Electrochemical and Solid-State Letters, 14 共3兲 D26-D29 共2011兲
D26
1099-0062/2010/14共3兲/D26/4/$28.00 © The Electrochemical Society
CVD of Pure Copper Films from Amidinate Precursor
Vladislav Krisyuk,a,b,z Lyacine Aloui,b Natalie Prud’homme,b Sergey Sysoev,a
François Senocq,b Diane Samélor,b and Constantin Vahlasb
a
Nikolaev Institute of Inorganic Chemistry, SB RAS, Novosibirsk 630090, Russia
CIRIMAT-ENSIACET, 31432 Toulouse Cedex 04, France
b
Copper共I兲 amidinate 关Cu共i-Pr-Me-AMD兲兴2 was investigated to produce copper films in conventional low pressure chemical vapor
deposition 共CVD兲 using hydrogen as reducing gas-reagent. Copper films were deposited on steel, silicon, and SiO2 /Si substrates
in the temperature range 200–350°C at a total pressure of 1333 Pa. The growth rate on steel follows the surface reaction between
atomic hydrogen and the entire precursor molecule up to 240°C. A significant increase of the growth rate at temperatures higher
than 300°C was attributed to thermal decomposition of the precursor molecule. It is shown that 关Cu共i-Pr-Me-AMD兲兴2 meets the
specifications for the metal organic chemical vapor deposition of Cu-based alloy coatings containing oxophilic elements such as
aluminum.
© 2010 The Electrochemical Society. 关DOI: 10.1149/1.3526142兴 All rights reserved.
Manuscript submitted October 2, 2010; revised manuscript received November 19, 2010. Published December 20, 2010.
The combination of high electrical and thermal conductivity,
high ductility, and antigerm properties makes copper and copper
alloy coatings promising materials for numerous applications. The
addition of alloying elements may become of great profit to tailor,
e.g., grain size, crystallographic orientation, and adhesion, and thus
functionality of Cu containing films. In particular, Al–Cu intermetallic compounds containing films present attractive properties for
manufacturing of interconnects for integrated circuits,1,2 corrosion
resistant coatings,3 and other applications. Al–Cu alloys can be approximants of ternary Al–Cu-TM 共transition metal兲 quasicrystals.4
Their processing by metal organic chemical vapor deposition
共MOCVD兲 allows functionalizing complex-in-shape surfaces, provided that suitable codeposition processes can be managed. Such
processes imply the use of compatible precursors for the involved
elements. In the case of Al–Cu films, the component metals have
different chemical properties and these properties govern the choice
of the appropriate precursors. Specifications for such precursors include oxygen- and halogen-free ligands, similar deposition conditions, and compatible gas phase and surface chemistries.
Most of the copper precursors contain oxygen and/or halogens in
their ligands5-11 or require oxygen containing coreactants for the
deposition of copper.12,13 Among the few oxygen- and halogen-free
ligands, copper共I兲 cyclopentadienyl phosphine derivatives were investigated in chemical vapor deposition 共CVD兲 processes.14-17 It
was revealed that besides direct advantages of these types of precursors, gas phase stability, morphology, and purity of the produced
films were unsatisfactory. For example, CpCuPEt3 is a thermally
fragile compound above 70°C, resulting in a narrow temperature
window between the sublimation and the deposition temperatures.17
It also yields films with nodular, often discontinuous, morphology,
which is characteristic of a Wolmer-Weber-type of growth.
Relatively air-stable copper共I兲 amidinates were successfully studied as alternative oxygen-free molecular compounds in atomic layer
deposition 共ALD兲, aiming at the preparation of pure copper
films,18-20 and in CVD of CuON films for subsequent reduction to
prepare ultrathin copper films.21 Based on these works, we recently
reported preliminary results on the CVD of copper共I兲
关Cu共i-Pr-Me-AMD兲兴2.22
The
N,N⬘-diisopropylacetamidinate
adopted process involved hydrogen as reducing gas-reagent and
showed promise for the processing of dense and pure copper films.
The present work further details the characteristics of the films and
also provides information on the gas phase behavior of the precursor
under CVD conditions. The simultaneous consideration of the gas
phase behavior of the precursor and of the characteristics of the
z
E-mail: [email protected]
films provides useful insight into the decomposition mechanisms
and thus allows monitoring the CVD process to obtain films with
targeted properties.
The MOCVD system employed for the present work has been
previously described.22 The vertical reactor was composed of a glass
tube containing an inductively heated stainless steel susceptor. A
showerhead above the susceptor ensured homogeneous flow of the
reactants on the surface of the substrates. Nitrogen was used as
carrier gas 共30 sccm兲 and dilution gas 共20 sccm兲. Hydrogen was
used as reducing gas-reagent 共50 sccm兲. 关Cu共i-Pr-Me-AMD兲兴2 was
heated in the evaporator at 95°C. The copper films were deposited
on steel substrates and in addition on Si共111兲 and on thermally oxidized silicon substrates 共140 nm SiO2兲. Depositions were performed
at different temperatures in the range 200–350°C. As a first approach to this process, pressure was maintained constant at 1333 Pa,
i.e., a technologically realistic value. The deposition time was 1 h in
all experiments.
关Cu共i-Pr-Me-AMD兲兴2 was provided by NanoMePS23 and was
purified by vacuum sublimation before use. Its vapor pressure was
measured by a flow method using He as carrier gas.24
Deposited phases were investigated by grazing incidence X-ray
diffraction with a Seifert XRD 3000 TT 共GE Sensing and Inspection
Technologies, USA兲 diffractometer using CuK␣ radiation. Elemental analysis of the films was performed with electron probe microanalysis 共EPMA兲 using a CAMECA SX50 共CAMECA, France兲
instrument. Surface morphology was investigated by scanning electron microscopy 共SEM兲 with a LEO 435 VP 共LEO Electron Microscopy Ltd, Cambridge, UK兲 and an FEG JEOL JSN6700F 共JEOL
Ltd, Japan兲 instrument. Electrical resistivity was measured in selected films deposited on silica with a SIGNATONE 共Bridge Technology, USA兲 4 points resistivity meter.
Preliminary results on vapor pressure measurement by the static
method previously reported22 revealed possible activation of precursor vapor leading to catalytic decomposition on the metallic walls of
the instrument. Such instability of the adsorbed copper共I兲 amidinate
on stainless steel and on nickel surfaces has already been reported in
the literature.25 To face this problem, the flow method was used
involving an all-quartz equipment. The results concerning the saturated vapor pressure over the solid until the melting temperature are
presented in Fig. 1. Squares correspond to mass loss of the compound in the evaporator. Dots correspond to measurement of the
amount of transferred material and were considered for data processing. Lines are linear regressions of the two sets of points.
These experiments revealed that the compound is partially decomposed during the transition into the gas phase over the entire
temperature range. In the longest-running experiments 共ca. 20 h兲, at
the lower temperature 共95°C兲 the quantity of the sublimed material
was 72% with respect to the initial amount. The yield of the second
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Electrochemical and Solid-State Letters, 14 共3兲 D26-D29 共2011兲
o
Temperature ( C)
2,0
150 145 140 135 130 125 120 115 110 105 100 95
100
10
P (Pa)
logP
1,0
0,5
0,0
-0,5
2,35
Growth rate
1,5
1
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
o
320 C
2,45
2,50
2,55
2,60
2,65
2,70
2,75
Ln(nm/h)
2
Ln(µg/cm /h)
o
350 C
o
300 C
o
280 C 260oC
o
240 C
o
220 C
o
200 C
0.0016
2,40
D27
0.0017
0.0018
0.0019
0.0020
0.0021
0.0022
-1
1/T (K )
-1
1000/T (K )
Figure 1. Results on vapor pressure measurement of 关Cu共i-Pr-Me-AMD兲兴2
by the flow method. Mass balance based on the amount of transferred material 共dots兲 and on the amount of the residue material 共squares兲. Lines: linear
regressions.
sublimation of the collected compound was 82%. The increase of
the yield of the sublimation process during the second run is attributed to the improvement of the purity of the source compound due
to the first sublimation. Vapor pressure measurements at high sublimation temperature 共140–145°C兲 were repeated twice using the
compound both as received and sublimed. The obtained values were
identical within the experimental error. Based on these results, the
Clausius—Clapeyron law for the vapor pressure dependence on
temperature was found to be
ln关P共Pa兲兴 = 36.42 − 13520.8/T共K兲
关1兴
in the temperature range 95–146°C. The enthalpy and entropy of
sublimation in this temperature range were assumed constant and
were found to be ⌬subH共具T典兲 = 112.42 ⫾ 2.18 kJ/mol and
⌬subS共具T典兲 = 206.98 ⫾ 5.47 J/共mol K兲, respectively. These results
represent the first reliable thermodynamic data for the solid—vapor
equilibrium of copper共I兲 amidinates. It appears that
关Cu共i-Pr-Me-AMD兲兴2 is relatively kinetically stable in inert conditions. From this perspective and considering its relatively long shelf
life in air, it can be a suitable precursor for CVD processes.
The maximum flow rate Qprec of 关Cu共i-Pr-Me-AMD兲兴2 in the
deposition chamber can be estimated from Eq. 2 by Hersee and
Ballingall26
Qprec = QN2,prec
Psat共Tsat兲
Preactor − Psat共Tsat兲
关2兴
where QN2,prec is the flow rate of nitrogen through the sublimator,
Psat共Tsat兲 is the saturated vapor pressure at the sublimation temperature Tsat, and Preactor is the operating pressure. Based on the results
presented in the previous section, the saturated vapor pressure of the
precursor in the adopted sublimation temperature of 95°C equals
0.735 Pa, Qprec equals ⬃1.7 ⫻ 10−2 sccm, and the molar fraction of
关Cu共i-Pr-Me-AMD兲兴2 in the input gas equals 10−4, i.e., a significantly low value. The H2/precursor ratio is high, attaining 2500.
At these conditions the obtained copper films are shiny metallic
and specular in reflection on polished substrates. XRD 共X-ray diffraction兲 and EPMA reveal that the copper films prepared in
200–350°C temperature range are composed of polycrystalline copper, with neither carbon nor nitrogen impurities. Preferred 共111兲 texture of the copper films deposited on Si共111兲 substrates was noticed,
but not on steel substrates.
The temperature dependence of the growth rate 共GR兲 was investigated for the films processed on steel substrates in the entire tem-
Figure 2. Arrhenius plot of the growth rate of Cu on steel substrates. Determination by weight gain 共triangles兲 and by EPMA 共squares兲.
perature range concerned by the present work, i.e., between 200 and
350°C. The GR was evaluated by two independent methods: directly by weight difference of the substrates after and before deposition and indirectly from thickness evaluation by EPMA using the
27
STRATAGEM software.
Five independent experiments were performed in the same conditions at 220°C, yielding a reproducible
value of the growth rate within a range of ⫾20%. The temperature
dependence of the growth rate on steel substrates is shown in Fig. 2
for both the methods. A close agreement is observed between the
two sets of data. An indirect estimation by EPMA systematically
underestimates the thickness of the films and subsequently the
growth rate. This is due to the assumption made running STRATAGEM, following which the volumic mass of the films equals that of
the bulk Cu, i.e., 8.92 g/cm3.28 Neither the porosity nor the surface
roughness is being considered in this case. Interestingly, the shift
between the values obtained from weight measurement and those
estimated by EPMA decreases with increasing deposition temperature. Based on the previous considerations, this is attributed to the
decrease of the porosity of the films.
A sharp linear increase of the GR is observed in the temperature
range up to 240°C; it refers to a surface reaction controlled regime.
The corresponding slope yields activation energy equal to ca.
65 kJ/mol. This value is remarkably close to the dissociation energy
of hydrogen on the copper surface.18 It is concluded that Cu deposition proceeds through the reduction reaction of the metal by hydrogen atoms on the surface. Dissociation of molecular hydrogen is
the rate limiting step in this process. This mechanism is coherent
with reports by Ma et al., who investigated the surface behavior of
copper共I兲 amidinate 关Cu共s-Bu-Me-AMD兲兴2 to optimize the ALD
process.25 The authors suggested that hydrogenation reactions on the
surface become apparent at pressures exceeding 10−6 Torr. In such a
case, the formation of pure copper films is expected to occur from
the reaction of hydrogen atoms with precursor molecules on the hot
surface before any decomposition of the molecule. The results reported in the present study indicate that the pure Cu films can be
obtained by a conventional CVD process controlled by such a
mechanism.
The observed stability of the GR in the temperature range between 240 and 300°C corresponds to a diffusion limited regime.
Interestingly, above 300°C the GR does not decrease as is typically
the case due to competitive phenomena such as desorption of precursor molecules from the surface or decomposition of the precursor
in the gas phase. A sharp increase of the GR is observed between
300 and 320°C, followed by a slight decrease at 350°C. A similar
behavior was also reported by Sahana et al. for the CVD of vanadium oxides, but the authors did not propose any unambiguous
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D28
Electrochemical and Solid-State Letters, 14 共3兲 D26-D29 共2011兲
1 µm
1 µm
(a)
(b)
1 µm
(c)
Figure 3. Surface SEM micrographs of
copper films prepared on steel substrates
at 共a兲 240°C, 共b兲 300°C, 共c兲 320°C, and
共d兲 350°C. Thickness of the films estimated from EPMA are 127, 166, 464, and
377 nm, respectively. Arrows point on
ridges on the substrate surface due to polishing.
1 µm
(d)
explanation.29 In the present case, this phenomenon is due to the
change of the deposition mechanism. It is assumed that thermally
activated dissociation of the precursor molecules on the surface is
initiated above 300°C which provides pure Cu at a higher yield than
the process prevailing at a lower temperature, which is limited by
the surface reaction between atomic hydrogen and the entire precursor molecule. This scheme is indirectly supported by the decomposition temperature of 关Cu共i-Pr-Me-AMD兲兴2 in nitrogen flow, which
was reported to be 300°C.19 In order to preserve self-limited reactivity in ALD processes using this precursor, the substrate temperature was recommended to be kept below this value. It would lead to
an increase in the growth-per-cycle and to a loss of saturated film
growth above 300°C in the ALD process. Unfortunately, ALD experiments in the literature refer to temperatures below 300°C.18,19 A
further increase of temperature presumably leads to significant decomposition of the molecules in the gas phase to fragments which
do not contribute to the film growth. Particular studies of the deposition in this temperature range are necessary in order to confirm the
proposed mechanisms. However, such a singular behavior can be
used to engineer the process with regard to targeted specifications.
For example, it appears that 320°C is the most appropriate temperature for a process running at a high growth rate. Indeed, a maximum
growth rate of 0.5 ␮m/h was obtained in the investigated conditions
still providing pure Cu films. This value is subjected to strong increase with increasing feeding rate of the precursor.
Figure 3 represents the SEM surface micrographs of the copper
films prepared at 240, 300, 320, and 350°C. The films present nodular smooth and uniform morphology throughout the surface of the
sample; morphology is only constrained by substrate roughness, as
shown in Fig. 3b. The grain size is approximately 0.2 ␮m for the
film in Fig. 3a 共ca. 0.1 ␮m for the films processed at 220°C兲. It
increases with increasing temperature and attains 1 ␮m for samples
processed at 320°C 共Fig. 3c兲 350°C 共Fig. 3d兲. XRD analysis in the
grazing mode revealed that the size of Cu crystallites also increases
with deposition temperature: from ⬍20 nm at 200°C to ca. 80 nm at
300°C.22 The simultaneous increase of the size of crystallites and
grains with increasing deposition temperature ultimately yields coalescence of the grains and consequently increase of the density of
the film. This is confirmed by the comparison between the results for
the growth rate obtained by the two methods, i.e., by weight gain
and EPMA measurements, as discussed in the previous paragraph. It
thus appears that the information obtained from the growth rate and
from the microstructure is coherent and indicates increasing film
density with increasing deposition temperature. A further confirmation of this trend is provided from electrical resistivity measurements. The Cu films processed on oxidized silicon for this purpose
present a looser morphology. The films deposited at 250°C with
thickness 130 nm present a resistivity of 6 ␮⍀ cm. The resistivity
of films deposited at 280°C with thickness 270 nm is 4 ␮⍀ cm, i.e.,
significantly lower. For comparison, the resistivity of the copper film
prepared by ALD from 关Cu共s-Bu-Me-AMD兲兴2 on glass substrate at
185°C reached 4.7 ␮⍀ cm at a thickness of 80 nm, corresponding
to a resistivity of 2.9 ␮⍀ cm at room temperature.20
Material and process specifications for MOCVD of Cu films depend on the targeted application. Most often they include high purity, high growth rate, low processing temperature, and probably
most importantly smooth and continuous microstructure. This latter
specification is difficult to meet due to the growth mechanism of the
Cu nucleus. The initial growth mode is Volmer—Weber 共island兲 one
as was illustrated by Liu et al.,8 which is followed by the Stranski—
Krastanov 共layer-by-layer plus island兲 mode.30 Significant efforts
have been made in order to meet the above specifications. However,
as mentioned in the introduction the investigated molecular compounds often contained oxygen in their ligands, and for this reason
they are incompatible with a targeted Al–Cu codeposition process.
The present work showed that 关Cu共i-Pr-Me-AMD兲兴2 meets the
specifications for the MOCVD of Cu-based alloy coatings containing oxophilic elements such as aluminum. Pure copper films were
obtained in the temperature range 200–350°C under a high
hydrogen/precursor molar ratio. The MOCVD process is typically
characterized by a kinetically controlled regime below 240°C, followed by diffusion limited one for depositions in the 240–300°C
range. A significant increase of the growth rate was noticed at temperatures higher than 300°C and was attributed to switching of the
predominant deposition reaction on the surface from hydrogen reduction to the thermally assisted decomposition of the precursor
molecule.
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Electrochemical and Solid-State Letters, 14 共3兲 D26-D29 共2011兲
Acknowledgments
We are indebted to Sophie Gouy and Philippe de Parseval, Observatoire Midi-Pyrénées, Toulouse for EPMA analyses. This work
was supported by the EC under contract no. NMP3-CT-2005500140, by the French Agence Nationale de la Recherche 共ANR兲
under contract no. NT05-3_41834, and by CNRS through a grant
awarded to V.K.
Centre National de la Recherche Scientifique assisted in meeting the
publication costs of this article.
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