Application of Triaryl Phosphate to Photosensitive Materials

Chapter 11

Application of Triaryl Phosphate to Photosensitive Materials Photoreaction Mechanism of Triaryl Phosphate Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 13, 2018 at 04:13:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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1

2

2

I. Naito , A. Kinoshita , Y. Okamoto , and S. Takamuku 1

Department of Photography, Kyushu Sangyo University, Matsukadai, Higashi-ku, Fukuoka 813, Japan ISIR, Osaka University, Mihogaoka, Ibaraki, Osaka 567, Japan 2

Five kinds of methoxy-substituted diaryl phosphate (DAP) and triaryl phosphates (TAP) were studied for application to positive-acting photo sensitive resist materials. Photoirradiation of these compounds with 254 nm light gave a biaryl and phosphoric acid (DAP) or its monoaryl ester (TAP) equivalently with good quantum yields. During the photoirradiation, a strong emission was detected. The emission spectrum has two emission maxima at 310 nm (monomerfluorescence)and around 350 nm (intra molecular excimerfluorescence).Photolysis of bis(4-methoxyphenyl) phosphate were carried out in methanol in the presence of oxygen as a quencher.

Because a quenching rate (Stern-Volmer constant; kq τ) of -1

the biaryl formation (214M ) agrees with that of the excimer fluorescence -1

(248 M ), the reaction is believed to proceed through the intramolecular excimer.

Phosphate-sensitized thin films of poly(4-trimethyl-siloxystyrene)

(PSSt) were made for the photosensitivity study. The most sensitive formulation was a 3- μ m-thick film of PSSt containing 5.0 wt/wt% tris(4-2

methoxyphenyl) phosphate that required a dose of 12 mJ cm at 254 nm. New positive-acting photosensitive resist materials consisting of a photo-acid generator and an acid-sensitive polymer have attracted our interest.*^) In reactions of these materials, the photoinitiator initially generates an acid in the thin film by irradiation, as shown in eq. 1. The polymer in the irradiated film undergoes an acid-catalyzed deprotection reaction by thermal treatment. A relief image can be formed in the nonirradiated polymer that remains after NOTE: This is Part II in a series. 0097-6156/94/0579-0139$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

140

POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

development, as shown in eq. 2.

Photoinitiator

hi/

Acid

(1)

+

Base-insoluble Polymer Η , Δ

Base-soluble Polymer (2)

Recently, we reported the photoreaction of TAP to yield a bisaryl compound and phosphoric acid monoaryl ester, equivalently, with good quantum yield A5) An emission spectrum of TAP exhibited two maxima around 310 and 350 nm, which were assigned to a monomer fluorecence and an intramolecular exicimer fluorescence, respectively. The photochemical reaction is proposed to proceed through the intramolecular excimer. Because phosphoric acid monoaryl ester is a strong acid, TAPs were studied for an application to positive-acting photosensitive resist materials.^) We synthesized methoxy-substituted bisaryl phosphates (DAPs) and TAPs to study the reaction mechanism and to prepare a highly sensitive film [ ( R,0 ) PO ( OR ); Ri = R2 = 4-methoxyphenyl ( TMP ); R = 4-methoxyphenyl, R = H ( DMP ); Rj = 4-methoxyphenyl, R2 = ethyl ( DMEP ); Rj = R = 3,4,5trimethoxyphenyl ( TTMP ); ^ = 3,4,5-trimethoxyphenyl, R = H ( DTMP ) ]. Photoirradiation of DAPs yields phosphoric acid, a strong and non-volatile acid. Poly(4-trimethylsiloxystyrene) (PSSt) was used as matrices for the sensitivity experiments. 2

2

{

2

2

2

CH 0 3

(TMP)

CH3O (TTMP)

( DMP)

I DMEP)

( DMEP)


11.

ΝΑΓΓΟ ET AL.

Application of TAP to Photosensitive Materials 141

CH 0

(-CH -CH-) - ( -CH -CH- )

3

2

m

2

n

(CH

( 77

mo!% )

(PSSt) Materials Syntheses of TMP and DMEP were described previously β) TTMP was obtained by the reaction of phosphorus trichloride with 3,4,5trimethoxyphenol, by the same method used for TMP.5) The product was identified by elemental analysis and by BR and NMR spectroscopies. The yield was 45.2 %. 4-Methoxyphenol ( 12.41 g ) and phosphorus trichloride ( 6.88 g ) were refluxed for 20 hours in dioxane (200mL). After dropwise addition of water ( 0.90 g ) in dioxane (20mL), the solution was refluxed for 2 hours. The solution was dried over anhydrous sodium sulfate, and then, dioxane was removed by evaporation. The product was purified using a neutral alumina column with 1,2-dichloroethane as the eluent, further purified by two recrystallizationsfrom1,2-dichloroethane (2.74g, yield; 37.6%), and identified by IR and NMR spectroscopies, and elemental analysis. DTMP was obtained by reacting phosphorus trichloride with 3,4,5trimethoxyphenol, by the same method used for DMP( yield: 21.4%). 4,4'-Dimethoxybiphenyl was purified by two recrystallizations of the commercial product from methanol. Commercial phenol was used after two distillations. Spectroscopic grade methanol, dioxane, and 1,2-dichloroethane were used without further purification. PSSt was obtained by treatung poly(4-hydroxystyrene) ( Maruzen Petrochemical Co., Ltd, Mn = 5 x l 0 ) with 1,1,1,3,3,3-hexamethyldisilazane. ) A highly substituted polymer was obtained by repeating the reaction. The conversion of the reaction was determined to be 77mol% by NMR spectroscopy. Other materials were used without further purification. 3

6

Photolysis of Pi$(4-metoyphwt) Phosphate DMP ( 198.94 mg ) was irradiated in methanol (500mL) with a N purge for three hours with a 20 W 2


142


low pressure Hg lamp. Filters were not used since DMP does not absorb light with a wavelength longer than 300 nm. After evaporation of the solvent, the products were dissolved in ether and washed several times with a 2 % sodium hydroxide aqueous solution. After drying the solution over anhydrous sodium sulfate, the ether was evaporated.

Sublimation yielded 66.34 mg of pure product,

which was identified as 4,4'-dimethoxybiphenyl by IR and NMR spectroscopies ( Yield: 64.8 % ). DMP ( ca. 200 mg ) was exposed in methanol ( 500 mL ) with a N purge 2

for three hours to 254 nm radiation. After evaporating the solvent, the remaining products were reacted with diazomethane in ether as previously reported.^)

The

product was identified to be trimethyl phosphate by gas chromatography (column: silicon grease OV-17) by comparison with a standard material.

Emission Spectrum Emission spectra of TAPs and DAPs were measured in degassed or nitrogen-saturated methanol by 254 nm light excitation ( d 5 4 = 2

nm

0.150 ± 0.005 ). The spectra of 4-methoxyphenol and 3,4,5-trimethoxyphenol were also measured. The quantum yield of the emission ( ρ ) was determined by means of integrated emission intensities and ^ reported ( φ

F

= 0.066 ) .

of phenol was previously

7)

Determination of Quantum Yield Determinations of reaction quantum yields were carried out using the 20 W low pressure Hg lamp and a Schott glass filter UG-5. The cycloreversion reaction of r-l,t-2,t-3,c-4-tetraphenylcyclobutane (to yield irans-stilbene; = 0.67 in 1-butyl chloride 8)) was used as the actinometer. The reactions were run to 10 % conversion. The yield of bisaryl was determined by using the gas chromatography on an OV-1 column. The acids were titrated with 0.01 Ν aqueous sodium hydroxide using a phenolphthalein indicator.

Quenching

The reactions of DMP were carried out in bubbling a

N2-O2

mixture and the effect of oxygen on the bisaryl formation was measured.

A

quenching of the emissions were also studied by using oxygen as a quencher. Emission intensities at 312 and 370 nm were monitored after bubbling a N - 0 2

gas mixture.


2

11.

ΝΑΓΓΟ ET AL.


Single Photon Counting The decay profiles of the emissions were monitored by the aid of a Horiba fluorescence lifetime measuring apparatus NAES-700F. A H2 flash lamp was used with a glass filter and solution filters (excitation: pulse width: ca. 1.5 ns, λ = ca. 280 nm, λ < 340 nm ). TAPs and DAPs were measured in methanol after N2 or N2-O2 mixed gas bubbling. The optical densities at 280 nm were adjustedto 0.60 (±0.01 ). Monitored traces were analyzed by using a program for analysis of triple exponential decay processes. m

θ χ

Sensitivity

Commercial pre-sensitized plates for printing ( Fuji FPS type)

were used as grained aluminium plates immediately after exposure and development. A solution of polymer (0.50 g in 20 mL of dioxane) and TAP or DAP (50mg) was spin-coated on a 1.00 χ 10'^m^ square grained aluminium or quartz plate. After drying under vacuum, the plate coated with 3.0 ( ± 0.02 ) g m 2 of the materials was used. The plates were pre-baked at 100 *C for 3 _

min, and then irradiated with 254 nm light.

The light intensity was monitored

with a Spectronics Joul-meter (DRC-100X) with a radiometer sensor (DIX-254). The exposed plates were heated at 100 *C for 5 min, and then dipped in an aqueous solution of tetrabutylammonium hydroxide (1.0% for 5min). The sensitivity of the film was determined as the minimum energy for satisfactory development of the irradiated polymer.

Results mû Pfeçu^sion, Photolysis of Arvl Phosphate

Figure 1 shows the absorption spectra of TAPs

and DAPs in methanol. Each compound has absorption maxima around 220 and 270 nm. Since the absorption tail ends around 300 nm, the photoreaction of TAPs was carried out by 254 nm irradiation in methanol. Photolysis of TMP and DMP yielded 4,4'-dimethoxybiphenyl as a main product [ φ = 0.17 ( TMP), 0.12 ( DMET ) ]. ) The photoirradiation of DMP was carried out with 254 nm light for four hours in methanol. The reaction produced acid and 4,4'-dimethoxybiphenyl (A) in good yields ( isolated yield: 33.3 % ). The quantum yield of the reaction was determined to be 0.05. Addition of diazomethane-ether solution to the reaction products gave trimethyl phosphate and bis(4-methoxyphenyl) methyl phosphate. These products are derived from 4


144


main reaction products must be A and phosphoric acid (B), as shown in eq. 3.

hp

(Ary!-0) PO(OH) 2

Aryi-Aryi + H P 0 3

in MeOH

(A) Emission Spectrum

(3)

4

(B)

During the photoirradiation of DMP, a strong emission

was detected. The emission spectra of DMP, DTMP, and TTMP were measured in degassed methanol by 254 nm excitation ( d254nm

=

0.150 ± 0.004 ). The

spectra exhibit two emission maxima at 310nm and around 350nm (DMP: 370 nm ), as shown in Figure 2. The spectra are not affected by dilution of the solution. Accordingly, the maximum at the shorter wavelength is assigned to the monomer fluorescence and the longer wavelength maximum to the intra-molecular excimer as previously assigned in the TMP and DMEP emission spectra.^) The fluorescence quantum yield ( ^. ) of phenol was reported to be 2 7

6.6 χ 10" . )

The values of TTMP, DMP, and DTMP were estimated by ¥

OCHs CHsO-^D^o'

Intramolecular Excimer comparison with the φψ value of phenol and the integrated emission intensities ( [ipdy), listed in Table I [ f : ç of the excimer, p ex

m o n o

: that of

the monomer ]. The spectra of the monomer and the excimer fluorescences have approximately the same shape as those of the phenol derivative and TMP, respectively.

Quantum Yield of Acid Formation TAPs produced phosphoric acid monoaryl ester upon photoirradiation with 254 nm light and the photoreaction of DAP yielded phosphoric acid. The yield of the acid was determined titremetrically Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

11.

NAITO ET AL.


Monomer Excimer

3

— 1

3

Ctf

r

1

500

Figure 2. Emission spectra of TMP ( 1 ), DMP ( 2 ), DMEP ( 3 ), TTMP ( 4 ), and DTMP ( 5 ) in methanol, λ . = 254 nm, d = 0.150 ±0.005. e x


146


Table I. Results of the emissions and the lifetimes of the excited states in methanol Compound

Monomer Fluorescence λ

(nm)

max TMP

305

A

Fmono 0.05

370

312 307

< 0.05

TTMP

312

5 χ 10"

4

ca.

r

( ns )

ex

0.12

4.2

11.4

— 4

3 χ 10"

355

( ns )

s

0.02

360

4 χ 10"

τ

Fex

> 0.02

350 4

Single Photon Counting

è

max 350

DMP

315

( nm )

0.01

DMEP DTMP

Excimer Fluorescence

έ

3.8

9.8

4.3

12.2

5.9

30.9

5.3

23.0

with 0.01 Ν aqueous sodium hydroxide solution. The quantum yields for the acid formation ( 6 . . ) are listed in Table II. The 6 . , value of DMP is acid acid about twice the value of the bisaryl formation. Phosphoric acid reacts with two r

r

molar equivalents of sodium hydroxide to yield Na2HPU4.

It is concluded that

the reactions of the each compound proceeded unimolecularly.

Quenching

The photoreactions of DMP were carried out with

N2-O2

mixed

gas bubbling. When the rates of the quantum yields of the biaryl formation in the absence and presence of oxygen ( φ Ι φ\ φ · in the absence, φ : in the 0

0

presence of 0 ) were plotted against oxygen concentration ( [ 0 ]saturated 2

2

2

χ ΙΟ" M in methanol at 25 °C 9) )

f

a

=

* -02

linear relationship was obtained, as shown

in Figure 3a. The quenching rate (kq r ) was determined to be 214M~*. The quenching of the DMP emissions was also carried out. The fluorescence

Table II. Quantum yields of the biaryl formation and the acid formation, and the sensitivity Compounds TMP DMP DMEP TTMP DTMP

φ

5 i s a r y I

0.17 0.05 0.12

φ

z

a c i d

0.19 0.11 0.14 0.02 0.06

E(mJmr ) 120 90 160 .1000 180



11. NAITO ET AL.

b)

a) 4

0

0 0

5 10 [O2]/10" M

5 10 [O2]/10" M

0

3

3

Figure 3. Stem-Volmer analysis of the quenching of the biaryl formation (a) and of the emission intensities (b) by oxygen. The ratio of the quantum yields in the absence to that in the presence of oxygen (a) and the ratios of IFs at A =312 nm ( monomer fluorescence, # ) and 370 nm ( excimer fluorescence, Ο ) in the absence of oxygen to that in the presence of of oxygen were plotted against the oxygen concentration.

intensities at 312 and 370 nm were measured instead of the monomer fluorescence and the excimerfluorescencequantum yields, respectively. The ratios of the fluorescence intensity in the absence of O 2 ( Ip ) versus that in the presence of 0

O 2 ( I F ) were plotted against the oxygen concentration, as shown in Figure 3b. Each plot gave a good linear relationship with a slope of 50.7 M~l (310nm), and 247 M~l (370nm).

Since the quenching rate determined for the excimer

fluorescence agreed with the quenching rate of bisaryl formation, the bisaryl formation reaction must proceed through the intramolecular excimer.

Single Photon Counting

Lifetimes of the excited singlet state and the

intramolecular excimer were measured in methanol by the aid of the single photon counting apparatus. Figure 4 shows a fluorescence decay profile of TMP. The decay profiles were analyzed using a program for three component processes. r

ex ^

The lifetimes of the excited singlet state (

w e r e

a D O l l t

4

n s

a n

r s

) and the excimer (

ns

d H » respectively, except for the r

e x

values of

TTMP ( 31 ns ) and DMP ( 23 ns ), listed in Table I. The quenching of the emissions of TMP and DMP by oxygen was carried out in methanol. Because


148


*

0

(3)

-75I

10

Ο

ιο

40

)0 Time/ ns

50

Fig. 4. Decay profiles of thefluorescenceof DMP(l) and the lamp intensity (2) measured in the single photon counting in methanol. A fitting plot (3) is also shown.

the excimer fluorescence was quenched completely at a low oxygen concentration, only a quenching rate constant for the monomer fluorescence was determined to 1

1

1

1

be 3.4 χ Î O ^ M " s" for TMP and 2.5 χ Ι Ο ^ Μ " s" for DMP.

Application of TAP to Photosensitive Materials To make a thin photosensitive film, the grained aluminum plate was spin-coated with solutions of a polymer and TAP or DAP. After drying under vacuum, the coated film ( film thickness : ca. 3 ^ m ) was heated at 100 °C (pre-baking) The irradiation was carried out using a low pressure Hg lamp. After photoirradiation, the film was heated at 100 °C ( post-baking ). A tetrabutylammonium hydroxide aqueous solution was used as the developer. The unit operations are shown in Figure 5. Because DAP is an acidic compound,^) psSt reacted with DAP without irradiation. Sensitivities ( Ε ) were determined as the minimum energy necessary to deprotect the polymer. The results are listed in Table II. The Ε value is inversely related to . The sensitivity of TMP-PSSt film is the highest acid

2

among the TAPs studied (E=12mJcnr ).



11. NAITO ET AL.

Photoinitiator

±

Polymer

Spin-coat Pre-baking Photoirradiation Post-baking ~

~

r

Polymer Photoinitiator%

PSSt 95wt/wt% 5 wt/wt%

Solvent

Dioxane

Film Thickness

-2 ( 3.0 ± 0.3 ) g m

100 °C

3 min

254 nm light

100 C e

5 min

~

Development

NBu OH Solution 15 °C 4

2% 10 min

Fig. 5. Materials and patterning conditions.

The photoreaction mechanism of TAP and DAP is concluded to be the following: the 254 nm irradiation of these compounds yields the intramolecular excimer by the way of the excited singlet state ( *TAP* ), as shown in eq. 4. The excimer yields the reaction products, a bisaryl and a phosphorus compound [Aryl-0-PC>3 ( TAP ) or H P 0 ( DAP ) ], as shown in eq. 5. The precursor of the reaction is 4

believed to be the intramolecular excimer on the basis of the agreement of the quenching rate of the biaryl formation with that of the excimer emission. The lifetimes of the excited singlet state and the intra-molecular excimer for TAP were determined to be about 4 and 11 ns, respectively. The phosphorus compound yielded by the reaction may abstract hydrogen atoms from the solvent to give phosphoric acid monoaryl ester or phosphoric acid, which are strong and nonvolatile acids, as shown in eq. 6. Through the application of TAPs and DAP to the photosensitive materials, the acid is generated in the polymer film by 254 nm light irradiation. Deprotection of the polymer, PSSt, is induced by the heat treatment, and finally, the pattern is formed in the non-irradiated polymer. In the case of DAP-PSSt film, since


150


DAPs are acidic compounds, the degradation proceeds without irradiation. The sensitivity of the thin TMP-PSSt film, the most sensitive film, is determined to be 12mJcm"2. This material can resolve 0.6 μ m line and space patterns.

TAP

1

TAP

— •

Excimer

Excimer — • Aryl-Aryl + Aryl-0-P0

(4) 3

(5)

Sovent

Ary!-0-P0

• Aryi-O-PO(OH)

3

2

(6)

Literature Cited 1 ) Thompson, L.F.; Willson, C.G.; Bowden, M.J. Edit, " Introduction to Microlithography, ACS Symposium Series 219",American Chemical Society, Wasington DC, ( 1983 ), P. 153. 2 ) Allen, N.S. Edit., " Photopolymerization and Photoimaging Science and Technology ", Elsevier Applied Science, London, ( 1989 ), p. 99. 3 ) Ueda, M.; Ito, H., J. Synthetic Org. Chem., Jpn. 1991, 49, 437. 4 ) Shin, M.; Yamamoto, M.; Okamoto, Y.; Takamuku, S., Phosphorus, Sulfur, and Silicon, 1991, 60, 1. 5 ) Naito, I.; Nakamura, Y.; Kinoshita, Α.; Okamoto, Y.; Takamuku, S., J. Imaging Sci. Eng., 1994, 38, 49. 6 ) Canington, W.C., J.P. 63-292128 ( 1983 ). 7 ) Förster, T.; Kasper, Κ., Z. Electrochem., 1955, 59, 976. 8 ) Naito, I.; Tashiro, K.; Kinoshita, Α.; Schnabel, W., J. Photochem., 1983, 23,73. 9 ) Murov, S.L.; Carmichael, I.; Hug, G.L., " Handbook of Photochemistry, second edition, Revised and

Expanded ", Marcel Dekker, Inc., ( 1993 ) p. 291.

10 ) The pH values of DMP and DTMP in pure water ( ca. 0.1 mM ) were measued to be 3.11 and 3.46, respectively. RECIEVED

October 21, 1994


Application of Triaryl Phosphate to Photosensitive Materials

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