DE102019201146A1 - Interferometric measuring arrangement - Google Patents

Abstract

The invention relates to an interferometric measuring arrangement in an optical system, in particular in an optical system for microlithography, at least one distance-measuring interferometer assigned to a component in the optical system for position determination, this interferometer comprising a measuring arm (101, 201, 301, 401, 501). , a reference arm (102, 202, 302, 402, 502) and a polarization beam splitter (110, 210, 310, 410, 510), wherein a of a light source (105, 205, 305, 405, 505) coupled light beam over the Polarization beam splitter is divided into a along the measuring arm and spread over one of the component associated first retroreflector (120, 220, 320, 420, 520) guided measuring beam and a propagating along the reference arm reference beam, wherein in the measuring arm, a first plane mirror (130, 230 , 330, 430, 530) which recalls the measurement beam coming from the first retroreflector ects.

Description

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to an interferometric measuring arrangement in an optical system, 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. The image of a mask (= reticle) illuminated by means of the illumination device is hereby projected onto a substrate (eg a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective in order to apply the mask structure to the photosensitive coating of the Transfer 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 operation of such projection lenses designed for EUV, in which mask and wafer are usually moved relative to one another in a scanning process, the positions of the mirrors, which are partially movable in all six degrees of freedom, must be set and maintained both with each other and with the mask or wafer with high accuracy to avoid, or at least reduce, aberrations and concomitant impairments to the imaging result. In this position determination, e.g. be demanded over a path length of 1 meter accuracies of the length measurement in the picometer (pm) range.

Various approaches are known in the prior art for interferometrically measuring the position of the individual objective mirrors as well as the wafer or the wafer stage and the reticle plane. To measure the position of a mirror in all six degrees of freedom six interferometers are required. One possible configuration is shown schematically in 9a shown. Marked are six measuring sections 905 with one each on a measuring frame 906 located starting point 904 and one on a mirror 901 located endpoint 903 , Like also in 9a indicated is located at the starting point 904 the measuring section 905 a polarization-dependent beam splitter 912 and retroreflector 913 constructed interferometer, and at the end point 903 the measuring section 905 one at the mirror 901 arranged retroreflector 911 , 9b serves to explain the simplified representation of the interferometer structure 9a , The separation of the two beams for the (to the retroreflector 911 extending) and the (to the retroreflector 913 running) reference arm is geometrically by exploiting the beam offset, which occurs when a retroreflector is used eccentrically in the optical beam path. Measuring and reference beam are present in linear mutually orthogonal polarization states, which by the polarization-dependent beam splitter 912 are defined.

In practice, when measuring the position of a component or a mirror, problems may result from movements of the "measurement targets" arranged on the mirror (which may be configured, for example, in the form of a retroreflector or a plane mirror) not only along the direction of the Measuring arm, but also in other of the six degrees of freedom can occur.
This in turn can result in the measuring beam and the reference beam, when combined on a detector, not propagating with sufficient accuracy parallel to one another and / or having a translatory offset perpendicular to the propagation direction. A relative tilt between the interfering wavefronts due to the non-parallel beam directions leads to a strip-like modulation of the intensity across the beam cross-section and thus to a contrast or signal loss. Shearing the two wavefronts due to the beam offset between the two interfering wavefronts also leads to a contrast or signal loss due to the reduced overlap area of the two wavefronts.

In this connection, the structure of the interferometer described above using retroreflectors (compared to the use of plane mirrors) has the advantage that a tilting of the measuring target does not lead to a tilting of the differential wavefront, since it is independent of the angular orientation of the retroreflector the beam direction in the reflection by the retroreflector according to 10a is inverted identical.

Lateral displacements not along the (in the in 10a and 10b drawn coordinate system in the z-direction) measuring axis take place, in particular Shifts along the y-direction, but have the consequence that at the output of the interferometer, an emigration of the measuring beam relative to the reference beam according to 10b takes place, which leads to a contrast or signal loss.

The variations in the degrees of freedom which do not act along the direction of the measuring arm (measuring axis) can, as a result, lead to a significant decrease in the signal contrast, up to a complete disappearance of the interference signal. In view of the high requirements to be placed on the beam direction deviation between the reference beam and the measuring beam (which may, for example, require deviations of less than 10μrad), ensuring that intentional or unintentional tilting or shifting of the measuring beam does not take effect when superposed to the interferogram , a challenging challenge.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an interferometric measuring arrangement in an optical system, 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 claim 1.

• - wenigstens ein einer Komponente in dem optischen System zur Positionsbestimmung zugeordnetes distanzmessendes Interferometer,
• - wobei dieses Interferometer einen Messarm, einen Referenzarm sowie einen Polarisationsstrahlteiler aufweist, wobei ein von einer Lichtquelle eingekoppelter Lichtstrahl über den Polarisationsstrahlteiler in einen sich entlang des Messarms ausbreitenden und über einen der Komponente zugeordneten ersten Retroreflektor geführten Messstrahl und einen sich entlang des Referenzarms ausbreitenden Referenzstrahl aufgeteilt wird; und
• - wobei im Messarm ein erster Planspiegel angeordnet ist, welcher den vom ersten Retroreflektor kommenden Messstrahl in sich zurückreflektiert.

An interferometric measuring arrangement in an optical system, in particular in an optical system for microlithography, comprises:

• at least one distance measuring interferometer associated with a component in the optical positioning system;
• wherein said interferometer comprises a measuring arm, a reference arm and a polarization beam splitter, wherein a light beam coupled in by a light source is split via the polarization beam splitter into a measuring beam propagating along the measuring arm and transmitted through one of the component first retroreflectors and a reference beam propagating along the reference arm becomes; and
• - Wherein a first plane mirror is arranged in the measuring arm, which reflects the coming of the first retroreflector measuring beam in itself.

The invention is based in particular on the concept of doubling the distance to be traveled within the measuring arm by placing a plane mirror in the measuring arm of a distance-measuring interferometer. Taking advantage of the principle of reversibility of the light path ensures in this way that the measuring beam with the reference beam without loss of contrast remains completely interfering even if lateral displacements occur on the part to be measured component or this component associated retroreflector, which not in the direction of the measuring arm ("In the measuring direction"), but run transversely to this.

In other words, it is achieved by the inventive use of a plane mirror in the measuring arm, which is reflected back regardless of lateral displacements of the component to be measured assigned retroreflector of the incident on said plane mirror measuring beam. This measuring beam thus travels along the identical path along the measuring arm back to the polarization beam splitter, with the result that the measuring beam and the reference beam can interfere on the detector side without a transverse translational offset and with parallel wavefronts and thus with maximum contrast. The variations in the degrees of freedom that do not act along the direction of the measuring arm (measuring axis) are thus completely eliminated in their effects on the interferometric measuring signal.

Lateral displacements of the retroreflector assigned to 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 no longer play any role in the distance measurement according to the invention. As a result, the interferometric 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 retroreflector is not possible or the associated expense should be avoided.

In this case, according to the invention, as explained in more detail below, for the separation of the input and output beams which is likewise required in the interferometer, it is consciously accepted that this separation must take place polarization-optically (in particular via lambda / 4 plates which will be described later) ,

The invention is equally feasible in homodyne interferometers as well as in heterodyne interferometers.

According to one embodiment, a mirror is arranged in the reference arm, which reflects the reference beam coming from the polarization beam splitter back into itself. This mirror can also be a plane mirror. In further embodiments, the mirror reflecting back the reference beam in the reference arm may also be a centrically or axially operated (and thus no lateral offset between incident and reflected beam generating) retroreflector.

According to one embodiment, a second retroreflector is arranged in the beam path of the reference beam between the above-mentioned second plane mirror and the polarization beam splitter. As a result, as a result, a differential construction can be realized, in which the design of the reference arm is analogous to that of the measuring arm, in order to detect extent changes between the measuring arm and reference arm. On the one hand, this embodiment has the advantage that atmospheric influences on the measurement result can be at least partially compensated and, furthermore, even uniform drift effects in the interferometer do not lead to a falsification of the measurement results obtained. Furthermore, in this way, when placing both retroreflectors on the component to be measured, the angle of a tilting movement performed by the component can also be determined.

In embodiments of the invention, a first lambda / 4 plate is arranged in the measuring arm.

In particular, this first lambda / 4 plate can be arranged in the beam path in the measuring arm between the first plane mirror and the first retroreflector. This arrangement is advantageous insofar as undesired, first-order polarization-optical effects of any defects present in the optical components (in particular of the retroreflector and the lambda / 4 plate) on the polarization state of the measuring beam and, consequently, cyclic or multiple passes of the measuring arm leading errors are minimized. However, the invention is not limited thereto, so that embodiments in which the lambda / 4 plate is arranged at a different position in the measuring arm (for example on the first retroreflector or between the first retroreflector and polarization beam splitter) should also be included in the present application ,

Furthermore, in embodiments of the invention, a second lambda / 4 plate is arranged in the reference arm. As a result, it can be taken into account that, as already explained, the separation of the input and output beams required in the interferometer in the context of the invention must be carried out polarization-optically. For this second lambda / 4-plate apply the o.g. Embodiments to the first lambda / 4-plate analog, so that in the presence of a second retroreflector and a second plane mirror in the reference arm, the second lambda / 4 plate is preferably disposed between the second plane mirror and the second retroreflector, but possibly also at other positions in Reference arm can be arranged.

According to a preferred embodiment, a measurement target is arranged on the component, the measurement target being designed as a plane mirror.

According to an alternative embodiment, the measurement target arranged on the component is designed as a retroreflector. According to another alternative embodiment, the measurement target is formed by the first retroreflector.

The measuring target, which is designed in particular as a plane mirror, is preferably arranged in the measuring arm such that the measuring beam coming from the polarization beam splitter layer is reflected by the measuring target in the direction of the first retroreflector.

It is preferably provided that the first retroreflector is arranged on the sides of a stationary part of the measuring arrangement.

According to one embodiment, six distance-measuring interferometers are assigned to the position-determining component in six degrees of freedom.

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.

list of figures

• 1-7 schematische Darstellungen zur Erläuterung unterschiedlicher Ausführungsformen der Erfindung;
• 8 eine schematische Darstellung zur Erläuterung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage;
• 9a-9b schematische Darstellungen zur Erläuterung einer möglichen Ermittlung der Lage eines Spiegels in sechs Freiheitsgraden; und
• 10a-10b schematische Darstellungen zur Erläuterung der Auswirkungen von Verkippungen (10a) bzw. Verschiebungen (10b) des Mess-Targets in einem Aufbau gemäß 9a und 9b.

Show it:

• 1-7 schematic representations for explaining different embodiments of the invention;
• 8th a schematic representation for explaining the possible structure of a designed for operation in the EUV microlithographic projection exposure apparatus;
• 9a-9b schematic diagrams for explaining a possible determination of the position of a mirror in six degrees of freedom; and
• 10a-10b schematic diagrams explaining the effects of tilting ( 10a) or shifts ( 10b) of the measuring target in a structure according to 9a and 9b ,

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1a-1b show schematic illustrations for explaining the structure and the mode of operation of an interferometric measuring arrangement in a first embodiment of the invention.

According to 1a the measuring arrangement has a polarization beam splitter 110 with a polarization beam splitter layer 110a on which of a merely indicated light source 105 generated and in the drawn coordinate system in the z-direction propagating light in one of the polarization beam splitter layer 110a reflected s-polarized partial beam and one through the polarization beam splitter layer 110a transmits transmitted p-polarized partial beam. In the illustrated embodiment - but without the invention being limited thereto - a heterodyne interferometer is realized by the measuring arrangement, in which two separate partial beams of different frequencies ( f ₁ . f ₂ ) and mutually orthogonal polarization states via the light source 105 coupled and through the polarization beam splitter layer 110a be separated.

The at the polarization beam splitter layer 110a Reflected s-polarized partial beam, hereinafter also referred to as "reference beam", passes through a lambda / 4 plate in the y-direction first 141 , is at a plane mirror 140 reflects and then passes through the lambda / 4 plate 141 a second time. Upon re-impinging on the polarization beam splitter layer 110a is the reference beam due to the double passage through the lambda / 4 plate 141 Accordingly, p-polarized, is now through the polarization beam splitter layer 110a transmits and hits a detector 135 , The from the polarization beam splitter 110 over the lambda / 4-plate 141 to the plane mirror 140 running interferometer arm is also referred to as "reference arm" below 102 designated.

The by splitting of the light source 105 in the z-direction incident light beam after transmission through the polarization beam splitter layer 110a produced p-polarized partial beam, hereinafter also referred to as "measuring beam" passes according to 1a via an off-axis retroreflector 120 also to a lambda / 4-plate 131 after passing through the measuring beam on a plane mirror 130 meets. After reflection on this plane mirror 130 the measuring beam passes through the lambda / 4 plate 131 a second time and passes, due to the double passage through the lambda / 4 plate 131 now s-polarized, in the opposite direction on the identical light path via the retroreflector 120 back to the polarization beam splitter layer 110a , There, the measuring beam is now reflected and also hits the detector 135 ,

It should be noted that in Fig. La-lb and in the other figures each on an identical light path back and forth or reflected back in itself rays are drawn separately only for better illustration.

Due to the interference of the measuring beam with the reference beam, the detector delivers 135 that for the position measurement of the retroreflector 120 or a retroreflector 120 associated component (in particular a mirror) required signal. The from the polarization beam splitter 110 to the retroreflector 120 extending interferometer is also referred to as "arm" 101 designated.

An inventively essential property of the measuring arrangement both of the embodiment described with reference to FIG. La-lb as well as in the following with reference to 2 to 4 described embodiments is that - in addition to the use of the retroreflector 120 Insensitivity to tilting movements achieved in a manner known per se - including lateral displacements of the retroreflector 120 , which do not run along the z-direction (in particular, for example, movements in the y-direction), for the interference achieved by the measuring beam and the reference beam contrast play no role, so in particular not to interference or signal loss on the part of the detector 135 comes.

As a result of the utilization of the principle of "reversibility of the light path" becomes namely even at said lateral displacement of the retroreflector 120 (as in 1b for a lateral displacement of the retroreflector 120 indicated by a distance "d" in the y-direction) of the plane mirror 130 coincident measuring beam reflected back in itself and runs in the reverse direction in the same way back to the polarization beam splitter layer 110a with the result that he continues to use the reference beam at the detector 135 can interfere with maximum contrast.

2 shows a schematic representation for explaining the structure and operation of a measuring arrangement according to the invention in a further embodiment, wherein compared to Fig. La-lb analog or substantially functionally identical components are designated by "100" reference numerals.

The embodiment of 2 differs from that of Fig. La-lb only by the presence of an additional deflection mirror 250 , via which it is achieved that the coupling of measuring and reference beam to the detector 235 again to the side takes place, from which also the beam injection from the light source 205 he follows. The realization of the deflecting mirror 250 can according to 2 in particular monolithic (eg by wringing on the polarization beam splitter 210 ) respectively.

5 shows a schematic representation for explaining further embodiments of an inventive interferometric measuring arrangement, wherein compared to 2 analogous or substantially functionally identical components are designated by "300" increased reference numerals. The embodiment of 5 is different from the one 2 in that instead of the retroreflector 220 a plane mirror 540 serves as a movable measurement target, with the first retroreflector 520 is arranged on the side of a stationary part of the measuring arrangement. The as a plane mirror 540 trained measurement target is presently arranged on the component (in particular the mirror).

According to 5 thus hits in the measuring arm 501 that of the polarization beam splitter layer 510a incoming measuring beam on the measuring target forming plane mirror 540 and is from this plane mirror 540 in the direction of the retroreflector located in the stationary part of the measuring arrangement 520 reflected. After said reflection at the retro reflector 520 the measuring beam passes over the plane mirror 540 to the analogy to 2 acting group of λ / 4 plate 531 and plane mirror 530 , The measuring beam becomes after reflection at the plane mirror 530 and twice the λ / 4 plate 531 again at the measuring target forming plane mirror 540 reflects and then passes on the same optical path in turn via the retroreflector located in the stationary part of the measuring arrangement 520 and the plane mirror 540 back to the polarization beam splitter layer 510a , Alternatively, the measurement target is designed as a retroreflector.

6 shows starting from the embodiment of 5 and from the direction of the measurement target forming plane mirror 540 some possible configurations differing in their geometric arrangement.

3 serves to explain the structure and operation of another embodiment, wherein 2 analogous or substantially functionally identical components are designated by "100" increased reference numerals.

The embodiment of 3 is different from the one 2 in that in the reference arm 302 instead of the plane mirror 240 out 2 a centric or axially operated retroreflector 340 is used. Through this retroreflector 340 is also ensured that of the polarization beam splitter layer 310a or the lambda / 4 plate 341 incident reference beam is reflected back in itself. The retro reflector 340 Alternatively, it may be solid or hollow, with the solid configuration permitting wringing and thus a monolithic construction. In the latter case is preferably in the measuring arm 301 a compensation plate 332 to compensate for the additional in the material of the retroreflector 340 arranged to be traveled optical path.

4 serves to explain the structure and operation of another embodiment of the invention, in turn to 3 Analogous or substantially functionally identical components with " 100 "Increased reference numerals are designated. According to 4 in this respect, one to the measuring arm 401 analogous design of the reference arm 402 , as the (via a deflecting mirror 485 deflected) reference beam also via a retroreflector 470 up to a plane mirror 490 with upstream lambda / 4-plate 491 and in an identical way in the reverse direction back to the polarization beam splitter layer 410a to be led. In this way, we realized in the result a differential construction, which in addition to an improved compensation of atmospheric effects also an angle measurement (when placing both in the measuring arm 401 located retroreflector 420 as well as in the reference arm 402 located retroreflector 470 on the same component).

7 shows a schematic representation of a possible principle arrangement of inventive interferometer for determining the position of a mirror 701 , At the mirror 701 in the exemplary embodiment, this is a mirror of a projection objective in a microlithographic projection exposure apparatus, wherein - without the invention being restricted to this - one for itself, for example US 6,864,988 B2 is based on known structure, in which both a load-bearing support structure 703 ("Force frame") as well as an independently provided for this measurement structure 704 ("Sensor frame") are present. According to 7 are both support structure 703 as well as measurement structure 704 independently of one another via mechanical connections (eg springs) acting as dynamic decoupling 705 respectively. 706 to a base plate or base 730 mechanically connected to the optical system. The mirror 701 in turn is about one mirror mounting 702 on the supporting structure 703 attached. In 7 schematically drawn are two inventive distance-measuring interferometer 710 and 720 whose measuring path 711 respectively. 721 from the measuring structure 704 to the mirror 701 runs.

8th shows a schematic representation of an exemplary designed for operation in EUV microlithographic projection exposure apparatus 800 , The interferometer (s) according to the invention can be used in this projection exposure apparatus for measuring the distance 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 realized in a mask inspection system or a wafer inspection system.

According to 8th has a lighting device of the projection exposure system 800 a field facet mirror 803 and a pupil facet mirror 804 on. On the field facet mirror 803 becomes the light of a light source unit, which is a plasma light source 801 and a collector mirror 802 includes, steered. In the light path after the pupil facet mirror 804 are a first telescope mirror 805 and a second telescope mirror 806 arranged. In the light path below is a deflection mirror 807 arranged, which reflects the radiation impinging on an object field in the object plane of a six mirror 851 - 856 comprehensive projection lens steers. At the location of the object field is a reflective structure-bearing mask 821 on a mask table 820 arranged, which is imaged by means of the projection lens in an image plane in which a substrate coated with a photosensitive layer (photoresist) 861 on a wafer table 860 located.

While the invention has been described in terms 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 embodiments 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.

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited patent literature

• US 6864988 B2 [0049]

Claims (10)

Interferometric measuring arrangement in an optical system, in particular in an optical system for microlithography, with At least one distance-measuring interferometer associated with a component in the optical positioning system, Wherein said interferometer comprises a measuring arm (101, 201, 301, 401, 501), a reference arm (102, 202, 302, 402, 502) and a polarization beam splitter (110, 210, 310, 410, 510), one of a light source (105, 205, 305, 405, 505) coupled via the polarization beam splitter (110, 210, 310, 410, 510) in a along the measuring arm (101, 201, 301, 401, 501) and a propagating via a light beam the component associated first retroreflector (120, 220, 320, 420, 520) guided measuring beam and a along the reference arm (102, 202, 302, 402, 502) propagating reference beam is divided; Wherein in the measuring arm (101, 201, 301, 401, 501) a first plane mirror (130, 230, 330, 430, 530) is arranged, which the coming of the first retroreflector (120, 220, 320, 420, 520) measuring beam reflected back in itself. Interferometric measuring arrangement according to Claim 1 , characterized in that in the reference arm (102, 202, 302, 402, 502) a mirror is arranged, which reflects back from the polarization beam splitter (110, 210, 310, 410, 510) coming reference beam. Interferometric measuring arrangement according to Claim 2 , characterized in that this mirror is a second plane mirror (140, 240, 490, 540). Interferometric measuring arrangement according to one of Claims 1 to 3 , characterized in that in the reference arm (302, 402), a second retroreflector (340, 470) is arranged. Interferometric measuring arrangement according to Claim 2 and 4 , characterized in that said second retroreflector (340) is the mirror which reflects back the reference beam from the polarization beam splitter (310). Interferometric measuring arrangement according to Claim 3 and 4 , characterized in that this second retroreflector (470) is arranged in the beam path of the reference beam between the second plane mirror (490) and the polarization beam splitter (410). Interferometric measuring arrangement according to one of the preceding claims, characterized in that a measurement target is arranged on the component, wherein the measurement target is formed as a plane mirror (540). Interferometric measuring arrangement according to Claim 7 , characterized in that the plane mirror (540) is arranged in the measuring arm (501) such that the measuring beam is reflected by the plane mirror (540) in the direction of the first retroreflector (520). Interferometric measuring arrangement according to Claim 8 , characterized in that the first retroreflector (520) is arranged on the side of a fixed part of the measuring arrangement. Interferometric measuring arrangement according to one of the preceding claims, characterized in that the component for position determination in six degrees of freedom six distance-measuring interferometers are assigned.

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