Purge gas flow control for high-precision film measurements using ellipsometry and reflectometry

An optical system for measuring characteristics of a samples using a broadband metrology tool in a purged gas flow environment, comprising:

• a light source configured to supply a beam of source radiation having at least one wavelength component in a VUV range and/or at least one wavelength component in a UV-Vis range;
• illumination optics adapted to illuminate a surface of the sample with the beam source of radiation;
• collection optics collecting signal radiation generated by interaction of the beam source radiation with the surface of the sample;
• a detector adapted to provide at least one output signal in response to the signal radiation collected from the collection optics, wherein the signal is related to a property of the surface of the sample;
• a purge gas source configured to supply a flow of purge gas to a beam path in the light source and/or illumination optics and/or sample and/or collection optics and/or detector;
• and
• a controller operably coupled to the detector and the purge gas source, wherein the controller is configured to control a flow rate of the purged gas flow in response to the output signal from the detector by adjusting a flow rate of the purge gas between a first flow rate for metrology measurements made when the source radiation is in the VUV range and a second flow rate for metrology measurements made when the source radiation is in the UV-Vis range.