Chapter 2 Principle
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 performed to make sure that the polymer structures were conformed to the expectancy.
Thus, polarized optical microscope is required for observing the polymer walls.
Optical microscope with the combination of an ocular and an object lens is the most popular instrument of observing a structure. A transmissive or reflective light can be chosen to observe the micro structure. Besides, a computer-controlled display (CCD) and a computer are usually equipped with the microscope and the picture of observation can be taken from the CCD. In display research, a polarized optical microscope is used to study the behavior of liquid crystal through a pair of polarizer,
especially. Although the magnifying power of general optical microscope has a limitation, a simple operating system still popularizes its application.
A desired angle of the polarized direction between two polarizers can be adjusted manually. In our experiment, a polarized optical microscope was utilized to observe the phase variation between FLC and monomers in encapsulation cells.
Chapter 4
Experimental Results and Discussions _________________________________________
4.1 Introduction
The electric field induced fabrication method of polymer wall structures was introduced in previous chapter. As mentioned before, fast process time and perfect separation quality are the main advantages of electric field induced method. This novel method can be utilized to form polymer walls in not only TNLC device but also FLC species. Therefore, electric field induced method was attempted to form polymer walls in TNLC devices. A polarized optical microscope was utilized to observe the phase separation and experimental results. The results will be shown in this chapter.
After fabricating polymer walls in TNLC device, the proposed process will also be evaluated in R2301 and R3206 FLC devices. The experimental results and comparisons between different materials will then be further discussed.
4.2 Experimental Results in TNLC devices
Polymer wall structure in TNLC device was fabricated by electric field induced method. The specification of AC voltage which was applied to induce phase separation of NOA65 and TNLC was shown in Tab. 4.1. According to the electric field induced process which discussed before, the temperature of polymerization was the most important in this method, and the temperature will greatly affect the result of polymer walls. In order to observe the phase separation temperature in TNLC devices,
a hot stage and polarized optical microscope was required. The temperature of the devices can be exactly controlled by the hot stage. As the observation by the microscope shown, the phase separation temperature of TNLC devices is 46oC
Tab. 4.1 Specification of applied AC voltage in TNLC device.
Parameter Condition
Waveform Square wave
Frequency 1k Hz
Amplitude 100 V (pk-pk)
The phase of TNLC material is nematic at 46 oC. After injecting the mixture of TNLC and NOA65 at 90oC, the temperature of the device was decreased to 46oC and AC voltage was applied to induce electric field between upper and lower ITO substrates. The electric filed can separate TNLC and NOA65 about 5 minutes. The polymer walls can be formed after polymerization by UV exposure at 46 oC. The experimental result of fabricating polymer walls in TNLC device is shown in Fig. 4.1.
The width of polymer walls is about 20 μm and the pixel area is about 200 by 200μ m. In pixel area, the nematic phase of liquid crystal can be clearly observed. The wall structure was formed in the gaps between pixels, and there are no monomers residual in pixel areas. Thus, the contrast ratio of the TNLC device can be maintained and will not be decreased by the residual of monomers in pixels. Fig. 4.2 shows another picture of TNLC device which was taken by an optical microscope with 45degree polarization. The dark and bright states of the TNLC device driven by AC voltage can be observed in Fig. 4.3.
Fig. 4.1 The structure of polymer walls in TNLC device.
Fig. 4.2 The structure of polymer walls in TNLC device at 45degree polarization.
(a)
(c)
(b)
(d)
(e)
Fig. 4.3 The TNLC device which contains polymer walls is driven by an AC voltage of (a) 0V, (b) 1V, (c) 2V, (d) 3V, and (e) 4V.
The temperature of polymerization is quite important, and it will greatly affect the quality of polymer walls. Fig. 4.4 shows the experimental result of the device which was polymerized at 50oC. The polymer walls are incomplete in this case, and there is no liquid crystal phase in pixel areas due to the residual of monomers in pixels.
The temperature of phase separation is 46oC, therefore, phase separation of TNLC and NOA65 is still incomplete at 60oC. If the device is exposed by UV light at this temperature, the formulation of polymer walls will be incomplete.
Fig. 4.4 The structure of imperfect polymer walls in TNLC device.
The final results will be affected by different polarization temperature. If the device is photo-polymerized at the temperature higher than phase separation temperature, polymer walls will not be formed in the gaps between pixels. However, monomers will be solidified in pixel areas and the liquid crystal phase of TNLC will also be destroyed, as shown in Fig. 4.5. The dark and bright states of these imperfect devices can not be observed easily. The contrast ratio of these devices is very poor.
(a) 70 oC (b) 65 oC
(c) 60 oC (d) 50 oC
Fig. 4.5 The photographs of TNLC device polymerized at (a) 70oC, (b) 65 oC, (c) 60 oC, and (d) 50 oC.
The process time of fabricating polymer walls in TNLC devices is about 30 minutes, and the quality of polymer walls in TNLC device is quite complete if the device was exposed by UV light at phase separation temperature. As the results, the electric field induced method can achieve a fabrication process of polymer walls with faster process time and better separation quality.
4.3 Experimental Results in FLC devices
The proposed process was firstly examined to form polymer walls in TNCL device by applying electric field, as shown before. After that, instead of TNLC material, FLC material was mixed with NOA65. The electric field induced process was utilized to form polymer walls in FLC devices. The result of polymer walls in FLC device will be shown in this section.
4.3.1 Polymer Structures in R2301 FLC devices
The FLC material used in this experiment was R2301 FLC. The phase transition of R2301 FLC was isotropic 86.8 oC – 84.8 oC chiral nematic 64.7 oC chiral smectic.
R2301 was firstly mixed with NOA65 as 20 % concentrations. In the beginning, a hot stage and a polarized optical microscope were utilized to observe the phase separation temperature of the mixture of R2301 and NOA65. The phase separation temperature was about 84 oC to 88 oC, as shown in Fig. 4.6. When the temperature of the FLC device is higher than 88 oC, R2301 FLC is in isotropic phase. As the temperature of the device decreased to 88 oC, the phase of R2301 FLC started transferring to chiral nematic phase, as shown in Fig. 4.6.
Fig. 4.6 The phase separation process of the mixture of R2301 FLC and NOA65. The temperature of the device is (a) 88 oC, (b) 87 oC, (c) 86 oC, (d) 85 oC, (e) 84 oC, and (f) 83oC.
(a) 88 oC (b) 87 oC
(c) 86 oC (d) 85 oC
(e) 84 oC (f) 83 oC
The observation of phase separation temperature can help us finding the photo-polymerization temperature more easily. After that, an AC voltage was applied to the FLC device in order to separate R2301 FLC and NOA65 as our expectation.
The specification of the AC voltage is shown in Tab. 4.2.
Tab. 4.2 Specification of applied AC voltage in R2301 FLC device.
Parameter Condition
Waveform Square wave
Frequency 2 kHz
Amplitude 120 V (pk-pk)
The viscosity of R2301 FLC is higher than that of TNLC at the same temperature.
Thus, the separation quality of R2301 FLC and NOA65 is not easy to be controlled, and the separation rate is slower than that of the TNLC device. In order to speed up the process of phase separation, the frequency of the AC voltage applied to induce phase separation was raised to 2k Hz. Besides, the amplitude of the AC voltage was also increased to 120 volt. Phase separation of R2301 and NOA65 can be successfully induced by applying AC voltage to the device. The photographs of separation process were shown in Fig. 4.7. The photographs in Fig. 4.7 were taken by a polarized optical microscope with different temperature.
Fig. 4.7 The phase separation process of the mixture of R2301 FLC and NOA65 with an AC voltage. The temperature of the device is (a) 89 oC, (b) 88 oC, (c) 87 oC, and (d) 86 oC.
(a1) Cross-polarized mode (a2) Non-polarized mode
(b1) Cross-polarized mode
(c1) Cross-polarized mode
(d1) Cross-polarized mode
(b2) Non-polarized mode
(c2) Non-polarized mode
(d2) Non-polarized mode
Fig. 4.7 The phase separation process of the mixture of R2301 FLC and NOA65 with an AC voltage. The temperature of the device is (e) 85 oC, (f) 84 oC, (g) 83 oC, and (h) 82 oC.
(e1) Cross-polarized mode
(f1) Cross-polarized mode
(g1) Cross-polarized mode
(h1) Cross-polarized mode (h2) Non-polarized mode (g2) Non-polarized mode (f2) Non-polarized mode (e2) Non-polarized mode
The phase separation of FLC and NOA65 induced by electric field is shown in Fig. 4.7. When the temperature of the FLC device was lower than 90 oC, the phase of FLC is transferring to chiral nematic phase. As decreasing of the temperature, more FLC molecules transfer to chiral nematic phase. This phase separation process is similar to the process shown in Fig. 4.6 which was applied with no AC voltage. The difference between the separation processes in Fig. 4.6 and Fig. 4.7 is AC voltage.
FLC device was applied with no AC voltage in Fig. 4.6 so that the phase separation of FLC and NOA65 was uniform in the device. Nevertheless, the process which NOA65 was separated to the gap between pixels as a network structure was due to the function of AC voltage. According to the principle of electric field induced phase separation method introduced before, FLC molecules were switched up and down by applying AC electric field and NOA65 were squeezed to the gap of pixels as polymer wall structures.
The best quality of the phase separation process was in 84 oC. However, while the temperature of the device was lower than 84 oC, the separation quality became incomplete, as shown in Fig 4.7(h). The reason is that the viscosity of FLC is larger than TNLC. Besides, the viscosity of FLC will be decreased in high temperature, so that phase separation will be induced more completely in high temperature.
Although NOA65 can be induced and separated to the gaps between pixels by applying an electric field, the polymer wall structures still can not be formed in R2301 FLC device. A UV light was utilized in photo-polymerization process in order to solidify NOA65. During the phase separation process, the R2301 FLC device must be avoided from UV exposure, or NOA65 will be polymerized before phase separation.
The structure of polymer wall in R2301 FLC device was shown in Fig. 4.8. This incomplete polymer structures resulted in lower transmittance of light source in the FLC device.
The switching property of R2301 FLC was affected by the polymer structure in pixel areas. Black and Bright states of the FLC device can not be clearly distinguished.
Fig. 4.9(a) is the bright state of the FLC device which contains incomplete polymer structures. Chiral nematic FLC is confined in small sections, while the dark area is the polymer structures in the device, as shown in Fig. 4.9. Although R2301 FLC and NOA65 can be separated in our device by the electric field induced method, nevertheless, the polymer wall structure still can not be formed successfully. After the phase separation process, the FLC device was then exposed by UV light. However, NOA65 was not photo-polymerized in R2301 FLC device after UV exposure.
Fig. 4.8 The incomplete polymer structures in R2301 FLC device.
Fig. 4.9 The state of incomplete polymer structures in R2301 FLC device.
(a) Cross-polarized mode and (b) 30o polarized mode.
(a)
(b)
The neat NOA65 generally can be photo-polymerized by UV exposure in 5 minutes. However, after mixing NOA65 with R2301 FLC material, NOA65 can not be photo-polymerized by UV exposure in R2301 FLC device. Although the device was exposed by UV light in more than 1 hour, NOA65 was still in liquid phase.
Several experiments were performed in order to solve the photo-polymerization issue in R2301 FLC device. The issues of this photo-polymerization are:
(1) Temperature effect: Photo-polymerization of NOA65 can not be induced in the temperature higher than 60 oC due to the side reaction between NOA65 and FLC.
(2) Electric field effect: Photo-polymerization of NOA65 was restrained by the electric field.
(3) Chemical reaction effect: R2301 FLC reacted with NOA65 or photo-initiators while blending.
In order to figure out the photo-polymerization issue, NOA65 was exposed by UV light at 90 oC without applying an electric field. As a result, NOA65 can be solidified by the UV exposure in 5 minutes, as shown in Fig. 4.10. Thus, the temperature effect was not the key issue in photo-polymerization problem.
Concerning the electric field effect, NOA65 was exposed by the UV light with the effect of an electric field, as shown in Fig. 4.11. NOA65 can be solidified even though it was functioned by an electric field.
According to these tests, chemical reaction between R2301 FLC and NOA65 was the most probable reason of the photo-polymerization issue. Base on this study, R2301 FLC was replaced by R3206 FLC to fabricate polymer walls in our device.
Fig. 4.10 Exposing NOA65 by UV light at 90 oC without applying an electric field.
Fig. 4.11 Exposing NOA65 by the UV light with the effect of an electric field.
4.3.2 Polymer Structures in R3206 FLC devices
R2301 FLC was estimated that it may react with NOA series monomers, so that R3206 was utilized to replace R2301 FLC material. R2301 was mixed with monomers as 20 % concentrations and blended at 100 oC. The phase transition of R3206 FLC was isotropic 107.0 oC – 105.4 oC chiral nematic78.4 oC chiral smectic.
The separation process of R3206 and monomers was the same with that in R2301 device. The specification of the AC voltag applied to induce phase separation of NOA65 and R3206 FLC is shown in Tab. 4.3.
NOA65 was firstly utilized to be blended with R3206 FLC. The process of phase separation between R3206 FLC and NOA65 was shown in Fig. 4.12. Black areas are NOA65 material, and the others are R3206 FLC. Unfortunately, NOA65 can not be concentrated in the gaps under applied AC voltage.
Tab. 4.3 Specification of applied AC voltage in R3206 FLC device.
Parameter Condition
Waveform Square wave
Frequency 500 Hz
Amplitude 120 volt (pk-pk)
Fig. 4.12 The phase separation process between R3206 FLC and NOA65 at (a) 80 oC, (b) 70 oC, and (c) 60 oC.
(a) 80 oC
(b) 70 oC
(c) 60 oC
Although NOA65 can not be separated to the gaps between pixels by switching R3206 FLC molecules in high frequency, photo-polymerization of NOA65 can be successfully induced in R3206 FLC device. Compared with R2301 FLC device, NOA65 can be solidified by UV exposure, as shown in Fig. 4.13. Black areas are polymer structures, and the others are R3206 FLC structures. This photograph showed that NOA65 can not be utilized to form polymer walls in R3206 FLC device.
Although NOA65 can not be separated to the gaps between pixels by switching R3206 FLC molecules in high frequency, photo-polymerization of NOA65 can be successfully induced in R3206 FLC device. Compared with R2301 FLC device, NOA65 can be solidified by UV exposure, as shown in Fig. 4.13. Black areas are polymer structures, and the others are R3206 FLC structures. This photograph showed that NOA65 can not be utilized to form polymer walls in R3206 FLC device.