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Master’s Thesis

Synthesis of Polyimide, Binder Polymer and Multifunctional Monomers for Patterning of Black

Pixel Define Layer of Organic Light Emitting Diodes

Sung-Hoon Park

Department of Materials Science and Engineering

Graduate School of UNIST

2019

Synthesis of Polyimide, Binder Polymer and Multifunctional Monomers for Patterning of Black Pixel Define Layer of Organic Light

Emitting Diodes

Sung-Hoon Park

Department of Materials Science and Engineering

Graduate School of UNIST

Synthesis of Polyimide, Binder Polymer and Multifunctional Monomers for Patterning of Black Pixel Define Layer of Organic Light

Emitting Diodes

A thesis

submitted to the Graduate School of UNIST

in partial fulfillment of the

requirements for the degree of

Master of Science

Sung-Hoon Park

01. 09. 2019

Approved by

______ _____

Advisor

Min-Sang Kwon

Synthesis of Polyimide, Binder Polymer and Multifunctional Monomers for Patterning of Black Pixel Define Layer of Organic Light

Emitting Diodes

Sung-Hoon Park

This certifies that the thesis/dissertation of Sung-Hoon Park is

approved.

01.09.2019

Abstract

Organic light emitting diode (OLEDs) is one of the main display for smart devices such as tablet

PCs, flexible devices and TVs. Currently a positive-tone photosensitive polyimide is used to pattern

the Pixel Define Layer (PDL) of OLEDs. However the use of positive-tone photosensitive polyimide

as photoresist to pattern PDL of OLED causes the reduced outdoor visibility of OLED devices Since

the fabricated PDL layer has yellowish brown[9][10] color thus reflecting ambient light. Therefore a ¼ λ

polarizing film and a black matrix pattern are used to reduce the reflection of the ambient light.

If a negative-tone black photoresist could be used in the patterning of PDL in OLED panel, the

outdoor visibility could not be reduced and also process of black matrix formation and ¼ λ polarizing

film could be removed. The negative-tone black photoresist for patterning of black PDL on OLED

panel consists of photoinitiator, photosensitizer , binder polymer, multifunctional monomers, black

millbase and polyimide as thermal stabilizer. The last component polyimide is not directly involved in

the photo-patterning of black PDL in OLED panel. However it has an important role of increasing the

thermal stability of the black PDL over 300ºC Since the whole OLED panel fabrication processes

exceed over 300ºC.

In this work, the synthesis of polyimides which are completely soluble in common organic solvent,

especially in propyleneglycol monomethylether acetate (PGMEA) was conducted first and applied in

the formulation of a negative-tone black photoresist. Secondly a new binderpolymer which has imide

linkage sidechain group was also studied for application to the negative-tone black photoresist.

Thirdly some new multifunctional monomers were synthesized and tested in the formulation of

negative-tone black photoresist for the patterning of black PDL of OLED panels.

I

Contents

Abstract________________________________________________________________________ I

List of Figures________________________________________________________________ III

List of Tables__________________________________________________________________V

1. Introduction

2. Experimental Methods

2-1 Polyimides as thermal stabilizer

2-1.1 Materials

2-1.2 Melt polymerization of polyimides with anhydrides end group

2-1.3 Melt polymerization of polyimides with monomines end group

2-2 Side chain polymides as binder polymer

2-2.1 Materials

2-2.2 Synthesis of styrene-type side-chain polyimides

2-2.3 Synthesis of acrylate-type side-chain polyimides

2-2.4 Synthesis of methacrylate-type side-chain polyimides

2-3 Synthesis of multifunctional monomers and analysis

2-3.1 Materials

2-3.2 Synthesis of acidic multifunctional monomer

3. Results and Discussions

3-1 Polyimides as thermal stabilizer

3-1.1 Synthesis of Polyimides by Melt Polymerization and Properties

3-1.2 Photolithography and thermal stability of pixel define layer patterns

3-2 Side chain polyimides as binder polymer

3-2.1 Synthesis of polyimide-based binder polymers

3-2.2 photolithography and thermal stability of black patterns with side chain polyimide

type binder polymer

3-3 Multifunctional monomers for black photoresists

3-3.1 Synthesis of multifunctional monomers and characterization

3-3.2 Photolithographic test of black pdl patterns obtained with multifunctional monomers

4. Conclusion

Acknowledgement

5. References

II

List of figures

Figure 1. Typical cross-sectional view of small size OLED panel____________________________ 3

Figure 2. The polyimides synthesized by melt polymerization with different monoanhydrides as

terminal groups.___________________________________________________________________ 5

Figure 3. Polyimides synthesized by melt polymerization with different monoamines as terminal

groups___________________________________________________________________________ 6

Figure 4. Synthetic scheme of styrene-type side-chain polyimides____________________________ 8

Figure 5. Synthetic scheme of acrylate-type side-chain polyimides____________________________9

Figure 6. Synthetic scheme of methacrylate-type side-chain polyimides_______________________10

Figure 7. Synthetic scheme of HM2___________________________________________________12

Figure 8. Synthetic scheme of HF2____________________________________________________12

Figure 9. Synthetic scheme of PT3____________________________________________________13

Figure 10. Synthetic scheme of PM6__________________________________________________ 13

Figure 11. Synthetic scheme of PF6___________________________________________________14

Figure 12. Structure of 6TAP-x______________________________________________________ 17

Figure 13. Structures, abbreviations and melting temperatures of monomers used in the melt

polymerization of polyimides________________________________________________________17

Figure 14. GPC chromatograms of T-10 and Y-10 polyimides_______________________________19

Figure 15. FT-IR spectra of BBP-3 and BBM-3 polyimides________________________________ 21

Figure 16. The optical microscopic images of black PDL patterns obtained with (a) Pt-0, (b) Pt-1, (c)

Pt-2, (d) Pt-5, (e) Pt-6 photoresists____________________________________________________ 24

Figure 17. TGA Thermograms of black thin layer of PDL patterns fabricated with photoresist (a) Pt-0

and (b) Pt-5______________________________________________________________________25

Figure 18. The optical microscopic images of black PDL patterns obtained with (a) PS-1 and (b) PS-2 photoresists (resolution: 10μm)______________________________________________________ 27

Figure 19. TGA thermograms of PDL patterns fabricated with photoresist (a) PS-0, (b) PS-2______ 27

Figure 20. GPC chromatogram of SEA-2 polyimide______________________________________31

Figure 21. FT-IR charts of SEA-2 polyimide and its starting material PSMA___________________31

III

Figure 22. The optical microscopic images of black PDL patterns obtained with (a) PR-0, (b) PR-1, (c)

PR-2 and (d) PR-3 black photoresist__________________________________________________ 33

Figure 23. TGA thermograms of black powder obtained from the black PDL patterns, (a) PR-0 and (b)

PR-3___________________________________________________________________________ 34

Figure 24. Mass spectrometry graphs of (a) PM6 and (b) PF6 at specific retention times of liquid

chromatography__________________________________________________________________ 35

Figure 25. FT-IR spectroscopic analysis of PM6 and PF6 multifunctional monomers____________ 36

Figure 26. The optical microscope images of black PDL patterns obtain with (a) PQ-0, (b) PQ-4, (c)

PQ-5, (d) PQ-6 photoresists_________________________________________________________ 38

IV

List of Tables

Table 1. Monomer combination, melt polymerization condition and solubility of resulting

polyimides_______________________________________________________________________16

Table 2. Molecular weights of melt polymerized MC6P series polyimides_____________________18

Table 3. GPC and TGA analyses of BBP and BBM series polyimides _______________________ 20

Table 4. Formulation of black photoresist and thermal stability of black PDL pattern____________ 23

Table 5. Formulation of black photoresist and thermal stability of black PDL pattern____________ 26

Table 6. Synthesis of side-chain polyimides for binder polymers of black photoresists___________ 30

Table 7. Formulation of black photoresists and thermal stability of black PDL pattern___________ 32

Table 8. Formulation of black photoresist with multifunctional monomers for PDL patterning_____37

V

- 1 -

1. Introduction

The organic light emitting (OLED) displays have become one of major displays due to their unique

advantages such as fast response time, high resolution, wide viewing angle, low power consumption

in addition to the light and slim panel.[1-4] The recent trends in display market such as foldable display,

automotive display and display for 5G telecommunication also prefer OLED displays to the thin film

transistor liquid crystal display (TFT-LCD) owing to the merits of OLED displays compared to the

TFT-LCD.[1][2] It is forecasted that foldable OLED displays alone will create 25 billion dollar market

by the year 2023. The automobile industry will adopt more OLED displays in the dashboard and

instrument panel as the electric vehicles is increasingly produced. These new demands for OLED

displays are also due the merits of OLED devices such as flexible display and better form factor of

OLED than liquid TFT-LCD displays. However, it is still necessary to improve the visibility of

OLEDs, especially when used outdoors.

The cross-sectional view of currently used OLED panel is shown in Figure 1.[5-8]

On top of TFT backplane, pixel define layer (PDL) patterns are formed currently by using positive-

tone photosensitive polyimide. The height of the PDL pattern is 3.0-3.2 µm and organic layers are

deposited by vacuum evaporation process utilizing the fine metal mask in the sunken part of the PDL.

After metal(Al) cathode is deposited, the whole area is passivated by using thin film encapsulation

(TFE) process.[3-5] The black matrix is patterned by using the black photoresist to cover except the

red(R), green(G) and blue(B) sub-pixels fabricated by the PDL patterning process. The cover window

is finally laminated by using pressure sensitive adhesive (PSA). If the PDL pattern could be formed

with a negative-tone black photoresist[11] by photolithographic process, the black matrix patterning

process can be eliminated. The ¼ λ polarizing film is also attached on top of the cover glass to reduce

the reflection of ambient light from the OLED panel in current OLED devices. These two processes

are required in the OLED panel fabrication in order to reduce the reflection of ambient light.This is

because the PDL layer formed by positive-tone photosensitive polyimide has yellowish brown color

[9][10] which could reflect ambient light.

So that the black PDL pattern fabricated by black photoresist could not only reduce the reflection of

ambient light, but also omit the black matrix patterning process and ¼ λ polarizing film. One reason

why a negative-tone black photoresist[11] is used instead of positive-tone photosensitive polyimide is

that incorporation of black pigment millbase in the positive-tone photosensitive polyimide could not

give a fine PDL pattern due to insufficient photosensitivity of the photosensitive polyimide solution.

Nevertheless, the negative-tone photoresist could give fine black PDL pattern up to 5x5µm size. This

is because the negative black photoresist is working through a photo-crosslinking mechanism of free

radical polymerization which has a chain polymerization character. Thus the basic composition of the

- 2 -

negative-tone black photoresist includes photoinitiator, photosensitizer, binderpolymer,

multifunctional monomer, solvent (PGMEA) and polyimide[5][6] as thermal stabilizer. The last

component is required in the black photoresist in order to increase the thermal stability of the resulting

PDL thin layer over 300℃ which is the high limit of process temperature to fabricate OLED panel.

This is why photosensitive polyimide is used in the positive-tone photoresist in the fabrication process

of OLED panels. The binder polymers are acrylate-type or cardo-type polymer with high acid value

over 120 which could be developed by tetramethyl ammonium hydroxide(TMAH) aqueous solution.

The binder polymer should have acrylate groups along the polymer chain which could take part in the

photo-crosslinking together with the multifunctional monomers, photoinitiator and photosensitizer

when irradiated by the UV light in the photolithographic process.[8][9]

In this work the synthesis of three components black photoresist such as polyimide[5][6] thermal

stabilizer, binderpolymer and multifunctional monomer were carried out from the view point of high

thermal stability of the resulting black PDL pattern and high definition of PDL pattern. After synthesis

of the three important ingredients, the formulation of black photoresist[7][8][9] and photolithographic

process to obtain fine PDL pattern were also examined.

- 3 -

Figure 1.

Typical cross-sectional view of small size OLED panel.

Out light

- 4 -

2. Experimental Methods

2-1 Polyimide as thermal stabilizer

Black negative photoresist consists of photosensitizer, photoinitiator, black millbase, binder

polymer for control of pattern property, multifunctional monomer for cross-linking and polyimide[5][6]

for increasing the thermal stability of patterned black pixel define layer on OLED devices. The

fabrication process temperature of black pixel define layer reaches about 300 oC. Positive photoresist

containing photosensitive polyimide has been used to achieve the high thermal stability but forms

non-black pixel define layer pattern. The polyimide in the black negative photoresist is necessary to

endure the process temperature as high as positive photoresist without degradation. All the

components of black negative photoresist should be soluble completely in the common organic

solvent such as propyleneglycol monomethylether acetate (PGMEA). Without meeting this condition,

polyimide cannot be used in the photoresist solution. Furthermore the result of black PDL pattern will

have residue, aggregates, deformation due to incomplete solubility of the ingredients in the photoresist

solution.[7-9]

2-1.1 Materials

Materials for the synthesis were commercial product and used without further purification. The

chemicals and abbreviations are as follows: N-Methyl-2-pyrrolidone (NMP); ethyl acetate (EtOAc);

propyleneglycol monomethylether acetate (PGMEA); dimethyl acetamide (DMAc); 5-(2,5-dioxo

tetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (MCDA); 2,2-Bis[4-(4-

aminophenoxy)phenyl]hexafluoropropane (BF6); 4-amino benzoic acid (4-AB); 2,2’-bis(3-amino-4-hydroxy-

phenyl)hexafluoropropane (AH6FP); 4,4’-(hexafluoroisopropylidene) dipthalic anhydride (6FDA); 3-

amino benzoic acid (3-AB); pthalic anhydride (PAn); trimellitic anhydride (TMA) and 4-

phenylethynylpthalic anhydride (PEPA).

- 5 -

2-1.2 Melt polymerization of polyimides with monoanhydrides end group

Powder form of monomers including diamine (AH6FP), monoanhydride (TMA or PEPA),

dianhydride (MCDA or 6FDA) was completely mixed and ground by using mortar. The powder

mixture in a stainless container was located in the middle of electric furnace and heated at 240 ºC for

10 minutes under N2 atmosphere. After 10 minutes, the product was taken out of electric furnace and

cooled down to room temperature. The polyimides are synthesized as shown in Figure 2.

Figure 2. The polyimides synthesized by melt polymerization with different monoanhydrides as

terminal groups.

- 6 -

2-1.3 Melt polymerization of polyimides with monoamines end group

Monomers including dianhydrides(MCDA), diamines(BF6) and monoamines(4-AB/3-AB) (molar

ratio 3:2:2) were well mixed using mortar and placed in a metal cup and then heated to 240°C in

electric oven with flowing of N2 gas for 10 to 60 minutes (Figure 3).

Figure 3. Polyimides synthesized by melt polymerization with different monoamines as

terminal groups

- 7 -

2-2 Side chain polyimides as binder polymer

The binder polymer is not only one of major component in the black photoresist in terms of weight

but also a significant component affecting the shape of PDL patterns during the development step in

the photolithographic process. New binder polymers with thermally stable imide linkages in the side

chain were synthesized and applied to the black photoresist formulation to pattern black PDL on

OLED panel.

2-2.1 Materials

Materials were commercial product and used without further purification. The abbreviations of

chemicals are as follows: poly(styrene-co-maleic anhydride), cumene terminated, Mn ~1600 (PSMA);

5-aminoisophthalic acid (AIPA); 3,5-bis(trifluoromethyl)aniline (6FAL); 4-aminostyrene (AS); 2-

hydroxylethylacrylate (2-HEA); glycidyl methacrylate (GMA); dimethyl acetamide (DMAc); 3,5-di-

tert-4-butylhydroxytoluene (BHT); tetrabutylphosphonium bromide (TBPB); propylene glycol

monomethyl ether acetate (PGMEA); pentaerythritol triacrylate (PETA).

- 8 -

2-2.2 Synthesis of styrene-type side-chain polyimides

First PSMA (20 mmol based on maleic anhydride units) was dissolved in 20 mL of DMAc, and

then AS, AIPA and 6FAL were added sequentially once an hour and stirred at room temperature as

shown in Fugure 4. After forming amic acid linkages between maleic anhydride units of PSMA and

three aromatic amines (AS, AIPA, 6FAL), the free radical polymerization inhibitor (BHT) and

chemical imidization agent (acetic anhydride, 10 mL) were added and the reaction mixture was

reacted for 4.5h at 100°C to give side-chain polyimides (SI-x).[12] After cooling down, the reaction

mixture was poured into excess water and the precipitates were collected by centrifugation. To the

precipitates EtOAc was added and the insoluble was removed by filtration. The organic solution was

washed with brine two times, dried with anhydrous Na2SO4 and evaporated to dryness. The solid

product was washed with diethyl ether multiple times to afford SI series side-chain polyimide as

brown powder.

Figure 4. Synthetic scheme of styrene-type side-chain polyimides.

- 9 -

2-2.3 Synthesis of acrylate-type side-chain polyimides

The starting material PSMA (20 mmol based on maleic anhydride units) was dissolved in 20 mL of

DMAc solvent. The mixture solution of acrylate monomer 2-HEA (10 mmol) and catalyst pyridine (1

mmol) were added to the PSMA solution and the reaction mixture was stirred at room temperature for

16h. Second, 6FAL (10 mmol) was added and stirred at room temperature for 1h. Third, acetic

anhydride (10 mL) was added and the mixture was stirred at 100 °C for 4.5h for the imidization

reaction (Figure 5). After reaction, the mixture was poured into excess amount water and the

subsequent work-up was the same as SI series in Section 2-2.2

Figure 5. Synthetic scheme of acrylate-type side-chain polyimides.

- 10 -

2-2.4 Synthesis of methacrylate-type side-chain polyimides

PSMA (20 mmol based on maleic anhydride unit) was dissolved in 20 mL of DMAc, and AIPA (6

mmol) and 6FAL (14 mmol) was added sequentially once an hour and stirred at room temperature.

Then the reaction mixture was brought up to 130 °C and stirred for 16h.[13] After cooling down, GMA

(4 mmol), TBPB (0.4 mmol) and BHT (0.04 mmol) were added and stirred at 90 C for 3h (Figure 6).

The reaction mixture was poured into water and the subsequent work-up was the same as SI series to

afford SEA-1.

For the SEA-2 synthesis, PSMA was dissolved in PGMEA (net. 30 wt%) and the reaction

proceeded as above without work-up.

Figure 6. Synthetic scheme of methacrylate-type side-chain polyimides.

- 11 -

2-3 Synthesis of multifunctional monomers synthesis and analysis

The negative-tone black photoresist is composed of photoinitiator, photosensitizer, binder polymer,

black pigment, dispersant and multifunctional monomer. Pentaerythritol tetraacrylate (PETA), which

is commercial multifunctional acrylate monomer, has been used for this purpose. However the

theraml stability of PETA is not high enough to withstand a temperature up to 300ºC[8][9] which is

occurring in the photolithographic patterning process of OLEDs.

In addition common acrylates are not soluble in aqueous base developer like tetramethylammonium

hydroxide (TMAH), resulting in difficulty in the development step. To solve this problem, we

designed and synthesized new acidic multifunctional monomers by esterification reaction and tested

them in the photolithographic process for black PDL patterns in the OLED panel to achieve better

development and higher thermal stability.

2-3.1 Materials

Materials were commercial product and used without further purification. Abbreviations of

compounds are as following: 4,4’-(hexafluoroisopropylidene) dipthalic anhydride (6FDA); 5-(2,5-

dioxo tetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (MCDA); trimellitic

anhydride (TMA); dimethyl acetamide (DMAc); N-Methyl-2-pyrrolidone (NMP); propyleneglycol

monomethylether acetate (PGMEA); tetrabutylphosphonium bromide (TBPB); dibutylhydroxytoluene

(BHT).

- 12 -

2-3.2 Synthesis of acidic multi functional monomers

Synthesis of HM2. 2-Hydroxylethylacrylate (1.277 g, 11 mmol), MCDA (1.391 g, 5 mmol),

PGMEA (3.498 g), TBPB (0.017 g, 0.05 mmol) and BHT (0.0024 g, 0.011 mmol) were mixed and

stirred at 60°C for 24h (Figure 7).

Figure 7. Synthetic scheme of HM2

Synthesis of HF2. 2-Hydroxylethylacrylate (1.277 g, 11 mmol), 6FDA (2.221 g, 5 mmol), PGMEA

(2.668 g), TBPB (0.017 g, 0.05 mmol) and BHT (0.0024 g, 0.011 mmol) were mixed and stirred at

60°C for 24h (Figure 8).

Figure 8. Synthetic scheme of HF2

- 13 -

Synthesis of PT3. PETA (3.281 g, 11 mmol), TMA (1.980 g, 5 mmol), DMAc (5.261 g), TBPB

(0.017 g, 0.05 mmol) and BHT (0.0024 g, 0.011 mmol) were mixed and stirred at 90°C for 24h

(Figure 9).

Figure 9. Synthetic scheme of PT3

Synthesis of PM6. PETA (3.281 g, 11 mmol), MCDA (1.391 g, 5 mmol), PGMEA (4.672 g), TBPB

(0.017 g, 0.05 mmol) and BHT (0.0024 g, 0.011 mmol) were mixed and stirred at 60°C for 24h.

(Figure 10)

Figure 10. Synthetic scheme of PM6

- 14 -

Synthesis of PF6. PETA (3.281 g, 11 mmol), 6FDA (2.221 g, 5 mmol), PGMEA (5.502 g), TBPB

(0.017 g, 0.05 mmol) and BHT (0.0024 g, 0.011 mmol) were mixed and stirred at 60°C for 24h

(Figure 11).

Figure 11. Synthetic scheme of PF6

After cooling down, all the resulting clear solutions shown in Figures 7 to 11 were directly

subjected to characterization and preparation of photoresist without purification. Synthesized

compounds were characterized by liquid chromatography mass spectrometry (LC-MS) and FT-IR

spectroscopy.

In LC-MS analysis, the sample solution was diluted by the eluent and injected into the column for

the subsequent procedure. In FT-IR analysis, the sample solution was first added into hexane and the

precipitating oil was mixed with dry KBr powder for the subsequent pellet preparation.

Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 thermogravimetric

analyzer with a programmed temperature at 10ºC/min. Fourier-transform infrared spectroscopy (FT-

IR) was performed on a Varian 670/620 spectrometer using KBr pellet method. The H1-NMR

spectroscopy was taken on a Bruker AVANCE III HD 400 MHz spectrometer.

- 15 -

3. Results and Discussions

3-1 Polyimides as thermal stabilizer

3-1.1 Synthesis of polyimides by melt polymerization and properties

The syntheses of polyimides by solution polymerization were carried out by two steps, first step

was to obtain polyamic acid and second one was imidization either by chemical or thermal methods. It

usually takes a long time to get final polyimides in powder form by solution polymerization process.

Therefore polyimides were synthesized by a melt polymerization method and two typical examples

were shown in Figure 2 and Figure 3.

In Table 1 are shown the reaction condition of polyimides by melt polymerization and the

relationship between the resulting polyimides and solubility in a common organic solvent such as

DMAC, NMP and PGMEA. The chemical structures and melting points of monoanhydride, diamine

and dianhydride monomers are shown in Figure 13. It was noted that combination of dianhydride and

diamine monomer is very inportant to meet the complete solubility in PGMEA common solvent used

in the preparation of black photoresist solution.

As shown in Table 1(a), the combinations of dianhydrides and diamines in the melt

polymerization of polyimides which give solubility in PGMEA solvent were found to be

(Dianhydrides; 6FDA, MCDA) and (Diamines; AH6FP, BF6, TFDB), while the combinations of

insolubility in PGMEA solvent(although soluble in such good solvents as DMAC and NMP) were

(Dianhydrides; BTDA, ODPA) and (Diamines; 4,4’-ODA, BAD, BAS). The main reason for the

insolubility of polyimides in PGMEA which is known to be a poor solvent is the formation of change

transfer complex formed by inter-molecular or intra-molecular interaction of dipole moments between

imide linkages. The presence of trifluoromethyl groups in 6FDA, AH6FP, BF6, TFDB monomers or

alicyclic structure in MCDA monomer can prevent the charge transfer complex formation by the

steric hinderance mechanism thus promote solubility in PGMEA solvent.

The effect of melt polymerization temperature and reaction time on the solubility of resulting

polyimide are shown in Table 1 (b). When the reaction temperature exceed 240 ºC or reaction time

over 10 min, the resulting polyimides exhibited insolubility or partial solubility (turbidity) in PGMEA

solvent. This could be resulted from the crosslinking reaction between phenolic -OH groups with

removal of water molecules in the 6TAP series polyimides shown in Figure 12.

- 16 -

Table 1. Monomer combination, melt polymerization condition and solubility of resulting

polyimides.

(a) Effect of combination of monomers on solubility

Polyimide

Sample

Monomers Melt polymerization Solubility

Dianhydride Diamine Monoanhydri

de

Temp

(ºC)

Temp

(min)

PGME

A

NMP DMA

C

B6OP-1 BTDA AH6FP PEPA 240 10 I S S

O6OP-1 ODPA AH6FP PEPA 240 10 I S S

D6MP-1 6FDA 4,4’-

ODA

PEPA 240 10 I S S

D6MP-2 6FDA BAS PEPA 240 10 I S S

D6MP-3 6FDA BAD PEPA 240 10 I S S

D6MP-4 6FDA BF6 PEPA 240 10 S S S

D6MP-5 6FDA TFDB PEPA 240 10 S S S

MC6P-1 MCDA AH6FP PEPA 240 10 S S S

6TAP-1 6FDA AH6FP PEPA 240 10 S S S

(b) Effect of melt polymerization condition on solubility

Polyimide

Sample

Monomers Melt polymerization Solubility

Dianhydri

de

Diamine Monoanhydri

de

Temp

(ºC)

Temp

(min)

PGME

A

NMP DMA

C

6TAP-2 6FDA AH6FP PEPA 280 10 I I I

6TAP-3 6FDA AH6FP PEPA 260 10 I I I

6TAP-4 6FDA AH6FP PEPA 250 10 I S S

6TAP-5 6FDA AH6FP PEPA 240 20 I S S

6TAP-6 6FDA AH6FP PEPA 240 30 I I I

- 17 -

Figure 12. Structure of 6TAP-x

Figure 13. Structures, abbreviations and melting temperatures of monomers used in the

melt polymerization of polyimides

CF3

F3C

OH

N

OH

F3C

CF3N

N

O

OO

O

CF3

F3C

OH

OH

NO

O

O

O

CF3

F3C

OH

OH

F3C

CF3N

N

O

OO

O

n

- 18 -

Of the melt polymerized polyimides in Table 1, D6MP-4, D6MP-5, 6TAP-1 and MC6P-1 samples

were completely soluble in PGMEA solvent. Of these we chose the MC6P series polyimides in order

to further investigate the effect of monoanhydrides on the molecular weights of resulting polyimides.

Table 2. Molecular weights of melt polymerized MC6P series polyimides

Polyimide

Samples

Monomer ratio (mmol) and melting temperatures of monomers

(Tm)

Mole weight

Mw; (GPC)

MCDA

172 ºC

AH6FP

243 ºC

PEPA

152 ºC

TMA

163-166 ºC

T-10 10 15 - 10 2,504

T-6 12 15 - 6 2,959

T-3 13.5 15 - 3 3,388

T-2 14 15 - 2 3,587

Y-10 10 15 10 - 1,976

Y-6 12 15 6 - 2,130

Y-3 13.5 15 3 - 2,450

Y-2 14 15 2 - 2,670

From Table 2 it is noted that the molecular weights (Mw) of T-series polyimides are higher than

those of Y-series polyimides. This may be explained by the comparison of melting temperature(Tm)

of monoanhydrides used. Since TMA has about 12 ºC higher Tm than PEPA, T series polyimides can

be more homogeneous during the melt polymerization. As the MCDA and TMA do not react with

each other until the temperature reach to 240 ºC melt polymerization temperature, they may give

narrow distribution of Mw in T-series polyimides compared to the Mw of Y series polyimides. The

typical GPC chromatograms of T-10 and Y-10 polyimides are shown in Figure 14.

- 19 -

Figure 14. GPC chromatograms of T-10 and Y-10 polyimides.

- 20 -

To identify the effect of monoamine and reaction time, we polymerized MCDA, BF6 and

aminobenzoic acids by melt polymerization method at 240 °C by varying reaction times from 10 to 60

min (Table 3). The structure of the resulting polyimides were shown in Figure 3. As a general trend,

when the reaction time was 30 min, both molecular weight (weight average, Mw) and thermal stability

(measured with thermal-gravimetric analysis, TGA) reached a maximum value as shown in Table 3.

If the reaction time was too short, the polymerization might not be complete, while if too long, the

resulting polyimides might undergo thermal degradation. Moreover, the experimental GPC Mw of

polyimides were much higher than the theoretical value (Mw=1,996), presumably due to stiff polymer

chain induced hydrodynamic volume increase. In case of our previously synthesized polyimide

thermal stabilizers 6TAP-1~6 series in Figure 12 and in Table 1 (b), when the melt polymerization

temperature exceeded 240°C or the reaction time exceeded 20 min, the resulting polyimide became

insoluble in PGMEA. However, BBP-x and BBM-x polyimides shown in figure 3 had lower

concentration of –COOH group and the absence of phenolic –OH group, so negligible dehydrative

cross-linking occurred during the melt polymerization reaction,[17] therefore the solubility in PGMEA

was still granted even if the reaction time was 60 min.

Table 3. GPC and TGA analyses of BBP and BBM series polyimides.

Compound

Code

Reaction

Time/min

GPC

Mw

1% WLT/°C

(TGA)a

5% WLT/°C

(TGA)

PGMEA

solubility

BBP-1 10 4,993 275 400

All soluble

BBP-2 20 4,562 326 398

BBP-3 30 4,745 338 411

BBP-4 40 3,486 235 391

BBP-6 60 4,238 252 391

BBM-1 10 4,121 287 383

All soluble

BBM-2 20 5,348 318 385

BBM-3 30 6,428 346 400

BBM-4 40 4,983 266 385

BBM-6 60 3,694 285 364aWLT: weight loss temperature

- 21 -

The FT-IR spectra of BBP-3 and BBM-3 in Figure 15 were quite similar in the typical imide

carbonyl group stretching (1,712 cm-1), aromatic -C=C- (1,502 cm-1) and aryl ether C-O stretching

(1,246 cm-1) absorption peaks while other specific functional group peaks were somewhat different.

Figure 15. FT-IR spectra of BBP-3 and BBM-3 polyimides.

- 22 -

3-1.2 Photolithography and thermal stability of pixel define layer patterns

The black photoresist is composed of photoinitiator, photosensitizer, binder polymer,

multifunctional monomer, black pigment dispersion in PGMEA solvent and polyimide as thermal

stabilizer. The binder polymer SR-6300T (SMS Co., Korea) was a commercial sample known as

cresol novolac epoxyacrylate dissolved in PGMEA at 30 wt%. An acrylate type monomer,

dipentaerythritol hexaacrylate (DPHA), was used as multifunctional monomer. The binder polymer

and multifunctional monomer are the main component of the photoresist which function as the photo-

patterning agents. However the thermal stability of the acrylate type compounds are not high enough

to endure the photolithographic process (about 300 ºC) and post-cure treatment. That is main reason

why polyimide is required as thermal stabilizer in the black photoresist formulation.

Another important major component of black photoresist is black pigment. In this experiment a

commercial lactam black (LT-1, 23 wt% in PGMEA obtained from SKC htm Co., Korea) was used.

The black pigment LT-1 is in a finely dispersed state (average particle size; 50 nm) in PGMEA

solvent. The compatibility of all other components of the black photoresist with the black pigment is

very important, otherwise micro-coagulums may be formed which will give saw-toothed or wavy type

patterns in the black pixel define layer.

The photolithographic process was as following. The black photoresist was spin coated on the wafer

(Si/SiO2 200 nm) with a stepwise speed of 300 rpm/5 sec, 500 rpm/5 sec, and 5,000 rpm/50 sec. After

soft bake at 120 ºC for 100 sec, the dry black photoresist thin film was exposed to UV irradiation (120

mJ/cm2) through the photomask. After hard bake at 120 ºC for 100 sec, the black PDL thin film was

developed with AZ-300 MIF developer (tetramethylammonium hydroxide 2 wt% in water, Bayer Co.,

Germany) for 120 sec followed by water rinsing and drying with N2 gun.

Of the polyimide thermal stabilizers in Table 1 and Table 2, six samples soluble in PGMEA solvent

were used in the black photoresist formulation as shown in Table 4.

- 23 -

Table 4. Formulation of black photoresist and thermal stability of black PDL pattern

Compnents/Photoresist samples Pt-0 Pt-1 Pt-2 Pt-3 Pt-4 Pt-5 Pt-6

PhotoinitiatorIrgacure 754 1 1 1 1 1 1 1

Irgacure TPO 6 6 6 6 6 6 6

Photosensitizer Darocur ITX 2 2 2 2 2 2 2

Binder Polymer

(30 wt% in

PGMEA)

SR-6300 (SMS Co.,

Korea)35 35 35 35 35 35 35

Multifunctional

MonomersDPHA 8 8 8 8 8 8 8

Polyimides (30

wt% in PGMEA)

6TAP-1 10

MC6P-1 10

D6MP-4 10

D6MP-5 10

T-10 10

Y-10 10

Black Millbase

(23 wt% in

PGMEA)

LT-1 (SKC htm. Co.,

Korea)35 35 35 35 35 35 35

Solvent PGMEA 5 5 5 5 5 5 5

Total wt% 100 100 100 100 100 100 100

Thermal Stability 1 wt% loss temp. (TGA) 282 - - - - 316 305

- 24 -

Black photoresist samples (Pt-1, Pt-3 and Pt-4) failed to give good patterns in the black pixel

define layer while other black photoresists exhibited fine PDL patterns. The rest of the black

photoresist (Pt- 2, 5, 6) including the reference black photoresist (Pt-0) produced good PDL patterns.

Typical PDL patterns are shown in Figure 16. Although all six polyimiedes used in the black

photoresist were completely soluble in PGMEA solvent, there were difference in the diamine

monomers in the D6MP-4 (BF6) and D6MP-5 (TFDB) polyimides as shown in Figure 13 and Figure

3. The polyimides which gave good PDL pattern had AH6FP as diamine monomer. The phenol -OH

group in AH6FP monomer could help the binder polymer which also have large amount of phenolic -

OH groups (acid value; 120) during the development stage by reacting with basic TMAH aqueous

solution, while the BF6 and TFDB diamines could not be developed without acidic group.

Figure 16. The optical microscopic images of black PDL patterns obtained with (a) Pt-0, (b)

Pt-1, (c) Pt-2, (d) Pt-5, (e) Pt-6 photoresists.

- 25 -

The effect of polyimides in the black photoresist on the thermal stability of the resulting PDL

patterns was investigated by TGA analysis. The black PDL patterns obtained on the silicon wafer was

scratched off by using sharp razor blade and then the 1 wt% loss temperatures were checked. As

shown in Figure 17 the black photoresist (Pt-5) exhibited higher 1 wt% loss temperatures than Pt-0

without polyimide as thermal stabilizer.

Figure 17. TGA Thermograms of black thin film layer of PDL patterns fabricated with

photoresist (a)Pt-0 and (b) Pt-5

- 26 -

The photolithographic evaluation and thermal stability of polyimides (BBP and BBM series in

Table 3) were also conducted by using the black photoresist formulation shown in Table 5. In this

experiments the fluorene-based cardo-type binder polymer which has both carboxylic group and

acrylate group in the main-chain was used instead of SR-6300 shown in Table 4. We prepared the

control sample (PS-0), sample with BBP-3 (PS-1) and sample with BBM-3 (PS-2) and subjected them

to 6,000 rpm spin-coating on silicon wafer (200 nm SiO2 on Si), soft-bake (110°C, 100 sec), UV

exposure (80 mJ), hard-bake (100°C, 120 sec) and development (2.3wt% tetrabutylammoniuum

hydroxide (TMAH) in water) successively.

Table 5. Formulation of black photoresist and thermal stability of black PDL pattern.

Components PS-0 PS-1 PS-2

Photo-initiator TPO 6 6 6

Photo-sensitizer Darocure ITX 2 2 2

Thermal stabilizer(30 wt% in DMAc)

BBP-3 10

BBM-3 10

Binder Polymer(45 wt% in PGMEA)

Cardo binder 21 21 21

Multifunctional MonomersPETAa 8 8 8

CEAOa (Aldrich) 2 2 2

Black Pigment(21 wt% in PGMEA)

LT-1 (SKC htm. Co., Korea) 28 28 28

Carbon Black 7 7 7

Solvent PGMEA 6 3 3

Total wt% 100 100 100

Thermal Stability1% WLT/°C (TGA) 315 301 323

5% WLT/°C (TGA) 357 343 361

aPETA: Pentaerythritol triacrylate.bCEAO: carboxylethylacrylate oligomer (n = 1~3).

- 27 -

Black photoresist PS-1 and PS-2 successfully gave fine 10x10 μm patterns as shown in Figure 18

without any residues, although the acid values of BBP-3 and BBM-3 were rather low. From the TGA

test results in Figure 19, BBM-3 polyimide positively contributed to the thermal stability of the black

pattern, which seemed to be due to the high molecular weight than that of BBP-3 as shown in Table 3.

To design a thermal stabilizer for negative black photoresist, it is important to balance the acid value

and thermal stability, since the acidic –OH and –COOH groups usually reduce the thermal stability of

black PDL layer. We found that the optimization of acidic group distribution (binder polymer, CEAO)

in black photoresist was important to give a fine pattern of black pixel define layer.

Figure 18. The optical microscopic images of black PDL patterns obtained with (a) PS-1 and (b)

PS-2 photoresists (resolution: 10 μm)

Figure 19. TGA thermograms of PDL patterns fabricated with photoresist (a) PS-0, (b) PS-2.

315℃ 99.00% 323℃ 99.00%

- 28 -

3-2 Side chain polyimides as binder polymer

The use of negative-tone black photoresist, instead of positive-tone photoresist based on

photosensitive polyimide, has merit of removing both the black matrix (BM) patterning process in the

fabrication of OLED panel and possibly, the 1/4λ polarizing film as well, by reducing the reflection of

ambient light from the OLED panel. The reason for using positive-tone photoresist based on

photosensitive polyimide is the high thermal stability of the resulting PDL made of polyimide thin

film which could withstand up to 300°C during the OLED panel fabrication process. However the

positive-tone photoresist based on photosensitive polyimide cannot be used to make black colored

photoresist due to limited penetration depth of the UV light in the photolithographic patterning of

black pixel define layer on OLED panel. The negative-tone black photoresist has deeper penetration

depth than positive-tone photoresist since the photolithographic patterning is carried out by

crosslinking reaction mechanism involving free radical chain polymerization among multifunctional

monomers and the binder polymer.

The binder polymer in the negative-tone black photoresist for the patterning of PDL on OLED panel

has to meet the following specifications. One is a good patterning function which could be achieved

by adequate amount of double bonds and carboxyl groups that take part in the crosslinking reaction

upon UV irradiation and development with aqueous alkaline solution, respectively. The other is high

thermal stability of binder polymer which corresponds to the photosensitive polyimide in the positive-

tone photoresist. According to this guideline three different binder polymers with thermally stable

imide linkages in the side chain of polymer have been synthesized.

- 29 -

3-2.1 Synthesis of side chain type polyimide binder polymers

(1) SI and SEI series

In the synthesis of SI series binder polymers, three aromatic amines were reacted with the maleic

anhydride units of PSMA as shown in Figure 4. The three amines (AIPA, 6FAL and AS) were used

first to give carboxyl groups necessary in the development step of photolithography, second to

increase the solubility of binder polymer in the PGMEA, common solvent in the photoresist, and third

to provide double bonds responsible for the photocrosslinking reaction, respectively. In the synthesis

of SI series chemical imidization has to be employed utilizing acetic anhydride under mild condition

of 100°C in order to avoid self-polymerization of AS by thermal initiation mechanism. The SI product

was also recovered by precipitation in excess water to remove the excess acetic anhydride followed by

filtration, drying and redissolution in PGMEA to be tested in the black photoresist.

In the SEI series 2-HEA was first reacted with maleic anhydride units of PSMA as shown in Figure

5 to generate both carboxyl groups and double bonds. Secondly aromatic amine (6FAL) was imidized

by chemical imidization method under mild condition in order not to crosslink the acrylate groups

attached first to the PSMA polymer chain. Since acetic anhydride was used for imidization, SEI has to

be worked-up in the same way as SI series.

(2) SEA series

SEA series were synthesized by two different processes as shown in Figure 6. First APIA and

6FAL amines were reacted with maleic anhydride groups of PSMA in DMAc solvent utilizing

thermal imidization at 130°C. The side-chain polyimide intermediate was reacted with GMA to give

SEA-1 binder polymer in which parts of carboxyl groups were converted to epoxymethacrylates. The

SEA-1 sample was recovered same as SI series in Section 2-2.2. In the second process SEA-2 was

synthesized same as SEA-1 by using PGMEA solvent instead of DMAc. However, SEA-2 product

was obtained in one-pot solution method without work-up to be tested directly as binder polymer in

the photolithographic patterning of black PDL in OLED panel.

As shown in Table 6 the yield of polyimides were not high (44~56%) due to weight loss during

precipitation in excess water since the polyimide products have relatively high carboxyl groups. Of

the synthesized binder polymers the GPC and FT-IR analyses of SEA-2 sample are shown in Figure

20 and Figure 21 respectively. The molecular weight of polyimides were higher than that of starting

material PSMA (Mn = 1,600g/mol). This would be explained by expansion of hydrodynamic volume

after imidization step which introduced bulky substituents in the side chain of relatively flexible

PSMA polymer. According to FT-IR charts, it was noted that almost all anhydrides were converted to

imides and methacrylate esters.

- 30 -

Table 6. Synthesis of side-chain polyimides for binder polymers of black photoresists.

Binder Polymer

Samples

Sample

CodesMolarities/mmol

Yield

(%)GPC

Mw

SI series

AS AIPA 6FAL

SI-1 4 6 10 52 4,716

SI-2 4 8 8 44 4,863

SEI series2-HEA 6FAL

SEI-1 10 10 53 4,637

SEA series

AIPA 6FAL GMA

SEA-1 6 14 4 56 4,735

SEA-2 6 14 6 - 4,819

- 31 -

Figure 20. GPC chromatogram of SEA-2 polyimide.

Figure 21. FT-IR charts of SEA-2 polyimide and its starting material PSMA.

- 32 -

3-2.2 Photolithography and thermal stability of black PDL patterns with side chain polyimide

type binder polymer

The black photoresist for patterning of black PDL on the OLED panel is composed of photoinitiator,

photosensitizier, multifunctional monomer, polyimide as thermal stabilizer, black millbase and binder

polymer. The binder polymer and multifunctional monomer are the main components of the

photoresist which affect the exact patterning of PDL through the photocrosslinking reaction in the UV

irradiated area and development of UV unirradiated area by aqueous alkaline solution.

In this work a commercial binder polymer SR-6300 (SMS Co., Korea) was used as a reference

binder polymer which is known as cresol novolac epoxyacrylate dissolved in PGMEA solvent at 30

wt%. As multifunctional monomer PETA, a commercial sample, was used which has functionality of

3. The thermal stability of the reference black photoresist (PR-0) was not high enough to endure the

photolithographic patterning of PDL and the subsequent post-cure treatment (up to 300 °C) as shown

in Table 7.

Table 7. Formulation of black photoresists and thermal stability of black PDL pattern.

.

Components/photoresist samples PR-0 PR-1 PR-2 PR-3

PhotoinitiatorsIrgacure 754 1 1 1 1

Irgacure TPO 6 6 6 6

Photosensitizer Darocure ITX 2 2 2 2

Binder polymer (30 wt% in PGMEA)

SR-6300 35

SI-2 35

SEI-1 35

SEA-2 35

Multifunctional monomer PETA 8 8 8 8

Polyimide thermal stabilizer (30 wt% in

PGMEA)Y-10 10 10 10 10

Black Millbase (23 wt% in PGMEA)

LT-1 (SKC

htm Co.,

Korea)

35 35 35 35

Solvent PGMEA 5 5 5 5

Total wt% 100 100 100 100

Thermal Stability1 wt% loss

temp. (TGA)282 - - 303

- 33 -

The photolithographic patterning of black PDL utilizing the side-chain polyimide type binder

polymers are shown in Figure 22. The black photoresists PR-0 and PR-3 exhibited fine patterns of

black PDL. However PR-1 and PR-2 showed irregular or wavy PDL patterns. These may be explained

as following. The PR-1 photoresist had SI-1 as binder polymer. Since SI-1 had styrenic groups as

sources of double bonds for the photo-crosslinking reaction, the adhesion to the silicon wafer was not

as good as the reference binder polymer (SR-6300) which has epoxyacrylate as source of double

bonds. In case of PR-2 black photoresist, the wavy PDL pattern seemed to be due to high acid value

of the binder polymer SEI-1 in which acrylate groups were introduced by reaction of 2-HEA with

maleic anhydride units of PSMA. The 2-HEA monomer had to be reacted at 10 mmol level for the

complete solubility of the resulting SEI-1 in PGMEA. The PR-3 black photoresist showed find PDL

pattern owing to the balance of good adhesion to the silicon wafer substrate and optimum acid value

from the benzoic acid type carboxyl groups.

Figure 22. The optical microscopic images of black PDL patterns obtained with (a) PR-0, (b)

PR-1, (c) PR-2 and (d) PR-3 black photoresist.

- 34 -

The effect of side-chain polyimide type binder polymer on the thermal stability of the resulting

PDL patterns was evaluated by TGA analysis. The black PDL patterns formed on the silicon wafer

was scratched off after photolithographic patterning by sharp razor blade and the fine black powder

was subjected to the TGA analysis. As shown in Table 7 and Figure 23 the PR-3 photoresist sample

with thermally stable side-chain polyimide SEA-2 as binder polymer gave higher 1 wt% loss

temperature than the PR-0 reference photoresist with commercial SR-300 acrylate type binder

polymer.

Figure 23. TGA thermograms of black powder obtained from the black PDL patterns, (a) PR-0

and (b) PR-3.

(a)PR-0 (b)PR-3

- 35 -

3-3 Multifunctional monomers for black photoresists

3-3.1 Synthesis of multifunctional monomers and characterization

To minimize the complexity of synthesis, we utilized one-pot reaction to synthesize the acidic

multifunctional monomers without work-up and obtained a concentrated solution that can be directly

applied to prepare black photoresists as described in section 2-3.2.

The reaction product solutions for example PM6 (Figure 10) and PF6 (Figure11) were analyzed

with negative LC-MS (Figure 24). Using polar mixture solvents as eluent, the molecular peak of PM6

appeared at retention time of 4.9-5.0 min ([M-H]¯: 859.2), while PF6 6.2-6.6 min ([M-H]¯: 1039.2).

These results demonstrate that the target acidic multifunctional monomers were successfully formed.

Figure 24. Mass spectrometry graphs of (a) PM6 and (b) PF6 at specific retention times of

liquid chromatography.

- 36 -

Moreover, PM6 and PF6 showed quite similar FT-IR spectra, since they share almost same active

functional groups for IR absorption (Figure 25). Functional groups detected including

carboxylic/alcoholic –OH stretching, carbonyl stretching, -CH2/-CH3 bending and ester C-O

stretching absorptions at 3500~3000, 1730, 1410 and 1186 cm-1, respectively.

Figure 25. FT-IR spectroscopic analysis of PM6 and PF6 multifunctional monomers.

3-3.2 Photolithographic tests of black PDL patterns obtained with multifunctional monomers

Negative-tone black photoresist formulations for the test of multifunctional monomers are shown in

Table 8. In the photoresist solution, one of the major components is black millbase (LT-2, 23 wt% in

PGMEA), which is responsible for a high optical density. Since the solvent for LT-2 is PGMEA, all

other components should have good solubility in PGMEA and be compatible with each other.

In a typical photolithographic process, the black photoresist was spin-coated on the wafer (200nm

SiO2 on Si) with a speed 6,000 rpm for 50 sec. After soft bake at 110ºC for 100 sec, the dry black

photoresist thin film was exposed to UV irradiation (80 mJ/cm2) through the photomask. After hard

bake at 100ºC for 120 sec, the black PDL thin film was developed with AZ-300 MIF developer (2 wt%

of TMAH in water, Bayer Co., Germany) for 60 sec followed by water rinsing and drying with N2 gun.

- 37 -

Table 8. Formulation of black photoresist with multifunctional monomers for PDL patterning.

Compnents/PhotoresistPQ-

0

PQ-

1

PQ-

2

PQ-

3

PQ-

4

PQ-

5

PQ-

6

PhotoinitiatorSPI-03 (Samyang,

Korea)6 6 6 6 6 6 6

Photosensitizer Darocure ITX 2 2 2 2 2 2 2

Thermal stabilizer SAMG-3a 5 5 5 5 5 5 5

Binder Polymer

(30 wt% in PGMEA)Cardo binder 21 21 21 21 21 21 21

Multifunctional

Monomers

100% PETA 8 8 8 8 8 8 8

50 wt%

in

PGMEA

HM2 3

HF2 3

PT3 3

PM6 3

PF6 2

100% CEAOb (Aldrich) 2

Black Millbase

(23 wt% in PGMEA)

LT-2 (SKC htm.

Co., Korea)52 52 52 52 52 52 52

Solvent PGMEA 6 3 3 3 3 4 4

Total wt% 100 100 100 100 100 100 100

aSAFG-3 is an acrylated polyimide serving as a thermal stabilizer.

bCEAO: carboxylethylacrylate oligomer (n = 1~3).

- 38 -

Figure 26. The optical microscope images of black PDL patterns obtained with (a) PQ-0, (b)

PQ-4, (c) PQ-5, (d) PQ-6 photoresists

In the photolithographic tests, the synthesized acidic multifunctional monomers were utilized

together with PETA in order to satisfy both cross-linking capability and developability with aqueous

base developer solution. When HM2, HF2 and PT3 were used as multifunctional monomers, the

patterns were lost during development due to insufficient cross-linking since the acrylate

functionalities were only 2 or 3. When only PETA was used (PQ-0) or CEAO was added (PQ-6),

over-crosslinking was found (Figure 26a, 26d) due to insufficient acidity since the number of –

COOH group was 0 or 1. When the number of double bonds and –COOH groups were well balanced

(PQ-4, PQ-5), good patterns were found (Figure 26b, 26c). Specifically PM6 resulted in better

pattern compared with PF6. This may be due to the –CF3 groups in PF6 which decreased the adhesion

to the wafer substrate.

- 39 -

4. Conclusion

The use of negative-tone black photoresist, instead of positive-tone photoresist based on

photosensitive polyimide, has merit of removing both the black matrix (BM) patterning process in the

fabrication of OLED panel and possibly, the 1/4λ polarizing film as well, by reducing the reflection of

ambient light from the OLED panel. The reason for using positive-tone photoresist based on

photosensitive polyimide is the high thermal stability of the resulting PDL made of polyimide thin

film which could withstand up to 300°C during the OLED panel fabrication process.

The basic composition of the negative-tone black photoresist includes photoinitiator,

photosensitizer, binderpolymer, multifunctional monomer, solvent (PGMEA) and polyimide[5][6] as

thermal stabilizer. The last component is required in the black photoresist in order to increase the

thermal stability of the resulting PDL thin layer over 300℃ which is the high limit of process

temperature to fabricate OLED panel. This is why photosensitive polyimide is used in the positive-

tone photoresist in the fabrication process of OLED panels. The binder polymers are acrylate-type or

cardo-type polymer with high acid value over 120 which could be developed by tetramethyl

ammonium hydroxide(TMAH) aqueous solution. The binder polymer should have acrylate groups

along the polymer chain which could take part in the photo-crosslinking together with the

multifunctional monomers, photoinitiator and photosensitizer when irradiated by the UV light in the

photolithographic process.[8][9]

In this work the synthesis of three components black photoresist such as polyimide[5][6] thermal

stabilizer, binderpolymer and multifunctional monomer were carried out from the view point of high

thermal stability of the resulting black PDL pattern and high definition of PDL pattern. Some

important results are as following.

First for the synthesis of polyimide thermal stabilizer both melt and solution polymerizations were

conducted. From the melt polymerization of polyimides it was found that the presence of

trifluoromethyl groups in 6FDA, AH6FP, BF6, TFDB monomers or alicyclic structure in MCDA

monomer can prevent the charge transfer complex formation by the steric hinderance mechanism thus

promote solubility in PGMEA solvent. From the solution polymerization of polyimides it was noted

that the combination of MCDA dianhydride, BF6 diamine and aminobenzoic acid monoamine

monomers gave high Mw polyimides which have complete solubility in PGMEA solvent. The

photolithographic tests of the polyimides showed that the resulting PDL patterns had high definition

up to 5x5 µm size and high thermal stability up to 346℃ as measured by TGA.

- 40 -

Second three side-chain polyimide type binder polymers were synthesized. Of these SEA series

binder polymer was found to give a fine pattern of black PDL pattern with thermal stability over

300℃.

Third five multifunctional monomers were synthesized with various combinations of carboxylic

groups and acrylate groups. Of these PM6 multifunctional monomer with 6 acrylates and 2 carboxyl

groups exhibited optimum property of high definition black PDL pattern and high black millbase

content up to 52% by weight.

- 41 -

Acknowledgement

This study was supported by the Technology Innovation Program of Industrial Strategic Technology

Development Program (10063289, Development of High Temperature Negative tone Photosensitive

Black Resin and Fabrication Process for Pol-less AMOLED Devices) funded by the Ministry of Trade,

Industry & Energy (MOTIE, Korea)

- 42 -

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