by Charles Annis, VP of manufacturing research, DisplaySearch
LCD prices are on a relentless downward trend. Lower prices are key to stimulating more demand and further growing the total market in terms of units as well as shifting demand to larger panel sizes. In order to maintain margins as prices fall, LCD producers constantly strive to lower costs. This trend is being exacerbated by the current weak economic environment and significant oversupply, causing prices to fall near cost levels, ever intensifying the focus on cost reduction.
Accounting for 15%-25% of total module costs, backlight unit and inverters are the single most expensive components of an LCD. For this reason, it is not surprising that the backlight assembly is an important focal point of cost reduction. In the backlit unit (BLU), CCFL lamps are the most expensive components, accounting for 30%-40% of the bill of materials — and in the case of LED-based BLUs, the lamps are substantially more expensive.
Increasing LCD panel transmissivity has now become an industrywide goal — not for the purpose of increasing on screen brightness, but rather to maintain brightness and reduce backlight lamps, inverters, and optical films in order to lower panel costs. Another benefit of improved transmissivity is power reduction. Not only is lower power consumption “green,” but in conventional LCDs the large majority of illumination generated by the backlight lamps is lost from polarizer absorption, shading by the TFT array aperture ratio, color filter, etc. so that only about 5% of the light emitted makes it to the front of the screen.
Even small improvements in transmissivity can lead to large gains in BLU lamp reduction. For example, a typical 400-nit panel with 5% transmissivity has 8000-nits luminance at the backlight. If transmissivity is increased from 5% to 10%, the same amount of front of panel luminance can be achieved with a 4000-nit backlight. In other words, the number of backlight lamps and inverters can be reduced by 50%.
There are a variety of technologies in production or in development by different panel manufacturers that target transmissivity improvements. Some of the most important include:
* Array process: “Cst (storage capacitor) -less” pixel design
* Array process: Low resistance bus line materials
* Array process: Super high aperture (SHA) with organic passivation
* Cell process: Optical alignment
* Cell process: Polymer sustained alignment (PSA)
* Color filter: Black matrix (BM) width reduction
* Color filter: Color filter-on-array (COA)
Typically storage capacitors (Cst) are fabricated at each pixel to hold voltage between writing data signals to the pixel. Without a Cst, voltage can leak and change the state of the 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 gate line area blocks illumination from the backlight. Samsung developed what it calls “Cst-Less” pixels, which eliminates the storage capacitor and increases aperture ratio. Samsung has not publicly disclosed all the details on how this was achieved, but it is likely though a combination of improved LC and TFT performance. The company claims that transmissivity can be increased up to 10% with the new pixel design.
Adoption of low resistance gate and data lines provides multiple benefits that include reduced RC delay that 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 aperture ratio without sacrificing performance. Since copper has a very low resistance, it is the material of choice. Already LG Display is mass-producing the majority of its larger panels using Cu, and over the next five years Cu and Cu-alloy application is expected to grow substantially.
SHA pixel designs use a thick organic passivation layer to move ITO away from the data line. Compared to conventional pixels where the ITO pixel electrode is only 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 in mass production for a while, but is tricky to implement.
Optical alignment and PSA are two methods that achieve proper liquid crystal pre-tilt angles without rubbing, protrusions, or patterned common electrodes. In optical alignment, UV exposure of a special polyimide film creates an anisotropic feature that generates the pre-tilt angle. Optical alignment promises multiple benefits, but has not been put into mass production due to materials longevity issues.
In PSA, 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. Because it eliminates a protrusion from the color filter side of the display, aperture ratio is increased and panel brightness can be improved by more than 20%; contrast ratio and LC switching speed are also improved. At the same time, costs are reduced because the protrusion mask step can be eliminated from the color filter process.
PSA polymerization process (right), conventional VA vs. PSA LCD (left) (Source: Left side Fujitsu, right side AUO)
PSA technology was originally developed by Fujitsu, and now AUO is bringing it to mass production, showing stunning examples of its technology (called “AMVA-III”) at this year’s FPD International. PSA offers multiple benefits, with minimum trade-offs. Several panel makers are expected to start mass production of PSA for both small/medium and large size LCDs over the next year.
In the color filter process, panel manufacturers are reducing the width of the black matrix (BM). Similar to reducing the width of gate and data lines, a thinner BM translates to a wider aperture ratio.
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 transmissivity by widening the aperture ratio as well as improve contrast. 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.
Lowering LCD costs are essential to maintain margins, and in the current environment, just to stay in business. Panel makers are applying a holistic strategy to cost reduction, which includes implementing new manufacturing technologies. Of these, improving panel transmission in order to lower backlight costs has emerged as a key trend. What technologies will be adopted will vary by panel maker, but those that both reduce cost and at the same time improve performance with minimal trade-offs will see the highest adoption rates. Particularly, LCDs adopting low-resistance bus lines, thinner BM, and PSA are already available in stores, and expect more in the near future.