Radial polarization-rotating optical arrangement and microlithographic projection exposure system incorporating said arrangement

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 (41, 42, 4i), a combination of birefringent quarter-wave plate 420 and a circular plate 430 is suggested in combination with a conical polarizer 21'. This arrangement is preferably utilized in the illumination portion of a microlithographic projection exposure system. It is important that the arrangement be mounted behind all asymmetric or polarizing component elements 103a.

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 European patent publications 0,602,923 and
 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 eacl case, the locally perpendicular polarized light component is
 intensely reflected.
 In European patent publication 0,602,923, 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 not
 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. 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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
 A polarization-rotating arrangement according to the invention is shown in
 FIGS. 1a 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.sup.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. 1b 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. 3b) are also possible. The number of the individual
 elements can then be in the area of 10.sup.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.degree. 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.sup.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.sub.2, with the conical
 angle.alpha. 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. 3a 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.degree. to the radius in the segment center as shown in FIG. 3b. 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. 4a 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. 6a.
 Planar plate 410 is a quarter-wave plate which, as FIG. 4b 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.degree. to the radius over the entire cross
 section as shown in FIG. 4c.
 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.sub.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.degree.. In this way, and as shown in FIG.
 4d, the radial polarization of the exiting light extends over the entire
 cross section.
 As in the embodiment of FIGS. 1a 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 .mu.m) 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.
 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. 1a and 1b and 4 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.