Probing atomic scale interface processes using x-rays and ions
Andrey Zameshin, a former PhD student in the Industrial Focus Group XUV Optics defended his thesis Wednesday November 14th 2018. His supervisor was prof.dr. F. Bijkerk from the Faculty of Science and Technology.
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. Although it is essential to study all processes occurring at the interfaces, there is no single analytical technique that allows to resolve buried interfaces with atomic resolution. 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). Together they will lead to a more comprehensive picture of the thin films and their interfaces.
GIXRR is widely used for analysis of multilayer films, but its representation of the interface lacks precision and flexibility, 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 method was validated on XRR data from LaN/B and LaN/La/B (with a La interlayer) multilayers. Features on the level of 0.3 nm were detected, which originated from the differences in the chemical state of the La interlayer. This experiment demonstrates the sensitivity of our approach to atomic scale interface features.
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. For example in the case of Ru films grown on B, C, B4C and Si, interface widths of 2.0 +/- 0.8 nm were determined. Further improvements to the accuracy of the analytical procedure were suggested to reduce uncertainties of the model parameters extracted.
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, with three different neutralization mechanisms responsible. A low work function matrix effect in La-based surfaces was caused by resonant neutralization from the conduction band of La to an excited state of He+ (C-RN). As a result, the characteristic velocity of He+ neutralization by La was found to depend on the work function.
A second type of matrix effect in La was so-called oscillatory matrix effect caused by quasiresonant charge transfer (qRCT) between La 5p and He 1s orbitals. It was found by accurate measurements of energy dependence of the He+ ion yield in a wide range of incident He+ energies (1 - 8 keV). A weak oscillatory structure was observed, the details of which depended on the La chemical state. This structure was attributed to a dampened quasiresonance between La 5p and He 1s levels. It is the first observation of s-p resonance in LEIS, while only s-d resonances were reported so far. The severe dampening of s-p resonance compared to s-d resonances is explained by the wider p-bands as compared to the d-bands. The difference in orbital symmetry is most probably not relevant for these resonances.
The last type of matrix effect was discovered in Ru-B, Ru-C and Ru-O mixtures, and was expressed as changes of characteristic velocity of each element due to the resonance between He 1s and electronic states from wide bands of the target material. These states can belong to wide continuous valence bands or to relatively wide non-valence bands. The former was demonstrated on Ru-B and Ru-C mixtures, but is applicable for all transition metal borides and carbides. The latter was demonstrated for O 2s states in a Ru-O mixture, but is applicable to all alkaline earth and rare earth metal hexaborides, as well as metal oxides and nitrides.
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.