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1 OPTICAL SYSTEM AND METHOD OF USING OPTICAL SYSTEM BACKGROUND [0001] Light generated by means of a radiation source can be used by exposure apparatuses for 5 semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus. [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 10 lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for 15 example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm. [0003] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure 20 which functions as a Bragg reflector, the multilayers may be alternatingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a mass- production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such 25 phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, 30 the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be 35 used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle/amplitude effects. 2 [0004] The lithographic apparatus may generate EUV radiation as a result of a forward propagating beam of light being incident on a droplet of fuel. There may be a plurality of optical elements configured to direct and/or transmit the forward propagating beam from the source to the droplet. In order to assess the effectiveness of the impact of the forward propagating beam on the droplet, the apparatus may 5 comprise a return beam detector. The forward propagating beam may be reflected off the droplet as a primary return beam. The return beam detector is configured to detect light of the primary return beam. [0005] The plurality of optical elements may include transmissive elements, for example windows and lenses. Light from the forward propagating beam may undesirably be reflected off one or more of these transmissive elements. These reflections may be referred to as ghost reflections. These ghost 10 reflections may travel back through the optical system towards the return beam detector. The return beam detector may therefore detect light which is not a true representation of the primary return beam. Instead the return beam detector may detect some combination of light of the primary return beam and light of ghost reflections. The measurements taken by the return beam detector may be used to assess effectiveness of the EUV generation and to adjust control inputs to the system if needed. As such, it is 15 desirable to be able to remove, distinguish and/or filter out the ghost reflections, such that a more accurate measurement of the primary return beam may be obtained. DETAILED DESCRIPTION [0006] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic 20 apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W. 25 [0007] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may 30 include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11. [0008] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that 35 purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The 3 projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a 5 different number of mirrors (e.g., six or eight mirrors). [0009] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W. [0010] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below 10 atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS. [0011] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning...