Polyimide Surface Degradation - American Chemical Society

Chapter 11

Polyimide Surface Degradation

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2015 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch011

X-ray Photoelectron Spectroscopic Study Under UV-Pulsed Laser Irradiation M. Chtaïb, E. M. Roberfroid, Y. Novis, Jean-Jacques Pireaux, and R. Caudano Facultés Universitaires Notre-Dame de la Paix, Laboratoire Interdisciplinaire de Spectroscopie Electronique, Rue de Bruxelles 61, B-5000 Namur, Belgium

Polyimide surface modifications under UV ArF (λ = 193 nm) laser i r r a d i a t i o n i n a i r were studied by X-ray Photoelectron Spectroscopy (XPS). Several chemical transformations, the nature of which changed with fluence (energy per pulse and unit area), were evidenced at the polymer surface. A sharp decrease i n the O/C and N/C signals was observed at fluence higher than 20mJ/cm and new carbon species appeared. At higher fluence (above 40 mJ/cm ) complete denitrification took place and the chemical nature of the surface was completely modified. 2

2

During the l a s t decade, there has been considerable i n t e r e s t i n studying the i n t e r a c t i o n between u l t r a v i o l e t r a d i a t i o n and polymers by the use of pulsed excimer l a s e r (1-4 ) . In f a c t , some a t t r a c t i v e a p p l i c a t i o n s i n microelectronics and surgery have been s u c c e s s f u l l y implemented (ϋ) , and f u r t h e r i n f o r m a t i o n s about the d i f f e r e n t mechanisms (photochemistry, thermal e f f e c t . . , ) involved at the polymer surface have been invoked i n order to e l u c i d a t e t h e i r r e l a t i v e c o n t r i b u t i o n s . More r e c e n t l y , the a t t e n t i o n has been focused on t h i s type of polymer surface m o d i f i c a t i o n s t o improve some surface p r o p e r t i e s l i k e the adhesion i n m e t a l l i z e d polymer s t r u c t u r e s . Studies of polymers such as p o l y m e t h y l m e t h a c r y l a t e (PMMA), polyethylenetherephthalate (PET) and p o l y t e t r a f l u o r o e t h y l e n e (PTFE) revealed that the chemical surface changes depend considerably on the l a s e r fluence , the nature of the environment surrounding the polymer during the treatments and the UV absorption c o e f f i c i e n t of the polymer ( 6, 7 8 ) . I t was found that the A b l a t i v e r

0097-6156/90/0440-O161$06.00/0 © 1990 American Chemical Society In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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METALLIZATION OF POLYMERS

P h o t o D e c o m p o s i t i o n (APD) t h a t o c c u r s above a w e l l - d e f i n e d ablation threshold u s u a l l y leads t o removal of polymer m a t e r i a l l e a v i n g a new f r e s h " c l e a n " s u r f a c e w i t h t h e same c o m p o s i t i o n as t h e u n t r e a t e d m a t e r i a l . However , i n t h e c a s e o f PET (£_) both composition and structural m o d i f i c a t i o n s o c c u r even f o r f l u e n c e s below the a b l a t i o n threshold. In t h i s work, we p r e s e n t t h e e f f e c t o f 193 nm p u l s e d UV h i g h i n t e n s i t y r a d i a t i o n on p o l y i m i d e , a p o l y m e r w e l l known f o r i t s a p p l i c a t i o n s i n s o l i d - s t a t e technology ( t h e r m a l s t a b i l i t y and d i e l e c t r i c p r o p e r t i e s ) . We have u s e d XPS i n o r d e r t o d e t e r m i n e t h e e v o l u t i o n o f c h e m i c a l s u r f a c e c o m p o s i t i o n v e r s u s l a s e r f l u e n c e and have o b t a i n e d some a t t r a c t i v e i n f o r m a t i o n s a b o u t t h e s e m o d i f i c a t i o n s i n d u c e d by UV l a s e r r a d i a t i o n . 2.

EXPERIMENTAL

P o l y i m i d e KAPTON f i l m s were i r r a d i a t e d u s i n g a LambdaP h y s i k EMG200 E x c i m e r l a s e r t u n e d t o 193 nm radiation (ArF) . The s a m p l e s were t r e a t e d i n a i r , a t different f l u e n c e s ; a l l r e c e i v e d 10 s h o t s a t t h e r e p e t i t i o n r a t e o f 10 Hz. S u r f a c e a n a l y s i s of the d i f f e r e n t samples was p e r f o r m e d u s i n g an ESCA-201 s y s t e m (SSL) w i t h an x-ray m i c r o s p o t f a c i l i t y (150 μπι) . 3. RESULTS AND

DISCUSSION

L i k e most a r o m a t i c p o l y m e r s , p o l y i m i d e ( f i g . 1) a b s o r b s s t r o n g l y UV r a d i a t i o n i n t h e w a v e l e n g t h r e g i o n b e l o w 200 nm ( a b s o r p t i o n c o e f f i c i e n t a =4.10 cm" ) (JLL. Therefore, t h e p e n e t r a t i o n d e p t h o f t h e UV l i g h t i n t h i s m a t e r i a l i s v e r y s m a l l : a b o u t two t h i r d s o f t h e r a d i a t i o n i n t e n s i t y i s a b s o r b e d i n t h e f i r s t 300 Â o f t h e p o l y i m i d e f i l m . The XPS peak a s s i g n m e n t s were p e r f o r m e d a c c o r d i n g t o literature results (10-12) and after surface characterization of u n t r e a t e d samples for comparison purpose. 5

1

In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

11.


CHTAÏB ET AL.

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The C l s c o r e l e v e l s i g n a l o f p r i s t i n e p o l y i m i d e ( f i g : 2) i s composed o f t h r e e peaks: A t 285.0 e V (1) o n e s e e s a s t r o n g s i g n a l due t o "pure" a r o m a t i c carbons (not bonded t o Ν o r O) i n t h e ODA r i n g s . T h e p e a k a t 2 8 6 . 2 e V ( 2 ) i s a t t r i b u t e d t o t h e o t h e r ODA c a r b o n s b o n d e d t o n i t r o g e n a n d o x y g e n (13 1 4 ) a s w e l l a s t o t h o s e l o c a t e d i n t h e c e n t r a l PMDA r i n g (JJ1). The l a s t carbon peak on t h e h i g h b i n d i n g e n e r g y s i d e a t a b o u t 289.0 e V (3) i s d u e t o c a r b o n y l g r o u p s C=0 o f t h e PMDA. The O l s s p e c t r u m c o n s i s t s o f t w o u n r e s o l v e d p e a k s : o n e c e n t e r e d a t 532.3 eV due t o c a r b o n y l o x y g e n a n d t h e o t h e r at 533.7 eV d u e t o o x y g e n atoms o f t h e e t h e r group (fig;3) . The N l s s i g n a l ( f i g : 4 ) c o n s i s t s o f a m a i n p e a k a t 4 0 0 . 8 eV d u e t o n i t r o g e n i m i d e l i n k i n g - u p t h e t w o PMDA a n d ODA p a r t s . T h e p e a k a t 3 9 9 e V i s p r o b a b l y f i n g e r p r i n t i n g some isoimide content into t h e m a t e r i a l (Hi). For f l u e n c e s u p t o 15 m J / c m , the intensities of d i f f e r e n t carbon f e a t u r e s as w e l l as those o f oxygen and n i t r o g e n remained constant. However m i n o r f l u c t u a t i o n s o f the i n t e n s i t y r a t i o s presented i n f i g : 5 c a nbe e x p l a i n e d by the desorption o f low molecular weight fragments s t i m u l a t e d even a t l o w f l u e n c e s . A s i m i l a r t e n d e n c y was p o i n t e d o u t f o r PET, a n o t h e r h i g h a b s o r b e n t polymer, i n which case i t has been attributed t o a competitive mechanism between d e s o r p t i o n o f l o w molecular weight fragments and surface r e a c t i o n w i t h t h e ambient atmosphere (8). F o rp o l y i m i d e , f u r t h e r a n a l y s i s i s necessary t o c o n f i r m t h i s f a c t , b u t i n t h e p r e s e n t w o r k , we f o c u s o u r a t t e n t i o n o n more d r a s t i c c h a n g e s t a k i n g p l a c e a t h i g h e r fluences (above 20mJ/cm ). According t o another study, the a b l a t i o n t h r e s h o l d f o r polyimide w i t h an A r F l a s e r i s a b o u t 25 mJ/cm (9) .


f

2

f

2

2

2

F o r f l u e n c e s h i g h e r t h a n 20 mJ/cm , a c l e a r d i m i n u t i o n of t h e r e l a t i v e c o n c e n t r a t i o n i n oxygen a n d n i t r o g e n i s observed ( f i g . 6) . T h e O/C a t o m i c r a t i o f a l l s f r o m 0.20 f o r t h e u n t r e a t e d p o l y i m i d e t o 0.04 a t 4 0 m J / c m and i s c h a r a c t e r i s t i c o f an important deoxydation induced by t h e UV r a d i a t i o n . These m o d i f i c a t i o n s a r e a l s o v i s i b l e i n t h e C l s s i g n a l where t h e i n t e n s i t i e s o f b o t h p e a k s c e n t e r e d a t 286.2 eV and 289.0 eV d e c r e a s e c o n s i d e r a b l y ( f i g : 2 c a n d 2 d ) . o t h e r new p e a k s a p p e a r e d i n t h i s XPS s i g n a l . The one c e n t e r e d a t 2 8 3 . 8 e V ( p e a k 4) h a s b e e n a t t r i b u t e d t o " g r a p h i t e " t y p e c a r b o n o r more p r e c i s e l y t o c r o s s - l i n k e d carbons chains coming from redeposited fragments o f a b l a t e d p o l y m e r i c m a t e r i a l . The i n t e n s i t y o f t h i s signal i s l a r g e a n d c e r t a i n l y c o n t r i b u t e s t o an o v e r e s t i m a t e d decrease i n t h e N/C a n d O/C r a t i o s . P e a k (5) a t a r o u n d 287.2 eV suggests t h e presence o f O-C-0 a n d f u n c t i o n a l i t i e s a t t h e s u r f a c e (16) , w h i l e t h e r e a r e no such components i n t h e s t r u c t u r a l formula (fig:l). The 2




Fig. 2 : C l s spectra of treated polyimide at d i f f e r e n t f l u e n c e s : r e f e r e n c e ( a ) , 3mJ/cm2(b), 15mJ/cm2(c), 20mJ/cm2(d), 40mJ/cm2(e), 50mJ/cm2(f).


CHTAIB ET AL.

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11.

410,0

405t0

400,0

390,0

39*0

Binding Energy (eV) F i g . 4 : N l s spectrum o f t r e a t e d polyimide a t d i f f e r e n t f l u e n c e s : r e f e r e n c e ( a ) , 15mJ/cm (b) , 20mJ/cm (c) , 4 0 m J / c m (d) 2

2

2


166


0.8

η


•

0

20

40

Fluence

60

Cphenyl/Ctot

100

80

(mj/cm2)

F i g . 5 : XPS i n t e n s i t y r a t i o o f t h e carbon species versus f l u e n c e .

different

small peak (6) a t h i g h b i n d i n g energy (289.8eV) i s t e n t a t i v e l y a s s i g n e d t o carbon of HO-£L=0 t y p e due t o c a r b o x y l i c f u n c t i o n a l i t i e s formed a t t h e s u r f a c e . At 40 mJ/cm d r a s t i c changes a r e a l s o o b s e r v e d i n t h e oxygen s i g n a l ( f i g . 3 ) . The i n t e n s i t i e s o f t h e two peaks i n t h e O l s r e g i o n d e c r e a s e a t t h e same r a t e ; b u t a new peak a p p e a r s a t low b i n d i n g e n e r g y . This i s consistent w i t h t h e a p p e a r a n c e o f t h e c a r b o n c a r b o x y l i c peak ( 6 ) . t h e e x t r a s t r u c t u r e a t low b i n d i n g e n e r g y a r o u n d 399 eV appears r e i n f o r c e d ( f i g . 4) . Such a s t r u c t u r e has been tentatively attributed to isoimide. However f r o m a n o t h e r s t u d y ( 1 2 ) , t h i s peak was a s s i g n e d t o h y d r o g e n a t e d n i t r o g e n o r t o C =N( p y r i d i n t y p e ) p r o d u c t s r e s u l t i n g f r o m t h e PI degradation. Hence i t a p p e a r s t h a t UV r a d i a t i o n i n d u c e s C-N bond r u p t u r e , c r e a t i n g n i t r o g e n atoms w i t h a c h e m i c a l environment d i f f e r e n t f r o m t h e ones o f t h e PMDA r i n g (imides). In a d d i t i o n to the decrease of the Ν integral i n t e n s i t y , t h e r e i s a l s o a change o f i t s c h e m i c a l n a t u r e . In f a c t t h e peak a t 399 eV ( i s o i m i d e ) i n c r e a s e s w h i l e t h e i n t e n s i t y o f t h e i m i d e peak (401.0 eV) ( F i g : 4) d e c r e a s e s d u r i n g l a s e r t r e a t m e n t s (15 t o 50 mJ/cm ) . The r a t i o Ν (399)/N(401) ( f i g . 7 ) o f t h e s e two peaks c l e a r l y shows an i n c r e a s e by a f a c t o r o f 10 f o r f l u e n c e s a r o u n d 50mJ/cm . We n o t e d t h a t a t v e r y h i g h f l u e n c e (96 mJ/cm ), t h e Nls s i g n a l completely disappears. 2

2

2

2


11.

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0.6 η

0

20

40

60

Fluence F i g . 6 : XPS i n t e n s i t y versus fluence.

80

100

(mj/cni2)

ratio

of Ols/Cls

and N l s / C l s


168



MQDEL The UV r a d i a t i o n i n d u c e s t h e r u p t u r e o f t h e i m i d e r i n g , t h u s c h a n g i n g t h e n e i g h b o u r h o o d o f c a r b o n and n i t r o g e n v i a chemical r e a c t i o n s with ambiant a i r (H2O, Ν 2 , θ 2 · . ·) · B e s i d e t h e UV bond b r e a k i n g a l o c a l i n c r e a s e o f s u r f a c e t e m p e r a t u r e , c a n a l s o c o n t r i b u t e by t h e r m a l damage t o t h e appearence o f carbon " g r a p h i t e " a t the s u r f a c e . From a l l t h e s e XPS i n f o r m a t i o n s i n o r d e r t o t a k e i n t o account the f a c t that a f t e r the l a s e r treatments s e v e r a l species like isoimide, C=N-,-CO,0-C-O,and carboxylic g r o u p s a r e p r e s e n t a t t h e s u r f a c e o f p o l y i m i d e , we p r o p o s e t h e f o l l o w i n g s u r f a c e r e a c t i o n u n d e r UV l a s e r r a d i a t i o n above t h e a b l a t i o n t h r e s h o l d i n p r e s e n c e o f a i r m o i s t u r e :

isoimide

SUMMARY The UV l a s e r i r r a d i a t i o n o f p o l y i m i d e i n a i r l e a d s t o d r a s t i c changes i n t h e polymer s u r f a c e c o m p o s i t i o n . At f l u e n c e s under 40 mJ/cm , an oxygen and n i t r o g e n d e p l e t e d surface i s created. At higher f l u e n c e s , the r a d i a t i o n completely modifies t h e n a t u r e o f t h e b o n d i n g between c a r b o n s , oxygen and n i t r o g e n atoms and seems t o i n d u c e new c a r b o x y l i c f u n c t i o n a l i t i e s and s u r f a c e g r a p h i t i z a t i o n . 2


11.

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169

ACKNOWLEDGMENT T h i s work was s p p o r t e d by t h e I n s t i t u t e Interface Science (IRIS)-Belgium.

f o r Research i n


LITERATURE CITED 1. Srinivasan,R.; Mayne-Banton, V; Appl. Phys. Lett. 1982, 41, 6, 576 2. Srinivasan,R; Laser Processing and Diagnostics, 1984, 343, Springer-Verlag, Berlin, Heidelberg 3. Srinivasan, R; Leigh, W.J; J. Am.Chem.Soc.,1982 ,104, 6784 4. Znotruo,Τ.A.; Laser Focus, mai 1978, 54, 5. Srinivasan,R.; Science, 1987, 234, 559 6. Lazare, S; Srinivasan, R; J. Phys. Chem.,1986,90,2124 7. Burrell, M.C.; Liu, Y.S.; Cole, H.S., J. Vac. S c i . Technol., 1986 A4 (6) 8. Chtaïb, M; Roberfroid, E.M; Novis, Y; Pireaux, J.J.; Caudano, R, to be published in JVST (December 89) 9. Sutclife, E; Srinavasan, R., J. Appl. Phys., 1986, 60, 9 10. Philipp, H.R; Cole, H.S.; Liu, Y.S.; Sitnik, T.A., Appl. Phys. Lett., 1986, 48, 2, 192 11. Leary,H.J.; Campbell, D.S., Surf. Int. Anal., 1979, 4, 3 12. Lamb, R. Ν.; Baxter, J.; Grunze, M; Kong, C.W.,.Untertl,W.N, Langmuir, 1988, 4, 249 13. Baise,A.I.; Buchwahlr, P.L., In Conference on Polyimide; Mittal, K.L., Ed.; E l l e n v i l l e , New York, 1982; p. 137 14. Peeling, J.; Clark, J.T., Polym. Degrad. Stabil.,1981, 3, 177 15. Leary H.J, Campbel D.S Surf, interface anal 1979 1 ,75 16. Momose, Y; Ikawa, K.; Satd, T., J. Appl. Polym. S c i . , 1987, 2715-2729 17. Anderson .S.G, Meyer.H.M .Atanasoska.L.J and Weaver.J.H, J.Vac.Sci.Technol. 1988 A6 38. RECEIVED January 17, 1990


Polyimide Surface Degradation - American Chemical Society

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