ATOM-PROBE STUDY OF Al-Nb INTERFACES

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ATOM-PROBE STUDY OF Al-Nb INTERFACES

K. Okuno, H. Yamashita, K. Oida, O. Nishikawa

To cite this version:

K. Okuno, H. Yamashita, K. Oida, O. Nishikawa. ATOM-PROBE STUDY OF Al-Nb INTERFACES.

Journal de Physique Colloques, 1987, 48 (C6), pp.C6-511-C6-516. �10.1051/jphyscol:1987684�. �jpa-

00226892�

JOURNAL D E PHYSIQUE

Colloque C6, suppl6ment au nO1l, Tome 48, novembre 1987

ATOM-PROBE STUDY OF AI-Nb INTERFACES

K. Okuno, H. Yamashita*, K. 0ida*and 0 . Nishikawa*

Department of Electrical Engineering, Nagasaki Institute of Applied Science, 536 Abamachi, Nagasaki 851-01, Japan

*~epartment of Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Abstract - Compositions of an A1-Nb interface with temperature were investigated by the high mass resolution time-of-flight atom-probe. The A1-Nb interfaces formed at room temperature exhibited an atomically abrupt interface and no mixed layer was found. The A1 atoms at the abrupt atomic interface desorbed as

~ 1 ions of a stronger evaporation field than that of the over- ~ + layer. This indicates that the evaporation field strength at the abrupt integfaces approximates to the evaporation field of Nb

(F2= 3.7 V/A). This indicates also that the A1-Nb binding is stronger than that of A1-A1 binding. An Al-Nb mixing layer was found after heating at 500 K. The composition of the mixed layer was not uniform and varied from Nb2A13 to NbA1, Nb3A12 and Nb3A1 with the A15 structure.

Microscopic characterizations at atomic interfaces due to metal- semiconductor and metal-metal interactions are increasing in im- portance. For example, A1 in Josephson junctions has been used as a buffer layer for the tunneling barrier layer (Al203) [l]. Al-Si and Al-GaAs have also been investigated about the characterizations in their atomic interfaces[21. On the otherhand the A1-Nb system is well known to form an alloy of the A15 metallurgical structure, as investi- gated in detail on the Ga-Mo[3]. The present work was performed to investigate the compositions of atomic interfaces of the A1-Nb with temperature, and the characterization at the interface of an Al- Nb oxide-Nb system using high mass resolution time-of-flight atom- probe (A-P) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987684

JOURNAL DE PHYSIQUE

I1 - EXPERIMENTAL

The compositions at the interface of A1-Nb with temperature were in- vestigated by A-P141. An A1 source with purity of 99.99

was vapour deposited on the Nb substrate surface in a subchamber with base pressure of the order of 10-7 Pa and the A-P analyses of A1-Nb are done at a base pressure of Pa.

111 - ALUMINUM -.NIOBIUM INTERFACES FORMED AT ROOM TEMPERATURE

The A1 vapour source was deposited on the Nb substrate surface at roam temperature(R.T), and a histogram analysis with the A-P of the Al-Nb interface is shown in Fig.1. Although field evaporated A1 ion is de- tected as ~ l + ion(F1= 1.9 v/&, A1 deposited on the substrate Nb sur- face was detected as ~1' and ~ 1 2 + (F2= 3.5 v/X) ions. A depth profile of the histogram analyzed with A-P is shown in Fig.2. The A1 atoms from the A1 overlayer deposited on the Nb substrate surface were ana-

Fig.1. A histogram analyzed with A-P of the A1-Nb inter- face formed at R.T.

Analyzing was done at R.T.

60-

I I 4 I J

.r?

AI deposited and detected

-b;,

-

at R.T ^$.f..

AI/ NbH0.74

_., ...-,-- .

Fig.2. Depth profile corre- sponding to the histogram of Fig-1.

No A1-Nb mixing is found. A1 at the

- abrupt interface is

desorbed as ~ 1 ~ '

ion.

lyzed a s ~ l + i o n s . This i s t h e same a s t h a t from t h e f i e l d d e s o r p t i o n of t h e A l . When t h e f i e l d d e s o r p t i o n has proceeded f u r t h e r , A 1 atoms a t t h e A1-Nb atomic i n t e r f a c e a r e desorbed a s ~ 1 i o n s with s t r o n g e r ~ + evaporation f i e l d t h a n ~ l + i o n s , and t h e d e s o r p t i o n f i e l d s t r e n g t h of t h e ~ 1 2 + i o n s i s i n c r e a s e d a b r u p t l y a t t h e A1-Nb atomic i n t e r f a c e , a s shown i n Fig.3. The v a l u e of t h e d e s o r p t i o n f i e l d s t r e n g t h a t t h e abgupt i n t e r f a c e approximates t o t h e evaporation f i e l d of Nb (F2= 3.5

This i n d i c a t e s t h a t t h e binding energy a t t h e a b r u p t i n t e r f a c e

of A1-Nb approximates t o t h e binding energy of t h e Nb-Nb. But no mixed l a y e r a t t h e i n t e r f a c e of A1-Nb was found a t R.T. Rather t h e i n t e r - f a c i n g s u r f a c e s of A1-Nb were c l e a r l y s e p a r a t e d .

I V - ALUMINUM - N I O B I U M INTERFACES FORMED AT 500 K

Nb metal i s a very r e a c t i v e metal with g a s atoms even i n r e s i d u a l g a s a t t h e o r d e r of t h e lo-* Pa a t R.T o r heated. Therfore, a f t e r t h e Nb t i p was heated a t 500

700 K, t h e Nb s u b s t r a t e s u r f a c e was f i e l d evaporated f o r s e v e r a l 10 atom t h i c k l a y e r s u n t i l a c l e a n Nb s u r f a c e i s obtained. A f t e r t h e A 1 vapour source was d e p o s i t e d on t h e s u b s t r a t e Nb s u r a c e a t R . T , t h e A1-Nb was heated f o r one minute a t a p r e s s u r e of Pa.

histogram and t h e depth p r o f i l e analyzed by A-P a r e shown i n Fig.4 and Pig.5, r e s p e c t i v e l y . Although t h e d e t e c t e d l i n e s of A 1 and Nb atoms were s e p a r a t e d c l e a r l y a t R.T, a s shown i n F i g . 2 ~ i n t h e h e a t i n g a t 500 K t h e i r d e t e c t i o n l i n e s were n o t c l e a r l y sepa- r a t e d . Therfore, a d i f f u s e d l a y e r of A 1 and Nb atoms a t t h e i n t e r f a c e was found by t h e h e a t i n g a t 500 K . Nb oxide was p r e s e n t e d a t t h e same time. Although Nb oxide was e a s i l y formed, no

oxide

found by t h e h e a t i n g a t 500 K. The composition of t h e mixed l a y e r a t t h e A1-Nb i n t e r f a c e can be determined p r e c i s e l y from t h e d e t e c t i o n r a t e s of t h e

and Nb atoms. Mixed l a y e r d e t e c t i o n l i n e s of

and Nb atoms a r e shown i n Fig.6. The s l o p e of d e t e c t e d r a t e i n t h e analyzed depth r e g i o n i n d i c a t e s t h e composition of t h e d i f f u s e d l a y e r . Analysis i n d i c a t e s t h a t t h e composition of t h e mixed l a y e r was n o t uniform and v a r i e d from Nb2A13 t o NbA1, Nb3A12 and Nb3A1 with t h e A15 s t r u c t u r e .

V

- ^ALUMINUM - N I O B I U M OXIDE INTERFACES FORMED AT R.T

Nb metal i s very a c t i v e and t h e o x i d a t i o n p r o c e s s i s easy. A f t e r t h e

-

2 ,

e g5

5 >

5 4-

-

g 83

>

Q

?? ^2-

- S

z ' ?

0

8

Al deposited and

- detected at R.T

Al/b&b.~+

Fig.3 ~ e s o r p t i o n

9

"

"

.

\ -

- f i e l d s t r e n g t h of t h e

1:

d e p o s i t e d on t h e Nb

7; s u b s t r a t e . The des-

- _OAl o r p t i o n f i e l d s t r e n g t h

of ~l atoms a t t h e

*Nb a b r u p t i n t e r f a c e

AI

*

NbH approximates t o evapo-

~d r a t i o n f i e l d of t h e

s u g s t r a t e Nb (F2= 3.7

,h'

& I I I I t

0 1 0 20 30 40 50 60 70 80-

Total number of ions

AI

bib + NbH 1

C6-5 14 JOURNAL DE PHYSIQUE

Nb specimen was electrolytically etched, the Nb tip was often covered with Nb oxide, even a non-heated Nb tip in an UHV. A-P analyses of the Nb tip covered with a Nb oxide layer are shown in Fig.7. After Nb

oxide ions NbO, Nb02 and Nb02Hn were detected, ~ b ~ + , ~ b 3 + and N ~ H ~ + ions of the substrate Nb were detected. This indicates that the Nb oxide layer and substrate Nb(Nb, NbH) are clearly separation. A1 atoms were deposited on the substrate Nb surface which was covered with a Nb oxide layer. The analyzed depth profiles are shown in Fig.8. Although the vapour deposited A1 atoms were detected at the beginning of the analyses, the diffusion of A1 atoms in the Nb oxide layer was also observed even at R.T. The result indicates that vapour deposited A1 on the Nb oxide layer forms a mixed layer. The following A1 ions were not detected in any other region of the oxide layer, as shown by saturated detection line in Fig.8. But several A1 ions were detected at the Nb oxide-Nb interface having passed throughout the Nb oxiae film. This phenomena can occur if the Nb oxide does not form as a contfnuous

Fig.4. A histogram analyzed with A-P of the Al-Nb inter- face formed after heating at 500 K.

Fig.5. Depth profile corresponding to the histogram of Fig.4.

A1-Nb mixing was

found after heating

at 500 K.

film, but is an amorphous structure and pin holed layer, as reported on the Ni/Si02/Si system[5].

I - CONCLUSION

The composition of the A1-Nb interface with temperature was investi- gated using a high mass resolution time-of-flight atom-probe. No inter-mixing layer was found at R.T. The binding energy of A1 at the abrupt interface is stronger than A1-A1 binding, as shown in Ga-Mo interfaces[3]. The A1 atoms at the abrupt interface were desorbed as

~ 1 ions with stronger evaporation field than that of ~ l + ~ + ions on the field evaporation from the Al. The desorption field strength of ~ 1 ~ + ion approximates to the evaporation field of Nb (F2= 3.7 v/& . ^{A mixed}

layer of A1 and Nb was found after heating at 500 K, but the compo- sition was not uniform. The exhibited compositions found were Nb2A13, NbA1, Nb3A12 and Nb3A1. Furthermore, some A1 atoms deposited 'on Nb oxide covered Nb surfaces were passed through the Nb oxide film. These

I I I I I

Heated at 500 K. 3 rnin

Detected at R.T

Fig.6. The compo- sition of the mixed layer was analyzed from the detected rate of A1 and Nb atoms. The compo- sitions are Nb2A13, NbA1, Nb3A12 and Nb3A1, respectively

Fig.7. Depth

profile analyzed

with A-P a Nb oxide

covered Nb tip. Nb

oxide and Nb

hydride layers are

clearly separated.

JOURNAL

DE PHYSIQUE

Pig.8. Depth profile analyzed with A-P of A1-Nb oxide-Nb in-

terfaces. A1 and Nb oxide mixing readily occurs even at R.T, and the some A1 atoms reached to the substrate Nb through the Nb oxide film.

result can be understand assuming that the oxide layer has many defects in its structure or is an amorphous state, with pin holes in the layer.

VII - ACKNOWLEDGEMENT

The author wishes to gratefully acknowledge the technical support of the members of the surface science laboratory, Tokyo Institute of Technology.

VIII - REFERENCES

[l] M. Gurvitch, M. A. Washinghton and H. A. Huggins, Appl. Phys. Lett

. 42 (1973) 472.

[ 2 ]

~.Jishikawa,

Kaneda, M. Shibata and E. Nomura, Phus. Rev. Lett

. 53 (1984) 1252.

131 0.Nishikawa and T. Utsumi, Appl. Phys. 44 (1973) 955.

(41

Nishikawa and K. Kurihara, M. Nachi, M. Konishi and M. Wada.

Rev. Sci. Instrum. 52 (1981) 810.

[5] M. Liehr, H. ~efakis, F. K. Legoues and G. W. Rubloff, Phys. Rev.

33 (1986) 5517.

-