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
  • G03F7/70058—Mask illumination systems
• • 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
  • G03F7/20—Exposure; Apparatus therefor
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F21—LIGHTING
  • F21V—FUNCTIONAL FEATURES OR DETAILS OF LIGHTING DEVICES OR SYSTEMS THEREOF; STRUCTURAL COMBINATIONS OF LIGHTING DEVICES WITH OTHER ARTICLES, NOT OTHERWISE PROVIDED FOR
  • F21V33/00—Structural combinations of lighting devices with other articles, not otherwise provided for
• • 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]
  • G03F7/70116—Off-axis setting using a programmable means, e.g. liquid crystal display [LCD], digital micromirror device [DMD] or pupil facets
• • 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
  • G03F7/70141—Illumination system adjustment, e.g. adjustments during exposure or alignment during assembly of illumination 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
  • G03F7/70258—Projection system adjustments, e.g. adjustments during exposure or alignment during assembly of projection system
  • G03F7/70266—Adaptive optics, e.g. deformable optical elements for wavefront control, e.g. for aberration adjustment or correction
• • 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
  • G03F7/70283—Mask effects on the imaging process
  • G03F7/70291—Addressable masks, e.g. spatial light modulators [SLMs], digital micro-mirror devices [DMDs] or liquid crystal display [LCD] patterning devices
• • 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
  • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
  • G03F7/70858—Environment aspects, e.g. pressure of beam-path gas, temperature
  • G03F7/70883—Environment aspects, e.g. pressure of beam-path gas, temperature of optical system
  • G03F7/70891—Temperature
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34

Definitions

• the invention generally relates to illumination systems for illuminating a mask in microlithographic exposure apparatus, and in particular to systems comprising an array of mirrors.
• a projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photore- sist.
• the illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example .
• the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution.
• the term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle .
• the angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be pro- jected onto the photoresist. For example, relatively large sized features may require a different angular distribution than- small sized features.
• the most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a system pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the system pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
• the illumination system usually comprises a mirror array (sometimes also referred to as faceted mirror) which directs the projection light produced by the EUV light source towards the system pupil sur- face so that a desired intensity distribution is obtained in the system pupil surface.
• WO 2005/026843 A2 proposes for a DUV illumination system to use a mirror array that illuminates the pupil surface.
• each of the mirrors can be tilted about two perpendicular tilt axes.
• a condenser lens arranged between the mirror array and the pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface.
• This known illumination sys- tern makes it possible to produce on the pupil surface a plurality of light spots, wherein each light spot is associated with one particular microscopic mirror and is freely movable across the pupil surface by tilting this mirror. It is also proposed to vary the size of the spots by using adaptive mir- rors having a mirror surface whose shape can be varied to a limited extent using suitable actuators, for example piezoelectric actuators.
• EP 0 532 236 Al discloses another correction device for a EUV projection objective of a microlithographic exposure apparatus.
• infrared radiation is directed on one of the large mirrors of the objective.
• the infrared light is controlled such that the shape of the mirror does not sub- stantially alter even under the impact of the high energy EUV projection light.
• heating or cooling devices are integrated into the mirror support for the same purpose .
• the mirror array comprising adaptive mirrors as disclosed in the aforementioned WO 2005/026843 A2 is particularly advantageous because additional reflective power may be added to correct for non-ideal optical properties of a subsequent condenser, or for aberrations caused by material defects and manufacturing tolerances.
• the use of piezoelectric actuators proposed in this document has some significant drawbacks. In order to achieve a desired curvature of the mirror surface, it is necessary to provide a large number of such actuators which adds to the system complexity. For example, a very large number of electrical leads have to be pro- vided for individually controlling the piezoelectric actuators. In a mirror array comprising several thousand mirror elements on a total area of less than 100 cm 2 the electrical wire density becomes critical. Apart from that it is difficult to obtain the desired surface shape of the mirror elements under varying temperature conditions.
• the invention thus exploits the effect that plates comprising materials having different coefficients of thermal expansion bend when the temperature changes, similar to bimetallic strips used for temperature controllers.
• the invention is furthermore based on the consideration that it is meanwhile possible to compute very accurately not only the temperature profile of mirror elements when heated or cooled at certain target areas, but also to predict the deformations occurring as a result of this temperature profile. In the context of the present invention this prediction has to take into ac- count bending forces produced by the different coefficients of thermal expansion. However, bending forces produced by a non-homogeneous temperature profiles in the mirror elements may be taken into account, too.
• the temperature induced mirror adaptation according to the present invention does not substantially add to the system com- plexity.
• the mirror elements comprise structures having a different coefficient of thermal expan- sion and being fixedly attached to one another.
• the structures are planar or curved structures having a pair of parallel surfaces.
• mirror elements typically comprise a mirror support and a reflective coating applied thereon, wherein both struc- tures have different coefficients of thermal expansion. Since the effect of bending as a result of temperature changes becomes larger the greater the difference between the coeffi- cients of thermal expansion is, the difference between the coefficients of thermal expansion should be substantial if a high sensitivity of the mirror elements to temperature changes is desired.
• Metals are a material class in which a wide variety of large coefficients of thermal expansion is available, and therefore the structures are made of metals in some embodiments.
• the mirror elements may be provided with an array of Peltier elements which are controlled such that only those Peltier elements that are arranged on a certain area, which may have the contour of a stripe or an ellipse, for example, are operated and cool the adjacent portions of the mirror elements .
• Target areas that can be varied individually for each mirror element may be more easily provided for if the radiation system comprises a secondary light source producing a radiation beam and a spatial light modulator that is configured to move the radiation beam over the target areas. In this case the target areas are "written" by a moving radiation beam. If desired, any arbitrary target area may be heated on any of the mirror elements by suitably controlling the spatial light modulator. Such a radiation system is particularly suitable in embodiments in which the target areas are line patterns. It may be necessary to provide more than one secondary light source and spatial light modulator in order to ensure that each mirror element is heated up with a sufficiently high re- fresh rate.
• Producing the astigmatic shape of the mirror elements with the help of the temperature control system is advantageous because it is difficult and costly to produce a large number of astigmatic mirror elements for the array. If a temperature control system is provided anyway for additionally varying the spot shapes in the system pupil surface, it is simpler and cheaper to use spherical or at least rotationally symmetrical mirrors and use the temperature control system also for producing the astigmatic shape required for allowing large deviation angles. However, it is to be understood that the mirror elements could have the required astigmatic effect also initially, i.e. not as a result of a deformation achieved with the help of the temperature control system.
• the illumination system comprises a mirror array which is arranged between a light source and a system pupil surface.
• the array comprises a plurality of mirror elements, wherein each mirror element is tiltably mounted with respect to a support structure and is configured to direct light produced by the primary light source towards the system pupil surface.
• the mirror elements have concave astigmatic mirror surfaces, i.e. the mirror elements have different focal lengths in two orthogonal planes. It is not mandatory to provide also a temperature control device.
• the mirror elements comprise heat barriers which have a lower coefficient of thermal conduction than the materials which are arranged on either side of the heat bar- riers.
• Such heat barriers ensure that the heat or the cold produced at the target areas and the adjacent material remains confined to this portion of the mirror elements over a longer period of time.
• the temperature difference between the target areas and the adjacent material on the one hand and the surrounding material on the other hand does not decrease too quickly. This makes it possible to reduce the refresh rate at which the target areas have to be heated or cooled by the temperature control device in order to ensure stable optical properties of the mirror elements.
• FIGS. 6a to 6d are different target areas subjected to heating radiation on a mirror element
• FIG. 7 is a top view on a mirror array illustrating an op- eration in which different target areas are associated with different groups of mirror elements
• FIG. 9 is a bottom view on a mirror support on which electrically conductive resistance wires used as heating members are applied;
• FIG. 10 is a perspective view of a mirror element according to an embodiment in which the mirror elements com- prise heat barriers;
• FIG. 12 is a top view of the mirror array shown in FIG. 11;
• FIG. 13 is an enlarged perspective view of a mirror element of the mirror array shown in FIG. 12;
• FIG. 14 is a perspective view of a mirror element according to an embodiment in which Peltier elements are used as cooling members.
• FIG. 1 is a perspective and highly simplified view of a projection exposure apparatus 10 that comprises an illumination system 12 for producing a projection light beam.
• the projec- tion light beam illuminates a field 14 on a mask 16 containing minute structures 18.
• the illuminated field 14 has approximately the shape of a ring segment.
• other, for example rectangular, shapes of the illuminated field 14 are contemplated as well.
• the illumination system 12 includes a housing 28 and a light source that is, in the embodiment shown, realized as an exci- mer laser 30.
• the excimer laser 30 emits projection light that has a wavelength of about 193 nm.
• Reference numeral 70 denotes a system pupil surface of the illumination system 12 that substantially defines the angular distribution of the light impinging on the mask 14.
• the system pupil surface 70 is usually plane or slightly curved and is arranged in or in immediate vicinity to the optical integrator 72.
• the optical integrator 72 substantially determines the basic geometry of the illuminated field 14 on the mask 16. Since the optical integrator 72 increases the range of angles considerably more in the X direction than in the scan direction Y, the illuminated field 14 has larger dimensions along the X direction than along the scan direction Y.
• a field stop objective 84 provides optical conjugation between the intermediate field plane 80 and the mask plane 86 in which the mask 16 is arranged.
• the field stop 82 is thus sharply imaged by the field stop objective 84 onto the mask 16.
• the field stop 82 and the field stop objective 84 may be dispensed with in other embodiments.
• the intensity distribution in the system pupil plane 70 translates into an angular distribution in the subsequent intermediate field plane 80. If the field stop objective 84 has a magnification of 1, the angular distribution and the intermediate field plane 80 appears again (in the absence of aberrations introduced by the field stop objective 84) in the mask plane 86; if the absolute value of the magnification is distinct from 1, the angular distribution is scaled up or down. Thus it is possible to vary the angular distribution in the mask plane 86 by modifying the intensity distribution in the system pupil plane 70 with the help of the mirror array 46.
• first condenser 58 is designed to produce not collimated, but diverging or converging light bundles.
• Non-ideal optical properties of the first condenser 58 may again result in a degraded intensity distribution in the system pupil surface 70, and thus a poorer angular distribution in the mask plane 86.
• the mirror elements M 13 of the mirror array 46 are adaptive.
• the shape of the mirror elements can be changed so as to alter their reflective power. By increasing or decreasing the reflective power, it is possible to vary the divergence of the reflected beams. If a mirror element M 1 -, directs a beam towards the centre of the first condenser 58, the necessary reflective power may have to be different if compared to a situation in which the mirror element M 1 -, directs a beam towards the circumference of the first condenser 58.
• a change of the shape of the mirror elements may also be advantageous if other undesired, including time variant, optical effects cause a degradation of the angular distribution in the mask plane 86.
• changes in the refractive power of the micro-lenses 40 may occur as a result of the heating caused by the absorption of projection light. Changes of the shape of the mirror elements M 1 -, may also be helpful in order to correct aberrations introduced by manufacturing tolerances .
• the illumination system 12 comprises, accommodated within its housing 28, a temperature control device 90 including, according to a first embodiment, an infrared laser source 92, which produces a laser beam 94.
• the temperature control device 90 further includes a spatial light modulator 96 which is configured to move the laser beam 94 over target areas on the mirror elements M 13 .
• the spatial light modulator 96 comprises a tiltable mirror 98 which can be tilted about two orthogonal axes with the help of suitable actuators.
• Such a tiltable mirror 98 may also be realized us- ing MEMS technology, as it is described in WO 2005/078506 A2 filed by Microvision.
• the spatial light modulator 96 comprises combinations of rotatable prisms or electro-optical elements that make it possible to change the direction of a light beam passing through the elements by varying a voltage applied to the elements.
• the spatial light modulator 96 is controlled in such a way that the laser beam 94 moves continuously or intermittently over target areas on the mirror elements M 13 .
• the wavelength of the laser light is selected such that it is almost completely, or at least by more than 80%, absorbed by the mirror elements. The absorbed laser light results in an increase of the temperature within the target areas to which the laser light has been directed by the spatial light modulator 96.
• FIG. 3 is an enlarged and perspective view of a single mirror element M 13 .
• the mirror element M 13 comprises a mirror support 100 and a reflective coating 102 which is applied on top of the mirror support 100.
• the reflective coating 102 may be formed by a plurality of thin layers having alternating refractive indices, for example.
• the reflective coating is specifically adapted to the wavelength of the projection light so as to ensure maximum reflection for this wavelength. For infrared radiation, as it is produced by the laser source 92, such reflective coatings are usually almost completely absorptive.
• the mirror support 100 may itself be formed by two or more individual parallel structures that are fixedly attached to one another, as is indicated with dashed lines in FIG. 3.
• Reference numeral 104 denotes a projection light area on the reflective coating 102 on which projection light impinges during operation of the illumination system 12. Adjacent along the X and the Y direction target areas 106a, 106b, 106c and 106d are indicated on which the laser beam 94 is di- rected. In other embodiments the projection light area 104 is substantially larger than shown in FIG. 3, and the target ar- eas 106a to 106d may partially or completely overlap the projection light area 104. Absorption of the infrared radiation in the target areas 106a to 106d results in an increase of the temperature in the vicinity to the target areas 106a to 106d.
• FIG. 4a the broken lines 110a and 110c indicate the temperature profiles produced solely by the infrared radiation impinging on the opposing target areas 106a and 106c, respectively.
• FIG. 4b the broken lines 110b and 11Od indicate the temperature profiles produced solely by the infrared radiation impinging on the opposing target areas 106b and 106d, respectively.
• the total temperature profiles obtained by adding the aforementioned profiles are indicated with dotted lines.
• the reflective coating 102 and the mirror support 100 have different coefficients of thermal expansion.
• a similar effect is achieved as with two bimetallic strips arranged in a crossing arrangement. Consequently, the mirror element Mj 0 starts bending with respect to two orthogonal bending axes extending along the X and Y direction when the temperature profiles shown in FIGS. 4a and 4b build up.
• FIG. 5 is a perspective view of the mirror array 46.
• a mirror element M 35 is shown in a position in which it has been tilted about two orthogonal tilt axes 56x and 56y.
• the laser beam 94 is directed onto a different mirror element M 65 .
• Dotted lines 94 ' indicate the laser beam 94 at a later point in time after it has been directed across the reflective coating 102 of the mirror element M 65 along a substantially straight line.
• various other deformations may be produced by heating target areas on the mirror elements Mi j . If the coefficient of thermal expansion of the reflective coating is not smaller but greater than the coefficient of ther- mal expansion of the mirror support 100, a temperature rise will result in a convex bending of the mirror element Mi j . It is also possible to manufacture the mirror element such that it has at room temperature a substantial curvature, and this curvature is reduced by increasing the temperature with the help of the temperature control device 90.
• FIGS. 6a to 6d show different target areas that may be produced on the mirror elements M 1 -,:
• FIG. 6a illustrates a target area 106-1 which has the geome- try of an elongated stripe symmetrically separating the square mirror surface in two equal halves. If the temperature is increased in the vicinity of this target area 106-1, the mirror element M 1 -, will bend only about a direction which is orthogonal to the longitudinal direction of the stripe. Such a deformation may be useful to correct astigmatic effects in the illumination system 12.
• the target area 106-2 shown in FIG. 6b is similar to the target areas 106a to 106d shown in FIG. 3. Consequently, the mirror element M 1 -, will similarly change its shape if infra- red radiation is directed to the target area 106-2. Since additional heat is produced at the centre of the mirror element M 1 -, the resulting curvature profile will be slightly different .
• the target areas shown in FIGS. 6a and 6b have the advantages that they may be described as line patterns which are simply to produce with the moving laser beam 94.
• FIG. 6c shows a target area 106-3 having the geometry of a circular disc. Such a disc will produce the most rotationally symmetrical bending forces.
• FIG. 6d shows a target area 106-4 which will also result in an at least substantially rotationally symmetric bending of the mirror element M 1 -,, but with a slightly different curvature profile as compared to the embodiments shown in FIGS. 3, 6b and 6c.
• the deformation of the mirror elements M ⁇ obtained by increasing the temperature in the target areas 106 also depends on any internal stress present in the mirror support 100 and the reflective coating 102. For example, it is possible to produce the layers forming the reflective coating 102 such that a mechanical stress remains after the manufacturing process. Such a stress may be released or increased by increasing the temperature in the layers. By suitably selecting the directions of the mechanical stress, it is possible to influence the deformations that are obtained after changing the temperature in the target areas 106.
• the mirror array 46 of this embodiment comprises 121 mirror elements Mij arranged in a rectangular grid pattern.
• the central mirror element Mi j shall not be deformed at all.
• Almost rotationally symmet- rical deformations shall be produced in all other mirror elements with the help of the temperature control device 90, wherein the curvature shall increase with increasing distance from the centre of the mirror array 46. Consequently, the target areas 106 are getting larger the more the mirror ele- ments Mi j are spaced apart from the centre.
• the laser beam 94 may also be used to modify the reflective properties of the reflective coating 102. If the laser beam 94 remains sufficiently long at a particular location on the mirror element Mi-,, the temperature will rise to an extent such that the reflectivity of the reflective coating decreases substantially. Such a deliberate reduction of the re- flectivity of the reflective coating 102 may be useful if the projection light, for example as a result of beam inhomoge- neities, has produced similar degradations in the area 104 exposed to projection light. The laser beam 94 may then ensure that a uniform (although lower) reflectivity is regained in a portion of the area 104.
• FIG. 8 is a perspective view of a mirror element M 1 -, according to another embodiment.
• the mirror element M 1 - is con- nected to a carrier structure 110 with the help of solid state articulations 112a, 112b, 112c and 112d.
• These articu ⁇ lations 112a, 112b, 112c and 112d are formed by bridges that remain in the carrier structure 110 after longitudinal slits 113 have been produced.
• actuators configured to tilt the mirror element M 1 -, around tilt axes 56x, 5 ⁇ y defined by opposing pairs of articulations 112a, 112c and 112b, 112d, respectively, are not shown in FIG. 8.
• the temperature control device 90 comprises a plurality of heating members 114 which connect the mirror element M 1 -, to a carrier plate 116 which is also defined by slits 113 in the carrier structure 110.
• the heating members 114 can be individually driven so that almost any arbitrary temperature profile can be produced in the mirror element Mi j .
• structures contained in the mirror element Mij and having different coefficients of thermal expansion ensure that the temperature profile produced in the mirror element Mi j results in a deformation that modifies the optical properties of the mirror element Mij.
• the heating members 114 are represented as small blocks that heat up if an electric voltage is applied.
• the heating members are formed by patterns of electrically conductive resistance wires, as is shown in the bottom view of a mirror support 100 of FIG. 9.
• heating wires 114 ' are arranged over the bottom surface of the mirror support 100 and can individually be connected to a voltage source so that various temperature profiles can be produced in the mirror element Mi j .
• the mirror elements Mij comprise two parallel heat barriers 118a, 118b that extend, in the embodiment shown, along a Y direction over the entire length of the mirror elements Mi j .
• the heat barriers 118a, 118b are parallel heat barriers that extend, in the embodiment shown, along a Y direction over the entire length of the mirror elements Mi j .
• first and second grooves 120a, 120b that may have been formed by etching in a mirror support 100. Between the first and second grooves 120a. 120b a lower and broader third groove 122 is formed. The coefficient of thermal expansion of the material 124 filling the third groove 122 and the surrounding material of the mirror support 100 is different.
• the refresh rate at which the laser beam 92 illuminates the target area 106 can be reduced.
• a similar effect is also achieved with materials which have a non-isotropic coefficient of thermal expansion.
• materials which have a non-isotropic coefficient of thermal expansion For example, if the material 124 below the target area 106- has a lower coefficient of thermal expansion in the XZ plane than along an orthogonal direction, heat will flow away faster along the Z direction, i.e. via the sockets 126 into the heat sink 128, than into the adjacent material of the mirror support 100. Crystalline materials which can be epitaxially grown often have such a non-isotropic coefficient of thermal conduction.
• the shape of the reflective coating may (before or after increasing the temperature with the help of the temperature control device 90) be planar, ro- tationally symmetrically curved, ellipsoidal or saddle shaped, for example.
• the mirror support is only a thin membrane so as to increase the sensibility of the mirror element to deformations induced by temperature changes .
• the temperature control device comprises cooling members such as channels within the mirror support through which a cooling fluid can flow.
• the invention may also be envisaged to use the invention in mirror arrays that are arranged in projection objectives of microlithographic exposure apparatus.
• the pro- jection objective does not include a primary light source, and the mirror elements do not direct light towards the system pupil surface.
• the mirror array may itself be arranged in a pupil surface of the projection objective.
• the above embodiments of the invention all relate to DUV illumination systems. However, as will become clear from the embodiment described below, the invention may also advantageously be used in EUV illumination systems.
• the wavelength of the projection light (also referred to as the operating wavelength) is below 50 nm, preferably below 25 nm, and most preferably between 13 and 14 nm. Since no transparent refractive materials are available at such short wavelengths, all optical elements (except stops of course) in EUV projection exposure apparatus are of the catoptric (i.e. re- flective) type.
• FIG. 11 is a schematic and not-to-scale meridional section through an EUV projection exposure apparatus which is denoted in its entirety by 210.
• the EUV projection exposure apparatus 210 comprises an illumination system 212 which illuminates a mask 216 which contains minute reflective structures and is arranged on a mask stage (not shown) . Projection light reflected from the mask 216 enters a projection objective 220 which images the reflective structures illuminated on the reticle 216 on a light sensitive layer 222 which is applied on a substrate 224 arranged on a substrate stage (not shown) .
• the illumination system 212 comprises a light source schematically indicated at 230 to which an axis of symmetry AX is associated.
• this axis of symmetry AX coincides with an optical axis of a concave mirror which is contained in the light source 230. If there is no such mirror, this axis AX is defined by symmetry properties of the projection light beam as such.
• the light emitted by the light source 230 impinges on a field defining mirror 272 which may comprise an array of mirror elements (sometimes also referred to as mirror facets) .
• the projection light After being reflected from the field defining mirror 272, the projection light impinges on a pupil defining mirror array 246.
• the pupil defining mirror array 246 comprises a plurality of concave mirror elements Mi j which initially are all spherical or at least rota- tionally symmetrical. This implies that the curvatures c a and C b of the mirror elements Mi j are identical in arbitrary pairs of orthogonal planes.
• Each mirror element Mi j is tiltably mounted with respect to a support structure, comprises a mirror support and a reflective coating and is configured to di- rect projection light towards a pupil surface of the illumination system 212.
• the EUV projection light is directed by the pupil defining mirror array 246 towards two mirrors Ml and M2 before it finally illuminates the mask 216.
• the illumination system 210 further comprises a temperature control device 290 which has, in the embodiment shown, essentially the same constitution as the temperature control device 90 shown in FIG. 2.
• the temperature control device 210 comprises an infrared laser source 292 which pro- prises a laser beam 294.
• the temperature control device 290 further includes a spatial light modulator 296 which is configured to move the laser beam 294 over target areas on the pupil defining mirror array 246.
• the spatial light modulator 296 comprises a mirror 298 which can be tilted about two orthogonal axes with the help of suitable actuators.
• the wavelength of the light produced by the laser source 292 is selected such that it is almost completely, or at least by more than 80%, absorbed by the target areas on the pupil defining mirror array 246.
• the remarkable feature of the pupil defining mirror array 246 is that the optical axes of the mirror elements Mi j form a very large angle ⁇ (in the embodiment shown ⁇ « 35°) with an optical axis of the field defining mirror 272 which immediately precedes the pupil defining mirror array 246 in a light propagation direction. If the preceding mirror comprises mirror elements, this condition applies individually for the op- tical axes of the mirror elements irrespective of their tilting position.
• the angle ⁇ is preferably greater than 20°, more preferably greater than 30°
• This angle ⁇ which is a measure for the beam deviation capability of the mirror array 246, is much larger than conventionally. In prior art EUV projection exposure apparatus the deviation angles ⁇ for all mirrors are kept as small as pos- sible.
• the prior art illumination systems attempt to keep the devia- tion angles ⁇ as small as possible because larger deviation angles will result in aberrations if the involved mirrors have spherical mirror surfaces.
• the production of aspherical, and in particular of rotationally asymmetrical, mirrors is difficult and costly, and this particularly applies if a fac- etted mirror comprises a large number of individual mirror elements, as is the case with the mirror arrays 246 and 272 shown in FIG. 11.
• the deviation angle ⁇ is allowed to be large, because aberrations that would be introduced by the initially spherical or rotationally symmetrical mirrors are corrected with the help of the temperature control device 290 in the following manner:
• the spatial light modulator 296 is controlled such that the laser beam 294 heats up the mirror elements Mi j in a specific manner that will result in a deformation of the mirror elements Mij.
• This deformation is determined such that the surface shape of the mirror elements Mj .j changes from rotationally symmetrical to astigmatic.
• the target area on each individual mirror element Mi j may have a geometry as it is shown in FIG. 6a, for example.
• the term "astigmatic" is used to indicate that the curvature of a mirror element Mi j is different in two orthogonal planes (see curvatures c a and C b in the enlarged cutout in FIG. 12) .
• the focal length f a of the mirror elements Mi j in one plane will be larger than the focal length f b in an orthogonal plane, as is illustrated in the perspective view of FIG. 13.
• This difference between the focal lengths may be as large as 1% (i.e. f a > 1.01-f b ) or even as large as 10%.
• R a R/cos( ⁇ )
• R b R-cos( ⁇ )
• the focal length of the mirror be large compared to the illuminated surface on the mirror.
• the mirror elements Mij of the field and pupil mirror arrays 272, 246 this condition is fulfilled, because their focal length is typically in the range of about 1 m, and the illuminated surface is small (typical diameters of the mirror elements are about a few millimeters up to a few centime ⁇ ters) .
• the mirrors Ml or M2 however, larger deviation angles could not be achieved even if they had astigmatic mirror surfaces.
• a larger deviation angle ⁇ makes it possible to arrange the light source 230 at a much more convenient height, as can be seen in FIG. 11.
• the axis of symmetry AX of the light source 230 runs substantially horizontally, and thus the light source 230 is arranged at the same height as the field defining mirror 272.
• the axis of symmetry AX forms an angle of less than 45°, preferably of less than 20°, with respect to a horizontal plane.
• FIG. 14 is a perspective view of a mirror element M ⁇ j according to another embodiment in which the temperature control device does not locally heat, but locally cools the mirror elements Mi j .
• the temperature control device comprises a plurality of micro Peltier elements 130 that connect the mirror support 100 to a heat sink 128.
• micro Peltier elements 130 are commercially available from MICROPELT GmbH, Freiburg, for example.
• Each micro Peltier element 130 can be operated individually, which is indicated in FIG. 14 by wiring 132, and thus the mirror support 100 can be locally cooled so as to produce a wide variety of different temperature distributions. Also in this embodiment structures having different coefficients of thermal expansion will cause a desired deformation of the mirror support 100. With the provision of the micro Peltier elements 130 an additional cooling system which may otherwise be necessary can be completely dispensed with.
• the mirror elements Mi j may be (additionally) heated and cooled (quasi-) simultaneously. This makes it possible to produce very large temperature differences and consequently large deformations of the mirror support.

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• 2008
  • 2008-09-29 EP EP08017088A patent/EP2169464A1/en not_active Withdrawn
• 2009
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