Sven Laux

Room-temperature deposition of indium tin oxide thin films with plasma ion-assisted evaporation

The electrical and optical properties of indium tin oxide (ITO) thin films deposited by plasma ion-assisted evaporation are reported. With deposition at room temperature using certain ion-source parameters ITO films with an electrical resistivity of 5×10−6 Ω m and an absorptance at 550 nm of less than 5% for 300 nm film thickness were obtained. Variation of oxygen pressure during deposition results in different band gap energies and different resistivities. The complex index of refraction was determined using a dispersion model suitable for explaining the particular properties of ITO in the visible and near infrared spectral range.

Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2 (111) substrates

Abstract The growth structure of MgF 2 and NdF 3 films grown on polished CaF 2 (111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF 2 and NdF 3 films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.

Ion-assisted deposition of oxide materials at room temperature by use of different ion sources.

Thin films of SiO(2), TiO(2), Ta(2)O(5), ZrO(2), and the mixed oxide H4 (Merck) have been deposited onto nonheated glass substrates by electron-beam evaporation in commercial coating plants. All depositions have been carried out with ion assistance provided by three different ion or plasma sources (end-hall, plasma, and cold-cathode sources). The optical film properties such as index of refraction, extinction coefficient, light scattering, and absorption have been examined by spectrophotometry, laser calorimetry, and total integrated light-scatter measurements. Surface morphology has been investigated by atomic force microscopy studies. Furthermore, films have undergone sand erosion tests for the determination of relative wear resistance. The film properties are compared for the three different ion sources.

Ion-assisted deposition processes: industrial network IntIon

The presented work is embedded in the research network “Integrative Ion Processes for Modern Optics”, called IntIon, consisting of 12 partners from the German optics industry and two research institutes. The main target of the IntIon network is the development of new process concepts on the basis of ion assisted deposition (IAD) for the industrial production of optical thin film components. Besides an improvement in efficiency, a major aim is concentrated on the optical characteristics for selected application fields with high economical potential. In this network, different ion and plasma sources are compared with regard to their qualification for ion assisted deposition processes. This work includes the characterization of the ion energy and ion current using Faraday-cup measurements. The selection of investigated coating materials includes a broad variety of standard and non-standard oxides. First results of the network will be presented for adapted deposition materials and different operation characteristics of ion sources.

Packing-density calculation of thin fluoride films from infrared transmission spectra.

Optical thin films of MgF(2) and NdF(3) have been made by molecular-beam deposition in ultrahigh vacuum. The film packing density was calculated from the water amount that filled the cavities of the films in air. For that the transmission in the IR spectral region with a characteristic water absorption band at a wave number of 3400 cm(-1) was measured. The influence on the packing density of a modification of thin-film morphology by stratification of very thin sublayers of different crystallizing fluorides is tested.

Antireflection coatings for UV radiation obtained by molecular-beam deposition

We have developed fluoride antireflection (AR) coatings on MgF(2) substrates for a wavelength of 248 nm by molecular-beam deposition. Transmission and laser-induced damage threshold of the samples were measured and atomic force microscope (AFM) investigations were carried out. We compare a 14-layer design for AR coatings with sublayer thicknesses of 12 nm with a conventional two-layer design with quarter-wavelength thicknesses. The laser-induced damage threshold of the 14-layer coating is slightly higher than that of the two-layer coating. The AFM surface images show that the 14-layer coating has a smoother surface than the two-layer coating.

Long-time radiation-resistant optical coatings for UV excimer laser applications

An example of a multilayer component for 248-nm excimer laser applications is explained in this contribution. By means of a polarizing beam splitter the potential of plasma- assisted deposition is shown. Such a coating shows a high reflecting and a high transmitting function for different polarization at a certain angle of incidence. High reflection requires a considerable number of layer pairs with a large total optical film thickness. Despite large film thickness, the optical absorption can be minimized and high transmission can be obtained. The laser induced damage threshold of such a coating is in the order of 2.5 J/cm2 for single shot measurements at 20 ns pulse length. The life time is greater than 5X109 shots at a fluence of 10mJ/cm2. Very stable deposition conditions must be maintained to produce these coatings within small tolerance of the optical function. It was found that plasma-assisted deposition is a more reliable technology for producing these coatings compared to classical evaporation.

Molecular beam deposition of fluorides

A new technique for the fabrication of optical interference layers is being introduced. MgF2 and NdF3 have been evaporated by molecular beam deposition (MBD) in an ultrahigh vacuum system. The fluoride films are used in optical interference coatings for optical components of high power excimer lasers. The growth conditions were varied by changing the remaining gas composition, variation of the substrate temperature from 50 degree(s)C to 300 degree(s)C, and interruption of grain growth. The optical losses of the films were measured by transmission spectroscopy and laser calorimetry, and the film morphology was investigated by transmission electron microscopy. It was shown, that a stratification of very thin (1 nm - 10 nm) layers of two different fluorides can avoid the growth of large grains and wide columns, and gives rise to smoother surfaces. The packing density of MBD-films is higher in comparison with fluoride films deposited under conventional high vacuum conditions.

Low-loss HR coatings on fused silica substrates for 193 nm micro-lithography applications

High reflective coatings for 193nm wavelength and 45° incidence were developed which combines the advantages of all-oxide and all-fluorides layer stacks. Using plasma-assisted evaporation very smooth and dense Al2O3/ SiO2 multilayers showing small light scatter were deposited onto fused silica substrates. In the same coating process followed metal fluoride stacks, which could reduce the resulting coating absorption at 193nm. The non-polarized reflectance of combined stacks at 193nm is R>98.5% at 45° and R>98.0% for an angle range of 42°-48°. As the number of fluoride layers could be drastically reduced compared to all-fluoride coatings any formation of micro-cracks could be avoided. The stress of the oxide/fluoride stacks was less than 40MPa.

Structure Evolution of NdF3 Optical Thin Films

The structure evolution characteristics of NdF 3 thin films has been analysed concerning texture and grain morphology. The structure could be classified according to the structure zone models, and the growth mode could be related to zone T and zone II. Contamination of the films by residual gases was found to inhibit the migration of grain boundaries and therefore, zone II does not appear even at high substrate temperatures in contaminated films. Stratification by MgF 2 interlayers may also result in a decrease of grain size and a smaller surface roughness. The correlation between deposition conditions, stratification, growth mode and optical properties is discussed.