02-05-2011, 09:43 AM
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Part1. LIQUID CRYSTAL DISPLAY CONSTRUCTION
Liquid crystal displays (LCDs) typically have three groups of assembly components: the cell,the module, and the monitor. The cell comprises the glass plates that contain the liquid crystalmaterial and the front and back polarizer filters. The module comprises the cell plus displaydrivers that control light and deliver host computer data to the cell, and a backlight assembly consisting of fluorescent lamps, light pipes, and associated diffusers and reflectors; all containedwithin a rigid sheet-metal structure. The monitor consists of the module plus an inverter topower the lamps, a display interface to the CPU, a plastic bezel and stand, and a power supply.
1.1 Liquid Crystal Chemistry
Liquid crystals are a set of complex organic compounds composed of elongated, rod-shapedmolecules that in their natural state are arranged in a loosely ordered fashion with their long axes parallel. They exist in many phases, with the most common being smectic (gel-like), nematic (most common for computer displays), and cholesteric (naturally rotating liquid crystal structures). There are hundreds of liquid crystal types from which to choose, depending on thephysical, electrical, and optical properties the user desires in a display. Typically, a flat panel display will contain a mixture of 10 or more of these compounds. Liquid crystal materials have two important features that make them useful in display applications: their molecules are “polar,” with one end being more electrically positive or negative than the other, much like a compass needle, to use a magnetic analogy; and they are able to conduct, bend, or twist rays of light along their axes depending on their orientation.
More on this property will be discussed in Part 2, Liquid Crystal Display Operation. Simply put, we use electronic devices to control liquid crystals to make them manipulate light. Figure 1shows the structure of a typical biphenyl type of liquid crystal molecule.
1.2 LCD Cell Construction
1.2.1 Substrates with Patterned Electrodes
An LCD cell is composed of two glass plates that are commonly coated with a very thin, metallic oxide layer known as indium tin oxide (ITO). Because the layer coating each glass substrate is so thin (only a few hundred angstroms), it is transparent; because it is made of an oxide of two metals, it is conductive.
Using conventional semiconductor photoimaging and etching techniques, these layers canbe patterned to form electrode structures. The electrodes may be patterned into 7-segmented numeric designs, as those commonly found in liquid crystal watches, or into a series of lines arranged along an x-y grid. In passive matrix-addressed cells, the two layers of ITO are patterned into tightly spaced parallel vertical traces on the front glass (columns) and horizontal traces on the back glass (rows). Figure 11 shows the construction of a 6 row x 7 column passive matrix cell, and its operation is described in section 2.2.1, Passive Matrix LCDs.
1.2.2 Molecular Alignment Layers
After patterning of the ITO layer, the surface of each glass plate is coated with an alignmentlayer, usually polyimide. That alignment layer is first baked and then polished or buffed to create microscopic parallel grooves on the surface of each plate. Although the grooves are all parallel, it is important to note that each plate has its grooves oriented in a different direction.
In subsequent processing, these grooves will cause the molecules of the liquid crystal material not only to “sheet” or wet the surfaces, but to line up parallel along the buffing direction as shown in Figure 2. What is not shown in Figure 2 is that the molecules do not lie exactly flat on the alignment layer but point up slightly from the surface at an angle of 2° to 5°. This “pretilt angle” is critical to the proper function of the display, but is also the cause of certain optical inconsistencies, as will be discussed in section 3.3.1, Viewing Angle. The plates are now ready for spacer application and assembly.
Figure 2.Molecular Alignment to a Buffed Surface. In their natural
state, liquid crystal molecules are arranged in a loosely ordered
fashion with their long axes parallel. The alignment layer surface
can be finely grooved by a polishing or buffing operation. When
liquid crystals are flowed onto this layer, their molecules line up
parallel along the grooves.
1.3 LCD Cell Assembly
During assembly, a sealing material is applied alongthe perimeter of one of the glass substrates, leaving a gap of a few millimeters at one corner, and then prebaked. Cell gap spacers, usually glass or plastic beads, are then applied by dry or wet spraying techniques. These beads are critical to the ultimate function of the display because they must maintain the spacing of the gap separating the two glass substrates at an optimum thickness of 4 to 5 microns, about 1/15 the thickness of a human hair. (In the Silicon Graphics 1600SW monitor display, the gap must be uniform across a 17.3-inch diagonal width!) The two glass plates are then oriented as shown in Figure 3 so that their respective buffing directions are at right angles to one another; they are then clamped and baked or exposed to ultraviolet radiation to set the sealant. This forms an empty package with an open port at one corner that is ready for the injection of the liquid crystal material.
Figure 3.Twisted-Nematic Alignment. A cell can be
constructed so that liquid crystals are sandwiched
between upper and lower plates with grooves pointing in
directions “a” and “b,” respectively. The molecules along
the upper plate point in direction “a,” and those along
the lower plate point in direction “b.” This forces the
liquid crystals into an overall 90° twisted-nematic state.
1.3.1 Filling and Sealing
Several cells are placed in a vacuum chamberin a fixture that suspends them on edge over a container of liquid crystal material. Air is exhausted from the chamber and the cells equilibrate to the surrounding vacuum through their fill ports. After the cells and the liquid crystal (LC) material in the reservoir have outgassed sufficiently, the plates are remotely lowered so that the fill ports are submerged. The LC material is injected by backfill pressure between the glass plates through the gap in the perimeter seal, which is then plugged with epoxy or more UV-cured adhesive. The filled and sealed cells are now ready for the addition of external optical elements and display drivers.
Part1. LIQUID CRYSTAL DISPLAY CONSTRUCTION
Liquid crystal displays (LCDs) typically have three groups of assembly components: the cell,the module, and the monitor. The cell comprises the glass plates that contain the liquid crystalmaterial and the front and back polarizer filters. The module comprises the cell plus displaydrivers that control light and deliver host computer data to the cell, and a backlight assembly consisting of fluorescent lamps, light pipes, and associated diffusers and reflectors; all containedwithin a rigid sheet-metal structure. The monitor consists of the module plus an inverter topower the lamps, a display interface to the CPU, a plastic bezel and stand, and a power supply.
1.1 Liquid Crystal Chemistry
Liquid crystals are a set of complex organic compounds composed of elongated, rod-shapedmolecules that in their natural state are arranged in a loosely ordered fashion with their long axes parallel. They exist in many phases, with the most common being smectic (gel-like), nematic (most common for computer displays), and cholesteric (naturally rotating liquid crystal structures). There are hundreds of liquid crystal types from which to choose, depending on thephysical, electrical, and optical properties the user desires in a display. Typically, a flat panel display will contain a mixture of 10 or more of these compounds. Liquid crystal materials have two important features that make them useful in display applications: their molecules are “polar,” with one end being more electrically positive or negative than the other, much like a compass needle, to use a magnetic analogy; and they are able to conduct, bend, or twist rays of light along their axes depending on their orientation.
More on this property will be discussed in Part 2, Liquid Crystal Display Operation. Simply put, we use electronic devices to control liquid crystals to make them manipulate light. Figure 1shows the structure of a typical biphenyl type of liquid crystal molecule.
1.2 LCD Cell Construction
1.2.1 Substrates with Patterned Electrodes
An LCD cell is composed of two glass plates that are commonly coated with a very thin, metallic oxide layer known as indium tin oxide (ITO). Because the layer coating each glass substrate is so thin (only a few hundred angstroms), it is transparent; because it is made of an oxide of two metals, it is conductive.
Using conventional semiconductor photoimaging and etching techniques, these layers canbe patterned to form electrode structures. The electrodes may be patterned into 7-segmented numeric designs, as those commonly found in liquid crystal watches, or into a series of lines arranged along an x-y grid. In passive matrix-addressed cells, the two layers of ITO are patterned into tightly spaced parallel vertical traces on the front glass (columns) and horizontal traces on the back glass (rows). Figure 11 shows the construction of a 6 row x 7 column passive matrix cell, and its operation is described in section 2.2.1, Passive Matrix LCDs.
1.2.2 Molecular Alignment Layers
After patterning of the ITO layer, the surface of each glass plate is coated with an alignmentlayer, usually polyimide. That alignment layer is first baked and then polished or buffed to create microscopic parallel grooves on the surface of each plate. Although the grooves are all parallel, it is important to note that each plate has its grooves oriented in a different direction.
In subsequent processing, these grooves will cause the molecules of the liquid crystal material not only to “sheet” or wet the surfaces, but to line up parallel along the buffing direction as shown in Figure 2. What is not shown in Figure 2 is that the molecules do not lie exactly flat on the alignment layer but point up slightly from the surface at an angle of 2° to 5°. This “pretilt angle” is critical to the proper function of the display, but is also the cause of certain optical inconsistencies, as will be discussed in section 3.3.1, Viewing Angle. The plates are now ready for spacer application and assembly.
Figure 2.Molecular Alignment to a Buffed Surface. In their natural
state, liquid crystal molecules are arranged in a loosely ordered
fashion with their long axes parallel. The alignment layer surface
can be finely grooved by a polishing or buffing operation. When
liquid crystals are flowed onto this layer, their molecules line up
parallel along the grooves.
1.3 LCD Cell Assembly
During assembly, a sealing material is applied alongthe perimeter of one of the glass substrates, leaving a gap of a few millimeters at one corner, and then prebaked. Cell gap spacers, usually glass or plastic beads, are then applied by dry or wet spraying techniques. These beads are critical to the ultimate function of the display because they must maintain the spacing of the gap separating the two glass substrates at an optimum thickness of 4 to 5 microns, about 1/15 the thickness of a human hair. (In the Silicon Graphics 1600SW monitor display, the gap must be uniform across a 17.3-inch diagonal width!) The two glass plates are then oriented as shown in Figure 3 so that their respective buffing directions are at right angles to one another; they are then clamped and baked or exposed to ultraviolet radiation to set the sealant. This forms an empty package with an open port at one corner that is ready for the injection of the liquid crystal material.
Figure 3.Twisted-Nematic Alignment. A cell can be
constructed so that liquid crystals are sandwiched
between upper and lower plates with grooves pointing in
directions “a” and “b,” respectively. The molecules along
the upper plate point in direction “a,” and those along
the lower plate point in direction “b.” This forces the
liquid crystals into an overall 90° twisted-nematic state.
1.3.1 Filling and Sealing
Several cells are placed in a vacuum chamberin a fixture that suspends them on edge over a container of liquid crystal material. Air is exhausted from the chamber and the cells equilibrate to the surrounding vacuum through their fill ports. After the cells and the liquid crystal (LC) material in the reservoir have outgassed sufficiently, the plates are remotely lowered so that the fill ports are submerged. The LC material is injected by backfill pressure between the glass plates through the gap in the perimeter seal, which is then plugged with epoxy or more UV-cured adhesive. The filled and sealed cells are now ready for the addition of external optical elements and display drivers.
