Organization of this Thesis

The thesis is organized as following: The principle of the phase separation, classification of liquid crystal, and fabrication method is presented in Chapter 2.After that, in Chapter 3, the novel fabrication process of the polymer walls in FLC device will be introduced in detail, and the major instruments used to measure the polymer walls, such as scanning electron microscope (SEM), will also be described. The experimental results, including the phase separation, polymer walls, and discussions, will be in Chapter 4. Finally, the conclusions of this thesis will be presented in Chapter 5.

Chapter 2

Principle _________________________________________

2.1 Liquid Crystal

Solid phase, liquid phase, and gaseous phase can express all the things in the world. The phase of matters can be changed upon different temperature and pressure of the matters. The LC phase is a distinct state of matter observed between the solid and liquid states. Therefore, LC phase is called mesophase, and the liquid crystal is also described as mesogen. There are several types of LC states, depending upon the amount of order in the material. From low temperature, the material begins in the crystalline (solid) state, it undergoes a phase change. The first LC phase is smectic phase, where there is layer-like arrangement as well as translational and rotational motion of the molecules. A further increase in temperature leads to the nematic phase, where the molecules rapidly diffuse out of the initial lattice structure and from the layer-like arrangement as well. At the highest temperatures, the material becomes an isotropic liquid where the motion of the molecules changes yet again. This section will explain the classifications of LC materials [10].

2.1.1 Nematic Phase

The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction, means the LC molecules will align along one direction. The molecules point vertically but are arranged with no

particular order, as shown in Fig 2.1. Nevertheless, if the nematic LC molecules are functioned by applying an electric field, the molecules will be aligned along the direction of the electric field due to the dipole effect, as shown in Fig 2.2. Nematic liquid crystal phase is the first one to be applied to LCD industry, such as twisted nematic (TN) type and super twisted nematic (STN) LCDs.

Fig. 2.1 Schematic diagram of the structure of nematic phase.

Fig. 2.2 The structure of nematic phase in the directional electric field.

2.1.2 Smectic Phase

The smectic state is another distinct mesophase of LC substances. In the smectic state, as shown in Fig 2.3, the molecules maintain the general orientation order of nematic phase, but also tend to align themselves in laminated layers or planes.

Smectic phase can be divided into several classifications by slight distinctions.

For example, the direction of the molecule is perpendicular to the smectic plane in the Fig. 2.3 The diagram of layer-like arrangement

of smectic phase.

smectic-A mesophase. However, in the smectic-C mesophase, molecules are arranged as in the smectic-A mesophase, but the director is at a constant tilt angle measured normally to the smectic plane, the comparison between smectic-A mesophase and smectic-C mesophase is shown in Fig 2.4.

2.1.3 Chiral Smectic-C Phase (Smectic-C*)

The smectic-C mesophase has a chiral state designated C* [11]. Consistent with the smectic-C, the director makes a tilt angle with respect to the smectic layer. The

Fig. 2.4 (a) the directions of molecules in smectic-A phase are normal to the layer, and (b) the molecules in smectic-C phase have a tilt angle θ from the axis.

difference is that this angle rotates from layer to layer forming a helical structure in the chiral smectic-C (Smectic-C*). In other words, the director of the smectic-C*

mesophase is not parallel or perpendicular to the layers, and it rotates from one layer to the next, as shown in Fig 2.5. Due to its ferroelectricity, this phase is also called FLC phase.

Fig. 2.5 Sketch of chiral smectic-C phase, molecule will align and switch as a cone structure. Black vectors are the polarization vectors of the molecules.

2.1.4 Chiral Nematic Phase (Cholesteric Phase)

The chiral nematic phase (N* phase) is also called Cholesteric phase [12]. This phase is typically composed of nematic mesogenic molecules containing a chiral center. The chiral center produces intermolecular forces that favor alignment between molecules at a slight angle to one another. This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those above and below. In this structure, the directors actually form in a continuous helical pattern about the layer normal as illustrated in Figure 2.6.

Fig. 2.6 Diagram of chiral nematic phase. Light with specific wavelength can be reflected by the liquid crystal film.

One of the important characteristic of the cholesteric mesophase is the pitch. The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix. A byproduct of the helical structure of the chiral nematic phase is its ability to selectively reflect light of wavelengths equal to the pitch length, so that the light will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum [13].

2.1.5 Isotropic Phase

The isotropic phase has properties that the liquid crystal molecules align disorderly. In the isotropic state, all directions are indistinguishable from each other.

In conclusion, this phase is more similar to liquid phase.

2.1.6 Ferroelectric Liquid Crystal (FLC)

When the LC molecule is chiraling, successive smectic C layers show a gradual change in the direction of tilt. As results, the director processes about the z axis from layer to layer, always lying on the surface of a hypothetical cone of angle 2θ as illustrated in Fig 2.7. The angle around the circle of precession is known as the azimuthal angle. Thus, a helical structure is created in the chiral smectic-C mesophase with the pitch being the distance along the z axis needed to reach the same molecular orientation [14].

Smectic-C* LC has a property about optical rotation. According to the asymmetry of the composition of the LC molecule, there are permanent dipole moments in smectic-C* liquid crystal. For a single layer of the smectic-C* phase, LC molecules point towards the same direction, and the director is at a constant tilt angle measured normally to the smectic plane. The dipole moments will not be canceled in smectic-C* liquid crystal phase. The dipole moments will result in a self-spontaneous molecular polarization, and the polarization vector is perpendicular to the molecule

Fig. 2.7 Helical structure of ferroelectric liquid crystal.

and contained in the smectic layer plane. Therefore, all possible directions for the vector are tangent to the circle of intersection of the cone with the plane. The polarization vector is the blue vector, as shown in Fig 2.7.

The phase of FLC will be changed by varying temperature. For example, R2301 FLC has the phase sequence chiral smectic C-64.7^oC-chiral nematic-85^oC- isotropic.

We can choose chiral smectic-C or chiral nematic phase in the FLC devices by controlling the temperature.

2.2 Phase Separation

In LCD technology, some applications require for combining the polymer structure in liquid crystal. For example, flexible display technique, polymer-dispersed liquid crystals (PDLC), and polymer stabilized ferroelectric liquid crystal (PSFLC) applications. As we know, in PDLC application, the liquid crystal molecules must be well mixed with the polymer materials. Besides, the monomers should be perfectly separated from LC material in order to generate polymer wall structures in flexible display technique and PSFLC application. In order to achieve these objectives, the phase separation method between the monomers and LC molecules was studied. This section will introduce the temperature induced phase separation method which liquid crystal molecules can be spread with the monomers spontaneously.

The temperature induced phase separation method was firstly proposed in 1988.

The mixture which is mixed by LC materials and monomers must be heated above the melting point of the monomers. The phase separation phenomenon will be induced during the cooling process [15-16]. LC materials can be separated out as a drop and scattered with the monomers at phase separation temperature. Recently, this method is

widely used in the fabrication process of polymer network and polymer wall structures.

2.3 Polymerization of Monomers

The current polymerization methods include step-growth polymerization, free radical polymerization, and photo-initiated polymerization. Step-growth method will have the defect about incomplete hardening of monomers. Therefore, the photo-initiated polymerization was utilized in our experiment. In order to initiate the polymerization by UV exposure, an additional photo-initiator is required in this proposed method. The concentrations of photo-initiator and exposure time will greatly affect the quality of polymer walls and liquid crystal molecules, this situation will be discussed in following sections.

2.4 Process of Polymer Walls

Polymer walls can improve the mechanical strength of the device, and were utilized in flexible devices and PSFLC displays recently. The fabrication processes of polymer walls in a FLC cell will be introduced in following sections. The current process requires a photomask to complete the polymer wall structures. Nevertheless, it suffered by the alignment and other problems, and the quality of the polymer walls is reduced. Consequently, we proposed an electric field induced method to settle these problems.

2.4.1 Process with a Photo-mask

The traditional fabrication process of polymer walls was proposed by NHK Corporation in last few years [7]. In this method, two substrates which were coated ITO electrodes and polyimide layer were used to compose a cell. The crisscross sections between upper and lower ITO patterns are pixels while the blank areas between pixels will expectantly be fabricated the polymer walls. The space between two substrates was controlled about 2 μm by spacers. The FLC mixture which was mixed with a monofunctional liquid crystalline acrylate monomer was filled into the space; the cell was heated and cooled down until the phase separation temperature very tardily. FLC molecules will be separated at this temperature from the mixture.

Fig. 2.8 Top view of the FLC device.

Significantly, a photomask was utilized to be an anti-dazzling screen which has the same patterns of the ITO pixels, as shown in Fig 2.8 and Fig 2.9. Perfectly overlapping the patterns of the cell and the photomask, light will be blocked by the opaque areas of the photomask. Finally, a laminated FLC cell was then irradiated with UV light through a photomask to form the polymer walls by polymerizing the monomers floating in UV exposed areas. Fig 2.10 is the flowing sketch of this fabrication process.

Fig. 2.9 Sketch of the photomask. Black parts are the opaque sections which can block UV light from irradiating.

Based on this temperature induced method, there will be some sufficient unreacted monomers must remain in the masked area. This situation results the poor separation quality of polymer molecules. Besides, the polymerization process will also take much time in forming polymer wall structures. Following, the novel

(a) (b)

(c) (d)

Fig. 2.10 Sketches of traditional temperature induced phase separation method under UV exposure, (a) initial condition (b) migration (c) aggregation, and (d) polymer wall formation.

separation method will be introduced. Some drawbacks of temperature induced method can be successfully improved.

2.4.2 Proposed Process with an Electric Field

The electric field induced method is that an electric field is applied to separate the monomers from ferroelectric liquid crystal [17-18], as shown in Fig 2.11. The electric field between upper and lower electrodes was generated by applying an AC voltage to the electrodes. The AC signal which is shown in Fig 2.12 should be square wave, high frequency, and high amplitude. As we mentioned before, the major axis of nematic liquid crystal molecule will follow the direction of the electric field.

Therefore, the direction of liquid crystal molecule can be switched up and down by turns in high frequency by controlling the waveform of the AC signal. The monomers which were mixed in the LC materials can be squeezed out of the areas where the LC molecules were being affected by an electric field. Compared to temperature induced phase separation method, the process time of this method can be shortened.

Figure 2.12 Waveform of AC signal

(a) (b)

(c) (d)

(e)

Fig. 2.11 The flow chart of the novel electric field induced phase separation method. (a) before progressing, (b) applied AC electric field, (c) phase separation, (d) photo-polymerization, and (e) formation of FLC cell with polymer walls.

Initially, both of the frequency and amplitude of the AC supplier were chosen as 1 kHz and 20 Volt. However, the preliminary experimental results of phase separation revealed that the separation quality and separation speed were not good. Thus, the comparisons of phase separation results were made by applying the AC voltage with different frequency and amplitude.

The key issue of the electric field induced method is that the temperature of separation process should be in the period of chiral nematic phase. Thus, ferroelectric liquid crystal will switch as our expectation and monomers can be squeezed to assigned areas. However, the polymerization process which was induced by UV exposure after the phase separation process can not be simply defined. Polymerization process of the polymer walls in TN devices can be done at the temperature which

Fig. 2.12 Waveform of the applied signal for driving the FLC devices.

liquid crystal is nematic phase. Generally speaking, the polymerized temperature was chosen as room temperature. Nevertheless, the polymerized temperature in FLC devices can not be room temperature due to that FLC is smectic phase at room temperature. The phase separation results can not be maintained at this temperature.

Thus, the polymerized temperature of FLC devices in our experiment will be shown and discussed in chapter4. With the electric field induced method, a high quality and rapid fabrication process of polymer walls can be attained.

2.4.3 Comparisons of these Processes

The conventional temperature induced phase separation is suffered some obvious defects, such as the quality of phase separation, longer process time, and alignment problem between the photomask and pixels. Tab. 2.1 lists their comparisons according to the separation quality, process time, and other properties. The major drawback of temperature induced method is the phase separation quality between monomers and ferroelectric liquid crystal. On the other hand, the electric field induced one can not compatible with the process of polymer network structures. Among these comparisons, the electric field induced method is the better choice of polymer wall structures than temperature induced one.

2.5 Summary

Polymer walls and polymer networks were utilized in PSFLC devices. In general, the conventional process of polymer walls required a photomask to be utilized as an anti-dazzling screen. However, poor separation quality and long process time were the drawbacks of this method. Therefore, a novel fabrication method which

used an electric field to induce phase separation between FLC and monomers was developed in this thesis. Via this electric field induced phase separation method, a

high quality and rapid fabrication process of polymer walls can be attained.

Process Temperature induced Electric field induced

Alignment photomask Ο Χ

Additional ac voltage Χ Ο

Shorten process time Χ Ο

Better phase separation quality

Χ Ο

Easier process Χ Ο

Compatibility with the

process of polymer network Ο Χ

Ο: Yes Χ: No

Tab. 2.1 Comparisons of temperature induced and electric field induced methods.

Chapter 3

Fabrication and Measurement Instruments _________________________________________

3.1 Introduction

A novel method to fabricate polymer walls in FLC devices was proposed in this thesis. The fabrication process will be also described in this chapter.

The fabrication sequences of the proposed method included the process of the substrate and the cell formulation. The commercial available indium-tin oxide (ITO) glass was cleaned by standard process in advance. After that, the semiconductor process including spin coating, lithography and etching was utilizing in order to obtain the desired pattern. The ITO pattern upon the glass was taken for the transparent electrode. Then the spin coating and rubbing techniques will be used to make the alignment layer.

The cell process joins the ITO substrates with accurate alignment. The cell gap was controlled about 2 um by spacers and the space between the two substrates was filled with the mixture of FLC and monomers. After cell process, an electric field between upper and lower ITO patterns which was applied by the function generator and a hot stage which was used to control the temperature of the device can induce the phase separation phenomena. Finally, a UV light was required in polymerization process.

Besides, the process of phase separation between FLC and monomers was observed by the polarized optical microscope and the performance of the polymer structures was measured by SEM. The major features of the above mentioned

instruments will be illustrated in this chapter.

3.2 Fabrication Process

The features of the novel fabrication process of polymer walls have been introduced in previous chapter. This part will describe the cell fabrication process which includes spin coating, lithography, and wet etching. Moreover, the phase separation process will also be explained.

3.2.1 Cell Fabrication

The detailed fabrication steps are listed below, and the substrate pretreatment is shown schematically in Fig 3.1.

(a) Substrate pretreatment:

(1) Substrate preparation: For the display application, the glass is widely used as a substrate. In the fabrication, the glass of about 1.1 mm thick was chosen.

And ITO was uniformly sputtered on the glass. Before the lithography process, ITO glass was cleaned by acetone and isopropyl alcohol.

(2) Lithography: First of all, positive photoresist was applied and coated on substrate. After soft baking, the ITO glass was exposed under ultra-violet (UV) light source through a mask. Consequently, the pattern on the mask was transformed to the positive photoresist after developing.

(3) Wet etching: After exposure and development, the substrate was etched.

Removing the photoresist, the patterned ITO substrate was obtained.

(4) Polyimide coating: Polyimide thin film was spin-coated upon the ITO layer

and baked by heating. Thereafter, the solid thin film was treated by rubbing technique to perform the substrate preparation.

Fig. 3.1 Flow of fabricating ITO electrodes. (a) Sputtering ITO material on the surface of the glass, (b) spin-coating Positive photoresist upon the ITO surface, (c) using lithography technique to obtain the latent image, (d) etching to produce the desired ITO pattern, and (e) eliminating the remaining photomask by acetone.

(a)

(b)

(c) (d)

(e)

(b) Cell process: Two crossed ITO glasses were glued with sealing gel containing spacers. The diameter of spacer with 2 um is required for maintaining the cell gap.

Finally, the space between two substrates was filled with the mixture of FLC and monomers.

3.2.2 Polymer Walls Process

One function generator is needed to apply the AC voltage between upper and lower ITO electrodes in polymer walls process, and one hot stage is used to accurately control the temperature of the cell. After cell fabrication, the temperature of the cell was risen to higher enough, which can keep FLC in isotropic phase. Thereafter, the AC voltage was applied continually and the temperature of the cell was reduced gradually until phase separation. Finally, the polymer wall structure was produced in desired area after UV exposing at the same time.

3.3 Measurement System

After the fabrication of the polymer wall structure, the inspection will be

After the fabrication of the polymer wall structure, the inspection will be