Impact of low temperature polysilicon on the AMLCD Market
01/01/1998
Impact of low temperature polysilicon on the AMLCD market
Julian G. Blake, James D. Stevens III, Eaton Flat Panel Equipment, Beverly, Massachusetts
Ross Young, DisplaySearch, Austin, Texas
Producing higher-performance displays at lower cost is a constant challenge for LCD manufacturers. In the 1980s, market demand forced a transition from passive twisted nematic displays to passive supertwisted nematic displays, and then to today`s amorphous silicon and low temperature polysilicon (LTPS) active-matrix liquid crystal displays (AMLCDs). LTPS technology has drawn the attention of many display manufacturers, because it has several potential advantages over amorphous and high-temperature polysilicon.
Compared to amorphous silicon, polysilicon has higher electron and hole mobilities. Device design rules can shrink, self-alignment features are possible, and pixel-charging time decreases, facilitating higher gray scale, color, and real-time video. Pixel aperture ratios become larger, producing brighter and lower power displays. The smaller size and improved performance of polysilicon thin-film transistors (TFTs) also make them suitable for higher-definition display applications.
AMLCDs and passive matrix liquid crystal displays (PMLCDs) are the dominant FPD technologies and account for nearly 90% (Fig. 1) [1] of the total FPD market, which is expected to reach $12.7 billion in 1997. AMLCDs, which include TFT LCDs and metal insulator metal (MIM) LCDs, have grown at a 73% compound annual growth rate (CAGR) from 1990 to 1996 and accounted for $5.85 billion or 60% of the total 1996 FPD market. Expected improvements in price and performance will enable TFTs, which account for 98% of the AMLCD market, to grow to 64% of the 2002 FPD market (Fig. 2). This projection excludes sales of FEDs, which are difficult to forecast. DisplaySearch forecasts AMLCDs to grow at a 24.6% CAGR from 1997 through 2002 to $22.6 billion, primarily as a result of notebook and LCD monitor sales.
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Figure 1. FPD forecast by technology: 1990-2002 (percentage of revenue basis).
The LCD market can be divided into three segments: notebook PC displays, desktop monitors, and small and medium-sized displays for consumer and automotive applications (Fig. 3). The notebook PC display segment currently accounts for 75% of TFT demand in dollars followed by small/medium displays at 18% and desktop monitors at 7%. TFTs currently represent about 67% of the 13.7 million unit notebook PC market and are expected to grow rapidly to nearly 90% in 2002 due to cost reductions resulting from implementation of larger substrate sizes and manufacturing process learning curve benefits. TFT notebook shipments will grow at a 27% CAGR to 30.5 million units. However, because prices are expected to continue to decline as a result of more economical production and an anticipated excess supply, revenues from the sales of TFT notebooks will grow only at a 16.3% CAGR from $5.6 billion to nearly $12 billion.
TFT notebook displays will continue to grow in size to as large as 15-in. (providing an equivalent viewable image to a 17-in. CRT monitor) and will improve in performance as manufacturers target the desktop PC replacement market with Mega-Laptop PCs. However, the 13.3-in. display for portable use will likely dominate the market for a considerable time. In addition, LCD manufacturers will increase resolution to XGA and even SXGA, and begin introducing full color 8-bit products to match the resolution and color depth of CRTs.
TFT-LCDs have been considered an attractive replacement for CRT desktop displays because they consume less power, occupy less desk space, are thinner and lighter, and produce less emissions. However, until recently, TFTs could not be manufactured at the sizes and visual performance levels desired by desktop users.
Some manufacturers, recognizing the opportunity that the monitor market represents, have developed LCD technology that matches the visual performance of CRTs on the desktop. Already, more than 80 TFT-LCD monitors are on the market, and TFT to CRT price ratios are falling to below 3?. Manufacturing in Generation 3.5 or 4 fabs will allow further reduction in price ratios to 1.4-2? because larger substrate sizes will give increased productivity. DisplaySearch expects the TFT-LCD monitor segment to grow at a 101% CAGR to 14.3 million units with 15-in. TFTs as the dominant size (Fig. 4). TFT-LCD monitor modules will grow at a 65% CAGR to $6.8 billion. TFT-LCD monitor producers are developing full-color displays as large as 30-in. with resolutions as high as QXGA (2048 ? 1536) and higher dots/in. than CRTs.
While the notebook and LCD monitor market segments are the two largest markets, the small and medium-sized display segment (0.7-8 in.) will also see significant growth. This segment includes displays for digital and video cameras, data and video projectors, wide-screen rear projection TVs, personal digital assistants (PDAs), pachinko machines, car navigation displays, video games, small TVs, head mounted displays, video cell phones, and a host of other consumer markets. As Fig. 1 shows, this segment will grow from $1 billion in 1996 to $3.5 billion in 2002, a 23% CAGR. The car navigation, PDA, and digital camera markets are all expected to enjoy >40% CAGR over the forecast period. Car navigation displays are growing from 5 in. to as large as 8 in. with increasing resolution. In Japan, car navigation systems have enjoyed rapid growth, and Europe is expected to be the next region to adapt them widely. PDAs are also expected to grow in size and migrate from passive STN displays to reflective TFT displays (Note: reflective AMLCD displays do not have backlights and consume much less power than conventional transmissive TFT-AMLCDs). Digital cameras, which are gaining market acceptance by PC users because of their ability to upload, modify, and store images, are migrating from 1.8- to 3-in. display sizes with resolutions advancing beyond QVGA (320 ? 240) and will likely grow to 4 in.
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Figure 2. FPD forecast by technology: 1990-2002 (revenue basis).
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Figure 3. TFT revenues by application.
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Figure 4. TFT-LCD monitor forecast.
Role of LTPS in AMLCD market
The small and medium-sized display segment is where most low temperature polysilicon developers will focus their initial penetration efforts. This market is presently shared by amorphous (a-Si) TFT-AMLCDs, metal insulator metal (MIM) AMLCDs, passive twisted nematic (TN) passive supertwisted nematic (STN), high temperature polysilicon AMLCDs, electroluminescent, and vacuum fluorescent displays. Many manufacturers believe that these rapidly growing market segments are all better suited for low temperature polysilicon technology. Projection, digital camera, video camera, and other small high resolution applications would benefit from the adoption of low temperature polysilicon display technology. Low temperature polysilicon offers higher resolution at lower cost and power consumption than amorphous silicon technology and equivalent resolution at a lower cost than high temperature polysilicon technology. Amorphous silicon`s material properties and associated low electron mobilities (0.5 cm2/Vs) require larger pixel TFTs. Aperture ratios for 5-in. VGA amorphous displays are less than one third of similarly sized low temperature polysilicon displays. Also, the aperture ratios for smaller VGA and higher resolution amorphous silicon displays can decline to the point where the display is unusable. Meanwhile, high temperature polysilicon panels are produced on small quartz substrates and cannot match the costs of low temperature polysilicon fabricated in reconditioned second and third generation lines. For example, a 1.3-in. SVGA high temperature polysilicon display is presently in the $300-400 range while a similarly sized low temperature polysilicon display is expected to be half the cost. Samsung Electronics has announced the development of a one million pixel LTPS 2.3-in. single-panel full-color projector. At the Japan Electronics `97 show, Sharp exhibited a 4.5-in. LTPS display, and Matsushita exhibited several 3- and 5-in. displays. Sanyo announced it will begin mass pr
In order to examine the cost implications of low temperature polysilicon vs. amorphous silicon, DisplaySearch conducted a Monte Carlo model simulation study for three different panel sizes (3, 6, and 12 in.) with XGA resolution. The study assumed second generation fabs using 360 ? 465 mm substrates with a 20% increase in equipment costs for the low temperature polysilicon fab to achieve the same throughput. Seventy-five percent effective yields were assumed for both technologies. However, because low temperature polysilicon technology will not initially achieve the same yields as amorphous silicon due to the additional masks, added process steps, and integrated driver circuitry, the study calculated the low temperature polysilicon yields required to result in an equivalent cost. Two different low temperature polysilicon driver integration scenarios were envisioned. In the first case, the row circuitry was fully integrated, and only switches and shift registers were integrated in the column circuitry. A single external driver IC that included the multiplexed DAC and amplifiers was mounted off panel. In the second case, all of the required driver circuitry was integrated on the panel as was the panel ASIC. The results of the study are shown in Fig. 5. DisplaySearch has recently completed a more detailed technical and cost study that modeled the cost benefits of low temperature polysilicon vs. amorphous silicon display technology using data from low temperature polysilicon producers and suppliers [2].
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Figure 5. Cost analysis: Low temperature p-Si vs. a-Si TFT-LCDs.
In the the 3-in. XGA display panel, the cost of partially integrated low temperature polysilicon display was 33% less than that of a similarly sized amorphous silicon display. The amorphous silicon display would also require black matrix on array and super high aperture technology in order to have a somewhat usable aperture ratio. The fully integrated driver low temperature polysilicon approach achieved costs 60% less than amorphous silicon. The yields for the two low temperature polysilicon cases could be as low as 14% and 8%, respectively, and still match the amorphous silicon costs. This large cost difference at 3-in. explains why manufacturers are actively targeting the smaller low temperature polysilicon displays. There is not only a performance advantage but a significant cost advantage as well.
The same is true for 6-in. displays. The partially integrated approach results in 22% lower costs, while the fully integrated approach results in a 40% cost advantage. Low temperature polysilicon yields would have to drop to 37% and 26%, respectively, for the amorphous silicon costs to be equivalent. Because of this cost advantage, manufacturers are targeting low temperature polysilicon for medium-sized display segment in applications such as car navigation, premium PDA, and sub-notebook computer displays.
At 12-in., the cost advantages resulting from the integrated driver circuitry are less visible. The gap narrows considerably as the driver costs became a smaller part of the total display cost equation and the higher array costs for low temperature polysilicon are amortized over fewer displays/plate. Partially integrated low temperature polysilicon panels fall to just 2.2% less than amorphous silicon, while fully integrated panel costs would be just 8.6% less. LTPS yields would need to be 72% in the case of the partially integrated panel, and 63% in the fully integrated panel compared to amorphous silicon yields of 75%.
However, this reduced benefit has not deterred some manufacturers from considering low temperature polysilicon for use in large size displays. Matsushita and Mitsubishi recently presented papers that argued that low temperature polysilicon will be better suited for large area, ultra high resolution displays for monitor applications. In 14-20-in. displays at QXGA resolution, they claim the aperture ratio of amorphous silicon would be insufficient because of the need to build larger TFTs to overcome the RC delays of longer electrodes and provide shorter pixel settling times. Both Toshiba and LG Electronics have exhibited 12.1-in. LTPS displays at recent electronics shows in Japan and Korea. LG, in a recently published paper [3], outlined the steps that they will take to make the use of low temperature polysilicon cost effective in large size displays. Briefly stated these consist of:
reducing the area occupied on the panel by the drive circuits thereby allowing the display to be built on the same size panel as an equivalent amorphous silicon display. This can be accomplished through the use of novel drive circuit designs, such as phased data drivers with demultiplexing switches, and the implementation of more aggressive photo-lithographic design rules;
simplifying the device structure of the thin-film transistors to reduce the number of process steps, reduce the required process equipment, and hopefully increase yields; and
improving the uniformity of the thin-film transistors through improvement of critical processes, such as the crystallization of the polysilicon film.
Whether or not low temperature polysilicon will be able to achieve these lower costs at the larger panel sizes will depend on the success of the display and equipment manufacturers in process and tool development.n
Acknowledgments
The authors wish to thank Barry E. Young of DisplaySearch for the preparation of the LCD cost model, and Daniel Farrin of Eaton Semiconductor Equipment Operations for the preparation of the graphs and drawings.
References
1. DisplaySearch, FPD Equipment and Materials Analysis and Forecast, (December 1997).
2. DisplaySearch, Status and Forecast of Low Temperature p-Si Displays, (February 1997).
3. H. Soh, et al, Proceedings, Semicon Korea `97, p. 86, (February 3-5, 1997).
Julian G. Blake received his AB degree from Amherst College, and his MAT and PhD degrees from Harvard University, where he worked on the optical properties of sputtered aSi:H and aSiGe:H films. He joined Eaton Corp. in 1984 and has been involved in process development and machine design of thin-film deposition, rapid thermal processing, and ion implant products. He is the technical director for Eaton Flat Panel Equipment, where he is responsible for thin-film transistor process development and machine design of ion implanter products for flat panel displays. Eaton Flat Panel Equipment, 108 Cherry Hill Drive, Beverly, MA 01915; ph 978/524-9229, fax 978/524-9224, e-mail [email protected].
James D. Stevens III received his BS in management engineering from Rensselaer Polytechnic Institute and his MS in operations research and applied statistics from Union College, Schenectady, NY. He joined Eaton Corp. Flat Panel Equipment as the worldwide sales and marketing manager in August 1995. Prior to joining Eaton, he served as director of marketing for Rodel and served over 30 years at IBM Microelectronics.
Ross Young was educated at Japan`s Tohoku University and the University of California at San Diego`s Graduate School of International Relations and Pacific Studies. He founded the FPD market research firm DisplaySearch in early 1996. Prior to that he held senior marketing positions in the FPD manufacturing equipment, FPD materials, and semiconductor equipment industries. DisplaySearch, 1937 Rue De St. Tropez, Suite #13, Austin, TX 78746; ph 512/329-9244, fax 512/347-0009, e-mail [email protected].