Characterization of EUV induced carbon films using laser-generated surface acoustic waves

The deposition of carbon layers on the surfaces of optics exposed to extreme ultraviolet (EUV) radiation has been observed in EUV lithography. It has become of critical importance to detect the presence of the carbon layer in the order of nanometer thickness due to carbon's extremely strong absorption of EUV radiation. Furthermore, the development of efficient cleaning strategies requires that the nature of these carbon layers is well understood. Here, we present experimental results on the detection and characterization of carbon layers, grown on Mo/Si EUV reflecting optics, by laser-generated surface acoustic waves (LG-SAW). It was found that SAW pulses with a frequency bandwidth of more than 220 MHz can be generated and detected for multilayer mirrors and LG-SAW is sensitive enough to detect EUV induced carbon layers less than 5 nm thick. It was inferred from the low Young's modulus (< 100 GPa) that the carbon layer induced by EUV illumination in these vacuum conditions is mechanically soft and polymeric in nature with a high percentage of hydrogen.