PROTECTED LENS COVER PLATE FOR AN OPTICAL METROLOGY DEVICE

摘要

A cover plate or lens for an optical metrology device that is positioned under a wafer during measurement is protected with a purge device. The purge device may include a ring that extends around a periphery of the cover plate or lens. The ring includes a plurality of apertures through which a purge gas or air is expelled over the surface of the cover plate or lens. Additionally or alternatively, one or more heating elements may be provided that extend around the periphery of the cover plate or lens. The heating elements heat the cover plate above a dewpoint temperature of contaminant vapor. A heat sensor may be used to monitor the temperature of the cover plate to control the heating elements and/or to compensate for optical changes of the cover plate caused by heating during measurement of a wafer.