Studies on effects of compatibilizers in over coating layer materials for TFT panels

Article abstract of DOI:10.1080/15421400600932462

Over coating layer (OCL) materials were prepared by mixing acrylate copolymer and epoxy compound. The shelf life of the OCL materials could be extended, because of suppression of the crosslinking reaction between the acrylic acid and epoxy groups during s

Full text of DOI:10.1080/15421400600932462

Mol. Cryst. Liq. Cryst., Vol. 458, pp. 89–95, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600932462
Studies on Effects of Compatibilizers in Over Coating
Layer Materials for TFT Panels
Tomonari Ogata
Kenji Mochida
Takamasa Nonaka
Seiji Kurihara
Department of Applied Chemistry and Biochemistry, Kumamoto
University, Kumamoto-shi, Kumamoto, Japan
Shuuichi Takahashi
AZ Electronic Materials Japan, Kakegawa-shi, Shizuoka, Japan
Over coating layer (OCL) materials were prepared by mixing acrylate copolymer
and epoxy compound. The shelf life of the OCL materials could be extended,
because of suppression of the crosslinking reaction between the acrylic acid and
epoxy groups during storage at room temperature. However, the microphase separ-
ation between the acrylate copolymer and the epoxy compound was caused during
the post-baking process, leading to the formation of rough surface, and conse-
quently decrease in the transparency. The compounds having longer alkyl chains
and acid group such as palmitic acid and 2-ethylhexyl maleate were found to
prevent the microphase separation.
Keywords: over coating layer; positive type photoresist
INTRODUCTION
Recently, the production and sales of liquid crystalline displays
(LCDs) are increasing remarkably in Flat Panel Display market. A
TFT array panel consists of many TFT devices in which the height
of each TFT device is in a range of 2-3 mm. The disorder of liquid crys-
tal molecules on the TFT devices sometimes causes a poor quality of
the TFT panel. Over coating layer (OCL) materials have been widely
Address correpondence to Seiji Kurihara, Department of Applied Chemistry and
Biochemistry, Kumamoto University, 2-39-1 Kurokami, Kumamoto-shi, Kumamoto
860-8555, Japan. E-mail: kurihara@gpo.kumamoto-u.ac.jp
89

90
T. Ogata et al.
FIGURE 1 TFT LCD panel bearing OCL.
studied for the TFT panel, because the order of liquid crystal mole-
cules can be improved by coating OCL materials on the TFT devices
(Fig. 1) [1,2].
A LCD display panel with the OCL film provides clear and highly
minute pictures. However, present OCL materials have short shelf life
due to a crosslinking reaction between epoxy units and methacrylic
acid units, because the most of the OCL materials developed have both
epoxy and methacrylic acid units in a same polymer main chain as side
groups. Therefore, we tried to extend the shelf life of the OCL
materials by separating the acrylic acid and epoxy units in the OCL
FIGURE 2 Images of rough surface of OCL film. (a) Optical microscope image
(Patterned line width is 30 mm); (b) SEM image.

Over Coating Layer Materials
91
materials. However, the decrease in the transparency of the TFT
panels was observed by using the modified OCL materials (Fig. 2).
The transparency was improved by using crosslinking accelerator,
but the shelf life was extremely shortened. In this study, we investi-
gated the mechanism of the decrease in the transparency, and tried
to improve the transparency without shortening of the shelf life of
the OCL materials.
EXPERIMENTAL
Preparation of OCL Resist Film
The OCL material used in this study were prepared by mixing acrylate
copolymer, epoxy compound and photoactive compound, separately.
The acrylate copolymer consisted of methyl methacrylate, 2-hydroxy-
propyl methacrylate and methacrylic acid, and its Mw was 20,000 as
measured by GPC, and the monomer ratio was 57, 30 and 13 wt%,
respectively. Tri-functional epoxy compound was used as the epoxy
compound for crosslinking. The photoactive compound (TS-150), diazo-
naphthoquinone derivative, was supplied by Toyo Gosei Industries.
The OCL films with a thickness of around 4.8 mm were prepared by
the spin coating using propylene glycol mono methyl ether acetate
(PEGMEA) as solvent on glass or silicon substrates. Soft-baking was
carried out by heating at 90ꢀC for 160 s on a hot plate.
Fabrication of OCL
High pressure mercury lamp equipped both heat-cutting and VIS-
cutting filters was used for exposure. After the exposure, the OCL
films were rinsed in 0.4 wt% tetramethyl ammonium hydroxide
(TMAH) aqueous solution for 80 sec. The bleaching process was car-
ried out using high pressure mercury lamp with a heat-cutting filter.
The post-baking was performed by heating in an oven at 220ꢀC for
60 min. The final film thickness of OCL films was around 3 mm. The
transmittance of the OCL films on glass was measured with a UV-
VIS spectrometer (Shimadzu; UV-1600PC). The surface of the OCL
films was investigated by means of optical microscope and scanning
electron microscope (SEM).
Prevention Ability for Roughing
Many kinds of compounds were examined in order to prevent the sur-
face roughing; fatty acids and their salts, alkyl amines, alkyl amides,

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T. Ogata et al.
FIGURE 3 Compatibilizers having both of carboxyl group and alkyl chain.
sulfonic acid, aromatic acids, surfactants and esters. Each compound
was added in the OCL material, and the transparency of the OCL
materials was examined by the same manner as described above.
Synthesis
Compounds having both long alkyl chain and carboxyl group were
synthesized via Williamson’s reaction as shown in Figure 3. All
reagents used in the synthesis were purchased and used without
further purification. The compounds synthesized were added to the
OCL material, and examined for the enhancement of the transparency.
RESULTS AND DISCUSSION
The transparency is one of the important parameters, and transmit-
tance higher than 90% at 400 nm is required for the OCL film with
3 mm. But the transmittance of the OCL films made by the above for-
mulation without any crosslinking accelerator was less than 20%.
Microscopic observations revealed that the low transmittance was
arose from the formation of rough surface of the OCL films (Fig. 2).
Actually, the corrugation with a height of 20 nm was observed by
atomic force microscope [3]. In addition, the transmittance of the

Over Coating Layer Materials
93
OCL films was improved by coating the acrylate copolymer solution on
the rough surface area of the OCL films, increasing the light transmit-
tance of OCL film from 20% to 90%. These results indicate that the
rough surface was caused only at top surface of the OCL films, and
the rough surface was due to a micro phase separation of main acry-
late copolymer and epoxy compound during the processes.
To prevent the microphase separation, many kinds of compounds
were added to the OCL materials. As given in Table 1, only fatty acid
was found to exhibit significant preventing ability for surface roughing
of the OCL films. To evaluate in detail the effects of the fatty acids,
some fatty acids and ester derivatives having different alkyl chains
were examined. All ester derivatives used did not show any prevention
effect as given in Table 2. Contrary to the ester derivatives, the fatty
acids prevented the decrease in the transparency. In particular, the
prevention effect of the fatty acids with longer alkyl chains was higher
than those with shorter alkyl chains. Both palmitic acid and stearic
acid provided clear OCL films after post-baking process by adding
even at 3.9 Â 10À2 M in the OCL film, whereas the OCL film added
with lauric acid or myristic acid was cloudy. From the point of solu-
bility in PGMEA and thermal stability, the lower concentration of
the fatty acid may be favored. The shelf life of novel OCL material con-
taining palmitic acid was longer than one month at room temperature.
Based on the results, it is assumed that combination of long alkyl
chain with carboxyl group is important for preventing ability of the
microphase separation in the OLC materials. That is, the fatty acids
TABLE 1 Prevention Abilities for Roughing of Various Types of Compounds
Condition of OCL film
Type of added
compound
Compound
Stearic acd
0.039 M
0.12 M
Fatty acid
clear
clear
Fatty acid salt
Sodium acetate
Sodium ocylate
Hexyl acetate
Octyl amine
Octadecane admie
Benzosulfonic acid
Benzoic acid
Phthalic acid
Terephthalic acid
CTAB
cloudy
cloudy
cloudy
insoluble
insoluble
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
insoluble
insoluble
cloudy
cloudy
cloudy
cloudy
clear
Ester
Alkyl amine
Alkyl amide
Sulfonic acid
Aromatic acid
Surfantant
SDS
cloudy

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T. Ogata et al.
TABLE 2 Prevention Abilities for Roughing of Esters and Fatty Acids
Conc. in
OCL film
(Â10À2 M)
Surface
condition of
OCL film
Solubility in
resist at 5ꢀC
Compound
Octyl acetate
Ethyl octylate
Amyl octylate
Methyl stearate
Ethyl stearate
Methyl laurate
Ethyl laurate
Lauric acid
12
12
12
12
12
12
12
3.9
12
3.9
12
3.9
12
3.9
12
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
cloudy
clear
cloudy
clear
clear
clear
clear
cloudy
cloudy
cloudy
good
good
good
good
good
good
good
Myristic acid
Palmitic acid
Stearic acid
good
good
good
good
clear
separated
having longer alky chains act as a compatibilizer for the system of the
acrylate copolymer and the epoxy compound. In order to develop the
compatibilizer with higher prevention ability as well as longer shelf
life, compounds having both carboxylic acid and longer alkyl chain
TABLE 3 Prevention Abilities for Roughing of Synthesized Compounds
Conc. in
OCL film
(Â10À2 M)
Surface
condition of
OCL film
Solubility in
resist at 5ꢀC
Compound
Benzoic acid
Palmitic acid
0.39
3.9
0.39
3.9
12
0.39
3.9
0.39
3.9
0.39
3.9
0.39
3.9
12
cloudy
cloudy
cloudy
clear
clear
cloudy
clear
cloudy
clear
cloudy
clear
clear
clear
good
separated
good
good
good
good
good
good
good
good
good
good
good
good
PhAc-10
PhAc-16
BPhAc-10
2-Ethylhexyl maleate
clear

Over Coating Layer Materials
95
in a molecule were synthesized from p-hydroxy phenylacetic acid (Fig.
3(a)). The prevention abilities of the compounds were given in Table 3.
PhAc-10 and BPhAc-10c showed better prevention ability than benzoic
acid. However, the prevention abilities of the compounds were not so
high compared to fatty acid such as palmitic acid. We assumed that
the insufficient abilities were due to the presence of benzene ring.
Thus, 2-ethylhexyl maleate was examined, because it has carboxyl
group, branched bulky alkyl chain, and double bond. As given in
Table 3, 2-ethylhexyl maleate showed good preventing effect on the
decrease in the transparency. Bulkiness is one of factors influencing
compatibilization of the acrylate copolymer and the epoxy compound.
In addition, the shelf life of the OCL materials containing 2-ethylhexyl
maleate was longer than one month.
CONCLUSIONS
The OCL materials showing long self life were prepared by mixing sep-
arately acrylic acid group and epoxy group. However, the microphase
separation between the acrylic copolymer and the epoxy compound
was brought about by the post-baking process, causing the formation
of rough surface of the OCL materials, and decrease in the trans-
parency. The compatibilizers having acid group and longer chains
such as palmitic acid and 2-ethylhexyl maleate, were found to prevent
the formation of the rough surface of the OCL material. The hydropho-
bicity, bulkiness and acid group are important for the compatibiliza-
tion of the acrylic acid and the epoxy compounds.
REFERENCES
[1] Takahashi, S., Kawato, S., & Noguchi, T. (2004). Japanese Patent, Toku-kai, 2004-
287326.
[2] Takahashi, S. (2005). Japanese Patent, Toku-kai, 2005-17666.
[3] Takahashi, S. (2005). Electronic Display technology, 2005, 86–90.