Sensitivity of Polymer Blends to Synchrotron Radiation - American

Chapter 13 Sensitivity of Polymer Blends to Synchrotron Radiation 2

J. A. Jubinsky1, R. J. Groele1, F. Rodriguez1, Y. M. N. Namasté, and S. K. Obendorf Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 19, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch013

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1School of Chemical Engineering, Olin Hall, Cornell University, Ithaca, NY 14853 Department of Textiles and Apparel, Martha Van Rensselaer Hall, Cornell University, Ithaca, NY 14853

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The sensitivity of poly(methyl methacrylate), PMMA, to x-rays is enhanced by the addition of poly(epichlorohydrin). The two polymers are miscible as shown by thermal analysis and by optical clarity of blends. Films of PMMA with 20 to 30% poly(epichlorohydrin) require only 1/2 to 1/4 the exposure to produce patterns compared to PMMA alone. Thinning-exposure curves also suggest a sensitivity increase of 3 or 4 times. Contrast suffers somewhat, but patterns are producible when a nitride-supported gold mask is used. When the resolving power of photolithography is to be exceeded, the two technologies that are most often called upon are electron-beam and x-ray lithography. For purposes of discussion, the lower limit of features made by optical (deep UV) methods may be placed at about 0.5 um. Electron beams currently can be used to produce features from 1.0 to 0.25 um. Since backscattering is not a problem with x-rays, the minimum dimensions obtainable with x-rays in thick films may well be smaller than those obtained with electron beams. Moreover, x-ray lithography is efficient for large volume production of chips. Electron-beam technology has been widely used for some time in mask-making and for customized, low-volume chip production, so a considerable body of information has been acumulated on the response of polymers to 20- to 50-keV electrons. A similar body of information does not exist on the response to x-rays. There are currently three types of x-ray sources available to researchers: anodic, in which high-energy electrons act on a metal target; plasma, in which IR or UV laser radiation acts on a metal target; and synchrotron, in which a beamline is attached to a high-energy electron storage ring. Anodic sources are the least expensive type, but they are from 100 to 10,000 times less intense than the other sources. 0097-6156/87/0346-0149$06.00/0 Φ 1987 American Chemical Society In Polymers for High Technology; Bowden, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 19, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch013

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As s o u r c e s t r e n g t h i s i n c r e a s e d a n d e x p o s u r e times reduced, s e n s i t i v i t y i s a l e s s urgent matter. When t h e response o f a g i v e n r e s i s t t o x - r a y s i s b e i n g measured, t h e w a v e l e n g t h o f t h e r a d i a t i o n h a s t o be t a k e n i n t o account a l s o . While s y n c h r o t r o n r a d i a t i o n i s i n t e n s e , t h e e n e r g y d i s t r i b u t i o n may n o t be c o n c e n t r a t e d i n t h e r e g i o n s c o r r e s p o n d i n g t o e f f i c i e n t a b s o r p t i o n by t h e atoms m a k i n g up t h e r e s i s t . S e v e r a l r e p o r t s have been p u b l i s h e d on x - r a y r e s i s t s in recent years. H a e l b i c h e t a l . ( 1 ) compared t h e p e r f o r m a n c e o f PMMA u s i n g s y n c h r o t r o n r a d i a t i o n w i t h t h a t u s i n g e-beam r a d i a t i o n . Yaakobi (4J e t a l . used a l a s e r - i o n x-ray source. The g e n e r a l t o p i c o f x - r a y r e s i s t s has been r e v i e w e d by Lane (3) a n d T a y l o r M ) . A v e r y o p t i m i s t i c view o f the f u t u r e f o r synchrotron-based l i t h o g r a p h y was t a k e n by W i l s o n ( 5 ) . More g u a r d e d p r e d i c t i o n s o f e v e n t u a l a p p l i c a t i o n o f x - r a y s have b e e n made by B r o e r s ( 6 ) a n d H e u b e r g e r ( 7 ) . P o l y m e t h y l m e t h a c r y l a t e ), (PMMA), i s one r e s i s t w h i c h i s e s p e c i a l l y f a v o r e d by r e s e a r c h e r s due t o i t s h i g h r e s o l u t i o n and c o n t r a s t . L i n e w i d t h s as s m a l l as 100 A have been p r o d u c e d w i t h an e x t r e m e l y h i g h d o s e o f x - r a y s ( 1 0 J/cm ) u s i n g PMMA. However, e v e n PMMA's n o r m a l s e n s i t i v i t y o f 6 0 0 - 1 0 0 0 mJ/cm i s t o o slow f o r commercial use. T h e r e have been many a t t e m p t s t o i m p r o v e t h e s e n s i t i v i t y o f PMMA t o e l e c t r o n beam i r r a d i a t i o n m a i n l y by c o p o l y m e r i z a t i o n ( 8 ) . I n t h e c u r r e n t p a p e r , we d e s c r i b e a n a l t e r n a t i v e a p p r o a c h whereby a s e n s i t i v e p o l y m e r i s p h y s i c a l l y b l e n d e d w i t h PMMA t o i n c r e a s e i t s s e n s i t i v i t y a n d y e t m a i n t a i n t h e good f i l m q u a l i t i e s a s s o c i a t e d w i t h PMMA a s a p o s i t i v e x - r a y r e s i s t . P o l y ( e p i c h l o r o h y d r i n ) , CO r u b b e r * ( H y d r i n ) , was chosen f o r v a r i o u s reasons. The one r e a s o n was t h a t CO has been shown t o be m i s c i b l e w i t h PMMA b y A n d e r s o n b a s e d upon d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) w h i c h showed o n l y one g l a s s t r a n s i t i o n t e m p e r a t u r e ( T ) f o r t h e b l e n d (9). Since Τ i s very s e n s i t i v e t o the d i s r u p t i o n o f the l o c a l s t r u c t u r e t h a t r e s u l t s when two p o l y m e r s a r e m i x e d , the e x i s t e n c e o f a s i n g l e g l a s s t r a n s i t i o n temperature i s a good i n d i c a t o r o f m i s c i b i l i t y ( 1 0 ) . R e c e n t l y , t h e m i s c i b i l i t y o f CO r u b b e r w i t h PMMA o v e r t h e e n t i r e range o f c o n c e n t r a t i o n s a n d m o l e c u l a r w e i g h t s has been c o n f i r m e d ( 1 1 ) . There i s a b r o a d e n i n g o f t h e g l a s s t r a n s i t i o n w h i c h r e a c h e s a maximum a t a b o u t 30% PMMA. However, e v e n a t t h a t c o n c e n t r a t i o n , t h e p o l y mers a r e t r u l y m i s c i b l e . S t u d i e s o f c o m p a t i b i l i t y o f CO r u b b e r a n d some c o p o l y m e r s have been r e p o r t e d w i t h o t h e r m e t h a c r y l a t e polymers and a l s o w i t h a c r y l a t e polymers o v e r a wide v a r i e t y o f c o n d i t i o n s ( 1 1 , 1 . 2 ) · A n o t h e r r e a s o n f o r b l e n d i n g w i t h CO i s t h a t CO c o n t a i n s c h l o r i n e w h i c h t e n d s t o enhance x - r a y a b s o r p t i o n , e s p e c i a l l y n e a r t h e a b s o r p t i o n edge o f 4 . 4 A. I t was hoped t h a t t h e i n c r e a s e i n a b s o r p t i o n w o u l d p r o d u c e more s e c o n d a r y *ASTM a b b r e v i a t i o n f o r t h i s

polymer.

In Polymers for High Technology; Bowden, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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e l e c t r o n s which would, i n t u r n , l e a d t o i n c r e a s e d s c i s s i o n i n g o f PMMA. Our e x p l o r a t o r y work i n d i c a t e d t h a t CO d e g r a d e s r a p i d l y on gamma r a d i a t i o n . Taylor e t a l . (13), using a v e r y h i g h m o l e c u l a r w e i g h t CO, h a s r e p o r t e d t h a t CO c r o s s l i n k s a p p r e c i a b l y when e x p o s e d t o Pd. x - r a y s . I n t h e p r e s e n t work no d e c r e a s e i n s o l u b i l i t y h a s been o b s e r v e d and o n l y a v e r y s l i g h t b r o a d e n i n g o f t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n h a s b e e n s e e n e v e n w h i l e t h e number average molecular weight decreased markedly. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 19, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch013

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Experimental

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CO r u b b e r ( H y d r i n 100) was o b t a i n e d f r o m t h e B . F . G o o d r i c h C h e m i c a l Company. B a s e d upon g e l p e r m e a t i o n c h r o m a t o g r a p h y (GPC) u s i n g t e t r a h y d r o f u r a n (THF) a s a c a r r i e r s o l v e n t , t h e CO h a s a p o l y s t y r e n e e q u i v a l e n t number a v e r a g e m o l e c u l a r w e i g h t (M ) o f 303 χ 10- a n d a w e i g h t a v e r a g e m o l e c u l a r w e i g h t (Λ ) o f 598 χ 10 . The g l a s s t r a n s i t i o n t e m p e r a t u r e i s -2u C L9). The PMMA u s e d was Rohm a n d Haas A-100 (M - 113 χ 10 , M - 213 χ 10 ) . Some a d d i t i o n a l b l e n d s were made u s i n g PMMA f r o m KTI (M - 360 χ 10 , M - 950 χ 10 ). T h r e e m i x t u r e s o f CO-PMMA were b l e n d e d t o g e t h e r f o r a n a l y s i s o f l i t h o g r a p h i c performance. Compositions o f 20, 33, a n d 50% CO b y w e i g h t were b l e n d e d b y d i s s o l u t i o n i n THF t o f o r m a 13% ( t o t a l s o l i d s ) s o l u t i o n . Cyclohexanone was added t o t h e b l e n d t o f o r m a 7% s o l u t i o n t o impart proper v i s c o s i t y and v o l a t i l i t y - f o r c a s t i n g f i l m s . In t h e c a s e o f PMMA w i t h M - 950 χ 10 , CO was d i s s o l v e d i n h o t c h l o r o b e n z e n e a n d t n e n m i x e d w i t h PMMA ( a l s o i n chlorobenzene). S o l v e n t c h o i c e was a m a t t e r o f convenience. The f i l m s were spun a t 1250 RPM f o r 1 m i n u t e o n t o three-inch s i l i c o n wafers. The w a f e r s were b a k e d a t 150 C f o r 1 h o u r i n a c o n v e c t i o n o v e n . Film thicknesses r a n g e d f r o m 0.8 t o 1.3 jum. P u r e PMMA f i l m s were a l s o p r e p a r e d u s i n g t h e same c a s t i n g s o l v e n t m i x t u r e a n d baking c o n d i t i o n s . G e l p e r m e a t i o n c h r o m a t o g r a p h y was u s e d t o d e t e r m i n e polymer m o l e c u l a r w e i g h t s . The model u s e d was a W a t e r s A s s o c i a t e s 201 HPLC w i t h 4 - j i S t y r a g e l c o l u m n s ( n o m i n a l p o r e s i z e s o f 500, 10 , 10 , a n d 10 A ) . THF was u s e d a s the c a r r i e r s o l v e n t . The m o l e c u l a r w e i g h t s were r e p o r t e d as p o l y s t y r e n e e q u i v a l e n t s w i t h t h e e x c e p t i o n o f PMMA f o r w h i c h a PMMA c a l i b r a t i o n s t a n d a r d was u s e d . G l a s s t r a n s i t i o n t e m p e r a t u r e s (T ) were m e a s u r e d u s i n g a P e r k i n E l m e r D i f f e r e n t i a l S c a n n i n g C a l o r i m e t e r (DSC) model DSC-2C. The x - r a y e x p o s u r e s were c a r r i e d o u t u s i n g s y n c h r o t r o n r a d i a t i o n d e l i v e r e d by t h e N a t i o n a l S y n c h r o t r o n L i g h t S o u r c e a t B r o o k h a v e n N a t i o n a l L a b o r a t o r i e s . The b e a m l i n e was b u i l t by IBM a n d h a s been d e s c r i b e d e l s e where ( 1_4, 1 ^ ) . I n summary, t h e s t o r a g e r i n g o p e r a t e s w i t h e l e c t r o n e n e r g i e s a t 750 MeV a n d a m a g n e t i c r a d i u s o f 1.91 m. The c u r r e n t i n t h e r i n g , w h i c h d e g r a d e s w i t h 5



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t i m e , r a n g e s between 7 5 a n d 2 5 0 mA. The a c t u a l power t h r o u g h t h e b e r y l l i u m w i n d o w a n d i n c i d e n t on t h e w a f e r r a n g e d f r o m 2 . 3 t o 7 . 5 mW/cm i n t h e p r e s e n t work. Samples were e x p o s e d t o s y n c h r o t r o n r a d i a t i o n t h r o u g h a g o l d - o n - b o r o n n i t r i d e mask a n d t h e n d e v e l o p e d u s i n g v a r i o u s m i x t u r e s o f i s o p r o p y l a l c o h o l (IPA) and methyl i s o b u t y l k e t o n e (MIBK). D e v e l o p m e n t was c o n d u c t e d u n d e r s t i r r e d c o n d i t i o n s u s i n g a constant temperature bath a t 23 C a n d was t e r m i n a t e d b y b l o w d r y i n g w i t h F r e o n g a s . F i l m t h i c k n e s s e s were m e a s u r e d u s i n g a T e n c o r A l p h a S t e p . E l e c t r o n beam e x p o s u r e s were p e r f o r m e d u s i n g a m o d i f i e d RCA model EMV-3 t r a n s m i s s i o n e l e c t r o n m i c r o scope. The a p e r t u r e was o p e n e d t o a l l o w t h e beam t o spread over t h e e n t i r e 3 - i n c h wafer g i v i n g a u n i f o r m exposure. The a c c e l e r a t i n g v o l t a g e was 5 0 keV, a n d t h e d o s e was m e a s u r e d u s i n g a f a r a d a y c u p ( 9 ) . Gamma i r r a d i a t i o n was c a r r i e d o u t a t C o r n e l l ' s Ward Reactor Laboratory, f w l k p o l y m e r s a m p l e s were i r r a d i a t e d under n i t r o g e n u s i n g CO s o u r c e a t a d o s e r a t e o f a b o u t 0.5 Mrad/hour. L a s e r i n t e r f e r o m e t r y was u s e d t o s t u d y t h e d i s s o l u tion rates of selected films ( 1 6 ,Γ 7 ) . A 6 3 2 . 8 nm wave l e n g t h beam o f u n p o l a r i z e d l i g E t f r o m a 2 mW He-Ne l a s e r was d i r e c t e d on a submerged w a f e r t h a t h a d b e e n c o a t e d w i t h a polymer f i l m . The a n g l e o f i n c i d e n c e was a p p r o x i mately 10 . R e f l e c t e d l i g h t from t h e p o l y m e r - s o l v e n t and p o l y m e r - s u b s t r a t e i n t e r f a c e s was d i r e c t e d t o w a r d a p h o t o c e l l w h i c h was c o u p l e d t o a c h a r t r e c o r d e r . A s t h e p o l y m e r s o l v e n t i n t e r f a c e b e g a n t o r e c e d e due t o d i s s o l u t i o n , c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e r e n c e from t h e i n t e r f a c e took p l a c e , and a s i n u s o i d a l o u t p u t was r e c o r d e d . By m e a s u r i n g t h e p e r i o d o f t h e s i n u s o i d a l c u r v e , t h e d i s s o l u t i o n r a t e was d e t e r m i n e d ( 1 6 , 1 7 ) .


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and D i s c u s s i o n

L i t h o g r a p h i c p e r f o r m a n c e o f t h e CO-PMMA b l e n d s was e v a l u a t e d by measuring t h e t h i c k n e s s o f r e s i s t r e m a i n i n g f o r v a r i o u s doses u s i n g a r b i t r a r i l y chosen development times. C o n t r a s t c u r v e s a r e shown f o r a 33% CO b l e n d a n d f o r PMMA i n F i g u r e 1. From t h e c o n t r a s t c u r v e s , t h i n n i n g c u r v e s were c o n s t r u c t e d b y p l o t t i n g t h e u n e x p o s e d n o r m a l i z e d t h i c k n e s s r e m a i n i n g v e r s u s t h e dose r e q u i r e d f o r complete d e v e l o p m e n t ( F i g u r e 2 ) . F o r l o w x - r a y d o s e s t h e 33% CO b l e n d e x h i b i t e d much l e s s t h i n n i n g t h a n PMMA. There i s a t r e n d toward i n c r e a s i n g s e n s i t i v i t y as t h e p e r c e n t a g e o f CO i s i n c r e a s e d ( F i g u r e 2 ) . F o r e x a m p l e , f o r 20% t h i n n i n g , t h e d o s e r e q u i r e d t o com p l e t e l y d e v e l o p t h e 3 3 a n d 50% CO b l e n d s a r e 2 a n d 3 t i m e s l e s s , r e s p e c t i v e l y , t h a n t h a t n e e d e d f o r PMMA. However, i n t h e h i g h d o s e r e g i m e where t h e a l l o w a b l e p e r c e n t a g e o f t h i n n i n g was r e d u c e d , t h e d i f f e r e n c e i n s e n s i t i v i t y narrowed. U n f o r t u n a t e l y , t h e enhanced s e n s i t i v i t y o f t h e CO b l e n d s o c c u r e d a t t h e e x p e n s e o f c o n t r a s t w h i c h was r e d u c e d b y 2 5 t o 40% when b l e n d i n g . No t r e n d w i t h r e s p e c t t o c o n t r a s t among t h e CO b l e n d s was



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Figure 1. C o n t r a s t curves f o r x-ray, flood-exposed f i l m s d e v e l o p e d one m i n u t e i n i n d i c a t e d m i x t u r e s o f i s o p r o p l y a l c o h o l a n d m e t h y l i s o b u t y l k e t o n e , ( a ) PMMA and ( b ) CO:PMMA, 1 : 2 . 1.0 r

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F i g u r e 2 . T h i n n i n g c u r v e s f o r PMMA a n d v a r i o u s b l e n d s c o n t a i n i n g ( a ) 5 0 % , ( b ) 3 3 % , ( c ) 2 0 % , a n d ( d ) 0% CO by weight. The numbers i n p a r e n t h e s e s g i v e t h e f r a c t i o n o f i s o p r o p y l a l c o h o l i n t h e s o l v e n t ( b a l a n c e o f MIBK). The gamma v a l u e ( c o n t r a s t ) a l s o i s g i v e n .



154


noticed. The 33% b l e n d a p p e a r e d t o have t h e b e s t c o n t r a s t , b u t i t was s t i l l s i g n i f i c a n t l y l o w e r t h a n t h a t o f t h e PMMA. The mechanism b e h i n d t h e i n c r e a s e i n t h e s e n s i t i v i t y of t h e b l e n d appears t o i n v o l v e t h e s c i s s i o n i n g and d i s s o l u t i o n p r o p e r t i e s o f CO. A d d i n g CO t o PMMA i n c r e a s e s t h e d i s s o l u t i o n r a t e o f t h e b l e n d a s compared t o PMMA. When t h e b l e n d i s e x p o s e d t o r a d i a t i o n , t h e CO s c i s s i o n s ( f a s t e r t h a n t h e PMMA) a n d i t s m o l e c u l a r w e i g h t i s decreased. The d e c r e a s e i n m o l e c u l a r w e i g h t o f t h e CO i s apparently responsible for the increased d i s s o l u t i o n rate of the blends. T h i s has been c o n f i r m e d by comparing t h e d i s s o l u t i o n r a t e s o f u n e x p o s e d C0-PMMA b l e n d s w i t h two d i f f e r e n t m o l e c u l a r w e i g h t s o f CO. A 33% ( b y w e i g h t ) CO b l e n d c o n t a i n i n g CO w i t h M - 8 6 , 0 0 0 d i s s o l v e d 8 t i m e s f a s t e r t h a n a b l e n d c o n t a i n i n g CO w i t h M - 3 0 3 , 0 0 0 . T h u s , t h e i n c r e a s e i n t h e d i s s o l u t i o n r a t e between t h e e x p o s e d a n d u n e x p o s e d r e g i o n s o f t h e b l e n d s a p p e a r s t o be p r i m a r i l y due t o t h e s c i s s i o n i n g o f t h e CO. The o r i g i n a l h y p o t h e s i s t h a t t h e CO e n h a n c e s s c i s s i o n i n g by s u p p l y i n g s e c o n d a r y e l e c t r o n s has n o t been s u p p o r t e d by t h i s e x p e r i m e n t . From t h e d a t a , i t a p p e a r s t h a t t h e enhancement o f d i s s o l u t i o n r a t e b y s c i s s i o n i n g o f CO may a c c o u n t t o t a l l y f o r t h e i n c r e a s e d s e n s i t i v i t y of the blends. T h i s p o i n t c o u l d be c l a r i f i e d b y a f u r t h e r experiment i n which the molecular weight o f the PMMA i n t h e b l e n d i s i n d e p e n d e n t l y m o n i t o r e d upon exposure· A t h i g h d o s e s , t h e a b i l i t y o f t h e CO t o i n f l u e n c e t h e d i s s o l u t i o n r a t e was d i m i n i s h e d s i n c e i t s a b s o l u t e m o l e c u l a r weight changed l e s s s i g n i f i c a n t l y . I t appears t h a t t h e m a i n mechanism f o r i n c r e a s i n g d i s s o l u t i o n a t h i g h e r d o s e s i s n o t t h e d e g r a d a t i o n o f CO, b u t t h e s c i s s i o n i n g o f PMMA. T h i s accounts f o rthe apparent s i m i l a r i t y i n b e h a v i o r o f PMMA a n d t h e b l e n d s a t h i g h d o s e s . Thus, t h e i n c r e a s e d s e n s i t i v i t y o f t h e b l e n d s i s r e a l i z e d o n l y i n the " f o r c e d d e v e l o p i n g " regimes. The r e l a t i v e r e s p o n s e o f CO/PMMA b l e n d s t o v a r i o u s f o r m s o f r a d i a t i o n c a n be compared. In the f i r s t place, gamma r a d i a t i o n o f b u l k s a m p l e s o f CO o r PMMA r e s u l t s i n v a l u e s o f G ( s ) - 5 . 1 s c i s s i o n s / 1 0 0 eV f o r CO compared t o 0 . 8 f o r PMMA ( F i g u r e 3 ) . I t i s i n t e r e s t i n g t h a t CO undergoes s c i s s i o n i n g w i t h l i t t l e c r o s s l i n k i n g (G(x) 0 . 5 a t most). I t i s presumably the ether l i n k a g e i n combination with t h e s u b s t i t u t e d c h a i n carbon which l e a d s to s c i s s i o n . Poly(ethylene oxide), i n contrast, c r o s s l i n k s r e a d i l y , even i n d i l u t e s o l u t i o n as l o n g as oxygen i s e x c l u d e d ( 1 8 ) . T a y l o r ( 1 1 ) found t h a t t h e d i s u b s t i t u t e d homolog, p o l y ( c i s - 1 , î ^ d i c h l o r o m e t h y l o x i r a n e ) , was u n e q u i v o c a l l y a s c i s s i o n i n g compound, b u t t h a t CO a p p e a r e d t o c r o s s l i n k somewhat. As mentioned e a r l i e r , t h i s may have b e e n due t o t h e h i g h e r m o l e c u l a r w e i g h t CO w h i c h he e m p l o y e d . The s e n s i t i v i t y o f CO t o e l e c t r o n s a n d x - r a y s was c h a r a c t e r i z e d b y c o m p a r i n g a b l e n d w i t h PMMA t o PMMA



13.

JUBINSKY ET A L .



Radiation

155

alone. I n a beam o f 50 keV e l e c t r o n s , a b l e n d w i t h a CO t o PMMA r a t i o o f 1 : 2 e x h i b i t s a s l o p e a b o u t 2 . 9 t i m e s t h a t o f PMMA i n a p l o t o f 1/M v e r s u s d o s e ( F i g u r e 4 ) . The p o l y d i s p e r s i t y , Μ /Μ , o f t h e b l e n d (shown on t h e same p l o t ) i n d i c a t e s t h a t t h e CO d e g r a d e d f a s t e r t h a n t h e PMMA. The p o l y d i s p e r s i t y o f n e a t PMMA r e m a i n s between 1 . 9 a n d 2 . 3 u n d e r t h e same e x p e r i m e n t a l conditions. Synchrotron r a d i a t i o n y i e l d e d a s i m i l a r pattern t o t h e e-beam r e s u l t when t h e same b l e n d a n d PMMA were compared ( F i g u r e 5 ) . The s l o p e f o r t h e b l e n d i s a b o u t 2 . 7 t i m e s t h e s l o p e f o r PMMA. W h i l e c h l o r i n e m i g h t be e x p e c t e d t o e n h a n c e t h e s e n s i t i v i t y t o x - r a y s more t h a n e i t h e r gamma r a d i a t i o n o r e l e c t r o n s , t h e p r e s e n t work d o e s n o t d e m o n s t r a t e t h i s effect. The most p r o b a b l e r e a s o n i s t h a t t h e s y n c h r o t r o n beam was o p e r a t i n g u n d e r c o n d i t i o n s where t h e i n t e n s i t y was n o t h i g h a t t h e a b s o r p t i o n edge f o r c h l o r i n e ( 4 . 4 A ) . The d i s s o l u t i o n b e h a v i o r o f CO-PMMA b l e n d s was e x a m i n e d f o r two m o l e c u l a r w e i g h t s o f PMMA ( T a b l e 1 ) . I n e a c h c a s e , t h e d i s s o l u t i o n r a t e was i n c r e a s e d b y a d d i t i o n o f CO. Two a d d i t i o n a l f e a t u r e s c a n be n o t e d . The a m p l i t u d e o f t h e o s c i l l a t i o n s i n r e f l e c t e d l i g h t i n t e n s i t y appears t o remain c o n s t a n t as t h e b l e n d s dissolve. T h i s would i n d i c a t e t h a t t h e u n d i s s o l v e d p o r t i o n o f the f i l m i s not undergoing s w e l l i n g or e x t r a c t i o n t o any n o t i c e a b l e e x t e n t . The s e c o n d f e a t u r e , o b s e r v a b l e e s p e c i a l l y i n t h e h i g h m o l e c u l a r w e i g h t PMMA b l e n d , i s a n i n c r e a s e d o f f s e t between t h e maximum a m p l i t u d e d u r i n g d i s s o l u t i o n and t h e r e f l e c t i o n from t h e bare wafer a f t e r d i s s o l u t i o n i s complete ( F i g u r e 6 ) . T h i s o f f s e t h a s been i n t e r p r e t e d a s r e p r e s e n t i n g a t r a n s i t i o n l a y e r on t h e s u r f a c e o f t h e d i s s o l v i n g f i l m (19).

The i n c r e a s e i n t h e o f f s e t means t h a t t h e CO-PMMA blend has a t h i c k e r s u r f a c e l a y e r . On t h e o t h e r hand, the f a c t t h a t t h e o f f s e t i s o n l y v e r y s l i g h t l y i n c r e a s e d by h i g h m o l e c u l a r w e i g h t CO w i t h t h e l o w e r m o l e c u l a r w e i g h t s o f PMMA means t h a t t h e t r a n s i t i o n l a y e r i s p r o b a b l y n o t due t o e n t a n g l e d CO m o l e c u l e s r e m a i n i n g on the s u r f a c e . Conclusions The s e n s i t i v i t y o f PMMA c a n be i n c r e a s e d b y a d d i n g CO i n a blend. The s e n s i t i v i t y o f t h e b l e n d i n c o m p a r i s o n t o t h e PMMA i n c r e a s e d u n d e r f o r c e d d e v e l o p i n g c o n d i t i o n s ( i . e . longer d e v e l o p i n g times f o r development o f lower doses). However, u n d e r s e v e r e c o n d i t i o n s , t h e c o n t r a s t o f t h e b l e n d s s u f f e r e d g r e a t l y w h i c h w o u l d make i t d i f f i c u l t t o f o r m good l i t h o g r a p h i c p a t t e r n s . A t 15 t o 20% t h i n n i n g , t h e 3 3 a n d 50% CO b y w e i g h t b l e n d s s t i l l show a 2 - t o 3 - f o l d i n c r e a s e i n s e n s i t i v i t y o v e r t h a t o f PMMA. However, t h i s o c c u r r e d a t t h e e x p e n s e o f a 2 5 t o 40% r e d u c t i o n i n c o n t r a s t .




I40r

Poly(epichloro hydri n)

20

M rad

30

F i g u r e 3. S e n s i t i v i t y t o gamma r a d i a t i o n ( C o b a l t • CO, G(s) = 5.1; Ο PMMA, G(S) = 0.8.

source).

F i g u r e 4. S e n s i t i v i t y t o 50 keV e l e c t r o n s f o r ( o ) PMMA and (· )C0:PMMA, 1:2. The s l o p e f o r the b l e n d i s 2.9 times t h a t f o r PMMA.


JUBINSKY ET A L .



Radiation


80 r

F i g u r e 5. S e n s i t i v i t y t o x - r a y s ( f l o o d e x p o s u r e t h r o u g h b e r y l i u m window) f o r same m a t e r i a l s a s i n f i g u r e 4. The s l o p e f o r t h e b l e n d i s 2.7 t i m e s t h a t f o r PMMA. P o l y d i s p e r s i t y f o r PMMA r a n g e s f r o m 1.9 t o 2.3 a t a l l d o s e s . T a b l e 1.

M

n

PMMA a l o n e D i s s . Rate f**

o f PMMA

360 χ 1 0 29 χ 1 0

D i s s o l u t i o n i n MIBK, 30°C

3

3

CO:PMMA, 1:4 D i s s . Rate f

0.031

0.80

0.27

0.61

0.14

0.97

0.75

0.94

* * O s c i l l a t i o n amplitude reduction f a c t o r (19), f - (a b ) / ( a - b + 2 s ) , where a a n d b a r e maximum a n d minimum r e f l e c t e d l i g h t i n t e n s i t i e s d u r i n g d i s s o l u t i o n and s i s the o f f s e t ( F i g u r e 6 ) .

1

minutes

—

Time

>

F i g u r e 6. D i s s o l u t i o n r a t e s m e a s u r e d b y i n t e r f e r o m e t r y u s i n g MIBK a t 30°C f o r ( a ) h i g h m o l e c u l a r w e i g h t PM/1A a n d ( b ) CO : PMMA, 1:4.



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Acknowledgments This work was supported in part by the office of Naval Research. Some of the work was performed at the National Research and Resource Facility for Submicrometer Structures at Cornell which is partially supported by the NSF under grant ECS-8200312. Special thanks go to Dr. Jerry Silverman (IBM Corporation) for his help with the x-ray exposures at Brookhaven National Laboratories and Mr. Howard Aderhold (Ward Reactor Laboratory, Cornell) for the gamma radiation exposures. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Haelbich, R. P., et a l . J. Vac. Sci. Tech. 1983, B1(4), 1072. Yaakobi, B., et al., Applied Physics Letters, 1983, 43, 686. Lane, J. G., Proc. SPIE, 1984 448, 119. Taylor, G. Ν., Solid State Tech. 1980, 23(5), 73. Wilson, A. D., Solid State Tech. 1986, 29(5), 249. Broers, A. N., Solid State Tech. 1985, 28(6), 119. Moreau, W. M., Optical Engineering 1983, 22(2), 18. Heuberger, Α., Solid State Tech. 1986, 29(2), 93. Anderson, C. C., Rodriguez, F., Proc. ACS Div. Polym. Matls,: Sci. & Eng. 1984, 51, 609. Olabisi, O., in Encyclo. Chem. Tech. 3rd, ed., Wiley, New York, 1982, Vol. 18, p. 443. Fernandes, A. C., Barlow, J. W., and Paul, D. R. J. Appl. Polym. Sci., 32, 5481 (1986). Fernandes, A. C., Barlow, J. W., and Paul, D. R., J. Appl. Polym. Sci., 32, 6073 (1986). Taylor, G. Ν., Coquin, G. Α., Somekh, S., Polym. Eng. Sci., 1977, 17, 420. Silverman, J. P., Haelbich, R. P., Grobman, W. D., Warlaumont, J. Μ., Proc. SPIE, 1983 393 99. Haelbich, R. P., Silverman J. P., Warlaumont, J. M., Nuc. Inst. Meth., 1984, 222, 291. Rodriguez, F., Krasicky, P. D., Groele, R. J., Solid State Tech., 1985 28(5), 125. Rodriguez, F., Groele, R. J., Krasicky, P. D., Proc. SPIE, 1985, 539, 14. van Brederode, R. Α., Rodriguez, F., Cocks, G. G., J Appl. Polym. Sci., 1968, 12, 2097. Krasicky, P. D., Groele, R. J. Jubinsky, J. Α., Rodriguez, F., Namaste, Υ. Μ. Ν., Obendorf, S. Κ., Proc. 7th Int. Conf. on Photopolymers, SPE, 1985, p. 237.

RECEIVED

April 8, 1987 In Polymers for High Technology; Bowden, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Sensitivity of Polymer Blends to Synchrotron Radiation - American

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