Porosity and optical properties of zirconia films prepared by plasma ion assisted deposition.

The porosity of zirconia films prepared by plasma ion assisted deposition has been investigated by means of optical (spectrophotometric) and nonoptical analytic techniques such as transmission electron microscopy, x-ray reflection, and energy dispersive x-ray spectroscopy. A discrimination between large (open) and small (closed) pores was achieved by means of measurement of the thermal and vacuum-to-air shift. Depending on the level of plasma assistance during film preparation, the porosity was found to vary between 30 vol. % and nearly 0 vol. %. With decreasing porosity, the surface roughness determined by atomic force microscopy tends to decrease as well.

Investigation of the refractive index repeatability for tantalum pentoxide coatings, prepared by physical vapor film deposition techniques.

Random effects in the repeatability of refractive index and absorption edge position of tantalum pentoxide layers prepared by plasma-ion-assisted electron-beam evaporation, ion beam sputtering, and magnetron sputtering are investigated and quantified. Standard deviations in refractive index between 4*10-4 and 4*10-3 have been obtained. Here, lowest standard deviations in refractive index close to our detection threshold could be achieved by both ion beam sputtering and plasma-ion-assisted deposition. In relation to the corresponding mean values, the standard deviations in band-edge position and refractive index are of similar order.

Estimation of the composition of coelectron-beam-evaporated thin-mixture films by making use of the Wiener bounds.

Material mixtures offer prospective possibilities for synthesizing coating materials with tailored optical constants. We present experimental results for mixture coatings of alumina/aluminum fluoride and alumina/hafnia deposited by electron beam evaporation. Thereby, the volume filling factors of the components are commonly estimated on the basis of deposition rates measured by quartz crystal microbalance. The interplay between the vapor fluxes from the two evaporation sources, the crosstalk between quartz crystal microbalances, and the influence of the plasma source on the tooling factors limit the accuracy of this estimation, and this has motivated us to develop an alternative approach. The general idea of our approach is based on the commonly high accuracy in thin-film optical constant determination using spectrophotometry. Therefore, these optical constants serve as a reliable input for a rather simple but robust evaluation procedure based on the concept of Wiener bounds. The consistency of the obtained results is illustrated by opposing the data to the elementary film composition estimated from energy-dispersive x-ray spectroscopy.

Observation of the waveguide resonance in a periodically patterned high refractive index broadband antireflection coating.

Grating waveguide structures have been prepared by the deposition of a high refractive index broadband antireflection coating onto a patterned fused silica substrate. Aluminum oxide and hafnium oxide as well as mixtures thereof have been used as coating materials. Optical reflection measurements combined with atomic force microscopy have been used to characterize the structures. Upon illumination with a TE wave, the best structure shows a narrow reflection peak located at 633 nm at an incidence angle of about 17°. The peak reflectance of that sample accounts for more than 89%. Off-resonance interference structures appear strongly suppressed in the spectrum between 450 and 800 nm because of the characteristics of the designed antireflection layer. The structure thus possesses a notch filter spectral characteristic in a broad spectral range.

Protected and enhanced aluminum mirrors for the VUV.

Aluminum layers protected with fluoride coatings have been deposited by evaporation and characterized with respect to their suitability as vacuum ultraviolet (VUV) mirrors. Optical characterization has been performed by spectrophotometry, while the surface quality of the layers has been judged by means of x ray reflection, scanning electron microscopy, and atomic force microscopy. In particular, protection with aluminum fluoride results in superior VUV reflection properties. VUV reflectance values between 80% and nearly 90% have been verified even two years after deposition and exposure to the atmosphere.

Deposition and spectral performance of an inhomogeneous broadband wide-angular antireflective coating.

Gradient index coatings and optical filters are a challenge for fabrication. In a round-robin experiment, basically the same hybrid antireflection coating for the visible spectral region, combining homogeneous refractive index layers of pure materials and linear gradient refractive index layers of material mixtures, has been deposited. The experiment involved three different deposition techniques: electron-beam evaporation, ion-beam sputtering, and radio frequency magnetron sputtering. The material combinations used by these techniques were Nb(2)O(5)/SiO(2), TiO(2)/SiO(2), and Ta(2)O(5)/SiO(2), respectively. The spectral performances of samples coated on one side and on both sides have been compared to the corresponding theoretical spectra of the designed profile. Also, the reproducibility of results for each process is verified. Finally, it is shown that ion-beam sputtering gave the best results in terms of deviation from the theoretical performance and reproducibility.