Apparatus for determining a distance, method for determining a distance, and lithography system

This application claims priority to German Patent Application No. DE 10 2020 213 326.3 filed October 22, 2020 incorporated by reference herein in their entirety to form a part of the present disclosure.

The invention relates to an apparatus for determining a distance, comprising an optical resonator, which has a plurality of resonant freguencies, and at least one radiation source, the spectrum of which comprises at least one target resonant freguency of the optical resonator.

The invention further relates to a method for determining a distance, according to which a target resonant freguency of an optical resonator, which has a plurality of resonant freguencies, is determined by means of radiation from a radiation source which is coupled into the optical resonator and the spectrum of which comprises at least the target resonant freguency.

Furthermore, the invention relates to a lithography system, in particular a projection exposure apparatus for the semiconductor industry.

The functionality of systems used to guide and shape radiation is based to a particular extent on a correct positioning of individual components of the system in relation to one another.

A tolerable extent of a deviation between a sought-after relative position of the components in relation to one another or in relation to a reference point depends, inter alia, on a wavelength of the radiation to be guided and shaped.

In particular when use is made of EUV (extreme ultraviolet) radiation, only small deviations of the relative position of the components in relation to one another or in relation to a reference point are tolerated on account of the short wavelength of the EUV radiation.

In this context, it may be the case that deviations of only a few picometres are tolerable.

Determining a position very precisely may also be relevant for other technical fields.

Apparatuses and methods for determining a distance with such high demands on accuracy known from the prior art are based on freguency measurements since freguencies can be captured with great precision using measurement technology.

The prior art has disclosed the analysis of freguencies of radiation in an optical resonator for the purposes of measuring a distance on the basis of a freguency measurement. The optical resonator influences the freguencies of the radiation situated in the optical resonator depending on a resonator length. In particular, the optical resonator has eigenfreguencies or resonant freguencies, which are adopted by radiation situated in the resonator. In this context, the resonant frequencies are those frequencies which are come across virtually exclusively in an unimpeded optical resonator.

DE 10 2018 208 147 A1 describes a measurement arrangement for the frequency-based determination of the position of a component.

DE 10 2019 201 146 A1 describes an interferometric measurement arrangement in an optical system.

The values of the resonant frequencies depend in particular on the resonator length. Therefore, information about the resonator length can be obtained by way of a suitable analysis of the resonant frequencies. This can realize a distance measurement.

For highly precise measurements, the use of a frequency comb is known from the prior art for analysing the resonant frequency.

However, the resonant frequencies of the optical resonator may also depend on other parameters than only the resonator length. In particular, it is known that resonant frequencies are dependent on a polarization and/or a form of spatial modes.

In this context, the dependence on additional parameters may lead to the formation of a fine structure. In this context, a single resonant frequency may be split into a plurality of resonant frequencies that under certain circumstances lie close to one another, at which radiation located in the optical resonator may be present.

In particular, a split of the resonant frequencies on account of birefringence at components of the optical resonator is known from practice. Therefore, a disadvantage of the prior art is that other resonant frequencies located very close to the resonant frequency to be examined may make a measurement more difficult within the scope of determining the resonator length from a resonant frequency.

The present invention is based on the object of developing an apparatus for determining a distance, which avoids the disadvantages of the prior art and, in particular, facilitates a reliable determination of a target resonant frequency.

According to the invention, this object is achieved by an apparatus having the features mentioned in Claim 1.

The present invention is further based on the object of developing a method for determining a distance, which avoids the disadvantages of the prior art and, in particular, reliably determines a target resonant frequency of the optical resonator. According to the invention, this object is achieved by a method having the features mentioned in Claim 25.

The present invention is further based on the object of developing a lithography system, in particular a projection exposure apparatus, which avoids the disadvantages of the prior art and, in particular, facilitates reliable and exact positioning of components of the lithography system, in particular of the projection exposure apparatus, in relation to one another and in relation to at least one reference point.

According to the invention, this object is achieved by a lithography system, in particular a projection exposure apparatus, having the features mentioned in Claim 45.

The apparatus according to the invention for determining a distance comprises an optical resonator, which has a plurality of resonant frequencies, and at least one radiation source. In this context, the spectrum of the radiation source comprises at least one target resonant frequency of the optical resonator. Further, provision is made for an isolation device to be provided in the beam path of the optical resonator, said isolation device isolating the target resonant frequency on the basis of the polarization from other resonant frequencies of the optical resonator.

The isolation of the target resonant frequency from in particular other resonant frequencies situated in the direct vicinity in the frequency domain, brought about by the isolation device, facilitates an advantageously accurate and reliable determination of the target resonant frequency, for example on account of an advantageously high signal-to-noise ratio.

In this case, in particular those resonant frequencies emerging from splitting from an original resonant frequency by birefringence at elements in the optical resonator are particularly close to the target resonant frequency in the frequency domain.

As a result, it is advantageous if a polarization direction is used as a selection parameter on the basis of which the target resonant frequency is isolated from other resonant frequencies.

The term optical resonator used in the context of the present invention should comprise all resonators for radiation, in particular for electromagnetic radiation. The reference to an optical resonator does not imply any restriction to, for example, light that is optically perceivable by humans. By way of example, the optical resonator can also be understood to be a microwave resonator.

In an advantageous development of the apparatus according to the invention, provision can be made for the at least one radiation source to be tuneable.

A particularly efficient formation of radiation at the target resonant frequency in the optical resonator arises if use is made of a tuneable radiation source. The frequency of the radiation emerging from the radiation source can be set in the case of a tuneable radiation source. Thus, in particular, provision can be made for the frequency of the radiation source to be set to the target resonant frequency. Thus, under certain circumstances, the radiation present in the optical resonator may almost exclusively have a frequency that corresponds to the target resonant frequency.

In an advantageous development of the apparatus according to the invention, provision can be made for a control loop to be provided, the latter being configured to stabilize the tuneable radiation source, wherein the tuneable radiation source is able to be stabilized at the target resonant frequency. By stabilizing the tuneable radiation source at the target resonant frequency by means of a control loop the radiation can advantageously be output stably by the radiation source at a frequency that equals the target resonant frequency.

As a result of this, the radiation is almost exclusively present at the target resonant frequency. In particular, the frequency of the radiation can stably remain at the target resonant frequency over time as a result thereof. In this case, it is particularly advantageous if control loop and radiation source are configured in such a way that they are able to be stabilized at the target resonant frequency following the isolation of the target resonant frequency according to the invention. That is to say that the isolation device should preferably isolate the target resonant frequency to a sufficient extent from other resonant frequencies in order to facilitate the stabilization of the tuneable radiation source at the target resonant frequency.

Further, there is no need for a direct readout of the radiation in the optical resonator as a result of stabilizing the tuneable radiation source to the target resonant frequency; instead, the frequency to which the tuneable radiation source has been set can be used for determining the target resonant frequency since the tuneable radiation source is stabilized at the target resonant frequency.

In an advantageous development of the apparatus according to the invention, provision can be made for the control loop to be configured according to the Pound-Drever-Hall technique.

A configuration of the control loop according to the Pound-Drever-Hall technique offers the advantage of the Pound-Drever-Hall technique facilitating particularly efficient and reliable stabilization.

Alternatively, provision can be made of other stabilization methods according to which the control loop can be configured.

In an advantageous development of the apparatus according to the invention, provision can be made for at least a short pulse radiation source and a beat analysis device to be provided for determining a beat frequency of a superposition signal formed by the superposition of the radiation from the tuneable radiation source on short pulse radiation from the short pulse radiation source in order to determine the frequency of the radiation from the radiation source. If the apparatus according to the invention has a short pulse radiation source, in particular a femtosecond laser for example, the short pulse radiation emanating from the short pulse radiation source can be superposed by radiation of the tuneable radiation source. The superposition signal arising as a result has a beat frequency, the analysis of which rendering it possible to determine the frequency of the radiation from the radiation source.

Sometimes, such a configuration of the apparatus according to the invention can be advantageous to form a frequency comb, by means of which it is possible to determine the resonator length particularly accurately.

In an advantageous development of the apparatus according to the invention, provision can be made for the optical resonator to have polarization eigenstates, which are adopted almost exclusively by the radiation at the respective resonant frequencies.

It is particularly advantageous if the optical resonator has polarization eigenstates. In a for example undisturbed optical resonator, radiation is almost exclusively present in one of the polarization eigenstates. In this case, the polarization eigenstates are clearly distinguishable or discrete. As a result, the isolation device can be designed in such a way, for example, that it isolates resonant frequencies from the target resonant frequency in accordance with the polarization eigenstates.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to separate the target resonant frequency from the other resonant frequencies.

Provision can be made for the isolation device to isolate the other resonant frequencies from the target resonant frequency by virtue of separating the other frequencies from the target resonant frequency in the frequency domain. This may mean that a frequency spacing between the target resonant frequency and the adjacent other resonant frequencies is increased. This can also be understood to be a shift of the target resonant frequency and the other resonant frequencies in relation to one another. In addition to such a separation of the resonant frequencies and the target resonant frequency, provision can also be made for the isolation device to isolate the target resonant frequency from the other resonant frequencies by virtue of the other resonant frequencies being suppressed. Thus, an isolation of the target resonant frequency from other resonant frequencies can also be achieved by virtue of the isolation device causing the optical resonator to be prevented from forming resonant frequencies which are located in such a vicinity of the target resonant frequency that a formation of the target resonant frequency would be impeded thereby.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to separate the target resonant frequency from the other resonant frequencies by at least one linewidth, preferably by at least twice the linewidth, of the target resonant frequency.

A separation of the target resonant frequency from the other resonant frequencies such that the target resonant frequency is separated from the other resonant frequencies by at least one linewidth, preferably by at least twice the linewidth, of the target resonant frequency is advantageous in that if line broadening of the target resonant frequency and/or the other adjacent resonant frequencies should occur, a clear distinction between the target resonant frequency and the other adjacent resonant frequencies is made possible. In general, a separation between the target resonant frequency and the other resonant frequencies that is as large as possible is advantageous.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to separate the target resonant frequency from the other resonant frequencies in such a way that a difference between the target resonant frequency and the other resonant frequencies is formed so as to be greater than a modulation frequency of the control loop.

If the difference between the target resonant frequency and the other resonant frequencies is advantageously greater than the modulation frequency of the control loop, it is possible, particularly in the case where the control loop is configured according to the Pound-Drever-Hall technique, to avoid that an adjacent resonant frequency influences an error signal of the control loop by virtue of having the same frequency as the target resonant frequency added with the modulation frequency of the control loop. At the frequency arising from the addition of the target resonant frequency and the modulation frequency, the control loop or the error signal of the control loop may react particularly sensitively to the occurrence of another resonant frequency.

It is particularly advantageous for the difference to be greater than 1.5-times the modulation frequency of the control loop.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to separate the target resonant frequency from the other resonant frequencies in such a way that a difference between the target resonant frequency and the other resonant frequencies corresponds at least approximately to a difference between adjacent resonant frequencies of spatial modes of radiation in the polarization eigenstate of the target resonant frequency.

The isolation device preferably operates by way of birefringence. This birefringence of the isolation device is preferably chosen to be so great that the frequency difference of the polarization eigenmodes is not only greater than a Fabry-Perot resonance width but also greater than the Pound-Drever-Hall modulation frequency. In particular, the frequency difference should preferably be greater than the Pound-Drever-Hall modulation frequency including a certain factor of for example 1 .5 or greater in order to take account of the outwardly falling edges of the Pound-Drever-Hall error signal. In the case of a frequency modulation, further sidebands may moreover occur at multiples of the Pound-Drever-Hall modulation frequency, and so a greater frequency difference should optionally be chosen. Advantageously, interferences in the measurement signal by coupling to the second polarization eigenmode can be suppressed in this way.

A formula-based description of the described circumstances arises from the following: For a resonator length L, the free spectral range of the optical resonator is given by Formula (1), where c denotes the speed of light.

FSR = — 2L (1)

A resonance width of the resonant frequencies is linked to the free spectral range FSR by way of the finesse F. Formula (2) represents these circumstances.

_ FSR ^l FWHM ~ _F

The birefringence 6 (measured in radians) leads to a frequency difference between the eigenstates or a frequency difference between the resonant frequencies. This frequency difference A is given by Formula (3).

A = ~FSR (3)

So that the resonant frequencies are spaced apart by at least one linewidth 6, the relationship specified in Formula (4) must therefore apply.

A > ^FWHM

Formula (5) arises from inserting Formulae (1), (2) and (3) into Formula (4).

It is particularly preferred if the frequency difference A is greaterthan the Pound-Drever-Hall frequency fpDH.

To satisfy this condition it is necessary for the condition formulated in Formula (6) to be satisfied.

— 2ir FSR > fpDH (6)

It is particularly advantageous if the condition formulated in Formula (7) is satisfied, wherein the aforementioned factor of 1 .5 or greater is included.

— FSR 1-5 fpDH (7) In a case where the isolation device leads to a linear birefringence the polarization eigenstates of the radiation in the optical resonator are likewise linear. However, in a general case, provision can be made for the polarization eigenstates to be circular and/or elliptical.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to separate the target resonant frequency from the other resonant frequencies in such a way that a difference between the target resonant frequency and the other resonant frequencies corresponds at least approximately to a difference between adjacent resonant frequencies of spatial modes of radiation in the polarization eigenstate of the target resonant frequency.

If the radiation in the optical resonator has different spatial modes, resonant frequencies can, apart from on the basis of the polarization, also be split on the basis of the respective spatial mode, which in turn leads to a fine structure.

It is advantageous if the isolation device separates the target resonant frequency from the other resonant frequencies in such a way that the separation corresponds at least approximately to a frequency spacing of resonant frequencies adjacent to the target resonant frequency, which separation arises from splitting a resonant frequency with a certain polarization eigenstate into the target resonant frequency and other resonant frequencies of other spatial modes. That is to say, the target resonant frequency is separated in such a way that the adjacent and interfering resonant frequency of a different polarization direction falls on a resonant frequency of a higher spatial mode of the target resonant frequency.

Such a shift is advantageous in that another adjacent resonant frequency shifted in that way only has a very small interaction with the target resonant frequency on account of the orthogonality of the spatial modes.

In an advantageous development of the apparatus according to the invention, provision can be made for the beat analysis device to have a frequency standard embodied as a gas cell.

To determine an absolute frequency, provision can be made for the beat analysis device to have a frequency standard. In this case, the use of a gas cell as frequency standard is particularly advantageous since gas cells facilitate the definition of a clear frequency line in a technically known and stable and hence reliable manner.

In an advantageous development of the apparatus according to the invention, provision can be made for the optical resonator to have at least one stationary resonator mirror and at least one deflection mirror.

In this case, the deflection mirror is set up to deflect or divert the radiation in the optical resonator.

By way of example, this can realize folded resonators. By way of example, a resonator folded into a V-configuration can be realized by virtue of the radiation emanating from a first stationary resonator mirror striking a plane deflection mirror and being deflected by the latter in the direction of a second stationary resonator mirror. Following this, the radiation reflected by the second stationary resonator mirror is deflected, in turn, by the deflection mirror to the first stationary resonator mirror.

Provision can also be made for only one resonator mirror from which the radiation emanates to be provided, wherein the at least one deflection mirror reflects the radiation in its direction of incidence and casts it back to the resonator mirror. As a result of this, it is possible, for example, for the optical resonator to maintain its functionality even in the case of slight variations in the angles from the direction of the resonator mirror and the at least one deflection mirror in relation to one another.

In particular, provision can be made for the optical resonator to be formed by a stationary resonator mirror in the form of a concave mirror and a first deflection mirror and a second, plane deflection mirror. In this case, the optical resonator cavity is formed by the resonator mirror and the plane mirror.

The use of a deflection mirror further offers the advantage of being able to vary a position of the deflection mirror, and hence the resonator length.

In an advantageous development of the apparatus according to the invention, provision can be made for the optical resonator to have a first stationary resonator mirror and a second stationary resonator mirror and at least one deflection mirror.

If the optical resonator has a first and a second stationary resonator mirror and the radiation situated in the optical resonator is reflected between the resonator mirrors by at least one deflection mirror, it is thus possible, for example, to advantageously vary a resonator length by changing the position of the deflection mirror.

In this case, the use of a deflection mirror is particularly advantageous if the deflection mirror is embodied in such a way that it always casts incident light back in the direction of the incidence, and consequently ensures the stability of the optical resonator.

In an advantageous development of the apparatus according to the invention, provision can be made for the isolation device to be embodied as a polarization-dependent retardation element.

A particularly advantageous and efficient way of isolating, in particular separating, the target resonant frequency from other resonant frequencies may lie in the use of a polarization-dependent retardation element as an isolation device. If the speed of propagation of radiation within the optical resonator is retarded on the basis of its polarization direction by the isolation device, a lower resonant frequency arises for the radiation retarded in this way on account of an increased circulation time through the optical resonator.

It is particularly advantageous in this case if the retardation element is formed from a birefringent material since a birefringent material leads to a polarization-dependent retardation of radiation passing through the material. This increases a circulation duration in the optical resonator of the radiation in one of the polarization eigenstates, as a result of which the frequency of the radiation is reduced.

In an advantageous development of the apparatus according to the invention, provision can be made for the retardation element to be embodied as a retardation coating on the deflection mirror.

A particularly advantageously implementable type of embodiment of the retardation element can be formed by a coating of the deflection mirror of the optical resonator, said coating being formed from a plurality of layers in such a way that the relative phase of reflected radiation that is s-polarized in relation to a plane of incidence and of reflected radiation that is p-polarized in relation to the plane of incidence is adjustable by way of a suitable choice of the layer thickness values and the refractive indices.

This can achieve a polarization-dependent retardation effect without the addition of further components and can hence achieve an isolation of the target resonant frequency from other resonant frequencies.

A retarding effect of an individual mirror in a corner reflector of a phase difference of 180°, which corresponds to metallic reflection, leads to an overall retardation of the corner reflector of 0°.

It can be shown that the overall retardation of the corner reflector about this zero has a gradient of

2V3 ® 3.5.

A necessary mirror phase difference as per Formulae (1) to (7) for a desired overall retardation can consequently be calculated easily.

A suitable choice of the layer thickness levels, of the material, in particular of the refractive indices, and of the number of layers for obtaining a mirror phase difference required for a desired overall retardation can be ascertained therefrom.

Preferably, a deflection mirror, in particular a corner reflector, is linearly birefringent when passed through twice in the case of an overall retardation unequal to zero such that the polarization eigenstates are linear polarization eigenstates.

An overall retardation arises from the retardation experienced by the radiation in the case of a complete circulation through the optical resonator. If a plurality of spatial modes build up, these circumstances can likewise be expressed in formulae.

The resonant frequencies of adjacent spatial modes for an optical resonator, which is formed for example from a plane mirror and a curved mirror and optional further folding mirrors, differ by a fraction of the free spectral range FSR. In the example shown in Figure 9, this fraction is slightly more than one fifth of the free spectral range FSR.

Using this, a required retardation can be calculated from a radius of the resonator mirror R and the resonator length L. A formula can also be specified for a general case of two curved resonator mirrors.

In an advantageous development of the apparatus according to the invention, provision can be made for the retardation element to be embodied as a retardation plate.

As an alternative and/or in addition thereto, provision can be made for a retardation plate to be arranged in the optical resonator as a retardation element or isolation device in order to isolate the target resonant frequency from other resonant frequencies in polarization-dependent fashion.

In an advantageous development of the apparatus according to the invention, provision can be made for the tuneable radiation source to have a polarization that corresponds to one of the polarization eigenstates.

If the radiation formed by the tuneable radiation source and brought into the optical resonator has one of the polarization eigenstates of the optical resonator, radiation at the target resonant frequency can be brought from the tuneable radiation source into the optical resonator, said radiation having a polarization direction that is particularly stable in the optical resonator.

This avoids the optical resonator being operated in a different polarization to a polarization eigenstate.

In an advantageous development of the apparatus according to the invention, provision can be made for the target resonant frequency to be the lowest resonant frequency.

Selecting the target resonant frequency from the resonant frequencies of the optical resonator in such a way that the target resonant frequency is the lowest resonant frequency is advantageous in that the lowest resonant frequency represents a fundamental frequency of the optical resonator. The radiation situated in the optical resonator can adopt the fundamental frequency in particularly stable fashion. In an advantageous development of the apparatus according to the invention, provision can be made for the at least one deflection mirror to be arranged and/or embodied such that an angle of incidence of the radiation on reflecting surfaces of the deflection mirror is greater than 0°, preferably greater than 10°, particularly preferably greater than 20°. Preferably, the at least one deflection mirror is arranged and/or embodied such that the angle of incidence of the radiation on reflecting surfaces of the deflection mirror is less than 80°.

Such a configuration can be particularly suitable for individual plane deflection mirrors.

By way of example, if the deflection mirror is embodied as a corner reflector, an angle of incidence of 53° to 58°, in particular 54.7°, in relation to the surface normal of the reflecting surfaces can be particularly advantageous as this corresponds to an incidence that is at least approximately parallel to an axis of symmetry of the corner reflector.

Even if the deflection mirror is embodied as a corner reflector, provision can be made in specific embodiments for the deflection mirror to be arranged and/or embodied such that the angle of incidence in relation to at least one surface normal of a reflecting surface is less than 53° orgreaterthan 58°, in particular lies within the aforementioned value ranges of the plane deflection mirror, since this facilitates a positioning of the deflection mirror at a multiplicity of angles in relation to the direction of propagation of the radiation.

An angle of incidence of the radiation in relation to the mirror normal which is greater than zero is advantageous in that an optical path length is increased by a retardation coating possibly arranged on the mirror or deflection mirror.

This can advantageously reinforce a retarding effect of the retardation coating and consequently reinforce the separation of the resonant frequencies.

In an advantageous development of the apparatus according to the invention, provision can be made for the at least one deflection mirror to be embodied as a corner-reflector reflector and/or as a corner cube and/or as a cat's eye mirror.

If the deflection mirror is formed in the aforementioned way, it is possible to resort to known efficient and reliable embodiments of the deflection mirror. As a result, the apparatus according to the invention can advantageously be designed simply.

In an advantageous development of the apparatus according to the invention, provision can be made for the polarization of the target resonant frequency to be the polarization for which the optical resonator has the greatest finesse and/or reflectivity. If the target resonant frequency is chosen as the resonant frequency forthe polarization of which the optical resonator has the greatest finesse and/or reflectivity, an advantageously small linewidth arises for the radiation at the target resonant frequency, as a result of which an isolation of the target resonant frequency from other resonant frequencies is advantageously possible in a simple manner. As a result, a stabilization of the frequency of the radiated-in light at the target resonant frequency can be implemented very accurately and precisely in turn.

Further, it may be advantageous to operate the optical resonator at that resonant frequency for which an overall reflectivity, and hence the finesse, is highest. For a deflection mirror or corner reflector and reflectivity values of the resonator mirrors and the deflection mirrors which have a value of at least approximately 1 , an overall reflectivity of the polarization eigenstates is given by Formulae (8) and (9). Formula (8) describes the overall reflectivity of a first polarization eigenstate.

Formula (9) describes the overall reflectivity of a second polarization eigenstate.

In this case, the overall polarization of the first eigenmode depends on the reflectivity of the mirrors in s- polarization (R _s ) and the reflectivity in p-polarization (R _P ). In particular, a dielectric mirror usually has a significantly higher reflectivity in the s-polarization than a p-polarization. Advantageously, optical resonators in many different geometries therefore have an optical path length which is longer for s-polarized radiation than for p-polarized radiation. Such a behaviour is counter to the usual behaviour of a dielectric mirror. However, a dielectric mirror is only optimized for a maximum reflectivity. Nevertheless, such a behaviour can be achieved by a suitable layer design.

In an advantageous development of the apparatus according to the invention, provision can be made for the polarization of the target resonant frequency to be the polarization for which the optical resonator has the greatest reflectivity and the longest optical path length.

Such a configuration is advantageous in that the radiation at the target resonant frequency has an advantageously small linewidth. The long optical path length yields an advantageously low resonant frequency and/or an advantageously pronounced and efficient isolation of the target resonant frequency from other resonant frequencies of the optical resonator.

In an advantageous development of the apparatus according to the invention, provision can be made for at least one part of the optical resonatorto be arranged at a component, the distance of which from a reference point should be determined. To determine a distance, the apparatus according to the invention can be used particularly advantageously if a part of the optical resonator is physically connected to a component, for example a projection exposure apparatus, in order to determine the distance thereof from another component or a reference point. Below, reference is made to a reference point which might also be a different component, for example. A resonator length and hence a value of a target resonant frequency can be defined by a position of the component or the distance thereof from a reference point. If the resonator length corresponds to the distance of the component from the reference point, the distance of the component from the reference point can be determined by determining the resonant frequency. In particular, provision can be made for a distance of the component from a reference point to be determined, with the reference point being an initial position of the component. Thus, in particular, it is possible to detect a change in the resonator length and hence a change in the distance. This advantageously facilitates an attainment of the determination of a distance from an initial point.

In an advantageous development of the apparatus according to the invention, provision can be made for the deflection mirror of the optical resonator to be arranged at the component.

If the deflection mirror of the optical resonator is arranged on the component, this is advantageous, in particular, in that the positions of inclination-sensitive elements of the resonator mirror can remain stationary while the deflection mirror changes its position with the component. Since the deflection mirror casts the light back into the incident direction, the optical resonator can be operated stably even in the case of a shift and a change in the position of the component.

A particularly preferred development of the apparatus according to the invention can consist of the optical resonator having a first stationary resonator mirror, a second stationary resonator mirror, a first deflection mirror which is embodied as a plane mirror and a second, stationary deflection mirror which is embodied as a corner reflector.

Here, provision can be made for the resonator mirrors, the plane mirror and the corner reflector to be arranged in such a way that the radiation emanating from the first resonator mirror is steered from the plane mirror to the corner reflector. Following this, the radiation is cast back with a spatial offset in its direction of incidence by the corner reflector and steered onto the second resonator mirror by the plane mirror.

Such a configuration is also referred to as double folded cavity and has the advantage of a compact structure with a high stability at the same time. The great stability arises from the fact that the corner reflector always casts the radiation back to the second resonator mirror for as long as the radiation emanating from the first resonator mirror strikes the first deflection mirror.

In this case, it can be particularly advantageous if the first, plane deflection mirror is arranged at the component. Advantageously, the first, plane deflection mirror can have a simple structure and can, in the case of a low weight and a small size, represent a small load on the component. The invention further relates to a method for determining a distance according to the preamble of Claim 25.

The method according to the invention for determining a distance provides for a target resonant frequency of an optical resonator, which has a plurality of resonant frequencies, to be determined by means of radiation from a radiation source which is coupled into the optical resonator and the spectrum of which comprises at least the target resonant frequency. The method according to the invention provides for the target resonant frequency to be isolated on the basis of its polarization from other resonant frequencies of the optical resonator.

The method according to the invention facilitates an accurate and reliable determination of a resonator length by virtue of determining a target resonant frequency. According to the invention, the target resonant frequency is advantageously determined accurately and reliably by virtue of the target resonant frequency being isolated from other resonant frequencies. As a result of isolating the target resonant frequency from the other resonant frequencies according to the invention, the signal-to-noise ratio with which the target resonant frequency can be determined, for example, is improved.

In an advantageous development of the method according to the invention, provision can be made for the radiation to be formed by a tuneable radiation source.

If the radiation situated in the optical resonator is formed by a tuneable radiation source, it is possible to form the radiation at the target resonant frequency. This advantageously reduces the formation of frequencies of the radiation which are not at the target resonant frequency in the optical resonator.

In an advantageous development of the method according to the invention, provision can be made for radiation at the target resonant frequency to be radiated into the optical resonator and stabilized at the target resonant frequency by means of a control loop which is configured to stabilize the tuneable radiation source.

Stabilizing the tuneable radiation source at the target resonant frequency of the optical resonator means that the control loop sets the tuneable radiation source in such a way that the latter emits radiation at a frequency corresponding to the target resonant frequency. Furthermore, should the frequency of the radiation emitted by the radiation source deviate from the target resonant frequency, the control loop causes the radiation source to be set in such a way that the frequency of the emitted radiation corresponds to the target resonant frequency again. Advantageously, the frequency to which the control loop sets the tuneable radiation source can be used to determine the target resonant frequency as a result thereof.

In an advantageous development of the method according to the invention, provision can be made for the control loop to be operated according to the Pound-Drever-Hall technique. Operating the control loop according to the Pound-Drever-Hall technique facilitates a reliable and fast stabilization of the tuneable radiation source at the target resonant frequency.

In an advantageous development of the method according to the invention, provision can be made for the radiation from the tuneable radiation source to be superposed on short pulse radiation from a short pulse radiation source and a beat frequency of a superposition signal formed thereby to be determined by means of the beat analysis device.

The target resonant frequency can be determined particularly accurately and reliably by virtue of radiation of the tuneable radiation source that has been stabilized at the target resonant frequency being superposed with short pulse radiation. By superposing the radiation of the tuneable radiation source on the short pulse radiation a superposition signal with a beat frequency arises. Analysing the beat frequency facilitates very accurate determination of the target resonant frequency in the case of a suitable choice of the parameters pulse duration and pulse frequency of the short pulse radiation source in comparison with the target frequency. The resonator length, for example, can also be determined very accurately by way of a very accurate determination of the target resonant frequency.

In an advantageous development of the method according to the invention, provision can be made for the beat frequency of the superposition signal to be determined by means of a beat analysis device which has a frequency standard preferably embodied as a gas cell.

An analysis of the beat frequency by means of a beat analysis device having a frequency standard is particularly preferred. By way of a frequency standard it is possible to determine an absolute target resonant frequency and hence an absolute resonator length.

In this case, gas cells are an advantageous realization for a frequency standard as they are simple and robust.

In other embodiments, the frequency standard can also be embodied as an optical resonator which is stabilized by means of a GPS (global positioning system) signal.

In an advantageous development of the method according to the invention, provision can be made for the radiation at the resonant frequencies to be present almost exclusively in polarization eigenstates of the optical resonator.

If the radiation in the optical resonator at the resonant frequencies is almost exclusively present in the polarization eigenstates, i.e., if the optical resonator is not operated outside of its polarization eigenstates, the radiation is present in discrete and distinguishable polarization states. As a result, an isolation according to the invention of the target resonant frequency from other resonant frequencies on the basis of the polarization of the radiation can be implemented particularly easily. Thus, in particular, it is necessary to isolate discrete and distinguishable resonant peaks in a resonant spectrum from the target resonant frequency.

In an advantageous development of the method according to the invention, provision can be made for the target resonant frequency to be separated from the other resonant frequencies.

A separation of the target resonant frequency from the other resonant frequencies, i.e., a shift of the target resonant frequency in relation to the other resonant frequencies in a frequency domain represents an advantageous manner of isolation since it is not necessary to intervene, for example in absorptive fashion, in the optical resonator but the resonant frequency only needs to be shifted by means of suitable measures.

In particular, frequency-dependent low-pass filters with an edge profile that would be necessary and/or suitable for isolating the target resonant frequency from the other resonant frequencies of the optical resonator are very difficult to produce.

In an advantageous development of the method according to the invention, provision can be made for the target resonant frequency to be separated from the other resonant frequencies by at least one linewidth, preferably by at least twice the linewidth, of the target resonant frequency.

A separation of the target resonant frequency from the other resonant frequencies by at least one linewidth, preferably at least twice the linewidth, of the target resonant frequency and/or the other resonant frequencies is advantageous in this case in that a determination of the target resonant frequency becomes more accurate, the further the other resonant frequencies are separated therefrom.

In this case, an advantageous lower limit for separation is represented by the linewidth of the target resonant frequency or of the adjacent other resonant frequencies. In particular, provision can be made for the greatest linewidth observed among the adjacent resonant frequencies to be used as a lower limit for a separation of the target resonant frequency from other resonant frequencies.

This ensures that, for example as a result of the control loop, components of other resonant frequencies are no longer able to impede the determination of the target resonant frequency in appreciable fashion.

In an advantageous development of the method according to the invention, provision can be made for the target resonant frequency to be separated from the other resonant frequencies in such a way that a difference between the target resonant frequency and the other resonant frequencies is formed so as to be greater than a modulation frequency of the control loop. If the target resonant frequency is separated from the other resonant frequencies by at least the modulation frequency of the control loop, incorrect influencing of the signal of the control loop by adjacent resonant frequencies can be reduced.

In particular, provision can be made for the difference between the target resonant frequency and the other resonant frequencies to be greater than or equal to 1 ,5-times the modulation frequency of the control loop. This can additionally prevent, for example in the case of a Pound-Drever-Hall technique, the other resonant frequency adjacent to the target resonant frequency from influencing a zero of the error signal of the Pound- Drever-Hall control loop at the modulation frequency.

Further, provision can be made for the difference between the target resonant frequency and the other resonant frequencies to be greater than or equal to the sum of the modulation frequency of the control loop and a linewidth, in particular a maximum linewidth, of the target resonant frequency and/or the other resonant frequencies.

In an advantageous development of the method according to the invention, provision can be made for the target resonant frequency to be separated from the other resonant frequencies in such a way that a difference between the target resonant frequency and the other resonant frequencies corresponds at least approximately to a difference between adjacent resonant frequencies of spatial modes of radiation in the polarization eigenstate of the target resonant frequency of the optical resonator.

A shift or separation of the target resonant frequency in relation to the other resonant frequencies such that the closest adjacent other resonant frequency comes to rest on the resonant frequency of a higher mode of the target resonant frequency with the same polarization has the advantage that this minimizes influencing of the target resonant frequency by the adjacent other resonant frequency since the higher spatial modes are orthogonal to one another.

In an advantageous development of the method according to the invention, provision can be made for a distance of a deflection mirror from at least one stationary resonator mirror to be determined.

Determining the distance between the deflection mirror and the at least one stationary resonator mirror can be implemented in this case by determining the resonator length. If the optical resonator consists only of a stationary resonator mirror and the deflection mirror, the distance between the deflection mirror and the stationary resonator mirror is given by half the resonator length.

In an advantageous development of the method according to the invention, provision can be made for a distance of a deflection mirror from a first stationary resonator mirror and/or a second stationary resonator mirror to be determined. As a result, a distance measurement between the deflection mirror and a resonator mirror can be advantageously translated into a measurement of the resonator length.

In an advantageous development of the method according to the invention, provision can be made for the target resonant frequency to be isolated from other resonant frequencies of the optical resonator by a retardation element.

The target resonant frequency can be isolated from other resonant frequencies of the optical resonator by a retardation element. In this case, the retardation element causes radiation in the optical resonator to be decelerated depending on the polarization direction. As a result, depending on its polarization, some of the radiation requires a longer time for circulation in the optical resonator and therefore has a lower frequency. In particular, provision can be made here for the target resonant frequency to be reduced in comparison with the other resonant frequencies by virtue of a retardation element decelerating the radiation which is at the target resonant frequency with a certain polarization.

As a result, an isolation according to the invention of the target resonant frequency from other frequencies can be achieved in advantageous fashion.

In an advantageous development of the method according to the invention, provision can be made for the radiation to be coupled into the optical resonator with a polarization corresponding to one of the polarization eigenstates.

Input coupling radiation with a polarization corresponding to one ofthe polarization eigenstates of the optical resonator is particularly advantageous; it is especially advantageous if this is the polarization eigenstate which the target frequency should have. What this can advantageously bring about is that almost exclusively radiation at the target resonant frequency in the desired polarization eigenstate is present in the optical resonator. As a result, other resonant frequencies build up only to a small extent, preferably not at all.

In an advantageous development of the method according to the invention, provision can be made for radiation at the lowest resonant frequency to be coupled into the optical resonator.

The lowest resonant frequency is the fundamental frequency of the optical resonator. Coupling in radiation at the lowest resonant frequency is therefore advantageous in that the fundamental mode or the fundamental frequency builds up most strongly in the optical resonator.

In an advantageous development of the method according to the invention, provision can be made for a distance of the deflection mirror from at least one of the resonator mirrors to be determined from the target resonant frequency, according to which the target resonant frequency is determined from the frequency of the radiation radiated into the optical resonator by the tuneable radiation source that has been stabilized at the target resonant frequency.

Determining a distance, for example from the deflection mirror to one of the resonator mirrors, can advantageously be implemented directly from a measurement of the target resonant frequency in that case. The target resonant frequency itself can advantageously be determined by reading the frequency at which the tuneable radiation source has been stabilized by the control loop.

In an advantageous development of the method according to the invention, provision can be made for radiation in the optical resonator to strike the at least one deflection mirror at an angle of incidence that is greater than 0°, preferably greater than 10°, particularly preferably greater than 20°. Preferably, the at least one deflection mirror is arranged and/or embodied such that the angle of incidence of the radiation on reflecting surfaces of the deflection mirror is less than 80°.

A long optical path length, which is caused by a large angle with respect to the surface normal, is advantageous in that an effect on the radiation by a coating, for example a coating of the deflection mirror, that acts as a retardation element is maximized. This can advantageously reduce, e.g., a coating thickness of the deflection mirror, which may lead to lower costs of the deflection mirror under certain circumstances.

In an advantageous development of the method according to the invention, provision can be made for at least one distance of a component from a reference point to be determined, wherein at least a part of the optical resonator is arranged at the component.

A distance of a component, for example of a projection exposure apparatus, from a reference point can be determined particularly accurately and precisely by the method according to the invention. This applies in particular if at least one part of the optical resonator is arranged at the component. In this case, a change in the position of the component leads to a change in the resonator length and hence to a change in the target resonant frequency. As a result, measuring the target resonant frequency allows conclusions to be drawn about a change in the distance of a component from a reference point. In particular, an absolute distance of the component from the reference point can be determined in the case of an absolute length measurement.

In an advantageous development of the method according to the invention, provision can be made for the part of the optical resonator arranged at the component to be the deflection mirror.

It is particularly advantageous if the deflection mirror as part of the optical resonator is arranged at the component since the deflection mirror casts incident light back in the direction of incidence, as a result of which the optical resonator can be operated particularly stably. The invention further relates to a lithography system, in particular a projection exposure apparatus.

The lithography system according to the invention, in particular a projection exposure apparatus for the semiconductor industry, has at least one component, in particular an optical element, particularly preferably a mirror. According to the invention, provision is made for at least one actual position of at least one of the components to be determined by means of the apparatus according to the invention and/or by means of the method according to the invention by virtue of determining a distance of the component from a reference point.

The lithography system according to the invention, in particular the projection exposure apparatus for the semiconductor industry, is particularly suitable for producing semiconductor structures by means of EUV light. By applying the apparatus according to the invention and/or the method according to the invention for the purposes of positioning components, in particular mirrors in relation to one another and/or in relation to a desired imaging plane, the lithography system according to the invention can advantageously be used to produce fine and precise semiconductor structures.

An advantageous development of the lithography system according to the invention can consist in that an adjustment device is provided for bringing the actual position of at least one of the components closer to at least one target position.

The adjustment device can calculate or correct the at least one distance from a target position on the basis of the actual position ascertained by means of the apparatus according to the invention or the method according to the invention. Alternatively, provision can likewise be made for the adjustment device to determine or correct the target position by virtue of adding a certain displacement length to the actual position.

The lithography system according to the invention, in particular the projection exposure apparatus for the semiconductor industry, which has at least one component, in particular an optical element, particularly preferably a mirror, can alternatively be characterized in that an apparatus according to the invention is provided which determines a distance of at least one of the components from a reference point as an actual position of the component.

Features described in conjunction with one of the subjects of the invention, specifically given by the apparatus according to the invention, the method according to the invention and the lithography system according to the invention, are also advantageously implementable for the other subjects of the invention. Advantages specified in conjunction with one of the subjects of the invention, specifically given by the apparatus according to the invention, the method according to the invention and the lithography system according to the invention, can also be understood to relate to the other subjects of the invention.

It should supplementarily be pointed out that terms such as "comprising", "having or "with" do not exclude other features or steps. Furthermore, terms such as "a(n)" or "the" which indicate single steps or features do not preclude a plurality of features or steps - and vice versa.

Exemplary embodiments of the invention are described in greater detail below with reference to the drawing.

The figures in each case show preferred exemplary embodiments in which individual features of the present invention are illustrated in combination with one another. Features of an exemplary embodiment are also able to be implemented independently of the other features of the same exemplary embodiment, and may readily be combined accordingly by a person skilled in the art to form further expedient combinations and sub-combinations with features of other exemplary embodiments.

In the figures, functionally identical elements are provided with the same reference signs.

Figure 1 shows an EUV projection exposure apparatus;

Figure 2 shows a DUV projection exposure apparatus;

Figure 3 shows a basic illustration of an apparatus for determining a distance;

Figure 3a shows a basic illustration of a double folded optical resonator;

Figure 4 shows a very approximate basic illustration of a spectrum of resonant frequencies of the optical resonator;

Figure 5 shows a further very approximate basic illustration of the spectrum of resonant frequencies of the optical resonator;

Figure 6 shows a basic illustration of a Pound-Drever-Hall error signal;

Figure 7 shows a basic illustration of a Pound-Drever-Hall error signal with higher spatial modes;

Figure 8 shows a further basic illustration of a Pound-Drever-Hall error signal; and

Figure 9 shows a basic illustration of a retardation.

Figure 1 shows by way of example the basic set-up of a lithography system or of an EUV projection exposure apparatus 400 for semiconductor lithography for which the invention can preferably find application. In particular, the invention can find application by virtue of at least one actual position of at least one component 102, 103, 104, 105, 106, 107, 108, 109, 140, 401 , 402, 403, 406, 407, 408, 411 , 412, 415, 416, 417, 418, 419, 420, preferably an optical element 18, 19, 20, 415, 416, 418, 419, 420, 108, particularly preferably a mirror, being determined by means of an apparatus according to any one of Claims 1 to 24 and/or by means of a method according to any one of Claims 25 to 44 by virtue of determining a distance of the component from a reference point or from another component.

An illumination system 401 of the projection exposure apparatus 400 comprises, besides a radiation source 402, an optical unit 403 for the illumination of an object field 404 in an object plane 405. A reticle 406 arranged in the object field 404 is illuminated, said reticle being held by a reticle holder 407, illustrated schematically. A projection optical unit 408, illustrated merely schematically, serves for imaging the object field 404 into an image field 409 in an image plane 410. A structure on the reticle 406 is imaged on a lightsensitive layer of a wafer 411 arranged in the region of the image field 409 in the image plane 410, said wafer being held by a wafer holder 412 that is likewise illustrated in part.

The radiation source 402 can emit EUV radiation 413, in particular in the range of between 5 nanometres and 30 nanometres, in particular 13.5 nm. Optically differently designed and mechanically adjustable optical elements are used for controlling the radiation path of the EUV radiation 413. In the case of the EUV projection exposure apparatus 400 illustrated in Figure 1 , the optical elements are embodied as adjustable mirrors in suitable embodiments, which are mentioned merely by way of example below.

The EUV radiation 413 generated by means of the radiation source 402 is aligned by means of a collector integrated in the radiation source 402 in such a way that the EUV radiation 413 passes through an intermediate focus in the region of an intermediate focal plane 414 before the EUV radiation 413 impinges on a field facet mirror 415. Downstream of the field facet mirror 415, the EUV radiation 413 is reflected by a pupil facet mirror 416. With the aid of the pupil facet mirror 416 and an optical assembly 417 having mirrors 418, 419, 420, field facets of the field facet mirror 415 are imaged into the object field 404.

Figure 2 illustrates an exemplary DUV projection exposure apparatus 100. The projection exposure apparatus 100 comprises an illumination system 103, a device known as a reticle stage 104 for receiving and exactly positioning a reticle 105, by which the later structures on a wafer 102 are determined, a wafer holder 106 for holding, moving and exactly positioning the wafer 102 and an imaging device, to be specific a projection lens 107, with a plurality of optical elements 108, which are held by way of mounts 109 in a lens housing 140 of the projection lens 107.

The optical elements 108 can be designed as individual refractive, diffractive and/or reflective optical elements 108, such as for example lens elements, mirrors, prisms, terminating plates and the like.

The basic functional principle of the projection exposure apparatus 100 provides for the structures introduced into the reticle 105 to be imaged onto the wafer 102. The illumination system 103 provides a projection beam 11 1 in the form of electromagnetic radiation, which is required for the imaging of the reticle 105 on the wafer 102. A laser, a plasma source or the like can be used as the source of this radiation. The radiation is shaped in the illumination system 103 by means of optical elements such that the projection beam 111 has the desired properties with regard to diameter, polarization, shape of the wavefront and the like when it is incident on the reticle 105.

An image of the reticle 105 is generated by means of the projection beam 1 11 and transferred from the projection lens 107 to the wafer 102 in an appropriately reduced form. In this case, the reticle 105 and the wafer 102 can be moved synchronously, so that regions of the reticle 105 are imaged onto corresponding regions of the wafer 102 virtually continuously during a so-called scanning process.

An air gap between the last optical element 108 or the last lens element and the wafer 102 can be replaced by a liquid medium having a refractive index of > 1 . The liquid medium can be high-purity water, for example. Such a set-up is also referred to as immersion lithography and has an increased photolithographic resolution.

Figure 3 shows a basic illustration of an apparatus 1 for determining a distance, comprising an optical resonator 2 having a multiplicity of resonant frequencies 3. The apparatus 1 further comprises at least one radiation source 4, the spectrum of which comprises at least one target resonant frequency 5 of the optical resonator 2. An isolation device 6 is provided in the beam path of the optical resonator 2, said isolation device isolating the target resonant frequency 5 on the basis of its polarization from other resonant frequencies 3 of the optical resonator 2 (see Figure 4).

The radiation source 4 is tuneable in the present exemplary embodiment.

In the exemplary embodiment of the apparatus 1 illustrated in Figure 3, provision is further made of a control loop 7 which is configured to stabilize the tuneable radiation source 4. In this case, the tuneable radiation source 4 is able to be stabilized at the target resonant frequency 5 or is stabilized at the target resonant frequency.

Some of the radiation reflected back from the optical resonator 2 is taken by means of a taking device 7a and analysed by means of a resonator radiation analysis device (not illustrated) for example belonging to the control loop 7.

Further, the control loop 7 in the present exemplary embodiment is configured according to the Pound- Drever-Hall technique.

The apparatus illustrated in the exemplary embodiment as per Figure 3 further comprises a short pulse radiation source 8 and a beat analysis device 9 for determining a beat frequency of a superposition signal which is formed by superposing short pulse radiation from the short pulse radiation source 8 on radiation of the tuneable radiation source 4. This configuration is set up to determine the frequency of the radiation of the tuneable radiation source 4.

In this context, a superposition device 9a is used to superpose radiation from the radiation source 4 on short pulse radiation formed by the short pulse radiation source 8.

In the illustrated exemplary embodiment, the optical resonator 2 has polarization eigenstates, which are almost exclusively adopted by the radiation at the respective resonant frequencies 3.

In the present exemplary embodiment, the isolation device 6 is set up in such a way that it separates the target resonant frequency 5 from the other resonant frequencies 3.

In particular, the isolation device 6 separates the target resonant frequency 5 from the other resonant frequencies 3 by preferably at least one linewidth of the target resonant frequency 5. In the exemplary embodiment, provision is preferably made for the isolation device 6 to separate the target resonant frequency 5 from the other resonant frequencies 3 by twice the linewidth or at least twice the linewidth of the target resonant frequency 5.

Likewise, the isolation device 6 is designed such that it separates the target resonant frequency 5 from the other resonant frequencies 3 in such a way that a difference between the target resonant frequency 5 and the other resonant frequencies 3 is formed so as to be greater than a modulation frequency 10 of the control loop 7.

Furthermore, the apparatus 1 represented in the exemplary embodiment illustrated in Figure 3 is designed in such a way that the isolation device 6 separates the target resonant frequency 5 from the other resonant frequencies 3 in such a way that a difference between the target resonant frequency 5 and the other resonant frequencies 3 corresponds at least approximately to a difference between adjacent resonant frequencies 3 of spatial modes of radiation in the polarization eigenstate of the target resonant frequency 5.

The beat analysis device 9 further has a frequency standard which is embodied as a gas cell.

In other embodiments, the frequency standard can also be embodied as an optical resonator which is stabilized by means of a GPS (global positioning system) signal.

In the illustrated exemplary embodiment, the optical resonator 2 has a first stationary resonator mirror 11 and a second stationary resonator mirror 12 and a deflection mirror 13.

In this case, provision can be made in particular for the first resonator mirror 11 and/or the second resonator mirror 12 to be provided as semi-transmissive mirrors. The use of semi-transmissive mirrors as resonator mirrors simplifies input coupling of radiation into the optical resonator 2 and/or output coupling of radiation from the optical resonator 2.

Alternatively, provision can also be made for the optical resonator 2 to have a stationary resonator mirror and a deflection mirror 13.

Further, provision can be made for the optical resonator 2 to have at least one stationary resonator mirror and at least one deflection mirror 13.

In the exemplary embodiment illustrated in Figure 3, the isolation device 6 is embodied as a polarizationdependent retardation element. In particular, the retardation element is embodied as a retardation plate.

Alternatively, provision can be made for the retardation element to be embodied as a retardation coating on the deflection mirror 13.

Radiation from the tuneable radiation source 4, which is coupled into the optical resonator 2, has a polarization in the illustrated exemplary embodiment which corresponds to one of the polarization eigenstates of the optical resonator 2.

In this case, the deflection mirror 13 is arranged and embodied such that an angle of incidence of the radiation on the deflection mirror 13 is greater than 0°. The angle of incidence can also be greater than 10° and, in particular, also greater than 20°. Preferably, the at least one deflection mirror 13 is arranged and/or embodied such that the angle of incidence of the radiation on reflecting surfaces of the deflection mirror 13 is less than 80°.

Such a configuration can be particularly suitable for individual plane deflection mirrors.

In the present exemplary embodiment, the deflection mirror 13 is embodied as a corner-reflector reflector. Alternatively, provision can be made for the deflection mirror to be embodied as a corner cube and/or as a cat's eye mirror.

If the deflection mirror 13 is embodied as a corner reflector like in the exemplary embodiment illustrated in Figure 3, an angle of incidence of 53° to 58°, in particular 54.7°, in relation to the surface normals of the reflecting surfaces can be particularly advantageous as this corresponds to an incidence that is at least approximately parallel to an axis of symmetry of the corner reflector.

In the present exemplary embodiment, the polarization of the radiation at the target resonant frequency 5 is the polarization at which the optical resonator 2 has the greatest finesse or reflectivity.

Additionally, the polarization is the polarization at which the optical resonator has the longest optical path length. Provision is made for at least part of the optical resonator 2, the deflection mirror 13 in the illustrated exemplary embodiment, to be arranged at a component 14, the distance of which should be determined from another component or from a reference point.

The component 14 is arranged at an adjustment device 15 in order to bring the actual position of the component 14 at least closer to a target position.

The component 14 whose distance should be determined from another component or from a reference point can be any component of the projection exposure apparatus 100, 400, in particular the components 102, 103, 104, 105, 106, 107, 108, 109, 140, 401 , 402, 403, 406, 407, 408, 411 , 412, 415, 416, 417, 418, 419, 420, in particular one of the optical elements 415, 416, 418, 419, 420, 108, in particular one of the mirrors of one of the EUV and DUV projection exposure apparatuses 100, 400 shown in Figures 1 and 2. The reference point or the other component can likewise be one of the components 102, 103, 104, 105, 106, 107, 108, 109, 140, 401 , 402, 403, 406, 407, 408, 411 , 412, 415, 416, 417, 418, 419, 420 of the projection exposure apparatuses shown in Figures 1 and 2.

The apparatus 1 illustrated in Figure 3 is particularly suitable for carrying out a method for determining a distance, wherein the target resonant frequency 5 of the optical resonator 2, which has a plurality of resonant frequencies 3, is determined by means of the radiation from the radiation source 4 which is coupled into the optical resonator 2, and the spectrum of which comprises at least the target resonant frequency 5. In the method, the target resonant frequency 5 is isolated on the basis of its polarization from other resonant frequencies 3 of the optical resonator 2.

Figure 3a shows a further possible embodiment of the optical resonator 2. In this case, the optical resonator 2 is embodied as a so-called double folded optical resonator or as double folded cavity. In the exemplary embodiment illustrated in Figure 3a, the optical resonator 2 has a first stationary resonator mirror 1 1 , a second stationary resonator mirror 12, a first, plane deflection mirror 13a and a second, stationary deflection mirror 13b, which is embodied as corner reflector. In this case, the first, plane deflection mirror 13a is arranged at the component 14 (not illustrated).

Further, the resonator mirrors 11 , 12, the plane deflection mirror 13a and the second deflection mirror 13b or corner reflector are arranged such that the radiation emanating from the first resonator mirror 11 is steered by the plane deflection mirror 13a onto the second deflection mirror 13b or corner reflector. Following this, the radiation is cast back with a spatial offset in its direction of incidence by the second deflection mirror 13b or the corner reflector and steered onto the second resonator mirror 12 by the plane deflection mirror 13a.

In the exemplary embodiment of the optical resonator 2 illustrated in Figure 3a, the isolation device 6 is preferably embodied as a coating (not shown) of the second deflection mirror 13b of the optical resonator 2, said coating being formed from a plurality of layers in such a way that the relative phase of reflected radiation that is s-polarized in relation to a plane of incidence and of reflected radiation that is p-polarized in relation to the plane of incidence is adjustable by way of a suitable choice of the layer thickness values and the refractive indices.

Figure 4 shows a very approximate basic illustration of a spectrum of resonant frequencies 3 of the optical resonator 2. A frequency is plotted on a horizontal X-axis while an intensity is plotted on a vertical Y-axis. In this case, the target resonant frequency 5 is so close to one of the adjacent resonant frequencies 3 that it is not possible to stabilize the tuneable radiation source 4 at the target resonant frequency 5.

Figure 5 shows a further approximate basic illustration of the resonant spectrum of the optical resonator 2, wherein the isolation device 6 is arranged in the beam path of the optical resonator 2 in such a way that the target resonant frequency 5 is isolated from the adjacent next resonant frequency 3 by virtue of being separated from the next adjacent resonant frequency 3. Said separation is obtained by the isolation device 6 by shifting the target resonant frequency 5 to lower frequency values. The frequency is plotted on the horizontal X-axis while the intensity is plotted on the vertical Y-axis.

In the exemplary embodiment, the target resonant frequency 5 is the lowest of the resonant frequencies 3 in the optical resonator 2.

In particular, the method for determining a distance can be carried out by virtue of determining a distance of the deflection mirror 13 from at least one of the resonator mirrors 11 , 12 using the target resonant frequency 5. In this case, the target resonant frequency 5 is determined from the frequency of the radiation, radiated into the optical resonator 2, from the tuneable radiation source 4 that has been stabilized at the target resonant frequency 5.

Figure 6 shows a basic illustration of an error signal of the Pound-Drever-Hall control loop 7. In this case, the error signal is plotted on a vertical Y-axis and a phase is plotted on the horizontal X-axis. In this case, the error signal has zeroes at the resonant frequency, which has the phase value of 0 on the X-axis, and at the modulation frequency 10 of the control loop 7, which occur on both sides of the phase value of 0.

It is particularly advantageous for the method for determining a distance to be designed such that the target resonant frequency 5 is separated from the other resonant frequencies 3 in such a way that a difference between the target resonant frequency 5 and the other resonant frequencies 3 is formed so as to be greater than the modulation frequency 10 of the control loop 7. In particular, provision is made for the difference to be equal to or greater than 1.5-times the modulation frequency 10 of the control loop 7.

Figure 7 shows a basic illustration of the Pound-Drever-Hall error signal for different spatial modes, denoted a to e, of the optical resonator 2. In this case, zeros of the error signal occur at resonant frequencies 3 of the respectively higher modes. Further zeros appear from the zeros of the edges, separated by the modulation frequency 10.

In this case, the Pound-Drever-Hall error signal is once again plotted on a vertical Y-axis while a phase angle is plotted on a horizontal X-axis. Illustrated are error signals of the zeroth, the first, the second, the third and the fourth higher spatial mode of the resonant frequency.

Figure 8 shows a basic illustration of a zeroth spatial mode, denoted as g, of the adjacent other resonant frequency 3, denoted as g, from the target resonant frequency 5, denoted as f, of the optical resonator 2. The other resonant frequency 3, denoted as g, is separated from the target resonant frequency 5 by the isolation device 6 in such a way that the difference between the target resonant frequency 5 and the other resonant frequency 3, denoted as g, at least approximately corresponds to the difference between adjacent resonant frequencies 3 of spatial modes, denoted by 1 and 2, of radiation in a polarization eigenstate of the target resonant frequencies 5, of the optical resonator 2.

As a result of the adjacent resonant frequency 3 having been placed on the frequency of a higher target resonant frequency 5, in this case of the first spatial mode, a build-up of the target resonant frequency 5 is reduced.

Figure 9 shows a basic illustration of a retardation of a deflection mirror 13, embodied as a corner reflector, which has been provided with an isolation device 6 in the form of a retardation coating. In this case, an overall retardation in degrees is plotted on a vertical Y-axis. A mirror retardation in degrees is plotted on a horizontal X-axis.

It is evident in this case that the overall retardation of the deflection mirror 13 embodied as corner reflector yields 0° for a retardation of the three individual mirrors of 180° in each case. Consequently, it is easy to calculate a required mirror phase difference for a desired overall retardation. Preferably, a deflection mirror 13 or corner reflector is linearly birefringent for a double pass in the case of an overall retardation unequal to 0 such that the eigenmodes of the radiation in the optical resonator 2 are linearly polarized.

List of reference signs:

1 Apparatus

2 Optical resonator

3 Resonant frequency

4 Radiation source

5 Target resonant frequency

6 Isolation device

7 Control loop

7a Taking device

8 Short pulse radiation source

9 Beat analysis device

9a Superposition device

10 Modulation frequency

11 First stationary resonator mirror

12 Second stationary resonator mirror

13 Deflection mirror

13a First deflection mirror

13b Second deflection mirror

14 Component

15 Adjustment device

100 Projection exposure apparatus

102 Wafer

103 Illumination system

104 Reticle stage

105 Reticle

106 Wafer holder

107 Projection lens

108 Optical element

109 Mount

111 Projection beam

140 Lens housing

400 Projection exposure apparatus Illumination system

Radiation source

Optical unit

Object field

Object plane

Reticle

Reticle holder

Projection optical unit

Image field

Image plane

Wafer

Wafer holder

EUV radiation

Intermediate focal plane

Field facet mirror

Pupil facet mirror

Optical assembly

Mirror

Mirror

Mirror