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1 INTERFEROMETER SYSTEM WITHOUT WALK-OFF, METHOD FOR USING AN INTERFEROMETER SYSTEM FIELD 5 [0001] The present disclosure relates to an interferometer system, and a method of using the interferometer system. The disclosure also relates to a lithographic apparatus including the interferometer system. BACKGROUND 10 [0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer). 15 [0003] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic 20 apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus 25 which uses, for example, radiation with a wavelength of 193 nm. [0004] Interferometers are used for various measurements in the lithographic apparatus and in other equipment related to the semiconductor fabrication process, including metrology. For instance, wavelength trackers may be used to control stability of laser beam inputs, while the position of moveable equipment such as wafer stages may be controlled up to nm level using one or more phase 30 tracking interferometers. [0005] The interferometer design used in known lithographic systems is typically based on a two-pass principle. Herein, the target is a flat mirror. Four-pass designs are also known. [0006] CN-218443724-U discloses a miniaturized laser interferometer characterized by comprising a miniaturized frequency-stabilized light source module, an interference light path 35 module, a corner reflector and a data processing module, wherein the interference light path module comprises: a miniaturized beam splitter, a miniaturized corner reflector, a glass slide and a miniaturized photodetector; the miniature frequency stabilization light source module generates a 2 frequency stabilization laser with narrow line width, and the frequency stabilization laser is divided into signal light and reference light with equal power by a miniature beam splitter; the signal light returns to the miniature beam splitter through a corner reflector mounted on the target; the reference light returns to the miniaturized beam splitter through the miniaturized corner reflector; the signal 5 light reflected by the corner reflector enters the interference light path module, interferes with the reference light to generate two paths of interference phase orthogonal signals, is incident on the miniaturized photoelectric detector, and inputs the interference signals obtained by detection into the data processing module. [0007] US-20030197870-A1 discloses an interferometer which returns parallel beams that are 10 subject to walk-off caused by reflector misalignment for an additional pass through the interferometer optics and thereby eliminates beam walk-off. A return reflector can be a plane mirror that directs returning beams to retrace paths through the interferometer optics to combine and exit along the axis of the input beam. Separation optics can separate the combined beam from the input beam. Alternatively, a return reflector such as an isosceles prism or a trapezoidal prism reflects and offsets 15 returning beams so that the combined beam is offset from the input beam. The return reflector more generally responds to a shift in incident beam position with a matching shift of the reflected beam in contrast to a retroreflector, which shifts a reflected beam in a direction opposite t...