Unlike their nematic cousins, ferroelectric liquid crystals exhibit a net dipole over the bulk of the material. Because of their electrical polarization properties, ferroelectric liquid crystals may switch very quickly under a DC field. Ferroelectrics are chiral smectic C devices, meaning that they have a layered structure with the molecules at some angle (the "cone angle") away from the layer normal, and that there is some inherent twist in the structure. That is, in an unconstrained system, the azimuthal direction in which the molecules tilt away from the layer normal will differ slightly from one layer to the next.
The most common configuration for a ferroelectric liquid crystal device is the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) configuration, in which the natural twist of the material is suppressed by the surface conditions. Such devices are commonly about 1 or 2 microns thick, and have a parallel rubbing configuration (figure 1). The first picture in this figure shows the smectic layer structure in the smectic A phase through which the liquid crystal passes as the display is cooled from isotropic. The second picture shows the chevron-layer structure that appears in such devices as the material forms the smectic C phase. As the molecules tip away from the layer normal on passage from A to C, the layers contract somewhat, forcing the chevron structure to form. The chevron structure allows the layer spacing to decrease while maintaining a fixed number of layers. As will be discussed in more detail below, the direction in which the apex of the chevron points is related to the pretilt direction of the surface, which is determined by the direction of rubbing.
When a DC voltage is applied across the two display substrates, the molecules rotate around the cone so that in the center region of the chevron, the direction the molecules are pointing changes in the plane of the cell by about 45°. Thus, the ferroelectric may act as a linear retarder whose direction can be switched very quickly from being along the polarization direction of incident light to being at 45° from the polarization direction of the light. If the material retards the light by a half wave, then the state of the light on reaching the exit polarizer may be changed from 0° to 90°, allowing for black-and white operation.
Figure 1. Layer structure in an SSFLC device.
Although they are geometrically very closely related, the cone angle to which the molecules tip away from the layer normal is not always equal to the chevron angle, defined as the angle to which the layers tip away from the cell normal direction. The relative values of the cone angle, the chevron angle, and the pretilt angle determine if the chevron apex is geometrically allowed to point along the rub direction (a "C2" chevron), opposite the rub direction (a "C1" chevron), or both. The details of the geometry are shown in figure 2.
Figure 2. Details of the geometry of various chevron structures in a ferroelectric LCD.
Note the cone drawn along the lower surface for each chevron type. This cone represents the set of all directions that are tipped exactly q (the cone angle) away from the smectic layer direction at the surface. In order for the chevron to be compatible with the given pretilt (a, shown in the center of the figure), the cone must intersect with the pretilt at some point. Otherwise, the surface conditions cannot be met without significant distortion of the chevron. The chevron angle in this figure is designated by d. As can be seen from the geometry of figure 2, the conditions for the different chevron types to be allowed is as follows:
C1: 0 < a < q + d
C2: 0 < a < q - d
For display devices, it is desired that only one type of chevron be present, because the presence of both types of chevrons in a display results in the formation of zig-zag defects, which appear at the intersection of domains with different chevron types. The two types of zig-zag defects are shown in Fig. 3.
Figure 3. Formation of zig-zag defects.
A microscope photograph of zig-zag defects viewed between crossed polarizers is shown in figure 4. The presence of zig-zag defects in a display pixel can drastically reduce the contrast of the pixel.
Figure 4. Zig-zag defects viewed between crossed polarizers.
Based on the understanding of chevron allowance conditions, Tsuboyama and other researchers at Canon determined that a high pretilt would be effective in eliminating the C2 structure, thus allowing for a uniform, high contrast device.
Unfortunately, high pretilt polyimides are not commonly available like low pretilt polyimides are. Thus, a further understanding of the parameters governing zig-zag defects was necessary. We decided to focus on surface topography as a possible initiator of zig-zag defects. A diagram of how an imperfect surface can effect the pretilt at a surface is shown in Fig. 5.
Figure 5. A ridge on a surface resulting in changes to the pretilt conditions. The pretilt condition on the right is effectively reversed.
In this study, we investigated the effects of surface topography on the formation of zigzag defects in SSFLC devices. Our concern was that a rough surface would be likely to reverse the pretilt direction in small areas of the surface, thus resulting in interspersed regions of chevrons pointing in opposite directions, and therefore in high densities of defects. We used an AFM to determine surface topography, and quantified the defect density through microphotography and image analysis. The results of this study may be found in the following references:
Effects of Surface Topography on formation of defects in SmC* devices explained using an alternative chevron description. P. Watson, P.J. Bos, J. Pirs. Phys. Rev. E, 56, 4, (1997). (Rapid Communication.).[PDF] [DjVu]
Effects of Surface Topography on Formation of Zig-Zag Defects in SSFLC Devices, P. Watson, P.J. Bos, J. Pirs. Society of Information Display International Symposium Digest of Technical Papers, p 743 (1997).
Effects of surface topography on formation of zig-zag defects in SSFLC devices, P.Watson, P.J.Bos, J. Pirs, ALCOM Technical Report VIII, p30 (1996)
An observation of the effect of the surface topography on the defect density in SmC* devices. J. Pirs, S. Kralj, S, Pirs, B. Marin, P. Watson, C. Hoke, P. Bos. 16th ILCC D1P.29 (1996)