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EP-4548039-B1 - METHOD TO DETERMINE AN ABSOLUTE POSITION OF A MOVABLE OBJECT, INTERFEROMETER SYSTEM, PROJECTION SYSTEM AND LITHOGRAPIC APPARATUS

EP4548039B1EP 4548039 B1EP4548039 B1EP 4548039B1EP-4548039-B1

Inventors

  • JANSEN, Maarten, Jozef
  • KOENEN, Willem, Herman, Gertruda, Anna

Dates

Publication Date
20260513
Application Date
20230525

Claims (15)

  1. A method to determine an absolute position of a first movable object using an interferometer system, said method comprising: providing a first beam and a second beam with a first light frequency from a first light source; providing a further first beam and a further second beam with a second light frequency from a second light source, wherein the second light frequency is a tunable light frequency; guiding the first beam and the further first beam along a first measurement axis to a first reflective surface of the first movable object to obtain a first interferometer signal and a further first interferometer signal, respectively, guiding the second beam and the further second beam along a second measurement axis to a second reflective surface of a second object to obtain a second interferometer signal and a further second interferometer signal, respectively, while changing the tunable light frequency of the second light source, detecting at at least one first detector the first interferometer signal and the further first interferometer signal, detecting at at least one second detector the second interferometer signal and the further second interferometer signal, characterised by determining on the basis of the first interferometer signal, the further first interferometer signal, the second interferometer signal and the further second interferometer signal a first count offset and/or a further first count offset using a non-linear equation, wherein the non-linear equation is based on the relationship: λ ls 2 / λ ls 1 = C ma 1 , ls 1 / C ma 1 , ls 2 = C ma 2 , ls 1 / C ma 2 , ls 2 , wherein λ ls2 is the second wavelength of the second light source, λ ls1 is the first wavelength of the first light source, C mal, ls1 is a total number of counts with respect to the first measurement axis and the first beam, C mal, ls2 is a total number of counts with respect to the first measurement axis and the further first beam, C ma2, ls1 is a total number of counts with respect to the second measurement axis and the second beam, C ma2, ls2 is a total number of counts with respect to the second measurement axis and the further second beam, and determining the absolute position of the first movable object on the basis of the first count offset and the first interferometer signal and/or on the basis of the further first count offset and the further first interferometer signal.
  2. The method of claim 1, wherein the step of determining the first count offset and/or the further first count offset comprises solving the non-linear equation: (ΔC ma1, ls2 (t) + offset ma1, ls2 ) * (ΔC ma2, ls1 (t) + offset ma2, ls1 ) = (ΔC ma2, ls2 (t) + offset ma2, ls2 ) * (ΔC ma1, ls1 (t) + offset ma1, ls1 ) wherein: ΔC ma1, ls2 (t) is a change in counts over time with respect to the first measurement axis and the further first beam measured with the further first interferometer signal, offset ma1, ls2 is the further first count offset with respect to the first measurement axis and the further first beam, ΔC ma2, ls1 (t) is a change in counts over time with respect to the second measurement axis and the second beam measured with the second interferometer signal, offset ma2, ls1 is a second count offset with respect to the second measurement axis and the second beam, ΔC ma2, ls2 (t) is a change in counts over time with respect to the second measurement axis and the further second beam measured with the further second interferometer signal, offset ma2, ls2 is a further second count offset with respect to the second measurement axis and the further second beam, ΔC ma1, ls1 (t) is a change in counts over time with respect to the first measurement axis and the first beam measured with the first interferometer signal, and offset ma1, ls1 is the first count offset with respect to the first measurement axis and the first beam.
  3. The method of claim 1 or 2, wherein the second object is movable.
  4. The method of claim 3, wherein the method comprises determining an absolute position of the second object, comprising determining a second count offset and/or a further second count offset, and determining the absolute position of the second movable object on the basis of the second count offset and the second interferometer signal and/or on the basis of the further second count offset and the further second interferometer signal.
  5. The method of claim 3 or 4, wherein the method comprises moving the first movable object in a first direction and moving the second object in a second direction, wherein the first direction and the second direction are opposite to each other.
  6. The method of any of the preceding claims, wherein the first light frequency is a tunable light frequency.
  7. The method of claim 6, wherein tuning of frequencies of the first light frequency and the second light frequency is performed in opposite directions.
  8. An interferometer system (100) to determine an absolute position of a first movable object (200), said interferometer system comprising: a first light source (101) arranged to provide a first beam and a second beam with a first light frequency; a second light source (107) arranged to provide a further first beam and a further second beam with a second light frequency, wherein the second light source is arranged to provide a tunable second light frequency, a first measurement axis (102) for guiding the first beam and the further first beam to a first reflective surface (201) of the first movable object (200) to obtain a first interferometer signal and a further first interferometer signal, respectively, a second measurement axis (104) for guiding the second beam and the further second beam to a second reflective surface (301) of a second object (300) to obtain a second interferometer signal and a further second interferometer signal, respectively, at least one first detector (103b) to detect the first interferometer signal and the further first interferometer signal, at least one second detector (103c) to detect the second interferometer signal and the further second interferometer signal, characterised by a processing device (106), wherein the processing device is arranged to determine on the basis of the first interferometer signal, the further first interferometer signal, the second interferometer signal and the further second interferometer signal a first count offset and/or a further first count offset using a non-linear equation, wherein the non-linear equation is based on the relationship: λ ls 2 / λ ls 1 = C ma 1 , ls 1 / C ma 1 , ls 2 = C ma 2 , ls 1 / C ma 2 , ls 2 , wherein λ ls2 is the second wavelength of the second light source, λ ls1 is the first wavelength of the first light source, C mal, ls1 is a total number of counts with respect to the first measurement axis and the first beam, C mal, ls2 is a total number of counts with respect to the first measurement axis and the further first beam, C ma2, ls1 is a total number of counts with respect to the second measurement axis and the second beam, C ma2, ls2 is a total number of counts with respect to the second measurement axis and the further second beam, and wherein the processing device is arranged to determine the absolute position of the first movable object on the basis of the first count offset and the first interferometer signal or on the basis of the further first count offset and the further first interferometer signal.
  9. The interferometer system of claim 8, wherein the processing device is arranged to determine the first count offset and/or the further first count offset by solving the non-linear equation: (ΔC ma1, ls2 (t) + offset ma1, ls2 ) * (ΔC ma2, ls1 (t) + offset ma2, ls1 ) = (ΔC ma2, ls2 (t) + offset ma2, ls2 ) * (ΔC ma1, ls1 (t) + offset ma1, ls1 ) wherein: ΔC ma1, ls2 (t) is a change in counts over time with respect to the first measurement axis and the further first beam measured with the further first interferometer signal, offset ma1, ls2 is the further first count offset with respect to the first measurement axis and the further first beam, ΔC ma2, ls1 (t) is a change in counts over time with respect to the second measurement axis and the second beam measured with the second interferometer signal, offset ma2, ls1 is a second count offset with respect to the second measurement axis and the second beam, ΔC ma2, ls2 (t) is a change in counts over time with respect to the second measurement axis and the further second beam measured with the further second interferometer signal, offset ma2, ls2 is a further second count offset with respect to the second measurement axis and the further second beam, ΔC ma1, ls1 (t) is a change in counts over time with respect to the first measurement axis and the first beam measured with the first interferometer signal, and offset ma1, ls1 is the first count offset with respect to the first measurement axis and the first beam.
  10. The interferometer system of claim 8 or 9, wherein the second object is movable.
  11. The interferometer system of claim 10, wherein the first movable object is movable in a first direction and the second object is movable in a second direction, wherein the first direction and the second direction are opposite to each other.
  12. The interferometer system of any of the claims 8-11, wherein the first light frequency is a tunable light frequency.
  13. The interferometer system of claim 12, wherein tuning of frequencies of the first light frequency and the second light frequency is performed in opposite directions.
  14. A projection system for optical lithography systems comprising the interferometer system of any of the claims 8-13.
  15. A lithographic apparatus, comprising the interferometer system of any of the claims 8-13.

Description

CROSS-REFERENCE TO RELATED APPLICATION The application claims priority of EP application 22182093.9 which was filed on 30 June, 2022. FIELD OF THE INVENTION The present invention relates to a method to determine an absolute position of a movable object using an interferometer system. The invention further relates to an interferometer system and to a projection system for optical lithography systems and/or a lithographic apparatus comprising such interferometer system. BACKGROUND ART A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. In embodiments of a lithographic apparatus, interferometer systems are used to determine the position of movable objects with high accuracy. Examples of these movable objects are the substrate support and movable optic elements, for example mirrors of the projection optics box. A drawback of most known interferometers is that an interferometer is only able to determine relative displacements of the movable object with respect to a reference location. In order to determine an absolute position of the movable object with respect to the reference location a separate zeroing sensor may be provided. This zeroing sensor is used to determine an absolute starting position of the movable object. Once this absolute starting position is known, the interferometer may determine a relative displacement of the movable object with respect to this absolute starting position in order to calculate an absolute position of the movable object. The zeroing sensor is normally mounted at a specific location at which the absolute starting position of the movable object may be determined. The absolute position of the movable object may therefore only be determined when the movable object is within a relatively small measurement range of the zeroing sensor. The measurement range of the zeroing sensor is typically close to the zeroing sensor, for example within a few centimeters of the zeroing sensor. Each time the measurement of the movable object is started using the interferometer, the movable target has to be brought back into the relatively small measurement range of the zeroing sensor of the position measurement system. This may not only be the case when the lithographic apparatus is started, but for example also when the movable object is shortly out of view of the interferometer, for example when passing behind another movable object. WO2019149515A1 discloses a method to determine an absolute position of a movable object with respect to a reference object using an interferometer system. The interferometer system comprises a measurement axis including a reflective measurement surface on the object and a reference axis including a reflective reference surface on the reference object. In this method a first beam and a second beam originating from a first light source are guided through the measurement axis and the reference axis, respectively. Similarly, a further first beam and a further second beam originating from a second light source are guided through the measurement axis and the reference axis, respectively. The light frequency of the second light source is tunable so that the light frequency of the second light source can be changed during the measurements. WO2019149515A1 provides an algorithm in which specific measurements values of the interferometer signals obtained from the measurements in the measurement axis and the reference axis are selected to determine the absolute position of the movable object. This algorithm requires that the length of the reference axis is stable, i.e. does not change during measurements. Moreover, the calculations are relatively complex and may require some specific selection c