Patent Grant
9436101
The disclosure relates to an optical arrangement, as well as a microlithographic projection exposure apparatus including such an arrangement.
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (=reticle) is illuminated via the illumination system, and is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range (wavelengths of for example about 13 nm or about 7 nm) due to the general lack of availability of suitable translucent refractive materials. Particularly in high-aperture projection objectives (for example with values of the numerical aperture of greater than 0.4), a small spacing (for example in the range of 20-50 mm) is present between the mirror arranged closest to the wafer and the wafer so that this mirror is comparatively thin (that is to say with low values for the ratio of thickness to diameter of for example 10 or more).
With thin mirrors of that kind, unwanted surface deformation phenomena can occur, for example due to the force of gravity or caused by mounting or coating effects. Mirror deformation phenomena caused by parasitic moments or forces or moments or forces which disturb static definiteness can be, for example, 50% of the surface deformation which is allowed for the mirrors in total.
Increased mirror dimensions, involved with increasing numerical apertures, to comply with the desired specifications, involve increasing natural frequencies to rigidly maintain the positions of the optical elements relative to each other even upon the occurrence of external vibration and to afford sufficient stiffness in relation to parasitic and mass-dependent moments or interference forces which are generally produced by screwing the mirrors after polishing.
For mirrors in positions other than the position that is last at the image plane side in the projection objective, when relatively large mirror diameters are involved, the attainable natural frequency is often limited independently of the mirror thickness. In particular for example natural frequencies which have been attained in an EUV projection objective equipped with six mirrors may not be adequate to sufficiently suppress interference influences due to external vibration to achieve the desired overlay specifications.
A large number of approaches using decoupling elements is known for resolving issues arising from mechanical stresses to optical elements such as mirrors or lenses.
It is known to provide decoupling elements in a kinematic three-point arrangement to achieve good reproducibility of the position by virtue of clearly defining the plane by three points.
WO 2005/054953 A2 discloses among other things a holding arrangement for an optical element in a projection objective. Between a holder and the optical element is a reinforcing element whose coefficient of thermal expansion substantially corresponds to that of the optical element.
US 2009/0122428 A1 discloses among other things optical elements of a projection exposure apparatus which are designed so that thermally induced deformation phenomena are reduced, for which purpose among other things a correction plate provided with a ribbed structure is also applied to the rear side of a mirror.
The disclosure provides an optical arrangement in a microlithographic projection exposure apparatus, which even with higher numerical apertures reduces or avoids detrimental effects with respect to the imaging properties due to parasitic forces or moments occurring at the optical elements.
In accordance with an aspect of the disclosure an optical arrangement in a microlithographic projection exposure apparatus includes:
In accordance with a first aspect the disclosure is based on the concept of producing decoupling elements used for decoupling the optical element from the surrounding carrier structure monolithically, that is to say from one and the same block of material as the optical element or its optically effective surface (for example a glass material such as quartz glass (SiO2), Zerodur®, ULE® or Clearceram®, that is to say applying the decoupling elements to the optical element precisely not in a separate joining step or using any connecting technology (for example using chemicals, adhesives, clamping connections etc). Consequently the shrinkage or stressing effects involved for example with adhesive connections and hardening thereof, as well as also instabilities in the connection under vacuum conditions, can be avoided.
The support element provides an interface between the optical element and a carrier structure of the projection exposure apparatus, wherein no stiffness is transmitted between the support element and the optical element, as a consequence of the three-point decoupling effect.
In accordance with the disclosure the potential increased complication and expenditure with respect to production technology, which is involved with a monolithic configuration for the decoupling elements, is deliberately accepted to reduce parasitic moments or forces and to take account the stiffness achieved in spite of large mirror diameters or small mirror thicknesses.
Thus in the arrangement according to the disclosure it is possible to provide a mirror that is sufficiently thin but nonetheless of sufficient stiffness. Joining technologies, such as, for example, adhesive and stress forces are unnecessary due to the monolithic configuration.
In accordance with the above aspect the decoupling elements are also formed monolithically with the support element so that no joining technologies also have to be applied between the decoupling elements and the support element and the application of stresses is also eliminated in that respect. The disclosure however is not limited to that configuration. Rather, the disclosure is also to be intended to embrace embodiments, in particular of a microlithographic projection exposure apparatus for EUV lithography, in which there is a monolithic configuration only between the optical element and the decoupling elements. Joining between the decoupling elements and the support element can be achieved via a separate joining technology.
In an embodiment the support element is connected to a carrier structure of the microlithographic projection exposure apparatus via mounting elements. The mounting elements are displaced in a peripheral direction with respect to the decoupling elements. Improved decoupling of the optical element (for example a mirror) from the carrier structure is achieved as a consequence of the displacement of the mounting elements forming the connection between the support element and the carrier structure, with respect to the decoupling elements providing the connection between the optical element and the support element.
In that way it is possible to achieve a further improvement in decoupling of parasitic holder influences insofar as the mounting elements or the coupling positions of the carrier structure are arranged at an increased or preferably maximum spacing from the joining locations or decoupling elements producing contact with the optical element. The maximum distance between the mechanical fixing to the carrier structure on the one hand and the decoupling elements or joining locations on the other hand affords a comparatively long or maximum travel distance and thus a relatively great suppression factor with respect to the transmission of any inevitable stresses from the mechanical connections between the carrier structure and the support element to the optical element.
That advantageous effect is also afforded if the connection between the optical element and the support element is not effected by monolithic decoupling elements, but by way of some other joining locations, for example when as described hereinafter the provision of decoupling elements which are monolithic (with the optical element) is not possible because of the mirror position.
Thus in a further aspect of the disclosure an optical arrangement of a microlithographic projection exposure apparatus includes:
In an embodiment the decoupling elements or the joining locations are respectively arranged at a substantially constant spacing from each other in the peripheral direction. Moreover in an embodiment the mounting elements can be respectively arranged at a substantially constant spacing from each other in the peripheral direction relative to the decoupling elements or the joining locations. That makes it possible to achieve a maximum spacing between the coupling positions of the carrier structure from the joining locations or decoupling elements making a contact with the optical element.
In an embodiment of the foregoing approaches the decoupling elements or the joining locations are respectively displaced relative to each other in a peripheral direction through an angle of 120°±30°, such as 120°±20°, for example 120°±10°. In addition preferably the mounting elements are respectively displaced in a peripheral direction through an angle of 60°±20°, such as 60°±10°, for example 60°±3°, with respect to the decoupling elements or with respect to the joining locations.
In an embodiment the support element is connected to the optical element by way of precisely three decoupling elements or joining locations. Such a configuration is found to be particularly advantageous insofar as a further increase in the number of decoupling elements or joining locations does not lead necessarily to an improvement in the stiffness properties or the natural frequencies achieved, but the deformation phenomena can increase due to parasitic moments in a detrimental fashion.
In an embodiment the mounting elements are configured so that they provide a statically defined mounting for the optical element (in which respect there are in particular no superfluous forces for holding the optical element in its position). For that purpose three bipods can be provided for example as mounting elements in per se known manner. Alternatively there can also be any other mounting arrangement resulting in static location of the optical element, for example a hexapod mounting configuration.
In an embodiment the optical element and the support element are made from materials with mutually different coefficients of thermal expansion. In that way it is possible to take account of different rises in temperature of the optical element and the support element, as can occur in operation under EUV conditions.
In a further embodiment the optical element and the support element are made from the same material, in particular quartz glass (SiO2), ULE®, Zerodur® or Clearceram®, whereby inter alia thermally induced deformation can be minimised.
In an embodiment the optical element is a mirror. The disclosure however is not restricted thereto. Rather, the term ‘optical element’ in the present application is basically used to embrace any optical elements, for example also refractive elements such as for example lenses or diffractive elements such as optical gratings.
In an embodiment the support element has at least one slot whereby (particularly with a tangential arrangement of a plurality of slots) additional flexibility is provided in the radial direction and at least partial decoupling in the radial direction can be implemented, which can be advantageous in particular in the case of temperature differences between the support element and the optical element or mirror.
In an embodiment the support element is arranged on an outside of the optical element, that is to say for example in the form of an outer ring with respect to the optical element.
In a further embodiment the support element can also be arranged on the surface of the optical element, facing away from an optically effective surface. That has the advantage that the support element can be produced with smaller outside dimensions (in the case of being in the form of a ring therefore for example with a smaller radius), whereby (glass) material is saved and weight is reduced. In addition in that way the support element can be provided with a higher degree of stiffness.
In an embodiment the optical element has a ratio of maximum diameter to maximum thickness of at least 5, in particular at least 10, in particular at least 15, further particularly at least 20, (wherein the diameter is respectively related to the surface which is optically used or acted upon by light). With such small thicknesses the disclosure is particularly advantageous insofar as the stiffness which can be achieved in accordance with the disclosure at small thicknesses is particularly useful.
In an embodiment the optical element has a central hole.
In an embodiment the optical element has measures for reducing the weight. In that case the edge portion can be increased for stiffening up the optical element in the direction of the optical axis of the system (that is to say in the direction of the thickness), whereby stiffness is not substantially reduced, with lower weight.
The disclosure further concerns a microlithographic projection exposure apparatus for EUV lithography including an illumination system and a projection objective, wherein the illumination system and/or the projection objective have an optical arrangement according to the disclosure having the above-described features. In an embodiment the optical element can be the mirror of the projection objective, such as the last optical element in the optical beam path.
In a further embodiment the optical element can be the mirror of the projection objective that is arranged most closely to the wafer. In addition the projection objective can have a numerical aperture of at least 0.3, in particular at least 0.4, in particular at least 0.45, further particularly at least 0.5. In that situation of use the disclosure is particularly advantageous insofar as—as discussed in the opening part of this specification—precisely in relation to comparatively high numerical apertures, in consideration of the desired small spacing between the mirror that is last at the image plane side and the wafer, the mirror is to be comparatively thin so that the stiffness which can be achieved in accordance with the disclosure with small thicknesses is particularly useful.
In addition the disclosure provides an optical arrangement in a microlithographic projection exposure apparatus including
The microlithographic projection exposure apparatus can be designed in particular for a working wavelength of less than 15 nm.
Further configurations of the disclosure are to be found in the description and the appendant claims.
The disclosure is described in greater detail hereinafter via embodiments by way of example illustrated in the accompanying drawings, in which:
A first embodiment of the disclosure is further described with reference to
Referring to
The arrangement 100 further includes a support element 120 which in the embodiment of
Although the embodiment illustrated shows a substantially round mirror the disclosure is not restricted thereto. Rather the mirror in this embodiment as in the further embodiments can also be of any other for example elliptical, rectangular, reniform or triangular geometry.
The decoupling elements 131-133 are sufficiently soft or elastic so that no transmission of deformation from the support element 120 to the optical element 110 takes place. The dimensioning of those decoupling elements 131-133 is to be suitably selected depending on the specific aspects or dimensions of the optical element 110 and the support element 120, wherein for typical diameters for the mirror in the range of between 300 and 600 mm, the values by way of example with respect to the length and width of the leg-like decoupling elements 131-133 can be in the range of between 5 and 20 mm.
In addition the optical arrangement 100 has fixing attachments 141, 142 and 143 for mounting elements (not shown in
A feature of the embodiment shown in
As can be seen from
The above-described relative arrangement can be implemented for example as shown in
The arrangement including the optical element 110 or mirror, of the (monolithic) decoupling elements 131-133, the support element 120 or outer ring and the fixing projections 141, 142 and 143 with the mounting elements mounted thereto such as for example bipods can be viewed as a dynamic system ‘M1-K1-M2-K2’. In that respect M1 denotes the mass of the optical element or mirror (in kg), K1 the stiffness of the decoupling elements 131-133 (in the main direction and in units of N/m), M2 the mass of the support element 120 or outer ring and K2 the stiffness of the mounting elements or bipods. For best possible decoupling of the mirror with respect to holder-induced parasitic influences K1 should be as small as possible in particular in the parasitic directions. In addition the natural frequency should be above 300 Hz. M1 and K1 are so dimensioned that they generate a natural frequency which is somewhat greater than the desired first natural frequency of the overall system (Hz) and can be for example in the range of between 350 Hz and 400 Hz. The ‘inner ring’ formed by the reinforced portion of the optical element, of the mass M2, should have adequate stiffness so that it behaves substantially like a rigid body, wherein the natural frequency generated by the total of the masses M1+M2 and K2 should be substantially greater than 300 Hz and for example be more than 600 Hz.
As indicated in
The embodiment of
Accordingly in the
Similarly to the embodiment of
Although a monolithic configuration of the decoupling elements 131-133 and 231-233 is implemented both with the optical element and also with the support element in the embodiments of
The optical arrangement 300 in
Unlike the
The configuration of
Also similarly to the embodiment of
As indicated in
For two actual embodiments by way of example of an optical arrangement, Table 1 shows characteristic values for two different mirror dimensions (Example No 1: maximum mirror thickness or height t=17.5 mm; Example No 2: maximum mirror thickness or height t=25 mm), wherein the dimensions of the ring forming the support element 320, with a ring width (that is to say difference between outside and inside diameter) a=30 mm and a ring height of h=60 mm, as well as the material (quartz glass, SiO2) were selected to be the same.
As can be seen from Table 1, both for Example No 1 and also for Example No 2, values for the first natural frequency (for the system consisting of mirror, support element or ring and a bipod set forming the basis for the arrangement by way of example) near to 400 Hz and mass values (for the arrangement consisting of the mirror and support element or ring) markedly below 10 kg are achieved, in which respect in addition for Example No 2 the deformation caused by gravity involves values below 500 nm. Table 1 also indicates some of the Zernike coefficients resulting upon development of the deformation acquired in Zernike polynomials, both for radial and also tangential moments, in which respect without exception as desired values below less than 0.5 nm are achieved and wherein the mirror in accordance with Example No 2, involving the greater thickness, has scarcely still any deformation phenomena. The parasitic moments also come from a bipod set which by way of example forms the basis of the configuration, with the usual mounting and production tolerances.
The projection exposure apparatus of
The projection objective 31 includes in total six mirrors M1-M6, wherein M5 denotes the mirror arranged closest to the wafer or the image plane 9 (in the illustrated embodiment at a spacing dw≈30 mm), having a through hole 19. M6 denotes the last mirror in relation to the optical beam path, having a through hole 18. A beam issuing from the object field 4 or reticle arranged in the object plane passes after reflection at the mirrors M1-M6 for producing an image of the structure to be imaged of the reticle, on to a wafer arranged in the image plane 9.
The last mirror M6 in relation to the optical beam path (having the relatively greatest radius) can be of a configuration according to the disclosure for example similar to the embodiment of
Even if the disclosure has been described by reference to specific embodiments numerous variations and alternative embodiments will be apparent to the man skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly it will be appreciated by the man skilled in the art that such variations and alternative embodiments are also embraced by the disclosure and the scope of the disclosure is limited only in the sense of the accompanying claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 045 163 | Sep 2009 | DE | national |
This application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2010/064090, filed Sep. 23, 2010, which claims benefit under 35 USC 119 of German Application No. 10 2009 045 163.3, filed Sep. 30, 2009 and under 35 USC 119(e) of U.S. Ser. No. 61/247,059, filed Sep. 30, 2009. International application PCT/EP2010/064090 is hereby incorporated by reference in its entirety.
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| Number | Date | Country | |
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| Parent | PCT/EP2010/064090 | Sep 2010 | US |
| Child | 13407466 | US |
