WO2015154994A1

WO2015154994A1 - Method and arrangement for actuating an element

Info

Publication number: WO2015154994A1
Application number: PCT/EP2015/056290
Authority: WO
Inventors: Gerald Rothenhöfer; Christian Kempter
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2014-04-07
Filing date: 2015-03-24
Publication date: 2015-10-15
Anticipated expiration: 2016-10-07
Also published as: JP6550398B2; DE102014206686A1; TWI684086B; JP2017513064A; US10025203B2; TW201610634A; US20170023869A1

Abstract

Method and arrangement for actuating an element The invention relates to a method and an arrangement (100) for actuating an element (140) in a system for microlithography. According to an aspect in at least one degree of freedom an actuator force is exerted on the element by means of at least two actuator components (110, 120), wherein said actuator components are driven independently of one another for generating the actuator force, and wherein said driving is effected in such a way that a thermal power (P_diss) introduced into the system on account of the generation of the actuator force by the actuator components deviates from a predefined constant value by not more than 20%.

Description

Method and arrangement for actuating an element

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application DE 10 2014 206 686.7 filed on April 7, 2014. The content of this application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the invention

The invention relates to a method and an arrangement for actuating an element in a system for microlithography.

The arrangement according to the invention is advantageously useable for example in an optical system, in particular a device for determining the position of structures on a mask or else in a microlithographic projection exposure apparatus (for example for actuating a mirror in an illumination unit or in a projection lens). The invention also advantageously useable in other systems, in particular in systems in which transient thermal loads lead to an impairment of the operating properties of the system and limiting said transient thermal loads is thus of major importance.

Prior art

In systems in which the exact positioning of an element with particularly high accuracy and setting speed is desirable, such as for example and in particular in optical systems, the exact and fast positioning is often at odds with the applicable restrictions with regard to the maximum permissible thermal loads in the system.

In this case, the transient (i.e. arising in a non-constant manner, or non- steady-state) thermal loads - associated e.g. with an actuation at varying positioning speeds - can be particularly problematic, which loads, for instance by way of temporally variable mechanical deformations induced by them in the relevant element and/or some other structure within the system can result in impairments of the operating properties of the system, said impairments being dependent on the operation of the actuator.

Known approaches for overcoming the problems brought about by transient thermal loads include e.g. thermally isolating the component(s) bringing about the relevant thermal loads from the rest of the system, or implementing active or passive cooling strategies. However, such approaches lead to an increased constructional outlay and, moreover, also do not always ensure a sufficiently fast or exact elimination of the transient thermal loads arising.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and an arrangement for actuating an element in a system for microlithography which enable exact and fast positioning whilst at least substantially avoiding the problems brought about by transient thermal loads.

Said object is achieved by means of the method and the arrangement according to the features of the independent claims.

In a method according to the invention for actuating an element in a system for microlithography, in at least one degree of freedom an actuator force is exerted on the element by means of at least two actuator components; wherein said actuator components are driven independently of one another for generating the actuator force; and - wherein said driving is effected in such a way that a thermal power introduced into the system on account of the generation of the actuator force by the actuator components deviates from a predefined constant value by not more than 20%. The invention is based on the concept, in particular, in the course of actuating an element in a system, of reducing or even completely eliminating the introduction of transient thermal loads into said system by virtue of the fact that an increased constant thermal load (which can also be designated as "nominal thermal load") is accepted by way of the predefinition of a constant value for the thermal power introduced into the system on account of the actuation, and by virtue of the fact that the actuator components are driven with permanent generation of said thermal power. This approach also proceeds from the consideration that regardless of a thermal load that is possibly increased but arises in a constant fashion in the process, the system can be operated at thermal equilibrium and in this respect, in particular, undesirable deformations that impair the operating properties (e.g. the optical properties of a microlithographic projection exposure apparatus that react particularly sensitively to such deformations) no longer take place.

In this case, the criterion according to which the thermal power introduced into the system by the actuator components deviates from the predefined constant value by not more than 20% is intended to express the fact that the invention should be deemed also already to encompass scenarios in which the constant value predefined for the thermal power introduced into the system is not always complied with exactly wherein, if appropriate, the occasional generation of transient loads can also still be afforded tolerance to a certain extent, depending on the predefined value of the thermal power.

In accordance with one embodiment, the driving of the actuator components is effected in such a way that a thermal power introduced into the system on account of the generation of the actuator force by the actuator components deviates from the predefined constant value by not more than 10%, in particular by not more than 5%. In accordance with one embodiment, the predefined constant value for the thermal power introduced into the system by the actuator components is chosen in such a way that said thermal power corresponds to the thermal power introduced into the system upon generation of the maximum permissible actuator force by the actuator components.

In accordance with one embodiment, the actuator components are coils to which electric current is applied for generating the actuator force.

In accordance with one embodiment, the driving of the actuator components is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element.

The invention also relates to a method for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force is exerted on the element by means of at least two actuator components; wherein said actuator components are driven independently of one another for generating the actuator force; and - wherein that said driving is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element. In accordance with one embodiment, the actuator components at least occasionally generate force components that counteract one another. The invention also relates to a method for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force is exerted on the element by means of at least two actuator components; wherein said actuator components are driven independently of one another for generating the actuator force; and wherein the actuator components at least occasionally generate force components that counteract one another.

In accordance with one embodiment, the actuator force is adjusted in amount and/or direction by varying said force components (in amount and/or direction).

In accordance with one embodiment, the actuator components are coils, wherein an electric current being applied to each of said coils, respectively, is at least occasionally varied (in amperage and/or current flow direction) in order to vary said force components.

In accordance with one embodiment, the driving of the actuator components is effected at least occasionally such that the thermal power introduced into the system by the actuator components at least partly compensates for a thermal disturbance present in the system, in particular on account of a heat source present elsewhere in the system.

In accordance with one embodiment, the system is an optical system.

The invention further also relates to an arrangement for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein said driving is effected in such a way that a thermal power introduced into the system on account of the generation of the actuator force by the actuator components deviates from a predefined value by not more than 20%.

The invention further also relates to an arrangement for actuating an element in a system for microhthography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein said driving is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element.

The invention further also relates to an arrangement for actuating an element in a system for microhthography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein the actuator components at least occasionally generate force components that counteract one another.

The invention further also relates to an optical system comprising an arrangement having the features described above, in particular a system for microhthography, such as e.g. a device for determining the position of structures on a mask or an optical system of a microlithographic projection exposure apparatus.

Further configurations of the invention can be gathered from the description and the dependent claims. The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

shows a basic schematic diagram for elucidating one possible construction of an actuating arrangement with which the method according to the invention is realizable; shows a control diagram for elucidating one possible embodiment of the method according to the invention; and show diagrams for elucidating the consideration - possible in embodiments of the invention - of position dependencies of the motor constants in an actuating arrangement according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 firstly shows a basic schematic diagram for elucidating one possible construction of an arrangement for actuating an element in a system, in which arrangement the method according to the invention is realizable.

In accordance with Fig. 1 , an actuator arrangement 100 for actuating an element 140 - which, merely by way of example, can be an optical element

(e.g. a mirror or a lens element) in an optical system - comprises a first coil 1 10, a second coil 120 and a permanent magnet 130. An electric current and i₂ can respectively be applied to the coils 1 10, 120 independently of one another, with the consequence that the respective coil 1 10, 120 in a given degree of freedom respectively exerts a force Fi and F₂ on the permanent magnet 130 or the element 140 mechanically coupled to the permanent magnet 130.

5

For the power introduced overall into the system by the coils 1 10, 120 owing to said application of current, which power hereinafter is also designated "dissipated power" P_di_SS, the following then holds true:

i o P_dlss. = Ri + R₂ (1 )

For the force F exerted overall on the element 140 by the coils 1 10, 120 the following holds true:

15 F = k_li_l + k₂i₂ (2)

In equations (1 ) and (2) above, Ri and R₂ denote the electrical resistances of the coils 1 10, 120, and ki and k₂ denote the motor constants (or actuator constants) of the actuator component respectively formed by the relevant 20 coil 1 10, 120 with the permanent magnet 130. In general, both the motor constants ki and k₂ and the electrical resistances of the coils 1 10, 120 are temperature-dependent, wherein the motor constants ki and k₂ additionally have a position dependence (in the sense of a dependence on the present position of the actuated element 140 or of the permanent magnet 130).

25 These dependencies, which will initially be disregarded in the following consideration, will be discussed in still greater detail.

In accordance with one aspect of the present invention, then, the electric currents and i₂ respectively applied to the coils 1 10, 120 are chosen (i.e.

30 the coils 1 10, 120 are driven), in such a way that the transient thermal loads brought about by this application of current in the system are reduced or even completely eliminated, if appropriate an increased constant thermal load ("nominal thermal load") being accepted. In accordance with one embodiment, in order to ascertain the electric currents and i₂, respectively, the dissipated power P_di_SS in equation (1 ) is predefined in such a way that it corresponds to the power P_max required for generating the maximum permissible force F_max of the actuator arrangement 100. In this case, said maximum force F_max of the actuator arrangement 100 corresponds to that force at which, during continuous operation or else upon a specific operating period being exceeded, a failure of the actuator arrangement 100 occurs for instance owing to the coils 1 10, 120 "burning away".

Solving the above-mentioned equation system comprising equations (1 ) and (2) yields:

The requirement that the equation system comprising (3) and (4) has real (and not just imaginary) solutions, that is to say that the respective root term is greater than zero, yields a limitation of the exerted actuator force as follows:

p 2 < ^ max ^l^l ⁺ ^ max ^l ^2 (5)

^1^2 from which the following maximum value and minimum value for the force generated by the actuator arrangement 100 are obtained: PmM² +P_maxRA²

max (6)

^Rl^R2

Provided that no force outside the above limit values is provided by the actuator arrangement 100, then, the relevant force can be exerted completely without generation of transient thermal loads in the system.

In accordance with the approach described above, the actuator arrangement 100 introduces the maximum power P_max (corresponding to the maximum thermal load) into the system at any time or permanently, wherein said maximum power P_max corresponds to that power which is required for generating the maximum permissible actuator force F_max mentioned above. This has the consequence that in operating phases which call for a lower force in comparison with the maximum permissible actuator force F_max, the coils 1 10, 120 at least partly "work against one another", that is to say that, for example, the first coil 1 10 exerts a force on the element 140 towards the left whereas the second coil 120 exerts a force on the element 140 towards the right), the requisite additional portions (i.e. which cancel one another out in terms of their effect with regard to the respective force components) of the electric currents and i₂ being used for generating heat. This additionally generated heat serves in turn to avoid transient thermal loads, for which purpose an increased level of the constant thermal load introduced into the system is accepted according to the invention.

In this regard, for instance in a simple example in an operating phase that demands the exertion of an actuator force on the element 140 amounting to half the maximum actuator force F_max, the currents and i₂ can be chosen in such a way that the first coil 1 10 generates a force Fi =

0.75^*F_max and the second coil 120 generates a force F₂ = -0.25^*F_max, wherein the current portions cancelling one another out with regard to their force action in the coils 1 10, 120 serve solely for generating heat in order that the thermal load introduced into the system is kept constant, as described above.

The invention in this case makes use of the fact that, on account of the at least two coils 1 10, 120 which are present in the actuator arrangement 100 per degree of freedom and to which electric current can be applied independently of one another, there are different possibilities for generating a specific required force on the respective element 100, wherein these possibilities differ from one another with regard to the application of current to the coils 1 10, 120. This application of current to the coils 1 10, 120 or the driving thereof, is then effected according to the invention in the above- described approach in such a way that although the thermal load introduced overall into the system is possibly increased (i.e. more thermal load is introduced than would be necessary per se for generating the required actuator force), in return as constant a thermal load as possible is maintained (or at least a reduction of the transient portion of the thermal load introduced overall into the system is obtained).

The invention is not restricted to the above-described power required for generating the maximum power P_max (corresponding to the power required for generating the maximum permissible force F_max of the actuator arrangement 100). In further embodiments, a power less than the above maximum power P_max can also be predefined (that is to say corresponding to a force which is less than the maximum permissible force F_max of the actuator arrangement 100). In this case, a complete elimination of transient thermal loads is possible only up to a threshold value of the generated force (i.e. only with the generation of a limited actuator force). Upon said threshold being exceeded, transient thermal loads are generated.

However, said transient thermal loads are still lower than the transient thermal loads which would be generated in the case of conventional driving of the coils 1 10, 120 entirely without the predefinition of a constant dissipated power P_di_SS or without current being applied to the coils 1 10, 120 in a manner at least partly cancelling one another out with regard to the force action thereof. In general, for the dissipated power P_di_SS introduced into the system by the actuator arrangement 100, it is also possible to predefine an arbitrary value which is less than the maximum permissible power P_max (at which a failure of the actuator arrangement 100 occurs e.g. owing to the coils 1 10, 120 "burning away"), although said power P_di_SS must be greater than the minimum power required for generating a desired force. With corresponding predefinition of the dissipated power P_di_SS introduced into the system, the equation system corresponding to equations (3), (4) has real solutions. If, as described above, the value of the dissipated power P_diss introduced into the system is set to the maximum permissible power P_max, the maximum permissible force F_max can be generated.

In accordance with a further aspect of the invention, the above-described predefinition of a power to be dissipated by the application of current to the coils 1 10, 120 can also be effected in order to introduce heat into the system in a targeted manner, wherein said heat can serve for taking account of or compensating for a thermal disturbance present in the system (e.g. owing to other heat sources in the system). In other words, the actuator arrangement 100 according to the invention can also be used in a targeted manner as heating, in order in this way to influence properties of the system and to compensate, by means of control engineering, for thermal disturbance sources in the system, for instance, without the requirement for an additional heating.

In this case, the concept according to the invention can be used to generate the heat required for said compensation "force-neutrally" in so far as the required actuator force on the element 140 is provided by the actuator arrangement 100 in each case without being altered. In contrast to the above-described approach of reducing transient thermal loads, the heat additionally arising during the operation of the actuator arrangement 100 is however not merely accepted here (and e.g. dissipated by means of a suitable cooling unit), but rather constitutes precisely a purposefully set variable used e.g. to compensate for a thermal disturbance generated elsewhere.

Fig. 2 shows a control diagram for elucidating a control that is possible according to the invention. A desired position 21 1 of the element 100 that is to be controlled in terms of its position is predefined as setpoint variable. The difference between said desired position 21 1 and an actual position 212 of the element 100, said actual position being ascertained in a sensor- aided manner, is fed to a position controller 201 , which calculates the required force F to be applied by the actuator arrangement 100. This required force corresponds to a necessary current from which the values of the currents and i₂ which are applied to the coils 1 10, 120 are calculated in turn in a unit 202 (with the realization of the concept according to the invention of predefining a specific constant power P_di_SS to be introduced into the system) with knowledge of the motor constants ki k₂, Ri and R₂. If the forces Fi and F₂ respectively generated in accordance with this driving of the coils 1 10, 120 or application of current thereto are of the same sign, both forces Fi and F₂ act completely on a mechanism 205 that carries the element 140 in an actuatable manner (i.e. are used for the positioning of the element 140) where e.g. the maximum actuator force F_max can be set. However, if the maximum actuator force F_max on the element 140 is not required e.g. in an operating phase, e.g. the force contribution Fi generated by the first coil 1 10 has a negative value and the force contribution F₂ generated by the second coil 120 has a positive value (or vice versa). Consequently, the coils 1 10, 120 partly work against one another, wherein only the difference force AF = Fi - F₂ influences the mechanism 205 or the positioning of the element 140, while the portions of the currents and i₂ that compensate for one another with regard to their force action on the element 140 serve only for the generation of heat (for the purpose of partly or completely eliminating transient thermal loads). Overall, therefore, in accordance with Fig. 2, the currents and i₂ in the coils 1 10, 120 are controlled in such a way that a desired force is exerted by the actuator arrangement 100 with the introduction of a predefined power Pdiss into the system.

As has already been mentioned and is illustrated by way of example in Fig. 3, generally the motor contants ki , k₂ are not position-independent, but rather have a position dependence with regard to the present position of the actuated element 140 or of the permanent magnet 130. Fig. 3 shows merely by way of example a regionally linear position dependence (between a position of -0.01 mm and a position of +0.01 mm), wherein the sum of the two motor constants ki , k₂ is furthermore constant in this example. Given a known characteristic e.g. in accordance with Fig. 3, it is possible to take account of said position dependence by means of a correspondingly modified setting of the currents , i₂, i.e. driving of the coils 1 10, 120, by solving the above equation system for each position of the element 140 (which is generally measured anyway during the operation of the system). Furthermore, it is possible to take account of thermal influences both on the motor contants ki , k₂ and on the electrical resistances Ri and R₂ (for instance in the case where the generation of transient thermal loads is partly allowed in a manner that is possible as described above), in each case by measuring the temperature and solving the above equation system for each position and each temperature. Taking account of the temperature dependence of the electrical resistances Ri and R₂ and the motor constants ki and k₂ is dispensable, however, if transient thermal loads are completely eliminated in accordance with the first-described approach, since then the system is permanently in thermal equilibrium.

In accordance with Fig. 4, different currents , i₂ arise for the different positions during the driving of the coils 1 10, 120, said currents being required in each case for generating a constant power P_di_SS introduced into the system. The diagram from Fig. 4 is applicable here merely by way of example for the position dependence described above with reference to Fig. 3 (other profiles arising, of course, in the case of different position dependence).

As is shown in the diagram from Fig. 5 for the above example of the position dependence of the motor constants ki and k₂, position-dependent limits arise for the achievable maximum force and the achievable minimum force. In the implementation, in each case the narrowest limit (in the example corresponding to a range of values of the permissible force between -1 .5 N and +1 .5 N) can be taken as a basis or a position- dependent limit value for the force can be taken as a basis with the consequence that at no point in time does the position controller 201 from Fig. 2 request a force that exceeds the force permissible for the relevant position.

As soon as the root term in the equation system comprising equations (3) and (4) is no longer positive, either the higher force demanded by the position controller 201 from Fig. 2 cannot be provided or the power P_di_SS rises with the consequence of generation of transient thermal loads. In the implementation, a possible momentary higher power requirement in comparison with the predefined power P_di_SS can furthermore be allowed (wherein a fault message can be generated in this case e.g. after a predefined time period (e.g. of 1 to 2 minutes) has been exceeded).

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are evident to the person skilled in the art, e.g. by combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying to the person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the appended patent claims and the equivalents thereof.

Claims

1 . Method for actuating an element in a system for microlithography,

• wherein in at least one degree of freedom an actuator force is exerted on the element by means of at least two actuator components;

• wherein said actuator components are driven independently of one another for generating the actuator force; and

• wherein said driving is effected in such a way that a thermal power (Pdiss) introduced into the system on account of the generation of the actuator force by the actuator components deviates from a predefined constant value by not more than 20%.

2. Method according to Claim 1 , characterized in that said driving is effected in such a way that a thermal power (Pdiss) introduced into the system on account of the generation of the actuator force by the actuator components deviates from the predefined constant value by not more than 10%, in particular by not more than 5%.

3. Method according to Claim 2, characterized in that the predefined constant value for the thermal power (Pdiss) introduced into the system by the actuator components is chosen in such a way that said thermal power corresponds to the thermal power (P_max) introduced into the system upon generation of the maximum permissible actuator force

(Fmax) by the actuator components.

4. Method according to any of Claims 1 to 3, characterized in that the actuator components are coils (1 10, 120) to which electric current is applied for generating the actuator force.

5. Method according to any of the preceding claims, characterized in that said driving is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element.

Method for actuating an element in a system for microlithography,

• wherein that said driving is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element.

Method according to any of the preceding claims, characterized in that the actuator components at least occasionally generate force components that counteract one another.

Method for actuating an element in a system for microlithography,

• wherein the actuator components at least occasionally generate force components that counteract one another.

Method according to claim 7 or 8, characterized in that the actuator force is adjusted in amount and/or direction by varying said force components.

Method according to any of claims 7 to 9, characterized in that the actuator components are coils (1 10, 120), wherein an electric current being applied to each of said coils (1 10, 120), respectively, is at least occasionally varied in order to vary said force components.

1 1 . Method according to any of the preceding claims, characterized in that said driving is effected at least occasionally such that the thermal power (Pdiss) introduced into the system by the actuator components at least partly compensates for a thermal disturbance present in the system, in particular on account of a heat source present elsewhere in the system.

12. Method according to any of the preceding claims, characterized in that the system is an optical system.

Arrangement for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein said driving is effected in such a way that a thermal power (Pdiss) introduced into the system on account of the generation of the actuator force by the actuator components deviates from a predefined value by not more than 20%.

Arrangement for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein said driving is effected at least occasionally such that the actuator components at least partly cancel one another out with regard to their contribution to the actuator force exerted on the element.

15. Arrangement for actuating an element in a system for microlithography, wherein in at least one degree of freedom an actuator force on the element is generatable by means of at least two actuator components, wherein said actuator components are driveable independently of one another for generating the actuator force, and wherein the actuator components at least occasionally generate force components that counteract one another.

16. Arrangement according to any of Claims 13 to 15, characterized in that said arrangement is designed to carry out a method according to any of Claims 1 to 12.

17. Optical system, characterized in that said optical system comprises an arrangement according to any of Claims 13 to 15.

18. Optical system according to Claim 17, characterized in that said optical system is a device for determining the position of structures on a mask or an optical system of a microlithographic projection exposure apparatus.