Protected lens cover plate for an optical metrology device

An apparatus comprising:

• a cover plate or lens positioned over optics of an optical metrology device, wherein the optical metrology device and the cover plate or lens are configured to be below a wafer during measurement of the wafer;
• one or more heating elements extending around a periphery of the cover plate or lens that heat the cover plate above a dewpoint temperature of contaminant vapor from the wafer;
• and
• a heat sensor mounted on the cover plate or lens, wherein the heat sensor monitors a temperature of the cover plate to compensate for optical changes to the cover plate due to the temperature during measurement of the wafer by the optical metrology device.

An integrated metrology module configured to be attached to a process tool, the integrated metrology module comprising:

• an optical metrology device;
• a wafer handling system for holding a wafer during measurement of the wafer by the optical metrology device;
• a cover plate or lens positioned between optics of the optical metrology device and the wafer held by the wafer handling system during measurement of the wafer by the optical metrology device;
• a purge device comprising a ring element extending around a periphery of the cover plate or lens, the ring element comprising a plurality of apertures through which a purge gas or air is expelled over a surface of the cover plate or lens that faces the wafer;
• one or more heating elements extending around the periphery of the cover plate or lens that heat the cover plate above a dewpoint temperature of contaminant vapor from the wafer;
• and
• a heat sensor mounted on the cover plate or lens, wherein the heat sensor monitors a temperature of the cover plate to compensate for optical changes to the cover plate due to the temperature during measurement of the wafer by the optical metrology device.