WO2015007298A1 - Appareil microlithographique et procédé de variation d'une répartition du rayonnement lumineux - Google Patents

Appareil microlithographique et procédé de variation d'une répartition du rayonnement lumineux Download PDF

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Publication number
WO2015007298A1
WO2015007298A1 PCT/EP2013/002114 EP2013002114W WO2015007298A1 WO 2015007298 A1 WO2015007298 A1 WO 2015007298A1 EP 2013002114 W EP2013002114 W EP 2013002114W WO 2015007298 A1 WO2015007298 A1 WO 2015007298A1
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WIPO (PCT)
Prior art keywords
optical element
diffractive optical
diffractive
projection light
relative position
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PCT/EP2013/002114
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English (en)
Inventor
Vladimir Kamenov
Toralf Gruner
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Carl Zeiss Smt Gmbh
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Application filed by Carl Zeiss Smt Gmbh filed Critical Carl Zeiss Smt Gmbh
Priority to PCT/EP2013/002114 priority Critical patent/WO2015007298A1/fr
Priority to TW103124451A priority patent/TW201510674A/zh
Publication of WO2015007298A1 publication Critical patent/WO2015007298A1/fr

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70216—Mask projection systems
    • G03F7/70308—Optical correction elements, filters or phase plates for manipulating imaging light, e.g. intensity, wavelength, polarisation, phase or image shift
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1838—Diffraction gratings for use with ultraviolet radiation or X-rays
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/208—Filters for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70058—Mask illumination systems
    • G03F7/70091—Illumination settings, i.e. intensity distribution in the pupil plane or angular distribution in the field plane; On-axis or off-axis settings, e.g. annular, dipole or quadrupole settings; Partial coherence control, i.e. sigma or numerical aperture [NA]
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70216—Mask projection systems
    • G03F7/70258—Projection system adjustments, e.g. adjustments during exposure or alignment during assembly of projection system
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70216—Mask projection systems
    • G03F7/70316—Details of optical elements, e.g. of Bragg reflectors, extreme ultraviolet [EUV] multilayer or bilayer mirrors or diffractive optical elements

Definitions

  • the invention generally relates to the field of microlitho- graphy, and in particular to objectives used in proj ection exposure apparatus or mask inspection apparatus .
  • the inven- tion is particularly concerned with correcting, or more gen- erally varying, a light irradiance distribution in a projec- tion light path in such objectives.
  • Microlithography is a technology for the fabrication of integrated circuits, liquid crystal displays and other micro- structured devices.
  • the process of microlithography in conjunction with the process of etching, is used to pattern fea- tures in thin film stacks that have been formed on a substrate, for example a silicon wafer.
  • the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) , vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light.
  • DUV deep ultraviolet
  • VUV vacuum ultraviolet
  • EUV extreme ultraviolet
  • the mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image correspond- ing to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.
  • a projection exposure apparatus typically includes an illumi- nation system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist.
  • the illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for ex- ample.
  • each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper.
  • each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference di- rection while synchronously scanning the substrate parallel or anti-parallel to this direction.
  • the ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification ⁇ of the projection lens.
  • mask (or reticle) is to be interpreted broadly as a patterning means.
  • Commonly used masks contain opaque, transparent or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example.
  • One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
  • the more devices can be produced on a single wafer the higher is the throughput of the production process.
  • the size of the structures that can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths.
  • the shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a wavelength of about 13 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6.9 nm.
  • An undesired irradiance variation in the image plane directly translates into CD variations, i.e. variations of the criti- cal dimensions. Irradiance variations in the pupil plane are more difficult to understand.
  • the amplitude part of the complex pupil transmission function describes the angular transmission properties of the objective, while the phase part of the pupil transmission function defines its aberrations.
  • imaging can be described by two Fourier transforms, namely one from the object plane to the pupil plane and one from the pupil plane to the image plane. Prior to the second Fourier transform, the complex pupil transmis- sion distribution must be multiplied by the optical transfer function (OTF) of the imaging system.
  • OTF optical transfer function
  • the OTF can be split into a phase term W describing the aberrations and an amplitude term A describing how the angular irradiance distribu- tion is affected by the objective. Both terms are generally functions of the pupil coordinates (i.e. of ray directions at field level) and of the field coordinates. This expresses that the amplitude of a light ray generally depends on the position where the light ray impinges in the field, and also on the direction of the light ray. Similar considerations apply also to the phase.
  • telecentricity denotes the mean direction of a light bundle emerging from or converging to a point in a field plane.
  • a non-telecentric objective overlay becomes a function of focus, because more light reaches a given point on the image plane from one side than from the other, with the result that if the wafer is moved up or down with respect to the image plane (thus defo- cusing the exposed image) , the image effectively moves horizontally.
  • pitch and orientation lines of different pitch require a different exposure dose to be printed at the same size.
  • Apodization is used for eliminating adverse effects that are associated with undesired variations of the irradiance distribution in the pupil plane.
  • the term apodization as used herein generally denotes a modification of the amplitude term A of the OTF by using a filter.
  • the term apodiza- tion is used in the art to denote an optical filtering of the transmittance in a pupil plane so as to suppress the energy of diffraction rings in an objective.
  • the apodization filter is used to correct the real irradiance distribution so that it approaches at least to some extent the ideal irradiance distribution.
  • no correction in this sense is required.
  • measures include, among others, modifications of the angular light distribution produced by the illumination system, or displacements of lenses contained in the objective or of the wafer.
  • the real irradiance distribution does not vary, it usually suffices to use an apodization filter having a fixed spatial filter function, i.e. an attenuation distribution that cannot be modified.
  • the real irradiance distribution often varies at least to some extent so that it is desirable to be able to vary the filter function of the apodization filter.
  • US 5,444,336 discloses a projection objective of a micro- lithographic projection exposure apparatus in which different grey filters can be inserted into the pupil plane of the objective.
  • the number of different filter functions is necessarily restricted.
  • US 2006/0092396 discloses a projection objective of a micro- lithographic projection exposure apparatus in which a vari- able apodization filter formed by an array of individually programmable elements, for example LCD cells, is arranged in a pupil plane of the objective. By controlling the elements of the array individually, the attenuation distribution of the apodization filter can be varied.
  • One drawback of this known approach is that it is difficult to finely adjust the attenuation produced by each element.
  • US 2010/0134891 Al discloses another variable apodization filter for an objective of a microlithographic projection exposure apparatus.
  • a reflective coating applied on a curved mirror surface is detuned so as to locally vary the coefficient of reflection of the mirror.
  • a similar approach is also described in US 7,791,711 B2. However, with this ap- proach it is difficult to ensure that the detuning can be completely reversed.
  • a drawback that is common to all prior art solutions is that the transmission filters attenuate the projection light by partial absorption. This creates heat in the transmission filters that may detrimentally affect its function. For example, the heat may result in a heat induced deformation of the transmission filter or of some of its components, or it may cause undesired refractive index changes inside the transmission filter. Heat created in the transmission filter may also adversely affect neighboring optical elements.
  • It is an object of the present invention to provide a micro- lithographic apparatus comprising a transmission filter that is configured to variably modify a light irradiance distribu- tion in a projection light path, and in particular in a pupil plane of an objective of the microlithographic apparatus.
  • the transmission filter shall be able to vary the transmittance in a fully reversible and locally resolved manner, and adverse effects associated with temperature changes shall be minimized.
  • a micro- lithographic apparatus and in particular a microlithographic projection exposure apparatus, comprising a light source that is configured to produce projection light having a center wavelength ⁇ , an illumination system that is configured to direct the projection light on a mask, and an objective that has an optical axis.
  • the objective is configured to produce an image of the mask in an image plane and comprises a transmission filter system that is configured to variably modify an irradiance distribution of projection light in a projection light path of the objective.
  • the transmission filter system comprises a transmission filter comprising a first diffractive optical element and a second diffractive optical element that is spaced apart, along the optical axis of the objective, from the first diffractive optical element by an axial distance ⁇ .
  • the transmission filter system further comprises a light absorbing element that is arranged outside the projection light path, and a drive that is configured to change the relative position between the first diffractive optical element and the second diffractive optical element between a neutral relative position, in which the combination of the first diffractive optical element and the second diffractive optical element diffracts a first fraction of projection light so that it impinges on the light absorbing ele- ment, and an active relative position, in which the combination of the first diffractive optical element and the second diffractive optical element diffracts a second fraction of projection light so that it impinges on the light absorbing element, wherein the second fraction is higher than the first fraction, which is preferably zero. At least in the active relative position the condition 0 ⁇ ⁇ 3 ⁇ holds.
  • the inventors have discovered that the diffractive effect produced by two diffractive optical elements can be finely adjusted by changing their relative position if the diffrac ⁇ tive optical elements are (at least in the active position) arranged in immediate proximity or even contact each other. If projection light, which is diffracted by the combination of the two diffractive optical elements, impinges on the light absorbing element, it will be absorbed and thus cannot contribute to the image formation in the image plane of the objective. Since the light absorbing element is arranged outside the projection light path, heat resulting from the absorption of projection light is produced outside the projec- tion light path. Therefore the heat cannot compromise the imaging quality of the objective.
  • the transmission filter of the apparatus is capable of varying its transmittance, but without producing heat in the projection light path that may adversely affect the image quality of the objective.
  • the light absorbing element may in fact be any structure hav ⁇ ing a surface that absorbs most of the projection light, preferably more than 90%. It is even possible to arrange the light absorbing element outside the objective, and to guide the diffracted projection light towards the light absorbing element with the help of mirrors. Then no heat caused by absorption is generated inside the objective.
  • the first and second diffractive optical element each comprises a plane plate that supports diffractive structures or in which the diffractive structures are integrated.
  • the plate may also be curved, or the diffractive structures may be applied on a curved surface of a lens or of a mirror.
  • the axial distance ⁇ is defined as the shortest distance be- tween the first and second diffractive optical element along the optical axis.
  • the first diffractive optical element comprises first diffractive structures and the second diffractive optical ele- ment comprises second diffractive structures.
  • the first diffractive structures In the neutral relative position the first diffractive structures may, in a projection along the optical axis, be arranged relative to the second diffractive structures in a staggered manner, whereas in the active relative position the first diffractive structures may be arranged relative to the second diffractive structures such that, in a projection along the optical axis, the first diffractive structures are arranged in line with the second diffractive structures. Then a minimum diffractive effect occurs in the neutral relative position, and a maximum diffractive effect may be observed in the active relative position.
  • the axial distance ⁇ is less than 3 ⁇ .
  • the axial distance may be greater than 3 ⁇ in the neutral position (because no diffractive effect is desired) , it may be preferred to keep the axial distance ⁇ in all relative positions smaller than 3 ⁇ .
  • the drive may be configured to vary the axial distance ⁇ between the first diffractive optical element and the second diffractive optical element. This variation of the axial distance may be used to switch between the active and the neutral relative position.
  • the axial distance ⁇ fixed and to have a drive that is configured to displace the first diffractive optical element in a plane that is at least substantially perpendicular to the optical axis. Then the change between the active and the neutral relative position is produced by displacing the first diffractive optical element in a direction perpendicular to the optical axis.
  • a second drive that is configured to displace the second diffractive optical element; what finally matters, however, is only the relative po- sition between the first and the second diffractive optical element and their correct arrangement in the projection light path .
  • the first diffractive optical element may have an optical surface facing the second diffractive optical element and supporting a first anti-reflection coating.
  • the second diffractive optical element may likewise have an optical surface facing the first diffractive optical element and supporting a second anti-reflection coating. With such anti-reflection coatings specular reflection at the optical interfaces at the gap can be reduced.
  • the light absorbing element - or if there is a plurality of light absorbing elements at least one of them - may be formed by an optical sensor that is configured to detect the first and second fraction of projection light that has been diffracted by the diffractive optical elements in the neutral and active relative position, respectively. Then it is possible to detect the fraction of projection light that is not transmitted by the transmission filter.
  • This is a significant advantage over prior art transmission filters in which it is not possible to directly measure the fraction of light that has been absorbed by the transmission filter.
  • any indirect measurement of the at- tenuating effect necessarily implies the measurement of transmitted projection light. In an online closed-loop control scheme that is performed during exposure, such a measurement inevitably results in light losses.
  • the transmission filter system comprises a control unit that is connected to the drive and to the optical sensor.
  • the control unit is configured to control the drive depending on signals received from the optical sensor. This makes it possible to control the relative position of the two diffractive optical elements in closed-loop control scheme. This may be particularly beneficial with a view to the fact that the diffractive effect very sensibly depends on minute changes of the relative position between the first and second diffractive optical element. A lateral displacement in the order of 100 nm typically separates the neutral from the active relative position, and therefore a simple feed-forward control may not be sufficient to accurately achieve the desired relative position.
  • the optical sensor may also be used to indirectly measure the irradiance distribution that shall be modified with the help of the transmission filter.
  • the control unit may then be configured to determine the irradiance distribution on the basis of the signals received from the optical sensor, to determine a deviation of the irradiance distribution from an ideal irradiance distribution, and to adjust the relative position of the first diffractive optical element and the second diffractive optical element depending on said deviation.
  • diffractive structures contained in the first diffractive optical element and the second diffractive optical element should have a maximum dimension perpendicular to the optical axis that is between 0.3 ⁇ and 3 ⁇ and preferably between 0.8 ⁇ and 1.2 ⁇ . If the diffractive structures have a higher or a lower refractive index than a surrounding medium, the diffractive elements will form as phase gratings. However, it is also possible to use transmission gratings in which the diffractive structures have a higher or lower coefficient of absorp- tion than a surrounding medium.
  • the second fraction i. e. the diffracted projection light in the active relative position
  • the second fraction may depend on the position where projection light impinges on the transmission filter.
  • the second fraction may depend only on the distance from the optical axis. This will result in a rotationally symmetric filter function. If, for example, the second fraction decreases with increasing distance from the optical axis, a filter function with a maximum attenuation at the center and a decreasing attenuation towards the circumference is obtained.
  • the distribution of height, width or fill factor of the diffractive structures may vary over surfaces of the first and the second diffractive optical element.
  • the transmission filter in a pupil plane of the objective for apodization purposes.
  • a position in an intermediate image plane for example, or an intermediate position between a field plane and a pupil plane, is also possible.
  • a method of varying an irradiance distribution of projection light in a projection light path of an objective comprising the following steps: a) arranging a first diffractive optical element and a second diffractive optical element in the projection light path; b) determining an ideal irradiance distribution of projection light in a plane of the objective; c) changing a relative position between the first diffractive optical element and the second diffractive optical element such that the combination of the first diffractive optical element and the second diffractive optical element diffracts a fraction of projection light on a light absorbing element, which is arranged outside the projection light path, thereby modifying the irradiance distribution in the plane such that it approaches the ideal irradiance distribution.
  • the axial distance between the first diffractive optical element and the second diffractive optical element should be less than 3 ⁇ .
  • the method comprises the additional steps of i) measuring an irradiance distribution in the pupil plane; ii) determining a deviation of the irradiance distribution measured in step i) from an ideal irradiance distribution; iii) adjusting the relative position of the first diffractive optical element and the second diffractive optical ele ⁇ ment depending on the deviation determined in step ii) .
  • the irradiance distribution in the pupil plane may be meas ⁇ ured indirectly in step i) by an optical sensor that detects projection light that has been diffracted by the first diffractive optical element and the second diffractive optical element .
  • Step b) may comprise the step of measuring an aberration and/or an intensity distribution in an image plane of an ob- jective, and computing the ideal irradiance distribution on the basis of the measured aberration and/or the measured intensity distribution.
  • an optical wavefront sensor for example, may be arranged in the image plane of the objective.
  • the ideal irradiance distribution may then be computed on the basis of simulations that may also take into account the specific structures contained in the mask to be projected.
  • the concept of being able of varying the diffractive effect only by displacing two diffractive optical elements relative to each other may also be used in a broader context, i. e. outside the application as a transmission filter.
  • Subject of the invention is therefore also a microlitho- graphic apparatus comprising a light source that is configured to produce projection light having a center wavelength ⁇ and an optical system having an optical axis and comprising a first diffractive optical element and a second diffractive optical element that are spaced apart from each other along the optical axis by an axial distance ⁇ .
  • the optical system further comprises a drive that is configured to change the relative position between the first diffractive optical ele- ment and the second diffractive optical element between a neutral relative position, in which the combination of the first diffractive optical element and the second diffractive optical element diffracts a first fraction of projection light, and an active relative position, in which the combina- tion of the first diffractive optical element and the second diffractive optical element diffracts a second fraction of projection light being higher than the first fraction. At least in the active relative position is 0 ⁇ ⁇ 3 ⁇ .
  • light is used herein to denote any electromagnetic radiation, in particular visible light, UV, DUV and VUV light.
  • light ray is used herein to denote light whose path of propagation can be described by a line.
  • light beam is used herein to denote a plurality of light rays.
  • a light beam usually has an irradiance profile across its diameter that may vary along the propagation path.
  • a single light beam can usually be associated with a single point or extended light source.
  • surface is used herein to denote any planar or curved surface in the three-dimensional space. The surface may be part of a body or may be completely separated therefrom.
  • optically conjugate is used herein to denote an imaging relationship between two points or two surfaces. Im- aging relationship means that a light bundle emerging from a point converges at the optically conjugate point.
  • field plane is used herein to denote a plane that is optically conjugate to the mask plane.
  • projection plane is used herein to denote a plane in which all light rays, which converge or diverge under the same angle in a field plane, pass through the same point.
  • the term “pupil plane” is also used if it is in fact not a plane in the mathematical sense, but is slightly curved so that, in a strict sense, it should be referred to as pupil surface.
  • projection light path is used herein to denote the entire space which may be exposed to projection light under any reasonable operating conditions defined by illumination setting and mask.
  • the projection light path is therefore primarily defined by optical parameters of the illumination system and the objective, for example the numerical aperture and the size of object and image field.
  • FIG. 1 is a schematic perspective view of a projection exposure apparatus in accordance with the present invention
  • FIG. 2 is a schematic meridional section through the apparatus shown in FIG. 1;
  • FIG. 3 is a meridional section through two diffractive optical elements in a neutral relative position; is a meridional section through the two diffractive optical elements in an active relative position; is meridional section through the two diffractive optical elements in an intermediate relative position; is an enlarged cross section through the two diffractive optical elements in the neutral relative position; is a graph illustrating the dependency of the specular transmittance on the angle of incidence for the neutral relative position shown in FIG. 6; is an enlarged cross section through the two diffractive optical elements in the active relative position; is a graph illustrating the dependency of the specular transmittance on the angle of incidence for the active relative position shown in FIG.
  • the dependency of the specular transmittance on the lateral displacement ⁇ for intermediate relative positions is a graph illustrating the dependency of the specular transmission on the lateral displacement ⁇ and the axial distance ⁇ ; is an enlarged cross section through two diffrac- tive optical elements supporting anti-reflection coatings in an intermediate relative position; is a graph illustrating, for the configuration shown in FIG. 15, the dependency of the specular transmittance on the lateral displacement ⁇ for intermediate relative positions; is a meridional section through an embodiment in which the height of the diffractive structures varies depending on the distance from the optical axis ; illustrates the filter function of the transmission filter shown in FIG.
  • 17 shows an alternative embodiment in which the fill factor of the diffractive structures varies depending on the distance from the optical axis; to 22 show meridional sections through another embodiment of a transmission filter in which the axial distance between two diffractive optical elements is varied between a neutral, an active and an intermediate relative position, respectively;
  • FIG. 23 is a flow diagram illustrating important method
  • FIG. 1 is a perspective and simplified view of a microlitho- graphic projection exposure apparatus 10 in accordance with the present invention.
  • the apparatus 10 comprises an illumination system 12 containing a light source LS which produces projection light having a central wavelength of 193 nm.
  • the projection light illuminates a field 14 on a mask 16 compris- ing a pattern 18 of fine features 19.
  • the illuminated field 14 has a rectangular shape.
  • other shapes of the illuminated field 14, for example ring segments, and also other central wavelengths, for example 157 nm or 248 nm, are contemplated as well.
  • a projection objective 20 having an optical axis OA and containing a plurality of lenses LI to L4 images the pattern 18 within the illuminated field 14 on a light sensitive layer 22, for example a photoresist, which is supported by a substrate 24.
  • the substrate 24, which may be formed by a silicon wafer, is arranged on a wafer stage (not shown in FIG. 1) such that a top surface of the light sensitive layer 22 is precisely located in an image plane of the projection objective 20.
  • the mask 16 is positioned by means of a mask stage (not shown in FIG. 1) in an object plane of the projection objective 20. Since the latter has a magnification ⁇ with
  • the mask 16 and the substrate 24 move along a scan direction which corresponds to the Y direction indicated in FIG. 1.
  • the illuminated field 14 then scans over the mask 16 so that patterned areas larger than the illuminated field 14 can be continuously imaged.
  • the ratio between the velocities of the substrate 24 and the mask 16 is equal to the magnification ⁇ of the projection objective 20. If the projection objective 20 does not invert the image ( ⁇ >0), the mask 16 and the substrate 24 move along the same direction, as this is indicated in FIG. 1 by arrows Al and A2.
  • the present invention may also be used with catadioptric projection objectives 20 having off-axis object and image fields, and with apparatus of the stepper type in which the mask 16 and the substrate 24 do not move during the projec- tion.
  • FIG. 2 is a schematic meridional section through the apparatus 10 shown in FIG. 1.
  • the mask stage denoted by 26 which supports and moves the mask 16 in the object plane 28 of the projection objective 20
  • the wafer stage denoted by 32 which supports and moves the substrate 24 in the image plane 30 of the projection objective 20, are schematically illustrated.
  • the projection objective 20 has an intermediate image plane 34.
  • a first pupil plane 36 is located be- tween the object plane 28 and the intermediate image plane
  • a second pupil plane 38 is located between the intermediate image plane 34 and the image plane 30 of the projection objective 20.
  • the projection objective 20 further includes a transmission filter system TFS comprising a variable transmission filter
  • the transmission filter 42 that is arranged in the first pupil plane 36 and comprises a first diffractive optical element 44 and a second diffrac- tive optical element 46.
  • the transmission filter 42 corrects, or more generally variably modifies, the light irradiance distribution in the first pupil plane 36. Due to its arrangement in one of the pupil planes 36, 38 of the projection objective 20, the transmission filter 42 may be used as an apo- dization filter that absorbs portions of projection light that would, if allowed to reach the image plane 30, deteriorate the quality of the image of the mask 16.
  • the transmission filter system TFS further comprises a drive 48 that is configured to displace the first diffractive opti- cal element 44 relative to the second diffractive optical element 46 parallel to the first pupil plane 36.
  • the transmission filter system TFS includes a control unit 50, which controls the drive 48 and is connected to an overall system control 52 of the apparatus 10, and a plurality of light ab- sorbing elements 54. These are arranged circumferentially around the projection light path and coated with a light absorbing layer 56. In this embodiment one or more light absorbing elements are formed by an optical sensor 58 configured to detect the irradiance of incident projection light and connected to the control unit 50.
  • FIG. 3 is a schematic meridional section through the first and second diffractive optical elements 44, 46.
  • the pitch p of the diffractive structures 60 is 400 nm.
  • the diffractive structures 60 of the first diffractive optical element 44 are arranged immediately adjacent the interstices that are formed between the diffractive structures 60 of the second diffractive optical element 46, and vice versa. Therefore, in a projection along the optical axis OA, the diffractive structures 60 of the first diffractive optical element 44 are arranged in a staggered manner with respect to the diffractive structures 60 of the second diffractive optical element 46.
  • the axial distance ⁇ i.e. the distance along the optical axis OA, between the first and second diffractive optical element 44, 46 is zero in this embodiment.
  • the first diffractive optical element 44 is in direct contact with the second diffractive optical element 46.
  • any residual transmission loss is mainly due to a non-zero coefficient of absorption of the materials, from which the diffractive optical elements 44, 46 are made, and Fresnel reflection losses at the optical interfaces. For that reason the relative position of the two diffractive optical elements 44, 46 as shown in FIG. 3 will be referred to in the following as neutral position.
  • the diffractive structures 60 of the two dif- fractive optical elements 44, 46 are arranged in line, i.e. one behind the other.
  • Such combined structures have significant diffractive effect on the projection light which has a similar wavelength as the width w of the diffractive structures 60. It can be shown that under these conditions a substantial fraction of the projection light is diffracted, as this is indicated by arrows PL, PL' and DL in FIG. 4.
  • DL denotes the fraction of diffracted light that is directed by the transmission filter 42 towards the absorbing elements 54 (see dotted lines in FIG. 2) and thus cannot contribute to the image formation on the light sensitive surface 22.
  • the diffractive effect of the transmission filter 42 simply by varying the lateral dis- placement ⁇ of the diffractive optical elements 44, 46 with the help of the drive 48.
  • the diffracted light DL In the active position or in the intermediate positions the diffracted light DL is not absorbed inside the transmission filter 42, but on the absorb- ing elements 54. Heat produced by absorption of the diffracted light DL cannot compromise the imaging quality of the projection objective 20, because the absorbing elements 54 are arranged outside the projection light path.
  • FIG. 6 shows a schematic cutout from the transmission filter 42 with the diffractive optical elements 44, 46 in a neutral relative position.
  • the diffractive optical elements 44, 46 are therefore phase gratings that produce diffractive effects not by affecting the intensity, but the phase of incident light.
  • phase gratings may be produced, for example, by locally exposing the surrounding material (in particular a S1O 2 glass) to a laser beam, an electron beam, an ion beam or another high-energy beam. Such beams result in a local compaction of the material that is associated with a rise of the refractive index. Additionally or alternatively, the material may be locally doped with certain atoms, as this is known in the art as such. It is also possible to locally apply coatings that affect the phase of the projection light, or to produce grooves or other grating structures by locally remov- ing material. Due to the minute size of the diffractive structures 60, electron lithography apparatus may be used to this end.
  • the surrounding material in particular a S1O 2 glass
  • FIG. 7 shows a graph which illustrates the dependency of the specular transmittance T s on the angle of incidence.
  • the specular transmittance T s is defined as the fraction of the projection light that passes through the transmission filter 42 without being diffracted.
  • the total transmittance T T is the sum of the specular transmittance T s and the diffuse transmittance T D being defined as the fraction of the projection light that is diffracted by the transmission filter 42.
  • FIG. 9 shows a graph which again illustrates the dependency of the specular transmittance T s on the angle of incidence. It can be seen in FIG. 9 that in the active position the specular transmittance T s is about 92% and depends only slightly on the angle of incidence a. Thus it is possible to reduce the irradiance of projection light PL by about 7% merely by changing the lateral displacement ⁇ .
  • FIG. 10 shows a schematic cutout from the transmission filter 42 in a state in which the diffractive optical elements 44, 46 are in an intermediate relative position.
  • the fraction of projection light diffracted towards the absorbing elements 54 is smaller as compared to the active position, but still sig- nificant.
  • the dependency of the specular transmittance T s on the angle of incidence is small (not shown) .
  • FIG. 11 shows a graph illustrating the dependency of the specular transmittance T s on the lateral displacement ⁇ for an angle of incidence of 0°. It can be seen in FIG. 11 that the specular transmittance T s smoothly increases from 92% for the active relative position (see FIG. 9) to about 99% for the inactive relative position (see FIG. 7) . IV.
  • the axial distance ⁇ between the diffractive optical elements 44, 46 is zero, i. e. the diffractive optical elements 44, 46 are in direct contact.
  • the diffractive optical elements 44, 46 are spaced apart along the optical axis OA.
  • FIG. 12 shows a schematic cutout from the transmission filter 42 in a state in which the diffractive optical elements 44, 46 are in an intermediate relative position.
  • the gap 64 between the diffractive optical elements 44, 46 is filled with air or any other gas having a refractive index of 1.
  • FIG. 13 shows a graph which illustrates the dependency of the specular transmittance T s on the lateral displacement ⁇ for an angle of incidence a of 0° (thin line) and 5° (dotted line) .
  • the dependency of the specular trans- raittance T s on the angle of incidence is somewhat stronger.
  • FIG. 14 shows a graph in which the dependency of the specular transmittance T s on the lateral displacement ⁇ and also on the axial distance ⁇ is illustrated.
  • Different grey tones represent different specular transmittances T s .
  • the dependency of the specular transmittance T s decreases with increasing axial distance ⁇ . This is also what one would expect because at larger axial distances ⁇ the two diffractive optical elements 44, 46 will increas- ingly be "seen” by the projection light as two distinct components that do not interact. Therefore the axial distance ⁇ should be as small as possible if a strong dependency on the axial displacement ⁇ (i. e. a high sensitivity of the transmission filter 42 on displacements) is desired, as this will usually be the case.
  • the dependency of the specular transmittance T s does not continuously decrease with increasing axial distance ⁇ , as one would expect.
  • FIG. 15 is a graph similar to FIG. 12 that illustrates another embodiment of a transmission filter 42. Again, there is an air gap 64 ⁇ between the two diffractive optical elements 44, 46.
  • the surface of the first diffractive optical element 44 that faces the second diffractive optical element 46 sup- ports a first anti-reflection coating 66 consisting of a plurality of thin layers 68 having alternating indices of refraction, and the surface of the second diffractive optical element 46 facing the first diffractive optical element 44 supports a second anti-reflection coating 70 that also consists of a plurality of thin layers 72 having alternating indices of refraction.
  • FIG. 16 shows a graph which illustrates, similar to FIG.
  • the provision of the anti- reflection coatings 66, 70 helps to suppress light losses due to Fresnel reflection at the optical surfaces on both sides of the air gap 64. Surprisingly this is only partially true. It can be shown that at least for certain anti-reflection coatings 66, 70 the specular reflectance is quite significant (about 1.5%) for a zero axial distance ⁇ , but drops off to very small values (about 0.07%) at certain axial distances ⁇ ⁇ , namely at about 36 nm and 125 nm. Generally it seems that the periodic dependency of the specular transmission T s on the axial distance ⁇ ⁇ , as it is observed for diffractive optical elements 44, 46 without an anti-reflection coating (see FIG. 14), is not present at all or at least less strong if an anti-reflection coating 66, 70 is applied to the diffractive optical elements 44, 46. VI.
  • the distribution of the diffractive structures 60 over the surfaces of the dif- fractive optical elements 44, 46 is uniform. It has been shown that with such uniform distribution it is possible to variably attenuate projection light passing through the transmission filter 42 in such a manner that the attenuation can be varied by displacing the diffractive optical elements 44, 46 in a direction that is perpendicular to the optical axis OA of the projection objective 20.
  • FIG. 17 is a schematic cross section through two diffractive optical elements 44, 46.
  • the height of the diffractive structures 66 varies over the surfaces of the diffractive optical elements 44, 46. More particularly, the height of the diffractive structures 60 has its maximum value at the center of the diffractive optical elements 44, 46, and if one moves radially towards the circumference of the diffractive optical elements 44, 46, it decreases continuously.
  • arrows PL' and DL denoting transmitted projection light and diffracted projection light, respectively.
  • FIG. 18 illustrates the two dimensional filter function that is obtained with the transmission filter 42 shown in FIG. 17. Darker areas represent a lower transmittance and thus a higher percentage of diffracted light. The same filter function is achieved with the transmission filter 42 which is shown in FIG. 19 in a schematic cross section. In this embodiment it is not the height of the diffractive structures 60, but the fill factor which varies radially across the surface of the diffractive optical elements 44, 46. Each group 72 of refractive structures 60 has the same effect, but the density of the groups 72 decreases radially, and therefore, on a macroscopic scale, the transmittance increases radially towards the circumference. It should be noted that FIG. 19 is not to scale and serves only to illus- trate the principle of varying the fill factor.
  • FIG. 20 illustrates an embodiment in which two diffractive optical elements 44, 46 are in a neutral position.
  • the axial distance ⁇ ⁇ is so large that the combination of the diffractive optical elements 44, 46 does not produce a significant diffractive effect. Consequently almost all projection light PL incident on the transmission filter 42 emerges from the second diffractive optical element 46 as transmitted projection light PL'.
  • the transmission filter 42 may be used for apodization purposes.
  • a measuring device 120 is arranged in the image plane 30 within the image field, as this is indicated in FIG. 2 by an arrow 122. Suitable measuring devices 120 are known in the art as such and typically comprise a CCD sensor and Fourier optics that translate angles in the image plane 30 into positions on the CCD sensor.
  • a mixed approach which uses measurements as well as simulations, may be used to quickly and accurately determine the angular light distribution in the image plane 30.
  • the angular light distribution determined by simulation and/or measurement is compared to an ideal angular light distribution.
  • This ideal angular light distribution is usually computed on the basis of the pattern 18 to be transferred on the light sensitive layer 22, the imaging proper- ties of the objective 20 and the angular light distribution of the projection light when it impinges on the mask 16.
  • the ideal angular light distribution is computed such that an optimum imaging of the pattern 18 on the light sensitive surface 22 is achieved. Optimization algorithms may be used to this end.
  • an algorithm computes a target transmittance distribu- tion of the transmission filter 42 that is required to transform the measured/simulated angular light distribution into the ideal angular light distribution.
  • This algorithm exploits that fact that the angular light distribution in the image plane 30 of the objective 20 corresponds to an irradiance distribution in the pupil plane 36 in which the transmission filter 42 is arranged.
  • the computational unit 50 of the transmission filter system TFS controls the drive 48 so as to arrange the dif- fractive optical elements 42, 44 relatively to each other such that they commonly produce the computed target transmit- tance distribution.
  • FIG. 23 is a flow diagram which summarizes important aspects of a method of varying a light irradiance distribution in a projection light path in the objective 20.
  • a first and a second diffractive optical ele- ment are arranged in a projection light path of the objective 20.
  • a second step S2 an ideal irradiance distribution of projection light in a plane of the objective 20 is determined.
  • a third step S3 the relative position between the first and the second diffractive optical element is changed so that the ideal irradiance distribution is approached.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Toxicology (AREA)
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  • Exposure And Positioning Against Photoresist Photosensitive Materials (AREA)
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Abstract

La présente invention concerne un appareil microlithographique comprenant une source de lumière produisant une lumière de projection présentant une longueur d'onde centrale λ et un objectif de projection présentant un axe optique et un système filtre à transmission. Ledit système filtre à transmission comprend un premier et un second élément optique à diffraction (44, 46). Un entraînement (48) est conçu de façon à modifier la position relative entre le premier et le second élément optique à diffraction entre une position active, dans laquelle l'association des deux éléments optiques à diffraction diffracte davantage de lumière de projection sur un élément d'absorption de la lumière que dans une position neutre. La distance axiale Δz entre les deux éléments optiques à diffraction (44, 46) remplit la condition 0 ≤ Δz ≤ 3λ.
PCT/EP2013/002114 2013-07-17 2013-07-17 Appareil microlithographique et procédé de variation d'une répartition du rayonnement lumineux WO2015007298A1 (fr)

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WO2018210691A1 (fr) * 2017-05-17 2018-11-22 Carl Zeiss Smt Gmbh Procédé d'exposition par projection et lentille de projection avec réglage de la transmission à la pupille
WO2019156203A1 (fr) * 2018-02-09 2019-08-15 ウシオ電機株式会社 Dispositif d'affichage d'image en couleurs, procédé de création de copie d'image en couleurs l'utilisant, et copie d'image en couleurs ainsi créée

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TWI596446B (zh) * 2016-04-08 2017-08-21 Luminous uniformity correction equipment and its correction method
EP3364247A1 (fr) * 2017-02-17 2018-08-22 ASML Netherlands B.V. Procédés et appareil de surveillance d'un processus de fabrication lithographique

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WO2018210691A1 (fr) * 2017-05-17 2018-11-22 Carl Zeiss Smt Gmbh Procédé d'exposition par projection et lentille de projection avec réglage de la transmission à la pupille
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JPWO2019156203A1 (ja) * 2018-02-09 2020-02-27 ウシオ電機株式会社 カラー画像表示装置、およびそれを用いたカラー画像複製物作成方法、その方法により作成されたカラー画像複製物

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