A simple modeling of carbon contamination on EUV exposure tools based on contamination experiments with synchrotron source

Contamination control of optics is one of critical issues for extreme ultraviolet (EUV) lithography. EUV irradiation under a carbon-containing environment causes carbon contaminations on mirror surfaces. We investigated irradiance dependency of contaminating rates of some contaminants using a synchrotron radiation of Saga Light Source (SAGALS). Decane's contaminating rate increased proportionally with irradiance, while perfluorohexane's contaminating rate was almost constant at a higher irradiance than 10 mW/cm2. We then introduced a simple model: contamination reaction occurs when photons are supplied onto contaminants which are supplied and adsorbed on mirrors, and the lesser of their supplying rates determines the contaminating rate. At a lower irradiance, since contaminants are sufficiently supplied, the photon supply determines the contaminating rate. At a higher irradiance, since photons are sufficiently supplied, the contaminant supply determines the contaminating rate, which is independent of irradiance and depends on contaminant's partial pressure. We also investigated irradiance dependency of cleaning rates of carbon contamination by oxidative gas and incorporated it into the model. We applied the contamination/cleaning model to an existing exposure tool, EUV1. The transmittance degradation history agreed well with the calculation.