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Effects of annealing on titanium dioxide structured films
Thin Solid Films 466 (2004) 92 – 96 www.elsevier.com/locate/tsf
Effects of annealing on titanium dioxide structured films M.J. Colgan a,*, B. Djurfors b, D.G. Ivey b, M.J. Brett a a b
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received 5 June 2003; received in revised form 8 December 2003; accepted 10 February 2004 Available online
Abstract Titanium dioxide is a material of interest for optically active structured films due to its high index of refraction and transparency across the visible wavelength range. A variety of films, consisting of different structures, were deposited by reactive electron beam evaporation using the glancing angle deposition process. Samples of these films were annealed at 500 jC for 3 hours in air. The structures of both as-deposited and annealed films were examined by scanning electron microscopy, X-ray diffractometry, and transmission electron microscopy. It was found that as-deposited films were amorphous with a very fine structure. The annealed films were found to be polycrystalline anatase and had lost their fine structure. In addition, the optical properties of the films were examined by spectrophotometric transmission measurements, wherein it was found that the annealing caused a significant transmission increase across the visible spectrum. D 2004 Published by Elsevier B.V. Keywords: Titanium dioxide; Annealing; Glancing angle deposition; Transmission electron microscopy
1. Introduction The optical anisotropy and activity of films deposited at oblique angles has been well studied by a number of groups [1 – 5]. From these studies have come proposals for a variety of applications ranging from diffraction gratings [6], rugate filters [7] and photonic bandgap crystals [8] to hybrid film/liquid crystal switching cells [9]. Although all have shown encouraging results, they all similarly suffer from the effects of Rayleigh scattering in the visible region. These effects are difficult to eliminate through control of the deposition process, as they are due to the inherently porous nature and small structure sizes found in obliquely deposited films. The effects of a post-deposition annealing step on structured films and their optical transmission were examined. The optical activity, for example rotation, of the films has been hypothesized to be due to the refractive index difference that may be found between the isolated structures and the air, or other material, filling the voids [10]. To maximize such effects, the refractive index of the structures should be
* Corresponding author. Tel.: +1-780-4927926; fax: +1-780-4922863. E-mail address: [email protected] (M.J. Colgan). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.02.019
chosen to provide maximum contrast to that of the filling material. To this end, titanium dioxide was chosen to form structures in air. Although dependent upon the method of deposition [11], titanium dioxide typically has one of the highest indices of refraction of standard optical materials and is highly transparent across the visible range. Titanium dioxide is also highly resistant to chemical, thermal and abrasive wearing. Films of this material will thus form robust, stable devices. An annealing step was examined because others’ work on normally deposited, ion-beamsputtered, dense films had shown annealing to be successful in promoting increased transmission [12].
2. Experimental details Titanium dioxide films were fabricated by electron beam evaporation of 3– 6 mm pieces of CERAC 99.9% pure rutile TiO2 in a vacuum system with base pressures less than 2.4 10 4 Pa. During the depositions, Air Liquide UHP oxygen was added close to the substrate holder until a partial pressure of 7.3 10 3 Pa, as measured by a Bayard – Alpert gauge at the diffusion pump mouth, was obtained. The depositions took place onto unheated 1VV 1VV indium tin oxide (ITO) coated glass substrates
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
provided by Philips Research Laboratories, as well as both metallized and bare silicon wafer witness samples held at oblique angles. The ITO coated glass was chosen as the substrate for the optical measurements to easily facilitate later work with electrically switched optical devices. The deposition rate, as measured by a quartz crystal oscillator positioned near the substrate holder and normal to the ˚ /s. The glancing angle deposition arriving flux, was 9 – 10 A (GLAD) process utilizes nearly collimated flux and controlled substrate motion to create films of isolated structures having diameters down to 50 nm. The GLAD process and equipment have been described in detail elsewhere [13,14] and will not be further discussed here. A number of films were deposited with the angle between the substrate normal and the incident flux direction varying between 83 and 87j. These films were comprised of a variety of structures, including slanted columns, vertical posts, helices and square spirals. After deposition, the substrates were allowed to cool in a nitrogen ambient and then stored in air. Two samples were taken from each deposition, one on ITO coated glass and other on silicon, and annealed in air at 500 jC for 3 h in a Thermolyne 48 000 muffle furnace. These samples were then allowed to cool overnight and again stored in air. Samples of as-deposited and annealed films on silicon were used for structural analysis. The samples were first cleaved and divided into two groups. The first group was coated with a thin layer of chromium and analyzed in a JEOL JSM6301FXV scanning electron microscope (SEM) with a field emission electron source running at 5 kV. The second group was left uncoated and analyzed in a Rigaku Rotaflex rotating anode X-ray diffractometer (XRD) with a thin film camera attachment. The Cu Ka X-ray incidence angle was set to 2j from the sample surface to minimize sampling of the substrate. The filament was set to run at 40 kV, with 100 mA of current. Samples were scanned between 5 and 90j at a rate of 2j/min. After XRD analysis, a scalpel was used to scrape areas of the samples off onto carbon coated transmission electron microscope (TEM) grids. Isopropyl alcohol was dripped onto the grids to disperse, and more evenly distribute, the structures. These samples were allowed to dry overnight and then examined in a JEOL 2010 TEM. The samples were imaged at 200 kV and crystal structure analysis was done using electron diffraction. Samples on ITO coated glass were allowed to age in air for several months so that their optical responses would more closely resemble that of a device in service. To ensure cleanliness of the samples, they were rinsed with isopropyl alcohol, then 20 MV de-ionized water, and finally dried in a gaseous nitrogen stream. To lessen the likelihood of excess water remaining absorbed in or on the films, they were allowed to dry in air overnight. The transmission spectra of the films in the visible region were acquired using a Perkin – Elmer Lambda 900 double beam spectrophotometer equipped with a General Purpose Optics Bench and integrating sphere. The effect of the substrate on the transmis-
93
sion measurements was corrected for by using the Autozero feature of the Perkin – Elmer software. This feature corrects the measured transmission spectra for reflection off of the front face of the substrate and absorption of light as it passes through the substrate by comparing the sample spectra to a standard spectrum measured from an uncoated substrate. An uncoated Philips ITO coated glass substrate was used as the baseline for as-deposited films and an uncoated Philips ITO coated glass substrate annealed under the same conditions as the films was used as the baseline for the annealed films. All spectra were acquired within two hours of each other and were found to have an error of F 0.2% at each wavelength measured. Previous measurements with this instrument indicated that adding desiccant and purging with dry nitrogen did not alter the transmission measured [15].
3. Results/discussion The deposition conditions and structures obtained are listed in Table 1. Although all depositions were performed onto unheated substrates, radiant heating occurs from the source melt and energy is also deposited from the condensation of the TiO2 vapor. Earlier work with GLAD using a thermal evaporation process had shown substrate temperatures of approximately 160 jC and it is expected that temperatures would be slightly higher for electron beam evaporation. When removed, a number of the substrates appeared to have a blue coloration, indicating an oxygen deficiency [16]. When observed after being stored in air overnight, all films appeared white, indicating an uptake of oxygen and a move towards fully stoichiometric TiO2. Other workers have observed similar effects in dense TiO2 films and found that the change in transmission caused by oxygen uptake was minimal after being stored in air for 48 h [17]. SEM analysis of as-deposited films indicated that the addition of oxygen during the deposition did not significantly affect the structures produced by the GLAD process. This was not unexpected as the mean free path of the TiO2 vapor was calculated to be greater than twice the separation of the crucible and the substrate holder when using the pressure indicated at the diffusion pump mouth [18]. SEM images of films C to F are shown in Fig. 1. Images of films
Table 1 Film structures and deposition conditions Film label
Structure
Deposition angle (j)
Thickness (nm)
A B C D E F
Slanted column Slanted column Slanted column Vertical posts Helices Square spirals
83 85 87 85 85 85
1830 1680 1330 1160 1300 1080
94
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
Fig. 2. XRD pattern of as-deposited and annealed samples of film C.
Fig. 1. Oblique SEM views of films (a) C, (b) D, (c) E and (d) F.
A and B appear very similar to those of film C, with small differences in the column inclination angles. The annealed samples showed little change when examined by SEM. In some samples there was a small decrease in the overall thickness of the film. This decrease was at all times less than 10% of the overall film thickness and in a number of films, no evidence of such densification could be observed. It should be noted that due to the highly oblique angle at which the substrate is held during deposition, there may be a variation of as much as 65% across a stationary 4 inch wafer, as calculated from the cosine law of emission [19]. In an attempt to minimize this effect on thickness measurements, silicon witness samples were taken from similar areas on the substrate holder, however, not all variation could be eliminated. Films deposited at oblique angles are not immune to the densification effect of annealing that has been reported by researchers working with normally deposited dense films [20]. In fact, in a recent experiment on yttrium oxide/ europium oxide films by a co-worker, annealing of a 50 nm europium oxide layer on top of a 400 nm film of yttrium oxide at 900 jC for 75 h resulted in a significant thickness reduction. The isolated columns remained, but the overall thickness of the film was found to decrease to 300 nm and the columns were significantly broadened. Thin films of titanium dioxide exist in three forms: amorphous, anatase and rutile. Anatase is the low temperature polymorph and rutile is the high temperature stable form. The XRD analysis, an example of which is shown in Fig. 2, indicated that all as-deposited films were amorphous, the usual state of TiO2 thin films deposited on unheated substrates by reactive evaporation [21]. The XRD of the annealed films, however, showed that the film had con-
verted to the polycrystalline anatase state. This is in agreement with the work of others who have found amorphous thin films of titanium dioxide transform to anatase at annealing temperatures above 350 jC, while a further transition to rutile requires temperatures above 800 jC [22]. The crystalline states of the as-deposited and annealed samples were independent of the structure of which the film consisted. The TEM analysis confirmed the XRD results, showing diffuse amorphous diffraction patterns for all as-deposited samples and polycrystalline anatase patterns for all annealed samples. The grains were commonly 30 100 nm, although there was a wide distribution of grain sizes, with a tendency in the helical film towards smaller grains. One dark field image of an annealed sample of film B taken from part of the (107) reflection for anatase is shown in Fig. 3. The corresponding polycrystalline diffraction pattern, with the
Fig. 3. TEM dark field image of film B showing (107) orientation, inset is the polycrystalline diffraction pattern.
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
Fig. 4. TEM bright field images of film B in its (a) as-deposited and (b) annealed states.
(107) reflection indicated, is shown in the inset of Fig. 3. Although TEM images of helices and square spirals are not shown, they exhibit similar structure and crystal form. Bright field TEM images show that the annealing process did structurally modify the films. As seen in Fig. 4a, the asdeposited structure had a fibrous appearance which has been previously described by others [23]. The annealed films, however, showed that the fibers had conglomerated, forming a structure more closely resembling a true solid column. This is illustrated in Fig. 4b. The optical spectra of the films showed significant absolute improvements in transmission at longer wavelengths for the majority of the annealed films. Fig. 5 shows the transmission spectra of films A and F. Film A exhibited the greatest overall improvement, while film F is composed of one of the most technologically interesting structures. Other films produced similar transmission spectra and showed similar improvements with annealing. The absolute improvement decreased so that the transmission of the annealed films at the low end of the wavelength range examined appeared to be very close to the transmission of the as-deposited films. In fact, when the improvement in transmission as a percentage of the transmission for the asdeposited films is examined, it can be seen that the improvement was somewhat higher at the lower wavelengths
95
for most films. From Fig. 4, it can be seen that only the small structures were eliminated by the annealing process. This translated to a smaller number of scattering centers, approaching the areal density of the individual structures, and an increase in the size of the scattering centers, to a size approaching that of the individual structures. The increase in scattering center size shifted the scattering from the Rayleigh regime, which applies when the scattering center size is less than approximately one tenth of the incident wavelength, into the Mie regime. In the Mie regime, there is increased scattering, but in a highly preferential forward direction [24]. When obtaining the optical spectra, the samples were placed as close as possible to the integrating sphere. The transmission measured in this way was the total transmission, as opposed to the direct transmission. It is expected that if the direct and diffuse transmission were measured, one would find a decrease in the direct transmission with annealing, accompanied by an increase in the diffuse transmission. Although the scattering increased, its directional nature resulted in a net increase in the transmitted light. It is also possible that the annealing resulted in more fully stoichiometric films, however, because the greatest source of optical loss in these films is scattering, as can be seen from the shape of the transmission spectra, slight decreases in absorption would not lead to such pronounced improvements in transmission.
4. Conclusions The effects of annealing on structured titanium dioxide films were examined. The structure of the films was characterized by SEM, XRD and TEM. The annealing was found to cause a transition from amorphous to polycrystalline anatase structure. It also caused the fibrous microstructure found in as-deposited amorphous films to disappear, resulting in a more solid structure for each column or other structure. This reduction in the number and size of scattering centers is hypothesized to be respon-
Fig. 5. Transmission spectra of (a) film A and (b) film F.
96
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sible for the increased optical transmission of the annealed films over the as-deposited films, which was observed by spectrophotometric measurements. In particular, a significant absolute improvement was observed at long wavelengths, while the percentage improvement was more significant at short wavelengths.
Acknowledgements The authors would like to gratefully acknowledge Scott Kennedy, Andy van Popta and Peter Hrudey for fruitful discussions and George Braybrook for the SEM images. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Informatics Circle of Research Excellence (iCORE) and Micralyne Inc.
References [1] [2] [3] [4]
R.M.A. Azzam, Appl. Phys. Lett. 61 (1992) 3118. I. Hodgkinson, Qh. Wu, Adv. Mater. 13 (2001) 889. Y. Taga, Mater. Sci. Eng. C C15 (2001) 231. F. Wang, A. Lakhtakia, R. Messier, Eur. Phys. J. Appl. Phys. 20 (2002) 91. [5] M.J. Brett, M.O. Jensen, J.C. Sit, S.R. Kennedy, K.D. Harris, D.J. Broer, Society of Vacuum Coaters 45th Annual Technical Conference, Orlando, USA, 2002, Society of Vacuum Coaters 45th Annual Technical Conference Proceedings (2002) 238.
[6] A.Y. Elezzabi, J.C. Sit, J.F. Holzman, K. Robbie, M.J. Brett, Electron. Lett. 35 (1999) 491. [7] K. Robbie, A.J.P. Hnatiw, M.J. Brett, R.I. MacDonald, J.N. McMullin, Electron. Lett. 33 (1997) 1213. [8] S.R. Kennedy, M.J. Brett, H. Miguez, O. Toader, S. John, Photon. Nanostruct. 1 (2003) 37. [9] J.C. Sit, D.J. Broer, M.J. Brett, Adv. Mater. 12 (2000) 371. [10] J.C.-H. Sit, Ph.D. Thesis, Department of Electrical and Computer Engineering, University of Alberta, Canada, 2002. [11] J.M. Bennett, E. Pelletier, G. Albrand, J.P. Borgogno, B. Lazarides, C.K. Carniglia, R.A. Schmell, T.H. Allen, T. Tuttle-Hart, K.H. Guenther, A. Saxer, Appl. Opt. 28 (1989) 3303. [12] W.-H. Wang, S. Chao, Opt. Lett. 23 (1998) 1417. [13] K. Robbie, M.J. Brett, U.S. Patent No. 5866204, February 2, 1999. [14] K. Robbie, J.C. Sit, M.J. Brett, J. Vac. Sci. Technol. B 16 (1998) 1115. [15] S.R. Kennedy, M.J. Brett, Appl. Opt. 42 (2003) 4573. [16] D.C. Cronemeyer, Phys. Rev. 113 (1959) 1222. [17] K.N. Rao, Opt. Eng. 41 (2002) 2357. [18] J.F. O’Hanlon, A User’s Guide to Vacuum Technology, Wiley, New York, 1989. [19] L.I. Maissel, R. Glang, Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. [20] V. Mikhelashvili, G. Eisentein, J. Appl. Phys. 89 (2001) 3256. [21] P. Lobl, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 72. [22] M.D. Wiggins, M.C. Nelson, C.R. Aita, J. Vac. Sci. Technol. A 14 (1996) 772. [23] M. Malac, R.F. Egerton, J. Vac. Sci. Technol. A 19 (2001) 158. [24] M. Born, E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Pergamon Press Ltd, London, 1970.
Effects of annealing on titanium dioxide structured films M.J. Colgan a,*, B. Djurfors b, D.G. Ivey b, M.J. Brett a a b
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received 5 June 2003; received in revised form 8 December 2003; accepted 10 February 2004 Available online
Abstract Titanium dioxide is a material of interest for optically active structured films due to its high index of refraction and transparency across the visible wavelength range. A variety of films, consisting of different structures, were deposited by reactive electron beam evaporation using the glancing angle deposition process. Samples of these films were annealed at 500 jC for 3 hours in air. The structures of both as-deposited and annealed films were examined by scanning electron microscopy, X-ray diffractometry, and transmission electron microscopy. It was found that as-deposited films were amorphous with a very fine structure. The annealed films were found to be polycrystalline anatase and had lost their fine structure. In addition, the optical properties of the films were examined by spectrophotometric transmission measurements, wherein it was found that the annealing caused a significant transmission increase across the visible spectrum. D 2004 Published by Elsevier B.V. Keywords: Titanium dioxide; Annealing; Glancing angle deposition; Transmission electron microscopy
1. Introduction The optical anisotropy and activity of films deposited at oblique angles has been well studied by a number of groups [1 – 5]. From these studies have come proposals for a variety of applications ranging from diffraction gratings [6], rugate filters [7] and photonic bandgap crystals [8] to hybrid film/liquid crystal switching cells [9]. Although all have shown encouraging results, they all similarly suffer from the effects of Rayleigh scattering in the visible region. These effects are difficult to eliminate through control of the deposition process, as they are due to the inherently porous nature and small structure sizes found in obliquely deposited films. The effects of a post-deposition annealing step on structured films and their optical transmission were examined. The optical activity, for example rotation, of the films has been hypothesized to be due to the refractive index difference that may be found between the isolated structures and the air, or other material, filling the voids [10]. To maximize such effects, the refractive index of the structures should be
* Corresponding author. Tel.: +1-780-4927926; fax: +1-780-4922863. E-mail address: [email protected] (M.J. Colgan). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.02.019
chosen to provide maximum contrast to that of the filling material. To this end, titanium dioxide was chosen to form structures in air. Although dependent upon the method of deposition [11], titanium dioxide typically has one of the highest indices of refraction of standard optical materials and is highly transparent across the visible range. Titanium dioxide is also highly resistant to chemical, thermal and abrasive wearing. Films of this material will thus form robust, stable devices. An annealing step was examined because others’ work on normally deposited, ion-beamsputtered, dense films had shown annealing to be successful in promoting increased transmission [12].
2. Experimental details Titanium dioxide films were fabricated by electron beam evaporation of 3– 6 mm pieces of CERAC 99.9% pure rutile TiO2 in a vacuum system with base pressures less than 2.4 10 4 Pa. During the depositions, Air Liquide UHP oxygen was added close to the substrate holder until a partial pressure of 7.3 10 3 Pa, as measured by a Bayard – Alpert gauge at the diffusion pump mouth, was obtained. The depositions took place onto unheated 1VV 1VV indium tin oxide (ITO) coated glass substrates
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
provided by Philips Research Laboratories, as well as both metallized and bare silicon wafer witness samples held at oblique angles. The ITO coated glass was chosen as the substrate for the optical measurements to easily facilitate later work with electrically switched optical devices. The deposition rate, as measured by a quartz crystal oscillator positioned near the substrate holder and normal to the ˚ /s. The glancing angle deposition arriving flux, was 9 – 10 A (GLAD) process utilizes nearly collimated flux and controlled substrate motion to create films of isolated structures having diameters down to 50 nm. The GLAD process and equipment have been described in detail elsewhere [13,14] and will not be further discussed here. A number of films were deposited with the angle between the substrate normal and the incident flux direction varying between 83 and 87j. These films were comprised of a variety of structures, including slanted columns, vertical posts, helices and square spirals. After deposition, the substrates were allowed to cool in a nitrogen ambient and then stored in air. Two samples were taken from each deposition, one on ITO coated glass and other on silicon, and annealed in air at 500 jC for 3 h in a Thermolyne 48 000 muffle furnace. These samples were then allowed to cool overnight and again stored in air. Samples of as-deposited and annealed films on silicon were used for structural analysis. The samples were first cleaved and divided into two groups. The first group was coated with a thin layer of chromium and analyzed in a JEOL JSM6301FXV scanning electron microscope (SEM) with a field emission electron source running at 5 kV. The second group was left uncoated and analyzed in a Rigaku Rotaflex rotating anode X-ray diffractometer (XRD) with a thin film camera attachment. The Cu Ka X-ray incidence angle was set to 2j from the sample surface to minimize sampling of the substrate. The filament was set to run at 40 kV, with 100 mA of current. Samples were scanned between 5 and 90j at a rate of 2j/min. After XRD analysis, a scalpel was used to scrape areas of the samples off onto carbon coated transmission electron microscope (TEM) grids. Isopropyl alcohol was dripped onto the grids to disperse, and more evenly distribute, the structures. These samples were allowed to dry overnight and then examined in a JEOL 2010 TEM. The samples were imaged at 200 kV and crystal structure analysis was done using electron diffraction. Samples on ITO coated glass were allowed to age in air for several months so that their optical responses would more closely resemble that of a device in service. To ensure cleanliness of the samples, they were rinsed with isopropyl alcohol, then 20 MV de-ionized water, and finally dried in a gaseous nitrogen stream. To lessen the likelihood of excess water remaining absorbed in or on the films, they were allowed to dry in air overnight. The transmission spectra of the films in the visible region were acquired using a Perkin – Elmer Lambda 900 double beam spectrophotometer equipped with a General Purpose Optics Bench and integrating sphere. The effect of the substrate on the transmis-
93
sion measurements was corrected for by using the Autozero feature of the Perkin – Elmer software. This feature corrects the measured transmission spectra for reflection off of the front face of the substrate and absorption of light as it passes through the substrate by comparing the sample spectra to a standard spectrum measured from an uncoated substrate. An uncoated Philips ITO coated glass substrate was used as the baseline for as-deposited films and an uncoated Philips ITO coated glass substrate annealed under the same conditions as the films was used as the baseline for the annealed films. All spectra were acquired within two hours of each other and were found to have an error of F 0.2% at each wavelength measured. Previous measurements with this instrument indicated that adding desiccant and purging with dry nitrogen did not alter the transmission measured [15].
3. Results/discussion The deposition conditions and structures obtained are listed in Table 1. Although all depositions were performed onto unheated substrates, radiant heating occurs from the source melt and energy is also deposited from the condensation of the TiO2 vapor. Earlier work with GLAD using a thermal evaporation process had shown substrate temperatures of approximately 160 jC and it is expected that temperatures would be slightly higher for electron beam evaporation. When removed, a number of the substrates appeared to have a blue coloration, indicating an oxygen deficiency [16]. When observed after being stored in air overnight, all films appeared white, indicating an uptake of oxygen and a move towards fully stoichiometric TiO2. Other workers have observed similar effects in dense TiO2 films and found that the change in transmission caused by oxygen uptake was minimal after being stored in air for 48 h [17]. SEM analysis of as-deposited films indicated that the addition of oxygen during the deposition did not significantly affect the structures produced by the GLAD process. This was not unexpected as the mean free path of the TiO2 vapor was calculated to be greater than twice the separation of the crucible and the substrate holder when using the pressure indicated at the diffusion pump mouth [18]. SEM images of films C to F are shown in Fig. 1. Images of films
Table 1 Film structures and deposition conditions Film label
Structure
Deposition angle (j)
Thickness (nm)
A B C D E F
Slanted column Slanted column Slanted column Vertical posts Helices Square spirals
83 85 87 85 85 85
1830 1680 1330 1160 1300 1080
94
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
Fig. 2. XRD pattern of as-deposited and annealed samples of film C.
Fig. 1. Oblique SEM views of films (a) C, (b) D, (c) E and (d) F.
A and B appear very similar to those of film C, with small differences in the column inclination angles. The annealed samples showed little change when examined by SEM. In some samples there was a small decrease in the overall thickness of the film. This decrease was at all times less than 10% of the overall film thickness and in a number of films, no evidence of such densification could be observed. It should be noted that due to the highly oblique angle at which the substrate is held during deposition, there may be a variation of as much as 65% across a stationary 4 inch wafer, as calculated from the cosine law of emission [19]. In an attempt to minimize this effect on thickness measurements, silicon witness samples were taken from similar areas on the substrate holder, however, not all variation could be eliminated. Films deposited at oblique angles are not immune to the densification effect of annealing that has been reported by researchers working with normally deposited dense films [20]. In fact, in a recent experiment on yttrium oxide/ europium oxide films by a co-worker, annealing of a 50 nm europium oxide layer on top of a 400 nm film of yttrium oxide at 900 jC for 75 h resulted in a significant thickness reduction. The isolated columns remained, but the overall thickness of the film was found to decrease to 300 nm and the columns were significantly broadened. Thin films of titanium dioxide exist in three forms: amorphous, anatase and rutile. Anatase is the low temperature polymorph and rutile is the high temperature stable form. The XRD analysis, an example of which is shown in Fig. 2, indicated that all as-deposited films were amorphous, the usual state of TiO2 thin films deposited on unheated substrates by reactive evaporation [21]. The XRD of the annealed films, however, showed that the film had con-
verted to the polycrystalline anatase state. This is in agreement with the work of others who have found amorphous thin films of titanium dioxide transform to anatase at annealing temperatures above 350 jC, while a further transition to rutile requires temperatures above 800 jC [22]. The crystalline states of the as-deposited and annealed samples were independent of the structure of which the film consisted. The TEM analysis confirmed the XRD results, showing diffuse amorphous diffraction patterns for all as-deposited samples and polycrystalline anatase patterns for all annealed samples. The grains were commonly 30 100 nm, although there was a wide distribution of grain sizes, with a tendency in the helical film towards smaller grains. One dark field image of an annealed sample of film B taken from part of the (107) reflection for anatase is shown in Fig. 3. The corresponding polycrystalline diffraction pattern, with the
Fig. 3. TEM dark field image of film B showing (107) orientation, inset is the polycrystalline diffraction pattern.
M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
Fig. 4. TEM bright field images of film B in its (a) as-deposited and (b) annealed states.
(107) reflection indicated, is shown in the inset of Fig. 3. Although TEM images of helices and square spirals are not shown, they exhibit similar structure and crystal form. Bright field TEM images show that the annealing process did structurally modify the films. As seen in Fig. 4a, the asdeposited structure had a fibrous appearance which has been previously described by others [23]. The annealed films, however, showed that the fibers had conglomerated, forming a structure more closely resembling a true solid column. This is illustrated in Fig. 4b. The optical spectra of the films showed significant absolute improvements in transmission at longer wavelengths for the majority of the annealed films. Fig. 5 shows the transmission spectra of films A and F. Film A exhibited the greatest overall improvement, while film F is composed of one of the most technologically interesting structures. Other films produced similar transmission spectra and showed similar improvements with annealing. The absolute improvement decreased so that the transmission of the annealed films at the low end of the wavelength range examined appeared to be very close to the transmission of the as-deposited films. In fact, when the improvement in transmission as a percentage of the transmission for the asdeposited films is examined, it can be seen that the improvement was somewhat higher at the lower wavelengths
95
for most films. From Fig. 4, it can be seen that only the small structures were eliminated by the annealing process. This translated to a smaller number of scattering centers, approaching the areal density of the individual structures, and an increase in the size of the scattering centers, to a size approaching that of the individual structures. The increase in scattering center size shifted the scattering from the Rayleigh regime, which applies when the scattering center size is less than approximately one tenth of the incident wavelength, into the Mie regime. In the Mie regime, there is increased scattering, but in a highly preferential forward direction [24]. When obtaining the optical spectra, the samples were placed as close as possible to the integrating sphere. The transmission measured in this way was the total transmission, as opposed to the direct transmission. It is expected that if the direct and diffuse transmission were measured, one would find a decrease in the direct transmission with annealing, accompanied by an increase in the diffuse transmission. Although the scattering increased, its directional nature resulted in a net increase in the transmitted light. It is also possible that the annealing resulted in more fully stoichiometric films, however, because the greatest source of optical loss in these films is scattering, as can be seen from the shape of the transmission spectra, slight decreases in absorption would not lead to such pronounced improvements in transmission.
4. Conclusions The effects of annealing on structured titanium dioxide films were examined. The structure of the films was characterized by SEM, XRD and TEM. The annealing was found to cause a transition from amorphous to polycrystalline anatase structure. It also caused the fibrous microstructure found in as-deposited amorphous films to disappear, resulting in a more solid structure for each column or other structure. This reduction in the number and size of scattering centers is hypothesized to be respon-
Fig. 5. Transmission spectra of (a) film A and (b) film F.
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M.J. Colgan et al. / Thin Solid Films 466 (2004) 92–96
sible for the increased optical transmission of the annealed films over the as-deposited films, which was observed by spectrophotometric measurements. In particular, a significant absolute improvement was observed at long wavelengths, while the percentage improvement was more significant at short wavelengths.
Acknowledgements The authors would like to gratefully acknowledge Scott Kennedy, Andy van Popta and Peter Hrudey for fruitful discussions and George Braybrook for the SEM images. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Informatics Circle of Research Excellence (iCORE) and Micralyne Inc.
References [1] [2] [3] [4]
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