Rubbing of polyimide (PI) and photo-induced alignment (photoalignment) are two
mainstream processes in liquid crystal (LC) alignment technology. Rubbing of PI is
commercially widely used in display fabrication, and is robust to temperature and moisture.
However, the brushing process introduces debris, electrostatic charges, non-uniform alignment
and mechanical damage. All these lead to degradation of LCD electro-optical properties or
production yield lost. In recent years, photoalignment technology has drawn increasing
attention due to its non-contact alignment process, high resolution and easily patterned
properties.
PI is usually used for displays with uniform alignment. Little investigation has been carried
out for patterned PI structures. By utilizing the theory of inhomoge...[
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Rubbing of polyimide (PI) and photo-induced alignment (photoalignment) are two
mainstream processes in liquid crystal (LC) alignment technology. Rubbing of PI is
commercially widely used in display fabrication, and is robust to temperature and moisture.
However, the brushing process introduces debris, electrostatic charges, non-uniform alignment
and mechanical damage. All these lead to degradation of LCD electro-optical properties or
production yield lost. In recent years, photoalignment technology has drawn increasing
attention due to its non-contact alignment process, high resolution and easily patterned
properties.
PI is usually used for displays with uniform alignment. Little investigation has been carried
out for patterned PI structures. By utilizing the theory of inhomogeneous surface theory, varied
LC pretilt angles can be realized by changing the proportion of homogenous and homeotropic
widths within one-pitch domain. With the help of E-beam lithography, we fabricated a patterned
alignment structure with varied pretilt angles. By designing the phase profile to be parabolic,
an LC lens is achieved. In comparison with the current methods to fabricate LC lenses, our
method has uniform electrodes and a uniform cell gap, which simplifies the fabrication process
to a great extent. Moreover, our LC lens requires low voltage driving, resulting in low power
consumption. Both positive and negative optical power LC lens can be fabricated. Such lenses
can be fabricated by mass production by means of nano-imprint technology.
Nowadays, the majority of display modes are either twist nematic (TN), in-plane switching
(IPS) or vertical alignment (VA). The advantages of VA mode include high contrast ratio (CR)
and fast response time compared to other modes. And its contrast ratio is insensitive to the
incident light wavelength, LC layer thickness and operating temperature. When VA mode is in
use, a multi-domain structure is normally needed since it provides a wide viewing angle. The
previous approaches included protrusion-controlled multi-domain vertical alignment (MVA),
patterned electrodes MVA, polymer stabilized (PS) VA and ultraviolet VA (UV
2A). All these
methods require complicated fabrication processes. In order to simply the process, a multi-layer
patterned thin film structure is designed and MVA can be realized by a single-step exposure
with photoalignment technology. The effectiveness of our design is verified by a photo-crosslinking
material, which provides vertical alignment.
Nowadays, the commonly used photo-induced vertical alignment material is crosslinking
material. Such materials require polarized deep UV light irradiation to generate uniform
alignment. However, the exposure dosage should be constrained within a certain value range,
otherwise it will bring degradation to the material, so that the alignment may be damaged.
In order to solve this problem, a new composite is demonstrated. By mixing SD1 and
vertical PI, photoaligned vertical alignment can be achieved through oblique non-polarized light
exposure. As SD1 can be photo-aligned by blue light, the light source module is much cheaper
than that of photo-crosslinking material. Moreover, the excess exposure dosage is no longer a
problem, since it will not bring damage to SD1 molecules. Due to the re-writable property of
SD1, patterned VA structures can be made for various optical devices. However, for display
application, we need to stabilize SD1. In chapter 5, we propose a method to realize this by
adding LC polymer. In order to make it suitable for mass production, the recipe is further
modified to increase the viscosity for rolling printing process by adding polyamic acid (PAA)
inside. The measurements show good electro-optical properties of the mixture, such as a high voltage holding ratio (VHR) and small residual direct current (RDC). The image sticking (IS)
parameter also shows it is as good quality as commercial PI. It has also been verified that our
VA recipe shows good photo-stability and thermal stability.
In summary, an LC lens by means of patterned vertical PI is demonstrated. And a multi-layer
structure is designed and fabricated to realize a multi-domain alignment structure through
one-step exposure. In addition, a composite of our own is introduced, which can provide photo-induced
vertical alignment. Patterned structures by utilizing the composite are achieved for
photonic applications and the stabilization method is demonstrated also for display applications.
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