Probing atomic scale interface processes using X-rays and ions

Interfaces between individual layers in thin films and multilayers affect mechanical, optical, electric and magnetic properties of the films. When the layer thicknesses approach the nanometer or even atomic scale, imperfections at the each of the interfaces cannot be ignored. They may consist of layer roughness, interdiffused material between the layers, and structures resulting from unwanted chemical interactions. This thesis is focused on the development of two separate, advanced analytical techniques with a high sensitivity to such interface processes: Grazing Incidence X-Ray Reflectivity (GIXRR) and in-vacuo Low Energy Ion Scattering (LEIS). The conventional approach to GIXRR analysis lacks flexibility in its representation of the interfaces, since a priori assumptions on the interface profile are required in the model. In this thesis we used a free-form, or a model-independent approach, where the entire structure, including interfaces, is divided into multiple sublayers. The composition of each sublayer is reconstructed independently. To direct the algorithm towards physically more correct smooth interface profiles, a regularization function was additionally introduced. This allows freedom of the interface profile and absence of unrealistic abrupt features in the profiles of optical constants. The sensitivity of our approach to atomic scale interface features was demonstrated experimentally. Unlike XRR, in-vacuo Low Energy Ion Scattering (LEIS) analysis is much less established for the analysis of buried interfaces. In this thesis, the limits of its usage are explored and expanded. The surface atomic fraction at every moment of thin film growth is found to be affected by two simultaneous phenomena: intermixing at interfaces and surface segregation. To separate these two processes, a phenomenological model of surface evolution was developed. As a result, by combining information from LEIS surface peaks and signals originating from deeper layers, interface transition effects were separated from surface segregation effects, and interface profile width as well as two segregation parameters were obtained. In addition, also matrix effects in LEIS, arising from specific material combinations in this thesis, were investigated. Three different types of matrix effects were found and described, with three different neutralization mechanisms responsible. Despite these matrix effects, a detailed quantification of surface composition was shown to be possible. The restrictions imposed by the matrix effects limit the accuracy of buried interface analysis by in vacuo LEIS, and more fundamental research on the behavior of matrix effects in LEIS is needed to further extend the limits of LEIS metrology. This thesis is a step in this direction

Angular and spectral bandwidth of Extreme UV multilayers near spacer material absorption edges

High resolution imaging systems for EUV range are based on multilayer optics. Current generation of EUV lithography uses broadband Sn LPP sources, which requires broadband mirrors to fully utilize the source power. On the other hand, there always remains a possibility to use FEL or synchrotron as EUV source. FEL can produce very bright narrowband EUV light of a tunable wavelength, and the spectral bandwidth of the mirror is no longer a restriction. In this paper we look at the consequences of switching to different wavelengths if FEL source is used. For instance, it is known that the reflectance of Mo/Si multilayers increases when approaching Si L-edge, and the spectral bandwidth drops. But the behavior of an angular bandwidth (and its relation with the spectral bandwidth) is usually left out. It is also sometimes assumed that these bandwidths are correlated. For a large aperture EUV optical system with diffraction-limited resolution angular acceptance of a mirror is also a very important parameter. We show that the angular bandwidth of several multilayer systems (Mo/Si, Mo/Be, Ru/Si, Ru/B, La/B) actually increases close to spacer absorption edges, opposite to what occurs with the spectral bandwidth. We study this effect and show that it is caused by an interplay of changing optical constants of respective materials used in these multilayer combinations. We also provide an experimental check of the angular bandwidth of Mo/Si multilayers at 13.5 and 12.6 nm, which confirms our calculations

W/B short period multilayer structures for soft x-rays

X-ray W/B multilayer mirrors with a period of 2.5 nm were deposited by dc magnetron sputtering and studied in comparison with W/Si multilayer systems of the same period. Transmission electron microscopy, grazing incidence X-Ray reflectivity, and x-ray photoelectron spectroscopy analysis revealed that the layer quality and interfaces of the W/B multilayers are not better than those of the W/Si multilayers. Strong intermixing between W and B is present, which leads to compound formation with little or no pure W left after the interaction: an optically unfavorable boride formation and an increased roughness result in a reduced reflectivity. The deposited W/B multilayer mirrors showed a reflectivity of 34.5% at 0.84 nm and angle of incidence 9.7°, compared to 40% obtained for W/Si multilayers. Ion polishing applied on the boron layers did not result in improvements of the reflectivity

Low Energy Ion Scattering from La surfaces

Low Energy Ion Scattering (LEIS) is a technique with ultimate sensitivity to only topmost atomic layer. We applied it together with XPS to studying the interaction of La and B layers at interfaces, which is crucial for improvement of properties of La/B-based multilayer mirrors for 6.7 nm wavelength light. Unusual drop of LEIS signal on La surface was observed, especially pronounced when interaction with O or B takes place. This behavior was found to be a rare matrix effect caused by low work functions of pure La as well as its compounds, which results in dominance of a resonant charge transfer mechanism, affecting LEIS signal. Since only La is present in the spectrum, matrix effect is formed by the sub-surface O and B atoms. Plotting normalized LEIS signal versus inversed velocity of the incident ions makes quantification of the surface composition possible

In-situ studies of silicide formation during growth of molybdenum-silicon interfaces

The growth development of nanometer thick Mo and Si layers was studied using in situ laser deflection and Low Energy Ion Scattering (LEIS). The growth stress obtained from changes in wafer curvature during growth is correlated to changes in the surface stochiometry monitored by LEIS. For Si on Mo, the compressive-tensile-compressive stress development could be explained by the formation of interfacial silicide compounds and the transition between these and the bulk growth of Si. For Mo on Si, a strong initial tensile stress due to silicide formation saturates upon reduced availability of free Si at the growing Mo surface, followed by a near instantaneous tensile increase in stress related to the amorphous-to-crystalline phase transition, which coincides with the end of the compound formation, as determined with LEIS