Relations between the optical properties and the microstructure of TiO2 thin films prepared by ion-assisted deposition

Thin Solid Films 359 (2000) 171±176 www.elsevier.com/locate/tsf

Relations between the optical properties and the microstructure of TiO2 thin ®lms prepared by ion-assisted deposition Y. Leprince-Wang a,*, D. Souche b, K. Yu-Zhang a, S. Fisson b, G. Vuye b, J. Rivory b a

Laboratoire de Physique des MateÂriaux DiviseÂes et Interfaces, Universite de Marne-la-ValleÂe, 5 Bd. Descartes, 77454 Marne-la-ValleÂe Cedex 2, France b Laboratoire d'Optique des Solides, UMR CNRS 7601, Universite P. et M. Curie, 4 place Jussieu, Case 80, 75252 Paris Cedex 05, France Received 14 April 1999; accepted in ®nal form 17 September 1999

Abstract Oxygen ion-assisted TiO2 thin ®lms have been studied by in situ visible spectroscopic ellipsometry (SE) and transmission electron microscopy (TEM). In¯uence of the substrate nature and the substrate temperature, the ion kinetic energy Ec and the ion/molecule ratio F i/F at was investigated on the microstructure and the optical properties of the ®lms. It is revealed that the refractive index n varies as a function of the average energy per TiO2 molecule, Ed ˆ Ec …Fi =Fat †. Conditions for obtaining dense ®lms with a high refractive index (n ~ 2.60 at l ˆ 0.45 mm) and a low extinction coef®cient k have been found for Ed . Edth (Edth ~50 eV). These dense ®lms are insensitive to moisture adsorption with a low surface roughness. Cross-sectional TEM has been mainly used for microstructure observation and phase identi®cation of the ®lms prepared under different evaporation conditions. Comparison is done in relationship with the optical property measurements. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Titanium dioxide; Ion bombardment; Transmission electron microscopy; Ellipsometry

1. Introduction Titanium dioxide TiO2 thin ®lms have applications either as an optical coating material or as a protective layer for very large scale integrated circuits, because of their high refractive index, excellent optical transmittance and good insulating properties. It is known that the growth morphology, the crystal structure and the stoichiometry of TiO2 ®lms are very sensitive to the deposition conditions. The parameters like oxygen partial pressure, substrate nature, substrate temperature and evaporation rate play important roles in the coating quality [1±3]. Thus, the optical properties of the TiO2 ®lms, such as refractive index, extinction coef®cient and scattering losses, are strongly process-dependent. Usually, the TiO2 ®lms prepared by physical vapor deposition (PVD) exhibit low packing density resulting in low refractive index values and high sensitivity to environment. Therefore, more energetic ion-activated technologies have been developed. Among them, the ion-assisted deposition (IAD) is largely used [4] because of its good adaptation to the coating chambers. * Corresponding author. Tel.: 133-160-957-276; fax: 133-160-957297. E-mail address: [email protected] (Y. Leprince-Wang)

In this paper, we focus our attention on the relationship between the microstructure and the optical properties of IAD TiO2 ®lms as a function of the ion assistance conditions. The results presented here have similar features to those reported in a previous paper on the IAD SiO2 ®lms [5], in particular with regard to the refractive index behavior in the visible range. However, the existence of the crystalline phases in TiO2, anatase or rutile, enlightened by the cross-sectional TEM observations allows encountering a larger range of situations than for the SiO2 ®lms. 2. Experimental details The ®lms under study have been evaporated by an electron gun in a UHV chamber prepumped down to a pressure of 10 28 Torr. The starting material is a commercial mixture of titanium oxides (OS50, Optron Ltd., Japan). The ion bombardment has been performed by a two-grid Kaufman-type source aimed at the substrate at an angle of incidence of 458. The ion gun was fed by a mixture of 90 vol.% oxygen and 10 vol.% argon gases which gave the total working pressure of 10 24 Torr during the process. In order to study the in¯uence of the substrate nature on the ®lm properties, three different substrates have been chosen: ¯oat glass, thermally grown SiO2 on Si, and fused

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silica. Two substrate temperatures Tsub have been also investigated: 1508C and 2508C. 1508C, because one of the interests of the IAD is to produce ®lms with good compacity at low temperature; 2508C, in order to determine the role of temperature on the crystallinity under ion bombardment. Concerning the in¯uence of the ion kinetic energy, two energy levels (150 eV and 300 eV) were studied; in each case the incident ion ¯ux density F i has been varied within the limits of the gun possibilities. The F i values were then measured both at the beginning and at the end of the deposition by a planar polarized probe. It should be noted that two parameters, the ion kinetic energy Ec and the ratio of the incident ion ¯ux density to the atom or molecule ¯ux density F i/F at, are commonly used to study the in¯uence on the microstructure and the phase composition of IAD thin ®lms [6,7]. Nevertheless, for simplicity's sake, we would discuss our results in terms of the average energy deposited per incident molecule, Ed ˆ Ec …Fi =Fat †, although Ed is not a universal parameter [7]. Therefore, the values of Ec and F i/F at (this quantity is calculated by assuming an average ®lm density of 3.8 g/cm 3) as well as Ed are indicated in Table 1, for each sample. The ®lms were characterized at ®rst by ellipsometry in two ways: during deposition, measurements were done at a single wavelength (l ˆ 0:45 mm) at an angle of incidence close to 758 (their optical thickness is of the order of one half wave, i.e. their physical thickness is around 100 nm); the growth of the ®lm is then represented as a trajectory in the (tanC , cosD) plane. After deposition, the SE measurements were performed between l ˆ 0:4 and 0.8 mm. Both sets of information are important and complementary. Models used for reproducing the SE spectra and the trajectories were reported in [8] (Fig. 2). They allow determination of the refractive index n and the thickness of the ®lm d by taking into account the presence of a super®cial layer and of a slight inhomogeneity of the index in the depth of the ®lm. The extinction coef®cient k mentioned in Table 1 was measured by photothermal de¯ection spectroscopy (PDS) at l ˆ 1:06 mm. For the TEM investigations, cross-sectional specimens were prepared by a sequence of mechanical thinning steps with the Tripod method [9]. This new preparation method allows a very large observation zone and a short ion milling time. In our case, according to the different substrates used

for TiO2 depositions, the samples of TiO2 deposited on thermally grown SiO2 on Si were tripod thinned until ~5 mm and then ion milled about 1 h; the samples of TiO2 deposited on the other substrates (¯oat glass and fused silica) are generally brittle and sensitive to ion bombardment. The tripod thinning should be stopped at a thicker stage (15±20 mm) and the cold ion milling procedure was necessary to prevent ion-bombardment artifacts. The microstructural observations and the structural characterizations were performed using a Topcon 002B transmission electron microscope operated at 200 kV acceleration voltage with Cs ˆ 1 mm. Complementary techniques were also used for ®lm characterizations: the TiO2 density was measured using X-ray re¯ectometry at grazing incidence, the ®lm stress measured by the bending of a ¯at thin disk of glass, and the root mean square (r.m.s.) roughness deduced from atomic force microscopy (AFM) experiments.

3. Results and discussion 3.1. Optical properties Fig. 1 shows the variation of the refractive index n as a function of the average energy Ed of the IAD TiO2 ®lms measured during deposition at l ˆ 0:45 mm [10]. Most of the ®lms were deposited at T sub ˆ 1508C. Two different regions are clearly identi®ed in Fig. 1 by an energy threshold Edth ~50 eV. For Ed , Edth , the refractive index n increases rapidly with Ed; for Ed . Edth , n remains roughly constant with a mean value of 2.60. Nevertheless, when Ed . 400 eV, the ®lms are damaged by ion bombardment and are no longer transparent. The results obtained from two ®lms deposited at T sub ˆ 2508C are also reported in Fig. 1 (samples T5 and T6). It can be seen that higher values of n are obtained by increasing the substrate temperature; but for high assistance level (Ec ˆ 300 eV, Ed ~400 eV), the ®lms become inhomogeneous and absorbing. Below Edth, ion bombardment induces a compaction of the ®lm, but the densi®cation is still incomplete and pores remain, leading to moisture absorption and producing an increase of n after venting. Above this threshold, the ®lms are stable and insensitive to their environment. A similar

Table 1 Characteristics of the TiO2 ®lms under study Sample

Substrate

Tsub (8C)

Ec (eV)

F i/F at

Ed (eV)

n at l ˆ 0:45 mm

r (g/cm 3)

k at l ˆ 1:06 mm

T1 T2 T3 T4 T5 T6

Float glass Float glass Float glass Thermal SiO2 Thermal SiO2 Fused silica

150 150 150 250 250 250

± 150 300 ± 150 300

± 0.40 1.60 ± 0.45 1.20

± 60 480 ± 68 360

2.36±2.39 a 2.60 2.63 2.45 2.63 2.65±2.80

3.45 3.76 3.80 ± 3.88 4.15

± ± ± ± 7.0 £ 10 23 2.6 £ 10 22

a

Ê) Roughness r.m.s. (A 7 3 3 13

When two values are indicated in the column of n, the ®rst one is the refractive index of the ®lm near the substrate, the second one near the air surface.

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model proposed by Movcham and Demchishin [11]; while the ®lms deposited on ¯oat glass were more related to zone 2 of the SZ model. The migration of Na ions from the ¯oat glass substrate towards the ®lm surface was put forward in order to explain these differences.

Fig. 1. Variation of the refractive index at l ˆ 0:45 mm of the IAD TiO2 ®lms measured in vacuum as a function of the average deposited energy for two ion kinetic energies. The arrows indicate the results after aging in air when an index increase has been detected. All ®lms have been deposited at T sub ˆ 1508C, except two of them labelled T5 and T6 for which T sub ˆ 2508C. For T6, only the index at the ®lm-substrate interface has been reported (see Table 1).

evolution of the refractive index as a function of Ed has been reported for SiO2 ®lms prepared under similar experimental conditions [5] and discussed in terms of porosity, the SiO2 structure remaining amorphous. In the case of TiO2 ®lms, changes are to be expected, not only in the ®lm porosity but also in their crystalline structure. The results obtained for four typical samples will be discussed in the following. Their characteristics are summarized in Table 1. Two ®lms prepared without ion assistance are also presented for comparison. 3.2. Microstructure As in the case of the optical results, it is worth discussing the microstructural properties of our ®lms deposited at two different substrate temperatures, separately. For T sub ˆ 1508C, the substrate is ¯oat glass and for at T sub ˆ 2508C, the substrate is SiO2 thermally grown on Si or fused silica. In fact, the crystalline phases and microstructure present in the ®lms depend on the nature of the substrate. By examination of the TiO2 ®lms deposited on various glass substrates at 3008C, Chen et al. [2] underlined that (1) the presence of Al2O3 in the substrate tends to enhance the rutile phase formation and to increase the extinction coef®cient of the ®lms; (2) the presence of Na2O tends to delay the rutile phase formation and to give a lower extinction coef®cient. On the other hand, our previous work on the PVD TiO2 ®lms deposited on Si, SiO2 and ¯oat glass at T sub ˆ 2508C shows different microstructures of ®lm growth [8]. The ®brous characteristic was observed for all ®lms deposited on Si or on SiO2, which corresponds to the zone T of the Structure Zone (SZ)

3.2.1. Films with substrate temperature Tsub 1508C Although the migration of Na was not observed for ¯oat glass at this temperature [10], TEM observation of the ®lm T1 (Fig. 2a) prepared without ion assistance shows large polycrystalline TiO2 grains (about 70 nm in diameter). Moire defaults are frequently observed in each grain bloc, signifying mediocre crystallinity of the ®lm. This microstructure characteristic can be related to the zone 2 described in the SZ model. Attention should also be paid to the interface structure: some amorphous zones were detected at the ®lm±substrate interface before the TiO2 crystalline grain growth, as observed in the previous work [8]. In fact, the ®rst growth stage of TiO2 thin ®lms is always amorphous [12]. Phase identi®cation performed by electron diffraction patterns results mainly in the anatase phase. Nevertheless, few spots appeared in the diffraction patterns are identi®ed as the rutile phase. As TiO2 ®lm is sensitive to electron irradiation [12], we are not certain that the random presence of the rutile phase detected by TEM observation is related to the deposition conditions. Fig. 2b shows the bright-®eld image and corresponding electron diffraction pattern taken from the sample T2 prepared with ion assistance. The Ed value is chosen just above Edth (Ec ˆ 150 eV, Ed ˆ 60 eV). Comparing to the non-assisted T1 sample, the IAD ®lm T2 exhibits a rather different microstructure: only very small crystallites have been detected in an amorphous matrix, as indicated by the arrow in Fig. 2b. The electron diffraction patterns con®rm the image analysis by the presence of the large diffuse rings and one partial, slightly discontinuous ring. However, the partial ring intensity of these crystallites is too weak to be used for phase identi®cation and for taking a dark-®eld image showing the small crystallite morphology better. One can also note the existence of a continuous layer of about 20 nm thickness at the ®lm±substrate interface, due to probably the ion bombardment at the substrate during evaporation preparation. Moreover, this IAD ®lm prepared with Ed around the Edth has its refractive index on the plateau shown in Fig. 1. Fig. 2c shows the TEM observation results of the sample T3 prepared with higher ion assistance energy (cf. Table 1). Its bright ®eld image reveals a similar microstructure to that of the T2 sample. Meanwhile, the crystallites become bigger so that more discontinuous rings can be seen in the corresponding diffraction patterns even if they are still superimposed on the diffuse amorphous rings. Phase identi®cation con®rms the presence of the rutile phase related to higher energy preparation conditions of the ®lm. Now, let us discuss the in¯uence of the energy level on the IAD TiO2 ®lm properties. As we mentioned above, ®lms

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Fig. 2. Cross-sectional TEM images and diffraction patterns of the TiO2 ®lms deposited at T sub ˆ 1508C on ¯oat glass. (a) Without ion assistance: sample T1. (b) Ec ˆ 150 eV, Ed ˆ 60 eV: sample T2. (c) Ec ˆ 300 eV, Ed ˆ 480 eV: sample T3.

with n~2:60 at l ˆ 0:45 mm and stable against moisture adsorption have been obtained at the substrate temperature of 1508C when Ed . Edth . In the case of sample T2 (Ed just above Edth), the measured ®lm density is close to the anatase one (rana ˆ 3:84 g cm 3) and the ®lm stress is found tensile as for the PVD sample T1. By contrast, when Ed increases further, the ®lm stress becomes compressive because of the implantation of forward recoils; this is the case for sample T3. From the microstructure comparison undertaking by TEM study, we prove a quite homogeneous amorphous structure of TiO2 for sample T2 and a less homogeneous

structure for sample T3. Obviously, too high energy assistance is rather unfavorable to obtain good ®lm structure because of the recrystallization of TiO2. This phenomenon can be explained by the relaxation processes taking place at the end of collision cascades. Our results are in agreement with the work of Ottermann et al. [13], who observed a correlation between density and stress, showing a change from tensile stress to compressive stress in the ®lm when the density reached 3.84 g/cm 3. On the other hand, our previous work show that the r.m.s. roughness is lower at Ec ˆ 300 eV (sample T3) than at Ec ˆ 150 eV (sample T2) due to ion-

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Fig. 3. Cross-sectional TEM images and diffraction patterns of the TiO2 ®lms deposited at T sub ˆ 2508C on SiO2 thermally grown on Si and on fused silica. (a) Without ion assistance: sample T4. (b) Ec ˆ 150 eV, Ed ˆ 68 eV: sample T5. (c) Ec ˆ 300 eV, Ed ˆ 360 eV: sample T6. (d) Dark-®eld images of sample T6.

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sputtering effects which are more important at higher kinetic energy [10]. 3.2.2. Films with substrate temperature Tsub 2508C Three samples were prepared at this temperature, either on SiO2 grown on Si (samples T4, T5) or on fused silica (sample T6). The results are presented, in the following, using the same way of comparison as used in the T sub ˆ 1508C case. That means, we compare ®rst the PVD sample (T4) with the IAD sample (T5) and then two IAD samples (T5 and T6) with different energy levels. In Fig. 3a, one recognizes the ®brous columnar growth morphology usually observed in the PVD samples (T4, for example). This microstructure characteristic can be related to the zone T of the SZ model. Small crystalline grains of anatase (,10 nm in diameter) are present. Fig. 3b shows the bright-®eld image and corresponding electron diffraction pattern taken from the sample T5 prepared with Ed just above Edth (Ec ˆ 150 eV, Ed ˆ 68). Comparing to the non-assisted T4 sample, the IAD sample T5 still keeps the ®brous columnar growth feature but the TiO2 grains are well crystallized. Anatase phase with random orientations has been identi®ed. It is interesting to notice a slight difference of microstructure over the ®rst 20 nm of ®lm growth. This inhomogeneity was not detected by ellipsometry measurements in which the spectra can indeed be reproduced without assuming an index gradient. However, the extinction coef®cient of the ®lms deposited at 2508C is one order of magnitude higher than for samples deposited at 1508C. Fig. 3c (bright-®eld image) and Fig. 3d (dark-®eld image) show the observation results of the sample T6 prepared with higher energy (Ec ˆ 300 eV, Ed ˆ 360 eV) on a fused silica substrate. Two distinguishing crystalline growth zones have been observed: (1) presence of a mountain-like and badly crystallized layer at the ®lm±substrate interface, whose thickness varies from 15 nm (in the valley of the `mountain') to 60 nm (at the summit of the `mountain'); (2) apparition of the large TiO2 crystals, identi®ed as rutile phase. The TEM observation results are consistent with those obtained by other characterization techniques. For example, the ®lm density measured by X-ray re¯ectometry is close to the rutile one (rrut ˆ 4:2 g cm 3) and the mean value of n measured by SE is higher than for the IAD ®lms deposited at T sub ˆ 1508C. In addition, a large extinction coef®cient measured by PDS and an increase of the r.m.s. roughness reduced from AFM observation have been found. All of these results help to interpret the ellipsometric spectra by introducing an index gradient in the depth of the ®lm. According to the modelling proposed in [8], n increases

from 2.65 to 2.80 from the bottom to the free surface of the ®lm. 4. Conclusion The microstructures of IAD TiO2 ®lms deposited at two substrate temperatures are compared under assistance conditions particularly around the Ed values higher than a threshold of about 50 eV per TiO2 molecule. Below this threshold, the dominant mechanism for the TiO2 ®lms densi®cation is the enhanced surface diffusion induced by ion collisions; above it, ion incorporation and recoil implantation are mostly responsible for the densi®cation. At low substrate temperature (T sub ˆ 1508C) and moderate assistance, ®lms with rather high refractive index (n~2:60 at l ˆ 0:45 mm) and low extinction coef®cient are obtained. They are homogeneous in depth and insensitive to moisture. TEM observations reveal different growth morphologies and phase transformation for the TiO2 ®lms according to the evaporation conditions. The surface roughness has been found relatively low; increase of the Tsub up to 2508C improves the crystallinity of the ®lms so that the TiO2 ®lms with higher refractive index values can be obtained. Meanwhile, the structural change from anatase to rutile phase detected at high assistance level and fused silica substrate gives unfavorable consequences on the quality of the ®lms for optical applications.

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