M. Alvisi

Structural and optical modification in hafnium oxide thin films related to the momentum parameter transferred by ion beam assistance

Abstract Hafnium oxide (HfO2) films were deposited on silica and glass substrates by ion (Xe+) assisted deposition with increasing ion momentum transfer to the growing film. The relationship among the ion momentum values, the crystalline phase and the refractive index (packing density) has been worked out by means of X-ray diffraction and spectrophotometric analysis. Compaction of the films by ion beam assistance is clearly evidenced by the changes in their microstructure. A three steps transition from a random monoclinic phase, via amorphous phase, up to an highly phase oriented (fiber texture), as a function of ion momentum, has been found.

High-reflectivity HfO 2 /SiO 2 ultraviolet mirrors

High-reflectivity dense multilayer coatings were produced for the ultraviolet spectral region. Thin-film single layers and UV mirrors were deposited by ion plating and plasma ion-assisted deposition high- energetic technologies. Optical characterizations of HfO2 and SiO2 single layers are made. The optical constants obtained for these two materials are presented. HfO2 and SiO2 mirrors with a reflectance of ∼99% near 250 nm are reported.

Structural and optical properties of silver thin films deposited by RF magnetron sputtering

Abstract In this work, we investigate the optical and the structural properties of Ag films by means of spectrophotometric techniques and X-ray diffraction methods in the low- and high-angle regimes. The coatings were grown by radio frequency magnetron sputtering on (100)-oriented silicon substrates. Several sets of samples were produced by varying the sputtering pressure (0.15–0.9 Pa) and changing the sputtering power supply in the range of 25–100 W. In particular, the dependence of the coating optical features, surface roughness, texture and residual stress on the parameter β, proportional to the momentum transferred to the growing film from the energetic particles bombarding it, was pointed out. The results indicate that the Ag films are polycrystalline with a moderate [111] texture and compressive in-plane stress was measured for all the investigated samples. The behavior of the stress with the momentum parameter can be summarized as follows: (i) the compressive stress increases in the range 0 1000 V2s/A.

Laser damage dependence on structural and optical properties of ion-assisted HfO2 thin films

Abstract Laser damage studies at 248 nm (KrF excimer laser) have been performed on HfO 2 films of 300 nm thickness deposited on silica substrates by the Xe ion-assisted electron beam evaporation technique. The assistance parameters (ion mass, energy and current density) have been adjusted to investigate the effect of the Xe-ion momentum transfer parameter P on the film optical and structural properties. Then, the dependence of laser damage fluence on film properties has been studied. Higher laser damage fluences have been found for the HfO 2 films deposited at lower P values and characterized by a fully crystalline structure with the grain of smaller size and randomly oriented.

Adhesion enhancement of optical coatings on plastic substrate via ion treatment

The main limitation in using plastics for optical components is the softness of their surface, which is responsible for low impact and abrasion resistance and for weak adhesion between the film and the substrate. Protecting the substrate with a hard film can increase the impact resistance of polymeric lenses. However, deposition of thin films on chemically cleaned plastic substrates has resulted in fast delamination of the coating. This paper discusses the problems associated with coated plastics, it presents the ion assistance as one of the solutions and it finally identifies the relevant parameters. A hard oxide layer with additional ion pre-treatment is proposed as protection for plastic optics. The adhesion between the film and the substrate has been evaluated by a scratch-test and the hardness has been calculated by measuring the radius of the stylus track.

High-performance deep-ultraviolet optics for free-electron lasers

Working with wavelengths shorter than the deep ultraviolet involves the development of dedicated optics for free-electron lasers with devoted coating techniques and characterizations. High-performance deep-ultraviolet optics are specially developed to create low-loss, high-reflectivity dielectric mirrors with long lifetimes in harsh synchrotron radiation environments. In February 2001, lasing at 189.7 nm, the shortest wavelength obtained so far with free-electron-laser oscillators, was obtained at the European Free-electron-laser project at ELETTRA Synchrotron Light Laboratory, Trieste, Italy. In July 2001, 330-mW extracted power at 250 nm was measured with optimized transmission mirrors. Research and development of coatings correlated to lasing performance are reported.

Deposition of SiO 2 films with high laser damage thresholds by ion-assisted electron-beam evaporation

SiO2 thin films (≈100 nm thick) with transmittivity and a laser damage threshold nearly equal to those of bulk material are deposited on silica substrates by the technique of ion-assisted electron-beam evaporation. The influence of film packing density on the laser damage threshold is investigated by the technique of photoacoustic probe beam deflection. It is shown that films with lower packing density may have a higher laser damage threshold and as a consequence better heat dissipation.

Influence of the assisting-ion-beam parameters on the laser-damage threshold of SiO2 films

Abstract Silica thin films have been deposited by a dual-ion-beam sputtering technique using argon or xenon ions mixed with oxygen ions in the assisting beam and the role of the assisting-ion-beam parameters on the laser-damage-threshold at 308 nm (XeCl laser) has been investigated. It is shown that a proper choice of these parameters allows a considerable increase of the laser-induced damage threshold. Lower damage thresholds (1 J/cm 2 ) were found for the xenon/oxygen assisted samples. Whereas, the highest damage threshold (8 J/cm 2 ) was found for the argon-ion assisted sample and its value was also much higher than that (2.9 J/cm 2 ) of the non-assisted films. Damage thresholds have been determined by the photoacoustic mirage technique and it is demonstrated that this technique can provide useful information on the mechanisms responsible for laser damage.

The momentum transfer parameter in argon-assisted carbon coatings

Abstract It is well known that energetic particle bombardment during diamond-like carbon film growth has a significant influence on the resulting film stress, optical transparency, hardness and chemical inertness. It has been commonly recognised that many of the desirable properties of carbon films arise from the sp 3 tetragonal bonding component. Therefore, it is essential to have a deeper knowledge of the role of ion energy on the evolution of the diamond-like phase. In this work, we propose a parameter ( P ) as a key for the understanding of the nucleation and growth mechanism, which takes into account the number of recoiled atoms and their transfer momentum by argon ion assistance, compared to all atoms present in the volume, described by ion range, plus the atoms coming from the graphite-sputtered target. Thin films of a-carbon were deposited by the dual ion beam sputtering technique. An argon ion beam sputters a graphite target and a second one assists the film during the growth. Sets of samples were produced with different momentum transferred by the assistance beam ( P ). A spectrophotometric analysis in the visible range is performed to determine the Tauc optical gap E g , the refractive index and the extinction coefficient. The results show that the films not assisted exhibit a lower density and consequently a low refractive index ( n =1.8 in the visible range), while increasing the momentum transfer parameter ( P ) the refractive index increases suggesting a densification of the material. The same trend is observed for the Tauc optical gap. In order to get further information on the evaluation of density due to the assistance, X-ray reflectivity measurements have been performed and correlated to the optical results.

Achromatic damage investigations on mirrors for UV-free electron lasers

Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localized spatial modification induced by the non-uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness.