Twisted Nematic (TN) LCDs
The basic twisted nematic LCD consists of a layer of liquid crystal material supported by two glass plates. The liquid crystal material is a mixture of long, cylindrically shaped molecules with different electrical and optical properties, depending on direction.
On the inner surfaces of the glass plates are transparent electrodes, which are patterned to form the desired visual image. The inner surfaces are coated with a polymer, which is rubbed so that the liquid crystal material at one surface lies perpendicular to the other. Across the film of liquid crystal, the molecules form a 90° twist.
On the outer surface of the glass plates, polarizers are placed so they are parallel to the liquid crystal orientation and perpendicular to each other. In the "off" state, light entering the first polarizer is guided by the liquid crystal layer twist to the second polarizer, through which it is transmitted. When the cell is energized, the LC material is aligned with the electric field; light transmitted through the first polarizer is blocked by the second polarizer, forming a dark image. The effect may be reversed if the polarizers are placed parallel to each other, and a light image on a dark background is formed.
Supertwist (STN) LCDs
Although twisted nematic LCDs may be driven in a time multiplexed fashion to increase the amount of information displayed, they are restricted in terms of reduced contrast and limited viewing angle. To achieve more highly multiplexed displays, supertwist technology is employed.
In this type of display, the LC material undergoes a greater than 90° twist from plate to plate; typical values range from 180 to 270°. The polarizers in this case are not mounted parallel to the LC at the surface but rather at some angle. The cell, therefore, does not work on a light "guiding" principle, as in twisted nematic LCDs, but instead on a birefringence principle. The position of the polarizers, the cell thickness, and the birefringence of the LC are carefully chosen to result in a particular color in the "off" state. Usually, this is a yellow-green to maximize the contrast ratio. The LC in the cell is "supertwisted" to give high multiplexibility. As the twist is increased, the LC molecules in the middle of the layer are aligned with the applied electric field by smaller changes in voltage. This gives rise to a very steep transmission vs. voltage curve, allowing up to 240-line multiplexing.
LCD Drive Considerations
Static Drive
A static drive (non-multiplexed) TN LCD has segments which can be modeled as capacitive elements with parallel and series resistive elements as shown in Fig. 1. The optical effect or contrast ratio produced by a segment depends on the RMS voltage applied across its capacitive element. The DC component of the drive signal must be minimized in order to prevent damage to the LCD. The most commonly used drive technique uses an oscillator which produces a square wave with 50% duty cycle. This signal is applied to the LCD backplane or common electrode, and to one input of each of the EX-OR gates.
When the other input of the gate is at logic ground, the gate output is identical to the backplane signal and the RMS voltage across the segment is zero, as shown in Fig. 2. When the input of the gate is high, the gate output is 180° out of phase from the backplane signal, and the RMS voltage across the segments is equal to the logic supply voltage. Note that it is important to use CMOS devices for the oscillator output and EX-OR gates, since CMOS outputs "swing very close to the rails", assuring that the DC component seen by the LCD is less than 100 mV.
The backplane should have a frequency of at least 30 Hz to prevent display flicker, and no more than 100 Hz to prevent ghosting. Ghosting is partial activation of segments which are being driven off.
This partial activation is a result of internal coupling due to isolation of the capacitive elements from the drive signals by the crossover spreading resistances. In general, as drive frequency and voltage increases, ghosting is more likely to occur. Ghosting problems are avoided by proper display design and choice of driving conditions. It is important that all unused segment lines be connected to the backplane line.
Multiplex Drive
A multiplexed LCD is partitioned as a matrix consisting of M rows and N columns. At each matrix location, an active element is formed, which is electrically equivalent to a lossy, non-linear capacitor. An example is shown in Fig. 3.
For example, a 6 digit, 7 segment display utilizing 3 backplanes would have the 42 segments arranged on a 3 x 18 matrix -- decimal points, symbols, or annunciators could be positioned on the 12 unused matrix locations. Matrix partitioning of display elements is also applicable to segmented alphanumeric, dot matrix, and bargraph displays. Amplitude varying, time synchronized waveforms are generated by the LCD driver and applied to the row and column lines of the display matrix (refer to the driver data sheets for detailed explanation of waveforms).
Segment contrast is a function of the RMS value of the (Fig. 4) backplane minus frontplane waveforms at that matrix location. Waveforms can be generated such that, at any point in the matrix, the resultant RMS voltage is either above the saturation voltage, or below the visual threshold voltage.
When viewed off-normal, the display will exhibit variations in the contrast vs. drive voltage characteristic. Von/Voff should be adjusted in order to optimize the viewing characteristics of the display for the LC material used and the viewing direction requirements of the application. Due to the correlation properties of the driver waveforms, the ratio of Von and Voff approaches unity as the multiplex order increases (number of backplanes increase).
The maximum acceptable multiplex order depends on the sharpness of the contrast vs. voltage curve, required operating temperature range, and the viewing cone requirements of the particular application.
As temperature increases, the contrast vs. drive voltage curve shifts to the left. In order to maintain optimum contrast over wide temperature variations, the drive level should be varied in order to compensate for this negative threshold temperature coefficient (dv/dt). (See Fig. 5.) A series combination of silicon diodes, each with a temperature coefficient of -2mV/°C can be used for compensation over a temperature range of about 0 to 50°C. Beyond this range, the display dv/dt curve is nonlinear and fixed compensation may result in less than optimum viewing characteristics.
Facility and Capabilities
Planar Systems has 60,000 square feet of facilities dedicated to the research, engineering, manufacturing, sales and service of its Liquid Crystal Display products.
Production quantities from 100 to 100,000 pieces, at competitive pricing, from a stable, growth oriented company that is totally committed to manufacturing excellence of standard and custom LCDs and modules. Planar has the capacity to produce large or small quantities of LCDs, satisfying orders ranging from 100 to 100,000 pieces. Many of the LCDs and LCD modules we procude are for custom applications with short lead-time requirements.
LCD Environmental Testing
All Planar LCD manufacturing operations are performed in a vertically integrated facility to assure total control of quality and customer delivery requirements.
The first, and perhaps most critical process in the manufacture of a high quality liquid crystal display is development of extremely accurate display artwork. Artwork for all Planar LCDs is developed on our computer-aided design system and the photographic masks are laser imaged for maximum possible accuracy. This leading edge process provides prompt reaction to custom designs and assures that we have the tight tolerance film masks that are critical for consistently superior LCDs.
Planar LCD production operations are performed in a cleanroom environment by skilled, highly trained personnel, supported by the most modern equipment and systems available.
The key to manufacturing high quality liquid crystal displays quickly and right the first time is to have absolute control of all processes employed. Processes are governed by our statistical process control system, from incoming materials to final inspection before shipment.
Our statistical control processes employ charting techniques such as X,R to monitor process parameters and products. P Charts are used extensively outside the manufacturing areas and for problem solving. All equipment is calibrated to MIL STD 45662 to ensure proper control of all ongoing processes. Process capability studies are carried out on a continuing basis for all operations and equipment to assure consistent quality.
All new equipment purchases are statistically verified to assure that the equipment will perform to, or exceed, specified quality standards.
Ongoing reliability tests are run to monitor the results of process changes as well as to supply data on all current processes and materials. Parts in process are selected to go through environmental tests which last 504 hours with randomly selected parts run to end of life.
