Electron-Beam-Induced Topographical, Chemical, and Structural

Oct 31, 2006 - Plasmon-Assisted, Spatially Resolved Laser Generation of Transition Metal Oxides from Liquid Precursors. Ivano Alessandri , Matteo Ferr...
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J. Phys. Chem. B 2006, 110, 23660-23668

Electron-Beam-Induced Topographical, Chemical, and Structural Patterning of Amorphous Titanium Oxide Films P. Kern,*,† Y. Mu1 ller,‡ J. Patscheider,§ and J. Michler† Empa, Materials Technology Laboratory, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland, Empa, Laboratory for Metallic Materials, U ¨ berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland, and Empa, Nanoscale Materials Science Laboratory, U ¨ berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland ReceiVed: July 6, 2006; In Final Form: September 20, 2006

Electrolytically deposited amorphous TiO2 films on steel are remarkably sensitive to electron beam (e-beam) irradiation at moderate energies at 20 keV, resulting in controlled local oxide reduction and crystallization, opening the possibility for local topographical, chemical, and structural modifications within a biocompatible, amorphous, and semiconducting matrix. The sensitivity is shown to vary significantly with the annealing temperature of as-deposited films. Well-defined irradiation conditions in terms of probe current IP (5 µA) and beam size were achieved with an electron probe microanalyzer. As shown by atomic force and optical microscopy, micro-Raman spectroscopy, wavelength-dispersive X-ray (WDX), and Auger analyses, e-beam exposure below 1 Acm-2 immediately leads to electron-stimulated oxygen desorption, resulting in a welldefined volume loss primarily limited to the irradiated zone under the electron probe and in a blue color shift in this zone because of the presence of Ti2O3. Irradiation at 5 Acm-2 (IP ) 5 µA) results in local crystallization into anatase phase within 1 s of exposure and in reduction to TiO after an extended exposure of 60 s. Further reduction to the metallic state could be observed after 60 s of exposure at approximately 160 Acm-2. The local reduction could be qualitatively sensed with WDX analysis and Auger line scans. An estimation of the film temperature in the beam center indicates that crystallization occurs at less than 150 °C, well below the atmospheric crystallization temperature of the present films. The high e-beam sensitivity in combination with the well-defined volume loss from oxygen desorption allows for precise electron lithographic topographical patterning of the present oxides. Irradiation effects leading to the observed reduction and crystallization phenomena under moderate electron energies are discussed.

1. Introduction The unusual properties of titanium dioxide (TiO2), such as its chemical and thermal stability, excellent optical transmittance in the visible and near-IR range, high refractive index, high dielectric constant, and n-type semiconducting properties as well as its photocatalytic activity, make thin TiO2 films attractive for wide applications: antireflextion coatings,1 multilayer coatings for optical filters,2 photocatalytic purifier for environmental polluants,3,4 humidity sensors,5 optical waveguides,6 solar cells,7 corrosion resistant barriers, and biomedical surfaces.8 The largest dielectric constant among simple metal oxides is attractive for capacitors or gate dielectrics in microelectronic device fabrication.9 TiO2 can crystallize in the polymorphs anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) and can also exist in an amorphous state (a-TiO2), the latter being clearly under-represented in the literature. Rutile is formed at temperatures above 800 °C and is the thermodynamically stable phase. Oxygen loss in TiO2 can lead to nonstoichiometric TiO2-x, to Ti2O3, Ti3O4, and TiO as well as to TinO2n-1 Magne´li phases.10 Accordingly, the physical properties of the titanium oxide can drastically change. For example, the monoxide TiO is reported to have almost metallic conductivity.11 The low resistance and * Author to whom correspondence should be addressed. Tel.: +41 33 228 36 26; fax: +41 33 228 44 90; e-mail: [email protected]. † Materials Technology Laboratory. ‡ Laboratory for Metallic Materials. § Nanoscale Materials Science Laboratory.

high efficiency as a silicon diffusion barrier make TiO a perspective future thin film material for low resistance contact metallization in microelectronics.12,13 Global changes in the oxidation state are obtained by thermal annealing in high vacuum or in reductive environment, which usually creates small quantities of Ti3+.14-16 Much higher quantities of mixed Ti oxidation states (Ti3+, Ti2+, Ti+) are observed after Ar+ bombardment.14,15 Local modifications in crystalline TiO2 films or powders have been obtained by ion implantation and laser and electron beam (e-beam) irradiation, while local functionalization in amorphous TiO2 have not been reported today. Even so, a-TiO2 can be obtained by plasma-enhanced chemical vapor depositon17,18 or by magnetron sputtering;19 crystalline films are mostly aimed at because of slightly better dielectric constants18 and probably also because TiO2 in the crystalline state is better investigated than in the amorphous state. After laser irradiation at 335 nm of rutile pellets, Le Mercier and co-workers20,21 found a drop in electrical resistance from 109 to 10 Ωcm. The authors found Ti3+ and Ti2+ species and pointed out the possible existence of Magne´li phases, which have recently raised considerable interest because of their electric properties.22 A significant increase in electrical conductivity was also observed after Ar-, Sn-, and W-ion implantation into rutile.23 Low-density (µAcm-2) e-beams below 1 kV in an Auger microscope16 induced an energy and dose-dependent controlled number of Ti3+ defects in TiO2 (110). Lower oxidation states than Ti3+ were not observed. Taking

10.1021/jp0642589 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/31/2006

Patterning of Amorphous Titanium Oxide Films use of the high probe current densities and accelerating voltages of transmission electron microscopes (TEM), several authors investigated the effect of e-beam irradiation on the oxidation state of crystalline TiO2. McCartney and co-workers24,25 observed formation of epitaxial TiO at current densities between 5 and 50 Acm-2 and reduction beyond the monoxide after irradiation at extremely high current densities (103-104 Acm-2) at 100-400 kV of rutile single-crystal samples. Irradiation resulted in extensive pitting in the observed zone. Contrary, TiO2 anatase powder did not show any noticeable structural or electronic changes upon TEM irradiation at 200 kV and 3 Acm-2.26 TEM e-beam heating effects upon irradiation of anodically formed oxide films on Al, Nb, Ta, and W demonstrated the possibility of local crystallization of as-formed amorphous oxides.27 With exception of the anodic TiO2 film, being already crystalline after anodization, all oxides could be locally crystallized. A significant drop of the crystallization temperature because of intense radiation damage was suggested. Amorphous Al2O3,28 ZrO2,29 SiO2,30 GeAu,31 GeSi,32 and ITO33 have since been crystallized under TEM e-beam exposure. Recently, we have discovered a significantly enhanced sensitivity of electrochemically deposited a-TiO2 films toward moderate (20 kV) e-beam irradiation in comparison to the abovereported observations on crystalline films.34 Using a wide range of beam currents from 10 nA to 24 µA, we found a rapid electron-stimulated oxygen desorption even at low currents, leading to well-defined cavities in the thin films. At larger probe currents, local crystallization to anatase structure was observed. In-situ measurements of the sample current and the global sample temperature upon exposure and an estimation of the additional temperature rise in the beam center were correlated to the observed reduction and crystallization phenomena. It was shown that local crystallization occurred at temperatures well below the film’s crystallization temperature under atmospheric conditions. In the present paper, an in-depth analysis of e-beam-induced chemical, topographical, and structural changes in amorphous TiOx (x ≈ 2) thin films is presented. To better understand the nature of previously reported physiochemical changes in terms of current density and dose, irradiations were performed under well-defined beam current, current density, and dose conditions using an electron probe microanalyzer and different electron microscopes. The resulting topographical, chemical, and structural changes in the film are analyzed by atomic force microscopy (AFM), optical microscopy, wavelength-dispersive X-ray analysis, as well as Auger and micro-Raman spectroscopy. The initial chemical state of the thin films is studied with X-ray photoelectron spectroscopy as a function of annealing temperatures. Physical and chemical effects are discussed as well as difficulties in measuring localized phenomena in thin films addressed. Finally, the feasibility of precise high-resolution topographical surface patterning of the present biocompatible films by e-beam lithography is explored. 2. Experimental Section Electrolytic Deposition of Titanium Oxide. Mirror-polished AISI 316L stainless steel disks (15-mm diameter, 1-mm thickness) were ultrasonically cleaned in acetone and ethanol and were activated in piranhia solution (H2SO4/H2O2 ) 3/1 vol %) followed by copious rinsing with ultrahigh-purity water (18 MΩcm) prior to deposition. Electrodeposition was performed in 0.05 M TiCl4 (Fluka) + 0.5 M H2O2 in mixed methanol/ water solvent (3/1 vol %) at pH 1.0 and 0 °C. Using a twoelectrode setup under galvanostatic control, a cathodic current

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23661 of -75 mAcm-2 was applied up to the desired charge of 5 and 8 C, corresponding to a film thickness of approximately 60 and 110 nm, respectively. These conditions have been shown to result in films with good thickness uniformity and low pore density.35 As-deposited uniform peroxotitanium hydrate films were thermally annealed at 150 °C and 250 °C with a heating and cooling rate of 6 °C/min and a holding time of 30 min. Thermally crystallized films were heated to 450 °C with 200 °C/h and 1-h holding time under argon protection.35 E-Beam Irradiation. The TiO2/steel samples were fixed to the sample holder with conductive silver glue. Residual contaminations were removed by careful plasma cleaning using air as feeding gas (Gala Instrumente, Plasma Prep 2). No effect on film properties was observed. Irradiations were performed at 20 keV with the following e-beam sources: (a) Electron probe microanalyzer (EPMA) JEOL JXA-8800RL, (b) scanning electron microscope (SEM) Hitachi 3600, (c) SEM Hitachi 4800 FEG, and (d) SEM Zeiss DSM 962. The EPMA was ideal for screening of e-beam intensity and dose effects within the irradiated oxide because of the large intensity range and its temporal stability as well as the possibility of independent adjustment of beam current and probe size. The defocusing of the beam probe through electron-optical lenses results in nearly uniform current density within the irradiated area. At 5 µA, a minimum beam size of approximately 2 µm was obtained. E-beam lithographic patterning defined by beam current, spot size, dwell time, increment, and field size was performed with the Hitachi 3600, equipped with a nanolithography tool (XeDraw 2, XENOS, Germany). The Hitachi 4800 FEG was used for higher resolution patterning. Both Hitachi microscopes delivered a maximum beam current of 5-10 nA. For Auger analyses, an intense irradiation on a spot of approximately 50 mm in diameter was performed with the Zeiss SEM with removed condenser aperture, delivering probe currents of up to 24 µA.34 After irradiation, deposited hydrocarbon contaminations were removed by careful oxygen plasma cleaning. Atomic Force and Optical Microscopy. AFM measurements were performed in noncontact mode on a Topometrix Explorer using uncoated Si tips (Ultrasharp NSC15/100). A camera allowed for rough positioning of the tip close to the irradiated area. Optical images of irradiated spots were acquired on a Zeiss Axiomat microscope. Micro-Raman. Laser Raman spectra were obtained in backscattering geometry with a Renishaw Ramascope 2000 using a 633-nm HeNe laser and a spot size of approximately 3 µm. Chemical Analyses. X-ray photoelectron spectroscopy (XPS) surveys and depth profiles were acquired on a Physical Electronics Quantum 2000 system using monochromatized Al KR radiation and 3 keV Ar-ions for sputtering. Wavelengthdispersive X-ray analysis (WDX) with the JEOL JXA-8800RL EPMA was performed at 20 keV and 40 nA beam current, defocused to 50 µm (2‚10-3 Acm-2) at nonirradiated spots and focused to approximately 1.5 µm (2.2 Acm-2) on irradiated spots allowing for local measurements within the spots after irradiation. The calibration was performed with stoichiometric TiO2 rutile powder (Fluka) and graphite. Quantification was based on a thin film model for the elements Ti, O, and C, carbon being assigned to a thin contamination layer. Qualitative Auger line scans through a spot previously irradiated in the Zeiss SEM at 24 µA during 600 s were performed on a Physical Electronics PHI 4300 Scanning Auger Microscope. The analyzed area was sputter cleaned (3 kV Ar+) for an equivalent removal of 2 nm carbon prior to line scan measurements. The differences of peak

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and background counts were measured for Ti (385/400 eV), O (515/532 eV), and C (276/255 eV) with 20 ms/step for 30 sweeps. 3. Results and Discussion 3.1. Theoretical Aspects of E-Beam-Induced Physiochemical Changes. E-beam irradiation of a surface can result in the following radiation damages: atomic displacement, e-beam sputtering, e-beam heating, radiolysis, electrostatic charging, and hydrocarbon contamination.37 The latter two phenomena can generally be reduced or avoided by establishing a good electrical contact as well as by sample pre- and posttreatment, for example, by plasma cleaning. For atomic displacement to happen, the maximum energy Emax (θ ) 180°), transferable by an electron of mass me (9.1‚10-28 g/atom) and kinetic energy E to a nucleus of mass ma, must be greater than the atomic displacement energy Ed of the irradiated species. For nonrelativistic energies in the SEM, Emax is given by

Emax ) 4E

me ma

(1)

In the present conditions with E ) 20 keV, ma(O) ) 2.66‚10-23 g/atom, and ma(Ti) ) 7.95‚10-23 g/atom, Emax ) 2.7 eV (O) and 0.9 eV (Ti), respectively. The displacement energy Ed for oxygen (44-50 eV38) is much higher than Emax, eliminating atomic displacement of O and even less of Ti as possible radiation damage. The threshold energy Es for e-beam sputtering of less-well-bound surface atoms is the sublimation energy per atom. Even though Es is smaller than Ed, this phenomenon is unlikely in the SEM because of the low incident beam energy.37 Radiolysis and beam heating, both due to inelastic electronelectron interactions, remain as possible damage mechanisms. The well-known Knotek-Feibelmann theory for radiolysis and oxygen desorption in transition-metal oxides upon e-beam irradiation describes a mechanism for converting the energy acquired by excited atomic electrons to kinetic energy and momentum of atomic nuclei.39,40 Shortly, the incident electrons create an inner-shell vacancy on the metal site. Since TiO2 has no valence electrons on the Ti side, the dominant channel for the 3p hole decay is an interatomic Auger decay from the oxygen atom. The oxygen then undergoes further Auger decay and is ejected into the vacuum as neutral atom or even positively charged oxygen ion in a highly repulsive Madelung potential. The oxygen desorption stops when the surface becomes sufficiently conductive (e.g., TiO) for the interatomic Auger decay being replaced by a faster conduction mechanism. The temperature in the center of a stationary electron beam because of e-beam heating can be written as Ttot ) TS + ∆T. The global sample temperature TS increases with exposure time and increasing probe current IP, as demonstrated by in-situ measurement of the temperature during e-beam irradiation.34 Upon irradiation at 24 uA during 60 s, TS increased from initially 70 °C to approximately 100 °C. In the present conditions using an EPMA e-beam current of 5 µA for a maximum of 60 s irradiation, a temperature increase of approximately 6 °C can be estimated assuming a linear dependence of TS with IP. Therefore, a global heating of the sample can be neglected and Ttot ≈ ∆T. With an acceleration voltage U and a beam current Ip, the additional temperature rise ∆T above substrate temperature in the beam center can be estimated by

∆T )

3UIP(1 - η) 2πκ(RE/2 + D)

(2)

η being the percentage of backscattered electrons (Fe: η ) 0.29), RE the electron range (Fe: RE ) 1.2 µm at 20 kV), D the probe diameter, and κ the thermal conductivity (steel: κ ) 22 Wm-1 K-1, TiO2: κ ) 11.7 Wm-1 K-1). For D , RE/2, eq 2 reduces to the formula given by a derivation in ref 40. Since the micrometer-sized electron interaction volume is situated inside the steel pellet, the values of Fe are used for κ in eq 2. Even though κ of the ca. 100-nm TiO2 top layer is a factor 2 smaller for pure TiO2 (anatase), the deviation from the calculated values is neglected because of the small thickness. In this work, we realized irradiation conditions covering pure reduction of the films to TiO2-x up to sufficient intensity necessary for local crystallization. 3.2. Electrolytically Deposited TiOx Films. Electrosynthesis of titanium oxide thin films in TiCl4 and H2O2 containing solutions has been proposed to occur via formation of a Tiperoxo-complex, which hydrolyzes as a result of electrogenerated base at the cathode, resulting in precipitation of peroxotitanium hydrate TiO3(H2O)x.35 As-deposited peroxo-titanium hydrate films on stainless steel are amorphous (a-TiO2) and can be transformed into crystalline TiO2 by heating at 450 °C, as shown recently by Raman measurements and glow-discharge optical emission spectroscopy (GD-OES).35 Under the vacuum conditions in the SEM, as-deposited films were found to crack immediately, probably because of the rapid loss of intrinsic humidity. This was also followed by a color change indicating a slight decrease in thickness. After heating at 150 °C or 250 °C, the films were stable in a vacuum, while still being amorphous. From DTA/TG measurements at a heating rate of 10 °C/min, the humidity and excess oxygen of as-deposited peroxo-titanium hydrate was shown to evaporate at around 130 °C with an ongoing mass loss up to 200 °C.36 An XPS study was performed to reveal bonding state and composition of the thin film. A survey of as-deposited TiOx (5 C) before surface sputtering is shown in Figure 1a. Besides the dominating Ti and O signals, the atmospheric carbon contamination and small amounts of N and Fe are present. A chloride contamination from the electrolyte is not found. The insert in Figure 1a shows the evolution of the Ti 2p spectra during XPS depth profiling through as-deposited TiOx. Sputtering with 3 keV Ar+ ions leads to oxide reduction visible already in the second spectrum after 15 s of sputtering and appearance of lower oxidation states than the initial Ti4+ state. No significant difference was observed with a sputter voltage of 1 keV. The high sensitivity of TiO2 to sputter reduction is a well-known problem. Le Mercier et al. have claimed to avoid this artifact by sputtering at 700 eV.21 As a result of the change of film chemistry of as-deposited films under vacuum conditions during XPS experiments, no significant difference was found between depth profiles of as-deposited (not shown) films and films heated to 150 °C in Figure 1b. Both profiles of films heated to 150 and 450 °C in Figure 1b show an O/Ti stoichiometry below 2, most probably because of oxygen loss from sputter reduction. The second measuring point after 15 s of sputtering and removal of atmospheric contaminations indicates an O/Ti ratio of 1.97 (150 °C) and 1.78 (450 °C), slightly decreasing inside the oxide film because of the sputter artifact. Previous quantitative GDOES depth profiles (not in a vacuum) of as-deposited films and films heated to 450 °C have shown oxygen excess in asdeposited films and TiO2 stoichiometry after heating to 450 °C,35 which is in general agreement with XPS results. In Figure 1b, the film heated to 150 °C has a higher O/Ti ratio and is somewhat thicker when defining the oxide thickness at Ti/Fe ) 1, indicative of intrinsic humidity or an excess of oxygen

Patterning of Amorphous Titanium Oxide Films

Figure 1. (a) XPS survey on as-deposited peroxotitanium hydrate thin film with mainly Ti and O and weak contribution of C, N, and Fe signals. The insert shows the 10 first Ti 2p spectra during depth profiling with the peaks shifting toward lower binding energies because of Ar+ sputter reduction. (b) XPS depth profiles through TiOx films annealed at 150 °C (gray) and 450 °C (black), indicating approximately TiO2 stoichiometry and additional loss of oxygen when heating above 150 °C. The insert represents a zoom of the residual C, N, and Fe contaminations within the oxide film.

still present in the film heated to 150 °C. XPS profiles of Tioxide films heated to 250 °C (not shown) were almost identical to films heated to 450 °C in Figure 1b, having lost possible humidity and excess oxygen. The film heated to 150 °C contains around 2 at-% N and 1.5-2.5 at-% C as contaminants (insert in Figure 1b), while heating to 450 °C results in a decrease of N (≈1 at-%) but an increase of the C contamination (≈5 at%). Moreover, the higher temperature enables diffusion of Fe from the substrate into the film (≈0.5 at-%), not present after heating to 150 °C. The origin of nitrogen, already present in as-deposited films, is not clear. 3.3. Localized Reduction and Crystallization. By changing the beam focus, a wide range of current densities from 0.8 to 159 Acm-2 could be covered with the EPMA, as shown in Table 1. The calculated ∆T values (eq 2) in the beam center of the irradiated TiO2 film on steel substrates indicate that sufficient heating for crystallization under atmospheric conditions of a-TiO2 (440 °C) is expected only under the most intense exposure condition N°4. Table 1 also summarizes the observed “dominating mechanism” upon e-beam irradiation in conditions N°1-4, which will be addressed in the following. The effect of the chosen substrate material on the expected e-beam heating is shown by the calculated ∆T values for the TiO2 film deposited

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23663 on indium tin oxide (ITO) and Si substrates (ITO: κ ) 8.7 Wm-1 K-1, Si: κ ) 149 Wm-1 K-1). However, no irradiation experiments were performed on TiO2/ITO and TiO2/Si. No differences are expected for Ti instead of steel as the substrate material, because of the almost identical thermal conductivity (Ti: κ ) 21.9 Wm-1 K-1). Optical Appearance. In-situ optical observation during EPMA irradiation conditions N°1-4 revealed an immediate color change from the initial oxide interference color to light or dark blue at the irradiated spots, indicating the formation of Ti2O3. A color change to blue has also been reported by Le Mercier et al.20,21 after laser irradiation (λ ) 355 nm) of rutile single crystals and powders. The authors attributed this to the formation of reduced species as a result of laser heating in reductive UHV environment, supported by XPS measurements indicating the presence of Ti3+ and some Ti2+ ions in a few angstro¨m-thick layer below the surface.21 Irradiating titanium pellets with Nd: YAG laser (λ ) 1064 nm) in air, del Pino et al.42 pointed out the correlation between local oxide composition and color, the ratio of TiO2, Ti2O3, TiO, and Ti2O being characteristic to a specific color. Blue zones were attributed to a strong presence of crystalline or amorphous Ti2O3.The ex-situ taken optical images of the irradiated spots in Figure 2b and c prior to plasma cleaning demonstrate a color change limited to the irradiated area, whereas after the most intense irradiation in Figure 2a, the blue appearance is visible around the irradiated spot (D ) 2.0 µm) because of the heat dissipation from the intensely heated beam center (Table 1) to the surrounding oxide. It is well-known that heating in UHV usually creates small quantities of Ti3+ defects.19 The ring visible in Figure 2b and c within the blue zone is due to hydrocarbon deposition during WDX analysis with a relatively low beam current of 40 nA. It is known that hydrocarbon contamination is generally more critical with relatively small beam currents.43 Topography. The reductive nature of e-beam irradiation is also evidenced by AFM imaging after plasma cleaning of the oxide irradiated at different current densities at 5 µA during 60 s, corresponding to an irradiation dose of 3‚10-4 C (Figures3 and 4). Electron-stimulated desorption (ESD) of oxygen leads to a well-defined volume loss in the thin film. Despite the plasma-cleaning step, considerable hydrocarbon contaminations remain in and around the spot irradiated at the highest intensity of around 160 Acm-2 (Figure 3a), the irradiated area having a diameter of approximately 2.0 µm. The profiles in Figure 4 represent a symmetric local thickness decrease of around 15 nm (condition N°1), 25 nm (N°3), and 100 nm (N°4) in the approximately 110-nm-thick TiO2 film, corresponding to a decrease of 13, 22, and 91% of the estimated film thickness. The uniformity of thickness decrease reflects the uniform energy distribution of the EPMA beam. Previous irradiations with Gaussian SEM probe intensities have led to Gaussian profiles within the oxide.34 The shape and lateral uniformity of the irradiated spots in Figure 4 do not indicate a major contribution of backscattered electrons to oxygen loss. The lateral extension of the symmetric volume losses in Figures 3 and 4 was considered as approximate e-beam probe size to calculate exposure current densities in Table 1. The large percentile thickness decrease in Figures 3a and 4a cannot be solely explained by oxygen desorption anymore but must involve partial melting/evaporation of Ti because of intense beam heating. The estimated film temperature in the beam center of roughly 600 °C (eq 2) seems too low for this to happen, considering the melting point of Ti (1668 °C). However, under vacuum conditions, thermal properties are considerably different as known from atmospheric conditions.27,29,34

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TABLE 1: Overview of EPMA E-Beam Irradiation Conditions Relevant for Figures 2-5a N°

IP (µA)

D (µm)

t (s)

J (Acm-2)

dose (C/cm-2)

∆T (°C)

dominating mechanism (steel substrate)

1 2 3 4

5 5 5 5

27.5 11.5 11.5 2.0

60 5 60 60

0.8 4.8 4.8 159

51 24 289 9549

55 (ITO: 139, Si: 8) 127 (ITO: 322, Si: 19) 127 (ITO: 322, Si: 19) 593 (ITO: 1499, Si: 88)

reduction (a-TiO2-x, a-Ti2O3) reduction (a-TiO2-x, a-Ti2O3) + crystallization (anatase) reduction (a-Ti2O3+ a,c-TiO) + crystallization (anatase) reduction (Ti) + (anatase + c-Ti2O3 around spot)

a IP ) probe current, D ) probe diameter, t ) irradiation time, J ) current density, ∆T ) additional temperature rise in probe center (approximately equivalent to the expected total temperature rise, see text). a ) amorphous, c ) crystalline. For comparison of TiO2 on the chosen steel substrate, the calculated ∆T for TiO2 present on indium tin oxide (ITO) and Si substrates is also given.

Figure 3. AFM images of irradiated zones in amorphous TiOx film (8 C deposition charge, 250 °C annealing). EPMA irradiations have been performed at (a) 159 Acm-2 for 60 s (N°4 in Table 1), (b) 4.8 Acm-2 for 60 s (N°3 in Table 1), and (c) 0.8 Acm-2 for 60 s (N°1 in Table 1). The volume loss is due to the electron-stimulated desorption of oxygen (oxide reduction) and is restricted to the irradiated zone, representing approximately the e-beam probe size. In Figure 3a, some hydrocarbon remains within cavity. Figure 2. Optical images of irradiated zones in amorphous TiOx film (8 C deposition charge, 250 °C annealing). EPMA irradiations have been performed at (a) 159 Acm-2 for 60 s (N°4 in Table 1), (b) 4.8 Acm-2 for 60 s (N°3 in Table 1), and (c) 0.8 Acm-2 for 60 s (N°1 in Table 1). The color change to blue indicates reduction of TiO2 to Ti2O3.

Chemical Analyses. The small cavity within the irradiated zone in Figure 3b and c is due to the additional oxygen desorption during local WDX analysis performed at 0.6 Acm-2 directly after e-beam irradiation experiments. It is therefore obvious that the local WDX measurement itself, performed at a current density between conditions N°1 and 3 (Table 1) to have an adequate signal-to-noise ratio, induces further oxygen loss and cannot truly sense the oxide composition after the main e-beam exposure. Moreover, the WDX results presented in Table 2 indicate a largely overrated O/Ti ratio which does neither agrees with XPS data in Figure 1 nor with previous GD-OES experiments.35 Considering the large penetration depth of injected electrons in the micrometer range with respect to the small oxide film thickness during WDX measurements, we assume that part of the error in the absolute ratio is due to a re-excitation of the oxide by electrons backscattered from the

Figure 4. AFM depth profiles of irradiated zones in Figure 3 after EPMA irradiation with (a) 159 Acm-2 for 60 s (N°4 in Table 1), (b) 4.8 Acm-2 for 60 s (N°3 in Table 1), and (c) 0.8 Acm-2 for 60 s (N°1 in Table 1).

substrate. The other part of the error arises from the quantitative WDX calibration with bulk material (TiO2 powder). In the quantitative calibration, the characteristic X-ray signals sensed by WDX analysis have to go through an iterative calculation, in which mainly effects caused by atomic number, absorption,

Patterning of Amorphous Titanium Oxide Films

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23665

TABLE 2: WDX Analysis Results on Nonirradiated Ti-Oxides (5 and 8 C Deposition Charge) Annealed at 200 and 250 °C (N°1,2)a N°

Ti-oxide

1 2 3 4 5

5C/200°C 8C/250°C 8C/250°C 8C/250°C 8C/250°C

t Ti O C Ip (Acm-2) (s) (at-%) (at-%) (at-%) O/Ti % O* 0 0 0.8 4.8 159

0 0 60 60 60

8.89 10.13 10.62 10.33 18.11

73.91 83.44 79.25 75.75 0.00

17.2 6.42 10.13 13.91 81.88

8.31 8.24 7.46 7.33 0.00

101 100 91 89 0

a (*) % O is calculated under assumption that N°2 corresponds to TiO2 stoichiometry ()100%). The difficulty in determining absolute O/Ti ratios is discussed in the text. WDX results after irradiations on the 250 °C annealed Ti-oxide (N°3-5) show a clear decrease of O/Ti ratio and reduction to metallic Ti after intense irradiation in N°5.

and fluorescence are corrected. These calculations are based on bulk conditions. In contrast to the powder material used for calibration of Ti and O, where the characteristic X-rays from oxygen incorporated in the metallic bulk are stronger absorbed, the X-rays from oxygen emitted in a thin titania film can hardly be absorbed. This results in an overcorrection of the oxygen amount when measuring thin oxide films. This effect is more important for the smaller element O compared to Ti. A reliable calibration would have to be done with an oxide of exactly known stoichiometry and a thickness similar to the oxide to be analyzed, which is very difficult to achieve. Nevertheless, a qualitative comparison of WDX data is straightforward. On the basis of XPS, the film heated at 250 °C (N°2 in Table 2) is supposed to have nearly TiO2 stoichiometry. Hence, a %O value of 100% is assigned to this film, with the other %O values calculated with respect to this reference. A significant hydrocarbon deposition during WDX measurements itself, also seen in Figure 2, explains the large amounts of C found in Table 2. The very low current density during WDX measurements with defocused beam (50 µm) on nonirradiated oxides (N°1,2) did not give rise to color change or measurable volume loss and therefore does not alter film composition during measurement. Table 1 indicates a slightly lower oxidation state of the film annealed at 250 °C with respect to the film heated to 200 °C. This suggests ongoing loss of excess oxygen above 200 °C, confirming the trend seen from XPS data in Figure 1b. The qualitative comparison of %O values shows a similar significant loss of oxygen after irradiation at 0.8 and 4.8 Acm-2, which is reflecting the reduction during WDX analysis itself. Absolutely no oxygen remains detectable after the intense irradiation N°5, indicating reduction to metallic state. Reduction beyond the monoxide phase has been reported by McCartney et al.,24 irradiating crystalline TiO2 with extremely high current densities of 103-104 Acm-2 at 100 keV under TEM conditions. However, to the authors’ best knowledge, this is the first time metallic Ti is reported after moderate e-beam exposure of TiO2 films on macroscopic substrates. Irradiated oxide spots were also analyzed with electrondispersive X-ray analysis (EDX), XPS, and infrared spectroscopy (IR). Using EDX, a reliable determination of the composition of the thin film was found critical because of the small differences in binding energy between TiLR (0.452 keV) and OKR (0.523 keV), adding to the problem of thin film analysis identical to WDX analysis as well as the problem of further oxide reduction during the analysis time itself. Similarly, a precise determination of the local oxidation state with XPS was not possible because of sputter reduction and limited lateral resolution, also being limited with IR, which was not able to trace any chemical change at irradiated spots. A clear chemical proof for the ongoing reduction was finally found from Auger

Figure 5. Qualitative Auger line scans through an ex-situ e-beamirradiated spot (see text for details) in amorphous TiOx (5 C deposition charge, 200 °C annealing). (a) Vertical scan, (b) horizontal scan. The inserts show an optical image of the irradiated spot with a brown center spot and a blue edge, indicative of the presence of TiO and Ti2O3.

analysis. Auger line scans through a previously irradiated spot (24 µA during 600 s, SEM Zeiss DSM) in approximately 80 nm of TiO2-x heated to 200 °C are presented in Figure 5. The optical images in Figure 5a and b show a large brown spot of approximately 50 µm in diameter, surrounded by a blue ring, the former indicating the presence of TiO and the latter being clear evidence for Ti2O3, respectively. The qualitative reduction can also be seen in the O/Ti ratio in Figure 5a and b, decreasing in the most intensively irradiated area confined by the brown spot. In-situ irradiations and analysis with the Auger microscope failed because of the low Auger beam current densities and the difficulties in irradiating a sufficiently large area allowing for detection of local reduction within the oxide. Structural Analysis. The Raman spectrum in Figure 6b after e-beam exposure under condition N°1 (Table 1) does not show any structural change within the film, the amorphous signal being nearly identical to the as-deposited oxide in Figure 6a. When increasing the current density to 4.8 Acm-2 corresponding to condition N°2, a strong anatase signal is observed after 5 s of irradiation in Figure 6d. Indeed, the anatase phase is already detectable after 1 s of exposure (not shown). For comparison, Figure 6c represents the very similar Raman spectra of a nonirradiated, thermally crystallized electrolytic TiOx film after heating at 450 °C/1 h, featuring the characteristic anatase modes at 146 (Eg), 394 (B1g), 515 (A1g + B1g), and 638 cm-1 (Eg).44,45 This film has previously been shown to be nanocrystalline by TEM imaging.35 Under exposure condition N°2, the global substrate heating during 5 s is considered negligible, while the estimated temperature rise of about 130 °C in the beam center according to eq 2 is far below the crystallization

23666 J. Phys. Chem. B, Vol. 110, No. 47, 2006

Figure 6. Laser micro-Raman spectra of electrolytic TiO2 film on AISI 316L (8 C deposition charge). (a) and (c) represent the nonirradiated film before annealing and after annealing at 450 °C, respectively. The other spectra originate from TiO2 films (8 C/250 °C) after EPMA irradiation under different conditions. (b) 0.8 Acm-2 for 60 s (N°1 in Table 1), (d) 4.8 Acm-2 for 5 s (N°2 in Table 1), (e) spot edge 4.8 Acm-2 for 60 s (N°3 in Table 1), (f) spot center 4.8 Acm-2 for 60 s (N°3 in Table 1), and (g) 159 Acm-2 for 60 s (N°4 in Table 1). The spectra give evidence for e-beam-induced reduction of amorphous TiO2 to Ti2O3 and TiO as well as for crystallization into anatase structure.

temperature of 440 °C for electrolytically deposited TiO2 films.36 This decrease in crystallization temperature is even more pronounced than previously reported for irradiation under SEM conditions at 2 Acm-2 during 600 s with additional substrate preheating at 250 °C,34 reaching an estimated temperature of about 270 °C in the beam center. It was shown that the crystallization was limited to the irradiated zone even after long and intense exposure. Under TEM conditions, Aladjem et al.27 have reported a drop of the crystallization temperature in anodic aluminum, niobium, tantalum, and tungsten films under intense irradiation probably because of generation of Frenkel defects. Strong TEM irradiation damage was found to enable crystallization of amorphous zirconia films kept under liquid helium temperature.29 Exposure during 60 s under condition N°3 (Table 1) leads to the Raman spectra in Figure 6e and 6f taken at the edge and in the center of the spot in Figure 3b. At the edge, the anatase signal is still present, yet the most intense peak at 147 cm-1 is weaker than in Figure 6d. Additionally, a broad band attributed to amorphous Ti2O3 (a-Ti2O3) appears in the range of 180360 cm-1. A similar band between 210 and 320 cm-1 has been identified as a-Ti2O3 by del Pino et al.42 after laser irradiation

Kern et al. of Ti. The spectrum in Figure 6f from the center of the same spot looks very different. The characteristic anatase peaks are still present but very weak, while the a-Ti2O3 band is more pronounced than in Figure 6e. The disappearance of anatase TiO2 under at least as intense irradiation conditions as at the edge of the spot is explained by the reduction of anatase into (crystalline) TiO in this area, the monoxide not being Raman active.46 The difference between edge and center is considered as the result of larger heat dissipation at the (colder) edge compared to the (hotter) center. In summary, Raman data indicates the presence of mainly a-Ti2O3 and TiO in the center and a-Ti2O3 and anatase at the edge of the spot irradiated under condition N°3. E-beam exposure at the most intense condition N°4 again reveals significant anatase contributions (Figure 6g) in the Raman laser targeted zone, mixed with three small modes at approximately 212, 267, and 338 cm-1. Three similar peaks have been attributed to β-Ti2O3.46 The spot size of about 3 µm in Raman measurements is slightly larger than the irradiated zone in Figures 3a and 4a. Moreover, since anatase has mainly been reduced to the monoxide already in condition N°3, and since WDX analysis has indicated total oxide reduction in this spot, the Raman signal in Figure 6g must originate from phase changes in the strongly heat affected zone outside the irradiated area rather than from the spot itself. As seen from the ∆T values in Table 1, the substrate material is expected to have an important effect on the beam heating and, hence, on local e-beam-induced crystallization. While no crystallization is expected on TiO2/Si even under irradiation condition N°4 because of the high thermal conductivity of Si, e-beam-induced heating is much more pronounced on ITO substrates where anatase formation might occur already under the weakest irradiation conditions N°1 (0.8 Acm-2), leading solely to oxide reduction on steel substrates. Therefore, local crystallization in a-TiO2 under smaller and more localized (focused) beam currents might be possible in TiO2/ITO. Summary of Analyses. Micro-Raman measurements are in general agreement with observations from optical microscopy, AFM profiling, WDX, and Auger analysis. E-beam exposure leads to an immediate film thickness decrease resulting from oxygen desorption. This reduction provokes a blue color shift because of formation of Ti2O3 mixed with TiO and possibly nonstoichiometricTiO2-x phases within the thin film. The reduction is at least qualitatively detectable by WDX and Auger analysis and is supported by evidence for Ti2O3 and TiO in Micro-Raman spectra. Parallel to the ongoing reduction, the possibility of local crystallization of a-TiO2 into anatase structure under short but intense e-beam irradiation is clearly shown. For this to happen, not only current density and exposure time but also the absolute probe current is supposed to play an important role. Estimations of the temperature in the e-beam center indicate that crystallization can occur at temperatures as low as around 150 °C, far below the crystallization temperature in a furnace at approximately 440 °C for the present films. This phenomenon is likely to be due to the intense radiation damage. Roddatis et al.29 have previously reported crystallization of amorphous zirconia films kept at liquid helium temperatures upon intense irradiation under TEM conditions. Similarly, a change of crystallization kinetics of amorphous aluminum, niobium, tantalum, and tungsten oxide films as a result of radiation damage in the TEM was observed.27 However, the high accelerating voltages under TEM irradiation conditions allow for generation of Frenkel defects through atomic displacement, which might act as nucleation centers for crystallization. This

Patterning of Amorphous Titanium Oxide Films is not possible under the present conditions because of the lower accelerating voltage (20 kV). We therefore suggest that the radiation effect in the present case is a sum of the following mechanisms: 1. Electron-stimulated oxygen desorption (radiolysis) because of inelastic electron scattering.39 2. Beam heating leads to increased rate of radiolysis.41 3. Temperature rise in reductive vacuum environment because of beam heating creates additional reduced Ti3+ defects.16 4. Local temperature rise in radiation damaged area triggers local crystallization. The present amorphous oxides are very sensitive to reduction even at low beam currents and implantation doses, and it seems reasonable to assume that crystallization requiring rather large absolute beam currents as well as current densities will hardly occur without significant reduction in the irradiated area. Therefore, we put forward that reduced crystalline TiO2-x phases may not be distinguished from stoichiometric anatase in Raman spectra, both showing the characteristic anatase signature observed in Figure 6 until the TiO monoxide forms, being Raman inactive because of its monoclinic low-temperature structure.47 At sufficiently large currents and current densities, local reduction to the metallic state can be achieved, as seen from WDX analysis. 3.4. E-Beam-Induced Topographical Patterning. Using electron lithography tools, the relatively high sensitivity of preheated electrolytic a-TiO2 films to e-beam reduction and related volume loss from oxygen desorption is attractive for precise topographical surface patterning with a lateral resolution primarily determined by the electron probe diameter and a height resolution in the nanometer range. Obviously, a topographical change will always be accompanied by a chemical reduction of the oxide or even by a structural change in case of sufficiently intense irradiation. The AFM images in Figure 7 represent two examples of e-beam lithography patterned a-TiO2 films on stainless steel annealed at 150 °C prior to irradiation. The films have an approximate thickness of 60 nm and a roughness of Ra ≈ 1.5 nm as determined by AFM. In Figure 7a, a probe current (Hitachi 3600 SEM) of 6 nA, (aperture 80 µm), a dwell time of 10-3 s, and an increment of 12.5 nm were chosen. Calculations of exposure dose and current density are hardly possible because of significant beam blanking time and lack of precise knowledge of the probe diameter. The circles with 1.5µm diameter in Figure 7a have an average depth of 13.0 ( 0.9 nm, as also shown in the profile of Figure 7c. The topography in Figure 7b was written alike but with 15-nm increment. The lines have a width of 1.5 µm and a depth of 8 nm. Considerably smaller topographies can be achieved using e-beam lithography, which was not the purpose of this study because of difficulties in ex-situ finding of the irradiated areas for AFM analysis. Using the same parameters, the pattern in Figure 7a was also written in a film cured at 200 °C instead of 150 °C, otherwise being identical. Interestingly, the lower annealing temperature resulted in clearly deeper features indicating higher sensitivity under the present low intensity irradiation conditions. The average depth of the circles in Figure 7a in a film annealed at 200 °C is 6.5 ( 0.9 nm (Figure 7c). As seen from XPS (Figure 1b) and WDX (Table 2) data, a lower annealing temperature results in slightly higher oxygen content in the films, which seems to facilitate e-beam-induced desorption of this oxygen. An example for topographical patterning with higher lateral resolution using a field emission gun (Hitachi 4800) is shown in Figure 8 on the same film as in Figure 7a. Point-wise irradiations were performed with 4.7 nA (aperture 100 µm, spot

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Figure 7. (a), (b) AFM images of SEM e-beam-induced topographical patterns (IP ) 6 nA) in amorphous TiOx films on steel annealed at 150 °C (5 C deposition charge). (c) AFM profile through pattern in Figure 7a and comparison to same pattern written in film annealed at 200 °C. The oxide with the lower annealing temperature has a higher sensitivity toward electron-stimulated oxygen desorption.

Figure 8. (a) AFM image after SEM e-beam irradiation (IP ) 4.7 nA, t ) 60 s) of an electrolytic TiOx film on steel annealed at 150 °C (5 C deposition charge). (b) Comparison of profiles after irradiation during 10 and 60 s, indicating nonlinear behavior of desorption kinetics.

size 1) during 1-60 s. A large spot size was chosen for increasing the probe current, clearly in detriment of the probe size. Figure 8a shows the surface after irradiation during 60 s. The profile in Figure 8b after 10 and 60 s of exposure reveals a Gaussian shape, reflecting the energy distribution of the beam

23668 J. Phys. Chem. B, Vol. 110, No. 47, 2006 current. No sign of sample drift during irradiation is observed. The fwhm values for the Gaussian fits are 75 nm (10 s) and 71 nm (60 s), respectively. The heights are 5.0 and 2.9 nm and the ratio of the volume loss for both irradiations is 1.6. These findings are in agreement with the proposed desorption mechanism, predicting that ESD of oxygen slows down with exposure time, as a result of increasing conductivity of the reduced oxide. 4. Conclusion The possibility of controlling on a local scale reduction and crystallization in electrolytically deposited amorphous TiOx films on well conductive steel substrates has been demonstrated. The oxygen loss changes film topography as well as chemistry and is explained by a radiolysis mechanism. In the present work, experimental evidence for this mechanism is obtained by a nonlinear desorption behavior when performing time-dependent irradiations, the oxygen desorption slowing down and finally stopping when the irradiated area becomes sufficiently conductive to stifle any further interatomic Auger decay. The e-beam sensitivity of the amorphous films was found to depend on the annealing temperature of as-deposited films. An increased sensitivity with lower annealing temperatures coincides with a higher oxidation state of the oxide films, facilitating oxygen desorption according to the discussed mechanism. The local changes in oxidation state and phase are strongly dependent on current density as well as irradiation dose. Current densities below 1 Acm-2 primarily lead to TiO2-x and Ti2O3 formation, while current densities around 5 Acm-2 additionally induce rapid local crystallization as well as oxide reduction to the monoxide state after about 60 s of irradiation. Reduction to the metallic state was observed when exposing at approximately 160 Acm-2 during 60 s. At this condition, the temperature in the beam center is expected to rise to around 620 °C. For crystallization to happen at a given current density, we expect the absolute beam current to have to exceed a threshold value, which is not the case for reduction, depending mainly on the current density. This threshold value for the beam current needs to be investigated in more detail. The reported effects are interesting for local modifications of oxidation state and phase within an otherwise uniform amorphous oxide known for its semiconducting and biocompatible properties. Using electron lithographic tools, nanometerprecise topographical patterns with the lateral resolution primarily given by the electron probe diameter can be achieved within reasonable exposure times. Furthermore, the local change in oxidation state and phase is expected to have a great influence on local electrical, optical, and catalytic properties of the irradiated oxide. Local changes of the electrical conductivity at irradiated spots within the present films are currently under investigation. Acknowledgment. The authors would like to thank the following people from EMPA Thun: Ch. Ja¨ggi for micro-Raman measurements, G. Bu¨rki for help with the Hitachi 4800 SEM, and V. Friedli and I. Utke for helpful discussions related to e-beam irradiation phenomena. Special thanks go to N. Xanthopoulos (EPFL, Lausanne, CH) in connection to Auger analysis. References and Notes (1) Bange, K.; Ottermann, C. R.; Anderson, O.; Jeschkowski, U.; Laube, M.; Feile, R. Thin Solid Films 1991, 197, 279. (2) Sawada, Y.; Taga Y. Thin Solid Films 1984, 116, L55. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69.

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