DIFFUSION MECHANISMS IN THE Cu/Sb THIN FILM SYSTEM

(1)

HAL Id: jpa-00230346

https://hal.archives-ouvertes.fr/jpa-00230346

Submitted on 1 Jan 1990

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

DIFFUSION MECHANISMS IN THE Cu/Sb THIN FILM SYSTEM

R. Halimi, A. Merabet

To cite this version:

R. Halimi, A. Merabet. DIFFUSION MECHANISMS IN THE Cu/Sb THIN FILM SYSTEM. Journal

de Physique Colloques, 1990, 51 (C1), pp.C1-501-C1-506. �10.1051/jphyscol:1990178�. �jpa-00230346�

(2)

DIFFUSION MECHANISMS IN THE Cu/Sb THIN FILM SYSTEM

R . HALIMI and A. MERABET*

DBpartement de Physique du Solide, Institut de Physique, UniversitB de Fonstantine, Constantine, AlgBrie

I.N.E.S. dlOptique et de MBcanique de PrBcision, SBtif, Algerie

RBsumB

-

L ' i n t e r d i f f u s i o n d a n s l e s f i l m s minces b i m B t a l l i q u e s d e Cu/Sb e s t BtudiBe p a r r 6 t r o d i f f u s i o n R u t h e r f o r d d a n s l ' i n t e r v a l l e d e t e m p k r a t u r e 150

-

300°C. Les p r o f i l s d e d i f f u s i o n s o n t a n a l y s e s 3 l ' a i d e du mod&le d e Whipple. Les v a l e u r s d e s c o e f f i c i e n t s d e d i f f u s i o n e n volume d e Sb d a n s Cu B t a n t p r i s e s d e l a rBfBrence/l2/, on t r o u v e l e s ener- g i e s d ' a c t i v a t i o n d e d i f f u s i o n p a r l e s j o i n t s d e g r a i n s s u i v a n t e s : ? , l 2 eV e t 1,26 eV pour l e s B c h a n t i l l o n s r e s p e c t i v e m e n t , (600

A

Sb s u r 650

d

Cu) e t (1800

d

Sb s u r 3100 Cul. I1 e s t B t a b l i que l e s c o e f f i c i e n t s d e d i f f u s i o n p a r l e s j o i n t s d e g r a i n s s o n t 1 0 3

-

106 f o i s p l u s i m p o r t a n t s que ceux de l a d i f f u s i o n en volume e x t r a p o l 6 s p a r t i r d e s mesures d e r a d i o t r a c e u r s u r d e s B c h a n t i l l o n s m a s s i f s . I 1 e s t Bgalement o b s e r v e que l a d i f f u s i o n e s t moins r a p i d e d a n s l e s 6 c h a n t i l l o n s r e l a t i v e m e n t Bpais.

A b s t r a c t

-

The i n t e r d i f f u s i o n o f b i m e t a l l i c Cu/Sb t h i n f i l m c o u p l e s is s t u d i e d u s i n g R u t h e r f o r d b a c k s c a t t e r i n g f o r a n n e a l i n g t e m p e r a t u r e s i n t h e r a n g e 150

-

300°C. Depth p r o f i l e s a r e analyzed u s i n g t h e Whipple's model. Taking t h e l a t t i c e d i f f u s i v i t i e s o f Sb i n Cu from Ref./l2/, f o l l o w i n g a c t i v a t i o n e n e r i e s f o r g r a i n boundary d i f f u s i o n a r e found : 1 , l 2 eV and 1 , 2 6 eV f o r r e s p e c t i v e l y (600

1

Sb on 650

d

Cu) and (1800

d

Sb on 3100

1

Cu) samples. I t i s e s t a b l i s h e d t h a t t h e g r a i n boundary d i f f u s i v i t i e s a r e 3

-

⁶

t i m e s l a r g e r t h a n t h e l a t t i c e d i f f u s i v i t i e s p r e d i c t e d by an e x t r a p o l a t i o n o f r a d i o a c t i - ve t r a c e r measurements o f bulk specimens. I t i s a l s o observed t h a t d i f f u s i o n is lower i n r e l a t i v e l y t h i c k samples.

1

-

INTRODUCTION

I n t e r d i f f u s i o n i n t h i n f i l m c o u p l e s h a s a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n because o f t h e i m - p o r t a n c e o f t h i n f i l m s i n m i c r o e l e c t r o n i c d e v i c e s /l

-

3/. Understanding t h e d i f f u s i o n pro- c e s s e s is a l s o v e r y i m p o r t a n t i n s u c h a r e a s o f i n v e s t i g a t i o n a s c r y s t a l l i z a t i o n , p r e c i p i t a - t i o n , oxydation and s u r f a c e s e g r e g a t i o n i n a l l o y s e t c . I t is known t h a t d i f f u s i o n o c c u r s much f a s t e r i n t h i n s o l i d f i l m s t h a n i n bulk m a t e r i a l s s i n c e t h e p r e s e n c e o f p o i n t and l i n e d e f e c t s , g r a i n boundaries and s t r a i n s c a n enhance t h e d i f f u s i o n p r o c e s s e s . I t is f o r t h i s r e a s o n t h a t d i f f u s i o n i n t h i n f i l m s can be observed a t t e m p e r a t u r e s w e l l below t h o s e a t which e q u i l i b r i u m v a c a n c i e s d i f f u s i o n is n e g l i g i b l e . Other f a c t o r s which can i n f l u e n c e t h e d i f f u - s i o n i n t h i n f i l m s a r e d e t a i l e d by Weaver /4/ and B a l l u f f i and B l a k e l y / 5 / .

D i f f u s i o n measurements with t h i n f i l m s r e q u i r e high d e p t h r e s o l u t i o n due t o t h e s m a l l f i l m t h i c k n e s s and r e l a t i v e l y s h o r t d i f f u s i o n l e n g t h s a t t h e t e m p e r a t u r e s o f i n t e r e s t . G e n e r a l l y , a n a l y t i c a l t e c h n i q u e s s u c h a s Auger e l e c t r o n s p e c t r o s c o p y (AES) /6/, secondary i o n mass spec- t r o m e t r y (SIMS) /7/ and R u t h e r f o r d b a c k s c a t t e r i n g s p e c t r o s c o p y (RBS) /8,9/ a r e a p p l i e d t o such s t u d i e s .

The o b j e c t i v e o f t h i s s t u d y is t o d e t e r m i n e g r a i n boundary d i f f u s i o n p a r a m e t e r s i n Cu/Sb t h i n f i l m s u s i n g t h e RBS t e c h n i q u e .

2

-

EXPERIMENTAL PROCEDURE

The t h i n f i l m c o u p l e s a r e p r e p a r e d by t h e r m a l e v a p o r a t i o n i n vacuum ( I O - ~ T o r r ) o f p u r e (99,99 L ) Cu and Sb m e t a l s . Copper f i l m s a r e d e p o s i t e d o n t o g l a s s s u b s t r a t e s a t 350°C and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990178

(3)

Cl-502 COLLOQUE DE PHYSIQUE

then antimony f i l m s a r e deposited onto copper a t 60°C. The two evaporation have been p e r f o r - med without breaking t h e vacuum. Two s e t s o f samples are evaporated : (600

8

Sb on 650 !/ Cu) f c r t h e f i r s t s e t and (1800

f!

Sb on 3100

A

Cu) f o r t h e second. The f i l m couples are annealed a t 150

-

300°C i n vacuum ( 5 . 1 0 - ~ Torr).

Analysis o f t h e samples was performed w i t h Rutherford backscattering spectrometry. I n t h e experiment, He+ ions w i t h energy 1,4 MeV c o l l i m a t i n g through a s l i t system are allowed t o h i t normally t h e specimen surface. The d e t e c t o r forms an angle of 170° w i t h respect t o t h e i n c i d e n t d i r e c t i o n . The detector s i g n a l s a r e converted i n t o an energy spectrum o f t h e back- s c a t t e r e d ions. The concentration p r o f i l e s are computed from t h e energy spectra.

Grain s i z e measurements and e l e c t r o n d i f f r a c t i o n p a t t e r n s are obtained u s i n g a 200 keV transmission e l e c t r o n microscope and have been described elsewhere /10/.

3

-

EXPERIMENTAL RESULTS AND DISCUSSION

Antimony d i f f u s i o n i n s i n g l e - c r y s t a l copper was s t u d i e d by t h e r a d i o a c t i v e - t r a c e r technique / l 1

-

13/. However i n t h i n s o l i d f i l m s , o n l y one study has been r e a l i z e d u s i n g x-ray d i f f r a c - t i o n / l O/.

Figure 1 shows energy backscattering spectra obtained from double l a y e r f i l m s o f 1800

fi

Sb and 3100

A

Cu before and a f t e r annealing a t various temperatures. The s a l i e n t feature o f t h i s f i g u r e i s immediately obvious from i n s p e c t i o n o f t h e spectra. There i s no i n t e r d i f f u s i o n between t h e Sb and Cu f i l m s i n an unannealed s t a t e , s i n c e t h e corresponding s p e c t r u m ( F i g . l ( l ) ) c o n s i s t s o f two very d i s t i n c t p a r t s : t h e Sb spectrum on t h e higher-energy s i d e and t h e Cu one i n t h e lower-energy side. Annealing o f samples a t temperatures from 150 t o 300°C increa- ses s i g n i f i c a n t l y t h e i n t e r d i f f u s i o n (Fig.1 ( 2 , 3 , 4 ) f o r example).

O r 8 1 r 0

ENERGY ( MeV )

Fig.1

-

Rutherford backscattering energy spectra from 1800

1

Sb on 3100

ff

Cu samples subjec- t e d t o various stages of annealing : (1) as-evaporated, ( 2 ) a f t e r 230 min a t 150°C, ( 3 ) a f t e r 195 min a t 200°C and (4) a f t e r 130 min a t 250°C.

I n order t o examine t h e d i f f u s i o n o f Sb i n Cu t h i n films, we have used spectra such as those i n f i g . 1 t o construct t h e d i f f u s i o n p r o f i l e s . Some o f the depth p r o f i l e s f o r t h e second s e t o f samples before and a f t e r annealing a r e shown in.Fig. 2. P r o f i l e broadenings a t the i n t e r - faces due t o d i f f u s i o n a r e c l e a r l y evident. I n many cases, long d i f f u s i o n " t a i l s " o f nearly

(4)

Fig. 2

-

D i f f u s i o n p r o f i l e s for Sb i n Cu t h i n f i l m s from the data o f Fig. 1.

I n t e r d i f f u s i o n i n t h i n f i l m s has most r e a d i l y been understood i n terms o f Whipple's theory o f d i f f u s i o n /14/, i n which g r a i n boundary d i f f u s i o n i s separated from bulk d i f f u s i o n . The g r a i n boundary d i f f u s i o n parameters can be determined by using Whipple's s o l u t i o n of the g r a i n boundary problem w i t h s e m i - i n f i n i t e s o l i d boundary conditions, assuming t h a t the d i f - fusion c o e f f i c i e n t s are independent o f concentration and t h a t d i f f u s i o n i n the g r a i n bounda- r i e s ( Db ) i s much f a s t e r than d i r e c t d i f f u s i o n through the g r a i n ( Dp

1.

According t o Le C l a i r e /15/, g r a i n boundary d i f f u s i v i t i e s , Db

,

can be calculated from the f o l l o w i n g equation :

where

c

i s the Sb concentration averaged a t t h e depth o f z from t h e o r i g i n a l Cu/Sb interface, t i s the d i f f u s i o n annealing time and ' S t h e g r a i n boundary width. This equation shows t h a t a p l o t o f i n

C

as a f u n c t i o n o f

r6b

i s a s t r a i g h t l i n e whose slops determines !Db

.

we confine ourselves on t h e data f o r Sb d i f f u s i n i n t o the Cu underlayer, i.e, t h e Cu sxde o f the depth p r o f i l e s . Some t y p i c a l p l o t s o f i n

!

^versus ²⁶¹⁵are shown i n Fig. 3.

d D b can then be obtained from the slopes o f "Whipple" p l o t s taken i n the d i f f u s i o n " t a i l "

region. The calculated values o f Db a t various temperatures, t a k i n g the bulk d i f f u s i v i t i e s obtained a t higherotemperatures from /12/ and extrapolated t o our temperature range and assuming

6

o f 10 A, are presented i n t a b l e 1 and shown i n Fig. 4. I f we assume an Arrhenius temperature dependence, t h e g r a i n boundar a c t i v a t i o n energies are : 1 ,l 2 eV f o r 600

W

Sb on 650

b.

Cu samples and 1,26 eV f o r l800

1

Sb on 3100

b.

Cu specimens.

According t o the Cu/Sb e q u i l i b r i u m phase diagram /16/, there are two stable i n t e r m e t a l l i c compounds ( Cu2 Sb and Cu Sb ) i n t h e considered temperature range. The s o l i d s o l u b i l i t y o f Sb i n Cu i s about 3 at9% a2 300°C and 1,5 a t % a t 250°C. The g r a i n boundary d i f f u s i o n c o e f f i c i e n t s o f Sb i n Cu are determined f o r s h o r t periodes o f heattreatmentsuch t h a t the s o l i d r e a c t i o n between the two previous m e t a l l i c f i l m s d i d n o t occur. Indeed, i t has been

found t h a t , depending of t h e annealing temperature, the r e a c t i o n was clearlyproduced a f t e r a c e r t a i n annealing time of samples. To get an idea, t h e r e a c t i o n was observed a f t e r about

(5)

Cl-504 COLLOQUE DE PHYSIQUE

10 min o f a n n e a l i n g a t 300°C o f 600 S b on 650

g

Cu sample, y i e l d i n g t o t h e formation and growth o f t h e Cu Sb p h a s e /17/. T h e r e f o r e , t h e d e c r e a s e k e s p e c t i v e l y i n c r e a s e ) o f Sb concen- t r a t i o n i n t h e 56 ( r e s p e c t i v e l y Cu) f i l m , ( F i g . 2, p l a t e a u o f p r o f i l e 41, c o u l d be a t t r i b u t e d t o t h e Cu2 Sb phase formation.

I t is worth n o t i n g t h a t t h e g r a i n boundary d i f f u s i v i t i e s v a l u e s measured h e r e f o r Sb d i f f u - s i o n i n Cu t h i n f i l m s a r e 3

-

6 t i m e s l a r g e r t h a n t h e l a t t i c e d i f f u s i v i t i e s p r e d i c t e d from bulk measurements. The a c t i v a t i o n e n e r g i e s f o r t h e g r a i n boundary d i f f u s i o n i n t h i n f i l m s a r e s u b s t a n t i a l l y lower t h a n t h a t f o r t h e volume d i f f u s i o n ( 1 , 9 eV). T h i s r e s u l t confirms t h a t thermal p r o c e s s e s i n t h i n f i l m s r e q u i r e t e m p e r a t u r e s lower t h a h t h o s e n e c e s s a r y f o r t h e same r e a c t i o n i n a b u l k sample, s u g g e s t i n g a g r a i n boundary d i f f u s i o n mechanism a s t h e dominant p r o c e s s .

Fig. 3

-

Whipple p l o t I n

c

v e r s u s z6/5 f o r 600

1

Sb on 650

1

Cu samples : ( 1 ) a s - d e p o s i t e d , ( 2 ) annealed a t 250°C f o r 1 0 min and ( 3 ) 300°C f o r 5 min.

Table 1. G r a i n boundary d i f f u s i o n p a r a m e t e r s o f Sb i n Cu t h i n f i l m s

Sample Temperature (OC) D, ⁽cm 2 /S ) D; ( cmZ/s E~ ( e ~ )

(6)

d( Cu )

=

3100

8

3,01.10-3 1,26 2 0,04

d( Sb )

=

1800

1

²⁵⁰ 1,66.10-l5

300 2,74.10-l4

Fig. 4

-

An Arrhenius p l o t o f t h e measured Db values f o r Sb d i f f u s i o n i n Cu t h i n f i l m s : (1 ) f i r s t s e t and ( 2 ) second s e t o f samples.

I t i s a l s o observed t h a t the d i f f u s i o n i n thinner samples ( the f i r s t s e t ) occurs more r a p i - d l y than i n t h e r e l a t i v e l y t h i c k e r samples ( t h e second s e t ). Indeed, t h e g r a i n boundary d i f f u s i v i t i e s i n t h e f i r s t case are about one order o f magnitude l a r g e r and t h e a c t i v a t i o n energy i s lower than t h e one i n the second case. This feature i s probably due t o the g r a i n s t r u c t u r e o o f t h e f i l m s . The as-deposited Cu g r a i n s have an average g r a i n s i z e o f about 1000

P\

and 3000 A r e s p e c t i v e l y f o r t h e f i r s t and t h e second s e t o f samples. On annealing, no s i g n i - f i c a n t g r a i n growth i s remarked.

It i s i n t e r e s t i n g t o underline t h a t t h e value (1,26 eV) o f t h e g r a i n boundary a c t i v a t i o n energy f o r the second s e t o f samples, i s i n agreement w i t h the value (1,28 eV) found from X-ray d i f f r a c t i o n measurements on ( 2000

X

Sb evaporated on 2700

a

^Cu ⁾samples /ID/. The a n a l y s i s technique used i n /10/ d i d n o t d i s t i n g u i s h between g r a i n boundary and l a t t i c e d i f - fusion, however, i t i s believed t h a t t h e a c t i v a t i o n energy represents t h e g r a i n boundary d i f f u s i o n .

ACKNOWLEDGEMENT

The authors wish t o thank Dr. F. Schwabe ( C.I.B.A.S., F r e i d r i s h

-

S c h i l l e r U n i v e r s i t y , Jena, G.D.R ) f o r RBS measurements.

(7)

COLLOQUE DE PHYSIQUE

REFERENCES

/l/ Poate, J.M., Tu, K.N. and Mayer, J.W., Thin Films

-

I n t e r d i f f u s i o n and Reactions (Wiley- Interscience, New York) 1978.

/2/ H a l l , P.M. and Morabito, J.M., Thin S o l i d Films

53

( 1978 ) 175.

/3/ Tu, K.N., Ann. Rev. Mater. Sci. 15 (1985) 147.

/4/ Weaver, C., i n Physics o f Thin ~ T m s , vo1.6, M.H. Francombe and R.W. Hoffmann ( Academic Press, New York ) 1971, p. 301.

/ 5 / B a l l u f f i , R.W. and Blakely, J.M., Thin S o l i d Films 25 (1975) 363.

/6/ Hall,

'P.M.,

Morabito, J.M. and Panousis, N.T., ~ h i n T o l i d Films

g

(1977) 341.

/7/ Dorner, P., Gust, W., Predel, B., R o l l , U., lodding, A. and Odelius, H., Philos. Mag.

9

(1984) 557.

/8/ Compisano, S.U., Costanzo, E. and Rimini, E., Philos. Mag.

2

(1977) 1333.

/9/ Schoen, J.M., Poate, J.M., Doherty, C.J. and Melliar-Smith, C.M., J. Appl. Phys.

(1979) 6910.

/10/Halimi, R., Thesis o f Ph.D., Minsk, URSS, 1984.

/11/ Inman, M.C., Barr, L.W., Acta Met.

8

(1960) 122.

/12/Gorbatchev, V.A., Klotsman, S.M., Rabovski, Ya.A., Talinski, V.K. and Timofeev, A.N., Fiz. Metal. i Metalloved.

2

(1973) 889.

/13/ Krautheim, G., Neidhart, A. and Reinhold, U., S o l i d State Commun.

2

(1980) 163.

/14/ Whipple, R.T.P., Philos. Mag.

42

(1954) 1225.

/15/Le Claire, A.D., B r i t . J. Appl. Phys.

14

(1963) 351.

/16/ N. Kh. Abricossov, Binary and Multicomponent Systems on Copper Base ( Metallurgia, Moscow, 1979).

/17/ R. Halimi and A. Merabet, Surface Sci., ( i n press).