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MicroMagazine.com

Flat-Panel Displays Achieving Dominance

Rapid Advancements, Cost Reductions Transforming the Display Market into a Flat World

by Bijan Moslehi

Bijan Moslehi, PhD, is chief technology officer and senior vice president, semiconductor technology research, for The Noblemen Group, a boutique investment banking, strategic advisory, and business development firm. Moslehi has 20 years' experience working in the semiconductor and semiconductor equipment industries. He can be reached at bmoslehi@noblemengroup.com.

Driven by rapidly declining prices and fast-paced major advances in products and quality, the large-area flat-panel display (FPD) markets have seen phenomenal growth. For the first time, FPDs surpassed CRTs in 2005, accounting for more than half of all displays. The lower weight, thinness, and form factor of FPDs have been the main reasons for their use in virtually all portable products and their increased adoption for PC monitors and TVs. Korea, Taiwan, Japan, and more recently China account for virtually all of the large-area FPD manufacturing.

The main FPD technologies include liquid-crystal display (LCD), organic light-emitting display (OLED), and plasma display panel (PDP). The primary applications of PDPs include large-area TVs, outdoor and indoor public/industrial digital signs, and computer monitors. For very large TV screens, rear-projection displays mainly based on either digital light processing or liquid crystal on silicon are also offered. Recently, PDPs have achieved a higher share of the TV market than projection systems.

In general, FPDs are composed of many small pixels. Each pixel contains three cells that provide the three primary red, green, and blue colors, respectively. Voltage is applied to the selected cells via a combination of rows of transparent electrodes located on the upper glass panel and columns of electrodes placed on the inner side of the lower panel. The transparent electrode is usually formed by deposition and patterning of a conductive film composed of indium–tin oxide (ITO). The response of each cell to the applied voltage leads to the emission or passage of light with the corresponding cell color. The light intensity is controlled by varying the applied voltage. The full range of colors is obtained through weighted combinations of red, green, and blue lights with various intensities.

Each FPD technology is based on the specific mechanism used for generating a response to the applied voltage or current flow. In self-emitting FPDs (such as OLEDs and PDPs), this physical phenomenon is also responsible for generating light inside the cells within each pixel. Other FPDs rely on backlighting (transmissive LCD), external/ambient sources of light (reflective LCD), or a combination of both (transflective LCD). Small reflective LCDs require little power and can be operated with a photovoltaic cell.

In PDPs, each cell is coated with red, green, or blue phosphors. Then the cavities in each cell are filled with a neon-xenon mixture or other inert gas. An applied voltage ionizes the gas in the cavity of each selected cell, and the resulting gas discharge leads to the emission of ultraviolet light. In response to the UV light, the phosphor emits light with the corresponding color, and the cells act like a group of miniature color fluorescent lights.

OLEDs are made of an organic light-emitting film sandwiched between a metal cathode in contact with an electron transport layer and a hole injection and transport layer in contact with an ITO anode (placed on the glass side of the panel). Different organic light-emitting materials generate the red, green, and blue colors in each cell. Since ITO film is expensive and brittle, flexible OLEDs require a suitable replacement for ITO as the transparent conductor.

In LCDs the cells are filled with liquid crystal and sandwiched between two electrodes, with each cell acting like a capacitor. In response to an applied charge, and in combination with two upper and lower polarizing filters, the liquid crystal controls the passage of light and allows various amounts of light provided by backlighting to pass through the cell. A color filter used on the front glass provides the red, green, and blue colors for each cell in each pixel. The backside glass contains the thin-film transistors (TFTs) for controlling each cell.

TFT-LCD panel manufacturing involves several sets of sequential processes. The TFT array process includes five or more masking steps and is used to fabricate the transistors on the substrate. It includes patterning, dry etch, deposition, wet etch and strip, wet cleaning, excimer laser annealing, and implant steps. Next in the cell process, the front substrate (with a color filter) is attached to the arrayed back substrate and the space between them is filled with liquid crystal. Finally, in the module assembly process, driver ICs, backlighting, and other components are attached and connected to the panel.

The FPD industry is extremely competitive and cost-sensitive, with growth in demand linked to lower panel prices. Thus, cost reduction is a major focus area. One very effective way to cut expenses is to increase the size of the mother glass, referred to as Generation, or G. The newer fabs use G7.5 (1950 mm × 2250 mm) and G7 (1870 mm × 2200 mm) glass, which followed the previous G6 (1500 mm × 1850 mm) substrates. The transition to G8 LCD glass (2160 mm × 2400 mm) will start in 2007. However, according to Corning, the top glass supplier, substrates of more than 3000 mm could face practical limits imposed by handling and transportation issues as well as fab- and operational-related height constraints.

Further cost control efforts involve reducing the price of materials and the number of process steps, simplifying the process, decreasing cycle time, and improving yields. For instance, implementation of the low-temperature polysilicon (LTPS) process (using laser anneal or other methods) would lead to the fabrication of transistors with ~100 times higher mobility than the conventional a-Si TFT. This would allow smaller TFTs and a higher aperture ratio. It would also enable the direct integration of LCD driver circuits onto the substrate and lower costs. In addition, this process would lead to higher luminance and contrast. LTPS is still an emerging process technology and requires serious attention to achieve good yields. Other process enhancements include reducing parasitic capacitance by using low-k dielectric materials and decreasing the resistance of the panel interconnect lines.

LCDs represent more than 80% of the FPD market, commanding a nearly 10× larger market than PDPs. Active-matrix LCD (AM-LCD), which is required for large-area displays, dominates the LCD segment. Here, each pixel is individually controlled through its own dedicated transistor switch. Otherwise, since the pixels are addressed in rows and columns, all pixels in the same row or column could be subject to a fraction of the applied voltage and change color or darken.

These transistor switches are typically fabricated on a deposited film of amorphous silicon using TFT technology. The cost of TFT-LCD panels has been steadily declining. In 1Q06, the price per diagonal inch of 15–20-in. LCD monitors ranged between $6 and $11—a significant drop from the $100 price point seen before 2001.

Over the course of this decade, TFT-LCD capacity should increase at least twentyfold. The industry has been going through a major ramp, with most LCD suppliers heavily investing in additional capacity. The new LCD fabs require investments of up to $3 billion, with recent and current spending on TFT-LCD manufacturing equipment estimated at between $9 billion and $10 billion.

LCD applications include notebook PCs, desktop PC monitors, LCD TVs, cell phones and other handheld devices, DVD players, and automotive displays, with the first five segments comprising more than 90% of the total market. LCD TVs, which account for a little over 10% of the market, are the next major growth area for TFT-LCD panels. Display Search projects that LCD TVs will constitute over one-third of the LCD market in five years, and iSuppli says that they will capture the largest share of the TV market by 2009. Displaybank believes that in five years, the price of 42-in. LCD TVs will drop below the magic $1000 mark.

OLEDs have very fast response time (~1000 times faster than LCDs). They are ~10 times thinner and ~5 times lighter than LCDs, and, unlike LCDs, they do not need backlighting. They also offer high contrast, excellent and wide viewing angle, low power consumption, high luminance, high luminous efficiency, and superior color. In addition, OLEDs can be made on various flexible substrates and fabricated using an ink-jet printing process. Furthermore, extensive efforts are under way to develop roll-print and other low-cost manufacturing methods. In many applications, OLEDs, and especially active-matrix OLEDs (AM-OLEDs), are seen as the ultimate replacement for LCDs.

However, OLEDs are still evolving, a process expected to take many years. Work is ongoing to improve yields, increase operating lifetimes to longer than 50,000 hours, resolve color-shifting issues, and reduce manufacturing costs. Since exposure to moisture can lead to the degradation of OLEDs, impervious panel materials (besides glass) are needed for flexible OLEDs to prevent this problem.

In some higher-priced systems with smaller displays, OLEDs have already started to displace LCDs. Applications include high-end cell phones, MP3 players, digital cameras, autos, and handheld devices, with volumes projected to exceed 100 million units by next year. In 2006, the OLED market could approach $1 billion and is projected to triple in five years. OLEDs will eventually be used in TVs, initially in sets of <14 in. While the feasibility of panel sizes up to 40 in. has been demonstrated, lower-cost TFT-LCD panels could delay the adoption of AM-OLEDs for larger displays.


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