U.S. patent application number 13/173207 was filed with the patent office on 2012-01-19 for projection exposure system, beam delivery system and method of generating a beam of light.
This patent application is currently assigned to CARL ZEISS SMT GMBH. Invention is credited to Vladimir Davydenko, Damian Fiolka, Matthias Kuss, Manfred Maul, Gerd Reisinger.
Application Number | 20120013878 13/173207 |
Document ID | / |
Family ID | 35840380 |
Filed Date | 2012-01-19 |
United States Patent
Application |
20120013878 |
Kind Code |
A1 |
Kuss; Matthias ; et
al. |
January 19, 2012 |
Projection Exposure System, Beam Delivery System and Method of
Generating a Beam of Light
Abstract
A beam delivery system of a projection exposure system comprises
a laser generating a beam of laser light from a plurality of
longitudinal laser modes in a cavity, wherein light generated by a
single longitudinal laser mode has an average line width
.lamda..sub.lat, wherein the laser light of the beam has, at each
of respective lateral positions of the beam, a second line width
.lamda..sub.lat corresponding to lateral laser modes, and wherein
the laser light of the beam has, when averaged over a whole cross
section thereof, a line width .lamda..sub.b corresponding to plural
lateral laser modes, and wherein
.lamda..sub.m<.lamda..sub.lat<.lamda..sub.b, and wherein an
optical delay apparatus disposed in the beam provides an optical
path difference .DELTA.l, wherein 0.8 .lamda. 0 2 ( 2 .DELTA.
.lamda. l ) < .DELTA. l < 1.8 .lamda. 0 2 ( 2 .DELTA..lamda.
l ) , ##EQU00001## wherein .lamda..sub.0 is an average wavelength
of the light of the first beam of laser light, and
.DELTA..lamda..sub.lat represents the second line width.
Inventors: |
Kuss; Matthias; (Sinningen,
DE) ; Fiolka; Damian; (Oberkochen, DE) ;
Reisinger; Gerd; (Oberkochen, DE) ; Maul;
Manfred; (Aalen, DE) ; Davydenko; Vladimir;
(Oberkochen, DE) |
Assignee: |
CARL ZEISS SMT GMBH
Oberkochen
DE
|
Family ID: |
35840380 |
Appl. No.: |
13/173207 |
Filed: |
June 30, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11792099 |
Jun 1, 2007 |
7995280 |
|
|
PCT/EP2005/012857 |
Dec 1, 2005 |
|
|
|
13173207 |
|
|
|
|
60632634 |
Dec 1, 2004 |
|
|
|
60676263 |
Apr 28, 2005 |
|
|
|
Current U.S.
Class: |
355/67 ; 359/577;
359/578 |
Current CPC
Class: |
B82Y 10/00 20130101;
G03F 7/70166 20130101; G03F 7/70583 20130101 |
Class at
Publication: |
355/67 ; 359/577;
359/578 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G02B 27/48 20060101 G02B027/48; G03B 27/72 20060101
G03B027/72; G02B 27/00 20060101 G02B027/00 |
Claims
1. (canceled)
2. A beam delivery system, comprising: a laser for generating a
beam of laser light from a plurality of longitudinal laser modes in
a cavity of the laser; an optical delay apparatus disposed in a
beam path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element disposed in the beam path of the closed loop, wherein the
phase changing element comprises a structured phase changing
surface having a plurality of projections and indentations of
amplitudes of more than 100 nm, and wherein at least one of widths
and heights of the projections and indentations have a random
distribution.
3. The beam delivery system according to claim 2, wherein lateral
extensions of the projections and indentations are smaller than
lateral extensions of laser light portions interacting with the
phase changing surface and originating from single longitudinal
laser modes in the cavity.
4. The beam delivery system according to claim 2, wherein lateral
extensions of the projections and indentations are smaller than
lateral extensions of coherence cells of the laser light at a
location of the phase changing surface.
5. The beam delivery system according to claim 2, wherein the
projections and indentations of the phase changing surface have a
stepped configuration.
6. The beam delivery system according to claim 5, wherein the
projections and indentations of the phase changing surface each
include surface portions oriented substantially parallel to a main
surface direction of the phase changing surface.
7. A beam delivery system, comprising: a laser for generating a
beam of laser light from a plurality of longitudinal laser modes in
a cavity of the laser; an optical delay apparatus disposed in a
beam path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element disposed in the beam path of the closed loop, wherein the
phase changing element comprises a structured phase changing
surface having a plurality of projections and indentations of
amplitudes of more than 100 nm, and wherein the projections and
indentations of the phase changing surface have a wedge
configuration.
8. The beam delivery system according to claim 7, wherein the
projections and indentations of the phase changing surface each
include surface portions inclined relative to a main surface
direction of the phase changing surface.
9. The beam delivery system; according to claim 8, wherein angles
of inclination of the projections are different between adjacent
projections.
10. The beam delivery system according to claim 8, wherein angles
of inclination of the projections and indentations are randomly
distributed.
11. The beam delivery system according to claim 7, wherein the
amplitudes of the projections and indentations are randomly
distributed.
12. A beam delivery system, comprising: a laser for generating a
beam of laser light from a plurality of longitudinal laser modes in
a cavity of the laser; an optical delay apparatus disposed in a
beam path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element comprising a phase changing surface disposed in the beam
path of the closed loop; and a surface wave generator for
generating surface acoustic waves propagating across a surface
portion of the phase changing surface exposed to the beam of laser
light traversing the optical delay apparatus.
13. The beam delivery system according to claim 2, wherein the
phase changing surface of the phase changing element is traversed
by the beam of laser light traversing the optical delay
apparatus.
14. The beam delivery system according to claim 2, wherein the
phase changing surface of the phase changing element provides one
of the plural reflective surfaces of the optical delay
apparatus.
15. The beam delivery system according to claim 2, wherein the
optical delay apparatus comprises a semi-reflective mirror
traversed by the beam of laser light traversing the optical delay
apparatus.
16. The beam delivery system according to claim 15, wherein a
portion of the beam of laser light traversing the optical delay
apparatus traverses the semi-reflective mirror to coincide with a
beam of laser light reflected from the semi-reflective mirror.
17. A beam delivery system, comprising: a laser for generating a
beam of laser light from a plurality of longitudinal laser modes in
a cavity of the laser; an optical delay apparatus disposed in a
beam path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element disposed in the beam path of the closed loop, wherein the
phase changing element comprises a structured phase changing
surface having a plurality of projections and indentations, and
wherein the optical delay element includes a prism made of a
CaF.sub.2 material having a crystal orientation such that a (100)
crystal plane is oriented under an angle of 45.degree. relative to
a surface of the prism.
18. A projection exposure system for imaging a patterning structure
onto a substrate, the system comprising: the beam delivery system
according to one claim 2; a projection optical system for imaging
an object plane into an image plane; a first mount for mounting the
patterning structure in a region of the object plane within a beam
path of the second beam of laser light generated by the beam
delivery system; and a second mount for mounting the substrate in a
region of the image plane of the projection optical system.
19. A projection exposure system for imaging a patterning structure
onto a substrate, the system comprising: the beam delivery system
according to one claim 7; a projection optical system for imaging
an object plane into an image plane; a first mount for mounting the
patterning structure in a region of the object plane within a beam
path of the second beam of laser light generated by the beam
delivery system; and a second mount for mounting the substrate in a
region of the image plane of the projection optical system.
20. A projection exposure system for imaging a patterning structure
onto a substrate, the system comprising: the beam delivery system
according to one claim 12; a projection optical system for imaging
an object plane into an image plane; a first mount for mounting the
patterning structure in a region of the object plane within a beam
path of the second beam of laser light generated by the beam
delivery system; and a second mount for mounting the substrate in a
region of the image plane of the projection optical system.
21. A projection exposure system for imaging a patterning structure
onto a substrate, the system comprising: the beam delivery system
according to one claim 17; a projection optical system for imaging
an object plane into an image plane; a first mount for mounting the
patterning structure in a region of the object plane within a beam
path of the second beam of laser light generated by the beam
delivery system; and a second mount for mounting the substrate in a
region of the image plane of the projection optical system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of generating a
beam of light, a beam delivery system and a projection exposure
system for imaging a patterning structure onto a light sensitive
substrate.
[0003] 2. Brief Description of Related Art
[0004] Lithographic processes are commonly used in the manufacture
of miniaturized structures, such as integrated circuits, liquid
crystal elements, micro-patterned structures and micro-mechanical
components.
[0005] A projection exposure system used for photolithography
generally comprises a projection optical system for imaging a
patterning structure, commonly referred to as a reticle, onto a
substrate, commonly referred to as a wafer. The substrate is coated
with a photosensitive layer, commonly referred to as a resist,
which is exposed with an image of the patterning structure using
imaging light. The imaging light is generated by a beam delivery
system illuminating the patterning structure with the imaging
light.
[0006] The beam delivery system comprises a laser light source,
such as an excimer laser, for producing the imaging light.
[0007] It has been observed that, due to the spatial coherence of
the laser light, interferences of the laser light result in a
non-homogeneous intensity of the imaging light in a plane where the
patterning structure is disposed. Such non-homogeneous distribution
of intensity of light, which is also known as speckle noise, may
result in a reduced imaging performance of the projection exposure
system.
SUMMARY OF THE INVENTION
[0008] The present invention has been accomplished taking the above
problems into consideration.
[0009] Embodiments of the present invention provide a method of
reducing a visibility of speckles in a projected image of a
projection exposure system.
[0010] Other embodiments of the present invention provide a beam
delivery system providing a beam of light having a reduced
coherence as compared to laser light directly emitted from a laser
of the beam delivery system.
[0011] Further embodiments of the present invention provide a
projection exposure system having an improved imaging performance
due to an improved homogeneity of light used for illuminating a
patterning structure to be imaged.
[0012] According to an embodiment of the invention, a method of
generating a beam of light, comprises: exciting a plurality of
longitudinal laser modes in a cavity of a laser and combining light
generated by the plurality of longitudinal laser modes to form a
first beam of laser light; separating the first beam of laser light
into at least one first partial beam and at least one second
partial beam; and combining the at least one first partial beam and
the at least one second partial beam to form a second combined beam
of laser light traversing a beam shaping optics to be incident on
an object plane; wherein the separating and combining includes
separating light of the longitudinal laser modes into at least
first light portions and second light portions and differently
manipulating the separated first and second light portions. By
differently manipulating first light portions and second light
portions, the first and second light portions will not identically
coincide in an object plane or image plane of a projection optical
system, such that speckle patterns generated by the first light
portions and speckle patterns generated by the second light
portions will not identically coincide in the object plane or will
experience an reduction due to an interference between the first
and second light portions.
[0013] According to an embodiment of the invention, a laser
generates a first beam of laser light, the first beam of laser
light is separated into a first partial beam and a second partial
beam, an optical path difference of the first partial beam is
provided relative to the second partial beam, and the first and
second partial beams are then combined to form a second beam of
laser light. The optical path difference .DELTA.l is greater than
about 0.8.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) and less
than about 1.8.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) wherein
.lamda..sub.0 is an average wavelength of the light generated by
the laser, and .DELTA..lamda..sub.lat is a line width of light
generated from a single lateral laser mode of the laser.
[0014] According to further exemplary embodiments herein, the
optical path difference .DELTA.l is greater than about
0.85.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) and less than
about 1.5.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat), and the
optical path difference .DELTA.l may be greater than about
0.9.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) and less than
about 1.24.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat)
[0015] According to an exemplary embodiment of the invention, the
combining of the first partial beam and the second partial beam is
performed such that cross sections of the first and second partial
beams are disposed adjacent to each other within a cross section of
the combined second beam.
[0016] According to a further exemplary embodiment, plural first
and second partial beams are combined such that their cross
sections are alternatingly disposed within the cross section of the
combined beam.
[0017] According to an exemplary embodiment, a beam path of the
first partial beam is laterally displaced relative to a beam path
of the second partial beam. Such displacement of the first and
second partial beams relative to each other may be greater than a
tenth of a distance corresponding to a width of a lateral laser
mode across the cross section of the first beam of laser light, and
less than the distance corresponding to the width of the lateral
laser mode.
[0018] According to an embodiment of the present invention, a beam
delivery system comprises a laser for generating a first beam of
laser light and an optical delay apparatus disposed in a beam path
of the first beam of laser light, wherein the optical delay
apparatus is configured to provide an optical path difference of a
first partial beam of the first beam of laser light relative to a
second partial beam of the first beam of laser light, wherein the
optical path difference is greater than about
0.8.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) and less than
about 1.8.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat).
[0019] According to an exemplary embodiment, the optical delay
apparatus comprises a stack of a plurality of first plates of a
transparent material, wherein the stack is disposed in the beam
path of the first beam of laser light such that plural first
partial beams of the first beam of laser light traverse the first
plates and that plural second partial beams traverse spaces between
adjacent first plates.
[0020] According to a further embodiment of the present invention,
a beam delivery system comprises a laser for generating a first
beam of laser light from a plurality of longitudinal laser modes in
a cavity of the laser; and an optical delay apparatus disposed in a
beam path of the first beam of laser light, wherein the optical
delay apparatus is configured to provide an optical path difference
of at least one first partial beam of the first beam of laser light
relative to at least one second partial beam of the first beam of
laser light; and wherein the optical delay apparatus comprises a
stack of a plurality of first plates of a transparent material
disposed at a distance from each other, wherein the first plates
are traversed by beam paths of plural first partial beams and
spaces between adjacent first plates are traversed by beam paths of
plural second partial beams.
[0021] In this embodiment, the optical path difference can be
greater than 1.8.lamda..sup.2/(2.DELTA..lamda..sub.lat).
[0022] According to an exemplary embodiment, the first plates are
each oriented substantially parallel to a direction of the light
traversing the optical delay apparatus.
[0023] According to a further exemplary embodiment, second plates
of transparent material are sandwiched between adjacent first
plates. A length of the first plates and/or a refractive index of
the material of the first plates differs from a length of the
second plates and a refractive index of the material of the second
plates, respectively.
[0024] According to an exemplary embodiment, the optical delay
apparatus comprises a third plate of transparent material disposed
in the beam path of the first beam of laser light such that
surfaces of the third plate are oriented transversely to the
direction of the first beam of laser light traversing the third
plate. The first partial beam directly traverses the third plate,
and the second partial beam is two or more times internally
reflected from surfaces of the third plate to be combined with the
first partial beam.
[0025] According to an exemplary embodiment, cross sections of the
first beam of laser light immediately upstream of the optical delay
apparatus and of the second beam of laser light immediately
downstream of the optical delay apparatus are substantially the
same.
[0026] According to a further embodiment of the present invention,
a beam delivery system comprises a laser for generating a beam of
laser light from a plurality of longitudinal laser modes in a
cavity of the laser; an optical delay apparatus disposed in a beam
path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element disposed in the beam path of the closed loop, wherein the
phase changing element comprises a structured phase changing
surface having a plurality of projections and indentations of
amplitudes of more than 100 nm.
[0027] According to an exemplary embodiment herein, lateral
extensions of the projections and indentations are smaller than
lateral extensions of coherent portions of laser light interacting
with the phase changing surface and originating from single
longitudinal laser modes in the cavity. This means that the lateral
extensions of the projections and indentations are smaller than
lateral extensions of coherent coherence cells of the laser light
of the beam at a location of the phase changing element.
[0028] Thus, light generated by one longitudinal laser mode
interacts with plural different projections and/or indentations
such that wave fronts of the laser modes are changed by each
interaction of the laser light with the phase changing surface
occurring in each traversal of the closed loop.
[0029] According to a still further embodiment of the invention, a
beam delivery system comprises a laser for generating a beam of
laser light from a plurality of longitudinal laser modes in a
cavity of the laser; an optical delay apparatus disposed in a beam
path of the beam of laser light, wherein the optical delay
apparatus comprises plural reflective surfaces arranged such that
the beam path comprises a closed loop; at least one phase changing
element comprising a phase changing surface disposed in the beam
path of the closed loop; and a surface wave generator for
generating surface acoustic waves propagating across a surface
portion of the phase changing surface exposed to the beam of laser
light traversing the optical delay apparatus.
[0030] The generated surface acoustic waves have an effect of
generating a structured phase changing surface such that different
portions of laser light generated by one single longitudinal laser
mode experience different phase changes when traversing the phase
changing surface.
[0031] In the above embodiments, the phase changing element has an
effect of artificially increasing an effective number of modes or
effective number of coherence cells of the light of a laser pulse.
Thus, an increased number of independently uncorrelated speckle
patterns is formed, wherein the increased number of speckle
patterns superimposed in the image plane results in a reduced
observable variation of light intensity in the image plane.
[0032] The laser may comprise an excimer laser, such as a KrF
laser, an ArF laser and an F.sub.2 laser.
[0033] According to a further exemplary embodiment, the laser
comprises a line narrowing module, having optical elements, such as
a prism and a reflective grating.
[0034] According to further exemplary embodiments, the beam
delivery system may comprise further optical elements, such as a
dispersion plate, a diffractive optical element, a refractive
optical element, and others.
[0035] According to an embodiment of the present invention, a
projection exposure system comprises a projection optical system
for imaging an object plane into an image plane, a first mount for
mounting a pattering structure in a region of the object plane, and
a second mount for mounting a substrate in a region of the image
plane of the projection optical system. The projection exposure
system further comprises a beam delivery system as illustrated
above for generating imaging light for illumination of the object
plane.
[0036] According to a further embodiment of the present invention,
a projection exposure system comprises a beam delivery system
including a laser for generating a beam of laser light from a
plurality of longitudinal laser modes in a cavity of the laser, and
an optical delay apparatus disposed in a beam path of the beam of
laser light; and a projection optical system for imaging a
patterning structure disposed in an object plane of the projection
optical system into an image plane thereof; wherein a beam of laser
light delivered by the beam delivery system to illuminate the
patterning structure disposed in the object plane has a
speckle-generated intensity variation of less than 2% across the
object plane.
[0037] Such reduced speckle contrast in the light illuminating the
patterning structure has a particular advantage in generating a
substantially uniform light intensity in the object plane of the
projection optical system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The foregoing as well as other advantageous features of the
invention will be more apparent from the following detailed
description of exemplary embodiments of the invention with
reference to the accompanying drawings. It is noted that not all
possible embodiments of the present invention necessarily exhibit
each and every, or any, of the advantages identified herein.
[0039] FIG. 1 is a schematic illustration of a projection exposure
system according to an embodiment of the invention;
[0040] FIG. 2 is a schematic illustration of a laser of a beam
delivery system of the projection exposure system shown in FIG.
1;
[0041] FIG. 3 is a graph illustrating line widths of laser light
generated by the laser shown in FIG. 2;
[0042] FIG. 4 is a schematic illustration of longitudinal and
lateral laser modes of the laser shown in FIG. 2;
[0043] FIG. 5 is a perspective view illustrating an optical delay
apparatus of the beam delivery system of the projection exposure
system shown in FIG. 1;
[0044] FIG. 6 is a graph illustrating an optical path difference
generated by the optical delay apparatus shown in FIG. 5;
[0045] FIG. 7 is a sectional view of a further embodiment of an
optical delay apparatus which may be used in the beam delivery
system of the projection optical system shown in FIG. 1;
[0046] FIG. 8 is a sectional view of a further embodiment of an
optical delay apparatus which may be used in the beam delivery
system of the projection optical system shown in FIG. 1;
[0047] FIG. 9 is a sectional view of a further embodiment of an
optical delay apparatus which may be used in the beam delivery
system of the projection optical system shown in FIG. 1;
[0048] FIG. 10 is a sectional view of a phase changing element of
the optical delay apparatus shown in FIG. 9;
[0049] FIG. 11 is a sectional view of a further example of a phase
changing element which may be used in the optical delay apparatus
shown in FIG. 9;
[0050] FIG. 12 is an elevational view of a further example of a
phase changing element which may be used in the optical delay
apparatus shown in FIG. 9;
[0051] FIG. 13 is a perspective view of a further embodiment of an
optical delay apparatus which may be used in the beam delivery
system of the projection optical system shown in FIG. 1;
[0052] FIG. 14 is a schematic illustration of a projection exposure
system according to a further embodiment of the invention; and
[0053] FIG. 15 is a perspective view of a further embodiment of an
optical delay apparatus which may be used in the beam delivery
system of the projection optical system shown in FIG. 1 or 2.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0054] In the exemplary embodiments described below, components
that are alike in function and structure are designated as far as
possible by alike reference numerals. Therefore, to understand the
features of the individual components of a specific embodiment, the
descriptions of other embodiments and of the summary of the
invention should be referred to.
[0055] FIG. 1 schematically illustrates a projection exposure
system 1. The projection exposure system 1 comprises a projection
optical system 3 comprising a plurality of lenses or mirrors for
imaging an object plane 5 of the projection optical system 3 onto
an image plane 7 of the projection optical system 3. A reticle 9 is
mounted by a reticle stage 11 such that a pattering structure
provided by the reticle 9 is disposed in the object plane 5. A
wafer 13 is mounted on a wafer stage 15 such that a light sensitive
resist provided on a surface of the wafer 13 is disposed in the
image plane 7. The patterning structure of the reticle 9 is
illuminated by a beam 17 of imaging light generated by a beam
delivery system 21.
[0056] The beam delivery system 21 comprises a laser light source
23 which is, in the present embodiment, an excimer laser, such as a
KrF laser, a ArF laser and a F.sub.2 laser. A beam 25 of laser
light generated by the laser light source 23 traverses an optical
delay apparatus 27, which will be illustrated in more detail below,
for reducing a coherence of the laser light. The beam 25 then
traverses a beam expander 29, which may comprise a refractive
optical element, a lens system 31, a refractive optical element 33,
such as a fly eye element, a further lens system 35, a diffractive
plate 37, a beam homogenizing apparatus 39, such as a glass rod,
and a further lens system 41 to be reflected from a mirror 43 to be
incident on the reticle 9. The optical elements 29, 31, 33, 35, 37,
39 and 41 are disposed such that a light intensity of the light of
the illuminating beam 17 is substantially constant across an
illuminated region of the reticle 9 and has a desired angular
distribution relative to the object plane 5.
[0057] The optical elements 29 to 41 illustrated so far may be of a
conventional arrangement for shaping the illuminating beam 17.
Further conventional arrangements of beam delivery systems are
known, for example, from U.S. Pat. No. 6,285,443 B1, U.S. Pat. No.
5,926,257 and U.S. Pat. No. 5,710,620, the contents of which are
incorporated herein by reference.
[0058] FIG. 2 is a schematic illustration of the excimer laser
light source 23. The laser light source 23 comprises a gas chamber
51 containing a gas which is excited by high voltage pulses to
produce excimer molecules emitting ultraviolet light.
[0059] The laser light source 23 further comprises a
semi-transparent mirror 53 which forms an exit window of the laser
which is traversed by the generated beam 25. The laser 23 further
comprises a line narrowing module 55, comprising an aperture 57,
two prisms 59 and a reflective grating 61. A further beam defining
aperture 63 is disposed between the gas chamber 51 and the exit
window 53.
[0060] The laser 23 may generate pulses of laser light at a
repetition rate of, for example, 4,000 Hz to 6,000 Hz, each pulse
having a duration of about 20 ns to about 150 ns. For example, 40
pulses are used for exposing each pattern onto the wafer. The laser
23 is a multi-mode laser supporting a plurality of lateral and
longitudinal laser modes. An average wavelength of the light 25
emitted from the laser is .lamda..sub.0. Due to the line narrowing
module 55, the distribution of wavelengths about the average
wavelength .lamda..sub.0 is a relatively narrow line width
.DELTA..lamda..sub.b as illustrated in FIG. 3 which shows a line 65
representing a distribution of light intensity I in dependence of
the wavelength .lamda.. The light 25 emitted from the laser 23
originates from plural longitudinal and lateral laser modes, each
having a line width smaller than the line width
.DELTA..lamda..sub.b of the light of beam 25. FIG. 3 shows a line
67 illustrating a spectral intensity distribution of an exemplary
laser mode having a line width .DELTA..lamda..sub.m.
[0061] FIG. 4 is a schematic illustration of a light pulse 71
generated by laser 23. The light pulse 71 is composed of light
generated from a plurality of longitudinal and lateral laser modes
within chamber 51, resulting in limited volumes 73 of coherent
light. In this simplified representation, each volume 73 of
coherent light has an extension l.sub.c(long) in the longitudinal
direction and an extension l.sub.c(lat) in both lateral directions.
Reference numeral 75 in FIG. 4 indicates plural coherence volumes
generated by different lateral modes at a same time, and reference
numeral 76 indicates coherence volumes generated by different
longitudinal modes at a same lateral position. The set of
longitudinal modes 76 contributing to light emitted at a particular
lateral position is also referred to as a lateral mode.
[0062] It should be noted that the illustration of FIG. 4 is very
schematic and only for illustrative purposes. In reality, a
coherence cell does not have a cubic shape as suggested by FIG. 4.
Further, different coherence volumes will overlap both in
longitudinal and lateral directions.
[0063] Light within a single coherence volume 73 is coherent light,
such that an interference pattern may be formed from light
originating from a single coherence cell 73. However, light from
one coherence cell superimposed with light from a different
coherence cell will not generate an interference pattern since a
coherence condition is not fulfilled between different cells.
[0064] The light of the exemplary coherence cell 73 indicated as a
hatched cell in FIG. 4 is assumed to have the spectral density as
illustrated by line 67 in FIG. 3 at a peak wavelength
.lamda..sub.1. The light from other coherence cells has different
peak wavelengths and may have slightly different line widths such
that the combined light of all coherence cells has a spectral
distribution as indicated by line 65 in FIG. 3.
[0065] Due to the geometry of the laser cavity 51 and a dispersion
of the line narrowing module 55, the average wavelength of the
light emitted by the laser will change across the cross section of
the beam 25. It appears that a spectral distribution of the laser
light generated by a single lateral mode formed of plural
longitudinal modes arranged in a line parallel to the direction of
the beam is narrower than the spectral distribution of the whole
beam and broader than the spectral distribution of the longitudinal
laser mode. Line 66 in FIG. 3 indicates a spectral distribution of
an exemplary lateral laser mode.
[0066] The following Table 1 illustrates data of an exemplary KrF
laser and an exemplary ArF laser.
TABLE-US-00001 TABLE 1 Excimer KrF ArF .lamda..sub.0 [nm] 248 193
.DELTA..lamda..sub.b [pm] 0.5 0.5 l.sub.b [cm] 6.15 3.72 N.sub.long
80-500 80-500 N.sub.lat 100 100 N.sub.tot 8,000-50,000 8,000-50,000
.DELTA..lamda..sub.lat [pm] 0.25 0.25 l.sub.lat [cm] 12.3 7.45
.DELTA..lamda..sub.m [pm] 0.125 0.125 l.sub.m [cm] 24.6 14.9
.lamda..sub.0 indicates the peak wavelength, .DELTA..lamda..sub.b
the line width of the laser light generated from the multitude of
laser modes forming the beam, N.sub.long indicates a number of
longitudinal modes, N.sub.lat a number of lateral modes and
N.sub.tot=N.sub.longN.sub.lat a resulting total number of laser
modes contributing to one pulse.
[0067] At each lateral position of the beam, the light is generated
from a lateral mode comprising plural longitudinal modes. A line
width of one single longitudinal mode is indicated by
.DELTA..lamda..sub.m, and the resulting line width of a lateral
mode at a particular lateral position is indicated by
.DELTA..lamda..sub.lat.
l.sub.m indicates a coherence length of the light from one single
laser mode calculated by the formula
l.sub.m=.lamda..sub.0.sup.2/2.DELTA..lamda..sub.m.
[0068] Table 1 further indicates comparative expressions
l.sub.lat=.lamda..sub.0.sup.2/2.DELTA..lamda..sub.lat, and
l.sub.b=.lamda..sub.0.sup.2/2.DELTA..lamda..sub.b.
[0069] The light from one single coherence cell 73 traverses the
beam delivery system 21 to be incident on the object plane 5. At
each location of the object plane 5, the incident light is composed
of light rays having traversed different paths through the beam
delivery system and having experienced slightly different optical
path lengths accordingly. The coherent light from one single
coherence cell 73 may thus generate an interference pattern, such
as a speckle pattern, in the object plane 5. The light intensity
will be modulated across the object plane, wherein a speckle
contrast may be as high as 100% which means that constructive
interference will take place at some locations and completely
destructive interference may take place at other locations.
[0070] Since light originating from different coherence cells will
not interfere with each other, each coherence cell will contribute
to an independent speckle pattern in the object plane. Such
independent patterns will result in an averaging of the light
intensities in the object plane such that the intensity modulation
is reduced by averaging by a factor 1/ {square root over
(N.sub.tot)} as compared to the modulation generated by the light
of one single coherence cell.
[0071] Even with such averaging, the light intensity distribution
may be not sufficiently constant in the object plane 5. Therefore,
the optical delay apparatus is disposed in the beam path of the
imaging light.
[0072] Moreover, it should be noted that, depending on the geometry
of the beam delivery system, it is possible that different
longitudinal modes may generate the same speckle patterns in the
object plane. In such situations, the number N of modes
contributing to the averaging is determined by the lower number
N.sub.lat of longitudinal modes rather than the total number
N.sub.tot of modes supported by the laser.
[0073] The optical delay apparatus 27 has a function of reducing
the coherence of the illuminating light and has a configuration as
shown in FIG. 5. The optical delay apparatus 27 comprises a
plurality of glass plates of a thickness d.sub.1 which are spaced
at a distance d.sub.2 from each other. The glass plates 81 are
mounted as a stack and fixed by two frames 83 (only one frame is
shown in FIG. 5) engaging the plates 81 at lateral sides 85
thereof. A broken line 87 illustrates a cross section of beam 25
incident on front surfaces 89 of plates 81. Flat main surfaces 91
of the plates 81 extend over a length L in a direction parallel to
the beam 25.
[0074] The beam 25 incident on the stack of plates 81 is separated
into first partial beams traversing the plates 81 and second
partial beams traversing the spaces between adjacent plates 81. The
first partial beams traversing the plates 81 experience an optical
delay or optical path length difference .DELTA.l=(n-1)L relative to
the second partial beams traversing the spaces between adjacent
plates 81 assuming a refractive index equal to 1 for the medium
between adjacent plates. The length L is chosen such that
.DELTA.l=.lamda..sub.0.sup.2/2.DELTA..lamda..sub.lat. Further, a
pitch d.sub.1+d.sub.2 of the stack is chosen such that it is about
equal to or less than a lateral extension of a lateral laser mode
in the incident beam 25. Assuming a square shaped beam cross
section, the lateral extension of a lateral laser mode is about the
diameter of the beam divided by {square root over (N.sub.lat)},
wherein {square root over (N.sub.lat)} is the number of lateral
modes of the beam. Thus, the coherent light from one single lateral
laser mode is separated into at least one first partial beam
experiencing the optical delay and at least one second partial beam
traversing the optical delay apparatus 27 without delay.
[0075] This is further illustrated in FIG. 6, in which a first line
95 represents a temporal intensity distribution of the non-delayed
first partial beam and a second line 97 indicates a temporal
intensity distribution of the delayed first partial beam.
[0076] It is apparent that each of the light pulses 95 and 97 may
form a speckle pattern in the object plane 5, such that the delay
apparatus 27 has a first effect of doubling the number of light
modes capable of forming an interference pattern in the object
plane. This first effect of the delay apparatus 27 reduces the
intensity modulation in the object plane 5 by a factor of 1/
{square root over (2)}.
[0077] Further, due to the temporal overlap of pulses 95 and 97,
light of the pulse 95 may interfere with the coherent light of the
pulse 97. However, the light traversing the plates 81 not only
experiences a delay by l.sub.lat relative to the light traversing
the spaces between plates 81, the light traversing the plates
further experiences a phase shift relative to the light not
traversing the plates 81. This phase shift is within a range from
zero to 2.pi., depending on the length L and the wavelength .lamda.
of the light. Since the wavelength .lamda. of the laser modes has a
random distribution about the peak wavelength .lamda..sub.0, the
resulting phase shifts which the light from the various laser modes
experiences from the optical delay apparatus 27 will also have a
random distribution. Therefore, a portion of the light intensities
of pulses 95 and 97 indicated as a hatched portion in FIG. 6
generates an interference pattern in the object plane 5 which is
different for each coherence cell such that a random averaging
takes place. Thus, a second effect of the optical delay apparatus
27 is to introduce random phase shifts between first and second
partial beams for further reducing the intensity modulation in the
object plane.
[0078] With the optical delay apparatus as illustrated above, it is
possible to reduce a coherence of the laser light to such an extent
that the intensity modulation in the object plane, and, thus, in an
image plane, of the projection exposure system due to speckles is
as low as 1%.
[0079] According to a further embodiment, an optical delay
apparatus as shown in FIG. 5 is disposed in the beam 25, wherein
the length L of the plates 81 is increased to generate
substantially greater optical delays or optical path length
differences .DELTA.L which are greater than
.DELTA..lamda..sub.0.sup.2/2.DELTA..lamda..sub.1at, such that a
substantial overlap between lines 95 and 97 in FIG. 6 does no
longer occur. Even without such overlap, this embodiment
significantly contributes to reducing a speckle contrast in the
object plane since the number of laser modes which will not
interfere with each other is increased.
[0080] According to a still further embodiment, plural optical
delay apparatuses of the type illustrated in FIG. 5 are disposed in
the beam path of beam 25. For example, a first optical delay
apparatus 27 may be disposed upstream of a second optical delay
apparatus 27 in the beam 25. The first and second delay apparatuses
may have different orientations of their plates 81 relative to the
beam 25. For example, the first delay apparatus may have its plates
81 oriented in a horizontal direction as illustrated in FIG. 5, and
the second delay apparatus may have its plate 81 oriented in a
vertical direction.
[0081] According to a further example, both the first and second
optical delay apparatuses have their plates oriented in a same
direction, wherein the plates of the second delay apparatus are
laterally displaced relative to the plates of the first optical
delay apparatus. Thus, portions of the laser light of the beam will
traverse only plates of the first optical delay apparatus, other
portions of the beam will traverse only plates of the second
optical delay apparatus, other portions will traverse plates of
both the first and second optical delay apparatus, and still other
portions of the beam will traverse none of the plates of the first
and second optical delay apparatuses.
[0082] Still further, it is possible that the first and second
optical delay apparatuses have plates of different lengths L.
[0083] FIG. 7 illustrates a further embodiment of an optical delay
apparatus 27a which may be used in the beam delivery system.
[0084] The optical delay apparatus 27a has a similar configuration
as that shown in FIG. 5, such that a plurality of parallel glass
plates 81a having a thickness d.sub.1 disposed at a distance
d.sub.2 from each other. The plates 81a have a length L.sub.1 in a
direction of an incident beam 25a. Further, glass plates 82 having
a thickness d.sub.2 are sandwiched between adjacent plates 81,
wherein front surfaces of the plates 82 are registered with front
surfaces 89a of the plates 81a. The plates 82 have a length L.sub.2
in the direction of the beam 25a which is less than the length
L.sub.1 of the plates 81a. The lengths L.sub.1 and L.sub.2 are
dimensioned such that the desired optical delay l.sub.m is
generated, such that L.sub.1 and L.sub.2 fulfill the relation:
l.sub.m=L.sub.1(n.sub.1-1)-L.sub.2(n.sub.2-1),
wherein [0085] n.sub.1 is a refractive index of the material of the
plates 81a, [0086] n.sub.2 is a refractive index of the material of
plates 82, and wherein a refractive index of the gas or vacuum
disposed in the void spaces between adjacent plates 81 is assumed
to be l for simplicity.
[0087] Further, the material of plates 82 may have a higher
extinction due to such as absorption and scattering for the laser
light as compared to the extinction of the material of plates 81a.
The lengths L.sub.1 and L.sub.2 are further determined such that
both first partial beams traversing the plates 81a and second
partial beams traversing the plates 82 experience a substantially
same extinction when traversing the optical delay apparatus
27a.
[0088] FIG. 8 illustrates a further embodiment of an optical delay
apparatus 27b. The optical delay apparatus 27b comprises a glass
plate 101 having a first main surface 103 and a second main surface
105 parallel to surface 103. Surface 103 is oriented transversely
in a beam 25b of incident laser light, wherein a surface normal of
surface 103 is oriented under an angle .alpha. relative to the
direction of the incident beam 25b. The surface 105 is a
semi-transparent surface separating the incident beam 25b into a
first beam 26 directly traversing the plate 101, and a second
partial beam 28 which is reflected from surface 105 and thereafter
reflected from surface 103 and then traverses surface 105 to be
superimposed with beam 26. A thickness d of the plate 101 is chosen
such that the partial beam 28 experiences a delay
.DELTA.l=.lamda..sub.0.sup.2/(2.DELTA..lamda..sub.lat) relative to
partial beam 26:
d = .DELTA. l 2 n , ##EQU00002##
wherein n is a refractive index of a material of plate 101.
[0089] Due to the angle .alpha., the partial beam 28 is laterally
displaced by an amount .delta.D relative to the partial beam 26.
Such displacement further contributes to reducing the modulation of
an interference pattern generated by light of partial beam 28
interfering with light of partial beam 26.
[0090] The displacement .delta.D is advantageously determined based
on a lateral extension of a lateral mode. According to a first
example the angle .alpha. fulfills the relation
0.5 arctan D 1 d N lat < .alpha. < 5 arctan D 1 d N lat .
##EQU00003##
According to further examples, the angle .alpha. fulfills the
relation
0.7 arctan D 1 d N lat < .alpha. < 2 arctan D 1 d N lat , or
##EQU00004## 0.8 arctan D 1 d N lat < .alpha. < 1.5 arctan D
1 d N lat . ##EQU00004.2##
Compared to a diameter D.sub.1 of the incident beam 25b, a diameter
D.sub.2 of combined partial beams 26, 28 is increased only by the
small value .delta.D.
[0091] FIG. 9 shows a further embodiment of an optical delay
apparatus 27c. The optical delay apparatus 27c comprises a prism
111 having five surfaces 113, 114, 115, 116 and 117.
[0092] Surface 113 is a semitransparent surface separating an
incident beam 25c into a first partial beam 26c directly reflected
from the surface 113 of the prism 111, and a second partial beam 30
which is refracted at the surface 113 and enters the bulk material
of the prism 111. Beam 30 is subsequently reflected from surfaces
114, 115, 116 and 117 of the prism 111 by internal reflection, and
is then again incident on surface 113 from the interior of the
prism 111. A portion of that beam traverses surface 113 and
coincides as a delayed beam 28c with beam 26c directly reflected at
surface 113. A beam path of beam 30 forms a closed loop within
prism 111.
[0093] The prism 111 may be made of a CaF.sub.2 material having a
crystal orientation such that a (100) crystal plane is oriented
under an angle .phi. of 45.degree. relative to the surface 113.
Such crystal orientation has an advantage in that an intrinsic
birefringence of the material has a reduced effect on the beam 30
traversing the material. If the light of incident beam 25c is
polarized by a polarizer 121, such as a half wave plate 121, the
delayed beam 28c has a substantially same polarization as the
directly reflected beam 26c.
[0094] A further plate like phase changing element 101c is disposed
in the beam path of beam 30 traversing the prism 111 such that beam
30 is substantially orthogonally incident on a surface 103c of
plate 101c.
[0095] FIG. 10 schematically shows a portion of an enlarged section
of plate 101c. Structured surface 103c of plate 101c is a stepped
surface such that projections 131 and indentation 133 are formed
and a thickness of the plate 101c varies across the surface. A
minimum thickness is b and a maximum thickness is b+a. Step
portions of equal thickness have a lateral dimension of s. Both the
height a of the steps and the width s of the step portions varies
across the surface of plate 101c. In the illustrated example, the
width s is within a range from 0,1 to 5,0 times a lateral extension
l.sub.c(lat) of a lateral laser mode in the beam 30 traversing the
plate 101c. The maximum height a of the stepped portions can amount
to some plural wavelengths of the laser light. For example, the
value of the height a may be in a range from 200 to 500 nm or as
high as some .mu.m. The projections 131 and indentations 133 can be
manufactured by lithographic methods, for example.
[0096] Further, the distribution of the individual heights of the
stepped portion and their widths s can have a random distribution
such that also the phase changing effect of plate 101c is a random
effect across the cross section of beam 28c.
[0097] The structured surface 103c has an effect that wavefronts of
the laser light traversing the surface 103c experience minute
deformations resulting in minute changes of propagation directions
of the light having traversed the surface 103c. If this light
traverses the surface 113 to coincide with the beam 26c directly
reflected from the surface 113, it will not be exactly coincident
with the directly reflected light. Thus, the combined beams 26, 28
include light generated by same longitudinal laser modes but
propagating in slightly different directions. This results in
different speckle patterns in the object plane disposed downstream
of the optical delay apparatus. Due to the enlarged number of
different speckle patterns, the uniformity of the light
distribution in the object plane will be significantly
increased.
[0098] Moreover, since a portion of the light of the beam 30 having
traversed the structured surface 103c a first time will be
reflected from the surface 113 and traverse the structured surface
103c a second time. This results in a further phase changing effect
on that light. A portion of that light will traverse the
semitransparent surface 113 and be combined with beam 26c, and a
further portion of that light will be reflected from surface 113c
and experience still further phase changing effects by traversing
the structured surface 103c, and so on.
[0099] FIG. 11 schematically shows a further example of a plate
101d having a structured phase changing surface 103d. The
structured surface 103d includes a plurality of projections 131d
and indentations 133d. The projections and indentations 131d, 133d
form a plurality of small prisms on the plate 101d which will cause
wavefront deviations of the laser light traversing the surface
103d. A characteristic dimension or extension of the prisms in a
lateral direction on the surface is less than, for example five to
ten times less than, a lateral extension Lc(lat) of a portion of
laser light originating from a same longitudinal laser mode.
Inclination angles .epsilon. of surface portions of the prisms
relative to a main surface direction of the surface 103d may
randomly vary from prism to prism. Still further amplitudes or
height differences between projections 131d and indentations 133d
may randomly vary from prism to prism.
[0100] In the embodiment shown in FIG. 9, the phase changing
surface 103c is traversed by the beam 30. It is, however, possible
that the structured phase changing surface is used as one of the
reflecting surfaces 113, 114, 115, 116 and 117 such that the beam
interacting with the structured surface is reflected there
from.
[0101] FIG. 15 is a perspective view schematically illustrating a
further embodiment of an optical delay apparatus 27d comprising a
prism for providing a closed-loop optical beam path similar to that
shown in FIG. 9. In this embodiment, a structured phase changing
surface 103d is provided on a reflecting surface 114d of the prism.
A beam of light traversing the closed loop is reflected from the
structured phase changing surface 103d by internal reflection. The
surface 103d is structured by a plurality of projections 131d and
indentations 133d forming prisms of random sizes and surface
orientations. The representation in FIG. 15 of the projections 131d
and indentations 133d is exaggerated with respect to a size of the
projections 131d and indentations 133d. In practice, the
projections 131d and indentations 133d are of a small size with
characteristic lateral extensions which are less that lateral
extensions of coherence cells of the laser light incident on and
reflected from the structured surface.
[0102] FIG. 12 is an elevational view of a reflective phase
changing surface wherein the structure of the reflective surface is
generated by surface acoustic waves. For this purpose, a surface
wave generator 141 including a plurality of interdigital electrodes
143 is provided on the surface 103e and connected to a
high-frequency generator 145. The substrate material of the plate
101e providing the surface 103e is made of a piezoelectric material
such that a high-frequency voltage generated by the high-frequency
generator 145 produces surface acoustic waves propagating in a
direction 147 across surface 103e. Broken lines 149 in FIG. 12
illustrate wavefronts of the surface acoustic waves, and a
rectangle 151 in FIG. 12 illustrates a portion of surface 103e in
which the beam 30 is incident on the surface 103e and may interact
with the surface acoustic waves 149.
[0103] FIG. 13 is a perspective view of a further optical delay
apparatus similar to that shown in FIG. 9. The optical delay
apparatus 27f shown in FIG. 13 differs from that shown in FIG. 9 in
that reflective surfaces 114f, 115f, and 116f are provided by
mirrors 114f, 115f, 116f rather than internal reflection surface
provided on a prism. A semitransparent surface 113f is provided as
a beam splitter on which beam 25f is incident, wherein a portion
26f of that beam traverses the beam splitter 113f and a portion 30f
is reflected from the beam splitter 113f. Subsequent reflections
from reflective surfaces 114f, 115f and 116f provide a closed loop
beam path of beam 30f such that the beam 30f is again incident on
the beam splitter 13f. A portion of that beam is reflected from the
beam splitter and coincides as beam 28f with beam 26f, whereas
another portion of the beam traverses the beam splitter 113f and
traverses the closed loop a second time or a greater number of
times. Any of the reflective surfaces 114f, 115f and 116f may be
formed as a structured surface of the type illustrated with
reference to FIGS. 10, 11 and 12 above. Still further, a plate
carrying such structured surface can be disposed between any of
mirrors 114f, 115f and 116f to generate phase changes when the beam
30f traverses such plate.
[0104] FIG. 14 is a schematic illustration of a further example of
a projection exposure system 1g in which the optical delay
apparatus as illustrated above can be incorporated. The projection
exposure system 1g comprises a beam delivery system 21g for
illuminating an object plane 5g of a projection exposure system 3g
such that the object plane 5g is imaged onto an image plane 7g.
[0105] The beam delivery system 21g comprises a laser light source
23g, such as an excimer laser. The laser 23g may include a beam
expanding optics such that a beam 25g emitted from the light source
23g is already an expanded beam. The beam 25g traverses an optical
delay apparatus 27g of the type illustrated above for reducing a
coherence of the laser light to reduce a speckle contrast generated
in the object plane 5g and in the image plane 7g, accordingly.
[0106] The laser light having traversed the optical delay apparatus
27g traverses a pupil shaping diffuser 32 which may be provided by
a diffraction grating, such as a computer generated hologram (CGH)
to define a shape and light distribution in a pupil plane 171
generated by a condenser lens 164.
[0107] A flies eye integrator 33g is disposed in the pupil plane
171. The light having traversed the flies eye integrator 33g
traverses a lens or lens system 163 such that a field plane 157 is
provided downstream of lens 163. Field plane 157 is imaged onto the
object plane 5g of the projection optical system 3g by a reticle
masking lens system 167, 169. A field stop 153 is disposed in field
plane 157 for defining that portion of the object plane 5g which is
illuminated with the laser light. Since the optical delay apparatus
27g of the type as illustrated above is disposed in the beam path
of the laser light, a speckle contrast generated in the object
plane 5g is effectively reduced to an amount which may be less than
2% or 1%.
[0108] According to further embodiments, an optical delay apparatus
as shown in any of FIGS. 5, 7 and 8 can be disposed in a beam path
of a type shown in FIG. 9, i.e. disposed in a closed loop path
formed by plural reflective surfaces. It should be noted that the
number of five reflections illustrated in combination with the
prism 111 in FIG. 9 is only an exemplary number. It is also
possible to use a lower number of reflections, such as three or
four reflections, or a higher number of more than five reflections
by suitably adjusting the relative angles of the reflecting
surfaces. Still further, it is also possible to dispose the plate
like phase changing element shown in FIG. 10 in the beam path of
the beam delivery system outside of a closed loop. For example, the
plate like phase changing element shown in FIG. 10 may be disposed
as the optical delay apparatus 27 in the beam delivery system shown
in FIG. 1.
[0109] It is envisaged to combine each of the above illustrated
embodiments with any other of the above illustrated embodiments
such that a combined embodiment may comprise one or more features
from one of the above illustrated embodiments and one or more
features of another of the above illustrated embodiments.
[0110] While the invention has been described with respect to
certain exemplary embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, the exemplary embodiments of
the invention set forth herein are intended to be illustrative and
not limiting in any way. Various changes may be made without
departing from the spirit and scope of the present invention as
defined in the following claims.
* * * * *