Research note

Irregular e�ect of chloride impurities on migration failurereliability: contradictions or understandable?

Ga bor Harsa nyi

Department of Electronics Technology, Technical University of Budapest, Budapest, H-1521, Hungary

Received 24 June 1999

Abstract

Metals can exhibit dendritic short-circuits caused by electrochemical migration in conductor±insulator structures,

which may result in failures and reliability problems in microcircuits. The phenomenon of electrochemical migrationhas been well known for several decades; the process is a transport of metal ions between two metallization stripesunder bias through a continuous aqueous electrolyte. Due to the electrodeposition at the cathode, dendrites anddendrite-like deposits are formed. Ultimately, such a deposit can lead to a short circuit in the device and can cause

catastrophic failure. Surface contaminants, especially ionic types, may have signi®cant in¯uences on the overallprocess. Clÿ contaminant has been investigated extensively; however, many contradictory statements were published.The role of these contaminants is rather complicated in in¯uencing the formation of migrated resistive shorts: the

various e�ects act against each other. Theoretical explanations are discussed and strengthened by experimentalresults in this paper. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Electrochemical migration; Dendritic growth; Reliability of microcircuits; Metallization failures; Ionic surface contami-

nants

1. Introduction

Recently, in connection with the production of high-

density interconnection systems in integrated circuits

and multichip modules (MCMs), the claim to conduc-

tor-systems with very high resolution and high re-

liability has emerged. The possibilities of integration

are determined not only by the technological bases but

also by those physical and chemical processes that can

cause resistive shorts between adjacent metallization

stripes during the operation [1]. One of these phenom-

ena is the electrochemical migration. This can be

de®ned as a transport of ions between two metalliza-

tion stripes under bias through an aqueous electrolyte.

Electrodeposition also occurs forming dendrites or

dendrite-like deposits. Ultimately, such a deposit can

lead to a short circuit in the device and can cause cata-

strophic failure. The conditions are: a ®lm of polar

liquid (usually water) to form an electrolyte, bias, and

operating time [2].

Migrated resistive shorts occur randomly in practice

and mainly under extreme conditions. However, a

number of electrical ®eld governed failure types are

correlated with migration. A device can operate for

many hundreds of hours under normal operating con-

ditions, and then, after a short exposure to special en-

vironmental conditions, fail [3,4].

The various models and theoretical explanations are

discussed in the next sections.

Microelectronics Reliability 39 (1999) 1407±1411

0026-2714/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0026-2714(99 )00079-7

www.elsevier.com/locate/microrel

E-mail address: Harsanyi@ett.bme.hu (G. Harsa nyi)

2. Contaminant-free electrochemical migration

The classical model of electrochemical migration hasbeen well known for several decades, however, a fewanomalous or newly discovered phenomena needed to

make some revisions and add supplementary models,thus, it is useful to recall the original descriptions.The phenomenon can be described as a transport of

metal ions from an anodic site, through an aqueouselectrolyte, toward a cathodic site where the electrode-position of the ions occurs forming dendrites [2].

Meanwhile, water decomposition processes may alsotake place. Silver is famous for its inclination to formmigratory shorts. Studies on silver migration havebeen made by many researchers among whom

Kohman et al. [2] investigated it very intensively andgave the ®rst explanation of the mechanism.At the anode, silver dissolves forming Ag+ ions:

Ag4Ag� � eÿ �1�Ag+ ions then combine with OHÿ ions to form AgOH

which is not stable and decomposes to Ag2O and H2Oaccording to the following equilibrium:

2Ag� � 2OHÿ , 2AgOH, Ag2O� H2O �2�The colloid precipitate of Ag2O can easily be reducedto metal Ag. Because of the relatively small solubility

product of AgOH, Ag+ ions do not precipitate fromthe solution after a short incubation period and movedirectly toward the cathode where their electrochemical

deposition occurs:

Ag� � eÿ4Ag �3�H+ ions also collect around the cathodic site where

their discharge maintains the electrical balance:

2H� � 2eÿ4H2 �4�The reason why silver is more susceptible to mi-

gration than other metals is that it is anodically verysoluble and its precipitates have also good solubility.The anodic dissolution behavior can be reduced by

alloying the silver with Pd and Pt [5,6]. At special tern-ary Pd±Pt±Ag compositions, the migration can practi-cally be eliminated [6].It has been proven in numerous further studies that

several other metals also show dendritic growth causedby the electrochemical migration, i.e., Cu, Pb, and Sn[4,7,8]. Although these metals have di�erent electrode

potential values and their products with water, mainlytheir hydroxides, have various solubility coe�cients,the basic mechanism of electrochemical migration is

the same as in the case of silver. Ni seems to havedi�erent mechanism since anodic dendritic growth canbe observed. The model was discussed elsewhere [9].

Returning to the theoretical model, the process con-sists of the following general steps:

. Metal-ion formation by an anodic corrosion thatmay either be direct electrochemical dissolution or amulti-step electrochemical±chemical process resulting

in precipitates as well.. Metal-ion migration through the electrolyte under

electrical ®eld toward the cathode.

. Electrochemical metal deposition at the cathodeforming dendrites or dendrite-like metallic deposits,which are growing toward the anode and may result

in short circuit bridges when reaching it.

3. Migration processes in the presence of Cl

3.1. The case of classical migration

Surface contaminants, especially ionic types mayhave a signi®cant in¯uence on the overall migrationprocess even in the case of metals following the dendri-

tic growth mechanism of the classical model. Clÿ con-taminant has been investigated extensively in variousresearch works, however, many contradictory state-ments were published.

One of the often-emphasized statements is that thepresence of ionic contaminants is essential for the mi-gration since the pure moisture ®lm has no ionic con-

ductivity. This is simply not true. Pure water hasrather low conductivity through the proton hoppingmechanism:

H2O� H2O�4H2O� � H2O �5�

But this is enough for the dendritic growth, as theresults of water drop (WD) tests performed generallywith de-ionized water have demonstrated many times.

Thus, the presence of ionic contaminants is not alwaysessential for the electrochemical migration [7±9]. Thereare cases, for example at gold, where this is true, butnot for the ionic conductivity, but for the complex

forming ability (see Section 2).Chloride is a common contaminant on device sur-

faces coming from the environment. The sources are as

follows:

. the salt spray of the seas;

. the dust particles in the air may contain Clÿ ions;

. human ®ngerprints contain NaCl salt;

. the air contains some Cl2 which may be dissolved inwater forming HCl which dissociates into ions;

. residual ¯uxes and adhesives used in electronicsassembly may also contain chlorides; and

. the breakdown of cleaning solvent molecules like tri-

G. HarsaÂnyi / Microelectronics Reliability 39 (1999) 1407±14111408

chloroethylene results chloride contamination as

well.

Thus, chloride may be present on device surfaces

both as alkalohalogenid salt or also in a form which

may be ionically dissolved into the moisture ®lm with-

out having any analytically recognizable ionic counter-

part.

The role of contaminants in in¯uencing the for-

mation of migrated resistive shorts is rather compli-

cated. Various e�ects act against each other. The most

important ones can be summarized as follows:

1. Ionic contaminants increase the conductivity of the

electrolyte formed by moisture condensation thus

they increase the overall speed of the process. It was

found at low concentration levels that the speed of

copper dendritic growth (measuring the dendrite

lengths data within a given time-frame before get-

ting shorts) is almost linearly proportional to the

chloride concentration [14]. The mentioned study

was performed on TAB tapes which have a material

system very similar to our case: copper foil attached

to kapton using adhesives containing phenolic

resins. The linear behavior may, however, be not

true at high concentration levels (see later) and at

zero level, at which the dendritic growth still occurs.

2. Chloride contaminants increase the ability of the

anode for dissolution. Clÿ ions are neutralized on

the surface of the anode and they can leave the elec-

trolyte forming Cl2 gas, but they may also form

chlorine oxides and/or chlorous acid destroying the

existing oxide ®lm on the anode surface or inhibit-

ing its formation which prevents the anode to have

a passivating ®lm on the surface. This increases the

speed of anodic dissolution hence, the metal ion

concentration inside the electrolyte, ®nally the speed

of dendrite formation.

3. Chloride ions can also form precipitates with metal

ions, for example AgCl, CuCl2, SnCl4, etc. This will

act against the dendritic growth, decreases its speed

or even stops it. This is why dendrite formation can

not be observed sometimes at high contamination

concentration levels.

4. The increased ionic conductivity reduces the e�ect

of electric ®eld on the cathodic deposition; metal

ions reach its surface due to the di�usion thus, a

continuous ®lm deposition replaces the dendritic

growth process at high contaminant concentration

levels.

5. Ionic contaminants are often present in inorganic

salt form on the dry device surfaces, and can act as

condensation sites for moisture adsorption because

of their hygroscopic nature. The total thickness of

water adsorbed on the surface of a clean, smooth

substrate is generally quite small, even at high

humidities. Practically, it remains under the level of

20 monolayers up to 90% RH [15]. In contrast, thepresence of water-soluble compound impurities may

cause the condensation of water at less than 100%RH. According to theoretical expectations and prac-tical measurements, the critical relative humidity

level where the condensation runaway takes place,corresponds to the equilibrium RH above the satu-rated salt solutions. For example, 82% at KCl, 75%

at NaCl, 69% at CuCl2, and only 11% at LiCl.Several studies have demonstrated the latter beha-vior [7,15]. The water uptake measured by weight

gain of clean and CuCl2 contaminated substrateshas well demonstrated, that a runaway can befound at about 70% RH on the contaminatedsample, however, the amount of adsorbed water is

also much bigger below this humidity level than atthe clean surface [15]. The water uptake can also befollowed by leakage current and migration failure

measurements. Using alumina substrates, gold elec-trodes and NaCl contamination, it was found thatthe presence of a continuous moisture ®lm should

be supposed above 75% RH levels.

3.2. The case of Au, Pd, and Pt

The application of `strongly precious' metals (Au,Pd, Pt) and their alloys had been considered the best

solution against migration problems, until migratedgold resistive shorts (MGRSs) were discovered andreported in several studies [3,7±11].

Gold migration requires the presence of a contami-nant (e.g. a halogen salt, which is a common contami-nant on device surfaces) in addition to moisture and

electrical bias. At ®rst, MGRSs were found when Ti/Pd/Au thin ®lm metallization systems were environ-mentally tested and examined. If the adsorbed ®lm of

water contains Clÿ ions (0.1±0.001 molar), gold dis-solves at the anode, and a soluble tetrachloro-goldcomplex is formed [8,11]:

Au� 4Clÿ4AuClÿ4 � 3eÿ �6�

The halogen contaminant induces the anodic dissol-ution, thus this process can be called contaminate

induced migration. The main problem of this model isthat the electrochemical process according to Eq. (5)leads to the formation of negative ions, therefore theirmigration toward the cathode can hardly be imagined.

The tetrachloro-gold complex ion might be unstableand further chemical reactions had to be supposed.The main forms of gold in solutions are complex ions

of the type [12]:

�Au�OH�xCly

��x�yÿ3�ÿ, �x� y < 4� �7�

G. HarsaÂnyi / Microelectronics Reliability 39 (1999) 1407±1411 1409

Among these complexes, positive ions may also be pre-sent (for example when x � y � 1), the migration of

which toward the cathode is possible.According to another theoretical explanation [13],

the following chemical `chain reaction' is also possible

in acidic media, which is present in the anode region(since the deposition of OHÿ ions results in an enrich-ment of H+ ions there):

AuClÿ4 � H�4H�AuCl4 �4HCl� AuCl34H�

� 4Clÿ � Au3� �8�

The resulted Au3+ ions are positive metal ions thatcan migrate toward the cathode and form dendritessimilarly to the classical model.

It was demonstrated in subsequent publications thatPd and Pt could also form dendrites [8,11] by a similarbehavior. Their anodic dissolution may follow the

reaction schemes as

Pd� 4Clÿ4PdCl2ÿ4 � 2eÿ �9�

Pt� 4Clÿ4PtCl2ÿ4 � 2eÿ �10�

Generally, the process of contaminant induced mi-gration can be summarized in the following steps:

. Primary negative complex-ion formation by ananodic corrosion induced by halogen contaminates(see Eqs. (6), (9) and (10)).

. A multi-step chemical process resulting metal ionsor secondary complex cations (see Fig. 1 and Eq.(8)).

. Cation migration through the electrolyte under elec-trical ®eld toward the cathode.

. Electrochemical deposition at the cathode formingmetallic dendrites.

4. Experimental

In order to clarify the real practical e�ect of Cl con-taminant on the migration process, experimental pro-cedures have been carried out on various thick ®lmmetallization systems ®red on alumina substrates hav-

ing a geometry of 4 mm length, line width 0.5 mm,spacing 0.2 mm.At ®rst, water drop (WD) tests have been performed

on the structures. A drop of water was placed and 5 Vdc bias was connected between the adjacent stripes andthe formation of dendrites was visually inspected

under microscope. The tests have been performedusing de-ionized (DI) water and NaCl solutions withseveral concentration levels: 1 mM, 10 mM, 100 mM,

and 1 M. The accelerating±inhibiting e�ect of thechloride-ion contaminant could be followed qualitat-

ively very well. The results are as follows:

1. At Au conductors and at its alloys with Pd and Pt,the migration process is accelerated by the Cl con-

taminant. The higher the contaminant concen-tration, the faster the short circuit formation.

2. At Ag-alloy conductors and base metals (Cu, or

conductors covered by SnPb solder), the qualitativeresults were practically independent from the con-ductor material but very much depending on the

Clÿ concentration level.* DI water: A rather slight dendritic growth pro-

cess can be observed with thin ®laments after a

long (30±60 s) incubation period. Short circuitscan be formed, but they also may be cut o�during drying. After drying, slight dendrites arepresent, mainly destroyed during the evaporation,

and hydroxide precipitates are also present.* 1 mM NaCl: The dendritic growth is fast, after a

short (10±20 s) incubation a dense system of den-

dritic ®laments is growing causing short circuits.After evaporation, the shorts can still bemeasured, the dendrites have a stable structure.

Precipitates are not characteristic.* 10 mM NaCl: The process is similar to the for-

mer one, di�erences are not characteristic, mainlydue to random e�ects.

* 100 mM NaCl: No observable dendritic growthstarts even after several minutes. Corrosion pre-cipitate ®lms are generally formed on the anode,

chloride salts for all likelihood.* 1 M NaCl: The short circuit formation returns,

however, a continuous ®lm deposition on the

cathode can be experienced rather than a dendri-tic growth process.

THB tests (458C/95% RH/10 V dc) were also per-

formed on thick ®lm samples. They were previouslyimmersed into NaCl solutions with various concen-trations (from 1 mM to 1 M) and then were dried out.

The surface concentration after drying was in the levelof 0.5 mg NaCl/cm2 when immersed previously into a 1mM solution. Fig. 1 shows mean time to failure data(MTTF) as a function of the contaminant concen-

tration. It is obvious, that these results correspondstrongly to those of the WD test.

5. Conclusions

Experimental results indicated that ionic surfacecontaminants may have contradictory e�ect on the for-

mation of migrated resistive shorts. In the case of Clcontaminant, two main groups of conductor materialscan be distinguished:

G. HarsaÂnyi / Microelectronics Reliability 39 (1999) 1407±14111410

1. `Strongly precious' metals, as Au, Pd, Pt and theiralloys show the halogen induced migration; they

loose their ability to resist against migration andshow decreasing lifetime with increasing contami-nation level.

2. Silver, its alloys and base metals, like Cu, Sn, Pb,etc. show an interesting behavior; while ionic con-taminants generally accelerate the migration process,a precipitate formation may hinder or even stop it

at medium concentration levels.

Acknowledgements

This research was supported by the European

Commission in the frame of INCO Copernicus ProjectNo. ERBIC 15CT 960743, as well as by the followingprojects in Hungary: OM FKFP 0300/1999 and

OTKA T030574.

References

[1] Illyefalvi-Vite z Zs, Ruszinko , J. Pinkola M. Recent

advancements in MCM-L imaging and via genaration by

laser direct writing. In: Proc. of the 48th Electronic

Components and Technology Conference. 1998. p. 144±

50.

[2] Kohman GT, Hermance HW, Downes GH. Silver mi-

gration in electrical insulation. Bell System Technical

Journal 1955;34:1115±47.

[3] Shumka A, Piety RR. Migrated-gold resistive shorts in

microcircuits. In: Proc. Int. Reliability Physics Symp.

1975. p. 93±8.

[4] Benson RC, Romenesko BM, Weiner JA, Nall BH,

Charles Jr HK. Metal Electromigration induced by

solder ¯ux residue in hybrid microcircuits. IEEE Trans

Comp, Hybrids, Manuf Technol 1988;CHMT-10:363±70.

[5] Coleman MV, Winster AE. Silver migration in thick-®lm

conductors and chip attachment resins. Microelectronics

Journal 1981;4:23±9.

[6] Naguib HM, MacLaurin BK. Silver migration and the

reliability of Pd/Ag conductors in thick-®lm dielectric

crossover structures. IEEE Trans Comp, Hybrids, Manuf

Technol 1979;CHMT-2:196±207.

[7] DerMarderosian A. The electrochemical migration of

metals. In: Proc. Int. Society of Hybrid Microelectronics.

1978. p. 134.

[8] Ripka G, Harsa nyi G. Electrochemical migration in

thick-®lm ICs. Electrocomp Sci Technol 1985;11:281±90.

[9] Harsa nyi G. Electrochemical processes resulting in

migrated short failures in microcircuits. IEEE

Transactions on Components, Packaging, and

Manufacturing Technology: Part A 1995;18(3):602±10.

[10] Grunthaner FG, Griswold TW, Clendening PJ.

Migratory gold resistive shorts: chemical aspects of fail-

ure mechanism. In: Proc. Int. Symp. Reliab. Physics.

1975. p. 99±106.

[11] Sbar NL. Bias humidity performance of encapsulated

and unencapsulated Ti±Pd±Au thin-®lm conductors in

an environment contaminated with Cl2. IEEE Trans

Parts, Hybrids, Packaging 1976;PHP-12:176±81.

[12] Benes P. On the state of manganese and gold traces in

aqueous solutions. J Inorg Nucl Chem 1967;2889±2898.

[13] Gaur JM, Schmid GM. Electrochemical behaviour of

gold in acidic chloride solutions. Electroanal Chem

Interfacial Electrochem. 1970;279±286.

[14] Nieman DA. E�ect of contamination on copper mi-

gration in TAB tape structures. In: Proc. of the

International Reliability Physics Symp. 1994. p. 87±92.

[15] Warren GW, Wynblatt P, Zamanzadeh M. The role of

electrochemical migration and moisture adsorption on

the reliability of metallized ceramic substrates. Journal of

Electronic Materials 1989;18(2):339±53.

Fig. 1. Main time to failure data vs. NaCl contamination level experienced during THB (458C/95% RH/5 V dc) tests performed on

thick ®lm samples using various conductors materials.

G. HarsaÂnyi / Microelectronics Reliability 39 (1999) 1407±1411 1411