ARTICLE IN PRESS
Microelectronics Journal 37 (2006) 1329–1334
www.elsevier.com/locate/mejo
Stress and resistivity analysis of electrodeposited gold films for
MEMS application
Subhadeep Kala,, A. Bagolinib, B. Margesinb, M. Zenb
a
Department of Chemistry, Indian Institute of Technology, Kharagpur,West Bengal 721302, India
b
ITC-irst, via Sommarive, 16, 38050 Trento, Italy
Received 31 March 2006; accepted 3 July 2006
Available online 22 August 2006
Abstract
Electroplated gold films have attracted much attention in recent years because of its desirable properties for microsystems applications
such as resistance to oxidation, low electrical resistance, overall chemical inertness and low processing temperature. In order to use gold
in microelectromechanical systems designs, systematic tests has to be conducted to characterize the material in terms of its electrical as
well as mechanical properties. In this paper, the stress and resistivity behavior of a nanometer-scale gold film with respect to the
deposition parameters and annealing condition is reported.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Thin films; Gold films; Grain growth
1. Introduction
The development of microelectromechanical systems
(MEMS) industry gave inputs to investigate new materials
due to importance of thin films. Most of the electronic
devices fabricated in MEMS use materials such as silicon
and many other thin films, which are not well characterized
regarding their electrical properties relevant to MEMS
application. Little research has been conducted on microscale gold, because it has not been considered earlier as an
engineering material.
Thin gold films are used in many MEMS devices because
of high electrical conductivity. Some of the applications
include radio-frequency (RF) MEMS which can operate at
gigahertz frequencies, allowing for large bandwidth and
extremely high signal-to-noise ratios [1] and in inertial
MEMS to increase mass of proof-mass for achieving higher
sensitivity in MEMS accelerometers. These films can also
be used in variable capacitor, chemical and biological
sensors [2,3], optical detectors etc. [4].
Corresponding author. Tel.: +91 3222 277749; fax: +91 3222 279388.
E-mail address: [email protected] (S. Kal).
0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.mejo.2006.07.006
Some basic requirements for Au films used in MEMS
applications are as follows [5]:
(1) High conductivity.
(2) Film–substrate adhesion should be strong enough to
prevent cracking, delamination and spallation.
(3) Residual stress developed should be minimum to
prevent deformation of MEMS structure.
(4) Films should be stable within a wide range of
temperature (from below zero to several hundred
degree Celsius).
(5) Films should be resistant to surface wears, corrosion
and oxidation.
Gold film is a polycrystalline film. The microstructural
characteristics of polycrystalline films control their properties, performance, and reliability in applications in wide
variety of electronic, magnetic, photonic, chemical, and
micromechanical devices and systems. Gold films have
columnar structure.
Adhesion of gold film on silicon is poor, so a layer of
chromium (Cr)is used before gold deposition, which
enhances the adhesion of the film with substrate. Owing
to this discrete layer of Cr, gold film has poor thermal
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S. Kal et al. / Microelectronics Journal 37 (2006) 1329–1334
stability because of the natural tendency of Cr to diffuse to
the film surface at higher temperatures. The knowledge of
diffusion process operating in gold film is important for
application and design of microelectronic devices, fabrication
and reliability of the thin-film package. The Cr transport may
manifest itself in development of undesirable characteristics,
such as decrease in electrical conductivity, generation of
internal stress and precipitation of phases in layers [6]. It was
found that Cr readily diffuses through the grain boundaries
of poly crystalline films to the surface during heating and
oxidizes to form Cr2O3. This diffusion process can be quite
extensive, with complete depletion of Cr adhesive layer and
formation of channeled grain boundaries that are occupied
with Cr2O3 and eventually formation of single crystals of
Cr2O3 at surface [6]. In addition to the depletion of the
adhesive layer, there is an increase in the electrical resistance
of the gold film during the Cr diffusion process, which may be
undesirable for thin-film conductors.
In oxidizing ambients, the out-diffusing species in gold
surface occur in an oxide form. The oxidized state is very
stable and is thought to act as a sink for the out-diffusing
species, which provides strong chemical driving force for
diffusion. Ambients such as oxygen and air result in
surface-oxide formation and enhanced Cr diffusion.
Besides Cr diffusion the other main factor controlling
the resistance of the film is its grain structure, which in turn
is controlled by the deposition parameters like current
density, electrolyte temperature, thickness of deposited
gold film and the annealing temperature.
The paper reports variation of stress and resistivity of
electroplated gold film with various deposition parameters
such as current density, deposition temperature of the
electrolyte, thickness of the gold film and annealing
temperature.
2. Experimental details
In this study a seedlayer of 3 nm Cr and then 51 nm Au
was deposited on a 4 in diameter silicon wafer by electron
beam evaporation. Initially the silicon substrates were
prepared by cleaning sequentially with H2SO4/H2O2 for
15 min at 90 1C, in HF 8% for 9 s at room temperature, in
NH4OH/H2O2 for 10 min at 70 1C and then in HCl/H2O2
for 10 min at 70 1C. This was followed by thermal
oxidation of the silicon wafer. The oxide layer was about
296.675 nm.
Gold was electrodeposited on these wafers having a
seedlayer of Cr/Au. Experiment was performed with an
experimental setup for low-volume plating, using 1.5 L
solution (Fig. 1). Neutral-based commercial electrolyte
solution ATOTECHTM gold potassium cyanide
[KAu(CN)2] was used.
During gold deposition the following parameters were
varied:
(a) The temperature of the electrolyte was varied in steps
of 45, 55, 65 and 75 1C.
Fig. 1. Setup of the electroplating deposition bath
(b) The current density during electrodeposition was
varied at intervals of 1, 2, 3, 5, 7 mA/cm2.
(c) The thickness of the deposited gold film was varied in
steps of 200, 300, 400 and 500 nm.
The stress in the plated gold films was calculated by
measuring wafer curvature and comparing it, before and after
deposition, using Stoney’s formula for thin films [7]. The
film’s curvature was measured by profilometer, KLA Tencor
P.15. at Load ¼ 200 mg and Scanning speed ¼ 1000 mm/s
The resistivity of the electroplated gold film was
measured by NAPSON TM Resistage R-8 instrument.
Estimation of the measurement error (absolute plus
random) of about 3% was obtained by a series of dedicated
tests.
The films were annealed at 100, 250 and 400 1C and the
stress and resistivity were measured.
3. Results and discussion
3.1. Stress dependence on current density
Stress variation of electrodeposited gold film on silicon
with current densities at different annealing temperatures is
shown in Fig. 2. The results show that the stress is low for
films plated at lower current densities (e.g. 1 mA/cm2) and
then it gradually increases till 4 mA/cm2 before it
decreases again irrespective of the annealing temperature.
The as-plated gold films (i.e. the unannealed films), which
are free from any Cr, as diffusion has not yet started, also
show this trend (Fig. 2). This behavior of stress variation
may be due to the variation in grain microstructure. The
electroplated Au deposition rate may be controlled by
adjusting the current density as (1) [8].
aF It ¼ m ¼ rTA
(1)
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S. Kal et al. / Microelectronics Journal 37 (2006) 1329–1334
as plated
450
105
100 C
250 C
400
wafer13 2mA/cm2
Cs primary beam
400 C
350
1331
109
108
Cs+
104
107
250
Counts/s
Stress (Mpa)
300
200
150
106
103
105
104
102
103
16
Cs O
Cs28Si
Cs52Cr
197
Cs Au
100
101
50
102
101
0
0
1
2
3
4
5
6
7
8
100
-50
0
2
Current Density (mA/cm )
100
(a)
Fig. 2. Stress variation at different annealing temperatures with varying
current density.
wafer5 3mA/cm2
(i) At higher temperature the grains rearrange and grow.
The film structure relaxes while coming back to room
temperature because of different thermal coefficients of
expansion between gold and silicon and an extrinsic
residual tensile stress is generated [9].
(ii) At higher temperature Cr diffusion through grain
boundaries starts, which affects the mechanical properties of the layer [6,10].
100
300
250
Cs+
104
109
108
107
16
Cs O
28
Cs Si
52
Cs Cr
Cs197Au
103
106
105
102
104
103
(2)
101
102
101
100
0
50
100
(b)
wafer1 7mA/cm2
150
200
Depth [nm]
250
100
300
Cs primary beam
105
Cs+
109
108
104
Counts/s
Thus the current density I/A is directly proportional to
the rate of deposition T/t, which may affect the microstructure size and distribution. This is also observed from
SIMS analysis, as shown in Figs. 3(a)–(c). The films
deposited at 2 and 3 mA/cm2 is very different in its oxygen
and Cr content as compared to those plated at 7 mA/cm2.
Cr and gold composition in the film changes with high
current density during deposition and as a consequence
film stress also changes.
Results from Fig. 2 also indicate that with increase in
annealing temperature the residual stress developed in the
multilayer increases, maintaining their as-plated trend.
This increase is mainly because:
150
200
Depth [nm]
Cs primary beam
105
Counts/s
where aF is the (mole weight/ valence)/96,500 g/C, I the
Plating current (A), t the Total plating time (s); m the total
mass of metal deposited (g), r the Density of plated
material (g/cm3]), T the Final plating thickness (cm) and A
the Total plating area (cm2).
Assuming 100% bath efficiency we can write, the law in
the following form:
I=A ¼ ðr=aF ÞðT=tÞ
50
107
Cs16O
28
Cs Si
52
Cs Cr
Cs197Au
103
106
105
104
102
103
102
101
101
100
0
(c)
50
100
150
200
Depth [nm]
250
100
300
Fig. 3. (a) SIMS analysis of film plated at 2 mA/cm2 and annealed
at 400 1C. (b) SIMS analysis of film plated at 3 mA/cm2 and annealed at
400 1C. (c) SIMS analysis of film plated at 7 mA/cm2 and annealed
at 400 1C.
3.2. Stress dependence on bath temperature
The effect of bath temperature during electrodeposition
of Au film on film stress has been measured and the results
are shown in Fig. 4. The stress variation of the films with
annealing temperature is also shown in the same figure
(Fig. 4). The results indicate that the stress in the films
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S. Kal et al. / Microelectronics Journal 37 (2006) 1329–1334
1332
as deposited
100C
250C
400C
500
450
400
400
350
350
Stress (MPa)
Stress(MPa)
450
as deposited
100C
250C
400C
500
300
250
200
300
250
200
150
150
100
100
50
50
0
240.0
0
40
50
60
Temerpature (C)
70
290.0
340.0
80
Fig. 4. Stress variation of the electroplated Au at different annealing
temperatures with varying bath temperature.
390.0 440.0 490.0
Thickness (nm)
540.0
590.0
Fig. 5. Stress variation at different annealing temperatures with varying
film thickness.
4.5E-08
3.3. Stress dependence on film thickness
4.0E-08
3.5E-08
Resistivity (Ω m)
deposited at constant current density of 3 mA/cm2 and
thickness 285 nm, increases slightly with increasing deposition temperature (bath temperature). This stress variation
can be considered as extrinsic. On annealing at higher
temperature their respective stress increases and attains a
uniform value. This confirms that the as-deposited stress
variation was merely extrinsic and therefore erased by
further annealing.
3.0E-08
2.5E-08
2.0E-08
1.5E-08
Measured values of stress variation with electrodeposited
gold film thickness at different annealing temperature are
shown in Fig. 5. The results show that in case of as-plated
film deposited at constant current density of 3 mA/cm2 and
bath temperature of 55 1C the stress is nearly constant at
different film thicknesses, though a decreasing trend can be
seen from thin to thick layer when annealed at higher
temperature. From the stress analysis, in the examined
thickness range we can deduce no relevant structure
variation.
But on annealing, Cr diffusion starts. For thin films it
diffuses through the plated gold easily (the diffusion rate is
same for all, as the grain microstructure is nearly same).
The diffused Cr reaches the surface and forms Cr2O3.
Thinner films quickly reach a higher Cr concentration, thus
inducing a change in mechanical properties so that the
extrinsic stress induced by heating increases.
3.4. Resistivity dependence on current density
Resistivity of the Au films deposited at various current
densities is shown in Fig. 6. Higher as-plated resistivity is
seen (Fig. 6) for films deposited at lower (1 mA/cm2) and
as deposited
1.0E-08
100C
250C
5.0E-09
400C
0.0E+00
0
2
4
6
8
current density (mA/cm2)
Fig. 6. Resistivity variation at different annealing temperatures with
varying current densities.
higher (7 mA/cm2) current densities. In polycrystalline
films (here electrodeposited Au films) the main cause of
resistance are grain boundaries and impurities. The asdeposited film, which is free from chromium diffusion, has
got impurities and moreover its resistivity is affected by
grain boundaries. By varying the current density while
plating we influence the grain microstructure, this is also
seen by stress measurements and SIMS analysis (Section
3.1). The increase in resisitivity for films plated at higher
and lower current densities may be because the grain size is
reduced and as a result grain boundaries increase, hence
the resistivity increases.
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S. Kal et al. / Microelectronics Journal 37 (2006) 1329–1334
Generally with increase in annealing temperature the
resistivity increases. But, for annealing at 100 1C the
resistivity remains the same as it was before annealing.
But after annealing at 250 1C there is a jump in resistivity
(such trends are also observed in case of stress Figs. 2–5).
Resistivity increases as there is Cr diffusion taking place at
250 1C. This sudden jump in resistivity after 250 1C suggests
that there is a certain threshold temperature or energy for
Cr diffusion to start.
After annealing at 400 1C, for films plated at current
densities between 2 and 5 mA/cm2 the resistivity does not
change much. But for films plated at higher current density
(e.g., 7 mA/cm2) and lower current density (e.g., 1 mA/cm2)
the resistivity decreases from the corresponding values
annealed at 250 1C. This supports our previous hypothesis
that at higher and lower current densities the grain size is
small. So there are large number of grain boundaries.
Hence Cr diffusion takes place faster in these cases. So at
higher temperature (4001C) all the Cr is diffused to the
surface and forms Cr2O and underneath pure gold will
show lower resistivity [11–13]. This phenomenon is much
faster for high deposition current density, confirming that
the layer has small grains that favor the diffusion of Cr.
3.5. Resistivity dependence on bath temperature
In Fig. 7 we see the resistivity variation with bath
temperature keeping other parameters constant (current
density ¼ 3 mA/cm2 and thickness of the deposited film
¼ 285 nm.). Here we see that there is no relevant change in
resistivity, indicating that the structure of the layer is not
affected much by plating temperature.
With increase in annealing temperature there is a slight
increase in the film resistance. Like in Fig. 6, here also we
see that annealing at 100 1C does not change the resistance
much. But then there is again a jump increase in resistivity
after annealing at 250 1C followed by no change for
annealing at 400 1C. Resistivity is proportional to Cr
diffusion, which in turn is proportional to annealing
temperature. This again justifies that there is a threshold
temperature to start Cr diffusion.
In this case we do not see a decrease in resistivity at the
extreme temperatures like in Fig. 6 because grain sizes are
not much affected by bath temperature (also shown by
stress measurements). So the rate of Cr diffusion is same
for all Au films deposited at various bath temperatures.
3.6. Resistivity dependence on film thickness
Fig. 8 shows the result of resistivity variation with
electrodeposited thickness of gold film at various annealing
temperatures. Little increase in resistivity is observed for
thicker Au films. This increase may be because of the fact
that thicker films have a longer deposition time and this
may yield a minor difference in the structure of the plated
gold with respect to thin films, due to grains reorganization.
There is a slight increase in resistance with increase in
annealing temperature. This is because the resistivity is
proportional to Cr diffusion, which in turn is proportional
to annealing temperature.
Similar to results in Figs. 6 and 7, a step jump in
resistivity is also observed after annealing at 250 1C. This is
again because the threshold temperature to start predominant Cr diffusion is nearly 250 1C.
3.7. Resistivity versus annealing temperatures for a
particular current density
The resistivity behavior of the gold films deposited at a
particular current density and then annealed at various
temperatures is shown in Fig. 9. Here we observe an initial
increase followed by a decrease in resistivity as the
annealing temperature is increased in case of films plated
at low (1 mA/cm2) and high (7 mA/cm2) current densities.
4.0E-08
4.0E-08
3.5E-08
3.5E-08
3.0E-08
resistivity (Ω m)
3.0E-08
resistivity (Ω m)
1333
2.5E-08
2.0E-08
as deposited
1.5E-08
100C
1.0E-08
as deposited
100C
250C
0.0E+00
50
70
Bath temperature (C)
1.5E-08
5.0E-09
400C
30
2.0E-08
1.0E-08
250C
5.0E-09
2.5E-08
90
Fig. 7. Resistivity variation at different annealing temperatures with
varying bath temperature.
0.0E+00
30.0
400C
230.0
430.0
Thickness (nm)
630.0
Fig. 8. Resistivity variation at different annealing temperatures with
varying film thickness.
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7mA/cm2
5mA/cm2
3mA/cm2
2mA/cm2
1mA/cm2
4.5E-08
Phase II
4.3E-08
resistivity (Ω m)
4.1E-08
Phase I
3.9E-08
3.7E-08
3.5E-08
3.3E-08
3.1E-08
The bath temperature does not have predominant effect
on the grain microstructure. However, electrodeposited
thickness of Au film influences the grain slightly and as a
result both resisitivity and stress change.
Cr diffusion influences the mechanical and electrical
properties of the film to a great extent. Cr diffusion is
directly proportional to the annealing temperature. But
there is a threshold temperature (or threshold energy)
before which diffusion does not start. This threshold
energy lies between 100–250 1C, as observed in the
experimental results.
2.9E-08
Acknowledgments
Phase III
2.7E-08
2.5E-08
0
100
200
300
400
annealing Temperature (C)
500
The authors greatly acknowledge the assistance of
F. Giacomozzi. We further acknowledge ITC-irst, Trento,
Italy for providing the experimental support.
Fig. 9. Resistivity variation of the deposited film with different annealing
temperatures at constant current density.
References
The initial rise in resistivity is due to Cr diffusion when
annealing at higher temperature.
Phase I: In this phase the resistivity does not change
much because the threshold temperature is not yet reached
to start Cr diffusion.
Phase II: In this phase chromium diffuses through gold
and there is a sharp increase in the resistivity regardless of
the plating current density.
Phase III:
Medium current density (2–5 mA/cm2): In this phase
resistivity does not change much. This is because at lower
current density the grain size is comparatively large so Cr
diffusion is not so fast as a result the chromium is still
present in the gold in an appreciable quantity.
High and low current density (1 and 7 mA/cm2): For high
and low current density in this phase we see a fall in the
resistivity. This is because at these current densities the
grain size is much smaller (also predicted by SIMS result as
well as stress measurement). As a result there are more
number of grain boundaries so Cr, diffusion is very fast in
these case. Thus at higher temperature Cr is diffused to the
surface entirely, forming Cr2O3 [6], leaving below relatively
pure gold of low resistivity.
4. Conclusion
Experimental results on stress and resistivity analysis of
electrodeposited gold reveals that, the grain microstructure
varies by varying the current density during deposition.
The results of resistivity measurements indicate that the
films deposited at lower and higher current densities (1 and
7 mA/cm2), the grain size is small as compared to those
films that are deposited at medium current densities (2,3
and 5 mA/cm2).
[1] C.L. Chang, P.Z. Chang, Innovative micromachined microwave
switch with very low insertion loss, Sensors Actuators A 79 (1) (2000)
71–75.
[2] R. Raiteri, G. Massimo, H.-J. Butt, P. Skadal, Micromechanical
cantilever-based biosensors, Sensors Actuators B 79 (2) (2001)
115–126.
[3] D.R. Baselt, B. Fruhbbereger, E. Klaassen, S. Cemalovic, C.L.
Britton, S.V. Pastel, Design and performance of a microcantileverbased hydrogen sensors, Sensors Actuators B 88 (2) (2003) 120–131.
[4] D.C. Miller, C.F. Herrmann, H.J. Maier, S.M. George, C.R. Stoldt,
K. Gall, Intrinsic stress development and microstructure evolution of
Au/Cr/Si multilayer thin films subject to annealing, Scr. Mater. 52
(2005) 873–879.
[5] Y. Fu, H. Du, W. Huang, S. Zhang, M. Hu, TiNi-based thin films in
MEMS applications: a review, Sensors Actuators A 112 (2004)
395–408.
[6] M.A. George, W.S. Glaunsinger, T. Thundat, S.M. Lindsay,
Electrical, spectroscopic, and morphological investigation of chromium diffusion through gold films, Thin Solid Films 189 (1990)
59–72.
[7] R.W. Hoffmann, Measurement Techniques for Thin Films, Electrochemical Society Inc., NewYork, 1967, p. 312.
[8] P.H. Lawyer, C.H. Fields, Film stress versus plating rate for pulseplated gold, HRL Laboratories, LLC, 3011, Malibu, CA 90265, 2001.
[9] B. Margesin, A. Bagolini, V. Guarnieri, F. Giacomozzi, A. Faes, R.
Pal, M. Decarli, Stress characterization of electroplating for low
temperature surface micromachining, DTIP for MEMS and
MOEMS, 5–7 May, 2003,Cannes, France.
[10] N.R. Moody, D.P. Adams, D. Medlin, T. Headley, N. Yang, A.
Volinsky, Effects of diffusion on interfacial fracture of gold
chromium hybrid microcircuit films, Int. J. Fract. 119/120 (2003)
407–419.
[11] A. Munitz, Y. Komem, The increase in electrical resistance of heattreated Au/Cr films, Thin Solid Films 71 (1980) 177–188.
[12] A. Munitz, Y. Komem, Structural and resistivity changes in heat
treated chromium–gold films, Thin Solid Films 37 (1976) 171–179.
[13] N.R. Moody, D.P. Adams, A.A. Volinsky, M.D. Kriese, W.W.
Gerberich, Annealing affects on interfacial fracture of gold-chromium films in hybrid microcircuits, In: Materials Reserch Society
Symposium. Proceedings 586 2000, pp. M5.2.1–11.