Much attention has focused on advances in TFT-LCD panel technology such as LED backlights, 120/240-Hz frame rates, wide viewing angle, and other technologies leading to performance improvements. But behind the scenes, TFT-LCD manufacturing continues to evolve, leading to increased manufacturing productivity, lower costs, and improved power efficiency. Recent developments include color filters, liquid-crystal alignment, and TFT design.

by Charles Annis and Paul Semenza

DESPITE THE FACT that TFT-LCDs use less power than similarly sized CRTs, the conversion to LCD TV has led consumers to choose larger and brighter TVs and is driving faster unit growth, which means the total power consumed by TVs is increasing. Worldwide, LCD TVs consumed an estimated 83 TWh of electricity in 2008, equivalent to the output of 11 nuclear power plants. That much electricity produced from coal-fired power plants would result in the creation of 66 million tons of CO₂. Much closer to home for electronics manufacturers, regulatory authorities in Asia, Europe, and the United States are writing and implementing power-consumption regulations for flat-panel TV sets.

At the same time, reducing costs is an essential goal for panel manufacturers to keep up with relentless price declines while trying to maintain positive margins. For larger TVs, average year-to-year cost changes during 2009–2015 are expected to range from –23% to –8%.

Backlight Emphasis

In conventional LCDs, most of the light generated by the backlight is lost due to polarizer absorption, shading by the TFT-array aperture, color-filter absorption, as well as other factors, so that only about 5% of the light emitted reaches the front of the screen. Thus, even small improvements in transmission can enable significant backlight lamp reduction. For example, a 400-nit panel with a 5% transmission rate requires an 8000-nit luminance at the backlight; if transmission is increased to 10%, the same amount of brightness can be achieved with a 4000-nit backlight.

The backlight unit is the most expensive component of a display and accounts for 90% of the power consumption in large LCDs. This is the case regardless of whether the backlight uses cold-cathode fluorescent lamps (CCFLs) or light-emitting-diode (LED) lamps. Because LEDs are still substantially more expensive than CCFLs, increasing transmission is critical to enabling wider LED adoption for LCD-TV applications and is therefore currently a key theme in LCD manufacturing. Given the transmission's potential to reduce both cost and power consumption, a large variety of manufacturing technologies that target transmission increases are being developed and adopted for mass production. Methods such as low-resistance bus lines, C_st-less pixels, SHA, OA, PSA, thinner black matrix, and color-filter-on-array (all explained later in this article) are being implemented in the array, cell, and color-filter processes to increase efficiency.

Array Processes

Storage-Capacitor-Less Pixel Design: In TFT-LCDs, storage capacitors (C_st) are typically fabricated at each pixel to hold the voltage between writing data signals to the pixel. Without a C_st, the voltage can leak and change the state of the liquid crystal (LC). To prevent this, conventional pixel designs use an extra gate metal or wider gate line to increase capacitance. The trade-off is that the extra area for the gate line blocks illumination from the backlight. Samsung developed what it calls "C_st-less" pixels, which eliminate the storage capacitor and increase aperture ratio. Samsung has not publicly disclosed all the details on how it was able to achieve this, but it is likely that it was though a combination of improved LC and TFT performance. The company claims that transmission can be increased up to 10% with new pixel design.

Low-Resistance Bus-Line Materials: Adoption of low-resistance gate and data lines enables a reduction in the RC delay, which improves performance and can potentially eliminate requirements for dual-scan driving. In addition, due to lower resistivity, the width of the bus line can be reduced, which increases the aperture ratio without sacrificing performance. Because copper has a very low resistance, it is the material of choice. LG Display is already mass producing the majority of its larger panels using copper, and over the next 5 years copper and copper-alloy applications are expected to grow substantially.

Super High Aperture (SHA) with Organic Passivation: SHA pixel designs use a thick organic passivation layer to move the indium tin oxide (ITO) pixel electrode further away from the data line. Compared to conventional pixels, in which the ITO pixel electrode is separated only by a thin passivation layer, TFT capacitance is reduced. This allows the ITO pixel electrode to be extended over the bus lines, increasing the aperture ratio at each pixel. The technology has been used in mass production for a while, but is tricky to implement.

Cell Processes

The performance of LCDs is strongly affected by the orientation of the LC molecules, and LC molecules can be aligned in a variety of ways, depending on the LC mode. The LC molecules should be aligned orderly in the OFF-state and with an appropriate orientation or pre-tilt angle to facilitate switching to the ON-state when the drive voltage is applied to the pixel. Conventional LC alignment is achieved through the use of a thin polyimide layer (about 100 nm) on both the array and color-filter (CF) side of the cells. In the twisted-nematic (TN) and in-plane switching (IPS) modes, rubbing is used to create a preferred alignment direction during the cell process. In the PVA mode, patterning of the CF ITO common electrode is used to align the array pixel ITO electrodes in order to create fringe fields. In the multi-domain viewing-angle (MVA) mode, a protrusion is patterned on the CF to physically align the molecules, and a slit is created on the TFT side of the ITO pixel electrode.

Each approach has advantages and dis-advantages. The viewing angle of the TN mode is limited. The TN and IPS modes offer higher transmittance, but rubbing is problematic because it generates particles and static electricity and is not easily scaled to larger substrates. The PVA and MVA modes are relatively fast and have good viewing-angle performance, but transmittance is lower, the LC material is more expensive, and the number of CF manufacturing steps must be increased.

For these reasons, developing new alignment technologies that enable higher transmittance, generate faster response times, avoid rubbing, improve contrast, allow a wide viewing angle, and offer more general productivity than conventional approaches have been long-standing goals for LCD makers. PSA and OA are two methods that achieve proper liquid-crystal pre-tilt angles without rubbing, protrusions, or patterned common electrodes.

Polymer-Sustained Alignment (PSA): Typically, MVA panels use a protrusion to aid alignment of the LC; however, protrusions require an extra mask step in the CF process, which drives up cost and increases manufacturing time. Furthermore, the inactive area of the protrusion restricts transmittance of the panel and creates some light leakage that reduces the dark level and contrast ratio.

In polymer-sustained alignment (PSA) – also known as polymer-stabilized alignment or phase-separated alignment) – a polymer-alignment layer is formed over a conventionally coated polyimide by mixing a UV-curable monomer into the LC. The monomer is then activated by UV radiation while applying an AC voltage. The monomer reacts with the polymer layer to form a surface that fixes the pre-tilt angle of the LC. By removing the protrusion and achieving excellent LC pre-tilt alignment, the aperture ratio is increased, light leakage reduced, and LC switching performance is improved. Because this eliminates a protrusion from the CF side of the display, the contrast ratio is increased and the panel brightness can be improved by more than 20%. At the same time, costs are reduced because the protrusion mask step can be eliminated from the CF process and backlight lamps can be reduced.

PSA technology was originally developed by Fujitsu and is being used in mass production by AU Optronics Corp. (AUO), which has reported the development of a "protrusion-less" MVA-LCD that increases transmittance and reduces cost and process time. The company is calling the technology Advanced MVA (AMVA). The key PSA-related AMVA improvements are achieved by adopting a new "fishbone"-shaped pixel electrode in the array process and polymer-sustained alignment in the cell process to eliminate the protrusions of the conventional MVA mode (Fig. 1). When a drive voltage is applied to the fishbone electrode, LC molecules align along the direction of each domain.

In addition to the fishbone electrode, the other key PSA process modification is to add a monomer to the LC, which is then polymerized by UV curing in the cell process. This provides LC molecules with the proper pre-tilt angle for smooth and fast switching. The pre-tilt angle is fixed by simultaneously applying a voltage and UV exposure (Fig. 2). The pre-tilt angle of the LC can be controlled by the curing recipe by varying the applied voltage and UV dose.

The performance of AMVA is strongly affected by the material science of the polymer-alignment layer, monomer mixture, and LC. AUO has developed unique and proprietary materials to enable this new technology.

 (a)    (b)

Fig. 1: Above is a top view of an AMVA3 pixel electrode; below is the cross section. Source: AUO, SID 2009.

Optical Alignment: Optical alignment (OA) is the process of the interaction of light with a material that generates light-induced anisotropic properties. The anisotropy is determined by the intrinsic characteristics of the material and the direction of the anisotropy is related to the incident angle of the polarized light. For the material researched here, photoalignment involves photon absorption and interaction on a molecular scale, and the effects are observed in terms of the parameters of LC tilt angle, alignment direction, and azimuthal anchoring energy.

In optical alignment, UV exposure through a mask made of a special polyimide film creates an anisotropic feature that generates the pre-tilt angle. There are a variety of materials that have been researched that include azobenzene dyes or azo-dyes, poly (vinyl cinnamate), coumarin dye, poly-siloxane based polymers, and polyimides with additives such as cyclobutane. The basic principle involves the absorption of photons in the 200–365-nm UV range by the material that causes alignment of the polymer chains forming the layer through several processes, including isomerization, dimerization, and decomposition.

Some manufacturers have pursued a photo-decomposition approach. The advantage of this approach is that polyimides similar to those used in conventional processing can be adopted. UV light of sufficient energy breaks the organic bonds within the polyimide molecules, which create an anisotropy in the LC molecule alignment or a pre-tilt angle of about 2° (Fig. 3). The actual mechanism and photochemistry is apparently not well understood, but some sources have suggested it is a type of photo-oxidation. Historically, optical alignment approaches have been susceptible to long-term stability problems where the pre-tilt angle degrades over time. Overcoming this problem appears to be more closely related to the material composition of the polyimide alignment film rather than the process itself.

Fig. 2: Steps 1–4 show the PSA polymerization process. Source: AUO, SID 2009.

Fig. 3: The OA process (left to right) results in LC molecules with a pre-tilt angle of about 2¼. Source: Sharp Corp., adapted by DisplaySearch.

The benefits of OA are quite similar to those for PSA. Additionally, OA is assumed to be highly productive and flexible, offering a wide process margin. The modifications to manufacturing are relatively simple. A photosensitive alignment film replaces the more conventional polyimides; the film is patterned on both the array and CF substrates in the cell process. Multi-domain structures can be achieved by patterning and the mask approach. The material is cured, then the substrates are conventionally aligned, scribed, and sent to module processing.

Sharp began mass-producing 32-in. eco-model TVs having OA in early 2009. In October 2009, the company announced that it will apply this technology to panel production at its plants in Sakai and Kameyama.¹ Refered to as UV2A (ultraviolet-induced multi-domain vertical alignment), the technology uses a combination of proprietary materials developed by Sharp and its partners with UV-exposure equipment and processing technologies. Sharp stated that its approach results in aperture ratios 20% larger than those of conventional panels.

Color Filter

Black-Matrix (BM) Width Reduction: In the CF process, panel manufacturers are reducing the width of the black matrix (BM). (Some width of black matrix is needed between the subpixels to block the light that leaks across from one subpixel its neighbor.) Similar to a reduction in the width of gate and data lines, a narrower BM translates into to a wider aperture ratio. Interestingly, the focus on improved transmission through thinner BM requirements has challenged other CF cost-saving technologies, such as ink-jet printing and BM ablation. Both of these have the potential to lower the total costs by reducing lithography-related-equipment and mask costs, but neither offer the same precise resolution and overlay performance as conventional photolithography that is required for very thin BM patterning.

Color Filter on Array (COA): COA is another CF-related manufacturing technology that moves RGB pixels from the common electrode glass to the array glass. There are multiple variations of this technology, but all increase transmission and improve contrast by widening the aperture ratio. However, moving color pixels to the array creates multiple process challenges – specifically, yield loss. For this reason, COA is not yet widely adopted. TMDisplay and Samsung are currently the two main producers of COA-based LCDs.

Greener Operation Equals More Green for Manufacturers: In order to continue profitable growth in an environment of continuous cost pressure and more-stringent environmental controls, TFT-LCD makers must simultaneously drive down manufacturing costs and reduce power consumption. The technologies that directly address these issues include those that aim to improve the historically low optical transmission of TFT-LCDs. Manufacturers that can bring such improvements into mass production will have a competitive advantage.

References

¹http://sharp-world.com/corporate/news/ 090916.html •