Corrosion of Microelectronics Devices

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Paper No.

6 8

The NACE International Annual Conference and Exposition

CORROSION OF MICROELECTRONICS

Bezad Bavarian, Kim S. Plourde

DEVICES

California State University, Northridge

Northridge, CA 91330

and

Mehrooz Zamanzaded

MATCO Associates

Pittsburgh, PA 15205

Abstract

Microelectronic devices are liable to failure due to different

Corrosion, and electrodeposition often occur due to moisture

electrochemical processes.

adsorption and presence of

a contaminant in the surrounding environment of these devices. The objective of this

investigation was to characterize the effects of contaminants, moisture levels on growth of

a dendrite-like structure which causing short between the interconnectors copper alloy in

a electronic circuit.

The dendrite growth rate was measured as function of cupric chloride, moisture levels,

and applied potential across the conducting lines. It was observed that the velocity of

dendrite growth can be measured,

but it is generally lower than the theoretical

predications.

Keywords: Microelectronic devices, Dendritic growth, Metallization,

Contaminants, Dendrite velocity, Cupric chloride

INTRODUCTION

Hardware reliability is defined as the probability that a system will perform without failure

for a defined period of time while operating in a specified environment. To recognize

Copyright

01996 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writ ing to NACE

International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this

paper are solely those of the author s and are not necessarily endorsed by the Association. Printed in the U.S.A.



corrosion as a possible source of device failure in microelectronic devices, thus impacting

reliability, it is necessary to be knowledgeable of the intended prepares and operational

environment of these devices. The vast majority of microelectronic devices manufactured

for military and/or aerospace applications are known to be subject to variable and often

severe environments l-3 . These hostile environments, including ambient relative humidity

combined with elevated temperatures, are responsible for the principal failure mechanisms

in electronic packages 2-5 . These failure mechanisms include oxidation/corrosion,

primarily in metals used both for conductors and for package enclosures, and deterioration

of polymers mainly reversion , This paper will be concerned with the corrosion failure

mechanism.

The predominant approach to controlling corrosion in the design of microelectronic devices

is to block electrolytic formation on corrodible surfaces or on dielectric surfaces connecting

two oppositely biased traces. The simplest way to block electrolytic formation on interior

module surfaces is to exclude moisture altogether. Hermetic packaging is one extremely

expensive approach to containing moisture. One of the major disappointments in many

hermetic packages has been the problem of a high quality sealing contaminants inside the

package, one of the worst contaminants being water vapor 5 . Water vapor either trapped

during the initial sealing or else outgassed from polymers inside the can, will attack the

metallization. Encapsulant are an economically competitive method commonly used to

protect microelectronic devices, and if used properly delay chemical attack for a given

period of time 6 . These encapsulant are usually polymers. The function of polymer

encapsulant is to chemically, electrically and mechanically isolate circuit elements from

their environments 5-7 . Because polymers are not absolute barriers, they serve to delay

failures and do not prevent them entirely 8 . Many actions are taken during the

manufacturing process of microelectronic devices to preventideter corrosion.

The

formation of passivating layers to protect the metal lization from oxidizing environments and

the use of diffusion barriers in the semiconductor-conductor interface region are common

industrial techniques. Rigorous contamination-free environments, as well as

manufacturer and test 8 methods, are employed in many microelectronic producing

companies. Because corrosion has not yet been eliminated in microelectronic devices, and

may not be for the foreseeable future, an investigation of the electrochemical failure

mechanisms of these devices was performed. This effort is an attempt to understand how

some of the basic environmental parameters common to most microelectronic devices

affect the mechanism of electrochemical corrosion.

The occurrence of corrosion in microelectronic devices continues to be an ongoing and

recurring problem in the electronics industry 6-8 . Its significance and impact on the

reliability of electronic equipment is becoming progressively greater, largely as a result of

the following trends 6-7 : -Designs requiring higher component density and faster signal

processing, resulting in smaller components with closer spacings and thinner metallic

sections ,

new requirements for low-resistance, electrical stable grounding paths and

electrical bonds to protect against stray electromagnetic radiation, and the exposure of the

electronics to more severe operational environments.

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Corrosion and electromigration are two critical phenomena that limit long-term reliability of

metal lization patterns in integrated circuits 9-1 2 . The thin film metal structures in

microelectronic devices that are subjected to corrosion include the electrical contacts to

the semiconductor, the interconnect lines, the bonding pads for wire or solder connections,

and the electrical leads to the components.

The results/conclusions contained in this paper suggest that, in lieu of totally preventing

the presence of moisture in a microelectronic devices due to technological or economical

reasons , concentrated efforts to influence and control operational environmental

parameters such as pH value and contaminant concentration levels present in the package

may result in long term gains in reliability of these devices.

Accelerated tests were performed on test samples that were constructed to simulate the

environment present in many microelectronic devices. No attempt was made to duplicate

the typical thin film metallic structure on ceramic substrate that is present in most

microelectronic devices. This is because, with rare exception, accelerated testing has not

been shown to measure the same corrosion reactions that occur in practice. Bailey and

Cvijanovich 8 have pointed out that accelerated testing is only valid if the mechanism that

causes the failures is precisely the same as that observed in the real world.

A very simple model representing localized electrolytic corrosion between adjacent

conductors was used in this series of experiments. Specimens tested in this study

consisted of a-alumina substrates with two parallel copper wires with an arm coming out of

the cathode toward the anode wire Figure 1 . Each test specimen was placed in an

electrochemical cell and the insulated wires leading from the substrate were connected to

a potentiostat for application of cathodic potential . A potentiostat technique was used for

all testing, as opposed to a constant potential bias technique, because of its ability to

provide a known, fixed cathodic potential.

Substrates in this study were completely immersed in bulk electrolyte of varying

composition at room temperature. Although a bulk solution may not be representative of

the operational environment of most microelectronic devices, even those exposed to very

humid environments, However, Yan l 4 has demonstrated that the properties of an

adsorbed water layer on a-alumina substrate should approach those of bulk water for

water films thicker than three water layers.

Electrochemical testing was performed in a 300 or 500 milliliter cell. The cell components

consisted of a glass beaker of varying size , test specimen, electrolyte, a reference

saturated calomel electrode SCE and a graphite counter electrode all potentials are

reported versus SCE . The substrate being tested was placed in the bulk electrolyte

between electrodes in the cell. Overpotential was quickly applied to the test electrode

upon immersion of the specimen, in order to prevent corrosion of the wires.

6 8 3



Immediately after immersion of the substrate into the bulk electrolyte, the resting or

corrosion potential Ecorr value of each specimen was measured and recorded. A

potentiostatic test was than pefformed, with an applied potential of-1.0 VSCE The wltant

current density were measured and plotted as a function of time.

The bulk electrolyte solutions used in this study consisted of 1 M sodium chloride

NaCl containing varying concentrations of dissolved copper added as CUCI to different

pH solutions. The majority of experiments were conducted at the pH 7.0 to pH 1.0, which

is representative of the environment in many microelectronic devices due to manufacturing

processes. This is also the region most likely to produce dendrite growth l 5-1 7 . The

choice of chloride as the contaminant was based on its seemingly ubiquitous presence in

the environment; a sufficient concentration of chloride was provided to the electrolyte to

ensure a roughly constant ion concentration and to promote/acclerate dendrite growth.

During the course of this series of experiments, it was quickly discovered that the volume

of the bulk electrolyte solution significantly affected the adhesion of the dissolution

products, making the dendritic growth process difficult to monitor or measure. In an

attempt to minimize this problem, the volume of bulk solution in the electrochemical cell

was decreased from 500 milliliters to 200 milliliters. The decrease in bulk electrolyte

volume tended to minimize dendrite adhesion problems. Two other important

environmental variables that affect corrosion were not addressed during these series of

experiments, temperature heat transfer and velocity fluid flow . In order to minimize the

effects of these two environmental variables on the results, all experiments were

performed at room temperature, and the bulk electrolytic solutions were not agitated in any

manner during testing.

Visual measurements of the apparently spacing between the two exposed wires on each

substrate were made using optical microscope prior to the initiation of testing with each

sample, and the results were recorded. In many instances, a photograph was also taken

of the wire spacing of the sample prior to testing.

After removal of the specimens from the bulk electrolyte, the resultant dendritic growth that

occurred at the closest spacing between the two wires was again visually measured and

recorded. Scanning electron microscopy SEM was also used to monitor the dendrite

growth. The scanning electron microscope was used to evaluate the nature and variation

of dendritic structure because of its advantages of ease of sample preparation and its

ability to allow direct examination of large samples.

RESULTS AND

DISCUSSIONS

An examination of the dendrite growth rate under experimental conditions offers a

direct method of quantifying the electromigration process in environments common to

microelectronic devices. With this is mind, the dendritic growth rate was measured in a

series of experiments as a function of copper ion concentration, time and pH value.

The influence of cupric ion concentration on dendrite growth at a constant potential of -1.0

628 4



volt SCE was examined through a series of experiments.

Figure 2 is a best-curve plot of dendritic growth versus time for two copper ion

concentrations.

The dendritic growth appears to be directly related to copper

concentration in the electrolyte. The .001 molar copper electrolyte produced a dendrite

that measured 3 E-4 centimeters in length after 30 minutes. The .005 molar copper ion

solution produced a dendrite 6 E-4 centimeters in length in 30 minutes.

The maximum velocity model of dendrite growth in electrochemical systems defines two

limiting growth modes, an exponential growth regime and a linear growth regime. Although

Figure 2 only covers a 30 minute interval, both concentrations appear to have a roughly

linear growth pattern. The relationship between the copper ion concentration in the bulk

electrolyte and the velocity of dendrite growth is shown in Figure 3. The increase in

velocity with copper concentration is approximately linear until .003M Cu ions

concentration, when the velocity plateaus at a constant rate. This may be the effects of

branching and secondary dendrite growth, an area that requires further study.

Figure 4 is the result of a series of experiments in which copper wire specimens were

immersed in a 1 molar NaCl electrolytic bulk solution containing a copper concentration of

.001 M, at varying pH levels, Zamanzadeh et al. 17-21 , predict that an acidic environment

is favor the growth of dendrites. As the pH value increases, dendritic growth length/rates

should decrease. With this in mind, the experiments represented in Figure 4 were

performed in the neutral to acidic range of the pH scale.

As anticipated, the dendritic growth rate was found to gradually increase with decreasing

pH values. However, this growth rate peaks at a pH value of 3, and than sharply declines.

This decrease may be attributed to the redissolution of the copper atoms at extremely low

pH values.

An induction period occurs prior to the formation of dendrites. Because the experimental

set-up used for this paper did not allow real-time monitoring/viewing of dendrite growth,

several consecutive experiments were run in series. Testing was initiated and stopped at

progressively larger one minute sequential intervals. The electrolytic solution used was a

1M NaCl solution, pH 3.0, containing .001 M cupric ion concentration. Substrates were

examined for dendritic growth at the end of each time frame. At a cathodic potential of 1.0

volts SCE , dendritic growth consistently appeared between three and five minute using

this technique. At no time was dendritic growth discovered before three minutes, a rough

indication of the presence of an induction period. Figure 5 shows the comparison of the

observed dendrite growth velocity with the theoretical growth velocity based on Bockris

model 22-23 . It showed be mentioned that the theoretical model is overestimating the

dendrite growth velocity because of ignoring the tree-like structure of a dendrite which led

to calculating velocity base on one arm dendrite formation which is unrealistic, Therefore,

the observed dendrite growth velocity is about two orders of magnitude lower than the

predicated values. The SEM observations also showed that dendrites are not branchless

as is assumed in the theory.

DENDRITE MORPHOLOGY

The morphology and forms of copper dendrites resulting from the above experiments were

examined with a scanning electron microscope at varying magnifications. Prior to

6 8 5



observing dendritic growth, the specimens were sputtered with a thin gold coating to

ensure the necessary conductivity for viewing. All growth depicted in the following pictures

occurred in electrolytic solutions of 1M NaCl, with an applied cathodic potential of -1.0

volts SCE . The magnification of each picture, as well as the copper concentration and

pH is listed for reference.

Figures 6 and 7 depict the growth of dendrites in more than one direction, and is from a

solution of pH 3.0, copper concentration was .001 M under an applied potential of 1.0

VSCE. The predominant growth of dendrites in all of the experiments was directly related

to the position of the substrate in reference to the electrodes in the cell.

Figure 7 is a higher magnification picture 720X view of a dendrite radius tip, taken from a

specimen grown in an electrolytic solution of pH 3.0, copper concentration .001 M. Note the

somewhat rounded appearance of the tip, which agrees with theoretical assumptions of a

spherical diffusion gradient at the surface. As pointed out earlier in this paper, the

dendrite growth velocity has been shown to be related to the tip radius of the growing

dendrite. The ability to measure or monitor the dendritic tip radius during growth is

required in order to relate experiment growth velocity values to theoretical growth velocity

values.

CONCLUSIONS

The problem of corrosion will continue to be a major concern in the microelectronics

industry for the foreseeable future. Although many different methods are currently being

used in industry to deter the exposure of metallization to moisture, each has its limitations.

With this fact in mind, the mechanisms of electrochemical failure in microelectronics

require further study and understanding to lessen the impact on the reliability of these

devices.

The results of the potentiostatic, and dendrite growth and morphology discussions

contained in this paper lead to the following conclusions concerning the corrosion and

failure of micromicroelectronic devices containing ceramic substrates, with copper

metallizations:

1 Acidic environments approximately pH 3.O favor the growth of dendrites. Lower PH

electrolytes that contain additional amounts of cupric ions have a significantly lower

corrosion bias potential.

2 Dendritic growth lengths appear to significantly increase as pH values are lowered

below the neutral portion of the pH scale, reaching the highest growth values at a pH of

3.0 and dropping sharply as pH approaches 1.0 due to the redissolution of deposited

copper atoms.

3 In bulk electrolytes, the growth velocity of the copper dendrites increased with

increasing copper concentration.

4 the comparison of the observed dendrite growth velocity with the theoretical growth

velocity showed that the theoretical model is overestimating the dendrite growth velocity by

ignoring the tree-like structure of a dendrite which led to calculating velocity base on one

6 8 6



arm dendrite formation which is unrealistic. Therefore, the observed dendrite growth

velocity is about two orders of magnitude lower than the predicated values.

The results obtained during this series of experiments agree with the majority of references

cited as source information. Although the majority of published work related to this topic

has been authored by electrochemists, an encouraging interest in the fundamental kinetics

of this process from a corrosion perspective appears to have started in the past ten years.

Future research should be directed toward both understanding the electrochemical failure

process as well as minimizing its impact on the reliability of microelectronic devices.

REFERENCES

1. R. Gehman, Electronic Packaging and Corrosion in Microelectronics, ASM International,

page 104, 1987

2. Mil-M-3851 O, Microcircuit, General Specification for, May 1986

3. Mil-Std-883, Test Methods and Procedures for Microelectronics, December 1984

4. J. DeVaney, Electronic Packaqin~ and Corrosion in Microelectronics, ASM International,

page 287, 1987

5. Metals Handbook, 13, ASM International, page 1107, 1987

6. M. Adams and D. Bowers, Electronic Packaqing and Corrosion in Microelectronics, ASM

International, page 15, 1987

7. R. Frankenthal, Properties of Electrodeposits-Their Measurement and Significance,

Sard, page 142, 1975

8. J. Gavrilovich, Electronic Packaqing

and Corrosion in Microelectronics, ASM

International, page 70, 1987

9. G, Hills, et al, Electroanal. Chem. Interracial Electro-them. 138, page 225, 1982

10. K. Popov, et al, J.

APPI. Electrochem., 9, page 527, 1979.

11. K.

Popov, et al, J.

APPI.

Electrochem., 10, page 299, 1980.

12. A. DerMardersian, Proceedincis of the 1978 International Microelectronics Svmposium.

page 134-141, September 1978.

13. R. Frankenthal and W. Becker, J. Electrochem Sot. Solid State Technol, 126, page

1718, 1979.

14. B. Yan, et al, Corrosion, Vol 43, page 118, 1987.

15. R. Augaki and T. Makino, Electrochem. Acts., Vol 26, page 1510, 1981.

16. R. Voss and M. Tomkiewicz, J. Electrochem. Sot., Vol 132, page 371-375, February

1985.

17. M. Zamanzadeh, et al, 170th Meeting of the Electrochemical Societv. page 173, 1986.

18. S. Meilink, et al, Corrosion, 44, 9, page 644-651, Sept 1988

19. J. Steppan, et al, J. Electrochem Soc:Solid State Sci and Technol., Vol 134, 1, page

176, January 1987.

20. M. Zamanzadeh, et al, Multilevel Metailization, Interconnection and Contact

Technolcwies, Electrochem. Sot., page 173, 1987

21. S.L. Meilink, m Zamanzadeh, G. W. Warren, and P. Wynblatt, CORROSION, Vol. 44,

no.9, 1988. p. 648-650.

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22. J. Bockris and N. Enyo, Trans. Faraday Sot., 58, page 1187, 1962

23. J. Bockris and A. Despic, Phvsical

Chemistrv . An AdvancedTreatise VOI W

Academic Press, page 64, 1970.

Anode

copper

~

Substrate

I

Figure 1: Schematic diagram of the specimen used to investigate the dendrite

growth for copper electrodes in

NaCl solution

A

A

A

’

I

‘

0.00IM

CU

’

0.005M

CU

- “ ‘ --- - --– –----–— --—------ ]

10

20

30

40

TitlW, mi~~

Figure 2: Dendrite length measurement in 1M NaCl solution of pH 3.0 for copper

electrodes under an 1.0 V=E applied potential.

628;8



4 000 E-5 —–

3.000 E-5 -

—

2.000 E-5 ~

1,000E-5 —

jE+ .

lE-3

2E-3 3E-3

4E-3

5E-3

6E-3

Cu Ions concentration

Figure

:

Correlation between dendrite growth velocity and copper ions

concentration in 1 M NaCl solution of pH 3.0 under an applied potential of

1.0vs~~

628 9



7 ‘---—----—”–”-”””——”-

——

6-

5

4–

a

3“

2

1

———

—’

—.———.—. .- ——. ——.

—-—

0123456 78

pH

Figure 4: pH dependency of the dendrite length

in 1 M NaCl solution under an

applied potential of 1.0

VscE.



10-2 ____

- observed velocity

-*Calculated velocity

8

10 3

—

.—

10-4

.—

e

10-5 _

O,OE+O 1.OE-3 2.OE-3

3,0 E-3 4,0 E-3 5,0 E-3 6,0 E-3

Copper ions Concentration, M/lit.

Figure 5: Comparison between the observed and calculated dendrite

growth

velocities in 1 M NaCl solution of pH 3.0 under an applied potential of

l.OV~CE

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Figure 6: SEM micrographs of the dendrite growth morphology observed for

copper electrodes in 1M NaCl + 0,005 M Cu ions of pH 3.0 solution

under an applied potential of 1.0 VscE , xl 60.

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Figure 7: SEM micrograph of the dendrite growth morphology observed

for

copper electrodes in 1M NaCl 0.005 M Cu ions of pH 3.0 solution

under an applied potential of 1.0 VSCE x700.

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Corrosion of Microelectronics Devices

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