POLARIZATION-MODULATING OPTICAL ELEMENT
A microlithography optical system includes a projection objective and an illumination system that includes a temperature compensated polarization-modulating optical element. The temperature compensated polarization-modulating optical element includes a first polarization-modulating optical element of optically active material, the first polarization-modulating optical element having a first specific rotation with a sign. The temperature compensated polarization-modulating optical element includes also includes a second polarization-modulating optical element of optically active material, the second polarization-modulating optical element having a second specific rotation with a sign opposite to the sign of the first specific rotation.
This application is a continuation of U.S. application Ser. No. 15/287,049, filed Oct. 6, 2016, which is a continuation of U.S. application Ser. No. 13/933,159, filed Jul. 2, 2013, which is a continuation of application U.S. application Ser. No. 12/201,767, filed Aug. 29, 2008, now U.S. Pat. No. 8,482,717, which is a continuation application of U.S. application Ser. No. 11/440,475, filed May 25, 2006, which is a Continuation-In-Part of, and claims priority under 35 U.S.C. §120 to, International Application PCT/EP2005/000320, having an international filing date of Jan. 14, 2005, which claimed the benefit of U.S. Ser. No. 60/537,327, filed on Jan. 16, 2004. U.S. application Ser. No. 11/440,475 also claims the benefit under 35 U.S.C. §119 of U.S. Ser. No. 60/684,607, filed on May 25, 2005. Each of these applications are incorporated herein by reference in their entirety.
FIELD OF THE INVENTIONThe invention relates to an optical element that affects the polarization of light rays. The optical element has a thickness profile and consists of or comprises an optically active crystal with an optical axis.
BACKGROUNDIn the continuing effort to achieve structures of finer resolution in the field of microlithography, there is a parallel pursuit of substantially three guiding concepts. The first of these is to provide projection objectives of very high numerical aperture. Second is the constant trend towards shorter wavelengths, for example 248 nm, 193 nm, or 157 nm. Finally, there is the concept of increasing the achievable resolution by introducing an immersion medium of a high refractive index into the space between the last optical element of the projection objective and the light-sensitive substrate. The latter technique is referred to as immersion lithography.
In an optical system that is illuminated with light of a defined polarization, the s- and p-component of the electrical field vector, in accordance with Fresnel's equations, are subject to respectively different degrees of reflection and refraction at the interface of two media with different refractive indices. In this context and hereinafter, the polarization component that oscillates parallel to the plane of incidence of a light ray is referred to as p-component, while the polarization component that oscillates perpendicular to the plane of incidence of a light ray is referred to as s-component. The different degrees of reflection and refraction that occur in the s-component in comparison to the p-component have a significant detrimental effect on the imaging process.
This problem can be avoided with a special distribution of the polarization where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the optical system have an approximately radial orientation relative to the optical axis. A polarization distribution of this kind will hereinafter be referred to as radial polarization. If a bundle of light rays that are radially polarized in accordance with the foregoing definition meets an interface between two media of different refractive indices in a field plane of an objective, only the p-component of the electrical field vector will be present, so that the aforementioned detrimental effect on the imaging quality is reduced considerably.
In analogy to the foregoing concept, one could also choose a polarization distribution where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the system have an orientation that is perpendicular to the radius originating from the optical axis. A polarization distribution of this type will hereinafter be referred to as tangential polarization. If a bundle of light rays that are tangentially polarized in accordance with this definition meets an interface between two media of different refractive indices, only the s-component of the electrical field vector will be present so that, as in the preceding case, there will be uniformity in the reflection and refraction occurring in a field plane.
Providing an illumination with either tangential or radial polarization in a pupil plane is of high importance in particular when putting the aforementioned concept of immersion lithography into practice, because of the considerable negative effects on the state of polarization that are to be expected based on the differences in the refractive index and the strongly oblique angles of incidence at the respective interfaces from the last optical element of the projection objective to the immersion medium and from the immersion medium to the coated light-sensitive substrate.
SUMMARYIn one aspect, the invention generally features a microlithography optical system that includes an illumination system, a projection objective and a temperature compensated polarization-modulating optical element in the illumination system. The temperature compensated polarization-modulating optical element includes a first polarization-modulating optical element that includes an optically active material. The first polarization-modulating optical element has a first specific rotation with a sign. The temperature compensated polarization-modulating optical element also includes a second polarization-modulating optical element comprising optically active material. The second polarization-modulating optical element has a second specific rotation with a sign opposite to the sign of the first specific rotation.
In another aspect, the invention generally features using the system described in the preceding paragraph to manufacturing a micro-structured semiconductor component.
In a further aspect, the invention generally features an optical element that includes a temperature compensated polarization-modulating element. The temperature compensated polarization-modulating element includes a first polarization-modulating optical element having a first thickness. The first polarization-modulating element includes an optically active material with a first specific rotation having a sign. The temperature compensated polarization-modulating element also includes a second polarization-modulating optical element having a second thickness different from said first thickness. The second polarization-modulating optical element includes an optically active material with a second specific rotation having a sign opposite the sign of the first specific rotation.
In an additional aspect, the invention generally a microlithography optical system that includes an illumination system, a projection objective and the optical element described in the preceding paragraph. The optical element is in the illumination system.
In another aspect, the invention generally features using the system described in the preceding paragraph to manufacturing a micro-structured semiconductor component.
In a further aspect, the invention generally features an optical system that has an optical axis. The optical system includes a first plane plate and a second plane plate. The first plane plate includes optically active quartz, and the first plane plate has a first thickness in the direction of the optical axis. The second plane plate includes optically active quartz. The second plane plate has a second thickness in the direction of the optical axis, and the second thickness being different from the first thickness.
In an additional aspect, the invention generally features a system that includes an illumination system, a projection objective and an optical system having an optical axis. The optical system includes a first plane plate and a second plane plate. The first plate includes optically active quartz. The first plane plate has a first thickness in the direction of the optical axis. The second plane plate includes optically active quartz. The second plane plate has a second thickness in the direction of the optical axis, and the second thickness being different from the first thickness.
In one aspect, the invention generally features an optical element that includes a support plate comprising optically active material, and at least two planar-parallel portions. Each of the at least two planar-parallel portions includes optically active material. When a first linearly polarized light ray passes through the optical element, a plane of oscillation of the first linearly polarized light ray is rotated by a first angle. When a second linearly polarized light ray passes through the optical element, a plane of oscillation of the second linearly polarized light ray is rotated by a second angle different from the first angle.
In another aspect, the invention generally features a microlithography optical system that includes an illumination system, a projection objective and the optical element described in the preceding paragraph. The optical element is in the illumination system.
In a further aspect, the invention generally features using the system described in the preceding paragraph to manufacturing a micro-structured semiconductor component.
In an additional aspect, the invention generally features an optical arrangement that includes a polarization-modulating optical element that includes a first optically active material having a first specific rotation with a sign. The polarization-modulating optical element has a first optical axis, and the polarization-modulating element has a first thickness profile that, as measured in the direction of the first optical axis, is variable. The optical arrangement also includes a compensation plate that includes a second optically active material having a second specific rotation with a sign opposite the sign of the first specific rotation. The compensation plate has a second thickness profile configured so that, when radiation passes through the optical arrangement, the compensation plate substantially compensates for angle deviations of the radiation that are caused by the polarization-modulating optical element.
In another aspect, the invention generally features a microlithography optical system that includes an illumination system, a projection objective and the optical arrangement described in the preceding paragraph. The optical arrangement is in the illumination system.
In a further aspect, the invention generally features using the system described in the preceding paragraph to manufacturing a micro-structured semiconductor component.
In certain embodiments, a polarization-modulating optical element is provided, which—with a minimum loss of intensity—affects the polarization of light rays in such a way that from linearly polarized light with a first distribution of the directions of the oscillation planes of individual light rays, the optical element generates linearly polarized light with a second distribution of the directions of the oscillation planes of individual light rays.
In some embodiments, an optical system is provided that has improved properties of the polarization-modulating optical element regarding thermal stability of the second distribution of oscillation planes (polarization distribution), and/or minimized influence of additional optical elements in the optical system to the polarization distribution after the light rays have passed these elements.
In some embodiments, a polarization-modulating optical element is provided which consists of or comprises an optically active crystal and which is shaped with a thickness profile that varies in the directions perpendicular to the optical axis.
A polarization-modulating optical element can have the effect that the plane of oscillation of a first linearly polarized light ray and the plane of oscillation of a second linearly polarized light ray are rotated, respectively, by a first and a second angle of rotation, with the first angle of rotation being different from the second angle of rotation. The polarization-modulating optical element can be made of an optically active material.
In some embodiments, one or more of the following desirable features can be provided.
In order to generate from linearly polarized light an arbitrarily selected distribution of linearly polarized light rays with a minimum loss of intensity, an optically active crystal with an optical axis is used as raw material for the polarization-modulating optical element. The optical axis of a crystal, also referred to as axis of isotropy, is defined by the property that there is only one velocity of light propagation associated with the direction of the optical axis. In other words, a light ray travelling in the direction of an optical axis is not subject to a linear birefringence. The polarization-modulating optical element has a thickness profile that varies in the directions perpendicular to the optical axis of the crystal. The term “linear polarization distribution” in this context and hereinafter is used with the meaning of a polarization distribution in which the individual light rays are linearly polarized but the oscillation planes of the individual electrical field vectors can be oriented in different directions.
If linearly polarized light traverses the polarization-modulating optical element along the optical axis of the crystal, the oscillation plane of the electrical field vector is rotated by an angle that is proportional to the distance traveled inside the crystal. The sense of rotation, i.e., whether the oscillation plane is rotated clockwise or counterclockwise, depends on the crystal material, for example right-handed quartz vs. left-handed quartz. The polarization plane is parallel to the respective directions of the polarization and the propagation of the light ray. In order to produce an arbitrarily selected distribution of the angles of rotation, it is advantageous if the thickness profile is designed so that the plane of oscillation of a first linearly polarized light ray and the plane of oscillation of a second linearly polarized light ray are rotated, respectively, by a first and a second angle of rotation, with the first angle of rotation being different from the second angle of rotation. By shaping the element with a specific thickness at each location, it is possible to realize arbitrarily selected angles of rotation for the oscillation planes.
Different optically active materials have been found suitable dependent on the wavelength of the radiation being used, specifically quartz, TeO2, and AgGaS2.
In an advantageous embodiment, the polarization-modulating optical element has an element axis oriented in the same direction as the optical axis of the crystal. In relation to the element axis, the thickness profile of the optical element is a function of the azimuth angle θ alone, with the azimuth angle θ being measured relative to a reference axis that intersects the element axis at a right angle. With a thickness profile according to this design, the thickness of the optical element is constant along a radius that intersects the element axis at a right angle and forms an azimuth angle θ with the reference axis.
In a further advantageous embodiment, an azimuthal section d(r=const.,θ) of the thickness profile d(r,θ) at a constant distance r from the element axis is a linear function of the azimuth angle θ. In the ideal case, this azimuthal section has a discontinuity at the azimuth angle θ=0. The linear function d(r=const.,θ) at a constant distance r from the element axis has a slope
wherein α stands for the specific rotation of the optically active crystal. At the discontinuity location for θ=0, there is an abrupt step in the thickness by an amount of 360°/α. The step at the discontinuity location can also be distributed over an azimuth angle range of a few degrees. However, this has the result of a non-optimized polarization distribution in the transition range.
In a further advantageous embodiment, an azimuthal section d(r=const.,θ) of the thickness profile d(r,θ) at a constant distance r from the element axis is a linear function of the azimuth angle θ with the same slope m but, in the ideal case, with two discontinuities at the azimuth angles θ=0 and θ=180°, respectively. At each discontinuity location, there is an abrupt step in the thickness by an amount of 180°/α. The two abrupt steps at the discontinuity locations can also be distributed over an azimuth angle range of a few degrees. However, this has the result of a non-optimized polarization distribution in the transition range.
In a further advantageous embodiment, an azimuthal section d(r=const.,θ) of the thickness profile d(r,θ) at a constant distance r from the element axis and in a first azimuth angle range of 10°<0<170° is a linear function of the azimuth angle θ with a first slope m, while in a second azimuth angle range of 190°<0<350°, the azimuthal section is a linear function of the azimuth angle θ with a second slope n. The slopes m and n have the same absolute magnitude but opposite signs. The magnitude of the slopes m and n at a distance r from the element axis is
With this arrangement, the thickness profile for all azimuth angles, including θ=0 and θ=180°, is a continuous function without abrupt changes in thickness.
In a further advantageous embodiment, the polarization-modulating optical element is divided into a large number of planar-parallel portions of different thickness or comprises at least two planar-parallel portions. These portions can for example be configured as sectors of a circle, but they could also have a hexagonal, square, rectangular, or trapezoidal shape.
In a further advantageous embodiment, a pair of first plan-parallel portions are arranged on opposite sides of a central element axis of said polarization-modulating optical element, and a pair of second plan-parallel portions are arranged on opposite sides of said element axis and circumferentially displaced around said element axis with respect to said first plan-parallel portions, wherein each of said first portions has a thickness being different from a thickness of each of said second portions.
In a further advantageous embodiment, a plane of oscillation of linearly polarized light passing through the polarization-modulating optical element is rotated by a first angle of rotation β1 within at least one of said first plan-parallel portions and by a second angle of rotation β2 within at least one of said second plan-parallel portions, such that β1 and β2 are approximately conforming or conform to the expression |β2−β1|=(2n+1)·90°, with n representing an integer.
In an advantageous embodiment, β1 and β2 are approximately conforming or conform to the expressions β1=90°+p·180°, with p representing an integer, and β2=q·180°, with q representing an integer other than zero. As will discussed below in more detail, such an embodiment of the polarization modulating optical element may be advantageously used in affecting the polarization of traversing polarized light such that exiting light has a polarization distribution being—depending of the incoming light—either approximately tangentially or approximately radially polarized.
The pair of second plan-parallel portions may particularly be circumferentially displaced around said element axis with respect to said pair of first plan-parallel portions by approximately 90°.
In a further advantageous embodiment, said pair of first plan-parallel portions and said pair of second plan-parallel portions are arranged on opposite sides of a central opening or a central obscuration of said polarization-modulating optical element.
Adjacent portions of said first and second pairs can be spaced apart from each other by regions being opaque to linearly polarized light entering said polarization-modulating optical element. Said portions of said first and second group can particularly be held together by a mounting. Said mounting can be opaque to linearly polarized light entering said polarization-modulating optical element. The mounting can have a substantially spoke-wheel shape.
In a further advantageous embodiment, the polarization-modulating optical element comprises a first group of substantially planar-parallel portions wherein a plane of oscillation of traversing linearly polarized light is rotated by a first angle of rotation β1, and a second group of substantially planar-parallel portions wherein a plane of oscillation of traversing linearly polarized light is rotated by a second angle of rotation, such that β1 and β2 are approximately conforming or conform to the expression |β2−β1|=(2n+1)·90°, with n representing an integer.
In a further advantageous embodiment, β1 and β2 are approximately conforming to the expressions β1=90°+p·180°, with p representing an integer, and β2=q·180°, with q representing an integer other than zero.
In a further advantageous embodiment, the thickness profile of the polarization-modulating optical element has a continuous surface contour without abrupt changes in thickness, whereby an arbitrarily selected polarization distribution can be generated whose thickness profile is represented by a continuous function of the location.
To ensure an adequate mechanical stability of the optical element, it is preferred to make the minimal thickness dmin of the polarization-modulating optical element at least equal to 0.002 times the element diameter D.
If the optically active material used for the optical element also has birefringent properties as is the case for example with crystalline quartz, the birefringence has to be taken into account for light rays whose direction of propagation deviates from the direction of the optical crystal axis. A travel distance of 90°/α inside the crystal causes a linear polarization to be rotated by 90°. If birefringence is present in addition to the rotating effect, the 90° rotation will be equivalent to an exchange between the fast and slow axis in relation to the electrical field vector of the light. Thus, a total compensation of the birefringence is provided for light rays with small angles of incidence if the distance traveled inside the crystal equals an integer multiple of 180°/α. In order to meet the aforementioned requirement for mechanical stability while simultaneously minimizing the effects of birefringence, it is of advantage if the polarization-modulating optical element is designed with a minimum thickness of
where N represents a positive integer.
From a manufacturing point of view, it is advantageous to provide the optical element with a hole at the center or with a central obscuration.
For light rays propagating not exactly parallel to the optical crystal axis, there will be deviations of the angle of rotation. In addition, the birefringence phenomenon will have an effect. It is therefore particularly advantageous if the maximum angle of incidence of an incident light bundle with a large number of light rays within a spread of angles relative to the optical crystal axis is no larger than 100 mrad, preferably no larger than 70 mrad, and with special preference no larger than 45 mrad.
In order to provide an even more flexible control over a state of polarization, an optical arrangement is advantageously equipped with a device that allows at least one further polarization-modulating optical element to be placed in the light path. This further polarization-modulating optical element can be an additional element with the features described above. However, it could also be configured as a planar-parallel plate of an optically active material or an arrangement of two half-wavelength plates whose respective fast and slow axes of birefringence are rotated by 45° relative to each other.
The further polarization-modulating optical element that can be placed in the optical arrangement can in particular be designed in such a way that it rotates the oscillation plane of a linearly polarized light ray by 90°. This is particularly advantageous if the first polarization-modulating element in the optical arrangement produces a tangential polarization. By inserting the 90°-rotator, the tangential polarization can be converted to a radial polarization.
In a further embodiment of the optical arrangement, it can be advantageous to configure the further polarization-modulating optical element as a planar-parallel plate which works as a half-wavelength plate for a half-space that corresponds to an azimuth-angle range of 180°. This configuration is of particular interest if the first polarization-modulating optical element has a thickness profile (r=const.,θ) that varies only with the azimuth angle θ and if, in a first azimuth angle range of 10°<θ<170°, the thickness profile (r=const.,θ) is a linear function of the azimuth angle θ with a first slope m, while in a second azimuth angle range of 190°<θ<350°, the thickness profile is a linear function of the azimuth angle θ with a second slope n, with the slopes m and n having the same absolute magnitude but opposite signs.
The refraction occurring in particular at sloped surfaces of a polarization-modulating element can cause a deviation in the direction of an originally axis-parallel light ray after it has passed through the polarization-modulating element. In order to compensate this type of deviation of the wave front which is caused by the polarization-modulating element, it is advantageous to arrange a compensation plate in the light path of an optical system, with a thickness profile of the compensation plate designed so that it substantially compensates an angular deviation of the transmitted radiation that is caused by the polarization-modulating optical element. Alternatively, an immersion fluid covering the profiled surface of the polarization-modulating element could be used for the same purpose.
Principally, in order to achieve the effect of compensating for the deviation in the direction of an originally axis-parallel light ray due to the polarization-modulating element, it would be possible to use a non-birefringent material such as CaF2 or fused silica as raw material for the compensation plate. However, significant drawbacks of such an optical arrangement are as follows: CaF2 is relatively difficult to handle during the manufacturing of the compensation plate, which usually makes it necessary to enhance its thickness e.g. up to 5 mm to achieve sufficient mechanical stability, leading to an enhancement of the space needed in the optical design. Fused silica is relatively sensitive to thermal compaction leading to local variations of the density and non-deterministic birefringence properties, which inadvertently modifies or destroys the polarization distribution after the optical arrangement. Furthermore, since the refraction indices of CaF2 or fused silica, on the one hand, and the optically active material of the polarization-modulating element, on the other hand, are not the same, the slopes in the thickness profiles of the compensation plate and the polarization-modulating element (or the “wedge angles”) in these elements, have to be different. With other words, the distance between the curved surfaces of these elements is not constant, which leads to a non-symmetric ray displacement for a light ray passing through the arrangement.
In order to avoid the above drawbacks while achieving a compensation for the deviation in the direction of an originally axis-parallel light due to the polarization-modulating element, an optical arrangement according to a further aspect of the present invention comprises
a polarization-modulating optical element having a first thickness profile and comprising a first optically active material with a first optical axis, wherein said first thickness profile, as measured in the direction of said optical axis, is variable,
and a compensation plate being arranged in the light path of the optical system and having a second thickness profile configured to substantially compensate for angle deviations of transmitted radiation which are caused by said polarization-modulating optical element,
wherein said compensation plate comprises a second optically active material with a specific rotation of opposite sign compared to said first optically active material.
Like in the embodiments of the polarization-modulating element discussed above, the first and second optically active materials could be solid or liquid optically active materials.
In an advantageous embodiment, the polarization-modulating optical element and the compensation plate are made of optical isomers. In a particular advantageous embodiment, the polarization-modulating optical element and the compensation plate are made of optically active crystalline quartz with clockwise and counterclockwise specific rotation. With other words, if the polarization-modulating optical element is made of R-quartz, the compensation plate is preferably made of L-quartz and vice versa. In such a combination of optically active materials with a specific rotation of opposite sign, a net change in the polarization direction of any linear polarized light ray will still occur, but is now depending on the value of the difference in the respective thicknesses being passed in these optically active materials. Such an embodiment has, in particular, the following advantageous effects:
- a) Since the refractive indices in the R-quartz and the L-quartz are substantially the same, the slopes in the thickness profiles of the compensation plate and the polarization-modulating element (or the “wedge angles” in these elements) may also be the same. In particular, both elements may be in direct contact to each other with their respective inclined or curved surface, in order to effectively forming a common or single optical element having the shape of a substantially plan-parallel plate. Alternatively, the compensation plate and the polarization-modulating element may also be arranged spaced apart from each other such that the distance between the inclined or curved surfaces of these elements is constant. As a consequence, any ray displacement that occurs for a light ray passing through the arrangement of the compensation plate and the polarization-modulating element will be symmetric.
- b) Any parts of the polarization-modulating element and the corresponding counter-part of the compensation plate can be substantially identical in geometry, making it possible to identically perform the corresponding manufacturing process (i.e. with the same programming of the tools used for the manufacturing procedure).
- c) Since the polarization-modulating optical element and the compensation plate are turning the direction of polarization of linear polarized light into opposite directions, temperature-induced modifications of the effective rotation of polarization in these elements will be at least partially compensated. In particular, any offset thickness of the compensation plate or the polarization-modulating element, respectively, will be without influence with regard to temperature changes, since the accompanying temperature-induced modifications of the effective rotation in one of the elements will be compensated by the opposed effective rotation in the other element.
- d) Since both the polarization-modulating optical element and the compensation plate are providing optical activity with a variable thickness profile, the respective slopes in the thickness profiles of the compensation plate and the polarization-modulating element may be reduced for each of these elements, if compared to the case where only the polarization-modulating optical element is made of optically active material. In particular, in the specific case where both the compensation plate and the polarization-modulating element have a substantially wedge-shaped cross-section, a tangential polarization distribution can be achieved with substantially half of the slope of the inclined surface of the compensation plate or the polarization-modulating element, respectively, if compared to the case where only the polarization-modulating optical element is made of optically active material. As a consequence of these reduced slopes, the space needed in the optical design is reduced and the manufacturing process is simplified due to less effort in the abrasive treatment of the respective raw materials used for making the compensation plate and the polarization-modulating element.
Polarization-modulating elements of the foregoing description, and optical arrangements equipped with them, are advantageously used in projection systems for microlithography applications. In particular, polarization-modulating elements of this kind and optical arrangements equipped with them are well suited for projection systems in which the aforementioned immersion technique is used, i.e., where an immersion medium with a refractive index different from air is present in the space between the optical element nearest to the substrate and the substrate.
According to a further aspect of the present invention, a polarization-modulating optical element is provided,
wherein a plane of oscillation of a first linearly polarized light ray and a plane of oscillation of a second linearly polarized light ray are rotated, respectively, by a first angle of rotation and a second angle of rotation in such a way that said first angle of rotation is different from said second angle of rotation;
wherein said polarization-modulating optical element comprises at least two planar-parallel portions that consist of an optically active material; and
wherein said planar-parallel portions are arranged on a support plate that consists of an optically active material.
As a consequence of making the support plate—like the at least two planar-parallel portions which are arranged thereon—of an optically active material, an enhanced durability of a wringing-connection between the support plate and said planar-parallel portions can be achieved, in particular under varying temperature conditions. The enhanced stability or durability of the wringing-connection particularly results from the fact that the support plate may be made from the same material and even with the same crystal orientation. Physical properties as the refraction numbers or expansion coefficients of the optically quartz material in the support plate and the plan-parallel portions can be made rotational symmetric with regard to the respective optical crystal axis. In particular, it can be achieved that the thermal expansion coefficients of the support plate and the plan-parallel portions (e.g. sector-shaped parts) are identical. Therefore, if the temperature changes in the direct environment of the contact region between the support plate and the planar-parallel portions (e.g. due to laser irradiation during the microlithography process or during applying antireflection coatings), the temperature increase and the thermal expansion are the same in the support plate and the planar-parallel portions, so that the risk of a temperature- or stress-induced loose of contact between these element is significantly reduced or eliminated, if e.g. compared to the use of a support plate made of CaF2 or fused silica (SiO2). A further advantage is that the use of an optical active material such as crystalline quartz avoids compaction effects as occurring e.g. in fused silica.
According to a further aspect the present invention relates to a projection system, comprising a radiation source, an illumination system operable to illuminate a structured mask, and a projection objective for projecting an image of the mask structure onto a light-sensitive substrate,
wherein said projection system comprises an optical system comprising an optical axis or a preferred direction given by the direction of a light beam propagating through the optical system,
the optical system comprising a temperature compensated polarization-modulating optical element described by coordinates of a coordinate system, wherein one preferred coordinate of the coordinate system is parallel to the optical axis or parallel to said preferred direction;
said temperature compensated polarization-modulating optical element comprising a first and a second polarization-modulating optical element, the first and/or the second polarization-modulating optical element comprising solid and/or liquid optically active material and a profile of effective optical thickness, wherein the effective optical thickness varies at least as a function of one coordinate different from the preferred coordinate of the coordinate system, in addition or alternative the first and/or the second polarization-modulating optical element comprises solid and/or liquid optically active material, wherein the effective optical thickness is constant as a function of at least one coordinate different from the preferred coordinate of the coordinate system;
wherein the first polarization-modulating optical element comprises optically active material with a specific rotation of opposite sign compared to the optically active material of the second polarization-modulating optical element.
Due to the presence of two polarization-modulating optical elements comprising optically active materials having specific rotations of opposite sign, the temperature effects in both these elements at least partially compensate each other, so that the combined system of these two elements has a reduced temperature dependence regarding the change of the polarization. Consequently, even under conditions of temperature variation, a change of the polarization state of light passing both elements is reduced, or the polarization state even remains unchanged. As a consequence of said compensation effect, a detrimental effect of temperature variations on the polarization state of light passing through said system can be reduced or avoided even with relatively large thicknesses (e.g. of several mm) of said elements.
The above compensation concept may particularly be used in a system of two plane plates with a first and a second thickness in the direction of the propagating light beam, said plates being made of optically active quartz with clockwise and counterclockwise specific rotation. Furthermore, according to the present invention said plane plates may have either substantially the same thickness or different thicknesses. In the case of substantial identical thicknesses of the two plane plates, a resulting effect of the whole arrangement of the two plates may be avoided, so that such arrangement is particularly suited e.g. as a polarization-neutral support of a micro-optical element such as a DOE. In the case of different thicknesses of the two plane plates, a resulting effect of the whole arrangement of the two plates can be achieved to provide a polarization-modulating arrangement having, due to the partial compensation effect mentioned before, a relatively weak sensitivity to temperature variations even if relatively large thicknesses for the plane plates are used in view of manufacturing aspects.
Therefore, according to a further aspect the present invention relates to an optical system comprising an optical axis or a preferred direction given by the direction of a light beam propagating through the optical system,
the optical system comprising a temperature compensated polarization-modulating optical element described by coordinates of a coordinate system, wherein one preferred coordinate of the coordinate system is parallel to the optical axis or parallel to said preferred direction;
said temperature compensated polarization-modulating optical element comprising a first and a second polarization-modulating optical element, the first and/or the second polarization-modulating optical element comprising solid and/or liquid optically active material and a profile of effective optical thickness, wherein the effective optical thickness varies at least as a function of one coordinate different from the preferred coordinate of the coordinate system, in addition or alternative the first and/or the second polarization-modulating optical element comprises solid and/or liquid optically active material, wherein the effective optical thickness is constant as a function of at least one coordinate different from the preferred coordinate of the coordinate system; wherein the first polarization-modulating optical element comprises optically active material with a specific rotation of opposite sign compared to the optically active material of the second polarization-modulating optical element;
wherein said first and second polarization-modulating optical elements are plane plates with a first and a second thickness in the direction of the propagating light beam, said plates being made of optically active quartz with clockwise and counterclockwise specific rotation; wherein the first and the second thickness are different from each other.
Preferably, the absolute value of the difference of the first and the second thickness is smaller than the thickness of the smaller plate.
According to a further aspect the present invention relates to an optical system having an optical axis, the optical system comprising a first plane plate with a first thickness in the direction of the optical axis and a second plane plate with a second thickness in the direction of the optical axis, wherein said first and second plane plates are made of optically active quartz with specific rotations opposite to each other; and wherein the first and the second thickness are different from each other.
According to a further aspect the present invention relates to a projection system comprising a radiation source, an illumination system operable to illuminate a structured mask, and a projection objective for projecting an image of the mask structure onto a light-sensitive substrate, wherein said projection system comprises an optical system having an optical axis, the optical system comprising a first plane plate with a first thickness in the direction of the optical axis and a second plane plate with a second thickness in the direction of the optical axis; wherein said first and second plane plates are made of optically active quartz with specific rotations opposite to each other.
Further advantageous aspects can be gathered from the following description and the appended claims.
The invention will hereinafter be explained in more detail with reference to the attached drawings, wherein:
Optically active crystals have at least one optical axis OA which is inherent in the crystal structure. When linearly polarized light travels along this optical axis OA, the plane of oscillation of the electrical field vector 206 is rotated by an angle β of proportionate magnitude as the distance d traveled by the light inside the crystal 202. The proportionality factor between distance d and angle of rotation is the specific rotation α. The latter is a material-specific quantity and is dependent on the wavelength of the light rays propagating through the crystal. For example in quartz, the specific rotation at a wavelength of 180 nm was measured as about α=(325.2±0.5)°/mm, at 193 nm α=323.1°/mm at a temperature of 21.6° C.
It is also important for the present invention, applying optically active materials in an illumination system and/or an objective of a projection optical system of e.g. a projection apparatus used in microlithography, that also the temperature dependency of the specific rotation is considered. The temperature dependency of the specific rotation α for a given wavelength is to a good and first linear approximation given by α(T)=α0(T0)+γ*(T−T0), where γ is the linear temperature coefficient of the specific rotation α. In this case α(T) is the optical activity coefficient or specific rotation at the temperature T and α0 is the specific rotation at a reference temperature T0. For optical active quartz material the value γ at a wavelength of 193 nm and at room temperature is γ=2.36 mrad/(mm*K).
Referring again to
Based on this property of an optically active crystal, it is possible to produce an arbitrarily selected linear polarization distribution by designing the polarization-modulating optical element 1 of
More general, alternative or in addition to the variation of the thickness d=d(x,y) of the polarization-modulating element, the specific rotation α may itself be dependent on the location within the modulating element such that α becomes an α(x,y,z) or α(r,θ,z), where x,y or r,θ are Cartesian or polar coordinates in a plane perpendicular to the element axis EA (or alternative to the optical axis OA) of the polarization-modulating element, as shown e.g. in
The polarization-modulating optical element 301 has a cylindrical shape with a base surface 303 and an opposite surface 305. The base surface 303 is designed as a planar circular surface. The element axis EA extends perpendicular to the planar surface. The opposite surface 305 has a contour shape in relation to the element axis EA in accordance with a given thickness profile. The optical axis of the optically active crystal runs parallel to the element axis EA. The reference axis RA, which extends in the base plane, intersects the element axis at a right angle and serves as the reference from which the azimuth angle θ is measured. In the special configuration illustrated in
In addition it should be mentioned that the polarization-modulating optical element 301 not necessary need to comprise a planar base surface 303. This surface in general can also comprise a contour shaped surface e.g. similar or equal to the surface as designated by 305 shown in
As described in connection with
The optical systems in accordance with the present invention advantageously modify respective planes of oscillation of a first linearly polarized light ray and a second linearly polarized light ray. Both light rays propagating through the optical system, and being at least a part of the light beam propagating through the optical system. The light rays are also passing the polarization-modulating optical element with different paths, and are rotated by a respective first and second angle of rotation such that the first angle is different of the second angle. In general the polarization-modulating optical element of the optical systems according to the present invention transforms a light bundle with a first linear polarization distribution, which enters said polarization-modulating optical element, into a light bundle exiting said polarization-modulating optical element. The exiting light bundle has a second linear polarization distribution, wherein the second linear polarization distribution is different from the first linear polarization distribution.
An azimuthal section of a polarization-modulating optical element 401 that is divided into sector-shaped portions has a stair-shaped profile in which each step corresponds to the difference in thickness d or optical effective thickness D between neighbouring sector elements. The profile has e.g. a maximum thickness dmax and a minimum thickness dmin. In order to cover a range of 0≦β≦360° for the range of the angle of rotation of the oscillation plane of linearly polarized light, there has to be a difference of 360°/α between dmax and dmin. The height of each individual step of the profile depends on the number n of sector elements and has a magnitude of 360°/(n·α). At the azimuth angle θ=0°, the profile has a discontinuity where the thickness of the polarization-modulating optical element 401 jumps from dmin to dmax. A different embodiment of the optical element can have a thickness profile in which an azimuthal section has two discontinuities of the thickness, for example at θ=0° and θ=180°.
In an alternative embodiment the profile has e.g. a maximum optical effective thickness Dmax and a minimum optical effective thickness Dmin, and the geometrical thickness d is e.g. constant, resulting in a variation of the specific rotation α of the individual segments 409 of the element 401. In order to cover a range of 0≦β≦360° for the range of the angle of rotation of the oscillation plane of linearly polarized light, there has to be a difference of 360°/d between αmax and αmin. The change of the specific rotation of each individual step of the profile depends on the number n of sector elements 409 and has a magnitude of 360°/(n·d). At the azimuth angle θ=0°, the profile has a discontinuity regarding the optical effective thickness where it jumps from Dmin to Dmax. It should be pointed out, that advantageously in this embodiment there is no discontinuity in the geometrical thickness d of the polarization-modulating element 401. Also the thickness profile of the optical effective thickness in which an azimuthal section has two discontinuities of the optical effective thickness can easily be realized, for example at θ=0° and θ=180°. To realize the defined changes in magnitude of the specific rotation of Δα=360°/(n·d)(if there a n angular segments 409 to form the element 401), the individual sector elements 409 are preferably made of or comprises cuvettes or cells, filled with an optical active liquid with the required specific rotation α. As an example, for the m-th sector element the specific rotation is α(m)=αmin+m*360°/(n·d), and 0≦m≦n. The required specific rotation e.g. can be adjusted by the concentration of the optical active material of the liquid, or by changing the liquid material itself
In a further embodiment the segments 409 of a polarization-modulating optical element 401 may comprise components of solid optically active material (like crystalline quartz) and cells or cuvettes filled with optically active material, and these components are placed behind each other in the light propagation direction. Alternative or in addition the cuvette itself may comprise optically active material like crystalline quartz.
The polarization-modulating optical element of the foregoing description converts linearly polarized incident light into a linear polarization distribution in which the oscillation planes of linearly polarized light rays are rotated by an angle that depends on the thickness (or optical effective thickness) of each individual sector element. However, the angle by which the direction of polarization is rotated is constant over an individual sector element. Thus, the distribution function for the directions of the oscillation planes of the individual field vectors takes only certain discrete values.
A continuous distribution of linear polarizations can be achieved with an optical element that has a continuously varying thickness (optical effective thickness) profile along an azimuthal section.
An example of a continuously varying thickness profile is illustrated in
The symbol a in this context stands for the specific rotation of the optically active crystal. As in the previously described embodiment of
A further embodiment of a polarization-modulating optical element which is shown in
Additionally it is mentioned that for certain special applications clockwise and counterclockwise optically active materials are combined in a polarization-modulating optical element.
As the slope of the thickness profile along an azimuthal section increases strongly with smaller radii, it is advantageous from a manufacturing point of view to provide a central opening 407 or a central obscuration in a central portion around the central axis of the circular polarization-modulating optical element.
It is furthermore advantageous for reasons of mechanical stability to design the polarization-modulating optical element with a minimum thickness dmin of no less than two thousandths of the element diameter. It is particularly advantageous to use a minimum thickness of dmin=N·90°/α, where N is a positive integer. This design choice serves to minimize the effect of birefringence for rays of an incident light bundle which traverse the polarization-modulating element at an angle relative to the optical axis.
According to an alternate embodiment not illustrated in
The sector-shaped parts 422 and 424 have a first thickness d1 which is selected so that the parts 422 and 424 cause the plane of oscillation of linearly polarized axis-parallel light to be rotated by 90°+p·180°, where p represents an integer. The sector-shaped parts 423 and 425 have a second thickness d2 which is selected so that the parts 423 and 425 cause the plane of oscillation of linearly polarized axis-parallel light to be rotated by q·180°, where q represents an integer other than zero. Thus, when a bundle of axis-parallel light rays that are linearly polarized in the y-direction enters the polarization-modulating optical element 421, the rays that pass through the sector-shaped parts 423 and 425 will exit from the polarization-modulating optical element 421 with their plane of oscillation unchanged, while the rays that pass through the sector-shaped parts 422 and 424 will exit from the polarization-modulating optical element 421 with their plane of oscillation rotated into the x-direction. As a result of passing through the polarization-modulating optical element 421, the exiting light has a polarization distribution which is exactly tangential at the centrelines 429 and 430 of the sector-shaped parts 422, 423, 424, 425 and which approximates a tangential polarization distribution for the rest of the polarization-modulating optical element 421.
When a bundle of axis-parallel light rays that are linearly polarized in the x-direction enters the polarization-modulating optical element 421, the rays that pass through the sector-shaped parts 423 and 425 will exit from the polarization-modulating optical element 421 with their plane of oscillation unchanged, while the rays that pass through the sector-shaped parts 422 and 424 will exit from the polarization-modulating optical element 421 with their plane of oscillation rotated into the y-direction. As a result of passing through the polarization-modulating optical element 421, the exiting light has a polarization distribution which is exactly radial at the centrelines 429 and 430 of the sector-shaped parts 422, 423, 424, 425 and which approximates a radial polarization distribution for the rest of the polarization-modulating optical element 421.
Of course the embodiment of the present invention according to
In order to produce a tangential polarization distribution from linearly polarized light with a wave length of 193 nm and a uniform direction of the oscillation plane of the electric field vectors of the individual light rays, one can use for example a polarization-modulating optical element of crystalline quartz with the design according to
The above mentioned data are based exemplarily for a specific rotation α of (325.2±0.5)°/mm. If the specific rotation α changes to 321.1°/mm, the value at 193 nm and at a temperature of 21.6° C., the thickness profile will change as follows: D(r,θ)=278.6 +180°−θ/180 °·557 μm for 0≦θ≦180° and r>10.5/2 mm D(r,θ)=278.6 +360°−θ/180 °·557 μm for 180≦θ≦360° and r>10.5/2 mm D(r,θ)=0 for r≦10.5/2 mm
The polarization-modulating optical element according to this embodiment has a central opening 407 with a diameter 10.5, i.e., one-tenth of the maximum aperture. The thickness maxima and minima, which are found at the discontinuities, are 830.26 μm and 276.75 μm, respectively for the first given example.
The embodiment of the foregoing description can be produced with a robot-polishing process. It is particularly advantageous to produce the polarization-modulating element from two wedge-shaped or helically shaped half-plates which are seamlessly joined together after polishing. If the element is produced by half-plates, it is easy and in some applications of additional advantage to use one clockwise and one counterclockwise optically active material like clockwise crystalline and counterclockwise crystalline quartz (R-quartz and L-quartz).
The embodiment of
The thickness d2 of the sector-shaped parts 452 and 453 is such that the orientation of polarization of linearly polarized light with normal incidence on the light entrance side of the polarization-modulating element 450 is rotated by an angle of 90°. The thickness d1 of the support plate 451 is such that the orientation of polarization of linearly polarized light with normal incidence on the light entrance side of the support plate 451 is rotated by an angle of q*180°, with q being an integer larger than zero. For use of synthetic, optically active crystalline quartz having a specific rotation of 323.1°/mm for a wavelength of 193 nm and a temperature of 21.6° C., this condition means that the thickness d of the support plate is dim q*557 μm. As a consequence, the support plate 451 behaves neutral with respect to the effect of the polarization-modulating element 450 in the sense that the polarization distribution of light entering the light entrance side of the support plate 451 with normal incidence is identical to the polarization distribution of this light after having passed the support plate 451, i.e. at the light exit side of the support plate 451.
A significant advantage of this embodiment using the above-described support plate 451 made of optically active quartz is that an enhanced durability of the wringing-connection between the support plate 451 and sector-shaped parts 452 and 453 can be achieved, in particular under varying temperature conditions. The enhanced stability or durability of the wringing-connection is a result from the fact that the support plate 451 and the sector-shaped parts 452 and 453 are not only made from the same material, but also have, in addition to the identity of materials, the same crystal orientation. In the crystal orientation according to the configuration shown in
In the embodiment illustrated in
Furthermore, although in the embodiment as shown in
In
Of course further possible embodiments for producing a radial polarization distribution are conceivable within the scope of the invention. For example, the further polarization-modulating optical element 621 can be connected to the polarization-modulating optical element 601. To allow a fast change-over from tangential to radial polarization, one could provide an exchange device that allows the further polarization-modulating element 621 to be placed in the light path and to be removed again or to be replaced by another element.
A tangential polarization distribution can also be produced with a polarization-modulating optical element that has a thickness profile in accordance with
An additional advantage of the present invention is that polarization-modulating optical elements or the optical system according to the present invention can be used for adjusting the polarization distribution and also for temperature compensation of the polarization distribution in a microlithography projection system as described in
Since the polarization distribution at the reticle is influenced by the various optical elements by e.g. tension-induced birefringence, or by undefined or uncontrolled changes of the temperature of individual optical elements, the polarization distribution can unpredictably or uncontrollably change over time. To correct such changes the temperature dependency of the specific rotation α of the polarization-modulating optical element can be used to control the magnitude of the polarization angles. The optical system according to an embodiment of the present invention preferably comprises a polarization control system for controlling the polarization distribution of the light beam which is propagating through the optical system. The polarization distribution of interest is at a predefined location in the optical system. The polarization control system comprises at least one heating or cooling device to modify the temperature and/or the temperature distribution of the polarization-modulating optical element to affect the polarization distribution of the light beam at the predefined location. Here the polarization-modulating optical element may have a varying or constant effective optical thickness.
In the case of a constant effective optical thickness the optical system comprises an optical axis or a preferred direction given by the direction of a light beam propagating through the optical system. The optical system additionally comprises a polarization-modulating optical element described by coordinates of a coordinate system, wherein one preferred coordinate of the coordinate system is parallel to the optical axis or parallel to said preferred direction. The polarization-modulating optical element comprises solid and/or liquid optically active material, wherein the effective optical thickness is constant as a function of at least one coordinate different from the preferred coordinate of the coordinate system. The optical system comprises further a polarization control system for controlling the polarization distribution of the light beam (propagating through the optical system) at a predefined location in the optical system, and the polarization control system comprises at least one heating or cooling device to modify the temperature and/or the temperature distribution of the polarization-modulating optical element to affect the polarization distribution of the light beam at the predefined location.
As an example, if the polarization-modulating optical element (as used e.g. in the optical system according to the present invention) is made of synthetic (crystalline) quartz, comprising a parallel plate or formed as a parallel plate, a thickness of 10 mm of such a plate will result in a change of polarization of 23.6 mrad/° C. or 23.6 mrad/K, equivalent to 1.35°/K, due to the linear temperature coefficient γ of the specific rotation α with γ=2.36 mrad/(mm*K). These data correspond to a wavelength of 193 nm. In such an embodiment, which is schematically shown in
The temperature control of the plate 901 can be done by closed-loop or open-loop control, using a temperature sensing device with at least one temperature sensor 902, 903 for determining the temperature of the plate 901 (or providing a temperature sensor value which is representative or equal to the temperature and/or the temperature distribution of the polarization-modulating optical element), at least a heater 904, 905, preferably comprising an infrared heater, for heating the plate by infrared radiation 906, and a control circuit 910 for controlling the at least one heater 904, 905. As an example of a temperature sensing device a infrared sensitive CCD-element with a projection optics may be used, wherein the projection optics images at least a part of the plate 901 onto the CCD-element such that a temperature profile of the viewed part of the plate 901 can be determined by the analysis of the CCD-element signals. The control circuit 910 may comprise a computer system 915 or may be connected to the computer or control system 915 of the microlithography projection system 833 (see
In a further preferred embodiment the temperature of the polarization-manipulating optical element 901 (the plate as shown in
Alternatively or in addition the heater or heating elements 904, 905 may be replaced or supplemented by one or more Peltier-elements 907, 908. The Peltier-element or elements are preferably connected to the control circuit 910 such that a control by open and/or closed loop control is possible. The advantage of the Peltier-elements is that also a controlled cooling of the polarization-manipulating optical element 901 can be achieved. Heating and cooling the optical element 901 at the same time result in complex temperature distributions in the polarization-modulating optical element 901, which result in complex polarization distributions of the light 950 propagating e.g. through the microlithography projection system 833, after passing the element 901. Of course, other heating and cooling means than the ones mentioned above can be used to achieve a required temperature profile or a required temperature of the polarization-modulating optical element 901.
The application of the plane plate 901 as polarization-modulating optical element 801 in the illumination system of a microlithography projection apparatus 833 (see
In a further preferred embodiment of the invention the state of the polarization of the light passed through the polarization-modulating element 901 or the optical system according to the present invention is measured. For this the polarization control system comprises a polarization measuring device providing a polarization value representative for or equal to the polarization or the polarization distribution of the light beam at the predetermined location in the optical system. Further, the control circuit controls the at least one heating or cooling device dependent on the temperature sensor value and/or the polarization value by open or closed loop control. The measured state of polarization is compared with a required state and in the case that the measured state deviates more than a tolerable value, the temperature and/or the temperature distribution of the polarizing-modulating element like the plane plate 901 is changed such that the difference between the measured and the required state of polarization becomes smaller, and if possible such small that the difference is within a tolerable value. In
The plane plate 901 used as polarization-modulating optical element or being a part of such element is especially appropriate to correct orientations of polarization states of the passed light bundles.
In a further embodiment of the present invention the plane plate 901 (comprising or consisting of optically active material), used as a polarization-modulating optical element, is combined with a plate 971 (see
In an additional embodiment of the present invention a polarization-modulating element or in general a polarizing optical element is temperature compensated to reduce any inaccuracy of the polarization distribution generated by the polarization-modulating element due to temperature fluctuations of said element, which for synthetic quartz material is given by the linear temperature coefficient γ of the specific rotation α for quartz (which is as already mentioned above γ=2.36 mrad/(mm*K)=0.15°/(mm*K)). The temperature compensation makes use of the realization that for synthetic quartz there exist one quartz material with a clockwise and one quartz material with a counterclockwise optical activity (R-quartz and L-quartz). Both, the clockwise and the counterclockwise optical activities are almost equal in magnitude regarding the respective specific rotations α. The difference of the specific rotations is less than 0.3%. Whether the synthetic quartz has clockwise (R-quartz) or counterclockwise (L-quartz) optical activity dependents on the seed-crystal which is used in the manufacturing process of the synthetic quartz.
R- and L-quartz can be combined for producing a thermal or temperature compensated polarization-modulating optical element 911 as shown in
In general any structured polarization-modulating optical element made of R- or L-quartz, like e.g. the elements as described in connection with
To generalize the above example of a temperature compensated polarization-modulating optical element 911, the present invention also relates to an optical system comprising an optical axis OA or a preferred direction 950 given by the direction of a light beam propagating through the optical system. The optical system comprising a temperature compensated polarization-modulating optical element 911 described by coordinates of a coordinate system, wherein one preferred coordinate of the coordinate system is parallel to the optical axis OA or parallel to said preferred direction 950. The temperature compensated polarization-modulating optical element 911 comprises a first 921 and a second 931 polarization-modulating optical element. The first and/or the second polarization-modulating optical element comprising solid and/or liquid optically active material and a profile of effective optical thickness, wherein the effective optical thickness varies at least as a function of one coordinate different from the preferred coordinate of the coordinate system. In addition or alternative the first 921 and/or the second 931 polarization-modulating optical element comprises solid and/or liquid optically active material, wherein the effective optical thickness is constant as a function of at least one coordinate different from the preferred coordinate of the coordinate system. As an additional feature, the first and the second polarization-modulating optical elements 921, 931 comprise optically active materials with specific rotations of opposite signs, or the first polarization-modulating optical element comprises optically active material with a specific rotation of opposite sign compared to the optically active material of the second polarization-modulating optical element. In the case of plane plates, preferably the absolute value of the difference of the first and the second thickness of the first and second plate is smaller than the thickness of the smaller plate.
In an additional embodiment of the present invention a polarization-modulating element comprises an optically active and/or optically inactive material component subjected to a magnetic field such that there is a field component of the magnetic field along the direction of the propagation of the light beam through the polarization-modulating element. The optical active material component may be constructed as described above. However, also optical inactive materials can be used, having the same or similar structures as described in connection with the optical active materials. The application of a magnetic field will also change the polarization state of the light passing through the optical active and/or optical inactive material due to the Faraday-effect, and the polarization state can be controlled by the magnetic field.
In the following, further embodiments of the present invention related to the above-described aspect of temperature-compensation are described with reference to
As a consequence, the temperature effects in both plates 11, 12 compensate each other, so that—due to the substantially identical thicknesses of the plates 11, 12—the polarization state of light passing both plates 11, 12 remains unchanged. The arrangement shown in
With reference to
As to the light beam “1” which passes plate 305 along the optical crystal axis, only circular birefringence and no linear birefringence occurs. As to light beam “2” which passes plate 25 not parallel to the optical crystal axis, the additional effect of linear birefringence occurs, with said linear birefringence reaching its maximum value if the light beam is perpendicular to the optical crystal axis, whereas the effect of circular birefringence decreases for increasing angular between the light beam and the optical crystal axis. Since the orientation of polarization is rotated between the light entrance surface and the light exit surface of plate 25 by ≈180° (which is approximately true also for beam “2” in spite of the decreasing circular birefringence if also considering the increased travelling path), said orientation of polarization is rotated by ≈90° after beam “2” has passed half of the thickness of plate 25. As a consequence, a polarity inversion (i.e. a reversal of the signs) occurs for the linear birefringence after beam “2” has passed half of the thickness of plate 25, so that the phase shifts collected due to linear birefringence while passing the first half of plate 25 are corrected back by the phase shifts collected due to linear birefringence while passing the second half of plate 25, which means that the effect of linear birefringence is almost nearly compensated for beam “2” after having passed the whole plate 25.
As to the light beam “1” which passes plates 32 and 31 along the optical crystal axis, only circular birefringence and no linear birefringence occurs. The orientation of polarization of beam “1” is rotated clockwise in plate 32 by 180° and is then rotated counterclockwise in plate 31 by 180°, so that the orientation of polarization beam 1 is effectively not rotated when passing the whole arrangement of plates 32 and 31. Light beam “2” enters plate 32 under an angle of incidence larger than zero and therefore experiences also the effect of linear birefringence, whereas the effect of circular birefringence decreases with increasing angle of incidence. As a consequence, the orientation of polarization for light beam 2 is rotated clockwise in plate 32 by less than 180° (see diagram 2b on the right side of
As described above with reference to
In the embodiment of
In the embodiment of
As a consequence of the weak sensitivity of the grey filter 40 or the diffractive optical element 50, said micro-optical elements are suitable to be used at a position in an illumination system where relatively large energy densities occur, since at such positions the avoidance of the above-discussed compaction effects and the compensation of the effects of temperature variations are particularly relevant. As an example, an optical arrangement or an optical element according to anyone of the embodiments described with reference to
Going back to the embodiments of the polarization-modulating optical element discussed before and having a variable thickness profile measured in the direction of the optical crystal axis, further advantageous embodiments are described in the following with reference to
As already mentioned above, the refraction occurring in particular at sloped surfaces of the polarization-modulating element can cause a deviation in the direction of an originally axis-parallel light ray after it has passed through the polarization-modulating element. In order to compensate for this type of deviation, it is advantageous to arrange a compensation plate in the light path of an optical system, with a thickness profile of the compensation plate designed so that it substantially compensates an angular deviation of the transmitted radiation that is caused by the polarization-modulating optical element. In an advantageous embodiment of the present invention which is explained in more detail in the following, said compensation plate is also made of an optically active material.
The optical arrangement according to
The shape of the polarization-modulating optical element 110 shown in
In the exemplarily embodiment, the polarization-modulating optical element 110 may be made of R-quartz, with the optical axis of the optically active crystal running parallel to the element axis. “R-quartz” means that the optically active quartz is turning the direction of polarization clockwise if seen through the optically active quartz towards the light source.
Furthermore, although the compensation plate 120 is shown, in
As can also be seen in
In the exemplarily embodiment with the polarization-modulating optical element 110 being made of R-quartz, the compensation plate 120 is preferably made of L-quartz. “L-quartz” means that the optically active quartz is turning the direction of polarization counter-clockwise if seen through the optically active quartz towards the light source. Of course the polarization-modulating optical element 110 can also be, vice versa, made of L-quartz, with the compensation plate 120 being made of R-quartz. More generally, the compensation plate 120 comprises an optically active material with a specific rotation of opposite sign compared to said first optically active material.
Furthermore, as already discussed before, the invention is not limited to the use of quartz or generally to the use of crystalline materials, so that both the polarization-modulating optical element and the compensation plate may also be replaced by one or more cuvettes of appropriate shape which are comprising an optically active liquid. In further more generalized embodiments, as has been already described above with reference to
To evaluate the effect of this thickness profile, it has to be considered that since the polarization-optical element 110 and the compensation plate 120 are turning the direction of polarization of linear polarized light into opposite directions, the relevant factor for the net effect on each light ray traversing the arrangement the polarization-optical element 110 and the compensation plate 120 parallel to the optical axis of each of these elements is the difference of the thicknesses d or optically effective thicknesses D being passed in the L-quartz or the R-quartz, respectively. Since this difference is just zero at the two crossing points of the solid lines C1, C2 with the dashed lines D1, D2, which occur for an azimuth angle of θ=90° as well as for an azimuth angle of θ=270°, a linearly polarized light ray passing the arrangement under an azimuth angle of θ=90° or θ=270° will leave the arrangement with the same orientation of polarization. This means that for a generation of a tangential polarization distribution as it has been explained above with reference to
For thicknesses 90°<θ<180°, the traveled distance in the R-quartz of the polarization-optical element is larger than the traveled distance in the L-quartz of the compensation plate, leading to a clockwise net-rotation of the direction of polarization. For thicknesses 0°<θ<90°, the traveled distance in the L-quartz of the compensation plate is larger than the traveled distance in the R-quartz of the polarization-optical element, leading to a counter-clockwise net-rotation of the direction of polarization.
Since both the polarization-modulating optical element 110 and the compensation plate 120 are rotating the direction of polarization into opposite directions, the slopes in the respective thickness profiles of the compensation plate and the polarization-modulating element may be reduced for each of these elements, if compared to a situation where only the polarization-modulating optical element is made of optically active material. More specifically and with reference to
A modification of the arrangement of
A further modification of the arrangement of
Various embodiments for a polarization-modulating optical element or for the optical systems according to the present invention are described in this application. Further, also additional embodiments of polarization-modulating optical elements or optical systems according to the present invention may be obtained by exchanging and/or combining individual features and/or characteristics of the individual embodiments described in the present application.
Claims
1. An illumination optical apparatus which illuminates an object having a pattern with illumination light, the illumination optical apparatus comprising:
- a polarization modulating member made of an optical material with optical activity, the polarization modulating member having a thickness that varies with respect to azimuthal positions about an optical axis of the illumination optical apparatus, the polarization modulating member being arranged around the optical axis so as to surround the optical axis, a direction of an optic axis of the optical material being substantially coincident with a direction of the optical axis of the illumination optical apparatus,
- wherein the polarization modulating member rotates a polarization direction of the illumination light so that the illumination light, being incident on the polarization modulating member in a linearly polarized state having the polarization direction along a single direction, is changed to a linearly polarized state having polarization directions being substantially coincident with an azimuthal direction about the optical axis at a pupil plane of the illumination optical apparatus, wherein the polarization modulating member has a region without optical activity in an area perpendicular to the optical axis, the region being on the optical axis, and
- wherein the polarization modulating member has substantially constant thicknesses in at least two radial directions that are not parallel to each other.
2. The illumination optical apparatus according to claim 1, wherein the illumination light is irradiated onto the object in a polarization state in which a principal component is S-polarized light.
3. The illumination optical apparatus according to claim 1, wherein a first thickness of the polarization modulating member in an optical path of a first part of the illumination light is different from a second thickness of the polarization modulating member in an optical path of a second part of the illumination light, and
- the first part of the illumination light passes through a first portion of the pupil plane away from the optical axis, and the second part of the illumination light passes through a second portion of the pupil plane away from the optical axis, the first and second portions being different from each other.
4. The illumination optical apparatus according to claim 3, wherein the first and second portions are included in an annular region about the optical axis.
5. An exposure method which exposes a substrate to light via an object having a pattern, the exposure method comprising:
- holding the substrate by the stage;
- illuminating the pattern with the light by using the illumination optical apparatus as defined in claim 1; and
- projecting an image of the pattern illuminated with the light onto the substrate held by the stage.
6. The exposure method according to claim 5, wherein the substrate is exposed to the light through liquid.
7. An exposure apparatus which exposes a substrate to light via an object having a pattern, the exposure apparatus comprising:
- a stage which holds the substrate,
- the illumination optical apparatus as defined in claim 1 which illuminates the pattern with the light; and
- a projection optical system which projects an image of the pattern illuminated with the light onto the substrate held by the stage, wherein the polarization modulating member has a region without optical activity in an area perpendicular to the optical axis, the region being on the optical axis, and
- wherein the polarization modulating member has substantially constant thicknesses in at least two radial directions that are not parallel to each other.
8. The exposure apparatus according to claim 7, wherein the substrate is exposed to the light through liquid.
9. The exposure apparatus according to claim 7, wherein the light is irradiated onto the object in a polarization state in which a principal component is S-polarized light.
10. The exposure apparatus according to claim 7, wherein a first thickness of the polarization modulating member in an optical path of a first part of the light is different from a second thickness of the polarization modulating member in an optical path of a second part of the light, and
- the first part of the light passes through a first portion of the pupil plane away from the optical axis, and the second part of the light passes through a second portion of the pupil plane away from the optical axis, the first and second portions being different from each other.
11. The exposure apparatus according to claim 10, wherein the first and second portions are included in an annular region about the optical axis.
12. The illumination optical apparatus according to claim 1, further comprising a raster plate arranged in an optical path of the illumination light.
Type: Application
Filed: Dec 19, 2016
Publication Date: Apr 13, 2017
Inventors: Damian Fiolka (Oberkochen), Markus Deguenther (Aalen)
Application Number: 15/383,136