U.S. patent application number 10/893542 was filed with the patent office on 2006-01-19 for ellipsometer, measurement device and method, and lithographic apparatus and method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Cristian Presura.
Application Number | 20060012788 10/893542 |
Document ID | / |
Family ID | 35599069 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060012788 |
Kind Code |
A1 |
Presura; Cristian |
January 19, 2006 |
Ellipsometer, measurement device and method, and lithographic
apparatus and method
Abstract
An ellipsometer includes an optical component and a detector.
The optical component has two birefringent parts in optical
communication via a border surface. Light incident on the border
surface is split into two reflected and two transmitted components.
The detector is configured to measure a property of at least three
out of the four components. Based on the measured properties, a
state of polarization of the incident light may be determined.
Inventors: |
Presura; Cristian;
(Veldhoven, NL) |
Correspondence
Address: |
Thomas D. Smith
7008 Landing Road
Oklahoma City
OK
73132
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
35599069 |
Appl. No.: |
10/893542 |
Filed: |
July 19, 2004 |
Current U.S.
Class: |
356/369 ;
356/399 |
Current CPC
Class: |
G01J 4/04 20130101; G01N
21/23 20130101; G01N 21/211 20130101 |
Class at
Publication: |
356/369 ;
356/399 |
International
Class: |
G01J 4/00 20060101
G01J004/00; G01B 11/00 20060101 G01B011/00 |
Claims
1. An ellipsometer comprising: an optical component including a
first birefringent part and a second birefringent part, said first
birefringent part having a first optical axis oriented along a
first direction and being connected along a border surface to said
second birefringent part, said second birefringent part having a
second optical axis oriented along a second direction different
from said first direction; and at least one detector, wherein said
optical component is configured to receive a beam of radiation
having a state of polarization, and wherein said border surface is
configured to split said beam into first and second components
reflected into said first birefringent part and third and fourth
components transmitted into said second birefringent part, and
wherein, for at least three of said four components, said at least
one detector is configured to measure a property of said
component.
2. The ellipsometer according to claim 1, said ellipsometer
comprising an evaluation unit configured to calculate said state of
polarization based on the measured properties of said at least
three components.
3. The ellipsometer according to claim 1, wherein said evaluation
unit comprises a processor.
4. The ellipsometer according to claim 1, wherein said property is
at least one of a phase and an intensity of said component.
5. The ellipsometer according to claim 1, wherein said first
birefringent part and said second birefringent part are connected
via at least one layer present in between.
6. The ellipsometer according to claim 5, wherein said at least one
layer includes a layer of material configured to equalize
transmitted and reflected optical power.
7. The ellipsometer according to claim 5, wherein at least one of
said at least one layer comprises air.
8. The ellipsometer according to claim 7, wherein said at least one
layer of air has a thickness of about one-half of the wavelength of
said radiation.
9. The ellipsometer according to claim 1, wherein said at least one
optical component is shaped as a prism.
10. The ellipsometer according to claim 1, wherein, for each of the
four components, said at least one detector is configured to
measure a property of said component, said property being an
intensity.
11. The ellipsometer according to claim 1, further comprising at
least one grating configured to transmit at least one of the group
consisting of said beam, a first of said four components, a second
of said four components, a third of said four components, and a
fourth of said four components.
12. A lithographic apparatus comprising an ellipsometer according
to claim 1.
13. A measurement device comprising an ellipsometer according to
claim 1 and a detection and evaluation unit configured to calculate
said state of polarization.
14. A lithographic apparatus comprising a measurement device
according to claim 13.
15. An ellipsometer according to claim 1, wherein said first
direction is substantially parallel to a direction of propagation
of said beam of radiation as received by said optical
component.
16. An ellipsometer according to claim 1, wherein a principal
wavelength of said beam of radiation is outside the visible
range.
17. An ellipsometer according to claim 1, wherein said second
birefringent part is aligned with respect to said first
birefringent part such that one of the third and the fourth
component beams has a direction of propagation substantially
parallel to a direction of propagation of said beam of radiation as
received by said optical component.
18. A method of measurement, said method comprising: directing a
measurement beam of radiation along an optical axis of a first
birefringent part to a border surface between the first
birefringent part and a second birefringent part, said second
birefringent part having an optical axis oriented along a different
direction from the optical axis of the first birefringent part; and
measuring a property of each of at least three of the group
consisting of a first component of the measurement beam, a second
component of the measurement beam, a third component of the
measurement beam, and a fourth component of the measurement beam,
wherein said first and second components are reflected from said
border surface into said first birefringent part, and wherein said
third and fourth components are transmitted from said border
surface into said second birefringent part.
19. The method of measurement according to claim 18, said method
comprising calculating a state of polarization of the measurement
beam based on the measured properties of said at least three
components.
20. The method of measurement according to claim 18, wherein said
measuring a property of each of at least three of the group
comprises measuring at least one of a phase and an intensity of
each of at least three of the group.
21. The method of measurement according to claim 18, wherein said
border surface includes a partially reflective coating.
22. The method of measurement according to claim 18, wherein said
border surface includes a layer of air.
23. The method of measurement according to claim 18, wherein said
measuring a property includes measuring an intensity of each of
said first, second, third, and fourth components.
24. The method of measurement according to claim 18, wherein a
principal wavelength of said measurement beam is outside the
visible range.
25. The method of measurement according to claim 18, wherein a
direction of propagation of said third component is substantially
parallel to the optical axis of the first birefringent part.
26. The method of measurement according to claim 18, said method
comprising determining a complex reflectivity of a surface of a
substrate based on a result of said detecting a property.
27. The method of measurement according to claim 26, wherein said
directing includes receiving the measurement beam as a reflection
from the surface of the substrate.
28. The method of measurement according to claim 18, said method
comprising: patterning a beam of radiation, and projecting the
patterned beam onto a target portion on a surface of a substrate,
wherein said projecting is based on a result of said detecting a
property.
29. The method of measurement according to claim 28, wherein said
directing includes receiving the measurement beam as a reflection
from the surface of the substrate.
30. An ellipsometer comprising: a first birefringent part; a second
birefringent part; and at least one detector, wherein said first
and second birefringent parts are arranged in optical communication
via a border surface such that a beam of radiation transmitted
through said first birefringent part is split into a first
component, a second component, a third component, and a fourth
component, said first and second components being reflected from
said border surface into said first birefringent part and said
third and fourth components being transmitted from said border
surface into said second birefringent part, and wherein said at
least one detector is configured to measure a property of each of
at least three of the first, second, third and fourth
components.
31. An ellipsometer according to claim 30, wherein said first
birefringent part has an optical axis oriented in a first
direction, and wherein said second birefringent part has an optical
axis oriented in a second direction different from said first
direction.
32. An ellipsometer according to claim 30, wherein said border
surface includes a partially reflective coating.
33. An ellipsometer according to claim 30, wherein said first
birefringent part is configured to be moveable with respect to said
second birefringent part to adjust a distribution of power among at
least two of the first, second, third, and fourth components.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to lithographic apparatus and
methods.
BACKGROUND
[0002] A lithographic apparatus is a machine that applies a desired
pattern onto a target portion of a substrate. Lithographic
apparatus can be used, for example, in the manufacture of
integrated circuits (ICs). In that circumstance, a patterning
structure, such as a mask, may be used to generate a circuit
pattern corresponding to an individual layer of the IC, and this
pattern can be imaged onto a target portion (e.g. including part
of, one or several dies) on a substrate (e.g. a silicon wafer) that
has a layer of radiation-sensitive material (resist). In general, a
single substrate will contain a network of adjacent target portions
that are successively exposed. Known lithographic apparatus include
so-called steppers, in which each target portion is irradiated by
exposing an entire pattern onto the target portion all at once, and
so-called scanners, in which each target portion is irradiated by
scanning the pattern through the projection beam in a given
direction (the "scanning"-direction) while synchronously scanning
the substrate parallel or anti-parallel to this direction.
[0003] 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, 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) or
a metrology or 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.
[0004] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of 365, 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.
[0005] The term "patterning structure" used herein should be
broadly interpreted as referring to a structure that can be used to
impart a beam of radiation 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 beam of radiation
may not exactly correspond to the desired pattern in the target
portion of the substrate. Generally, the pattern imparted to the
beam of radiation will correspond to a particular functional layer
in a device being created in the target portion, such as an
integrated circuit.
[0006] Patterning structures may be transmissive or reflective.
Examples of patterning structures 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; in this manner, the reflected beam is
patterned.
[0007] The support structure supports, i.e. bears the weight of,
the patterning structure. It holds the patterning structure in a
way depending on the orientation of the patterning structure, the
design of the lithographic apparatus, and other conditions, such as
for example whether or not the patterning structure is held in a
vacuum environment. The support can be using mechanical clamping,
vacuum, or other clamping techniques, for example electrostatic
clamping under vacuum conditions. The support structure may be a
frame or a table, for example, which may be fixed or movable as
required and which may ensure that the patterning structure 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
structure".
[0008] The term "projection system" used herein should be broadly
interpreted as encompassing various types of projection system,
including refractive optical systems, reflective optical systems,
and catadioptric optical systems, as appropriate for example for
the exposure radiation being used, or for other factors such as the
use of an immersion fluid or the use of a vacuum. Any use of the
term "lens" herein may be considered as synonymous with the more
general term "projection system".
[0009] The illumination system may also encompass various types of
optical components, including refractive, reflective, and
catadioptric optical components for directing, shaping, or
controlling the projection beam of radiation, and such components
may also be referred to below, collectively or singularly, as a
"lens".
[0010] 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.
[0011] The lithographic apparatus may also be of a type wherein the
substrate is immersed in a liquid having a relatively high
refractive index, e.g. water, so as to fill a space between the
final element of the projection system and the substrate. Immersion
liquids may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the first element of
the projection system. Immersion techniques are well known in the
art for increasing the numerical aperture of projection
systems.
[0012] Ellipsometers are typically used for determining the complex
reflectivity of a surface. Such a surface may be, for example, the
top of a wafer, an alignment marker/structure on a wafer, or a
piece of human tissue. Interpretation of the measured complex
reflectivity yields information about the properties of the
respective surface.
[0013] While existing apparatus allow for only one-dimensional
measurements, the inventors note that it may be possible in the
future to perform measurements in two dimensions, enabling to
determine mutual distances between, for example, alignment markers.
Ellipsometers may thus be used in a lithographic apparatus. In
particular, ellipsometers are employed in connection with (wafer)
alignment.
[0014] Presently available ellipsometers (also named
scatterometers) are generally slow. This slowness may cause a long
measurement time in any application, and it may be a critical
problem for in-line metrology applications for scatterometers. The
inventors have realized that this long measurement time is
generally primarily related to the technical design of the
measurement unit of an ellipsometer. Typically, the measurement of
the elliptical polarized light is done using measurement devices
that include moveable parts such as rotating analyzers, or rotating
retarders. Also, photo-elastic modulators are commonly used. These
photo-elastic modulators can also be considered as measurement
devices containing moveable parts.
[0015] An example of an ellipsometer can be found, for example, at
http://www.instant-analysis.com/patents/polarization patent.htm (as
available on May 2, 2004). This document discloses a special
analyzing mask consisting of four polarizers in different positions
covering six photo detectors and a waveplate that covers two photo
detectors. Monochromatic light is incident on the analyzing plate
and subsequently hits the photo detectors. The electric signals
generated by the incident light on the photo detectors are used, in
conjunction with a suitable computer, to measure the polarization
state of the monochromatic light incident on the analyzing plate.
To assess the polarization state of the monochromatic light, it is
desirable to obtain information about the degree-of-polarization,
azimuth angle and ellipticity of the monochromatic light. The
analyzing mask as described above, however, and the number of
required detectors, makes this ellipsometer relatively complex. A
minimum number of four detectors may be required in a conventional
ellipsometer.
SUMMARY
[0016] An ellipsometer according to one embodiment of the invention
includes an optical component including at least a first part being
connected along a border surface to a second part, the first part
and the second part being birefringent. The first part has a first
optical axis oriented along a first direction and the second part
has a second optical axis oriented along a second direction, the
first direction being different from the second direction. The
optical component is arranged in use to receive a beam of radiation
on the border surface, the beam having a state of polarization. The
border surface is arranged to split the beam into a first and a
second component which are, when the ellipsometer is in use,
reflected into the first part and a third and a fourth component
which are, when the ellipsometer is in use, transmitted into the
second part. The at least one detector is arranged to measure a
property of at least three out of the four components to allow
calculating the state of polarization. In one application, such a
device is used to enable a swift measurement of the state of
polarization of incident light.
[0017] An ellipsometer according to another embodiment of the
invention, includes at least one optical component including at
least a first birefringent part and a second birefringent part, the
first birefringent part having a first optical axis oriented along
a first direction and being connected along a border surface to the
second birefringent part, the second birefringent part having a
second optical axis oriented along a second direction different
from the first direction. The ellipsometer also includes at least
one detector. The at least one optical component is configured to
receive a beam of radiation, having a state of polarization, on
said border surface, and the border surface is configured to split
the beam into a first and a second component reflected into the
first birefringent part and to split the beam into a third and a
fourth component transmitted into the second birefringent part. The
at least one detector is configured to measure a property of at
least three out of the four components to calculate the state of
polarization.
[0018] In an embodiment of the invention, there is provided a
measurement device including an ellipsometer as described above and
a detection and evaluation unit configured to calculate the state
of polarization.
[0019] A device manufacturing method according to an embodiment of
the invention includes providing a substrate; providing a beam of
radiation using an illumination system; using a patterning
structure to impart the beam of radiation with a pattern in its
cross-section; and projecting the patterned beam of radiation onto
a target portion of the substrate, wherein an ellipsometer as
described above is used to characterize a surface of the
substrate.
[0020] A device manufacturing method according to another
embodiment includes determining the complex reflectivity of a
surface of a substrate; providing a beam of radiation; patterning
the beam of radiation; and projecting the patterned beam of
radiation onto a target portion on the surface of said substrate,
wherein the determination of the complex reflectivity of the
surface is done with an ellipsometer as described herein.
[0021] A device manufacturing method according to a further
embodiment of the invention includes providing a substrate;
providing a beam of radiation using an illumination system; using a
patterning structure to impart the beam of radiation with a pattern
in its cross-section; and projecting the patterned beam of
radiation onto a target portion of the substrate, wherein a
measurement device as described above is used to characterize a
surface of the substrate.
[0022] A method according to an embodiment includes providing a
beam of radiation; patterning the beam of radiation with a pattern
in its cross-section; projecting the patterned beam of radiation
onto a target portion of a surface of a substrate, and measuring a
complex reflectivity of the surface of the substrate using a
measurement device as described herein.
[0023] A method of measurement according to an embodiment of the
invention includes directing a measurement beam of radiation along
an optical axis of a first birefringent part to a border surface
between the first birefringent part and a second birefringent part,
the second birefringent part having an optical axis oriented along
a different direction from the optical axis of the first
birefringent part. The method also includes measuring a property of
each of at least three of the group consisting of first and second
components of the measurement beam reflected from said border
surface into the first birefringent part and third and fourth
components of the measurement beam transmitted from said border
surface into the second birefringent part.
[0024] An ellipsometer according to another embodiment of the
invention includes a first birefringent part, a second birefringent
part, and at least one detector. The first and second birefringent
parts are arranged in optical communication via a border surface
such that a beam of radiation transmitted through said first
birefringent part is split into a first component, a second
component, a third component, and a fourth component, said first
and second components being reflected from said border surface into
said first birefringent part and said third and fourth components
being transmitted from said border surface into said second
birefringent part. The at least one detector is configured to
measure a property of each of at least three of the first, second,
third and fourth components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] 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:
[0026] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0027] FIG. 2 shows a principle of operation of an
ellipsometer;
[0028] FIG. 3 shows an ellipsometer according to an embodiment of
the present invention; and
[0029] FIG. 4 is a graphic representation of a matrix of rotation
according to an embodiment of the invention.
DETAILED DESCRIPTION
[0030] Embodiments of the present invention include an ellipsometer
with improved design characteristics (i.e. a relatively simple
ellipsometer) that is configured to measure the polarization state
of incoming monochromatic light in a swift, easy and reliable way
using only a minimum number of detectors.
[0031] FIG. 1 schematically depicts a lithographic apparatus
according to an embodiment of the invention. The apparatus includes
an illumination system (illuminator) IL configured to provide a
beam PB of radiation (e.g. UV radiation or EUV radiation), a first
support structure (e.g. a mask table) MT configured to support a
patterning structure (e.g. a mask) MA and connected to first
positioning device PM configured to accurately position the
patterning structure with respect to the projection system
("lens"), item PL. The apparatus also includes a substrate table
(e.g. a wafer table) WT configured to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioning device
PW configured to accurately position the substrate with respect to
the projection system ("lens"), item PL, the projection system
(e.g. a refractive projection lens) PL being configured to image a
pattern imparted to the projection beam PB by a patterning
structure MA onto a target portion C (e.g. comprising one or more
dies) of the substrate W.
[0032] 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).
[0033] The illuminator IL receives a beam of radiation 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 including for example suitable directing mirrors and/or a
beam expander. In other cases the source may be an integral part of
the 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.
[0034] The illuminator IL may include an adjusting structure AM
configured to adjust the angular intensity distribution of the
beam. Generally, at least the outer and/or inner radial extent
(commonly referred to as a-outer and a-inner, respectively) of the
intensity distribution in a pupil plane of the illuminator can be
adjusted. In addition, the illuminator IL generally includes
various other components, such as an integrator IN and a condenser
CO. The illuminator provides a conditioned beam of radiation,
referred to as the beam of radiation PB, having a desired
uniformity and intensity distribution in its cross-section.
[0035] The beam of radiation PB is incident on the mask MA, which
is held on the mask table MT. Having traversed the mask MA, the
beam of radiation PB passes through the lens PL, which focuses the
beam onto a target portion C of the substrate W. With the aid of
the second positioning device PW and position sensor IF (e.g. an
interferometric device), the substrate table WT can be moved
accurately, e.g. so as to position different target portions C in
the path of the beam PB. Similarly, the first positioning device 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 beam PB, e.g. after mechanical retrieval from a
mask library, or during a scan. In general, movement of the object
tables MT and WT will be realized with the aid of a long-stroke
module (coarse positioning) and a short-stroke module (fine
positioning), which form part of the positioning device PM and PW.
However, 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.
[0036] The depicted apparatus can be used in the following
preferred modes:
[0037] 1. In step mode, the mask table MT and the substrate table
WT are kept essentially stationary, while an entire pattern
imparted to the beam of radiation is projected onto a target
portion C at once (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.
[0038] 2. In scan mode, the mask table MT and the substrate table
WT are scanned synchronously while a pattern imparted to the beam
of radiation 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 is 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.
[0039] 3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning structure, and the
substrate table WT is moved or scanned while a pattern imparted to
the projection beam is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning structure 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 structures, such as a programmable mirror array of a
type as referred to above.
[0040] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0041] With reference to FIG. 2, a general principle of operation
of an ellipsometer 8 is explained. Such an ellipsometer 8 may be
present, for example, near the wafer stage in a lithographic
apparatus (see FIG. 1) or in the alignment unit of a dual stage
scan apparatus. The ellipsometer 8 includes an optical component
(or combination of optical components), shown schematically as a
rectangle indicated by reference numeral 2. A monochromatic beam of
light 15 enters the optical component 2. The beam 15 may be
received from a surface under investigation. In the optical
component 2, a set of measurements may be carried out with
different combinations of polarizers and/or waveplates in different
subsequent positions. Obtaining such measurements may require
physical movements of these components, which may be a time
consuming operation. Alternatively, it is also possible to split
the beam 15 in several components that pass through combinations of
polarizers and/or waveplates at the same time. However, this may
render an ellipsometer based on this principle relatively
complex.
[0042] Either such method may be used to provide measurement
signals (e.g. from one or more photo detectors receiving the beam
and/or components thereof) as referred to with respective reference
numerals 4, 10, 12 and 14 in FIG. 2. The measurement signals 4, 10,
12 and 14 are input to a detection and evaluation unit 30. The
detection and evaluation unit 30 (which may include, for example, a
number of photo detectors and/or a suitably programmed computer or
processor) evaluates the measurement signals 4, 10, 12 and 14 and
provides the state of polarization 6 of the incoming beam 15.
[0043] In FIG. 3, an ellipsometer 100 according to an embodiment of
the present invention is shown. Similar reference numerals as used
in connection with FIG. 2 refer to similar parts. The ellipsometer
in FIG. 3 includes a prism 1. This prism 1 may be employed as the
optical component 2 in FIG. 2. The prism 1 includes a first part 3
and a second part 5. The first part 3 has an uniaxial (or "optic"
or "optical") axis 17 and the second part 5 has an uniaxial axis
29.
[0044] The first part 3 and the second part 5 are connected to each
other along a border surface or border surface 27. It will be
appreciated that the term "connected" as used herein to indicate a
relation between the first part and the second part does not
necessarily refer to a fixed connection. It should be understood
that if the first part 3 and the second part 5 are placed in each
other's vicinity (e.g. such that optical communication from the
first part to the second part may occur), the invention will also
function. At the border surface 27, the monochromatic beam 15 of
light is split into four parts or components 7, 9, 11 and 13 that
hit four respective detectors 31, 33, 35 and 37 located at
respective locations 19, 21, 23 and 25. The detectors 31, 33, 35
and 37 are connected to the detection and evaluation unit 30.
[0045] The ellipsometer functions in the following way. The prism 1
is oriented in such a way that the uniaxial axis 17 of the first
part 3 is substantially along the direction of propagation of the
incoming monochromatic beam 15. Such orientation may require a
suitable calibration beforehand. The beam 15 may include visible
light and/or radiation with a frequency outside (e.g. above or
below) the visible part of the electromagnetic spectrum. The beam
15 impinges on a front surface or "face" 16 of prism 1 and enters
the prism 1. The front surface 16 and the border surface 27 are
oriented at a predetermined angle with respect to each other.
[0046] As the beam 15 is directed substantially along the uniaxial
axis 17, it will remain substantially unsplit while passing through
the first part 3. When the beam 15 hits the border surface 27,
parts 11 and 13 of the beam 15 will be transmitted to the second
part 5 of the prism 1, and parts 7 and 9 of the beam 15 will be
reflected backwards into the first part 3. The orientation of the
uniaxial axis 17 of the first part 3 of the prism 1 is different
from the orientation of the uniaxial axis 29 of the second part 5
of the prism 1. The second part 5 may be positioned in such a way
with respect to the first part 3 that the transmitted beam 11 will
be exactly aligned with the beam 15. However, principles of the
invention may also be applied to arrangements in which this
particular alignment is not present.
[0047] The part 3 and the part 5 are birefringent. This means, as
will be apparent to the skilled person, that the beam 15 after
being reflected by the border surface 27 will be propagate at an
angle relative to the uniaxial axis 17, and will be split in two
linearly polarized states that are mutually orthogonal, an ordinary
or "o"-part 9 and an extraordinary or "e"-part 7. The transmitted
part of the beam 15 at the border surface 27 will be at an angle
relative to uniaxial axis 29 and is split into an ordinary or
"o"-part 11 and an extraordinary or "e"-part 13. The definitions of
"ordinary" and "extraordinary" are known to the skilled person.
[0048] The parts 7, 9, 11 and 13 subsequently hit the four
respective detectors 31, 33, 35 and 37 located at respective
locations 19, 21, 23 and 25. These detectors 31, 33, 35 and 37
measure the intensities of the parts 7, 9, 11 and 13. They send
output signals indicating these four intensities to the detection
and evaluation unit 30. This unit may be constructed as an
evaluation unit comprising, e.g., a processor configured to
evaluate expressions as described herein. From the intensities and
the polarization of the o- and e-parts, the detection and
evaluation unit 30 determines the polarization of the incoming beam
15 in accordance with calculation rules known to the skilled
person. The o-part 9 is y polarized at 0.degree. compared with the
beam 15, the e-part 7 is (x+y) polarized at 90.degree. compared
with the beam 15, the o-part 11 is (x-y) polarized at -45.degree.
compared with the beam 15 and the e-part 13 is (x+y) polarized at
45.degree. compared with the beam 15. Alternatively, the phase of
the parts 7, 9, 11 and 13 of the beam 15 may be used, instead of
(and/or in addition to) the intensities, to obtain the state of
polarization of the beam 15.
[0049] In addition, the border surface 27 may be coated with a
suitable substance to equalize the transmitted and reflected
optical power, i.e. to divide the total input power of beam 15 over
the reflected beams 7, 9 and the transmitted beams 11, 13 (such as,
e.g. 50%-50%) such that all beams have enough power to allow easy
detection. For example, a TBP reflective coating (a broadband
partially reflective coating that reflects up to 50%, e.g. as
marketed by LINOS Aktiengesellschaft, Goettingen, Del.) may be
used.
[0050] Also, as a "fill up" material at the location of the border
surface 27 (a layer of) air may be considered. In an embodiment of
the invention, the resulting "air gap" may be in the order of half
the wavelength used (e.g. a few hundred nanometers) in between the
first part 3 and the second part 5. In such case, there may be no
direct mechanical contact between first part 3 and the second part
5.
[0051] In another embodiment of the invention, air may be combined
with a coating. For example, one may fill the gap in between the
first part 3 and the second part 5, partly, e.g. with a substance
such as a TBP reflective coating mentioned above, combined with an
air gap of about 5 micrometer. In this case, it may be desirable to
avoid mechanical contact between the first part 3 and the second
part 5, since e.g. mechanical contact may cause stresses in the
substance/coating that may deform it and/or have an effect on
properties (e.g. of reflection) of the coating.
[0052] For purpose of illustration, an example is given below
describing how the polarization of an incoming beam of radiation 15
can be calculated from the intensities measured by the detectors
31, 33, 35 and 37 (see also FIG. 3).
[0053] With reference to the prism 1, as shown in FIG. 3, the axes
{x,y,z} are also indicated. Elliptically polarized light incident
on the face 16 is given by two components in the {xy} plane:
E.sub.x=E.sub.a sin(.omega.t) E.sub.y=E.sub.b
sin(.omega.+.delta.)
[0054] In principle, the above relation states that elliptically
polarized light can be seen as a sum of two linearly polarized
beams, which have a phase difference .delta. and an de ratio R: R =
E a E b . ##EQU1##
[0055] The ellipsometer may be applied to measure R and
.delta..
[0056] In complex terms, the two incident amplitudes may be written
as: E.sub.x=E.sub.ae.sup.j.omega.t E.sub.y=E
E.sub.be.sup.j(.omega.t+.delta.) where j= {square root over (-1)}.
These beams "travel" unchanged until the border surface 27. There,
parts 7, 9 of the incident beam 15 are reflected and parts 11, 13
are transmitted. First the reflected parts 7, 9 are considered.
[0057] If the border surface 27 is coated, the reflection at the
coated border surface 27 may be described by a reflection
coefficient r. This coefficient is a property of the coating, and
depends usually on the orientation of the polarization of the light
and the incident angle. In addition, the reflection coefficient is
a complex number.
[0058] Due to the particular orientation of the axis 17 of the
first part 3 of the prism 1, it can be seen that the part 9 of the
incident beam 15, also referred to below as ON
[0059] The coating layer 27 also has some transmission
coefficients, which are dependent as well on the polarization and
angle of incidence, and also are complex numbers. In this example,
only the numbers that present interest are used, namely t.sub.+45
and t.sub.-45. Here, t.sub.+45 refers to the transmission
coefficient of the part 13 of the beam 15 polarized at 45.degree.
and t.sub.-45 refers to the transmission coefficient of the part 11
of the beam 15 polarized at -45.degree..
[0060] Then, the amplitudes of the transmitted parts 11, 13, also
referred to below as OR and OS, respectively, can be written as:
E.sub.OR=t.sub.-45E.sub.-45 E.sub.OS=t.sub.+45E.sub.+45.
[0061] This gives the intensities: I.sub.R=|t.sub.-45|.sup.2
E.sub.-45.sup.2 I.sub.S=|t.sub.45|.sup.2 E.sub.45.sup.2.
[0062] Here, see FIG. 3, I.sub.R refers to the intensity as
measured by the detector 35 and I.sub.S refers to the intensity as
measured by the detector 37. The components for the incident fields
at the interface E.sub.+45, E.sub.-45, are not written here
explicitly because they are calculated from the known incident
fields E.sub.a, E.sub.b, and the corresponding phase .delta.. This
can be done using a matrix of rotation, as shown in FIG. 4.
[0063] The matrix is written as: [ E + 45 E - 45 ] = [ cos
.function. ( 45 .times. .degree. ) sin .function. ( 45 .times.
.degree. ) - sin .function. ( 45 .times. .degree. ) cos .function.
( 45 .times. .degree. ) ] .function. [ E x E y ] = [ E x + E y - E
x + E y ] , ##EQU2## in which a factor of 1/2 2 has been omitted in
front of the right hand matrix, as it cancels in the ratio of the
intensities on the detectors 35 and 37, written as: I R I S = t -
45 t 45 2 .times. E - 45 E 45 2 = t - 45 t 45 2 .times. - E x + E y
E x + E y 2 . ##EQU3##
[0064] The complex initial formulas for E.sub.x and E.sub.y are now
replaced to obtain: (polarization perpendicular on the plane of
incidence) is the ordinary beam, and the part 7, also referred to
below as OM (polarization in the plane of incidence) is the
extraordinary beam. The reflection coefficients of the coating at
the border surface 27 for the ON (initially 0.sup.0, or y
component) and OM (initially 90.degree., or x component) parts are
taken as r.sub.0 and r.sub.90.
[0065] The amplitudes of the OM and ON parts become then:
E.sub.OM=r.sub.90E.sub.ae.sup.j.omega.t
E.sub.ON=r.sub.0E.sub.be.sup.j(.omega.t+.delta.)
[0066] and the intensities of the two parts 7, 9 are given by:
I.sub.M=|r.sub.90|.sup.2 E.sub.a.sup.2 I.sub.N=|r.sub.0|.sup.2
E.sub.b.sup.2.
[0067] Here (see FIG. 3) I.sub.M refers to the intensity of part 7
as measured by the detector 31 and I.sub.N refers to the intensity
of part 9 as measured by the detector 33. The first R value can
then be calculated as: R = E a E b = r 0 r 90 .times. I M I N ( 1 )
##EQU4##
[0068] It should be noted that only the absolute reflection
coefficients of the coated material at the border surface 27 enter
into the formula (1) in the example above.
[0069] In order to calculate .delta., the transmitted parts 11, 13
of the beam 15, and R, which can be calculated from the relation as
derived above, may be used.
[0070] The second part 5 of the prism 1 may be oriented in such a
way as to choose the polarization of the transmitted ordinary beam
11 (also referred to below as OR) in the x+y direction to be
-45.degree. and the one of the extraordinary beam 13 (also referred
to below as OS) in the -x+y direction to be 45.degree.. In such a
case, the orientation of the optical axis 29 can be determined
considering that it must be perpendicular to the polarization of
the ordinary axis (which is x+y). Thus, it must be confined in the
plane determined by the vectors (x-y) and (z). One reasonable
option is the direction (x-y+z). I R I S = t - 45 t 45 2 .times. -
E a .times. e j .function. ( .omega. .times. .times. t ) + E b
.times. e j .function. ( .omega. .times. .times. t + .delta. ) E a
.times. e j .function. ( .omega. .times. .times. t ) + E b .times.
e j .function. ( .omega. .times. .times. t + .delta. ) 2 = t - 45 t
45 2 .times. - R + e j.delta. R + e j.delta. 2 ##EQU5## which may
be further simplified as: I R I S = t - 45 t 45 2 .times. R 2 - 2
.times. R .times. .times. cos .times. .times. .delta. + 1 R 2 + 2
.times. R .times. .times. cos .times. .times. .delta. + 1
##EQU6##
[0071] For the above formula one finds immediately the value of the
phase difference .delta.: cos .times. .times. .delta. = R 2 + 1 2
.times. R .times. R ' - 1 R ' + 1 .times. .times. where .times. :
.times. .times. R ' = I R I S .times. t - 45 t 45 2 . ( 2 )
##EQU7##
[0072] It should be noted that, in the above calculations, real
numbers have been used for the reflection coefficients and
transmission coefficients. In general, however, these coefficients
may as well be tensors. Hence, these coefficients may be matrices.
Then, the result will not be given by the formulas (1) and (2)
above, but by a system of equations which must be solved
accordingly. Formulas (1) and (2) in this case are only
approximations.
[0073] It can thus be seen that the properties of the elliptically
polarized light (R and .delta.) can be measured using formula (1)
for R and formula (2) for .delta., once the transmission t and
reflection r coefficients of the coating are known, and if the
intensities of all four detectors 31, 33, 35 and 37 are measured,
to calculate the ratios I.sub.M/I.sub.N and I.sub.S/I.sub.R. It is
even possible to dispense with one detector and use a total number
of three detectors to obtain the properties of the elliptically
polarized light.
[0074] To explain this idea further, reference is again made to
FIG. 4. Let us suppose that we have only three detectors,
corresponding to polarizations along the X-,Y-, and X'-directions
(beams 7, 9, and 11). In the following, the values of the
reflection coefficients or transmission coefficients are not taken
into account. The ratio of the intensities, as measured by a first
detector corresponding to the X direction and a second detector
corresponding to the Y direction, gives one parameter of the
ellipse, namely: R=Ex/Ey= (Ix/Iy), see formula (1). The incoming
elliptically polarized light is a coherent combination of two
linearly polarized beams, one along the X-direction and one along
the Y-direction. The ratio Ex/Ey of the amplitudes is one parameter
calculated in ellipsometry.
[0075] In order to completely characterize the elliptically
polarized beam, however, the phase between the two linearly
polarized components Ex and Ey is needed as well. But the phase
difference cannot be determined by measuring only the intensities
Ix and Iy. This can be seen by observing that by changing the phase
difference between the two linearly polarized components Ex and Ey,
the intensities Ix and ly remain the same. However, the intensity
measured along a different direction (such as X') may change. For
example, let us consider the case Ex=Ey. If the phase difference
between Ex and Ey is 0, then linearly polarized light along X' is
obtained (if X' is oriented at 45 degrees with respect to X, see
FIG. 4), and thus Ex'= (2)Ex. If the phase difference between Ex
and Ey is 180 degrees, light polarized along Y' is obtained and
thus Ex'=0. In other words, Ex' varies from 0 to (2)Ex according to
the phase difference between Ex and Ey, and this can be measured
with a third detector measuring the intensity along the direction
X'.
[0076] While in the above description, the intensity has been used
as a measured quantity, the same properties of the ellipticity can
be obtained when using the phase as a measured quantity. To measure
the phases, it may be desirable to mix up the reflected and
transmitted beams 7, 9, 11, 13, and create thus effectively an
interferometer. This result can be realized for example by feeding
up the beams 11 and 13 into a single detector, after the
polarization of beam 11 has been rotated 90 degrees, with, for
example, a half wave plate.
[0077] The angle between the front face 16 on which beam 15 is
incident and the border surface 27 can be designed such that the
angle between the o-part 9 and the e-part 7 is maximized. In that
configuration, the distance between the detectors 31, 33 can be
relatively large. This predetermined angle may vary largely with
tens of degrees. A mathematical relation between the predetermined
angle and the angle between the o- and e-parts exists. However, a
typical interval for the angle is 0.degree.-30.degree. (for a prism
1 made of calcite), resulting in a split between the o- and e-parts
of 5.degree. to 15.degree..
[0078] If the first part 3 and the second part 5 are placed one on
top of the other, the reflected part 9 and 7 of the beam 15 may
have a low intensity, e.g. less than 10%. This may be valuable in
some cases, e.g. if the beam 15 is polarized in the shape of an
elongated ellipse. In this case, the prism 1 may be adjusted to
have about the same intensity on the four detectors 31, 33, 35 and
37. For example, adjustments can be made to the coating used and/or
to the angles of the prism 1.
[0079] It will be appreciated that only three intensities may be
sufficient to calculate the polarization of the incoming beam 15.
However, the fourth intensity, which may also be obtained, can be
used for calibration or error control purposes (e.g. misalignment
of the incoming beam 15 with reference to the uniaxial axis 17 ). A
relation, namely that the sum of the four intensities measured
equates to some constant, exists among the four intensities as
measured irrespective of the polarization of the incoming beam
15.
[0080] It will be appreciated that an ellipsometer according to an
embodiment of the invention can be used to perform broadband
measurements, e.g. using a grating in front of a detector. A
grating 39 may be present in front of the face 16 as shown in FIG.
3. Additionally, or alternatively, a grating may be positioned in
front of one or more of the individual detectors 31, 33, 35 and
37.
[0081] The size of the prism 1 may be in the order of centimeters
in an embodiment of the invention. It will be appreciated that an
ellipsometer according to an embodiment as described herein may be
employed in both lithographic apparatus of a transmissive type (see
FIG. 1) and of a reflective type.
[0082] According to an embodiment of the invention, at least one of
a phase and an intensity of a component is measured. The phase and
the intensity are properties from which the state of polarization
may be easily obtained.
[0083] One or more detectors may be configured to measure a
property of the four components such as intensity. The four
intensities may be used to correct for an undesired tilt and/or to
control a correct alignment.
[0084] In a further embodiment of the invention, the first part and
the second part are joined together with at least one layer present
in between. The layer can be designed to approximately equalize the
transmission and reflection coefficient as required.
[0085] In yet another embodiment of the invention, at least one of
the at least one layer includes air. A layer of air may reduce the
amount of mechanical contact between the two parts of the optical
component.
[0086] In a further embodiment of the invention, the ellipsometer
further includes at least one grating, which is configured to
transmit at least one of the beam and the four components. Such an
arrangement may be applied to make broadband measurements
feasible.
[0087] While specific embodiments have been described above, it
will be appreciated that the invention may be practiced otherwise
than as described. In addition, embodiments also include computer
programs (e.g. one or more sets or sequences of instructions) to
control a lithographic apparatus to perform a method as described
herein, and storage media (e.g. disks, semiconductor memory)
storing one or more such programs in machine-readable form. The
description is not intended to limit the invention.
* * * * *
References