Browsing by Author "Vutova K."
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Item Application of electron beam melting method for recycling of tantalum scrap(2024-10-01) Vutova K.; Stefanova V.; Iliev P.The paper presents a study on the refining efficiency of tantalum scrap when applying the electron beam melting method to recycle tantalum material. Thermodynamic analysis and experimental tests were carried out at different technological regimes to determine the optimal conditions for obtaining tantalum with high purity. The behavior of the controlled impurities (W, Nb, Mn, Ce, Cr, Mo, Al, Fe, etc) and the influence of the thermodynamic and kinetic technological parameters (temperature, melting time) on the refining efficiency of the studied material were investigated. At a temperature of 3700 K and a melting time of 10 min, the highest overall degree of refinement of 98.82% and the highest tantalum purity of 99.987% were achieved.Item Behaviour of Impurities during Electron Beam Melting of Copper Technogenic Material(2022-02-01) Vutova K.; Stefanova V.; Vassileva V.; Kadiyski M.The current study presents the electron beam melting (EBM) efficiency of copper techno-genic material with high impurity content (Se, Te, Pb, Bi, Sn, As, Sb, Zn, Ni, Ag, etc.) by means of thermodynamic analysis and experimental tests. On the basis of the calculated values of Gibbs free energy and the physical state of the impurity (liquid and gaseous), a thermodynamic assessment of the possible chemical interactions occurring in the Cu-Cu2O-Mex system in vacuum in the temperature range 1460–1800 K was made. The impact of the kinetic parameters (temperature and refining time) on the behaviour and the degree of removal of impurities was evaluated. Chemical and metallographic analysis of the obtained ingots is also discussed.Item Comparative study of the sidewall shape and proximity effect of the AR-N 7520 negative electron beam resist(2022-04-04) Kostic I.; Vutova K.; Koleva E.; Bencurova A.; Konecnikova A.; Andok R.This paper deals with the performance of the AR-N7520 (Allresist) negative electron beam resist (EB resist) which was selected as an etch mask for the fabrication of gratings on GaAs substrates. The developed resist sidewall shape is crucial for this purpose. The required near-to-vertical sidewall shape can be achieved by optimizing the electron beam lithography (EBL) process based on experimental investigations and computer simulations. The sidewall shape dependence on the EBL parameters (exposure dose, resist pattern, etc.) and the proximity effect are studied.Item Dependence of PMMA electron beam resist sidewall shape on exposure dose and resist thickness(2021-11-02) Andok R.; Vutova K.; Koleva E.; Bencurova A.; Kostic I.The influence of electron beam lithography parameters (such as electron energy, resist thickness, the exposure dose) on the resist sidewall shape (profile) was studied for the PMMA (polymethyl-methacrylate) positive resist. The profile of the positive tone resist PMMA was investigated depending on varying exposure doses for the resist thicknesses 600 and 1300 nm, and electron energy 30 keV. Simulation results based on measurements along the resist profile depth are presented and discussed. The results obtained contribute to the knowledge on electron scattering in resist/substrate in electron beam lithography for the case of field emission cathode and Gaussian intensity distribution, and to the development and approval of models for the prediction and precise control of resist profiles in thick PMMA layers for 3D proximity effect simulation, the bilayer resist system, and the lift-off method.Item Effect of electron beam method on processing of titanium technogenic material(2019-06-01) Vutova K.; Vassileva V.; Stefanova V.; Amalnerkar D.; Tanaka T.This study reveals the efficiency of the electron beam processing of titanium technogenic material with a high level of impurities and the quality of the obtained metal in correlation to process parameters which are discussed. The influence of the beam power and melting time on the composition variation, morphologies, hardness of metal samples and mass losses is investigated. Based on the different technological parameters, the removal efficiency of impurities is also discussed, and the corresponding experiments are carried out in order to make a comparison. Different thermal process conditions are realized during the single-melt operation. Chemical and metallographic analyses are performed, and the results are discussed. The hardness of the titanium decreases by prolonging the time of the electron beam processing. A maximal overall removal efficiency of 99.975% is seen at 5.5 kW beam power for a 40 min melting time and the best purification of Ti (99.996%) is achieved.Item Effects of process parameters on electron beam melting technogenic materials for obtaining rare metals(2018-10-19) Vutova K.; Vassileva V.; Koleva E.; Stefanova V.; Amalnerkar D.In this paper, effects of electron beam melting process parameters (e-beam power, refining time, etc.) and the number of melting operations for fulfilling the requirements concerning the composition and the structure of metals obtained after e-beam processing are studied. Results obtained at electron beam melting of molybdenum and tungsten technogenic materials are presented and discussed. The refining efficiency is evaluated and effective technological regimes (process parameters) for production of metals which meet specific requirements for high-purity and good-quality structure after electron beam melting of Mo and W technogenic materials are proposed.Item Electron beam melting efficiency at multiple hafnium e-beam processing(2024-01-01) Markov M.; Stefanova V.; Vutova K.; Vassileva V.; Tanaka T.; Kakugawa K.The method of electron beam melting and vacuum refining has clear advantages over other metallurgical methods since it enables manufacturing of refractory and chemically active metals. This study focuses on the efficiency of removing impurities from technogenic hafnium under multiple electron beam melting. Assessments are performed on the efficiency of double and triple e-beam melting processing of refractory metal hafnium. The influence of different e-beam melting technological modes on the refining effectiveness is investigated. A highest hafnium purity of 99.2% was achieved after double and triple e-beam refinements of the investigated materials, with the highest process efficiency reaching 61.58% and 51.07%, respectively.Item Heat distribution simulation in electron-beam surface modification of 316L stainless steel samples(2020-06-02) Koleva E.; Tsonevska T.; Koleva L.; Vutova K.; Mladenov G.; Naplatanova M.; Trushnikov D.; Varushkin S.A time-dependent heat model is implemented for the simulation of the heat distribution in 316L stainless steel samples during electron-beam irradiation. The influence is studied of the electro-beam power and processing time on the temperature field produced in the samples. The simulation results are compared with real experimental data of electron-beam surface modification of 0.5-mm-thick 316L stainless steel samples using the ELIT-60 installation in the Institute of Electronics BAS. The simulation and the experimental results show good agreement.Item Investigation of tantalum recycling by electron beam melting(2016-11-21) Vutova K.; Vassileva V.; Koleva E.; Munirathnam N.; Amalnerkar D.P.; Tanaka T.Investigations are carried out and obtained experimental and theoretical data for tantalum scrap recycling by electron beam melting (EBM) is presented in this paper. Different thermal treatment process conditions are realized and results are discussed. A chemical analysis is performed and refining mechanisms for electron beam (EB) refining of Ta are discussed. For the performed experiments the best purification of Ta (99.96) is obtained at 21.6 kW beam power for a melting time of 3 min. A statistical approach is applied for estimation of the material losses and the liquid pool characteristics based on experimentally-obtained data. The aim is to improve the EBM and choosing optimal process conditions, depending on the concrete characteristic requirements. Model-based quality optimization of electron beam melting and refining (EBMR) processes of Ta is considered related to the optimization of the molten pool parameters, connected to the occurring refining processes, and to minimal material losses. Optimization of the process of EBM of Ta is based on overall criteria, giving compromised solutions, depending on the requirements concerning the quality of the performed products. The accumulated data, the obtained results, and the optimization statistical approach allow us to formulate requirements on the process parameters.Item Model-based multicriterial optimization of electron beam melting and refining of copper(2022-04-04) Koleva E.; Koleva L.; Vassileva V.; Vutova K.; Tsonevska T.Experimental results on the quality of electron beam melting and refining of copper samples by implementing the ``method of washers``are analyzed by using an empirical model estimation approach. The influence is studied of the process parameters - melting power and time of refining, on the residual content of impurities, the refining rate, the overall removal efficiency and the material losses. A graphical user interface is designed and developed for calculation, investigation and multicriterial optimization of the quality of the refined copper samples.Item Modeling approaches for electron beam lithography(2018-10-19) Koleva E.; Vutova K.; Asparuhova B.; Kostic I.; Cvetkov K.; Gerasimov V.In this paper, a study based on the mathematical modelling, applying different process simulation tools (CASINO, TREM, SELID) for characterization of PMMA resist and for the improvement of the resolution concerning the critical dimensions of nano-patterning by electron beam lithography (EBL) is presented. Data for important EBL characteristics (energy deposition function, proximity effect parameters, solubility rate, etc.) are obtained by applying different approaches (Monte Carlo methods, regression models, etc.).Item Optimisation criteria for the process electron beam lithography of negative AR-N7520 resists(2023-01-01) Koleva E.; Kostic I.; Andok R.; Vutova K.; Bencurova A.; Konecnikova A.Experimental investigation of negative electron resist AR-N 7520 profiles using an electron beam lithography system ZBA23 (Raith) is performed at variation of the exposure doses and the exposure patterns. The form of the obtained after the exposure resist profiles is investigated and optimized. Artificial neural networks for the dependence of the overall geometry of the obtained resist profiles on process parameters are trained, tested and validated. Several overall geometry quality criteria for the shape of the developed resist profile cross-sections are defined. An approach, based on the defined overall quality characteristics and multicriterial parameter optimization, is proposed and implemented for fulfillment of the technological requirements for the produced resist profile dimensions.Item Patterning of structures by e-beam lithography and ion etching for gas sensor applications(2014-01-01) Durina P.; Bencurova A.; Konecnikova A.; Kostic I.; Vutova K.; Koleva E.; Mladenov G.; Kus P.; Plecenik A.This work deals with a method of preparation of nanometer structures for a gas detector based on e-beam lithography and ion etching of a thin TiO2 film. The aim was the fabrication of a gas sensor with meander or comb structures of nanometer dimensions. This is of importance since both the size of the contacts and the layer thickness affect the sensor's sensitivity.Item PMMA resist profile and proximity effect dependence on the electron-beam lithography process parameters(2020-06-02) Kostic I.; Vutova K.; Koleva E.; Bencurova A.The study reveals the influence of the electron-beam lithography parameters (such as the exposure dose, resist thickness, depth) on the resist profile shape in the case of the PMMA (polymethyl-methacrylate) positive resist. The experiments are performed using an Elphy Quantum (Raith) e-beam lithography control system installed on an Inspect F50 (FEI) scanning electron microscope with a field emission cathode and a Gaussian intensity distribution. Profiles developed in the PMMA using the MIBK:IPA 1:3 developer and simulation results based on measurements along the resist profile depth for the case of 30-keV electron energy are presented and discussed. The results contribute to the knowledge on electron scattering in the resist/substrate in electron-beam lithography and assist in the development and approval of simulation tools for prediction and control of resist profiles in thick PMMA layers for lift-off nanopatterning.Item Recycling of Technogenic CoCrMo Alloy by Electron Beam Melting(2022-06-01) Vutova K.; Stefanova V.; Vassileva V.; Atanasova-Vladimirova S.In the current work, the possibility of the recycling of technogenic CoCrMo material by electron beam melting is investigated. The influence of thermodynamic and kinetic parameters (temperature and melting time) on the behavior of the main components of the alloy (Co, Cr, and Mo) and other elements (Fe, Mn, Si, W, and Nb) present in it, and on the microstructure of the ingots obtained after e-beam processing is studied. The vapor pressure of the alloy is determined taking into account the activities of the main alloy components (Co, Cr, and Mo). The relative volatility of the metal elements present in the alloy was also evaluated. An assessment of the influence of the temperature and the retention time on the degree of elements removal from CoCrMo technogenic material was made. The results obtained show that the highest degree of refining is achieved at 1860 K and a residence time of 20 min. The conducted EDS analysis of the more characteristic phases observed on the SEM images of the samples shows distinct micro-segregation in the matrix composition.Item Simulation and experimental study on developed profiles in the positive polymer resist PMMA(2018-10-19) Koleva E.; Vutova K.; Kostic I.Theoretical and experimental study on developed profiles in the positive tone PMMA (polymethyl-methacrylate) resist and e-beam lithography process parameters was performed. E-beam lithography control system Elphy Quantum (Raith) installed on Scanning Electron Microscope Quanta FEG (FEI) with a field emission cathode and Gaussian intensity distribution has been used for the conducted experiments. Simulation results obtained by different models concerning the geometry of the developed resist profiles in PMMA were compared and verified with experimental data for improvement characterization of PMMA resist. The estimated models presented are useful for prediction and optimization of the developed resist profiles at electron beam lithography.Item Study and comparison of resist characteristics for different negative tone electron beam resists(2023-01-01) Andok R.; Vutova K.; Bencurova A.; Kostic I.; Koleva E.In this work, four types of negative electron beam resists are investigated. Electron beam lithography (EBL) experiments were conducted using EBL system ZBA23 (Raith) with the variable-shaped electron beam cross-section at 40 keV electron energy. Important electron beam resist characteristics such as sensitivity, dissolution rate, aspect ratio and sidewall developed profiles in the chemically amplified resist (CAR) SU-8 2000, non-CARs ma-N 2410 and ARN-7520, and inorganic negative resist HSQ XR-1514 are studied and compared. This study was motivated by the selection of a suitable resist for practical use such as large area gratings fabrication for optoelectronics.Item Study of the new CSAR62 positive tone electron-beam resist at 40 keV electron energy(2016-04-07) Andok R.; Bencurova A.; Vutova K.; Koleva E.; Nemec P.; Hrkut P.; Kostic I.; Mladenov G.One of the few «top-down» methods for nano-device fabrication is the electron-beam lithography, which allows flexible patterning of various structures with a nanoscale resolution down to less than 10 nm. Thinner, more etching durable, and more sensitive e-beam resists are required for the better control, linearity, and uniformity of critical dimensions of structures for nano-device fabrication. Within the last decade, researchers have made significant efforts to improve the resolution of the nanoscale e-beam lithography. The resist material properties are an important factor governing the resolution. Only the e-beam resist ZEP 520 of the Japanese manufacturer ZEON is characterized by relatively good properties and thus meets most users' expectations. This paper deals with the investigation and simulation of the characteristics of the new less-expensive AR-P 6200 (CSAR 62) positive e-beam resist (available since May 2013, manufactured by Allresist GmbH company).Item Study of the Possibility of Recycling of Technogenic Hafnium during Electron Beam Refining(2022-12-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The possibility of removing metallic (such as Zr, Fe, Cr, and Zn) impurities and non-metallic (such as [O] and C) impurities from technogenic hafnium through single and double refining in the conditions of electron beam melting (EBM) has been studied. The influence of thermodynamic and kinetic parameters on the degree of removal of these impurities from the base metal under vacuum conditions and within a temperature interval of 2500 K to 3100 K is defined. The relative volatility of metal impurities and the stability of the oxides and carbides present in the base metal are evaluated. The possibility for complete removal of Fe, Cr, Zn, [O], and C during EBM is shown. In the case of double refining, at a temperature of 2700 K for 20 min, the maximum degree of removal of Zr is 46.8%, the achieved highest hafnium purity is 99.004%, and the overall effectiveness of the refining of hafnium from impurities is 53%. There is a correlation between the degree of removal of Zr and the micro-hardness of the Hf ingots obtained after EBM. The weight losses vary in the ranges of 1.5–5.8% and 1–8% under the studied single and double refining processes, respectively.Item Study on Hardness of Heat-Treated CoCrMo Alloy Recycled by Electron Beam Melting(2023-04-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The hardness of heat (thermally) treated CoCrMo ingots, recycled by electron beam melting and refining (EBMR) of a technogenic CoCrMo material (waste from the dental technology) under different process conditions (temperature and residence time) is examined. The heat treatment consists of two-step heating up to temperatures of 423 K and 1343 K and retention times of 40 and 60 min, respectively. The influence of various loads (0.98 N, 1.96 N, 2.94 N, 4.9 N, and 9.8 N) on the hardness of the CoCrMo alloy, recycled by EBMR, before and after heat treatment is studied. It has been found that regardless of the EBMR process conditions, the obtained samples after heat treatment have similar hardness values (between 494.2 HV and 505.9 HV) and they are significantly lower than the hardness of the specimens before the heat treatment. The highest hardness (600 HV) is measured in the alloy recycled at 1845 K refining temperature for 20 min. This is due to the smaller crystal structure of the resulting alloy and the higher cobalt content. The results obtained show that the heat treatment leads to considerable changes in the microstructure of the CoCrMo ingots recycled by EBMR. With the increase of the e-beam refining temperature, after the heat treatment, the grains’ size increases and the grains’ shape indicates an incomplete phase transition from γ-fcc to ε-hcp phase. This leads to a slight increase in the hardness of the alloy.