EP3803511A1

EP3803511A1 - Measuring assembly for the frequency-based determination of the position of a component

Info

Publication number: EP3803511A1
Application number: EP19724359.5A
Authority: EP
Inventors: Matthias Manger; Andreas KÖNIGER; Alexander Vogler
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2018-05-24
Filing date: 2019-04-30
Publication date: 2021-04-14
Also published as: KR20200143482A; US11274914B2; KR102501932B1; CN112204471A; US20210080244A1; JP7159352B2; CN118151496A; CN112204471B; JP2021524033A; DE102018208147A1; WO2019223968A1

Abstract

The invention relates to a measuring assembly for the frequency-based determination of the position of a component, in particular in an optical system for microlithography, having at least one optical resonator, said resonator comprising a stationary, first resonator mirror, a movable measuring target associated with the component and a stationary, second resonator mirror, the second resonator mirror being formed by an inverting mirror (130, 330, 430, 530) which reflects back into itself a measuring beam coming from the measuring target.

Description

Measuring arrangement for frequency-based

Position determination of a component

The present application claims the priority of German Patent Application DE 10 2018 208 147.6, filed on May 24, 2018. The content of this DE application is incorporated by reference into the present application text by "incorporation by reference".

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a measuring arrangement for the frequency-based position determination of a component, in particular in an optical system for microlithography.

State of the art

Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection objective. In this case, the image of a mask (= reticle) illuminated by means of the illumination device is projected by means of the projection objective onto a substrate (eg a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective. to transfer the mask structure to the photosensitive coating of the substrate.

In EUV-designed projection exposure equipment, i. at wavelengths below 15 nm (e.g., about 13 nm or about 7 nm), mirrors are used as optical components for the imaging process due to the lack of availability of suitable transparent refractive materials.

In the operation of such projection lenses designed for EUV, in which the mask and wafer are usually moved relative to one another in a scanning process, the positions of the mirrors, which are in some cases movable in all six degrees of freedom, have to be adjusted to each other as well as to mask or wafer with high accuracy and maintained in order to avoid or at least reduce aberrations and the associated impairment of the imaging result. In this position determination, e.g. be required over a path length of 1 meter accuracy of the length measurement in the picometer (pm) range.

Various approaches are known in the prior art for measuring the position of the individual objective mirrors as well as the wafer or the wafer stage and the reticle plane. Besides interferometric measurement arrangements, the frequency-based position measurement using an optical resonator is also known.

In a conventional structure shown by way of example in FIG. 12 taken from DE 10 2012 212 663 A1, a resonator 152 in the form of a Fabry-Perot resonator comprises two resonator mirrors 154 and 155, of which the first resonator mirror 154 is connected to a reference element 140 in FIG In the form of a measuring frame fixedly connected to the housing of the projection lens of the projection exposure apparatus, and the second resonator mirror 155 (as a "measuring target") is fastened to an EUV mirror M to be measured with respect to its position. The actual distance measuring device comprises a radiation source 156 that can be tuned with regard to its optical frequency generates a coupling radiation 158, which passes through a beam splitter 162 and is coupled into the optical resonator 152. In this case, the radiation source 156 is controlled by a coupling device 160 so that the optical frequency of the radiation source 156 is tuned to the resonance frequency of the optical resonator 152 and thus coupled to this resonance frequency. Coupling radiation 158 coupled out via a beam splitter 162 is analyzed by means of an optical frequency measuring device 164, which may comprise, for example, a frequency comb generator 132 for the highly accurate determination of the absolute frequency. If the position of the EUV mirror M changes in the x direction, the resonant frequency of the optical resonator 152 also changes with the distance between the resonator mirrors 154 and 155 and thus - due to the coupling of the frequency of the tunable radiation source 156 to the resonant frequency of the resonator 152 - and also the optical frequency of the coupling-in radiation 158, which in turn is registered directly with the frequency measuring device 164.

Essential for the functionality of an optical resonator in the distance measurement, e.g. According to FIG. 12, on the one hand, the measuring beam within the optical resonator can perform as large a number of circulations as possible within the resonator (without leaving the cavity formed by the resonator), so that eigenmodes can be formed in the resonator , Also essential is the ability to couple the external radiation field (= "coupling field") present at the input of the resonator section to the mode field of the optical resonator (= "resonator field"). The coupling efficiency characteristic of said coupling is defined here by the overlap integral between the coupling field and the resonator field, so that, in order to achieve a high coupling efficiency, coupling field and resonator field must match as well as possible in all relevant parameters.

In practice, the use of an optical resonator for distance measurement in the measurement of the position of a component or a mirror can now result in problems in that movements of the measurement target arranged on the mirror (which, for example, in the form of a retroreflector or mirror) a plane mirror can be configured) not only along the actual measuring direction, but also in other of the total of six degrees of freedom can occur. Such (parasitic) movements not taking place along the measuring direction, eg intentional or unintentional tilting or lateral displacements of the measuring target, can lead to an "emigration" of the main beam on which the modes of the resonator are simultaneously "threaded" , in position and angle takes place with the result that a sufficient coupling of the resonator field is no longer given to the coupling field.

In view of the high requirements to be placed on the beam direction deviation (which may require, for example, that angle deviations in the jet vector of the main jet amount to less than 0.1 mrad), the guarantee that tilting or lateral displacements of the measuring target do not become effective in the frequency-based position determination demanding challenge.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a measuring arrangement for the frequency-based position determination of a component, in particular in an optical system for microlithography, which enables a highly accurate position determination while avoiding the problems described above.

This object is achieved according to the features of independent patent claim 1.

A measuring arrangement for the frequency-based position determination of a component, in particular in an optical system for microlithography, comprises: at least one optical resonator, this resonator having a stationary first resonator mirror, a movable measuring target associated with the component, and a stationary second resonator mirror,

- Wherein the second resonator is formed by a reversing mirror, which reflects back a coming from the measurement target measurement beam in itself.

According to one embodiment, the resonator further comprises a retroreflector, which reverses the measuring beam parallel-offset identically in its direction. This retroreflector may be designed as a cube-corner retroreflector (hollow or vitreous retro-reflector) or as a cat-eye retro-reflector (for example with a Fourier lens with a mirror arranged in its focal plane).

The invention is based in particular on the concept of repeatedly traversing the path to be traveled by the measuring beam in an optical resonator by placing a reversing mirror. By utilizing the principle of reversibility of the light path, it is ensured in this way that lateral displacements or tiltings on the part to be measured or of the measurement target assigned to this component, which do not act solely in the measuring direction, do not become effective in the frequency-based position determination or without Affect the measurement result.

In other words, it is achieved by the inventive use of a reversing mirror in the measuring arm, which is reflected back into the measurement beam, regardless of lateral displacements or tiltings of the measuring object associated with the component to be measured, of the measuring beam arriving at said reversing mirror. This measuring beam travels the identical route over the measuring target with the result that variations in the degrees of freedom that do not act along the direction of the measuring arm (measuring axis) are completely eliminated in their effects on the measurement. Lateral displacements or tiltings of the measuring target associated with the component to be measured, transversely to the measuring direction (which are not directly detected by the distance measurement and can therefore also be referred to as "parasitic movements") thus play no role in the distance measurement according to the invention more. As a result, the measuring arrangement according to the invention has an increased insensitivity with respect to the said parasitic movements with the result that a highly accurate position measurement can also be realized in scenarios in which a stable control of the position of said measuring target is not possible or the associated measurement Effort should be avoided.

According to one embodiment, the measurement target is formed by a retroreflector.

According to a further embodiment, the measurement target is formed by a plane mirror.

According to one embodiment, the measuring arrangement has a polarization-optical beam splitter. In this case, in particular as described in more detail below, a vertical incidence on a measuring target designed as a plane mirror can be achieved by folding the beam path directly onto the optical axis using the polarization-optical beam splitter.

According to one embodiment, a measuring beam coming from the polarization-optical beam splitter hits the measuring target perpendicularly.

According to one embodiment, the measuring arrangement has an optical group with two lenses in Kepler arrangement. According to one embodiment, the optical group in a common focal plane of these two lenses has a mirror with an opening which reflects back the beam path returning from the measurement target.

According to one embodiment, the retroreflector is designed to be polarization-preserving.

According to one embodiment, the first resonator mirror has a curvature such that a light field present in the resonator is stably enclosed.

According to one embodiment, the first resonator mirror is designed as a cat's eye mirror. In this case, this mirror is preferably arranged defocused relative to the focal planes of a lens in order to produce a wavefront curvature required for the field inclusion in the resonator.

According to one embodiment, the measuring arrangement has at least one tunable laser stabilized on a resonator mode of the optical resonator.

According to one embodiment, the measuring arrangement has a control loop which is configured to stabilize the tunable laser according to the Pound-Drever-Hall method.

According to one embodiment, the measuring arrangement has at least one femtosecond laser for determining the frequency of the laser radiation of the at least one tunable laser.

According to one embodiment, the measuring arrangement further has a frequency standard, in particular a gas cell.

According to one embodiment, the measuring arrangement for realizing an absolute length measurement has two different resonator modes with known frequency spacing of the optical resonator stabilizable, tunable laser. In this case, each of these two tunable lasers can be assigned a beat frequency analyzer unit.

As a result of the design with two tuneable lasers that can be stabilized to different resonator modes with known frequency spacing of the optical resonator, as explained in more detail below, an otherwise existing ambiguity problem can be taken into account, which is the spectrum of the beat frequencies that represents a periodic diamond pattern eg between a tunable laser stabilized on a resonator mode and a femtosecond laser with respect to the counting direction of the passages through cell boundaries in the diamond pattern. The laser frequencies of the two tunable lasers show at the o.g. Embodiment of the invention namely two entangled grid of beat frequencies, on the basis of which said counting direction ambiguity can be eliminated as further described below.

According to one embodiment, the measuring arrangement has an acousto-optical modulator for realizing a frequency shift in a partial beam branched off from the laser beam generated by the tunable laser.

According to one embodiment, the component for position determination in six degrees of freedom is assigned six optical resonators for frequency-based length measurement.

In one embodiment, the component is a mirror.

According to one embodiment, the optical system is a microlithographic projection exposure apparatus.

Further embodiments of the invention are described in the description and the dependent claims. The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Show it:

Figure 1-11 schematic representations to illustrate different

Embodiments of the invention;

FIG. 12 shows a schematic illustration for explaining a conventional construction of a measuring arrangement for frequency-based position measurement;

Figure 13 is a schematic representation for explaining the possible

Construction of a microlithographic projection exposure system designed for operation in the EUV;

FIG. 14 shows a schematic illustration for explaining a possible realization of measuring sections according to the invention on a mirror in a structure with a load-bearing supporting structure and independently provided measuring structure; and

FIG. 15 shows a schematic illustration for explaining a possible determination of the position of a mirror in six degrees of freedom. DETAILED DESCRIPTION PREFERRED

EMBODIMENTS

FIGS. 1 a - 1 b show schematic representations for explaining the structure and mode of operation of a measuring arrangement in exemplary embodiments of the invention.

1 a, after entering the resonator, a measurement beam strikes a unit 101 (the structure and operation of which will be described in more detail with reference to FIGS. 6-11) and an optical fiber 102 through a fixed curved resonator mirror 110 after passing through a free - Space path A off-axis on a retroreflector 120 (as a measurement target) and is reflected parallel offset back. After passing through a free space path B, the measuring beam is reflected back into itself by means of a reversing mirror 130 which is perpendicular to the beam propagation direction and without beam offset. After again passing through the free space sections B and A, including the retroreflector 120, the measuring beam in turn strikes the stationary curved resonator mirror 110, so that the circulation closes.

Since, following reflection at the inverted mirror (= "recirculation mirror") 130 perpendicular to the beam propagation direction, the measuring beam returns in an identical manner, the principle of "reversibility of the optical path" becomes the one with a transverse displacement of the measuring target Retroreflektors 120 compensated compensated beam offset to zero.

The embodiment shown in FIG. 1 b differs from that of FIG. 1 a only in that, instead of the fixed curved resonator mirror 110, a stationary "cat's eye optic" consisting of a Fourier lens 112 with a mirror 113 arranged in its focal plane is used. In order to generate the wavefront curvature required for inclusion in the resonator, this mirror 113 is arranged in a defocused manner with respect to the focal plane of the lens. In order to understand the further remarks, the expanded formalism of the paraxial matrix optics is briefly introduced below, and with this, fundamentals of the optics of resonators are then presented. The extension of formalism involves the consideration of beam offsets and beam bends, which inevitably occur in measuring resonators for position determination. The general transfer matrix of an optical system or of a subsystem consisting of spherically curved and / or planar elements (mirrors and plates) is in this formalism

The entries A, B, C, D describe the paraxial beam propagation parameters of a system which is rotationally symmetrical about the optical axis (propagation axis), if appropriate after corresponding unfolding of the nominal deflecting reflections. The attached column with the one entry at the last position makes it possible to describe the rotational symmetry-breaking effect of elements which cause a beam offset and / or a beam tilt. The parameters t _x , t _y are the translational displacements perpendicular to the optical axis, which corresponds here to the z-axis. The parameters f _c , f _n denote the angles (in radians) of the beam dips. For a concatenated optical system of K sections, the transfer matrix results

M = MM, (2) by flanking the elementary transfer matrices M _L , by matrix multiplication. The elementary transfer matrices from which all subsequent measuring resonators are composed, are:

Freiraumausbreitungsstrecke by distance z:

Passage through lens with focal length f:

• Retroreflector with offset (s, s _y ) to the optical axis:

Beam offset around (s _x , s _y ): • beam bending around (q _c , q _g ) (in radians):

In a resonator, the rays travel through the optical path several times, ideally infinitely high finesse, even infinitely often. An n-fold pass means an n-fold successive switching of the simple resonator path accordingly

R _n = R ⁿ M_ _Q (8)

By an own decomposition of the simple distance matrix accordingly

M V = diag

If one obtains the matrix of eigenvectors V = (Ei v ₂ v ₃ v ₄ v ₅ ) _U nd obtains the corresponding eigenvalues m = (m ₁ , m ₂ , m ₃ , m ₄ , m ₅ ).

It can be generally shown that for the 2x2 sub-transfer matrix the determinant for the case that the indices of refraction at the entrance and at the exit of the route are identical is always identical to one. So that applies = AD - BC = 1 and only three of the four entries are independent. The

Eigenvalues of the distance matrix M_ are based on elementary calculation

Bi = 1 (1 1) and

The associated eigenvectors are

and

Thus, for the beam vector R _n, after n-passing through the resonator section is obtained wherein the input beam R _{0 is represented} by its components R _{k 0} , k - 1,2, 3, 4, 5 with respect to the eigenvectors.

The stability of an optical resonator requires that the beam vector is always limited in any number of rounds. This in turn requires that the two eigenvalues m _{2 3} and m _{4 5 are} also limited, accordingly This requirement, in turn, translates directly into the stability condition

\ g \ = \ (A + D) / 2 \ <l (18) where the so-called stability parameter is defined by g = (A + D) / 2. For a stable resonator path, the two eigenvalues and the associated eigenvectors inevitably become complex and then form mutually conjugate pairs accordingly

and

with the substitution cos (0) - g. Thus one obtains for the beam vector after n-times traversed route

+ Exp (-in0) (v _x _R _{x o} + V _y _R _{y 0)}

(22)

From this, the oscillating and limited in the amplitude behavior of a bound beam in the resonator is explicitly visible.

A Gaussian ray in fundamental mode (TEM00) is completely described by the complex ray parameter q. This combines the two beam-large radius of curvature R and beam size w. He is defined as follows through his reciprocal:

1 _ 1 i l

q R nw ²

(23) where l is the wavelength of the light field. The propagation of the beam parameter is in the formalism of the transfer matrices by the expression

given. In this case, q _out denotes the output-side beam parameter and q _in denotes the input-side beam parameter.

The stable modes of a resonator must satisfy two stationarity conditions. The stationarity of the main ray R _c ("Chief Ray"), along which the light field propagates, firstly requires L · ^ KL · (25)

The solution for the principal ray just corresponds to the eigenvector of the resonator path to the eigenvalue m ₁ = 1 corresponding to L · = Ei _> where v _{t is given} in the above section.

Secondly, the stationarity of the complex beam parameter of the radiation field propagating along the main beam requires

This equation has two solutions for the eigen-beam parameter. They are explicit

Thus, finally, as a result at the input of the resonator path for the wavefront curvature radius R _{m of} the eigenmodes, the expression is obtained

4 + {A - D) ² - Ag ²

R _m = 1 / real (l / q _{+ /} _)

2C (A - D)

(28) and for the beam size of the eigenmodes the expression

FIG. 1 c shows a derived equivalent circuit diagram for the embodiments of FIGS. 1 a-1 b for the simple resonator path for the description in the expanded formalism of the paraxial matrix optics. The corresponding transfer matrix is exemplary for the case of a curved fixed resonator mirror 110 according to FIG. 1 a

K = KFS (L) RR (S) KFS (L 'KFSW) KRR (S) FSO K _L ens (R / 2) Here L denotes the variable distance between the stationary curved resonator mirror 110 and the measurement target forming retroreflector 120, V the variable distance between the fixed plane inverted mirror (= "recirculation mirror") 130 and the movable retroreflector 120, R the radius of curvature of the curved resonator mirror 110 and (s _x , s _y ) the transverse displacement of the retroreflector 120 to the optical axis (which extends in the drawn coordinate system in the z-direction).

Because of the identical disappearance of the first four entries in the last column of the transfer matrix, R _c = (O, O, O, O, I) ^{7 '} applies to the jet vector of the main jet. As required, the main beam is thus independent of the migration of the retroreflector 120 forming the measurement target. The effective resonator length is L _eff = L + V. A displacement of the retroreflector 120 forming the measurement star in the measuring direction about AL results in AL _eff = 2 AL. The satisfaction of the stability condition requires L + V <R <oo. The parameters of the TEMOO eigenmodes result from the equations above

R _m = R and Fig. 2 shows in a further schematic representation of a diagram for explaining the inventive concept. In the left part of Fig. 2, a conventional optical resonator with fixed resonator mirror 10 and measuring target 20 is indicated. According to the invention (as indicated in the right-hand part of FIG. 2) a recirculation optic 230 (realized in FIG. 1 by the reversing mirror 130) is provided between the stationary resonator mirror 210 and the measuring target 220.

3a-3b show schematic illustrations for explaining further embodiments of a measuring arrangement according to the invention, wherein in comparison to FIGS. 1a-1b analogous or substantially functionally identical components are designated by reference numerals increased by "200". The embodiments of FIGS. 3a-3b differ from those of FIGS. 1a-1b in that, instead of the retroreflector 120, a plane mirror 340 serves as a movable measurement target, wherein the retroreflector 320 is arranged on the side of the stationary part of the resonator.

The transfer matrix of the distance which is unfolded by the nominal angle is exemplary for the embodiment of the curved solid resonator mirror 310 according to FIG. 3a

Therein, L denotes the variable distance between the stationary curved resonator mirror 310 and the movable plane mirror 340, the variable distance between the fixed retroreflector 320 and the movable plane mirror 340, L "denotes the variable distance between the stationary envelope. sweeping mirror 330 and the movable plane mirror 340 and R the curvature radius of the curved stationary resonator mirror 310th

Due to the identical disappearance of the first four entries in the last column of the transfer matrix according to (2) applies to the beam vector of the main beam R. = (O, O, O, O, I) ^{7 '} . The main beam is thus - as desired - also here independent of the emigration of the measurement target. The effective resonator length is L _eff = L + 2L '+ L ", and AL _eff = 4 AL follows from a shift of the plane mirror 340 forming the measurement target in the measurement direction about AL. Fulfillment of the stability condition requires L + 2L' + L "<R <oo. For parameters of the TEMOO eigenmodes we obtain R _m = R and

The embodiment shown in FIG. 3 b differs from that of FIG. 3 a again (analogously to FIGS. 1 a - 1 b) only in that, instead of the fixed curved resonator mirror 310, a fixed "cat eye optic" comprising a Fourier lens 312 in its focal plane arranged mirror 313 is used.

Fig. 3c shows, starting from the embodiments of Figs. 3a-3b and from the direction of the measuring target forming plane mirror 340, some possible configurations differing in their geometrical arrangement.

4a-4b show schematic representations for explaining further embodiments of a measuring arrangement according to the invention, again analogous or essentially functionally identical components having reference numbers increased by "100" being compared to FIGS. 3a-3b.

According to FIG. 4 a, a measuring beam in turn passes through a unit 401 (the structure and mode of operation of which will be described in more detail with reference to FIGS. 6-11) and an optical fiber 402 into the resonator through the curved fixed resonator mirror 410 (with mirror surface 411) and hits after passing through a free space path on a polarization optical beam splitter 450, which has a beam splitter layer 450a. The p-polarized component of the measuring beam is transmitted, whereas the s-component is reflected out of the resonator and thus destroyed. The now p-polarized beam is transformed into a circularly polarized beam by means of a lambda / 4 plate 460 and passes through a further free space path up to the plane mirror 440 forming the measurement target. There it is reflected back and again passes through the lambda / 4-plate 460, whereby it is transformed into a linearly polarized beam with 90 ° rotation in relation to the original p-polarization, ie into an s-polarized beam.

The now s-polarized beam is completely reflected at the polarization-optical beam splitter 450 and introduced into the (eg monolithically attached) retroreflector 420. There, the beam is reflected back with a parallel offset and again deflected at the beam splitter layer 450a in the direction of the plane target 440 forming the measurement target. When it passes through the lambda / 4 plate, the beam is once again circularly polarized and after a free-space path arrives at the plane mirror 440 forming the measurement target, where it is again reflected back. After repeated passage through the lambda / 4 plate, it again assumes the original p-polarization state, passes through the beam splitter layer 450a without deflection and finally reaches the fixed reversing mirror 330. From there, the entire optical path is identical in the reverse order, so that the beam at the end of a run again in its original position and with the same inclination on the curved stationary resonator mirror 410 impinges. This closes the circle and the next circulation is initiated with the reflection at the curved resonator mirror 410. It is hereby assumed that the retroreflector is designed in such a way that the polarization of the beam after the passage is maintained, which can be achieved by coating with a suitably designed optical multilayer coating system on the mirror surfaces. The embodiment shown in FIG. 4 b differs from that of FIG. 4 a again (analogously to FIGS. 1 a - 1 b) only in that, instead of the stationary curved resonator mirror 410, a fixed "cat eye optic" comprising a Fourier lens 412 In its focal plane arranged, defined defocused mirror 413 is used.

As a result, in contrast to FIGS. 3a-3b, in each case a nominally perpendicular incidence is achieved on the measuring target 440 designed as a plane mirror, by using the polarization-optical beam splitter 450 the "retroreflector beam path" directly onto the optical axis is folded.

5a-5b show schematic representations for explaining further embodiments of a measuring arrangement according to the invention, again analogous or substantially functionally identical components having reference numerals increased by "100" being compared with FIGS. 4a-4b.

In the embodiments of FIGS. 5a-5b, instead of the retroreflector, an optical group 520 of two lenses 521, 523 in Kepler arrangement (afocal arrangement) is used. In the common focal plane of these two lenses 521, 523 - the so-called spatial filter plane - there is a mirror 522 (also referred to as a retina mirror) with a central opening, which throws back the beam path returning from the plane target 540 forming the measurement target, if the plane mirror 540 has a sufficiently large angle of attack. The transfer matrix of the unfolded nominal system (at which the nominal angle of attack of the plane mirror 540 is folded out) is

Here, L denotes the variable distance between the output side lens 523 and the measurement target plane mirror 540, and F and F ₂ denote the focal lengths of the two lenses 521, 523. q = (q _c , q _n ) represents the inclination deviations of the measurement target planing mirror 540 over its nominal values. The underlying paraxial replacement scheme for the arrangement with plane mirror 530 and optical group 520 shown in FIG. 5 c is shown in FIG. 5 d. The output-side lens 523, together with the (retina) mirror 522 in its focal plane forms a functional retroreflector in the form of a cat's eye. In this case, the focal plane of the first lens 521 is selected as the input-side reference plane. The transfer matrix shows the property of retroreflection in the form of the identical disappearance of its entries M _{S 1} and M _{5 3.}

Furthermore, in accordance with FIGS. 5a-5b, with reference to the optical beam path following the above-described optical group 520 (ie at its "system output"), analogously to the embodiments described above, a plane mirror 530 as recirculation optics is inserted, which throws the beam path back into itself ,

The transfer matrix of the unfolded nominal cavity or of the optical resonator according to FIGS. 5a-5c is

The variables contained therein are already defined above, except for the distance l between the plane mirror 530 and the input-side lens 521. As a result of the recirculation via the plane mirror 530, the input and output are identical, and the disappearance of the first four entries of the last column indicates that that the targeted robustness is achieved with respect to parasitic tilting of the measurement target 540 forming the measurement target.

The optics described above are completed according to Fig. 5a-5c to an optical resonator by input side with a curved mirror 510 (FIG. 5a) or alternatively with a "cat's eye optics" of a Fourier lens 512 with arranged in its focal plane mirror 513 (as shown in FIG. 5b) is completed. The transfer matrix for the simple passage passage of such a resonator for the curved mirror embodiment according to FIG. 5a is

wherein both the curved resonator mirror 510 and the recirculation plane mirror 530 are located in the focal plane of the input-side lens 521 of the optical group 520. Due to the identical disappearance of the first four entries in the last column of the transfer matrix, R _c = (O, O, O, O, I) ^{7 '} applies to the beam vector of the main beam. As is desired, the main beam is therefore independent of the migration of the plane mirror 540 forming the measurement target. The effective resonator length is L _eff = 4 (L-F ₂ ) and is counted from the output focal plane of the output-side lens 523. A displacement of the plane mirror 540 forming the measurement target in the measuring direction about AL is followed by AL _eff = 4 AL. The fulfillment of the stability condition requires L _eff - 4 (L - F ₂ ) <RF ₂ ² / F ² <oo.

By virtue of the imaging properties of the optical group 520 (which forms an effective Kepler telescope), the radius of curvature of the input-side resonator mirror 510 is transformed into an effective radius of curvature R _eff = R FH Fl. The scaling factor corresponds to the depth scale of the afocal optics.

In all the embodiments described above with reference to Figures 1 -5, any retroreflector present may also be configured in a "cat's eye configuration" (i.e., with a Fourier optic or lens having a mirror disposed in its focal plane). This allows for the fact that the losses in the optical resonator typically have to be limited to a maximum of 0.1% -0.5%, which is made more difficult when designing the retroreflector with a plurality of reflection surfaces due to the plurality of reflections occurring.

It is further assumed that the retroreflector is designed such that the polarization of the beam is maintained after the passage. The property of the polarization maintenance of the retroreflector can be achieved by coating by means of a suitably designed optical multilayer coating system on the mirror surfaces.

In the following implementation concepts of a frequency-based length or position measurement are described with reference to the schematic representations of Fig. 6-9.

6 shows a diagram for explaining the principle known per se, according to which a tunable laser 601 follows a frequency of a resonator 602 via a suitable control circuit (in the illustrated example according to the Pound-Drever-Hall method), so that the ultimate to be measured Length L of the resonator 602 is coded as the frequency of the tunable laser 601.

In Fig. 6, the area surrounded by the dashed line corresponds to the unit "501" of Fig. 5 (or the units "102", "301" and "401" in Fig. 1, Fig. 3 and Fig. 4).

The arrangement according to FIG. 6 comprises a Faraday isolator 605, an electro-optical modulator 606, a polarization-optical beam splitter 607, a lambda / 4-plate 608, a photodetector 609 and a low-pass filter 610. For frequency measurement, part of the signal from the tunable laser 601 emitted laser light via a beam splitter 603 coupled and supplied to an analyzer 604 for frequency measurement. The actual frequency measurement in the analyzer 604 can be made, for example, by comparison with a frequency reference (for example, as explained below, an fs frequency comb of a femtosecond laser).

According to FIG. 7, in a further development of the principle of the frequency-based length measurement described above, regulation of two tunable lasers 701, 702 (eg also according to the pound-three-fill method) can be performed on two different resonator modes known to their mode index spacing respectively. The beat frequency fbeat = Af = f ₂ -f of the signal obtained by superposing the radiation of the two lasers 701, 702 on a photodetector 703 is determined via a frequency counter 704. The sought length L of the resonator can then according to

L = c / 2 Aq / f _beat (35), where Aq denotes the mode spacing in the frequency comb of the resonator. The mode spacing Aq can be obtained, for example, by tuning one of the two laser frequencies, starting from a common output frequency and counting the traversed reflection minima of the frequency comb of the resonator. FIG. 8 is illustrative of the principle of frequency-based length measurement based on the beat between a tunable laser 801 stabilized on a resonator mode of a resonator 802 and a femtosecond laser 803. The beat between the laser beams of the tunable laser 801 and the femtosecond laser 803 becomes realized by their superposition on a fast photodetector 805. From the analysis of the beat signal, which comprises a superposition of a plurality of simultaneous beats, the individual beating frequencies are extracted. According to FIG. 8, a frequency standard 806 (eg in the form of a gas cell, in particular approximately an acetylene gas cell in the S and C telecommunications frequency bands around 1500 nm) is also provided to eliminate ignorance of the frequency comb index. Downstream of the frequency standard 806 are a photodetector 810 and a signal analyzer 811.

From knowledge of the individual beat frequencies as well as knowledge of the mode indices, the desired frequency of the tunable laser 801 can be reconstructed according to FIG.

The carrier envelope frequency (comb offset frequency) of the femtosecond laser 803 is given by and can be measured with the aid of a non-linear, so-called f-2f interferometer and kept constant via a control loop or eliminated via an optically non-linear process. The comb offset frequency f _ceo and

the pulse repetition frequency f _rep = - lie in the radio frequency range and can be measured with high precision and stabilized on atomic clocks. The wide optical spectrum of this femtosecond laser 803 comprises a multiplicity of sharp lines with a constant frequency spacing f _rep f _k = fceo + kf _rep , k EM »1 (37) where k denotes the comb index.

The numerous possible beat frequencies between a tunable laser with frequency f _x to be determined and a femtosecond laser precisely known in its parameters are generally

An exemplary spectrum of beat frequencies between a tunable laser stabilized to a resonator mode and a femtosecond laser as a function of resonator length change is shown in FIG. 9a. It is a periodic diamond pattern along both axes, which can also be called a beat raster. A basically resulting ambiguity must be eliminated in analogy to the counting distance-measuring interferometry by gapless counting the passages through cell boundaries in the diamond pattern, starting from a fixed starting position by zeroing. The remaining uncertainty with regard to the counting direction and the elimination of this uncertainty will be discussed below with reference to FIG. 10.

Fig. 10 shows an extension of the structure of Fig. 8, wherein to Fig. 8 analog or substantially functionally identical components are designated by reference numerals increased by "200".

Referring to FIG. 10, in addition to the first tunable laser 1001, a second tunable laser 1012 with photodetector 1008 and associated beat frequency analyzer unit 1009 is integrated with the measurement system. The second laser 1012 also becomes a selected resonator mode of the optical resonator so that the frequency of the laser beam generated by the second tunable laser 1012 is: h = L + FSR (L) Aq. (39)

FSR (L) = c / 2L designates the so-called free spectral range, which corresponds to the frequency spacing between adjacent modes in the mode comb of the resonator.

The laser frequencies of the lasers 1001 and 1012 of FIG. 10 have two restricted rasters of beat frequencies (analogous to the diamond pattern shown schematically in FIG. 9b), by which an otherwise given "directional ambiguity" with respect to the counting direction (in counting the With the aid of the laser beam generated by this further laser 1012 and coupled to the frequency comb of the optical resonator, the solution of the uniqueness problem with respect to the counting direction succeeds, since with the aid of the additional information in FIG Form of the frequencies of the second beat grid always clearly the counting direction can be determined (see Fig. 9b). It is possible in an advantageous manner, the absolute length of the optical resonator according to c

L = (40

² k 02,1)

2 Li at any time to determine directly and so the absolute (connection) value for the further incremental counting according to

to obtain. In the case of incremental counting, the change in the offset index S _g , which also includes the known guoy phase, can be neglected that the relative frequency change is directly related to a relative change in length. With knowledge of the previously determined absolute length, it is possible to calculate directly from the relative change in length the absolute change in length of interest. As a result, with the construction proposed in FIG. 10, a frequency-based length measurement is realized.

Basically, the two o.g. Beat signals are also superimposed additively fed to a single common beat analyzer, but then the beat frequencies of both rasters coincide and the separation and assignment of the grid in the presence of measurement errors is at least difficult or in extreme cases is no longer possible.

FIG. 11 shows an alternative embodiment to FIG. 10, wherein components analogous or substantially functionally identical to FIG. 10 are designated by reference numerals increased by "100".

11, dispensing with the second tunable laser 1012 from FIG. 10, a further laser beam for generating a further shifted beat raster is realized in that a partial beam is branched off from the tunable laser 1101 that can be stabilized on the resonator comb and is shifted by an acousto-optic modulator (AOM) 1114 in its frequency by the value f _aom . This partial beam with a frequency f ₂ = f + f _{aom which} is coupled in its frequency to the tunable laser 1101 is likewise made to _bounce on a photodetector 1112 with a branched beam of the femtosecond laser 1103. The beat signal obtained in this case is analyzed by means of another beat frequency analyzer unit 1113 in its frequency composition. Here, too, it is fundamentally possible to add the two beat signals additively to a single common beat frequency analyzer unit.

13 shows a schematic representation of an exemplary microlithographic projection exposure apparatus designed for operation in the EUV. 1300. The measuring arrangement according to the invention can be used in this projection exposure apparatus for the distance measurement of the individual mirrors in the projection objective or in the illumination device. However, the invention is not limited to the application in systems designed for operation in the EUV, but also in the measurement of optical systems for other operating wavelengths (eg in the VUV range or at wavelengths less than 250nm) feasible. In other applications, the invention can also be implemented in a mask inspection system or a wafer inspection system.

According to FIG. 13, a lighting device of the projection exposure apparatus 1300 has a field facet mirror 1303 and a pupil facet mirror 1304. On the field facet mirror 1303, the light of a light source unit comprising a plasma light source 1301 and a collector mirror 1302 is directed. In the light path after the pupil facet mirror 1304, a first telescope mirror 1305 and a second telescope mirror 1306 are arranged. A deflecting mirror 1307 is arranged downstream of the light path, which deflects the radiation impinging on it onto an object field in the object plane of a projection objective comprising six mirrors 1351-1356. At the location of the object field, a reflective structure-carrying mask 1321 is arranged on a mask table 1320, which is imaged by means of the projection lens into an image plane in which a photosensitive layer (photoresist) -coated substrate 1361 is located on a wafer table 1360.

Without limiting the invention to this, it is possible to use, for example, a structure known per se from eg US Pat. No. 6,864,988 B2, in which both a load-bearing support structure 1403 ("force frame") and an independently provided measuring structure 1404 ("sensor frame") available. According to FIG. 14, both supporting structure 1403 and measuring structure 1404 are mechanically connected to a base plate or base 1430 of the optical system independently of one another via mechanical connections (eg springs) 1405 or 1406 acting as dynamic decoupling. The mirror 1401 in turn is attached via a mirror attachment 1402 to the support structure 1403. Shown schematically in FIG. 14 are two measurement sections 1411 and 1421 measured via optical resonators according to the invention, which extend from the measurement structure 1404 to the mirror 1401.

In order to measure the position of a mirror in all six degrees of freedom, six optical resonators according to the invention for frequency-based length measurement are required, one possible configuration being shown schematically in FIG. 15. Shown are six measuring sections 1505, each with a starting point 1504 located on a measuring frame 1506 and an end point 1503 located on a mirror 1501.

Although the invention has also been described by means of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and / or exchange of features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative forms of embodiment are intended to be embraced by the present invention, and the scope of the invention is limited only in terms of the appended claims and their equivalents.

Claims

claims

1. Measuring arrangement for the frequency-based position determination of a component, in particular in an optical system for microlithography, with

• at least one optical resonator, said resonator having a fixed first resonator mirror, a component of the associated mobile measurement target and a stationary second resonator mirror;

Wherein the second resonator mirror is formed by a reversing mirror (130, 330, 430, 530) which reflects back a measuring beam coming from the measuring target.

2. Measuring arrangement according to claim 1, characterized in that the resonator further comprises a retroreflector (120), which reverses the measuring beam parallel-offset identically in its direction.

3. Measuring arrangement according to claim 2, characterized in that the measurement target is formed by the retroreflector.

4. Measuring arrangement according to claim 1 or 2, characterized in that the measurement target is formed by a plane mirror (340, 440, 540).

5. Measuring arrangement according to one of the preceding claims, characterized in that it comprises a polarization-optical beam splitter (450).

6. Measuring arrangement according to claim 5, characterized in that a measuring beam coming from the polarization-optical beam splitter (450) perpendicularly impinges on the measuring target.

7. Measuring arrangement according to one of the preceding claims, characterized in that it comprises an optical group (520) with two lenses (521, 523) in Kepler arrangement.

8. Measuring arrangement according to claim 7, characterized in that the optical group (520) in a common focal plane of these two lenses (521, 523) has a mirror (522) with an opening, which reflects the return from the measurement target beam path.

9. Measuring arrangement according to one of the preceding claims, characterized in that the retroreflector is configured polarization-preserving.

10. Measuring arrangement according to one of the preceding claims, characterized in that the first resonator mirror has a curvature such that a light field present in the resonator is stably enclosed sen.

11. Measuring arrangement according to one of the preceding claims, character- ized in that the first resonator mirror is designed as a cat's eye mirror.

12. Measuring arrangement according to one of the preceding claims, characterized in that it comprises at least one stabilized on a resonator of the optical resonator, tunable laser (601, 801, 1001, 1101).

13. Measuring arrangement according to claim 12, characterized in that it has a control loop which is configured to stabilize the tunable laser (601, 801, 1001, 1101) according to the Pound-Drever-Hall method.

14. Measuring arrangement according to claim 12 or 13, characterized in that it has at least one femtosecond laser (803, 1003, 1103) for determining the frequency of the laser radiation of the at least one tunable laser (601, 801, 1001, 1101).

15. Measuring arrangement according to one of the preceding claims, characterized in that it further comprises a frequency standard (806, 1006, 1106), in particular a gas cell.

16. Measuring arrangement according to one of claims 1 to 15, characterized in that the latter has two tunable lasers (1001, 1012) which can be stabilized to different resonator modes with a known frequency spacing of the optical resonator in order to realize an absolute length measurement.

17. Measuring arrangement according to claim 16, characterized in that each of these two tunable lasers (1001, 1012) is associated with a beat frequency analyzer unit (1005, 1009).

18. Measuring arrangement according to one of claims 1 to 15, characterized in that it comprises an acousto-optic modulator (11 14) for realizing a frequency shift in a laser beam generated by at least one tunable laser (1101). branched partial beam has.

19. Measuring arrangement according to one of the preceding claims, characterized in that the component for position determination in six degrees of freedom six optical resonators for frequency-based length measurement are assigned.

20. Measuring arrangement according to one of the preceding claims, characterized in that the component is a mirror.

21. Measuring arrangement according to one of the preceding claims, characterized in that the optical system is a microlithographic projection exposure apparatus.