Liquid Crystal Display (FAQ) Frequently Asked Question Fundamental Liquid Crystal Display Technology: a multi-part introduction for the basic understanding of Liquid Crystal Displays Version 1.00 August 18, 1993 By Scott M. Bruck This FAQ should be printed in 9 POINT MONACO FONT 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. Part I. Liquid Crystal Display Fundamentals: 1) A general discussion of how liquid crystal displays work. 2) A basic introduction to the chemistry, structure, and properties of liquid crystals used in displays. 3) An overview of display structure, assembly, and related technology is summarized. Part II. Addressing Technology: Passive and Active Matrix Displays: 1) The differences between Active and Passive matrix display addressing techniques is examined 2) Color technology applicable to liquid crystal displays is discussed. Part III: New Technology and Questions Answered: 1) State of the art displays being prototyped are described. 2) A list of TFT display manufacturers is summarized and a list of what notebook computers use what TFT display. 3) Solutions to questions proposed on Internet news groups not covered in the body of the LCD FAQ text. LCD FAQ Part I: Liquid Crystal Display Fundamentals 1.0 General Characteristics and LCD Modes Liquid Crystal Displays (LCDs) are categorized as non- emissive 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. 1.1 LCD Cell Switching and Fundamentals 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 1.2 Liquid Crystal Material 1.21 Fundamentals 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. 1.22 Liquid Crystal Alignment 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 non- uniform 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. 1.23 Liquid Crystal Display Names and classes 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 1.24 The TN Liquid Crystal Response Curve The most prominent feature of the TN response curve is the central linear region between the two flat areas (Zone II). Zone I describes the white color of the display when no electric field is applied. In other words, the display will transmit virtually all the introduced light. On the other hand, in Zone III, the display will diffuse light and appear dark. The middle region can display gray scale or an image somewhere between White and Black. The key point here is that you must be able to very carefully control the voltage applied to the LC cell and maintain it for one duty cycle (before that pixel is addressed again) in order to produce accurate colors. For this reason, this type of LC material is primarily used for active matrix LCDs. COMMONLY ASKED QUESTION NOTE: because the LC material is partially twisted in the gray scale area, when looking at a display at an off angle the colors tend to shift and sometimes invert due to birefringence. COMPUTER APPLICATION NOTE: The TN response curve does not have to be utilized for gray scale, in order to make a simpler display, improve viewing angle, and use cheaper IC drivers; the Apple Powerbook 170's TFTs (thin film transistors) drive the TN response curve directly into region 3. This gives all the speed/contrast advantages of a TFT display and cheaper manufacturing cost, but provides no gray scale. 1.25 The STN Liquid Crystal Response Curve The Key to understanding the STN curve is simply that due to the addressing method applied, only a small amount of voltage is available to change the LC material from transmittance to a dispersion state. For this reason, the shape of the curve has nearly a 90 degree shift between Zone I and Zone II regions; in other words, it goes ballistic and nearly straight up ! This property allows the LC material to shift from white to black at its threshold voltage (VT) without being concerned with partial transmission (gray scale). Furthermore, the 90 Degree curve shape means that gray scale is not available from the LC material itself and the driving circuits must provide the necessary fixes for levels of gray. STN displays inherently have a yellow on blue appearance (anyone remember the old Zenith Laptops ?). Because many individuals found the yellow and blue appearance undesirable, a number of techniques were developed to convert the STN image to a black on white scheme. DSTN, developed by Sharp Corporation, was the first commercial black and white conversion of the STN display and refers to Double Super Twisted Nematic. DSTN displays are actually two distinct STN filled glass cells glued together. The first is a LCD display as described previously, the second is a glass cell without electrodes or polarizers filled with LC material for use as a compensator which increases contrast and gives the black on white appearance. The drawbacks are a heavier module, a more expensive manufacturing process, and a more powerful backlighting system. FCSTN is Film Compensated STN and is now the most commonly used STN display technology on the market. FSTN, monochrome STN, and Polymer film STN are all standard STN displays with a polymer film applied to the glass as a compensation layer instead of the second cell as in the case of the DSTN. This simpler and more importantly cost effective method provides the preferred black on white image for this display technology. However, once again, this design lowers the transmittance of light and requires a more powerful back lighting system. COMMONLY ASKED QUESTION NOTE: Why are STN displays slow ? Due to the method used to address passive matrix (STN/DSTN) displays and the high density of pixels required for standard VGA displays, the liquid crystal material must respond to an extremely small change in voltage. In developing these materials for this voltage characteristic, there was a reduction in the switching speed. A slow display can best be illustrated by the tendency of the cursor to "submerge" or disappear when rapidly moved across the screen. Another common example is the blurring of images when they quickly move across the display as in the case of high speed games. A fast display is less than 40 milliseconds, most STN type displays are between 200 and 250 milliseconds. However, some new LC mixtures are reaching 150 millisecond speeds. COMMONLY ASKED QUESTION NOTE: What is Contrast ? Contrast is defined as the ratio of black to white, more simply put, how black is black when next to a white or clear pixel. In terms of numbers, passive matrix LCDs are usually able to produce a contrast ratio of approximately 13 - 20:1; in real terms you get a set of different grays and blues but no true blacks. 100% ^ | Figure 6 T | R | ******** A | * N | * S |<-Zone I----> *<-----------Zone II---------------------> M | * I | * T | * T | * A | * N | * C | * E | * | * | * | * | * | ************************** 0% -------------------------------------------------------> Vt Threshold Voltage Applied Voltage 1.3 Liquid Crystal Display Assembly Once the switching devices or electrodes have been fabricated on the glass halves and the polyimide film has been applied & rubbed, spacer balls (usually 4 to 8 micrometers [1 X 10 - 6 meters] in diameter) are sprayed on one half of the display. Spacer balls are used to insure that the glass plates remain a certain distance apart over the entire area of the display; this is also known as cell gap. If the cell gap is not uniform, an image will appear different from one end of the display to the other. If the spacer balls are not applied correctly, they will collect and the user will be able see them as strange areas of non-uniform dust or distortion. (Single spacer balls are too small to see and they are not black dots.) If the Display has a very large cell gap, when you apply slight pressure to the display by touching it with your finger, you will see the image change and the LC material shift under the glass. Doing this does not damage the display, but take care when bringing any sharp objects, such as pen or pencils, near the screen; it is very easy to damage the polymer film and or polarizers on the display. The two glass panels are then aligned and glued together with an epoxy. During panel assembly, if dirt is trapped between the two glass plates, you most likely will see these as annoying spots on the display. During the application of the glue, one corner is left open. In a vacuum chamber, the liquid crystal material is drawn into the display through the open corner. Upon completion, the remaining hole is filled with another epoxy. The LC material will align itself to the grooves in the polyimide and spread out around the spacer balls. After final assembly, excess glass is cut and driver ICs are mounted. The finished display is mounted onto a backlight assembly (also known as an inverter assembly) and encased in metal. There are a number of methods for backlighting a LC display. STN displays usually have a side, top, or bottom lighting system. In simple terms, this is where the fluorescent tube is mounted. For example, in a side-lit display one or two fluorescent tubes will be located at the left and or right edges of the display. A fluorescent tube normally 4 mm in diameter is used. This is dispersed by a plastic plate around the entire area of the display. A dispersion plate looks like a white sheet with small holes; each of the holes provides a small point of light. On top of the dispersion plate, a diffuser is placed. A diffuser takes the numerous points of light and uniformly spreads it out over the entire area of the display. The net effect is providing a backlighting source around 4 or 5 mm thick ! An Active matrix display, especially color modules, transmit much less of the incident light and require more elaborate backlighting systems. An active matrix TFT display has a matrix fabricated on one piece of glass; the metal lines and transistor elements are not transparent and block a significant percentage of light. In order to obtain higher contrast, newer displays incorporate what is called a black matrix. This is a black film that surrounds the pixel elements (this can be on the matrix, but is usually around the color filters); although this yields higher contrast, it also reduces brightness. Further complicating this, the polarizers and the color filters reduce the output to less than 5% of the incident light. As a result, most backlighting systems designed for active matrix based displays usually consist of 4 or 5 four mm tubes placed directly behind the display with a diffuser plate to insure uniform irradiation. Therefore, they are called backlit. This method of lighting makes the display slightly larger, heavier, and greatly increases power consumption. The final metal encased display is called a display module or sub-assembly and this is what the end user or notebook manufacturer receives. References: Solid State Technology, December 1988, page 65 Amorphous Silicon Technology, Chapter 3 page 77 High-resolution panels target laptop computers, EDN, April 23, 1992 A. Miyaji, M. Yamaguchi, A. Toda, H. Mada, and S. Kobayashi, Control and Elimination of Disclinations in Twisted Nematic Liquid Crystal Displays, IEEE Trans. E. D. Vol. ED-24, No.7, 1977, pg. 811 Kaneko, E., Liquid Crystal TV Displays: Principles and Applications of Liquid Crystal Displays, KTK Scientific Publishers Flat Panel Display 1993 (Japanese Publication) from Nikkei Electronics Proceedings of SID; May 1993 Various articles from SID Information Display and Electronic Engineering Times Magazine Acknowledgments: I would like to thank Mike Schuster (SCHUSTER@PANIX.COM) for commenting on clearness and general understanding while compiling this FAQ. Written By Scott M. Bruck SBRUCK@EM.DRL.MEI.CO.JP 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. Copyright 1992-93 by Scott M. Bruck, Osaka 573 Japan 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. Every effort was made to insure the validity of this document.