1. Introduction

Polarizers are widely used as one of the key optical components in the various display devices such as liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), and LCD projection displays [1–7]. Dichroic polarizers like an H-sheet polarizer [8], which have an anisotropically absorptive characteristic, are favorably used in the LCDs and the OLEDs owing to the requirement of low reflection of ambient light. On the other hand, a wire-grid polarizer (WGP), which has a reflective facial characteristic, is normally used in LCD projection displays due to the requirement of the high temperature stability [6,7]. Since the absorptive dichroic polarizers and the reflective WGPs are the popular optical components in the display applications, polarizers can be classified into the two types according to their facial characteristics, the absorptive and the reflective.

The absorptive polarizer has many advantages in a low reflective feature and a high polarization extinction ratio (PER), defined as the ratio of the transmittance of transverse magnetic (TM) waves to that of transverse electric (TE) waves - the TM waves are defined as the electric fields of the incident light are perpendicular to the absorption axis of the polarizer, which result in passing through the polarizer without a considerable reduction of the light intensity, while the TE waves are predominantly absorbed owing to the electric fields of the light oscillating to the direction of the absorption axis of the polarizer. The H-sheet polarizer, the most typical absorptive one, is fabricated by stretching the base polymer of polyvinyl alcohol (PVA) incorporated with Iodine grains aligned in a specific direction [8]. However, the typical polarizer needs triacetylcellulose (TAC) layers to protect the PVA film and also needs an adhesive layer to laminate [9]. Owing to these additional layers, the thickness of the conventional absorptive polarizer reaches over 100 µm. It is one of the issues for the development of the slim mobile devices in the flat panel display industry. To overcome the thickness demerit of the conventional polarizer and to operate the in-line manufacturing process without the polarizer-film lamination process, a coatable thin-film polarizer by photocrosslinking of a highly ordered smectic guest-host system was reported [10]. Even though the thickness of the coatable polarizer was reduced to 5 µm compared with that of the H-sheet polarizer to be around 100 µm, the thickness of the coatable polarizer is still thick compared with that of the WGP [7,11–15], and its performance is still far behind compared with that of the H-sheet polarizer [10]. Furthermore, the aforementioned and reported polymer-based polarizers are very weak to heat [8–10]. Therefore, in order to preclude organic materials in the polarizers from degrading, the applications of the polarizers are limited to be used under moderate temperature conditions.

On the contrary, reflective-type polarizers like a WGP can be used in harsh temperature conditions. For example, the reflective metal WGP is currently used for the LCD projection system in which some of the components near the lamp are required to endure almost up to 1000 °C [16]. And also, the reflective polarizers are appropriate for the recycling of light in the LCDs to enhance the brightness [17,18]. By inserting the reflective polarizer between the backlight unit and the bottom absorptive polarizer of the LCD panel, the light-extraction enhancement was reported to be around 1.3 times higher than that without the WGP [17]. However, the reflective polarizers are not suitable for the display applications, as far as the contrast between the brightest and the darkest colors of the display is concerned [19]. Due to the inherent reflection from a shiny nano-patterned metal surface [11–15], the metal WGP could seriously deteriorate the contrast of the displays like the LCDs and the OLEDs. So the LCDs currently use the absorptive film-type polarizer and the OLEDs use the same type of polarizer together with a quarter-wave retardation film to obtain a circular polarizer.

In order to overcome the weak points of each type of polarizers, a new type of optically bifacial polarizer is needed, deviating from the conventional absorptive dichroic polarizer and from the reflective WGP. If one side of a WGP is absorptive and the other side is reflective, and if the polarizer is composed of inorganic materials neither iodine nor dichroic dye, we can solve all the above-mentioned problems of each type of polarizers. Besides, due to the optically bifacial characteristic, i.e., the absorptive front and the reflective back, one sheet of the new-type polarizer can have the same functions of two sheets of the conventional polarizers, the metal WGP and the bottom polarizer, in the light recycling system of the LCD devices [17]. And also, it can considerably reduce the total thickness of the OLED device, if the coatable quarter-wave retardation layer is combined with the absorptive feature of the bifacial WGP.

In this paper, we suggest a new concept of the bifacial WGP with an absorptive metal-dielectric composite layer under a reflective metal layer, where linearly periodic nano-patterns are formed consecutively, and show the experimental results to reduce drastically the reflection from the absorptive side of the bifacial WGP in comparison with that from the reflective metal of the common WGP. By analyzing the depth profile of the element ratio on the metal-dielectric composite layer, we show that the metal-dielectric composite layer inserted between the metal layer and the glass substrate can impressively play a role of a considerable reduction of reflection due to the elimination of the optical boundary. The optical properties of the fabricated bifacial WGPs are reported including the optically dual character on the reflection - an exceptionally low level of reflection from the absorptive metal-dielectric composite side and a high reflection from the metal side, the equivalent or relatively high transmittance in comparison with the common WGPs, and a commercially-available level of high PERs.

 

Fig. 1. Schematic of a bifacial thin film WGP with nano-patterned metal and graded metal-dielectric composite layers

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2. Design

A schematic of the bifacial thin-film WGP is shown in Fig. 1, which consists of nano-patterned metal and graded metal-dielectric composite layers on a glass substrate. The role of the graded metal-dielectric composite layer, which is positioned between the glass substrate and the metal layer, is to give an absorptive function to a reflective WGP. By varying the composition ratio of the metal to the dielectric material along the y-axis in the composite layer illustrated in the extended part of Fig. 1, the complex refractive index of the layer can be controlled to have a gradual change in the y direction [20]. When the graded composite layer is optimally designed and fabricated, its thickness and the reflection of the WGP can be suppressed dramatically. The role of the periodic nano-wire patterns of the graded composite layer on the metal layer is to give a function of a polarizer, which is similar to the typical metal WGPs.

For the graded composite layer, various combinations of metals and dielectric materials can be used. In consideration of the etching selectivity between the Al metal layer and the metal-dielectric composite layer in the nano-patterning process, Ti and SiO₂ were selected as the materials for the metal-dielectric composite. Figure 2 shows the depth profile of the graded Ti-SiO₂ layer on the glass substrate by the dynamic secondary ion mass spectroscopy. While the content of the SiO₂ in the composite layer is gradually decreasing from the glass substrate to the air side, that of the Ti is gradually increasing. According to the theoretical analysis on the metal-dielectric composite, the effective complex dielectric constant can be decided from the composition ratio of the metal to the dielectric material in the metal-dielectric composite layer [19]. The reflectance depends on the difference and the sum of the complex refractive indices of two materials on the boundary [21], as expressed in Eq. (1),

where ñ₁ and ñ₂ are the complex refractive indices of the material 1 and the material 2, respectively.

Considering a metal-dielectric interface, there is a big discrepancy between the complex refractive indices of the materials - both the real and the imaginary parts, which results in high reflectance as predicted in Eq. (1). In the graded metal-dielectric composite layer consisting of Ti and SiO₂, however, its effective refractive index is gradually varied to the deposition direction of the composite layer, so the optical interface is eliminated. As a result, the extremely low reflectance can be achieved in the gradual metal-dielectric composite layer relatively to the metal-dielectric interface. The reflectance on the SiO₂-rich side of the Ti-SiO₂ composite layer faced with the glass substrate is about 10 % or less in the visible wavelength range as shown in Fig. 3, which is available for an absorptive-type polarizer. Since the reflectance on the Ti-rich side of the composite layer, lower than 30 % in the visible range as shown in Fig. 3, is too low for a reflective-type polarizer, a highly reflective metal layer should be deposited on the Ti-rich side of the composite layer. The reflective function of the bifacial WGP can be provided by an Al layer on the composite layer. The transmittance of the composite layer only on the glass substrate is approximately 40 % in the all visible wavelength range as shown in Fig. 3. That is, more than 40 % of each polarization of the incidence waves will pass through the line-patterned composite layer on the glass substrate. Therefore, there must be another layer on the composite layer to obstruct the incidence light before patterning the layers. The Al layer and the composite layer contribute together to the absorptive function of the bifacial WGP. To validate the reduction of the reflectance from the metal-dielectric interface, the reflection of the graded Ti-SiO₂ composite layer inserted between an Al layer and a glass substrate was measured and was compared with the reflection of an Al-deposited glass as shown in Fig. 4. The thicknesses of the graded Ti-SiO₂ and the Al layers were 100 nm and 300 nm, respectively.

 

Fig. 2. A depth profile of the Ti-SiO₂ graded composite layer by dynamic secondary ion mass spectroscopy

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Fig. 3. Measured reflectances and tranmittances on the SiO₂-rich and Ti-rich sides of the 100 nm-thick unpatterned Ti-SiO₂ graded composite layer only between an Al layer and a glass substrate

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Fig. 4. Measured reflectances on the unpatterned samples with and without the Ti-SiO₂ graded composite layer between an Al layer and a glass substrate.

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When the Ti-SiO₂ composite layer on the Al layer, a two-dimensionally isotropic absorption medium to the normal incidence on the surface, is patterned together with the overlying Al layer in a sub-wavelength scale of period, the line-patterned graded composite layer with the Al wire-grid layer acts as an anisotropic absorptive medium to the normal incidence [12], which is necessary for the high transmittance of the TM polarized light and for the inhibition of the transmission of the TE polarized waves. If the line-patterned graded composite layer between the glass substrate and Al wire-grid layer is thinner than the optimum thickness for its absorptive function, the reflection from the composite layer will be unexpectedly increased because of an abrupt effective-refractive-index change in the relatively-thin gradual composite layer and because of a relatively big discrepancy between the complex refractive indices of the composite layer and the Al layer. On the contrary, if the patterned composite layer is relatively thick to the optimal level, the transmittance of the TM waves passing through the thick graded-composite layer will be deteriorated owing to the increase of the absorptive region. If the thickness of the graded composite layer is controlled in a reasonable degree, the polarized light perpendicular to the line patterns, the TM waves, can propagate through the graded composite layer as well as Al-wire grids without any significant reduction of the light intensity. Most of the TE light parallel to the line patterns, in contrast to the TM light, is absorbed in the line-patterned multi-layers composed of the Ti- SiO₂ composite and the Al layer, which results in the dramatic reduction of reflection. As the anisotropy in transmittance through the patterned Ti-SiO₂ composite layer with the Al wire-grid layer comes from the anisotropic structure of the line patterning, the anisotropy in absorption in the line patterned Ti-SiO₂ composite layer with the Al wire-grid layer comes from the same origin also [22].

3. Experiment

Ti and SiO₂ were deposited by e-beam evaporation on the Eagle2000 glass substrate (made by Samsung Corning) and deposition rates were controlled to form an absorptive graded composite layer with a thickness of 100 nm. After depositing 290 nm- and 185 nm-thick Al layers for 230 nm- and 144 nm-period structures, respectively, on the graded composite layer, a SiO₂ layer with a thickness of 100 nm was deposited as a hard mask for Al-layer etching. An ultraviolet (UV) negative photoresist with a thickness of 100 nm on the SiO₂ layer was coated. After the Ar-ion laser-interference lithographic process with a 450 mW power at the operating wavelength of 365.8 nm and the UV-photoresist developing process in a diluted Clariant AZ300 MIF solution (AZ300 MIF:DI water=1:1) for 25 seconds at room temperature, we could obtain the photoresist line-grating patterns with periods of 230 nm and 144 nm by controlling the lithography conditions. The photoresist patterns were transferred consecutively to the SiO₂ hard-mask, the Al-metal, and the graded Ti-SiO₂ composite layers by changing the reactive-ion etching conditions. The SiO₂ layer could be patterned with CHF₃ and Ar gases in the 50 mTorr vacuum chamber and the Al layer was etched by using BCl₃ and Cl₂ gases on the 5 mTorr pressure at the 150 W RF power. After dry etching of the graded composite layer with O₂ and CF₄ gases, the bifacial WGPs with the periodic nano-patterns were obtained. Figures 5(a) and 5(b) show the microstructures of the nano-patterned bifacial WGPs obtained by Scanning Electron Microscopy (SEM). The average widths of the bifacial WGPs are 138 nm and 86 nm for 230 nm- and 144 nm-period structures, respectively, each of which is set to the duty cycle of 60% for the periods of the nano-line patterns.

 

Fig. 5. Scanning electron microscope images of the bifacial WGPs with patterned-line periods of (a) 230 nm and (b) 144 nm on the glass substrate.

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4. Results and discussion

The transmittances of the WGPs for the TE- and TM-polarized incidence waves have been characterized in the visible wavelength range by using a UV/VIS spectrophotometer (Perkin Elmer Lambda950) with a depolarizer and Glan-Taylor polarizers before sample holders, and their PERs can be calculated easily by dividing the transmittance of the incident TM wave by that of the TE wave. On the contrary, Reflection on the each side of the WGPs has been measured for unpolarized incidence waves, because the reflection measurement system in the UV/VIS spectrophotometer was not compatible with the Glan-Taylor polarizer module supporting the linearly polarized incidence wave. Nevertheless, the tendency of the reflection on the WGPs from the linearly polarized incidence waves could be described by analyzing on the measured data of each transmission for the linearly polarized incidence waves and of the total reflection of the WGPs. Figures 6(a) and 6(b) show the measured transmittances and reflectances of the obtained bifacial thin film WGPs with the 230 nm- and 144 nm-period line patterns, respectively.

The Reflectance on the absorptive side of the 230 nm-period WGP is 10 % to 15 % in the visible wavelength range, while that from the Al-grid side of the WGP is 20 % to 35 % as shown in Fig. 6(a). The reflectance on the absorptive side of the 144 nm-period WGP is less than 10 % in the short visible-wavelength range below 500 nm and is less than 25 % in the long visible-wavelength range over 600 nm, while the reflectance on the Al-grid reflective side of the WGP is over than 43 % in the all visible-wavelength range as shown in Fig. 6(b). Considering the reflectances from the Al-grid reflective sides of the bifacial WGPs, the reflection from their absorptive sides is reduced significantly, which shows the graded Ti-SiO₂ layer successfully eliminates the reflection from the metal-glass interface. An obvious distinction between the reflectances on the two sides of each WGP clearly shows its optically bifacial characteristics. The reflectance on the Al-grid side of the 144 nm-period WGP, as also shown in Fig. 6(b), is over 43 % as the fact in the commercially-available reflective-type WGPs as shown in the Reference [23].

 

Fig. 6. Transmittances and reflectances of the bifacial WGPs with (a) 230 nm- and (b) 144 nm-period grid patterns: T_X-nm indicates the transmittance of the WGPs with X nm-period line patterns (X=230 or 144). R_{X-nm_Ti-SiO2} and R_{X-nm_Al} represent the reflectances on the patterned Ti-SiO₂ graded-composite-layer and the Al-grid sides of the bifacial WGPs, respectively.

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Fig. 7. Polarization extinction ratios of the bifacial WGPs with nano-grid patterns of 144 nm and 230 nm periods

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The transmittance of the WGP with the 144 nm-period line patterns is over than 40 % in the all visible wavelength range in Fig. 6(b), but that of the 230 nm-period WGP is 25 % to 35 % as shown in Fig. 6(a). The transmission difference between the 144 nm- and 230 nm-period WGPs results from several reasons as analyzed theoretically and experimentally in the References [13] and [14]. The spectral distinction of the transmission between the two WGPs in the short visible wavelength region results principally from the difference of the wire-grid periods as explained in Reference [14]. The wire-grid shapes, the duty-cycles of the wire grids, and the fabrication imperfection, may cause the reduction of the transmittance of the WGPs.

The PER values for the bifacial WGPs with pattern periods of 230 nm and 144 nm are 70 and 500 at the wavelength of 550 nm, respectively, as shown in Fig. 7. The PER is well known to have strong dependence upon the line-pattern period of the WGP, because it is calculated by dividing the TM-wave transmittance by the TE-wave transmittance and the transmittances of the linearly polarized incidence waves depend on the wire-grid periods. The relative difference between the TE-wave transmittances of the 230 nm- and 144 nm-period WGPs is wider than that of the TM waves as shown in Fig. 8(b). Therefore, for the relatively good quality of the polarizer, elimination of the TE-wave transmission through the design optimization and the development of the fabrication of the WGP is one of the key issues. The PER value of the 230 nm-period WGP in the short wavelength range is extremely small owing to the relatively high transmission of the TE incidence wave. The PER value of 500 on the 144 nm-period WGP is comparable to those of the commercially available absorptive H-sheet polarizer and the reflective metal WGP [23].

 

Fig. 8. (a). Linear- and (b) log-scale transmittances of the bifacial WGPs with 230 nm- and 144 nm-period wire-grid patterns for the TE- and TM-polarized incidence waves.

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Fig. 9. Photograph of an 8 inch-sized bifacial WGP with 144 nm-period grid patterns under the two conventional H-sheet polarizers; The polarization axes of the H-sheet polarizers are parallel (left) and perpendicular (right) to that of the bifacial WGP.

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From more than 80 % of the transmission of the TM incidence wave and less than 1 % of that of TE wave on the 144 nm-period WGP as shown in Figs. 8(a) and 8(b), most of the reflection on the Al wire-grid side of the 144 nm-period WGP as shown in Fig. 6(b) results essentially from the reflection of the TE incidence wave.

Figure 9 shows the photograph of the fabricated 8-inch-sized bifacial thin-film WGP with the patterning period of 144 nm under the two conventional H-sheet polarizers. When the polarization axes of the WGP and the H-sheet polarizer are parallel to each other, approximately half of the unpolarized incident light - most of the TM wave - propagates through the bifacial WGP and passes dominantly through the H-sheet polarizer as shown on the left in Fig. 9. However, if the polarization axes of the two polarizers are perpendicularly aligned to each other, transmission of the polarized light passing through the WGP will be drastically suppressed by the absorptive H-sheet polarizer, depending on the PERs of the H-sheet polarizer and the bifacial WGP. Since the PERs of the fabricated bifacial WGP and the H-sheet polarizer are around to be 500 and over 500, respectively, the linearly polarized light, which has passed through the bifacial WGP and which is parallel to the polarization axis of the H-sheet polarizer, is blocked to below 0.2 % as the right sample in Fig. 9.

5. Conclusion

We have demonstrated the optically bifacial thin-film WGPs with the wire-grid patterns, the periods of which are set to be 230 nm and 144 nm. One side of the bifacial WGPs has a reflective feature by Al metal-wire grids of common WGPs and the other side shows an absorptive peculiarity from the Ti-SiO₂ metal-dielectric composite layer with a depth profile of a gradually-varied composition ratio. The thin film polarizer with the 144 nm-period wire-grid patterns has shown the exceptionally low reflectance below 15 % and the high transmittance over 40 %, and its PER value has reached around 500 at the wavelength of 550 nm which is comparable to conventional polarizers. Since the metal-dielectric composite materials have high temperature stability, the bifacial WGP can be used even at the high temperature applications like an LCD projection system. Since the designed WGP has the optically dual characters, absorptive on one side and reflective on the other surface, it has a possibility to reduce a number of optical components in the light recycling system in LCDs and can be used to enhance the brightness without severely deteriorating the contrast of display devices like OLEDs. With the combined advantages of the conventionally absorptive and the reflective polarizers, the new type of the bifacial thin-film WGP can be used for vast areas of the flat panel display applications.