Dependence of contamination rates on key parameters in EUV optics

Optics contamination remains one of the challenges in extreme ultraviolet (EUV) lithography. Dependence of contamination rates on key EUV parameters was investigated. EUV tools have optics at different illumination angles. It was observed that at shallower angles, the carbon contamination rate and surface roughness was higher on the optics surface. This is a concern in EUV optics as higher roughness would increase the scattering of the EUV radiation. Secondary ion time of flight mass spectrometer (TOF-SIMS) data indicated that the carbon contamination film might be a polymer. Three chemical species were used to investigate the dependence of polymerization and reactivity on the contamination rate. Acrylic acid was found to have a measurable contamination rate above background compared to propionic acid and methyl methacrylate. Secondary electron dissociation is one of the mechanisms considered to be a cause for the growth of the carbon contamination film. Multiple experiments with two substrates having different secondary electron yields were performed. The substrate with the higher secondary electron yield was found to give a higher contamination rate.