Photoelectron Spectroscopies Applied to PolymerâMetal Interactions

Chapter 2

Photoelectron Spectroscopies Applied to Polymer—Metal Interactions Steven P. Kowalczyk T. J. Watson Research Center, IBM Corporation, Box 218, Yorktown Heights, NY 10598

The a p p l i c a t i o n of photoelectron spectroscopy (PES) f o r the i n v e s t i g a t i o n of polymer-metal i n t e r f a c e s i s discussed i n t h i s c h a p t e r . The information o b t a i n a b l e from both c o r e - l e v e l spectra and valence-band spectra is briefly d e s c r i b e d . The approach of model compounds to study s p e c i f i c i n t e r a c t i o n s i s shown to be a useful a i d to the understanding of polymer-metal reactivity. Emphasis i s given to a number of experimental aspects r e l e v a n t to polymer-metal i n t e r f a c e s t u d i e s , such as sample preparation and problems such as beam induced damage. Some of the advantages of synchrotron r a d i a t i o n and small spot x - r a y sources are demonstrated. Most of the illustrative examples will come from polyimide-metal i n t e r f a c e studies d i r e c t e d at i n v e s t i g a t i n g the r o l e of interfacial chemistry in adhesion at these i n t e r f a c e s and the non-equivalence of polymer-on-metal and metal-on-polymer i n t e r f a c e s . P h o t o e l e c t r o n spectroscopy (PES) has become an important and widely used tool i n m a t e r i a l science ( 1 - 3 ) . I t has been a p a r t i c u l a r l y f r u i t f u l technique for the i n v e s t i g a t i o n of polymers ( 4 - 9 ) . In t h i s r e v i e w , we w i l l focus on the a p p l i c a t i o n of photoelectron spectroscopy to the i n v e s t i g a t i o n of the i n t e r f a c e s between metals and polymers. These s t u d i e s are d i r e c t e d p r i m a r i l y to understand the r o l e of i n t e r f a c i a l chemistry i n the adhesion between metals and polymers. Two a s p e c t s , which w i l l be emphas i z e d here, are the experimental approaches i n PES s t u d i e s of polymer/metal i n t e r f a c e s and the types of information a c c e s s i b l e from the PES experiments. The experimental emphasis w i l l be on p r e p a r a t i o n of a p p r o p r i a t e samples for polymer/metal i n t e r f a c e s t u d i e s , p r a c t i c a l problems 0097-6156/90/0440-O010$07.50A) © 1990 American Chemical Society

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Photoelectron Spectroscopies

11

such as charging and beam induced damage, and the u s e f u l ness of synchrotron r a d i a t i o n and small area x - r a y sources. The i n f o r m a t i o n a v a i l a b l e from c o r e - l e v e l spect r a i n c l u d i n g s a t e l l i t e s and valence-band spectra w i l l be d i s c u s s e d . The examples used to i l l u s t r a t e the above w i l l p r i m a r i l y come from s t u d i e s of i n t e r f a c i a l chemistry and i t s c o n t r i b u t i o n to adhesion at p o l y i m i d e / m e t a l i n t e r faces. Polyimide i s a polymer of intense c u r r e n t i n t e r est i n the m i c r o e l e c t r o n i c i n d u s t r y as a t h i n f i r m d i e l e c t r i c constant and a number of other a t t r a c t i v e prope r t i e s (10-14). P h o t o e l e c t r o n spectroscopy c o n s i s t s of i r r a d i a t i n g the sample being i n v e s t i g a t e d with a monoenergetic source of photons and energy a n a l y z i n g the emitted photoelectrons ( 1 5 ) . The two key features of the PES experiment are that the measured energies are very s e n s i t i v e to the chemical environment ( c h e m i c a l - s h i f t phenomenon) (15) and the analyzed photoelectrons have mean free paths on the order of 20A (surface s e n s i t i v i t y ) ( 1 6 , 1 7 ) . The f i r s t f e a t u r e makes c o r e - l e v e l spectra such a powerful and widely used t o o l to o b t a i n chemical i n f o r m a t i o n , w h i l e the second feature makes the PES technique a potent surface s e n s i t i v e spectroscopy and n e c e s s i t a t e s u l t r a high vacuum (UHV) techniques and care i n sample p r e p a r a t i o n but a l l o w s the study of i n t e r f a c e s and surfaces i n a s t r a i g h t f o r w a r d and w e l l - c h a r a c t e r i z e d manner. We w i l l s t a r t by d i s c u s s i n g several important experimental aspects r e l e v a n t to the a p p l i c a t i o n of PES i n polymer-metal interface studies. EXPERIMENTAL Charging. Most polymers are i n s u l a t i n g and thus present the problem of charging ( 1 8 ) . While t h i s presents problems i n o b t a i n i n g absolute b i n d i n g e n e r g i e s , i n many cases t h i s i s not a s e r i o u s hindrance as the spectra often can be used as f i n g e r p r i n t s or can be referenced to a f i d u c i a l such as the valence-band maximum or a prominent well known c o r e - l e v e l f e a t u r e . Homogeneous charging besides l e a d i n g to b i n d i n g energy s h i f t s can s i g n i f i c a n t l y broaden spectra thus s e r i o u s l y degrading r e s o l u t i o n and i n h i b i i t i n g i n t e r p r e t a t i o n . A sample with nonuniform morphology often r e s u l t s i n inhomogeneous charging which can f u r t h e r degrade spectra by severe d i s t o r t i o n of lineshapes to the extent t h a t any i n t e r p r e t a t i o n i s p r o b l e m a t i c . These e f f e c t s are often a l l e v i a t e d or minimized by charge n e u t r a l i z a t i o n to compensate f o r the surface charge by f l o o d i n g the surface with low energy e l e c t r o n s ( 1 8 ) . While t h i s approach i s often u s e f u l , p o s s i b l e damage induced by the e l e c t r o n beam must be c a r e f u l l y monitored f o r i n o r g a n i c m a t e r i a l s . It is also p o s s i b l e to over compensate f o r the charging induced binding energy s h i f t . The use of t h i n f i l m s approximatel y 1000A t h i c k can minimize charging e f f e c t s . Figure 1

METALLIZATION OF POLYMERS

2. KOWALCZYK


13

shows a carbon Is spectrum from a 1900A t h i c k p o l y i m i d e (PMDA-ODA) f i l m obtained without the use of charge neutra l i z a t i o n but nonetheless with very good r e s o l u t i o n . Beam Induced E f f e c t s . One problem i n the study of organic f i l m s i s beam induced damage. The photograph of Figure 2 shows an i n s i t u grown p r v o m e l l i t i c d i a n h y d r i d e (PMDA) f i l m and an o x y d i a n i l i n e (ODA) f i l m a f t e r long exposure (^15 hours) to the monochromatized ΑΙ Κα x - r a y source of a Hewlett Packard 5950A e l e c t r o n spectrometer. The f i l m s were*~10,000A t h i c k . The ODA f i l m c l e a r l y shows the pat t e r n of the 1mm by 5mm x - r a y beam, w h i l e the p a t t e r n i n the PMDA f i l m i s more complex. A f t e r exposure to a i r dur ing storage and f o r t r a n s f e r , these f i l m s were s t u d i e d with a Surface Science Instruments s m a l l - s p o t x - r a y photo e l e c t r o n (XPS) spectrometer. Figure 3 shows the C Is spectra obtained w i t h a small spot x - r a y beam ( 15OJJ) from two spots on the surface of the PMDA f i l m , one near the center of the d i s c o l o r e d area and the other near one edge. No change i n l i n e s h a p e i s observed across the sample but there i s a smooth s h i f t of observed b i n d i n g energy with the minimum b i n d i n g energy obtained at the center of the damage area and i n c r e a s i n g b i n d i n g energy towards e i t h e r side. 0 Is spectra r i g i d l y s h i f t s i n b i n d i n g energy with the C Is spectra and a l s o does not undergo any lineshape change. F i n a l l y there i s no v a r i a t i o n of the C to 0 r a t i o across the sample. Thus the e x p l a n a t i o n f o r the beam ef f e c t s i n the PMDA f i l m i s that the x - r a y beam induces heating with a thermal g r a d i e n t across the sample w i t h the temperature highest at the center where the x - r a y beam im pinges the sample, and lower towards the edges which are not d i r e c t l y i l l u m i n a t e d by the x - r a y s . This heating i s s u f f i c i e n t to evaporate PMDA from the surface under i r radiation. Because of the thermal g r a d i e n t , there i s the i n t e r f e r e n c e f r i n g e p a t t e r n on the f i l m r e f l e c t i n g v a r i a t i o n i n f i l m t h i c k n e s s due to d i f f e r e n t evaporation r a t e s across the f i l m . The f i l m t h i c k n e s s v a r i a t i o n produces the v a r i a t i o n i n charging across the sample and the r e s u l t i n g U-shape curve i n b i n d i n g energy as a f u n c t i o n of p o s i t i o n across the sample with the g r e a t e s t charging at the edges where the f i l m i s t h i c k e s t due to the l e a s t amount of evaporation a r i s i n g from i t s lower temperature. Time dependent charging as well as lineshape d i s t o r t i o n could be observed during some experiments on the i n s i t u grown PMDA f i l m s as the f i l m t h i c k n e s s was changing dur ing the measurement. A r i s e i n system pressure (*vOne order of magnitude) a l s o could be observed upon x - r a y i r r a d i a t i o n of the sample. While t h i s c o n t i n u o u s l y produces a fresh c l e a n s u r f a c e , i t makes the experiements d i f f i c u l t ! Thus experiments on a sample l i k e PMDA and other s i m i l a r monomers would be best performed at low tempera tures . ODA presents a d i f f e r e n t beam induced e f f e c t . Small spot XPS a n a l y s i s across the sample does not show any


ure 2. Photograph of x - r a y beam induced damage i n PMDA and (b) ODA f i l m s .

2ΘΘ.0

(eV)

Figure 3. Carbon Is s p e c t r a o b t a i n by small spot a n a l y s i s (15Cjk) from two p o s i t i o n s on a p r e v i o u s l y beam dam aged PMDA f i l m .

B i n d i n g Energy

278.0

16


v a r i a t i o n i n c o r e - l e v e l binding energy f o r any p a r t i c u l a r feature. However, there i s a change i n the 0 Is l i n e s h a p e and a v a r i a t i o n i n the C/0 and N/0 r a t i o s with both r a t i o s i n c r e a s i n g . This behavior suggests photo-induced bond c l e a v a g e , probably at the ether l i n k a g e though d e t a i l i n t e r p r e t a t i o n would r e q u i r e f u r t h e r c o n t r o l l e d experiments without ambient exposure. Both uncured PMDA-ODA polyimide precursor (polyamic a c i d ) and f u l l y cured PMDA-ODA ( p o l y imide) d i d not e x h i b i t e i t h e r of the above e f f e c t s upon s i m i l a r exposures. The above i l l u s t r a t e s the u t i l i t y of small area x - r a y beams and why techniques with intense beams such as Auger e l e c t r o n spectroscopy have l i m i t e d u t i l i t y i n s t u d i e s on these m a t e r i a l s . Ion beams are often u t i l i z e d to prepare clean s u r faces f o r PES s t u d i e s or f o r depth p r o f i l i n g through a sample. This causes problems i n polymer s t u d i e s as the surface can be c h e m i c a l l y degraded as has been demonstrated i n the case of p o l y i m i d e ( 1 9 , 2 0 ) . This e f f e c t , howe v e r , has been used to increase metal/polymer adhesion, w h i l e the exact mechanism ( c h e m i c a l , mechanical) f o r the improved adhesion for the metals to polyimide i s not y e t completely understood ( 2 1 , 2 2 ) . Angle Dependence. The e f f e c t i v e escape depth i n a PES experiment can be s u b s t a n t i a l l y decreased by using a low e l e c t r o n e x i t angle geometry, i f one has a s u f f i c i e n t angle r e s o l v e d e l e c t r o n a n a l y z e r ( 2 3 ) . The e f f e c t i v e escape depth v a r i e s as the s i n # o f e l e c t r o n e x i t a n g l e , 0 , the angle between the e x i t i n g e l e c t r o n and the sample s u r face. The surface s e n s i t i v i t y can e a s i l y be increased by an order of magnitude. This can be very useful for d e t e r mining the o r i e n t a t i o n of an organic monolayer on a m e t a l . As an example using a modestly a n g l e - r e s o l v e d a n a l y z e r (30° acceptance angle) a monolayer of d o d e c a n e t h i o l , an alkane chain with S at one end, was a p p l i e d to a gold surface using the method of Whitesides et a l ( 2 4 ) . The C/Au and S/Au r a t i o s were i n v e s t i g a t e d as a f u n c t i o n of e x i t a n g l e . We observe t h a t the Au/S r a t i o i s n e a r l y cons t a n t upon v a r y i n g the e x i t a n g l e , w h i l e the C/Au r a t i o v a r i e s by more than a f a c t o r of 3 upon v a r i a t i o n of the e x i t angle with the r a t i o i n c r e a s i n g to lower (more surface s e n s i t i v e ) e x i t a n g l e s . This i s c o n s i s t e n t with the s t r u c t u r e o f a monolayer of the molecules being o r i e n t e d n e a r l y p e r p e n d i c u l a r and attached to the Au surface v i a the S end of the m o l e c u l e . Much work on t h i s i n t e r e s t i n g c l a s s of compounds has been c a r r i e d out r e c e n t l y by G. M. Whitesides and c o l l a b o r a t o r s (see reference 24 and r e f e r ences t h e r i n ) . This type of measurement would g r e a t l y b e n e f i t from the use of higher angular r e s o l u t i o n . Experiments with angular r e s o l u t i o n of 2° to 6° are q u i t e feasible (23). Photon Source. There are three main types of photon sources used i n PES. The most common l a b o r a t o r y source

2. KOWALCZYK


17

i s an x - r a v tube with e i t h e r AT (1486.6 eV) or Mg (1256.6 eV) Κα. r a d i a t i o n . Al sources have an advantage as they can be monochromatized to achieve s u p e r i o r r e s o l u t i o n and e l i m i n a t e bremstalung and s a t e l l i t e l i n e s from PES spectra. A l s o , a l Kc*- x - r a y s have been focussed to small spot s i z e , ~ 1 0 0 M ( 2 5 ) . The other common l a b o r a t o r y source i s the r a r e gas ( u l t r a v i o l e t ) discharge lamp, u t i l i z i n g p r i m a r i l y the He I (21.1 èV) or He II (40.8 eV) resonance l i n e s . These have the disadvantage that r e l a t i v e l y few core l e v e l s of i n t e r e s t ( e . g . C I s , D I s , Ν I s , or F Is) can be accessed nor can the e n t i r e valence band be measured. Synchrotron r a d i a t i o n provides a v a r i a b l e range of photon energies (most commonly i n the range of ~20-1000 eV with the c u r r e n t generation of synchrotrons and a v a i l a b l e monochromators). This a l l o w s advantage to be taken of the energy dependence of the e l e c t r o n i n e l e a s t i c mean path f o r increased surface s e n s i t i v i t y . Figure 4 shows a S i (111) surface covered with a t h i n l a y e r of the polyimide p r e c u r s o r , p o l y a m i c a c i d taken at two d i f f e r ent photon e n e r g i e s . The enhancement of the reacted c h e m i c a l l y s h i f t e d S i i n t e n s i t y at higher binding energy to the unreacted S i can be seen i n the spectrum obtained at the lower more surface s e n s i t i v e photon energy. Another aspect of synchrotron r a d i a t i o n i s the energy de pendence of the p h o t o i o n i z a t i o n cross s e c t i o n . This i s p a r t i c u l a r l y useful i n valence-band s t u d i e s where v a r i o u s l e v e l s can be modulated by v a r i a t i o n of the photon energy and w i l l be discussed i n a l a t t e r s e c t i o n . Sample P r e p a r a t i o n . The best method to study i n t e r f a c e s i n v o l v e s preparing the i n t e r f a c e i n s i t u with a t h i n overl a y e r such t h a t core l e v e l s from both sides of the i n t e r face can be observed (see Figure 5 ) . The i n t e r f a c e can be b u i l t up from a submonolayer coverage upto several l a y e r s to form a f u l l y developed ( " b u r r i e d " ) i n t e r f a c e . The metal-on-polymer i n t e r f a c e has been the most s t u d i e d i n t e r f a c e as metals can c o n v e n i e n t l y be deposited by evap o r a t i o n i n s i t u i n a c o n t r o l l a b l e f a s h i o n i n a UHV system (26-33). In the case of p o l y i m i d e , Cu and Cr have been the most studied metals but other metals i n c l u d i n g N i , Co, A l , Au, Ag, Ge, Ce, C s , and S i have been s t u d i e d . The best experimental arrangement i n c l u d e s a UHV system with a load lock i n t r o d u c t i o n chamber, a p r e p a r a t i o n chamber with e v a p o r a t o r s , heating c a p a b i l i t i e s , e t c . , and a sepa r a t e a n a l y s i s chamber. A l l the chambers are separated by gate valves and the samples are t r a n s f e r r e d between cham bers under vacuum. A l t e r n a t i v e metal d e p o s i t i o n sources such as o r g a n o m e t a l l i c chemical vapor d e p o s i t i o n are prom i s i n g and such techniques p o s s i b l y can lead to d i f f e r e n t i n t e r f a c e formation than obtained by metal e v a p o r a t i o n ( 3 4 ) . For polymer f i l m p r e p a r a t i o n , e s p e c i a l l y for the i n v e s t i g a t i o n of polymer-on-metal i n t e r a c t i o n s , a molecular beam d e p o s i t i o n (MBD) technique i s necessary. This t e c h nique was f i r s t demonstrated f o r polyimide by Salem and co»

18


1.0 - clean Si(lll) ..PAA/Si(m) 0.8

0.6

et,

1 0.4 a

62

64

66

68

Kinetic energy (eV) 1.0

Τ • clean Si(111) ..PAA/Si(111)

0.8

a ρ

0.6

ce 0.4

0.2

0.0 160

162

164

166

168

17C

Kinetic energy (eV)

Figure 4 . S i 2p a f t e r d e p o s i t i o n of ^ 4 A o f polyamic a c i d on S i ( l l l ) . The upper spectrum was obtained with hv • 167 eV and the lower spectrum with hv = 267 eV. The S i 2ph s p i n - o r b i t component has been subtracted for c l a r i t y .

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Figure 5. Schematic of an i n t e r f a c e sample s u i t a b l e fo PES i n t e r f a c e a n a l y s i s , where A and Β are e i t h e r a polymer or metal of i n t e r e s t .

20


workers (35) and a p p l i e d to UHV i n t e r f a c e s t u d i e s by Grunze and co workers ( 3 6 - 3 9 ) . This technique i n v o l v e s the use of molecular beams of PMDA and ODA, which form polyamic a c i d on the s u r f a c e , which upon c u r i n g (heating to 400*) c y c l o i m i d i z e s the polyamic a c i d to form p o l y i mide. The t a b l e shows a comparison of energy s p l i t t i n g s and i n t e n s i t y of the f i v e main features i n the C Is spectrum between a MBD polyimide (PMDA-ODA) f i l m and a spun film. This technique a l l o w s the study of the polymer-onmetal i n t e r f a c e f o r m a t i o n . I t a l l o w s the metal surface to be c o n t r o l l a b l y prepared i n UHV p r i o r to c o a t i n g with the polymer. Thus an a t o m i c a l l y c l e a n surface can be prepared or a p a r t i c u l a r oxide surface grown p r i o r to polymer overgrowth. This technique e l i m i a t e d the use of s o l v e n t s , which can introduce other c o m p l i c a t i n g e f f e c t s . Table I .

Comparison between spun and MBE PI

samp!e

MBE

spun

Peak

E(eV)

I

E(eV)

I

PI

0.00

36.1

0.00

36.1

P2

1.03

34.2

1.03

36.1

P3

1.66

8.2

1.66

6.4

P4

4.04

13.8

4.04

12.4

P5

6.4,8.1

7.6

6.5,8.3

8.9

Spun-on polymer f i l m s can be used as s u b s t r a t e s f o r i n v e s t i g a t i o n of metal-on-polymer i n t e r f a c e s . Good p o l y imide surfaces can be prepared from as r e c e i v e d , f r e s h l y spun f i l m s . However, polyimide i s known to be very suscept i b l e to water uptake. Heating to 350*in UHV appears to regenerate the polyimide i f i t has been exposed to ambient f o r any length of t i m e . For polymer-on-metal s t u d i e s when using spun-on p o l y i m i d e , the metal surface i s mostly l i m i t e d to a n a t i v e oxide covered surface r a t h e r than atomica l l y c l e a n metal surfaces as these i n t e r f a c e s are not usua l l y prepared under vacuum. This i s nonetheless of great i n t e r e s t as such metal surfaces are t e c h n o l o g i c a l l y important s u r f a c e s . One must be c a u t i o u s about spun t h i n f i l m s of 50A or l e s s necessary f o r i n t e r f a c e s t u d i e s as they may not be p i n h o l e f r e e . Solvent used i n spun-on f i l m s themselves may cause important e f f e c t s ( 4 0 - 4 2 ) . Again t h i s i t s e l f i s of i n t e r e s t . Comparison of spun f i l m s with MBD f i l m s can help to i s o l a t e the r o l e of s o l v e n t s i n r e a c t i o n s during the cure of the polymer ( 4 0 ) . Peel t e s t s , where the force to peel a s t r i p of one m a t e r i a l (metal) from another (polymer) i s measured, i s a common technique to o b t a i n a measure of adhesion s t r e n g t h .

2. KOWALCZYK


21

Both peeled surfaces subsequently can be analyzed by PES to determine the p o i n t of f a i l u r e ( 4 3 ) . This can be use ful to d i s t i n g u i s h between adhesisve and cohesive f a i l u r e as well as the presence of contamination at the p o i n t of failure. It i s l i m i t e d in obtaining d e t a i l e d information about the i n i t i a l i n t e r f a c i a l chemistry as the peel i s normally performed e x - s i t u . Surface m o d i f i c a t i o n p r i o r to m e t a l l i z a t i o n i s ano ther important area of r e s e a r c h . For example, p r e s p u t t e r ing can be an important treatment f o r adhesion enhancement (21,22) but contamination e f f e c t s due to r e d e p o s i t i o n ef f e c t s are common. Wet chemical treatments and dry (gas phase) etchings are other areas of pretreatments under active current i n v e s t i g a t i o n (44,45). INFORMATION FROM PES C o r e - l e v e l S p e c t r a : One-hole S t a t e s , S a t e l l i t e s , Relaxation. C o r e - l e v e l spectra have been the most u t i l i z e d f e a ture of PES i n v e s t i g a t i o n s of polymers. The f i r s t stage of polymer/metal i n t e r f a c e s t u d i e s i s to o b t a i n and under stand the spectra of the m a t e r i a l s p r i o r to i n t e r f a c e f o r mation. Figure 6 shows C Is spectra from two d i f f e r e n t p o l y i m i d e s , PMDA-ODA and BPDA-PDA ( b i s p h e n y l d i a n h y d r i d e phenyldiamine). These two polyimides have d i f f e r e n t chem i c a l s t r u c t u r e s which produce very d i f f e r e n t c h a r a c t e r i s tic core-level spectra. PMDA-ODA polyimide has been well c h a r a c t e r i z e d and the main features of the c o r e - l e v e l spectra are well understood ( 4 6 , 4 7 ) . The d i f f e r e n t r i n g s t r u c t u r e ( d i f f e r e n t number of carbonyl groups on the anhydride r i n g s ) between PMDA-ODA and BPDA-PDA leads to d i f f e r e n t e l e c t r o n charge d i s t r i b u t i o n thus the s h i f t s between the anhydride and amine r i n g s i s d i f f e r e n t i n the two p o l y i m i d e s . Figure 6 a l s o shows the C Is spectrum of the polyamic e s t e r of PMDA-ODA, an a l t e r n a t i v e precursor to polyamic a c i d f o r the p r e p a r a t i o n of PMDA-ODA p o l y i mide. The polyamic e s t e r spectra i s of course q u i t e d i f f e r e n t than the polyimides e s p e c i a l l y i n the carbonyl r e g i o n . However, upon c u r i n g , the polyimide formed i s s i m i l a r to the one form from the PMDA-ODA polyamic a c i d (com pare Figures 6a and 6 d ) . Figure 7 f o l l o w s the i n t e r f a c e formation between Cr and p o l y i m i d e , both i n the case of Cr on polyimide and polyimide ( i n i t i a l l y polyamic a c i d ) on C r . The C Is spec t r a show s i g n i f i c a n t changes as the i n t e r f a c e developes. There i s a c t u a l l y a remarkable correspondence between the two types of i n t e r f a c e s ( 4 8 ) . Ν Is and 01s spectra a l s o e x h i b i t s i m i l a r s t r i k i n g correspondence. These r e s u l t s tend to support the r e a c t i o n v i a the carbonyl moiety (32) than the d e l o c a l i z e d Cr arene complex formation i n t e r p r e tation (30). C o r e - l e v e l spectra of the metal g e n e r a l l y have been l i t t l e u t i l i z e d i n polymer/metal i n t e r f a c e s t u d i e s . Metal core l e v e l s are often much broader than the C I s , 0 I s , or

22


290.3

Binding energy (eV)

282.8

293.0

Binding energy (eV)

283.0

294.0

Binding energy (eV)

284.0

292.5

Binding energy (eV)

282.5

Figure 6. Carbon Is spectra from two d i f f e r e n t p o l y i mides: (a) PMDA-ODA, (b) BPDA-PDA, (c) Polyamic e s t e r , and (d) Cured polyamic e s t e r .

2. KOWALCZYK

a) PAA on Cr -30 λ

23


F\

b) Annealed -22Â

/1 \ 1 ' 1 r »

1

1 «»» Λ /'r ι» * ^1 f »\ \\

3 c) PAA on Cr -80 Â

d) Annealed /I 1

L·

Λ

ΙΛ

-40 λ

ι

l

Λ\ r /

\

t) PAA on Cr >80Â

f) Annealed -80 À

i

r 294

290

È

V

Λ \ f ί» 1 ; »

Λ /

*

Λ Γ» » ι fill i l l / ι» \

\

/

' » V

/ \V 286

282

290

286

BINDING ENERGY (Relative to E ) p

282

294

290

286

282

BINDING ENERGY Relative to t (eV) f

Figure 7. C Is spectra from polyamic a c i d on C r , cured polyamic a c i d on Cr (Reproduced from Ref. 4 8 . Copyright 1989 American Chemical S o c i e t y . ) and Cr on polyimide i n t e r f a c e s . (Reproduced with permission from Ref. 32. Copyright 1987 American P h y s i c a l S o c i e t y . )

24


Ν Is l e v e l s and the chemical s h i f t s a l s o tend to be s m a l l er(48,49). Figure 8 shows Cr 2p spectra where l i t t l e change i s observed upon polyamic a c i d a b s o r p t i o n although corresponding C Is and 0 Is spectra (see Figure 7) show disappearance of the carbonyl f u n c t i o n a l group. Upon c u r ing there i s evidence of c o n s i d e r a b l e broadening on the high binding energy side of the Cr 2p peak, evidence for o x i d a t i o n but not very much d e t a i l e d i n f o r m a t i o n can be deduced from t h i s spectrum, w h i l e again the corresponding C I s , 0 I s , and Ν Is show the fragmentation of polyimide the formation of c a r b i d e s , o x i d e s , and n i t r i d e s ( 4 8 ) . Figure 4, which shows the S i 2p s p e c t r a , i s one case where d e t a i l e d s t r u c t u r e about chemical i n f o r m a t i o n i s observed from the non-polymer c o r e - l e v e l spectrum. S i +1, +2, and +3 s t a t e s can be c l e a r l y seen a f t e r s p i n - o r b i t deconvolution (48). I t should a l s o be noted that the i n t e r p r e t a t i o n of c o r e - l e v e l s h i f t s f o r small coverages of metal i s f u r t h e r complicated by s i z e e f f e c t s . In any d e t a i l e d i n t e r p r e t a t i o n of c o r e - l e v e l s p e c t r a , the e n t i r e s p e c t r a l d i s t r i b u t i o n must be taken i n t o ac count to o b t a i n a complete c h a r a c t e r i z a t i o n . The C Is spectrum from an ODA f i l m i s shown i n Figure 9. This spectrum besides e x h i b i t i n g the two main features c o r r e s ponding to the two types of i n e q u i v a l e n t carbons i n ODA, e x h i b i t s weaker s t r u c t u r e at about 6 eV higher binding energy. These features are shake-up s t r u c t u r e s due to m u l t i e l e c t r o n t r a n s i t i o n s during the photoemission pro cess from a deep core l e v e l , e . g . yt t o n * t r a n s i t i o n s . These t r a n s i t i o n s which i n v o l v e the highest occupied and lowest unoccupied molecular o r b i t a l s give f u r t h e r i n f o r mation concerning the e l e c t r o n i c s t r u c t u r e of these mater ials. Shakeups can have important e f f e c t s on the observed i n t e n s i t y d i s t r i b u t i o n . In the case of PMDA the i n t e n s i t y r a t i o of the two C Is peaks i s not the 1.50 expected by s t o i c h i o m e t r y (6 aromatic carbons/4 carbonyl carbons) but 1.42. Likewise f o r the r a t i o between the b r i d g i n g 0 and the carbonyl oxygen i s 1.7 and not the s t o i c h i o m e t r i c 2 . 0 . Recent s e l f - c o n s i s t e n t extended basis set c a l c u l a t i o n s which have considered m u l t i e l e c t r o n e f f e c t s , r e l a x a t i o n , e t c . , have given these r a t i o s as 1.41 and 1.8, q u i t e com p a t i b l e with the experimental values ( 5 0 ) . The missing i n t e n s i t y i s accounted f o r by c o n s i d e r i n g the complete photoemission process ( 5 0 ) . Another c o n s i d e r a t i o n i n i n t e r p r e t a t i o n and data treatment i s the i n e l a s t i c back ground ( 5 1 , 5 2 ) , which we w i l l o n l y mention here. Valence-band S p e c t r a : E l e c t r o n Density of S t a t e s . Valence band s p e c t r a , which i n the case of polymers are very molec u l a r - l i k e as there i s l i t t l e d i s p e r s i o n i n the bands, have been l e s s used, e s p e c i a l l y i n polymer-metal i n t e r f a c e studies. They o f f e r p o t e n t i a l l y much deeper understanding of i n t e r a c t i o n s but are harder to o b t a i n and need to be c l o s e l y coupled to t h e o r e t i c a l work f o r i n t e r p r e t a t i o n ( 5 3 ) . Recent t h e o r e t i c a l s t u d i e s of polyimide has shown

2. KOWALCZYK


Figure 8. (a) Comparison of Cr 2p spectra of Cr metal with-~30A of polyamic a c i d coverage and (b) the same a f t e r cure of polyamic a c i d . The dotted spectrum i n each case i s from the i n i t i a l Cr metal s u r f a c e .

25

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good agreement with p h o t o e l e c t r o n spectra ( 5 3 ) . The next step i s to apply t h i s i n f o r m a t i o n to the p o l y i m i d e / m e t a l systems: (a) f i r s t l y , to observe which valence (molecular) l e v e l s of the polymer are a l t e r e d upon i n t e r f a c e formation and (b) to perform more s o p h i s t i c a t e d c a l c u l a t i o n s d i r e c t l y on the polymer-metal system to see the molecular o r b i t a l - m e t a l v a l e n c e - l e v e l i n t e r a c t i o n and the new v a l e n c e level structure. Here synchrotron r a d i a t i o n may provide an important advantage. The technique used i n adsorbate s t u d i e s on metals to enhance adsorbate emission r e l a t i v e to metal emission i s q u i t e a p p l i c a b l e . This i n v o l v e s using c r o s s - s e c t i o n modulation ( 5 4 - 5 7 ) . For example at c e r t a i n e n e r g i e s , the C 2p d e r i v e d l e v e l s from the polymer may be more intense than the metal d band, w h i l e at another energy, these i n t e n s i t y r a t i o s maybe r e v e r s e . The v a r i a t i o n of the C/metal r a t i o n as f(hv) can be q u i t e s i g n i f i c a n t ( 5 6 , 5 7 ) . The c r o s s - s e c t i o n v a r i a t i o n f o r p o l y i mide can be seen i n Figure 10 which shows the polyimide valence-band spectra i n the range of 80

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