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
  • G03F7/708—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
• • 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

Definitions

• Projection exposure apparatus for semiconductor lithography comprising a device for the thermal manipulation of an optical element
• the invention relates to a projection exposure apparatus for semiconductor lithography comprising a device for the thermal manipulation of an optical element.
• manipulators For the correction of image aberrations in projection objectives for semiconductor lithography, two basic types of manipulators, positioned manipulators and deformation manipulators, are used according to the current state of the art.
• position manipulators the sensitivity of an optical element to its position change relative to other optical elements is utilized in order to achieve the desired correction of the wavefront and thus of the image aberration in the case of a suitable displacement or a suitable tilting.
• deformation manipulators the sensitivities of optical elements to mechanical deformation are utilized in order to obtain the desired correction effect.
• the correction possibilities are limited to low radial and azimuthal orders in the case of position manipulators.
• deformation manipulators afford the possibility of also adjusting higher azimuthal orders due to the arrangement of a suitable number of actuators, they are nonetheless also restricted to low orders for radial corrections. This stems from the fact, in particular, that the mechanical forces and moments required for the mechanical deformation are subject to certain limitations.
• An alternative to the mechanical manipulation which is presented above and which, in particular, does not permit corrections of higher radial orders consists in thermally manipulating an optical element, such as a lens, for example.
• an optical element such as a lens
• the temperature dependence of the refractive index of the material of the optical element is utilized in order to achieve the desired correction of the wavefront and thus of the image aberration by means of a suitable temperature distribution across the optical element.
• Conventional concepts for the thermal manipulation of an optical element consist either in heating or cooling said optical element from its edge or else in directly influencing the temperature distribution in the optical element itself, for example by means of thin heating wires arranged in the optically active region of the optical element.
• the US patent application US 2005/0018269 Al proposes compensating for the inhomogeneous, region-by-region heating - caused by operation - of the optical elements used in a semiconductor lithography apparatus by locally heating the optical elements by means of a laser at those locations at which the optical elements used have applied to them electromagnetic radiation of lower intensity than at other locations. In this way, the inhomogeneous irradiation of the optical elements is compensated for and homogeneous heating of the optical elements used takes place as a result.
• the projection exposure apparatus according to the invention for semiconductor lithography exhibits an optical element having a front side, which is suitable for the reflection of electromagnetic radiation, and a rear side, wherein thermal actuators for influencing the optical properties of the optical element are provided, which act on the optical element from the rear side thereof.
• This arrangement of thermal actuators has the effect that the thermal actuators are situated in a region of the optical element which is not reached by the electromagnetic useful radiation to which the optical element is exposed during operation as intended.
• the limitations existing when arranging thermal actuators on an optical element operated in transmission are inapplicable by virtue of this measure.
• the solution according to the invention makes it possible to arrange thermal actuators on the optical element across the entire cross section thereof.
• the spatial frequencies that can be realized in the temperature distributions are essentially limited by the resolution capability of the actuator arrangement and, if appropriate, the thickness of the optical element.
• different temperature distribution functions can thus be configured depending on the image aberration to be corrected.
• only the thickness of the optical element, and not the diameter thereof, is crucial for the time constants for setting a temperature distribution. This has the effect that, in comparison with optical elements whose temperature distribution is set from the edge, time constants which are shorter by approximately one to two orders of magnitude are possible, which constitutes a considerable advantage from the standpoint of control technology.
• the optical element can be for example just an individual mirror, in particular a metal-coated individual mirror, on the rear side of which the thermal actuators are arranged.
• the desired correction of the optical properties of the mirror is achieved exclusively by the change in shape of the reflective side of the mirror, which change is thermally induced from the rear side.
• the thermal actuators are arranged at the rear side of the optical element in such a way that they are not in mechanical contact with the rear side of the optical element.
• a thin gap can remain between the thermal actuator and the rear side of the optical element. This ensures that the thermal actuators are arranged in a manner mechanically decoupled from the optical element to be manipulated, whereby parasitic mechanical effects such as can be brought about for example by vibrations or mechanical stresses in the arrangement or by the actuators are effectively precluded.
• the thermal actuators can also - in particular for the cases in which they are embodied as Peltier elements or resistance heating elements - be in direct mechanical contact with the rear side of the optical element, whereby an effective heat transfer is ensured.
• a gas- or liquid-filled gap can remain between the thermally active area of the actuators, which are embodied as Peltier elements, for example, and the rear side of the optical element; in this case, the gas or the liquid serves as a transmission medium for the thermal energy to be transmitted into the optical element.
• the gases or liquids used can be chosen such that they have a high thermal conductance in order to ensure an efficient heat transfer between the thermal actuator and the optical element.
• helium is most appropriate as gas, for example, on account of its high thermal conductivity.
• the thermal actuators can be gas nozzles which are arranged in the region of the rear side of the optical element and are directed at said rear side and are suitable for locally applying gas bursts or a continuous gas stream having the desired temperature to the rear side of the optical element.
• This embodiment of the invention ensures a comparatively fast drivability of the optical element.
• This solution has the advantage in common with the realization of the thermal actuators as Peltier elements that the optical element can be locally heated and also cooled either through the choice of the polarity of the connection of the Peltier elements or through the choice of the temperature of the gas burst.
• the realization of the thermal actuators in such a way that the latter are suitable for emitting electromagnetic radiation for absorption onto the rear side of the optical element also has advantages for some fields of application.
• One of these advantages is that a completely contactless coupling of the thermal actuators to the rear side of the optical element can be achieved in this way.
• the thermal actuators can be realized e.g. as LEDs or laser diodes, in particular with an emission spectrum in the infrared range.
• the thermal actuators if appropriate embodied as electromagnetic emitters
• the thermal actuators can be arranged as an array on or in the region of the rear side of the optical element. In this case, the desired distribution of the thermal energy input into the optical element is achieved by means of a suitable driving of the thermal actuators.
• a particularly advantageous variant of the configuration of the optical element consists in the fact that a substrate that is at least partly transparent to the useful wavelength is adjacent to the front side of the optical element.
• This measure has the effect that the electromagnetic useful radiation passes through the region in front of the optical element twice.
• the correction of the wavefront is achieved not only by the geometrical change in the reflective surface of the optical element, but also by the optical properties of the substrate that vary on account of the changing temperature.
• the density and thus the refractive index of the substrate are altered by the changing temperature, whereby an improved response of the entire arrangement to the thermal driving can be achieved.
• the optical effect of the thermal manipulation according to the invention is further enhanced and the substrate can therefore be chosen to be comparatively thin.
• the realization of the optical element as a so-called Mangin mirror is particularly advantageous. This is achieved by virtue of the fact that a refractive optical element, such as a lens, for example, is adjacent to the substrate.
• the Mangin mirror can also be achieved by for example a lens being provided with a reflection layer on one side, whereby a particularly simple embodiment of the optical element according to the invention can be realized.
• the thermal drivability of the optical element according to the invention can be further improved by virtue of the fact that a planar, if appropriate cooled, element acting as a heat sink is arranged in the region of the rear side.
• a planar, if appropriate cooled, element acting as a heat sink is arranged in the region of the rear side.
• only local heating is effected against the continuous cooling of the planar element. It thereby becomes possible to realize the desired temperature distributions along the rear side of the optical element as rapidly as possible.
• the invention exhibits considerable improvement potential.
• the optical element can be a mirror having a multilayer layer arranged on its front side and a substrate arranged on its rear side.
• the thermal actuators can be suitable for emitting electromagnetic radiation which can at least partly pass through the substrate from the rear side of the mirror and can be at least partly absorbed by an absorption layer arranged between substrate and multilayer layer. This can be achieved for example by choosing ULE (Ultra Low Expansion Titanium Silicat glass material by Corning) or Zerodur (by Schott) for the substrate material, and by the actuators being suitable for emitting electromagnetic radiation in the visible or ultraviolet spectral range. Both ULE and Zerodur are substantially transmissive to electromagnetic radiation in the visible or ultraviolet spectral range.
• the absorption layer can contain a lacquer layer, a glass or else a metal powder.
• the thickness of the absorption layer can be between 5 ⁇ m and 15 ⁇ m. It can have, on the side facing the substrate, a roughness of between 0.05 and 0.5 rms or even of between 0.1 and 0.3 rms with the band range of each decade between 0.1 micrometer and 10 millimeters.
• the definition of the (lateral) band ranges of an area is defined by means of a 2-dimensional Fourier decomposition.
• the RMS in a band range is then the standard deviation in this band range.
• a definition of the bandwidth and of the RMS of the microroughness according to ISO 10010 is also used.
• the absorption layer can have different absorption coefficients in regions; as an alternative or in addition, means for the controlled spatial distribution of the electromagnetic radiation emitted by the actuators can be present.
• the first problem in EUV semiconductor lithography systems is that it is necessary to reckon with the occurrences of thermal loads distributed comparatively inhomogeneously on the individual mirrors of the projection objective.
• This stems firstly from the fact that for higher-aperture EUV systems (numerical aperture ⁇ 0.3) provision is made for operating them for the imaging of general two-dimensional structures for example with illumination settings such as e.g. an annular (ring-shaped) setting or a dipole setting.
• the setting is understood to mean the distribution of the intensity of the electromagnetic radiation which is used for the illumination of the mask to be imaged, the reticle. Particularly in the case of mirrors near the pupil, this leads to inhomogeneous, setting-dependent thermal loads.
• the powers of the EUV radiation sources which are used in EUV projection exposure apparatuses will increase further in the future, thus giving rise to higher input intensities in the projection objective and hence an enlargement of the peak-to- valley values of the thermal loads on the individual mirrors.
• ever greater inhomogeneous thermal loads are impressed on the mirrors of an EUV projection objective.
• the frequency of the cleaning processes will increase as the EUV intensity increases, since the EUV radiation has the effect that gaseous hydrocarbons are dissociated in the projection objective and a film of carbon deposits on the reflective layers, that is to say the multilayers, of the mirrors.
• inhomogeneous thermal loads described lead to inhomogeneous deformations of the mirrors primarily on account of the inhomogeneous thermal deformation of the substrate, resulting in additional imaging aberrations. Since the imaging aberrations thus induced are based on inhomogeneous deformations, these aberrations can generally be rectified only with difficulty, or not at all, by means of conventional, for example mechanical, manipulators since such manipulators have only a limited number of degrees of freedom.
• the second problem that can be solved by the described embodiment of the invention is that the spatial position of that plane in the multilayer layer which can conceptually be assumed to be a location of the reflection of the wavefront is not always optimized toward a wavefront having an ideal, spherical form.
• the spatial position of the EUV multilayer layer it is also possible to refer to the layer figure. It is desirable to be able to modify the layer figure even after the coating of the mirror substrate with the multilayer, in particular including in the cases where the projection exposure apparatus or the projection objective is already in use in the exposure of wafers. Firstly, actually during the production of the multilayer layer on the substrate it is not possible to ensure that the layer figure is realized within a desired tolerance range.
• the invention makes it possible to realize at least in part a compensation of the abovementioned sources of aberrations by means of a homogenization of the thermal load in the mirror or in the mirror substrate and by means of a modification of the layer figure.
• figure 1 shows a first simple embodiment of the invention
• figure 2 shows an embodiment of the invention in which different variants for locally influencing the temperature of the optical element are shown by way of example;
• figure 3 shows a projection exposure apparatus according to the invention for semiconductor lithography
• figure 4 shows a variant of the invention for the application for a multilayer mirror for EUV lithography.
• Figure 1 shows a first embodiment of the invention in a cross- sectional illustration.
• the optical element 1 is realized as a simple plane mirror having a reflective front side 2 and thermal actuators 4 embodied as Peltier elements that are arranged on the rear side 3 of the optical element 1.
• the thermal actuators 4 are selectively driven by the controller 5, such that a desired temperature distribution across the entire area of the optical element 1 can be achieved through the corresponding choice of the control voltages for each individual actuator 4.
• the thermal actuators 4 can also be embodied as resistance heating elements.
• Figure 2 shows, likewise in a cross-sectional illustration, an exemplary arrangement depicting various possibilities for locally influencing the temperature of the optical element 1.
• the optical element 1 illustrated in figure 2 has a substrate 6 arranged on its front side 2, which substrate is substantially transparent to the useful radiation, that is to say the radiation which is incident on the optical element 1 during operation as intended.
• This procedure has the effect that in addition to the thermally caused deformation of the reflective surface arranged on the front side 2 of the optical element 1, a further thermally induced effect for influencing the optical properties of the optical element 1 additionally occurs:
• a further intensification of the effect of the temperature change can be achieved by the further optical element 14 being arranged as an alternative or in addition to the substrate 6 indirectly or directly at the front side 2 of the optical element 1, said further optical element being embodied as a lens in the example in figure 2.
• Various thermal actuators are arranged at the rear side 3 of the optical element 1.
• a Peltier element 41 is arranged in the region of the rear side 3 of the optical element 1; in this case, the Peltier element 41 is driven by the controller 5.
• a heat sink 9 embodied as a planar plate is arranged.
• the gap can also be filled by a liquid.
• the heat sink 9 continuously dissipates heat from the region of the rear side 3 of the optical element 1; that is to say that the desired temperature for example in the region of the Peltier element 41 is effected merely by the Peltier element 41 being heated in a manner controlled in open-loop or closed-loop fashion.
• the gas nozzle 44 is arranged as a further variant of a thermal actuator in figure 2, said gas nozzle being directed at the heat sink 9 in the present example.
• the gas nozzle 44 is connected to the gas supply 8, which is likewise driven by the controller 5.
• the gas nozzle 44 can be embodied in pivotable fashion, such that the gas stream emerging from the gas nozzle 44 can be directed onto predetermined locations on the rear side 3 of the optical element 1.
• a further thermal actuator embodied as an LED 42, is furthermore arranged at the rear side 3 of the optical element 1, wherein the electromagnetic radiation emitted by the LED is directed into the desired regions of the rear side 3 of the optical element 1 by the deflection element 10, which - like the LED 42 as well - is controlled by the controller 5.
• the beam direction is indicated by the dashed arrow 15.
• a plurality of LEDs 43 arranged as an array are positioned at the rear side 3 of said optical element, by means of the targeted driving of which LEDs a desired temperature distribution can be set.
• the LEDs 43 arranged as an array are also driven by the controller 5.
• a coating 13 is arranged in the region to which the electromagnetic radiation emitted by the LEDs 42 and 43 is applied, said coating having a high absorptance for the electromagnetic radiation emitted by the LEDs 42 and 43. The efficiency of the thermal driving of the optical element 1 by electromagnetic radiation is further improved in this way.
• Figure 3 illustrates a projection exposure apparatus 16 for semiconductor lithography into which the device described is integrated. It serves for the exposure of structures onto a substrate coated with photosensitive materials, said substrate generally being composed predominantly of silicon and being referred to as a wafer 17, for the production of semiconductor components, such as e.g. computer chips.
• the projection exposure apparatus 16 essentially comprises an illumination device 18, a device 19 for receiving and exactly positioning a mask provided with a structure, a so- called reticle 20, which is used to determine the later structures on the wafer 17, a device 21 for retaining, moving and exactly positioning precisely said wafer 17, and an imaging device, namely a projection objective 22, having a plurality of optical elements 23 that are borne by means of mounts 24 in an objective housing 25 of the projection objective 22.
• the basic functional principle provides for the structures introduced into the reticle 20 to be imaged onto the wafer 17; the imaging is generally performed in demagnifying fashion. After an exposure has taken place, the wafer 17 is moved further in the arrow direction, with the result that a multiplicity of individual fields each having a structure prescribed by the reticle 20 are exposed on the same wafer 17.
• the step-by-step advancing movement of the wafer 17 in the projection exposure apparatus 16 the latter is often also referred to as a stepper.
• the illumination device 18 provides a projection beam 26 required for the imaging of the reticle 20 on the wafer 17, for example light or a similar electromagnetic radiation.
• a laser or the like can be used as a source for this radiation.
• the radiation is shaped in the illumination device 18 by means of optical elements in such a way that the projection beam 26, upon impinging on the reticle 20, has the desired properties with regard to diameter, polarization, shape of the wavefront and the like.
• the projection objective 22 has a multiplicity of individual refractive, diffractive and/or reflective optical elements 23, such as e.g. lenses, mirrors, prisms, terminating plates and the like.
• the device 50 according to the invention is integrated in the projection objective 22.
• Figure 4 shows an embodiment of the invention in which the optical element 100 is embodied as a multilayer mirror, in particular for an EUV projection exposure apparatus.
• the multilayer mirror 100 exhibits the front side 102 embodied as a multilayer.
• the multilayer is embodied as an alternating layer sequence of approximately 50 to 100 molybdenum and silicon layers. It is arranged on the substrate 106, which is composed of ULE or Zerodur, for example, wherein the absorption layer 104 is situated between the substrate 106 and the multilayer mirror 102, which absorption layer can have a thickness of 5 to 15 ⁇ m and can be formed for example by means of a lacquer, a metal powder or else a glass.
• electromagnetic radiation is applied to said optical element, which radiation is at least partly transmitted by the substrate 106 and thus reaches the absorption layer 104, where the portions transmitted by the substrate 106 are substantially absorbed and thus contribute to heating of the absorption layer 104.
• an electromagnetic radiation is applied to the rear side
• the spatial distribution of the electromagnetic radiation precisely counteracts the initially inhomogeneous heating of the substrate 106.
• This is indicated schematically in figure 4 by a different density of the arrows (not designated by reference symbols) .
• the desired spatial distribution of the electromagnetic radiation can be achieved by means of the structural unit 105 illustrated schematically in figure 4 - comprising actuators and means for the controlled spatial distribution of the electromagnetic radiation emitted by the actuators.
• an individual radiation source can be used as a thermal actuator, wherein the electromagnetic radiation emitted by the individual radiation source is distributed over the rear side 103 of the optical element 100 by means of a suitable imaging optical unit.
• the imaging optical unit can involve for example lenses, mirrors or a combination of lenses and mirrors.
• An individual orientable mirror is also conceivable; it may likewise be advantageous to direct the electromagnetic radiation proceeding from an individual radiation source firstly onto a multimirror array, which then produces the desired spatial radiation distribution on the rear side 103 of the optical element 100 or in the absorption layer
• the desired temperature distribution in the absorption layer 104 and thus in the substrate 106 by means of the absorption properties of the absorption layer 104 following a desired spatial distribution, such that, even in the case of homogeneous thermal irradiation of the rear side 103 of the optical element, across the absorption layer 104, a temperature gradient arises which at least partly compensates for the inhomogeneous heating of the substrate 106.
• the solution illustrated above in particular with reference to figure 4 makes it possible to effectively combat both the outlined problem of the setting of the layer figure and that of the inhomogeneous heating of the mirror substrate as a result of spatially inhomogeneous thermal loads.
• the setting of the desired layer figure can be achieved e.g. by choosing the absorption layer 104 with a thickness and a coefficient of thermal expansion such that just the thermal expansion of the absorption layer 104 alone suffices to set the layer figure as desired.
• a comparatively thin absorption layer having a lower coefficient of thermal expansion can be employed, in principle. Mixed forms of the two variants are also conceivable.
• the absorption layer 104 can also be omitted and the multilayer can be directly heated through the substrate 106 from the rear side of the multilayer mirror.
• This embodiment of the invention is primarily suitable for achieving any homogeneous temperature regulation of the substrate 106 and hence a reduction of the thermally induced deformations of the substrate 106.
• a projection exposure apparatus (16) for semiconductor lithography comprising a device for the thermal manipulation of an optical element (1,100) of the projection exposure apparatus (16), wherein the optical element (1,100) has a front side (2) for the reflection of electromagnetic radiation and a rear side (3,103), characterized in that thermal actuators (4) for influencing the optical properties of the optical element (1,100) are present, which act on the optical element (1,100) from the rear side (3,103) thereof.
• thermo actuators (4) are Peltier elements (41) .
• thermo actuators (4) are gas nozzles (44) which are suitable for locally applying a predetermined temperature to the rear side (3) of the optical element ( 1 ) .
• thermo actuators (4) are suitable for emitting electromagnetic radiation for adsorption onto the rear side (3,103) of the optical element (1,100) .
• thermo actuators (4) are lasers, or LEDs (42), the emission spectrum of which lies in the infrared spectral range.
• deflection elements (10) are present which are suitable for directing the radiation emitted by at least one thermal actuator (4) or the gas stream emitted by at least one thermal actuator directionally onto predetermined positions on the rear side (3) of the optical element (1) .
• thermo actuators (4) are arranged as an array and can be driven selectively.
• the projection exposure apparatus (16) according to any of the preceding embodiments, characterized in that a substrate that is at least partly transparent to the useful wavelength, in particular a lens (14), is adjacent to the front side (2) of the optical element (1) .
• the projection exposure apparatus (16) according to any of the preceding embodiments, characterized in that the optical element is a mirror (100) having a multilayer layer (102) arranged on its front side and a substrate (106) arranged on its rear side (103) . 14.
• the projection exposure apparatus (16) according to any of embodiments 14-16, characterized in that the absorption layer (104) contains a lacquer layer.
• the projection exposure apparatus (16) according to any of embodiments 14 to 16, characterized in that the absorption layer (104) contains aluminum.
• the (104) has, on the side facing the substrate (106), a roughness of between 0.05 and 0.5 rms, preferably between 0.1 and 0.3 rms, in all decades from 0.1 micrometer to 10 millimeters.
• the projection exposure apparatus (16) according to any of the embodiments 14 to 22, characterized in that the absorption layer (104) has different absorption coefficients in regions.
• the projection exposure apparatus (16) according to any of the embodiments 14 to 23, characterized in that means for the controlled spatial distribution of the electromagnetic radiation emitted by the actuators are present.

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• 2009
  • 2009-06-04 WO PCT/EP2009/004006 patent/WO2009152959A1/en active Application Filing
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