Chemical Interaction of Hydrogen Radicals (H*) with Transition Metal Nitrides.

Transition metal nitrides (TMNs) are reported as protective coatings in reactive hydrogen environments. Although the permeation of H2 through TMN coatings is well reported, their reducibility in H* environments is less investigated. In this work, we categorize the interaction of H* with ambient exposed TiN, ZrN, HfN, VN, NbN, and TaN thin films at 700 °C into three classes. We find that in TiN and VN samples, H*-induced reduction was limited to the surface (≈ top 2 nm). Significant denitridation was observed in ZrN and HfN samples beneath the surface, along with an increase in the transition metal oxide (TMOx) fraction. Denitridation was observed in NbN and TaN samples as well, but the increase in the TMOx content was less than for ZrN and HfN. We propose a model in three steps: hydrogenation, formation of volatile species, and diffusion of subsurface atoms to the surface. We show that the interaction of H* with TiN, ZrN, HfN, VN, NbN, and TaN with partially oxidized surfaces can be explained using the preferred hydrogenation pathway (based on the work functions) and the thermodynamic driver for forming volatile species (NH3 and H2O; based on the change in Gibbs free energy).

Determination of optical constants of thin films in the EUV.

The determination of fundamental optical parameters is essential for the development of new optical elements such as mirrors, gratings, or photomasks. Especially in the extreme ultraviolet (EUV) and soft x-ray spectral range, the existing databases for the refractive indices of many materials and compositions are insufficient or are a mixture of experimentally measured and calculated values from atomic scattering factors. Since the physical properties of bulk materials and thin films with thicknesses in the nanometer range are not identical, measurements need to be performed on thin layers. In this study we demonstrate how optical constants of various thin film samples on a bulk substrate can be determined from reflection measurements in the EUV photon energy range from 62 eV to 124 eV. Thin films with thickness of 20 nm to 50 nm of pure Mo, Ni, Pt, Ru, Ta, and Te and different compositions of NixAlx, PtTe, PtxMo, RuxTax, Ru3Re, Ru2W, and TaTeN were prepared by DC magnetron sputtering and measured using EUV reflectometry. The determination optical constants of the different materials are discussed and compared to existing tabulated values.

Bifunctional catalytic effect of Mo2C/oxide interface on multi-layer graphene growth.

The role of the Mo2C/oxide interface on multi-layer graphene (MLG) nucleation during a chemical vapor deposition (CVD) process is investigated. During the CVD process, MLG growth is only observed in the presence of a Mo2C/SiO2 interface, indicating that the chemical reactions occurring at this interface trigger the nucleation of MLG. The chemical reaction pathway is explained in four steps as (1) creation of H radicals, (2) reduction of the oxide surface, (3) formation of C-C bonds at O-H sites, and (4) expansion of graphitic domains on the Mo2C catalyst. Different Mo2C/oxide interfaces are investigated, with varying affinity for reduction in a hydrogen environment. The results demonstrate a catalyst/oxide bifunctionality on MLG nucleation, comprising of CH4 dehydrogenation by Mo2C and initial C-C bond formation at the oxide interface.

Nb Texture Evolution and Interdiffusion in Nb/Si-Layered Systems.

In this paper, we present a detailed study on the microstructure evolution and interdiffusion in Nb/Si-layered systems. Interlayer formation during the early stages of growth in sputter-deposited Nb-on-Si and Si-on-Nb bilayer systems is studied in vacuo using a high-sensitivity low-energy ion-scattering technique. An asymmetric intermixing behavior is observed, where the Si-on-Nb interface is ∼2× thinner than the Nb-on-Si interface, and it is explained by the surface-energy difference between Nb and Si. During Nb-on-Si growth, the crystallization of the Nb layer occurs around 2.1 nm as-deposited Nb thickness with a strong Nb(110)-preferred orientation, which is maintained up to 3.3 nm as-deposited Nb thickness. A further increase in the Nb layer thickness above 3.3 nm results in a polycrystalline microstructure with a reduced degree of texture. High-resolution cross-sectional transmission electron microscopy imaging is performed on Nb/Si multilayers to study the effect of the Nb layer texture on interdiffusion during low-temperature annealing. Nb/Si multilayers with amorphous 2 nm Nb layers and strongly textured 3 nm thick Nb layers, with limited grain-boundary pathways for diffusion, show no observable interdiffusion during annealing at 200 °C for 8 h, whereas in a Nb/Si multilayer with polycrystalline 4 nm thick Nb layers, a ∼1 nm amorphous Nb/Si interlayer is formed at the Si-on-Nb interface during annealing.

Nanoscale Transition Metal Thin Films: Growth Characteristics and Scaling Law for Interlayer Formation.

A comprehensive study on the growth of nanoscale transition metal-on-transition metal (TM-on-TM) systems is presented. The near room-temperature intermixing and segregation phenomena during growth are studied in vacuo using high-sensitivity low-energy ion scattering. The investigated TM-on-TM systems are classified into four types according to the observed intermixing and segregation behavior. Empirical rules are suggested to qualitatively predict the growth characteristics of any TM-on-TM system based on the atomic size difference, surface-energy difference, and enthalpy of mixing between the film and substrate atoms. An exponential trend is observed in the effective interface width as a function of the surface-energy difference between the film and substrate layers, with a subtrend based on the crystal structure of the TM layers. A semiempirical model that accurately describes the experimental data is presented. It serves as a scaling law to predict the effective interface width and the minimum film thickness required for full film coverage in TM-on-TM systems in general. The ability to predict the growth characteristics as well as the interface width for any TM-on-TM system significantly contributes to the process of finding the best material combination for a specific application, where layer growth characteristics are implicitly considered when selecting materials based on their functional properties.

Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold.

The durability of grazing- and normal-incidence optical coatings has been experimentally assessed under free-electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10% of the single-shot damage threshold. The experiment was performed at FLASH, the Free-electron LASer in Hamburg, using 13.5 nm extreme UV (EUV) radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20° and 10° grazing incidence, respectively. Mo/Si periodical multilayer structures were tested in the Bragg reflection condition at 16° off-normal angle of incidence. The exposed areas were analysed post-mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X-ray photoelectron spectroscopy. The analysis revealed that Ru and Mo/Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV-induced oxidation of the surface.

Mo/Si multilayer-coated amplitude-division beam splitters for XUV radiation sources.

Amplitude-division beam splitters for XUV radiation sources have been developed and extensively characterized. Mo/Si multilayer coatings were deposited on 50 nm-thick SiN membranes. By changing the multilayer structure (periodicity, number of bilayers, etc.) the intensity of the reflected and transmitted beams were optimized for selected incident radiation parameters (wavelength, incident angle). The developed optical elements were characterized by means of XUV reflectometry and transmission measurements, atomic force microscopy and optical interferometry. Special attention was paid to the spatial homogeneity of the optical response and reflected beam wavefront distortions. Here the results of the characterization are presented and improvements required for advanced applications at XUV free-electron lasers are identified. A flatness as low as 4 nm r.m.s. on 3 × 3 mm beam splitters and 22 nm r.m.s. on 10 × 10 mm beam splitters has been obtained. The high-spatial-frequency surface roughness was about 0.7-1 nm r.m.s. The middle-spatial-frequency roughness was in the range 0.2-0.8 nm r.m.s. The reflection and transmission of the beam splitters were found to be very homogeneous, with a deviation of less than 2% across the full optical element.

Structural properties of subnanometer thick Y layers in extreme ultraviolet multilayer mirrors.

We studied the structure and optical properties of B(4)C/Mo/Y/Si multilayer systems. Using extended x-ray absorption fine structure measurements at the Y and Mo K-edge, the structure of the subnanometer thick Y layer and the underlying Mo layer were analyzed. It was found that even a 0.2 nm thick Y layer significantly reduced silicon diffusion toward Mo, thus reducing Mo silicide formation. Hard x-ray reflectometry showed that the difference in average interface roughness of the B(4)C/Mo/Y/Si multilayer structure compared to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures was negligible. Soft x-ray reflectometry showed optical improvement of B(4)C/Mo/Y/Si with respect to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures.

Microstructure of Mo/Si multilayers with B4C diffusion barrier layers.

The growth behavior of B(4)C interlayers deposited at the interfaces of Mo/Si multilayers was investigated using x-ray photoemission spectroscopy, x-ray reflectivity, and x-ray diffraction measurements. We report an asymmetry in the formation of B(4)C at the B(4)C-on-Mo interface compared to the B(4)C-on-Si interface. X-ray photoelectron spectroscopy (XPS) depth profiling shows that for B(4)C-on-Mo the formed stoichiometry is close to expectation (4:1 ratio), while for B(4)C-on-Si it is observed that carbon diffuses from the B(4)C interfaces into the multilayer, resulting in nonstochiometric growth (>4:1). As a result, there is a discrepancy in the optical response near 13.5 nm wavelength, where B(4)C-on-Mo behaves according to model simulations, while B(4)C-on-Si does not. The as-deposited off-stoichiometric B(4)C-on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of B(4)C at the Mo-Si interfaces is connected to the structure of the layers onto which B(4)C is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form Si(X)B(Y) and Si(X)C(Y) compounds. The low formation enthalpy of Si(X)C(Y) ensures C depletion of any B(X)C(Y) interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual B(4)C formation. In contrast to the off-stoichiometric B(4)C growth on top of Si, B(4)C grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for Mo(X)B(Y) and Mo(X)C(Y) formation and the limited diffusion depth into the (poly)-crystalline Mo surface.