U.S. patent application number 11/635787 was filed with the patent office on 2008-06-12 for scatterometer, a lithographic apparatus and a focus analysis method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Ronald Franciscus Herman Hugers.
Application Number | 20080135774 11/635787 |
Document ID | / |
Family ID | 39149380 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080135774 |
Kind Code |
A1 |
Hugers; Ronald Franciscus
Herman |
June 12, 2008 |
Scatterometer, a lithographic apparatus and a focus analysis
method
Abstract
To detect whether a substrate is in a focal plane of a
scatterometer, a cross-sectional area of radiation above a certain
intensity value is detected both in front of and behind a back
focal plane of the optical system of the scatterometer. The
detection positions in front of and behind the back focal plane
should desirably be equidistant from the back focal plane along the
path of the radiation redirected from the substrate so that a
simple comparison may determine whether the substrate is in the
focal plane of the scatterometer.
Inventors: |
Hugers; Ronald Franciscus
Herman; (Eindhoven, 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: |
39149380 |
Appl. No.: |
11/635787 |
Filed: |
December 8, 2006 |
Current U.S.
Class: |
250/393 ;
250/492.2 |
Current CPC
Class: |
G03F 9/7026 20130101;
G03F 7/70625 20130101 |
Class at
Publication: |
250/393 ;
250/492.2 |
International
Class: |
G01T 1/00 20060101
G01T001/00 |
Claims
1. A scatterometer configured to measure a property of a substrate,
the apparatus comprising: a high numerical aperture lens configured
to project radiation onto the substrate and to project radiation
redirected from the substrate towards a back focal plane of the
high numerical aperture lens or towards a conjugate of a front
focal plane of the high numerical aperture lens; a first detector
configured to detect a cross-sectional area of the redirected
radiation having an intensity above a first value; and a second
detector configured to detect a cross-sectional area of the
redirected radiation having an intensity above a second value,
wherein the first detector is arranged in front of the back focal
plane, between the high numerical aperture lens and the back focal
plane, and the second detector is arranged behind the back focal
plane, or the first detector is arranged in front of the conjugate
of the front focal plane, between the high numerical aperture lens
and the conjugate of the front focal plane, and the second detector
is arranged behind the conjugate of the front focal plane.
2. The scatterometer of claim 2, further comprising an angle
detector configured to detect an angle resolved spectrum of the
redirected radiation.
3. The scatterometer of claim 1, wherein the first detector and the
second detector are arranged equidistant from the back focal plane
or the conjugate of the front focal plane along an optical path of
the redirected radiation.
4. The scatterometer of claim 1, further comprising a comparator
configured to comparing the cross-sectional area of the redirected
radiation having an intensity above the first value detected by the
first detector and the cross-sectional area of the redirected
radiation having an intensity above the second value detected by
the second detector.
5. The scatterometer of claim 1, further comprising a first
reflector configured to reflect the redirected radiation towards
the first detector.
6. The scatterometer of claim 5, wherein the first reflector
comprises a partially reflective mirror.
7. The scatterometer of claim 1, further comprising a second
reflector configured to reflect the redirected radiation towards
the second detector.
8. The scatterometer of claim 7, wherein the second reflector
comprises, a partially reflective mirror.
9. The scatterometer of claim 1, wherein the first detector
comprises a plurality of first sub-detectors.
10. The scatterometer of claim 1, wherein the second detector
comprises a plurality of second sub-detectors.
11. The scatterometer of claim 1, wherein the first value is the
same as the second value.
12. The scatterometer of claim 1, wherein the first value is
greater than the second value.
13. A lithographic apparatus comprising: a substrate table
configured to hold a substrate; a system configured to transfer a
pattern onto the substrate; and a scatterometer configured to
measure a property of a substrate, the apparatus comprising: a high
numerical aperture lens configured to project radiation onto the
substrate and to project radiation redirected from the substrate
towards a back focal plane of the high numerical aperture lens or
towards a conjugate of a front focal plane of the high numerical
aperture lens, a first detector configured to detect a
cross-sectional area of the redirected radiation having an
intensity above a first value, and a second detector configured to
detect a cross-sectional area of the redirected radiation having an
intensity above a second value, wherein the first detector is
arranged in front of the back focal plane, between the high
numerical aperture lens and the back focal plane, and the second
detector is arranged behind the back focal plane, or the first
detector is arranged in front of the conjugate of the front focal
plane, between the high numerical aperture lens and the conjugate
of the front focal plane, and the second detector is arranged
behind the conjugate of the front focal plane.
14. A focus analysis method for detecting whether a substrate is in
the focal plane of a lens, the method comprising: projecting
radiation through a high numerical aperture lens and onto the
substrate; detecting a first cross-sectional area of radiation
redirected by the substrate and passing through the high numerical
aperture lens, having an intensity above a first value, the
detecting the first cross-sectional area of the redirection
radiation occurring between the high numerical aperture lens and a
back focal plane of the high numerical aperture lens or between the
high numerical aperture lens and a conjugate of a front focal plane
of the high numerical aperture lens; and detecting a second
cross-sectional area of the redirected radiation having an
intensity above a second value, the detecting the second
cross-sectional area of the redirected radiation occurring,
respectively to the first detector, behind the back focal plane or
behind the conjugate of the front focal plane.
15. The method of claim 14, further comprising comparing the first
cross-sectional area of the redirected radiation and the second
cross-sectional area of the redirected radiation.
16. The method of claim 14, further comprising detecting angles of
a spectrum of redirected radiation.
17. A device manufacturing method, comprising: projecting a
patterned beam of radiation onto a substrate; and detecting whether
a substrate is in the focal plane of a lens, the detecting
comprising: projecting radiation through a high numerical aperture
lens and onto the substrate, detecting a first cross-sectional area
of radiation redirected by the substrate and passing through the
high numerical aperture lens, having an intensity above a first
value, the detecting the first cross-sectional area of the
redirection radiation occurring between the high numerical aperture
lens and a back focal plane of the high numerical aperture lens or
between the high numerical aperture lens and a conjugate of a front
focal plane of the high numerical aperture lens, and detecting a
second cross-sectional area of the redirected radiation having an
intensity above a second value, the detecting the second
cross-sectional area of the redirected radiation occurring,
respectively to the first detector, behind the back focal plane or
behind the conjugate of the front focal plane.
18. The method of claim 17, further comprising comparing the first
cross-sectional area of the redirected radiation and the second
cross-sectional area of the redirected radiation.
19. A control system configured to control a lithographic
apparatus, the control system embodying executable instructions
configured to carry out a focus analysis method for detecting
whether a substrate is in the focal plane of a lens, the method
comprising: projecting radiation through a high numerical aperture
lens and onto the substrate; detecting a first cross-sectional area
of radiation redirected by the substrate and passing through the
high numerical aperture lens, having an intensity above a first
value, the detecting the first cross-sectional area of the
redirection radiation occurring between the high numerical aperture
lens and a back focal plane of the high numerical aperture lens or
between the high numerical aperture lens and a conjugate of a front
focal plane of the high numerical aperture lens; and detecting a
second cross-sectional area of the redirected radiation having an
intensity above a second value, the detecting the second
cross-sectional area of the redirected radiation occurring,
respectively to the first detector, behind the back focal plane or
behind the conjugate of the front focal plane.
20. The control system of claim 19, wherein the method further
comprises comparing the first cross-sectional area of the
redirected radiation and the second cross-sectional area of the
redirected radiation.
Description
FIELD
[0001] The present invention relates to a method of inspection
usable, for example, in the manufacture of devices by a
lithographic technique and to a method of manufacturing devices
using a lithographic technique.
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] To determine features of a substrate, such as its alignment,
a beam is typically redirected off the surface of the substrate,
for example at an alignment target, and an image is created on a
camera of the redirected beam. By comparing a property of the beam
before and after it has been redirected by the substrate, a
property of the substrate may be determined. This can be done, for
example, by comparing the redirected beam with data stored in a
library of known measurements associated with a known substrate
property.
SUMMARY
[0004] When detecting features of a pattern, the pattern should be
in the focal plane of the optics. A method for determining whether
a pattern on a substrate is in focus is the so-called "knife edge"
method described in U.S. patent application publication no. US
2006-0066855, which document is hereby incorporated in its entirety
by reference. However, this method may complicated and require
complex parts.
[0005] It is desirable, for example, to provide a method and
apparatus for detecting whether the substrate is in focus.
[0006] According to an aspect of the invention, there is provided a
scatterometer configured to measure a property of a substrate, the
apparatus comprising:
[0007] a high numerical aperture lens configured to project
radiation onto the substrate and to project radiation redirected
from the substrate towards a back focal plane of the high numerical
aperture lens or towards a conjugate of a front focal plane of the
high numerical aperture lens;
[0008] a first detector configured to detect a cross-sectional area
of the redirected radiation having an intensity above a first
value; and
[0009] a second detector configured to detect a cross-sectional
area of the redirected radiation having an intensity above a second
value,
[0010] wherein the first detector is arranged in front of the back
focal plane, between the high numerical aperture lens and the back
focal plane, and the second detector is arranged behind the back
focal plane, or the first detector is arranged in front of the
conjugate of the front focal plane, between the high numerical
aperture lens and the conjugate of the front focal plane, and the
second detector is arranged behind the conjugate of the front focal
plane.
[0011] According to an aspect of the invention, there is provided a
lithographic apparatus comprising:
[0012] a substrate table configured to hold a substrate;
[0013] a system configured to transfer a pattern onto the
substrate; and
[0014] a scatterometer configured to measure a property of a
substrate, the apparatus comprising: [0015] a high numerical
aperture lens configured to project radiation onto the substrate
and to project radiation redirected from the substrate towards a
back focal plane of the high numerical aperture lens or towards a
conjugate of a front focal plane of the high numerical aperture
lens, [0016] a first detector configured to detect a
cross-sectional area of the redirected radiation having an
intensity above a first value, and [0017] a second detector
configured to detect a cross-sectional area of the redirected
radiation having an intensity above a second value, [0018] wherein
the first detector is arranged in front of the back focal plane,
between the high numerical aperture lens and the back focal plane,
and the second detector is arranged behind the back focal plane, or
the first detector is arranged in front of the conjugate of the
front focal plane, between the high numerical aperture lens and the
conjugate of the front focal plane, and the second detector is
arranged behind the conjugate of the front focal plane.
[0019] According to a further aspect of the invention, there is
provided a focus analysis method for detecting whether a substrate
is in the focal plane of a lens, the method comprising: [0020]
projecting radiation through a high numerical aperture lens and
onto the substrate; [0021] detecting a first cross-sectional area
of radiation redirected by the substrate and passing through the
high numerical aperture lens, having an intensity above a first
value, the detecting the first cross-sectional area of the
redirection radiation occurring between the high numerical aperture
lens and a back focal plane of the high numerical aperture lens or
between the high numerical aperture lens and a conjugate of a front
focal plane of the high numerical aperture lens; and [0022]
detecting a second cross-sectional area of the redirected radiation
having an intensity above a second value, the detecting the second
cross-sectional area of the redirected radiation occurring,
respectively to the first detector, behind the back focal plane or
behind the conjugate of the front focal plane.
[0023] According to a further aspect of the invention there is
provided a device manufacturing method comprising the focus control
method described above. The focus control method described above
may be implemented using a control system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] 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:
[0025] FIG. 1a depicts a lithographic apparatus;
[0026] FIG. 1b depicts a lithographic cell or cluster;
[0027] FIG. 2 depicts a scatterometer;
[0028] FIG. 3 depicts a further scatterometer and the general
operating principle of measuring an angle resolved spectrum in the
pupil plane of a high-NA lens;
[0029] FIGS. 4a and 4b depict arrangements according to an
embodiment of the invention;
[0030] FIG. 5 depicts an further arrangement according to an
embodiment of the invention;
[0031] FIGS. 6A and 6C depicts patterns of radiation detected on
the detector when the substrate is in and out of focus;
[0032] FIGS. 7A and 7B depict detectors according to an embodiment
of the invention; and
[0033] FIGS. 8 to 10 depict further detectors according to an
embodiment of the invention.
DETAILED DESCRIPTION
[0034] FIG. 1a schematically depicts a lithographic apparatus. The
apparatus comprises: [0035] an illumination system (illuminator) IL
configured to condition a radiation beam B (e.g. UV radiation or
EUV radiation); [0036] 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; [0037] 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
[0038] a projection system (e.g. a refractive projection lens
system) PL 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.
[0039] 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.
[0040] The support structure 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."
[0041] 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.
[0042] 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.
[0043] 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".
[0044] 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).
[0045] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more support
structures). In such "multiple stage" machines the additional
tables and/or support structures may be used in parallel, or
preparatory steps may be carried out on one or more tables and/or
support structures while one or more other tables and/or support
structures are being used for exposure.
[0046] 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.
[0047] Referring to FIG. 1a, 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.
[0048] 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.
[0049] 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 patterning device MA, the radiation beam B passes
through the projection system PL, 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. 1a) can be used to accurately position
the patterning device 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 support structure 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 support
structure MT may be connected to a short-stroke actuator only, or
may be fixed. Patterning device MA and substrate W may be aligned
using patterning device 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 patterning device MA, the patterning device
alignment marks may be located between the dies.
[0050] The depicted apparatus could be used in at least one of the
following modes:
[0051] 1. In step mode, the support structure 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.
[0052] 2. In scan mode, the support structure 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 support structure MT may be determined by
the (de-)magnification and image reversal characteristics of the
projection system PL. 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.
[0053] 3. In another mode, the support structure 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.
[0054] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0055] As shown in FIG. 1b, the lithographic apparatus LA
(controlled by a lithographic apparatus control unit LACU) forms
part of a lithographic cell LC, also sometimes referred to as a
lithocell or lithocluster, which also includes apparatus to perform
one or more pre- and post-exposure processes on a substrate.
Conventionally these include one or more spin coaters SC to deposit
a resist layer, one or more developers DE to develop exposed
resist, one or more chill plates CH and one or more bake plates BK.
A substrate handler, or robot, RO picks up a substrate from
input/output ports I/O1, I/O2, moves it between the different
process devices and delivers it to the loading bay LB of the
lithographic apparatus. These devices, which are often collectively
referred to as the track, are under the control of a track control
unit TCU which is itself controlled by the supervisory control
system SCS, which also controls the lithographic apparatus via
lithographic apparatus control unit LACU. Thus, the different
apparatus may be operated to maximize throughput and processing
efficiency.
[0056] In order that the substrate that is exposed by the
lithographic apparatus is exposed correctly and consistently for
each layer of resist, it is desirable to inspect an exposed
substrate to measure one or more properties such as whether changes
in alignment, rotation, etc., overlay error between subsequent
layers, line thickness, critical dimension (CD), etc. If an error
or change is detected, an adjustment may be made to an exposure of
one or more subsequent substrates, especially if the inspection can
be done soon and fast enough that another substrate of the same
batch is still to be exposed. Also, an already exposed substrate
may be stripped and reworked--to improve yield- or
discarded--thereby avoiding performing an exposure on a substrate
that is known to be faulty. In a case where only some target
portions of a substrate are faulty, a further exposure may be
performed only on those target portions which are good. Another
possibility is to adapt a setting of a subsequent process step to
compensate for the error, e.g. the time of a trim etch step can be
adjusted to compensate for substrate-to-substrate CD variation
resulting from the lithographic process step.
[0057] An inspection apparatus is used to determine one or more
properties of a substrate, and in particular, how one or more
properties of different substrates or different layers of the same
substrate vary from layer to layer and/or across a substrate. The
inspection apparatus may be integrated into the lithographic
apparatus LA or the lithocell LC or may be a stand-alone device. To
enable most rapid measurements, it is desirable that the inspection
apparatus measure one or more properties in the exposed resist
layer immediately after the exposure
[0058] The one or more properties of the surface of a substrate W
may be determined using a sensor such as a scatterometer such as
that depicted in FIG. 2. The scatterometer comprises a broadband
(white light) radiation projector 2 which projects radiation onto a
substrate W. The reflected radiation is passed to a spectrometer
detector 4, which measures a spectrum 10 (i.e. a measurement of
intensity as a function of wavelength) of the specular reflected
radiation. From this data, the structure or profile giving rise to
the detected spectrum may be reconstructed by a processing unit,
e.g. by Rigorous Coupled Wave Analysis and non-linear regression or
by comparison with a library of simulated spectra as shown at the
bottom of FIG. 2. In general, for the reconstruction, the general
form of the structure is known and some parameters are assumed from
knowledge of the process by which the structure was made, leaving
only a few parameters of the structure to be determined from the
scatterometry data. Such a scatterometer may be configured as a
normal-incidence scatterometer or an oblique-incidence
scatterometer. A variant of scatterometry may also be used in which
the reflection is measured at a range of angles of a single
wavelength, rather than the reflection at a single angle of a range
of wavelengths.
[0059] A scatterometer for measuring one or more properties of a
substrate may measure, in the pupil plane 11 of a high numerical
aperture lens, the properties of an angle-resolved spectrum
reflected from the substrate surface W at a plurality of angles and
wavelengths as shown in FIG. 3. Such a scatterometer may comprise a
radiation projector 2 configured to project radiation onto the
substrate W and a detector 18 configured to detect the reflected
spectra. The pupil plane is the plane in which the radial position
of radiation defines the angle of incidence and the angular
position defines azimuth angle of the radiation. The detector 14 is
placed in the pupil plane of the high numerical aperture lens. The
numerical aperture of the lens may be high and desirably is at
least 0.9 or at least 0.95. An immersion scatterometer may even
have a lens with a numerical aperture over 1.
[0060] An angle-resolved scatterometer only measures the intensity
of scattered radiation. However, a scatterometer may allow several
wavelengths to be measured simultaneously at a range of angles. The
properties measured by the scatterometer for different wavelengths
and angles may be the intensity of transverse magnetic- and
transverse electric-polarized radiation and/or the phase difference
between the transverse magnetic- and transverse electric-polarized
radiation.
[0061] Using a broadband radiation source (i.e. one with a wide
range of radiation frequencies or wavelengths--and therefore of
colors) is possible, which gives a large etendue, allowing the
mixing of multiple wavelengths. The plurality of wavelengths in the
broadband desirably each has a bandwidth of .delta..lamda. and a
spacing of at least 2.delta..lamda. (i.e. twice the wavelength
bandwidth). Several "sources" of radiation may be different
portions of an extended radiation source which have been split
using, e.g., fiber bundles. In this way, angle resolved scatter
spectra may be measured at multiple wavelengths in parallel. A 3-D
spectrum (wavelength and two different angles) may be measured,
which contains more information than a 2-D spectrum. This allows
more information to be measured which increases metrology process
robustness. This is described in more detail in U.S. patent
application publication no. US 2006-0066855, which document is
hereby incorporated in its entirety by reference.
[0062] A scatterometer that may be used with an embodiment of the
present invention is shown in FIG. 3. The radiation of the
radiation projector 2 is collimated using lens system 12 through
interference filter 13 and polarizer 17, reflected by partially
reflective surface 16 and is focused onto substrate W via a
microscope objective lens 15. The reflected radiation is then
transmitted through partially reflective surface 16 into a CCD
detector 18 in the back projected pupil plane 11 in order to have
the scatter spectrum detected. The pupil plane 11 is at the focal
length of the lens system 15. A detector and high aperture lens are
placed at the pupil plane. The pupil plane may be re-imaged with
auxiliary optics since the pupil plane of a high-NA lens is usually
located inside the lens.
[0063] A reference beam is often used, for example, to measure the
intensity of the incident radiation. To do this, when the radiation
beam is incident on the partially reflective surface 16 part of it
is transmitted through the surface as a reference beam towards a
reference mirror 14. The reference beam is then projected onto a
different part of the same detector 18.
[0064] The pupil plane of the reflected radiation is imaged on the
CCD detector, which may have an integration time of, for example,
40 milliseconds per frame. In this way, a two-dimensional angular
scatter spectrum of the substrate target is imaged on the detector.
The detector may be, for example, an array of CCD or CMOS
sensors.
[0065] One or more interference filters 13 are available to select
a wavelength of interest in the range of, say, 405-790 nm or even
lower, such as 200-300 nm. The interference filter(s) may be
tunable rather than comprising a set of different filters. A
grating could be used instead of or in addition to one or more
interference filters.
[0066] The target on substrate W may be a grating which is printed
such that after development, the bars are formed of solid resist
lines. The bars may alternatively be etched into the substrate. The
target pattern is chosen to be sensitive to a parameter of
interest, such as focus, dose, overlay, chromatic aberration in the
lithographic projection apparatus, etc., such that variation in the
relevant parameter will manifest as variation in the printed
target. For example, the target pattern may be sensitive to
chromatic aberration in the lithographic projection apparatus,
particularly the projection system PL, and illumination symmetry
and the presence of such aberration will manifest itself in a
variation in the printed target pattern. Accordingly, the
scatterometry data of the printed target pattern is used to
reconstruct the target pattern. The parameters of the target
pattern, such as line width and shape, may be input to the
reconstruction process, performed by a processing unit, from
knowledge of the printing step and/or other scatterometry
processes.
[0067] FIG. 4a depicts an arrangement according to an embodiment of
the invention in which radiation is projected through the high
numerical aperture lens 15 and through a focusing lens 21. The
radiation is then projected onto a first detector 30 and a second
detector 31. As described below, each of the detectors detects an
amount (or cross-sectional area) of radiation above a predetermined
intensity level. Each of the detectors may comprise one or more
photodiodes, CCDs or CMOS. In this embodiment, the detectors are at
least partially transmissive such that radiation is transmitted
through the detectors and onto one or more further optical
elements. The detectors are desirably arranged equidistant along
the optical path from the back focal plane of the high numerical
aperture lens 15 or equidistant from a conjugate of the substrate
plane, shown as the dashed line in FIG. 4a. Assuming no
transmissive losses between the detectors, if the substrate is in
focus the cross-sectional area of the radiation above a
predetermined intensity level (the spot size) will be the same at
both detectors, as shown in both columns in FIG. 6a, and a simple
comparator can be used to determine whether the substrate is in
focus. However, if the substrate is out of focus by being too far
from the high numerical aperture lens, the spot size will be
greater in the first detector (shown in the left column in FIG. 6b)
than the second detector (shown in the right column in FIG. 6b).
Conversely, if the substrate is out of focus by being to close to
the high numerical aperture lens, the spot size in the second
detector (shown in the right column in FIG. 6c) will be greater
than that in the first detector (shown in the left column in FIG.
6c).
[0068] A further arrangement according to an embodiment of the
invention is shown in FIG. 4b. In this embodiment, a partially
transmissive mirror 22 is placed in the path of the beam after the
high numerical aperture lens 15. The partially transmissive mirror
22 deflects a portion of the radiation towards a focus branch which
includes the focusing lens 21 together with first detector 30 and
second detector 31. In this embodiment the first detector 30 and
second detector 31 are placed either side and desirably equidistant
of a conjugate of the substrate plane (a conjugate of the front
focal plane of the high numerical aperture lens). The second
detector need therefore not be partially transmissive. As an
alternative to the transmissive mirror, a beam splitter could also
be used.
[0069] The predetermined intensity levels measured on the first and
second detectors may not be the same. For example, if there are
transmissive losses between the first and second detectors, the
predetermined intensity level above which radiation is measured may
be greater for the first detector than the second detector. Some
calibration may be required to determine the desired predetermined
intensity levels.
[0070] Although these examples have just a first detector 30 and a
second detector 31, each of the first detector and second detector
could be divided into a plurality of sub-detectors as shown in
FIGS. 7a and 7. FIG. 7a depicts the first detector divided into a
plurality of first sub-detectors, 32, 33, 34 and FIG. 7b depicts
the second detector divided into a plurality of second
sub-detectors 37, 38, 39. The focus area is then given by:
(I.sub.32+I.sub.34+I.sub.38-(I.sub.33+I.sub.37+I.sub.39)
[0071] where I.sub.32 is the amount of radiation above a first
predetermined intensity level incident on sub-detector 32, I.sub.37
is the amount of radiation about a second predetermined intensity
level incident on sub-detector 37, etc.
[0072] Although FIGS. 7a and 7b depict the first and second
detectors divided into sub-detectors along a horizontal direction,
the detectors could be divided into sub-detectors in any number of
ways. For example, FIG. 8 depicts a detector divided into
sub-detectors 42, 43, 44 along a vertical direction. FIG. 9 depicts
a detector divided into sub-detectors 51 to 59 in a grid
arrangement and FIG. 10 depicts a detector divided into
sub-detectors 62, 63, 64 in concentric circles.
[0073] FIG. 5 depicts a further arrangement of the detectors shown
in FIG. 4. In this embodiment, mirrors are used to project the
radiation onto the detectors. A partially reflective mirror 35
allows part of the radiation to pass through and onto first
detector 30 while the remaining radiation reflects towards a second
mirror 36 which reflects at least part of the radiation onto the
second detector 31. The second mirror 36 may be either fully
reflective or partially reflective and the second detector 31 may
be transmissive to allow the radiation to be projected onto further
optics. Again, the detectors are desirably arranged equidistant
along the path of the radiation from the back focal plane of the
high numerical aperture lens 15 or equidistant from a conjugate of
the substrate plane.
[0074] Although the detectors are desirably arranged equidistant
along the path of the radiation from the back focal plane of the
high numerical aperture lens 15 or from a conjugate of the
substrate plane, they need not be. If they are not equidistant from
the back focal plane or a conjugate of the substrate plane, a
calculation, rather than a simple comparison, may determine whether
the relative spot sizes on the detectors indicate that the
substrate is in focus or out of focus.
[0075] This method can be used in conjunction with an other,
conventional focus detection method. For example, one or more
different focus detection methods may occupy different optical
branches.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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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