3.1.

Microstructures

Figure 1 shows the morphology of the serial bideposition ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) film for $t_1=7\mathrm{nm}$ and $t_{\mathrm{Ta}2\mathrm{O}5}=1500\mathrm{nm}$. Figures 1(a) and 1(b) show the SEM images of the cross-sectional views parallel to the deposition plane of the ${\mathrm{Ta}}_2{\mathrm{O}}_5$ 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 ${\mathrm{Ta}}_2{\mathrm{O}}_5$ flux deposition plane.

Fig. 1

SEM Image of (a) cross section (b) top view of the sample of ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) serial bideposition film.

This surface morphology can be observed even on a ${\mathrm{Ta}}_2{\mathrm{O}}_5$ 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.

3.2.

Optical Properties

The birefringence and transmittance loss of the ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) serial bideposition film at the wavelength of 550 nm as a function of thickness $t_1$ with $t_{\mathrm{Ta}2\mathrm{O}5}=1500\mathrm{nm}$ are shown in Fig. 2, in which the plot of $t_1=1500\mathrm{nm}$ is equivalent to the film prepared by conventional oblique deposition. Transmittance loss was calculated by subtracting the sum of transmittance and reflectance from 100%.

Fig. 2

$\mathrm{\Delta }n$ and transmittance loss as a function of thickness $t_1$.

As for the birefringence, $\mathrm{\Delta }n$ 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 Wu⁴ explained that structural growth occurs in plate-like 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 $\mathrm{\Delta }n$ for a packing density $p=0.60$ using equation for form birefringence.¹⁴ This value coincides with an increase in the $\mathrm{\Delta }n$ of serial bideposition ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) 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 $t_1=6\mathrm{nm}$, and it seems to be unchanged for $t_1>10\mathrm{nm}$. For $t_1<10\mathrm{nm}$, 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 $t_1<10\mathrm{nm}$. The transmittance loss for both orthogonal polarizations is decreased. This result shows that the serial bideposition films can not only achieve larger $\mathrm{\Delta }n$ but also decrease the transmittance loss compared with conventional obliquely deposited films.

3.3.

Influence of TiO₂ Addition

The optical transmittance of ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=0-100\mathrm{wt}\%$) serial bideposition films as a function of ${\mathrm{TiO}}_2$ 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 $t_1=7\mathrm{nm}$ and $t_{\mathrm{Ta}2\mathrm{O}5}=6000\mathrm{nm}$ were selected so that the influence of additive on transmittance could be recognized easily. As seen in the figure, the transmittance is maximized at $x=5–15$ and decreases at both $x<5$ and $x>30$; particularly, the transmittance is improved in the wavelength range of 51–430 nm at $x=5–15$, respectively.

Fig. 3

${\mathrm{TiO}}_2$ composition dependence of transmittance of ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=0-100\mathrm{wt}\%$) serial bideposition films.

Saitoh et al. revealed that a low concentration of ${\mathrm{TiO}}_2$ doping in a ${\mathrm{Ta}}_2{\mathrm{O}}_5$ 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 ${\mathrm{Ti}}^{4+}$ ions, which are substitutionally incorporated into ${\mathrm{Ta}}^{5+}$ sites in the film.⁸ This suggests that the transmittance improvement of serial bideposition films at lower ${\mathrm{TiO}}_2$ doses is due to a decrease in the number of oxygen vacancies in the $\mathrm{Ta}\text{-}\mathrm{O}x$.

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 ${\mathrm{TiO}}_2$ to ${\mathrm{Ta}}_2{\mathrm{O}}_5$ can reduce the resulting film’s transmittance loss. The serial bideposition film with a lower dose of ${\mathrm{TiO}}_2$ in ${\mathrm{Ta}}_2{\mathrm{O}}_5$ is potentially applicable to wave plates in applications requiring high transmittance.

3.4.

Properties of Wave Plate Application

Figures 4(a) and 4(b) show the retardation and transmittance/reflectance spectra of ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) serial bideposition films with antireflection coatings for $t_1=7\mathrm{nm}$ and $t_{\mathrm{Ta}2\mathrm{O}5}=800\mathrm{nm}$. The retardation of 115 nm at $\lambda =460\mathrm{nm}$, 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 $\lambda =430–510\mathrm{nm}$ were more than 99% and less than 0.5%, respectively.

Fig. 4

(a) Retardation and (b) Transmittance/reflectance spectra of ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) serial bideposition film with antireflection coating for $t_1=7\mathrm{nm}$ and $t_{\mathrm{Ta}2\mathrm{O}5}=800\mathrm{nm}$.

Figure 5 shows the angle dependence of the retardation of a serial bideposition film at $\lambda =460\mathrm{nm}$. 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.

Fig. 5

Measurement angle dependence of retardation of serial bideposition ${\mathrm{Ta}}_2{\mathrm{O}}_5(100-x)+{\mathrm{TiO}}_2(x)$ ($x=5\mathrm{wt}\%$) film.

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).

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.

3.5.

Wave Plate Reliability

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.

Fig. 6

Transmittance of serial bideposition films as a function of time under humidity condition.

Figure 7 shows changes in the transmittance with time under irradiation from a light intensity of $0.6\mathrm{W}/{\mathrm{mm}}^2$ (i.e., light resistance test). For comparison, a conventional organic wave plate was tested as well.

Fig. 7

Transmittance of wave plates as a function of time in light resistance test.

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.