Abstract
An optical assembly of a projection exposure apparatus for semiconductor lithography comprises an optical element and an actuator for deforming the optical element. The actuator is subjected to a bias voltage by a controller that is present. A projection exposure apparatus for semiconductor lithography comprises an optical assembly. A method for operating an actuator for deforming an optical element for semiconductor lithography comprises subjecting the actuator to a bias voltage by a controller.
Claims
1. An optical assembly, comprising: an optical element; and an actuator configured to deform the optical element, wherein: the actuator is configured so that, when the actuator is in a zero position and the actuator is subjected to a bias voltage, the actuator deflects; and the bias voltage is such that a change in shape of the actuator due to a change in temperature is compensated by a reduction in a sensitivity of the actuator due to the change in temperature.
2. The optical assembly of claim 1, wherein the optical assembly comprises a compensation element configured to compensate a change in geometry due to the change in temperature of the actuator.
3. The optical assembly of claim 2, wherein a thermal expansion of the compensation element is different from a thermal expansion of the actuator.
4. The optical assembly of claim 3, wherein a sign of the thermal expansion of the compensation element is opposite to a sign of the thermal expansion of the actuator.
5. The optical assembly of claim 3, wherein the thermal expansion of the compensation element is negative.
6. The optical assembly of claim 3, wherein the compensation element is between the optical element and the actuator.
7. The optical assembly of claim 3, wherein the actuator comprises the compensation element.
8. The optical assembly of claim 3, wherein the actuator comprises layers comprising an electrostrictive material, and the layers alternate with layers comprising the compensation element.
9. The optical assembly of claim 3, wherein the compensation element comprises a plurality of layers.
10. The optical assembly of claim 9, further comprising electrodes are between the layers.
11. The optical assembly of claim 10, wherein the electrodes are configured so that no electric field forms across the layers of the compensation element during use of the optical assembly.
12. The optical assembly of claim 11, wherein the actuator is configured so that the compensation element is embedded in the form of a plurality of individual elements in the material of the actuator.
13. The optical assembly of claim 3, wherein the actuator and the compensation element are formed together so that at least one of the following holds: when the temperature changes in an effective direction of the actuator, a difference in geometry is less than 5 ppm/K from a target value; when there is a change in temperature, an expansion corresponding to the change in geometry of a component part connected to the actuator and/or to the compensation element arises; and the actuator and the compensation element compensate an expansion of a component part connected to the actuator and/or to the compensation element.
14. The optical assembly of claim 1, wherein the actuator is configured so that, when the actuator undergoes a transverse deformation, the optical element is deformed.
15. The optical assembly of claim 1, wherein the actuator comprises at least one member selected from the group consisting of electrostrictive elements, piezoelectric elements, and magnetostrictive elements.
16. The optical assembly of claim 1, wherein the actuator comprises layers.
17. The optical assembly of claim 1, wherein the optical element comprises a mirror.
18. The optical assembly of claim 1, wherein the optical assembly comprises a plurality of individual actuators.
19. An apparatus, comprising: the optical assembly of claim 1, wherein the apparatus is a projection exposure apparatus for semiconductor lithography.
20. A method, comprising applying a bias voltage to an actuator to deform an optical element of a projection exposure apparatus for semiconductor lithography.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] Exemplary embodiments and variants of the disclosure are explained in more detail below with reference to the drawing, in which:
[0042] FIG. 1 shows a basic construction of a projection exposure apparatus;
[0043] FIG. 2 shows a basic construction of an optical assembly;
[0044] FIG. 3 shows a basic mode of action of an actuator;
[0045] FIGS. 4A-4D show a schematic illustration of different variants of a structure of an actuator and of a compensation element;
[0046] FIG. 5 shows a diagram for illustrating an action of a compensation element;
[0047] FIGS. 6A-6B show a schematic illustration of possible electrode arrangements of actuators; and
[0048] FIGS. 7A-7D show a diagram for illustrating an effect of a change in temperature on the compensation element and the sensitivity of the electrostrictive effect.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0049] FIG. 1 shows by way of example the basic construction of a microlithographic EUV projection exposure apparatus 1 in which the disclosure can find application. An illumination system of the projection exposure apparatus 1 has, in addition to a light source 3, an illumination optical unit 4 for the illumination of an object field 5 in an object plane 6. EUV radiation 14 in the form of optical used radiation generated by the light source 3 is aligned via a collector, which is integrated in the light source 3, in such a way that it passes through an intermediate focus in the region of an intermediate focal plane 15 before it is incident on a field facet mirror 2. Downstream of the field facet mirror 2, the EUV radiation 14 is reflected by a pupil facet mirror 16. With the aid of the pupil facet mirror 16 and an optical assembly 17 having mirrors 18, 19 and 20, field facets of the field facet mirror 2 are imaged into the object field 5.
[0050] A reticle 7 arranged in the object field 5 and held by a schematically illustrated reticle holder 8 is illuminated. A merely schematically illustrated projection optical unit 9 serves for imaging the object field 5 into an image field 10 in an image plane 11. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 12, which is arranged in the region of the image field 10 in the image plane 11 and held by a likewise partly represented wafer holder 13. The light source 3 can emit used radiation for example in a wavelength range of between 5 nm and 120 nm.
[0051] The disclosure can likewise be used in a DUV apparatus, which is not illustrated. A DUV apparatus is set up in general like the above-described EUV apparatus 1, wherein mirrors and lens elements can be used as optical elements in a DUV apparatus and the light source of a DUV apparatus emits used radiation in a wavelength range from 100 nm to 300 nm.
[0052] FIG. 2 shows the basic construction of an optical assembly 30 in which a mirror 31 having an actuator matrix 46 is illustrated. The mirror 31 is for example part of the projection optical unit 9 described in FIG. 1. The actuator matrix 46 comprises a plurality actuators 33 arranged in the manner of matrix, which are arranged on the rear side 32 of the mirror, i.e. on the opposite side of the mirror 31 from the optically active side. As a result of the actuators 33 being deflected, the rear side 32 of the mirror is deformed, with the result that the optically active face of the mirror 31 is also deformed on account of the stiffness of the mirror 31. As a result of the deformation of the optically active mirror surface, the imaging properties of the mirror 31 are changed, with the result that imaging aberrations of the projection optical unit can be compensated. An optically active face is understood here to be a face which, during normal operation of the associated apparatus, is subjected to used radiation, i.e. radiation used for imaging and exposure.
[0053] FIG. 3 schematically shows the basic construction of an actuator 33 without the attachment to the rear side of the mirror. The electrostrictive actuator material 39 is arranged between two electrodes 36, 37 as an actuator layer 34, wherein the first electrode is in the form of a voltage electrode 36 and the second electrode is in the form of a neutral electrode 37 or neutral conductor. As a result of the application of a voltage between the voltage electrode 36 and neutral electrode 37, an electrostrictive effect is brought about, which causes a change in length L of the actuator material 39 in a first direction and a transverse contraction Q, i.e. a contraction of the material 39 in a second direction perpendicular to the first direction. In FIG. 3, the shape of the actuator 33 without the action of an electric field is illustrated by dashed lines. For the deformation of the mirror 31 illustrated in FIG. 2, for example the transverse contraction or transverse deformation Q of the actuator 33 can be used. In this case, the actuator 33 is operated such that the force exerted thereby is exerted substantially along the contact face between the mirror 31 and the actuator 33 and not normally thereto.
[0054] FIGS. 4A to 4D show different variants of a construction of an optical assembly 30 and of the actuator 33, in each of which an actuator 33 and a thermal compensation element are illustrated.
[0055] FIG. 4A shows a first variant, in which an optical assembly 30 having an actuator 33 and a compensation plate 41 is illustrated. The compensation plate 41 is arranged between the actuator 33 and the rear side 32 of the mirror 31 and has a negative thermal expansion. If the mirror 31, the compensation plate 41 and the actuator 33 are subjected to an increase in temperature, the actuator 33 widens in the direction parallel to the rear side 32 of the mirror, whereas the negative thermal expansion of the compensation plate 41 in this case brings about a reduction in the width of the compensation plate 41. Given a suitable choice of the thermal expansion of the actuator 33 and of the compensation plate 41 and taking the stiffnesses of the actuator 33 and of the compensation plate 41 into consideration, the resultant movement between the compensation plate 41 and the rear side 32 of the mirror is equal to zero. This is only the case, however, when the mirror 31 itself does not exhibit any thermal expansion. The thermal expansion and the stiffnesses of the actuator 33 and of the compensation plate 41 are advantageously set such that they jointly have a thermal expansion corresponding to the thermal expansion of the mirror 31.
[0056] FIG. 4B shows a further variant of a construction of the actuator 33, in which an actuator 33 having integrated compensation elements 40 is illustrated. The actuator 33 and the compensation element 40 each comprise a plurality of layers 34, 42, which are layered alternately on one another. As already described with reference to FIG. 4A, the thermal expansion, or the coefficient of thermal expansion defined as the material constant, and the stiffnesses of the actuator layers 34 and of the compensation layers 42 are configured such that they correspond in total to the thermal expansion of the mirror. The actuator layer 34 may comprise an electrostrictive material, for example lead magnesium niobate, a piezoelectric material or a magnetostrictive material. The compensation layer 42 may comprise for example Ba.sub.0.2Sr.sub.0.8Zn.sub.2Si.sub.2O.sub.7 or zirconium tungstate Zr[WO.sub.4].sub.2.
[0057] In the variant, shown in FIG. 4C, of a construction of an actuator 33 having an integrated compensation element 40, a radial layer construction of actuator layers 34 and compensation layers 42 is illustrated. The procedure for the configuration of the actuator is analogous to the procedure described with reference to FIGS. 4A and 4B. FIG. 4D shows a further variant of a construction of an actuator 33 having a compensation element 40, which is embedded in the form of compensation beads 43 in the electrostrictive material 39 of the actuator 33. Here too, the procedure for the configuration of the thermal expansion of the actuator 33 is analogous to the procedure in FIGS. 4A to 4C.
[0058] FIG. 5 shows a diagram for illustrating the mode of action of the compensation element, in which the travels of an actuator with and without a compensation element at different temperatures are plotted over the value E of the electric field strength. Here, the dot-dashed line represents the travel of an actuator without a compensation element at a temperature X, for example 22° Celsius. The dashed line represents the travel of an actuator with a compensation element, which is designed such that it has a thermal expansion of 0, with the result that the travel applies for the temperature X and the temperature Y, which is about 20 Kelvin higher. The solid line represents the travel of the actuator without a compensation element at the temperature Y. At the temperature X, the resultant travel L.sub.O0res of the actuator without a compensation element is greater than the resultant travel L.sub.Kres of the actuator with a compensation element, this being attributable to the stiffness of the compensation element, which has to be deformed by the actuator. If the resultant travels are considered at a temperature Y that is 20 Kelvin higher than the temperature X, the travel L.sub.O0res of the actuator without a compensation element is already not equal to zero without an applied electric field. An electric field strength E.sub.k is already involved in order to compensate the change in length brought about by the change in temperature. As a result, the resultant travel L.sub.O20res is reduced at a maximum available electric field to a value which is lower than the resultant travel L.sub.Kres, which remains constant, of the actuator with a compensation element. The travel available for correcting imaging aberrations is therefore greater for the actuator with a compensation element than for the one without a compensation element.
[0059] FIGS. 6A and 6B show two different arrangements of electrodes 36, 37, in each of which an optical assembly 30 having an actuator 33 with a layered construction made up of electrostrictive layers 35 and compensation layers 42 is illustrated.
[0060] In FIG. 6A, the voltage electrodes 36 and neutral electrodes 37 are arranged between the alternating electrostrictive layers 35 of the actuator 33 and the compensation layers 42 of the compensation element 40 such that the compensation layer 42 is enclosed either by two voltage electrodes 36 or two neutral electrodes 37. This has the result that, in the compensation layers 42, no electric field is applied and, as a result, no reaction is brought about on account of an electric field in the compensation layers 42. The actuator 33 is connected to the rear side 32 of the mirror 31 via an adhesive layer 44 made from an adhesive exhibiting shear stiffness. If an electric field is applied in the electrostrictive layers 35 via the electrodes 36, 37, the actuator 33 expands perpendicularly to the layers 35, 42 and contracts on account of the transverse contraction in the direction of the layer planes. As a result, the rear side 32 of the mirror contracts via the adhesive layer 44, causing the formation of a bulge 47 on the optically active mirror top side 45. The effective direction of the actuator, which is illustrated as an arrow in FIG. 6A, is thus perpendicular to the transverse contraction of the actuator 33. The voltage electrodes 36 are connected via an attachment 38 to an open-loop and/or closed-loop controller (not illustrated). The neutral electrodes 37 are connected to the ground wire (not illustrated). The compensation layers 42 are arranged such that they form the capping layer for the actuator 33, i.e. protect the electrodes 36, 37 from mechanical contact. As a result, it is possible to dispense with the usually capping layers in the construction of the actuator 33.
[0061] FIG. 6B shows an arrangement of the voltage electrodes 36 and neutral electrodes 37, which are arranged such that an electric field is also applied in the compensation layers 42. This results in a greater spacing between the electrodes 36, 37 and thus, via a weaker electric field, in lower sensitivity of the actuator. A reaction of the compensation layers 42 on account of the electric field is, if present, taken into consideration when controlling the actuator 33. The voltage electrodes 36 are, as in FIG. 6A, likewise attached via an attachment 38 to an open-loop and/or closed-loop controller (not illustrated). The electrostrictive layers 35 may comprise for example lead magnesium niobate ceramics (PMN) and the compensation layers 42 may comprise for example barium strontium zinc silicon oxide. The actuator 33 is likewise, as already described with reference to FIG. 6A, connected to the rear side 32 of the mirror 31 by an adhesive layer 44 exhibiting shear stiffness, wherein the illustration in FIG. 6B shows a non-deflected optical assembly 30.
[0062] FIGS. 7A to 7D each show a diagram for illustrating the effect of the change in temperature on the compensation element and the sensitivity of the electrostrictive effect.
[0063] FIG. 7A shows the change in shape of the actuator with a compensation element at a temperature of 20° Celsius, 40° Celsius and 60° Celsius. The change in shape ε.sub.thermal of the actuator with the compensation element is independent of the voltage and negative on account of the negative thermal expansion of the compensation element. In other words, FIG. 7A shows the voltage-independent contribution of the coefficient of thermal expansion, which is negative in the example shown. The thermal change in shape ε.sub.thermal is plotted in arbitrary units over the voltage. Note that FIG. 7A does not illustrate the coefficient of linear thermal expansion but rather a real thermally induced contraction of compensation layers of an actuator with increasing temperature independently of a voltage applied to the actuator.
[0064] FIG. 7B shows the electrostrictive extension or deformation of the actuator over the applied voltage at a temperature of 20° Celsius, 40° Celsius and 60° Celsius. The gradient of the curves decreases with increasing temperature, and so the change in shape per voltage unit is, at a constant voltage (at a constant spacing d of the electrodes), different for different temperatures. Since, in the exemplary embodiment described, the transverse contraction of the actuator is used with longitudinal expansion as manipulated variable, the change in shape with increasing voltage is likewise negative. Only the electrostrictive portion is shown in FIG. 7B—for this reason, the straight lines that represent this portion all start at the same position on the y-axis, this not being the case in reality.
[0065] FIG. 7D shows only the situation in which the actuator is operated with a bias voltage in its zero position, and for this reason the x-axis has been shifted accordingly. (Note: possibly adapting the illustration here may be misleading. Possibly even omit FIG. 7D and just verbally discuss FIG. 7C).
[0066] FIG. 7C now shows the total change in shape of the actuator by the thermal and the electrostrictive effect at a temperature of 20° Celsius, 40° Celsius and 60° Celsius. Note that the effects compensate one another at a particular voltage, meaning that the curves for different temperatures intersect. Thus, FIG. 7C shows the real relationships as a superimposition of both effects. Depending on the temperature, the actuator starts at a voltage of 0 V in different expansion states, as is apparent from the y-axis portions in FIG. 7C. On account of the different electrostrictive expansions at different temperatures, there is, however, a voltage region, indicated in a lightly shaded manner in the figure, in which the three curves shown intersect. In this voltage region, the deformation of the actuator is more or less independent of the ambient temperature. However, the different sensitivity of the actuator to voltage changes at different temperatures is still to be noted, as is immediately clear from the different gradients of the individual straight lines. As a result, the actuator thus displays, given a suitable choice of the bias voltage, a largely minimized temperature drift.
[0067] FIG. 7D now shows the change in shape of the actuator for different temperatures, as finds application jointly with the optical element. The actuator is mounted under a bias voltage in the neutral position of the optical element in the form of a mirror, that is to say the position in which the surface of the mirror corresponds to its target shape. If the bias voltage is reduced to zero, the actuator expands and the mirror surface deforms. If the projection exposure apparatus is now put into operation, the actuator is controlled with the determined bias voltage and the surface corresponds to its target shape, independently of the temperature of the mirror. The application of the bias voltage is reflected in the figure in that a new zero point of the y-axis—namely the desired actuator deformation is set at the target shape of the surface. The adaptation of the shape of the mirror surface can now be set more or less independently of the temperature via the voltage applied to the actuator. Only the temperature-dependent sensitivity of the electrostrictive effect is, in general, taken into consideration, as already mentioned above, in dependence on the desired properties in terms of precision of the shape of the mirror surface.
LIST OF REFERENCE SIGNS
[0068] 1 Projection exposure apparatus [0069] 2 Field facet mirror [0070] 3 Light source [0071] 4 Illumination optical unit [0072] 5 Object field [0073] 6 Object plane [0074] 7 Reticle [0075] 8 Reticle holder [0076] 9 Projection optical unit [0077] 10 Image field [0078] 11 Image plane [0079] 12 Wafer [0080] 13 Wafer holder [0081] 14 EUV radiation [0082] 15 Intermediate field focal plane [0083] 16 Pupil facet mirror [0084] 17 Assembly [0085] 18 Mirror [0086] 19 Mirror [0087] 20 Mirror [0088] 30 Optical assembly [0089] 31 Mirror [0090] 32 Rear side of the mirror [0091] 33 Actuator [0092] 34 Actuator layer [0093] 35 Electrostrictive layer [0094] 36 Voltage electrode [0095] 37 Neutral electrode [0096] 38 Voltage electrode attachment [0097] 39 Electrostrictive material [0098] 40 Compensation element [0099] 41 Compensation plate [0100] 42 Compensation layer [0101] 43 Compensation bead [0102] 44 Adhesive layer [0103] 45 Mirror top side [0104] 46 Actuator matrix [0105] 47 Bulge [0106] 48 Bias voltage [0107] L Change in length [0108] Q Transverse contraction [0109] L.sub.Kres Resultant change in length at maximum electric field with compensation element [0110] L.sub.O0res Resultant change in length at maximum electric field without compensation element at ΔT=0K [0111] L.sub.O20 Change in length without compensation element at ΔT=20K [0112] L.sub.O20res Resultant change in length at maximum electric field without compensation element at ΔT=20K [0113] E.sub.K Electric field strength for compensation of the change in length [0114] ε.sub.thermal Extension/change in length on account of change in temperature [0115] ε.sub.electrostrictive Extension/change in length on account of electrostrictive effect [0116] ε.sub.total Total extension/change in length (temperature and electrostrictive effect) [0117] V Voltage
