Oblique evaporation

Most (but not all) LCDs require that the liquid crystal be uniformly oriented i.e., that the LC exist in the form of a single domain in either the OFF or ON condition. The required alignment can be brought about by imparting to the confining surfaces a unidirectional sense, either by rubbing or by the oblique evaporation of SiO or other compounds (Kahn, 1977). A variety of boundary conditions can therefore be set up, including some that result in a metastable behavior of the enclosed liquid crystal phase. 

The surface anisotropy, which is necessary to induce a certain direction of liquid crystal alignment, can be produced in many different ways, such as rubbing, oblique evaporation, oblique ionic bombardment, surface bombardment... 

Fig. 7.11. The device described by Barberi et al. (a) The tuned oblique evaporation of SiO gives three stable states for the surface director N parallel to the surface and normal to the evaporation plane, and the symmetrically positioned O and O, with polar tilt S and azimuthal tilt 45°. (b) Two such plates are assembled making the projections of N of the top plate and O of the bottom plate as well as O of the top plate and N of the bottom plate parallel, (c) This makes two splayed bulk director configurations possible, azimuthally separated by an angle of 45°. These states have opposite signs for the splay-induced fiexoelectric polarization perpendicular to the surface, (d) The device can be switched between the stable dark and bright states depending on the polarity of the applied pulse.

Fig. 10.20 Schemes of the planar homogeneous alignment of a nematic by an obliquely evaporated thin film of SiO (a) and homeotropic alignment by a monolayer of surfactant molecules (b)...

A tilted orientation of molecules at a given angle to the surface is achieved using layers of SiO produced by oblique evaporation at a very large angle (80-90 deg) between the normal to the surface and the direction to the SiO source. Tilted orientation of the nematic liquid crystal molecules can also be achieved by using photosensitive films irradiated by obliquely incident light. 

There are two important differences between r.f. plasma deposited Si02 films and obliquely evaporated SiO films. 

CB, E-8, E-18, ZLI-1132 and ZLI-1083 were used, as shown in Table 1. The other was that on the twisted nematic samples, in which 7CB was used. Pretilts below 10 have been realized by the double evaporation of SiO having an appropriate incidence angle,6 those with medium value, say, 20-40 by a single 85 oblique evaporation and those with a high tilt, about 50 , by an application of the surfactant followed by a single 85 oblique evaporation of SiO The thickness of all samples was about 8 ym. 

In order to express the surface energy, it is assumed that there is an easy axis on the surface or interface. The existence of the easy axis is the only respect in which the surface differs from the bulk. The interaction between the LC and the interface in contact with it may be mainly due to the van der Waals-Lifshitz forces,which are related to the imaginary part of the dielectric permittivity at frequencies above the infrared, i.e., light absorption. In this paper, the easy axis is defined by a unit vector whose direction coincides with the optical axis of the anisotropic surface. For a single oblique evaporation of 85 , it was found that the optical axis tilted from the surface by about 5 . ... 

In order to make a more qualitative discussion, it will be necessary to find a method to measure the coefficients k and Att, which determine the value of a, and to measure the surface order parameter S . Furthermore, the direction of the easy axis must be determined for the oblique evaporated surface exactly... 

MBBA SiO oblique evaporation Electric saturation field for the Prederiks transition 10 (in the vicinity of Tni) [43]... 

FIGURE 7.25. (a) Schematic picture of zig-zag defects. Zig-zag defects are avoided by preparing (b) the preferred orientation of chevrons (left) or uniform tilt (right) of FLC layers. Arrows in (b) indicate the direction of the oblique evaporation of orienting layers. 

One of the important aligning techniques, which allows us to avoid zigzag defects between two adjacent smectic layers bent in opposite directions (chevrons), remains the oblique evaporation of silicon monoxide [141-144] (Fig. 7.25). Zig-zag defects are avoided by the promotion [25] of only one possible bend or tilt of the smectic layers due to a specific oblique director orientation at the boundaries (Fig. 7.25). Samples with the antiparallel evaporation direction contain uniformly tilted layers. Those with parallel directions exhibit the chevron structure of tilted layers free of zig-zag defects. Reference [135] shows that in the case of parallel evaporation the variation of the evaporation angle results in different chevron bend angles. 

Oblique Obliquely evaporate SiO High tilt angles near 80° [2]... 

In the oblique evaporation process, a micro columnar structure is realized on the substrate surface, due to the self shadowing effect as shown in Fig. 3.3.1. When a nematic liquid crystal contacts such a surface, elastic deformation of the liquid crystal along the surface induces an interaction energy between the surface and the nematic material. This is thought to be the driving force for alignment of the nematic director. 

The surface structure of the oblique evaporated film changes with the evaporation angle (the angle between evaporation beam and substrate normal). The structure of stepwise grooves and the array of columns change their angle and density. 

The surface structure of the oblique evaporated film is illustrated schematically as in Fig. 3.3.2. There are many grooves and columns, the features of which vary with evaporation angle and change in the nematic alignment direction and pretilt angle. 

Fig. 3.3.1 SEM image for the columnar structure of an SiO oblique evaporated film surface.

Fig. 3.4.2 The temperature dependence of the magnitude of the liquid crystal order parameter on grooved surfaces with various pitches (4). Solid lines show the cases using grating surfaces with pitches of 0.5, 0.75, 1, and 2 pm. The broken line shows the case for the surface made by SiO oblique evaporation. The lines, (a) and (b), show the values in the bulk for the LC cells using grating surfaces with pitches of 0.5 and 2 pm, respectively.

Fig. 3.7.3 Liquid crystal molecular orientation on obliquely evaporated SiO surfaces (a) without treatment by surface active agent, (b) with treatment by surface active agent.

Finally, although they are not described in this chapter, we must note some important and interesting studies such as those of the oblique evaporation method [43], the twisted FLC [44], an oblique layered structure that exhibits an analogue grey scale [45], etc. 

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