Optical properties and microstructures of inorganic wave plates prepared by serial bideposition

Abstract. We investigated the optical characteristics and microstructures of wave plates composed of Ta2O5(100−x)+TiO2(x) and prepared by the so-called serial bideposition technique. While a single-layer film prepared by conventional oblique deposition technique has a tilted columnar structure (i.e., tilted optical axis), a serial bideposition film has a narrow, long columnar structure; this ensures that the optical axis of the film is along the quasinormal to the substrate, thus reducing haze. The influence of using additives with Ta2O5 was investigated as well. It was found that additive TiO2 improves optical transmittance at shorter wavelengths. For verifying the advantage of this type of wave plates, quarter wave plates with optimized TiO2 content were fabricated and their optical performance and reliability were evaluated against those of organic-type wave plates. The results show that the inorganic wave plate prepared by serial bideposition is advantageous for applications where high-transmittance and high-temperature durability are essential.

Abstract. We investigated the optical characteristics and microstructures of wave plates composed of Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ and prepared by the so-called serial bideposition technique. While a single-layer film prepared by conventional oblique deposition technique has a tilted columnar structure (i.e., tilted optical axis), a serial bideposition film has a narrow, long columnar structure; this ensures that the optical axis of the film is along the quasinormal to the substrate, thus reducing haze. The influence of using additives with Ta 2 O 5 was investigated as well. It was found that additive TiO 2 improves optical transmittance at shorter wavelengths. For verifying the advantage of this type of wave plates, quarter wave plates with optimized TiO 2 content were fabricated and their optical performance and reliability were evaluated against those of organic-type wave plates. The results show that the inorganic wave plate prepared by serial bideposition is advantageous for applications where high-transmittance and high-temperature durability are essential. © The Authors. Published by SPIE under a Creative Commons Attribution

Introduction
The demand for optical devices with high-light resistance and thermal durability is increasing because requirements such as increased luminescence and miniaturization of liquid crystal display (LCD) projectors. Polymer-based wave plates are widely used in LCD projectors. They contain organic materials that degrade easily under long, intense exposure to light from a superhigh-pressure mercury lamp. Therefore, wave plates of inorganic materials with high-light resistance and high-thermal durability are required for those applications. It is well known that obliquely deposited thin films can be applied to wave plates. [1][2][3][4] The oblique deposition can lead to the formation of birefringent layers on any substrate at low cost without size limitation. Because these wave plates are composed of inorganic materials, superior thermal durability is expected.
High transmittance and exact control of retardation value are extremely important for using wave plates with obliquely deposited films in practical optical instruments. Several researchers have reported solutions 2,4 for improving the transmittance, which depends on the smaller birefringent, Δn, and, occasionally, on film haze. 1 Several studies have reported that the retardation films prepared by the serial bideposition technique 4-6 have nanostructural growth perpendicular to the substrate, and can therefore be used for increasing Δn. Film thickness could decrease at high values of Δn. Moreover, Motohiro et al. 2 suggested that the columnar growth normal to the substrate can reduce polarization-dependent reflection, which occurs owing to the interface of bilayered retardation films 1 and is obstructive in some applications. 7 Meanwhile, Motohiro and Taga 1 explained that the nature of haze can be attributed to agglutination of the columns; therefore, if columnar structures are considerably smaller than the wavelength of light, the resulting haze value is very low.
Serial bideposition films consist of numerous individual thin layers. Hence, it is possible that haze can be reduced by inhibiting column agglutination. However, there have been few reports on the relationship between the birefringent and the transmittance of serial bideposition films.
In addition, we noticed the effect of the composition of serial bideposition films on their optical properties. The birefringence values of several materials' oblique deposition films were reported, 1,4 and large Δn were obtained for Ta 2 O 5 , TiO 2 , ZrO, and CeO 2 when deposited at lower temperatures. Furthermore, Motohiro and Taga mentioned that obliquely deposited Ta 2 O 5 films show high transmittance in the visible spectrum range. 1 In contrast, there have been few attempts to investigate the effect of element addition on the optical properties of obliquely deposited film.
Several studies reported that the addition of TiO 2 to Ta 2 O 5 thin films prepared by chemical vapor deposition 8 or radio frequency magnetron sputtering deposition 9 can improve the films' insulating properties. It is considered that this improvement occurs owing to the compensation of oxygen vacancies 8 contained in Ta 2 O 5 films evaporated at lower temperatures. [10][11][12][13] We consider that this result implies the addition of TiO 2 to Ta 2 O 5 films could improve the films' transmittance by compensating for oxygen vacancies with serial bideposition films.
In this paper, we present the microstructures and optical properties of serial bideposition films. Then, we clarify the effect of Ta 2 O 5 with TiO 2 additive. Finally, to verify the advantages of the above configuration, we demonstrate an inorganic quarter wave plate with high transmittance, thermal durability, and light resistance. The novelty of this paper is in production of very practical inorganic wave plates prepared by the serial bideposition technique.

Experiments
Serial bideposition films of Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (0≦x≦100 wt%) were formed on a glass substrate with an EB source placed at a source-to-substrate distance of 1000 mm under a pressure of 5 × 10 −4 Pa. During evaporation, deposition angle, α, measured from the surface normal, was fixed to 70 deg, while the azimuth was rapidly switched by 180 deg with each deposition of a 5-1600-nm-thick layer, t 1 . Serial bideposition films with a total thickness, t Ta2O5 , of 800-1600 nm were prepared by repeating 0-200 cycles of each deposition, and subsequently annealing at 200°C in air. Passivation layers of SiO 2 and antireflection coatings of SiO 2 ∕Nb 2 O 5 are formed on the serial bideposition films using a magnetron sputtering system.
Microstructures and compositions of serial bideposition films were investigated by scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry, respectively. The transmittance and reflectance were measured using a double-beam spectrophotometer in the wavelength range of 400-700 nm with polarized light. For reflectance measurements, the angle of incidence of light was 5 deg, and the spectra of specularly reflected light were acquired. In contrast, transmittance spectra were measured for normal incidence. Diffuse scattering in the off-specular or off-normal directions was negligible. The birefringence was evaluated using a phase-retardation measurement system with a rotating analyzer.
Humidity resistance was evaluated by measuring the transmittance of the serial bideposition films, which were maintained at 60°C and 90% R.H for different storage times and then maintained at 23°C for 24 h. Light resistance properties were evaluated by measuring the transmittance of serial bideposition films irradiated with a light intensity of 0.6 W∕mm 2 from a highpressure mercury lamp for different storage times and then maintained at 23°C for 24 h. Figure 1 shows the morphology of the serial bideposition Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 5 wt%) film for t 1 ¼ 7 nm and t Ta2O5 ¼ 1500 nm. Figures 1(a) and 1(b) show the SEM images of the cross-sectional views parallel to the deposition plane of the Ta 2 O 5 flux and the top view, respectively. It is observed that the serial bideposition film is composed of narrow, long, columnar structures, and that many bundles and gaps derived from the columnar structures were along the normal to the Ta 2 O 5 flux deposition plane.

Microstructures
This surface morphology can be observed even on a Ta 2 O 5 film prepared by conventional oblique deposition. In the case of serial bideposition, the shadowing area lies alternately in front of and behind the columns, and, accordingly, the intercolumn gaps are widened, and bundles are formed more visibly. 6 Therefore, the degree of anisotropy of the surface morphology is considerably higher than that of a conventional obliquely deposited film.

Optical Properties
The birefringence and transmittance loss of the Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 5 wt%) serial bideposition film at the wavelength of 550 nm as a function of thickness t 1 with t Ta2O5 ¼ 1500 nm are shown in Fig. 2, in which the plot of t 1 ¼ 1500 nm is equivalent to the film prepared by conventional oblique deposition. Transmittance loss was calculated by subtracting the sum of transmittance and reflectance from 100%.
As for the birefringence, Δn increases with decreasing t 1 when t 1 is less than 70 nm, whereas it remains same as that of the conventional oblique deposition film when t 1 is more than 70 nm. With serial bideposition, Hodgkinson and Wu 4 explained that structural growth occurs in platelike columns perpendicular to both the deposition plane and the substrate when individual layers are thin. In the case of such a structure, Hodgkinson et al. estimated a 100% increase in Δn for a packing density p ¼ 0.60 using equation for form birefringence. 14 This value coincides with an increase in the Δn of serial bideposition Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 5 wt%) films, as shown in Fig. 1. In our case, it is considered that the plate-like columns grow when t 1 is less than 10 nm.
The transmittance loss is the least at around t 1 ¼ 6 nm, and it seems to be unchanged for t 1 > 10 nm. For t 1 < 10 nm, agglutination of the columnar structures of a stack of individual  thin layers whose directions alternate could be prevented, thus reducing haze. Moreover, it is considered that the narrow, long columns standing normal to the substrate instead of being inclined, as with conventional oblique deposition, reduced the amount of light reflected off the interface. As the result, the transmittance loss is decreased because t 1 < 10 nm. The transmittance loss for both orthogonal polarizations is decreased. This result shows that the serial bideposition films can not only achieve larger Δn but also decrease the transmittance loss compared with conventional obliquely deposited films.

Influence of TiO 2 Addition
The optical transmittance of Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 0 − 100 wt%) serial bideposition films as a function of TiO 2 composition is shown in Fig. 3, in which the rhombuses, squares, and triangles indicate the transmittance averaged over the wavelength ranges of 680-600, 590-520, and 510-430 nm, respectively. Thickness values of t 1 ¼ 7 nm and t Ta2O5 ¼ 6000 nm were selected so that the influence of additive on transmittance could be recognized easily. As seen in the figure, the transmittance is maximized at x ¼ 5-15 and decreases at both x < 5 and x > 30; particularly, the transmittance is improved in the wavelength range of 51-430 nm at x ¼ 5-15, respectively.
Saitoh et al. revealed that a low concentration of TiO 2 doping in a Ta 2 O 5 film by chemical vapor deposition could reduce the leakage current of the film. 8 In that case, they explained that oxygen vacancies are possibly compensated for by Ti 4þ ions, which are substitutionally incorporated into Ta 5þ sites in the film. 8 This suggests that the transmittance improvement of serial bideposition films at lower TiO 2 doses is due to a decrease in the number of oxygen vacancies in the Ta-Ox.
Although the exact mechanism responsible for this improvement is yet to be found and we need further experiments to reveal the same, it is considered that adding a small amount of TiO 2 to Ta 2 O 5 can reduce the resulting film's transmittance loss. The serial bideposition film with a lower dose of TiO 2 in Ta 2 O 5 is potentially applicable to wave plates in applications requiring high transmittance. Figures 4(a) and 4(b) show the retardation and transmittance/reflectance spectra of Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 5 wt%) serial bideposition films with antireflection coatings for t 1 ¼ 7 nm and t Ta2O5 ¼ 800 nm. The retardation of 115 nm at λ ¼ 460 nm, which corresponds to a quarter wave plate for the wavelength, is achieved. In the wavelength range of 430-510 nm, the transmittance and reflectance averaged over λ ¼ 430-510 nm were more than 99% and less than 0.5%, respectively. Figure 5 shows the angle dependence of the retardation of a serial bideposition film at λ ¼ 460 nm. Rotation around the fast axis increases the retardation, whereas rotation around the slow axis decreases the retardation. In addition, angle dependence is symmetrical about 0 deg. In the case of conventional obliquely deposited films, angle dependence is asymmetric around 0 deg. 1,3 In the case of a serial bideposition film, angle dependence is symmetric because narrow, long columnar structures perpendicular to the substrate are formed, as shown in Fig. 1(b).

Properties of Wave Plate Application
This angle dependence of serial bideposition films corresponds to the homogeneous biaxial indicatrix. Although serial bideposition films are composed of numerous individual thin layers, they can be considered as a homogeneous single layer with an optical axis parallel to the substrate because each layer of the stack has a thickness that is a fraction of the operating wavelength.
This result offers the following advantage. Optical devices are preferred to have left-right symmetric optical properties when they are used for optical instrument using luminance fluxes that have a definite incidence angle such as LCD projectors. Asymmetric optical properties lead to uneven luminance, and hence, the quality of the projected image deteriorates. Therefore, serial bideposition films with symmetric properties are applicable to wave plates mounted inside actual optical instruments. Figure 6 shows the change in the averaged transmittances of serial bideposition films with or without the passivation layer at a temperature of 60°C and relative humidity of 90% (i.e., humidity resistance test). The transmittance decreases monotonically with time without the passivation layer and is lowered by 20% after 500 h, whereas it remains unchanged with the passivation layer. Figure 7 shows changes in the transmittance with time under irradiation from a light intensity of 0.6 W∕mm 2 (i.e., light resistance test). For comparison, a conventional organic wave plate was tested as well.

Wave Plate Reliability
Our inorganic wave plate with a serial bideposition film showed significant/superior robustness to irradiation, and there was no degradation in its optical characteristics or appearance.  Conversely, the transmittance of the organic wave plate deteriorated rapidly, and it finally burned out after 200 h of irradiation. As a result, it is shown that our fully inorganic wave plates with a serial bideposition film is advantageous for applications in which it is used under either/both high-humidity or high-power light irradiation.

Conclusion
We investigated the optical characteristics and microstructures of serial bideposition Ta 2 O 5 ð100 − xÞ þ TiO 2 ð1 − xÞ (0≦x≦100 wt%) films. The dense, narrow, and long columns that grow quasiperpendicular to the substrate give rise to higher birefringence and lower transmittance loss compared with conventional obliquely deposited films.
Additionally, we investigated the effect of TiO 2 addition to Ta 2 O 5 . It was found that a small amount of additive can improve the film's optical transmittance, especially at shorter wavelengths.
We demonstrated a fully inorganic quarter wave plate with the serial bideposition film of Ta 2 O 5 ð100 − xÞ þ TiO 2 ðxÞ (x ¼ 5 wt%), which achieved both high transmittance and low reflectance at λ ¼ 430-510 nm, as well as symmetric angle dependence of retardation. Finally, we showed the superior durability of our wave plate under both high-humidity and high-power light irradiations. It is expected that the developed wave plates can be applied to applications requiring high transmittance and long-term operation stability under high temperatures.

Acknowledgments
The evaluation of optical properties and microstructures presented in this manuscript was performed by Nobuyuki Koike. The fabrication of wave plates and relativity experiments was performed by Nobuyuki Koike and Koji Sasaki. Naoki Hanashima and Takatoshi Yamada worked on data analysis. Akio Takada and Motofumi Suzuki provided the valuable suggestion and discussion through the entire process of the research.