High Power Metrology Lasers in Interferometry

摘要

This disclosure relates to the application of high power metrology lasers in displacement interferometry, in particular in a lithographic apparatus, mask and wafer metrology applications. Helium-Neon (HeNe) lasers with an operable wavelength of around 633 nm and output power on the order of 1mW are widely used in displacement interferometry applications in e.g., lithography machines (including DUV and EUV), mask inspection and mask writing equipment. For long distance displacement interferometry a stabilized laser source is typically used, with a coherence length that is larger than the measured optical pathlength difference. Zeeman-stabilized and two-mode frequency-stabilized HeNe lasers have shown instabilities on the order of ～100 kHz and a long term stability of better than a few MHz. HeNe lasers with internal (or external) iodine reference cells stabilized to Doppler-free molecular hyperfine lines of iodine may achieve an instability down to sub-pm levels per meter over periods of hours (1).