This research disclosure relates to a lithographic apparatus. In particular, it relates to a metrology apparatus for use in a lithographic apparatus. The lithographic apparatus may he an extreme ultraviolet (EUV) lithographic apparatus. Photolithography is a process of transferring a pattern to a radiation-sensitive substrate by exposing the substrate to radiation having the pattern. Radiation used in a lithographic apparatus may be extreme ultraviolet (EUV) radiation. EUV radiation has a wavelength in the range 4-20 nm, for example 13.5 nm. One known source of EUV radiation illuminates fuel droplets with laser radiation provided by a laser. Typically, tin (Sn) is used as a fuel. Fuel can be provided to the source using an in-line refill system. It may be beneficial to monitor the in-line refill system or a portion thereof, for example monitoring the temperature and/or temperature variations of a portion of the system. Such monitoring may be performed with a metrology apparatus.
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