This research disclosure relates to a radiation source. In particular, it relates to a laser-produced plasma (LPP) radiation source. Such an LPP source may provide radiation to a lithographic apparatus. Photolithography is a process of transferring a pattern to a radiation-sensitive substrate by exposing the substrate to radiation having the pattern. The substrate may be a resist-coated silicon wafer. The pattern may be imparted onto a beam of radiation using a reticle. Radiation used in a lithographic apparatus may be extreme ultraviolet (EUV). EUV radiation has a wavelength in the range 4-20 nm. for example 13.5 nm. EUV lithography is used in printing integrated circuits. One known source of EUV radiation is shown schematically in Figure 1. This EUV radiation source comprises a droplet generator that is arranged to produce a stream of fuel droplets along a fuel droplet trajectory and a laser arranged to produce a laser beam that intersects this trajectory. Typically, tin is used as a fuel. The laser beam is incident on the fuel droplets and at least partially converts the droplets into a plasma at a primary focus. The plasma subsequently emits EUV radiation.
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