The cathode ray tube (CRT), a vacuum device employing an electron gun and a fluorescent screen was first invented in 1897, and became the basis of the first televisions that were introduced in 1934. The same technology was also used for computer monitors, and the CRT dominated the TV and monitor industry up through the 1990s. The CRT was finally displaced with the introduction of flat panel screens. Today there are several flat panel technologies, including Liquid Crystal Displays (LCD), Plasma Displays and Organic Light Emitting Diode Displays (OLED). The most popular display is the LCD, and its applications include TVs, monitors, notebook computers, tablets, cell phones, watches, calculators and appliances, to name just a few.
LIQUID CRYSTAL DISPLAYS
The basic components of an LCD include the backlight unit, an incoming polarizer, an array of thin film transistors, an indium tin oxide electrode, the liquid crystal, a color filter and a final polarizer which controls the output of light. The technology has become very affordable as a result of mass production techniques which include the processing of many devices at once on glass panels that can be larger than three meters on a side.
Figure 1. A liquid crystal display showing improvements in brightness and integration (elimination of DBEF and film polarizer) by using a wire grid polarizer (WGP)
JET AND FLASH IMPRINT LITHOGRAPHY FOR DISPLAYS
Jet and Flash Imprint lithography (J-FIL), has the ability to enhance the performance of flat panel displays in a variety of ways. Possible applications for this industry include Wire Grid Polarizers (WGP), photonic crystals to enhance the light output of an OLED device, antireflection films, plasmonic color filters and patterned retarder layers. The WGP is particularly interesting, since the integration of a high brightness/high contrast polarizer into an LCD has the potential to both improve the display performance and decrease fabrication cost of the LCD device (fig.1). Until now, WGPs have been limited to applications requiring small area polarizers (approximately 25mm x 25mm), such as high end projectors. By employing a roll based imprint technique, it is now possible to scale the technology and apply J-FIL to large glass panels. MII has used a plate-to-roll or roll-to-plate approach, which gives the user the ability to pattern WGPs on either a flexible polymer film or directly on a glass substrate. Nanopatterned film is shown in Figure 2.
Figure 2: J-FIL technology has been adapted for flexible film imprinting (plate-to-roll) and large area rigid substrate imprinting (roll-to-plate) so that all display types can be patterned at low cost.
WIRE GRID POLARIZER PERFORMANCE
Wire grid polarizers typically are fabricated by patterning high density line/space arrays with pitches less than 120nm and transferring the pattern into an underlying aluminum film. Shown in Figure 3 is the imprint resist after patterning (Figures 3a and 3b) and the aluminum features after reactive ion etching (Figure 3c). The pitch of these structures is 100nm.
Figure 3. a) Top down view of 50nm lines after imprinting. b) Cross sectional view of the imprinted lines. c) Aluminum lines after pattern transfer.
Polarizer performance is shown in Figure 4. Figures 4a and 4b compare the performance between a conventional iodine-based film polarizer (shown in red) and a 100nm pitch WGP (shown in blue) for both polarizer transmission (Figure 4a) and extinction ratio (Figure 4b). The extinction ratio (or contrast ratio) is the ratio of the transmitted transverse magnetic and transverse electric components of the incoming light. For both graphs, the WGP demonstrates superior performance. Higher light transmission has the positive effect of reducing power consumption and improving battery lifetime of mobile devices.
Figure 4. Transmission and Extinction Ratio as a function of wavelength. The wire grid polarizer outperforms the film polarizer.
The ability to print time after time, without creating defects or needing to rework the imprint template is a critical manufacturing requirement of any imprint process and is one aspect of what differentiates J-FIL from other imprinting technologies. Shown in Figure 5 are two examples of imprint longevity. Figure 5a depicts the imprinting of a 300nm pitch concentric line pattern across the length of the web. Figure 5b shows the imprint quality of an imprinted wire grid pattern after more than 1000 consecutive imprints.
Figure 5. a) Imprinting of a 300nm concentric line pattern across the length of the web. B) Imprint quality of an imprinted wire grid polarizer pattern after more than 1000 consecutive imprints.
The integration of a WGP represents one example of how J-FIL can positively impact the Cost of Ownership of the display industry. Many other opportunities exist as well. Finally, it should be noted that roll-based Jet and Flash Imprint Lithography for large area devices such as displays can also play a role in markets such as photovoltaics and nanoparticle drug delivery.
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