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Submitted on 1 Jan 1978
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STRAIN RELAXATION EFFECTS IN Pb-ALLOY JOSEPHSON TUNNEL JUNCTIONS
S. Basavaiah, M. Murakami, C . J . Kircher
To cite this version:
S. Basavaiah, M. Murakami, C . J . Kircher. STRAIN RELAXATION EFFECTS IN Pb-ALLOY
JOSEPHSON TUNNEL JUNCTIONS. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1247-
C6-1249. �10.1051/jphyscol:19786551�. �jpa-00218040�
STRAIN RELAXATION EFFECTS IN Pb-ALLOY JOSEPHSON TUNNEL JUNCTIONS
S. Basavaiah, M. Murakami and C.J. Kircher
IBM T.J. Watson Research Center, Yovktown Heights, New York, 10598, U.S.A.
Abstract.- Strain relaxation has been measured in Pb-alloy films on Si substrates during cooling to 4.2K. The amount of strain relaxed was found to be strongly dependent on the film thickness h increa- sing from about 0.2 to 0.6 % as h was increased from 0.2 to 1 urn. The failure of Josephson tunnel junctions made with similar Pb-alloy films was shown to occur by the formation of a microbridge of about 1000 A size through the tunnel barrier. It is proposed that the microbridge forms due to strain relaxation in the junction electrodes.
INTRODUCTION.- Josephson tunnel junctions fabricated with Pb-alloy electrodes are of interest for logic and memory circuits/1-2/. Junctions with Pb-In-Au base electrodes and Pb-Au counterelectrodes have been reported to withstand several hundred thermal cycles between 300 and 4.2 K before failure due to shorting of the tunnel barriers/3-4/. The films are strained during thermal cycling due to the differen- ce in thermal expansion coefficients of the Pb-alloy films and the Si substrate. In the work reported he- re, the strain in Pb-alloy films has been measured to determine the level of strain relaxation and its relation to junction cyclability. The electrical characteristics of partially failed junctions have been investigated to gain insight into the nature of their failure.
STRAIN RELAXATION.- The films used for strain mea- surements were prepared on oxidized Si wafers using the Au(4 %) + Pb + In(12 %) type (composition in wt. %) that exhibited improved device cyclability
/4/. The elastic strain in the films was determined from X- ray diffraction measurements which were ca- ried out by attaching substrates to a liquid He cold stage of an X-ray diffractometer/5/. The films were found to have i> 0 % strain after a short period of storage at room temperature. As they are cooled from 300 to 4.2 K, the films are strained. The average elastic strain normal to the film surface is appro- ximated by E33 = (d-d )/d , where d is a measured (222) interplanar spacing normal to the films surfa-
ce and d is a strain-free value calculated using the reported thermal expansion coefficients for pure Pb. In figure 1, £33 values are shown for the Pb- alloy films as a function of total film thickness.
The value of e' the theoretical maximum strain max
expected if no strain relaxation occurs, was cal- culated using a biaxial strain model and pure Pb values for the elastic constants and thermal expan- sion coefficients, since the latter quantities are not available for the alloys/5/. The amount of strain relaxed in thus Ae = e' - £33. It is seen
max
from figure 1 that Ae is ^ 0.5 % for a film thick- Fig. 1 : Strain at 4.2 K in Pb-In-Au films as a func- tion of film thickness.
JOURNAL D E PHYSIQUE Colloque C€, supplément au n" 8, Tome 39, août 1978, page C6-1247
Résumé.- La relaxation de contrainte a été mesurée, pendant un refroidissement à 4.2 K, dans les couches minces des alliages de plomb déposées sur des supports de silicium. Le pourcentage de la relaxation s'est trouvé être fortement dépendant de l'épaisseur h des couches, augmentant de
^ 0,2 % à 0,6 % lorsque h était augmenté de 0,2 à 1 ym. On montre que la dégradation des jonctions tunnel Josephson fabriquées avec des couches minces d'alliage de plomb est due à la formation d'un micropont d'environ 1000 A à travers la jonction tunnel. La formation de ces microponts peut-être attribuée à une contrainte de relaxation dans les électrodes de jonction.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786551
ness h = 0.5 Llm. A s i m i l a r A e v a l u e was observed f o r a Pb-Au a l l o y f i l m . The v a l u e of Ae d e c r e a s e s w i t h d e c r e a s i n g h , b u t does not reach 0 even f o r t h e t h i n n e s t f i l m s s t u d i e d . This s t r a i n r e l a x a t i o n de- forms t h e j u n c t i o n e l e c t r o d e s t h u s p o s s i b l y c a u s i n g j u n c t i o n f a i l u r e . The cycling-induced f a i l u r e s a r e t h u s expected t o d e c r e a s e w i t h d e c r e a s i n g A e , i . e . w i t h d e c r e a s i n g e l e c t r o d e f i l m t h i c k n e s s e s . This i s i n f a c t t h e c a s e , a s r e p o r t e d r e c e n t l y : Josephson j u n c t i o n s prepared w i t h s i m i l a r Pb-alloy f i l m s exhi- b i t e d % 4 X r e d u c t i o n i n t h e number of cycling-in- duced f a i l e u r e s when t h e e l e c t r o d e f i l m t h i c k n e s s e s were reduced by Q 2 X from 4000 i / 5 0 0 0
11
t o 17002700 11/41. The p r e s e n t r e s u l t s i n d i c a t e t h a t a s i - g n i f i c a n t amount of s t r a i n r e l a x a t i o n i s s t i l l oc- c u r i n g f o r t h e devices with t h e t h i n n e r e l e c t r o d e s . Thus, s i g n i f i c a n t f u r t h e r improvements i n d e v i c e c y c l a b i l i t y should be a c h i e v a b l e i f t h i s s t r a i n r e - l a x a t i o n could be e l i m i n a t e d .
JUNCTION BEHAVI0UR.- J u n c t i o n s 25 ym x 25 ym i n si- ze were prepared u s i n g 4000 t h i c k base e l e c t r o d e s of Pb-In(8 %)-Au(4 %) and 5000
i
t h i c k t o p e l e c t r o - des of Pb-Au(l.7 %). The s u b s t r a t e s were o x i d i z e d S i wafers covered w i t h i n s u l a t e d Nb ground p l a n e s . Control l i n e s were prepared over t h e j u n c t i o n s t o allow s u p p r e s s i o n of t h e Josephson c u r r e n t . The f a - b r i c a t i o n process i s d e s c r i b e d i n more d e t a i l s e l s e - w h e r e / l / . Following i n i t i a l t e s t i n g , j u n c t i o n I-V c h a r a c t e r i s t i c s were remeasured a f t e r i n t e r v a l s of 10 c y c l e s between 300 and 4.2 K/3/. Figure 2 d e p i c t s t h e I - V c h a r a c t e r i s t i c s of a t y p i c a l j u n c t i o n a s a f u n c t i o n of t h e number of thermal c y c l e s N. No chan-Fig. 2 : I-V c h a r a c t e r i s t i c s of a t y p i c a l Pb-alloy Josephson j u n c t i o n a f t e r v a r i o u s amounts of thermal c y c l i n g t o 4.2 K
ges were n o t i c e s f o r N < 40 i n t h i s c a s e . For
NL
50 excess c u r r e n t s I were observed. Iex i s t h e d i f f e -e x
r e n c e between I(V) measured a t N and I(V) f o r N = 1 . Iex was observed t o be l i n e a r l y p r o p o r t i o n a l t o V f o r V > 0.2 mV, t o i n c r e a s e w i t h N and t o b e only weakly temperature-dependent. The z e r o v o l t a g e cur- r e n t could n o t b e suppressed by t h e s m a l l magnetic f i e l d s t h h t a r e s u f f i c i e n t t o s u p p r e s s t h e Josephson c u r r e n t s . These p r o p e r t i e s of I s u g g e s t t h a t i t i s
ex
due t o t h e formation of a superconducting microbridge between t h e j u n c t i o n e l e c t r o d e s . F i g u r e 3 shows I
ex a t 1 mV) a s a f u n c t i o n of N. A f t e r I i s f i r s t ob-
e x
F i g . 3 : J u n c t i o n excess c u r r e n t Iex a t V=l mV a s a f u n c t i o n of t h e number of thermal c y c l e s N f o r junc- t i o n s with d i f f e r i n g amounts of s i n g l e p a r t i c l e tun- n e l i n g c u r r e n t I
TUN '
s e r v e d , i t i s found t o i n c r e a s e i n approximate pro- p o r t i o n t o N ~ , a s shown i n f i g u r e 3 f o r t h r e e d i f - f e r e n t j u n c t i o n s . The power l a x dependence of I i s
e x n o t now understood. However, one c a e x t r a p o l a t e t h i s dependence o t o b t a i n I v a l u e s expected f o r N < N
e x i'
t h e number of c y c l e s a t which an I i s f i r s t detec- e x
t e d . For t h e j u n c t i o n w i t h I I mA i n f i g u r e 3 TUN
an Iex of 100 yA would b e expected a t N = 40 i f N . = 50. Since t h i s could have e a s i l y been d e t e c t e d
( f i g u r e 2 a ) , i t appears t h a t an a b r u p t change i n t h e j u n c t i o n ( e . g . a l o c a l r u p t u r e of t h e t u n n e l b a r r i e r ) h a s occured between N = 40 and 50 t o pro- duce a microbridge. For pure Pb j u n c t i o n s , such an a b r u p t change h a s been observed d u r i n g t h e f i r s t cy- c l e / 6 / . Thus, excess c u r r e n t does n o t seem t o deve- l o p a s a g r a d u a l d e g r a d a t i o n of t h e e n t i r e j u n c t i o n beginning a t N = 0 . The s i z e of t h e microbridge can be e s t i m a t e d from i t s c r i t i c a l c u r r e n t and r e s i s t a n - c e l e v e l s . For example, i n f i g u r e 2 a t 100 c y c l e s t h e c r i t i c a l c u r r e n t i s 2.8 mA and t h e r e s i s t a n c e i s 0.11 O . C r i t i c a l c u r r e n t d e n s i t i e s of t h e e l e c - trodes. Vere % 3 x 10' ~ / c m ~ and r e s i s t i v i t i e s a t 10 K of t h e ' b a s e and c o u n t e r electi-ode f i l m s were
6.5 and 0.5 pa-cm r e s p e c t i v e l y . Assuming a uniform References c u r r e n t d i s t r i b u t i o n and t h a t t h e m i c r o b r i d g e i s
composed o f t h e base e l e c t r o d e m a t e r i a l , t h e diame- / I / Greiner,J.H., Basavaiah,S. Ames,I.J., J. Vac.
t e r of t h e m i c r o b r i d z e i s found t o b e 600 and 1000 S c i . Technol.
2
(1974) 81-
from the resistivity and critical current density
1 2 1
See f o r example, H e r r e l l , D . J . , IEEEJ.
S o l i d S t a t e C i r c u i t s , (1975) 360, and Zappe,H.H., r e s p e c t i v e l y . I f t h e m i c r o b r i d g e c u r r e n t d i s t r i b u - IEEE J. S o l i d S t a t e C i r c u i t s , ( 1 975) 12 t i o n i s non-uniform, a l a r g e r d i a m e t e r would b e ob- / 3 / Basavaiah,S., G r e i n e r , J . H . , J . Appl. Phys.5
t a i n e d , p o s s i b l y s e v e r a l thousand
%.
These s i z e s a r e (1977) 4630comparable t o t h e g a i n s i z e s and s u r f a c e roughness / 4 / L a h i r i , S . K . , Basavaiah,S., J. Appl. Phys. May (1 978)
of t h e f i l m s , a c c o u n t i n g f o r t h e f a c t t h a t we have
/ 5 / Murakami,M., Acta. Met.
6
(1978) 175 n o t y e t been a b l e t o d i r e c t l y observe t h e f a i l u r e161 Zappe,H.H., Phys. Rev.
g
(1975) 2535 l o c a t i o n s on t h e s h o r t e d d e v i c e s .CONCLUSIONS.- S t r a i n r e l a x a t i o n o c c u r s i n t h e junc- t i o n e l e c t r o d e s d u r i n g thermal c y c l i n g . I t i s b e l i e - ved t o o c c u r by d i s l o c a t i o n motion a n d / o r h i l l o c k f o r m a t i o n which deform t h e e l e c t r o d e s 14-51 and s t r e s s t h e t u n n e l b a r r i e r . These r e s u l t s s u g g e s t t h a t when a s u f f i c i e n t l y l a r g e l o c a l s t r e s s b u i l d s up, t h e t u n n e l b a r r i e r i s r u p t u r e d and a m i c r o b r i d g e forms. The s i z e of t h e m i c r o b r i d g e grows w i t h addi- t i o n a l c y c l i n g .
ACKNOWLEDGEMENTS.- The a u t h o r s wish t o e x p r e s s t h e i r g r a t i t u d e f o r t h e e x c e l l e n t t e c h n i c a l h e l p r e n d e r e d by R. Drake, A. Ginzberg, S. S i n g e r and V. Tom. Help- f u l d i s c u s s i o n s w i t h E. H a r r i s , S. L a h i r i , and J.H.
G r e i n e r a r e v e r y much a p p r e c i a t e d .
