Fundamental Liquid Crystal Display Technology: an introduction for the basic understanding of LCDs
Date: August 18, 1993
Copyright (c) 1992-93 by Scott M. Bruck
Copyright Notice: This document is copyright by Scott M. Bruck. It may be distributed freely electronically in its complete form including references and copyright notices. This document may not be included in any publication without written permission from the author.
Please address any questions or comments to the EMAIL address listed below:
Written By Scott M. Bruck <firstname.lastname@example.org>
Matsushita Electric Industrial Co., Ltd.
Liquid Crystal Display Development Center
Development Group #1
Moriguchi-Shi, Osaka 570 JAPAN
In no way does this document represent the views or policies of Matsushita Electric Industrial Co., Ltd.
Every effort was made to insure the validity of this document.
Since the introduction, rapid decline in price, and increased availability of notebook computers capable of operating Graphical User Interfaces (GUIs--MacOS and Windows), there has been an increased interest in flat panel display technology. A notebook/palmtop computer requires a light weight, durable, and reliable display. Liquid Crystal Display technology has met these requirements and as a result, virtually all notebook computers are equipped with some form of LCD. This FAQ is intended to address the general confusion concerning LCDs that has arisen recently by explaining the technology, operation, and characteristics of this important display device.
Liquid Crystal Displays (LCDs) are categorized as nonemissive display devices, in that respect, they do not produce any form of light like a Cathode Ray Tube (CRT). LCDs either pass or block light that is reflected from an external light source or provided by a back/side lighting system. There are two modes of operation for LCDs during the absence of an electric field (applied Power); a mode describes the transmittance state of the liquid crystal elements. Normal White mode: the display is white or clear and allows light to pass through and Normal Black Mode: the display is dark and all light is diffused. Virtually all displays in production for PC/Workstation use are normal white mode to optimize contrast and speed.
A simplified description of how a dot matrix LCD display works is as follows: A twisted nematic (TN) LC display consists of two polarizers, two pieces of glass, some form of switching element or electrode to define pixels, and driver Integrated Circuits (ICs) to address the rows and columns of pixels. To define a pixel (or subpixel element for a color display), a rectangle is constructed out of Indium Tin Oxide -- a semi-transparent metal oxide (ITO) and charge is applied to this area in order to change the orientation of the LC material ( change from a white pixel to a dark pixel). The method utilized to form a pixel in passive and active matrix displays differs and will be described in later sections. Figure 1 illustrates a cross sectional view of a simple TN LC display. Figure 2 depicts a dot matrix display as viewed without its metal module/case exposing the IC drivers.
Looking directly at the display the gate or row drivers are located either on the left or the right side of the display while the data or column drivers are located on the top (and or bottom) of the display. New thin display module technology mounts the ICs on conductive tape that allows them to be folded behind the display further reducing the size of the finished module. An IC will address a number of rows or columns, not just 1 as pictured in figure 2.
Figure 1: Cross Section of a Simple LC Display
viewer ///////////////////////////////////// Polarizer _____________________________________ glass ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Liquid Crystal _____________________________________ glass \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ Polarizer backlight
Figure 2: LCD panel and IC driver locations
________________________________________ | | | IC IC | Source/Column ICs | | | | | | |IC---------------------Pixel | | | |IC <---- Gate Line/Row IC | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* An IC driver will address a number of row/column lines and not just the single pixel in the diagram above
Polarizers are an integral part of a LCD display, possessing the unique property of only passing light if it is oriented in a specific (oriented) direction. To utilize this phenomena in TN LC displays, the bottom polarizer orients incoming light in one direction. The oriented light passes through the LC material and is either unaltered or "bent" 90 degrees. Depending on the orientation of the top polarizer, this light will either pass through or be diffused. If the light is diffused, it will appear as a dark area. Figure 3 is a simple illustration of the sequence of events that occur when light passes through a simple twisted nematic LC display.
Figure 3: Polarized Light and its use in a TN LC display
Light (unoriented) will be defined as: !#$%&|- Polarizer Orientation is defined by: ( $ or # ) ($ polarizer will only pass $ light) (# polarizer will only pass # light) THEREFORE: Light Polarizer result LC (90 result Polarizer Image Input type passed degree passed type output twist) !##$%%&|-> | # | -> ## -> ~~~~~ -> $$$$ ->| # | ------> Black !##$%%&|-> | # | -> ## -> ~~~~~ -> $$$$ ->| $ | ------> White
Please note, I am not a chemist, so I will keep this section as simple and concise as possible. Liquid crystals encompass a broad group of materials that posses the properties of both a solid and a liquid. More specifically, they are a liquid with molecules oriented in one common direction (having a long range and repeating pattern-- definition of a crystal), but have no long range order in the other two directions. For example, in figure 4 all the lines are oriented in the Y direction (up and down), but they posses no common ordering in the x direction (disorder is assumed in the Z direction). To more easily visualize this, think of figure 4 as one thin slice (one layer of molecules to be exact) of a block of material. If you examined another slice, the molecules would still be oriented in the Y direction, but they would be in different positions along the X-axis. By stacking millions of these thin slices, the Z direction is built up and as a result of the change in relative position on the x-axis, the Z direction has no long range order.
^ Y | Figure 4 | | | | |||| ||| || | ||||| | | |||||||| ||||||||||||| |||| | | | |||||| ||||| |||| |||| | ||||||| | |------------------------------------------------------> X
* The Z direction is coming out of the page toward the reader
The liquid crystals used for display technology are thermotropic liquid crystals; they exhibit desired characteristics over a specific temperature range. This is the primary reason why LCDs do not operate properly when they are too cold or too warm. If liquid crystals are too cold, they will not twist and the display will not form an image. If the display is too warm, the resistance of the liquid crystal material changes and this alters the properties of the display and performance suffers. Liquid crystal material for display use is normally referred to as TN (STN, DSTN, MSTN, and etc.) or Twisted Nematic--sometimes known as TNFE or Twisted Nematic Field Effect. It is called TWISTED since the crystals are twisted 90 degrees (or more for STN) from the top piece of glass to the bottom piece of glass. (TN usually refers only to a 90 degree twist.) Field Effect (a direct correlation is the semiconductor MOSFET), refers to the LC material's ability to align parallel or perpendicular to an applied electric field. As a result, using twisted or untwisted liquid crystal and two polarizers; an applied electric field can force the LC material into a particular alignment effectively diffusing or passing light through the top polarizer.
As a note of interest, polarizers are also one of the major reasons that LC displays require bright back lighting. The polarizers and liquid crystal materials absorb more than 50% of the incident light. As a result, even though the actual display is a very low power device, the power hungry back lighting makes a LCD module one of the primary causes of short battery life in notebook computers. Due to the fact that the LC material has optical properties and effectively bends light, the problem of viewing angle effects occur. When the user is not directly in front of the display the image can disappear or seem to invert (dark images become light and light images become dark). However, LC material and polarizer technology is rapidly improving and that improvement is showing up in brighter displays with greater viewing angles.
Liquid crystals must be aligned to the top and bottom pieces of glass in order to obtain the desired twist. In other words, the 90 degree twist is formed by anchoring the liquid crystal on one glass plate and forcing it to twist across the cell gap (the distance between the two glass plates) when contacting the second plate. Furthermore, The actual image quality of the display will be dependent on the surface alignment of the LC material. The method currently used for aligning liquid crystals was developed by the Dai-Nippon Screening (English= Big Japan Screening) Company. The process consists of coating the top and bottom sheets of glass with a Polyimide based film. The top piece of glass is coated and rubbed in a particular orientation; the bottom panel/polyimide is rubbed perpendicular (90 degrees for TN displays) with respect to the top panel. It was discovered that by rubbing the polyimide with a cloth, nanometer (1 X 10 - 9 meters) size grooves are formed and the liquid crystals align with the direction of the grooves. It is common that when assembling a TN LC cell, it will be necessary to eliminate patches of nonuniform areas. The two parameters required to eliminate the nonuniformities and complete the TN LC display are pretilt angle and cholesteric impurities. TN LC cells commonly have two problems that affect uniformity following assembly: reverse tilt and reverse twist. Reverse tilt is a function of the applied electrical field and reverse twist is common when no electrical field is applied. Reverse twist is eliminated by the introduction of cholesteric additives and reverse tilt is eliminated by introducing a pre-tilt angle to the LC material. The pre-tilt angle also determines what direction the LC molecules will rotate when an electrical field is applied. Pre-tilt angle can be visualized by considering the normal position of the LC molecule to be flat against the glass plate, by anchoring one edge and forcing the other upward by a specific number of degrees, a pretilt angle is established.
Before discussing the different types of LC displays the topic of Birefringence must be explained. When a light ray strikes a crystal ( or crystal-like material), it will be split into two separate light beams; with one beam perpendicular (offset by 90 degrees) from the other. Since the beams travel different paths, they reach the viewer's eyes at slightly different times. This is an essential point, it may cause the color or polarity of the display to change when viewed at angles where the viewer may see both rays.
For active matrix displays, in order to maximize contrast and gray scale reproducibility, Twisted Nematic (TN) is utilized. This material is twisted 90 degrees from the top to bottom glass panels. STN or Super Twisted Nematic is chemically distinct from TN and the twist angle is usually greater than 200 degrees. Furthermore, due to the large twist angle, the actual alignment of the polarizers for STN LCDs are not perpendicular, but adjusted to find the best direction (rotation) for optimum display characteristics. The STN material is rotated in a way so the change from transmission to dispersion is very abrupt and therefore can respond quickly to small changes in voltage. Figure 5 illustrates the response characteristics of a TN curve and Figure 6 shows the response characteristics of a STN curve which will further clarify these points.
100% | Figure 5 | Typical Response of a Normal White TN Display T | R |************ A | * N | * S |<-- Zone I-> * M | * I | * T | * T | * A | * N | <----Zone II--- * ---------> C | * E | * <--Zone III--> | * | * | ************* 0% |________________________________________________________> Vt (Threshold Voltage) Applied Voltage