Title: Radial polarization-rotating optical arrangement and microlithographic projection exposure system incorporating said arrangement
Abstract: An optical arrangement is disclosed wherein an entering beam is converted into an exiting beam having a total cross section of light which is linearly polarized essentially in the radial direction by rotation. For this purpose, rasters of half-wave plates (
Patent Number: 6,885,502 Issued on 04/26/2005 to Schuster
| Inventors:
|
Schuster; Karl-Heinz (Königsbronn, DE)
|
| Assignee:
|
Carl-Zeiss-Stiftung (Heidenheim, DE)
|
| Appl. No.:
|
140895 |
| Filed:
|
May 9, 2002 |
Foreign Application Priority Data
| Sep 23, 1995[DE] | 195 35 392 |
| Current U.S. Class: |
359/485; 359/489; 359/495; 359/497; 355/71 |
| Intern'l Class: |
G02B 005//30; G03B 027//72 |
| Field of Search: |
359/485,489,495,497
355/71
|
References Cited [Referenced By]
U.S. Patent Documents
| 3213753 | Oct., 1965 | Rogers.
| |
| 3719415 | Mar., 1973 | Rawson.
| |
| 3935444 | Jan., 1976 | Zechnall et al.
| |
| 4286843 | Sep., 1981 | Reytblatt.
| |
| 4755027 | Jul., 1988 | Schäfer.
| |
| 4899055 | Feb., 1990 | Adams.
| |
| 5365371 | Nov., 1994 | Kamon.
| |
| 5375130 | Dec., 1994 | Shih.
| |
| 5436761 | Jul., 1995 | Kamon.
| |
| 5467166 | Nov., 1995 | Shiraishi.
| |
| 5548427 | Aug., 1996 | May.
| |
| 5559583 | Sep., 1996 | Tanabe.
| |
| 5677755 | Oct., 1997 | Oshida et al.
| |
| 5715084 | Feb., 1998 | Takahashi et al.
| |
| 6191880 | Feb., 2001 | Schuster.
| |
| 6229647 | May., 2001 | Takahashi et al.
| |
| 6392800 | May., 2002 | Schuster.
| |
| 6404482 | Jun., 2002 | Shiraishi.
| |
| 2001/0022687 | Sep., 2001 | Takahashi et al.
| |
| Foreign Patent Documents |
| 1572195 | Mar., 1970 | DE.
| |
| 3523641 | Dec., 1986 | DE.
| |
| 0419257 | Mar., 1991 | EP.
| |
| 0602923 | Jun., 1994 | EP.
| |
| 0608572 | Jun., 1994 | EP.
| |
| 01 143216 | Jun., 1989 | JP.
| |
| 05-226225 | Sep., 1993 | JP.
| |
| 06 160628 | Jun., 1994 | JP.
| |
| 07 176476 | Nov., 1995 | JP.
| |
| 07 307268 | Nov., 1995 | JP.
| |
Other References
"Efficient radially polarized laser beam generation with a double interferometer",
by S. Tidwell et al, Applied Optics, vol. 32, No. 27, 1993, pp. 5222 to 5228.
|
Primary Examiner: Shafer; Ricky D.
Attorney, Agent or Firm: Ottesen; Walter
Parent Case Text
This is a divisional of patent appication Ser. No. 09/730,778 filed Dec. 7,
2000 now U.S. Pat. No. 6,392,800, which is a divisional of patent application Ser.
No. 09/352,408, filed Jul. 14, 1999, now U.S. Pat. No. 6,191,880 B1, which is a
continuation of application Ser. No. 08/717,902, filed Sep. 23, 1996, now abandoned.
Claims
1. A microlithographic projection exposure system comprising:
a light source defining an optical axis and transmitting a light beam along said
axis toward an object;
a reticle mounted on said axis for receiving said light beam and transmitting
said light beam down said optical axis;
a catadioptric projection objective mounted on said optical axis downstream of
said reticle; and,
said catadioptric projection objective including a polarizing beam splitter and
a radial polarization-rotating optical arrangement mounted downstream of said polarizing
beam splitter generating a radially orientated linear polarization on the total
cross section of said light beam directed toward said object.
2. The microlithographic projection exposure system of claim 1, wherein said
radial polarization-rotating optical arrangement comprises:
an optical structure for receiving an entering light beam;
said entering light beam having a linear polarization (P) in a predetermined
direction;
said optical structure being adapted to convert said entering light bean into
an exiting light beam wherein said direction of said linear polarization (P) is
rotated essentially over the entire cross section of said exiting light beam;
said entering light beam defining an optical axis (A) and said optical structure
including a frame and more than four half-wave plates disposed in a raster, segment
or facet configuration;
said half-wave plates having respective preferred directions so arranged that
each half-wave plate deflects the polarization direction of the penetrating linear
polarized light in the direction of a radius which cuts through the corresponding
half-wave plate and is directed to said optical axis;
a reflective polarizer having a conical surface shaped polarizing surface or
a conical-frustrum surface shaped polarizing surface; and,
said half-wave plates being mounted in the beam path of the light reflected at
said reflection polarizer.
Description
FIELD OF THE INVENTION
The invention relates to an optical arrangement which converts an entering light
beam into an exiting light beam having light which is linearly polarized in the
entire cross section essentially in radial direction.
BACKGROUND OF THE INVENTION
It is necessary to provide projection exposure systems with a very high numerical
aperture in order to achieve the highest resolutions in microlithography. Light
is coupled into the resist layer at very large angles. When this light is coupled
in, the following occur: light losses because of reflection at the outer resist
boundary layer and deterioration of the resolution because of lateral migration
caused by reflections at the two boundary layers of the resist to the wafer and
to the air (formation of standing waves).
The degree of fresnel reflection is then dependent upon the angle between the
polarization direction and the reflection plane. The reflection vanishes when light
having an electrical field oscillating parallel to the incident angle incidents
at the brewster angle. This provides for optimal in-coupling into the resist while
at the same time providing maximum suppression of the standing waves.
However, disturbances occur for light which is linearly polarized in one
direction as described in U.S. Pat. Nos. 5,715,084 and 6,229,647 and U.S. Patent
Application Publication US 2001/0022687 A1 as well as in European patent publication
0,608,572. Accordingly, the apparatus disclosed in these publications generate
circularly polarized light which is coupled into the resist as the equivalent of
unpolarized light. In this way, homogeneity is achieved over the entire image.
However, a loss of efficiency is accepted because in each case, the locally perpendicular
polarized light component is intensely reflected.
In U.S. Pat. Nos. 5,715,084 and 6,229,647 and U.S. Patent Application Publication
2001/0022687 A1, it is alternatively suggested that linearly polarized light should
be orientated in one direction relative to the orientation of a pattern to be imaged
as already disclosed in German patent publication 1,572,195. The penetration via
a multiple reflection takes place in the longitudinal direction of the structures
and not in the direction of the critical resolution. The efficiency of the in-coupling
or the reflection at the resist surface, is however no homogeneous.
The effect of the polarization on the reflection at the resist layers and the
significance of the fresnel coefficients is described in U.S. Pat. No. 4,899,055
directed to a method for measuring thickness of thin films.
U.S. Pat. No. 5,365,371 discloses a projection exposure apparatus for microlithography
wherein a radially directed linear polarization of the light is introduced in order
to prevent disturbances because of standing waves in the resist when generating
images therein. Two different polarization elements are given, namely, a radial
polarization filter composed of a positive cone and a negative cone. This filter
is utilized in transmission and effects radial polarization for the reflection
because of the fresnel equations. However, it is not disclosed how a complete polarization
of the transmitted light is achieved. In the description of U.S. Pat. No. 5,365,371
and in claim 3 thereof, it is required in addition that both parts have different
refractive indices. The transmitted part must then however be deflected and cannot
pass in a straight line.
U.S. Pat. No. 5,436,761 has a disclosure identical to that of U.S. Pat. No.
5,365,371 referred to above and includes a single claim wherein no condition is
given for the indices of refraction. Furthermore, in claim 4 of U.S. Pat. No. 5,365,371,
a plate having segments of radially orientated polarization filter foils is given
as is known from U.S. Pat. No. 4,286,843 (see FIG. 19 and column 9, lines 60 to 68).
Both polarizers are polarization filters, that is, they lead to high light loss
and are suitable only for an incoming light beam which is unpolarized or circularly
polarized because, otherwise, an intense nonhomogeneity of the intensity would
occur over the cross section of the exiting light beam.
In the example of FIG. 1 of U.S. Pat. No. 5,365,371, the deflecting mirror 17
causes a partial polarization and therefore the light beam exiting from the polarizer
21 is nonhomogeneous.
U.S. Pat. No. 5,365,371 discloses that the radial polarizer lies in the pupillary
plane of the projection objective. A position of the radial polarizer in the objective
is problematical because there, the tightest tolerances for an optimal image quality
must be maintained.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an optical arrangement which permits
a homogeneous coupling of light into optical boundary surfaces with high aperture
and with low loss and low scattered light. It is another object of the invention
to provide such an arrangement wherein the efficiency and the homogeneity of the
exiting light beam are optimized.
Projection exposure apparatus are provided which permit maximum use of
the advantages of radial linear polarization with minimum disturbance of the imaging
and minimum complexity with respect to assembly.
The invention is directed to an optical arrangement which includes: an optical
structure for receiving an entering light beam; the entering light beam having
a linear polarization (P) in a predetermined direction; and, the optical structure
being adapted to convert the entering light beam into an exiting light beam wherein
the direction of the linear polarization (P) is, however, not subtractively selected
but is instead rotated essentially over the entire cross section of the exiting
light beam.
In this connection, it is noted that normal polarizers effect a selection. Thus,
a polarization direction is permitted to pass and the orthogonals are, for example,
removed from the light beam by reflection, refraction and absorption. Accordingly,
unpolarized light yields a maximum of 50% linear polarized light. When linear polarized
light enters a polarizer at an angle to the direction of polarization, the projection
of the polarization vector is selected to the polarization direction for through
passage and the orthogonals are eliminated. In contrast, in the optical arrangement
of the invention, the direction of the linear polarization is actually rotated.
Advantageous embodiments are disclosed which provide different ways
of generating the desired polarization distribution. One embodiment includes ring
aperture illumination wherein the incident light at low angles (for which low angles
the reflectivity is only slightly dependent upon polarization) is suppressed.
Another embodiment is directed to the integration of a radially polarizing
optical arrangement into a microlithographic projection exposure system.
In this system, the possibilities of the optics are fully utilized and an improvement
in the homogeneity and in the efficiency of coupling light into the resist layer
is achieved because the reflection at the resist layer is reduced uniformly. However,
uniform reduction is also achieved at all lenses arranged downstream of the polarizing
element. For the light incident at large angles (up to the brewster angle), the
effect is the greatest especially where the light intensity (peripheral decay)
is at the lowest. The disturbances of the resolution because of scattered light,
even at the resist wafer boundary layer, are homogenized and reduced.
An arrangement close to start of the beam path is advantageous because the disturbances
caused by stress-induced birefringence at all downstream lenses is minimized and
made symmetrical.
For this reason, it is also advantageous for polarization filters (in addition
to the preferred polarization-rotating elements) when these elements are mounted
in the illuminating system.
In another embodiment, the polarization-rotating elements are mounted at any
desired
location in a projection illuminating system which is characterized by improved
homogeneity and a much higher efficiency compared to the state of the art.
In another embodiment, a reduction and homogenization of the scattered light
occurs
at each lens of the system (even with a low angle of incidence).
On the other hand, asymmetrical optical elements change the state of polarization
and can therefore only be arranged downstream when a reflecting layer having phase
correction is utilized. This is especially the case for deflecting mirrors such
as for shortening the structural length or as provided in catadioptric projection
objectives. If a totally-reflecting prism is utilized as a deflecting element,
then a precisely adapted phase-retarding plate must be mounted downstream or the
totally reflecting boundary layer must be coated with a phase-correcting layer.
Polarizing optical elements such as polarization beam splitters and quarter-wave
plates are also disturbing.
The invention is also directed to a microlithographic projection exposure system
incorporating a radially polarizing optical arrangement. More specifically, the
microlithographic projection exposure system of the invention includes: a light
source defining an optical axis and transmitting a light beam along the optical
axis; an optical structure arranged on the optical axis for receiving the light
beam; the entering light beam having a linear polarization (P) in a predetermined
direction; and, the optical structure being adapted to convert the entering light
beam into an exiting light beam wherein the direction of the linear polarization
(P) is rotated essentially over the entire cross section of the exiting light beam.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the drawings wherein:
FIG. 1
a is a plan view of a radially polarization-rotating optical arrangement
of a raster of half-wave plates for linearly polarized incident light;
FIG. 1
b shows the polarization directions of the light beam exiting from
the arrangement of FIG. 1
a;
FIG. 2 is an elevation view, in section, of a radially polarizing optical arrangement
having a conical-frustrum reflector having a brewster angle for circularly polarized
or non-polarized incident light;
FIG. 3
a is an arrangement incorporating a conical-frustrum reflector
and segmented half-wave plates for complete utilization of circularly-polarized
light or non-polarized light;
FIG. 3
b is a plan view of the arrangement of FIG. 3
a as viewed
from the light exit end thereof;
FIG. 4
a is a radial polarization-rotating optical arrangement having
a plate with a central-symmetrical stress-induced birefringence;
FIG. 4
b is a plan view of the quarter-wave plate of the arrangement of
FIG. 4
a;
FIG. 4
c is a plan view of the compressive-strain plate of the arrangement
of FIG. 4
a;
FIG. 4
d is a plan view of the birefringent 450 plate corresponding
to the arrangement of FIG. 4
a;
FIG. 5 is a schematic representation of a microlithographic projection exposure
system incorporating a radially polarizing optical arrangement in the illumination
portion thereof;
FIG. 6 is a schematic representation of a catadioptric projection objective
having a radially polarizing optical arrangement of the invention incorporated
therein; and,
FIG. 7 is a schematic representation of a microlithographic projection exposure
system incorporating the radially polarization-rotating optical arrangement of
a half raster of wave plates as shown in FIGS. 1
a and 1
b.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
A polarization-rotating arrangement according to the invention is shown in FIGS.
1
a and
1b as it is suitable especially in combination with
a honeycomb condenser for the conversion of linearly polarized light. This arrangement
is especially suited for lasers as a light source. The beam cross section is subdivided
into a multiplicity of facets (
11,
12,
1i) which is,
in each case, made of a half-wave plate of birefringent material. Each facet
1i
corresponds to a honeycomb element of the honeycomb condenser. The facets
1i
are preferably cemented to the honeycomb or placed in wringing contact therewith.
For extreme radiation loads, the facets can be separately held and coated to prevent
reflection. The honeycomb condensers conventional for microlithographic projection
exposure systems have about 10
2 honeycomb elements and the number of
the facets is the same.
The main axes (
21,
22,
2i) of the facets (
11,
1i) are each aligned in the direction of the angle bisector between
the polarization direction of the entering linearly polarized light and the radius
(which is aligned to the particular optical axis A of the light beam and of the
honeycomb condenser) through the center of each facet
1i. In this
way, each half-wave plate facet
1i effects the rotation of the polarization
direction in the direction of the above-mentioned radius. FIG. 1
b shows
this effect. Here, the entry surfaces (
41,
42,
4i)
of the honeycomb condenser are shown with the polarization directions (
31,
32,
3i) of the particular component beams which are all aligned radially.
The raster with hexagonal facets
1i is only one embodiment which
is especially adapted for the combination with a honeycomb condenser. Other rasters
and especially fan-shaped sector subdivisions of the half-wave plates (see FIG.
3
b) are also possible. The number of the individual elements can then be
in the area of 10
1.
A reduction of the total degree of reflection at an optical boundary surface
compared
to unpolarized light takes place so long as the component of the light, which is
polarized perpendicularly to the plane of incidence, is less than the component
of the parallelly polarized light. This boundary case is achieved with only four
90° sectors having half-wave plates so that, preferably, more half-wave plates
are arranged in the cross section of the light beam especially in the order of
magnitude of 10 to 10
2 facets or sectors.
In contrast to the known radial polarizers with sectors as shown in U.S. Pat.
Nos. 4,286,843 and 5,365,371, the polarization is filtered out with an insignificant
amount of loss; instead, the light is changed at minimal loss in its polarization
direction via birefringent elements.
The arrangement shown in FIG. 2 effects a continuous radial direction of the
linear polarization for entering unpolarized or circularly polarized light
40.
This arrangement is a polarization filter and is basically known from U.S. Pat.
No. 5,365,371 but is new with respect to its details.
The conical frustrum
20 has a through bore and is made of a transparent
material, such as glass FK5, quartz glass or CaF
2, with the conical
angle a corresponding to the brewster angle and a dielectric reflection coating
on the conical surface
21. The component
45 of the light beam
40
is polarized perpendicularly to the incident plane and is therefore completely
reflected. The transmitted beam
4p is polarized completely parallel
to the incident plane and is therefore everywhere linearly polarized radially to
the optical axis A. The conical frustrum
20 is adapted for an annular aperture
illumination and ensures the shortest structural length. A complete cone is also
suitable. The conical frustrum
20 is supplemented by a suitable hollow cone
22 to form a cylinder ring whereby the reflective conical surface
21
is protected and the entire structure is easier to mount. The conical frustrum
20 and the hollow cone
22 have the same index of refraction so that
the light passing therethrough does so without refraction at the conical surface
21, which is in contrast to U.S. Pat. No. 5,365,371.
FIG. 3
a shows, in section, a further embodiment of that shown in FIG.
2 wherein the reflective component
4s is also utilized so that an
arrangement with a substantially lower than 50% light loss is achieved because
the polarization is effectively rotated and not filtered.
A transparent part
30 having a conical surface
31 is mounted about
the conical frustrum
20′ having the conical surface
21′
corresponding to FIG. 2 (with an extending cylindrical extension portion). The
transparent part
30 has a reflective cone surface
31 parallel to
the conical surface
21′. A ring of segments (
5i,
5k)
of half-wave plates is mounted on the exit surface
32 of the part
30.
The main axes (
6i,
6k) of the segments are at 45°
to the radius in the segment center as shown in FIG. 3
b. In this way, and
as described with respect to FIG. 1, the radial linear polarization is effected
also of the light
4s reflected at the conical surface
21′
in the beam
4r parallel to the axis. The effected increase of the
light-conductance value is often desired at least for laser light sources. It is
important that the arrangement is suitable for unpolarized incident light. By omitting
or adding optical glass, the optical path of conical frustrum
20′
and transparent part
30 can be adapted.
An arrangement for continuously generating radially linear polarized light is
shown in FIGS. 4
a to
4d. Here, the arrangement is for linearly
or circularly polarized light at the input with reduced structural length in the
direction of the optical axis. It is especially suitable for annular aperture optics.
An annular beam of uniformly linear polarized light
41 impinges on a stack
of three planar plates (
410,
420,
430) as shown in section
in FIG. 4
a. Planar plate
410 is a quarter-wave plate which, as FIG.
4
b shows, circularly polarizes the through-passing light. If the entering
beam is already circularly polarized, then the plate
410 can be omitted.
A plate
420 follows and can, for example, be made of glass or quartz glass.
The plate
420 is under centrally-symmetrical pressure stress and has therefore
stress-induced birefringence. Thickness, material and stress are so selected that
the plate
420 is a quarter-wave plate in the outer region touched by the
annular beam
41 but with radial symmetry so that the circularly polarized
entering light is linearly polarized and with the polarization direction at 45°
to the radius over the entire cross section as shown in FIG. 4
c.
Such a pressure stress always accompanies thermal expansion and temperature
gradients when cooling or a compensating thermal treatment in circularly round
glass plates (or plates of quartz glass, berylliumfluoride, CaF
2 et
cetera). The pressure stress is normally minimized with the longest possible cooling.
Via deliberate cooling, the desired pressure stress can be generated within wide
limits and therefore the desired stress-induced birefringence is generated in the
exterior region.
A third plate
430 follows which has circular birefringence and rotates
the
polarization direction by 45°. In this way, and as shown in FIG. 4
d,
the radial polarization of the exiting light extends over the entire cross section.
As in the embodiment of FIGS. 1
a and
1b, this embodiment
affords the advantage of being especially thin and, as shown in the embodiment
of FIG. 2, has the advantage that precise radial polarization is provided without
complex assembly of many facets or segments. The main advantage is also the high
efficiency because the polarization is rotated and not selected. If, in lieu of
an annular beam
41, a complete beam is transmitted through the arrangement,
then the core area is simply not influenced.
FIG. 5 is a schematic showing a complete microlithographic projection exposure
system with a radially polarizing optical arrangement
55 which is here in
the form of a conical-frustrum polarizer according to FIG.
2. Except for
this element and its mounting, all components and their arrangement are conventional.
A light source
51, for example, an i-line mercury discharge lamp having
mirror
52, illuminates a diaphragm
53. The i-line mercury lamp is
tuned to the i-line (atomic emission spectral line of mercury having a wavelength
of 358 nm) and is conventionally used in microlithography. An objective
54
(for example, a zoom axicon objective as disclosed in German patent publication
4,421,053) follows and makes possible various adjustments, especially the selection
of an annular aperture.
The conical-frustrum polarizer
55, which is suitable for unpolarized entering
light, is followed by: a honeycomb condenser
56 and a relay and field optic
57. These parts together serve to optimize illumination of the reticle
58
(the mask) which is imaged by the projection objective
59 at a reduced scale
and with the highest resolution (below 1 μm) on the resist film
60
of the wafer
61. The numerical aperture of the system lies in the range
of values above 0.5 to 0.9. Annular apertures between 0.7 and 0.9 are preferred.
The radial polarization of the light after leaving the conical-frustrum polarizer
55 causes the effect of the stress-induced birefringence to be rotationally
symmetrical with respect to the optical axis at all of the following optical elements
(
56,
57,
58,
59). The effect is the greatest at the
entrance into the resist film
60 where the largest inlet angles occur and
therefore optimal transmission and minimum reflection are achieved. The sensitive
beam path in the projection objective
59 is undisturbed.
The embodiment of the polarizing optical arrangement
55 is not limited
to the embodiment of FIG.
2. Especially all polarization-rotating arrangements
can be used and, if needed, a polarizer or birefringent plate can be mounted forward
of the arrangement for adaptation. Also, a polarization-rotating optical arrangement
55 can be placed at other locations in the overall configuration.
As noted above, all polarization-rotating arrangements can be used and FIG. 7
shows an embodiment of a microlithographic projection exposure system incorporating
the radially polarization-rotating optical arrangement
62 shown in detail
in FIGS. 1
a and
1b.
This is especially true when deflection mirrors without phase correction or
polarizing elements, such as polarization beam splitters, are used. Then, the polarization-rotating
optical arrangement according to the invention is placed behind these elements
as viewed in light flow direction. One embodiment is shown in FIG. 6 in the context
of a catadioptric projection objective.
FIG. 6 corresponds completely to FIG. 1 of European patent publication 0,602,923
having polarizing beam splitter
103, concave mirror
106, lens groups
(
102,
105,
108) and quarter-wave plate
104. The polarization-rotating
optical element
107 is, however, not a quarter-wave plate for circular polarization
and therefore uniform deterioration of the coupling in of light into the resist
109, as described initially herein with respect to European patent publication
0,602,923. The polarization-rotating optical element
107 also is not a means
for aligning the uniform linear polarization to a preferred direction of the pattern
on the reticle
101. Rather, a radial polarization-rotating optical arrangement
107 is provided in FIG.
6.
The embodiments of FIGS. 1
a,
1b and
4a are
the best suited here because of the small amount of space available. The advantage
is clear, namely, independently of the pattern of the individual case, optimal
scatter light suppression and uniform efficiency of the incoupling of light into
the resist
109 is achieved.
The radial polarizing optical arrangement
107 is mounted as close as possible
behind the deflecting mirror
103a in the almost completely collimated
beam path, that is, in a range of moderate angles and divergences of the light
rays. Small angles are important for a trouble-free functioning of the birefringent
elements. The best effect is achieved when the exit plane of the polarization-rotating
elements lies in a plane of the illumination or projection system which is fourier-transformed
to the image plane or in a plane equivalent thereto.
The use of the polarization-rotating optical arrangement, which generates a radially
orientated linear polarization on the total beam cross section, is not limited
to microlithography.
It is understood that the foregoing description is that of the preferred embodiments
of the invention and that various changes and modifications may be made thereto
without departing from the spirit and scope of the invention as defined in the
appended claims.
*
