High reflectance ta-C coatings in the extreme ultraviolet

J I Larruquert,P J Martin,J A Méndez,L V Rodríguez-De Marcos,A Bendavid

doi:10.1364/oe.21.027537

Abstract

The extreme ultraviolet (EUV) reflectance of amorphous tetrahedrally coordinated carbon films (ta-C) prepared by filtered cathodic vacuum arc was measured in the 30-188-nm range at near normal incidence. The measured reflectance of films grown with average ion energies in the ~70-140-eV range was significantly larger than the reflectance of a C film grown with average ion energy of ~20 eV and of C films deposited by sputtering or evaporation. The difference is attributed to a large proportion of sp3 atom bonding in the ta-C film. This high reflectance is obtained for films deposited onto room-temperature substrates. The reflectance of ta-C films is higher than the standard single-layer coating materials in the EUV spectral range below 130 nm. A self-consistent set of optical constants of ta-C films was obtained with the Kramers-Krönig analysis using ellipsometry measurements in the 190-950 nm range and the EUV reflectance measurements. These optical constants allowed calculating the EUV reflectance of ta-C films at grazing incidence for applications such as free electron laser mirrors.

Highlights

Many applications demand efficient mirrors for the extreme ultraviolet (EUV; here it refers to wavelengths in the 10-100-nm range; the 100-200-nm range will be referred to as the far ultraviolet, FUV)
Aluminum is an exception to this rule, with a high reflectance down to its plasma wavelength at 83 nm [1]; Al slightly oxidizes in contact with the atmosphere, which destroys its EUV reflectance and which is unavoidable since no protective coating is available that is transparent below ~105 nm, the cutoff of LiF [2]
This paper reports on the EUV reflectance of tetrahedrally coordinated carbon films (ta-C) films deposited by filtered cathodic vacuum arc (FCVA)

Summary

Introduction

Many applications demand efficient mirrors for the extreme ultraviolet (EUV; here it refers to wavelengths in the 10-100-nm range; the 100-200-nm range will be referred to as the far ultraviolet, FUV). The mirror substrate must be kept close to room temperature during coating deposition With this limitation, sputter-deposited coatings of SiC [6,7], along with B4C [8], provide a reflectance of ~30-35% in the 60-100-nm range after the material has been exposed to the atmosphere for extensive periods. Carbon films can be deposited onto room-temperature substrates either by evaporation [27,28] or sputtering techniques [29]; their reflectance is substantially smaller than that of bulk or CVD diamond. Other than EUV reflectance, ta-C films have remarkable properties, such as mechanical hardness, chemical inertness, ultrasmoothness, and thermal stability, which are comparable with those of natural diamond These are valuable properties for a mirror coating that may have to work in adverse conditions, such as in space or in a FEL.

Experimental techniques

EUV reflectance

Optical constants of ta-C films

Conclusions