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In this paper, we have fabricated for the first time films with different orientations of principal axes but similar principal indices and also films with different principal refractive indices but the same orientations of principal axes. During deposition, the deposition angle and the substrate sweep angle are varied to control separately the porosity (refractive principal indices) and column tilt angle (orientations of principal axes) of a birefringent thin film.
©2008 Optical Society of America
1. Introduction
Films with various nano-structures have been fabricated, with the feasibility of their new optical applications demonstrated as well [1–4]. The original typical nano-structured thin film is a tilted columnar thin film that is made by glancing angle deposition (GLAD) [5]. The column angle (the angle between column growth direction and substrate normal) and porosity of the thin film increase with the deposition angle (the angle between the deposited vapor flux and the substrate normal). The porosity is inversely proportional to the index of refraction. GLAD technology enables an optical thin film to be fabricated with a refractive index that is not restricted to that of the material. A rare film with a refractive index like that of air has been made and used as an antireflection coating [2]. Although the tilt columnar structured thin film exhibits very weak anisotropic biaxial optical properties, recent research has shown that the weak anisotropy yields a strong optical signal associated with enhanced polarization conversion [6]. It has been shown that the optical constants, including principal indices, column angle and thickness influence the polarization conversion sensitively [7]. In this paper, the substrate sweep technique (also called the Phisweep technique) is applied to sculpture an anisotropic thin film with designated principal indices and orientation of principal axes.
The traditional GLAD method limits the design of an anisotropic thin film to having a certain tilt principal axes and principal indices of refraction within a particular range. Once the tilt angle α is fixed, both the columnar growth direction and porosity are also fixed [8]. In this work, the Phisweep technique [9] is employed to fabricate anisotropic thin films with the same column angle but different porosities and films with similar principal indexes but different column angles. In previous studies [10], Phisweep technique is only applied in improving the columnar structures and preventing fan-out phenomenon. However, our originality is using Phisweep technique to sculpture various birefringent optical thin films and measure their anisotropic optical constants.
The relationship between the column tilt angle β and the deposition angle α is given by the tangent rule [11] and Tait’s rule [12]. The tangent rule is empirical, and is given by Eq. (1), where E is a constant that depends on coating material. Tait’s rule is developed by flux vector analysis, and is given by Eq. (2).
Figure 1 depicts Tait’s rule. It indicates that when the deposition angle is fixed, both the column angle and the porosity are fixed too. A fixed porosity necessitates a fixed refractive index.
The Phisweep technique is one in which the substrate periodically rotates back and forth around the substrate normal (ψ-axis) during the deposition process, as shown in Fig. 2. The sweep angle is defined as γ. According to the flux vector analysis [10], the relationship among deposition angle α, column angle β and sweep angle γ is given by Eq. (3)
Figure 3 plots the column angle β as a function of deposition angle α for various sweep angles γ. The column angle and the film porosity can be controlled separately by the Phisweep technique. Figure 3 indicates the possible deposition angles and sweep angles to have a particular column angle. Those squares in Fig. 3 are associated with films with fixed column angles but different refractive indices. Those circles in Fig. 3 also indicate that controlling the sweep angle at a specified deposition angle yields films with different column angles but similar refractive indices.
Fig. 3. The column angle β as a function of deposition angle α for various sweep angle γ in Phisweep technique.
2. Experiment
In our experiments, two sets of anisotropic MgF2 thin films on the BK7 substrates in the thermal coater are prepared using the Phisweep technique. The first set consists of two films that have the same column angle but different refractive indexes. The second set consists of two films that have similar refractive indices but different column angles. The chamber was pumped to a base pressure of 4×10-6 Pa prior to each deposition. The deposition rate is maintained at 1.5 nm per second. A quartz thickness monitor set next to the substrate is used to measure the deposition rate and the thickness of the film is controlled. The angle of inclination of the columns of an anisotropic thin film is determined from the scanning electron micrograph (SEM). The anisotropic thin film is arranged in a BK7 prism/anisotropic thin film/air configuration, and its polarization conversion reflectance Rsp (intensity ratio of incident s-polarized light to reflected p-polarized light) is measured versus incident angle at wavelength λ=632.8 nm. The optical constants of the anisotropic thin films are determined from the polarization conversion reflectance angular spectrum. The orientation of principal axes of an anisotropic thin film is defined according to reference 7. For a typical low refractive anisotropic thin film with optical constants (n1=1.321, n2=1.320, n3=1.326, β=30° and thickness d=900nm), there are two peaks in the polarization conversion reflectance angular spectrum when the incident plane is vertical to the deposition plane. The global reflectance maximum Rmax occurs at the incident angle αmax. One of the principal indices n1 and thickness d can be measured by s-polarized light incident on the film that is isotropic when the incident plane is coincident with the deposition plane. Other optical constants including n2, n3 and β can be determined by the three characteristics of polarization conversion reflectance spectrum (Rmax, αmax and αpp) via triangle convergence method [7]. The thickness error 0.1nm would cause the variation of (Rmax, αmax and αpp) as (Δαmax=5.06×10-4 deg, ΔRmax=1.69×10-4 deg, Δαpp=1.27×10-3 deg) which would cause the errors in measured principal indices (n2, n3) as (Δn2=10-4, Δn3=1.3×10-4).
3. Result and discussion
One of the two samples for the first set is prepared at a deposition angle α of 70 deg and a substrate sweep angle γ of 45 deg. The column angle β of the anisotropic MgF2 thin film with column angle is 39 deg. The other anisotropic MgF2 thin film with the same column angle β=39 deg is prepared at a deposition angle of 80 deg and a sweep angle of 60 deg. The sweep pitch q is 30 nm, and represents the increase in thickness during the substrate, which pauses at each outer extreme of the sweep curve. Figure 4 presents the cross-section SEM images. Table 1 presents the measured principal refractive indices and the column angle of each sample. The two samples have the same column angle but the averages of their principal indices are different. Under the same coating parameters without substrate sweeping, the principal indices (n1, n2, n3) and column angles β of anisotropic thin films deposited at 70 deg and 80 deg are measured as (1.270, 1.268, 1.290, β=43deg) and (1.204, 1.202, 1.223, β=47deg) respectively. Although the porosity (the average of principal refractive indices) of an anisotropic thin film can be made to increase (decrease) by increasing the deposition angle, the column angle (principal axes tilt angle) can be varied by changing the substrate sweep angle.
Fig. 4. Cross-section SEM images of the MgF2 anisotropic thin films: The sample (a) is deposited at α=70° with γ=45°. The sample (b) is deposited at α=80° and γ=60°.
Table 1. Optical constants of the MgF2 columnar thin films. Both (a) and (b) have the same column angle, but their principal indices are in different ranges.
For the second set, an anisotropic MgF2 thin film with column angle β=34 deg is prepared at a deposition angle α of 70 deg and a substrate sweep angle γ of 60 deg. The other anisotropic MgF2 thin film with similar refractive indices is fabricated by the Phisweep method at a deposition angle α of 70 deg, a sweep angle γ of 75 deg and a sweep pitch q of 30 nm. Figure 5 presents the cross-section SEM images. Table 2 presents the measured principal refractive indices and column angle of each sample. The principal indices of the two samples are similar but their column angles are different. Because the deposition angles are the same, the porosity (average of principal indices) is similar for both samples. The different substrate sweep angles lead to different column angles (principal axes tilt angle).
Fig. 5. Cross-section SEM images of the MgF2 anisotropic thin films: The sample (a) is deposited at α=70° with γ=60°. The sample (b) is deposited at α=70° and γ=75°.
Table 2. Optical constants of the MgF2 columnar thin films measured by polarization conversion. Both (a) and (b) have similar refractive indices, but their column angles are different.
Fig. 6. The polarization conversion angular spectra of the MgF2 anisotropic thin films: The sample (a) is deposited at α=70° with γ=60°. The sample (b) is deposited at α=80° and γ=60°. The sample (c) is deposited at α=70° with γ=60°. The sample (d) is deposited at α=70° and γ=75°.
4. Conclusion
In this work, the Phisweep technique is employed to sculpture various anisotropic MgF2 films. Adding the sweep angle parameter to the conventional GLAD technique has enabled the porosity and the column angle to be controlled separately. Accordingly, an anisotropic thin film could be fabricated with a designated orientation of the principal axes and principal refractive indices. The polarization conversion angular spectra for the four samples (a), (b), (c) and (d) in the prism coupling system (BK7/film/air) with deposition angle vertical to the plane of incidence are presented in Fig. 6. It shows that the tiny change of anisotropy of a columnar thin film would change the reflected optical signal intensively. Therefore we can conclude that the finely sculptured anisotropic films can be applied in novel optical coatings in the future, such as used in polarization conversion filters and polarizer filters.
References and links
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