EP1721217A2

EP1721217A2 - System for reducing the coherence of laser radiation

Info

Publication number: EP1721217A2
Application number: EP05707555A
Authority: EP
Inventors: Nils Dieckmann; Manfred Maul; Damian Fiolka
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2004-02-26
Filing date: 2005-02-22
Publication date: 2006-11-15
Also published as: WO2005083511A2; WO2005083511A3; US7593095B2; US20070206381A1; JP4769788B2; JP2007528595A; KR20060129502A; KR101109354B1

Abstract

The invention relates to a system for reducing the coherence of a wave front-emitting laser radiation (10), especially for a projection lens for use in semiconductor lithography, wherein a first partial beam (10a) of a laser beam (10) incident on a surface (11) of a resonator body (9) is partially reflected. A second partial beam (10b) penetrates the resonator body (9) and emerges from the resonator body (9) at least approximately in the area of entry after a plurality of total internal reflections. The two partial beams (10a and 10b) are then passed on jointly to an illumination plane. The resonator body (9) is adapted, in addition to splitting the laser beam into partial beams (10a, 10b), to modulate the wave fronts of at least one partial beam (10b) during a laser pulse. The partial beams (10a, 10b) reflected on the resonator body (9) and penetrating the resonator body are superimposed downstream of the resonator body (9). The resonator body (9, 9') is provided with a phase plate (12) having different local phase distribution.

Description

System for reducing the coherence of laser radiation

The invention relates to a system for reducing the coherence of a laser radiation emitting wavefronts, in particular for a projection objective in semiconductor lithography, a first partial beam being partially reflected by the laser beam impinging on a surface of a resonator body and a second partial beam entering the resonator body and after several total reflections at least approximately in the area of the entry point, it emerges from the resonator body and is passed on together with the first partial beam to an illumination plane. The invention also relates to a projection exposure system with a laser as a light source, an illumination system and a projection lens.

In the case of projection lenses in semiconductor lithography, it is necessary to illuminate the mask, also called the reticle, as homogeneously as possible by means of an illumination system. However, there is a problem with the use of pulsed laser light in a large temporal coherence of the laser radiation, as a result of which the homogeneity is disturbed by speckle, ie differences in light / dark. It has therefore already been proposed to divide the laser beam when it hits the resonator body by arranging a resonator body in the form of a prism with three corners, namely into a partially reflected partial beam and a second partial beam that enters the resonator body after corresponding total reflections emerges from the resonator body in the area of the entry point and is then reunited with the reflected partial beam. In this way, the laser pulses are practically "chopped" into several partial beams, which arrive in succession at the illumination level, for example the reticle level in the case of use in semiconductor lithography. The purpose is to Make the time interval between two pulses so large that it is greater than the so-called temporal coherence of the laser radiation. This means that the beams are no longer capable of interference, ie they can no longer form interference. This measure should result in an improvement in lighting homogeneity.

With regard to the prior art, reference is made to EP 1 107 089 A2, US Pat. No. 6,238,063 B1 and the Patent Abstract of Japan 01198759A.

The present invention has for its object to provide a system with which the lighting homogeneity can be improved even further.

According to the invention, this object is achieved in that the resonator body is designed such that, in addition to the division into partial beams, the wave fronts of at least one partial beam are modulated during a laser pulse, the partial beams reflected on the resonator body and those entering the resonator body after the resonator body are superimposed and the resonator body is provided with a phase plate with different local phase distribution.

In addition to the known division of the laser beam into partial beams which are no longer coherent in time, the wave fronts of the laser radiation are also modulated according to the invention. This leads to different wave fronts (local phase distributions) during a single laser pulse. In other words: By overlaying different speckle patterns during a pulse, a significant increase in lighting homogeneity can be achieved by averaging over several speckle distributions. The combination of temporally staggered wave fronts according to the invention, which is additionally modulated and thus receives different phase distributions, allows a very high degree of homogeneity to be achieved by means of averaged speckle patterns. A phase plate, which is attached in the resonator body, is suitable for this.

In an advantageous embodiment of the invention, it can be provided that the resonator body is designed as a prism with at least five corners. It was found that, in comparison to a resonator body with 3 corners, at the generally used wavelengths of the laser beams, in particular in the VUV range (vacuum ultraviolet spectral range) or shorter, deflection angles which are too steep result in the prism, so that it does not Total reflections occur, but partial exits with corresponding light losses. When using a prism with at least 5 corners at least four reflections are generated, in ^■ usually minimum angle of 37 degrees are adhered to NEN kön- so that it always comes to total reflections. Calcium fluoride in particular has been found to be a material for the resonator body according to the invention. Of course, other materials such as magnesium fluoride and quartz are also possible for this.

Various other measures are conceivable for modulating a wavefront.

If, according to a development of the invention, the thickness of the phase plate is different, then spatially offset wavefronts result. Different changes in thickness - in relation to a direction transverse to the beam direction - should occur at a distance which corresponds to the order of magnitude of the spatial coherence length of the laser radiation which should be modulated through. In an advantageous embodiment of the invention it can be provided that the phase plate is designed as a diffractive optical element (DOE) which is operated in the zeroth diffraction order. A diffractive optical element in the first or even a higher diffraction order is normally used. However, in order to achieve the wavefront modulation, the diffractive optical element (DOE) in the zeroth diffraction order will be used, where the laser light passes through unbroken.

Another option is to use a diffuser as a phase plate.

The modulation of a wavefront according to the invention can also be achieved in that the resonator body, e.g. the prism with at least five corners is asymmetrical. This can e.g. by an asymmetrical, i.e. non-mirror-symmetrical formation of at least one side of the prism.

Another solution, or a combination with an asymmetrical resonator body, is that the beam guidance of the laser beam is selected such that the center of gravity beam hits the resonator body off-center. In this case, modulations of the wavefront are also created when the partial beam circulates in the resonator body.

If the surface on which the laser beam strikes is provided with a divider layer which is designed in such a way that the entry angle can be changed, the possible uses can be increased further, since the angle dependency can be at least partially reduced. The entry angle can thus be varied through the divider layer and thus also the ratio of the reflected partial beam and that entering the resonator body Partial beam.

In order to also achieve wavefront modulations, e.g. the divider layer can be of different thicknesses and / or not homogeneous. A dielectric layer with a division ratio of 33: 67 or 1/3: 2/3 can advantageously be used as the divider layer.

Exemplary embodiments of the invention are shown in principle below with reference to the drawings.

It shows:

FIG. 1 shows a schematic structure of a projection exposure system with a light source, an illumination system and a projection lens;

FIG. 2 pentagonal prism as a resonator body with a phase plate;

3a shows a cross section through the phase plate according to FIG. 2;

FIG. 3b shows a cross section through a further embodiment of a phase plate;

Figure 4 shows a pentagonal prism with two asymmetrical sides; and

Figure 5 shows a pentagonal prism with a divider layer on one surface.

As can be seen from FIG. 1, a projection exposure system 1 has a light source 2 in the form of a laser

Illumination system 3 for illuminating a field in a Level 4, in which a structure-bearing mask 4a is arranged, and a projection objective 5 for imaging the structure-bearing mask 4a in level 4 onto a light-sensitive substrate 6. The projection lens 5 has a plurality of optical elements 7 in its housing 8. The projection exposure system 1 is used to produce semiconductor components, such as computer chips.

In the present exemplary embodiment, a resonator body 9, 9 'is arranged between the laser 2 and the illumination system 3 in order to reduce the, in particular temporal, coherence of a laser radiation 10 from the laser 2.

FIG. 2 shows a pentagonal prism as resonator body 9,

For the sake of simplicity, only the focus beam of the incident laser beam bundle 10 is shown and in the following only the laser beam 10 is spoken of. In practice, of course, there is a beam of rays. A lambda / 2 plate 18 is used to adjust the degree of polarization (between unpolarized and linearly polarized) of the laser beam 10. The laser beam 10 impinging on a surface 11 of the prism 9 is divided into a first partial beam 10a which reflects on the surface 11 and a partial beam 10b which enters the resonator body 9 and there after several total reflections at the entry point again out of the resonator body 9 emerges and is combined there with the first partial beam 10a. Both partial beams are then fed to an illumination level, in this case level 4 with the structure-bearing mask. There are four total reflections in the 5-sided prism shown. In the case of a prism with more corners, there are correspondingly more total reflections.

According to FIG. 2, a phase plate 12 protrudes into the prism 9 is introduced into the prism 9 in such a way that the circulating partial beam 10b, which approximately meets perpendicularly with a front surface 13 of the phase plate 12, must penetrate it. The phase plate 12 causes a different local phase distribution. For this purpose, the phase plate 12 has a different thickness, as can be seen from the enlarged cross-sectional representations according to FIGS. 3a and 3b. As can be seen, the different thicknesses of the phase plate 12 vary in width s in the transverse direction to the beam direction. The greatest width s should be - depending on the laser used and its wavelength - in the order of the spatial coherence length of the laser radiation used. For excimer lasers, the value for s is between 0.05 and 1 mm, whereby a possible beam expansion increases the value accordingly.

The difference in thickness between the maximum and the minimum thickness of the plate should be of the order of a few wavelengths of the light used. At the wavelengths mentioned above, this should be between a = 200 to 500 nm.

The base thickness of the phase plate b can be in the range of 500 μm.

The distribution of the width differences s and the thickness differences a should be as random as possible, so that a relatively random phase distribution on the wavefront is also obtained. In this way, the optical path lengths differ locally and, in the case of the reunited partial beams 10a and 10b after the resonator body 9, correspondingly different partial beams are obtained, which are also modulated with respect to the wavefront. In this way, the individual pulses can be so short in their duration and in phase distribution that there is no longer any interference capability. In the embodiment according to FIG. 3b, the structure of the phase plate 12 is made tapered or prism-like, whereby an improved beam expansion is achieved.

When using a 5-sided prism, reflection angles are greater than 37 degrees, which means that total reflections occur inside. The exemplary embodiment shown was designed so that all total reflection angles are identical and are approximately 55 degrees.

Calcium fluoride can be used as the material for the prism. The crystal orientation of the CaF ₂ prism 9 is selected such that the first (100) crystal plane forms an angle of 45 ° with the light entry plane of the entry or surface 11 and is perpendicular to the side surface 14. The second (100) crystal plane lies parallel to the side surface 14 of the prism 9.

In this way, the --- intrinsic birefringence, which is significant at the wavelength of 157 nm and 193 nm, does not affect the polarization of the circulating beam if the light at the entrance obex surface 11 is linearly polarized and the direction of oscillation of the electrical Field strength vector is parallel (p-polarized) or perpendicular (s-polarized) with respect to the plane of incidence. The light emerges from the prism 9 again in s or p polarization.

If the incident beam 10 consists of unpolarized light, then the crystal orientation is not important and the prism 9 made of CaF ₂ can be oriented as desired.

A corresponding possibility consists in producing the block from MgF ₂ (transparent at 157 nm and 193 nm, strongly birefringent). It is about. unpolarized Light at the entrance, then the crystal orientation with regard to the prism surfaces can also be chosen arbitrarily.

If the light at the entrance plane or entrance surface 11 is linearly polarized (s- or p-polarized) and the light is to emerge again linearly polarized, then the crystal orientation must be chosen so that the direction of oscillation of the incident electric field strength vector parallel to the fast ( Direction with extraordinary refractive index) or slow (direction with ordinary refractive index) crystal axis.

In this case there is no birefringence and the polarization state is retained.

A diffractive optical element can be used as the phase plate 12, which is optimized to the zero order of diffraction, in which incident light passes undeflected.

A diffusing screen can also be used as the phase plate 12.

FIG. 4 shows a prism in a pentagonal shape, an offset being present or the prism body being asymmetrical. As can be seen, one side, namely the first side 15, on which the partial beams 10b impinge are shifted downward by the distance d. This disturbs the mirror symmetry, which also applies to the following page 16. In this way, it is also achieved that the reflected partial beam 10a and the partial beam 10b circulating in the prism 9 ', and thus finally all partial beams, are offset in time and space from one another. The distance d can be of the order of 0.1 mm.

If necessary, a phase plate 12 (shown in dashed lines in FIG. 4) can also be used 2, which is even more variable with respect to the modulation of wave fronts.

FIG. 5 shows an embodiment of a prism 9, also in a pentagonal shape, a divider layer 17 being applied to the entrance surface 11. The divider layer can e.g. a dielectric layer with a split ratio of 1/3: 2/3. On the one hand, the splitter layer has the task of influencing the entry angle of the partial beam 10b into the prism 9. This can be chosen according to the design and the material of the divider layer as desired.

If the divider layer also has a different thickness, modulation of the wavefront is also achieved in a manner similar to that of the phase plate (shown in broken lines in FIG. 5). The same is possible through an inhomogeneous or non-homogeneous formation of the divider layer.

The exemplary embodiments shown in FIGS. 2, 4 and 5 for generating different wave fronts can be used both separately and in any combination with one another. For example, it is also possible to combine all three measures, i.e. to arrange a phase plate 12 in an asymmetrical prism 9 'according to FIG. 4 and additionally also to provide the entrance surface 11 with a divider layer 17 which, moreover, has a different thickness and / or is also not homogeneous.

Claims

Claims:

1. System for reducing the coherence of a laser radiation emitting wavefronts, in particular for a projection objective in semiconductor lithography, wherein a first partial beam is partially reflected by the laser beam striking a surface of a resonator body and a second partial beam enters the resonator body and after several total reflections at least approximately in the area of the entry point, exits the resonator body again and is passed on together with the first partial beam to an illumination plane, characterized in that the resonator body (9, 9 ') is designed such that, in addition to the division into partial beams (10a, 10b) the wave fronts of at least one partial beam (10b) are modulated during a laser pulse, the partial beams (10a, 10b) reflected on the resonator body (9,9 ') and entering into the resonator body (9,9') superimposed on the resonator body (9,9 ') w earth and the resonator body (9, 9 ') is provided with a phase plate (12) with different local phase distribution.

2. System according to claim 1, characterized in that the phase plate (12) for the passage of the second partial beam (10b) of the laser beam has different thicknesses transverse to the beam direction.

3. System according to claim 2, characterized in that the differences in thickness are between 200 and 500 nm.

4. System according to claim 2, characterized in that the different thicknesses of the phase plate (9) vary in the transverse direction in a width s, which is in the order of magnitude of the spatial coherence length of the laser radiation at the entrance plane (11).

5. System according to claim 4, characterized in that 0.05 <s <1 mm applies to the width s.

6. System according to claim 1, characterized in that the phase plate (12) is designed as a diffractive optical element (DOE), which is optimized to the zeroth diffraction order.

7. System according to claim 1, characterized in that a diffuser is provided as the phase plate (12).

8. System according to claim 1, characterized in that the resonator body (9,9 ') is designed as a prism with at least five corners.

9. System according to claim 1, characterized in that the reflection angles in the resonator body (9,9 ') are at least 37 degrees.

10. System according to claim 1, characterized in that the optical path length of the second partial beam (10b) in the resonator body (9,9 ') is a multiple of the coherence length.

11. System according to claim 1, characterized in that the light impinging on the resonator body (9,9 ') in a ratio of 1/3 to 2/3 with respect to the first reflected partial beam ^" (10a) and the second in the re sonator body (9,9 ') circumferential beam (10a, 10b) is divided.

12. System according to claim 1, characterized in that at wavelengths of the laser beam (10) of 157 nm or less, calcium fluoride is used as the resonator body (9,9 ').

13. System according to claim 12, characterized in that calcium fluoride is chosen in a crystal orientation such that the first (100) crystal plane forms an angle of 45 degrees with the plane of the surface that the laser beam strikes and perpendicularly on a side surface lies, whereby the second (100) crystal plane lies parallel to this side surface.

14. System according to claim 1, characterized in that the direction of polarization of the laser beam impinging on the resonator body (9, 9 ') can be rotated relative to the plane of incidence to set a state of polarization.

15. System according to claim 14, characterized in that the degree of polarization between unpolarized and linearly polarized is adjustable.

16. System according to claim 15, characterized in that a lambda / 2 plate (18) is used to adjust the polarization state.

17. System according to claim 8, characterized in that the prism (9 ') is asymmetrical.

18. System according to claim 17, characterized in that the prism (9 ') is provided with at least one asymmetrical side.

19. System according to claim 1, characterized in that the position of the center of gravity beam of the laser beam (10) striking the resonator body (9, 9 ') is off-center.

20. System according to claim 1, characterized in that the resonator body (9 ') is asymmetrical and that the focus beam of the laser beam (10) is hits the center of the resonator body (9 ').

21. System according to claim 1, characterized in that the surface (11) of the resonator body (9,9 '), on which the laser beam (10) impinges, is provided with a divider layer (17) in such a way that it defines the entry angle of the influences the partial beam (10b) entering the resonator body (9,9 ').

22. ^" System according to claim 21, characterized in that the divider layer (17) has a different thickness.

23. System according to claim 21, characterized in that the divider layer (17) is not formed homogeneously.

24. System according to one of claims 21, 22 or 23, characterized in that the divider layer (17) has a dielectric layer.

25. Projection exposure system for semiconductor lithography with a laser as a light source, an illumination system, an illumination plane with a mask and with a projection lens, the laser beam emitting a surface of a resonator body being partially reflected with a first partial beam to reduce the coherence of a wave front. and wherein a second partial beam enters the resonator body and after several total reflections at least approximately in the area of the entry point it emerges from the resonator body and is passed on together with the first partial beam to an illumination plane, characterized in that the resonator body (9,9 ') is designed in such a way that, in addition to the division into partial beams (10a, 10b), the wave fronts of at least one partial beam (10b) are modulated during a laser pulse. body (9,9 ') reflected and the partial beams (10a, 10b) entering the resonator body (9,9') are superimposed after the resonator body (9,9 ') and the resonator body (9,9') with a phase plate (12) is provided with different local phase distribution.

26. Projection exposure system according to claim 25, characterized in that the phase plate (12) is designed as a diffractive optical element (DOE) which is optimized to the zero order of diffraction.

27. Projection exposure system according to claim 25, characterized in that a diffusing screen is provided as the bias plate (12).

28. Projection exposure system according to claim 25, characterized in that the resonator body (9, 9 ') is designed as a prism with at least five corners.

29. Projection exposure system according to claim 25, characterized in that the optical path length of the second partial beam (10b) in the resonator body (9,9 ') is a multiple of the temporal coherence length.

30. Projection exposure system according to claim 25, characterized in that calcium fluoride is provided as the resonator body (9, 9 ') at wavelengths of the laser beam (10) of 157 nm or less.

31. Projection exposure system according to claim 26, characterized in that the resonator body (9, 9 ') is provided with a phase plate (12) with different local phase distribution.

32. Projection exposure system according to claim 25, characterized in that the prism (9 ') is asymmetrically shaped. forms is.

33. Projection exposure system according to claim 25, characterized in that the position of the focus beam of the laser beam (10) striking the resonator body (9, 9 ') is off-center.

34. Projection exposure system according to claim 25, characterized in that the surface (11) of the resonator body (9, 9 ') on which the laser beam (10) impinges is provided with a divider layer (17) in such a way that it covers the entry angle of the partial beam (10b) entering the resonator body (9,9 ').

35. Projection exposure system according to claim 34, characterized in that the divider layer has a different thickness and / or is not homogeneous.

36. Projection exposure system according to claim 34 or 35, characterized in that the divider layer has a dielectric layer.