Ralph Kurt

Molecular contamination mitigation in EUVL by environmental control

Abstract EUVL tools operate under vacuum conditions to avoid absorption losses. Under these conditions, the MoSi multilayer mirrors are contaminated, resulting in reduced reflection and thus throughput. We report on experiments on MoSi mirrors exposed to EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ∼0.016 nm/h per mW/mm 2 of EUV radiation under conditions expected for a high throughput EUVL system. This oxidation can to a large extent be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A carbon growth rate of 0.25 nm/h was found for a hydrocarbon pressure of 10 −9 mbar Fomblin. We demonstrate that carbonisation can be suppressed by admitting oxygen during electron gun exposure.

Radiation-induced carbon contamination of optics

In this work molecular contamination of optical components such as e.g. reflective mirrors for extreme ultraviolet (EUV) lithography was experimentally studied using irradiation with keV electrons. Since secondary electrons are known to initiate most of the surface chemistry, radiation with EUV photons or electrons often gives similar results. Due to presence of hydrocarbon molecules in a vacuum system, carbon growth will occur at each irradiated surface. The carbon growth rate for various molecules was measured by in-situ Auger electron spectroscopy and Rutherford backscattering spectroscopy (RBS) analyzing the influence of the two main parameters being radiation dose and partial pressure of hydrocarbons. The latter was varied over a large range from 10-5 until 10-11 mbar. Furthermore the structure of the resulting carbon films was analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicating the formation of a nanocrystalline graphite-like structure independent of the original hydrocarbon molecule.

Prevention of MoSi multilayer reflection loss in EUVL tools

Extreme ultraviolet lithography requires vacuum conditions in the optical train. In order to maintain sufficient energy throughput, reflection reduction of multilayer mirrors due to contamination has to be minimized. We report on oxidation and carbonization experiments on MoSi mirrors under exposure with EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ~0.015 nm/h per mW/mm2 of EUV radiation under vacuum conditions that are typical for a high throughput EUVL system, I.e. 10-6 mbar H2O. This oxidation can to a large extend be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A deposition rate of 0.25 nm/h was found when the hydrocarbon pressure of Fomblin was reduced to 10(superscript -9 mbar. We demonstrate that carbonization can be suppressed by admitting oxygen during electron gun exposure.