Atmospheric Molecular Contamination Control with Local Purging

A method for determining a thickness correction for a thickness measurement of a thin film of a wafer, comprising the steps of:

• establishing a correlation between relative humidity and measured thickness of a thin film of a wafer;
• acquiring an initial humidity level measurement of an environment proximate to a measurement location of a portion of a surface of the wafer;
• making a thickness measurement of the thin film of the wafer at the measurement location of the portion of the surface of the wafer with an optical system, wherein the portion of the surface of the wafer is locally purged with purging gas when the thickness measurement is made;
• acquiring a subsequent humidity level measurement of the environment proximate to the measurement location, wherein the subsequent humidity level measurement is related to the thickness measurement of the thin film of the wafer;
• calculating a differential between the initial humidity level measurement and the subsequent humidity level measurement;
• and
• determining a thickness correction for the thickness measurement of the thin film of the wafer utilizing the established correlation between relative humidity and measured thickness and the calculated differential between the initial humidity level measurement and the subsequent humidity level measurement.