WO2015120977A1

WO2015120977A1 - Bearing element and system for mounting an optical element

Info

Publication number: WO2015120977A1
Application number: PCT/EP2015/000290
Authority: WO
Inventors: Jens PROCHNAU; Dirk Schaffer
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2014-02-14
Filing date: 2015-02-11
Publication date: 2015-08-20
Also published as: DE102014202737A1

Abstract

A bearing element for a bearing system for mounting an optical element, in particular in a lighting device for a microlithographic EUV projection exposure apparatus, comprising a bipod unit with at least one carrier unit (48, 50) that extends between a first coupling end (52) and a second coupling end (56). The carrier unit comprises a support element (66) and at least one function element (64). The function element (64) is a thermal conducting element (65), wherein the support element (66) and the thermal conducting element (65) have different coefficients of thermal conductivity in an operating temperature range, such that the thermal conducting element (65) has an predominant thermal conducting function in comparison to the support element (66). The support element (66) is supported by the thermal conducting element (65), in order to mechanically reinforce the thermal conducting element (65). The thermal conducting element (65) comprises two joints in the form of flexural elements (60).

Description

BEARING SYSTEM AND SYSTEM FOR STORING AN OPTICAL ELEMENT

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a bearing element for a bearing system for supporting an optical element, with a) a first coupling end, to which it can be coupled to the optical element; b) a second coupling end, on which it can be coupled to a support structure; wherein c) the bearing element comprises a bipod unit with at least one support unit, which extends between the first coupling end and the second coupling end; d) the support unit comprises a support element, which predominantly has a support function, and at least one functional element which, in comparison to the support element predominantly has a different function than a support function.

In addition, the invention relates to a system for storing an optical element in semiconductor clean rooms or in vacuum, in particular in a lighting device or a projection lens for a microlithographic EUV projection exposure apparatus, with at least one bearing element, each at a first coupling end with the optical element and at a two-

CONFIRMATION COPY coupling end with a support structure is coupled. 2. Description of the Related Art

By means of microlithographic projection exposure equipment, structures arranged on a mask are transferred to a photosensitive layer such as a photoresist or the like. For this purpose, the projection exposure apparatus comprises an illumination device with a light source and an illumination system, which processes the projection light generated by the light source and directs it to the mask. The illuminated by the illumination device mask is imaged by a projection lens on the photosensitive layer.

The shorter the wavelength of the projection light, the smaller the structures can be defined on the photosensitive layer by means of the projection exposure system. For this reason, projection light in the extreme ultraviolet spectral range, so-called EUV radiation, whose mean wavelength is 13.5 nm is increasingly used today. Such projection exposure systems are therefore often referred to simply as EUV projection exposure systems.

Since there are no optical materials having sufficiently high transmittance for such short wavelengths, an EUV projection exposure apparatus comprises reflecting optical elements in the form of mirrors. By means of the mirrors arranged in the illumination device of the EUV projection exposure apparatus, the projection light is directed onto the mask and these are illuminated. With the help of the mirror of the associated projection lens, the illuminated mask is correspondingly imaged onto the photosensitive layer. To accomplish this with the required accuracy, the mirrors must be precisely aligned in all six degrees of freedom.

For a precise alignment and storage of mirrors in a projection lens, hexapod systems are known, among others, in which an optical element is mounted on a support structure by means of three bipods. The bipods comprise two of the above-mentioned support units and usually have at the same time during the storage of the optical element at least the task of exercising at least one additional function in addition to the support function. This primarily includes the further function of dissipating heat from the optical element to the support structure, from where the heat can finally be dissipated. But also an electrical insulation between the support structure and the optical element or its cooling may be desired or required as a further function of the bipods.

The interior of an EUV lighting device or an EUV projection lens is usually evacuated, in particular, there is a high vacuum. For this reason - and in principle, when a lighting device is used in a semiconductor clean room - the use of drive means for changing the position of a mirror, e.g. in the form of actuators, micrometer screws or differential threading device, not or only very limited possible. In the case of actuators, complicated encapsulations are generally necessary in order to prevent the outgassing of actuator materials. Again, hexapod systems are used for precise alignment and storage of mirrors.

The support structure is usually stationary in the housing of the illumination device or the projection lens installed and may also be formed by the housing or a frame of the lighting device or the projection lens itself.

In particular, in EUV systems prevail higher operating temperatures, which is why it can cause temperature changes during operation on the bipods, which can cause a change in length of the support units of the bipods due to a thermal expansion of the bipod material. In addition, changes in length on the bipods can also lead to rigid body displacements and to deformations of the optical element.

Overall, this changes the position and orientation of the optical element coupled to the bipod in space, whereby the required exact alignment of the components is no longer guaranteed.

The extent of the change in length of the bipods is significantly dependent on the material of the bipods and its coefficient of thermal expansion coefficients, which is hereinafter also referred to as CTE (Coefficient of Thermal Expansion) and should take into account both the coefficient of linear expansion and the expansion coefficients.

There are attempts to fabricate the bipods mainly from a low CTE ceramic material in the operating temperature range of the projection exposure equipment. Only bending areas of the bipods are made of a metallic material. That is, in this case, the support unit between the first and the second coupling end of the bearing element is made of such ceramic material and connected via metallic bending elements at opposite ends to the coupling ends of the bearing element. Due to the poor thermal conductivity of ceramic materials, however, no or only a very small amount of heat can be dissipated from the optical element to the support structure.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a bearing element and a system of the type mentioned, which take into account this idea.

This object is achieved in a bearing element of the type mentioned above in that e) the functional element is a heat conducting element and the

Support element and the heat-conducting in an operating temperature range have such different thermal conductivity coefficients that the heat-conducting element in comparison to the support element predominantly has a heat conducting function; f) the support member is supported by the heat conducting member to mechanically strengthen the heat conducting member; g) the heat-conducting element has two joints in the form of bending elements; h) the heat conducting element of a metal material, in particular of an alloy of 64% iron and 36% nickel, and the support member of a ceramic material, in particular Zerodur® or a cordierite is.

As in the case of the bipods explained and established at the outset, the bearing element comprises a bipod unit with a first support unit and a second support unit. The invention Relying on the recognition that different tasks to be performed by the support unit can be decoupled from each other and can be effected by different components, without this adversely affecting the precise alignment and storage of the optical element.

The heat-conducting element carries the support element, so that the support element forms a kind of mechanical reinforcement of the heat-conducting element.

As mentioned above, it is particularly important to dissipate heat from the optical element. Therefore, the functional element is a heat-conducting element, wherein the support element and the heat-conducting element have such different thermal conductivity coefficients in an operating temperature range that the heat-conducting element predominantly has a heat-conducting function compared to the support element. This opens up the possibility of reducing a temperature-dependent change in length in particular of the support unit of the bearing element and at the same time to ensure that a sufficient amount of heat can be derived from the optical element.

In known bearing elements, the dissipation of heat from the optical element is simultaneously effected by the support element. However, it is desirable that the support element learns as possible no changes in length with temperature changes. Although this can be achieved if the support element is made of materials with a small CTE. Then, however, the thermal conductivity is usually low. According to the invention can now be selected for the support element such a material, since the heat transfer is ensured independently by the heat conducting element.

It has proved to be particularly effective when the heat conducting element of a metal material, in particular of an alloy of 64% iron and 36% nickel, and the support member is of a ceramic material, in particular Zerodur® or a cordierite.

In particular, in the use of ceramic materials for the support element, the bearing element is also less sensitive to magnetic fields, so that deformations by magnetostriction can turn out lower.

The support element and the heat-conducting element preferably differ in their properties relevant to the function at least by a factor of 5. Relevant parameters are in particular the rigidity, the modulus of elasticity, the electrical resistance, the thermal resistance or the thermal conductivity.

It is favorable if the support element encompasses a bridge part which extends between at least one first attachment region and a second attachment region of the bearing element.

For an effective function, it is advantageous if the heat-conducting element provides the first and the second fastening region for the bridge part.

The support element and the heat-conducting element preferably form a parallel structure.

The positional stability of the optical element can be improved if there are decoupling means by which changes in the length and / or the volume of the support element or of the heat-conducting element are decoupled from the respective other element. In particular, should in the case of a Wärmeleitelements length changes produced there do not affect the support element, which mainly the location and orientation of the optical element.

This can advantageously be effected by the decoupling means being formed by hinge regions of the support element or of the heat-conducting element.

It is also beneficial if there are coolant to cool the bearing element.

The cooling means are preferably provided by cooling ribs on the support element. Alternatively, active cooling by means of a coolant can also be provided.

In a system for supporting an optical element of the type mentioned in the introduction, the above-mentioned object is achieved in that the at least one bearing element according to one of claims 1 to 9 is formed. Consequently, the bearing element has some or all of the features explained above with the advantages mentioned above.

It is particularly advantageous if three Hexapod forming bipod units are present.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in more detail with reference to the drawings. In these show:

FIG. 1 shows a perspective view of a schematic microlithographic EUV projection exposure apparatus with a lighting unit and a projection lens; FIG. 2 shows a partial section of the EUV projection exposure apparatus of FIG. 1, wherein the illumination unit schematically shows two storage devices each carrying a mirror;

Figure 3 is a schematic perspective view of a

Hexapod formed bearing device of Figure 2 with three bipod units;

FIG. 4 shows a perspective detailed view of a bipod unit with two carrying units, each comprising a support element and a heat-conducting element;

FIG. 5 a perspective detail view of the bipod unit without support elements;

6 shows a perspective detail view of a modified bipod unit with cooling means.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically shows a microlithographic EUV projection exposure apparatus 10, which comprises an illumination device 12 and a projection objective 14.

With the projection objective 14, reflecting structures 16 arranged on a mask 18 are transferred to a photosensitive layer 20. The photosensitive layer 20 is usually a photoresist and is located on a wafer 22 or other substrate.

For transmitting the reflective structures 16 of the mask 18 to the photosensitive layer 20, the mask 18 is illuminated by the illumination device 12 with EUV radiation 24 illuminated. The illumination device 12 generates EUV radiation, which in the present exemplary embodiment has a center wavelength of 13.5 nm and a spectral half-width of approximately 1%, so that the majority of the EUV radiation leaving the illumination device 24 has wavelengths between 13.36 nm and 13.64 nm.

The illumination device 12 lights on the underside of the mask 18 from a stationary field 26, which corresponds to a ring segment in the present embodiment. The projection lens 14 generates on the wafer 22 a reduced image 28 of the structures 16 which are illuminated on the mask 18 in the field 26.

The projection objective 14 is designed for a scanning operation in which the mask 18 and the wafer 22 are moved in opposite directions in a manner known per se with speeds predetermined by the magnification of the projection objective 14. This is indicated in FIGS. 1 and 2 by the arrows PI and P2.

The illumination device 12 comprises a housing 30, in which a light source 32 to be recognized in FIG. 2 is arranged for the EUV radiation 24. The EUV radiation 24 generated by the light source 32 is projected onto the mask 18 by means of optical elements in the form of mirrors 34. In the highly schematic representation of FIG. 2, only two mirrors 34 and no intermediate focus are shown.

Each mirror 34 is connected to a bearing means 36 associated with it, which is arranged in the housing 30 and serves to align the respective mirror 34 in space and to hold the EUV radiation 24 with the required accuracy on the field 26 for illumination the mask 18 hits. Corresponding mirrors 34, each with a bearing device 36, are also located in the projection objective 14. By means of the bearing devices 36, the mirrors 34 of the projection objective 14 are aligned in space such that the EUV radiation 24 strikes the wafer 22 with the required accuracy. Accordingly, the following explanations apply to mirrors 34 and bearing devices 36 of both the illumination device 12 and the projection objective 14.

Such a bearing device 36 is shown very schematically in FIG. 3 and comprises a support structure 38, which in the present exemplary embodiment is designed as a base plate 40. The optical element, i. In the present case, the mirror 34 is coupled to the base plate 40 via bearing elements 42. The bearing elements 42 are formed as a bipod unit 44, three of which define a hexapod system 46, which is thus supported by the support structure 38. The three bipod units 44 are preferably of identical construction and are preferably positioned at the vertices of an equilateral triangle.

These Bipodeinheiten 44 in turn each comprise a first support unit 48 and a second support unit 50, which are connected at one end via a first coupling end 52 acting as a common bearing socket 54 to a V-shaped arrangement and with the mirror 34. Thus, the support units 48, 50 each include the bearing pedestal 54 first

Coupling end 52 and form a leg of the "V".

At a distance from the bearing base 54 second coupling end 56, the support units 48, 50 are each coupled to the base plate 38. The respective second coupling end 56 is shown in FIGS. 4 to 6 in the form of a connection socket 58 to recognize.

The bearing elements 42 in the form of bipod units 44 will now be described first with reference to FIGS. 4 and 5 and with reference to the first carrying unit 48. The statements made above apply mutatis mutandis to the second support unit 50 accordingly.

As can be seen in Figures 4 and 5, the support unit 48 extends between the first coupling end 52 and its associated second coupling end 56 and is movably connected to each coupling end 52 and 56 via joints in the form of bending elements 60 so that there in each case a bending region 62 of the bipod unit 44 is defined.

The support unit 48 comprises a functional element 64 and a support element 66. In an operating temperature range, these have different thermal conductivity coefficients such that the functional element 64 is a heat-conducting element 65 and predominantly has a heat-conducting function compared to the support element 66.

For this purpose, the heat-conducting element 65 is formed from a metal material. It is present as a metallic Wärmeleitarm 68, which has the shape of a metal strip 70 in the present embodiment.

As the metal material, in particular a known under the brand name Invar® iron-nickel alloy with 64% iron and 36% nickel or alloys based thereon are also suitable. The latter can, for example, have a cobalt content.

Accordingly, the support element 66 has predominantly a support function in comparison to the heat-conducting element 65. The support element 66 is made of a ceramic material, whereby in particular so-called "zero-CTE ceramics" as the glass-ceramic Zerodur® or cordierite come into question, which have a particularly small coefficient of thermal expansion. In the present embodiment, the guide arm 68 carries the support member 66. The flexures 60 to the bearing base 54 and the connection base 58 are connected to the guide arm 68. The support member 66 as it forms a mechanical reinforcement of the heat-conducting element 65 in the form of the Leitarms 68; the heat-conducting element has a smaller modulus of elasticity than the supporting element 66 in addition to the larger coefficient of thermal expansion.

In the case of known bearing elements, such a reinforcement is lacking and only the functional element 64 acting here as the heat-conducting element 65 is present, which consequently also alone provides the support function for the optical element 34.

In the present embodiment, the support member 66 includes a bridge portion 72 which extends between a first attachment portion 74 and a second attachment portion 76 of the bearing member 42. These attachment areas 74, 76 are provided by the guide arm 68. There, the Leitarm 68 and the bridge portion 72 can be secured by screws, bonding or clamping together. In a modification, the support element 66 can also carry the functional element 64, the latter then being fastened to the support element 66 in a corresponding manner.

The longitudinal axes of the Leitarms 68 and the bridge portion 72 run in the same direction, so that the heat-conducting element 65th and the support element 66 as a whole form a parallel structure 78. Apart from the geometry in the attachment regions 74 and 76, the first and the support element 64, 66 thus extend substantially parallel.

In operation, depending on the prevailing temperature and the respective thermal expansion coefficients of the components, the length and / or the volume of the heat-conducting element 64 and of the support element 66 varies to varying degrees. If the force acting on the attachment portions 74 and 76 by such changes in length and / or volume is too great, mechanical breakage may occur between the support member 66 and the heat conducting member 65 at the attachment portions 74, 76. In order to prevent this, only decoupling means 80 shown in FIG. 5 are present, by means of which changes in the length and / or the volume of the support element 66 or of the heat-conducting element 64 are decoupled from the respective other element.

For this purpose, the heat-conducting element 65 comprises two hinge regions 82, which enable its deflection in the radial direction with respect to the longitudinal axis of the heat-conducting element 65, which is indicated in FIG. 5 as a dashed line without reference symbols. The hinge portions 82 are specifically generated in the metal strip 70 by radial slots 84 which are machined from opposite directions radially outward in the metal strip 70, so that at lateral edge regions of the opposite edges of the metal strip 70 remains only in the joint areas 82 material.

When the metal strip 70 now extends in the direction between the attachment regions 74, 76, the hinge regions 82 can each migrate into the direction away from the slots 84, so that the metal strip 70 can rotate without the distance between the attachment regions. chen 74, 76 changes.

In this training, with a material combination of Invar® with 64% iron and 36% nickel for the heat-conducting element

65 and Zerodur® for the support member 66 a thermal expansion of the functional units 48, 50 can be achieved, which is about 60% less than without the reinforcement by the support member

66 made of Zerodur®.

FIG. 6 now shows a modified bearing element 42, in which coolant 86 is additionally present in order to cool it. In the embodiment shown in Figure 6, the coolant 86 are designed as passive cooling. For this purpose, the support member 66 carries a plurality of cooling fins 88, of which only two bear a reference numeral.

Alternatively, the cooling of the bearing elements 42 can also be effected by an active cooling device with the aid of a cooling medium.

Claims

Patent claims

1. Storage element for a storage system for storing an optical element, with a) a first coupling end (52) at which it can be coupled to the optical element (34); b) a second coupling end (56), at which it can be coupled to a support structure (38); wherein c) the bearing element (42) comprises a bipod unit (44) with at least one support unit (48, 50) which extends between the first coupling end (52) and the second coupling end (56); d) the support unit (48, 50) comprises a support element (66), which predominantly has a support function, and at least one functional element (64), which, in comparison to the support element (66), predominantly has a function other than a support function, characterized that e) the functional element (64) is a heat-conducting element (65) and the support element (66) and the heat-conducting element (65) have such different thermal conductivity coefficients in an operating temperature range that the heat-conducting element (65) predominantly compared to the support element (66). has a heat conduction function; f) the support element (66) is carried by the heat-conducting element (65) in order to mechanically reinforce the heat-conducting element (65); g) the heat-conducting element (65) has two joints in the form of bending elements (60); h) the heat-conducting element (65) is made of a metal material, in particular an alloy of 64% iron and 36% nickel, and the support element (66) is made of a ceramic material, in particular Zerodur® or a cordierite.

Bearing element according to claim 1, characterized in that the supporting element (66) and the heat-conducting element (65) differ in their properties relevant to their function, in particular the stiffness, the modulus of elasticity, the electrical resistance, the thermal resistance or the thermal conductivity, at least by Differentiate by factor 5.

Bearing element according to claim 1 or 2, characterized in that the support element (66) comprises a bridge part (72) which extends between at least a first fastening region (74) and a second fastening region (76) of the bearing element (42).

Bearing element according to claim 3, characterized in that the heat-conducting element (65) provides the first and second fastening areas (74, 76) for the bridge part (72).

Bearing element according to one of claims 1 to 4, characterized in that the support element (66) and the heat-conducting element (65) form a parallel structure (78).

6. Bearing element according to one of claims 1 to 5, characterized in that decoupling means (80) are present, through which changes in the length and / or the volume of the support element (66) or the heat-conducting element (65) from the other element (65 , 66) are decoupled.

7. Bearing element according to claim 6, characterized in that the decoupling means (80) are formed by joint regions (82) of the support element (66) or the functional element (64).

8. Bearing element according to one of claims 1 to 7, characterized in that coolant (86) is present to cool the bearing element (42).

9. Bearing element according to claim 8, characterized in that the coolant (86) is provided by cooling fins (88) on the support element (66).

10. System for storing an optical element (34) in

Semiconductor clean rooms or in a vacuum, in particular in a lighting device or a projection lens for a microlithographic EUV projection exposure system, with at least one bearing element (42), which is each connected to the optical element (34) at a first coupling end (52) and at a second coupling end (56) can be coupled to a support structure (38), characterized in that the at least one bearing element (42) according to one of Claims 1 to 9 is designed.

System according to claim 10, characterized in that there are three bipod units (44) forming a hexapod.