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

Abstract

Ongoing endurance testing of Ru-capped multilayer mirrors (MLMs) at the NIST synchrotron facility has revealed that the damage resulting from EUV irradiation does not always depend on the exposure conditions in an intuitive way. Previous exposures of Ru-capped MLMs to EUV radiation in the presence of water vapor demonstrated that the mirror damage rate actually decreases with increasing water pressure. We will present results of recent exposures showing that the reduction in damage for partial pressures of water up to 5x10 -6 Torr is not the result of a spatially uniform decrease in damage across the Gaussian intensity distribution of the incident EUV beam. Instead we observe a drop in the damage rate in the center of the exposure spot where the intensity is greatest, while the reflectivity loss in the wings of the intensity distribution appears to be independent of water partial pressure. (See Fig. 1.) We will discuss how the overall damage rate and spatial profile can be influenced by admixtures of carbon-containing species (e.g., CO, CO 2 , C 6 H 6 ) at partial pressures one-to-two orders of magnitude lower than the water vapor partial pressure. An investigation is underway to find the cause of the non-Gaussian damage profile. Preliminary results and hypotheses will be discussed. In addition to high-resolution reflectometry of the EUV-exposure sites, the results of surface analysis such as XPS will be presented. We will also discuss how the bandwidth and time structure of incident EUV radiation may affect the rate of reflectivity degradation. Although the observations presented here are based on exposures of Rucapped MLMs, unless novel capping layers are similarly characterized, direct application of accelerated testing results could significantly overestimate mirror lifetime in the production environment.

Accelerated lifetime metrology of EUV multilayer mirrors in hydrocarbon environments

“…Our earlier accelerated testing experiments focused on photo-induced reaction of adsorbed water on Ru-capped MLMs and showed that the rate of reflectivity loss was a very complicated function of dose at high partial pressures of water. [14] These tests also indicated that ambient hydrocarbon species present at much lower concentrations than the admitted water were playing a key role.…”

Tracking electron-induced carbon contamination and cleaning of Ru surfaces by Auger electron spectroscopy

“…The experiments were performed at the materials characterization laboratory IMPACT at CMUXE that hosts an ultrahigh vacuum (UHV) chamber equipped with in situ diagnostic tools such as XPS, AES, EUVR, etc.. 17,21 To create a chamber condition similar to a EUVL system, 6 we did not bake the UHV chamber, giving a base pressure of $1.8 Â 10 À8 Torr. Since the residual gases in the chamber are known to control the growth of carbon and oxidation of the target surface, 22 a residual gas analyzer (RGA-100) was used to identify the gaseous components. It reveals a partial pressure of water on the order of $2.2 Â 10 À9 Torr with respect to nitrogen, as well as different background hydrocarbons such as methane, acetone, ethyl alcohol, methyl alcohol, benzene, toluene, and methane in conjunction with hydrogen.…”

Ni-Al Alloys as Alternative EUV Mask Absorber

“…The substrate consists of a low thermal expansion material (LTEM) glass plate with a Mo/Si ML mirror coating on front side, and a conductive backside coating for electrostatic chucking. A thin ruthenium (Ru) capping layer protects the mirror against oxidation of the top Si layer, and against the cleaning of organic contaminants by hydrogen reduction [1]. Currently, tantalum (Ta) is the main component of most common absorber materials developed for commercial EUV photomasks.…”