A LIGNMENT OF SSFLC

2.4.1 Smectic Layer Structures

The alignment of an FLC material is strongly related to the surface pre-treatment of an LC cell. Under varying alignment conditions, FLC devices present different smectic layer structures as shown in Fig. 2-4. In a homeotropic cell, FLC molecules are vertically aligned and the smectic layer normal is formed parallel to the surface normal, as shown in Fig. 2-4(a). In a homogenous cell with antiparallel alignment, a tilted smectic layer structure is presented as shown in Fig. 2-4(b) [20].In a homogenous cell with parallel alignment, in most SSFLCs, a chevron shaped layer structure is formed, as shown in Fig. 2-4(c). This special layer structure is confirmed by X-ray measurement [21]. The formation of the chevron layer structure is explained by the difference in layer spacing between the SmC* and SmA phases. When a homogeneous cell cools down from the SmA to the SmC* phase, FLC molecules tend to tilt at an angle (layer tilted angle) as shown in Fig. 2-5. The layer spacing in the SmA phase is designated as dA. When the SSFLC cell cools down in the transition from the SmA to the SmC* phase, the layer spacing shrinks from dA to dC. Thus the smectic layers tilt from both top and bottom substrates and form the vertical chevron structure. The relation between dA and dC is expressed as d_C d_A cos_C, where _C is the layer tilted angle in the SmC* phase.

Fig. 2-4. Smectic layer structures in various alignment layers. (a) Homeotropic;

(b) Tilted; (c) Chevron [18].

Fig. 2-5. Origin of the chevron layer structure [18].

2.4.2 Chevron Layer Structure

Two classifications shown in Fig. 2-6 define FLC molecular orientation in the chevron structure. The first relates to the tilting direction of the chevron structure and the surface pre-tilt. When the chevron structure tilts towards the rubbing direction, the molecular orientation is named the C2 state. Conversely, when the chevron structure tilts opposite to the rubbing direction it is the C1 state.

Fig. 2-6. Illustration of C1 and C2 states, distinguished by directions between the chevron layer structure and the surface pre-tilt [22].

The second classification is based on the optical properties of an SSFLC device viewed through a polarizing optical microscope (POM). This classification defines two different alignment states viz. the uniform state (U)

and the twisted state (T) [23-24]. When an SSFLC cell is rotated, the uniform state shows extinction positions, while the twisted state shows only positions of colouration without any extinction.

Fig. 2-7. Schematic illustration of the geometrical conditions for C1, C2 states.

α, 2θc, and δ are pretilt angles, cone angles, and layer tilt angles respectively [22].

The conditions required for existence of the C1 and C2 states are shown in Fig.2-7 [22]. In a cell with low pretilt angle surface pre-treatment, both C1 and C2 states exist. In a cell with high pretilt angle surface pre-treatment, only the C1 state exists. In the C2 state the FLC director cannot lie on the switching cone. Based on these criteria, when an SSFLC device cools down in the transition from the SmA to SmC* phase, only the C1 state presents because the cone angle and layer tilt angle near the SmA phase are still very small. At lower temperatures as _c increases gradually, the C2 state starts to present. Finally, the C1 and C2 states coexist and form the zigzag defect as shown in Fig. 2-8.

Zigzag defects form when the chevron structures of C1 and C2 states point to opposite directions as shown in Figs. 2-6(a-b). Zigzag defects limit

display applications for SSFLC devices while the alignment defects reduce the contrast ratio.

Fig. 2-8. Zigzag defects of a surface stabilized ferroelectric liquid crystal in the chevron geometry.

2.4.3 Zigzag Defect Free SSFLC Devices Zigzag Free C1 Structure

As previously mentioned in section 2.4.3, the chevron structure of the C1 state tends to exist in a cell with a high pre-tilt alignment surface. This is a precondition for the existence of a zigzag-free C1. Several groups attempted to achieve zigzag-free C1 structures [22, 25-26].Canon achieved a zigzag-free C1 structure by using polyimide film with a high pre-tilt angle of 18° [27].

However, the polar interaction between FLC’s Ps and alignment layers caused a twisted C1 state in the parallel aligned cell with no extinction position [25]. A cross-rubbing alignment (±10° with respect to the rubbing direction) can improve the twisted C1 state and achieve a high contrast bi-stable C1 SSFLC.

A high pre-tilt SiOx alignment in which the pre-tilt angle was close to the chevron angle in the SmC* phase [26] also achieved a zigzag-defect free C1 state. Good bistability and a large memory angle, resulted in high brightness.

Zigzag Free C2 Structure

Zigzag-free C2 structures were developed in the cells by applying very low pre-tilt angle alignment pre-treatments [28-33]. Specifically, a very smooth polyimide RN-1199 (from Nissan Chemical) was applied at a pretilt angle of about 1° [34]. In C2 configuration, the splay-twist elastic deformation energy is lower than that of C1. The FLC director in the C2 configuration lies at the top or bottom of the switching cone, nearly parallel to the rubbing direction. A bistable states, negating availability of intermediate states. Based on bistable switching, SSFLC displays with gray scales were first developed by applying spatial and temporal dither techniques [35]. An LC display, employing the spatial dither technique, reduces resolution while increasing the number of sub-pixels. In addition, control of the divided sub-pixels requires doubling of, the number of data lines and electrodes which, in turn, slows down signal writing speed. In contrast, the downside of the temporal dither technique is the