Study of Oxidation of Thin Ti Film Controlled by Reduction Processes in Water Vapor Plasma∗

Changes of microstructure, phase composition and surface morphology were investigated for 200-700 nm thick Ti films, which were deposited on Si substrates using magnetron sputtering technique and treated at pressure of 5-10 Pa by water vapor plasma with the plasma processing power in the range of 20-300 W. Depth profiling of oxygen and hydrogen atoms across the thickness of plasma treated Ti films, surface height nanotopography and four-point probe resistivity measurements have been performed complementary to the analysis of phase and compositional changes registered by XRD and EDS techniques, respectively. It is shown that the different reduction/oxidation state on the surface can be maintained by coordinated adjustment in plasma power. Due to strong coupling of adsorbed water clusters to hydrophilic titanium oxide, a fast and complete transformation of Ti into TiO2 accompanied by film cracking and spontaneous lifting in the form of “popping off” discrete blisters was observed. Analysis of the experimental results is used to obtain important insights on the behavior of water molecules adsorbed on the reduced titania surfaces exposed to water vapor plasma at room temperature. [DOI: 10.1380/ejssnt.2012.613]


I. INTRODUCTION
Ultimate challenge in catalysis is to formulate materials that catalyze useful reactions by O 2 or water vapor.Effective catalysts would have a number of significant uses including the formulation of fabrics, coatings, and other materials that could catalytically clean the air and water by oxidative decontamination of the offending agents using solely the environment friendly elements.Titanium dioxide represents as an effective photocatalyst for self-cleaning surfaces (windows, mirrors), odor, bacteria, germs, fungus elimination, sterilization, antifogging films, and cheap solar cells [1].
Reduced surfaces of TiO 2 possess chemisorptive and catalytic properties different from stoichiometric surfaces [2,3].The reason for the difference between oxidized and reduced surfaces is probably the different electronic configuration of the ions, and the lower degree of coordinative unsaturation in the fully oxidized oxides.Reduction of titania leads to the formation of oxygen vacancies on the surface.The equilibrium number of oxygen vacancies depends on the concentration of adsorbed water molecules according to the reaction equation: Ti 4+ + O 2− + H 2 H 2 O + VO + Ti 3+ + e − .The oxygen vacancies and the Ti 3+ ions are most likely located on the titania surface.The electrons are delocalized in the bulk and occupy the conduction band thus causing decreased IR transmittance and increased electrical conductivity [4].Some of these factors lead to the enhanced activity of the surface to adsorbed water molecules.The neutral water molecules arriving to the surface from the water vapor oxidize Ti atoms as follows: Ti x O y + (2x − y) H 2 O = xTiO 2 + (2x − y) H 2 .These possible oxidation reactions are highly thermodynamically favorable [5].
Removal of lattice oxygen can be achieved by many reducing agents.The common ones include hydrogen, carbon monoxide, ammonia gas, and hydrocarbons.It is known that titania coating exposed to ultraviolet light have the extraordinary property of complete wettability for water [6].
Because of the large numbers of significant variables, the surface properties of partially reduced titanium oxides may be quite different and dependent on the details of the reduction process even for the same degree of reduction.Water vapor plasma is an environment providing an added opportunity to realize controllable oxidation/reduction processes.Water vapor plasma can give many radicals and atoms as well as some ions into the surface.Ultraviolet irradiation and ion bombardment occur during exposition in plasma [7], then oxygen layers with non-stoichiometric ratio would form on the stoichiometric TiO 2 layer [8].
Many attempts have been made to understand the titanium oxidation mechanism by modifying the surface properties, controlling working gas composition or adding various elements.However, this understanding is not complete.This paper includes the study of phase and compositional changes in Ti film driven by primary reduction/oxidation processes on the surface using water vapor plasma and aims to deepen understanding about the behavior of water molecules on the surface of reduced titanium oxides.

II. EXPERIMENTAL
The thin Ti films were deposited using magnetron sputtering system onto the crystalline Si (111) substrates (10×20 mm 2 ).A circular magnetron was installed in the vacuum chamber of a PVD-75 model vacuum device.Pure titanium (99.99 at.%) of 76 mm diameter and 3 mm thickness was used as a target material and distanced 35 mm away from substrate holder.Pure argon was used as the sputtering gas.The discharge parameters have been controlled using a variable DC power supply (3 kV, 500 mA).Prior the deposition, the target was plasma cleaned in order to remove surface oxide layers.Sputtering time was chosen in order to obtain 200-700 nm thick Ti films for sputtering power supply of 100 W (200 mA, 500 V), that is corresponding to a power density of 1.2 W/cm 2 .The thickness of the films was measured using nanoprofilometer revealing the height of the step with an accuracy of ±10 nm.
After Ti film deposition the chamber was evacuated up to 10 −3 Pa, the vapor of distilled water was injected until the steady state pressure of 5-10 Pa was reached and the plasma generator was activated at a power of 20-300 W. The plasma density of 2×10 10 cm −3 and the electron temperature of 1.5-1.6 eV were measured at a power of 150 W by a Langmuir probe.Without an applied axial magnetic field, ionization degree increases linearly with dissipated power.With an axial magnetic field, ionization degree jumps to a maximum value at about 300 W and then saturates with power.The nominal kinetic energies of 10-20 eV were enhanced by negative bias voltage of 200-250 V.The ion current density directed to the sample was verified in the range from 0.01 mA/cm 2 up to 10 mA/cm 2 .The samples were located on the water cooled holder and the temperature of the substrates was always starting at room temperature, rising to 50 • C or less at the end of experiment.
The structure and phase composition of the samples were registered by the Bruker X-ray diffractometer (Bruker D8).The measurements were performed in the range 2θ = 20 • -70 • using Kα radiation of Cu cathode in steps of 0.01 • .All X-ray diffraction peaks were indexed using International Centre for Diffraction Data (ICDD) current Powder Diffraction File (PDF-2 2006).EVA software used to determine peak positions and intensities and used to search-match or compare with all (or subset/restrictions) of the ICDD PDF-2 database.The surface views before and after water vapor plasma treatment were investigated by the scanning electron (SEM, JEOL JSM-5600) and optical (Nikon Eclipse Lv150) microscopies.The elemental analysis and chemical characterization of treated films were carried out by the energy dispersive X-ray spectroscopy (EDS, Bruker Quad 5040).The distribution profiles of O in Ti films were measured using X-ray photoelectron spectroscopy (XPS) equipped with an ion gun by using focused 0.5 keV Ar + ion beam sputtering at the incidence angle of 45 degrees from surface normal and rotated around the measured spot.The ion current density was around 0.5 µAcm −2 .Hydrogen distribution profiles were measured by glow discharge optical emission spectroscopy (GDOES, Spectruma Analytic GMBH) equipped with radio frequency generator and plasma source with an inner anode diameter of 2.5 mm.The optical part of spectrometer consisted of a polychromator which led reliable and time resolved depth profiles.Electrical conductivity of plasma treated Ti films was measured using four-point probe method (Jandel universal probe).Surface height topography of Ti films was analyzed using the nanoprofilometer (AMBIOS XP 200).

A. XRD analysis
All samples that had been treated using water vapor plasma were subsequently analyzed by XRD technique.The X-ray diffraction patterns of Ti film as-deposited and after treatment at different ion current densities for different treatment durations are shown in Fig. 1(a

B. XPS Depth Profiling
Figure 2 includes XPS profiles of main elements (titanium, oxygen and silicon) across the thickness of Ti film treated under low-flux ion irradiation for 20 min.It is seen that the near-surface region of 120 nm thickness is oxidized stoichiometrically, as the O and Ti concentrations are distributed homogeneously and equals to 66 at.% and 34 at.%,respectively.Under this layer, O concentration drastically decreases and becomes variable from 3 at.% to 12 at.%.

C. X-ray Compositional Analysis (EDS)
The EDS results were compared with the color change in dependence of plasma treatment parameters.After water vapor plasma treatment, a wide range of colors appear on the surface of Ti film affected by inhomogeneous radiation field.Figure 3 includes optical microscopy surface view of Ti film treated at the boundary of magnetically focused plasma beam.It has been reported that film color change manifest the reduced oxygen content.For example, it is reported [9] that the white color of TiO 2 changes to navy blue after treatment by H 2 plasma due to generation of surface oxygen vacancy sites.In the present work, it has been registered that the metallic color of 240 nm thick Ti film changes into navy blue after treatment at 300 W during 5 min corresponding to the value of O/Ti ratio of 1.3-1.5 measured by EDS.The rose layer has an O/Ti ratio of 1.7 and has been registered for samples treated at 200 W for treatment time in the range between 20 and 60 min.Samples treated at low-flux ion radiation have a light beige coloration with a gray tinge underneath.Stoichiometrically oxidized titanium has a solid beige coloration.The thin film local areas covered by water drops were completely oxidized and lifted in the form of "popping off" discrete blisters due to accomodation of hydrogen at the film-substrate interface (white local areas, Fig. 3).

D. GDOES Depth Profiling
The atomic hydrogen dissolves in the TiO 2 lattice and diffuses through the oxide layer into the bulk and, taking into account titanium's high affinity for hydrogen, is absorbed by the titanium.Figure 4 includes the GDOES depth distribution profiles of H atoms in Ti film treated for 10 min at fixed dissipated power (300 W) for different ion current densities: curve 1 -10 mA/cm 2 , curve 2 -1-2 mA/cm 2 and curve 3 -1 µA/cm 2 .It is seen that the quantity of H in the bulk of Ti film increases as radiation intensity increases.The maximum concentration of H is located at the near surface region.Presumably, it is because of high concentration of hydroxyl groups •OH on the surface related to the film superhydrophilicity.Concentration of H in the bulk does not exceed the limit of solubility and does not depend on the radiation intensity (Fig. 4).

E. Surface Topography Profiling
Studies of surface topography have shown that asdeposited Ti film on the silicon substrate has the root  mean squared roughness of 2-3 nm (Fig. 5(a)).Surface roughness increases after the plasma treatment: 12 nmfor 20 min at ion current density equal to 1 µA/cm 2 , and 43 nm -for 20 min at 1 mA/cm 2 .Additionally, it was registered that the initially flat surface topography becomes periodically bumpy with height amplitude equal to about 6 nm and a period equal to 15-18 µm after treatment at 50 W for 60 min (Fig. 5(b)).As plasma dissipated power increases, the treated surface is subjected to the development of blisters and flakes.The film treated at 300 W for 5 min, contains randomly distributed small holes throughout the entire film thickness with an average diameter of about 10-12 µm (Fig. 5(c)).As a confirmation, the holes of the same size were registered in SEM (Fig. 6(a)).
After plasma treatment at lower plasma generation power of 250 W for 20 min the optical microscopy surface view (Fig. 6(b)), includes white circular areas which may be the manifestation of completely oxidized titanium before lift-off processing surrounded by matrix of hydrogenated TiO 2 (black [10]).

F. Four-Point Probe Resistivity Method
Surface resistivity of treated Ti thin films was measured using four-point probe method and resistivity calculations were made using the equation where V is the voltage, I is the current and k = π ln 2 = 4.53.The experimental results are summarized in Fig. 7.
It is seen that the surface resistivity of Ti film sharply increases from 84 Ω/2 for the untreated sample to 1020 Ω/2 for the plasma treated at 20 W for 60 min.It supports the above made suggestion that the stoichiometric TiO 2 on the surface of Ti film is formed at low power dissipation.It is in agreement with the experimental results obtained by XRD (Fig. 1(a), curves 2 and 3) and XPS (Fig. 2).It discloses that the oxidation process dominates in the range of low plasma densities (plasma dissipated power of about 20 W).The surface resistivity starts to decrease as plasma dissipated power increases.Resistivity decreases up to 635 Ω/2 after treatment at 50 W for 60 min, and up to 211 Ω/2 after treatment at 200 W for 5 min.It indicates that with the increase of dissipated power the reduction of TiO 2 starts resulting in the formation of two-layered structure of reduced-TiO 2 /TiO 2 .
The degree of a reduction has a steady state limit.For example, the surface resistivity is equal to 170 Ω/2 after treatment at 200 W for 20 min and 185 Ω/2 at 300 W for 20 min.The XRD data show that 240 nm thick Ti film is completely transformed into TiO 2 after plasma treatment at 200 W for 20 min.Peaks of titanium suboxides have not been registered.It indicates that the thickness of the reduced-TiO 2 layer on the surface of TiO 2 is small and not observable by XPS and XRD.

IV. DISCUSSIONS
As mentioned above, the oxidation of Ti film in water plasma provides controlled conditions in which the surface stoichiometry ratio may be verified.Presumably, the reduction goes according to the nucleation model, which is based on the suggestion that surface oxygen ions are preferentially removed from the surface of oxidized titanium by energetic ions arriving from plasma leaving behind anion vacancies.When the concentration of vacancies reaches a critical value, the vacancies are annihilated by rearrangement of the lattice with the eventual formation of lower oxide layer [11].It results in changes of film optical properties (Fig. 3) and surface electrical resistivity (Fig. 7).
An oxygen deficient surface with suboxides is highly reactive.The adsorbed water molecules quickly hydrate the surface leading to the increased hydroxyl group density on the oxide surface.The concentration of hydroxyl groups on the surface depends on the oxidation state of titanium oxide.Taking into account hydrophilic properties of reduced titanium oxide, the adsorbed water drops form thin water island layers on the surface which are registered in surface views obtained by SEM and optical microscopies (Figs. 3 and 6, respectively).Irradiation of the surface area covered by monomolecular water layer by particles arriving from plasma leads to the split of water molecules into their atomic components of hydrogen and oxygen [12,13].It results that the metallic titanium beneath the water island layer is oxidized much faster than the titanium uncovered by water layer (Fig. 6(b)).Surface topography height profiles (Figs.5(a-c)) and surface views obtained by optical microscopy and SEM (Figs. 3 and Fig. 6(a)) show that local areas of Ti film covered by water island layers are lifted in the form of "popping off" discrete blisters after exposition in high density plasma (power dissipation of 300 W).
Hydrogen permeation through titanium oxide film is a very complex process involving interfacial charge transfer, trapping, and transport, thus is inherently influenced by properties of the oxide, such as the chemical composition and structure [14].Several studies [15,16] have shown that diffusion of hydrogen in TiO 2 is much slower than in Ti metal.Hydrogen distribution profiles (Fig. 4) show that: (i) H transportation kinetics from the surface into the bulk increases as ion radiation intensity increases, (ii) the bulk H concentration does not exceed the solubility limit and does not depend on the intensity of radiation, and (iii) the high H surface concentration is in agreement with the assumption about the presence of high concentration of hydroxyls.

V. CONCLUSIONS
Water vapor plasma processing offers modified Ti oxidation thermodynamics and kinetics over conventional, thermal oxidation, with new surface properties of TiO 2 .The presented experimental results show that water vapor plasma is an environment for Ti oxidation under conditions of controllable surface reduction.It was registered that adsorbed water clusters, taking into account superhydrophilic properties of reduced titanium oxide, converge to thin water island layers.The water spreading quickly hydrates the surface leading to increased hydroxyl group density.Transport kinetics of water constituents depends on the oxidation state of titanium which may be modified verifying plasma intensity radiation.Oxygen and hydrogen atoms, generated on the surface, move through the upper-most TiO 2 layer and reach the metallic Ti which is oxidized and hydrogenated.As a result, oxidation rate of titanium is significantly enhanced, while hydrogen, taking into account its lower affinity to Ti, forms only α phase without strong chemical bonds.
FIG.2: XPS depth profiles of oxygen, titanium and silicon across Ti film treated at low-flux (0.01 mA/cm 2 ) ion irradiation for 20 min.