Haochuan Li

Optical and structural performance of the Al(1%wtSi)/Zr reflection multilayers in the 17–19nm region

Two kinds of Al/Zr (Al(1%wtSi)/Zr and Al(Pure)/Zr) multilayers for extreme ultraviolet (EUV) optics were deposited on fluorine doped tin oxide coated glass by using direct-current magnetron sputtering technology. The comparison of the two systems shows that the Al(1%wtSi)/Zr multilayers have the lowest interfacial roughness and highest reflectivity. Based on the X-ray diffraction, the performance of the two systems is determined by the crystallization of Al layer. To fully understand the Al(1%wtSi)/Zr multilayer, we built up a two-layer model to fit situation of the AFM images, and simulate the grazing incident x-ray reflection-measurements of multilayers with various periods (N = 10, 40, 60, 80). Below 40 periods, the roughness components are lowered. After 40 periods, both surface and interfacial roughness increase with the period number, and decrease the reflectance. According to transmission electron microscope images, the model can represent the variable structure of the system.

Comparison of Mg-based multilayers for solar He II radiation at 30.4 nm wavelength

Mg-based multilayers, including SiC/Mg, Co/Mg, B(4)C/Mg, and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a 30.4 nm wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of B(4)C/Mg and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a 30.4 nm wavelength.

Asymmetrical diffusion at interfaces of Mg/SiC multilayers

Interfacial structure of Mg/SiC multilayers as extreme ultra-violet reflectors was studied along with Mg/Si and Mg/C multilayers by means of x-ray reflectometry, x-ray diffraction, x-ray photoemission spectroscopy, and transmission electron microscopy. The interfacial diffusion in the Mg/SiC multilayer is found asymmetrical as the interlayers formed at SiC-on-Mg interfaces (2.5 nm) are much thicker than those at Mg-on-SiC interfaces (1.0 nm). Contrary asymmetry is found in the Mg/Si and Mg/C multilayers. An explanation of this phenomenon is suggested based on the investigation results. Our findings may result in improved reflectance of Mg/SiC multilayers by inserting diffusion barriers at the more diffused interfaces.

Thermal stability of Mg/Co multilayer with B 4 C, Mo or Zr diffusion barrier layers

The efficiency of B(4)C, Mo and Zr barrier layers to improve thermal stability of Mg/Co multilayer up to 400 °C is investigated. Multilayers were deposited by direct current magnetron sputtering and characterized using X-ray and extreme ultraviolet reflection. The results suggest that B(4)C barrier layer is not effective due to drastic diffusion at Mg-B(4)C interface. Although introducing Mo barriers improves the thermal stability from 200 to 300 °C, it increases the interface roughness and thus degrades the optical performances. On the contrary, Zr barriers can significantly increase the thermal stability of Mg/Co up to 400 °C without optical performance degradation. Thus, Mg/Zr/Co/Zr is suitable for EUV applications requiring both optimal optical performances and heat resistance.

Zr/Mg multilayer mirror for extreme ultraviolet application and its thermal stability

Zr/Mg multilayer mirror was proposed for extreme ultraviolet (EUV) spectral range and deposited by magnetron sputtering. Its thermal stability during annealing up to 600 °C was evaluated by EUV reflection measurements, x-ray analyses, and transmission electron microscopy and found superior to that of Y2O3/Mg, SiC/Mg, and Co/Mg. The reflectance of as-deposited Zr/Mg multilayer is 30.6% at wavelength of 30.4 nm. The reflectance slightly decreases with annealing temperature when not above 500 °C and eventually drops to 15.1% at 600 °C. The degradation of performance is attributed to roughening induced by strain relaxation rather than interdiffusion or alloy compound formation.

Ni Layer Thickness Dependence of the Interface Structures for Ti/Ni/Ti Trilayer Studied by X-ray Standing Waves

X-ray standing waves generated by periodic multilayers have been used to characterize the interface microstructures of Ti/Ni/Ti trilayers based on the X-ray reflectivity (XRR) and grazing incidence X-ray fluorescence (GIXRF) methods. For the Ni layer having thickness of 1.7 nm, it is observed that the roughness of Ti-on-Ni interface is 0.64 nm and that of Ni-on-Ti interface is 0.40 nm, which can be explained by an additional roughness on the Ti-on-Ni interface induced by the nucleation of Ni crystallites when the Ni layer thickness is at the amorphous-to-crystalline transition region. For the Ni layer thickness of 3.3 nm beyond this transition region, the roughness of Ti-on-Ni interface is 0.42 nm and Ni-on-Ti interface is 0.46 nm, which is consistent with the expectation on wetting and dewetting conditions.

Thermal effects on Co/Mo2C multilayer mirrors studied by soft x-ray standing wave enhanced photoemission spectroscopy

Here is presented the spectroscopic study of the evolution of the first buried interfaces of a B4C capped Co/Mo2C multilayer mirror induced by thermal treatment up to 600°C. This kind of study is typically performed to simulate the response of multilayer optics working in extreme conditions, as for instance when irradiated by new high brilliance sources as Free Electron Lasers. In fact, the efficiency of multilayers is related to the optical contrast between the alternating high and low density layers, and then to the degree of interdiffusion and the creation or evolution of interface compounds. The analysis has been performed at the Co L23 edge with different soft x-ray spectroscopic techniques including diffuse and specular reflectivity, total electron and fluorescent yield at the BEAR beamline at Elettra (Trieste) (http://www.elettra.trieste.it/elettra-beamlines/bear.html). The presentation is focused on the spectroscopic results obtained by soft x-ray standing wave enhanced photoemission (XSW) from the Mo 3d, B 1s, C 1s, O 1s core levels by using a photon energy close to the Co L23 edge and corresponding to the first Bragg peak of the multilayer. The experimental results have been compared with simulations to obtain information both on the chemical state (e.g. oxidation state) and interface morphology in terms of profiles of distribution of elements and interdiffusion of B, oxidized B and C in the interface region. In summary, it is possible to conclude in favour of a good stability of the multilayer in the investigated temperature range, as confirmed by the good performance in terms of reflectivity. These results confirm the usefulness of XSW for this kind analysis of multilayer optics.

Investigation of the x-ray reflectivity of the Co/Mo2C system upon thermal treatment

The Co-based multilayers have been shown promising optical mirrors for application in the EUV and soft x-ray ranges. Most multilayer systems cannot attain the reflectivity and resolution requirements assumed by theory because of interdiffusion and roughness. Therefore, it is necessary to find out the excellent material possessing optical performance in the EUV and soft x-ray ranges and propose solution to eliminate the interface imperfections or find out new efficient combinations. Here we propose a new system, namely the periodic Co/Mo2C multilayer. The multilayer systems are prepared by the magnetron sputtering and characterized by x-ray reflectivity at 8048 eV (Cu Kα emission) and with synchrotron radiation in the soft x-ray range at 778 eV. The measurements are used in order to determine the structural parameters (thickness, roughness and density) of the layers. The simulated reflectivity at 11° grazing angle with s-polarized is calculated to be 45% at 778 eV, if there is no interaction between the layers and no interfacial roughness, while experimentally reflectivity is limited to 25%. The relationship between the reflectivity and annealing up to a temperature of 600°C is also investigated. It shows that the Co/Mo2C multilayer is able to work up to 600°C. First the reflectivity increases to 27% at 300°C. After the reflectivity slightly decreases to 25% at 500°C and then we observe a reflectivity drop to 20% at 600°C. Relationship between the structural parameters and the reflectivity values is deduced from the fit of the experimental curves.

Thermal stability of Co/C multilayers

The structural and interface changes induced by thermal annealing in Co/C multilayers were investigated. Co/C multilayers with period thickness of 4.1 nm and bi-layer number of 20 were deposited by direct current magnetron sputtering. We characterized all samples by using x-ray reflectivity, x-ray diffuse scattering, zero-field nuclear magnetic resonance spectroscopy and x-ray diffraction. The results indicate that Co and C atoms mixed during deposition and then after annealing both atoms separated from their mixed region. The annealing process also causes a sharp increase of roughness at interfaces, which can be attributed to the crystallization of Co layers.

Improved thermal stability of Mg/Co multilayer by introducing Zr barrier layer

High reflective multilayer mirrors are widely used as optical elements for applications such as extreme ultraviolet (EUV) microscopy, high harmonic femtosecond chemistry, solar astrophysics imaging, and synchrotron radiation. Mg-based multilayers, such as Mg/SiC, Mg/Y2O3 and Mg/Co are promising in the wavelength of 25-40 nm for Mg L3 absorption edge is located at 25 nm. Mg/Co has narrower bandwidth and better thermal stability. In applications such as synchrotron radiation and solar imaging, multilayers mirrors must endure high heat loads. Thus, we investigated the thermal stability of Mg/Co multilayer and then introduced barrier layer to improve thermal stability in this paper. The interface structures evolution was studied by using X-ray reflection/scatter technique. Mg/Co multilayer can be stable when not heated above 300°C. B4C and Zr layers were inserted into Mg/Co multilayer as interface barrier layer to improve thermal stability. According to the measured results, B4C barrier layer is not suitable for Mg/Co multilayer, mainly due to the diffusion between Mg and B4C. The introduction of Zr can significantly improve the thermal stability of Mg/Co up to 400°C without reducing EUV reflectance. Thus, introduction of Zr barrier layer is an efficient method to improve the thermal stability of Mg/Co multilayer for EUV applications such as astronomical observation and synchrotron radiation.