Higg-resolution and color liquid crystal displays. David H. Ahl.
The technology of liquid crystal displays (LCD) is just 13 years old. Yet in that short period of time the sales of LCD devices have surpassed the sales of all other display technology combined, save one, the venerable cathode ray tube (CRT). Now, however, with the advent of large area, high density LCDs and full color, it appears that even the CRT may be in danger of being overtaken by the LCD. What are the advances that have made this possible?
Initially, LCDs were small--use in watches was and is still the most common application--and resolution was relatively low. Early displays of the mid-and late 70's had fewer than 100 pixels (picture elements or dots) in an area of five square centimeters.
The basic technology for most LCDs is known as twisted-nematic (TN). Liquid crystals are sandwiched between polarizers which are placed at 90-degree angles to one another (see Figure 1). When the current is off, the crystals fall into a pattern of layers, twisting through 90 degrees from the bottom to the top layer, parallel to the plane of the display. In this orientation, light may be transmitted (actually reflected) through the crystals. To the viewer, this appears white or light gray. When the current is on, the crystals are forced perpendicular to the plane of the display, and the polarized light passes around the crystals and is absorbed by the top polarizers, this appears black (or dark).
Electrodes invisible to the eye are deposited on the two surfaces of the glass between which the liquid crystals reside (see Figure 2). One layer of glass contains the X lines, sometimes called the data lines; while the other surface contains the Y lines, sometimes called the scanning lines. This is the approach used in most LCDs as it is the lowest cost method of construction and permits a fairly large display to be constructed.
Unfortunately, the resolution of such a display is limited because as more electrode lines are etched into the glass, the contrast ratio and viewing angle worsens. Figure 3 shows a monochrome 200 x 640 pixel display--about the upper limit of this technology. This is equivalent in size and cost to about a 9" high-resolution monochrome CRT display. Displays such as this are found on the Data General/One and other recently announced systems. Active Matrix Displays
To overcome the resolution, contrast, and viewing angle limitations of standard TN technology, two methods are currently under development, both of which use active elements in the display itself. In these methods, there is one active element for each pixel in the display, and each element holds the data for its corresponding pixel.
One method is known as a Metal-Insulator-Metal (MIM) structure device (see Figure 4). A MIM element consists of thin tantalum-pentoxide layer (insulator) between two metal layers. This device has characteristics similar to a bidirectional diode. In other words, below a certain threshold voltage, the device acts as an insulator; above the threshold, current flows freely.
Since a MIM device has only two states (on and off), it is quite suitable for a monochrome computer display, but not for a television display which requires levels of gray. Since electrode lines are engraved in only one glass surface, MIM LCDs can produce high-contrast, high-resolution images that can be viewed at relatively wide angles.
Figure 5 shows an experimental MIM LCD developed by Epson and Suwa Seikosha. It has a 240 x 250 pixel matrix in an area of about 3.7" x 3.9".
The second active matrix LCD technology uses nearly invisible thin film transistors (TFT) deposited on one glass surface. Normally, transistors are fabricated on a single crystal wafer. However researchers developed a method for depositing them oin the glass substrate (see Figure 6).
When a TFT is turned on, a charge is stored in the TFT that will continue to drive the liquid crystal materials corresponding to that pixel until the next signal is received. Since the TFT acts as a memory device rather than a switching device or a diode, it can hold a charge to produce various levels of gray and is thus suitable for television use. Furthermore, there are no electrodes etched into the glass at all, thus reducing the limitations on resolution and viewing angle. On the other hand, the fabrication process is quite complicated, and the largest commercial TFT devices produced thus far are only about 2" square.
A further advantage to TFT technology is that it can be used to construct integrated circuits for the display driver and interface right on the same glass substrate, thus lowering the overall cost of the device. Color Graphics
Theoretically, any of the above LCD fabrication technologies could be used to produce a color display with the addition of red, green, and blue filters. However, because of its high resolution, gray scales, and wide viewing angle, the TFT technology is the most suitable.
Figure 7 shows the structure of a portion of a color display. Between the upper glass and the common electrode is an additional layer containing a checker-board pattern of colored filters, one for each pixel, alternating red, green, and blue. Unlike monochrome LCDs, which utilize reflected light, a color LCD must have a white light source behind the display.
The LCD layer acts as a shutter and transmits all, a portion, or none of the filtered light through each pixel.
This is the technique that has been used in the Epson Elf pocket color television (see Figures 8 and 9), the first commercial color LCD television. It has a resolution of 220 x 240 pixels (52,800 pixels) in an area 2" square.
An experimental color LCD 3.4" x 2.6" (4.25" diagonal) has been constructed with 230,400 pixels, more than four times the number on the television display (see title illustration). This display is only about 15% smaller than a 5" CRT; thus we can expect to see it available for both computer and television applications. Down the Road
In the near future, 400 x 640 monochrome LCDs will be widely available at prices rivaling those of CRTs. And within the next five years engineers expect to be able to produce very high-resolution monochrome LCDs (over 1000 x 1000 pixels) at reasonable cost.
Furthermore, it is likely that 200 x 640 color LCDs will be introduced within a few years. However, until production levels increase substantially, such color displays will cost considerably more than their CRT counterparts. On the other hand, if enough people want flat screen TVs for their homes, production levels could increase rapidly, and such displays could be commonplace on both television sets and computer displays by 1990.