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Abstract
Barrier ribs in the plasma display panel(PDP) function to maintain the discharge space between the glass plates as well as to prevent optical crosstalk.
Patterning of barrier ribs is one of unique processes for making PDP. In this work photosensitive barrier rib pastes were prepared by incorporating binder polymer, solvent, functional monomers photoinitiator, and barrier rib powder of which surface was treated with fumed silica particles.
Study on the function of materials for the barrier rib paste were undertaken. After optimization of paste formulation and photolithographic process, it was found that photolithographic patterning of barrier ribs with photosensitive barrier rib green sheet could be used in the fabrication of high resolution PDP.
1. Introduction
Several methods are available for the fabrication of barrier ribs on the plasma display panel(PDP) including screen printing, sandblasting and photolithographic method.
Currently sandblast and etching method are widely used in the industry for the formation of b a r r i e r r i b s for PDP . A s t h e s i z e o f P D P increase ove r 60 inches and high resolution ( X G A g r a d e ) i s n e e d e d , h o w e v e r , photolithographic method is considered to be the method of choice for barrier rib patterning.
In this work, the formulation of photosensitive barrier rib paste for use in the green sheet form and the photolithographic process of patterning barrier ribs of PDP were carried out.
2. Experimental 2.1 Material.
Hydroxypropyl cellulose(Aldrich Chemical Co., MW 80,000) which is soluble in water was used as binder polymer for photosensitive barrier rib paste.
3-Methoxy-3-methyl butanol(3MMB) was used as solvent(Tb = 175 ℃) to dissolve HPC binder polymer. UV curable monomers include p e n t a e r y t h r i t o l t r i a c r y l a t e ( P E T A ) , t r i m e t h y l o l p r o p a n e t r i a c r y l a t e ( T M P T A ) , t r i m e t h y l o l p r o p a n e e t h o x y t r i a c r y l a t e
Photolithographic Method of Patterning Barrier Ribs for PDP by Green Sheet
Lee Soon Park, Dong Gyu Jang, Young June Hur*, Sung Ho Lee*, Duck Gon Kim, Young Hwan Kwon**
Department of Polymer Science, Kyungpook National University1370 Sankyuk-Dong, Buk-gu, Daegu, 702-701, Korea
*Advanced Display Manufacturing Research Center, Kyungpook National University 1370 Sankyuk-Dong, Buk-gu, Daegu, 702-701, Korea
**Department of Chemical Engineering, Daegu University,
Jinryang, Gyeongsan, Gyeongbuk, 712-714, Korea
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(TMPEOTA) in addition to some oligomers.
Photoinitiator(HSP -188) was purchased from SK - UCB Co. and used as received. Barrier rib powder whose surface is treated with fumed silica particles, has an approximate composition of PbO 60.0, SiO2
11, Al2O3 29.0wt. Nano sized fumed silica was purchased from Degussa. Co.
.
2.2 Preparation of photosensitive barrier rib paste
.The components of photosensitive barrier rib paste were mixed with the aid of mechanical stirrer and three roll mill(Exact Co., Germany) according to the process described below. First binder polymer(HPC) was dissolved in the 3MMB solvent. UV curable monomers and photoinitiator were added to this solution and the resulting mixture was stirred for 2hr.
To this mixture was added barrier rib powder and the slurry was mixed with mechanical stirrer for 10- 30 min. The mixture slurry was then placed on the three roll mill and ground for 2-3 hr until a homogeneous paste with desired rheological property was obtained. In the case where barrier rib powder was surface treated with nano-size silica particles, the barrier rib powder was mixed with fumed silica(Dmin = 12nm) in the ball mill and then heat treated in the oven(50~150℃). The mixture powder was re-pulverized to the designated particle size distribution using the ball miller.
2.3 Fabrication of Photosensitive Barrier rib Green Sheet
For fabrication of green sheet, barrier rib paste was coated to about 200~250㎛ thickness on the poly(ethylene terephthalate)(PET) base film coated with release agent. This coated film was dried in the
IR oven at 110 for several minutes and then ℃ covered with PET cover film by lamination. Figure 1 shows the fabrication process of barrier rib green sheet.
Figure 1. Fabrication of barrier rib green sheet.
2.4 Barrier rib patterning by photolithographic method.
The process of obtaining fine pattern of barrier rib by photolithographic method with green sheet is shown in Fig.2.
First, cover film was removed and the paste layer was laminated on the rear panel of PDP on which address elect was patterned.
The photomask was placed on top of the dryed barrier rib layer laminated and then irradiated with a UV lamp to a total dose of 300-500mJ/cm2. After UV Exposure, the PET base film was peeled off.
The unirradiated barrier rib was developed with alkaline water at 1.0 kg/cm2 pressure for 20-30 sec and then dried in the 110℃ oven for 10 min. After drying the PDP panel was sintered in the high temperature oven up to 550 for 30min. Barrier rib ℃ patterns without any binder polymer and other organic components were obtained through this firing process
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Fig. 2. Formation of PDP barrier rib by photolithographic process
3. Results and Discussion
3.1Photosensitivity of UV Curable Monomer / Oligomer System.
The fine patterning of barrier rib is dependent on many parameters both involved in the formulation of photosensitive barrier rib paste and photolithographic processes. One of the important factors controlling the photosensitivity of barrier rib paste is the selection of UV curable monomer/oligomer system.
In order to compare the photosensitivity of UV curable monomer/oligomer system, photosensitive vehicles which do not contain barrier rib powder was prepared and their photolithographic properties were evaluated. The photosensitive vehicles prepared are shown in Table 1.
Table 1. Photosensitivity of curable System
Sample No.
Monomer /Weight(g )
Oligomer /Weight(
g)
C=C (mol e)
Conver sion (%)
PRM-1 - EB600
10
0.05
7 38.0
PRM-2 TPGDA 2.5
EB600 7.5
0.04
6 44.3
PRM-3 PETA
10 - 0.10
1 58.8
PRM-4 TMPTA
10 - 0.10
1 69.1
PRM-5 TMPEOT
A10 - 0.07
0 81.8
Table 2. Photolithographic Process Conditions
Process Parameter Optimum Condition Screen
printing
Viscosity(cps) Mask(mesh No.)
27,500 cps 325
Drying Temp/Time 110℃/20min Exposure Dose
Gap
300mJ/cm2 300µm Development Medium/Time H2O/35sec
※Typical Photosensitive Barrier Rib Paste :
Binder polymer(16.1%), Solvent(63.3%), Monomer(16.1%)
Photoinitiator(1.6%), Additive(0.7%), Powder(50.0%)
3.2 Photosensitive Barrier Rib Paste Formulation and Photolithographic Process
.Using the photosensitive vehicle based on Table 1, various photosensitive barrier rib pastes were formulated with a fixed amount of barrier rib
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powder(70%wt). The photolithographic processes such as screen printing, drying, UV exposure, and development were tested utilizing the photosensitive barrier rib paste incorporated in the green sheet and process parameters are summarized in Table 2.
In the photolithographic method of patterning barrier ribs for PDP, it is desirable to obtain a fine pattern of barrier ribs with one time exposure of UV light. Since the photosensitive barrier rib paste has inorganic powders over 60% weight, these particles scatter UV light during exposure resulting in under- cut in the barrier ribs or washing out of barrier ribs after development. It was found that the surface treatment of barrier rib powder with nano-size silica particles as shown in Fig. 3 greatly improve the photosensitivity in the photolithographic patterning of barrier ribs, probably by introducing light propagation channels among the barrier ribs particles.
( A ) ( B )
Fig 3. Barrier rib powder Before(A) and after(B) surface treatment
4.Conclusion
Both photosensitive barrier rib paste formulation and photolithographic process were developed for the fine patterning of barrier rib with high resolution.
The photosensitive barrier rib paste and
photolithographic process are especially suitable for large size (over 60 inch) and high resolution (XGA grade) plasma panel display.
5. Acknowledgement
This work was supported by Grant No.
10016632 from New Generation Growth Engines project of the Ministry of commerce, industry &
energy in the republic of Korea.
