Advances in multilayer reflective coatings for extreme-ultraviolet lithography

Multilayer mirror coatings which reflect extreme ultraviolet (EUV) radiation are a key enabling technology for EW Lithography but must meet stringent requirements in terms of film quality, stability, and thickness control across multiple optical elements up to 300 mm in diameter. Deposition technology has been dramatically improved to meet those specifications for thickness control and repeatability over large curved optical substrates. Coating uniformity was improved to +/-0.055% peak-to-valley (P-V) on 140-mm flats and +/-0.1% P-V across 160 mm curved substrates. The run-to-run reproducibility of the: reflectance peak wavelength was improved to 0.13% on flats to enable fabrication of wavelength-matched sets of optics. Multilayers with reflectances of 67.5% at 13.42 nm and 70.2% at 11.34 nm are typically achieved for Mo/Si and Mo/Be multilayers, respectively. Also, we have recently achieved a reflectance of 70.1% at 13.5 nm for a Mo/Si multilayer deposited with a modified process. The reflectance and stress of these multilayers appear to be stable relative to the requirements for application to EUV lithography. These improvements in EW multilayer mirror technology enable us to meet the stringent specifications for coating the large optical substrates for our next-generation EUV lithography system. The primary remaining issues are improving the run-to-run wavelength repeatability on curved optics to realize the maximum optical throughput, and verifying long-term stability of the multilayers within the environment of a production EUV lithographic system.