IRRW$ ELSEVIER
Thin Solid Films 300 (1997) 113-121
Microstructure modification of amorphous titanium oxide thin films during annealing treatment Nicolas Martin a, Christophe Rousselot a, Daniel Rondot a, Franck Palmino b, Rend Mercier ° Laboratoire de Mdtrologie des Interfaces Techniques, 4 place Tharradin, F-25200 Montbdliard, France b Equipe d'Electronique des Solides, Laboratoire de Physique et de M#trologie des OsciItateurs, 4 place Tharradin, F-25200 Montb~liard, France c Laboratoire d'Electrochimie des Syst~mes Microdispers&, Route de Gray, F-25000 Besancon Cedex, France
Received 15 July 1996; accepted31 October1996
Abstract
Thin films of titanium oxide have been deposited on (100) silicon wafers and on quartz substrates by reactive r.f. magnetron sputtering from a 99.6% pure Titanium target. Amorphous and overoxidised coatings (TiO2.2) have been obtained from this technique. The influence of the post-deposition annealing between 300 °C and 1100 °C on the structural and optical properties and on the surface morphology has been investigated. The results of X-ray diffraction showed that films annealed from 300 to 500 °C have an anatase crystalline structure whereas those annealed at 1100 °C have a futile crystalline structure. Optical analyses showed that UV-Vis transmission spectra are strongly modified by the annealing temperature and refractive index of TiOx layers also changes. Atomic force microscopy measurements corroborate optical and structural analyses and showed that the surface of the coatings can have various appearances and morphologies for the annealing temperatures investigated. © 1997 Elsevier Science S.A. Keywords: Atomicforce microscopy(AFM); Opticalproperties;Phase transitions;Physicalvapourdeposition(PVD)
1. Introduction
Titanium dioxide is one of the most extensively studied transition-metal oxides [1-6]. The preparation of TiO 2 thin films have been received the greatest attention during the last decade because of its remarkable optical and electronic properties (refractive index, dielectric constant . . . . ). Many different procedures for the preparation of thin films of titanium oxide materials are reported in the literature [7-9]. With all these deposition methods, TiO 2 films can be made with largely varying structural and optical properties. R.f. magnetron sputtering can be used to deposit this type of coating. In addition this tec.hnique is of special interest because it is an industrial process applicable tO large-area deposition and because high quality films can be achieved even at low substrate temperatures. TiO 2 exists in three different crystalline phases: anatase, rutile (both tetragonal) and brookite (orthorhombic). When the substrate temperature during deposition is low, amorphous TiO 2 films are often observed [7,10]. Only rutile and anatase have been observed in thin films up to now. Ruffle being the most dense (4.25 g cm -3) and the most thermodynamically stable, several experimental and theoretical investigations have been carried out about it. In 0040-6090/97/$17.00 © 1997 ElsevierScienceS.A. All rights reserved. PII S0040-6090(96)09510-7
contrast to the extensive studies on ruffle phase, very little is known about anatase and brookite phases. In this paper, we report a study of the effect of the annealing temperature on some optical and structural properties and on the surface morphology of titanium oxide films prepared by r.f. magnetron sputtering using an argon + oxygen mixture. The coatings have been deposited on (100) silicon wafers and quartz substrates and they have been annealed in air for 1 h and at various temperatures (300-1100 °C). The optical properties have been measured with UV-Vis spectrophotometer and the structural properties have been determined by X-ray diffraction (XRD). The surface morphology of TiO x layers have been analyzed using atomic force microscopy (AFM). Rutherford backscattering spectroscopy (RBS) measurements revealed overoxidised films (TiO x with x = 2.2) with the oxygen partial pressure used for the preparation [11]. These analyses showed that the annealing temperature has a strong influence on the structure of the films. When the annealing temperature increases, anatase is observed at 300 °C and then is transformed in futile between 700 and 900 °C. In addition, optical measurements showed that annealing can modify the density of TiO x layers and can change their refractive index and the position of absorption
114
N. Martin et a L / Thin Solid Fibns 300 (1997) t]3-12t
edge. The AFM studies have been correlated with the evolution of optical and structural properties from the estimation of the root mean square (RMS) roughness of the films.
The AFM micrographs were achieved with a Digital Instruments' Nanoscope III with an AFM tapping mode. The RMS roughness of the surface was calculated (with flatting corrections) on the z scale as a standard deviation Rq with the same device.
2. Experimental details 3. Results and discussion Titanium oxide fitms were prepared with a r.f. magnetron sputtering system SCM 650 (Alcatel). The deposit system is a stainless-steel vacuum chamber of 100 dm 3. The background pressure in the process chamber was 5 × 10 -5 Pa using a turbomolecular pump backed by a mechanical pump. High purity argon (N60) and oxygen (N45) were used as sputtering and reactive gases respectively. The target was a titanium disk (purity, 99.6%) of 20 cm diameter and was powered by a r.f. generator at a frequency of 13.56 MHz. The flow rates of argon and oxygen were controlled with a mass flowmeter MKS and pressures were measured with Pirani and Penning pressure gauges. The discharge was generated at a constant r.f. power of 1000 W and the flow rate of Ar and O 2 were kept at a constant value of 30 sccm (standard cubic centimeter per minute) and 4 sccm respectively in order to have experimental conditions close similar to those used in a previous paper [11]. So, argon and oxygen partial pressures were kept to 9.0X 10 -1 Pa and 6.0 × 10 .2 Pa respectively for the deposition. The optimum target-substrate distance was determined to 60 mm. Before each run, the chamber was pumped down to 5 X 10 .5 Pa and the target was pre-sputtered in a pure Argon atmosphere for 10 rain in order to remove the surface oMde layer on the target. Afterwards, Oxygen was introduced into the chamber and the target was sputtered in an argon + oxygen mixture. Stoichiometry of titanium oxides deposited on (100) silicon wafers substrates was determined by Rutherford backscattering spectroscopy (RBS) using a 2 MeV 4He ion beam. A simulated spectrum has been compared with the experimental one and corrections, adjustments of concentrations and compositions have been carried out in order to obtain the best agreement between the experimental and the theoretical spectrum. The thickness of the films was measured using a Taylor-Hobson Talysurf profilometer and the optical properties were determined from the UV-Vis transmission spectrum of the films deposited on quartz substrates using a UV-Vis Secomam $750 spectrophotometer. According to the Swanepoel's method [12], analyses of the optical transmittance of a transparent film revealed refractive index n, absorption coefficient o~, extinction coefficient k, thickness and optical bandgap Eg. X-ray diffraction (XRD) measurements were carried out using a Siemens-Inel computer-controlled diffractometer. Cobalt Koe radiation (k K = 1.78892 A) was used from a X-ray tube with normal focus.
3.1. X-ray diffraction measurements Fig. 1 shows the XRD patterns of 4000 ,~ thick films deposited on (100) silicon wafers and annealed at different temperatures. By comparing our results with powder diffraction ASTM card files (futile, ASTM card # 21-1276; anatase, ASTM card # 21-1272) and spectra in Table 1, it is found that annealed coatings consist of anatase and rutile phases. It can be seen that the films exhibit an amorphous structure before annealing. This kind of coatings has ever been observed by several authors using deposition methods with a low substrate temperature [7,10,13]. One can suppose that the as-deposited films are amorphous (or weakly crystallized) due to the low energy and the low mobility of particles impinging on the " e N d " substrate (slow surface diffusion). Lob1 et al. [7] have drawn a qualitative diagram showing the experimental conditions for the occurrence of amorphous, anatase or rutile titanium dioxide films (Fig. 2). Substrate temperature and energy of the particles impinging on the substrate are the relevant parameters. The first traces of long-range order appear when the coatings are annealed at 300 °C. Calcihation in air at temperatures between 300 and 700 °C induces the appearance of diffraction peaks at the positions corresponding to the Anatase structure. In addition, the films show a random orientation at 300 °C and have a preferred orientation along the (101) direction when the annealing temperature reaches 700 °C. From the literature [14-17], single-phase anatase has been obtained by reactive sputtering with similar experimental conditions and the same type of preferred orientation along the (101) crystal plane has been observed. The well-developed ruffle structure has been produced from the post-deposition annealing in air at 900 or 1100 °C. As illustrated in Fig. 1, (110) planes seem to be the preferential orientation. This kind of structure is usually obtained for titanium dioxide films prepared at high substrate temperature or after a post-deposition annealing [10,18,19]. So, the anatase-rutile transformation has been observed between 700 and 900 °C. From Exarhos [20], for TiO 2 films made by r.f. sputtering, the amount of ruffle increases strongly when the annealing temp.erature reaches 700 °C. This is approximately the temperature where the transformation from anatase to rutile occurs under equilibrium conditions [7]. Batffston et al. [9] have studied the anneal-
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N. Martin et at./Thin Solid Films 300 (1997) 113-121
Table 1 Comparison with anatase and rutile powders clearly shows phase transformation. The increase of the annealing temperature causes crystallization of the films. Anatase crystalline structure can be observed from 300 °C until 700 °C. Then, first crystals of rutile appear and the material is completely transformed to rutile when the temperature reaches 1100 °C. Anatase
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3.533 3.250 2.487
2.431 2.378 2.332
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1.493 1.481
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1.647
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1.508
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2.299 2.193 2.058 1.898
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Fig. 3. Influence of the annealing temperature on the tetragonal structure of titanium oxide films deposited by reactive r,f. sputtering magnetron, Evolution of celt parameters of futile and anatase phases.
iV. Martin et al. / Thin Solid Films 300 (1997) 113-121
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3.2. Optical properties
Fig. 6. The evolution of absorption coefficient with wavelength of titanium oxide films as deposited and for various annealing temperatures until 700 °C.
From the method of Swanepoel described in a previous pap.er [11], optical properties of transparent films can be determined [11,21-23]. So, refractive index n, absorption coefficient a, extinction coefficient k and optical bandgap Eg of titanium oxide layers deposited on quartz substrates have been determined in the visible region by simple straightforward calculations using the transmission UV-Vis spectrum alone. Fig. 4 shows the transmission spectrum of annealed coatings (6000 A thick) in air for 1 h and for different temperatures. It can be seen that the transmittance of the fflrns decreases weakly when the materials are calcined up to 700 °C. Since XRD analyses did not show a change of structure until 700 °C but only the presence of anatase phase, it can be assumed that coatings become denser and denser with the annealing temperature. Then, refractive index has been determined in order to show the densification (since the optical transmission of coatings annealed at 900 °C and 1100 °C is too weak in the visible region,
Swanepoel's method does not allow to calculate refractive index and the other optical properties). Fig. 5 illustrates the evolution of refractive indexes as a function of wavelength for titanium oxide films annealed in air at different temperatures and as deposited (ellipsometric measurements were performed to determine refractive index of few samples in order to cross check the data obtained by optical transmission; good agreement has been obtained between Swanepoel's method and ellipsometry [11]). As can be seen, refractive index increases with temperature. From the density and refractive index of bulk rutile and anatase (%ut = 2.90 and nana 2.49 for hN~ = 589 nm; densityR~t = 4.2 and densitYA~a = 3.2), this effect can be attributed to a densification of the layers. The absorption coefficient and extinction coefficient have been also calculated from refractive index values with the method described in Ref. [11]. Figs. 6 and 7 show the variations of these optical properties with the wavelength and with the annealing temperature. It is seen that =
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118
N. Martin et al. / Thin Solid Films 300 (1997) 113-121
the main tendency of both coefficients is to increase with calcination. So, the evolution of extinction coefficient can be related to the variation of transmittance. The transmittance decreases and this results in the increase in the extinction coefficient. It can be supposed that the compactness of titanium oxide films changes from a porous structure into a dense structure. This means that the packing density of the films increases when the annealing temperature increases to reach a maximum at 700 °C. With regard to the layers heat-treated at 900 and t 100 °C, optical properties like refractive index or extinction coefficient cannot be calculated by the method described by Swanepoel. In fact, absorption is too strong and the interference fringes disappear. Then, if we take into account XRD measurements, the futile phase has been obtained for high annealing temperatures. It can be assumed that layers become denser and denser, but the sudden decrease of transmission and the phase evolution observed at 900 °C suggest that the major contribution comes from the phase transformation. This may be responsible for the changes in structure of the material but also for the increased scattering losses due to the surface modifications. This point of view will be corroborated with AFM measurements in the next section. Since TiO 2 is a semiconductor with a la'ge bandgap [3,19,24], the optical bandgap Eg can be determined from absorption coefficient oe. The sharp decrease in the transparency of the thin films in the UV region is caused by the fundamental light absorption of the semiconductor. The absorption coefficient oe, which depends on the wavelength A, can be obtained by using the following relationship [25,261 T=
(1 - R ) 2 e -~'t
(1)
where T is the transmittance, R is the reflectance, a is the absorption coefficient and d is the film thickness. Eq. (1) is of course, only valid close to the optical bandgap of the material under the condition exp(2oed)>>R 2. When scattering effects are neglected, the absorption coefficient may be expressed by ~
(h,o-~)
m
(2)
where Eg is the optical bandgap. For the indirect allowed transitions m = 2, but, for the direct forbidden transitions m = 1.5 [24]. Both experimental results and theoretical calculations suggest that TiO 2 has a direct forbidden gap (3.03 eV), which is almost degenerated with an indirect allowed transition [3,27]. Due to the weak strength of the direct forbidden transition, the indirect allowed transition dominates in the optical absorption just above the absorption edge [15]. Then, the absorption coefficient above the threshold of fundamental absorption follows the (E - E~)2 energy dependence characteristic of indirect allowed transit.ions as illustrated by the c~i/2 versus photon energy (E) plot in Fig. 8. The extrapolated optical absorption gaps of
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films have been determined. It can be seen that the optical bandgap decreases (3.33 to 3.25 eV) when the annealing temperature increases. Radecka et al. [10] obtained similar evolution of Eg with the temperature. Then, it can be supposed that the variation of density, and the structural modifications may be responsible for changes in the shape of the fundamental absorption edge. However, since it is generally accepted that the electronic disorder resulting in the electronic conductivity of titanium dioxide is introduced by oxygen non-stoichiometry, it should be interesting to measure the composition of the films after the heat treatment for different temperatures. This point will be investigated in another paper.
3.3. Surface morphology by AFM Atomic force microscopy is sometimes used to study the surface topography of thin films [28-30] and is commonly used to observe the atomic-scale surface structure of materials like titanium dioxide [3t-35]. To this aim, AFM micrographs of titanium oxide films surface deposited on (100) silicon wafers have been carried out. The influence of the annealing temperature on the surface morphology has been studied. It can be seen in Fig. 9 that appearance of the surface depends strongly on the thermal treatment. Layers weakly heated (less than 700 °C) are made up of nodules. The size of these nodules depends little on the temperature as it can be seen in Fig. 9(b) and 9(c). Fig. 9(a) shows the surface of sample as deposited. It is also composed of nodules 30-50 nm diameter. When coatings ,are annealed at temperatures up to 700 °C the surface morphology appears quasi-similar as untreated films. When the temperature reaches 900 °C, there is a distinct change of the surface structure. Fig. 9(d) shows that previous nodules observed now have facets. In addition, the size of the particles increases strongly up to 100-200
N. Martin et a l . / Thin Solid Films 300 (1997) ]13-121
119
Fig. 9. Surface morphology by AFM of titanium oxide films deposited by reactive r.f. sputtering magnetron. Coatings have been annealed in air at different temperatures for 1 h. (a) as-deposited, (b) 300 °C, (c) 700 °C, (d) 900 °C, (e) 1100 °C, (f) 1100 °C.
120
N. Martin et al. / Thin Solid Films 300 (1997) t t3-t21
Thus, similarly as optical and structural results, a transition zone in terms of annealing temperature can be defreed. Until 700 °C, the film structure changes gradually from porous into compact and dense. If the temperature passes 900 °C, surface topography changes and futile titanium oxide crystals appear. An intermediate region is then defined for annealing temperatures contained between 700 and 900 °C.
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200
400
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Annealingtemperature (~C) Fig. t0. The influence of annealing temperature on RMS roughness (Rq) of TiO x layers. RMS as calculated (with flatting corrections) on the z scale from the AFM images.
nm and they seem to be less coalesced than those observed at lower temperatures. For a 1100 °C annealing temperature, materials become more and more organized and the surface is made up of small crystals. Terraces and faces can be observed in Fig. 9(e) and 9(f). The grain size exceeds 500 nm and large grains well crystallized and separated can be easily distinguished. In order to support the AFM observations, RMS roughness measurements of annealed films have been carried out (with flattening corrections) on z scale as a standard deviation Rq with the AFM device. As can be seen in Fig. 10, the RMS follows a similar evolution as previous microscopic observations. RMS values decrease from 4.2 to .2.8 nm when the annealing temperature increases until 500 °C. Then, if the temperature increases further, the RMS increases as well from 700 °C and reaches 20 nm for I I00 °C. S o , the strong increase of RMS when the annealing temperature is higher than 700 °C corroborates the results obtained with the AFM micrographs. Moreover, if XRD results are taken into account, it can be noticed that rutile phase has been clearly determined for annealing temperatures higher than 900 °C. Thus, crystals observed from AFM are manly composed of futile phase. Concerning the decrease of RMS until 500 °C, this phenomenon can be correlated with the evolution of optical properties previously obtained in Section 3.2. Since the anatase phase has been obtained from XRD analyses between 300 and 700 °C and particles size does not clearly change, it can be supposed that the decreasing of RMS in this range of temperature is chiefly due to an increase of the coalescence and the diffusion among the grains. Then, this supports UV-Vis spectroscopy results. An increase .of refractive index of titanium oxide films with annealing temperature has been noticed. RMS measurements and optical analyses are in agreement and can be correlated to the evolution of refractive index.
Reactive r.f. sputtering magnetron was used to deposit overoxidised TiO2.2 thin films. At the substrate temperature of 150 °C as-sputtered films were amorphous (or weakly crystallized). Coatings were annealed in air for 1 h. Structural studies showed that Anatase phase was developed until 700 °C. Optical properties such as refractive index or extinction coefficient can be influenced in a wide range by the annealing temperature. A densification of the layers was observed when the heat-treatment increased. AFM micrographs and RMS measurements supported this observation. The optical bandgap was determined for coatings annealed until 700 °C. The absorption edge of the films moved towards lower photon energies as the annealing temperature increased. A reduction of Eg from 3.33 to 3.25 eV was noticed. This evolution is not yet clearly explained and other investigations are in progress especially about the stoichiometry of annealed films. Annealing in air up to 1100 °C for 1 h induced the complete anatase-rutile transformation. Small crystals (rutile from XRD) were observed by AFM and a strong increase of RMS roughness was measured. The appearance of this stable phase was also correlated with optical transmission spectrum, which was strongly affected by post deposition annealing particularly when the temperature passed 900 °C. A transition zone in terms of annealing temperature was defined. A temperature range where the transformation from anatase to mtile occurs was characterized between 700 and 900 °C. Evolution in optical and structural properties were noticed. So, annealing in air in this temperature range caused crystallization of both the anatase and rutile phase. Thus, using an appropriate annealing procedure, a single titanium oxide phase structure or a mixture of anatase and rutile can be obtained.
Acknowledgements The authors would like to thank C.T.I.T.S. (Centre de Transfert Industriel en Traitement de Surface - Montb~liard) and D.U.P.M. (District Urban du Pays de Montb~liard) for technical and financial support.
N. Martin et aL / T h i n Solid Films 300 (1997) t13-121
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