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TFT LCDs Produced on Soda Lime Glass Substrates

One of the biggest news stories at FPD International held Oct 24-26 in Yokohama, Japan, was Samsung’s display of the five 19″ SXGA modules (shown below), fabricated on soda-lime glass substrates. Display specifications include

  • Resolution: 1280 × RGB × 1024
  • Brightness: 300 cd/m2
  • Color Gamut: 72% NTSC (CIE 1931)
  • Contrast Ratio: >1000:1
  • Substrate: 1100 × 1250 mm 

The panels appeared the same as any other monitor modules; normal thickness and with no obvious defects. The Korean Samsung employee staffing the booth was not able to answer any detailed questions on the process, except to say that development was complete and mass production ready to begin. He recommended contacting Samsung marketing in Korea for more information.

Figure 1: Samsung Soda-Lime TFT LCD Exhibit
 Samsung Soda-Lime TFT LCD Exhibit
Source: Photo taken by DisplaySearch at FPD International 2007/10/30

Typically, LCDs are produced on high-performance non-alkali glass developed specifically for LCD applications. Soda-lime glass is most commonly used for window panes and containers, and in some cases it is used for less demanding electronics applications like PDPs and STNs.

Corning’s latest glass substrate technology, EAGLE XG for LCDs, is less dense, has much lower thermal expansion, has a higher strain point, is environmentally friendly, has excellent surface quality, dimensional stability, only very small inclusions and has no optical distortion compared to soda-lime glass. Non-alkali glass is, all around, superior to soda-lime glass for making LCDs, with one major exception: it is much more expensive. A square meter of Gen6 non-alkali glass currently sells for around ¥6,122. With two glass substrates used for each LCD module, that means glass accounts for about 12-13% of the total sales cost of a 32″ VA type LCD TV manufactured by a first tier maker in Q4’07.

Not only is glass one of the more expensive materials that goes into making a display, the high profit margin around 50% enjoyed by leading glass suppliers perpetually irks many other participants of the LCD supply chain, particularly panel suppliers. But up to now, panel makers have had no choice because it’s not just a matter of melting sand. Making non-alkali glass is capital intensive and is very technically and logistically challenging—and to date, there have been no alternatives.

About a year ago, we started hearing rumors that panel makers were developing soda-lime capable processes and added soda-lime to our manufacturing technology roadmap, with mass production starting in 2011/2012. So DisplaySearch, and as far as we have heard, just about everybody, including leading glass makers, leading CVD makers, competing panel makers, etc., were all surprised to see Samsung’s demonstration. It was even more surprising to hear that the panels were made on an existing production line and that the company is claiming the process is ready for mass production.

Samsung was not offering any technical details on its soda-lime process, but some assumptions can be made. A low-temperature process had to be adopted to minimize thermal expansion, which can create severe pattern overlay problems and other issues. The TFT LCD CVD process is typically >300º C, and although this can be lowered, usually there is a trade off in film quality and TFT performance, especially at temperatures below 200º C. Samsung presented the research shown below at IMID 2007, which suggests they have substantially improved the performance and stability of TFTs at low-process temperatures. 

Figure 2: TFT Performance & Stability with Conventional vs. Samsung’s Improved Low Temp Process
Figure 2	TFT Performance & Stability with Conventional vs. Samsung’s Improved Low Temp Process 
Source: Peter Shin, Senior VP Samsung Electronics, IMID 2007/10/30

Discussing other potential issues with LCD equipment and materials makers, one vendor suggested that potentially over time sodium migration could be a problem. That is, the soda-lime panels may look good now, but over time they could degrade and TFT reliability could become an issue due to sodium migration. A good barrier layer might prevent this sort of contamination from the glass, but it could require an extra process step and add cost.

One commentator suggested that soda-lime glass is optimally manufactured at 2 mm and that this would cause displays to be heavier and reduce transmissivity, which would add transportation cost and require extra backlight power. However, one source that should know said that the Samsung panels were fabricated on soda-lime substrates less than 1 mm thick and that even 0.5 mm was possible.

Doing a simple cost analysis comparing non-alkali to soda lime, without knowing the detailed cost of the soda-lime glass used by Samsung or the process modifications required (since PDP type soda-lime substrates are about half the price of equivalent LCD non-alkali substrates), LCD module costs could be reduced by around 6% or so. That is, 32″ VA LCD total costs could be reduced from ¥32,751 to about ¥30,627. Although the potential cost savings may not be as dramatic as some might have expected, 6% is a lot in the highly competitive LCD manufacturing industry.

Over the past week, we have heard many comments on how important this demonstration was and how it might change the LCD industry. Even though the sales guy at the Samsung booth suggested they would start mass production in the near future, other knowledgeable people have implied that Samsung does not have any firm near-term manufacturing plans. It could be that the demonstration was intended mainly to show off the company’s technical prowess and also as a warning shot to glass makers: lower prices or we will push soda-lime production technology. Even though the demonstration caught us by surprise, we still assume the other panel makers are not as far along in developing soda-lime compatible processes, and even Samsung may still face some technical challenges. Until more information becomes available, it may be premature to throw away that old DisplaySearch manufacturing technology roadmap, soda-lime mass production in 2011/2012 may yet prove a reasonable assumption. But regardless of when soda-lime actually starts mass production, the technology will put pressure on glass makers and help to keep LCD costs on a continuous downward curve. 

  • Ki-Beom Kim

    The Empire strikes back?

    This post is very interesting both in terms of strategic aspect and technical aspect.
    LCD substrate glass (Non-Alkali, N/A) market has long been supplier market because of limited capa. of major glass companies. Now it seems like that SEC got a new card to deal with them. ( I am not sure this is just bluffing or not.)

    Several technical issues were already mentioned in article. However, I have two additional concerns. (Although SEC people already solved those issues, these possibly can affect to the cost and throughput.)
    First, float glass made from Tin bath require surface polishing. Is polishing process applicable for the substrate size larger than G5 (Substrate: 1100 × 1250 mm )? Second, soda-lime glass is softer than N/A glass. This may be a problem for the substrate transfer due to lager glass sagging.

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