Metal Information Center

Barrier Rib Forming by Photosensitive Paste for Plasma Display Panel

March 29, 2007 – 6:11 am

Abstract

On the rear panel in Plasma Display Panel, 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.  Various barrier rib forming routes were proposed including screen printing, sandblasting, lift-off, rolling of green tape, and photosensitive paste.  In this study, a rear panel forming process by photosensitive paste was presented.  Photosensitive barrier rib pastes were prepared by incorporating glass powders, binder polymer, functional monomer, photoinitiator, additives, and solvent.  After optimization of the paste formula and photolithographic process, a barrier rib could be obtained with a high resolution of 150μm height and 30μm width, with a pitch of 150μm.
Keywords

Plasma Display Panel, Barrier Rib, Photolithography, Photosensitive Paste, Fine Pattern
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.

AZoJomo - The AZO Journal of Materials Online - Schematic figure of various barrier rib forming processes

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).

AZOJomo - THe AZO Journal of Materials Online - Problems of barrier rib forming by conventional photosensitive paste process

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].

AZoJomo - The AZO Journal of Materials Online - New barrier rib forming process by present photosensitive paste

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.

AZoJomo - The AZO Journal of Materials Online - Schematic image of photosensitive paste process

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.