WO2006111185A1

WO2006111185A1 - Method for connecting two elements and optical component

Info

Publication number: WO2006111185A1
Application number: PCT/EP2005/004950
Authority: WO
Inventors: Claudia Ekstein; Joachim Hoga; Eric Eva; Hubert Holderer
Original assignee: Carl Zeiss Smt Ag
Priority date: 2005-04-16
Filing date: 2005-05-06
Publication date: 2006-10-26
Also published as: DE112005003536T5

Abstract

In a method for connecting two elements (1, 2) at least one of which consists of fused silica, wherein both elements (1, 2) are connected to each other through fusion bonding in a gas atmosphere (5), preferably at temperatures between 300° C and 1000° C, the hydrogen portion in the gas atmosphere (5) is adjusted such that reduction of the hydrogen content of the fused silica first and/or second element(s) (1, 2) is substantially prevented. An optical component, in particular, a terminating element (6) for a projection objective of a microlithography projection exposure apparatus is produced in accordance with the method.

Description

Method for connecting two elements and optical component

Prior Art

The invention relates to a method for connecting two elements, at least one of which consists of fused silica, wherein both elements are connected through fusion bonding in a gas atmosphere, and also an optical component produced in accordance with the method.

An optical component is conventionally produced through connecting two fused silica elements at a contact point or joint without additional materials such as e.g. glue, solder or liquids, since glue swelling, glue creeping or irregularly distributed solder can cause deformation and hence imaging faults. On the other hand, the joint should be waterproof and stable over a long time to maintain fitting of the joined parts and prevent changes in the optical performance, i.e. in the imaging properties of the component in the optical path.

Two optical materials are conventionally connected without additional materials by wringing the two elements or connecting them by so-called "fusion bonding". Wringing is a connection of two optical materials, wherein the joined surfaces are held only by molecular attractive forces. Wringing is a "detachable" connection which can be partially or completely released (under the influence of moisture or wedge effects).

In fusion bonding, a connection which was previously wrung at one or more contact points is heated to several 100⁰C in a gas atmosphere in a furnace, in the extreme case up to just below the glass transition temperature of fused silica, thereby forming siloxane and silane compounds, and is subsequently cooled. This connection is far more solid than a mere wrung connection. The solidity generally increases with increasing maximum temperature and dwell time. However, the fused silica is thereby usually modified such that the optical performance can no longer be guaranteed. Moreover, the hydrogen contained in the fused silica is outgassed, which reduces the service life of optical components which are produced in accordance with such a method and are used in lithography optics, under irradiation with light of a wavelength range of between approximately 150nm and 250nm, e.g. 193nm. Conventional bonding methods moreover also generate bubbles in the joint which bear the risk of later delaminating.

Object of the invention

It is the object of the invention to provide a method of the above-mentioned type for permanently connecting the two elements without reducing their service life or impairing their optical properties, and an optical component produced in accordance with this method.

Summary of the invention

This object is achieved by a method wherein a hydrogen portion in the gas atmosphere is adjusted such that reduction of the hydrogen content of the first and/or second element(s) is substantially prevented, and by an optical component produced according to this method.

Introduction of a hydrogen portion into the gas atmosphere prevents reduction of the hydrogen content of the elements during heating due to outgassing of hydrogen. Such a reduction is detrimental, since the hydrogen content is usually adjusted during production of the individual elements such that maximum service life of the materials is ensured. The reduction of the hydrogen content could result in increased induced absorption, compaction and microchannels when the materials are irradiated with ultraviolet radiation, e.g. with wavelengths of 248 nm or 193 nm. Reduction of the original hydrogen content up to 25 %, in special cases up to 50 % is still to be considered as a substantially prevented reduction of the hydrogen content.

It is therefore very important to maintain the optical performance during the entire bonding process to obtain optimum performance during later operation, i.e. to maintain the initial optical properties and the service life of the optical materials, from which the elements are made. It is clear, that the inventive method permits connection also of more than two, in particular, of three or four elements.

In an advantageous variant, the hydrogen portion of the gas atmosphere is adjusted such that the hydrogen content of the first and/or second element(s) remains substantially constant during the method. The hydrogen portion of the gas atmosphere is selected such that the temperature-dependent diffusing out and doping of hydrogen during the method cancels out when the method is terminated, such that the hydrogen content in the elements remains substantially unchanged.

In a further variant, the hydrogen portion of the gas atmosphere is adjusted such that the hydrogen content of the first and/or second element(s) is increased during the method. This is advantageous, in particular, if the hydrogen content of the elements was selected to be smaller during production than is advantageous for their optimum performance and service life. In this case, the optimum hydrogen content is adjusted only during the bonding process of the two elements.

In a further development of this variant, the first and/or second element(s) substantially contain(s) no hydrogen before the method is started. In this variant, the complete hydrogen content of the elements is supplied to the two materials not until after termination of the phases of the connecting process which require a high temperature, which is advantageous, in particular, for materials having a low OH content. A hydrogen content smaller by a factor of 10 compared to a hydrogen content provided for a later application is defined as substantially hydrogen-free hydrogen content.

The manufacturer of such materials usually loads the hydrogen in a cold state, i.e. at low temperatures, to reduce formation of silane and siloxane compounds which increases with a low OH content. If a cold-loaded material of this type with a very high initial content would be subjected to the connecting process (referred to as bonding process in the following), part of the hydrogen would diffuse out and a further part of the hydrogen would react into SiH. The initial hydrogen content would have to be that high that 2 x 10¹⁶ molecules/cm³ are still left in the material after diffusing out. Since the high temperatures usually act on the optical material for several days, that part of the hydrogen which reacts to SiH would, even with this minimum amount of hydrogen due to the small OH content, still be that large that an unacceptable amount of SiH is formed. It is therefore more favourable to load hydrogen only during the bonding process.

In an advantageous further development, the hydrogen portion is added to the gas atmosphere during cooling, which is preferred when the temperature has fallen below 500⁰C. Adding the hydrogen portion at relatively low temperatures is advantageous, in particular, for optical materials having a small OH content (see above). Such materials have less compaction and less polarization-inducing birefringence than materials with a higher OH content. However, these materials generate more silane and siloxane compounds at high temperatures if they are loaded with hydrogen. Silane (SiH) is reversibly split under laser irradiation, wherein the decomposition products show a strong and broad-band absorption around 215 nm. This causes transmission hysteresis and possibly other undesired reactions of the optical materials to laser irradiation. For this reason, the hydrogen is loaded only during cooling i.e. after termination of that part of the bonding process which requires high temperatures, such that formation of silane and siloxane compounds can be reduced.

In an alternative further development, the hydrogen portion of the gas atmosphere is added after cooling. In this variant, too, the hydrogen is loaded only at a time when no further heating of the elements to high temperatures is required.

In a preferred variant, the gas atmosphere is under high pressure which facilitates doping of hydrogen into the elements. The furnace in which the bonding process is carried out, may preferably be an autoclave, i.e. a pressure container which can be sealed in an air-tight and vapor-tight manner.

In a further preferred variant, an inert gas is added to the gas atmosphere. The inert gas, being a further component of the gas atmosphere, keeps the materials to be connected free from contamination. In a particularly preferred variant, the first and second elements are wrung in a previous process step, preferably in a protective gas atmosphere. Through wringing, the elements are connected to each other already at the common contact point which facilitates the subsequent bonding process. Through the protective gas atmosphere, pre-cleaning with a solvent or through UV burning, it can be ensured that the contact points on the surfaces of the elements are free from contaminations which prevents formation of bubbles which could cause possible later delaminating.

In a further advantageous variant, the hydrogen portion of the gas atmosphere at normal pressure is higher than 5%, preferably higher than 9%. Such a hydrogen portion of the gas atmosphere is advantageous to effectively compensate for diffusion of hydrogen out of the fused silica through approximately identical doping. The hydrogen portion (partial pressure) in the gas atmosphere required for this purpose may be determined, like the temperature development, through finite element simulations from the required hydrogen content in the fused silica, the hydrogen homogeneity, the maximum admissible SiH content and the geometry of the parts.

In a particularly preferred variant, the temperature curves during heating and cooling are selected such that delaminating of the elements is reduced. Through suitable selection of the temperature curves, in particular, through introducing additional ramps and dwell times, water which forms bubbles in the material can be eliminated thereby preventing delaminating and improving the rigidity and impermeability of the connection.

In a preferred variant, the maximum dwell temperature, i.e. the temperature which is obtained between heating and cooling, is between 100⁰C and the transition temperature of the fused silica, preferably between 300⁰C and 1000⁰C, with particular preference between 300⁰C and 700⁰C, in particular between 300⁰C and 500⁰C. When this temperature has been reached, the two elements can bond at their contact points. In a further preferred variant, the maximum dwell temperature is maintained only for a short period such that, in particular, formation of silane and siloxane compounds can be reduced.

In one variant of the method, slight deformations of the elements are corrected in a subsequent process step through ion beam figuring or other finishing methods. Through surface or locally effective finishing, deformations of a few nm on the elements can still be corrected.

In a preferred variant, the chemical composition of the elements is monitored during the method e.g. by measuring the contamination on so-called getter plates (dummy plates of quartz) in the furnace used as dirt trap in the method, after termination of the process. This ensures that the chemical composition of the optical materials does not change.

The invention also relates to an optical component, in particular, a terminating element for a projection objective of a microlithography projection exposure apparatus, with two interconnected elements, at least one of which consists of fused silica, produced in accordance with the inventive method. The optical component consists of two elements which are normally both produced from fused silica to ensure a uniform temperature expansion coefficient of the optical component.

In a particularly preferred embodiment, the second element is a holder element for holding the first element. In this case, the first element serves as optical element, i.e. it is introduced into the optical path and transmits radiation in a defined manner while the second element meets a purely mechanical or shielding function and is at most hit by the scattered light. This provides a certain degree of freedom to optimize costs or improve the bonding result. The hydrogen content of e.g. the holder element is not important for the mechanical and thermo-mechanical properties or for the quality of the bonding connection. For this reason, rejected material formed during the fused silica production process can e.g. be used, which does not meet the specified hydrogen range and therefore may not be used in an optical application. The optical homogeneity and number of bubbles (in as far as bubbles are not directly adjacent to the bonding surface) are also not critical for the holder element. For this reason raw glass may be used which is obtained from the same synthesis process as the glass for optical elements, which is however, not further refined, e.g. formed or annealed, after sintering. These expensive processes are usually carried out subsequently to adjust the optical properties such as index homogeneity and stress birefringence and to eliminate bubbles.

In a preferred embodiment, the first and second elements consist of synthetic fused silica, preferably having the same OH content. The use of synthetic fused silica having the same OH content for both elements is based on the consideration that the OH content has a strong influence on the CTE (heat expansion coefficient) and the viscosity as a function of the temperature. The use of the same OH content ensures that the cooling rates act similarly on both elements, producing no or only little tension. If the second element is a holder element, the only "optical" requirement is basically that it does not release any easily diffusing substances such as e.g. Na⁺ to the optical element during the bonding process, which have a negative influence on the transmission of the optical element. For this reason, a synthetic fused silica is preferably used for the holding element, which has only little metallic impurities.

In an alternative embodiment, the first element consists of synthetic fused silica and the second element of technical fused silica. The use of technical quartz (i.e. from rock crystal or molten material from oscillator crystal production waste) for the holder element is advantageous if it is ensured that doping of metals into the optical element is limited to the region below the bonding surface, i.e. no impurities dope into the optical free diameter of the optically active element. Technical quartz is much cheaper and mechanically more stable than synthetic fused silica.

In a further particularly preferred embodiment, the first element has a OH content of less than 500 ppm, preferably less than 300 ppm, with particular preference less than 100 ppm. A low OH content reduces compaction and polarization-induced birefringence. Only the inventive method ensures that such a material can be bonded at the temperatures required for this purpose without reducing the service life of the optical component formed thereby.

In a further preferred embodiment, the first element has a hydrogen content in a range between 2 x 10¹⁵ and 5 x 10¹⁷ molecules/cm³, preferably between 15 x 10¹⁵ and 1 x 10¹⁷ molecules/cm³. This range ensures on the one hand that a hydrogen content in the optical element is not fallen below, which would produce increased induced absorption and compaction and on the other hand that the amount of hydrogen present is not too large thereby preventing increased SiH formation and rarefaction.

In an advantageous embodiment, the second element is doped with at least one element of the group consisting of rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, boron, and aluminium. Synthetic fused silica can be doped already during production through adding respective compounds to the synthesis flame or doping during sintering, e.g. with metals. To maximally reduce contamination of the first (optical) element during bonding, the alkaline metals of lower atomic number (Li, Na, K) should be avoided. Rb, Cs, alkaline earth metals or members of the third main group such as boron and aluminium are e.g. suited. Technical quartz may alternatively be used for the holder element, which is stabilized by the impurities (softening point technical = Homosil 1730⁰C, synthetic = Suprasil 1600⁰C, source: Broschϋre Quarzglas fur die Optik (leaflet fused silica for optics), Heraeus Quarzglas GmbH, Hanau 1994).

In a preferred embodiment, the second element has an OH content of more than 500 ppm, in particular of more than 1000 ppm. A soot process is required to produce fused silica with OH contents of less than 500 ppm. If this OH content is exceeded, fused silica produced in a direct deposition process may also be used instead of the soot process, which is generally less expensive than fused silica produced through a soot process. In the above case, the holder element has a substantially higher OH content than the optical element (typically < 500 ppm). OH-rich fused silica is considerably more viscous during heating than OH-poor glass. With a thick optical element and a holder element relatively thin-walled compared thereto, e.g. a support ring, the support ring may adapt to the optical element, i.e. it is not deformed or bent during bonding and cooling. SiOH groups moreover play a large role in bonding, since a SiOH group bonds more easily with Si-O-Si or Si-H through release of hydrogen compared to two directly bonding Si-O-Si chains and also since Si-OH groups can be directly bonded through hydrogen bridge bonding or covalent bonds in the form of Si-O- O-Si. If glass with < 300 ppm or even < 100 ppm has to be used for an optical element due to compaction or optical homogeneity, the support ring may have more OH groups thereby obtaining a higher solidity at the same bonding temperature, or obtaining a more solid connection at a lower temperature. In an alternative embodiment, the second element has a smaller OH content than the first element. For a thin optical element and a comparably relatively thick holder element, it may be desired that the holder element is rigid and is minimally deformed during bonding thereby always offering a very flat support surface to which the rather instable optical element adjusts.

If the optical component is a terminating element, the second element is formed as support ring and the first element as terminating plate or terminating lens. While the terminating plate or terminating lens serves as optical element, the support ring has a purely mechanical function. The above presented material types can be used for the latter which optimizes the bonding process and reduces the production costs of the terminating element.

The invention also concerns a projection objective in a microlithography projection exposure apparatus for imaging a structure on a light-sensitive substrate, with a terminating element of this type, and a microlithography projection exposure apparatus comprising such a projection objective, wherein an immersion liquid is disposed between the terminating element and the light- sensitive substrate.

The connections between two elements which form the terminating element of a projection objective for immersion lithography must meet particularly high demands, since the terminating element is directly exposed to the immersion liquid and glued joints would swell under the influence of water. With the use of the inventive method, a tight connection can be produced without using additional materials to ensure that these demands can be fulfilled. 50

10

Further features and advantages of the invention can be extracted from the following description of an embodiment of the invention, with reference to the figures of the drawing which show inventive details, and from the claims. The individual features can be realized individually or collectively in arbitrary combination in a variant of the invention.

Drawing

The schematic drawing shows one embodiment which is explained in the following description.

Fig. 1 shows an optical terminating element for a projection objective which consists of a terminating plate and a support ring in a furnace with a gas atmosphere; and

Fig. 2 shows a schematic illustration of the wafer-side end of a projection objective of a microlithography projection exposure apparatus with the terminating element of Fig. 1 which is in contact with an immersion liquid.

Fig. 1 shows a terminating plate 1 of fused silica as first element and a support ring 2 of fused silica as second element which are brought together in a furnace 3. The furnace 3 has a gas atmosphere 5 which comprises an inert gas to prevent contamination during the production of a connection between the support ring 2 and the terminating plate 1 (bonding process). Further gases may be supplied to or discharged from the furnace 3 during the bonding process, wherein the supply and discharge lines for the gases are not illustrated in Fig. 1. So-called getter plates, i.e. dummy plates of quartz, may also be introduced into the furnace 3. Impurities which form during the process are precipitated thereon. After termination of the process, these show the degree of contamination which permits later quality control, e.g. with respect to the transmission properties.

The support ring 2 is wrung to the terminating plate 1 at contact points 4 in a protective gas atmosphere before introduction into the furnace 3. The support ring 2 and the terminating plate 1 together form an optical terminating element 6 of a projection objective for microlithography as soon as these have been permanently connected by the bonding process. A terminating lens may e.g. be used as alternative to the terminating plate 1.

Two method variants for producing a solid connection between the support ring 2 and the terminating plate 1 using "fusion bonding" are described below. Which variant is used depends substantially on the OH content of fused silica of the terminating plate 1.

With fused silica having a high OH content (e.g. 280 ppm OH of weight), the hydrogen is usually introduced during the synthesis of the fused silica, i.e. in a hot state which can be realized e.g. through adjusting the flame stochiometry for directly deposited fused silica or during the soot process through sintering of the quartz powder in a hydrogen atmosphere. If the hydrogen is introduced in a hot state, a certain portion of silane (SiH) is automatically produced, i.e. the hydrogen reacts with the silicon of the glass matrix. OH thereby acts as getter i.e. the more OH groups a glass contains, the less SiH is formed with the same hydrogen content and identical thermal history.

A high OH content therefore reduces formation of silane. For this reason, the terminating plate 2 and the support ring 1 can be heated to high temperatures without the danger of excessive silane formation. In this case, one must merely prevent a reduction in the hydrogen content of the fused silica, since optical elements are provided with an optimum hydrogen content already during production.

Towards this end, the gas atmosphere 5 in the furnace 3 is adjusted such that it contains a permanent hydrogen portion of e.g. 10% (vol.) at one atmosphere. The hydrogen content of 10% is adjusted to the lithographical optical material used in the present case and can also assume other, smaller or larger values. One essential feature of the method is the partial pressure of the hydrogen, i.e. the absolute percentage of the hydrogen portion depends on the pressure in the gas atmosphere 5.

As an alternative to the use of a fixed hydrogen portion in the gas atmosphere 5, the hydrogen portion may also be adjusted to the temperature curves, the maximum temperature and the initial content of the introduced elements during the process. In the present case with a selected hydrogen portion of 10% in the gas atmosphere 5, hydrogen doping into and diffusing out of the support ring 2 and of the terminating plate 1 are balanced such that the hydrogen content of the terminating plate 1 set by the producer can substantially be maintained also during connection.

To perform the first variant of the method, the furnace 3 is heated from 20⁰C to a temperature of approximately 800⁰C. This temperature is kept only for a short time to reduce SiH formation. The furnace 3 is subsequently cooled, wherein the cooling rates are not too fast and the annealing time is optimized. The final element 6 produced during the bonding process has a hydrogen content of approximately 2 x 10¹⁶ molecules/cm³ at least in the terminating plate 1 which ensures that defects such as compaction or microchannels which can form in the fused silica during irradiation with ultraviolet laser light of e.g. 193 nm, are saturated, and at the same time occurrence of rarefaction and SiH formation is prevented.

With materials having a OH content of less than 100 ppm, e.g. 20 or 40 ppm, compaction and polarization-induced birefringence are considerably reduced compared to materials with a higher OH content. Such materials, however, usually require that the producers load the hydrogen in the cold state to prevent or reduce formation of SiH. These materials are thereby loaded with hydrogen under pressure at less than 500⁰C for several weeks after the final tension- releasing annealing.

To perform the second method variant for glasses with a lower OH content, the material for the terminating plate 1 is finely annealed by the producer in contrast to the first variant, but is delivered without being charged with hydrogen. The hydrogen content of the support ring 2 is not that important since it serves as holder element and meets only a mechanical function. The bonding process is carried out as described above, wherein the hydrogen portion of the gas atmosphere 5 is increased from 0% to 10% only during the cooling phase, at the earliest when the temperature has fallen below 500⁰C, by adding hydrogen to the purge gas. The furnace 3 may be a highly pressurized autoclave which facilitates loading with hydrogen.

In the second method variant, the loading of the producer which also depends on the geometry of the part to be loaded basically is carried through later on with the connected elements. In the second method variant, the hydrogen may be loaded alternatively only after cooling.

After taking the terminating element 6 out of the furnace 3, slight deformations (on a nm scale) which were generated during the bonding process, can be corrected on the surfaces using "ion beam figuring" or other final polishing methods.

The optical terminating element 6 joined in the above-described manner, is mounted to a projection objective 7 of a microlithography projection exposure apparatus shown in Fig. 2 which images a structure (mask), which is not illustrated, onto a light-sensitive substrate 8. With such exposure systems which are operated e.g. at a wavelength of 193 nm, it has proven to be advantageous to introduce an immersion liquid 9, e.g. water, between the final element 6 and the substrate 8. The larger numerical aperture produced in this manner permits imaging of smaller structures with the exposure apparatus than is possible with use of air or vacuum as medium between projection objective 7 and substrate 8.

The final element 6 is in contact with the immersion liquid 9 on the wafer side which poses particular requirements e.g. with regard to impermeability, to the connection between support ring 2 and terminating plate 1. These requirements are met by the connection of the terminating element 6 produced using one of the two above-described method variants. Further modifications and variants of the method in addition to the two above-described methods are, of course, possible, wherein the joint meets these requirements without using additional materials. Different optical materials may be used for the support ring 2 and the terminating plate 1 if these have a similar temperature expansion coefficient.

The fused silica material used in this case for the final plate 1 is generally synthetic fused silica with an OH content of less than 500 ppm, in particular, less than 300 ppm and with particular preference less than 100 ppm. The hydrogen content of the final plate 1 is typically in a region between 2 x 10¹⁵ and 5 x 10¹⁷ molecules/cm³, preferably between 15 x 10¹⁵ and 1 x 10¹⁷ molecules/cm³.

For the material used for the support ring 2 more freedom is offered since it is not used as optical element. The support ring may also consist of synthetic fused silica, wherein, in particular, the OH content corresponds to that of the terminating plate 1 such that its temperature expansion coefficient does not differ from that of the terminating plate 1.

One substantial requirement for the support ring 2 is that it does not discharge any diffusing substances such as e.g. Na⁺ ions to the terminating plate 1 or the immersion liquid 9. Synthetic fused silica is suited for this purpose, which generally has very few metallic impurities. Technical fused silica may alternatively also be used if doping can be kept that small that the substances do not dope into the optical free diameter of the final plate 1 which can be ensured e.g. through stabilizing the technical fused silica through certain metallic impurities.

The hydrogen content of the support ring 2 plays a minor role. Its OH content can either correspond to that of the terminating plate 1 or may be more or less. In the first case e.g. synthetic fused silica, which was produced in a direct precipitation process may be used which saves costs and optimizes the bonding process. In the latter case, if the terminating plate 1 is thin and the support ring 2 is relatively thick compared thereto, the latter may offer a stable and very flat support surface during the bonding process, to which the instable final plate 1 adjusts.

It is mainly essential that the hydrogen content set in the terminating plate 1 ensures a high optical performance and service life of the terminating element 6. In particular, if a terminating element 6 with a terminating plate 1 are used, which consist of a material which has only a small compaction and an OH content of less than 100 ppm, the loss in imaging contrast can be reduced and preservation of the polarization of the radiation can be improved.

Claims

1. Method for connecting two elements (1, 2) at least one of which consists of fused silica, wherein both elements (1, 2) are connected through fusion bonding in a gas atmosphere (5), characterized in that a hydrogen portion in the gas atmosphere (5) is adjusted such that reduction in the hydrogen content of the first and/or second element(s) (1; 2) is substantially prevented.

2. Method according to claim 1, wherein the hydrogen portion of the gas atmosphere (5) is adjusted such that the hydrogen content of the first and/or second element(s) (1, 2) remains substantially constant during the method.

3. Method according to claim 1, wherein the hydrogen portion of the gas atmosphere (5) is adjusted such that the hydrogen content of the first and/or second element(s) (1, 2) is increased during the method.

4. Method according to claim 3, wherein the first and/or the second element(s) (1, 2) are substantially free from hydrogen before performing the method.

5. Method according to claim 3 or 4, wherein the hydrogen portion is added to the gas atmosphere (5) during cooling, preferably when the temperature drops below 500⁰C.

6. Method according to claim 3 or 4, wherein the hydrogen portion of the gas atmosphere (5) is added after cooling.

7. Method according to any one of the preceding claims, wherein the gas atmosphere (5) is under high pressure.

8. Method according to any one of the preceding claims, wherein the gas atmosphere (5) comprises an inert gas.

9. Method according to any one of the preceding claims, wherein in a preceding method step, both elements (1, 2) are wrung, preferably in a protective gas atmosphere.

10. Method according to any one of the preceding claims, wherein the hydrogen portion of the gas atmosphere (5) at normal pressure is more than 5%, preferably more than 9%.

11. Method according to any one of the preceding claims, wherein the temperature curves during heating and cooling are selected such that delaminating of the elements (1, 2) is reduced.

12. Method according to any one of the preceding claims, wherein the maximum dwell temperature is between 100⁰C and the transition temperature of the fused silica, preferably between 300⁰C and 1000⁰C, particularly preferred between 300⁰C and 700⁰C, in particular, between 300⁰C and 500⁰C.

13. Method according to any one of the preceding claims, wherein the maximum dwell temperature is maintained only for a short time period.

14. Method according to any one of the preceding claims, wherein, in a subsequent method step, slight deformation of the elements (1, 2) is corrected through ion beam figuring or other finishing methods.

15. Method according to any one of the preceding claims, wherein the chemical composition of the elements (1, 2) is controlled during the method.

16. Optical component, in particular, terminating element (6) for a projection objective (7) of a microlithography projection exposure apparatus, with two interconnected elements (1, 2) at least one of which consists of fused silica, produced according to the method according to any one of the preceding claims.

17. Optical component according to claim 16, wherein the second element (2) is a holder element for holding the first element (1).

18. Optical component according to claim 16 or 17, wherein the first and second elements (1, 2) consist of synthetic fused silica, preferably having the same OH content.

19. Optical component according to claim 16 or 17, wherein the first element (1) consists of synthetic fused silica and the second element (2) consists of technical fused silica.

20. Optical component according to any one of the claims 16 through 19, wherein the first element (1) has an OH content of less than 500 ppm, preferably less than 300 ppm, with particular preference less than 100 ppm.

21. Optical component according to any one of the claims 16 through 20, wherein the first element (1) has a hydrogen content in a range between 2 x 10¹⁵ and 5 x 10¹⁷ molecules/cm³, preferably between 15 x 10¹⁵ and 1 x 10¹⁷ molecules/cm³.

22. Optical component according to any one of the claims 16 through 21, wherein the second element (2) is doped with at least one element of the group which consists of rubidium, caesium, beryllium, magnesium, calcium, strontium, barium, boron and aluminium.

23. Optical component according to any one of the claims 16 through 22, wherein the second element (2) has an OH content of more than 500 ppm, in particular, of more than 1000 ppm.

24. Optical component according to any one of the claims 16 through 22, wherein the second element (2) has a smaller OH content than the first element

(I)-

25. Terminating element (6) according to any one of the claims 16 through 24, characterized in that the second element (2) is a support ring and the first element (1) is a terminating plate or terminating lens.

26. Projection objective (7) in a microlithography projection exposure apparatus for imaging a structure on a light-sensitive substrate (8) having a terminating element (6) according to claim 25.

27. Microlithography projection exposure apparatus with a projection objective (7) according to claim 26, wherein an immersion liquid (9) is disposed between the terminating element (6) and the light-sensitive substrate (8).