Etienne Quesnel

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Effect of systematic errors in spectral photometric data on the accuracy of determination of optical parameters of dielectric thin films

The determination of optical parameters of thin films from experimental data is a typical task in the field of optical-coating technology. The optical characterization of a single layer deposited on a substrate with known optical parameters is widely used for this purpose. Results of optical characterization are dependent on not only the choice of the thin-film model but also on the quality of experimental data. The theoretical results presented highlight the effect of systematic errors in measurement data on the determination of thin-film parameters. Application of these theoretical results is illustrated by the analysis of experimental data for magnesium fluoride thin films.

Optical and microstructural properties of MgF2 UV coatings grown by ion beam sputtering process

The optical, mechanical, and microstructural properties of MgF2 single layers grown by ion beam sputtering have been investigated by spectrophotometric measurements, film stress characterization, x-ray photoelectron spectroscopy (XPS), x-ray diffraction, and transmission electron microscopy. The deposition conditions, using fluorine reactive gas or not, have been found to greatly influence the optical absorption and the stress of the films as well as their microstructure. The layers grown with fluorine compensation exhibit a regular columnar microstructure and an UV-optical absorption which can be very low, either as deposited or after thermal annealings at very low temperatures. On the contrary, layers grown without fluorine compensation exhibit a less regular microstructure and a high ultraviolet absorption which is particularly hard to cure. On the basis of calculations, it is shown that F centers are responsible for this absorption, whereas all the films were found to be stoichiometric, in the limit o...

Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes

Abstract MgF 2 is a current material for the optical applications in the UV and deep UV range. Nevertheless, modern applications still require improvement of the optical and structural quality of the deposited layers. In the present work, the composition and microstructure of MgF 2 single layers grown on Si [100] substrate by physical vapour deposition (PVD) and ion beam sputtering (IBS) processes, were analyzed and compared. Experiments were carried out using X-ray photoelectron spectroscopy (XPS) in depth profile, grazing angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). Both layers exhibited a good stoichiometry and a low level of contamination. The sample grown by IBS revealed a more homogeneous and regular columnar microstructure than the other one.

Investigation of the surface micro-roughness of fluoride films by spectroscopic ellipsometry

Spectroscopic ellipsometry is a sensitive and reliable diagnostic tool for the optical properties of thin films and multi-layer coatings. In this paper we have derived new formulas permitting qualitative and quantitative analysis of the surface micro-roughness in the case where the refractive index of the film is close to that of the substrate. Theoretical results are applied to the developing of experimental data for lanthanum fluoride and magnesium fluoride thin films.

UV-optical and microstructural properties of MgF2-coatings deposited by IBS and PVD processes

In high quality otpical coating systems for the DUV-spectral range, MgF2 is one of the preferred deposition materials. MgF2-coatings exhibit relatively low optical losses as well as high stability and laser induced damage thresholds. In the present joint research effort of several European laboratories, the potentiality of MgF2 is evaluated in respect to the production of improved optical coatings for applications in laser technology and semiconductor lithography. For this purpose, single layers of MgF2 were deposited on superpolished fused silica and CaF2-substrates by ion beam sputtering, boat and e-beam evaporation in different laboratories. Besides photometric inspections, the samples were characterized by an optical scatter measurement facility at 193 nm and 633 nm. The structural properties were assessed using AFM, XRD, and adapted TEM-techniques invovling conventional thinning methods for the layers. For the measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed.

Laser damage studies on MgF2 thin films

The results of laser damage studies performed at 248 nm (KrF excimer laser) on MgF2 thin films deposited by different techniques (electron-beam evaporation, thermal boat evaporation, and ion-beam sputtering) on fused silica and CaF2 substrates are presented. We find that the films deposited on CaF2 substrates by the electron-beam evaporation technique present the highest damage threshold fluence (9 J/cm2). The photoacoustic (PA) beam deflection technique was employed, in addition to microscopical inspection, to determine laser damage fluences. We confirm, by scanning electron microscopy analysis of the damaged spots, the capability of the PA technique to provide information on the mechanisms leading to damage. The dependence of both laser damage fluence and damage morphology on the film deposition technique, as well as on the film substrate, is discussed.

Optical characterization of materials desposited by different processes: the LaF3 in the UV-visible region

The optical characterization of materials in thin film phase is a standard task in the field of coating technology. There are experimental circumstances where the accurate comparison between several deposition processes (for the same material) is important. In these cases, several sets of substrates are coated at the different deposition plants. The samples will be subsequently analyzed using, if the plants are at different locations, different spectrophometers and finally the results of all the optical characterizations will be compared. The aim of this work is to present the results of a global procedure for the optical characterization of LaF3 in the UV-visible region, deposited at three different plants. We have used R and T spectrophotometric data and we have assumed the following model for the optical characterization: n(λ)=n0+n1/λ2, k(λ)=k0 exp(k1/(λ). Our method characterizes all the samples from the same deposition process by a single set of parameters (instead of a set for each sample), using all the available measurements to determine them in a single numerical fitting, without a significant loss in the quality of the fittins. This procedure reduces the number of parameters and makes the comparison between different deposition processes more clear. By using similar results obtained for MgF2 the optical characterization of stacks (manufactured using MgF2 and LaF3) is also presented.

New procedure for the optical characterization of high-quality thin films

The optical characterization of materials in thin film phase is a standard task in the field of coating technology. One typical problem is the optical characterization of a single layer of material deposited on a well-known substrate. Provided the physical model considered for the modeling is correct and the available experimental date (usually spectrophotometric or ellipsometric spectra) are accurate, a precise optical characterization is quite straightforward. However, there are experimental circumstances where several samples have been coated under very well defined conditions. As when they have been obtained in the same coating run, so that no differences are expected due to the positions of each individual sample inside the chamber during the deposition process. The aim of this work is to present an improved procedure for the optical characterization of the material deposited under the very well controlled conditions explained above. The basis of our method is to use the a priori information about the identical nature of all the samples, introducing all the spectrophotometric and/or ellipsometric data available from optical properties of the layers, as compared with the separate (individual) characterization of the samples. We will illustrate our procedures for MgF2 films in the range 200-800 nm.

Laser damage threshold of MgF2 thin films by photoacoustic beam deflection

The photoacoustic beam deflection technique at (lambda) equals 248 nm was employed for the measurement of the laser damage threshold of single films of magnesium fluoride deposited on superpolished calcium fluoride and fused silica substrates. Different samples were investigated. All films have similar thickness but are deposited by various techniques in different laboratories. The samples are fully characterized, both from the optical and the structural point of view, in the framework of a European Project on UV-coatings. The results of these measurements are reported, along with the data on laser damage threshold, to find out which are the most significant parameters for laser applications.