Liquid Crystal Display (LCD)

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Background

Liquid crystal displays (LCDs) consist of liquid crystals that are activated by electric current. They are used most frequently to display one or more lines of alpha-numeric information in a variety of devices: fax machines, laptop computer screens, answering machine call counters, scientific instruments, portable compact disc players, clocks, and so forth. The most expensive and advanced type—active matrix displays—are even being used as screens for handheld color TVs. Eventually, they may be widely used for large screen, high-definition TVs.

The basis of LCD technology is the liquid crystal, a substance made of complicated molecules. Like water, liquid crystals are solid at low temperatures. Also like water, they melt as you heat them. But when ice melts, it changes into a clear, easily flowing liquid. Liquid crystals, however, change into a cloudy liquid very different from liquids like water, alcohol, or cooking oil. At slightly higher temperatures, the cloudiness disappears, and they look much like any other liquid.

When the liquid crystal is a solid, its molecules are lined up parallel to one another. In the intermediate cloudy phase (liquid), the molecules still retain this more or less parallel orientation. As in any liquid, the molecules are free to move around, but they tend to "line up" in one direction, reflecting light and causing a cloudy appearance. Higher temperatures tend to agitate the molecules and thus make the liquid clear.

In an LCD, an electric current is used to switch segments of liquid crystals from a transparent phase to a cloudy phase, each segment forming part of a number or letter. The segments can also be in the shape of tiny dots or pixels, and the can be arranged in rows and columns. They are turned on and off individually to either block or allow polarized light to pass through. When the light is blocked, a dark spot is created on the reflecting screen.

There are two general types of LCDs: passive matrix, and the newer active matrix (AMLCDs). Brighter and easier to read, active matrix displays use transistors behind each pixel to boost the image. The manufacturing process for AMLCDs, however, is much trickier than that for passive matrix LCDs. As many as 50 percent of those made must now be thrown out because of imperfections. One imperfection is enough to ruin an AMLCD. This makes them very expensive to manufacture.

Raw Materials

A working LCD consists of several components: display glass, drive electronics, control electronics, mechanical package, and power supply. The display glass —between which the liquid crystals lie—is coated with row and column electrodes and has contact pads to connect drive electronics (electric current) to each row and column electrode. The drive electronics are integrated circuits that supply current to "drive" the row and column electrodes. The control electronics are also integrated circuits. They decode and interpret the incoming signals—from a laptop computer, for example—and send them to the drive electronics. The mechanical package is the frame that mounts the printed circuit boards for the drive and control electronics to the display glass. This package

power supply is an electronic circuit that supplies current to the LCD. Equipment makers who use LCDs often purchase the power supplies separately.

also strengthens and protects the display glass and anchors the entire display to the device using the LCD, whether it is a laptop computer, a fax machine, or another device. Finally, theis an electronic circuit that supplies current to the LCD. Equipment makers who use LCDs often purchase the power supplies separately.

In all LCDs, the liquid crystal is sandwiched between two pieces of glass or transparent plastic called substrates. Just any glass will not do. If the glass has many sodium or other alkali ions, they can move to the glass surface, combine with any moisture that is there, and alter the electric field pattern and liquid crystal alignment. To eliminate that, LCD makers either use borosilicate glass, which has few ions, or they apply a layer of silicon dioxide to the glass. The silicon dioxide prevents the ions from touching any moisture. An even simpler solution is to use plastic instead of glass. Using plastic also makes the display lighter. However, inexpensive plastics scatter light more than glass, and they may react chemically with liquid crystal substances.

Most LCDs today also use a source of light coming from the rear of the display (backlight), such as a fluorescent light, to make the liquid crystal appear darker against the screen when in its cloudy phase. LCD makers also use sheets of polarizer material to enhance this effect.

The Manufacturing
Process

Making passive matrix LCDs is a multi-step process. The surface and rear glass of the display is first polished, washed, and coated with silicon dioxide (SiO 2 ). Next, a layer of indium tin oxide is evaporated onto the glass and etched into the desired pattern. A layer of long chain polymer is then applied to allow the liquid crystals to align properly, followed by a sealing resin. The spacers next are put into place, and the glass sandwich is filled with the liquid crystal material.

Preparing the glass substrates

• 1 First, the two glass substrates must be cut to the proper size, polished, and washed. Cutting can be done with a diamond saw or scribe, while polishing involves a process called lapping, in which the glass is held against a rotating wheel that has abrasive particles embedded in it. After being washed and dried, the substrates are coated with a layer of silicon dioxide.

Making the electrode pattern

• 2 Next, the transparent electrode pattern must be made on the substrates. This is done by completely coating both front and rear glass surfaces with a very thin layer of indium tin oxide. Manufacturers then make a mask of the desired pattern, using either a silk-screening or photolithography process. They apply the finished mask to the fully coated glass, and areas of indium tin oxide that are not needed are etched away chemically.
• 3 Alternatively, finer definition can be achieved by using glass that has a layer of etching-resistant, light-sensitive material (called photoresist) above the indium tin oxide film. A mask with the desired pattern is placed over the glass, and the glass is bombarded with ultraviolet light. This light causes the resistive layer it shines on to lose its resistance to etching, allowing the chemicals to eat away both the exposed photoresist and the indium tin oxide below it, thus forming the pattern. The unnecessary photoresist that remains can then be removed with other chemicals. A second variety of resistive film resists etching only after it is exposed to ultraviolet light; in this case, a negative mask of the pattern must be used. Regardless of which method is used, the patterns on the two substrates are designed to overlap only in specific places, a design that ensures that the thin strips of indium tin oxide conveying voltage to each element have no electrode positioned directly opposite that might show up while the cell is working.

Applying the polymer

• 4 After the electrode pattern is in place, the substrates must be coated with a polymer. The polymer allows the liquid crystals to align properly with the glass surface. Polyvinyl alcohol, polyamides, and some silanes can be used. Polyamides are the most popular agents, because polyvinyl alcohol is subject to moisture problems, and silanes produce a thin, unreliable coating.
• 5 After coating the glass, manufacturers then stroke the polymer coat in a single direction with soft material. This can result in small parallel grooves being etched into the polymer, or it may simply stretch the polymer coat. In any case, this process forces the liquid crystals to lie parallel to the direction of the stroke. The crystals may be aligned another way, by evaporating silicon oxide onto the glass surface at an oblique angle. This procedure is used to make most digital watch displays but is not convenient for making large-scale displays. It also does not yield the low-tilt angle possible with the previous method.
• 6 If LCD makers want to align liquid crystals perpendicular to the glass surface, another technique is used: coating the glass with an amphophilic material. This is material whose molecules display affinity for water at one end of the molecule and repulsion from water at the other end. One end—the affinity end—adheres to the glass surface while the other end—the repulsing end—points into the liquid crystal area, repelling the liquid crystals and forming them into an alignment that is perpendicular to the glass surface.

Applying the sealant and injecting
the liquid crystal

• 7 A sealing resin is next applied to the substrates, followed by plastic spacers that will give the liquid crystal cell the proper thickness. Next, the liquid crystal material is injected into the appropriate area between the two glass substrates. The thickness of the LCD cell is usually restricted to 5-25 micrometers. Because proper thickness is crucial for cell operation and because spacers don't always achieve uniform thickness, LCD makers sometimes put appropriately sized glass fibers or beads in the liquid crystal material. The beads or fibers cannot be seen by the naked eye. They help hold the cell at the proper thickness while the sealant material is setting.
• 8 To make LCDs more visible, polarizers are added. These are usually made from stretched polyvinyl alcohol films that have iodine in them and that are sandwiched between cellulose acetate layers. Colored polarizers, made using dye instead of iodine, are also available. Manufacturers glue the polarizer to the glass using an acrylic adhesive and cover it with a plastic protective film. They can make reflective polarizers, which also are used in LCDs, by incorporating a simple metal foil reflector.

Final assembly

• 9 After the polarizer film is attached, the unit is allowed to age. Finally, the finished glass display assembly is mounted to the circuit boards containing the control and drive electronics. Then, the entire package is ready to be mounted to the device using the LCD—laptop computer, fax machine, clock, etc.

Active Matrix LCD Manufacture

The process used to make an active matrix LCD (AMLCD) is quite similar to that used for passive matrix LCDs, although it is more complex and more difficult. Generally, the steps of SiO 2 coating, indium tin oxide application, and the photoresist etching are replaced by a host of other steps.

In the case of AMLCDs, each LCD component has to be changed to work properly with the thin film transistor and electronics used to boost and clarify the LCD image. Like their passive matrix brethren, active matrix displays are sandwiches consisting of several layers: a polarizing film; a sodium barrier film (SiO 2 ), a glass substrate incorporating a black matrix, and a second sodium barrier film; a color filter and a color filter overcoat made of acrylic/urethane; a transparent electrode; an orientation film made of polyamide; and the actual liquid crystal material incorporating plastic/glass spacers to maintain proper LCD cell thickness.

Quality Control

LCDs—especially those for laptop computer displays—-are made under highly controlled conditions in a clean room environment to maximize yield. "Clean rooms" have special air filtering devices designed to keep all dust particles out of the room, and workers inside the room must wear special clothing. Nonetheless, many LCDs have to be discarded because of imperfections. This is particularly true of AMLCDs, which currently have a rejection rate of approximately 50 percent. To minimize the rejection rate, each active device is inspected and as many are repaired as possible. In addition, active matrix assemblies are inspected immediately after the photoresist etching step and again after the liquid crystal material is injected.

The Future

The future is clearly with active matrix LCDs, even though the current rejection rate is very high and the manufacturing process is so expensive. Gradual improvements are expected in the manufacturing process of AMLCDs, and in fact companies are already beginning to offer inspection and repair equipment that may cut the current rejection rate from 50 percent down to around 35 percent.

But the real boost to LCD manufacturing technology may come from all the money that companies are pouring into the research and development process on large screen, AMLCD displays for the long-awaited high-definition television technology.

Where To Learn More

Books

Chandrasekhar, S. Liquid Crystals, 2nd ed. Cambridge University Press, 1993.

Collins, Peter J. Liquid Crystals: Nature's Delicate Phase of Matter. Princeton University Press, 1991.

Doane, J. W., ed. Liquid Crystal Displays and Applications. SPIE-International Society for Optical Engineering, 1990.

Drzaic, P. S., ed. Liquid Crystal Materials, Devices, and Applications. SPIE-International Society for Optical Engineering, 1992.

Kaneko, D. Liquid Crystal TV Displays. Kluwer Academic Publishers, 1987.

O'Mara, William C. Liquid Crystal Flat Panel Display: Manufacturing Science and Technology. Van Nostrand Reinhold, 1993.

Periodicals

Curran, Lawrence. "Kopin, Sarnoff Team in Advanced LCD Effort." Electronics. August 10, 1992, p. 11.

Fitzgerald, Michael. "Display Standards Elusive." Computerworld. December 21, 1992, p. 27.

Fleischmann, Mark. "Wall-Size TV from Tiny LCDs." Popular Science. June, 1991, p. 94.

Kinnaman, Daniel E. "LCD Panels: The Next Generation." Technology & Learning. March, 1993, p. 44.

Robinson, Gail M. "Display Systems Leap Forward: New Technologies Offer Designers More Choices Than Ever in CRTs, LCDs, EL and More." Design News. February 13, 1989, p. 52.

Woodard, Ollie C., Sr. and Tom Long. "Display Technologies." Byte. July, 1992, p. 158.

— Edward J. Stone

Flat panel display manufacturing

LCD Equipment
Crystec Technology Trading GmbH

Flat panel display manufacturing.

On this page, we explain the working principle of passive and active liquid crystal displays (LCDs) and their main production steps. You can find the related equipment descriptions, when you follow the links in the text. Together with our partners in Japan and Korea, we can offer equipment for almost all production steps, even if these machines are not listed in detail on our web pages.

Working principle

Liquid crystals

Liquid crystals are organic molecules that have crystal-like properties but that are liquid at normal temperatures. Because the intermolecular forces are weak, the molecules can be oriented by weak electromagnetic fields. The liquid crystal molecules used in LCDs also have an optical anisotropy (different indices of refraction for different axes of the molecule) that is used to create visible images. Depending on the orientation of the molecules, the panel is either transparent or dark.

Setup of a typical LCD panel: 1 - polarizer, 2 - glass substrate, 3 - seal, 4 - spacer, 5 - ITO, 6 - hard coat, 7 - polyimide, 8 - TFT

Passive LCDs

A passive matrix LCD is composed of several layers. The main parts are two glass plates, connected by seals. The polarizer is applied to the front glass plate in order to polarize the incoming light in a single direction. The light then passes through the front glass sheet. An Indium Tin Oxide (ITO) layer is used as an electrode. A passivation layer, sometimes called hard coat layer, based on SiOx is coated over the ITO to electrically insulate the surface. Polyimide is printed over the passivation layer to align the liquid crystal fluid. The liquid crystal fluid is sensitive to electric fields and changes orientation when an electric field is applied. The liquid crystal is also optically active and rotates the polarization direction of the incoming light. The thickness of this layer is determined by spacers, which keep the two glass plates in a fixed distance. When there is no electric potential from the front piece of glass to the rear piece of glass, the polarised light is rotated 90° as it passes through the liquid crystal layer. When an electric potential is applied from one plate to the other plate the light is not rotated. After the light has passed through the liquid crystal, it passes through another polyimide layer, another hard coat layer, the rear ITO electrode, and the rear glass. When it reaches the rear polarizer it is either transmitted through or absorbed, depending on whether or not it has been rotated 90°.
This technology is now also used for manufacturing of smart windows.

Active matrix LCDs

The dominant active matrix technology is using thin-film transistors (TFT) of either amorphous or polycrystalline silicon applied to the rear LCD glass plate. While the amorphous silicon TFTs are easier to produce and therefore are used for most large displays nowadays, the poly-silicon TFTs show the better performance, but require a higher deposition temperature. They are produced in tube furnaces and therefore only small displays can be manufactured, using poly-silicon technology.

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Coloured LCDs

In coloured LCDs, color filters are applied to the inside of the front glass sheet. Three colours red, blue, green and a black matrix are used.

Manufacturing Process

The front glass plate and the rear glass plates are produced in different production lines. In most cases several (4-6) displays are produced on one glass plate. The rear glass plates is the substrate for the TFT production in case of active matrix LCDs. On top of the ITO layer, the transistors are created by a serious of PECVD and sputter steps. Then hard coat, polyimide and spacers are applied.

The front glass plate wears the colour filter layers, same as the rear glass plate ITO, hard coat and polyimide and the sealing.

In the assembly machine, the two glass plates are aligned, combined and fixed together, using UV hardened polymer spots. Nowadays this process is performed under vacuum conditions in the so called ODF-Process. Then the raw panels are pressed together and heated in order to cure the seals and create a stable panel structure. Then the large panels are scribed and broken to the final display dimensions. The edges are ground. Now the singularised displays are filled with liquid crystal liquid and the opening in the seal is closed. The polarizers are applied to both sides. The display is ready. Following steps are mounting of electronic and packing.

Here you can see an integrated LCD-manufacturing line from Joyo. 1- Loader, 2 - Wet cleaner, 3 - PI coater, 4 - Inspection, 5 - Rubbing, 6 - US cleaner, 7 - After rubbing cleaner, 8 - Spacer Spray, 9 - Spacer Checker, 10 - Ag Dispenser, 11 - Seal Dispenser, 12 - Pre-cure oven, 13 - Assembly machine, 14 - Hot press oven, 15 - Alignment checker, 16 - Unloader

Beside the fully automatic in-line production lines, many machines are also available as single machines or integrated in smaller production clusters.

TFT production on the rear glass

TFT formation consists of several vacuum process steps, using  PECVD for deposition of a-Si and the gate dielectric insulation layer and sputtering equipment for data and scan metal lines as well as for ITO layers. A typical process step series is: Deposition of gate metal (Ta, Al, MoTa), patterning, anode oxidation Ta2O5, deposition of silicon nitride, patterning, deposition of a-Si for the electrode, patterning, deposition of source and data line (Ti, Al), patterning, deposition of pixel electrode (ITO), patterning, passivation, patterning. Some companies use pre-coated ITO substrates; thus, the first step is to pattern and etch the layer.

For high performance displays a poly-silicon deposition step is used instead of the a-Si deposition. The poly-silicon deposition is done under low pressure in a tube furnace. This furnace is similar to equipment used also in the semiconductor industry.

In order to structure the various layers, the listed patterning steps use common lithographic equipment like resist coaters, steppers and dry or wet etching equipment. Dry etching can provide much better line-width control, but wet etching is the faster and cheaper method because it is a batch process.

Colour filter application on the front glass

The cover-plate colour-filter process is extremely important; it can be a very expensive process because of high materials cost and low yield. Colour filters can be applied by several methods to the front glass. Dye or pigment filter material can be spinned on the glass, which is a simple technology but produces a rather high amount of expensive waste material. A doctor blade technology can be used to deposit colour filter material on the glass. Much less waste is created. In both cases a cure process has to follow the deposition process. The third possibility is to apply colour filter foils to the front glass. The colour filters are overcoated by a protection layer.

ITO deposition

Indium tin oxide ITO is usually deposited by sputtering technology.

Hard coat

The passivation layer consisting of SiOx and SOG is printed on the substrate using flexo printer technology and is then cured and annealed in a furnace.

Polyimide (PI) layers

The polyimide layer is printed on the substrates, using flexo printing technology. The polyimide requires a proper cure process using inert gas. This can be done in clean convection ovens or on hot plates. Good temperature uniformities are required in order to create homogeneous polyimide properties.

Rubbing

Polyimde layer rubbing is necessary in order to create a proper LC alignment towards the PI surface. The rubbing is aligned parallel to the in the polarizer direction.

Spacer

In order to create a uniform distance between both glass plates, spacers are created on one substrate. Nowadays litho spacer are used in most cases. In the past spacers have been sprayed on the substrate. These consist of small glass or plastic balls. Three main processes can be used: Dry spray which is used for high throughput and large display manufacturing, semi-dry spacer spray with is the best method for medium and small displays and not so high throughput. Wet spacer spray is not used very often anymore but gives a very nice spacer uniformity and low numbers of spacer clusters.

Seal deposition and cure

For large factories screen printing is the best method of seal deposition. High throughput and high performance can be combined with this method. For smaller production volume and higher design flexibility, seal dispensing is the best way. The seal material has to be pre-cured in an oven before the substrate glass plates is forwarded to the assembly machine. After cell assembly the final cure of the seal happens in a hot press oven. The panels are combined to panel stacks, pressed and cured in an oven. Alternatively, panels can be pressed and cured one by one.

Contact creation

The external contact are produced by printing Ag paste contacts on the substrate glass, using screen printing technology. Dispensing of silver paste is also possible.

Cell Assembly

In the cell assembly machine, both glass plates are aligned and combined. The position of the glass plates against each other is fixed by UV hardened polymer spots in the cell assembly machine. Cell assembly can also be performed under vacuum conditions when necessary.

Hot press oven

As described above already, the seal has to be final cured after the cell assembly process. This has to be done under pressure in order to make sure that the seal thickness is properly related to the spacer diameter and the calculated liquid crystal thickness can be reached with low tolerances. Hot press ovens are available as a batch process tool and as a single panel press oven. The batch oven requires previous collection of panels and preparation of a larger pile of panels which are presses all together. The pressed pile of wafers is then cured in a clean convection oven. The single panel hot press oven is easier to integrate in automatic lines and works continuously.

Liquid crystal fluid filling.

The LCD fill method is a vacuum application and today no more necessary, since in the ODF process, the filling occurs during cell assembly. The liquid crystal displays are placed in a vacuum chamber mounted above the liquid crystal fluid. The chamber is then pumped down and the empty panel is evacuated. The fill ports are lowered into the trough and the chamber is brought back to atmospheric pressure. The atmospheric pressure forces the liquid crystal fluid into the display. After filling the panels the hole in the sealing is closed in a separate process step.
Alternatively, the liquid crystals can be dispensed on the lower glass plate before cell assembly. This technology is called ODF-technology and it requires a Vacuum Assembly Machine.

Polarizer attachment.

After proper surface cleaning the polarizer foils are attached in parallel to the rubbing direction of the related polyimide layer to the front side and the back side of the LCD panel. This is the last step of the main LCD fabrication.

Cleaning

Several cleaning steps are necessary during the LCD manufacturing process: Initial glass plate cleaning, cleaning before spacer spray (after rubbing), cleaning before attachment of polarizers, etc. Ultrasonic cleaning is used frequently for these applications.

Inspection

Inspection of process results is required after several production phases. Most important however is the final inspection. In many cases this final inspection is done manually. However, automatic inspection machines are available now also.

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