Chapter 1 Introduction
1.5 Organization of this thesis
The thesis is organized as following: the theory of multi-domain vertical alignment displays with optical films is presented in Chapter 2. Additionally, this chapter also represents the phase retardation at oblique incidence. In Chapter 3, the pixel designs and simulations are introduced in detail. The fabrication process of array and the measurement instruments used to characterize the performance of display system are described in Chapter 4. After that, the measurement results and the discussion are presented in Chapter 5. Finally, the conclusions and future works are given in Chapter 6.
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Chapter 2
Principles of LCDs with Optical Films
This chapter covers the principle of multi-domain vertical alignment (MVA-b) liquid crystal cell used for our pixel design and also the principle of optical films at oblique incidence. First, the optical properties of VA cells will be discussed. Second, in order to design pixels for multi-view 3D displays, which involved oblique incidence, phase retardation at oblique incidence will then be considered. Third, the MVA-b LCDs used for conventional 3D displays are discussed. Finally, a brief summary will be given.
2.1 Optical properties of VA cells
Vertical alignment of liquid crystals with negative Δε is widely used in display applications. In a vertically aligned LC cell, the LC director is perpendicular to the surfaces of the cell. When the voltage is turned OFF, most of the liquid crystal molecules align vertically to the substrate. When an electric field is applied to the LC cell, the LC molecules are re-aligned by the electric field. In order to understand the transmission of LCDs, Jones matrix method[15] is needed to describe the propagation of light through the display. The formula for transmission of VA cell sandwiched between a pair of crossed polarizers is derived below[16]. When a plane wave is incident to a uniaxial liquid crystal layer, there are two transmission propagation modes, the ordinary mode and the extraordinary mode, which are mutually orthogonal. The refractive index of uniaxial LC for ordinary and extraordinary wave are denoted as n o and n , respectively as shown in Eq.2-1 and Eq.2-2: e
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The wave will undergo a phase retardation Γ due to the different propagation velocities of the extraordinary and ordinary rays inside the LC as shown in Eq.2-3:
d
where d is the thickness of the plate and λ is the wavelength of the light beam.
We assume an incident beam of polarized light. A factor of 2
1 must be included
for unpolarized light. We then derive the transmission properties of a general VA-LCD.
Referring to Fig. 2-1, the input and output polarization states are given by Eq.2-4
⎥⎦
Fig. 2-1 Schematic drawing showing the orientation of the polarizer axes, LC directors of a general VA-LCD in the xy plane.
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The transmitted intensity can be written
ent
For VA-LCD sandwiched between a pair of crossed polarizers under unpolarized light,
=0
φ , then the Eq.2-4 should be rewritten to Eq.2-5:
⎟⎠ the phase retardation of the LC cell.
2.2 Phase retardation at oblique incidence
The phase retardation may depend on the direction of propagation due to the fact that refractive index for the extraordinary mode is related to the direction of propagation. In this section, we discuss the dependence of the phase retardation on the direction of incidence.
Referring to Fig. 2-2, we assume a plate of homogeneous and uniaxially birefringent medium with its c axis parallel to the plate surface. Let θe, θo be the ray angles and ne(θ), n be the eigen indices of refraction for the rays. o
The phase retardation is given by Eq. 2-7 AC
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where k is the wave number in vacuum.
Using the continuity condition (Snell’s law)
o
Fig. 2-2 Phase retardation at oblique incidence.
then Eq.2-7 should be rewritten as Eq.2-9:
d
Fig. 2-3 shows a plate of uniaxial crystal, whose c axis is parallel to the surface.
For small angles of incidence θ <<1, the phase retardation (Eq.2-9) can be written as Eq.2-10:
where Γ is the phase retardation at normal incidence, θ is the angle between o incidence and the normal to the plate, and φ is the angle between the c-axis and the projection of the incident wave vector onto the surface of the medium.
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In a c plate of uniaxial crystal, whose c axis is perpendicular to the surface, the phase retardation can be written as Eq.2-11:
θ sin2
2 o e
e o
o n n
n n + Γ
=
Γ (2-11)
θ
c φ
Fig. 2-3 Phase retardation of a plate at oblique incidence.
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2.3 Conventional MVA-b LCDs
2.3.1 Pixel
Vertical alignment LC cell offers excellent contrast ratio and low operation voltage. However, a major problem is that its viewing angle is limited to ± 20°. To improve the viewing angle, several approaches to achieve multi-domain vertical alignment (MVA-b) have been developed[17],[18],[19]. Fig. 2-4(a) shows the MVA-b for mobile device, and Fig. 2-4(b) shows the cross-section of the MVA-b pixel. The protrusion (bump) was formed before coating the alignment layer. The domains are automatically controlled by the fringe fields generated near the edges of the pixel electrode and slope of bump without rubbing.
CF
TFT Fringe field
bump BM
ITO
(a) (b)
Fig. 2-4 (a) MVA-b pixel layout and (b) cross-section of the MVA-b pixel.
When the voltage is turned OFF, most of the liquid crystal molecules align themselves normal to the substrate, but those positioned above the bump incline slightly towards the substrate due to the slope of the bump beneath them. Since the
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contrast ratio is determined by the light leakage in the voltage-off state, the light leakage near the bump is blocked by BM to enhance the contrast ratio. When the voltage is turned ON, the molecules on the sloped protrusions and those on the edge of pixel electrode initially start tilting.
2.3.2 Optical films
The MVA-b cell has the advantage of wider viewing angle. A typical optical film for MVA-b display consist of two crossed polarizers, two quarter-wave plates, and negative c plates, as shown in Fig. 2-5[20]. In general, a quarter-wave plate can convert a linearly polarized light into an elliptically polarized light and vice versa. The role of negative c plate is to significantly improve the contrast ratio.
Fig. 2-5 The optical configuration of conventional MVA-b mode.
For a beam of light with normal incidence, the phase retardation is zero for the propagation along the c axis. For a beam of light with an oblique incidence, the VA cell thus exhibits birefringence. The phase retardation increases with the angle of incidence θ . Adding a negative c plate with its c axis aligned along the z axis does not create any
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additional phase retardation for the normal incident light. Moreover, MVA-b cell can be compensated by negative c plate for light at oblique incidence, as shown in Fig. 2-6.
no
no
ne
‐ C plate θ
Fig. 2-6 Phase retardation with negative c plates.
2.4 Summary
The VA and MVA-b mode LCDs have been briefly discussed in this chapter. We presented the fundamental theory of conventional LCDs with optical films. The basic working principle and the brief numerical calculation of VA LCDs were presented.
Moreover, the formulas of phase retardation at oblique incidence for a plate and c plate were described. Additionally, the conventional pixel design of MVA-b LCDs was discussed. In this thesis work, these fundamental working principles are employed for designing the special pixel used for our proposed 3D display to enhance the brightness.
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Chapter 3
Pixel Design and Simulation for High Transmittance of 3D Displays
This chapter covers pixel design and simulation results. First, the issues of conventional 3D displays are discussed. Second, three parameters which affect the brightness will be considered. After that, the concept of proposed pixel design and simulation results will be shown respectively.
3.1 Issues of conventional 3D displays
As mentioned in Chapter 1, there are still many limitations for different kinds of 3D technologies. For multi-view autostereoscopic 3D displays with slanted lenticular lenses, they have the issue of the higher crosstalk compared to parallax barrier.
However, a drawback of conventional multi-view slanted parallax barrier 3D displays is lower transmittance with unbalanced brightness for each viewing zone. This is due to the fact that the pixel design of conventional LCDs was optimized for normal viewing direction. In a conventional 3D display with many viewing zones, image transmittance away from the normal direction is too low. Here we designed a pixel array based on a 6-view slanted parallax barrier 3D display, the pixels can be considered to be comprised of 6 kinds of pixels, as shown in Fig. 3-1. The schematic drawing of conventional parallax barrier display with unbalanced brightness issue is shown in Fig. 3-2. Fig. 3-3(a) shows the transmittance curve of a conventional liquid crystal display with parallax barrier. The light from pixel 1 is too low in viewing zone
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1. If there are more than 6 viewing zones, the transmittance at a large viewing angle is significantly lowered. Therefore, unbalanced brightness issue for each viewing zone occurs in conventional 3D displays.
The design concept is that each kind of pixel has its own transmittance curve whose maximum transmittance can be optimized within its own viewing zone. For instance, the maximum transmittance of pixel 1 is in viewing zone 1, as shown in Fig.
3-3(b). More light passes through the LC cell within viewing zone 1. Other kinds of pixels can also be optimized for their viewing zones, the transmittance at wide viewing angles can be increased, and the difference in brightness between each viewing zone can be minimized.
6 5 4 3 2 1
Fig. 3-1 6-view slanted parallax barrier 3D display.
5 3
Fig. 3-2 Schematic drawing of a 6 views parallax barrier display.
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Fig. 3-3 Transmittance of (a) conventional LC and (b) proposed LC.
3.2 Parameters affecting brightness of 3D displays
There are three major parameters which affect the brightness of 3D displays. The configuration of a typical autostereoscopic 3D display is shown in Fig. 3-4. When considering the brightness of displays, the first parameter is the transmittance curve of the liquid crystal cell. We thus proposed a single domain VA with different LC alignment for different kinds of pixels, the detailed results are discussed in section 3.3.
The second parameter is the aperture ratio of the pixel array, which is considered in section 3.4. The aperture ratio can be increased by using a single domain VA cell without a bump. The third parameter that should be considered is the aperture ratio of parallax barrier in section 0. By considering these three parameters, total brightness can be increased. The total brightness, which is determined by these three parameters, is written as
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Fig. 3-4 The configuration of the parallax barrier 3D display.
3.3 Single domain VA
In conventional LCDs, the wide viewing angle is a critical issue to be improved.
Many researchers focused on wide viewing angle issue for either large panel liquid crystal displays or handheld displays, as depicted in Fig. 3-5(a). One of the wide viewing angle technologies is multi-domain VA. However, when combining with parallax barriers to form a 3D display, most of the light from pixel is blocked by parallax barriers, the light can only pass through at specific viewing zone, as shown in Fig. 3-5(b). Therefore, the objective of our design is to enhance the brightness at specific viewing angles, as shown in Fig. 3-5(c). The reason to use single domain VA is that single domain VA can generate higher transmittance than MVA-b mode in a specific viewing zone, as illustrated in Fig. 3-6. Fig. 3-7 shows that each kind of pixels has a transmittance curve optimized for its own viewing zone in 6-view 3D display.
Under above-mentioned design concept, each pixel has maximum transmittance in its
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viewing zone, so the total transmittance in large viewing angle can be increased.
Furthermore, the difference in brightness between each view can be minimized. We proposed two methods that could generate a peak of transmittance curve at specific viewing angles with different gray levels in the following section.
Conventional Display Conventional Display + Parallax Barriers
Single‐domain Display
(a) (b) (c)
Fig. 3-5 Viewing angle of (a) conventional displays, (b) conventional 3D displays, and (c) single domain displays.
Single domain Multi‐domain
LC alignment
Fig. 3-6 The difference in brightness between multi-domain VA and single domain VA
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Fig. 3-7 Each kind of pixel can be optimized for specific viewing zone.
3.3.1 Changing the thickness of negative c plate VA
The optical configuration of single domain vertical alignment is shown in Fig. 3-8. The total phase retardation is given by Eq.3-2:
)
where deff is the effective thickness of the c plates.
The second term of Eq.3-2 is caused by negative c plate. By changing the thickness of the negative c plate, we can generate the maximum transmittance in a specific viewing zone. Fig. 3-9 shows an example of this method.
Fig. 3-8 The optical configuration of single-domain VA.
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Fig. 3-9 Maximum transmittance can be generated at a specific viewing angle by changing the thickness of the negative c plate.
Assume that the maximum transmittance is at viewing angle θA. From Eq.2-6, the maximum transmittance can be generated at viewing angle θB by increasing the thickness of the c plate. Fig. 3-10 shows the simulated transmittance profile with various kinds of pixels with different LC alignment directions and c plate thicknesses by DIMOS[21]. The transmittance at viewing zone 6 (-9°) was enhanced with different gray levels compared to that of the conventional pixel layout. The difference in brightness between each viewing zone was minimized. However, it is difficult to fabricate due to the different thickness of c plate for different kinds of pixel. Therefore, we proposed the second method, Normally White Single Domain Vertically Alignment, to enhance transmittance at each viewing zone.
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Multi-domain VA Transmittance profile (Bright state) Single-domain VA Transmittance profile (Bright state) Single-domain VA Transmittance profile (Gray level state)
-9° -6° -2°
(a) View 6 (-9°) (b) View 5 (-6°) (c) View 4 (-2°)
2° 6° 9°
(d) View 3 (2°) (e) View 2 (6°) (f) View 1 (9°) Fig. 3-10 Transmittance profiles of single-domain VA and conventional MVA-b for
different kinds of pixels.
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3.3.2 Normally White Single Domain VA
The optical configuration of Normally White Single Domain VA with different azimuth angles is shown in Fig. 3-11. According to section 2.2, the light at oblique incidence in optical films, the maximum transmittance can be generated in a specific viewing zone by rotating the angle Φ. Fig. 3-13 shows a simulated transmittance profile with various kinds of pixels with different LC alignment directions (Fig. 3-12).
Fig. 3-11 The optical configuration of Normally White Single Domain VA.
3 4 5 6 2
1
6 5 4 3 2 1
3 4 5 6 2
1
3 4 5 6 2
1
110 104 95 275 281 290 Φ
Fig. 3-12 The LC alignment directions for various pixels.
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Multi-domain VA Transmittance profile (Bright state) Single-domain VA Transmittance profile (Bright state) Single-domain VA Transmittance profile (Gray level state)
(a) View 6 (-9°) (b) View 5 (-6°) (c) View 4 (-2°)
(d) View 3 (2°) (e) View 2 (6°) (f) View 1 (9°) Fig. 3-13 Transmittance profiles of Normally White Single Domain VA and
conventional MVA-b for different kinds of pixels.
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The transmittance comparison of each view is shown in Fig. 3-14. The relative improvement in transmittance on the bright state is 7%. Due to the fact that the total viewing range of a 6-view slanted parallax barrier 3D display is only 18°(-9°~ 9°), the transmittance is not enhanced enough compared to that of a conventional one. If number of views increase, our design also can generate the maximum transmittance at wide viewing points. Fig. 3-14 also shows the transmittance at large viewing points (~30°), the transmittance could be enhanced by more than 24%. Furthermore, the difference in brightness between each view would be minimized.
0.2 0.22 0.24 0.26 0.28 0.3 0.32 0.34
‐30 ‐20 ‐9 ‐6 ‐2 2 6 9 20 30
Tr an sm it tan ce (a. u .)
Viewing angle(deg.)
6‐view slanted barrier
Conventional (Bright state) Single Domain (Bright state)
Fig. 3-14 The transmittance comparison of Normally White Single Domain VA and conventional MVA-b for each view.
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3.4 Pixel array design
A parallax barrier 3D display with conventional MVA-b pixel design is shown in Fig. 3-15. There are two major opaque components, including storage capacitor (Cst) and bump, within the barrier slit (pink region), so the aperture ratio of the pixel within the pink region is low. We proposed a pixel layout which increases the brightness of 3D display and suppresses the crosstalk issue. The brightness can be increased due to bumpless pixel with slanted storage capacitor. Beside, the crosstalk issue can be suppressed due to pixel with slanted storage capacitor.
Fig. 3-15 A parallax barrier 3D display with conventional MVA-b pixel design.
3.4.1 Bumpless
The conventional MVA-b pixel design was discussed in section 2.3. The bump is needed to achieve multi-domain alignment. Due to the fact that the light leakage near the bump is large, there should be a black matrix to block the light around the bump.
According to section 3.3.2, transmittance of each kind of pixel in its own viewing zone could be enhanced by using Normally White Single Domain VA. Due to the fact that the LC mode is single domain in our design, there is no bump in the pixel layout.
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Therefore, the aperture ratio can be increased by removing the bump.
In the proposed Normally White Single Domain VA method, each kind of pixels has its own ITO slit direction which can control LC alignment. The direction of LC pre-alignment will be the same as that of ITO slit by PSA technology[ 22 ]. Consequently, different kinds of pixels have their own LC pre-alignment. Fig. 3-16 shows the simulation results of the conventional MVA-b mode and NW single domain VA for the 6th pixel with an azimuthal angle of LC alignment 110° by using ExpertLCD[23]. The aperture ratio of pixel is increased by a factor of 1.16 compared to that of the conventional pixel.
AR=45% AR=52%
(a) (b)
Fig. 3-16 The 6th pixel layout of (a)conventional 2.83” MVA-b mode (b) NW single domain VA with azimuth angle of ITO fine slit 110°.
3.4.2 Slanted storage capacitor
According to Fig. 3-15, the second significant opaque component is storage capacitor which was not optimized for slanted parallax barrier. We modified the shape of the storage capacitor from rectangular to slanted structure. After that, the slanted storage capacitor is rearranged to hide behind the parallax barrier in order to increase
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the aperture ratio of pixel within the barrier slit (pink area), as shown in Fig. 3-17.
Fig. 3-17 A parallax barrier using proposed pixel with slanted storage capacitor.
In addition, the crosstalk can be minimized due to optimizing the position of storage capacitor. Fig. 3-18 shows viewing zone 3 of a conventional 3D display. In the viewing zone 3, there is interfering light, called crosstalk[24], from pixel 2 and pixel 4.
By optimizing the position of storage capacitor to block the light leakage from pixel 2, crosstalk can be minimized.
Fig. 3-18 The viewing zone 3 of 3D displays using conventional and proposed pixels.
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3.5 Aperture ratio of parallax barrier
This is a joint work between classmates in National Tsing Hua University (NTHU). The parallax barriers were designed by NTHU. The aperture ratio of the parallax barrier is determined by the barrier slit size. The barrier slit size can be increased by Normally White Single Domain VA with slanted storage capacitor (section 3.3.2 and section 3.4), as shown in Fig. 3-19. From Fig. 3-20, the aperture ratio of parallax barrier has a 60% improvement compared to that of conventional 3D displays based on the same crosstalk condition[25].
Fig. 3-19 The modified pixel layout.
Aperture ratio of barrier (NTHU)
8.9% 14.3%
~60%
Fig. 3-20 Conventional and modified pixel layouts.
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3.6 Discussion
Our task is to design a 6-view 2D/3D switchable display based on single domain VA for the same resolution in 2D and 3D images. The reason for the same resolution in 2D and 3D images is shown in Fig. 3-21. In 2D mode, as shown in Fig. 3-21(a), the six types of pixels are written by the same image signals. In 3D mode, as shown in Fig.
3-21(b), the six types of pixels are written by six types of image signals, respectively.
In order to get acceptable 2D images, the resolution of images in 2D mode should be the same as that of images shown in conventional LCDs. Consequently, the pixel size of proposed method should be smaller than conventional pixel size. For a 6-view 3D display, the pixel size should be 1/6 compared to conventional pixel size, as shown in Fig. 3-21(c). However, it is not easy to increase the pixel density due to requirement of more driving control lines and ICs. Therefore, for fabrication, we used multi-domain instead of single domain VA to keep the advantage of wide viewing angle and high resolution in 2D mode. The multi-domain mode with slanted storage capacitor ( Fig. 3-22) has been fabricated and will be discussed in Chapter 5.
1 1 1 1
2D Image(multi-domain) 3D Image(single domain)
4 3 2 1
Fig. 3-21 Illustration of 2D/3D switchable display with the same resolution.
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Fig. 3-22 Pixel layout for fabrication.
3.7 Summary
Three parameters, which include transmittance of LC cells, the aperture ratio of
Three parameters, which include transmittance of LC cells, the aperture ratio of