Chapter 2 Principle
2.3 Summary
A simple expression can be derived for the maximum emission efficiency based on ray optics and Snell’s law. Cathode can be set as a perfect reflector and the emission can be isotropic distribution. For the typical value of refractive index of the organic layer (n = 1.7), the maximum output coupling efficiencies for isotropic and in-plane dipoles correspond to a value of 26 % and 42 %, respectively [15]. Although, the critical angle analyzed from Snell’s law at the interface between the top-glass substrate and the air is about 42 degrees, the incident angle larger than 36 degrees causes the transmittance less than 90 % from Fresnel equations. Therefore, considering the Fresnel equations is necessary for establishing a simulation model of the appropriate pyramidal-ALEL.
Chapter 3
Fabrication and Measurement Instruments
3.1 Introduction
A preliminary structure will be used to confirm the features of the microlens array. The embodiment including several fabrication processes will be shown in the following sections, and all the fabrication process, technologies, instruments and a preliminary structure will be introduced in this chapter.
First, the semiconductor fabrication process including mask layout, spin coating, exposure and development were preceeded. Besides, the mold was subsequently filled with the thermal-cured elastomer to replicate the microlenses. Then, the characteristics and performance of the fabricated structure, such as similarity to the geometric design and light efficiency were measured by typical measurement systems, such as optical microscope (OM), scanning electron microscope (SEM), chromameter, and conoscope. The major features of the above mentioned instruments will be illustrated in this chapter.
3.2 Fabrication Process
The entire fabrication processes we utilized to fabricate the microoptical components include the typical VLSI fabrication processes and plastic modeling replication techniques.
First, we determined the feature parameters, like the height of each ALEL pixel (LEL_H), the tilt angle of pyramidal pixel (TILT) and the spacing between two adjacent pixels (LEL_S), need to be designed properly for practical component fabrication. Then, we transfer the desired structures into multi-pattern for mask layout. Third, the microoptical structure on
microoptical structure into thermal-cured elastomer to replicate the microlenses. The processes are shown schematically in Fig. 3.1.
Required parameters:
LEL_H, TILT, LEL_S, Pixel size
Generate the mask patterm in CADs sofeware
Lithographic mask
Lithography and Etching
Replication by plastic molding
Profile evaluation
Fig. 3. 1 Flow chart of array light enhancing layer fabrication process.
3.2.1 Semiconductor Fabrication Process
A prototype is fabricated to characterize the features of the microlens array structure. The semiconductor processes including cleaning, coating PR, UV exposure with G-Line (436 nm) stepper, develop, fixing, RIE etching and KOH etching will be proceeded to fabricate the desired structure on Si-wafer at semi-conductor research center (SRC).
The detail processes are shown in Fig. 3.2. First step is initial-cleaning. Wet-cleaning processes are necessary to obtain an ultra clean wafer surface for subsequent fabrication. Then a 0.5 µm thick layer of SiNx will be deposited by plasma enhanced chemical vapor on wafer.
Next, the wafer is placed on a vacuum chuck in the coater and the photoresist is dropped onto the center of the wafer. A uniform and thin photoresist layer can be coated on the wafer
surface after spinning the wafer. The following step is exposure. The mask pattern is transferred onto the wafer. The exposed wafer is loaded into the development system after exposure. Consequently the desired structure will show up in the photoresist. Etching is then applied to get the desired structures from the developed pattern to a wafer.
Non-photoresist-covered SiNx will be removed during RIE etching. Then the remained photoresist will be stripped by acetone. Finally, the regions of silicon will be etched by KOH and the fabricated microoptical structure will appear on the Si wafer.
(A) Initial Clean
H.M.D.S.
(C) Coating H.M.D.S.
SiNx
(B) Deposit ing SiNx
Photoresist (PR)
(D) Coating Photoresist
(F) Developing & Fixing
(G) RIE Etching
(H) PR removing
(E) Exposure
(I) KOH Etching 4" Dummy Wafer
Fig. 3.2 Semiconductor fabrication process of microoptical structure.
3.2.2 Replication
The plastic material for replication is considered to have a similar refraction index as optical glass. We use poly(dimethylsiloxane) (PDMS), a thermal-cured elastomer, as the material. Fig. 3.3 shows an overview of the major replication technology.
(a)
(b)
(c)
Fig. 3.3 ALEL replication process flow (a) filling the mold with PDMS (b) removing bubbles by vacuum process, and (c) curing.
The fabricated mold was pretreated by an anti-sticking layer (1H, 1H, 2H, 2H-perfluorodecyl trichlorosilane) to prevent the mold adhering to PDMS during the replication process. To fabricate the microlenses, the mold was subsequently filled with PDMS. Then we remove the bubbles from the microstructure by using vacuum chamber. The PDMS is cured at 50 °C for 3 hr leading to a flexible sheet that can easily be peeled off from the mold. The thickness of the PDMS layer ranges from 1 to 3 mm.
3.3 Measurement System
After the fabrication, we must make sure the fabricated components meet the design goal.
First, the geometric similarity to the designed structure is checked by the optical microscope (OM) and scanning electron microscope (SEM). Then, the optical performances of the fabricated components and designed ALEL on OLED panel are measured by the chromameter CS-100 and conoscope system, the detailed descriptions are shown below.
3.3.1 Scanning Electron Microscope (SEM)
A scanning electron microscope (SEM) is an essential instrument to measure the accuracy and fidelity of the fabricated microstructure, as shown in Fig. 3.4. The instrument scans electrons reflecting onto a fluorescent screen across the target where the image is captured by a camera and enlarged. Electrons are much smaller than atoms, so a scanning electron microscope paints a razor-sharp image of the target, and the feature variation of few angstrom can be observed. SEM is useful for mapping details of objects that optical microscopes can not resolve. Using the electromagnetic lenses to focus the accelerated electron beam, the diameter of electron beam can be converged to the dimension of 10 µm.
The secondary electrons are generated where the focused accelerated electrons bombard the sample. Detecting the secondary electrons can determine the location of bombardment.
Simultaneously, the focusing electron beam scans the surface of sample, with the aid of
-3
scanning coil, to map the feature of measured area.
HITACHI S-4000 SEM was utilized to measure the quality of our fabricated microstructure elements. The line width, etching depth, and aperture size can be measured accurately.
Fig. 3.4 Photograph of SEM.
3.3.2 Measurement Systems for light efficiency
3.3.2.1 Chromameter CS-100
The color and brightness of image are the important properties of system we want to evaluate. In order to get these information, a colormeter is necessary. We choose the Minolta Chroma Meter CS-100, which utilizes three high-sensitivity silicon photo cells, which are filtered to match Commission International de I’Eclairage (CIE) standard observer response. Chromaticity coordinates (x,y) and illumination (Y) as well as color temperature in Kelvin (K) are calculated from the three cells’ measurements. With this compact reliable color analyzer, we can easily get the Chromaticity coordinates and
3.3.2.2 Conoscope
Conoscope is a measurement system which utilizes Fourier transform lens to transfer the light beams emitted (or reflected) from the test area of different angles to the CCD array, as shown in Fig. 3.5. Every light beam emitted from the test area with a θ incident angle will be focused on the focal plane at the same azimuthal angle and at a position x = F(θ).
Therefore, the angular characteristics of the sample are thus measured simply and quickly, without any mechanical movement. Particularly, which kind of light source, i.e. collimated or diffuse illumination, is provided depending on the needs.Besides, by equipped with a fast photometer system and a high-sensitivity spectrometer, its functions are extended to comprise not only the simultaneous measurement of luminance and chromaticity versus viewing direction, evaluation of the data yields, i.e. luminance contras ratio, grey-scale inversion and reduction, color shift and many more characteristics, but also the spectra and temporal luminance variances.
C: cone of converging elementary parallel beams A: variable aperture Fig. 3.5 Schematic diagram of Conoscope.
Chapter 4
Simulated Results and Discussions
4.1 Introduction
In chapter 2, we had analyzed the transmittance at the interface between the top-glass substrate and air to confirm that the Fresnel equations are necessary for modelling a pyramidal ALEL. We established a simulation model to characterize the features of the array light-enhancing layer on OLED panel.
First, the parameters of pyramidal ALEL, including the height of each ALEL pixel, the tilt angle of pyramidal pixel and the spacing between two adjacent pixels, were designed and optimized. After that, the alignment issue of the array light-enhancing layer on OLED panel was also discussed.
4.2 Simulation Software
The optical simulator Advanced Systems Analysis Program (ASAPTM), developed by Breault Research Organization (BRO) was used to optimize the array light-enhancing layer on OLED panel and simulate its light efficiency.
4.3 Simulation description
4.3.1 Optical Model
In order to consider the whole effect of the array light-enhancing layer on an OLED panel, all the design and optimization were carried out under the complete system framework.
bottom glass substrates, as shown in Fig. 4.1. Ribs and insulators were set with 60 % transmittance. Since the organic layers, including the electron injection layer, the electron transport layer, the active layer (the emitting layer), the hole transport layer and the hole injection layer, are much thinner (~0.15 µm) than other layers, they are assumed as one “source layer” in our simulation, as shown in Fig. 4.2. Metal cathode was set with 90 % reflectance.
Fig. 4.1 Schematic diagram of OLED structure.
Glass Substrate
Cathode
Electron Transport Layer Electron Injection Layer
Hole Transport Layer Hole Injection Layer
Anode
Emitting Layer Organic Layers
Fig. 4.2 Schematic diagram for organic layers of OLED structure.
Lights from active layer was assumed to emit in the form of pixel array and was set as an isotropic light source. The array light-enhancing layer consists of the base substrate and microlens array. The pixel size of ALEL, as shown in Fig. 4.3, was set to fit one of OLED panel. The size of ALEL and OLED panel are W×L. In addition, a flat detector with the diameter of 3cm was located at a distance of 3.5cm above the OLED panel, as shown in Fig.
4.4.
1 Pixel size of OLED Panel 1 Pixel size of
OLED Panel
1 Pixel size of ALEL
1 Pixel size of
ALEL Alignment
Fig. 4.3 Schematic diagram for designing pixel size of light source and ALEL.
L
OLED panel ALEL
W
Detector 3 cm
Fig. 4.4 Schematic diagram of measurement.
Light efficiency is defined as the detected flux over the generated one. Furthermore, gain factor was defined as the ratio of the light efficiency of an OLED panel with ALEL to that of a panel without ALEL, which were served as the bases of the optimizations.
(1) = detected flux light efficiencyof OLED panel with ALEL
Gain Factor =
light efficiencyof OLED panel
light efficiencyof OLED panel with ALEL Gain Factor
light efficiencyof OLED panel
=
4.3.2 Parameters
In order to obtain to maximum gain factor, several variables, as shown in Fig. 4.5, such as the height of each ALEL pixel (LEL_H), the tilt angle of pyramidal pixel (TILT) and the spacing between two adjacent pixels (LEL_S), need to be designed properly. A simulation tool, ASAPTM, with the Fresnel equations assigned was used to perform the following simulation.
LEL_H
Fig. 4.5 Illustration of parameters for designing array light-enhancing layer.
4.4 Results and discussions
4.4.1 Tilt angle and spacing of pyramidal pixel
First, the relationship between the gain factor and the tilt angle (TILT) will be discussed.
Height and spacing are kept constant, and TILT is changed from 90° to the minimum angle where the two slopes meet at the intersection point, and then the relationship between the gain factor and TILT is formed. Besides, spacing is kept as different constants to represent the
relationship between the gain factor and spacing. The green sub-pixel light source is turned on.
From the simulated results, as illustrated in Fig. 4.6, the maximal gain factor can be derived at 50° to 55° and smaller spacing.
1.0 1.2 1.4 1.6 1.8 2.0
40 50 60 70 80 90
TILT (degrees)
Ga in Fa c to r
Spacing = 10 µm Spacing = 25 µm Spacing = 40 µm
Fig. 4.6 Illustration of the relationship between gain factor and TILT. The height is 125 µm.
4.4.2 Height of pyramidal pixel
Next, the relationship between the gain factor and the height is discussed. From the simulated results, as illustrated in Fig. 4.7, the curve of gain factor becomes stable when the height is higher than 100 µm. Since the lower height can bring higher yield rate during the demolding process, the optimized height is set as around 150 µm.
1.0 1.2 1.4 1.6 1.8 2.0
0 50 100 150 200 250
Height (µm)
G a in F acto r
TILT = 40 TILT = 50 TILT = 60 o o o
Fig. 4.7 Illustration of the relationship between gain factor and Height. The spacing is 10 µm.
4.4.3 Opening area ratio
We further broaden the analysis of the relationship between gain factor and the parameters of ALEL. Opening area ratio is defined as the sum of the flat areas A over the pixel size of ALEL, as illustrated in Fig. 4.8.
Fig. 4.8 Definition of opening area ratio.
The relationship between the gain factor and the parameters of ALEL, as illustrated in Figs. 4.6 and 4.7, will be transferred to the relationship between the gain factor and the opening area ratio.
First, the relative curves of gain factor versus opening area ratio are obtained by converting the gain-TILT curve (Fig. 4.6). Maximal opening area ratios, as illustrated in Fig.
4.9, are different at various spacing. Therefore, the relationship between the gain factor and TILT is still the suitable method to obtain proper directionalities.
1.0 1.2 1.4 1.6 1.8 2.0
0 20 40 60 80 100
Opening area ratio (%)
G a in F acto r
Spacing= 10 µm Spacing= 25 µm Spacing= 40 µm
Fig. 4.9 Illustration of the relationship between gain factor and opening area ratio (transfromed from gain-TILT curve). The height is 125 µm.
Moreover, the relationship between gain factor and height was also transferred to the relative curves of gain factor versus opening area ratio. From the results, as illustrated in Fig. 4.10, it can be concluded that smaller opening area ratio produces higher gain factor. In addition, the gain factor is similar when opening area ratio is smaller than 30 % at the optimized TILT angle of 50°. It could be the reference for designing lower height of ALEL to bring higher yield rate during the demolding process.
1.0 1.2 1.4 1.6 1.8 2.0
0 20 40 60 80 100
Opening area ratio (%)
Ga in Fa c to r
TILT = 40 TILT = 50 TILT = 60 o o o
Fig. 4.10 Illustration of the relationship between gain factor and opening area ratio (transferred by various height). The spacing is 10 µm.
4.4.4 Alignment issue
In our design, the pixel size of ALEL is set to fit the one OLED pixel. As a result, the alignment issue should be considered in the mass production. Fig. 4.11 shows the result of gain factor loss versus ALEL shift. ALEL shift is defined as the same shift on X and Y axes as illustrated in Fig. 4.12. The space between adjacent sub-pixels on OLED panel is about 40 µm. When ALEL are 60 µm and 90 µm shifted, as illustrated in Figs. 4.13 (c) and (d), their regions of the pixel spacing of ALEL crossing the green sub-pixel source are similar. There is about 15 % loss of gain factor when ALEL is shifted 90 µm along X and Y axis. The gain factor losses of 60 µm and 90 µm misalignments are also approximated. Therefore, the pixel
Fig. 4.11 Illustration of the relationship between gain factor loss a spacing of ALEL crossing light source is the main factor of alignment.
nd ALEL shift.
0 5 10 15 20 25
0 30 60 9
X & Y Shift (µm) Ga in Fa c to r L o ss ( % )
Spacing = 40 µm , TILT = 55Height =125 µm,
o
0
OLED panel
ALEL
Y
X
OLED panel
ALEL
Y
X
Fig. 4.12 Illustration of ALEL misalignment.
Pixel spacing (a) (b) (c) (d)
Fig. 4.13 Schematic diagram of ALEL shift on X and Y axis: (a) ideal case, (b) 30 µm, (c) 60 µm, and (d) 90 µm.
4.5 Comparison
The structures of the other two prior arts, as illustrated in table 4.1, are listed here for comparison. The dimension of our proposed ALEL pixel was about 300 µm, which fitted the pixel of light source. Other variables were determined by optimized results and process limitation. The optimal pixel spacing was equal to Dr. Wei’s result and the height was larger than the other two prior arts. As a result, the gain factor of our proposed structure is as high as 1.70.
Table 4.1 Gain factor of OLED panels with various microlens geometry.
Journal Mao-Kuo Wei,
OPTICS EXPRESS, No 23, Vol. 12, 2004
S. Moller, J. Appl. Phys.,
V9, 2002
NCTU
Spherical geometry Arc geometry pyramid-shaped geometry (55o)
Microlen Dimension (µm) 100 10 ~ 300
Pixel Spacing (µm) 11 ~ 0 15
Height (µm) 50 5 200
ALEL Material PMMA PDMS PDMS
Gain factor 1.56 1.50 1.70
Structure
4.6 Summary
We have designed and optimized the array light-enhancing layer on an OLED panel.
From the mentioned analyses, thus the structure of ALEL has the distinguished capability to eliminate the total internal reflection (TIR) effect, thus, light efficiency and gain factor of OLED panel can be increased significantly. Optimized pyramidal ALEL yields a gain factor of 1.7. A pyramidal ALEL on OLED panel produces higher light efficiency than other structures in our simulation results.
Additionally, according to the simulated results, three important features of design rules for ALEL on OLED panel can be derived:
In ideal alignment cases, the gain factor can be optimized with (a) Narrower pixel spacing
(b) Tilt angle of 50º to 55º
(c) Opening area ratio smaller than 30 %.
For alignment issue, the area of pixel spacing cross light source was the main factor of the gain factor loss.
Chapter 5
Experimental Results and Discussions
5.1 Introduction
Based on the simulation and fabrication presented in the previous chapters, the experimental results and discussions will be shown in this chapter. Optical microscope (OM) and SEM were utilized to examine the mold and ALEL. The optical performances of a Chung Hwa Picture Tube, Ltd (CPT) OLED panel with ALEL, such as luminance, were measured by Chromameter CS-100 and Conoscope. Finally, the gain factors of ALEL-attached OLED panels with two different pixel sizes are compared.
5.1.1 Mold examination
Optical microscope was used to examine the fabricated structures of silicon mold. The spacing between two adjacent pixels and the pixel size were checked, as illustrated in Figs. 5.1 (a) and (b). The height (H) of each ALEL pixel was also inspected as illustrated in Fig. 5.2 (a).
Wet etching rate on the {111} crystal lattice direction, as illustrated in Fig. 5.2 (b), is 400 times slower than the {100} direction [22]. Since the angle between {100} and {111} crystal lattice is well-know as 54.74º, which fits our optimized TILT, the relationship between the height of the ALEL pixel (H) and the width of the slope (W) can be derived by the following equation:
H = W tan (54.74º ) (1)
Spacing between two adjacent pixels
15 µm 40 µm
(a)
Pixel size
Pixel size of ALEL : Pixel size of OLED panel1:1 0.5 : 1 0.2 : 1
(b)
Fig. 5.1 Photographs of fabricated structure on molds from optical microscope (a) different spacing between two adjacent pixels and (b) different pixel size.
Height of ALEL pixel
W
H
H = W tan (54.74º )
W
H
{111}
{100 }
54.74o
Si-mold
{111}
{100 }
54.74o
Si-mold
(a) (b)
Fig. 5.2 Illustrations of fabricated structure on molds from optical microscope (a) height of ALEL pixel and (b) cross-section and crystal lattice direction of a silicon mold.
5.1.2 Replication examination
Optical microscope and SEM were used to examine the replicated ALEL. The overviews of ALEL are shown in Figs. 5.3 and 5.4, while various variables of ALEL are illustrated in Fig.
5.5.
Fig. 5.3 A photograph of ALEL.
Fig. 5.4 SEM photographs of ALEL.
~
Tilt angle of pyramidal pixel
TILT
TILT ~~
Tilt angle of pyramidal pixel
TILT TILT
(a)
LEL_S LEL_S
Spacing between two adjacent pixels
15 µm 30 µm 40 µm
(b)
(d)
Fig. 5.5 Photographs of ALEL by optical microscope (a) tilt angle of ALEL pixel, (b) various spacing between two adjacent pixels, (c) various height of two ALEL pixel, and (d) various pixel size.
5.1.3 Measurement System
The measurement system was illustrated in Fig. 5.6. The optical performance, the luminance of ALEL region at the direction normal to CPT OLED panel, was measured by Chromameter CS-100 and Conoscope in a dark room.
Detector
( MINOCTA Chroma Meter CS-100 ) OLED Panel (CPT)
Power Supply
Detector
( MINOCTA Chroma Meter CS-100 ) OLED Panel (CPT)
Power Supply
(a)
Detector (Conoscope)
(b)
Fig. 5.6 Photographs of measurement systems (a) Chromameter CS-100 and (b) Conoscope.
5.2 Results and discussions
5.2.1 Alignment issue
5.2.1.1 Moire pattern
According to the measured luminance, we studied the alignment issue first. Several alignment cases, as illustrated in table 5.1, are listed here for comparison. Free from moire pattern is the desired optical performance as shown in the left alignment of the case 1. Moire pattern are serious in the cases 2 and 3. We checked the periods of pixel size of ALEL and the pixel size of the OLED panel, they were different along x and y axes. Therefore, the
According to the measured luminance, we studied the alignment issue first. Several alignment cases, as illustrated in table 5.1, are listed here for comparison. Free from moire pattern is the desired optical performance as shown in the left alignment of the case 1. Moire pattern are serious in the cases 2 and 3. We checked the periods of pixel size of ALEL and the pixel size of the OLED panel, they were different along x and y axes. Therefore, the