Reducing the operating wavelength in advanced photolitho- graphy while maintaining the lithography machine’s produc- tivity has been a traditional way to enable improved imaging for the last 20 years. The transition from 13.5 nm to 6.5 to 6.9 nm optical lithography offers a possibility to combine high imaging capabilities using a manageable process win- dow. It is shown that around 6.6 nm wavelength, the high- est reflectance is obtained with multilayer mirrors based on lanthanum as a reflector and boron as a spacer material. Boron is the preferred spacer material for this wavelength because of the close proximity to the boron K-absorption edge.
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