New nanostructured thin-film materials prepared with plasma processing
The main aim of the scientific activity of the research team is the research and development of new nanostructured thin-film materials and of new plasma sources for their preparation and for surface modification of the materials. These materials with exceptionally high application potential and the plasma sources are of essential significance for some exceptionally important branches of industry (flexible electronics, high-temperature electronics and optoelectronics, telecommunication systems, and aerospace, automotive and optical industry), in which they contribute to enhancement of useful properties of products, to the advancement of new technologies, development of new energy sources, reduced energy demands of the equipment, and are also important for environmental technologies and bio or medical applications. To produce a new generation of these materials, the research team solves essential problems in the area of discharge plasma physics, plasma chemistry, thin-film physics, and solid-state physics. The research team uses current or brand new plasma technologies that allow for production of materials with unique physical and functional properties and that are at the same time exceptionally environmentally friendly. The group has experts on preparation of new thin-film materials especially using the method of reactive magnetron sputtering, experts on characterization of structure and properties of the films produced and on their computer simulations, and experts on diagnostics of the non-equilibrium discharge plasma and its computer modeling. These experts take part in research projects that fall into two key areas:
A. New nanostructured thin-film materials
The main aim is the research and development of oxide- and oxynitride-based thin-film materials with unique physical and functional properties. The attention is focused on fundamental aspects of high-rate magnetron deposition of densified stoichiometric oxides for potential applications in microelectronics (high dielectric constant and low leakage current) and optical applications (high index of refraction, controlled electrical conductivity, the thermochromic effect and low extinction coefficient). In case of brand new oxynitride-based materials, the focus is on preparation and characterization of the properties of these materials with continuously tuned elemental composition and structure for potential use as photoactive, electrochromic or biocompatible coatings. Combination of oxide or oxynitride films with precious metals or their nanoclusters is important also in the area of conductometric gas sensors or water splitting under visible light. Therefore attention is also focused on the study of these films from the point of view of their microstructure stabilization and influence of precious metals on the electrical resistance, reactivity and photoactivity.
Another aim in this area is also research into multi-component, multi-layer or graded coatings based on multi-element nitrides, borides, carbides, oxides or metal alloys with controlled architecture. The emphasis is placed on the explanation of the physical basis for the processes leading to the preparation of these films with optimized nanostructure, properties and architecture. Attention is given to complex interactions at grain boundaries of individual phases or at individual layer interfaces. The aim is to prepare multifunctional films with a unique combination of several properties (high hardness, high oxidation resistance and thermal stability, high resistance to cracking, low stress, low wear rate, high optical transparency, high or low electrical and thermal conductivity or anisotropic thermal conductivity, antibacterial activity, etc.).
B. New plasma sources for film deposition and surface modification
The main aim is especially the design, optimization, and clarification of fundamental aspects of the operation of new magnetron systems and methods of various types that are being designed or constructed in the NTIS center laboratories. Examples of the new plasma sources and methods are:
- a new method to control high-power pulsed magnetron sputtering for the deposition of stoichiometric oxide films and oxynitride-based films with continuously tuned elemental composition,
- a new type of magnetron for a fast film deposition based on simultaneous sputtering, sublimation and evaporation of the target cathode,
- a new two-functional magnetron for film deposition and improved cleaning of the substrate surface before the actual deposition.
The research team has in the new building of the NTIS center several top-class laboratories of high international level for intense and systematic research in the area of plasma physics and plasma technologies. It is a total of 14 laboratories with a total area of about 540 m2 equipped mainly with 14 vacuum deposition systems for the preparation of thin-film materials and surface modification in electrical discharges of various kinds, 8 systems for complex discharge plasma diagnostics and 22 modern analytical instruments for complex characterization of the films produced. More in the section Laboratory equipment.
Original results
In recent years the members of the research team have achieved several original results that have led to a further enhanced international prestige of the group. The results obtained are an important contribution to the advancement of knowledge in the area of research into a new type of nanostructured thin-film materials and in the area of new plasma sources for film deposition.
Thermochromic coatings
The coatings are based on materials which exhibit a strong dependence of their electronic structure on the temperature. They are semiconducting below the thermochromic transition temperature, and metallic above it. The closing of the band gap and the subsequent sharp increase of the free charge carrier concentration affects many properties, ranging from the transmittance in the infrared through the electrical conductivity to the thermal conductivity. This leads to numerous applications, including the hottest one: coatings on smart windows which automatically become opaque for the infrared radiation above the desired temperature. This allows one to achieve significant energy savings, at preserved (contrary to other similar coatings) transmittance of the windows in the visible.
Thermochromic coatings based on VO2 and V1-xWxO2
- new technique allowing one to prepare VO2 at exceptionally industry-friendly conditions: onto amorphous glass substrate, at no substrate bias voltage, at a temperature lowered to 300 °C
- control of the thermochromic transition temperature (using doping by tungsten) in a wide range including the room temperature, at preserved values of all other properties
- design and preparation of multilayers combining the thermochromic layer with antireflection layers in order to maximize both the integral visible light transmittance (Tlum) and modulation of the integral solar energy transmittance (ΔTsol)
Publications:
High-temperature coatings
A crucial factor of many industrial sectors is ability to operate at high temperatures in various, more or less, aggressive environments. Hence, new advanced high-temperature materials with heat-resistant capabilities are being extensively developed.
Recently, several groups of high-temperature coatings have been developed and prepared in our laboratories. All coatings were deposited by reactive pulsed dc magnetron co-sputtering at a floating potential and a substrate temperature of 350°C. Using the floating potential (i.e., the ability to prepare well-densified films without any substrate bias) improves the application potential of the coatings due to a simplified deposition process and a decreased ion-induced compressive stress.
Amorphous Si-B-C-N coatings
- high hardness, low electrical and thermal conductivity, low dielectric constant, low thermal expansion coefficient, high optical transparency, low residual stress, smooth surface, very good adhesion to various substrates
- extraordinary thermal stability of the properties and the structure in inert gases (1600°C) and oxidation resistance in air (1500°C)
- potential applications: in high-temperature microelectronics and optoelectronics or in passive protection of aircraft and spacecraft active sensors and systems, optical devices and new materials such as carbon fibers, nanowires, and nanotubes
Nanocrystalline Zr-B-C-N and Hf-B-Si-C(-N) coatings
- very high hardness, high wear resistance, high electrical and thermal conductivity, low residual stress, smooth surface
- high oxidation resistance and high thermal stability of the properties in air (up to 950°C)
- potential applications: in high-temperature harsh-environment capacitive sensors or in active protection of high-speed cutting tools, turbine blades and vanes, wing leading edges and nose cones of hypersonic vehicles
Publications:
Flexible protective coatings
The resistance to cracking in bending or under loading of protective coatings is of vital importance when they are deposited onto flexible materials or onto materials subjected to subsequent forming processes. Therefore, our research team has paid attention to several kinds of flexible protective coatings.
Highly transparent Zr-Al-O and Zr-Si-O coatings
Highly optically transparent inorganic coatings are crucial for the mechanical and chemical protection of soft plastics against scratching and ambient environment. If such coatings are in addition resistant to cracking, they are very attractive for using in flexible display devices.
The coatings based on tough tetragonal zirconia phase stabilized in an amorphous matrix were prepared by reactive pulsed dc magnetron co-sputtering in a dual configuration at a floating potential and a substrate temperature of 500°C.
- high hardness and resistance to cracking in bending and under high-load indentation
- high optical transparency, high refractive index
- potential applications: protection of touch screens and all other plastic products.
Publications:
Antibacterial Al-Cu-N and Zr-Cu-N coatings
Antibacterial coatings, which efficiently kill bacteria on their surfaces, are important in hospital environment or public transportation. The antibacterial coatings should exhibit not only high antibacterial activity but also enhanced resistance to mechanical damages.
The coatings based on hard metal nitride (ZrN, AlN) and metallic copper were prepared by reactive pulsed dc magnetron co-sputtering in a dual configuration at a substrate temperature of 400°C or 450°C.
- high hardness and resistance to cracking in bending and under high-load indentation
- strong antibacterial activity to kill E. Coli bacteria without the need of UV activation
- potential applications: hospital equipment, public transportation, cash and ticket machines, furniture in restaurants, theatres or schools.
Publications:
Hard hydrophobic coatings
Hydrophobicity of materials is desirable for various applications where water repellent surfaces are required. It is often achieved by surface modification, e.g. by attaching organic polymers or, in case of superhydrophobicity, by patterning the surface. Such surface modifications are, however, not suitable for harsh environment applications as polymers decompose chemically and/or thermally and surface patterns are destroyed mechanically. Therefore, there is need for hard materials that are intrinsically hydrophobic. Although most ceramics are hydrophilic, oxides and nitrides of various low-electronegativity metals (such as Nd, Zr, Y, and La) turn out to be hydrophobic hard ceramics. Coatings of these oxides and nitrides were prepared by dc reactive sputtering using an unbalanced magnetron at a floating potential and substrate temperature of 300°C.
- high hardness and scratch resistance
- high water-droplet contact angle without surface modifications
- potential applications: self-cleaning surfaces, surgical tools, turbine blades, ship hulls with low resistance
Publication:
High-rate high power impulse maganetron sputtering of oxides and oxynitrides
A very important parameter for thin-film preparation is the deposition rate, i.e., the rate of thin-film growth. High-rate deposition of thin films with desired properties is the key for prospective industrial applications. Especially in the case of reactive high power impulse magnetron sputtering of dielectric films (reactive HiPIMS), only low deposition rates have been reported so far.
Recently, we have developed our own solution to high-rate reactive HiPIMS. This solution includes (i) an optimization of reactive gas inlet geometry and (ii) a design of pulsed reactive gas flow control. Using this approach we were able to produce densified stoichiometric oxides at very high deposition rates as well as oxynitrides with tunable elemental composition and properties.
High-rate reactive HiPIMS of ZrO2, Ta2O5 and HfO2 coatings
- several times higher deposition rates (up to 120 nm/min for ZrO2, 125 nm/min for Ta2O5 and 345 nm/min for HfO2 films) than reported in the literature
- high density, high optical transparency, high refractive index, low extinction coefficient
- potential applications: optical coatings, dielectric layers, wear resistant coatings
Publications:
High-rate reactive HiPIMS of Ta-O-N coatings
- very high deposition rate (up to 195 nm/min)
- continuously tunable nitrogen and oxygen content in the films in spite of the different reactivity of oxygen and nitrogen gas with tantalum
- continuously tunable optical band gap and electrical resisitivity of the films
- potential applications: visible light photocatalyts for water splitting, semiconductor devices
Computer simulations of the growth, structures and properties of thin-film materials
The experimental research is supported by atomic-scale computer simulations. The simulations allow us to explain the complex relationships between preparation conditions, composition, crystalline or amorphous structure, electronic structure and properties of novel functional materials, to predict which materials will have the desired properties, and to define pathways for the preparation of desired materials. Different state-of-the-art simulation algorithms combined with different levels of theory describing the interatomic interactions (ranging from classical interaction potentials to ab-initio) and implemented in different software packages (LAMMPS, PWscf, CPMD, ...) are being used in this context.
Simulations of amorphous materials, e.g. CNx, Si-B-C-N or M(transition metal)-Si-B-C-N
- explanation of the role of individual constituent elements in the atomic and electronic structures (bonding preferences, coordinations, localization of electronic states, role of Si in relieving the compressive stress caused by Ar implantation, ...)
- predictions of trends of (mechanical, electrical, ...) properties and thermal stability in a wide range of compositions
- prediction of the maximum achievable N content (which is limited by the formation of N2 molecules) as a function of the rest of the composition
Publications:
J. Houska: Maximum N content in a-CNx by ab-initio simulations. Acta Materialia 174 (2019) 189-194.
Simulations of crystalline solid solutions, e.g. M-B-C-N, M1‑M2-N or Al-Cu-O
- predictions of exact crystalline structures: lattice parameters, distribution of atoms and vacancies, number of vacancies
- calculations of total energies and using the energies in order to predict the stability x metastability in a wide range of compositions
- predictions of mechanical properties (bulk modulus, shear modulus, ductility, ...) in a wide range of compositions
Publications:
Simulation of the atom-by-atom growth of various phases of e.g. TiO2, ZrO2, Al2O3 and ZrCu
- disentanglement of crystal nucleation and crystal growth, identification which preparation conditions (atom energies, temperature, Ar bombardment) lead to uninterrupted crystal growth and which lead to amorphization of rutile TiO2, anatase TiO2, α-Al2O3, γ-Al2O3, etc., subsequent defining of pathways for the preparation of desired phases
- detailed characterization of the growing films when they are amorphous, e.g. metallic glasses ZrCu
- development of own interaction potentials - contribution to the methodology of reliable film growth simulations
Publications:
Computer simulation of magnetron discharges
The physics of plasma discharges and plasma-surface interactions is a complex field including several branches of physics such as fluid dynamics, electromagnetics, solid state physics and chemical physics (reaction kinetics). Computer simulations are needed to solve problems arising from practical applications for thin-film deposition. The results are used to support experimental findings and to evaluate variables which cannot be easily measured.
The primary focus of our simulation work is on magnetron sputtering and the deposition of thin films. Recently, we have theoretically studied the processes relevant to high-power impulse magnetron sputtering (HiPIMS) discharges and developed new models for HiPIMS discharges. We use custom-made codes based on the global (volume-averaged) approach and on particle (Monte Carlo) methods.
Phenomenological equilibrium model for HiPIMS
- theoretical analysis of the sputtered particles pathways in a HiPIMS discharge
- explanation of a decrease of power normalized deposition rate in HiPIMS discharges
- evaluation of the probability of ionization and return of sputtered atoms and the degree of ionization of sputtered atoms in the flux onto substrate for specific discharge conditions
Volume-averaged model of HiPIMS
- non-stationary model using (i) ionization zone above the target racetrack and (ii) bulk plasma zone
- calculation of the time evolution of the spatially averaged densities of discharge species (including sputtered atoms and ions) and their fluxes onto the target and substrate
- evaluation and explanation of qualitative relations between the discharge and deposition characteristics and process input parameters
- validation of theoretical models for discharge processes (e.g. secondary electron emission, transport of sputtered atoms, etc.) when supported with experimentally measured discharge characteristics
Publications:
Parametric model for reactive HiPIMS
- non-stationary model of the target and substrate processes during HiPIMS in a reactive atmosphere, including a parametric description of discharge processes (reactive gas ionization and dissociation, ionization and return of sputtered atoms onto the target, gas rarefaction)
- fast evaluation of the target and substrate state during pulses for given process input parameters (target power density, pulse length, oxygen partial pressure, etc.)
- explanation of the correlations between process parameters and deposition characteristics, such as deposition rate and film composition
