The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "The invention relates to production methods for mirror elements and to mirror elements comprising a reflective coating for the EUV wavelength range and a substrate.
"It is known that the density of many materials, especially materials containing silicon, changes under irradiation with high-energy radiation. That effect is referred to in the literature as 'compaction'. For applications in extreme environments (reactors, outer space) in particular, studies were carried out and those effects were quantitatively determined a long time ago (see W. Primak, Nucl. Sci. Eng., 65, 141, 1978: 'Radiation Behaviour of Vitreous Silica' and
"It has been found that the change in volume or density in silicon dioxide typically attains, after sufficiently long irradiation, a saturation value on the order of about 2%-3% within the penetration depth reached by the radiation. The penetration depths of the high-energy types of radiation considered therein were typically in a range of from 0.5 .mu.m to about 10 .mu.m or more.
"Comparable effects are also known in microlithography, especially for the VUV wavelength range; however, owing to the relatively low level of interaction of the VUV light with the optical material used, especially quartz glass, the changes in volume in that case are as a rule in the ppm range and therefore, typically, no saturation value is attained, the optical materials used in that case being completely penetrated by the radiation.
"A method is known from U.S. Pat. No. 6,205,818 B1 by which quartz glass (SiO.sub.2) is to be made insensitive to compacting caused by long-term irradiation with UV laser radiation. The method provides for the quartz glass material to be pre-compacted by being exposed to high-energy radiation or by being pre-treated by hot isostatic pressing (HIP). The high-energy radiation is said to make a compaction of between about 10 ppm and 100 ppm possible, while a change in volume of the entire quartz glass body of from about 0.1% to about 3% is said to be achievable by hot isostatic pressing.
"Since microlithography will have to rely in future on the EUV wavelength range in order to obtain a further increase in resolution and since, owing to their coating, the mirrors used in that case are capable of reflecting only about 70% of the incident light and consequently absorb about 30% of the incident light, materials having a low coefficient of thermal expansion are normally used as substrate material for such mirrors. Such so-called 'low expansion materials' are, for example, Zerodur.RTM., ULE.RTM. or Clearceram.RTM.. Those materials normally have an amorphous silicate glass content of above about 50% and, in extreme cases, of even 100%. For a projection exposure system to be capable of functioning on a long-term basis it is necessary to ensure, therefore, that the energy absorbed in the substrate material during operation does not lead to changes in the substrate and thus to degradation of the mirror surface. In other words, it is necessary to ensure that changes of any kind in the shape or roughness of the surface, which can lead to a no longer tolerable increase in aberrations or stray light, do not occur."
As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventor's summary information for this patent application: "An object of the invention to provide production methods for mirror elements so that the mirror elements exhibit no change or only a negligibly small change in surface shape on long-term irradiation with EUV. It is a further object of the invention to provide mirror elements of that kind and an EUV projection exposure system for microlithography having such mirror elements.
"In accordance with one aspect, that object is attained by a method for the production of a mirror element that has a reflective coating for the EUV wavelength range and a substrate, the method comprising: pre-compacting the substrate by hot isostatic pressing, and applying the reflective coating to the pre-compacted substrate, wherein either the pre-compacting of the substrate is performed until a saturation value of the compaction of the substrate by long-term EUV irradiation is reached, or, for further compaction, the pre-compacted substrate is irradiated, especially homogeneously, with ions and/or with electrons in a surface region in which the coating has been or will be applied.
"In accordance with the invention it is proposed that pre-compacting of a mirror substrate of an EUV mirror be carried out by hot isostatic pressing. The penetration depth of the EUV radiation impinging on the mirror element when in use, and hence the volume of the substrate in which a change in density may occur as a result of EUV irradiation, is admittedly relatively small as a rule (typically some 100 nm). The inventor has found that it is nevertheless advantageous for the entire substrate to be pre-compacted by hot isostatic pressing since such a pressure treatment can be carried out relatively quickly and inexpensively in the case of the materials used as the substrate.
"In one variant, there is selected as the substrate material a doped glass material or a glass ceramic, which may be selected in particular from the group comprising: ULE.RTM., Zerodur.RTM. and Clearceram.RTM.. Such substrate materials have a low coefficient of thermal expansion which may, for example, be at most |0.5.times.10.sup.-7| 1/K in a range of 0.degree. C. to 50.degree. C. To produce such a low coefficient of thermal expansion (CTE), doped glass or glass ceramic materials are typically used--for example, as mentioned above, ULE.RTM., Clearceram.RTM. or Zerodur.RTM.. Glass ceramic materials having the low coefficient of thermal expansion indicated above consist as a rule of a crystalline phase and a glass phase. The crystalline phase has a negative coefficient of expansion which can precisely be compensated for by the positive coefficient of expansion of the glass phase. Glass materials having a low CTE are as a rule doped glasses, for example TiO.sub.2-doped quartz glass (ULE). It will be appreciated that undoped glass, for example undoped quartz glass (fused silica), may alternatively also serve as substrate material. The materials mentioned above have an amorphous silicate glass content of more than about 50 wt. % and are therefore especially suitable for pre-compaction by hot isostatic pressing.
"In the hot isostatic pressing, (initial and holding) temperatures of between about 1100.degree. C. and about 1300.degree. C., preferably between 1150.degree. C. and 1250.degree. C., have proved to be especially favourable. It will be appreciated that it is not imperative for a single temperature to be maintained during the hot isostatic pressing; rather, where appropriate, it is possible, for example, for cooling to take place from a maximum temperature in several temperature stages.
"In a further variant, the pressure in the hot isostatic pressing is selected to be between 20 MPa and 250 MPa, preferably between 50 MPa and 150 MPa. The use of that pressure range has been found to be especially advantageous for creating a high degree of pre-compaction.
"In one variant, the holding time in the hot isostatic pressing is selected to be between 0.5 hour and 5 hours, preferably between 2 hours and 4 hours. It has been found that a sufficient pre-compaction can be achieved even when relatively short holding times are used.
"Especially in the case of materials having a high silicate glass content of more than 90 wt. % (such as in the case of ULE.RTM.), the hot isostatic pressing may be carried out substantially as described in the above-mentioned U.S. Pat. No. 6,205,818 B1, which regarding that aspect is incorporated into the present application by reference.
"The substrate may be compacted by the hot isostatic pressing by at least 1%, preferably by at least 1.5%, and in particular by at least 3%. Especially when the compaction is in the range of about 2%-3% or more, the saturation value of the compaction by long-term EUV irradiation can be reached and therefore the substrate, once pre-compacted, cannot be further compacted by the EUV radiation. It is, however, possible that the saturation value cannot be reached in the hot isostatic pressing or can be reached only with excessively long holding times which may, for example, be in the range of several days.
"For that reason, for further compaction the pre-compacted substrate may be irradiated, especially homogeneously, with ions and/or with electrons in a surface region in which the coating has been or will be applied. With this irradiation, a surface region extending from the surface of the substrate over a small depth, typically in the range of several micrometres, can be additionally compacted, so that the saturation value of the density change is reached at least in that region. In this case, the pre-compaction using the inexpensive hot isostatic pressing process permits the irradiation times of the ion or electron irradiation to be considerably reduced and thereby allows any form change possibly occurring as a result of the ion or electron irradiation to be kept as small as possible.
"The irradiation is advantageously carried out with high-energy ions having an energy of between 0.2 MeV and 10 MeV at a total particle density of from 10.sup.14 to 10.sup.16 ions per cm.sup.2 and/or with high-energy electrons having a dose of between 10 J/mm.sup.2 and 2000 J/mm.sup.2 at energies of between 10 KeV and 20 KeV. The irradiation may be carried out in this case especially as described in the Applicant's US 61/234815, which is incorporated in this Application by reference. It will be appreciated that, before and/or after the irradiation, additional processing steps, especially smoothing steps at the surface of the substrate, may be carried out, for example as described in the Applicant's US 61/234815.
"In one variant, the irradiation is carried out until there is obtained in the surface region a density that is at least 0.5%, preferably at least 1%, in particular at least 1.5% higher than the density of the remainder of the substrate. Together with the change in density obtained on pre-compaction of the substrate it is possible in this case for the saturation value of the compaction to be attained in an especially simple manner. The additionally compacted surface region extends as a rule in this case to a depth of about 5 .mu.m from the surface of the substrate, the exact value depending on the ion or electron energy which is typically so selected that the compacted surface region extends as least as far as the penetration depth of the EUV radiation on use of the mirror.
"A further aspect of the invention relates to a mirror element comprising: a reflective coating for the EUV wavelength range, and a substrate, wherein the substrate is pre-compacted by hot isostatic pressing. Either the entire substrate is pre-compacted to a saturation value of the compaction of the substrate by long-term EUV irradiation, or a surface region of the substrate that extends beneath the coating has a density that is at least 0.5%, preferably at least 1%, in particular at least 1.5%, higher than that of the remainder of the substrate. The density of the pre-compacted substrate material markedly exceeds the density of the substrate material attained in a conventional production process (without pre-compaction).
"As explained above, the material of the substrate is typically a doped glass material or a glass ceramic having a low coefficient of thermal expansion, especially ULE.RTM., Zerodur.RTM. or Clearceram.RTM.. Apart from the low thermal expansion of those substrate materials they have the additional advantage that they have a high silicate glass content (about 50 wt. % or more). With that material, a considerable degree of pre-compaction can be achieved by hot isostatic pressing with relatively short holding times.
"In one embodiment, the material of the substrate is a quartz glass doped with TiO.sub.2, in particular ULE.RTM., an initial density of the substrate before compaction being 2.21 g/cm.sup.3. That initial density is typically obtained in ULE.RTM. that has been produced by a conventional production process.
"Especially when the saturation value of the density change is not yet reached by hot isostatic pressing, the resistance of the substrate to EUV irradiation can be increased if a surface region of the substrate extends beneath the coating, which surface region has a density that is at least 0.5% higher, preferably at least 1% higher, in particular at least 1.5% higher than that of the remainder of the substrate and which surface region has been obtained through high-energy ion or electron irradiation. As described above, it is advantageous if that surface region extends to a depth of about 5 .mu.m from the surface of the substrate. By homogeneous irradiation with ions and/or electrons it is possible to achieve a homogeneous compaction of the substrate in that surface region.
"A further aspect of the invention is implemented in an EUV projection exposure system for microlithography, comprising an illumination system and a projection system having at least one mirror element for the EUV wavelength range as described above. In such an EUV projection exposure system, the surface shape of the mirror elements designed as described above changes on EUV irradiation only negligibly during the useful life of the system, and therefore no appreciable surface deformations that might lead to an increase in aberrations or stray light occur any longer.
"Further features and advantages of the invention will be apparent from the following description of illustrative embodiments of the invention with reference to the Figures of the drawings, which show details essential to the invention, and from the claims. The individual features may be implemented individually or a plurality thereof may be implemented in any desired combination in a variant of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
"Illustrative embodiments are shown in the schematic drawings and explained in detail in the following description. In the drawings:
"FIG. 1 is a schematic illustration of an embodiment of an EUV lithography system having a plurality of mirror elements,
"FIG. 2 is a schematic illustration of a pressure chamber for the hot isostatic pressing for pre- compacting a mirror substrate, and
"FIG. 3 is an illustration of a mirror element for the EUV projection exposure system of FIG. 1 with the substrate of FIG. 2."
For additional information on this patent application, see: CLAUSS, Wilfried. Mirror Elements for Euv Lithography and Production Methods Therefor. Filed
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