Full text of DOI:10.1063/1.114596

Calorimetric study of the energetics and kinetics of interdiffusion in
Cu/Cu6Sn5 thinfilm diffusion couples
K. F. Dreyer, W. K. Neils, R. R. Chromik, D. Grosman, and E. J. Cotts
Citation: Appl. Phys. Lett. 67, 2795 (1995); doi: 10.1063/1.114596
View online: http://dx.doi.org/10.1063/1.114596
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v67/i19
Published by the American Institute of Physics.
Related Articles
Evaluation and modeling of lanthanum diffusion in TiN/La2O3/HfSiON/SiO2/Si high-k stacks
Appl. Phys. Lett. 101, 182901 (2012)
Influence of catalytic gold and silver metal nanoparticles on structural, optical, and vibrational properties of silicon
nanowires synthesized by metal-assisted chemical etching
J. Appl. Phys. 112, 073509 (2012)
Chemical and structural investigations of the interactions of Cu with MnSiO3 diffusion barrier layers
J. Appl. Phys. 112, 064507 (2012)
Thermal tweezers for nano-manipulation and trapping of interacting atoms or nanoparticles on crystalline
surfaces
J. Chem. Phys. 137, 114701 (2012)
Thermal conductance at atomically clean and disordered silicon/aluminum interfaces: A molecular dynamics
simulation study
J. Appl. Phys. 112, 054305 (2012)
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 13 Nov 2012 to 132.210.244.226. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Calorimetric study of the energetics and kinetics of interdiffusion
in Cu/Cu6Sn5 thin-film diffusion couples
K. F. Dreyer, W. K. Neils, R. R. Chromik, D. Grosman, and E. J. Cotts
Department of Physics, Binghamton University, State University of New York,
Binghamton, New York 13902-6016
͑Received 28 March 1995; accepted for publication 1 September 1995͒
Differential scanning calorimetry was used to characterize the energetics and kinetics of
interdiffusion in solder/metal diffusion couples. The heat of formation of Cu3Sn from Cu6 Sn5 and
Cu thin films was found to be ⌬HrϭϪ4.3Ϯ0.3 kJ/mol, similar to the results of previous
measurements on bulk samples. We have seen that the nucleation of Cu3Sn begins at temperatures
near 360 K, but that the nucleation and initial growth of Cu3Sn is not a well-defined Arrhenius
process in these diffusion couples. Later portions of our differential scanning calorimetry scans were
identified with diffusion-limited growth of Cu3Sn. From these calorimetry data we have estimated
2
˜
the averaged interdiffusion coefficient, D(cm /s͒ϭD0 exp (ϪE/kbT), where kb is Boltzmann’s
constant and D0ϭ3.2ϫ10Ϫ2 cm2/s and Eϭ0.87 eV/atom. © 1995 American Institute of Physics.
As dimensions in electronic packages decrease, the im-
portance of characterizing interdiffusion in solder/metal
couples on a submicron length scale increases.1–12 A number
of studies have been performed on interdiffusion in Cu/Sn
and Cu/PbSn composites, generally in bulk diffusion
couples.1–4,6 In the present work we investigate the forma-
tion of Cu3Sn in Cu/Cu6Sn5 thin-film multilayers5 using dif-
ferential scanning calorimetry.8–11 The heat of formation of
Cu3Sn and an estimate of the effective interdiffusion coeffi-
cient in Cu3Sn are found. In addition, the nucleation of the
Cu3Sn intermetallic layer is examined.
The heat flow versus temperature for a Cu/Cu6Sn5 mul-
tilayered composite of 92 nm bilayer thickness is shown in
Fig. 1. A significant heat flow signal is first observed at a
temperature of approximately 360 K. Between temperatures
of approximately 380 and 420 K the heat flow signal remains
almost constant. The large peak in the heat flow data at a
temperature near 440 K corresponds to previous reports of
temperatures where Cu3Sn was observed by different means
to begin to grow in Cu/Sn diffusion couples.1–6
X-ray diffraction analysis indicated that the DSC signals
observed in Figs. 1–3 correspond to the growth of Cu3Sn.
After these anneals, all peaks observed in x-ray diffraction
scans were indexed to the equilibrium Cu3Sn phase. In an
effort to determine whether the low-temperature shoulder on
the observed peaks in DSC traces ͑cf. Figs. 1 and 2͒ was also
associated with the growth of Cu3Sn, different composites
were heated once to a temperature of 425 K and cooled to
Cu/Sn multilayer thin-film samples were prepared by
thermal evaporation in an evacuated chamber with a base
pressure of 2ϫ10Ϫ7 Torr prior to evaporation. Alternating
layers of Cu and Sn were deposited on a glass slide located
approximately 20 cm above the molten evaporants. The glass
slide was located on a water, or liquid-nitrogen, cooled sub-
strate holder. Deposition rates ͑approximately 0.8 nm/s͒ were
monitored using a quartz crystal rate monitor. The thin-film
composites were fabricated with an average stoichiometry of
Cu3Sn, and a Cu/Sn bilayer thickness of either 29, 42, or 92
nm. ͑Subsequent references to the bilayer thickness of a
composite refer to the Cu/Sn bilayer thicknesses, even after
transformation to a Cu/Cu6Sn5 couple͒. After deposition, the
composites were held at room temperature for at least 24 h to
allow the formation of Cu6Sn5.5 X-ray diffraction analysis,
using Cu K␣ radiation in a standard ␪-2␪ geometry, was
performed to ensure that all the Sn layers had been con-
sumed in the formation of Cu6Sn5. The samples were then
hermetically sealed in Al pans in a He atmosphere at a pres-
sure of 10 Pa. Each Cu/Cu6Sn5 composite sample was heated
in a differential scanning calorimeter ͑a Perkin-Elmer DSC-2
interfaced to a microcomputer͒. The samples were heated at
a constant rate of 20 K/min from a temperature of 290 K to
the desired isotherm temperature. For a full heat, the sample
was heated at a constant rate to a temperature between 550
and 600 K. All anneals were repeated three times and the
data of the third anneal were subtracted from the data of the
first for data analysis. The reacted Cu/Sn composites were
characterized by x-ray diffraction analysis.
FIG. 1. The heat flow as a function of temperature for a constant scan rate
of 20 K/min, measured by means of differential scanning calorimetry. The
sample was a multilayered, Cu/Cu6Sn5 complete with a 92 nm bilayer thick-
ness and an average stoichiometry of Cu3Sn produced by evaporation of the
two metals and room-temperature annealing.
Appl. Phys. Lett. 67 (19), 6 November 1995
0003-6951/95/67(19)/2795/3/$6.00
© 1995 American Institute of Physics
2795
Downloaded 13 Nov 2012 to 132.210.244.226. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

where ⌬Hr is the enthalpy per mole of atoms for the reac-
tion. Integration between temperatures of 360 and 600 K of
the observed heat flow signals during full heats resulted in an
average value for ⌬HrϭϪ4.3Ϯ0.3 kJ/mol. Using previ-
ous observations13 of the heats of formation from Cu and Sn
of bulk Cu3Sn and Cu6Sn5, an estimated value, ⌬Hr,est , can
be calculated. These heats of formation were ⌬Hf(Cu3Sn͒
ϭϪ7.7Ϯ0.5 kJ/mol
and
⌬Hf(Cu6Sn5)ϭϪ5.8
Ϯ0.6 kJ/mol, respectively.13 The estimate yields: ⌬Hr,est
ϭ⌬Hf(Cu3Sn)Ϫ0.55⌬Hf(Cu6Sn5)ϭϪ4.5Ϯ0.8 kJ/mol,
in good agreement with our measured value.
An observed variation with heating rate of the heat flow
curves indicates that the growth of the Cu3Sn phase in these
samples is not dictated by simple one-dimensional, diffusion-
limited growth kinetics6–8,12 until later times in the reactions.
In Fig. 2 the heat flow as a function of temperature is dis-
played for 42 nm bilayer, Cu/Cu6Sn5 composites, each
heated at a different rate. For one-dimensional growth in a
planar geometry the heat flow, dH/dt, is directly propor-
tional to the rate of growth, dx/dt, of the Cu3Sn layer thick-
ness:
dH
dt
⌬Hr dx
´
ϭA␳
,
͑2͒
M
dt
FIG. 2. The heat flow as a function of temperature for different scan rates,
measured by means of differential scanning calorimetry. The samples were
42 nm bilayers prepared similarly to those in Fig. 1. Four different scans are
presented, corresponding to heating rates of 10, 20, 40, and 80 K/min. The
maximum signal amplitude occurred at higher temperatures for higher scan
rates. The results of a simulation of such DSC data are provided for similar
samples and heating rates in the inset figure. The simulations assume
diffusion-limited growth and use the reaction constants of Ref. 2.
where A is the interfacial area in the Cu/Cu6Sn5 composite,
and ␳ and M are, respectively, the density and molar mass of
the growing phase.9 For diffusion-limited growth
dx k2
ϭ
,
͑3͒
dt 2x
where x is the thickness of the growing layer, and k2 is a
room temperature. X-ray diffraction analysis of these
samples revealed Bragg peaks corresponding to Cu or to
Cu6Sn5, with new Bragg peaks indexed to the Cu3Sn phase.
It was concluded that the heat flow data of Fig. 1, as well as
those of numerous other calorimetry scans of Cu/Cu6Sn5
composites over this temperature range, correspond to the
growth of Cu3Sn. We consider the reaction forming Cu3Sn
from Cu and Cu6Sn5:
reaction constant. At a given temperature, k2 can be related
˜
to the averaged interdiffusion coefficient, D:
2
˜
k ϭ2G␤⌬C•D,
͑4͒
where ⌬C is the concentration difference across the growing
intermetallic layer, and G␤ is related to the concentration
change across the interfaces.6–10 Previous simulations12 and
experimental results8–10 of full heat DSC data for diffusion-
limited growth in planar layers have shown ͓cf. Eq. ͑3͔͒ that
at a given temperature, dH/dt is greater for samples heated
at greater rates. For diffusion-limited growth ͓Eq. ͑3͔͒ these
DSC traces should never cross each other, in contrast to the
data of Fig. 2. This is illustrated for diffusion-limited growth
in the inset of Fig. 2, where DSC traces are simulated for the
sample and heating rates of Fig. 2 using a previously deter-
mined form for the reaction constant.2
1
1
Cu Sn ϩ9Cu͒ϭ Cu Sn͒, ⌬Hr ,
͑1͒
͑
͑
6
5
3
20
4
If nucleation of the Cu3Sn phase, and its lateral growth,
has not resulted in an essentially planar layer of Cu3Sn
across the entire Cu/Cu6Sn5 interface, deviations from the
one-dimensional, diffusion-limited growth kinetics of Eq. ͑3͒
would be expected.7–12 In Fig. 3 the heat flow for an iso-
therm at 430 K is displayed. A vertical line has been added to
the plot to indicate the beginning of the isotherm. The mag-
nitude of the heat flow continued to increase for a significant
length of time during the isotherm for this sample, and nu-
merous other samples. This is clearly inconsistent with Eq.
͑4͒, where k2 is constant at a given temperature, while
x is increasing. This observation is also inconsistent with
FIG. 3. The heat flow as a function of temperature for a constant scan rate
of 20 K/min to a temperature of 430 K, followed by an isothermal anneal,
measured by means of differential scanning calorimetry. The sample was
similar to that of Fig. 1.
2796
Appl. Phys. Lett., Vol. 67, No. 19, 6 November 1995
Dreyer et al.
Downloaded 13 Nov 2012 to 132.210.244.226. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

slope determined from a linear fit to our data, the activation
energy was found to be 0.87 eV/atom. Also plotted in Fig. 4
are values of k2 determined from previous, classical meas-
urements2 of interdiffusion in bulk Cu/Cu6Sn5 diffusion
couples at higher temperatures. Good agreement is found
between these two different measurements; the activation en-
ergy determined from Onishi’s work2 was 0.82 eV/atom. A
linear fit to the combined data sets for the bulk Cu/Cu6Sn5
diffusion couples and for our thin-film Cu/Cu6Sn5 diffusion
couples is included in Fig. 4, which has a slope correspond-
ing to an activation energy of 0.79 eV/atom. We can also
compare these results to a previous measurement5 of the ac-
tivation energy of 0.99 eV/atom made by Rutherford back-
˜
scattering on thin films. We calculate values of D from our
measured values of k2 using the equilibrium values1 of ⌬C,
2
˜
and G␤ . We find from our DSC data D(cm /s)
ϭD0 exp(ϪE/kbT), where D0ϭ3.2ϫ10Ϫ2 cm2/s, and
Eϭ0.87 eV/atom, consistent with previous results.1–3
Differential scanning calorimetry studies of interdiffu-
sion in Cu/Cu6Sn5 diffusion couples have shown that the
driving force for interdiffusion is similar for both thin-film
and bulk diffusion couples. We have seen that the nucleation
of Cu3Sn begins at temperatures near 360 K, but that the
nucleation and initial growth of Cu3Sn is not a well-defined
Arrhenius process in these diffusion couples. At later times
we observed diffusion-limited growth of Cu3Sn at rates con-
sistent with those observed previously in classical diffusion
experiments.
FIG. 4. A plot of the logarithm of the reaction parameter, k2, vs the inverse
temperature multiplied times one thousand. ͑᭺͒ Values of k2 were calculated
from plots of the square of the integrated heat flow vs temperature. The heat
flow was measured by differential scanning calorimetry for 92 nm bilayer
composites similar to those of Fig. 1. The data of Fig. 3 were analyzed, as
were that of similar samples annealed at lower temperatures. ͑᭝͒ Values of
k2 obtained from Ref. 2, a previous study of diffusion in bulk samples.
We gratefully acknowledge discussions with James
Clum and the support of the National Science Foundation,
DMR-9202595, and DUE-9452604.
interface controlled growth kinetics,7 in which case the rate
would be constant as a function of time. However, these
observations are consistent with a continuing formation of
the Cu3Sn phase along the Cu/Cu6Sn5 interface.
1 Z. Mei, A. J. Sunwoo, and J. W. Morris, Metall. Trans. A 23, 857 ͑1992͒.
2 M. Onishi and H. Fujibuchi, Trans. JIM 16, 539 ͑1975͒.
3 P. T. Vianco, K. L. Erickson, and P. L. Hopkins, J. Electron. Mater. 23, 721
͑1994͒.
At long times during isothermal anneals the heat flow is
observed to decrease with time, consistent with diffusion-
limited growth kinetics.7–12 Integration of Eq. ͑3͒ with re-
spect to time indicates that x2ϭk2t. Using Eq. ͑2͒, we deter-
mine values of x2 from our measurement of the square of the
integrated heat flow, H2. Plots of H2 as a function of time
reveal straight line region corresponding to the portion of the
DSC curves where the magnitude of the heat flow signal is
decreasing ͑e.g., Fig. 3͒. From linear fits to these regions we
determined values of k2. In Fig. 4 we plot the logarithm of
values of k2 as a function of inverse temperature. Over the
temperature range of our data we observe a linear variation
of the logarithm of k2 with inverse temperature, consistent
with thermally activated ͑Arrhenius͒ interdiffusion. From the
4 A. K. Bandyopadhyay and S. K. Sen, J. Appl. Phys. 67, 3681 ͑1990͒.
5 K. N. Tu and R. D. Thompson, Acta Metall. 30, 947 ͑1982͒.
6 The Mechanics of Solder Alloy Interconnects, edited by D. R. Frear, S. N.
Burchett, H. S. Morgan, and J. H. Lau ͑Van Nostrand Reinhold, New
York, 1994͒.
7
¨
U. Gosele and K. N. Tu, J. Appl. Phys. 53, 3252 ͑1982͒.
8 D. Grosman and E. J. Cotts, Phys. Rev. B 48, 5579 ͑1993͒.
9 E. J. Cotts, in Thermal Analysis in Metallurgy, edited by R. D. Shull and
A. Joshi ͑Minerals, Metals, and Mining Society, Warrendale, PA, 1992͒,
pp. 299–328.
10 B. E. White, M. E. Patt, and E. J. Cotts, Phys. Rev. B 42, 11017 ͑1990͒.
11 E. Ma, L. A. Clevenger, and C. V. Thompson, J. Mater. Sci. 7, 1350
͑1992͒.
12 R. J. Highmore, R. E. Somekh, J. E. Evetts, and A. L. Greer, J. Less-
Common Met. 140, 353 ͑1988͒.
13 O. Kubaschewski and J. A. Catterall, Thermochemical Data of Alloys
͑Pergammon, New York, 1956͒.
Appl. Phys. Lett., Vol. 67, No. 19, 6 November 1995
Dreyer et al.
2797
Downloaded 13 Nov 2012 to 132.210.244.226. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions