This article was originally published in the DisplaySearch Monitor, January 2012, by Charles Annis, DisplaySearch.
Annis shares 10 trends in liquid crystal display (LCD) manufacturing, covering LTPS, IGZO, alignment technologies, metallization techs, 2µm resolution lithograhy patterns on Gen 8 glass, and more.
The LCD industry faces heady challenges. Pure play LCD makers have lost money 5 quarters in a row and it looks to continue. The equipment market is expected to drop a record 63% in 2012. Fab utilization remains stuck in the 70% range. However, LCD makers have continuously improved manufacturing technology, and are applying revolutionary new technologies. Adoption of new manufacturing technology in 2012 will make the highest-quality, lowest-cost flat panel displays (FPDs) available to consumers worldwide, especially in smartphones and tablets.
|Figure 1. Top 10 LCD manufacturing technology trends of 2012. Source: DisplaySearch TFT LCD Process Roadmap Report.|
1: Most LCDs are produced using amorphous silicon thin-film transitors (a-Si TFTs). Although a-Si suffers from poor mobility, it is a low-cost semiconductor material acceptable for many applications. However, as FPD performance has continued to increase over time, the need for higher mobility backplanes has grown. The main reasons for this are to reduce TFT size for super-high resolution small/medium LCDs to increase transmission and battery life as well as to provide sufficient current to drive active-matrix organic light emitting diode (AMOLED) devices.
Low-temperature polysilicon (LTPS) has been in mass production for more than 10 years, a great technology in need of appropriate applications. High-resolution LCDs and AMOLEDs are a substantial growth opportunity for LTPS. In 2012, LTPS manufacturing will take off as SMD, Sharp, and Toshiba all ramp up new Gen 5.5/6 LTPS fabs and as SMD begins production on its Gen 8 LTPS pilot line.
2: Indium gallium zinc oxide (IGZO) offers mobility performance somewhere between a-Si and LTPS. Although it is a less mature technology than LTPS, IGZO processes are quite similar to conventional a-Si, with only a marginal capital cost add (around 20%) compared to 2X the additional capital required to produce high-performance LTPS panels. Sharp started IGZO pilot production late in 2011, and LG Display and Samsung are expected to follow in 2012.
|Figure 2. Equipment spending by technology. Source: DisplaySearch Q4’11 Quarterly FPD Supply/Demand and Capital Spending Report.|
3: Polymer stabilized alignment (PSA) and optical alignment (OA) are the two main technologies to improve performance of the alignment process. Both simultaneously improve image quality — mainly by improving contrast — and lower costs by improving transmission. Production of OA will increase significantly in 2012, as Sharp applies it to FFS-type panels and licenses its VA technology to other manufacturers.
4: Advanced resolution exposure refers to the pattering of very fine features in the FPD array. Conventional photolithography for Gen 5 and larger substrates has historically been limited to 3µm at best. In 2012, leading FPD lithography tool vendors are expected to release next-generation tools that enable 2µm resolution on glass sizes up to Gen 8. Market forces are driving the push to higher resolution pattering:
- Increase aperture ratio for super high resolution displays
- Complicated AMOLED pixel designs
- Narrow pixel electrode patterns for PSA and FFS to increase transmission
- Novel pixel designs such as short channel TFTs
5: The most important trend in liquid crystal is the continuous shift towards FFS as the LC mode of choice for mobile applications, particularly for those that adopt touch. FFS, only a few years ago, seemed like it would become a niche technology compared to conventional IPS and VA. However, because it offers superior transmission, off-axis viewing, and resistance to touch mura, FFS continues to gain share not only in mobile applications but also in some large-area applications.
|Figure 3. LC Mode by TFT Capacity (000 m²). Source: DisplaySearch TFT LCD Process Roadmap Report.|
6: Super high aperture (SHA) ratio pixel designs typically use an extra organic planarization layer in the array process to planarize the device and increase the vertical gap between the pixel ITO and bus lines. This reduces unwanted capacitive coupling and enables the pixel electrode to be extended over the gate and data lines without causing cross-talk or affecting image quality — thus increasing aperture area. Higher transmission can lower backlight costs by reducing LEDs, brightness enhancement film, etc. Despite a yield trade-off and additional costs to implement, SHA has grown rapidly since 2009. It is now commonly applied to higher resolution mobile products and also, in many cases, to large-area LCDs. About 25% of all LCDs now adopt an SHA process.
7: Low resistance metallization now refers to copper. Cu has the lowest resistivity of any of the other bus line metals that have been used historically to manufacture LCDs, with several benefits:
- Thinner gate and source line, which can help increase transmission
- Reduces RC delay issues
- May reduce costs by eliminating dual-scan driver drivers
- The major trade-off is reduced yield. LG Display implemented it for large-scale commercial production in Gen 6.
Its IP position has made it difficult for other manufacturers to adopt. Regardless, Cu adoption has grown rapidly since 2009 as various alternatives have been developed. Several top-tier LCD manufacturers are now using copper, though some are still in the development stage.
8: Color filter on array (COA) is a technology that was developed many years ago, but has been widely adopted only since 2009. COA moves the RGB color patterns from the opposite glass to the array glass, with several benefits:
- Improved contrast
- Increased aperture ratio (the thick organic color resist enables the same sort of high aperture pixel designs as SHA by allowing the pixel electrode to be extended over the bus line)
- Reduced BM width
- Reduced alignment errors between array and opposite glass issue
- Possible improvement in cell process curing performance
- Like many new manufacturing technologies, the trade-off in implementing is yield. In 2011, LG Display became the third top-tier manufacturer to implement COA in mass production of large-area LCDs, and further growth is expected in 2012.
|Figure 4. COA concept. Source: DisplaySearch TFT LCD Process Roadmap Report, and Samsung.|
9: The key trend related to glass is no longer size increases — it is reducing thickness. Historically, glass substrate size growth was the most important trend in LCD manufacturing. Through Gen 8, a new glass size was introduced every one or two years. However, this trend has slowed significantly due to endemic over-supply and high capital costs of larger fabs. Motivations to adopt thin glass vary by small/medium and large-area applications. For small/medium, reducing thickness enables a thinner, lighter LCD required for mobile applications. For large-area LCDs, reducing glass costs has been an important target for panel makers. In 2012, 0.4mm glass for =Gen 5 and 0.5mm for +Gen 8 is expected to grow dramatically.
10: Black matrix (BM) width reduction has been an ongoing trend for several years and is forecast to continue in 2012. The main benefit is an improvement in transmission by increasing the pixel aperture area. Here are some examples:
- 25µm BM width = 60% aperture
- 15µm BM width = 75% aperture
- 10µm BM width = 80% aperture
|Figure 5. BM width reduction. Source: DisplaySearch TFT LCD Process Roadmap Report.|
An increase in brightness is the most common target for manufacturing technologies. This is not due to panel makers trying to increase device brightness, but because brightness can be traded off to lower costs or power consumption. Resolution is the second most common target, mainly because both smartphones and tablets are rapidly driving the mobile market. These applications are also pushing reduced weight and thickness. Also read: Mobile drives display materials development in 2012
More information about current LCD manufacturing trends can be found in the newly released TFT LCD Process Roadmap Report. The report focuses on key current industry trends such as LTPS, oxide semiconductors like IGZO, super high resolution displays, FFS, optical alignment, and other technologies related to smart phones, tablets, and Apple, as well as large-area displays for TVs and other applications. Learn more about the report from DisplaySearch LLC, an NPD Group Company, at www.displaysearch.com.