CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 18154460.2 which was filed on 2018 Jan. 31 and EP application 18197177.1 which was filed on 2018 Sep. 27 which are incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

A first aspect of the present invention relates to a wavelength tracking system, a method to calibrate a wavelength tracking system and a lithographic apparatus.

A second aspect of the present invention relates to a method to determine an absolute position of a movable object with respect to a reference location using an interferometer system. The second aspect of the invention further relates to an interferometer system to carry out such method, and 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, interferometers 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 mirror elements of the projection optics box.

In certain types of lithographic apparatus this position measurement takes place in a conditioned space, in particular a space with reduced pressure. This space with reduced pressure is also indicated as vacuum space. The vacuum space may be subdivided in multiple sub-spaces between which gas locks are provided to avoid that particles will move from one sub-space to another sub-space. For example, the substrate support may be arranged in a first vacuum sub-space and the movable mirror elements of the projection optics box may be arranged in a second vacuum sub-space.

The gas lock that is provided between the first vacuum sub-space and the second vacuum sub-space may be configured to supply H2-gas as a particle avoidance medium from the first vacuum sub-space to the second sub-space. Over time, the concentration of H2 in the first vacuum subspace and the second vacuum sub space may vary, due to flow restriction variations, for example due to a moving substrate support. Variation in H2 concentration, also referred to as local H2 pressure variation, will induce refractive index changes as seen by the position measurement interferometers. These changes in the refractive index, if not compensated, will result in position measurement errors that will have a significant effect on the measurement accuracy of the position measurement interferometers. The sensitivity of the position measurement interferometers may for example be about 1.4 nm/m/Pa H2 pressure.

Furthermore, each projection optics box mirror elements may require a separate laser source because of required noise level per axis. The basic wavelength stability of a laser source, may not be sufficient to achieve the desired accuracy.

Both effects, refractive index changes due to variation in H2 concentration as well as wavelength stability of the multiple laser sources can be compensated by using a wavelength tracking system. In such wavelength tracking system a stabile cavity is used as a local actual wavelength reference. In the wavelength tracker an interferometer is measuring the difference in length between a first reflection path to a first reflection surface within the cavity and a second reflection path to a second reflection surface at the entrance of the cavity. Due to the difference in length between the first reflection path and the second reflection path, a wavelength or refractive index change will be measured. Since the cavity is stable, i.e. the first reflection surface and the second reflection surface do not move with respect to each other, the measurements do not comprise the effects of changes in the mutual positions of the first reflection surface and the second reflection surface, and can therefore be used to determine the wavelength or change in wavelength.

In the wavelength tracking system an optical differential interferometer is used. Such optical differential interferometer requires a relatively complex structure with multiple polarizing beam splitters, corner cubes, etc. This results in a relatively long optical path with long glass lengths.

Another 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 is 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.

SUMMARY OF THE INVENTION

It is an object of a first aspect of the invention to provide a wavelength tracking system that is of relatively simple construction requiring a relatively short optical path, or at least to provide an alternative wavelength tracking system. It is a further object the first aspect of the invention to apply such wavelength tracking system in a lithographic apparatus.

It is an object of the second aspect of the invention to provide an improved method to determine an absolute position of a movable object with respect to a reference location using an interferometer system, that may take into account movements of the movable object during measurements to determine the absolute position of the movable object.

According to an aspect of the invention, there is provided a wavelength tracking system comprising:

a wavelength tracking unit with reflection surfaces at stabile positions providing a first reflection path with a first path length and a second reflection path with a second path length, wherein the first path length is substantially larger than the second path length, and

an interferometer system, wherein the interferometer system comprises:a beam splitter to split a light beam in a first measurement beam and a second measurement beam,at least one optic element to guide the first measurement beam, at least partially, along the first reflection path and the second measurement beam, at least partially, along the second reflection path,a first light sensor arranged at an end of the first reflection path to receive the first measurement beam and to provide a first sensor signal on the basis of the first measurement beam,a second light sensor arranged at an end of the second reflection path to receive the second measurement beam and to provide a second sensor signal on the basis of the second measurement beam, and

a processing unit to determine a wavelength or change in wavelength on the basis of the first sensor signal and the second sensor signal.

According to an aspect of the invention, there is provided a method to calibrate a wavelength tracking system according to the invention, comprising the steps of:

moving the wavelength tracking unit in a measurement direction relative to the interferometer system,

obtaining the first sensor signal from the first sensor and the second sensor signal of the second sensor,

determining non-linearity of the first reflection path of the wavelength tracking system on the basis of the first sensor signal, and/or

determining non-linearity of the second reflection path of the wavelength tracking system on the basis of the second sensor signal.

According to an aspect of the invention, there is provided a lithographic apparatus comprising:

a conditioned space,

a movable object arranged within the conditioned space,

an interferometer position measurement system to measure a position of the movable object within the conditioned space, and

the wavelength tracking system according to the invention,

wherein the interferometer position measurement system is arranged to receive a wavelength or change in wavelength as determined by the wavelength tracking system and to compensate measurements of the interferometer position measurement system for the wavelength or change in wavelength.

According to an aspect of the invention, there is provided method to determine an absolute position of a movable object with respect to a reference location using an interferometer system, wherein said interferometer system comprises:

a first light source arranged to provide a first beam and a second beam with a first light frequency;

a second light source arranged to provide a further first beam and a further second beam with a second light frequency, wherein the second light frequency is a tunable light frequency;

said method comprising the steps of:projecting simultaneously the first beam and the further first beam along a measurement path on a reflective surface of the movable object and projecting the second beam and the further second beam along a reference path on a reference mirror while changing the tunable light frequency of the second light source, the reference path having a fixed length,determining a first measurement phase value based on the first beam, a second measurement phase value based on the further first beam, a first reference phase value based on the second beam and a second reference phase value based on the further second beam,determining the absolute position based on the first measurement phase value, the second measurement phase value, the first reference phase value and the second reference phase value.

According to an aspect of the invention, there is provided an interferometer system to determine a position of a movable object having a reflective measurement surface, comprising:

a first light source to provide a first beam and a second beam with a first light frequency,

a second light source to provide a further first beam and a further second beam with a second light frequency, wherein the second light frequency is a tunable light frequency,

a reflective reference surface,

a light sensor,

wherein the interferometer system is arranged to perform the method according to the invention.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising the interferometer system according to the invention.

