Background

The applications of liquid crystals (LCs) in optical devices have been widely investigated for decades due to their anisotropic electrical permittivity and magnetic susceptibility [1]. Especially in the area of information display, nematic liquid crystals (NLCs) have received considerable attention due to its promising electro-optical properties. Furthermore, surface alignments of liquid crystals are essential in liquid crystal displays (LCDs). It determines the boundary condition for the molecular orientation at the surface. Currently, the mechanical rubbing is the most conventional method of surface alignment due to its low cost and reliable alignment ability. As a polymer with convincing thermal and mechanical properties, the polyimide (PI) is so far the most favorable alignment material in the conventional rubbing method owing to its high transparency, superior adhesion and chemical stability [2]. However, the mechanical rubbing method which employs a velvet rubbing process on the PI-coated substrate has some drawbacks such as leaving debris and electrostatic charges on the rubbed surfaces. Also, it becomes increasingly difficult to maintain uniformity as the substrate size of LCD gets larger rapidly in industry. Multi-domain or high pretilt angle alignment cannot be easily achieved either.

In order to enhance the qualities of LC products, several contact-free alignment methods such as the photoalignment [3-15], oblique evaporation [16-20] and ion beam alignment [21-27] techniques have been vastly investigated in the past decades. Besides, intense studies have been carried out to develop the alignment materials suitable for each alternative method. These techniques and the related alignment mechanisms are introduced in the following sections.

1.1.1 Photoalignment

In 1991, Gibbons et al. first reported a method for LC alignment that uses

polarized laser light in visible range [3]. A rewritable ability was also discovered by subsequently illuminating the silicone PI copolymer doped with a diazodiamine dye. So far, three kinds of photoreactive polymers have been extensively studied as the photoalignment layer including photo-decomposable polymers [6-9], photocrosslinkable polymers [4,10-12], and photo-isomerizable polymers [13-15]. The corresponding photoreactions have been confirmed as the selective degradation, dimerization reaction (or crosslinking), and isomerization mechanism. Either a parallel or a perpendicular aligning direction can be obtained in these treatments depending on the type of polymer or reaction mechanism. It is worth noticing that the photoreaction of the PI has received considerable attention because of its being widely used in LCDs industries already [6,9,15]. However, long exposure time or high dosage of ultraviolet (UV) irradiation is required to achieve significant effect of surface alignment due to the low photoreactive efficiency of PI.

1.1.2 Oblique evaporation

In 1972, Janning demonstrated a promising method to align LCs using the silicon monoxide or gold film obliquely deposited on the substrates [16]. The angular deposit causes the film to grow in a preferred direction. Only a very thin film with thickness of 70 Å is required for surface alignment. It is interesting that various deposited materials show different results. For example, a copper will give homeotropic alignment, while chromium, platinum, and aluminum align LC to the direction of deposit. In 1980, Uchida et al. investigated the relationship between the pretilt angle and the evaporation conditions including the incidence angle and film thickness [19]. They proposed two methods of varying the pretilt angle in the range of 0° to 30° from the homeotropic alignment. In 1982, Hiroshima proposed another evaporation procedure to obtain a wide tilt range of 0° to 60° from surface normal [20]. Without change of incidence angle, the azimuth of SiO beam changes continuously during deposition. The tilt angle can be controlled by choosing the azimuthal distribution of deposition.

In the oblique evaporation process, a micro columnar structure is realized on the substrate surface due to the self shadowing effect. This columnar structure agrees with the growth process of the surface structure model suggested by van de Waterbeemd [21].

The relationship among the surface topology and LC orientation could be explained well by the columnar structure model proposed by Goodman-van de Waterbeemd [22].

At a deposition angle of ca. 50°, homogeneous alignment with a 0° pretilt angle is realized due to the formed grooves perpendicular to the incidence plane of beam.

1.1.3 Ion beam alignment

In 1998, Chaudhari et al. from IBM research group found that the LCs can be aligned on the PI surface exposed to a low energy and neutral argon ion beam [23].

They also successfully realized this non-contact alignment technology by integrating low energy ion beam equipments and diamond-like carbon (DLC) thin films into LCD manufacturing processes [24]. In 2001, Stöhr et al. demonstrated that the anisotropical changes of carbon double or triple bonds caused by ion bombardment are responsible for introducing the surface orientational anisotropy [26]. That means any amorphous carbon layer with directional nature of sp² and sp bonds can induce the alignment of LCs. Besides carbon, also a great variety of other materials, such as SiN_x, SiC, SiO₂, Al2O3, CeO2, SnO2, ZnTiO2 and InTiO2 can be used as alignment materials.

Over the past few years, several studies devoted to ion-beam bombarded DLC and PI films have also been reported [27-29]. One of the most remarkable results is that the homeotropic alignments can be obtained by using fluorinated DLC thin films as the alignment layer and the pretilt angle can also be controlled by choosing different ion-beam parameters or the concentration of fluorine doped in DLC films [29]. In addition, both homogeneous and homeotropic alignments can be obtained with the same kind of organic alignment layers bombarded by ion beams with different energies [27,28]. This remarkable ability of controlling the alignment modes makes the ion-beam alignment method potentially useful in LC-based applications, especially in LCDs industry.

1.1.4 Alignment mechanism

For a conventional rubbing method, the preferential alignment of polyimide chain segments along the rubbing direction is formed. The epitaxial effects are suggested responsible for the LC alignment [30]. Another mechanism suggested by Berreman is

that the LC molecules prefer to align with the microgrooves created by rubbing process, so as to the total surface free energy is reduced [31]. However, LC alignment also occurs on surfaces of disordered polymers. Most recently, Stöhr et al.’s results suggest that LC alignment only requires a statistically significant preferential bond orientation at the polymer surface, without the necessity of crystalline or quasi-crystalline order [32].

A general directional interaction model was proposed in which the LC direction is guided by a π orbital interaction between the LC molecules and the anisotropic polymer surface. Even materials without translational order, i.e. amorphous materials, may have orientational order because of the strong directional nature of unsaturated carbon bonds [33]. Rubbing, UV irradiation, and ion beam bombardment are examples of methods that can produce orientational order.