Applications of Photoinitiated Cationic Polymerization to the

1

Downloaded by 80.82.77.83 on December 26, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch001

A p p l i c a t i o n s o f Photoinitiated C a t i o n i c P o l y m e r i z a t i o n to the Development o f N e w Photoresists J. V. CRIVELLO Corporate Research & Development Center, General Electric Company, Schenectady, NY 12301 The recent development of several new classes of highly efficient photoinitiators for cationic polymerization makes possible their application in the design of novel photoresists. The con cepts on which these imaging processes are based are set forth in this article. Although both positive and negative working photoresists based on photoinduced condensation and free-radical chemistry are wellknown, cationic polymerization chemistry has received l i t t l e attention for the fabrication of photoresists. The recent devel opment of several new classes of practical photoinitiators for cationic polymerization has now made i t possible to utilize this chemistry in a number of ways to produce highly sensitive photo resists (1-6). The facile synthesis of onium salts I-III to gether with their ready structural modification to manipulate +

+

A r I X"

Ar S X

I

II

2

3

Ar-C-CH -SR X 2

2

III

their spectral absorption characteristics make them very attrac tive for photoresist applications. Using the above photoinitia tors together with monomers which undergo cationic polymerization, acid catalyzed polymerization and polymers which undergo acid catalyzed depolymerization, one can prepare a variety of unique and novel photoresists. In the present paper, several examples of these concepts are demonstrated. Results and Discussion Mechanism of Photolysis. When diaryliodonium (I) and triarylsulfonium (II) salts are irradiated at wavelengths from 200-300 nm, 0097-6156/84/0242-0003S06.00/0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

4

P O L Y M E R S IN ELECTRONICS

they undergo i r r e v e r s i b l e p h o t o l y s i s w i t h r u p t u r e of a c a r b o n i o d i n e o r c a r b o n - s u l f u r bond a s shown i n E q u a t i o n s 1 and 2 . Ar I

+

X"

î^—>

Ar S

+

X"

—

2

3

>

Ar-

+

Α Π * Χ"

(1)

Ar-

+

A r ^ t χ"

(2)

I n t e r a c t i o n of the r e s p e c t i v e c a t i o n - r a d i c a l s w i t h s o l v e n t or monomer, R - H , r e s u l t s i n t h e r e l e a s e o f a p r o t o n and i n t h e f o r m a t i o n of p r o t o n i c a c i d s as d e p i c t e d i n E q u a t i o n s 3-7. + ΑΠ-

R-H

>

+ ΑΠ-Η X

A r l - H X"

>

Arl

+


+

Ar st

+

2

R-H

2

2Ar S+

+

2

+

> Ar S

+

+

£

H 0

> Ar SO

2

2

R-

(3)

HX

> A r S - H X~

Ar S -H X" 2

+

+

(4) +

R-

(5)

HX +

(6)

Ar S

+

2

2H

+

(7)

D i a l k y l p h e n a c y l s u l f o n i u m s a l t s ( I I I ) , i n c o n t r a s t , undergo r e v e r s i b l e d i s s o c i a t i o n on p h o t o l y s i s w i t h g e n e r a t i o n o f y l i d s and protonic acids (6). 0 H + Ar-C-CH -SR 2

2

X

-

h

V

^

. >

0 II Syr Ar-C-CH -S^

+

2

HX

(8)

i Ar-C-CH=S^ R A l t h o u g h o n i u m s a l t s I - I I I a r e e f f i c i e n t l y p h o t o l y z e d on i r r a d i a t i o n , they d i s p l a y l i t t l e tendency toward thermal decomposition even when e x p o s e d t o e l e v a t e d t e m p e r a t u r e s (150°C) f o r l o n g p e r i o d s of time (1-2 h o u r s ) . This excellent thermal s t a b i l i t y a l l o w s r e s i s t s c o n t a i n i n g t h e s e p h o t o i n i t i a t o r s t o be s u b j e c t e d to l o n g bake c y c l e s w i t h o u t danger of premature d e g r a d a t i o n of their sensitivity. Photosensitization. The c h i e f a b s o r p t i o n bands f o r onium s a l t s I - I I I l i e b e l o w 230 nm i n t h e u l t r a v i o l e t r e g i o n . P h o t o l y s i s at longer wavelengths i s , t h e r e f o r e , i n e f f e c t i v e . However, the p h o t o l y s i s o f t h e s e compounds c a n be r e a d i l y p h o t o s e n s i t i z e d t o r e s p o n d t o l o n g w a v e l e n g t h uv and t o v i s i b l e l i g h t . The mechanism of p h o t o s e n s i t i z a t i o n i n v o l v e s an e l e c t r o n t r a n s f e r p r o c e s s

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

1.

CRIVELLO

Photoinitiated

Cationic

5

Polymerization

b e t w e e n t h e p h o t o s e n s i t i z e r and t h e o n i u m s a l t a s shown i n E q u a t i o n s 9-12 f o r d i a r y l i o d o n i u m s a l t s ( 7 , 8 ) . ρ '

*

+

Ar I

+

X

2

[Ρ···ΑΓ Ι 2

+

J

^

P

-

,

>[Ρ···ΑΓ Ι

+

-,*

X ]

(10)

Ar I-

(ID

2

Χ ]*

pt Χ "

>

-

Ar I-

;>Ar-

2


(9)

*

+

+

2

(12)

ΑΠ

T y p i c a l p h o t o s e n s i t i z e r s f o r d i a r y l i o d o n i u m s a l t s a r e condensed r i n g a r o m a t i c h y d r o c a r b o n s , d i a r y l k e t o n e s , and a c r i d i n i u m d y e s . Condensed r i n g a r o m a t i c h y d r o c a r b o n s a r e p a r t i c u l a r l y e f f e c t i v e photosensitizers for triarylsulfonium s a l t s . The u s e o f p h o t o s e n s i t i z e r s i n onium s a l t p h o t o l y s i s p e r m i t s t h e p h o t o i m a g i n g p r o c e s s e s i n d u c e d b y t h e s e compounds t o be o p t i m a l l y f i t t e d t o the s p e c i f i c i r r a d i a t i o n source used f o r t h e i r exposure. P h o t o s e n s i t i z e r s c a n a l s o be u s e d t o g r e a t a d v a n t a g e i n multilevel photoresists. A t y p i c a l system i n c o r p o r a t e s a l o n g w a v e l e n g t h uv o r v i s i b l e l i g h t p h o t o s e n s i t i z e d r e s i s t on t h e t o p l e v e l and a deep u v s e n s i t i v e r e s i s t on t h e b o t t o m . Both r e s i s t s may be b a s e d on c a t i o n i c p h o t o i n i t i a t o r s o r one may b e o f t h i s t y p e w h i l e t h e o t h e r may be a c o n v e n t i o n a l r e s i s t m a t e r i a l . Photoimaging Processes

B a s e d on Onium S a l t s

C a t i o n i c P o l y m e r i z a t i o n . P r o t o n i c a c i d s such as HBF^, H P F ^ , H S b F ^ , e t c . , d e r i v e d f r o m t h e p h o t o l y s i s o f onium s a l t s I - l I I a r e w e l l known i n i t i a t o r s o f c a t i o n i c p o l y m e r i z a t i o n (9). In equa t i o n s 13-15 i s shown t h e p r o p o s e d mechanism o f c a t i o n i c p o l y m e r i z a t i o n u s i n g a t r i a r y l s u l f o n i u m s a l t and a t y p i c a l monomer, M . Photolysis Ar S Q

X"

+

— R-H

3

>

Ar S

+

0

ArH

2

+

HX, . Q

(13)

C S ;

Initiation M

+

HX

+

nM

( g )

v

N

HM X"

(14)

+

Propagation HM

H(M) M x " (15) η V i r t u a l l y a l l known t y p e s o f c a t i o n i c a l l y p o l y m e r i z a b l e monomers a r e p o l y m e r i z e d u s i n g onium s a l t p h o t o i n i t i a t o r s . I n c l u d e d among t h e s e monomers a r e e p o x i d e s , c y c l i c e t h e r s , mono a n d p o l y f u n c t i o n a l v i n y l compounds, s p i r o e s t e r s , s p i r o c a r b o n a t e s and c y c l i c +

X"

>

+


6

POLYMERS IN ELECTRONICS


silicones. U s i n g s u c h monomers, a w i d e v a r i e t y o f n e g a t i v e w o r k i n g r e s i s t s c a n be d e v e l o p e d . The most s u c c e s s f u l o f t h e s e t o d a t e h a v e b e e n p h o t o r e s i s t s b a s e d on epoxy r e s i n s ( 1 0 ) . Solid m u l t i f u n c t i o n a l novolac-epoxy r e s i n s are p a r t i c u l a r l y a t t r a c t i v e p r e c u r s o r s f o r use i n the f a b r i c a t i o n of p h o t o r e s i s t s because the presence of m u l t i p l e epoxide f u n c t i o n a l i t y i n t h e s e r e s i n s e n hances t h e i r apparent photoresponse. S i m i l a r l y , v i n y l ether d i f u n c t i o n a l monomers a r e e x c e p t i o n a l l y r e a c t i v e i n p h o t o i n i t i a t e d

(16)

c a t i o n i c p o l y m e r i z a t i o n , and p h o t o r e s i s t s b a s e d on t h e s e m a t e r i a l s h a v e a l s o been i n v e s t i g a t e d .

(17)

A c i d C a t a l y z e d C o n d e n s a t i o n P o l y m e r i z a t i o n s . The s t r o n g p r o t o n i c a c i d s p r o d u c e d by t h e p h o t o l y s i s o f o n i u m s a l t s I - I I I c a n a l s o be employed t o c a t a l y z e t h e c o n d e n s a t i o n o f p h e n o l i c , m e l a m i n e , and u r e a f o r m a l d e h y d e r e s i n s . V e r y d u r a b l e p h o t o r e s i s t s b a s e d on t h e s e i n e x p e n s i v e and r e a d i l y a v a i l a b l e r e s i n s c a n be made. Such r e s i s t s g e n e r a l l y r e q u i r e a p o s t b a k e p r i o r t o d e v e l o p m e n t t o com p l e t e t h e c o n d e n s a t i o n and t o enhance image f o r m a t i o n . A c i d C a t a l y z e d R e a r r a n g e m e n t s . A n example o f a p h o t o r e s i s t b a s e d on a n a c i d c a t a l y z e d r e a r r a n g e m e n t i s t h e d i a r y l i o d o n i u m s a l t p h o t o i n d u c e d c y c l i z a t i o n o f c i s - l , 4 - p o l y i s o p r e n e shown i n E q u a t i o n 18. T h i s f a c i l e c y c l i z a t i o n w h i c h has been r e p o r t e d p r e v i o u s l y (11) by n o n - p h o t o c h e m i c a l p r o c e s s e s r e s u l t s i n a p o l y c y c l i c p o l y m e r whose p h y s i c a l p r o p e r t i e s and s o l u b i l i t y c h a r a c t e r i s t i c s are c o n s i d e r a b l y d i f f e r e n t than the i n i t i a l polymer. E x p l o i t a t i o n o f t h e s e d i f f e r e n c e s i n t h e e x p o s e d and u n e x p o s e d r e g i o n s o f t h e polymer f i l m p e r m i t t h e i r use as e i t h e r p o s i t i v e or n e g a t i v e tone resists.


1.

CRIVELLO

Photoinitiated

Cationic

1

Polymerization

A n o t h e r type of a c i d c a t a l y z e d rearrangement i n d u c e d by d i a r y l i o d o n i u m and t r i a r y l s u l f o n i u m s a l t s was r e c e n t l y r e p o r t e d b y I t o and W i l l s o n o f IBM (12) and i s d e p i c t e d i n E q u a t i o n 1 9 .


- f c H

-ÇH-)-

h

(6)

_^CH„-CHV

v

(Ο)

A ^ S + A s ^

0-C- -C(CH ) 0

3

CH

=SH

+

3

+

OH

3

(19)

0 Imagewise e x p o s u r e o f t h e s a l t s d i s s o l v e d i n t h e p o l y m e r f i l m generates a l o c a l c o n c e n t r a t i o n of s t r o n g p r o t o n i c a c i d s . Upon b a k i n g a t 100°C f o r a few s e c o n d s , t h e l a b i l e e s t e r g r o u p i s c a t a l y t i c a l l y decomposed t o i s o b u t y l e n e , c a r b o n d i o x i d e a n d p o l y p-hydroxystyrene. D e p e n d i n g on t h e n a t u r e o f t h e d e v e l o p i n g s o l v e n t , e i t h e r n e g a t i v e o r p o s i t i v e t o n e images a r e f o r m e d . I t o and W i l l s o n r e p o r t t h a t the r e s o l u t i o n c a p a b i l i t y of t h i s r e s i s t i s a b o u t 0 . 7 5 ym. C a t i o n i c D e p o l y m e r i z a t i o n . Many p o l y m e r s c o n t a i n i n g h e t e r o a t o m s i n t h e i r m a i n c h a i n a r e known t o u n d e r g o f a c i l e d e p o l y m e r i z a t i o n when t r e a t e d w i t h a c a t i o n i c i n i t i a t o r ( 1 3 ) . Such polymers u n d e r go r e i n i t i a t i o n a t t h e c h a i n ends o r a l o n g t h e b a c k b o n e f o l l o w e d by a " b a c k b i t i n g " p r o c e s s w h i c h e l i m i n a t e s monomers and c y c l i c oligomers. N e g a t i v e r e s i s t s u s i n g onium s a l t p h o t o i n i t i a t o r s c a n be d e v i s e d b a s e d on t h i s c o n c e p t . A n e x a m p l e i s shown i n Equations 20-23: H 1+ hv w0-CH -CH -0-CH - CH -0^v > ^wO-CH -CH -0-CH C H - 0 ^ (20) Ar S 2

2

2

2

2

2

2

2

3

H .wO-CH„

-

C H

2-£7? 2H

CH.

2

2

(21)

/

^0-CH -CH -0-CH -CH 2

A/vO-CH -CH -OH 2

Si

2

2

+

r~\

^vO-CH„-CH

-0 2

0 \_y


8

POLYMERS IN ELECTRONICS

/wvO-CH -CH -q 2

0

2

->

r^ss- 0 - C H - C H + 2

2

+

0

Applications of Photoinitiated Cationic Polymerization to the

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