Juan I. Larruquert

Reflectance enhancement in the extreme ultraviolet and soft x rays by means of multilayers with more than two materials

Sub-quarterwave multilayer coatings with more than two different materials are shown to provide a reflectance enhancement compared with the standard two-material multilayer coatings when reflectance is limited by material absorption. A remarkable reflectance enhancement is obtained when the materials in the multilayer are moderately absorbing. A simple rule based on the material optical constants is provided to select the most suitable materials for the multilayer and to arrange the materials in the correct sequence in order to obtain the highest possible reflectance. It is shown that sub-quarterwave multilayers generalize the concept of multilayers, of which the standard two-material multilayers are a particular case. Various examples illustrate the benefit of sub-quarter-wave multilayer coatings for highest reflectance in the extreme ultraviolet. Applications for sub-quarterwave multilayer coatings are envisaged for astronomy in the extreme ultraviolet (EUV) and soft x rays and also for future EUY lithography.

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.

Far‐ultraviolet absolute reflectometer for optical constant determination of ultrahigh vacuum prepared thin films

An absolute reflectometer is described, which can determine the optical constants of thin films prepared and maintained in ultrahigh vacuum (UHV) in the 30–200 nm far‐ultraviolet (FUV) region. The optical constants are calculated with Fresnel equations from reflectance measurements as a function of the incidence angle, which can be continuously varied from near‐normal to grazing incidence. Corrections for the surface roughness effects of both the deposited film and the substrate on the reflectance in the calculation of optical constants were incorporated. The surface roughness was determined by atomic force microscopy. An innovative feature of the instrument is an oxidation chamber containing an atomic oxygen source attached to the reflectometer. Inside this chamber the FUV reflectance degradation of materials, caused by oxidation processes in low earth orbits, can be simulated on freshly prepared UHV samples. Another improvement of the reflectometer is the capability of performing the reflectance measure...

Determination of optical constants of scandium films in the 20-1000 eV range

The transmittance of thin films of Sc deposited by evaporation in ultrahigh vacuum conditions has been investigated in the 20-1000 eV spectral range. Transmittance measurements were performed in situ on Sc layers that were deposited over grids coated with a C support film. Transmittance measurements were used to obtain the extinction coefficient of Sc films at each individual photon energy investigated. These data, along with the data available in the literature for the rest of the spectrum, were used to obtain the refractive index of Sc by means of the Kramers-Krönig analysis. Sum-rule tests indicated an acceptable consistency of the data.

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.

Optical constants of electron-beam evaporated boron films in the 6.8-900 eV photon energy range

The optical constants of electron-beam evaporated boron from 6.8 to 900 eV were calculated through transmittance measurements of boron thin films deposited onto carbon-coated microgrids or LiF substrates in ultrahigh-vacuum conditions. In the low-energy part of the spectrum the measurements were performed in situ on freshly deposited samples, whereas in the high-energy range the samples were exposed to the atmosphere before the measurements. The extinction coefficient was calculated directly from the transmittance data, and a Kramers-Kronig analysis that combined the current data with data from the literature was performed to determine the dispersive part of the index of refraction. Finally, two different sum-rule tests were performed that indicated the good consistency of the data.

New layer-by-layer multilayer design method

A new layer-by-layer multilayer design method is presented. The method is demonstrated mathematically and makes possible the optimization of the multilayer for the highest possible reflectance either at normal incidence or at nonnormal incidence for s- or p-polarized radiation. With the current method multilayers can be designed regardless of the number of different materials used. The optimum layer thickness is determined by means of functions suitable for implementation in a computer code. The new multilayer design method is fast and accurate.

Triple-wavelength, narrowband Mg/SiC multilayers with corrosion barriers and high peak reflectance in the 25-80 nm wavelength region

We have developed new, Mg/SiC multilayer coatings with corrosion barriers which can be used to efficiently and simultaneously reflect extreme ultraviolet (EUV) radiation in single or multiple narrow bands centered at wavelengths in the spectral region from 25 to 80 nm. Corrosion mitigation was attempted through the use of Al-Mg or Al thin layers. Three different multilayer design concepts were developed and deposited by magnetron sputtering and the reflectance was measured at near-normal incidence in a broad spectral range. Standard Mg/SiC multilayers were also deposited and measured for comparison. They were shown to efficiently reflect radiation at a wavelength of 76.9 nm with a peak reflectance of 40.6% at near-normal incidence, the highest experimental reflectance reported at this wavelength for a narrowband coating. The demonstration of multilayer coatings with corrosion resistance and multiple-wavelength EUV performance is of great interest in the development of mirrors for space-borne solar physics telescopes and other applications requiring long-lasting coatings with narrowband response in multiple emission lines across the EUV range.

Sub-quarter-wave multilayer coatings with high reflectance in the extreme ultraviolet

Multilayer coatings with a small number of layers were designed and prepared to provide an increase in normal-incidence reflectance in the extreme ultraviolet compared with the reflectance of available single-layer coatings, namely, SiC, B4C, and Ir. Multilayers were designed to produce coatings with the highest possible reflectance at 91.2 and at 58.4 nm. At these wavelengths all the materials absorb radiation strongly, but still a reflectance enhancement can be obtained by means of sub-quarter-wave multilayer coatings with two or more different materials. Sub-quarter-wave multilayer coatings based on Al, MgF2, diamondlike carbon, B4C, SiC, and Ir showed higher reflectance than single-layer coatings of SiC and B4C not only at the target wavelength but in a wide band ranging from 50 nm to the 121.6-nm H Lyman-alpha line. Multilayer coatings suffered some reflectance degradation over time. However, after approximately 80-100 days of aging in a desiccator, the reflectance for the multilayer coatings was greater than for the single-layer coatings.

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.