EXAFS STUDY OF COPPER-HAFNIUM MULTILAYERS

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EXAFS STUDY OF COPPER-HAFNIUM MULTILAYERS

S. Heald, J. Tranquada, B. Clemens, J. Stec

To cite this version:

S. Heald, J. Tranquada, B. Clemens, J. Stec. EXAFS STUDY OF COPPER-HAFNIUM MULTILAYERS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-1061-C8-1064.

�10.1051/jphyscol:19868206�. �jpa-00226115�

EXAFS STUDY OF COPPER-HAFNIUM MULTILAYERS

S.M. HEALD, J.M. TRANQUADA, B.M. CLEMENS* and J.P. STEC*

Brookhaven National Laboratory, Upton, NY 11973, U.S.A.

"physics Department, General Motors Research, Warren, MI 48090-9055, U.S.A.

ABSTRACT.

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EXAFS measurements have been made on a number of Cu-Hf m u l t i l a y e r s with modulation wavelengths of 112 t o 20 close-packed monolayers. C l e a r i n d i c a t i o n s a r e found f o r a n amorphous i n t e r f a c i a l compound. F i t t i n g of t h e EXAFS s p e c t r a i n d i c a t e t h a t t h e i n t e r f a c i a l r e g i o n s a r e about 8-10 l a y e r s t h i c k , and c o n t a i n n e a r l y equal numbers of l a y e r s of Hf and Cu. The r e s u l t s a r e compared t o x-ray d i f f r a c t i o n s t u d i e s of t h e same samples.

INTRODUCTION

The s t r u c t u r e of s o l i d s t a t e i n t e r f a c e s has important i m p l i c a t i o n s i n understanding s o l i d s t a t e r e a c t i o n s , t h i n f i l m adhesion and s t a b i l i t y . EXAFS measurements can be u s e f u l i n providing such s t r u c t u r a l information p a r t i c u l a r l y f o r p o l y c r y s t a l l i n e o r d i s o r d e r e d f i l m s where t h e i n f o r m a t i o n from d i f f r a c t i o n measurements may be l i m - i t e d . The problem i n a p p l y i n g EXAFS-is o b t a i n i n g a s i g n i f i c a n t i n t e r f a c e component i n t h e EXAFS s i g n a l . One approach i s t o use glancing a n g l e s t o l i m i t the x-rays t o t h e i n t e r f a c e r e g i o n [ I ] . I n t h i s paper we d e s c r i b e a second approach, t h e use of m u l t i l a y e r s t o i n c r e a s e t h e i n t e r f a c e c o n t r i b u t i o n t o t h e s i g n a l . By measuring t h e m u l t i l a y e r system a t s e v e r a l l a y e r t h i c k n e s s e s the r e l a t i v e c o n t r i b u t i o n of t h e i n t e r f a c e component i s v a r i e d and can thereby be i s o l a t e d . I n metal m u l t i l a y e r s t h e d i f f r a c t i o n from t h e d i s o r d e r e d i n t e r f a c e i s o f t e n masked by the s t r o n g d i f - f r a c t i o n i n t e n s i t y from t h e c r y s t a l l i n e l a y e r s . EXAFS i s s u i t e d t o t h i s type of study s i n c e t h e r e l a t i v e s e n s i t i v i t y t o d i s o r d e r e d and ordered s t r u c t u r e s is ap- proximately equal. Recently, t h e a p p l i c a t i o n of EXAFS t o m u l t i l a y e r f i l m s has been demonstrated f o r Nb-Zr m u l t i l a y e r s [ 2 ] .

The system chosen f o r study i s the Cu-Hf system which has a r e l a t i v e l y l a r g e s i z e mismatch (11%) s o t h a t c o h e r e n t i n t e r f a c e s a r e n o t expected. T h i s i s s i m i l a r t o o t h e r l a t e t r a n s 1 t i o n m e t a l - e a r l y t r a n s i t i o n metal systems which have been i n v e s t i - g a t e d [ 3 ] . Unlike s e v e r a l such systems s o l i d s t a t e r e a c t i o n does n o t r e s u l t i n amorphous phase formation i n t h e Cu-Hf system 131. I n v e s t i g a t i o n s of the i n t e r f a c e s t r u c t u r e should be u s e f u l i n determining what the r o l e of t h e i n t e r f a c e s t r u c t u r e p l a y s i n f a i l u r e t o observe s o l i d s t a t e formation of t h e amorphous a l l o y .

EXPERIMENT AND RESULTS

Compositionally modulated Cu-Hf m u l t i l a y e r samples of wavelength of 0.5 t o 20 mono- l a y e r s were prepared on o x i d i z e d s i l i c o n wafers and kapton s u b s t r a t e s by magnetron s p u t t e r i n g . Here and elsewhere i n t h i s paper t h e composition modulation wavelength d e s i g n a t i o n is i n terms of t h e number of c l o s e packed planes of e a c h c o n s t i t u e n t p e r l a y e r . Due t o t h e i r d i f f e r i n g l a t t i c e parameters a l a y e r of Cu atoms c o n t a i n s a b o u t 1.5 times t h e number of atoms a s a Hf l a y e r . The s u b s t r a t e s were r o t a t e d under two independently c o n t r o l l e d sources t o produce a composition modulation wavelength of any d e s i r e d value. The magnetron s o u r c e s were o p e r a t e d a t 2.0 mTorr

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868206

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of argon which was monitored by a high s t a b i l i t y c a p a c i t a n c e manometer. X-ray d i f f r a c t i o n was performed on samples i n t r a n s m i s s i o n and r e f l e c t i o n geometries.

D e t a i l s of t h e s e measurements w i l l be published elsewhere. I n summary, samples w i t h composition modulation wavelength o f 1/2 and 5 looked completely amorphous w i t h a s i n g l e broad d i f f r a c t i o n maxima i n t r a n s m i s s i o n and r e f l e c t i o n geometries.

Some a n i s o t r o p y was evidenced i n a sample w i t h 7 monolayers by i n c r e a s e d i n t e n s i t y of t h e d i f f r a c t i o n maxima i n t r a n s m i s s i o n and r e f l e c t i o n geometries. The d i f f r a c - t i o n maxima s e p a r a t e d i n t o two d i s t i n c t peaks a t a composition modulation wave- l e n g t h of 10 i n r e f l e c t i o n geometry, and c r y s t a l l i n e peaks were superimposed on an amorphous background i n t r a n s m i s s i o n geometry. Small a n g l e d i f f r a c t i o n i n r e f l e c - t i o n geometry showed s e v e r a l s h a r p maxima f o r samples w i t h a composition modulation wavelength of 5 monolayers of each c o n s t i t u e n t o r g r e a t e r . These peaks a r e e v i - dence of composition modulation, and were used t o determine the composition mod- u l a t i o n wavelength, which compared f a v o r a b l y w i t h t h a t determined from d e p o s i t i o n c o n d i t i o n s . No small a n g l e peaks were observed f o r t h e 1 / 2 monolayer sample which i s a homogeneous amorphous a l l o y , i n d i s t i n g u i s h a b l e from one produced by co-deposition.

A l l t h e EXAFS measurements were made i n t r a n s m i s s i o n by s t a c k i n g s e v e r a l l a y e r s of sample. Most measurements were made with the x-ray p o l a r i z a t i o n p a r a l l e l t o t h e m u l t i l a y e r s , a l t h o u g h f o r t h e 7 and 20 monolayer samples t h e Hf edge measurements were a l s o made w i t h t h e p o l a r i z a t i o n r o t a t e d 60° o u t of t h e planes. No p o l a r i z a - t i o n dependence was found. Measurements were made f o r both the Cu and Hf edges, and analyzed u s i n g s t a n d a r d techniques. Examples of F o u r i e r transforms of t h e Hf d a t a a r e shown i n Fig. 1.

The transforms f o r t h e samples w i t h a composition modulation of 7 o r l e s s a l l look t h e same w i t h t h e f i r s t s h e l l s h i f t e d - 0 . 6 A t o s m a l l e r R from t h e Hf metal s t a n - dard. For the 10 monolayer sample a s i g n i f i c a n t s i g n a l due to Hf metal begins to appear which grows a s t h e l a y e r t h i c k n e s s i n c r e a s e s . S i m i l a r r e s u l t s were found f o r t h e Cu edge d a t a , although i n t h a t c a s e t h e r e i s much more o v e r l a p between t h e amorphous phase and metal transform peaks.

On t h e b a s i s of t h e s e r e s u l t s a very simple model p r e s e n t s i t s e l f . The m u l t i l a y e r i n t e r f a c e is a t h i n l a y e r of amorphous m a t e r i a l which i s surrounded by bulk metal i f t h e l a y e r s a r e t h i c k enough. To t e s t t h i s model l e a s t s q u a r e s f i t s of the d a t a were attempted u s i n g t h e bulk m e t a l s and t h e 112 monolayer samples a s s t a n d a r d s . E x c e l l e n t f i t s were o b t a i n e d a s i l l u s t r a t e d i n Fig. 2 f o r t h e 20 monolayer case.

The Cu edge d a t a were f i t using one parameter d e s c r i b i n g the r a t i o of amorphous t o metal phase. For t h e Hf edge some improvement i n t h e f i t could be obtained i f an a d d i t i o n a l c o n t r i b u t i o n t o the Hf metal Debye-Waller f a c t o r was included. The r e s u l t s of f i t t i n g t o t h i s simple model a r e summarized i n Table I, where the r e - s u l ts a r e a l s o used t o e s t i m a t e t h e number of metal l a y e r s involved i n t h e amor- phous i n t e r f a c e . The number of l a y e r s t i e d up i n t h e amorphous i n t e r f a c e i s ap- proximately the same f o r Hf and Cu except a t l a r g e r l a y e r t h i c k n e s s e s where more Cu l a y e r s tend t o be involved. Thus, i n terms of t h e number of atoms t h e i n t e r f a c e c o n t a i n s subs t a n t i a l l y more Cu than Hf

.

The s u c c e s s of t h i s simple model is somewhat s u r p r i s i n g s i n c e the i n t e r f a c e might be expected t o have a composition g r a d i e n t w i t h varying n e a r neighbor environments.

Some i n d i c a t i o n of t h i s was seen f o r t h e 10 monolayer Hf edge d a t a where t h e f i t s were s i g n i f i c a n t l y i n f e r i o r . T h i s seemed t o be due t o a d i s t o r t i o n of the very

t h i n Hf metal l a y e r by the surrounding amorphous m a t e r i a l . I n g e n e r a l , however, t h e f i t s a r e e x c e l l e n t i n d i c a t i n g t h a t e i t h e r the amorphous phase EXAFS i s r e l - a t i v e l y i n s e n s i t i v e t o composition o r t h a t t h e r e is a p r e f e r r e d composition i n t h e - amorphous l a y e r . A d d i t i o n a l s t u d i e s on Hf-Cu amorphous compounds of v a r i o u s com- p o s i t i o n s a r e planned t o answer t h e s e q u e s t i o n s . I n i t i a l a t t e m p t s a t analyzing t h e Cu environment i n t h e amorphous phase i n d i c a t e t h a t t h e r e may be some l o c a l o r d e r - i n g w i t h t h e Cu n e a r e s t neighbors being predominantly Bf. F u r t h e r work i s needed f o r more d e f i n i t i v e r e s u l t s .

1 / 2 monolayers, b) 5 monolayers, c ) 10 d e s c r i b e d i n t e x t ( p o i n t s ) f o r t h e 20 monolayers, d) 14 monolayers, e ) 20 nonolayer Cu-Hf m u l t i l a y e r .

monolayers, f ) hafnium metal.

The o b s e r v a t i o n of d i s o r d e r e d i n t e r f a c e s of a p p r e c i a b l e s i z e i s c o n s i s t e n t w i t h t h e l a r g e s i z e mismatch between the c o n s t i t u e n t s , and i s evidence f o r c o n s i d e r a b l e atomic m o b i l i t y during t h e d e p o s i t i o n process. P r e v i o u s work on Ni-Ti demonstrated t h e importance of i n t e r f a c i a l d i s o r d e r f o r s o l i d s t a t e formation of amorphous a l - l o y s [4]. The f a i l u r e t o observe amorphous phase growth d u r i n g s o l i d s t a t e reac- t i o n i n Cu-Hf i m p l i e s t h a t k i n e t i c s of growth p l a y a c r u c i a l r o l e i n amorphous phase growth, i . e . , amorphous i n t e r f a c e s a r e a n e c e s s a r y b u t n o t a s u f f i c i e n t con- d i t i o n f o r amorphous phase growth. A p r e v i o u s l y published p r e d i c t i v e scheme f o r s o l i d s t a t e formation of amorphous a l l o y s proposed the importance of two param- e t e r s , the thermodynamic d r i v i n g f o r c e f o r t h e r e a c t i o n , and t h e volume r a t i o , which c o r r e l a t e s w i t h r a p i d d i f f u s i o n [ 3 ] . The Cu-Hf system has a small d r i v i n g

f o r c e and a r a t h e r l a r g e volume r a t i o r e l a t i v e t o systems which e x h i b i t amorphous phase formation by s o l i d s t a t e r e a c t i o n . T h i s may e x p l a i n why t h e growth k i n e t i c s a r e n o t conducive t o amorphous phase growth.

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T a b l e I

F i t t i n g R e s u l t s f o r t h e Model D e s c r i b e d i n t h e T e x t

Layer Thickness F r a c t i o n of Amorphous Phase Amorphous Layers

Hf Edge Cu Edge Hf Cu

ACKNOWLEDGMENTS

T h i s work was performed a t beam l i n e X - 1 1 a t t h e NSLS and is s u p p o r t e d i n p a r t by t h e D i v i s i o n of M a t e r i a l s S c i e n c e o f DOE under C o n t r a c t Nos. DE-AS05-80-ERl0742 and DE-AC02-76CH00016.

REFERENCES

111

[ 2 ] T. Claeson, J. B. Boyce, W. P. Lowe, and T. H. G e b a l l e , Phys. Rev. B

2 ,

131 B. M. Clemens, Phys. Rev. B E (1986).

[41 B. M. Clemens, J. Appl. Phys. ( i n p r e s s , 1986).