Petr Bábor

Focused ion beam fabrication of spintronic nanostructures: an optimization of the milling process

Focused ion beam (FIB) milling has been used to fabricate magnetic nanostructures (wires, squares, discs) from single magnetic layers (Co, permalloy) and spin-valve (permalloy/Cu/Co) multilayers (thicknesses 5-50 nm) prepared by ion beam sputtering deposition. Milled surfaces of metallic thin films typically exhibit residual roughness, which is also transferred onto the edges of the milled patterns. This can lead to domain wall pinning and influence the magnetization behaviour of the nanostructures. We have investigated the milling process and the influence of the FIB parameters (incidence angle, dwell time, overlap and ion beam current) on the roughness of the milled surface. It has been found that the main reasons for increased roughness are different sputter yields for various crystallographic orientations of the grains in polycrystalline magnetic thin films. We have found that the oblique ion beam angle, long dwell time and overlap < 1 are favourable parameters for suppression of this intrinsic roughness. Finally, we have shown how to determine the ion dose necessary to mill through the whole thin film up to the silicon substrate from scanning electron microscopy (SEM) images only.

Highly Sensitive Detection of Surface and Intercalated Impurities in Graphene by LEIS

Low-energy ion scattering (LEIS) is known for its extreme surface sensitivity, as it yields a quantitative analysis of the outermost surface as well as highly resolved in-depth information for ultrathin surface layers. Hence, it could have been generally considered to be a suitable technique for the analysis of graphene samples. However, due to the low scattering cross section for light elements such as carbon, LEIS has not become a common technique for the characterization of graphene. In the present study we use a high-sensitivity LEIS instrument with parallel energy analysis for the characterization of CVD graphene transferred to thermal silica/silicon substrates. Thanks to its high sensitivity and the exceptional depth resolution typical of LEIS, the graphene layer closure was verified, and different kinds of contaminants were detected, quantified, and localized within the graphene structure. Utilizing the extraordinarily strong neutralization of helium by carbon atoms in graphene, LEIS experiments performed at several primary ion energies permit us to distinguish carbon in graphene from that in nongraphitic forms (e.g., the remains of a resist). Furthermore, metal impurities such as Fe, Sn, and Na located at the graphene-silica interface (intercalated) are detected, and the coverages of Fe and Sn are determined. Hence, high-resolution LEIS is capable of both checking the purity of graphene surfaces and detecting impurities incorporated into graphene layers or their interfaces. Thus, it is a suitable method for monitoring the quality of the whole fabrication process of graphene, including its transfer on various substrates.

Real-time observation of self-limiting SiO2/Si decomposition catalysed by gold silicide droplets

The thermal decomposition of thin SiO2 layers on silicon substrates draws significant attention due to its high technological importance in the semiconductor industry and in all relevant fields where silicon is employed as a substrate or part of an active device. Understanding of the underlying processes on silicon surfaces is therefore of fundamental importance. Here we show that the presence of gold silicide (AuSi) catalytically enhances the decomposition of SiO2 layers on a Si substrate, which proceeds via void nucleation under the positions of Au nanoparticles and subsequent lateral growth of the void. Our real-time secondary electron microscopy data reveal that the presence of a AuSi droplet within the void enhances the reaction rate due to an increased pre-exponential factor of the rate limiting step (i.e., SiO desorption at temperatures beyond 700 °C). While the SiO2 is decomposed the silicon surface in the open voids is covered by an Au monolayer. Consequently, as the void grows, the AuSi droplet is depleted of gold and the reaction rate enhancement is terminated when the supply of gold stops. Hence, the size of the pits is determined by the initial size of the Au nanoparticle. Our work thus provides insight into Au-enhanced SiO2 decomposition and its self-limiting nature offers a way for the preparation of nanoscale features with nanometer precision.

High-resolution characterization of hexagonal boron nitride coatings exposed to aqueous and air oxidative environments

Hexagonal boron nitride (h-BN) is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H2O2 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H2O2. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10–15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multilayer (not monolayer) h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.

Polycrystalline Silicon Layers with Enhanced Thermal Stability

We report on a new method of external gettering in silicon substrate for semiconductor applications. The proposed method is based on the deposition of a multilayer system formed by introducing a number of thin buried silicon oxide layers into the thick polycrystalline silicon layer deposited on the wafer backside. Oxide films of a few nanometer thicknesses significantly retard both the grain growth and subsequent loss of the gettering capability of the polycrystalline silicon layer during high temperature annealing. The mechanisms of the grain growth and the influence of the embedded oxide layers on the gettering function in the multilayer system are discussed. We used scanning electron microscopy and transmission electron microscopy for the characterization of the multilayer system, and intentional contamination for demonstration of the gettering properties.