INTERACTION OF DISLOCATIONS AND GRAIN BOUNDARIES IN Al FILMS

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INTERACTION OF DISLOCATIONS AND GRAIN BOUNDARIES IN Al FILMS

G. Radnóczi, P. Barna

To cite this version:

G. Radnóczi, P. Barna. INTERACTION OF DISLOCATIONS AND GRAIN BOUNDARIES IN Al FILMS. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-429-C4-433. �10.1051/jphyscol:1985447�.

�jpa-00224698�

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JOURNAL DE PHYSIQUE

Colloque C*, supplement au n ° 4 , Tome 46, avril 1985 page C4-429

INTERACTION OF DISLOCATIONS AND GRAIN BOUNDARIES IN Al FILMS

G. Radnoczi and P.B. Barna

Research Institute for Technical Physics of the Hungarian Academy of Sciences, P.O. Box 78, H-1S25 Budapest, Hungary

Résumé - Les joints de grains des couches minces d'aluminium déposées sous vide sont facettés, ils contient bien des dislocations. Les dislocations sont produites par les tensions thermiques. Elles passent d'un grain à l'autre. Les irrégulari- tés de surface au-prês des joints de grains produisent la formation de segments de dislocations au-dessous des collines sur les couches d'aluminium.

Abstract - The grain boundaries of vacuum deposited Al films are faceted and contain many dislocations. The dislocations originate from thermal stresses mostly and move from boundary to boundary. Surface irregularities at grain boundaries resulted in the formation of dislocation segments under the hills on Al films.

INTRODUCTION

In the course of the investigation of the surface growth morphology of Al films it was found that grain boundaries (GB) may have different configurations on the film surface. There can be grooves or hills at the GBs depending on the preparation conditions, namely on the nature of the substrate and especially on the extent of impurities [1]. The different growth mechanisms resulting in different growth morphologies also lead to different defect structures. Dislocations created during coalescence or generated by thermal stresses interact with the GBs, producing the apparent dislocation structure of the films. Dislocation processes taking place near GBs are discussed in the present paper.

PREPARATION OF THE FILMS

Al films of 0.1-1.5 um thickness were prepared by vacuum deposition on to air-cleaved mica and NaCl substrates at temperatures of 2O0-4O0°C and at deposition rates of 1-5 nm/s as measured by a quartz micro- balance. The pressure of residual gases was less than 5 x 1 0- 5 Pa. The substrates were degassed for 30 min at the deposition temperature be- fore deposition. Al of 99.99 purity was evaporated from tungsten double spiral sources.

The films were floated off from NaCl substrates and thinned by Ag ion milling from their substrate side to preserve the surface growth morphology. Films prepared on mica were investigated together with their substrate. The samples for TEM studies were prepared by ion milling from the substrate side. The surfaces of some samples were shadowed by Pt to study the surface growth morphology, simultaneously with the bulk structure.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985447

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C4-430 JOURNAL DE PHYSIQUE

STRUCTURE OF ALUMINIUM FILMS

The g r a i n s i z e o f t h e f i l m s was t y p i c a l l y 2-4 wn, b u t g r a i n s a s l a r g e a s 1 0 pm c o u l d e a s i l y b e found.

The f i l m s e x h i b i t e d s t r o n g (111) t e x t u r e and t h e a z i m u t h a l m i s o r i e n - t a t i o n s between g r a i n s were a l m o s t randomly d i s t r i b u t e d on mica ( t h e r e was some p r e f e r e n c e f o r a ( l l 0 > ~ 1 11 (11.0) ,mica o r i e n t a t i o n ) . ^{On NaC1,}

many t w i n and 30° [ill] tilt m i s o r i e n t a t i o n s were found.

The G B s have a w e l l o r d e r e d s t r u c t u r e c o n s i s t i n g of p l a n e f a c e t s , b u t t h e y a l s o c o n t a i n many d i s l o c a t i o n s ( F i g . 1 1 , s i m i l a r t o b u l k specimens C 2 1.

F i g , 1. GB f a c e t s and d i s l o c a t i o n s a r e s e e n i n a d a r k f i e l d image o f an A 1 g r a i n . The f o i l i s t i l t e d 350 a b o u t t h e C1101 d i r e c - t i o n . The o p e r a t i n g r e f l e c t i o n i s g=220.

-

D i s l o c a t i o n s a r e p r e s e n t i n t h e b u l k o f t h e g r a i n s , t o o . S l i p t r a c e s o f d i s l o c a t i o n s h a v i n g c r o s s e d t h e g r a i n s c a n b e s e e n v e r y o f t e n

( F i g . 2 , 3 ) . They a r e v i s i b l e due t o s t r a i n f i e l d s c r e a t e d b o t h i n t h e s u r f a c e o x i d e f i l m C31 and a l s o i n t h e mica s u b s t r a t e . I n t h i s l a t t e r c a s e t h e s t r a i n f i e l d c a n b e v i s u a l i z e d , u s i n g t h e mica r e f l e c t i o n s

( F i g . 3 ) . I n b o t h c a s e s most o f t h e s l i p t r a c e s e x t e n d a c r o s s t h e en- t i r e g r a i n . D i s l o c a t i o n s a r e m o s t l y g e n e r a t e d a t GBs and t h e G B s a r e a l s o e f f e c t i v e b a r r i e r s f o r t h e i r movement. From F i g . 3 t h e d e n s i t y of s l i p t r a c e s d o e s n o t seem t o b e uniform, b u t t h i s n o n u n i f o r m i t y d o e s n o t e x t e n d from g r a i n t o g r a i n . Thus GB d i s l o c a t i o n s o u r c e s work p r i m a r i l y i n one o f t h e a d j a c e n t g r a i n s o n l y . S l i p t r a c e s a r e i n d i f - f e r e n t s l i p s y s t e m s ( F i g . 2 and 3b, marked by a r r o w s ) . Thus, one may c o n c l u d e t h a t d i s l o c a t i o n s o u r c e s i n G B s c a n work on d i f f e r e n t s l i p systems i n t h e same g r a i n .

S l i p t r a c e s i n a l l t h r e e (111) s l i p p l a n e s n o t p e r p e n d i c u l a r t o t h e C l l l l ( f i l m normal) d i r e c t i o n a r e p r e s e n t i n most g r a i n s .

The n a t u r e o f t h e d i s l o c a t i o n s can b e e s t a b l i s h e d from a s l i p t r a c e a n a l y s i s . A s c a n b e s e e n i n F i g . 2, s l i p t r a c e s a r e c o n n e c t e d t o t h e d i s l o c a t i o n s i n G B s s o most of t h e s e d i s l o c a t i o n s can b e i d e n t i f i e d a s c a p t u r e d l a t t i c e d i s l o c a t i o n s . They o r i g i n a t e d u r i n g c o o l i n g of t h e f i l m from t h e r m a l s t r e s s e s which a r e due t o d i f f e r e n c e s i n t h e t h e r m a l e x p a n s i o n c o e f f i c i e n t s o f t h e f i l m and s u b s t r a t e . Another p o s s i b l e s o u r c e o f t h e s e d i s l o c a t i o n s i s t h e p r e p a r a t i o n of t h e f i l m s f o r t r a n s - m i s s i o n e l e c t r o n microscopy (by f l o a t i n g and t h i n n i n g ) , when t h e i n - t e r n a l s t r e s s e s c a n a l s o deform t h e f i l m .

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F i g . 2 . D i s l o c a t i o n s i n t h e GB a r e c a p t u r e d

l a t t i c e d i s l o c a t i o n s ⁿ

a s r e l e a l e d by s l i p t r a c e s i n t h e g r a i n

F i g . 3. S l i p t r a c e s i n t h e A 1 on mica f i l m e x t e n d from one A 1 GB t o t h e o t h e r .

( a ) B r i g h t f i e l d image w i t h a 11.0 mica r e f l e c t i o n o p e r a t i n g and (b) t h e d a r k f i e l d image of a r e a marked i n ( a ) w i t h a 1 1 . 0 mica r e f l e c t i o n

THE I N T E R A C T I O N OF DISLOCATIONS AND G R A I N BOUNDARIES

Captured l a t t i c e d i s l o c a t i o n s a r e mobile i n t h e G B s a s t h e y move away from t h e l i n e o f i n t e r s e c t i o n o f t h e s l i p p l a n e w i t h t h e GB. T h i s can b e concluded from t h e u n p a r a l l e l n a t u r e o f GB d i s l o c a t i o n images i n F i g . 1. The f a d i n g of d i s l o c a t i o n images (A i n F i g . 1) a s a r e s u l t of t h e e x t e n s i o n of d i s l o c a t i o n c o r e s may a l s o be t h e r e s u l t o f d i s - l o c a t i o n movement o r t h e s p l i t t i n g o f t h e i r c o r e s C41.

A s p e c i a l c a s e of t h e i n t e r a c t i o n of G B s w i t h l a t t i c e d i s l o c a t i o n s was observed i n A 1 f i l m s having GB h i l l s C l l . I n t h i s c a s e a d i s l o c a - t i o n approaching t h e GB should i n c r e a s e i t s l e n g t h i n t h e r e g i o n o f t h e h i l l . I n s t e a d , ( d o t t e d i n t e r v a l AB i n F i g . 4a) a d i s l o c a t i o n segment CA i s l e f t under t h e h i l l .

T h i s means t h a t i n t h e c a s e of s u r f a c e i r r e g u l a r i t i e s ( e . g . h i l l s ) t h e s u r f a c e p i t of t h e d i s l o c a t i o n c a n b e s t o p p e d a t t h e edge of t h e h i l l , and i n s t e a d o f remaining s t r a i g h t , d i s l o c a t i o n s become k i n k e d a n d d i s l o c a t i o n segments a r e c r e a t e d .

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C4-tr32 JOURNAL DE PHYSIQUE

Fig. 4. Schematic illustration of a dislocation approaching a GB (1-4) and producing a dislocation segment CA under the GB hill (a, b). Slip traces stopped at the edge of the GB hill and con- tinued as dislocation segments under the hill in an A1 film deposited on NaCl (c) .

Therefore, one can assume that

where ECA and EAB are the energies of dislocation segments CA and AB respectively, and ECB is the energy of the strain field in the slip trace CB.

Another possible cause of the formation of these dislocation configurations is the pinning of dislocations by impurity phase obstacles C11 at the edge of the hill.

Similar processes may take place not only at GB hills but also at other surface irregularities (e.g. growth hills on the surface of the grains). Further, we may suppose that the same happens at curved in- ternal interfaces (phase or grain boundaries) if certain conditions are satisfied. These conditions cannot exactly be formulated at present.

Apparently, the edge of the surface irregularity must have some stop- ping efficiency that retards the movement of the surface pits of dislocations. The irregularity must also have some minimal height so that eq. (1) can be satisfied.

Rewriting eq. (1) yields

where LCA, LAB, LCB are the lengths of the intervals CAI AB, CB, respectively (Fig. 4a), while ed and eSt are the dislocation and slip trace energies per unit length, respectively.

Assuming

ed (AB) " ^{ed (AC)}

L~~ - - L~~

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one o b t a i n s t h e f o l l o w i n g f o r t h e l i m i t i n g h e i g h t o f t h e i r r e g u l a r i -

I n o u r A 1 f i l m s LAB/LCA = - 0 . 1 - 0 . 2

From t h i s v a l u e w e c a n e s t i m a t e t h e r a t i o o f eSt/ed.

One f i n d s eSt&(0.8 - 0 . 9 ) e d , whikh seems t o b e q u i t e r e a s o n a b l e . The segment CA u n d e r t h e h i l l c a n move ( s l i p ) i n i t s (111) p l a n e

( F i g . 4b, c ) , which i s p a r a l l e l t o t h e f i l m p l a n e , and i n t h i s way even d i s l o c a t i o n w a l l s c a n form u n d e r t h e h i l l .

CONCLUSIONS

A g e n e r a l i z a t i o n o f t h e i n t e r a c t i o n o f d i s l o c a t i o n s w i t h GB h i l l s o f f e r s a n i n t e r p r e t a t i o n f o r t h e f o r m a t i o n of d i s l o c a t i o n segments between t h e i n t e r s e c t i o n p o i n t s o f a s l i p t r a c e w i t h c u r v e d i n t e r n a l o r e x t e r n a l s u r f a c e s . The s u r f a c e morphology a t GBs c a n p l a y an a c t i v e r o l e i n t h e f o r m a t i o n of t h e f i n a l d i s l o c a t i o n s t r u c t u r e o f . t h i n f i l m s a s w e l l a s even b u l k specimens.

REFERENCES

1. BARNA, P . B . , RADN6CZ1, G . , and REICHA, F.M., I n p r o c - of T h i n F i l m s Conf. Stockholm, August 1984.

2. POND, R.C.: Proc. Roy. Soc. A357 (1977) 471.

3 . MADER, S. and CHAUDHARI, P.: J. Vac. S c i . Technol. 6 N4 (1969) 615.

4. PUMPHREHEY, P.H. and GLEITER, H . : P h i l . Mag. (1974) 593.