Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
G389
0013-4651/2001/148共7兲/G389/9/$7.00 © The Electrochemical Society, Inc.
Surface Chemistry Studies of Copper Chemical Mechanical
Planarization
J. Hernandez,a P. Wrschka,b and G. S. Oehrleinc,*, z
Department of Physics, University at Albany, State University of New York, Albany, New York 12222, USA
Surface chemistry studies of the chemical mechanical planarization 共CMP兲 of copper are presented in this paper. Blanket copper
samples were polished with an acidic alumina-based slurry which contains an organic acid salt 共phthalic acid salt兲 and an oxidizer
(H2O2兲. Surface studies using X-ray photoelectron spectroscopy 共XPS兲 were performed on copper samples after chemical etching
or CMP in order to determine the effect that different polishing parameters 共i.e., pH and oxidizer concentration兲 have on the copper
surface. XPS studies were also done on samples that were passively soaked in an acidic slurry mixture containing different
concentrations of H2O2 to determine how the chemical action alone affects the removal of copper. The etching results revealed that
a cuprous oxide (Cu2O) forms on the surface of etched metal while polished samples showed CuO and Cu共OH兲2. The effect of
these copper oxide films on the removal of copper in passive etching and chemical mechanical polishing is discussed.
© 2001 The Electrochemical Society. 关DOI: 10.1149/1.1377595兴 All rights reserved.
Manuscript submitted January 21, 2000; revised manuscript received March 3, 2001. Available electronically June 8, 2001.
Copper is poised to replace aluminum as the main on-chip interconnect material in integrated circuits 共ICs兲.1 The benefits of switching to copper are the low resistivity and superior resistance to electromigration offered by this metal which translates into higher chip
speeds, lower IC failure rates, reduced power consumption and
fewer chip processing steps.2-4 Implementation of copper into ICs,
however, has been delayed in part because of the lack of understanding of the chemical mechanical polishing 共CMP兲 step, which provides the planarity necessary to build multilevel interconnect
schemes. Chemical mechanical polishing is used to pattern the copper lines by removing the overburden leftover after the deposition/
fill of the patterned interlevel dielectric in a damascene/multilevel
processing scheme. CMP removes material by rotating a metal or
dielectric coated wafer and pressing its active surface against a rotating polymer-based pad filled with abrasive slurry particles.
Chemical reagents in the slurry mixture are added to enhance the
removal rate of the particular material being polished and/or to passivate the low lying regions on the wafer. Planarity of the surface
results from high points on the sample being polished at a higher
rate than the polish rate for lower lying ones.
Although the basic principles of CMP are understood, the process remains unoptimized for copper CMP because the exact mechanism of copper removal during polishing and the necessary surface
chemical processes are still unknown. In an effort to better understand how copper is removed during CMP, initial studies were performed in the present work to examine the effects that the chemical
component alone has on the copper. Copper samples were exposed
to various slurry mixtures of different pH containing different oxidizer concentrations without mechanical action for certain exposure
times. X-ray photoelectron spectroscopy 共XPS兲 was used to determine how the different components of the slurry modified the copper surface and how these modifications affect the removal or etching of copper. Samples were also polished and changes of the metal
surface induced by the mechanical action were determined. The effects of these surface modifications on the chemical mechanical polish rates are discussed.
Experimental
Polishing tool.—The chemical mechanical polishing results described in this paper were obtained using a Cybeqd 3900 planarization tool with a wafer carrier that was modified to polish partial
copper samples 2.3 ⫻ 2.3 cm in size. The down force on the wafer
or sample is set by air pressure applied uniformly to the wafer carrier via a tube connected to an air supply source. In this study, the
pressure and the linear polishing velocity were kept constant at 19
kPa and 26 m/min, respectively.
Samples and thickness determination.—Polishing was performed
on 2.3 ⫻ 2.3 cm samples cleaved from a 200 mm diam Cu-coated
Si wafer with a 50 nm thick Ta liner. Experiments were also done on
full-uncleaved wafers. The thickness of the copper films were obtained from four-point probe measurements of the sheet resistance at
10 fixed points on the full wafer or at the center of the cleaved
samples. For the blanket selectivity studies, ex situ single wavelength (␭ ⫽ 632.8 nm) ellipsometry was used to determine the
thickness of SiO2 on Si across the center of a 150 mm wafer.
Slurry mixture.—All experiments were done using a Rodele
IC1400 k-groove pad and an experimental slurry 共hereafter referred
to as slurry A-P兲. Slurry A-P consists in its premixed form of 3.1
wt % fumed alumina particles with a median particle diam of 220
nm, and a phthalic acid salt which is commonly used in metal CMP
slurries as a buffering or complexing agent.5-7 Hydrogen peroxide
(H2O2) was added to the premixed slurry to oxidize the copper and
enhance its removal. For the oxidizer concentration experiments, the
concentration of the hydrogen peroxide was varied between 0.0 and
7.5 vol % by diluting 30 vol % H2O2 with deionized water. The pH
of the final slurry mixture 共abrasive⫹organic acid⫹hydrogen peroxide兲 was kept constant or varied by adding nitric acid (HNO3) or
ammonium hydroxide (NH4OH) in small amounts to lower or raise
the pH, respectively. Exposing the copper to very low concentrations of HNO3 should not increase the oxidation rate substantially8
and for pH values ⬍7, most of the ammonium hydroxide dissociates
9,10
as NH⫹
Hence, the
4 , and no stable copper complexes are formed.
presence of these species in the slurry should have no affect on the
copper removal rate. In all slurry mixtures, the abrasive particle
concentration was kept constant.
For all polishing runs, the slurry was continually agitated to prevent agglomeration of the solids and pumped to the polishing platen
at a flow rate of 300 mL/min.
* Electrochemical Society Active Member.
a
Present address: Speedfam-IPEC, CMP Process Engineering Department, Chandler, Arizona 85226.
Present address: Infineon Technologies, Hopewell Junction, New York 12533.
c
Present address: Department of Materials Science and Engineering, University of
Maryland, College Park, Maryland 20742.
z
E-mail: [email protected]
b
Sample preparation.—After each polishing run, i.e., as soon as
the pressure on the wafer was released, the samples were rinsed with
d
e
Cybeq Nano Technologies, Menlo Park, CA 94025.
Rodel Incorporated, Newark, DE 19713.
G390
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
deionized water, carefully removed from the sample holder and
dried with compressed N2. After drying the sample, it took about 5
min before the sample was placed into the load-lock chamber of the
surface analysis tool.
For the dipping experiments, the copper samples were prepolished and then exposed to dilute sulfuric acid 共pH 2.35兲 for 3 min to
remove any native oxides present on the surface. The samples were
then rinsed for 5 s in deionized water, exposed to the organic acid
salt-hydrogen peroxide mixture for 15 s, rinsed in deionized water
for 5 s, and dried with compressed N2 for 5 s. After drying the
sample, it took about 20 s to place it in the load-lock chamber of the
surface analysis tool.
Surface analysis.—In order to determine the chemical composition of the copper surface after dipping and CMP, samples were
placed in an ultrahigh vacuum chamber and analyzed using XPS.
The base pressure of the surface analysis chamber of the XPS system was ⬃10⫺9 Torr during the runs. Photoelectrons were excited
by using nonmonochromatized Mg K␣ X-ray irradiation 共1253.6
eV兲, and photoelectron spectra were taken at electron emission
angles of 90 and 15° with respect to 共w.r.t兲 the sample surface. The
resolution of the X-ray source is ⬃1 eV.
Results
Technological issues.—It is essential that in any CMP process,
the higher features of a sample are polished at a higher rate than the
lower features if planarization of the surface is to result. To achieve
planarization of the metal, the pad has to be rigid enough so that it
does not bend into the recessed areas of the sample and the slurry
has to be formulated such that chemical reagents in the mixture do
not etch the low features during CMP. In this work, the focus is on
the polishing and planarization performance of the slurry mixture.
The alumina-based acidic slurry used in the present study incorporating hydrogen peroxide (H2O2) as an oxidizer and a phthalic
acid salt as a pH buffer shows excellent removal and planarization
behavior. Figure 1 shows the results of polishing blanket 150 mm
diam copper-coated silicon wafers at load pressures in the range of
19-33 kPa and wafer linear velocities between 26 and 48 m/min.
High copper removal rates varying between 600 and 1100 nm/min
with oxide selectivities between 120 and 210 are observed for this
CMP process. Copper removal rates of 300 nm/min or greater are
desirable in IC manufacturing where process time and wafer
throughput are a major concern11,12 while blanket metal to oxide
selectivities greater than 50 are considered optimal.12 A high metal
to SiO2 removal selectivity is required when polishing damascene
structures to prevent thinning of the interlevel dielectric 共ILD兲 during the overpolish step. To verify that the excellent polishing results
observed on blanket wafers are also seen on patterned copper
samples, damascene structures were polished with the same slurry
mixture used above for blanket CMP. Figure 2 shows a Cu damascene structure before and after polishing. A detailed study13 of
these structures after CMP with the above slurry has shown minimal
erosion of the dielectric (SiO2兲, low etching and dishing of the Cu
lines, and no corrosion of the copper wires. However, this slurry
mixture was not effective at removing the Ta adhesion/diffusion
barrier liner between the Cu and SiO2. Figure 3 shows that a Ta
residue is still present after polishing Cu damascene structures with
slurry A-P ⫹ 7.5% H2O2. In this case, the liner removal is purely
mechanical and the damascene structures must be overpolished considerably to completely remove the Ta across the sample. The erosion of the field SiO2 is not substantial, however, because of the high
Cu to SiO2 removal rate selectivity during polishing 共see Fig. 1b兲.
Surface analysis: Cu etching and Cu CMP.—After the polishing
performance of slurry A-P was determined, copper samples were
dipped and polished in slurry mixtures containing different oxidizer
concentrations and pH, and subsequently examined using XPS. In
the dipping study, the samples were not polished or mechanically
abraded in any way either during the experiment or during the post-
Figure 1. 共a兲 Cu Removal rate and 共b兲 Cu/SiO2 removal selectivity as a
function of wafer linear velocity at three different wafer load pressures. In
these experiments, 150 mm sputtered Cu and thermal oxide wafers were
polished with an IC1400 pad and slurry A-P ⫹ 7.5% H2O2.
cleaning. The goal of this work is to determine how the slurry chemistry and process parameters modify the copper surface during CMP
and to find the effect that these modifications have on the overall
removal mechanism.
Copper etching.—Figure 4 shows the effect of hydrogen peroxide concentration in the slurry on the copper etch rate. Initially, the
Cu etch rate increases as the H2O2 concentration is increased from 0
to 2.0 vol % H2O2. However, above 2.0 vol % H2O2, the etch rate
begins to drop very rapidly and levels off to a value of about 13
nm/min for concentrations of hydrogen peroxide greater than 5.0 vol
%. These results are similar to those obtained by Zeidler et al.14 and
Wang et al.15 using a similar slurry chemistry. This etching behavior is due to an active to passive transition of the copper surface as
the oxidizer concentration in the slurry is increased. For peroxide
concentrations less than 2.0 vol %, the copper is active and the
etching is controlled by the oxidation rate of the metal surface.
Above 2.0 vol % H2O2, the copper is in its passive state and the
etching of the metal is likely controlled by ionic diffusion through a
passivation layer.16
In order to establish the chemical composition of the passivation
layer and better understand the etch rate behavior of Fig. 4, copper
samples were exposed to the individual chemical components
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
G391
Figure 4. Etch rate of copper vs. H2O2 concentration in a slurry at a pH of
3.89. The error bars are less than the size of the data marker.
Figure 2. 共a兲 Unpolished copper damascene structure; 共b兲 Cu damascene
structure that was polished at a wafer load pressure of 19 kPa and wafer
linear velocity of 26 m/min with an IC1400 pad using slurry A-P
⫹ 7.5% H2O2.
Figure 3. A copper damascene structure that was polished with slurry A-P
⫹ 7.5 vol % H2O2 showing incomplete removal of the Ta.
present in the slurry and analyzed using XPS. Surface analysis results of samples dipped in 7.5 vol % H2O2 only, A-P only and
A-P ⫹ 7.5 vol % H2O2 are shown in Fig. 5. The XPS spectra of the
‘‘as received’’ Cu surface is also shown. The Cu 2p3/2 XPS spectra
revealed that the ‘‘as received’’ copper surface 共Fig. 4a兲 consists
mainly of cupric (Cu2⫹) oxides as indicated by the shake-up satellite structure centered at a binding energy of 943 eV. The presence
of this satellite structure has been attributed to charge transfer transitions from the ligands 共O2⫺ ions for CuO兲 into the unfilled (d9)
valence level of the Cu2⫹ ion.17 For cuprous oxides (Cu1⫹), which
have a filled (d10) ground state configuration, such transition cannot
occur and no satellites are seen. A linear background subtraction and
Gaussian peak fitting of the XPS spectrum further revealed that the
cupric species present on the surface are CuO and Cu共OH兲2 as indicated by the peaks at binding energies 933.8 and 934.75 eV, respectively. These values agree very well with those given in the
literature18-22 for the binding energy of Cu metal and its oxidation
states 共see Table I兲. It is not clear whether the peak at 932.67 eV is
due to metallic copper or Cu2O because the Cu 2p3/2 binding energies and peak widths of these two species are very similar. The
broader peak shape of the cupric species as compared to the
Cu/Cu2O peak is due to coupling between unpaired electrons 共multiplet splitting兲 in the paramagnetic cupric ions.17,23
The Cu sample dipped in the A-P slurry shows only a very small
etch rate 共⬃1.7 nm/min兲. The XPS data of Fig. 5b reveals no CuO or
Cu共OH兲2 and the peak centered at 932.67 eV is due to either Cu
metal, Cu2O or both. Since there are no cupric species on the surface, the presence of Cu metal and Cu⫹ may be deduced from the
LMM Auger spectrum. The two peaks centered at binding energies
335 and 337 eV are comparable to those given in Table I for Cu0
and Cu2O. Thus, the results indicate that the chemical reagents in
the A-P slurry dissolve the initial CuO and Cu共OH兲2 layers present
on the Cu surface but not the Cu2O. The Cu2O most likely passivates the Cu and prevents or limits its dissolution.
Figure 5c shows the surface composition of the Cu sample after
dipping into 7.5 vol % H2O2 in water. A strong peak centered at
933.8 eV and a broad satellite peak centered at 944 eV indicates the
presence of a cupric layer. Fitting of the data reveals the absence of
Cu共OH兲2 on the surface indicating that the initial copper hydroxide
layer was either dissolved or converted to CuO by the peroxide.
From angle-resolved XPS studies, the absence of the Cu2O is likely
due to the thickness of the CuO layer being greater than the XPS
sampling depth of electrons 共⬃8 nm兲.24 Measurements of the Cu
etch rate 共⬍1 nm/min兲 indicate little dissolution of the Cu metal in
7.5 vol % H2O2.
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
G392
Figure 5. XPS Cu 2p3/2 and Auger LMM spectra of copper samples: 共a兲 as received Cu, 共b兲 Cu dipped in slurry A-P only, 共c兲 Cu dipped in 7.5 vol % H2O2
only, 共d兲 Cu dipped in slurry A-P ⫹ 7.5 vol % H2O2. XPS measurements taken at 90° w.r.t. the copper surface.
Figure 5d shows the combined effect of the premixed slurry
chemistry and the oxidizer on the Cu surface. The symmetry of the
peak at 932.67 eV and the lack of satellite features in the XPS
spectrum indicate that no cupric species are present. The Auger
spectrum reveals that a thin Cu2O exists on the Cu surface. Although
these results are similar to what was found above for the A-P slurry,
it differs in that the etch rate of copper is much higher 共⬃13 nm/
min兲 for this slurry mixture.
The results of the above XPS studies are summarized in Fig. 6
and explained below. When the as received sample is exposed to the
A-P slurry containing the organic acid salt only, the copper oxide
and copper hydroxide films are dissolved and possibly complexed
by the phthalate anions (R-COO⫺) to form Cu共II兲 phthalate in solution. No etching of the metal occurs because there is no oxidation
of the Cu or Cu1⫹ to the Cu2⫹ state and the Cu2O is not dissolved at
the pH 共⬃4兲 of the A-P slurry. If the as received sample is exposed
to an oxidizer only 共7.5 vol % H2O2兲, the copper hydroxide film
initially present on the surface is either converted to CuO or goes
into solution because of the low pH of the H2O225 and the Cu/Cu2O
is oxidized to CuO. Since the hydrogen peroxide has a pH ⬃4, the
CuO is not dissolved25 and a thick CuO layer is formed. There is no
etching of the copper because the thick CuO passivates the metal
and prevents further oxidation.
For the case when both slurry components 共phthalic acid salt and
hydrogen peroxide兲 are mixed in solution, the copper is etched be-
cause there is continuous oxidation of Cu and Cu2O to the Cu2⫹
state and subsequent complexation of these cations by the phthalate
ions.
The conclusion that Cu2⫹ or a cupric complex is an etching
reaction product was inferred from changes in the color of the etching solution during the oxidizer concentration experiments of Fig. 1.
At low percent of H2O2 in the slurry, it was noted that the color of
the etching solution went from being initially colorless to having a
light blue hue during the experiment while at higher oxidizer concentrations no visible changes from the initial conditions were observed. The light blue color denotes the presence of Cu2⫹ ions in the
etching solution.25
The effects of varying the oxidizer concentration in the slurry on
Cu2O formation are shown in Fig. 7, where the intensity of Cu2O
relative to the Cu metal is plotted with the error bars indicating the
variability in the data. The Cu samples used in this experiment were
exposed to the same range of hydrogen peroxide concentrations in
Table I. Values of the Cu 2p3Õ2 binding energies and Auger LMM
transition energies for Cu metal, CuO, Cu„OH…2, and Cu2O
taken from literature.19-22
Oxidation
state
Metallic copper Cu0
Cu2O
CuO
Cu共OH兲2
Cu 2p3/2
Binding energy
共eV兲
Cu 2p3/2
Peak widths
共eV兲
Auger
LMM energy
共eV兲
932.67
932.6
933.7
934.9
1.1
1.2
-
334.95
336.80
335.70
337.0
Figure 6. Schematic summarizing the surface analysis results of Fig. 5.
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
G393
Figure 7. Ratio of the Cu2O to Cu metal XPS intensity for samples that
were dipped in slurry mixtures with a constant pH 共⬃4兲 containing different
H2O2 concentrations. XPS measurements taken at 15° w.r.t. the copper surface.
the slurry as in the experiment of Fig. 4. Since contributions from
cupric species were absent when the sample was exposed to an
oxidizer-organic acid salt mixture, the relative contributions of
Cu2O and metallic copper to the spectrum can be determined from
the Cu共L3M45M45兲 Auger spectra. Peak height was used as measure
of spectral intensity since no major changes in peak shapes or linewidths should occur with changes in overlayer (Cu2O)
thickness.20,26 Figure 7 shows that the intensity of Cu2O relative to
the Cu metal increases linearly with percent of H2O2 at first and
starts to saturate at peroxide concentrations of 5.0 vol %. These
surface analysis results are consistent with the etch rate data of Fig.
4. The oxide passivates the metal surface and drastically reduces the
metal etch rate at high oxidizer concentrations.
The above study was done at a constant pH of 4. In order to
observe the effects of H⫹ concentration in the slurry on the removal
of copper, the etch rate of copper was measured as a function of pH
at two different percentages of H2O2 concentrations. Figure 8 shows
that the Cu etch rates at 2.0 and 7.5 vol % H2O2 in slurry decrease
with increasing pH. At 2.0 vol % H2O2, the drop in the etch rate
with increasing pH can be explained by the increase in the amount
of Cu2O present on the Cu surface, as shown by the Auger
Figure 8. Etch rate vs. pH at 2.0 and 7.5 vol % H2O2 slurry A-P.
Figure 9. XPS spectra of copper samples that were dipped in slurry mixtures with a constant H2O2 concentration 共2.0 vol % in slurry A-P兲 at various
pH values 共a兲 Auger L3M45M45 spectra, 共b兲 Cu 2p spectra. XPS measurements taken at 15° w.r.t. the copper surface.
(L3M45M45) data of Fig. 9a. That there is a greater amount of Cu2O
on the surface at higher pH values is deduced from the growth of the
Cu2O Auger peak 共binding energy ⬃337 eV兲 with increasing pH.
The greater amount of cuprous oxide protects or passivates the metal
surface and reduces the oxidation rate of the metal at higher pH
values. Unlike the results of Fig. 7 where the spectral heights of the
Auger spectra where used to determine the amount of Cu2O present
on the surface, for the pH experiments this was not possible since
cupric species were also detected on the copper surface at pH of 5.0.
The presence of these cupric species is indicated by the shoulder at
934.7 eV and the broad satellite peaks seen on the Cu 2p spectrum
of Fig. 9b. A similar but less dramatic scenario is seen when the
copper is exposed to a slurry mixture containing 7.5 vol % H2O2,
where there is very little change in both the etch rate and in the
amount of Cu2O present on the surface 共see Fig. 10a兲 with varying
pH. At a pH of 5.0, the Cu 2p spectrum of Fig. 10b indicates the
presence of cupric oxide/hydroxide species and thus it is unclear
what percentage of Cu2O is present on the metal surface at this pH
value since it is not possible to distinguish between Cu2O and
Cu共OH兲2 in the Auger spectrum.
The difference in etch rates between the samples exposed to the
two oxidizer concentrations is most likely due to a thicker and more
uniform Cu2O layer present on the surface when copper is exposed
to the slurry mixture containing 7.5 vol % H2O2. It is not clear why
the etch rates at pH of 5.0 are similar for both oxidizer concentrations, but the presence of CuO and Cu共OH兲2 at this slurry pH in both
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Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
Figure 12. XPS Cu 2p3/2 spectra for samples polished with different slurry
A-P mixtures of pH ⬃ 3.9 containing 共vol %兲 共a兲 H2O2 7.5; 共b兲 H2O2 2.0;
and 共c兲 H2O2 0.5. The samples were polished at a pressure of 21 kPa and
linear velocity of 26 m/min.
Figure 10. XPS spectra of copper samples that were dipped in slurry mixtures with a constant H2O2 concentration 共7.5 vol % in slurry A-P兲 at various
pH values 共a兲 Auger L3M45M45 spectra, 共b兲 Cu 2p spectra. XPS measurements taken at 15° w.r.t. the copper surface.
cases may provide an additional layer of protection to more effectively reduce the oxidation of the metal.
Figure 11. Removal rate of copper vs. H2O2 concentration in slurry at a pH
of 3.9. Samples were polished at a pressure of 21 kPa and linear velocity of
26 m/min for 30 s.
Copper CMP.—The Cu etching study showed that the removal of
copper is controlled by the presence of a Cu2O film which becomes
passivating at high oxidizer concentrations and high pH. The question is if for CMP conditions, the removal of copper is controlled by
an abrasion repassivation process whereby the Cu2O passive film is
continually abraded by the mechanical action allowing the oxidefree surface to quickly oxidize or repassivate.
The effect of the mechanical action on the copper removal behavior was investigated by polishing samples at 19 kPa, 26 m/min
for 30 s with the same oxidizer-organic acid salt slurry mixtures
used in the etching experiments. Figure 11 shows the results of this
study. At low peroxide concentrations, the removal rate exhibits a
linear behavior with percent of H2O2 ; at higher concentrations, the
rate becomes less dependent on oxidizer concentration in the slurry.
The weak variation of the Cu removal rate at high oxidizer concentrations might be caused by the rate at which the passivation layer
on the metal surface is removed and reformed, and this idea was
therefore examined by XPS.
In order to determine the surface modifications induced by the
mechanical action, the copper surface was analyzed with XPS after
CMP. Figure 12 shows the XPS spectra at 90° w.r.t. the surface for
three samples that were polished using three different concentrations
of H2O2 共0.5, 2.0, and 7.5 vol %兲 in the slurry. In contrast with the
results of the etching study, the samples that were polished showed
large traces of cupric species on the surface as indicated by the
presence of an intense shake-up satellite structure centered at a binding energy of 944 eV. It is not clear whether Cu2O is also present
since it is not possible to distinguish between cupric and cuprous
oxide in the Auger spectra when both of these are present and between metallic copper and Cu2O in the Cu 2p3/2 spectra. Peak fitting
of the Cu 2p3/2 spectra with multiple Gaussian functions revealed
that both CuO and Cu共OH兲2 are present, with CuO existing only at
high oxidizer concentrations. The XPS intensitiesf of the cupric
films as a function of H2O2 concentration in the slurry and the peak
f
A linear function was used to remove the background of inelastically scattered
electrons from the Cu 2p3/2 spectra. Only the intensity of the CuO and Cu共OH兲2 in the
primary peak were used in the plot of Fig. 13 and these intensities were normalized to
the total intensity of the primary peak. The intensities of the CuO and Cu共OH兲2 in the
satellite structure were not used.
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
Figure 13. Normalized XPS intensities as a function of H2O2 concentration
for Cu samples after CMP. The squared and circle points show the intensity
of CuO and Cu共OH兲2, respectively.
fitting parameters are shown in Fig. 13 and Table II, respectively.
The error in the intensities is similar to what was shown for the
static etching study. The presence of cupric species on these samples
appears to be induced by the polishing process itself since no cupric
films were observed in the etching experiments that employed the
same oxidizer-organic acid slurry mixture. The cupric film formation may be the result of enhanced oxidation due to the temperature
increases 共⬃5°C at 19 kPa and 26 m/min兲 that the sample experiences during polishing or the formation of defects within the copper
creating regions of high stress 共high energy兲. The high energy state
of the stressed metal, which may exhibit a high intensity of broken
bonds, lowers the energy barrier to oxidation.27 If the concentration
of cupric ions exceeds the saturation limit, the cations might react
with water molecules or other dissolved species in the slurry to form
solid cupric oxide species on the metal surface.28 The Cu removal
behavior of Fig. 11 might be influenced by whether or not a CuO
layer is present on the metal surface during polishing. The possible
effect of these cupric films on the copper removal rate is discussed
in the Discussion section on Cu CMP.
The above study was done at a constant pH. Decreasing the pH
should increase the solubility of the cupric films on the metal
surface.30 Figure 14 shows how the removal rate varies with pH at
oxidizer concentrations of 2.0 and 7.5 vol %. The removal rate of
copper at 7.5 vol % H2O2 drops with increasing pH while it increases with pH at 2.0 vol % H2O2. Figure 15a shows that for the
2.0 vol % H2O2 slurry mixture, there is no CuO present on the
copper surface at low pH values while there is some cupric oxide at
a pH of 5. In contrast, for the 7.5 vol % H2O2 mixture 共Fig. 15b兲,
CuO is present at all pH values and the amount comparable to that
of Cu共OH兲2. The role of these cupric films in the polishing of copper
is discussed below.
Table II. Cu 2p3Õ2 spectra best fit values for the binding energies
and peak widths for Cu metal, CuO, Cu„OH…2, and Cu2O.
Oxidation
state
Metallic copper Cu0
Cu2O
CuO
Cu共OH兲2
a
Cu 2p3/2
Binding energy 共eV兲
Cu 2p3/2
Peak widths 共eV兲
932.67
932.67
933.8
934.75
1.2
1.2
2.6a
2.6a
Best fit value obtained from a Cu sample with a thick CuO overlayer
共⬎XPS sampling depth ⬃8 nm兲.
G395
Figure 14. Removal rate vs. pH at 2.0 and 7.5 vol % H2O2 in slurry A-P.
Discussion
In this section, the role that the surface reaction layers on the
metal surface have on the etching and polishing of copper is discussed.
Cu etching.—The reactions that take place at the metal/liquid
interface increase the oxidation state of the copper leading to the
production of copper ions which are then complexed by the phthalate anions in the slurry mixture. The most likely reactions describing this process are25,29
Cu共s兲 → Cu2⫹ ⫹ 2e⫺
关1兴
Cu共s兲 → Cu⫹ ⫹ 1e⫺
⫹
关2兴
⫹
2Cu ⫹ H2O → Cu2O ⫹ 2H
2⫹
Cu
⫺
⫹ R-COO → Cu共II兲 phthalate
关3兴
关4兴
It should be noted that Reactions 1 and 2 are half-reactions and
require a corresponding reduction reaction to act as an electron sink.
Oxidizing agents such as hydrogen peroxide or dissolved oxygen are
usually present in the slurry mixture to act as electron acceptors.
When the concentration of H2O2 in the slurry is varied, Fig. 4 indicates that Cu etches more rapidly at low oxidizer concentrations.
Although the results of Fig. 7 show that Cu2O is present on the
surface when the concentration of hydrogen peroxide in the slurry is
low, the XPS measurements do not allow us to answer the question
if the cuprous overlayer is continuous. It is likely that the formation
of Cu2O layers at low H2O2 concentration is incomplete, with the
copper having only patches 共islands兲 of Cu2O on its surface which
offer no protection against oxidation. Hence, at low oxidizer concentrations, the metal is in its active state and the etching of metal is
limited by the rate of the oxidation reaction, which is a function of
peroxide concentration in the slurry.
At higher concentrations 共⬎2.0 vol %兲, however, a uniform
Cu2O layer is likely present that limits the oxidation reaction and
restrains the etching of the copper. As the oxidizer concentration is
increased, the thickness of the cuprous overlayer increases and the
oxidation reaction is further inhibited. The thick cuprous layer acts
as a boundary layer preventing chemical reactants or products 共i.e.,
copper ions兲 from diffusing to or away from the metal/solution interface. Since the dissolution of Cu2O increases with increasing
H⫹ concentration,25 the higher etch rate at lower pH values for 2.0
vol % H2O2 seen in Fig. 8 is due to the greater amount of reactive
sites that are available for reaction. At 7.5 vol % H2O2, it is the
thinning of the Cu2O layer that leads to slightly higher ionic diffu-
G396
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
Figure 16. Schematic summarizing the Cu etching results and mechanism of
Cu removal.
Figure 15. Normalized XPS intensities as a function of pH for Cu samples
after CMP with H2O2 共a兲 2.0 共vol %兲 and 共b兲 7.5 共vol %兲 in slurry. The
squared and circle points show the intensity of CuO and Cu共OH兲2, respectively.
sion rates and therefore higher etch rates. The schematic in Fig. 16
summarizes the surface mechanisms that are consistent with the dissolution rates and surface analysis results obtained for static etching
of Cu in this slurry chemistry.
Cu CMP.—When the Cu sample is polished a different scenario
is present 共see Fig. 17兲. An abundance of cupric species are formed
which are either dissolved and complexed in solution and/or react
with dissolved species in the slurry to form a solid cupric layer on
the metal surface. At low concentrations of H2O2, the concentration
of cupric ions produced as a result of the applied stress and the
oxidizing nature of the slurry is high but most of the Cu2⫹ ions are
complexed by the phthalate anions (R-COO⫺). Because of the
abundance of cupric ions produced during polishing, not all of the
cupric ions are complexed and some react with dissolved species or
water molecules in the slurry to form a copper hydroxide layer.
However, this film is either porous or incomplete and does not act as
an effective barrier to the diffusion of chemical reactants to or cupric species away from the copper surface. Hence, the removal rate
of copper is limited primarily by the oxidation rate of the metal,
which is dependent on oxidizer concentration.
At high peroxide concentrations, the cupric ion concentration is
much higher than at low oxidizer concentrations while the initial
R-COO⫺ concentration remains the same. In this case, an overabundance of Cu2⫹ ions during polishing leads to more cupric species
being produced than can be complexed or swept away by the flowing slurry. This results in excess Cu2⫹ ions, leading to the formation
of solid Cu共OH兲2 and CuO layers on the metal surface. At a pH of
3.9, the CuO layer is less soluble than the Cu共OH兲2 and its presence
on the copper surface during CMP becomes a limiting factor for the
removal of copper at high oxidizer concentrations.
Varying the value of the slurry pH has the effect of changing the
solubility of any cupric films that are present on the metal surface.
At low pH values the cupric films dissolve more easily, with CuO
being less soluble than Cu共OH兲2. Changing the pH of the slurry also
changes the complexation ability of the phthalic acid. Kummert
et al.30 have shown that in the pH range of 3-5, the dissociation of
phthalic acid increases with pH. The greater concentration of
phthlate anions in the slurry at higher pH values would allow for
greater complexation of the Cu2⫹ ions, increasing the solubility of
the cupric films in solution. This result implies that the removal of
Cu might not be limited exclusively by the presence of boundary
layers; the complexation ability of the organic acid in the slurry may
play a more critical role in certain circumstances. The removal rate
behavior of Fig. 14 can be explained by these pH induced effects. At
low oxidizer concentration, the XPS results show that the primary
species present during Cu CMP is Cu共OH兲2. Although the dissolution of the Cu共OH兲2 is high in the pH region examined, the phthalic
acid dissociation decreases with decreasing pH, resulting in less
phthalate anions present in solution at low pH values. Hence, the
removal rate at 2.0 vol % H2O2 in slurry drops with decreasing pH
because the complexation of the cupric species decreases. The Cu
removal rate as a function of pH at low oxidizer concentrations is
thus limited by the formation of cupric complexes. At higher oxidizer concentrations, a CuO film which is less soluble than the
Cu共OH兲2 is also present on the metal surface during polishing. In
this case, the removal rate drops with increasing pH because of the
decrease in CuO solubility as the H⫹ ion concentration in the slurry
decreases at higher pH values. Thus, at high oxidizer concentrations
and varying pH, the removal rate is likely limited by the solubility
of the CuO film that forms on the metal surface during CMP.
A summary of the limiting steps in Cu CMP as discussed above
is shown in Fig. 17. The removal of copper during CMP appears to
be a combination of two processes: 共i兲 mechanically enhanced oxidation of the metal and 共ii兲 dissolution of the oxidation reaction
products. For constant mechanical action, the oxidation reaction is
limited by the oxidizer concentration while the solubility of oxidation products 共cupric species兲 is limited by the pH of the slurry
mixture. This removal mechanism is much different than that observed for other metals polished with the same slurry chemistry used
Journal of The Electrochemical Society, 148 共7兲 G389-G397 共2001兲
G397
Figure 17. Schematic summarizing the Cu removal process and its rate limiting steps after chemical mechanical polishing with an acidic alumina-based slurry
containing a phthalic acid salt and hydrogen peroxide.
here. Wrschka et al.,31 for example, have shown that for aluminum
CMP, an Al2O3 passivation layer is present on the metal surface
during polishing. Since the slurry chemistry does not dissolve the
aluminum oxide and the film quickly reforms on the highly reactive
aluminum surface, the removal mechanism is determined by the
growth and abrasion of the Al2O3 layer. In Cu CMP, the removal of
copper is determined by the oxidation of the Cu metal and by the
solubility of a cupric film.
Conclusions
The present work has shown that the removal of copper during
etching or chemical mechanical polishing is affected by the presence
of copper oxides on the surface. Surface analysis revealed the presence of Cu2O on the copper surface after dipping Cu samples in a
slurry containing hydrogen peroxide and a phthalic acid salt. The
thickness of the Cu2O film increased with increasing oxidizer concentration reducing the etch rate of the copper at sufficiently high
concentrations. For polishing of copper, surface analysis revealed
the presence of CuO and Cu共OH兲2 on the metal surface. The amount
of CuO on the surface increased with increasing hydrogen peroxide
concentration in the slurry limiting the removal of Cu at high oxidizer concentrations. When the pH of the slurry mixture was varied,
the polish rate decreased with increasing pH at high oxidizer concentrations and increased with increasing pH at low oxidizer concentrations. This difference in polishing behavior with hydrogen
peroxide concentration has been attributed to the presence of a CuO
film which is present at 7.5 vol % H2O2 but not at 2.0 vol % H2O2.
The removal of copper during CMP for the particular slurry used in
this study appears to be determined by the oxidation rate of the Cu
metal and by the solubility of a cupric film.
Acknowledgments
The authors would like to thank Rodel, Inc., for providing the
pads for this work. A special gratitude goes to Xiang Wang for his
help in running the CMP tool and to Theo Standaert for his stimulating discussions and for helping the authors decipher the XPS data
seen in this paper.
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