Introduction
On the rear panel in Plasma Display Panel (PDP), barrier ribs were formed in stripe and matrix arrangements to maintain the discharge space between the two glass plates as well as to prevent electrical and optical cross talking between the adjacent cells, and also to provide the additional areas for phosphor coatings on their side walls, which contribute to the improvements in contrast as well as luminance of the device. The width of the barrier rib used in a PDP is typically in the range 30-80μm and its height is 110 –140μm. The distance between the barrier rib centers (subpixel pitch, pitch) is 150 and 420μm for 42 inch (107 cm) full-spec. High Definition Television (HDTV), 1920×1080 pixels) and Video Graphic Adaptor (VGA), 852×480 pixels) grade PDPs, respectively. Ribs with finer dimensions have been required for devices of higher resolution.
Various barrier rib forming routes were proposed and examined including screen printing [1], sandblasting [2], lift-off, rolling of green tape [3], and photosensitive paste [4-6] as shown in Figure 1. Among the routes, sandblasting and photosensitive paste were currently used in most PDP- manufacturing lines.
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Figure 1. Schematic figure of various barrier rib forming processes.
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In photosensitive paste process, a glass substrate is coated with a photosensitive glass paste and the patterning is carried out by a conventional photolithographic process. Conventional photosensitive paste process was, however, not yet in practical use as a rib forming though photosensitive silver paste has been widely used for the formation of sustain, bus, and address electrodes [7, 8].
By the conventional photosensitive paste process, a thickness of the pattern attained by one time exposure is as thin as 30μm as shown in Figure 2. Therefore, it has a big shortcoming to repeat the application of paste, drying and exposure for 4-10 times to form a rib of a required height, i.e.200μm (about 140μm after firing).
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Figure 2. Problems of barrier rib forming by conventional photosensitive paste process.
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In the present work, this shortcoming of repeating the exposure and development was solved by the examinations of fabrication process, namely paste compositions and glass powders design. The chemical composition, particle size, and distribution of glass powders and firing conditions were investigated. As a result, photosensitive paste process was established which could form the required rib pattern by one time application, drying, exposure, and development as shown in Figure 3 [9, 10].
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Figure 3. New barrier rib forming process by present photosensitive paste.
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Photosensitive rib pastes were prepared by incorporating the glass powders, binder polymer, functional monomer, photo initiator, additives, and solvent. The effects of the paste components and photolithographic processes on rib forming were examined. The effects of the processing parameters such as the exposure energy and the gap between the coated film and photo mask, and the development conditions on the height, width, pitch, and cross section of the ribs were investigated.
Experimental
Photosensitive Paste Preparation
The photosensitive paste process is schematically shown in Figure 4. Photosensitive pastes are composed of inorganic glass powders and photosensitive organic components including acrylic binder polymer, ultraviolet (UV) curable functional monomer, photo initiator, plasticizer, additives, and solvent.
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Figure 4. Schematic image of photosensitive paste process.
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Inorganic glass powder was produced by glass makers and its chemical composition was within the B2O3-SiO2-Al2O3-Li2O-BaO-MgO system. The average particle sizes of the glass powders, measured using laser diffraction particle size analysis (Microtrack) were 1.0-4.0μm.
Pastes for rib forming were prepared by kneading the glass powders and organic constituents, i.e., solvent, polymer binder, monomer, plasticizer, and other additives.
First, binder polymer was dissolved in γ-BL (butyrolactone) solvent with mechanical stirrer to the extent which did not have any gel or coaglulums. Monomers, photo initiator, plasticizer, and additives were added to this solution, and the resulting mixture was stirred. To this mixture, the slurry and glass powder were added and mixed with a mechanical stirrer. Total slurry of desired formulation was then kneaded by the three roll mill (Exact Co., Germany) for 5 -10 times to achieve the uniform dispersion and desired viscosity.
Printing
Photosensitive paste was screen printed over the whole surface of the glass substrate of high strain point (Asahi Glass PD-200, 125 mm square and 2.8 mm thickness). The stainless steel screen of 325 mesh (25μm in opening) was used. Printing and drying were repeated over 10 times so that the thickness reached 200μm. After leveling at room temperature for 5-10 min., the coated film on glass substrate was dried at 80˚C to remove the solvent.
Barrier Rib Patterning by Photolithographic Method
Using glass photo masks with three different pitches (360, 230, and 130μm) of chromium stripes, the coated substrate was exposed to UV irradiation (wave length: 365, 405, and 436 nm). Light source was ultra high voltage mercury lamp and the amount of exposure is varied from 200-700 mJ/cm2.
Exposed substrates are immersed into a weak alkali aqueous solution of 0.1-0.5 % for 10-30 sec. to dissolve the unexposed portion, and then dried at 80˚C to obtain the barrier rib formed substrate. Barrier rib formed substrate was fired in an air atmosphere using a box type furnace. Temperature was heated up to 580˚C at a rate of 80 -200˚C/h and kept at that temperature for 15 min to sinter.
Barrier rib patterns without any binder polymer and other organic components were obtained through this firing process.
Cross section, top view, and the defects such as disconnection of fired patterns were examined using scanning electron microscopy (SEM) and optical microscope.
Results and Discussion
The patterning of barrier rib is dependent on many factors and parameters involved in both the formulation of photosensitive pastes and photolithographic processes. One of the important factors controlling the photosensitivity and photo polymerization of barrier rib paste is the selection of UV curable monomer/polymer system and photoinitiator. Perfect photo polymerization throughout the bottom part of the barrier rib is a key factor to obtain the good patterning without disconnection and curving.
Figure 5 shows the SEM images of cross section and whole view of barrier ribs. Each of the ribs with 360, 230 and 130μm pitches was formed by one time exposure and development. In order to form a rib pattern of 200μm height, the UV scattering by photosensitive paste should be avoided. The ribs with high resolution (XGA) of 150μm height, 30μm width, and 150μm pitch after firing could be formed by the optimization of paste formula and photolithographic process as shown in Figure 6. Any defect such as the disconnection, peeling, and curving of ribs was not detected.
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Figure 5. SEM image of rib pattern of 360, 230 and 130μm pitches by photosensitive process and their applications.
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Figure 6. SEM image of rib pattern of 150μm height, 30μm width, and 150μm pitch by photosensitive paste.
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In addition, two different types of the cross section of ribs could be formed by selecting the paste formula and exposure energy to obtain the uniform phosphor layer thickness for high brightness as shown in Figure 7.
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Figure 7. SEM image of rib pattern of two different cross sections.
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Conclusions
The barrier ribs with high resolution (XGA) of 150μm height, 30μm width, and 150μm pitch after firing were achieved by the optimization of paste formulation and photolithographic process. The photosensitive paste process is especially suitable for high resolution plasma display panel.
References
1. A. Sabel, “Plasma Displays,” IEEE Trans.Plasma Sci., 19, [6] (1991)1032-47.
2. T. Shinoda, M. Wakitani, T. Nakano, and N. Awaji, “Development of Panel Structure for a High-Resolution 21-in.-Diagonal Full-Color Surface-Discharge Plasma Displays Panel,” IEEE Trans. Electr. Devices, 47, [1] (2000)77-81.
3. Y.-S. Kim, S.-Y. Lee, S.-K. Hong and H.-J. Jeon, “Formation of Barrier Ribs for Plasma Display Panel via Roll Forming of Green Tapes,”J.Am.Ceram.Soc., 84, [7] 1470-74(2001).
4. L.S. Park, S.W. Jeong, S.H. Paek, S.W. Yun, “Photolithographic Method of Patterning Barrier Ribs for PDP ”, IDMC 2002, pp.171-74.
5. T. Masaki, G. Moriya, K. Horiuchi and Y.Iguchi, ,“Plasma Display and Method of Manufacturing the Same,” US Patent No.6043604 (28 March 2000).
6. S.H.Paek, H.S. Choi and L.S.Park, “Patterning Barrier Ribs of PDP by Transparent Soft Mold”, IMID 2002 DIGEST, pp.639-42..
7. L.P.Drozadyk, J.H.Choi, and M.Tsuchiya, “Photopatternable Conductor Tapes for PDP Applications” SID 99 DIGEST, pp.1044-47.
8. T. Masaki, A. Yoshimura and K. Iwanaga, “Photosensitive Conductive Pastes,” Denshizairyo, Japan, (1996) pp.1-7.
9. T.Masaki and Y.Iguchi, “Barrier Rib Formation for PDP with Photosensitive glass paste,” in the Proceeding of Japan Ceramic Society Meeting, Ceramic Society of Osaka, (2000) pp.33-35.
10. K. Iwanaga, T. Masaki, and I. Iguchi, “Photosensitive Paste, a Plasma Display, and a Method for the Production Thereof,” US Patent No.6197480 (6 March 2001).
Contact Details
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Takaki Masaki
Department of New Material Science and Engineering
Halla University
Wonju 200-712
Korea
Email: [email protected]
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Yuichiro Iguchi
Department of PDP Technologhy
Toray Industies, Inc
Otsu,Shiga 520-0842
Japan
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Soo-Jong Kim
Small & Medium Business Administration
856-8, Toigye-dong
Chunchon-City
Kangwon-do, 200-161
Korea
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