Flat panel display manufacturing.
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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.

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.

LCD setup

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°.

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.

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. 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.

Integrated production line

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 spray

In order to create a uniform distance between both glass plates, spacers are sprayed on one 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. 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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