| knowing lcd |
What is
a Liquid Crystal Display?
Simply put,
a liquid crystal display (LCD) is a parallel plate
capacitor with a dielectric between the plates.
However, it's not quite that simple. First we
select glass coated with a transparent metal
coating for the electrodes of the display. The
glass is usually a soda lime type but in some
instances it can be a more expensive borosilicate
type. The transparent metal coating can be any
thin layer of conductive material, such as gold,
silver or tin. In order to keep the cost down and
the have a reasonable process window with a highly
transparent coating, the industry has been using
indium-tin oxide (ITO) as the preferred electrode
material.
Photoresist is then put on top
of the transparent metal coating and a
photolithographic process is used to image
patterns in the photoresist. These patterns can be
segments of numeric digits or they can be icons
that represent various functions. The exposed
patterns are then developed and the glass is sent
through an acid bath where the excess metal
coating is removed. 
The remaining photoresist is
stripped off and the patterned segment and common
plane electrodes have been generated.
After
the electrodes have been made, an insulating layer
is put on top of them. This is a silicon dioxide
layer that is used to seal the electrode surface,
act as an electrical barrier, and index match the
electrodes and the glass. The next layer to be
applied is the liquid crystal alignment layer.
This is usually a polyimide type material and has
been chosen for its environmental stability in
high moisture and heat. More importantly is its
ability to cause the molecules of liquid crystal
to align their long axis in the direction in which
the polymer has been buffed.
We then buff
the two halves of the display at right angles to
one another and since the liquid crystal molecules
like to arrange themselves parallel to one
another, we cause a helical structure to be formed
between the two electrode faces. This helical
structure is a 90 degree rotation of the liquid
crystal molecules from one side of the display to
the other. After the polymer is buffed, a glue
ring or seal is printed on the glass. This is a
thermoset epoxy type material with a very high
glass transition temperature.
We then apply
a small spot of conductive epoxy to connect the
common plane electrode on the top piece of glass
to the segment plane which is on the bottom piece
of glass. This conductive epoxy is called a
crossover dot.
To make the display uniform
in appearance, spacers are then applied. These are
usually glass rods that have the desired diameter
to produce a fixed gap between the glass plates.
Depending on the liquid crystal used, this gap can
be between 6 and 8 microns. The two halves of the
display are then heat sealed together. A very
thin, uniform, flat and empty bottle has been
formed.
A liquid crystal is put inside this
bottle by using a vacuum filling technique. The
liquid crystal (dielectric material of our
capacitor) is selected for it's various physical
properties. The application may call for a liquid
crystal fluid that has a very low operating
voltage or the display may be used outdoors and
require a very wide temperature range. Display
manufacturers have developed several liquid
crystal mixtures to fulfill most
applications.
Once the liquid crystal has
been put inside the display and the port opening
has been sealed, a polarizer is put on the front
and an analyzer (another polarizer) is put on the
back. If everything has gone as planned, we will
have a device that can present the desired
information to an observer. In other words, you
will have a device similar to what is seen in
calculators and gas pumps. With a little
imagination, many variations of displays can be
made.
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