Daniel Mathys

Structural and optical properties of titanium aluminum nitride films (Ti1−xAlxN)

Titanium aluminum nitride films (Ti1−xAlxN) have been deposited by reactive magnetron cosputtering. Elemental compositions of these films have been determined by core level photoelectron spectroscopy. Scanning electron microscopy reveals a columnar film growth. This is also reflected by the topography of film surfaces as studied by atomic force microscopy. By x-ray diffraction a crystalline atomic structure is revealed. Single phase samples can be obtained, consisting of the substitutional solid solution (Ti, Al)N. Crystallites show preferential orientation. The optical properties of these films have been investigated by spectrophotometry in the UV-VIS-NIR wavelength range. Depending on the elemental composition, the optical constants vary from metallic to dielectric behavior. For film compositions with x<0.5 typical features are a tunable transmission maximum and reflection minimum in the visible spectral range, a high infrared reflection, and a low infrared absorption. Due to these optical properties, T...

Preparation and characterization of TiN–Ag nanocomposite films

Abstract Thin nanostructured films of TiN–Ag are deposited by a plasma vapour process consisting of co-sputtering of Ti and Ag from three magnetrons in an Ar–N 2 gas mixture. The coatings are characterized by in situ photoelectron spectroscopy, energy-filtered transmission electron microscopy and scanning electron microscopy. The dependence of the film structure and silver cluster distribution on total silver content, substrate biasing and substrate temperature was investigated.

Optical characterization of alignment and effective refractive index in carbon nanotube films

We have characterized the thickness and effective refractive index of carbon nanotube forests by fitting reflectance measurements in the visible and near infrared ranges. The measurements were performed with polarized light. An effective medium layer consisting of a mixture of graphite and air was used to simulate the nanotube film. The proposed model accurately described the behaviour of the reflected s-polarized component (Rs), which allowed for the precise determination of the thickness and porosity of the films, in very good agreement with SEM measurements of film thickness. The p-polarized component (Rp), on the other hand, could not be described in terms of the developed model. In badly aligned samples, where there is a mixture of Rs and Rp behaviour, the model fails to fit the Rs component as well. This effect can therefore be taken as an indirect indication of lack of alignment in the samples.

The effect of low temperature deuterium plasma on molybdenum reflectivity

Metallic first mirrors (FMs) are foreseen to play a crucial role for all optical diagnostics in ITER. It is highly important for the FMs to maintain a good reflectivity both in erosion and deposition zones in the harsh ITER environment. Molybdenum mirrors, which are important candidates for the FMs, exhibit a reflectivity spectrum different from that of bulk molybdenum after exposure to low temperature (4–5 eV) deuterium plasma. This difference is mainly due to the presence of deuterium and deuterium-induced defects in the metal. The results presented show that these reflectivity changes are similar for single and nanocrystalline molybdenum mirrors. Moreover, exposure of magnetron sputtered nanocrystalline molybdenum films to deuterium plasma revealed that after a certain deviation of the spectrum has been reached, the reflectivity remains constant upon further exposure. Exposures were carried out in a range of fluences corresponding to up to 18 ITER discharges in equatorial ports and 38 discharges in the upper ports in the first wall positions. Constant conditions of −200 V bias and 150 °C temperature were maintained on the samples. Further exposures performed in a tokamak result in reflectivity changes that are comparable to those obtained with deuterium plasma exposure. No mechanical damage, such as blistering and increase in roughness, is observed on the coated molybdenum films upon any of the mentioned exposures. The complex permittivity of the exposed molybdenum is determined from ellipsometry measurements and corroborated with core and valence level photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy and surface resistivity measurements.

Surface Modifications Induced by High Fluxes of Low Energy Helium Ions

Several metal surfaces, such as titanium, aluminum and copper, were exposed to high fluxes (in the range of 1023 m−2s−1) of low energy (<100 eV) Helium (He) ions. The surfaces were analyzed by scanning electron microscopy and to get a better understanding on morphology changes both top view and cross sectional images were taken. Different surface modifications, such as voids and nano pillars, are observed on these metals. The differences and similarities in the development of surface morphologies are discussed in terms of the material properties and compared with the results of similar experimental studies. The results show that He ions induced void growth and physical sputtering play a significant role in surface modification using high fluxes of low energy He ions.

Towards plasma cleaning of ITER first mirrors

To avoid reflectivity losses in ITER’s optical diagnostic systems, on-site cleaning of metallic first mirrors via plasma sputtering is foreseen to remove deposit build-ups migrating from the main wall. In this work, the influence of aluminium and tungsten deposits on the reflectivity of molybdenum mirrors as well as the possibility to clean them with plasma exposure is investigated. Porous ITER-like deposits are grown to mimic the edge conditions expected in ITER, and a severe degradation in the specular reflectivity is observed as these deposits build up on the mirror surface. In addition, dense oxide films are produced for comparisonswithporousfilms. Thecomposition,morphologyandcrystalstructureofseveralfilmswerecharacterizedbymeans of scanning electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction and secondary ion mass spectrometry. The cleaning of the deposits and the restoration of the mirrors’ optical properties are possible either with a Kaufman source or radio frequency directly applied to the mirror (or radio frequency plasma generated directly around the mirror surface). Accelerating ions of an external plasma source through a direct current applied onto the mirror does not remove deposits composed of oxides. A possible implementation of plasma cleaning in ITER is addressed.

Rhodium and silicon system: II. Rhodium silicide formation.

Detailed characterizations of rhodium/silicon films prepared by co-deposition using magnetron sputtering have been carried out on silicon substrates at room temperature up to 900 degrees C. The properties of the films were investigated using XPS/UPS, XRD, SIMS, SEM and AFM techniques. It should be emphasized that XPS/UPS measurements are carried out without breaking the vacuum to avoid any contamination of the film. Up to 500 degrees C an interdiffusion between the oxidized silicon wafer and the deposited Rh/Si film occurred leading to hole formation in the entire film at 900 degrees C. Diffraction patterns for the compounds Rh(2)Si, Rh(5)Si(3), RhSi and Rh(3)Si(4) were measured. Upon annealing the covalent character is increased and for the samples forming the compound RhSi the valence band structure is markedly changed. Depth profiling (XPS and SIMS) reveals a stable composition in the bulk of the film. For these measurements the silicon-rich alloy in the interfacial layer is probably an effect of sputtering, by implanting the Rh atoms into the silicon substrate. A previously reported negative shift for the compound Rh(5)Si(3) could be connected to the sample preparation, as sputtering of the surface is reducing the silicon content and inducing a glassy state. For the first phase Rh(2)Si formed on the rhodium-rich side the shift in binding energy is unclear, for all the other compounds encountered in this work a positive shift relative to pure rhodium was found.

Morphological changes of tungsten surfaces by low-flux helium plasma treatment and helium incorporation via magnetron sputtering

The effect of helium on the tungsten microstructure was investigated first by exposure to a radio frequency driven helium plasma with fluxes of the order of 1 × 10(19) m(-2) s(-1) and second by helium incorporation via magnetron sputtering. Roughening of the surface and the creation of pinholes were observed when exposing poly- and nanocrystalline tungsten samples to low-flux plasma. A coating process using an excess of helium besides argon in the process gas mixture leads to a porous thin film and a granular surface structure whereas gas mixture ratios of up to 50% He/Ar (in terms of their partial pressures) lead to a dense structure. The presence of helium in the deposited film was confirmed with glow-discharge optical emission spectroscopy and thermal desorption measurements. Latter revealed that the highest fraction of the embedded helium atoms desorb at approximately 1500 K. Identical plasma treatments at various temperatures showed strongest modifications of the surface at 1500 K, which is attributed to the massive activation of helium singly bond to a single vacancy inside the film. Thus, an efficient way of preparing nanostructured tungsten surfaces and porous tungsten films at low fluxes was found.

Changing TiN film morphology by “plasma biasing”

The influence of the substrate potential with respect to the plasma on the morphology of reactively sputtered TiN thin films on Si(100) has been investigated. It is well known that the film quality with respect to grain size and distribution can be improved by applying a negative substrate bias to increase energetic ion bombardment. For large-area applications, however, a grounded substrate is very much desirable. Therefore, a technique has been developed to deposit films with comparably improved morphology on grounded substrates by means of a so-called “plasma electrode.” Grain size and distribution have been analyzed by top- and side-view scanning electron microscopy. To adjust the parameters for the TiN deposition we have used in situ photoelectron spectroscopy as the process control.

Supramolecular Organization of Collagen Fibrils in Healthy and Osteoarthritic Human Knee and Hip Joint Cartilage.

Cartilage matrix is a composite of discrete, but interacting suprastructures, i.e. cartilage fibers with microfibrillar or network-like aggregates and penetrating extrafibrillar proteoglycan matrix. The biomechanical function of the proteoglycan matrix and the collagen fibers are to absorb compressive and tensional loads, respectively. Here, we are focusing on the suprastructural organization of collagen fibrils and the degradation process of their hierarchical organized fiber architecture studied at high resolution at the authentic location within cartilage. We present electron micrographs of the collagenous cores of such fibers obtained by an improved protocol for scanning electron microscopy (SEM). Articular cartilages are permeated by small prototypic fibrils with a homogeneous diameter of 18 ± 5 nm that can align in their D-periodic pattern and merge into larger fibers by lateral association. Interestingly, these fibers have tissue-specific organizations in cartilage. They are twisted ropes in superficial regions of knee joints or assemble into parallel aligned cable-like structures in deeper regions of knee joint- or throughout hip joints articular cartilage. These novel observations contribute to an improved understanding of collagen fiber biogenesis, function, and homeostasis in hyaline cartilage.