Modeling radiation-induced carbon contamination of extreme ultraviolet optics

Abstract

A comprehensive model of radiation-induced carbon contamination of extreme ultraviolet (EUV) optics is presented. The mathematical model describes the key processes that contribute to the deposition of a carbon film on a multilayer optic when the optic is exposed to EUV radiation in the presence of residual hydrocarbons. These processes include the transport of residual hydrocarbons to the irradiated area, molecular diffusion across the optic surface, and the subsequent dissociation or “cracking” of the hydrocarbon by both direct EUV ionization and secondary electron excitation. Model predictions of carbon growth are compared to measurements taken on optics exposed to EUV in the presence of residual hydrocarbons. Model estimates of hydrocarbon film growth under various conditions of hydrocarbon partial pressures and EUV power demonstrate the sensitivity of film growth to varying operating conditions. Both the model and experimental data indicate that the predominant cause of hydrocarbon dissociation is bond breaking by direct photon absorption, rather than by dissociation processes caused by exposure to secondary electrons. Detailed predictions for carbon deposition for a variety of conditions of EUV power and hydrocarbon pressure are reported. The model successfully predicts that light hydrocarbons (<∼100amu) pose a negligible risk to EUV optics, in general agreement with the experiment. Calculations also predict that modest increases in substrate temperature, on the order of 30°C, will substantially reduce optic contamination by increasing hydrocarbon desorption from the surface. Model investigation of surface diffusion indicates that, while surface diffusion is an important surface phenomenon for light gases, for the heavier hydrocarbons that contribute substantially to contamination, surface diffusion is not an important transport phenomenon.

Characterization of carbon contamination under ion and hot atom bombardment in a tin-plasma extreme ultraviolet light source

“…Even in vacuum conditions, carbon atoms are deposited onto a surface when residual hydrocarbons dissociate during interaction with energetic photons [1]. The carbon atoms can then accumulate on surfaces, which is undesirable for reflective optics because the carbon layer absorbs radiation.…”

“…Off-line removal of EUV-induced carbon contamination also reduces the duty cycle of EUVL, which is undesirable [1,6,7], and ultimately, increases the operating cost of the EUVL process.…”

IonCCD Detector for Miniature Sector-Field Mass Spectrometer: Investigation of Peak Shape and Detector Surface Artifacts Induced by keV Ion Detection

“…The O1s at 530.5 eV is due to oxide formation on the surface [64]. The C1s peak at 284.5 eV is due to hydrocarbon contamination [65][66][67][68]. Comparison of spectra for the unexposed and exposed surfaces reveals an overall decrease in photoelectron yield.…”

Critical parameters influencing the EUV-induced damage of Ru-capped multilayer mirrors

“…For most materials δ is approximately 1 nm for the ~10 eV electrons that dominate the photoelectron spectrum. 10,11 So the SEY will be determined primarily by the EUV intensity 1-2 nm nanometers below the mirror surface. 12 For the resonant case depicted in Fig.…”