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Study of porosity of titanium oxide films by X-ray photoelectron spectroscopy and IR transmittance
Thin Solid Films, 239 (1994) 117-122
117
Study of porosity of titanium oxide films by X-ray photoelectron spectroscopy and IR transmittance Li-Jian M e n g *a, Carlos P. Moreira de S/t b and M. P. dos Santos a "Physics Department, University of Minho, 4719 Braga Codex (Portugal) bCEMUP, 4100 Porto (Portugal) (Received June 22, 1993; accepted September 8, 1993)
Abstract Titanium oxide films were deposited on glass substrates by d.c. reactive magnetron sputtering at different oxygen partial pressures (3.4 x 10 -4 mbar to 3 x 10 -3 mbar) and total pressures (2 x 10 -3 mbar to 2 x 10 -2 mbar). Two resolved peaks in the Ols spectra of XPS were observed. These two peaks were attributed to the O - T i bonding and the O - H bonding. The O - H bonding component increased as the oxygen partial pressure and total pressure were increased. The near IR transmittance measurements showed that all films had an absorption near 2.8 lain, and the absorption increased as the total pressure and oxygen partial pressure were increased. These phenomena were related to the porosity of the film. The films prepared by d.c. reactive magnetron sputtering generally have a columnar structure with many pores. In air some of these pores are filled with water, and this results in the appearence of the O - H bonding component and the absorption around 2.8 lain. The increase of the O - H bonding component and the absorption around 2.8 lam indicates the increase of the film porosity.
1. Introduction Dense films with low porosity are desired for good environmental reliability. Moisture penetrating through the pores alters the optical and mechanical properties of the coatings. The optical effects are due to filling of the pores with water, which has a h i g h e r refractive index (1.33) than air, and due to changes in the polarizability of the grain boundary surfaces due to adsorption of polar molecules. Moisture also induces compressive or tensile stress, depending on the material and grain boundary surface chemistry. This stress can lead to catastrophic mechanical failure of the coating [1, 2]. Therefore, study of film porosity is of special interest. X - R a y photoelectron spectroscopy (XPS) gives the possibility to detect the different chemical states of an element by measuring the binding energy with a high energy resolution. The difference in the binding energy in the photon emission spectrum can b e c o r r e l a t e d directly with the difference in the chemical states of the elements. Therefore XPS has been used widely to analyze film composition and the chemical bonding of the elements [3-9]. Titanium oxide films deposited by various methods have been extensively studied for use as optical coatings
*On leave from Changchun Institute of Physics Chinese Academy of Sciences, Changchun 130021, People's Republi of China.
0040-6090/94/$7.00 SSDI 0040-6090(93)02909-W
for visible and near IR optics [10-14]. The optical and structural properties of titanium oxide films prepared by the d.c. reactive magnetron sputtering method have been found to be quite sensitive to the 'deposition parameters; specifically, the oxygen partial pressure, total pressure and the substrate temperature during deposition [ 15-17]. In this paper, the variation of the porosity o f the films prepared at different oxygen partial pressures and total pressures was studied using XPS and near IR transmittance methods.
2. Experimental details Titanium oxide films were deposited on glass substrates by d.c. reactive magnetron sputtering in a mixed argon and oxygen atmosphere. The target was titanium metal of 99.6% purity. The substrate was without extra heating and biasing during the deposition processes. For samples N1, N5 and N i l , the total pressure was kept at 8 x 10-3 m b a r and the oxygen partial pressures were 3 x 10 -3 mbar, 6 x 10 -4 m b a r and 3.4 x 10 -4 m b a r respectively. For samples N 15, N 17 and N 19, the oxygen partial pressure was kept at 6 x 10- 4 m b a r and the total pressures were 2 x 10-3 mbar, 6 x 10-3 m b a r and 2 x 10 -2 m b a r respectively. Details of the sample preparation can be found in other papers [16, 17]. The transmittance of the films was measured using a Shimadzu UV-3101PC computer-controlled spectro-
© 1 9 9 4 - Elsevier Sequoia. All rights reserved
118
L. J. Meng et al. / Porosity of 7702 films •
photometer. It can measure the transmittance of the near IR region. The surface topology and cross-sectional structure of the films were observed using scanning electron microscopy (SEM). In order to prevent charge build-up, a thin gold film was sputtered on the surface of the sample before doing SEM. XPS analysis was performed with a VG Escalab 200A electron analyser operating under computer control using VGS 5250 Datasystem software. The samples were placed in the vacuum system and pumped down to the 10 -9 mbar range before the measurement. The pressure of the vacuum system was steady during the measurement. Photoelectrons were excited over a nominal 10 mm x 10 mm area with AI K~t radiation (1486.6 eV). Photoelectrons were detected with a hemispherical analyser with an input lens positioned normal to the surface. Spectrometer pass energy was 20 eV for detailed region spectra. The atomic concentrations of the various elements were computed from measured peak areas (Cls, Ols and Ti2p3/2) using VGS 5250 software with the following sensitivity factors: C, 1; O, 2.85; Ti, 5.22. Linear background subtraction and gaussian peak deconvolutions were performed for Ols photoelectron peaks using VGS 5250 software.
3. Results
Detailed X-ray photoelectron region spectra of the Ols, Ti2p and Cls regions of sample N15 are shown in Fig. 1. It is clear that all the peaks shift to higher energies owing to charge effect [18]. The carbon peaks are from the surface contamination, because after 5 rain of Ar ion bombardment they disappear. Peak positions were calibrated by taking the Cls peak (284.6 eV) as a reference, because many authors have suggested that the Cls peak produced by adsorbed, adventitious carbon could be assigned a fixed value and all other binding energies could be referenced to it [18]. The results were listed in Table 1. It can be seen from Table 1 that the peak positions after calibration are in agreement with literature values, which, give O ls and Ti2p3/2 binding energies relative to TiO2 species of 530 eV and 458.6 eV respectively [19]. From Fig. 1 it can be seen that the Ols spectra consist of two peaks. The deconvolution of the O ls spectra has been done for all the samples as shown in Fig, 2. The peak positions and the calculated atomic concentrations of the elements were summarized in Table 1. The two deconvoluted peaks observed in the Ols spectra near 530 and 532 eV binding energy can be attributed to the O-Ti (O1) and O - H (02) bonding respectively [19, 20]. It can be seen that the component corresponding to the O - H bonding increases as the oxygen partial and total pressure are increased.
°
"
•
•
.
i
•
,
'•o
290
280
300
0 la
~
|
m
t
t
I
520
535
550
tl/45
t¢60
tt75
Bindirx:j Enerqy (eV) Fig. 1. XPS spectra of a titanium oxide film (sample Ni5). Figures 3 and 4 show the transmittance of the films prepared at different oxygen partial and total pressures. It is dear that the films have an absorption peak near 2.8 pm and the absorption increases as the oxygen partial find total pressures are increased. This absorption can be attributed to the absorption of the water. Therefore, the variation of this absorption reflects the variation of the amount of water in the films and further the variation of the film porosity.
4. Discussion
The deconvoluted XPS Ols spectra as shown in Fig. 2 show that the resolved peaks are separated by about 2 eV. The positions of the two resolved peaks in the Ols spectra are located at 530 eV and 532 eV respectively. The lower energy peak located at 530 eV (O1) corresponds to O-Ti bonding, while the higher energy peak located at 532 eV (02) may be assigned to OH (531.5 eV) and/or H20 (533 eV) species, indicating the presence of hydrated oxides [5]. The calculated atomic concentrations for films prepared at different oxygen partial and total pressures were listed in Table 1. The compositions given in Table 1 probably have
L. J. Meng et al. / Porosity of TiO2films
119
TABLE 1. Deposition conditions and XPS and near IR absorption data of titanium oxide films (data in parentheses are the calculated atomic concentration)
Oxygen pressure (mbar) Total pressure (mbar) Ti2p3/2 CIs Ols Ol O2 O2/(OI + 02) Cw ( x 10-4nm -1) Refractive index (500 nm)
NI
N5
NIl
Nl5
NI7
NI9
3 x l0 -3
3.4 x l0 -4
458.6 (14%) 284.6 (36%)
6 x l0 -( 8 x l 0 -3 458.6 (16%) 284.6 (31%)
458.6 (16°/o) 284.6 (31%)
2 x l 0 -3 458.6 (18%) 284.6 (29%)
6 x l0 -4 6 x l 0 -3 458.7 (16%) 284.6 (33%)
2 x l 0 -2 458.7 (15%) 284.6 (34%)
529.9 (31%) 532.1 (19%) 0.38 1.85 2.37
529.9 (36%) 531.7 (17%) 0.32 1.46 2.39
529.9 (37%) 531.8 (16%) 0.30 1.20 2.42
529.5 (38%) 531.7 (15%) 0.28 0.74 2.48
530 (34%) 532 (17%) 0.33 1.20 2.41
530 (31%) 531.7 (19%) 0.38 1.68 2.14
01 1 , t!vvvvv A O1
N15
01
Nll ~ 0 2
0
. . . . . . .
i . . . . . . . . .
, . . . . . . . . .
i . . . . . . . . .
i . . . . . . . . .
i . . . . . . . . .
~
2,50 7,50 12,50 17,50 2250 2750 32,50 Wavellm~ (rim) I
520
530
Binding Energy
540 (eV)
520
I
|
530
Binding Energy
540
Fig. 3. Transmission spectra of titanium oxide films prepared at different oxygen partial pressures.
(eV)
Fig. 2. Ols spectra with the two resolved O bonding components for titanium oxide films prepared at different oxygen partial and total pressures.
some uncertainties in the absolute values, but their variations are significant. From Table 1 it can be seen that the 0 2 component increases as the oxygen partial and total pressures are increased; that means an increase o f the water component in the films, because the 0 2 component is related to water. In order to prove that the 0 2 component is not from surface contamination, sample N 15 was bombarded by Ar ions for about 5 min and then XPS analysis was done again. The result shows that the carbon peak disappeared but the 0 2 component was still there. However, it is unavoidable that the surface contamination also has some contribution to the 0 2 component. Although Ar ion bombardment can clean the sample surface, it also
produces some negative effects, e.g. the variation of surface topography and the occurrence of preferential sputtering. These effects will also affect the measurement of the 0 2 component. For these reasons, we did not clean the sample surfaces by Ar ion bombardment. In general, metallic oxide films prepared by d.c. reactive magnetron sputtering have a columnar structure with many pores. In air some of these pores are filled with water, and this results in the appearance of 0 2 component in the Ols spectra. The increase of 0 2 component indicates the increase of the porosity of the films. That means the film porosity increases as the oxygen partial and total pressures are increased. The porosity arises from the limited adatom mobility, which, for a given species, depends both on the energy of the sputtered species incident on the growth surface and on the substrate temperature. Experiments indicate that high temperatures and energetic sputtered species
L. J. Meng et al. [ Porosity of TiO 2 films
120
where P is the packing density, which can be obtained by using the equation [14]:
100
n f2 _ 1
1p
nm 2 -
n2+2--nm2+2
N19
0
....... :......... :......... :......... :......... :......... : 250
750
1250 1750 2250 2750 3250 Wavellmon~ (am)
Fig. 4. Transmission spectra of titanium oxide films prepared at different total pressures. (e.g. ions) promote dense structures, whereas high background gas pressures, by causing loss of energy of sputtered species via gas-phase scattering, cause higher porosities. Therefore, the variation of the 0 2 component in the O l s spectra reflects correctly the variation of the film porosity. Although the XPS measurement results only give the information in a depth of about 3 nm, the films have a columnar structure and the pores go through the whole film depth. Therefore, the XPS measurement results can reflect the film porosity properties. The ratio O2/(O1 + 02) can be related to the film porosity to some extent. The film porosity may be written as follows:
Ol Porosity = A Ol + 0 2
(!)
where O l and 0 2 are the atomic concentrations of O1 and 0 2 components as shown in Table 1, and A can be considered as a corrective factor. The ratios of O2/(O1 + 0 2 ) for the films prepared at different deposition conditions have been given in Table 1. In addition, the film porosity can also be written as follows: Porosity = 1 - P TABLE 2.
(2)
nw 2 - 1 + ~ a ( 1
--P)
(3)
where nr, nm and nw are the refractive indices of the film, bulk material and the water respectively, a is a coefficient which indicates the relative proportion of the pores filled with water, and P is the film packing density. The refractive index of the films prepared at different oxygen partial and total pressures has been given in Table 1 [16, 17]. The nm and nw are 2.57 and 1.33 respectively [21]. Coefficient a was taken as 1, which means all the pores were filled with water. By using these parameters and eqns. (2) and (3), the film porosities were calculated and the results were listed in Table 2. It can be seen that the film porosity decreases as the oxygen partial and total pressures are increased. The parameter A can be obtained by comparing eqns. (1) and (2), and the results were shown in Table 2. It can be seen that A is different for different samples. However, it should be noted that a large difference only occurs with samples N15 and N19, and the difference is small for the rest. If we rule out the values of N 15 and N I 9 and take the average value of the others, then the film porosities can be calculated by eqn.(1). The results were listed in Table 2. The transmittance spectrum of the water-containing films exhibits an absorption band located near 2.8 ~tm as shown in Figs. 3 and 4, which can be used for determination of the amount of water in the film. A relative measure of water in the film Cw can be calculated using the equation [2]: In (To~T) Cw - - dr
(4)
and the film porosity can be obtained using the following equation: Porosity = BCw
(5)
where T and To are respectively the optical tance at 2.8 ~ n of the film in ambient and of film if it has no water. B is a parameter which to the water absorption coefficient at 2.8 gm
transmitthe same is related and dr is
Porosity of the films estimated by different methods
Film porosity estimated from refractive index Film porosity estimated from XPS Film porosity estimated from IR transmittance Parameter A Parameter B ( x 102 nm)
N1
N5
Nil
NI5
NI7
NI9
0.10 0.10 0.11 0.26 5.41
0.09 0.08 0.09 0.28 6.16
0.07 0.08 0.07 0.23 5.83
0.04 0.07 0.04 0.14 5.40
0.08 0.08 0.07 0.24 6.67
0.24 0.10 0.10 0.63 14.29
L. J. Meng et al. / Porosity of TiO2films
the film thickness. The transmittance of the film without water was obtained by reconstructing the transmittance curve at the water absorption peak by using adjacent interference extrema. By comparing eqns. (2) and (5), the parameter B can be obtained as shown in Table 2. It can be seen that the B value of sample N19 is very different from the others. By taking the average value of five similar B values, the film porosity can be calculated via eqn. (5). The film porosity calculated by this method is also shown in Table 2. It can be seen that the film porosity calculated by water absorption has a similar variation trend to those obtained from XPS and the refractive index. Comparing the porosity obtained by measuring XPS and IR transmittance with that obtained from the refractive index, it can be found that the large divergence occurs for sample NI9. The porosity calculated from the refractive index is much larger than that obtained from XPS and IR transmittance. This can be explained as follows: the pores in the films are only partially filled with water. From Fig. 5 it can be seen that the pores in N I9 seem to be larger than those in the other samples. Some pores may be too large to act as capillaries, and these pores did not fill with water, which results in the small porosity in the XPS and IR measurements, because the porosity is measured by calculating the amount of water. Figure 5 shows the surface and cross-section structures for the films deposited at different oxygen partial and total pressures. As can be seen from Fig. 5, the film structure changes from more dense into more porous as the oxygen partial and total pressures are increased.
i Fig. 5. Scanning electron micrographs of titanium oxide films prepared at different oxygen partial and total pressures: upper row, surfaces; lower row, cross-sections.
121
These observations are in accordance with the discussion above.
5. Conclusions The porosities of titanium oxide films prepared by d.c. reactive magnetron sputtering at different oxygen partial and total pressures have been studied by analysis of their XPS and IR transmittance. All the Ols spectra of XPS have two peaks. The peak located at lower binding energy is related to T i - O bonding and the peak located at higher binding energy is related to O - H bonding. The film which has a higher O - H bonding component has a more porous structure. The near IR transmittance of the film shows an absorption band near 2.8 lxrn. The film porosity has been related to this absorption band. The film which has stronger absorption has a more porous structure. All the results show that the film porosity increases as the oxygen partial and total pressure are increased.
Acknowledgment L. J. Meng is thankful to the Orient Foundation for providing a scholarship.
References 1 H. Sankur, Thin Solid Films, 218 (1992) 161. 2 H. Sankur and W. Gunning, J. Appl. Phys., 66 (1989) 807. 3 S. R. Kumar, R. B. Gore. S. K. Kulkarni and R. K. Pandey, Thin Solid Films, 208 (1992) 161. 4 A. J. Nelson and H. Aharoni, J. Vac. Sci. Technol. A, 5 (1987) 231. 5 L. Avalle, E. Santos, E. Leiva and V. A. Macagno, Thin Solid Films, 219 (1992) 7. 6 N. Ozer, Thin Solid Films, 214 (1992) 17. 7 L. G. Mar, P. Y. Timbrell and R. N. Lamb, Thin Solid Films, 223 (1993) 341. 8 J. S. Kim, H. A. Marzouk, P. J. Reucroft and C. E. Hamrin, Thin Solid Films, 217 (1992) 133. 9 C. Agashe, M. G. Takwale, V. G. Bhide, S. Mahamuni and S. K. Kulkarni, J. AppL Phys., 70 (1991) 7382. 10 M. G. Krishna, K. N. Rao and S. Mohan, J. Appl. Phys., 73(1) (1993) 434. I1 S. Pongratz and A. Zoller, J. Vac. Sci. Technol. A, 10(4) (1992) 1897. 12 M. H. Suhail, G. M. Rao and S. Mohan, J. Appl. Phys., 71(3) (1992) 1421. 13 K. A. Vorotilov, E. V. Orlova and V. I. Petrovsky, Thin Solid Films, 207 (1992) 180. 14 G. Atanassov, R. Thielsch and D. Popov, Thin Solid Films, 223 (1993) 288. 15 L. J. Meng, M. Andritschky and M. P. dos Santos, Thin Solid Films, 223 (1993) 242.
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L. J. Meng et al. / Porosity of TiO 2 films
16 L. J. Meng and M. P. dos Santos, Thin Solid Films, 226 (1993) 22. 17 L. J. Meng and M. P. dos Santos, Appl. Surf. Sci., 68(1993) 319. 18 D. Briggs and M. P. Seah (eds.), Practical Surface Analysis by Auger and XPS, Wiley, Chichester, 1987. 19 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and
G. E. Muilenberg (¢ds.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1979. 20 M. E. Levin, M. Salmeron, A. T. Bell and G.A. Somorjai, Surf. Sci., 195 (1988) 429. 21 G. V. Samsonov, The Oxide Handbook, IFI-Plenum, New York, 1982.
117
Study of porosity of titanium oxide films by X-ray photoelectron spectroscopy and IR transmittance Li-Jian M e n g *a, Carlos P. Moreira de S/t b and M. P. dos Santos a "Physics Department, University of Minho, 4719 Braga Codex (Portugal) bCEMUP, 4100 Porto (Portugal) (Received June 22, 1993; accepted September 8, 1993)
Abstract Titanium oxide films were deposited on glass substrates by d.c. reactive magnetron sputtering at different oxygen partial pressures (3.4 x 10 -4 mbar to 3 x 10 -3 mbar) and total pressures (2 x 10 -3 mbar to 2 x 10 -2 mbar). Two resolved peaks in the Ols spectra of XPS were observed. These two peaks were attributed to the O - T i bonding and the O - H bonding. The O - H bonding component increased as the oxygen partial pressure and total pressure were increased. The near IR transmittance measurements showed that all films had an absorption near 2.8 lain, and the absorption increased as the total pressure and oxygen partial pressure were increased. These phenomena were related to the porosity of the film. The films prepared by d.c. reactive magnetron sputtering generally have a columnar structure with many pores. In air some of these pores are filled with water, and this results in the appearence of the O - H bonding component and the absorption around 2.8 lain. The increase of the O - H bonding component and the absorption around 2.8 lam indicates the increase of the film porosity.
1. Introduction Dense films with low porosity are desired for good environmental reliability. Moisture penetrating through the pores alters the optical and mechanical properties of the coatings. The optical effects are due to filling of the pores with water, which has a h i g h e r refractive index (1.33) than air, and due to changes in the polarizability of the grain boundary surfaces due to adsorption of polar molecules. Moisture also induces compressive or tensile stress, depending on the material and grain boundary surface chemistry. This stress can lead to catastrophic mechanical failure of the coating [1, 2]. Therefore, study of film porosity is of special interest. X - R a y photoelectron spectroscopy (XPS) gives the possibility to detect the different chemical states of an element by measuring the binding energy with a high energy resolution. The difference in the binding energy in the photon emission spectrum can b e c o r r e l a t e d directly with the difference in the chemical states of the elements. Therefore XPS has been used widely to analyze film composition and the chemical bonding of the elements [3-9]. Titanium oxide films deposited by various methods have been extensively studied for use as optical coatings
*On leave from Changchun Institute of Physics Chinese Academy of Sciences, Changchun 130021, People's Republi of China.
0040-6090/94/$7.00 SSDI 0040-6090(93)02909-W
for visible and near IR optics [10-14]. The optical and structural properties of titanium oxide films prepared by the d.c. reactive magnetron sputtering method have been found to be quite sensitive to the 'deposition parameters; specifically, the oxygen partial pressure, total pressure and the substrate temperature during deposition [ 15-17]. In this paper, the variation of the porosity o f the films prepared at different oxygen partial pressures and total pressures was studied using XPS and near IR transmittance methods.
2. Experimental details Titanium oxide films were deposited on glass substrates by d.c. reactive magnetron sputtering in a mixed argon and oxygen atmosphere. The target was titanium metal of 99.6% purity. The substrate was without extra heating and biasing during the deposition processes. For samples N1, N5 and N i l , the total pressure was kept at 8 x 10-3 m b a r and the oxygen partial pressures were 3 x 10 -3 mbar, 6 x 10 -4 m b a r and 3.4 x 10 -4 m b a r respectively. For samples N 15, N 17 and N 19, the oxygen partial pressure was kept at 6 x 10- 4 m b a r and the total pressures were 2 x 10-3 mbar, 6 x 10-3 m b a r and 2 x 10 -2 m b a r respectively. Details of the sample preparation can be found in other papers [16, 17]. The transmittance of the films was measured using a Shimadzu UV-3101PC computer-controlled spectro-
© 1 9 9 4 - Elsevier Sequoia. All rights reserved
118
L. J. Meng et al. / Porosity of 7702 films •
photometer. It can measure the transmittance of the near IR region. The surface topology and cross-sectional structure of the films were observed using scanning electron microscopy (SEM). In order to prevent charge build-up, a thin gold film was sputtered on the surface of the sample before doing SEM. XPS analysis was performed with a VG Escalab 200A electron analyser operating under computer control using VGS 5250 Datasystem software. The samples were placed in the vacuum system and pumped down to the 10 -9 mbar range before the measurement. The pressure of the vacuum system was steady during the measurement. Photoelectrons were excited over a nominal 10 mm x 10 mm area with AI K~t radiation (1486.6 eV). Photoelectrons were detected with a hemispherical analyser with an input lens positioned normal to the surface. Spectrometer pass energy was 20 eV for detailed region spectra. The atomic concentrations of the various elements were computed from measured peak areas (Cls, Ols and Ti2p3/2) using VGS 5250 software with the following sensitivity factors: C, 1; O, 2.85; Ti, 5.22. Linear background subtraction and gaussian peak deconvolutions were performed for Ols photoelectron peaks using VGS 5250 software.
3. Results
Detailed X-ray photoelectron region spectra of the Ols, Ti2p and Cls regions of sample N15 are shown in Fig. 1. It is clear that all the peaks shift to higher energies owing to charge effect [18]. The carbon peaks are from the surface contamination, because after 5 rain of Ar ion bombardment they disappear. Peak positions were calibrated by taking the Cls peak (284.6 eV) as a reference, because many authors have suggested that the Cls peak produced by adsorbed, adventitious carbon could be assigned a fixed value and all other binding energies could be referenced to it [18]. The results were listed in Table 1. It can be seen from Table 1 that the peak positions after calibration are in agreement with literature values, which, give O ls and Ti2p3/2 binding energies relative to TiO2 species of 530 eV and 458.6 eV respectively [19]. From Fig. 1 it can be seen that the Ols spectra consist of two peaks. The deconvolution of the O ls spectra has been done for all the samples as shown in Fig, 2. The peak positions and the calculated atomic concentrations of the elements were summarized in Table 1. The two deconvoluted peaks observed in the Ols spectra near 530 and 532 eV binding energy can be attributed to the O-Ti (O1) and O - H (02) bonding respectively [19, 20]. It can be seen that the component corresponding to the O - H bonding increases as the oxygen partial and total pressure are increased.
°
"
•
•
.
i
•
,
'•o
290
280
300
0 la
~
|
m
t
t
I
520
535
550
tl/45
t¢60
tt75
Bindirx:j Enerqy (eV) Fig. 1. XPS spectra of a titanium oxide film (sample Ni5). Figures 3 and 4 show the transmittance of the films prepared at different oxygen partial and total pressures. It is dear that the films have an absorption peak near 2.8 pm and the absorption increases as the oxygen partial find total pressures are increased. This absorption can be attributed to the absorption of the water. Therefore, the variation of this absorption reflects the variation of the amount of water in the films and further the variation of the film porosity.
4. Discussion
The deconvoluted XPS Ols spectra as shown in Fig. 2 show that the resolved peaks are separated by about 2 eV. The positions of the two resolved peaks in the Ols spectra are located at 530 eV and 532 eV respectively. The lower energy peak located at 530 eV (O1) corresponds to O-Ti bonding, while the higher energy peak located at 532 eV (02) may be assigned to OH (531.5 eV) and/or H20 (533 eV) species, indicating the presence of hydrated oxides [5]. The calculated atomic concentrations for films prepared at different oxygen partial and total pressures were listed in Table 1. The compositions given in Table 1 probably have
L. J. Meng et al. / Porosity of TiO2films
119
TABLE 1. Deposition conditions and XPS and near IR absorption data of titanium oxide films (data in parentheses are the calculated atomic concentration)
Oxygen pressure (mbar) Total pressure (mbar) Ti2p3/2 CIs Ols Ol O2 O2/(OI + 02) Cw ( x 10-4nm -1) Refractive index (500 nm)
NI
N5
NIl
Nl5
NI7
NI9
3 x l0 -3
3.4 x l0 -4
458.6 (14%) 284.6 (36%)
6 x l0 -( 8 x l 0 -3 458.6 (16%) 284.6 (31%)
458.6 (16°/o) 284.6 (31%)
2 x l 0 -3 458.6 (18%) 284.6 (29%)
6 x l0 -4 6 x l 0 -3 458.7 (16%) 284.6 (33%)
2 x l 0 -2 458.7 (15%) 284.6 (34%)
529.9 (31%) 532.1 (19%) 0.38 1.85 2.37
529.9 (36%) 531.7 (17%) 0.32 1.46 2.39
529.9 (37%) 531.8 (16%) 0.30 1.20 2.42
529.5 (38%) 531.7 (15%) 0.28 0.74 2.48
530 (34%) 532 (17%) 0.33 1.20 2.41
530 (31%) 531.7 (19%) 0.38 1.68 2.14
01 1 , t!vvvvv A O1
N15
01
Nll ~ 0 2
0
. . . . . . .
i . . . . . . . . .
, . . . . . . . . .
i . . . . . . . . .
i . . . . . . . . .
i . . . . . . . . .
~
2,50 7,50 12,50 17,50 2250 2750 32,50 Wavellm~ (rim) I
520
530
Binding Energy
540 (eV)
520
I
|
530
Binding Energy
540
Fig. 3. Transmission spectra of titanium oxide films prepared at different oxygen partial pressures.
(eV)
Fig. 2. Ols spectra with the two resolved O bonding components for titanium oxide films prepared at different oxygen partial and total pressures.
some uncertainties in the absolute values, but their variations are significant. From Table 1 it can be seen that the 0 2 component increases as the oxygen partial and total pressures are increased; that means an increase o f the water component in the films, because the 0 2 component is related to water. In order to prove that the 0 2 component is not from surface contamination, sample N 15 was bombarded by Ar ions for about 5 min and then XPS analysis was done again. The result shows that the carbon peak disappeared but the 0 2 component was still there. However, it is unavoidable that the surface contamination also has some contribution to the 0 2 component. Although Ar ion bombardment can clean the sample surface, it also
produces some negative effects, e.g. the variation of surface topography and the occurrence of preferential sputtering. These effects will also affect the measurement of the 0 2 component. For these reasons, we did not clean the sample surfaces by Ar ion bombardment. In general, metallic oxide films prepared by d.c. reactive magnetron sputtering have a columnar structure with many pores. In air some of these pores are filled with water, and this results in the appearance of 0 2 component in the Ols spectra. The increase of 0 2 component indicates the increase of the porosity of the films. That means the film porosity increases as the oxygen partial and total pressures are increased. The porosity arises from the limited adatom mobility, which, for a given species, depends both on the energy of the sputtered species incident on the growth surface and on the substrate temperature. Experiments indicate that high temperatures and energetic sputtered species
L. J. Meng et al. [ Porosity of TiO 2 films
120
where P is the packing density, which can be obtained by using the equation [14]:
100
n f2 _ 1
1p
nm 2 -
n2+2--nm2+2
N19
0
....... :......... :......... :......... :......... :......... : 250
750
1250 1750 2250 2750 3250 Wavellmon~ (am)
Fig. 4. Transmission spectra of titanium oxide films prepared at different total pressures. (e.g. ions) promote dense structures, whereas high background gas pressures, by causing loss of energy of sputtered species via gas-phase scattering, cause higher porosities. Therefore, the variation of the 0 2 component in the O l s spectra reflects correctly the variation of the film porosity. Although the XPS measurement results only give the information in a depth of about 3 nm, the films have a columnar structure and the pores go through the whole film depth. Therefore, the XPS measurement results can reflect the film porosity properties. The ratio O2/(O1 + 02) can be related to the film porosity to some extent. The film porosity may be written as follows:
Ol Porosity = A Ol + 0 2
(!)
where O l and 0 2 are the atomic concentrations of O1 and 0 2 components as shown in Table 1, and A can be considered as a corrective factor. The ratios of O2/(O1 + 0 2 ) for the films prepared at different deposition conditions have been given in Table 1. In addition, the film porosity can also be written as follows: Porosity = 1 - P TABLE 2.
(2)
nw 2 - 1 + ~ a ( 1
--P)
(3)
where nr, nm and nw are the refractive indices of the film, bulk material and the water respectively, a is a coefficient which indicates the relative proportion of the pores filled with water, and P is the film packing density. The refractive index of the films prepared at different oxygen partial and total pressures has been given in Table 1 [16, 17]. The nm and nw are 2.57 and 1.33 respectively [21]. Coefficient a was taken as 1, which means all the pores were filled with water. By using these parameters and eqns. (2) and (3), the film porosities were calculated and the results were listed in Table 2. It can be seen that the film porosity decreases as the oxygen partial and total pressures are increased. The parameter A can be obtained by comparing eqns. (1) and (2), and the results were shown in Table 2. It can be seen that A is different for different samples. However, it should be noted that a large difference only occurs with samples N15 and N19, and the difference is small for the rest. If we rule out the values of N 15 and N I 9 and take the average value of the others, then the film porosities can be calculated by eqn.(1). The results were listed in Table 2. The transmittance spectrum of the water-containing films exhibits an absorption band located near 2.8 ~tm as shown in Figs. 3 and 4, which can be used for determination of the amount of water in the film. A relative measure of water in the film Cw can be calculated using the equation [2]: In (To~T) Cw - - dr
(4)
and the film porosity can be obtained using the following equation: Porosity = BCw
(5)
where T and To are respectively the optical tance at 2.8 ~ n of the film in ambient and of film if it has no water. B is a parameter which to the water absorption coefficient at 2.8 gm
transmitthe same is related and dr is
Porosity of the films estimated by different methods
Film porosity estimated from refractive index Film porosity estimated from XPS Film porosity estimated from IR transmittance Parameter A Parameter B ( x 102 nm)
N1
N5
Nil
NI5
NI7
NI9
0.10 0.10 0.11 0.26 5.41
0.09 0.08 0.09 0.28 6.16
0.07 0.08 0.07 0.23 5.83
0.04 0.07 0.04 0.14 5.40
0.08 0.08 0.07 0.24 6.67
0.24 0.10 0.10 0.63 14.29
L. J. Meng et al. / Porosity of TiO2films
the film thickness. The transmittance of the film without water was obtained by reconstructing the transmittance curve at the water absorption peak by using adjacent interference extrema. By comparing eqns. (2) and (5), the parameter B can be obtained as shown in Table 2. It can be seen that the B value of sample N19 is very different from the others. By taking the average value of five similar B values, the film porosity can be calculated via eqn. (5). The film porosity calculated by this method is also shown in Table 2. It can be seen that the film porosity calculated by water absorption has a similar variation trend to those obtained from XPS and the refractive index. Comparing the porosity obtained by measuring XPS and IR transmittance with that obtained from the refractive index, it can be found that the large divergence occurs for sample NI9. The porosity calculated from the refractive index is much larger than that obtained from XPS and IR transmittance. This can be explained as follows: the pores in the films are only partially filled with water. From Fig. 5 it can be seen that the pores in N I9 seem to be larger than those in the other samples. Some pores may be too large to act as capillaries, and these pores did not fill with water, which results in the small porosity in the XPS and IR measurements, because the porosity is measured by calculating the amount of water. Figure 5 shows the surface and cross-section structures for the films deposited at different oxygen partial and total pressures. As can be seen from Fig. 5, the film structure changes from more dense into more porous as the oxygen partial and total pressures are increased.
i Fig. 5. Scanning electron micrographs of titanium oxide films prepared at different oxygen partial and total pressures: upper row, surfaces; lower row, cross-sections.
121
These observations are in accordance with the discussion above.
5. Conclusions The porosities of titanium oxide films prepared by d.c. reactive magnetron sputtering at different oxygen partial and total pressures have been studied by analysis of their XPS and IR transmittance. All the Ols spectra of XPS have two peaks. The peak located at lower binding energy is related to T i - O bonding and the peak located at higher binding energy is related to O - H bonding. The film which has a higher O - H bonding component has a more porous structure. The near IR transmittance of the film shows an absorption band near 2.8 lxrn. The film porosity has been related to this absorption band. The film which has stronger absorption has a more porous structure. All the results show that the film porosity increases as the oxygen partial and total pressure are increased.
Acknowledgment L. J. Meng is thankful to the Orient Foundation for providing a scholarship.
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