國 立 交 通 大 學
電子物理系
博 士 論 文
直流離子濺鍍機於液晶配向的應用
及其配向特性之研究
Study on the Alignment Properties of Liquid Crystals on
the Surfaces treated by a Direct-Current Ion Sputter
研 究 生：吳信穎
指導教授：趙如蘋 教授
直流離子濺鍍機於液晶配向的應用
及其配向特性之研究
Study on the Alignment Properties of Liquid Crystals on
the Surfaces treated by a Direct-Current Ion Sputter
研 究 生：吳信穎
S t u d e n t：Hsin-Ying Wu
指導教授：趙如蘋
A d v i s o r：Ru-Pin Pan
國 立 交 通 大 學
電子物理系
博 士 論 文
A ThesisSubmitted to Department of Electrophysics College of Science
National Chiao Tung University in Partial Fullfillment of the Requirements
for the Degree of Doctor of Philosophy
in Electrophysics
Feb 2009
Hsinchu, Taiwan, Republic of China
直流離子濺鍍機於液晶配向的應用
及其配向特性之研究
學生：吳信穎
指導教授：趙如蘋 博士
國立交通大學電子物理系
摘
要
液晶配向技術早已廣泛的應用在液晶顯示科技領域。在這段期間，由於傳統 摩刷配向方法本身所具有的缺點限制了顯示相關技術品質的提昇，因此有許多新 的配向技術陸續被開發出來。本論文係利用一台構造簡單的直流離子濺鍍機來產 生離子源，以提供液晶配向處理之用。利用電漿放電的不同區域來進行表面處理， 能提供不同的液晶配向效果。分別利用偏光顯微鏡與聚光干涉儀來鑒別所組合成 之液晶盒的配向效果，並且進一步量測液晶盒的表面定向錨定強度與預傾角。此 外，利用原子力/磁力顯微鏡、表面化學成分分析儀、紫外/可見光譜儀、超導量 子干涉磁量儀及接觸角量測系統深入探討造成表面配向與預傾角的機制是本論文 的研究重點。 首先，我們發現不同的離子束能量可以使液晶產生兩種配向效果，包括水平 配向及垂直配向。經證實於高能量離子轟擊的狀況下，會在配向膜表面堆積一定 量的氧化鐵。也因此，我們提出一種利用單一步驟濺鍍所形成的薄膜來產生垂直 配向方法。此種薄膜經由表面分析證實是由許多顆粒大小約三十奈米的粒子所組 成，且表面具有磁化強度的異向性。由表面錨定強度與飽和磁化強度的比較結果， 我們推測垂直配向的效果是起因於磁性薄膜所產生的磁場。因此我們假設一個簡 單的表面模型，描述兩兩相鄰但所具備磁矩的方向相反的磁區所產生的磁場空間 分布。經由與液晶間的力矩作用所產生的能量計算，進一步推得表面錨定能的大小並與實驗量測值做比較。 此外，我們利用偏振紫外光照射與離子轟擊一種新型帶有含氟羰基側鏈的感 光性配向膜。並觀察含氟側鏈的不同濃度對於液晶配向效果的影響。發現經由子 外光照射過的表面有明顯較大的預傾角。相反地，離子轟擊過後的表面所得到的 預傾角很小。經由表面分析證實，含 CF2 的側鏈結構含量的確在影響表面預傾角 的大小上扮演重要的角色。此外，在經離子轟擊過後的表面發現含氧量的大幅增 加，這也解釋了極性表面能的增加現象。在這份工作中，表面的極性力被證實是 影響液晶預傾角的主要因素。 最後，我們利用輝光放電區對聚醯亞胺膜表面進行處理，同時濺鍍氧化鐵於 其上。發現在所加偏壓低於七百伏特的條件下，所處理過的薄膜表面對液晶產生 品質與穩定度皆不錯的水平配向效果。此外，預傾角能透過調變偏壓或處理時間 來改變其大小。其產生預傾角的機制也在本論文探討的範疇內。
Study on the Alignment Properties of Liquid Crystals on
the Surfaces treated by a Direct-Current Ion Sputter
Student: Hsin-Ying Wu
Advisor: Dr. Ru-Pin Pan
Department of Electrophysics
National Chiao Tung University
ABSTRACT
Liquid crystal alignment technologies have been widely used in liquid crystal display industry in the past decades. Meanwhile, numerous alignment methods have been intensely developed to replace the conventional rubbing method due to its intrinsic shortcoming. In this thesis, the liquid crystal alignments on surfaces treated by ion bombardments are achieved by utilizing a direct-current ion sputter with diode-type electrodes. Various conditions of plasma discharge offer a variety of alignment effects. The alignment effects of nematic liquid crystal on plasma/ion-treated surfaces are characterized using polarizing optical microscope and conoscope. The surface polar anchoring strength and pretilt angle have also been studied. Furthermore, the corresponding mechanisms of pretilt angle and alignment effect are studied using atomic/magnetic force microscope (AFM/MFM), x-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-VIS), super conducting quantum interference device (SQUID), and apparatus of contact angle measurement.
First of all, we found that two alignment modes, homogeneous alignment and homeotropic alignment, can be induced by bombardments of ion accelerated by different electric bias. According to the discovery of iron oxide on the surface bombarded by high-energy ion, a method for homeotropic alignment of liquid crystals
using a one-step, ion beam sputtering on glass substrates is proposed. The surveyed surface morphology reveals that the films are composed of granular nanoparticles with dimensions around 30 nm. Anisotropy of magnetization is also found on the sputtered ferric films. Polar anchoring strength and saturation magnetization of the coated films of different thicknesses are compared with each other. Accordingly, we deduce that the homeotropic alignment is achieved probably due to the orientation of the diamagnetic nematogenic molecules in the magnetic field caused by the γ-Fe2O3 ferrimagnetic thin
films. Therefore, a simple model of alternatively distributed magnetic moments with opposite direction is proposed. The profile of magnetic field strength near the surface is then evaluated and taken into the calculation of polar anchoring strength.
Besides, surface treatments accomplished through the ultraviolet irradiation and ion bombardment on a newly developed copolymer materials with fluorinated carbonyl groups of different mole fraction and their effects on liquid crystal surface alignments have been studied. The surface alignment with high pretilt angle is achieved by the ultraviolet treatment. The induced pretilt angle by ion bombardments on this photo-reactive polymer film, however, is relatively small. It is confirmed that the content of CF2 grafted side chains plays a dominant role while determining the pretilt
angle. A significant increase of oxygen content has also been found to be responsible for the increase of polar surface free energy in ion bombardments. Consequently, the polar force contributed to the surface tension has been proved to dominate the induced pretilt angle.
Finally, we demonstrate that the polyimide surfaces are treated by oblique plasma irradiation with simultaneous coating of maghemite nanoparticles. Homogeneous alignments of liquid crystals with good quality and reliability are obtained on the polyimide surfaces treated with plasma energy lower than 700 V. The pretilt angle can be controlled by different plasma energies and treating time. The mechanism in determining the pretilt angle has also been discussed.
誌 謝
轉眼間，博士班的研究生涯即將進入尾聲，非常感謝我的指導老
師 趙如蘋 教授在這段過程中，適時的給予指導與鼓勵，讓我受益匪
淺。老師的包容在研究上讓我有足夠的空間來自由發揮，才能夠讓我
擁有今天這些成果。過程中所獲得的滿足與喜悅也是支撐我走完這段
路的動力。謝謝老師！在這邊，也感謝這幾年來給實驗室帶來歡笑的
學弟妹們，以及你們在實驗上的幫忙。
最後，僅將這份成果獻給我親愛的父母與女友，你們的善體人意
讓我無後顧之憂，才能平穩地走過這一切。有你們真好，謝謝你們！！
信穎
Table of Contents
中文摘要 ………...……… i Abstract ………...………… iii 誌謝 ………...……… v Table of Contents ………... vi List of Figures ………...……… xList of Tables ………...……….... xviii
1. Introduction ………... 1
1.1 Background ……….. 1
1.1.1 Photoalignment ………. 1
1.1.2 Oblique Evaporation ……… 2
1.1.3 Ion Beam Alignment ……… 3
1.1.4 Alignment mechanism ………. 3
1.2 Overview of this thesis ………. 4
References ………... 6
2. Controllable Alignment Modes of Nematic Liquid Crystals on Argon Ion Beam Bombarded Polyimide Films ...………. 8
2.1 Overview ……….. 8
2.2 Ion beam treatment ………... 8
2.2.1 Ion source ………..………... 8
2.2.2 Ion beam conditions ………. 9
2.3 Experiments ……….. 9
2.3.1 Sample preparation ………... 10
2.3.2 Alignment characterization ……….. 10
2.3.3 Surface analyses ………... 10
2.4.1 Alignment characterization ……….. 12 2.4.2 Surface morphologies ………... 13 2.4.3 XPS analyses ……… 14 2.5 Concluding remarks ………. 18 References ………... 20 Figures ………... 22 Tables ………... 47
3. Liquid Crystalline alignment on the Ion Beam Sputtered Magnetic Thin Film ………. 50
3.1 Overview ……….. 50
3.2 Experiments ……….. 50
3.2.1 Ion beam treatment ………... 51
3.2.2 Sample preparation ………... 51
3.2.3 Alignment characterization ……….. 51
3.2.4 Surface analyses ………... 52
3.2.5 Magnetic properties ……….. 52
3.3 Results and discussion ……….. 52
3.3.1 Film characterization ……… 53 3.3.2 Alignment characterization ……….. 53 3.3.3 Magnetic properties ……….. 54 3.3.4 Surface morphology ………. 56 3.3.5 Theoretical analyses ………. 57 3.4 Concluding remarks ………. 60 References ………... 62 Figures ………... 63 Tables ………... 102
4. Liquid Crystal Alignments on the Fluorinated Copolymer Films Treated by
Ion Beam Bombardment and Ultraviolet Irradiation ....……….. 103
4.1 Overview ……….. 103 4.2 Experiments ……….. 103 4.2.1 Surface treatments ……… 104 4.2.2 Sample preparation ………... 105 4.2.3 Alignment characterization ……….. 105 4.2.4 Surface analyses ………... 105
4.3 Results and discussion ……….. 106
4.3.1 Alignment characterization ……….. 106
4.3.2 XPS analyses ……… 106
4.3.3 Surface free energy ………... 108
4.3.4 Depth-dependence of fluorine content ………. 109
4.4 Concluding remarks ………. 110
References ………... 112
Figures ………... 113
Tables ………... 128
5. Tilt Alignment of Liquid Crystals on the Polyimide Surface Treated by Plasma Irradiation with Simultaneous Coating of Magnetic Nanoparticles ……..……. 130
5.1 Overview ……….. 130
5.2 Experiments ……….. 130
5.2.1 Plasma beam treatment ………. 130
5.2.2 Sample preparation ………... 131
5.2.3 Alignment characterization ……….. 131
5.2.4 Surface analyses ………... 131
5.3.1 Alignment characterization ……….. 132 5.3.2 Surface morphologies ………... 132 5.3.3 Surface energy ……….. 133 5.3.4 XPS analyses ……… 134 5.4 Concluding remarks ………. 136 References ………... 138 Figures ………... 139 Tables ………... 151 6. Concluding Remarks ………..……… 152 6.1 Looking back ……… 152 6.2 Looking forward ………... 154
Appendix A Ion sources ……….………... 156
Appendix B Polar anchoring strength measurement ……….... 162
Appendix C Simulation of magnetic field distribution ……… 171
Appendix D Estimation of the magnetic field strength for liquid crystal alignment ……….. 178
List of Figures
Figure 2.2.1 Structural view of the DC ion-beam sputter operated in coating
mode ……….... 20
Figure 2.2.2 Sketch of argon ion-beam bombardment: (a) The space
distribution of glow discharge in a diode sputter operated in etching mode, and (b) the arrangement of glass substrates in the
sputter ……….. 20
Figure 2.3.1 Procedures of cleaning process ………... 21
Figure 2.3.2 A Shirley background computed from a Fe 2p spectrum ………... Figure 2.4.1 The POM photographs of NLC 5CB cells treated by different ion
beam conditions: (a) homogeneous alignment, and (b) homeotropic alignment. (Inset: conoscopic pattern)……… 22 Figure 2.4.2 Pretilt angle vs. the ion beam bombarding time with Vb of 560 V
(top) and 1120 V (bottom) ………... 22 Figure 2.4.3 Polar anchoring strength Wp of 5CB cells determined for (a)
various τ with θion=60° and (b) various θion with τ=8 min for all
with Vb=560 V ………. 23
Figure 2.4.4 AFM images of the PI films treated for different τ: (a) As-deposited, (b) 6 min, (c) 14 min, and (d) 30 min with ion
energy =560 V. The films here give homogeneous
alignments ………... 23
Figure 2.4.5 Roughness of the surfaces treated with ion energy Vb=560 V for
various τ ……….………. 24
Figure 2.4.6 AFM images of the PI films treated by different ion energies: (a) 560 V, (b) 840 V, and (c) 1120 V with τ=5 min ……….. 25
Figure 2.4.7 Primary structure of polyimide ………….……….. 26
Figure 2.4.8 Survey spectra for the untreated and ion-beam treated PI films with various Vb, τ=5 min, θion=60°, and Jion=255 μA/cm2 ……… 26
Figure 2.4.9 Multiplex spectra for the untreated and ion-beam treated PI films with various Vb, τ=5 min, θion=60°, and Jion=255 μA/cm2 ………. 27 Figure 2.4.10 Deconvolution of the (a) C1s and (b) O1s spectra of the untreated
and treated PI films with =1120 V ………..…………..………. 28
Figure 2.4.11 Fraction of the components contributing to the (a) C1s and (b) O1s
core-level spectra of untreated and ion-beam treated PI films as a
Figure 2.4.12 Survey spectra for the untreated and ion-beam treated PI films with different τ, =560 V, θion=60°, and Jion=255 μA/cm2 …….. 30 Figure 2.4.13 Multiplex spectra for the untreated and ion-beam treated PI films
with different τ, =560 V, θion=60°, and Jion=255 μA/cm2 ..…… 31 Figure 2.4.14 Deconvolution of the (a) C1s and (b) O1s spectra for the untreated
and treated PI films with τ=8 min ………..…... 32
Figure 2.4.15 Fraction of the components contributing to the (a) C1s and (b) O1s
core-level spectra of untreated and ion-beam treated PI films as a
function of τ ………..……….. 33
Figure 2.4.16 Survey spectra for the untreated and ion-beam treated PI films with different θion, Vb=840 V, Jion=458 μA/cm2, and τ=5 min …… 34 Figure 2.4.17 Multiplex spectra for the untreated and ion-beam treated PI films
with different θion, Vb=840 V, Jion=458 μA/cm2, and τ=5 min …… 35 Figure 2.4.18 Deconvolution of the C1s spectra of the treated PI films with θion
of 40°, 60°, and 80°………...…..… 36
Figure 2.4.19 Deconvolution of the O1s spectra of the treated PI films with θion of 40°, 60°, and 80° ... 37 Figure 2.4.20 Fraction of the components contributing to the (a) C1s and (b) O1s
core-level spectra of untreated and ion-beam treated PI films as a
function of θion ……….... 38
Figure 2.4.21 Survey spectra scanned with x-ray monochromatic sources of the Mg Kα and Al Kα lines for the PI surface treated with Vb=1120 V,
θion=60°, Jion=255 μA/cm2, and τ=5 min ……….... 39 Figure 2.4.22 The Fe 2p spectrum in multiplex mode of the film treated by
Vb=1120 V, Jion=255 μA/cm2, θion=60°, and τ=5 min ………. 40 Figure 2.4.23 Deconvolution of the Fe 2p3/2 spectrum of (a) IB-etched PI film
and (b) IB-etched ITO film with ion beam condition: Vb=1120 V, Jion=255 μA/cm2, θion=60°, and τ=5 min ………..…….. 41 Figure 2.4.24 Photographs of an ion beam treated NLC cell placed between
crossed-polarizers. The left side of the substrates was covered with fused silica plate while being ion-beam treated. The Vb and
are 560 V and 20 min, respectively ……….…..……. 42
Figure 3.2.1 Procedures of etching process for the patterned-ITO ………. 58
Figure 3.3.1 Deconvolution of the Fe 2p3/2 spectra of ITO-coated substrates
treated in (a) etching mode and (b) coating mode with ion beam condition: Vb=1120 V, Jion=255 μA/cm2, incidence angle of (a) 60° and (b) 0°, and treating time of 5 min. (c) Deconvolution of the O 1s spectrum of ITO-coated substrates treated in coating mode with the same energy, current density, and treating time.
The incidence angle is 0° .………..…. 60
Figure 3.3.2 The UV/VIS absorbance spectrum of iron oxide thin film with ion condition: Vb=1120 V, Jion=255 μA/cm2, incidence angle of 0°, and coating time of 20 min. Inset: the optical constants of hematite (α-Fe2O3) ……….……... 61
Figure 3.3.3 The UV/VIS transmittance spectra of γ-Fe2O3 films with different
thickness of 63.6, 106, 212, 424, and 636 nm ……….... 62 Figure 3.3.4 (a) Top-view and (b) tilt-view of the MLC-6608 cells with
γ-Fe2O3 and DMOAP coating under crossed-polarizers; (c)
top-view (without polarizers) and (d) tilt-view of the 5CB cells coated with different thickness of γ-Fe2O3 films. Inset: the
conoscopic pattern of the MLC-6608 cells with γ-Fe2O3 coating... 63
Figure 3.3.5 Normalized phase retardation of MLC-6608 cells at different applied voltage. Inset: the measured curves of transmittance
versus applied voltage ………. 64
Figure 3.3.6 Temperature dependence of the magnetization measured in the direction parallel to the surface for the iron oxide film with thickness of 636 nm in an applied field of 300 Oe ………. 65 Figure 3.3.7 Magnetization curves for a γ-Fe2O3 film with thickness of 636 nm
at T=10 K and 300 K in the magnetic field parallel to the film
surface ………... 66
Figure 3.3.8 Hysteresis loops measured in the direction (a) perpendicular and (b) parallel to the surfaces for sputtered γ-Fe2O3 films with
thickness of 212 nm (¡), 424 nm (z), and 636 nm (S) ………... 67 Figure 3.3.9 Thickness-dependence on (a) Ms and (b) Mnet of the γ-Fe2O3
films. The vertical and horizontal components of M are labeled as
(S) and (z), respectively ……….. 68
Figure 3.3.10 Polar anchoring strength Wp and the saturation magnetization Ms
as a function of γ-Fe2O3 film thickness ……….. 69
Figure 3.3.11 Hysteresis loops measured for an as-sputtered γ-Fe2O3 film (−z−)
and a thermally-treated γ-Fe2O3 film (−{−) with thickness of 212
nm. Two randomly directions on each film are selected for
measurements ………. 70
Figure 3.3.12 Hysteresis loops measured in the direction parallel to the surface (a) for as-sputtered γ-Fe2O3 films with thickness of 106 and 424
nm and (b) a 106 nm-thick γ-Fe2O3 film with thermal treatment at
Figure 3.3.13 AFM images taken at three different positions on the γ-Fe2O3 film
surface with thickness of 42.4 nm ……….. 72
Figure 3.3.14 AFM images taken at three different positions on the γ-Fe2O3 film
surface with thickness of 106 nm ………... 73
Figure 3.3.15 AFM images taken at three different positions on the γ-Fe2O3 film
surface with thickness of 212 nm ………... 74
Figure 3.3.16 AFM images taken at three different positions on the γ-Fe2O3 film
surface with thickness of 424 nm ………... 75
Figure 3.3.17 AFM images taken at three different positions on the γ-Fe2O3 film
surface with thickness of 848 nm ………... 76
Figure 3.3.18 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with thickness of 42.4 nm …………..…….. 77
Figure 3.3.19 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with thickness of 106 nm ………...…... 78
Figure 3.3.20 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with thickness of 212 nm ……….. 79
Figure 3.3.21 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with thickness of 424 nm ……….. 80
Figure 3.3.22 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with thickness of 848 nm ……….. 81
Figure 3.3.23 Surface morphology image of as-sputtered γ-Fe2O3 film with
thickness of 169.6 nm: (a) Top-view image; (b) top-view phase
image; (c) three-dimensional image ………... 82
Figure 3.3.24 Surface morphology of a γ-Fe2O3 film with thickness of 169.6
nm: (a) Three-dimensional image; (b) top-view phase image; (c) surface profiles for particle size analyses ………... 83 Figure 3.3.25 Thickness-dependence of particle size for as-sputtered (z) and
annealed ({) γ-Fe2O3 films ……….... 84
Figure 3.3.26 AFM images taken at two different positions on the γ-Fe2O3 film
surface with coating time of 2 min and beam energy of 840 V ….. 85 Figure 3.3.27 AFM images taken at three different positions on the γ-Fe2O3 film
surface with coating time of 10 min and beam energy of 840 V .... 86 Figure 3.3.28 AFM images taken at two different positions on the annealed
γ-Fe2O3 film surface with coating time of 2 min and beam energy
of 840 V ……….. 87
Figure 3.3.29 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with coating time of 10 min and beam
energy of 840 V ……….. 88
Figure 3.3.30 AFM images taken at two different positions on the γ-Fe2O3 film
Figure 3.3.31 AFM images taken at three different positions on the γ-Fe2O3 film
surface with coating time of 10 min and beam energy of 1400 V .. 90 Figure 3.3.32 AFM images taken at two different positions on the annealed
γ-Fe2O3 film surface with coating time of 2 min and beam energy
of 1400 V ……….. 91
Figure 3.3.33 AFM images taken at three different positions on the annealed γ-Fe2O3 film surface with coating time of 10 min and beam
energy of 1400 V ………. 92
Figure 3.3.34 Thickness-dependence of size of particle for as-sputtered (solid) and annealed (open) γ-Fe2O3 films coated by beam energies of
840 V (circle) and 1400 V (triangle) ………... 93
Figure 3.3.35 SEM images taken for γ-Fe2O3 films with thickness of (a) 106 nm
(60,000X), (b) 318 nm (60,000X), and (c) 636 nm (60,000X), and γ-Fe2O3 films with thickness of (d) 212 nm (190,000X), (e) 424
nm (22,000X) and (f) 424 nm (85,000X) for 60-second thermal
annealing ………... 94
Figure 3.3.36 A simple model describing the spatial distribution of the induced magnetic field by the γ-Fe2O3 film: (a) imaginary surface
structure; (b) Hm(x, y) at z=5 nm; (c) Hm(x, y) at z=20 nm; (d)
Hm(z) at different (x, y) ………... 95
Figure 3.3.37 Surveyed images for γ-Fe2O3 film with 212 nm thick by (a) AFM
and (b) MFM ………... 96
Figure 4.1.1 Chemical structure of the fluorinated polymer MAPhM-F8.…….. 109 Figure 4.2.1 Schematic diagram of the LPUVL irradiation system………. 109 Figure 4.3.1 Photographs of 5CB cells taken with crossed polarizers in the
dark state. The cells are aligned with surface-treated x0, x0.5, and
x0.67 films by LPUVL irradiation and IB bombardment …………. 110
Figure 4.3.2 Pretilt angle of LC cells with MAPhM-F8 films, X0 (S), X1/2
(z), and X2/3 (), treated by LPUVL irradiations and IB
bombardments for different exposure times ………... 111
Figure 4.3.3 Survey spectra for the untreated and IB-treated X2/3 films with
different bombarding times ………. 112
Figure 4.3.4 Survey spectra for the untreated and LPUVL-irradiated X2/3 films
with different exposure times ………. 113
Figure 4.3.5 Variation of chemical composition of the (a) X1/2 and (b) X2/3 film
surfaces treated by LPUVL irradiation and IB bombardment with respect to the treating time for elements: C (), O (z), N (S), F
(T), and Fe (¡) ……….. 114
Figure 4.3.6 XPS spectra for X2/3 films treated by both noncontact methods
with different exposure times. The C1s spectra for (a)
LPUVL-irradiated films and (b) IB bombarded films and (c) the
Figure 4.3.7 Logarithm scaled pretilt angle as a function of CF2/C ratio for the
LPUVL (z) and IB (S) treatments ………... 116
Figure 4.3.8 Surface energies of MAPhM-F8 films including X0 (), X1/2 (z),
and X2/3 (S) treated by LPUVL irradiations and IB
bombardments for different treating times ………. 117
Figure 4.3.9 Pretilt angle (circle) and atomic ratio of oxygen content (triangle) as a function of surface energy for all the samples treated by LPUVL irradiation (open) and IB bombardment (solid) with
different treating times ……….... 118
Figure 4.3.10 Total surface energy (circle), polar surface energy
(triangle), and dispersion surface energy (square) are plotted as a function of pretilt angle in logarithm scale for all the samples treated by LPUVL irradiation (open) and IB bombardment (solid)
with different treating times ………. 119
Figure 4.3.11 Depth-dependencies of C (), O (z), N (S) and F (T) contents in the LPUVL-irradiated X2/3 films with treating time: (a) 1 min
and (b) 10 min ………. 120
Figure 4.3.12 Depth-dependencies of (a) F and (b) CF2 contents in the
LPUVL-irradiated X2/3 films with treating time of 1 min (S) and
10 min (z) ……….. 121
Figure 4.3.13 (a) Number of F8 monomer assumed in each layer and (b) the evaluated fluorine content for the as-deposited film (S) and films treated by LPUVL-irradiation for 1 min (z) and 10 min (¡) …... 122 Figure 4.3.14 Stability of pretilt angle on the X0 (square), X1/2 (circle), X2/3
(triangle) films treated by LPUVL-irradiation for 1 min (solid)
and 10 min (open) ………... 123
Figure 5.2.1 Spatial distribution of glow discharge in the diode-type ion beam
system operated in the coating mode ……….. 135
Figure 5.3.1 The POM photographs of LC cells treated by different Vb: 420 V
and 560 V for 20 min ………..……….... 135
Figure 5.3.2 Pretilt angle θp is plotted as a function of (a) τ for 5CB cells with surfaces treated by plasma beam with different Vb: 420 V (), 560 V (z), and 700 V (S) and (b) Vb for 10 min treatments …… 136
Figure 5.3.3 Surface roughness of the PI films treated with different (a) τ for Vb of 420 V (S) and 560 V (z), and (b) Vb with τ=10 min ……... 137
Figure 5.3.4 SEM photographs of the PI film surfaces treated with different (a) Vb for τ=10 min and (b) τ for Vb of 420 V and 560 V ………….... 138
Figure 5.3.5 Size of aggregation on the PI surfaces treated by Vb of 420 V (z) and 560 V (S) is plotted as a function of τ ……..………. 139 Figure 5.3.6 Pretilt angle is plotted as a function of the size of aggregation for
Vb of 420 V (z) and 560 V (S) ………... 140
Figure 5.3.7 Surface energy is plotted with treating time for Vb of (a) 420 V
and (b) 560 V ………... 141
Figure 5.3.8 Surface energies of the PI films treated with different Vb for 10
min ………...………..……. 142
Figure 5.3.9 Content of C (), O (z), N (S), and Fe (T) are plotted as a function of (a) τ with the bias of 560 V and (b) Vb for 20 min
irradiation ………..……….. 143
Figure 5.3.10 Composition of O1s core level signal as a function of (a) τ with the bias of 560 V and (b) Vb for 20 min irradiation ……… 144 Figure 5.3.11 Relation between the dispersive surface energy and the content of
C-O-Fe bond for surfaces treated by different Vb and τ …..……... 145 Figure 5.3.12 Modes of ligand coordination to the iron oxide surface and modes
of coordination through COOH groups ……….. 146
Figure A1 Schematic drawing of ion beam system utilized in (a) Chaudhari et al.’s, (b) Hwang et al.’s, and (c) Gwag et al.’s works …………. 157 Figure A2 The normal glow discharge in neon in a 50 cm tube at pressure of
1 torr ……… 158
Figure A3 Schematic representation of self-sustained plasma induced in a
planar diode ………. 159
Figure A4 Potential distribution in a dc glow discharge process ………. 160 Figure B1 Schematic representation of the displacement of component
angles ………...……… 162
Figure B2 Experimental setup for measuring the optical transmittance of the LC cell: P1 and P2, polarizers; PD1 and PD2, photo-detectors; BS, beam splitter; DMM, digital multi-meter; Lock-in, Lock-in amplifier. (The light source is a He-Ne laser with wavelength 632.8nm and the two polarizers are Glan-Thompson type) ……… 164 Figure B3 The extreme values of intensity measured as a function of the
applied voltage. The sample is IB-aligned ( DC voltage of 560 V, current density of 255 μA/cm2
, irradiation time of 8 min, and angle of incidence of 60° with thickness of LC layer being 32.4
μm ……….... 165
Figure B4 Dependence of / plotted against . The
solid line represents the best linear fit to the data (a) from 34.0 V to 88.8 V and yields 6.6 10 J/m2 of a rubbed 5CB cell and (b) 20.2 V to 60.0 V and yields 3.2 10 J/m2 of an ion-beam bombarded 5CB cell with condition of 560 V, 255
μA/cm2, 60° and 90 sec for ion energy, current density, angle of
incidence and treating time, respectively ……….... 166
Figure B5 Voltage-dependent capacitance of a VA cell filled with MLC-6608 168 Figure B6 Illustration of the (a) intercept extrapolation method and (b) slope
fitting method. A MLC-6608 cell aligned by γ-Fe2O3 film with
thickness of 212 nm is used as an example ………. 169
Figure C1 Equivalent current-loops of squarely-arrayed magnetic domains ... 171
Figure C2 Z-dependence of Hz(x, y, z) ……….… 174
Figure C3 Z-dependence of Hx(x, y, z) ……….… 175
Figure C4 X-dependence of (a) Hx and (b) Hz components at the position of
(y=L, z=2 nm) ………. 176
Figure C5 X-dependence of (a) Hx and (b) Hz components at the position of
(y=L, z=5 nm) ………. 177
Figure D1 Schematic drawing of director distribution of a LC cell …………. 178 Figure D2 Field-dependence of director distribution in a LC cell aligned with
Wp=2×10-4 J/m2 ………... 179
Figure D3 A close look at director distribution in nearby region of surface in a LC cell aligned with Wp=2×10-4 J/m2 ……….. 180 Figure D4 Schematic drawing of LC director distribution in surface region... 181
Figure D5 Director distribution φ(z) in SR at H=0.5 T ……… 182
Figure D6 Wv(z) in SR evaluated for two strength of H: 0.35 and 0.50 T …… 182 Figure D7 Wv(z) in SR at H=0.5 T with three different Wp ……….. 183 Figure D8 Director distribution φ(z) in SR with ξ=4 nm at H=0.5 T for (a)
List of Tables
Table 2.4.1 Intensities of chemical bonds convoluted to the C1s spectrum
of surfaces treated by different energies of ion beam ………….... 47 Table 2.4.2 Intensities of chemical bonds convoluted to the O1s spectrum
of surfaces treated by different energies of ion beam ………. 47 Table 2.4.3 Intensities of chemical bonds convoluted to the C1s spectrum
of surfaces treated by different bombarding time ………... 48 Table 2.4.4 Intensities of chemical bonds convoluted to the O1s spectrum
of surfaces treated by different bombarding time ………... 48 Table 2.4.5 Intensities of chemical bonds convoluted to the C1s spectrum
of surfaces treated by different angle of incidence ………. 49 Table 2.4.6 Intensities of chemical bonds convoluted to the O1s spectrum
of surfaces treated by different angle of incidence ………. 49 Table 3.3.1 Comparison of polar anchoring strength of the γ-Fe2O3 and
DMOAP films ………. 102
Table 3.3.2 Horizontal and vertical components of the magnetization induced by the γ-Fe2O3 films with different thickness. (Unit: emu/g) ……. 102
Table 3.3.3 Magnetization of the γ-Fe2O3 films with or without thermal
annealing. (Unit: emu/g) ………. 102
Table 4.3.1 Intensities of chemical bonds convoluted to the C1s spectrum
of X0 surfaces treated by LPUVL irradiations and IB
bombardments ………. 128
Table 4.3.2 Intensities of chemical bonds convoluted to the C1s spectrum
of X1/2 surfaces treated by LPUVL irradiations and IB
bombardments ………. 128
Table 4.3.3 Intensities of chemical bonds convoluted to the C1s spectrum
of X2/3 surfaces treated by LPUVL irradiations and IB
bombardments ………... 129
Table 5.3.1 Intensities of chemical bonds convoluted to the O1s spectrum
of surfaces treated by diode plasma with Vb=560 V for various τ... 151 Table 5.3.2 Intensities of chemical bonds convoluted to the O1s spectrum
of surfaces treated by different Vb for 20 min ………. 151
Chapter 1
Introduction
1.1 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
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 SiNx, 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.
1.2 Overview of this thesis
In this thesis, the content is divided into four subjects related to the ion beam alignment techniques. The first coming, in Chap. 2, is titled as the “controllable alignment modes of nematic liquid crystals on argon ion beam bombarded polyimide films”. We will demonstrate that the polyimide surface treated by argon ion beams can give both homogeneous and homeotropic alignments with the same ion beam apparatus but varying the energy of ion beam and the bombarding time.
In Chap. 3, demonstration of a method for liquid crystal surface alignment by using a one-step, ion beam sputtering on glass substrates is followed with the identification of the unexpectedly coated material, i.e. iron-oxide. In subsequent two chapters, two alignment methods of inducing high pretilt angle are presented. In Chap. 4, surface treatments accomplished through the ultraviolet irradiation and ion beam bombardment on the newly developed copolymer materials with fluorinated carbonyl groups of different mole fraction and their effects on liquid crystal surface alignments have been studied. In Chap. 5, the polyimide surfaces are treated by oblique plasma beam irradiation with simultaneous coating of maghemite nanoparticles. For each of them, the mechanism in determining the pretilt angle has also been discussed.
References
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15. B. Park, Y. Jung, H. H. Choi, H. K. Hwang, Y. Kim, S. Lee, S. H. Jang, M. A. Kakimoto, and H. Takezoe, Jpn. J. Appl. Phys. 37, 5663 (1998).
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17. W. Urbach, M. Boix, and E. Guyon, Appl. Phys. Lett. 25, 479 (1974). 18. D. Armitage, J. Appl. Phys. 51, 2552 (1980).
19. T. Uchida, M. Ohgawara, and M. Wada, Jpn. J. Appl. Phys. 19, 2127 (1980). 20. K. Hiroshima, Jpn. J. Appl. Phys. 21, L761 (1982).
21. J. G. W. van de Waterbeemd and G. W. van Oosterhout, Philips Res. Rep. 22, 375 (1967).
22. L. A. Goodman, J. T. Mcginn, C. H. Anderson and F. Digeronimo, IEEE Trans. Electron Devices ED-24, 795 (1977).
23. P. Chaudhari, J. Lacey, S. C. Alan Lien, and J. L. Speidell, Jpn. J. Appl. Phys. 37, L55 (1998).
25. J. P. Doyle et al., Nuclear Instruments and Methods in Physics Research B 206, 467 (2003).
26. J. Stöhr, M. G. Samant, J. Lüning, A. C. Callegari, P. Chaudhari, J. P. Doyle, J. A. Lacey, S. A. Lien, S. Purushothaman, and J. L. Speidell, Science 292, 2299 (2001). 27. S. H. Lee, K. H. Park, J. S. Gwag, T. H. Yoon, and J. C. Kim, Jpn. J. Appl. Phys. 42,
5127 (2003).
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30. J. M. Geary, J. W. Goodby, A. R. Kmetz, and J. S. Patel, J. Appl. Phys. 62, 4100 (1987).
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Chapter 2
Controllable Alignment Modes
of Nematic Liquid Crystals
on Argon Ion Beam Bombarded Polyimide Films
2.1 Overview
In this work, we study the alignment properties of nematic liquid crystal (NLC) on the polyimide (PI) films treated by argon ion beams with various energies and bombarding times by using a commercial diode sputter as the ion beam source. The structural design and operating principle of a diode type ion sputter will be described in detail in Sec. 2.2. We find that there exist two alignment modes, homeotropic and homogeneous alignments, which can be controlled by varying the energy of ion beams and the bombarding time. The pretilt angle is measured as a function of the bombarding time. We have also studied the surface morphology change of the PI films by using atomic force microscope (AFM) and the chemical change by using the surface sensitive x-ray photoelectron spectroscopy (XPS). The alignment mechanism is then discussed depending on the results of AFM and XPS in Sec. 2.4.
2.2 Ion beam treatment
It is entirely different from the usage of an independent ion source (Kaufman type) in Chaudhari et al.’s excellent work [1,2] that a diode type source is adopted in our studies. More details about these two types of ion sources are introduced in Appendix A. The ion beam conditions used in this study are then addressed in Sec. 2.2.2.
2.2.1 Ion source
In this research, a direct-current (DC) ion sputter (model IB-2 from EIKO Engineering Co., Ltd.) has been used for ion-beam treatment on PI surface. A schematic structure is plotted in Fig. 2.2.1. A cylindrical vacuum chamber with diameter of 130
mm and height of 100 mm is connected to a direct drive oil rotary vacuum pump (GVD-101, ULVAC) through the exhaust pipeline. The ultimate pressure of 15 mTorr can be achieved. The working gas, argon, is introduced into the chamber through a gas inlet equipped with a precision micro gas control valve. The clearance between the electrodes is 35 mm. The diameters of upper and lower electrodes are 50 mm and 52 mm, respectively. This ion-beam sputter can be used either as a coating or etching device, depending on the polarity of the voltage. Its operational principles are the same as those of a diode sputter as described in Sec. 2.2.2.
2.2.2 Ion beam conditions
The etching mode is selected in this work. Figure 2.2.2(a) shows the spatial distribution of glow discharge in our ion coater under the etching mode. The cathode dark space has the most energetic ions and provides the main region for ion-beam treatment. Limited to the scale of bombarding area mentioned above, the size of substrates has to be controlled such that they are totally immersed in this region as shown in Figure 2.2.2(b). The arrows indicate the direction of incident ion beam. In our system, the incidence angle θion, bombarding time τ, current density Jion and the energy of ions are all controllable. The energy of ions can be varied by changing the DC bias between electrodes. It is difficult to quantitate the real ion energy in our diode plasma system; therefore, a DC bias is used instead and labeled as . It should be noticed that the sample stages with different angles are insulated from the lower electrode. Before each ion-beam process, the chamber is pumped down to a base pressure of 30 mTorr and then argon gas is fed into the chamber to a target pressure. To obtain the required ion current density, the total pressure of the chamber has to be adjusted between 50 and 180 mTorr when the applied dc voltage is changed.
2.3 Experiments
The indium-tin-oxide coated glasses with size of 20 mm × 10 mm are used as the substrates. Before polymer coating, each substrate have to be cleaned using a standard process shown in the flowchart of Fig. 2.3.1.
2.3.1 Sample preparation
After cleaning, the substrates are spin coated with the polyimide SE-130B (Nissan Chemical Industries, Ltd.), which is commonly used as an alignment agent in super twisted nematic LCD. The spin rate is 2000 rpm for the first 15 seconds and 4000 rpm for the 25 seconds afterward. The substrates are then pre-baked at 70°C for 15 minutes and cured at 180°C for another one hour. This thermal treatment process is chosen according to the company’s instruction for achieving a pretilt angle of 3° by conventional rubbing method. Two substrates are bombarded in ion beam chamber simultaneously and then combined with a 23 μm Mylar spacer in between with anti-parallel alignment direction to form an empty cell. The nematic liquid crystal 4′-n-pentyl-4-cyanobiphenyl (5CB, Merck) with a nematic range between 24.0°C and 35.3°C is filled into the empty cell for alignment characterization.
2.3.2 Alignment characterization
After annealing, the alignment modes of the LC cells are characterized with conoscope and polarizing optical microscope (POM). The pretilt angle of 5CB molecules near the surface is measured by using the “crystal rotation method” [3]. The experimental determination of polar anchoring strength is also carried out using the high-electric-field methods [4-6]. The process for measurement and theoretical analysis has been described in detail in Appendix B.
2.3.3 Surface analyses
In addition, the surface morphologies of ion-beam treated PI films are characterized by using a DINS3a atomic force microscope with tapping mode. To study the possible reactions caused by ion-beam bombardment, the compositions of chemical bonds of the polyimide film after ion-beam treatment at various conditions are also analyzed by using XPS (PHI-1600 from Physical Electronics, Inc.). For XPS, a PHI dual-anode x-ray monochromatic source for the x-ray irradiation of Mg Kα (1253.6 eV) and Al Kα (1486.6 eV) and a PHI 10-360 precision energy analyzer are used. Only the Mg Kα line is used in this work unless it is mentioned otherwise. The incident angle of the x-ray is 36° from the sample normal and the photoelectrons are detected at the angle
of 45° from the sample normal. The base pressure during acquirement is below 5.0×10-9
Torr. The anode voltage is set at 15 kV (x-ray power 250 W).
Due to the surface charging effect, the measured binding energy (BE) of targeting core level of element will be shifted by ca 5 eV to a higher value in this work. The C1s photoelectrons are those most generally adopted for referencing purposes [7]. A binding energy of 284.7±0.2 eV is used for this level in this work. Furthermore, if results from different laboratories are to be compared, the spectrometers should be properly calibrated [8]. After the correction of binding energy scale, the Shirley background [9] is subtracted from the XPS spectrum I of each element by using the packaged software MultiPak (version 8.0, ULVAC-PHI, Inc.). The essential feature of the Shirley algorithm is the iterative determination of a background using the areas marked A1 and A2 in Fig. 2.3.2 to compute the background intensity at BE:
where A1 and A2 represent the enclosed areas by the spectrum and background curves. Clearly, S(BE) is initially unknown, therefore, the calculation of a Shirley background from spectral data is an iterative procedure. That is, the integrated areas A1 and A2 for each point on the S(BE) have to be initially determined using an approximation to S(BE), then repeat the process using the evaluated S(BE) as input to improve the computed values for A1 and A2 to achieve being self-consistent eventually.
After background subtraction, a deconvolution is carried out by fitting the spectra to multiple peaks Ii whereas each are described as a Gaussian-Lorentzian sum function, i.e.
2 √ 2
√ 4 2
1
1 4
where Ii (BE) is the intensity of i-th component with peak center at BE which is given by , is the area of peak, is the full width at half maximum of peak and is the mixing ratio (1 for pure Gaussian, 0 for pure Lorentzian). In the process of deconvolution using the commercial software PeakFit (version 4.11, SYSTAT Software Inc.), parameters a2 and a3 are shared between each component peaks. According to the
BE specified in the literatures by which is initially given for each considered bonding, , and are then obtained after best fitting of experimental data whereas is usually slightly different from the given value.
After multiple-peaks fitting, the ratio of chemical composition is further evaluated using the fitted parameter of each component peak. For instance, assume n peaks are convoluted to the core-level signal of element; each fitted of them is labeled as Ai. Therefore, the total intensity A of considered spectrum should be equal to the sum of
each component signal, i.e. ∑ . Moreover, the composition ratio of each
chemical bond related to the considered element j then can be simply obtained by
, ∑ ,
, 100 % .
If m number of elements is considered, moreover, the ratio of each component element Cj can be evaluated by
∑ ,
∑ ∑ , 100 %
2.4 Results and discussion
2.4.1 Alignment characterization
We have observed both of the homeotropic and homogeneous alignments with the ion bombarded films. Figure 2.4.1 shows the POM photographs of 5CB cells put in between the two crossed polarizers. The substrates treated by argon ion beam with various energies exhibit good alignment qualities. In Fig. 2.4.1(a), the homogeneous alignment of 5CB is obtained by using substrates bombarded with the following ion beam condition: the DC bias of 560 V and bombarding time of 40 min. The working pressure is 120 mTorr. The easy axis of 5CB lies in the incidence plane of ion beam and tilts toward the direction of incident ion beam. These alignment properties agree with the results reported by an IBM group [1]. In Fig. 2.4.1(b), we show the homeotropic alignment obtained with another set of ion beam condition: the DC bias of 1120V and bombarding time of 7 min. The working pressure is 50 mTorr The conoscopic pattern shown in the inset further indicates that the cell is homeotropically aligned.
to the bombarding time. In the case of homogeneous alignment ( =560 V), the pretilt angle approaches to a maximum value about 6.43° and then decreases with the increasing bombarding time. However, the pretilt angle of the homeotropic cell ( =1120 V) is determined as 90° and remains almost the same while increasing the bombarding time.
The polar anchoring strength Wp of 5CB cells are also determined as a function of bombarding time and angle of incidence with Vb=560 V. Figure 2.4.3(a) shows an optimized Wp of ca 1.6×10-3 J/m2 is found for the surfaces bombarded for 6 to 8 min. It is followed with an abrupt drop of Wp for surfaces bombarded with duration longer than 8 min. We believe that the etching ability of ion bombardment is responsible for this resultant behavior. In Fig. 2.4.3(b), an angle-dependency of Wp shows that lower values are found in the range of 40° to 60°.
2.4.2 Surface morphologies
To investigate the possible alignment mechanism for liquid crystals on treated PI films, the surface topology are surveyed using AFM. A series of roughness with respect to different bombarding time and applied bias of 560 V is measured. All the treated substrates induce the homogeneous alignments. As seen in Fig. 2.4.4, the surface morphology becomes rough as bombarding time increases. The mean roughness of the surfaces treated for 0, 6, 14, 30 min is 0.2 nm, 1.7 nm, 1.8 nm, and 3.6 nm, respectively. Figure 2.4.5 shows a linear fit of roughness versus bombarding time. Compare the results here with the measured pretilt angle, no strong connection between them is found.
Figure 2.4.6 also shows the AFM images of the PI surfaces but bombarded by various ion energies. The mean roughness of the surfaces treated with Vb of 560 V, 840 V, and 1120 V is 1.2 nm, 1.4 nm, and 0.3 nm respectively. Obviously, the roughness of PI surface bombarded by high energy ion (Vb=1120 V) is much smaller than it is by lower energy treatment. It is expected because the higher energy ions have better etching ability and reduces the roughness more. Accordingly, there is not a straightforward relation between the alignment modes and surface topologies induced by ion beam bombardment with different ion energy either.
Although the generation of micro-scaled directional grooves is one of the major alignment mechanisms for conventional rubbing method, the groove structures are not found on the ion-beam treated surfaces. Therefore, the mechanism for aligning liquid crystals is predominately related to the intermolecular interactions.
2.4.3 XPS analyses
The XPS is used to further investigate the changes of chemical bonds on ion-beam treated SE-130B films which possess the typical chemical structure of PI drawn in Fig. 2.4.7 [10-12]. A series of conditions including different ion energies, incidence angles, and bombarding times are chosen for comparison in the ion-beam bombardments. We will begin with the issue of ion beam energy.
Four kinds of samples treated by ion beam bombardments with various Vb, bombarding time of 5 min, incidence angle of 60°, and current density of 255 μA/cm2
are scanned in survey mode with the step size of 1 eV and the analyzer pass energy of 117.4 eV. The survey spectra shown in Fig. 2.4.8 reveal that a significant amount of iron appears on the treated surface with ion beam energy higher than 560 V. The Fe 2p and Fe 3p core-level signals are located in the ranges of 705-735 eV and 53-60 eV, respectively [13]. We speculate that this iron contamination comes from the electrodes of ion beam system. More discussions on these unexpected results will be given later on. In addition, the Si2s signal probably comes from the glass substrate. Figure 2.4.9 shows
the core-level 1s signals of carbon (C), oxygen (O), and nitrogen (N) elements scanned in multiplex mode with the step size of 0.2 eV and the analyzer pass energy of 23.5 eV. As we can see, the signals of C1s and N1s are dramatically reduced by increasing the ion
beam energy in the treatments. However, the peak position of O1s signal moves to a
lower binding energy. And the increase of peak intensity indicates that the extra bonds have been newly formed. The deconvolutions are further carried out for C1s and O1s
signals and their fitting results are plotted in Fig. 2.4.10.
For core-level C1s signal, it is composed of five main signals, C-C/C-H (284.56
eV), C-N (285.68 eV), C-O-C (286.29 eV), N-C=O (288.6 eV) and O-C=O (287.4 eV), and a signal contributed from the shakeup satellite at BE=291.30 eV described in [10]. The shakeup satellites are usually shown on the higher BE side of the core-level spectra
of aromatic and unsaturated polymer [14,15]. As shown in Fig. 2.4.10(a), a significant decrease of signal from C-C/C-H, C-N, and C-O-C bonds are found in the high-energy ion beam treatments. The intensity of shakeup satellite also disappears after ion beam treatment with energy of 1120 V. This indicates that the aromatic components like benzene ring and pyrrolidine are destroyed by ion beam bombardments. Similar trend of variation for carbonyl group is also found in O1s spectra as shown in Fig. 2.4.10(b). It
should be noticed that two bonds, C-O-Fe (531.1 eV) and Fe2O3 (529.77 eV), related to
the iron element are newly formed on the surfaces after high energy ion-beam treatments. Their appearances cause the movement of envelope of spectrum to a lower BE side.
To quantitate the variation of signals, the intensity of each chemical bond convoluted to a core-level spectrum is obtained by the integration of its fitted curve, i.e. the mentioned Gaussian-Lorentzian sum function in Sec. 2.3.3, and summarized in Table 2.4.1 and Table 2.4.2. Besides, the integrated intensity is further divided by the overall intensity of the corresponding spectrum to obtain the composition ratio. Figure 2.4.10 shows the composition of each bond as a function of ion beam energy. We can find out that the shakeup satellite in Fig. 2.4.11(a) is gradually reduced with the raise of ion beam energy. In addition, the iron-related bonds appear to become larger as the ion energy increases, as shown in Fig. 2.4.11(b). In Table 2.4.1 and Table 2.4.2, another chemical reaction should be noticed is that the re-oxidization of the dangling bonds occurs after the ion beam bombardments. It causes the increase of oxygen content. However, most contribution is made from the formation of iron oxide on the treated surface for the cases with the ion energy higher than 560 V.
Furthermore, the impact of different bombarding times of the ion beam treatments with ion energy of 560 V, incidence angle of 60°, and current density of 255 μA/cm2 on
the chemical bonding of treated surface is discussed as well. Figure 2.4.12 shows the survey spectra of PI surfaces treated for different bombarding times, 0 min, 2 min, and 8 min. It is not surprising that the iron content is raised by an increase in length of bombarding time. Figure 2.4.13 shows the raw spectra of C1s, O1s, and N1s scanned in
the multiplex mode. The deconvoluted signals of C-O-C and C-C/C-H bonds are obviously attenuated for 8 min treatment, as seen in Fig. 2.4.14(a). No significant
reduction but a movement to a lower BE of signal is found for shakeup satellites. The same behavior as remarked before is also observed in the O1s spectrum, as shown in Fig.
2.4.14(b). The intensities of chemical bonds convoluted to the C1s and O1s spectra are
also organized in Table 2.4.3 and Table 2.4.4. The decrease of intensity of C-N bond means that a part of the backbones of PI are broken into pieces; meanwhile, the dangling bonds react with oxygen to form the carbonyl groups, N-C=O and O-C=O. Similar results can also be concluded in Fig. 2.4.15(a). Moreover, a dramatic increase of the content of Fe2O3 and C-O-Fe bonds after ion beam bombardment is reconfirmed in
Fig. 2.4.15(b).
Next, ion beam conditions of ion energy of 840 V, current density of 458 μA/cm2,
bombarding time of 5 min and different angles of incidence are discussed. The survey spectra of surfaces treated with incidence angle of 40°, 60°, and 80° are shown in Fig. 2.4.16. All the bombarded surfaces have the iron contamination without exception. The relative intensities of the fine scanned spectra shown in Fig. 2.4.17 suggest that a smaller angle of incidence offers a better etching ability. Deconvolutions of C1s and O1s
spectra have been accomplished further and shown in Fig. 2.4.18 and Fig. 2.4.19. The fitted intensity of each chemical bond is listed in Table 2.4.5 and Table 2.4.6. We can find out that the best etching effect is given by incidence angle of 60°; on the other hand, more iron contaminations are also found. A similar remark can be concluded according to the calculated composition ratio as shown in Fig. 2.4.20.
Now, we turn to look deeply into how the iron element leads to the homeotropic alignment of liquid crystal. As it has been noticed above that the iron element could be found in the sample if treated by ion beam with energy higher than 560 eV. Figure 2.4.21 shows two survey spectra scanned by using the Mg Kα and Al Kα lines as the x-ray monochromatic sources for the PI surface treated by beam energy of 1120 V, incidence angle of 60°, beam current density of 255 μA/cm2, and treating time of 5 min.
It should be mentioned that the BE of auger signals remain the same no matter what the x-ray source is. The survey spectrum by Al Kα line has been shifted about +230 eV for comparison with that by Mg Kα line. In other words, a shift of BE about −230 eV for primary signals of the O1s and Fe2p3 levels when x-ray source is changed from Mg Kα
LMM [16]. For a detail study of the Fe peaks, the multiplex mode scanning of Fe 2p region on the high energy IB-treated surface is carried out. The spectrum is shown in Fig. 2.4.22. According to the literatures [17,18], the shake-up satellite line at 718.2 eV is characteristic of Fe3+ in Fe2O3. Further, the narrow peak at 710.4 eV of the Fe 2p
spectrum indicates that no Fe2+ iron oxidation state exists. In other words, the possibility of forming Fe3O4 (magnetite) in the film can be excluded. The film should be composed
of the Fe3+ oxides, Fe2O3 only. The spectrum of the IB-etched ITO film reveals the same
profile. Structurally, however, there are four possible types of Fe2O3. Two of them,
α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite) are common and widespread in soils [19].
Of the two, only the γ-Fe2O3 has permanent magnetic moments.
An interesting result that the homeotropic alignment is still achieved even on a clean substrate without PI coating after the high-energy IB treatment was discovered in addition. To further identify the coated films, the XPS Fe 2p3/2 signals of IB-etched PI
and ITO films are analyzed in detail as shown in Fig. 2.4.23(a) and Fig. 2.4.23(b), respectively. The measured XPS data are smoothed and the Shirley background is subtracted. Then a deconvolution is done by fitting the spectra to multiple Gaussian peaks. The Fe 2p3/2 envelope of the measured spectra is well fitted by using peaks
constrained to the multiplets calculated for the iron compound γ-Fe2O3 by Gupta and
Sen [20]. The splitting of four Fe3+ 2p3/2 multiplet peaks with binding energies at 709.8,
710.8, 711.8, and 713.0 eV in Fig. 2.4.23 are due to the inclusion of electrostatic interactions and spin-orbit coupling in theoretical calculation [21]. The presence of satellite peak has been ascribed to the shake-up processes [22]. We conclude, therefore, that the treated substrate is coated with an iron oxide of γ-Fe2O3.
It is well known that the LC molecules can be reorientated by electric and magnetic field due to their anisotropic electrical permittivity and magnetic susceptibility [23]. The mechanism of homeotropic LC alignment might be ascribed to the magnetic field
induced by the γ-Fe2O3 or/and the intermolecular interactions at the
liquid-crystalline-maghemite interface.
Finally, another issue concerned in this work is that the bond-breaking effects of PI films to the LC alignment induced by ion beams here are similar to that induced by polarized ultraviolet (UV) light irradiation [24,25]. They are both attributed to the
intermolecular interactions between the liquid crystals and the unbroken polyimide chains. To confirm that the alignment in this work is indeed induced by ion beam rather than the UV irradiation in the ion beam chamber, we have formed a cell with substrates partially covered by a fused silica plate while performing the ion-beam treatment. At the covered area the UV light can transmit through the silica plate while the ion beam is blocked. The pictures of this cell between crossed-polarizers are shown in Fig. 2.4.24, only the uncovered area (right hand side) shows good alignment. Since no alignment effect appears in the untreated area (left hand side), we can conclude that the alignment effects are not caused by the UV light from plasma discharge.
2.5 Concluding remarks
We have demonstrated that both homeotropic and homogeneous alignments can be obtained with the same ion beam apparatus and polyimide by varying the ion beam energy or the bombarding time. Both of the homeotropic and homogeneous cells have uniform and high contrast under the crossed polarizer. AFM images of polyimide surfaces treated by ion beams show no microgroove structure but only with roughness changing with bombarding time and ion-beam energy. There is no strong connection between the measured pretilt angle and the roughness. The XPS results show that the main structures including the C-N, C-O-C bonds, and the aromatic rings of PI films are significantly destroyed in the ion beam bombardments and the formation of N-C=O bonds is considered as the neutralization of dangling bonds when exposed to the air. We deduce that the anisotropic destruction to the aromatic groups of PI film dominate the alignment mechanism, as mentioned in Stöhr et al.’s works [26]. The alignment effects are attributed to the intermolecular interactions between the liquid crystals and the unbroken polyimide chains, or more precisely, the benzene rings [27]. Besides, an unexpected contamination of iron element is detected on the bombarded PI film surface. No matter the ion beam condition of how long or what angle the PI film treated with, ion beam energy higher than 560 V always makes it happen. Even the substrate without PI coating is treated by high-energy IB treatments. A homeotropic alignment of LC can be achieved as well. Through the XPS analyses, we have confirmed that the coated
material is an iron oxide of γ-Fe2O3 which has intrinsic magnetism. As a result, we
speculate that the magnetic field induced by γ-Fe2O3 or/and the intermolecular
interaction at the liquid-crystalline-maghemite interface give rise to the resultant homeotropic alignment of LC. Moreover, the possibility of UV light induced alignment effect has been excluded in our ion beam treatment process.
