A purge system for an optical metrology tool is disclosed. The metrology
tool includes an optics plate for supporting the measurement optics. A
movable stage supports a wafer below the optics plate. Inert purge gas is
injected between the lower surface of the optics plate and the upper
surface of the wafer. The gas flow functions to stabilize and homogenize
the ambient in the measurement region. The gas flow also serves to clear
the measurement area of absorbing species which is particular useful for
measurements using vacuum ultraviolet light.
