WO2003009021A1

WO2003009021A1 - Delay element made from a cubic crystal and corresponding optical system

Info

Publication number: WO2003009021A1
Application number: PCT/DE2002/002392
Authority: WO
Inventors: Jürgen HARTMAIER; Damian Fiolka; Markus Zenzinger; Birgit Mecking; Olaf Dittmann; Toralf Gruner; Vladimir Kamenov; Martin Brunotte
Original assignee: Carl Zeiss Smt Ag
Priority date: 2001-07-18
Filing date: 2002-06-25
Publication date: 2003-01-30
Also published as: DE10133842A1; JP2005509184A; CN1555502A; DE10293197D2; EP1407299A1

Abstract

Centimetre-thick sheets or lenses made from calcium fluoride or barium fluoride with beam propagation in the direction of the < 110 > crystal direction or a main axis equivalent thereto, used as delay elements for the deep ultraviolet are disclosed, embodied in a stress-free form. The above is particularly suitable for microlithography at 157 nm.

Description

Description Delay element made of cubic crystal and optical systems with it

The invention relates to a delay element made of cubic crystal and optical systems with such delay elements.

Delay elements of this type are known, for example, from US Pat. No. 6,191,880 B. There, a delay element is described in the form of a delay plate, that is to say a birefringent plate, which causes a phase shift between two orthogonally polarized transmitted beams and can be designed, for example, as a λ / 4 or λ / 2 plate. The plate consists of calcium fluoride, which exhibits stress birefringence due to external forces or the manufacturing process. Nothing is said about crystal orientation there.

Due to their symmetry, cubic crystals generally do not have birefringence.

Manufacturing-related residual birefringence of optical elements made of calcium fluoride is known from US Pat. No. 6,201,634 B1.

Classic birefringent crystals, such as magnesium fluoride, have such a high birefringence that only very thin plates are required, but which pose technological problems, such as DE 197 04 936 A (US Ser. No. 09 / 017,159) and US 6,084,708 B can be found in the applicant. Thicker retardation plates with a path difference (n + 1/4), which are possible and customary per se, ie λ-th order retardation plates, are thicker, but need the same narrow thickness tolerance and have a much smaller angle tolerance for the light.

Many other known materials for delay elements are not available in the ultraviolet range from 200 to 150 nm and below because of too high absorption.

Calcium is known from the Internet publication "Preliminary Determination of an Intrinsic Birefringence in CaF ₂ " by John H. Burnett, Eric L. Shirley, and Zachary H. Levine, NIST Gaithersburg MD 20899 USA (distributed on May 7, 2001) Fluoride single crystals also have non-stress-induced, i.e. intrinsic birefringence. The measurements presented there show that when the beam propagates in the direction of the <110> crystal axis, a birefringence of (6.5 ± 0.4) nm / cm occurs at a wavelength of λ = 156.1 nm. Applicant's measurements show 11 nm / cm. The birefringence in the other crystal axis directions, however, is small.

It is an object of the invention to provide an alternative type of delay element which is suitable for wavelengths in the range from 200 to 150 nm and below and which allows very exact function with a moderate production outlay. Inexpensive optical systems with such delay elements are also to be provided.

High-quality delay elements for this wavelength range are required, for example, in projection exposure systems in microlithography, in particular in connection with catadioptric projection objectives. They are essential for projection lenses with polarization beam splitters as quarter-wave delay elements between the beam splitter and the concave mirror. With other types with deflecting mirrors with approx. 90 ° deflection, the reflection leads close to that Brewster angles to polarization-dependent reflectivities, which must be compensated.

This object is achieved by a delay element with the features of claim 1. Advantageous further developments are specified in the dependent claims. The wording of all claims is incorporated by reference into the content of the description.

According to the invention, the residual birefringence of fluoride crystal material with intrinsic birefringence, in particular calcium fluoride, which is maximal when the beam passes parallel to the <110> crystal axis or parallel to an equivalent main axis of the crystal, and which has hitherto been regarded as a problem of lens systems made from this material was specifically used as a mechanism of action for retarders. Because of the relatively low birefringence, the element can be several millimeters or several centimeters thick. However, the absolute thickness is also important for very precise delays only in an area which is problem-free for the production of optical elements.

In addition to this intrinsic birefringence, the voltage birefringence due to manufacturing conditions in the claimed direction has a relatively high value according to US Pat. No. 6,201,634B. The thickness of such a delay element with a desired delay, for example as a quarter-wave retarder, can be determined from the measured value of the birefringence of the specific material batch and thus take into account both causes of the birefringence.

In addition, it was found by the inventors that barium fluoride single crystal likewise has such birefringence, but with approximately twice the value of approximately 25 nm / cm. That is also Barium fluoride suitable in the same orientation and has the advantage of about half the thickness.

It is clear that all other crystals, if they have a similar birefringence, are equally suitable. However, these values are currently not known for other fluoride crystals which are transparent in the deep ultraviolet. Only for strontium fluoride is information known in US Pat. No. 6,201,634 B regarding the stress birefringence caused by the manufacture.

Compared to extremely thin MgF ₂ delay plates, the use of CaF ₂ or BaF _{2 has} the advantage that the thicknesses can be in the cm range. This extremely simplifies the manufacture of the delay elements.

Further embodiments are the subject of the subclaims.

Important designs of the delay elements are the half-wave plates and the quarter-wave plates, the embodiment according to the invention made of relatively weakly birefringent materials being particularly suitable for representing zero-order plates. In these, the path difference is equal to (0 + 1/4) λ or (0 + 1/2) λ, so an ineffective path difference of a multiple of the wavelength is not additionally introduced. In the case of plates made of magnesium fluoride, this is unavoidable in order to achieve manageable plate thicknesses, but causes a reduction in the angle acceptance.

Stress-free storage is possible. This means, for example, that a delay plate with normal frames, such as are also used for lenses, filter plates and the like, can be stored. Complex devices for the homogeneous application of force, such as, for example, according to US Pat. No. 6,084,708 B, are eliminated, as are problems of holding particularly thin elements.

Designs in which a delay plate carries a functional surface are particularly advantageous. Without an effective influence on the deceleration or the polarization rotation, one or both end faces can be provided with a structure that has a refractive or diffractive effect. Fresnel lenses, zone plates, refractive or diffractive grid plates and the like with structural heights up to the millimeter range can thus be provided without an additional component. Such components can be used, for example, in the lighting system of a microlithography

Projection exposure system at the same time the polarization distribution can be influenced and the light conductance can be increased.

One or both end surfaces can also be spherical or aspherical or curved as a free-form surface, so that the retardation plate can simultaneously contribute to the correction of an optical system.

For example, a considerably curved meniscus can also serve as the delay element according to the invention if the light path only corresponds sufficiently precisely to the desired delay over the entire cross section. One or both interfaces or end surfaces can also have a considerable curvature, so that the delay element can form a, preferably meniscus-shaped, lens. The delay element can thus also have positive or negative refractive power. The integration of the decelerating effect, which is in the foreground here, with a lens effect can be used for material-saving, constructively favorable designs. Such lenses can also be used in purely dioptric optical systems, particularly useful in microlithography projection lenses or lighting systems, and in catadioptric systems.

The intrinsic birefringence of the materials mentioned has its maximum value in <110> crystal directions. For rays that run through the material at an angle to <110> directions, the amount of intrinsic birefringence shows a parabolically decreasing course with increasing angle, while the axes of the intrinsic birefringence approximately maintain the direction. This fact can be used to even out the delay effect over the entire irradiated area. For this purpose, in the case of a delay element with two optical surfaces, the shape of the optical surfaces and the installation position of the delay element can be adapted to one another such that the light path of rays within the delay element between the optical surfaces is greater, the greater the angle between the beam and the optical axis or a <110> direction of the delay element. This means that beams with a larger angle to the <110> direction have to cover a longer light path, so that the retarding effect that results from the product between intrinsic birefringence and light path becomes approximately uniform over the entire effective area.

This concept will be explained later on the basis of exemplary embodiments of catadioptric projection lenses, in which a delay element comprises a lens or lens group made of <110> -oriented fluoride crystal which is arranged in the vicinity of the concave mirror and which is overall meniscus-shaped and has negative refractive power. A lens or lens group of this type arranged in the vicinity of the pupil can have a largely constant or only slightly varying retarding effect over the entire pupil. The integration of a delay element with a lens element by fabricating a lens element (provided with refractive power) from a <1 10> -oriented single crystal with intrinsic birefringence (e.g. calcium fluoride or barium fluoride single crystal) can be useful with all catadioptric or dioptric projection lenses. A suitably dimensioned lens or lens group with the retarding effect of a λ / 4 plate can be used, for example, in systems with a polarization-selective beam splitter as a (functionally necessary) retarder between the beam splitter and the concave mirror and / or elsewhere on a projection lens, for example between the object plane and the beam splitter and / or between beam splitter and image plane.

The above and further features emerge from the claims and also from the description and the drawings, the individual features being realized individually or in groups in the form of subcombinations in embodiments of the invention and in other fields and being advantageous and capable of being protected by themselves Can represent designs.

Fig. 1 shows a section of a catadioptric

Projection objective in which an embodiment of a meniscus-shaped delay element with a negative refractive power is arranged between a beam splitter surface and a concave mirror; and

FIG. 2 is a schematic illustration of the catadioptric objective part of a projection objective with a physical beam splitter. The invention is first explained in more detail on the basis of exemplary embodiments of quarter-wave plates in comparison to designs made of magnesium fluoride.

A quarter-wave plate of zero order for the wavelength 157nm has a thickness of 39 mm made of calcium fluoride with a delay of 10nm / cm and a thickness of 15.7 mm made of barium fluoride with a delay of 25nm / cm. In the event of deviations in the delay in a batch of material - for example due to stress birefringence due to the manufacture - the required thickness changes in proportion to the deviation in the delay. Plates of this thickness can be produced in the typical dimensions of lenses, currently in microlithography optics up to approx. 300 mm. They can be captured or stored using the technology available for lenses.

A corresponding zero-order quarter-wave plate for 157 nm made of magnesium fluoride has a thickness of only 5.5 μm (compare US Pat. No. 6,084,708 B). The problem of stable storage can be solved by cracking onto a thicker element, such as a beam splitter prism. The production of such a thin crystal plate with diameters over 100 mm remains a problem (see DE 197 04 936 A). A twentieth-order quarter-wave plate also has a thickness of approximately 0.22 mm. The deviation from the exact quarter-wave delay due to thickness variation has exactly the same relationship for zero or higher order plates. With magnesium fluoride, therefore, a thickness deviation of just 0.5μm results in an unusable phase deviation of approx. 20%.

In the zero-order delay plate made of calcium fluoride according to the invention, an error in the phase of 2% is likewise corresponding to a thickness error of likewise 2%. Due to the thickness of 39 mm this is 2% but 0.8 mm. The normal manufacture optical elements is much more accurate, so thickness is not a manufacturing problem at all. The same applies to the barium fluoride quarter wave plate, which is about half as thick. There is therefore no motive for using higher-order plates, although of course they are also possible.

This permissible thickness tolerance now enables the end surfaces of the retardation plate to be processed as functional surfaces with a refractive or diffractive effect. The exit surface is preferably suitable for this purpose, since within the retardation plate the light should propagate largely in the axial direction (i.e. essentially parallel to <110>).

Up to a sine of the aperture angle (numerical aperture) of 0.2, the loss of the linear degree of polarization remains below 2% for a 157 nm half-wave plate made of calcium fluoride, and it remains below 0.1% up to NA 0.15.

A zero-order half-wave plate made of magnesium fluoride allows a numerical aperture of up to 0.4 with the same quality. With higher-order plates, however, the angular acceptance decreases rapidly, with a twentieth-order half-wave plate it is only NA 0.1.

In contrast to magnesium fluoride, the materials of the delay plates according to the invention therefore offer greater angular acceptance in real terms. At these angles, the birefringence, which varies several times over the azimuth angle, does not yet play a role either, owing to the birefringence properties which differ for other main axes and which can be disadvantageous especially for lenses. In conventional optical designs, especially in lighting systems and projection lenses of microlithography, planar plates are not provided for retardation plates, but rather single millimeter thick plates, or they are provided as a negligible thin layer on beam splitter prisms and the like. In all areas of these designs, however, where the beam angles are in the range mentioned above, the centimeter-thick flat plates can be incorporated into the design with the help of experts who can easily make corrections. It complies with the fact that the end faces, as mentioned above, are even accessible to a certain extent as functional and correction means.

The applicant's EP 1 102 100 A shows a microlithographic catadioptric projection objective with a polarizing beam splitter cube, on which the beam path is largely collimated. A quarter-wave plate is required between this and the concave mirror. As a thick plate according to the invention, it can be separated and derived, for example, from the thick, almost plano-convex lens in front of the concave mirror, also with a derivation for a wavelength of 157 nm.

Another embodiment of a catadioptric projection lens is explained with reference to FIG. 1, in which a delay element 17 in the form of a λ / 4 retarder that has been passed twice is arranged between the beam splitter 15 and the concave mirror 16. This is a lens made of <11 reoriented calcium fluoride crystal which is arranged in the vicinity of the concave mirror and which is overall meniscus-shaped and has negative refractive power. The negative lens 17 arranged near the pupil has a double function. On the one hand, it supports the chromatic correction of the projection lens as an optical lens together with the concave mirror 16. At the same time, it acts as a λ / 4 delay element with a largely constant or only slightly varying delay effect over the entire pupil. It has been recognized that a largely constant distribution of the delay over the pupil can be achieved if the (axial) thickness d of the delay element (in the z direction) is optimized as a function of the radial distance x from the optical axis in such a way that the light path of rays within the delay element between light entry and light exit becomes larger, the larger the angle _{\ n} between the beam and the optical axis of the delay element or the <1 10> direction running parallel to it. The adjustment is ideally such that the increase in thickness largely or completely compensates for the parabolic drop in intrinsic birefringence in the event of a deviation from the <1 10> direction.

The beam splitter 15 can be a geometric beam splitter with one or more deflecting mirrors or a physical beam splitter with a beam splitter surface that is effective in a polarization-selective manner.

To determine the ideal curvature in the central region of the delay element, consider a bundle of rays 18 in the center of the delay element 17. Make the condition for all rays that the optical path length in the material is λ / 4. This defines a surface that is in two-dimensional space through the equations

X = (λ / 4 * sin (αin)) / Δn (α _in ) and Z ≡ d (x) = (λ / 4 ^* cos ( _in )) / Δn (αin)

Δn is the refractive index difference between the medium surrounding the delay element (normally air) and the material of the delay element, <_ the angle between the optical axis or the <110> axis and the beam 18 in question and d (x) the thickness as a function of the radius x of the delay element. This calculation results in an approximately parabolic course of the thickness in the radial direction of the delay element, which is approximately realized in the negative meniscus lens 17, taking into account the curvatures of the entrance surface and the exit surface, which are ideal for optical reasons.

If the resulting lens thickness is considered to be unfavorable, it is also possible to distribute the delay over a plurality of delay lenses or combinations of delay lenses and delay plates, the total thickness of which can be determined, for example, according to the above (see FIG. 2).

In order to be able to optimally benefit from this aspect of the invention, the combined lens / delay element should be arranged in an area of incidence that is as small as possible. Ideally, the maximum angle of incidence in air should not be greater than approx. 39 °, since otherwise a crystallographic four-wave ripple of the deceleration as a function of the crystal direction can be noticeable. It is also favorable if the curvature of the lens is made smaller the smaller the angle _m is. The sum of the lens thicknesses should approximately correspond to the corresponding thickness of a λ / 4 delay element made of the material. Small corrections to the overall thickness to adjust the retarding effect can be advantageous. For example, it may be more advantageous if the retarding effect for marginal rays is set more precisely than for central rays. This can lead to a homogenization of the intensity distribution after double passage through the delay element. The aspect of the invention also permits corrective measures in the event that the ideal total thickness determined is too large or too small. For example, a weakening of the delay is possible if two <110> -cut lenses of approximately the same thickness are rotated relative to one another by 45 ° with respect to the <110> axis. If the total thickness is too small, an additional, plane-parallel plate made of <110> -oriented material can be provided, for example. It is particularly important to ensure that the inclination of the rays is not too great.

An embodiment of a catadioptric projection objective with a polarization-selective beam splitter 20 in the form of a beam splitter cube is explained with reference to FIG. 2. In this embodiment, a polarization rotating device 23 with the effect of a λ / 4 radar is arranged between the beam splitter 20 and the concave mirror 21. The multi-part delay element 23 consists of two negative meniscus lenses 24, 25, each of which consists of <110> -oriented calcium fluoride crystal. The total axial thickness of the lenses in the central region near the axis corresponds to the corresponding thickness of a λ / 4 retardation plate (for example approx. 36 mm for calcium fluoride at a working wavelength of 157 nm) and increases parabolically in the radial direction in order to equalize the retardation effect over the entire lens cross section in the range of Pupil arranged lenses 24, 25 to achieve.

The projection lens is designed for operation with circularly polarized input light and has a λ / 4 plate 47 between object plane 26 and beam splitter 20 for converting the input light into light which is s-polarized with respect to beam splitter surface 28. This plate 27 can consist, for example, of <110> oriented calcium fluoride. The light passes through the two lenses 24, 25 and is due to whose retarding effect is converted into circularly polarized light, which is reflected by the concave mirror 21 and runs back through the delay device 23. After passing through the retardation lenses 24, 25 again, the light is p-polarized with respect to the beam splitter layer 28 and passes through it with little loss in the direction of a deflecting mirror 29 which deflects the light in the direction of the object plane. It is hereby explained by way of example that the λ / 4 redarder, which is necessary for such systems, between

Beam deflection device 20 and concave mirror can be formed by one or more lenses with a suitable retarding effect. The conventionally required λ / 4 plate between the beam splitter and the concave mirror can thus be omitted.

The cited documents should also be part of this application in full. The invention is particularly advantageous at 157 nm and in its surroundings, since the intrinsic birefringence is particularly high here, but it also finds meaningful use in the 193 nm microlithography systems and other optical systems, for example inspection systems.

Claims

claims

1. Delay element, in particular in a microlithography projection exposure system, characterized in that the

Delay element consists of calcium fluoride crystal or barium fluoride crystal and the optical axis of the delay element points approximately in the direction of the <110> crystal axis or an equivalent main crystal axis.

2. Delay element according to claim 1, wherein the delay element is a delay plate, in particular a λ / 2 plate or a λ / 4 plate, preferably zero order.

3. Delay element according to one of claims 1 or 2, wherein the delay element is a delay plate and the delay plate has a thickness variation of up to 2% and / or up to 1 mm.

4. Delay element according to one of the preceding claims, wherein the delay element is a delay plate and at least one end surface, in particular the light exit surface is provided with a refractive or diffractive structure or shape.

5. Delay element according to one of the preceding claims, wherein the delay element has a diameter in the range of 50 to 300 mm.

6. Delay element according to one of the preceding claims, wherein it is mounted stress-free.

7. Delay element according to one of claims 1, 5 or 6, characterized in that it is designed as a lens element (17, 24, 25) with positive or negative refractive power, preferably as a meniscus-shaped lens, in particular with negative refractive power.

8. Delay element according to one of the preceding claims, in which the delay element (17) has two optical surfaces, the shape of the optical surfaces and the installation position of the delay element being matched to one another such that the light path of rays (18) within the delay element between the optical surfaces, the larger the angle between a beam passing through and the optical axis of the

Delay element is.

9. Delay element according to one of the preceding claims, which is a lens (17) made of a cubic crystal material with intrinsic birefringence, in which the thickness as a function of the radius has an approximately parabolic course with radially increasing thickness.

10. Catadioptric projection lens, in particular in a microlithography projection exposure system, with at least one delay element according to one of claims 1-9.

11. Catadioptric projection lens for imaging a pattern arranged in an object plane of the projection lens into the image plane of the projection lens, with between the A catadioptric objective part with a concave mirror (16, 21) and a beam splitter (15, 20) with a beam splitter surface is arranged on the object plane and the image plane, with a delay device (17, 23) with the effect of a λ / 4 between the beam splitter surface and the concave mirror -Plate is arranged, and wherein the delay device has at least one delay element designed as a lens (17, 24, 25), which consists of an intrinsic birefringence cubic crystal material, in particular of calcium fluoride crystal or barium fluoride crystal, the optical axis of the delay element approximately in the direction of one <110> crystal axis of the crystal is aligned.

12. Projection lens according to claim 11, characterized in that at least one delay element (17, 24, 25) is designed as a meniscus-shaped lens, in particular with a negative refractive power.

13. Projection lens according to one of claims 11 or 12, in which at least one delay element (17, 24, 25) has two optical surfaces, the shape of the optical surfaces and the installation position of the delay element being matched to one another such that the light path of rays within the delay element between the optical surfaces, the greater the angle between a beam passing through and the optical axis of the

Delay element is.

14. Projection lens according to one of claims 11 to 13, wherein in the delay element (17, 24, 25) the total thickness as

Function of the radius has an approximately parabolic course with radially increasing thickness.

15. Projection lens according to one of claims 1 1 to 14, in which the delay device (17, 23) is arranged in the vicinity of a pupil plane of the projection lens, in particular in the vicinity of the concave mirror (16, 21).

16. Projection objective according to one of claims 11 to 15, in which no λ / 4 plate is arranged between the beam splitter and the concave mirror (16, 21).

17. Microlithography projection exposure system, comprising an illumination system and a projection objective, which images a structure-bearing mask on a light-sensitive substrate, characterized in that it has at least one delay element according to one of claims 1 to 9.

18. Microlithography projection exposure system according to claim 17, characterized in that the lighting system has a delay element according to one of claims 1 to 9.

19. A method for producing semiconductor components with a microlithography projection exposure system according to claim 17 or 18.