GB2476984A

GB2476984A - Wire grid polarizer

Info

Publication number: GB2476984A
Application number: GB1000794A
Authority: GB
Inventors: Alexandra Baum; Allan Evans
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2010-01-19
Filing date: 2010-01-19
Publication date: 2011-07-20
Also published as: GB201000794D0

Abstract

A wire grid polarizer comprises a plurality of wires 3 disposed in a plane and extending in a first direction. Each of the wires 3 comprises a first portion 3a and a set of second portions 3b which are connected to the first portion 3a. The second portions extend away from the first portion in a second direction perpendicular to the first direction and the plane and are spaced apart in a third direction which is perpendicular to the first and second directions.

Description

Wire Grid Polarizer

Technical Field:

The present invention relates to a wire grid polarizer (WGP). Such a design may show improved broadband transmission, e.g. across the whole visible wavelength range (VIS).

This is, for example, required for the application of WGPs as polarizing elements in a liquid crystal display (LCD) to guarantee uniform color balance of the display.

Background of the Invention:

A wire grid polarizer consists of an array of aligned metal structures as shown in Figure 1.

Such a grid of wires with spacing much smaller than the wavelength is a reflective polarizer for electromagnetic waves of this wavelength. Progress in precision manufacturing has enabled wire grid polarizers at optical and even UV wavelengths. A typical design that acts as a polarizer in the visible wavelength range may have the following geometry related to Figure 1: square profile metal wires 1 (typically Aluminum or Silver) with periodicity 4 of 100 nm, wire width 5 of 50 nm, wire thickness 6 of 200 nm.

The wires are located on a substrate 2 and are embedded in a material 3, which may be air. Figure 2 shows the optical performance of this example design with the aluminium wires on a glass substrate in air. The structure was simulated with the finite difference time domain (FDTD) software FDTD Solutions by Lumerical Solutions Incorporated. It can be seen that while the polarization direction perpendicular to the wires (p-polarization) is mainly transmitted, the polarization direction parallel to the wires (s-polarization) is mainly reflected. The ratio of transmitted p-polarized and s-polarized light is the extinction ratio of the polarizer (ERTpIT5).

In liquid crystal displays, the small thickness and durability at elevated temperatures of wire grid polarizers would allow their integration inside the liquid crystal cell, resulting in more compact devices and improved contrast. Iodine polarizers (iodine doped stretched polymer), which are conventionally used in LCDs, cannot be used in-cell because they are thick (typically -20.tm) and not robust against solvents and processing temperatures (-200°C). Figure 3 shows the schematic of LCDs, comprising of a backlight unit 7, a first polarizer 8, a lower substrate 9 with pixel electrodes 10 to address the liquid crystal 11, a second substrate 14 with a common electrode 12 and color filters 13 and a second polarizer 15. The different positions for the polarizers as external polarizers (Figure 3a) or internal polarizers (Figure 3b) are depicted. The polarization state of light in a LCD panel is usually controlled by two external polarizers, placed outside the substrate glass. It is well known that placing the polarizers inside the substrates has many advantages, such as reduced display thickness, improved robustness and the possibility to use of birefringent plastic substrates; however, internal polarizers are more difficult to manufacture.

Figure 3c and Figure 3d show the combination of an internal "clean-up" polarizer 16 with two external polarizers 8 and 15. The internal polarizer reduces the depolarization that is caused by scattering of light in the color filters or on the pixel electrodes (Jones et al. US006124907). The overall contrast of the display can be substantially improved even for a clean-up polarizer with relatively low extinction ratio. In Figure 3c, the clean-up polarizer is located between the color filters 13 and the common electrode 12, and has its transmission axis oriented parallel to the top polarizer 15. Light incident on the internal polarizer is partially transmitted or blocked according to its final polarization state at the exit polarizer, so less depolarization can occur. In Figure 3d, the clean-up polarizer is located in the plane of the pixel electrodes 10 with its transmission axis oriented parallel to the bottom polarizer 8, so the depolarized light is blocked by the clean-up polarizer.

The use of wire grid polarizers in liquid crystal displays has been considered for more than 20 years, for example by Grinberg (US4688897, 1985), where the wire grid serves as polarizer, reflector and pixel electrode in a reflective LCD. More recently, Sergan (J.

Opt. Soc. Am. 19, 1872, 2002) used reflective wire grid polarizers in a twisted nematic LCD. Lee et al. demonstrated a stereoscopic LC display based on patterned wire grid polarizers (SID paper 8.4 2006). Ge et al. developed a transflective LCD (AppI. Phys. Lett. 92, 051109, 2008) and demonstrated light "recycling" from the LCD backlight (AppI.

Phys. Lett. 93, 121104, 2008) both using the reflections from a wire grid.

However, as shown in Figure 2, the transmission through a conventional wire grid polarizer and its extinction ratio usually degrade quickly towards the short wavelength end of the spectrum. This is a disadvantage for many applications where broadband polarizing properties are required. In a LCD, the polarizers should ideally have uniform transmission and extinction ratio across the whole visible wavelength range from 400-750 nm to guaranty uniform color balance of the display.

The wavelength dependency of the wire grid polarizer transmission can be reduced by: 1) Very small structure sizes (-40 nm period for visible wavelength range); 2) Wire profiles or additional layers that reduce interference effects between the metal I dielectric interfaces (e.g. triangular or rounded shapes; antireflection layers underneath the metal); 3) Near field coupling of several wire elements that favors certain wavelengths (e.g. as in double layer gratings) and compensate for the reduced transmission at low wavelengths.

The method to reduce the periodicity makes the structures truly sub-wavelength in the VIS and shifts the onset of resonances to shorter wavelengths. Structure sizes below 100 nm period are increasingly difficult to manufacture and, therefore, a design with larger period but sufficient broadband properties is preferred.

Interference effects between the metal I dielectric interfaces can cause wavelength dependencies, which can be reduced by wire profiles that avoid parallel surfaces. The third option uses the coupling between back-to-back wire elements in the optical path. By employing the additionally introduced wavelength dependency, the performance can be tuned by choosing parameters that compensate for other wavelength dependencies.

Several modified wire cross sections were suggested to date to improve the WGP broadband behaviour, such as tapered wires in U57046442B2 (Suganuma et al., Enplas Corp., 2004) and Tsai et al. (AppI. Opt. 47(15), 2008, p. 2882), and concavo-convex wires in US0242187A1 (Yamaki et al., 2006).

U56122103 (Perkins et al., Moxtek, 2000) achieved improved broadband behaviour by inserting a low refractive index layer underneath the metal wires.

Flanders et al. (J. Vac. Sci. Technol. 19(4), 1981, p. 892), U57227684B2 (Wang et al., NanoOpto Corp., 2002) and Wang et al. (Opt. Lett. 30(2), 2005, p.195) suggested structures, which resemble an L-shape, achieved by metal deposition on high-aspect-ratio dielectric gratings. U57227684B2 and Wang et al. optimized the WGP performance in the near infrared wavelength range by anti reflection coatings that are coupled to the WGP core structure. U57227684B2 and Wang et al. focus on the coupling of those structures to anti reflection layers.

All above methods show improved transmission but none of the them demonstrates uniform transmission across the whole visible range.

Double wire layers, as described in US7158302B2 (Chiu et al., 2004), US0183025A1 (Asakawa et al., 2006), Yang et al. (Opt. Expr. 15(15), 2007, p. 9510) and Ekinci et al. (Opt. Expr. 14(6), 2006, p. 2323) introduce an additional wavelength dependency that favors certain wavelengths. Mainly narrow band filters were realized to date. The possibility of improving broadband behaviour through this effect is mentioned by Ekinci et al.; however, no further steps towards an optimization were taken.

In summary, there are applications where the small thickness and high durability of a wire grid polarizer would be very beneficial; however, a broadband polarizing performance is required too. This is true for a liquid crystal display where uniform color distribution is important. To date, no wire grid polarizer has been proposed or demonstrated showing uniform transmission properties across the whole visible wavelength range.

Summary of the Invention:

According to a first aspect of the invention, there is provided a wire grid polariser as defined in the appended claim 1.

According to a second aspect of the invention, there is provided a liquid crystal device as defined in the appended claim 8.

According to third and fourth aspects of the invention, there are provided methods as defined in the appended claims 11 and 13, respectively.

Embodiments of the invention are defined in the other appended claims.

It is possible to provide wire grid polarizer designs which give the opportunity to tune transmission properties depending on the wavelength, and this may result in improved broadband transmission in the visible wavelength range. In the case of a wire profile which comprises a base unit together with a multiplicity of narrow, vertical units (e.g. two), the performance of the WGP can be tuned by adjusting the height and width of those vertical "legs'. Changing the height of the "legs" alters the properties and supported fields inside the cavity, formed by the vertical structures. It is possible to provide: increased extinction ratio (ER) compared to the individual or in-series (spaced apart further apart) components of the design; modification of the wavelength dependency of either transmission or ER; reduced wavelength dependency realized for larger period structures (e.g. 100 nm instead of 40 nm period)

Brief Description of the Drawings

Figure 1 is a schematic view of a conventional wire grid polarizer; Figure 2 represents the optical performance of a conventional wire grid polarizer; Figure 3 illustrates an a) external polarizers in a conventional LCD, and b) in-cell polarizer in a conventional LCD; Figure 4 is a cross sectional schematic view of an example wire grid polarizer design according to the present invention; Figure 5 is a graph representing the optical performance (transmission T, reflection R for s-, p-polarization and extinction ratio ER) for a WGP structure with base only (p=1 OOnm, wlp=0.5, t=4Onm, normal incidence); Figure 6 is a graph representing the optical performance (transmission T, reflection R for s-, p-polarization and extinction ratio ER) for a WGP structure with same the base as in Figure 5 together with two vertical 65nm x 7nm "legs" (p=lOOnm, w!p=0.5, t=4Onm, normal incidence); Figure 7 is a graph representing a comparison of the optical performance (transmission T, reflection R for s-, p-polarization and extinction ratio ER) of an example WGP as a function of the height of the vertical structures for two different base thicknesses: 23nm and 46nm (A=525nm, plOOnm, w/p0.5); Figure 8 is a graph representing the WGP optical performance (transmission T, reflection R for s-, p-polarization and extinction ratio ER) with homogenized extinction ratio (p=lOOnm, w/p=0.5, base thickness 23nm, vertical unit height 4Onm, vertical unit width lOnm); Figure 9 is a graph representing the WGP optical performance (transmission T, reflection R for s-, p-polarization and extinction ratio ER) of an example design, optimized for high and uniform transmission in the visible wavelength range (p=lOOnm, wlp=0.3, base thickness 40nm, vertical unit height 75nm, vertical unit width 1 Onm); Figure 10 represents a process for fabricating a WGP according to the present invention using a high aspect ratio grating, conformal and directional material deposition and a lift-off process; Figure 11 is a graph representing the WGP optical performance for a base unit with rounded edges as it may result from a deposition process (p=lOOnm, wIpO.3, base thickness 4Onm, vertical unit height 6Onm, vertical unit width 11 nm); and Figure 12 represents a process for fabricating a WGP according to the present invention using restricted angle deposition onto a surface relief grating situated on a moving belt; also shown are various possible example structures that may be generated by this method using different relief profiles.

Detailed Description of the Invention

The present invention will now be described in detail with reference to the figures, wherein like reference numerals are used to refer to like elements throughout.

The present embodiments are based on the near-field coupling of several wire elements that allow wavelength tuning and, therefore, balancing of the transmission properties.

Embodiment 1: In a first embodiment of the invention, a wire grid polarizer design as schematically depicted in Figure 4 is presented. The polarizer consist of wires with a cross sectional structure 3 of a mainly horizontal base unit 3 a) of width 5 and height 6 and a multiplicity of wires extending mainly vertically from that base unit (further denoted as "legs") 3 b) with width 18 and height 17. The light is preferably incident from the direction 17. In the preferred arrangement of this invention, the base unit is combined with two "legs" as illustrated in Figure 4.

Figure 5 shows the optical performance of an example structure that consists only of a base unit 6. The graph shows the transmission T, the reflection R for s-, and p-polarized light, respectively, and the extinction ratio ER = Tp/Ts for a WGP structure with period 4 p=lOOnm, width 5 w=5Onm and thickness 6 t=4Onm at normal incidence as a function of wavelength. It can be seen that the ER is small since the metal thickness is low. In Figure 6, the optical performance is shown for the same base unit as in Figure 6, but in conjunction with an example of two vertical legs of 7nm width 18 and 65nm height 17.

The combination of the base unit with the two vertical structures enables influencing the wavelength dependency of the transmission. In addition, the extinction ratio is increased (here by a factor of 10) and the ER of the combined structure is found to be larger than the ER of the individual structures or the individual structures in series (with a gap between the horizontal and vertical structures, which prevents near field interaction between the components).

Embodiment 2: In a second embodiment of the invention, the dimensions of the structure described in Embodiment 1 are adjusted to cause increased transmission at specific wavelengths.

Preferably, the vertical length of the "legs" 17 is adjusted to achieve this.

Figure 7 demonstrates how the design parameters are optimized to provide increased transmission at certain wavelengths, which can enable tuning of the wavelength dependency of the performance. By varying the height 17 of the vertical structures, the supported fields in the cavity are modified and the WGP performance is modulated.

Choosing a leg height of 6Onm, for a base height of 46nm produces a maximum in Tp of close to 0.8 for the chosen wavelength of 525nm, and so enables shifting the transmission within certain limits. Figure 7 indicates also that the location of the transmission maxima and minima is independent of the thickness of the base unit and is, therefore, shown to be mainly affected by the "leg" height 17.

Embodiment 3: In a third embodiment of the invention, the dimensions of the structure described in Embodiment 1 are adjusted to cause increased transmission for short wavelengths, for example the blue end of the visible spectrum near 400nm. This results in a more uniform transmission characteristic across the spectrum, as is shown by comparing Figure 6 and Figure 9. Preferably, the vertical length of the "legs" 17 is adjusted to achieve this. This design uses the same base unit as embodiment 1 (4Onm thick), but with longer and slightly wider vertical legs (75nm compared to 65nm long; 1 Onm compared to 7nm wide) It is important to note that the periodicity of the example design is lOOnm, which is by about a factor of two larger compared to a WGP with rectangular wire cross section that shows comparably neutral transmission. The larger periodicity is an advantage for the manufacturability of these structures and enables manufacturing at reduced cost due to lower structure density.

Embodiment 4: In a fourth embodiment of the invention, the dimensions of the structure described in Embodiment 1 are adjusted to cause increased extinction ratio for specific wavelengths.

Preferably, the vertical length of the "legs" 17 is adjusted to achieve this. As an example, the extinction ratio for the central part of the visible wavelength range (-550nm) can be increased in order to level out the wavelength dependency of the extinction ratio.

For conventional wire grid polarizer designs, the extinction ratio increases drastically with wavelength, as was shown in Figure 2. Figure 8 illustrates that by changing the structure dimension, the extinction ratio can be adjusted. Here, a vertical leg height of 4Onm (with 23nm thick base, lOnm leg width) affects the dependency of extinction ratio with wavelength so that a maximum ER occurs at 500nm wavelength. Although, the total ER in this particular arrangement is low, it provides additional design freedom which can be useful for certain applications where the influence of the extinction ratio is most critical.

Embodiment 5: This embodiment is a specific application of the present invention to a liquid crystal display. The broadband wire grid polarizer can, for example, be used as clean-up polarizer in connection with an additional external polarizer, as shown in Figure 3c and 3d. This reduces the depolarization by other display components and thus enhances the contrast of the LCD while the loss of contrast through additional reflections of ambient light is minimized.

In order to optimize the performance for the application as in-cell polarizer, which requires high transmission Tp of about 90%, the duty cycle can be reduced and the width of the vertical "legs" can be increased. An example of the resulting performance of a WGP with duty cycle 03, base thickness 4Onm, vertical leg length 75nm vertical leg width lOnm, which would be used in a LCD is presented in Figure 9. The transmission lies between 90-92% for the entire visible wavelength range. The extinction ratio varies between ER=31 (at 400nm wavelength) and ER=248 (650nm wavelength), which is sufficient for the in-cell use of the WGP in addition to the external polarizers for contrast improvement of an LCD.

Embodiment 6: This embodiment contains two examples for the illustrative fabrication of the broadband wire grid polarizer design of the present invention.

Figure 10 illustrates the fabrication of a wire grid structure according to the present invention by a combination of conformal and directional deposition of a suitable conducting material onto a suitable surface relief grating 20 and a lift-off process of surplus material. The preferred order of the process is that a first deposition step 21 produces a conformal material layer 22 on the grating. A second deposition step 23 is directional and mainly deposits material on the top and the bottom of the grating 24. In a final step 25, the material on the top of the grating teeth is lifted off by a method similar to micro-contact printing, in which a material with high affinity to the deposited material (adhesive layer or tuned chemically affinity) is brought into close contact with the grating surface. The remaining material 26 has a cross section similar to the one described in the present invention.

Figure 11 shows the optical performance of a structure with a base unit that has rounded edges as it may result from a deposition process. Although the extinction ratio is slightly reduced compared to the same structure with rectangular cross section, it is demonstrated that also a structure with "imperfect" layout, as may result from a real manufacturing process, can be tuned to show broadband transmission.

Figure 12 shows a process for fabricating a WGP according to the present invention using a restricted angle deposition technique. A high aspect ratio surface relief grating 30 is situated on a moving belt 34. The deposition source 27 is shielded by a baffle 28 to restrict the material to certain angles. Therefore, suitable conducting material is deposited onto the sides and the top of the surface relief; however, the bottom of the grating structure is shielded from material deposition. Figure 13 illustrates various possible example structures that may be generated by this method using different relief profiles, e.g. rectangular 32 or slanted gratings (trapezoidal 33 and triangular 34) cross sections.

Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.

Claims

CLAIMS: 1. A wire grid polariser comprising a plurality of wires disposed in a plane and extending in a first direction, each of the wires comprising a first portion and a set of second portions which are connected to the first portion, extend away from the first portion in a second direction perpendicular to the first direction and to the plane, and are spaced apart in a third direction which is perpendicular to the first and second directions.
2. A polariser as claimed in claim 1, in which the wires are spaced apart with a constant pitch.
3. A polariser as claimed in claim 1 or 2, in which each set comprises two of the second portions.
4. A polariser as claimed in any one of the preceding claims, in which, for each wire, the outer surfaces, in the third direction, of the set of second portions are substantially coplanar with the other surfaces, in the third direction, of the first portion.
5. A polariser as claimed in any one of the preceding claims, in which the ratio of the pitch of the wires to the width, in the third direction, of the wires is greater than or equal to two.
6. A polariser as claimed in any one of the preceding claims, in which the height, in the second direction, of the second portions is greater than the height, in the second direction, of the first portions.
7. A polariser as claimed in any one of the preceding claims, in which the width, in the third direction, of the first portions is greater than or equal to four times the width, in the third direction, of the second portions.
8. A liquid crystal device including at least one polariser as claimed in any one of the preceding claims.
9. A device as claimed in claim 8, in which the at least one polariser is disposed between substrates of the device.
10. A device as claimed in claim 8 or 9, comprising a liquid crystal display.
11. A method of making a polariser as claimed in any one of claims 1 to 7, comprising providing a surface relief grating, depositing a metal on all of the surfaces of the surface relief, and removing the metal from upper surfaces of the surface relief.
12. A method as claimed in claim 11, in which the grating has surface relief features which are of substantially rectangular cross-section.
13. A method of making a polariser as claimed in any one of claims 1 to 7, comprising providing a surface relief grating and directionally depositing a metal substantially only on top and side surfaces of the surface relief.
14. A method as claimed in claim 13, in which the grating has surface relief features whose cross-sections are one of rectangular, trapezoidal and triangular.