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Correlation between microstructure and the optical properties of TiO2 thin films prepared on different substrates
ELSEVIER
Thin Solid Films 307 (t997) 38-42
Correlation between microstructure and the optical properties of TiO a thin films prepared on different substrates Y. Leprince-Wang a,U,,, K. Yu-Zhang b, V. Nguyen Van a, D. Souche a, j. Rivory a a Laboratoire d'Optique des Solides, URA CNRS 781, Universitd P. et M. Curie, Boite 80, 4 Place Jussieu, 75252 Paris cedex 05, France b Ddpartement de Physique, Universitd de Marne la Vallde, 2 rue de la Butte Vet~e, 93166 Noisy le Grand cedex, France
Received 15 January 1997; accepted 13 May I997
Abstract High refractive index TiO 2 thin films have been deposited by electron-beam evaporation on different substrates: Si (111) wafers, thermal SiO2, fused silica and float glass. Optical properties and growth morphology of the evaporated layers have been characterized by in situ spectroscopic ellipsometry and by transmission electron microscopy. Trajectories cos A = f(tang") and spectroscopic ellipsometry measurements give a coherent description of fitm growth. The necessity of taking into account the presence of a surface layer less dense than the main part of the film is established. For thicker samples (optical thickness higher than half of the wavelength) index gradients are revealed and evaluated. The columnar structure is found in all samples, but differences in column size and fibre packing are observed on TEM images. The influence of the nature of the substrate on the morphology and then the optical properties of the films is not well established. © 1997 Elsevier Science S.A. Keywords: Ellipsometry; Optical coatings; Titanium oxide; TEM
1. Introduction Titanium dioxide TiO 2 films have many unusual dielectric properties which make them suitable for wide applications. For example, they own excellent optical transmittance, high refractive index and good durability. All these attractive features can be used for multilayer optical coatings. Many papers are devoted to the comparison of the optical properties of TiO 2 films produced by various techniques. It is recog-nized that, in the case of classical reactive evaporation (i.e. without ion assistance), a large dispersion of values for the refractive index and the extinction coefficient is observed, caused by small changes in the process conditions. The influence on the optical properties of deposition parameters has been studied by several authors in the case of reactive evaporation [1,2], of dc reactive magnetic sputtering [3,4] etc. 0 2 pressure, evaporation rate, substrate temperature were the main parameters able to influence the packing fraction, the crystallinity of
* Corresponding author. Universit6 de Marne-La-Vall6e, Bat. M2-G26, 2, rue de la Butte Verte, 93166 Noisy-Le-Grand Cedex, France. 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 2 9 3 - 9
the films and then their optical properties. The influence of the substrate nature has not been studied; for optical applications only transparent substrates (glass, silica) were used. In the present work, the growth of TiO 2 films on various substrates is followed by in situ ellipsometry; the optical properties of the films are discussed in terms of microstructure with the help of transmission electron microscopy (TEM) images on sample cross-sections.
2. Experimental details TiO 2 films were deposited by electron beam evaporation in a ultrahigh vacuum chamber (MECA 2000). The base pressure was 3 × 10 .9 Torr. The starting material (mixture of Ti305 and Ti40 7 (OS50, OPTRON, Japan)), was evaporated from a 9 kW electron-beam gun (TEMESCAL, model SFIH-270-1, U.K.). The partial pressure of Oz, the deposition rate controlled by a quartz micro balance, as well as the substrate temperature are indicated on Table 1 for each film. Various substrates were used: Si wafers, thermal SiO 2 films about 110 nm thick on Si, fused silica and float glass
Y. Leprince-Wang et a I . / T h i n Solid Films 307 (I997) 3 8 - 4 2
Table 1 Characteristics of the TiO 2 films under study. When two values are indicated in the last column, the first one is the refractive index of the film near the substrate(n~), the second one near the surface (na~~) Sample
Substrate
02 pressure (10 -4 Ton')
Substrate temperature (°C)
T1 2"9_ T3 T4 T5 T6
Si Si thermal SiO 2 fused SiO 2 float glass float glass
1,5 1.5 1.0 1.5 1.0 1.0
250 250 °,250 250 250 150
Deposition rate (rim/s)
Thickness (nm)
Refractive index n at 3. = 450 nm
0.06 0.05 0.19 0.12 0.06 0.06
62.7 76 86.4 110 116 1 i4
2.34 2.42 2.465 2.36-2,31 2.67-2.38 2.22-2.27
TI T2 T3 T4 T5 T6
plates. Substrate temperature was controlled by means of a calibrated thermocouple placed on the substrate holder. The surface temperature of Si substrates can be checked by ellipsometry [5]; for silica and glass substrates, the actual temperature can be a few tens of a degree lower than indicated by the thermocouple. A rotating polarizer spectroscopic ellipsometer (SOPRA, France) records the ellipsometric parameters tantY and cos zi during film growth through stress-free windows under an angle of incidence close to 75 °. The differential phase retardation A, and the amplitude ratio tank~ are related to the Fresnel coefficients by: t a l 2 k l r = l r p l / l r s [ , A = 6 p - 6s, where rp and r~ are the complex Fresnel reflection coefficients for the p and s polarizations respectively, and 6p, (3S are the phase change for the two polarizations. For TEM investigations, the specimens were prepared by a cross-sectional technique similar to that used in the reference [6]. Microstructural observation and structural characterization have been performed using a Topcon 002B transmission electron microscope at 200 kV acceleration voltage.
3. Results and discussion
3.1. Optical properties Dielectric films are characterized by ellipsometry in two ways; during deposition, measurements are done at a single wavelength (3, = 450 nm), the growth of the film is then represented as a trajectory in the (tan~, cos A) plane. After deposition, spectroscopic measurements (SE) are performed between 400 and 800 nm. Both information are
39
important and complementary. The trajectory reflects the history of the film which could suffer transformation along its growth (recrystallisation for example); nevertheless the details of this evolution are not necessarily observable in the finished film. So, in some cases, inconsistencies between the two analyses could appear. The trajectories at )t = 450 nm of samples whose characteristics are tbund in Table 1, are d)awn on Fig. 1. The aspect of these trajectories are different from sample to sample, it is due to the different substrates and also to different growth modes. The starting point of the growth (ellipsometric parameters of the bare substrate) S is the same for samples T1 and T2, for samples T5 and T6 respectively. The interpretation of the trajectory is done by visual comparison with simulation, but not by a fitting procedure. Two steps of the growth are informative for proposing a model: when the optical thickness is a quarter of the wavelength (QW), the trajectory of a film of constant refractive index should come back on itself (it is quite the case in T1) and in any case the coordinates of this point give a mean value of the index of the film at this step. For almost all the films, the trajectory does not return on itself and the opening of the two branches can be interpreted as due to the growth of a surface layer with an index lower than the index of the underlying layer. If the deposited film is thick enough, so that its optical thickness is half of the wavelength (HW), the ellipsometric parameters of a homogeneous film is one of the substrate (absentee layer). The distance between the points HW and S is an indication of the inhomogeneity of the film. Mistakes between index inhomogeneity and residual absorption are possible in some cases. By looking again at Fig. 1, it appears that, for samples T1, T2, and T3 a model consisting of a main layer with a constant index nf growing from 0 to d f together with a surface layer of index /'/surf< /lf growing from 0 to d~ is satisfactory to reproduce the experimental results (Fig. 2(a)). For the three thicker samples T4, T5, and T6 examination of the trajectory around the HW point, lets assume that the films are slightly inhomogeneous in the case of T4 and T6~strongly inhomogenous in the case of T5. With information at a single wavelength, it is hazardous to propose a model of variation of the refractive index along the growth. We preferred to analyze first the SE measurements by assuming, in a first attempt, the film is homogeneous, then by describing it as a stack of a few numbers of sublayers (Fig. 2(b)). The refi'active index of each sublayer is considered as one of a mixture of TiO~ and void in the framework of the Bruggman effective medium approximation (EMA); for TiO,, the data file of rutile (ordinary index) is used [7]. The thickness and the void volume fraction of each sublayer are determined by a standard optimisation procedure. A surface layer is enclosed in the model; its index is fixed at a given value (/Z~urf = 1.8-2.0), its thickness only is fitted. In the examples of Fig. 1, a close correspondence
40
Y, Leprince-Wang et a l . / Thin Solid Fihns 307 (1997) 38-42
between the parameters resulting in the analysis of the SE results (Tables 2 and 3) and the parameters needed for reproducing the trajectory is observed. In the case of inhomogeneous films, it is assumed that, after the growth of the first layer on the substrate, a second layer grows on the top of the first one which is not modified and so on. A continuous variation of the refractive index would be a better representation than a step-wise variation, but the differences are not detectable on the trajectory. If we define the quantity An = n s - n ~ as the difference between the index of the sublayer in contact with the substrate and one of the outer sublayer and ( n ) as the mean value of the index, one remarks that An is positive for sample T4 and An is negative for sample T6. Accord-
hag to other authors [3,4,8], we think that these differences reflect the influence of the substrate temperature more than its nature. At 150°C (sample T6) ( n ) = 2.24 and An = -0.05. At 250°C (sample T4) ( n ) b = 2 . 3 4 and An = + 0.05. Due to the increase of the substrate temperature, ( n ) increases and the index gradient changes its sign. Sample T5 exhibits values of ( n ) and sign of An in relation with the deposition temperature but the magnitude of A n (An = 0.3) is abnormally high. A new phenomenon could be responsible of this inhomogeneity: the migration of Na from the float glass plate to the surface of the film at 250°C, which is clearly observed by XPS. Nevertheless, the role of Na in the formation of the film is not understood.
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tan q-' Fig. 1. In situ ellJpsometric data at h = 450 nm for six TiO 2 films (continuous line) and calculation (dots): (a) T1, (b) T2, (c) T3, (d) T4, (e) T5 and (f) T6.
Y. Leprince-Wang et a I . / T h i n SoTi8 Fihns 307 (1997) ,38-42
Surfacelayer(ds~lr f , nvlrf) Mainlayer(df, nf)
3.2.
Microstructure
Fig. 3 presents micrographs obtained by T E M on cross-sections of four previously studied samples. Although samples T1 and T2 were evaporated in the same conditions, it was found that the value of their refractive index is different (n = 2.35 for T1, n = 2.42 for T2). Fig. 3(a, b) show that both films exhibit the well k n o w n c o l u m n a r growth, but for T1 the structure is fibrous, leading to a h o m o g e n e o u s porous material with a small roughness, while sample T2 consists of well crystallized columns; T2 is denser than T1, but also rougher, as c o n f m n e d by atomic force microscopy ( A F M ) pictures
(a) Surfacelayer(dsnrf, nsurf) Sublayer 1 (d1, hi) Sublayer2 (d2, n2) SubIayer3 (d3, n3)
(b)
[9,10].
Fig. 2. Schematic diagram of the models used for reproducing the SE spectra and the trajectories of six TiO2 films: (a) model for T1, T2 and T3; (b) model for T4, T5 and T6.
S a m p l e T-3 (Fig. 3(c)) deposited on thermal SiO~ exhibits also a c o l u m n a ~ habit. The presence of large crystallites is detected. One can remark the absence of the fibrous structure on the first stage of growth (about 10 n m thick), it seems to suggest that thin layer is amorphous. The roughness of the surface is comparable to the one of sample T2. This image does not allow to explain clearly the origin of the slight index increase when going from T2 to T3: a reason would be that the packing of the c o l u m n s is denser in T3 than in T2, although T3 is not well crystallized as T2. In the case of T5 (Fig. 3(d)), the c o l u m n a r structure is always present, but the crystallinity of the film is improved; the surface roughness is also increased with respect to the other samples, in agreement with SE results. Electron diffraction pattern showed that all four thin films have the same crystalline structure, i.e. anatase,
Table 2 Parameters for samples T1, T2 and T3 in the model of Fig. 2(a) Sample
T1
T2
T3
dsurf, nsurf
0.5 rim, 1.8 62.7 nm, 2.34
2.4 nm, 1.8 73 nm, 2.42
4.0 rim, 2.0 82.4 rim, 2.465
d e, nf
41
Table 3 Parameters for samples T4, T5 and T6 in the model of Fig. 2(b) Sample
T4
T5
T6
dsurf, nsurf d I, n I d 2, n 2 d 3, n 3
4.5 nm, 2.0 44.4 nm, 2.308 44.5 nm, 2.318 44.5 nm, 2.365
17 nm, 2.0 30 ran, 2.383 38 nm, 2.455 30 rim, 2.675
3.4 nm, 2.0 38 nm, 2.272 37.7 nm, 2.267 38 nm, 2.224
(a)
(b) :
..
.2_22
v?
Fig. 3. Cross-secdonaI TEM images taken on TiO 2 films deposited at 250°C: (a) sample T1 on a Si wafer, d = 62.7 nm; (b) sample T2 on a Si wafer, d = 76 rim; (c) sample T3 on a thick thermal SiO2 layer, d = 86.4 nm and (d) sample T5 on float glass, d = 116 nm.
42
Y. Leprince-Wang et al. / Thin Solid Films 307 (1997) 38-42
although we do not exclude the co-existence of the mtile phase, in agreement with the literature [11].
4. Conclusion W e have performed a detailed characterization of TiO 2 films by in situ ellipsometl3, and TEM. W e have emphasized the relationship between microstructure and optical properties and the complementarity between TEM and SE. The substrate temperature is an important parameter for the value of the refractive index, for the magnitude and the sign of the index gradient. The influence of the nature of the substrate is not clearly established, except in cases where an interaction between film and substrate is revealed (Na migration for T i O ; on float glass at 250°C). The tendency to obtain a slightly increased value of the refractive index when TiO 2 is deposited on thermal SiO, with respect to Si and fused silica is underlined.
References [1] H.K. Pulker, G. Paesold, E. Ritter, Appl. Opt, 15 (1976) 2986. [2] K. Balasubramanian, X.F. Han, K.H. Guenther, AppI. Opt. 32 (1993) 5594. [3] M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 (1992) 1421. [4] Li-jian Meng, M. Andritschky, M.P. dos Santos, Thin Solid Films, (1993) pp. 223, 242. [5] G. Vuye, S. Fisson, V. Nguyen Van, Y. Wang, J. Rivory, F. Abel,s, Thin Solid Films 233 (1993) 166. [6] K. Yu-Zhang, G. Boisjolly, J. Rivory, L. Kilian, C. Colliex, Thin Solid Films 253 (1994) 299. [7] M.W. Ribarsky, in: E.D. Palik (Ed.), Handbook of Optical Constants of Solids IL Academic Press (1991) p. 795. [81 V. Nguyen Van, A. Brunet-Bruneau, S, Fisson, J.M. Fdgerio, G. Vuye, Y. Wang, F. Abel,s, J. Rivory, M. Berger, P. Charon, Applied Optics 35 (1996) 5540. [9] V. Nguyen Van, S. Fisson, LM. Frigerio, J. Rivory, G. Vuye, Y. Wang, F. Abel,s, Thin Solid Films 253 (1994) 257. [10] Y. Wang, thesis of the Universit6 Pierre et Made Curie (1995). [11] M. Lottiaux, C. Boulesteix, G. Nihoul, F. Vamier, F. Flory, R. Galindo, E. Pelletier, Thin Solid Films 170 (1989) 107.
Thin Solid Films 307 (t997) 38-42
Correlation between microstructure and the optical properties of TiO a thin films prepared on different substrates Y. Leprince-Wang a,U,,, K. Yu-Zhang b, V. Nguyen Van a, D. Souche a, j. Rivory a a Laboratoire d'Optique des Solides, URA CNRS 781, Universitd P. et M. Curie, Boite 80, 4 Place Jussieu, 75252 Paris cedex 05, France b Ddpartement de Physique, Universitd de Marne la Vallde, 2 rue de la Butte Vet~e, 93166 Noisy le Grand cedex, France
Received 15 January 1997; accepted 13 May I997
Abstract High refractive index TiO 2 thin films have been deposited by electron-beam evaporation on different substrates: Si (111) wafers, thermal SiO2, fused silica and float glass. Optical properties and growth morphology of the evaporated layers have been characterized by in situ spectroscopic ellipsometry and by transmission electron microscopy. Trajectories cos A = f(tang") and spectroscopic ellipsometry measurements give a coherent description of fitm growth. The necessity of taking into account the presence of a surface layer less dense than the main part of the film is established. For thicker samples (optical thickness higher than half of the wavelength) index gradients are revealed and evaluated. The columnar structure is found in all samples, but differences in column size and fibre packing are observed on TEM images. The influence of the nature of the substrate on the morphology and then the optical properties of the films is not well established. © 1997 Elsevier Science S.A. Keywords: Ellipsometry; Optical coatings; Titanium oxide; TEM
1. Introduction Titanium dioxide TiO 2 films have many unusual dielectric properties which make them suitable for wide applications. For example, they own excellent optical transmittance, high refractive index and good durability. All these attractive features can be used for multilayer optical coatings. Many papers are devoted to the comparison of the optical properties of TiO 2 films produced by various techniques. It is recog-nized that, in the case of classical reactive evaporation (i.e. without ion assistance), a large dispersion of values for the refractive index and the extinction coefficient is observed, caused by small changes in the process conditions. The influence on the optical properties of deposition parameters has been studied by several authors in the case of reactive evaporation [1,2], of dc reactive magnetic sputtering [3,4] etc. 0 2 pressure, evaporation rate, substrate temperature were the main parameters able to influence the packing fraction, the crystallinity of
* Corresponding author. Universit6 de Marne-La-Vall6e, Bat. M2-G26, 2, rue de la Butte Verte, 93166 Noisy-Le-Grand Cedex, France. 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 2 9 3 - 9
the films and then their optical properties. The influence of the substrate nature has not been studied; for optical applications only transparent substrates (glass, silica) were used. In the present work, the growth of TiO 2 films on various substrates is followed by in situ ellipsometry; the optical properties of the films are discussed in terms of microstructure with the help of transmission electron microscopy (TEM) images on sample cross-sections.
2. Experimental details TiO 2 films were deposited by electron beam evaporation in a ultrahigh vacuum chamber (MECA 2000). The base pressure was 3 × 10 .9 Torr. The starting material (mixture of Ti305 and Ti40 7 (OS50, OPTRON, Japan)), was evaporated from a 9 kW electron-beam gun (TEMESCAL, model SFIH-270-1, U.K.). The partial pressure of Oz, the deposition rate controlled by a quartz micro balance, as well as the substrate temperature are indicated on Table 1 for each film. Various substrates were used: Si wafers, thermal SiO 2 films about 110 nm thick on Si, fused silica and float glass
Y. Leprince-Wang et a I . / T h i n Solid Films 307 (I997) 3 8 - 4 2
Table 1 Characteristics of the TiO 2 films under study. When two values are indicated in the last column, the first one is the refractive index of the film near the substrate(n~), the second one near the surface (na~~) Sample
Substrate
02 pressure (10 -4 Ton')
Substrate temperature (°C)
T1 2"9_ T3 T4 T5 T6
Si Si thermal SiO 2 fused SiO 2 float glass float glass
1,5 1.5 1.0 1.5 1.0 1.0
250 250 °,250 250 250 150
Deposition rate (rim/s)
Thickness (nm)
Refractive index n at 3. = 450 nm
0.06 0.05 0.19 0.12 0.06 0.06
62.7 76 86.4 110 116 1 i4
2.34 2.42 2.465 2.36-2,31 2.67-2.38 2.22-2.27
TI T2 T3 T4 T5 T6
plates. Substrate temperature was controlled by means of a calibrated thermocouple placed on the substrate holder. The surface temperature of Si substrates can be checked by ellipsometry [5]; for silica and glass substrates, the actual temperature can be a few tens of a degree lower than indicated by the thermocouple. A rotating polarizer spectroscopic ellipsometer (SOPRA, France) records the ellipsometric parameters tantY and cos zi during film growth through stress-free windows under an angle of incidence close to 75 °. The differential phase retardation A, and the amplitude ratio tank~ are related to the Fresnel coefficients by: t a l 2 k l r = l r p l / l r s [ , A = 6 p - 6s, where rp and r~ are the complex Fresnel reflection coefficients for the p and s polarizations respectively, and 6p, (3S are the phase change for the two polarizations. For TEM investigations, the specimens were prepared by a cross-sectional technique similar to that used in the reference [6]. Microstructural observation and structural characterization have been performed using a Topcon 002B transmission electron microscope at 200 kV acceleration voltage.
3. Results and discussion
3.1. Optical properties Dielectric films are characterized by ellipsometry in two ways; during deposition, measurements are done at a single wavelength (3, = 450 nm), the growth of the film is then represented as a trajectory in the (tan~, cos A) plane. After deposition, spectroscopic measurements (SE) are performed between 400 and 800 nm. Both information are
39
important and complementary. The trajectory reflects the history of the film which could suffer transformation along its growth (recrystallisation for example); nevertheless the details of this evolution are not necessarily observable in the finished film. So, in some cases, inconsistencies between the two analyses could appear. The trajectories at )t = 450 nm of samples whose characteristics are tbund in Table 1, are d)awn on Fig. 1. The aspect of these trajectories are different from sample to sample, it is due to the different substrates and also to different growth modes. The starting point of the growth (ellipsometric parameters of the bare substrate) S is the same for samples T1 and T2, for samples T5 and T6 respectively. The interpretation of the trajectory is done by visual comparison with simulation, but not by a fitting procedure. Two steps of the growth are informative for proposing a model: when the optical thickness is a quarter of the wavelength (QW), the trajectory of a film of constant refractive index should come back on itself (it is quite the case in T1) and in any case the coordinates of this point give a mean value of the index of the film at this step. For almost all the films, the trajectory does not return on itself and the opening of the two branches can be interpreted as due to the growth of a surface layer with an index lower than the index of the underlying layer. If the deposited film is thick enough, so that its optical thickness is half of the wavelength (HW), the ellipsometric parameters of a homogeneous film is one of the substrate (absentee layer). The distance between the points HW and S is an indication of the inhomogeneity of the film. Mistakes between index inhomogeneity and residual absorption are possible in some cases. By looking again at Fig. 1, it appears that, for samples T1, T2, and T3 a model consisting of a main layer with a constant index nf growing from 0 to d f together with a surface layer of index /'/surf< /lf growing from 0 to d~ is satisfactory to reproduce the experimental results (Fig. 2(a)). For the three thicker samples T4, T5, and T6 examination of the trajectory around the HW point, lets assume that the films are slightly inhomogeneous in the case of T4 and T6~strongly inhomogenous in the case of T5. With information at a single wavelength, it is hazardous to propose a model of variation of the refractive index along the growth. We preferred to analyze first the SE measurements by assuming, in a first attempt, the film is homogeneous, then by describing it as a stack of a few numbers of sublayers (Fig. 2(b)). The refi'active index of each sublayer is considered as one of a mixture of TiO~ and void in the framework of the Bruggman effective medium approximation (EMA); for TiO,, the data file of rutile (ordinary index) is used [7]. The thickness and the void volume fraction of each sublayer are determined by a standard optimisation procedure. A surface layer is enclosed in the model; its index is fixed at a given value (/Z~urf = 1.8-2.0), its thickness only is fitted. In the examples of Fig. 1, a close correspondence
40
Y, Leprince-Wang et a l . / Thin Solid Fihns 307 (1997) 38-42
between the parameters resulting in the analysis of the SE results (Tables 2 and 3) and the parameters needed for reproducing the trajectory is observed. In the case of inhomogeneous films, it is assumed that, after the growth of the first layer on the substrate, a second layer grows on the top of the first one which is not modified and so on. A continuous variation of the refractive index would be a better representation than a step-wise variation, but the differences are not detectable on the trajectory. If we define the quantity An = n s - n ~ as the difference between the index of the sublayer in contact with the substrate and one of the outer sublayer and ( n ) as the mean value of the index, one remarks that An is positive for sample T4 and An is negative for sample T6. Accord-
hag to other authors [3,4,8], we think that these differences reflect the influence of the substrate temperature more than its nature. At 150°C (sample T6) ( n ) = 2.24 and An = -0.05. At 250°C (sample T4) ( n ) b = 2 . 3 4 and An = + 0.05. Due to the increase of the substrate temperature, ( n ) increases and the index gradient changes its sign. Sample T5 exhibits values of ( n ) and sign of An in relation with the deposition temperature but the magnitude of A n (An = 0.3) is abnormally high. A new phenomenon could be responsible of this inhomogeneity: the migration of Na from the float glass plate to the surface of the film at 250°C, which is clearly observed by XPS. Nevertheless, the role of Na in the formation of the film is not understood.
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05
0.6
tan q-' Fig. 1. In situ ellJpsometric data at h = 450 nm for six TiO 2 films (continuous line) and calculation (dots): (a) T1, (b) T2, (c) T3, (d) T4, (e) T5 and (f) T6.
Y. Leprince-Wang et a I . / T h i n SoTi8 Fihns 307 (1997) ,38-42
Surfacelayer(ds~lr f , nvlrf) Mainlayer(df, nf)
3.2.
Microstructure
Fig. 3 presents micrographs obtained by T E M on cross-sections of four previously studied samples. Although samples T1 and T2 were evaporated in the same conditions, it was found that the value of their refractive index is different (n = 2.35 for T1, n = 2.42 for T2). Fig. 3(a, b) show that both films exhibit the well k n o w n c o l u m n a r growth, but for T1 the structure is fibrous, leading to a h o m o g e n e o u s porous material with a small roughness, while sample T2 consists of well crystallized columns; T2 is denser than T1, but also rougher, as c o n f m n e d by atomic force microscopy ( A F M ) pictures
(a) Surfacelayer(dsnrf, nsurf) Sublayer 1 (d1, hi) Sublayer2 (d2, n2) SubIayer3 (d3, n3)
(b)
[9,10].
Fig. 2. Schematic diagram of the models used for reproducing the SE spectra and the trajectories of six TiO2 films: (a) model for T1, T2 and T3; (b) model for T4, T5 and T6.
S a m p l e T-3 (Fig. 3(c)) deposited on thermal SiO~ exhibits also a c o l u m n a ~ habit. The presence of large crystallites is detected. One can remark the absence of the fibrous structure on the first stage of growth (about 10 n m thick), it seems to suggest that thin layer is amorphous. The roughness of the surface is comparable to the one of sample T2. This image does not allow to explain clearly the origin of the slight index increase when going from T2 to T3: a reason would be that the packing of the c o l u m n s is denser in T3 than in T2, although T3 is not well crystallized as T2. In the case of T5 (Fig. 3(d)), the c o l u m n a r structure is always present, but the crystallinity of the film is improved; the surface roughness is also increased with respect to the other samples, in agreement with SE results. Electron diffraction pattern showed that all four thin films have the same crystalline structure, i.e. anatase,
Table 2 Parameters for samples T1, T2 and T3 in the model of Fig. 2(a) Sample
T1
T2
T3
dsurf, nsurf
0.5 rim, 1.8 62.7 nm, 2.34
2.4 nm, 1.8 73 nm, 2.42
4.0 rim, 2.0 82.4 rim, 2.465
d e, nf
41
Table 3 Parameters for samples T4, T5 and T6 in the model of Fig. 2(b) Sample
T4
T5
T6
dsurf, nsurf d I, n I d 2, n 2 d 3, n 3
4.5 nm, 2.0 44.4 nm, 2.308 44.5 nm, 2.318 44.5 nm, 2.365
17 nm, 2.0 30 ran, 2.383 38 nm, 2.455 30 rim, 2.675
3.4 nm, 2.0 38 nm, 2.272 37.7 nm, 2.267 38 nm, 2.224
(a)
(b) :
..
.2_22
v?
Fig. 3. Cross-secdonaI TEM images taken on TiO 2 films deposited at 250°C: (a) sample T1 on a Si wafer, d = 62.7 nm; (b) sample T2 on a Si wafer, d = 76 rim; (c) sample T3 on a thick thermal SiO2 layer, d = 86.4 nm and (d) sample T5 on float glass, d = 116 nm.
42
Y. Leprince-Wang et al. / Thin Solid Films 307 (1997) 38-42
although we do not exclude the co-existence of the mtile phase, in agreement with the literature [11].
4. Conclusion W e have performed a detailed characterization of TiO 2 films by in situ ellipsometl3, and TEM. W e have emphasized the relationship between microstructure and optical properties and the complementarity between TEM and SE. The substrate temperature is an important parameter for the value of the refractive index, for the magnitude and the sign of the index gradient. The influence of the nature of the substrate is not clearly established, except in cases where an interaction between film and substrate is revealed (Na migration for T i O ; on float glass at 250°C). The tendency to obtain a slightly increased value of the refractive index when TiO 2 is deposited on thermal SiO, with respect to Si and fused silica is underlined.
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