We investigated the thermal stability of e-beam deposited Mo/B4C/Si/B4C layered systems, with and without ion assistance during the growth of the B4C diffusion barrier layers. The thermal stability was investigated by in situ thermal annealing during grazing incidence X-ray reflection. By studying partially treated B4C barrier layers, we found that the improvement in thermal stability is caused by an enhanced density of the B4C layer.

Multilayered Extreme UV mirrors present unprecedented fundamental questions to solid state diffusion, requiring understanding of diffusion phenomena on length scales of only few picometers. Using x-ray diffraction applied in situ during thermal annealing, we have investigated diffusion processes in Mo/Si multilayers at the very initial stage of compound interface formation. Grazing incidence x-ray reflectometry resolves picometer structural changes in the multilayer structure as a function of time and temperature and reveals diffusion limited compound interface growth. The temperature dependence of the measured diffusion constants exhibits Arrhenius-like diffusion behaviour with an increasing activation energy during interface growth that may explain the range of activation energies reported for this system in literature. The method and results presented here are relevant for understanding and controlling diffusion processes on a picometer length scale, with potential spin-off to semiconductor and other thin film applications.

The internal structure of Mo/Si multilayers is investigated during and after thermal annealing. Multilayer period compaction is shown to result from diffusion induced MoSi2 interlayer growth, reducing optical contrast and changing the reflected wavelength. We focus on early-stage interface growth observed at relatively low temperatures (100 °C - 300 °C), determining diffusion constants from parabolic interface growth laws. Diffusion constants obey Arrhenius-type behavior, enabling temperature scaling laws. Using the methods developed, we compare results on Mo/Si based multilayers designed for enhanced thermal stability and discuss their relevant diffusion behavior. Arrhenius-type behavior can be observed in all multilayers studied here, and demonstrates reduction of diffusion rates over several orders of magnitude. The method described here is of general interest for any multilayer application that is subjected to enhanced thermal loads and demonstrates the enormous technology gain that this type of optics has experienced the last decade.

We investigated the damage mechanism of MoN/SiN multilayer XUV optics under two extreme conditions: thermal annealing and irradiation with single shot intense XUV pulses from the free-electron laser facility in Hamburg - FLASH. The damage was studied "post-mortem" by means of X-ray diffraction, interference-polarizing optical microscopy, atomic force microscopy, and scanning transmission electron microscopy. Although the timescale of the damage processes and the damage threshold temperatures were different (in the case of annealing it was the dissociation temperature of Mo2N and in the case of XUV irradiation it was the melting temperature of MoN) the main damage mechanism is very similar: molecular dissociation and the formation of N2, leading to bubbles inside the multilayer structure.

We present a low temperature diffusion study on the formation of intermixing zones between periodic, nanometer thick films consisting of Mo and Si. An in-situ X-ray diffraction method at pm-accuracy was developed, including a model that explains the period change observed by diffusion limited interface growth. Experiments were carried out on Mo/Si multilayered films in the temperature range of 100-275 degrees C, resulting in the determination of diffusion coefficients. Temperature scaling showed Arrhenius-type behavior of the diffusion constant over the entire temperature range, with an activation energy of 0.5 eV. (C) 2010 Elsevier B.V. All rights reserved.

We investigate the influence of the Mo crystalline state (quasi-amorphous or crystalline) on the thermal stability of Mo/Si thin film multilayers with B4C diffusion barrier layers at either of the two interfaces. We find that multilayers containing amorphous Mo layers are more stable than those containing crystalline layers. This observation is in contrast to the case where Si3N4 diffusion barriers are used. Using X-ray diffraction, X-ray reflection and X-ray photo-electron spectroscopy we show that this difference can be attributed to the dissociation of B4C followed by diffusion of B in Mo. Due to the favorable thermodynamic properties of MoxBy compounds, the boron atoms react with the Mo layer, forming a MoxBy layer that effectively improves the multilayer thermal resistance.

We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.

Thermally induced diffusion through the Si-on-Mo interface of multilayers with either amorphous or polycrystalline Mo layers has been investigated using grazing incidence and wide angle x-ray reflectometry. Diffusion through the Mo-on-Si interface was reduced by applying a diffusion barrier, allowing us to probe the diffusion at the opposite, Si-on-Mo interface. We found that diffusion through this interface is much slower for polycrystalline Mo than for amorphous Mo layers. The reason for this difference might be the larger defect concentration in amorphous Mo as compared to crystalline Mo.

Diffusion behaviour at the interfaces of multilayered thin film structures generally leads to intermixing and subsequent compound formation. For multilayer systems based on the Mo/Si material combination, it is well known that diffusion induced silicide formation leads to a loss of optical contrast as well as a change in the reflected wavelength. Often, diffusion barriers are applied to reduce diffusion speeds, slowing down, but not stopping the deterioration of these systems at elevated temperatures. This paper intends to provide an overview of in-depth diffusion analysis in several state-of-the-art multilayer systems. We report here on a method of increasing the optical contrast between thin films, as applied to the Mo/Si system. Using physical vapour deposition, multilayer structures were grown on Si substrates mounted on an actively (liquid-N2) cooled substrate holder. The low energy of the deposited atoms, combined with the reduced substrate temperature, leads to a significantly reduced interaction between Mo and Si. The first experiments reported here show a reduction of the total silicide interlayer at both interfaces by 0.9 nm. This reduced interface width was maintained upon subsequent warming up of the samples to room temperature, suggesting that the atoms deposited in the interaction zones are “frozen in” during subsequent deposition of atoms. This meta-stable structure is destroyed at elevated temperatures, where diffusion speeds up and a strong increase in the silicide interface width is observed. To study the diffusion kinetics at elevated temperatures in more detail, we developed a novel method based on Grazing Incidence X-ray Reflectometry (GIXR). Using an in-situ analysis of the GIXR data during thermal annealing, it is possible to determine multilayer period changes with a sub-pm accuracy. This method enables determination of the diffusion constants in temperature ranges that are inaccessible by means of traditional diffusion studies. The data show that already at 100°C, there is a small but observable diffusion induced change in the Mo/Si multilayer period. An Arrhenius-type plot of the obtained diffusion constants shows that diffusion in the 100°C-275°C temperature range can be described with a single proportionality factor and a single activation energy, signifying a single physical process to be at the base of interface diffusion. Finally, well above 300°C and depending on the exact structure and application of diffusion barriers, a strong acceleration of the diffusion is generally observed. Using low energy ion scattering (LEIS), we investigated diffusion of Mo and Si atoms through a diffusion barrier at 500°C. The LEIS data show several distinct diffusion regions and reveal an instantaneous acceleration of the diffusion rate by an order of magnitude. X-ray photoemission spectroscopy and transmission electron microscopy analysis clearly show that it is not the “chemical breakdown” of the applied diffusion barrier, but crystallization of the interface region that leads to the observed dramatic increase in diffusion rate. The research presented here is particularly relevant for coatings applied at high fluency photon sources such as next-generation pulsed EUV sources and X-ray Free Electron Lasers. PXRMS 2010, R. vd Kruijs

Stenotrophomonas maltophilia strain W81, isolated from the rhizosphere of field-grown sugar beet, produced the extracellular enzymes chitinase and protease and inhibited the growth of the phytopathogenic fungus Pythium ultimum in vitro. The role of these lytic enzymes in the interaction between W81 and P. ultimum was investigated using Tn5 insertion mutants of W81 incapable of producing extracellular protease (W81M1), extracellular chitinase (W81M2) or the two enzymes (W81A1). Lytic enzyme activity was restored in W81A1 following introduction of a 15 kb cosmid-borne fragment of W81 genomic DNA. Incubation of P. ultimum in the presence of commercial purified protease or cell-free supernatants from cultures of wild-type W81, the chitinase-negative mutant W81M2 or the complemented derivative W81A1(pCU800) resulted in hyphal lysis and loss of subsequent fungal growth ability once re-inoculated onto fresh plates. In contrast, commercial purified chitinase or cell-free supernatants from cultures of the protease-negative mutant W81M1 or the chitinase-and protease-negative mutant W81A1 had no effect on integrity of the essentially chitin-free Pythium mycelium, and did not prevent subsequent growth of the fungus. in soil microcosms containing soil naturally infested by Pythium spp., strains W81, W81M2 and W81A1(pCU800) reduced the ability of Pythium spp. to colonize the seeds of sugar beet and improved plant: emergence compared with the untreated control, whereas W81A1 and W81M1 failed to protect sugar beet from damping-off. Wild-type W81 and its mutant derivatives colonized the rhizosphere of sugar beet to similar extents. It was concluded that the ability of S. maltophilia W81 to protect sugar beet from Pythium-mediated damping-off was due to the production of an extracellular protease.