Computer display technologies are developing at a dramatic rate. Until recently, cathode ray tubes dominated the industry, but flat-panel displays are becoming increasingly more cost effective and thus more common in the home and the workplace. Fueling increased demand for such displays are their characteristically low weight, compactness, and clear, bright screens.
Indeed, it is estimated that flat-panel displays will capture about 50% of the display market by the year 2004, with dollar shipments approaching $26 billion.1 Active-matrix liquid-crystal displays (LCDs), with their high resolution and reliability, are expected to account for 62% of the flat-panel-display market. This should boost demand for the ultraviolet (UV) excimer-laser technology used to produce low-temperature polysilicon thin-film transistors, which are key display components.
How liquid-crystal displays work
The principle of LCDs is based on the properties of liquid crystals, which are transparent organic polymers that can orient polarized light according to the orientation of the liquid crystal molecules. Apply an electric field, and these molecules rotate locally.
In an LCD, a back light at the rear of the display illuminates the viewing face through the liquid crystal and polarizing films on either side of the crystal. Applying a voltage to the crystals causes them to twist. This, in turn, rotates the polarization of light passing through the crystals. Thus the back light can pass through the panel only when a voltage is on or off-depending on the polarization of the polarizing films.
FIGURE 1. In this active-matrix LCD, each pixel is driven by a thin-film transistor. (Courtesy Labor fur Bildschirmtechnik, University of Stuttgart)
Individual cells (or pixels) of a display can be addressed-that is, to apply a local electric field to the liquid crystal-by either the passive-matrix or active-matrix LCD technique. With passive addressing, transparent electrodes are patterned in lines on each of the two plates enclosing the liquid crystal. In such a configuration, the electrodes are oriented 90° from one another. The voltage pulses transmitted along these row-and-column electrodes combine at the crosspoint, and the pixels are addressed. Because of the complexity of the addressing scheme and the response time of the liquid crystals, such LCDs are too slow for use in displays larger than 10-in. diagonal.
More promising is the active-matrix LCD, where each pixel includes a metallic electrode driven by a thin-film transistor (see Fig. 1). This type of LCD works in applications that require display sizes ranging from 1 to 5 in. for video cameras and projection light valves to 10 to 15 in. for notebook computers and up to 25 in. for TV screens. In such a system, the front panel is not patterned and acts as a ground electrode. The rear substrate is deposited with a matrix of thin-film transistors and interconnect lines. This way of addressing the pixels simplifies the display electronics.
Building active-matrix displays
Most thin-film transistors for active-matrix LCDs are still built by depositing amorphous silicon (a-Si) on glass substrates. However, because of its higher electron and hole mobilities, polycrystalline silicon (p-Si) is beginning to challenge a-Si. Indeed, the smaller size of p-Si thin-film transistors makes them suitable for high-definition-display applications. The enhancement of the pixel aperture ratio produced with p-Si also allows brighter displays with lower power consumption.
The most significant advantage, though, is the integration of driver circuitry, which accounts for 5% to 30% of the total panel cost and thus influences overall display cost dramatically. The performance of p-Si thin-film transistors allows placing integrating driver circuits at the outside edge of the display, reducing the number of interconnects and increasing the reliability. For example, Toshiba (Tokyo, Japan) recently reduced the number of internal connections on a 12.1-in. extended graphic array display by 95%. In the near future, this trend is expected to lead to the use of so-called system-on-panel technology.
Extended graphic array displays (1024 x 768 pixels) have been available since 1996. This year, the ongoing demand for higher resolution will drive display manufacturers to bring out superextended graphic array displays (1280 x 1024 pixels). By next year, ultraextended graphic array (1600 x 1200 pixels) display resolution should be available. For such ranges of resolutions, the layout of p-Si thin-film transistors is all the more desirable as their price is expected to fall drastically within the two next years.
Annealing amorphous silicon for TFTs
A thin-film transistor (TFT) consists of layers of insulators and metals deposited on a quartz substrate and patterned to yield the drain/source/gate structure. The thermal treatment that turns the a-Si into p-Si takes place just after the deposition of an a-Si layer typically 40 to 100 nm thick.
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Such a treatment can be performed by solid-phase crystallization. The a-Si layer is processed in a high-temperature environment. The resulting p-Si is called high-temperature polysilicon. However, the cost of the quartz substrates limits displays to smaller than 4-in. diagonal for video-camera or projection light valves.
Nowadays, excimer UV lasers appear to be the only means to anneal a thin silicon film to turn it into polysilicon while still keeping the substrate at ambient temperature, thus enabling the use of a low-cost glass substrate. Indeed, the 308-nm output of a xenon chloride (XeCl) excimer laser is absorbed by a-Si within a few nanometers (typically 10 nm). The radiation is then turned into heat, leading to the melting of the a-Si. The incident energy density can easily be adjusted to ensure a complete melt of the film without damaging the underlying layers. During cooling, the silicon solidifies into the desired microcrystalline phase (see Fig. 2).
There is currently a fairly high level of understanding of laser annealing of Si deposited on glass, based on 20 years of investigation of the process. One technique now considered mature for industrial production applications is the low-temperature polysilicon process, and excimer laser annealing of a-Si is widely acknowledged as the preferred way toward manufacturing low-cost, reliable, and high-resolution active-matrix LCDs.
SAELC annealing technique
SOPRA manufactures the SAELC (single-area excimer laser crystallization) annealing machine, which includes a VEL (very large excimer laser) xenon chloride laser with a wavelength of 308 nm, coupled with a motion stage and high-performance optical setup. The standard workstation unit holds Si-coated glass substrates up to 680 x 880 mm or larger.
The laser source delivers up to 15 J/pulse onto the panel to be treated, at a repetition rate of 1 Hz. Enough energy is generated to treat an area 25 cm2 at 600 mJ/cm2 per pulse. Panels can be treated with only a few laser shots, which enhances the lifetime of critical parts of the laser unit and helps lower the cost of ownership.
Both the energy distribution within the laser spot and the shot-to-shot stability of the laser impact the uniformity of the annealing process of a Si-coated panel. The laser light, focused onto the panels through a "fly´s eye" type beam homogenizer, yields a flat energy distribution over a large area, with a typical uniformity over the spot of better than 2.5%.
The system´s long pulse duration (200 ns) also promotes a low cooling rate of the melted silicon, which facilitates the growth of large grains. Grain size is closely related to the field-effect mobility of the thin-film transistors (mobility describes how strongly an applied electric field will influence the motion of an electron). Optimizing both the grain size and the uniformity of the grains depends on random nucleation. Large, homogeneous grains are ob tained over a wide area with only a few laser irradiations without requiring any additional processing or devices such as a heating and vacuum chamber.
Since 1995, various LCD manufacturers have made polysilicon thin-film transistors with the SAELC annealing technique. Among them are ´top-gate´ type transistors, which were obtained by annealing an a-Si layer about 50 to 80 nm thick. The n-mobility of these devices kept increasing over the years, as the overall process efficiency was adjusted to produce the TFT structure on the substrate. The treatment in air and at room temperature currently leads to TFTs with mobility of 200 cm2/Vs.
Future market projections
Due to last year's Asian crisis, lower than expected market penetration of polysilicon thin-film-transistor displays, and reduced demand for notebook computers, LCD manufacturers postponed investments in low-temperature polysilicon production. Most, however, continued development efforts to improve the low-temperature polysilicon process. Indeed, while originally dedicated to 4- to 8-in. displays, the process is now gaining ground in manufacturing displays up to 20 in.
In 1996, a Japanese LCD manufacturer began fabrication of a line of 4-in. displays, followed by 8-in. models. This year, two other major Japanese manufacturers began producing 8-in. and 13-in. active-matrix LCDs, respectively, with low-temperature polysilicon production. These developments were probably fueled by anticipation that LCD demand could exceed the world production capacity sometime this year.2
The potential market is becoming real, and the excimer laser annealing technique is perhaps the most promising process to allow manufacturers to comply with both the technical and financial requirements and the demand for active-matrix LCDs based on polysilicon technology in the coming years.
REFERENCES
- OLE 58, 52 (Jan. 1999).
- Display Technology Report 41(8), 1 (Aug. 1998).