High reflectance ta-C coatings in the extreme ultraviolet.

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.

General theory of sub-quarterwave multilayers with highly absorbing materials.

A general theory of multilayers with enhanced reflectance has been developed based on the superposition of sub-quarterwave layers of various highly radiation-absorbing materials. The theory has been developed by second-order expansion of the multilayer reflectance with respect to the optical-constant differences between the materials in the multilayer. The current paper completes and improves the theory that was developed in a previous paper [J. Opt. Soc. Am. A 18, 1406 (2001)] by including the case of nonnormal incidence and general radiation polarization and by providing more-accurate film thickness values of the optimized multilayer than with the previous theory. The theory provides an accurate approach to the design of a new concept of multilayer coatings with more than two materials. The new multilayers are adequate to enhance the reflectance of the materials particularly in the far and the extreme ultraviolet.

Reflectance enhancement with sub-quarterwave multilayers of highly absorbing materials.

A new theory of multilayers with enhanced normal reflectance has been developed based on the superposition of a few layers of various different radiation-absorbing materials. Every layer in the multilayer had a subquarterwave optical thickness. The theory was developed for materials with small refractive-index differences, although it is also valid in some cases for materials with large refractive-index differences. Reflectance enhancements were obtained in a very broad band and over a wide range of incidence angles. The theory is particularly suited to designing multilayers with enhanced reflectance in the extreme ultraviolet for wavelengths above 50 nm. In this spectral region the reflectance of single layers of all materials is relatively low, and standard multilayers are not possible because of the high absorption of materials.

Multilayer coatings for narrow-band imaging in the extreme ultraviolet.

Multilayer coatings for the extreme ultraviolet with high reflectance at a spectral band of interest and zero reflectance at another band to be suppressed have been designed and prepared. Multilayer coatings were designed to maximize normal-incidence reflectance at the O(+) 83.4-nm spectral line and to suppress at the same time radiation at the 121.6-nm hydrogen Lyman alpha line. Fresh Al/MgF(2)/Mo multilayer coatings resulted in high reflection/suppression ratios at the above wavelengths. The coatings also exhibited a dip in reflectance at 102.6-nm Lyman beta. The coatings showed some slow but steady degradation over time that could be partially eliminated by sample cleaning. Al/MgF(2)/Mo multilayer coatings protected with a thin ion-beam-deposited C film were found to give a high reflection/suppression ratio with lower degradation over time.

Preservation of far-UV aluminum reflectance by means of overcoating with C60 films.

Thin films of C(60) were investigated as protective coatings of Al films to preserve their far-UV (FUV) reflectance by inhibition or retardation of their oxidation. Two methods were used for the overcoating of Al films with approximately one monolayer of C(60): (1) deposition of a multilayer film followed by temperature desorption of all but one monolayer and (2) direct deposition of approximately one-monolayer film. We exposed both types of sample to controlled doses of molecular oxygen and water vapor and measured their FUV reflectance before and after exposure to evaluate the achieved protection on the Al films. The whole process of sample preparation, reflectance measurement, sample heating, and oxidation was made without breaking vacuum. Results show that a C(60) monolayer protected Al from oxidation to some extent, although FUV reflectance of unprotected Al films was never exceeded. FUV optical constants of C(60) films and the FUV reflectance of the C(60) film as deposited and as a function of exposure to O(2) were also measured.

Optical properties of hot-pressed B4C in the extreme ultraviolet.

Hot-pressed B(4)C is found to have a high normal reflectance in the extreme-UV spectral region above 49 nm. This reflectance is comparable with or higher than chemical-vapor-deposited SiC in the spectral region from 49 to 92 nm. Reflectance measurements as a function of the angle of incidence yielded the optical constants of B(4)C in the spectral range 49-121.6 nm.

Reflectance measurements and optical constants in the extreme ultraviolet for thin films of ion-beam-deposited SiC, Mo, Mg2Si, and InSb and of evaporated Cr.

Reflectance measurements and optical constants of thin films of ion-beam-deposited SiC, Mo, Mg(2)Si, and InSb and of evaporated Cr have been measured in the extreme-ultraviolet (EUV) spectral region from 49.0 to 200.0 nm. In this spectral region no optical constant data were available for materials deposited by ion-beam deposition. We compared our data with those for bulk samples and for thin films prepared by different techniques. The goal of this research has been to study candidate materials for multilayer coatings in the EUV.

Multilayer coatings with high reflectance in the extreme-ultraviolet spectral range of 50 to 121.6 nm.

Multilayer coatings with three layers were designed to yield an increase in normal-incidence reflectance in the extreme ultraviolet over that of the available single-layer coatings. Multilayer coatings based on Al, MgF(2), and either SiC or B(4)C were demonstrated to have higher reflectance than single layers of SiC and B(4)C in the spectral region from 57.9 nm to the H Lyman-alpha line (121.6 nm) and above. The increase in reflectance was higher at wavelengths close to 121.6 nm. Reflectance degraded slightly over time in the same way as for single layers. After a few months, multilayer coatings maintained higher reflectance than their single-layer counterparts.

Optical constants of aluminum films in the extreme ultraviolet interval of 82-77 nm.

Measurements of the extreme ultraviolet (EUV) reflectance of unoxidized aluminum films versus the angle of incidence in the interval of 82-77 nm, just below the aluminum plasma wavelength (83 nm), are presented. The continuum of helium was used as a radiation source for the first time in EUV reflectometry, to our knowledge. The surface roughness of substrates and samples was characterized by atomic force microscopy. The complex refractive index of unoxidized aluminum was obtained from reflectance measurements at each wavelength for the first time in this spectral range. Data on the refractive index, the dielectric constant, and the energy loss function in the above interval are shown together with our previous data obtained in an interval of 82.6-113.5 nm. Current results on the refractive index show a good match with the data in the literature calculated through the Kramers-Kronig analysis, the largest differences being in the imaginary part of the refractive index at the shortest wavelengths.

Far-ultraviolet reflectance measurements and optical constants of unoxidized aluminum films.

The far-UV reflectance of thin unoxidized aluminum films prepared and maintained in ultra-highvacuum conditions was measured versus the angle of incidence, and the complex refractive index was obtained from those measurements on several wavelengths from 82.6 to 113.5 nm. Measurements were made on two perpendicular planes of incidence to deal with the unknown of the polarization state of the radiation beam. The surface roughness was characterized by atomic force microscopy. The refractive index is obtained for the first time, to our knowledge, from direct optical measurements in this spectral range. Current results match well the former values in the literature that were calculated through the Kramers-Kronig analysis by using in the above interval reflectances estimated from electron-energy-loss spectra and from optical measurements on surfaces of unstated roughness.

Far-UV reflectance of UHV-prepared Al films and its degradation after exposure to O(2).

The far-ultraviolet reflectance of aluminum films prepared and maintained under ultrahigh-vacuum conditions was measured for wavelengths ranging from 82.6 to 120.0 nm. The degradation of the reflectance after exposure to controlled doses of molecular oxygen was also studied. The degradation rate proved to be higher for wavelengths less than 120 nm than previous measurements made at 121.6 nm.

Empirical relations among scattering, roughness parameters, and thickness of aluminum films.

Experimental measurements of the angular distribution of scattering and scanning electron microscopy pictures of thin aluminum films were used to relate the total integrated scattering and the statistical parameters of the surface roughness to the film thickness.