Rafael Alvarez

• PhD
• PostDoc Position at Materials Science Institute of Seville

The morphology of numerous nanocolumnar thin films deposited by the magnetron sputtering technique at oblique geometries and at relatively low temperatures has been analyzed for materials as different as Au, Pt, Ti, Cr, TiO2, Al, HfN, Mo, V, WO3 and W. Despite similar deposition conditions, two characteristic nanostructures have been identified dep...

The morphology and void connectivity of thin films grown by a magnetron sputtering deposition technique at oblique geometries were studied in this paper. A well-tested thin film growth model was employed to assess the features of these layers along with experimental data taken from the literature. A strong variation in the film morphology and pore...

The low temperature formation of monoclinic VO2 crystal domains in nanocolumnar vanadium/oxygen thin films prepared by magnetron sputtering at oblique angles is analyzed. The synthesis procedure involved the deposition of amorphous nanocolumnar VO1.9 thin films at room temperature and its subsequent annealing at temperatures between 250ºC and 330ºC...

It is demonstrated that, besides classical nanocolumnar arrays, the oblique angle geometry induces the growth of singular structures in the nanoscale when using wisely designed patterned substrates. Well-ordered array of crosses, cylindrical nanorods or hole structures arranged in square or hexagonal regular geometries are reported as examples, amo...

We demonstrate the existence of stoichiometric variations at the nanoscale when growing nanocolumnar SiOx thin films by reactive magnetron sputtering deposition at oblique angles. Results show stoichiometric variations in the range 0.3 < x < 1.3 when growing a SiO0.5 thin film. This agrees with results from a numerical growth model that obtains a s...

Nanostructuration and 2D patterning of thin films are common strategies to fabricate biomimetic surfaces and components for microfluidics, microelectronics or photonic applications. This work presents the fundamentals of a surface nanotechnology procedure for laterally tailoring the nanostructure and crystalline structure of thin films that are pla...

Maximizing the electrochromic response of tungsten oxide-based systems demands highly porous electrode layers that facilitate the incorporation of electrolyte cations during the reduction process. In this work, amorphous and porous WO3 thin films were grown on indium tin dioxide glass substrates by magnetron sputtering at oblique angles at two diff...

The scientific interest in the growth of nanostructured surfaces and thin films by means of physical vapor deposition (PVD) techniques has undoubtedly increased in the last decade [...]

The operational behavior of packed‐bed plasma reactors depends on the dimension, shape, and chemical properties of the pellets used as moderators, but little information exists about the influence of their specific dielectric properties. Herein, we comparatively study the electrical behavior of a packed‐bed reactor filled with pellets of either die...

We report on the morphological disparity of the columnar growth in W thin films sputter-deposited by oblique angle deposition. Oriented tungsten thin films (400 ± 50 nm thick) are prepared using a tilt angle α of 80° and a sputtering pressure of 0.25 Pa. Inclined columns (β = 38 ± 2°) are produced and the microstructure is observed by scanning elec...

The nano-porosity embedded into the tilted and separated nanocolumns characteristic of the microstructure of evaporated thin films at oblique angles has been critically assessed by various variants of the positron annihilation spectroscopy. This technique represents a powerful tool for the analysis of porosity, defects and internal interfaces of ma...

We experimentally analyze different growth regimes of Ti thin films associated to the existence of kinetic energy-induced relaxation mechanisms in the material's network when operating at oblique geometries. For this purpose, we have deposited different films by evaporation and magnetron sputtering under similar geometrical arrangements and at room...

Based on an already tested laboratory procedure, a new magnetron sputtering methodology to simultaneously coat two-sides of large area implants (up to ~15 cm2) with Ti nanocolumns in industrial reactors has been developed. By analyzing the required growth conditions in a laboratory setup, a new geometry and methodology have been proposed and tested...

Traditionally porous silicon based photonic structures have been prepared by electrochemically etching of silicon. In this work, porous multilayers of nanocolumnar SiOx and SiO2 thin films acting as near infrared (NIR) 1D-photonic nanostructures are prepared by magnetron sputtering deposition at oblique angles (MS-OA). Simultaneous control of poros...

Although thin film porosity is the basis of many optical sensors, it can be deleterious for a stable optical behavior of passive optical elements due to the condensation of water and other vapors in their pores. This paper proposes a new strategy for the magnetron sputtering (MS) fabrication of environmentally tight SiO2–TiO2 porous multilayers. Th...

In this work we analyze a phenomenon that takes place when growing magnetron sputtered porous/compact multilayer systems by alternating the oblique angle and the classical configuration geometries. We show that the compact layers develop numerous fissures rooted in the porous structures of the film below, in a phenomenon that amplifies when increas...

The selective incorporation of deposition species with preferential directionality is analyzed during the growth of TiO2 thin films by magnetron sputtering. Using wisely-designed collimators, tilted nanocolumnar morphologies are grown in a ballistic deposition regime, i.e. when most deposition species arrive at the film surface along well-defined p...

This paper reports a novel cholesterol biosensor based on nanostructured platinum (Pt) thin films prepared by Magnetron Sputtering (MS) in an oblique angle (OAD) configuration. Pt thin films were deposited onto a gold screen-printed electrode and characterized using Rutherford Back Scattering (RBS), Scanning Electron Microscopy (SEM), X-ray Diffrac...

The fabrication of black-gold coatings using sputtering is reported here. Glancing angle deposition with a rotating substrate is needed to obtain vertical nanostructures. Enhanced light absorption is obtained in the samples prepared in the ballistic regime with high tilt angles. Under these conditions the diameter distribution of the nanostructures...

Front Cover: The Authors present a technique to grow thin films with composition raging from pure metallic to full stoichiometric by magnetron sputtering, using the oblique angle configuration. The film stoichiometry is tuned by tilting the substrate with respect to the target, and using low amounts of reactive gases. This method has been tested by...

Target poisoning in reactive magnetron sputtering deposition of thin films is an undesired phenomenon, well known for causing a drastic fall of the process efficiency. We demonstrate that when this technique is operated at oblique angles, films with composition raging from pure metallic to stoichiometric compound can be grown in non-poisoned condit...

Porous thin films grown at oblique angles by evaporation techniques are formed by tilted nanocolumnar structures which, depending on the material type and growth conditions, associate along certain preferential directions, giving rise to large domains. This arrangement, commonly denoted as bundling association, is investigated in the present work b...

The deposition of SiOx () compound thin films by the reactive magnetron sputtering technique at oblique angles is studied from both theoretical and experimental points of view. A simple mathematical formula that links the film stoichiometry and the deposition conditions is deduced. Numerous experiments have been carried out to test this formula at...

The growth of Ti thin films by the magnetron sputtering technique at oblique angles and at room temperature is analysed from both experimental and theoretical points of view. Unlike other materials deposited in similar conditions, the nanostructure development of the Ti layers exhibits an anomalous behaviour when varying both the angle of incidence...

The growth of nanoporous layers by plasma-assisted deposition techniques is strongly mediated by the ion fluxes in the reactor. To analyze their influence we have deposited different nanostructured thin films by the magnetron sputtering technique at oblique angles, modulating the ion fluxes in the plasma by tuning the frequency of the electromagnet...

Bacterial colonization and biofilm formation on orthopedic implants is one of the worst scenarios in orthopedic surgery, in terms of both patient prognosis and healthcare costs. Tailoring the surfaces of implants at the nanoscale to actively promote bone bonding while avoiding bacterial colonization represents an interesting challenge to achieving...

In the last few years, several papers have appeared showing the capabilities of electron Rutherford backscattering spectrometry (eRBS) to quantify the H content at surfaces. The basis of the H detection in this technique relies on the difference in recoil energy of the incident electrons depending on the mass of the atoms located at the surface tha...

The formation of the porous structure in dc magnetron sputtered amorphous silicon thin films at low temperatures is studied when using helium and/or argon as the processing gas. In each case, a-Si thin films were simultaneously grown at two different locations in the reactor which led to the assembly of different porous structures. The set of four...

The growth of nanostructured physical vapor deposited thin films at oblique angles is becoming a hot topic for the development of a large variety of applications. Up to now, empirical relations, such as the so-called tangent rule, have been uncritically applied to account for the development of the nanostructure of these thin films even when they d...

We describe here the deposition of thin films using magnetron sputtering at oblique angles. General relations between the deposition rates of the films and experimental parameters, such as gas pressure or substrate tilt angles, are deduced and experimentally tested. The model also permits the direct determination of the thermalization mean free pat...

The framework to describe the out-of-equilibrium free electrons in cold plasmas is developed assuming the electron entropy is defined through the Boltzmann H-theorem. Our theory explains why the Saha-Boltzmann relation among higher-lying excited states by means of the electron kinetic temperature is fulfilled, even when free electrons are far from...

The growth of ultra-smooth amorphous thin films induced by low-energy (below 1 keV) ion-assistance processes is studied. The relative contribution of ion-induced smoothening effects is analysed by means of a Monte Carlo model and experimental data. In general, highly rough granular or ultra-smooth (with roughness below one monolayer) films are prod...

We have deposited TiO2 and SiO2 thin films by techniques as different as plasma-enhanced chemical vapor deposition (PECVD) and reactive magnetron sputtering under experimental conditions where highly directional deposition fluxes are avoided. The results indicate that whatever the deposition technique employed or even the precursor gas in the PECVD...

Growth regimes of gold thin films deposited by magnetron sputtering at oblique angles and low temperatures are studied from both theoretical and experimental points of view. Thin films were deposited in a broad range of experimental conditions by varying the substrate tilt angle and background pressure, and were analyzed by field emission scanning...

This article aims toward a full description of the wetting conversion from superhydrophobicity to superhydrophilicity under illumination with UV light of high-density ZnO nanorods surfaces by (i) following the evolution of the clusters and superstructures formed by the nanocarpet effect as a function of the water contact angle (WCA); (ii) character...

Growth of amorphous SiO2 thin films deposited by reactive magnetron sputtering at low temperatures has been studied under different oxygen partial pressure conditions. Film microstructures varied from coalescent vertical column-like to homogeneous compact microstructures, possessing all similar refractive indexes. A discussion on the process respon...

This commentary addresses the problem of determining surface roughness values and their use to assess the wetting behavior of surfaces. For very rough surfaces it is shown that depending on the observation scale by atomic force microscopy (AFM) quite different RMS roughness values can be obtained and that only the values taken at saturation can be...

The microstructural features of amorphous TiO2 thin films grown by the electron beam physical vapour deposition technique at oblique angles have been experimentally and theoretically studied. The microstructural features of the deposited films were characterized by considering both the column tilt angle and the increase in the column thickness with...

We show that the tilt angle of nanostructures obtained by glancing angle sputtering is finely tuned by selecting the adequate argon pressure. At low pressures, a ballistic deposition regime dominates, yielding high directional atoms that form tilted nanocolumns. High pressures lead to a diffusive regime which gives rise to vertical columnar growth....

The influence of isotropically directed deposition flux on the formation of the thin film microstructure at low temperatures is studied. For this purpose we have deposited TiO2 thin films by two different deposition techniques: reactive magnetron sputtering, in two different experimental configurations, and plasma enhanced chemical vapor deposition...

Morphological evolution of ZrO ₂ thin films deposited during pulsed laser deposition of Zr in O ₂ atmosphere has been experimentally studied at two different film deposition temperatures, 300 and 873 K. The roughness exponent, α , the growth exponent, β , the coarsening exponent, 1/z , and the exponent defining the evolution o...

This paper presents calculations of electromagnetic field distributions with different microwave-driven devices. Results are obtained by using a two-dimensional numerical simulation tool that solves Maxwell's equations for an axis-symmetric TM mode, by considering a harmonic time description. The tool is applied to a coaxial waveguide, a circular w...

This paper presents simulation results for a microwave (2.45GHz) plasma reactor, operated by an axial injection torch (AIT). The study gives a two-dimensional description of the AIT-reactor system, based on an electromagnetic model (that solves Maxwell's equations adopting a time-harmonic description, yielding the distribution of the electromagneti...

This paper presents simulation results for a microwave plasma torch (MPT, at 2.45 GHz). The particular device under study couples the MPT (connected to a coaxial waveguide) to a cylindrical reactor chamber, where it produces helium plasma at atmospheric pressure. The study gives a 2D description of the MPT-reactor system, based on an electromagneti...

This work deals with the two-dimensional electromagnetic modelling of a microwave-driven plasma reactor, operated by an axial injection torch (AIT). The model solves Maxwell's equations, which are discretized using a finite difference scheme within staggered grids, adopting a time-harmonic description at fixed 2.45 GHz excitation frequency. The stu...

The study of the electromagnetic (em) field with a microwave-driven reactor excited by an axial injection torch (TIA) provides direct information about the power coupled to the device. A numerical simulation tool has been developed to calculate the 2-D em fields with a TIA-reactor system in the presence of plasma. The field images are used to monit...

This work reports simulation results for a microwave (2.45 GHz) plasma reactor, operated by an axial injection torch (TIA). The study is based on a two-dimensional electromagnetic (EM) and hydrodynamic description of the TIA-reactor system. The EM model [1] solves Maxwell's equations, yielding the distribution of the EM fields and the averaged powe...

This paper presents a two-dimensional electromagnetic model for a microwave (2.45 GHz) plasma reactor operated by an axial injection torch. The model solves Maxwell’s equations, adopting a harmonic time description and considering the collision dispersion features of the plasma. Perfect-conductor boundary conditions are satisfied at the reactor wal...

This work reports simulation results for a microwave (2.45 GHz) plasma reactor, operated by an axial injection torch (TIA). The study gives a two-dimensional description of the TIA-reactor system, based on an electromagnetic model (that solves Maxwell's equations adopting a harmonic time description, yielding the distribution of the electromagnetic...

This work models the 2D electromagnetic field distribution, produced within a metallic reactor by a microwave driven Axial Injection Plasma Torch. The coaxial torch imposes a TEM mode to excite the device, which leads to the development of a symmetric TM mode inside the cylindrical reactor. Maxwell's equations were discretized on a variable grid, f...

The use of the Stark broadening of Balmer lines spontaneously emitted by atmospheric-pressure plasmas as a method to determine both the electron density and temperature in high-pressure plasmas is discussed in this paper. This method is applied to argon and helium plasmas produced in microwave discharges. Especially for Ar plasmas, valuable and rel...

The population density of several excited states has been obtained spectroscopically in a helium plasma sustained by a torch device at atmospheric pressure as a function of the radius in the plasma for different conditions of microwave power and plasma gas flow. The ground-state atom density is determined from the gas temperature, which is deduced...

In this paper, the radial distributions of emissivity of the species present in the helium–nitrogen, helium–oxygen and helium–argon plasmas sustained by the axial injection torch at atmospheric pressure are studied by optical emission spectroscopy using the Abel inversion procedure. The spectrum was scanned for atomic and ionic lines together with...

The effect of the gas flow-rate on the radial structure of a torch-like helium microwave plasma has been studied by measuring the radial distributions of emission intensity of spectral lines from helium and from the air that surrounds the discharge. It has been found that increasing the gas flow-rate leads to an increase of the maximum of the spect...

The radial dependence of the electron density and the gas temperature, two major parameters of the microwave helium plasma produced at atmospheric pressure by the axial injection torch, is presented here. An intensified charge coupled device in combination with an image rotation system allowed to perform laterally resolved emission intensity measur...

In this work the radial distributions of the emission coefčient of some atomic and ionic argon lines in the thin helium plasma (∼1 mm diameter) produced by the axial injection torch are presented as a function of the supplied microwave power and the argon concentration. Radial distributions were obtained using an Abel inversion method on laterally...

A helium microwave plasma torch was studied and optimised as a destruction system of volatile organic compounds. Attention was focused on trichloroethylene as a prototypical volatile organic compound, which is used technologically and which poses known health risks. The dependence of the destruction efficiency on the plasma conditions was obtained...

This work shows the experimental characterization of a helium atmospheric pressure microwave plasma produced by the Axial Injection Torch (TIA design) at 2.45 GHz. The axial and radial description of the discharge is made under different microwave power and gas flow-rate conditions. For that, the electron and gas temperature and the electron densit...

Three different methods, which includes the 486.1 nm H(I) line method, the 667.8 nm He(I) line method and the 447.1 nm He(I) line method, for the determination of the electron density were investigated. At every radial position, the 447.1 nm line spectrum was found to be fitted to the sum of two Voigt functions, on for each component of the line, w...

A method for the radial analysis of the plasma torch produced by the axial injection torch device at atmospheric pressure is proposed. The method uses a fast high-resolution acquisition set-up including an intensified charge coupled device and an image rotation system, as well as a new data processing procedure based on the Abel inversion technique...

This work presents the use of a collisional radiative model for the characterization of a helium plasma far from thermodynamic equilibrium. This model allows to calculate the value of the electron temperature using the population of the excited states obtained from absolute line intensity measurements. This method was applied for the characterizat...

This work presents the radial characterisation of a helium plasma produced by the “Torch à Injection Axiale” by means of the determination of the electron density and electron temperature. Through the Abel inversion technique on the experimental data, the radial distribution of absolute intensity along the flame is obtained, from which we can study...