Ultraviolet Light Induced Reactions in Polymers

9 Applications of Radiation Sensitive Polymer Systems W . M O R E A U and N . V I S W A N A T H A N

Downloaded by CORNELL UNIV on July 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch009

I B M Corp., Systems Products Division, E . Fishkill, N.Y. 12533

Over the l a s t f i v e years, s i g n i f i c a n t advances have been made i n the r a d i a t i o n processing of commercial products. The products range i n size from cured films on automobiles to picosecond microelectronic devices 500A° i n three dimensional s i z e . Four major techno l o g i c a l areas encompass the scope of the applied tech nologies, ( c f . , Table I ) . These include; (1) the r a d i a t i o n curing or a l t e r a t i o n of coatings and f i b e r s , (2) the photoproduction of lithographic p r i n t i n g plates, (3) the processing of f a b r i c a t i n g e l e c t r o n i c devices with r e s i s t s , and (4) the commercial development of photodegradable p l a s t i c s . The treatment of photoconductive polymers for xerography w i l l not be reviewed but i s treated i n several reviews (see bibliography). In this report we w i l l survey the catagories of a p p l i c a tion by examining the chemical classes of polymeric systems, the processing sequences, and the e f f i c i e n c y of the various systems. The exposure of polymeric systems has involved the full electromagnetic spectrum (Table I I ) . The i r r a d i a tion wavelength i s usually i n excess of the bond energy of the carbon bonds or i n excess of the a c t i v a t i o n energy of chemical reaction. High energy x-rays, gamma, ion, electron, u l t r a v i o l e t and i n f r a r e d beams have been used to degrade, polymerize, or c r o s s l i n k polymeric f i l m s . The most widely used wavelengths are the 3000-5000A region of mercury lamps for the expo sure of polymer f i l m s . The thicknesses of exposed films range from Angstroms to thousands of microns (cf., Table I I I ) . 0

107 Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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TABLE I


R a d i a t i o n P r o c e s s i n g o f polymers Degradation

Polymerization

Grafting

Crosslinking

Polymer

Fiber treatment

Negative resists

Positive resists

Ink C u r i n g

Membrane

Ink curing

Photodegrading plastics

Coating curing

"Casing"

Coating curing Dental c o a t i n g s; Heat shrinks

Plastic weathering Paint weathering Polymer analysis

Synthesis

TABLE I I Radiation source

Exposure Sources of Polymer Approximate Energy Wavelength A° Kcal 3

290 x l O 6 28 x l O 5 35 xlO

X-Rays

1-10

Gamma(1MEV)

0.01

Ε-Beam(100KV)

0.008

E-beam(lOeV)

1200

250

UV UV

1000-3000 3000-4000

190 100

UV

4000-5000

60

Laser

3000-6000

70

3

10.6 μ

Laser ION(IOOKV) ION (lOev)

0.13 1000

2

xlO 300

6

Systems Application Resist exposure Polymer synthesis Resist exposure Plasma processing UV c u r i n g Ink C u r i n g Lithography Photoresists Graphic A r t s Holography Resist exposure Plastic Evaporation Resist Etching Plasma Chem. treatment o f films, fibers, membranes·

Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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The p o l y m e r i c systems are u s u a l l y composed of a polymer which imparts the m a j o r i t y of p h y s i c a l p r o p e r t i e s and a c t i n i c a d d i t i v e s . In simple systems such as c u r i n g f i l m s or e l e c t r o n beam r e s i s t s , the polymer i s a l s o the r a d i a t i o n s e n s i t i v e s p e c i e s . In most c a s e s , the f o r m u l a t i o n s behave s i m i l a r i l y i n t h e i r response to h i g h energy i r r a d i a t i o n . P r a c t i c a l l y any polymer can be made r a d i a t i o n s e n s i t i v e by d i r e c t exposure to i o n i z i n g e n e r g i e s or by f o r m u l a t i o n w i t h a d d i t i v e s such as f r e e r a d i c a l p r e c u r s o r s . Thermally s e n s i t i v e poly mers are a l s o l i k e l y to undergo a s i m i l a r r e a c t i o n when exposed. The p r o c e s s i n g of polymer f i l m s i n v o l v e s wet and dry t e c h n i q u e s . Some systems are s o l v e n t l e s s l a m i n a t e s such as dry r e s i s t s . In most cases one-dimen s i o n a l f i l m s r e q u i r e o n l y c o a t i n g and e x p o s u r e . For m u l t i d i m e n s i o n a l images, s o l v e n t s are used to form images by d i s s o l u t i o n of unwanted a r e a s . A general t r e n d f o r economic and e c o l o g i c a l purposes w i l l i n v o l v e s o l v e n t l e s s c o a t i n g s and dry p r o c e s s i n g . The i n c r e a s e d i n t e r e s t i n energy s e n s i t i v e p o l y mers p r o b a b l y e v o l v e s from the shortcomings of the c o n v e n t i o n a l image r e c o r d i n g m e d i a - s i l v e r h a l i d e emulsion. I t i s both d i f f i c u l t and e x p e n s i v e to a p p l y emulsion f i l m s as p r o t e c t i v e l a y e r s or use as p r i n t i n g p l a t e s or e t c h r e s i s t s . S y n t h e t i c polymers are one to two o r d e r s of magnitude l e s s r a d i a t i o n s e n s i t i v e than h a l i d e e m u l s i o n , but t h e i r unique p r o p e r t i e s of chemi c a l and p h y s i c a l r e s i s t a n c e e a s i l y overcome t h i s d i s advantage, ( c f . , T a b l e I V ) . R a d i a t i o n s e n s i t i v e f i l m s are a v a i l a b l e from a wide source of cheap s y n t h e t i c monomers or polymers. The r e s o l u t i o n of p o l y m e r i c f i l m s g r e a t l y exceeds t h a t of s i l v e r e m u l s i o n . Images as m i c r o s c o p i c as 50A° l i n e s and spaces (200,000 lines/mm.) have been r e c o r d e d i n r e s i s t f i l m s by exposure i n a s c a n n i n g e l e c t r o n m i c r o s c o p e . M o l e c u l a r a d d i t i v e s such as diazonium s a l t s or o r g a n i c h a l i d e s can be added to produce c o l o r e d images of h i g h o p a c i t y . P o l y m e r i c systems can be e n v i s i o n e d as meeting a m a j o r i t y of t e c h n o l o g i c a l needs f o r i n f o r m a t i o n r e c o r d i n g and i n the f a b r i c a t i o n of m i n i a t u r e e l e c t r o n i c or m e c h a n i c a l components. In the a p p l i e d t e c h n o l o g i e s , the c u r i n g of p a i n t s or i n k s by e l e c t r o n beam or u l t r a v i o l e t l i g h t has


UV

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TABLE I I I Exposure P e n e t r a t i o n i n P r o c e s s i n g


Exposing Radiation

Depth o f Penetration

Plasma (UV E m i s s i o n ) Plasma(1-lOeV) Ion (60KV) E-Beam(20KV) Ε-Beam(1MEV) X-Rays (60KV) UV (3650A ) 0

Polymer T h i c k n e s s

1-10 / i 0.005-0.05 2 8 150 10 1-125 p

μ μ μ μ

Polymers

Surface μ

Treatment

S u r f a c e Treatment 0.1 - 1 μ 0.1 - 2 μ 25 - 200 μ 0.5 - 2.0 μ 0.1 - 10

μ

T a b l e IV Comparison of C o n v e n t i o n a l S i l v e r Emulsion and U n c o n v e n t i o n a l Polymer Image R e c o r d i n g Silver

Emulsion F i l m

Higher c o s t Higher p r o p r i e t a r y formulations Precoated films Thickness r e s t r i c t i o n R e s o l u t i o n to Ija Wet C h e m i c a l P r o c e s s High speed (lO'^cm^/erg) Poor c h e m i c a l r e s i s t a n c e Poor p h y s i c a l r e s i s t a n c e One or two d i m e n s i o n a l images Quantum a m p l i f i c a t i o n 200-1000 nm

sensitivity

Polymeric

Films

Low cost Wide p a t e n t base Easily applied Molecular films R e s o l u t i o n to .00]yu Dry and Wet P r o c e s s i n g Slow speed (lO'^cm^/erg Excellent resistance Excellent resistance Multi-dimensional images Low quantum a m p l i fication 200-500 nm s e n s i tivity



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f i n a l l y been a c c o m p l i s h e d on an i n d u s t r i a l s c a l e . The e l e c t r o n beam c u r i n g of c o a t i n g s has f o r some time a t t r a c t e d a g r e a t d e a l of a t t e n t i o n s i n c e i t i s c l a i m e d t h a t c o a t i n g s can be f u l l y cured i n a f r a c t i o n of a JL second. The c o a t i n g s a r e p r i m a r i l y based on the c r o s s l i n k i n g o f a c r y l i c or epoxy r e s i n s i n a p o l y u r e t h a n e or p o l y e s t e r base ( i . e . , m o d i f i e d p a i n t s ) . E l e c t r o n beam c u r i n g has reached f u l l y commercial s t a g e s i n the USA and Europe.— P r o t e c t i v e coatings have been e l e c t r o c u r e d on automotive f i n i s h e s , wood p r o d u c t s , p l a s t i c p r o d u c t s , and s t r u c t u r a l m e t a l s . The p r o d u c t s u s u a l l y have to be f l a t f o r the most e f f i c i e n t processing. The h i g h s e n s i t i v i t y of s o l v e n t l e s s c o a t i n g s c o u p l e d w i t h more e f f i c i e n t s o u r c e s has enabled p r o c e s s speeds to a t t a i n 50-100 f e e t per ο

minute S i n c e the e l e c t r o n beam (150 KV) p e n e t r a t e s and c u r e s f i l m s up to 50 m i l s i n t h i c k n e s s e s , the f i l m s can be h i g h l y pigmented ( p r e f e r a b l y w i t h h i g h atomic number a d d i t i v e s to enhance e l e c t r o n absorption). The u l t r a v i o l e t c u r i n g of i n k s and c o a t i n g s has a l s o r e c e i v e d new a t t e n t i o n . The renewed i n t e r e s t i n c u r i n g i n k s has been i n f l u e n c e d by e c o l o g i c a l and eco nomic c o n s i d e r a t i o n s T h e i n k s can be f o r m u l a t e d as s o l v e n t f r e e systems which a r e s e n s i t i z e d to mercury 3650A i r r a d i a t i o n . The c u r i n g energy i s o n l y a f r a c t i o n of t h a t needed w i t h heat c u r a b l e systems. The i n k f o r m u l a t i o n s u s u a l l y c o n t a i n an u n s a t u r a t e d monomer or prepolymer, a r a d i c a l p r e c u r s o r and/ or an a c t i n i c s e n s i t i z e r , a polymer v e h i c l e base, and pigmentation. Recent f o r m u l a t i o n s i n c l u d e p o l y u n s a t u r a t e d p o l y e n e s , and p o l y a c r y l a t e s w i t h t h i o l or ben z o i n r a d i c a l s o u r c e s . ! The a c i d or h y d r o x y l c o n t e n t of the r e s i n base i s u s u a l l y v a r i e d to a d j u s t the water o r o l e o p h i l l i c w e t t i n g a c t i o n of the c o m p o s i t i o n . A l t h o u g h the p e n e t r a t i o n range of u l t r a v i o l e t l i g h t i s o n l y about 25 microns (1-2 m i l s ) , the c u r i n g time i s i n the second range and throughputs approach the speeds of e l e c t r o n beam cured f i l m s . The e l e c t r o n and u l t r a v i o l e t c u r i n g of r a d i a t i o n can be compared as follows. E l e c t r o n c u r i n g has a wider range of a p p l i c a t i o n and i s s u i t a b l e f o r t h i c k e r , pigmented systems. E l e c t r o n beam s o u r c e s a r e more e x p e n s i v e to o p e r a t e and not r e a d i l y adapted to a s m a l l i n d u s t r i a l s c a l e or f o r 0



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small parts. Both t e c h n i q u e s overcome the s o l v e n t p o l l u t i o n of heat c u r a b l e c o a t i n g s but a r e s t i l l r e s t r i c t e d to m a i n l y f l a t s u r f a c e s . In the p r i n t i n g i n d u s t r y one of the a p p l i c a t i o n s of p h o t o p o l y m e r i z a t i o n i s i n the area of g e n e r a t i n g l i t h o g r a p h i c p r i n t i n g p l a t e s . As i s commonly known i n a l l l i t h o g r a p h i c systems, the p r i n t i n g p l a t e s c o u l d be generated by p o s i t i v e l y or n e g a t i v e l y a c t i n g r e s i s t systems. The p r i n t i n g p l a t e s can be b r o a d l y c l a s s i f i e d as S u r f a c e p l a t e s or Deep e t c h p l a t e s . In the former c l a s s , the p h o t o p o l y m e r i s e d c o a t i n g i s a hard r e s i d u e on the developed p l a t e ; and the p o l y m e r i s e d m a t e r i a l i s u s u a l l y o l e o p h i l l i c and h y d r o phobic. The background ( u s u a l l y a m e t a l s u b s t r a t e ) i n c o n t r a s t i s u s u a l l y h y d r o p h i l l i c and o l e o p h o b i c . In a c t u a l p r a c t i c e , the background m e t a l l a n d s a r e p e r i o d i c a l l y d e s e n s i t i z e d w i t h a s u s p e n s i o n of c o l l o i d a l p a r t i c l e s i n an a c i d i c medium. The Deep e t c h p l a t e s , on the o t h e r hand, a r e u s u a l l y etched m e t a l p l a t e s u s i n g a n e g a t i v e l y working p h o t o r e s i s t and an e t c h i n g l a c q u e r . In the i n d u s t r y the Deep e t c h p l a t e s have found wider usage i n view of t h e i r m e c h a n i c a l s t a b i l i t y towards p r i n t i n g more copies. From the m a t e r i a l s p o i n t of view, the p r i n t i n g i n d u s t r y has used a l l p o s s i b l e c o m p o s i t i o n s of photopolymer i z a b l e p r e p o l y m e r s . Commonly used systems a r e m a t e r i a l s w i t h u n s a t u r a t e d m o i e t i e s as e s t e r s , k e t o n e s , e t h e r s , and m o d i f i c a t i o n s of t h e s e . Of these c l a s s e s , b r o a d e s t p a t e n t coverage seems to c e n t e r around a c r y l i c e s t e r s and d e r i v a t i v e s o f a c r y l a t e systems, e s p e c i a l l y i n the a r e a s of s u r f a c e p r i n t i n g p l a t e s . The major emphasis i n t h i s type of l i t h o g r a p h i c p l a t e s seems to be on the m e c h a n i c a l s t a b i l i t y of the p o l y m e r i z e d image to s u s t a i n r e p e a t e d impacts w i t h o u t s e r i o u s l o s s of r e s o l u t i o n and d e f o r m a t i o n of the images. From t h i s p o i n t of view, i n c r e a s e d m e c h a n i c a l s t r e n g t h ob t a i n e d by extraneous c r o s s l i n g i n g agents have found the w i d e s t a p p l i c a b i l i t y to o b t a i n r i g i d s t r u c t u r e s . On the other hand, i n the areas of Deep e t c h p l a t e s , the emphasis seems to be on the s t a b i l i t y and e t c h r e s i s t a n c e of the images ( t o w i t h s t a n d c h e m i c a l a g e n t s ) n e c e s s i t a t e the use of c h e m i c a l l y s t a b l e c o m p o s i t i o n s (such as c y c l i z e d i s o p r e n e , e p o x i d i s e d e s t e r s ) . Ad d i t i o n a l f a c t o r s such as a d h e s i o n of the p h o t o r e s i s t



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to the m e t a l p l a t e a f f e c t the q u a l i t y of the f i n a l image. The s e n s i t i z e r s and i n i t i a t o r s used i n the p r i n t i n g p l a t e i n d u s t r i e s are b a s i c a l l y w e l l known s e n s i t i z e r s i n the p h o t o c h e m i c a l f i e l d . Most of these have s t r o n g a b s o r p t i o n i n the near U V - v i s i b l e s o u r c e s , Hg lamps occupying the major r o l e i n p r e s e n t - d a y t e c h n o logy. T a b l e V i s a comprehensive summary of some im p o r t a n t s e n s i t i z e r s and prepolymers used i n the p h o t o lithographic f i e l d . From p r o c e s s i n g p o i n t of view, emphasis has been p l a c e d h e a v i l y on s u b s t i t u t i n g hard p l a s t i c s i n the p l a c e of metals used i n the p a s t , w i t h the major ad vantage of reduced c o s t s i n v o l v e d i n f a b r i c a t i o n . Al most every p o s s i b l e p h y s i c a l and c h e m i c a l p r o p e r t y has been experimented w i t h to o b t a i n the n e c e s s a r y c o n t r a s t between the exposed and unexposed areas of the l i t h o g r a p h i c p l a t e s . P h y s i c a l p r o p e r t y d i s t i n c t i o n s based on a d h e s i o n , thermal p r o p e r t i e s ( m e l t i n g p o i n t , f o r example) have paved the way f o r the long sought a f t e r dry p r o c e s s i n g t e c h n i q u e s . The t r e n d seems to be i n the d i r e c t i o n of d e p a r t i n g from wet c h e m i c a l p r o c e s s i n g (such as s o l u b i l i t y methods) techniques. I t should a l s o be noted t h a t emphasis seems to be i n the d i r e c t i o n of a v o i d i n g p r o c e s s i n g s t e p s under darkroom c o n d i t i o n s . S e v e r a l commercial c o m p o s i t i o n s a r e o b t a i n a b l e i n the l a m i n a t e form, u s u a l l y w i t h an i n e r t oxygen i n s e n s i t i v e top f i l m to a v o i d quenching e f f e c t s caused by oxygen d u r i n g expo sures · To improve the m e c h a n i c a l s t r e n g t h and hardness c a l l f o r the use of c u r i n g agents ( u s u a l l y a c t i v a t e d by a bake step a f t e r development) i n the p h o t o p o l y merizing composition. T y p i c a l c u r i n g agents mentioned have been a l k y l h y d r o p e r o x i d e s , b i f u n c t i o n a l amines, a c i d s , e t c . The l a s t c l a s s of compounnds have been the most p o p u l a r a d d i t i v e s to the epoxy based polymer compositions. A d d i t i o n a l l y , the use of i n e r t f i l l e r s i s a l s o known. A r e c e n t example of newer c l a s s e s of compounds i n p h o t o s e n s i t i v e p r i n t i n g p l a t e i n d u s t r y i s the use of s u l f e n y l c a r b o x y l a s e s and a r y l s u l f o n y l d i a z o methane d e r i v i t i v e s I n the same r e p o r t , examples of n e g a t i v e working d i a z o i n d o l e based polymers are shown.



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POLYMERS

The concepts of p o s i t i v e or n e g a t i v e systems are by no means a s t r i c t c l a s s i f i c a t i o n s i n c e m o d i f i c a t i o n s of the o r i g i n a l pre-polymer w i t h a d d i t i v e s , w i t h s u i t a b l e changes i n the development p r o c e s s e s can be u t i l i z e d to o b t a i n p o s i t i v e r e s i s t s w h i l s t the unmodified material i s a negative r e s i s t . For example, t e t r a c h l o r o d i a z o c y c l o p e n t a d i e n e s i n g e n e r a l are p o l y m e r i s a b l e by UV l i g h t to o b t a i n n e g a t i v e r e s i s t systems. By u s i n g these m a t e r i a l s w i t h novolak type ( p h e n o l formaldehyde f o r example) r e s i n s , and u s i n g aqueous d e v e l o p e r s , one can o b t a i n a p r e f e r e n t i a l s o l u b i l i t y of the exposed a r e a s , a p o s i t i v e working system. In t h i s c o n t e x t i t s h o u l d be mentioned t h a t most of the newer m a t e r i a l s s u f f e r from the d i s a d v a n t a g e of h a v i n g a b s o r p t i o n peaks around 3000A , a r e g i o n r e s t r i c t e d f o r use o n l y w i t h q u a r t z o p t i c s and d e a r t h of h i g h i n t e n s i t y l i g h t s o u r c e s . The photo- and e l e c t r o n ( i o n and x - r a y ) beam r e s i s t s a r e d i v i d e d i n t o two c l a s s e s ( c f . , F i g . 1) of s o l u b i l i z a t i o n i n d e v e l o p i n g an image. A p o s i t i v e r e s i s t i s more s o l u b l e i n the d e v e l o p e r i n r e g i o n s i r r a d i a t e d w h i l e a n e g a t i v e r e s i s t i s i n s o l u b i l z e d i n the exposed r e g i o n s . The s o l u b i l i t y d i f f e r e n c e s between the exposed and unexposed r e g i o n s are used to form t h r e e d i m e n s i o n a l images which p r o t e c t d e s i r e d m a t e r i a l s d u r i n g the e t c h i n g of the s u b s t r a t e . The p h o t o r e s i s t s are exposed through a mask by c o n t a c t or p r o j e c t i o n p r i n t i n g w i t h the 3650, 4050, and 4350A mercury l i n e s . E l e c t r o n beams 5-30KV and x-rays 210A° have a l s o been employed. The p o s i t i v e r e s i s t r e produces the mask or image i n a d i r e c t p o s i t i v e r e p l i c a t i o n w h i l e a n e g a t i v e r e s i s t reproduces an image r e v e r s a l of the opaque p o r t i o n s of the mask. 0

0

The r e s i s t a r e about one to two microns t h i c k f o r s i l i c o n semiconductor f a b r i c a t i o n of two to ten micron d e v i c e s and about t h r e e to t w e n t y - f i v e micron t h i c k f o r p r i n t e d c i r c u i t board p a c k a g i n g . The r e s i s t s are m a i n l y used i n s u b t r a c t i v e masking f o r e t c h i n g by c h e m i c a l s o r , most r e c e n t l y , by plasmas of f l u o r i d e ions. S i n c e p o s i t i v e r e s i s t s are s o l u b l e i n o r g a n i c s o l v e n t s , they a r e e x c l u s i v e l y used i n an a d d i t i v e c y c l e to d e f i n e metal lands i n a l i f t - o f f scheme. Most of the r e s i s t s are i n the l i q u i d form, but r e c e n t l y s o l v e n t l e s s dry f i l m r e s i s t f o r t h i c k f i l m ap plications have been i n t r o d u c e d . 2 Dry f i l m s p r o t e c t p l a t i n g h o l e s i n c i r c u i t boards.


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TABLE V List

o f commonly used p r e p o l y m e r s i n p h o t o l i t h o g r a p h y

Acrylates Acrylonitriles A l l y l Esters Cellulose esters Epoxies E t h y l e n i c (unsaturated) hydrocarbons Poly-hydroxy e s t e r s ( e g : P e n t a e r y t h r i t o l tetramethacrylate,Ethylene Glycol diacrylate) Polyolefins V i n y l compound^esters,acids) V i n y l i d e n e compounds.


Common

Sensitizers

Class

Typical

Examples

Benzoin,Quinones,Benzophenone Diazonium compounds, Mercaptans , D j ^ u l f i ^ ç s Per a c i d s Fe - Fe Silver Halides, CBr ,HgBr Eosin/Amine,Cyanine dyes Methylene b l u e Metal a l k y l s M(CO) ZnO Benzophenone,Benzothiazoles Nitroaromatics E o s i n dyes.

C a r b o n y l compounds Azo compounds O r g a n i c S u l f u r Cpds, Redox systems Halogen compounds Dyes

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The k e t e n e i n t e r m e d i a t e r e a c t s w i t h w a t e r p r e s e n t i n the p h e n o l i c polymer base or i n subsequent a l k a l i n e developer to form a s o l u b l e indene c a r b o x y l i c a c i d . R e c e n t w o r k by R u s s i a n W o r k e r s h a s r e v e a l e d a more r e a s o n a b l e e x p l a n a t i o n o f t h e p h o t o i n s o l u b i l i z a t i o n o f a p h e n o l i c r e s i n by t h e d e c o m p o s e d d i a z o q u i none. The k e t e n e i n t e r m e d i a t e was f o u n d t o be g r a f t e d onto the p h e n o l i c r e s j n through the f o r m a t i o n of a c a r b o x y l i c a c i d ester-

The g r a r t e d p o l y m e r i s s o l u b l e i n t h e a l k a l i n e ph-12 developer. The u n e x p o s e d p h e n o l i c r e s i n p r o t e c t e d by t h e i n s o l u b l e n a p t h o q u i n o n e d i a z i d e i s u n a f f e c t e d by the d e v e l o p e r . Since the polymers are p h o t o s e n s i t i v e t o 5500A°, t h e y c o u l d e v e n t u a l l y be u s e f u l s p i n o f f s as photodegradable polymers s o l u b l e i n a l k a l i n e s o i l s . In c o n t r a s t to dry f i l m p l a t i n g or e t c h i n g nega t i v e r e s i s t s s u c h as D u p o n t s R i s t o n ^ n o d r y f i l m p o s i t i v e r e s i s t s h a v e b e e n m a r k e t e d . — The p o s i t i v e resists offer versatile processing alternatives. In F i g u r e 1, t h e a d d i t i v e l i f t - o f f o r s u b t r a c t i v e e t c h process i s o u t l i n e d . Since the p o s i t i v e r e s i s t s are s o l u b l e i n o r g a n i c s o l v e n t s , m e t a l f i l m s c a n be p a t t e r n e d and " l i f t e d o f f " i n u n d e s i r e d r e g i o n s . Nega t i v e r e s i s t s are not s o l u b l e i n o r g a n i c s o l v e n t s . H a r s h s t r i p p e r s s u c h as a l k a l i o r s t r o n g a c i d s a r e used. T h e s e a g e n t s w o u l d a t t a c k m e t a l s s u c h as c o p p e r or aluminum d e p o s i t e d over n e g a t i v e r e s i s t s . N e g a t i v e r e s i s t s have been e x c l u s i v e l y used i n lithographic plate production. They have been a d o p t e d to e l e c t r o n i c manufacture of p r i n t e d c i r c u i t boards and m i c r o s c o p i c s e m i - c o n d u c t o r d e v i c e s . Micro e l e c t r o n i c grade n e g a t i v e r e s i s t s f r e e of p a r t i c l e s or m e t a l i o n s have been e x c l u s i v e l y marketed. Negative r e s i s t s h a v e b e e n f o r m u l a t e d on a l l a s p e c t s o f p h o t o p o l y m e r i z a t i o n , c r o s s l i n k i n g , and i n s o l u b l i z i n g r e a c t i o n s o f u n s a t u r a t e d monomers o r p o l y m e r s . Since h i g h molecular weight polymers or c r o s s l i n k e d i n s o l u b l e polymers are d e s i r e d to produce a " n e g a t i v e " (image area exposed i s i n s o l u b l e i n d e v e l o p e r ) , v a r i o u s !


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s e n s i t i z e r s and v i n y l o r a l l y l based p o l y m e r s have been used. Three s u c c e s s f u l r e s i s t s e x e m p l i f y b a s i c p a t h s t o o b t a i n an i n s o l u b l e h i g h p o l y m e r . These a r e based on t r i p l e t — s e n s i t i z e d c r o s s l i n k i n g o f p o l y v i n y l c i n n a m a t e . O v e r 300 p a t e n t s h a v e b e e n d i s c l o s e d on t h i s r e a c t i o n . CH 6

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In t h i n f i l m s ^ 2 j i , t h e s y s t e m i s l i m i t e d t o 3650A e x posure and i s n o t s u i t a b l e f o r p r o j e c t i o n p r i n t i n g . R e c e n t l y , t h e s p e c t r a l s e n s i t i v i t y of" t h e c o n j u g a t i o n a s s o c i a t e d w i t h a z i d e chromophore has been extended to 4350A for projection exposure.— In very thin layers 5000A , t h e p r e s e n c e o f oxygen w i l l quench t h e n i t r e n e c r o s s l i n k i n g by c o n v e r s i o n i n t o a n i t r o s o derivative. — P r e p o l y m e r s e s p e c i a l l y d i , t r i , and t e t r a f u n c t i o n a l a c r y l a t e s o r d i a l l y l p t h a l a t e s s e n s i t i z e d by t r i p l e t o r r a d i c a l photos e n s i t i z e r s form t h e t h i r d c l a s s o f n e g a t i v e r e s i s t s . These r e s i s t s a r e a l s o u s e f u l as examples o f r a d i a t i o n c u r a b l e c o a t i n g s , paints, or inks. The b e n z o i n - s e n s i t i z e d i n s o l u b i l i z a t i o n of d i a l l y p t h a l a t e p r e p o l y m e r s and t h e t - b u t y l a n t h r a q u i o n e sensitized gelation of pentaerythritol acrylates - t Q examples o f t h i c k f i l m wet and d r y n e g a t i v e r e s i s t s . — The s y s t e m s a r e o x y g e n s e n s i t i v e . Rapid 0

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POLYMERS

i n s o l u b i l i z a t i o n i s achieved through s p a t i a l poly merization i n a l l dimensions. I t i s e x p e c t e d t h a t t h e r m o s e t epoxy p r e p o l y m e r s w i l l be e v e n t u a l l y u s e d as h i g h s p e e d n e g a t i v e r e s i s t s o n c e t h e s h e l f l i f e c a n be e x t e n d e d . Heat s e n s i t i v e p o l y m e r s c o u l d be a c t i v a t e d by a d d i n g t h e p h o t o s e n s i t i z e r immediately before exposure. H i g h l y s e n s i t i v e n e g a t i v e r e s i s t s a r e more d i f f i c u l t t o p r o cess s i n c e i n s o l u b l e r e s i d u e s a r i s i n g from "dark" p o l y m e r i z a t i o n c a n n o t be t o l e r a t e d i n t h e d e v e l o p e d (unexposed) r e g i o n s . Solventless compositions w i l l e v e n t u a l l y be r e q u i r e d f o r h i g h s p e e d n e g a t i v e resists. I d e a l l y , t h e r m a l l y s t a b l e (up t o 300°C) b u t r a d i a t i o n s e n s i t i v e polymers are needed i n the f i e l d of t h r e e d i m e n s i o n a l i m a g e r e c o r d i n g . I n c o m p a r i s o n o f p o s i t i v e and n e g a t i v e r e s i s t s , t h e n a t u r e o f t h e s u b s t r a t e and e t c h i n g c o n d i t i o n s e v e n t u a l l y d i c t a t e the c h o i c e of r e s i s t s . In T a b l e VI i s a comparison of a commercial p o s i t i v e d i a z o q u i o n e r e s i s t with a commercial negative r e s i s t . The p o s i t i v e r e s i s t s have h i g h e r r e s o l u t i o n but s l o w e r s p e e d and l e s s e t c h r e s i s t a n c e . S i n c e g l a s s and m e t a l f i l m s w i t h a m u l t i t u d e of e t c h a n t s or processes are used, b o t h r e s i s t s are w i d e l y used i n s e m i c o n d u c t o r manufacture. H y d r o p h i l l i c and h y d r o p h o b i c s u r f a c e s are e n c o u n t e r e d . A u n i v e r s a l r e s i s t with adhesion and h i g h e t c h r e s i s t a n c e b u t s o l u b l e i n o r g a n i c s o l v e n t s c a n be e n v i s i o n e d as t h e n e x t g e n e r a t i o n material. In t h e s e m i c o n d u c t o r t e c h n o l o g y , a major e f f o r t t o p r o d u c e m i c r o c o m p u t e r c i r c u i t s w i t h enormous memories has been u n d e r t a k e n i n the l a s t decade. T h i s has i n c r e a s e d t h e demand f o r t h e d e v e l o p m e n t o f c i r c u i t s w i t h d i m e n s i o n s n e a r t h e w a v e l e n g t h o f l i g h t 0.5/1. H i g h e r r e s o l u t i o n e x p o s u r e u s i n g X - r a y s , i o n beams and e l e c t r o n beams h a v e b e e n i n v e s t i g a t e d . S i n c e the e l e c t r o n beam c a n be a c c u r a t e l y moved by d e f l e c t i o n i n e q u i p m e n t s u c h as i n a s c a n n i n g e l e c t r o n m i c r o s c o p e , v a r i o u s r e s i s t s b a s e d on d e g r a d i n g o r c r o s s l i n k i n g polymers have been i n v e s t i g a t e d . The n e g a t i v e e l e c t r o n beam r e s i s t s as shown i n T a b l e V I I a r e b a s e d on v i n y l p o l y m e r s —

with

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TABLE V I

Comparison o f P o s i t i v e and N e g a t i v e Positive Diazoquinone

Negative Bis-Azide 0

S p e c t r a l Sens. 3650-5000A Oxygen Sens. no Exposure ergs/cm 2 χ 10 Resolution/mm 800 l i n e s Etch Resistance a c i d but n o t alkali· Strip organic solvents Lift-Off Yes


2

Resists

β

0

3650A yes 4 χ 10 300 l i n e s acids& a l k a l i s 5

acids No

TABLE V I I Electron

Beam

Polymers

POSITIVE - DEGRADING TYPE Min. Dose Coul/cm 2

POLYMETHYL METHACRYLATE POLY T-BUTYL METHACRYLATE POLYMETHYL ISOPROPENYL KETONE CROSSLINKED POLYMETHYL METHACRYLATE POLY OLEFIN SULFONES POLY HEXYL ISOCYANATE DIAZOQUINONE PHOTORESIST

5

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NEGATIVE - INSOLUBILIZING TYPE

AZIDE PHOTORESIST POLY DIALLYL ORTHO PHTHALATE POLYSILOXANES EPOXIDIZED RUBBERS POLYGLYCIDYL METHACRYLATE

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C r o s s l i n k i n g s i d e c h a i n s o f R s u c h as e p o x y , o l e f i n , or c o n j u g a t e d k e t o n e s have a l s o been i n v e s t i g a t e d . Traditionally a l l c o a t i n g s c u r e d b y h e a t , MEV e l e c t r o n s , gamma r a y s , o r UV l i g h t may f i n d u s e f u l a p p l i c a t i o n s as h i g h s p e e d ( t h i n f i l m ( O . l - l . O / i ) ) e l e c t r o n beam r e s i s t s . The r e s o l u t i o n o f t h e n e g a t i v e e l e c t r o n beam r e s i s t s a t l-2/ι r a n g e i s n o t a s g o o d as t h e e q u i v a l e n t p o s i t i v e e l e c t r o n beam r e s i s t s . Since no p h o t o s e n s i t i z e r s a r e n e e d e d , i t c a n be e x p e c t e d t h a t t h o u s a n d s o f v i n y l p o l y m e r s c o u l d be f o r m u l a t e d into negative r e s i s t s . Since polymers w i t h t e t r a s u b s i t u t e d centers are d i f f i c u l t t o s y n t h e s i z e , two m a i n t y p e s o f p o s i t i v e r e s i s t s have been d i s c l o s e d . P o l y m e t h y l m e t h a c r y l a t e ^ o r p o l y m e t h y l i s o p r o p e n y l k e t o n e ^ h i c h undergo s i d e c h a i n e l i m i n a t i o n and s u b s e q u e n t main c h a i n f r a c t u r e have b e e n u s e d as p o s i t i v e e l e c t r o n beam r e s i s t s ( c f . Table V I I ) . The i n c o r p o r a t i o n o f a weak l i n k i n t h e m a i n c h a i n o f a o l e f i n s u l f o n e c o p o l y m e r s u c h as p o l y - 1 butene s u l f o n e i s a r e c e n t example o f polymers w i t h high G value ( ^and high s e n s i t i v i t y 10" coul/sq. cm. needed t o r e a d and w r i t e p a t t e r n s i n m i n u t e t h r o u g h p u t times.— The l o w c e i l i n g t e m p e r a t u r e (y*+25° C) o f some highly radiation sensitive sulfones severely limits the p r a c t i c a l p r o c e s s i n g of r e s i s t s . Polymer f i l m s w h i c h a r e d e p o s i t e d as c o l d f i l m s a t l o w t e m p e r a t u r e by gas p h a s e r e a c t i o n s i n c l u d i n g r a d i a t i o n p r o c e s s i n g s u c h as p l a s m a o r UV, c a n be e n v i s i o n e d as t h e n e x t generation of s o l v e n t l e s s r e s i s t s of high s e n s i t i v i t y . The e q u i p m e n t f o r p r o c e s s i n g p o l y m e r s p e r f o r m s t h r e e m a j o r s t e p s : c o a t - d r y , e x p o s e , and d e v e l o p - r e a d out. C o a t i n g f i l m s g r e a t e r t h a n a m i c r o n i s accom p l i s h e d b y p a i n t t e c h n i q u e s s u c h as r o l l , d i p , s p r a y , or laminate. T h i n c o a t i n g s s u c h as i n m i c r o e l e c t r o n i c s a r e a p p l i e d by s p i n n i n g t h e o b j e c t and applying the r e s i s t solution. U l t r a t h i n films are a p p l i e d b y p o l y m e r i z a t i o n o f a c t i v a t e d monomers o n t o the s u b s t r a t e . S o l v e n t c o a t i n g s c a n b e d r i e d by c o n d u c t i o n , c o n n e c t i o n o r o t h e r f o r m s o f e n e r g y s u c h as i n f r a r e d , microwave, or i n d u c t i o n h e a t i n g . After d r y i n g t h e f i l m s c a n be f l o o d e x p o s e d b y m e r c u r y v a p o r l a m p s o r s p r a y e d w i t h KV e l e c t r o n s . T h i n r e s i s t f i l m s are contacted p r i n t e d w i t h l i g h t or d i g i t a l l y patterned w i t h s u b m i c r o n e l e c t r o n beams. The i m a g e s a r e u s u a l l y d e v e l o p e d by o r g a n i c o r a l k a l i n e s o l v e n t s by s t a t i c o r spray processing. The e f f i c i e n c y o f t h e e x p o s u r e s o u r c e i s a c r i t i c a l c o s t and t h r u p u t f a c t o r . U l t r a v i o l e t sources output up t o 1 0 % o f t h e i n p u t e n e r g y w h i l e 150 KV e l e c t r o n c u r e s o u r c e s can be 70% e f f i c i e n t . To r e c o r d h i g h l 0

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121

r e s o l u t i o n images, p o i n t s o u r c e s are used but at a s a c r i f i c e o f low o u t p u t i n t e n s i t y and e f f i c i e n c y . E l e c t r o n beams a r e t h e most a c c u r a t e l y c o n t r o l l e d t h a n d e f l e c t e d l i g h t , i o n , or x - r a y s . The p r o c e s s i n g e q u i p m e n t i n r e s i s t t e c h n o l o g y has advanced from the t r a d i t i o n a l b a t c h tanks f o r d e v e l o p ing, e t c h i n g , and s t r i p p i n g t o f u l l y a u t o m a t e d c o a t i n g , b a k i n g , e x p o s i n g d e v e l o p i n g , e t c h i n g , and s t r i p p i n g systems. The a u t o m a t i o n o f t h i c k r e s i s t f o r p r i n t e d c i r c u i t s was a c h i e v e d i n t h e l a t e s i x t i e s and t h e automation of t h i n f i l m s r e s i s t f o r semiconductor p r o c e s s i n g has b e e n made c o m m e r c i a l l y a v a i l a b l e . — The e x p o s u r e o f r e s i s t s has s w i t c h e d from c o n t a c t t o p r o x i m i t y to p r o j e c t i o n p r i n t i n g i n a e f f o r t to extend mask l i f e . M o r e d u r a b l e masks o f chrome o r i r o n o x i d e on g l a s s h a v e o v e r c o m e t h e l i m i t e d l i f e o f s i l v e r e m u l s i o n masks. I n t h e f u t u r e e l e c t r o n beams a r e e n v i s i o n e d t o r e p l a c e masked e x p o s u r e . Besides u s i n g the r e s i s t s to e t c h copper boards, m e t a l p a r t s , o r s i l i c o n s u r f a c e s , new a p p l i c a t i o n s have been d i s c l o s e d . S p a t i a l images have been h o l o g r a p h i c a l l y recorded i n r e s i s t s . — The r e s i s t s y s t e m s of s i l o x a n e s c a n be c o n v e r t e d a f t e r i m a g i n g by o x i d a t i o n i n t o p a s s i v a t e d g l a s s f o r d i r e c t f o r m a t i o n of insulated c i r c u i t s . — The r e s i s t c a n be f i l l e d w i t h glass£2 o r m e t a l s — and f i r e d t o f o r m c i r c u i t s directly. I n one a p p l i c a t i o n , t h e r e s i s t s a r e f i l l e d w i t h e l e c t r o l e s s p l a t i n g s e n s i t i z e r f o r d e p o s i t i o n of metal f i l m s . — The r e s i s t s c a n be u s e d i n t h e l i t h o g r a p h i c s e n s e as r e l i e f p r i n t i n g p l a t e s . — The c h e m i c a l m i l l i n g of s m a l l machine p a r t s i s a economical means o f mass p r o d u c t i o n . The g e n e r a l i m p r o v e m e n t s i n v a r i o u s p r o p e r t i e s s u c h as c h e m i c a l and t h e r m a l s t a b i l i t y are d e s i r a b l e f o r e x t e n s i v e f u t u r e a p p l i cations. The c o m m e r c i a l s u p p l i e r s o f r e s i s t h a v e e s t a b l i s h e d e n g i n e e r i n g l a b s to e x p e r i e n c e the customer a p p l i c a t i o n s on m u l t i f a r i o u s i n s u l a t o r o r c o n d u c t i v e s u b s t r a t e s and t o i m p r o v e by e x p e r i e n c e t h e performance of r e s i s t f o r m u l a t i o n s . In the l a s t c a t e g o r y o f r a d i a t i o n s e n s i t i v e p o l y m e r s , the development of e c o l o g i c a l l y sound p h o t o d e g r a d a b l e p o l y m e r s has been a n o t e w o r t h y application. Polymers have been t r a d i t i o n a l l y f o r m u l a t e d w i t h u l t r a v i o l e t absorbers or a n t i - o x i d a n t s to prevent o x i d a t i v e , p h o t o c h e m i c a l , t h e r m a l , o r biodégradation d u r i n g p r o c e s s i n g or i n consumer p r o d u c t s . Consumer p a c k a g i n g p r o d u c t s have been s e n s i t i z e d to photod e g r a d e by t h e i n c o r p o r a t i o n o f k e t o n e g r o u p s t o a b s o r b 3000A s u n l i g h t e x p o s u r e . ^ The N o r r i s h e l i m i n a t i o n of t h e a d j a c e n t a l k a n e g r o u p s i n e t h y l e n e c a r b o n mon o x i d e c o p o l y m e r s w i t h c h a i n s c i s s i o n i s p r o p o s e d as a 0



122

UV


l i k e l y d e g r a d a t i o n mechanism. In a r e a l i s t i c sense, v e r y l i t t l e 2537 A° r a d i a t i o n i s a v a i l a b l e f r o m t h e p o l l u t e d atomsphere f o r photodecomposition. The p o l y m e r i c a d d i t i v e s i n c o m m e r c i a l p l a s t i c s h a v e t o be eliminated i f reasonable degradation i s to occur. The b i o d e g r a d a b l e s y s t e m s a r e more e f f e c t i v e t h a n p h o t o degradation s i n c e uniform exposure i s d i f f i c u l t t o achieve f o r waste m a t e r i a l s . Perhaps, a combination o f b i o - , p h o t o - , o r h e a t d e g r a d a b l e p l a s t i c s w i l l be produced f o r food p r o d u c t s . Up t o t h i s p o i n t we h a v e surveyed the v a r i o u s a p p l i c a t i o n s of r a d i a t i o n s e n s i t i v e p o l y m e r s , t h e c o m p o s i t i o n s and c h e m i c a l p a r a m e t e r s and b r i e f l y e x a m i n e d t h e p r o c e s s i n g s equences. F i n a l l y , the comparison of the e f f i c i e n c y of v a r i o u s r a d i a t i o n sources i s of imminent i n t e r e s t . Each a p p l i c a t i o n r e q u i r e s a p a r t i c u l a r c o m b i n a t i o n o f t h e m a t e r i a l a b s o r p t i o n and t h e r a d i a t i o n s o u r c e . I n o r d e r t o a s s e s s t h e e f f i c i e n c y o f each o f t h e s y s t e m s , s e v e r a l f a c t o r s h a v e t o be t a k e n i n t o c o n s i d e r a t i o n . N o t a b l e among t h e s e a r e t h e t h i c k n e s s o f t h e f i l m , t h e a b s o r p t i o n c r o s s s e c t i o n , and t h e s u b s e q u e n t c h e m i c a l r e a c t i o n s i n t h e exposed phase. As n o t e d b e f o r e i n T a b l e I I , t h e s p e c t r u m o f e n e r g i e s used r a n g e f r o m h i g h e n e r g y (MeV) e l e c t r o n s t o l o w e r energy (KeV) e l e c t r o n s and x - r a y s t o t h e l o w e s t e n e r g y (eV) u l t r a v i o l e t r a y s . I n some c a s e s t h e a b s o r p t i o n c r o s s s e c t i o n s a r e d e p t h d e p e n d e n t f u n c t i o n s as t y p i f i e d by e l e c t r o n p e n e t r a t i o n i n t o p o l y m e r s ( c f . , f i g u r e 2 ) . H e n c e t h e c h o i c e o f e l e c t r o n beam e n e r g i e s i s s t r i c t l y a m a t t e r o f the p e n e t r a t i o n range d e s i r e d i n t h e e x p o s e d medium. F o r t h i n r e s i s t f i l m s ( l - 4 j i ) , 5-30kV e l e c t r o n beams a r e u s e d i n s e m i c o n d u c t o r fabrication. The c u r i n g o f t h i c k e r f i l m s (lO-1000/ι) r e q u i r e s a h i g h e r e n e r g y ( 0 . 0 5 - 2 M e V ) e l e c t r o n beam. The i m a g e r e a d o u t e f f i c i e n c y ( a s i m p l e c o n t r a s t f u n c t i o n f o r l i t h o g r a p h i c r e s i s t ) c a n be r e l a t e d t o the r a t i o of the c h e m i c a l or p h y s i c a l p r o p e r t i e s of the exposed t o unexposed m o l e c u l a r w e i g h t s . A r e l a t i v e comparison i s given i n Table V I I I of v a r i o u s exposable compositions. The g r e a t e s t c h a n g e o r c o n t r a s t i n m a t e r i a l p r o p e r t i e s o c c u r s b e t w e e n a monomer (M) a n d a p o l y m e r . ( P z ) The n M — t P n ^ P z — » P*, r a d i a t i o n i n t i a t e d p o l y m e r i z a t i o n has t h e h i g h e s t e f f i c i e n c y s i n c e a m p l i f i c a t i o n (up t o 1 0 ^ ) o c c u r s a f t e r i n i t i a t i o n by c o n v e n t i o n a l p o l y m e r i z a t i o n a t l o w ( ^ 5 k c a l . ) a c t i v a t i o n e n e r g i e s . As t h e r e a c t i o n proceeds to high c o n v e r s i o n , the propagating i n t e r m e d i a t e s a r e d e a c t i v a t e d by t h e v i s c o u s m e d i a .



9.

MOREAU

AND

viswANATHAN


S e c o n d l y , l o w v i s c o s i t y monomers c a n n o t f o r m f i l m s u s e f u l i n c o a t i n g o r i n k c u r i n g a n d as r e s i s t s . Thus, the a m p l i f i c a t i o n and c o n t r a s t g a i n i s l o s t by u s i n g p r e p o l y m e r s Pn t o o b t a i n f i l m f o r m i n g p r o p e r t i e s . From a photochemical viewpoint, a polymerizable composition would have a h i g h speed b u t f o r p r a c t i c a l i m a g i n g c o n t r a s t , t h e c r o s s l i n k i n g r e a c t i o n i s t h e second b e s t . Although c r o s s l i n k i n g (Pz—•Poo) o r s c i s s i o n i n g ( P o o — * P z ) h a v e a b o u t t h e same G v a l u e s ( 0 . 1 - 1 0 ) a n d a c t i v a t i o n e n e r g i e s , (10-40 k c a l / m o l e ) , a g r e a t e r change i n p h y s i c a l p r o p e r t i e s f o r e q u i v a l e n t doses occurs i n going from a uncross l i n k e d t o a c r o s s l i n k e d polymer i n s t e a d o f l o w e r i n g t h e m o l e c u l a r w e i g h t by degradation. O n l y two c r o s s l i n k s p e r c h a i n a r e n e e d e d to achieve i n f i n i t e m o l e c u l a r weight. By t h e same reasoning, the selective degradation of crosslinks i n a polymer network would form a h i g h speed " p o s i t i v e " r e s i s t or photodegradable f i l m . The c r o s s l i n k i n g o f p o l y m e r s c a n b e a c h i e v e d b y t h r e e major r e a c t i o n s : ( 1 ) p o l y m e r i z a t i o n o f m u l t i f u n c t i o n a l monomers ( e . g . , t e t r a a c r y l a t e s ) , ( 2 ) s e l f c r o s s l i n k i n g p o l y m e r s ( e . g . , p o l y v i n y l c i n n a m a t e s ) and (3) c r o s s l i n k i n g a g e n t s ( e . g . , b i s a z i d e s ) . For the c h o i c e o f e x p o s u r e s o u r c e and c o s t , i t a p p e a r s t h a t h i g h r e s o l u t i o n imaging f o r s m a l l exposed areas i s a c h i e v a b l e b y u l t r a v i o l e t o r l o w KV e l e c t r o n s f o r s u b micron dimensions. I n f i g u r e I I I , the exposure costs f o r l a r g e r a r e a s a r e c o n s i d e r e d t o be l o w e r t h a n thermal curing. I n t h i n f i l m s f o r m i c r o e l e c t r o n i c s (lyu) t h e e x p o s u r e s y s t e m s c a n b e c o m p a r e d i n T a b l e I X . As c a n be s e e n , t h e t y p i c a l u l t r a v i o l e t ( 3 6 5 0 A ) e x p o s u r e i s a b s o r b e d much more e f f i c i e n t l y t h a n e l e c t r o n o r x - r a y s . H o w e v e r , t h e h i g h e r e n e r g y e l e c t r o n beam c a n be f o c u s e d to submicron dimensions w h i l e t h e x-ray exposure t h r o u g h a mask h a s a l a r g e d e p t h o f f o c u s . From t h e m a t e r i a l s v i e w p o i n t , i t i s o f i n t e r e s t t o examine t h e thermodynamics o f r a d i o l y s i s o f p o l y m e r s . S i n c e gamma r a d i o l y s i s d a t a i s r e a d i l y a v a i l a b l e , p o l y m e r s c a n be c o m p a r e d as i n T a b l e X. N o t e t h a t m a t e r i a l s having a l a r g e heat of p o l y m e r i z a t i o n tend to c r o s s l i n k under r a d i o l y s i s . T h e same p o l y m e r s a r e t h e r m a l l y r e s i s t a n t t o d e g r a d a t i o n . Degrading polymers h a v e l o w c e i l i n g t e m p e r a t u r e s (^150°C) a n d l o w h e a t s of p o l y m e r i z a t i o n . The G v a l u e s ( c h e m i c a l e v e n t s / 1 0 0 e v a b s o r b e d ) a r e a l s o i n d i c a t i o n s of the tendency of a polymer to degrade o r i n s o l u b i l i z e under any form o f r a d i a t i o n i n e x c e s s o f bond e n e r g i e s . In Table X I , a compilation of t h e G v a l u e s i s n o t e d a l o n g w i t h t h e expended energy f o r each bond b r o k e n o r formed. I n the case of 0



124

U V L I G H T INDUCED REACTIONS IN P O L Y M E R S

TABLE Image IMAGING PROPERTY S o l u b i l i t y Rate Volatilization Rate Tackiness Adhe s i o n Adsorption Viscosity Solidification Permeabili ty Transmi ssion Re f r a c t i o n

VIII

Differentiation TYPE OF Positive

IMAGE Negative Dec Dec Dec Inc Dec Inc Inc Dec Dec Inc

Inc inc

Inc Dec Dec Inc Inc Dec

COST COMPARISON OF CURING i—

I Figure S. Cost comparison of curing

ι ι ι ι ι 50 100 130 200 PROCESSED AREA SQUARE METERS 1 METER Wl DTH


9.

MOREAU

AND


viswANATHAN

TABLE Efficiency


E-Beams,Ion beams(20-50KV) X-Rays,Gamma (50KV-1MEV) UV ( t y p . 3 6 5 0 A ° )

Typical absorption percent 4 - 1 1

0.5-1

50

25

Radiation Effect "~

Overall efficiency percent 0.01 0.0005 12.5

X

of Crosslinking

Poly ethylene Crosslink -Propylene -Methyl acrylate -Isobutene Scission -Methyl methacrylate -alpha-Methyl styrene from

Reaction efficiency percent 1

- 0.01

TABLE

Polymer

IX

of Radiation Reactions i n Films

Energy System

Comparison

125

Vs

Scission

of

Polymers

Heat o f PolymnΓ Kcal/mole

Monomer y i e l d on pyrolysis

22 16.5 19

0.025 2.0 2.0

10 13

20.0 100.0

9

100.0

D.E.Roberts,J.Res.Natl.Bur.Std.,

%

44,221,(1950)


UV


126


POLYMERS

d e g r a d i n g p o l y m e r s , t h e s u l f o n e s expend the l e a s t energy (12.5ev/bond) which i s s t i l l i n excess of t h e b o n d e n e r g y ( 4 - 5 e v ) . C o m p e t i n g r e a c t i o n s s u c h as e m i s s i o n , c h a r g i n g , and t h e r m a l d i s s i p a t i o n o f t h e absorbed e n e r g i e s reduce the e f f e c t i v e n e s s of absorbed u l t r a v i o l e t e l e c t r o n , x - r a y , o r i o n beams. F r o m T a b l e X I , i t c a n be s e e n t h a t p o l y m e r s h a v e s p e c t r u m o f d o s e s n e c e s s a r y t o o b t a i n maximum r e a d o u t efficiency. We w i l l e x a m i n e i n t h e f o l l o w i n g e x a m p l e t h e i m a g i n g o f a p o s i t i v e e l e c t r o n beam r e s i s t as a f u n c t i o n o f dose and m o l e c u l a r w e i g h t - s o l u b i l i t y changes· A p o s i t i v e r e s i s t s y s t e m c a n be o f e i t h e r two types. The c l a s s i c a l d i a z o q u i n o n e s y s t e m r e p r e s e n t s a photochemical rearrangement r e a c t i o n which i s the basis of commercial p h o t o r e s i s t s . Scissioning or d e g r a d a t i o n o f a p o l y m e r c h a i n by l i g h t o r e l e c t r o n s i s a l a t e r example of s o l u b i l i t y i n d u c e d change. We w i l l examine t h i s change i n d e t a i l . A p o s i t i v e e l e c t r o n beam r e s i s t i m a g e i s d e v e l o p e d by i m m e r s i o n i n a s o l v e n t w h i c h d i s s o l v e s the e x p o s e d r e g i o n a t a r a t e ( S f ) w h i c h i s f a s t e r ( a p p r o x . 10X) t h a n t h e u n e x p o s e d r a t e ( S i . The r a t e of d i s s o l u t i o n o f a l i n e a r p o l y m e r i s r e l a t e d t o i t s molecular weight M by t h e U b e r r e i t e r f u n c t i o n ( 2 6 ) :

s

*

k

M"

0

[1 ]

1

where q i s t h e s o l v e n t - p o l y m e r s o l u b i l i t y p a r a m e t e r . I n f i g u r e 4, t h e i n c r e a s e i n s o l u b i l i t y o r d e c r e a s e in viscosity i s i l l u s t r a t e d f o r a corresponding decrease i n molecular weight. In a p o s i t i v e r e s i s t t h e image r e a d o u t e f f i c i e n c y f o r a p a r t i c u l a r e x p o s u r e dose i s d e f i n e d as t h e r a t i o of s o l u b i l i t y r a t e s :

The exposure

image r e a d o u t c a n be d e r i v e d d o s e by t h e e q u a t i o n ( 2 7 ) : SR

= ( 1 + |

Ultraviolet Light Induced Reactions in Polymers

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