Stanislav Průša

Study of aerosols generated by 213 nm laser ablation of cobalt-cemented hard metals

Cobalt-cemented hard metals present an example of samples with a complicated matrix consisting of components differing in chemical and physical properties and with extremely low volatility of all components. The purpose of this study was to compare particle formation of a set of real samples with similar matrices but different content of major components. The laser ablation process was studied using a Q-switched quintupled (213 nm) nanosecond Nd:YAG laser. Five samples of Co-cemented tungsten carbides, actually WC-TiC-(Ta,Nb)C-Co with a varied content of main constituents, were selected as representatives of a family of 15 miscellaneous tungsten carbide hard metal products. Physical and chemical properties vary over this specimen selection and therefore the effect on particle size formation and distribution was expected. The size distributions by number of ablated particles in different size ranges were measured using an optical aerosol spectrometer. The results proved the relationship between particle formation and sample composition. The structure of laser generated particles and the properties of ablation-craters were additionally studied by scanning electron microscopy (SEM). Spherical particles in the diameter range of 0.25–2 μm and μm-sized agglomerates composed of primary nano-particles were observed. The W and Co content in the aerosol particles was determined by energy dispersive X-ray spectroscopy (EDS). The volumes of ablation craters were measured by an optical profilometer. The laser ablation study of selected Co-cemented tungsten carbide hard metals indicates a similar total volume of formed particles with composition-dependent particle-size distributions.

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.

DESIGN OF THE ENTRANCE ION OPTICS FOR SIMS AND LEIS IN SITU MONITORING OF DEPOSITION PROCESSES

Abstract A six-lens-entrance ion optics and its retractable vacuum housing for ion beam analytical instrument was designed. Based on an electrostatic energy analyser and mass spectrometer the instrument will allow us to carry out in situ monitoring of ion beam deposition (IBD) processes and relevant technologies via SIMS, low energy ion scattering (LEIS) and energy analysis of charge-exchange ions. To optimize the entrance optics, the computer code SIMION 4.0 was used.