal
includes two segments having global extreme values and a local
segment between the two segments, and wherein the alignment system
is arranged to select a local extreme value in the local segment as
the selected extreme value.
7. The lithographic apparatus according to claim 1, wherein the
alignment mark comprises n groups of m pairs of lines and spaces,
each of the n groups having a periodicity p, and wherein the
detected alignment signal includes 2n-1 characteristic signal
segments and a plurality of global extreme values, and wherein each
of the plurality of global extreme values has substantially the
same amplitude, and wherein the selected extreme value is one of
the plurality of global extreme values.
8. The lithographic apparatus according to claim 1, wherein the
alignment mark comprises: a first alignment mark structure in a
direction of a scribe lane of the substrate, and a second alignment
mark structure comprising a set of n groups of m pairs of lines and
spaces, each of the n groups having a periodicity p in a direction
at a angle in the range of from zero to ninety degrees, inclusive,
to the scribe lane direction, and wherein the alignment system is
arranged to detect the alignment signal while scanning the
alignment mark at the angle to the scribe lane direction.
9. The lithographic apparatus according to claim 1, wherein a
length of the alignment mark is substantially equal to eighty
microns, a width of the alignment mark is substantially equal to
forty microns, and a periodicity of the alignment mark is
substantially equal to one micron.
10. The lithographic apparatus according to claim 1, wherein the
alignment system is arranged to obtain an initial position of the
alignment mark for aligning the illumination spot with the
alignment mark.
11. The lithographic apparatus according to claim 1, wherein the
combiner system comprises a set of optical elements configured to
transfer two images of the illuminated alignment mark without
spatial filtering of the images, rotate the images 180.degree.
relatively to each other, and combine the two images.
12. The lithographic apparatus according to claim 1, wherein the
detection system comprises a detector configured to detect an
alignment signal from the combined images and a signal analyzer
configured to determine a unique alignment position by selecting
one of a plurality of extreme values in the detected alignment
signal.
13. An alignment system for aligning a substrate comprising: a spot
source configured to illuminate an alignment mark on the substrate
with an illumination spot; a combiner system configured to transfer
two images of the illuminated alignment mark without spatial
filtering of the images, rotate the images 180.degree. relatively
to each other, and combine the two images; and a detection system
configured to detect an alignment signal from the combined images
and to determine a unique alignment position by selecting one of a
plurality of extreme values in the detected alignment signal.
14. The alignment system according to claim 13, wherein the length
of a line segment passing through the center of the illumination
spot and terminating at the periphery of the illumination spot is
less than the length of the alignment mark in a direction of a
scribe lane of the substrate.
15. The alignment system according to claim 13, wherein an area of
the illumination spot is less than an area of the alignment
mark.
16. The alignment system according to claim 13, wherein the
alignment mark comprises N pairs of lines and spaces with a
periodicity p, and wherein the detected alignment signal has a
plurality of global extreme values having substantially equal
amplitudes, and wherein the selected extreme value is one of the
plurality of global extreme values.
17. The alignment system according to claim 16, wherein a width of
the illuminating spot, as measured through the center of the
illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces.
18. The alignment system according to claim 13, wherein the
alignment mark comprises a combination of a first set of N.sub.1
pairs of lines and spaces with a periodicity p, and a second set of
N.sub.2 pairs of lines and spaces with the periodicity p, and
wherein the first and second sets are separated by a space.
19. The alignment system according to claim 18, wherein the space
separating the first and second sets has a width greater than one
of said spaces of the first set.
20. The alignment system according to claim 18, wherein N.sub.2 is
less than N.sub.1, and wherein the detection system is configured
to select the largest extreme value in a segment of the detected
alignment signal corresponding to the second set of N.sub.2 pairs,
and wherein the detection system is configured to select the one of
a plurality of extreme values using the selected largest extreme
value as a reference.
21. The alignment system according to claim 20, wherein a width of
the illuminating spot, as measured through the center of the
illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces, and wherein the detected alignment signal includes a
plurality of global extreme values having substantially equal
amplitudes.
22. The alignment system according to claim 18, wherein N.sub.2 is
equal to N.sub.1, and wherein the detected alignment signal
includes two segments having global extreme values and a local
segment between the two segments, and wherein the alignment system
is arranged to select a local extreme value in the local segment as
the selected extreme value.
23. The alignment system according to claim 22, wherein a width of
the illuminating spot, as measured through the center of the
illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces, M being less than (N.sub.1+N.sub.2).
24. The alignment system according to claim 13, wherein the
alignment mark comprises n groups of m pairs of lines and spaces,
each of the n groups having a periodicity p, and wherein the
detected alignment signal includes 2n-1 characteristic signal
segments and a plurality of global extreme values, and wherein the
selected extreme value is one of the plurality of global extreme
values.
25. The alignment system according to claim 24, wherein each of the
plurality of global extreme values has substantially the same
amplitude.
26. The alignment system according to claim 13, wherein the
alignment mark comprises: a first alignment mark structure in a
direction of a scribe lane of the substrate, and a second alignment
mark structure comprising a set of n groups of m pairs of lines and
spaces, each of the n groups having a periodicity p in a direction
at a angle in the range of from zero to ninety degrees, inclusive,
to the scribe lane direction, and wherein the alignment system is
arranged to detect the alignment signal while scanning the
alignment mark at the angle to the scribe lane direction.
27. The alignment system according to claim 13, wherein a length of
the alignment mark is substantially equal to eighty microns, a
width of the alignment mark is substantially equal to forty
microns, and a periodicity of the alignment mark is substantially
equal to one micron.
28. The alignment system according to claim 13, wherein the
alignment system is arranged to obtain an initial position of the
alignment mark for aligning the illumination spot with the
alignment mark.
29. The alignment system according to claim 13, wherein the
combiner system comprises a set of optical elements configured to
transfer two images of the illuminated alignment mark without
spatial filtering of the images, rotate the images 180.degree.
relatively to each other, and combine the two images.
30. The alignment system according to claim 13, wherein the
detection system comprises a detector configured to detect an
alignment signal from the combined images and a signal analyzer
configured to determine a unique alignment position by selecting
one of a plurality of extreme values in the detected alignment
signal.
31. A device manufacturing method comprising: projecting a
patterned beam of radiation onto a substrate; illuminating an
alignment mark in a scribe lane on the substrate with an
illumination spot, the alignment mark comprising a plurality of
lines and spaces; transferring the images of the illuminated
alignment mark without spatial filtering of the images; combining
two images of the alignment mark that are rotated by 180.degree.
relative to each other; and detecting an alignment signal from the
combined images and determining a unique alignment position by
selecting an extreme value in the detected alignment signal.
32. The device manufacturing method according to claim 31, wherein
the dimension of the illumination spot is smaller than the length
of the alignment mark in a scribe lane direction.
33. The device manufacturing method according to claim 31, wherein
an area of the illumination spot is less than an area of the
alignment mark.
34. The device manufacturing method according to claim 31, wherein
the alignment mark comprises N pairs of lines and spaces with a
periodicity p, and wherein the detected alignment signal includes a
plurality of global extreme values having substantially equal
amplitudes, and wherein the selected extreme value is one of the
plurality of global extreme values.
35. The device manufacturing method according to claim 34, wherein
a width of the illuminating spot, as measured through the center of
the illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces.
36. The device manufacturing method according to claim 31, wherein
the alignment mark comprises a combination of a first set of
N.sub.1 pairs of lines and spaces with a periodicity p, and a
second set of N.sub.2 pairs of lines and spaces with the
periodicity p, and wherein the first and second sets are separated
by a space.
37. The device manufacturing method according to claim 36, wherein
the space separating the first and second sets has a width greater
than one of said spaces of the first set.
38. The lithographic apparatus according to claim 36, wherein
N.sub.2 is less than N.sub.1, and wherein the detection system is
configured to select the largest extreme value in a segment of the
detected alignment signal corresponding to the second set of
N.sub.2 pairs, and wherein the detection system is configured to
select the one of a plurality of extreme values using the selected
largest extreme value as a reference.
39. The device manufacturing method according to claim 38, wherein
a width of the illuminating spot, as measured through the center of
the illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces, and wherein the detected alignment signal includes a
plurality of global extreme values having substantially equal
amplitudes.
40. The device manufacturing method according to claim 36, wherein
N.sub.2 is equal to N.sub.1, and wherein the detected alignment
signal includes two segments having global extreme values and a
local segment between the two segments, and wherein the alignment
system is arranged to select a local extreme value in the local
segment as the selected extreme value.
41. The device manufacturing method according to claim 40, wherein
a width of the illuminating spot, as measured through the center of
the illumination spot and in the direction of a scribe lane of the
substrate, is substantially equivalent to M pairs of lines and
spaces, M being less than (N.sub.1+N.sub.2).
42. The device manufacturing method according to claim 31, wherein
the alignment mark comprises n groups of m pairs of lines and
spaces, each of the n groups having a periodicity p, and wherein
the detected alignment signal includes 2n-1 characteristic signal
segments and a plurality of global extreme values, and wherein the
selected extreme value is one of the plurality of global extreme
values.
43. The device manufacturing method according to claim 42, wherein
each of the plurality of global extreme values has substantially
the same amplitude.
44. The device manufacturing method according to claim 31, wherein
the alignment mark comprises: a first alignment mark structure in a
direction of a scribe lane of the substrate, and a second alignment
mark structure comprising a set of n groups of m pairs of lines and
spaces, each of the n groups having a periodicity p in a direction
at a angle in the range of from zero to ninety degrees, inclusive,
to the scribe lane direction, and wherein the alignment system is
arranged to detect the alignment signal while scanning the
alignment mark at the angle to the scribe lane direction.
45. The device manufacturing method according to claim 31, wherein
a length of the alignment mark is substantially equal to eighty
microns, a width of the alignment mark is substantially equal to
forty microns, and a periodicity of the alignment mark is
substantially equal to one micron.
46. The device manufacturing method according to claim 31, said
method comprising obtaining an initial position of the alignment
mark for aligning the illumination spot with the alignment
mark.
47. A method of aligning a substrate, said method comprising:
illuminating an alignment mark on the substrate with a spot, said
alignment mark including an alternating plurality of lines and
spaces; rotating at least one of two images of the illuminated mark
with respect to the other such that a rotation of one of the images
with respect to the other is substantially equal to
one-hundred-eighty degrees to produce a combined image, said
combined image being based on said two images subsequent to said
rotating; detecting an alignment signal based on the combined
image, said alignment signal including a plurality of locally
extreme values having substantially equal amplitudes; and selecting
one of a plurality of locally extreme values in the detected
alignment signal.
48. An alignment system comprising: a spot source configured to
illuminate an alignment mark on a substrate with an illumination
spot; a plurality of optical elements configured to rotate at least
one of two images of the illuminated mark with respect to the other
such that a rotation of one of the images with respect to the other
is substantially equal to one-hundred-eighty degrees to produce a
combined image, said combined image being based on said two images
subsequent to said rotating; a detector configured to detect an
alignment signal from the combined image, said detected alignment
signal including a plurality of locally extreme values having
substantially equal amplitudes; and a signal analyzer configured to
determine a unique alignment position by selecting one of a
plurality of locally extreme values in the detected alignment
signal.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a lithographic apparatus,
an alignment system, e.g. for use in a lithographic apparatus, and
a method for manufacturing a device.
BACKGROUND
[0002] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
[0003] An important step in a typical lithographic process is
aligning the substrate to the lithographic apparatus so that the
image of the mask pattern is projected at the correct position on
the substrate. Semiconductor, and other, devices manufactured by
lithographic techniques may require multiple exposures to form
multiple layers in the device, and it is may be essential that
these layers line up correctly. As ever smaller features are
imaged, overlay requirements, and hence the accuracy required of
the alignment process, become stricter.
[0004] In one known alignment system, described in EP-A-0,906,590
which document is hereby incorporated by reference, marks on the
substrate comprise two pairs of reference gratings, one X and one
Y, with the two gratings of the pair having slightly different
periods. The gratings are illuminated with spatially coherent light
and the diffracted light is collected and imaged on a detector
array, the different diffraction orders having been separated so
that corresponding positive and negative orders interfere. Each
detector in the array comprises a reference grating and a photo
detector. As the substrate is scanned, the output of the detector
varies sinusoidally. When the signals from both gratings of a pair
peak simultaneously, the mark is aligned. This type of system
provides a large dynamic range and by using high diffraction
orders, is relatively insensitive to mark asymmetry. However, the
need to provide two gratings with different periods increases the
amount of space required for the alignment marks on the substrate.
It is desirable to minimize the amount of such "silicon real
estate" devoted to alignment marks and therefore not available for
production of devices, or for other purposes.
[0005] Another known alignment system, described in EP-A-1,148,390
which document is hereby incorporated by reference, uses a compact
self-referencing interferometer to generate two overlapping images
rotated over +90.degree. and -90.degree. which are then made to
interfere in a pupil plane. An optical system and (optional)
spatial filter selects and separates the first order beams and
re-images them on a detector. The system described in
EP-A-1,148,390 utilizes a special technique, also described as
self-referencing on center of symmetry of an alignment mark. Also,
this alignment system only uses the envelope of the detected signal
to determine the correct alignment position.
SUMMARY
[0006] A lithographic apparatus according to one embodiment
comprises an illumination system configured to condition a
radiation beam; a support constructed to support a patterning
device, the patterning device being capable of imparting the
radiation beam with a pattern in its cross-section to form a
patterned radiation beam; a substrate table constructed to hold a
substrate; a projection system configured to project the patterned
radiation beam onto a target portion of the substrate; and an
alignment system configured to illuminate an alignment mark on the
substrate with an illumination spot, the alignment mark comprising
a plurality of lines and spaces. The alignment system comprises a
combiner system configured to transfer two images of the
illuminated alignment mark without spatial filtering of the images,
rotate the images 180.degree. relatively to each other, and combine
the two images, and a detection system configured to detect an
alignment signal from the combined images and to determine a unique
alignment position by selecting one of a plurality of extreme
values in the detected alignment signal.
[0007] According to another embodiment, an alignment system for
aligning a substrate comprises a spot source configured to
illuminate an alignment mark on the substrate with an illumination
spot; a combiner system configured to transfer two images of the
illuminated alignment mark without spatial filtering of the images,
rotate the images 180.degree. relatively to each other, and combine
the two images; and a detection system configured to detect an
alignment signal from the combined images and to determine a unique
alignment position by selecting one of a plurality of extreme
values in the detected alignment signal.
[0008] A device manufacturing method according to a further
embodiment comprises projecting a patterned beam of radiation onto
a substrate; illuminating an alignment mark in a scribe lane on the
substrate with an illumination spot, the alignment mark comprising
a plurality of lines and spaces; transferring the images of the
illuminated alignment mark without spatial filtering of the images;
combining two images of the alignment mark that are rotated by
180.degree. relative to each other; detecting an alignment signal
from the combined images; and determining a unique alignment
position by selecting an extreme value in the detected alignment
signal.
[0009] A method of aligning a substrate according to a further
embodiment comprises illuminating an alignment mark on the
substrate with a spot, said alignment mark including an alternating
plurality of lines and spaces; rotating at least one of two images
of the illuminated mark with respect to the other such that a
rotation of one of the images with respect to the other is
substantially equal to one-hundred-eighty degrees to produce a
combined image, said combined image being based on said two images
subsequent to said rotating; detecting an alignment signal based on
the combined image, said alignment signal including a plurality of
locally extreme values having substantially equal amplitudes; and
selecting one of a plurality of locally extreme values in the
detected alignment signal.
[0010] An alignment system according to a further embodiment
comprises a spot source configured to illuminate an alignment mark
on a substrate with an illumination spot; a plurality of optical
elements configured to rotate at least one of two images of the
illuminated mark with respect to the other such that a rotation of
one of the images with respect to the other is substantially equal
to one-hundred-eighty degrees to produce a combined image, said
combined image being based on said two images subsequent to said
rotating; a detector configured to detect an alignment signal from
the combined image, said detected alignment signal including a
plurality of locally extreme values having substantially equal
amplitudes; and a signal analyzer configured to determine a unique
alignment position by selecting one of a plurality of locally
extreme values in the detected alignment signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0012] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0013] FIG. 2 shows a schematic view of an alignment system of the
lithographic apparatus of FIG. 1;
[0014] FIG. 3 shows a schematic view of an alignment mark according
to an embodiment of the present invention;
[0015] FIG. 4 shows a graph of a detected intensity signal versus
position of the alignment system associated with the alignment mark
shown in FIG. 3;
[0016] FIG. 5 shows a schematic view of an alignment mark according
to a second embodiment of the present invention;
[0017] FIG. 6 shows a graph of a detected intensity signal versus
position of the alignment system associated with the alignment mark
shown in FIG. 5;
[0018] FIG. 7 shows a schematic view of an alignment mark according
to a third embodiment of the present invention;
[0019] FIG. 8 shows a graph of a detected intensity signal versus
position of the alignment system associated with the alignment mark
shown in FIG. 7;
[0020] FIG. 9 shows a schematic view of an alignment mark according
to a fourth embodiment of the present invention;
[0021] FIG. 10 shows a graph of a detected intensity signal versus
position of the alignment system associated with the alignment mark
shown in FIG. 9;
[0022] FIG. 11 shows a schematic view of an alignment mark
according to a fifth embodiment of the present invention;
[0023] FIG. 12 shows a schematic view of the alignment mark of FIG.
11 as perceived by the alignment system;
[0024] FIG. 13 shows a graph of a detected intensity signal versus
position of the alignment system associated with the alignment mark
shown in FIG. 11.
DETAILED DESCRIPTION
[0025] At least some embodiments of the invention may be applied to
provide a lithographic apparatus which comprises an alignment
system that delivers an accurate and unambiguous alignment
position.
[0026] In at least one application of a lithographic apparatus and
alignment system according to an embodiment of the invention, no
separate sensor nor marks are needed solely for capturing.
[0027] FIG. 1 schematically depicts a lithographic apparatus
according to one embodiment of the invention. The apparatus
comprises:
[0028] an illumination system (illuminator) IL configured to
condition a radiation beam B (e.g. UV radiation or radiation).
[0029] a support structure (e.g. a mask table) MT constructed to
support a patterning device (e.g. a mask) MA and connected to a
first positioner PM configured to accurately position the
patterning device in accordance with certain parameters;
[0030] a substrate table (e.g. a wafer table) WT constructed to
hold a substrate (e.g. a resist coated wafer) W and connected to a
second positioner PW configured to accurately position the
substrate in accordance with certain parameters; and
[0031] a projection system (e.g. a refractive projection lens
system) PS configured to project a pattern imparted to the
radiation beam B by patterning device MA onto a target portion C
(e.g. comprising one or more dies) of the substrate W.
[0032] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
[0033] The support structure supports, i.e. bears the weight of,
the patterning device. It holds the patterning device in a manner
that depends on the orientation of the patterning device, the
design of the lithographic apparatus, and other conditions, such as
for example whether or not the patterning device is held in a
vacuum environment. The support structure can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure may be a frame or a table,
for example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
[0034] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0035] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0036] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0037] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0038] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more mask tables).
In such "multiple stage" machines the additional tables may be used
in parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for
exposure.
[0039] The lithographic apparatus may also be of a type wherein at
least a portion of the substrate may be covered by a liquid having
a relatively high refractive index, e.g. water, so as to fill a
space between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
[0040] Referring to FIG. 1, the illuminator IL receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system BD comprising, for example, suitable directing
mirrors and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0041] The illuminator IL may comprise an adjuster AD for adjusting
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0042] The radiation beam B is incident on the patterning device
(e.g., mask MA), which is held on the support structure (e.g., mask
table MT), and is patterned by the patterning device. Having
traversed the mask MA, the radiation beam B passes through the
projection system PS, which focuses the beam onto a target portion
C of the substrate W. With the aid of the second positioner PW and
position sensor IF (e.g. an interferometric device, linear encoder
or capacitive sensor), the substrate table WT can be moved
accurately, e.g. so as to position different target portions C in
the path of the radiation beam B. Similarly, the first positioner
PM and another position sensor (which is not explicitly depicted in
FIG. 1) can be used to accurately position the mask MA with respect
to the path of the radiation beam B, e.g. after mechanical
retrieval from a mask library, or during a scan. In general,
movement of the mask table MT may be realized with the aid of a
long-stroke module (coarse positioning) and a short-stroke module
(fine positioning), which form part of the first positioner PM.
Similarly, movement of the substrate table WT may be realized using
a long-stroke module and a short-stroke module, which form part of
the second positioner PW. In the case of a stepper (as opposed to a
scanner) the mask table MT may be connected to a short-stroke
actuator only, or may be fixed. Mask MA and substrate W may be
aligned using mask alignment marks M1, M2 and substrate alignment
marks P1, P2. Although the substrate alignment marks as illustrated
occupy dedicated target portions, they may be located in spaces
between target portions (these are known as scribe-lane alignment
marks). Similarly, in situations in which more than one die is
provided on the mask MA, the mask alignment marks may be located
between the dies.
[0043] The depicted apparatus could be used in at least one of the
following modes:
[0044] 1. In step mode, the mask table MT and the substrate table
WT are kept essentially stationary, while an entire pattern
imparted to the radiation beam is projected onto a target portion C
at one time (i.e. a single static exposure). The substrate table WT
is then shifted in the X and/or Y direction so that a different
target portion C can be exposed. In step mode, the maximum size of
the exposure field limits the size of the target portion C imaged
in a single static exposure.
[0045] 2. In scan mode, the mask table MT and the substrate table
WT are scanned synchronously while a pattern imparted to the
radiation beam is projected onto a target portion C (i.e. a single
dynamic exposure). The velocity and direction of the substrate
table WT relative to the mask table MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
[0046] 3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning device, and the
substrate table WT is moved or scanned while a pattern imparted to
the radiation beam is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning device is updated as required after each
movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
[0047] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0048] In applications including alignment of a wafer in a
lithographic apparatus, an alignment mark according to an
embodiment of the present invention can be used for both coarse and
fine wafer alignment. Such a use may result in less space on the
wafer (real estate) being required for alignment purposes because
one doesn't need separate marks for both phases. The fine wafer
alignment will lead to a correct determination of alignment
position without any ambiguity which is present in certain state of
the art alignment systems. No additional gratings with a different
periodicity, used especially for capturing, may be needed,
resulting in less required real estate. Throughput costs of
operational use (i.e. time spent for alignment of wafers) are not
negatively influenced, as no additional capture gratings or marks
have to be scanned. Also, the need for a separate capture sensor in
a lithographic apparatus is eliminated.
[0049] The present apparatus and method can be advantageously
applied when the alignment system is an alignment system not using
spatial filtering. As a result of not using spatial filtering
techniques in the alignment, sharp images are possible from the
alignment mark for further processing. An example of such an
alignment system not using spatial filtering is described in patent
application EP-A-1,148,390, which description is incorporated
herein by reference. The alignment system comprises an alignment
sensor that is self referencing to the center of symmetry of any
mark having 180.degree. of symmetry. The image of the alignment
mark is divided into two images, which are then rotated 180.degree.
with respect to each other, and are then coherently recombined with
an interferometer. With proper phasing of the interferometer paths,
the recombined image will interfere constructively or destructively
in amplitude. A prism may be used to form the two images, rotate
the two images with respect to each other, and interferometrically
recombine the two images.
[0050] In FIG. 2, a simplified schematic diagram is shown of an
alignment system 10 as used in the lithographic apparatus according
to one embodiment of the present invention. Alignment system 10
comprises an illumination source 12, such as a laser, providing
electromagnetic radiation 13, to a beam splitter 14. A portion of
the electromagnetic radiation is reflected off coating 16 to
illuminate an alignment mark or target 18. The alignment mark 18 is
placed on or within a photosensitive substrate or wafer 20. The
photosensitive substrate 20 is placed on a stage 22. The stage 22
may be scanned in the direction indicated by arrow 24.
Electromagnetic radiation diffracted from the alignment mark 18
passes through the beam splitter 14 and is collected by the image
rotation device 26. It should be appreciated that a good quality
image need not be formed, but that the features of the alignment
mark 18 should be resolved. The image rotation device 26 may be any
appropriate set of optical elements, and is preferably a
combination of prisms, that forms two images of the alignment mark
18, rotates one of the images with respect to the other one hundred
and eighty degrees, and then recombines the two images. The optical
ray passing through the center of rotation established by the image
rotation device 26, defines the sensor alignment axis 27. Detector
28 receives the electromagnetic radiation from the image rotation
device 26. The detector 28 then provides signals to the signal
analyzer 30. The signal analyzer 30 is coupled to the stage 22 such
that the position of the stage is known (e.g. using a position
sensor 15) when the center of alignment mark 18 is determined.
Therefore, the position of the alignment mark 18 is very accurately
known with reference to the stage 22. Accordingly, the location of
the center of the alignment target 18 is known substantially
exactly relative to a reference position. Additionally, the center
of the mark may be very accurately determined even with a
relatively poor image.
[0051] It is noted that the embodiment depicted uses a beam
splitter 14 to direct the illuminating beam towards the alignment
mark 18, and to direct the image from the mark 18 towards the image
rotation device 26. It will be apparent to the skilled person, that
other (optical) arrangements may be used to obtain the same result
of illuminating the alignment mark 18 on the wafer 20 and detecting
an image of the alignment mark 18.
[0052] An optical arrangement as shown in FIG. 2 is described in
more detail with reference to a number of embodiments in the
European patent application EP-A-1,148,390, which description is
incorporated herein by reference. This document describes
embodiments of an alignment system with a coherent illumination
source, interferometric combination of an image pair, and a
detection system with detection based on interferometric
properties.
[0053] The detection system or detector 28 may be based on
intensity measurement of the combined image, e.g. a detector on
which the combined image is projected. The combination of the
rotated images can be regarded as a convolution measurement method,
as the images of the illuminated alignment mark 18 are overlaid on
each other when the alignment mark 18 moves with respect to the
illumination spot 7 (see below). In conventional alignment systems,
reference gratings are used on which a periodic alignment mark is
imaged, after which only certain grating orders are used for
detection. As no pupil filtering (or spatial filtering) is present
in an alignment system 10 according to at least some embodiments of
the present invention, all image information can be used. This has
as a further advantage that a capturing method according to at
least some embodiments of the present invention does not need any
intermediate steps: both fine wafer alignment and coarse wafer
alignment (when necessary) use the same kind of illumination and
detection. No intermediate steps like order filtering when using
gratings as alignment mark are necessary.
[0054] The dimension of the illumination spot 7 is smaller than the
length of the alignment mark 18 in a scribe lane direction in an
embodiment of the present invention. This will provide sufficient
signal data characteristics for an accurate and unambiguous
alignment. However, it is also possible that the illumination spot
7 is larger than the length of the alignment mark 18 in the scribe
lane direction.
[0055] A number of different alignment mark types may be used with
the present alignment system 10, and a number of different types
are described below.
[0056] In a particular embodiment the alignment mark 18 comprises N
pairs of lines and spaces with a predetermined periodicity p, the
illuminating spot 7 having a dimension in the scribe lane direction
equivalent to M pairs of lines and spaces, resulting in a detected
signal having a predetermined number of global extremes (e.g.
(2N-1)-2(M-1)) with substantially equal value, and the specific one
of extreme values is one of the global extremes. This embodiment
allows to use edge detection, but with a mesa type signal envelope.
All of the alignment mark 18 is used for (fine) alignment. As an
example, the middle global minimum can be selected.
[0057] In a further set of embodiments, the alignment mark 18
comprises a combination of a first set of N.sub.1 pairs of lines
and spaces with a predetermined periodicity, and a second set of
N.sub.2 pairs of lines and spaces with the predetermined
periodicity, the first and second set being separated by a space
having a predetermined distance. The space is e.g. equal to an
integer multiple of the periodicity pitch. Such a space may be used
to provide an additional feature in the alignment mark 18,
resulting in a distinct characteristic in the detected alignment
signal, which can further aid in determining an unambiguous
alignment position.
[0058] E.g., in a specific embodiment wherein N.sub.2<N.sub.1,
the illuminating spot 7 has a dimension in the scribe lane
direction equivalent to M pairs of lines and spaces, resulting in a
detected signal having a predetermined number of global extremes
with substantially equal value, and the second set of N.sub.2 pairs
causes a specific additional signal segment, in which the alignment
system is further arranged to first select the largest extreme
value in the additional signal segment, and then select the
specific one of extreme values using the first selected top as a
reference. This additional segment is easy to detect from the
associated additional characteristic in detected signal, thus
providing a more robust alignment system.
[0059] In another embodiment wherein N.sub.2=N.sub.1, the
illuminating spot 7 has a dimension in the scribe lane direction
equivalent to M pairs of lines and spaces, M being smaller than
(N.sub.1+N.sub.2), resulting in a detected signal having two
segments of global extremes and a local segment between the two
segments, and the alignment system is further arranged to select a
local extreme in the local segment as the specific one of extreme
values.
[0060] In a further embodiment of the alignment mark 18, sub
segmentation of a periodic grating is used, which may result in a
very specific pattern in the detected signal. The alignment mark 18
comprises n groups of m pairs of lines and spaces with a
predetermined periodicity, resulting in an alignment signal having
2n-1 characteristic signal segments, and the specific one of
extreme values is one of the global extremes. The characteristic
signal segments may e.g. comprise a diamond shape of falling minima
followed by rising minima. The illumination spot 7 has a dimension
in the scribe lane direction corresponding to k groups, k being an
integer, resulting in a very predictable expected shape of the
detected alignment signal.
[0061] In a further embodiment of the present invention the
alignment mark 18 comprises a first alignment mark structure in a
scribe lane direction, and a second alignment mark structure
comprising a set of n groups of m pairs of lines and spaces with a
predetermined periodicity in a direction at a angle between zero
and ninety degrees to the scribe lane direction (e.g.
perpendicular), and the alignment system is arranged to scan the
alignment mark at the angle to the scribe lane direction and to
determine the specific one of extreme values. This allows to
capture on the cross segmentation, saving scribe lane space, as no
separate marks are needed for X and Y direction alignment. Again,
optionally the illumination spot has a dimension in the scribe lane
direction corresponding to k groups.
[0062] In FIG. 3, a first embodiment of an alignment mark 18 is
shown. The alignment mark 18 is positioned in a scribe lane 5 on a
wafer 20 (of which die parts 6 are shown on both sides of the
scribe lane 5). In the FIG. 3, also the scribe lane direction
X.sup.+, and the direction Y.sup.+ perpendicular to the scribe lane
direction are indicated.
[0063] In a typical exemplary embodiment, the width of the scribe
lane 5 is about 40 .mu.m. The length of the alignment mark 18 is
about 80 .mu.m, and comprises a large number of lines and spaces
e.g. with a pitch of about 1 .mu.m. In general, the alignment mark
18 has a length l and a plurality of N line/space pairs or mark
elements with a pitch p. Thus, the number of line/space pairs N may
also be determined by dividing the length l by the pitch p.
[0064] The alignment system 10 produces an illumination spot 7 on
the wafer, and, due to the movement of the wafer stage 22, the
alignment mark 18 will travel in the direction indicated by the
arrow in FIG. 3 relative to the illumination spot 7. The
illumination spot 7 has a dimension which generally corresponds to
the scribe lane width, but is smaller than the length of the
alignment mark 18. In other words, the alignment mark 18 is longer
than the dimension of the illumination spot 7 in the scribe lane
direction X.sup.+.
[0065] When the alignment mark 18 moves under the illumination spot
7 in the scribe lane 5, the detector 28 will provide an intensity
signal as shown in FIG. 4. FIG. 4 shows an intensity signal value
of the detector 28 versus the position of the wafer stage 22 in the
X-direction. As the image rotation device 26 and detector 28
operate as a convolution detector, the signal will vary from
substantially zero when the illumination spot 7 is not illuminating
the alignment mark 18 to a maximum intensity value when the lines
and spaces of the two images are not aligned (i.e. the line of one
image overlaps a space of the other image). When a line of one
image overlaps a line of the other image, destructive interference
will cause a decrease of the detected intensity signal. Due to the
convolution type of signal detection, an alignment mark 18 having N
line/space pairs will result in an intensity signal having 2N-1
minimum values. Depending on how many line/space pairs of the two
formed images overlap, the intensity of the signal will be higher.
A global maximum value is obtained when the illumination spot 7 is
entirely over the alignment mark 18 (illumination spot 7
illuminates a maximum number of lines).
[0066] In an ideal embodiment, the illumination spot 7 has straight
leading and trailing edge in the scribe lane direction, which would
result in illumination of an integer number of lines of the
alignment mark 18. However, in more practical embodiments, the
illumination spot 7 will be a rounded or even substantially round
spot. When the alignment mark 18 is `hit` by the illumination spot
7, this will result in the fact that initially, not the entire
line(s) of the alignment mark 18 will be illuminated. However, this
will only result in the amplitude of the detected signal being
somewhat lower. The further description below is valid for both the
ideal embodiments and the practical embodiments of the illumination
spot 7.
[0067] When an alignment mark 18 as depicted in FIG. 3 is used, the
resulting detected intensity signal will have characteristic as
shown in FIG. 4: Of the total of 2N-1 extremes, a limited number of
extremes will have the global minimum value, resulting in the
mesa-type plot of FIG. 4. The slope of the envelope of the detected
signal will depend on the sensitivity of the detector 28, and on
the dimension of the illumination spot 7 (the more light hits the
mark 18, the larger the output signal of the detector can be). Note
that when the dimension of the illumination spot 7 in scribe lane
direction is large enough to cover the entire alignment mark 18,
the resulting detected signal will have a pyramid shaped
envelope.
[0068] When the alignment mark 18 is a periodic mark having N pairs
of lines and spaces (embodiment shown in FIG. 3), the middle
position of the alignment mark 18 with respect to the reference of
the alignment system (e.g. alignment axis 27 as shown in FIG. 2),
can be determined by the position Xa of the middle global minimum
in the intensity plot. When the illumination spot dimension in the
scribe lane direction corresponds to the dimension of M line/space
pairs (M<N), in general, there will be (2N-1)-2(M-1) global
extreme values (peaks) in the intensity plot.
[0069] It would also be possible to take the (2N-1)/2th minimum
value of all minimum values (peaks) present in the intensity plot.
However, due to noise and other artifacts, one or more of the small
value extremes at the start and end slope of the intensity plot may
be missed, resulting in an incorrect alignment position Xa. By only
looking at the global minimum values (i.e. the lows of the plot in
the mesa-like part of the envelope of the plot of FIG. 4), it is
possible to prevent this inaccuracy.
[0070] The number of extremes in the plot shown in FIG. 4 do not
correspond to the exemplary dimensions shown in FIG. 3 or to the
example dimensions described above for clarity reasons. When the
pitch of the alignment mark 18 is e.g. 1 .mu.m as described above,
the distance between two global minimum values in the plot of FIG.
4 will be 0.5 .mu.m.
[0071] Apart from picking the right top from the plot of FIG. 4, it
is also possible to use other characteristic features of this plot,
such as the slopes of the envelope of the plot of FIG. 4, or the
initial part of the detected signal without extremes.
[0072] In FIG. 5, a further embodiment of the alignment mark 18 is
shown, in which the alignment mark 18 comprises a first part 21,
having N.sub.1 pairs of lines and spaces, and a second part 23,
which comprises a set of N.sub.2 lines and spaces (e.g. N.sub.2=3,
second part 23 is a triplet), in which the first and second part
are separated by a space of at least one line and space pair. The
alignment mark 18 according to this embodiment can also be regarded
as having a length l, and having a plurality of line/space pairs
with a pitch p, in which a number of line/space pairs are left out.
Still, the detected signal will have a total of 2N-1 minima, in
which N=l/p, and N.sub.1+N.sub.2<N.
[0073] This alignment mark 18 will result in a detected signal
which is shown in FIG. 6. In this plot, a characteristic feature
caused by the triplet is visible: a diamond like envelope with
decreasing and increasing slope of the various minima. It may be
relatively easy to detect the deepest minimum of this diamond-like
envelope, and from this, the position where the alignment axis 27
of the illumination spot 7 is exactly over the center of the
triplet second part 23 is known. From this position, again, the
correct top of the set of global minima can be selected, depending
on the dimensions and properties of the alignment mark 18.
[0074] For the embodiments as described in this description, it is
also possible to use correlation techniques to determine the right
alignment position of the alignment mark 18 with respect to the
lithographic apparatus. It may then be desirable to take the
complete signal as obtained by the alignment system (i.e. the plots
as exemplified in FIG. 4, 6, 8, 10 and 13) and correlate or match
this signal with a predetermined signal (pattern), and to detect
the proper aligned position from this correlation.
[0075] When the total length of the alignment mark 18 in the
embodiment shown in FIG. 5 is equal to the length of the alignment
mark 18 in FIG. 3 (using the same pitch), the total number of
global minimum values will be smaller, but still it is possible to
provide an unambiguous alignment position.
[0076] The embodiment shown in FIG. 5 may suffer from the fact that
the characteristic signal envelope caused by the second part 23 has
a relatively low amplitude, which may make proper capturing
problematic. To solve this, the embodiment shown in FIG. 7 is
proposed. Here, the alignment mark 18 comprises two identical parts
25, e.g. each having n pairs of lines and spaces with a
predetermined pitch p (e.g. 1 .mu.m), which are separated in the
middle by a space region 27, having a dimension equal to a number
of lines and spaces with the same predetermined pitch p. In other
words, the alignment mark 18 may be regarded as a number of N line
and space pairs (N>2n), wherein a number of line and space pairs
are omitted in the middle. It is noted that in an alternative
embodiment, a so called phase jump mark 18 may be used also, in
which the two mark parts 25 have the same period, but a different
phase. A phase jump mark 18 is discussed in European patent
application EP-A-1,434,103, which is incorporated herein by
reference.
[0077] This type of mark 18 will result in a signal of the detector
28 as shown in FIG. 8. The missing lines in the middle of the
alignment mark 18 will result in a drop in the size of the minima
of the detected signal. When the number of pairs of lines and
spaces left out is not too high (e.g. 3 or 4 pairs), the resulting
plot will have distinct differences with the plot as shown in FIG.
4. The number of missing lines in relation to the dimension of the
illumination spot 7 in the scanning direction X.sup.+ will
determine which effect will be visible. The alignment position Xa
can then be determined by choosing the local extreme of the
envelope in the affected area of the signal plot of FIG. 8.
[0078] By determining a local extreme, it is easier to determine
the correct alignment position Xa when the detected signal
comprises noise (e.g. by contamination of alignment mark 18, or
non-zero scanning offset where the illumination spot 7 also
illuminates structures on the die 6 next to the scribe lane 5).
[0079] An even further embodiment of the present invention is shown
in FIG. 9. This embodiment will probably make the capture mechanism
even more robust. Here, the alignment mark 18 is a so called
"periodic grating with sub segmentation", having a number of
line/space groups 29 (nine groups being shown in the embodiment of
FIG. 9), with a first pitch distance p, in which each line/space
group 29 comprises a number (e.g. three as shown in FIG. 9:
triplets) of line and space pairs with a second pitch distance
p.sub.2. When the first pitch distance p is a multiple integer of
the second pitch distance p.sub.2, the resulting intensity signal
from the detector 28 will be as shown in FIG. 10. A number of
diamond like envelopes will result, in which the number of minima
depends on the number of line/space pairs in each line/space group,
and the distance between each group. Again, when l is the length of
the total alignment mark 18, the total number N of present or non
present line/space pairs is equal to l/p, which will result in a
total of 2N-1 minima (peaks) in the intensity signal. Also for the
second periodicity structure, having a pitch of p.sub.2, there will
be 2(l/p.sub.2)-1 characteristic features in the intensity
signal.
[0080] The embodiment shown in FIG. 9 has nine groups 29 of
triplets, and the illumination spot 7 has a dimension in the
X.sup.+ direction covering four groups as a maximum. This will
result in the intensity profile as shown in FIG. 10, having
seventeen series of rising and falling maxima, each series having
five minima, resulting in 85 peaks. The alignment mark of FIG. 9
comprises nine sub groups of each 3 line/space pairs, each sub
group being spaced by a distance equal to two line/space pairs.
Thus, in this case, N=l/p=8.times.5+3=43, and 2.times.43-1=85 peaks
are present in the intensity signal, in 2.times.9-1=17
characteristic signal features.
[0081] Of the seventeen series, eleven series are showing a global
minimum. In this case, the alignment position Xa can easily be
determined by picking the middle minimum.
[0082] A further embodiment of the present invention uses a
conventional alignment mark 18 augmented with a further alignment
mark structure having a periodic structure with sub segmentation.
An example of this embodiment is shown in FIG. 11, which shows the
alignment mark 18 positioned in the scribe lane 5 of a wafer in
between dies 6. In the X direction of the scribe lane 5, the
alignment mark 18 comprises a plurality of lines and spaces 32
having a set periodicity (same pitch). In the direction
perpendicular to that (Y direction), the alignment mark 18
comprises a cross-segmentation, which comprises a number of
line/space groups 33. In this example, each group 33 comprises
three lines with a predetermined pitch, and between each group 33
is a predetermined distance. The person skilled in the art will
understand that more groups 33 may be provided, as well as a
smaller or larger number of line/space pairs per group 33. Also, it
will be apparent that the cross-segmentation may also be under a
different angle than perpendicular. In general, the
cross-segmentation may be under an angle between substantially zero
and ninety degrees.
[0083] When the scribe lane is scanned by the illumination spot 7
in the Y.sup.+-direction, the alignment system 10 will actually
perceive an alignment mark 18 comparable to the alignment mark 18
as described with reference to FIG. 9 (see FIG. 12). The resulting
intensity signal as detected by detector 28 against the wafer stage
position is shown in FIG. 13. The signal shows a double period,
where the finer period is given by the fine separation of the
lines, while the coarser period is given by the separation of the
groups. Again, a number of diamond-like envelopes (of a series of
rising and falling minimum values) results, of which in this case
the lowest peak in the middle diamond-like envelope is chosen to
indicate the alignment position Ya in the Y.sup.+ direction. It is
however noted that a possibility exists that scan direction and
trajectory of the alignment system are aligned such that the
detected signal shows no features as discussed above.
[0084] This embodiment would e.g. allow to perform a coarse wafer
alignment on a set of two alignment marks 18, each having a
periodic structure (possibly with sub-segmentation) in both the X-
and Y-direction. The two structures in X-direction will provide an
initial alignment in Y-direction, and the two structures in
Y-direction will provide an initial alignment in X-direction. These
initial alignments provide a coarse wafer alignment grid, from
which the initial position for fine wafer alignment may be
determined using the same scan results.
[0085] To be able to use the alignment method as described with
reference to the above exemplary embodiments of the alignment mark
18, it may be necessary to have a proper starting position of the
illumination spot 7 with respect to the scribe lane 5. Therefore,
the alignment system 10 may be further arranged to obtain an
initial position of the alignment mark 18 for aligning the
illumination spot 7 with the alignment mark 18. Initial positioning
may already be obtained when the initial wafer load accuracy of the
lithographic apparatus is sufficiently accurate. Alternatively, the
initial positioning may be obtained using conventional coarse wafer
alignment methods, e.g. diagonal scan techniques (see e.g. the
description of such techniques in European patent application
EP-A-1,434,103, which description is incorporated herein by
reference).
[0086] The graphs of detected signals of FIGS. 4, 6, 8, 10 and 13
show the extremes of the detected signals as minimum values
(peaks). Depending on the specific structure of the alignment
sensor, the detected signal may be the result of destructive
interference of the two mutually rotated images, thus resulting in
minimum values as extremes. However, for the skilled person it will
be clear that alternatives are possible (e.g. resulting in a
constant value minus the signal as shown in the mentioned figures),
and that in the above description the term minimum (or minimum
value) may also be a maximum (or maximum value) according to the
specific circumstances.
[0087] A lithographic apparatus according to one embodiment
comprises an illumination system configured to condition a
radiation beam; a support constructed to support a patterning
device, the patterning device being capable of imparting the
radiation beam with a pattern in its cross-section to form a
patterned radiation beam; a substrate table constructed to hold a
substrate; a projection system configured to project the patterned
radiation beam onto a target portion of the substrate; and an
alignment system which is configured to illuminate an alignment
mark on the substrate with an illumination spot, the alignment mark
comprising a plurality of lines and spaces, the alignment system
comprising a combiner system which is configured to transfer two
images of the illuminated alignment mark without spatial filtering
of the images, rotate the images 180.degree. relatively to each
other, and combine the two images, and a detection system which is
configured to detect an alignment signal from the combined images
and to determine a unique alignment position by selecting a
specific one of extreme values in the detected alignment signal.
The determination of the unique (i.e. unambiguous) alignment
position may e.g. be accomplished using signal matching
techniques.
[0088] According to another embodiment, an alignment system for
aligning a substrate, e.g. in a lithographic apparatus, comprises
an illuminator system which is configured to illuminate an
alignment mark on the substrate with an illumination spot, the
alignment mark comprising a plurality of lines and spaces, a
combiner system which is configured to transfer two images of the
illuminated alignment mark without spatial filtering of the images,
rotate the images 180.degree. relatively to each other, and combine
the two images, and a detection system which is configured to
detect an alignment signal from the combined images and to
determine a unique alignment position by selecting a specific one
of extreme values in the detected alignment signal.
[0089] A device manufacturing method according to a further
embodiment comprises projecting a patterned beam of radiation onto
a substrate; illuminating an alignment mark in a scribe lane on the
substrate with an illumination spot, the alignment mark comprising
a plurality of lines and spaces, transferring the images of the
illuminated alignment mark without spatial filtering of the images,
combining two images of the alignment mark that are rotated by
180.degree. relative to each other; and detecting an alignment
signal from the combined images, and determining a unique alignment
position by selecting a specific one of extreme values in the
detected alignment signal.
[0090] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
[0091] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0092] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 355, 248, 193,
157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g.
having a wavelength in the range of 5-20 nm), as well as particle
beams, such as ion beams or electron beams.
[0093] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive, reflective, magnetic, electromagnetic and
electrostatic optical components.
[0094] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0095] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
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