1.Electron beam melting and refining of metals in vacuum
2.Electron beam welding
3.Electron and ion beam lithography
4.Evaluation of intense electron beam quality
5. Deposition and study of thin films
6. New materials and technologies for renewable energy sources
1. Electron beam melting and refining of metals in vacuum
1.1. New materials regeneration through electron beam melting
We continued our experimental and theoretical studies of electron beam melting that is a key method for obtaining new materials for micro- and nano-electronics, for which high purity and quality are demanded. Using 60 kW equipment ELIT-60, electron beam melting of tungsten carbide and W-Mo alloy was performed at different technological regimes (beam power and casting velocity). It was found that for effective e-beam melting of tungsten carbide at comparatively small changes in the proportion tungsten/ carbon, more appropriate are regimes at low e-beam power and temperatures near to the melting temperature of the carbide.
The thermal transfer processes are important for production of metal blocks with good quality (structure, surface and composition). Computer simulation of the heat transfer processes at electron beam melting and refining (EBMR) is a tool for better understanding, studying and control of these processes. The heat exchange at different interfaces between the casting ingot and both the water-cooled crucible and the pulling mechanism is studied. The temperature variations of the thermal conductivity and the heat capacity for the refractory metal Ta and for Cu - low-melting-temperature metal, are estimated and are taken into account in the proposed heat model. Using the heat model, numerical experiments are made at different beam powers and casting velocities for investigation of the heat transfer in Ta and in Cu. Useful for the practice data about geometry of the liquid pool, crystallization front and thermal streams is obtained.
The obtained results show that the heat stream through the bottom of the Cu ingot, for casting velocities up to 6 mm/min, is the main part of the energy losses for the investigated conditions. The lower energy losses through the other ingot boundaries contribute to more efficient EBMR process. These conditions are appropriate to form a flat crystallization front (liquid/solid boundary) which is connected with the quality of the performed pure ingot – this front permits formation of dendrite structures parallel to the ingot axis and uniform impurity displacement toward the ingot top surface. It is shown that flat crystallization front shape for Cu, heated by 20-30 kW, can be obtained for casting velocity up to 6 mm/min. Calculated and experimentally obtained crystallization front forms are compared and a good correspondence is observed.
The calculated data about molten pools, heat flows and liquid/solid boundaries can be used for optimization and control of the EBMR process parameters as well as to improve the quality of the produced pure materials.
A chapter devoted to the EBMR technologies were published in a book on applications of EB technologies, published in English, еds. M.Nemtanu, M.Brasoveanu, publ. Research Signpost/Transworld Research Network.
1.2. Integrated information media for simulation and control of the processes of EBMR
During the year, the development was continued of integrated information media for simulation and control of the processes of EBMR of metals and alloys. The project is implemented by a joint team from IE BAS and the Department of Automation of the University of Chemical Technology and Metallurgy. The GERAM standard referent architecture and UML diagrams for various real situations were presented and discussed. In the cases of EBMR of Ta, Ti and Hf, the data base of the information media includes kinetic coefficients and parameters of relative weight for determination of diffusion or mass transport due to molten metal mixing with impurity components, its evaporation from molten metal surface, and due to chemical reactions.
The practical processing of refractory metals and alloys by EBMR is characterized by a considerable amount of unknown data and parameters. It is known that educated neuron nets can be used as a universal approximation that is characterized by a low sensitivity to errors. On this basis we proposed methodology for development of neuron net models for evaluation of EBMR processing quality. The first models of this type were developed; after education with experimental data, these neuron models were validated by independent experimental data from the EBMR process studied. The results were in good agreement.
The project was implemented with the participation of students of the University of Chemical Technology and Metallurgy (six MS and four BS theses were defended).
2. Electron beam welding
In a book chapter, in a journal article for scientific reviews for mechanical engineering and a presentation, made at the specialized conference on Electron Beam Welding (EBW) in St. Petersburg, Russian Federation, were generalized our before presented results, for applying a thermal model of EBW for heating the sample by a linear moving heat source, as well as the developed in the laboratory and implemented by us statistical approach at prognostication of the geometry characteristics of the obtained welding joints. In the report in St. Petersburg, an expert computer system helping the operators decision making, applicable for personnel education, as well as for computer control and optimization of the technological process was presented. As examples regimes for welding of stainless steel 1Н18NT and Steel 45, based on experimental data sets, are shown and discussed. In other two scientific reports in this field the implementation of artificial neuron networks at prognostication of the EBW results are discussed and some problems at the application of statistical models at the quality improvement of the process by robust methods are also discussed.
The microstructure changes and mechanical properties of heat-treated Al-Si alloys resulting from additional alloying with Fe, Co, Ni, Cr by hybrid electron beam treatment techniques are investigate. These techniques perform an additional alloying in the zone treated, which has a substantial effect on the physical and mechanical properties of the materials processed.
The study of influence of electron beam oscillation patterns travel speed and additional alloying in weld zone on the melting efficiency, weld geometry, structure and mechanical properties welds of Al – Mg alloys are investigate. Electron beam welding process was done to connect details of up to 25 mm in thickness with beam oscillation (perpendicular of weld direction, 45o from weld direction, along of the weld direction and elliptic) using a 15kW Leybold Heraeus welding equipment. Welding without beam oscillation was also done for comparison. Melting efficiency is determined from the measured welding seams cross section for a specified electron beam power and travel speed.
In two chapters of a monograph and a review paper on the thermal processes and quality improvement of electron beam welding (EBW) we presented our conclusion that the geometrical dimensions of the weld during EBW cannot be predicted due to the fact that the spatial and angular distributions of the beam power in the interaction zone are unknown and, therefore, neglected. All known thermal models use a distribution of the thermal source operating in the welding bath that is not based on real data. As a minimum, it does not take in account the distances between the electron gun and the welded work-pieces, as well as the position of the beam focus in relation to the sample surface.
The formulae and graphics developed by us permit exact calculations of the weld parameters at various regimes of EBW of thin materials; we also discussed possible applications of our computer models for evaluation of the beam penetration depth.
Simulation and optimization of the EBW process were achieved through the use of regression models, or the introduction of desirability function in utilizing neuron nets educated on the basis of preliminarily obtained experimental data. Further, we studied models for the weld dimensions average values and dispersion in view of choosing EBW regimes with stable and reproducible results under mass production conditions.
The EBW process was studied of materials having various physical and chemical properties. The shape and quality of EB welds of Al samples of thickness exceeding 25 mm were studied from the point of view of the active zone position relative to the samples welded. It was found that defects, such as pores, cracks and non-uniformity of the weld root, decrease in the case of beam oscillations with amplitude 2 mm and frequency 1000 Hz. A model was proposed with the purpose of investigating the heating and cooling processes during surface modification of steels by a scanning electron beam. The results of the numerical experiments demonstrated that the rates of these processes increase as the sample motion speed (at constant beam power) and the beam power (constant sample motion speed) are raised. Therefore, the efficiency of carbon steels hardening by a scanning electron beam is higher in the case of raising the sample motion speed.
3. Electron and ion beam lithography
Nanoelectronics, nanomaterials and fabrication of nanoelectronic structures with electron lithography
3.1. Nanoelectronics and nanomaterials
During 2011 the second book of the monograph “Nanoelectronics” was published by the Ukrainian Publishing house Avers in the frame of the intergovernmental collaboration project with Ukraine – our partner is the Faculty of Electronics of the Kiev Polytechnic Institute (KPI), and coordinator of the project from Ukrainian side is the first pro-rector of KPI acad. Uri Yakimenko. This book presents discussions on the results of the investigation of carbon nano-materials, nano-structured silicon and silicon composites, the self arrangement of nano-size materials, their properties and applications. Functional devices of photonics, bio- crio- and molecular- electronics, magneto-electronics and spintronics, sensors and mechanical electro-nano-components etc. are also considered. The authors from Bulgarian-Ukrainian team – G. Mladenov and E. Koleva were awarded by the Ministry of Science and Education of Ukraine, The Academy of Pedagogic Sciences of Ukraine and Kiev Chamber of Commerce and Industry with diplomas for exceptional creative contribution in the educational process.
The collaboration with the Hiroshima Institute of Technology (HIT), Japan is also in this field of research. A common Chapter (in the book “Practical Aspects and Applications of Electron Beam Irradiation”) was published as a result of this long term collaboration of which Prof. Takeshi Tanaka is a coordinator from Japan.
3.2. Nanosructuring by electron beam lithography
Nonlinear solubility behavior of polymer resist SAL 110 and of the important negative oligomer resist hydrogen silsesquioxane (HSQ) was investigated. The experimental data are obtained at different conditions of the development process, varying the time of development and the temperature of the used developer solution for thin and tick HSQ resist layer. Peculiarities of HSQ nonlinear development process due to the complicated mechanism of the resist removal from soluble resist areas are considered and hypothesis for explanation of HSQ dissolution process is proposed.
During the year one colleague of our team (Assoc.Prof. Y. Georgiev) was working in Ireland on nanofabrication and particularly on electron beam lithography (EBL) of various structures on silicon (Si) and silicon on insulator (SOI) substrates. Using a Jeol 6000FS EBL system he took part in the development of a new procedure for mix-and-match lithography on small chips (down to 10´10 mm). In this way, a number of Si-nanowire (NW) field effect transistors on SOI substrates, applicable for sensing devices, have been fabricated. The typical NW dimensions are: widths between 50 and 70 nm, height of 70 nm, and length of 10 mm. EBL process for fabrication of very short Si NWs (lengths between 500 nm and 2 mm) connecting two large contact pads was developed. This process has allowed the fabrication of such NWs with homogeneous widths along their whole length due to the proper compensation of the proximity effect. The NW widths of these short NWs were varied in a wide range from about 15 nm up to 105 nm. These structures can further be used for fabrication of suspended NWs and, hence, for advanced beyond CMOS devices.
Utilizing a double layer resist (PMMA bottom layer and HSQ top layer), EBL process was applied for EBL-directed silver-assisted etching of vertical Si NWs. Using this process, arrays of nanopillars have been fabricated in the double layer resist with diameters of 30-35 nm, pitch of 100 nm, and heights of about 150 nm. These arrays can be used as masks for EBL-directed silver-assisted etching of vertical Si NWs for sensing or for new photovoltaic (PV) applications.
In the field of nanolithography by means of electron beams, we studied the nonlinear behavior of polymer and olygomer resists development, instead of the generally accepted linear local development of irradiated or nonirradiated areas of electron resist layers. Based on experimentally obtained dependencies of the development rate on the exposure dose for positive polymer resists or for negative olygomer resist type HSQ and on computer simulation for selected developers and resist thickness, the developed relief structure in the resist layer was predicted. The computer simulation is based on an original time-dependent scheme taking into account the type of developer and its temperature.
The scheme makes use of the multiciphered experimental dependencies of the solubility rate on the exposure dose at various other conditions (time, depth).
Using experimental data obtained in Germany, the maximal aspect ratio (AR) during electron lithography of HSQ and the supercritical resist drying (SRD) using carbon dioxide were evaluated. The results show that the critical dimension of the nanostructures developed is about 1/3 of the dimensions of olygomer or polymer aggregates of the resist studied (for the most frequently used electron resists, this critical dimensions is 20 nm for PMMA and 10 nm for HSQ). The width of the pitch between two nanostructures has also a critical dimension limited by the surface tension. In the case of width values lower than this critical dimension, the surface roughness plays a considerable role. This roughness is connected with the mentioned dimension of the resist material aggregates.
We analyzed our data on creating sub-50 nm conventional metal gates and T and Г-shaped gates for SiGe/Si heterofield-effect transistors. Contrast curves were measured for two-layered resists from PMMA of various molecular weights under specific conditions of exposure and development. For conventional gates of Ti and Pt/Au, we obtained minimal widths of, respectively, 40 nm and 60 nm. For T and Г-shaped gates, in the case of Ti, we obtained 35 − 40 nm at a vertical part of 180 nm and head of 550 and 300 nm, respectively; for Pt/Au these dimensions were: 35 − 40 nm at a vertical part of 160 nm and head of 550 and 400 nm. The real SiGe/Si heterofield effect-transistors with a Pt/Au gate exhibit a higher frequency of 145 GHz at room temperature.
4. Evaluation of intense electron beam quality
The electron beam emittance is chosen as a suitable parameter for standardization of the electron optical technology systems. The evaluation of this parameter is a condition for achieving good quality, repeatability and reproducible performance of electron beam welds. This parameter forms the basis for transferring a concrete technology from one machine to another which will minimize the experimental test volume as well as will extend the capability of the expert systems to choose the process regimes of specific welds. The methods for intense beam emittance evaluation were analyzed. A new method for emittance estimation based on the use of a measuring slit in a non-movable plate, and another method using the changes of the beam focusing current during the beam profiles measurement were developed. The methods are modification of a method proposed earlier by us for emittance determination utilizing three beam profile measurements along the beam axis.
As a practical implementation of the measuring devices we developed two beam-analyzing devices: (i) with one line slit and (ii) with seven radial slits. The modulation plates with these slits are situated at the input of a Faraday cup.
5. Deposition and study of thin films
5.1. Study of thin SiC films prepared by rapid thermal annealing (RTA)
During the year carbon films with thickness of 50 and 300 Ǻ were deposited by RF sputtering at 0.5 Pa pressure in a gas mixture of 30% H2 and Ar atmosphere on p-type (100) silicon substrates. Further, the samples were submitted to RTA in vacuum of 5.10-5 Torr (6.7.10-3Pa) for 3 min in temperature of 800 and 1400°C. Raman spectra of the layers were been taken before and immediately after the RTA. Raman spectra shows peak in the range of 1500-1550 cm-1 characterizing amorphous carbon for sample before RTA. The Raman spectrum of the layer after RTA at 1400°C for 3 min show peaks in the range of 910-1050 cm-1 and 775-850 cm-1 which is indication for existence of hexagonal and cubic SiC polytypes, respectively. Typically, the cubic phase is considerably smaller in quantity. Calculated as a percent with respect to the basic phase (hexagonal SiC) is about 14%. So, we can assume that the cubic SiC phase is distributed in the basic matrix of hexagonal silicon carbide. Raman spectrum of the SiC layer subjected to RTA at 800°C for 3 min shows lower intensity of the peaks, which indicate that both phases increase with increasing the annealing temperature.
Analysis of the electrical cross conductance of the carbon and SiC layers deposited onto silicon substrates were carried out for samples with thickness of 300Ǻ because of their good homogeneity. For as-deposited sample I-V curve shows that it behave as an additional resistance switched in series of the rectifying contact between the carbon layer and the silicon substrate. After the RTA procedure at 1400°C for 3 min, the SiC layer behaves as a semiconductor with a large density of defects.
We have comparative studied non-equilibrium carrier dynamics by light-induced transient grating (LITG) technique in bulk and 3C-SiC layers grown by sublimation epitaxy on 6H-SiC (0001)Si substrate with and without 3C-SiC seed layer and bulk 3C-SiC layer grown by chemical vapour deposition (CVD) on undulant Si substrate.
The results revealed that the low carrier scattering rate in the bulk CVD layer and bulk sublimation layer determined high carrier mobilities, especially at lower temperatures. In the 3C-SiC layer, the decrease of mobility and increase of carrier lifetime with temperature were attributed to the charge state of defects, being dependent on temperature and excitation.
We continued our studies on silicon carbide as it is widely considered to be a strategic semiconductor for high-temperature, high-power, and high-frequency device applications. The most common polytypes of SiC are 4H, 6H, and 3C, the latter being the only cubic phase. As a consequence, it has specific advantages over the hexagonal forms that make it desirable for certain device applications.
We studied the growth of 3C-SiC on 6H-SiC(0001) substrates at source temperature of 2000°C and various temperature gradients (5 − 8°C/mm).
The morphology investigation by optical microscopy in #Fnsmission mode revealed complete 3C-SiC coverage with no 6H-SiC inclusions for a sample grown at temperature gradient of 8°C/mm.
For this sample, the 2-theta-omega scan showed high intensity of the 111 peak which is related with complete 3C-SiC coverage. The AFM image of the same sample showed some additional lines related to overlapping of domains.
Two p-doped 3C-SiC layers grown on 6H-SiC (0001) by means of sublimation epitaxy were studied electrically using C-V, deep level transmission spectroscopy (DLTS) and admittance spectroscopy. The net doping concentration was measured as 3x1017 cm−3 and 1.5x1017 cm−3, respectively, in agreement with the SIMS (secondary ion mass spectroscopy) and LTPL (low temperature photoluminescence) results. It was found that all incorporated Al is electrically activated during the growth. DLTS spectra taken on several contacts of both samples reveal many peaks, which are not repeatedly observed, indicating an inhomogeneous defect distribution in the samples. Furthermore, admittance spectroscopy and DLTS confirmed the unintentional incorporation of boron during the growth. The presence of electrically active extended defects was detected by DLTS. The dependence of the corresponding dips on the filling pulse length is in agreement with the model proposed by Figielski.
We also studied the nonequilibrium carrier dynamics by light-induced transient grating technique in 3C-SiC layers grown by sublimation epitaxy on 6H-SiC, directly and on seed 3C-SiC layer. The results revealed that the use of a seed layer reduces the amount of structural defects and improves slightly the photoelectrical parameters. The temperature dependences of the carrier mobility pointed to the reduced amount of ionized impurities in the seeded layer and the efficient screening of carrier scattering centres in the heteroepitaxial layer.
6. New materials and technologies for renewable energy sources
In 10 scientific reports, presented on 5 scientific forums (3 in Bulgaria, 2 abroad) the tendencies and the problems are analyzed, new solutions and approaches are discussed for solving of the practical problems of future energy power technologies.
Important for our country conclusions are made:
- - The goal Bulgaria to reach 16% extension of the renewable energy sources up to 2020 year is lowered due to a wrong economical concept.
- - At the end of the decade in Bulgarian energy system instabilities will appear and its transformation into an intelligent energy system should be planned earlier - from now.
- - It is not correct, as it is done in our country, to make an accent in the development of the renewable energy sources to be put only on large photovoltaic and wind parks, which present independent investments. Investments should be made in the thermo-mechanical conversion of the solar energy combined in existing or new steam-turbine generators. There are prognoses for the rapid development of the conversion of the thermal energy in the Mediterranean countries (North Africa, the Near East and South Europe) and Bulgaria cannot stay isolated from these energy projects
- - As Scientific priorities in the research and development of new energy sources are indicated: the development of third generation photovoltaic batteries using nanotechnologies and nanomaterials, the transition of energetics and the power engineering to pure solar fuels and mainly hydrogen, the thermo-chemical and photo-chemical conversion of the solar energy into hydrogen, the fuel cells as pure generators of electrical energy and hydrogen and also joining the transport sector to the intelligent energy system
The conclusions made by us are our expert evaluation, which are important for our country, according the appeal of the Chairman of BAS – acad. Sabotinov.
The work preformed and the publications in this area were also a training for scientists from the laboratory on the subjects of the future energy technologies, on which for first time in our country an International symposium “Advanced Solutions in Applied Energy Technologies” was held 19-21.09.2011 in Sofia, organized by Union of Electronics, Electrical Engineering and Telecommunications in Bulgaria (CEEC), Institute of Electronics at Bulgarian Academy of Sciences (IE BAS), Federation of Scientific and Technical Unions in Bulgaria (FNTS), Union of Power Engineers in Bulgaria (CEB), corr. Member of BAS G. Mladenov – Chairman of the Organizing Committee, Prof. K. Vutova and Assoc. Prof. E. Koleva – members of the Organizing Committee. The accents of the symposium were on: the generation and the energy storage, hydrogen energetics and fuel cells, nanomaterials and nanotechnologies in power devices. 11 scientific reports from 7 Bulgarian and 13 foreign scientists were made, 2 Bulgarian, one German companies and the Bulgarian Industrial Cluster for Electric Vehicles we presented.
The design of an automatic control system of the vacuum system of available expensive installation for electron beam welding, evaporation and surface modification of materials is an important first step toward the development of integrated complete control system for the processes of thermal treatment, movement of the electron beam and movement of samples and other components by a manipulator.
The work of the development of the automation control system is performed at the following stages: Detailed description of the actions (sequence, conditions for action) necessary to set the installation into work regime, its stopping and the appearance of emergency or break-down situations (lack of electricity, water etc); description of the available control mechanisms (valves, gauges, pumps, cocks, etc.), which are manually turned on and off; work on the project for automation control system. At the design stage a comparison for different variants for choice of new components (PLC, electrical and mechanical components, valves etc) is made, as well as the possibilities for the available components in the computer-based automation control system. Work is done also on the development of a program for the Siemens Simatic S7-300 programmed controller.
Financed by the National Science Fund
1. DO 02-200 New materials regeneration through electron beam melting and refining of refractory and reactive metals and alloys in vacuum.
2. BIn-5 New materials regeneration through electron beam melting and refining of refractory metal wastes in vacuum, with the Centre for Materials for Electronics technology (C-MET), Hyderabad, India, under Bulgarian-Indian inter-governmental programme of cooperation in science and technology
3. NTS 01-193 New materials, nano-dimensional devices and electron control systems for/or created by beam technologies, with the Kiev Polytechnic Institute (KPI), Ukraine, under Bulgarian-Ukrainian inter-governmental programme of cooperation in science and technology
4. DNTS/Slovakia01/10 “Robust Lithography of Submicron and Nano-dimensional Structures”, with the Institute of Informatics, Slovak Academy of Sciences, under Bulgarian-Slovak inter-governmental programme of cooperation in science and technology.
5. VUI 307 Integrated information medium for modeling and control of the process of electron beam melting and refining of metals
EU, NATO and other international organizations projects
1. Contract No MRTN-CT-2006-035735 Promoting and structuring a multidisciplinary academic-industrial network through the heteropolytype growth, characterization and application of 3C-SiC on hexagonal substrates, Dr. Gabriel Ferro, Coordinator, 2008 – 2010 Financed by the European Community through the Marie Curie RTN MANSiC project
Other scientific institutions
1. Computer simulation of the processes of electron, ion and X-ray irradiation of electronic materials - Hiroshima Institute of Technology, Hiroshima, Japan.
2. Generation and use of intense electron beam created by plasma emitter electron gun - Institute of High Current Electronics, Russian Academy of Sciences, Siberian Branch.
3. “Investigation of nanosystems and new materials by neutron scattering”, with the LNP, JINR, Dubna
Financed by the BAS
1.“Pure metal obtaining, thin film deposition by e-beam evaporation and computer simulation of nano-lithography processes”.
2. “Physical and heat processes at electron beam welding”.