Electromigration: Investigation of heterogeneous systems

Microelectron. Reliab., Vol. 33, No. 8, pp. 1141-1157, 1993. Printed in Great Britain.

0026-2714/9356.00+.00 © 1993 Pergamon Press Ltd

ELECTROMIGRATION: INVESTIGATION OF HETEROGENEOUS SYSTEMS B. VANHECKE, I L. DE SCHEPPER, 2 W. DE CEUNINCK, 2 V. D'HAEGER, 2 M. D'OLIESLAEGERS,2 E. BEYNE, l J. ROGGEN l and L. STALS 2 1Materials and Packaging Division, IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium and /Material Physics Division, Institute for Material Research, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium

(Received for publication 28 March 1992)

A b s t r a c t ; - E l e c t r o m i g r a t i o n is a p h e n o m e n o n w h e r e a t o m s a r e d r i v e n f r o m t h e i r l a t i l c e p o s i t i o n s d u e to a n e l e c t r i c c u r r e n t . I n g e n e r a l two t y p e s of e l e c t r o m i g r a t i o n systems c a n be d i s t i n g u i s h e d : the heterogeneous system, in which electromigration failure o c c u r s a t t h e i n t e r f a c e o f t w o d i s t i n c t p a r t s on t h e i n t e r c o n n e c t i o n while in the second, homogeneous case the failure occurs within the i n t e r c o n n e c t i o n itselfT h e f i r s t t y p e o f e l e c t r o m i g r a t i o n is r e p o r t e d in thin s t u d y a n d off-chip g o l d b a l l b o n d s on alnmirtiunl m e t a l l i z a t i o n a r e u s e d as a n example. I n - s i t u e l e c t r i c a l m e a s u r e m e n t s of the r e s i s t a n c e c h a n g e as a function of time with different temperature and current stresses are performed. A good understanding resistance

change

of the kinetics of the

could be obtained

which helps in the

characterisation of the processes active during the d e g r a d a t i o n of the interconnections.

1. I N T R O D U C T I O N

E l e c t r o m i g r a t i o n is a t r a n s p o r t phenomenon which occurs when the c u r r e n t p a s s i n g through a conductor provokes a change in the atomic distribution of this conductor. The best way to describe the phenomenon is by the theory of electron wind [1]. According to this theory the electrons will collide with the atoms of the conductor a n d will p a s s t h e i r m o m e n t u m onto these atoms, much like wind pulling on the leaves of a tree. This will provoke lattice vibrations 1141

1142

B. VANHECKE et al.

t h a t can not be a t t r i b u t e d to t h e r m a l excitations. In specific cases the lattice vibrations will be large enough to produce a t r a n s p o r t of atoms in the conductor.

Another

atomic

transport

effect

which

occurs

in

h e t e r o g e n e o u s s y s t e m s even without c u r r e n t stresses, is called migration. This t r a n s p o r t phenomenon is the r e s u l t of differences in chemical p o t e n t i a l when two m a t e r i a l s are p u t together. In order to o b t a i n an e q u i l i b r i u m situation, atoms will diffuse or migrate

and

i n t e r m e t a l l i c s are

formed.

In

some

material

combinations, such as Au-A1, the atomic t r a n s p o r t will be l a r g e r in one direction t h a n in the other. This creates divergences in atomic transport. The diffusion from Au into A1 is faster t h a n the other w a y a r o u n d and so a n e t t m a s s t r a n s p o r t from Au into A1 will occur.

D e t e r i o r a t i o n of i n t e r c o n n e c t i o n s by e l e c t r o m i g r a t i o n or m i g r a t i o n can occur when the atomic t r a n s p o r t or flux is not uniform over t h e i n t e r c o n n e c t i o n s t r u c t u r e . C u r r e n t crowding, g r a i n b o u n d a r i e s , differences in diffusion coefficient and l a t t i c e defects can cause such n o n - u n i f o r m i t y . W h e n d i v e r g e n c e s in atomic flux exist, their will be sites in the lattice where more atoms are being driven a w a y t h a n atoms t h a t are arriving. This will cause a depletion of m a t t e r and a void will be formed. This void can grow until it has the size comparable to t h a t of the interconnection and an open circuit is formed. Opposed to depleten, accumulation of atoms occurs as well to form hillocks and whiskers which can ultimately lead to short circuits.

2. LITERATURE STUDY

In l i t e r a t u r e some very interesting results on heterogeneous systems have been obtained. All the articles t h a t are discussed here, describe gold ball bonds on a l u m i n i u m metallizations which is the well known interconnection technique in micro-electronics.

F r o m e l e c t r i c a l t e s t i n g e x p e r i m e n t s , a s q u a r e root time

Electromigration investigation dependency of the relative resistance change AR/R o could be observed [2,3]. Relative resistance change is defined as:

APJRo (t) =

R(t) - R o Ro

With R(t) the resistance of the sample at time t, and R o the resistance at t--0. At high temperatures this parabolic behaviour changes rapidly in a large increase in resistance change AR/R o [2]. No effect of addition of Si and Cu to the aluminium metallization could be observed [2]. On the other hand, a dependency of the alumlnium layer thickness on the resistance change was found [3]. It was noticed that thicker layers at high temperatures (higher than 150°C) yielded better behaviour thin thinner layers against a steep increase in resistance. All the measurements stated here were performed ex-situ or on very discrete time intervals. This distorts the image of resistance change kinetics.

Also a relation could be seen between electrical failure (open contacts) and the formation of voids [4]. These voids on the other hand will not necessarily lead to mechanical failure [4] which is possible when the voids form a ring around the foot of the ball bond and not underneath the ball bond. In that case an electrically open circuit still has a reasonable mechanical strength. Above 200°C large voids had been noticed which can explain the large increase in resistance observed [2]. It was also observed that voids form in the Au rich phases of the intermetallic structure [4]. In another article voids were observed in the Au5A12 and the AuA12 phases [5] These are only two of the five existing phases of Au and Al. The occurrence and the formation rates of the different phases has been studied in some extent [5,3,6]. A square root time dependency was observed and growth rate constants for the different phases were established. In one study [2] the resistivity of the different intermetallic phases was measured. Increase in resistance of the total bond was attributed to formation of these intermetallic phases u n d e r n e a t h the ball [3]. Another explanation for the increase in resistance could be found in the oxidation of the intermetallic phases [7].

1143

B. VANHECKE et al.

1144

8. R X ' P E R I M E N T A L S E T - U P

In order to measure the resistance changes of a Au ball bond on Al-based metallizations, samples were p r e p a r e d as follows. Three s t r i p s of m e t a l l i z a t i o n were m o u n t e d in a 18 pin c e r a m i c DIL package in order to perform three different m e a s u r e m e n t s at exactly the same t e m p e r a t u r e and c u r r e n t conditions. The m e t a l l i z a t i o n consisted of a 1.1~m thick layer of Al-l%Si or A l - l % S i - l % C u on an oxidized Si substrate. On each strip four contacts were bonded. A1 bond wires were formed at the outer contact points (contacts 1 and 3) in order to prevent interactions with the metal film at these locations (growth of i n t e r m e t a l l i c s and voids). The two r e m a i n i n g contacts formed a double Au ball bond in the centre of the strips. The upper ball bond (contact 4) conducted a c u r r e n t t h r o u g h the Au/A1 interface during the m e a s u r e m e n t or d u r i n g ageing u n d e r current stress, while the other ball (contact 2) was used to m e a s u r e the voltage across the Au/A1 interface (figure 1). With this set-up a fourp o i n t m e a s u r e m e n t of the r e s i s t a n c e of t h e Au/A1 i n t e r f a c e u n d e r n e a t h the ball bond was performed. In order to exclude the influence

of t h e r m o - e l e c t r i c

contact

potentials,

resistance

m e a s u r e m e n t s a r e p e r f o r m e d with both positive a n d n e g a t i v e c u r r e n t directions. F o r the m e a s u r e m e n t of a c u r r e n t s t r e s s e d sample, the stress c u r r e n t was switched off a n d a small c u r r e n t ( l m A ) was used.

These samples were then mounted in a specially developed

I

v

F i g u r e 1: Schematic r e p r e s e n t a t i o n of test structure used in fourp o i n t m e a s u r e m e n t s of gold b a l l bonds on a l u m i n i u m m e t a l l i s a t i o n s . The negative c u r r e n t configuration is defined as the configuration where the electrons enter from the ball into the m e t a l l i z a t i o n layer; in the positive c u r r e n t c o n f i g u r a t i o n the electrons move from the metallization into the ball.

Electromigration investigation

1145

high stability furnace [8] and aged under t e m p e r a t u r e and current stress.

Another p a r a m e t e r t h a t has been investigated is the effect of doping in the metallization on resistance change. Therefore four types of metallization were used: All%Si 1.1ttm and A11%Sil%Cu 1.1tLm using one process of s p u t t e r i n g and All%Si 1.2pm and All%Si+As (implanted at 50KeV to a concentration of 1.0.1016 atoms per cm2) 1.2pro using another process. The Cu doping was examined since in some literature beneficial effects of Cu had been stated [9] while in others no influence of Cu on the resistance change has been detected [2]. This ambiguity in the results needed to be clarified.

Also the explanation from H a a g [7] for the resistance change due to oxidation of the interconnection intermetallics has been investigated by comparing the resistance change of balls aged in He and in air atmosphere.

In order to investigate the intermetallics growth as a function of time and current density, cross sections of the ball bonds had been made and E D X mapping of the contact region was done.

4. E X P E R I M E N T A L R E S U L T S

4.1. Temperature stress measurements:

A

set of AII%Si

temperature

l.l~m

samples

was

put

through

a

stress experiment in which the samples were

submitted to high temperatures ranging from 250°C to 350°C. N o D C current was sent through the contacts during the ageing. Only low A C currents of l m A were used for the measurement of the resistance. The resistance was measured in-situ at the ageing temperature. This allows us to follow the resistance change with m u c h more detail than is possible with ex-situ measurements since m u c h more data points are acquired during the ageing.

1146

B. VANHECKEet al. 1000.00

r~

f

I°°°I ,

d

looqi

O.

Ic

°

/

1 0 ~ 0

I0

20

30

40

50

60

70

time (h) Figure 2: Relative resistance change AR/Ro for gold ball bonds on All%Si metallization at 4 different temperatures (a: 250°C, b:290°C, c:310°C, d:350°C)

In figure 2 a set of measurements at different temperatures is given. One can easily see the dependency of temperature on the change of resistance. Higher temperatures of ageing will lead to faster increases of resistance. The interpretation of the different phases in the resistance change curve will be discussed in paragraph 5.

4.2. C u r r e n t s t r e s s m e a s u r e m e n t s :

A similar type of experiments is done on both All%Si 1.1~m and A11%Si 1.2tim samples but now an electricalD C current from 0 to 800 m A

is passed through the ball bond interconnection. Both

current directions are being investigated. W e define as a negative current, the current in which the electrons m o v e from the ball (bond 3) into the metallization. For positive currents the electrons move in the opposite direction. In figure 3 the relative resistance change of the ball bonds on AI1%Si 1.1~m metallization as a function of time for different current stresses can be seen. It is clear that for high negative currents the resistance increase in

much faster than for positive currents. It even can be seen at first sight t h a t positive currents

yield a p p r o x i m a t e l y the same

resistance change rate as samples aged without c u r r e n t stress

Electromigration investigation

1147

i000.

900-I

800-

I I

700-

,-800

i

600-

r

A

\ o~

I

500-

400-

300-

200-

./

me

400

/ I00-

I

50 time

(h)

Figure 3: Relative resistance change measurement of A11%Si 1.1.m samples under different current stresses at 300°C. Negative current stresses yield a faster increase in resistance.

(True Ohm). For All%Si 1.2~tm samples the behaviour is similar at first sight. For high positive and negative currents approximately the same resistance changes are observed. Nevertheless higher resistance increases are observed for low currents (-50 to 50 inA) as can be seen in figure 4. This is due to the differences in processes.

4.3. Effect of doping on resistance change:

First a set of measurements were performed on A11%Si 1.2~ra samples with As doping. The samples were aged at 300°C under current stress. When we compare the results (figure 5) with those of undoped metallizations, we can notice a slightly faster decay for As doped samples. Here also a clear current density and current direction dependency can be observed. High negative currents

1148

B. VANHECKEet

al.

1000

900-

800-

7o0.

I

600v

500\

400300-

200~800 i00-

O20

25 time

30

35

40

(h)

Figure 4: Relative resistance change measurement of AI1%Si 1.2.m samples under different current stresses at 300°C. A similar current dependency can be observed except for low current densities (currents of -50 to 50 mA) where the resistance change is much faster.

break down the interconnection rapidly while samples under high positive currents keep on working for a longer period of time.

When we m e a s u r e samples with additional Cu in the aluminium metallization aged under the same conditions as the previous samples, we obtain curves of resistance change that show only minor resistance increases over periods of time wherefore the samples without Cu addition already reached high resistance values (figure 6).

We also found that samples aged at higher temperatures and with high c u r r e n t densities showed better resistance change results with than without Cu. At elevated temperatures with high

Electromigration investigation

1149

900800700600-

,-40O

/o

A d#

\

500-

4~o0

400300.

200.

100.

O. time (h) Figure 5: Relative resistance change measurement of AII%Si+As 1.2~m samples under different current stresses at 300°C. Samples with As doping in the metallization show a slightly faster increase in resistance than samples with an undoped metallization

negative c u r r e n t s the resistance nevertheless exhibited a steep increase after some hours.

4.4. EDX mapping of cross sections:

Cross sections of ball bond samples aged at 300°C with current stress of +400 and -400mA are viewed in a SEM a p p a r a t u s a n d analyzed with EDX mapping. With the obtained data the different Au-A1 phases of the interface can be established. In figure 7 we see the representations of the different samples with and without Cu doping (1.1gin metallizations) and with different current stresses after 50000 seconds (almost 14 hours).

I t can be seen t h a t the addition of Cu to the metallization

1150

B. VANHECKE et al. r e s u l t s in a more balanced interdiffusion of Au a n d A1 across t h e original interface. In the right p a r t of figure 7 A1-Au i n t e r m e t a l l i c p h a s e s a r e not only found below b u t also above the o r i g i n a l interface. This can be explained by the fact t h a t the addition of Cu strongly r e t a r d s the formation of Au/A1 intermetallic phases due to the p i n n i n g of the A1 atoms n e a r the grain boundaries by the Cu atoms. At the same time, this explains why void formation is also r e t a r d e d , since K i r k e n d a h l voids are formed as a r e s u l t of the unbalance in diffusion velocity of Au and A1.

5. DISCUSSION

5.1. T e m p e r a t u r e dependency W h e n no c u r r e n t s t r e s s is a p p l i e d to the s a m p l e s , no

i000

I

I

F

I

I

I

I

I

I

900800-

700-

600v

500-

400-

300-

-6;0[.,

200-

i00-

0 .

| 5

I i0

I 15

P 20

I 25

time

I 30

F 35

I 40

f 45

(h)

Figure 6: Relative resistance change measurement of A11%Si1%Cu 1.1.m samples under different current stresses at 300°C. At this temperature no drastic increase of the resistance could be observed over the same time period in which undoped samples exhibited resistance changes of several 1000%.

50

Electromigration investigation

1151

electromigration will occur. Only migration will be able to deteriorate the interconnection structure. Migration is a diffusion process a n d

diffusion is highly t e m p e r a t u r e

dependent,

so

temperature dependence of migration and of the resistance change caused by this migration is obvious. The higher the temperature, the faster the migration of atoms from Au into A1 and vice versa. The faster the migration, the faster the intermetallic phase formation, the faster the resistance of the interconnection will change. This is exactly what can be seen in the m e a s u r e m e n t s which have been done.

With the measurements performed at different temperatures one is able to calculate the activation energy of a process. This is possible if the sample resistance obeys the Arrhenius law: AR C t exp ( - ~ ) Ro-

where C is a constant, t is the time, k is Boltzmann's constant, T is absolute temperature. We use here the simplest form of the A r r h e n i u s law a l t h o u g h several different (non-linear) time dependencies were observed. Then one has to establish for a certain value of the resistance change (e.g. aR/R = 100%), called the failure

AI 1~Si

AI 1%Si1%Cu

+400rnA~

_

_

_

true ohm

--400 _

mA

_

legend: I ~ "°

1~ ''2.°

D ''*°'

[]"'"

I ~ ''2'''s

I]#*'

Figure 7: EDX mapping on cross sections of Au ball bonds on samples with and without Cu under different current stresses.

I-] '''"2

1152

B. VANHECKE et al.

criterion,

the

time needed to reach this value for each temperature.

The time is called the time to failure (TrF). ma

rlq~ =-~oR 1100%exp( C

~)

with D~ R [ lOO% the failure criterion. Plotting In(TTF) against the inverse of the temperature, one can obtain the activation energy E a of the process from the slope of the curve.

S a

ln(TrF) = ~-~ + In (a)

where a is a constant. In

figure 8 the a p p a r e n t activation

energy of Au ball bonds on A11%Si 1.1gm and AII%Sil%Cu 1.1~m metallizations is plotted against the failure criterion. The activation energy rises with time since during the ageing of the ball bonds,

4

.I-

6-

O

4-

O 0 .-4 aJ

3-

----[]---AII%SiI%Cu ---O--- A l l % S i

:> .,4 aJ u

2.

1.

0 .1

"

~

~

1.0 failure

0

criteria

Figure 8: Apparent activation energy of A11%Si and A11%Si1%Cu samples for different failure criteria.

.0

Electromigration investigation processes acting on the samples will change. First the processes with lowest activation energy will occur. A low activation energy means that the process can be easily activated. W h e n these first processes have reached an equilibrium situation, other processes can start to occur with other activation energies. F o r m this graph it can be seen that the process with the lowest activation energy is still active for samples with C u while those without C u have changed process a long time earlier.

This is also what is noticed in the curve for the resistance change. Three distinct phases in the curve could be observed. First there is a parabolic phase which can be attributed to interdiffusion of A u and Al underneath the ball. This is a diffusion process perpendicularly to the interface of the two metals which is k n o w n to have a square root time dependency. This process in A11%Si samples reaches an equilibrium state w h e n all the metallization underneath the ball is transformed into intermetallic phases. Secondly a linear part in the curve can be observed. This can be attributed to the diffusion in the plane of the metallization a w a y from the ball. Here the diffusion process is cylindrically.The third part shows a fast increase of the resistance. This is the result of the growth of voids and cracks in the interface leading to an open circuit.

The diffusion in All%Sil%Cu metallization is, as was stated before, retarded and thus the process with the lowest activation energy (at the beginning of the resistance change) will need more time to reach an equilibrium state. In figure 8 we can see that the activation energy of these samples maintains a low value while that for All%Si samples rises very fast.

5.2. Influence of Oxygen on resistance change:

As stated before, Haag reported a possible explanation of the resistance increase of Au ball bonds on A1 metallizations by oxidation of the intermetallic phases of the interface. A set of ageing experiments is performed on Au ball bonds on All%Si

1153

1154

B. VANHECKEet al. 1.1~m metallization under different atmospheres. Both inert He atmosphere and Air are used. The differences in resistance change for both atmospheres (figure 9) are small compared to those reported earlier [7]. There seems to be no reason to believe that oxidation is the major process that changes the resistance.

5.3. Current stress dependency

It is known that for most materials the direction of the moving atoms due to electromigration is the same as that of the moving electrons of the current [1] which is also the case for Au and A1. It thus can be understood that the interdiffusion of Au and A1 can be assisted or obstructed by the electromigration depending on the direction

of the

current.

In

the

case

of our

samples,

electromigration due to a negative current will help the normally 1000

'

'Y 'I'

'

'

'

'

'

'

900-

800-

700-

-800 He -800 Air

600A dO v

\

500-

..v'oHe .,..," ......'""

400-

...."'"'

+(~00

Ai¢

300-

200. i00.

O. time

(h)

Figure 9: Relative resistance change of A11%Si samples in air and in inert He atmosphere. No major difference between both situations has been observed.

Electromigration investigation

1155

p r e s e n t m i g r a t i o n , since b o t h t r a n s p o r t p r o c e s s e s a r e i n t h e s a m e direction, while a positive c u r r e n t will o b s t r u c t t h e i n t e r d i f f u s i o n or m i g r a t i o n . I t c a n t h a n be u n d e r s t o o d t h a t s a m p l e s a g e d u n d e r p o s i t i v e c u r r e n t s t r e s s will d e c a y less f a s t t h a n t h o s e a g e d u n d e r n e g a t i v e c u r r e n t stress.

A l s o i n t h e EDX m a p p i n g s of t h e b a l l b o n d cross s e c t i o n s we c a n s e e t h e i n f l u e n c e o f t h e c u r r e n t s t r e s s d i r e c t i o n on t h e f o r m a t i o n of i n t e r m e t a l l i c s . As s t a t e d e a r l i e r t h e i n t e r d i f f u s i o n o f A u a n d Al u n d e r p o s i t i v e c u r r e n t s t r e s s is m o r e b a l a n c e d w h i c h r e t a r d s t h e f o r m a t i o n of voids a n d cracks.

I f we plot t h e t i m e to f a i l u r e (TTF) a s a f u n c t i o n o f c u r r e n t d e n s i t y for different v a l u e s of AR/R, we find a curve a s i n figure 10. T h e f i g u r e s h o w s a n i n c r e a s i n g c u r v e for i n c r e a s i n g c u r r e n t d e n s i t i e s (from n e g a t i v e to positive) for t h e d i f f e r e n t s a m p l e s all

40

(o)

/

30

/

/

/A,\o///~'\ \

,,,0//AI 1°/oSi 1°/. Cu 1.1/J.m /-

/

\

/

//

\ ",,0/'/

/"

20

AI1%Si 1.2/J.m~ ~ ~

V

I--I--

~

A

0

I

-800

--400

AIl%Si+As 1.2Nm

I

]

]

0

400

800

20

(b) A1%Si 1.1~m 10

0 -- 8 0 0

. . . . •"-=-"-='"-:"'"~.................... 9 ............ ~,v. .~..................... 9 --400

0

400

800

Current. (mA)

Figure 10: Time to failure (TI'F) as a function of the current stress for different samples for a failure criterion of a R / R = 100%. The higher TTF for samples with Cu d o p i n g can easily be seen in this graph. (a: at 300°C, b: the A11%Si 1.1mm s a m p l e s at different temperatures)

1156

B. VANHECKE

et al.

using the same failure criterion (AR/R = 100%). F r o m this curve one

can

see

the

minor

m e t a l l i z a t i o n s (1.1 a n d

difference

between

both

All%Si

1.2~m) which w a s d i s c u s s e d before.

F u r t h e r m o r e we observe a s l i g h t l y worse r e s i s t a n c e c h a n g e behaviour

for

the

All%Si+As

sample

while

doping

the

metallization with Cu increases in a r e m a r k a b l e way the TTF. The same can be said for samples aged at higher t e m p e r a t u r e s under c u r r e n t stress. H i g h e r t e m p e r a t u r e s reduce the TTF a n d a g a i n addition of Cu leads to longer lifetimes for the samples.

6. CONCLUSIONS

The analysis was possible due to the fact that a high resolution in the resistance change kinetics was obtained with high accuracy. This high accuracy was the benefit of using high accuracy measurement equipment and especially a high stability furnace. With this furnace w e were able to exclude large fluctuations in temperature (and thus in resistance due to TCR).

It was observed that the resistance change of A u ball bonds on AI based metallizations was highly influenced by temperature and current density. Higher temperatures yielded faster increases in the resistance while the decay of the contacts was accelerated with negative current densities and retarded with positive currents.

C o n t r a r y to some other results an influence of the addition of Cu to the metallization was observed. Cu atoms r e t a r d the diffusion of Au into A1 a t a given t e m p e r a t u r e . This also allows a more balanced interdiffusion so t h a t the nett mass flux from Au into A1 will decrease or vanish. The atomic flux divergence t h a t existed d i s a p p e a r s and void formation is stopped or retarded.

A t h e o r e t i c a l model has also been p r e s e n t e d which can explain the somewhat strange current density dependency of the r e s i s t a n c e change as a function of time. The model combines different

diffusion

processes

interconnection interface.

which

are

at

work

at

the

Electromigrationinvestigation ACKNOWLEDGMENTS The authors would like to thank M. Van De Peer and E. Thoonen for their help and technical assistance in the preparation of the samples and in the execution of the measurements.

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