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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.

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

Publication: J. Vlček, P. Calta, P. Steidl, P. Zeman, R. Čerstvý, J. Houška, J. Kohout: Pulsed reactive magnetron sputtering of high-temperature Si-B-C-N films with high optical transparency. Surf. Coat. Technol., 226 (2013) 34-39.

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: 

J. Kohout, J. Vlček, J. Houška, P. Mareš, R. Čerstvý, P. Zeman, M. Zhang, J. Jiang, E.I. Meletis, Š. Zuzjaková: Hard multifunctional Hf–B–Si–C films prepared by pulsed magnetron sputtering. Surf. Coat. Technol., 257 (2014) 301-307.

M. Zhang, J. Jiang, J. Houška, J. Kohout, J. Vlček, E. I. Meletis: A study of the microstructure evolution of hard Zr–B–C–N films by high-resolution transmission electron microscopy. Acta Mater., 77 (2014) 212-222.

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:

J. Musil, J. Sklenka, R. Čerstvý: Transparent Zr-Al-O oxide coatings with enhanced resistance to cracking. Surf. Coat. Technol., 206 (2012) 8-9. 

J. Musil, J. Sklenka, J. Procházka: Protective over-layer coating preventing cracking of thin films deposited on flexible substrates. Surf. Coat. Technol., 240 (2014) 275-280.

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:

J. Musil, J. Blažek, K. Fajfrlík, R. Čerstvý: Flexible antibacterial Al–Cu–N films.Surf. Coat. Technol., 264 (2015) 114-120.

J. Musil, M. Zítek, K. Fajfrlík, R. Čerstvý: Flexible antibacterial Zr–Cu–N films resistant to cracking, J. Vac. Sci. Technol. A. 34 (2016) 021508.

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:

S. Zenkin, Š. Kos, J. Musil: Hydrophobicity of Thin Films of Compounds of Low-Electronegativity Metals. J. Am. Ceram. Soc., 97 (2014) 2713-2717.

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

Publication: J. Rezek, J. Vlček, J. Houška, R. Čerstvý: High-rate reactive high-power impulse magnetron sputtering of Ta–O–N films with tunable composition and properties. Thin Solid Films, 566 (2014) 70-77.

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. 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, ...)
  • predictions of trends of (mechanical, electrical, ...) properties and thermal stability in a wide range of compositions
  • explanation of the role of implanted Ar and the role of Si in relieving the compressive stress caused by Ar implantation

Publications:

J. Houška, J. Vlček, S. Hřeben, M.M.M. Bilek and D.R. McKenzie: Effect of B and the Si/C ratio on high-temperature stability of novel Si-B-C-N materials. Europhys. Lett. 76 (2006) 512-518.

J. Houška, Š. Kos: SiBCN materials for high-temperature applications: Atomistic origin of electrical conductivity. J. Appl. Phys. 108 (2010) 083711-1/083711-7.

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: 

J. Houška, J. Kohout, P. Mareš, R. Čerstvý, J. Vlček: Dependence of structure and properties of hard nanocrystalline conductive films MBCN (M = Ti, Zr, Hf) on the choice of metal element. Thin Solid Films. 586 (2015) 22-27.

V. Petrman, J. Houška: Trends in formation energies and elastic moduli of ternary and quaternary transition metal nitrides. J. Mater. Sci., 48 (2013) 7642-7651.

Simulation of the atom-by-atom growth of various phases of e.g. TiO2, ZrO2 or Al2O3

  • 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, ...
  • defining pathways for preparation of desired phases, considering different conditions needed for crystal nucleation and for crystal growth
  • development of own interaction potentials - contribution to the methodology of reliable film growth simulations

Publications:

J. Houška: Pathway for a low-temperature deposition of α-Al2O3: a molecular-dynamics study. Surf. Coat. Technol., 235 (2013) 333-341.

J. Houška: Force field for realistic molecular dynamics simulations of ZrO2 growth. Comput. Mater. Sci., 111 (2016) 209-217.

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

Publication: J. Vlček, K. Burcalová: A phenomenological equilibrium model applicable to high-power pulsed magnetron sputtering. Plasma Sources Sci., 19 (2010) 1-12.

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:

T. Kozák, A.D. Pajdarová: A non-stationary model for high power impulse magnetron sputtering discharges. J. Appl. Phys., 110 (2011) 1033031-10330311.

T. Kozák, J. Vlček: Effect of the voltage pulse characteristics on high-power impulse magnetron sputtering of copper. Plasma Sources Sci. Technol., 22 (2013) 015009-1-015009-9. 

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

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