Etude des oxynitrures de silicium en couches minces déposées par pulvérisation en vue d'applications optiques

This work is based n the study of silicon oxynitride thin films, deposited by RF magnetron sputtering, to realize low losses optical multilayers (absorption, sputtering).At first, we have analysed the variations of the optical and physicochemical properties of the oxynitrides monolayers due to the deposition parameters : the gas partial pressures, the RF power, the target composition, the gas mixture. So, by describing the whole oxynitrides domain from the oxyde to the nitride, we have shown the linear variation of the refractive index as well as the strict substitution of oxygen atoms by nitrogen atoms : this is the proof of a simple mechanism of formation. Moreover, thanks to IR spectrophotometric analyses, a model of the oxynitrides amorphous structure has been proposed and confirmed by two approximation methods.At last, we have studied more precisely the absorption (photothermal deflection) and the scattering (scatterrometer CASI) levels of the monolayers and the multilayers synthetized from silicon oxynitrides (antireflective stacks, mirrors). A comparison with the optical performances of classical stacks made of oxide, deposited by ion beam sputtering, have been made and we have proposed some solutions to decrease the two sources of losses.Ce travail repose sur l'étude des couches minces d'oxynitrures de silicium obtenues par pulvérisations radio-fréquence magnétron réactive en vue de réaliser des multicouches optiques de faibles pertes (absorption, diffusion).Tout d'abord, nous avons analysé les variations des propiétés optiques et physicochimiques des couches d'oxynitrures liées aux différents paramètres du bâti de dépôt : les pressionspartielles des gaz, la puissance RF, la nature de la cible et des gaz.Ainsi, lorsque l'on décrit toute la gamme des oxynitrures de l'oxyde au nitrure, nous avons en particulier mis en évidence l'évolution quasi linéaire de l'indice sur un domaine relativement important ainsi que la substitution rigoureuse des atomes d'oxygène par les atomes d'azote : ceci est la preuve d'un mécanisme simple de formation. De plus, grâce à des analyses par spectrophotométrie IR, un modèle de la structure amorphe des oxynitrures a été proposé : (pseudo-binaire oxyde-nitrure) et vérifié par deux méthodes d'approxmation. Enfin, une étude plus particulière de l'absorption (photothermie) et de la diffusion( diffusomètre CASI) a été menée sur les couches et sur les multicouches synthétisés à partir des oxynitrures (Antireflets, Miroirs). Une comparaison avec les performaces des empilements classiques d'oxydes réalisés par pulvérisation par faisceaux d'ions a pu être faite et nous avons ainsi proposé des solutions pour optimiser les deux sources de pertes

Mechanical loss in state-of-the-art amorphous optical coatings

We present the results of mechanical characterizations of many different high-quality optical coatings made of ion-beam-sputtered titania-doped tantala and silica, developed originally for interferometric gravitational-wave detectors. Our data show that in multi-layer stacks (like high-reflection Bragg mirrors, for example) the measured coating dissipation is systematically higher than the expectation and is correlated with the stress condition in the sample. This has a particular relevance for the noise budget of current advanced gravitational-wave interferometers, and, more generally, for any experiment involving thermal-noise limited optical cavities.Comment: 31 pages, 14 figure

Material loss angles from direct measurements of broadband thermal noise

International audienceWe estimate the loss angles of the materials currently used in the highly reflective test-mass coatings of interferometric detectors of gravitational waves, namely Silica, Tantala, and Ti-doped Tantala, from direct measurement of coating thermal noise in an optical interferometer testbench, the Caltech TNI. We also present a simple predictive theory for the material properties of amorphous glassy oxide mixtures, which gives results in good agreement with our measurements on Ti-doped Tantala. Alternative measurement methods and results are reviewed, and some critical issues are discussed

Correlated evolution of structure and mechanical loss of a sputtered silica film

Energy dissipation in amorphous coatings severely affects high-precision optical and quantum transducers. In order to isolate the source of coating loss, we performed an extensive study of Raman scattering and mechanical loss of a thermally-treated sputtered silica coating. Our results show that loss is correlated with the population of three-membered rings of Si-O$_4$ tetrahedral units, and support the evidence that thermal treatment reduces the density of metastable states separated by a characteristic energy of about 0.5 eV, in favour of an increase of the states separated by smaller activation energies. Finally, we conclude that three-fold rings are involved in the relaxation mechanisms only if they belong to more complex chain-like structures of 10 to 100 tetrahedra.Comment: 5 pages, 3 figure

Measurement of Thermal Noise in Multilayer Coatings with Optimized Layer Thickness

A standard quarter-wavelength multilayer optical coating will produce the highest reflectivity for a given number of coating layers, but in general it will not yield the lowest thermal noise for a prescribed reflectivity. Coatings with the layer thicknesses optimized to minimize thermal noise could be useful in future generation interferometric gravitational wave detectors where coating thermal noise is expected to limit the sensitivity of the instrument. We present the results of direct measurements of the thermal noise of a standard quarter-wavelength coating and a low noise optimized coating. The measurements indicate a reduction in thermal noise in line with modeling predictions.Comment: 8 pages, 14 figure

Measurements of mechanical thermal noise and energy dissipation in optical dielectric coatings

In recent years an increasing number of devices and experiments are shown to be limited by mechanical thermal noise. In particular sub-Hertz laser frequency stabilization and gravitational wave detectors, that are able to measure fluctuations of 1E-18 m/rtHz or less, are being limited by thermal noise in the dielectric coatings deposited on mirrors. In this paper we present a new measurement of thermal noise in low absorption dielectric coatings deposited on micro-cantilevers and we compare it with the results obtained from the mechanical loss measurements. The coating thermal noise is measured on the widest range of frequencies with the highest signal to noise ratio ever achieved. In addition we present a novel technique to deduce the coating mechanical losses from the measurement of the mechanical quality factor which does not rely on the knowledge of the coating and substrate Young moduli. The dielectric coatings are deposited by ion beam sputtering. The results presented here give a frequency independent loss angle of (4.70 $\pm$ 0.2)x1E-4 with a Young's modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. For as-deposited silica, a weak frequency dependence (~ f^{-0.025}) is observed in this frequency range, with a Young's modulus of 70 GPa and an internal damping of (6.0 $\pm$ 0.3)x1E-4 at 16 kHz, but this value decreases by one order of magnitude after annealing and the frequency dependence disappears.Comment: Accepted for publication in Phys. Rev.

Effect of heating treatment and mixture on optical properties of coating materials used in gravitational-wave detectors

The interferometer mirrors of Gravitational-Wave Detectors (GWD) are Bragg reflectors made of alternate amorphous silica (SiO2) and titania-doped tantala (TiO2:Ta2O5) layers as low- and high-refractive index material, respectively. A thermal treatment is usually performed to reduce both mechanical losses and NIR optical absorptions of the coatings. We present a spectroscopic ellipsometry (SE) investigation of the effect of annealing and Ti:Ta mixing on Ta2O5 coatings deposited under conditions similar to those adopted for building up mirrors of GWDs. The broad-band analysis covers both the NIR and the fundamental absorption threshold region. The data show an evident annealing-induced reduction of the fundamental optical absorption broadening. Modelling the data through the Cody-Lorentz formula confirms that NIR absorption are below the SE sensitivity and shows a notable annealing-induced reduction of so-called Urbach tails. Titania-doping of tantala slightly reduces the Urbach energy. After the heating treatment the resulting Urbach energy of the doped material is lower than the one of annealed pure tantala. The observed reduction of Urbach tails is important because it parallels the reduction of so-called internal friction observed in mechanical measurements. So that SE emerges as a convenient tool for an agile diagnostic of both optical and mechanical quality of amorphous oxide coatings

Observation of a Correlation Between Internal friction and Urbach Energy in Amorphous Oxides Thin Films

We have investigated by spectroscopic ellipsometry (SE, 190-1700 nm) the optical properties of uniform, amorphous thin films of Ta2O5 and Nb2O5 as deposited and after annealing, and after so-called "doping" with Ti atoms which leads to mixed oxides. Ta2O5 and Ti:Ta2O5 are currently used as high-index components in Bragg reflectors for Gravitational Wave Detectors. Parallel to the optical investigation, we measured the mechanical energy dissipation of the same coatings, through the so-called "loss angle" \u3d5\u2009=\u2009Q-1, which quantifies the energy loss in materials. By applying the well-known Cody-Lorentz model in the analysis of SE data we have been able to derive accurate information on the fundamental absorption edge through important parameters related to the electronic density of states, such as the optical gap (Eg) and the energy width of the exponential Urbach tail (the Urbach energy EU). We have found that EU is neatly reduced by suitable annealing as is also perceptible from direct inspection of SE data. Ti-doping also points to a minor decrease of EU. The reduction of EU parallels a lowering of the mechanical losses quantified by the loss angle \u3d5. The correlation highlights that both the electronic states responsible of Urbach tail and the internal friction are sensitive to a self-correlation of defects on a medium-range scale, which is promoted by annealing and in our case, to a lesser extent, by doping. These observations may contribute to a better understanding of the relationship between structural and mechanical properties in amorphous oxides

Characterization of Core Optics in Gravitational-Wave Detectors: Case Study of KAGRA Sapphire Mirrors

We report the characterization of superlow-loss optics used in the second-generation gravitational-wave detectors currently in operation. The sapphire test-mass mirrors in the KAGRA detector are introduced as an example, but the techniques here are common to all detectors. In this work, we discuss mainly the surface topography obtained by interferometric techniques and the optical properties obtained with special setups

A new method of probing mechanical losses of coatings at cryogenic temperatures

A new method of probing mechanical losses and comparing the corresponding deposition processes of metallic and dielectric coatings in 1-100 MHz frequency range and cryogenic temperatures is presented. The method is based on the use of extremely high-quality quartz acoustic cavities whose internal losses are orders of magnitude lower than any available coatings nowadays. The approach is demonstrated for Chromium, Chromium/Gold and a multilayer tantala/silica coatings. The ${\rm Ta}_2{\rm O}_5/{\rm Si}{\rm O}_2$ coating has been found to exhibit a loss angle lower than $1.6\times10^{-5}$ near 30 {\rm MHz} at 4 {\rm K}. The results are compared to the previous measurements