Processing induced material interactions determining the reliability of LTCC multichip modules

Research note

Processing induced material interactions determining thereliability of LTCC multichip modules

GaÂ bor HarsaÂ nyi

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

Received 13 July 1999

Abstract

Metals can exhibit dendritic short-circuit growth caused by electrochemical migration in conductor±insulatorstructures, which may result in failures and reliability problems in microcircuits. The classical model of

electrochemical migration has been well known for several decades. This process is a transport of metal ionsbetween two metallization stripes under bias through a continuous aqueous electrolyte. Due to the electrochemicaldeposition at the cathode, dendrites and dendrite-like deposits are formed. Ultimately, such a deposit can lead to a

short circuit in the device and can cause catastrophic failure. Recent investigations have demonstrated that not onlymetallic components, but also oxides from the isolating layers can take part in the formation of migrated shorts,after a chemical reduction process. Material design aspects need to clarify the correlation between material

composition, processing, chemical bonding state, and electrochemical migration failure rate in isolating compounds:this is the scope of the present study. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Recently, in connection with the production of high-

density interconnection systems in integrated circuitsand multichip modules (MCMs), the claim to conduc-tor-systems with very high resolution and high re-

liability has emerged. There is also a great demand onintegrating embedded passive components (includingmultilayer capacitors) into the interconnection sub-

strate. The possibilities of integration are determinednot only by the technological bases but also by thosephysical and chemical processes that can cause resistiveshorts between adjacent metallization stripes or layers

during the operation. One of these phenomena is theelectrochemical migration. This can be de®ned as atransport of ions between two metallization stripes or

layers under bias through an aqueous electrolyte. Elec-

trochemical deposition also occurs forming dendrites

or dendrite-like deposits. Ultimately, such a deposit

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

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

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

operating time [1].

Migrated resistive shorts occur randomly in practice

and mainly under extreme conditions. A device can op-

erate for many hundreds of hours under normal oper-

ating conditions, and then, after a short exposure to

special environmental conditions, fail [2,3].

The classical model of electrochemical migration has

been well known for four decades [4±6], however, sev-

eral anomalous phenomena have initiated to perform

some revisions and to add supplementary models to

the conventional one. A recent discovery in this ®eld

was that metal ions forming dendrites can originate

not only from the metallic but even from the isolating

Microelectronics Reliability 40 (2000) 339±345

0026-2714/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PII: S0026-2714(99 )00207-3

www.elsevier.com/locate/microrel

E-mail address: [email protected] (G. HarsaÂ nyi).

Fig. 1. In situ X-ray di�raction analysis during heating (source CuKa): (a) Bi2O3/BaTiO3; (b) PbO/BaTiO3; (c) Fe2O3/BaTiO3; (d)

CuO/BaTiO3.

G. HarsaÂnyi / Microelectronics Reliability 40 (2000) 339±345340

Fig. 1 (continued)

G. HarsaÂnyi / Microelectronics Reliability 40 (2000) 339±345 341

compounds of the layers, assuming that these constitu-

ents can chemically be reduced [7]. In some cases, alsothe reduction site may run through the dielectric ®lmresulting dendrite-like conductive ®laments without

any ion formation and migration processes [8]. Thislatter phenomenon can be called ``virtual migration''.It has a much lower activation energy level than the

former ones because the real ionization and ionicmovement steps are avoided. The failure rates depend

on the reducibility that means the chemical bondingstate of the given compound. Material design aspectsneed to clarify the correlation between them.

In glass±ceramic based electronic components,packages, and interconnection systems, such as thick-

®lm hybrid circuits and low temperature co-®red cer-amic (LTCC) based multichip modules (MCMs), nu-merous composite (conducting and dielectric) materials

are applied containing various secondary constituentssuch as metal oxides for inorganic binder, dye andother purposes. Several types of these oxides can

chemically easily be reduced, for example by hydrogen,thus, they show the ability for metallic dendrite for-

mation. Till now, the materials having been found totake part in dendrite formation corresponding to thismodel are the oxides of copper, bismuth, lead and iron

[9]. Other materials may also show this ability. Mostdangerous are those oxides types that remain in thestate of oxides after the heat treatment (sintering, ®r-

ing) of the structures. Thus, at reactive type oxides,which take part in the physio±chemical reactions

during ®ring, the migration lifetime is determined bythe excess amount of metal-oxide. Lifetime data maybe improved by composition and technology changes.

At non-reactive type oxides, which do not take part inany reaction during ®ring, no improvement of lifetimedata can be expected by changing the composition and

the ®ring parameters.The models of various migration failure mechanisms

and evidences for the ``reduced oxide type'' as well as``virtual'' migration have been described in earlier stu-dies [7±12].

Thick-®lm and LTCC dielectrics for integrated ca-pacitor purposes are compound materials containing

not only a high permittivity ferroelectric powder, suchas BaTiO3 [13], but various secondary constituents asinorganic binders and dyes, respectively. Therefore,

special dielectric materials were produced for the ex-perimental purposes with controlled amounts of Bi2O3,PbO, CuO and Fe2O3 additives for achieving a better

understanding of failure mechanisms and for separ-ating the migration behavior of various oxide constitu-

ents. Originally, they have the following roles in thepastes. Bi2O3 is a common reactive binder componentin conductor pastes and ferroelectric paste or ``tape''

type materials. It provides a good adhesion due to theformation of various compounds during ®ring [14,15].

A similar behaviour is expected from PbO and CuO.On the other hand, Fe2O3 is used as a simple non-reac-

tive dye in these dielectric materials.The present work concentrates on the real chemical

state of critical compounds investigated by in-situ X-

ray di�ractometry during heat annealing. The objectiveis to study the processing induced chemical reactionsof those auxiliary oxide type additives that, after their

processing, are inclined to chemical reduction in moist-ure condensation induced electrochemical processesand subsequent metallic dendritic growth during device

operation. The reliability of the ready structures isdetermined by the chemical interactions during ®ring.The interaction of the additives with the functionaldielectrics, such as BaTiO3, has the greatest import-

ance, since the latter one is used as a ferroelectric com-pound in thick-®lm and embedded-LTCC-capacitordielectrics.

2. Reactivity of additive oxides

In order to get a better understanding of chemicalprocesses, in situ X-ray di�raction spectra have beenperformed during heat annealing on various powder

mixtures. The application of real thick-®lm pastes orLTCC compositions was not possible, because of theirrelatively low additive contents. Real printed thick-®lmsamples were also not suitable for the analysis because

the large X-ray peaks of alumina substrates disturb theevaluation of the spectra. Therefore, 1 : 1 molar ratiohigh-purity Bi2O3/BaTiO3, PbO/BaTiO3, Fe2O3/

BaTiO3 and CuO/BaTiO3 powder samples were pre-pared by mixing. The sample powder was pressed ontoa heated Pt element for making X-ray di�raction (XD)

spectra using a CuKa source. The XD spectra (madeby a Siemens D 500 di�ractometer) are shown in Fig.1. The most important results can be summarized as

follows:

1. In the case of Bi2O3/BaTiO3 powder (see: Fig. 1a),several strong chemical changes can be detected

when heated up from 650 to 8508C, up to the typi-cal thick-®lm ®ring temperature, X-ray peaks of sev-eral new compounds, such as: Bi12TiO20, andBaBi4Ti4O15 can be recognized.

2. In the case of PbO/BaTiO3 powder (see: Fig. 1b),chemical changes can be detected again at 7008C:the peaks of Pb2Ti2O6 can be recognized.

3. In the case of Fe2O3/BaTiO3 powder (see Fig. 1c),no reaction has been found under 9508C.

4. The situation is very similar with the CuO/

BaTiO3 powder mixture (see Fig. 1d): there is noreaction below 9508C.

Accordingly, Bi2O3 and PbO react inside the dielectrics


with the ferroelectric compound and they may disap-

pear from the ®lm if the temperature program is

appropriate for completing the chemical reaction. The

chemical reaction between Bi2O3 and BaTiO3 was

described already elsewhere [13], however, the postu-

lated compound (Ba2Bi4Ti5O18) was not the same that

has been detected here. On the other hand, no reaction

has occurred in Fe2O3/BaTiO3 and CuO/BaTiO3 mix-

tures, respectively, if applying a conventional thick-®lm

®ring cycle. Thus, any modi®cation of the ®ring par-

ameters can not alter the composition. These XD

analysis results con®rm some preliminary investi-

gations published elsewhere [16].

High temperature chemical reactions of the men-

tioned oxides with Al2O3 may also have an important

role in determining the ®nal composition. There is

enough information in the literature to have an im-

agination about these processes. Bi2O3 starts to form

new compounds with alumina above 6008C resulting

good adhesion between ®lm and substrate [15]. CuO

forms CuAlO2, typically starting at 9508C, but also hy-

pothesized to occur somewhat below this temperature

[17,18]. No reaction has been found in PbO/Al2O3 and

Fe2O3/Al2O3 mixtures [15,17].

3. Capacitor sample fabrication

Thick-®lm capacitors were chosen as model systems

for life test investigations. The samples were produced

by traditional thick-®lm technology using a non-mi-

grating Au conductor composition for electrode pur-

poses. The ®lms were printed onto 96% alumina

substrates using a 200-mesh stainless steel screen for

conductor layers, 160- and 200-mesh screens for the

two layers of the dielectrics, respectively. The ®lms

were dried at 1508C for 15 min and ®red in air within

a conventional belt furnace, at a standard 8508C pro-

®le, over a 60 min cycle time. The time at peak tem-

perature was approximately 10 min. The two printed

layers of the dielectric paste were co-®red, separately

from both conductor layers. The ®red thickness was

about 12±15 mm for the conductor and 50±60 mm for

the dielectric layers. The samples contained traditional

square shaped, 3.32 � 3.32 mm2 (130 mil sq) capacitor

structures.

The components of the pastes used in the exper-

iments were: BaTiO3, the main constituent of most ca-

pacitor dielectrics, a small amount of glass frit, Bi2O3,

PbO, Fe2O3, or CuO additives, respectively in various

concentrations, and organic vehicle.

4. Accelerated life tests

Thermal Humidity Bias (THB) testing was per-

formed on the thick-®lm capacitor samples for up to4000 h which meant an accelerated life test at 95%relative humidity (RH), under 10 V dc bias at the tem-

perature of 408C. The purpose of this experiment wasto estimate mean time to failure data of the sampleswith various materials and to ®nd a correlation

between their composition, processing, and migrationshort circuit failure formation ability. The empiricaltime dependency of the migration failure rates showedstatistically a close correlation with a log-normal distri-

bution, according to the expectations [5,11]. Thus,main time to failure data were estimated using leastsquare ®tting in a log-normal scale.

Fig. 2 shows main time to failure data as a functionof the metal-oxide content of the dielectrics. It isobvious that migration processes are strongly depen-

dent on the Bi2O3, CuO and PbO concentrations, butless in¯uenced by the Fe2O3 content.A similar behavior was found when examining the

in¯uence of the high temperature processing par-

ameters. Table 1 summarizes failure rate results after a1000 h THB test as a function of the ®ring pro®le:peak temperature and peak time. For Bi2O3, CuO and

Fig. 2. Thick-®lm capacitor lifetime values as a function of

the composition of the dielectrics (THB test performed at

95% RH, 408C, 10 V dc)


PbO, the migration behavior can be improved byincreasing the peak temperature and time. On theother hand, changing these ®ring parameters can notin¯uence the migration behavior of Fe2O3.

From these results, considering also the XD analysisspectra, a general conclusion can be drawn:

. At reactive type oxides (e.g. at Bi2O3, CuO, and

PbO), which take part in the physio±chemical reac-tions during ®ring, the migration lifetime is deter-mined by the excess amount of metal-oxide

(remaining in the form of oxide after ®ring). Life-time data can be improved by composition and pro-cessing changes.

. At non-reactive type oxides (e.g. at Fe2O3), whichare not involved in any reaction during ®ring, life-time data can not be improved signi®cantly by chan-ging the composition and ®ring parameters,

respectively. The use of these oxide-types in practicalstructures should be avoided.

5. Conclusions

. Investigating short circuit failure processes causedby the electrochemical migration of reduced isolatingcompounds in thick-®lm capacitors, a strong in¯u-

ence of the composition and processing parametershas been found.

. In situ XD spectra during heat annealing indicate

that the determining factor is the availability andcompleteness of those chemical reactions, in whichthe critical, easily reducible compound may partici-

pate.. Thus, there are two types of metal-oxide constitu-

ents

Reactive type oxides, which take part in the phy-sio±chemical reactions during ®ring. The migrationlifetime is determined by the excess amount ofmetal-oxide (remaining in the form of oxide after ®r-

ing). Lifetime data can be improved by compositionand technology parameter changes (e.g. Bi2O3, CuO,and PbO).

Non-reactive type oxides do not take part in anyreaction during ®ring. Lifetime data can not beimproved by changing the composition and the ®r-

ing parameters. The use of this oxide-types in thepractical structures should be avoided (e.g. Fe2O3).

. A wide range of materials and processes should beinvestigated in the near future to understand their

behavior in this aspect to optimize compositions andprocessing parameters in order to improve the re-liability of the structures where they are used. For

example, the low decomposition energy experiencedat laser annealed AlN ceramics [19] suggests to in-vestigate the possibility of virtual migration on these

substrate types as well.

Acknowledgements

This work was supported by the Hungarian

National Scienti®c Research Fund, OTKA, projectNo. F007365, T030574 and in part by the EU foundedINCO-COPERNICUS project No. ERBIC 15C

960743. The X-ray di�raction investigations were per-formed at the Florida International University in theframe of the COBASE program (an NSF grant coordi-

nated by the National Research Council, USA). Theauthor greatly appreciates the help of Prof. W. KinzyJones and Liana Pernes in connection with this work.

Table 1

Failure rates of thick-®lm capacitors after THB test with various dielectrics and ®ring parameters

Oxide type Metal-oxide/ BaTiO3 ratio

(%)

Failure rates (%) (1000 h, 408C, 95% RH, 10 V dc)

Firing peak temperature (8C) / peak time (min)

800/5 800/10 850/5 850/10 900/5 900/10

Bi2O3 1 90210 80215 1022 0 0 0

0.5 50210 1525 0 0 0 0

PbO 3 100 9525 2024 0 0 0

6 100 100 100 9525 4025 0

CuO 6 9921 9822 3025 0 0 0

12 9921 9921 100 9921 60210 2023

Fe2O3 1.25 100 9822 100 100 9524 9822

0.6 80215 90210 80210 80215 90210 90210


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Processing induced material interactions determining the reliability of LTCC multichip modules