TFT manufacturing on the rear glass plate
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.
