production is more important than cost per unit area. The expectation is that display (die) sizes can be made smaller to reduce cost while still increasing performance functionality, as in VLSI experience.

Table 1. New Display Technologies

TECHNOLOGY

SIZE

Passive matrix LCD

Simple multiplex

Active addressing

Large/small

Active matrix LCD

Amorphous silicon

Polycrystalline silicon

Transfer silicon

Large/small

Thin film electro-luminescent displays

Passive matrix

Active matrix

Large

Small

Digital micromirror devices

Small

Plasma displays

Large

Field emission displays

Large

Most of the interest in flat displays is for direct view displays. Thus cost per unit of display area is critical for a practicable process. Typical display sizes are about 10 inches diagonally for video graphics array (VGA) displays with 480 × 640 resolution. The market desires larger, 15-inch diagonal, and higher-resolution, 768 × 1024 and 1024 × 1280, displays but as yet few applications can afford them because of high manufacturing costs.

The remainder of this section identifies, for each of the display technologies listed in Table 1, the current and potential future roles of plasma processing.

Passive Matrix Liquid Crystal Display

There are two basic types of LCD configurations: passive matrix and active matrix. Low-cost large and small LCDs are typically of the passive matrix type. Passive matrix LCDs consist simply of two transparent substrates having transparent electrodes. The substrates are separated by a gap of about 10 µm filled with the liquid crystal material. One substrate contains column electrodes, and the other contains the row electrodes. Picture elements (pixels) are defined by the intersection of these orthogonal electrodes. Today such LCDs are produced without any plasma processing. Potentially, plasma dry etching could replace the wet etching techniques currently used to define the electrode structures, but this is not currently pursued because of the higher cost of plasma processes. However, the environmental impact of liquid waste may eventually change the cost balance.

Active Matrix Liquid Crystal Display

The active matrix liquid crystal display (AMLCD) is the flat display technology of choice for high-performance applications—both large-area, direct-view displays and small-area, projection displays. In active matrix LCDs, one substrate contains the active matrix and the other acts as a ground plane and is electrically featureless. The active matrix consists of an array of isolated pixels connected to a matrix addressing structure via thin film, field-effect transistors. Gates of these thin film transistors (TFTs) are organized as the matrix rows, and the sources are organized as the matrix columns.

Although the geometry of these TFTs is large compared with current VLSI chips, these 2- to 10-mm geometries do warrant precision processing. Plasma dry etching techniques similar to those used in the semiconductor industry are commonly used where wet chemical etching does not suffice. An R&D opportunity exists for increasing the etch rates and uniformity of large-area etching processes.

Silicon is the material from which these TFTs are fabricated. Two silicon forms are typically used: hydrogenated amorphous silicon (aSi) and high-temperature polycrystalline silicon (pSi). For special applications, transfer crystalline silicon (xSi) is also being developed.



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