DE102011005840A1 - A controllable multi-mirror arrangement, optical system with a controllable multi-mirror arrangement and method for operating a controllable multi-mirror arrangement - Google Patents

Abstract

A multiple mirror arrangement has a support structure (120) which supports a plurality of individual mirror elements (110). A mirror element has a mirror substrate (112) which has a front surface (114) coated with a reflective coating (118) and a rear surface (116) opposite the front surface at a distance and facing the carrier structure. A temperature control device is provided for generating a lateral, stationary temperature gradient in a temperature-controllable zone (150) of the substrate located between the front surface and the rear surface in response to signals from a control device.

Description

BACKGROUND

Technical area

The invention relates to a controllable multiple mirror arrangement according to the preamble of claim 1, an optical system with a controllable multiple mirror arrangement according to the preamble of claim 12 and to a method for operating a controllable multiple mirror arrangement according to the preamble of claim 14.

Description of the Prior Art

For the production of semiconductor components and other finely structured components, predominantly microlithographic projection exposure methods are used today. In this case, masks (reticles) or other pattern generating means are used, which carry or form the pattern of a structure to be imaged, for. B. a line pattern of a layer (bearing) of a semiconductor device. The pattern is positioned in a projection exposure apparatus between a lighting system and a projection lens in the region of the object surface of the projection lens and illuminated with an illumination radiation provided by the illumination system. The radiation changed by the pattern passes through the projection lens as projection radiation, which images the pattern onto the substrate to be exposed coated with a radiation-sensitive layer.

The pattern is illuminated by means of an illumination system which forms from the radiation of a primary radiation source a pattern directed illumination radiation characterized by certain illumination parameters and impinging on the pattern within an illumination field of defined shape and size.

As a rule, different illumination modes (so-called illumination settings) are used, which can be characterized by different local intensity distribution of the illumination radiation in a pupil surface of the illumination system, depending on the type of structures to be imaged.

There are different possibilities to set the desired intensity distribution of the illumination radiation in a pupil surface of the illumination system or a corresponding angular distribution of the illumination light in the illumination field. In general, an illumination system has a pupil shaping unit for receiving radiation from a primary radiation source and for generating a variably adjustable two-dimensional intensity distribution in the pupil surface of the illumination system.

Some concepts envisage using in the pupil shaping unit a multi-mirror array (MMA) controllable mirror assembly having a plurality of discrete mirror elements carried by a common support structure that can be independently tilted to selectively change the angular distribution of the radiation incident on the entirety of the mirror elements in such a way that the desired spatial illumination intensity distribution results in the pupil surface.

In order to be able to set the geometric reflection properties of a controllable mirror arrangement in a targeted manner, a controllable mirror arrangement generally has, for each mirror element, an actuator arrangement coupled to the mirror element for controllably changing the position of the mirror element relative to the support structure carrying the mirror elements. Furthermore, a control device for controlling actuating movements of the actuator assembly is provided. In the absence of control signals, the mirror elements assume their respective zero position. Under the control of the control device, the orientation and / or the position of the mirror surface of the mirror element can be selectively changed starting from the zero position. An actuator arrangement has one or more selectively activatable actuator elements whose actuation or activation lead to the adjusting movement of the actuator arrangement.

From the WO 2009/100856 A1 Multi-mirror arrangements are known which have a plurality of individual mirror elements which are each connected to an actuator in such a way that they can be tilted separately from each other by at least one tilting axis. In one embodiment, the actuators are designed as electrically controllable piezo actuators.

Also in the patent US 6,977,718 B1 are shown multiple mirror assemblies whose individual mirror elements can be tilted by means of piezoelectric actuators.

The patent application US 2008/0100816 A1 shows illumination systems for microlithography, in which a multiple mirror arrangement is used, whose individual mirror elements can be tilted with the aid of piezoelectric or electrostatic actuators.

The patent application US 2006/0103908 A1 shows multiple mirror arrays, the individual Mirror elements using magnetic actuators or Lorentz actuators can be tilted.

The patent US Pat. No. 6,428,173 B1 shows microelectromechanical structures (MEMS) that are designed so that a single mirror element or many mirror elements of a mirror assembly can be selectively moved in response to a selective thermal actuation or activation of thermally activatable actuator elements. For the thermal activation of an actuator element, an electric current can be passed through at least part of the actuator element in order to heat it directly electrically. Indirect thermal activation by means of an external heater is mentioned as an alternative.

The patent application WO 2010/003527 A1 shows multi-mirror arrays with individually tiltable mirror elements, which are supported by a support structure which is divided into two or more mutually tiltable portions. As a result, at least two of the mirror elements in their zero position have a different orientation of their mirror surface.

The US 2007/0165202 A1 (corresponding WO 2005/026843 A2 ) as well as the WO 2008/131928 A1 or the EP 1 262 836 A1 show examples of the use of multi-mirror arrays in illumination systems for microlithographic projection exposure equipment operating in the deep ultraviolet (DUV) range.

The use of multiple mirror arrays in extreme ultraviolet radiation (EUV) radiation illumination systems is disclosed, for example, in the patent US 6,977,718 B1 or in the patent applications DE 2007 041 004 A1 and DE 10 2008 009 600 A1 shown. It is in the DE 10 2008 009 600 A1 proposed to use two controllable multi-mirror arrays in an illumination system, one of which serves as a field facet mirror and the other as a pupil facet mirror.

The requirements for the accuracy of the orientation of the individual mirror surfaces of a mirror arrangement are very high, especially in applications in the field of microlithography. Typical requirements for the accuracy of the orientation can be on the order of one or a few microradians (μrad) both at room temperature and in the heated state during operation (1 radian = 1 rad = 180 ° / π). This angular accuracy typically corresponds to required positioning accuracies in the range of a few nanometers, wherein these accuracies should preferably also be maintained under the influence of temperature variations, such as may be generated during the operation of a projection exposure apparatus by the electromagnetic radiation used in the projection exposure. A misorientation of one or more mirror surfaces of a mirror arrangement can have different causes. Misorientations may already occur, for example, during assembly of a mirror arrangement within an optical system. Also, over the life of the optical system, due to repetitive thermal cycling, relaxation effects can occur which cause an irreversible change in orientation. During operation, there may be reversible misalignments, for example due to temperature changes. Misorientations of the mirror surfaces affect the geometrical reflection behavior of the mirror arrangement and can, for. B. adversely affect the angular distribution of the radiation in the reflected beam from the mirror array.

There is therefore a need for controllable mirror arrangements which permit, in the long term, the most exact possible adjustment of the geometric-optical properties of a beam bundle influenced by the mirror arrangement.

TASK AND SOLUTION

It is an object of the invention to provide a multi-mirror controllable arrangement in which the problems described above can be reduced or avoided. In particular, disadvantages should be avoided or reduced, which may result from misalignments of mirror elements.

To achieve this object, the invention provides a controllable multi-mirror arrangement with the features of claim 1. Furthermore, an optical system with a controllable multi-mirror arrangement with the features of claim 12 and a method for operating a controllable multi-mirror arrangement with the features of claim 14 are provided.

Advantageous developments are specified in the dependent claims. The wording of all claims is incorporated herein by reference.

A multi-level controllable mirror assembly according to the claimed invention has a support structure supporting a plurality of individual mirror elements. A mirror element has a mirror substrate having a front surface coated with a reflective coating and a rear surface spaced from the front surface at a distance facing the support structure. The reflection-coated front surface forms the (reflective) mirror surface of the single mirror element. The front surface or mirror surface may be flat, so that the mirror element forms a plane mirror. The front surface may also be convex or concave in one or more directions. The front surface is prepared with optical quality and carries the reflection coating whose material or material combination and structure determines the optical reflection properties of the mirror element, in particular its reflectance. For example, the reflective coating may be constructed to have a high reflectance for extreme ultraviolet (EUV) radiation. It may also be a deep ultraviolet (DUV) reflection reflective coating.

The mirror arrangement includes a tempering device for generating a lateral, stationary temperature gradient in a temperature-controllable zone of the substrate lying between the front surface and the rear surface in response to signals of a control device. The term "lateral temperature gradient" here denotes a temperature gradient or a temperature gradient whose essential component or its temperature gradient vector is directed at least approximately perpendicular to a surface normal of the front surface. The temperature gradient can be generated and / or maintained by a heat flow whose current direction is substantially parallel to the front surface. The tempering device is designed so that within the mirror substrate, a thermal imbalance in the lateral direction (transverse direction) can be actively maintained. For this purpose, for example, one side of the mirror substrate can be strongly heated, while the opposite side is only slightly or not heated or even cooled. In this case, a temperature gradient or a temperature gradient sets in from the warmer to the colder side. Due to the thermal expansion of the substrate material results within the temperature-controlled zone on the warmer side, a greater thermal expansion than on the colder side, the extent of thermal expansion from the warmer to the colder side without cracks decreases continuously. The non-uniform thermal expansion in the region of the temperature-controllable zone causes the shape of the mirror substrate to change in a manner that can be defined by specifying the temperature gradient, resulting in a first approximation global tilting of the front surface of the mirror substrate with respect to the rear surface about a (virtual) tilting axis, which is substantially perpendicular to the direction of heat flow and more or less perpendicular to a surface normal of the front surface. This thermally induced change in shape of the mirror substrate is completely reversible and, in principle, repeatable as often as desired with a reproducible result.

By setting a stationary temperature gradient within the mirror substrate, spatially separated areas can thus thermally expand to different extents, resulting in effective tilting or rotation of the front surface serving as a mirror surface with respect to the rear surface. The correlation between the orientation of the temperature gradient and the resulting tilt of the front surface relative to the back surface is determined essentially by the system geometry, by the substrate material and by and the type of asymmetric introduction of heat energy into the mirror substrate.

In order to achieve a sufficiently strong tilting of the mirror surface with respect to the rear surface even at moderate temperature gradients, it is provided in some embodiments that the mirror substrate consists at least in the region of the temperature-controllable zone of a substrate material which at room temperature (20 ° C) has a thermal expansion coefficient of at least 1 · 10 ^-6 K ^-1 . Preferably, materials having good thermal conductivity and a thermal expansion coefficient of at least 10 × 10 ^-6 K ^-1 are used. As a substrate material, for example, a semiconductor material, such. As silicon, or silicon carbide, or a metallic material such. As aluminum, tungsten or copper can be used. When using materials with good thermal conductivity can be achieved that when heating of tempering the temperature at the occupied with a possibly temperature-sensitive reflection coating front surface does not exceed a tolerable temperature limit of z. B. 200 ° C increases. High thermal expansion coefficients contribute to the fact that tilting of sufficient size can be achieved even at relatively moderate temperature differences.

In preferred embodiments, the substrate dimensions, the substrate material and the tempering device are matched to one another such that a sensitivity S of the order of magnitude of 1 μrad / K is achieved for the thermally induced tilting. At a sensitivity S = 1 μrad / K, a temperature difference ΔT = 1 K between opposite sides of the substrate produces a tilt of the front surface of 1 μrad. The sensitivity S is preferably in the range between 0.3 μrad / K and 90 μrad / K, in particular between 1 μrad / K and 30 μrad / K.

Preferably, between the temperature-controllable zone and the front surface is not substantially by the tempering temperature-controlled intermediate area. The heat flow generated by the tempering device runs in the absence of further heat sources or heat sinks substantially within the temperature-controlled zone, so that hardly any heat flows into adjacent zones. If there is a sufficiently large intermediate region between the temperature-controllable zone and the front surface, the side surfaces of which are not heated, it can be achieved that the shape of the front surface is practically unchanged by the temperature control. It does not come to thermally induced deformations of serving as a mirror surface front surface, so that the change in the effect of the heated mirror element exclusively results from the change in the orientation of the mirrored front surface. The temperature-controlled zone can, for. B. located centrally between the front surface and the rear surface, possibly also closer to the rear surface than on the front surface.

In some embodiments, a cooling device is provided for actively cooling the support structure. The support structure may include, for example, coolant channels through which a liquid or gaseous cooling fluid is passed to dissipate heat from the support structure. An electric cooling by means of Peltier elements or the like is possible. With sufficiently good heat conduction contact between the mirror substrate and the support structure, the support structure thus acts as a heat sink and heat is dissipated from the mirror substrate away from the front surface toward the rear surface. The cooling device, in interaction with the tempering device, causes the heat optionally introduced by the tempering device to flow away predominantly in the direction of the rear surface of the mirror substrate, so that the asymmetrical deformation of the front surface by the tempering device is negligible. In addition, the cooling device helps to remove heat from the mirror substrate, which may result from absorption of the radiation incident on the reflective coating in the region of the reflective front side of the mirror element. This can contribute to the long-term stability of the optical properties of the mirror element.

The mirror substrate can be connected directly to the carrier structure, preferably flat. It may be made of the same material as the support structure and possibly integrally formed therewith. An integral design allows a particularly good heat transfer between mirror substrate and support structure. The mirror arrangement can be produced for example by a lithography process. In some embodiments, a thermally conductive intermediate structure is disposed between the support structure and the mirror substrate. This can serve as a thermal / mechanical interface between the material of the support structure and the possibly deviating material of the mirror substrate. The intermediate structure should allow the best possible heat transfer between mirror substrate and support structure, it is therefore preferably extended flat. With the aid of an intermediate structure, in some embodiments during the production of the mirror arrangement, an adjustment of the orientation of the mirror substrate relative to the carrier structure is possible before the mirror substrate is fixed to the carrier structure. The intermediate structure may, for. As an adhesive layer or a solder layer or be formed by such. It may also be provided a mechanical fixation structure that maximizes the contact surface to the support structure. The intermediate structure may alternatively or additionally comprise an adjustable mechanical hinge arrangement.

The tempering device can be constructed in different ways. In some embodiments, the mirror substrate has side surfaces and at least one of the side surfaces in the region of the temperature-controlled zone, an electrothermal transducer is provided. The term "electrothermal transducer" refers to a tempering element which can be heated or cooled by means of electric current. This may be, for example, a resistance heating element or a Peltier element. Whereas in a resistance heating element a direct or alternating current passed through leads to heating (temperature increase), a Peltier element can optionally serve as a heating element (for increasing the temperature) or as a cooling element (for lowering the temperature), depending on the polarity of the applied voltage. Preferably, there is a flat contact between the electrothermal transducer and the side surface of the mirror substrate, so that large amounts of heat are introduced into the mirror substrate per unit time or can be removed from the mirror substrate via the contact surface.

Preferably, electrothermal transducers are mounted in pairs on the mirror substrate, wherein a first electrothermal transducer of a first pair is disposed on a first side surface and a second electrothermal transducer of the first pair is disposed on a second side surface opposite in a first direction. As a result, the heat flow in the region between the electrothermal transducers is particularly well controlled and it can be adjusted by targeted heating and / or cooling of the electrothermal transducer of a pair different degrees of temperature along the first direction.

Preferably, at least a second pair of electrothermal transducers are provided on opposite side surfaces of the Mirror substrate are arranged in a second direction which is transverse, in particular perpendicular to the first direction defined by the first pair of electrothermal transducers. The converters of the pairs can be controlled independently of each other. As a result, two or more differently oriented virtual tilting axes are possible, so that the front surface can be tilted in virtually any direction relative to the rear surface.

In some embodiments, the effect of the cooling means of the support structure is sufficient to allow the orientation of the mirror substrate solely by the use of electrothermal transducers without heat sink functionality. In such embodiments, a paired arrangement of the transducers is not required. For any control of the orientation of the mirror substrates, three or more transducers can be used.

The tempering device may be the only manipulator device for influencing the orientation of the reflective front surfaces of the mirror elements of the mirror arrangement. Upon activation of the tempering device, its orientation can be adjusted with high accuracy in the μrad range for each mirror surface. This makes it possible, for example, a fine adjustment of the individual mirror surfaces of the mirror assembly relative to each other. In this way, for example, a desired reflection geometry of the entire mirror arrangement can be set and maintained for a long time. It is also possible, with the aid of the tempering device, to dynamically change the relative orientations of the mirror surfaces of the mirror elements. For example, in the case of mirror substrates having typical lateral dimensions of between 1 mm and 10 mm, relaxation times or switching times of between 1 sec and 10 sec can easily be achieved if necessary.

In some embodiments, to achieve the required orientation accuracies, it is sufficient to control the tempering device on the basis of default values, for example from a lookup table (open loop control). In other embodiments, the multi-mirror assembly is incorporated into a control loop with which the adjustment of the desired orientation of individual mirror surfaces of the mirror elements can be monitored and regulated. For this purpose, in some embodiments, a measuring device connected to the control device is provided for determining the orientation of the front surface and for generating measurement signals representing the orientation of the front surface, wherein the control device is configured to control the tempering device on the basis of the measurement signals. As a result, an exact orientation of the individual mirror elements is possible even under changing operating conditions.

The multiple mirror arrangement can be designed overall as a plane mirror. In this case, the mirror surfaces of all mirror elements are as accurately as possible in a common plane when the tempering device is switched off. The support structure may have a plane surface for supporting the mirror elements. It is also possible to design the multi-mirror arrangement as a whole as a curved mirror, in which the entirety of all the mirror surfaces is nominally on a common concave or convex curved surface. In any case, misalignments can be corrected from the desired orientation by means of tempering.

The invention also relates to an optical system having a plurality of optical elements, characterized in that the optical system includes at least one multiple mirror array of the type described in this application. The optical system can, for. Example, be an optical system for a projection exposure system for microlithography. In particular, it may be the illumination system of a projection exposure apparatus. The mirror arrangement, as an element of a pupil shaping unit as a spatially resolving light modulating device, can contribute to setting a predefinable local illumination intensity distribution in a pupil surface of the illumination system. Alternatively or additionally, the mirror arrangement z. B. also be used as a facet mirror with nominally invariable relative orientation of the individual mirror.

The invention also relates to a method of operating a steerable mirror assembly having a support structure supporting a plurality of individual mirror elements, a mirror element comprising a mirror substrate having a front surface coated with a reflective coating and a rear surface spaced from the front surface, which faces the support structure. The method is characterized by the creation of a lateral stationary temperature gradient in a temperature-controllable zone of the substrate located between the front surface and the back surface in response to signals from a controller.

These and other features will become apparent from the claims but also from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in an embodiment of the invention and in other fields be realized and advantageous and protectable Can represent versions. embodiments are shown in the drawings and are explained in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

1 shows a detail of an embodiment of a controllable multi-mirror arrangement;

2 shows a schematic vertical section through two mirror elements of a multi-mirror assembly, wherein the left mirror element is shown in a thermally unactivated neutral position and the right mirror element in a thermally activated position with tilted mirror surface.

3 shows an experimentally determined diagram for the time dependence of the tilting of a mirror surface with differently strong asymmetric heating of the mirror substrate;

4 shows an experimentally determined diagram for the dependence of the tilting of a mirror surface of the electric heating power;

5 schematically shows an EUV microlithography projection exposure apparatus with a controllable mirror assembly according to one embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In 1 is a schematic, oblique perspective view of a section of an embodiment of a controllable multi-mirror array (multi-mirror array, MMA) 100 shown. The multiple mirror assembly has a plurality of individual mirror elements 110 at the front of a common support structure 120 are attached and worn by them. The support structure consists in the example of a torsion-resistant material of high thermal conductivity, such as. As silicon, silicon carbide (SiC) or a metal such as aluminum.

Each of the mirror elements has a cuboid mirror substrate 112 , Which in the example case in each case as a monolithic block of a thermally relatively highly conductive substrate material having a thermal expansion coefficient α of more than 1 · 10 ^-6 K ^-1 . In the example, the mirror substrate consists of a metallic material (α> 20 · 10 ^-6 K ^-1 ), it can also be made of a semiconductor material, such. B. silicon. The mirror substrate has on its support structure 120 opposite side a flat front surface 114 and at the opposite side of the support structure, a likewise flat rear surface 116 , The vertical distance between the front surface and the rear surface determines the substrate thickness, the z. B. can be between 1 mm and 10 mm. The front surface and the rear surface each have a rectangular, in particular substantially square shape with edge lengths in the order of magnitude of the substrate thickness.

The front surface is of optical quality, ie machined with very low surface roughness and with a multilayer reflective coating 118 coated, which in the example case for radiation from the extreme ultraviolet (EUV) has a reflective effect and, for example, exchange pairs of molybdenum / silicon or ruthenium / silicon and optionally may have additional layers.

The mirror substrates are not directly on the support structure in the example 120 attached, but are of an intermediate structure 130 supported, which is disposed between the support structure and the mirror substrate and forms a thermal / mechanical interface. The intermediate structure may comprise a layer (eg adhesive layer or solder material layer) or a block of a good heat-conducting material, which differs from the material of the mirror substrate and / or the material of the carrier structure.

The support structure 120 , which has a serving as a mounting surface for the mirror elements, for example, flat front, can be actively cooled by means of a cooling device. These are in the support structure 120 Coolant channels 122 provided in the mounted state of the mirror assembly to a coolant source 125 ( 2 ) are connected and can be flowed through during operation of the mirror assembly with cooling fluid or other cooling fluid.

The reflective surfaces provided with the front surfaces of the mirror substrates form the respective square mirror surfaces of the mirror assembly. These mirror surfaces are distributed in the manner of a two-dimensional matrix in the form of rows and columns largely full-surface over a larger area, each between adjacent mirror surfaces small distances on the order of a few micrometers can remain. The mirrored front surfaces (mirror surfaces) in their entirety form a faceted mirror surface or a facet mirror whose geometrical reflection properties are determined by the orientation of the individual mirror surfaces.

The number of mirror elements (individual mirrors or facets) may vary depending on the application. It can z. B. between 1000 and 10000 individual mirror elements may be provided, but possibly much more, z. B. between 50000 and 1 million or more, or less than 1000, z. B. between 100 and 1000.

The orientation of the individual planar mirror surfaces can in the example case by the orientation of their respective surface normal 113 to be discribed. The orientation of the surface normals is the same for all surfaces of the mirror surface in the case of a flat mirror surface. In the case of likewise possible concavely or convexly curved mirror surfaces, the orientation can be described, for example, by the orientation of the surface normal in the center of the mirror or by the mean orientation of the surface normals of the curved mirror surface.

In each of the mirror elements, the orientation of the reflective front surface relative to that of the support structure 120 facing rear surface 116 can be changed steplessly and in different tilting directions by tilting. For this purpose, a tempering is provided, which now mainly in connection with the in 1 right front shown mirror element as well as in connection with 2 is explained in more detail.

2 shows a schematic vertical section through two mirror elements 110 . 110 ' a multi-mirror assembly, wherein the left mirror element 110 ' in a thermally unactivated neutral position and the right mirror element 110 is shown in a thermally activated position with tilted mirror surface.

The tempering device is a thermoelectric device capable of being located between the front surface 114 and the back surface 116 lying temperature-controlled zone 150 to build a running between opposite side surfaces of the mirror substrate lateral temperature gradient and maintain stationary if necessary for a predetermined period of time. In 2 the upper and lower limits of the temperature-controlled zone are indicated by dashed lines. To generate lateral temperature gradients, electrothermal transducers WX1, WX2, WY1 and WY2 in the form of areally applied resistance heating elements are provided on the side surfaces of the mirror substrate. The transducers are all connected via electrical lines to a common control device 160 the tempering device electrically connected and can by the control device 160 each be supplied with electrical energy independently.

Resistive heating elements WX1 and WX2, respectively, which form a first pair of electrothermal transducers, are respectively applied to the two side faces opposite to each other along a first direction (x-direction). Two further electrothermal transducers in the form of flat resistive heating elements WY1 and WY2, respectively, which together form a second pair of electrothermal transducers, are applied to the mirror surfaces oriented perpendicular thereto, opposite a second direction (y-direction). The resistance heating elements applied over a large area on the side surfaces each have a flat rectangular shape in each case and extend essentially over the entire width of the side surfaces, while the height of the electrothermal transducers (measured parallel to the z direction) corresponds only to a fraction of the height of the mirror substrate, for example, a maximum of 50% or a maximum of 30%. The electrothermal transducers are mounted in the example approximately centrally between the front surface and the rear surface. The temperature-controlled zone 150 essentially corresponds to that partial volume of the mirror substrate which is enclosed by the electrothermal transducers.

Between the temperature-controlled zone 150 and the specular front surface is over the entire cross section of the mirror substrate, a distance which corresponds in the example about the order of magnitude (in the z direction) of the temperature-controlled zone. This intermediate area 155 is outside the temperature-controlled zone and is not or only slightly heated by heating the resistance heating by them. The distance to the front surface is so large that the front surface of practically any temperature changes in the temperature-controlled zone remains virtually unaffected.

The function of this arrangement is now mainly related to 2 explained. This shows a section in the xz plane through two mirror elements that are on the common support structure 120 are applied and located in the beam path of an optical system, for example, a lighting system for a projection exposure system. The incident on the mirror surfaces Nutzlichtstrahlung within the optical system is indicated by arrows 210 symbolizes.

In the mirror element shown on the left 110 ' the electrothermal transducers are not energized on all four sides of the substrate, so throughout the temperature-controlled zone 150 essentially the same temperature T ₁ prevails. During operation of the mirror assembly, the useful light strikes 210 on the reflective mirror surface and is reflected there. Part of the incident radiation is typically absorbed into the reflective coating, creating heat in the area of the front surface. This is discharged substantially through the substrate in the direction of the cooled support structure. In the substrate, therefore, a temperature gradient substantially perpendicular to the front surface (approximately parallel to the surface normal) present, which leads to a first heat flow W1 in the direction of the support structure.

If now the orientation of the reflective front surface of a mirror element relative to the rear surface is changed, the mirror substrate of this mirror element is heated asymmetrically with the aid of the associated tempering device. In the example of the mirror element shown on the right 110 For this purpose, for example, a heating current is passed through the electrothermal transducer WX1 shown on the right, causing it to heat up. The in the first direction (x-direction) opposite electrothermal transducer WX2 is not supplied with electricity, so that there is no electrical heat. The electric heater in the area of the right heating element WX1 generated inside the adjoining part of the volume of the substrate, a temperature increase .DELTA.T relative to the opposite side, so that there the elevated temperature T ₁ + .DELTA.T sets. This difference in temperature between the opposite sides of the mirror substrate creates a temperature gradient across the mirror substrate and results in a second relatively high temperature side heat flow W2 from the relatively lower temperature side. The heat flow runs predominantly within the temperature-controlled zone 150 , A heat flow in the direction of the front surface is largely prevented by the existing between this and the cooled rear surface temperature cases, so that the front surface is virtually unaffected by the introduced with the aid of the electrothermal transducer second heat flow W2.

The lateral temperature gradient within the temperature-controllable zone leads to an asymmetric thermal expansion of the substrate material in the region of the temperature-controllable zone. In this case, the amount of thermal expansion in the vicinity of the hotter electrothermal transducer WX1 is particularly strong and decreases continuously in the direction of the opposite side. By acting in all spatial directions thermally induced expansion of the substrate material of the warmer side (near WX1) associated side of the reflective mirror surface is raised relative to the opposite side, so that adjusts a tilting of the reflective front surface relative to the stationary remaining rear surface. The virtual tilt axis associated with this tilt substantially perpendicular to the generated temperature gradient, d. H. parallel to the y-direction.

In a corresponding manner, a tilting of the reflective front surface in the opposite direction can be generated. It is also possible to produce a tilting of the mirror surface about a virtual tilting axis directed parallel to the x-direction. For this purpose, the two in the y-direction opposite transducer elements WY1, WY2 are asymmetrically energized with the aid of the control device, so that, for example, the substrate in the vicinity of in 1 Front shown transducer element WY1 heated more than in the region of the opposite transducer element WY2. This would be a tilt in 1 to the rear.

Each of the transducers mounted on a mirror substrate is independent of the other transducers mounted on the mirror substrate by means of the control means 160 controllable. As a result, the heating power of all converters can be set independently. This in turn makes it possible for the front surface to be tilted in virtually any direction, ie, around arbitrarily oriented virtual tilt axes. For example, the in 1 heated visible on the visible front side converters WX1 and WY1, while the respective opposite transducers WX2, WY2 remain de-energized, so will set a lateral temperature gradient within the mirror substrate, the main direction between the x-direction and the y-direction. If, for example, WY1 and WX1 are heated to the same extent, the heat flow will run essentially parallel to the bisector midway between the y and x directions across the substrate so that a virtual tilt axis results parallel to the perpendicular bisector.

The tempering device is incorporated in the embodiment in a control loop, which allows to set and maintain the desired tilted position of the individual mirror surfaces regardless of environmental conditions very accurately. The control loop comprises an optical measuring device operating according to the autocollimation principle 170 , which on each of the monitored mirror elements a measuring beam 172 directed, which runs back to the measuring device after reflection on the monitored mirror surface and carries information about the current orientation of the reflective mirror surface. From this information, the orientation of the front surface representing measuring signals are generated, which are the control device 140 be returned, which controls the tempering on the basis of these measurement signals (closed loop control).

The actuation principle explained here by way of example makes it possible to set the orientation of the mirror surfaces with an accuracy in the range of 1 microrad (μrad) or less, wherein actuation times in the range of a few seconds, eg. B. of less than 10 s, can be achieved.

The following information may be useful in designing mirror arrays. In preferred embodiments, the substrate dimensions, the substrate material and the tempering device are matched to one another such that a sensitivity S of the order of magnitude of 1 μrad / K is achieved for the thermally induced tilting. At a sensitivity S = 1 μrad / K, a temperature difference ΔT = 1 K between opposite sides of the substrate produces a tilt of the front surface of 1 μrad. The sensitivity S is preferably in the range between 0.3 μrad / K and 90 μrad / K, in particular between 1 μrad / K and 30 μrad / K. Temperature differences down to the order of .DELTA.T = 1 K are control technology controlled with sufficient accuracy and reasonable effort. Temperatures up to the order of T = 200 ° C or are with different tempering, z. B. SMD or SMC resistors or -Heizbändern easily generated and are sufficiently low to prevent overheating of the reflective layer. Accordingly, lateral temperature differences between the side surfaces up to the order of ΔT = 100 K are well feasible. Considering further typical substrate dimensions with a ratio of substrate height to substrate width in the vicinity of 1, the sensitivity S is in a first approximation equal to the thermal expansion coefficient α. Under these conditions, for a substrate material with a thermal expansion coefficient of 1 · 10 ^-6 K ^-1, a tilting of 1 μrad would be produced at a lateral temperature gradient of ΔT = 1 K. The adjustment range for silicon (α = 2.6 10 ^-6 K ^-1 ) and ΔT = 200 K is then approximately 520 μrad. For metals, such as. As aluminum (α ≈ 23 10 ^-6 K ^-1 ) tilting up to the mrad range would be possible, in which case the accuracy of the temperature control should possibly be below 1 K.

In an experimental embodiment, the multi-mirror assembly had a coolable support structure on which approximately cubic mirror substrates with edge lengths of about 5 mm were applied. On some mirror substrates, similar to in 2 shown on the right, on two opposing mirror surfaces in the x direction resistance heating elements applied flat. The orientation of the reflecting front surface was determined with an autocollimation system. One of the opposing resistance heating elements was successively acted upon by gradually increasing electrical power. The resulting tilting of the specular front surface about the virtual tilt axis parallel to the y direction was optically measured. The results of this calibration are in the 3 and 4 shown.

3 shows a diagram in which on the abscissa the time t [s] and on the ordinate of the tilt angle of the mirror surface (in μrad) is indicated. Both the tilting in the x-direction (about a virtual tilting axis parallel to the y-direction, tilting angle TY) and the tilting perpendicular thereto (about a virtual tilting axis parallel to the x-direction, tilt angle TX) were measured. 4 shows a diagram in which the achieved tilt angle TY (in μrad) as a function of the electric heating power P [W] is shown in these experiments. There were successively set five different heating power (namely about 0.5 W, about 1.5 W, about 2.5 W, about 4 W and about 5.5 W). Between each heating phase, the tempering was switched off again.

The temporal course of the tilt positions is 3 refer to. When the lowest heating power was switched on, a tilt angle of approx. 5 μrad turned on after less than 10 s, which then remained largely constant as long as the heating power remained switched on. After switching off the heat output, the mirror substrate also returned within a few seconds to the original shape, in which the reference tilt angle (0 μrad) was present. After a break (about 1 min), the next higher heating power of 1.5 W was introduced. Within less than 10 s, the corresponding tilt angle TY of about 12 μrad was established, which remained more or less constant with slight fluctuations until the heating power was switched off again. This procedure was repeated for the next higher heat outputs. As can be seen from the steep flanks of the curve TY in 3 results, the tilting position associated with a heating power respectively set very rapidly (after less than 10 s) and then remained at a more or less constant level, the measured fluctuations in the tilting position being within the measuring accuracy of the measuring system (about 1 μrad) , When the heat output was switched off, the tilt was completely reset, which shows that the process is completely reversible. In the example, up to a heating power of about 4 W showed a more or less linear relationship between the introduced heating power and the extent of tilting achieved thereby. For higher heating power then results in a deviation from the linearity.

In 3 It can also be seen that the tilting of the mirror surface takes place almost exclusively in the desired direction, namely around a virtual tilting axis which runs perpendicular to the generated temperature gradient. This can be seen on the course of the curve TX, which shows the corresponding tilting about a virtual tilting axis extending parallel to the x-axis. Although slight power deviations from the desired orientation are shown when the heating powers are switched on However, they are at least an order of magnitude below the level of the desired tilt in the direction perpendicular thereto. In the example, the deviations from the target orientation (with TX = 0 μrad) are in the range of a maximum of 2 to 3 μrad).

The actuation principle exemplified here, which is based on a specifically adjustable shape change of the mirror substrate, allows adjustments of the orientation of a mirror surface relative to the back surface of a mirror substrate with highest accuracy in the range of μrad. The achievable tilt angles in the experimental example were on the order of a few tens of μrad, e.g. B. up to about 40 μrad. A set tilt position could be set and maintained at an accuracy of less than 15%, in particular less than 10% of the set tilt angle. In absolute terms, the adjustment accuracy was typically less than 10 μrad (at the highest absolute tilt angles), typically even less than 5 μrad, or even less than 2 μrad. This accuracy of adjustment is also referred to herein as resolution of the multi-mirror arrangement in the angular space.

A multiple mirror arrangement of the type described here by way of example can be used in different ways.

In some embodiments, the adjustment is used exclusively for adjustment purposes. A goal of this possibility of use is for each of the individual mirrors of a mirror assembly at any time desired for the individual level zero position, d. H. its target orientation, with the highest accuracy set and maintain. This can be used, for example, in the case of a facet mirror, which should have a defined relative orientation of its individual mirror facets that is invariable during operation (ideally). If such a facet mirror is installed in an optical system, the installation can lead to misalignments of individual mirror elements. These misalignments can be corrected with the aid of the tempering device shown here by individual mirrors are tilted relative to other mirrors by uneven heating of the corresponding mirror substrates so that they again assume their desired orientation (zero position). The tempering device serves as a control system for variable adjustment of the reference position or zero position of each individual mirror element. The thermally adjusted tilting can be maintained virtually indefinitely over long time intervals and further corrected if necessary.

In particular, in applications where dynamic tilting of individual mirror elements in the range of a maximum of 50 to 1400 μrad are desired, a mirror assembly with tempering can also be used for the dynamic change of the geometric reflection properties of a multi-mirror arrangement.

5 shows the essential optical components of a microlithography projection exposure apparatus 500 , The system includes a lighting system 510 and a projection lens 530 and becomes with the radiation of a light source 514 operated. The light source 514 may be, inter alia, a laser plasma source or a discharge source. Such light sources generate radiation 520 in the EUV range, that is to say with wavelengths between 5 nm and 15 nm. In order for the illumination system and the projection objective to be able to work in this wavelength range, they are constructed with components which are reflective for UV radiation.

The of the light source 514 outgoing radiation 520 is done by means of a collector 515 collected and into the lighting system 510 directed. The lighting system here comprises a mixing unit 512 , a telescope optics 516 and a field-forming mirror 518 , The projection lens 530 serves to create an object field 552 in the object plane 550 of the projection lens on a picture field 562 in the picture plane 560 of the projection lens. The projection lens has six mirrors here.

The mixing unit consists essentially of two facet mirrors 570 . 580 , The first facet mirror 570 is arranged in a plane of the illumination system that is the object plane 550 is optically conjugated. It is therefore also called a field facet mirror. The second facet mirror 580 is arranged in a pupil plane of the illumination system, which is optically conjugate to a pupil plane of the projection lens. It is therefore also referred to as a pupil facet mirror. With the help of the pupil facet mirror 580 and the downstream imaging optical assembly, the telescope optics 516 and the grazing incidence field-forming mirror 518 includes the individual specular facets (individual mirrors) of the first facet mirror 570 in the object field 552 displayed. EUV projection exposure systems with this or similar basic structure are z. B. from the WO 2009/100856 A1 the disclosure of which is incorporated herein by reference.

Each of the facet mirrors has a support structure which carries a multiplicity of individual mirror elements, ie individual mirrors. Their reflecting front surfaces form the facets (mirror surfaces) of the facet mirror.

At least one of the facet mirrors 570 . 580 may include a tempering device of the type described herein to ensure the accuracy of the orientation of the individual mirrors under different operating conditions and over the life of the plant away.

In the example, only the pupil facet mirror is 580 equipped with a tempering device. This is to a control device 582 connected, which controls the thermally induced tilting of the individual mirrors. The orientation of the individual mirrors is determined by a measuring device connected to the control device 584 optically monitored without contact. The pupil facet mirror should have a temporally invariable reflection geometry or emission characteristic. If, in the course of operation, the desired nominal orientation of individual or all facets of the pupil mirror changes, a compensation or adjustment can be carried out by means of the tempering device incorporated in a control loop by tilting mirror surfaces in order to obtain the desired orientation of each of the individual mirrors Adjustment accuracy of ± 1 μrad or less to be kept permanently within the tolerance ranges.

In other embodiments, only the field facet mirror is equipped with a tempering device. Likewise, both the field facet mirror and the pupil facet mirror can be equipped with a tempering device.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2009/100856 A1 [0008, 0076]
• US 6977718 B1 [0009, 0015]
• US 6428173 B1 [0012]
• WO 2010/003527 A1 [0013]
• US 2007/0165202 A1 [0014]
• WO 2005/026843 A2 [0014]
• WO 2008/131928 A1 [0014]
• EP 1262836 A1 [0014]
• DE 2007041004 A1 [0015]
• DE 102008009600 A1 [0015, 0015]

Claims (14)

A multiple mirror assembly comprising: a support structure ( 120 ) containing a plurality of individual mirror elements ( 110 ), wherein a mirror element is a mirror substrate ( 112 ), which one with a reflection coating ( 118 ) coated front surface ( 114 ) and one of the front surface in a distance opposite rear surface ( 116 ), which faces the support structure, characterized by a tempering device for generating a lateral stationary temperature gradient in a lying between the front surface and the rear surface temperature-controlled zone ( 150 ) of the substrate in response to signals from a controller ( 160 ). A multi-mirror array according to claim 1, wherein the mirror substrate ( 112 ) at least in the region of the temperature-controllable zone consists of a substrate material which at room temperature (20 ° C) has a thermal expansion coefficient of at least 1 · 10 ^-6 K ^-1 , wherein the substrate material is preferably a semiconductor material or a metallic material. Multiple mirror arrangement according to claim 1 or 2, wherein substrate dimensions, the substrate material and the tempering device are matched to one another such that for a thermally induced tilting of the front surface ( 114 ) opposite the back surface ( 116 ) a sensitivity S in the range between 0.3 μrad / K and 90 μrad / K, in particular between 1 μrad / K and 30 μrad / K is present, wherein at a sensitivity S = 1 μrad / K a temperature difference ΔT = 1 K between opposite side surfaces of the Substrate generated a tilt of the front surface by 1 μrad. Multiple mirror arrangement according to one of the preceding claims, wherein between the temperature-controlled zone ( 150 ) and the front surface ( 114 ) an intermediate region which can not be tempered by the tempering device ( 155 ), wherein the temperature-controllable zone ( 150 ) preferably midway between the front surface and the rear surface or closer to the rear surface than the front surface. A multiple mirror arrangement according to any one of the preceding claims, wherein a cooling means is provided for actively cooling the support structure. A multiple mirror arrangement according to any one of the preceding claims, wherein between the support structure and the mirror substrate a thermally conductive intermediate structure ( 130 ), wherein the intermediate structure preferably comprises or is formed by an adhesive layer or a solder layer. A multiple mirror device according to any one of the preceding claims, wherein the mirror substrate has side surfaces and at least one of the side surfaces in the region of the temperature-controllable zone, an electrothermal transducer (WX1, WX2, WY1, WY2) is arranged. A multiple mirror arrangement according to claim 7, wherein electrothermal transducers are arranged in pairs on the mirror substrate ( 112 ), wherein a first electrothermal transducer (WX1) of a first pair (WX1, WX2) is disposed on a first side surface and a second electrothermal transducer (WX2) of the first pair is disposed on a second side surface opposite in a first direction. A multiple mirror assembly according to claim 8, wherein at least a second pair (WY1, WY2) of electrothermal transducers are provided on opposite side surfaces of the mirror substrate in a second direction transverse to, in particular perpendicular to, the first pair of electrothermal transducers (WX1, WX2) defined first direction. A multiple mirror device according to claim 8 or 9, wherein the transducers (WX1, WX2, WY1, WY2) of the pairs are independently controllable. Multiple mirror arrangement according to one of the preceding claims, characterized by a control device ( 160 ) connected measuring device ( 170 ) for determining the orientation of the front surface ( 114 ) and for generating measurement signals representing the orientation of the front surface, wherein the control device is configured to control the tempering device on the basis of the measurement signals An optical system having a plurality of optical elements, characterized in that the optical system comprises at least one multiple mirror arrangement according to one of claims 1 to 11. An optical system according to claim 12, wherein said optical system is an optical system of a projection exposure apparatus ( 500 ) for microlithography, preferably a lighting system ( 512 ) Projection exposure system. A method of operating a controllable multi-mirror array having a support structure supporting a plurality of individual mirror elements, wherein a mirror element comprises a mirror substrate having one with a Reflective coating coated front surface and having a front surface in a spaced opposite rear surface facing the support structure, characterized by the generation of a lateral stationary temperature gradient in lying between the front surface and the rear surface temperature-controlled zone of the substrate in response to signals of a control device.

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