U.S. patent application number 11/707041 was filed with the patent office on 2007-06-21 for polarization rotator and a crystalline-quartz plate for use in an optical imaging system.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Michael Gerhard.
Application Number | 20070139636 11/707041 |
Document ID | / |
Family ID | 7685481 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070139636 |
Kind Code |
A1 |
Gerhard; Michael |
June 21, 2007 |
Polarization rotator and a crystalline-quartz plate for use in an
optical imaging system
Abstract
A polarization rotator and crystalline quartz plate for use with
an optical imaging system. The system has several imaging optical
components (L1-L16) sequentially arranged along an optical axis
(16), a means for creating radially polarized light arranged at a
given location in that region extending up to the last of said
imaging optical components, and a crystalline-quartz plate
employable in such a system. A polarization rotator (14) for
rotating the planes of polarization of radially polarized light and
transforming same into tangentially polarized light, particularly
in the form of a crystalline-quartz plate as noted above, is
provided at a given location within a region commencing where those
imaging optical components that follow said means for creating
radially polarized light in the optical train are arranged. The
optical imaging system is particularly advantageous when embodied
as a microlithographic projection exposure system.
Inventors: |
Gerhard; Michael; (Aalen,
DE) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
CARL ZEISS SMT AG
Oberkochen
DE
|
Family ID: |
7685481 |
Appl. No.: |
11/707041 |
Filed: |
February 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11336945 |
Jan 23, 2006 |
7199864 |
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11707041 |
Feb 16, 2007 |
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10883849 |
Jul 6, 2004 |
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11336945 |
Jan 23, 2006 |
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10145138 |
May 15, 2002 |
6774984 |
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10883849 |
Jul 6, 2004 |
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Current U.S.
Class: |
355/71 ;
355/67 |
Current CPC
Class: |
G03F 7/70241 20130101;
G02B 13/143 20130101; G02B 27/286 20130101; G03F 7/70966 20130101;
G03B 27/72 20130101; G03F 7/70566 20130101 |
Class at
Publication: |
355/071 ;
355/067 |
International
Class: |
G03B 27/72 20060101
G03B027/72 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2001 |
DE |
101 24 566.1 |
Claims
1. A projection exposure system, comprising: a plurality of imaging
optical elements disposed along an optical beam path extending from
a light source through an illumination system and a projection
lens; and a crystalline quartz plate arranged in the optical beam
path which rotates a polarization of an imaging beam passing along
said optical beam path by the optical activity of the crystalline
quartz plate.
2. The projection exposure system according to claim 1, wherein the
crystalline quartz plate is disposed at a location in the beam path
where the light rays propagate at least substantially parallel to
an optical axis of the system.
3. The projection exposure system according to claim 1, wherein the
crystalline quartz plate is disposed with its optical axis parallel
to an optical axis of the system.
4. The projection exposure system according to claim 1, wherein a
crystal axis of the crystalline quartz plate is at least
approximately normal to the plane of the crystalline quartz
plate.
5. The projection exposure system according to claim 1, wherein the
crystalline quartz plate has a thickness of 500 .mu.m or less.
6. The projection exposure system according to claim 5, wherein the
crystalline quartz plate has a thickness of 200 .mu.m or less.
7. The projection exposure system according to claim 1, wherein the
crystalline quartz plate outputs tangentially polarized light.
8. The projection exposure system according to claim 2, wherein the
crystalline quartz plate is disposed within the projection lens,
along the optical beam path.
9. The projection lens according to claim 8, wherein said
crystalline quartz plate is disposed at a pupillary plane of the
projection lens.
10. The projection lens according to claim 8, wherein said
crystalline quartz plate is disposed between a pupillary plane of
the projection lens and an image plane of the projection lens.
Description
[0001] This is a continuation of application Ser. No. 11/336,945
filed Jan. 23, 2006, which in turn is a continuation of application
Ser. No. 10/883,849 filed Jul. 6, 2004, which in turn is a
divisional application of application Ser. No. 10/145,138 filed May
15, 2002 (now U.S. Pat. No. 6,774,984). The entire disclosures of
the prior applications, application Ser. Nos. 11/336,945,
10/883,849, and 10/145,138, are considered part of the disclosure
of the present continuation application and are hereby incorporated
by reference. These Applications claim priority from German Patent
Application No. 101 24 566.1, filed on May 15, 2001, which is also
incorporated in this application by reference.
FIELD OF AND BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a polarization rotator and
crystalline quartz plate for use with an optical imaging system
having several imaging optical components arranged in succession
along an optical axis, means for creating radially polarized light
arranged within that region extending up to the last of said
imaging optical components, and a crystalline-quartz plate
employable on such a system.
[0004] 2. Description of the Related Art
[0005] German laid-open publication DE 195 35 392 A1 discloses an
optical imaging system of said type in the form of a
microlithographic projection exposure system having, e.g., an
i-line mercury discharge lamp as a light source. Said system's
employment of radially polarized light for exposing wafers was
intended to improve the coupling of said light into the layer of
photoresist, particularly at very large angles of incidence, while
simultaneously achieving maximum suppression of any standing waves
that might be caused by reflections at the inner and outer
interfaces of said photoresist. Various types of radial polarizers
that employ birefringent materials were mentioned as prospective
means for creating radially polarized light. That radial polarizer
chosen was arranged within that region that followed said system's
final phase-correcting or polarizing optical element in the optical
train in order that the degree of radial polarization attained
prior to incidence on said wafers would remain unchanged. In the
event that a catadioptic optical system were employed as said
system's projection lens, the radial polarizer involved should be
preferably arranged, e.g., following said optical system's final
deflecting mirror. Otherwise, it might be arranged, e.g., within
the preceding illumination system of the projection exposure
system.
[0006] Radially polarized light, i.e., light that is linearly
polarized parallel to its plane of incidence on an interface, is,
in general, preferable in cases involving imaging optics, e.g., the
imaging optics of microlithographic projection exposure systems,
since radially polarized light allows employing highly effective
antireflection coatings on their imaging optical components,
particularly their lenses, which is a matter of major importance,
particularly in the case of microlithographic projection exposure
systems with high numerical apertures and at short wavelengths,
e.g., wavelengths falling in the UV spectral range, since there are
few coating materials that are suitable for use in that spectral
range. On the other hand, tangentially polarized light, i.e., light
that is linearly polarized orthogonal to the plane of incidence of
an imaging light beam on the respective interfaces of the lenses,
or similar, involved, should preferably be employed for
illumination in order to allow creating the best possible
interference-fringe contrasts when imaging objects on, e.g.,
wafers. In order to allow same, the older German patent application
100 10 131.3 proposed employing a tangentially polarizing element
arranged in the vicinity of a pupillary plane of the projection
lens, or within the illumination system that precedes same in the
optical train that may be assembled from segmented birefringent
plates instead of the radial polarizer of German patent disclosure
DE 195 35 392 A1.
OBJECTS OF THE INVENTION
[0007] The invention is based on the technical problem of providing
a polarization rotator and a crystalline quartz plate for use in an
optical imaging system of the type mentioned at the outset that
will both allow comparatively highly antireflective coatings on its
optics, which will minimize disturbing reflected stray light, and
be capable of yielding an exiting light beam that will allow
creating high-contrast interference fringes on an image plane.
SUMMARY OF THE INVENTION
[0008] According to one formulation, the invention solves these and
other objects by providing a polarization rotator and a crystalline
quartz plate for use in an optical imaging system, in particular a
microlithographic projection exposure system, that includes several
imaging optical elements arranged one after the other along an
optical axis, and a radial polarizer radially polarizing light
transiting said optical imaging system and arranged at a location
ahead of the final imaging optical element. The polarization
rotator transforms the radially polarized light into tangentially
polarized light and is arranged at a location following that
imaging optical element that follows said radial polarizer in the
optical train. The invention additionally addresses these objects
by providing a crystalline-quartz plate configured as a
polarization rotator, wherein a crystal axis of said plate is at
least approximately normal to the plane of said plate.
[0009] The optical imaging system according to the invention is
characterized therein that it both provides a means for creating
radially polarized light with which at least part of the imaging
optical components of said system operate, and provides a
polarization rotator for rotating the planes of polarization of
said radially polarized light and for transforming same into
tangentially polarized light in order to yield light that will be
tangentially polarized in an imaging plane. Said polarization
rotator is arranged following at least one, and preferably several,
or even all, of the imaging optical components of said system.
[0010] A consequence of said measures according to the invention is
that all imaging optical components of said system that are
situated between said means for creating radially polarized light
and said polarization rotator may operate with radially polarized
light, for which they may be highly effectively antireflection
coated. In particular, a conventional type of radial polarizer
situated at an arbitrary location in the beam path between said
light source, i.e., a location ahead of said system's first imaging
optical component, and said system's final imaging optical
component, but ahead of said polarization rotator, may serve as
said means for creating radially polarized light. Said polarization
rotator will simultaneously transform said radially polarized
light, which is preferable for the imaging optical components
involved, into tangentially polarized light that will then be
incident on said image plane, which will allow creating
high-contrast interference fringes thereon. Since said polarization
transformation is effected by rotating planes of polarization, the
associated intensity losses may be held to low levels.
[0011] Under another embodiment of the invention, a plate having an
optically active material is employed as said polarization rotator.
Optically active materials are known to rotate the planes of
polarization of transmitted light, where the angles through which
same are rotated will be proportional to the thicknesses of said
materials and the constants of proportionality involved will
increase as the wavelengths involved decrease. Under another
embodiment of the invention, a crystalline-quartz plate serves as
said polarization rotator. Although said crystalline-quartz plate
will also have birefringent properties, suitably dimensioning and
orienting said plate will allow maintaining same at levels so low
that the desired polarization rotation will not be significantly
altered by the optical activity of said crystalline quartz, at
least not in cases involving UV-light, e.g., light having
wavelengths of about 157 nm or less.
[0012] Under beneficial other embodiments of the invention in which
said optical imaging system is a microlithographic projection
exposure system, said polarization rotator for rotating the planes
of polarization of radially polarized light and transforming same
into tangentially polarized light is arranged within a section of
said system's projection lens where the beam path is approximately
parallel to its optical axis, in particular, in a pupillary plane,
or within a section lying between a pupillary plane and an image
plane of same containing, e.g., a wafer to be illuminated. In the
case of the first of said arrangements, arranging said polarization
rotator in said pupillary plane has the advantage that the
approximately normal incidence of light on said polarization
rotator yields a high optical activity of same and the effects of
off-axis illumination of same, such as birefringence effects, will
remain minimal. On the other hand, arranging said polarization
rotator closer to said image plane has the advantages that those
imaging optical elements situated between said pupillary plane and
said polarization rotator will also be penetrated by radially
polarized light and that employment of a smaller polarization
rotator will be sufficient.
[0013] In the case of the crystalline-quartz plate according to the
invention, the crystal axis of said plate is oriented approximately
parallel to the normal to its surface.
[0014] A crystalline-quartz plate having said orientation is
particularly well-suited to employment as a polarization rotator on
optical imaging systems according to the invention.
[0015] Under a modified embodiment of the invention, the thickness
of said crystalline-quartz plate is 500 .mu.m or less and
preferably about 200 .mu.m or less. Plates that thin are
particularly suitable for accomplishing the polarization-rotation
function on optical imaging systems according to the invention when
operated at far-UV wavelengths of 157 nm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A beneficial embodiment of the invention is depicted in the
accompanying drawings and will be described below. Said drawings
depict:
[0017] FIG. 1: a schematic drawing of a microlithographic
projection exposure system that has a means for creating radially
polarized light arranged within its illumination system and a
polarization rotator for rotating the planes of polarization of
same and for transforming same into tangentially polarized light
arranged within its projection lens and
[0018] FIG. 2: a detailed drawing of the projection lens depicted
in FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIEMENTS
[0019] FIG. 1 depicts a conventional microlithographic projection
exposure system that is similar to that cited in German patent
disclosure DE 195 35 392 A1, except for the arrangement of its
polarization rotator within its projections lens. Light from a
light source (1), e.g., an i-line mercury discharge lamp, that
emits illuminating UV-radiation at a desired wavelength focused by
a mirror (2) illuminates an aperture stop (3) that is followed by a
lens (4) such as, in particular, a zoom lens, and that allows
making various adjustments, in particular, choosing a desired
circular aperture. Instead of a mercury discharge lamp, a laser
light source emitting at a wavelength of around 260 nm or less,
e.g., at 157 nm, may be employed as said light source (1), in which
case said mirror (2) will be superfluous.
[0020] A radial polarizer (5) that transforms unpolarized incident
light into radially polarized light is arranged following said lens
(4). Said radial polarizer (5) may be, e.g., a truncated-cone
polarizer having the configuration described in German patent
laid-open publication DE 195 35 392 A1, which performs said
transformation without causing significant light losses. The
resultant largely radially polarized light then travels from said
radial polarizer (5) to a honeycomb condenser (6) and a relay and
field lens (7) that follows same in the optical train. The latter
components collectively serve to provide optimal illumination of a
mask (8), which is also termed a "reticle," bearing the pattern to
be imaged. A projection lens (9), which has been configured as a
reducing lens and follows said components in the optical train,
images said pattern, which lies in the object plane of said
projection lens (9), onto a film of photoresist (10) on a wafer
(11) lying in the image plane of said projection lens (9) with
ultrahigh spatial resolution, preferably with a spatial resolution
of better than 1 .mu.m. The numerical aperture of said system
should preferably exceed 0.5, in particular, should preferably
range from 0.7 to 0.9.
[0021] FIG. 2 schematically depicts a prospective configuration of
said projection lens (9), which has numerous lenses (L1-L16). Since
many of the lens arrangements typically employed on projection
lenses of that type are known, those lenses (L1-L16) that have been
depicted in FIG. 2 are to be interpreted as representing lenses
typically employed on said conventional types of lens arrangements
and have thus been symbolically indicated by rectangles that, of
course, are not intended to represent their true geometric shapes.
In order to clarify the operation of said projection lens (9), the
paths of the principal rays (12a, 13a) and the marginal rays (12b,
13b) of the imaging beams (12, 13) associated with a central point
(8a) of said mask and a point (8b) near the edge of said mask,
respectively, have been schematically indicated.
[0022] The distinctive feature of the projection lens depicted in
FIG. 2 is its arrangement of a polarization rotator (14) that, in
the case of this particular example, is situated right after a
pupillary plane (15) of said projection lens where a typical
aperture stop is arranged. Said polarization rotator (14) has been
designed to rotate the planes of polarization of incident radially
polarized light and transform same into tangentially polarized
light. A thin crystalline-quartz plate whose crystal axis (17),
which has been schematically indicated in FIG. 2, is oriented
approximately parallel to the optical axis of said projection lens
may be employed in an exemplary embodiment of said polarization
rotator, where said crystal axis (17) of said crystalline-quartz
plate is oriented approximately orthogonal to the plane of said
plate, i.e., approximately parallel to the normal to its
surface.
[0023] Crystalline quartz is known to be optically active and,
unlike the case of normal birefringence, rotates the planes of
polarization of incident light, regardless of their original
orientations, due to its optical activity. Another advantage of
optically active materials is that they create no double images.
The angle of rotation for a given material will be proportional to
its thickness, where the constant of proportionality involved will
vary with its temperature and be largely determined by the
wavelength involved. In the case of the application involved here,
it is particularly beneficial that said constant of proportionality
markedly increases with decreasing wavelength and is several times
greater for wavelengths falling within the UV spectral range, e.g.,
the wavelength range 150 nm to 260 nm, than for visible light. This
is the reason why it will be sufficient to employ a very thin
crystalline-quartz plate whose thickness is only around 500 .mu.m,
and preferably 200 .mu.m or less, in order to produce the desired
rotation in cases where UV-radiation is employed on
microlithographic projection illumination systems. Since
birefringence effects will not simultaneously significantly
increase at shorter wavelengths, the ratio of the aforesaid desired
function of said optical activity to any disturbing birefringence
effects will be correspondingly improved at short wavelengths
falling within the UV spectral range.
[0024] Arranging said polarization rotator (14) near said pupillary
plane (15) or at some other location in the beam path where light
rays propagate parallel to, or at a small angle of inclination with
respect to, said optical axis (16) has the advantage that light
rays incident on same will be approximately normal to its surface,
in which case the ratio of said desirable function of said optical
activity to said, in the case of the example considered here,
undesirable, birefringence effects of crystalline quartz, will be
particularly large. In the case of that particular location of said
polarization rotator (14) shown in FIG. 2, eleven of said sixteen
lenses (L1-L16) of said projection lens and the entire optical
train of said illumination system, commencing with said radial
polarizer (5), will lie within that portion of the beam path where
light is largely radially polarized. This will allow providing
highly effective antireflection coatings on the lenses involved,
while said polarization rotator (14) will provide light incident on
said wafer (11) that has the desired, largely tangential,
polarization.
[0025] Alternatively, said polarization rotator (14) may also be
positioned at any arbitrary, location along said optical axis (16)
of said system, but should preferably be positioned as close as
possible to said image plane or said wafer (11) in order to ensure
that as many as possible of said imaging optical components will be
penetrated by radially polarized light. Relocating said
polarization rotator (14) from the vicinity of said pupillary plane
(15) to a location closer to said wafer (11) will allow choosing a
smaller diameter for said polarization rotator (14), while
providing that at least some of those lenses (L12-L16) situated
between the indicated location of said polarization rotator (14)
and said wafer (11) will still be irradiated by radially polarized
light. However, the divergence, i.e., the maximum angle of
inclination with respect to said optical axis (16), of the beam
incident on said polarization rotator (14) will then increase.
[0026] The ratio of the strength of said optical activity to that
of said birefringence effects will decrease with increasing angle
of incidence, which will slightly worsen the effects due to said
crystalline-quartz material's birefringence. However, decisions
regarding the maximum angles of incidence that may be tolerated may
be made based on the particular applications to be involved. Those
decisions will also depend upon the extent to which light has been
radially polarized with respect to the optical axis of the crystal
of said polarization rotator (14) prior to its arrival at same,
since, in the ideal case of totally radially polarized light, no
birefringence effects will occur, even for high beam divergences,
i.e., at large angles of incidence on same. However, said ideal
case will usually be unachievable in actual practice, since light
supplied by said illumination system will not be perfectly radially
polarized and slight departures from perfect radial polarization
will occur due to stress-induced birefringence in said lenses.
Nevertheless, fairly high beam divergences may be tolerated due to
the resultant very high degrees of optical activity, particularly
at short UV-wavelengths, and said polarization rotator (14) might
even be positioned between the last of said lenses (L16) and said
wafer (11). The latter placement of said polarization rotator (14)
has the particularly beneficial advantage that all imaging optical
components of said optical imaging system will be able to operate
with radially polarized light and said polarization rotator (14)
will no longer need to be incorporated into said projection lens,
i.e., may be positioned outside same.
[0027] The foregoing description of a beneficial sample embodiment
makes it clear that an optical imaging system according to the
invention will allow achieving high-quality imaging largely free of
the disturbing effects of stray light by providing that a large
majority of said imaging optical components, preferably at least
2/3 thereof, will be irradiated by radially polarized light for
which said imaging optical components have highly effective
antireflection coatings. Said optical imaging system will also be
capable of providing a largely tangentially polarized beam that
will allow creating high-contrast interference fringes, such as
those that will be of benefit when same is employed as, e.g., a
microlithographic projection illumination system for exposing
photoresists on wafers, at its image plane.
* * * * *