Patterned and photo-induced vertical alignment for liquid crystal photonics and display applications

Abstract

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 (UV²A). 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.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology