OPTICAL ELEMENT, OPTICAL ARRANGEMENT, AND METHOD FOR MANUFACTURING AN OPTICAL ELEMENT

Abstract

An optical element comprises a substrate and an optical surface formed on the substrate. At least one fluid-tight sealed chamber is embedded in the substrate and has a rheological fluid introduced therein for deforming the optical surface. An optical arrangement, such as an EUV lithography system, comprises at least one optical element as described above and a field generating device for generating an electromagnetic field. The electromagnetic field can be a time-varying electromagnetic field. The electromagnetic field can be a magnetic field. The electromagnetic field passes through the at least one chamber which contains the rheological fluid. A method for producing an optical element designed as described above is also provided.

Claims

1. An optical element, comprising: a substrate comprising a member selected from the group consisting of glass and a glass ceramic; an optical surface supported by the substrate; a fluid-tight sealed chamber embedded in the substrate; and a rheological fluid in the fluid-tight sealed chamber.

2. The optical element of claim 1, wherein the optical element comprises a plurality of fluid-tight sealed chambers embedded in the substrate, and each fluid-tight sealed chamber contains the rheological fluid.

3. The optical element of claim 1, wherein the rheological fluid comprises a member selected from the group consisting of a magneto-rheological fluid and an electro-rheological fluid.

4. The optical element of claim 1, wherein the substrate comprises first and second partial bodies connected along a connecting surface, the second partial body supports the optical surface, and the fluid-tight sealed chamber is adjacent the connecting surface.

5. The optical element of claim 4, wherein the chamber defines a depression in the first partial body, and the depression is adjacent to the connecting surface.

6. The optical element of claim 4, wherein the connecting surface extends along a side of the second partial body remote from the optical surface.

7. The optical element of claim 6, wherein: the optical surface is convexly preformed; and under the action of a field on the rheological fluid, the optical surface converts to a neutral state from which a bidirectional deformation of the optical surface is implemented.

8. The optical element of claim 1, wherein the substrate comprises a fluid-tightly sealed channel connecting the fluid-tightly sealed chamber to a surface of the substrate.

9. The optical element of claim 1, further comprising a reflective coating supported by the substrate, wherein the optical surface is supported by the reflective coating.

10. The optical element of claim 1, wherein a surface of the substrate remote from the optical surface has a recess extending into a region of the chamber, the surface of the substrate is configured to have a field generating device inserted therein so that the field generating device is configured to act on the rheological fluid.

11. An optical arrangement, comprising: an optical element according to claim 1; and a field generating device configured to generate an electromagnetic field that passes through the chamber.

12. The optical arrangement of claim 11, wherein the electromagnetic field comprises a time-varying electromagnetic field.

13. The optical arrangement of claim 11, wherein the electromagnetic field comprises a magnetic field.

14. The optical arrangement of claim 11, wherein the field generating device comprises a coil.

15. The optical arrangement of claim 14, wherein the coil is in a recess in the substrate.

16. The optical arrangement of claim 11, wherein the field generating device comprises a plurality of permanent magnets disposed in a Halbach arrangement, and the Halbach arrangement is in a recess in the substrate.

17. The optical arrangement of claim 16, wherein the Halbach arrangement defines a core of a coil.

18. The optical arrangement of claim 11, further comprising a control device configured to control the field generating device to adjust deformation of the optical surface.

19. An apparatus, comprising: an illumination system; and a projection lens, wherein: the apparatus is lithography apparatus; at least one member selected from the group consisting of the illumination system and the projection lens comprises an optical arrangement; and the optical arrangement comprises: an optical element according to claim 1; and a field generating device configured to generate an electromagnetic field that passes through the chamber.

20. A method of making an optical element comprising a substrate comprising a member selected from the group consisting of glass and a glass ceramic, an optical surface supported by the substrate, and a fluid-tight sealed chamber embedded in the substrate, the method comprising: a) disposing a rheological fluid in the chamber; and b) after a), fluid-tightly closing off the chamber.

21.-23. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Exemplary embodiments are depicted in the schematic drawings and are explained in the following description. In the drawings:

[0042] FIG. 1A shows a schematic representation of an EUV lithography apparatus comprising an illumination system and a projection lens;

[0043] FIG. 1B shows a schematic representation of a DUV lithography apparatus comprising an illumination device and a projection lens;

[0044] FIGS. 2A-2D show schematic sectional representations of an optical element in the form of an EUV mirror comprising a substrate in which magneto-rheological fluid filled chambers are embedded;

[0045] FIGS. 3A-3D show schematic representations of a field generating device for generating a magnetic field which passes through the chambers of the optical element in FIGS. 2A-2D in order to deform the optical surface of the latter.

DETAILED DESCRIPTION

[0046] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0047] An optical arrangement in the form of an EUV lithography apparatus 40 is shown schematically in FIG. 1A. It has an EUV light source 1 for generating EUV radiation, which has a high energy density in an EUV wavelength range below 50 nm, for example between approx. 5 nm and approx. 15 nm. The EUV light source 1 may for example take the form of a plasma light source for generating a laser-induced plasma or be formed as a synchrotron radiation source. In the former case, for example, a collector mirror 2 may be used, as shown in FIG. 1A, in order to focus the EUV radiation of the EUV light source 1 into an illumination beam 3 and in this way increase the energy density further. The illumination beam 3 serves for the illumination of a structured object M via an illumination system 10, which in the present example has four reflective optical elements 13 to 16.

[0048] The structured object M may be for example a reflective mask, which has reflective and non-reflective, or at least much less reflective, regions for producing at least one structure on the object M. Alternatively, the structured object M may be a plurality of micro-mirrors, which are arranged in a one-dimensional or multi-dimensional arrangement and which are possibly movable about at least one axis, in order to set the angle of incidence of the EUV radiation 3 on the respective mirror.

[0049] The structured object M reflects part of the illumination beam 3 and shapes a projection beam 4, which carries the information about the structure of the structured object M and is radiated into a projection lens 20, which generates an image of the structured object M or of a respective partial region thereof on a substrate W. The substrate W, for example a wafer, comprises a semiconductor material, for example silicon, and is disposed on a mounting, which is also referred to as a wafer stage WS.

[0050] In the present example, the projection lens 20 has four reflective optical elements 21 to 24 (mirrors) for generating an image of the structure that is present at the structured object M on the wafer W. The number of mirrors in a projection lens 20 typically lies between four and eight; however, only two mirrors may also possibly be used.

[0051] In order to achieve a high imaging quality in the imaging of a respective object point OP of the structured object M onto a respective image point IP on the wafer W, relatively stringent expected properties are to be made in respect of the surface shape of the reflective optical elements (mirrors) 21 to 24; and the position or the alignment of the optical elements 21 to 24 in relation to one another and in relation to the object M and the substrate W also involves precision in the nanometer range.

[0052] In order to respond to imaging aberrations within the projection lens 20, for example due to an incorrect alignment of the optical elements 21 to 24, due to manufacturing errors, and/or due to temperature-related deformations during operation, it is possible to counteract the unwanted deformation of the optical elements 21 to 24 via a first field generating device 17a, which typically comprises a plurality of electromagnets or coils 5 for generating a location-dependent variable magnetic field. FIG. 1A depicts the first field generating device 17a only in the region of the optical element 21 of the projection lens 20, but it is also possible to provide a respective field generating device for a plurality of optical elements 21 to 24 or even for all optical elements. In FIG. 1A, a second field generating device 17b with coils 5 is also arranged at the optical elements 13 to 16 of the illumination system 10, and so corrections can also be made in the illumination system 10.

[0053] FIG. 1B shows a schematic view of a DUV projection exposure apparatus 100, which comprises a beam shaping and illumination device 102 and a projection lens 104. In this case, DUV stands for “deep ultraviolet” and denotes a wavelength of the working light of between 30 nm and 370 nm. The DUV projection exposure apparatus 100 comprises a DUV light source 106. For example, an ArF excimer laser that emits radiation 108 in the DUV range at for example 193 nm, may be provided as the DUV light source 106.

[0054] The beam shaping and illumination device 102 illustrated in FIG. 1B guides the DUV radiation 108 onto a photomask 120. The photomask 120 is formed as a transmissive optical element and may be arranged outside the beam shaping and illumination device 102 and the projection lens 104. The photomask 120 has a structure of which a reduced image is projected onto a wafer 124 or the like via the projection lens 104.

[0055] The projection lens 104 has a number of lens elements 128, 140 and/or mirrors 130 for projecting an image of the photomask 120 onto the wafer 124. In this case, individual lens elements 128, 140 and/or mirrors 130 of the projection lens 104 may be arranged symmetrically in relation to the optical axis 126 of the projection lens 104. It should be noted that the number of lens elements and mirrors of the DUV projection exposure unit 100 is not restricted to the number shown. More or fewer lens elements and/or mirrors may also be provided. Furthermore, the mirrors are generally curved on their front side for beam shaping.

[0056] An air gap between the last lens element 140 and the wafer 124 may be replaced by a liquid medium 132 which has a refractive index of >1. The liquid medium 132 may be high-purity water, for example. Such a set-up is also referred to as immersion lithography and has an increased photolithographic resolution.

[0057] With the help of field generating devices (not depicted here) embodied analogously to the field generating devices 17a,b shown above in the context of FIG. 1A, it is also possible to correct undesired deformations of the lens elements 128, 140 and/or mirrors 130 of the projection lens 104 in the case of the DUV lithography apparatus 100 shown in FIG. 1B. The same applies to the optical elements of the beam shaping and illumination device 102.

[0058] FIGS. 2A-d show the structure of the first optical element 21 in the projection system 20 of the EUV lithography apparatus 40 from FIG. 1A in a schematic representation. The optical element 21 comprises a substrate 30 made of a material with a low coefficient of thermal expansion, for example Zerodur®, ULE® or Clearceram®, and a coating 31 that reflects the EUV radiation.

[0059] The reflective coating 31 has a number of layer pairs (not depicted here) with alternating layers made of a high refractive index layer material and a low refractive index layer material. As a result of the typically periodic structure of the reflective coating 31 (i.e., with pairs of layers of identical thickness), it is possible to reflect short-wavelength EUV radiation with a wavelength in the nm range (e.g., at 13.5 nm). In this case, the layers made of the high refractive index material are silicon and the layers made of the low refractive index material are molybdenum. Other material combinations such as molybdenum and beryllium, ruthenium and beryllium or lanthanum and B4C, for example, are likewise possible.

[0060] Should the reflective optical element 21 be operated not in the EUV lithography apparatus 40 shown in FIG. 1A but with imaging light at wavelengths longer than 150 nm, for example in the DUV lithography apparatus 100 shown in FIG. 1B, then the reflective coating 31 generally likewise has a plurality of individual layers that consist of materials having different refractive indices in alternation. In this case, however, it may optionally also be possible to dispense with a multi-layer coating, that is to say the reflective coating may be formed from a single layer (e.g., made of aluminum) only.

[0061] An optical surface 32 on which the EUV radiation 3 is incident is formed on the upper side of the reflective coating 31 facing the surroundings. In order to change the optical properties of the optical element 21, more precisely in order to correct wavefront aberrations that arise during imaging with the projection lens 20, a plurality of fluid-tightly sealed chambers 33 are embedded in the substrate 30 and have introduced therein a magneto-rheological fluid 34 for deforming the optical surface 32, as will be described further below.

[0062] As is apparent from FIGS. 2A-d, the respective chamber 33 is completely filled with the magneto-rheological fluid 34. The total of five chambers 33 in FIGS. 2A-c is chosen in exemplary fashion, that is to say a greater or optionally a smaller number of chambers 33 may also be introduced into the substrate 30. The arrangement of the chambers 33 in the substrate 30 can be regular, that is to say a two-dimensional grid arrangement of chambers 33 can be formed in the substrate 30. However, an irregular arrangement of the chambers 33 in the substrate 30 is also possible. The lateral spacing of the chambers 33 and the width of the chambers 33 are determined on the basis of the spatial resolution with which the optical surface 32 is to be deformed. The width of the chambers 33 or their size may vary across the substrate 30, depending in each case on the locally desired spatial resolution. The height of the chambers 33 determines the area with which the magneto-rheological fluid 34 acts on the substrate and thus determines the force that is transmitted to the substrate 30 from the magneto-rheological fluid.

[0063] The substrate 30 comprises a first partial body 30a and a second partial body 30b, which are interconnected along a common connecting surface 35. A respective chamber 33 forms a cuboid depression in the first partial body 30a in the examples shown in FIGS. 2A-c. In the example shown in FIG. 2D, a cuboid depression is formed in each of the two partial bodies 30a, 30b, the cuboid depressions together forming the chamber 33. The depressions are formed by mechanical processing, for example by milling or grinding, in the material of the first partial body 30a or of the two partial bodies 30a, 30b. To form the substrate 30, the first partial body 30a provided with the depressions and the second partial body 30b optionally likewise provided with depressions are interconnected along a connecting surface 35 which extends outside of the chambers 33. The two partial bodies 30a, 30b are connected along the connecting surface 35 by a bonding process without the use of a joining agent. The bonding process can be, for example, fusion bonding, silicate bonding, or direct bonding.

[0064] Before or after the two partial bodies 30a, 30b are connected, bores 36 connecting a respective chamber 33 to a surface 37 of the first partial body 30a that forms the back side of the substrate 30 remote from the optical surface 32 are introduced in the first partial body 30a. The magneto-rheological fluid 34 is introduced or filled into the chamber 35 through the bores or channels 36. After the magneto-rheological fluid 34 has been filled, the channels 36 are fluid-tightly sealed, for example by being filled with rubber, for example FFKM, in order to close-off the channels 36 in the manner of a plug.

[0065] The three optical elements 21 shown in FIGS. 2A-d are each depicted with an undeformed optical surface 32, that is to say without a magnetic field being generated with the aid of the field generating device 17b. The optical elements 21 differ from one another in terms of the geometry or the surface shape of the optical surface 32: The optical element 21 shown in FIG. 2A has a flat optical surface 32 and the connecting surface 35 is likewise flat. The optical element 21 shown in FIG. 2B has a concavely curved undeformed surface 32 and the connecting surface 35 is likewise curved. In the optical elements 21 shown in FIGS. 2A,b, the thickness of the second, planar partial body 30b of the substrate 30 is therefore substantially constant. The first partial body 30a likewise has a constant, albeit greater thickness than the second partial body 30b on which the optical surface 32 is formed.

[0066] In the optical element 21 shown in FIG. 2C, the connecting surface 35 is flat like in the optical element 21 shown in FIG. 2A, but the optical surface 32 is convexly curved, with the result that the thickness of the second partial body 30b varies depending on the location. The optical surface 32 of the optical element 21 in FIG. 2C is preformed, that is to say it has a curvature that deviates from a flat geometry. The preformed optical surface 32 in FIG. 2C can be deformed into an undeformed, flat state via the field generating device 17b. The preforming of the optical surface 32 is advantageous since the magneto-rheological fluid 34 can be used only to introduce pressure or shear forces into the substrate 30 for deforming the optical surface 32. The preforming makes it possible to convert the optical surface 32 into a neutral, for example flat initial state by the application of a magnetic field 38 (cf. FIG. 3A), from which initial state the optical surface 32 can be deformed both in the positive and in the negative direction; that is to say it is possible to concavely or convexly deform the optical surface 32 by increasing or reducing the field strength of the magnetic field 38.

[0067] In the optical element 21 shown in FIG. 2D, the optical surface 32 likewise has convex curvature. Unlike what was described in FIG. 2C, the curved surface 32 of the optical element 21 in FIG. 2D has not been preformed in order to be converted into a neutral, for example flat state with the aid of the field generating device 17b. Instead, the curved surface 32 of the optical element 21 in FIG. 2D is a free-form surface which forms an initial state for a unidirectional deformation of the optical surface 32. Like the optical element 21 shown in FIG. 2C, the optical element 21 shown in FIG. 2D also has a second partial body 30b with a non-constant thickness.

[0068] FIG. 3A shows a detailed representation of the field generating device 17b, which is embodied to generate the magnetic field 38. The coils 5 each comprise an iron core 39 and form an electromagnet. The current through a respective coil 5 and hence the field strength of the magnetic field 38 which passes through a respective chamber 33 can be adjusted on an individual basis, that is to say individually for each coil 5, with the aid of a control device 40. As depicted in FIG. 3A by double-headed arrows 41, the penetration of the magneto-rheological fluid 34 with the magnetic field 38 leads to a shear stress on the substrate 30, which causes the section of the optical surface 32 located above the respective chamber 33 to deform, with the degree of deformation depending on the field strength of the magnetic field 38 in the respective chamber 33.

[0069] The deformation of the optical surface 32 with the aid of the field generating device 17b can, for example, be implemented semi-actively at predetermined time intervals in order to set a new, constant magnetic field and in this way minimize wavefront aberrations caused by machine errors, for example in order to set a figure of the optical surface 32. In the case of the semi-active deformation, the control device 40 can apply a time-varying current to the respective coil 5 during the respective adjustment, in order to generate a time-varying magnetic field 38. The semi-active adjustment of the deformation can be implemented, for example, when the exposure is paused.

[0070] For correcting a dynamic behavior of the EUV lithography apparatus 40, for example for compensating for wafer topology errors, there can also be an active, for example continuous control or adjustment of the deformation during the exposure of the wafer W (within the exposure time). In this case, for example, feedforward control can also be implemented on the basis of a measurement of the wafer topology errors.

[0071] However, a passive, temporally constant deformation of the optical surface 32 is also possible, with a current that is constant over time being generated by the field generating device 17b. A passive deformation is also possible if the field generating device 17b is not designed to adjust the strength of the magnetic field 38, for example if it only comprises permanent magnets.

[0072] The efficiency of generating the magnetic field 38 can be increased if the magnetic field lines are impressed with the rheological liquid 34 very close to the respective chamber 33. For this purpose, a horseshoe-shaped iron core 39 is provided in the field generating device 17b shown in FIG. 3B, the horseshoe-shaped iron core extending into two recesses 41, which extend from the back side 37 of the substrate 30 into the region of the chamber 33, to be precise on opposite sides of the respective chamber 33. The horseshoe-shaped iron core 39 is used to generate a magnetic field with field lines 42 which run substantially parallel to the X-direction of an XYZ coordinate system, with the X-direction being aligned perpendicular to the thickness direction Z of the substrate 30. It goes without saying that the impressing of the magnetic field lines in the vicinity of a respective chamber 33 can also be implemented in a different way vis-à-vis the use a horseshoe-shaped iron core 39.

[0073] Such an example is depicted in FIG. 3C: In order to minimize stray magnetic fields, the field generating device 17b comprises a plurality of permanent magnets 43 arranged in a Halbach arrangement 44. Like the iron core 43 depicted in FIG. 3B, the Halbach arrangement 44 of the permanent magnets 43 is also formed in horseshoe-shaped (or C-shaped) fashion and is inserted into two recesses 41, which extend from the back side 37 of the substrate 30 into the region of the chamber 33, to be precise on opposite sides of the respective chamber 33. In the Halbach arrangement 44, the individual permanent magnets 43, in terms of their magnetization direction, are rotated through 90° with respect to one another, as a result of which improved guidance of the field lines 41 within the chamber 33 is realized.

[0074] A shielding 45, which in the example shown is formed from a Mu-metal and extends into the two recesses 41, is attached between the chamber 33 remote side of the permanent magnets 43 and the substrate 30. The shielding 45 serves to shield the outside and optionally adjacent chambers 33 in the substrate 30 from the field lines 41, in order thus to avoid interference. The shielding 45 is adapted to the geometry of the Halbach arrangement 44 and is likewise horseshoe-shaped or C-shaped in the example shown.

[0075] In the case of the Halbach arrangement 44 shown in FIG. 3C, the optical element 21 forms a passively deformable mirror, that is to say a constant, fixedly predetermined magnetic field is generated by the field generating device 17b. After measuring the optical arrangement, for example the optical arrangement of the EUV lithography system 40, in the field (or at the end customer), the desired surface figure of the optical element 21 can be adjusted via an individual set of permanent magnets 43 per chamber 33 in order to improve system performance. It goes without saying that such a purely passive deformation can also be implemented if the permanent magnets 43 are not arranged in a Halbach arrangement 44.

[0076] In the case of the example shown in FIG. 3D, the Halbach arrangement 44 comprising the permanent magnets 43 is combined with a coil 5 comprising an iron core, or the Halbach arrangement 44 forms the iron core of the coil 5. A combination of a plurality of coils 5 with the Halbach arrangement 44 is also possible. When the field generating device 17b is implemented in the manner shown in FIG. 3D, the permanent magnets 44 predefine the magnetic field and the at least one coil 5 comprising an iron core can change the magnetic field 38 in a time-variable manner by adapting the current flow through the coil 5. In the example shown in FIG. 3D, there is also a shielding 45 made of a Mu-metal in order to shield the outside from the magnetic field lines 41.

[0077] Although the examples above have been described in connection with a magneto-rheological fluid 34, other rheological fluids, for example electro-rheological fluids, can also be introduced into the respective chambers 33 in order to deform the optical surface 32 through the action of an electric field. In this case, the field generating device 17b is embodied to generate a time-constant or time-varying electric field. For this purpose, the field generating device 17b can comprise electrodes, for example in the form of two capacitor plates, which are each inserted in a cutout 41 and between which the chamber 33 is arranged. Instead of the flat or (spherically) concavely or convexly curved surface 32, (initially undeformed) aspheric surfaces or free-form surfaces 32 can naturally also be deformed in the manner described above in order to correct wavefront aberrations. It goes without saying that the optical surfaces 32 of optical elements not embodied to reflect EUV radiation can also be deformed in the manner described above.