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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.
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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.
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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
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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
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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
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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
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page 104, 1987
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page 287, 1987
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International, page 15, 1987
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Sard, page 142, 1975
8. J. Gavrilovich, Electronic Packaqing
and Corrosion in Microelectronics, ASM
International, page 70, 1987
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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.
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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~~
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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.
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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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