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PhD Defense Małgorzata Pachecka

Metal diffusion and chemical processes of mono and bi-layer thin films

Małgorzata Pachecka is a PhD student in the Industrial Focus Group XUV Optics. Her supervisor is prof.dr. F. Bijkerk from the Faculty of Science and Technology.   

The physics and chemistry of Sn etching from optical and di-electrical surfaces must be understood in order to ensure that cleaning is complete and that the process does not damage the surface of the elements. We proposed the use of the relative electronegativities of the surface material and Sn to predict the degree to which electrons are available for bonding. This allows the success or failure of Sn etching from a thin metallic surface material to be predicted. It was found that Sn can be completely etched from transition metals with electronegativity values that are lower than or equal to Sn (𝝌Sn-𝝌M≥0). We observed incomplete Sn etching when Sn was initially deposited onto materials with an electronegativity value higher than that of Sn (𝝌Sn-𝝌M<0). This shows that electronegativity is a useful predictor for hydrogen etching of Sn from metal surfaces. Furthermore, for high electronegativity surfaces, which we predict to prevent complete etching, the remaining amount of Sn after etching depends on the electronegativity of the surface onto which Sn was deposited. This indicates that even Sn that is separated from the surface by several atomic layers is still influenced by the surface.

Metals with high electronegativities form weak oxides, such as ruthenium, while those with low electronegativity form strong oxides (e.g., scandium and aluminium). This leaves open the question of the role of surface oxidation in tin adhesion. To investigate the influence of oxidation, the removal of Sn from metallic and oxidized Sc thin films was studied. We have shown that tin can be fully etched from a metallic Sc surface as well from Sc thin films with complete and incomplete oxidation. Furthermore, our results show that the presence of Sc2O3 on a Sc thin film significantly slows the etch rate. For Sn etching from oxidized Sc, we propose that etching is slowed by electrons being trapped at defect sites in the Sc2O3 layer. This effect is strengthened when Sc metal is present below the Sc2O3, due to the built-in electric field spanning the oxide, generated by the difference in work function between Sc and Sn.

In this thesis, catalytic dissociation of CO2 was studied as a precursor for hydrogen control studies. Carbon dioxide adsorption and dissociation on a Ru single crystal surface was studied. Our results show that CO2 adsorption on a Ru(0001) surface results in partial dissociation, with CO2 and CO present on the surface. Furthermore, the dissociation of CO2 appears to be irreversible and is already observed at 85 K. The observed vibrational modes at 660 and 2343 cm-1 correspond to stretch modes of linear, undisturbed CO2, while a vibrational feature, which is visible only for higher CO2 coverages, at 1580cm-1 is tentatively assigned to an asymmetric stretching mode of a bent CO2 species. The adsorption  and desorption temperatures of CO2 on Ru are relatively low. By annealing the surface at 120 K—just above the peak desorption temperature of CO2—it was observed that the rate of CO2 dissociation was increased and that the CO restructures to a weaker bond between the surface and CO. The partial dissociation of CO2 is an important first step, since CO and H may react to form water and volatile hydrocarbons. This may then reduce the hydrogen concentration at the surface, which slows hydrogen penetration into the mirror.

Scandium oxide (Sc2O3) is a possible candidate for high-k dielectric applications. However, the stability of the dielectric layer is also very important: metal atoms from the gate electrode should not penetrate into or through the oxide layer. In this thesis, metal diffusion studies were carried out to test if Sc2O3 layers of various thicknesses act as a diffusion barrier. Scandium oxide diffusion barrier properties were tested using a MIM structure, consisting of Ru, Sc2O3, and Sn. We observed complete Sn etching from 0.71, 1.1 and 1.56 nm of Sc2O3. It was demonstrated that even 0.5 nm Sc2O3 forms a closed oxide layer that prevents the diffusion of Sn through to the Ru layer. The Sn etching time was observed to depend on the Sc2O3 barrier thickness and increased with increasing Sc2O3 thickness. This is explained by the formation of a MIM junction between the Ru, Sc2O3 and Sn, where the built-in potential enhances diffusion of electrons from the Ru and Sc2O3 defect states to the Sn. This diffusion of electrons becomes less efficient for larger barrier thicknesses. We use a simple model to show that, for thicker layers of Sc2O3, the etch rate is limited by charge diffusion, while electron tunneling may contribute significantly for thin layers of Sc2O3.