Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70216—Mask projection systems
  • G03F7/70225—Optical aspects of catadioptric systems, i.e. comprising reflective and refractive elements
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/30—Polarising elements
  • G02B5/3083—Birefringent or phase retarding elements
  • G02B5/3091—Birefringent or phase retarding elements for use in the UV

Definitions

• the invention relates to a retardation plate with a birefringent crystal plate, which has an entry face and an exit face for incident and emerging light, respectively.
• retardation plates refers to optically birefringent plane-parallel plates, which generally consist of an optically uniaxial crystal.
• the surfaces of the retardation plate are parallel to the optical axis of the crystal, so that a normally incident wave is split into two waves oscillating mutually orthogonally with a phase difference dependent on the plate thickness.
• Behind the retardation plate the light is combined to form a polarisation state which depends on the plate thickness. If, for example, this thickness is chosen so that the phase difference corresponds to one quarter of the wavelength of the incident light, then the retardation plate is referred to as a quarter-wave plate, which converts linearly polarised light into elliptically or circularly polarised light, and vice versa.
• the phase difference introduced between the polarisation directions by the retardation plate is a half wavelength
• this is referred to as a half-wave plate, which, for example, can be used to invert the handedness of elliptically or circularly polarised light .
• Retardation plates are used, for example, in catadioptric projection objectives of microlithographic projection illumination systems. Such systems are nowadays operated with such short-wave ultraviolet light that very many birefringent crystals are no longer viable as a material for the retardation plates owing to excessive adsorption.
• Magnesium fluoride is in principle suitable for this wavelength range, but it has such a high birefringence that very stringent requirements need to be placed on the manufacturing tolerances. Indeed, even very minor deviations from the intended thickness lead to a noticeable deviation from the desired phase difference between the orthogonal polarisation directions.
• Such zeroth-order retardation plates are in fact so thin that both their production and their handling in optical instruments entail significant problems.
• Zeroth-order retardation plates are generally preferred because their function depends less strongly on the angle at which the light strikes the retardation plate. This aspect is of particular importance in the aforementioned projection objectives, since these often have a numerical aperture of more than 0.3, so that large angles of incidence can occur.
• the retardation plate is intended to have a high transparency in the ultraviolet radiation range, to be simple to produce and to handle, and furthermore to be usable even in wide-aperture optical systems .
• the crystal plate consists of an alkaline-earth metal fluoride, in particular of fluorspar, and its optical axis is aligned at least approximately in the direction of the ⁇ 110> crystal axis or of a principal crystal axis equivalent thereto, and by the fact that a form- birefringent layer structure is applied to the entry and/or exit face.
• the invention is based, on the one hand, on the fact that very many alkaline-earth metal fluoride crystals, for example fluorspar crystals (CaF 2 ) or barium fluoride crystals (BaF 2 ) have an intrinsic birefringence for beam propagation in the direction of the ⁇ 110> crystal axis.
• the birefringence for beam propagation along the other crystal axis directions is small. Since these crystals have a high transparency in the ultraviolet wavelength range, they are suitable in particular for use in projection objectives of microlithographic projection illumination systems.
• Such a retardation plate is therefore also suitable for very wide-aperture objectives in projection illumination systems .
• the form-birefringent layer structure may be configured as a periodic sequence of at least two layers with alternating refractive indices.
• the thicknesses of the layers must then be smaller than the wavelength for which the retardation plate is designed.
• the thicknesses of the layers are advantageously less than 1/5 or even 1/10 of this wavelength.
• the smaller the thicknesses of the layers are compared with the wavelength of the incident light the more the layer structure acts as a homogeneous uniaxial birefringent medium for incident light. It is furthermore preferable for all the layers to have the same thickness.
• Figure 1 represents a disc-shaped retardation plate in a section along its symmetry axis
• Figure 2 shows a refractive-index ellipsoid for a layer structure which is part of the retardation plate shown in Figure 1.
• Figure 1 shows a retardation plate, denoted overall by 10, in a section along its symmetry axis.
• the retardation plate 10 has a fluorspar crystal plate 12, whose optical axis indicated by 11 is aligned at least approximately in the direction of the ⁇ 110> crystal axis.
• the lower layer structure 16 consists of a sequence of six dielectric layers 161, 162, ..., 166 with an alternating refractive index.
• the layers 161, 163 and 165 have a first refractive index ⁇
• the layers 162, 164 and 166 have a second refractive index n 2 which is different from the refractive index i.
• All the layers 161, 162, ..., 166 have the same thickness d, which, in the exemplary embodiment being represented, is 1/10 of the wavelength of the incident light.
• the thickness d is only about 15 nm.
• the thickness of the individual layers 161 to 166 is consequently represented on a significantly exaggerated scale in Figure 1.
• the lower layer structure 16 is form-birefringent because of the alternating sequence of layers 161 to 166 with high and low refractive index. This means that the lower layer structure 16 has a differing refractive index, depending on the polarisation direction of the light, for light incident obliquely to the layer planes.
• Figure 2 shows a refractive- index ellipsoid for the lower layer structure 16. It is clear from this that light which is polarised parallel to the layer planes is exposed to the refractive index n 0 for the ordinary beam, whereas light which is polarised perpendicularly to the layer planes is exposed to the refractive index n e for the extraordinary beam, with n e ⁇ n 0
• the lower layer structure 16 Since light incident normally on the layer structure is always polarised parallel to the layer planes, the lower layer structure 16 is not birefringent for such a light beam. However, the larger the angle is between the layer planes and the light passing through, the stronger is the birefringent effect of the lower layer structure 16 - at least for unpolarised or circularly polarised light.
• the upper layer structure 14 is constructed precisely like the lower layer structure 16, so that the comments made above correspondingly apply here.
• the birefringent effect of the upper and lower layer structures 14 and 16, as well as the fluorspar crystal plate 12, is illustrated highly schematically for two linearly polarised light beams 22 and 24.
• the light beam 22 in this case strikes the entry face 18 of the retardation plate 10 in such a way that it passes normally through the upper layer structure 14. Owing to this normal transmission, as mentioned above, the light beam 22 is not exposed to any birefringence in the upper layer structure 14. As a consequence of this, splitting of the wavefronts does not take place there either.
• the incident wave is split in the way typical of birefringence into an ordinary wave and an extraordinary wave, which are respectively illustrated in Figure 1 as dashed and dotted wavefronts.
• This splitting of the wavefronts, and the concomitant increase in the phase difference ends as soon as the wavefronts enter the lower layer structure 16, since the beam 22 is not exposed to any birefringence there.
• the emerging beam 22 has the desired phase difference of lambda/4 or lambda/2, corresponding to the thickness of the layer 12, between the two mutually orthogonally polarised components .
• the second beam 24 is inclined relative to the first beam 22 in such a way that it strikes the entry face 18 of the retardation plate 10 at a large angle.
• both the upper and lower layer structures 14 and 16 have a strongly birefringent effect, whereas the fluorspar crystal plate 12 lying in-between is hardly at all birefringent for this angle of incidence.
• the layer structures 14 and 16 are configured in such a way that the overall splitting of the wavefronts, that is to say the phase difference introduced by the retardation plate 10 for the different polarisation directions, corresponds approximately in the case of the beam 24 incident obliquely to the optical axis 11 to the phase difference which has been introduced by the retardation plate 10 for the beam 22 incident normally to the optical axis 11.
• the retardation plate 10 makes it possible to produce an approximately constant phase difference for light beams over a large range of angles of incidence.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Polarising Elements (AREA)

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• 2003
  • 2003-02-14 JP JP2004565923A patent/JP2006513443A/ja active Pending
  • 2003-02-14 EP EP03708098A patent/EP1583988A1/en not_active Withdrawn
  • 2003-02-14 WO PCT/EP2003/001475 patent/WO2004063777A1/en not_active Ceased
  • 2003-02-14 AU AU2003212243A patent/AU2003212243A1/en not_active Abandoned
• 2005
  • 2005-07-15 US US11/182,599 patent/US20060014048A1/en not_active Abandoned

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