U.S. patent application number 14/281346 was filed with the patent office on 2014-09-11 for tunable wavelength illumination system.
This patent application is currently assigned to ASML Holding N.V.. The applicant listed for this patent is ASML Holding N.V., ASML Netherlands B.V.. Invention is credited to Keith William ANDRESEN, Arie Jeffrey DEN BOEF, Earl William EBERT, JR., Harry SEWELL, Sanjeev Kumar SINGH.
Application Number | 20140253891 14/281346 |
Document ID | / |
Family ID | 43103053 |
Filed Date | 2014-09-11 |
United States Patent
Application |
20140253891 |
Kind Code |
A1 |
DEN BOEF; Arie Jeffrey ; et
al. |
September 11, 2014 |
TUNABLE WAVELENGTH ILLUMINATION SYSTEM
Abstract
A lithographic apparatus comprises an alignment system including
a tunable narrow pass-band filter configured to receive a
broad-band radiation and to filter the broad-band radiation into
narrow-band linearly polarized radiation. The tunable narrow
pass-band filter is further configured to modulate an intensity and
wavelength of the narrow-band radiation and to provide a plurality
of pass-band filters at a same time or nearly the same time. The
alignment system further includes a relay and mechanical interface
configured to receive the narrow-band radiation and to adjust a
profile of the narrow-band radiation based on physical properties
of alignment targets on a substrate. The adjusted narrow-band
radiation is focused on the alignment targets using a focusing
system.
Inventors: |
DEN BOEF; Arie Jeffrey;
(Waalre, NL) ; EBERT, JR.; Earl William; (Oxford,
CT) ; SEWELL; Harry; (Ridgefield, CT) ;
ANDRESEN; Keith William; (Wilton, CT) ; SINGH;
Sanjeev Kumar; (Danbury, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Holding N.V.
ASML Netherlands B.V. |
Veldhoven
Veldhoven |
|
NL
NL |
|
|
Assignee: |
ASML Holding N.V.
Veldhoven
NL
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
43103053 |
Appl. No.: |
14/281346 |
Filed: |
May 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13898973 |
May 21, 2013 |
8730476 |
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14281346 |
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12751479 |
Mar 31, 2010 |
8508736 |
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13898973 |
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61168095 |
Apr 9, 2009 |
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Current U.S.
Class: |
355/55 ;
359/285 |
Current CPC
Class: |
G03F 7/70141 20130101;
G03F 9/7065 20130101; G01B 11/14 20130101; G02F 1/116 20130101;
G03F 7/70191 20130101 |
Class at
Publication: |
355/55 ;
359/285 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G02F 1/11 20060101 G02F001/11 |
Claims
1. An alignment system comprising: a tunable narrow pass-band
filter configured to: receive broad-band radiation, and filter the
broad-band radiation into narrow-band radiation; a relay and
mechanical interface configured to: receive the narrow-band
radiation, and adjust a profile of the narrow-band radiation based
on a physical property of a plurality of alignment targets on a
substrate; and a focusing system configured to focus the adjusted
narrow-band radiation on the plurality of alignment targets.
2. The alignment system of claim 1, wherein the tunable narrow
pass-band filter is configured to filter the broad-band radiation
into narrow-band linearly polarized radiation.
3. The alignment system of claim 1, wherein the tunable narrow
pass-band filter is further configured to select a plurality of
narrow-band wavelength set points at a same time or nearly the same
time.
4. The alignment system of claim 1, wherein the tunable narrow
pass-band filter is further configured to modulate intensity and
wavelength of the broad-band radiation.
5. The alignment system of claim 1, wherein the tunable narrow
pass-band filter is further configured to provide a plurality of
pass-band filters at a same time or nearly the same time.
6. The alignment system of claim 1, wherein the tunable narrow
pass-band filter is an Acousto-Optical Tunable Filter (AOTF).
7. The alignment system of claim 1, wherein the narrow-band
radiation has a mean wavelength of approximately 980 nm.
8. The alignment system of claim 1, wherein the broad-band
radiation ranges from 450 nm to 2500 nm.
9. A method to align a substrate in a lithographic apparatus, the
method comprising: filtering, using a tunable narrow pass-band
filter, a broad-band radiation to generate a narrow-band polarized
radiation; receiving, using a relay and mechanical interface, the
narrow-band polarized radiation from the tunable narrow pass-band
filter; adjusting a profile of the narrow-band polarized radiation;
and focusing the adjusted narrow-band polarized radiation on an
alignment target on the substrate.
10. The method of claim 9, further comprising: selecting, using the
tunable narrow pass-band filter, a plurality of narrow-band
wavelength set points at a same time or nearly the same time.
11. The method of claim 9, further comprising: modulating, using
the tunable narrow pass-band filter, intensity and wavelength of
the broad-band radiation.
12. The method of claim 9, further comprising: providing, using the
tunable narrow pass-band filter, a plurality of pass-band filters
at a same time or nearly the same time.
13. The method of claim 9, wherein the tunable narrow pass-band
filter is an Acousto-Optical Tunable Filter (AOTF).
14. A lithographic apparatus comprising: a tunable narrow pass-band
filter configured to: receive a broad-band radiation, and filter
the broad-band radiation into narrow-band radiation; a relay and
mechanical interface configured to: receive the narrow-band
radiation, and adjust a profile of the narrow-band radiation based
on physical properties of alignment targets on a substrate; and a
focusing system configured to focus the adjusted narrow-band
radiation on the plurality of alignment targets.
15. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter is configured to filter the broad-band
radiation into narrow-band linearly polarized radiation.
16. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter is further configured to select a plurality
of narrow-band wavelength set points at a same time or nearly the
same time.
17. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter is further configured to modulate intensity
and wavelength of the broad-band radiation.
18. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter is further configured to provide a
plurality of pass-band filters at a same time or nearly the same
time.
19. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter is an Acousto-Optical Tunable Filter
(AOTF).
20. The lithographic apparatus of claim 14, wherein the tunable
narrow pass-band filter comprises a library of spectral filters.
Description
[0001] This patent application is related to U.S. application Ser.
No. 13/898,973 and U.S. application Ser. No. 12/751,479 and U.S.
Provisional Patent Application No. 61/168,095, which are
incorporated by reference herein in their entireties.
BACKGROUND
[0002] 1. Field of Invention
[0003] The present invention generally relates generally to an
illumination system of the type used in lithographic apparatus for
semiconductor wafer manufacture.
[0004] 2. Related Art
[0005] A lithographic apparatus 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 carried out by imaging the pattern using a UV radiation
beam 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. Another lithographic
system is an interferometric lithographic system where there is no
patterning device, but rather a light beam is split into two beams,
and the two beams are caused to interfere at a target portion of
substrate through the use of a reflection system. The interference
causes lines to be formed on at the target portion of the
substrate.
[0006] Lithography apparatus may use an alignment system for
detecting the position of alignment marks on a wafer and align the
wafer using the alignment marks to ensure accurate exposure from a
mask. Alignment systems typically have their own illumination
source. The signal detected from the illuminated alignment marks
can be affected by how well the illumination wavelengths are
matched to the physical or optical characteristics of the alignment
marks, or physical or optical characteristics of materials in
contact with or adjacent to the alignment marks. The aforementioned
characteristics can vary depending on the processing steps used.
Phase-grating alignment systems commonly offer a set of discrete,
relatively narrow band illumination wavelengths in order to
maximize the quality and intensity of alignment mark signals
detected by the alignment system. The specific discrete wavelengths
are often limited to the types of sources commercially
available.
[0007] While a selection of discrete wavelengths allows flexibility
to choose a wavelength that improves the alignment signal for a
given set of alignment mark and other local characteristics as
described earlier, certain lithographic processes and/or alignment
marks may require an illumination wavelength that falls outside the
discrete wavelengths that are generated by conventional alignment
systems. If the optimal narrow band of radiation required for a
particular alignment mark or lithographic process falls in between,
or outside of, a set of discrete set point options, the alignment
performance will be adversely affected, perhaps to the level that
alignment is not possible. This limitation reduces the flexibility
to modify lithographic processes and/or alignment marks. Methods
and systems are needed to overcome the above mentioned
deficiencies.
BRIEF SUMMARY OF THE INVENTION
[0008] This section is for the purpose of summarizing some aspects
of the present invention and to briefly introduce some preferred
embodiments. Simplifications or omissions may be made to avoid
obscuring the purpose of the section. Such simplifications or
omissions are not intended to limit the scope of the present
invention.
[0009] It is desirable to provide a lithographic apparatus which
reduces the aforementioned problems.
[0010] Consistent with the principles of the present invention as
embodied and broadly described herein, the present invention is
described in part by various embodiments. According to one
embodiment of the present invention, there is provided an alignment
system for a lithographic apparatus. The alignment system includes
a radiation source configured to convert narrow-band radiation into
continuous, flat and broad-band radiation. An acoustically tunable
narrow pass-band filter is coupled to the radiation source and is
configured to filter the broad-band radiation into narrow-band
linearly polarized radiation. The narrow-band radiation may be
focused on alignment targets of a wafer so as to enable alignment
of the wafer. In one embodiment, the target is a grating. In
another embodiment, the target may be one that is used in a pattern
recognition system. The filter is configured to modulate an
intensity and wavelength of radiation produced by the radiation
source and to have multiple simultaneous pass-bands. The radiation
source may comprise a fiber amplifier configured to generate high
intensity short pulse radiation with a high repetition rate. The
radiation source may also comprise photonic crystal fibers coupled
to the fiber amplifier and configured to generate the continuous,
flat and broad spectrum of radiation from the high intensity short
pulse radiation. The radiation source has high spatial coherence
and low temporal coherence.
[0011] Another embodiment of the present invention provides a
method for aligning a wafer. A first high intensity short-pulse
radiation is generated and propagated through a non-linear device
to generate a second continuous, broad and flat spectrum radiation.
The second radiation is acoustically filtered to generate
narrow-band linearly polarized radiation. The method may also
comprise illuminating an alignment target with the narrow-band
radiation to enable alignment of a wafer. The filtering may further
comprise modulating an intensity and wavelength of the second
radiation and generating multiple simultaneous pass-band filters.
In an embodiment, the first radiation is high intensity short pulse
radiation with a high repetition rate and the second radiation has
high spatial coherence and low temporal coherence.
[0012] A further embodiment of the invention provides an alignment
system that includes an illumination source that is tunable to
desired narrow-band wavelengths over a continuous broad spectral
range. The illumination source includes a tunable filter that
selects only a desired wavelength set point up to a few or several
nanometers wide within an available spectral tuning range by
blocking out-of-band wavelengths to a level that does not adversely
effect alignment for detecting a position of alignment mark on a
wafer to align the wafer using the alignment mark. The illumination
source further includes an optics system configured to cover the
continuous broad spectral range of the illumination source, wherein
the alignment mark having a relatively narrow spectral band over
which an alignment mark signal is above a predetermined acceptable
threshold and the desired wavelength set point substantially
matches the relatively narrow spectral band.
[0013] Further features and advantages of the invention, as well as
the structure and operation of various embodiments of the present
invention, are described in detail below with reference to the
accompanying drawings. It is noted that the invention is not
limited to the specific embodiments described herein. Such
embodiments are presented herein for illustrative purposes only.
Additional embodiments will be apparent to persons skilled in the
relevant art(s) based on the teachings contained herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated herein and
form a part of the specification, illustrate one or more
embodiments of 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.
[0015] FIG. 1A is a schematic representation of an example
lithographic apparatus.
[0016] FIG. 1B is a schematic representation of an example
alignment system according to an embodiment of the invention.
[0017] FIGS. 2A-E illustrate example alignment marks.
[0018] FIG. 3 is a schematic representation of a conventional
illumination system.
[0019] FIG. 4 is a schematic representation of an illumination
system according to an embodiment of the invention.
[0020] FIG. 5 further illustrates an example illumination system
according to an embodiment of the invention.
[0021] FIG. 6 illustrates an exemplary filter according to an
embodiment of the invention.
[0022] FIG. 7 is an example flowchart illustrating steps performed
according to an embodiment of the invention.
[0023] One or more embodiments of the present invention will now be
described with reference to the accompanying drawings. In the
drawings, like reference numbers can indicate identical or
functionally similar elements. Additionally, the left-most digit(s)
of a reference number can identify the drawing in which the
reference number first appears.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The invention will be better understood from the following
descriptions of various "embodiments" of the invention. Thus,
specific "embodiments" are views of the invention, but each does
not itself represent the whole invention. In many cases individual
elements from one particular embodiment may be substituted for
different elements in another embodiment carrying out a similar or
corresponding function. The scope of the invention is not limited
to the disclosed embodiment(s). The invention is defined by the
claims appended hereto.
[0025] The embodiment(s) described, and references in the
specification to "one embodiment", "an embodiment", "an example
embodiment", etc., indicate that the embodiment(s) described can
include a particular feature, structure, or characteristic, but
every embodiment cannot necessarily include the particular feature,
structure, or characteristic. Moreover, such phrases are not
necessarily referring to the same embodiment. Further, when a
particular feature, structure, or characteristic is described in
connection with an embodiment, it is understood that it is within
the knowledge of one skilled in the art to effect such feature,
structure, or characteristic in connection with other embodiments
whether or not explicitly described.
[0026] Embodiments of the invention can be implemented in hardware,
firmware, software, or any combination thereof. Embodiments of the
invention can also be implemented as instructions stored on a
machine-readable medium, which can be read and executed by one or
more processors. A machine-readable medium can include any
mechanism for storing or transmitting information in a form
readable by a machine (e.g., a computing device). For example, a
machine-readable medium can include read only memory (ROM); random
access memory (RAM); magnetic disk storage media; optical storage
media; flash memory devices; electrical, optical, acoustical or
other forms of propagated signals (e.g., carrier waves, infrared
signals, digital signals, etc.), and others. Further, firmware,
software, routines, instructions can be described herein as
performing certain actions. However, it should be appreciated that
such descriptions are merely for convenience and that such actions
in fact result from computing devices, processors, controllers, or
other devices executing the firmware, software, routines,
instructions, etc.
[0027] FIG. 1A schematically depicts an embodiment of lithographic
apparatus suitable for use with the invention. Other arrangements
of lithographic apparatus are also suitable for use with the
invention. This exemplary lithographic apparatus includes: an
illumination system (illuminator) L configured to condition a
radiation beam B (e.g., UV radiation, DUV radiation, etc.), 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; 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 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.
[0028] 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.
[0029] 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 include a frame RF 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."
[0030] 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.
[0031] 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.
[0032] Further, in an interferometric lithographic system there is
no patterning device, but rather a light beam is split into two
beams, and the two beams are caused to interfere at a target
portion of substrate through the use of a reflection system. The
interference causes lines to be formed on at the target portion of
the substrate.
[0033] 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."
[0034] 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).
[0035] 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.
[0036] Referring to FIG. 1A, the illuminator L receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate elements, 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 L 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 L,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0037] The illuminator L 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 L may comprise various
other components, such as an integrator N 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.
[0038] 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.
[0039] The depicted apparatus could be used in at least one of the
following modes:
[0040] 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.
[0041] 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.
[0042] 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.
[0043] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0044] FIG. 1B is a schematic diagram illustrating an example
alignment system 10. Alignment system 10 comprises a coherent
illumination source 12, such as a laser, providing electromagnetic
radiation 13, to a beamsplitter 14. A portion of the
electromagnetic radiation is reflected off coating 16 to illuminate
an alignment mark or target 18. The alignment mark or target 18 may
have one hundred and eighty degree symmetry. By one hundred and
eighty degree symmetry, it is meant that when the alignment mark 18
(also referred to as a "target") is rotated one hundred and eighty
degree about an axis of symmetry perpendicular to the plane of the
alignment mark 18, the alignment mark is substantially identical to
the unrotated alignment mark. The axis for which this is true is
called the axis of symmetry. The alignment mark 18 is placed on a
substrate or wafer 20 that may be coated with a radiation-sensitive
film.
[0045] The substrate 20 is placed on a stage 22. The stage 22 may
be scanned in the direction indicated by arrow 24. Electromagnetic
radiation reflected from the alignment mark 18 passes through the
beamsplitter 14 and is collected by the image rotation
interferometer 26. It should be appreciated that a good quality
image need not be formed, but that the features of the alignment
mark should be resolved. The image rotation interferometer 26 may
be any appropriate set of optical-elements, and is preferably a
combination of prisms, that form two images of the alignment mark,
rotate one of the images with respect to the other one hundred and
eighty degrees and then recombines the two images
interferometrically so that when aligned with the alignment target
18, the electromagnetic radiation will interfere either in a
polarization sense or in an amplitude sense, constructively or
destructively, making readily detectable the center of the
alignment mark 18. The optical ray passing through the center of
rotation established by the interferometer, 26, defines the sensor
alignment axis 27.
[0046] Detectors 28 receive the electromagnetic radiation from the
image rotation interferometer 26. The detectors 28 then provide
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 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. Alternatively, the location
of the alignment sensor 10 may be known such that the center of the
alignment mark 18 is known with reference to the alignment sensor
10. Accordingly, the exact location of the center of the alignment
target 18 is known relative to a reference position.
[0047] FIGS. 2A-E illustrate plan views of examples of different
possible alignment marks.
[0048] It should be appreciated that FIGS. 2A-E are only examples
of different alignment marks and that many alignment marks may be
utilized in practicing the present invention that can readily be
determined by one skilled in the art.
[0049] FIG. 2A illustrates a square grid checkerboard alignment
mark 18A (also known as a "target"). The target 18A is comprised of
a plurality of two types of optically different squares, 34 and 36.
The two types of squares may be differentiated by pattern,
reflectance (amplitude and/or phase), or any combination of these.
Alignment mark 18A functions primarily like two linear gratings
oriented at right angles with respect to each other; one at an
angle of +45 degrees with respect to the orientation of edge or
line 32 and the other at an angle of +45 degrees with respect to
edge or line 32.
[0050] FIG. 2B illustrates a diamond shaped alignment mark 18B. The
alignment mark 18B is comprised of a plurality of vertical equally
spaced lines 40 having spaces 38 there between.
[0051] FIG. 2C illustrates another alignment mark 18C. The
alignment mark 18C has a plurality of lines 44 separated by spaces
42. The spaces 42 are of different spacing or dimensions.
Therefore, the lines 44 have a different pitch or period. The
different periods of lines 44 are symmetrical about a central line
46.
[0052] FIG. 2D illustrates another alignment target 18D. Alignment
target 18D has alternating lines, which may be spaces 39 and lands
41. The spaces 39 and the lands 41 are angled forty-five degrees
with respect to the longitudinal axis of the alignment target
18D.
[0053] FIG. 2E illustrates another alignment target 18E. Alignment
target 18E has alternating lines, which may be spaces 45 and lands
43. The spaces 45 and the lands 43 are angled forty-five degrees
with respect to the longitudinal axis of the alignment target
18E.
[0054] The signal detected from the illuminated alignment marks 18
can be affected by how well the illumination wavelengths are
matched to the physical or optical characteristics of the alignment
marks, or physical or optical characteristics of materials in
contact with or adjacent to the alignment marks. Improved alignment
mark signals that carry accurate information about the position of
the marks can enhance overlay performance of the lithographic
tool.
[0055] FIG. 3 illustrates an example of a conventional alignment
illumination source 12'. Illumination source 12' comprises a
4-color LASER Module Assembly (LMA) 30 and a Polarized Multiplexer
(PMUX) 31. LMA 30 is configured to generate four distinct lasers.
For example, LMA 30 may generate a 532 nm green wavelength, a 633
nm red wavelength, a 780 nm near infrared wavelength and an 850 nm
far infrared wavelength beam of radiation. Polarized multiplexer 31
is configured to multiplex the four LASERs generated by LMA 30 into
a single polarized beam 13 that serves as the illumination source
for alignment system 10. However, CLMA 30 generates a green LASER
that has a higher noise level. However, the color options of LMA 30
are limited to four colors with no bandwidth tunability options.
The specific discrete wavelengths are often limited to the types of
sources commercially available such as 532 nm laser, 632 nm HeNe
laser, 635 nm SLD (Super Luminescent Diode) or Infra Red (IR) laser
diodes.
[0056] While a selection of discrete wavelengths allows flexibility
to choose a wavelength that improves the alignment signal for a
given set of alignment marks and other local characteristics as
described earlier, there may be unique lithographic mark or process
characteristics that have only a narrow spectral band over which
acceptable alignment mark signals are possible. If this optimal
narrow band falls in between, or outside of, the set of discrete
set point wavelength options conventionally available, the
alignment performance will be adversely affected, perhaps to the
level that alignment is not possible. This reduces flexibility to
modify a lithographic process or alignment mark to enhance a
product.
[0057] Another conventional approach has been to use a broad band
illumination in order to, on average, improve the alignment signal.
Broadband illumination requires optics to be corrected over the
broad spectral range in use. This requires complex optical and
coating designs that are typically more expensive, difficult to
align, and are less radiometrically efficient. While alignment
systems using discrete wavelength set points also need to be
designed to operate over a wide spectral range, they don't need to
do so simultaneously. Therefore embodiments presented herein
provide a fully tunable source of radiation for alignment systems
as described below.
[0058] FIG. 4 illustrates an exemplary illumination source 12''
according to an embodiment of the invention. Illumination source
12'' comprises a broadband tunable radiation source (BTRS) 51
coupled to a relay and mechanical interface 53. In an embodiment
BTRS 51 includes a supercontinuum source and an
Acousto-Optical-Tunable-Filter (AOTF). Relay and mechanical
interface 53 are configured to adjust a profile of the radiation
beam emitted from BTRS 51. According to an embodiment, illumination
source 12'' that can be tuned to specific narrow-band wavelengths
over a continuous, flat and broad spectral range. Tuning can be
accomplished at the lithographic system level. This tunability
allows the selection of wavelengths that fall in a spectral gaps
that lies between, or falls outside of, conventional discrete
wavelength set points. to tune the wavelength for signal strength.
This tunability also allows for tuning the alignment wavelength to
the most stable alignment offset, which may be at a place different
from the strongest available diffraction signal.
[0059] Some users may have a set of fixed processes and do not need
a continuous tunable range. However, their processes may require a
set of discrete alignment-system wavelength set points not
currently available due to the limited selection of narrow-band
illumination source types. For this situation, the desired
tunablity can be achieved for a wide range of discrete set points
by filtering the broadband source along with filters such as
Rugate, dielectric and/or holographic filters. The bandwidth of a
given set point can be adjusted to suit the application
requirements. If the filter in use is an AOTF, multiple adjacent
narrow-band set point wavelengths can be selected simultaneously.
Additional filters can be used in conjunction with AOTF or a
mechanism may be added that manipulates optics to achieve bandwidth
adjustment.
[0060] The embodiments presented herein utilize a broadband source,
such as an arc lamp or a Supercontinuum source. A means of tunable
filtering selects only the desired wavelength set point, typically
up to a few or several nanometers wide. The filtering mechanism for
the broadband source is configured to block out-of-band wavelengths
to a level that will have no adverse effect on alignment system
functions. One such implementation would be the use of an
Acousto-Optical Tunable Filter (AOTF) in conjunction with a
Supercontinuum source. In an embodiment, the available spectral
tuning range can cover from 450 nm to 2500 nm and will be limited
only by the availability of the source, tuning mechanics and the
optical design of the alignment system. An example embodiment using
a Supercontinuum source in conjunction with an AOTF is described
below.
[0061] FIG. 5 further illustrates illumination system 12''
according to an embodiment of the invention. Illumination system
12'' includes fiber amplifier 50, photonic crystal fiber 52, AOTF
54 and relay and mechanical interface 53. Fiber amplifier 50 and
photonic crystal fiber 52 may be part of a Supercontinuum source
56.
[0062] Supercontinuum source 56 uses Supercontinuum generation that
causes narrow-band radiation 58 from a source radiation, such as
fiber amplifier 50, to be converted to radiation with a continuous,
broad and flat spectral bandwidth that has low temporal coherence
while maintaining high spatial coherence of source radiation 58.
For example, a narrow-band radiation of 980 nm having a bandwidth
of a few nanometers may be converted into continuous, flat and
broad spectrum of radiation with high spatial coherence and a
bandwidth ranging from 450 nm to 2500 nm. In flat spectrum
radiation, such as radiation 57, the spectral density of intensity
for each wavelength in the spectrum is constant. In continuous
spectrum radiation, all wavelengths in a range or wavelengths, for
example 450 nm to 2500 nm, are present. Radiation 57 has a high
degree of spatial coherence and can be used as a point source i.e.
the radiation can be focused in a diffraction limited point, which
is one of the advantages of the invention since phase-grating
alignment sensors typically require point source radiation.
Spectral broadening may be accomplished by propagating optical
pulses of radiation 58 through a strongly nonlinear device, such as
photonic crystal fibers 56. Photonic crystal fibers 52 have
chromatic dispersion characteristics which allow for a strong
nonlinear interaction over a significant length of fiber. Even with
fairly moderate input powers, very broad spectra are achieved which
leads to generation of a rainbow of colors. In some cases, tapered
fibers can also be used instead of photonic crystal fibers 52.
[0063] Fiber amplifier 50 is configured to provide pumped high
intensity short pulse radiation 58 at a high repetition rate to
photonic crystal fibers 52. Radiation 58 may have a higher radiance
than thermal white light sources. For example, fiber amplifier 50
may be configured to generate pumped Erbium radiation in a
narrow-band with a mean 980 nm wavelength in approximately 5
picosecond (ps) pulses and with a repetition rate of 80 Mhz. Erbium
is used to dope fibers in fiber amplifier 50 so as to modify
optical properties of the fibers and cause the fibers to act as
optical amplifiers.
[0064] The physical processes behind supercontinuum generation in
photonic crystal fibers 52 are based on chromatic dispersion and
length of the fibers in fiber amplifier 50 (or other nonlinear
medium), the pulse duration of radiation 58 generated by fiber
amplifier 50, the initial peak power and the pump wavelength of
radiation 58. When femtosecond pulses are used as source radiation
58, spectral broadening can be dominantly caused by self-phase
modulation by photonic crystal fibers 52. When pumping with
picosecond or nanosecond pulses of radiation 58, Raman scattering
and four-wave mixing are implemented using photonic crystal fiber
52. The spatial coherence, with respect to the cross-spectral
density, of output radiation 57 is usually very high, particularly
if photonic crystal fibers 52 are a single-mode fiber. The high
spectral bandwidth typically results in low temporal coherence.
This kind of coherence is important for the generation of frequency
combs in photonic crystal fibers 56, and it may or may not be
achieved depending on parameters such as the seed pulse duration
and energy, fiber length, and fiber dispersion. In an embodiment
the photonic crystal fibers convert the narrow-band radiation 58 of
980 nm into continuous, broad and flat band radiation ranging from
450 nm to 2500 nm.
[0065] Output radiation 57 is desirably tuned using Acousto-Optical
Tunable Filter (AOTF) 54. AOTF is a an electronically tunable
narrow passband acoustic filter configured to modulate intensity
and wavelength of radiation 57. AOTF 54 is configured to provide
multiple simultaneous passband filters. In an embodiment, AOTF 54
is configured to generate upto eight simultaneous passbands. AOTF
54 may be based on Bragg diffraction in a volume medium. Operation
of AOTF 54 is described in further detail below with respect to
FIG. 6. AOTF 54 generates tuned narrow-band linearly polarized
radiation 59 that is fed into relay and mechanical interface 53.
Relay and mechanical interface 53 are configured to adjust a
profile of radiation 59 and generate radiation beam 13 that is
focused on alignment targets 18.
[0066] Consistent with an embodiment of the present invention, the
desired wavelength set point of the tunable filter such as AOTF 54
can be dynamically set such that this desired wavelength set point
matches the relatively narrow spectral band of the alignment mark
to which it provides the alignment mark signal above the
predetermined acceptable threshold. In this way, quick fine tuning
by the alignment system can be provided, e.g., on the fly
tuning
[0067] FIG. 6 illustrates an exemplary AOTF 54 according to an
embodiment of the invention. AOTF 54 includes an anistropic
bifringement crystal 64, piezoelectric transducer 62 and acoustic
absorber 65. Crystal 64 has an optical axis 67, an acoustic walkoff
angle 69 and an extraordinary wave walkoff angle 63. Piezoelectric
transducer 62 and acoustic absorber 65 are coupled to crystal 64 on
opposite ends of optical axis 67.
[0068] In an embodiment, piezoelectric transducer 62 is configured
to receive a radio frequency signal 60 that is applied to
anisotropic crystal 64. The resultant periodic acoustical wave
propagates along optical axis 67 through the volume of crystal 64.
The acoustic wave creates a periodic pattern of alternating
high/low refractive index in crystal 64. The resulting periodic
index modulation approximates a Bragg diffraction grating such that
a limited spectral band of input radiation 57 is diffracted.
Incident non-polarized radiation 57 splits into orthogonally
polarized diffracted ordinary wave 68, extraordinary first order
wave 61 and un-diffracted zero.sup.th order waves that comprise
narrow-band linearly polarized radiation 59. The diffracted
spectral passband varies as a function of the applied acoustic
frequency. The intensity of the passband wavelengths varies as a
function of amplitude of radio frequency control signal 60.
[0069] The desired wavelength set point can be selected by various
methods, including but not limited to one or more of manual,
automatic, or user-assisted. In manual mode, a user may input a
desired set point wavelength directly. Based on the selected
wavelength a corresponding signal 60 is applied to radio frequency
input 60. In automatic mode, such as during a calibration,
alignment wavelength can be optimized by continuously monitoring
alignment signals as a function of illumination wavelength and
selecting the set point wavelength that maximizes signal quality or
meets predetermined specifications by adjusting radio frequency
input 60. In user assisted mode, a user my control one or more
parameters, such as radio frequency input 60, used in the automated
process. The set point wavelength can also be downloaded at the
beginning of wafer lot via a process recipe file. This allows
different wavelengths to be used for different wafer lot
processing.
[0070] FIG. 7 is an example flowchart 70 illustrating steps
performed to generate a tunable wavelength source for an alignment
system according to an embodiment of the invention. Flowchart 70
will be described with continued reference to the example operating
environment depicted in FIGS. 1-6. However, flowchart 70 is not
limited to these embodiments. Note that some steps shown in
flowchart 70 do not necessarily have to occur in the order
shown.
[0071] In step 72, high intensity pumped radiation with short
pulses and a high repetition rate is generated. For example, pico
or nanosecond pulsed Erbium radiation 58 is generated by fiber
amplifier 50.
[0072] In step 74, the high intensity short-pulse radiation is
propagated through a non-linear optical medium to generate
radiation having a broad and flat spectrum. For example, radiation
58 is propagated through photonic crystal fiber 52 to generate
radiation 57 that has a broad and flat spectrum. In an example the
spectrum ranges from 450 nm to 2500 nm.
[0073] In step 76, the broad and flat spectrum radiation is
filtered using a multi-passband filter to generate narrowband
linearly polarized radiation. For example, broad and flat spectrum
radiation 57 is filtered using AOTF 54 to generate narrow-band
linearly polarized radiation 59.
[0074] In step 78, a profile of the narrowband linearly polarized
is to adjusted based on physical properties of an alignment target
so as to provide higher order diffraction from the alignment
target. The resulting radiation is used to illuminate alignment
targets of a wafer. For example relay and mechanical interface 53
is used to adjust the profile of the narrow-band linearly polarized
radiation 59 to generate radiation 13 that is used to illuminate an
alignment target P1/P2 on wafer W of FIG. 1.
[0075] 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.
[0076] 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, 248, 193, 157
or 126 nm) or extreme ultraviolet radiation.
[0077] The term "lens," where the context allows, may refer to any
one or combination of various types of optical components,
including refractive and reflective optical components.
[0078] 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 embodiments
of 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. Further, the machine readable
instruction may be embodied in two or more computer programs. The
two or more computer programs may be stored on one or more
different memories and/or data storage media.
CONCLUSION
[0079] 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.
[0080] It is to be appreciated that the Detailed Description
section, and not the Summary and Abstract sections, is intended to
be used to interpret the claims. The Summary and Abstract sections
can set forth one or more, but not all exemplary embodiments of the
present invention as contemplated by the inventor(s), and thus, are
not intended to limit the present invention and the appended claims
in any way.
* * * * *