Photosensitive Poly(benzoxazole) Basedon Poly(o-hydroxy amide), Dissolution Inhibitor,Thermoacid Generator, and Photoacid Generator

Tomohito OGURA, Ko-ta YAMAGUCHI, Yuji SHIBASAKI, and Mitsuru UEDAy

Department of Organic & Polymeric Materials, Graduate School of Science & Engineering,

Tokyo Institute of Technology, 2-12-1-H120 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

(Received November 28, 2006; Accepted December 23, 2006; Published February 9, 2007)

ABSTRACT: A positive-type photosensitive polybenzoxazole (PSPBO) based on a poly(o-hydroxy amide) (PHA),

a dissolution inhibitor (DI) 9,9-bis(4-tert-butoxycarbonyloxyphenyl)fluorene (t-BocBHF), a thermoacid generator

(TAG) isopropyl p-toluenesufonate (ITS), and a photoacid generator (PAG) (5-propylsulfonyloxyimino-5H-thio-

phene-2-ylidene)-(2-methylphenyl)acetonitrile (PTMA) has been developed. ITS was easily prepared by reaction of

p-toluenesulfonyl chloride with isopropyl alcohol. The dissolution behavior of the PSPBO system was studied in rela-

tion to PTMA loadings, and post exposure baking (PEB) temperature and time. The PSPBO consisting of PHA

(73wt%), t-BocBHF (18wt%), ITS (7.5wt%), and PTMA (1.5wt%) exhibited a sensitivity of 33mJ/cm2 and a con-

trast of 5.1 when exposed to 365 nm light (i-line) and developed with an aqueous alkaline developer, 2.38wt% tetra-

methylammonium hydroxide solution (TMAHaq)/2.5wt% iso-propanol (i-PrOH). A clear positive image with 3 mmfeatures was produced by contact-printing and converted into a poly(benzoxazole) (PBO) pattern upon heating at

250 �C for 20min. Thus, the ITS is effective as a TAG for improving the sensitivity and low temperature cyclization

of PHA into the PBO as well to meet practical requirement in the industry. [doi:10.1295/polymj.PJ2006174]KEY WORDS Functional Polymers / Polymer Synthesis / Gel / Tetrahydrofuran / Infrared

Spectroscopy /

Photosensitive polybenzoxazoles (PSPBOs)1–5 areused as buffer coatings to protect bare chips fromstresses induced by fillers or thermal mismatches be-tween a passivation layer and molding materials.These compounds possess high mechanical strength,thermal stability, and low dielectric constants. Theyhave been developed to simplify processes in whicha precursor of polybenzoxazoles (PBOs), poly(o-hy-droxy amide) (PHA), is used as a polymer matrix.The phenolic hydroxyl groups in PHA promote solu-bility of 2.38wt% tetramethylammonium hydroxidesolution (TMAHaq), used as the aqueous alkaline de-veloper. Previously, PHA obtained from 4,40-(hexa-fluoroisopropylidene)bis(o-aminophenol) (6FAP) and4,40-oxybis(benzoic acid) derivatives was used be-cause of its high transparency at a wavelength of365 nm (i-line). A conventional PSPBO was formulat-ed by adding the photo-sensitizer diazonaphthoqui-none (DNQ) to PHA.6

The sensitivity of photopolymers is important in thedesign of photoresist materials. The conventionalPSPBO based on DNQ, however, possesses low sensi-tivity (>100mJ/cm2) and a strong absorbance at365 nm, and is difficult to use for thick patterns. Toovercome these problems, a chemically amplifiedtechnique was introduced to develop PSPBOs cou-pled with a photoacid generator (PAG),7 for which

9,9-bis(4-tert-butoxycarbonyloxyphenyl)fluorene (t-BocBHF) was used as a dissolution inhibitor (DI).8

This three-component, positive-type, alkaline-devel-opable PSPBO showed the sensitivity and contrastof 34mJ/cm2 and 5.8, respectively with i-line expo-sure, to produce a 20 mm image in 10 mm thick film.A positive image of PHA is converted into that ofPBO by thermal treatment at 350 �C. This high-tem-perature process is not applicable to conventionalelectronic applications containing one or more ther-mally unstable organic components, such as glass-ep-oxy resin or glass-bis(maleimide)-triazine resins thatmay be present in a built-up or package board. Thus,a significant decrease in the cyclization temperature ofPHA allows versatile development of a PSPBO sys-tem, producing more useful applications in the micro-electronics industry.Recently, sulfonic acids were found to work as ef-

fective catalysts for low-temperature cyclization ofPHA.9 This finding was applied to develop a low-tem-perature curable PSPBO coupled with a PAG [5-pro-pylsulfonyloxyimino-5H-thiophene-2-ylidene-(2-meth-ylphenyl)acetonitrile] (PTMA), for which DNQ orpartially tert-butoxycarbonyl (t-Boc)-protected PHAwas used as the DI or polymer matrix.10,11 The formerPSPBO, however, cannot be applied for thick patterns,and t-Boc groups must be introduced to PHA through

yTo whom correspondence should be addressed (Tel/Fax: +81-3-5734-2127, E-mail: mueda@polymer.titech.ac.jp).

245

Polymer Journal, Vol. 39, No. 3, pp. 245–251 (2007)

#2007 The Society of Polymer Science, Japan

polymeric reactions in the latter PSPBO, which istroublesome. A more convenient method that is wide-ly applicable and provides easy resist formulation in-volves a four-component system consisting of PHA, aPAG, a DI, and a thermoacid generator (TAG) basedon acid-labile sulfonates.For practical use of these acids, development of

latent acid catalysts for low-temperature cyclizationof PHA is more desirable. Latent catalysts are com-pounds stable under ambient conditions, which func-tion only after specific external stimulation such asheating or photo-irradiation. Sulfonate derivativesare known to function as latent acid generators bothby thermal treatment and photo irradiation.12,13 Re-cently, Arimitsu et al. demonstrated a novel photopro-liferation for a chemically amplified photoresist inorder to improve the sensitivity.14 We anticipatedthat such a latent thermo-sensitive acid catalyst willenhance both storage stability and processability ofPHA solutions because of non-reactivity at storagetemperatures, while the cyclization of PHA is promot-ed within a specific range of elevated temperatures.This report describes the successful development of

an alkaline-developable, chemically amplified posi-tive-type PSPBO based on a PHA, DI (t-BocBHF),TAG (isopropyl p-toluenesufonate, ITS), and PAG(PTMA). A patterning process and subsequent low-temperature cyclization using PSPBO is shown inScheme 1.The formulation of PSPBO is simple, involving the

addition of t-BocBHF, ITS, and PTMA to a PHA so-lution. The PSPBO solution is spin-coated and bakedin the usual way. Then, the film is exposed to UV lightto produce propanesulfonic acid from PTMA. Uponpost-exposure bake (PEB) treatment of the PSPBOfilm, propanesulfonic acid catalyzes decompositionof ITS to yield propene and p-toluenesulfonic acid.

These sulfonic acids induce a cascade of deprotectionreactions of t-BocBHF to produce 9,9-bis(4-hydroxy-phenyl)fluorene (BHF). The exposed and baked filmthen can be developed with 2.38wt% TMAHaq toprovide a positive image. The subsequent low-temper-ature cyclization into the PBO pattern at 250 �C pro-ceeds with p-toluenesulfonic acid produced from ther-mal decomposition of ITS. As a consequence, thedirect formulation of chemically amplified PSPBOsimplifies the photolithographic process, and thefollowing thermal treatment at 250 �C will promotePSPBO for a variety of applications.

EXPERIMENTAL

MaterialsThe PHA derived from 6FAP and 4,40-oxybis(ben-

zoic acid) derivatives was prepared as described pre-viously.10 The number- and weight-average molecularweights (Mn and Mw) of PHA were 7,400 and 16,000(Mw=Mn 2.2), respectively, as measured by gel perme-ation chromatography (GPC) with polystyrene stand-ards. The photo-acid generator PTMA was donatedby Ciba Specialty Chemicals and stored in a refriger-ator. t-Boc BHF was prepared from BHF and di-tert-butyl dicarbonate.8 2-Methoxyethanol and otherchemicals were used as received unless otherwisenoted.

Synthesis of Isopropyl Methanesufonate (IMS).15

Pyridine (8.15mL, 100mmol) was added dropwiseover 15min to a solution of isopropanol (3.83mL,50.0mmol) and methanesulfonyl chloride (3.23mL,41.7mmol) in dichloromethane (100mL) at 0 �C.The resulting solution was stirred at 0 �C for 1.5 h,then at room temperature for 1.5 h, followed by dilu-tion with dichloromethane (200mL). The solutionwas washed successively with 1M hydrochloric acidsolution and aqueous 20% potassium hydrogen car-bonate solution, dried over anhydrous magnesium sul-fate, filtered, and concentrated at reduced pressureusing a rotary evaporator to afford a oil which wasdistilled under reduced pressure to yield IMS (3.1 g,53%) as a clear, colorless liquid. Infrared (IR) (KBr,v, cm�1): 1350 and 1176 cm�1 (–SO3–).

1H NMR(CDCl3, �, ppm): 4.98 (m, 1H, CH(CH3)2), 3.02 (s,3H,CH3), 1.45 (d, 6H, CH(CH3)2).

Synthesis of Isopropyl p-Toluenesufonate (ITS).16

This compound was prepared from isopropanol (8mL,104mmol) and p-toluenesulfonyl chloride (8mL, 104mmol) as described above. The product was a clear,colorless liquid (5.50 g, 49%). IR (KBr, v, cm�1):1361 and 1176 cm�1 (–SO3–).

1H NMR (CDCl3, �,ppm): 7.81 (d, 2H, ArH), 7.36 (d, 2H, ArH), 4.76(m, 1H, CH(CH3)2), 2.47 (s, 3H, CH3), 1.29 (d, 6H,CH(CH3)2).

H

R SO3 H

CH3(CH 2)2SO3

R SO3R'∆

F3C CF3N

O

N

O nO

< 250 oCR SO3 H

HN

HO

F3C CF3 HN

OHO

OOn

OO

O

O

O

O DITAG

HO OH

DI

+ +

H+

PBO

TAG

∆ TAG

PHA

PHA

PTMA

Coating

Exposure & PEB

Development

Curing

Unexposed area

Exposed area

acid generator)PTMA (Photo

hν

Scheme 1. Patterning process and subsequent low-tempera-

ture cyclization using positive-type PSPBO.

T. OGURA et al.

246 Polym. J., Vol. 39, No. 3, 2007

Thermal Decomposition of TAGs0.5 g of IMS or ITS was placed in a flask, heated at

100, 125, and 150 �C under nitrogen for 1, 5, 10, and20min. The aliquot of each sample was transferredinto a NMR tube, dissolved in CDCl3, and the degreeof the decomposition was determined from the inte-gral ratio of the methyl group of IMS or ITS and thatof the corresponding sulfonic acid.

Degree of Cyclization from PHA into PBOThe PHA or the photosensitive polymer film spin-

cast on a silicon wafer was baked at 100 �C for5min as a mimic of PEB. The thickness of eachfilm was approximately 1.2mm. The film was heatedon a hotplate at a series of temperatures from 150 to250 �C for 15min each. The PBO film was preparedby heating at a temperature up to 350 �C; the filmwas held at that temperature for 1 h under nitrogenas a reference. Absorption intensities on the IR spec-trum at 1045 cm�1 (A1045) assignable to C–O stretchof benzoxazole group and at 609 cm�1 (A609) assigna-ble to silicon wafer as invariable standard was ob-tained, and the degree of cyclization was determinedusing the following eq 1,11

Cyclization [%] ¼ ðA1045=A609½samp� � A1045=A609½init�Þ=ðA1045=A609½PBO� � A1045=A609½init�Þ � 100; ð1Þ

where the subscripts between the brackets followingA1045=A609 indicate the state of the film; e.g., [samp]is the polymer sample at each heating temperaturelevel (150–250 �C); [init] is initially pre-baked poly-mer film at 110 �C in air; [PBO] is the fully curedPBO at 350 �C for 1 h under nitrogen.

Dissolution RateDI, TAG, and PTMA were added to a PHA solution

in cyclohexanone to construct a photosensitive poly-mer. The polymer film was spin-cast from the solution(15wt% concentration) on a silicon wafer and pre-baked at 100 �C for 5min, then exposed to a filteredsuper-high-pressure mercury lamp at 365 nm (i-line),followed by post-exposure baking at 80–140 �C for3min. The exposed film was developed with 2.38wt% TMAHaq/2.5wt% iso-propyl alcohol (i-PrOH)at 25 �C to determine dissolution rate (A/sec).

PhotosensitivityThe polymer film of 1.2-mm thickness on a silicon

wafer was exposed to light at a wavelength of 365nm, developed with 2.38wt% TMAHaq/2.5wt% i-PrOH at 25 �C for 33 s, and rinsed in water. A charac-teristic curve was obtained by plotting normalizedfilm thickness as a function of exposure dose (mJ/cm2). The image exposure was obtained as a contactprint.

MeasurementsIR spectra were obtained using a Horiba FT-720

spectrophotometer. The 1H NMR spectra were re-corded on a Bruker GPX300 (300MHz) spectrometer.Number- and weight-averaged molecular weights (Mn

and Mw) were determined by GPC using a HitachiLaChrom with two polystyrene gel columns (TSKGELs; GMHHR-M) at 40 �C in tetrahydrofuran(THF) at a flow rate of 1.0mL/min, calibrated withpolystyrene standards. The film thickness on sili-con wafers was measured with a Veeco InstrumentDektak3 surface profiler. Scanning electron micro-scope (SEM) photos were taken with a Technex LabTiny SEM 1540 instrument with 15 kV acceleratingvoltage for imaging. Pt/Pd was spattered on a filmprior to obtaining the SEM pictures.

RESULTS AND DISCUSSION

Synthesis of TAGTAG is required for compatibility with PHA and

good thermal stability during pre-baking treatment.It decomposes near 150 �C to produce a sulfonic acid.While TAG decomposes easily with a photo-generat-ed acid, the sensitivity of PSPBO is improved with asmall amount of PAG. To satisfy these requirementsfor TAG, its design was based on sulfonate com-pounds able to decompose thermally with the produc-tion of stable carbocation species, giving olefins andsulfonic acids. TAGs, IMS, and ITS were preparedby reaction of isopropyl alcohol with methanesulfonylchloride and p-toluenesulfonyl chloride in the pres-ence of pyridine in dichloromethane.

Thermal Decomposition of TAGsThe relation between the decomposition percentage

of TAGs and heating time at each temperature isshown in Figures 1 and 2. The thermal decompositionbehavior of TAGs was followed by 1H NMR spec-troscopy. IMS is stable at 100 �C, and decomposescompletely upon thermal treatment at 125 �C for

0

20

40

60

80

100

0 10 20 30

100°C

125°C

150°C

Time (min)

Dec

ompo

sitio

n (%

)

Figure 1. Relationship between the decomposition percentage

of IMS and the heating time at each temperature.

Positive Type Photosensitive Poly(benzoxazole)

Polym. J., Vol. 39, No. 3, 2007 247

20min. At 150 �C, complete decomposition occurredwithin 5min. In contrast, ITS was more stable thanIMS, and decomposed completely within 10min at150 �C. No mixture of TAGs and the correspondingsulfonic acids was observed in the 1H NMR spectra,which suggests that the thermo-generated sulfonicacid promotes decomposition of TAGs.

Degree of Cyclization of PHA into PBO by IR Spec-troscopyThe effect of TAGs on low-temperature cyclization

of PHA to PBO was investigated using IR spectrosco-py. The PHA solutions containing 10wt% TAGs in 2-methoxyethanol were spin-coated on a silicon wafer,pre-baked at 100 �C for 5min, and heated at each tem-perature for 15min. A fully cured PBO film was pre-pared for each sample by increasing the temperatureto 350 �C and then holding it for 1 h under nitrogen.IR spectra of the film are shown in Figure 3 afterpre-baking and heating at 250 or 350 �C (as the refer-ence). At a temperature of 250 �C or higher, the spec-tra exhibit characteristic absorption bands at 1045 and1620 cm�1 corresponding to a benzoxazole ring. Aband at 1654 cm�1 due to the amide group and a bandnear 3400 cm�1 due to the hydroxyl group of PHAdisappear. Eventually, the IR spectrum obtained afterthermal treatment at 250 �C for 15min is identical tothat of a fully cured PBO film (350 �C for 1 h). Thus,

PHA is readily converted to the corresponding PBO inthe presence of 10wt% TAG. The degree of cycliza-tion in the presence of ITS is less than that using IMSin the temperature range of 150–200 �C, but becomesgreater than that using IMS above 210 �C (Figure 4).This behavior may be explained by high mobility atlow temperatures and easy volatilization at highertemperatures of IMS compared to ITS. Based on theseresults, ITS was selected as the TAG for further inves-tigation. The mechanism of acid-catalyzed cyclizationinvolved sulfonic acid acceleration of the phenolic hy-droxyl group attack on an amide carbonyl unit throughprotonation of the oxygen of the amide group. Thedegree of cyclization was calculated from the eq 1as described in the Experimental section and is shownin Figure 4. An absorbance band at 1045 cm�1 (A1045)assignable to C–O stretching of a benzoxazole groupoccurred in the IR spectrum upon an increase in tem-perature, while the band at 609 cm�1 (A609) assignableto the silicon wafer was used as a non-interactivestandard.

Lithographic Evaluation of the PHA/DI/TAG/PAGSystemThe PHA matrix polymer was prepared from 6FAP

and 4,40-oxybis(benzoyl chloride) according to amethod reported previously.9 The Mn and Mw ofPHA were 7,400 and 16,000, respectively.Preliminary optimization studies of processing con-

ditions and composition ratios of DI, TAG, and PAGto PHA for formulation of new PSPBOs were con-ducted using t-BocBHF and PTMA as the DI anda PAG, respectively, according to a previous report.8

Films were obtained by spin-casting a diluted solutionof PHA, DI, TAG, and PTMA in cyclohexanone on asilicon wafer, and then pre-baked at 100 �C for 5minin air. This photosensitive polymer film was irradiatedwith UV light (100mJ/cm2) at 365 nm (i-line) using afiltered super-high-pressure mercury lamp, baked afterexposure at a set temperature for 3min, and developedwith TMAHaq/2.5wt% i-PrOH at 25 �C.

0

20

40

60

80

100

0 10 20 30

100°C125°C150°C

Time (min)

Dec

ompo

sitio

n (%

)

Figure 2. Relationship between the decomposition percentage

of ITS and the heating time at each temperature.

40080012001600-1)

Abs

orba

nce

(a.u

.)

A

B

C

Wavenumbers (cm

Figure 3. FT-IR spectra of PHA films containing 10wt%

IMS on a silicon wafer: (A) reference film baked at 350 �C for

1 h without IMS, (B) baked film at 250 �C for 15min, (C) as-made

film.

0

20

40

60

80

100

150 175 200 225 250

Temperature (oC)

Cyc

lizat

ion

(%)

Figure 4. Degree of cyclization of PHA containing 10wt%

TAGs into PBO at different temperatures. IMS ( ), ITS ( ),

and no-additive ( ).

T. OGURA et al.

248 Polym. J., Vol. 39, No. 3, 2007

Post-exposure baking (PEB) temperature is crucialfor chemically amplified resist systems because diffu-sion of the generated acids in the films is a key proc-ess. To clarify the dissolution behaviors of PSPBOfilms containing t-Boc BHF, ITS, and PTMA in bothexposed and unexposed areas, the dissolution ratewas estimated by the change in film thickness beforeand after development. As shown in Figure 5, a largedifference in dissolution (dissolution contrast, DC) ofapproximately 800 times between the exposed andunexposed areas in 2.38wt% TMAHaq/2.5wt% i-PrOH was obtained for PEB at a temperature rangeof 120 �C, indicating that the photo-generated acid de-composes t-Boc BHF effectively to give BHF by PEBtreatment. PEB temperatures greater than 130 �C mayinduce thermal decomposition of t-Boc BHF, resultingin greater solubility in 2.38wt% TMAHaq/2.5wt%i-PrOH.The effect of PEB time on dissolution rate of the

film was investigated as shown in Figure 6. PEB times

of 3–4min were adequate to produce a large DC be-tween exposed and unexposed areas. Further PEBtreatment may induce decomposition of TAG, result-ing in no DC between the exposed and unexposedareas.The effect of PTMA loading of PSPBO on the dis-

solution rate of the film is shown in Figure 7. Evenwith 1wt% loading of PTMA to PSPBO, a largeDC was obtained, indicating that the photo-generatedacid catalyzes a cascade of ITS decomposition, fol-lowed by effective deprotection of t-Boc BHF byPEB.Based on the studies involving PTMA loadings,

and PEB temperature and time, a PSPBO consistingof PHA (73wt%), t-Boc BHF (18wt%), ITS (7.5wt%), and PTMA (1.5wt%) was formulated. Thephotosensitivity curve of resist films 1.2mm thick isshown in Figure 8. The resist containing t-Boc BHFand ITS possessed outstanding sensitivity (D0) of33mJ/cm2, and good contrast (�0) of 5.1.Figure 9 shows the scanning electron micrograph of

a contact-printed image obtained with the system de-

1

10

100

1000

90 100 110 120 130

Unexposed

Exposed

PEB temperature (oC)

Dis

solu

tion

rate

(Å

/sec

)

Figure 5. Effect of PEB temperature on dissolution rate

for PHA/t-Boc BHF/ITS/PTMA (73/18/7.5/1.5wt/wt/wt/wt)

resist system in exposed ( ) and unexposed ( ) areas. The i-line

exposure and PEB time were fixed at 100mJ/cm2 and 3min,

respectively.

1

10

100

1000

1 2 3 4 5 6 7

Unexposed

Exposed

PEB time (min)

Dis

solu

tion

rate

(Å

/sec

)

Figure 6. Effect of PEB time on dissolution rate for PHA/

t-Boc BHF/ITS/PTMA (73/18/7.5/1.5wt/wt/wt/wt) resist sys-

tem in exposed ( ) and unexposed ( ) areas. The i-line exposure

and PEB temperature were fixed at 100mJ/cm2 and 120 �C,

respectively.

1

10

100

1000

10000

0 2 4 6

Unexposed

Exposed

PTMA to PHA content (wt%)

Dis

solu

tion

rate

(Å

/sec

)

Figure 7. Effect of PTMA loading on PHA for dissolution

rate for PHA/t-Boc BHF/ITS/PTMA resist system in exposed

( ) and unexposed ( ) areas. The i-line exposure, PEB tempera-

ture, and time were 100mJ/cm2, 120 �C, and 3min, respectively.

00.10.20.30.40.50.60.70.80.9

1

1 10 100Exposure dose (mJ/cm2)

Nor

mal

ized

film

thic

knes

s

Figure 8. Characteristic photosensitive curve for the PHA/

t-Boc BHF/ITS/PTMA (73/18/7.5/1.5wt/wt/wt) resist system.

The PEB temperature and time were 120 �C and 3min, respec-

tively.

Positive Type Photosensitive Poly(benzoxazole)

Polym. J., Vol. 39, No. 3, 2007 249

scribed: the resist layer was exposed to 50mJ/cm2,post-baked at 115 �C for 3min, and developed with2.38wt% TMAHaq/2.5wt% i-PrOH at 25 �C. Aclear, positive pattern with an 3 mm feature was ob-served using a 3mm thick film.

Low-Temperature Cyclization of PHAThe printed pattern was converted to a PBO pattern

without deformation by heating at temperatures up to150 �C for 10min and 250 �C for 20min under nitro-gen (Figure 10). The formation of PBO was con-firmed by IR spectroscopy, in which the characteristicoxazole ring absorption appeared at 1616 cm�1 andabsorptions due to hydroxyl and carbonyl groups at3300 and 1650 cm�1, respectively, in the PHA spec-trum disappeared.The resist system was used to produce a thick pat-

tern. The 20-mm image was made in 10 mm thick filmby PEB at 115 �C for 3min after exposure to 70mJ/cm2 of i-line, followed by developing with 2.38wt%TMAHaq/2.5wt% i-PrOH at 25 �C for 110 s. Ade-quate resolution and edge sharpness can be seen inFigure 11.

CONCLUSION

Two TAGs (IMS and ITS), which improve thesensitivity and transparency at the i-line region ofPSPBO, and accomplish low-temperature cyclizationof PHA to PBO, were prepared from isopropyl alcohol

and methanesulfonyl chloride or p-toluenesulfonylchloride. ITS satisfied the requirements for TAG,and was used for formulation of a positive-type alka-line developable PSPBO based on a PHA, t-Boc BHF,and PTMA. The new resist system showed high sensi-tivity and contrast of 33mJ/cm2 and 5.1, respectively,with i-line exposure. Furthermore, the clear positiveimage after development was converted to a PBOimage by low-temperature treatment. A thick patternalso was obtained with the PSPBO system. This newformulation method provides a more efficient andversatile process compared to the standard route thatrequires large exposure doses and high cyclizationtemperatures.

Acknowledgment. This work was financially sup-ported by a Grant-in-Aid for Science Research(No. 18350059) from the Japanese Ministry of Educa-tion, Science, Sports, and Culture, which is gratefullyacknowledged.

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Figure 9. SEM image of positive-pattern 3-mm-thick film

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Figure 10. SEM image of PBO pattern cured at 250 �C for

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Figure 11. SEM image of positive-pattern 10-mm-thick film

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